avalanche photodiode

Abstract

An avalanche photodiode of the present application has, in order, a buffer layer (2), a multiplication layer (3), a light absorption layer (5), a window layer (6, 7), and a contact layer (8) layered on a semiconductor substrate (1). A p-type region (9) is formed by doping an impurity in the window layer (6, 7). The band gap of the window layer (6, 7) is larger than that of the light absorption layer (5). The window layer (6, 7) has a first window layer (6) and a second window layer (7) formed on the first window layer (6) and having a faster diffusion speed of impurities than the first window layer (6). The first window layer (6) is an InP layer doped with Ru, Rh, or Os.

Description

Technical Field

[0001] This disclosure relates to an avalanche photodiode (APD). Background Technology

[0002] Avalanche photodiodes are used in optical communication, particularly in receiving devices for long-distance transmission. When light is incident on an avalanche photodiode, photocarriers consisting of electrons and holes are generated within the InGaAs light-absorbing layer. Electrons multiply due to the avalanche effect as they pass through the AlInAs multiplication layer. This amplifies the received optical signal.

[0003] In the case where the p-type region of an avalanche photodiode is formed by Zn diffusion, an AlInAs window layer with slow Zn diffusion is disposed between the InGaAs light-absorbing layer and the InP window layer (for example, see Patent Document 1). This allows control over the depth of Zn diffusion, forming the desired pn junction.

[0004] Patent Document 1: Japanese Patent No. 4956944

[0005] However, crystalline materials with high Al content have higher thermal resistance compared to InP. Therefore, sometimes the temperature rise caused by localized heat generated when photocarriers are produced during light incidence deteriorates the electrical properties. Summary of the Invention

[0006] This disclosure is made to solve the aforementioned problems, and its purpose is to obtain an avalanche photodiode that can improve heat dissipation.

[0007] The avalanche photodiode disclosed herein comprises: a semiconductor substrate; a buffer layer, a multiplication layer, a light-absorbing layer, a window layer, and a contact layer sequentially deposited on the semiconductor substrate; and a p-type region in the window layer doped with impurities, the band gap of the window layer being larger than that of the light-absorbing layer, the window layer having a first window layer and a second window layer, the second window layer being formed on the first window layer, and the diffusion rate of the impurities being faster than that of the first window layer, the first window layer being an InP layer doped with Ru, Rh, or Os.

[0008] This disclosure improves heat dissipation compared to existing techniques using AlInAs window layers by employing InP window layers doped with Ru, Rh, or Os. This results in avalanche photodiodes with excellent temperature characteristics. Attached Figure Description

[0009] Figure 1 This is a cross-sectional view showing the avalanche photodiode of Embodiment 1.

[0010] Figure 2 This is a cross-sectional view of an avalanche photodiode, representing a comparative example.

[0011] Figure 3 This is a graph showing the temperature near the multiplication layer of the avalanche photodiode in Embodiment 1 and the comparative example.

[0012] Figure 4 This is a graph showing the temperature near the multiplication layer in the avalanche photodiode of Embodiment 1 and the comparative example when the film thickness of the window layer is changed.

[0013] Figure 5 This is a cross-sectional view showing the avalanche photodiode of Embodiment 2.

[0014] Figure 6 This is a cross-sectional view showing the avalanche photodiode of embodiment 3.

[0015] Figure 7 This is a cross-sectional view showing a modified example of the avalanche photodiode according to Embodiment 3.

[0016] Figure 8 This is a cross-sectional view showing the avalanche photodiode of embodiment 4.

[0017] Figure 9 This is a cross-sectional view showing a modified example of the avalanche photodiode according to Embodiment 4. Detailed Implementation

[0018] The avalanche photodiode of the embodiment will be described with reference to the accompanying drawings. The same or corresponding components are labeled with the same reference numerals, and sometimes repeated descriptions are omitted.

[0019] Implementation method 1.

[0020] Figure 1 This is a cross-sectional view showing the avalanche photodiode of Embodiment 1. An n-type InP buffer layer 2, an i-type AlInAs avalanche multiplication layer 3, a p-type AlInAs electric field mitigation layer 4, an n-type InGaAs light-absorbing layer 5, a ruthenium (Ru)-doped semi-insulating Ru-doped InP window layer 6, an n-type InP window layer 7, and a p-type InGaAs contact layer 8 are sequentially deposited on an n-type InP substrate 1. The band gaps of the Ru-doped InP window layer 6 and the n-type InP window layer 7 are larger than those of the n-type InGaAs light-absorbing layer 5.

[0021] The carrier concentration of the n-type InP buffer layer 2 is 1–5 × 10⁻⁶. 18 cm -3 The thickness of the i-type AlInAs avalanche multiplication layer 3 is 0.1–0.5 μm. The thickness of the p-type AlInAs electric field mitigation layer 4 is 0.5–1 × 10⁻⁶ μm.18 cm -3 The film thickness is 0.05–0.15 μm. The carrier concentration of the n-type InGaAs light-absorbing layer 5 is 1–5 × 10⁻⁶. 15 cm -3 The film thickness is 1–1.5 μm. The doping concentration of Ru-doped InP window layer 6 is 0.1–1.0 × 10⁻⁶. 18 cm -3 The film thickness is 0.05–1 μm. The carrier concentration of the n-type InP window layer 7 is 0.1–5 × 10⁻⁶. 15 cm -3 The film thickness is 0.5–1 μm. The carrier concentration of the p-type InGaAs contact layer 8 is 1–5 × 10⁻⁶. 18 cm -3 The film thickness is 0.1–0.5 μm.

[0022] The report demonstrates that in Ru-doped InP, Zn diffuses almost non-existently even when in contact with the doped InP layer. Therefore, the diffusion rate of Zn in the n-type InP window layer 7 is faster than that in the Ru-doped InP window layer 6. Utilizing this property, Zn is doped into the n-type InP window layer 7, where Zn diffusion is faster, to form a p-type region 9. The depth of the p-type region 9 is controlled by the Ru-doped InP window layer 6, where Zn diffusion is slower. This allows for the acquisition of the desired pn junction shape.

[0023] A concentric p-type InGaAs contact layer 8 is formed on the p-type region 9. The upper surface, excluding the p-type InGaAs contact layer 8, is covered by a surface protective film 10. The surface protective film 10 is made of SiNx and also serves as an anti-reflective film. A p-type electrode 11 is formed on the p-type InGaAs contact layer 8. The p-type electrode 11 is made of materials such as Au and Zn. An n-type electrode 12 is formed on the back side of the n-type InP substrate 1. The n-type electrode 12 is made of materials such as Au, Ge, and Ni.

[0024] Next, the manufacturing method of the avalanche photodiode according to this embodiment will be described. Using metal-organic vapor phase (MOVPE) growth at a growth temperature of approximately 600°C, an n-type InP buffer layer 2, an i-type AlInAs avalanche multiplication layer 3, a p-type AlInAs electric field mitigation layer 4, an n-type InGaAs light-absorbing layer 5, a Ru-doped InP window layer 6, an n-type InP window layer 7, and a p-type InGaAs contact layer 8 are sequentially grown on an n-type InP substrate 1. Alternatively, molecular beam epitaxy (MBE) or similar methods can be used as the crystal growth method.

[0025] A SiOx film is formed on the wafer surface by sputtering or other methods to create a circular pattern mask with a diameter of 50 μm. Zn is diffused into the circular portion not covered by the mask to form a p-type region 9. Next, etching is performed so that the p-type InGaAs contact layer 8 remains only as concentric circles with a width of 2.5 to 5.0 μm on the p-type region 9. Then, after a surface protective film 10 is formed on the wafer surface, the surface protective film 10 is removed only from the upper part of the p-type InGaAs contact layer 8. A p-type electrode 11 is formed on the p-type InGaAs contact layer 8. Finally, the back side of the n-type InP substrate 1 is ground to form an n-type electrode 12.

[0026] Next, the operation of the avalanche photodiode of this embodiment will be explained. A reverse bias voltage is applied externally with the n-type electrode 12 side as + and the p-type electrode 11 side as -. In this state, light in the 1.3μm or 1.5μm band, which is the optical communication band, is incident from the p-type electrode 11 side into the p-type region 9. The light is absorbed by the n-type InGaAs light-absorbing layer 5, generating electron-hole pairs as photocarriers. Electrons move towards the n-type electrode 12 side, and holes move towards the p-type electrode 11 side. When the reverse bias voltage is sufficiently high, electrons are ionized in the i-type AlInAs avalanche multiplication layer 3 to generate new electron-hole pairs. The newly generated electrons and holes together cause further ionization. This leads to avalanche multiplication, which causes electrons and holes to avalanche multiply. Therefore, the heat source of the avalanche photodiode during avalanche operation is concentrated near the i-type AlInAs avalanche multiplication layer 3.

[0027] Next, the effects of this embodiment will be compared with those of a comparative example for explanation. Figure 2 This is a cross-sectional view of an avalanche photodiode representing a comparative example. In the comparative example, an n-type AlInAs window layer 13 is used instead of the Ru-doped InP window layer 6 in Embodiment 1. Other structures are the same, and the manufacturing methods are almost identical. A p-type region 9 is formed in the n-type InP window layer 7, where Zn diffusion is fast, and the depth of the p-type region 9 is controlled by the n-type AlInAs window layer 13, where Zn diffusion is slow. The thermal resistance of AlInAs is 10 [W / mK] compared to 68 [W / mK] for InP, representing a difference of approximately 7 times. Therefore, the heat dissipation of the device structure depends on how thin the overall AlInAs film thickness is reduced. In this embodiment, by using the Ru-doped InP window layer 6, heat dissipation is improved compared to the comparative example using the n-type AlInAs window layer 13. As a result, an avalanche photodiode with excellent temperature characteristics can be achieved.

[0028] Figure 3This is a graph showing the temperature near the multiplication layer of the avalanche photodiode in Embodiment 1 and the comparative example. In the calculation, the film thickness of the n-type AlInAs window layer 13 in the comparative example was set to 1 μm, and the film thickness of the Ru-doped InP window layer 6 in Embodiment 1 was set to 1 μm, with other layers having the same structure. It is assumed that light incident generates a heat source of 0.6 W in the i-type AlInAs avalanche multiplication layer 3, and the substrate side is in contact with an ideal heat sink, defined as a state of 0 K. For the temperature near the i-type AlInAs avalanche multiplication layer 3 under this condition, the heat conduction equation was analytically solved to calculate the temperature from the heat sink as a reference. The temperature near the i-type AlInAs avalanche multiplication layer 3 was 281 K in the comparative example, while it was reduced to 246 K in Embodiment 1, representing an improvement in heat dissipation of approximately 14%.

[0029] Figure 4 This graph shows the temperature near the multiplication layer in the avalanche photodiode of Embodiment 1 and the comparative example when the thickness of the window layer is changed. If the n-type AlInAs window layer 13 is also thinned to 0.05 μm in the comparative example, the temperature can be improved to almost the same as in Embodiment 1 when the Ru-doped InP window layer 6 has a thickness of 1 μm. However, to control the depth of the p-type region 9, the thickness of the n-type AlInAs window layer 13 needs to be 0.5 μm or more. On the other hand, even if the thickness of the Ru-doped InP window layer 6, which has low thermal resistance, is varied between 0.05 μm and 1 μm, the temperature change is only about 1%, and the effect of the thickness is almost negligible. Therefore, in Embodiment 1, even if the window layer is relatively thick, there is no problem with the temperature characteristics, and thus the p-type region 9 is easily formed. Furthermore, whether it is the n-type AlInAs window layer 13 or the Ru-doped InP window layer 6, if the film is made thicker, it will affect the high-speed response; therefore, there is an upper limit to the thickness of the window layer.

[0030] Alternatively, a doping concentration of 1.0 × 10⁻⁶ can be used. 18 cm -3 A Rh (rhodium) or Os (osmium) InP layer was used instead of the Ru-doped InP window layer 6. The Rh-doped InP layer was as thermally stable as the Ru-doped InP layer. It was reported that even when Zn was in contact with the doped InP layer, Zn hardly diffused in the Rh-doped InP. Because the carrier trapping energy level in the Rh-doped InP was deeper than that in the Ru-doped InP, the resulting p-type region 9 and the temperature characteristics of the avalanche photodiode were further stabilized. The same was true in the Os-doped InP layer.

[0031] Implementation method 2.

[0032] Figure 5This is a cross-sectional view showing the avalanche photodiode of Embodiment 2. The film thickness is 0.01–0.1 μm, and the carrier concentration is 1–5 × 10⁻⁶. 15 cm -3 An n-type InGaAsP transition layer 14 is inserted between the n-type InGaAs light-absorbing layer 5 and the Ru-doped InP window layer 6. Other structures are the same as in Embodiment 1.

[0033] By inserting an n-type InGaAsP transition layer 14, the band discontinuity between InGaAs and InP can be mitigated. Therefore, the high-speed response of photocarriers generated during light incidence can be improved.

[0034] If the material of the n-type InGaAsP transition layer 14 is made of In 1-x Ga x As y P 1-y The expression (0 < x < 1, 0 < y < 1) indicates that the desired bandgap InGaAsP layer can be obtained by controlling the composition of x and y. For example, when x = 0.28 and y = 0.61, the bandgap λ (Eg) of the n-type InGaAsP transition layer 14 is 1.2 eV.

[0035] Similar to InP, InGaAsP has lower thermal resistance compared to AlInAs. Furthermore, from the viewpoint of high-speed response, the n-type InGaAsP transition layer 14 is deposited to a relatively thin thickness of 0.1 μm or less. Therefore, the addition of the n-type InGaAsP transition layer 14 does not deteriorate heat dissipation. Additionally, since the InGaAsP layer is lattice-matched with InP, it can be grown more easily. Moreover, the same effects as in Embodiment 1 can be obtained.

[0036] Furthermore, the n-type InGaAsP transition layer 14 is not limited to a single InGaAsP layer, but can also be composed of multiple layers. However, it is preferable to use In... 1-x Ga x As y P 1-y The wavelength determined by the band gap gradually shortens from the n-type InGaAs light-absorbing layer 5 towards the Ru-doped InP window layer 6. In this case, it is more effective to make the band gap change continuously rather than in a stepwise manner. In addition, the material of the transition layer is not limited to InGaAsP. As long as the band gap is between InGaAs and InP, other materials composed of Al, Ga, In, As, P, Sb, etc. can also be expected to achieve the same effect.

[0037] Implementation method 3.

[0038] Figure 6This is a cross-sectional view of the avalanche photodiode according to Embodiment 3. The p-type region 9 is stepped, having a first p-type region 9a and a second p-type region 9b that is deeper than the first p-type region 9a. The other structures are the same as in Embodiment 1.

[0039] Next, the manufacturing methods of the first p-type region 9a and the second p-type region 9b will be described. First, a SiOx film is formed on the wafer surface by sputtering or the like, forming a first mask with a circular opening of 50 μm in diameter. Zinc is diffused through the circular opening not covered by the first mask to form the second p-type region 9b. At this time, the depth of Zn diffusion in the second p-type region 9b is controlled within the n-type InP window layer 7.

[0040] Next, a second mask with a circular opening of 30 μm in diameter is formed on it. Zinc diffuses through the circular opening that is not covered by the second mask to form the first p-type region 9a. At this time, the depth of Zn diffusion in the first p-type region 9a is controlled within the Ru-doped InP window layer 6.

[0041] As explained above, in this embodiment, the p-type region 9 is formed in a stepped shape. In this case, the pn junction capacitance C is represented by C = εS1 / d1 + εS2 / d2 + εS1 / d1. Here, the dielectric constant is set to ε [F / m], and the pn junction area of ​​the first p-type region 9a is set to S1 [m²]. 2 Let the area of ​​the pn junction in the second p-type region 9b be S2[m]. 2 Let d1[m] be the distance from the lower end of the n-type InGaAs light-absorbing layer 5 to the first p-type region 9a (depletion layer distance), and let d2[m] be the distance from the lower end of the n-type InGaAs light-absorbing layer 5 to the second p-type region 9b (depletion layer distance).

[0042] Since d1 > d2, the pn junction area can be reduced. This reduces the capacity of the region receiving light from the avalanche photodiode. Therefore, compared to Embodiment 1, the high-speed responsiveness of photocarriers generated during light incidence is improved. Furthermore, if the p-type region 9 in Embodiment 1 is simply reduced to decrease the pn junction area, the effective light-receiving region becomes narrower, and the ease of use of the avalanche photodiode deteriorates. In contrast, in this embodiment, by forming the p-type region 9 in a stepped shape, high-speed responsiveness can be achieved without reducing the effective light-receiving region. In addition, the same effects as Embodiment 1 can be obtained.

[0043] Figure 7This is a cross-sectional view showing a modified example of the avalanche photodiode according to Embodiment 3. Multiple pairs of Ru-doped InP window layers 6 (with slow Zn diffusion) and n-type InP window layers 7 (with fast Zn diffusion) are alternately stacked. This allows the p-type region 9 to be formed into two or more levels. Since the window layers are composed of InP with low thermal resistance, heat dissipation is almost independent of the film thickness of the InP window layers, thus enabling such control.

[0044] Implementation method 4.

[0045] Figure 8 This is a cross-sectional view showing the avalanche photodiode of Embodiment 4. In this embodiment, instead of the Ru-doped InP window layer 6 of Embodiment 1, a surface carrier concentration of 0.1 to 1.0 × 10⁻⁶ is formed on the boundary between the n-type InGaAs light-absorbing layer 5 and the n-type InP window layer 7 on the n-type InGaAs light-absorbing layer 5. 12 cm -2 The Siδ-doped layer 15. Other structures are the same as in Embodiment 1.

[0046] The Siδ-doped layer 15 is formed by two-dimensionally depositing Si on the n-type InGaAs light-absorbing layer 5 crystal by supplying Si raw materials but not In, Ga, etc. The carrier concentration of the Siδ-doped layer 15 is expressed as the surface carrier concentration (cm²). -2 )express.

[0047] Since the Si in the Siδ-doped layer 15 prevents the diffusion of Zn from the p-type region 9, Zn remains in the Siδ-doped layer 15 and does not reach the n-type InGaAs light-absorbing layer 5. Thus, the desired pn junction shape can be obtained.

[0048] Furthermore, the Siδ-doped layer 15 is a two-dimensional layer with a thickness on the order of angstroms. Therefore, it is assumed that the formation of the Siδ-doped layer 15 will not affect heat dissipation.

[0049] Furthermore, the Siδ-doped layer 15 can be fabricated relatively easily using readily available Si, without the need for novel materials like ruthenium. Siδ doping is a technique used in HEMT devices, where the surface carrier concentration can be relatively easily controlled between 0.01 and 10.0 × 10⁻⁶ by adjusting the temperature or material supply. 12 cm -2 The range.

[0050] Figure 9 This is a cross-sectional view showing a modified example of the avalanche photodiode according to Embodiment 4. Multiple pairs of Siδ-doped layers 15 and n-type InP window layers 7 are alternately layered. This allows the p-type region 9 to be formed into two or more layers. Therefore, the pn junction area can be reduced, decreasing the capacity of the region receiving light from the avalanche photodiode.

[0051] Furthermore, in embodiments 1 to 4, AlInAs was used as the multiplication layer, but AlGaInAs can also be used as the material for the multiplication layer. The material for the multiplication layer can be any semiconductor that matches the InP lattice and has an electron ionization rate greater than the hole ionization rate; AlInAs / AlGaInAs superlattice, InGaAsP, AlAsSb, or AlGaAsSb can also be used.

[0052] Furthermore, when using AlInAs in the multiplication layer, the easily grown AlInAs layer can be used as a buffer layer to suppress impurity diffusion from the InP substrate or improve crystal quality. However, the increased film thickness due to the high Al content deteriorates heat dissipation.

[0053] Furthermore, when using InP as the multiplication layer, heat dissipation is superior compared to using AlInAs, but noise characteristics are poor. Additionally, this hole-multiplying avalanche photodiode requires a protective ring structure within the device to prevent localized electric field concentration. Therefore, the manufacturing method is unique and presents manufacturing difficulties.

[0054] Furthermore, the diffusion of Zn in the p-type region 9 was explained, but as long as the atom imparts p conductivity, Cd or Be can be used instead of Zn. Zn diffusion methods can include solid-phase diffusion using ZnO or gas-phase diffusion using a crystal growth furnace.

[0055] Furthermore, a surface incident structure in which the light to be detected is incident from the p-type electrode 11 side into the p-type region 9 has been described. However, conversely, the same effect can be expected even in a back incident structure in which the light is incident from the n-type InP substrate 1 side, and in an end-face incident structure in which the light is incident from the end face of the n-type InGaAs light absorption layer 5.

[0056] Explanation of reference numerals in the attached figures

[0057] 1…n-type InP substrate (semiconductor substrate); 2…n-type InP buffer layer (buffer layer); 3…i-type AlInAs avalanche multiplication layer (multiplication layer); 5…n-type InGaAs light absorption layer (light absorption layer); 6…Ru-doped InP window layer (first window layer); 7…n-type InP window layer (second window layer); 8…p-type InGaAs contact layer (contact layer); 9…p-type region; 9a…first p-type region; 9b…second p-type region; 14…n-type InGaAsP transition layer (transition layer); 15…Siδ-doped layer.

Claims

1. An avalanche photodiode, characterized in that, have: Semiconductor substrate; A buffer layer, a multiplication layer, a light-absorbing layer, a window layer, and a contact layer are sequentially deposited on the semiconductor substrate; and In the p-type region of the window layer, impurities are doped. The band gap of the window layer is larger than that of the light absorption layer. The window layer has a first window layer and a second window layer, the second window layer being formed on top of the first window layer, and the diffusion rate of the impurities is faster than that of the first window layer. The first window layer is an InP layer doped with Ru, Rh, or Os. The window layer does not have an AlInAs layer.

2. The avalanche photodiode according to claim 1, characterized in that... The impurities are Zn, Cd, or Be.

3. The avalanche photodiode according to claim 1 or 2, characterized in that, It also includes a transition layer inserted between the light-absorbing layer and the window layer. The transition layer is composed of a material with a band gap between the material of the light-absorbing layer and InP.

4. An avalanche photodiode, characterized in that, have: Semiconductor substrate; A buffer layer, a multiplication layer, a light-absorbing layer, a window layer, and a contact layer are sequentially deposited on the semiconductor substrate; and In the p-type region of the window layer, impurities are doped. The band gap of the window layer is larger than that of the light absorption layer. A Siδ-doped layer is formed at the boundary between the light-absorbing layer and the window layer. The window layer does not have an AlInAs layer.

5. The avalanche photodiode according to any one of claims 1 to 4, characterized in that, The p-type region has: a first p-type region and a second p-type region with a depth greater than that of the first p-type region.

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