Semiconductor photodetectors

Abstract

To provide a semiconductor light-receiving device capable of operating at a high speed.SOLUTION: A semiconductor light-receiving device comprises an input optical waveguide, a tapered optical waveguide, and a light detection part. The input optical waveguide comprises a first core and a first clad layer. The tapered optical waveguide comprises a second core and a second clad layer. The light detection part comprises a light absorption layer, a first group III-V compound semiconductor layer of a first conductivity type, and a second group III-V compound semiconductor layer of the first conductivity type. The second group III-V compound semiconductor layer has a higher dopant concentration than the first group III-V compound semiconductor layer. The second core has a first thickness. The light absorption layer has a second thickness. The second thickness is smaller than the first thickness.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a semiconductor light receiving device.

Background Art

[0002] Patent Document 1 discloses a light receiving device having an input waveguide formed on a substrate, a photodiode formed on the substrate, and a tapered waveguide formed on the substrate. The tapered waveguide connects the input waveguide and the photodiode. The width of the tapered waveguide widens from the input end connected to the input waveguide toward the output end connected to the photodiode.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The present disclosure provides a semiconductor light receiving device that can operate at high speed.

Means for Solving the Problems

[0005] A semiconductor photodetector according to one aspect of the present disclosure comprises an input optical waveguide provided on a substrate, a tapered optical waveguide provided on the substrate and connected to the input optical waveguide, and a photodetector provided on the substrate and connected to the tapered optical waveguide, wherein the input optical waveguide comprises a first core and a first cladding layer, the first core is disposed between the substrate and the first cladding layer, the tapered optical waveguide has a width that increases from the input optical waveguide toward the photodetector, the tapered optical waveguide comprises a second core and a second cladding layer, the second core is disposed between the substrate and the second cladding layer, and the second core is optically connected to the first core The photodetector comprises a light-absorbing layer, a first III-V compound semiconductor layer of a first conductivity type, and a second III-V compound semiconductor layer of a first conductivity type, wherein the light-absorbing layer is disposed between the substrate and the first III-V compound semiconductor layer, the first III-V compound semiconductor layer is disposed between the light-absorbing layer and the second III-V compound semiconductor layer, the light-absorbing layer is optically coupled to the second core, the second III-V compound semiconductor layer has a dopant concentration higher than that of the first III-V compound semiconductor layer, the second core has a first thickness, and the light-absorbing layer has a second thickness, the second thickness being smaller than the first thickness. [Effects of the Invention]

[0006] This disclosure provides a semiconductor photodetector capable of high-speed operation. [Brief explanation of the drawing]

[0007] [Figure 1] Figure 1 is a schematic perspective view showing a semiconductor photodetector according to one embodiment. [Figure 2] Figure 2 is a cross-sectional view along the line II-II in Figure 1. [Figure 3] Figure 3 is a schematic cross-sectional view showing a semiconductor photodetector according to another embodiment. [Figure 4] Figure 4 is a schematic cross-sectional view showing a semiconductor photodetector according to another embodiment. [Figure 5] Figure 5 is a graph showing an example of the relationship between the thickness of the light-absorbing layer and the 3dB bandwidth. [Figure 6] Figure 6 is a graph showing an example of the relationship between the thickness of the light-absorbing layer and the 3dB bandwidth. [Modes for carrying out the invention]

[0008] [Description of Embodiments in this Disclosure] First, embodiments of this disclosure will be listed and described. (1) comprising an input optical waveguide provided on a substrate, a tapered optical waveguide provided on the substrate and connected to the input optical waveguide, and a photodetector provided on the substrate and connected to the tapered optical waveguide, wherein the input optical waveguide comprises a first core and a first cladding layer, the first core is disposed between the substrate and the first cladding layer, the tapered optical waveguide has a width that increases from the input optical waveguide toward the photodetector, the tapered optical waveguide comprises a second core and a second cladding layer, the second core is disposed between the substrate and the second cladding layer, the second core is optically coupled to the first core, and the photodetector is A semiconductor photodetector comprising a light-absorbing layer, a first-type III-V compound semiconductor layer, and a second-type III-V compound semiconductor layer, wherein the light-absorbing layer is disposed between the substrate and the first-type III-V compound semiconductor layer, the first-type III-V compound semiconductor layer is disposed between the light-absorbing layer and the second-type III-V compound semiconductor layer, the light-absorbing layer is optically coupled to the second core, the second-type III-V compound semiconductor layer has a dopant concentration higher than that of the first-type III-V compound semiconductor layer, the second core has a first thickness, and the light-absorbing layer has a second thickness, the second thickness being smaller than the first thickness.

[0009] According to the above semiconductor photodetector, the light absorption layer of the photodetector can be made thinner. Since a thin light absorption layer shortens the carrier travel time, the above semiconductor photodetector can operate at high speed.

[0010] (2) In the above (1), the first III-V compound semiconductor layer may have a third thickness, the second III-V compound semiconductor layer may have a fourth thickness, and the third thickness may be smaller than the fourth thickness. In this case, the first III-V compound semiconductor layer of the photodetector can be made thinner. Therefore, the series resistance of the photodetector can be reduced.

[0011] (3) In the above (2), the third thickness may be smaller than the second thickness.

[0012] (4) In any one of the above (1) to (3), at the interface between the tapered optical waveguide and the photodetector, the upper surface of the second cladding layer may be closer to the substrate than the lower surface of the second III-V compound semiconductor layer. In this case, it becomes difficult for light to leak from the second core of the tapered optical waveguide to the second III-V compound semiconductor layer of the photodetector. Therefore, light absorption by the second III-V compound semiconductor layer of the photodetector can be suppressed.

[0013] (5) In any one of the above (1) to (4), the first cladding layer may have a fifth thickness at the input end face of the input optical waveguide and a sixth thickness at the output end face of the input optical waveguide, and the sixth thickness may be smaller than the fifth thickness. In this case, the first cladding layer becomes thicker at the input end face of the input optical waveguide. Therefore, the loss of light input to the input end face can be reduced.

[0014] (6) In any one of the above (1) to (5), the second thickness may be 500 nm or less.

[0015] [Details of Embodiments of the Present Disclosure] Hereinafter, embodiments of the present disclosure will be described in detail while referring to the accompanying drawings. In the description of the drawings, the same or equivalent elements are denoted by the same reference numerals, and duplicate descriptions are omitted. An XYZ coordinate system is shown in the drawings as necessary. The X-axis direction, Y-axis direction, and Z-axis direction are orthogonal (for example, perpendicular) to each other.

[0016] FIG. 1 is a perspective view schematically showing a semiconductor light-receiving device according to an embodiment. FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1. FIG. 2 shows an XZ cross-section orthogonal to the Y-axis direction. The semiconductor light-receiving device 100 shown in FIGS. 1 and 2 may be used in an optical communication device having a modulation rate of, for example, 130 GBaud or more. The semiconductor light-receiving device 100 includes an input optical waveguide 20 provided on a substrate 10, a tapered optical waveguide 30 provided on the substrate 10, and a photodetection unit 40 provided on the substrate 10. The tapered optical waveguide 30 is connected to the input optical waveguide 20. The photodetection unit 40 is connected to the tapered optical waveguide 30. The input optical waveguide 20, the tapered optical waveguide 30, and the photodetection unit 40 are arranged in order along the optical axis direction (X-axis direction). The input optical waveguide 20 and the tapered optical waveguide 30 may be in contact with each other. The tapered optical waveguide 30 and the photodetection unit 40 may be in contact with each other. The light input to the input optical waveguide 20 travels through the input optical waveguide 20 and the tapered optical waveguide 30 and is detected by the photodetection unit 40.

[0017] The substrate 10 may be a semi-insulating group III-V compound semiconductor substrate. The substrate 10 may be a semi-insulating indium phosphide (InP) substrate.

[0018] The input optical waveguide 20 may have a mesa structure. The input optical waveguide 20 extends along the X-axis direction. The input optical waveguide 20 has a width W1 (length in the Y-axis direction) orthogonal to the X-axis direction. The input optical waveguide 20 has a height in the Z-axis direction orthogonal to the main surface of the substrate 10.

[0019] The input optical waveguide 20 comprises a first core 22 and a first cladding layer 24. The first core 22 is disposed between the substrate 10 and the first cladding layer 24. The thickness of the first core 22 may be 200 nm or more, or 600 nm or less. The thickness of the first cladding layer 24 may be 800 nm or less. The input optical waveguide 20 may further comprise a III-V compound semiconductor layer 26 of a second conductivity type (e.g., n-type). The second conductivity type is the opposite conductivity type to the first conductivity type (e.g., p-type). The III-V compound semiconductor layer 26 is disposed between the substrate 10 and the first core 22. The III-V compound semiconductor layer 26 may be a buffer layer or a cladding layer. The thickness of the III-V compound semiconductor layer 26 may be 500 nm or less. Adjacent members of the substrate 10, III-V compound semiconductor layer 26, first core 22, and first cladding layer 24 may be in contact with each other.

[0020] The first core 22 may be an i-type (undoped) III-V compound semiconductor layer. The first core 22 may contain gallium indium arsenide phosphide (GaInAsP). The first cladding layer 24 may contain i-type InP. The III-V compound semiconductor layer 26 may contain second-conductivity InP.

[0021] The tapered optical waveguide 30 may have a mesa structure. The tapered optical waveguide 30 has a width that increases from the input optical waveguide 20 toward the photodetector 40. The tapered optical waveguide 30 has a width W2 (length in the Y-axis direction) at the input end face connected to the input optical waveguide 20. Width W2 is greater than width W1. The difference between width W2 and width W1 may be 100 nm or more. Higher-order modes can be excited by the difference between width W2 and width W1. As a result, light leakage in the Z-axis direction can be suppressed at the interface between the tapered optical waveguide 30 and the photodetector 40. The tapered optical waveguide 30 has a width W3 (length in the Y-axis direction) at the output end face connected to the photodetector 40. Width W3 is greater than width W2. The photodetector 40 may have the same width as width W3.

[0022] The tapered optical waveguide 30 comprises a second core 32 and a second cladding layer 34. The second core 32 is disposed between the substrate 10 and the second cladding layer 34. The second core 32 is optically coupled to the first core 22. The second core 32 has a first thickness D1. The first thickness D1 may be 600 nm or less, or 200 nm or more. The thickness of the second cladding layer 34 may be 800 nm or less. The tapered optical waveguide 30 may further comprise a second conductivity type III-V compound semiconductor layer 36. The III-V compound semiconductor layer 36 is disposed between the substrate 10 and the second core 32. The III-V compound semiconductor layer 36 may be a buffer layer or a cladding layer. The thickness of the III-V compound semiconductor layer 36 may be 500 nm or less. Adjacent members of the substrate 10, III-V compound semiconductor layer 36, second core 32, and second cladding layer 34 may be in contact with each other.

[0023] The example of the material included in the second core 32 may be the same as the example of the material included in the first core 22. The example of the material included in the second cladding layer 34 may be the same as the example of the material included in the first cladding layer 24. The example of the material included in the III-V compound semiconductor layer 36 may be the same as the example of the material included in the III-V compound semiconductor layer 26.

[0024] The photodetector 40 may be a PIN photodiode. The photodetector 40 may have a mesa structure. The photodetector 40 comprises a light-absorbing layer 42, a first-conductivity type first III-V compound semiconductor layer 44, and a first-conductivity type second III-V compound semiconductor layer 45. The light-absorbing layer 42 is disposed between the substrate 10 and the first III-V compound semiconductor layer 44. The light-absorbing layer 42 is optically coupled to the second core 32. The first III-V compound semiconductor layer 44 is disposed between the light-absorbing layer 42 and the second III-V compound semiconductor layer 45. The first III-V compound semiconductor layer 44 may be a cladding layer. The second III-V compound semiconductor layer 45 may be a contact layer. The second III-V compound semiconductor layer 45 has a dopant concentration higher than that of the first III-V compound semiconductor layer 44. The dopant concentration of the second III-V compound semiconductor layer 45 is 1 × 10⁻¹⁶. 18 cm -3 That's fine too.

[0025] The light-absorbing layer 42 has a second thickness D2. The second thickness D2 is smaller than the first thickness D1 of the second core 32. The second thickness D2 may be 500 nm or less, or 200 nm or more. The first III-V compound semiconductor layer 44 may have a third thickness D3. The third thickness D3 may be 300 nm or less, or 100 nm or more. The second III-V compound semiconductor layer 45 may have a fourth thickness D4. The fourth thickness D4 may be 400 nm or less, or 100 nm or more. The third thickness D3 may be smaller than the fourth thickness D4. The third thickness D3 may be smaller than the second thickness D2 of the light-absorbing layer 42.

[0026] The light-absorbing layer 42 may be an i-type (undoped) III-V compound semiconductor layer. The light-absorbing layer 42 may contain gallium indium arsenide (GaInAs). The example of the material contained in the first III-V compound semiconductor layer 44 may be the same as the example of the material contained in the first cladding layer 24. The second III-V compound semiconductor layer 45 may contain GaInAs.

[0027] The second III-V compound semiconductor layer 45 may have an XY cross-section perpendicular to the thickness direction (Z-axis direction) of the second III-V compound semiconductor layer 45. The area of ​​the XY cross-section of the second III-V compound semiconductor layer 45 is 8 μm². 2 The above is also acceptable, or 120 μm 2 The following may also apply: The XY cross-section of the second III-V compound semiconductor layer 45 may have a rectangular shape. In the XY cross-section of the second III-V compound semiconductor layer 45, the length along the X-axis may be 4 μm or more, or 12 μm or less. In the XY cross-section of the second III-V compound semiconductor layer 45, the length along the Y-axis may be 2 μm or more, or 10 μm or less.

[0028] The photodetector 40 may further include a second conductivity type III-V compound semiconductor layer 46. The III-V compound semiconductor layer 46 is disposed between the substrate 10 and the light absorption layer 42. The III-V compound semiconductor layer 46 may also be a buffer layer. The thickness of the III-V compound semiconductor layer 46 may be 300 nm or less. Examples of materials included in the III-V compound semiconductor layer 46 may be the same as examples of materials included in the III-V compound semiconductor layer 26.

[0029] The photodetector 40 may further include an i-type III-V compound semiconductor layer 48. The III-V compound semiconductor layer 48 is disposed between the III-V compound semiconductor layer 46 and the light absorption layer 42. The III-V compound semiconductor layer 48 may be a buffer layer. The thickness of the III-V compound semiconductor layer 48 may be 300 nm or less. The III-V compound semiconductor layer 48 may contain GaInAsP.

[0030] The semiconductor photodetector 100 may further comprise a second conductivity type III-V compound semiconductor layer 12. The III-V compound semiconductor layer 12 is disposed between the substrate 10 and the photodetector 40. The III-V compound semiconductor layer 12 may be disposed between the substrate 10 and the input optical waveguide 20. The III-V compound semiconductor layer 12 may be disposed between the substrate 10 and the tapered optical waveguide 30. The III-V compound semiconductor layer 12 may be a contact layer. The III-V compound semiconductor layer 12 has a dopant concentration higher than that of the III-V compound semiconductor layer 46. The dopant concentration of the III-V compound semiconductor layer 12 is 1 × 10⁻¹⁶. 17 cm -3 The above is also acceptable. Adjacent members of the substrate 10, III-V compound semiconductor layer 12, III-V compound semiconductor layer 46, III-V compound semiconductor layer 48, light absorption layer 42, first III-V compound semiconductor layer 44, and second III-V compound semiconductor layer 45 may be in contact with each other.

[0031] The semiconductor photodetector 100 may further include a first electrode 50. The first electrode 50 is connected to a second III-V compound semiconductor layer 45. The junction area between the first electrode 50 and the second III-V compound semiconductor layer 45 is 80 μm². 2 The following is also possible: The semiconductor photodetector 100 may further include a second electrode connected to the III-V compound semiconductor layer 12. Wiring may be connected to the first electrode 50 and the second electrode, respectively. A reverse bias voltage may be applied between the first electrode 50 and the second electrode.

[0032] In the semiconductor photodetector 100, the series resistance between the first electrode 50 and the second electrode may be 60 ohms or less, or 40 ohms or less. When the third thickness D3 of the first III-V compound semiconductor layer 44 is small, the series resistance of the semiconductor photodetector 100 becomes small.

[0033] In the semiconductor photodetector 100, the capacitance between the first electrode 50 and the second electrode may be 40 fF (femtofarads) or less, or 10 fF or more. Reducing the cross-sectional area of ​​the XY section of the second III-V compound semiconductor layer 45 reduces the capacitance of the semiconductor photodetector 100.

[0034] The semiconductor photodetector 100 allows for a thinner light absorption layer 42 in the photodetector 40. A thinner light absorption layer 42 shortens the carrier travel time, enabling the semiconductor photodetector 100 to operate at high speed. The 3dB bandwidth of the semiconductor photodetector 100 may be 50GHz or higher.

[0035] If the third thickness D3 of the first III-V compound semiconductor layer 44 is smaller than the fourth thickness D4 of the second III-V compound semiconductor layer 45, the first III-V compound semiconductor layer 44 of the photodetector 40 can be made thinner. Therefore, the series resistance of the semiconductor photodetector 100 can be reduced.

[0036] Figure 3 is a schematic cross-sectional view showing a semiconductor photodetector according to another embodiment. The semiconductor photodetector 100A shown in Figure 3 has the same configuration as the semiconductor photodetector 100, except that it includes an input optical waveguide 120 and a tapered optical waveguide 130 instead of the input optical waveguide 20 and the tapered optical waveguide 30. The input optical waveguide 120 has the same configuration as the input optical waveguide 20, except that it includes a first cladding layer 124 instead of the first cladding layer 24. The first cladding layer 124 is thinner than the first cladding layer 24. The tapered optical waveguide 130 has the same configuration as the tapered optical waveguide 30, except that it includes a second cladding layer 134 instead of the second cladding layer 34. The second cladding layer 134 is thinner than the second cladding layer 34. The second cladding layer 134 may have the same thickness as the first cladding layer 124.

[0037] At the interface between the tapered optical waveguide 130 and the photodetector 40, the upper surface 134U of the second cladding layer 134 is closer to the substrate 10 than the lower surface 45L of the second III-V compound semiconductor layer 45. As a result, the second cladding layer 134 does not come into contact with the second III-V compound semiconductor layer 45.

[0038] The semiconductor photodetector 100A provides the same effects as the semiconductor photodetector 100. Furthermore, light leakage from the second core 32 of the tapered optical waveguide 130 to the second III-V compound semiconductor layer 45 of the photodetector 40 is reduced. Therefore, light absorption by the second III-V compound semiconductor layer 45 of the photodetector 40 can be suppressed.

[0039] Figure 4 is a schematic cross-sectional view showing a semiconductor photodetector according to another embodiment. The semiconductor photodetector 100B shown in Figure 4 has the same configuration as the semiconductor photodetector 100A, except that it includes an input optical waveguide 220 instead of the input optical waveguide 120. The input optical waveguide 220 has the same configuration as the input optical waveguide 120, except that it includes a first cladding layer 224 instead of the first cladding layer 124. The first cladding layer 224 has a fifth thickness D5 at the input end face 224a of the input optical waveguide 220 and a sixth thickness D6 at the output end face 224b of the input optical waveguide 220. The sixth thickness D6 is smaller than the fifth thickness D5. The sixth thickness D6 may be the same as the thickness of the second cladding layer 134. The input optical waveguide 220 may have a first portion including the output end face 224b and a second portion including the input end face 224a. The first portion of the input optical waveguide 220 may have a constant thickness. The second portion of the input optical waveguide 220 may have a thickness that increases from the output end face 224b toward the input end face 224a. Recesses may be formed on the surface of the first cladding layer 224. Such recesses can be formed by photolithography and wet etching.

[0040] The semiconductor photodetector 100B provides the same effects as the semiconductor photodetector 100A. Furthermore, since the first cladding layer 224 is thicker at the input end face 224a of the input optical waveguide 220, the loss of light input to the input end face 224a can be reduced.

[0041] A simulation was performed on a semiconductor photodetector having the same structure as semiconductor photodetector 100. This simulation is not limiting to the present disclosure. The simulation was performed using the following equation (1).

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[0042] In equation (1), f1 represents the 3dB bandwidth value of the semiconductor photodetector. f2 is expressed by the following equation (2).

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[0043] In equation (1), f3 is expressed by the following equation (3).

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[0044] Figures 5 and 6 are graphs illustrating examples of the relationship between the thickness of the light-absorbing layer and the 3dB bandwidth. The horizontal axis represents the thickness of the light-absorbing layer (μm). The vertical axis represents the 3dB bandwidth (GHz). Figure 5 shows the simulation results when the series resistance Rs of the photodiode is 50Ω. Figure 6 shows the simulation results when the series resistance Rs of the photodiode is 20Ω. In Figures 5 and 6, 10fF, 20fF, 30fF, 40fF, and 50fF represent the capacitance C of the photodiode. Each curve shows the simulation results for each value of the photodiode capacitance C. In Figures 5 and 6, the baseline REF shows the simulation results assuming that the photodiode capacitance C is 0fF.

[0045] As shown in Figures 5 and 6, it can be seen that the 3dB bandwidth increases as the capacitance of the photodiode decreases. Furthermore, it can be seen that the 3dB bandwidth increases as the series resistance of the photodiode decreases. Additionally, it can be seen that the 3dB bandwidth increases as the thickness of the light-absorbing layer decreases.

[0046] While preferred embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to the above embodiments. [Explanation of symbols]

[0047] 10... Circuit board 12…III-V compound semiconductor layer 20... Input optical waveguide 22…First Core 24…First cladding layer 26…III-V compound semiconductor layer 30... Tapered optical waveguide 32…Second Core 34…Second cladding layer 36…III-V compound semiconductor layer 40... Light detection unit 42…Light-absorbing layer 44…First III-V compound semiconductor layer 45…Second III-V compound semiconductor layer 45L…Bottom surface 46…III-V compound semiconductor layer 48…III-V compound semiconductor layer 50…1st electrode 100... Semiconductor photodetectors 100A... Semiconductor photodetector 100B... Semiconductor photodetector 120... Input optical waveguide 124...First cladding layer 130... Tapered optical waveguide 134...Second cladding layer 134U…Top surface 220... Input optical waveguide 224...First cladding layer 224a...Input end face 224b…Output end face REF…Reference line

Claims

1. An input optical waveguide provided on the substrate, A tapered optical waveguide is provided on the substrate and connected to the input optical waveguide, A photodetector is provided on the substrate and connected to the tapered optical waveguide, Equipped with, The input optical waveguide comprises a first core and a first cladding layer, the first core being disposed between the substrate and the first cladding layer. The tapered optical waveguide has a width that increases from the input optical waveguide toward the photodetector. The tapered optical waveguide comprises a second core and a second cladding layer, the second core being disposed between the substrate and the second cladding layer, and the second core being optically coupled to the first core. The photodetector comprises a light-absorbing layer, a first III-V compound semiconductor layer of a first conductivity type, and a second III-V compound semiconductor layer of a first conductivity type, wherein the light-absorbing layer is disposed between the substrate and the first III-V compound semiconductor layer, the first III-V compound semiconductor layer is disposed between the light-absorbing layer and the second III-V compound semiconductor layer, the light-absorbing layer is optically coupled to the second core, and the second III-V compound semiconductor layer has a dopant concentration higher than that of the first III-V compound semiconductor layer. The second core has a first thickness, and the light-absorbing layer has a second thickness, the second thickness being smaller than the first thickness. A semiconductor photodetector in which, at the interface between the tapered optical waveguide and the photodetector, the upper surface of the second cladding layer is closer to the substrate than the lower surface of the second III-V compound semiconductor layer.

2. The first III-V compound semiconductor layer has a third thickness, The semiconductor photodetector according to claim 1, wherein the second III-V compound semiconductor layer has a fourth thickness, and the third thickness is smaller than the fourth thickness.

3. The semiconductor photodetector according to claim 2, wherein the third thickness is smaller than the second thickness.

4. The semiconductor photodetector according to any one of claims 1 to 3, wherein the first cladding layer has a fifth thickness at the input end face of the input optical waveguide and a sixth thickness at the output end face of the input optical waveguide, the sixth thickness being smaller than the fifth thickness.

5. The semiconductor photodetector according to any one of claims 1 to 3, wherein the second thickness is 500 nm or less.

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