Semiconductor light-receiving element, optical line terminating device, multi-level intensity modulation transmitting / receiving device, digital coherent receiving device, radio-over-fiber system, spad sensor system, and lidar device

Abstract

A semiconductor light-receiving element (100) according to the present disclosure comprises: a substrate (1); an n-type semiconductor layer (2) formed on the substrate (1); a multiplier layer (3) formed on the n-type semiconductor layer (2); a p-type electric field relaxation layer (4) formed on the multiplier layer (3); a light-absorbing layer (5) formed on the p-type electric field relaxation layer (4); a first window layer (7) formed on the light-absorbing 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 with a certain periodicity; 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

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

[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 mutually communicate digital information and data centers that store and process data. Optical communication is used for communication networks and communication within data centers. Optical communication has made remarkable progress in recent years in increasing speed and capacity. With the development of optical communication, photodiodes (PDs) and avalanche photodiodes (APDs) that can provide high receiving sensitivity are required as optical communication receivers.

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

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

[0005] JP 3-050875 A JP 2005-159002 A JP 2008-311562 A JP 2016-178234 A JP 2016-178293 A JP 2018-63975 A JP 2019-54145 A JP 2019-68019 A

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

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

[0008] 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 to shorten the carrier transit time. However, reducing the thickness of the light absorption layer causes a problem of reduced light sensitivity.

[0009] When the absorption coefficient of the light absorption layer is a and the thickness of the light absorption 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 equation (1), for example, if a = 10,000 / cm and W = 1 μm, the quantum efficiency η is 63%. Incidentally, the light receiving sensitivity S (A / W) is given by S = η λ (nm) / 1240, so for light with a wavelength of 1.3 μm, the light receiving sensitivity is 0.66 A / W.

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

[0012] In equation (2), Vav is the average saturated 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 becomes higher than 63%, but the response band becomes lower than 29.8 GHz. In order to improve this trade-off between the response band and the 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] The element structures disclosed in Patent Documents 2 to 8 utilize resonance by a photonic crystal layer, and therefore, as shown in Figure 3 of Patent Document 6, for example, the photonic crystal layer acts like a filter only within a specific narrow wavelength range. Therefore, to obtain high receiving sensitivity at a desired wavelength, high precision is required in crystal growth and processing. Furthermore, because semiconductor photodetectors are not temperature-controlled during actual use, there is a risk that the resonant wavelength may change by 10 nm or more with changes in environmental temperature, resulting in a significant change in receiving sensitivity.

[0016] In recent years, attempts have been made to expand the wavelength range in which high receiving 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, thereby achieving high receiving sensitivity over a wide wavelength range.

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

[0018] Furthermore, because the InGaAs layer that constitutes the optical absorption layer has a small band gap, carriers generated by light absorption recombine on the surface of the holes, resulting in a problem of reduced receiver sensitivity. Furthermore, in order to generate multiple resonant modes, it is necessary to strengthen the optical confinement in the optical absorption layer, which requires a structure in which the optical absorption layer is sandwiched between low-refractive-index layers. However, 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 Related to the Multiplication Layer in APDs APDs, which are semiconductor photodetectors used in PON systems, have a device 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 approximately 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 receiver sensitivity.Furthermore, the greater the ratio of the ionization rates of electrons and holes, the shorter the multiplication time in the multiplication layer, resulting in a wider 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 performance of the APD. The multiplication layer of an APD for optical communication uses a compound semiconductor material such as InAlAs or InP.

[0022] When InAlAs is selected as the material for the multiplication layer, the difference between the ionization rates of electrons and holes is larger than when InP is used. In InP, the ionization rate of holes is higher than that of electrons, and the ionization rate of holes is approximately 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 higher than that of holes, and the ionization rate of electrons is approximately five times that of holes. Therefore, since the receiver 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, in a PON system, a wide response band and high receiving sensitivity are required for the APD, which is a semiconductor light receiving element. However, unlike a PD, an APD has 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, there is a problem in that it is difficult to achieve the wide response band of 37.5 GHz or more required for 50G-PON systems when conventional APDs are used.

[0025] As mentioned above, APDs, which are semiconductor light-receiving elements used in optical communications, are required to operate over an even wider response band. Patent Document 1 describes an APD that uses a superlattice as a multiplication layer, but because each layer of the superlattice has a thickness of 5 to 10 nm, it acts as a quantum well that reflects the band gap of each layer. When the thickness of each layer in the stack exceeds several nm, energy unevenness reflecting the band gap of each layer is created, hindering the movement of carriers and reducing their travel speed.

[0026] In a 50G-PON system, the response bandwidth 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), i.e., the receiving device on the subscriber side.

[0027] Furthermore, in the optical line terminal (OLT), i.e., the receiving device on the central office side, a semiconductor optical amplifier (SOA) is required to compensate for the lack of receiving sensitivity of the semiconductor light receiving element, or an SOA must be integrated 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, increasing the number of branches of the optical signal output from the OLT is being considered to reduce costs. However, even in this case, it is necessary to integrate an SOA in the EML on the transmitting side of the OLT and ONU to increase the optical output, which causes problems such as increased power consumption of the transmitter and increased 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 and power-hungry DSPs and SOAs into the ONU and OLT, but this results in problems such as increased power consumption and increased costs.

[0031] The present disclosure has been made to solve the above-mentioned problems, and has as its object to provide a semiconductor photodetector that has high receiving sensitivity over a wide wavelength range and operates over a wide response band.

[0032] The semiconductor light receiving element according to the present disclosure comprises: a substrate; an n-type semiconductor layer formed on the 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 absorption layer formed on the p-type electric field buffer layer; a first window layer formed on the light absorption 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 at least formed on the two-dimensional periodic structure; and a surface electrode at least formed on the surface protective insulating film.

[0033] The optical line terminal device according to the present disclosure comprises the above-mentioned semiconductor photodetector; an optical multiplexer / demultiplexer that inputs an optical signal to the semiconductor photodetector; an amplifier circuit that amplifies an electrical signal output from the semiconductor photodetector; a clock and data recovery circuit connected to the amplifier circuit that recovers clock data from the amplified electrical signal; and a forward error correction circuit connected to the clock and data recovery circuit that corrects errors in the clock data.

[0034] The multilevel intensity modulation transceiver according to the present disclosure includes the above-described semiconductor photodetector that receives an optical signal that has been intensity-modulated into multiple levels; an amplifier circuit that amplifies an electrical signal output from the semiconductor photodetector; an analog-to-digital conversion circuit that is connected to the amplifier circuit and converts the amplified electrical signal into a digital signal; and a digital signal processing circuit that is connected to the analog-to-digital conversion circuit and processes the digital signal.

[0035] The radio-on-fiber system according to the present disclosure includes: a light source that emits an analog-modulated optical signal; the above-described semiconductor photodetector that receives the analog-modulated optical signal; a transmission path that transmits the analog electrical signal output from the semiconductor photodetector to an antenna; and an antenna that is connected to the transmission path and emits the analog electrical signal as a radio wave signal.

[0036] The digital coherent receiving device according to the present disclosure comprises the semiconductor photodetector described above, a polarization separator that separates the polarizations of a polarization multiplexed optical signal whose intensity and phase are modulated, a 90-degree hybrid that splits and combines the optical signal output from the polarization separator, and a digital signal processing circuit that is connected to the 90-degree hybrid and processes digital signals.

[0037] A SPAD (Single Photon Avalanche Diode) sensor system according to the present disclosure includes: a SPAD sensor configured with the above-described semiconductor light-receiving element; 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; and a photoelectron measurement circuit that measures an electrical signal output from the SPAD sensor.

[0038] The LIDAR device according to the present disclosure comprises a light source that emits light in pulses, the above-mentioned semiconductor light receiving element that receives light that is 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, and a ranging circuit that calculates distance based on the electrical signal amplified by the amplifier circuit.

[0039] The semiconductor photodetector according to the present disclosure employs an element structure that includes 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 reception sensitivity.

[0040] According to 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 of the present disclosure, the semiconductor photodetector element of the present disclosure is used as the semiconductor photodetector element, thereby achieving the effect of obtaining each device and system with excellent performance.

[0041] 1A and 1B are diagrams showing the wavelength dependence of the refractive index in an InAs / AlAs digital alloy structure and an InAlAs random alloy structure. 1C are diagrams showing the wavelength dependence of the absorption coefficient in an InAs / AlAs digital alloy structure and an InAlAs random alloy structure. 1D are cross-sectional views showing the device structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to the first embodiment. 4A and 4B are top views showing the device structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to the first embodiment. 5A and 5B are cross-sectional views and top views, respectively, showing the manufacturing process of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to the first embodiment. 6A and 6B are cross-sectional views and top views, respectively, showing the manufacturing process of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to the first embodiment. 7A and 7B are cross-sectional views and top views, respectively, showing the manufacturing process of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to the first embodiment. 8A and 8B are cross-sectional views and top views, 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. FIGS. 9A and 9B are cross-sectional views and top views, 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. FIGS. 10A and 10B are cross-sectional views and top views, 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. 11 is a diagram illustrating a refractive index distribution between points P and Q in FIG. 11. 12 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. 13 is a diagram illustrating a refractive index distribution between points P and Q in FIG. 14. 14 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 modification of the first embodiment. 1A and 1B are cross-sectional views showing the 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, and a fourth modification of the first embodiment.28A to 28C are schematic diagrams showing the electric field dependence of the electron dead space in an InAlAs multiplication layer. 28A to 28C are schematic diagrams showing the electric field dependence of the electron dead space in an InAlAs multiplication layer. 28B are schematic diagrams showing the electric field dependence of the electron dead space in an InAlAs multiplication layer. ... 30A to 30D are conceptual diagrams showing the ionization rates in the multiplication layer and the electric field relaxation layer, where FIG. 30A is for the multiplication layer having a random alloy structure, FIG. 30B is for the multiplication layer having a digital alloy structure, FIG. 30C is for the multiplication layer having a partially disordered digital alloy structure, and FIG. 30D is a conceptual diagram showing the ionization rate when a thick electric field relaxation layer and a multiplication layer having a digital alloy structure are combined. A cross-sectional view showing the element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to a third embodiment. A cross-sectional view showing the 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. A cross-sectional view showing the element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to a second modification of the third embodiment. A cross-sectional view showing the element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to a third ... fourth modification of the third embodiment. A configuration diagram showing an optical line terminal (OLT) of a 50G-PON system relating to embodiment 4.42A and 42B are conceptual diagrams illustrating received waveforms of the multi-level intensity modulation transceiver according to the fifth embodiment. FIGS. 43A and 43B are conceptual diagrams illustrating the operation of a PD at high optical input. FIGS. 43A and 43B are conceptual diagrams illustrating the operation of an APD at high optical input. FIGS. 43B are diagrams illustrating the residence times of electrons and holes for each material constituting the multiplication layer. FIGS. 43C are diagrams illustrating the configuration of a radio-on-fiber system according to a sixth embodiment. FIGS. 43D are diagrams illustrating the configuration of a radio-on-fiber system as a comparative example. FIGS. 43E are diagrams illustrating the configuration of a digital coherent receiving device according to a seventh embodiment. FIG. 49A is a conceptual diagram showing a waveform of a digital coherent receiving device as a comparative example, and FIG. 49B is a conceptual diagram showing a waveform of a digital coherent receiving device according to embodiment 7. It is a diagram showing the configuration of a SAPD sensor system according to embodiment 8. 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 embodiment 8. It is a diagram showing the calculation of the difference between the quenching electric field and the Geiger mode electric field for each configuration of the multiplication layer. It is a diagram showing the configuration of a LIDAR device according to embodiment 9. FIG. 54A is a conceptual diagram showing the received waveform of an APD of a LIDAR device as a comparative example, and FIG. 54B is a conceptual diagram showing the received waveform of an APD of a LIDAR device according to embodiment 9.

[0042] First Embodiment <Characteristics of the Semiconductor Photodetector (APD) According to the First Embodiment> Before describing the specific structure of the semiconductor photodetector 100 according to the first embodiment, the digital alloy structure, which is a structural feature of the semiconductor photodetector 100 according to the first embodiment, will be described below.

[0043] The inventors have found 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 layers and two atomic layers of AlAs layers are repeatedly stacked, has an InAs / AlAs layer structure with a substantially uniform composition ratio as a whole. 0.52 Al 0.48 It was discovered that the refractive index of the material is lower than that of a random alloy structure made of In. 0.52 Al 0.48 As is simply expressed as InAlAs.

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

[0045] Furthermore, 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 approximately the same, the band structure changes due to the periodicity of the InAs and AlAs layers, which causes periodicity in the wavelength dependence of the optical absorption coefficient. Furthermore, at band edge wavelengths of 850 nm or higher, the absorption coefficient of the InAs / AlAs digital alloy structure drops sharply. This phenomenon is thought to result in a small refractive index 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 when the composition is approximately the same. The inventors have utilized this phenomenon to propose applying an InAs / AlAs digital alloy structure to the multiplication layer of an APD for optical communication that uses a two-dimensional periodic structure.

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

[0048] On the other hand, in an APD, to strongly confine light in a light absorption layer with 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, a multiplication layer with a digital alloy structure has a smaller refractive index than a multiplication layer with a random alloy structure. Therefore, a multiplication layer with a digital alloy structure has more light confinement in the light absorption layer by the multiplication layer than a multiplication layer with a random alloy structure. Furthermore, the complex resonance of light generated by the application of a two-dimensional periodic structure has a synergistic effect of realizing improved receiving sensitivity over a wide wavelength range.

[0049] Other advantages of applying an InAs / AlAs digital alloy structure to the multiplication layer are described 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 receiver 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 electron ionization rate and β is the hole ionization rate. 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 communication.

[0051] When the multiplication layer is composed of an InAlAs layer, the ionization rate of holes is lower in a digital alloy structure than in a random alloy structure, so the ionization rate ratio k can be made smaller. Furthermore, since the dead space length of holes is long in an InAs / AlAs digital alloy structure, the hole ionization rate β 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 an InAs / AlAs digital alloy structure is applied to the multiplication layer, excess noise generated during multiplication is reduced, resulting in higher receiver sensitivity, and the multiplication time in the multiplication layer is shortened, resulting in a wider bandwidth.

[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] 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 refers to 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 refers to 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 a layer thickness of 0.1 to 1.0 μm; an InAs / AlAs multiplication layer 3 (hereinafter referred to as an i-type InAs / AlAs digital alloy structure multiplication layer 3) having a digital alloy structure 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 stacked multiple times; and a carrier concentration of 0.1 to 50×10 17 cm -3The semiconductor device includes a p-type InP field relaxation layer 4 having a thickness of 10 to 70 nm, an i-type InGaAs light absorption layer 5 having 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 having a thickness of 0.1 to 3.0 μm, a p-type InAlAs second window layer 8 having a thickness of 0.1 to 3.0 μm and a two-dimensional periodic structure 70, a p-type InGaAs contact layer 9, and a SiN surface 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 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 to make it easier to understand the structure viewed from above.

[0055] Instead of InP, the i-type InP first window layer 7 may be made of InAlAs, InAlGaAs, or InGaAsP. The i-type InP first window layer 7 may be made of a plurality of 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 p-type InAlAs second window layer 8 described above has a configuration in which a plurality of holes 11, which penetrate the p-type InAlAs second window layer 8 from the surface to the bottom surface 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 openings of the holes 11 on the surface side may have a circular, rectangular, triangular, or other shape, but may also have other shapes. Alternatively, as illustrated 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 circular portions of the p-type InAlAs second window layer 8 after etching. The remaining portions 8s may have a circular shape, rectangular, triangular, or other shape, but may also have other shapes. In the following first to third embodiments, the holes 11, i.e., recesses, may be read as remaining portions 8s, i.e., protrusions. The semiconductor light-receiving element 101 has a two-dimensional periodic structure 70 made of remaining portions 8s.

[0059] In the two-dimensional periodic structure 70, the ratio of the diameter of the holes 11 to the constant period is preferably within a range of 10% to 80%. The depth of the holes 11 (height when the holes are considered to be 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. 3, or may be located midway through the p-type InAlAs second window layer 8.

[0061] A Zn-diffused p-type region 12 is selectively formed below the p-type InGaAs contact layer 9 and in the region surrounded by the p-type InGaAs contact layer 9. The 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 within the surface of the rear surface side of the n-type InP substrate 1, which faces the portion surrounded by the p-type InGaAs contact layer 9 on the front surface side. Instead, an anti-reflection film 40 made of a SiN film or the like is formed within the surface of the rear surface side of the n-type InP substrate 1, which faces 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 composed 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 composed 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 intermingle, 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 InAs layers (layer thickness: 2 atomic layers, approximately 0.6 nm) and AlAs layers (layer thickness: 2 atomic layers, approximately 0.6 nm) are alternately stacked. However, the thicknesses of the InAs layers and AlAs layers may be in the range of 2 to 6 atomic layers, respectively. The reason for specifying 6 atomic layers or less is that it is desirable for the stacked structure of the InAs layers and AlAs layers not to function as a quantum well structure. In other words, 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.

[0066] Furthermore, the number of atomic layers in each layer of the i-type InAs / AlAs digital alloy structure multiplication layer 3 is preferably 2 to 4 atomic layers, with 2 atomic layers being optimal. This is because the thinner the atomic layer thickness of each layer, the greater the effect of reducing the ionization rate ratio k due to the digital alloy structure. Furthermore, 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, as this reduces the number of shutter switches during crystal growth by molecular beam epitaxy (MBE). Considering the above factors, it can be said that the number of atomic layers in each layer of the i-type InAs / AlAs digital alloy structure multiplication layer 3 is preferably in the range of 2 to 6 atomic layers. Similarly, from the viewpoint of productivity, the entire multiplication layer does not have to have an InAs / AlAs digital alloy structure. Instead, a part of the multiplication layer may have an InAs / AlAs digital alloy structure and the rest may have 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 (two atomic layers) / AlAs layer (two atomic layers) is 85.

[0068] Considering the affinity with InP constituting the n-type InP buffer layer 2, it is preferable to make the 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 formed by alternately stacking InAs layers and AlAs layers 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 However, as the conductivity type of the InAs / AlAs digital alloy structure multiplication layer, the carrier concentration 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, which is 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 in 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 alternately stacking two types of i-type InAlGaAs layers with different compositions multiple times while changing the layer thickness, thereby changing the equivalent band gap. The carrier concentration of the i-type InAlGaAs / InAlAs graded layer 6 is 5×10 17 cm -3 or 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 field buffer layer 4 and the i-type InGaAs light absorption layer 5.

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

[0073] <Method for Manufacturing Semiconductor Photodetector 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] By using MOVPE or MBE, a semiconductor having a carrier concentration of 1 to 5×10 is formed on an n-type InP substrate 1.18 cm -3 The n-type InP buffer layer 2 having a thickness of 0.1 to 1 μm is grown by crystal growth.

[0075] On the n-type InP buffer layer 2, a carrier concentration of 5×10 17 cm -3 The multiplication layer 3 having an i-type InAs / AlAs digital alloy structure and 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 x 10 18 cm -3 The p-type InP field buffer layer 4 having a thickness of 10 to 70 nm is crystal-grown. Examples of p-type dopants for the p-type InP field buffer layer 4 include Be and Zn.

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

[0078] Furthermore, the carrier concentration is 5×10 17 cm -3 an i-type InAlGaAs / InAlAs graded layer 6 having a thickness of 5 to 50 nm and a carrier concentration of 5×10 17 cm -3 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 grown by crystal growth. 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, since they will be converted to p-type by selective diffusion in a later step.

[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 the solid or vapor 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 methods such as dry etching or wet etching. An InP layer with a thickness of approximately 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 etching depth control. The cross-sectional view in FIG. 6A and the top view in 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 will become the light receiving portion, i.e., the light receiving region, so as to be arranged two-dimensionally at a constant period. The cross-sectional view of Fig. 7A and the top view of Fig. 7B show the state in which the plurality of holes 11 have been formed. The top view of Fig. 7B shows an example of a configuration in which the holes 11 are arranged two-dimensionally in a lattice pattern, as an example of a configuration in which the holes 11 are arranged two-dimensionally at a constant period. The plurality of holes 11 are provided in the p-type InAlAs second window layer 8. A structure in which the plurality of holes 11 are arranged two-dimensionally 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, the bottom surface of the hole 11 does not have to reach the i-type InGaAs light absorption layer 5, and 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 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 placed at the outermost surface, dark current is likely to increase.

[0084] The first and second window layers can both be made of i-type InP layers, which are resistant to oxidation. In this case, etching of the InP second window layer around the periphery of the p-type InGaAs contact layer 9 is unnecessary. Conversely to the above configuration, the first window layer may be made of an InAlAs layer and the second window layer may be made of InP. The reason for using different materials for the first and second window layers is to improve controllability of the depth of the hole 11 (or the height, if the hole is left as the remaining portion), i.e., the etching depth. An etching stopper layer may be provided between the first and second window layers to improve controllability of the etching depth.

[0085] Next, a SiN surface protective insulating film 10 is formed on the entire surface of the front surface 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. The cross-sectional view in Fig. 9A and the top view in Fig. 9B show the state after the ohmic contact has been formed.

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

[0087] Next, a p-type electrode 32 is formed on the ring-shaped etched p-type InGaAs contact layer 9 and on the SiN surface protective insulating film 10 on the inner periphery 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 is made of a metal material such as Ti and Au, or Pt. The p-type electrode 32 also functions as a metal reflective film. Specifically, the p-type electrode 32 is made of a multilayer film such as Ti / Au, Ti / Pt / Au, Ti / Au / Pt / Au, Ti / Au / Ti / Pt / Au, or Pt / Ti / Au / Ti / Pt / Au, in that order from the semiconductor layer side.

[0089] After the processing of the front surface side is completed, 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 film of n-type electrode material is formed on the entire rear surface of the n-type InP substrate 1, and then the n-type electrode material formed on the rear surface side opposite 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 of the n-type InP substrate 1 opposite the portion surrounded by the p-type InGaAs contact layer 9 on the front surface side. This completes the method for manufacturing the semiconductor photodetector according to the first embodiment.

[0091] Incident light 90 incident through the antireflection film 40 formed on the back surface of the n-type InP substrate 1 is perpendicularly incident on the i-type InGaAs light absorption layer 5 provided on the front surface 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 in the range of 5 μm to 1 mm, and when the light-receiving region is rectangular, its long side is in the range of 5 μm to 1 mm.

[0092] <Operation of Semiconductor Light-Receiving Element According to 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 semiconductor light receiving element 100 passes through antireflection film 40 and enters the interior of semiconductor light receiving element 100. Incident light 90 passes through n-type InP substrate 1, and after a portion of incident light 90 is absorbed in i-type InGaAs light absorption layer 5, it reaches i-type InP first window layer 7 and p-type InAlAs second window layer 8.

[0094] The p-type InAlAs second window layer 8 has a plurality of holes 11 arranged in a lattice pattern, i.e., a two-dimensional periodic structure 70, which causes optical resonance 91. 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: approximately 2.0) and the InAlAs layer (refractive index: approximately 3.25) is large, so optical resonance 91 easily occurs. Furthermore, because the InGaAs layer has a higher refractive index (refractive index: approximately 3.6) than the InP layer (refractive index: approximately 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 positions A to D in FIG. 12 are assumed. When the APD is in operation, multiple resonances occur from positions A to D. In multiple resonances, light resonates simultaneously at each resonance point from positions A to 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 wavelengths where optical resonance 91 occurs, but complex resonance increases the light receiving sensitivity over an even wider wavelength range. The reason for this is that the equivalent refractive index differs at the resonance points from position A to position D, and therefore 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 at 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 receiving sensitivity over a wide wavelength range of several tens of nanometers or more.

[0097] Changing the period (period: the distance between the centers of the holes 11) of the holes 11, which are arranged two-dimensionally periodically in a lattice pattern, changes the wavelength at which complex resonance occurs. Therefore, it is sufficient to set the period of the holes 11 so that complex resonance occurs at a desired wavelength. The period of the holes 11 is roughly a 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 configuration and the aperture ratio of the holes 11, and falls within a range of 2.8 to 3.6. The period of the holes 11 may be set within a 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, while 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 aperture 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 × 10%) and a maximum of 343 nm (= 571 nm × 60%). Here, the diameter of the hole 11 is a definition that applies when the opening of the hole 11 is circular. If the opening of the hole 11 has a shape other than a circle, such as a rectangle, it may be defined as the length of the diagonal, or if the opening of the hole 11 has a shape other than a circle, it may be defined as the length of the long side.

[0100] In the plurality of holes 11 arranged two-dimensionally periodically in the x and y directions, i.e., in a lattice pattern, it is desirable that the period in the x direction and the period in the y direction be 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, polarization dependency of the receiving sensitivity occurs. From the viewpoint of reducing the polarization dependency of the receiving sensitivity, it is desirable that the shape of the opening of the holes 11 be close to a circle. However, even if the shape of the opening of the holes 11 is a polygon such as a triangle or a rectangle, or an ellipse, it is also possible to apply it to the two-dimensional periodic structure 70. Furthermore, it is also possible to arrange multiple patterns close to each other, such as arranging triangular or rectangular holes close to each other, and have them function as a single hole.

[0101] The advantages of the semiconductor photodetector 100 according to the first embodiment over the semiconductor photodetector described in Non-Patent Document 1 are described below. The semiconductor photodetector described in Non-Patent Document 1 has holes periodically arranged in a lattice pattern in a light absorption layer made of germanium (Ge). Because electron-hole pairs generated by light absorption recombine on the surface of the holes, there is a problem of reduced reception sensitivity. Furthermore, because 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 device structure described in Non-Patent Document 1 is applied to an APD, a reduction in reliability is a particular concern due to the application of a high electric field to the APD.

[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. Therefore, electrons and holes generated in the i-type InGaAs light absorption layer 5 do not diffuse to the holes 11. Furthermore, 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 achieve 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, light incident from the 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 light that has 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, contributing to the optical resonance and thereby improving the receiving sensitivity.

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

[0105] The above-mentioned phenomenon will be explained in detail using 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 at 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 (horizontally) of the light absorption layer is high, resulting in a resonance point occurring 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 high refractive index of the InAlAs random alloy structure multiplication layer causes the resonance point of the light absorption layer to be biased toward the semiconductor substrate, resulting in light dissipation. As a result, in the case of an InAlAs random alloy structure multiplication layer, the optical confinement rate in the light absorption layer is reduced.

[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, and is approximately the same as that of the InP layer that constitutes the first window layer, so that leakage of light into the multiplication layer is reduced, as shown in Figure 1. In other words, when an InAs / AlAs digital alloy structure is applied to the multiplication layer, resonated light is efficiently confined in the light absorption layer, making it easier for complex resonance to occur, thereby realizing an APD with high receiving sensitivity over a wide wavelength range.

[0107] <Effects of 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 internally. Furthermore, by applying an InAs / AlAs digital alloy structure to the multiplication layer, the amount of light confined in the light absorption layer is increased, thereby achieving the effect of enabling high reception sensitivity of the semiconductor photodetector over a wide wavelength range due to complex resonance. Also, a semiconductor photodetector that can achieve low noise and a wide bandwidth can be obtained. Therefore, it is possible to obtain an APD with sufficient reception sensitivity even in applications requiring a response bandwidth of 25 Gbps or more.

[0108] 13 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor photodetector 110 according to the first modification of the first embodiment. The semiconductor photodetector 110 according to the first modification of the first embodiment is characterized in that the first window layer is made up of multiple layers with different refractive indices.

[0109] In the device structure of the semiconductor photodetector 110 according to the first modification of the first embodiment, the configuration of each layer from the n-type InP substrate 1 to the i-type InAlGaAs / InAlAs graded layer 6 is the same as that of the semiconductor photodetector 100 according to the first embodiment, and therefore a description thereof will be omitted.

[0110] Formed in this order on the i-type InAlGaAs / InAlAs graded layer 6 of the semiconductor light-receiving device 110 according to the first modification of the first embodiment are 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 concentrations 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 are each 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 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 configuration 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 in the semiconductor photodetector 100 according to the first embodiment, i.e., 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 , which show the manufacturing method of the semiconductor photodetector 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. However, in the semiconductor photodetector 110 according to the first modification of the first embodiment, the i-type InP second window layer 8a is composed of an InP layer, and therefore this removal is not necessary. The reason for this is that in the semiconductor photodetector 100 according to the first embodiment, the InAlAs layer constituting the p-type InAlAs second window layer 8 is susceptible to degradation due to surface oxidation near the boundary 13 of the Zn-diffused p-type region 12 to which an electric field is applied, whereas in the semiconductor photodetector 110 according to the first modification of the first embodiment, the InP layer constituting the i-type InP second window layer 8a is less susceptible to degradation due to oxidation.

[0114] <Operation of Semiconductor Photodetector (APD) According to Modification 1 of First Embodiment> As with the first embodiment, the semiconductor photodetector 110 according to Modification 1 of the first embodiment also generates complex resonance at the resonance points as shown in Fig. 14 . In the refractive index profile of the semiconductor photodetector 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 highest, the proportions of light intensity at positions C and D in Fig. 14 are large as optical resonance points. In order to obtain high receiver sensitivity over a wide wavelength range by complex resonance, it is also necessary to ensure optical resonance at positions A and B in Fig. 14 .

[0115] In the refractive index profile shown in Fig. 15, the refractive index of the i-type InAlAs first window layer 7b is higher than in the refractive index profile 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 the difference in refractive index when the first window layer 7b is an InAlGaAs layer (refractive index: 3.454) with a composition wavelength λg of 1240 nm is 0.25.

[0116] In other words, in the device structure of the semiconductor light receiving device 110 according to the first modification of the first embodiment, the first window layer is configured as a two-layer structure of the i-type InP first window layer 7a and the i-type InAlAs first window layer 7b, and as a result, a refractive index difference of 0.05 to 0.25 is obtained, and as a result, complex resonance is likely to occur. Note that, in the case of the refractive index profile shown in Figure 15, the digital alloy structure multiplication layer (solid line) has greater light confinement in the light absorption layer than the random alloy structure multiplication layer (dotted line), and therefore the effect of improving the receiver sensitivity in complex resonance is greater.

[0117] Effect of Modification 1 of First Embodiment As described above, according to the semiconductor photodetector according to Modification 1 of First Embodiment, a first window layer made up of multiple layers is provided to increase the refractive index of the upper layer, a two-dimensional periodic structure is provided in the second window layer, and the semiconductor layer is covered with a SiN surface protective insulating film and a p-type electrode to reflect light internally. Furthermore, by applying an InAs / AlAs digital alloy structure to the multiplication layer, the amount of light confined in the light absorption layer is increased, thereby achieving both an improvement in the amount of light confined in the light absorption layer and complex resonance. This has the effect of enabling the semiconductor photodetector to have high reception sensitivity over a wide wavelength range, 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 requiring a response bandwidth of 25 Gbps or more.

[0118] Modification 2 of First Embodiment. <Features of Semiconductor Photodetector (APD) According to Modification 2 of First Embodiment> Figure 16 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor photodetector 120 according to Modification 2 of First Embodiment. The semiconductor photodetector 100 according to First Embodiment shown in Figures 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 First Embodiment is characterized in that it does not include a second window layer, but instead includes only an i-type InP window layer 7c, and has a plurality of holes 11a formed 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 in 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 circular, rectangular, triangular, or the like, but may also be other shapes.

[0120] In the two-dimensional periodic structure 70a, the ratio of the diameter of the holes 11a to the constant 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 First Embodiment> In the semiconductor light receiving element 120 according to Modification 2 of First Embodiment, similarly to the semiconductor light receiving element 100 according to First Embodiment, complex resonance occurs corresponding to the period of the plurality of holes 11 a 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 receiving sensitivity. The advantage over the semiconductor light receiving element 100 according to First Embodiment is that the semiconductor light receiving element 120 does not have a second window layer, and therefore the overall layer thickness of the epitaxial crystal growth layer can be made thinner.

[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. Because selective etching of the insulating film and the semiconductor layer is easy, the bottoms of the holes 11a can be accurately stopped at the surface of the semiconductor layer. This provides the advantage of making the depths of the multiple holes 11a that make up the two-dimensional periodic structure 70a uniform (or the heights, if the holes are left as remaining portions).

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

[0125] Effect of Modification 2 of First Embodiment As described above, the semiconductor photodetector according to Modification 2 of First Embodiment provides a lattice-like two-dimensional periodic structure in the SiN surface protective insulating film on the i-type InP window layer. Light is incident from the semiconductor substrate side, and the semiconductor layer is covered with the SiN surface protective insulating film and the p-type electrode, thereby reflecting the light internally. This provides the effect of achieving high reception sensitivity of the semiconductor photodetector over a wide wavelength range through complex resonance. It also provides the effect of providing a semiconductor photodetector that can achieve low noise and a wide bandwidth. In addition, compared to the first embodiment, the depth (or height, if the hole is considered the remaining portion) of the holes provided in the SiN surface protective insulating film is uniform, and the refractive index difference can be increased, which facilitates optical resonance and improves light absorption efficiency. As a result, a semiconductor photodetector that can achieve high reception sensitivity, low noise, and a wide bandwidth can be achieved. Therefore, an APD with sufficient reception sensitivity can be obtained even for applications requiring a response bandwidth of 25 Gbps or more.

[0126] Modification 3 of First Embodiment. <Features of Semiconductor Photodetector (APD) According to Modification 3 of First Embodiment> Figure 17 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor photodetector 130 according to Modification 3 of First Embodiment. The semiconductor photodetector 100 according to First Embodiment shown in Figures 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 First Embodiment is characterized in that it does not include a second window layer, but only includes an i-type InP window layer 7c, and has holes 11b 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 in the range of 50 nm to 500 nm. The material of the p-type electrode 32a may be Ti / Pt / Au instead of 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 shapes 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 the constant 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.

[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 First Embodiment> In the semiconductor light receiving element 130 according to Modification 3 of First Embodiment, similar to the semiconductor light receiving element 100 according to First Embodiment, complex resonance occurs corresponding to the period of the plurality of holes 11 b formed in a lattice pattern, and the light intensity in the light absorption layer increases over a wide wavelength range, resulting in high receiving sensitivity. The advantage over the semiconductor light receiving element 100 according to First Embodiment is that the thickness of the epitaxial crystal growth layer can be made thinner because there is no second window layer.

[0132] Dry etching or wet etching is generally used to form the holes 11b in the p-type electrode 32a, which is made of a metal film such as Ti / Au. Because selective etching of the SiN surface protective insulating film 10 below the p-type electrode 32a is easy, the bottom of the holes 11b can be accurately stopped at the surface of the SiN surface protective insulating film 10. This has the effect of making the depths of the multiple holes 11b that make up the two-dimensional periodic structure 70b uniform (or the heights, if the holes are left as remaining portions).

[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, i.e., air, becomes large, there is also an effect that the resonance effect is more easily obtained.

[0134] <Effects of Modification 3 of First Embodiment> As described above, according to the semiconductor photodetector according to Modification 3 of First Embodiment, a lattice-like two-dimensional periodic structure is provided in the p-type electrode on the SiN surface protective insulating film, and light is incident from the semiconductor substrate side. This provides the effect of enabling high reception sensitivity of the semiconductor photodetector over a wide wavelength range through complex resonance. In addition, compared to the first embodiment, the depth (or height, if the hole is considered to be the remaining portion) of the hole provided in the p-type electrode is uniform, and the refractive index difference can be increased, making it easier for optical resonance to occur, thereby improving light absorption efficiency. As a result, a semiconductor photodetector with high reception sensitivity, low noise, and a wide bandwidth can be realized. Therefore, it is possible to obtain an APD with sufficient reception sensitivity even in applications requiring a response bandwidth of 25 Gbps or more.

[0135] Variation 4 of First Embodiment. <Features of Semiconductor Photodetector (APD) According to Variation 4 of First Embodiment> Figure 18 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor photodetector 140 according to Variation 4 of First Embodiment. The semiconductor photodetector 100 according to the first embodiment shown in Figures 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 Variation 4 of 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 in 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 the constant 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.

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

[0140] <Function of Modification 4 of First Embodiment> In the semiconductor light receiving element 140 according to Modification 4 of First Embodiment, similarly to the semiconductor light receiving element 100 according to First Embodiment, complex resonance occurs corresponding to the period of the plurality of holes 11 c formed in a lattice pattern, and the light intensity in the light absorption layer increases over a wide wavelength range, resulting in high receiving sensitivity. The advantage over the semiconductor light receiving element 100 according to First Embodiment is that the thickness of the epitaxial crystal growth layer can be made thinner 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. This method has the advantage that the hole 11c can be formed simultaneously with the process of forming the ring-shaped ohmic contact, eliminating the need for a separate process of forming the hole 11c.

[0142] When etching the p-type InGaAs contact layer 9 a, selective etching with respect to the i-type InP window layer 7 c that is in contact with the p-type InGaAs contact layer 9 a on the lower side is easy, so the bottoms of the holes 11 a can be accurately stopped on the surface of the i-type InP window layer 7 c, which has the effect of making the depths of the multiple holes 11 c that make up the two-dimensional periodic structure 70 a uniform (or the heights, if the holes are left as remaining parts).

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

[0144] Effect of Modification 4 of First Embodiment As described above, according to the semiconductor photodetector according to Modification 4 of 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 for complex resonance, thereby enabling the semiconductor photodetector to have high reception sensitivity over a wide wavelength range, and also enables low noise and a wide bandwidth. In addition, compared to the first embodiment, the depth of the holes provided in the p-type InGaAs contact layer (or the height, if the holes are considered the remaining portion) is uniform, making it easier for optical resonance to occur and improving the light absorption efficiency. As a result, a semiconductor photodetector can be realized that has high reception sensitivity, 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.

[0145] Modification 5 of First Embodiment. <Features of a Semiconductor Photodetector (APD) According to Modification 5 of First Embodiment> FIG. 19 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor photodetector 150 according to Modification 5 of First Embodiment. In the first embodiment and Modifications 1 to 4 of the embodiments, the p-type region is formed by selective diffusion of Zn. On the other hand, in the semiconductor photodetector 150 according to Modification 5 of First Embodiment, as shown in FIG. 19 , p-type doping is performed on each 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 during epitaxial crystal growth, thereby forming a mesa-type APD having a mesa structure (hereinafter simply referred to as a mesa). Examples of p-type dopants that can be used include Be, Zn, Mg, and C. The doping concentration of each layer is 5×10 17 cm -3 or more, 1 × 10 18 cm -3The degree is suitable.

[0146] <Function of Modification 5 of First Embodiment> When light is incident on the lattice-like two-dimensional periodic structure 70 from a vertical direction, 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 outside the boundary 13 of the Zn-diffused p-type region 12. In this case, the light that has reached outside 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, where they are extracted as a current signal. Since the time it takes for carriers to diffuse in the region where no electric field is applied is longer than in the region where an electric field is applied, the frequency response deteriorates. As a result, there is a risk of a problem in that a trailing edge 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 the semiconductor light-receiving element 150 according to the fifth modification of the first embodiment, the APD has a mesa-type element structure, and therefore an electric field is applied across the entire region inside the mesa structure. As a result, no tailing occurs in the pulse response waveform, and the frequency response does not deteriorate.

[0149] As described above, the semiconductor photodetector according to the fifth modification of the first embodiment has a mesa-type device structure, and an electric field is applied across the entire region inside the mesa structure. This prevents the pulse response waveform from trailing, and therefore the frequency response does not deteriorate. This provides an advantage of realizing a semiconductor photodetector that has high reception sensitivity, low noise, and a wide bandwidth. 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] Modification 6 of First Embodiment <Features of Semiconductor Photodetector (APD) According to Modification 6 of First Embodiment> Fig. 20 is a cross-sectional view showing the element structure of a back-illuminated APD, which is an example of a semiconductor photodetector 160 according to Modification 6 of the first embodiment.

[0151] <Structure of Semiconductor Photodetector According to Modification 6 of First Embodiment> A back-illuminated APD, which is an example of a semiconductor photodetector 160 according to Modification 6 of the first embodiment, includes an Fe-doped semi-insulating InP substrate 1a and an Fe-doped semi-insulating InP substrate 1a, which has 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 formed by alternately stacking 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) multiple times, and an n-type InP buffer layer 2a having a carrier concentration of 0.1 to 1.0 μm and an i-type InAs / AlAs digital alloy structure multiplication layer 3 having a carrier concentration of 0.1 to 50×10 17 cm -3 a p-type InP field relaxation layer 4 having a thickness of 10 to 70 nm, an i-type InGaAs light absorption layer 5 having a thickness of 0.1 to 2.0 μm, an i-type InAlGaAs / InAlAs graded layer 6, a p-type InP first window layer 7d having a thickness of 0.1 to 3.0 μm, a p-type InAlAs second window layer 8 having a thickness of 0.1 to 3.0 μm and a two-dimensional periodic structure 70, a ring-shaped p-type InGaAs contact layer 9, a plurality of holes 11 two-dimensionally arranged in the p-type InAlAs second window layer 8, and a plurality of holes 11 exposed at the bottom of the holes 11. the n-type InP substrate 1 includes an n-type electrode 31a formed on the exposed n-type InP buffer layer 2a and the SiN surface protective insulating film 10a formed on the side surfaces of the p-type InP first window layer 7d and the p-type InAlAs second window layer 8; an n-type electrode 31a formed on the exposed n-type InP buffer layer 2a and the SiN surface protective insulating film 10a formed on the side surfaces of the n-type InP buffer layer 2a; 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 of the n-type InP substrate 1.

[0152] <Method of Manufacturing a Semiconductor Light-Receiving Element According to Modification 6 of First Embodiment> An Fe-doped InP substrate 1a is formed on the substrate 1a.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, the i-type InAs / AlAs digital alloy structure multiplication layer 3, the p-type InP electric field relaxation layer 4, the i-type InGaAs light absorption layer 5, and the i-type InAlGaAs / InAlAs graded layer 6 are successively grown by crystal growth on the n-type InP buffer layer 2a.

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

[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 ring-shaped p-type InGaAs contact layer 9 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 is removed from the portion where the ohmic contact is to be formed, to form a p-type electrode 32 and an n-type electrode 31 a. 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 10 a 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, which is formed by etching down to 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 to the n-type electrode 31a. Furthermore, 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 First Embodiment> In the semiconductor light-receiving element 160 according to Modification 2 of the first embodiment, when light is incident vertically on the lattice-like two-dimensional periodic structure 70, light is generated that travels horizontally. The side surfaces of the mesa are covered with the SiN surface protective insulating film 10a and the n-type electrode 31a, and therefore function as reflective mirrors for this horizontally traveling light. Therefore, the light traveling horizontally is reflected back inward by the side surfaces of the mesa and absorbed by the i-type InGaAs light absorption layer 5, thereby improving the receiving sensitivity.

[0158] The semiconductor photodetector 160 according to the second modification of the first embodiment uses an Fe-doped semi-insulating InP substrate 1a that has smaller optical absorption loss than the n-type InP substrate 1, and therefore the receiving sensitivity is further improved.

[0159] As described above, in the semiconductor photodetector 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, which enables light traveling horizontally to be returned to the interior, thereby achieving the effect of further improving the receiving sensitivity of the semiconductor photodetector. 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] Seventh Modification of First Embodiment Fig. 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 a seventh modification of the first embodiment. In the first embodiment and modifications 1 to 6 of the first embodiment, light is incident from the back surface. On the other hand, the semiconductor light-receiving device 170 according to the seventh modification of the first embodiment 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] Effect of Modification 7 of First Embodiment According to the semiconductor photodetector of Modification 7 of First Embodiment, by providing a two-dimensional periodic structure in the second window layer and further applying an InAs / AlAs digital alloy structure to the multiplication layer, the amount of light confined in the light absorption layer is increased, thereby enabling high reception sensitivity over a wide wavelength range through complex resonance. In addition, by providing an anti-reflection film on the entire facet, even higher reception sensitivity is possible, resulting in an edge-illuminated semiconductor photodetector that is capable of achieving lower noise and a wider bandwidth. Therefore, it is possible to obtain an APD with sufficient reception sensitivity even in applications requiring a response bandwidth of 25 Gbps or more.

[0162] 22 is a cross-sectional view showing the 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 substantially 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 to satisfy the following formula (3): t=λ / (4·n) (3) where n is the refractive index of each layer and λ is the wavelength of the incident light.

[0166] Furthermore, the thickness of each layer constituting the n-type DBR layer 14 may be an odd multiple of the layer thickness t, i.e., 1t, 3t, 5t, .... The reflectance of the n-type DBR layer 14 can be changed by changing the composition of the InAlGaAs layer and the number of InAlGaAs / 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 layers 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 layers 14 is 101.6 nm. Therefore, the total thickness of the pair of n-type DBR layers 14 is 197.8 nm.

[0168] The number of InAlGaAs / InP pairs in the n-type DBR layer 14 is preferably 2 to 20, and the total thickness of the n-type DBR layer 14 is preferably 0.4 μm 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 types of layers to be stacked, the more pairs are required.

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

[0170] <Function of Second Embodiment> The semiconductor photodetector 180 according to the second embodiment achieves the same functions and effects as the semiconductor photodetector 100 according to the first embodiment, and also achieves 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 incident through the anti-reflection coating 40 reaches the n-type DBR layer 14. The n-type DBR layer 14 reflects light, but transmits wavelengths that satisfy the resonance conditions of each layer located above the n-type DBR layer 14. As a result, a vertical cavity is formed between the n-type DBR layer 14 and the p-type electrode 32, causing optical resonance.

[0172] The vertically 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 graph showing the dependency of the reflectivity and quantum efficiency of the DBR layer on the number of pairs of the DBR layer in a back-illuminated APD, which is an example of the semiconductor photodetector 180 according to the second embodiment. Here, the wavelength of incident light is 1300 nm, the refractive index n of the InAlGaAs layer is 3.38, the thickness is 96.2 nm, and the refractive index n of the InP layer is 3.20, the thickness is 101.6 nm. The refractive index of the i-type InGaAs light absorption layer 5 is 3.595, the thickness is 542 nm, and the absorption coefficient is 12716 / cm. The reflectivity of the two-dimensional periodic structure 70 and the p-type electrode 32 is 80%. In other words, the two-dimensional periodic structure 70 and the p-type electrode 32 function as a mirror with a reflectivity 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 were placed between the n-type DBR layer 14 and a mirror with a reflectivity of 80%. In Fig. 23, the vertical axis represents the reflectivity 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 , the reflectivity of the n-type DBR layer 14 increases monotonically as the number of pairs in the n-type DBR layer 14 increases. 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-type DBR layer 14 exceeds 8 pairs, the amount of light transmitted through the n-type DBR layer 14 decreases, and therefore the amount of light resonating in the cavity formed by the n-type DBR layer 14, the two-dimensional periodic structure 70, and the p-type 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 but is transmitted through. Therefore, the semiconductor light-receiving element described in Patent Document 2 does not have a vertical cavity formed by the two-dimensional periodic structure 70 and the p-type electrode 32 and n-type DBR layer 14 as disclosed in the present disclosure, and therefore does not achieve improved reception sensitivity due to optical resonance in the vertical direction.

[0178] <Effects of Second Embodiment> As described above, the semiconductor photodetector according to the second embodiment has an n-type DBR layer below the multiplication layer, a two-dimensional periodic structure in the second window layer, and a vertical cavity formed by covering the two-dimensional periodic structure with a SiN surface protective insulating film and a p-type electrode. This provides the advantage of providing a semiconductor photodetector with high reception sensitivity by allowing light to be incident from the semiconductor substrate side. Furthermore, by applying an InAs / AlAs digital alloy structure to the multiplication layer, the amount of light confined in the light absorption layer is increased, resulting in complex resonance that enables high reception sensitivity over a wide wavelength range, and a semiconductor photodetector 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.

[0179] Modification 1 of Second Embodiment. Figure 24 is a cross-sectional view showing the device structure of a back-illuminated PD, which is an example of a semiconductor photodetector 190 according to Modification 1 of Second Embodiment. The configuration in which the n-type DBR layer 14 is provided below the multiplication layer 3 is also possible in Modifications 1 to 6 of First Embodiment, and the same functions and effects are obtained. The semiconductor photodetector 190 according to Modification 1 of Second Embodiment is characterized in that the i-type InAs / AlAs digital alloy structure multiplication layer 3 and the p-type InP field relaxation layer 4 are removed from the semiconductor photodetector 180 according to Second Embodiment to form a PD. The semiconductor photodetector 190 according to Modification 1 of Second Embodiment also achieves substantially the same effects as the semiconductor photodetector 180 according to Second Embodiment.

[0180] Modification 2 of Embodiment 2. Fig. 25 is a cross-sectional view showing the device 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] 26 is a cross-sectional view showing the device 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 210 according to Modification 3 of Embodiment 2 are reversed, with the substrate side being p-type and the front surface side being n-type, compared to the semiconductor light-receiving element 200 according to Modification 2 of Embodiment 2.

[0182] <Structure of Semiconductor Photodetector According to Modification 3 of Second Embodiment> A back-illuminated APD, which is an example of a semiconductor photodetector 210 according to Modification 3 of 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 carrier concentration of 1 to 5×10, which are sequentially formed on the Fe-doped semi-insulating InP substrate 1a. 18 cm -3 The 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 absorption layer 5 having a thickness of 0.1 to 2.0 μm; and the carrier concentration of 0.1 to 50×10 17 cm -3a p-type InP field relaxation layer 4 having a thickness of 10 to 70 nm; an i-type InAs / AlAs digital alloy structure multiplication layer 3 formed by alternately stacking multiple i-type AlAs layers (for example, two atomic layers, about 0.6 nm thick) and i-type InAs layers (for example, two atomic layers, about 0.6 nm thick); an n-type InP first window layer 7e having a thickness of 0.1 to 3.0 μm; an n-type InAlAs second window layer 8c having a 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 two-dimensionally arranged in the n-type InAlAs second window layer 8c. the n-type InGaAs contact layer 9c and the SiN surface protective insulating film 10a surrounded by the n-type InGaAs contact layer 9c; and an anti-reflection film 40 formed on the back surface of the Fe-doped semi-insulating InP substrate 1a.

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

[0184] <Method for Manufacturing a Semiconductor Photodetector According to Modification 3 of Embodiment 2> A p-type InP buffer layer 2b is crystal-grown to a thickness of 0.1 to 1 μm on an Fe-doped semi-insulating InP substrate 1a. The carrier concentration is 1 to 5×10 18 cm -3 The p-type DBR layer 14b is formed by crystal growth of p-type InAlGaAs layers and p-type InP layers alternately stacked in this order. The i-type InAlGaAs / InAlAs graded layer 6, i-type InGaAs light absorption layer 5, p-type InP field relaxation layer 4, i-type InAs / AlAs digital alloy structure multiplication layer 3, n-type InP first window layer 7e, n-type InAlAs second window layer 8c, and n-type InGaAs contact layer 9c are successively formed by crystal growth 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 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 those of the semiconductor light-receiving element 150 according to the fifth modification of the first embodiment, but the 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 outer periphery of the ring-shaped n-type InGaAs contact layer 9c is etched away 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 down to 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. Furthermore, 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 Second Embodiment> N-type semiconductors have less optical absorption loss than p-type semiconductors. In semiconductor photodetector 210 according to Modification 3 of Second Embodiment, the conductivity type is upside down compared to semiconductor photodetector 200 according to Modification 2 of Second Embodiment. Therefore, the first window layer in which the resonance point is formed is n-type instead of p-type, which has the excellent effect of reducing optical absorption loss in n-type InP first window layer 7e.

[0190] As described above, in the semiconductor photodetector according to 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, which reduces the light absorption loss caused by the first window layer and thereby provides an advantage of realizing a semiconductor photodetector with high reception sensitivity. Therefore, it is possible to obtain an APD with sufficient reception sensitivity even in applications requiring a response bandwidth of 25 Gbps or more.

[0191] In each of the above-described embodiments and modifications, the multiplication layer has been described as having 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-described semiconductor light-receiving elements.

[0192] If a wideband APD of 37.5 GHz or more can be realized, next-generation high-speed PON systems can be realized without using DSPs and SOAs. In the case of PDs, where it is relatively easy to widen the response bandwidth, the response bandwidth is limited by: (1) the RC time constant (R is the element resistance, and C is the element capacitance) and (2) the carrier transit time (the time it takes for electrons or holes to transit through the depletion layer). In APDs, the response bandwidth is further limited by: (3) the multiplication time (the time it takes for electrons and holes to multiply in a chain reaction within the multiplication layer, which increases in proportion to the multiplication factor).

[0193] While a PD can achieve the above-mentioned 37.5 GHz bandwidth, an APD requires a multiplication time, making it difficult to achieve the desired bandwidth if the multiplication factor is increased. The multiplication time TM is expressed by the following equations (4) to (6): Multiplication time TM = multiplication factor M / GB product (4) GB product = 1 / (2πNkτav) (5) That is, multiplication time TM = 2πNkMτav (6)

[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 it takes for electrons and holes to travel through the multiplication layer. Therefore, by reducing the ionization rate ratio k, it is possible to shorten the multiplication time TM. 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. Furthermore, in order to reduce the ionization rate ratio k, a digital alloy structure has been proposed in which semiconductor layers of different compositions are alternately stacked in a cycle of 1 to 6 atomic layers. However, even in the digital alloy structure, it is difficult to make the ionization rate ratio k zero 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 formed by alternately stacking two-atom InAs layers and two-atom AlAs layers, and analyzed the multiplication characteristics. As a result, they discovered that the distance that carriers travel through the multiplication layer to become ionized is longer than that of an APD with an InAlAs multiplication layer made of a normal bulk crystal, i.e., an InAlAs random alloy structure multiplication layer. The distance that carriers travel through the multiplication layer to become ionized is called the dead space.

[0197] Because 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 typical bulk crystal, when the multiplication layer is thinned to a level of several tens of nanometers, the holes cannot be ionized, resulting in a decrease in the ionization rate ratio k. However, when the multiplication layer is thinned to a level of several tens of nanometers, a new problem arises: a higher electric field must be applied to the multiplication layer to obtain the desired multiplication factor, which increases leakage currents such as tunnel currents. In other words, an increase in tunnel current increases noise generated in the APD. On the other hand, the inventors' analysis discovered that, because the dead space is unusually large in the digital alloy structure compared to the random alloy structure, the ionization rate ratio k = 0 even when the multiplication layer is 100 nm or thick.

[0198] In other words, the inventors have discovered for the first time that by constructing the multiplication layer of an APD with a digital alloy structure, it is possible to achieve an ionization rate ratio k = 0 while suppressing the tunneling current. Specifically, they have found that in a 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 that the dead space effect is dramatically improved particularly when the multiplication layer thickness is in the range of 60 to 130 nm. In other words, 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 a multiplication layer having the digital alloy structure of the present disclosure. At present, no research institute has reported that thinning the multiplication layer of an APD having a digital alloy structure multiplication layer is more effective in reducing the ionization rate ratio k than thinning the multiplication layer of an APD made of conventional materials.

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

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

[0201] The thickness of the i-type InAs / AlAs digital alloy structure multiplication layer is in the 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 in the range of 40 nm to 170 nm. Furthermore, considering the typical degree of variation of 20% in layer thickness during fabrication of the semiconductor light receiving element 100, the thickness of the i-type InAs / AlAs digital alloy structure multiplication layer is more preferably in the 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 discovered that using a digital alloy structure multiplication layer, as in the APDs according to the first and second embodiments, enhances the dead space effect, i.e., the effect of reducing the ionization rate ratio k. FIG. 27 is a graph showing the electric field dependence of the electron dead space in an InAlAs multiplication layer. The inventors analyzed the electron multiplication characteristics of the digital alloy structure multiplication layer and found that, as shown in the graph in 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 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, which leads to a rapid increase in tunnel current and an increase in noise.

[0204] On the other hand, in the InAs / AlAs digital alloy structure 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 voltage of 1.5 V / V, the dead space length is approximately 85 nm, and therefore the ionization rate ratio k can be made close to zero even if the thickness of the multiplication layer is approximately 1.5 times the dead space (approximately 130 nm). Therefore, the effect of the tunnel current is small in the APDs according to the first and second embodiments.

[0205] In addition, in the multiplication layer with an InAs / AlAs digital alloy structure, 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 -6 In the case of a multiplication layer having a dead space length of about 50 nm, the thickness of the multiplication layer must be reduced to 75 nm, as shown in the graph of Fig. 27. In other words, the thickness of the multiplication layer having an InAs / AlAs digital alloy structure can be made thicker than that of the multiplication layer having an InAlAs random alloy structure.

[0206] Fig. 29 is a graph 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, 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 Figure 29, in an InAlAs random alloy multiplication layer, the effect of reducing the ionization rate ratio k due to dead space cannot be achieved unless the multiplication layer thickness is 80 nm or less. On the other hand, if the multiplication layer thickness is thinner than 80 nm, the tunneling current increases sharply, resulting in tunnel breakdown. When the multiplication layer thickness is around 60 nm, both the reduction in the ionization rate ratio k and the limitation of the tunneling current are barely achieved, but the thickness margin is only a few nanometers, making it extremely difficult to reliably manufacture APDs. Furthermore, the ionization rate ratio k is also large, at 0.12. In other words, with a conventional InAlAs random alloy multiplication layer, it is difficult to apply the effect of reducing the ionization rate ratio k by thinning the layer 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 multiplication factor range of 1 to 10. For the same ionization rate ratio k, the layer 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 in FIG. 29 , if the lower limit of the thickness of the multiplication layer for which the tunnel current is 1 μA in an APD with a pn junction diameter of 20 μm is 40 nm, the layer thickness range of 40 nm to 170 nm is the optimum range for an InAs / AlAs digital alloy structure multiplication layer, and layer thicknesses within this range can be fabricated with sufficient reproducibility.

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

[0212] Furthermore, in order to control the ionization rate ratio k to 0.05 or less in an 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 Figure 29. Furthermore, in order to achieve a tunnel current of 1 μA or less and an ionization rate ratio k of approximately zero, the thickness of the multiplication layer is optimally in the range of 60 nm to 130 nm. If a margin of 10 nm is set during the fabrication of an APD, the thickness of the multiplication layer is preferably set in the range of 70 nm to 120 nm.

[0213] Furthermore, as shown in Figure 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, calculated by dividing the minimum dead space length of 50 nm by the maximum multiplication layer thickness of 170 nm. As the ratio increases, the ionization rate ratio k decreases, but it cannot exceed 100%. This is because multiplication no longer occurs 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 but less than 100%. Furthermore, since the multiplication layer thickness in this prototype was 120 nm, the experimentally confirmed optimal range was 42% (= 50 nm / 120 nm) to 75% (= 90 nm / 120 nm).

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

[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 equations (7) and (8). Note that equation (7) represents the condition for the difference in dead space length, and equation (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 to not affect noise, and the thicker the multiplication layer, the more the tunnel current decreases. As shown in the conceptual diagram of Figure 28C, the condition for the difference in dead space length is set as shown in the above-mentioned formula (8) because when the layer thickness of the multiplication layer is equal to or less than the dead space length of holes, holes are no longer multiplied and the ionization rate ratio k = 0.

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

[0218] On the other hand, in the case of an InAs / AlAs digital alloy structure multiplication layer, the values ​​at which the ionization rate ratio k begins to decrease as the multiplication layer is made thinner are De of approximately 80 nm and Dh of approximately 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 is a multiplication layer thickness that satisfies the condition that 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 both are the same.

[0219] Specifically, the inventors found that 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, 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 field relaxation layer, i.e., 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 rates in the multiplication layer and the electric field relaxation layer, where Fig. 30A is for an InAlAs random alloy structure multiplication layer, Fig. 30B is for an InAs / AlAs digital alloy structure multiplication layer, Fig. 30C is for a partially disordered InAs / AlAs digital alloy structure multiplication layer, and Fig. 30D is a conceptual diagram showing the ionization rates when a thick electric field relaxation layer and an InAs / AlAs digital alloy structure multiplication layer are combined. Compared to the InAlAs random alloy structure multiplication layer shown in Fig. 30A, the InAs / AlAs digital alloy structure multiplication layer shown in Fig. 30B has a longer dead space length, but due to dopant diffusion from the electric field relaxation layer, the partially disordered InAs / AlAs digital alloy structure multiplication layer has a shorter dead space length, 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 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 equation (9), N is the impurity concentration, t is time, D is the diffusion constant, x is position, and F is the external force acting on the diffusion. Examples of materials for the field buffer layer include InP, an InAlAs random alloy structure, and an InAs / AlAs digital alloy structure. Examples of p-type dopants for the field buffer layer include Be and Zn. Considering the p-type dopant, a combination of a Be-doped p-type InP 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 itself and the InAs / AlAs digital alloy structure multiplication layer. The potential barrier corresponds to F in equation (9).

[0224] When the thickness of the electric field relaxation layer varies, the carrier concentration of the electric field relaxation layer is set to 2×10 so that the variation in the amount of electric field relaxation, that is, the variation in the product of the layer thickness and the carrier concentration, does not increase. 18 cm -3 When InAlAs is used as the material for the field relaxation layer, Zn doping is optimal, and the carrier concentration is 2×10 18 cm -3 The optimum is 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=WqN / ε (10) In formula (10), W is the layer 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. When the electric field relaxation amount ΔE is constant, if the carrier concentration of the electric field relaxation layer is increased, the layer 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 Figure 30D, when the thickness of the electric field buffer layer becomes 1.5 times or more the dead space length of the electric field buffer layer, multiplication occurs in the electric field buffer layer. As shown in Figure 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 must 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 must 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] <Effects of the Digital Alloy Structure Multiplication Layer of the Semiconductor Photodetector (APD) According to the First and Second Embodiments> First, a first effect of the semiconductor photodetector 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 equation (11), where frc is the bandwidth limit due to the RC time constant, ftr is the bandwidth limited by the carrier transit time, and fm is the bandwidth limit due to the multiplication time. 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 therefore is 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 equation (12), the transit time ftr of the carriers includes the transit time in the light absorption layer plus the transit time in 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. That is, the maximum bandwidth occurs when frc = ftr. Substituting frc = ftr, Equation (12) is expressed as the following Equation (13): fc = ftr / √2 (13)

[0232] The 3 dB bandwidth ftr determined by the transit time is expressed by the following equation (14): ftr=3.5Vav / (2πWt) (14)

[0233] In equation (14), Vav is the average saturated transit 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 Furthermore, 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 the calculated ftr into equation (14), ftr = 59.6 GHz. Furthermore, substituting the calculated ftr into equation (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 considerations 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 descriptions of devices, systems, etc., the APD having the InAs / AlAs digital alloy structure multiplication layer according to the first and second embodiments of the present disclosure will be 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 therefore 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 photodetector. However, the semiconductor photodetectors according to the first and second embodiments exhibit excellent device characteristics as a semiconductor photodetector even when the multiplication layer has an InAlAs random alloy structure.

[0237] 31 is a cross-sectional view showing the device 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 is 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 can further reduce noise and have a wide bandwidth can be obtained.

[0239] 32 is a cross-sectional view showing the device 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] Although the semiconductor photodetector 230 according to the first modification of the third embodiment has a multiplication layer formed of an InAlAs random alloy structure, a lattice-like two-dimensional periodic structure 70a is provided in the SiN surface protective insulating film 10a on the i-type InP window layer 7c. Light is incident from the semiconductor substrate side, and the light is internally reflected by the SiN surface protective insulating film 10a and the p-type electrode 32 covering the semiconductor layer. This complex resonance enables the semiconductor photodetector to have high reception sensitivity over a wide wavelength range, as well as low noise and a wide bandwidth. Additionally, the depth (or height, if the hole is considered the remaining portion) of the holes 11a formed in the SiN surface protective insulating film 10a is more uniform than in the semiconductor photodetector 220 according to the third embodiment, and the refractive index difference can be increased, which facilitates optical resonance and improves light absorption efficiency. As a result, a semiconductor photodetector with high reception sensitivity, low noise, and a wide bandwidth can be realized.

[0241] 33 is a cross-sectional view showing the device 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] Although the semiconductor light-receiving element 240 according to the second modification of the third embodiment has a multiplication layer formed of an InAlAs random alloy structure, a lattice-like two-dimensional periodic structure 70b is provided on the p-type electrode 32a on the SiN surface protective insulating film 10, and light is incident from the semiconductor substrate side. This improves the reception sensitivity of the semiconductor light-receiving element over a wide wavelength range due to complex resonance. Additionally, the depth (or height, if the hole is considered the remaining portion) of the hole 11b provided in the p-type electrode 32a is more uniform than in the semiconductor light-receiving element 220 according to the third embodiment, and the refractive index difference can be increased, making it easier for optical resonance to occur, thereby improving the light absorption efficiency. As a result, a semiconductor light-receiving element with high reception sensitivity, low noise, and a wide bandwidth can be realized.

[0243] 34 is a cross-sectional view showing the device 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 photodetector 245 according to the third modification of the third embodiment, although the multiplication layer is constructed with an InAlAs random alloy structure, a lattice-like two-dimensional periodic structure 70c is provided in the p-type InGaAs contact layer 9a on the i-type InP window layer 7c, and light is incident from the semiconductor substrate side. This allows for complex resonance, thereby enabling the semiconductor photodetector to have high reception sensitivity over a wide wavelength range, as well as low noise and a wide bandwidth. In addition, the depth (or height, if the hole is considered the remaining portion) of the holes 11c provided in the p-type InGaAs contact layer 9a is more uniform than in the semiconductor photodetector 220 according to the third embodiment, making resonance more likely to occur and improving light absorption efficiency. As a result, a semiconductor photodetector with high reception sensitivity, low noise, and a wide bandwidth can be realized.

[0245] 35 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor photodetector 247 according to a fourth modification of the third embodiment. The semiconductor photodetector 247 according to the fourth modification of the third embodiment differs from the semiconductor photodetector 180 according to the second embodiment 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, although the multiplication layer is constructed with an InAlAs random alloy structure, 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] 36 is a configuration diagram showing an optical line terminal (OLT) 260 of a 50G-PON system according to embodiment 4. The optical line terminal 260 includes an FEC (Forward Error Correction) 261, a driver amplifier 262, a light source 263, a WDM (Wavelength Division Multiplexing) 264, a CDR (Clock Data Recovery) 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] The DA-APD of the present disclosure refers to an APD in which a lattice-like two-dimensional periodic structure is provided above the light absorption layer and the multiplication layer has an InAs / AlAs digital alloy structure, as described in the above-mentioned first embodiment, modifications 1 to 7 of the first embodiment, second embodiment, and modifications 2 and 3 of the second embodiment.

[0249] 37 is a configuration diagram illustrating an optical network unit (ONU) of a 50G-PON system according to embodiment 4. The optical network unit 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 a comparative example includes a forward error correction (FEC) circuit (FEC) 251, a driver amplifier 252, a light source 253, a wavelength division multiplexing (WDM) circuit (WDM) 254, a digital signal processing circuit (DSP) 255, an analog-to-digital converter (ADC) 256, an analog-to-digital conversion circuit (ADC), a burst TIA 257, and a conventional APD 258.

[0251] As shown in the optical line terminal 250a of a 50G-PON system, which is a comparative example, shown in Figure 38, the 50G-PON system, which is a comparative example, required digital bandwidth compensation, that is, a DSP 255. On the other hand, in a 50G-PON system using the DA-APD of the present disclosure, digital bandwidth compensation is not required. In other words, as shown in the optical line terminal (ONU) of a 50G-PON system according to embodiment 4 shown in Figure 37, by using the DA-APD 275 of the present disclosure, a wide response band and high receiving sensitivity are possible, which enables simplification of the DSP circuit, power saving, and reduction of the output power of the SOA.

[0252] In order to increase the number of branches in a PON system and eliminate the need for an SOA, it is necessary to improve the S / N 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 S / N ratio must be improved by at least 3 dB. The S / N ratio of a receiver using an APD is expressed by the following equation (15): S / N ratio = Iph 2 ・M 2 / (2q(Iph+Id)M 2 ・F・B +4Kb・T・Ft・B / Rt) (15)

[0253] In equation (15), I is the photocurrent of the APD, M is the multiplication factor, q is the unit charge, I is the dark current to be multiplied, F is the excess noise factor of the APD, B is the bandwidth, K is the Boltzmann constant, T is the absolute temperature, F is the noise figure of the amplifier, and R is the input resistance. The left term of the denominator represents the shot noise of the APD, and the right term 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 multiplication factor is set to maximize the S / N ratio. If we replace the amplifier thermal noise term with the APD shot noise term, the S / N ratio can be expressed by the following equation (16): S / N ratio=Iph / (4q F B) (16)

[0255] The excess noise factor F is given by the following equation (17): F = M(1-(1-k) x ((M-1) 2 / M2 )) (17)

[0256] In the case of a conventional InAlAs random alloy multiplication layer, thinning it to the point where the dead space effect appears (approximately 70 nm) increases the tunnel current and deteriorates the noise characteristics, as described above. To minimize the tunnel current, the pn junction diameter must be reduced to a few microns or less, but this requires strict alignment precision when focusing light onto the APD. While the noise characteristics are inferior to those of a digital alloy multiplication layer, a random alloy multiplication layer can achieve some noise reduction by thinning the multiplication layer in this way. However, the small pn junction diameter imposes limitations on productivity and other factors. For this reason, systems are designed with the ionization rate ratio k set to 0.2 for an unthinned InAlAs multiplication layer. When the ionization rate ratio k = 0.2 and the multiplication factor is 12x, the excess noise factor 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 multiplication layer, i.e., the DA-APD of the present disclosure, is used as the semiconductor light-receiving element. In the case of the InAs / AlAs digital alloy structure multiplication layer of the present disclosure, the dead space effect functions even with a layer thickness of 100 nm or more, making it possible to apply it 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 approximately half that of a conventional APD. As a result, the DA-APD of the present disclosure improves the S / N ratio by 3 dB.

[0258] Generally, in a 50G-PON system, inserting one stage of a 2-stage demultiplexer to increase the number of branches increases the loss by 3 dB. Therefore, by applying the DA-APD of the present disclosure as a semiconductor photodetector, it becomes possible to insert one more stage of demultiplexer into the 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. An 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 network unit (ONU) of a 50G-PON system according to embodiment 4. The optical network unit 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 effects of the semiconductor photodetector according to the present disclosure will now be further explained. In an APD having an InAs / AlAs digital alloy structure multiplication layer according to the present disclosure, the multiplication time in equation (6) becomes nearly zero by controlling the thickness of the multiplication layer within a predetermined range and setting the ionization rate ratio k to zero. As a result, the response bandwidth of the APD does not deteriorate even when the multiplication factor is increased. In other words, in the DA-APD according to the present disclosure, the response bandwidth is limited only by the RC time constant and the carrier transit time, as in conventional PDs. This makes it possible to widen the response bandwidth required for 50G-PON systems, enabling reception without digital bandwidth compensation by a DSP.

[0262] Furthermore, when the ionization rate ratio k approaches zero, excess noise that deteriorates receiver sensitivity is suppressed, eliminating the need for optical signal amplification by an SOA. Furthermore, even in PON systems other than 50G-PON systems, it becomes possible to achieve a higher number of branches than before. As a result, PON systems can be made lower cost and more energy-efficient.

[0263] <Effects of Fourth Embodiment> As described above, according to the optical line terminal of the fourth embodiment, the DA-APD of the present disclosure is used as the semiconductor photodetector, and therefore an optical line terminal that can increase the transmission distance of optical signals and reduce power consumption can be obtained.

[0264] Embodiment 5 Fig. 41 is a diagram showing the configuration of a multi-level intensity modulation transmitting / receiving apparatus 300 according to embodiment 5. Also, Fig. 42A and Fig. 42B are conceptual diagrams showing 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 that uses PAM (Pulse Amplitude Modulation), which is a multilevel intensity modulation method. In the transmitting section, a digital signal generated by a DSP 301 is converted to an analog signal by a DAC 302a, amplified by 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] On the other hand, 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. After being amplified in the Linear-TIA 306, the light is 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 Embodiment 5> The PAM-based multilevel intensity modulation transmitter / receiver 300 is required to receive not only binary signals of 1 and 0 such as NRZ (None Return to Zero) and RZ (Return to Zero), but also four values ​​of different optical signal intensities, for example, 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 determine the quality of the received waveform in PAM4. TDECQ is calculated using 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 dependent 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), it is necessary that: (1) the eye opening at each level is uniform, and (2) the noise at each level is low.

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

[0270] Furthermore, PAM requires a good dynamic range because it must receive signals ranging from low to high intensity. In other words, even if the optical input power Pin increases, a good dynamic range can be achieved if the decrease in Iph / Pin is small. 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 degradation occurs 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 for the degradation of APD linearity. Figures 43A and 43B are conceptual diagrams 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 occurs, as if a voltage drop occurs due to 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 equation (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 Rsc can be expressed by the following equation (20), where Td is the time it takes for electrons and holes generated by light absorption to pass through the depletion layer: 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 Td it takes for electrons and holes to pass through the depletion layer. Therefore, it is possible to reduce the resistance Rsc by increasing the transport speed of electrons and holes and reducing Td.

[0276] In the DA-APD disclosed herein, the absorption of the light-absorbing layer is high due to optical resonance, making it possible to thin the light-absorbing layer, which in turn reduces the resistance Rsc. As a result, the eye opening becomes uniform, and TDECQ satisfies the specified value. Furthermore, it becomes possible to increase the transmission distance and reduce the drive current of the transmitting laser.

[0277] The use of the DA-APD of the present disclosure will be described below. First, the operation of an APD at high light input will be described. FIG. 44 is a conceptual diagram illustrating the operation of an 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. The occurrence of this space charge effect reduces the multiplication factor of the APD and deteriorates its linearity. As described above, the deterioration of the linearity of the APD is caused by the series resistance Rsc, so it is necessary to reduce the residence time Tdm of electrons and holes in the depletion layer. In particular, as 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 equation (21). Residence time Tdm = multiplication time = 2πNkMτav (21)

[0278] In equation (21), N is the Emmons coefficient (which is loosely dependent 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. N is 0.55, 0.83, 1.1, and 2.0 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), respectively.

[0279] Figure 45 shows the residence time Tdm of electrons and holes for each material constituting the multiplication layer. In the InAs / AlAs digital alloy structure multiplication layer, the residence time Tdm within 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, resulting in improved linearity and dynamic range in the InAs / AlAs digital alloy structure multiplication layer.

[0280] As a result, while the eye opening of PAM4 was non-uniform with a conventional APD as shown in the conceptual diagram of Fig. 42B, the eye opening is uniform with the DA-APD of the present disclosure 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 even in a PAM transceiver, which makes it possible to increase the transmission distance of optical signals and reduce the drive current of the transmitting laser.

[0281] <Effects of Fifth Embodiment> As described above, the multi-level intensity modulation transceiver according to the fifth embodiment uses the DA-APD of the present disclosure as the semiconductor light-receiving element, and therefore has the effect of providing a multi-level intensity modulation transceiver that can increase the transmission distance of optical signals and reduce power consumption.

[0282] Sixth Embodiment Fig. 46 is a schematic diagram showing the configuration of a radio-on-fiber (RoF) system 400 according to a 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 path 402 such as an optical fiber cable, a DA-APD 403 according to 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 according to 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 signals to an antenna 404 that is located at a distance from an electrical signal source. Furthermore, since no analog-to-digital or digital-to-analog conversion is performed during transmission, the system has the advantages of a simple configuration and low power consumption.

[0285] <Functions and Effects of the Radio-on-Fiber System According to the Sixth Embodiment> In the radio-on-fiber system 450 of the comparative example shown in FIG. 47, if the signal is attenuated during transmission through the optical fiber cable, the signal cannot be amplified by the PD 406, and therefore there is a problem that a sufficient radio wave 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, causing the multiplication factor to saturate and making it impossible to ensure the dynamic range. This not only results in an insufficient amplitude of the electrical signal, but also in distortion of the analog signal. As a result, it is difficult to apply a conventional APD to a radio-on-fiber system 450 such as 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 due to optical resonance is large, so it is possible to make the optical absorption layer thinner, and thus 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 it is possible to output a strong radio wave signal with good linearity even over a long optical transmission distance.

[0288] Furthermore, in the DA-APD 403 utilizing optical resonance according to the present disclosure, as shown in FIG. 45, the residence time Tdm of electrons and holes in the multiplication layer is short, thereby suppressing changes in the electric field distribution within the multiplication layer. As a result, a response with excellent linearity can be obtained over a wide dynamic range. In other words, since the DA-APD 403 according to 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-APD403 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 desirable to use it with a multiplication factor in the range of 1.2 to 5. Furthermore, considering the loss of the optical fiber and the fact that the quantum efficiency of the APD is about 80% rather than 100%, it is optimal to use it with a multiplication factor of 2 to 3 to compensate for such loss.

[0290] Effect of 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 that can output a strong radio wave signal even if the optical transmission distance is long.

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

[0292] In digital coherent communications, optical signals modulated in both phase and intensity 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 laser light are multiplexed, and the signal light is further separated into orthogonal components (I, Q) and output. The four optical signals, i.e., the orthogonal I and Q components for each polarization, total four optical signals, are incident on four balanced detectors 505 arranged in 90-degree hybrid devices 503a, 503b, each of which is made up of two DA-APDs 505a of the present disclosure connected in series. The electrical signals output from the balanced detectors 505 are input to the DSP 506. The digital coherent receiving device 500 according to the seventh embodiment has the above configuration.

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

[0295] Conventional balanced detectors use PDs as semiconductor light-receiving elements for receiving signal light. On the other hand, the DA-APD 505a disclosed herein can multiply signals, thereby enabling the local oscillation to be suppressed. Furthermore, when a conventional APD is used, as shown in the conceptual diagram of FIG. 44 , 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 an inability to ensure a sufficient dynamic range. This not only results in insufficient electrical signal amplitude, but also in distortion of the analog signal. As a result, as shown in the conceptual diagram of FIG. 49A , in the comparative example, the interval between waveforms A1 and B1 becomes narrow, distorting the intensity signal of the constellation waveform, making it difficult to apply an 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, which makes it 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, and therefore a constellation waveform with excellent linearity can be obtained over a wide dynamic range.

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

[0298] The multiplication factor of the DA-APD 505a of the present disclosure can be used within a range of 1.2 to 10. However, as the multiplication factor increases, the signal becomes distorted, so it is desirable to use a multiplication factor within a range of 1.2 to 5.

[0299] <Effects of 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 that receives an optical signal, and therefore it is 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] 50 is a schematic diagram showing the configuration of a SPAD sensor system according to embodiment 8. A SPAD sensor system 600 includes a photoelectron 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 photons but also as highly sensitive light-receiving elements. However, they require constant cycling from the Quenching voltage (described below) to the Geiger mode voltage (described below) (B). The cycling cycle is on the order of nanoseconds to microseconds. If the cycling cycle between the Quenching voltage (A) and the Geiger mode voltage (B) can be shortened, the response speed of the SPAD can be increased.

[0302] In other words, when the DA-APD of the present disclosure is used in a SPAD, it becomes possible to switch between A: Quenching voltage and B: Geiger mode voltage, which have a high response speed, and it is possible to improve the response band of the SPAD sensor 602.

[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, which is made up 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, electrons are 6 The generated electrons flow as a current and are sent to 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 according to a comparative example, and FIG. 51B is a conceptual diagram showing the multiplication characteristics of a SAPD sensor system according to embodiment 8. Continuous application of an electric field equal to or greater than the avalanche breakdown electric field to the multiplication layer causes excessive current to flow. Therefore, after photons are detected, the voltage applied to the SPAD sensor 602 is quickly reduced to weaken the electric field in the multiplication layer. This is called quenching. As shown in the comparison of the multiplication characteristics of the SPAD sensor in the conceptual diagrams of FIGS. 51A and 51B, the voltage is reduced from B: Geiger-mode voltage to A: quenching voltage to stop chain multiplication. Then, the voltage is increased again from A: quenching voltage to B: Geiger-mode voltage, enabling the sensor to receive incident photons with high sensitivity.

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

[0307] <Functions 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 to count 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] The passive quenching circuit 603 makes it possible to reduce the resistance value connected in series to the SPAD sensor 602, thereby increasing the response speed of the SPAD sensor 602. Furthermore, the active quenching circuit 603 reduces the voltage amplitude, making it possible to simplify the drive circuit and save power, and also to widen the response band.

[0309] In the DA-APD utilizing resonance of the present disclosure, the light absorption layer has a high absorption amount, making it possible to make the light absorption layer thinner. Thinning the light absorption layer reduces the resistance Rsc, which also reduces the breakdown voltage. Using the DA-APD of the present disclosure in a SPAD makes it possible to reduce the difference between the quenching voltage and the Geiger mode voltage, i.e., the applied voltage difference, thereby improving the response band and simplifying the quenching circuit and reducing power consumption.

[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 FIGS. 28A and 28B , the dead space length is long, so multiplication does not occur at low electric fields. However, as the electric field increases, the dead space length shortens, causing a rapid increase in the multiplication factor, leading to breakdown. In APDs with InAlAs random alloy structure multiplication layers and APDs with thick InAs / AlAs digital alloy structure multiplication layers, if the voltage at which the dark current exceeds 10 μA is defined 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 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 buffer 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 and the electric field of the multiplication layer are proportional.

[0312] 52 is a diagram showing the calculated 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 exceptionally low. The InAs / AlAs digital alloy structure multiplication layer (thinned DA) of the present disclosure has a superlattice structure similar to the InAs / AlAs digital alloy structure multiplication layer of the present disclosure, but has a layer thickness of 200 nm or more, and the electric field is 120 kV / cm lower than that of the non-thinned InAs / AlAs digital alloy structure multiplication layer.

[0313] <Effects of Eighth Embodiment> As described above, according to the SPAD sensor system of the eighth embodiment, the DA-APD of the present disclosure is used in the SPAD sensor, and therefore the difference between the quenching electric field and the Geiger mode electric field, that is, the applied voltage difference, can be reduced, thereby achieving the effect of obtaining a SPAD sensor system that enables an improvement in the response band, simplification of the quenching circuit, and power saving.

[0314] Embodiment 9. Fig. 53 is a diagram showing the configuration of a LIDAR (Light Detection and Ranging: LiDAR) device according to embodiment 9. Fig. 54A is a conceptual diagram showing the received waveform of the APD of a LIDAR device that is a comparative example, and Fig. 54B is a conceptual diagram showing the received waveform of the 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 according to 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 pulsed light emitted from a 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 light output of 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. Therefore, the receiving sensitivity of the semiconductor light-receiving element needs to be increased. Therefore, in the LIDAR device 700 according to the ninth embodiment, the DA-APD 702 according to the present disclosure is used as the semiconductor light-receiving element with a high multiplication factor.

[0317] The detected optical 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 as shown in the conceptual diagrams of Figures 54A and 54B, the time when the intensity of the pulse signal exceeds a preset discrimination line is determined to be the arrival time. The timing at which the optical pulse is emitted 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 them by the speed of light and dividing the result by 2. Another method that can be used is to emit frequency-modulated light and calculate the distance from the frequency difference between the emitted wave and the returning reflected wave.

[0318] <Functions and Effects of the LIDAR Device According to Embodiment 9> Because the reflectivity of the object 705 is not necessarily high and the reflection direction varies, it is necessary to detect weak light using an APD. With a conventional APD, as shown in the conceptual diagram of FIG. 54A , when a voltage is set to achieve high multiplication and the APD is operated, the multiplication time becomes long, and the current pulse width output from the APD becomes wide. Furthermore, the tunnel current increases, making it difficult to distinguish between light pulses. Even with a method of calculating distance from the frequency difference between the emitted wave and the reflected wave, frequency discrimination becomes difficult.

[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 the optical output of the light source 701, thereby saving power and improving safety for the eyes.

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

[0321] Effect of Ninth Embodiment As described above, according to the LIDAR device of the ninth embodiment, the reflected light from an object is received by the DA-APD of the present disclosure, which has the effect of enabling distance measurement of a distant object, enabling power consumption of the light source to be reduced, and further providing a LIDAR device that is highly safe for the eyes.

[0322] While 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 variations not illustrated are conceivable within the scope of the technology of the present disclosure, including, for example, cases where at least one component is modified, added, or omitted, and even cases where at least one component is extracted and combined with a component of another embodiment.

[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 field relaxation layer, 4 p-type InP 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 optical 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 light receiving element, 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 Multi-level intensity modulation transmitter / receiver, 302a DAC, 310, 501 Optical fiber cable, 306 Linear-TIA, 400, 450 Radio-on-fiber system, 402 Transmission line, 404 Antenna, 406 PD, 500 Digital coherent receiver, 501 Optical fiber cable,502 Polarization 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. A semiconductor light-receiving element comprising: a substrate; an n-type semiconductor layer formed on the 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 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 arranged two-dimensionally at a constant period; a surface protective insulating film at least formed on the two-dimensional periodic structure; and a surface electrode at least formed on the surface protective insulating film.

2. A semiconductor light-receiving element comprising: a substrate; an n-type semiconductor layer formed on the 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 and having a two-dimensional periodic structure in which a plurality of holes or remnants are arranged two-dimensionally at a constant period; and a surface electrode formed at least on the two-dimensional periodic structure.

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

4. A semiconductor light-receiving element comprising: a substrate; an n-type semiconductor layer formed on the 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 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 at least formed on the two-dimensional periodic structure; and a surface electrode at least formed on the surface protective insulating film.

5. A semiconductor light-receiving element comprising: a substrate; a p-type semiconductor layer formed on the substrate; a light absorption 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 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 at least formed on the two-dimensional periodic structure; and a surface electrode at least formed on the surface protective insulating film.

6. A semiconductor light-receiving element comprising: a substrate; a p-type semiconductor layer formed on the substrate; a light absorption 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 and having a two-dimensional periodic structure in which a plurality of holes or remnants are arranged two-dimensionally at a constant period; and a surface electrode formed at least on the two-dimensional periodic structure.

7. A semiconductor photodetector comprising: an InP substrate; a p-type semiconductor layer formed on the substrate; a light absorption 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; and a surface electrode formed on the surface protective insulating film, the surface electrode having a two-dimensional periodic structure in which a plurality of holes or remnants are arranged two-dimensionally at a constant period.

8. A semiconductor light-receiving element comprising: a substrate; a p-type semiconductor layer formed on the substrate; a light absorption 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 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 at least formed on the two-dimensional periodic structure; and a surface electrode at least formed on the surface protective insulating film.

9. The semiconductor light-receiving element according to claim 1 or 5, wherein the first window layer is composed of two or more types of semiconductor layers having different refractive indices.

10. The semiconductor light-receiving element according to claim 9, wherein the difference in refractive index between each of the two or more semiconductor layers is 0.05 or more and 0.25 or less.

11. The semiconductor light-receiving element according to any one of claims 2 to 4 and 6 to 8, wherein the window layer is composed of two or more types of semiconductor layers having different refractive indices.

12. The semiconductor light-receiving element according to claim 11, wherein the difference in refractive index between each of the two or more semiconductor layers is 0.05 or more and 0.25 or less.

13. The semiconductor photodetector according to any one of claims 1 to 4, wherein the n-type semiconductor layer is a DBR layer.

14. The semiconductor photodetector according to any one of claims 5 to 8, wherein the p-type semiconductor layer is a DBR layer.

15. The semiconductor photodetector according to claim 13 or 14, wherein the reflectance of the DBR layer is 1% or more and 40% or less.

16. The semiconductor photodetector according to any one of claims 13 to 15, characterized in that the number of pairs in the DBR layer is 2 pairs or more and 14 pairs or less.

17. A semiconductor photodetector according to any one of claims 1 to 4, characterized in that it has a mesa structure outside the outer periphery of the surface electrode, in which each semiconductor layer above the n-type semiconductor layer is removed.

18. A semiconductor light-receiving element according to any one of claims 5 to 8, characterized in that it has a mesa structure outside the outer periphery of the surface electrode, in which each semiconductor layer above the p-type semiconductor layer is removed.

19. The semiconductor light-receiving element according to claim 17 or 18, wherein a surface protective insulating film and an electrode are provided on the side surface of the mesa structure.

20. The semiconductor light-receiving element according to claim 19, wherein an anti-reflection film is provided on the side surface of said mesa structure.

21. The semiconductor photodetector according to any one of claims 1 to 20, wherein the multiplication layer has a digital alloy structure.

22. The semiconductor photodetector according to claim 21, 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.

23. The semiconductor photodetector according to claim 22, 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.

24. The semiconductor photodetector according to claim 21, 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.

25. A semiconductor light-receiving element according to any one of claims 1 to 24, characterized in that the period of the holes or remaining portions is 347 nm or more and 571 nm or less.

26. A semiconductor light-receiving element according to any one of claims 1 to 25, wherein the diameter of the opening or remainder of the hole is 35 nm or more and 343 nm or less.

27. A semiconductor light-receiving element comprising: a substrate; an n-type semiconductor layer formed on the substrate; a light-absorbing layer formed on the n-type semiconductor 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 at least formed on the two-dimensional periodic structure; and a surface electrode at least formed on the surface protective insulating film.

28. An optical line termination device comprising: a semiconductor photodetector according to any one of claims 1 to 27; an optical multiplexer / demultiplexer for inputting an optical signal to said semiconductor photodetector; an amplifier circuit for amplifying an electrical signal output from said semiconductor photodetector; a clock and data recovery circuit connected to said amplifier circuit for recovering clock data from said amplified electrical signal; and a forward error correction circuit connected to said clock and data recovery circuit for correcting errors in said clock data.

29. An optical line termination device comprising: a semiconductor photodetector according to any one of claims 1 to 27; an optical multiplexer / demultiplexer that inputs an optical signal to the semiconductor photodetector; an amplifier circuit that amplifies an electrical signal output from the semiconductor photodetector; an analog / digital conversion circuit connected to the amplifier circuit that converts the amplified electrical signal into a digital signal; a digital signal processing circuit connected to the analog / digital conversion circuit that processes the digital signal; and a forward error correction circuit connected to the digital signal processing circuit that corrects errors in the digital signal.

30. A multi-level intensity modulation transmitting / receiving device comprising: a semiconductor photodetector according to any one of claims 1 to 27, which receives a multi-level intensity modulated optical signal; an amplifier circuit which amplifies an electrical signal output from the semiconductor photodetector; an analog / digital conversion circuit connected to the amplifier circuit, which converts the amplified electrical signal into a digital signal; and a digital signal processing circuit connected to the analog / digital conversion circuit, which processes the digital signal.

31. A radio-on-fiber system comprising: a light source that emits an analog-modulated optical signal; a semiconductor photodetector according to any one of claims 1 to 27 that receives the analog-modulated optical signal; a transmission path that transmits the analog electrical signal output from the semiconductor photodetector to an antenna; and an antenna connected to the transmission path that radiates the analog electrical signal as a radio wave signal.

32. A digital coherent receiving device comprising: a semiconductor photodetector according to any one of claims 1 to 27; a polarization separator that separates the polarizations of a polarization multiplexed optical signal whose intensity and phase are modulated; a 90-degree hybrid that splits and combines the optical signals output from the polarization separator; and a digital signal processing circuit connected to the 90-degree hybrid that processes digital signals.

33. A SPAD sensor system comprising: a SPAD sensor constructed using a semiconductor light-receiving element according to any one of claims 1 to 26; a quenching circuit that repeatedly applies a voltage equal to or greater than the breakdown voltage and a voltage less than the breakdown voltage to said SPAD sensor; and an optoelectronic measurement circuit that measures the electrical signal output from said SPAD sensor.

34. A lidar device comprising: a light source that emits pulsed light or frequency-modulated light; a semiconductor light-receiving element according to any one of claims 1 to 27 that receives light that is 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; and a distance measuring circuit that calculates distance based on the electrical signal amplified by the amplifier circuit.

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