Photodiode and method of forming the same

Abstract

Various embodiments may relate to a photodiode. The photodiode may include a first germanium-containing region. The photodiode may include also a second germanium- containing region. The photodiode may further include a barrier between the first germanium- containing region and the second germanium-containing region. The photodiode may additionally include a first doped region in contact with the first germanium-containing region, the first doped region being of a first electrical conductivity type. The photodiode may also include a second doped region in contact with the second germanium-containing region, the second doped region being of a second electrical conductivity type different from the first electrical conductivity type.

Description

PHOTODIODE AND METHOD OF FORMING THE SAMECROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore application No. 10202302199W filed August 2, 2023, the contents of it being hereby incorporated by reference in its entirety for all purposes.TECHNICAL FIELD

[0002] Various embodiments of this disclosure may relate to a photodiode. Various embodiments of this disclosure may relate to a method of forming a photodiode.BACKGROUND

[0003] Imaging and detection involving short-wave infrared (SWIR), ranging from wavelengths of 1 pm to 3 pm, can enable unique and important applications in defense, industry and research as a complement to conventional silicon (Si)-based technologies, which operate with visible and near-infrared (NIR) wavelengths (0.4 pm - 1 pm). SWIR wavelengths can benefit high-resolution night vision and imaging in adverse weather conditions for eye-safe uses, such as automotives. The longer wavelengths of SWIR light compared to visible and NIR light can also facilitate deeper imaging depth sensitivity in industrial vision. SWIR imaging has represented a market of USS 429 million in 2021 , and is expected to increase significantly at a compound annual growth rate (CAGR) of 45.4% to ~US$ 4 billion by year 2027. Expected key drivers for the market growth include industrial and 3-dimensional (3-D) sensing cameras, which have expanding potential applications, such as facial recognition and item recognition in smartphones.

[0004] Most of current SWIR imagers rely on indium gallium arsenide (TnGaAs) material. A conventional single InGaAs-based detector device may exhibit high quantum efficiency (-80% - 90%) and low dark current (-2 nA / cm2) at room temperature for wavelengths from 0.9 pm to 1.7 pm. However, this material is incompatible with current complementary metal -oxide- semiconductor (CMOS) foundry manufacturing, which limits its scalability to larger-diameter wafers, and thus low-cost use. Colloidal quantum dot (CQD) technologies have been explored recently as a low-cost alternative for InGaAs-based imagers. However, although simple and scalable to high-volume manufacturing, the relevant processes pertaining to CQD technologies have yet to be able to be fully adopted by foundries for CMOS integration.

[0005] Germanium (Ge) is a renowned CMOS-compatible material and exhibits decent absorption for photons up to wavelengths of -1 .6 pm. This may open the possibility for large- volume manufacturing and low-cost applications of SWIR imagers. However, Ge photodiodes (PDs) typically exhibit 5 orders of magnitude higher dark current noise, compared to InGaAs and CQD, hindering their use for imagers in sensitive low-light environment.SUMMARY

[0006] Various embodiments may relate to a photodiode. The photodiode may include a first gennanium-containing region. The photodiode may include also a second gennanium- containing region. The photodiode may further include a barrier between the first germanium- containing region and the second germanium-containing region. The photodiode may additionally include a first doped region in contact with the first germanium-containing region, the first doped region being of a first electrical conductivity type. The photodiode may also include a second doped region in contact with the second germanium-containing region, the second doped region being of a second electrical conductivity type different from the first electrical conductivity type.

[0007] Various embodiments may relate to a method of forming a photodiode The method may include forming a first germanium-containing region. The method may also include forming a second germanium-containing region. The method may further include forming a barrier between the first germanium-containing region and the second germanium-containing region. The method may additionally include forming a first doped region in contact with the first germanium-containing region, the first doped region being of a first electrical conductivity type. The method may also include forming a second doped region in contact with the second germanium-containing region, the second doped region being of a second electrical conductivity type different from the first electrical conductivity type.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.FIG. 1 is a general illustration of a photodiode according to various embodiments.FIG. 2 is a general illustration of a method of forming a photodiode according to various embodiments.FIG. 3A shows a cross-sectional side schematic of a photodiode according to various embodimentsFIG. 3B shows a cross-sectional side schematic of another photodiode according to various embodiments.FIG. 4A shows a cross-sectional side schematic of a photodiode according to various embodiments.FTG. 4B shows a top planar schematic of the photodiode shown in FIG. 4A according to various embodiments.FIG. 4C shows a cross-sectional side schematic of another photodiode according to various embodiments.FIG. 4D shows a top planar schematic of the photodiode shown in FIG. 4C according to various embodimentsFIG. 5A shows the simulated electric field in the photodiode according to various embodiments under reverse bias (-1 V bias).FIG. 5B shows an enlarged view of the electric field at the highlighted box of the photodiode according to various embodiments in FIG. 5A.FIG. 5C shows the band energy diagram of an amorphous silicon carbide (a-SiC) barrier with adjoining germanium containing regions of the photodiode according to various embodiments under a reverse bias voltage of -1 V.FIG. 5D shows the band energy diagram of the amorphous silicon carbide (a-SiC) barrier with adjoining germanium containing regions of the photodiode according to various embodiments under a reverse bias voltage of -3 V.FIG. 6A shows a plot of current (in amperes or A) as a function of voltage (V) illustrating the simulated dark current performance of the photodiode according to various embodimentsFIG. 6B shows a plot of current (in amperes or A) as a function of voltage (V) illustrating the simulated effect of background (BG) doping concentration of the Ge-containing regions of the photodiode according to various embodiments on the dark currentFIG. 6C shows a plot of current (in amperes or A) as a function of voltage (V) illustrating the simulated effect of threading dislocation density (TDD) of the Ge-containing regions of the photodiode according to various embodiments on the dark current.FTG. 6D shows a plot of current density (in amperes per square centimeters or A / cm2) as a function of voltage (in volts or V) comparing the dark current densities of a conventional germanium (Ge)) photodiode (PD), a photodiode without an amorphous silicon carbide (SiC) and a photodiode with an amorphous SiC barrier according to various embodiments.FIG. 7A shows a plot of external quantum efficiency EQE (in percent or %) as a function of wavelength (in micrometers or pm) illustrating the simulated external quantum efficiency (EQE) of the photodiode according to various embodiments with wavelength at -3 V.FIG. 7B shows a plot of external quantum efficiency EQE (in percent or %) as a function of voltage (in volts or V) illustrating the simulated external quantum efficiency (EQE) of the photodiode according to various embodiments with reverse bias voltage at wavelengths of 900 nm, 1 ,350 nm and 1 ,550 nm.FIG. 8 shows a table illustrating the performance of a photodiode according to various embodiments.DESCRIPTION

[0009] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments

[0010] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and / orcombinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0011] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0012] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance, e g. within 10% of the specified value.

[0013] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0014] By “comprising” it is meant including, but not limited to, whatever follows the word “comprising” Thus, use of the term “comprising” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[0015] By “consisting of’ it is meant including, and limited to, whatever follows the phrase “consisting of’. Thus, the phrase “consisting of’ indicates that the listed elements are required or mandatory, and that no other elements may be present.

[0016] Embodiments described in the context of one of the photodiodes are analogously valid for the other photodiodes, embodiments described in the context of a method are analogously valid for a photodiode, and vice versa.

[0017] Various embodiments may address issues faced by conventional imagers and / or photodiodes. Various embodiments may have a dark current noise lower than conventional Ge photodiodes Various embodiments may relate to a photodiode in which the dark current density is reduced by about 4 orders of magnitude, reaching about 1 uA / cm2. Various embodiments may have a reasonably high external quantum efficiency (EQE), e g., 6% - 10% under wavelengths of 900 nm - 1,550 nm. Various embodiments may pave the way towards a high-performance, low-noise CMOS-compatible SWIR imager for diversified sensing and imaging applications.

[0018] FIG. 1 is a general illustration of a photodiode according to various embodiments. The photodiode may include a first germanium-containing region 102. The photodiode may include also a second germanium-containing region 104. The photodiode may further include a barrier 106 between the first germanium-containing region 102 and the second germanium- containing region 104. The photodiode may additionally include a first doped region 108 in contact with the first germanium-containing region 102, the first doped region 108 being of a first electrical conductivity type. The photodiode may also include a second doped region 110 in contact with the second germanium-containing region 104, the second doped region 110 being of a second electrical conductivity type different from the first electrical conductivity type.

[0019] In other words, various embodiments may relate to a photodiode including a barrier 106 separating germanium-containing regions 102, 104. The photodiode may further include a first doped region 108 in contact with the germanium-containing region 102, as well as a second doped region 1 10 in contact with the germanium-containing region 104. The first doped region 108 and the second doped region 110 may be oppositely doped. The barrier 106 may be spaced from the first doped region 108 by the germanium-containing region 102, and may be spaced from the second doped region 110 by the germanium-containing region 104.

[0020] For avoidance of doubt, FIG. 1 seeks to illustrate some features of a photodiode according to various embodiments, and is not intended to limit, for instance, the shape, size, orientation, dimensions etc. of these features. For instance, while FIG. 1 illustrates the regions or layers 102, 104, 106, 108, 110 to be rectangular, various embodiments may include regions or layers of any suitable shape. Also, as further elaborated below, in various embodiments, the barrier 106 together with the germanium-containing regions 102, 104 may extend horizontally,while in various other embodiments, the barrier 106 together with the germanium-containing regions 102, 104 may extend vertically.

[0021] In various embodiments, the first germanium-containing region 102 may have an atomic ratio of silicon (Si) to germanium Ge) selected from a range from 0 to 0.6. The first germanium-containing region 102 may have or include SixGei-x, wherein x may be of any suitable value selected from a range from 0 to 0.6. Similarly, in various embodiments, the second germanium-containing region 104 may have an atomic ratio of silicon (Si) to germanium Ge) selected from a range from 0 to 0.6. The second germanium-containing region 104 may have or include SixGei-x, wherein x may be of any suitable value selected from a range from 0 to 0.6. In other words, the first germanium-containing region 102 and / or the second germanium-containing region 104 may include or consist of silicon-germanium (SiGe) or germanium (Ge, i.e., when x = 0 for SixGei-x).

[0022] In various embodiments, the atomic ratio of Si to Ge of the first germanium- containing region 102 and the atomic ratio of Si to Ge of the second germanium-containing region 104 may be substantially equal or same. In various other embodiments, the atomic ratio of Si to Ge of the first germanium-containing region 102 and the atomic ratio of Si to Ge of the second germanium-containing region 104 may be different.

[0023] In various embodiments, the first doped region 102 and the second doped region 104 adjoining the barrier 106 may be of the same electrical conductivity type (i.e., both regions 102, 104 may be n-type or p-type). In various embodiments, the barrier 106 may not be doped.

[0024] In various embodiments, the first electrical conductivity type (of the first doped region 108) may refer to being p-type, while the second electrical conductivity type (of the second doped region 110) may refer to being n-type. In various other embodiments, the first electrical conductivity type (of the first doped region 108) may refer to being n-type, while the second electrical conductivity type (of the second doped region 110) may refer to being p-type.

[0025] Generally speaking and in the current context in relation to regions 102, 104, 108 and 110, a n-type region may include any suitable dopant, e.g., a Group V element such as phosphorous (P) or arsenic (As) or a Group VI element such as sulfur (S) or selenium (Se). A p-type region may include any suitable dopant, e.g., a Group III element such as boron (B) or aluminum (A).

[0026] In various embodiments, the first germanium-containing region 102 and / or the second germanium-containing region 104 may have a doping concentration selected from a range from 1016cm'3to 1018cm'3. Generally speaking, a higher background doping concentration may result in a lower minority carrier concentration for a lower dark current.

[0027] The first doped region 108 and / or the second doped region 110 may have a doping concentration equal to or greater than 1019cm'3.

[0028] In various embodiments, the first germanium-containing region 102 and / or the second germanium-containing region 104 may have a threading dislocation density (TDD) value of less than 5>< 1016cm'2, e.g, less than or equal to 108cm'2, e.g., less than or equal to 106cm'2. A lower TDD value may result in a lower dark current.

[0029] In various embodiments, the barrier 106 (alternatively referred to as “barrier layer”) may include a semiconductor material or a dielectric material. For instance, the barrier 106 may include a material selected from a group consisting of silicon carbide (SiC), titanium oxide (TiCh), zinc oxide (ZnO) and gallium oxide (GazCh). The barrier 106 may serve to reduce the dark current, without compromising the photocurrent generation and collection. The barrier 106 may be included in an intrinsic region between the first doped region 108 and the second doped region 110.

[0030] The material included in the barrier 106 may be selected to have reduced or minimal offset with either the conduction band or the valence band of the germanium-containing regions 102, 104 for maintaining the photocurrent and quantum efficiency. In various embodiments, abandgap of the barrier 106 may be wider than a bandgap of the first germanium-containing region 102 and / or a bandgap of the second germanium-containing region 104.

[0031] In various embodiments, a valence band of the barrier 106 may have a lower energy level than a valence band of the first germanium-containing region 102 and / or the second germanium-containing region 104. A conduction band of the barrier 106 may have an equivalent or lower energy level than a conduction band of the first germanium-containing region 102 and / or the second germanium-containing region 104.

[0032] In various other embodiments, a conduction band of the barrier 106 may have a higher energy level than a conduction band of the first germanium-containing region 102 and / or the second germanium-containing region 104. A valence band of the barrier 106 may have an equivalent or higher energy level than a valence band of the first germanium-containing region 102 and / or the second germanium-containing region 104.

[0033] The barrier 106 may be of any suitable thickness. In various embodiments, the barrier 106 may have a thickness selected from a range of values from 1 nm to 50 nm e.g., 1 nm to 10 nm.

[0034] In various embodiments, the photodiode may include a substrate such that the first germanium-containing region 102, the second germanium-containing region 104 and the barrier 106 are over the substrate. The substrate may be a semiconductor substrate such as a silicon substrate or a silicon-on-insulator (SOI) substrate. In various embodiments, the first doped region 108 or the second doped region 110 may be the substrate or may be part of the substrate.

[0035] In various embodiments, the barrier 106 may extend in parallel to a surface of the substrate. The barrier 106 may be included in the intrinsic region of a vertically stacked p-i-n junction of the photodiode. The vertically stacked p-i-n junction may extend parallel to the surface of the substrate.

[0036] Tn various other embodiments, the barrier 106 may extend in a direction substantially perpendicular to a surface of the substrate. The barrier 106 may be included in the intrinsic region of a lateral p-i-n junction of the photodiode. The lateral p-i-n junction may extend in a direction substantially perpendicular to the surface of the substrate.

[0037] In various embodiments, the first germanium-containing region 102 (and / or the second germanium-containing region 104) may have one or more portions extending into the substrate, e.g., extending into or through the silicon oxide (SiOz) layer of the substrate. The one or more portions into the SiOz layer (alternatively referred to SiOz windows) may help trap threading dislocations, thereby providing a high quality overlying portion with low TDD adjoining the barrier 106.

[0038] In various embodiments, the photodiode may include a first electrode in contact with the first doped region. Likewise, the photodiode may include a second electrode in contact with the second doped region.

[0039] In various embodiments, the photodiode may have a dark current density less than 10'1mA / cm2, e.g., less than 10'2mA / cm2, e.g., less than or equal to 10'3niA / cm2during operation. The photodiode may be configured to detect, sense, measure or determine shortwave infrared (SWIR) light. The short-wave infrared (SWIR) light may have a wavelength selected from a range from 1 pm to 3 pm.

[0040] FIG. 2 is a general illustration of a method of forming a photodiode according to various embodiments. The method may include, in 202, forming a first germanium-containing region The method may also include, in 204, forming a second germanium-containing region. The method may further include, in 206, forming a barrier between the first germanium- containing region and the second germanium-containing region. The method may additionally include, in 208, forming a first doped region in contact with the first germanium-containing region, the first doped region being of a first electrical conductivity type. The method may alsoinclude, in 210, forming a second doped region in contact with the second germanium- containing region, the second doped region being of a second electrical conductivity type different from the first electrical conductivity type.

[0041] For avoidance of doubt, FIG. 2 is not intended to limit the sequence of the various steps. For instance, step 202 may occur before, after or at the same time as step 204.

[0042] Tn various embodiments, the first germanium-containing region and / or the second germanium-containing region may be formed by depositing silicon-germanium (SiGe) or germanium (Ge) over a substrate using a chemical vapor deposition (CVD) technique, such as ultra-high vacuum chemical vapor deposition (UHV-CVD) or remote plasma chemical vapour deposition (RPCVD) etc. The CVD technique may be carried out such that the first germanium- containing region and / or the second germanium-containing region may have a threading dislocation density (TDD) value of less than 5><1015cm'2, e.g, less than or equal to 108cm'2, e.g., less than or equal to 106cm'2.

[0043] In various embodiments where the barrier extends in parallel to a surface of a substrate, the photodiode may be formed by wafer bonding and layer transfer. In various other embodiments where the barrier extends in a direction substantially perpendicular to a surface of a substrate, the photodiode may be formed by selective epitaxy.

[0044] Various embodiments may adopt the following approaches to achieve reduction of dark current density by 4 orders of magnitude: (1) a 60 atomic (at.) % silicon (Si) composition (but can be in the range from 0 at. % to 60 at. %) into germanium (Ge) to form SixGei-x(where x is selected from a range from 0 to 0.6) alloy; (2) incorporating a higher doping concentration (can be p-type or n-type) at a level of 1015- 1018cm'J, (3) the epitaxial growth of SixGei-xon or over Si or silicon-on-insulator (SOI) using chemical vapor deposition (CVD) techniques (e.g., UHV-CVD, RPCVD, etc ); and (4) insertion of a thin barrier (e g., silicon carbide (SiC), titanium dioxide (DO2), zinc oxide (ZnO), gallium oxide (Ga2O3)) into the device structure asa current blocking layer to reduce the dark current, without compromising the photocurrent generation and collection.

[0045] Regarding (1), incorporating silicon may help reduce the intrinsic carrier concentration for dark current reduction. In relation to (2), the higher background doping may benefit a lower minority carrier concentration for a lower dark current. On (3), using epitaxial growth of SixGei.x on or over Si or SOI using a suitable CVD technique may control the threading dislocation density (TDD) of the grown material to be less than 5x 105cm'2. A lower TDD may reduce the number of carrier generation centers for a lower dark current. Further, in relation to (4), the barrier may have a wider electronic bandgap than that of SixGei-x, thereby hindering the carrier flow in the device due to the induced electronic band offsets. In order to maintain the photocurrent and quantum efficiency, the material of the barrier may be selected to have a minimal band offset with either the conduction band or valence band of SixGei-x. The photo-generated carriers may thus not be blocked, and may flow through the barrier via the band with the minimal offset.

[0046] FIG. 3 A shows a cross-sectional side schematic of a photodiode according to various embodiments FIG. 3B shows a cross-sectional side schematic of another photodiode according to various embodiments. As illustrated in both FIGS. 3A-B, each photodiode may include a first germanium-containing region 302, a second germanium-containing region 304 and a barrier 306 between the first germanium-containing region 302 and the second germanium- containing region 304. The first germanium-containing region 302 of the photodiode in FIG. 3A is on a silicon (Si) substrate 308, which may be heavily doped and may be referred to as a first doped region (e.g., a n+Si region) of the photodiode.

[0047] The first germanium-containing region 302 of the photodiode in FIG. 3B may be on a silicon layer 308a of a SOI substrate (which also includes a further silicon layer 308b, and a silicon oxide layer 308c between the silicon layer 308a and the silicon layer 308b). The siliconlayer 308a may be heavily doped and may be referred to as a first doped region (e g., a n+Si region) of the photodiode.

[0048] On the other hand, in both FIGS. 3A-B, each photodiode may include a second doped region 310 (e.g., a p“ Ge region) on the second germanium-containing region 304. The first doped region 308, 308a may be in contact with electrodes 312 (anodes), while the second doped region 310 may be in contact with electrodes 314 (cathodes). An encapsulation structure or layer 316 including silicon oxide (SiCh) may be formed over the substrate (308 or 308a-c), covering the barrier 306, regions 302, 304, 310 and electrodes 312, 314.

[0049] The barrier layer 306 may lie within the p-i-n junction of the photodiode in the intrinsic (i-) region, and may extend horizontally in the vertically stacked p-i-n junction.

[0050] Tn various embodiments, the width or diameter (in case of a circular mesa) IV of the SiGe regions may be of any one value selected from a range from 4 pm to 40 pm. The thickness (?) of the SixGei-x stack (including layers 302, 304, 306 and 310) may be of any one value selected from a range from 1 pm to 3 pm. The spacing (7) between the doped n+region 308, 308a and the doped p+region 310 may be of any one value selected from a range from 1 pm to 3 pm. The thickness or width (w) of the barrier 306 may be of any one value selected from a range from 1 nm to 50 nm. The doping concentration in the doped regions 308, 308a, 310 may be greater than 1019cm'3, i.e., > 1019cm'3. The doping concentration of regions 302, 304 may be about 1017cm'3. The barrier 306 may include a material such as silicon carbide (SiC), titanium oxide (TiCh), zinc oxide (ZnO) or gallium oxide (GazCh).

[0051] The photodiode may be realized by wafer bonding and layer transfer. The germanium-containing layers 302, 304, the barrier 306 and the second doped region 310 may be formed first on a carrier silicon (Si) substrate, and may be bonded onto the n+Si substrate 308 (as shown in FIG. 3A) or the SOI substrate 308a-c (as shown in FIG. 3B). The carrier silicon (Si) substrate may subsequently be removed, and chemical-mechanical polishing maybe carried out The carrier Si substrate removal followed by chemi cal -mechanical polishing may result in a low TDD in the SixGei-x regions.

[0052] FIG. 4A shows a cross-sectional side schematic of a photodiode according to various embodiments. FIG. 4B shows a top planar schematic of the photodiode shown in FIG. 4A according to various embodiments. The photodiode may include a first germanium-containing region 402, a second germanium-containing region 404 and a barrier 406 between the first germanium-containing region 402 and the second germanium-containing region 404. The photodiode may also include a first doped region 408 in contact with the first germanium- containing region 402, the first doped region 408 being of a first electrical conductivity type (e.g., n“). The photodiode may further include a second doped region 410 in contact with the second germanium-containing region 404, the second doped region 410 being of a second electrical conductivity type (e.g., p+) different from the first electrical conductivity type. The photodiode may additionally include a first electrode 412 (anode) in contact with the first doped region 408, and a second electrode 414 (cathode) in contact with the second doped region 410.

[0053] The regions 402, 404, 408, 410, the barrier 406 and the electrodes 412, 414 may be formed over a substrate which may include a SOI substate (with layers 418a-d), and a silicon oxide (SiOz) layer 418e over the SOI substrate. The barrier 406 may extend vertically (perpendicular to the surface of the substrate) within the lateral p-i-n junction. The germanium- containing regions 402, 404 may include portions 402’, 404’ extending through the silicon oxide (SiOz) layer 418e as shown in FIG. 4A to contact silicon (Si) layer 418a, 418b, respectively, of the SOI substrate

[0054] In order to form the structure shown in FIGS. 4A-B, the SOI substrate may first be patterned The silicon layer 418e may be formed over the SOI substrate. The germanium- containing regions 402, 404 may then be formed by selective epitaxy from the silicon oxide (SiOz) layer 418e. The portions 402’, 404’ through the silicon oxide (SiOz) layer 418e(alternatively referred to Sit)? windows) may help trap threading dislocations, thereby providing a high quality overlying portion with low TDD adjoining the barrier 406. The barrier 406 may be formed after the germanium-containing regions 402, 404 have been formed.

[0055] FIG. 4C shows a cross-sectional side schematic of another photodiode according to various embodiments. FIG. 4D shows a top planar schematic of the photodiode shown in FIG. 4C according to various embodiments. The regions 402, 404, 408, 410, the barrier 406 and the electrodes 412, 414 in FIGS. 4C - D may be similar to that shown in FIGS. 4A -B, and are denoted with the same reference numerals. However, as shown in FIG. 4C, the regions 402, 404, 408, 410, the barrier 406 and the electrodes 412, 414 may be formed over a silicon (Si) substrate 418f, and a silicon oxide (SiOz) layer 418g over the silicon substrate 418f. The germanium-containing regions 402, 404 may include portions 402’, 404’ extending through the silicon oxide (SiOz) layer 418g as shown in FIG. 4C to contact the silicon (Si) substrate 418f. The germanium-containing regions 402, 404 may then be formed by selective epitaxy from the silicon oxide (SiOz) layer 418g. The portions 402’, 404’ through the silicon oxide (SiOz) layer 418g (alternatively referred to SiOz windows) may help trap threading dislocations, thereby providing a high quality overlying portion with low TDD adjoining the barrier 406.

[0056] In various embodiments, in relation to FIGS. 4A-D, the width or diameter (in case of a circular mesa) L ’ may be of any one value selected from a range from 4 pm to 40 pm. The thickness ( / ) of the SixGei.xstack (i.e., regions 402, 404) may be of any one value selected from a range from 1 pm to 3 pm. The spacing (Z) between the doped n“ region 408 and the doped p+region 410 may be of any one value selected from a range from 1 pm to 3 pm. The thickness or width (w) of the barrier 406 may be of any one value selected from a range from 1 nm to 50 nm, e.g., 1 nm to 10 nm. The doping concentration in the doped regions 408, 410 may be greater than 1019cm'3, i.e., > 1019cm'3. The width of the SixGei-xgrowth window (w’) may be of any one value selected from a range from 0.3 pm to 2 pm. The thickness of the growth window (t”)may be of any one value selected from a range from 1 gm to 3 gm. The doping concentration of regions 402, 404 may be about 10l zcm'3. The barrier 406 may include a material such as silicon carbide (SiC), titanium oxide (TiCh), zinc oxide (ZnO) or gallium oxide (GazCh)

[0057] Various embodiments may provide a thin barrier or barrier layer in the intrinsic region of a Ge-based p-i-n junction. The barrier may be on the path of current flow in the p-i-n junction, which may result in dark current reduction Unlike conventional approaches in which the barrier layer was inserted between the metal contacts and Ge, where a significant electric field can be induced across, various embodiments may avoid the excess electric field directly across the barrier as well as the induced noise to the photodiode. FIG. 5A shows the simulated electric field in the photodiode according to various embodiments under reverse bias (-1 V bias). FIG 5B shows an enlarged view of the electric field at the highlighted box of the photodiode according to various embodiments in FIG. 5 A. As seen in FIG. 5A, the electric field may mainly be induced across the regions where doping gradients were present, beyond the barrier where there was no induced electric field. This may avoid the electric field-induced defects and trap states to device operation.

[0058] All simulations in FIGS. 5A-B are based on an amorphous silicon carbide (SiC) barrier (w = I nm) in a lateral p-i-n junction (shown in FIGS. 4A-B). It is believed that same or similar results may be expected for the structures illustrated in FIGS. 3 A-B. It can be observed that the electric field was mainly induced at areas where doping gradients present (i.e., at N+ / P and P+ / P intersections). There was no observable electric field at the barrier in the middle of the PN junction This indicates that the current flow over the barrier can be dominated by carrier diffusion and tunnelling, instead of drift. This may suggest a low dark current less prone to electric field-related traps and defect states.

[0059] FIG. 5C shows the band energy diagram of an amorphous silicon carbide (a-SiC) barrier with adjoining germanium containing regions of the photodiode according to variousembodiments under a reverse bias voltage of -1 V. FIG. 5D shows the band energy diagram of the amorphous silicon carbide (a-SiC) barrier with adjoining germanium containing regions of the photodiode according to various embodiments under a reverse bias voltage of -3 V. FIGS. 5C-D show that by using an amorphous silicon carbide (a-SiC) barrier, the conduction band offset may be negligible for the photo carriers to pass through, while the valence band off set may block the current for dark current reduction.

[0060] Simulation on the dark current of the photodiode has also been performed. FIG. 6A shows a plot of current (in amperes or A) as a function of voltage (V) illustrating the simulated dark current performance of the photodiode according to various embodiments. FIG. 6A shows the effect of SixGei-xalloying on the dark current of a photodiode without the barrier. A 60% Si composition was selected to balance the external quantum efficiency (EQE) and dark current of the photodiode. The dark current was reduced by -2 orders of magnitude at reverse bias voltage of -1 V.

[0061] FIG. 6B shows a plot of current (in amperes or A) as a function of voltage (V) illustrating the simulated effect of background (BG) doping concentration of the Ge-containing regions of the photodiode according to various embodiments on the dark current. FIG 6B shows simulated dark currents at 3 different background (BG) doping concentrations, i.e., 1015cm'J, 1017cm'3and 1018cm'3. The dark current was reduced by an order of magnitude with the increase of BG doping from 1015to 1017cm'3. Further increasing the BG doping to 1018cm'3increased carrier tunnelling via the junction, resulting in a more prominent dark current increase with respect to bias voltage.

[0062] FIG. 6C shows a plot of current (in amperes or A) as a function of voltage (V) illustrating the simulated effect of threading dislocation density (TDD) of the Ge-containing regions of the photodiode according to various embodiments on the dark current. Decreasing the TDD from 1 x 108to 1 * 10ficm'2resulted in more than 2 orders of magnitude of reduction ofthe simulated dark current. As such, bonding and layer transfer to form the structures shown in FIGS. 3A-B and selective Ge epitaxy to form the structure shown in FIGS. 4A-B for reducing the TDD may therefore play an important role in the reduction of dark current

[0063] The aggregate effect of all the above factors on the dark current may be illustrated in FIG. 6D. FIG. 6D shows a plot of current density (in amperes per square centimeters or A / cm2) as a function of voltage (in volts or V) comparing the dark current densities of a conventional germanium (Ge)) photodiode (PD), a photodiode without an amorphous silicon carbide (SiC) and a photodiode with an amorphous SiC barrier according to various embodiments. In total, the dark current can be reduced by ~4 orders of magnitude, in which the use of a barrier (SiC) can contribute to ~2 orders of the reduction.

[0064] The external quantum efficiency (EQE) of the photodiode was also studied FIG. 7A shows a plot of external quantum efficiency EQE (in percent or %) as a function of wavelength (in micrometers or pm) illustrating the simulated external quantum efficiency (EQE) of the photodiode according to various embodiments with wavelength at -3 V. EQE decreases with increasing wavelength as the material absorption coefficient decreases. The resonating spectrum was from the Fabry-Perot resonance between the top and bottom surfaces of SiGe (SiGe thickness was set as 1 pm in simulation). A higher EQE may be achieved with a thicker SiGe or a resonant cavity structure.

[0065] FIG. 7B shows a plot of external quantum efficiency EQE (in percent or %) as a function of voltage (in volts or V) illustrating the simulated external quantum efficiency (EQE) of the photodiode according to various embodiments with reverse bias voltage at wavelengths of 900 nm, 1,350 nm and 1,550 nm. As shown in FIG. 7B, EQE increases with increasing reverse bias and saturates at — 3 V. The saturated EQE can reach ~11%, 9% and 4% at 900 nm,1,350 nm and 1,550 nm, respectively.

[0066] FIG. 8 shows a table illustrating the performance of a photodiode according to various embodiments.

[0067] Various embodiments may involve features using a subset of the 4 approaches as highlighted above, instead of all 4 approaches. Various embodiments may relate to a photodiode with any one, any two or any three of the above 4 approaches.

Claims

Claims1. A photodiode comprising: a first germanium-containing region; a second germanium-containing region; a barrier between the first germanium-containing region and the second germanium-containing region; a first doped region in contact with the first germanium-containing region, the first doped region being of a first electrical conductivity type; and a second doped region in contact with the second germanium-containing region, the second doped region being of a second electrical conductivity type different from the first electrical conductivity type.

2. The photodiode according to claim 1, wherein the first germanium-containing region has an atomic ratio of silicon to germanium selected from a range from 0 to 0.6; and wherein the second germanium-containing region has an atomic ratio of silicon to germanium selected from a range from 0 to 0.6.

3. The photodiode according to claim 1, wherein the barrier comprises a semiconductor material or a dielectric material.

4. The photodiode according to claim 1 , wherein the barrier comprises a material selected from a group consisting of silicon carbide, titanium oxide, zinc oxide and gallium oxide.

5. The photodiode according to claim 1, wherein the first germanium-containing region and the second germanium- containing region have a doping concentration selected from a range from 1015cm'3to 1018cm'3.

6. The photodiode according to claim 1, wherein the first germanium-containing region and the second germanium- containing region have a threading dislocation density (TDD) value of less than 5xl016cm'2.

7. The photodiode according to claim 1, wherein a bandgap of the barrier is wider than a bandgap of the first germanium-containing region or a bandgap of the second germanium- containing region.

8. The photodiode according to claim 1, wherein a valence band of the barrier has a lower energy level than a valence band of the first germanium-containing region or the second germanium- containing region.

9. The photodiode according to claim 8, wherein a conduction band of the barrier has an equivalent or lower energy level than a conduction band of the first germanium-containing region or the second germanium-containing region.

10. The photodiode according to claim 1, wherein a conduction band of the barrier has a higher energy level than a conduction band of the first germanium-containing region or the second germanium-containing region.1 1 . The photodiode according to claim 10, wherein a valence band of the barrier has an equivalent or higher energy level than a valence band of the first germanium-containing region or the second germanium-containing region.

12. The photodiode according to claim 1, further comprising: a substrate such that the first germanium-containing region, the second germanium-containing region and the barrier are over the substrate.

13. The photodiode according to claim 1, wherein the barrier extends in parallel to a surface of a substrate.

14. The photodiode according to claim 1, wherein the barrier extends in a direction substantially perpendicular to a surface of a substrate15. A method of forming a photodiode, the photodiode comprising: forming a first germanium-containing region;forming a second germanium-containing region; and forming a barrier between the first germanium-containing region and the second germanium-containing region; forming a first doped region in contact with the first germanium-containing region, the first doped region being of a first electrical conductivity type; and forming a second doped region in contact with the second germanium- containing region, the second doped region being of a second electrical conductivity type different from the first electrical conductivity type.

16. The method according to claim 15, wherein the first germanium-containing region and the second germanium- containing region are formed by depositing silicon-germanium or germanium over a substrate using a chemical vapor deposition technique.

17. The method according to claim 15, wherein the barrier extends in parallel to a surface of a substrate.

18. The method according to claim 17, wherein the photodiode is formed by wafer bonding and layer transfer.

19. The method according to claim 15, wherein the barrier extends in a direction substantially perpendicular to a surface of a substrate20. The method according to claim 19, wherein the photodiode is formed by selective epitaxy.

Images