Waveguide-type light-receiving element

Abstract

A waveguide type light receiving element has a ridge-shaped structure (7) with a light absorbing layer (4) provided on a semiconductor substrate (1). A semiconductor buried layer (8) having a lower refractive index than the light absorbing layer (4) buries a side surface of the light absorbing layer (4). A semiconductor layer (13) having a refractive index between the light absorbing layer (4) and the semiconductor buried layer (8) is provided between the side surface of the light absorbing layer (4) and the semiconductor buried layer (8). The refractive index of the semiconductor layer (13) is set to n3, the wavelength of incident light (15) is set to λ, and the thickness of the semiconductor layer (13) in the lateral direction is in the range of -30% to +20% of λ / (4×n3).

Description

Technical Field

[0001] This disclosure relates to waveguide-type light-receiving elements for use in optical fiber communication and the like. Background Technology

[0002] With the significant increase in communication capacity, there is a growing demand for high-capacity communication systems. Therefore, high-speed optical communication equipment is required. One of the key factors determining the response speed of the semiconductor light-receiving element used in optical communication equipment, namely the photodiode (PD), is the CR time constant. This CR time constant is determined by the capacitance and resistance of the semiconductor light-receiving element. To improve the response speed, the CR time constant needs to be reduced. Therefore, reducing the element capacitance is crucial.

[0003] To reduce component capacitance, waveguide-type light-receiving elements are used, for example, to achieve high-speed response above 40 GHz. This is a structure where light is incident from the side of the epitaxial layer, unlike the usual surface-incident structure, allowing for individual optimization of sensitivity and bandwidth. Therefore, it becomes a structure suitable for high-speed operation.

[0004] Waveguide-type photodetectors are broadly classified into two types. One type is the additively sensing photodetector. In this type, an optical waveguide is formed to a split end face. Light is incident into this waveguide and guided to a light-absorbing layer formed at a distance of several μm or more from the incident portion. In this light-absorbing layer, the transient light seeping from the guiding layer along the layer thickness direction is photoelectrically converted. Therefore, the photoelectric conversion is indirect, and the concentration of photocurrent near the incident end face is mitigated, resulting in the advantage that even with high-intensity incident light, it is difficult to cause a degradation in response speed. On the other hand, since the photoelectric conversion is performed on the light seeping from the guiding layer along the layer thickness direction, there is also the disadvantage that it is difficult to obtain high sensitivity in principle.

[0005] To address this issue, structures have been proposed that involve directly incident light onto the light-absorbing layer or embedding the light-absorbing layer within an Fe-doped InP layer. Because light is directly incident onto the light-absorbing layer through a window layer, these structures achieve high sensitivity even with a relatively short waveguide length. Furthermore, capacitance can be reduced, making it easier to balance high sensitivity with high-speed response.

[0006] However, a refractive index difference arises at the interface between the light-absorbing layer and the semiconductor buried layer, causing light reflection. The reflected light from the light-receiving element, when coupled to the optical fiber, returns towards the light-emitting element, making the operation of the light-emitting element unstable. Therefore, it is desirable to minimize light reflection within the light-receiving element. In response, a structure has been proposed to reduce reflection at the coupling interface by inserting a dielectric film with a refractive index intermediate between the two optical waveguides with different refractive indices (see, for example, Patent Document 1). Figure 7 ).

[0007] Patent Document 1: Japanese Patent Application Publication No. 2006-106587

[0008] However, since it is difficult to stack the desired semiconductor layer on the dielectric film, it is practically impossible to manufacture waveguide-type light-receiving elements with a dielectric film inserted at the interface between the light-absorbing layer and the semiconductor buried layer. Summary of the Invention

[0009] This disclosure was made to solve the aforementioned problems, and its purpose is to obtain a waveguide-type light-receiving element that can reduce light reflection within the element.

[0010] The waveguide-type light-receiving element disclosed herein is characterized by comprising: a semiconductor substrate; a ridge structure disposed on the semiconductor substrate, including at least a light-absorbing layer; a semiconductor buried layer that buries the side of the light-absorbing layer and has a refractive index lower than that of the light-absorbing layer; and a semiconductor layer disposed between the side of the light-absorbing layer and the semiconductor buried layer, having a refractive index between the light-absorbing layer and the semiconductor buried layer, wherein the refractive index of the semiconductor layer is set to n3, the wavelength of the incident light is set to λ, and the lateral thickness of the semiconductor layer is in the range of -30% to +20% of λ / (4×n3).

[0011] In this disclosure, a semiconductor layer having a refractive index between the light-absorbing layer and the semiconductor buried layer is disposed between the side of the absorbing layer and the semiconductor buried layer, and the lateral thickness of the semiconductor layer is set to be in the range of -30% to +20% of λ / (4×n³). As a result, incident light is reflected at the front and rear sides of the semiconductor layer, interfering with each other and canceling each other out. Consequently, light reflection within the device is reduced. Therefore, since the returned light becomes smaller, the operation of the light-emitting element is stabilized. Attached Figure Description

[0012] Figure 1 This is a cross-sectional view showing the waveguide-type light-receiving element of Embodiment 1.

[0013] Figure 2 This is a perspective view of the waveguide-type light-receiving element in Embodiment 1.

[0014] Figure 3 It is along Figure 2 Sectional view I-II.

[0015] Figure 4 This is a cross-sectional view of a waveguide-type light-receiving element for a comparative example.

[0016] Figure 5 It represents the reflectivity of light inside the element relative to... Figure 1 The figure shows the calculated results of the lateral thickness of the semiconductor layer in the structure.

[0017] Figure 6 It represents reflectivity relative to Figure 1 A graph showing the calculated refractive index of the semiconductor layer in the structure.

[0018] Figure 7 This is a cross-sectional view showing the waveguide-type light-receiving element of embodiment 4.

[0019] Figure 8 This is a cross-sectional view showing the waveguide-type light-receiving element of embodiment 4.

[0020] Figure 9 It represents the reflectivity of light inside the element relative to... Figure 7 and Figure 8 The figure shows the calculated results of the lateral thickness of the semiconductor layer in the structure.

[0021] Figure 10 It represents reflectivity relative to Figure 7 and Figure 8 A graph showing the calculated refractive index of the semiconductor layer in the structure. Detailed Implementation

[0022] The waveguide-type light-receiving element of the embodiment will be described with reference to the accompanying drawings. Identical or corresponding components are labeled with the same reference numerals, and instances of repeated description are omitted.

[0023] Implementation method 1.

[0024] Figure 1 This is a cross-sectional view showing the waveguide-type light-receiving element of Embodiment 1. Figure 1 This is a cross-sectional view along the light incident direction. An n-type contact layer 2, an n-type cladding layer 3, a light-absorbing layer 4 made of InGaAs, a p-type cladding layer 5, and a p-type contact layer 6 are sequentially stacked on an InP substrate 1. A ridge structure 7, including at least the light-absorbing layer 4, is disposed on the InP substrate 1. A semiconductor buried layer 8, having a lower refractive index than the light-absorbing layer 4, is buried on both sides of the ridge structure 7, i.e., the sides of the light-absorbing layer 4. A p-type electrode metal 9 is disposed on the p-type contact layer 6.

[0025] A back metal 10 is provided on the entire back surface or a portion of the back surface of the InP substrate 1. An anti-reflective film 11 is provided on at least the light-incident portion of the light-incident surface 14. The surface side, excluding the contact layer, is covered by a passivation film 12. Alternatively, the back metal 10, anti-reflective film 11, and passivation film 12 may be omitted.

[0026] A semiconductor layer 13 made of InGaAsP is disposed between the side of the light-absorbing layer 4 and the semiconductor buried layer 8. Specifically, semiconductor layers 13 are disposed between the front side of the light-absorbing layer 4 on the light-incident surface 14 and the semiconductor buried layer 8 on the light-incident surface 14, and between the rear side of the light-absorbing layer 4 on the opposite side of the light-incident surface 14 and the semiconductor buried layer 8 behind it. The refractive index n3 of the semiconductor layer 13 is the refractive index between the light-absorbing layer 4 and the semiconductor buried layer 8. Incident light 15 is incident from the light-incident surface 14. The wavelength of the incident light 15 is set as λ, and the lateral thickness d1 of the semiconductor layer 13 is set to approximately λ / (4×n3).

[0027] Figure 2 This is a perspective view of the waveguide-type light-receiving element in Embodiment 1. Figure 3 It is along Figure 2 Sectional view I-II. Figure 3 This is a cross-sectional view perpendicular to the direction of light incidence. In the portion of the ridge structure 7 that is not included in the light-absorbing layer 4, an n-type electrode metal 16 connected to the n-type contact layer 2 is provided.

[0028] The crystal growth methods for each semiconductor layer of the aforementioned waveguide-type light-receiving element can include liquid phase growth (LPE), vapor phase growth (VPE), especially metal-organic vapor phase growth (MO-VPE), and molecular beam epitaxy (MBE).

[0029] After the semiconductor layers are crystallized and grown using the above method, an insulating mask is formed using conventional photolithography. Then, using dry or wet etching methods such as reactive ion etching (RIE), the portion of the semiconductor layer not covered by the insulating mask is etched up to the middle of the n-type cladding layer 3. Afterwards, using methods such as MO-VPE, the semiconductor layer 13 and the semiconductor buried layer 8 are crystallized and grown in the etched portion.

[0030] The passivation film 12 is formed by etching the unwanted portions of the insulating film after the insulating film is deposited using methods such as plasma-enhanced chemical vapor deposition (PE-CVD) or sputtering, with the mask remaining only in the desired areas using conventional photolithography. Next, a portion of the area where the semiconductor layer 13 and the semiconductor buried layer 8 are crystallized is etched directly above the n-type contact layer 2 using dry etching or wet etching methods such as RIE.

[0031] The p-type electrode metal 9 and n-type electrode metal 16 can be formed by depositing materials such as Ti, Pt, and Au using methods such as electron beam evaporation or sputtering with the mask open only in the desired area using conventional photolithography techniques, and then removing the unwanted metal portions. Alternatively, the p-type electrode metal 9 and n-type electrode metal 16 can also be formed by wet etching of the unwanted metal portions after the metal has been deposited over the entire surface, with the mask remaining only in the desired area using conventional photolithography techniques.

[0032] The back metal 10 can be formed by inverting the InP substrate 1 and, using conventional photolithography techniques, depositing a film of materials such as Ti, Pt, or Au using methods such as electron beam evaporation or sputtering, with the mask only open in the desired area, and then removing the unwanted metal. Alternatively, the back metal 10 can also be formed by wet etching of the unwanted metal after the metal has been deposited over the entire surface, with the mask remaining only in the desired area using conventional photolithography techniques. The anti-reflective film 11 is formed by evaporation or sputtering on the end face while the chip is cleaved.

[0033] The effects of this embodiment will be explained in comparison with the comparative example. Figure 4 This is a cross-sectional view of a waveguide-type light-receiving element representing a comparative example. In this comparative example, the semiconductor layer 13 is not provided. When light is incident on the light-incident surface 14, the incident light passes through the semiconductor buried layer 8 and reaches the light-absorbing layer 4. At this time, due to the refractive index difference between the semiconductor buried layer 8 and the light-absorbing layer 4, light reflection occurs. Furthermore, light that is not completely absorbed by the light-absorbing layer 4 passes through and reaches the semiconductor buried layer 8 behind the light-absorbing layer 4, but light reflection also occurs at the interface between the light-absorbing layer 4 and the semiconductor buried layer 8. As a result, the amount of light returning to the light-incident surface 14 increases.

[0034] In contrast, in this embodiment, a semiconductor layer 13 having a refractive index between the light-absorbing layer 4 and the semiconductor buried layer 8 is provided on the side of the light-absorbing layer 4 buried in the semiconductor buried layer 8. Moreover, the lateral thickness d1 of the semiconductor layer 13 is set to approximately λ / (4×n3).

[0035] Figure 5 It represents the reflectivity of light inside the element relative to... Figure 1The graph shows the calculated results of the lateral thickness of the semiconductor layer 13 in the structure. The horizontal axis represents the value of the lateral thickness d1 of the semiconductor layer 13 normalized to λ / (4×n3). In the calculation, the wavelength λ of the incident light 15 is set to 1550nm, the refractive index of the semiconductor buried layer 8 is set to 3.17, the real part of the refractive index of the light absorbing layer 4 is set to 3.67, the imaginary part is set to -0.084, and the refractive index n3 is set to 3.4. It can be seen that if the lateral thickness d1 of the semiconductor layer 13 is approximately λ / (4×n3), the reflection of light inside the device can be sufficiently reduced.

[0036] For example, if the wavelength λ of the incident light 15 is set to 1550 nm, the refractive index of the semiconductor buried layer 8 is set to 3.17, and the refractive index of the light-absorbing layer 4 is set to 3.67, then in the comparative example, the reflectivity at the interface between the front side of the semiconductor buried layer 8 and the front side of the light-absorbing layer 4 is 22.6 dB. In this embodiment, if the refractive index n3 is set to 3.3 and the thickness d1 is set to 117 nm, the reflectivity becomes -29.1 dB. Therefore, the reflectivity can be reduced by 6.5 dB, and the reflected light can be reduced accordingly. Similarly, the reflection at the interface between the rear side of the light-absorbing layer 4 and the rear semiconductor buried layer 8 is -22.1 dB in the comparative example and -28.6 dB in this embodiment, which reduces the reflectivity by 6.5 dB.

[0037] However, according to Figure 5 It is known that if the lateral thickness d1 of the semiconductor layer 13 is within the range of -30% to +20% of λ / (4×n3), the reflectivity is reduced to below 0.01%, which is practically harmless. Therefore, the lateral thickness of the semiconductor layer 13 is set to be within the range of -30% to +20% of λ / (4×n3). As a result, the incident light 15 is reflected at the front and rear sides of the semiconductor layer 13, interfering with each other and canceling each other out. Consequently, the reflection of light inside the element can be reduced. Therefore, since the returned light becomes smaller, the operation of the light-emitting element is stabilized.

[0038] Furthermore, while existing techniques exist for inserting dielectric films at the coupling interface of two optical waveguides with different refractive indices, it is difficult to stack the desired semiconductor layer on the dielectric film. In contrast, in this embodiment, the semiconductor buried layer 8 can be epitaxially grown on the semiconductor layer 13 using conventional semiconductor processes. Therefore, waveguide-type light-receiving elements having the semiconductor layer 13 can be manufactured. For example, in the case of the MO-VPE method, since the lateral growth rate of the semiconductor layer 13 can be controlled by gas flow rate or pressure, temperature, etc., the layer thickness control, which is difficult in the deposition of the dielectric film, is also high.

[0039] Furthermore, the InP substrate 1 is preferably a semi-insulating substrate doped with Fe or the like. The material of the n-type contact layer 2 can also be InGaAs, InP, InGaAsP, AlInAs, AlGaInAs, or combinations thereof. The material of the n-type cladding layer 3 can also be InP, InGaAsP, AlInAs, AlGaInAs, or combinations thereof. The material of the light-absorbing layer 4 is not limited to InGaAs, as long as it is a material that generates charge carriers when light is incident, i.e., a material with a small band gap for incident light. It can also be InGaAsP, InGaAsSb, or combinations thereof. The material of the p-type cladding layer 5 can also be InP, InGaAsP, AlInAs, AlGaInAs, or combinations thereof. The material of the p-type contact layer 6 can also be InGaAs, InP, InGaAsP, AlInAs, AlGaInAs, or combinations thereof. The material of the semiconductor buried layer 8 can be InP, InGaAsP, or doped with Fe or Ru. To mitigate band discontinuities, band discontinuity mitigation layers composed of InGaAsP, AlGaInAs, etc., can be provided between the epitaxial layers or between the p-type electrode metal 9 and the epitaxial layers. The passivation film 12 can also be made of SiO2, SiN, SiON, or combinations thereof. As a waveguide-type light-receiving element, any material can be used in each layer as long as the required characteristics for operation are obtained; the range of materials is not limited.

[0040] As p-type dopants that impart conductivity to group III-V semiconductor crystals, group II atoms such as Be, Mg, Zn, and Cd can be used. As n-type dopants, group VI atoms such as S, Se, and Te can be used. As amphoteric impurities that function as dopants of any conductivity type through semiconductor crystallization, group IV atoms such as C, Si, Ge, and Sn can be used. Additionally, atoms such as Fe and Ru act as insulating dopants that suppress conductivity and become semi-insulating (SI) type.

[0041] Implementation method 2.

[0042] In Embodiment 1, the reflectivity is reduced by setting the lateral thickness d1 of the semiconductor layer 13 to approximately λ / (4×n³). However, more precisely, the light-absorbing layer 4 is made of a light-absorbing material, therefore the imaginary part of the refractive index of the light-absorbing layer 4 is not zero. Therefore, as... Figure 5As shown, when the thickness d1 is λ / (4×n3) = 114 nm, the reflectivity is 0.014%. However, if the thickness d1 is shortened to 105 nm, the reflectivity can be reduced to 0.003%. That is, when the lateral thickness d1 of the semiconductor layer 13 is shorter than λ / (4×n3), the reflectivity becomes minimal. Therefore, in this embodiment, the lateral thickness d1 of the semiconductor layer 13 is shortened to λ / (4×n3), and is set to a thickness where the reflectivity of the incident light 15 is minimal. This further reduces the reflectivity.

[0043] Implementation method 3.

[0044] Figure 6 It represents reflectivity relative to Figure 1 The graph shows the calculated refractive index of the semiconductor layer in the structure. In the calculation, the wavelength λ of the incident light 15 was set to 1550 nm, the thickness d1 was set to λ / (4×n3), the refractive index n1 of the semiconductor buried layer 8 was set to 3.17, and the refractive index n2 of the light absorbing layer 4 was set to 3.67. It can be seen that the reflectivity becomes minimal when the refractive index n3 of the semiconductor layer 13 is 3.41. Therefore, in this embodiment, the refractive index n3 of the semiconductor layer 13 is set to a value that minimizes the reflectivity of the incident light 15. Specifically, when the refractive index n3 is set to (n1×n2)^0.5, the reflectivity can be minimized, thus reducing the reflectivity.

[0045] Implementation method 4.

[0046] Figure 7 and Figure 8 This is a cross-sectional view showing the waveguide-type light-receiving element of embodiment 4. Figure 7 It is a cross-sectional view along the direction of light incidence. Figure 8 This is a cross-sectional view perpendicular to the light incident direction. In Embodiment 1, the semiconductor layer 13 is in contact with the light-absorbing layer 4, but in this embodiment, the semiconductor buried layer 8 is also disposed between the side of the light-absorbing layer 4 and the semiconductor layer 13. The lateral spacing d2 between the side of the light-absorbing layer 4 and the semiconductor layer 13 is set to approximately λ / (2×n1). Other structures are the same as in Embodiment 1, and the lateral thickness d1 of the semiconductor layer 13 is set to approximately λ / (4×n3).

[0047] After the first semiconductor buried layer 8 is grown on both sides of the ridge structure 7 formed by etching such as MO-VPE, the semiconductor layer 13 and the second semiconductor buried layer 8 are sequentially crystallized and grown, thereby realizing the structure of this embodiment. Other formation methods are the same as in Embodiment 1.

[0048] Figure 9 It represents the reflectivity of light inside the element relative to... Figure 7 and Figure 8 The graph shows the calculated results of the lateral thickness of the semiconductor layer 13 in the structure. The horizontal axis represents the value of the lateral thickness d1 of the semiconductor layer 13 normalized to λ / (4×n3). In the calculation, the wavelength λ of the incident light 15 is set to 1550nm, the refractive index of the semiconductor buried layer 8 is set to 3.17, the real part of the refractive index of the light absorbing layer 4 is set to 3.67, the imaginary part is set to -0.084, and the refractive index n3 is set to 3.4. It can be seen that if the lateral thickness d1 of the semiconductor layer 13 is approximately λ / (4×n3), the reflection of light inside the device can be sufficiently reduced.

[0049] As described above, in the comparative example, the reflectivity at the interface between the front semiconductor buried layer 8 and the front side of the light-absorbing layer 4 was 22.6 dB. In this embodiment, if the refractive index n3 is 3.3, the thickness d1 is 117 nm, and the spacing d2 is 244 nm, the reflectivity becomes -29.2 dB. Therefore, the reflectivity can be reduced by 6.6 dB, and the reflected light can be reduced accordingly. Similarly, the reflectivity at the interface between the rear side of the light-absorbing layer 4 and the rear semiconductor buried layer 8 was -22.1 dB in the comparative example and -28.5 dB in this embodiment, which reduces the reflectivity by 6.4 dB.

[0050] However, according to Figure 9 It is known that if the lateral thickness d1 of the semiconductor layer 13 is within the range of -30% to +20% of λ / (4×n3), the reflectivity can be reduced to less than 0.01%, which is practically harmless. Therefore, the lateral thickness of the semiconductor layer 13 is set within the range of -30% to +20% of λ / (4×n3). As a result, the incident light 15 is reflected at the front and rear sides of the semiconductor layer 13, interfering with each other and canceling each other out. Consequently, the reflection of light inside the element can be reduced. Therefore, since the returned light becomes smaller, the operation of the light-emitting element is stabilized.

[0051] Furthermore, in this embodiment, a semiconductor buried layer 8 is also disposed between the side surface of the light-absorbing layer 4 and the semiconductor layer 13. Therefore, the semiconductor buried layer 8, which has a smaller refractive index (i.e., a smaller band gap) than the semiconductor layer 13, is in contact with the side surface of the light-absorbing layer 4. As a result, leakage current flowing through the side surface of the light-absorbing layer 4 can be reduced, and long-term reliability can be improved.

[0052] Implementation method 5.

[0053] In embodiment 4, the reflectivity is reduced by setting the lateral thickness d1 of the semiconductor layer 13 to approximately λ / (4×n³). However, more precisely, the light-absorbing layer 4 is made of a light-absorbing material, therefore the imaginary part of the refractive index of the light-absorbing layer 4 is not zero. Therefore, as... Figure 9As shown, when the thickness d1 is λ / (4×n3) = 114 nm, the reflectivity is 0.012%, but if the thickness d1 is shortened to 105 nm, the reflectivity can be reduced to 0.003%. That is, when the lateral thickness d1 of the semiconductor layer 13 is shorter than λ / (4×n3), the reflectivity becomes minimal. Therefore, in this embodiment, the lateral thickness d1 of the semiconductor layer 13 is shortened to λ / (4×n3), and is set to a thickness where the reflectivity of the incident light 15 is minimal. This further reduces the reflectivity.

[0054] Implementation method 6.

[0055] Figure 10 It represents reflectivity relative to Figure 7 and Figure 8 The graph shows the calculated refractive index of the semiconductor layer in the structure. In the calculation, the wavelength λ of the incident light 15 is set to 1550 nm, the thickness d1 is set to λ / (4×n3), the refractive index n1 is set to 3.17, and the refractive index n2 is set to 3.67. It can be seen that the reflectivity becomes minimal when the refractive index n3 of the semiconductor layer 13 is 3.417. Therefore, in this embodiment, the refractive index n3 of the semiconductor layer 13 is set to a value that minimizes the reflectivity of the incident light 15. In addition, in Embodiment 3, the reflectivity is minimized when the refractive index n3 is set to (n1×n2)^0.5, but in this embodiment, if the calculation is performed in the same way, a larger refractive index n3 can further reduce the reflectivity. This is unique when the imaginary part of the refractive index of the light-absorbing layer 4 is not zero.

[0056] Explanation of reference numerals in the attached figures:

[0057] 1…InP substrate (semiconductor substrate); 4…light absorption layer; 7…ridge structure; 8…semiconductor buried layer; 13…semiconductor layer.

Claims

1. A waveguide-type light-receiving element, characterized in that, have: Semiconductor substrate; A ridge-shaped structure disposed on the semiconductor substrate includes at least a light-absorbing layer; A semiconductor buried layer that buries the side of the light-absorbing layer and has a lower refractive index than the light-absorbing layer; as well as A semiconductor layer is disposed between the side of the light-absorbing layer and the buried semiconductor layer, and has the refractive index between the light-absorbing layer and the buried semiconductor layer. The refractive index of the semiconductor layer is set to n3, the wavelength of the incident light is set to λ, and the lateral thickness of the semiconductor layer is in the range of -30% to +20% of λ / (4×n3). The semiconductor buried layer is also disposed between the side of the light-absorbing layer and the semiconductor layer. The refractive index of the semiconductor buried layer is set to n1, and the lateral spacing between the side of the light absorption layer and the semiconductor layer is λ / (2×n1).

2. The waveguide-type light-receiving element according to claim 1, characterized in that, The imaginary part of the refractive index of the light-absorbing layer is not zero. The refractive index of the semiconductor layer is set to n3, the wavelength of the incident light is set to λ, and the lateral thickness of the semiconductor layer is set to be shorter than λ / (4×n3) and the reflectivity of the incident light is minimized.

3. The waveguide-type light-receiving element according to claim 1 or 2, characterized in that, The refractive index of the semiconductor layer is set to a value at which the reflectivity of the incident light becomes a minimum.

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