Semiconductor photodetector

Abstract

To provide a semiconductor photodetector that enables high-speed operation while suppressing cost increases. [Solution] The semiconductor photodetector comprises a substrate 10, a semiconductor laminate 20 formed on a first region RB of the substrate, and a first electrode 40 and a second electrode 50 electrically connected to the semiconductor laminate 20, wherein the semiconductor laminate is In x Ga 1-x The optical fiber includes a first conductivity type light-absorbing layer 24 containing As, and a second conductivity type different from the first conductivity type, located on the opposite side of the substrate from the light-absorbing layer and bonded to the light-absorbing layer. The first electrode is connected to a first portion of the semiconductor laminate that is on the substrate side of the light-absorbing layer and is of the first conductivity type, and the second electrode is connected to a second portion of the semiconductor laminate that is on the opposite side of the substrate from the light-absorbing layer and is of the second conductivity type. The In composition x in the light-absorbing layer is 0.55 or greater, and the thickness of the light-absorbing layer is 1.8 μm or less.

Description

[Technical Field]

[0001] The present disclosure relates to a semiconductor light-receiving element. [Background Art]

[0002] Patent Document 1 describes a photodiode. This photodiode includes a slope reflection portion formed on an InP substrate, a light-receiving portion formed of a p-electrode, a diffraction grating, and an InGaAs light absorption layer, and an n-electrode. Light incident in a direction perpendicular to the surface is totally reflected by the slope reflection portion, the optical path is converted to an obliquely upward direction, and the light is incident on the light absorption layer in the light-receiving portion from an obliquely downward direction. After the obliquely incident light propagates through the light absorption layer, it is reflected in a direction opposite to an incident direction by the diffraction grating provided on an upper portion of the light-receiving portion and the p-electrode, and is absorbed again by the light absorption layer. [Prior Art Literature] [Patent Literature]

[0003] [Patent Document 1] Japanese Unexamined Patent Publication No. 2009-117499 [Summary of Invention] [Problem to be Solved by Invention]

[0004] By the way, in the above technical field, further increase in operating speed is desired. To achieve this, it is conceivable to shorten the travel distance of electrons by thinning the light absorption layer. However, thinning the light absorption layer causes a decrease in sensitivity. In contrast, the photodiode described in Patent Document 1 increases the effective thickness of the absorption layer by allowing light to propagate along an oblique optical path with respect to the light absorption layer. According to this configuration, it is considered that a decrease in sensitivity caused by thinning the light absorption layer can be suppressed and higher speed can be achieved.

[0005] However, in the photodiode described in Patent Document 1, in order to form an oblique optical path with respect to the light absorption layer, processing for forming the slope reflection portion on the substrate is required, which may increase the cost.

[0006] This disclosure aims to provide a semiconductor photodetector that can achieve high speed while suppressing cost increases. [Means for solving the problem]

[0007] The semiconductor photodetector according to this disclosure is a semiconductor photodetector for receiving incident light in at least one wavelength band of the 1.3 μm band, the 1.55 μm band, and the 1.6 μm band, and generating an electrical signal in response to the incident light, comprising a substrate, a semiconductor laminate formed on a first region of the substrate, and a first electrode and a second electrode electrically connected to the semiconductor laminate, wherein the semiconductor laminate is In x Ga 1-x The optical absorbing layer comprises a first conductivity type light-absorbing layer containing As, a first conductivity type buffer layer provided between the substrate and the light-absorbing layer, and a second region of a second conductivity type different from the first conductivity type, located on the opposite side of the substrate from the light-absorbing layer and bonded to the light-absorbing layer. The first electrode is connected to a first portion of the semiconductor laminate that is located on the substrate side of the light-absorbing layer, and the second electrode is connected to a second portion of the semiconductor laminate that is located on the opposite side of the substrate from the light-absorbing layer. The In composition x in the light-absorbing layer is 0.55 or greater, and the thickness of the light-absorbing layer is 1.8 μm or less.

[0008] This semiconductor photodetector is designed for light in optical communication wavelength bands such as the 1.3 μm band (O-band (Original-band)), the 1.55 μm band (C-band (Conventional-band)), and the 1.6 μm band (L-band (Long-wavelength-band)). In this semiconductor photodetector, the light absorption layer provided on the substrate is In x Ga 1-x It contains As. And the In composition x of the light-absorbing layer is 0.55 or greater (and less than 1). Thus, in the light-absorbing layer, In x Ga 1-xIf the In composition x of As is set to 0.55 or higher, the absorption coefficient is improved compared to, for example, the case where the In composition x is 0.53 (for example, in the 1.55 μm band, the absorption coefficient is improved by about twofold by setting the composition x to 0.62). Therefore, even if the thickness of the light absorption layer is thinned to 1.8 μm or less, a decrease in sensitivity can be avoided. In other words, higher speeds become possible. Furthermore, with this semiconductor photodetector, there is no need to form a separate configuration (for example, the bevel reflection portion of the photodiode described in Patent Document 1 above) to achieve higher speeds. Therefore, with this semiconductor photodetector, higher speeds are possible while suppressing an increase in cost.

[0009] In the semiconductor photodetector according to this disclosure, the buffer layer may include a strain relaxation layer having a lattice constant between the lattice constant of the substrate and the lattice constant of the light absorption layer. In this case, the crystallinity of the semiconductor stack is improved.

[0010] In the semiconductor photodetector according to this disclosure, the buffer layer may include a plurality of strain relaxation layers arranged such that the lattice constants gradually approach the lattice constant of the light absorption layer as one moves from the substrate toward the light absorption layer. Alternatively, the buffer layer may include strain relaxation layers in which the lattice constants continuously approach the lattice constant of the light absorption layer as one moves from the substrate toward the light absorption layer. In these cases, the crystallinity of the semiconductor stack is reliably improved.

[0011] In the semiconductor photodetector according to this disclosure, the semiconductor stack includes a cap layer of a first conductivity type containing InAsP, provided on the light absorption layer on the side opposite to the substrate relative to the light absorption layer, and a contact layer of a first conductivity type containing InGaAs, provided on the cap layer on the side opposite to the substrate relative to the light absorption layer, and the second region is formed extending from the contact layer through the cap layer to the light absorption layer, and the second portion to which the second electrode is connected may be the surface of the second region formed on the contact layer. In this case, the contact resistance of the second electrode is reduced.

[0012] In the semiconductor photodetector according to this disclosure, the semiconductor stacked portion may include a first semiconductor layer of a first conductivity type disposed between the substrate and the light-absorbing layer, and a second semiconductor layer of a first conductivity type disposed between the first semiconductor layer and the light-absorbing layer, having an impurity concentration lower than that of the first semiconductor layer. In this case, further speed increases can be achieved by reducing capacitance.

[0013] In the semiconductor photodetector according to this disclosure, the semiconductor stacked portion may include a third semiconductor layer provided between the light-absorbing layer and the capping layer, having a band gap between the band gap of the light-absorbing layer and the band gap of the capping layer. In this case, the abrupt change in the band gap between the capping layer and the light-absorbing layer is suppressed, which can make it difficult to extract carriers.

[0014] In the semiconductor photodetector according to this disclosure, at least one layer of the buffer layer may be semi-insulated by Fe doping. In this case, crystallinity is improved.

[0015] In the semiconductor photodetector according to this disclosure, the In composition x in the light absorption layer is 0.57 or higher, and the thickness of the light absorption layer may be 1.2 μm or less. Furthermore, the In composition x in the light absorption layer may be 0.59 or higher, and the thickness of the light absorption layer may be 0.7 μm or less. In these cases, further thinning of the light absorption layer can lead to increased speed.

[0016] In the semiconductor photodetector according to this disclosure, the substrate may include a semi-insulating semiconductor. In this case, capacitance can be reduced.

[0017] In the semiconductor photodetector according to this disclosure, the substrate includes an insulator or a semi-insulating semiconductor, and the semiconductor laminate may be directly bonded to the substrate. By constructing the semiconductor photodetector by directly bonding the substrate and the semiconductor laminate as separate components, it becomes possible to increase the aperture size and reduce costs by manufacturing optical components using inexpensive materials. [Effects of the Invention]

[0018] According to this disclosure, it is possible to provide a semiconductor photodetector that can be made faster while suppressing cost increases. [Brief explanation of the drawing]

[0019] [Figure 1] Figure 1 is a schematic side view showing a laser device according to one embodiment. [Figure 2] Figure 2 is a plan view of the semiconductor photodetector shown in Figure 1. [Figure 3] Figure 3 is a cross-sectional view taken along the line III-III in Figure 2. [Figure 4] Figure 4 is a cross-sectional view along the line IV-IV in Figure 2. [Figure 5] Figure 5 is a cross-sectional view along the VV line in Figure 2. [Figure 6] Figure 6 is a graph illustrating the relationship between the composition of the light-absorbing layer and its absorption coefficient. [Figure 7] This is a schematic side view showing the configuration of an optical device in a modified form. [Modes for carrying out the invention]

[0020] One embodiment will be described in detail below with reference to the drawings. In each drawing, the same or corresponding elements are denoted by the same reference numeral, and redundant explanations may be omitted.

[0021] Figure 1 is a schematic side view showing an optical device according to one embodiment. As shown in Figure 1, the optical device A includes a semiconductor photodetector 1. The optical device A is intended to target light in wavelength bands used for optical communication, such as the 1.3 μm band (O-band (Original-band)), the 1.55 μm band (C-band (Conventional-band)), and the 1.6 μm band (L-band (Long-wavelength-band)), and convert this light into an electrical signal for output. The 1.3 μm band is, for example, a wavelength range of 1.26 μm to 1.36 μm. The 1.55 μm band is, for example, a wavelength range of 1.53 μm to 1.565 μm. The 1.6 μm band is, for example, a wavelength range greater than 1.565 μm and less than or equal to 1.625 μm. Furthermore, light in the wavelength band used for communication is light that has a peak within the wavelength range of any of the above wavelength bands (i.e., wavelengths other than the peak may be outside the wavelength range of the above wavelength bands).

[0022] Therefore, the semiconductor photodetector 1 is also intended for the above wavelength range, and is designed to receive incident light L with a wavelength belonging to at least one wavelength range within that range, and to generate an electrical signal in response to the incident light. The semiconductor photodetector 1 is mounted on submount A1. The light L is guided by optical fiber A4 and focused toward the light-receiving portion of the semiconductor photodetector 1 by lens A3.

[0023] The electrical signal generated by the semiconductor photodetector 1 is input to the transimpedance amplifier (TIA) A5 via electrode pads (schematically shown as hatching in Figure 1, etc.) and wires provided on the submount A1. The signal is then converted into a voltage by the transimpedance amplifier A5 and output to the outside. In this case, the semiconductor photodetector 1 is mounted on the submount A1 with the back surface 10b of the substrate 10 (described later) facing the lens A3 and optical fiber A4. In other words, the semiconductor photodetector 1 is used as a back-side incidence type.

[0024] Figure 2 is a plan view of the semiconductor photodetector shown in Figure 1. Figure 3 is a cross-sectional view taken along line III-III in Figure 2. As shown in Figures 2 and 3, the semiconductor photodetector 1 comprises a substrate 10, a semiconductor laminate 20, a first electrode 40 (cathode in this case), and a second electrode 50 (anode in this case).

[0025] The substrate 10 includes a semi-insulating semiconductor. Here, the substrate 10 is a semi-insulating semiconductor substrate made of, for example, InP. The substrate 10 includes a surface 10a and a back surface 10b opposite to the surface 10a. The substrate 10 also includes a plurality of regions RA, region RB (first region), and region RC arranged sequentially along the surface 10a and the back surface 10b. Region RB is the region between region RA and region RC, and is the region where the semiconductor laminate 20 is provided. More specifically, region RB includes a central region RB1 and regions RB2 located on both sides of region RB1 (regions RA and RC sides). Here, the back surface 10b of the substrate 10 is the incident surface of light L, and a lens RL for focusing light L is formed thereon. The lens RL is formed so as to partially overlap region RB2 with region RB1 at the center.

[0026] As described above, the semiconductor stacked portion 20 is formed on region RB of the substrate 10 and is a semiconductor mesa protruding from the surface 10a. The semiconductor stacked portion 20 is a first conductivity type (here it is N-type, and as an example N + It includes a buffer layer 30 of type RB1. The buffer layer 30 is provided so as to overlap region RB2 with region RB1 at its center. Here, the semiconductor stacked portion 20 is in contact with the surface 10a of the substrate 10 at the buffer layer 30.

[0027] The layers of the semiconductor stack 20 other than the buffer layer 30 are provided in the portions that overlap with region RB1 of the buffer layer 30 when viewed from a direction intersecting the surface 10a. The buffer layer 30 has a first portion 31 that is exposed from the other layers of the semiconductor stack 20 (and the protective film 60 described later) when viewed from a direction intersecting the surface 10a, and a junction with the first electrode 40 is formed in this first portion 31. The buffer layer 30 contains, for example, InP. As an example, the buffer layer 30 contains N+ made of InP.

[0028] The semiconductor stacked portion 20 includes buffer layers 21, 22, 23, a light absorbing layer 24, a cap layer 25, and a contact layer 26 stacked on a buffer layer 30 in order from the substrate 10 side. The buffer layers 21 and 22 are of a first conductivity type (e.g., N + type). The buffer layer 23 is of a first conductivity type (e.g., N - type). The buffer layers 21, 22, and 23 contain InAsP. As an example, the buffer layer 21 is N + -InAs 0.05 P, the buffer layer 22 is N + -InAs 0.10 P, and the buffer layer 23 is N - -InAs 0.15 P (alternatively, N - -InGaAsP).

[0029] Thereby, the buffer layers 21, 22, 23 function as strain relaxation layers having a lattice constant between the lattice constant of the substrate 10 and the lattice constant of the light absorbing layer 24. That is, the semiconductor stacked portion 20 includes a plurality of strain relaxation layers (step layers) arranged such that the lattice constant gradually approaches the lattice constant of the light absorbing layer 24 as going from the substrate 10 toward the light absorbing layer 24.

[0030] Further, the buffer layer 23 is arranged closer to the light absorbing layer 24 side than the buffer layers 21 and 22, and has an impurity concentration lower than the impurity concentration of the buffer layers 21 and 22. Therefore, the semiconductor stacked portion 20 includes: a first semiconductor layer (the buffer layer 21 or the buffer layer 22) arranged between the substrate 10 and the light absorbing layer 24; and a second semiconductor layer (the buffer layer 23) that has an impurity concentration lower than the impurity concentration of the first semiconductor layer and is arranged between the first semiconductor layer and the light absorbing layer 24.

[0031] The light absorbing layer 24 is of a first conductivity type (e.g., N - type). The light absorbing layer 24 contains InGaAs. Here, the light absorbing layer 24 is N - -In xGa 1-x It consists of As. The In composition x of the light absorption layer 24 is 0.55 or greater (and less than 1). For example, here the In composition x is 0.59. The thickness of the light absorption layer 24 (thickness along the stacking direction of the semiconductor laminate 20) is 1.8 μm or less, and here, as an example, it is 0.7 μm. The light absorption layer 24 may be a mixed crystal absorption layer of Al, P, Sb, N, other materials and InGaAs, with a band gap of 0.72 eV or less. The proportion of Al, P, Sb, and N (or other materials) mixed with InGaAs can be, for example, 5% or less, or 10% or less.

[0032] The cap layer 25 is a first conductivity type (e.g., N - The cap layer 25 has a type. The cap layer 25 contains InAsP. For example, the cap layer 25 is N - -InAs 0.15 It consists of P. The contact layer 26 is a first conductivity type (e.g., N - The contact layer 26 has a type. The contact layer 26 contains InGaAs. For example, the contact layer 26 is N - -Consists of InGaAs

[0033] The semiconductor stacked portion 20 contains a second conductivity type (here, P + A semiconductor region (second region) 27 of type P is formed. The semiconductor region 27 can be formed, for example, by impurity diffusion or ion implantation. The semiconductor region 27 extends from the top surface 20a of the semiconductor stacked portion 20 toward the substrate 10. Here, the top surface 20a of the semiconductor stacked portion 20 (the surface facing away from the substrate 10) is the surface of the contact layer 26. And P + The semiconductor region 27 of the type is formed to extend from the contact layer 26 through the cap layer 25 to the light absorption layer 24.

[0034] Here, the semiconductor region 27 is also formed within the light absorption layer 24. In the example where the thickness of the light absorption layer 24 is 0.7 μm, the semiconductor region 27 is defined as a range of approximately 0.2 μm on the cap layer 25 side of the light absorption layer 24. In other words, in this example, a 0.5 μm thick N- Region and P with a thickness of 0.2 μm + The region is included, and its boundaries are formed. + The end of the region is, for example, when the P-type impurity concentration is 1 × 10⁻⁶. 17 / cm 3 The position is as follows. However, N - Region and P + The boundary with the region may be formed externally on the light-absorbing layer 24.

[0035] In the above example, N + The type is defined as an N-type impurity concentration of 1 × 10⁻⁶. 17 / cm 3 It means "above a certain degree." - The type refers to the impurity concentration of N-type, which is 8 × 10⁻⁶. 15 / cm 3 It is below a certain level, meaning it is relatively low compared to the N+ type. Also, P + The type refers to a concentration of P-type impurities of 1 × 10⁻⁶. 17 / cm 3 It means exceeding a certain degree.

[0036] Here, the semiconductor photodetector 1 is equipped with a protective film 60. The protective film 60 is, for example, an insulating film. A portion of the top surface 20a of the semiconductor laminate 20, and the side surface 20s of the semiconductor laminate 20 extending from the periphery of the top surface 20a toward the substrate 10 are covered by the protective film 60. On the other hand, the remaining portion of the top surface 20a of the semiconductor laminate 20, in this case P + The surface of the semiconductor region 27 of the type is exposed from the protective film 60. A second electrode 50 is formed on the portion of the top surface 20a that is exposed from the protective film 60, and a junction is formed between the second electrode 50 and the semiconductor region 27 (contact layer 26). In other words, the second electrode 50 is connected to the second portion (semiconductor region 27) of the second conductivity type in the semiconductor stacked portion 20, which is located on the side of the substrate 10 relative to the light absorption layer 24. On the other hand, the first electrode 40 is connected to the first portion 31 (the portion of the buffer layer 30 exposed from the protective film 60) of the first conductivity type in the semiconductor stacked portion 20, which is located on the substrate 10 side relative to the light absorption layer 24.

[0037] Figure 4 is a cross-sectional view along the line IV-IV in Figure 2. As shown in Figures 2 and 4, a semiconductor laminate 70 is formed on the surface 10a of the substrate 10 via a buffer layer 30. The structure of the semiconductor laminate 70 is P + Except for the absence of a specific semiconductor region 27, the configuration is the same as that of the semiconductor stack 20, except for the buffer layer 30. The semiconductor stack 70 is entirely covered by a protective film 60.

[0038] In this configuration, the second electrode 50 extends from the top surface 20a of the semiconductor stacked portion 20 to the top surface 70a of the semiconductor stacked portion 70 (the surface facing away from the substrate 10), and forms an anode pad 55 on the top surface 70a. That is, an anode pad 55 is formed on the top surface 70a of the semiconductor stacked portion 70, electrically connected to the second electrode 50 via a protective film 60.

[0039] Figure 5 is a cross-sectional view along the VV line in Figure 2. As shown in Figures 2 and 5, semiconductor stacked portions 80 and 90 are formed on the surface 10a of the substrate 10 via a buffer layer 30. The structure of the semiconductor stacked portions 80 and 90 is P + Except for the absence of a specific type of semiconductor region 27, the configuration is the same as that of the semiconductor stack 20, except for the buffer layer 30. The semiconductor stacks 80 and 90 are entirely covered by a protective film 60. Here, the first electrode 40 extends from the portion bonded to the buffer layer 30 to the top surface 90a of the semiconductor stack 90 (the surface facing away from the substrate 10), and a cathode pad 45 is formed on the top surface 90a.

[0040] Specifically, a cathode pad 45 is formed on the top surface 90a of the semiconductor stacked portion 90, electrically connected to the first electrode 40 via a protective film 60. On the other hand, a dummy pad 100 is formed on the top surface 80a of the semiconductor stacked portion 80, via a protective film 60. As shown in Figure 2, the cathode pad 45 (and the semiconductor stacked portion 90) are formed in pairs so as to sandwich the anode pad 55 (and the semiconductor stacked portion 70), and the dummy pad 100 (and the semiconductor stacked portion 80) are also formed in pairs.

[0041] In optical device A, the semiconductor photodetector 1 is mounted on submount A1 with the surface 10a of the substrate 10 facing towards submount A1, that is, with the back surface 10b of the substrate 10 facing away from submount A1. As a result, the pair of cathode pads 45, anode pad 55, and pair of dummy pads 100 are connected to the respective electrode pads provided on submount A1. Consequently, the cathode pads 45 and anode pads 55 are connected to electrodes on submount A1 that are electrically connected to the transimpedance amplifier A5.

[0042] As explained above, the semiconductor photodetector 1 is designed for light in the wavelength bands used for optical communication, such as the 1.3 μm band, the 1.55 μm band, and the 1.6 μm band. In the semiconductor photodetector 1, the light absorption layer 24 provided on the semi-insulating semiconductor substrate 10 is In x Ga 1-x It contains As. And the In composition x of the light-absorbing layer 24 is 0.55 or greater (and less than 1). Thus, in the light-absorbing layer 24, In x Ga 1-x If the In composition x of As is set to 0.55 or higher (graph G2 in Figure 6), the absorption coefficient is improved compared to, for example, the case where the In composition x is 0.53 as shown in graph G1 in Figure 6 (in the example in Figure 6, it is improved by about two times in the 1.55 μm band). Note that graph G0 in Figure 6 shows the case when a light absorption layer made of InGaAsP is used.

[0043] Therefore, even if the thickness of the light-absorbing layer 24 is reduced to 1.8 μm or less, a decrease in sensitivity can be avoided. In other words, higher speeds become possible. Furthermore, the semiconductor photodetector 1 does not require the formation of any additional configuration (for example, the oblique reflection portion of the photodiode described in the above-mentioned Patent Document 1) to achieve higher speeds. Thus, the semiconductor photodetector 1 enables higher speeds while suppressing an increase in cost. However, from the perspective of higher speeds, the semiconductor photodetector 1 may be configured such that an oblique optical path is formed in the light-absorbing layer 24 with respect to the thickness direction of the light-absorbing layer 24.

[0044] As shown in graphs G1 and G2 of Figure 6, increasing the In composition x of the light-absorbing layer 24 extends the wavelength of the absorption edge. However, the semiconductor photodetector 1 is characterized not by extending the wavelength of the absorption edge, but by improving the absorption coefficient in the target wavelength bands (the 1.3 μm, 1.55 μm, and 1.6 μm bands used for optical communication mentioned above). In other words, the semiconductor photodetector 1 enables thinning of the light-absorbing layer 24 and high-speed operation by improving the absorption coefficient in the target wavelength bands. To put it another way, in the semiconductor photodetector 1, the combination of the target wavelength band and the specified In composition x and thickness of the light-absorbing layer 24 is important. Furthermore, as the light-absorbing layer 24 is thinned, the influence of the absorption coefficient on the light-receiving sensitivity becomes more pronounced, so a configuration that achieves both a constrained response and high light-receiving sensitivity, as in the semiconductor photodetector 1, is effective.

[0045] On the other hand, if the In composition x of the light-absorbing layer 24 is changed (increased), the difference between the lattice constant of the light-absorbing layer 24 and the lattice constant of the substrate 10 tends to increase, which may lead to deterioration of crystallinity when growing the light-absorbing layer 24 on the substrate 10.

[0046] Therefore, in the semiconductor photodetector 1, the semiconductor stacked portion 20 has buffer layers 21 to 23 that function as strain relaxation layers having a lattice constant between the lattice constant of the substrate 10 and the lattice constant of the light absorption layer 24. This improves the crystallinity of the semiconductor stacked portion 20 including the light absorption layer 24. In particular, in the semiconductor photodetector 1, the buffer layers 21 to 23 function as a plurality of strain relaxation layers arranged so that their lattice constants gradually approach the lattice constant of the light absorption layer 24 as you move from the substrate 10 toward the light absorption layer 24. As a result, the crystallinity of the semiconductor stacked portion 20 is reliably improved.

[0047] Furthermore, in the semiconductor photodetector 1, the semiconductor stacked portion 20 includes a first conductivity type cap layer 25 containing InAsP, which is provided on the light absorption layer 24 on the side opposite to the substrate 10 relative to the light absorption layer 24, and a first conductivity type contact layer 26 containing InGaAs, which is provided on the cap layer 25 on the side opposite to the substrate 10 relative to the light absorption layer 24. The second conductivity type semiconductor region 27 is formed extending from the contact layer 26 through the cap layer 25 to the light absorption layer 24. The portion to which the second electrode 50 is connected is the surface of the semiconductor region 27 formed on the contact layer 26. This reduces the contact resistance of the second electrode 50.

[0048] Furthermore, in the semiconductor photodetector 1, the semiconductor stacked portion 20 includes a first semiconductor layer (buffer layer 21 or buffer layer 22) of a first conductivity type disposed between the substrate 10 and the light absorption layer 24, and a second semiconductor layer (buffer layer 23) of a first conductivity type disposed between the first semiconductor layer and the light absorption layer 24, having an impurity concentration lower than that of the first semiconductor layer. Therefore, further speed increases can be achieved by reducing capacitance.

[0049] Furthermore, in the semiconductor photodetector 1, the substrate 10 includes a semi-insulating semiconductor. If a conductive substrate is used as the substrate 10, the substrate and the semiconductor laminate 20 are electrically conductive and therefore at the same potential. In that case, capacitive coupling between the anode and cathode occurs even through the protective film 60 (insulating film), and no reduction in capacitance can be expected. In contrast, in the semiconductor photodetector 1, the growth layer that becomes the semiconductor laminate 20 can be electrically isolated by etching or other means to a semi-insulating or insulating substrate 10. As a result, capacitive coupling can be prevented, and a reduction in capacitance can be achieved. The substrate 10 can be semi-insulated, for example, by doping InP or GaAs with Fe or the like. Since InP has a matching lattice constant with InGaAs, a well-crystalline InGaAs layer can be directly grown on the semi-insulating substrate 10.

[0050] The embodiments described above illustrate one aspect of the present disclosure. Therefore, the present disclosure can be modified at will without limiting it to the embodiments described above. Modifications will now be described.

[0051] Figure 7 shows a modified example of the mounting method for the semiconductor photodetector. In the optical device B shown in Figure 7(a), the semiconductor photodetector 1 is mounted on the glass substrate B1 such that the back surface 10b of the substrate 10 faces the glass substrate B1, and is electrically connected directly by wires to the transimpedance amplifier A5, which is similarly mounted on the glass substrate B1. The lens A3 and optical fiber A4 are arranged on the side of the glass substrate B1 opposite to the side on which the semiconductor photodetector 1 is mounted, and light L is incident on the semiconductor photodetector 1 from the back surface 10b side via the glass substrate B1. Furthermore, in the optical device C shown in Figure 7(b), the lens A3 is omitted compared to the optical device A shown in Figure 1. In addition, the semiconductor photodetector 1 may be directly mounted on the transimpedance amplifier A5.

[0052] As described above, various configurations are possible for the mounting method of the semiconductor photodetector 1 and the optical device including the semiconductor photodetector 1. In the above examples, the semiconductor photodetector 1 was used as a back-side incidence type. However, the semiconductor photodetector 1 may also be configured as a front-side incidence type. In this case, an opening should be formed so that the light-receiving portion is exposed to the second electrode 50 provided on the light-receiving portion (semiconductor region 27).

[0053] Furthermore, in the above example, the thickness of the light-absorbing layer 24 was given as 0.7 μm, and the In composition x of the light-absorbing layer 24 was given as 0.59. However, the thickness of the light-absorbing layer 24 does not need to be 1.8 μm or less, and the In composition x does not need to be 0.55 or more. In particular, the In composition x of the light-absorbing layer 24 may be 0.57 or more, and the thickness of the light-absorbing layer may be 1.2 μm or less. Moreover, the In composition x of the light-absorbing layer 24 may be 0.59 or more, and the thickness of the light-absorbing layer 24 may be 0.7 μm or less. In these cases, speed can be increased by further thinning of the light-absorbing layer.

[0054] Examples of combinations of each wavelength band, the thickness of the optical absorption layer 24, and the In composition x of the optical absorption layer 24 are listed below. Note that, for example, (5) below can be constructed similarly not only in the C-band band, but also in the O-band and L-band bands.

[0055] (1) C-band. Sensitivity: 0.86 A / W or higher. Cutoff frequency: 20GHz or higher (for 28GB, etc.). Absorption layer thickness: 1.5 μm (Breakdown: N - Area 1.3μm, P + area 0.2μm). In composition x:x=0.55.

[0056] (2) C-band. Sensitivity: 0.90 A / W or higher. Cutoff frequency: 20GHz or higher (for the 28GB high-sensitivity model). Absorption layer thickness: 1.5 μm (Breakdown: N - Area 1.3μm, P + area 0.2μm). In composition x:x=0.57.

[0057] (3) C-band. Sensitivity: 0.80 A / W or higher. Cutoff frequency: 30GHz or higher (for 56GB, etc.). Absorption layer thickness: 1.2 μm (Breakdown: N - Area 1.0μm, P + area 0.2μm). In composition x:x=0.57.

[0058] (4) C-band. Sensitivity of 0.85 A / W or higher. Cutoff frequency: 30GHz or higher (for the 56GB high-sensitivity model). Absorption layer thickness: 1.2 μm (Breakdown: N - Area 1.0μm, P + area 0.2μm). In composition x:x=0.59.

[0059] (5) C-band. Sensitivity of 0.7 A / W or higher. Cutoff frequency: 45GHz or higher (for 96GB, etc.). Absorption layer thickness: 0.7 μm (Breakdown: N - Area 0.5μm, P + area 0.2μm). In composition x:x=0.59.

[0060] (6) C-band. Sensitivity of 0.90 A / W or higher. Cutoff frequency: 16GHz or higher (for 25GB, etc.). Absorption layer thickness: 1.8 μm (Breakdown: N - Area 1.6μm, P + area 0.2μm). In composition x:x=0.55.

[0061] (7) C-band. Sensitivity of 0.93 A / W or higher. Cutoff frequency: 16GHz or higher (for 25GB, etc.). Absorption layer thickness: 1.8 μm (Breakdown: N - Area 1.6μm, P + area 0.2μm). In composition x:x=0.57.

[0062] Furthermore, in the semiconductor photodetector 1, the semiconductor stacked portion 20 may include a third semiconductor layer provided between the light absorption layer 24 and the cap layer 25, having a band gap between the band gap of the light absorption layer 24 and the band gap of the cap layer 25. The third semiconductor layer is a first conductivity type (for example, N - It has the type N - -It consists of InAsGaP. In this case, the rapid change in the band gap between the capping layer 25 and the light-absorbing layer 24 is suppressed, which would make it difficult to extract carriers.

[0063] Furthermore, in the semiconductor photodetector 1, at least one of the buffer layers 21-23,30 may be semi-insulated and thickened by Fe doping. In this case, crystallinity is improved.

[0064] Furthermore, buffer layers 21-23 and 30 may contain InGaAsP (or be composed of InGaAsP) in order to increase the band gap and improve transmittance in the 1.3 μm, 1.55 μm, and 1.6 μm bands, rather than being limited to InAsP. In addition, each layer of the semiconductor stacked portion 20 may contain other elements such as Al.

[0065] Furthermore, in the semiconductor photodetector 1, buffer layers 21-23 were used in which the lattice constant changed in steps as one moved from the substrate 10 toward the light absorption layer 24. However, a strain relaxation layer (buffer layer) may also be used in which the lattice constant continuously approaches the lattice constant of the light absorption layer 24 as one moves from the substrate 10 toward the light absorption layer 24. In the above example, the semiconductor photodetector 1 has a cap layer 25 and a contact layer 26 stacked sequentially on the light absorption layer 24, and the second electrode 50 was connected to the surface of a semiconductor region 27 formed on the contact layer 26. However, in the semiconductor photodetector 1, the cap layer 25 may be omitted, and the contact layer 26 may be formed directly on the light absorption layer 24. Even in this case, the contact resistance of the second electrode 50 can be reduced.

[0066] Furthermore, from the perspective of increasing speed, the above-described light-absorbing layer 24 may be applied to a waveguide-type semiconductor photodetector. In a waveguide-type semiconductor photodetector, a ridge waveguide is formed on a semi-insulating InP substrate, and a light-receiving section including the light-absorbing layer 24 is formed within the ridge waveguide. In this way, even in a waveguide type, by employing a light-absorbing layer 24 with improved absorption, it is possible to shorten the length of the light-receiving surface along the direction of the waveguide extension and reduce capacitance. Also, even with the same thickness, the response is improved by increasing the electron travel speed.

[0067] Furthermore, in the semiconductor photodetector 1, the substrate 10 may be removed by etching or polishing, for example, and the semiconductor laminate 20 may be bonded to a substrate made of an insulator such as quartz or a semi-insulating semiconductor other than InP (for example, gallium arsenide). In other words, in the semiconductor photodetector 1, the substrate 10 may be configured separately from the semiconductor laminate 20 and may be bonded to the substrate 10 (for example, directly). By manufacturing the semiconductor photodetector 1 by configuring the substrate 10 and the semiconductor laminate 20 as separate components and bonding them together, it becomes possible to increase the aperture size and reduce costs by fabricating optical components using inexpensive materials.

[0068] Furthermore, when joining the substrate 10 and the semiconductor laminate 20, which are constructed as separate components, direct bonding or bonding using resin can be employed. When resin is used to bond the substrate 10 and the semiconductor laminate 20, depending on the properties of the resin, there is a possibility that it may absorb light in the target wavelength band, but this possibility is eliminated with direct bonding.

[0069] Furthermore, the semiconductor photodetector according to this disclosure is a semiconductor photodetector for receiving incident light in at least one wavelength band of the 1.3 μm band, 1.55 μm band, and 1.6 μm band, and generating an electrical signal in response to the incident light, comprising a substrate, a semiconductor laminate formed on a first region of the substrate, and a first electrode and a second electrode electrically connected to the semiconductor laminate, wherein the semiconductor laminate is In x Ga 1-x The optical absorbing layer comprises a first conductivity type light-absorbing layer containing As, a first conductivity type buffer layer provided between the substrate and the light-absorbing layer, and a second region of a second conductivity type different from the first conductivity type, located on the opposite side of the substrate from the light-absorbing layer and bonded to the light-absorbing layer. The first electrode is connected to a first portion of the semiconductor laminate that is located on the substrate side of the light-absorbing layer and is of the first conductivity type. The second electrode is connected to a second portion of the semiconductor laminate that is located on the opposite side of the substrate from the light-absorbing layer and is of the second conductivity type. The In composition x in the light-absorbing layer is 0.55 or greater, and the thickness of the light-absorbing layer may be 1.5 μm or less. [Explanation of Symbols]

[0070] 1...Semiconductor photodetector, 20...Semiconductor stacked portion, 21,22...Buffer layer (strain relaxation layer, first semiconductor layer), 23...Buffer layer (strain relaxation layer, second semiconductor layer), 24...Light absorption layer, 25...Cap layer, 26...Contact layer, 27...Semiconductor region (second portion), 31...First portion, 40...First electrode, 50...Second electrode.

Claims

1. A semiconductor photodetector for receiving incident light in at least one wavelength band of 1.3 μm, 1.55 μm, and 1.6 μm, and generating an electrical signal in response to the incident light, circuit board and A semiconductor laminate formed on the first region of the substrate, The first electrode and the second electrode are electrically connected to the semiconductor stack portion, Equipped with, The aforementioned semiconductor stacked portion is In ｘ Ga １－ｘ A first-type conductivity light-absorbing layer containing As, The first conductive buffer layer provided between the substrate and the light-absorbing layer, A second region of a second conductivity type different from the first conductivity type is located on the opposite side of the substrate from the light absorption layer and bonded to the light absorption layer, Includes, The first electrode is connected to the first portion of the semiconductor stack that is located on the substrate side relative to the light absorption layer, The second electrode is connected to the second portion of the semiconductor laminate that is located on the side of the light-absorbing layer away from the substrate, and is of the second conductivity type. The In composition x in the light-absorbing layer is 0.55 or greater. The thickness of the light-absorbing layer is 1.8 μm or less. Semiconductor photodetector.

2. The buffer layer includes a strain relaxation layer having a lattice constant between the lattice constant of the substrate and the lattice constant of the light absorption layer. The semiconductor photodetector according to claim 1.

3. The buffer layer includes a plurality of strain relaxation layers arranged such that their lattice constants gradually approach the lattice constant of the light absorption layer as they move from the substrate toward the light absorption layer. The semiconductor photodetector according to claim 2.

4. The buffer layer includes a strain relaxation layer whose lattice constant is continuously changed as it moves from the substrate toward the light absorption layer, so as it moves toward the light absorption layer, The semiconductor photodetector according to claim 2.

5. The aforementioned semiconductor stacked portion is A first conductive cap layer containing InAsP is provided on the light absorption layer on the side opposite to the substrate with respect to the light absorption layer, A first conductive contact layer containing InGaAs is provided on the cap layer on the side opposite to the substrate relative to the light absorption layer, Includes, The second region is formed extending from the contact layer through the cap layer to the light absorption layer, The second portion to which the second electrode is connected is the surface of the second region formed in the contact layer. A semiconductor photodetector according to any one of claims 1 to 4.

6. The aforementioned semiconductor stacked portion is The first semiconductor layer of the first conductivity type is disposed between the substrate and the light absorption layer, A second semiconductor layer of the first conductivity type having an impurity concentration lower than that of the first semiconductor layer and disposed between the first semiconductor layer and the light absorption layer, including, A semiconductor photodetector according to any one of claims 1 to 5.

7. The semiconductor stacked portion includes a third semiconductor layer provided between the light-absorbing layer and the cap layer, having a band gap between the band gap of the light-absorbing layer and the band gap of the cap layer. The semiconductor photodetector according to claim 5.

8. At least one layer of the buffer layer is semi-insulated by Fe doping. A semiconductor photodetector according to any one of claims 1 to 7.

9. The In composition x in the light-absorbing layer is 0.57 or higher. The thickness of the light-absorbing layer is 1.2 μm or less. A semiconductor photodetector according to any one of claims 1 to 7.

10. The In composition x in the light-absorbing layer is 0.59 or higher. The thickness of the light-absorbing layer is 0.7 μm or less. A semiconductor photodetector according to any one of claims 1 to 8.

11. The substrate includes a semi-insulating semiconductor. A semiconductor photodetector according to any one of claims 1 to 10.

12. The substrate includes an insulator or a semi-insulating semiconductor. The semiconductor stacked portion is bonded to the substrate. A semiconductor photodetector according to any one of claims 1 to 10.

13. It is either a back-side incidence type or a front-side incidence type. A semiconductor photodetector according to any one of claims 1 to 12.

14. The light-absorbing layer consists of a single semiconductor layer containing InGaAs. A semiconductor photodetector according to any one of claims 1 to 13.

15. The second region is formed throughout the interior of the light-absorbing layer, The thickness of the region of the first conductivity type in the light absorption layer is 0.5 μm or more. A semiconductor photodetector according to any one of claims 1 to 14.

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