Avalanche photodiode

Abstract

An avalanche photodiode (100) is provided with: a semiconductor substrate (1) having a first surface; and an avalanche multiplication layer (3), an electric field control layer (4), a first transition layer (5), a second transition layer (6), and a light absorption layer (7), which are sequentially laminated on the first surface from the semiconductor substrate side. The second transition layer and the electric field control layer each contain a dopant of the first conductivity type. The first transition layer does not contain a dopant, or contains a dopant of a first conductivity type or a second conductivity type different from the first conductivity type. The carrier concentration of the first transition layer is lower than the carrier concentration of the second transition layer.

Description

Technical Field

[0001] The present disclosure relates to avalanche photodiodes. Background Art

[0002] In the field of optical communications, especially in receiving-side devices for long-distance transmission, avalanche photodiodes are used. Avalanche photodiodes comprise an absorption layer, an electric field control layer, and an avalanche multiplication layer. When light strikes the absorption layer, electron-hole pairs (photogenerated carriers) are generated within the absorption layer. Electrons or holes that reach the avalanche multiplication layer are avalanche-multiplied by the avalanche effect. Therefore, avalanche photodiodes can amplify the incident light and extract it as a signal.

[0003] In the field of optical communications, communication speeds are increasing, and avalanche photodiodes are required to have high-speed responsiveness capable of receiving signals with baud rates exceeding 50 GHz.

[0004] On the other hand, a typical avalanche photodiode has a characteristic that the higher the gain (multiplication factor), the lower the receiving sensitivity in the high-frequency band. This is because the material constituting the avalanche multiplying layer in the avalanche photodiode has a gain-bandwidth product value that is unique to the material.

[0005] In order to achieve a relatively high gain, such as 10 dB, while also improving response speed in an avalanche photodiode, it is desirable to use a material that achieves a large gain-bandwidth product for the avalanche multiplying layer. Non-Patent Document 1 below reports that, under high-gain driving conditions, an avalanche photodiode with an avalanche multiplying layer made of AlGaAsSb exhibits a high gain-bandwidth product of 424 GHz.

[0006] Prior art literature

[0007] Non-patent literature

[0008] Non-patent document 1: Shiyu Xie, et al. "InGaAs / AlGaAsSb avalanche photodiode with high gain-bandwidth product", Optics Express vol. 24, No. 21 (2016) 24242. Summary of the Invention

[0009] However, when the energy band barrier at the heterojunction interface formed between the light-absorbing layer and the avalanche multiplying layer is large, photocarriers tend to be trapped at the heterojunction interface, resulting in a problem of reduced response speed. The present inventors have confirmed that this problem is particularly pronounced when an avalanche photodiode having an avalanche multiplying layer made of a material capable of achieving a large gain-band product is driven under conditions designed to achieve a gain lower than the rated gain (hereinafter referred to as low-gain conditions).

[0010] A main object of the present disclosure is to provide an avalanche photodiode that can suppress a decrease in response speed even under low-gain conditions.

[0011] The avalanche photodiode disclosed herein comprises: a semiconductor substrate having a first surface; and an avalanche multiplying layer, an electric field control layer, a first transition layer, a second transition layer, and a light absorption layer stacked sequentially on the first surface from the semiconductor substrate side. The second transition layer and the electric field control layer each contain a dopant of the first conductivity type. The first transition layer contains no dopant or contains a dopant of the first conductivity type or a second conductivity type different from the first conductivity type. The carrier concentration of the first transition layer is lower than the carrier concentration of the second transition layer.

[0012] According to the present disclosure, it is possible to provide an avalanche photodiode capable of suppressing a decrease in response speed even under low-gain conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a cross-sectional view for explaining the avalanche photodiode according to the first embodiment.

[0014] Figure 2 This is a flowchart of the method for manufacturing the avalanche photodiode according to the first embodiment.

[0015] Figure 3 This is a graph showing calculation results regarding the relationship between the multiplication factor (gain) and the bandwidth of the avalanche photodiode according to the first embodiment.

[0016] Figure 4A This is a schematic diagram for explaining the stacked structure of the avalanche photodiode according to the first embodiment.

[0017] Figure 4B Schematic diagram showing the electric field intensity distribution of the avalanche photodiode according to the first embodiment.

[0018] Figure 5A This is a schematic diagram for explaining the stacked structure of the avalanche photodiode according to the second embodiment.

[0019] Figure 5B It shows Figure 5A Schematic diagram of the electric field intensity distribution of the avalanche photodiode shown.

[0020] Figure 6A This is a schematic diagram for explaining a modified example of the stacked structure of the avalanche photodiode according to the second embodiment.

[0021] Figure 6B It shows Figure 6A Schematic diagram of a modified example of the electric field intensity distribution of the avalanche photodiode shown.

[0022] Figure 7 It is a cross-sectional view for explaining an avalanche photodiode according to a comparative example.

[0023] Figure 8 yes Figure 7 The energy band diagram of the avalanche photodiode according to the comparative example is shown.

[0024] Figure 9 is shown with Figure 7 The graph shown is a calculation result regarding the relationship between the multiplication factor (gain) and the frequency band of the avalanche photodiode according to the comparative example.

[0025] Figure 10A Is used to illustrate Figure 7 FIG. 1 is a schematic diagram of a stacked structure of an avalanche photodiode according to a comparative example.

[0026] Figure 10B It shows Figure 10A Schematic diagram of the electric field intensity distribution of the avalanche photodiode shown. DETAILED DESCRIPTION

[0027] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. In the following drawings, identical or corresponding parts are denoted by the same reference numerals, and their description will not be repeated.

[0028] Implementation method 1.

[0029] <Structure of Avalanche Photodiode>

[0030] like Figure 1 As shown, the avalanche photodiode 100 involved in embodiment 1 includes a semiconductor substrate 1, a buffer layer 2, an avalanche multiplying layer 3, an electric field control layer 4, a first transition layer 5, a second transition layer 6, a light absorption layer 7, a window layer 8, a contact region 9, a protective film 10, a contact layer 11, a first electrode 12, and a second electrode 13.

[0031] In the avalanche photodiode 100, the electric field control layer 4, the second transition layer 6, and the contact region 9 have a first conductivity type, and the semiconductor substrate 1, the buffer layer 2, and the window layer 8 have a second conductivity type different from the first conductivity type. In this specification, "having a conductivity type" means that the n-type carrier concentration or the p-type carrier concentration is 1×10 16 atoms / cm 3 For example, the first conductivity type is p-type and the second conductivity type is n-type.

[0032] In the avalanche photodiode 100, the avalanche multiplying layer 3, the first transition layer 5, and the light absorbing layer 7 are i-type semiconductors that do not contain dopants (hereinafter sometimes referred to as undoped). In this specification, "doping does not contain dopants that are intentionally added, and the n-type carrier concentration or the p-type carrier concentration is less than 1×10 16 atoms / cm 3 .

[0033] Semiconductor substrate 1 is, for example, an n-type compound semiconductor substrate. The material constituting semiconductor substrate 1 is, for example, InP. Semiconductor substrate 1 has a first surface 1A and a second surface 1B located opposite first surface 1A. A buffer layer 2, an avalanche multiplying layer 3, an electric field control layer 4, a first transition layer 5, a second transition layer 6, a light absorption layer 7, and a window layer 8 are stacked sequentially on first surface 1A from the semiconductor substrate 1 side.

[0034] The buffer layer 2 is, for example, an epitaxial layer made of an n-type compound semiconductor. The material constituting the buffer layer 2 is, for example, AlGaAsSb. The carrier concentration of the buffer layer 2 is, for example, 5×10 18 atoms / cm 3 The thickness of the buffer layer 2 is, for example, not less than 0.10 μm and not more than 1.00 μm.

[0035] The avalanche multiplying layer 3 is an epitaxial layer composed of an i-type compound semiconductor. Preferably, the material constituting the avalanche multiplying layer 3 is a compound semiconductor containing antimony (Sb). More preferably, the material constituting the avalanche multiplying layer 3 is AlGaAsSb. The avalanche multiplying layer 3 may also have a thickness of 0.1×10 15 atoms / cm 3 Above and 8×10 15 atoms / cm 3 The thickness of the avalanche multiplying layer 3 is, for example, not less than 0.05 μm and not more than 0.50 μm.

[0036] The electric field control layer 4 is, for example, an epitaxial layer made of a p-type compound semiconductor. The material constituting the electric field control layer 4 is, for example, AlGaAsSb. The carrier concentration of the electric field control layer 4 is, for example, 2×10 17 atoms / cm 3 The thickness of the electric field control layer 4 is, for example, not less than 0.01 μm and not more than 0.20 μm.

[0037] The first transition layer 5 is an epitaxial layer composed of an i-type quaternary compound semiconductor mixed crystal. Preferably, the first transition layer 5 is a graded layer whose composition ratio changes in the stacking direction. The compound semiconductor mixed crystal constituting the first transition layer 5 is, for example, AlGaAsSb. In this case, the composition ratio of the compound semiconductor mixed crystal constituting the first transition layer 5 is preferably adjusted so that the aluminum (Al) composition ratio decreases as it moves away from the electric field control layer 4 in the above-mentioned stacking direction. In other words, the composition ratio of the compound semiconductor mixed crystal constituting the first transition layer 5 is preferably adjusted so that the aluminum (Al) composition ratio decreases as it approaches the second transition layer 6 in the above-mentioned stacking direction. The thickness of the first transition layer 5 is, for example, not less than 0.01 μm and not more than 0.20 μm.

[0038] The second transition layer 6 is an epitaxial layer composed of a p-type quaternary compound semiconductor mixed crystal. That is, the carrier concentration of the first transition layer 5 is lower than the carrier concentration of the second transition layer 6. The compound semiconductor mixed crystal constituting the second transition layer 6 is different from the compound semiconductor mixed crystal constituting the first transition layer 5. As a result, a band discontinuity (heterogeneous barrier) is formed between the first transition layer 5 and the second transition layer 6. Preferably, the second transition layer 6 is a graded layer in which the composition ratio changes in the stacking direction. The compound semiconductor mixed crystal constituting the second transition layer 6 is, for example, InGaAlAs. In this case, the composition ratio of the compound semiconductor mixed crystal constituting the second transition layer 6 is preferably adjusted so that the aluminum (Al) composition ratio increases the further away from the light absorption layer 7 in the stacking direction. In other words, the composition ratio of the compound semiconductor mixed crystal constituting the second transition layer 6 is preferably adjusted so that the aluminum (Al) composition ratio decreases as it moves away from the first transition layer 5 in the stacking direction. The thickness of the second transition layer 6 is, for example, not less than 0.01 μm and not more than 0.20 μm.

[0039] Furthermore, the composition ratios of the compound semiconductor mixed crystal constituting the first buffer layer 5 and the compound semiconductor mixed crystal constituting the second buffer layer 6 may be constant in the stacking direction.

[0040] The light absorbing layer 7 is an epitaxial layer composed of an i-type compound semiconductor. The material constituting the light absorbing layer 7 is, for example, InGaAs. The thickness of the light absorbing layer 7 is, for example, not less than 0.30 μm and not more than 2.50 μm.

[0041] The window layer 8 is an epitaxial layer composed of an n-type compound semiconductor. The material constituting the window layer 8 is, for example, InP. The carrier concentration of the window layer 8 is, for example, 3×10 16 atoms / cm 3 The thickness of the window layer 8 is, for example, 0.50 μm or more and 2.00 μm or less. The window layer 8 may be an epitaxial layer composed of an i-type compound semiconductor.

[0042] Contact region 9 is a p-type region formed by diffusing a p-type dopant from the upper surface of window layer 8 into light absorbing layer 7 in the stacking direction. The lower end of contact region 9 is formed within light absorbing layer 7. The p-type dopant diffused into contact region 9 is, for example, zinc (Zn).

[0043] A protective film 10 is formed on the upper surface of the window layer 8. The protective film 10 is formed to cover the area of ​​the upper surface of the window layer 8 where the contact layer 11 is not formed. Preferably, the protective film 10 is configured to function as a passivation film and also as an antireflection film. The material constituting the protective film 10 includes, for example, silicon nitride (SiN). Preferably, the protective film 10 is configured such that its thickness d, refractive index n, and wavelength λ of incident light satisfy the relationship d = λ / 4 / n.

[0044] The contact layer 11 is formed by patterning an epitaxial layer made of a p-type compound semiconductor formed on the upper surface of the window layer 8 . The contact layer 11 is electrically connected to the contact region 9 .

[0045] The first electrode 12 is formed on the upper surface of the contact layer 11 and is electrically connected to the contact region 9 via the contact layer 11. The first electrode 12 is a so-called p-side electrode. The planar shape of each of the contact layer 11 and the first electrode 12 is, for example, an annular shape.

[0046] The second electrode 13 is formed on the second surface 1B of the semiconductor substrate 1 and is electrically connected to the buffer layer 2 via the semiconductor substrate 1. The second electrode 13 is a so-called n-side electrode.

[0047] <Method for Manufacturing Avalanche Photodiode>

[0048] Below, refer to Figure 2 An example of a method for manufacturing the avalanche photodiode 100 will be described. First, a semiconductor substrate 1 having a first surface 1A is prepared (step S1).

[0049] Second, a buffer layer 2, an avalanche multiplying layer 3, an electric field control layer 4, a first transition layer 5, a second transition layer 6, a light absorption layer 7, and a window layer 8 are sequentially formed on the first surface 1A from the semiconductor substrate 1 side. Furthermore, an epitaxial layer, before being patterned into the contact layer 11, is formed on the upper surface of the window layer 8 (step S2). The method for forming each layer is not particularly limited, and examples include MOCVD (metalorganic chemical vapor deposition) and MBE (molecular beam epitaxy).

[0050] Third, the p-type dopant is diffused into a portion of each of the light absorbing layer 7 and the window layer 8 when viewed from above, thereby forming the contact region 9 in the light absorbing layer 7 and the window layer 8 (step S3). The method for diffusing the p-type dopant is, for example, selective thermal diffusion. In this case, an insulating film having an opening formed only in the area where the contact region 9 is to be formed is formed on the upper surface of the epitaxial layer before being patterned into the contact layer 11. Thereafter, the p-type dopant is diffused using this insulating film as a mask. The planar shape of the opening is, for example, circular. After the diffusion is completed, the insulating film is removed.

[0051] Fourthly, the epitaxial layer is patterned to form the contact layer 11 (step S4).

[0052] Fifth, a protective film 10 is formed on the upper surface of the window layer 8. The protective film 10 is formed, for example, by forming a film so as to cover the contact layer 11 and then removing a portion formed on the upper surface of the contact layer 11 (step S5).

[0053] Sixth, the first electrode 12 is formed on the upper surface of the contact layer 11 (step S6 ).

[0054] Seventh, after the second surface 1B is formed by polishing the surface of the semiconductor substrate 1 opposite to the first surface 1A, the second electrode 13 is formed on the second surface 1B (step S7 ).

[0055] As described above, the avalanche photodiode 100 can be manufactured.

[0056] <Avalanche Photodiode Operation>

[0057] The following describes the operation of the avalanche photodiode 100. A reverse bias voltage is applied externally to the avalanche photodiode 100, with the second electrode 13 side being positive and the first electrode 12 side being negative. This reverse bias voltage is set so that when electrons generated by incident light in the light absorption layer 7 reach the avalanche multiplication layer 3, avalanche multiplication occurs in the avalanche multiplication layer 3. When light is incident on the light absorption layer 7 while this reverse bias voltage is applied, electron-hole pairs are generated in the light absorption layer 7. The generated electrons pass through the electric field control layer 4 and reach the avalanche multiplication layer 3, to which a high electric field is applied that causes avalanche multiplication. This causes avalanche multiplication in the avalanche multiplication layer 3, and the electrons that reach the avalanche multiplication layer 3 are avalanche-multiplied. Thus, when light is incident on the light absorption layer 7, a large current flows through the avalanche photodiode 100. When light is not incident on the light absorbing layer 7 , avalanche multiplication does not occur in the avalanche multiplication layer 3 , and no current flows in the avalanche photodiode 100 .

[0058] Furthermore, when the intensity of light incident on the avalanche photodiode 100 is weak, the avalanche photodiode 100 is driven under conditions that achieve the rated gain, and a voltage that achieves the rated gain is applied to the avalanche multiplying layer 3. Compared to this driving state, when the intensity of light incident on the avalanche photodiode 100 increases, it is necessary to drive the avalanche photodiode 100 under low gain conditions.

[0059] <Effects of Avalanche Photodiodes>

[0060] The effects of the avalanche photodiode 100 will be described below based on comparison with a comparative example. Figure 7 The avalanche photodiode 200 according to the comparative example shown differs from the avalanche photodiode 100 only in that it includes a third transition layer 61 composed of an i-type compound semiconductor mixed crystal instead of the second transition layer 6 composed of a p-type compound semiconductor mixed crystal. In other words, the avalanche photodiode 200 differs from the avalanche photodiode 100 in that it includes two undoped graded layers (the first transition layer 5 and the third transition layer 61) composed of different compound semiconductor mixed crystals between the light absorption layer 7 and the avalanche multiplication layer 3. The semiconductor mixed crystal constituting the third transition layer 61 is, for example, InGaAlAs. Figure 8 yes Figure 7 The energy band diagram of the avalanche photodiode 200 is shown.

[0061] In the avalanche photodiode 200, the first transition layer 5 and the third transition layer 61 are provided to reduce the energy band discontinuity (heterojunction barrier) between the light absorbing layer 7 and the avalanche multiplying layer 3. Therefore, the energy band discontinuity between the light absorbing layer 7 and the avalanche multiplying layer 3 in the avalanche photodiode 200 is smaller than that of an avalanche photodiode that does not include the first transition layer 5 and the third transition layer 61 between the light absorbing layer 7 and the avalanche multiplying layer 3. On the other hand, in the avalanche photodiode 200, the formation of a heterojunction interface between the light absorbing layer 7 and the avalanche multiplying layer 3, specifically, between the first transition layer 5 and the third transition layer 61, is unavoidable.

[0062] like Figure 8 As shown, at the heterojunction interface between the first transition layer 5 and the third transition layer 61, the energy levels of the valence band and the conduction band become discontinuous, forming a band discontinuity (heterobarrier). This band discontinuity causes photocarriers generated by light absorption or avalanche multiplication to be retained at the heterojunction interface, which is the main cause of the slowing of the photoresponse, in other words, the reduction of receiving sensitivity in the high-frequency band. It is difficult to achieve a first transition layer 5 and a third transition layer 61 that can reduce the size of this band discontinuity to a level that can suppress the retention of carriers at the heterojunction interface and that is lattice-matched with the semiconductor substrate 1. Therefore, in the avalanche photodiode 200, the response speed is reduced due to the band discontinuity described above. The present inventors have confirmed from the calculation results shown below that this problem is particularly significant under low-gain conditions.

[0063] Figure 9 Graph showing the calculation results of the gain (multiplication factor) dependence of the frequency band of the avalanche photodiode 200. The multiplication factor is changed by changing the bias voltage. The frequency band is calculated by calculating the transient response of the photocurrent to short pulse incident light using a semiconductor simulator and performing Fourier transform on the waveform. Figure 9 As shown, under the condition of a multiplication factor of less than 10, a tendency was observed that the frequency band became lower as the multiplication factor decreased. This tendency was particularly pronounced under the condition of a multiplication factor of 6 or less.

[0064] Figure 9 This tendency differs from that of an avalanche photodiode with an InAlAs multiplication layer. In an avalanche photodiode with an InAlAs multiplication layer, the low-gain band exhibits a flat characteristic limited by the CR time constant. The mechanism by which the band decreases with decreasing multiplication rate in avalanche photodiode 200 is considered as follows.

[0065] Figure 10A is a schematic diagram of the stacked structure of the avalanche photodiode 200. Figure 10B The electric field intensity distribution of the avalanche photodiode 200 corresponding to the multiplication factor is shown. Figure 10BIn FIG. 1 , the solid line represents the electric field intensity distribution under the condition of a multiplication factor of 4, the dotted line represents the electric field intensity distribution under the condition of a multiplication factor of 10, and the single-dot chain line represents the position of the interface 15. When a reverse bias voltage is applied to the avalanche photodiode 200, a depletion layer is formed from the avalanche multiplying layer 3 to the portion other than the contact region 9 in the light absorption layer 7, and a depletion layer is generated. Figure 10B The electric field intensity distribution shown by the solid line and the dotted line is small in the undepleted semiconductor substrate 1, the buffer layer 2, and the contact region 9.

[0066] The location where photocarriers are likely to be retained is the interface 15 between the first transition layer 5 and the third transition layer 61, where the energy band discontinuity is large. Figure 10B As shown, the electric field intensity B of the interface 15 when the multiplication factor is 4 is smaller than the electric field intensity C of the interface 15 when the multiplication factor is 10. If the electric field intensity B of the interface 15 when the multiplication factor is 4 is not large enough to eliminate the retention of photocarriers, and the electric field intensity C of the interface 15 when the multiplication factor is 10 is large enough to eliminate the retention of photocarriers, then Figure 9 As shown, the frequency band when the multiplication ratio is 4 is lower than the frequency band when the multiplication ratio is 10.

[0067] The above calculation results suggest that even an avalanche photodiode that exhibits a high gain-bandwidth product under high gain conditions may experience reduced reception sensitivity and response speed in a high-frequency band under low gain conditions.

[0068] In contrast, the avalanche photodiode 100 according to the first embodiment includes an i-type first buffer layer 5 and a p-type second buffer layer 6 between the light absorption layer 7 and the avalanche multiplication layer 3 .

[0069] Figure 3 is a graph showing the calculation results of the gain (multiplication factor) dependence of the avalanche photodiode 100 on the frequency band. Figure 3 The calculation method used is the same as that used to obtain Figure 9 The calculation method used is the same as above. Figure 3 As shown, in the avalanche photodiode 100, no tendency was observed in which the frequency band decreased as the multiplication ratio decreased under the condition of a multiplication ratio lower than 10. The frequency bands under the condition of a multiplication ratio of 4 and the frequency bands under the condition of a multiplication ratio of 6 were both at least equal to the frequency band under the condition of a multiplication ratio of 10. The mechanism by which the tendency of the frequency band decreasing as the multiplication ratio decreased, which was observed in the avalanche photodiode 200, is eliminated in the avalanche photodiode 100 is considered as follows.

[0070] Figure 4A is a schematic diagram of the stacked structure of the avalanche photodiode 100. Figure 4BThe electric field intensity distribution of the avalanche photodiode 100 under the condition of a multiplication factor of 4 is shown. In the avalanche photodiode 100, when a reverse bias voltage is applied, a depletion layer is formed from the avalanche multiplication layer 3 to the portion outside the contact region 9 in the light absorption layer 7. In the avalanche photodiode 100, the location where photocarriers are easily trapped is the interface 14 between the first transition layer 5 and the second transition layer 6, where the band discontinuity is large. Figure 4B In FIG, the dot-dashed line indicates the position of the interface 14. The second transition layer 6 is doped to p-type, so the electric field intensity of the second transition layer 6 is greater than the electric field intensity of the light absorbing layer 7 and greater than the electric field intensity of the third transition layer 61. As a result, Figure 4B As shown, the electric field intensity A of the interface 14 when the multiplication factor is 4 is greater than Figure 10B The electric field intensity B at the interface 15 when the multiplication factor is 4 in the avalanche photodiode 200 is shown. Therefore, in the avalanche photodiode 100, by adjusting the carrier concentration and thickness of the second buffer layer 6, the electric field intensity A at the interface 14 when the multiplication factor is 4 can be increased to a level that eliminates the retention of photocarriers. As a result, in the avalanche photodiode 100, a decrease in response speed can be suppressed even under low-gain conditions.

[0071] Furthermore, in the avalanche photodiode 200, the band gap of the undoped light-absorbing layer 7 is small, and the electric field intensity at the interface 15 is approximately the same as the electric field intensity in the light-absorbing layer 7. Therefore, dark current due to tunneling current is likely to occur. In contrast, in the avalanche photodiode 100, the electric field intensity at the interface 14 is greater than the electric field intensity in the undoped light-absorbing layer 7. Therefore, the aforementioned dark current is less likely to occur than in the avalanche photodiode 200.

[0072] Implementation method 2.

[0073] The avalanche photodiode according to the second embodiment has substantially the same structure and effects as the avalanche photodiode 100 according to the first embodiment, but differs from the avalanche photodiode 100 in that the first transition layer 5 has the first conductivity type or the second conductivity type. The following mainly describes the differences between the avalanche photodiode 200 and the avalanche photodiode 100.

[0074] Figure 5A Schematic diagram of the stacked structure of an avalanche photodiode including an n-type first transition layer 5n. Figure 5B Show Figure 5A The electric field intensity distribution of the avalanche photodiode is shown in Figure 2. Figure 5B In FIG, a dashed line indicates the position of the interface 14. The carrier concentration of the first transition layer 5n is lower than the carrier concentration of the second transition layer 6.

[0075] Figure 6ASchematic diagram of the stacked structure of an avalanche photodiode including a p-type first buffer layer 5p. Figure 6B Show Figure 6A The electric field intensity distribution of the avalanche photodiode is shown in Figure 2. Figure 6B In FIG, a dashed line indicates the position of the interface 14. The carrier concentration of the first transition layer 5p is lower than the carrier concentration of the second transition layer 6.

[0076] like Figure 5B As shown, the electric field strength at the interface 14 between the first transition layer 5n and the second transition layer 6 can be greater than Figure 10B The electric field intensity at the interface 15 between the first transition layer 5 and the third transition layer 61 in the avalanche photodiode 200 shown in FIG. Figure 6B As shown, the electric field strength at the interface 14 between the first transition layer 5p and the second transition layer 6 can be greater than Figure 10B The electric field intensity at the interface 15 between the first buffer layer 5 and the third buffer layer 61 in the avalanche photodiode 200 is shown. Therefore, the avalanche photodiode according to the second embodiment can also achieve the same effects as the avalanche photodiode 100 according to the first embodiment.

[0077] Furthermore, if Figure 5B As shown, the electric field intensity at the interface 14 between the first transition layer 5n and the second transition layer 6 is greater than the electric field intensity of the undoped light absorbing layer 7. Figure 6B As shown, the electric field intensity at the interface 14 between the first buffer layer 5p and the second buffer layer 6 is greater than that of the undoped light absorbing layer 7. Therefore, the avalanche photodiode according to the second embodiment is less likely to generate the aforementioned dark current than the avalanche photodiode 200.

[0078] In an avalanche photodiode having a first transition layer 5n, it is preferable that the first relationship of |T2×C2|>|T1×C1| holds true. T1 represents the thickness of the first transition layer 5n, C1 represents the carrier concentration of the first transition layer 5n, T2 represents the thickness of the second transition layer 6, and C2 represents the carrier concentration of the second transition layer 6.

[0079] like Figure 5B As shown, the electric field intensity at the interface between the electric field control layer 4 and the first transition layer 5n is lower than the electric field intensity at the interface 14. Therefore, if the first transition layer 5n is too thick or the carrier concentration in the first transition layer 5n is too high, the electric field intensity at the interface between the electric field control layer 4 and the first transition layer 5n may decrease to the same level as the electric field intensity at the light absorption layer 7, thereby hindering carrier transport. In contrast, when the first relationship above holds, the electric field intensity at the interface between the electric field control layer 4 and the first transition layer 5n is higher than the electric field intensity at the light absorption layer 7, and thus carrier transport is less likely to be hindered.

[0080] In an avalanche photodiode having a first transition layer 5p, the second relationship preferably holds: |T3×C3|>|T2×C2|>|T1×C1|. T1 represents the thickness of the first transition layer 5p, C1 represents the carrier concentration of the first transition layer 5p, T2 represents the thickness of the second transition layer 6, C2 represents the carrier concentration of the second transition layer 6, T3 represents the thickness of the electric field control layer 4, and C3 represents the carrier concentration of the electric field control layer 4.

[0081] like Figure 6B As shown in FIG. 1 , the electric field intensity at the interface between the electric field control layer 4 and the first transition layer 5p is greater than the electric field intensity at the interface 14. Therefore, when the thickness of the first transition layer 5p is too thick or the carrier concentration of the first transition layer 5p is too high, the change in the electric field intensity of the first transition layer 5p ( Figure 6B If the slope of the solid line in FIG1 becomes greater than the change in the electric field intensity of the second transition layer 6, the above-mentioned effect achieved by increasing the bonding strength at the interface 14 will be impaired. In contrast, when the second relational expression is satisfied, the change in the electric field intensity of the first transition layer 5p becomes less than the change in the electric field intensity of the second transition layer 6, thereby ensuring the above-mentioned effect.

[0082] <Modification>

[0083] The avalanche photodiode 100 according to Embodiment 1 or the avalanche photodiode according to Embodiment 2 can be modified as follows.

[0084] The materials constituting each layer are not limited to the above-mentioned materials. In addition, each layer having a conductivity type may also have a conductivity type different from the above-mentioned conductivity type.

[0085] For example, the buffer layer 2 may be made of InP. The avalanche multiplying layer 3 may be made of AlInAs or AlAsSb. The electric field control layer 4 may be made of AlGaInAs. The light absorption layer 7 may be made of InGaAsP. The window layer 8 may be made of InGaAsP, InGaAsP, AlGaInAs, or AlInAs.

[0086] Alternatively, the buffer layer 2 may be a contact layer made of n-type InGaAs or the like, the semiconductor substrate 1 may be a semi-insulating substrate such as an Fe-doped substrate, and the second electrode 13 may be provided on the surface side of the semiconductor substrate 1 .

[0087] As described above, the embodiments of the present disclosure are described, but various modifications can be made to the above embodiments. In addition, the scope of the present disclosure is not limited to the above embodiments. The scope of the present disclosure is indicated by the claims, and it is intended to include all changes within the meaning and scope equivalent to the claims.

[0088] (Explanation of Reference Numerals)

[0089] 1: semiconductor substrate; 1A: first surface; 1B: second surface; 2: buffer layer; 3: avalanche multiplication layer; 4: electric field control layer; 5, 5n, 5p: first transition layer; 6: second transition layer; 7: light absorption layer; 8: window layer; 9: contact area; 10: protective film; 11: contact layer; 12: first electrode; 13: second electrode; 14: interface; 100: avalanche photodiode.

Claims

1. An avalanche photodiode comprising: a semiconductor substrate having a first surface; and An avalanche multiplying layer, an electric field control layer, a first transition layer, a second transition layer, and a light absorbing layer are sequentially stacked on the first surface from the semiconductor substrate side. The second transition layer and the electric field control layer each contain a dopant of the first conductivity type. The first transition layer does not contain dopants, or contains dopants of the first conductivity type or a second conductivity type different from the first conductivity type. The carrier concentration of the first transition layer is lower than the carrier concentration of the second transition layer.

2. The avalanche photodiode according to claim 1, wherein: The material constituting the avalanche multiplying layer is a compound semiconductor containing antimony (Sb).

3. The avalanche photodiode according to claim 1 or 2, wherein: The material constituting the first transition layer is a first semiconductor mixed crystal, The material constituting the second transition layer is a second semiconductor mixed crystal different from the first semiconductor mixed crystal.

4. The avalanche photodiode according to claim 3, wherein: The first semiconductor mixed crystal is AlGaAsSb, The second semiconductor mixed crystal is InGaAlAs.

5. The avalanche photodiode according to any one of claims 1 to 4, wherein: The first transition layer contains the second conductivity type dopant, When the thickness of the first transition layer is set to T1, the carrier concentration of the first transition layer is set to C1, the thickness of the second transition layer is set to T2, and the carrier concentration of the second transition layer is set to C2, the relationship |T2×C2|>|T1×C1| holds.

6. The avalanche photodiode according to any one of claims 1 to 4, wherein: The first transition layer contains the first conductive type dopant, When the thickness of the first transition layer is set to T1, the carrier concentration of the first transition layer is set to C1, the thickness of the second transition layer is set to T2, the carrier concentration of the second transition layer, the thickness of the electric field control layer is set to T3, and the carrier concentration of the electric field control layer is set to C3, the relationship |T3×C3|>|T2×C2|>|T1×C1| holds.

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