Semiconductor photodetectors, optical line terminals, multilevel intensity modulation transceivers, digital coherent receivers, radio-over-fiber systems, SPAD sensor systems, and lidar devices

Abstract

The semiconductor light receiving element (100) of the present disclosure comprises a substrate (1), an n-type semiconductor layer (2) formed on the substrate (1), a multiplication layer (3) formed on the n-type semiconductor layer (2), a p-type electric field buffer layer (4) formed on the multiplication layer (3), a light absorption layer (5) formed on the p-type electric field buffer layer (4), a first window layer (7) formed on the light absorption layer (5), a second window layer (8) formed on the first window layer (7) and having a two-dimensional periodic structure (70) in which a plurality of holes (11) or remnants (8s) are two-dimensionally arranged at a constant period, a surface protective insulating film (10) formed at least on the two-dimensional periodic structure (70), and a surface electrode (32) formed at least on the surface protective insulating film (10).

Description

[Technical field]

[0001] The present disclosure relates to a semiconductor photodetector, an optical line terminal, a multilevel intensity modulation transceiver, a digital coherent receiver, an optical fiber radio system, a SPAD sensor system, and a LIDAR device.

[0002] Along with the progress of digital transformation that utilizes digital information, there has been remarkable development of communication networks that communicate digital information with each other and data centers that store and process data. Optical communication is used for communication networks and communication within data centers. In recent years, optical communication has made remarkable progress in increasing speed and capacity. With the progress of optical communication, photodiodes (PD) and avalanche photodiodes (APD), which provide high receiving sensitivity, are required as optical communication receivers.

[0003] Passive Optical Networks (PON) are the main method used in access networks that connect optical communication subscribers. PON systems started with G(E)-PON systems that transmit signals of 1 to 2 Gbps, and are expected to increase in the future with 10G-EPON and XG-PON systems that transmit signals of 10 Gbps.

[0004] Furthermore, the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is currently studying the 50G-PON system, a next-generation high-speed PON system, and it is expected that 50Gbps-class transmission will also be put to practical use in access networks in the future. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 3-050875 [Patent Document 2] JP 2005-159002 A [Patent Document 3] JP 2008-311562 A [Patent Document 4] JP 2016-178234 A [Patent Document 5] JP 2016-178293 A [Patent Document 6] JP 2018-63975 A [Patent Document 7] JP 2019-54145 A [Patent Document 8] JP 2019-68019 A [Non-patent literature]

[0006] [Non-Patent Document 1] Jinwen Song, et al., “High-efficiency and high-speed germanium photodetector enabled by multiresonant photonic crystal” De Gruyter Nanophotonics 20200455 (2020) [Non-Patent Document 2] Jiyuan Zheng, et al.,“Digital Alloy InAlAs Avalanche Photodiodes”,JOURNAL OF LIGHTWAVE TECHNOLOGY,VOL.36,NO.17,SEPTEMBER 1,pp.3580-3585,2018 Summary of the Invention [Problem to be solved by the invention]

[0007] (1) Issues regarding the light absorption layer in APDs PDs and APDs used in high-speed optical communications use InGaAs as the material for the optical absorption layer, which has a high absorption coefficient in the 1.3 μm and 1.55 μm bands, which are optical communication wavelengths. For example, in the 1.3 μm band, a high absorption coefficient of 10,000 / cm or more can be obtained.

[0008] In order to broaden the response band of PDs and APDs, it is necessary to reduce the thickness of the InGaAs layer that constitutes the light absorption layer and shorten the carrier transit time. However, when the thickness of the light absorption layer is reduced, a problem occurs in that the light receiving sensitivity decreases.

[0009] When the absorption coefficient of the light absorbing layer is a and the thickness of the light absorbing layer is W, the quantum efficiency η (=number of absorbed photons / number of incident photons) is expressed by the following formula (1). η=1-exp(-a W) (1)

[0010] In formula (1), for example, if a = 10000 / cm and W = 1μm, the quantum efficiency η is 63%. Incidentally, the photosensitivity S (A / W) is given by S = η·λ (nm) / 1240, so in the case of light with a wavelength of 1.3μm, the photosensitivity is 0.66A / W.

[0011] On the other hand, the 3 dB bandwidth ftr, which is determined by the time it takes for carriers to travel through the optical absorption, is expressed by the following equation (2). ftr=3.5Vav / (2πW) (2)

[0012] In equation (2), Vav is the average saturation transit velocity of electrons and holes. For example, if the light absorption layer is made of InGaAs, Vav = 5.35 × 10 6 In the case where Vav and W are 1 μm, ftr=29.8 GHz is obtained by substituting Vav and W into equation (2).

[0013] Therefore, if the thickness W of the light absorption layer is made thicker than 1 μm, the quantum efficiency η will be higher than 63%, but the response band will be lower than 29.8 GHz. In order to improve this trade-off between the response band and quantum efficiency, it is necessary to increase the absorption amount of the light absorption layer.

[0014] As a method for increasing the absorption amount of a light absorption layer in a PD, a method of resonating incident light using a so-called photonic crystal layer has been proposed, as disclosed in Patent Documents 2 to 8. The photonic crystal layer is also called a two-dimensional periodic structure.

[0015] In the element structures disclosed in Patent Documents 2 to 8, the resonance of the photonic crystal layer is utilized, so that the photonic crystal layer acts like a filter only within a specific narrow wavelength range, as shown in FIG. 3 of Patent Document 6, for example. Therefore, in order to obtain high reception sensitivity at a desired wavelength, high precision is required for crystal growth and processing. In addition, since the temperature of the semiconductor light receiving element is not adjusted during actual use, there is a risk of a problem in that the resonance wavelength changes by 10 nm or more with changes in the environmental temperature, resulting in a large change in reception sensitivity.

[0016] In recent years, attempts have been made to expand the wavelength range in which high reception sensitivity can be obtained in PDs that use photonic crystal layers. For example, Non-Patent Document 1 reports an element structure in which the light confinement in the vertical direction of the optical absorption layer is strengthened to generate multiple resonance modes, and the multiple resonance modes resonate in combination to obtain high reception sensitivity in a wide wavelength range.

[0017] The semiconductor photodetector described in Non-Patent Document 1 relates to a PD with an absorption layer of germanium (Ge) formed on a SiO2 layer on a silicon (Si) substrate. It is difficult to apply the element structure of Non-Patent Document 1 to an APD with high reception sensitivity that has an absorption layer of InGaAs on an InP substrate. This is because, in the semiconductor photodetector of Non-Patent Document 1, holes are provided in the Ge light absorption layer to form a PD with a photonic crystal, but since a high electric field is applied to an APD, providing holes in, for example, the InGaAs light absorption layer increases the dark current, which is a problem.

[0018] Furthermore, because the InGaAs layer that constitutes the optical absorption layer has a small band gap, carriers generated by optical absorption recombine on the surface of the hole, resulting in a problem of reduced reception sensitivity. Also, in order to generate multiple resonance modes, it is necessary to strengthen the optical confinement in the optical absorption layer, and a structure is applied in which the optical absorption layer is sandwiched between low refractive index layers, but in APDs, it is difficult to lower the refractive index of the multiplication layer adjacent to the optical absorption layer. For these reasons, there is a strong demand for an optimal device structure for APDs that uses InGaAs on an InP substrate as an absorption layer.

[0019] (2) Issues regarding the multiplication layer in APDs APDs, which are semiconductor photodetectors used in PON systems, have an element structure consisting of a light absorption layer (InGaAs), an electric field buffer 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 to multiply, or ionize, the electrons and holes generated in the light absorption layer. The electric field buffer layer functions to weaken the electric field so that the high electric field of the multiplication layer is not applied to the light absorption layer. Incidentally, the ionization rate of electrons is expressed as α, and the ionization rate of holes as β.

[0020] In an APD, the greater the ratio of the ionization rates of electrons and holes, the smaller the excess noise generated during multiplication and the higher the receiving sensitivity. Furthermore, the greater the ratio of the ionization rates of electrons and holes, the shorter the multiplication time in the multiplication layer, and the wider the response band.

[0021] The ionization rate ratio k of electrons and holes is defined as k = β / α. When electrons are injected into the multiplication layer, the smaller the ionization rate ratio k, the better the APD performance. Compound semiconductor materials such as InAlAs or InP are used for the multiplication layer of APDs for optical communications.

[0022] When InAlAs is selected as the material for the multiplication layer, the difference between the ionization rates of electrons and holes is greater than that of InP. In InP, the ionization rate of holes is greater than that of electrons, and the ionization rate of holes is about twice that of electrons. On the other hand, when InAlAs is selected as the material for the multiplication layer, the ionization rate of electrons is greater than that of holes, and the ionization rate of electrons is about five times that of holes. Therefore, since the reception sensitivity is higher when InAlAs is used as the multiplication layer, InAlAs is more suitable than InP as the material for the multiplication layer of an APD.

[0023] As mentioned above, PON systems require APDs, which are semiconductor light-receiving elements, to have a wide response band and high reception sensitivity. However, unlike PDs, APDs have a problem in that the time required for multiplication, that is, the multiplication time, increases as the multiplication factor increases, resulting in a decrease in the response band at high multiplication factors.

[0024] Although APDs with a multiplication layer made of InAlAs, which is used in optical communications, have a wider response band than APDs made of other semiconductor materials, the response band remains at approximately 20 GHz when the multiplication factor is 6 or more. In other words, the wide response band of 37.5 GHz or more required for 50G-PON systems is difficult to achieve when conventional APDs are used.

[0025] As described above, APDs, which are semiconductor light receiving elements used in optical communications, are required to operate in an even wider response band. Patent Document 1 describes an APD that uses a superlattice as a multiplication layer, but since the thickness of each layer of the superlattice is 5 to 10 nm, it acts as a quantum well that reflects the band gap of each layer. If the thickness of each layer in the stack is several nm or more, energy unevenness that reflects the band gap of each layer is generated, which hinders the movement of carriers and reduces the traveling speed.

[0026] In a 50G-PON system, the response bands of the semiconductor light emitting element and the semiconductor light receiving element, as well as the optical output of the semiconductor light emitting element and the receiving sensitivity of the semiconductor light receiving element are insufficient. For this reason, it is being considered to provide a digital bandwidth compensation circuit using a digital signal processor (DSP) after the APD in the optical network unit (ONU), that is, the receiving device on the subscriber side.

[0027] Furthermore, in the optical line terminal (OLT), i.e., the receiving equipment on the central office side, a semiconductor optical amplifier (SOA) is required to compensate for the lack of receiving sensitivity of the semiconductor photodetector, and it is also necessary to integrate an SOA into the electro-absorption modulated laser diode (EML) on the transmitting side of the ONU to increase the optical output.

[0028] However, DSPs and SOAs consume a lot of power, which increases costs, and there are fears that the replacement of existing PON systems with 50G-PON systems will not progress.

[0029] In existing PON systems other than the next-generation high-speed PON system, it has been considered to increase the number of splits of the optical signal output from the OLT in order to reduce costs. However, in this case, it is necessary to increase the optical output by integrating an SOA in the EML on the transmitting side of the OLT and ONU, which causes problems such as an increase in the power consumption of the transmitter and an increase in costs.

[0030] As described above, in order to compensate for the limitations of the receiving sensitivity and response bandwidth of semiconductor photodetectors, transceivers have been designed that incorporate expensive, power-hungry DSPs and SOAs into the ONUs and OLTs. However, this brings about problems of increased power consumption and costs.

[0031] The present disclosure has been made to solve the above-mentioned problems, and has an object to provide a semiconductor light-receiving element that has high receiving sensitivity over a wide wavelength range and operates over a wide response band. [Means for solving the problem]

[0032] The semiconductor light receiving element according to the present disclosure comprises: An InP substrate, an n-type semiconductor layer formed on the InP substrate; a multiplication layer made of a compound semiconductor formed on the n-type semiconductor layer; a p-type electric field buffer layer formed on the multiplication layer; a light absorbing layer formed on the p-type electric field buffer layer; a first window layer formed on the light absorbing layer; A two-dimensional periodic structure is formed on the first window layer, in which a plurality of holes or remainders are two-dimensionally arranged at a constant period. The first window layer is made of a different material. A second window layer; and a surface protective insulating film formed at least on the two-dimensional periodic structure; At least a surface electrode formed on the surface protective insulating film; Equipped with.

[0033] The optical line terminal according to the present disclosure comprises: The semiconductor light receiving element described above, an optical multiplexer / demultiplexer that inputs an optical signal to the semiconductor light receiving element; an amplifier circuit for amplifying an electrical signal output from the semiconductor light receiving element; a clock data recovery circuit connected to the amplifier circuit and configured to recover clock data from the amplified electrical signal; and a forward error correction circuit connected to the clock data recovery circuit for correcting an error in the clock data.

[0034] A multi-level intensity modulation transmitting / receiving device according to the present disclosure comprises: The semiconductor light receiving element described above for receiving an optical signal intensity-modulated into multiple values; an amplifier circuit for amplifying an electrical signal output from the semiconductor light receiving element; an analog / digital conversion circuit connected to the amplifier circuit and converting the amplified electrical signal into a digital signal; A digital signal processing circuit is connected to the analog / digital conversion circuit and processes the digital signal.

[0035] The radio-over-fiber system according to the present disclosure comprises: a light source that emits an analog modulated optical signal; The semiconductor light receiving element described above for receiving the analog-modulated optical signal; a transmission path for transmitting an analog electrical signal output from the semiconductor light receiving element to an antenna; and an antenna connected to the transmission line for emitting the analog electrical signal as a radio wave signal.

[0036] A digital coherent receiving device according to the present disclosure includes: The semiconductor light receiving element described above, a polarization splitter that splits the polarization of the intensity- and phase-modulated polarization multiplexed optical signal; a 90-degree hybrid that splits and combines the optical signals output from the polarization splitter; and a digital signal processing circuit connected to the 90-degree hybrid device for processing a digital signal.

[0037] The SPAD (Single Photon Avalanche Diode) sensor system according to the present disclosure includes: A SPAD sensor constituted by the semiconductor light receiving element described above; a quenching circuit for repeatedly applying a voltage equal to or greater than a breakdown voltage and a voltage equal to or less than the breakdown voltage to the SPAD sensor; and an optoelectronic measurement circuit that measures the electrical signal output from the SPAD sensor.

[0038] The LIDAR device according to the present disclosure comprises: A light source that emits light in a pulsed manner; the semiconductor light receiving element described above for receiving light emitted from the light source and reflected by an object; an amplifier circuit for amplifying an electrical signal output from the semiconductor light receiving element; and a distance measuring circuit that calculates a distance based on the electrical signal amplified by the amplifier circuit. Effect of the Invention

[0039] The semiconductor photodetector according to the present disclosure employs an element structure that is composed of a two-dimensional periodic structure formed above a light absorption layer and an electrode that covers the two-dimensional periodic structure, thereby achieving the effect of obtaining a semiconductor photodetector that operates over a wide response band and has high receiving sensitivity.

[0040] The optical line terminal device, multi-level intensity modulation transceiver device, digital coherent receiver device, optical fiber radio system, SPAD sensor system, and LIDAR device according to the present disclosure have the advantage that the semiconductor photodetector element according to the present disclosure is used as the semiconductor photodetector element, thereby providing devices and systems with excellent performance. [Brief description of the drawings]

[0041] [Figure 1] FIG. 1 is a diagram showing the wavelength dependence of the refractive index in an InAs / AlAs digital alloy structure and an InAlAs random alloy structure. [Diagram 2]FIG. 1 is a diagram showing the wavelength dependence of the absorption coefficient in an InAs / AlAs digital alloy structure and an InAlAs random alloy structure. [Diagram 3] 1 is a cross-sectional view illustrating an element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to a first embodiment. [Figure 4] 4A and 4B are top views illustrating the device structure of a back illuminated APD, which is an example of the semiconductor light receiving element according to the first embodiment. [Diagram 5] 5A and 5B are a cross-sectional view and a top view, respectively, illustrating a manufacturing process of a back illuminated APD, which is an example of the semiconductor light receiving element according to the first embodiment. [Figure 6] 6A and 6B are a cross-sectional view and a top view, respectively, illustrating a manufacturing process of a back illuminated APD, which is an example of the semiconductor light receiving element according to the first embodiment. [Figure 7] 7A and 7B are a cross-sectional view and a top view, respectively, illustrating a manufacturing process of a back illuminated APD, which is an example of the semiconductor light receiving element according to the first embodiment. [Figure 8] 8A and 8B are a cross-sectional view and a top view, respectively, illustrating a manufacturing process of a back illuminated APD, which is an example of the semiconductor light receiving element according to the first embodiment. [Figure 9] 9A and 9B are a cross-sectional view and a top view, respectively, illustrating a manufacturing process of a back illuminated APD, which is an example of a semiconductor light receiving element according to the first embodiment. [Figure 10] 10A and 10B are a cross-sectional view and a top view, respectively, illustrating a manufacturing process of a back illuminated APD, which is an example of a semiconductor light receiving element according to the first embodiment. [Figure 11] 2 is a diagram illustrating main resonance points of a two-dimensional periodic structure in a back-illuminated APD, which is an example of a semiconductor light-receiving element according to the first embodiment. FIG. [Figure 12] 12 is a diagram showing the refractive index distribution between points P and Q in FIG. 11. FIG. [Figure 13] 1 is a cross-sectional view illustrating an element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to a first modification of the first embodiment. [Figure 14]11 is a diagram illustrating main resonance points of a two-dimensional periodic structure in a back-illuminated APD, which is an example of a semiconductor light-receiving element according to Modification 1 of the first embodiment. FIG. [Figure 15] 15 is a diagram showing the refractive index distribution between points P and Q in FIG. 14. FIG. [Figure 16] 10 is a cross-sectional view illustrating an element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to Modification 2 of the first embodiment. FIG. [Figure 17] 11 is a cross-sectional view illustrating an element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to a third modification of the first embodiment. FIG. [Figure 18] 11 is a cross-sectional view illustrating an element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to Modification 4 of the first embodiment. FIG. [Figure 19] 13 is a cross-sectional view illustrating an element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to a fifth modification of the first embodiment. FIG. [Figure 20] 13 is a cross-sectional view illustrating an element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to a sixth modification of the first embodiment. FIG. [Figure 21] 13 is a cross-sectional view illustrating a device structure of an edge-illuminated APD, which is an example of a semiconductor light-receiving device according to Modification 7 of the first embodiment. FIG. [Figure 22] 10 is a cross-sectional view illustrating an element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to a second embodiment. [Figure 23] 13 is a graph showing the dependency of the reflectance and quantum efficiency of a DBR layer on the number of pairs in a back-illuminated APD, which is an example of a semiconductor light-receiving element according to the second embodiment. FIG. [Figure 24] 11 is a cross-sectional view illustrating an element structure of a back-illuminated PD, which is an example of a semiconductor light-receiving element according to a first modification of the second embodiment. FIG. [Diagram 25] 11 is a cross-sectional view illustrating an element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to Modification 2 of the second embodiment. FIG. [Figure 26] 11 is a cross-sectional view illustrating an element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to Modification 3 of the second embodiment. FIG. [Figure 27]FIG. 13 is a diagram showing the electric field dependence of the electron dead space in an InAlAs multiplication layer. [Figure 28] 28A to 28C are conceptual diagrams showing the ionization rates of electrons and holes. [Figure 29] FIG. 13 is a graph showing the dependence of the ionization rate ratio and the tunnel current on the thickness of the multiplication layer. [Diagram 30] Figures 30A to 30D are conceptual diagrams showing the ionization rate in the multiplication layer and electric field relaxation layer, with Figure 30A being a conceptual diagram showing the ionization rate in the case of a multiplication layer having a random alloy structure, Figure 30B being a conceptual diagram showing the ionization rate in the case of a multiplication layer having a digital alloy structure, Figure 30C being a conceptual diagram showing the ionization rate in the case of a multiplication layer having a partially disordered digital alloy structure, and Figure 30D being a conceptual diagram showing the ionization rate in the case of a combination of a thick electric field relaxation layer and a multiplication layer having a digital alloy structure. [Diagram 31] 11 is a cross-sectional view illustrating an element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to a third embodiment. FIG. [Diagram 32] 13 is a cross-sectional view illustrating an element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to a first modification of the third embodiment. FIG. [Diagram 33] 13 is a cross-sectional view illustrating an element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to Modification 2 of Embodiment 3. FIG. [Diagram 34] 13 is a cross-sectional view illustrating an element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to Modification 3 of Embodiment 3. FIG. [Diagram 35] 13 is a cross-sectional view illustrating an element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to Modification 4 of the third embodiment. FIG. [Diagram 36] FIG. 11 is a configuration diagram illustrating an optical line terminal (OLT) of a 50G-PON system according to a fourth embodiment. [Figure 37] FIG. 11 is a configuration diagram showing an optical network unit (ONU) of a 50G-PON system according to a fourth embodiment. [Figure 38] FIG. 1 is a configuration diagram illustrating an optical line terminal (OLT) of a 50G-PON system as a comparative example. [Figure 39]FIG. 13 is a diagram illustrating a configuration of an optical line terminal (OLT) of a 50G-PON system according to a fourth embodiment. [Diagram 40] A diagram showing the configuration of an optical network unit (ONU) of a 50G-PON system relating to embodiment 4. [Diagram 41] FIG. 13 is a diagram illustrating a configuration of a multilevel intensity modulation transmitting / receiving device according to a fifth embodiment. [Diagram 42] 42A and 42B are conceptual diagrams showing received waveforms of a multilevel intensity modulation transmitting / receiving device according to embodiment 5. FIG. [Diagram 43] 43A and 43B are conceptual diagrams illustrating the operation of a PD when a high optical input is applied. [Diagram 44] 1 is a conceptual diagram illustrating the operation of an APD when a high optical input is applied. [Diagram 45] FIG. 2 is a diagram showing the residence times of electrons and holes for each material constituting the multiplication layer. [Figure 46] FIG. 13 is a diagram illustrating a configuration of a radio-on-fiber system according to a sixth embodiment. [Figure 47] FIG. 1 illustrates a configuration of a radio-on-fiber system as a comparative example. [Figure 48] FIG. 13 is a diagram illustrating a configuration of a digital coherent receiving device according to a seventh embodiment. [Figure 49] FIG. 49A is a conceptual diagram showing waveforms of a digital coherent receiving device which is a comparative example, and FIG. 49B is a conceptual diagram showing waveforms of a digital coherent receiving device according to the seventh embodiment. [Figure 50] FIG. 23 is a diagram illustrating a configuration of a SAPD sensor system according to an eighth embodiment. [Figure 51] FIG. 51A is a conceptual diagram showing the multiplication characteristics of a SAPD sensor system as a comparative example, and FIG. 51B is a conceptual diagram showing the multiplication characteristics of a SAPD sensor system according to the eighth embodiment. [Figure 52] FIG. 13 is a diagram showing the calculated difference between the quenching electric field and the Geiger mode electric field for each multiplication layer configuration. [Diagram 53] FIG. 23 is a diagram illustrating the configuration of a LIDAR device according to a ninth embodiment. [Figure 54]FIG. 54A is a conceptual diagram showing a received waveform of the APD of a LIDAR device as a comparative example, and FIG. 54B is a conceptual diagram showing a received waveform of the APD of a LIDAR device according to embodiment 9. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] Embodiment 1 <Features of the semiconductor photodetector (APD) according to the first embodiment> Before describing the specific structure of the semiconductor light receiving element 100 according to the first embodiment, the digital alloy structure, which is a structural feature of the semiconductor light receiving element 100 according to the first embodiment, will be described below.

[0043] The inventors have reported that an InAs / AlAs digital alloy structure (also called an atomic layer superlattice, ALSL: Atomic Layer Super Lattice, Non-Patent Document 2), in which two-atomic layers of InAs and two-atomic layers of AlAs are repeatedly stacked, has a uniform composition ratio of In as a whole. 0.52 Al 0.48 We found that the refractive index of the random alloy structure made of In is lower than that of the random alloy structure made of As. 0.52 Al 0.48 As is simply expressed as InAlAs.

[0044] Fig. 1 is a diagram showing the wavelength dependence of the refractive index in an InAs / AlAs digital alloy structure and an InAlAs random alloy structure, and Fig. 2 is a diagram showing the wavelength dependence of the absorption coefficient in an InAs / AlAs digital alloy structure and an InAlAs random alloy structure. Fig. 1 shows that in the wavelength range of 1200 nm to 1600 nm used in optical communications, the InAs / AlAs digital alloy structure has a smaller refractive index than the InAlAs random alloy structure.

[0045] Also, from Figure 2, it can be seen that the InAs / AlAs digital alloy structure has peaks in the absorption coefficient near 650 nm and 800 nm, and the absorption coefficient is improved by 1.5 times. On the other hand, a decrease in the absorption coefficient is observed near 720 nm. This is thought to be because, even if the overall composition ratio is roughly the same, the band structure changes due to the periodicity of the InAs and AlAs layers, and periodicity also occurs in the wavelength dependence of the optical absorption coefficient. Also, at wavelengths above 850 nm, which is the band edge wavelength, the absorption coefficient of the InAs / AlAs digital alloy structure drops sharply. From this phenomenon, it is thought that the refractive index becomes smaller in the wavelength range from 1200 nm to 1600 nm.

[0046] As described above, it is clear that the refractive index can be reduced by replacing the random alloy structure with a digital alloy structure even with approximately the same composition. The inventors have devised the use of this phenomenon to apply an InAs / AlAs digital alloy structure to the multiplication layer of an APD for optical communications that uses a two-dimensional periodic structure.

[0047] In PDs that use a two-dimensional periodic structure, it is possible to increase the receiving sensitivity by confining light in the light absorption layer. PDs that increase the proportion of light confined in the light absorption layer are called Photo-Trapping-Enhanced PDs.

[0048] On the other hand, in an APD, in order to strongly confine light in the light absorption layer, which has a relatively large refractive index, it is necessary to make the refractive index of the multiplication layer adjacent to the light absorption layer as small as possible. As described above, the multiplication layer having a digital alloy structure has a smaller refractive index than the multiplication layer having a random alloy structure. Therefore, in the multiplication layer having a digital alloy structure, the light confinement in the light absorption layer by the multiplication layer is increased compared to the multiplication layer having a random alloy structure, and further, the complex resonance of light generated by the application of the two-dimensional periodic structure produces a synergistic effect that improves the receiving sensitivity over a wide wavelength range.

[0049] Other advantages of applying the InAs / AlAs digital alloy structure to the multiplication layer are explained below. In an APD, the larger the ratio of the ionization rates of electrons and holes, the smaller the excess noise generated during multiplication and the higher the reception sensitivity. Furthermore, the larger the ratio of the ionization rates of electrons and holes, the shorter the multiplication time in the multiplication layer, resulting in a wider bandwidth.

[0050] The ionization rate ratio k of electrons and holes is defined as k = β / α, where α is the ionization rate of electrons and β is the ionization rate of holes. When electrons are injected into the multiplication layer, the smaller the ionization rate ratio k, the better the APD performance. Therefore, InAlAs, which has a small ionization rate ratio k, is often used for the multiplication layer of APDs for optical communications.

[0051] When the multiplication layer is made of an InAlAs layer, the ionization rate of holes is lower in the digital alloy structure than in the random alloy structure, so that the ionization rate ratio k can be made smaller. Furthermore, since the dead space length of holes is long in the InAs / AlAs digital alloy structure, the ionization rate β of holes can be made almost zero by thinning the multiplication layer to about 130 nm or less. In other words, it is possible to make k=0. Therefore, when the InAs / AlAs digital alloy structure is applied to the multiplication layer, the excess noise generated during multiplication is reduced, so that the receiving sensitivity is increased, and the multiplication time in the multiplication layer is shortened, so that the bandwidth is broadened.

[0052] As described above, by further applying a multiplication layer with a digital alloy structure to an APD with a two-dimensional periodic structure, the complex resonance of light is enhanced, making it possible to simultaneously achieve high receiving sensitivity and a wide bandwidth.

[0053] <Element structure of semiconductor light receiving element according to the first embodiment> 3 and 4A are a cross-sectional view and a top view showing the element structure of a back-illuminated APD, which is an example of the semiconductor light receiving element 100 according to embodiment 1. In the following description, the upper side means the direction in which the semiconductor layers are stacked from the surface of the semiconductor substrate, that is, the stacking direction, and the lower side means the direction opposite to the stacking direction.

[0054] The back-illuminated APD, which is an example of the semiconductor light-receiving element 100 according to the first embodiment, includes an n-type InP substrate 1 and a semiconductor layer having a carrier concentration of 1 to 5×10 18 cm -3 and an n-type InP buffer layer 2 having a thickness of 0.1 to 1.0 μm; an InAs / AlAs multiplication layer 3 having a digital alloy structure in which an i-type AlAs layer (for example, a thickness of two atomic layers, about 0.6 nm) and an i-type InAs layer (for example, a thickness of two atomic layers, about 0.6 nm) are alternately laminated multiple times (hereinafter referred to as an i-type InAs / AlAs digital alloy structure multiplication layer 3); 17 cm -3 The p-type InP field relaxation layer 4 has a thickness of 10 to 70 nm, an i-type InGaAs light absorption layer 5 has a thickness of 0.1 to 2.0 μm, an i-type InAlGaAs / InAlAs graded layer 6, an i-type InP first window layer 7 has a thickness of 0.1 to 3.0 μm, a p-type InAlAs second window layer 8 has a thickness of 0.1 to 3.0 μm and has a two-dimensional periodic structure 70, a p-type InGaAs contact layer 9, and a SiN surface layer formed on the i-type InP first window layer 7 and the p-type InAlAs second window layer 8. The semiconductor device is composed of a surface protective insulating film 10, a Zn-diffused p-type region 12 formed in a part of the i-type InAlGaAs / InAlAs graded layer 6 and in the i-type InP first window layer 7, a plurality of holes 11 arranged two-dimensionally in the p-type InAlAs second window layer 8, an n-type electrode 31 and an anti-reflection film 40 formed on the back surface side of the n-type InP substrate 1, and a p-type electrode 32 formed on the p-type InGaAs contact layer 9 and on the SiN surface protective insulating film 10 surrounded by the p-type InGaAs contact layer 9. Note that the p-type electrode 32 is omitted in Figures 4A and 4B so that the structure viewed from above can be easily understood.

[0055] The i-type InP first window layer 7 may be made of InAlAs, InAlGaAs, or InGaAsP instead of InP. The i-type InP first window layer 7 may be made of multiple semiconductor layers instead of a single InP layer. The n-type InP substrate 1 is an example of a substrate, and a substrate other than the n-type InP substrate 1 may be used.

[0056] The p-type InAlAs second window layer 8 may be formed using InAlGaAs, InGaAsP, or InP instead of InAlAs. Also, the p-type InAlAs second window layer 8 may be formed of multiple semiconductor layers instead of a single InAlAs layer.

[0057] The two-dimensional periodic structure 70 provided in the above-mentioned p-type InAlAs second window layer 8 has a configuration in which a plurality of holes 11, which penetrate from the surface to the bottom surface of the p-type InAlAs second window layer 8 and expose the i-type InP first window layer 7 provided with the Zn-diffused p-type region 12 on the bottom surface side, are two-dimensionally arranged at a constant period.

[0058] An example of the two-dimensional periodic structure 70 is a structure in which holes 11 are arranged in a lattice pattern in a p-type InAlAs second window layer 8. The shape of the opening on the front side of the holes 11 may be circular, rectangular, triangular, or the like, but may have other shapes. Alternatively, as exemplified in the semiconductor light receiving element 101 according to the first embodiment in FIG. 4B, the two-dimensional periodic structure 70 may be formed by leaving the p-type InAlAs second window layer 8 in a circular shape during etching. The shape of the remaining portions 8s may be rectangular, triangular, or the like in addition to a circular shape, but may have other shapes. In the following first to third embodiments, the holes 11, i.e., the recesses, may be read as the remaining portions 8s, i.e., the protrusions. The semiconductor light receiving element 101 has a two-dimensional periodic structure 70 made of the remaining portions 8s.

[0059] In the two-dimensional periodic structure 70, the ratio of the diameter of the holes 11 to a certain period is preferably within a range of 10% to 80%. The depth of the holes 11 (height when the holes are the remainder) is preferably within a range of 100 nm to 1000 nm.

[0060] The bottom of the hole 11 may be located at a position where the i-type InP first window layer 7 is exposed, as in the example shown in FIG.

[0061] A Zn-diffused p-type region 12 is selectively formed under the p-type InGaAs contact layer 9 and in a region surrounded by the p-type InGaAs contact layer 9. A boundary 13 of the Zn-diffused p-type region 12 is indicated by a dotted line in Fig. 3. In Fig. 3, the Zn-diffused p-type region 12 reaches the i-type InAlGaAs / InAlAs graded layer 6. However, the Zn-diffused p-type region 12 may reach into the i-type InP first window layer 7 or the i-type InGaAs light absorption layer 5.

[0062] The n-type electrode 31 is not formed in the surface of the back surface side facing the portion surrounded by the p-type InGaAs contact layer 9 on the front surface side of the n-type InP substrate 1. Instead, an anti-reflection film 40 made of a SiN film or the like is formed in the surface of the back surface side of the n-type InP substrate 1 facing the portion surrounded by the p-type InGaAs contact layer 9 on the front surface side of the n-type InP substrate 1.

[0063] The n-type electrode 31 is made of a multilayer film of AuGe / Ni / Au, Ti / Pt / Au, or a combination of AuGe / Ni / Au and Ti / Pt / Au from the side of the n-type InP substrate 1. The p-type electrode 32 is made of a multilayer film of Ti / Au, Ti / Pt / Au, etc. The n-type InP buffer layer 2 is also called an n-type semiconductor layer.

[0064] Silicon (Si) is optimal as the n-type dopant for the n-type InP buffer layer 2. This is to prevent the n-type impurity from diffusing from the n-type InP buffer layer 2 into the i-type InAs / AlAs digital alloy structure multiplication layer 3, causing disorder in the digital alloy structure. Here, disorder refers to the phenomenon in which the compositions of the layers in the digital alloy structure mix together, resulting in a random alloy structure with an average composition.

[0065] As described above, the i-type InAs / AlAs digital alloy structure multiplication layer 3 is composed of semiconductor layers in which an InAs layer (layer thickness: 2 atomic layers, approximately 0.6 nm) and an AlAs layer (layer thickness: 2 atomic layers, approximately 0.6 nm) are alternately laminated in this order. However, it is sufficient that the layer thicknesses of the InAs layer and the AlAs layer are each in the range of 2 atomic layers or more and 6 atomic layers or less. The reason for specifying 6 atomic layers or less is that it is desirable for the laminated structure of the InAs layer and the AlAs layer not to function as a quantum well structure. In other words, the digital alloy structure is formed by alternately laminating two types of semiconductor layers, each made of a different semiconductor material, at a period of 2 atomic layers to 6 atomic layers.

[0066] Furthermore, the number of atomic layers of each layer of the i-type InAs / AlAs digital alloy structure multiplication layer 3 is preferably 2 to 4 atomic layers, and is optimally 2 atomic layers. The reason for this is that the thinner the atomic layer thickness of each layer, the greater the effect of reducing the ionization rate ratio k by the digital alloy structure. In addition, when considering not only the performance as a semiconductor light receiving element but also productivity, a layer thickness of 4 to 6 atomic layers is also preferable, which reduces the number of shutter switching times during crystal growth by molecular beam epitaxy (MBE). In view of the above factors, it can be said that the number of atomic layers of each layer of the i-type InAs / AlAs digital alloy structure multiplication layer 3 is preferably in a period of 2 to 6 atomic layers. Similarly, from the viewpoint of productivity, the entire multiplication layer does not have to be an InAs / AlAs digital alloy structure, but a part of the multiplication layer may be an InAs / AlAs digital alloy structure, and the remaining part may be an InAlAs random alloy structure.

[0067] The thickness of the i-type InAs / AlAs digital alloy structure multiplication layer 3 is suitably in the range of 40 nm to 170 nm, but may be 300 nm or less. For example, if the thickness of the i-type InAs / AlAs digital alloy structure multiplication layer is 100 nm, the number of repetitions of the InAs layer (2 atomic layers) / AlAs layer (2 atomic layers) is 85.

[0068] Considering the affinity with InP constituting the n-type InP buffer layer 2, it is preferable to make the layer thickness of only the first AlAs layer of the i-type InAs / AlAs digital alloy structure multiplication layer 3 three atomic layers or more thick. Alternatively, the i-type InAs / AlAs digital alloy structure multiplication layer 3 may be laminated by alternately forming an InAs layer and an AlAs layer in this order.

[0069] The conductivity type of the i-type InAs / AlAs digital alloy structure multiplication layer 3 is i-type, and the carrier concentration is 1×10 17 cm -3 The following is an example. However, the conductivity type of the InAs / AlAs digital alloy multiplication layer is 5×10 17 cm -3 It may be p-type or n-type as follows:

[0070] In addition to the multiplication layer having an InAs / AlAs digital alloy structure, for example, a digital alloy structure made of InAlAsSb, a material system containing antimony (Sb), can also be applied as the multiplication layer of the semiconductor light receiving element of the present disclosure.

[0071] The i-type InAlGaAs / InAlAs graded layer 6 is a layer in which the band gap is gradually changed by changing the InAlGaAs composition, and each layer thickness is within the range of 5 nm to 50 nm. The InAlGaAs composition may be changed stepwise, and the band gap is intermediate between that of an InP layer and an InGaAs layer. Alternatively, the i-type InAlGaAs / InAlAs graded layer 6 may be formed by changing the equivalent band gap by alternately stacking two types of i-type InAlGaAs layers with different compositions multiple times while changing the layer thickness. The carrier concentration of the i-type InAlGaAs / InAlAs graded layer 6 is 5×10 17 cm -3or less, and if the carrier concentration is low, p-type or n-type may be used instead of i-type. Note that the i-type InAlGaAs / InAlAs graded layer 6 is not necessarily required and may be omitted. Also, an i-type InAlGaAs graded layer may be inserted between the p-type InP electric field relaxation layer 4 and the i-type InGaAs light absorption layer 5.

[0072] The p-type InP electric field buffer layer 4 may be a p-type InAlAs electric field buffer layer having a random alloy structure, or a p-type electric field buffer layer having an InAs / AlAs digital alloy structure.

[0073] <Method of Manufacturing Semiconductor Light-Receiving Element According to First Embodiment> A back-illuminated APD, which is an example of the semiconductor photodetector 100 according to the first embodiment, can be realized by using an epitaxial crystal growth method such as metal organic vapor phase epitaxy (MOVPE) or MBE on an n-type InP substrate 1. A method for manufacturing the semiconductor photodetector 100 according to the first embodiment will be described below.

[0074] Using MOVPE or MBE, a semiconductor having a carrier concentration of 1 to 5 × 10 18 cm -3 The n-type InP buffer layer 2 having a thickness of 0.1 to 1 μm is crystal-grown.

[0075] On the n-type InP buffer layer 2, a carrier concentration of 5×10 17 cm -3 The i-type InAs / AlAs digital alloy structure multiplication layer 3 having a thickness of 40 to 170 nm is crystal-grown.

[0076] On the i-type InAs / AlAs digital alloy structure multiplication layer 3, a carrier concentration of 1×10 16 ~5×10 18 cm -3 The p-type InP electric field buffer layer 4 having a thickness of 10 to 70 nm is crystal-grown. Examples of the p-type dopant for the p-type InP electric field buffer layer 4 include Be and Zn.

[0077] On the p-type InP field buffer layer 4, a carrier concentration of 1×10 17 cm -3 The i-type InGaAs light absorbing layer 5 having a thickness of 0.1 to 2.0 μm is crystal-grown.

[0078] Furthermore, the carrier concentration is 5×10 17 cm -3 The i-type InAlGaAs / InAlAs graded layer 6 has a thickness of 5 to 50 nm and a carrier concentration of 5×10 17 cm -3 As described above, the i-type InP first window layer 7 having a thickness of 0.1 to 3.0 μm, the p-type InAlAs second window layer 8 having a thickness of 0.1 to 3.0 μm, and the p-type InGaAs contact layer 9 are successively crystal-grown. Since they will be converted to p-type by selective diffusion in a later process, at this point, the p-type InAlAs second window layer 8 and the p-type InGaAs contact layer 9 may be either i-type or n-type.

[0079] After the crystal growth is completed, the surface other than the light-receiving region and the p-type InGaAs contact layer 9 surrounding the light-receiving region in a ring shape is covered with an insulating film, and Zn is selectively diffused in solid or gas phase using this insulating film as a diffusion mask to form the Zn-diffused p-type region 12. The cross-sectional view in Fig. 5A and the top view in Fig. 5B show the state after Zn diffusion.

[0080] The p-type InGaAs contact layer 9 is processed into a ring shape surrounding the light receiving region by removing unnecessary portions using a method such as dry etching or wet etching. An InP layer with a thickness of about 50 nm may be provided as an etching stopper layer between the p-type InGaAs contact layer 9 and the p-type InAlAs second window layer 8 to improve the controllability of the etching depth. The cross-sectional view of FIG. 6A and the top view of FIG. 6B show the state after the p-type InGaAs contact layer 9 has been processed into a ring shape.

[0081] A plurality of holes 11 are formed in a region that becomes the light receiving portion, that is, in the light receiving region, so as to be two-dimensionally arranged at a constant period. The cross-sectional view of FIG. 7A and the top view of FIG. 7B show a state in which a plurality of holes 11 are formed. The top view of FIG. 7B shows an example of a configuration in which a plurality of holes 11 are two-dimensionally arranged in a lattice shape as an example of a configuration in which a plurality of holes 11 are two-dimensionally arranged at a constant period. The plurality of holes 11 are provided in a p-type InAlAs second window layer 8. A structure in which a plurality of holes 11 are two-dimensionally arranged at a constant period is called a two-dimensional periodic structure 70.

[0082] The hole 11 is set to a depth such that the bottom surface thereof reaches the i-type InP first window layer 7. That is, the i-type InP first window layer 7 provided with the Zn-diffused p-type region 12 is exposed at the bottom surface of the hole 11. However, it is sufficient that the bottom surface of the hole 11 does not reach the i-type InGaAs light absorption layer 5, and it may be located, for example, inside the i-type InP first window layer 7 or inside the p-type InAlAs second window layer 8.

[0083] After the multiple holes 11 are formed, the p-type InAlAs second window layer 8 exposed at the outer periphery of the ring-shaped p-type InGaAs contact layer 9 is etched away. The cross-sectional view of Fig. 8A and the top view of Fig. 8B show the state after the p-type InAlAs second window layer 8 has been etched away. The reason for etching the p-type InAlAs second window layer 8 exposed at the outer periphery of the p-type InGaAs contact layer 9 is that a high electric field is applied to the boundary 13 of the Zn-diffused p-type region 12, and if the InAlAs layer, which is easily oxidized, is made the outermost surface, dark current is likely to increase.

[0084] Both the first and second window layers can be made of i-type InP layers that are difficult to oxidize. In this case, etching of the InP second window layer at the outer periphery of the p-type InGaAs contact layer 9 is not necessary. Conversely to the above-mentioned configuration, the first window layer can be made of an InAlAs layer and the second window layer can be made of InP. The reason for using different materials for the first and second window layers is that the controllability of the depth of the hole 11 (the height when the hole is left as the remaining part), that is, the etching depth, can be improved. The controllability of the etching depth can be improved by providing an etching stopper layer between the first and second window layers.

[0085] Next, a SiN surface protective insulating film 10 is formed on the entire surface of the front side as a surface protective film, and then the SiN surface protective insulating film 10 on the ring-shaped p-type InGaAs contact layer 9 is removed to form an ohmic contact part. The cross-sectional view of Fig. 9A and the top view of Fig. 9B show the state after the ohmic contact part is formed.

[0086] The SiN surface protective insulating film 10 is an example of an insulating film. The surface protective film may be an insulating film of other film types, such as an SiO2 film, a SiON film, or an insulating film made of an organic material. Furthermore, a film configuration in which a SiN film and a SiO2 film are stacked may be used.

[0087] Next, a p-type electrode 32 is formed on the p-type InGaAs contact layer 9 etched into a ring shape and on the SiN surface protective insulating film 10 on the inner circumference side of the ring. The cross-sectional view of Fig. 10A and the top view of Fig. 10B show the state after the p-type electrode 32 is formed.

[0088] The p-type electrode 32 uses Ti and Au or Pt as a metal material. The p-type electrode 32 also functions as a metal reflective film. Specifically, it is made up of a multilayer film such as Ti / Au, Ti / Pt / Au, Ti / Au / Pt / Au, Ti / Au / Ti / Pt / Au, Pt / Ti / Au / Ti / Pt / Au, etc., in that order from the semiconductor layer side.

[0089] After completion of processing on the front surface side, an n-type electrode 31 and an anti-reflection film 40 are formed on the rear surface of the n-type InP substrate 1. First, a material for the n-type electrode is deposited on the entire rear surface of the n-type InP substrate 1, and the material for the n-type electrode deposited on the rear surface side facing the portion of the n-type InP substrate 1 surrounded by the p-type InGaAs contact layer 9 on the front surface side is removed, thereby forming the n-type electrode 31.

[0090] Furthermore, an anti-reflection film 40 made of a SiN film or the like is formed on the portion from which the material of the n-type electrode has been removed, that is, on the back surface side facing the portion surrounded by the p-type InGaAs contact layer 9 on the front surface side of the n-type InP substrate 1. This completes the method for manufacturing the semiconductor light-receiving element according to the first embodiment.

[0091] Incident light 90 entering through the antireflection film 40 formed on the back surface side of the n-type InP substrate 1 is perpendicularly incident on the i-type InGaAs light absorption layer 5 provided on the front surface side of the n-type InP substrate 1. When the light receiving region surrounded by the p-type InGaAs contact layer 9 is circular, its diameter is within the range of 5 μm to 1 mm, and when the light receiving region is rectangular, the size of the long side is within the range of 5 μm to 1 mm.

[0092] <Function of the semiconductor light receiving element according to the first embodiment> The operation of the APD, which is an example of the semiconductor light receiving element 100 according to the first embodiment shown in FIG. 3, will be described below.

[0093] Incident light 90 incident on the back surface side of the semiconductor photodetector 100 passes through the antireflection film 40 and enters the inside of the semiconductor photodetector 100. The incident light 90 passes through the n-type InP substrate 1, and after a portion of the incident light 90 is absorbed in the i-type InGaAs light absorption layer 5, it reaches the i-type InP first window layer 7 and the p-type InAlAs second window layer 8.

[0094] In the p-type InAlAs second window layer 8, a plurality of holes 11 formed in a lattice shape, i.e., a two-dimensional periodic structure 70, are formed, and therefore optical resonance 91 occurs. A SiN surface protective insulating film 10 is embedded inside the holes 11. In the wavelength band of 1.3 μm to 1.6 μm used in optical communications, the difference in refractive index between the SiN film (refractive index: about 2.0) and the InAlAs layer (refractive index: about 3.25) is large, and therefore optical resonance 91 occurs easily. In addition, since the refractive index of the InGaAs layer is higher (refractive index: about 3.6) than that of the InP layer (refractive index: about 3.2), optical resonance 91 also occurs between the i-type InGaAs light absorption layer 5 and the i-type InP first window layer 7.

[0095] FIG. 11 shows the main resonance points in the two-dimensional periodic structure 70. FIG. 12 shows the refractive index distribution between points P and Q in FIG. 11. The positions where the light intensity increases due to the resonance 91 of light are called resonance points. For example, resonance points from position A to position D in FIG. 12 are assumed. When the APD is in operation, multiple resonance occurs from position A to position D. In multiple resonance, light resonates simultaneously at each resonance point from position A to position D, but the light intensity at each resonance point differs depending on the wavelength. For example, the light intensity at positions A and B, which are resonance points, is strong at a certain wavelength, while the light intensity at position C, which is a resonance point, is strong at a different wavelength.

[0096] The light receiving sensitivity of the semiconductor light receiving element is high at the wavelength where the optical resonance 91 occurs, but the light receiving sensitivity becomes high in an even wider wavelength range due to complex resonance. The reason for this is that the equivalent refractive index is different at the resonance points from position A to position D, and therefore the optical resonance 91 occurs at different wavelengths. For example, in the case of position A, which is the resonance point, the center of the resonance point is in the first window layer, which has a low refractive index, so the resonance wavelength is shorter than that of position D, where the center of the resonance point is in the light absorption layer, which has a high refractive index. In this way, since complex resonance can be generated in the semiconductor light receiving element 100 according to the first embodiment, it is possible to obtain a semiconductor light receiving element with high reception sensitivity in a wide wavelength range of several tens of nanometers or more.

[0097] Changing the period (the distance between the centers of the holes 11) of the holes 11, which are arranged two-dimensionally and periodically in a lattice pattern, changes the wavelength at which complex resonance occurs. Therefore, the period of the holes 11 can be set so that complex resonance occurs at a desired wavelength. The period of the holes 11 is roughly the value obtained by dividing the wavelength at which complex resonance occurs by the equivalent refractive index at the resonance point.

[0098] In optical communications, wavelengths of 1250 to 1600 nm are used. For example, if the wavelength at which complex resonance occurs is 1600 nm and the equivalent refractive index is 3.2, the period of the holes 11 is 500 nm (= 1600 nm / 3.2), and if the wavelength is 1250 nm, it is 391 nm (= 1250 nm / 3.2). The equivalent refractive index varies depending on the layer structure and the aperture ratio of the holes 11, and is a value within the range of 2.8 to 3.6. The period of the holes 11 may be set within the range of 347 nm (= 1250 nm / 3.6) to 571 nm (= 1600 nm / 2.8).

[0099] If the opening of the hole 11 is small, it becomes difficult to process it, whereas if the opening of the hole 11 is large, the physical strength of the remaining portion other than the hole 11 decreases. For this reason, the opening ratio of the hole 11 (=diameter of the hole 11 / period of the hole 11) is preferably 10% to 60%. Therefore, the diameter of the opening of the hole 11 may be set within a range of a minimum of 35 nm (=347 nm x 10%) and a maximum of 343 nm (=571 nm x 60%). Here, the definition of the diameter of the hole 11 is applied when the opening of the hole 11 is circular. If the opening of the hole 11 is a shape other than a circle, for example, if it is a rectangle, it may be defined as the length of the diagonal, and if it is a triangle, it may be defined as the length of the long side.

[0100] In the multiple holes 11 arranged two-dimensionally and periodically in the x and y directions, that is, in a lattice pattern, it is desirable that the period in the x direction and the period in the y direction are approximately the same value. The reason for this is that if the period in the x direction and the period in the y direction are different, the polarization dependency of the reception sensitivity occurs. From the viewpoint of reducing the polarization dependency of the reception sensitivity, it is desirable that the shape of the opening of the hole 11 is close to a circle. However, even if the shape of the opening of the hole 11 is a polygon such as a triangle or a rectangle, or an ellipse, it is possible to apply it as the two-dimensional periodic structure 70. Furthermore, multiple patterns may be arranged closely together, such as holes having a triangular or rectangular shape, to function as one hole.

[0101] The advantages of the semiconductor light receiving element 100 according to the first embodiment over the semiconductor light receiving element described in Non-Patent Document 1 will be described below. In the semiconductor light receiving element described in Non-Patent Document 1, holes are provided in a light absorption layer made of germanium (Ge) that are periodically arranged in a lattice pattern, and therefore electron-hole pairs generated by light absorption recombine on the surface of the holes, resulting in a problem of reduced reception sensitivity. In addition, since the Ge layer is exposed at the bottom of the holes, there is also a problem of increased dark current and reduced reliability. When the element structure described in Non-Patent Document 1 is applied to an APD, a high electric field is applied to the APD, and therefore a reduction in reliability is of particular concern.

[0102] On the other hand, in the APD, which is an example of the semiconductor photodetector 100 according to the first embodiment, the holes 11 are formed in the p-type InAlAs second window layer 8, which has a larger band gap than the i-type InGaAs light absorption layer 5, so that the electrons and holes generated in the i-type InGaAs light absorption layer 5 do not diffuse to the holes 11. Also, because no electric field is applied to the p-type InAlAs second window layer 8, problems such as an increase in dark current or a decrease in reliability do not occur. Therefore, the semiconductor photodetector 100 according to the first embodiment can realize a semiconductor photodetector with higher reception sensitivity, lower dark current, and higher reliability than the conventional semiconductor photodetector described in Non-Patent Document 1.

[0103] Furthermore, in the semiconductor photodetector described in Non-Patent Document 1, the light incident from the front surface that cannot be absorbed by the Ge light absorption layer, which is a photonic crystal, is transmitted to the substrate side. On the other hand, in the APD, which is an example of the semiconductor photodetector 100 according to the first embodiment, the SiN surface protective insulating film 10 and the p-type electrode 32 are formed on the upper part of the p-type InAlAs second window layer 8, so that the light transmitted through the p-type InAlAs second window layer 8 is reflected by the SiN surface protective insulating film 10 and the p-type electrode 32 and returned to the i-type InGaAs light absorption layer 5 again to contribute to the optical resonance, thereby improving the receiving sensitivity.

[0104] Next, the function of the i-type InAs / AlAs digital alloy structure multiplication layer 3 will be described. In the i-type InAs / AlAs digital alloy structure multiplication layer 3, low noise and a wide bandwidth can be expected due to the small ionization rate ratio k, and high reception sensitivity can also be achieved by applying the InAs / AlAs digital alloy structure to the multiplication layer of the semiconductor light receiving element 100 according to the first embodiment. The reason why high reception sensitivity is possible will be described below. In order to increase reception sensitivity, it is necessary to confine light as much as possible in the light absorption layer. As described above, when the resonance point exists in the light absorption layer, high reception sensitivity can be achieved. That is, since the light absorption layer has the highest refractive index, most light components are confined in the light absorption layer.

[0105] The above phenomenon will be specifically explained with reference to FIG. 12. FIG. 12 shows the refractive index distribution between points P and Q at position A in FIG. 11. The dotted line in FIG. 12 shows the refractive index of the InAlAs random alloy structure multiplication layer, and the solid line shows the refractive index of the InAs / AlAs digital alloy structure multiplication layer. Consider the case where a resonance point exists in the light absorption layer, as shown in positions C and D in FIG. 11. In FIG. 12, when the multiplication layer is configured with an InAs / AlAs digital alloy structure, the refractive index of the semiconductor layers on both sides (horizontal direction) of the light absorption layer is high, so that a resonance point occurs near the center of the light absorption layer. On the other hand, when the multiplication layer is configured with an InAlAs random alloy structure, as shown in FIG. 1, the refractive index of the InAlAs random alloy structure multiplication layer is high, so that the resonance point of the light absorption layer is biased toward the semiconductor substrate side, and light is dissipated. As a result, in the case of an InAlAs random alloy structure multiplication layer, the light confinement rate in the light absorption layer decreases.

[0106] On the other hand, when an InAs / AlAs digital alloy structure is applied to the multiplication layer, the refractive index of the InAs / AlAs digital alloy structure is low, approximately the same as that of the InP layer that constitutes the first window layer, so there is less leakage of light into the multiplication layer, as shown in Figure 1. In other words, when an InAs / AlAs digital alloy structure is applied to the multiplication layer, resonating light is efficiently trapped in the optical absorption layer, making it easier for complex resonance to occur, thereby realizing an APD with high receiving sensitivity over a wide wavelength range.

[0107] <Advantages of the First Embodiment> As described above, according to the semiconductor photodetector of the first embodiment, a two-dimensional periodic structure is provided in the second window layer, light is incident from the semiconductor substrate side, and the semiconductor layer is covered with a SiN surface protective insulating film and a p-type electrode to reflect the light inward, and an InAs / AlAs digital alloy structure is applied to the multiplication layer to increase the amount of light confined in the light absorption layer, which has the effect of enabling the semiconductor photodetector to have high reception sensitivity over a wide wavelength range due to complex resonance, and also has the effect of providing a semiconductor photodetector that can achieve low noise and a wide bandwidth. Therefore, it is possible to obtain an APD with sufficient reception sensitivity even in applications that require a response bandwidth of 25 Gbps or more.

[0108] Variation 1 of the first embodiment <Features of Semiconductor Photodetector (APD) According to Modification 1 of First Embodiment> 13 is a cross-sectional view illustrating an element structure of a back-illuminated APD, which is an example of a semiconductor light receiving element 110 according to Modification 1 of Embodiment 1. The semiconductor light receiving element 110 according to Modification 1 of Embodiment 1 is characterized in that the first window layer is composed of multiple layers having different refractive indices.

[0109] In the element structure of the semiconductor photodetector 110 according to the first modification of the first embodiment, the configurations of the layers from the n-type InP substrate 1 to the i-type InAlGaAs / InAlAs graded layer 6 are the same as those of the semiconductor photodetector 100 according to the first embodiment, and therefore the description thereof will be omitted.

[0110] On the upper side of the i-type InAlGaAs / InAlAs graded layer 6 of the semiconductor light receiving element 110 according to the first modification of the first embodiment, there are sequentially formed an i-type InP first window layer 7a having a thickness of 0.1 to 3.0 μm, an i-type InAlAs first window layer 7b having a thickness of 0.1 to 3.0 μm, an i-type InP second window layer 8a having a thickness of 0.1 to 3.0 μm, and a p-type InGaAs contact layer 9. The i-type InAlAs first window layer 7b may be formed of an i-type InAlGaAs layer.

[0111] The carrier concentration of the i-type InP first window layer 7a, the i-type InAlAs first window layer 7b, and the i-type InP second window layer 8a is 5×10 17 cm -3 When the first window layer 7b is made of an InAlGaAs layer, the composition wavelength λg of the InAlGaAs layer is 50 to 100 nm shorter than the wavelength of the light incident on the APD, so that no light absorption occurs in the first window layer 7b.

[0112] The p-type InGaAs contact layer 9, the SiN surface protective insulating film 10, the n-type electrode 31 formed on the back surface side of the n-type InP substrate 1, and the p-type electrode 32 formed on the SiN surface protective insulating film 10 surrounded by the p-type InGaAs contact layer 9 have the same configurations as those of the semiconductor light receiving element 100 according to the first embodiment shown in Fig. 3. The two-dimensional periodic structure 70 also has the same configuration as that of the semiconductor light receiving element 100 according to the first embodiment.

[0113] The Zn-diffused p-type region 12 is formed in the same region as that of the semiconductor light-receiving element 100 according to the first embodiment, that is, in the region shown in the cross-sectional view of FIG. 13. Meanwhile, in the cross-sectional view of FIG. 6A and the top view of FIG. 6B showing the manufacturing method of the semiconductor light-receiving element 100 according to the first embodiment, the p-type InAlAs second window layer 8 outside the outer periphery of the ring-shaped p-type InGaAs contact layer 9 is removed, but in the case of the semiconductor light-receiving element 110 according to the first modification of the first embodiment, the i-type InP second window layer 8a is composed of an InP layer, so there is no need to remove it. The reason for this is that in the semiconductor light-receiving element 100 according to the first embodiment, the InAlAs layer constituting the p-type InAlAs second window layer 8 is easily deteriorated by surface oxidation near the boundary 13 of the Zn-diffused p-type region 12 to which an electric field is applied, whereas in the case of the semiconductor light-receiving element 110 according to the first modification of the first embodiment, the InP layer constituting the i-type InP second window layer 8a is not easily deteriorated by oxidation.

[0114] <Function of Semiconductor Photodetector (APD) According to Modification 1 of First Embodiment> In the semiconductor light receiving element 110 according to the first modification of the first embodiment, complex resonance occurs at the resonance point as shown in Fig. 14, similarly to the first embodiment. In the refractive index profile of the semiconductor light receiving element 110 shown in Fig. 15, the i-type InAlAs first window layer 7b has a higher refractive index than the i-type InP first window layer 7a and the i-type InP second window layer 8a, but a lower refractive index than the i-type InGaAs light absorption layer 5. Therefore, since the refractive index of the light absorption layer is the highest, the proportion of the light intensity at positions C and D in Fig. 14 is high as the light resonance point. In order to obtain high reception sensitivity in a wide wavelength range by complex resonance, it is necessary to ensure the light resonance at positions A and B in Fig. 14 as well.

[0115] In the refractive index distribution shown in Fig. 15, the refractive index of the i-type InAlAs first window layer 7b is higher than that shown in Fig. 12, and therefore complex resonance is likely to occur even at positions A and B in Fig. 14. Specifically, for a wavelength of 1300 nm, the difference in refractive index between the i-type InP first window layer 7a (refractive index: 3.204) and the i-type InAlAs first window layer 7b is 0.05, and when the first window layer 7b is an InAlGaAs layer (refractive index: 3.454) with a composition wavelength λg of 1240 nm, the difference in refractive index is 0.25.

[0116] That is, in the element structure of the semiconductor light receiving element 110 according to the first modification of the first embodiment, the first window layer is a two-layer structure of the i-type InP first window layer 7a and the i-type InAlAs first window layer 7b, so that a refractive index difference of 0.05 to 0.25 is obtained, and complex resonance is likely to occur. Note that, in the case of the refractive index distribution shown in Fig. 15, the digital alloy structure multiplication layer (solid line) has a larger light confinement in the light absorption layer than the random alloy structure multiplication layer (dotted line), and therefore the improvement effect of the reception sensitivity in complex resonance is larger.

[0117] <Effects of Modification 1 of Embodiment 1> As described above, according to the semiconductor light receiving element according to the first modification of the first embodiment, a first window layer made of multiple layers is provided to increase the refractive index on the upper layer side, a two-dimensional periodic structure is provided in the second window layer, the semiconductor layer is covered with a SiN surface protective insulating film and a p-type electrode to reflect light inward, and an InAs / AlAs digital alloy structure is applied to the multiplication layer to increase the amount of light confined in the light absorption layer, thereby realizing both an improvement in the amount of light confined in the light absorption layer and complex resonance, thereby achieving an effect of enabling the semiconductor light receiving element to have high reception sensitivity over a wide wavelength range, and also achieving an effect of obtaining a semiconductor light receiving element that can achieve low noise and a wide bandwidth. Therefore, an APD with sufficient reception sensitivity can be obtained even in applications requiring a response bandwidth of 25 Gbps or more.

[0118] Variation 2 of embodiment 1 <Features of Semiconductor Photodetector (APD) According to Modification 2 of First Embodiment> Fig. 16 is a cross-sectional view showing an element structure of a back-illuminated APD which is an example of a semiconductor photodetector 120 according to Modification 2 of Embodiment 1. The semiconductor photodetector 100 according to Embodiment 1 shown in Figs. 3 and 4A is provided with a p-type InAlAs second window layer 8 having a plurality of holes 11 arranged in a lattice pattern. On the other hand, the semiconductor photodetector 120 according to Modification 2 of Embodiment 1 is characterized in that no second window layer is provided, only an i-type InP window layer 7c is provided, and a plurality of holes 11a are provided in a SiN surface protective insulating film 10a, thereby forming a lattice-like two-dimensional periodic structure 70a.

[0119] The thickness of the SiN surface protective insulating film 10a is within the range of 50 nm to 500 nm. A SiO2 film may be used instead of the SiN surface protective insulating film 10a. The shape of the opening of the hole 11a may be a circle, a rectangle, a triangle, or the like, but may also be another shape.

[0120] In the two-dimensional periodic structure 70a, the ratio of the diameter of the holes 11a to a certain period is preferably within a range of 10% to 80%. The depth of the holes 11a (or the height when the holes are the remainder) is preferably within a range of 100 nm to 1000 nm.

[0121] A p-type electrode 32 is disposed on the ring-shaped p-type InGaAs contact layer 9 and on the SiN surface protective insulating film 10a inside the ring.

[0122] <Function of Modification 2 of Embodiment 1> In the semiconductor light receiving element 120 according to the second modification of the first embodiment, similarly to the semiconductor light receiving element 100 according to the first embodiment, complex resonance occurs corresponding to the period of the plurality of holes 11a arranged in a lattice pattern, and the light intensity in the light absorption layer increases over a wide wavelength range, resulting in an effect of obtaining high reception sensitivity. The advantage of the semiconductor light receiving element 120 over the semiconductor light receiving element 100 according to the first embodiment is that the semiconductor light receiving element 120 does not have a second window layer, and therefore the total layer thickness of the epitaxial crystal growth layer can be made thin.

[0123] The holes 11a are generally formed in an insulating film such as a SiN film or a SiO2 film by dry etching or wet etching. Since selective etching of the insulating film and the semiconductor layer is easy, the bottom of the holes 11a can be accurately stopped at the surface of the semiconductor layer. This provides the effect of making the depths of the holes 11a constituting the two-dimensional periodic structure 70a uniform (the heights, if the holes are left as remaining parts).

[0124] Furthermore, the hole 11a is filled with the p-type electrode 32. For example, if a metal film made of Ti / Au is used as the p-type electrode 32, the refractive index difference between the p-type electrode 32 and the SiN surface protective insulating film 10a becomes large, which has the effect of making it easier to obtain the resonance effect.

[0125] <Effects of Modification 2 of Embodiment 1> As described above, according to the semiconductor light receiving element according to the second modification of the first embodiment, a lattice-shaped two-dimensional periodic structure is provided in the SiN surface protective insulating film on the i-type InP window layer, light is incident from the semiconductor substrate side, and the light is reflected inward by covering the semiconductor layer with the SiN surface protective insulating film and the p-type electrode, thereby achieving an effect of enabling the semiconductor light receiving element to have high reception sensitivity in a wide wavelength range by complex resonance, and also achieving an effect of obtaining a semiconductor light receiving element that can achieve low noise and wide bandwidth. In addition, compared to the first embodiment, the depth of the hole provided in the SiN surface protective insulating film (height when the hole is the remaining part) becomes uniform, and the refractive index difference can be increased, so that optical resonance occurs easily, and the light absorption efficiency is improved. As a result, an effect of realizing a semiconductor light receiving element that can achieve high reception sensitivity, low noise, and wide bandwidth is achieved. Therefore, it is possible to obtain an APD with sufficient reception sensitivity even in applications that require a response bandwidth of 25 Gbps or more.

[0126] Variation 3 of embodiment 1 <Features of Semiconductor Photodetector (APD) According to Modification 3 of First Preferred Embodiment> 17 is a cross-sectional view showing an element structure of a back-illuminated APD which is an example of a semiconductor photodetector 130 according to Modification 3 of Embodiment 1. The semiconductor photodetector 100 according to Embodiment 1 shown in FIGS. 3 and 4A is provided with a p-type InAlAs second window layer 8 having a plurality of holes 11 arranged in a lattice pattern. On the other hand, the semiconductor photodetector 130 according to Modification 3 of Embodiment 1 is characterized in that no second window layer is provided, only an i-type InP window layer 7c is provided, and holes 11b are provided in the p-type electrode 32a on the SiN surface protective insulating film 10, thereby forming a lattice-like two-dimensional periodic structure 70b.

[0127] The thickness of the p-type electrode 32a is within the range of 50 nm to 500 nm. The material of the p-type electrode 32a may be Ti / Pt / Au in addition to Ti / Au.

[0128] An example of the two-dimensional periodic structure 70b is a structure in which holes 11b are arranged in a lattice pattern in the p-type electrode 32a. The opening shape of the holes 11b may be circular, rectangular, triangular, or the like, but may also be other shapes.

[0129] In the two-dimensional periodic structure 70b, the ratio of the diameter of the holes 11b to a certain period is preferably within a range of 10% to 80%. The depth of the holes 11b (height when the holes are the remainder) is preferably within a range of 100 nm to 1000 nm.

[0130] As in the first embodiment, a p-type electrode 32a is provided on the ring-shaped p-type InGaAs contact layer 9 to form an ohmic contact portion.

[0131] <Function of Modification 3 of Embodiment 1> In the semiconductor light receiving element 130 according to the third modification of the first embodiment, similarly to the semiconductor light receiving element 100 according to the first embodiment, complex resonance occurs corresponding to the period of the plurality of holes 11b formed in a lattice shape, and the light intensity in the light absorption layer increases over a wide wavelength range, resulting in high reception sensitivity. The advantage over the semiconductor light receiving element 100 according to the first embodiment is that the thickness of the epitaxial crystal growth layer can be made thin because there is no second window layer.

[0132] The holes 11b are generally formed in the p-electrode 32a made of a metal film such as Ti / Au by dry etching or wet etching. Since selective etching of the SiN surface protective insulating film 10 below the p-electrode 32a is easy, the bottom of the holes 11b can be accurately stopped on the surface of the SiN surface protective insulating film 10. This provides the advantage that the depths of the multiple holes 11b constituting the two-dimensional periodic structure 70b (heights when the holes are the remaining parts) can be made uniform.

[0133] It is also possible to form the hole 11b in the p-type electrode 32a by using lift-off. Furthermore, since the difference in refractive index between the p-type electrode 32a and the hole 11b, that is, between the p-type electrode 32a and the hole 11b, that is, between the p-type electrode 32a and the air, is large, there is also an effect that the resonance effect is more easily obtained.

[0134] <Effects of Modification 3 of Embodiment 1> As described above, according to the semiconductor light receiving element according to the third modification of the first embodiment, a two-dimensional periodic lattice structure is provided on the p-type electrode on the SiN surface protective insulating film, and light is incident from the semiconductor substrate side, so that the semiconductor light receiving element can have a high reception sensitivity in a wide wavelength range by complex resonance. In addition, compared to the first embodiment, the depth of the hole provided in the p-type electrode (height when the hole is the remaining part) is uniform, and the refractive index difference can be made large, so that light resonance occurs easily, and the light absorption efficiency is improved. As a result, a semiconductor light receiving element that has high reception sensitivity, low noise, and can be made wideband can be realized. Therefore, it is possible to obtain an APD with sufficient reception sensitivity even in applications that require a response band of 25 Gbps or more.

[0135] Variation 4 of embodiment 1 <Features of Semiconductor Photodetector (APD) According to Modification 4 of First Preferred Embodiment> 18 is a cross-sectional view showing an element structure of a back-illuminated APD which is an example of a semiconductor photodetector 140 according to the fourth modification of the first embodiment. The semiconductor photodetector 100 according to the first embodiment shown in FIGS. 3 and 4A is provided with a p-type InAlAs second window layer 8 having a plurality of holes 11 arranged in a lattice pattern. On the other hand, the semiconductor photodetector 140 according to the fourth modification of the first embodiment is characterized in that no second window layer is provided, but only an i-type InP window layer 7c is provided, and a plurality of holes 11c are provided in the p-type InGaAs contact layer 9a on the i-type InP window layer 7c, thereby forming a lattice-like two-dimensional periodic structure 70c.

[0136] The thickness of the p-type InGaAs contact layer 9a is within the range of 50 nm to 500 nm.

[0137] An example of the two-dimensional periodic structure 70c is a structure in which holes 11c are arranged in a lattice pattern in the p-type InGaAs contact layer 9a. The opening shape of the holes 11c may be circular, rectangular, triangular, or the like, but may also be other shapes.

[0138] In the two-dimensional periodic structure 70c, the ratio of the diameter of the holes 11c to a certain period is preferably within a range of 10% to 80%. The depth of the holes 11b (or the height when the holes are the remainder) is preferably within a range of 100 nm to 1000 nm.

[0139] An ohmic contact is formed on the p-type InGaAs contact layer 9a by providing a p-type electrode 32. Note that the SiN surface protective insulating film 10b does not need to be removed from a portion of the p-type InGaAs contact layer 9a where the hole 11c is not provided.

[0140] <Function of Modification 4 of Embodiment 1> In the semiconductor light receiving element 140 according to the fourth modification of the first embodiment, similarly to the semiconductor light receiving element 100 according to the first embodiment, complex resonance occurs corresponding to the period of the plurality of holes 11c formed in a lattice shape, and the light intensity in the light absorption layer increases over a wide wavelength range, resulting in high reception sensitivity. The advantage over the semiconductor light receiving element 100 according to the first embodiment is that the thickness of the epitaxial crystal growth layer can be made thin because there is no second window layer.

[0141] The hole 11c is generally formed in the p-type InGaAs contact layer 9a by dry etching or wet etching. Since the hole 11c can be formed simultaneously in the process of forming the ring-shaped ohmic contact, there is an advantage that a separate process of forming the hole 11c is not required.

[0142] When etching the p-type InGaAs contact layer 9a, selective etching with the i-type InP window layer 7c adjacent to the lower side can be easily performed, so that the bottom of the hole 11a can be accurately stopped on the surface of the i-type InP window layer 7c. This provides the effect of making the depth (height, if the holes are the remaining parts) of the multiple holes 11c constituting the two-dimensional periodic structure 70a uniform.

[0143] A p-type electrode 32 is provided in a portion of the p-type InGaAs contact layer 9a where the hole 11c is not formed, thereby forming an ohmic contact with the p-type InGaAs contact layer 9a. This increases the contact area between the p-type InGaAs contact layer 9a and the p-type electrode 32, thereby making it possible to reduce the ohmic resistance of the semiconductor light-receiving element 140.

[0144] <Advantages of Modification 4 of Embodiment 1> As described above, according to the semiconductor light receiving element according to the fourth modification of the first embodiment, a lattice-like two-dimensional periodic structure is provided in the p-type InGaAs contact layer on the i-type InP window layer, and light is incident from the semiconductor substrate side. This allows the semiconductor light receiving element to have high reception sensitivity in a wide wavelength range by complex resonance, and also allows low noise and wide bandwidth. In addition, compared to the first embodiment, the depth of the hole provided in the p-type InGaAs contact layer (height when the hole is the remaining part) is uniform, so that optical resonance is more likely to occur, and the light absorption efficiency is improved. As a result, a semiconductor light receiving element that has high reception sensitivity, low noise, and can be made wide bandwidth can be realized. Therefore, it is possible to obtain an APD with sufficient reception sensitivity even in applications that require a response bandwidth of 25 Gbps or more.

[0145] Modification 5 of the first embodiment <Features of Semiconductor Photodetector (APD) According to Modification 5 of First Preferred Embodiment> FIG. 19 is a cross-sectional view showing an element structure of a back-illuminated APD, which is an example of a semiconductor light receiving element 150 according to the fifth modification of the first embodiment. In the first embodiment and the first to fourth modifications of the embodiments, the p-type region is formed by selective diffusion of Zn. On the other hand, in the semiconductor light receiving element 150 according to the fifth modification of the first embodiment, as shown in FIG. 19, each layer of the p-type InP first window layer 7d, the p-type InAlAs second window layer 8b, and the p-type InGaAs contact layer 9b is p-doped during epitaxial crystal growth to form a mesa-type APD having a mesa structure (hereinafter also simply referred to as a mesa). As the p-type dopant, Be, Zn, Mg, C, etc. can be used. The doping concentration of each layer is 5×10 17 cm -3 That's 1 x 10 18 cm -3 The degree is suitable.

[0146] <Function of Modification 5 of Embodiment 1> When light is vertically incident on the lattice-like two-dimensional periodic structure 70, light 92 is generated that travels horizontally. The light 92 traveling horizontally is absorbed by the i-type InGaAs light absorption layer 5 and gradually attenuates. In the case of the semiconductor light receiving element 100 according to the first embodiment, a portion of the light 92 traveling horizontally may reach the outside of the boundary 13 of the Zn-diffused p-type region 12. In this case, the light that has reached the outside of the boundary 13 of the Zn-diffused p-type region 12 is also absorbed, generating electron and hole carriers.

[0147] Because no electric field is applied outside the boundary 13 of the Zn-diffused p-type region 12, the generated carriers diffuse slowly and return to the Zn-diffused p-type region 12, and are extracted as a current signal. The time it takes for carriers to diffuse in the region where no electric field is applied is longer than that in the region where an electric field is applied, so the frequency response is degraded. As a result, there is a risk of a problem in that tailing occurs in the pulse response waveform of the APD.

[0148] On the other hand, in the back-illuminated APD shown in FIG. 19, which is an example of a semiconductor photodetector 150 according to the fifth modification of the first embodiment, an electric field is applied to the entire region inside the mesa structure due to the mesa element structure. As a result, no tailing occurs in the pulse response waveform, and the frequency response does not deteriorate.

[0149] <Effects of Modification 5 of Embodiment 1> As described above, the semiconductor light receiving element according to the fifth modification of the first embodiment has a mesa-type element structure, and an electric field is applied over the entire region inside the mesa structure, so that no tailing occurs in the pulse response waveform and the frequency response does not deteriorate, thereby achieving an effect of realizing a semiconductor light receiving element that has high reception sensitivity, low noise, and is capable of wideband operation. Therefore, even in applications requiring a response bandwidth of 25 Gbps or more, a pulse response waveform with an excellent S / N ratio can be obtained, and an APD with sufficient reception sensitivity can be obtained.

[0150] Variation 6 of the first embodiment <Features of Semiconductor Photodetector (APD) According to Modification 6 of First Preferred Embodiment> FIG. 20 is a cross-sectional view illustrating the device structure of a back-illuminated APD, which is an example of a semiconductor light-receiving device 160 according to the sixth modification of the first embodiment.

[0151] <Structure of Semiconductor Light Receiver According to Modification 6 of First Embodiment> The back-illuminated APD, which is an example of the semiconductor light-receiving element 160 according to the sixth modification of the first embodiment, includes an Fe-doped semi-insulating InP substrate 1a and a semiconductor layer having a carrier concentration of 1 to 5×10 18 cm -3 and an n-type InP buffer layer 2a having a thickness of 0.1 to 1.0 μm; an i-type InAs / AlAs digital alloy structure multiplication layer 3 in which an i-type AlAs layer (for example, a layer thickness of two atomic layers, about 0.6 nm) and an i-type InAs layer (for example, a layer thickness of two atomic layers, about 0.6 nm) are alternately laminated multiple times; and 17 cm -3the p-type InP electric field relaxation layer 4 having a thickness of 10 to 70 nm, the i-type InGaAs light absorption layer 5 having a thickness of 0.1 to 2.0 μm, the i-type InAlGaAs / InAlAs graded layer 6, the p-type InP first window layer 7d having a thickness of 0.1 to 3.0 μm, the p-type InAlAs second window layer 8 having a thickness of 0.1 to 3.0 μm and a two-dimensional periodic structure 70, the p-type InGaAs contact layer 9 having a ring shape, a plurality of holes 11 two-dimensionally arranged in the p-type InAlAs second window layer 8, and the p-type InP first window layer 7d, the p-type InAlAs second window layer 8, and a SiN surface protective insulating film 10a formed on the surfaces and side surfaces of each layer; an n-type electrode 31a formed on the n-type InP buffer layer 2a exposed on the surface and on the SiN surface protective insulating film 10a formed on the side surfaces; a p-type electrode 32 formed on the ring-shaped p-type InGaAs contact layer 9 and on the SiN surface protective insulating film 10a surrounded by the p-type InGaAs contact layer 9; and an anti-reflection film 40 formed on the back surface side of the n-type InP substrate 1.

[0152] <Method of manufacturing semiconductor light receiving element according to modification 6 of embodiment 1> On the Fe-doped InP substrate 1a, a semiconductor having a carrier concentration of 1 to 5 × 10 18 cm -3 An n-type InP buffer layer 2a having a thickness of 0.1 to 1.0 μm is grown on the n-type InP buffer layer 2a. The n-type InP buffer layer 2a functions as both a conductive layer and a contact layer. As in the semiconductor light receiving element 100 according to the first embodiment, an i-type InAs / AlAs digital alloy structure multiplication layer 3, a p-type InP electric field relaxation layer 4, an i-type InGaAs light absorption layer 5, and an i-type InAlGaAs / InAlAs graded layer 6 are successively grown by crystal growth on the n-type InP buffer layer 2a.

[0153] A p-type InP first window layer 7d is grown by crystal growth on the i-type InAlGaAs / InAlAs graded layer 6, in the same manner as in the semiconductor photodetector 150 according to the fifth modification of the first embodiment. Furthermore, a p-type InAlAs second window layer 8 and a p-type InGaAs contact layer 9 are grown by crystal growth in this order, in the same manner as in the semiconductor photodetector 100 according to the first embodiment.

[0154] Next, the p-type InGaAs contact layer 9 is processed into a ring shape, and a plurality of holes 11 are formed in the p-type InAlAs second window layer 8 so as to be arranged in a lattice pattern, thereby providing a two-dimensional periodic structure 70 in the p-type InAlAs second window layer 8. The outside of the outer periphery of the p-type InGaAs contact layer 9 processed into a ring shape is etched and removed until the bottom surface reaches the n-type InP buffer layer 2a.

[0155] After etching, the entire surface is covered with a SiN insulating film, and the SiN insulating film in the portion where the ohmic contact portion is to be formed is removed to form a p-type electrode 32 and an n-type electrode 31a. The p-type electrode 32 is formed on the ring-shaped p-type InGaAs contact layer 9 and on the SiN surface protective insulating film 10a surrounded by the p-type InGaAs contact layer 9.

[0156] The n-type electrode 31a is formed so as to cover the side surface of the mesa formed by etching until it reaches the n-type InP buffer layer 2a, via the SiN surface protective insulating film 10a provided on the side surface of each semiconductor layer. Alternatively, the electrode covering the side surface of the mesa may be an electrode independent of the n-type electrode 31a formed on the n-type InP buffer layer 2a, without being connected thereto. The electrode on the side surface of the mesa may be formed simultaneously with the p-type electrode 32.

[0157] <Function of Modification 6 of Embodiment 1> In the semiconductor light receiving element 160 according to the second modification of the first embodiment, when light is incident vertically on the lattice-like two-dimensional periodic structure 70, light traveling horizontally is generated. The side surface of the mesa is covered with the SiN surface protective insulating film 10a and the n-type electrode 31a, and therefore functions as a reflecting mirror for this horizontally traveling light. Therefore, the light traveling horizontally is reflected back to the inside by the side surface of the mesa and absorbed by the i-type InGaAs light absorption layer 5, improving the receiving sensitivity.

[0158] In the semiconductor light receiving element 160 according to the second modification of the first embodiment, the Fe-doped semi-insulating InP substrate 1a, which has smaller optical absorption loss than the n-type InP substrate 1, is used, and thus the receiving sensitivity is further improved.

[0159] <Advantages of Modification 6 of First Embodiment> As described above, in the semiconductor light receiving element according to the sixth modification of the first embodiment, a reflective mirror made of a SiN surface protective insulating film and an n-type electrode is provided on the side surface of the mesa, so that light traveling in the horizontal direction can be returned to the inside, thereby improving the receiving sensitivity of the semiconductor light receiving element. Therefore, it is possible to obtain an APD with sufficient receiving sensitivity even in applications requiring a response bandwidth of 25 Gbps or more.

[0160] Variation 7 of the first embodiment 21 is a cross-sectional view showing the device structure of an edge-illuminated APD, which is an example of a semiconductor light receiving device 170 according to Variation 7 of Embodiment 1. In Embodiment 1 and Variations 1 to 6 of Embodiment 1, light is incident from the back surface. On the other hand, the semiconductor light receiving device 170 according to Variation 7 of Embodiment 1 shown in FIG. 21 is characterized in that an anti-reflection film 41 is provided on the entire end face of the side portion of the mesa, and incident light 90 is incident from the end face.

[0161] <Advantages of the Seventh Modification of the First Embodiment> In the semiconductor photodetector according to the seventh modification of the first embodiment, a two-dimensional periodic structure is provided in the second window layer, and an InAs / AlAs digital alloy structure is applied to the multiplication layer, thereby increasing the amount of light confined in the light absorption layer, and as a result, high reception sensitivity can be achieved over a wide wavelength range by complex resonance. In addition, an anti-reflection film is provided on the entire facet, which enables even higher reception sensitivity, and an end-illuminated semiconductor photodetector that enables lower noise and a wider bandwidth can be obtained. Therefore, an APD with sufficient reception sensitivity can be obtained even for applications requiring a response bandwidth of 25 Gbps or more.

[0162] Embodiment 2 <Element structure of semiconductor photodetector (APD) according to the second embodiment> 22 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor photodetector 180 according to the second embodiment. The semiconductor photodetector 180 according to the second embodiment has a carrier concentration of 1 to 5×10 between an n-type InP buffer layer 2 and an i-type InAs / AlAs digital alloy structure multiplication layer 3. 18 cm -3 The semiconductor light receiving element 100 according to the first embodiment is characterized in that it has an n-type DBR layer 14 (Distributed Bragg Reflector: DBR) formed by alternately stacking n-type InAlGaAs layers and n-type InP layers in this order. The configuration other than the n-type DBR layer 14 is almost the same as that of the semiconductor light receiving element 100 according to the first embodiment.

[0163] The n-type DBR layer 14 also functions as a buffer layer. Therefore, the n-type InP buffer layer 2 is not necessary. The n-type DBR layer 14 is formed by a crystal growth method such as MOCVD or MBE. The n-type DBR layer 14 is also called an n-type semiconductor layer.

[0164] The i-type InP first window layer 7 may be replaced by an i-type InAlAs first window layer, and conversely, the p-type InAlAs second window layer 8 may be replaced by a p-type InP second window layer.

[0165] The thickness t of each layer constituting the n-type DBR layer 14 may be set so as to satisfy the following formula (3), where n is the refractive index of each layer and λ is the wavelength of the incident light. t=λ / (4 n) (3)

[0166] Furthermore, the thickness of each layer constituting n-type DBR layer 14 may be an odd multiple of layer thickness t, i.e., 1t, 3t, 5t, .... The reflectance of n-type DBR layer 14 can be changed by the composition of the InAlGaAs layer and the number of InAlGaAs layer / InP layer pairs.

[0167] For example, if the wavelength of the incident light is 1300 nm and the composition wavelength λg of InAlGaAs is 1100 nm, the refractive index n is 3.38 and the thickness of the InAlGaAs layers constituting one pair of n-type DBR layer 14 is 96.2 nm. Since the refractive index n of InP is 3.20, the thickness of the InP layers constituting one pair of n-type DBR layer 14 is 101.6 nm. Therefore, the thickness of one pair of n-type DBR layer 14 is 197.8 nm, which is the sum of the two layers.

[0168] The number of InAlGaAs / InP pairs in n-type DBR layer 14 is preferably 2 to 20, and the total thickness of n-type DBR layer 14 is preferably 0.4 to 4 μm. However, the number of pairs depends on the refractive index of each layer to be stacked, and the smaller the difference in refractive index between the two layers to be stacked, the more pairs are required.

[0169] The n-type DBR layer 14 may be any of the pairs InGaAs / InP, InGaAsP / InP, InGaAsP / InAlGaAs, InAlGaAs / InAlAs, and InGaAs / InAlAs, other than the above-mentioned pair InAlGaAs / InP.

[0170] <Function of the Second Embodiment> The semiconductor light receiving element 180 according to the second embodiment has the same functions and effects as the semiconductor light receiving element 100 according to the first embodiment, and in addition, has the function and effect of further improving the receiving sensitivity. The function of the n-type DBR layer 14 will be described below with reference to FIG.

[0171] 22, incident light 90 entering through anti-reflection film 40 reaches n-type DBR layer 14. n-type DBR layer 14 reflects light, but transmits wavelengths that satisfy the resonance conditions of each layer located above n-type DBR layer 14. As a result, a vertical cavity is formed between n-type DBR layer 14 and p-type electrode 32, causing optical resonance.

[0172] The vertical resonant light confined by the n-type DBR layer 14 and the p-type electrode 32 enhances the optical resonance caused by the two-dimensional periodic structure 70 formed by the plurality of holes 11 arranged in a lattice pattern in the p-type InAlAs second window layer 8, i.e., the resonant light shown in Fig. 22. As a result, the receiving sensitivity of the semiconductor photodetector 180 is improved.

[0173] 23 is a diagram showing the dependency of the reflectance and quantum efficiency of the DBR layer on the number of pairs in a back-illuminated APD, which is an example of the semiconductor light receiving element 180 according to the second embodiment. Here, the wavelength of the incident light is 1300 nm, the refractive index n of the InAlGaAs layer is 3.38, the layer thickness is 96.2 nm, the refractive index n of the InP layer is 3.20, and the layer thickness is 101.6 nm. The refractive index of the i-type InGaAs light absorption layer 5 is 3.595, the layer thickness is 542 nm, and the absorption coefficient is 12716 / cm. The reflectance of the two-dimensional periodic structure 70 and the p-type electrode 32 is 80%. That is, the two-dimensional periodic structure 70 and the p-type electrode 32 function as a mirror with a reflectance of 80%.

[0174] The efficiency (quantum efficiency) of absorption due to cavity resonance was calculated using a model in which an InGaAs light absorption layer, an InAs / AlAs digital alloy structure multiplication layer, and an InP window layer are placed between the n-type DBR layer 14 and a mirror with a reflectance of 80%. In Fig. 23, the vertical axis represents the reflectance of the n-type DBR layer 14 and the quantum efficiency of light absorbed in the InGaAs light absorption layer, and the horizontal axis represents the number of pairs in the n-type DBR layer 14.

[0175] 23, as the number of pairs in the n-DBR layer 14 increases, the reflectance of the n-DBR layer 14 increases monotonically. On the other hand, the quantum efficiency increases up to 8 pairs, but decreases when the number of pairs exceeds 8. This is because when the number of pairs constituting the n-DBR layer 14 exceeds 8 pairs, the amount of light transmitting through the n-DBR layer 14 decreases, and therefore the amount of light resonating in the cavity formed by the n-DBR layer 14, the two-dimensional periodic structure 70, and the p-electrode 32 decreases.

[0176] To obtain a quantum efficiency of 80% or more, the reflectance of the n-type DBR layer 14 is preferably 1% or more and 40% or less. The number of pairs constituting the n-type DBR layer 14 is preferably 2 pairs or more and 14 pairs or less. To obtain a higher quantum efficiency of 85% or more, the reflectance of the n-type DBR layer 14 is preferably 5% or more and 33% or less, and the number of pairs constituting the n-type DBR layer 14 is more preferably 4 pairs or more and 12 pairs or less.

[0177] Patent Document 2 discloses a semiconductor light receiving element using a photonic crystal and a DBR layer, but the DBR layer only functions to return light that is incident from the surface and is not absorbed by the light absorption layer and is transmitted through. Therefore, the semiconductor light receiving element described in Patent Document 2 does not have a two-dimensional periodic structure 70 as in the present disclosure and a vertical cavity consisting of the p-type electrode 32 and the n-type DBR layer 14, and therefore does not achieve an improvement in receiving sensitivity due to optical resonance in the vertical direction.

[0178] <Advantages of the second embodiment> As described above, according to the semiconductor light receiving element of the second embodiment, an n-type DBR layer is provided under the multiplication layer, a two-dimensional periodic structure is provided in the second window layer, and the two-dimensional periodic structure is covered with a SiN surface protective insulating film and a p-type electrode to form a vertical cavity, so that a semiconductor light receiving element with high reception sensitivity can be obtained by making light incident from the semiconductor substrate side. In addition, by applying an InAs / AlAs digital alloy structure to the multiplication layer, the amount of light confined in the light absorption layer is increased, so that a semiconductor light receiving element can be obtained that can achieve high reception sensitivity over a wide wavelength range by complex resonance, and can further achieve low noise and wide bandwidth. Therefore, an APD with sufficient reception sensitivity can be obtained even in applications that require a response bandwidth of 25 Gbps or more.

[0179] Variation 1 of embodiment 2 24 is a cross-sectional view showing an element structure of a back-illuminated PD, which is an example of a semiconductor photodetector 190 according to a first modification of the second embodiment. The configuration in which the n-type DBR layer 14 is provided below the multiplication layer 3 is also possible in the first to sixth modifications of the first embodiment, and the same action and effect can be obtained. The semiconductor photodetector 190 according to the first modification of the second embodiment is characterized in that the i-type InAs / AlAs digital alloy structure multiplication layer 3 and the p-type InP electric field relaxation layer 4 are removed from the semiconductor photodetector 180 according to the second embodiment to form a PD. The semiconductor photodetector 190 according to the first modification of the second embodiment also has substantially the same effect as the semiconductor photodetector 180 according to the second embodiment.

[0180] Variation 2 of embodiment 2 Fig. 25 is a cross-sectional view showing an element structure of a back-illuminated APD which is an example of a semiconductor photodetector 200 according to Modification 2 of Embodiment 2. The semiconductor photodetector 200 according to Modification 2 of Embodiment 2 has a configuration in which the n-type InP buffer layer 2a in the semiconductor photodetector 160 according to Modification 6 of Embodiment 1 shown in Fig. 20 is replaced with an n-type DBR layer 14a. Higher receiving sensitivity can be obtained by confining light in both the horizontal and vertical directions.

[0181] Variation 3 of embodiment 2 26 is a cross-sectional view showing an element structure of a back-illuminated APD, which is an example of a semiconductor light receiving element 210 according to Modification 3 of Embodiment 2. The semiconductor light receiving element 210 according to Modification 3 of Embodiment 2 is characterized in that the conductivity types of the semiconductor light receiving element 200 according to Modification 2 of Embodiment 2 are reversed, that is, the substrate side is p-type and the front surface side is n-type.

[0182] <Structure of Semiconductor Light Receiver According to Modification 3 of Embodiment 2> A back-illuminated APD, which is an example of a semiconductor light-receiving element 210 according to the third modification of the second embodiment, includes an Fe-doped semi-insulating InP substrate 1a, a p-type InP buffer layer 2b having a thickness of 0.1 to 1.0 μm, and a p-type InP buffer layer 2c having a carrier concentration of 1 to 5×10. 18 cm -3The p-type DBR layer 14b is formed by alternately stacking p-type InAlGaAs layers and p-type InP layers, the i-type InAlGaAs / InAlAs graded layer 6, the i-type InGaAs light absorbing layer 5 having a thickness of 0.1 to 2.0 μm, and the carrier concentration of 0.1 to 50×10 17 cm -3 The multiplication layer 3 is an i-type InAs / AlAs digital alloy structure multiplication layer 3 in which an i-type AlAs layer (for example, a layer thickness of two atomic layers, about 0.6 nm) and an i-type InAs layer (for example, a layer thickness of two atomic layers, about 0.6 nm) are alternately laminated multiple times, an n-type InP first window layer 7e having a layer thickness of 0.1 to 3.0 μm, an n-type InAlAs second window layer 8c having a layer thickness of 0.1 to 3.0 μm and a two-dimensional periodic structure 70d, an n-type InGaAs contact layer 9c having a ring shape, and a plurality of holes arranged two-dimensionally in the n-type InAlAs second window layer 8c. the n-type InP first window layer 7e and the n-type InAlAs second window layer 8c exposed at the bottom of the hole 11d, a SiN surface protective insulating film 10a formed on the surfaces of the n-type InP first window layer 7e and the n-type InAlAs second window layer 8c exposed at the bottom of the hole 11d and on the side surfaces of each layer, a p-type electrode 32b formed on the p-type DBR layer 14b exposed at the surface and on the SiN surface protective insulating film 10a formed on the side surfaces, an n-type electrode 31b formed on the ring-shaped n-type InGaAs contact layer 9c and on the SiN surface protective insulating film 10a surrounded by the n-type InGaAs contact layer 9c, and an antireflection film 40 formed on the back surface side of the Fe-doped semi-insulating InP substrate 1a.

[0183] Instead of the p-type InP buffer layer 2b, an n-type or i-type InP buffer layer may be provided, but the InP buffer layer is not necessarily required.

[0184] <Method of manufacturing semiconductor light receiving element according to modification 3 of embodiment 2> On the Fe-doped semi-insulating InP substrate 1a, a p-type InP buffer layer 2b is crystal-grown to a thickness of 0.1 to 1 μm. 18 cm -3A p-type DBR layer 14b is formed by alternately stacking a p-type InAlGaAs layer and a p-type InP layer in this order by crystal growth. An i-type InAlGaAs / InAlAs graded layer 6, an i-type InGaAs light absorption layer 5, a p-type InP electric field relaxation layer 4, an i-type InAs / AlAs digital alloy structure multiplication layer 3, an n-type InP first window layer 7e, an n-type InAlAs second window layer 8c, and an n-type InGaAs contact layer 9c are successively crystal grown on the p-type DBR layer 14b.

[0185] The p-type DBR layer 14b differs only in conductivity type from the n-type DBR layer 14 of the semiconductor light receiving element 200 according to the second embodiment, and the layer thickness and constituent materials of each layer of the pair are the same. The n-type InP first window layer 7e, the n-type InAlAs second window layer 8c, and the n-type InGaAs contact layer 9c differ in conductivity type from the semiconductor light receiving element 150 according to the fifth modification of the first embodiment, but the layer thickness and carrier concentration of each layer are the same.

[0186] Next, the n-type InGaAs contact layer 9c is processed into a ring shape, and a plurality of holes 11d are formed in the n-type InAlAs second window layer 8c so as to be arranged in a lattice pattern, thereby providing a two-dimensional periodic structure 70d in the n-type InAlAs second window layer 8c. The outside of the outer periphery of the n-type InGaAs contact layer 9c processed into a ring shape is etched and removed until the bottom surface reaches the p-type DBR layer 14b.

[0187] After etching, the entire surface is covered with a SiN film, and the SiN film is removed from the portion where the ohmic contact portion is to be formed, to form a p-type electrode 32a and an n-type electrode 31b. The n-type electrode 31b is formed on the ring-shaped n-type InGaAs contact layer 9c.

[0188] The p-type electrode 32a is disposed so as to cover the side surface of the mesa formed by etching until it reaches the p-type DBR layer 14b, i.e., the side surface of each semiconductor layer, via the SiN surface protective insulating film 10a also provided on the side surface of each semiconductor layer. Alternatively, the electrode covering the side surface of the mesa may be an independent electrode that is not connected to the p-type electrode 32a. Also, the electrode on the side surface of the mesa may be formed simultaneously with the n-type electrode 31b.

[0189] <Function of Modification 3 of Embodiment 2> An n-type semiconductor has less optical absorption loss than a p-type semiconductor. In the semiconductor light receiving element 210 according to the third modification of the second embodiment, the conductivity type is inverted from that of the semiconductor light receiving element 200 according to the second modification of the second embodiment. Therefore, the first window layer in which the resonance point is formed is n-type instead of p-type, which provides the excellent effect of reducing the optical absorption loss in the n-type InP first window layer 7e.

[0190] <Effects of Modification 3 of Embodiment 2> As described above, according to the semiconductor photodetector of the third modification of the second embodiment, the conductivity type of the first window layer in which the resonance point is formed is n-type instead of p-type, and therefore the light absorption loss by the first window layer can be reduced, thereby providing an advantage that a semiconductor photodetector with high reception sensitivity can be realized. Therefore, an APD with sufficient reception sensitivity can be obtained even in applications requiring a response bandwidth of 25 Gbps or more.

[0191] In each of the above-mentioned embodiments and the modified examples of each embodiment, the multiplication layer is configured with an i-type InAs / AlAs digital alloy structure. The following describes in more detail the digital alloy structure multiplication layer, which is one of the structural features of each of the above-mentioned semiconductor light receiving elements.

[0192] If a wideband APD of 37.5 GHz or more can be realized, the next generation high-speed PON system can be realized without using a DSP or SOA. In the case of a PD, which has a relatively easy response bandwidth, the response bandwidth is as follows: (1) RC time constant (R is the element resistance, C is the element capacitance) (2) Carrier transit time (the time it takes for an electron or hole to travel through the depletion layer) In addition, the APD is limited by (3) It is also limited by the multiplication time (the time it takes for electrons and holes to multiply in a chain reaction in the multiplication layer, which increases in proportion to the multiplication factor).

[0193] Although a PD can achieve the above-mentioned 37.5 GHz bandwidth, an APD requires a multiplication time, so increasing the multiplication factor makes it difficult to achieve the desired bandwidth. The multiplication time TM is expressed by the following equations (4) to (6). Multiplication time TM=multiplication rate M / GB product (4) GB product=1 / (2πNkτav) (5) In other words, Multiplication time TM=2πNkMτav (6) It becomes.

[0194] Here, GB product is the product of the multiplication factor and the response bandwidth, k is the ionization rate ratio, N is a coefficient that is loosely dependent on the ionization rate ratio k, and τav is the average time that electrons and holes travel through the multiplication layer. Therefore, it is possible to shorten the multiplication time TM by reducing the ionization rate ratio k. In particular, to realize a high-speed PON system, it is necessary to make the multiplication time TM approach zero, that is, to make the ionization rate ratio k approach zero.

[0195] In order to make the ionization rate ratio k zero, various compound semiconductors have been proposed as materials for the multiplication layer. In addition, in order to reduce the ionization rate ratio k, a digital alloy structure has been proposed in which semiconductor layers with different compositions are alternately stacked in a cycle of 1 to 6 atomic layers. However, it is difficult to make the ionization rate ratio k zero in the digital alloy structure unless the structure is optimized. The digital alloy structure is described in Non-Patent Document 2.

[0196] Therefore, in order to reduce the ionization rate ratio k of the digital alloy structure, the inventors fabricated an APD with a digital alloy structure multiplication layer using a multiplication layer in which two-atom-layered InAs layers and two-atom-layered AlAs layers are alternately stacked, and as a result of analyzing the multiplication characteristics, they discovered that the distance that carriers travel in the multiplication layer until they are ionized is longer than that of an APD with an InAlAs multiplication layer made of normal bulk crystal, that is, an InAlAs random alloy structure multiplication layer. The distance that carriers travel in the multiplication layer until they are ionized is called the dead space.

[0197] Since the length of the dead space (hereinafter referred to as the dead space length) is longer for holes than for electrons, in the case of an InAlAs random alloy structure made of a normal bulk crystal, when the thickness of the multiplication layer is thinned to a level of several tens of nm, the ionization rate ratio k decreases because the holes cannot be ionized. However, when the thickness of the multiplication layer is thinned to a level of several tens of nm, a new problem occurs in that a leak current such as a tunnel current increases because a higher electric field needs to be applied to the multiplication layer in order to obtain a desired multiplication factor. In other words, an increase in the tunnel current increases the noise generated in the APD. On the other hand, the inventors' analysis discovered that the ionization rate ratio k=0 in the digital alloy structure even when the multiplication layer has a thickness of 100 nm or more because the dead space is unusually large in the digital alloy structure compared to the random alloy structure.

[0198] That is, the inventors have found for the first time that it is possible to make the ionization rate ratio k=0 while suppressing the tunnel current by forming the multiplication layer of the APD with a digital alloy structure. Specifically, it has been found that in the multiplication layer having the digital alloy structure of the present disclosure, the ionization rate ratio k drops sharply at a layer thickness of 170 nm or less, and in particular, the dead space effect improves dramatically when the layer thickness of the multiplication layer is in the range of 60 to 130 nm. That is, the inventors have demonstrated that the ionization rate ratio k=0, which was impossible to achieve with a multiplication layer having a random alloy structure or a multiplication layer having a thick digital alloy structure, can be achieved by applying the multiplication layer having the digital alloy structure of the present disclosure. At present, no research institute has reported that the thinning of the multiplication layer of an APD having a digital alloy structure has a higher effect of reducing the ionization rate ratio k than the thinning of the multiplication layer of an APD made of a conventional material.

[0199] The following describes an APD, which is an example of a semiconductor light receiving element according to embodiments 1 and 2. One of the features of the semiconductor light receiving elements according to embodiments 1 and 2 is that the multiplication layer has a digital alloy structure.

[0200] One example of an i-type InAs / AlAs digital alloy structure is a digital alloy structure in which i-type AlAs layers (for example, a layer thickness of two atomic layers, approximately 0.6 nm) and i-type InAs layers (for example, a layer thickness of two atomic layers, approximately 0.6 nm) are alternately stacked multiple times.

[0201] The thickness of the i-type InAs / AlAs digital alloy structure multiplication layer is within a range of 40 nm to 1000 nm. However, in order to increase the dead space effect in the i-type InAs / AlAs digital alloy structure multiplication layer, the thickness of the i-type InAs / AlAs digital alloy structure multiplication layer may be within a range of 40 nm to 170 nm. Furthermore, considering the typical degree of variation in layer thickness during fabrication of the semiconductor light receiving element 100, which is 20%, the thickness of the i-type InAs / AlAs digital alloy structure multiplication layer is more preferably within a range of 50 nm to 140 nm.

[0202] The operation of an APD having an i-type InAs / AlAs digital alloy structure multiplication layer will be described below. The inventors have found that the use of a digital alloy structure multiplication layer, as in the APDs according to the first and second embodiments, enhances the dead space effect, that is, the effect of reducing the ionization rate ratio k. FIG. 27 is a diagram showing the electric field dependence of the electron dead space in an InAlAs multiplication layer. As a result of analyzing the electron multiplication characteristics in a digital alloy structure multiplication layer, the inventors have revealed that, as shown in the graph of FIG. 27, the InAs / AlAs digital alloy structure multiplication layer of the present disclosure has a longer dead space than the conventional InAlAs random alloy structure multiplication layer.

[0203] 28A to 28C are conceptual diagrams showing the ionization rates of electrons and holes, respectively, where FIG. 28A shows the case of electron ionization, FIG. 28B shows the case of hole ionization, and FIG. 28C shows the ionization rate when the multiplication layer is thinned. In a conventional InAlAs random alloy structure multiplication layer, as shown in the graph of FIG. 27, the dead space length is about 45 nm, so the layer thickness of the multiplication layer needs to be thinned to about 1.5 times the dead space (about 70 nm). However, thinning the multiplication layer to 70 nm increases the electric field in the multiplication layer, and the tunnel current increases sharply, resulting in increased noise.

[0204] On the other hand, in the InAs / AlAs digital alloy multiplication layer of the APD according to the first and second embodiments, as shown in the graph of FIG. 27, the reciprocal of the applied electric field is 1.47×10 -6 In the case of a dc current density of 1.0 V, the ionization rate ratio k can be made close to zero even if the thickness of the multiplication layer is about 1.5 times (about 130 nm) the thickness of the dead space, because the dead space length is about 85 nm, and therefore the effect of the tunnel current is small in the APDs according to the first and second embodiments.

[0205] In addition, in the InAs / AlAs digital alloy structure multiplication layer, the dead space is highly dependent on the applied electric field. For example, when the reciprocal of the applied electric field is 1.27×10 -6In the case of cm / V, the dead space length is about 50 nm, so the multiplication layer needs to be thinned to 75 nm, as shown in the graph in Figure 27. In other words, the thickness of the InAs / AlAs digital alloy structure multiplication layer can be made thicker than that of the InAlAs random alloy structure multiplication layer.

[0206] Fig. 29 is a diagram showing the layer thickness dependence of the ionization rate ratio and the tunnel current on the multiplication layer. The inventors fabricated APDs having an InAs / AlAs digital alloy structure multiplication layer and an InAlAs random alloy structure multiplication layer, respectively, measured the ionization rate ratio k, and further plotted the results in Fig. 29 together with the measurement results of References 1 and 2 described in Fig. 29. References 1 and 2 in Fig. 29 are as follows:

[0207] (1)Reference 1 Yuan Yuan,et al. “Temperature dependence of the ionization coefficients of InAlAs and AlGaAs digital alloys”pp.794,Vol.6,No.8 / August 2018 / Photonics Research (2) Literature 2 Wenyang Wang,et al. “Characteristics of thin InAlAs digital alloy avalanche photodiodes” pp.3841,Vol.46,No.16 / 15 August 2021 / Optics Letters

[0208] As shown in FIG. 29, in the InAlAs random alloy multiplication layer, the effect of reducing the ionization rate ratio k by the dead space cannot be realized unless the thickness of the multiplication layer is 80 nm or less. On the other hand, if the thickness of the multiplication layer is made thinner than 80 nm, the tunnel current increases rapidly and a tunnel breakdown occurs. When the thickness of the multiplication layer is about 60 nm, the reduction of the ionization rate ratio k and the limitation of the tunnel current are barely compatible, but the margin of the layer thickness is only a few nm, making it extremely difficult to stably manufacture APDs. In addition, the ionization rate ratio k is large at 0.12. In other words, with the conventional InAlAs random alloy multiplication layer, it is difficult to apply the effect of reducing the ionization rate ratio k by making the layer thinner to APDs.

[0209] On the other hand, in the InAs / AlAs digital alloy structure multiplication layer of the present disclosure, as discovered by the inventors, the dead space is large, and therefore, as the multiplication layer is made thinner, the ionization rate ratio k starts to decrease to 0.1 or less at a layer thickness of 170 nm, as shown in Fig. 29. Here, the ionization rate ratio k is determined from the measured value of the multiplication noise, and is the minimum value of the ionization rate ratio in the range of multiplication factors 1 to 10. For the same ionization rate ratio k, the thickness of the InAs / AlAs digital alloy structure multiplication layer is more than twice as large as that of the InAlAs random alloy structure multiplication layer.

[0210] As shown in the graph of FIG. 29, in an APD with a pn junction diameter of 20 μm, if the lower limit of the thickness of the multiplication layer for which the tunnel current is 1 μA is set to 40 nm, the optimum thickness range for an InAs / AlAs digital alloy structure multiplication layer is 40 nm or more and 170 nm or less, and the thickness within this range can be fabricated with sufficient reproducibility.

[0211] The thickness of the InAs / AlAs digital alloy multiplication layer, which can sufficiently reduce the ionization rate ratio k due to the dead space effect, is the reciprocal of the applied electric field of 1.47×10 -6In the case of InAs / AlAs digital alloy structure multiplication layer, the thickness is approximately twice the length of the dead space, and considering that the length of the dead space is 85 nm as shown in FIG. 29, 170 nm, which is twice the length of the dead space, is a suitable upper limit for the thickness of the InAs / AlAs digital alloy structure multiplication layer.

[0212] Moreover, in order to control the ionization rate ratio k to 0.05 or less in the InAs / AlAs digital alloy structure multiplication layer, the thickness of the multiplication layer is preferably 150 nm or less, as shown in the graph of Fig. 29. Furthermore, in order to achieve a tunnel current of 1 µA or less and an ionization rate ratio k of approximately zero, the optimum thickness of the multiplication layer is in the range of 60 nm to 130 nm. If the margin during the fabrication of the APD is 10 nm, the thickness of the multiplication layer is preferably set in the range of 70 nm to 120 nm.

[0213] Also, from FIG. 27, the length of the dead space is preferably 50 nm to 90 nm. The ratio of the length of the dead space to the thickness of the multiplication layer is preferably 29% or more, which is the minimum dead space length of 50 nm divided by the maximum multiplication layer thickness of 170 nm. As the ratio increases, the ionization rate ratio k becomes smaller, but it cannot exceed 100%. This is because multiplication does not occur when the ionization rate ratio k exceeds 100%. Therefore, in principle, the ratio of the length of the dead space to the thickness of the multiplication layer is preferably 29% or more and less than 100%. Furthermore, since the thickness of the multiplication layer in this prototype was 120 nm, the optimum range confirmed experimentally is 42% (= 50 nm / 120 nm) to 75% (= 90 nm / 120 nm).

[0214] The inventors have considered the reason why the ionization rate ratio k=0 can be achieved in the InAs / AlAs digital alloy structure multiplication layer of the present disclosure, whereas the ionization rate ratio k=0 could not be achieved in the conventional InAlAs random alloy structure multiplication layer.

[0215] If the electron dead space length is De and the hole dead space length is Dh, the conditions for achieving the ionization rate ratio k = 0 are expressed by the following formulas (7) and (8). Note that formula (7) represents the condition for the difference in the dead space lengths, and formula (8) represents the condition for the tunnel current. Dhe=Dh―De>0 (7) Dh>Tmin (8)

[0216] Here, Dhe is the difference between the dead space lengths of holes and electrons. Tmin is the minimum thickness of the multiplication layer at which the tunnel current becomes small enough that it does not affect noise, and the thicker the multiplication layer, the smaller the tunnel current becomes. As shown in the conceptual diagram of Figure 28C, when the thickness of the multiplication layer becomes equal to or smaller than the dead space length of holes, holes are no longer multiplied and the ionization rate ratio k = 0, so the condition for the difference in the dead space length is set as shown in the above formula (8).

[0217] 27 and 29, in the case of the InAlAs random alloy structure multiplication layer, the ionization rate ratio k starts to decrease as the multiplication layer is thinned, at values ​​De of about 40 nm and Dh of about 80 nm. Since the pn junction diameter is 20 μm and the tunnel current is set to 100 nA or less, the minimum layer thickness Tmin=90 nm, and therefore the InAlAs random alloy structure does not satisfy the tunnel current condition, and it is impossible to achieve the ionization rate ratio k=0.

[0218] On the other hand, in the case of an InAs / AlAs digital alloy structure multiplication layer, the ionization rate ratio k starts to decrease as the multiplication layer is thinned at values ​​De of about 80 nm and Dh of about 170 nm, and when the pn junction diameter is 20 μm and the tunnel current is 100 nA or less, the minimum layer thickness Tmin = 90 nm, so there exists a multiplication layer thickness that satisfies the condition of the ionization rate ratio k = 0. Note that the minimum layer thickness Tmin is the same for the InAs / AlAs digital alloy structure multiplication layer and the InAlAs random alloy structure multiplication layer because the band gaps of the two are the same.

[0219] Specifically, in the case of a random alloy structure, De is about 40 nm and Dh is about 80 nm, whereas in the case of a digital alloy structure, the inventors found that De is about 80 nm and Dh is about 170 nm.

[0220] In the InAs / AlAs digital alloy structure multiplication layer, the difference in lattice constant between the InAs (lattice constant = 0.606 nm) and AlAs (lattice constant = 0.566 nm) that make up the superlattice is very large at 6.55%, which means that the dopants in the electric field relaxation layer, that is, the impurities, may diffuse into the InAs / AlAs digital alloy structure multiplication layer during the fabrication process, causing disorder within the multiplication layer.

[0221] 30A to 30D are conceptual diagrams showing the ionization rate in the multiplication layer and the electric field relaxation layer, where Fig. 30A shows the ionization rate in the case of the InAlAs random alloy structure multiplication layer, Fig. 30B shows the ionization rate in the case of the InAs / AlAs digital alloy structure multiplication layer, Fig. 30C shows the ionization rate in the case of the partially disordered InAs / AlAs digital alloy structure multiplication layer, and Fig. 30D shows the ionization rate in the case of a combination of a thick electric field relaxation layer and an InAs / AlAs digital alloy structure multiplication layer. Compared with the InAlAs random alloy structure multiplication layer shown in Fig. 30A, the dead space length of the InAs / AlAs digital alloy structure multiplication layer shown in Fig. 30B is long, but the dead space length of the partially disordered InAs / AlAs digital alloy structure multiplication layer due to dopant diffusion from the electric field relaxation layer is short as shown in Fig. 30C.

[0222] In order to avoid the influence of disorder in the InAs / AlAs digital alloy structure multiplication layer, the selection of the material and dopant of the electric field relaxation layer and the doping concentration are important. The impurity diffusion equation is expressed by the following equation (9). dN / dt=D(d 2 N / d 2 x)-F (9)

[0223] In formula (9), N is the impurity concentration, t is time, D is the diffusion constant, x is the position, and F is the external force acting on the diffusion. Examples of materials for the electric field buffer layer include InP, an InAlAs random alloy structure, and an InAs / AlAs digital alloy structure. Examples of p-type dopants for the electric field buffer layer include Be and Zn. Considering the p-type dopant, a combination of a Be-doped p-type InP electric field buffer layer and an InAs / AlAs digital alloy structure multiplication layer is preferable. This is because Be has a small diffusion constant D and also forms a potential barrier between the InAs / AlAs digital alloy structure multiplication layer. The potential barrier corresponds to F in formula (9).

[0224] In order to prevent the variation in the amount of electric field relaxation, that is, the product of the layer thickness and the carrier concentration, from increasing when the thickness of the electric field relaxation layer varies, the carrier concentration of the electric field relaxation layer is set to 2×10 18 cm -3 The following is preferable. When InAlAs is used as the material for the electric field relaxation layer, Zn doping is optimal, and the carrier concentration is 2×10 18 cm -3 The following is optimal. Note that 2×10 18 cm -3 When the impurity concentration is higher than 5×10, the inactive impurities increase and diffusion becomes more likely. 18 cm -3 The following is required:

[0225] The electric field relaxation amount ΔE is expressed by the following formula (10). ΔE=W q N / ε (10) In equation (10), W is the thickness of the electric field relaxation layer, q is the elementary charge, N is the carrier concentration of the electric field relaxation layer, and ε is the dielectric constant. If the electric field relaxation amount ΔE is constant, when the carrier concentration of the electric field relaxation layer is increased, the thickness of the electric field relaxation layer must be reduced in inverse proportion to the carrier concentration.

[0226] The carrier concentration N of the electric field relaxation layer is 5×10 18 cm-3 In order to prevent the dead space length from being shortened due to impurity diffusion into the multiplication layer, the carrier concentration of the field relaxation layer is set to 5×10 18 cm -3 The electric field relaxation layer must have a thickness of 10 nm or more.

[0227] On the other hand, as shown in Fig. 30D, when the electric field buffer layer becomes thicker than 1.5 times the dead space length of the electric field buffer layer, multiplication occurs in the electric field buffer layer. As shown in Fig. 27, in the random alloy structure, the dead space length is 45 nm or less, so the layer thickness of the electric field buffer layer made of the random alloy structure needs to be 70 nm or less. On the other hand, in the InAs / AlAs digital alloy structure, the dead space length is 85 nm or less, so the layer thickness of the electric field buffer layer made of the digital alloy structure needs to be 130 nm or less.

[0228] The lengths of the dead spaces shown in FIGS. 30A to 30D have the following relationship: dead space (FIG. 30A)<dead space (FIG. 30D)<dead space (FIG. 30C)<dead space (FIG. 30B).

[0229] <Effect of the digital alloy structure multiplication layer of the semiconductor photodiode (APD) according to the first and second embodiments> First, a first effect of the semiconductor light receiving element according to the first and second embodiments will be quantitatively described below. The 3 dB bandwidth fc of a conventional APD is expressed by the following formula (11), where the bandwidth limitation due to the RC time constant is frc, the bandwidth limited by the carrier transit time is ftr, and the bandwidth limitation due to the multiplication time is fm. fc_APD=1 / ((1 / frc) 2 +(1 / ftr) 2 +(1 / fm) 2 ) 0.5 (11)

[0230] On the other hand, the 3 dB bandwidth fc of the APD having the InAs / AlAs digital alloy structure multiplication layer according to the first and second embodiments has an ionization rate ratio k close to zero, and is therefore limited only by the RC time constant and the carrier transit time according to equations (4), (5), and (6), and can be expressed by the following equation (12). fc_APD=1 / ((1 / frc) 2 +(1 / ftr) 2 ) 0.5 (12) In formula (12), the transit time ftr of a carrier includes the transit time through the light absorption layer plus the transit time through the multiplication layer.

[0231] Since the RC time constant is inversely proportional to the sum of the thicknesses of the light absorption layer and the multiplication layer, while the transit time is directly proportional, equation (11) has a maximum value. In other words, the maximum bandwidth occurs when frc=ftr. Substituting frc=ftr, equation (12) can be expressed as the following equation (13). fc=ftr / √2 (13)

[0232] Moreover, the 3 dB bandwidth ftr determined by the running time is expressed by the following equation (14). ftr=3.5Vav / (2πWt) (14)

[0233] In equation (14), Vav is the average saturation velocity of electrons and holes, and Wt is the total thickness of the light absorption layer and the multiplication layer. For example, in the case of InGaAs, Vav is 5.35×10 6 In addition, if the thickness of the multiplication layer is 100 nm and the thickness of the light absorption layer is 400 nm, then Wt = 500 nm.

[0234] Vav=5.35×10 6Substituting cm / s and Wt=500 nm into formula (14) gives ftr=59.6 GHz. Substituting the calculated ftr into formula (13), the 3 dB bandwidth of the APD having the InAs / AlAs digital alloy structure multiplication layer according to the first and second embodiments is 42.2 GHz. Therefore, it is clear from the above consideration that the APD having the InAs / AlAs digital alloy structure multiplication layer according to the first and second embodiments can meet the bandwidth of 37.5 GHz required for a 50G-PON system. In the following description of the device, system, etc., the APD having the InAs / AlAs digital alloy structure multiplication layer according to the first and second embodiments of the present disclosure is referred to as the DA-APD of the present disclosure.

[0235] As described above, the semiconductor photodetector according to the first and second embodiments further includes a digital alloy structure multiplication layer whose layer thickness is controlled within a preset range, and thus has the effect of providing a semiconductor photodetector that operates in a wider response band and has high receiving sensitivity.

[0236] Embodiment 3 In the first and second embodiments, an APD having an InAs / AlAs digital alloy structure multiplication layer has been described as an example of a semiconductor light receiving element. However, the semiconductor light receiving elements according to the first and second embodiments exhibit excellent element characteristics as a semiconductor light receiving element even when the multiplication layer has an InAlAs random alloy structure.

[0237] 31 is a cross-sectional view showing an element structure of a back-illuminated APD, which is an example of a semiconductor photodetector 220 according to embodiment 3. The semiconductor photodetector 220 according to embodiment 3 differs from the semiconductor photodetector 100 according to embodiment 1 only in that the i-type InAs / AlAs digital alloy structure multiplication layer 3 is an i-type InAlAs random alloy structure multiplication layer 3a.

[0238] In the semiconductor photodetector 220 according to the third embodiment, although the multiplication layer is constructed with an InAlAs random alloy structure, a two-dimensional periodic structure 70 is provided in the p-type InAlAs second window layer 8, and light is incident from the semiconductor substrate side and reflected inward by covering the semiconductor layer with the SiN surface protective insulating film 10 and the p-type electrode 32. As a result, the amount of light confined in the i-type InGaAs light absorption layer 5 is increased, and as a result, high receiving sensitivity can be achieved over a wide wavelength range due to complex resonance, and a semiconductor photodetector that enables low noise and wide bandwidth can be obtained.

[0239] Variation 1 of embodiment 3 32 is a cross-sectional view showing an element structure of a back-illuminated APD which is an example of a semiconductor photodetector 230 according to Modification 1 of Embodiment 3. The semiconductor photodetector 230 according to Modification 1 of Embodiment 3 differs from the semiconductor photodetector 120 according to Modification 2 of Embodiment 1 only in that the i-type InAs / AlAs digital alloy structure multiplication layer 3 is an i-type InAlAs random alloy structure multiplication layer 3a.

[0240] In the semiconductor light receiving element 230 according to the first modification of the third embodiment, the multiplication layer is formed of an InAlAs random alloy structure, but the SiN surface protective insulating film 10a on the i-type InP window layer 7c is provided with a lattice-shaped two-dimensional periodic structure 70a, and light is incident from the semiconductor substrate side, and the light is reflected inward by covering the semiconductor layer with the SiN surface protective insulating film 10a and the p-type electrode 32. This allows the semiconductor light receiving element to have high reception sensitivity in a wide wavelength range by complex resonance, and further allows low noise and wide bandwidth. In addition, the depth (height when the hole is the remaining part) of the hole 11a provided in the SiN surface protective insulating film 10a is more uniform than that of the semiconductor light receiving element 220 according to the third embodiment, and the refractive index difference can be made larger, so that light resonance occurs more easily, and the light absorption efficiency is improved. As a result, a semiconductor light receiving element that has high reception sensitivity, low noise, and wide bandwidth can be realized.

[0241] Variation 2 of embodiment 3 33 is a cross-sectional view showing an element structure of a back-illuminated APD which is an example of a semiconductor photodetector 240 according to Modification 2 of Embodiment 3. The semiconductor photodetector 240 according to Modification 2 of Embodiment 3 differs from the semiconductor photodetector 130 according to Modification 3 of Embodiment 1 only in that the i-type InAs / AlAs digital alloy structure multiplication layer 3 is an i-type InAlAs random alloy structure multiplication layer 3a.

[0242] In the semiconductor light receiving element 240 according to the second modification of the third embodiment, the multiplication layer is formed of an InAlAs random alloy structure, but the p-type electrode 32a on the SiN surface protective insulating film 10 is provided with a lattice-like two-dimensional periodic structure 70b, and light is incident from the semiconductor substrate side, so that the semiconductor light receiving element has a high reception sensitivity in a wide wavelength range due to complex resonance. In addition, the depth (height when the hole is the remaining part) of the hole 11b provided in the p-type electrode 32a is more uniform than that of the semiconductor light receiving element 220 according to the third embodiment, and the refractive index difference can be made larger, so that light resonance occurs more easily, and the light absorption efficiency is improved. As a result, a semiconductor light receiving element that has high reception sensitivity, low noise, and can be made wider bandwidth can be realized.

[0243] Variation 3 of embodiment 3 34 is a cross-sectional view showing an element structure of a back-illuminated APD which is an example of a semiconductor photodetector 245 according to Modification 3 of Embodiment 3. The semiconductor photodetector 245 according to Modification 3 of Embodiment 3 differs from the semiconductor photodetector 140 according to Modification 4 of Embodiment 1 only in that the i-type InAs / AlAs digital alloy structure multiplication layer 3 is an i-type InAlAs random alloy structure multiplication layer 3a.

[0244] In the semiconductor light receiving element 245 according to the third modification of the third embodiment, the multiplication layer is formed of an InAlAs random alloy structure, but the p-type InGaAs contact layer 9a on the i-type InP window layer 7c is provided with a lattice-shaped two-dimensional periodic structure 70c, and light is incident from the semiconductor substrate side, so that the semiconductor light receiving element can have high reception sensitivity in a wide wavelength range by complex resonance, and can further reduce noise and widen the bandwidth. In addition, the depth (height when the hole is the remaining part) of the hole 11c provided in the p-type InGaAs contact layer 9a is more uniform than that of the semiconductor light receiving element 220 according to the third embodiment, so that resonance occurs more easily, and the light absorption efficiency is improved. As a result, a semiconductor light receiving element that has high reception sensitivity, low noise, and can be made wideband can be realized.

[0245] Variation 4 of embodiment 3 35 is a cross-sectional view showing an element structure of a back-illuminated APD which is an example of a semiconductor photodetector 247 according to Modification 4 of Embodiment 3. The semiconductor photodetector 247 according to Modification 4 of Embodiment 3 differs from the semiconductor photodetector 180 according to Embodiment 2 only in that the i-type InAs / AlAs digital alloy structure multiplication layer 3 is an i-type InAlAs random alloy structure multiplication layer 3a.

[0246] In the semiconductor photodetector 247 according to the fourth modification of the third embodiment, the multiplication layer is constructed with an InAlAs random alloy structure, but an n-type DBR layer 14 is provided below the multiplication layer, a two-dimensional periodic structure 70 is provided in the p-type InAlAs second window layer 8, and the two-dimensional periodic structure 70 is covered with the SiN surface protective insulating film 10 and the p-type electrode 32 to form a vertical cavity. Therefore, by making light incident from the semiconductor substrate side, a semiconductor photodetector with high reception sensitivity can be obtained.

[0247] Embodiment 4 36 is a configuration diagram showing an optical line terminal (OLT) 260 of a 50G-PON system according to the fourth embodiment. The optical line terminal 260 includes a forward error correction (FEC) 261, a driver amplifier 262, a light source 263, a wavelength division multiplexing (WDM) 264, a clock data recovery (CDR) 265 which is a clock and data recovery circuit, a limiting amplifier 266, a burst TIA 267, and a DA-APD 268 of the present disclosure.

[0248] Note that the DA-APD of the present disclosure refers to an APD having a lattice-like two-dimensional periodic structure on the upper side of a light absorption layer and an InAs / AlAs digital alloy structure as a multiplication layer, as described in the above-mentioned first embodiment, first to seventh variations of the first embodiment, second embodiment, and second and third variations of the second embodiment.

[0249] 37 is a configuration diagram showing an optical line terminal (ONU) of a 50G-PON system according to embodiment 4. The optical line terminal 270 includes a WDM 271, a light source 272, a driver amplifier 273, an FEC 274, a DA-APD 275 of the present disclosure, a TIA 276, a limiting amplifier 277, and a CDR 278.

[0250] 38 is a configuration diagram showing an optical line terminal (OLT) 250a of a 50G-PON system as a comparative example. The optical line terminal 250a as the comparative example includes a forward error correction (FEC) circuit FEC251, a driver amplifier 252, a light source 253, a wavelength division multiplexing (WDM) circuit WDM254, a digital signal processing circuit DSP255, an analog-to-digital converter (ADC) circuit ADC256, a burst TIA257, and a conventional APD258.

[0251] As shown in the optical line terminal 250a of the 50G-PON system in the comparative example shown in Fig. 38, the 50G-PON system in the comparative example required digital bandwidth compensation, that is, the DSP 255. On the other hand, in the 50G-PON system using the DA-APD of the present disclosure, digital bandwidth compensation is not required. That is, as shown in the optical line terminal (ONU) of the 50G-PON system according to the fourth embodiment shown in Fig. 37, if the DA-APD 275 of the present disclosure is used, a wide response band and high receiving sensitivity are possible, so that it is possible to simplify the DSP circuit, save power, and reduce the output power of the SOA.

[0252] To increase the number of branches in a PON system and eliminate the need for an SOA, it is necessary to improve the signal-to-noise ratio of the receiver and increase the receiving sensitivity. For example, if an optical demultiplexer is added to increase the number of branches from the current level, the amount of light will be halved, so the signal-to-noise ratio must be improved by at least 3 dB. The signal-to-noise ratio of a receiver using an APD is expressed by the following equation (15). SN ratio=Iph 2 M 2 / (2q(Iph+Id)M 2 ·F·B +4Kb T Ft B / Rt) (15)

[0253] In equation (15), Iph is the photocurrent of the APD, M is the gain, q is the unit charge, Id is the dark current to be multiplied, F is the excess noise factor of the APD, B is the bandwidth, Kb is the Boltzmann constant, T is the absolute temperature, Ft is the noise figure of the amplifier, and Rt is the input resistance. The term on the left of the denominator represents the shot noise of the APD, and the term on the right of the denominator represents the thermal noise of the amplifier.

[0254] To simplify equation (15), we assume that Id is sufficiently smaller than Iph, and that the APD shot noise term and the amplifier thermal noise term are equal when the gain is at the maximum SNR. If we then replace the amplifier thermal noise term with the APD shot noise term, the SNR can be expressed by the following equation (16). SN ratio=Iph / (4q F B) (16)

[0255] Moreover, the excess noise factor F is given by the following equation (17). F = M(1-(1-k)·((M-1) 2 / M 2 )) (17)

[0256] In the case of the conventional InAlAs random alloy structure multiplication layer, when the layer is thinned to the extent that the dead space effect appears (about 70 nm), the tunnel current increases and the noise characteristics deteriorate as described above. In order to minimize the tunnel current, it is necessary to reduce the pn junction diameter to several μm or less, but in this case, the alignment accuracy when focusing the light on the APD becomes strict. Although it is possible to reduce noise to a certain extent with the random alloy structure multiplication layer by thinning the multiplication layer in this way, although the noise characteristics are inferior to those of the digital alloy structure multiplication layer, there are restrictions from the viewpoint of productivity, etc., because the pn junction diameter is small. For this reason, the system is designed with the ionization rate ratio k for the non-thinned InAlAs multiplication layer set to 0.2. When the ionization rate ratio k=0.2 and the multiplication factor is 12 times, the excess noise coefficient F=3.9.

[0257] On the other hand, in the 50G-PON system according to the fourth embodiment, an APD with an InAs / AlAs digital alloy structure as a multiplication layer, that is, the DA-APD of the present disclosure, is applied as a semiconductor light receiving element. In the case of the InAs / AlAs digital alloy structure multiplication layer of the present disclosure, the dead space effect works even with a layer thickness of 100 nm or more, so it can be applied to an APD. In this case, the ionization rate ratio k=0, and when the multiplication factor is 12 times, the excess noise factor F=1.9. Therefore, the excess noise is about half that of the conventional APD. As a result, the DA-APD of the present disclosure is applied, and the signal-to-noise ratio is improved by 3 dB.

[0258] Generally, in a 50G-PON system, inserting one stage of a 2-way demultiplexer to increase the number of branches increases the loss by 3 dB. Therefore, by applying the DA-APD disclosed herein as a semiconductor photodetector, it becomes possible to insert one more stage of demultiplexer into a 50G-PON system.

[0259] 39 is a diagram illustrating a configuration of an optical line terminal (OLT) of a 50G-PON system according to embodiment 4. The optical line terminal 260a of the 50G-PON system includes an FEC 261, a driver amplifier 262, a light source 263, a WDM 264, a DSP 265a, an ADC 266a, a burst TIA 267, and a DA-APD 268 of the present disclosure.

[0260] 40 is a diagram illustrating a configuration of an optical line terminal (ONU) of a 50G-PON system according to the fourth embodiment. The optical line terminal 260b of the 50G-PON system includes an FEC 261, a driver amplifier 262, a light source 263, a WDM 264, a DSP 265a, an ADC 266a, a TIA 267a, and a DA-APD 268 of the present disclosure.

[0261] The effect of the semiconductor light receiving element according to the present disclosure will be further explained. In the APD having the InAs / AlAs digital alloy structure multiplication layer according to the present disclosure, the multiplication time in formula (6) becomes almost zero by controlling the layer thickness of the multiplication layer within a preset range and setting the ionization rate ratio k to zero. As a result, the response band of the APD does not deteriorate even if the multiplication factor is increased. In other words, in the DA-APD according to the present disclosure, the response band is limited only by the RC time constant and the carrier travel time, as in the conventional PD. Therefore, it is possible to widen the response band required for the 50G-PON system, and reception is possible without digital band compensation by DSP.

[0262] In addition, when the ionization rate ratio k approaches zero, excess noise that deteriorates receiver sensitivity is suppressed, making it unnecessary to amplify the optical signal using an SOA. Furthermore, even in PON systems other than 50G-PON systems, it becomes possible to achieve more branching than before. As a result, PON systems can be made low cost and power-saving.

[0263] <Advantages of the Fourth Embodiment> As described above, according to the optical line terminal of embodiment 4, the DA-APD disclosed herein is used as the semiconductor photodetector, thereby achieving the effect of obtaining an optical line terminal that can increase the transmission distance of optical signals and reduce power consumption.

[0264] Embodiment 5. Fig. 41 is a diagram illustrating a configuration of a multi-level intensity modulation transmitting / receiving apparatus 300 according to embodiment 5. Fig. 42A and Fig. 42B are conceptual diagrams illustrating received waveforms of the multi-level intensity modulation transmitting / receiving apparatus 300 according to embodiment 5.

[0265] The multilevel intensity modulation transmitting / receiving device 300 is a multilevel intensity modulation transmitting / receiving device using a PAM (Pulse Amplitude Modulation) method, which is a multilevel intensity modulation method. In the transmitting section, a digital signal generated in a DSP 301 is converted to an analog signal in a DAC 302a, amplified in a driver amplifier 303, and driven by a light source 304 consisting of a DFB laser or EML to emit an optical signal to an optical fiber cable 310.

[0266] Meanwhile, in the receiving section, the light passes through the optical fiber cable 310 and the optical system and enters the DA-APD 305, which is the semiconductor light receiving element of the present disclosure, where the optical signal is converted into a current and multiplied. The optical signal is then amplified in the Linear-TIA 306, converted into a digital signal in the ADC 302b, and signal processing is performed by the DSP 301.

[0267] <Functions and Effects of the Multilevel Intensity Modulation Transmitter / Receiver According to the Fifth Embodiment> In the PAM-based multilevel intensity modulation transmitting / receiving device 300, it is necessary to receive not only binary signals of 1 and 0 such as NRZ (None Return to Zero) and RZ (Return to Zero), but also, for example, four values ​​of different optical signal intensity in PAM4 (Pulse Amplitude Modulation-4). An example of a PAM4 received waveform is shown in the conceptual diagram of FIG. 42A. An index called TDECQ (Transmitter Dispersion and Eye Closure Quaternary) is used to judge the quality of the received waveform in PAM4. TDECQ is calculated by the following equation (18). TDECQ(dB)=10·log(OMA / (6·Qt·R)) (18)

[0268] In equation (18), the optical modulation amplitude (OMA) is the total amplitude from level 0 to level 3, Qt is a value that depends on the SER (Symbol Error Rate) defined by the IEEE (Institute of Electrical and Electronics Engineers), and R is the additional noise value required to achieve the SER value. TDECQ (dB) is defined as, for example, 3 dB or less. To reduce TDECQ (dB), (1) The eye opening at each level must be uniform. (2) Low noise at each level is necessary.

[0269] In order to ensure uniform eye openings for each of the four levels of optical signal strength, the semiconductor photodetector must have excellent linearity. Here, a semiconductor photodetector has good linearity when the photocurrent Iph increases in proportion to the optical input power Pin. In other words, the linearity is good if Iph / Pin is constant even if the optical input power Pin changes.

[0270] In addition, PAM needs to receive signals with low to high intensity, so it needs to have a good dynamic range. In other words, even if the optical input power Pin increases, if the drop in Iph / Pin is small, the dynamic range is good. As shown in the conceptual diagram of the received waveform in Figure 42B, if the linearity and dynamic range deteriorate, the eye opening formed between level 2 and level 3 deteriorates.

[0271] In the case of PDs and APDs, the linearity deteriorates when the photocurrent increases with an increase in optical input, which causes an increase in the number of holes and electrons traveling in the multiplication layer and the light absorption layer, changing the electric field distribution in the multiplication layer and the light absorption layer. This phenomenon is called the space charge effect.

[0272] The inventors have studied a model of degradation of the linearity of an APD. Figures 43A and 43B are conceptual diagrams for explaining the operation of a PD when a high optical input is applied. As shown in the conceptual diagram of Figure 43A, when the optical input increases and the photocurrent increases, a space charge effect acts as if a voltage drop occurs due to the series resistance and no voltage is applied to the pn junction. This voltage drop reduces the multiplication factor. This is because the generated electrons and holes affect the electric field distribution, as shown in the conceptual diagram of Figure 43B. The series resistance Rli that degrades the linearity of an APD is expressed by the following formula (19). Rli = Rsc + Rd + Rlo (19)

[0273] In equation (19), Rsc is the resistance due to the space charge effect, Rd is the element resistance, and Rlo is the load resistance. Rd and the load resistance are usually several tens of Ω, but Rsc can be several hundreds of Ω or more.

[0274] The inventors have found that, when the time it takes for electrons and holes generated by light absorption to pass through the depletion layer is Td, Rsc can be expressed by the following formula (20). Rsc=W Td / (2εS) (20)

[0275] In equation (20), W is the thickness of the depletion layer, ε is the dielectric constant, and S is the pn junction area. The resistance Rsc due to the space charge effect is proportional to the time it takes for electrons and holes to pass through the depletion layer, Td. Therefore, it is possible to reduce the resistance Rsc by increasing the speed at which electrons and holes travel and reducing Td.

[0276] In the DA-APD disclosed herein, the light absorption layer has a high absorption amount due to the resonance of light, so it is possible to make the light absorption layer thinner, and the resistance Rsc can be reduced by making the light absorption layer thinner. As a result, the eye opening becomes uniform, so that the TDECQ satisfies the specified value. Furthermore, it is possible to increase the transmission distance and reduce the driving current of the transmission laser.

[0277] The case where the DA-APD of the present disclosure is used will be described below. First, the operation of the APD at high light input will be described. FIG. 44 is a conceptual diagram for explaining the operation of the APD at high light input. When a large number of electrons and holes are generated in the multiplication layer, the electric field in the multiplication layer of the APD changes, that is, the so-called space charge effect occurs. Due to the occurrence of this space charge effect, the multiplication factor of the APD decreases and the linearity deteriorates. As described above, since the deterioration of the linearity of the APD is caused by the series resistance Rsc, it is necessary to reduce the residence time Tdm of the electrons and holes in the depletion layer. In particular, when the multiplication factor increases, the residence time Tdm in the multiplication layer increases. Tdm is the same as the so-called multiplication time, and is expressed by the following formula (21). Residence time Tdm=multiplication time=2πNkMτav (21)

[0278] In equation (21), N is the Emmons coefficient (which depends gently on the ionization rate ratio k), M is the multiplication factor, and τav is the average time it takes for electrons and holes to travel through the multiplication layer. The one-way transit time for carriers to traverse the multiplication layer is excluded from the residence time Tdm. When the ionization rate ratio k=0.5 (InP), 0.2 (InAlAs), 0.1 (Si), and 0 to 0.001 (InAs / AlAs digital alloy structure), N is 0.55, 0.83, 1.1, and 2.0, respectively.

[0279] Figure 45 shows the residence time Tdm of electrons and holes for each material that composes the multiplication layer. In the InAs / AlAs digital alloy structure multiplication layer, the residence time Tdm in the multiplication layer is dramatically reduced. In other words, since electrons and holes are quickly discharged from the multiplication layer, the space charge effect in the multiplication layer is suppressed, and as a result, the linearity and dynamic range are improved in the InAs / AlAs digital alloy structure multiplication layer.

[0280] As a result, while the conventional APD had a non-uniform PAM4 eye opening as shown in the conceptual diagram of Fig. 42B, the DA-APD of the present disclosure has a uniform eye opening as shown in the conceptual diagram of Fig. 42A, making it possible for TDECQ to satisfy the specified value. Therefore, when the DA-APD of the present disclosure is used, an APD can be used in a PAM transceiver as well, making it possible to increase the transmission distance of an optical signal and reduce the drive current of the transmitting laser.

[0281] <Advantages of the Fifth Embodiment> As described above, according to the multi-level intensity modulation transceiver of embodiment 5, the DA-APD disclosed herein is used as the semiconductor photodetector, thereby achieving the effect of obtaining a multi-level intensity modulation transceiver that can increase the transmission distance of an optical signal and reduce power consumption.

[0282] Embodiment 6 Fig. 46 is a schematic diagram showing the configuration of a radio on fiber system 400 (Radio on fiber: RoF) according to the sixth embodiment. Fig. 47 is a schematic diagram showing the configuration of a radio on fiber system 450, which is a comparative example. The radio on fiber system 400 includes a light source 401, a transmission line 402 such as an optical fiber cable, a DA-APD 403 of the present disclosure, and an antenna 404.

[0283] In a radio-on-fiber system 400 according to the sixth embodiment, an analog electrical amplitude signal is input to a light source 401 such as an LD and converted into an optical amplitude signal. The converted optical amplitude signal is transmitted through an optical fiber cable, i.e., a transmission line 402. The transmitted optical amplitude signal is multiplied and converted into an electrical amplitude signal using a DA-APD 403 of the present disclosure. The converted electrical amplitude signal is transmitted to an antenna 404 and radiated as a radio wave signal.

[0284] The radio-on-fiber system 400 according to the sixth embodiment can efficiently supply a signal to the antenna 404 that is located at a distance from the electric signal source. In addition, since no analog-to-digital or digital-to-analog conversion is performed during transmission, the system has a simple configuration and consumes little power.

[0285] <Functions and Effects of Radio on Fiber System According to Sixth Embodiment> In the comparative example radio-on-fiber system 450 shown in FIG. 47, if a signal is attenuated during transmission through the optical fiber cable, the signal cannot be amplified by the PD 406, and therefore a sufficient radio signal cannot be emitted from the antenna.

[0286] Furthermore, when a conventional APD is used, as shown in the conceptual diagrams of Figures 43A and 43B, an increase in the number of electrons and holes in the multiplication layer changes the electric field distribution, resulting in saturation of the multiplication factor and making it impossible to ensure the dynamic range. This causes problems such as not only being unable to obtain a sufficient amplitude of the electrical signal, but also distorting the analog signal. As a result, it is difficult to apply a conventional APD to the radio-on-fiber system 450 of the comparative example.

[0287] On the other hand, in the DA-APD of the present disclosure used in the radio-on-fiber system 400 according to the sixth embodiment, the amount of light absorbed by the optical absorption layer is large due to optical resonance, so that the optical absorption layer can be made thin, and the resistance Rsc can be reduced by making the optical absorption layer thinner. As a result, a response with good linearity can be obtained over a wide dynamic range, and a large current amplitude can be obtained. In this way, the radio-on-fiber system 400 is configured using the DA-APD of the present disclosure, so that it is possible to output a strong radio wave signal with good linearity even if the optical transmission distance is long.

[0288] Furthermore, in the DA-APD403 utilizing the optical resonance of the present disclosure, as shown in FIG. 45, the residence time Tdm of electrons and holes in the multiplication layer is short, so that the change in the electric field distribution in the multiplication layer is suppressed. As a result, a response with excellent linearity can be obtained over a wide dynamic range. In other words, since the DA-APD403 of the present disclosure is used to multiply a signal, the original signal can be reproduced and a large current amplitude can be obtained.

[0289] The DA-APD 403 of the present disclosure can be used with a multiplication factor in the range of 1.2 to 10. However, since a larger multiplication factor causes signal distortion, it is preferable to use a multiplication factor in the range of 1.2 to 5. In addition, considering the loss of the optical fiber and the quantum efficiency of the APD, which is about 80% rather than 100%, it is optimal to use a multiplication factor of 2 to 3 to compensate for such loss.

[0290] <Advantages of the Sixth Embodiment> As described above, according to the radio-on-fiber system of the sixth embodiment, the DA-APD of the present disclosure is used to configure the radio-on-fiber system, which has the effect of providing a radio-on-fiber system capable of outputting a strong radio signal even if the optical transmission distance is long.

[0291] Embodiment 7 48 is a schematic diagram illustrating a configuration of a digital coherent receiving device 500 according to the seventh embodiment. The digital coherent receiving device 500 according to the seventh embodiment is characterized in that it uses a DA-APD 505a according to the present disclosure.

[0292] In digital coherent communication, optical signals in which both phase and intensity are modulated are polarization-multiplexed and transmitted through optical fibers. In a digital coherent receiving device 500, an optical signal input from an optical fiber cable 501 is first polarization-separated by a polarization separator 502. After polarization separation, each polarized signal light is input to a 90-degree hybrid device 503a and a 90-degree hybrid device 503b, respectively. Meanwhile, a laser light locally emitted from a semiconductor laser 504 is separated into two signals with a phase shift of 90 degrees from each other.

[0293] The signal light and the laser light are multiplexed, and the signal light is further separated into orthogonal components (I, Q) and output. The four optical signals, i.e., four optical signals in total, each consisting of orthogonal I and Q components for each polarization, are respectively incident on four balanced detectors 505 arranged in 90-degree hybrid devices 503a, 503b, each consisting of two DA-APDs 505a of the present disclosure connected in series as a pair. The electrical signal output from the balanced detector 505 is input to the DSP 506. The digital coherent receiving device 500 according to the seventh embodiment has the above-mentioned configuration.

[0294] <Function of the digital coherent receiving device according to the seventh embodiment> FIG. 49A is a conceptual diagram showing waveforms of a digital coherent receiving device which is a comparative example, and FIG. 49B is a conceptual diagram showing waveforms of a digital coherent receiving device according to the seventh embodiment.

[0295] In the conventional balanced detector, a PD was used as a semiconductor light receiving element that receives signal light. On the other hand, when the DA-APD505a of the present disclosure is used, it is possible to multiply the signal, so that it is possible to suppress the local light to a small value. In addition, when the conventional APD is used, as shown in the conceptual diagram of FIG. 44, when the number of electrons and holes in the multiplication layer increases, the electric field distribution changes, so that the multiplication factor is saturated and the dynamic range cannot be secured. For this reason, not only is it not possible to obtain a sufficient amplitude of the electric signal, but there is also a problem that the analog signal is distorted. As a result, as shown in the conceptual diagram of FIG. 49A, in the comparative example, the interval between the waveform A1 and the waveform B1 becomes narrow, and the intensity signal of the constellation waveform is distorted, so that it is difficult to apply the APD.

[0296] On the other hand, in the DA-APD that utilizes the optical resonance of the present disclosure and is used in the digital coherent receiving device 500 according to the seventh embodiment, the light absorption layer has a large absorption amount, so that it is possible to make the light absorption layer thinner. By making the light absorption layer thinner, it becomes less susceptible to the influence of the resistance Rsc due to the space charge effect, so that a constellation waveform with excellent linearity over a wide dynamic range can be obtained.

[0297] Furthermore, in the DA-APD 505a utilizing the optical resonance of the present disclosure, as shown in Fig. 45, the residence time Tdm of electrons and holes in the multiplication layer is short, so that the change in the electric field distribution in the multiplication layer is suppressed. As a result, as shown in the conceptual diagram of Fig. 49B, when the DA-APD 505a of the present disclosure is used, the interval between waveform A and waveform B becomes wider, and a constellation waveform with excellent linearity over a wide dynamic range is obtained. In other words, even if the signal is multiplied by the APD, the original signal can be reproduced, and a large current amplitude can be obtained.

[0298] The gain of the DA-APD 505a of the present disclosure can be within a range of 1.2 to 10. However, since a larger gain causes signal distortion, it is preferable to use the gain within a range of 1.2 to 5.

[0299] <Advantages of the Seventh Embodiment> As described above, according to the digital coherent receiving device of the seventh embodiment, the DA-APD of the present disclosure is applied as a semiconductor photodetector for receiving an optical signal, thereby making it possible to reduce the drive current of the local light (laser), that is, to reduce the power consumption of the digital coherent receiving device.

[0300] Embodiment 8 50 is a schematic diagram illustrating a configuration of a SPAD sensor system according to embodiment 8. A SPAD sensor system 600 includes an optoelectronic measurement circuit 601, a SPAD sensor 602 including a DA-APD according to the present disclosure, and a quenching circuit 603.

[0301] SPADs can be used not only to count the number of photons but also as highly sensitive light receiving elements. However, they require constant cycling from A: Quenching voltage, which will be described later, to B: Geiger mode voltage, which will be described later. The cycling period is on the order of nanoseconds to microseconds. If the cycling period between A: Quenching voltage and B: Geiger mode voltage can be shortened, it is possible to increase the response speed of the SPAD.

[0302] In other words, when the DA-APD of the present disclosure is used in a SPAD, switching between A: Quenching voltage and B: Geiger mode voltage with high response speed becomes possible, and the response band of the SPAD sensor 602 can be improved.

[0303] Furthermore, when the DA-APD 505a utilizing the resonance of the present disclosure is used in the SPAD sensor system 600, photons incident on the SPAD sensor system 600 are absorbed in the light absorption layer of the SPAD sensor 602 consisting of the DA-APD utilizing the resonance of the present disclosure, generating pairs of electrons and holes, and the electrons flow into the multiplication layer. An electric field approximately 10% higher than the avalanche breakdown electric field is applied to the multiplication layer.

[0304] This state is called the Geiger mode. In the Geiger mode, the electrons are 106 The electrons generated flow as a current and are passed through the photoelectron measurement circuit 601. If the current generated by one photon is known in advance, it is possible to count the number of photons incident on the SPAD sensor system 600.

[0305] FIG. 51A is a conceptual diagram showing the multiplication characteristics of a SAPD sensor system as a comparative example, and FIG. 51B is a conceptual diagram showing the multiplication characteristics of a SAPD sensor system according to the eighth embodiment. If an electric field equal to or greater than the avalanche breakdown electric field is continuously applied to the multiplication layer, an excess current will flow, so the voltage applied to the SPAD sensor 602 after detecting a photon is quickly reduced to weaken the electric field of the multiplication layer. This is called quenching. That is, as shown in the comparison of the multiplication characteristics of the SPAD sensor in the conceptual diagrams of FIG. 51A and FIG. 51B, the voltage is reduced from B: Geiger mode voltage to A: quenching voltage to stop the chain multiplication, and then the voltage is increased again from A: quenching voltage to B: Geiger mode voltage to make it possible to receive the incident photons with high sensitivity.

[0306] The quenching circuit 603 that controls the voltage can be a passive circuit or an active circuit. In a passive circuit, when a current flows due to a photon incident on the SPAD sensor 602, a voltage drop occurs in a resistor connected in series to the SPAD sensor 602, and the voltage applied to the SPAD sensor 602 decreases. In other words, the quenching circuit 603 operates to repeatedly apply to the SPAD sensor 602 a voltage equal to or greater than the breakdown voltage and a voltage less than the breakdown voltage.

[0307] <Actions and Effects of the SPAD Sensor System According to the Eighth Embodiment> The SPAD sensor system 600 according to the eighth embodiment can be used not only for counting the number of photons but also as a semiconductor light receiving element with high reception sensitivity. However, it is necessary to constantly repeat the process from B: Geiger mode voltage to A: quenching voltage. The repetition period is on the order of nanoseconds to microseconds. If the difference between A: quenching voltage and B: Geiger mode voltage can be reduced, the repetition period can be shortened, and the response speed of the SPAD sensor system 600 can be increased.

[0308] In the passive quenching circuit 603, it is possible to reduce the resistance value connected in series to the SPAD sensor 602, thereby increasing the response speed of the SPAD sensor 602. In addition, in the active quenching circuit 603, the voltage amplitude is reduced, which allows for simplification of the drive circuit and power saving, and further allows for a wider response band.

[0309] In the DA-APD utilizing the resonance of the present disclosure, the light absorbing layer has a large absorption amount, so it is possible to make the light absorbing layer thinner. By making the light absorbing layer thinner, the resistance Rsc can be reduced, and the breakdown voltage can also be reduced. When the DA-APD of the present disclosure is used in a SPAD, the difference between the quenching voltage and the Geiger mode voltage, that is, the applied voltage difference, can be reduced, so that the response band can be improved and the quenching circuit can be simplified and power saving can be achieved.

[0310] The effect of using the InAs / AlAs digital alloy structure of the present disclosure as a multiplication layer will be described below. In the InAs / AlAs digital alloy structure multiplication layer of the present disclosure, as shown in the conceptual diagrams of FIG. 28A and FIG. 28B, the dead space length is long, so multiplication does not occur at a low electric field. However, as the electric field is increased, the dead space length becomes shorter, so the multiplication factor increases rapidly and leads to breakdown. In an APD with an InAlAs random alloy structure multiplication layer and an APD with an InAs / AlAs digital alloy structure multiplication layer having a thick multiplication layer, if the voltage at which the dark current exceeds 10 μA is set as the breakdown voltage, the multiplication factor at 90% of the breakdown voltage exceeds 10 times. On the other hand, in the InAs / AlAs digital alloy structure multiplication layer of the present disclosure, the multiplication factor at a voltage at 90% of the breakdown voltage is 10 times or less.

[0311] The voltage required for breakdown depends on the device structure, such as the thickness of the light absorption layer and the carrier concentration of the electric field relaxation layer, so here we verify the effect using the quantifiable electric field of the multiplication layer. Note that above the reach-through voltage (up to 12 V), the voltage applied to the SPAD sensor 602 is proportional to the electric field of the multiplication layer.

[0312] 52 is a diagram showing the calculation of the difference between the quenching electric field and the Geiger mode electric field for each material constituting the multiplication layer. It can be seen that in the InAs / AlAs digital alloy structure multiplication layer of the present disclosure, the difference between the quenching electric field and the Geiger mode electric field of each multiplication layer is 170 kV / cm, which is peculiarly low. The InAs / AlAs digital alloy structure multiplication layer (thinned DA) of the present disclosure has an electric field 120 kV / cm lower than that of the InAs / AlAs digital alloy structure multiplication layer having a superlattice structure similar to the InAs / AlAs digital alloy structure multiplication layer of the present disclosure but a layer thickness of 200 nm or more that is not thinned.

[0313] <Advantages of the Eighth Embodiment> As described above, according to the SPAD sensor system of embodiment 8, the DA-APD of the present disclosure is used in the SPAD sensor, so that the difference between the quenching electric field and the Geiger mode electric field, i.e., the applied voltage difference, can be reduced, resulting in an effect of obtaining a SPAD sensor system that enables an improved response band, a simplified quenching circuit, and reduced power consumption.

[0314] Embodiment 9 Fig. 53 is a diagram illustrating a configuration of a LiDAR (Light Detection And Ranging) device according to embodiment 9. Fig. 54A is a conceptual diagram illustrating a received waveform of an APD of a LiDAR device that is a comparative example, and Fig. 54B is a conceptual diagram illustrating a received waveform of an APD of a LiDAR device 700 according to embodiment 9.

[0315] A LIDAR device 700 according to the ninth embodiment includes a light source 701, a DA-APD 702 of the present disclosure, a TIA 703, and a distance measurement circuit 704. The light source 701 emits pulsed light (hereinafter referred to as pulsed light) or frequency-modulated light.

[0316] In the LIDAR device 700 according to the ninth embodiment, the distance to the object 705 is calculated by measuring the time it takes for the pulsed light emitted from the light source to hit the object 705 and return to the semiconductor light receiving element. An LD or the like is used as the light source 701. To measure long distances, the amount of light from the LD needs to be increased, but an upper limit is set for the amount of light emitted from the LD for eye safety reasons. For this reason, it is necessary to increase the reception sensitivity of the semiconductor light receiving element. Therefore, in the LIDAR device 700 according to the ninth embodiment, the DA-APD 702 disclosed herein is used as the semiconductor light receiving element with a high multiplication factor.

[0317] The detected light pulse is multiplied by the DA-APD 702 of the present disclosure and converted into a current pulse. It is then amplified by the TIA 703 and input to the distance measurement circuit 704, and the time when the intensity of the pulse signal exceeds a preset discrimination line is determined as the arrival time, as shown in the conceptual diagrams of Figures 54A and 54B. The timing of the emission of the light pulse from the light source 701 is input as a signal to the distance measurement circuit 704, and the distance to the object 705 can be calculated by multiplying the time difference between the two by the speed of light and dividing the result by 2. A method is also used in which frequency-modulated light is emitted and the distance is calculated from the frequency difference between the emitted wave and the reflected wave.

[0318] <Functions and Effects of the LIDAR Device According to the Ninth Embodiment> Since the reflectance of the object 705 is not necessarily high and the reflection direction varies, it is necessary to detect weak light with an APD. In a conventional APD, as shown in the conceptual diagram of FIG. 54A, when the voltage is set to achieve high multiplication, the multiplication time becomes long and the current pulse width output from the APD becomes wide. In addition, the tunnel current increases, making it difficult to distinguish the optical pulse. Even in a method of calculating the distance from the frequency difference between the emitted wave and the reflected wave, it becomes difficult to distinguish the frequency.

[0319] On the other hand, the DA-APD utilizing the optical resonance of the present disclosure used in the LIDAR device 700 according to the ninth embodiment can obtain high reception sensitivity even in cases where the reception sensitivity of a conventional APD is insufficient. As a result, not only can it measure the distance to a distant object 705, but it can also reduce power consumption by reducing the optical output of the light source 701, and it is also safer for the eyes.

[0320] Furthermore, in the DA-APD 505a utilizing the optical resonance of the present disclosure, even with a high multiplication factor of 20 or more, the tunnel current does not increase as described in the explanation of FIG. 29, so that it is easy to identify weak light. Also, as shown in FIG. 45, since the residence time in the multiplication layer is short, a current pulse with a large peak intensity is obtained as shown in the conceptual diagram of FIG. 54B, so that the identification sensitivity is high. As a result, not only is it possible to measure the distance to a distant object, but the optical output of the light source can be reduced, so that power consumption is reduced, and further, safety to the eyes is improved.

[0321] <Effects of the 9th embodiment> As described above, according to the LIDAR device of embodiment 9, the reflected light from an object is received by the DA-APD of the present disclosure, which makes it possible to measure the distance to a distant object, reduces the power consumption of the light source, and provides a LIDAR device that is also safer for the eyes.

[0322] Although the present disclosure describes various exemplary embodiments, the various features, aspects, and functions described in one or more embodiments are not limited to application to a particular embodiment, but may be applied to the embodiments alone or in various combinations.

[0323] Therefore, countless modifications not illustrated are assumed within the scope of the technology of the present disclosure, including, for example, modifying, adding, or omitting at least one component, and further, extracting at least one component and combining it with a component of another embodiment. [Explanation of symbols]

[0324] 1 n-type InP substrate, 1a Fe-doped semi-insulating InP substrate, 2, 2a n-type InP buffer layer, 2b p-type InP buffer layer, 3 i-type InAs / AlAs digital alloy structure multiplication layer, 3a i-type InAlAs random alloy structure multiplication layer, 4 p-type InP electric field relaxation layer, 4 p-type InP electric field relaxation layer, 5 i-type InGaAs light absorption layer, 6 i-type InAlGaAs / InAlAs graded layer, 7, 7a i-type InP first window layer, 7b i-type InAlAs first window layer, 7c i-type InP window layer, 7d p-type InP first window layer, 7e n-type InP first window layer, 8, 8b p-type InAlAs second window layer, 8a i-type InP second window layer, 8c n-type InAlAs second window layer, 8s remainder, 9, 9a, 9b p-type InGaAs contact layer, 9c n-type InGaAs contact layer, 10, 10a, 10b SiN surface protective insulating film, 11, 11a, 11b, 11c, 11d hole, 12 Zn-diffused p-type region, 13 boundary, 14, 14a n-type DBR layer, 14b p-type DBR layer, 31, 31a, 31b n-type electrode, 32, 32a, 32b p-type electrode, 40, 41 anti-reflection film, 70, 70a, 70b, 70c, 70d two-dimensional periodic structure, 90 incident light, 91 light resonance, 92 light traveling in the horizontal direction, 100, 101, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 245, 247 Semiconductor photodetector, 250a, 260, 260a, 260b, 270 Optical line terminal, 251, 261, 274 FEC, 252, 262, 273, 303 Driver amplifier, 253, 263, 272, 304, 401, 701 Light source, 254, 264, 271 WDM, 255, 265a, 301, 506 DSP, 256, 266a, 302b ADC, 258 APD, 257, 267 Burst TIA, 265, 278 CDR, 266, 277 Limiting amplifier, 267a, 276, 703 TIA, 268, 275, 305, 403, 505a, 702 DA-APD, 300 Multilevel intensity modulation transmitter / receiver, 302a DAC, 310, 501 Optical fiber cable, 306 Linear-TIA, 400, 450 Radio-over-fiber system, 402 Transmission line, 404 Antenna, 406 PD, 500 Digital coherent receiver, 501 Optical fiber cable, 502Polarization separator, 503a, 503b 90-degree hybrid, 504 Semiconductor laser, 505 Balanced detector, 600 SPAD sensor system, 601 Optoelectronic measurement circuit, 602 SPAD sensor, 603 Quenching circuit, 700 Lidar device, 704 Distance measurement circuit, 705 Object

Claims

1. An InP substrate; an n-type semiconductor layer formed on the InP substrate; a multiplication layer made of a compound semiconductor formed on the n-type semiconductor layer; a p-type electric field buffer layer formed on the multiplication layer; a light absorbing layer formed on the p-type electric field buffer layer; a first window layer formed on the light absorbing layer; a second window layer formed on the first window layer and having a two-dimensional periodic structure in which a plurality of holes or remnants are two-dimensionally arranged at a constant period; a surface protective insulating film formed at least on the two-dimensional periodic structure; a surface electrode formed on the surface protective insulating film; A semiconductor light receiving element comprising:

2. A semiconductor photodetector as described in claim 1, characterized in that the multiplication layer has a digital alloy structure.

3. The semiconductor photodetector according to claim 1, wherein the multiplication layer has a random alloy structure made of InAlAs.

4. An InP substrate; an n-type semiconductor layer formed on the InP substrate; a multiplication layer made of a compound semiconductor formed on the n-type semiconductor layer; a p-type electric field buffer layer formed on the multiplication layer; a light absorbing layer formed on the p-type electric field buffer layer; a window layer formed on the light absorbing layer; a surface protective insulating film formed on the window layer and having a two-dimensional periodic structure in which a plurality of holes or remnants are two-dimensionally arranged at a constant period; At least a surface electrode formed on the two-dimensional periodic structure; A semiconductor light receiving element comprising:

5. A semiconductor photodetector as described in Claim 4, characterized in that the multiplication layer has a digital alloy structure.

6. The semiconductor photodetector according to claim 4, wherein the multiplication layer has a random alloy structure made of InAlAs.

7. an InP substrate; an n-type semiconductor layer formed on the InP substrate; a multiplication layer formed on the n-type semiconductor layer; a p-type electric field buffer layer formed on the multiplication layer; a light absorbing layer formed on the p-type electric field buffer layer; a window layer formed on the light absorbing layer; a surface protective insulating film formed on the window layer; a surface electrode formed on the surface protective insulating film and having a two-dimensional periodic structure in which a plurality of holes or remnants are two-dimensionally arranged at a constant period; A semiconductor light receiving element comprising:

8. An InP substrate; an n-type semiconductor layer formed on the InP substrate; a multiplication layer formed on the n-type semiconductor layer; a p-type electric field buffer layer formed on the multiplication layer; a light absorbing layer made of a compound semiconductor formed on the p-type electric field buffer layer; a window layer formed on the light absorbing layer; a contact layer formed on the window layer and having a two-dimensional periodic structure in which a plurality of holes or remnants are two-dimensionally arranged at a constant period; a surface protective insulating film formed at least on the two-dimensional periodic structure; a surface electrode formed on the surface protective insulating film; A semiconductor light receiving element comprising:

9. A semiconductor photodetector as described in Claim 8, characterized in that the multiplication layer has a digital alloy structure.

10. The semiconductor light-receiving element according to claim 8, wherein the multiplication layer has a random alloy structure made of InAlAs.

11. An InP substrate; a p-type semiconductor layer formed on the InP substrate; a light absorbing layer formed on the p-type semiconductor layer; a p-type electric field buffer layer formed on the light absorption layer; a multiplication layer made of a compound semiconductor formed on the p-type electric field buffer layer; a first window layer formed on the multiplication layer; a second window layer formed on the first window layer and having a two-dimensional periodic structure in which a plurality of holes or remnants are two-dimensionally arranged at a constant period; a surface protective insulating film formed at least on the two-dimensional periodic structure; a surface electrode formed on the surface protective insulating film; A semiconductor light receiving element comprising:

12. The semiconductor photodetector according to claim 11, wherein the multiplication layer has a digital alloy structure.

13. The semiconductor light-receiving element according to claim 11, wherein the multiplication layer has a random alloy structure made of InAlAs.

14. An InP substrate; a p-type semiconductor layer formed on the InP substrate; a light absorbing layer formed on the p-type semiconductor layer; a p-type electric field buffer layer formed on the light absorption layer; a multiplication layer made of a compound semiconductor formed on the p-type electric field buffer layer; a window layer formed on the multiplication layer; a surface protective insulating film formed on the window layer and having a two-dimensional periodic structure in which a plurality of holes or remnants are two-dimensionally arranged at a constant period; At least a surface electrode formed on the two-dimensional periodic structure; A semiconductor light receiving element comprising:

15. The semiconductor light-receiving element according to claim 14, wherein the multiplication layer has a digital alloy structure.

16. The semiconductor light-receiving element according to claim 14, wherein the multiplication layer has a random alloy structure made of InAlAs.

17. an InP substrate; a p-type semiconductor layer formed on the InP substrate; a light absorbing layer formed on the p-type semiconductor layer; a p-type electric field buffer layer formed on the light absorption layer; a multiplication layer formed on the p-type electric field buffer layer; a window layer formed on the multiplication layer; a surface protective insulating film formed on the window layer; a surface electrode formed on the surface protective insulating film and having a two-dimensional periodic structure in which a plurality of holes or remnants are two-dimensionally arranged at a constant period; A semiconductor light receiving element comprising:

18. An InP substrate; a p-type semiconductor layer formed on the InP substrate; a light absorbing layer formed on the p-type semiconductor layer; a p-type electric field buffer layer formed on the light absorption layer; a multiplication layer made of a compound semiconductor formed on the p-type electric field buffer layer; a window layer formed on the multiplication layer; a contact layer formed on the window layer and having a two-dimensional periodic structure in which a plurality of holes or remnants are two-dimensionally arranged at a constant period; a surface protective insulating film formed at least on the two-dimensional periodic structure; a surface electrode formed on the surface protective insulating film; A semiconductor light receiving element comprising:

19. The semiconductor light-receiving element according to claim 18, wherein the multiplication layer has a digital alloy structure.

20. The semiconductor light-receiving element according to claim 18, wherein the multiplication layer has a random alloy structure made of InAlAs.

21. 14. The semiconductor light-receiving element according to claim 1, wherein the first window layer is made up of two or more types of semiconductor layers having different refractive indices.

22. 22. The semiconductor light-receiving element according to claim 21, wherein the difference in refractive index between the semiconductor layers in the two or more kinds of semiconductor layers is 0.05 or more and 0.25 or less.

23. 21. The semiconductor light-receiving element according to claim 4, wherein the window layer is made up of two or more semiconductor layers having different refractive indices.

24. 24. The semiconductor light-receiving element according to claim 23, wherein the difference in refractive index between the semiconductor layers in the two or more kinds of semiconductor layers is 0.05 or more and 0.25 or less.

25. 11. The semiconductor light-receiving element according to claim 1, wherein the n-type semiconductor layer is a DBR layer.

26. 21. The semiconductor light-receiving element according to claim 11, wherein the p-type semiconductor layer is a DBR layer.

27. 26. The semiconductor light-receiving element according to claim 25, wherein the reflectance of the DBR layer is 1% or more and 40% or less.

28. 28. The semiconductor light-receiving element according to claim 27, wherein the number of pairs of the DBR layer is 2 pairs or more and 14 pairs or less.

29. A semiconductor light-receiving element as described in Claim 26, characterized in that the reflectivity of the DBR layer is 1% or more and 40% or less.

30. A semiconductor light-receiving element as described in Claim 29, characterized in that the number of pairs of the DBR layer is 2 pairs or more and 14 pairs or less.

31. 11. The semiconductor light-receiving element according to claim 1, further comprising a mesa structure in which semiconductor layers above the n-type semiconductor layer are removed outside the outer periphery of the surface electrode.

32. 21. The semiconductor light-receiving element according to claim 11, further comprising a mesa structure outside the outer periphery of the surface electrode, in which semiconductor layers above the p-type semiconductor layer are removed.

33. 32. The semiconductor light-receiving element according to claim 31, wherein a surface protection insulating film and an electrode are provided on a side surface of the mesa structure.

34. A semiconductor light-receiving element as described in claim 32, characterized in that a surface protective insulating film and an electrode are provided on the side surface of the mesa structure.

35. 34. The semiconductor light-receiving element according to claim 33, wherein an anti-reflection film is provided on a side surface of the mesa structure.

36. A semiconductor light-receiving element as described in claim 34, characterized in that an anti-reflection film is provided on the side surface of the mesa structure.

37. The semiconductor photodetector according to any one of claims 2, 5, 9, 12, 15 and 19, characterized in that the digital alloy structure is formed by alternately stacking two types of semiconductor layers, each made of a different semiconductor material, at a period of 2 to 6 atomic layers.

38. 38. The semiconductor light-receiving element according to claim 37, wherein the two types of semiconductor layers are any combination of an InAs layer and an AlAs layer, an InAlAs layer and an InGaAs layer, and an InAlGaAs layer and an InAlAsSb layer having different composition ratios.

39. 20. The semiconductor light-receiving element according to claim 2, wherein the multiplication layer has an InAs / AlAs digital alloy structure and a thickness of the multiplication layer is 60 nm or more and 130 nm or less.

40. 21. The semiconductor light-receiving element according to claim 1, wherein the pitch of the holes or the remaining portions is 347 nm or more and 571 nm or less.

41. 21. The semiconductor light-receiving element according to claim 1, wherein the diameter of the opening or the remainder of the hole is 35 nm or more and 343 nm or less.

42. A semiconductor light-receiving element according to claim 1, wherein the light absorption layer is InGaAs.

43. The semiconductor light receiving element according to any one of claims 1 to 20, an optical multiplexer / demultiplexer that inputs an optical signal to the semiconductor light-receiving element; an amplifier circuit that amplifies the electrical signal output from the semiconductor light-receiving element; a clock data recovery circuit connected to the amplifier circuit and recovering clock data from the amplified electrical signal; a forward error correction circuit connected to the clock data recovery circuit for correcting an error in the clock data; An optical line terminal comprising:

44. The semiconductor light receiving element according to any one of claims 1 to 20, an optical multiplexer / demultiplexer that inputs an optical signal to the semiconductor light-receiving element; an amplifier circuit that amplifies the electrical signal output from the semiconductor light-receiving element; an analog-to-digital conversion circuit connected to the amplifier circuit and converting the amplified electrical signal into a digital signal; a digital signal processing circuit connected to the analog-to-digital conversion circuit and processing the digital signal; a forward error correction circuit connected to the digital signal processing circuit for correcting errors in the digital signal; An optical line terminal comprising:

45. a semiconductor light-receiving element according to any one of claims 1 to 20, which receives a multilevel intensity-modulated optical signal; an amplifier circuit that amplifies the electrical signal output from the semiconductor light-receiving element; an analog-to-digital conversion circuit connected to the amplifier circuit and converting the amplified electrical signal into a digital signal; a digital signal processing circuit connected to the analog-to-digital conversion circuit and processing the digital signal; A multilevel intensity modulation transmitting and receiving device comprising:

46. a light source that emits an analog modulated optical signal; a semiconductor light-receiving element according to any one of claims 1 to 20, which receives the analog-modulated optical signal; a transmission path for transmitting the analog electrical signal output from the semiconductor light receiving element to an antenna; an antenna connected to the transmission line and configured to emit the analog electrical signal as a radio wave signal; A radio-on-fiber system comprising:

47. The semiconductor light receiving element according to any one of claims 1 to 20, a polarization separator that separates the polarizations of the intensity- and phase-modulated polarization multiplexed optical signal; a 90-degree hybrid that splits and combines the optical signals output from the polarization splitter; a digital signal processing circuit connected to the 90-degree hybrid device and processing a digital signal; A digital coherent receiving device comprising:

48. a SPAD sensor configured by the semiconductor light receiving element according to any one of claims 1 to 20; a quenching circuit that repeatedly applies a voltage equal to or greater than a breakdown voltage and a voltage less than the breakdown voltage to the SPAD sensor; an optoelectronic measurement circuit that measures the electrical signal output from the SPAD sensor; A SPAD sensor system comprising:

49. a light source that emits pulsed or frequency-modulated light; a semiconductor light-receiving element according to any one of claims 1 to 20, which receives light emitted from the light source and reflected by an object; an amplifier circuit that amplifies the electrical signal output from the semiconductor light-receiving element; a distance measuring circuit that calculates a distance based on the electrical signal amplified by the amplifier circuit; A lidar device comprising:

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