Semiconductor light receiving element

Abstract

This semiconductor light-receiving element (100) is characterized by being provided with: a semiconductor substrate (1); an n-type buffer layer (2) formed on the semiconductor substrate (1); a multiplication layer (3) formed on the n-type buffer layer (2); a p-type electric field relaxation layer (4) formed on the multiplication layer (3); and a light absorption layer (5) formed on the p-type electric field relaxation layer (4), and any one layer, any two layers or any three layers of the n-type buffer layer (2), the multiplication layer (3) and the p-type electric field relaxation layer (4) are configured from a digital alloy structure.

Description

Technical Field

[0001] The present disclosure relates to a semiconductor light-receiving element. Background Art

[0002] As digital transformation using digital information progresses, communication networks that exchange digital information and data centers that accumulate and process data have developed significantly. Optical communications are used in communication networks and data centers, and in recent years, they have made particularly outstanding progress in terms of speed and capacity. Among them, avalanche photodiodes (APDs) that can achieve high receiving sensitivity are used on the receiving side of optical communications.

[0003] APDs are roughly divided into two types: the surface-incident type in which light is incident from the upper surface of the chip, and the waveguide type in which light is incident from the end face of the chip. For the surface-incident type APD, the diameter of the light receiving part is about 10 to 100 μm and the light is easily coupled to the optical fiber. However, in order to increase the light receiving sensitivity, the light absorption layer needs to be thickened to several μm, and the time for electrons and holes to move in the light absorption layer becomes longer, so the high-speed responsiveness is poor. On the other hand, the waveguide type APD is a structure that absorbs light while propagating the light incident from the end face of the chip to the waveguide-shaped light absorption layer, so high receiving sensitivity can be obtained even if the light absorption layer is thinned to less than 1 μm.

[0004] As mentioned above, waveguide APDs have excellent high-speed responsiveness and are therefore the most important light-receiving devices in high-speed communications above 25 Gbit / sec. In recent years, waveguide APDs have been required to be even faster and more sensitive, so in order to achieve even higher sensitivity, it is necessary to efficiently couple the incident light to the APD's optical waveguide.

[0005] Patent Document 1: Japanese Patent Application Publication No. 2007-311455

[0006] Non-patent literature 1: Jiyuan Zheng et al., "Digital Alloy InAlAs Avalanche Photodiodes", JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL.36, NO.17, SEPTEMBER 1, pp.3580-3585, 2018

[0007] APD is mainly composed of a light absorption layer (InGaAs), an electric field relaxation layer (InP or InAlAs), and a multiplication layer (InP or InAlAs). A high electric field of about 800 kV / cm is applied to the multiplication layer, which multiplies (ionizes) the holes and electrons generated in the light absorption layer. The electric field relaxation layer has the function of weakening the electric field so that the high electric field of the multiplication layer is not applied to the light absorption layer.

[0008] Usually, the electric field relaxation layer and multiplication layer of the waveguide type APD use compound semiconductors such as InAlAs and InAlGaAs, which have a refractive index n higher than the semiconductor material InP that constitutes the substrate and cladding. In the APD, the greater the difference between the ionization rate of holes and electrons, the smaller the excess noise generated during multiplication, and the higher the receiving sensitivity. The difference between the ionization rate of electrons and holes in InAlAs is larger than that in InP. By the way, in InP, the ionization rate of holes is greater than that of electrons, that is, the ionization rate of holes is about twice that of electrons. On the other hand, in InAlAs, the ionization rate of electrons is greater than that of holes, that is, the ionization rate of electrons is about 5 times that of holes.

[0009] Therefore, the receiving sensitivity is higher when InAlAs is used as the multiplication layer of the APD than when InP is used. Therefore, InAlAs is more suitable than InP for the multiplication layer of the waveguide type APD.

[0010] Among the light absorption layer (InGaAs), electric field relaxation layer (InAlAs), multiplication layer (InAlAs), buffer layer (InAlAs) and cladding layer (InP) located in the upper and lower directions that constitute the waveguide APD, the light absorption layer has the highest refractive index n, followed by the electric field relaxation layer, the multiplication layer and the buffer layer. Light has the property of being mostly confined to the layer with a high refractive index n when propagating in a waveguide. That is, light is mostly confined in the light absorption layer with the highest refractive index n, but light is also confined in the electric field relaxation layer, the multiplication layer and the buffer layer with a refractive index n second highest, so the light distribution is asymmetric in the upper and lower directions. As a result, there is a problem of reducing the amount of light confined in the light absorption layer (light confinement rate). In addition, in the above, the upper and lower directions refer to the direction perpendicular to the substrate surface, with the light absorption layer as the starting point and the direction away from the substrate as the upper direction, and the direction close to the substrate as the lower direction.

[0011] As in the semiconductor light-receiving element described in Patent Document 1, in order to improve the light confinement rate, the idea of ​​sandwiching a light-absorbing layer with InAlGaAs (light confinement layer) with a high refractive index n has also been proposed. However, since the electrons and holes spend extra time moving in the light confinement layer, there is a problem of impaired high-speed responsiveness. In addition, since the thermal conductivity of layers with a high refractive index n such as InAlGaAs or InGaAsP is low and the heat dissipation is poor, there is a concern that the semiconductor light-receiving element will deteriorate when the input intensity of light is high. Summary of the invention

[0012] The present disclosure has been made to solve the above-mentioned problems, and an object of the present disclosure is to obtain a semiconductor light-receiving element having high light-receiving sensitivity and excellent high-speed responsiveness.

[0013] The semiconductor light receiving element disclosed in the present invention comprises:

[0014] Semiconductor substrates;

[0015] An n-type buffer layer is formed on the semiconductor substrate;

[0016] a multiplication layer formed on the n-type buffer layer;

[0017] A p-type electric field relaxation layer formed on the multiplication layer; and

[0018] A light absorbing layer is formed on the p-type electric field relaxation layer.

[0019] Any one, any two or three of the n-type buffer layer, the multiplication layer and the p-type electric field relaxation layer are formed of a digital alloy structure.

[0020] According to the semiconductor light-receiving element involved in the present disclosure, it is possible to obtain a semiconductor light-receiving element with high light-receiving sensitivity and excellent high-speed responsiveness because any one, any two or three layers among the n-type buffer layer, the multiplication layer and the p-type electric field relaxation layer are composed of a digital alloy structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a cross-sectional view showing the element structure of the semiconductor light-receiving element according to the first embodiment.

[0022] Figure 2 It is a cross-sectional view showing the element structure of a semiconductor light-receiving element according to a comparative example.

[0023] Figure 3 This is a diagram showing the relationship between the refractive index n and the wavelength in DA-InAlAs, InAlAs as a bulk crystal, and InP.

[0024] Figure 4 This is a cross-sectional view showing the element structure of the semiconductor light-receiving element according to the second embodiment.

[0025] Figure 5 This is a cross-sectional view showing the element structure of the semiconductor light-receiving element according to the third embodiment.

[0026] Figure 6 This is a cross-sectional view showing the element structure of the semiconductor light-receiving element according to the third embodiment.

[0027] Figure 7 This is a cross-sectional view showing the element structure of the semiconductor light-receiving element according to the fourth embodiment.

[0028] Figure 8It is a cross-sectional view showing the element structure of the semiconductor light-receiving element involved in the first variation of the fourth embodiment.

[0029] Fig. 9 This is a cross-sectional view showing the element structure of the semiconductor light-receiving element according to the second variation of the fourth embodiment.

[0030] Fig.10 This is a cross-sectional view showing the element structure of the semiconductor light receiving element involved in the third variation of the fourth embodiment.

[0031] Fig.11 This is a cross-sectional view showing the element structure of the semiconductor light-receiving element according to the fifth embodiment.

[0032] Fig.12 This is a cross-sectional view showing the element structure of the semiconductor light-receiving element according to the first variation of the fifth embodiment.

[0033] Fig.13 This is a cross-sectional view showing the element structure of the semiconductor light receiving element involved in the second variation of the fifth embodiment.

[0034] Fig.14 This is a cross-sectional view showing the element structure of the semiconductor light-receiving element according to the sixth embodiment.

[0035] Fig.15 It is a cross-sectional view showing the element structure of the semiconductor light-receiving element involved in the first variation of the sixth embodiment. DETAILED DESCRIPTION

[0036] Implementation Method 1

[0037] <Element Structure of Semiconductor Light Receiving Element According to Embodiment 1>

[0038] Figure 1 1 is a cross-sectional view showing the device structure of the semiconductor light-receiving device 100 according to Embodiment 1. As an example of the semiconductor light-receiving device 100 according to Embodiment 1, a waveguide type APD is cited.

[0039] The semiconductor light-receiving element 100 according to the first embodiment is composed of a plurality of layers having a carrier concentration of 1 to 5×10 18 cm －3The n-type InAlAs buffer layer 2 (hereinafter referred to as the n-type DA-InAlAs buffer layer 2) has a digital alloy structure composed of multiple layers of alternately stacked n-type AlAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) and n-type InAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) with an overall layer thickness of 0.1 to 1.0 μm, and an overall layer thickness of 0.05 to 0.4 μm and a carrier concentration of 5×10 17 cm －3 The following i-type InAlAs multiplication layer 3 (hereinafter referred to as i-type DA-InAlAs multiplication layer 3) is composed of a digital alloy structure formed by alternately stacking multiple i-type AlAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) and i-type InAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) and has a carrier concentration of 0.1 to 10×10 17 cm －3 The p-type InAlAs electric field relaxation layer 4 (hereinafter referred to as the p-type DA-InAlAs electric field relaxation layer 4) has an overall layer thickness of 10 to 100 nm and is composed of a digital alloy structure formed by alternating multiple p-type AlAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) and p-type InAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm), an i-type InGaAs light absorption layer 5 with a layer thickness of 0.1 to 1.0 μm, a p-type InP cladding layer 6 with a layer thickness of 1 to 3 μm, an n-type electrode 31 formed on the back side of the n-type InP substrate 1, and a p-type electrode 32 formed on the p-type InP cladding layer 6.

[0040] Regarding the digital alloy structure, for example, a digital alloy structure in which AlAs layers and InAs layers are alternately stacked a plurality of times will be referred to as DA-InAlAs.

[0041] A transition layer with a thickness of 0.1 μm or less and made of a compound semiconductor material such as InAlGaAs or InGaAsP may be provided between the p-type DA-InAlAs electric field relaxation layer 4 and the i-type InGaAs light absorption layer 5. The InAlGaAs or InGaAsP constituting the transition layer has an intermediate band gap energy value with respect to the band gap energy of InAlAs constituting the p-type DA-InAlAs electric field relaxation layer 4 and the band gap energy of InGaAs constituting the i-type InGaAs light absorption layer 5. By providing this transition layer, the accumulation of electrons and holes at the heterojunction interface can be prevented.

[0042] In addition, for the same purpose as the above-mentioned transition layer, a transition layer with a thickness of 0.1 μm or less made of a compound semiconductor material such as InAlGaAs or InGaAsP may be provided between the i-type InGaAs light absorbing layer 5 and the p-type InP cladding layer 6. The InAlGaAs or InGaAsP constituting the transition layer has an intermediate band gap energy value with respect to the band gap energy of InGaAs constituting the i-type InGaAs light absorbing layer 5 and the band gap energy of InP constituting the p-type InP cladding layer 6. By providing this transition layer, it is possible to prevent the accumulation of electrons and holes at the heterojunction interface.

[0043] In the semiconductor light-receiving element 100 involved in the first embodiment, DA-InAlAs is used in all layers of the n-type DA-InAlAs buffer layer 2, the i-type DA-InAlAs multiplication layer 3, and the p-type DA-InAlAs electric field relaxation layer 4, but even if DA-InAlAs is applied to only a part of the layers, a certain degree of effect can be obtained. For example, as the n-type buffer layer, n-type InP as a bulk crystal or n-type InAlAs as a bulk crystal may be used, and conversely, as the multiplication layer or the electric field relaxation layer, InAlAs as a bulk crystal may be used, or DA-InAlAs may be applied only to the n-type buffer layer.

[0044] In the semiconductor light-receiving element 100 involved in the first embodiment, as an example of DA-InAlAs, a structure is formed by alternately stacking multiple AlAs layers (layer thickness is 2 atomic layers, about 0.5 nm) and InAs layers (layer thickness is 2 atomic layers, about 0.5 nm), but it is considered that as long as the thickness of the AlAs layer and the InAs layer is in the range of more than 1 atomic layer and less than 10 atomic layers, the refractive index is reduced due to the regularity of the atomic layer arrangement unique to digital alloys. However, in the case where the thickness of the AlAs layer and the InAs layer is 1 atomic layer, since the period is too small, the reduction degree of the refractive index n is small, which is close to the case of random atomic arrangement. In addition, in the case where the thickness of the AlAs layer and the InAs layer exceeds 8 atomic layers, the energy band discontinuity between the layers begins to appear, which begins to affect the movement of holes, so the response speed is reduced, and it is preferably more than 2 atomic layers and less than 8 atomic layers. If the thickness exceeds 6 atomic layers, each layer functions as a quantum well and unnecessary light absorption occurs. Therefore, the thickness of the AlAs layer and the InAs layer is preferably not less than 2 atomic layers and not more than 6 atomic layers, respectively.

[0045] That is, the thickness of the AlAs layer in DA-InAlAs is preferably within the range of N times (2≦N≦6) the thickness of a single atomic layer, and the thickness of the InAs layer is preferably within the range of M times (2≦M≦6) the thickness of a single atomic layer.

[0046] In addition, if the thickness of the AlAs layer and the InAs layer is more than 2 atomic layers and less than 4 atomic layers (2≦M, N≦4), from the perspective of crystal growth, the thickness of each layer is thin and sufficient relative to the critical film thickness, so crystal defects caused by the lattice mismatch of each layer are not easily generated. Ideally, 2 atomic layers are preferred, which is the most sufficient relative to the critical film thickness. Figure 3 This is a graph showing the relationship between the refractive index n and the wavelength in DA-InAlAs (the thickness of the AlAs layer and the InAs layer is 2 atomic layers), InAlAs as a bulk crystal, and InP. By setting the thickness of each layer of DA-InAlAs to 2 atomic layers, the refractive index n is greatly reduced. As a result, Figure 3 As shown, the refractive index n of DA-InAlAs is roughly consistent with the refractive index n of InP. That is, it can be seen that the refractive index n of DA-InAlAs is significantly lower than the refractive index n of InAlAs as a bulk crystal. In addition, the above-mentioned roughly consistent means that, for example, the refractive index difference Δn between the refractive index n of DA-InAlAs and the refractive index n of InP is suitable to be in the range of 0≦Δn≦0.02.

[0047] As a digital alloy structure other than DA-InAlAs, DA-InAlGaAs formed by alternately growing InAlzGa(1-z)As (layer thickness of more than 2 atomic layers and less than 6 atomic layers, Al composition ratio z) and InAlz'Ga(1-z')As (layer thickness of more than 2 atomic layers and less than 6 atomic layers, Al composition ratio z') multiple times can also be similarly applied to each layer such as the multiplication layer. In addition, in a digital alloy structure composed of a quaternary compound semiconductor such as InAlAsSb and a compound semiconductor material system to which Sb is added, it can also be similarly applied to each layer such as the multiplication layer.

[0048] In addition, for the i-type DA-InAlAs multiplication layer 3, the layer thickness of the i-type DA-InAlAs multiplication layer 3 is 0.05 to 0.4 μm and the number of stacking times when 2 atomic layers of AlAs layers (0.5 nm) and 2 atomic layers of InAs layers (0.5 nm) are alternately stacked multiple times is preferably in the range of more than 100 times and less than 800 times.

[0049] In the element structure of APD, if the refractive index n of the cladding (InP) in the up and down directions relative to the waveguide is made substantially the same as the refractive index n of the electric field relaxation layer, the multiplication layer and the n-type buffer layer and only the refractive index n of the light absorption layer is increased, the light distribution can be concentrated in the light absorption layer, that is, the confinement rate of light relative to the light absorption layer can be increased.

[0050] However, if the multiplication layer is made of the same compound semiconductor material as the cladding layers in the upper and lower directions, namely InP, there is the problem of increased noise as described above. The inventors have conducted in-depth research on countermeasures to this problem and have discovered for the first time that by making the bulk crystal of the ternary compound semiconductor, namely InAlAs, into a digital alloy structure, the refractive index n of the multiplication layer is reduced to become substantially the same as the refractive index n of InP, which is a binary compound semiconductor.

[0051] As a specific example of the digital alloy structure, a layer composed of InAlAs is described below. In general, Al, In, and As have a certain composition ratio, and Al atoms and In atoms are randomly arranged, which is also called a bulk crystal or a random alloy structure.

[0052] On the other hand, the structure of forming InAlAs by alternately stacking AlAs, which is a binary compound semiconductor, and InAs, which is also a binary compound semiconductor, multiple times with a layer thickness of several atomic layers (the layer thickness of each layer is more than 2 atomic layers and less than 6 atomic layers: the layer thickness of each layer is 0.5 to 1.8 nm) is also called digital alloy (Digital Alloy), pseudo alloy (Pseudoalloy), etc. Among them, digital alloy is described in non-patent document 1. In addition, pseudo alloy is described in the specification of U.S. Patent No. 6326650.

[0053] As described above, the digital alloy structure has a structure in which multiple semiconductors with different physical property values ​​are stacked at a layer thickness of several atomic layers and each constituent atom is artificially arranged. As a structure similar to the digital alloy structure, there are stacked structures such as multiple quantum well structure (MQW: Multiple Quantum Well) or superlattice structure (SL: Superlattice). However, the essential difference between these structures and the digital alloy structure is that in each layer, each constituent atom is randomly arranged while maintaining a certain composition ratio.

[0054] In a general multi-quantum well structure or superlattice structure, each layer has its own band structure. On the other hand, since the digital alloy structure is a periodic stacked structure formed by alternately stacking several atomic layers, it does not exhibit the properties of each layer of the AlAs layer and the InAs layer, and has a band structure close to that of a bulk crystal, that is, InAlAs, which has an averaged composition ratio. In the digital alloy structure, since the constituent atoms are regularly stacked and the periodicity of the atoms is maintained, which is not present in the random alloy structure, it is believed that the refractive index n is lower than that of the bulk crystal.

[0055] In addition to the above-mentioned binary compound semiconductors, namely, the stacked structure of AlAs and InAs, the digital alloy structure can also be composed of InGaAs formed by alternately stacking InAs and GaAs, which are also binary compound semiconductors, InAlGaAs formed by alternately stacking ternary compound semiconductors, namely, InAlAs and InGaAs, with a layer thickness of several atomic layers, and InAlGaAs formed by alternately stacking quaternary compound semiconductors, namely, InAlzGa(1-z)As and InAlz'Ga(1-z')As, with a layer thickness of several atomic layers.

[0056] The stacked structure formed by alternately stacking a binary compound semiconductor, i.e. AlAs, and a ternary compound semiconductor, i.e. (InzGa(1-z)As), and the stacked structure formed by alternately stacking a ternary compound semiconductor, i.e. InzGa(1-z)As, and a quaternary compound semiconductor, i.e. InAlz'Ga(1-z')As, can also be applied to the digital alloy structure.

[0057] InGaAs formed by stacking a binary compound semiconductor, i.e., GaAs or InAs, and a ternary compound semiconductor, i.e., InzGa(1-z)As, and a stacked structure formed by stacking a binary compound semiconductor, i.e., InAs, GaAs or AlAs, and a quaternary compound semiconductor, i.e., InAlz'Ga(1-z')As, can also be applied to the digital alloy structure. Here, the same can be applied when Al constituting the above-mentioned compound semiconductor material is replaced by P.

[0058] The present invention can be similarly applied to compound semiconductor materials to which Sb is added (InAlAsSb) etc. Alternatively, a quinary compound semiconductor, InAlGaAsP, which is a stack of quaternary compound semiconductors, InAlGaAs or InGaAsP may be used.

[0059] As a digital alloy structure applied to the semiconductor light-receiving element 100 involved in embodiment 1, a combination of compound semiconductor materials such as those described above can be appropriately selected, and a periodic stacked structure formed by alternately stacking multiple layers with a thickness of several atomic layers can show a reduction in the refractive index n relative to the bulk crystal. As described above, as a digital alloy structure, a stacked structure consisting of a layer thickness of more than 2 atomic layers and less than 6 atomic layers is considered to be optimal. This is because, if the stacked structure is composed of a layer thickness of more than 7 atomic layers, each layer functions as a quantum well, and there is a concern that an undesirable situation such as unnecessary light absorption will occur.

[0060] Although there are concerns about crystal strain in digital alloy structures, since each layer is at most a few atomic layers thick, even if the lattice mismatch of each layer is plus or minus a few percent, as long as the cumulative lattice mismatch is small, crystal growth of a stacked structure is possible.

[0061] As a digital alloy structure, the thickness of each layer can also be a period of 1 atomic layer, but in the epitaxial crystal growth of the digital alloy structure, it is necessary to open and close the shutter of the epitaxial crystal growth device or switch the raw material gas every several atomic layers, and the time required for epitaxial crystal growth is long. Therefore, from the perspective of productivity, it is preferably at a level of several atomic layers, for example, more than 2 atomic layers and less than 6 atomic layers. In addition, in the case of 1 atomic layer, there is also a problem that the reduction of the refractive index n is small due to the small period.

[0062] U.S. Patent No. 6,326,650, U.S. Patent No. 6,436,784, U.S. Patent No. 7,045,833, U.S. Patent No. 6,437,362, etc. disclose APDs that achieve noise reduction or high-speed responsiveness improvement by applying a periodic superlattice structure or MQW structure with a thickness of several nanometers or more to the multiplication layer.

[0063] On the other hand, in the present disclosure, the phenomenon of the reduction of the refractive index n was discovered for the first time through experiments with respect to the digital alloy structure, and the light distribution in the optical waveguide was successfully controlled by applying the digital alloy structure to the layer constituting the waveguide type APD. That is, it can be said that the working principle of the digital alloy structure of the present disclosure is different from that of the superlattice structure or MQW structure disclosed in the above-mentioned prior art documents.

[0064] First, the above-mentioned prior art documents utilize the discontinuity of the band gap generated between each layer of the superlattice structure, etc., and operate based on the principle of ionizing electrons and holes. Therefore, unless a periodic superlattice structure is set with each layer having a thickness of more than several nanometers, no band gap discontinuity will be generated.

[0065] On the other hand, compared to the above-mentioned prior art documents, if the digital alloy structure in which the thickness of each layer is set to a period of more than 2 atomic layers and less than 6 atomic layers as shown in the present disclosure is applied, then as described in non-patent document 1, there will no longer be a separate band gap for each layer, but a uniform band gap that is suspected to be the same as that of a bulk crystal. That is, in the above-mentioned prior art documents, the ionization effect of electrons and holes caused by the desired band gap discontinuity does not appear. That is, the working principles of the superlattice structure used for APD in the above-mentioned prior art documents and the digital alloy structure of the present disclosure are different. In addition, in the above-mentioned prior art documents, no phenomenon of a decrease in the refractive index n was found, and there was no record of being able to control the light distribution in the optical waveguide by combining with a waveguide-type APD.

[0066] Next, the difference between the semiconductor light receiving element 100 according to the non-patent document 1 and the embodiment 1 is described. Figure 1 (c) shows the cross-sectional structure of the APD, but the first difference is that it has no light absorption layer, and the multiplication layer with a large band gap absorbs the 543nm wavelength laser. The digital alloy layer has a carrier concentration of up to 1×10 19 cm －3 InGaAs layers, however, due to the high carrier concentration, the diffusion length is short, and the carriers generated by light absorption recombine instantly, so they cannot be extracted as photocurrent. In addition, due to the high carrier concentration, an electric field cannot be applied, so it cannot function as a light absorption layer.

[0067] Second, the APD disclosed in Non-Patent Document 1 is not a waveguide APD because the refractive index is low in the digital alloy layer and high in the vertical contact layer portion of the digital alloy layer, and light incident from the end face is lost in the vertical contact layer portion.

[0068] Third, the APD disclosed in non-patent document 1 is made to facilitate the incidence of light in order to measure noise, and is therefore considered to be a surface-incident APD in which light is incident from the top and bottom directions of the chip. This is because, compared to the surface-incident APD, the tolerance of the incident position of light in the waveguide APD is more than one order of magnitude smaller, so noise cannot be accurately measured. In addition, due to the second reason mentioned above, the APD disclosed in non-patent document 1 is not a waveguide APD. In summary, it can be seen that unlike the semiconductor light receiving element 100 involved in embodiment 1 which is a waveguide APD, the APD disclosed in non-patent document 1 Figure 1 The APD disclosed in (c) is a surface-incident APD.

[0069] Here, it should be pointed out that the present disclosure first clarified that problems would arise when the digital alloy structure is applied to the multiplication layer of the surface-incident APD where light is incident from the top and bottom of the chip. Figure 3 As shown in FIG. 1 , since the refractive index n is reduced in the multiplication layer of the digital alloy structure, the refractive index difference between the light absorption layer and the electric field relaxation layer becomes larger. When light is incident from the top and bottom of the chip, multiple reflections, interference or resonance are caused by the refractive index difference in the multiplication layer part, so the light sensitivity changes according to the wavelength and phase of the incident light, which becomes a problem.

[0070] For example, consider the case where the wavelength of light often used in optical communications is 1300nm. The semiconductor material with an optical thickness T of one quarter of the wavelength that causes interference or resonance. When the refractive index is set to n,

[0071] T=1300 / 4 / n(nm)····(1).

[0072] according to Figure 3 , the refractive index n of DA-InAlAs at a wavelength of 1300nm is about 3.2. Substituting into equation (1), T = 101.6nm. The multiplication layer of APD used in high-speed transmission of 10Gbit / sec or 25Gbit / sec or higher usually has a layer thickness of about 100nm, but the value of T = 101.6nm, which is close to the resonance, makes the wavelength dependence of the light sensitivity extremely large.

[0073] On the other hand, in the semiconductor photosensitive element 100 involved in embodiment 1, since it is constructed so that light is incident on the multiplication layer of the digital alloy structure from the end face direction, it has the advantage of not causing the wavelength dependence of the photosensitivity of the photosensitivity caused by interference, which becomes a problem when the digital alloy structure is applied to the multiplication layer of the surface-incidence APD.

[0074] That is, the inventors discovered for the first time that the refractive index decreases in the digital alloy structure, and pointed out in principle that the wavelength dependence of the photosensitivity increases in the surface-incident APD of non-patent document 1 in which the digital alloy structure is set as the multiplication layer. In addition, it was discovered for the first time that by adopting a waveguide-type APD in which the digital alloy structure is set as the end-face incidence of the multiplication layer, the wavelength dependence of the photosensitivity can be solved and high sensitivity can be achieved, thereby completing the semiconductor photosensitive element involved in the present disclosure.

[0075] In the above description, among the layers constituting the semiconductor light-receiving element 100 involved in Embodiment 1, the n-type DA-InAlAs buffer layer 2, the i-type DA-InAlAs multiplication layer 3, and the p-type DA-InAlAs electric field relaxation layer 4 formed by the digital alloy structure, the specific semiconductor materials, layer thickness, etc. constituting the digital alloy structure are described in detail. However, it is not limited to the semiconductor materials exemplified. That is, the multiplication layer, the electric field relaxation layer, and the n-type buffer layer may also have the following structure.

[0076] The i-type multiplication layer is composed of a digital alloy structure formed by alternating multiple times of first multiplication constituent layers and second multiplication constituent layers, wherein the first multiplication constituent layer is composed of a layer thickness of M times (2≦M≦6) of a single atomic layer, and the second multiplication constituent layer is composed of a layer thickness of N times (2≦N≦6) of a single atomic layer and has a band gap energy smaller than that of the first multiplication constituent layer.

[0077] The p-type electric field relaxation layer is composed of a digital alloy structure formed by alternating multiple layers of a first electric field relaxation layer and a second electric field relaxation layer, wherein the first electric field relaxation layer is composed of a layer thickness of P times (2≦P≦6) of a single atomic layer, and the second electric field relaxation layer is composed of a layer thickness of Q times (2≦Q≦6) of a single atomic layer and has a band gap energy smaller than the band gap energy of the first electric field relaxation layer.

[0078] The n-type buffer layer is composed of a digital alloy structure formed by alternating multiple times of stacking a first buffer constituent layer and a second buffer constituent layer, wherein the first buffer constituent layer is composed of a layer thickness of R times (2≦R≦6) of a single atomic layer, and the second buffer constituent layer is composed of a layer thickness of S times (2≦S≦6) of a single atomic layer and has a band gap energy smaller than the band gap energy of the first buffer constituent layer.

[0079] <Method for Manufacturing Semiconductor Light Receiving Element According to Embodiment 1>

[0080] Hereinafter, a method for manufacturing the waveguide type APD as an example of the semiconductor light receiving element 100 according to the first embodiment will be described.

[0081] Through metal organic chemical vapor deposition (MOVPE: Metal Organic Vapor Phase Epitaxy) or molecular beam epitaxy (MBE: Molecular Beam Epitaxy) Epitaxial crystal growth methods such as epitaxial growth and epitaxy are used to grow, on the surface of an n-type InP substrate 1, an n-type DA-InAlAs buffer layer 2 having an overall layer thickness of 0.1 to 1.0 μm, which is composed of an n-type AlAs layer consisting of 2 atomic layers and an n-type InAs layer consisting of 2 atomic layers alternately stacked multiple times; an i-type DA-InAlAs multiplication layer 3 having an overall layer thickness of 0.05 to 0.4 μm, which is composed of an i-type AlAs layer consisting of 2 atomic layers and an i-type InAs layer consisting of 2 atomic layers alternately stacked multiple times; a p-type DA-InAlAs electric field relaxation layer 4 having an overall layer thickness of 10 to 100 nm, which is composed of a p-type AlAs layer consisting of 2 atomic layers and a p-type InAs layer consisting of 2 atomic layers alternately stacked multiple times; an i-type InGaAs light absorption layer 5 having a layer thickness of 0.1 to 1.0 μm; and a p-type InP cladding layer 6 having a layer thickness of 1 to 3 μm are sequentially crystal-grown.

[0082] After the crystal growth, a metal material such as Ti / Au is formed on the surface of the p-type InP cladding layer 6 by vapor deposition or the like, and then patterned to form the p-type electrode 32 .

[0083] After the processing of the front side of the wafer is completed, the back side of the n-type InP substrate 1 is ground, and a metal material such as AuGeNi is deposited by evaporation to form the n-type electrode 31. The wafer is separated into individual chips by cleavage, thereby completing the waveguide APD.

[0084] The above is the method for manufacturing the waveguide type APD as an example of the semiconductor light receiving element 100 according to the first embodiment.

[0085] <Operation of the semiconductor light-receiving element according to the first embodiment>

[0086] Figure 3 This is the result of actually measuring the wavelength dependence of the refractive index n of InP, InAlAs (bulk crystal) and DA-InAlAs. In the wavelength band longer than the absorption edge wavelength of the semiconductor, the longer the wavelength, the more monotonically the refractive index n decreases. Since the absorption edge wavelength of InAlAs (bulk crystal) is 860nm and the absorption edge wavelength of InP is 920nm, in the wavelength band of 1000nm to 1600nm, the longer the wavelength, the lower the refractive index n of each semiconductor.

[0087] At a wavelength of 1300nm, which is often used in short-distance optical communications, the refractive index n of InP is 3.204, the refractive index n of InAlAs (bulk crystal) is 3.254, and the refractive index n of DA-InAlAs is 3.212. In addition, at a wavelength of 1550nm, which is often used in long-distance optical communications, the refractive index n of InP is 3.166, the refractive index n of InAlAs (bulk crystal) is 3.219, and the refractive index n of DA-InAlAs is 3.169. That is, at any wavelength of 1300nm and 1550nm, the refractive index n of DA-InAlAs is lower than the refractive index n of InAlAs (bulk crystal) and is roughly consistent with the refractive index n of InP.

[0088] As described above, the inventors first discovered that the refractive index n of the digital alloy structure is smaller than the refractive index n of the bulk crystal (random alloy structure). In the digital alloy structure, since atoms are regularly arranged compared to the bulk crystal (random alloy structure), it is believed that the refractive index n is lower.

[0089] exist Figure 2, as a comparative example, a cross-sectional structure of a waveguide type APD, which is an example of a general semiconductor light receiving element 300 composed of a normal bulk crystal of a non-digital alloy structure, is shown. The semiconductor light receiving element 300 is composed of an n-type InAlAs buffer layer 2a, an i-type InAlAs multiplication layer 3a, a p-type InAlAs electric field relaxation layer 4a, an i-type InGaAs light absorption layer 5, a p-type InP cladding layer 6, an n-type electrode 31 formed on the back side of the n-type InP substrate 1, and a p-type electrode 32 formed on the p-type InP cladding layer 6, which are sequentially formed on an n-type InP substrate 1.

[0090] from Figure 2 As shown in the light distribution of the propagating light, it can be seen that the incident light reflects the refractive index distribution of the waveguide and expands in the direction of the i-type InAlAs multiplication layer 3a and the n-type InAlAs buffer layer 2a in addition to the i-type InGaAs light absorption layer 5. Therefore, the light distribution of the propagating light is attracted to the i-type InAlAs multiplication layer 3a side, and the light confinement rate of the i-type InGaAs light absorption layer 5 is reduced. In other words, due to the influence of the expansion of the light distribution of the propagating light, the light confinement coefficient of the i-type InGaAs light absorption layer 5 becomes smaller. Therefore, there is a problem that it is difficult to further improve the receiving sensitivity.

[0091] On the other hand, in the waveguide type APD as an example of the semiconductor light receiving element 100 involved in the first embodiment, the refractive index n of the n-type DA-InAlAs buffer layer 2, the i-type DA-InAlAs multiplication layer 3, and the p-type DA-InAlAs electric field relaxation layer 4 respectively composed of DA-InAlAs is substantially the same as the refractive index n of InP constituting the p-type InP cladding layer 6. Therefore, the refractive index n is increased only in the i-type InGaAs light absorbing layer 5 composed of InGaAs. Figure 1 As shown, the light distribution of the propagating light is symmetrical in the vertical direction with the i-type InGaAs light absorbing layer 5 as the center.

[0092] As mentioned above, in Figure 2 In the comparative example shown, the light distribution of the propagating light is attracted to the i-type InAlAs multiplication layer 3a side, so that the light confinement rate of the i-type InGaAs light absorption layer 5 is reduced, but in Figure 1 In the semiconductor light receiving element 100 according to the first embodiment shown, the light distribution is symmetrical in the vertical direction, so the light confinement rate of the i-type InGaAs light absorbing layer 5 is increased, thereby achieving an effect of obtaining high light receiving sensitivity.

[0093] <Effects of Embodiment 1>

[0094] As described above, according to the semiconductor light receiving element involved in embodiment 1, at least the multiplication layer and the electric field relaxation layer are composed of a digital alloy structure, so that high light receiving sensitivity can be obtained. In addition, the SN ratio is improved by the high receiving sensitivity, and the receiving sensitivity is also improved.

[0095] In addition, according to the semiconductor light receiving element involved in embodiment 1, since the light confinement rate of the light absorbing layer is high, the waveguide length can be shortened, the pn junction capacitance can be reduced, and the effect of achieving high speed is achieved. In addition, since the light confinement rate of the light absorbing layer is high, the thickness of the light absorbing layer can be reduced, and the movement time of electrons and holes is shortened, and the effect of achieving a faster response can be achieved.

[0096] Implementation Method 2

[0097] <Element Structure of Semiconductor Light Receiving Element According to Embodiment 2>

[0098] Figure 4 2 is a cross-sectional view showing the device structure of the semiconductor light-receiving device 110 according to Embodiment 2. As an example of the semiconductor light-receiving device 110 according to Embodiment 2, a waveguide-type APD having an optical waveguide portion is cited.

[0099] The semiconductor light receiving element 110 according to the second embodiment includes a light receiving portion 110 a formed on a common n-type InP substrate 1 , and an optical waveguide portion 110 b in contact with the light receiving portion 110 a via a connecting portion 13 .

[0100] The light receiving portion 110a has the same structure as the layers constituting the semiconductor light receiving element 100 according to Embodiment 1. The structure of each layer has been described in Embodiment 1, and thus will be omitted here.

[0101] The optical waveguide portion 110b is composed of an InP cladding layer (not shown in the figure because it is composed of the same InP as the n-type InP substrate 1) formed on the n-type InP substrate 1, an optical waveguide layer 11 composed of a quaternary compound semiconductor, i.e., InGaAsP or InAlGaAs, having a band gap wavelength shorter than the wavelength of light to be guided, and an upper InP cladding layer 12. For example, in the case of guiding light with a wavelength of 1300 nm, in the optical waveguide layer 11 composed of InGaAsP, the band gap wavelength is preferably in the range of about 1000 nm to 1200 nm.

[0102] The InP constituting the upper InP cladding layer 12 may be of n-type or p-type conductivity or i-type. The i-type InGaAs light absorbing layer 5 of the light receiving portion 110a and the light waveguide layer 11 of the light waveguide portion 110b are connected so that their heights as viewed from the bottom surface of the element are substantially the same.

[0103] When the composition wavelength of InGaAsP is 1150nm and lattice matching with InP, when the composition ratio of InGaAsP is expressed by In(1-x)Ga(x)As(y)P(1-y), x=0.18, y=0.41. The refractive index n of InGaAsP is given by 3.4+0.256y-0.095y^2, so it is about 3.5. On the other hand, since the refractive index n of InGaAs is about 3.6, the refractive index n of InGaAsP and InGaAs is greater than the refractive index n of the InP cladding, which is 3.2. Figure 4 As shown in the light distribution, in the light waveguide portion 110 b , light is mainly confined in the light waveguide layer 11 , and in the light receiving portion 110 a , light is mainly confined in the i-type InGaAs light absorbing layer 5 .

[0104] Since the refractive index difference Δn with respect to InP is 0.3 with respect to InGaAsP constituting the optical waveguide layer 11 and is 0.4 with respect to InGaAs constituting the i-type InGaAs light absorption layer 5, the optimum layer thickness of the InGaAsP optical waveguide layer 11 is 0.4 / 0.3=4 / 3 times that of the InGaAs constituting the i-type InGaAs light absorption layer 5. Since the layer thickness of InGaAs constituting the i-type InGaAs light absorption layer 5 is in the range of about 0.15 to 0.9 μm, when the layer thickness of the InGaAsP optical waveguide layer 11 is set to about 0.2 to 1.2 μm, the light distribution of the InGaAsP optical waveguide layer 11 is substantially consistent with the light distribution of the i-type InGaAs light absorption layer 5. That is, if the layer thickness dp of the InGaAsP optical waveguide layer 11 and the layer thickness da of the i-type InGaAs light absorbing layer 5 are set to satisfy the following formula (2), the light distribution and the equivalent refractive index can be made substantially the same between the light receiving portion 110a and the optical waveguide portion 110b.

[0105] Δnp∶Δna=da∶dp····(2)

[0106] In formula (2), Δna is the refractive index difference between the i-type InGaAs light absorption layer 5 and the p-type InP cladding layer 6, Δnp is the refractive index difference between the InGaAsP optical waveguide layer 11 and the upper InP cladding layer 12, da is the layer thickness of the i-type InGaAs light absorption layer 5, and dp is the layer thickness of the InGaAsP optical waveguide layer 11.

[0107] <Operation of the semiconductor light-receiving element according to the second embodiment>

[0108] exist Figure 4 The left end of FIG. 1 shows the refractive index distribution of the optical waveguide portion 110 b. Figure 4 The right end of shows the refractive index distribution of the light receiving portion 110a. Figure 4As shown in the light distribution of the incident light, the light incident from the lens or optical fiber to the semiconductor light receiving element 110 is symmetrical in the vertical direction. Therefore, when the light distribution of the propagating light propagating in the optical waveguide portion 110b is also symmetrical in the vertical direction, the light coupling efficiency becomes high. Figure 2 As shown, the light distribution of the propagating light of the semiconductor light receiving element 120 as a comparative example reflects the asymmetric refractive index distribution in the up and down directions and is still asymmetric in the up and down directions. Therefore, assuming that the light receiving portion 110a is set to the structure of the semiconductor light receiving element 120 as a comparative example, coupling loss occurs at the connecting portion 13 where the optical waveguide portion 110b contacts the light receiving portion 110a.

[0109] On the other hand, in the case of the semiconductor light receiving element 110 according to the second embodiment, Figure 4 As shown in the light distribution of the light receiving portion 110a, the light distribution is symmetrical in the vertical direction, so the coupling loss at the connection portion 13 can be reduced. In addition, if the layer thickness and refractive index n of the InGaAsP optical waveguide layer 11 are set to satisfy the above formula (2), the light distribution of the optical waveguide portion 110b can be made substantially consistent with the light distribution of the light receiving portion 110a, and the coupling loss at the connection portion 13 can be further reduced.

[0110] In addition, since the equivalent refractive indexes of the optical waveguide portion 110b and the light receiving portion 110a can be made roughly consistent using a simple formula such as formula (2), the element design becomes easy and the design accuracy is improved. When the equivalent refractive indexes of the optical waveguide portion 110b and the light receiving portion 110a can be made consistent, the reflected return light from the connecting portion 13 can be reduced, so that the sensitivity fluctuation or noise caused by light interference unique to the integrated device can be reduced. As long as the reflected return light can be reduced, the light receiving sensitivity of the APD can be improved, and the deviation of the light receiving sensitivity between the elements can also be reduced.

[0111] <Effects of Embodiment 2>

[0112] As described above, according to the semiconductor photosensitive element involved in embodiment 2, in the element structure connecting the optical waveguide part and the photosensitive part, by applying the digital alloy structure to the buffer layer, the multiplication layer and the electric field relaxation layer of the photosensitive part, the light distribution of the optical waveguide part can be made roughly consistent with the light distribution of the photosensitive part, and the coupling loss at the connection part caused by the mismatch of the light distribution can be reduced, thereby improving the coupling efficiency of the optical waveguide part and the photosensitive part, thereby improving the photosensitivity of the APD.

[0113] In addition, the equivalent refractive index at the connection portion can be easily made substantially consistent between the optical waveguide portion and the light receiving portion, and reflection at the connection portion is reduced, thereby achieving the effect of reducing fluctuations in receiving sensitivity and noise caused by light interference that are unique to integrated devices.

[0114] Implementation 3

[0115] <Element Structure of Semiconductor Light Receiving Element According to Embodiment 3>

[0116] Figure 5 2 is a cross-sectional view showing the device structure of the semiconductor light-receiving device 120 according to Embodiment 3. As an example of the semiconductor light-receiving device 120 according to Embodiment 3, a waveguide-type APD having an optical waveguide portion is cited.

[0117] The semiconductor light receiving element 120 according to the third embodiment includes a light receiving portion 120 a formed on a common n-type InP substrate 1 , and an optical waveguide portion 120 b in contact with the light receiving portion 120 a via a connecting portion 13 .

[0118] The light receiving portion 120a is formed on the n-type InP substrate 1 and has a carrier concentration of 1 to 5×10 18 cm －3 The n-type InAlAs buffer layer 2b with a thickness of 0.1 to 1.0 μm and serving as an optical waveguide layer, the overall thickness of 0.05 to 0.4 μm and a carrier concentration of 5×10 17 cm －3 The i-type DA-InAlAs multiplication layer 3 is formed by alternately stacking multiple i-type AlAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) and i-type InAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm), and the carrier concentration is 0.1 to 10×10 17 cm －3 The p-type DA-InAlAs electric field relaxation layer 4 is formed by alternating multiple p-type AlAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) and p-type InAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) with an overall layer thickness of 10 to 100 nm, an i-type InGaAs light absorption layer 5 with a layer thickness of 0.1 to 1.0 μm, a p-type InP cladding layer 6 with a layer thickness of 1 to 3 μm, an n-type electrode 31 formed on the back side of the n-type InP substrate 1, and a p-type electrode 32 formed on the p-type InP cladding layer 6.

[0119] In addition, in the above structure, the n-type InAlAs buffer layer 2b can also be composed of InAlGaAs or InGaAsP which is a bulk crystal. In addition, in the above structure, the n-type InAlAs buffer layer 2b refers to a layer which also serves as an optical waveguide layer and is arranged on the lower side of the i-type DA-InAlAs multiplication layer 3, that is, on the side of the n-type InP substrate 1, and is a layer having a higher refractive index n than the n-type InP cladding layer (which also serves as the n-type InP substrate 1). In the above structure, an optical waveguide layer can also be provided on the upper side of the n-type InAlAs buffer layer 2b. For example, the n-type InAlAs buffer layer 2b, the InAlGaAs optical waveguide layer, and the i-type DA-InAlAs multiplication layer 3 can also be formed in sequence on the InP cladding layer.

[0120] Since multiplication does not usually occur in the electric field relaxation layer, it does not necessarily need to be composed of DA-InAlAs, and a bulk crystal with a low refractive index n, that is, p-type InP, can also be used as the electric field relaxation layer.

[0121] A transition layer with a thickness of about 0.1 μm or less, such as InAlGaAs or InGaAsP, may be provided between the p-type DA-InAlAs electric field relaxation layer 4 and the i-type InGaAs light absorption layer 5 to prevent the accumulation of electrons and holes. In addition, for the same purpose, a transition layer with a thickness of about 0.1 μm or less, such as InAlGaAs or InGaAsP, may be provided between the i-type InGaAs light absorption layer 5 and the p-type InP cladding layer 6 to prevent the accumulation of electrons and holes.

[0122] The optical waveguide portion 120b is composed of an n-type InP cladding layer (not shown in the figure because it is composed of the same InP as the n-type InP substrate) formed on the n-type InP substrate 1, an optical waveguide layer 11 composed of a quaternary compound semiconductor, i.e., InGaAsP or InAlGaAs, having a band gap wavelength shorter than the wavelength of light to be guided, and an upper InP cladding layer 12. For example, in the case of guiding light with a wavelength of 1300 nm, in the InGaAsP optical waveguide layer 11, the band gap wavelength is preferably in the range of about 1000 nm to 1200 nm.

[0123] The InP constituting the upper InP cladding layer 12 may be of n-type or p-type conductivity or i-type. The n-type InAlAs buffer layer 2b of the light receiving portion 120a and the optical waveguide layer 11 of the optical waveguide portion 120b are connected in such a manner that the heights thereof as viewed from the bottom surface of the element are substantially the same. That is, the propagation light guided to the optical waveguide layer 11 of the optical waveguide portion 120b is incident on the vicinity of the n-type InAlAs buffer layer 2b located below the i-type DA-InAlAs multiplication layer 3.

[0124] In the semiconductor light-receiving element 120 involved in Embodiment 3, as an example of DA-InAlAs, a stacked structure of an AlAs layer (as an example, a layer thickness of 2 atomic layers, about 0.5 nm) and an InAs layer (as an example, a layer thickness of 2 atomic layers, a layer thickness of about 0.5 nm) is used, but the thickness of the InAs layer and the AlAs layer only needs to be in the range of 1 atomic layer or more and 10 atomic layers or less. However, in the case where the thickness of the AlAs layer and the InAs layer is 1 atomic layer, the degree of reduction in the refractive index n is small. On the other hand, in the case where the thickness of the AlAs layer and the InAs layer exceeds 6 atomic layers, since the energy band discontinuity between the layers occurs, as DA-InAlAs, a stacked structure consisting of layers with a thickness of 2 atomic layers or more and 6 atomic layers or less is preferably used.

[0125] As a digital alloy structure other than DA-InAlAs, DA-InAlGaAs formed by alternately stacking multiple layers of InAlzGa(1-z)As (layer thickness is more than 2 atomic layers and less than 6 atomic layers, Al composition ratio z) and InAlz'Ga(1-z')As (layer thickness is more than 2 atomic layers and less than 6 atomic layers, Al composition ratio z') can also be similarly applied to each layer such as the multiplication layer. In addition, in a digital alloy structure composed of a compound semiconductor material system to which Sb is added, such as InAlAsSb, it can also be similarly applied to each layer such as the multiplication layer.

[0126] The refractive index n of the n-type InAlAs buffer layer 2b composed of bulk crystals is about 3.3. On the other hand, when the composition wavelength of InGaAsP constituting the InGaAsP optical waveguide layer 11 is 1080nm and is lattice-matched with InP, when it is set to In(1-x)Ga(x)As(y)P(1-y), x=0.13, y=0.30. The refractive index n of InGaAsP with this composition ratio is given by 3.4+0.256y-0.095y^2, which is about 3.5. Since the refractive index n of both the InGaAsP optical waveguide layer 11 and the n-type InAlAs buffer layer 2b is greater than the refractive index n of the InP cladding layer, that is, 3.2, if Figure 5 As shown in the light distribution, the light is confined in the vicinity of the connection portion 13 where the light receiving portion 120a and the optical waveguide portion 120b are in contact.

[0127] In order to suppress the reflection of the propagating light at the connection portion 13, it is necessary to make the equivalent refractive index of the light receiving portion 120a and the optical waveguide portion 120b as consistent as possible. Since the refractive index difference Δn with respect to InP is 0.3 for InGaAsP constituting the InGaAsP optical waveguide layer 11 and 0.1 for InAlAs constituting the n-type InAlAs buffer layer 2b, the optimal layer thickness of the InGaAsP optical waveguide layer 11 is 0.1 / 0.3=1 / 3 times the layer thickness of the InAlAs layer constituting the n-type InAlAs buffer layer 2b.

[0128] Since the thickness of the n-type InAlAs buffer layer 2b is in the range of about 0.3 to 0.9 μm, when the thickness of the InGaAsP optical waveguide layer 11 is set in the range of 0.1 to 0.3 μm, the light distribution of the InGaAsP optical waveguide layer 11 is substantially consistent with the light distribution of the n-type InAlAs buffer layer 2b. That is, if the thickness of the InGaAsP optical waveguide layer 11 and the thickness of the n-type InAlAs buffer layer 2b are set to satisfy the following formula (3), it is easy to make the light distribution and the equivalent refractive index substantially consistent between the light receiving portion 120a and the optical waveguide portion 120b.

[0129] Δnp: Δnb = db: dp (3)

[0130] In formula (3), Δnp is the refractive index difference between the InGaAsP optical waveguide layer 11 and the upper InP cladding layer 12, Δnb is the refractive index difference between the n-type InAlAs buffer layer 2b and the InP cladding layer, db is the layer thickness of the n-type InAlAs buffer layer 2b, and dp is the layer thickness of the InGaAsP optical waveguide layer 11.

[0131] <Operation of the semiconductor light-receiving element according to the third embodiment>

[0132] use Figure 5 The function of the semiconductor light receiving element 120 involved in the third embodiment is described. The propagation light propagating from the optical waveguide portion 120b is incident on the n-type InAlAs buffer layer 2b of the light receiving portion 120a through the connecting portion 13. The light distribution in the n-type InAlAs buffer layer 2b is formed to be substantially consistent with the light distribution in the InGaAsP optical waveguide layer 11 of the optical waveguide portion 120b, that is, it satisfies the formula (3). As a result, the coupling loss at the connecting portion 13 can be suppressed. In addition, since the equivalent refractive index of the light receiving portion 120a and the optical waveguide portion 120b is also substantially consistent, the reflected return light that becomes a problem in the integrated device is reduced. If the refractive index n of the multiplication layer is high, radiation loss and reflected return light will be generated in the connecting portion 13, but in the semiconductor light receiving element 120 involved in the third embodiment, since a digital alloy structure with a low refractive index n is used, the radiation loss and reflected return light at the connecting portion 13 can be reduced.

[0133] The light incident on the n-type InAlAs buffer layer 2b propagates while gradually transferring to the i-type InGaAs light absorption layer 5 with a higher refractive index n through evanescent coupling. The advantage of the evanescent coupling type APD is that the light absorption at the connection portion 13 will not become too large, and as the light travels, the light is gradually absorbed in the light absorption layer, so the photocurrent density is averaged relative to the travel direction of the waveguide. When the photocurrent density becomes the same in the waveguide direction, even if a large amount of light is incident, the space charge effect is suppressed and the multiplication rate will not decrease. That is, the receiving dynamic range of the APD is improved.

[0134] On the other hand, the conventional evanescent coupling type APD uses a bulk crystal with a large refractive index n, that is, InAlAs, in the multiplication layer, so that the leakage of light into the light absorption layer becomes larger. As a result, the light absorption near the connection portion, that is, the photocurrent density, becomes larger, and the dynamic range is limited. However, by setting the structure as the semiconductor light receiving element 120 involved in the third embodiment, the receiving dynamic range can be greatly improved.

[0135] For example, Figure 6 As shown in FIG. 1 , the same effect of improving the reception dynamic range is obtained in the element structure in which the optical waveguide portion is not connected. Figure 6 The waveguide type APD as an example of the semiconductor light receiving element 130 shown in FIG. 1 is similar to the semiconductor light receiving element 130 except that the semiconductor light receiving element 130 does not have an optical waveguide portion. Figure 5 That is, the semiconductor light receiving element 130 has the same structure as the light receiving portion 120 a of the semiconductor light receiving element 120 .

[0136] exist Figure 6 In the case of the semiconductor light receiving element 130 shown in the figure, light from a lens or an optical fiber is incident on the n-type InAlAs buffer layer 2b or the optical waveguide layer (not shown) located below the i-type DA-InAlAs multiplication layer 3. Figure 5 Similarly to the element structure shown, the semiconductor light receiving element 130 is set as an i-type DA-InAlAs multiplication layer 3 with a low refractive index n, so that the light incident from the end face slowly propagates into the i-type InGaAs light absorption layer 5, so that the reduction in the multiplication rate when light with high light intensity is incident is suppressed, thereby achieving the effect of improving the receiving dynamic range.

[0137] <Effects of Embodiment 3>

[0138] As described above, according to the semiconductor photosensitive element involved in embodiment 3, a digital alloy structure with a low refractive index is used in the multiplication layer, and an evanescent waveguide-type APD is configured in which light is incident on the buffer layer which also serves as an optical waveguide layer on the lower side of the multiplication layer, thereby suppressing the radiation loss of light and reflected return light at the connection between the optical waveguide portion and the photosensitive portion. By applying a digital alloy structure with a low refractive index to the multiplication layer, the light slowly moves toward the light absorption layer, thereby suppressing the light absorption near the connection portion, and suppressing the reduction in the multiplication rate when large light is incident, thereby achieving an effect of improving the receiving dynamic range.

[0139] Implementation 4

[0140] <Element Structure of Semiconductor Light Receiving Element According to Embodiment 4>

[0141] Figure 7 1 is a cross-sectional view showing the element structure of the semiconductor light receiving element 140 according to Embodiment 4. A waveguide type APD is cited as an example of the semiconductor light receiving element 140 according to Embodiment 4. In Embodiment 4, the element structure is obtained by inverting the pn junction upside down relative to the semiconductor light receiving element 100 according to Embodiment 1.

[0142] The semiconductor light-receiving element 140 according to the fourth embodiment is composed of a semiconductor substrate 1a and a semiconductor substrate 1a having a carrier concentration of 0.1 to 10×10 18 cm －3 The p-type InP cladding layer 7, which also serves as an optical waveguide layer, has a thickness of 0.1 to 10 μm, the i-type InGaAs light absorption layer 5 has a thickness of 0.1 to 1.0 μm, and the carrier concentration is 0.1 to 10×10 17 cm －3 The p-type DA-InAlAs electric field relaxation layer 4 is composed of a plurality of p-type AlAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) and a p-type InAs layer (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) alternately stacked multiple times with an overall layer thickness of 10 to 100 nm, and a p-type DA-InAlAs electric field relaxation layer 4 is composed of a total layer thickness of 0.05 to 0.4 μm and a carrier concentration of 5×10 17 cm －3 The following i-type DA-InAlAs multiplication layer 3 is formed by alternately stacking multiple i-type AlAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) and i-type InAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm), and the carrier concentration is 0.5 to 10×10 18 cm －3 The substrate 1a is composed of an n-type InP cladding layer 6a having a thickness of 1 to 3 μm, a p-type electrode 32a formed on the back side of a semi-insulating InP substrate 1a, and an n-type electrode 31a formed on the n-type InP cladding layer 6a.

[0143] Since multiplication does not usually occur in the electric field relaxation layer, it does not necessarily need to be composed of DA-InAlAs, and a bulk crystal with a low refractive index n, that is, p-type InP, can also be used as the electric field relaxation layer.

[0144] The p-type InP cladding layer 7 also serves as a conductive layer. Instead of the p-type InP cladding layer 7, a p-type cladding layer composed of p-type InGaAsP or p-type InGaAs, or a p-type cladding layer composed of a stacked structure of p-type InP, p-type InGaAsP, p-type InGaAs, or p-type InAlGaAs may be used.

[0145] The p-type electrode 32a is electrically connected to the p-type InP cladding layer 7. For example, a method of partially removing a lower layer portion below the p-type InP cladding layer 7 to obtain contact can be considered.

[0146] Modification 1 of Implementation Example 4

[0147] <Element Structure of Semiconductor Light Receiving Element According to Modification 1 of Embodiment 4>

[0148] Figure 8 1 is a cross-sectional view showing the device structure of a semiconductor light receiving device 150 according to Modification 1 of Embodiment 4. As an example of the semiconductor light receiving device 150 according to Modification 1 of Embodiment 4, a waveguide type APD having an optical waveguide portion is cited.

[0149] The semiconductor light receiving element 150 according to the first modification of the fourth embodiment includes a light receiving portion 150 a formed on a common semi-insulating InP substrate 1 a and an optical waveguide portion 150 b connected to the light receiving portion 150 a via a connecting portion 13 .

[0150] The light receiving portion 150a has the same structure as the layers constituting the semiconductor light receiving element 140 according to Embodiment 4. Since the structure of each layer has been described in Embodiment 4, it will be omitted here.

[0151] The optical waveguide portion 150b is composed of an InP cladding layer (not shown in the figure because it is composed of the same InP as the semi-insulating InP substrate 1a) formed on the semi-insulating InP substrate 1a, an optical waveguide layer 11 composed of a quaternary compound semiconductor, i.e., InGaAsP or InAlGaAs, having a band gap wavelength shorter than the wavelength of light to be guided, and an upper InP cladding layer 12. For example, in the case of guiding light with a wavelength of 1300nm, in the case of the InGaAsP optical waveguide layer 11, the band gap wavelength is preferably in the range of 1000nm to 1200nm.

[0152] The InP constituting the upper InP cladding layer 12 may be of n-type or p-type conductivity or i-type. The i-type InGaAs light absorbing layer 5 of the light receiving portion 150a and the InGaAsP light waveguide layer 11 of the light waveguide portion 150b are connected so that their heights as viewed from the bottom of the element are substantially the same.

[0153] Modification 2 of Implementation Method 4

[0154] <Element Structure of Semiconductor Light Receiving Element According to Modification 2 of Embodiment 4>

[0155] Fig. 9 2 is a cross-sectional view showing the device structure of the semiconductor light receiving element 160 according to the second modification of the fourth embodiment. As an example of the semiconductor light receiving element 160 according to the second modification of the fourth embodiment, a waveguide type APD having an optical waveguide portion is cited.

[0156] The semiconductor light receiving element 160 according to the second variation of the fourth embodiment includes a light receiving portion 160a formed on a common semi-insulating InP substrate 1a and an optical waveguide portion 160b connected to the light receiving portion 160a via a connecting portion 13. Figure 5 The semiconductor light receiving element 120 according to the third embodiment shown in the figure similarly has an element structure in which an optical waveguide portion and an evanescent coupling type light receiving portion are connected.

[0157] The light receiving portion 160a has a structure substantially similar to that of the layers constituting the semiconductor light receiving element 140 according to the fourth embodiment, except that an n-type InAlAs optical waveguide layer 8 is provided on the upper side of the i-type DA-InAlAs multiplication layer 3. The n-type InAlAs optical waveguide layer 8 has a layer thickness of about 0.1 to 1 μm and a carrier concentration of 0.1 to 10×10 18 cm －3 The n-type optical waveguide layer may be a layer composed of n-type InGaAsP or n-type InAlGaAs, or a stacked structure of n-type InGaAsP or n-type InAlGaAs.

[0158] The semiconductor light receiving element 160 according to the second variation of the fourth embodiment is similar to the semiconductor light receiving element 140 according to the fourth embodiment, in which the upper surface is the n-type electrode 31a, and the p-type electrode 32a is electrically connected to the p-type InP cladding layer 7 on the lower side of the i-type InGaAs light absorbing layer 5. Similar to the semiconductor light receiving element 120 according to the third embodiment, the InGaAsP light waveguide layer 11 of the light waveguide portion 160b is in contact with the n-type InAlAs light waveguide layer 8 of the light receiving portion 160a.

[0159] Modification 3 of Implementation Method 4

[0160] <Element Structure of Semiconductor Light Receiving Element According to Modification 3 of Embodiment 4>

[0161] Fig.10 1 is a cross-sectional view showing the device structure of a semiconductor light receiving element 170 according to Modification 3 of Embodiment 4. As an example of the semiconductor light receiving element 170 according to Modification 3 of Embodiment 4, a waveguide type APD having an optical waveguide portion is cited.

[0162] The semiconductor light receiving element 170 according to the third modification of the fourth embodiment has an element structure in which the optical waveguide portion 160 b is removed from the semiconductor light receiving element 160 according to the second modification of the fourth embodiment and only the light receiving portion 160 a is provided.

[0163] The semiconductor light receiving element 170 according to the third modification of the fourth embodiment has a structure in which light is incident on the n-type InAlAs optical waveguide layer 8 from the end surface.

[0164] <Operations of semiconductor light-receiving elements according to the fourth embodiment and modifications 1 to 3 of the fourth embodiment>

[0165] In the waveguide APD, in order to obtain a high-speed response and a wide receiving dynamic range, the carrier concentration of the electric field relaxation layer is set in such a way that a high electric field is applied to the light absorption layer. Since the band gap of the light absorption layer is the narrowest among the semiconductor materials used in the waveguide APD (in the case of InGaAs, the band gap wavelength is 1670nm), tunnel current and generation-recombination current occur, and the dark current increases. In order to reduce the generation of recombination current, it is preferred to stack InGaAs constituting the light absorption layer on the lattice-matched crystal.

[0166] In embodiments 1 to 3, the i-type InGaAs light absorption layer 5 is crystal-grown on the upper portion of the i-type DA-InAlAs multiplication layer 3 composed of a digital alloy structure. When forming the digital alloy structure, layers to which compressive strain and tensile strain are applied are alternately stacked to allow the crystal to grow in a manner that is almost strain-free as a whole. The digital alloy structure composed of InAlAs is, for example, a semiconductor layer formed by alternately stacking multiple InAs layers (layer thickness of 2 atomic layers, about 0.5 nm) and AlAs layers (layer thickness of 2 atomic layers, about 0.5 nm).

[0167] The lattice spacing of InAs is 0.618nm, and the lattice spacing of AlAs is 0.566nm. The lattice spacing is offset from 0.588nm of InP as the substrate, so compressive strain and tensile strain are applied alternately to each layer. When strain is applied alternately, the strain of the i-type DA-InAlAs multiplication layer 3 as a whole is offset, but sometimes residual strain is left due to the deviation of the layer thickness during crystal growth. If the InGaAs layer crystal with a narrow band gap constituting the i-type InGaAs light absorption layer 5 is grown on the crystal with residual strain, there is a concern that crystal defects and dislocations will occur, which are the source of the generation of composite current. That is, if the i-type InGaAs light absorption layer 5 is provided on the upper side of the layer composed of the digital alloy structure, there is a concern that the manufacturing deviation of the dark current will increase.

[0168] On the other hand, in the semiconductor light receiving elements 140, 150, 160, and 170 according to Embodiment 4 and Modifications 1 to 3 of Embodiment 4, since the digital alloy structure crystals constituting the i-type DA-InAlAs multiplication layer 3 and the p-type DA-InAlAs electric field relaxation layer 4 are grown after the crystal growth of the i-type InGaAs light absorbing layer 5, the i-type InGaAs light absorbing layer 5 can be crystal-grown on the InP layer without strain. Therefore, in the i-type InGaAs light absorbing layer 5, the crystal defects and dislocations that are the source of the recombination current are reduced, thereby achieving the effect of reducing the manufacturing variation of the dark current of the semiconductor light receiving element.

[0169] <Effects of Embodiment 4 and Modifications 1 to 3 of Embodiment 4>

[0170] As described above, according to the semiconductor light receiving element involved in the fourth embodiment and the modified examples 1 to 3 of the fourth embodiment, in addition to the effects achieved by the semiconductor light receiving element involved in the first to third embodiments, since the light absorbing layer that is prone to dark current can be crystal-grown not on the upper side of the digital alloy structure that is prone to residual strain but on the cladding layer that also serves as the optical waveguide layer without strain, the effect of reducing the manufacturing variation of the dark current can be achieved. In addition, since crystal defects and dislocations are suppressed in the light absorbing layer, the recombination of carriers generated by light absorption can be prevented, thereby achieving the effect of improving quantum efficiency.

[0171] Implementation method 5

[0172] <Element Structure of Semiconductor Light Receiving Element According to Embodiment 5>

[0173] Fig.11 2 is a cross-sectional view showing the device structure of the semiconductor light-receiving device 180 according to Embodiment 1. As an example of the semiconductor light-receiving device 180 according to Embodiment 5, a waveguide type APD is given.

[0174] The semiconductor light-receiving element 180 according to the fifth embodiment is composed of a plurality of layers having a carrier concentration of 1 to 5×10 18 cm －3 The n-type InAlAs buffer layer 2c has a thickness of 0.1 to 1.0 μm, and the layer thickness is 0.05 to 0.4 μm and the carrier concentration is 5×10 17 cm －3 The following i-type InAlAs multiplication layer 3c has a carrier concentration of 0.1 to 10×10 17 cm －3 The p-type DA-InAlAs electric field relaxation layer 4 is composed of a digital alloy structure with an overall layer thickness of 10 to 100 nm, which is formed by alternating multiple p-type AlAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) and p-type InAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm), an i-type InGaAs light absorption layer 5 with a layer thickness of 0.1 to 1.0 μm, a p-type InP cladding layer 6 with a layer thickness of 1 to 3 μm, an n-type electrode 31 formed on the back side of the n-type InP substrate 1, and a p-type electrode 32 formed on the p-type InP cladding layer 6.

[0175] The semiconductor light receiving element 180 according to the fifth embodiment is characterized in that only the electric field relaxation layer is formed of a digital alloy structure.

[0176] Modification 1 of Implementation Example 5

[0177] <Element Structure of Semiconductor Light Receiving Element According to Modification 1 of Embodiment 5>

[0178] Fig.12 1 is a cross-sectional view showing the device structure of a semiconductor light receiving device 190 according to Modification 1 of Embodiment 5. As an example of the semiconductor light receiving device 190 according to Modification 1 of Embodiment 5, a waveguide type APD is cited.

[0179] The semiconductor light receiving element 190 according to the first modification of the fifth embodiment is composed of a plurality of semiconductor layers having a carrier concentration of 1 to 5×10 18 cm －3 The n-type InAlAs buffer layer 2c has a layer thickness of 0.1 to 1.0 μm, the overall layer thickness is 0.05 to 0.4 μm, and the carrier concentration is 5×10 17 cm －3The following is an i-type DA-InAlAs multiplication layer 3 composed of a digital alloy structure formed by alternately stacking multiple i-type AlAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) and i-type InAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm), and a carrier concentration of 0.1 to 10×10 17 cm －3 It is composed of a p-type InAlAs electric field relaxation layer 4c with a thickness of 10 to 100 nm, an i-type InGaAs light absorption layer 5 with a thickness of 0.1 to 1.0 μm, a p-type InP cladding layer 6 with a thickness of 1 to 3 μm, an n-type electrode 31 formed on the back side of the n-type InP substrate 1, and a p-type electrode 32 formed on the p-type InP cladding layer 6.

[0180] The semiconductor light receiving element 190 according to the first modification of the fifth embodiment is characterized in that only the multiplication layer is composed of a digital alloy structure.

[0181] Modification 2 of Implementation Example 5

[0182] <Element Structure of Semiconductor Light Receiving Element According to Modification 2 of Embodiment 5>

[0183] Fig.13 2 is a cross-sectional view showing the device structure of a semiconductor light receiving device 200 according to Modification 2 of Embodiment 5. As an example of the semiconductor light receiving device 200 according to Modification 2 of Embodiment 5, a waveguide type APD is cited.

[0184] The semiconductor light receiving element 200 according to the second modification of the fifth embodiment is composed of a plurality of layers having a carrier concentration of 1 to 5×10 18 cm －3 The n-type DA-InAlAs buffer layer 2 has a total layer thickness of 0.1 to 1.0 μm and is composed of a digital alloy structure formed by alternately stacking multiple n-type AlAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) and n-type InAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm), a layer thickness of 0.05 to 0.4 μm and a carrier concentration of 5×10 17 cm －3 The following i-type InAlAs multiplication layer 3c has a carrier concentration of 0.1 to 10×10 17 cm －3 It is composed of a p-type InAlAs electric field relaxation layer 4c with a thickness of 10 to 100 nm, an i-type InGaAs light absorption layer 5 with a thickness of 0.1 to 1.0 μm, a p-type InP cladding layer 6 with a thickness of 1 to 3 μm, an n-type electrode 31 formed on the back side of the n-type InP substrate 1, and a p-type electrode 32 formed on the p-type InP cladding layer 6.

[0185] The semiconductor light receiving element 200 according to the second modification of the fifth embodiment is characterized in that only the buffer layer is composed of a digital alloy structure.

[0186] In the semiconductor light receiving elements 180, 190, 200 according to the fifth embodiment, the first variant of the fifth embodiment, and the second variant of the fifth embodiment, a transition layer having a thickness of 0.1 μm or less and made of a compound semiconductor material such as InAlGaAs or InGaAsP may be provided between the p-type DA-InAlAs electric field relaxation layer 4 or the p-type InAlAs electric field relaxation layer 4c and the i-type InGaAs light absorption layer 5. The InAlGaAs or InGaAsP constituting the transition layer has a band gap energy value intermediate between the band gap energy of InAlAs constituting the p-type DA-InAlAs electric field relaxation layer 4 or the p-type InAlAs electric field relaxation layer 4c and the band gap energy of InGaAs constituting the i-type InGaAs light absorption layer 5. By providing the transition layer, the accumulation of electrons and holes at the heterojunction interface can be prevented.

[0187] In addition, for the same purpose as the above-mentioned transition layer, a transition layer with a thickness of 0.1 μm or less made of a compound semiconductor material such as InAlGaAs or InGaAsP may be provided between the i-type InGaAs light absorbing layer 5 and the p-type InP cladding layer 6. The InAlGaAs or InGaAsP constituting the transition layer has an intermediate band gap energy value with respect to the band gap energy of InGaAs constituting the i-type InGaAs light absorbing layer 5 and the band gap energy of InP constituting the p-type InP cladding layer 6. By providing this transition layer, it is possible to prevent the accumulation of electrons and holes at the heterojunction interface.

[0188] In the semiconductor light-receiving element 100 involved in the first embodiment, the DA-InAlAs structure is used in all layers of the n-type DA-InAlAs buffer layer 2, the i-type DA-InAlAs multiplication layer 3, and the p-type DA-InAlAs electric field relaxation layer 4, but in the semiconductor light-receiving elements 180, 190, and 200 involved in the fifth embodiment, the first variant of the fifth embodiment, and the second variant of the fifth embodiment, the DA-InAlAs structure is applied to any one of the buffer layer, the multiplication layer, and the electric field relaxation layer. In addition, the structure in which the DA-InAlAs structure is applied to a part of each buffer layer, multiplication layer, and electric field relaxation layer is also included in the fifth embodiment, the first variant of the fifth embodiment, and the second variant of the fifth embodiment.

[0189] <Operations of semiconductor light-receiving elements according to the fifth embodiment, the first modification of the fifth embodiment, and the second modification of the fifth embodiment>

[0190] When the refractive index is different between the lower layer (substrate side) and the upper layer (upper surface side) of the light absorbing layer, the light propagation mode is asymmetric in the vertical direction as described in Embodiment 1. The degree of asymmetry in the vertical direction depends on the following formula (4), and the larger the value of formula (4), the more the light propagation mode is biased toward the substrate side.

[0191] Γa·Δna·Ta+Γb·Δnb·Tb+Γc·Δnc·Tc···(4)

[0192] In formula (4), Γa, Γb, Γc are the light confinement coefficients of the electric field relaxation layer, multiplication layer, and buffer layer, respectively; Δna, Δnb, Δnc are the differences between the refractive index of the electric field relaxation layer, multiplication layer, and buffer layer and the refractive index of InP, respectively; Ta, Tb, Tc are the layer thicknesses of the electric field relaxation layer, multiplication layer, and buffer layer, respectively.

[0193] If the value of equation (4) is made as small as possible, the asymmetry of the light propagation mode in the vertical direction is reduced. Therefore, by making any one of the electric field relaxation layer, the multiplication layer, and the buffer layer or a part of any one of the layers into a digital alloy structure, any one of Δna, Δnb, and Δnc can be reduced.

[0194] <Effects of Embodiment 5, Modification 1 of Embodiment 5, and Modification 2 of Embodiment 5>

[0195] As described above, according to the semiconductor light receiving element involved in Embodiment 5, Variation 1 of Embodiment 5, and Variation 2 of Embodiment 5, since any one of the electric field relaxation layer, the multiplication layer, and the buffer layer, or a part of any one of the electric field relaxation layer, the multiplication layer, and the buffer layer is composed of a digital alloy structure, the propagation mode of light in the waveguide type APD is symmetrical in the vertical direction, thereby achieving an effect of obtaining high light receiving sensitivity. In addition, the SN ratio is improved by the high receiving sensitivity, thereby achieving an effect of improving the receiving sensitivity.

[0196] The semiconductor photosensitive elements involved in embodiment 5, variant example 1 of embodiment 5, and variant example 2 of embodiment 5 are less effective in reducing the asymmetry of the light propagation mode than embodiment 1 in which the electric field relaxation layer, the multiplication layer, and the buffer layer are all set as digital alloy structures. However, the number of element switching times in the MOVPE device or the MBE device used for the crystal growth of each layer is reduced, so the crystal growth time can be shortened, thereby achieving the effect of reducing the consumption of the MOVPE device or the MBE device.

[0197] Implementation 6

[0198] <Element Structure of Semiconductor Light Receiving Element According to Embodiment 6>

[0199] Fig.142 is a cross-sectional view showing the device structure of the semiconductor light-receiving device 210 according to Embodiment 6. As an example of the semiconductor light-receiving device 210 according to Embodiment 6, a waveguide type APD is cited.

[0200] The semiconductor light-receiving element 210 according to the sixth embodiment is composed of a plurality of semiconductor layers having a carrier concentration of 1 to 5×10 18 cm －3 The n-type DA-InAlAs buffer layer 2 has a total layer thickness of 0.1 to 1.0 μm and is composed of a digital alloy structure formed by alternately stacking multiple n-type AlAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) and n-type InAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm), a layer thickness of 0.05 to 0.4 μm and a carrier concentration of 5×10 17 cm －3 The following i-type InAlAs multiplication layer 3c has a carrier concentration of 0.1 to 10×10 17 cm －3 The p-type DA-InAlAs electric field relaxation layer 4 is composed of a digital alloy structure with an overall layer thickness of 10 to 100 nm, which is formed by alternating multiple p-type AlAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) and p-type InAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm), an i-type InGaAs light absorption layer 5 with a layer thickness of 0.1 to 1.0 μm, a p-type InP cladding layer 6 with a layer thickness of 1 to 3 μm, an n-type electrode 31 formed on the back side of the n-type InP substrate 1, and a p-type electrode 32 formed on the p-type InP cladding layer 6.

[0201] The semiconductor light-receiving element 210 according to the sixth embodiment is characterized in that only the buffer layer and the electric field relaxation layer are formed of a digital alloy structure.

[0202] Modification 1 of Implementation Example 6

[0203] <Element Structure of Semiconductor Light Receiving Element According to Modification 1 of Embodiment 6>

[0204] Fig.15 1 is a cross-sectional view showing the device structure of the semiconductor light receiving element 220 according to the first modification of the sixth embodiment. As an example of the semiconductor light receiving element 220 according to the first modification of the sixth embodiment, a waveguide type APD is cited.

[0205] The semiconductor light receiving element 220 according to the first modification of the sixth embodiment is composed of a plurality of semiconductor layers having a carrier concentration of 1 to 5×10 18 cm －3The n-type DA-InAlAs buffer layer 2 has a total layer thickness of 0.1 to 1.0 μm and is composed of a digital alloy structure formed by alternately stacking multiple n-type AlAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) and n-type InAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm), and has a total layer thickness of 0.05 to 0.4 μm and a carrier concentration of 5×10 17 cm －3 The following is an i-type DA-InAlAs multiplication layer 3 composed of a digital alloy structure formed by alternately stacking multiple i-type AlAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm) and i-type InAs layers (as an example, the layer thickness is 2 atomic layers, about 0.5 nm), and a carrier concentration of 0.1 to 10×10 17 cm －3 It is composed of a p-type InAlAs electric field relaxation layer 4c with a thickness of 10 to 100 nm, an i-type InGaAs light absorption layer 5 with a thickness of 0.1 to 1.0 μm, a p-type InP cladding layer 6 with a thickness of 1 to 3 μm, an n-type electrode 31 formed on the back side of the n-type InP substrate 1, and a p-type electrode 32 formed on the p-type InP cladding layer 6.

[0206] The semiconductor light receiving element 220 according to the first variation of the sixth embodiment is characterized in that only the buffer layer and the multiplication layer are formed of a digital alloy structure.

[0207] In the semiconductor light receiving elements 210 and 220 according to Embodiment 6 and Variation 1 of Embodiment 6, a transition layer having a thickness of 0.1 μm or less and made of a compound semiconductor material such as InAlGaAs or InGaAsP may be provided between the p-type DA-InAlAs electric field relaxation layer 4 or the p-type InAlAs electric field relaxation layer 4c and the i-type InGaAs light absorption layer 5. The InAlGaAs or InGaAsP constituting the transition layer has a band gap energy value intermediate between the band gap energy of InAlAs constituting the p-type DA-InAlAs electric field relaxation layer 4 or the p-type InAlAs electric field relaxation layer 4c and the band gap energy of InGaAs constituting the i-type InGaAs light absorption layer 5. By providing the transition layer, the accumulation of electrons and holes at the heterojunction interface can be prevented.

[0208] In addition, for the same purpose as the above-mentioned transition layer, a transition layer with a thickness of 0.1 μm or less made of a compound semiconductor material such as InAlGaAs or InGaAsP may be provided between the i-type InGaAs light absorbing layer 5 and the p-type InP cladding layer 6. The InAlGaAs or InGaAsP constituting the transition layer has an intermediate band gap energy value with respect to the band gap energy of InGaAs constituting the i-type InGaAs light absorbing layer 5 and the band gap energy of InP constituting the p-type InP cladding layer 6. By providing this transition layer, it is possible to prevent the accumulation of electrons and holes at the heterojunction interface.

[0209] In the semiconductor light receiving element 100 involved in the first embodiment, the DA-InAlAs structure is used in all layers of the n-type DA-InAlAs buffer layer 2, the i-type DA-InAlAs multiplication layer 3, and the p-type DA-InAlAs electric field relaxation layer 4, but in the semiconductor light receiving element 210 involved in the sixth embodiment, the DA-InAlAs structure is applied to two layers, namely the buffer layer and the electric field relaxation layer, and in the semiconductor light receiving element 220 involved in the first variation of the sixth embodiment, the DA-InAlAs structure is applied to two layers, namely the buffer layer and the multiplication layer. In addition, the structure in which DA-InAlAs is applied to a part of each buffer layer, multiplication layer, and electric field relaxation layer is also included in the sixth embodiment and the first variation of the sixth embodiment.

[0210] <Effects of Embodiment 6 and Modification 1 of Embodiment 6>

[0211] As described above, according to the semiconductor light receiving element involved in Embodiment 6 and Modification 1 of Embodiment 6, since the two layers of the buffer layer and the electric field relaxation layer or the two layers of the buffer layer and the multiplication layer, or a part of the two layers of the buffer layer and the electric field relaxation layer or a part of the two layers of the buffer layer and the multiplication layer are composed of a digital alloy structure, the propagation mode of light in the waveguide type APD is symmetrical in the vertical direction, thereby achieving an effect of being able to obtain high light receiving sensitivity. In addition, the SN ratio is improved by the high receiving sensitivity, thereby achieving an effect of improving the receiving sensitivity.

[0212] The semiconductor photosensitive element involved in embodiment 6 and variant example 1 of embodiment 6 has a smaller effect of reducing the asymmetry of the light propagation mode than embodiment 1 in which the electric field relaxation layer, the multiplication layer, and the buffer layer are all set as digital alloy structures, but the number of element switching times in the MOVPE device or MBE device used for the crystal growth of each layer is reduced, so the crystal growth time can be shortened, thereby achieving the effect of reducing the consumption of the MOVPE device or MBE device.

[0213] The present disclosure describes various illustrative embodiments and examples, but various features, methods, and functions described in one or more embodiments are not limited to application in specific embodiments and can be applied to the embodiments alone or in various combinations.

[0214] Therefore, numerous modifications not shown in the examples are conceivable within the technical scope disclosed in the present specification, including, for example, the modification, addition or omission of at least one constituent element, and the extraction and combination of at least one constituent element with constituent elements of other embodiments.

[0215] Description of Reference Numerals

[0216] 1...n-type InP substrate; 2...n-type DA-InAlAs buffer layer; 2a, 2b, 2c...n-type InAlAs buffer layer; 3...i-type DA-InAlAs multiplication layer; 3a, 3c...i-type InAlAs multiplication layer; 4...p-type DA-InAlAs electric field relaxation layer; 4a, 4c...p-type InAlAs electric field relaxation layer; 5...i-type InGaAs light absorption layer; 6, 7...p-type InP cladding layer; 6a...n-type InP cladding layer; 8.. .n-type InAlAs optical waveguide layer; 11...optical waveguide layer; 12...upper InP cladding layer; 31, 32a...n-type electrode; 31a, 32...p-type electrode; 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 300...semiconductor light receiving element; 110a, 120a, 150a, 160a...light receiving portion; 110b, 120b, 150b, 160b...optical waveguide portion.

Claims

1. A semiconductor light receiving element, characterized in that: have: Semiconductor substrates; An n-type buffer layer is formed on the semiconductor substrate; a multiplication layer formed on the n-type buffer layer; A p-type electric field relaxation layer formed on the multiplication layer; as well as a light absorbing layer formed on the p-type electric field relaxation layer, Any one, any two or three of the n-type buffer layer, the multiplication layer and the p-type electric field relaxation layer are formed of a digital alloy structure.

2. A semiconductor light receiving element, characterized in that: have: Semiconductor substrates; A p-type cladding layer formed on the semiconductor substrate; a light absorbing layer formed on the p-type cladding layer; A p-type electric field relaxation layer formed on the light absorption layer; a multiplication layer formed on the p-type electric field relaxation layer; as well as An n-type cladding layer is formed on the multiplication layer, Either one or both of the multiplication layer and the p-type electric field relaxation layer are formed of a digital alloy structure.

3. The semiconductor light receiving element according to claim 1, wherein: In the case where the n-type buffer layer is composed of a digital alloy structure, the n-type buffer layer is a digital alloy structure formed by alternately stacking a first buffer constituent layer and a second buffer constituent layer for multiple times, wherein the first buffer constituent layer is composed of a layer thickness of R times the single atomic layer, 2≦R≦6, and the second buffer constituent layer is composed of a layer thickness of S times the single atomic layer and has a band gap energy smaller than the band gap energy of the first buffer constituent layer, 2≦S≦6.

4. The semiconductor light receiving element according to claim 1 or 3, characterized in that: have: a light receiving portion formed on the semiconductor substrate and having at least the n-type buffer layer, the multiplication layer, the p-type electric field relaxation layer and the light absorption layer; and The optical waveguide portion is formed on the semiconductor substrate so as to be in contact with the light receiving portion and includes at least an optical waveguide layer for guiding incident light to the light absorbing layer.

5. The semiconductor light receiving element according to claim 1 or 3, characterized in that: have: a light receiving portion formed on the semiconductor substrate and having at least the n-type buffer layer, the multiplication layer, the p-type electric field relaxation layer and the light absorption layer; and The optical waveguide portion is formed on the semiconductor substrate so as to be in contact with the light receiving portion and includes at least an optical waveguide layer that guides incident light toward the n-type buffer layer.

6. The semiconductor light receiving element according to claim 2, wherein: The semiconductor light receiving element further includes an n-type optical waveguide layer which is formed between the multiplication layer and the n-type cladding layer and has a refractive index higher than that of the multiplication layer.

7. The semiconductor light receiving element according to claim 2, characterized in that: have: a light receiving portion formed on the semiconductor substrate and comprising at least the p-type cladding layer, the light absorption layer, the p-type electric field relaxation layer, the multiplication layer and the n-type cladding layer; and The optical waveguide portion is formed on the semiconductor substrate so as to be in contact with the light receiving portion, and includes at least an optical waveguide layer that guides incident light to the light absorbing layer.

8. The semiconductor light receiving element according to claim 2, wherein: have: a light receiving portion formed on the semiconductor substrate and comprising at least the p-type cladding layer, the light absorption layer, the p-type electric field relaxation layer, the multiplication layer, an n-type optical waveguide layer, and the n-type cladding layer; and The optical waveguide portion is formed on the semiconductor substrate so as to be in contact with the light receiving portion, and includes at least an optical waveguide layer for guiding incident light to the n-type optical waveguide layer.

9. The semiconductor light receiving element according to any one of claims 1 to 8, characterized in that In the case where the multiplication layer is composed of a digital alloy structure, the multiplication layer is a digital alloy structure formed by alternately stacking a first multiplication constituent layer and a second multiplication constituent layer for multiple times, wherein the first multiplication constituent layer is composed of a layer thickness of M times the single atomic layer, 2≦M≦6, and the second multiplication constituent layer is composed of a layer thickness of N times the single atomic layer and has a band gap energy smaller than the band gap energy of the first multiplication constituent layer, 2≦N≦6.

10. The semiconductor light receiving element according to any one of claims 1 to 9, characterized in that In the case where the p-type electric field relaxation layer is composed of a digital alloy structure, the p-type electric field relaxation layer is a digital alloy structure formed by alternately stacking a first electric field relaxation constituent layer and a second electric field relaxation constituent layer for multiple times, wherein the first electric field relaxation constituent layer is composed of a layer thickness of P times that of a single atomic layer, 2≦P≦6, and the second electric field relaxation constituent layer is composed of a layer thickness of Q times that of a single atomic layer and has a band gap energy smaller than the band gap energy of the first electric field relaxation constituent layer, 2≦Q≦6.

11. The semiconductor light receiving element according to claim 9, wherein: The first multiplication constituent layer is composed of an AlAs layer, and the second multiplication constituent layer is composed of an InAs layer.

12. The semiconductor light receiving element according to claim 10, wherein: The first electric field relaxing layer is composed of an AlAs layer, and the second electric field relaxing layer is composed of an InAs layer.

13. The semiconductor light receiving element according to claim 3, characterized in that The first buffer constituent layer is composed of an AlAs layer, and the second buffer constituent layer is composed of an InAs layer.

14. A semiconductor light receiving element, characterized in that: have: InP substrate; An InAlAs multiplication layer is formed on the InP substrate, and is composed of a digital alloy structure formed by alternately stacking multiple AlAs layers and InAs layers, and the refractive index difference between the AlAs layer and the InAs layer is greater than 0 and less than 0.02, wherein the AlAs layer is composed of a layer thickness of M times that of a single atomic layer, 2≦M≦6, and the InAs layer is composed of a layer thickness of N times that of a single atomic layer, 2≦N≦6; a p-type InAlAs electric field relaxation layer, formed on the InAlAs multiplication layer, and composed of a digital alloy structure formed by alternately stacking multiple AlAs layers and InAs layers, the refractive index difference between the digital alloy structure and the refractive index of InP is greater than 0 and less than 0.02, wherein the AlAs layer is composed of a layer thickness of M times that of a single atomic layer, 2≦M≦6, and the InAs layer is composed of a layer thickness of N times that of a single atomic layer, 2≦N≦6; and The InGaAs light absorbing layer is formed on the p-type InAlAs electric field relaxing layer.

15. The semiconductor light receiving element according to any one of claims 1 to 14, characterized in that: The semiconductor light receiving element is a waveguide type APD into which light is incident from an end face.

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