Light-receiving element

Abstract

This light-receiving element comprises: a p-type contact layer (102) composed of a p-type semiconductor formed on a substrate (101); a light-absorbing layer (103) composed of a semiconductor formed on the p-type contact layer (102); and an n-type contact layer (104) composed of an n-type semiconductor formed on the light-absorbing layer (103). Furthermore, the light-receiving element comprises a p-electrode (121) connected to the p-type contact layer (102), and an n-electrode (122) connected to the n-type contact layer (104). The n-type contact layer (104) is formed in an area smaller than that of the light-absorbing layer (103) and is disposed in the formation region of the light-absorbing layer (103).

Description

Light receiving element

[0001] The present invention relates to a light receiving element.

[0002] Light receiving elements are used in a wide range of fields, including optical communications and sensing. For example, photodiodes (PDs) for communications, which require high-speed operation, avalanche photodiodes (APDs), and Geiger-mode APDs, which require high gain, are in practical use.

[0003] For example, a Geiger-mode APD used as a single-photon detector has a multiplication layer, a p-type field control layer (P-FC layer), an i-type light absorption layer, a p-type light absorption layer, and a p-type contact layer stacked on an n-type contact layer (Non-Patent Document 1). The p-type field control layer is used to increase the electric field strength of the multiplication layer more than that of the i-type light absorption layer.

[0004] In the APD of Non-Patent Document 1, a P-type region is formed by ion implantation, which has the drawback that the operating area of ​​the device and the thickness of the multiplication layer vary due to the diffusion of the implanted ions, making them difficult to control. Also, although the APD of Non-Patent Document 1 is a hole injection type, an electron injection type is preferable from the viewpoint of carrier transport speed.

[0005] An electron injection APD that can solve the above-mentioned problems and can be fabricated with high alignment accuracy will be described with reference to FIG. 8 . This APD includes an n-type contact layer 302 formed on a substrate 301, a multiplication layer 303 formed on the n-type contact layer 302, and a p-type field control layer 304 formed on the multiplication layer 303. The APD also includes an i-type light absorption layer 305a formed on the p-type field control layer 304, a p-type light absorption layer 305b formed on the i-type light absorption layer 305a, and a p-type contact layer 306 formed on the p-type light absorption layer 305b. The APD also includes an n-electrode 321 connected to the n-type contact layer 302 and a p-electrode 322 connected to the p-type contact layer. The multiplication layer 303, p-type field control layer 304, i-type light absorption layer 305a, p-type light absorption layer 305b, and p-type contact layer 306 are formed in the shape of a rectangular column in a plan view.

[0006] In this APD, the leakage current increases when an electric field is applied to the side of the pillar-shaped element. In the structure of the prior art, the electric field strength is generated at the center and side of the pillar-shaped element. In particular, APDs that operate in Geiger mode are designed to have a high electric field strength during operation, and the effect of increased leakage current on the side of the element is significant.

[0007] J. Zhang et al., "Advances in InGaAs / InP single-photon detector systems for quantum communication", Light: Science & Applications, vol. 4, no. e286, 2015.

[0008] As mentioned above, conventional photodiodes have a problem in that the leakage current increases when an electric field is applied to the side of the columnar element. This increase in leakage current leads to an increase in noise (dark counts) when the APD is operating in Geiger mode, and reduces reliability.

[0009] The present invention has been made to solve the above problems, and has as its object to reduce the leakage current of a light-receiving element.

[0010] The light-receiving element according to the present invention comprises a p-type contact layer made of a p-type semiconductor formed on a substrate, a light-absorbing layer made of a semiconductor formed on the p-type contact layer, an n-type contact layer made of an n-type semiconductor formed on the light-absorbing layer, a p-electrode connected to the p-type contact layer, and an n-electrode connected to the n-type contact layer, wherein the n-type contact layer is formed to have an area smaller than that of the light-absorbing layer and is arranged inside the region where the light-absorbing layer is formed.

[0011] As described above, according to the present invention, the n-type contact layer disposed on the light absorption layer is formed to have an area smaller than that of the light absorption layer and is disposed inside the region where the light absorption layer is formed, thereby reducing the leakage current of the light receiving element.

[0012] FIG. 1 is a cross-sectional view schematically showing the configuration of a light-receiving element according to a first embodiment of the present invention. FIG. 2 is an explanatory diagram for explaining the electric field distribution in the light-receiving element. FIG. 3 is a characteristic diagram showing the relationship between the terrace width and the lateral resistance of the p-type contact layer 102. FIG. 4 is a cross-sectional view schematically showing the configuration of a light-receiving element according to a second embodiment of the present invention. FIG. 5 is a cross-sectional view schematically showing the configuration of a light-receiving element according to a third embodiment of the present invention. FIG. 6 is a cross-sectional view schematically showing the configuration of a light-receiving element according to a fourth embodiment of the present invention. FIG. 7 is a cross-sectional view schematically showing the configuration of a light-receiving element according to a fifth embodiment of the present invention. FIG. 8 is a cross-sectional view schematically showing the configuration of a light-receiving element.

[0013] Hereinafter, a light receiving element according to an embodiment of the present invention will be described.

[0014] First Embodiment First, a light receiving element according to a first embodiment of the present invention will be described with reference to FIG.

[0015] This light-receiving element first includes a p-type contact layer 102 made of a p-type semiconductor formed on a substrate 101, a light-absorbing layer 103 made of a semiconductor formed on the p-type contact layer 102, and an n-type contact layer 104 made of an n-type semiconductor formed on the light-absorbing layer 103. It also includes a p-electrode 121 connected to the p-type contact layer 102 and an n-electrode 122 connected to the n-type contact layer 104.

[0016] The substrate 101 may be made of, for example, InP doped with Fe to give it high resistance. The p-type contact layer 102 may be made of, for example, p-type InGaAsP. The light absorption layer 103 may have a stacked structure of a p-type light absorption layer 103a made of p-type InGaAs and an i-type light absorption layer 103b made of i-type InGaAs. The n-type contact layer 104 may be made of n-type InGaAsP. The p-electrode 121 and the n-electrode 122 may have a three-layer stacked structure of a titanium layer, a platinum layer, and a gold layer.

[0017] The n-type contact layer 104 is formed to have an area smaller than that of the light absorption layer 103 and is disposed inside the formation region of the light absorption layer 103. Each of the p-type contact layer 102, the light absorption layer 103, and the n-type contact layer 104 is formed in a columnar (e.g., rectangular or cylindrical) element shape, and the element diameter of the columnar n-type contact layer 104 is smaller than the element diameter of the columnar light absorption layer 103. Here, the p-type contact layer 102, the light absorption layer 103, and the n-type contact layer 104 can each have a common central axis in the substrate normal direction.

[0018] The above-described configuration reduces the electric field generated on the side surface of the light absorption layer 103, thereby reducing the leakage current. Furthermore, breakdown due to the edge electric field can be suppressed.

[0019] Here, the width (terrace width) of the region (terrace) 131 where the light absorption layer 103 is wider than the n-type contact layer 104 can be greater than 1 μm and smaller than 40 μm. In other words, the difference (terrace width) between the distance from the center to the side surface of the columnar n-type contact layer 104 (element radius) and the distance from the center to the side surface of the columnar light absorption layer 103 (element radius) can be greater than 1 μm and smaller than 40 μm.

[0020] In this example, the photodiode includes a p-type field control layer 105 formed on the light absorption layer 103, a multiplication layer 106 formed on the p-type field control layer 105, an n-type field control layer 107 formed on the multiplication layer 106, and an edge field relaxation layer 108 formed on the n-type field control layer 107. The multiplication layer 106 is formed on and in contact with the p-type field control layer 105, and the n-type field control layer 107 is formed on and in contact with the multiplication layer 106. In this example, the edge field relaxation layer 108 is formed on and in contact with the n-type field control layer 107. Furthermore, a diffusion barrier layer 109 is provided between the p-type contact layer 102 and the light absorption layer 103. The n-type contact layer 104 is formed on the edge field relaxation layer 108. This photodiode is an avalanche photodiode.

[0021] The p-type field control layer 105 can be made of a semiconductor such as p-type InGaAsP. The multiplication layer 106 can be made of a semiconductor such as i-type InP. The n-type field control layer 107 can be made of a semiconductor such as n-type InP. The edge field relaxation layer 108 can be made of a semiconductor such as i-type InP whose bandgap energy is larger than that of the light absorption layer 103. The diffusion barrier layer 109 can be made of p-type InP.

[0022] In the first embodiment, the layers above the light absorption layer 103 are formed to have an area smaller than that of the light absorption layer 103 and are arranged inside the region where the light absorption layer 103 is formed. Furthermore, the layers above the multiplication layer 106 are formed to have an area smaller than that of the multiplication layer 106 and are arranged inside the region where the multiplication layer 106 is formed. In this example, the p-type field control layer 105 and the multiplication layer 106 are formed to have the same area, and the n-type field control layer 107 and the edge field relaxation layer 108 are formed to have the same area. Furthermore, the n-type contact layer 104 is formed to have an area smaller than that of the edge field relaxation layer 108 and is arranged inside the region where the edge field relaxation layer 108 is formed. These layers can share a central axis in the substrate normal direction.

[0023] With the above-described configuration, it is possible to reduce the electric field generated on the side surface of the multiplication layer 106 in addition to the side surface of the light absorption layer 103, and it is possible to suppress breakdown due to the edge electric field.

[0024] For example, an APD with high sensitivity and low dark current has thick multiplication layer 106 and light absorption layer 103. For example, the total thickness of i-type light absorption layer 103b, p-type field control layer 105, multiplication layer 106, n-type field control layer 107, and edge field reduction layer 108 exceeds 1 μm. As shown in FIG. 2 , considering a model in which electric field lines 141 spread at a 45-degree angle, it is desirable to set the width of terrace 131 to 1 μm or more in order to reduce the electric field strength on the side surfaces.

[0025] On the other hand, if the width of the terrace 131 exceeds 40 μm, the series resistance of the element (the lateral depth of the p-type contact layer 102) increases by about 10 times compared to when the width is 1 μm, limiting operation at high repetition rates, as shown in FIG. 3. For this reason, it is desirable that the width of the terrace 131 be 40 μm or less.

[0026] Next, a brief description will be given of a method for manufacturing the above-described photodetector (avalanche photodiode). First, on a semi-insulating InP substrate 101, p-type InAlGaAs (p-type contact layer 102), p-type InP (diffusion barrier layer 109), p-type InGaAs (p-type light absorption layer 103a), undoped InGaAs (i-type light absorption layer 103b), p-type InGaAsP (p-type field control layer 105), undoped InP (multiplication layer 106), n-type InP (n-type field control layer 107), undoped InP (edge ​​field relaxation layer 108), and n-type InGaAsP (n-type contact layer 104) are sequentially deposited by epitaxial growth. These layers can be formed by the well-known metalorganic vapor phase epitaxy (MOVPE) method.

[0027] Next, an n-electrode 122 is formed on the n-type InGaAsP layer. For example, a resist mask pattern having an opening in the region that will become the n-electrode 122 is formed, and a three-layer film of a titanium layer, a platinum layer, and a gold layer is formed on top of this by electron beam evaporation. After this, the resist mask pattern is removed (lift-off), thereby forming the n-electrode 122 that makes an ohmic contact with the n-type InGaAsP layer (n-type contact layer 104).

[0028] Next, the n-type InGaAsP layer is patterned by known lithography and etching (wet etching) techniques to form the n-type contact layer 104 .

[0029] Next, the undoped InP and n-type InP layers are patterned by the same lithography and etching techniques as described above to form the edge field relaxation layer 108 and the n-type field control layer 107 .

[0030] Next, the undoped InP and p-type InGaAsP layers are patterned by the same lithography and etching techniques as described above to form the multiplication layer 106 and the p-type field control layer 105 .

[0031] Next, the undoped InGaAs, p-type InGaAs, and p-type InP layers are patterned by the same lithography and etching techniques as described above to form the i-type light absorption layer 103b, the p-type light absorption layer 103a, and the diffusion barrier layer 109. This patterning exposes a portion of the p-type contact layer 102 around the diffusion barrier layer 109.

[0032] Finally, a p-electrode 121 is formed on the p-type contact layer 102 exposed by the patterning. The p-electrode 121 has a three-layer structure of a titanium layer, a platinum layer, and a gold layer. As with the p-electrode 121, the p-electrode 121 can be formed by electron beam evaporation and lift-off.

[0033] Second Embodiment Next, a light receiving element according to a second embodiment of the present invention will be described with reference to FIG.

[0034] This light-receiving element first includes a p-type contact layer 102 made of a p-type semiconductor formed on a substrate 101, a light-absorbing layer 103 made of a semiconductor formed on the p-type contact layer 102, and an n-type contact layer 104 made of an n-type semiconductor formed on the light-absorbing layer 103. It also includes a p-electrode 121 connected to the p-type contact layer 102 and an n-electrode 122 connected to the n-type contact layer 104.

[0035] In the second embodiment, the n-type contact layer 104 is also formed to have a smaller area than the light absorbing layer 103 and is disposed inside the region where the light absorbing layer 103 is formed. The p-type contact layer 102, the light absorbing layer 103, and the n-type contact layer 104 are each formed in a columnar (e.g., rectangular or cylindrical) element shape, and the element diameter of the columnar n-type contact layer 104 is smaller than the element diameter of the columnar light absorbing layer 103. Here, the p-type contact layer 102, the light absorbing layer 103, and the n-type contact layer 104 can each share a central axis in the substrate normal direction. Furthermore, the width of the region (terrace 131) where the light absorbing layer 103 is wider than the n-type contact layer 104 can be greater than 1 μm and less than 40 μm.

[0036] Also in the second embodiment, a p-type electric field control layer 105 is formed on the light absorption layer 103, a multiplication layer 106 is formed on the p-type electric field control layer 105, an n-type electric field control layer 107 is formed on the multiplication layer 106, and an edge electric field relaxation layer 108 is formed on the n-type electric field control layer 107. Furthermore, a diffusion barrier layer 109 is provided between the p-type contact layer 102 and the light absorption layer 103. The n-type contact layer 104 is formed on the edge electric field relaxation layer 108. This light receiving element is an avalanche photodiode.

[0037] In the second embodiment, the diffusion barrier layer 109, the light absorption layer 103, the p-type field control layer 105, the multiplication layer 106, the n-type field control layer 107, and the edge field relaxation layer 108 are formed to have the same area. These layers can share a central axis in the substrate normal direction.

[0038] According to the second embodiment, the n-type contact layer 104 is formed to have an area smaller than that of the light absorbing layer 103 and is disposed inside the region where the light absorbing layer 103 is formed, thereby reducing the electric field generated on the side surface of the light absorbing layer 103 and reducing the leakage current. Furthermore, breakdown due to the edge electric field can be suppressed.

[0039] Third Embodiment Next, a light receiving element according to a third embodiment of the present invention will be described with reference to FIG.

[0040] This light-receiving element first includes a p-type contact layer 102 made of a p-type semiconductor formed on a substrate 101, a light-absorbing layer 103 made of a semiconductor formed on the p-type contact layer 102, and an n-type contact layer 104 made of an n-type semiconductor formed on the light-absorbing layer 103. It also includes a p-electrode 121 connected to the p-type contact layer 102 and an n-electrode 122 connected to the n-type contact layer 104.

[0041] In the third embodiment, the n-type contact layer 104 is also formed to have a smaller area than the light absorbing layer 103 and is disposed inside the region where the light absorbing layer 103 is formed. The p-type contact layer 102, the light absorbing layer 103, and the n-type contact layer 104 are each formed in a columnar (e.g., rectangular or cylindrical) element shape, and the element diameter of the columnar n-type contact layer 104 is smaller than the element diameter of the columnar light absorbing layer 103. Here, the p-type contact layer 102, the light absorbing layer 103, and the n-type contact layer 104 can each share a central axis in the substrate normal direction. Furthermore, the width of the region (terrace 131) where the light absorbing layer 103 is wider than the n-type contact layer 104 can be greater than 1 μm and less than 40 μm.

[0042] Also in the third embodiment, a p-type electric field control layer 105 is formed on the light absorption layer 103, a multiplication layer 106 is formed on the p-type electric field control layer 105, an n-type electric field control layer 107 is formed on the multiplication layer 106, and an edge electric field relaxation layer 108 is formed on the n-type electric field control layer 107. Furthermore, a diffusion barrier layer 109 is provided between the p-type contact layer 102 and the light absorption layer 103. The n-type contact layer 104 is formed on the edge electric field relaxation layer 108. This light receiving element is an avalanche photodiode.

[0043] In the third embodiment, the diffusion barrier layer 109, the light absorption layer 103, the p-type field control layer 105, and the multiplication layer 106 are formed to have the same area. In the third embodiment, the layers above the multiplication layer 106 are formed to have an area smaller than that of the multiplication layer 106 and are arranged inside the formation region of the multiplication layer 106. In this example, the n-type field control layer 107 and the edge field relaxation layer 108 are formed to have the same area. Furthermore, the n-type contact layer 104 is formed to have an area smaller than that of the edge field relaxation layer 108 and is arranged inside the formation region of the edge field relaxation layer 108. These layers can share a central axis in the substrate normal direction.

[0044] According to the third embodiment, the n-type contact layer 104 is formed to have an area smaller than that of the light absorption layer 103 and is disposed inside the region where the light absorption layer 103 is formed, thereby reducing the electric field generated on the side surfaces of the light absorption layer 103 and reducing leakage current. Also, breakdown due to edge electric fields can be suppressed. Furthermore, the layers above the multiplication layer 106 are formed to have an area smaller than that of the multiplication layer 106, thereby reducing the electric field generated on the side surfaces of the multiplication layer 106 in addition to the side surfaces of the light absorption layer 103, thereby suppressing breakdown due to edge electric fields.

[0045] Fourth Embodiment Next, a light receiving element according to a fourth embodiment of the present invention will be described with reference to FIG.

[0046] This light-receiving element first includes a p-type contact layer 102 made of a p-type semiconductor formed on a substrate 101, a light-absorbing layer 103 made of a semiconductor formed on the p-type contact layer 102, and an n-type contact layer 104 made of an n-type semiconductor formed on the light-absorbing layer 103. It also includes a p-electrode 121 connected to the p-type contact layer 102 and an n-electrode 122 connected to the n-type contact layer 104.

[0047] In the fourth embodiment, the n-type contact layer 104 is also formed to have a smaller area than the light absorbing layer 103 and is disposed inside the region where the light absorbing layer 103 is formed. The p-type contact layer 102, the light absorbing layer 103, and the n-type contact layer 104 are each formed in a columnar (e.g., rectangular or cylindrical) element shape, and the element diameter of the columnar n-type contact layer 104 is smaller than the element diameter of the columnar light absorbing layer 103. Here, the p-type contact layer 102, the light absorbing layer 103, and the n-type contact layer 104 can each share a central axis in the substrate normal direction. Furthermore, the width of the region (terrace 131) where the light absorbing layer 103 is wider than the n-type contact layer 104 can be greater than 1 μm and less than 40 μm.

[0048] Also in the fourth embodiment, a p-type electric field control layer 105 is formed on the light absorption layer 103, a multiplication layer 106 is formed on the p-type electric field control layer 105, an n-type electric field control layer 107 is formed on the multiplication layer 106, and an edge electric field relaxation layer 108 is formed on the n-type electric field control layer 107. Furthermore, a diffusion barrier layer 109 is provided between the p-type contact layer 102 and the light absorption layer 103. The n-type contact layer 104 is formed on the edge electric field relaxation layer 108. This light receiving element is an avalanche photodiode.

[0049] In the fourth embodiment, first, the diffusion barrier layer 109 and the light absorption layer 103 are formed to have the same area. Furthermore, in the fourth embodiment, the layers above the light absorption layer 103 are formed to have an area smaller than that of the light absorption layer 103 and are arranged inside the formation region of the light absorption layer 103. In this example, the p-type field control layer 105, the multiplication layer 106, the n-type field control layer 107, the edge field reduction layer 108, and the n-type contact layer 104 are formed to have the same area. Each of these layers can have a common central axis in the substrate normal direction.

[0050] In the fourth embodiment, the n-type contact layer 104 is formed to have an area smaller than that of the light absorbing layer 103 and is disposed inside the region where the light absorbing layer 103 is formed, thereby reducing the electric field generated on the side surface of the light absorbing layer 103 and reducing the leakage current. In addition, breakdown due to the edge electric field can be suppressed.

[0051] Fifth Embodiment Next, a light receiving element according to a fifth embodiment of the present invention will be described with reference to FIG.

[0052] This light-receiving element first includes a p-type contact layer 102 made of a p-type semiconductor formed on a substrate 101, a light-absorbing layer 103 made of a semiconductor formed on the p-type contact layer 102, and an n-type contact layer 104 made of an n-type semiconductor formed on the light-absorbing layer 103. The element also includes a p-electrode 121 connected to the p-type contact layer 102 and an n-electrode 122 connected to the n-type contact layer 104. The element also includes an electron transit layer 110 formed on the light-absorbing layer 103. The electron transit layer 110 can be made of a semiconductor such as i-type InP, for example.

[0053] The n-type contact layer 104 is formed to have an area smaller than that of the light absorption layer 103 and is disposed inside the formation region of the light absorption layer 103. Each of the p-type contact layer 102, the light absorption layer 103, and the n-type contact layer 104 is formed in a columnar (e.g., rectangular or cylindrical) element shape, and the element diameter of the columnar n-type contact layer 104 is smaller than the element diameter of the columnar light absorption layer 103. Here, the p-type contact layer 102, the light absorption layer 103, and the n-type contact layer 104 can each have a common central axis in the substrate normal direction.

[0054] The width (terrace width) of the terrace 131 where the light absorption layer 103 is wider than the n-type contact layer 104 can be greater than 1 μm and smaller than 40 μm. In other words, the difference (terrace width) between the distance from the center to the side surface of the columnar n-type contact layer 104 (element radius) and the distance from the center to the side surface of the columnar light absorption layer 103 (element radius) can be greater than 1 μm and smaller than 40 μm.

[0055] The above-described configuration reduces the electric field generated on the side surfaces of the light absorption layer 103, thereby reducing leakage current. It also makes it possible to suppress breakdown due to edge electric fields. In this example, the electron transit layer 110 is formed to have a smaller area than the light absorption layer 103 and is disposed inside the region where the light absorption layer 103 is formed. Furthermore, the n-type contact layer 104 is formed to have a smaller area than the electron transit layer 110 and is disposed inside the region where the electron transit layer 110 is formed. These configurations reduce the electric field strength on each side surface.

[0056] As described above, according to the embodiment of the present invention, the n-type contact layer disposed on the light absorption layer is formed to have an area smaller than that of the light absorption layer and is disposed inside the formation region of the light absorption layer, thereby making it possible to reduce the leakage current of the light receiving element.

[0057] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0058] 101...substrate, 102...p-type contact layer, 103...light absorption layer, 103a...p-type light absorption layer, 103b...i-type light absorption layer, 104...n-type contact layer, 105...p-type electric field control layer, 106...multiplication layer, 107...n-type electric field control layer, 108...edge electric field relaxation layer, 109...diffusion barrier layer, 121...p electrode, 122...n electrode.

Claims

1. A photodetector comprising: a p-type contact layer made of a p-type semiconductor formed on a substrate; a light absorption layer made of a semiconductor formed on said p-type contact layer; an n-type contact layer made of an n-type semiconductor formed on said light absorption layer; a p-electrode connected to said p-type contact layer; and an n-electrode connected to said n-type contact layer, wherein said n-type contact layer is formed with an area smaller than said light absorption layer and is positioned inside the region where said light absorption layer is formed.

2. A light-receiving element according to claim 1, further comprising an electron transit layer made of a semiconductor formed on said light absorption layer.

3. A light-receiving element according to claim 1, comprising: a p-type electric field control layer made of a p-type semiconductor formed on the light absorption layer; a multiplication layer made of a semiconductor formed on the p-type electric field control layer; an n-type electric field control layer made of an n-type semiconductor formed on the multiplication layer; and an edge electric field relaxation layer formed on the n-type electric field control layer and made of a semiconductor having a band gap energy larger than that of the light absorption layer, wherein the n-type contact layer is formed on the edge electric field relaxation layer.

4. A photodetector according to claim 3, wherein the layer above the multiplication layer is formed to have an area smaller than that of the multiplication layer and is disposed inside the region where the multiplication layer is formed.

5. A light-receiving element according to claim 4, wherein the n-type contact layer is formed to have an area smaller than that of the edge electric field relaxation layer and is arranged inside the region where the edge electric field relaxation layer is formed.

6. A light-receiving element according to claim 3, wherein the layer above the light-absorbing layer is formed to have an area smaller than that of the light-absorbing layer and is disposed inside the region where the light-absorbing layer is formed.

7. A photodetector according to claim 6, wherein the layer above the multiplication layer is formed to have an area smaller than that of the multiplication layer and is disposed inside the region where the multiplication layer is formed.

8. A photodiode according to any one of claims 1 to 7, wherein the width of the region where the light absorption layer is wider than the n-type contact layer is 1 μm or more and 40 μm or less.

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