Optical device and imaging system

Abstract

The present invention relates to an optical device and an imaging system. The optical device includes: a substrate having a first material; an absorption region having a second material different from the first material, the absorption region configured to absorb photons and generate photo-carriers, the photo-carriers including electrons and holes generated in response to the absorbed photons; a first well region surrounding the absorption region and disposed between the absorption region and the substrate, the first well region doped with a first polarity; one or more switches respectively controlled by a corresponding control signal, the one or more switches respectively configured to collect at least a portion of the photo-carriers based on the corresponding control signal and provide the portion of the photo-carriers to a corresponding readout circuit; and one or more isolation structures in the substrate, the one or more isolation structures including one or more insulator isolations, implant isolations, or heterojunction isolations.

Description

[0001] This application is a divisional application of the application with national application number 201880027480.7 (international application number PCT / US2018 / 020262, international filing date February 28, 2018, invention title "High-Speed Optical Sensing Device II").

[0002] Cross-Reference to Related Applications

[0003] This application claims the benefit of U.S. Provisional Patent Application No. 62 / 465,139, filed February 28, 2017; U.S. Provisional Patent Application No. 62 / 479,322, filed March 31, 2017; U.S. Provisional Patent Application No. 62 / 504,531, filed May 10, 2017; U.S. Provisional Patent Application No. 62 / 485,003, filed April 13, 2017; U.S. Provisional Patent Application No. 62 / 511,977, filed May 27, 2017; U.S. Provisional Patent Application No. 62 / 534,179, filed July 18, 2017; U.S. Provisional Patent Application No. 62 / 561,266, filed September 21, 2017; U.S. Provisional Patent Application No. 62 / 613,054, filed January 3, 2018; and U.S. Provisional Patent Application No. 62 / 617,317, filed January 15, 2018, all of which are hereby incorporated by reference in their entirety. Technical Field

[0004] This application relates to detecting light using a photodetector. Background Art

[0005] Light propagating in free space or an optical medium is coupled to a photodetector that converts the optical signal into an electrical signal for processing. Summary of the Invention

[0006] According to an innovative aspect of the objectives described in the present invention, light reflected by a three-dimensional object can be detected by a photodetector of an imaging system. This photodetector can convert the detected light into charge. Each photodetector may include two sets of switches for collecting charge. The charge collection process controlled by the two sets of switches can be switched sequentially, such that the imaging system can confirm the phase information of the sensed light. The imaging system can use the phase information to analyze the characteristics associated with the three-dimensional object, including depth information or material composition. The imaging system can also use the phase information to analyze the characteristics associated with: eye tracking, body posture recognition, three-dimensional object scanning / video recording, motion tracking, and / or augmented / virtual reality applications.

[0007] Generally speaking, the innovative aspects of the objectives described in the present invention can be implemented in an optical device; the optical device includes: a semiconductor substrate; a germanium-silicon layer coupled to the semiconductor substrate, the germanium-silicon layer including a photodetector region configured to absorb photons and generate photocarriers from the absorbed photons; one or more first switches controlled by a first control signal, the one or more first switches being configured to collect at least a portion of the photocarriers based on the first control signal; and one or more second switches controlled by a second control signal, the one or more second switches being configured to collect at least a portion of the photocarriers based on the second control signal, wherein the second control signal is different from the first control signal. The one or more first switches include a first p-doped region located in the germanium-silicon layer, the first p-doped region being controlled by the first control signal; and a first n-doped region included in the germanium-silicon layer, the first n-doped region being coupled to a first readout integrated circuit. The one or more second switches include a second p-doped region in the germanium-silicon layer, the second p-doped region being controlled by the second control signal; and a second n-doped region located in the germanium-silicon layer, the second n-doped region being coupled to a second readout integrated circuit.

[0008] This embodiment and other embodiments may each optionally include one or more of the following features. The germanium-silicon layer may include a third n-doped region and a fourth n-doped region, at least a portion of the first p-doped region may be formed in the third n-doped region, and at least a portion of the second p-doped region may be formed in the fourth n-doped region. The germanium-silicon layer may include a third n-doped region, at least a portion of the first p-doped region and a portion of the second p-doped region may be formed in the third n-doped region. The semiconductor substrate may include a third p-doped region and one or more n-doped regions, the germanium-silicon layer may be disposed above the third p-doped region, and the third p-doped region may be electrically shorted to the one or more n-doped regions.

[0009] The first control signal may be a fixed bias voltage, and the second control signal may be a variable bias voltage that biases the fixed bias voltage of the first control signal. The photons absorbed by the germanium-silicon layer may be reflected from the surface of a three-dimensional object; the portions of the photocarriers collected by the one or more first switches and the portions of the photocarriers collected by the one or more second switches may be used by a time-of-flight (ToF) system to analyze the depth information or material composition of the three-dimensional object.

[0010] Another innovative aspect of the object described by the present invention can be implemented in an optical device; the optical device includes: a semiconductor substrate; an absorption layer, coupled to the semiconductor substrate, the absorption layer includes a photodetector region configured to absorb photons and generate photocarriers from the absorbed photons; one or more first switches, controlled by a first control signal, the one or more first switches being configured to collect at least a portion of the photocarriers based on the first control signal; and one or more second switches, controlled by a second control signal, the one or more second switches being configured to collect at least a portion of the photocarriers based on the second control signal, the second control signal being different from the first control signal. The one or more first switches include: a first p-doped region located in the semiconductor substrate and controlled by the first control signal; and a first n-doped region located in the semiconductor substrate, the first n-doped region being coupled to a first readout integrated circuit. The one or more second switches include: a second p-doped region located in the semiconductor substrate, where the second p-doped region is controlled by the second control signal; and a second n-doped region located in the semiconductor substrate, where the second n-doped region is coupled to a second readout integrated circuit.

[0011] This embodiment and other embodiments may each optionally include one or more of the following features. The semiconductor substrate may include a third n-doped region and a fourth n-doped region, at least a portion of the first p-doped region may be formed in the third n-doped region, and at least a portion of the second p-doped region may be formed in the fourth n-doped region. The semiconductor substrate may include a third n-doped region, at least a portion of the first p-doped region and a portion of the second p-doped region may be formed in the third n-doped region. The semiconductor substrate may include one or more p-well regions.

[0012] The first control signal may be a fixed bias voltage, and the second control signal may be a variable bias voltage that is a fixed voltage bias to the first control signal. The photons absorbed by the absorption region may be reflected from the surface of a three-dimensional object, and the portions of the photocarriers collected by the one or more first switches and the portions of the photocarriers collected by the one or more second switches may be used by a time-of-flight system to analyze the depth information or material composition of the three-dimensional object.

[0013] Another innovative aspect of the object described by the present invention can be implemented in an optical device; the optical device includes: a semiconductor substrate; an absorption layer, coupled to the semiconductor substrate, the absorption layer includes a photodetector region configured to absorb photons and generate photocarriers from the absorbed photons; one or more first switches, controlled by a first control signal, the one or more first switches are configured to collect at least a portion of the photocarriers based on the first control signal; and one or more second switches, controlled by a second control signal, the one or more second switches are configured to collect at least a portion of the photocarriers based on the second control signal, the second control signal being different from the first control signal. The one or more first switches include: a plurality of first p-doped regions located in the semiconductor substrate and controlled by the first control signal; and a plurality of first n-doped regions located in the semiconductor substrate, wherein the plurality of first n-doped regions are coupled to a first readout integrated circuit. The one or more second switches include: a plurality of second p-doped regions located in the semiconductor substrate, wherein the plurality of second p-doped regions are controlled by the second control signal; and a plurality of second n-doped regions located in the semiconductor substrate, wherein the plurality of second n-doped regions are coupled to a second readout integrated circuit.

[0014] This embodiment and other embodiments may each optionally include one or more of the following features. The semiconductor substrate may include a third n-doped region, and at least a portion of the plurality of first p-doped regions and a portion of the plurality of second p-doped regions may be formed in the third n-doped region. The plurality of first p-doped regions and the plurality of second p-doped regions may be arranged in a finger-like pattern along a first plane of the semiconductor substrate, and the plurality of first n-doped regions and the plurality of second n-doped regions may be arranged in a finger-like pattern along a second plane of the semiconductor substrate, the second plane being different from the first plane. Each p-doped region in the plurality of first p-doped regions may be disposed above a corresponding n-doped region in the plurality of second n-doped regions, and each p-doped region in the plurality of second p-doped regions may be disposed above a corresponding n-doped region in the plurality of second n-doped regions. The semiconductor substrate may include one or more p-well regions.

[0015] The first control signal may be a fixed bias voltage, and the second control signal may be a variable bias voltage that is a fixed voltage bias to the first control signal. The photons absorbed by the absorption region may be reflected from the surface of a three-dimensional object; the portion of the photocarriers collected by the one or more first switches and the portion of the photocarriers collected by the one or more second switches may be used by a time-of-flight system to analyze the depth information or material composition of the three-dimensional object.

[0016] Yet another innovative aspect of the object described by the present invention can be implemented in a time-of-flight system; the time-of-flight system includes: a light source; and an image sensor, which includes a plurality of pixels fabricated on a semiconductor substrate, and each of the pixels includes a germanium-silicon layer coupled to the semiconductor substrate. The germanium-silicon layer includes: a light detector region configured to absorb photons and generate photo-carriers from the absorbed photons; one or more first switches controlled by a first control signal, the one or more first switches being configured to collect at least a portion of the photo-carriers based on the first control signal; and one or more second switches controlled by a second control signal, the one or more second switches being configured to collect at least a portion of the photo-carriers based on the second control signal, the second control signal being different from the first control signal.

[0017] This embodiment and other embodiments may each optionally include one or more of the following features. The light source is configured to emit light pulses having a duty cycle of less than 50% but maintaining the same energy amount per light pulse.

[0018] Yet another innovative aspect of the object described by the present invention can be implemented in an optical device; the optical device includes: a semiconductor substrate; a germanium-silicon layer coupled to the semiconductor substrate, the germanium-silicon layer including a light detector region configured to absorb photons and generate photo-carriers from the absorbed photons; one or more first switches controlled by a first control signal, the one or more first switches being configured to collect at least a portion of the photo-carriers based on the first control signal; and one or more second switches controlled by a second control signal, the one or more second switches being configured to collect at least a portion of the photo-carriers based on the second control signal, the second control signal being different from the first control signal. The one or more first switches include: a first p-doped region located in the germanium-silicon and controlled by the first control signal; and a first n-doped region located in the semiconductor substrate, the first n-doped region being coupled to a first readout integrated circuit. The one or more second switches include: a second p-doped region located in the germanium-silicon layer and controlled by the second control signal; and a second n-doped region located in the semiconductor substrate, wherein the second n-doped region is coupled to a second readout integrated circuit.

[0019] This embodiment and other embodiments may each optionally include one or more of the following features. The germanium-silicon layer may include a third n-doped region and a fourth n-doped region, at least a portion of the first p-doped region may be formed in the third n-doped region, and at least a portion of the second p-doped region may be formed in the fourth n-doped region. The germanium-silicon layer may include a third n-doped region, at least a portion of the first p-doped region and a portion of the second p-doped region may be formed in the third n-doped region. The semiconductor substrate may include one or more p-well regions.

[0020] The first control signal can be a fixed bias voltage, and the second control signal can be a variable bias voltage that biases the fixed voltage of the first control signal. The photons absorbed by the absorption region can be reflected from the surface of the three-dimensional object; the portions of the photo carriers collected by the one or more first switches and the portions of the photo carriers collected by the one or more second switches can be used by the time-of-flight system to analyze the depth information or material composition of the three-dimensional object.

[0021] Another innovative aspect of the object described in the present invention can be implemented in an optical device; the optical device includes: a semiconductor substrate; a first light absorption region supported by the semiconductor substrate, the first light absorption region includes germanium, and is configured to absorb photons and generate photo carriers from the absorbed photons; a first layer supported by at least a portion of the first light absorption region and the semiconductor substrate, the first layer is different from the first light absorption region; one or more first switches controlled by a first control signal, the one or more first switches are configured to collect at least a portion of the photo carriers based on the first control signal; and one or more second switches controlled by a second control signal, the one or more second switches are configured to collect at least a portion of the photo carriers based on the second control signal, and the second control signal is different from the first control signal. The one or more first switches include: a first control contact coupled to a first control region of the first layer, where the first control region is controlled by the first control signal; and a first readout contact coupled to a first readout region of the first layer, where the first readout region is coupled to a first readout integrated circuit. The one or more second switches include: a second control contact coupled to a second control region of the first layer, where the second control region is controlled by the second control signal; and a second readout contact coupled to a second readout region of the first layer, where the second readout region is coupled to a second readout integrated circuit.

[0022] Embodiments of the optical device may each optionally include one or more of the following features. For example, the semiconductor substrate can include a recess, and at least a portion of the first light absorption region can be embedded in the recess. The first layer can be a silicon layer or a silicon-germanium layer. The first layer can include materials compatible with complementary metal oxide semiconductor (CMOS) processes.

[0023] The first light absorption region can be formed of germanium or silicon-germanium.

[0024] In some embodiments, the first readout region can include a first n-doped region, and the second readout region can include a second n-doped region. The first light absorption region can support the first readout region and the second readout region, and the first light absorption region can support the first control region and the second control region. The first control region can include a first p-doped region, and the second control region can include a second p-doped region. The first light absorption region can include: a third n-doped region located below the first control region of the first layer and in contact with the first p-doped region; and a fourth n-doped region located below the second control region of the first layer and in contact with the second p-doped region. The first light absorption region can include a third p-doped region and a fourth p-doped region.

[0025] In some embodiments, the first light absorption region can include: a third n-doped region located below the first control region of the first layer, and a fourth n-doped region located below the second control region of the first layer.

[0026] In some embodiments, the semiconductor substrate can support the first readout region and the second readout region, and the semiconductor substrate can support the first control region and the second control region. The first control region can include a first p-doped region, and the second control region can include a second p-doped region. The semiconductor substrate can include: a third n-doped region located below the first control region of the first layer and in contact with the first p-doped region; and a fourth n-doped region located below the second control region of the first layer and in contact with the second p-doped region. The semiconductor substrate can include a third p-doped region and a fourth p-doped region.

[0027] In some embodiments, the semiconductor substrate can include: a third n-doped region located below the first control region of the first layer, and a fourth n-doped region located below the second control region of the first layer.

[0028] In some embodiments, the one or more first switches can further include: a third control contact coupled to the first control region of the first layer, wherein the first light absorption region supports a third control region, and the third control region is controlled by a third control signal; and a fourth control contact coupled to the fourth control region of the first layer, wherein the first light absorption region supports a fourth control region, and the fourth control region is controlled by a fourth control signal. The third control region can include a third p-doped region, and the fourth control region can include a fourth p-doped region. The semiconductor substrate can include: a third n-doped region located below the first control region of the first layer and in contact with the first p-doped region: and a fourth n-doped region located below the second control region of the first layer and in contact with the second p-doped region. The first light absorption region can include: a fifth n-doped region located below the third control region of the first layer and in contact with the third p-doped region; and a sixth n-doped region located below the fourth control region of the first layer and in contact with the fourth p-doped region. The semiconductor substrate can further include a fifth p-doped region and a sixth p-doped region.

[0029] In some embodiments, a semiconductor substrate may include: a third n-doped region located in a first control region of a first layer and contacting a lower portion of a first p-doped region; and a fourth n-doped region located below a second control region of the first layer and contacting a second p-doped region. The first light absorption region may include: a fifth n-doped region located below a third control region of the first layer, and a sixth n-doped region located below a fourth control region of the first layer.

[0030] In some embodiments, an optical device may include a first bipolar junction transistor and a second bipolar junction transistor. The first bipolar junction transistor may include: a first electron emitter supported by the semiconductor substrate; a first p-doped region, and a first n-doped region. The second bipolar junction transistor may include a second electron emitter supported by the semiconductor substrate; a second p-doped region, and a second n-doped region.

[0031] Another innovative aspect of the object described in the present invention can be implemented in an optical device; the optical device includes: a semiconductor substrate; a first light absorption region supported by the semiconductor substrate, the first light absorption region being configured to absorb photons and generate photo carriers from the absorbed photons; one or more first switches controlled by a first control signal, the one or more first switches being configured to collect at least a portion of the photo carriers based on the first control signal; one or more second switches controlled by a second control signal, the one or more second switches being configured to collect at least a portion of the photo carriers based on the second control signal, wherein the second control signal is different from the first control signal; and an anti-doped region formed in a first region of the first light absorption region, the anti-doped region including a first dopant and having a first net carrier doping concentration that is lower than a second net carrier concentration of a second portion of the first light absorption region. The one or more first switches include: a first control contact coupled to a first control region, wherein the first control region is controlled by the first control signal; and a first readout contact coupled to a first readout region, wherein the first readout region is coupled to a first readout integrated circuit. The one or more second switches include: a second control contact coupled to a second control region, wherein the second control region is controlled by the second control signal; and a second readout contact coupled to a second readout region, wherein the second readout region is coupled to a second readout integrated circuit.

[0032] Embodiments of the optical device may optionally include one or more of the following features. For example, in operation, the anti-doped region can reduce leakage current transferred between the first control contact and the second control contact compared to a comparable optical device without the anti-doped region.

[0033] In some embodiments, the first light absorption region supports the first control region, the first readout region, the second control region, and the second readout region; the counter-doped region includes at least a portion of the first control region, the first readout region, the second control region, and the second readout region. The first readout region may include a first n-doped region, and the second readout region may include a second n-doped region. The first control region may include a first p-doped region, and the second control region may include a second p-doped region. The optical device may further include: a third n-doped region in contact with the first p-doped region; and a fourth n-doped region in contact with the second p-doped region, wherein a first lateral separation between the third n-doped region and the fourth n-doped region is less than a second lateral isolation between the first p-doped region and the second p-doped region.

[0034] The first light absorption region may include germanium or germanium silicon. The first dopant of the counter-doped region is selected from the group consisting of phosphorus, arsenic, antimony, and fluorine. The doping concentration of the counter-doped region may be between 2*10 13 / cm 3 and 5*10 14 / cm 3 range. The doping concentration of the counter-doped region may be greater than the defect concentration of germanium or germanium silicon.

[0035] In some embodiments, the optical device may include a first reflector, and the semiconductor substrate supports the first reflector. The first reflector can be one or more of a metal mirror, a dielectric mirror, and a distributed Bragg reflector. The optical device can further include a second reflector, and the semiconductor substrate supports the second reflector; wherein the first reflector and the second reflector are located on opposite sides of the first light absorption region. The optical device can further include a first anti-reflection layer, and the semiconductor substrate supports the first anti-reflection layer; wherein the first reflector and the first anti-reflection layer are located on opposite sides of the first light absorption region.

[0036] In some embodiments, the optical device can further include a lens, and the semiconductor substrate supports the lens. The lens can be integrally formed on the semiconductor substrate. The optical device can further include a spacer layer, and the semiconductor substrate supports the spacer layer, wherein in a direction perpendicular to the substrate surface, the spacer layer is disposed between the first light absorption region and the lens. The optical device can further include a second anti-reflection layer, and the semiconductor substrate supports the second anti-reflection layer, and the second anti-reflection layer is disposed between the semiconductor substrate and the lens. The refractive index of at least a portion of the second anti-reflection layer can be greater than 1.8. The second anti-reflection layer can include a high-k material compatible with a complementary metal oxide semiconductor (CMOS) process.

[0037] In some embodiments, the optical device may further include a first layer supported by at least a portion of the first light absorption region and the semiconductor substrate, and the first layer is different from the first light absorption region.

[0038] Yet another innovative aspect of the object described by the present invention can be implemented in an optical device; the optical device includes: a semiconductor substrate; a first light absorption region supported by the semiconductor substrate, the first light absorption region being configured to absorb photons and generate photo carriers from the absorbed photons; one or more first switches controlled by a first control signal, the one or more first switches being configured to collect at least a portion of the photo carriers based on the first control signal; and one or more second switches controlled by a second control signal, the one or more second switches being configured to collect at least a portion of the photo carriers based on the second control signal, wherein the second control signal is different from the first control signal. The one or more first switches include: a first p-doped region located in the first light absorption region, wherein the first p-doped region is controlled by the first control signal and has a first p-dopant concentration; a second p-doped region located in the first light absorption region and contacting at least a first portion of the first p-doped region, wherein the second p-doped region has a second p-dopant concentration lower than the first p-dopant concentration; and a first n-doped region located in the first light absorption region, wherein the first n-doped region is coupled to a first control integrated circuit and has a first n-dopant concentration. The one or more second switches include: a third p-doped region located in the first light absorption region, wherein the third p-doped region is controlled by the second control signal and has a third p-dopant concentration; a fourth p-doped region located in the first light absorption region and contacting at least a first portion of the third p-doped region, wherein the fourth p-doped region has a fourth p-dopant concentration lower than the third p-dopant concentration; and a second n-doped region located in the first light absorption region, wherein the second n-doped region is coupled to a second control integrated circuit and has a second n-dopant concentration.

[0039] Embodiments of the optical device may each optionally include one or more of the following features. For example, in operation, compared to an optical device without the second and fourth p-doped regions, the second p-doped region may reduce a first dark current transmitted between the first p-doped region and the first n-doped region, and the fourth p-doped region may reduce a second dark current transmitted between the third p-doped region and the second n-doped region.

[0040] In some embodiments, the one or more first switches can further include a third n-doped region located in the first light absorption region and contacting at least a portion of the first n-doped region, where the third n-doped region has a third dopant concentration lower than the first dopant concentration. The one or more second switches can further include a fourth n-doped region located in the first light absorption region and contacting at least a portion of the second n-doped region, where the fourth n-doped region has a fourth dopant concentration lower than the second dopant concentration. In operation, compared to an optical device without the third and fourth n-doped regions, the third n-doped region can reduce a first dark current that is transmitted between the first p-doped region and the first n-doped region, and the fourth n-doped region can reduce a second dark current that is transmitted between the third p-doped region and the second n-doped region.

[0041] In some embodiments, the first light absorption region includes germanium or germanium-silicon. The optical device can further include a first layer supported by the first light absorption region, where the first layer is different from the first light absorption region. The one or more first switches can further include a fifth n-doped region that contacts a second portion of the first p-doped region; the one or more second switches can further include a sixth n-doped region that contacts a second portion of the third p-doped region.

[0042] Another innovative aspect of the object described by the present invention can be implemented in an optical device; the optical device includes: a semiconductor substrate; a first light absorption region supported by the semiconductor substrate, the first light absorption region being configured to absorb photons and generate photo-carriers from the absorbed photons; one or more first switches controlled by a first control signal, the one or more first switches being configured to collect at least a portion of the photo-carriers based on the first control signal; and one or more second switches controlled by a second control signal, the one or more second switches being configured to collect at least a portion of the photo-carriers based on the second control signal, where the second control signal is different from the first control signal. The one or more first switches include: a first p-doped region located in the first light absorption region, where the first p-doped region is controlled by the first control signal and has a first p-dopant concentration; a first n-doped region located in the first light absorption region, where the first n-doped region is coupled to a first readout integrated circuit and has a first n-dopant concentration; and a first trench located between the first p-doped region and the first n-doped region. The one or more second switches include: a second p-doped region located in the first light absorption region, where the second p-doped region is controlled by the second control signal and has a second p-dopant concentration; a second n-doped region located in the first light absorption region, where the second n-doped region is coupled to a second readout integrated circuit and has a second n-dopant concentration; and a second trench located between the second p-doped region and the second n-doped region.

[0043] Embodiments of the optical device may each optionally include one or more of the following features. For example, in operation, the first trench can reduce a first dark current that is transferred between the first p-doped region and the first n-doped region, compared to a comparable optical device that does not have the first and second trenches, and the second trench can reduce a second dark current that is transferred between the second p-doped region and the second n-doped region.

[0044] In some embodiments, the first light absorption region includes germanium or germanium silicon. The one or more first switches may further include: a third p-doped region that is located in the first light absorption region and contacts at least a first portion of the first p-doped region, where the third p-doped region has a third p-dopant concentration that is lower than the first p-dopant concentration; and a third n-doped region that is located in the first light absorption region and contacts at least a portion of the first n-doped region, where the third n-doped region has a third n-dopant concentration that is lower than the first n-dopant concentration. The one or more second switches can further include: a fourth p-doped region that is located in the first light absorption region and contacts at least a first portion of the second p-doped region, where the fourth p-doped region has a fourth p-dopant concentration that is lower than the second p-dopant concentration; and a fourth n-doped region that is located in the first light absorption region and contacts at least a portion of the second n-doped region, where the fourth n-doped region has a fourth n-dopant concentration that is lower than the second n-dopant concentration. In operation, the third n-doped region and the third p-doped region can reduce the first dark current that is transferred between the first p-doped region and the first n-doped region, compared to an optical device that does not have the third and fourth n-doped regions and the third and fourth p-doped regions, and the fourth n-doped region and the fourth p-doped region can reduce the second dark current that is transferred between the second p-doped region and the second n-doped region.

[0045] In some embodiments, the optical device may further include a first layer, where the first light absorption region supports the first layer, and the first layer is different from the first light absorption region and covers the first trench and the second trench. The one or more first switches may further include a fifth n-doped region that contacts a second portion of the first p-doped region. The one or more second switches may further include a sixth n-doped region that contacts a second portion of the second p-doped region.

[0046] In some embodiments, at least a portion of the first trench and the second trench is filled with a dielectric material.

[0047] Embodiments of the optical device may each optionally include one or more of the following features. For near-infrared wavelengths, germanium is an efficient absorption material. When using an inefficient absorption material (e.g., silicon), germanium can reduce the problem of slower generation of photo carriers deep in the substrate. For photodetectors with n- and p-doped regions fabricated at different depths, the transport distance of photo carriers is limited by the depth rather than the width of the absorption material. Therefore, even when using an effective absorption material with a short absorption length, the distance between the p- and n-doped regions can be made shorter, so that a small bias voltage can establish a strong electric field, thereby increasing the operating speed. For such a photodetector, two sets of switches can be inserted and configured to be arranged in a lateral interdigital pattern to collect photo carriers of different optical phases for the ToF system. The increased operating speed allows the use of a higher modulation frequency in the ToF system, thereby obtaining a higher depth resolution. In the ToF system, the peak intensity of the light pulse increases as its duty cycle decreases, which can improve the signal-to-noise ratio (and depth accuracy) while keeping the energy consumption of the ToF system unchanged. If the operating speed is increased so that the duty cycle of the light pulse can be shortened without distorting the pulse shape, the above situation can be achieved. In addition, when germanium is used as the absorption region, light pulses with wavelengths greater than 1 μm can be used. Longer wavelength NIR light (e.g., 1.31 μm, 1.4 μm, 1.55 μm) is generally considered safer for the human eye, so light pulses with longer wavelengths can be output at a higher intensity while meeting the eye safety requirements, thereby improving the signal-to-noise ratio (even depth accuracy).

[0048] Details of one or more embodiments are described in the accompanying drawings and the following embodiments. Other potential features and advantages will become apparent from the embodiments, the drawings, and the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1A 、 1B 1C and 1D are examples of switched photodetectors.

[0050] Figure 2A 、 2B 2C and 2D are examples of switched photodetectors.

[0051] Figure 3A 、 3B 3C and 3D are examples of switched photodetectors.

[0052] Figure 4A 、 4B 4C, 4D and 4E are examples of switched photodetectors.

[0053] Figures 5A - 5C is an example of a photodetector.

[0054] Figures 5D - 5KIt is an example of a switching photodetector.

[0055] Figures 6A - 6B It is an example of a switching photodetector.

[0056] Figures 7A - 7B It is a cross-sectional view of an example structure integrating a lens and a photodetector.

[0057] Figures 8A - 8C It is an example of a switch of a switching photodetector.

[0058] Figures 9A - 9E It is an example of an electrical terminal of a switching photodetector.

[0059] Figures 10A - 10I It is an example structure of a photodetector having an absorption region and a substrate.

[0060] Figures 11A - 11F It is a top view and a side view of an example of a switching photodetector.

[0061] Figures 12A - 12H It is a top view and a side view of an example of a switching photodetector.

[0062] Figures 13A - 13G It is a top view and a side view of an example of a switching photodetector.

[0063] Figures 14A - 14B It is a top view of an example of a switching photodetector.

[0064] Figures 15A - 15G It is a side view of an example structure of sensor pixel isolation.

[0065] Figures 16A - 16J It is a cross-sectional view of an example structure of a photodetector.

[0066] Figures 17A - 17E It is a cross-sectional view of an example structure for surface modification of an absorption region.

[0067] Figures 18A - 18G It is a top view and a side view of an example of a switching photodetector.

[0068] Figures 19A - 19H It is a top view and a side view of an example of a switching photodetector.

[0069] Figures 20A - 20L It is a top view and a side view of an example of a switching photodetector.

[0070] Figures 21A - 21F It is a top view and a side view of an example of a switching photodetector.

[0071] Figures 22A - 22D It is a top view and a side view of an example of a switching photodetector.

[0072] Figures 23A - 23B Top view and side view of an example of a switching photodetector.

[0073] Figures 24A - 24G Top view and side view of an example of a switching photodetector.

[0074] Figures 25A - 25H Top view and side view of an example of a switching photodetector.

[0075] Figure 26 Example of a unit cell of a rectangular photodetector.

[0076] Figure 27 Example of a rectangular switching photodetector with phototransistor gain.

[0077] Figure 28A Block diagram of an example of an imaging system.

[0078] Figures 28B - 28C Example of a technique for using an imaging system to determine object characteristics.

[0079] Figure 29 Example of a flowchart for using an imaging system to determine object characteristics.

[0080] In the various figures, like reference numerals and names indicate like devices. It should also be understood that each of the exemplary embodiments shown in the figures is only an illustrative representation and is not necessarily drawn to scale. Detailed Description

[0081] A photodetector can be used to detect an optical signal and convert the optical signal into an electrical signal that can be further processed by other circuits. In time-of-flight (ToF) applications, depth information of a three-dimensional object can be determined by the phase difference between a transmitted optical pulse and a detected optical pulse. For example, a two-dimensional array of pixels can be used to reconstruct a three-dimensional image of the three-dimensional object, where each pixel can include one or more photodetectors for obtaining the phase information of the three-dimensional object. In some embodiments, ToF applications use a light source having a wavelength in the near-infrared (NIR) range. For example, the wavelength of a light-emitting diode (LED) can be 850 nm, 940 nm, 1050 nm, or from 1.3 μm to 1.6 μm. Some photodetectors can use silicon as an absorption material, but silicon is not an efficient absorption material for NIR wavelengths. Specifically, photo carriers can be generated deep in a silicon substrate (e.g., at a depth greater than 10 μm), and these photo carriers slowly drift and / or diffuse to the junction of the photodetector, which results in a reduction in the operating speed of the device. Second, to minimize power consumption, a small voltage amplitude is typically used to control the operation of the photodetector. For a large absorption region (e.g., with a diameter of 10 μm), a small voltage amplitude can only form a small transverse / longitudinal electric field across the entire large absorption region, which affects the drift speed of photo carriers sweeping across the absorption region. Therefore, the operating speed of the device is further limited. For ToF applications using NIR wavelengths, a photodetector using germanium-silicon (GeSi) as an absorption material solves the technical problems discussed above. In this application, the term "photodetector" can be used interchangeably with the term "optical sensor". In this application, the term "germanium-silicon (GeSi)" refers to a GeSi alloy ranging from 99% germanium (i.e., 1% silicon) to 1% germanium (i.e., 99% silicon). In this application, the GeSi material can be formed by blanket epitaxial growth technology, selective epitaxial growth technology, or other applicable technologies. Second, an absorption layer containing a GeSi layer can be formed as a planar, mesa top, or trench bottom surrounded by an insulator (e.g., oxide, nitride), a semiconductor (e.g., silicon, germanium), or a combination thereof. In addition, a strained superlattice structure or multiple quantum well structure including multiple layers such as alternating layers of germanium-silicon with different compositions can be used for the absorption layer. Also, a silicon layer or germanium-silicon layer with a low germanium concentration (e.g., <10%) can be used to passivate the surface of a germanium-silicon layer with a high germanium concentration (e.g., >50%), which can reduce the dark current or leakage current on the surface of the germanium-silicon layer with a high germanium concentration.

[0082] Figure 1Ais an example of the switching photodetector 100, where the switching photodetector 100 is used to convert an optical signal into an electrical signal. The switching photodetector 100 includes an absorption layer 106 fabricated on a substrate 102. The substrate 102 can be any suitable substrate on which semiconductor devices can be fabricated. For example, the substrate 102 can be a silicon substrate. The absorption layer 106 includes a first switch 108 and a second switch 110.

[0083] Generally, the absorption layer 106 receives the optical signal 112 and converts the optical signal 112 into an electrical signal. The absorption layer 106 can be intrinsic, p-type, or n-type. In some embodiments, the absorption layer 106 can be formed of a p-type germanium-silicon material. The absorption layer 106 is selected to have a high absorption coefficient within a desired wavelength range. For NIR wavelengths, the absorption layer 106 can be a GeSi mesa, where germanium-silicon absorbs photons in the optical signal 112 and generates electron-hole pairs. The material composition of germanium and silicon in the GeSi mesa can be selected for a specific technology or application. In some embodiments, the absorption layer 106 is designed to have a thickness t. For example, for wavelengths of 850 nm or 940 nm, in order to have a high quantum efficiency, the thickness of the GeSi mesa can be about 1 μm. In some embodiments, the surface of the absorption layer 106 is designed to have a specific shape. For example, the GeSi mesa can be circular, square, or rectangular, depending on the spatial profile of the optical signal 112 on the top surface of the germanium-silicon mesa. In some embodiments, the absorption layer 106 is designed to have a lateral dimension d for receiving the optical signal 112. For example, the germanium-silicon plane can be circular or rectangular, where d ranges from 1 μm to 50 μm.

[0084] The first switch 108 and the second switch 110 are fabricated in the absorption layer 106. The first switch 108 is coupled to a first control signal 122 and a first readout circuit 124. The second switch 110 is coupled to a second control signal 132 and a second readout circuit 134. Generally, the first readout circuit 124 or the second readout circuit 134 determines to collect electrons or holes according to the control of the first control signal 122 and the second control signal 132.

[0085] In some embodiments, the first switch 108 and the second switch 110 are fabricated for collecting electrons. In such a case, the first switch 108 includes a p-doped region 128 and an n-doped region 126. For example, the p-doped region 128 can have a p+ doping, where the activated dopant concentration can be as high as that achievable by the fabrication process. For example, when the absorption layer 106 is germanium and doped with boron, it is about 5x10 20 cm -3 . In some embodiments, the doping concentration of the p-doped region 128 can be lower than 5x10 20 cm -3, so as to simplify the manufacturing complexity at the cost of increasing the contact resistance. The n-doped region 126 may have an n+ doping, where the activated dopant concentration can be as high as achievable by the manufacturing process; for example, when the absorption layer 106 is germanium and doped with phosphorus, it is about 1x10 20 cm -3 . In some embodiments, the doping concentration of the n-doped region 126 can be less than 1x10 20 cm -3 , so as to simplify the manufacturing complexity at the cost of increasing the contact resistance. The distance between the p-doped region 128 and the n-doped region 126 can be designed based on the manufacturing process design rules. Generally speaking, the closer the distance between the p-doped region 128 and the n-doped region 126, the higher the switching efficiency of generating photo carriers. However, reducing the distance between the p-doped region 128 and the n-doped region 126 may increase the dark current associated with the PN junction between the p-doped region 128 and the n-doped region 126. Therefore, the distance can be set depending on the performance of the switching photodetector 100. The second switch 110 includes a p-doped region 138 and an n-doped region 136. The p-doped region 138 is similar to the p-doped region 128, and the n-doped region 136 is similar to the n-doped region 126.

[0086] In some embodiments, the p-doped region 128 is coupled to the first control signal 122. For example, the p-doped region 128 can be coupled to a voltage source, where the first control signal 122 can be an AC voltage signal from the voltage source. In some embodiments, the n-doped region 126 is coupled to the readout circuit 124. The readout circuit 124 can be a three-transistor configuration, which is composed of a reset gate, a source follower, and a select gate, or any suitable circuit for processing charges. In some embodiments, the readout circuit 124 can be fabricated on the substrate 102. In some other embodiments, the readout circuit 124 can be fabricated on another substrate and integrated with the switching photodetector 100 or co-packaged together through wafer / wafers bonding or stacking.

[0087] The p-doped region 138 is coupled to the second control signal 132. For example, the p-doped region 138 can be coupled to a voltage source, where the second control signal 132 can be an AC voltage signal, and its phase is opposite to that of the first control signal 122. In some embodiments, the n-doped region 136 can be coupled to the readout circuit 134. The readout circuit 134 can be similar to the readout circuit 124.

[0088] The first control signal 122 and the second control signal 132 are used to control the collection process of the electrons generated by absorbing photons. For example, when the first control signal 122 is different from the second control signal 132, an electric field is formed between the p-doped region 128 and the p-doped region 138, and the free electrons drift to the p-doped region 128 or the p-doped region 138 according to the direction of the electric field. In some embodiments, the first control signal 122 may be fixed at a voltage value V i , and the second control signal 132 may vary between V i ±ΔV. The bias value determines the drift direction of the electrons. Accordingly, when one of the switches (e.g., the first switch 108) is "on" (i.e., the electrons drift to the p-doped region 128), the other switch (e.g., the second switch 110) is "off" (i.e., the electrons are blocked by the p-doped region 138). In some embodiments, the first control signal 122 and the second control signal 132 may have different voltage values.

[0089] Generally, the difference between the Fermi levels of the p-doped region and the n-doped region forms an electric field between these two regions. In the first switch 108, an electric field is formed between the p-doped region 128 and the n-doped region 126. Similarly, in the second switch 110, an electric field is formed between the p-doped region 138 and the n-doped region 136. When the first switch 108 is "on" and the second switch 110 is "off", the electrons drift to the p-doped region 128, and the electric field between the p-doped region 128 and the n-doped region 126 further moves the electrons to the n-doped region 126. The readout circuit 124 can then process the charge collected in the n-doped region 126. Conversely, when the second switch 110 is on and the first switch 108 is off, the electrons drift to the p-doped region 138, and the electric field between the p-doped region 138 and the n-doped region 136 further moves the electrons to the n-doped region 136. The readout circuit 134 can then process the charge collected in the n-doped region 136.

[0090] In some embodiments, a voltage can be applied between the p-doped region and the n-doped region of the switch to operate the switch in an avalanche mechanism to increase the sensitivity of the dual-switch photodetector 100. For example, in the case where the germanium-silicon layer 106 contains germanium-silicon, when the distance between the p-doped region 128 and the n-doped region 126 is about 100 nm, a voltage below 7V can be applied to establish avalanche gain between the p-doped region 128 and the n-doped region 126.

[0091] In some embodiments, the substrate 102 can be coupled to an external control 116. For example, the substrate 102 can be coupled to electrical ground or a default voltage that is lower than the voltage of the n-doped regions 126 and 136. In some other embodiments, the substrate 102 can be floating or not coupled to any external control.

[0092] Figure 1B is an example of a switching photodetector 160, where the switching photodetector 160 is used to convert an optical signal into an electrical signal. The switching photodetector 160 is similar to Figure 1A the switching photodetector 100 shown, but the first switch 108 and the second switch 110 further include an n-well region 152 and an n-well region 154, respectively. Additionally, the absorption region 106 can be a p-doped region. In some embodiments, the doping level range of the n-well regions 152 and 154 can be 10 15 cm -3 to 10 17 cm -3 and the doping level range of the absorption region 106 can be 10 14 cm -3 to 10 16 cm -3 .

[0093] The arrangement of the p-doped region 128, the n-well region 152, the p-doped absorption region 106, the n-well region 154, and the p-doped region 138 forms a PNPNP junction structure. Generally, the PNPNP junction structure reduces the leakage current from the first control signal 122 to the second control signal 132, or the leakage current from the second control signal 132 to the first control signal 122. The arrangement of the n-doped region 126, the p-doped absorption region 106, and the n-doped region 136 forms an NPN junction structure. Generally, the NPN junction structure reduces the charge coupling from the first readout circuit 124 to the second readout circuit 134, or the charge coupling from the second readout circuit 134 to the first readout circuit 124.

[0094] In some embodiments, the p-doped region 128 is completely formed within the n-well region 152. In some embodiments, the p-doped region 128 is partially formed within the n-well region 152. For example, a portion of the p-doped region 128 can be formed within the n-well region 152 by implanting p-dopants, and another portion of the p-doped region 128 can be formed within the absorption layer 106 by implanting p-dopants. Similarly, in some embodiments, the p-doped region 128 is completely formed within the n-well region 154. In some other embodiments, the p-doped region 138 is partially formed within the n-well region 154. In some embodiments, the depths of the n-well regions 152 and 154 are shallower than the depths of the p-doped regions 128 and 138.

[0095] Figure 1C is an example of a switching photodetector 170, where the switching photodetector 170 is used to convert an optical signal into an electrical signal. The switching photodetector 170 is similar to Figure 1A the switching photodetector 100 shown, but the absorption layer 106 further includes an n-well region 156. Additionally, the absorption region 106 can be a p-doped region. In some embodiments, the doping level range of the n-well region 156 can be 1015 centimeter -3 to 10 17 centimeter -3 , the doping level range of the absorption layer 106 can be 10 14 centimeter -3 to 10 16 centimeter -3 .

[0096] The arrangement of the p-doped region 128, the n-well region 156, and the p-doped region 138 forms a PNP junction structure. Generally speaking, the PNP junction structure reduces the leakage current from the first control signal 122 to the second control signal 132, or from the second control signal 132 to the first control signal 122. The arrangement of the n-doped region 126, the p-doped absorption layer 106, and the n-doped region 136 forms an NPN junction structure. Generally speaking, the NPN junction structure reduces the charge coupling from the first readout circuit 124 to the second readout circuit 134, or from the second readout circuit 134 to the first readout circuit 124. In some embodiments, when the n-well region 156 is deep, the arrangement of the n-doped region 126, the p-doped region 106, the n-well region 156, the p-doped absorption region 106, and the n-doped region 136 can form an NPNPN junction structure to further reduce the charge coupling from the first readout circuit 124 to the second readout circuit 134, or from the second readout circuit 134 to the first readout circuit 124.

[0097] In some embodiments, the p-doped regions 128 and 138 are completely formed within the n-well region 156. In some embodiments, the p-doped regions 128 and 138 are partially formed within the n-well region 156. For example, a part of the p-doped region 128 can be formed within the n-well region 156 by implanting p-dopants, and another part of the p-doped region 128 can be formed within the absorption layer 106 by implanting p-dopants. In some embodiments, the depth of the n-well region is shallower than the depths of the p-doped regions 128 and 138.

[0098] Figure 1D is an example of the switch photodetector 180, where the switch photodetector 180 is used to convert an optical signal into an electrical signal. The switch photodetector 180 is similar to Figure 1A the switch photodetector 100 shown, but the switch photodetector 180 further includes a p-well region 104 and n-well regions 142 and 144. In some embodiments, the doping level range of the n-well regions 142 and 144 can be 10 16 centimeter -3 to 10 20 centimeter -3 , the doping level range of the p-well region 104 can be 10 16 centimeter -3 to 10 20 centimeter -3 .

[0099] In some embodiments, the absorption layer 106 may not completely absorb the photons in the incident optical signal 112. For example, if the GeSi mesa does not completely absorb the photons in the incident NIR optical signal 112, the NIR optical signal 112 may penetrate into the silicon substrate 102, and the silicon substrate 102 may absorb the photons that penetrate into it and generate optically-generated carriers with slow recombination in the deep part of the silicon substrate 102. These optically-generated carriers with slow recombination have a negative impact on the operating speed of the switching photodetector. Secondly, the optically-generated carriers generated in the silicon substrate 102 can be collected by adjacent pixels, which causes signal crosstalk between unwanted pixels. In addition, the optically-generated carriers generated in the silicon substrate 102 cause the substrate 102 to be charged, which raises reliability issues of the switching photodetector.

[0100] To remove the optically-generated carriers with slow recombination, the switching photodetector 180 may include a connection that shorts the n-well regions 142, 144 to the p-well region 104. For example, the p-well region 104 and the n-well regions 142, 144 can be connected to form this connection by a salicide process or depositing a metal pad. The short circuit between the n-well regions 142, 144 and the p-well region 104 allows the optically-generated carriers generated in the substrate 102 to recombine at the short circuit node, thereby improving the operating speed and / or reliability of the switching photodetector. In some embodiments, to reduce the device dark current, the p-well region 104 is used to protect or reduce the electric field around the interface defects between the absorption layer 106 and the substrate 102.

[0101] Although Figures 1A - 1D not shown, in some embodiments, the optical signal may enter the switching photodetector from the back side of the substrate 102 of the switching photodetector. One or more optical devices (e.g., microlenses or optical waveguides) may be fabricated on the back side of the substrate 102 to focus, collimate, defocus, filter, or otherwise manipulate the optical signal.

[0102] Although Figures 1A - 1D not shown, in some other embodiments, the first switch 108 and the second switch 110 may alternatively be fabricated to collect holes instead of electrons. In this case, the p-doped regions 128 and 138 will be replaced by n-doped regions, the n-doped regions 126 and 136 will be replaced by p-doped regions, the n-well regions 142, 144, 152, 154, and 156 will be replaced by p-well regions, and the p-well region 104 will be replaced by an n-well region.

[0103] Although Figures 1A - 1DNot shown, but in some embodiments, the absorption layer 106 may be bonded to the substrate after the switching photodetectors 100, 160, 170, and 180 are formed. The substrate may be any material that allows the optoelectronic signal 112 to be transmitted to the switching photodetector. For example, the substrate may be a polymer or glass. In some embodiments, one or more optical devices (e.g., microlenses or light guides) may be fabricated on the carrier substrate to focus, collimate, defocus, filter, or otherwise manipulate the optical signal.

[0104] Although Figures 1A - 1D Not shown, but in some embodiments, the switching photodetectors 100, 160, 170, and 180 may be bonded (e.g., by metal-metal bonding, oxide-oxide bonding, hybrid bonding) to a second substrate that includes a control signal circuit and / or a readout circuit and / or a phase-locked loop (PLL) and / or an analog-to-digital converter circuit. A metal layer may be deposited on top of the switching photodetector to act as a reflector to reflect the optical signal incident from the back side of the substrate 102. Adding a mirror-like metal layer can increase the absorption efficiency (quantum efficiency) of the absorption layer 106. For example, by adding a reflective metal layer, the absorption efficiency of the photodetector operating at long NIR wavelengths in the range of 1.0 μm to 1.6 μm can be significantly improved. An oxide layer may also be included between the metal layer and the absorption layer to increase the reflectivity. The metal layer may also act as a bonding layer during the wafer bonding process. In some embodiments, one or more switches similar to the first switch 108 and the second switch 110 can be added to link the control signal / readout circuit.

[0105] Although Figures 1A - 1D not shown in, but in some embodiments, the absorption layer 106 may be partially or fully embedded or recessed in the substrate 102 to mitigate the surface topography and facilitate fabrication. The foregoing technique is described in U.S. Patent Publication No. US20170040362A1, titled "Germanium-Silicon Light Sensing Apparatus".

[0106] Figure 2A is an example of a switching photodetector 200, where the switching photodetector 200 is used to convert an optical signal into an electrical signal. The first switch 208 and the second switch 210 are fabricated on the substrate 202. The switching photodetector 200 includes an absorption layer 206 fabricated on the substrate 202. The substrate 202 may be any suitable substrate on which semiconductor devices can be configured. For example, the substrate 202 may be a silicon substrate.

[0107] Generally, the absorption layer 206 receives the optical signal 212 and converts the optical signal 212 into an electrical signal. The absorption layer 206 is similar to the absorption layer 106. The absorption layer 206 can be intrinsic, p-type, or n-type. In some embodiments, the absorption layer 206 can be formed of a p-type GeSi material. In some embodiments, the absorption layer 206 may include a p-doped region 209. The p-doped region 209 can repel the photoelectrons transferred from the absorption region 206 to the substrate 202, thereby increasing the operation speed. For example, the p-doped region 209 may have a p+ doping, where the concentration of the p+ dopant can be as high as the degree achievable by the manufacturing process. For example, when the absorption layer 206 is germanium and doped with boron, it is about 5x1020 cm-3. In some embodiments, the doping concentration of the p-doped region 209 can be lower than 5x1020 cm-3 in order to simplify the manufacturing complexity at the cost of increasing the contact resistance. In some embodiments, the p-doped region 209 can be a graded p-doped region.

[0108] The first switch 208 and the second switch 210 can be fabricated in the substrate 202. The first switch 208 is coupled to the first control signal 222 and the first readout circuit 224. The second switch 210 is coupled to the second control signal 232 and the second readout circuit 234. Generally, the first readout circuit 224 or the second readout circuit 234 determines whether to collect electrons or holes according to the control of the first control signal 222 and the second control signal 232. The first control signal 222 is similar to the first control signal 122, and the second control signal 232 is similar to the second control signal 132; the first readout circuit 224 is similar to the first readout circuit 124, and the second readout circuit 234 is similar to the second readout circuit 134.

[0109] In some embodiments, the first switch 208 and the second switch 210 are fabricated to collect the electrons generated in the absorption region 206. In such a case, the first switch 208 includes a p-doped region 228 and an n-doped region 226. For example, the p-doped region 228 may have a p+ doping, where the concentration of the activated dopant can be as high as the degree achievable by the manufacturing process. For example, when the substrate 202 is silicon and doped with boron, it is about 2x1020 cm-3. In some embodiments, the doping concentration of the p-doped region 228 can be lower than 2x1020 cm-3 in order to simplify the manufacturing complexity at the cost of increasing the contact resistance. The n-doped region 226 may have an n+ doping, where the concentration of the activated dopant can be as high as the degree achievable by the manufacturing process. For example, when the substrate 202 is silicon and doped with phosphorus, it is about 5x1020 cm-3. In some embodiments, the doping concentration of the n-doped region 226 can be lower than 5x1020 cm- 3, so as to simplify the manufacturing complexity at the cost of increasing the contact resistance. The distance between the p-doped region 228 and the n-doped region 226 can be designed based on the design rules of the manufacturing process. Generally speaking, the closer the distance between the p-doped region 228 and the n-doped region 226, the higher the switching efficiency of generating photo carriers. The second switch 210 includes a p-doped region 238 and an n-doped region 236. The p-doped region 238 is similar to the p-doped region 228, and the n-doped region 236 is similar to the n-doped region 226.

[0110] In some embodiments, the p-doped region 228 is coupled to the first control signal 222, and the n-doped region 226 is coupled to the readout circuit 224. The p-doped region 238 is coupled to the second control signal 232. The n-doped region 236 is coupled to the readout circuit 234. The first control signal 222 and the second control signal 232 are used to control the collection process of electrons generated by absorbing photons. For example, when the absorption layer 206 absorbs photons in the optical signal 212, electron-hole pairs are generated and drift or diffuse into the substrate 202. When a voltage is applied, if the first control signal 222 is biased relative to the second control signal 232, an electric field is formed between the p-doped region 228 and the p-doped region 238, and free electrons drift from the absorption layer 206 to the p-doped region 228 or the p-doped region 238 according to the direction of the electric field. In some embodiments, the first control signal 222 can be fixed at a voltage value Vi, and the second control signal can vary between Vi±ΔV. The bias value determines the drift direction of the electrons. Accordingly, when one of the switches (e.g., the first switch 208) is turned "on" (i.e., electrons drift to the p-doped region 228), the other switch (e.g., the second switch 210) is turned "off" (i.e., electrons are blocked by the p-doped region 238). In some embodiments, the first control signal 222 and the second control signal 232 can be different voltage values from each other.

[0111] In the first switch 208, an electric field is formed between the p-doped region 228 and the n-doped region 226. Similarly, in the second switch 210, an electric field is formed between the p-doped region 238 and the n-doped region 236. When the first switch 208 is turned "on" and the second switch 210 is turned "off", electrons drift to the p-doped region 228, and the electric field between the p-doped region 228 and the n-doped region 226 further moves the electrons to the n-doped region 226. Then the readout circuit 224 is enabled to process the charge collected by the n-doped region 226. Conversely, when the second switch 210 is turned "on" and the first switch 208 is turned "off", electrons drift to the p-doped region 238, and the electric field between the p-doped region 238 and the n-doped region 236 further moves the electrons to the n-doped region 236. The readout circuit 234 can then process the charge collected by the n-doped region 236.

[0112] In some embodiments, a voltage can be applied between the p-doped region and the n-doped region of the switch to operate the switch in an avalanche mechanism to increase the sensitivity of the switched photodetector 200. For example, when the substrate 202 comprises GeSi, a voltage of less than 7V can be applied to establish avalanche gain between the p-doped region 228 and the n-doped region 226 when the distance between the p-doped region 228 and the n-doped region 226 is about 100nm.

[0113] In some embodiments, the p-doped region 209 can be coupled to an external control 214. For example, the p-doped region 209 can be coupled to ground. In some embodiments, the p-doped region 209 can be floating or uncoupled to any external control. In some embodiments, the substrate 202 can be coupled to an external control 216. For example, the substrate 202 can be coupled to electrical ground or a default voltage that is lower than the voltage of the n-doped regions 226 and 236. In some other embodiments, the substrate 202 can be floating or uncoupled to any external control.

[0114] Figure 2B is an example of a switched photodetector 250, where the switched photodetector 250 is used to convert an optical signal into an electrical signal. The switched photodetector 250 is similar to Figure 2A the switched photodetector 200 shown, but the first switch 208 and the second switch 210 further comprise an n-well region 252 and an n-well region 254, respectively. Additionally, the absorption layer 206 can be a p-doped region and the substrate 202 can be a p-doped substrate. In some embodiments, the doping levels of the n-well regions 252 and 254 can range from 1015 cm-3 to 1017 cm-3, and the doping level of the substrate 202 can range from 1014 cm-3 to 1016 cm-3.

[0115] The arrangement of the p-doped region 228, the n-well region 252, the p-doped substrate 202, the n-well region 254, and the p-doped region 238 forms a PNPNP junction structure. Generally, the PNPNP junction structure reduces the leakage current from the first control signal 222 to the second control signal 232, or from the second control signal 232 to the first control signal 222. The arrangement of the n-doped region 226, the p-doped substrate 202, and the n-doped region 236 forms an NPN junction structure. Generally, the NPN junction structure reduces the charge coupling from the first readout circuit 224 to the second readout circuit 234, or from the second readout circuit 234 to the first readout circuit 224.

[0116] In some embodiments, the p-doped region 228 is formed entirely within the n-well region 252. In some embodiments, the p-doped region 228 is partially formed within the n-well region 252. For example, a portion of the p-doped region 228 may be formed within the n-well region 252 by implanting p-dopants, and another portion of the p-doped region 228 may be formed within the substrate 202 by implanting p-dopants. Similarly, in some embodiments, the p-doped region 238 is formed entirely within the n-well region 254. In some embodiments, the p-doped region 238 is partially formed within the n-well region 254. In some embodiments, the depth of the n-well region 252 is shallower than the depths of the p-doped regions 228 and 238.

[0117] Figure 2C is an example of a switched photodetector 260, where the switched photodetector 260 is used to convert an optical signal into an electrical signal. The switched photodetector 260 is similar to Figure 2A the switched photodetector 200 shown, but the substrate 202 further includes an n-well region 244. Additionally, the absorption layer 206 may be a p-doped region, and the substrate 202 may be a p-doped substrate. In some embodiments, the doping level range of the n-well region 244 may be from 10^15 cm^-3 to 10^17 cm^-3, and the doping level ranges of the absorption layer 206 and the substrate 202 may be from 10^14 cm^-3 to 10^16 cm^-3.

[0118] The arrangement of the p-doped region 228, the n-well region 244, and the p-doped region 238 forms a PNP junction structure. Generally, the PNP junction structure reduces the leakage current from the first control signal 222 to the second control signal 232, or from the second control signal 232 to the first control signal 222. The arrangement of the n-doped region 226, the p-doped substrate 202, and the n-doped region 236 forms an NPN junction structure. Generally, the NPN junction structure reduces the charge coupling from the first readout circuit 224 to the second readout circuit 234, or from the second readout circuit 234 to the first readout circuit 224. In some embodiments, if the depth of the n-well region 244 is deep, the arrangement of the n-doped region 226, the p-doped substrate 202, the n-well region 244, the p-doped substrate 202, and the n-doped region 236 may form an NPNPN junction structure, which further reduces the charge coupling from the first readout circuit 224 to the second readout circuit 234, or from the second readout circuit 234 to the first readout circuit 224. In some embodiments, the n-well region 244 also effectively reduces the potential energy barrier perceived when electrons flow from the absorption layer 206 to the substrate 202.

[0119] In some embodiments, the p-doped regions 228 and 238 are formed entirely within the n-well region 244. In some other embodiments, the p-doped regions 228 and 238 are partially formed within the n-well region 244. For example, a portion of the p-doped region 228 may be formed within the n-well region 244 by implanting p-dopants, and another portion of the p-doped region 228 may be formed within the substrate 202 by implanting p-dopants. In some embodiments, the depth of the n-well region 244 is shallower than the depths of the p-doped regions 228 and 238.

[0120] Figure 2D is an example of a switched photodetector 270 that converts an optical signal into an electrical signal. The switched photodetector 270 is similar to Figure 2A the switched photodetector 200 shown, but further includes one or more p-well regions 246 and one or more p-well regions 248. In some embodiments, the one or more p-well regions 246 and the one or more p-well regions 246 may be part of an annular structure; the annular structure surrounds the first switch 208 and the second switch 210. In some embodiments, the doping level range of the one or more p-well regions 246 and 248 may be from 10^15 cm^-3 to 10^20 cm^-3. The one or more p-well regions 246 and 248 may be used for isolating photoelectrons from adjacent pixels.

[0121] Although Figures 2A - 2D not shown, in some embodiments, the optical signal may enter the switched photodetector from the back side of the substrate 202 of the switched photodetector. One or more optical devices (e.g., microlenses or light guides) may be fabricated on the back side of the substrate 202 to focus, collimate, defocus, filter, or otherwise manipulate the optical signal.

[0122] Although in Figures 2A - 2D not shown, in some embodiments, the first switch 208 and the second switch 210 may alternatively be fabricated to collect holes instead of electrons; in this case, the p-doped region 228, the p-doped region 238, and the p-doped region 209 will be replaced by n-doped regions, the n-doped regions 226 and 236 will be replaced by p-doped regions, the n-well regions 252, 254, and 244 will be replaced by p-well regions, and the p-well regions 246 and 248 will be replaced by n-well regions.

[0123] Although Figures 2A - 2D not shown, in some embodiments, the absorption layer 206 may be bonded to the substrate after fabricating the switched photodetectors 200, 250, 260, and 270. The carrier substrate may be any material that allows the optical signal 212 to be transmitted to the switched photodetector. For example, the substrate may be a polymer or glass. In some embodiments, one or more optical devices (e.g., microlenses or light guides) may be fabricated on the carrier substrate to focus, collimate, defocus, filter, or otherwise manipulate the optical signal.

[0124] Although Figures 2A - 2D not shown, in some embodiments, the switching photodetectors 200, 250, 260, and 270 may be bonded (e.g., by metal-metal bonding, oxide-oxide bonding, hybrid bonding) to a second substrate including a control signal circuit and / or a readout circuit and / or a phase-locked loop and / or an analog-to-digital converter circuit. A metal layer may be deposited on the top of the switching photodetector for use as a reflector to reflect the optical signal incident from the back side of the substrate 202. Adding a mirror-like metal layer can increase the absorption efficiency (quantum efficiency) of the absorption layer 206. For example, by adding a reflective metal layer, the absorption efficiency of the photodetector operating at long NIR wavelengths in the range of 1.0 μm to 1.6 μm can be significantly improved. An oxide layer may be included between the metal layer and the absorption layer to increase the reflectivity. The metal layer can also be used as a bonding layer during the wafer bonding process. In some embodiments, one or more switches similar to the first switch 208 and the second switch 210 can be added to link the control signal / readout circuit.

[0125] Although Figures 2A - 2D not shown, in some embodiments, the absorption layer 206 may be partially or fully embedded or recessed in the substrate 202 to mitigate the surface topography and facilitate fabrication. The foregoing technique is described in U.S. Patent Publication US20170040362A1.

[0126] Figure 3A is an example of a switching photodetector 300 for converting an optical signal into an electrical signal. In Figure 3A , the first switches 308a and 308b, and the second switches 310a and 310b are fabricated on the substrate 302 in a vertical configuration. One feature of the switching photodetector 100 or the switching photodetector 200 is that the larger the optical window size d, the longer the optoelectronic transport time for electrons to drift or diffuse from one switch to another. This affects the operating speed of the switching photodetector. The switching photodetector 300 can further improve the operating speed through vertically arranged p-doped regions and n-doped regions. Through such a vertical arrangement, the optoelectronic transport distance will be limited by the thickness t of the absorption layer (e.g., about 1 μm), rather than by the optical window size d of the absorption layer (e.g., about 10 μm). The switching photodetector 300 includes an absorption layer 306 fabricated on the substrate 302. The substrate 302 can be any suitable substrate on which semiconductor devices can be configured. For example, the substrate 302 can be a silicon substrate.

[0127] Generally, the absorption layer 306 receives the optical signal 312 and converts the optical signal 312 into an electrical signal. The absorption layer 306 is similar to the absorption layer 206. The absorption layer 306 can be intrinsic, p-type, or n-type. In some embodiments, the absorption layer 206 can be formed of a p-type germanium-silicon material. In some embodiments, the absorption layer 306 can include a p-doped region 309; the p-doped region 309 is similar to the p-doped region 209.

[0128] The first switches 308a and 308b and the second switches 310a and 310b are fabricated in the substrate 302. It should be noted here that although Figure 3A two first switches 308a and 308b and two second switches 310a and 310b are shown, the number of the first switches and the second switches can be more or less. The first switches 308a and 308b are coupled to the first control signal 322 and the first readout circuit 324, and the second switches 310a and 310b are coupled to the second control signal 332 and the second readout circuit 334.

[0129] Generally, the first readout circuit 324 or the second readout circuit 334 determines to collect electrons or holes under the control of the first control signal 322 and the second control signal 332. The first control signal 322 is similar to the first control signal 122, the second control signal 332 is similar to the second control signal 132, the first readout circuit 324 is similar to the first readout circuit 124, and the second readout circuit 334 is similar to the second readout circuit 134. In some embodiments, the first switches 308 and 308b and the second switches 310a and 310b are fabricated to collect electrons generated by the absorption layer 306. In such a case, the first switches 308a and 308b each include p-doped regions 328a, 328b and n-doped regions 326a, 326b. For example, the p-doped regions 328a and 328b may have p+ doping, where the activated dopant concentration can be as high as achievable by the fabrication process. For example, when the substrate 302 is silicon and doped with boron, it is about 2x1020 cm-3. In some embodiments, the doping concentration of the p-doped regions 328a and 328b may be lower than 2x1020 cm-3 in order to simplify the fabrication complexity at the cost of increasing the contact resistance. The n-doped regions 326a and 326b may have n+ doping, where the activated dopant concentration can be as high as achievable by the fabrication process. For example, when the substrate 302 is silicon and doped with phosphorus, it is about 5x1020 cm-3. In some embodiments, the doping concentration of the n-doped regions 326a and 326b may be lower than 5x1020 cm-3 in order to simplify the fabrication complexity at the cost of increasing the contact resistance. The distance between the p-doped region 328a and the n-doped region 326a can be designed based on the manufacturing process design rules. For example, the distance between the p-doped region 328a and the n-doped region 326a can be controlled according to the energy associated with the implanted dopants. Generally, when the distance between the p-doped regions 328a / 328b and the n-doped regions 326a / 326b is closer, the switching efficiency of the generated photo carriers is higher. The second switches 310a and 310b each include p-doped regions 338a and 338b, and n-doped regions 336a and 336b. The p-doped regions 338a / 338b are similar to the p-doped regions 328a / 328b, and the n-doped regions 336a / 336b are similar to the n-doped regions 326a / 326b.

[0130] In some embodiments, the p-doped regions 328a and 328b are coupled to the first control signal 322. The n-doped regions 326a and 326b are coupled to the readout circuit 324. The p-doped regions 338a and 338b are coupled to the second control signal 332. The n-doped regions 336a and 336b are coupled to the readout circuit 332. The first control signal 322 and the second control signal 332 are used to control the collection process of the electrons generated by absorbing photons. For example, when the absorption layer 306 absorbs photons in the optical signal 312, electron-hole pairs are generated and drift or diffuse to the substrate 302. When a voltage is applied, if the first control signal 322 is biased with respect to the second control signal 332, an electric field is established between the p-doped region 309 and the p-doped regions 328a / 328b or 338a / 338b, and the free electrons drift from the absorption layer 306 to the p-doped regions 328a / 328b or 338a / 338b according to the direction of the electric field. In some embodiments, the first control signal 322 can be fixed at a voltage value Vi, and the second control signal 332 can vary between Vi±ΔV. The bias value determines the drift direction of the electrons. Accordingly, when a set of switches (e.g., the first switches 308a and 308b) are turned "on" (i.e., the electrons drift towards the p-doped regions 328a and 328b), the other set of switches (e.g., the second switches 310a and 310b) are turned "off" (i.e., the electrons are blocked by the p-doped regions 338a and 338b). In some embodiments, the first control signal 322 and the second control signal 332 can have different voltage values.

[0131] In each of the first switches 308a / 308b, an electric field system is established between the p-doped regions 328a / 328b and the n-doped regions 326a / 326b. Similarly, in each of the second switches 310a / 310b, an electric field is established between the p-doped regions 338a / 338b and the n-doped regions 336a / 336b. When the first switches 308a and 308b are turned "on" and the second switches 310a and 310b are turned "off", electrons drift to the p-doped regions 328a and 328b, and the electric field between the p-doped region 328a and the n-doped region 326a further moves the electrons to the n-doped region 326a. Similarly, the electric field between the p-doped region 328b and the n-doped region 326b also moves the electrons to the n-doped region 326b. The readout circuit 324 can then process the charges collected by the n-doped regions 326a and 326b. Conversely, when the second switches 310a and 310b are turned "on" and the first switches 308a and 308b are turned "off", electrons drift to the p-doped regions 338a and 338b, and the electric field between the p-doped region 338a and the n-doped region 336a further moves the electrons to the n-doped region 336a. Similarly, the electric field between the p-doped region 338b and the n-doped region 336b also moves the electrons to the n-doped region 336b. The readout circuit 334 can then process the charges collected by the n-doped regions 336a and 336b.

[0132] In some embodiments, a voltage can be applied between the p-doped region and the n-doped region of the switch to operate the switch in an avalanche mechanism to increase the sensitivity of the switched photodetector 300. For example, in the case where the substrate 302 comprises GeSi, when the distance between the p-doped region 328a and the n-doped region 326a is about 100 nm, a voltage of less than 7 V can be applied to establish avalanche gain between the p-doped region 328a and the n-doped region 326a.

[0133] In some embodiments, the p-doped region 309 can be coupled to an external control 314. For example, the p-doped region 309 can be coupled to electrical ground. In some embodiments, the p-doped region 309 can be floating or uncoupled to any external control. In some embodiments, the substrate 302 can be coupled to an external control 316. For example, the substrate 302 can be coupled to electrical ground. In some embodiments, the substrate 302 can be floating or uncoupled to any external control.

[0134] Figure 3B is an example of a switched photodetector 360, where the switched photodetector 360 is used to convert an optical signal into an electrical signal. The switched photodetector 360 is similar to Figure 3AThe switching photodetector 300, but the switching photodetector 360 further includes an n-well region 344. In addition, the absorption region 360 can be a p-doped region, and the substrate can be a p-doped substrate. In some embodiments, the doping level range of the n-well region 344 can be from 10^15 cm^-3 to 10^17 cm^-3, and the doping level range of the substrate 302 can be from 10^14 cm^-3 to 10^16 cm^-3.

[0135] The arrangement of the p-doped region 328a, the n-well region 344, and the p-doped region 338a forms a PNP junction structure; similarly, the arrangement of the p-doped region 328b, the n-well region 344, and the p-doped region 338b forms another PNP junction structure. Generally, the PNP junction structure reduces the leakage current from the first control signal 322 to the second control signal 332, or from the second control signal 332 to the first control signal 322. The arrangement of the n-doped region 326a, the p-doped substrate 302, and the n-doped region 336a forms an NPN junction structure; similarly, the arrangement of the n-doped region 326b, the p-doped substrate 302, and the n-doped region 336b forms an NPN junction structure. Generally, the NPN junction structure reduces the charge coupling from the first readout circuit 324 to the second readout circuit 334, or from the second readout circuit 334 to the first readout circuit 324. In some embodiments, the n-well region 344 also effectively reduces the potential energy barrier perceived when electrons flow from the absorption layer 306 to the substrate 302.

[0136] In some embodiments, the p-doped regions 328a, 338a, 328b, and 338b are completely formed within the n-well region 344. In some other embodiments, the p-doped regions 328a, 338a, 328b, and 338b are partially formed within the n-well region 344. For example, a portion of the p-doped region 328a can be formed by implanting p-dopants in the n-well region 344, and another portion of the p-doped region 328a can be formed by implanting p-dopants in the substrate 302. In some embodiments, the depth of the n-well region is shallower than the depths of the p-doped regions 328a, 338a, 328b, and 338b.

[0137] Figure 3C An example switching photodetector 370 for converting an optical signal into an electrical signal. The switching photodetector 370 is similar to Figure 3AShown is a switched photodetector 300, but further includes one or more p-well regions 346 and one or more p-well regions 348. In some embodiments, one or more p-well regions 346 and one or more p-well regions 348 can be part of an annular structure surrounding the first switches 308a, 308b and the second switches 310a, 310b. In some embodiments, the doping level range of one or more p-well regions can be from 10^15 cm^-3 to 10^20 cm^-3. One or more p-well regions 246 and 248 can be used for isolation of photoelectrons from adjacent pixels.

[0138] Figure 3D is a cross-sectional view of an example switched photodetector 380. In Figure 3D it, the p-doped regions 328a, 328b of the first switches 308a, 308b and the p-doped regions 338a, 338b of the second switches 310a, 310b can be arranged in a finger-like pattern on a first plane 362 of a substrate 302. Figure 3D Further shown is that the n-doped regions 326a and 326b of the first switches 308a and 308b and the n-doped regions 336a and 336b of the second switches 310a and 310b can be arranged in a finger-like pattern on a second plane 364 of the substrate 302.

[0139] Although Figures 3A - 3D not shown, in some embodiments, an optical signal can enter the switched photodetector from the back side of the substrate 302 of the switched photodetector. One or more optical devices (e.g., microlenses or light guides) can be fabricated on the back side of the substrate 302 to focus, collimate, defocus, filter, or otherwise manipulate the optical signal.

[0140] Although Figures 3A - 3D not shown, in some embodiments, the first switches 308a and 308b and the second switches 310a and 310b can alternatively be fabricated to collect holes instead of electrons; in this case, the p-doped regions 328a and 328b, the p-doped regions 338a and 338b, and the p-doped region 309 will be replaced by n-doped regions, the n-doped regions 326a and 326b and the n-doped regions 336a and 336b will be replaced by p-doped regions, the n-well region 344 will be replaced by a p-well region, and the p-well regions 346 and 348 will be replaced by n-well regions.

[0141] Although Figures 3A - 3DNot shown, but in some other embodiments, the absorption layer 306 may be bonded to the substrate after the switch photodetectors 300, 360, 370, and 380 are fabricated. The substrate can be any material that allows the optical signal 312 to be transmitted to the switch photodetector. For example, the substrate can be a polymer or glass. In some embodiments, one or more optical devices may be fabricated on the carrier substrate to focus, collimate, defocus, filter, or otherwise manipulate the optical signal 312.

[0142] Although Figures 3A - 3D Not shown, but in some embodiments, the switch photodetectors 300, 360, 370, and 380 may be bonded (e.g., by metal-metal bonding, oxide-oxide bonding, hybrid bonding) to a second substrate having a circuit that includes a control signal circuit and / or a readout circuit and / or a phase-locked loop and / or an analog-to-digital converter circuit. A metal layer may be deposited on top of the switch photodetector to act as a reflector to reflect the optical signal incident from the back side of the substrate 302. Adding a mirror-like metal layer can increase the absorption efficiency (quantum efficiency) of the absorption layer 306. For example, by adding a reflective metal layer, the absorption efficiency of the photodetector operating at long NIR wavelengths in the range of 1.0 μm to 1.6 μm can be significantly improved. An oxide layer may be included between the metal layer and the absorption layer to increase the reflectivity. The metal layer can also serve as a bonding layer during the wafer bonding process. In some embodiments, one or more switches similar to the first switch 308a (or 308b) and the second switch 310a (or 310b) may be added to interface with the control signal / readout circuit.

[0143] Although Figures 3A - 3D Not shown, but in some embodiments, the absorption layer 306 may be partially or fully embedded or recessed in the substrate 302 to mitigate the surface topography and facilitate fabrication. The foregoing technique is described in U.S. Patent Publication US20170040362A1.

[0144] Figure 4A is an example of a switch photodetector 400, where the switch photodetector 400 is used to convert an optical signal into an electrical signal. The switch photodetector 400 includes an absorption layer 406 fabricated on a substrate 402. The substrate 402 can be any suitable substrate on which semiconductor devices can be configured. For example, the substrate 402 can be a silicon substrate. The absorption layer 406 includes a first switch 408 and a second switch 410.

[0145] Generally, the absorption layer 406 receives the optical signal 412 and converts the optical signal 412 into an electrical signal. The absorption layer 406 can be intrinsic, p-type, or n-type. In some embodiments, the absorption layer 406 can be formed of a p-type GeSi material. The absorption layer 406 is selected to have a high absorption coefficient within a desired wavelength range. For NIR wavelengths, the absorption layer 406 can be a GeSi mesa, where GeSi absorbs photons in the optical signal 412 and generates electron-hole pairs. The material composition of germanium and silicon in the GeSi mesa can be selected for a particular technology or application. In some embodiments, the absorption layer 406 is designed to have a thickness t. For example, for wavelengths of 850 nm or 940 nm, the thickness of the GeSi mesa can be approximately 1 μm to have a sufficiently large quantum efficiency. In some embodiments, the surface of the absorption layer 406 is designed to have a specific shape. For example, the GeSi mesa can be circular, square, or rectangular, depending on the spatial profile of the optical signal 412 on the surface of the GeSi mesa. In some embodiments, the absorption layer 106 is designed to have a lateral dimension d for receiving the optical signal 412. For example, the GeSi mesa can be circular or rectangular, where d ranges from 1 μm to 50 μm.

[0146] The first switch 408 and the second switch 410 are fabricated in the absorption layer 406 and the substrate 402. The first switch 408 is coupled to the first control signal 422 and the first readout circuit 424. The second switch 410 is coupled to the second control signal 432 and the second readout circuit 434. Generally, the first readout circuit 424 or the second readout circuit 434 determines to collect electrons or holes under the control of the first control signal 422 and the second control signal 432.

[0147] In some embodiments, the first switch 408 and the second switch 410 can be fabricated to collect electrons. In such a case, the first switch 408 includes a p-doped region 428 implanted in the absorption layer 406 and an n-doped region 426 implanted in the substrate 402. For example, the p-doped region 428 can have a p+ doping, where the activated dopant concentration can be as high as achievable by the fabrication process, such as about 5×1020 cm−3 when the absorption layer 106 is germanium and doped with boron. In some embodiments, the doping concentration of the p-doped region 428 can be lower than 5×1020 cm−3 to simplify the fabrication complexity at the cost of increasing the contact resistance. The n-doped region 426 can have an n+ doping, where the activated dopant concentration can be as high as achievable by the fabrication process, such as about 5×1020 cm−3 when the substrate 402 is silicon and doped with phosphorus. In some embodiments, the doping concentration of the n-doped region 426 can be lower than 5×1020 cm−3 to simplify the fabrication complexity at the cost of increasing the contact resistance. The distance between the p-doped region 428 and the n-doped region 426 can be designed based on the manufacturing process design rules. Generally, the closer the distance between the p-doped region 428 and the n-doped region 426, the higher the switching efficiency of the generated photo-carriers. The second switch 410 includes a p-doped region 438 and an n-doped region 436, where the p-doped region 438 is similar to the p-doped region 428, and the n-doped region 436 is similar to the n-doped region 426.

[0148] In some embodiments, the p-doped region 428 is coupled to a first control signal 422. For example, the p-doped region 448 can be coupled to a voltage source, where the first control signal 422 can be an AC voltage signal from the voltage source. In some embodiments, the n-doped region 426 is coupled to a readout circuit 424. The readout circuit 424 can be a three-transistor configuration consisting of a reset gate, a source follower, and a select gate, a circuit including four or more transistors, or any circuit suitable for processing charge. In some embodiments, the readout circuit 424 can be fabricated on the substrate 402. In some other embodiments, the readout circuit 424 can be fabricated on another substrate and integrated or co-packaged with the switched photodetector 400 using chip / wafer bonding or stacking techniques.

[0149] The p-doped region 438 is coupled to a second control signal 432. For example, the p-doped region 438 can be coupled to a voltage source; where the second control signal 462 can be an AC voltage signal and its phase is opposite to that of the first control signal 122. In some embodiments, the n-doped region 436 is coupled to a readout circuit 434, and the readout circuit 434 can be similar to the readout circuit 424.

[0150] The first control signal 422 and the second control signal 432 are used to control the collection process of electrons generated by absorbing photons. For example, if the first control signal 422 biases the second control signal 432, an electric field is formed between the p-doped region 428 and the p-doped region 438, and free electrons drift to the p-doped region 428 or the p-doped region 438 according to the direction of the electric field.

[0151] In some embodiments, the first control signal 422 can be fixed at a voltage value Vi, and the second control signal 432 can vary between Vi±ΔV. The bias value determines the drift direction of the electrons. Accordingly, when a switch (e.g., the first switch 408) is turned "on" (i.e., electrons drift to the p-doped region 428), another switch (e.g., the second switch 410) is turned "off" (i.e., electrons are blocked by the p-doped region 438). In some embodiments, the first control signal 422 and the second control signal 432 can have different voltage values.

[0152] Generally, the difference (before equilibrium) between the Fermi levels of the p-doped region and the n-doped region forms an electric field between these two regions. In the first switch 408, an electric field is formed between the p-doped region 428 and the n-doped region 426. Similarly, in the second switch 410, an electric field is formed between the p-doped region 438 and the n-doped region 436. When the first switch 408 is turned "on" and the second switch 410 is turned "off", electrons drift to the p-doped region 428, and the electric field between the p-doped region 428 and the n-doped region 426 further carries the electrons to the n-doped region 426. Then the readout circuit 424 can be enabled to process the charge collected by the n-doped region 426. On the other hand, when the second switch 410 is turned "on" and the first switch 408 is turned "off", electrons drift to the p-doped region 438, and the electric field between the p-doped region 438 and the n-doped region 436 further carries the electrons to the n-doped region 436. The readout circuit 434 can then process the charge collected by the n-doped region 436.

[0153] In some embodiments, the substrate 402 is coupled to an external control 416. For example, the substrate 402 can be coupled to electrical ground, or a preset voltage lower than the voltage of the n-doped regions 426 and 436. In some embodiments, the substrate 402 can be floating or not coupled to any external control.

[0154] Figure 4B is an example of a switched photodetector 450, where the switched photodetector 450 is used to convert an optical signal into an electrical signal. The switched photodetector 450 is similar to Figure 4AThe illustrated switched photodetector 400, but the first switch 408 and the second switch 410 further include an n-well region 452 and an n-well region 454, respectively. Additionally, the absorption region 406 can be a p-doped layer and the substrate 402 can be a p-doped substrate. In some embodiments, the doping level range of the n-well region 452 can be from 10^15 cm^-3 to 10^17 cm^-3, and the doping level range of the substrate 402 can be from 10^14 cm^-3 to 10^16 cm^-3.

[0155] The arrangement of the p-doped region 428, the n-well region 452, the absorption region 406, the n-well region 454, and the p-doped region 438 forms a PNPNP junction structure. Generally, the PNPNP junction structure reduces the leakage current from the first control signal 422 to the second control signal 432, or from the second control signal 432 to the first control signal 422.

[0156] The arrangement of the n-doped region 426, the p-doped substrate 402, and the n-doped region 436 forms an NPN junction structure. Generally, the NPN junction structure reduces the charge coupling from the first readout circuit 424 to the second readout circuit 434, or from the second readout circuit 434 to the first readout circuit 424.

[0157] In some embodiments, the p-doped region 428 is completely formed within the n-well region 452. In some other embodiments, the p-doped region 428 is partially formed within the n-well region 452. For example, a portion of the p-doped region 428 can be formed by implanting p-dopants in the n-well region 452, and another portion of the p-doped region 428 can be formed by implanting p-dopants in the absorption layer 406. Similarly, in some other embodiments, the p-doped region 438 is completely formed within the n-well region 454. In some embodiments, the p-doped region 438 is partially formed within the n-well region 454. In some embodiments, the depths of the n-well regions 452 and 454 are shallower than the depth of the p-doped region.

[0158] Figure 4C is an example of a switched photodetector 460, where the photodetector 460 is used to convert an optical signal into an electrical signal. The switched photodetector 460 is similar to Figure 4A the illustrated switched photodetector 400, but the absorption layer 406 further includes an n-well region 456. Additionally, the absorption region 406 can be a p-doped region and the substrate 402 can be a p-doped substrate. In some embodiments, the doping level range of the n-well region 456 can be from 10^15 cm^-3 to 10^17 cm^-3, and the doping level ranges of the absorption layer 406 and the substrate 402 can be from 10^14 cm^-3 to 10^16 cm^-3.

[0159] The arrangement of the p-doped regions 428, n-well region 456, and p-doped region 438 forms a PNP junction structure. Generally, the PNP junction structure reduces the leakage current from the first control signal 422 to the second control signal 432, or from the second control signal 432 to the first control signal 422.

[0160] The arrangement of the n-doped regions 426, p-doped absorption layer 406, and n-doped region 436 forms an NPN junction structure. Generally, the NPN junction structure reduces the charge coupling from the first readout circuit 424 to the second readout circuit 434, or from the second readout circuit 434 to the first readout circuit 424.

[0161] In some embodiments, the p-doped regions 428 and 438 are completely formed within the n-well region 456. In some other embodiments, the p-doped regions 428 and 438 are partially formed within the n-well region 456. For example, a portion of the p-doped region 428 can be formed by implanting p-dopants into the n-well region 456, and another portion of the p-doped region 428 can be formed by implanting p-dopants into the absorption layer 406. In some embodiments, the depth of the n-well region is shallower than the depths of the p-doped regions 428 and 438.

[0162] Figure 4D is an example of the switching photodetector 470 for converting an optical signal into an electrical signal. The switching photodetector 470 is similar to Figure 4C the switching photodetector 460 shown, but its n-well region 458 is formed to extend from the absorption region 406 to the substrate 402. Additionally, the absorption region 406 can be a p-doped region, and the substrate 402 can be a p-doped substrate. In some embodiments, the doping level range of the n-well region 456 can be from 10^15 cm^-3 to 10^17 cm^-3. The doping level ranges of the absorption layer 406 and the substrate 402 can be from 10^14 cm^-3 to 10^16 cm^-3.

[0163] The arrangement of the p-doped regions 428, n-well region 458, and p-doped region 438 forms a PNP junction structure, which can further reduce the leakage current from the first control signal 422 to the second control signal 432, or from the second control signal 432 to the first control signal 422. The n-doped regions 426, p-doped substrate 402, n-well region 458, p-doped substrate 402, and n-doped region 436 form an NPNPN junction structure, which reduces the charge coupling from the first readout circuit 424 to the second readout circuit 434, or from the second readout circuit 434 to the first readout circuit 424. In some embodiments, the n-well region 458 can effectively reduce the potential barrier perceived by electrons when flowing from the absorption layer 406 to the substrate 402.

[0164] Figure 4Eis an example of a switched photodetector 480, where the photodetector 480 is used to convert an optical signal into an electrical signal. The switched photodetector 480 is similar to Figure 4A the switched photodetector 400 shown, but the switched photodetector 480 further includes one or more p-well regions 446 and one or more p-well regions 448. In some embodiments, one or more of the p-well regions 446 and one or more of the p-well regions 448 can be part of an annular structure; the annular structure surrounds the first switch 408 and the second switch 410. In some embodiments, the doping level range of the p-well regions 446 and 448 can be from 10^15 cm^-3 to 10^20 cm^-3. The one or more p-well regions 246 and 248 can be used as isolation for photoelectrons from adjacent pixels.

[0165] Although Figures 4A - 4D not shown, in some embodiments, the optical signal can enter the switched photodetector from the back side of the substrate 402 of the switched photodetector. One or more optical devices (e.g., microlenses or light guides) can be fabricated on the back side of the substrate 402 to focus, collimate, defocus, filter, or otherwise manipulate the optical signal.

[0166] Although Figures 4A - 4E not shown, in some embodiments, the first switch 408 and the second switch 410 can alternatively be fabricated to collect holes instead of electrons; in this case, the p-doped regions 428 and 438 will be replaced by n-doped regions, the n-doped regions 426 and 436 will be replaced by p-doped regions, the n-well regions 452, 454, 456, and 458 will be replaced by p-well regions, and the p-well regions 446 and 448 will be replaced by n-well regions.

[0167] Although Figures 4A - 4E not shown, in some embodiments, the absorption layer 406 can be bonded to the substrate after the switched photodetectors 400, 450, 460, 470, and 480 are formed. The substrate can be any material that allows the optoelectronic signal 412 to be transmitted to the switched photodetector. For example, the substrate can be a polymer or glass. In some embodiments, one or more optical devices (e.g., microlenses or light guides) can be fabricated on the carrier substrate to focus, collimate, defocus, filter, or otherwise manipulate the optical signal.

[0168] Although Figures 4A - 4ENot shown, but in some embodiments, the switching photodetectors 400, 450, 460, 470, and 480 may be bonded (e.g., by metal-metal bonding, oxide-oxide bonding, hybrid bonding) to a second substrate that includes a control signal circuit and / or a readout circuit and / or a phase-locked loop and / or an analog-to-digital converter circuit. A metal layer may be deposited on top of the switching photodetector for use as a reflector to reflect the optical signal incident from the back side of the substrate 402. Adding a mirror-like metal layer can increase the absorption efficiency (quantum efficiency) of the absorption layer 406. For example, by increasing the reflective metal layer, the absorption efficiency of the photodetector operating at long NIR wavelengths in the range of 1.0 μm to 1.6 μm can be significantly improved. An oxide layer may be included between the metal layer and the absorption layer to increase the reflectivity. The metal layer may also be used as a bonding layer during the wafer bonding process. In some embodiments, one or more switches similar to the first switch 408 and the second switch 410 can be added to link the control signal / readout circuit.

[0169] Although Figures 4A - 4E not shown, but in some embodiments, the absorption layer 406 may be partially or fully embedded or recessed in the substrate 402 to mitigate the surface topography and facilitate fabrication. The foregoing technique is described in U.S. Patent Publication US20170040362A1.

[0170] Figure 5A is an example of a switching photodetector 500, where the switching photodetector 500 is used to convert an optical signal into an electrical signal. The switching photodetector 500 includes an absorption layer 506 fabricated on a substrate 502 and a first layer 508 formed above the absorption layer 506 and the substrate 502. The substrate 502 may be similar to the foregoing substrate 102, and the absorption layer 506 may be similar to the foregoing absorption layer 106 and may be formed, for example, of Ge or GeSi, with the concentration of Ge in the range of 1 - 99%. The background doping polarity of the Ge or GeSi absorption layer 506 may be p-type and the doping level range may be 10 14 cm -3 to 10 16 cm -3 . The background doping level may depend on, for example, the explicit incorporation of doping or material defects introduced during the formation of the absorption layer 506. The absorption layer 506 of the switching photodetector 500 has a mesa structure, and the substrate 502 supports this mesa structure. Although in the illustrated example, the sidewalls may be vertical, the sidewall profile of the mesa structure may vary depending on the characteristics of the growth and fabrication process of the absorption layer 506.

[0171] The first layer 508 covers the upper surface and the side surfaces of the absorption layer 506 and the portion of the substrate 502 that forms the upper surface of the absorption layer 506. The first layer 508 can be formed of materials compatible with CMOS processes, such as amorphous silicon, polycrystalline silicon, epitaxially grown silicon, alumina family (e.g., Al 2 O 3 ), silica family (e.g., SiO2), germanium oxide family (e.g., GeO2), GeSi family (e.g., Ge0.4Si0.6), silicon nitride family (e.g., Si3N4), high-k materials (e.g., HfOx, ZnOx, LaOx, LaSiOx), and any combination thereof. The first layer 508 present on the surface of the absorption layer 506 can have different effects. For example, the first layer 508 can act as a surface passivation layer for the absorption layer 506, thereby reducing the dark current or leakage current generated by defects present on the surface of the absorption layer 506. In the case of a germanium (Ge) absorption layer or a germanium-silicon (GeSi) absorption layer 506, surface defects can be a significant source of dark current or leakage current, which increases the noise intensity in the photocurrent generated by the switching photodetector 500. By forming the first layer 508 on the surface of the absorption layer 506, the dark current or leakage current can be reduced, thereby reducing the noise amount of the photodetector 500. In other examples, the first layer 508 can adjust the Schottky barrier between the contacts formed on the switching photodetector 500 and the absorption layer 506 and / or the substrate 502. The barrier adjustment effect will be described in detail later.

[0172] Figure 5B is an example of a switching photodetector 510 for converting an optical signal into an electrical signal. The switching photodetector 510 is similar to the switching photodetector 500 shown in Figure 5A , but differs in that the absorption layer 506 is partially embedded in a recess formed in the substrate 502, and the switching photodetector 510 further further includes a spacer 512. The spacer 512 can be a dielectric material, such as various oxides and nitrides that separate the sidewalls of the absorption layer 506 from the substrate 502. In some embodiments, the spacer 512 can be omitted, and the embedded portion of the absorption layer 506 can directly contact the surface of the recess formed in the substrate 502, such as the

[110] sidewall of a silicon substrate. The embedding technique is described in U.S. Patent Publication US20170040362A1.

[0173] Figure 5C is an example of a switching photodetector 520 for converting an optical signal into an electrical signal. The switching photodetector 520 is similar to the switching photodetector 510 shown in Figure 5B , but differs in that the absorption layer 506 is completely embedded in a recess formed in the substrate 502. The embedding technique is described in U.S. Patent Publication US20170040362A1.

[0174] Figure 5D is an example of the switching photodetector 530 for converting an optical signal into an electrical signal. The switching photodetector 530 is similar to Figure 5B the switching photodetector 510 illustrated in Figure 1A but is different in that the first switch 532 and the second switch 542 are fabricated in the absorption layer 506 and the first layer 508. The first switch 532 is similar to Figure 1A the first switch 108 shown in Figure 1A but further includes a first readout contact 535 coupled to the first n-doped region 534 and a first control contact 538 coupled to the first p-doped region 537. Similarly, the second switch 542 is similar to Figure 1A the second switch 110 shown in

[0175] but further includes a second readout contact 545 coupled to the second n-doped region 544 and a second control contact 548 coupled to the second p-doped region 547. The first p-doped region 537 and the second p-doped region 547 can be control regions and the first n-doped region 534 and the second n-doped region 544 can be readout regions. The first readout contact 535 and the second readout contact 545 are connected to a readout circuit corresponding to the readout circuits 124 and 134 shown in

[0176] Figure 5E is an example of the switching photodetector 550, where the switching photodetector 550 is used to convert an optical signal into an electrical signal. The switching photodetector 550 is similar to Figure 5DThe illustrated switching photodetector 530, except that the first switch 532 and the second switch 542 further include n-well regions 539 and 549 and p-well regions 536 and 546, respectively. Adding the n-well regions and the p-well regions can adjust the electronic and / or optical characteristics of the switching photodetector 550. In some embodiments, the doping levels of the n-well regions 539 and 549 and the p-well regions 536 and 546 can range from 10^15 cm^-3 to 10^17 cm^-3.

[0177] The arrangement of the p-well region 537, the n-well region 539, the p-type absorption layer 506, the n-well region 549, and the p-doped region 547 forms a PNPNP junction structure. Generally, the PNPNP junction structure reduces the leakage current from the first control signal 122 to the second control signal 132, or from the second control signal 132 to the first control signal 122. The arrangement of the n-doped region 534, the p-well region 536, the p-type absorption layer 506, the p-well region 546, and the n-doped region 544 forms an NPN junction structure. Generally, the NPN junction structure reduces the charge coupling from the first readout circuit 124 to the second readout circuit 134, or from the second readout circuit 134 to the first readout circuit 124.

[0178] In some embodiments, the p-doped region 537 is completely formed within the n-well region 539. In some other embodiments, the p-doped region 537 is partially formed within the n-well region 539. For example, a portion of the p-doped region 537 can be formed by implanting p-dopants in the n-well region 539, and another portion of the p-doped region 537 can be formed by implanting p-dopants in the absorption layer 506. Similarly, in some embodiments, the p-doped region 547 is completely formed within the n-well region 549. In some other embodiments, the p-doped region 547 is partially formed within the n-well region 549. In some embodiments, the n-well regions 539 and 549 form a continuous n-well region that includes at least a portion of the p-doped regions 537 and 547.

[0179] In some embodiments, the n-doped region 534 is completely formed outside the p-well region. In some other embodiments, the n-doped region 534 is partially formed within the p-well region 536. For example, a portion of the n-doped region 534 can be formed by implanting n-dopants in the p-well region 536, and another portion of the n-doped region 534 can be formed by implanting n-dopants in the absorption layer 506. Similarly, in some embodiments, the n-doped region 544 is completely formed outside the p-well region 546. In some other embodiments, the n-doping 544 is partially formed within the p-well region 546.

[0180] Although Figure 5D and 5EA switched photodetector with a partially embedded absorption region 506 is shown, but the same structure can be used in a switched photodetector 500 with a non-embedded absorption layer 506 and a photonic switch detector 520 with a fully embedded absorption layer 506 to achieve a similar effect.

[0181] Although for ease of illustration, the n-well regions 539 and 549 and the p-well regions 536 and 546 are drawn together; in actual implementation, these well regions can be implemented independently or in any combination.

[0182] Figure 5F is an example of a switched photodetector 560 for converting an optical signal into an electrical signal. The switched photodetector 560 is similar to Figure 5D the switched photodetector 530 shown in, but differs in that the p-doped regions 537 and 547 corresponding to the switches 532 and 542 are omitted. As a result, the first control contact 538 and the second control contact 548 form a Schottky junction with the first layer 508. A Schottky junction is an electrical junction formed between a metal and a semiconductor when the semiconductor is not intentionally doped or doped with a moderate impurity concentration, e.g., below 1x1015 cm-3. The region 562 passing through the first layer 508 and the absorption layer 506 between the first control contact 538 and the second control contact 548 is marked as a leakage current path, which will be described in detail with reference to Figure 5G for details.

[0183] Figure 5G is an example of an energy band diagram 570 of the leakage current path formed between the control contacts 538 and 548. The energy band diagram 570 shows the different energy levels of charge carriers such as electrons 572 and holes 574 at different positions in the leakage current path. The vertical axis corresponds to the energy level E, and the horizontal axis corresponds to the position x along the leakage current path formed between the control contacts 538 and 548. An example scenario is shown here where the potential of the first control contact 538 is higher than the potential of the second control contact 548 (e.g., the voltage of the first control signal 122 is lower than the voltage of the second control signal 132). The potential difference is represented by the downward slope of the entire energy band diagram from the first control contact 538 to the second control contact 548. The shown energy level and position relationship is for illustrative purposes only and may not represent actual values.

[0184] The electron barrier 573 and the hole barrier 575 are examples of Schottky barriers. A characteristic of a Schottky junction is the presence of a Schottky barrier, which is the potential energy barrier that electrons 572 and holes 574 need to overcome to pass through the Schottky junction. The values of the barriers 573 and 575 can vary depending on the work functions of the materials of the contacts 538 and 548 and the material of the first layer 508. Thus, by selecting a suitable combination of contact and first layer materials, the desired electron barrier 573 and hole barrier 575 can be set.

[0185] The electron 572 must overcome the electron potential barrier 573 between the first control contact 538 and the first layer 508. By providing a sufficiently high electron potential barrier 573, the potential of the control signal 122 supplied to the first control contact cannot overcome the potential barrier 573. Thereby, the electron potential barrier 573 can block the flow of electrons 572 to the absorption layer 506. In the case where electrons 572 may cross the absorption layer 506 and be transferred to the first layer 508 adjacent to the second control contact 548 due to statistical fluctuations in the thermal energy of the electrons 572 ("thermionic emission") or quantum tunneling of the electrons 572, the electrons 572 can flow through the absorption layer 506 to the first layer 508 adjacent to the second control contact 548. Another electron potential barrier appears at the junction between the absorption layer 506 and the first layer 508, which further prevents the transfer of electrons to the second control contact 548, thereby reducing the leakage current generated by the transfer of electrons 572 from the first control contact 538 to the second control contact 548.

[0186] Similarly, the hole 574 must overcome the potential barrier 575 between the second control contact 548 and the first layer 508. By providing a sufficiently high hole potential barrier 575, the potential of the control signal 132 supplied to the second control contact is sufficient to overcome the potential barrier 575. Thereby, the hole potential barrier 575 can block the transfer of holes 574 to the absorption layer 506. In the case where holes 574 may cross the absorption layer 506 and be transferred to the first layer 508 adjacent to the first control contact 538 due to statistical fluctuations in the thermal energy of the holes 574 ("thermionic emission") or quantum tunneling effect of the holes 574, the holes 574 can flow through the absorption layer 506 to the first layer 508 adjacent to the first control contact 538. Another hole potential barrier appears at the junction between the absorption layer 506 and the first layer 508, which can further block the transfer of holes 574 to the first control contact 538, thereby reducing the leakage current generated by the transfer of holes from the second control contact 548 to the first control contact 538.

[0187] When light irradiates the absorption layer 506, the photons 576 of the light can be absorbed by electrons in the valence band of the absorption layer 506, thereby creating electron-hole pairs as indicated by the vertical arrows beside the photons 576. The electrons in the electron-hole pairs form a photocurrent that is to be read by the readout circuit 124 and / or 134 corresponding to the readout contacts 535 and / or 545 and do not flow to the control contacts 538 and 548. In this case, the potential barrier formed by the junction between the first layer 508 and the absorption layer 506 can prevent this flow, thereby improving the photocurrent collection efficiency of the readout circuit.

[0188] When the first layer 508 (e.g., amorphous silicon, polysilicon, crystalline silicon, or silicon germanium) is inserted between the control contacts 538, 548 and the absorption layer 506 (e.g., GeSi mesa), the Schottky barrier of the metal semiconductor junction is changed so that the electrons or holes injected into the first layer 508 by the contacts 538 and 548 can be partially shielded. The energy consumption of a ToF pixel, such as the switching photodetector described herein, depends in part on the partial leakage current flowing between the two control contacts 538 and 548 connected to the two control circuits. In this way, by partially shielding the electrons or holes injected by the contacts 538 and 548, the energy consumption of the ToF pixel can be greatly reduced.

[0189] Figure 5H is an example of a switch photodetector 580, wherein the switch photodetector 580 is used to convert an optical signal into an electrical signal. The switch photodetector 580 is similar to Figure 5F The switch photodetector 560 is shown in FIG. 1 , but the difference is that the switch photodetector 580 further includes n-well regions 539 and 549 and p-well regions 536 and 546, and has been described with reference to FIG. Figure 5E The structures and effects of n-well regions 539 and 549 and p-well regions 536 and 546 are described. In addition, n-well regions 539 and 549 overlap with portions of first layer 508 below control contacts 538, 548, which increases the voltage drop within absorption layer 506. Increasing the voltage drop within absorption layer 506 increases the electric field strength established within absorption layer 506, which in turn increases the ability of photogenerated electrons to be captured by readout circuitry 124 and / or 134 through corresponding readout contacts 535 and / or 545.

[0190] Figure 5I is an example of a switch photodetector 582 for converting an optical signal into an electrical signal. The switch photodetector 582 is similar to Figure 5E The switching photodetector 550 is shown, but differs in that the first switch 532 is located on the substrate 502 and adjacent to the left side of the absorption region 506, and the second switch 542 is located on the substrate 502 and adjacent to the right side of the absorption region 506. The operation of the switching photodetector 582 is similar to the operation of the switching photodetector described above. However, compared to the electrical contacts formed between the contacts and the Ge or GeSi absorption layer 506, the electrical contacts formed between the contacts, such as the read contacts 535 and 545 or the control contacts 538 and 548, and the silicon substrate 502 generally have a low dark current or leakage current (e.g., because the substrate 502 has fewer material defects than the absorption layer 506), and the overall dark current or leakage current can be lower. Figure 5EThe configuration of the illustrated photodetector 550 is reduced. Additionally, since the switching configuration is on the substrate 502, the photo-generated carriers generated in the absorption region 506 due to light absorption can flow from the absorption region 506 to the substrate 502 before reaching the readout circuits 124 and 134. Based on the specific geometries of the absorption region 506 and the spacer 512 and the differences in their materials, the optical carriers can be conducted through the spacer 512, flow around the periphery of the spacer 512, or a combination thereof.

[0191] In some embodiments, the p-doped regions 537 and 547 can be omitted in a configuration similar to Figure 5F the illustrated configuration. Although the n-well regions 539 and 549 and the p-well regions 536 and 546 are illustrated together for purposes of illustration, these wells can be omitted, can be implemented independently, or can be implemented in any combination.

[0192] Figure 5J is an example of a switched photodetector 586 for converting an optical signal into an electrical signal. The switched photodetector 586 is similar to Figure 5I the illustrated switched photodetector 582, but differs in that the corresponding p-doped regions 537 and 547 of the switches 532 and 542 are omitted; whereby, the first control contacts 538 and 548 form a Schottky junction with the first layer 508. The effect of the Schottky junction has been described with respect to Figures 5F - 5H Since the geometry of the modified switched photodetector 586 is related to the switched photodetector 506, Figure 5G the illustrated energy band diagram 570 still applies to region 562 in the switched photodetector 586, and the potential barrier formed in the first layer 508 is now formed by the corresponding first layer 508, substrate 502, and spacer 512.

[0193] Although the n-well regions 539 and 549 and the p-well regions 536 and 546 are illustrated together for purposes of illustration, these wells can be omitted, can be implemented independently, or can be implemented in any combination.

[0194] Figure 5K is an example of a switched photodetector 588, where the switched photodetector 588 is for converting an optical signal into an electrical signal. The switched photodetector 588 is similar to Figure 5IThe illustrated switched photodetector 582, except that the first switch 532 further includes a second p-doped region 537a, a third control contact 538a coupled to the second p-doped region 537a, and a second n-well region 539a in contact with the second p-doped region 537a, and the second switch 542 further includes a second p-doped region 547a, a fourth control contact 548a coupled to the second p-doped region 547a, and a second n-well region 549a in contact with the second p-doped region 547a. The second p-doped regions 537a and 537b are respectively similar to the second p-doped regions 537 and 547. The second n-well regions 539a and 549a are respectively similar to the second n-well regions 539 and 549. The third control contact 538a is similar to the first control contact 538, and the fourth control contact 548a is similar to the second control contact 548. The third control contact 538a is connected to the first control signal 122, and the fourth control contact 548a is connected to the second control signal 132.

[0195] The first control contact 538 and the associated doped regions do not directly contact the absorption region 506. Therefore, the electric field established inside the absorption region 506 when the first control signal 122 is applied to the first control contact 538 is Figure 5E weaker than the electric field established when the illustrated switched photodetector 550 has the first control contact 538 directly contacting the absorption layer 506. By adding the third and fourth control contacts 538a and 548a and the associated doped regions, the carrier collection control efficiency of the switched photodetector 586 can be Figure 5I increased compared to the illustrated switched photodetector 582 and be similar to the carrier collection control efficiency of the Figure 5E illustrated switched photodetector 550; however, moving the contacts to the substrate 502 can still partially retain the advantage of reducing the dark current or leakage current. In addition, the large electric field in the absorption region can increase the bandwidth of the photodetector, speed up the switching of the first switch 532 and the second switch 542, and the additional control contacts 538a and 548a can also increase the operating speed of the photodetector 584.

[0196] Although the illustrated third control contact 538a and fourth control contact 548a are shown as sharing control signals 122 and 132 with the first control contact 538 and second control contact 548, in some embodiments, contacts 538a and 548a may be coupled to control signals different from the first control signal 122 and second control signal 132. For example, the control signal provided to the third control contact 538a may be less than the first control signal 122 provided to the first control contact 538 because, due to the proximity of the second p-doped region 537a to the carriers generated by the absorption region 506, the control signal provided to the third control contact 538a can have a greater effect on the photo-generated carriers than the first control signal 122 provided to the first control contact 538; the same mechanism applies to the control signal supplied to the fourth control contact 548a.

[0197] In some embodiments, the second p-doped regions 537a and 547a may be omitted to form a Schottky junction, the effects of which can be referenced in Figures 5F - 5H the description. For ease of drawing, the n-well regions 539 and 549 and the p-well regions 536 and 546 are drawn together; however, in actual implementation, these wells may be implemented independently or in any combination.

[0198] Although Figures 5D - 5K multiple structures of a switched photodetector having a partially embedded absorption layer 506 are described, the structures may also be applied to a switched photodetector having a fully protruding absorption layer 506 such as Figure 5A the illustrated structure and to a switched photodetector having a fully embedded absorption layer 506 structure such as Figure 5C the illustrated structure and achieve similar effects.

[0199] In Figures 5A - 5K the switched photodetector described may be incorporated into a front-side illumination (FSI) image sensor or a back-side illumination (BSI) image sensor. In the FSI configuration, light enters the photodetector from the top of the first layer 508. In the BSI configuration, light enters the photodetector from the bottom of the substrate 502.

[0200] The control regions (e.g., p-doped regions 537 and 547) and the readout regions (e.g., n-doped regions 534 and 544) may have different heights. For example, for the switching photodetectors 530, 550, 560, and 580 and for any structure where the control regions and readout regions are located in the absorption region 506, a portion of the absorption region 506 corresponding to the readout region or the control region may be etched, and the readout region or the control region may be formed on the etched portion, so as to form a vertical offset between the control region and the readout region. Similarly, for the switching photodetectors 582, 586, and 588 and for any structure where the control regions and readout regions are located in the substrate 502, a portion of the substrate 502 corresponding to the readout region or the control region may be etched, and the readout region or the control region may be formed on the etched portion, so as to form a vertical offset between the control region and the readout region.

[0201] In some embodiments, a lens may be disposed on the optical path of the incident light. The lens may be, for example, a microsphere lens or a Fresnel zone plate (FZP) lens. In other examples, for the silicon substrate 502, the lens may be directly formed on the substrate 502 by etching the substrate 502. The detailed structure of the lens will be described with reference to Figures 7A - 7B for a detailed description.

[0202] In some embodiments, the interfaces between the absorption layer 506 and the spacers 512 may be doped with n- or p-type dopants respectively to enhance the electrical isolation of holes and electrons. In some embodiments, the interface between the absorption layer 506 and the substrate 502 (e.g., the bottom interface) may be doped with n- or p-type dopants to enhance the electrical isolation of holes and electrons.

[0203] Figure 6A is an example of a switching photodetector 600 for converting an optical signal into an electrical signal. The switching photodetector 600 includes a substrate 502, an absorption region 506, a first switch 532, a second switch 542, and an anti-doping region 610. The anti-doping region 610 is disposed within the absorption region 506, and the first switch 532 and the second switch 542 are disposed on the absorption layer 506; the substrate 502, the absorption region 506, the first switch 532, and the second switch 542 have been Figure 5D described.

[0204] The anti-doped region 610 is part of the absorption region 506, where a doping species is doped to reduce the net carrier concentration of the absorption region 506. An undoped semiconductor material has a certain concentration of charge carriers that can contribute to current conduction, even in the absence of dopants - this is called the intrinsic carrier concentration of the semiconductor. The absorption region 506 is typically formed of a semiconductor material, such as silicon, Ge, or an alloy of both, and has an associated intrinsic carrier concentration. The intrinsic carrier concentration can vary depending on different factors, such as the material preparation method and the defect level (defect concentration). Material preparation methods include, for example, epitaxial growth, chemical vapor deposition (CVD), metalorganic chemical vapor deposition (MOCVD), and physical vapor deposition (PVD), and different material preparation methods can produce different material defect levels. Generally, the more material defects, the higher the intrinsic carrier concentration. For example, bulk crystalline germanium can have an intrinsic p-type carrier concentration of about 2*1013 cm-3 at room temperature, while epitaxially grown germanium can have a higher order of magnitude of about 5*1014 cm-3 of intrinsic p-type carrier concentration. Depending on the material properties and the type of defects, the semiconductor material can be p-type or n-type.

[0205] Reducing the leakage current of a switching photodetector, such as the switching photodetector 600, to reduce power consumption is important for ToF pixels. Part of the leakage current of the switching photodetector comes from the leakage current flowing between the control regions, such as the current flowing between the p-doped regions 537 and 547. One way to reduce such current is to reduce the net carrier concentration of the absorption region 506 between the p-doped regions 537 and 547. The net carrier concentration is the concentration of carriers available for current conduction and can be determined by combining the intrinsic carrier concentration and the external carrier concentration contributed by impurities. By appropriately selecting the electrical properties, type, and concentration of the impurities, the intrinsic carrier concentration can be compensated, or "anti-doped" with dopants to make the semiconductor material have a lower net carrier concentration. Typically, when the intrinsic and net carriers have the same polarity, i.e., both p-type or n-type, the leakage current between the control regions is proportional to the net carrier concentration.

[0206] The type of dopant in the anti-doped region 610 can be selected according to different factors, such as the material forming the absorption region 506 or the type of dopant in the absorption region 506. For example, Ge epitaxially grown on the Si substrate 502 is generally a p-type material. In this case, n-type dopants such as phosphorus, arsenic, antimony, and fluorine can be used to dope the anti-doped region 610. Doping can be performed in different ways, including implantation, diffusion, and in-situ doping during material growth. In some cases, dopants such as fluorine can passivate defects. The passivated defects will no longer serve as a source of charge carriers, and the net carrier concentration of the absorption region 506 doped with fluorine can be reduced and become more intrinsic.

[0207] The dopant concentration for the anti-doping region 610 can be selected based on the intrinsic carrier concentration of the absorption region 506. For example, epitaxially grown germanium with an intrinsic carrier concentration of approximately 5×10¹⁴ cm⁻³ can be doped with an anti-doping concentration of approximately 5×10¹⁴ cm⁻³ to reduce the intrinsic carriers towards the intrinsic carrier concentration of bulk crystalline Ge of approximately 2×10¹³ cm⁻³. Generally, the anti-doping concentration can be in the range of 1×10¹³ cm⁻³ to 1×10¹⁶ cm⁻³. In some embodiments, different regions of the anti-doping region 610 can have different dopant concentrations. For example, regions near the material interface, such as the bottom surface of the absorber 506, can have a higher intrinsic carrier concentration due to an increased defect level, and a high anti-doping level can provide better compensation. In some embodiments, the anti-dopant concentration can be higher than the intrinsic carrier concentration of the absorption region 506; in such a case, the polarity of the absorption region 506 can be changed from p-type to n-type, or from n-type to p-type.

[0208] Although the illustrated anti-doping region 610 is capable of completely covering the n-doped regions 534 and 544 and the p-doped regions 537 and 547; however, the anti-doping region 610 can also cover only the p-doped regions 537 and 547, or the n-doped regions 534 and 544. Secondly, although the illustrated anti-doping region 610 is a continuous region, the anti-doping region 610 can also be two or more separate regions. Additionally, although the illustrated anti-doping region 610 is part of the absorption region 506, the anti-doping region 610 can also span across the entire absorption region 506.

[0209] In some embodiments, the anti-doping region 610 can act as a dopant diffusion inhibitor and be used to provide a structure with a steep junction profile. The structure with a steep junction profile between the anti-doping region 610 and the p-doped regions 537 and 547 can reduce the leakage current, thereby reducing the power consumption of the ToF pixel. For example, in the Ge absorption region 506, fluorine doping can inhibit the diffusion of phosphorus dopants in the n-doped region 534.

[0210] Generally, the anti-doping region 610 can be implemented in different embodiments of the switched photodetector to reduce the leakage current between the control regions.

[0211] In some embodiments, the p-doped regions 537 and 547 can be omitted, but this results in the formation of a Schottky junction structure, the effects of which have been described with respect to Figures 5F - 5H are described.

[0212] Figure 6B is an example of a switched photodetector 620 for converting an optical signal into an electrical signal. The switched photodetector 620 is similar to Figure 6AThe illustrated switched photodetector 600, except that the first switch 532 and the second switch 542 further respectively include n-well regions 612 and 614. Adding the n-well regions can modify the electrical and / or optical characteristics of the switched photodetector 620. In some embodiments, the doping levels of the n-well regions 612 and 614 can range from 10 15 cm -3 to 10 17 cm -3 . In some embodiments, the n-well regions 612 and 614 can extend from the upper surface of the absorption region 506 to the lower surface of the anti-doping region 610, or the interface between the absorption layer 506 and the substrate 502.

[0213] The p-doped region 537, the n-well region 612, the anti-doping region 610, the n-well region 614, and the p-doped region 547 are arranged to form a PNINP junction structure. Generally, the PNINP junction structure reduces the leakage current from the first control signal 122 to the second control signal 132, or from the second control signal 132 to the first control signal 122.

[0214] In some embodiments, the p-doped region 537 is completely formed within the n-well region 612. In some other embodiments, the p-doped region 537 is partially formed within the n-well region 612. For example, a portion of the p-doped region 537 can be formed within the n-well region 612 by implanting p-dopants, and another portion of the p-doped region 537 can be formed within the anti-doping region 610 by implanting p-dopants. Similarly, in some embodiments, the p-doped region 547 is completely formed within the n-well region 614. In some other embodiments, the p-doped region 547 is partially formed within the n-well region 614. In some embodiments, the n-well regions 612 and 614 form a continuous n-well region that simultaneously includes at least a portion of the p-doped regions 537 and 547.

[0215] The operating speed or bandwidth of a switched photodetector is an important performance parameter for applications and is beneficial for high-speed optical detection, such as ToF detection. One of the characteristics of a switched photodetector that can affect its bandwidth is the physical size of the photodetector, such as the area of the photodetector that can receive light. Reducing the area of the switched photodetector, for example, results in a reduction in device capacitance, a reduction in carrier flow time, or a combination of the two, which generally leads to an increase in the bandwidth of the switched photodetector. However, reducing the detection area of the switched photodetector can also result in a reduction in the amount of light measured by the switched photodetector (i.e., the number of photons). For example, for light of a predetermined light intensity per unit area, reducing the area of the detector reduces the amount of light it can detect.

[0216] For applications that benefit from both high bandwidth and high detection efficiency, such as ToF detection, adding a microlens in front of the photodetector can be advantageous. The microlens can focus the incident light onto the photodetector, thereby allowing a small-area photodetector to detect incident light over an area larger than its own. For example, a combination of a properly designed microlens and a spacer layer (SL) can separate the microlens from the photodetector by the effective focal length of the microlens, so that the incident light can be focused onto the diffraction limit point, which is on the order of the square of the optical wavelength of the incident light. Such a scheme can allow for reducing the area of the photodetector while alleviating the potential drawbacks of the reduction in the area of the photodetector.

[0217] Figure 7A A cross-sectional view showing an exemplary structure 700 integrating a silicon lens into a photodetector is shown. Structure 700 includes a donor wafer 710 and a carrier wafer 730. Donor wafer 710 includes a plurality of pixels 720a - 720c (collectively referred to as pixels 720), vias 714, metal pads 716, and a first bonding layer 712; carrier wafer 730 includes a second bonding layer 732. Donor wafer 710 and carrier wafer 730 are bonded to each other through first bonding layer 712 and second bonding layer 732. Substrate 710 can be similar to Figure 5A the substrate 502 shown, and absorption region 706 can be similar to Figures 5A - 5K the absorption region 506 shown.

[0218] Pixels 720a - 730c include absorption regions 706a - 706c and microlenses 722a - 722c (collectively referred to as microlenses 722). Microlenses 722 are convex lenses integrated in or on donor wafer 710. In applications that benefit from high light collection efficiency, such as ToF detection, additional microlenses 722 may be beneficial. The convex structure of microlenses 722 can focus the light incident on microlenses 722 onto absorption region 706, which can improve the light collection efficiency of pixels 720 and thus improve pixel performance. Configuring pixels 720 with microlenses 722 on the back side of donor wafer 710 can be referred to as a back-illuminated technology.

[0219] The characteristics of the microlens 722 affect its performance, including its geometric parameters and the materials it is made of. The microlens 722 is generally implemented in a plano-convex structure; one surface of the microlens 722 facing the incident light is convex and has a radius of curvature, and the other surface that is bonded to the donor wafer 710 (the microlens 722 is bonded in or integrated on the donor wafer 710) is flat. The plano-convex microlens 722 can be fabricated by standard semiconductor process technologies. The microlens 722 can have a height HL and a diameter DL, and can be isolated from the surface of the lens face of the absorption region 706 by a height HO. In some embodiments, the range of HL can be from 1 to 4 μm, the range of HO can be from 8 to 12 μm, the range of HA can be from 1 to 1.5 μm, and the range of DL can be from 5 to 15 μm. In some embodiments, for the spherical microlens 722, its radius of curvature can be set such that the focal length is approximately equal to HO to achieve optimal focusing of light to the absorption region 706. The dimensions of the focal length and the radius of curvature can be determined by different simulation techniques, such as: Beam Propagation Method (BPM) and Finite Difference Time Domain (FDTD) techniques. In some embodiments, the microlens 722 is an aspherical lens.

[0220] The microlens 722 can be formed from different materials and processes. Generally, a variety of materials that are transparent to the wavelengths to be detected by the pixel 720 can be used to fabricate the microlens 722. For example, the microlens 722 can be made of materials with a medium to high refractive index (e.g., greater than 1.5), such as: crystalline silicon, polysilicon, amorphous silicon, silicon nitride, polymers, or combinations thereof. At visible light wavelengths, polymer materials are generally used to make microlenses. At NIR wavelengths, silicon is mostly used to make microlenses; this is because silicon is relatively transparent to NIR wavelengths and has a high refractive index (about 3.5 at a wavelength of 1000 nm), so silicon is a suitable lens material for NIR wavelengths. In addition, silicon has high absorption for visible light (e.g., less than 800 nm), and the silicon microlens can prevent a significant amount of visible light from entering the absorption region 706, which is beneficial for applications that detect NIR wavelengths (e.g., ToF detection). The crystalline silicon microlens 722 can be fabricated by patterning and etching the surface of the donor wafer 710, which is typically a crystalline silicon wafer. In other examples, polysilicon or amorphous silicon can be deposited on the surface of the donor wafer 710 and fabricated in a similar manner by patterning and etching. The formation of the lens by etching the crystalline silicon donor wafer 710 or etching the polysilicon or amorphous silicon deposited on the donor wafer 710 is an exemplary method for integrally forming the microlens 722 on the donor wafer 710.

[0221] The patterning process of the microlens 722 can be performed, for example, using grayscale lithography. In grayscale lithography, the features to be patterned (e.g., microlenses) are exposed using a local grading of the exposure dose, which transfers the graded thickness on the photomask for development. For example, the photomask can be patterned to have a shape similar to the microlens 722. Then, through semiconductor etching techniques, such as plasma-based directional etching techniques, the patterned shape is transferred to the bottom of the material (e.g., crystalline silicon donor wafer 710) to complete the fabrication of the microlens 722. In some embodiments, the grading mask of the locally exposed area can be achieved, for example, by changing the fill factor of the sub-wavelength features on the photomask.

[0222] The absorption region 706 can be similar to the absorption region 506 described with respect to Figure 5A The carrier wafer 730 can include different electronic circuits coupled to the pixels 720. For example, the electronic circuits can be coupled through structures such as vias 714. The vias 714 can be coupled to metal pads 716 and bonded to external electronic devices through, for example, wires.

[0223] The carrier wafer 730 and the donor wafer 710 can be bonded or mechanically attached to each other through techniques. For example, the first bonding layer 712 and the second bonding layer 732 can be oxides (e.g., silicon dioxide) and are bonded to each other using an oxide-oxide bonding technique. In other examples, the first bonding layer 712 and the second bonding layer 732 can be metals (e.g., copper) and are bonded to each other using a metal-metal bonding technique. In another example, the first bonding layer 712 and the second bonding layer 732 can be a combination of metal and oxide (e.g., silicon oxide and copper) and are bonded to each other using a hybrid bonding technique.

[0224] Figure 7B A cross-sectional view showing an example structure 740 integrating a silicon lens into a photodetector is shown. The structure 740 includes a microlens 742, an antireflection coating (ARC) layer 744, a spacer layer 746, a first layer 748, a second layer 750, a silicon layer 752, and a photodetector 754. The microlens 742 supports the ARC layer 744, and the spacer layer 746 supports the microlens layer 742. The silicon layer 752 can support the photodetector 754, or the photodetector 754 can be formed in the silicon layer 752. The first layer 748 and the second layer 750 can be intermediate layers between the silicon layer 752 and the spacer layer 746.

[0225] An ARC layer 744 is provided to reduce the reflectivity of light incident on the microlens 742. For example, the ARC layer 744 can be designed to have a refractive index that is the square root of the refractive index of the microlens 742 and, at the same time, have a thickness equivalent to a quarter of the incident wavelength. In some embodiments, the ARC layer 744 can be formed of silicon dioxide. In some embodiments, the ARC layer 744 can be a multi-layer ARC formed of a multi-layer structure.

[0226] The structure 740 can be equivalent to an integrated lens in a backside illumination (BSI) image sensing structure. For example, the silicon layer 752 can be a silicon substrate, such as Figure 7A the substrate 710 shown or Figure 5D the substrate 502 shown; the photodetector 754 can be, for example, Figure 5D the switching photodetector 530. The interface between the silicon layer 752 and the second layer 750 can correspond to Figure 5D the bottom surface of the substrate 502 opposite the absorption region 506. In a BSI configuration, the second layer 750 formed on the silicon layer 752, such as the back side of the substrate 502, can include various structures and layers typically fabricated on a sensor wafer illustrated in a BSI diagram; examples of such structures and layers include an ARC layer for reducing the interface reflection of light on the silicon layer 752 and a metal grid, such as a tungsten grid, for blocking light from entering the silicon layer 752 except for the region for receiving light, such as the bottom of the microlens 742. The first layer 748 can be a thin layer that enhances the adhesion of the spacer layer 746 to the second layer 750 and is used to increase the fabricability and reliability of the structure 740, etc. The material of the first layer 748 can be, for example, various dielectric materials (e.g., SiO2, SiON, and SiN) or polymers. In some embodiments, the first layer 748 can be omitted depending on the interaction between the second layer 750 and the spacer layer 746 (e.g., when the spacer layer 746 can have good adhesion to the second layer 750).

[0227] The structure 740 can be fabricated by the following method: providing a sensor wafer including a silicon layer 752, a photodetector 754, and a second layer 750, and sequentially depositing a first layer 748, a spacer layer 746, a microlens 742, and an ARC layer 744; then patterning and etching to expose a metal pad, which is similar to Figure 7A the metal pad 716 shown. The microlens 742 can be patterned and etched by Figure 7A the technique shown for fabricating the microlens 722. Although the ARC layer 744 shown here is limited to the surface of the microlens 742, generally, the ARC layer 744 can extend to other surfaces, such as the side surface of the microlens 742 or the upper surface of the spacer layer 746.

[0228] Various device characteristics for structure 740 at a specific implementation when the operating wavelength is 940 nm are given as an example. The microlens 742 has a refractive index of 1.5316, a radius of curvature of 6 μm, a height of 4 μm, and a diameter DL of 10 μm. The ARC layer 744 is formed of SiO2 and has a refractive index of 1.46 and a thickness of 160.96 nm at a wavelength of 940 nm. The spacer layer 746 has a refractive index of 1.5604 and a thickness of 10 μm. The first layer 748 has a refractive index of 1.5507 and a thickness of 60 nm. The second layer 750 has a tungsten grid and an ARC layer for the silicon layer 752. Although the foregoing parameters have been provided, the characteristics of structure 740 are still adjusted, for example, in accordance with different operating wavelengths, materials, and the size of the photodetector 754.

[0229] In some embodiments, the second layer 750 may be referred to as the "top layer" because the second layer 750 is formed on top of the silicon substrate of a BSI image sensor and can be adjusted to improve the overall optical performance of structure 740. As previously mentioned, the second layer 750 includes a metal grid embedded in a dielectric layer, such as a tungsten grid embedded in SiO2. When light enters the silicon layer 752 directly from air, the SiO2 layer can act as an ARC layer. However, since the refractive indices of the additional microlens 742, spacer layer 746, and first layer 748 are much greater than the refractive index of air (about 1.0), SiO2 may not effectively reduce the optical reflection at the interface between the silicon layer 752 and the stacked first layer 748 and / or spacer layer 746.

[0230] Table 1 shows the simulation parameters corresponding to structure 740 and the calculated transmittance, with the layers and thicknesses modified and / or approximated to enable structure 740 to achieve the desired transmittance in different implementations.

[0231]

[0232] Referring to Table 1, the second layer 750 corresponding to Case 1 includes standard single-layer SiO2, and the transmission rate obtained by simulation is about 79%. For applications that require maximizing incident light, an incident light loss of, for example, 21% may be unacceptable. By adding a Si3N4 layer between the SiO2 layer and the silicon layer 752, the problem of reduced transmission rate can be alleviated. By adding about 121 nm of Si3N4, the transmission rate can be increased to about 97.6%. Thus, the intermediate layer can be referred to as an ARC layer. Generally, various optically transparent materials with a refractive index greater than that of SiO2 can be used to replace Si3N4. Example materials include SiON, SiN, Al2O3, HfO2, ZrO2, La2O3, and high-k materials (e.g., materials with a high dielectric constant) that are compatible with the CMOS process. Suitable materials can have a refractive index higher than, for example, 1.6, 1.7, 1.8, 1.9, or 2.0. The thickness of the material should be adjusted to an odd multiple of one-quarter of the light wavelength entering the material.

[0233] Adding Si3N4 or a high-k material directly on the silicon layer 752 may cause an increase in the dark current of the photodetector 754, which is, for example, due to an increase in surface defects at the silicon-Si3N4 interface compared to the silicon-SiO2 interface. In some embodiments, the increased dark current can be alleviated by inserting a second layer of SiO2 between the Si3N4 layer and the silicon layer 752. The thickness of the inserted second layer of SiO2 ranges from 10 nm to 50 nm, and the corresponding transmission rate is increased to about 97.1% to 85%. Thus, inserting a thin SiO2 layer of, for example, 10 nm can help reduce the increase in dark current while maintaining a high optical transmission rate.

[0234] As previously mentioned, a low leakage current flowing through the control region of the switched photodetector is an important performance parameter because a low leakage current can reduce the power consumption of the device containing the photodetector. Another important performance parameter is the dark current flowing between the readout circuit and the control region of the switched photodetector. The dark current contributes to the noise in the signal measured by the switched photodetector and reduces the signal-to-noise ratio of the measured ToF signal.

[0235] Figure 8A is an example of the switch 800 of the switched photodetector. The switch 800 can be used as the first or second switch in various switched photodetectors. As regarding Figure 5AAs described, the switch 800 is formed in the absorption region 506 having the first layer 508. The switch 800 includes an n-doped region 802, a readout contact 804 coupled to the n-doped region 802, a lightly doped n-well region 806, a p-doped region 812, a control contact 814 coupled to the p-doped region 812, a lightly doped p-well region 816, and an n-well region 818. A spacing distance S is provided between the edges of the n-doped region 802 and the p-doped region 812. The n-doped region 802 and the p-doped region 812 can be similar to Figure 5E the first n-doped region 534 and the first p-doped region 537. The n-well region 818 can be similar to Figure 5E the n-well region 539 in Figure 5E . The readout contact 804 and the control contact 814 can be similar to

[0236] the first readout contact 535 and the first control contact 538 in

[0237] The sources of the dark current of the lateral PIN diode formed by the control region (p-doped region 812), the absorption region 506 (undoped / intrinsic), and the readout region (n-doped region 802) include Shockley-Read-Hall (SRH) generation and interband tunneling. Surface defects that appear on the surface of the absorption region 506 may affect SRH generation. Adding the first layer 508 reduces some of the surface defects, which can reduce the dark current due to SRH generation. Increasing the distance S between the n-doped region 802 and the p-doped region 812 can also reduce the dark current, for example, due to the reduction of the electric field between the n-doped region 802 and the p-doped region 812, which also reduces the SRH generation rate between the two regions. For example, the distance S should be maintained above 400 nm. However, increasing the distance S can narrow the bandwidth of the photodetector, for example, due to the increased carrier transit time. Adding the lightly doped n-well region 806, the lightly doped p-well region 816, or a combination thereof can help overcome such a trade-off.The doping concentrations of the lightly doped regions 806 and 816 are lower than those of the n-doped region 802 and the p-doped region 812, respectively. For example, the doping concentrations of the lightly doped regions 806 and 816 can be on the order of 1*10^17 cm^-3, which is lower than the doping concentrations of the n-doped region 802 and the p-doped region 812 on the order of 1*10^19 cm^-3. The presence of the lightly doped regions can reduce the problem of doping concentration discontinuity between the doped regions 802, 812 and the absorption region 506, and the lightly doped regions can have a doping concentration of 1*10^15 cm^-3 or lower. By providing regions with intermediate doping concentrations, the electric field values at the edges of the doped regions 802 and 812 can be weakened. Weakening the electric field values can reduce interband tunneling, thereby reducing the dark current between the two doped regions 802 and 812. In addition, the dark current contributed by SRH can be reduced. Generally, the doping concentrations of the lightly doped regions 806 and 816 can be adjusted according to different factors, such as the geometry of the switch, the doping concentrations of the doped regions 802 and 812, and the doping concentration of the absorption region 506.

[0238] Figure 8B is an example of the switch 820 of the switched photodetector. The switch 820 is similar to Figure 8A the switch 800 shown, but differs in that the lightly doped regions 806 and 816 are replaced by a trench 822 formed in the absorption region 506, and the trench 822 is filled with a dielectric filler 824. The trench 822 filled with the dielectric filler 824 can reduce the dark current.

[0239] The dielectric filler 824 is generally an electrically insulating material having a dielectric constant lower than that of the surrounding absorption region 506. The electric field can penetrate through the region with a lower dielectric constant more easily than through a region with a higher dielectric constant. By placing the trench 822 filled with the filler near the doped regions 802 and 812, some of the high electric field regions around the doped regions 802, 812 and in the depletion regions ("space charge regions") around the doped regions 802, 812 are brought into the dielectric filler 824; accordingly, SRH generation and / or interband tunneling in the absorption region 506 are reduced. In addition, unlike the germanium absorption region 506, the dielectric filler 824, such as SiO2, is an insulator and does not contribute to SRH generation and / or interband tunneling. Therefore, the dark current generated by SRH generation and / or interband tunneling caused by the high electric field at the edges of the doped regions 802 and 812 can be reduced.

[0240] The trench 822 can be formed by etching the absorption region through dry etching (e.g., plasma etching) or wet etching (e.g., liquid chemical bath) techniques. The depth of the trench 822 etched can be similar to the depths of the doped regions 802 and 812 (e.g., 10 - 200 nm). The trench 822 at least partially overlaps with the high electric field region around the n-doped region 802 or the p-doped region 812. In some embodiments, the trench 822 cuts into the doped regions 802 and 812, removing a portion of the n-doped region 802 and the p-doped region 812. Once the trench 822 is formed, the first layer 508 can be deposited over the trench 822 to passivate the defects that appear on the surface of the trench 822. For the germanium absorption region 806, the first layer 508 can be, for example, amorphous silicon, polycrystalline silicon, or a combination thereof. Thereafter, the trench 822 is filled with a dielectric filler 824, and the dielectric filler 824 can be, for example, SiO2. The dielectric filler 824 should be cleaned to have no significant concentration of impurities to avoid generating dark current.

[0241] In some embodiments, the depth of the trench can be deeper than the depths of the doped regions 802 and 812. For example, the depths of the doped regions 802 and 812 can be about 100 nm, and the depth of the trench can reach 200 nm to reduce SRH generation and / or interband tunneling. In some embodiments, a reduction of greater than 50% in SRH generation and / or interband tunneling around the doped regions 802 and 812 can be observed.

[0242] Figure 8C is an example of the switch 830 of the switched photodetector. The switch 830 is similar to Figure 8A the switch 800 shown, but further includes Figure 8B the trench 822 and the dielectric filler 824. Implementing both the low-doped regions 806 and 816 and the trench 822 can further reduce SRH generation and / or interband tunneling compared to implementing the low-doped region 806, 816, or the trench 822 alone.

[0243] Generally, the reduction in dark current achieved using the low-doped regions 806 and 816, or the trench 822, depends on the specific design of the switch and the overall design of the switched photodetector that includes the switch. Thus, even though Figure 8C the switch shown includes both the low-doped regions 806 and 816 and the trench 822, the key to implementing the low-doped region, the recess, or their combination still depends on the specific design of the switch in the switched photodetector. Additionally, although only an example of a single trench 822 is shown, generally, the trench 822 can be divided into two or more trenches.

[0244] Although Figures 8A - 8C the embodiments include the first layer 508 and the n-well region 818, in some embodiments, the first layer 508 and / or the n-well region 818 can be omitted.

[0245] So far, different switching photodetectors and switches in the switching photodetectors have been described. Now, different structures and devices of the switching photodetectors will be described.

[0246] The switching photodetectors are generally fabricated on a substrate, such as substrates 102, 202, 302, 402, and 502. The substrate is the material on which the switching photodetector is fabricated and carried. A semiconductor wafer is an example of a substrate. The substrate can be part of the switching photodetector; however, generally, the substrate can only provide a mechanical platform on which the switching photodetector is fabricated. The substrate can be made of different materials, such as silicon, germanium, compound semiconductors (e.g., III-V, II-VI), silicon carbide, glass, and sapphire. The substrate can contain different layers therein. For example, a silicon-on-insulator (SOI) substrate includes a silicon base layer, an insulator layer (e.g., SiO2) on the silicon base layer, and a silicon device layer on the insulator layer. The SOI can additionally include device layer-insulator layer pairs. For example, a dual SOI wafer includes two device layer-insulator layer pairs.

[0247] The switching photodetector includes an absorption layer, which is used to absorb incident light and convert the incident light into charge carriers. Absorption layers 106, 206, 306, 406, and absorption regions 506, 706 are examples of the absorption layer. The absorption region can be formed by various absorption materials that absorb light at the operating wavelength of the switching photodetector. Example materials for the absorption region include silicon, germanium, IV-IV semiconductor alloys (e.g., GeSn, GeSi), group III-V compound semiconductors (e.g., GaAs, InGaAs, InP, InAlAs, InGaAlAs), and other materials in groups III, IV, and V of the periodic table. In some embodiments, the absorption region can be a region in the substrate. For example, a region in a silicon substrate can be used as the absorption region for visible light.

[0248] In some embodiments, the absorption region can be defined in the light-absorbing material by a change in material composition (e.g., different GeSi compositions), doping a region of the absorption material (e.g., a reverse-doped region), or by forming an optical window for light to pass through (e.g., tungsten grid openings in a BSI image sensor).

[0249] The absorbing material can be deposited on a substrate. For example, the absorbing material can be deposited in a blanket fashion on the substrate. In some embodiments, the absorbing material can be deposited on an intermediate layer formed on the substrate. Generally, the intermediate layer can be selected based on the absorbing material, the substrate, or both. Such an intermediate layer can enhance the fabricability of the device and / or enhance the device performance. Examples of materials for the intermediate layer include silicon, graded GeSi composite materials, graded III-V materials, germanium, GaN, and SiC. A graded material is a material that changes its material composition along at least one direction. For example, in a graded GeSi material, its Ge composition varies from 1% at one end to 99% at the other end. Generally, the compositions at the beginning and the end can be set according to, for example, the composition of the substrate and the absorbing material.

[0250] In some embodiments, the material of the absorbing layer can be epitaxially grown on the intermediate layer in one or two steps. For example, the absorbing layer (e.g., Ge, GeSi) can be deposited on an intermediate layer having an opening that opens to the substrate (e.g., a crystalline silicon substrate) below. When the absorbing material is deposited on a substrate with a mismatched lattice constant, a multi-step growth process can improve the material quality (e.g., reduce the number of material defects). Such a multi-step growth process is described in U.S. Patent No. 9,786,715, titled "High Efficiency Wide Spectrum Sensor", which is hereby incorporated by reference in its entirety.

[0251] Figures 9A - 9D An example of an electrical terminal for a switching photodetector is shown. Referring to Figure 9A , the electrical terminal 900 includes a region 902, a contact metal 904, and a doped region 906. The region 902 is the material on which the electrical terminal 900 is formed and can correspond to the absorbing region (e.g., the absorbing region 506) or the substrate (e.g., the substrate 502). The doped region 906 can be a p-type (acceptor) doped region or an n-type (donor) doped region based on the type of dopant. The doped region 906 is typically doped to a high doping concentration (e.g., 1×1019 to 5×1020 cm−3) to allow an ohmic contact to be formed between the contact metal 904 and the region 902. Such an amount of doping concentration can be referred to as "degenerate doping".

[0252] The contact metal 904 is a metallic material and contacts the region 902 through the doped region 906. The contact metal can be selected from different metals or alloys based on the material of the region 902 and the dopant in the doped region 906. Examples include Al, Cu, W, Ti, Ta-TaN-Cu stacks, Ti-TiN-W stacks, and various metal silicides.

[0253] Referring to Figure 9B the electrical terminal 910 is similar to Figure 9AThe illustrated electrical terminal 900, except that the doped region 906 is omitted. The contact metal 904 is directly assembled in region 902 without the doped region 906, which can form a Schottky contact, an ohmic contact, or an intermediate characteristic between the two, depending on various factors including the material of region 902, the contact metal 904, and the impurity or defect level of region 902.

[0254] Reference Figure 9C , the electrical terminal 920 is similar to Figure 9B the illustrated electrical terminal 910, except that a dielectric layer 922 is inserted between the contact metal 904 and region 902. For example, for the crystalline germanium region 902, the dielectric layer 922 can be amorphous silicon, polycrystalline silicon, or silicon germanium. In other examples, for the crystalline silicon region 902, the dielectric layer 922 can be amorphous silicon, polycrystalline silicon, or silicon germanium. Inserting the dielectric 922 can form a Schottky contact, an ohmic contact, or an intermediate characteristic that is a combination of the two.

[0255] Reference Figure 9D , the electrical terminal 930 is similar to Figure 9B the illustrated electrical terminal 910, but different in that an insulating layer 932 is inserted between the contact metal 904 and region 902. The insulating layer 932 blocks the direct current conduction from the contact metal 904 to region 902, but after applying a voltage to the contact metal 904, an electric field can be formed in region 902. The formed electric field can attract or repel charge carriers into region 902. The insulating layer 932 can be SiO2, Si3N4, or a high-k material.

[0256] The switch of the switch photodetector, for example Figure 5D the illustrated first switch 532 includes a carrier control terminal and a carrier collection (readout) terminal. The carrier control terminal is a terminal for directing the optically generated carriers in region 902 in a certain direction (e.g., toward the carrier collection terminal) by applying a control voltage through, for example, an external bias circuit. The operation of the carrier control terminal has been described in Figure 1A in relation to the control signals 122 and 132. Different types of electrical terminals can be used to implement the carrier control terminal. For example, the electrical terminals 900, 910, 920, and 930 can be used to implement the carrier control terminal.

[0257] The carrier collection terminal is a terminal for collecting the optical carriers in region 902. The carrier collection terminal can be configured to collect electrons (e.g., n-type doped region 906) or holes (e.g., p-type doped region). The carrier collection terminal has been described in relation to Figure 1A the readout circuits 124 and 134 in. Different types of electrical terminals can be used to implement the carrier collection terminal. For example, the electrical terminals 900, 910, and 920 can be used to implement the carrier collection terminal.

[0258] The number of carrier control terminals and carrier collection terminals can be adjusted, for example, based on the performance of the target device. For example, a switching photodetector may have the following exemplary structures: two carrier control terminals paired with two carrier collection terminals, two carrier control terminals paired with one carrier collection terminal, four carrier control terminals paired with two carrier collection terminals, and four carrier control terminals paired with four carrier collection terminals. Generally, the number of carrier control terminals and carrier collection terminals in a switching photodetector is greater than 1.

[0259] When a switching photodetector includes more than two control terminals, various combinations of the electrical terminals can be used. For example, combinations of ohmic, Schottky / ohmic terminals (such as terminals 900 and 920), ohmic and insulating (such as, terminals 900 and 930), insulating and Schottky / ohmic (such as, terminals 930 and 920), and ohmic, Schottky / ohmic, and insulating terminals (such as, terminals 900, 920, 930).

[0260] When implementing two or more carrier collection terminals in a switching photodetector, a combination of ohmic and Schottky / ohmic terminals (such as, terminals 900 and 920) can be used.

[0261] The electrical terminals can have different shapes based on different considerations such as fabricability and device performance. Figure 9E An exemplary top view showing electrical terminals of different shapes. The shape of terminal 940 can be rectangular, triangular, circular, polygonal, or a combination thereof. The corners of the terminal can be sharp or rounded. Doped regions, metal silicides, contact metals, or a combination thereof can be used to define the shape.

[0262] The absorption region and the substrate can be configured in different structures, and the absorption region can have different shapes based on different considerations such as fabricability and device performance. Refer to Figures 10A - 10I which shows various structures of the absorption region and the substrate. Specifically, in Figure 10A structure 1000 includes a substrate 1002 and an absorption region 1004 protruding from the upper surface of the substrate 1002; the substrate 1002 can be similar to the substrate 502 described with respect to Figure 5D and the absorption region 1004 can be similar to the absorption region 506 described with respect to Figure 5D Structure 1000 can be fabricated by depositing the absorption region 1004 on the substrate 1002 and then etching the absorption region 1004 to form the protruding structure.

[0263] Refer to Figure 10B structure 1010 is similar to Figure 10AThe structure 1000 shown, but further includes an intermediate layer 1006 between the absorption layer 1004 and the substrate 1002. The intermediate layer can act as a buffer layer to facilitate the growth of the absorption layer 1004 over the substrate 1002. The structure 1010 can be fabricated by depositing the intermediate layer 1006 on the substrate 1002, depositing the absorption layer 1004 on the intermediate layer 1006, and then etching the absorption layer 1004 and the intermediate layer 1006 to form a protruding structure.

[0264] Reference Figure 10C , the structure 1020 is similar to Figure 10A the structure 1000 shown, but its absorption layer 1004 is partially embedded in the substrate 1002. The structure 1020 can be fabricated by forming a recess in the substrate 1002 and depositing the absorption layer 1004 in the recess. The structure 1020 can be fabricated as follows: deposit a sacrificial layer over the substrate 1002, etch the deposited sacrificial layer to form a recess in the substrate 1002, selectively deposit the absorption material, remove the absorption material deposited outside the recess by performing a planarization step such as chemical mechanical polishing (CMP), and finally remove the sacrificial layer by selective etching such as wet chemical etching.

[0265] Reference Figure 10D , the structure 1030 is similar to Figure 10C the structure 1020 shown, but its absorption layer 1004 is completely embedded in the substrate 1002. The structure 1030 can be fabricated by forming a recess in the substrate 1002, depositing a selective layer of the absorption material over the substrate 1002, and removing the absorption material deposited outside the recess by performing a planarization step such as a CMP step.

[0266] Reference Figure 10E , the structure 1040 is similar to Figure 10D the structure 1030 shown, but an intermediate layer 1006 is inserted between the absorption layer 1004 and the substrate 1002 in its recess. The structure 1040 can be fabricated by forming a recess in the substrate 1002, depositing a conformal layer of the intermediate layer 1006, depositing a blanket layer of the absorption material over the intermediate layer 1006, and removing the absorption material and the intermediate layer deposited outside the recess by performing a planarization step such as a CMP step.

[0267] Reference Figure 10F , the structure 1050 is similar to Figure 10EThe structure 1040 shown, but with the second intermediate layer 1008 replacing the first intermediate layer 1006 between the sidewalls of the absorption region 1004 and the sidewalls of the recesses in the substrate 1002. The structure 1050 can be fabricated as follows: by forming recesses in the substrate 1002, depositing a conformal layer of the second intermediate layer 1008, and performing anisotropic blanket etching to remove the second intermediate layer 1008 along the vertical surfaces, depositing a conformal layer of the first intermediate layer 1006, performing anisotropic blanket etching to remove the first intermediate layer 1006 along the non-vertical surfaces, depositing a selective layer of the absorption material, and removing the absorption material and the first intermediate layer deposited outside the recesses by performing a planarization step (e.g., a CMP step). In an exemplary embodiment, the first intermediate layer 1006 can be formed of SiO2 and the second intermediate layer 1008 can be formed of GeSi.

[0268] Reference Figure 10G , the structure 1060 is similar to Figure 10A the structure 1000 of, but includes a layered intermediate layer 1062 in which the absorption region 1004 is embedded. The layered intermediate layer 1062 includes an opening 1064 extending to the substrate 1002 and a recess 1066 in which the absorption region 1004 is embedded. The absorption region 1004 contacts the substrate 1002 through the opening 1064. The structure 1060 can be fabricated as follows: by forming and depositing an intermediate layer on the substrate 1002, etching an opening 1064 through the entire thickness of the intermediate layer, etching a recess 1066 in the intermediate layer, depositing the absorption region 1004 on the layered intermediate layer 1062, and removing the absorption material deposited outside the recess 1066 by performing a planarization step (e.g., a CMP step).

[0269] Reference Figure 10H , the structure 1070 is similar to Figure 10G the structure 1060 of, but includes a second intermediate layer 1072 in which the recess 1066 is formed. The structure 1070 can be fabricated as follows: by depositing the first intermediate layer 1062 on the substrate 1002, depositing the second intermediate layer 1072, etching the first intermediate layer 1062 and the second intermediate layer 1072 to form an opening 1064, etching a recess 1066 in the second intermediate layer 1072, depositing the absorption layer 1004, and removing the absorption material deposited outside the recess 1066 by performing a planarization step (e.g., a CMP step).

[0270] Reference Figure 10I , the structure 1080 is similar to Figure 10Estructure 1040, but includes an opening 1084 formed in the intermediate layer 1006. The absorption region 1004 contacts the substrate 1002 through the opening 1084. The structure 1080 can be fabricated in the following manner: by forming a recess in the substrate 1002, depositing a conformal layer of the intermediate layer 1006, etching the opening 1084, depositing a blanket layer of the absorption material above the intermediate layer 1006, and removing the absorption material and the intermediate layer deposited outside the recess by performing a planarization step such as a chemical mechanical polishing (CMP) step.

[0271] The absorption region, the carrier control terminal, and the carrier collection terminal can be configured in different structures according to different considerations such as fabricability and device performance. Figures 11A - 11B A top view and a side view of an exemplary switched photodetector 1100 are shown, where the carrier control terminal and the carrier collection terminal are assembled on a substrate, and a portion of the substrate is the absorption region. In this example, the switched photodetector 1100 includes a substrate 1102, an absorption region 1104, a carrier collection terminal 1106, and a carrier control terminal 1108. The absorption region 1104 is a region in the substrate 1102. For example, for a silicon substrate 1102, the absorption region 1104 is formed in silicon and is used to absorb visible light. The absorption region 1104 can have different shapes, such as a square in the top view of the photodetector 1100. The absorption region 1104 can extend from the upper surface of the substrate 1102 to a desired depth below the aforementioned upper surface. For example, the absorption region 1104 can extend up to 1 μm, 2 μm, 3 μm, 5 μm, or 10 μm below the upper surface of the substrate 1102. An adjacent pair of the carrier collection terminal 1106 and the carrier control terminal 1108 forms a switch. The absorption region 1104 is disposed between the adjacent pair of the carrier collection terminal 1106 and the carrier control terminal 1108. In some embodiments, the adjacent pair of the carrier collection terminal and the carrier control terminal are symmetrically configured around the absorption region 1104 (e.g., on opposite sides or four sides of the absorption region 1104). Such a symmetric configuration can improve the matching of the carrier control and collection performance of the pair of two switches.

[0272] Figures 11C - 11F A top view and a side view of a switched photodetector are shown, where the absorption region is formed of a material different from that of the substrate. Refer to Figures 11C - 11D , the switched photodetector 1120 includes a substrate 1102, an absorption region 1124, a carrier collection terminal 1106, and a carrier control terminal 1108. Figure 11C A top view of the switched photodetector 1120 is shown, and Figure 11D A side view of the switched photodetector 1120 is shown. The switched photodetector 1120 is similar to Figures 11A - 11BThe illustrated switched photodetector 1100, except that the absorption region 1124 of the switched photodetector 1120 is made of a material different from that of the substrate 1102. For example, the absorption region 1124 can be formed of germanium, and the substrate 1102 can be a silicon substrate. The absorption region 1124 is completely embedded in a recess formed in the substrate 1102. Although the details of the embedded structure are not shown, the embedded absorption region 1124 can be implemented, for example, as described with respect to Figures 10D - 10F and Figure 5C described.

[0273] Referring Figure 11E , the switched photodetector 1130 is similar to Figures 11C - 11D the illustrated switched photodetector 1120, except that the absorption region 1124 is partially embedded in the substrate 1102. Although the details of the embedded structure are not shown, the partially embedded absorption region 1124 can be implemented as described in Figure 10C and Figure 5B described.

[0274] Referring Figure 11F , the switched photodetector 1140 is similar to Figures 11C - 11D the illustrated switched photodetector 1120, except that the absorption region 1124 completely protrudes above the substrate 1102. Although the details of the fully protruding structure are not shown, the fully protruding absorption region 1124 can be implemented as described in Figures 10A - 10B and Figure 5A described.

[0275] In some structures of switched photodetectors, the carrier collection terminal, the carrier control terminal, or both can be assembled in the absorption region. For the sake of brevity, the detailed implementation details of the substrate, the absorption region, the carrier control terminal, and the carrier collection terminal will be omitted. Figures 12A - 12B The top view and side view of an example of the switched photodetector 1200 are shown, where the carrier collection terminal is assembled on the substrate, and the carrier control terminal is assembled in the absorption region. The switched photodetector 1200 includes a substrate 1202, an absorption region 1204, a light receiving region 1205, a carrier collection terminal 1206, and a carrier control terminal 1208. The light receiving region 1205 can indicate that the input light is incident on a part of the absorption region 1204 and can be physically different from the remaining part of the absorption region 1204. For example, the combination of a light shield (e.g., a tungsten grid) and a microlens can block and focus the incident light on the light receiving region 1205. The carrier collection terminal 1206 is assembled on the substrate 1202, and the carrier control terminal 1208 is assembled at a position on the absorption region 1204 that does not overlap with the light receiving region 1205. For the switched photodetector 1200, the absorption region 1204 completely protrudes. The absorption region 1204 can be partially embedded as shown in Figure 12C for the switched photodetector 1220, or for the switched photodetector 1230 asFigure 12D The fully embedded shown.

[0276] Figures 12E - 12F Top and side views showing an example of the switching photodetector 1240, where the carrier collection terminal and the carrier control terminal are assembled on the absorption region. The switching photodetector 1240 is similar to Figures 12A - 12B the switching photodetector 1200 shown, but the difference is that the carrier collection region 1260 is now assembled on the absorption region 1204 at a position that does not overlap with the light receiving region 1205. For the switching photodetector 1240, the absorption region 1204 is fully protruding. For the switching photodetector 1240, the absorption region 1204 is fully protruding. The absorption region 1204 can be partially embedded as shown for the switching photodetector 1250, or fully embedded as shown for the switching photodetector 1250. Figure 12G shown, or fully embedded as shown for the switching photodetector 1250 Figure 12H The fully embedded shown.

[0277] Although Figures 12A - 12H the light receiving region 1205 shown does not overlap with the carrier collection terminal or the carrier control terminal, in fact, the light receiving region 1205 can overlap with at least a part of the carrier control terminal, at least a part of the carrier receiving terminal, and at least a part of various n-doped regions or p-doped regions. For example, such an overlap can occur in pixels applying FSI and BSI configurations.

[0278] In the switching photodetectors of some embodiments, each switch may include more than 1 carrier collection terminal, more than 1 carrier control terminal, or the number of both of the foregoing is more than 1. For the sake of brevity, the detailed implementation details of the substrate, the absorption region, the carrier control terminal, and the carrier collection terminal are omitted. Figures 13A - 13G Top view showing an example of a switching photodetector having a switch including a carrier control terminal or a carrier collection terminal. In Figure 13A , the switching photodetector 1300 includes a substrate 1302, an absorption region 1304, a light receiving region 1305, a substrate carrier collection terminal 1306, a substrate carrier control terminal 1308, and an absorber carrier control terminal 1309. The substrate carrier collection terminal 1306 is a carrier control terminal assembled on the substrate, such as the substrate 1302. The substrate carrier control terminal 1308 is a carrier control terminal assembled on the substrate, such as the substrate 1302. The absorber carrier control terminal 1309 is a carrier control terminal assembled on the absorption region, such as the absorption region 1304. The effects and implementation details of combining the absorber carrier control terminal 1309 and the substrate carrier control terminal 1308 have been described with respect to Figure 5K . In some embodiments, the substrate carrier collection terminal 1306, the substrate carrier control terminal 1308, and the absorber carrier control terminal 1309 will repeatedly appear in Figure 13Bin the second column.

[0279] In Figure 13B the switch photodetector 1310 is similar to Figure 13A the switch photodetector 1300 shown, except that the substrate carrier control terminal 1308 is omitted, and a pair of terminals 1306 and 1309 in the second column are added. The second pair of control and collection terminals adjacent to the first pair of control and collection terminals can operate independently of or in combination with the first pair of control and collection terminals.

[0280] In Figure 13C the switch photodetector 1320 is similar to Figure 13B the switch photodetector 1310 shown, except that the substrate carrier collection terminal 1306 on one side of the light receiving region 1305 is removed. The combination of a pair of absorber carrier control terminals 1309 and the corresponding substrate carrier collection terminal 1306 on one side of the light receiving region 1305 can act as a switch.

[0281] In Figure 13D the switch photodetector 1330 is similar to Figure 13B the switch photodetector 1310 shown, except that the substrate carrier collection terminal 1306 is moved to the absorption region 1304 to serve as the absorber carrier collection terminal 1307.

[0282] In Figure 13E the switch photodetector 1340 is similar to Figure 13D the switch photodetector 1330 shown, except that the absorber carrier collection terminal 1307 on one side of the light receiving region 1305 is removed. The combination of a pair of absorber carrier control terminals 1309 and the corresponding absorber carrier collection terminal 1307 on one side of the light receiving region 1305 can act as a switch.

[0283] In Figure 13F the switch photodetector 1350 is similar to Figure 13D the switch photodetector 1330 shown, except that the absorber carrier collection terminal 1307 on one side of the light receiving region 1305 is removed. The combination of a pair of absorber carrier control terminals 1309 and the corresponding absorber carrier control terminal 1309 on each side of the light receiving region 1305 can act as a switch.

[0284] In Figure 13G the switch photodetector 1360 is similar to Figure 13DThe illustrated switched photodetector 1330, except that four pairs of absorption carrier collection and control terminals 1307 and 1309 are symmetrically disposed around the light receiving region 1305. Any pair of terminals 1307 and 1309 can act as a switch. Each switch can be operated individually or in cooperation with other switches. For example, the east and west switches can be controlled to act as a first switch, and the south and north switches can be controlled to act as a second switch. In other examples, the east and south switches can be controlled to act as a first switch, and the west and north switches can be controlled to act as a second switch.

[0285] Although Figures 13A - 13G the illustrated light receiving region 1305 does not overlap with the carrier collection terminal or the carrier control terminal, generally, the light receiving region 1305 can overlap with at least a portion of the carrier control terminal, at least a portion of the carrier receiving terminal, and at least a portion of various n-doped regions or p-doped regions. For example, such overlap can occur in pixels for both FSI and BSI configurations.

[0286] For a switch having two or more carrier control terminals, an independent control bias voltage can be applied to each carrier control terminal, or biased with a single bias voltage to carrier control terminals shorted to each other. Figures 14A - 14B A top view showing an example of a switched photodetector having a switch including a plurality of carrier control terminals. In Figure 14A it, the switched photodetector 1400 is similar to Figure 13A the switched photodetector 1300. The substrate carrier collection terminal 1306, the substrate carrier control terminal 1308, and the absorber carrier control terminal 1309 on the left side of the light receiving region 1305 form a first switch 1410, and the substrate carrier collection terminal 1306, the substrate carrier control terminal 1308, and the absorber carrier control terminal 1309 on the right side of the light receiving region 1305 form a second switch 1420.

[0287] In switches 1410 and 1420, the substrate carrier control terminal 1308 and the absorber carrier control terminal 1309 can be shorted together and biased with a single bias voltage, or can be biased with separate control bias voltages. For example, the substrate carrier control terminal 1308 of the first switch 1410 is biased with voltage VB1, and the absorber carrier control terminal 1309 is biased with voltage VA1; similarly, the substrate carrier control terminal 1308 of the second switch 1420 is biased with bias voltage VB2, and the absorber carrier control terminal 1309 is biased with voltage VA2. In some embodiments, control terminals near the light receiving region 1305, such as the absorber carrier control terminal 1309, can be biased with control voltages VA1 and VA2 respectively to direct the photo-generated carriers in the light receiving region 1305 towards the substrate carrier collection terminals 1306 biased with voltages Vc1 and Vc2, as shown. At the same time, the substrate control terminal 1308 can be biased to voltages Vb1 and Vb2 to establish a high electric field between the substrate carrier control terminal 1308 and the substrate carrier collection terminal 1306. When the electric field between terminals 1308 and 1306 is high enough, an avalanche multiplication region can be established between terminals 1308 and 1306, providing avalanche gain to the photo-generated carriers directed by the absorber carrier control terminal 1309 towards the substrate carrier collection terminal 1306. Thereby, the photo-generated carriers can be multiplied due to avalanche gain, which can increase the photocurrent signal generated by the switched photodetector 1400.

[0288] In Figure 14B the switched photodetector 1430 is similar to Figure 14A the switched photodetector 1400 shown, but differs in that the substrate carrier collection terminal 1306 is repositioned on the absorption region 1304 as the absorber carrier collection terminal 1407, and the substrate carrier control terminal 1308 is repositioned on the absorption region 1304 as the absorber carrier control terminal 1409. The effects of different biases on the terminals are similar to those described with respect to Figure 14A above.

[0289] Although the light receiving region 1305 shown in Figures 14A - 14B does not overlap with the carrier collection terminal or the carrier control terminal, generally, the light receiving region 1305 can overlap with at least a portion of the carrier control terminal, at least a portion of the carrier receiving terminal, and at least a portion of various n-doped or p-doped regions. For example, such overlap can occur in pixels applying both FSI and BSI configurations.

[0290] In an embodiment of a typical image sensor, a plurality of sensor pixels (e.g., photodetectors) may be arranged in an array to allow the image sensor to capture an image having a plurality of image pixels. To provide a high packing density, a plurality of sensor pixels are typically arranged close to each other on a common substrate. For a semiconductor substrate such as a p-doped silicon substrate, the proximity of sensor pixels to each other may cause electrical and / or optical crosstalk between the sensor pixels, which may, for example, reduce the signal-to-noise ratio of the sensor pixels. In such cases, electrical isolation between sensor pixels can be enhanced by introducing various isolation structures.

[0291] Figures 15A - 15G is a side view of an exemplary structure for sensor pixel isolation. In Figure 15A , an exemplary configuration 1500 includes a substrate 1502, sensor pixels 1510a and 1510b (collectively referred to as sensor pixels 1510), and an isolation structure 1506. Sensor pixels 1510a, 1510b include corresponding absorption regions 1504a and 1504b. Each imager pixel 1510 may be a switched photodetector, such as Figures 5A - 5K the switched photodetector shown. For clarity, details of the sensor pixels 1510 are omitted.

[0292] The isolation structure 1506 can increase the electrical isolation between sensor pixels 1510a and 1510b. In configuration 1500, the isolation structure extends from the upper surface of the substrate 1502 to a predetermined depth inside the substrate 1502. In some embodiments, the isolation structure 1506 is a doped region doped with a p-type dopant or an n-type dopant. Doping of the isolation structure 1506 can create a potential energy barrier due to a bandgap offset that impedes current across the isolation configuration 1506, and enhance the electrical isolation between pixels 1510a and 1510b. In some embodiments, the isolation structure 1506 is a trench filled with a semiconductor material different from the substrate 1502. An interface between two different semiconductors of the substrate 1502 and the isolation structure 1506 can create a potential energy barrier due to a bandgap offset that impedes current across the isolation configuration 1506, and enhance the electrical isolation between pixels 1510a and 1510b.

[0293] In some embodiments, the isolation structure 1506 is a trench filled with a dielectric or an insulator. The isolation structure 1506 filled with a low-conductivity dielectric or insulator can provide a region with a high resistance between sensor pixels 1510a and 1510b, impede current across the isolation structure 1506, and enhance the electrical isolation between pixels 1510a and 1510b.

[0294] Although only a single isolation structure 1506 is shown, generally, multiple isolation structures 1506 may be configured in each adjacent pair of image sensing pairs 1510. For example, in a two-dimensional array of sensor pixels 1510, a single sensor pixel 1510 may be surrounded by the four nearest neighboring sensor pixels 1510. In such a case, the isolation structures 1506 may be assembled along the four nearest neighboring interfaces. In some embodiments, the isolation structure 1506 may be a continuous structure surrounding the sensor pixel 1510. The isolation configuration 1506 may be shared at the interfaces between the pixels 1510.

[0295] Figure 15B An example of an exemplary configuration 1520 is shown, where the configuration 1520 is similar to Figure 15A configuration 1500 of, but differs in that the absorption regions 1504a and 1504b are fully embedded in the substrate 1502.

[0296] Figure 15C An exemplary configuration 1530 is shown, where the configuration 1530 is similar to Figure 15A configuration 1500 of, but differs in that the isolation structure 1506 extends from the upper surface of the substrate 1502 through the entire depth of the substrate 1520 to the lower surface of the substrate 1502. The configuration 1530 may remove the alternative flow paths between the image sensors 1510 that turn the isolation structure 1506 and may improve the electrical isolation between the sensor pixels 1510.

[0297] Refer to Figure 15D , the exemplary configuration 1540 is similar to Figure 15C configuration 1530 of, but differs in that the absorption regions 1504a and 1504b are fully embedded in the substrate 1502.

[0298] Refer to Figure 15E , the exemplary configuration 1550 includes a substrate 1502, sensor pixels 1510a, 1510b (collectively referred to as sensor pixels 1510), and isolation structures 1556a and 1556b (collectively referred to as isolation structure 1556). The isolation structures 1556a and 1556b are similar to Figure 15A the isolation structure 1506 described, but differ in that the isolation structure 1556 is configured over a portion of the substrate 1502 and directly under the corresponding absorption layer 1504. The isolation structure 1556 configured between the absorption region 1504 and the substrate 1502 may help confine the photo-generated carriers in the absorption region 1504 and may help reduce the photo-generated carriers leaking into the substrate 1502. For example, the sensor pixels 1510a and 1510b may be implemented as Figure 5DThe switch photodetector 530 therein has all electrical terminals assembled on the absorption region 1504. In such a case, the electrical isolation provided by the isolation structure 1556 (e.g., a thin p-doped layer) can improve the photocurrent collection efficiency and / or the bandwidth of the sensor pixel 1510.

[0299] Reference Figure 15F , the example configuration 1560 is similar to Figure 15E the configuration 1550, but the difference is that the absorption regions 1504a and 1504b are completely embedded in the substrate 1502, and the isolation structure 1556 partially or completely surrounds the absorption region 1504. For the isolation structure 1556 formed of an insulator or a dielectric, the isolation structure 1556 may include an opening located under the absorber and partially surrounding the embedded absorption region 1504. For the isolation structure 1556 that is a doped region, the isolation structure 1556 can be a continuous structure that completely surrounds the embedded absorption region 1504 without an opening.

[0300] Although the isolation structure that is a doped region, a dielectric material, or an insulator has been described, generally, the isolation structure can be a combination of these implementations. Reference Figure 15G , the example configuration 1570 is similar to Figure 15A the configuration 1500 shown, but the difference is that the isolation structure 1506 includes a first isolation structure 1576 and a second isolation structure 1577. The first isolation structure 1576 can be a trench filled with a semiconductor material different from the substrate 1502, or a trench filled with a dielectric or an insulator. The second isolation structure 1577 can be a doped region doped with a p-type dopant or an n-type dopant. Compared with implementing a single isolation structure, implementing an isolation structure 1504 with both different materials and doped regions can further improve the electrical isolation between the sensor pixels 1510. In some embodiments, doped isolation can be used to form the second isolation structure 1577, while material isolation by trench filling can be used to form the first isolation structure 1576 where the doped isolation is shallower than the material isolation.

[0301] The light detection efficiency of a photodetector, such as a switch photodetector, can be improved by adding various structures that adjust the characteristics of the photodetector. For example, mirrors, dielectric layers, and antireflection coating (ARC) layers can be added individually or simultaneously to achieve different effects, including increasing the light absorption efficiency through the absorption region, creating an optical resonant cavity, and / or changing the spectral response of the photodetector. Figures 16A - 16J A cross-sectional view of an example structure for improving the detection efficiency of a photodetector. Reference Figure 16A , the example configuration 1600 includes a substrate 1602, an absorption region 1604, and a metal mirror 1606. The absorption region 1604 forms the photodetector. The metal mirror 1606 reflects the incident light.

[0302] As shown in the figure, the optical signal 1605 is incident on the absorption region 1604 from the top, which can be regarded as a front-side illumination (FSI) configuration. In such a structure, the optical signal 1605 may not be completely absorbed by the absorption region 1604, and a part of the optical signal 1605 may pass through the absorption region 1604. The light passing through the absorption region 1604 without being absorbed by the absorption region 1604 may reduce the light detection efficiency of the photodetector. By assembling the metal mirror 1606 on the lower surface of the substrate 1602 to reflect the optical signal 1605 passing through the absorption region 1604, the optical signal 1605 passing through the absorption region 1604 can be reflected back to the absorption region 1604 and pass through the absorption region 1604 again, improving the detection efficiency.

[0303] The portion of the optical signal 1605 absorbed by the absorption region 1604 can be a function of the light absorption coefficient of the absorption region 1604, the thickness of the light absorption region 1604 along the light incident direction (e.g., along the vertical direction), and the wavelength of the optical signal 1605.

[0304] The metal mirror 1606 can be formed of different optically reflective metals, such as copper, aluminum, gold, and platinum. The metal mirror 1606 can have a reflectivity higher than 50%, 60%, 70%, 80%, 90%, or 95% at the operating wavelength of the photodetector of the configuration 1600. The thickness of the metal mirror 1606 can be greater than the skin depth of the metal. For example, the metal mirror 1606 can have a thickness in the range from 50 nm to 500 nm.

[0305] Reference Figure 16B , the exemplary configuration 1610 is similar to Figure 16A the configuration 1600, but the difference is that the configuration 1610 further includes a dielectric layer 1608 disposed between the substrate 1602 and the metal mirror 1606. The dielectric layer 1608 can change the optical reflection spectrum of the metal mirror 1606. For example, through thin-film interference caused by the dielectric layer 1608 (e.g., SiO2 layer), the reflectivity of the metal mirror 1606 (e.g., aluminum layer) for light incident thereon at certain wavelengths can be increased (e.g., the reflectivity can be increased from less than 90% to greater than 97%), while the reflectivity of the incident light at other wavelengths may be reduced.

[0306] Reference Figure 16C ,the exemplary configuration 1620 is similar to Figure 16AConfiguration 1600, except that the metal mirror 1606 in Configuration 1600 is replaced by a dielectric mirror 1626. The dielectric mirror 1626 can be a single-layer dielectric film or a stack of various dielectric films. The dielectric mirror 1626 can be formed of various dielectric materials, such as SiO2, Si3N4, SiON, and Si. At the operating wavelength of the photodetector in Configuration 1620, the dielectric mirror can have a reflectivity of up to 50%, 60%, 70%, 80%, 90%, or 95%. The dielectric mirror 1626 can have a thickness in the range of 50 nm to 4000 nm.

[0307] Reference Figure 16D , Example configuration 1630 is similar to Figure 16C Configuration 1620, except that the dielectric mirror 1626 in Configuration 1620 is replaced by a distributed Bragg reflector (DBR) 1632. The DBR mirror comprises a first dielectric layer 1634 and a second dielectric layer 1636 alternately stacked on top of each other. The second dielectric layer 1636 has a refractive index different from that of the first dielectric layer 1634. The first dielectric layer 1634 and the second dielectric layer 1636 can have a thickness corresponding to a quarter of the operating wavelength in the respective dielectric materials. The reflectivity and the reflection bandwidth can depend on the thickness, the refractive indices of the first dielectric layer 1634 and the second dielectric layer 1636, and the number of first-second layer pairs.

[0308] Reference Figure 16E , Example configuration 1640 includes a substrate 1602, an absorption layer 1604, and an ARC layer 1648. The ARC layer 1648 can reduce the reflection of the optical signal 1605 when incident on the absorption region 1604. The ARC layer 1648 can be similar to Figure 7B the ARC layer 744 shown in

[0309] Reference Figure 16F , Example configuration 1650 is similar to Figure 16A Configuration 1600, except that the metal mirror 1606 is now assembled on the upper surface of the substrate 1602 on the side of the absorption region 1604. The optical signal 1605 is now incident on the absorption region 1604 through the lower surface of the substrate 1602, which can be referred to as a backside illumination (BSI) configuration. The effect of the metal mirror 1606 is similar to that described in Figure 16A

[0310] Reference Figure 16G , Example configuration 1660 is similar to Figure 16BConfiguration 1610 shown, but with the difference that the dielectric layer 1608 and the metal mirror 1606 are assembled on the upper surface of the substrate 1602 on the side of the absorption region 1604 at this time. The optical signal 1605 is incident on the absorption region 1604 through the lower surface of the substrate 1602 at this time, which can be referred to as a backside illumination (BSI) configuration. The effects of the dielectric layer 1608 and the metal mirror 1606 are similar to those in Figure 16B described effects.

[0311] Refer to Figure 16H , the exemplary configuration 1670 is similar to Figure 16C configuration 1620, but with the difference that the dielectric mirror 1626 is assembled on the upper surface of the substrate 1602 on the side of the absorption region 1604 at this time. The optical signal 1605 is incident on the absorption region 1604 through the lower surface of the substrate 1602 at this time, which can be referred to as a backside illumination (BSI) configuration. The effect of the dielectric mirror 1626 is similar to that in Figure 16C described effects.

[0312] Refer to Figure 16I , the exemplary configuration 1680 is similar to Figure 16D configuration 1630 shown, but with the difference that the DBR mirror 1632 is assembled on the upper surface of the substrate 1602 on the side of the absorption region 1604 at this time. The optical signal 1605 is incident on the absorption region 1604 through the lower surface of the substrate 1602 at this time, which can be referred to as a backside illumination (BSI) configuration. The effect of the DBR mirror 1632 is similar to that in Figure 16D described effects.

[0313] Refer to Figure 16J , the exemplary configuration 1690 is similar to Figure 16E configuration 1640 shown, but with the difference that the ARC layer 1648 is assembled on the lower surface of the substrate 1602 on the side opposite to the absorption region 1604 at this time. The optical signal 1605 is incident on the absorption region 1604 through the lower surface of the substrate 1602 at this time, which can be referred to as a backside illumination (BSI) configuration. The effect of the ARC layer 1648 is similar to that in Figure 16E described effects.

[0314] Generally speaking, mirror structures such as the metal mirror 1606, the dielectric layer 1608, the dielectric mirror 1626, and the DBR mirror 1632 can be fabricated using different methods. For example, the mirror structure can be directly deposited on the substrate 1602. Optionally or additionally, the mirror structure can be fabricated on a separate substrate and bonded to the substrate 1602 using wafer bonding technology.

[0315] Although various embodiments are shown having a metal mirror 1606, a dielectric layer 1608, a dielectric mirror 1626, and a DBR mirror 1632 on the lower or upper surface of the substrate 1602, in practice, the structures can be implemented on both sides of the substrate 1602 simultaneously. For example, the DBR mirror 1632 can be implemented on both sides of the substrate 1602, which can establish an optical resonance cavity around the absorption region 1604 and change the spectral response of the photodetector. In other examples, the ARC layer 1648 can be implemented on the upper surface of the substrate 1602 and combined with the mirror structure on the lower surface of the substrate 1602 (e.g., configurations 1600, 1610, 1620, and 1630) to further improve the light detection efficiency of the photodetector. Generally, mirrors, such as the metal mirror 1606, the dielectric layer 1608, the dielectric mirror 1626, and the DBR mirror 1632, can partially reflect and partially transmit.

[0316] The surface of the absorption region can be modified in different ways to adjust various performance characteristics of light detection. Example modifications of the surface of the absorption region further include: addition of doped regions, introduction of hetero elements, change of material composition, introduction of the topography of the surface of the absorption region, and deposition of dielectric or semiconductor materials. Example performance characteristics include: light absorption efficiency, optical absorption spectrum, carrier absorption efficiency, dark current or leakage current, operating power of the photodetector, and bandwidth of the photodetector.

[0317] Figures 17A - 17E A cross-sectional view of an example structure for surface modification of the absorption region. In Figure 17A it, the surface-modified absorption region 1700 includes a germanium-silicon-based absorption region 1704 and a surface modification layer 1706. The germanium-silicon-based absorption region 1704 can be the absorption region of a switching photodetector; examples of the switching photodetector are Figure 5D the switching photodetector 530 shown.

[0318] The germanium-silicon-based absorption region 1704 can be a SixGe1-x compound with a varying composition (X). For example, the range of variation of the composition (X) can be from 0.01 where the germanium-silicon-based absorption region 1704 has characteristics close to Ge to 0.99 where the germanium-silicon-based absorption region 1704 has characteristics close to silicon. The composition of the germanium-silicon-based absorption region can affect its light absorption efficiency for light of the absorption wavelength and simultaneously affect the overall optical absorption spectrum. For example, compared to the high silicon composition corresponding to a high (X) composition, the high Ge concentration corresponding to a low (X) composition can absorb a large amount of near-infrared wavelengths (e.g., greater than 1 μm).

[0319] The surface modification layer 1706 can modify the optical and / or electrical properties of the germanium-silicon-based absorption region 1704 and the photodetector having such an absorption region 1704. The surface modification layer can be formed of various materials, such as amorphous silicon, polycrystalline silicon, epitaxially grown silicon, SiYGe1-Y compounds with variable composition (Y), GeZSn1-Z compounds with variable composition (Z), and any combination thereof.

[0320] In some embodiments, for a germanium-silicon-based absorption region 1704 having a SixGe1-x composition, the surface modification layer 1706 can be a SiYGe1-Y layer, where the composition (Y) is different from the composition (X). For example, when the composition (X) is higher than the composition (Y), the surface modification layer 1706 has a higher absorption coefficient for long wavelengths compared to the germanium-silicon-based absorption region 1704. In such a case, incident light of long wavelengths can be largely absorbed in the surface modification layer 706 without penetrating deeply into the germanium-silicon-based absorption region 1704. By absorbing the incident light at the surface of the germanium-silicon-based absorption region 1704, the bandwidth of the photodetector containing the absorption region 1704 can be increased because the diffusion of photo-generated carriers in the light absorption region 1704 is reduced. In some embodiments, for a pure germanium absorption region 1704 (i.e., X = 0), the surface modification layer 1706 can be a SiYGe1-Y layer. In some embodiments, the composition of the surface modification region 1706 and the germanium-silicon-based absorption region 1704 can vary along a direction (e.g., the vertical direction) to form a graded GeSi absorption region 1704. The grading of the GeSi composition can further increase the bandwidth of the photodetector. In some embodiments, the surface modification layer 1706 can be multilayered. For example, a GeSi layer can be deposited on top of the germanium-silicon-based absorption region 1704 to provide passivation and another silicon layer can be deposited on top of the GeSi layer for further passivation.

[0321] In some embodiments, the surface modification layer 1706 can be a germanium-tin alloy GeZSn1-Z with variable composition (Z). Increasing the tin in the surface modification layer 1706 can improve the optical absorption efficiency at long wavelengths, such as beyond the germanium bandgap (about 1.55 μm), where the absorption efficiency of pure germanium generally drops significantly beyond this bandgap.

[0322] Reference Figure 17B, the surface modification layer 1710 includes a germanium-silicon-based absorption region 1704 and a first doping region 1712. In some embodiments, the first doping region 1712 may be doped with a p-type or n-type dopant. The p-type or n-type dopant can change the electrical properties of the absorption region 1704. For example, due to the first doping region 1712, photo-generated electrons (or holes) can be repelled from the surface, thereby avoiding surface recombination, which provides high absorption efficiency when the first doping region 1712 has a p-type (or n-type) dopant. In some embodiments, the first doping region 1712 may be doped with impurities such as silicon or tin to adjust the optical properties of the absorption region 1704.

[0323] Reference Figure 17C , the surface modification layer 1720 is similar to the surface modification layer 1710, but differs in that it further includes a second doping region 1722. The second doping layer 1722 may be similar to the first doping region 1712 or have a different polarity, depth, or width, whereby photo-generated carriers can be attracted by the second doping region 1722 and repelled by the first doping region 1712.

[0324] Reference Figure 17D , the surface modification layer 1730 includes a germanium-silicon-based absorption region 1704 and a dielectric well 1732. The dielectric well 1732 may be filled with different dielectrics such as SiO2, Si3N4, and high-k materials. When the dielectric well is configured within a PN junction or sandwiched between surface electrical terminals, it can reduce the dark current or leakage current, reduce the operating power of the photodetector, and / or increase the bandwidth of the photodetector.

[0325] Reference Figure 17E , the switching photodetector 1740 includes a surface-modified Ge absorption layer 1710 as Figure 17B shown. The switching photodetector 1740 is similar to Figure 1B the switching photodetector 160 shown, but differs in that it further includes a surface modification layer 1706 and Figure 11A the carrier collection terminal 1106 and the carrier control terminal 1108. Adding the surface modification layer 1706 can improve various performance characteristics of the switching photodetector 1740, such as: light absorption efficiency, light absorption bandwidth, carrier collection efficiency, dark current or leakage current, operating power of the photodetector, and bandwidth of the photodetector.

[0326] Although separate embodiments of surface modification of the absorption region are shown, generally, surface modification can be implemented in different combinations to achieve the desired effects. For example, the surface modification layer 1706 can be implemented in combination with the first doped region 1712 and / or the second doped region 1722. In another example, the surface modification layer 1706 can be implemented in combination with the dielectric well 1732. In yet another example, the surface modification layer 1706 can be implemented in combination with the first doped region 1712 and / or the second doped region 1722 and the dielectric well 1732.

[0327] Various doped regions and wells such as p-doped regions and wells and n-doped regions and wells can be configured at different positions in the absorption region, the substrate, or the intermediate layer to change the performance characteristics of the device. Exemplary performance characteristics include: light absorption efficiency, light absorption spectrum, carrier collection efficiency, dark current or leakage current, the operating power of the photodetector, and the photodetector bandwidth.

[0328] The depth of the doped regions and wells can be determined based on various considerations such as manufacturability and device performance. One or more doped wells and regions can be connected to a voltage or current source. One or more doped wells and regions can also be not connected to a voltage or current source (i.e., floating) and / or connected to each other (i.e., shorted).

[0329] Figures 18A - 18B A top view and a side view showing an example of the switching photodetector 1800 are presented. The switching photodetector 1800 is similar to Figure 1B the switching photodetector 160 shown, and further includes Figure 11A the carrier collection terminal 1106 and the carrier control terminal 1108 shown. As previously described in Figure 1B the n-well regions 152 and 154 can reduce the leakage current from the first control signal 122 to the second control signal 132 and can reduce the charge coupling between the n-doped regions 126 and 136. Reducing the leakage current can reduce the operating power of the switching photodetector 1800.

[0330] Figures 18C - 18D A top view and a side view showing an example of the switching photodetector 1820 are presented. The switching photodetector 1800 is similar to Figures 18A - 18B the switching photodetector 1800 of Figure 2D but further includes a p-well region 1822. The p-doped region 1822 can be similar to

[0331] In some cases, the n-doped regions 126 and 136 cannot completely absorb the photo-generated carriers in the absorption region 106. In such cases, the photo-generated carriers may reach the material surface where there are material defects between the substrate 102 and the absorption region 106. The material defects can trap the photo-generated carriers and release these carriers after a period of time; the n-doped regions 126 and 136 then collect the carriers. The phenomenon that the material defects at the interface intercept, release the carriers and the subsequent n-doped regions 126 and 136 collect the carriers may reduce the bandwidth of the switching photodetector 1800 due to the time delay effect of intercepting and releasing the carriers. Therefore, adding the p-well region 1822 can slow down the reduction of the bandwidth because it can prevent the carriers from being collected by the n-doped regions 126 and 136 by not allowing the photo-carriers to reach the interface between the absorption region 106 and the substrate 102.

[0332] Figure 18E A top view showing an example of the switching photodetector 1830. The switching photodetector 1830 is similar to Figures 18C - 18D the switching photodetector 1820, but further includes a p-well region 1832. The p-well region 1832 is similar to the p-well region 1822. The combination of the p-well regions 1822 and 1832 surrounds the corresponding n-doped regions 126 and 136, which can further prevent the carriers from being absorbed by the n-doped regions 126 and 136 by preventing the photo-generated carriers from reaching the interface between the absorption region 106 and the substrate 102. Although the separate p-well regions 1822 and 1832 are shown here, the p-well regions 1822 and 1832 can be combined into a "C"-shaped region surrounding the corresponding n-doped regions.

[0333] Figures 18F - 18G A top and side view showing an example of the switching photodetector 1840. The switching photodetector 1840 is similar to Figures 18A - 18B the switching photodetector 1800 shown, but differs in that it omits the n-well regions 152 and 154 and includes a p-well region 1842. The p-well region 1842 can be similar to Figure 2D the p-well regions 246 and 248 shown. The p-well region 1842 surrounds the absorption region 106 embedded in the substrate 102. The p-well region 1842 can block the photo-generated electrons in the absorption region 106 from reaching the substrate 102. Compared with the switching photodetector 1800, such a block can increase the collection efficiency of the photo-generated carriers of the switching photodetector 1840. The p-doped region 1842 can be formed in the absorption region 106, the substrate 102, the intermediate layer between the absorption region 106 and the substrate, or a combination thereof.

[0334] Although the separate embodiments of the n-well regions 152 and 154, and the p-well regions 1822, 1832 and 1842 are shown, generally, the n-well regions and the p-well regions can be combined to achieve the desired effect.

[0335] So far, specific implementations of several elements of the switching photodetector and various arrangements of the elements have been introduced. Next, the combination of the foregoing elements will be described. The combinations described herein are not a complete enumeration of all combinations.

[0336] Figures 19A - 19B A top view and a side view showing an example of the switching photodetector 1900 are presented. The switching photodetector 1900 is similar to Figure 1A the switching photodetector 100, but differs in that the absorption region 106 of the switching photodetector 1900 is completely embedded in the substrate 102 and further includes Figure 11A the carrier collection terminal 1106 and the carrier control terminal 1108. The light receiving region 1205 has been described with respect to Figures 12A - 12B The presence of the p-doped regions 128 and 138 results in the formation of an ohmic contact at the interface between the carrier control terminal 1108 and the absorption region 106.

[0337] Figures 19C - 19D A top view and a side view showing an example of the switching photodetector 1910 are presented. The switching photodetector 1910 is similar to Figures 19A - 19B the switching photodetector 1900 shown, but differs in that the p-doped regions 128 and 138 are omitted. The omission of the p-doped regions 128 and 138 results in the formation of a Schottky junction at the interface between the carrier control terminal 1108 and the absorption region 106.

[0338] Figures 19E - 19F A top view and a side view showing an example of the switching photodetector 1920 are presented. The switching photodetector 1910 is similar to Figures 19A - 19B the switching photodetector 1900 shown, but differs in that the p-doped regions 128 and 138 are added and carrier control terminals 1108 are added on each side of the light receiving region 1205.

[0339] Figures 19G - 19H A top view and a side view showing an example of the switching photodetector 1930 are presented. The switching photodetector 1930 is similar to Figures 19E - 19F the switching photodetector 1920 shown, but differs in that the p-doped regions 128 and 138 are omitted. The omission of the p-doped regions 128 and 138 results in the formation of a Schottky junction at the interface between the carrier control terminal 1108 and the absorption region 106.

[0340] Figures 20A - 20B A top view and a side view showing an example of the switching photodetector 2000 are presented. The switching photodetector 2000 is similar to Figures 19A - 19B the switching photodetector 1900 shown, but differs in that Figure 10I the intermediate layer 1006 shown is added. As in Figure 10IAs described, the intermediate layer 1006 has an opening to the substrate 102, the absorption region 106 is filled into the opening of the substrate 102, and the opening is formed by the intermediate layer 1006. In some embodiments, the intermediate layer 1006 can be SiO2, SiNx, AlOx, or any oxide or nitride-based insulator.

[0341] Figures 20C - 20D Top and side views showing an example of the switching photodetector 2010. The switching photodetector 2010 is similar to Figures 19A - 19B the switching photodetector 2000 shown, but differs in that Figures 20A - 20B the intermediate layer 1006 is replaced by another intermediate layer 2012. The material of the intermediate layer 2012 is similar to that of the intermediate layer 1006, but differs in that the intermediate layer 2012 is a uniform layer that spans the upper surface of the substrate 102 and has an opening to the substrate. The absorption region 106 is embedded in the opening of the intermediate layer 2012. In some embodiments, the intermediate layer 2012 can be SiO2, SiNx, AlOx, or any oxide or nitride-based insulator.

[0342] Figures 20E - 20F Top and side views showing an example of the switching photodetector 2020. The switching photodetector 2020 is similar to Figures 20C - 20D the switching photodetector 2010 shown, but differs in that the p-doped regions 128 and 138 are omitted. The omission of the p-doped regions 128 and 138 causes a Schottky junction to form at the interface between the carrier control terminal 1108 and the absorption region 106.

[0343] Figures 20G - 20H Top and side views showing an example of the switching photodetector 2030. The switching photodetector 2030 is similar to Figures 20C - 20D the switching photodetector 2010 shown, but differs in that Figures 20C - 20D the intermediate layer 2012 is replaced by another intermediate layer 2032. The intermediate layer 2032 is similar to Figures 20C - 20D the intermediate layer 2012, but differs in that the intermediate layer 2032 has a first opening 2034 to the substrate 102 and a second opening 2036 larger than the first opening 2034, and the opening of the second opening 2036 faces the upper surface of the intermediate layer 2032.

[0344] Figures 20I - 20J Top and side views showing an example of the switching photodetector 2040. The switching photodetector 2040 is similar to Figures 20G - 20H the switching photodetector 2030 shown, but differs in that the p-doped regions 128 and 138 are omitted. The omission of the p-doped regions 128 and 138 causes a Schottky junction to form at the interface between the carrier control terminal 1108 and the absorption region 106.

[0345] Figures 20K - 20L Top and side views showing an example of the switching photodetector 2050. The switching photodetector 2050 is similar to Figures 20G - 20H the switching photodetector 2030 shown, but differs in that n-well regions 152 and 154 are added. The n-well regions 152 and 154 have been described with respect to Figure 1B this.

[0346] Figures 21A - 21B Top and side views showing an example of the switching photodetector 2100. The switching photodetector 2100 is similar to Figures 19A - 19B the switching photodetector 1900 shown, but differs in that the n-doped regions 126 and 136, the p-doped regions 128 and 138, the carrier collection terminals 1106 and the carrier control terminals 1108 are moved from the absorption region 106 to the substrate 102. Such terminals 1106 and 1108 may be referred to as substrate carrier collection terminals and substrate carrier control terminals.

[0347] Figures 21C - 21D Top and side views showing an example of the switching photodetector 2110. The switching photodetector 2110 is similar to Figures 21A - 21B the switching photodetector 2100 shown, but differs in that the absorber p-doped regions 2128 and 2138 and the absorber carrier control terminal 2108 are assembled in the absorption region 106. The substrate carrier absorption terminal 1106, the substrate carrier control terminal 1108, and the absorber carrier control terminal 2108 may be similar to the substrate carrier collection terminal 1306, the substrate carrier control terminal 1308, and the absorber carrier control terminal 1309 described in Figure 14A and have a similar effect.

[0348] Figures 21E - 21F Top and side views showing an example of the switching photodetector 2120. The switching photodetector 2120 is similar to Figures 21C - 21D the switching photodetector 2110 shown, but differs in that the absorber p-doped regions 2128 and 2138 are omitted. The omission of the absorber p-doped regions 2128 and 2138 results in the formation of a Schottky junction at the interface between the absorber carrier control terminal 2108 and the absorption region 106.

[0349] Figures 22A - 22B Top and side views showing an example of the switching photodetector 2200. The switching photodetector 2200 is similar to Figures 18F - 18G the switching photodetector 1840 shown, but differs in that Figures 18A - 18B the n-well regions 152 and 154 in

[0350] Figures 22C - 22DTop and side views showing an example of the switching photodetector 2210. The switching photodetector 2210 is similar to Figures 21C - 21D the switching photodetector 2110, but differs in that Figures 18A - 18B the n-well regions 152 and 154 in

[0351] Figure 23A Top view showing an example of the switching photodetector 2300 and Figure 23B side view showing an example of the switching photodetector 2300 along line AA. The switching photodetector 2300 is similar to Figures 21C - 21D the switching photodetector 2110 shown, but differs in that a p-well region 2302 is added at the interface between the absorption region 106 and the substrate 102. The p-well region 2302 can help to mitigate the carrier collection and release at the junction between the absorption region 106 and the substrate 102, which has been described with respect to Figures 18C - 18D that of

[0352] Figures 24A - 24B Top and side views showing an example of the switching photodetector 2400. The switching photodetector 2400 is similar to Figures 18C - 18D the switching photodetector 1820, but differs in that the n-well regions 152 and 154 are omitted.

[0353] Figure 24C Top view showing an example of the switching photodetector 2410. The switching photodetector 2410 is similar to Figure 18E the switching photodetector 1830, but differs in that Figure 18E the p-well regions 1822, 1832 in

[0354] Figures 24D - 24E Top and side views showing an example of the switching photodetector 2420. The switching photodetector 2420 is similar to Figures 24A - 24B the switching photodetector 2400 shown, but differs in that dielectric wells 2422 are added in the n-doped regions 126 and 136. The dielectric wells 2422 are similar to Figure 17D the dielectric wells 1732 shown in

[0355] Figures 24F - 24G Top and side views showing an example of the switching photodetector 2430. The switching photodetector 2430 is similar to Figures 24D - 24EThe switching photodetector 2420, except that the dielectric well 2422 is moved from the n-doped regions 126 and 136 to the p-doped regions 128 and 138. The depth of the dielectric well 2422 can be less than, equal to, or greater than the depth of the p-doped region 128. Generally, the dielectric well 2422 can be disposed at any position between the n-doped region 126 and the p-doped region 128 and between the n-doped region 136 and the p-doped region 138.

[0356] Figures 25A - 25B A top view and a side view showing an example of the switching photodetector 2500 are shown. The switching photodetector 2500 is similar to Figures 19A - 19B the switching photodetector 1900 shown, except that Figure 16F the shown metal mirror 1606 is added as the metal mirror 2502, where the metal mirror 2502 is disposed on the upper surface of the absorption region 106 provided with the carrier collection terminal 1106 and the carrier control terminal 1108. The metal mirror 2502 can be assembled above the light receiving region 1205. In some embodiments, the metal mirror 2502 can be implemented by the first metal layer (M1) or the second metal layer (M2) in the CMOS process, or a combination thereof.

[0357] Figures 25C - 25D A top view and a side view showing an example of the switching photodetector 2510 are shown. The switching photodetector 2510 is similar to Figures 25A - 25B the switching photodetector 2500 shown, except that the p-doped regions 128 and 138 are omitted. The omission of the p-doped regions 128 and 138 results in the formation of a Schottky junction at the interface between the carrier control terminal 1108 and the absorption region 106.

[0358] Figures 25E - 25F A top view and a side view showing an example of the switching photodetector 2520 are shown. The switching photodetector 2520 is similar to Figures 20K - 20L the switching photodetector 2050 shown, except that Figure 16F the shown metal mirror 1606 is added as the metal mirror 2502, where the metal mirror 2502 is disposed on the upper surface of the absorption region 106 provided with the carrier collection terminal 1106 and the carrier control terminal 1108. The metal mirror 2502 can be assembled above the light receiving region 1205. In some embodiments, the metal mirror 2502 can be implemented by the first metal layer (M1) or the second metal layer (M2) in the CMOS process, or a combination thereof.

[0359] Figures 25G - 25H A top view and a side view showing an example of the switching photodetector 2530 are shown. The switching photodetector 2530 is similar to Figures 18F - 18G the switching photodetector 1840 shown, except thatFigure 16F The metal mirror 1606 shown is used as the metal mirror 2502, where the metal mirror 2502 is disposed on the upper surface of the absorption region 106 provided with the carrier collection terminal 1106 and the carrier control terminal 1108. The metal mirror 2502 can be assembled above the light receiving region 1205. In some embodiments, the metal mirror 2502 can be implemented by the first metal layer (M1) or the second metal layer (M2) in the CMOS process, or a combination thereof.

[0360] In an embodiment of a typical image sensor, a plurality of sensor pixels (e.g., switched photodetectors) are arranged in an array to allow the image sensor to capture an image having a plurality of image pixels. When viewing the image sensor from the top, square sensor pixels having the same size on both sides allow a simple two-dimensional array. However, for some applications, such as ToF, some sensor pixels may not be square but rectangular. For example, in Figure 1B the switched photodetector 160 has two carrier control terminals (e.g., p-doped regions 128 and 138) and two carrier collection terminals (e.g., n-doped regions 126 and 136). These four terminals are generally arranged along a line, which results in a rectangular sensor pixel being longer in the shape of the line along which the terminals are arranged (e.g., Figure 18A the switched photodetector 1800 shown).

[0361] Such rectangular sensor pixels may make it difficult in terms of the effective arrangement of pixels, which is, for example, due to design rules related to semiconductor fabrication in a foundry. Design rules limit various minimum spacings of features such as doped regions, doped wells, dielectric wells, and germanium absorption regions. One way to improve the compactness and symmetry is to establish a unit cell of a photodetector that includes four rectangular photodetectors. Figure 26 An example of a unit cell of a rectangular photodetector is shown. The unit cell 2600 includes Figure 18A the four switched photodetectors 1800 shown and four isolation structures 2602 respectively surrounding the switched photodetectors 1800. The isolation structures 2602 have been described with respect to Figures 15A - 15D the description. The unit cell 2600 can improve the compactness and symmetry of the sensor pixels on the rectangular unit cell.

[0362] Figure 27 A top view of an example of a rectangular switched photodetector 2700 having a photosensitive transistor gain is shown. The switched photodetector 2700 is similar to Figure 18AThe illustrated switched photodetector 1800, but with the difference that an electron emitter 2710 is added on the substrate 102. The electron emitter 2710 can be similar to the n-doped regions 126 and 136. The rectangular shape of the switched photodetector 1800 allows a photocurrent integrating capacitor (e.g., a floating diffusion capacitance) to be coupled to a bipolar junction transistor (BJT) 2720 formed by the n-doped regions 126 and 136, the p-doped regions 128 and 138, and the electron emitter 2710, resulting in an NPN BJT. When appropriately biased, the BJT 2720 can provide a phototransistor gain in response to an incident optical signal, which can improve the optical-to-photocurrent conversion efficiency of the photodetector 2700. For example, the BJT 2720 can be biased as follows: the n-doped regions 126 and 136 are biased between 1V and 3V, the p-doped regions 128 and 138 are biased between 0V and 1V, and the electron emitter 2710 is biased at a lower bias than the corresponding n-doped regions 126 and 136.

[0363] Generally, the electron emitter 2710 and / or the n-doped regions 126 and 136 should be biased to an external voltage, or short-circuited to the p-doped region through a metal connection to allow electrons to be emitted from the electron emitter 2710.

[0364] Although various embodiments of the switched photodetector with a specific combination and arrangement of n-type and p-type regions and wells have been described, generally the polarities of the doped regions and wells can be reversed and similar operations and functions can be achieved. For example, all instances of p-wells and p-doped regions can be respectively converted to n-wells and n-doped regions, and all n-wells and n-doped regions can be correspondingly converted to p-wells and p-doped regions.

[0365] Figure 28A An example imaging system 2800 for determining the characteristics of a target object 2810 is shown. The target object 2810 can be a three-dimensional object. The imaging system 2800 can include a transmitter unit 2802, a receiver unit 2804, and a processing unit 2806. Generally, the transmitter unit 2802 emits light 2812 towards the target object 2810; the transmitter unit 2802 can include one or more light sources, control circuits, and / or optical elements. For example, the transmitter unit 2802 can include one or more NIR LEDs or lasers; the emitted light 2812 can be collimated by a collimating lens to propagate in free space.

[0366] Generally, the receiver unit 2804 receives the reflected light 2814 reflected by the target object 2810. The receiver unit 2804 may include one or more photodiodes, control circuits, and / or optics. For example, the receiver unit 2804 may include an image sensor; wherein, the image sensor includes pixels fabricated on a semiconductor substrate. Each pixel may include one or more switched photodetectors for detecting the reflected light 2814, and the reflected light 2814 may be focused onto the switched photodetectors. Each switched photodetector may be the switched photodetector disclosed in the present application.

[0367] Generally, the processing unit 2806 processes the photo-generated carriers generated by the receiver unit 2804 and determines the characteristics of the target object 2810. The processing unit 2806 may include control circuits, one or more processors, and / or a computer storage interface that can store instructions for determining the characteristics of the target object 2810. For example, the processing unit 2806 may include: a readout circuit and a processor, wherein the processor may process the information related to the collected photo-generated carriers to determine the characteristics of the target object 2810. In some embodiments, the characteristics of the target object 2810 may be the depth information of the target object 2810. In some embodiments, the characteristics of the target object 2810 may be the material composition of the target object 2810.

[0368] Figure 28B An exemplary technique for determining the characteristics of the target object 2810 is shown. The transmitter unit 2802 may emit light pulses 2812 modulated at a frequency fm with a duty cycle of 50%. The receiver unit 2804 may receive the reflected light pulses 2814 with a phase shift of Φ. The switched photodetectors are controlled such that the readout circuit 1 reads out the charge Q1 whose collected phase is synchronized with the emitted light pulse, and the readout circuit 2 reads out the charge Q2 whose collected phase is opposite to the emitted light pulse. In some embodiments, the distance D between the imaging system 2800 and the target object 2810 can be obtained by the following formula:

[0369]

[0370] where c is the speed of light.

[0371] Figure 28CAnother exemplary technique for determining characteristics of a target object 2810 is shown. A transmitter unit 2802 can transmit optical pulses 2812 modulated at a frequency fm with a duty cycle of less than 50%. By reducing the duty cycle of the optical pulses by a factor N, but at the same time increasing the intensity of the optical pulses 2812 by a factor N, the signal-to-noise ratio of the received reflected optical pulses 2814 can be improved while maintaining substantially the same energy consumption for the imaging system 2800. This is possible with an increase in the device bandwidth, such that the duty cycle of the optical pulses can be shortened without distorting the pulse shape. A receiver unit 2804 can receive the reflected optical pulses 2814 with a phase shift of Φ. A multi-gate photodiode is controlled such that a readout circuit 1 reads out the charge Q1' with the collected phase synchronized with the transmitted optical pulses, and a readout circuit 2 reads out the charge Q2' with the collected phase having a time delay with respect to the transmitted optical pulses. In some embodiments, the distance D between the imaging system 2800 and the target object 2810 can be obtained by the following formula:

[0372]

[0373] Figure 29 An example of a flowchart 2900 for determining object characteristics by an imaging system is shown. The process 2900 can be performed by a system such as the imaging system 2800.

[0374] The system receives reflected light (2902). For example, a transmitter unit 2802 can transmit NIR optical pulses 2812 towards a target object 2810. A receiver unit 2804 can receive the reflected NIR optical pulses 2814 reflected by the target object 2810.

[0375] The system determines phase information (2904). For example, the receiver unit 2804 can include an image sensor, where the image sensor includes a plurality of pixels fabricated on a semiconductor substrate. Each pixel can include one or more switched photodetectors for detecting the reflected optical pulses 2814. The type of the switched photodetectors can be the switched photodetectors disclosed in the present application, where the phase information can be determined by referring to Figure 28B or Figure 28C the techniques described above.

[0376] The system determines object characteristics (2906). For example, a processing unit 2806 can use the techniques described in Figure 28B or Figure 28C the above to determine the depth information of the object 2810 based on the phase information.

[0377] In some embodiments, an image sensor includes pixels fabricated on a semiconductor substrate, where each pixel may include one or more switched photodetectors 100, 160, 170, 180, 200, 250, 260, 270, 300, 360, 370, 380, 400, 450, 460, 470, and 480 for detecting reflected light as shown in Figure 28A and Figure 28B The isolation between these pixels can be achieved based on insulator isolation such as an oxide layer or a nitride layer, or based on implant isolation such as using p-type or n-type regions to block signal electrons or holes, or based on an intrinsic built-in energy barrier such as using a germanium-silicon heterointerface.

[0378] A number of specific implementations have been described. However, it should be understood that various modifications can be made without departing from the spirit and scope of the present disclosure. For example, various forms of the above-described processes can be used, in which certain steps can be rearranged, added, or deleted.

[0379] For ease of illustration and description, various specific implementations may have been discussed in terms of two-dimensional cross-sections. However, their three-dimensional variations and derivatives should also be covered by the present disclosure, provided that there is a corresponding two-dimensional cross-section in the three-dimensional structure.

[0380] Although this specification contains many details, these details should not be construed as limiting, but rather as specific feature descriptions for particular embodiments. In the context of different embodiments, certain features described in this specification can also be implemented together in a single embodiment. Conversely, the various features described in the context of a single embodiment can also be implemented separately in multiple embodiments, or in any suitable sub-combination. Moreover, although the above-described features may act in certain combinations, even if initially claimed as such, in some cases, one or more features in the claimed combination can be deleted from this combination, and the claimed combination can be a variation of a particular sub-combination or a combination of sub-combinations.

[0381] Similarly, although the figures show their operations in a specific order, this should not be construed as meaning that such operations must be performed in the specific order or sequence shown, or that all shown operations must be performed to obtain the desired result. In certain cases, multitasking and parallel processing are advantageous. Also, the separation of the various system components in the above embodiments should not be construed as meaning that such separation must be implemented in all embodiments, but rather that the process components and systems can generally be integrated together in a single software product or packaged into multiple software products.

[0382] Accordingly, specific embodiments have been described. Other embodiments are within the scope of the following claims. For example, the operations listed in the claims can be performed in a different order and still obtain the desired result.

Claims

1. An optical device, characterized in that, comprising: a substrate having a first material; an absorption region having a second material different from the first material, the absorption region being configured to absorb photons and generate photo - carriers, the photo - carriers including electrons and holes generated in response to the absorbed photons; a first well region surrounding the absorption region and disposed between the absorption region and the substrate, wherein the first well region is doped with a first polarity; one or more switches respectively controlled by a corresponding control signal, the one or more switches being respectively configured to collect at least a portion of the photo - carriers based on the corresponding control signal and provide the portion of the photo - carriers to a corresponding read - out circuit; and one or more isolation structures in the substrate, the one or more isolation structures including one or more of insulator isolation, implant isolation, or heterojunction isolation.

2. The optical device according to claim 1, characterized in that, at least one of the one or more isolation structures extends from an upper surface of the substrate and extends from the upper surface to a predetermined depth.

3. The optical device according to claim 1, characterized in that, the first well region is doped to block at least a portion of the electrons or at least a portion of the holes generated in the absorption region from entering the substrate.

4. The optical device according to claim 1, characterized in that, the first well region is formed in the absorption region, the substrate, or between the absorption region and the substrate, or a combination thereof.

5. The optical device according to claim 1, characterized in that, the absorption region is at least partially embedded in the substrate.

6. The optical device according to claim 1, characterized in that, further comprising a first layer covering an upper surface of the absorption region.

7. The optical device according to claim 1, characterized in that, the substrate has a p - doped region and an n - doped region, and the p - doped region is electrically short - circuited with the n - doped region.

8. The optical device according to claim 1, characterized in that, each of the one or more switches includes a corresponding control contact on the absorption region and a dielectric layer between the corresponding control contact and the absorption region.

9. The optical device according to claim 1, characterized in that, the first well region is an annular structure.

10. The optical device according to claim 1, characterized in that, further comprising a bonding layer between the absorption region and the read - out circuit.

11. An imaging system, characterized in that, comprising: a transmitter unit capable of emitting light; and a receiver unit including a plurality of pixels formed on a substrate having a first material, each pixel comprising: an absorption region having a second material different from the first material, the absorption region being configured to absorb photons and generate photo - carriers, the photo - carriers including electrons and holes generated in response to the absorbed photons; A first well region, surrounding the absorption region and disposed between the absorption region and the substrate, wherein the first well region is doped with a first polarity; One or more switches, each controlled by a respective control signal, the one or more switches being respectively configured to collect at least a portion of the photo - carriers based on the respective control signal and to provide the portion of the photo - carriers to a respective readout circuit; And One or more isolation structures in the substrate, the one or more isolation structures including one or more of insulator isolation, implant isolation, or heterojunction isolation.

12. The imaging system according to claim 11, wherein, The first well region is doped to block at least a portion of the electrons or at least a portion of the holes generated in the absorption region from entering the substrate.

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