Avalanche photodiode

Abstract

To provide an avalanche photodiode that reduces dark current and enables fast response. [Solution] The semiconductor comprises a first semiconductor layer having a p-type conductivity, a barrier layer having a p-type conductivity laminated on one side of the first semiconductor layer, a light-absorbing layer having a p-type conductivity laminated on the side of the barrier layer opposite to the first semiconductor layer, a second semiconductor layer having a p-type conductivity laminated on the side of the light-absorbing layer opposite to the barrier layer, a multiplier layer having a multiplier opposite to the light-absorbing layer of the second semiconductor layer, and a third semiconductor layer having an n-type conductivity laminated on the side of the multiplier layer opposite to the second semiconductor layer. The first semiconductor layer, barrier layer, light-absorbing layer, second semiconductor layer, and multiplier layer form a first mesa, the third semiconductor layer forms a second mesa, the second mesa is located inside the first mesa in a plan view and protrudes beyond the first mesa in the thickness direction, the doping concentration of the barrier layer is equal to or greater than the doping concentration of the light-absorbing layer, and the doping concentration of the second semiconductor layer is equal to or less than the doping concentration of the light-absorbing layer.

Description

[Technical Field]

[0001] This disclosure relates to an avalanche photodiode. [Background technology]

[0002] In avalanche photodiodes, the presence of a mesa can sometimes narrow the electric field (for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2012-54478 [Overview of the project] [Problems that the invention aims to solve]

[0004] When operating an avalanche photodiode, a reverse bias voltage is applied to deplete the semiconductor layer of the avalanche photodiode. However, when the light-absorbing layer is depleted, the dark current increases. Therefore, the objective is to provide an avalanche photodiode that reduces the dark current and enables a fast response. [Means for solving the problem]

[0005] The avalanche photodiode according to this disclosure comprises: a first semiconductor layer having a p-type conductivity; a barrier layer having a p-type conductivity and laminated on one side of the first semiconductor layer; a light-absorbing layer having a p-type conductivity and laminated on the side of the barrier layer opposite to the first semiconductor layer; a second semiconductor layer having a p-type conductivity and laminated on the side of the light-absorbing layer opposite to the barrier layer; a multiplier layer having an n-type conductivity and laminated on the side of the multiplier layer opposite to the second semiconductor layer; wherein the first semiconductor layer, the barrier layer, the light-absorbing layer, the second semiconductor layer and the multiplier layer form a first mesa; the third semiconductor layer forms a second mesa; the second mesa is located inside the first mesa in a plan view and protrudes more than the first mesa in the thickness direction; the doping concentration of the barrier layer is equal to or greater than the doping concentration of the light-absorbing layer; and the doping concentration of the second semiconductor layer is equal to or less than the doping concentration of the light-absorbing layer. [Effects of the Invention]

[0006] According to this disclosure, it is possible to provide an avalanche photodiode that reduces dark current and enables a fast response. [Brief explanation of the drawing]

[0007] [Figure 1] Figure 1 is a plan view illustrating an avalanche photodiode according to the first embodiment. [Figure 2] Figure 2 is a cross-sectional view illustrating an avalanche photodiode. [Figure 3] Figure 3 illustrates a simulation of the band structure of an avalanche photodiode. [Figure 4] Figure 4 illustrates a simulation of the electric field in an avalanche photodiode. [Figure 5] Figure 5 illustrates a simulation of electric current. [Modes for carrying out the invention]

[0008] [Description of Embodiments in this Disclosure] First, the contents of the embodiments of this disclosure will be listed and explained.

[0009] One embodiment of the present disclosure is an avalanche photodiode comprising: (1) a first semiconductor layer having a p-type conductivity; a barrier layer having a p-type conductivity laminated on one surface of the first semiconductor layer; a light-absorbing layer having a p-type conductivity laminated on the surface of the barrier layer opposite to the first semiconductor layer; a second semiconductor layer having a p-type conductivity laminated on the surface of the light-absorbing layer opposite to the barrier layer; a multiplier layer having an n-type conductivity laminated on the second semiconductor layer opposite to the light-absorbing layer; wherein the first semiconductor layer, the barrier layer, the light-absorbing layer, the second semiconductor layer and the multiplier layer form a first mesa; the third semiconductor layer forms a second mesa; the second mesa is located inside the first mesa in a plan view and protrudes more than the first mesa in the thickness direction; the doping concentration of the barrier layer is equal to or greater than the doping concentration of the light-absorbing layer; and the doping concentration of the second semiconductor layer is equal to or less than the doping concentration of the light-absorbing layer. The light-absorbing layer is less prone to depletion, resulting in a lower electric field. The second mesa narrows the electric field, further reducing the electric field on the sides of the light-absorbing layer. Dark current can be reduced. An energy barrier is formed because the p-type doping concentration of the barrier layer is greater than or equal to that of the light-absorbing layer. The energy is lower because the p-type doping concentration of the second semiconductor layer is less than or equal to that of the light-absorbing layer. Electrons are less likely to move towards the barrier layer and more likely to move towards the second semiconductor layer. (2) In (1) above, the doping concentration of the light-absorbing layer is 1.0 × 10 16 cm -3 The above is 1.0 × 10 19 cm -3 The following are also acceptable: The light-absorbing layer is less prone to depletion, and dark current can be reduced. Lattice defects can be prevented. (3) In (1) or (2) above, the doping concentration of the light-absorbing layer may be higher closer to the barrier layer and lower closer to the second semiconductor layer. The light-absorbing layer is less likely to be depleted, and the electric field applied to the light-absorbing layer becomes lower. Dark current can be reduced. The bands are tilted from the light-absorbing layer toward the second semiconductor layer. Electrons move more easily. (4) In any of (1) to (3) above, the device comprises a first electric field control layer having a p-type conductivity and laminated between the second semiconductor layer and the multiplier layer, and a second electric field control layer having an n-type conductivity and laminated between the multiplier layer and the third semiconductor layer, wherein the first semiconductor layer, the barrier layer, the light absorption layer, the second semiconductor layer, the first electric field control layer, the multiplier layer, and the second electric field control layer may form the first mesa. A high electric field is applied to the multiplier layer. Electrons accelerated by the electric field drift to the multiplier layer, causing avalanche multiplication. High sensitivity can be obtained. The electric field can be narrowed by the second mesa. The electric field on the sides of the multiplier layer and the light absorption layer becomes lower. Edge breakdown can be prevented. Dark current can be reduced. (5) In any of (1) to (4) above, the second semiconductor layer has a gradient layer and a drift layer, the gradient layer and the drift layer have a p-type conductivity, the gradient layer is laminated on the side of the light absorption layer opposite to the barrier layer, the drift layer is laminated on the side of the gradient layer opposite to the light absorption layer, and the doping concentration of the gradient layer and the drift layer may be less than or equal to the doping concentration of the light absorption layer. Because the bands are gradient in the gradient layer and the drift layer, electrons generated in the light absorption layer move more easily. (6) In (5) above, the light-absorbing layer may be made of indium gallium arsenide, the drift layer and the multiplier layer may be made of aluminum gallium arsenide antimony, and the gradient layer may be made of indium aluminum gallium arsenide antimony. (7) In any of (1) to (6) above, an electron transport layer is laminated between the multiplier layer and the third semiconductor layer, the electron transport layer is undoped, and the electron transport layer and the third semiconductor layer may form the second mesa. Edge breakdown can be prevented. Dark current can be reduced.

[0010] [Details of the embodiments of this disclosure] Specific examples of avalanche photodiodes according to the embodiments of this disclosure will be described below with reference to the drawings. However, this disclosure is not limited to these examples and is intended to include all modifications within the meaning and scope of the claims as indicated by the claims.

[0011] Figure 1 is a plan view illustrating an avalanche photodiode (APD) 100 according to the first embodiment. The insulating film, which will be described later, is visible through the view. Figure 2 is a cross-sectional view illustrating the avalanche photodiode 100, showing a cross section along line AA in Figure 1. The avalanche photodiode 100 is used, for example, for detecting near-infrared light.

[0012] As shown in Figure 1, the avalanche photodiode 100 has a mesa 10 (first mesa), a mesa 13 (second mesa), an electrode 12, an electrode 14, a pad 16, a pad 18, and a substrate 20. The planar shape of the substrate 20 is, for example, a square. The top surface of the substrate 20 is parallel to the XY plane. The length of one side L0 is, for example, 400 μm. The Z axis is in the thickness direction of the substrate 20. The X, Y, and Z axes are orthogonal to each other.

[0013] In a plan view, mesa 10 and mesa 13 are circular. Mesa 10 and mesa 13 are arranged, for example, concentrically. Mesa 10 is larger than mesa 13. Mesa 13 is located inside mesa 10. The diameter D1 of mesa 10 is, for example, 280 μm. The diameter D2 of mesa 13 is, for example, 240 μm.

[0014] Electrodes 12 and 14 have an annular shape. Electrode 14 is provided on mesa 13. The portion of mesa 13 inside electrode 14 functions as a light-receiving region 11. The diameter D3 of the light-receiving region 11 is, for example, 200 μm. Electrode 12 is provided outside mesa 10 and mesa 13, surrounding mesa 10 and mesa 13. Pad 16 is electrically connected to electrode 12. Pad 18 is electrically connected to electrode 14.

[0015] As shown in Figure 2, the avalanche photodiode 100 includes a substrate 20, a contact layer 22 (first semiconductor layer), an electron barrier layer 24, a light absorption layer 26, a light absorption layer 28, a light absorption layer 30, a gradient layer 32 (second semiconductor layer), a drift layer 34 (second semiconductor layer), an electric field control layer 36 (first electric field control layer), a multiplication layer 38, an electric field control layer 40 (second electric field control layer), an electron transport layer 42, a buffer layer 44, a buffer layer 46, a buffer layer 48, a buffer layer 50, and a contact layer 52 (third semiconductor layer).

[0016] A contact layer 22 is laminated on one side of the substrate 20. The central part of the contact layer 22 protrudes in the Z-axis direction more than the outer edge of the contact layer 22. An electron barrier layer 24, a light absorption layer 26, a light absorption layer 28, a light absorption layer 30, a gradient layer 32, a drift layer 34, an electric field control layer 36, a multiplier layer 38, an electric field control layer 40, and an electron transport layer 42 are laminated in this order in the central part of the contact layer 22. The layers from the central part of the contact layer 22 to the electron transport layer 42 form a mesa 10.

[0017] The central part of the electron transport layer 42 protrudes in the Z-axis direction more than the outer periphery of the electron transport layer 42. Buffer layers 44, 46, 48, 50, and contact layer 52 are stacked in this order on the central part of the electron transport layer 42 to form a mesa 13. Mesa 10 protrudes in the Z-axis direction more than the upper surface of the substrate 20. Mesa 13 protrudes in the Z-axis direction more than mesa 10. The height of mesa 13 relative to the upper surface of mesa 10 is, for example, 3.2 μm.

[0018] The outer peripheral portion of the contact layer 22 is located outside the mesa 10 and the mesa 13 in the XY plane. The insulating film 54 covers the outer peripheral portion of the contact layer 22, the side surface and the upper surface of the mesa 10, and the side surface and the upper surface of the mesa 13. An opening is provided in the portion of the insulating film 54 that covers the outer peripheral portion of the contact layer 22. An opening is provided in the portion of the insulating film 54 that covers the upper surface of the mesa 13.

[0019] The electrode 12 is provided on the outer peripheral portion of the contact layer 22 and is electrically connected to the contact layer 22 through the opening of the insulating film 54. The electrode 14 is provided on the mesa 13 and is electrically connected to the contact layer 52 through the opening of the insulating film 54.

[0020] The substrate 20 is, for example, a semi-insulating semiconductor substrate and is formed of indium phosphide (InP) doped with iron (Fe). The contact layer 22 is formed of, for example, p+-type indium gallium arsenide ((p+)-In x Ga 1-x As, x = 0.53). The thickness of the contact layer 22 is, for example, 200 nm or more and 2000 nm or less. The p-type semiconductor layer such as the contact layer 22 is doped with beryllium (Be). The doping concentration in the contact layer 22 is, for example, 1.0×10 18 cm -3 or more and 5.0×10 19 cm -3 or less.

[0021] The electron barrier layer 24 is formed of, for example, p+-type aluminum indium arsenide ((p+)-Al x In 1-x As, x = 0.48). The thickness of the electron barrier layer 24 is, for example, 20 nm or more and 500 nm or less. The Be doping concentration of the electron barrier layer 24 is, for example, 1.0×10 17 cm -3 or more and 5.0×10 18 cm -3 or less.

[0022] The light absorption layer 26 is, for example, (p+)-In x Ga1-x It is formed of As(x=0.53). The light-absorbing layers 28 and 30 are made of, for example, p-In x Ga 1-x It is formed of As (x=0.53). The thickness of the light-absorbing layer 26 is, for example, 200 nm or more and 1000 nm or less. The thicknesses of the light-absorbing layer 28 and the light-absorbing layer 30 are, for example, 50 nm or more and 500 nm or less. The doping concentration of Be in the light-absorbing layer 26 is, for example, 5.0 × 10⁻⁶. 17 cm -3 The above is 2.5 × 10 18 cm -3 The following applies: The doping concentration of Be in the light-absorbing layer 28 is, for example, 1.0 × 10⁻⁶. 17 cm -3 The above is 1.0 × 10 18 cm -3 The following applies: The doping concentration of Be in the light-absorbing layer 30 is, for example, 1.0 × 10⁻⁶. 16 cm -3 The above is 5.0 x 10 17 cm -3 The following applies:

[0023] The gradient layer 32 is formed of, for example, p-type indium aluminum gallium arsenide antimony ((p-)-InAlGaAsSb), and may also be a chirped superlattice layer. The thickness of the gradient layer 32 is, for example, 50 nm or more and 500 nm or less. The doping concentration of Be in the gradient layer 32 is, for example, 5.0 × 10⁻⁶. 15 cm -3 The following applies: The drift layer 34 is, for example, p-type aluminum gallium arsenide antimony ((p-)-Al x Ga 1-x As y S 1-y It is formed with x=0.85, y=0.56. The thickness of the drift layer 34 is, for example, 20 nm or more and 1000 nm or less. The doping concentration of Be in the drift layer 34 is, for example, 5.0 × 10 15 cm -3 The following applies: The gradient layer 32 and the drift layer 34 may be given the above-mentioned doping concentration by impurities introduced during the manufacturing process, without intentional doping.

[0024] The electric field control layer 36 is, for example, p-Al x Ga 1-x As y S 1-y It is formed at (x=0.85, y=0.56). The thickness of the electric field control layer 36 is, for example, 20 nm or more and 700 nm or less.

[0025] The multiplier layer 38 is, for example, (p-)-Al x Ga 1-x As y S 1-y It is formed with (x=0.85, y=0.56). The thickness of the multiplier layer 38 is, for example, 50 nm or more and 1000 nm or less. Intentional doping is not required for the multiplier layer 38.

[0026] The electric field control layer 40 is, for example, n-type Al x Ga 1-x As y S 1-y It is formed at (x=0.85, y=0.56). The thickness of the electric field control layer 40 is, for example, 20 nm or more and 700 nm or less. The electric field control layer 40 is doped with, for example, gallium and tellurium (GaTe).

[0027] The electron transport layer 42 is, for example, undoped Al x In 1-x It is formed of As(x=0.48). The thickness of the electron transport layer 42 is, for example, 100 nm or more and 3500 nm or less. The doping concentration of the electron transport layer 42 is, for example, 1.0 × 10⁻⁶ 16 cm -3 The following applies:

[0028] Buffer layers 44, 46, 48, and 50 are, for example, (n+)-Al x In 1-xIt is formed of As (x=0.48). The thickness of buffer layer 44, buffer layer 46, and buffer layer 48 is, for example, 20 nm or more and 300 nm or less. The thickness of buffer layer 50 is, for example, 100 nm or more and 1000 nm or less. Buffer layers 44, buffer layer 46, buffer layer 48, and buffer layer 50 are doped with, for example, silicon (Si). The doping concentration in buffer layer 44 is, for example, 0.1 × 10⁻⁶. 17 cm -3 The above is 5.0 x 10 17 cm -3 The following applies: The doping concentration in buffer layer 46 is, for example, 1.0 × 10⁻⁶. 17 cm -3 The above is 5.0 x 10 17 cm -3 The following applies: The doping concentration in buffer layer 48 is, for example, 0.5 × 10⁻⁶. 18 cm -3 The above is 5.0 x 10 18 cm -3 The following applies: The doping concentration in buffer layer 50 is, for example, 1.0 × 10⁻⁶. 18 cm -3 The above is 5.0 x 10 18 cm -3 The following applies:

[0029] Contact layer 52 is, for example, (n++)-In x Ga 1-x It is formed of As(x=0.53). The thickness of the contact layer 52 is, for example, 300 nm. The Si doping concentration in the contact layer 52 is, for example, 2.0 × 10⁻⁶ 18 cm -3 The above is 3.0 × 10 19 cm -3 The following applies: The semiconductor layer of the avalanche photodiode 100 may be formed from a compound semiconductor other than those mentioned above.

[0030] The insulating film 54 is a passivation film and is made of an insulator such as silicon nitride (SiN). The thickness of the insulating film 54 is, for example, 100 nm or more and 1000 nm or less. Electrodes 12 and 14 are made of metal.

[0031] (Manufacturing method) For example, contact layers 22 to 52 are epitaxially grown sequentially on one surface of the substrate 20 using molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD). Impurities such as Be or Si can be added by supplying them along with the source gas.

[0032] A mesa 13 is formed by etching from the contact layer 52 to partway through the electron transport layer 42. A mesa 10 is formed by etching from the outer periphery of the electron transport layer 42 to partway through the contact layer 22. An insulating film 54 is deposited by plasma CVD (Plasma Enhanced Chemical Vapor Deposition). Openings are formed in the portion of the insulating film 54 that covers the upper surface of the mesa 10 and the portion that covers the upper surface of the contact layer 22 by etching. Electrodes 12 and 14 are formed by vacuum deposition and lift-off. An avalanche photodiode 100 is formed.

[0033] The avalanche photodiode 100 detects light such as infrared light. When using the avalanche photodiode 100, a negative voltage is applied to electrode 12 and a positive voltage is applied to electrode 14. In the Z-axis direction, a p-type contact layer 22 and light absorption layer, an i-type electron transport layer 42, a p-type buffer layer and contact layer 52 are arranged to form a nip (negative-intrinsic-positive) junction. Depletion regions extend to mesa 10 and mesa 13.

[0034] Light incident from the light-receiving region 11 is absorbed by the light-absorbing layers 26, 28, and 30. By absorbing light, the light-absorbing layers generate carriers (electron-hole pairs). The holes move toward the contact layer 22. Electrons are blocked by the electron barrier layer 24 and have difficulty moving toward the contact layer 22. On the other hand, the electric field applied in the depletion region causes electrons to move toward the electrode 14 and is output as a photocurrent. The electrons collide with atoms in the multiplier layer 38, generating even more carriers. This improves sensitivity.

[0035] Figure 3 illustrates a simulation of the band structure of the avalanche photodiode 100. The horizontal axis represents the position of the avalanche photodiode 100 in the Z-axis direction. Position 0 is the surface of the contact layer 52. As the numerical position increases, the position shifts downward in Figure 2. The vertical axis represents energy. The dotted line represents the energy Ev of the valence band. The solid line represents the energy Ec of the conduction band. The dashed line represents the layer boundary. The simulation was performed with a punch-through voltage Vpt of 34V and a breakdown voltage Vbr of 60V.

[0036] The band gaps of the electron barrier layer 24, gradient layer 32, drift layer 34, and electron transport layer 42 are larger than the band gap of the light absorption layer. The valence band energy Ev is nearly flat from the contact layer 22 to the light absorption layer. The conduction band energy Ec of the light absorption layer is nearly flat. The conduction band energy Ec of the electron barrier layer 24 is higher than the energy Ec of the light absorption layer and the contact layer 22. In other words, an energy barrier is formed in the electron barrier layer 24 in the conduction band.

[0037] From the light absorption layer to the contact layer 52, the energies Ec and Ev decrease. In the gradient layer 32 and drift layer 34 adjacent to the light absorption layer 30, the energy Ec slopes downward. Between the electric field control layer 36 and the electric field control layer 40, the energy changes more sharply. From the electron transport layer 42 to the contact layer 52, the energy changes more gradually.

[0038] The light-absorbing layer absorbs light, exciting electrons into the conduction band. The electrons are blocked by the energy barrier of the electron barrier layer 24 and have difficulty moving toward the contact layer 22. Because the energy Ec decreases from the light-absorbing layer toward the contact layer 52, electrons move easily from the light-absorbing layer toward the contact layer 52. Because the valence band is tilted, holes move easily toward the contact layer 22.

[0039] Figure 4 illustrates a simulation of the electric field in the avalanche photodiode 100. The horizontal axis represents position, and the vertical axis represents the electric field. The simulation was performed with Vpt = 35V and Vbr = 60V. Compared to other semiconductor layers, high electric fields are applied to the electric field control layer 36, the multiplication layer 38, and the electric field control layer 40. The electric field is particularly high in the multiplication layer 38. Electrons accelerated by the high electric field enter the multiplication layer 38, enabling avalanche multiplication. On the other hand, the electric field in the light absorption layer is almost zero. Because there is almost no electric field applied to the light absorption layer with a small band gap, the dark current can be reduced.

[0040] Figure 5 illustrates a simulation of current. The horizontal axis represents the voltage applied to the avalanche photodiode 100. The vertical axis represents the current. The dotted line represents the photocurrent in the comparative example. The dashed line represents the dark current in the comparative example. The solid line represents the photocurrent in the embodiment. The dashed line represents the dark current in the embodiment. In the comparative example, an undoped InGaAs photoabsorption layer is provided on top of the p-type photoabsorption layer. The photocurrent is similar in the embodiment and the comparative example. On the other hand, the dark current in the embodiment is lower than that of the comparative example. In the comparative example, the dark current increases due to the depletion of the undoped photoabsorption layer. The electric field may be increased to extract carriers from the photoabsorption layer and for a fast response. The higher the electric field, the greater the dark current.

[0041] According to this embodiment, the avalanche photodiode 100 has a p-type light-absorbing layer 26, a light-absorbing layer 28, and a light-absorbing layer 30. The p-type light-absorbing layer is less prone to depletion than the i-type semiconductor layer. As a result, the electric field applied to the p-type light-absorbing layer is low, almost zero, as shown in the example in Figure 4. As shown in Figure 5, the dark current can be reduced. The photocurrent is about the same as in the comparative example where an undoped light-absorbing layer is provided. As a result, the sensitivity of the avalanche photodiode 100 is improved.

[0042] As shown in Figure 2, the avalanche photodiode 100 has mesa 10 and mesa 13. Mesa 13 protrudes from mesa 10 in the Z-axis direction and is located inside mesa 10 in a plan view. The electric field is narrowed by mesa 13, and the electric field on the side surface of mesa 10 decreases. The side surface of mesa 10 includes the side surface of the light absorption layer. Because the electric field on the side surface of the light absorption layer decreases, the dark current can be reduced. As shown in Figure 1, in all directions of the XY plane, mesa 13 protrudes outside mesa 10. The electric field across the entire outer surface of the light absorption layer decreases. The dark current can be reduced.

[0043] A p-type electron barrier layer 24, three p-type light-absorbing layers, a p-type gradient layer 32, and a drift layer 34 are stacked in order. The doping concentration of the electron barrier layer 24 is greater than or equal to the doping concentration of the light-absorbing layers. As shown in Figure 3, an energy barrier is formed in the conduction band of the electron barrier layer 24. Electrons generated in the light-absorbing layers cannot overcome the energy barrier and are less likely to move toward the contact layer 22, while they are more likely to move toward the n-type semiconductor layer.

[0044] The doping concentrations of the gradient layer 32 and the drift layer 34 are less than or equal to the doping concentration of the light absorption layer, for example, they may be 1 / 10 or less, or even 1 / 100 or less, of the doping concentration of the light absorption layer. When the light absorption layer, gradient layer 32, and drift layer 34 are joined, the bands slope downward from the light absorption layer toward the gradient layer 32 and the drift layer 34. A band structure like that shown in Figure 3 is formed, making it easier for carriers to move. As shown in Figure 5, the photocurrent becomes sufficiently large. The avalanche photodiode 100 is capable of high sensitivity and high-speed operation. The electric field may be increased to extract carriers from the light absorption layer and for high-speed response. Because the electric field applied to the light absorption layer is low, the dark current can be reduced.

[0045] The layers from the contact layer 22 to the electric field control layer 36 are p-type semiconductor layers. The multiplication layer 38 is an undoped semiconductor layer. The layers from the buffer layer 44 to the contact layer 52 are n-type semiconductor layers. A nip junction is formed along the Z-axis in the avalanche photodiode 100. Since the p-type doping concentration of the electron barrier layer 24 is greater than or equal to the doping concentration of the light absorption layer, an energy barrier is formed in the conduction band. The band is gradient from the p-type light absorption layer 26 to the n-type contact layer 52. Electrons generated in the p-type light absorption layer move towards the n-type contact layer 52. Holes move towards the p-type contact layer 22. Photocurrent output is possible. No energy barrier is generated in the valence band, and the band is almost flat from the light absorption layer to the contact layer 22. Due to the electric field near the boundary between the light absorption layer and the gradient layer 32, holes are easily extracted and move towards the n-type semiconductor layer.

[0046] A semiconductor layer with a p-type doping concentration lower than that of the light-absorbing layer can be provided between the light-absorbing layer 30 and the electric field control layer 36. As shown in Figure 4, electron drift can be enabled by tilting the band from the light-absorbing layer to the semiconductor layer. For example, as shown in Figure 2, a gradient layer 32 and a drift layer 34 are stacked between the light-absorbing layer 30 and the electric field control layer 36. The doping concentrations of the gradient layer 32 and the drift layer 34 are less than or equal to the doping concentration of the light-absorbing layer, for example, 5 × 10⁻⁶. 15 cm -3The following may be 1×10 16 cm -3 or less. The inclined layer 32 and the drift layer 34 may not be intentionally doped, for example.

[0047] When the doping concentration of the light absorption layer is low, the light absorption layer becomes depleted and the electric field increases. When the doping concentration is high, depletion of the light absorption layer can be prevented and the electric field can be reduced. However, lattice defects may occur in the light absorption layer and the semiconductor layer laminated thereon. The doping concentrations of the light absorption layer 26, the light absorption layer 28, and the light absorption layer 30 are, for example, 1.0×10 16 cm -3 or more and 1.0×10 19 cm -3 or less. The light absorption layer becomes less likely to be depleted. By reducing the electric field applied to the light absorption layer, the dark current can be reduced. Lattice defects can be prevented. The doping concentration of the light absorption layer is 1.0×10 17 cm -3 or more, 5.0×10 17 cm -3 or more, 1.0×10 18 cm -3 or less, or 5.0×10 18 cm -3 or less may also be acceptable.

[0048] The doping concentration of the light-absorbing layer may be higher closer to the electron barrier layer 24 and lower closer to the gradient layer 32. For example, the doping concentration may be varied among the three light-absorbing layers. The light-absorbing layer 26 is laminated on the upper surface of the electron barrier layer 24. The doping concentration of the light-absorbing layer 26 is higher than that of the light-absorbing layers 28 and 30. The doping concentration of the light-absorbing layer 28 is higher than that of the light-absorbing layer 30. The doping concentration of the light-absorbing layer 30 is the lowest among the three light-absorbing layers. As shown in Figure 3, the bands smoothly slope from the light-absorbing layer 30 to the gradient layer 32 and the drift layer 34. As shown in Figure 4, an electric field is applied near the boundary between the light-absorbing layer and the gradient layer 32, but there is almost no electric field applied to most of the light-absorbing layer. Dark current can be reduced. The number of light-absorbing layers may be three or more, or three or fewer. Within a single light-absorbing layer, the doping concentration may be gradually changed in the thickness direction.

[0049] An n-type field control layer 36, a multiplier layer 38, and a p-type field control layer 40 are stacked between the drift layer 34 and the electron transport layer 42. The field control layer 36, multiplier layer 38, and field control layer 40 are contained within the mesa 10. As shown in Figure 4, the electric field applied to the multiplier layer 38 can be increased. Electrons accelerated by the high electric field drift to the multiplier layer 38, causing avalanche breakdown. High sensitivity can be obtained. The electric field is narrowed by the mesa 13, and the electric field on the side of the mesa 10 decreases. Edge breakdown of the multiplier layer 38 can be prevented. Dark current can be reduced.

[0050] As shown in Figure 4, the electric field is maximum at the multiplier layer 38 and decreases as you move from the multiplier layer 38 towards the electric field control layer 36, drift layer 34, and gradient layer 32. Therefore, the electric field in the light absorption layer is low, which reduces dark current.

[0051] As an example, the electron barrier layer 24 is formed of AlInAs. The light absorption layer is formed of p-type InGaAs. The drift layer 34, the electric field control layer 36, the multiplication layer 38, and the electric field control layer 40 are formed of AlGaAsSb. The inclined layer 32 is formed of InAlGaAsSb. Lattice matching is possible between these layers. For example, Be is used as the dopant for the p-type semiconductor layer. For example, Si is used as the dopant for the n-type semiconductor layer.

[0052] The electron barrier layer 24 is Al x In 1-x In addition to As, InP, Al x In 1-x As, Al x Ga y In 1-x-y As, AlAs y Sb 1-y 、Al x Ga 1-x As y Sb 1-y etc. may be formed. The light absorption layer is formed of a semiconductor having a narrower bandgap than the electron barrier layer 24.

[0053] An electron traveling layer 42 is laminated between the electric field control layer 40 and the buffer layer. The electron traveling layer 42 is included in the mesa 13. Since the electric field is constricted by the mesa 13, the electric field on the side surface of the mesa 10 becomes low. The dark current can be reduced. Edge breakdown can also be prevented. The electron traveling layer 42 may be thicker than the light absorption layer or the like. Since the side surface of the mesa 10 is separated from the contact layer 52 and the electrode 14, the electric field applied to the side surface can be lowered. The dark current can be reduced and edge breakdown can be prevented. The electron traveling layer 42 is, for example, undoped. As shown in FIG. 3, the band is inclined from the electron traveling layer 42 toward the n-type buffer layer and the contact layer 52. Electrons can easily move toward the contact layer 52.

[0054] The embodiment may also be applied to an array-type avalanche photodiode. In an array-type avalanche photodiode, multiple mesas are arranged, for example, in a two-dimensional grid. A pair of mesa 10 and mesa 13 functions as a single photodiode. The photodiodes are separated by recesses between the mesas. The embodiment may also be applied to each photodiode.

[0055] Although embodiments of this disclosure have been described in detail above, this disclosure is not limited to these specific embodiments, and various modifications and changes are possible within the scope of the gist of this disclosure as described in the claims. [Explanation of Symbols]

[0056] 10, 13 Mesas 11 Light receiving area 12, 14 electrodes 16, 18 pads 20 circuit boards 22, 52 Contact layer 24 Electron barrier layer 26, 28, 30 Light-absorbing layer 32 Graded layer 34 Drift Layer 36, 40 Electric field control layer 38 Multiplier Layers 42 Electron transport layer 44, 46, 48, 50 Buffer layers 100 avalanche photodiodes

Claims

1. A first semiconductor layer having a p-type conductivity, A barrier layer having a p-type conductivity is laminated on one surface of the first semiconductor layer, A light-absorbing layer having a p-type conductivity is laminated on the side of the barrier layer opposite to the first semiconductor layer, A second semiconductor layer is laminated on the side of the light-absorbing layer opposite to the barrier layer, A multiplier layer stacked opposite to the light-absorbing layer of the second semiconductor layer, The multiplier layer comprises a third semiconductor layer having an n-type conductivity, which is stacked opposite to the second semiconductor layer, The first semiconductor layer, the barrier layer, the light absorption layer, the second semiconductor layer, and the multiplication layer form a first mesa. The third semiconductor layer forms a second mesa. The second mesa is located inward of the first mesa in a plan view and protrudes more than the first mesa in the thickness direction. The doping concentration of the barrier layer is equal to or greater than the doping concentration of the light-absorbing layer. An avalanche photodiode in which the doping concentration of the second semiconductor layer is less than or equal to the doping concentration of the light-absorbing layer.

2. The doping concentration of the light-absorbing layer is 1.0 × 10⁻⁶ １６ cm －３ The above is 1.0 x 10 １９ cm －３ The avalanche photodiode according to claim 1, which is as follows:

3. The avalanche photodiode according to claim 1 or claim 2, wherein the doping concentration of the light-absorbing layer is higher closer to the barrier layer and lower closer to the second semiconductor layer.

4. A first electric field control layer having a p-type conductivity is laminated between the second semiconductor layer and the multiplication layer, The device comprises a second electric field control layer having an n-type conductivity, which is laminated between the multiplier layer and the third semiconductor layer. The avalanche photodiode according to claim 1 or 2, wherein the first semiconductor layer, the barrier layer, the light absorption layer, the second semiconductor layer, the first electric field control layer, the multiplier layer, and the second electric field control layer form the first mesa.

5. The aforementioned second semiconductor layer has a gradient layer and a drift layer, The gradient layer and the drift layer have a p-type conductivity. The gradient layer is laminated on the side of the light-absorbing layer opposite to the barrier layer. The drift layer is laminated on the side of the gradient layer opposite to the light-absorbing layer. The avalanche photodiode according to claim 1 or claim 2, wherein the doping concentrations of the gradient layer and the drift layer are less than or equal to the doping concentration of the light absorption layer.

6. The aforementioned light-absorbing layer is formed of indium gallium arsenide. The drift layer and the multiplication layer are formed of aluminum gallium arsenide antimony. The avalanche photodiode according to claim 5, wherein the gradient layer is formed of indium aluminum gallium arsenide antimony.

7. The device comprises an electron transport layer laminated between the multiplication layer and the third semiconductor layer, The aforementioned electron transport layer is undoped, The avalanche photodiode according to claim 1 or 2, wherein the electron transport layer and the third semiconductor layer form the second mesa.

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