Patent Description:
Semiconductors increasingly are being used for various applications. Some applications use multiple semiconductors in close proximity to one another. The operation of semiconductors, such as photodiodes, may result in electrical and/or optical crosstalk between photodiode pixels. The crosstalk may be a source of noise that may increase as pixel pitch decreases. Fabrication of photodiodes to reduce crosstalk may be inefficient due to a need for electrical passivation of materials with significant electric fields, and due to a risk of noise resulting from some material used to facilitate electrical passivation. There is a need for an efficient design of low-noise photodiodes with small pixel pitch.

The document entitled "<NPL>, discusses the design, fabrication, and performance of focal plane arrays (FPAs) for use in <NUM>-D LADAR imaging applications requiring single photon sensitivity.

Certain implementations will now be described more fully below with reference to the accompanying drawings, in which various implementations and/or aspects are shown. However, various aspects may be implemented in many different forms and should not be construed as limited to the implementations set forth herein; rather, these implementations are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers in the figures refer to like elements throughout. Hence, if a feature is used across several drawings, the number used to identify the feature in the drawing where the feature first appeared will be used in later drawings.

Example embodiments described herein provide certain systems, methods, and devices for reducing electrical and optical crosstalk in photodiodes.

Photodiodes are semiconductor devices that may convert light into electrical current. Photodiodes may be elements of sensors, and may be arranged in an array in which pixels of the photodiode array may have a p-n junction. A p-n junction may be formed when p-type and n-type materials are arranged in contact with each other. Current may flow from one type of material to the other to create a diode.

Buried p-n junctions may be used in photodiodes, such as in avalanche photodiodes (APDs), which may refer to photodiodes that may generate large electrical current signals in response to receiving a low-powered optical signal. An APD may be biased by applying a voltage across the APD, resulting in a significant (e.g., non-zero) electrical field. Free electrical carriers may be generated in an absorption layer of the APD and injected into a multiplication region (e.g., avalanche region) of the APD. The absorption layer may absorb energy from light to generate free charge carriers. The multiplication layer may be a region of the APD in which the free charge carriers multiply to generate detectable electrical current. The free carriers may accelerate in the multiplication layer, allowing them to create additional free carriers (e.g., a process referred to as "impact ionization"), and the additional free carriers also may accelerate (e.g., due to the electric field presence) to create more free carriers. This multiplication of free carriers may be referred to as avalanche multiplication. When light enters a photodiode, electron-hole pairs may be generated when the energy from the light exceeds a band gap energy. An electric field may cause the electrons to drift to the n-type material and the holes (e.g., electron holes) to drift to the p-type material. The stronger the electric field, the more drift of the electrons and holes.

An active material of a photodiode may refer to a material having a finite, non-zero electric field. A non-active or passive material may refer to a material in which no electric field is present (e.g., an electric field of zero). The rate at which an electric field decreases (e.g., to zero) from an active region to a passive region may depend on the doping used. For example, an electrical field of one photodiode may drop off at a distance closer to a diffused region than in another photodiode because of the doping used in the photodiodes.

The use of buried p-n junctions in a photodiode may facilitate electrical passivation, but may result in a remaining presence of an absorber material that may contribute to increased noise associated with crosstalk effects. A non-zero electric field (e.g., caused by an applied voltage across a photodiode) may be present in at least a portion of absorber material. For example, the absorber material may at least partially surround an active p-n junction. The further away from the active p-n junction, the weaker the electric field of the absorber material may be.

Some photodiodes use a "mesa" structure to create an active region. A mesa structure on a semiconductor may refer to an area where material of the semiconductor has not been removed (e.g., etched away), resulting in a flat-topped surface that rises above a surrounding semiconductor substrate. Mesa structures for semiconductors may be formed by etching away non-active material and leaving active material for the mesa with non-planar passivation. To reduce crosstalk, non-active absorber material may be etched away because the non-active material may contribute to crosstalk between photodiode pixels. The crosstalk may be a source of noise that increases rapidly with decreasing pixel pitch (e.g., the density of pixels in a cluster of photodiode array pixels). Mesa formation, however, may require electrical passivation of surfaces having high electric fields, resulting in fabrication challenges and inefficiencies. For example, forming a mesa structure by etching into an active region where the electric field strength is greater than zero or etching into an inactive region too close to the active region may require passivation of material even though crosstalk may be reduced by removing some of the material.

Therefore, there is a need to design a low-noise photodiode array with small pixel pitch.

The use of a mesa structure may be combined with a buried p-n junction to form a photodiode array with reduced crosstalk and noise. The photodiode array may be formed by removing absorber material from a device having a buried p-n junction to mitigate crosstalk while maintaining high-quality passivation. For example, the more material etched away or otherwise removed, the more crosstalk may be reduced. However, the removal of absorber material further away from an active region may result in the remaining absorber material being within the active region and having a non-zero electric field, or being in close proximity to material having a non-zero electric field, thereby necessitating passivation. The surface electric field intensities may be reduced and may be tuned by adjusting the distance between the active absorber material and the removed absorber material. Removal of absorber material may be achieved using geometric configurations such as trenches, pits, end caps, wells, and troughs as described further herein, and may allow for a significant reduction in the dark carrier generation, which manifests as dark current in a linear-mode APD or as the dark count rate (e.g. the average rate of counts without incident light) of a single-photon avalanche diode. An active region of the photodiode array may be formed by etching away at least a portion of the absorber region to form a mesa structure with non-planar passivation, and the active region may be formed by dopant diffusion to create a buried p-n junction with planar passivation. To optimize passivation by avoiding a need to passivate material with a high electric field, at least some absorber material around the active p-n junction may not be removed by etching. Etching further away from the p-n junction and leaving more absorber material may result in significant crosstalk, but etching closer to the p-n junction and leaving less (if any) absorber material may result in a need to passivate material having significant electric field strength. Therefore, the hybrid use of a mesa structure and a buried p-n junction may balance a goal of reducing crosstalk by etching absorber material with a goal of avoiding a need to passivate materials with a significant electric field.

The electric field strength of the photodiode array may be tailored based on the amount of absorber material that is left around the buried p-n junction after etching away some of the absorber material. For example, rather than etching away all of the non-active absorber material around the active material in a photodiode array, passivation may be achieved by leaving some absorber material as a buffer between the exposed portions of active material, but not so much absorber material that significant crosstalk results. The optimized amount of etching may answer a question of how far into or how close to the active region to etch in order to minimize both passivation and crosstalk. The distance from the diffused region at which absorber material may be etched may be dependent on the electric field strength at the distance, and based on any need to passivate any material that remains after etching.

In some focal plane arrays, carrier diffusion time constants may have relaxed requirements. For example, for some InGaAs PIN detectors (e.g., un-doped detectors) operating at video rates (e.g., <NUM>-<NUM>), collection times for free carriers may be on the order of microseconds or milliseconds. In such cases, no absorber material needs to be removed from a photodiode array, and all of the absorber material in a pixel area of the photodiode array may contribute to the overall fill factor when the pixel dimension is on the order of a minority carrier diffusion length. In contrast, for applications requiring faster time constants, there may be drawbacks to leaving absorber material within an effective pixel because the absorber material may have slow time constants for carrier collection. Light detection and ranging (LIDAR) applications are among applications that may require fast time constants. Telecommunications also may require fast time constants, and may require high linearity. For example, analogue telecommunications receivers may experience significant waveform distortion with slow time constants.

To address the need for fast time constants in some applications, an optical use may ensure that no photons reach the non-active (i. field-free) absorber material of a photodiode array. However, when photons are generated within a semiconductor itself, it may not be feasible to ensure that the photons do not reach the absorber material. Such "crosstalk" photons generated during a very large avalanche detection event may be absorbed directly in a neighboring pixel active region (e.g., "optical crosstalk") or absorbed in a non-active region where the generated electrical carriers may diffuse to a neighboring active region (e.g., "diffusive crosstalk"). This issue may be a problem for applications requiring fast time constants. For example, an InGaAs absorber material may have long carrier diffusion lengths (e.g., <NUM> - <NUM>) and lifetimes (e.g., <NUM> - <NUM>). Because carriers may be long-lived and may diffuse relatively long distances, the carriers may migrate long distances outside of the active regions of the photodiode array to be collected and trigger dark counts at hundreds of nanoseconds or microseconds later, giving rise to false detection events much later than the time corresponding to the actual arrivals of signal photons.

In one or more embodiments, a photodiode array may include pits, trenches, end caps, and/or troughs. For example, an avalanche photodiode array may include multiple layers of materials, such as a buffer layer, an absorber layer above the buffer layer, active and multiplication/avalanche regions above the absorber layer, and troughs etched from the active region to the buffer layer. The troughs may allow for a portion of the top surface of the absorber layer to be exposed and for a portion of the buffer to be exposed, forming a mesa structure in which the active material forms the top of the mesa, with some absorber material left around the active material, and some absorber material etched away to form the mesa with part of the buffer exposed at a lower level. The photodiode array may have multiple active regions surrounded by absorber material, and some absorber material etched away to expose the buffer layer and form a mesa structure. The active material may include a buried p-n junction to leverage the electrical passivation facilitated by the buried p-n junction, and some of the noisy absorber material may be etched away from around the buried p-n junction to reduce crosstalk.

In one or more embodiments, a hybrid design of photodiode arrays that uses both buried p-n junctions and an etched mesa structure may support Geiger-mode operations. In Geiger-mode operations, APDs in an array may be biased above a breakdown voltage to allow a photon to trigger an avalanche.

The above descriptions are for purposes of illustration and are not meant to be limiting. Numerous other examples, configurations, processes, etc., may exist, some of which are described in greater detail below. Example embodiments will now be described with reference to the accompanying figures.

<FIG> illustrates example photodiode structures, in accordance with one or more example embodiments of the present disclosure.

Referring to <FIG>, a photodiode structure <NUM> is shown two-dimensionally from a top view. The photodiode structure <NUM> may be a planar-diffused structure with a non-active material <NUM> (e.g. material having zero electric field strength) surrounding active material <NUM> (e.g., material having non-zero electric field strength). The non-active material <NUM> may be removed (e.g., etched away) to form a photodiode structure <NUM> (e.g., shown from a top view). The photodiode structure <NUM> may include the active material <NUM> of the photodiode structure <NUM>, along with a buffer layer <NUM> (e.g., exposed by removing the entirety of the non-active material <NUM> from the photodiode structure <NUM>). The photodiode structure <NUM> therefore may include no non-active absorber material surrounding the active material <NUM>, so the photodiode structure <NUM> may require significant passivation of the active material <NUM>.

Referring to <FIG>, a photodiode structure <NUM> is shown two-dimensionally from a top view. The photodiode structure <NUM> may be a planar-diffused structure with the non-active material <NUM> (e.g. material having zero electric field strength) surrounding the active material <NUM> (e.g., material having non-zero electric field strength). Unlike the photodiode structure <NUM> of <FIG>, some of the non-active material <NUM> may remain after etching. The buffer layer <NUM> may be exposed where portions of the non-active material <NUM> have been removed. In the photodiode structure <NUM>, the non-active material <NUM> forms a rectangular region around the active material <NUM>. In this manner, the remaining non-active material <NUM> after etching may result in an area with zero electric field, thereby avoiding a difficult process of passivating material having a high electric field strength.

Still referring to <FIG>, a photodiode structure <NUM> is shown two-dimensionally from a top view. The photodiode structure <NUM> may be a planar-diffused structure with the non-active material <NUM> (e.g. material having zero electric field strength) surrounding the active material <NUM> (e.g., material having non-zero electric field strength). Unlike the photodiode structure <NUM> of <FIG>, some of the non-active material <NUM> may remain after etching. The buffer layer <NUM> may be exposed where portions of the non-active material <NUM> have been removed. In the photodiode structure <NUM>, the non-active material <NUM> forms a circular region around the active material <NUM>. In this manner, the remaining non-active material <NUM> after etching may result in an area with zero electric field, thereby avoiding a difficult process of passivating material having a high electric field strength.

Referring to <FIG>, the removal of at least a portion of the non-active material <NUM> may result in a mesa structure. As shown in <FIG>, the active material <NUM> may be formed by using a buried p-n junction, which may be above the buffer layer <NUM>. The buffer layer <NUM> with a higher layer formed by the active material <NUM> may result in a mesa structure with a p-n junction. The photodiode structure <NUM> and the photodiode structure <NUM> may refer to APDs with an array of photodiodes.

<FIG> illustrates a portion <NUM> of photodiode structures, in accordance with one or more example embodiments of the present disclosure.

Referring to <FIG>, the portion <NUM> may be similar to the photodiode structure <NUM>. For example, the portion <NUM> may have electrical traces (e.g., trace <NUM>, trace <NUM>, trace <NUM>) with anode contact points (e.g., contact point <NUM> of the trace <NUM>, contact point <NUM> of the trace <NUM>, contact point <NUM> of the trace176). The anode contact points may be positioned within respective diffused regions (e.g., the contact point <NUM> may be positioned within diffused region <NUM>; the contact point <NUM> may be positioned within the diffused region <NUM>; and the contact point <NUM> may be positioned within diffused region <NUM>). The diffused regions may refer to multiplication/avalanche regions where avalanching may occur. Plateaus (e.g., plateau <NUM>, plateau <NUM>, plateau <NUM>, plateau <NUM>, plateau <NUM>, and plateau <NUM>) may be created by removal (e.g., etching) of material, resulting in the formation of trenches (e.g., trench <NUM>) between the plateaus (e.g., the trench <NUM> may be further into the page than the plateaus).

Where the trench <NUM> is etched, and therefore where the plateaus are formed relative to the diffused regions, represents a selection of the distance d1 (e.g., a distance between the diffused region <NUM> and the trench <NUM>) to minimize crosstalk and optimize electrical passivation. For example, the smaller the distance dl, the more likely the electrical field where the trench <NUM> is formed may be greater than zero, therefore requiring more significant electrical passivation. The larger the distance dl, the more likely the electrical field where the trench <NUM> is formed may be zero. In this manner, the distance d1 is a factor in the electrical field strength of the sides of the plateaus formed by the etching that results in the trench <NUM>.

The cross-section lines are the basis for <FIG>.

<FIG> illustrates an example homojunction p-n photodiode <NUM>, in which one or more example embodiments of the present disclosure may be used.

Referring to <FIG> , the homojunction p-n photodiode <NUM> may include a p-diffused region <NUM> at least partially positioned within an n-absorber layer <NUM>. The n-absorber layer <NUM> may be positioned on a buffer layer <NUM>. Where the homojunction p-n photodiode <NUM> is active (e.g., exhibits an electric field E > <NUM>) may depend a distance d1 from the p-diffused region <NUM>. As shown by a graph <NUM>, the electric field E of the homojunction p-n photodiode <NUM> may be active up until a distance d2, and then inactive beyond the distance d2. In this manner, when the n-absorber layer <NUM> is removed (e.g., etched) at a location within the distance d1 from the p-diffused region <NUM>, the remaining n-absorber layer <NUM> may be active, and therefore may require passivation. When the n-absorber layer <NUM> is removed at a location outside of (e.g., greater than) the distance d1 from the p-diffused region <NUM>, the remaining n-absorber layer <NUM> may be inactive, and therefore may avoid the need for passivation. However, more of the n-absorber layer <NUM> remaining after etching may result in greater noise due to crosstalk effects. In this manner, the location at which the n-absorber layer <NUM> may be removed may be a balancing act that maximizes the amount of the n-absorber layer <NUM> removed to reduce crosstalk effects while minimizing passivation that may be necessitated by removing too much of the n-absorber layer <NUM> (e.g., such that the remaining n-absorber layer <NUM> is active). If the n-absorber layer <NUM> is removed within the active region and exposes a surface with a higher electric field intensity, as indicated by the graph <NUM>, then a higher quality passivation layer will be needed to adequately passivate this exposed active region surface.

<FIG> illustrates an example heterojunction p-i-n photodiode <NUM>, in which one or more example embodiments of the present disclosure may be used.

Referring to <FIG> , the heterojunction p-i-n photodiode <NUM> may include a p+ diffused region <NUM> positioned at least partially within a n- cap layer <NUM> and an n- absorption layer <NUM> (e.g., which may be positioned below the n- cap layer <NUM>), and a n+ buffer layer <NUM> positioned below the n- absorption layer <NUM>. Where the heterojunction p-i-n photodiode <NUM> is active (e.g., exhibits an electric field E > <NUM>) may depend a distance d3 and/or the distance d4 from the p<+> diffused region <NUM>. As shown by a graph <NUM>, the electric field E of the heterojunction p-i-n photodiode <NUM> may be constant up until the distance d3, and then the E field may decline from d3 to d4, beyond which the heterojunction p-i-n photodiode <NUM> may become inactive. In this manner, when the n- cap layer <NUM> and the n- absorption layer <NUM> are removed (e.g., etched) at a location within the distance d3 or d4 from the p+ diffused region <NUM>, the remaining n- cap layer <NUM> and n- absorption layer <NUM> may be active, and therefore may require passivation. When the n- cap layer <NUM> and the n- absorption layer <NUM> are removed at a location outside of (e.g., greater than) the distance d4 from the p+ diffused region <NUM>, the remaining n- cap layer <NUM> and n-absorption layer <NUM> may be inactive, and therefore may avoid the need for passivation. However, more of the n- cap layer <NUM> and n- absorption layer <NUM> remaining after etching may result in greater noise due to crosstalk effects. In this manner, the location at which the n- cap layer <NUM> and n- absorption layer <NUM> may be removed may be a balancing act that maximizes the amount of the n- cap layer <NUM> and n- absorption layer <NUM> removed to reduce crosstalk effects while minimizing passivation that may be necessitated by removing too much of the n- cap layer <NUM> and n- absorption layer <NUM> (e.g., such that the remaining n- cap layer <NUM> and n- absorption layer <NUM> are active).

<FIG> illustrates an example heterojunction avalanche photodiode (APD) <NUM>, in which one or more example embodiments of the present disclosure may be used.

Referring to <FIG> , the heterojunction APD <NUM> may include a p+ diffused region <NUM> at least partially positioned within a cap layer <NUM>, and the cap layer <NUM> may be positioned above an n- absorption layer <NUM>. Below the n- absorption layer <NUM>, the heterojunction APD <NUM> may include an n+ buffer layer <NUM>. Avalanching may occur within the cap layer <NUM> (e.g., at a portion <NUM> of the cap layer <NUM> proximal enough to the p+ diffused region <NUM>, where the electric field strength may be strong enough to give rise to avalanche gain). The portion <NUM> of the cap layer <NUM> may be referred to as an avalanche region, which may extend to the n-absorption layer <NUM>.

Where the heterojunction APD <NUM> is active (e.g., exhibits an electric field E > <NUM>) may depend a distance d5 and/or the distance d6 from the p+ diffused region <NUM>. As shown by a graph <NUM>, the electric field E of the heterojunction APD <NUM> may be constant up until the distance d5 (e.g., the edge of the avalanche region), and then the E field may decline from d5 to d6, beyond which the heterojunction APD <NUM> may become inactive. In this manner, when the cap layer <NUM>, the n- absorption layer <NUM>, and the n+ buffer layer <NUM> are removed (e.g., etched) at a location within the distance d5 or d6 from the p+ diffused region <NUM>, the remaining cap layer <NUM>, n-absorption layer <NUM>, and n+ buffer layer <NUM> may be active, and therefore may require passivation. When the cap layer <NUM>, the n- absorption layer <NUM>, and the n+ buffer layer <NUM> are removed at a location outside of (e.g., greater than) the distance d6 from the p+ diffused region <NUM>, the cap layer <NUM>, n- absorption layer <NUM>, and n+ buffer layer <NUM> may be inactive, and therefore may avoid the need for passivation. However, more of the cap layer <NUM>, n- absorption layer <NUM>, and n+ buffer layer <NUM> remaining after etching may result in greater noise due to crosstalk effects. In this manner, the location at which the cap layer <NUM>, n- absorption layer <NUM>, and n+ buffer layer <NUM> may be removed may be a balancing act that maximizes the amount of the cap layer <NUM>, n-absorption layer <NUM>, and n+ buffer layer <NUM> removed to reduce crosstalk effects while minimizing passivation that may be necessitated by removing too much of the cap layer <NUM>, n-absorption layer <NUM>, and n+ buffer layer <NUM> (e.g., such that the cap layer <NUM>, n-absorption layer <NUM>, and n+ buffer layer <NUM> are active). If the n- absorption layer <NUM> is removed within the active region and exposes a surface with a higher electric field intensity, as indicated by the graph <NUM>, then a higher quality passivation layer will be needed to adequately passivate this exposed active region surface.

<FIG> illustrates a graph <NUM> showing average dark count rate for photodiode devices, in accordance with one or more example embodiments of the present disclosure.

Referring to <FIG>, the dark count rate is shown on the vertical axis with a metric of hertz (Hz), and overbias is shown on the horizontal axis with a metric of volts (V). The dark count rate <NUM> of an un-optimized device (e.g., a device without the hybrid mesa structure with buried p-n junction and selective removal of non-active material) may experience a significantly higher dark count rate than the dark count rate <NUM> experienced by an optimized device (e.g., a device using the hybrid mesa structure with buried p-n junction and selective removal of non-active material), such as a device using the photodiode structure <NUM> or the photodiode structure <NUM> of FIG. For example, an optimized device may experience a thirty-time reduction in dark count rate when compared to an un-optimized device. In this manner, the graph <NUM> shows benefits of using enhanced methods and structures to reduce electrical and optical crosstalk in photodiodes.

<FIG> illustrates an example top view of a photodiode array <NUM>, in which one or more example embodiments of the present disclosure may be used.

Referring to <FIG> , the photodiode array <NUM> may include traces <NUM> (e.g., metal traces) and one or more trenches <NUM> (e.g., indentions), which may be dry-etched or otherwise fabricated. The traces <NUM> may include conductive material (e.g., copper or otherwise) that allow for the flow of electricity. The one or more trenches <NUM> may prevent optical crosstalk as well as diffusive crosstalk. The photodiode array <NUM> may be an APD or other type of photodiode.

<FIG> illustrates an example side cross-section view of the photodiode array <NUM> of <FIG>, in which one or more example embodiments of the present disclosure may be used.

Referring to <FIG> , the photodiode array <NUM> may include an anode <NUM> (e.g., a charged electrode in one of the traces <NUM>, through which bias voltage is applied from the outside) at least partially covering a portion a diffused region <NUM> (e.g., a degenerately doped region where the electric field strength is zero except within a proximity, such as around <NUM> nanometers, of a p-n junction <NUM>). The diffused region <NUM> may include the p-n junction <NUM> (e.g., a buried p-n junction). The diffused region <NUM> may extend at least partially into a cap layer <NUM> (e.g., an i-InP cap, or another material). Avalanching may occur within the cap layer <NUM> (e.g., at a portion <NUM> of the cap layer <NUM> proximal enough to the diffused region <NUM>, where the electric field strength may be strong enough to give rise to avalanche gain). The portion <NUM> of the cap layer <NUM> may be referred to as an avalanche region. Below the cap layer <NUM> may be an absorber layer <NUM> (e.g., an I-InGaAS absorber material or another material), and below the absorber layer <NUM> may be a buffer layer <NUM> (e.g., an n+-InP material or another material). An electrical carrier <NUM> created by photon absorption in the absorber layer <NUM> may diffuse along a path <NUM>. The electrical carrier <NUM> may diffuse to the avalanche region <NUM>(e.g., diffusive crosstalk).

In one or more embodiments, crosstalk can be particularly troubling in applications with fast time constants. An InGaAs absorber material (e.g., used in the absorber layer <NUM>), for example, may allow for long carrier diffusion lengths and lifetimes (e.g., diffusion lengths of ~<NUM> - <NUM>, lifetimes from ~<NUM> - <NUM>). Because carriers (e.g., the electrical carrier <NUM>) may be long-lived and may diffuse quite far, carriers may migrate from long distances outside the avalanche region <NUM> to be collected and trigger dark counts at <NUM> of nanoseconds or even microseconds later, giving rise to false detection events much later than the time corresponding to true signal photon arrivals.

In one or more embodiments, at least some of the absorber layer <NUM> may be removed (e.g., etched away) in order to mitigate diffusive crosstalk. The absorber layer <NUM> that may be at least partially removed from the photodiode array <NUM> may include the one or more trenches <NUM> of <FIG>. The optimized amount of the absorber layer <NUM> removed may answer a question of how far into or how close to the active region to etch in order to both optimize passivation and minimize crosstalk. The distance from the avalanche region <NUM> at which the absorber layer <NUM> may be etched may be dependent on the electric field strength at the distance, and based on any need to passivate any material that remains after etching. For example, <FIG> represents complete removal of the non-active portion of the absorber layer <NUM> (e.g., corresponding to the non-active material <NUM>), exposing the buffer layer <NUM> (e.g., corresponding to the buffer layer <NUM>) below. represents partial removal of the non-active portion of the absorber layer <NUM>, leaving some of the absorber layer <NUM> (e.g., corresponding to the non-active material <NUM>), and exposing some of the buffer layer <NUM> (e.g., corresponding to the buffer layer <NUM>) below. The lateral distance from the avalanche region <NUM> to the buffer layer <NUM> may be based on the electric field strength at the edges of the unetched material at the closest location of the exposed buffer layer <NUM> to the avalanche region <NUM> due to the at least partial removal of the absorber layer <NUM>. The buried p-n junction <NUM> may be passivated electrically, but electric field intensity extending laterally from the p-n junction <NUM> may require leaving some of the absorber layer <NUM>, which may result in crosstalk as represented by the path <NUM> of the electrical carrier <NUM>. Therefore, the photodiode array <NUM> may benefit from removing some of the active portion of the absorber layer, but not so much that the exposed surfaces experience an electric field intensity that it too large to passivate.

As shown by graph <NUM>, the electric field strength E within the avalanche region is non-zero, and outside the avalanche region may be non-zero, but declining toward zero as the lateral distance from the p-n junction <NUM> approaches the distance d8.

<FIG> illustrates example avalanching pixels of the photodiode array <NUM> of <FIG>, in which one or more example embodiments of the present disclosure may be used.

Referring to <FIG> , the top view of the photodiode array <NUM> of <FIG> is shown in more detail with both optical crosstalk and diffusive crosstalk. Photons (e.g., photon <NUM>, photon <NUM>, photon <NUM>) may be emitted by an avalanche (e.g., of an avalanching pixel <NUM>) may result in electrical carriers <NUM> created by photon absorption. The result may be optical crosstalk at one or more pixels <NUM> (e.g., carrier generation within the active regions of individual pixels). The avalanching pixel <NUM> may represent an initial avalanching pixel that may generate electrical carriers <NUM>, which may follow a path <NUM> to induce a subsequent avalanche at pixel <NUM> - a manifestation of diffusive crosstalk in which the pixel <NUM> may collect the electrical carriers <NUM>). In a region <NUM> where photon absorption may create diffusion carriers, diffusive crosstalk may occur as the electrical carriers <NUM> diffuse along the path <NUM>. In this manner, the avalanche at the pixel <NUM> may result in secondary avalanches at the pixel <NUM>. In one or more embodiments, to reduce such optical and diffusive crosstalk, the photodiode array <NUM> may benefit from removal of at least some of the absorber layer <NUM> of <FIG> as described above.

In one or more embodiments, the buried p-n junction <NUM> of <FIG> may be used in the photodiode array <NUM>, such as in APDs. Free electrical carriers (e.g., the electrical carrier <NUM>) may be generated in the absorber layer <NUM> of the APD and injected into the avalanche region <NUM> of the APD. The absorber layer <NUM> may absorb energy from light to generate free charge carriers. The avalanche region <NUM> may be a region of the APD in which the free charge carriers multiply to generate detectable electrical current. The free carriers may accelerate in the avalanche region <NUM>, allowing them to create additional free carriers (e.g., a process referred to as "impact ionization"), and the additional free carriers also may accelerate (e.g., due to the electric field presence) to create more free carriers. This multiplication of free carriers may be referred to as avalanche multiplication. When light enters the photodiode array <NUM>, electron-hole pairs may be generated when the energy from the light exceeds a band gap energy. An electric field may cause the electrons (e.g., the electrical carrier <NUM> of <FIG>) to drift to the substrate, and the holes (e.g., electron holes) to drift to the p-type material of the p-n junction <NUM>. The stronger the electric field, the more drift of the electrons and holes.

7A illustrates an example top view of a photodiode array <NUM>, in accordance with one or more example embodiments of the present disclosure.

Referring to <FIG>, the photodiode array <NUM> may be similar to the photodiode array <NUM> of <FIG>. For example, the photodiode array <NUM> may include one or more trenches <NUM> (e.g., similar to the one or more trenches <NUM> of <FIG>), and pits <NUM> (e.g., removed portions down to the buffer layer <NUM> of <FIG>), and traces <NUM> The part of the absorber layer <NUM> of <FIG> that may be removed from the photodiode array <NUM> (or the photodiode array <NUM>) may include any of the one or more trenches <NUM>, and any of the pits <NUM>.

In one or more embodiments, material around the traces <NUM> may be etched away or otherwise removed to generate the mesa structure as shown in <FIG>. The etching or other removal of material may generate the one or more trenches <NUM> and/or the pits <NUM>.

<FIG> illustrates an example side cross-section view of the portion <NUM> of photodiode structures of <FIG>, in accordance with one or more example embodiments of the present disclosure.

Referring to <FIG>, the portion <NUM> represented by the cross-section view may include the contact point <NUM> of <FIG>, at least partially disposed over a portion of the diffused region <NUM>. The diffused region <NUM> may include a p-n junction <NUM> (e.g., a buried p-n junction such as the p-n junction <NUM> of <FIG>). Below the diffused region <NUM> may be an avalanche region <NUM> (e.g., similar to the avalanche region <NUM> of <FIG>). The diffused region <NUM> may at least partially extend into a cap layer <NUM> (e.g., similar to the cap layer <NUM> of <FIG>). Below the cap layer <NUM> may be an absorber layer <NUM> (e.g., similar to the absorber layer <NUM> of <FIG>), and below the absorber layer <NUM> may be a buffer layer <NUM> (e.g., similar to the buffer layer <NUM> of <FIG>). The trench <NUM> may extend (e.g., via etching) to the buffer layer <NUM> (e.g., through the cap layer <NUM> and through the absorber layer <NUM>). The trench <NUM> may be formed by etching away at least some of the cap layer <NUM> and the absorber layer <NUM>. The distance d1 from the avalanche region <NUM> at which the trench <NUM> is etched may be determined by the presence of the electrical field at the distance d1. The greater the distance dl, the less electrical passivation may be required for any remaining material between the avalanche region <NUM> and the trench <NUM>, and the easier the fabrication of the portion <NUM> may be as a result, but the more noise from crosstalk may be present (e.g., as explained with respect to <FIG>). The smaller the distance dl, the greater the likelihood of a non-zero electrical field presence that may require electrical passivation, resulting in a more difficult fabrication. In this manner, the distance d1 may be selected to reduce crosstalk while also minimizing the need for electrical passivation.

In one or more embodiments, the buried p-n junction <NUM> may be used in a photodiode array, such as in APDs. Free electrical carriers (e.g., the electrical carrier <NUM> of <FIG>) may be generated in the absorber layer <NUM> of the APD and injected into the avalanche region <NUM> of the APD. The absorber layer <NUM> may absorb energy from light to generate free charge carriers. The avalanche region <NUM> may be a region of the APD in which the free charge carriers multiply to generate detectable electrical current. The free carriers may accelerate in the avalanche region <NUM>, allowing them to create additional free carriers (e.g., a process referred to as "impact ionization"), and the additional free carriers also may accelerate (e.g., due to the electric field presence) to create more free carriers. This multiplication of free carriers may be referred to as avalanche multiplication. When light enters the portion <NUM>, electron-hole pairs may be generated when the energy from the light exceeds a band gap energy. An electric field may cause the electrons (e.g., the electrical carrier <NUM> of <FIG>) to drift to the buffer layer <NUM>, and the holes (e.g., electron holes) to drift to the p-type material of the p-n junction <NUM>. The stronger the electric field, the more drift of the electrons and holes.

Referring to <FIG> and <FIG>, the diffused region <NUM> and the diffused region <NUM> may form a mesa structure with the buried p-n junction <NUM> and the buried p-n junction <NUM>, respectively (e.g., the plateau <NUM> of <FIG>). The removal of the absorber layer <NUM> and the absorber layer <NUM> may mitigate crosstalk while maintaining high-quality passivation. For example, the more material etched away, the more crosstalk may be reduced, but the removal of absorber material may result in the remaining absorber material having a non-zero electric field or being in close proximity to remaining absorber material having a non-zero electric field, thereby necessitating passivation. The surface electric field intensities may be reduced and may be tuned by adjusting the distance between the active absorber material and the removed absorber material. Removal of non-active material (e.g., the cap layer <NUM>, the absorber layer <NUM>, the cap layer <NUM>, the absorber layer <NUM>) may be achieved using geometric configurations such as trenches, pits, end caps, wells, and troughs (e.g., the trench <NUM>), and may allow for a significant reduction in the dark count rate (e.g., the average rate of counts without incident light) when compared to some existing photodiodes. The diffused region <NUM> and the diffused region <NUM> may be formed by etching away at least a portion of the surrounding material(s) to form a mesa structure with non-planar passivation, and the diffused region <NUM> and the diffused region <NUM> may be formed by dopant diffusion to create the buried p-n junction <NUM> and the buried p-n junction <NUM>, respectively, with planar passivation. To optimize passivation by avoiding a need to passivate material with a high electric field, at least some non-active material around the active p-n junction may be removed by etching. Etching further away from the p-n junction and leaving more absorber material may result in significant crosstalk, but etching closer to the p-n junction and leaving less (if any) absorber material may result in a need to passivate material having significant electric field strength (e.g., as shown in <FIG>). Therefore, the hybrid use of a mesa structure and a buried p-n junction may balance a goal of reducing crosstalk by etching non-active material with a goal of avoiding a need to passivate materials with a significant electric field.

<FIG> illustrates a flow diagram for a process <NUM> for forming a photodiode, in accordance with one or more example embodiments of the present disclosure.

At block <NUM>, a first layer for a photodiode may be formed, the first layer having passive material with no electrical field. For example, the first layer may include a buffer layer (e.g., the buffer layer <NUM> of <FIG>, the buffer layer <NUM> of <FIG>, the buffer layer <NUM> of <FIG>, the buffer layer <NUM> of <FIG>). The first layer may include an n+-InP material or another material on which one or more additional layers may be arranged.

At block <NUM>, a second layer for the photodiode may be formed (e.g., the non-active material <NUM> of <FIG>) above the first layer, the second layer having absorber material. The second layer may include a non-active absorber layer (e.g., the absorber layer <NUM> of <FIG>, the absorber layer <NUM> of <FIG>). When the photodiode is a homojunction p-n photodiode (e.g., the homojunction p-n photodiode <NUM> of <FIG>), the second layer may include a non-active absorber layer (e.g., the n-absorber layer <NUM> of <FIG>). When the photodiode is a heterojunction p-i-n photodiode (e.g., the heterojunction p-i-n photodiode <NUM> of <FIG>), the second layer may include an n- absorption layer (e.g., the n- absorption layer <NUM> of <FIG>). When the photodiode is a heterojunction APD (e.g., the heterojunction APD <NUM> of <FIG>), the second layer may include an n- absorption layer (e.g., the n- absorption layer <NUM> of <FIG>). An electric field strength in the second layer may depend on a distance from a location of the second layer to an active region formed at block <NUM>.

At block <NUM>, a diffused region having a buried p-n junction for the photodiode may be formed. The diffused region may be in the form of a mesa in which a portion of the diffused region is above at least a portion of at least one of the first layer or the second layer. The diffused region may include an active region and the second layer. For example, referring to <FIG>, the p-diffused region <NUM> may be at least partially positioned within the n-absorber layer <NUM>. Referring to <FIG>, the p+ diffused region <NUM> may be positioned at least partially within the n- cap layer <NUM> and the n- absorption layer <NUM>. Referring to <FIG>, the p+ diffused region <NUM> may be at least partially positioned within the cap layer <NUM>, and the cap layer <NUM> may be positioned above the n- absorption layer <NUM>. Referring to <FIG>, the diffused region <NUM> may include the buried p-n junction <NUM> within the cap layer <NUM>, above the absorber layer <NUM>. Referring to <FIG>, the diffused region <NUM> may include the buried p-n junction <NUM> within the cap layer <NUM>, above the absorber layer <NUM>. Within the diffused region, the electric field strength may be non-zero. The electric field strength of the second layer may be zero or non-zero depending on the distance from the diffused region to the second layer (see <FIG> and the respective electric field graphs, for example).

At block <NUM>, an active region for the photodiode may be determined. The active region may refer to a region in which the electrical field strength is non-zero, and the active region may include the buried p-n junction and may include surrounding material where the electric field is present.

At block <NUM>, the photodiode may be etched through the second layer to the first layer (e.g., the trench <NUM> of <FIG> and <FIG>). The distance (e.g., the distance d1 of <FIG>) from the diffused region at which the photodiode is etched to remove at least some of the second layer may be determined in a manner that reduces crosstalk and the need for electrical passivation. For example, rather than etching away all of the non-active second layer around the diffused region, passivation may be achieved by leaving some of the second layer, but not so much of the second layer that significant crosstalk results. The optimized amount of etching may answer a question of how far into or how close to the diffused region to etch in order to minimize both passivation and crosstalk. The distance from the diffused region at which the second layer may be etched may be dependent on the electric field strength at the distance, and based on any need to passivate any material that remains after etching.

At block <NUM>, the etching of the second layer may form a plateau (or mesa) structure in which at least a portion of the diffused region is at least partially above at least a portion of the first layer and/or the second layer. As shown in <FIG>, etching the trench <NUM> may result in at least a portion of the absorber layer <NUM> and the buffer layer <NUM> below the diffused region <NUM>. The top views of <FIG> show the active material <NUM> above the buffer layer <NUM> (e.g., the active material <NUM> coming out of the page more than the buffer layer <NUM>) in the form of a plateau or mesa. The plateau or mesa formed by etching may be a result of removed non-active material that, when not removed, may contribute to electrical and optical crosstalk in the photodiode. The plateau or mesa may be formed by removing enough material around the diffused region to reduce crosstalk, and may allow for some material around the diffused region to remain as long as the electrical field strength within the remaining material around the diffused region is less than a threshold electrical field strength so as to avoid difficult electrical passivation of the remaining material around the diffused region.

The descriptions of the figures are not meant to be limiting.

A method for forming a photodiode may include: forming a first layer comprising passive material, the passive material having no electric field; forming a second layer comprising an absorbing material, the second layer above the first layer; forming a diffused region comprising a buried p-n junction; determining an active region comprising the buried p-n junction and having an electric field greater than zero; etching through the second layer to the first layer, the etching performed at a distance of fifteen microns or less from the buried p-n junction (or another distance); and forming a plateau structure based on the etching. At the distance, an electric field of the second layer is zero or non-zero. The photodiode may be an APD. The distance may be determined based on a passivation associated with the photodiode and/or based on the electric field of the active region. The method may include forming a third layer above the second layer, wherein the buried p-n junction is disposed in the second layer and in the third layer. The etching may be from the third layer to the first layer. The method may include forming a third layer above the second layer, wherein the buried p-n junction is disposed in the third layer and is above the second layer.

A photodiode may include a first layer with passive material, the passive material having no electric field. The photodiode may include a second layer with an absorbing material, the second layer above the first layer. The photodiode may include a diffused region with a buried p-n junction. The photodiode may include an active region with the buried p-n junction and having an electric field greater than zero. The photodiode may include a plateau structure based on etching through the second layer to the first layer, the etching performed at a distance of fifteen microns or less from the buried p-n junction. At the distance, an electric field of the second layer is zero or non-zero. The photodiode may be an APD. The distance may be determined based on a passivation associated with the photodiode and/or based on the electric field of the active region. The photodiode may include a third layer above the second layer, wherein the buried p-n junction is disposed in the second layer and in the third layer. The etching may be from the third layer to the first layer. The photodiode may include a third layer above the second layer, wherein the buried p-n junction is disposed in the third layer and is above the second layer.

As used herein, unless otherwise specified, the use of the ordinal adjectives "first," "second," "third," etc., to describe a common object, merely indicates that different instances of like objects are being referred to and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.

It is understood that the above descriptions are for purposes of illustration and are not meant to be limiting.

Although specific embodiments of the disclosure have been described, one of ordinary skill in the art will recognize that numerous other modifications and alternative embodiments are within the scope of the disclosure. For example, any of the functionality and/or processing capabilities described with respect to a particular device or component may be performed by any other device or component. Further, while various illustrative implementations and architectures have been described in accordance with embodiments of the disclosure, one of ordinary skill in the art will appreciate that numerous other modifications to the illustrative implementations and architectures described herein are also within the scope of this disclosure.

Claim 1:
An avalanche photodiode (<NUM>, <NUM>), comprising:
a first layer (<NUM>; <NUM>; <NUM>) comprising passive material;
a second layer (<NUM>; <NUM>; <NUM>) comprising an absorbing material, the second layer (<NUM>; <NUM>; <NUM>) being on the first layer (<NUM>; <NUM>; <NUM>);
a plateau structure (<NUM>) comprising a diffusion region (<NUM>; <NUM>) having an active region (<NUM>), the active region (<NUM>) comprising a buried p-n junction and at least a first portion of the second layer (<NUM>; <NUM>; <NUM>);
wherein the passive material has no electric field and the active region (<NUM>) has an electric field greater than zero, further to applying voltage across the photodiode,
wherein the plateau structure (<NUM>) is based on an absence of a second portion of the second layer (<NUM>; <NUM>; <NUM>), said second portion of the second layer (<NUM>; <NUM>; <NUM>) being removed at a distance greater than zero from the buried p-n junction;
characterized in that
the distance is such that the second portion of the second layer which is removed comprises a part of the active region which has an electric field greater than zero.