Photodetector and Optoelectronic Integrated Device

Abstract

To provide a photodetector that achieves both light receiving sensitivity and high-speed operation.SOLUTION: A photodetector includes a light input layer 102 formed of a first semiconductor on a substrate 100 and transparent to the light of the wavelength used, and a light absorption layer 104 formed on the light input layer with a second semiconductor having a bandgap smaller than that of the first semiconductor. The light absorption layer includes a first polar region 105 doped with impurities of the first conductivity type, a second polar region 106 doped with an impurity of a second conductivity type different from the first conductivity type, and an undoped region sandwiched between the first polar region and the second polar region in the direction horizontal to the substrate, and between the first polar region and the second polar region, there is a region with a higher effective refractive index than the rest of the light absorption layer.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present disclosure relates to a photodetector and an optical integrated device.

Background Art

[0002] In order to cope with the increase in communication capacity, research and development in the field called silicon photonics have attracted attention. By forming an optical circuit on a silicon (Si) substrate or a silicon-on-insulator (SOI) substrate, integration with an electronic circuit formed on the same substrate becomes possible. With an optical integrated circuit, improvement in the conversion efficiency between electrical signals and optical signals and realization of a small-sized, large-capacity, and low-power-consumption optical transceiver are expected.

[0003] Among optical integrated circuits, waveguide portions such as optical multiplexing / demultiplexing and modulation are required to have the property of not absorbing light in order to suppress excessive loss. On the other hand, a photodetector that converts light into electricity requires the property of absorbing light. As a combination that satisfies these requirements, a configuration using germanium (Ge) for the photodetector and Si for other waveguide portions and using near-infrared light with a wavelength of 1.2 μm to 1.6 μm is promising. Light in this wavelength band is transparent to Si and is absorbed by Ge.

[0004] When extracting photocarriers from a Ge photodetector connected to an Si waveguide, the configuration of FIG. 1 can be adopted. By doping a p-type impurity into the Si waveguide (Si-WG) joined to the Ge photodetector (Ge-PD) and doping an n-type impurity on the upper part of the Ge-PD, a rectifying action works. When a reverse bias is applied such that the n-type becomes a high potential with respect to the p-type, among the photocarriers generated by light absorption in Ge, electrons are pulled out to the n electrode and holes are pulled out to the p electrode. Since this configuration has a heterointerface between Si and Ge and a PIN structure is formed in the direction perpendicular to the substrate, it is called a hetero-junction vertical PIN structure.

[0005] A stripe-shaped Ge photodetector formed on a Si island of an SOI (Silicon-on-Insulator) substrate is known (see, for example, Patent Document 1). This Ge photodetector includes a heterojunction in which a high-concentration p-type impurity region and an n-type impurity region are arranged in a direction horizontal to the substrate.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] Regardless of whether the PIN structure is perpendicular or horizontal to the substrate, in a configuration including a heterojunction in the movement path of photo carriers, the photo carriers pass through a hetero interface where the energy bands are discontinuous. In the configuration of FIG. 1, when holes pass through the Si / Ge hetero interface, due to the barrier generated in the valence band, a part of the holes accumulates at the interface. The holes accumulated at the interface shield the externally applied voltage and cause a decrease in the electric field strength of the depletion layer. As a result, the response characteristics of the photodetector deteriorate and high-speed operation is hindered.

[0008] An object of the present disclosure is to provide a photodetector that achieves both light reception sensitivity and high-speed operation.

Means for Solving the Problems

[0009] In one embodiment, the photodetector is formed of a first semiconductor on a substrate and has a light input layer that is transparent to light of the wavelength to be used, and a light absorption layer formed of a second semiconductor having a smaller bandgap than the first semiconductor on the light input layer. The light absorption layer has, in a direction horizontal to the substrate, a first polarity region doped with impurities of a first conductivity type, a second polarity region doped with impurities of a second conductivity type different from the first conductivity type, and an undoped region sandwiched between the first polarity region and the second polarity region, and has a region with an effective refractive index higher than that of other parts of the light absorption layer between the first polarity region and the second polarity region.

Advantages of the Invention

[0010] A photodetector that achieves both light reception sensitivity and high-speed operation is realized.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4A

Figure 4B

Figure 4C

Figure 4D

Figure 5

Figure 6A

Figure 6B

Figure 7A

Figure 7B

Figure 7C

Figure 7D

Figure 7E

Figure 7F

Figure 8

Figure 9

Figure 10

Figure 11

Embodiments for Carrying Out the Invention

[0012] Before explaining the specific configuration of the photodetector of the embodiment, the technical problems occurring in the hetero-junction type photodetector will be explained in more detail, and then the problems occurring in the homo-junction type photodetector will be explained.

[0013] FIG. 2 is a diagram showing the accumulation of holes at the Si / Ge hetero interface. At the interface between Si and Ge, the energy level of the valence band Ev drops on the Si side and rises on the Ge side. This energy band discontinuity becomes a potential barrier for the holes generated by light absorption in Ge.

[0014] Holes can cross this barrier with a certain probability due to the quantum tunneling effect, but some accumulate at the interface. The accumulated holes shield the applied voltage from the outside, and the electric field strength of the depletion layer decreases. In the steady state, the transport speed of the photocarriers in the depletion layer is approximately proportional to the electric field strength. The decrease in the electric field strength of the depletion layer causes the carrier transport speed to decrease. The greater the accumulation of carriers, the greater the shielding effect and the greater the rate of decrease in the transport speed. Ultimately, the decrease in the carrier transport speed leads to a deterioration in the conversion rate of the optical input to the electrical signal, that is, the response characteristics of the photodetector.

[0015] Since the degree of deterioration of the response speed of the photodetector depends on the amount of photo carriers, the higher the intensity of the incident light, the more the response speed deteriorates. In an optical receiver for coherent transmission, local light emission is introduced into the receiving front-end circuit simultaneously with the signal light to perform heterodyne or homodyne detection. There is a tendency to increase the intensity of the local light emission in order to increase the reception sensitivity, and the bandwidth deterioration when the light intensity is high becomes a particular problem.

[0016] Figure 3 shows the evaluation results of the response characteristics of the heterojunction photodetector of Figure 1 actually fabricated. The horizontal axis is the frequency (GHz), the vertical axis is the relative response intensity (dB), and the response characteristics are evaluated at a plurality of bias voltages. The frequency at which the response intensity decreases by 3 dB from the response intensity to a low-frequency (near 0 GHz) signal is called the response bandwidth of the photodetector. In the figure, the 3 dB down line is shown as a horizontal broken line with the horizontal axis. (a) of Figure 3 is the evaluation result when the optical input intensity is weak, and (b) of Figure 3 is the evaluation result when the optical input intensity is strong.

[0017] It can be seen that when the optical input intensity is strong, the response bandwidth tends to become narrow and the response speed decreases. It can also be seen that by increasing the absolute value of the bias voltage, the deterioration of the response characteristics is alleviated. This is because when the absolute value of the bias voltage is increased, the ratio of photo carriers having energy that can exceed the energy barrier at the Si / Ge interface by the electric field potential increases. Although increasing the applied voltage can alleviate the accumulation of photo carriers, there is a limit to the bias voltage that can be applied to the photodetector when actually mounting the photodetector in an optical integrated circuit and using it as a receiver. Realistically, it is difficult to apply a high voltage sufficient to prevent response deterioration.

[0018] As a measure to alleviate these problems, it is conceivable to use a homojunction PIN-PD. In the case of a homojunction, there is no potential barrier associated with an energy band discontinuity in the movement path of photo carriers, so local accumulation of photo carriers does not occur. Since there is almost no decrease in the electric field strength of the depletion layer, bandwidth deterioration is less likely to occur even when the optical incident intensity is high. However, there are problems peculiar to homojunctions.

[0019] Figures 4A to 4D show problems that can occur in a homojunction photodetector. A Ge-PD that absorbs light of the operating wavelength is connected to a Si-WG that is transparent to the operating wavelength. In a homojunction Ge-PD, high-concentration impurity regions of opposite polarities are provided in a direction horizontal or lateral to the substrate inside the light absorption layer. The Ge-PD has a high-concentration n-type impurity region (denoted as "n+" in the figure) and a p-type impurity region (denoted as "p+" in the figure) with an undoped Ge layer sandwiched therebetween. In this specification, these high-concentration impurity regions in the light absorption layer are referred to as "polarity regions". The n-type polarity region is connected to the n electrode (n-ELC), and the p-type polarity region is connected to the p electrode (p-ELC).

[0020] When light is incident on the polarity region, a phenomenon called free carrier absorption occurs. Ideally, it is desirable that light be absorbed only by the undoped Ge layer. Free carrier absorption is not interband absorption that generates new photocarriers, but rather light absorption that gives the energy of photons to carriers that already exist. Since free carrier absorption does not contribute to an increase in photocurrent, it becomes so-called "ineffective absorption".

[0021] The light reception sensitivity, which is one of the important performance indicators of a photodetector, is given by the ratio of the output current to the incident light intensity. One measure to increase the light reception sensitivity is to suppress ineffective absorption. In a homojunction, the relationship between the light intensity distribution and the polarity region becomes a problem in order to minimize ineffective absorption.

[0022] Immediately after light is incident from the Si-WG to the undoped light absorption region of the Ge-PD, as shown in Figure 4A, light B1 is distributed between the n-type polarity region and the p-type polarity region. However, as shown in Figure 4B, the light B2 that has propagated a certain distance spreads over the entire Ge layer and enters the polarity region. When the polarity region is formed deeper to increase the electric field strength as shown in Figure 4C, in particular, the overlap between the light B3 and the polarity region becomes large, and ineffective absorption in the polarity region is likely to occur. As a result, the light reception sensitivity decreases.

[0023] To avoid the ineffective absorption that occurs in FIGS. 4B and 4C, as shown in FIG. 4D, it is conceivable to keep the polar region in the shallow region of the light absorption layer. Although the ineffective absorption is suppressed in the configuration of FIG. 4, the intensity distribution of light B4 moves away from the polar region. The electric field strength (schematically indicated by the dashed arrow in the figure) generated by the potential difference between the polar regions of opposite polarities becomes weak in the region where light B4 propagates, and the response speed decreases. Since the decrease in the electric field strength causes a decrease in the carrier transport speed, it is difficult to achieve both high-speed operation and high light reception sensitivity in the homojunction structure shown in FIGS. 4A to 4D.

[0024] <First Embodiment> FIG. 5 is a schematic diagram of the photodetector 10 according to the first embodiment. FIG. 5(a) is a top view, and FIG. 5(b) is a cross-sectional view taken along the line X-X'. As the coordinate axes, the stacking direction or the film thickness direction is the Z direction, and the plane in which the X and Y directions are orthogonal to the Z direction is the X-Y plane. In the embodiment, a configuration of a photodetector is provided in which ineffective absorption in the polar region is suppressed while maintaining efficient electric field application.

[0025] The photodetector 10 is provided on the dielectric layer 101 on the substrate 100 and has a light input layer 102 and a light absorption layer 104. The light input layer 102 is formed as an extension of the optical waveguide 120 and guides the propagating light to the light absorption layer 104. For light confinement, the entire optical waveguide 120 and the photodetector 10 are surrounded by the dielectric layers 101 and 116. When the dielectric layers 101 and 116 are formed of the same material, they finally become an integral layer, but from the viewpoint of the layer structure with respect to the light input layer 102 and the light absorption layer 104, the dielectric layer covering the upper surface of the light absorption layer 104 is referred to as the "dielectric layer 116".

[0026] Assuming light in the wavelength range of 1.2 μm to 1.6 μm (near-infrared region) used for optical communication, the optical waveguide 120 and the light input layer 102 are formed of a first semiconductor that is transparent to light of the above wavelength. The light absorption layer 104 is formed of a second semiconductor that absorbs light of the above wavelength. The first semiconductor is, for example, Si having a bandgap that does not absorb light of the above wavelength. The second semiconductor has a narrower bandgap than Si and is Ge, SiGe, GeSn, etc. that absorb light of the above wavelength.

[0027] Ge and Sn are Group IV materials like Si, and have the advantage of being less affected by contamination in the manufacturing process compared to the case of using III-V compound semiconductors. However, the material of the light absorption layer 104 is not limited to these materials. Other semiconductor materials with appropriately designed band gaps may be used as long as they can absorb light with wavelengths in the range of 1.2 μm to 1.6 μm.

[0028] The Si optical waveguide 120 and the light input layer 102 can be fabricated as part of an optical integrated circuit using silicon photonics technology. The dielectric layers 101 and 116 are layers of a material with a large refractive index difference with respect to the first and second semiconductors, for example, silicon dioxide (SiO2). When using an SOI substrate, the substrate 100 may be a Si substrate, and the dielectric layer 101 may be the BOX (Buried Oxide) layer of the SOI substrate. In this case, the Si layer (so-called SOI layer) on the BOX layer is patterned to form the optical waveguide 120 and the light input layer 102.

[0029] The optical waveguide 120 may include an optical waveguide 121 with a constant width and a tapered waveguide 122 whose width continuously increases toward the light input layer 102. Light incident on the light input layer 102 from the optical waveguide 120 is coupled to the light absorption layer 104 by evanescent optical coupling. Since Ge has a higher refractive index than Si, the light guided by the Si optical waveguide 120 is coupled into Ge by its evanescent field and absorbed within Ge.

[0030] The light absorption layer 104 has a first conductivity type polarity region 105 and a second conductivity type polarity region 106 inside. The polarity region 105 is doped, for example, with a high concentration of n-type impurities, and the polarity region 106 is doped with a high concentration of p-type impurities. Between the polarity regions 105 and 106 with opposite polarities is the undoped light absorption layer 104. This forms a lateral PIN structure. Assuming the light propagation direction is the Y direction, the PIN structure is arranged in a direction horizontal to the substrate 100 in the X-Z plane orthogonal to the Y direction. Since the PIN structure is inside the Ge light absorption layer 104 and has no heterointerface in the current path, it is a homojunction type PIN structure.

[0031] In an embodiment, the ineffective absorption in the polar regions 105 and 106 is minimized, and the photocurrent is efficiently extracted. To achieve this, the light incident on the light absorption layer 104 is efficiently confined between the polar regions 105 and 106, and the spread of light to the polar regions 105 and 106 is suppressed. Specifically, in the region between the polar regions 105 and 106, the film thickness of at least one of the light input layer 102, the light absorption layer 104, and the dielectric layer 116 is partially changed.

[0032] In the first embodiment, a recess 103 is formed in the light input layer 102 to partially increase the film thickness of the light absorption layer 104. When the upper surface of the light absorption layer 104 is flat, the film thickness of the light absorption layer 104 in the region corresponding to the recess 103 increases by the amount by which the film thickness of the light input layer 102 becomes thinner in the recess 103. Light is confined in the light absorption layer 104 that is partially thickened between the polar regions 105 and 106, and ineffective absorption in the polar regions 105 and 106 is suppressed. The light confined between the polar regions 105 and 106 receives a sufficient electric field strength, and the photocarriers generated by light absorption are efficiently extracted. Thereby, both the light reception sensitivity and the response speed can be achieved.

[0033] FIGS. 6A and 6B are diagrams showing the effects of the configuration of FIG. 5. When the upper and lower sides of the light absorption layer 104 with an increased film thickness in the recess 103 are reversed, the light absorption layer 104 becomes a so-called rib waveguide, and light is confined in the rib portion with a large film thickness.

[0034] In FIG. 6A, in the state immediately after light is coupled from the light input layer 102 to the light absorption layer 104, light 130A exists in the space where the light absorption layer 104 has become thick due to the recess 103. Here, consider the effective refractive indices of the three layers of the light input layer 102, the light absorption layer 104, and the dielectric layer 116. The effective refractive index is proportional to the space ratio (space occupancy ratio) occupied by the medium.

[0035] In a direction perpendicular to the substrate 100, since the refractive index of the Ge optical absorption layer 104 is greater than that of the Si optical input layer 102 and also greater than that of the SiO2 dielectric layer 116, the light 130A is confined in the optical absorption layer 104. In the horizontal direction, the effective refractive index of the portion where the film thickness of the optical absorption layer 104 is thick is higher than that of the regions on both sides of the recess 103 where the film thickness is relatively small, and the light 130A is confined in the region with a large film thickness. The light 130A is less likely to spread to the polar regions 105 and 106 formed in the regions with a low effective refractive index.

[0036] In order to efficiently confine the light 130A, it is desirable that the change in the effective refractive index due to the film thickness change is sufficiently large. For example, it is desirable to change the film thickness at a rate of 1 / 10 or more of the wavelength of the light 130A. When the wavelength of the light 130A is 1500 nm, the change in the film thickness of the optical absorption layer 104 increasing within the recess 103 is desirably 150 nm or more. This is because if the film thickness change is less than 1 / 10 of the wavelength of the light 130A, the difference in the effective refractive index becomes insufficient, and there is a risk that the lateral confinement of light becomes insufficient.

[0037] With the above configuration, even after the light 130B propagates a certain distance within the optical absorption layer 104 as shown in FIG. 6B, the spread of the light 130B over the entire optical absorption layer 104 is suppressed. The light 130B has a dominant intensity distribution in the central portion of the optical absorption layer 104, that is, in the region between the polar regions 105 and 106, in a plane orthogonal to the direction of light propagation. Since the polar regions 105 and 106 can be separated from the distribution of the light 130B, the useless absorption due to free carrier absorption can be suppressed, and the light receiving sensitivity can be kept high.

[0038] Returning to FIG. 5, in order to extract the photocarriers generated in the optical absorption layer 104 as a photocurrent, an electric field is applied from the electrodes 111 and 112 to the optical absorption layer 104. The electrode 111 is electrically connected to the n-type polar region 105 through the plug electrode 107 formed in the dielectric layer 116. The electrode 112 is connected to the p-type polar region 106 through the plug electrode 108 formed in the dielectric layer 116.

[0039] By applying a positive bias voltage to electrode 111 and a negative bias voltage to electrode 112, a potential difference is generated between polarity regions 105 and 106, and polarity region 105 is in a reverse bias state. Electrons generated in light absorption layer 104 are drawn out from electrode 111, and holes are drawn out from electrode 112. Since ineffective absorption in polarity regions 105 and 106 is minimized, a photocurrent corresponding to the light incident amount is extracted from electrodes 111 and 112, and high light reception sensitivity is realized. Also, an electric field of sufficient intensity is applied to the photo carriers, and the response speed is maintained high.

[0040] Since lights 130A and 130B (hereinafter sometimes collectively referred to as "light 130") are separated from polarity regions 105 and 106, polarity regions 105 and 106 can be arranged to a deep position of light absorption layer 104. Thereby, an electric field is efficiently applied to a region where the light distribution is dominant, and both an improvement in carrier transport speed and light reception sensitivity can be achieved.

[0041] From the viewpoint of applying a sufficient electric field to light absorption layer 104, it is desirable that the depths of polarity regions 105 and 106 reach a depth of 1 / 2 or more of the average film thickness of light absorption layer 104. By confining light 130 in a region with a strong electric field intensity, the photo carrier generation region coincides with the region with a strong electric field intensity. The carrier transport time is shortened, and high-speed operation is realized.

[0042] From the viewpoint of separating the distributions of polarity regions 105 and 106 and light 130, it is desirable that the width of the region where the film thickness of light absorption layer 104 is thick, that is, the width of recess 103, is narrower than the width of the undoped region between polarity regions 105 and 106. The width of recess 103 is a dimension in the (X) direction orthogonal to both the film thickness (Z) direction and the light traveling (Y) direction (see FIG. 5). By controlling the film thickness with an appropriate width to adjust the effective refractive index distribution, the spread of light 130 is suppressed, and deterioration of light reception sensitivity due to free carrier absorption in polarity regions 105 and 106 is suppressed.

[0043] <Fabrication process of photodetector> Figs. 7A to 7F are manufacturing process diagrams of the photodetector of the embodiment. In each of Figs. 7A to 7F, a top view (a) and an X-X' cross-sectional view (b) are shown in parallel. In the following description, the materials, sizes, forming methods, etc. of each component are examples and do not limit the present disclosure.

[0044] In Fig. 7A, on the dielectric layer 101 on the substrate 100, an optical input layer 102 and an optical waveguide 120 connected to the optical input layer 102 are formed of Si. When using a SOI substrate, as an example, the thickness of the Si substrate serving as the substrate 100 is 750 μm, the thickness of the SiO2 layer used as the dielectric layer 101 is 2 μm, and the thickness of the Si layer (so-called SOI layer) on the SiO2 layer is 250 nm. The optical waveguide 120 and the optical input layer 102 are formed, for example, by patterning the SOI layer by electron beam (EB) lithography and inductively coupled plasma (ICP) etching.

[0045] The optical waveguide 120 may include an optical waveguide 121 and a tapered waveguide 122. The optical waveguide 121 may be connected to other optical elements (such as a 90° hybrid optical mixer, a wavelength filter, etc.) of the optical integrated circuit. The tapered waveguide 122 continuously widens from the optical waveguide 121 toward the optical input layer 102. The width of the optical waveguide 121 is, for example, 500 nm. In the tapered waveguide 122, the width may be widened to 1 μm. The size of the optical input layer 102 in the light propagation direction (Y direction) is, for example, 35 μm, and the size in the width direction (X direction) is, for example, 10 μm.

[0046] In Fig. 7B, a resist mask 124 that exposes only a partial region of the optical input layer 102 is formed by photolithography. The width of the opening 125 of the resist mask 124 is 1 μm. From the opening 125, the Si of the optical input layer 102 is removed to a depth of 100 nm by dry etching to form a recess 103.

[0047] In FIG. 7C, the resist mask 124 is removed, and a SiO2 film 114 is formed over the entire surface with a thickness of 20 nm. Although the SiO2 film is transparent, for ease of understanding the manufacturing process, in the top view (a), the optical input layer 102 and the optical waveguide 120 covered by the SiO2 film 114 are shown by dotted lines. A resist mask is formed on the SiO2 film 114 by photolithography, and the SiO2 film 114 is patterned by dry etching using the resist mask to form a mask for Ge growth. Due to the opening 115 formed in the mask of the SiO2 film 114, a partial region of the Si optical input layer 102 is exposed. Then, the resist mask is removed.

[0048] In FIG. 7D, in the region not covered by the SiO2 film 114 (inside the opening 115), a Ge layer is epitaxially grown by low-pressure chemical vapor deposition (LP-CVD) to form the optical absorption layer 104. The thickness of the Ge layer is 500 nm. The size of the optical absorption layer 104 is 30 μm in the optical propagation direction (Y direction) and 8 μm in the width direction (X direction).

[0049] In FIG. 7E, a resist mask is formed by photolithography to expose a part of the optical absorption layer 104, and boron (B) is ion-implanted to form an n-type polarity region 105. The impurity concentration is, for example, 1E20 [cm-3]. Then, the resist mask is removed, and again, a resist mask is formed by photolithography to expose a different region of the optical absorption layer 104. Phosphorus (P) is ion-implanted to form a p-type polarity region 106. Then, the resist mask is removed.

[0050] In FIG. 7F, a SiO2 film is formed to a thickness of 1 μm as the dielectric layer 116 over the entire surface by CVD method, and the implanted impurities are activated by annealing. For example, the impurities in the Ge layer are activated by annealing for about 10 seconds in an atmosphere of 600 °C. Thereafter, contact holes reaching the polar regions 105 and 106 are formed in the dielectric layer 116 by photolithography and dry etching. Subsequently, an aluminum (Al) film is formed over the entire surface by sputtering, and the Al film is processed by photolithography and dry etching. Thereby, the electrodes 111 and 112 connected to the polar regions 105 and 106 are formed. The Al film filling the contact holes becomes the plug electrodes 107 and 108.

[0051] In the above process, the photodetector 10A with a partially thickened light absorption layer 104 is completed. The photodetector 10A is fabricated simultaneously with other optical waveguides and optical devices of the optical integrated circuit on the SOI substrate. Here, the formation procedure of other optical devices is omitted.

[0052] <Second Embodiment> FIG. 8 is a schematic cross-sectional view of the photodetector 20 according to the second embodiment. In the second embodiment, without providing a recess in the light input layer, the film thickness is partially increased on the upper surface of the light absorption layer.

[0053] The photodetector 20 is provided on the dielectric layer 201 on the substrate 200, and includes a light absorption layer 204 and a light input layer 202 that guides light to the light absorption layer 204. The entire photodetector 20 is surrounded by the dielectric layers 101 and 216. The light input layer 202 is formed of a first semiconductor that is transparent to light of the wavelength used, and the light absorption layer 204 is formed of a second semiconductor that absorbs light of the wavelength used. As an example, the first semiconductor is Si and the second semiconductor is Ge.

[0054] The light absorption layer 204 has a first conductivity type polarity region 205 and a second conductivity type polarity region 206 inside. The polarity region 205 is doped, for example, with a high concentration of n-type impurities, and the polarity region 206 is doped with a high concentration of p-type impurities. Between the polarity regions 205 and 206 with opposite polarities to each other is the undoped light absorption layer 204. The photodetector 20 has a homojunction PIN structure that does not have a heterointerface in the current path.

[0055] The light absorption layer has a protrusion 215 between the polarity regions 205 and 206. Due to this protrusion 215, the thickness of the light absorption layer 204 becomes partially thick, and a rib-type waveguide is formed. The effective refractive index in the region where the film thickness is increased by the protrusion 215 is higher than the effective refractive index in the regions where the polarity regions 105 and 106 on both sides are formed. The light coupled from the light input layer 202 to the light absorption layer 204 is efficiently confined between the polarity regions 205 and 206. The spread of light to the polarity regions 205 and 206 is suppressed, and ineffective absorption is suppressed.

[0056] A positive bias is applied to the polarity region 205 via the electrode 211 and the plug electrode 207, and a negative bias is applied to the polarity region 206 via the electrode 212 and the plug electrode 208. The photo carriers generated in the light absorption layer 204 receive a sufficient electric field strength between the polarity regions 205 and 206 and are drawn out to the corresponding electrodes 211 and 212.

[0057] The protrusion 215 of the light absorption layer 204 is formed by dry etching the epitaxially grown Ge layer in FIG. 7D without forming the recess 103 of the first embodiment. As an example, by etching the regions on both sides along the Y direction of the epitaxially grown Ge layer by 100 nm, a protrusion 215 with a height of 100 nm can be formed on the upper surface of the Ge layer. Even in the configuration of the second embodiment, it is possible to achieve both light reception sensitivity and response speed.

[0058] <Third Embodiment> FIG. 9 is a cross-sectional schematic view of the photodetector 30 of the third embodiment. In the third embodiment, the film thickness of the dielectric layer covering the photodetector is partially changed.

[0059] The photodetector 30 is provided on the dielectric layer 301 on the substrate 300, and has a light absorption layer 304 and a light input layer 302 that guides light to the light absorption layer 304. The entire photodetector 30 is surrounded by the dielectric layers 301 and 316. The light input layer 302 is formed of a first semiconductor that is transparent to light of the wavelength used, and the light absorption layer 304 is formed of a second semiconductor that absorbs light of the wavelength used. As an example, the first semiconductor is Si and the second semiconductor is Ge.

[0060] The light absorption layer 304 has a first conductivity type polarity region 305 and a second conductivity type polarity region 306 inside. The polarity region 305 is doped with a high concentration of, for example, n-type impurities, and the polarity region 306 is doped with a high concentration of p-type impurities. Between the polarity regions 305 and 306 with opposite polarities to each other is the undoped light absorption layer 304. The photodetector 30 has a homojunction PIN structure that does not have a heterointerface in the current path.

[0061] The dielectric layer 316 covering the light absorption layer 304 has a protrusion 315 at a portion corresponding to the undoped region between the polarity regions 305 and 306 of the light absorption layer 304. Here, consider a four-layer slab of the light input layer 302, the light absorption layer 304, the dielectric layer 316, and the air layer. The total effective refractive index of the region where the dielectric layer 316 is thicker compared to the air layer is higher than the total effective refractive index of the region where the dielectric layer 316 is thinner. Therefore, the light coupled from the light input layer 302 to the light absorption layer 304 is confined to the region with a high effective refractive index between the polarity regions 305 and 306. The spread of light to the polarity regions 305 and 306 is suppressed, and ineffective absorption is suppressed.

[0062] A positive bias is applied to the polarity region 305 via the electrode 311 and the plug electrode 307, and a negative bias is applied to the polarity region 306 via the electrode 312 and the plug electrode 308. The photocarriers generated in the light absorption layer 304 receive a sufficient electric field strength between the polarity regions 305 and 306 and are drawn out to the corresponding electrodes 311 and 312.

[0063] The protrusion 315 of the dielectric layer 316 is formed by forming the dielectric layer 316 that covers the entire surface after FIG. 7E without forming the recess 103 of the first embodiment, and then dry-etching this dielectric layer 316. For example, an SiO2 layer may be formed over the entire surface as the dielectric layer 316, and a protrusion with a height of 500 nm may be formed by dry-etching the SiO2 layer by 500 nm. Even with the configuration of the third embodiment, both the light reception sensitivity and the response speed can be achieved.

[0064] <Fourth Embodiment> FIG. 10 is a schematic cross-sectional view of the photodetector 40 of the fourth embodiment. In the fourth embodiment, by combining the first embodiment and the second embodiment, the thickness of the light absorption layer is partially increased.

[0065] The photodetector 40 is provided on the dielectric layer 401 on the substrate 400, and includes a light absorption layer 404 and a light input layer 402 that guides light to the light absorption layer 404. The entire photodetector 40 is surrounded by the dielectric layers 401 and 416. The light input layer 402 is formed of a first semiconductor that is transparent to light of the wavelength used, and the light absorption layer 404 is formed of a second semiconductor that absorbs light of the wavelength used. As an example, the first semiconductor is Si and the second semiconductor is Ge.

[0066] The light absorption layer 404 has a first conductivity type polarity region 405 and a second conductivity type polarity region 406 inside. The polarity region 405 is doped with a high concentration of, for example, n-type impurities, and the polarity region 406 is doped with a high concentration of p-type impurities. Between the polarity regions 405 and 406 with opposite polarities to each other is an undoped light absorption layer 404. The photodetector 40 has a homojunction PIN structure that does not have a heterointerface in the current path.

[0067] The optical input layer 402 has a recess 403, and the optical absorption layer 404 has a protrusion 415 in the region between the polar regions 405 and 406. In the optical absorption layer 404, since the film thickness of the undoped regions of the polar regions 405 and 406 is large, the effective refractive index of this portion is large compared to the effective refractive indices on both sides. The light coupled from the optical input layer 402 to the optical absorption layer 404 is confined in the region with a high effective refractive index between the polar regions 405 and 406. The spread of light to the polar regions 405 and 406 is suppressed, and ineffective absorption is suppressed.

[0068] A positive bias is applied to the polar region 405 via the electrode 411 and the plug electrode 407, and a negative bias is applied to the polar region 406 via the electrode 412 and the plug electrode 408. The photo carriers generated in the optical absorption layer 404 receive a sufficient electric field strength between the polar regions 405 and 406 and are drawn out to the corresponding electrodes 411 and 412. With the configuration of the fourth embodiment, both the light reception sensitivity and the response speed can be achieved.

[0069] FIG. 11 is a schematic diagram of an optical integrated device 50 for a transceiver using the photodetector of the embodiment. The optical integrated device 50 includes an optical transmitter TX and an optical receiver RX. The optical integrated device 50 is for a coherent transmission transceiver application, and the locally generated light input from the port P2 is supplied to the optical transmitter TX and the optical receiver RX.

[0070] In the optical receiver RX, the signal light received at the port P1 is separated by the polarization beam splitter PBS into two orthogonal polarization waves. One polarization wave is input to the 90° hybrid optical mixer 51a. The other polarization wave has its polarization axis rotated by 90 degrees by the polarization rotator PR and is input to the 90° hybrid optical mixer 51b.

[0071] The 90° hybrid optical mixer 51a detects the input signal light (polarization) by local light emission and outputs four lights with a phase shift of 90° each. The four lights are detected by the photodetectors 10-1 to 10-4. The 90° hybrid optical mixer 51b detects the input signal light (polarization) by local light emission and outputs four lights with a phase shift of 90° each. The four lights are detected by the photodetectors 10-5 to 10-8.

[0072] Balanced photodiodes may be formed by the photodetectors 10-1 and 10-2, and the photodetectors 10-3 and 10-4, respectively. In this case, the difference between the outputs of the photodetectors 10-1 and 10-2 is taken out as the in-phase (I) component of the first polarization, and the difference between the outputs of the photodetectors 10-3 and 10-4 is taken out as the quadrature phase (Q) component of the first polarization.

[0073] Similarly, balanced photodiodes may be formed by the photodetectors 10-5 and 10-6, and the photodetectors 10-7 and 10-8, respectively. In this case, the difference between the outputs of the photodetectors 10-5 and 10-6 is taken out as the in-phase (I) component of the second polarization, and the difference between the outputs of the photodetectors 10-7 and 10-8 is taken out as the quadrature phase (Q) component of the second polarization.

[0074] The polarization beam splitter PBS, the polarization rotator PR, and the 90° hybrid optical mixers 51a and 51b are included in the optical circuit connected to the photodetectors 10-1 to 10-8. In each of the photodetectors 10-1 to 10-8, the ineffective absorption is suppressed, the electric field application efficiency is improved, the light reception sensitivity is high, and high-speed operation is possible. The photocurrents output from the photodetectors 10-1 to 10-8 are amplified by an electric circuit including a transimpedance amplifier and input to the subsequent digital signal processor (DSP).

[0075] The optical transmitter TX has optical modulators 53a and 53b. The optical modulators 53a and 53b are Mach-Zehnder interferometer type IQ modulators in this example. The local light emission serving as a carrier is modulated by the data signals input from the DSP to each of the optical modulators 53a and 53b. The output light of the optical modulator 53b is rotated by the polarization rotator PR and then multiplexed with the output of the optical modulator 53a by the polarization beam combiner PBC. The multiplexed light is output from the port P3 as the signal light in the polarization multiplexing method.

[0076] The optical integrated device 50 is not limited to the polarization multiplexing type transceiver applications, and may be used in QPSK method or multi-value QAM method optical transceiver applications. The plurality of optical detectors 10 may be arranged not only as a one-dimensional array but also as a two-dimensional array. In any case, due to the high light reception sensitivity and operating speed, high-speed and high-sensitivity optical detection operations are possible.

[0077] Needless to say, the same effect can be obtained by using the optical detectors 20, 30, or 40 instead of the optical detector 10. Also, when using an SiGe optical detector or a GeSn optical detector instead of the Ge optical detector, the same optical confinement structure can suppress the ineffective absorption and enable efficient electric field application.

Explanation of Signs

[0078] 10, 10A, 20, 30, 40 Optical detectors 50 Optical integrated device 100, 200, 300, 400 Substrates 101, 116, 201, 216, 301, 316, 401, 416 Dielectric layers 102, 202, 302, 402 Optical input layers 103, 403 Recesses 104, 204, 304, 404 Optical absorption layers 105, 106, 205, 206, 305, 306, 405, 406 Polar regions 120 Optical waveguide 130, 130A, 130B Light 215, 315, 415 Protrusions

Claims

1. An optical input layer formed of a first semiconductor on a substrate and transparent to light of a use wavelength, An optical absorption layer formed of a second semiconductor having a smaller bandgap than the first semiconductor on the optical input layer, having, The optical absorption layer has, in a direction horizontal to the substrate, a first polarity region doped with impurities of a first conductivity type, a second polarity region doped with impurities of a second conductivity type different from the first conductivity type, and an undoped region sandwiched between the first polarity region and the second polarity region, and has a region having a higher effective refractive index than other portions of the optical absorption layer between the first polarity region and the second polarity region. The optical absorption layer has, between the first polarity region and the second polarity region, a region with a relatively large film thickness, the width of the region with a relatively large film thickness is narrower than the width of the undoped region, and the difference between the thickness of the region with a relatively large film thickness and the maximum thickness of the first polarity region and the second polarity region is 1 / 10 or more of the use wavelength, an optical detector.

2. The optical input layer has a recess at a position corresponding to the region with a relatively large film thickness of the optical absorption layer, the optical detector according to claim 1.

3. The optical absorption layer has a protrusion in the region with a relatively large film thickness, the optical detector according to claim 1 or 2.

4. An optical input layer formed of a first semiconductor on a substrate and transparent to light of a use wavelength, An optical absorption layer formed of a second semiconductor having a smaller bandgap than the first semiconductor on the optical input layer, having, The optical absorption layer has, in a direction horizontal to the substrate, a first polarity region doped with impurities of a first conductivity type, a second polarity region doped with impurities of a second conductivity type different from the first conductivity type, and an undoped region sandwiched between the first polarity region and the second polarity region, and has a region having a higher effective refractive index than other portions of the optical absorption layer between the first polarity region and the second polarity region. The light absorption layer has a region with a relatively large film thickness partially between the first polarity region and the second polarity region. The light input layer is a photodetector having a recess at a position corresponding to the region with a relatively large film thickness of the light absorption layer.

5. A photodetector formed of a first semiconductor on a substrate, having a light input layer transparent to light of a wavelength of use, a light absorption layer formed of a second semiconductor having a smaller bandgap than the first semiconductor, on the light input layer, a dielectric layer covering the light absorption layer, and having The light absorption layer has, in a direction horizontal to the substrate, a first polarity region doped with impurities of a first conductivity type, a second polarity region doped with impurities of a second conductivity type different from the first conductivity type, and an undoped region sandwiched between the first polarity region and the second polarity region. Between the first polarity region and the second polarity region, there is a region having a higher effective refractive index than other portions of the light absorption layer. The dielectric layer is a region covering between the first polarity region and the second polarity region of the light absorption layer, and partially has a larger film thickness. The width of the region with the larger film thickness is narrower than the width of the undoped region. Photodetector.

6. The first polarity region and the second polarity region reach a depth of at least 1 / 2 of the average thickness of the light absorption layer from the surface of the light absorption layer. The photodetector according to any one of claims 1 to 5.

7. The first semiconductor is Si, and the second semiconductor is Ge, SiGe, or GeSn. The photodetector according to any one of claims 1 to 6.

8. A first electrode electrically connected to the first polarity region and a second electrode electrically connected to the second polarity region, and a reverse bias is applied between the first polarity region and the second polarity region. The photodetector according to any one of claims 1 to 7.

9. A photodetector according to any one of claims 1 to 8, an optical circuit optically connected to the photodetector, and an optical integrated device having the same.

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