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 (110) according to the present disclosure comprises: an n-type InP substrate (1); an n-type semiconductor layer (2a) formed on the n-type InP substrate (1); a multiplication layer (13) formed on the n-type semiconductor layer (2a); a p-type electric field relaxation layer (14) formed on the multiplication layer (13); and a light absorption layer (13) formed on the p-type electric field relaxation layer (14) and comprising an InAs / GaAs digital alloy structure with alternating layers of InAs layers and GaAs layers stacked at a period of two to six atomic layers.

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-A-3-050875, JP-A-1-255282, JP-A-6-097483

[0006] 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 PDs and APDs: PDs and APDs used in high-speed optical communications use InGaAs as the light absorption layer, which has a high absorption coefficient in the 1.3 μm and 1.55 μm wavelength bands used in optical communications. For example, in the 1.3 μm band, a high absorption coefficient of 10,000 / cm or more can be achieved.

[0008] In order to broaden the response band of PDs and APDs, it is necessary to reduce the thickness of the InGaAs layer that constitutes the light absorption layer and shorten the carrier transit time. However, 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 α 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(−αW) (1)

[0010] In equation (1), for example, if α = 10000 / 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 When the wave length is cm / s and W=1 μm, substituting these values ​​into equation (2) gives ftr=29.8 GHz.

[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 coefficient of the light absorption layer.

[0014] (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 αe, and the ionization rate of holes is expressed as βh.

[0015] 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.

[0016] The ionization rate ratio k of electrons and holes is defined as k = βh / αe. 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.

[0017] When InAlAs is selected as the material for the multiplication layer, the difference between the ionization rates of electrons and holes is greater than when InP is used. In InP, the ionization rate of holes is greater 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 greater 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.

[0018] As mentioned above, in a PON system, APDs, which are semiconductor light-receiving elements, are required to have a wide response band and high receiving sensitivity. However, unlike PDs, APDs have a problem in that the time required for multiplication, i.e., the multiplication time, increases as the multiplication factor increases, resulting in a decrease in the response band at high multiplication factors.

[0019] 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.

[0020] As mentioned above, PDs and APDs, which are semiconductor light-receiving elements used in optical communications, are required to operate over an even wider response band. Patent Publication 1 describes an APD using a superlattice, but the superlattice is applied to the multiplication layer and the electric field relaxation layer, not the electron transit layer. Furthermore, because each layer 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 exceeds several nanometers, energy unevenness reflecting the band gap of each layer occurs, hindering carrier transit and reducing the transit speed.

[0021] In a 50G-PON system, the response bandwidth of the semiconductor light-emitting element and the semiconductor light-receiving element, 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] The present disclosure has been made to solve the above-mentioned problems, and has an object to provide a semiconductor light-receiving element that operates over a wide response band and has high receiving sensitivity.

[0027] The semiconductor light-receiving element according to the present disclosure comprises an InP substrate, an n-type semiconductor layer formed on the InP substrate, a multiplication layer formed on the n-type semiconductor layer, a p-type electric field buffer layer formed on the multiplication layer, and a light-absorbing layer formed on the p-type electric field buffer layer and having a digital alloy structure.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] According to the semiconductor light receiving element according to the present disclosure, at least the light absorption layer has a digital alloy structure, and therefore, an effect is achieved in that a semiconductor light receiving element that operates in a wide response band and has high receiving sensitivity is obtained.

[0035] 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.

[0036] 9A to 9C are diagrams showing the optical absorption spectra of an InAs / AlAs digital alloy structure and an InAlAs random alloy structure.

[0033] FIG. 9B is a diagram showing the relationship between the lattice constant of each constituent material and the amount of strain relative to InP.

[0034] FIG. 9C is a diagram showing the results of estimating the effects on the optical absorption spectra of an InAs / GaAs digital alloy structure and an InGaAs random alloy structure.

[0035] FIG. 9B is a cross-sectional view showing the device structure of a front-illuminated PD, which is an example of a semiconductor light-receiving element according to the first embodiment.

[0036] FIG. 9C is a cross-sectional view showing the device structure of an edge-illuminated PD, which is an example of a semiconductor light-receiving element according to the first embodiment.

[0037] FIG. 9C is a cross-sectional view showing the device structure of a front-illuminated APD, which is an example of a semiconductor light-receiving element according to the second embodiment.

[0038] FIG. 9C is a diagram showing the electric field dependence of the electron dead space in an InAlAs multiplication layer.

[0039] FIG. 9A to 9C are diagrams showing the ionization rates of electrons and holes.

[0039] FIG. 9C is a diagram showing the layer thickness dependence of the ionization rate ratio and the tunnel current. 11A to 11D are diagrams showing the ionization rates in the multiplication layer and the electric field relaxation layer, with FIG. 11A being for a random alloy structure multiplication layer, FIG. 11B being for a digital alloy structure multiplication layer, FIG. 11C being for a partially disordered digital alloy structure multiplication layer, and FIG. 11D being for a combination of a thick electric field relaxation layer and a digital alloy structure multiplication layer. It is a cross-sectional view showing the element structure of a front-illuminated PD, which is an example of a semiconductor light-receiving element according to a third embodiment. It is a cross-sectional view showing the element structure of a front-illuminated PD, which is an example of a semiconductor light-receiving element according to the third embodiment. It is a cross-sectional view showing the element structure of a front-illuminated APD, which is an example of a semiconductor light-receiving element according to a fourth ... fifth embodiment. It is 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 sixth embodiment. Fig. 10 is a cross-sectional view showing the element structure of a front-illuminated PD which is an example of a semiconductor light-receiving element according to embodiment 7. Fig. 11 is a cross-sectional view showing the element structure of a front-illuminated PD which is an example of a semiconductor light-receiving element according to embodiment 7. Fig. 12 is a cross-sectional view showing the element structure of a front-illuminated APD which is an example of a semiconductor light-receiving element according to embodiment 8.12. A cross-sectional view showing the element structure of a front-illuminated APD, which is an example of a semiconductor photodetector according to an eighth embodiment. 13. A cross-sectional view showing the element structure of a front-illuminated APD, which is an example of a semiconductor photodetector according to a ninth embodiment. 14. A cross-sectional view showing the element structure of a back-illuminated APD, which is an example of a semiconductor photodetector according to a tenth embodiment. 15. A cross-sectional view showing the element structure of a back-illuminated APD, which is an example of a semiconductor photodetector according to an eleventh embodiment. 16. A configuration diagram showing an optical line terminal (OLT) of a 50G-PON system according to a twelfth embodiment. 17. A configuration diagram showing an optical line terminal (ONU) of a 50G-PON system according to a twelfth embodiment. 18. A configuration diagram showing an optical line terminal (OLT) of a 50G-PON system, which is a comparative example. 19. A diagram showing the configuration of an optical line terminal (OLT) of a 50G-PON system according to a twelfth embodiment. 20. A diagram showing the configuration of an optical line terminal (ONU) of a 50G-PON system according to a thirteenth embodiment. 31A and 31B are diagrams illustrating received waveforms of a multilevel intensity modulation transceiver according to a thirteenth embodiment. FIGS. 32A and 32B are diagrams illustrating the operation of a PD at high optical input. FIGS. 32A and 32B are diagrams illustrating the operation of an APD at high optical input. FIGS. 32B and 32C are diagrams illustrating the operation of an APD at high optical input. FIGS. 32B and 32C are diagrams illustrating the residence times of electrons and holes for each material constituting the multiplication layer. FIGS. 32A and 32B are diagrams illustrating the configuration of a radio-on-fiber system according to a fourteenth embodiment. FIGS. 32B and 32C are diagrams illustrating the configuration of a radio-on-fiber system as a comparative example. FIGS. 32B and 32C are diagrams illustrating the configuration of a digital coherent receiving device according to a fifteenth embodiment. FIGS. 38A and 38B are diagrams illustrating waveforms of a digital coherent receiving device according to a comparative example. FIGS. 38A and 38B are diagrams illustrating waveforms of a digital coherent receiving device according to the fifteenth embodiment. FIGS. 38A and 38B are diagrams illustrating the configuration of a SAPD sensor system according to a sixteenth embodiment. FIGS. 38A and 38B are diagrams illustrating the multiplication characteristics ... Figure 43A is a diagram showing the received waveform of the APD of a LIDAR device as a comparative example, and Figure 43B is a diagram showing the received waveform of the APD of the LIDAR device according to embodiment 17.

[0037] First Embodiment <Characteristics of the Semiconductor Photodetector (PD) According to First Embodiment> Before describing the specific structure of the semiconductor photodetector according to the first embodiment, the following will first describe a digital alloy structure light absorption layer, which is a structural feature of the semiconductor photodetector according to the first embodiment. Note that although the semiconductor photodetector according to the first embodiment is a PD, both will be described together, including the APD, which is a semiconductor photodetector described in the second embodiment and thereafter.

[0038] 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 1), 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 absorption coefficient of the light absorption layer is higher than that of a random alloy structure made of In. 0.52 Al 0.48 As is simply expressed as InAlAs.

[0039] The optical absorption spectra of the InAs / AlAs digital alloy structure and the InAlAs random alloy structure are shown in Figure 1. The InAs / AlAs digital alloy structure has a steeper rise in the absorption coefficient at the absorption edge near 850 nm than the InAlAs random alloy structure.

[0040] Furthermore, it was found that the InAs / AlAs digital alloy structure exhibited peaks in the absorption coefficient near 800 nm and 650 nm, with the absorption coefficient increasing by 1.5 times. On the other hand, a decrease in the absorption coefficient was observed near 720 nm. This is thought to be due to the band structure changing based on the periodicity of the InAs / AlAs layers, even though the overall composition ratio was roughly the same, and this also resulted in periodicity in the wavelength dependence of the optical absorption coefficient.

[0041] In this way, it was found that even with almost the same composition, it is possible to increase the absorption coefficient by changing the random alloy structure to a digital alloy structure. 0.53 Ga 0.47We considered applying an InAs / GaAs digital alloy structure to the As random alloy structure light absorption layer. 0.53 Ga 0.47 As will be simply expressed as InGaAs.

[0042] 2 is a diagram showing the relationship between the lattice constant of each constituent material and the amount of strain relative to InP. As shown in FIG. 2, just as the composition ratios of the InAlAs random alloy structure and the InAs / AlAs digital alloy structure are almost the same, the composition ratio of the InGaAs random alloy structure is almost the same as that of the InAs / GaAs digital alloy structure. In other words, it is thought that the absorption coefficient can be increased by changing the InGaAs random alloy structure to an InAs / GaAs digital alloy structure.

[0043] Figure 3 shows the results of calculations of the effects of an InAs / GaAs digital alloy structure and an InGaAs random alloy structure on the optical absorption spectrum. Line A in Figure 3 shows the wavelength on the horizontal axis of the InAlAs random alloy structure in Figure 1 multiplied by 1.92 (=1.46 / 0.76), which is the band gap ratio between InAlAs (Eg = 1.46 eV) and InGaAs (Eg = 0.76 eV). Furthermore, the vertical axis value of line A in Figure 3, i.e., the absorption coefficient, is multiplied by 0.37 so that the absorption coefficient of the InGaAs random alloy structure measured at a wavelength of 1.3 μm (12716 / cm) matches the absorption coefficient value of line A in Figure 3 at 1.3 μm.

[0044] Like line A, line B in Figure 3 is obtained by multiplying the wavelength on the horizontal axis of the InAs / GaAs digital alloy structure in Figure 1 by 1.92 and the absorption coefficient on the vertical axis by 0.37. As shown in Figure 3, line B is expected to have an increased absorption coefficient compared to line A at the 1.3 μm and 1.55 μm wavelengths used in optical communications. If line B is considered to represent the predicted absorption spectrum of an InAs / GaAs digital alloy structure, improving light-receiving sensitivity can be expected by using a digital alloy structure for the light absorption layer.

[0045] As shown in Figure 3, the InAs / GaAs digital alloy structure is expected to have a 1.5-fold increase in absorption coefficient at 1.55 μm wavelength compared to the InGaAs random alloy structure. When the absorption coefficient is increased by 1.5 times, the thickness of the optical absorption layer required to obtain the same quantum efficiency is reduced to 67% (= 1 / 1.5) according to equation (1). When the thickness of the optical absorption layer in the InAs / GaAs digital alloy structure is 67% of the thickness of the optical absorption layer in the InGaAs random alloy structure, the 3 dB bandwidth ftr, which is determined by the time it takes for carriers to travel through the optical absorption layer, can be increased by 1.5 times according to equation (2). In other words, by using an optical absorption layer with an InGaAs random alloy structure, the 3 dB bandwidth ftr is improved by 1.5 times compared to the conventional optical absorption layer with an InGaAs random alloy structure.

[0046] On the other hand, when the thickness of the light absorption layer is fixed at 1 μm, the InAs / GaAs digital alloy structure and the InGaAs random alloy structure have the same ftr. However, by using the InAs / GaAs digital alloy structure, the quantum efficiency improves from 43% for the InAs / GaAs digital alloy structure to 57%. Here, the light absorption layer thickness is 1 μm, and the absorption coefficient of the InGaAs random alloy structure at a wavelength of 1.55 μm is 5578 / cm shown in FIG. 3, and the absorption coefficient of the InAs / GaAs digital alloy structure is 8530 / cm shown in FIG. 3.

[0047] As described above, by applying the digital alloy structure to the light absorption layer, it is possible to achieve a broader response band and higher efficiency, that is, higher light receiving sensitivity.

[0048] <Element Structure of Semiconductor Photodetector (PD) According to First Embodiment> Fig. 4 is a cross-sectional view showing the element structure of a front-illuminated PD, which is an example of the semiconductor photodetector 100 according to the first embodiment. Fig. 5 is a cross-sectional view showing the element structure of an edge-illuminated PD, which is an example of the semiconductor photodetector 100a according to the first embodiment.

[0049] The front-illuminated PD, which is an example of the semiconductor photodetector 100 according to the first embodiment, includes an n-type InP substrate 1 and a semiconductor layer having a carrier concentration of 1 to 5×1018 cm -3 and an n-type InP buffer layer 2 having a thickness of 0.1 to 1.0 μm and a carrier concentration of 5×10 17 cm -3 an i-type InP electron transit layer 3 having a thickness of 0.1 to 1.0 μm and a carrier concentration of 5×10 17 cm -3 an i-type InAlGaAs graded layer 4 having a thickness of 5 to 50 nm and a carrier concentration of 1×10 17 cm -3 a light absorbing layer having a digital alloy structure in which i-type InAs layers (for example, a layer thickness of two atomic layers, about 0.6 nm) and i-type GaAs layers (for example, a layer thickness of two atomic layers, about 0.6 nm) are alternately stacked multiple times, and the layer thickness is 50 to 2000 nm (hereinafter referred to as i-type InAs / GaAs digital alloy structure light absorbing layer 5); and a carrier concentration of 5×10 17 cm -3 an i-type InAlGaAs / InAlAs graded layer 6 having a layer thickness of 5 to 50 nm and a carrier concentration of 5×10 17 cm -3 The semiconductor device is composed of a p-type InP window layer 7 having a thickness of 0.1 to 3.0 μm, a p-type InGaAs contact layer 8, an n-type electrode 31 formed on the back surface side of the n-type InP substrate 1, and a p-type electrode 32 formed on the p-type InGaAs contact layer 8.

[0050] A p-type InAlAs window layer may be used instead of the p-type InP window layer 7. The n-type InP buffer layer 2 is also called an n-type semiconductor layer.

[0051] The semiconductor light-receiving element 110 according to the first embodiment shown in FIG. 5 has a layer structure similar to that of the semiconductor light-receiving element 100, but further includes an Fe-doped semi-insulating InP buried layer 20 formed at least on the end surface onto which the incident light 90 is incident.

[0052] Silicon (Si) is optimal as the n-type dopant for the n-type InP buffer layer 2. This is to prevent n-type impurities from diffusing from the n-type InP buffer layer 2 into the i-type InAs / GaAs digital alloy structure light absorption layer 5, 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.

[0053] As described above, the i-type InAs / GaAs digital alloy structure light absorption layer 5 is composed of semiconductor layers in which InAs layers (layer thickness: 2 atomic layers, approximately 0.6 nm) and GaAs layers (layer thickness: 2 atomic layers, approximately 0.6 nm) are alternately stacked in this order. However, the layer thicknesses of the InAs layers and GaAs 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 GaAs layers not to function as a quantum well structure. In other words, the digital alloy structure is composed of two types of semiconductor layers, each made of a different semiconductor material, alternately stacked in a cycle of 2 to 6 atomic layers.

[0054] Furthermore, the number of atomic layers in each layer of the i-type InAs / GaAs digital alloy structure light absorption layer 5 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 switching times 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 / GaAs digital alloy structure light absorption layer 5 is preferably in the range of 2 to 6 atomic layer periods. Similarly, from the viewpoint of productivity, the entire light absorption layer does not have to have an InAs / GaAs digital alloy structure. Instead, part of the light absorption layer may have an InAs / GaAs digital alloy structure and the rest may have an InGaAs random alloy structure.

[0055] The thickness of the i-type InAs / GaAs digital alloy structure light absorption layer 5 is within a range of 50 to 2000 nm. For example, if the thickness of the i-type InAs / GaAs digital alloy structure light absorption layer 5 is 500 nm, the number of repetitions of the InAs layer (two atomic layers) / GaAs layer (two atomic layers) is 417.

[0056] Considering the affinity with InP constituting the n-type InP buffer layer 2, it is preferable to make the layer thickness of only the first GaAs layer of the i-type InAs / GaAs digital alloy structure light absorption layer 5 three atomic layers or more. Alternatively, the i-type InAs / GaAs digital alloy structure light absorption layer 5 may be laminated by alternately forming InAs layers and GaAs layers in this order.

[0057] The conductivity type of the InAs / GaAs digital alloy structure light absorption layer is i-type, and the carrier concentration is 1×10 17 cm -3 However, as the conductivity type of the InAs / GaAs digital alloy structure light absorption layer, the carrier concentration is 5×10 17 cm -3 It may be p-type or n-type as follows.

[0058] In addition to the light absorption layer having an InAs / GaAs digital alloy structure, an InAlGaAs digital alloy structure in which InAlAs / InGaAs, InAlxGa(1-x)As (layer thickness of 2 to 6 atomic layers, Al composition ratio X) and InAlyGa(1-y)As (layer thickness of 2 to 6 atomic layers, Al composition ratio Y) are alternately stacked, as shown in Figure 2, which shows the relationship between the lattice constant of each constituent material and the amount of strain relative to InP, can also be used as the light absorption layer of the present disclosure. Furthermore, a digital alloy structure made of InAlAsSb, a material system to which antimony (Sb) is added, can also be used as the light absorption layer of the semiconductor light-receiving element of the present disclosure.

[0059] The i-type InAlGaAs graded layer 4 and the i-type InAlGaAs / InAlAs graded layer 6 are layers in which the band gap is gradually changed by changing the InAlGaAs composition, and each layer thickness is in the range of 5 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. The carrier concentration is 5×10 17 cm -3 The i-type InAlGaAs graded layer 4 and the i-type InAlGaAs / InAlAs graded layer 6 are not necessarily required and may be omitted.

[0060] In an example of the device structure of the semiconductor photodetector 100 shown in FIG. 4, two types of i-type InAlGaAs layers with different compositions are alternately stacked multiple times on an i-type InAs / GaAs digital alloy structure light absorption layer 5, thereby forming an i-type InAlGaAs / InAlAs graded layer 6.

[0061] <Method for Manufacturing Semiconductor Photodetector According to First Embodiment> A front-illuminated PD, which is an example of the semiconductor photodetector 100 according to the first embodiment, can be realized by using 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.

[0062] 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.

[0063] On the n-type InP buffer layer 2, a carrier concentration of 5×10 17 cm -3 The i-type InP electron transport layer 3 having a thickness of 0.1 to 1 μm is grown by crystal growth.

[0064] On the i-type InP electron transport layer 3, a layer having a thickness of 5 nm to 50 nm and a carrier concentration of 5×10 17 cm-3 The following i-type InAlGaAs graded layer 4 is grown by crystal growth.

[0065] On the i-type InAlGaAs graded layer 4, a layer having a carrier concentration of 1×10 17 cm -3 The i-type InAs / GaAs digital alloy structure light absorption layer 5 is crystal grown to have a thickness of 50 to 2000 nm or less. That is, the i-type InAs / GaAs digital alloy structure light absorption layer 5 is formed by alternately growing crystals of a GaAs layer (having a thickness of two atomic layers, approximately 0.6 nm) and an InAs layer (having a thickness of two atomic layers, approximately 0.6 nm) in this order from on the n-type InP buffer layer 2.

[0066] 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 p-type InP window layer 7 and the p-type InGaAs contact layer 8 having a thickness of 0.1 to 3.0 μm are grown sequentially by crystal growth.

[0067] After the crystal growth is completed, a p-type electrode 32 is formed on the surface of the p-type InGaAs contact layer 8, and an n-type electrode 31 is formed on the back surface of the n-type InP substrate 1. The p-type electrode 32 of the PD is made of metal materials such as Ti and Au. A voltage is applied to the PD in the reverse direction, and the operating voltage is 0V to 10V.

[0068] In the case of the front-illuminated PD shown in Figure 1, incident light 90 is incident perpendicularly to the i-type InAs / GaAs digital alloy structure light absorption layer 5. The diameter of the PD's light receiving section when it is circular, or the length of the long side when it is rectangular, is in the range of 5 μm to 1 mm. An anti-reflective coating (not shown) is applied to the incident surface of the PD.

[0069] In the case of the edge-illuminated PD shown in Figure 5, incident light 90 is incident parallel to the i-type InAs / GaAs digital alloy structure light absorption layer 5. From the viewpoint of reliability, the edge portion is covered with an insulating film, an organic film, or a semiconductor layer. In the edge-illuminated PD shown in Figure 5, an Fe-doped semi-insulating InP burying layer 20 is formed on the edge. The thickness of the Fe-doped semi-insulating InP burying layer 20 is in the range of 100 nm to 5 μm in the incident direction.

[0070] <Function of Semiconductor Photodetector (PD) According to First Embodiment> A PD will be described as an example of the semiconductor photodetector 100 according to the first embodiment shown in Figures 4 and 5. Note that the APDs according to the second and subsequent embodiments will also be described.

[0071] In Figure 3, line A represents an InGaAs random alloy structure light absorption layer, and line B represents an InAs / GaAs digital alloy structure light absorption layer. According to Figure 3, the absorption coefficient at 1.3 μm is 12716 / cm for line A and 16108 / cm for line B. The absorption coefficient at 1.55 μm is 5578 / cm for line A and 8530 / cm for line B.

[0072] When an InAs / GaAs digital alloy structure light absorption layer is used as the light absorption layer for PDs and APDs, the improvement effect is particularly significant for high-speed PDs and APDs of 25 G or higher. For surface-illuminated PDs and APDs, the optimal thickness of the light absorption layer is 500 to 1000 nm. When the light absorption layer thickness is 500 to 1000 nm, the light receiving sensitivity in the 1.3 μm wavelength band is 0.49 A / W (quantum efficiency 47%) to 0.75 A / W (quantum efficiency 72%) for an InGaAs random alloy structure light absorption layer. On the other hand, when an InAs / GaAs digital alloy structure light absorption layer is used, the sensitivity is 0.58 A / W (quantum efficiency 55%) to 0.84 A / W (quantum efficiency 80%), which is expected to be a significant improvement.

[0073] Furthermore, in the 1.55 μm wavelength band, the InGaAs random alloy structure light absorption layer has a quantum efficiency of 0.30 A / W (quantum efficiency 24%) to 0.53 A / W (quantum efficiency 43%), whereas the InAs / GaAs digital alloy structure light absorption layer has a quantum efficiency of 0.43 A / W (quantum efficiency 35%) to 0.72 A / W (quantum efficiency 57%), which is expected to be an even greater improvement than the 1.3 μm band.

[0074] For edge-illuminated high-speed PDs and APDs with a power of 25 G or higher, the optimal thickness of the optical absorption layer is 200 to 500 nm. For edge-illuminated high-speed PDs and APDs, increasing the absorption coefficient allows for a shorter waveguide length, thereby reducing the p-n junction capacitance. For example, in the 1.55 μm wavelength band, the absorption coefficient of an InAs / GaAs digital alloy structure optical absorption layer is approximately 1.5 times greater than that of an InGaAs random alloy structure optical absorption layer. Therefore, the same light receiving sensitivity can be obtained with a waveguide length approximately 1.5 times shorter, resulting in a reduction in the p-n junction capacitance by approximately 1.5 times. Reducing the p-n junction capacitance allows for a broader response band.

[0075] As explained above, by using an InAs / GaAs digital alloy structure light absorption layer as the light absorption layer of a PD or APD, it is possible to increase the sensitivity and widen the response band of the PD or APD. Therefore, it is possible to obtain a PD or APD with sufficient receiving sensitivity even for applications with a response band of 25 Gbps or more.

[0076] In the InAs / GaAs digital alloy structure light absorption layer of the present disclosure, the InAs layers and GaAs layers are alternately repeated with extremely thin layer thicknesses of 2 to 6 atomic layers (0.6 to 1.8 nm), and therefore a quantum well structure is not formed between the InAs layers and the GaAs layers, and the band gap of the InAs / GaAs digital alloy structure itself is approximately the same as that of InGaAs.

[0077] For example, if the thickness of the InAs layer is several nanometers or more, the absorption edge will be at a wavelength of 3 μm, which is close to the bandgap wavelength of InAs, resulting in different physical properties. Patent Document 1 discloses an example in which the light absorption layer has a stacked structure of an AlGaAsSb wide-gap layer (thickness: approximately 0.1 μm) and a short-period (m = 1 to 2) superlattice narrow-gap layer (thickness: approximately 0.1 μm) made of (InAs)m / (GaAs)m. However, p-layers and n-layers are arranged to the left and right of the stacked surface of the light absorption layer, and only electrons travel through the (InAs)m / (GaAs)m layer, which also functions as a multiplication layer.

[0078] When a p-layer and an n-layer are arranged to the left and right of the stacking plane of the light absorption layer as in Patent Document 1, the distance between them becomes wide, weakening the electric field, and the mechanism is such that electrons and holes travel in separate layers, which generates an electric field distribution and makes it easy for nonlinear operation to occur when a high optical input is applied.In addition, there is also the problem that even if an attempt is made to pass a current in the vertical direction of the stacking plane of the light absorption layer, the current does not flow because it is sandwiched between wide-gap layers.

[0079] On the other hand, in the PD and APD according to the present disclosure, the p-layer and n-layer are arranged above and below the stacking surface of the light absorption layer, and therefore the InAs / GaAs digital alloy structure has a structure in which electrons and holes travel short distances in opposite directions above and below within the same layer, ensuring linear operation.

[0080] Patent Document 2 discloses a layered structure of InGaAs and InAlAs, and also describes that the InGaAs layer forms a quantum well (well) with a layer thickness of 10 nm, and that disordering is caused by heat treatment to change the bandgap wavelength determined by the InGaAs well.

[0081] On the other hand, in the semiconductor light receiving element of the present disclosure, since each of the InAs layer and the GaAs layer is composed of a thin film of 2 to 6 atomic layers, no well is formed in the InGaAs layer. Also, since the bandgap wavelength is the same as that of InAlGaAs, which has the same ratio of Al and Ga, the bandgap wavelength does not change even when heat treatment is applied.

[0082] Patent Document 3 also shows an example in which the bandgap wavelength is changed by changing the layer thickness of an InGaAs layer, which also functions as a well, from 5.5 nm to 8 nm. On the other hand, in the semiconductor light-receiving element of the present disclosure, the InAs layer and the GaAs layer are each thin films of 2 to 6 atomic layers (0.6 to 1.8 nm), and therefore no well is formed as an InGaAs layer, and therefore the bandgap wavelength does not change.

[0083] When the layers constituting the light absorption layer form a quantum well structure, the absorption coefficient increases at wavelengths corresponding to the quantum level, but the absorption coefficient significantly decreases at wavelengths around the wavelength corresponding to the quantum level. Furthermore, the formation of a quantum well structure shortens the bandgap wavelength, thereby decreasing the absorption coefficient at longer wavelengths. Furthermore, polarization dependence of the absorption coefficient occurs, making it difficult for electrons and holes to be ejected from the well. Therefore, it is important not to form a quantum well structure by setting the layer thickness to a few atomic layers, as in the semiconductor light-receiving element of the present disclosure.

[0084] <Effects of First Embodiment> As described above, the semiconductor photodetector according to the first embodiment has an i-type InAs / GaAs digital alloy structure light absorption layer, which provides an effect of providing a semiconductor photodetector that operates over a wide response band and has high reception sensitivity.

[0085] Embodiment 2. <Element Structure of Semiconductor Photodetector (APD) According to Embodiment 2> Fig. 6 is a cross-sectional view showing the element structure of a front-illuminated APD, which is an example of a semiconductor photodetector 110 according to embodiment 2. Fig. 7 is a cross-sectional view showing the element structure of an edge-illuminated APD, which is an example of a semiconductor photodetector 110a according to embodiment 2.

[0086] The semiconductor light-receiving element 110 according to the second embodiment includes an n-type InP substrate 1 and a semiconductor layer having a carrier concentration of 1 to 5×10 18 cm -3 and an n-type InAlAs buffer layer 2a having a thickness of 0.1 to 1.0 μm and a carrier concentration of 1×10 17 cm -3 an i-type InAlAs multiplication layer 13 having a layer thickness of 50 to 500 nm and a carrier concentration of 1×10 16 ~5 x 1018 cm -3 and a p-type InP field relaxation layer 14 having a layer thickness of 10 to 70 nm, and a carrier concentration of 1×10 17 cm -3 an i-type InAs / GaAs digital alloy structure light absorption layer 5 having a thickness of 50 to 2000 nm, in which i-type InAs layers (for example, a layer thickness of two atomic layers, about 0.6 nm) and i-type GaAs layers (for example, a layer thickness of two atomic layers, about 0.6 nm) are alternately stacked multiple times; and a carrier concentration of 5×10 17 cm -3 an i-type InAlGaAs / InAlAs graded layer 6 having a layer thickness of 5 to 50 nm and a carrier concentration of 5×10 17 cm -3 The semiconductor device is composed of a p-type InP window layer 7 having a layer thickness of 0.1 to 3.0 μm, a p-type InGaAs contact layer 8, an n-type electrode 31 formed on the back surface side of the n-type InP substrate 1, and a p-type electrode 32 formed on the p-type InGaAs contact layer 8.

[0087] A p-type InAlAs window layer may be used instead of the p-type InP window layer 7. The n-type InAlAs buffer layer 2a is also called an n-type semiconductor layer.

[0088] The semiconductor light-receiving element 110a according to the second embodiment shown in FIG. 7 has a layer structure similar to that of the semiconductor light-receiving element 110, but further includes an Fe-doped semi-insulating InP buried layer 20 formed at least on the end surface onto which the incident light 90 is incident.

[0089] Silicon (Si) is the optimum n-type dopant for the n-type InAlAs buffer layer 2a. The n-type InAlAs buffer layer 2a may have either a random alloy structure or a digital alloy structure.

[0090] As described above, the i-type InAs / GaAs digital alloy structure light absorption layer 5 is configured of semiconductor layers in which InAs layers (layer thickness: 2 atomic layers, approximately 0.6 nm) and GaAs layers (layer thickness: 2 atomic layers, approximately 0.6 nm) are alternately stacked in this order. However, it is sufficient that the thicknesses of the InAs layers and GaAs layers are each in the range of 2 to 6 atomic layers. The reason for specifying 6 atomic layers or less is that it is desirable that the stacked structure of the InAs layers and GaAs layers does not function as a quantum well structure.

[0091] Furthermore, the number of atomic layers in each layer of the i-type InAs / GaAs digital alloy structure light absorption layer 5 is preferably between two and four atomic layers, with two atomic layers being optimal. The reason for this is that the thinner the atomic layer thickness of each layer, the greater the effect of reducing the ionization rate ratio k 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 switching times during crystal growth by MBE. Considering the above factors, it can be said that the number of atomic layers in each layer of the i-type InAs / GaAs digital alloy structure light absorption layer 5 is preferably in the range of 2 to 6 atomic layer periods.

[0092] The thickness of the i-type InAs / GaAs digital alloy structure light absorption layer 5 is in the range of 50 nm to 2000 nm. For example, if the thickness of the i-type InAs / GaAs digital alloy structure light absorption layer 5 is 500 nm, the number of repetitions of the InAs layer (two atomic layers) / GaAs layer (two atomic layers) is 417.

[0093] In consideration of the affinity with InAlAs constituting the n-type InAlAs buffer layer 2a, it is preferable to make the thickness of only the first GaAs layer of the i-type InAs / GaAs digital alloy structure light absorption layer 5 three atomic layers or more thick. Alternatively, the i-type InAs / GaAs digital alloy structure light absorption layer 5 may be stacked by alternately forming InAs layers and GaAs layers in this order.

[0094] The conductivity type of the i-type InAs / GaAs digital alloy structure light absorption layer 5 is i-type, and the carrier concentration is 1×10 17 cm -3However, as the conductivity type of the i-type InAs / GaAs digital alloy structure light absorption layer 5, the carrier concentration is 5×10 17 cm -3 It may be p-type or n-type as follows.

[0095] In addition to the light absorption layer having an InAs / GaAs digital alloy structure, an InAlGaAs digital alloy structure in which InAlAs / InGaAs, InAlxGa(1-x)As (layer thickness of 2 to 6 atomic layers, Al composition ratio X) and InAlyGa(1-y)As (layer thickness of 2 to 6 atomic layers, Al composition ratio Y) are alternately stacked, as shown in Figure 2, which shows the relationship between the lattice constant of each constituent material and the amount of strain relative to InP, can also be used as the light absorption layer of the present disclosure. Furthermore, a digital alloy structure made of InAlAsSb, a material system to which antimony (Sb) is added, can also be used as the light absorption layer of the semiconductor light-receiving element of the present disclosure.

[0096] The i-type InAlGaAs / InAlAs graded layer 6 is a layer in which the band gap is gradually changed by changing the composition of InAlGaAs, and each layer thickness is in the range of 5 to 50 nm. The composition of InAlGaAs may be changed stepwise, and the band gap is intermediate between that of an InP layer and an InGaAs layer. The carrier concentration is 5×10 17 cm -3 The i-type InAlGaAs / InAlAs graded layer 6 is not necessarily required and may be omitted.

[0097] A layer having an intermediate band gap, such as InAlGaAs or InGaAsP, with a thickness of 0.1 μm or less may be provided between the p-type InP electric field buffer layer 14 and the i-type InAs / GaAs digital alloy structure light absorption layer 5. This is because it is possible to prevent accumulation of electrons and holes at the heterojunction interface.

[0098] In an example of the device structure of the semiconductor photodetector 110 shown in FIG. 6, two types of i-type InAlGaAs layers with different compositions are alternately stacked multiple times on an i-type InAs / GaAs digital alloy structure light absorption layer 5, thereby forming an i-type InAlGaAs / InAlAs graded layer 6.

[0099] The p-type electrode 32 of the APD, which is an example of a semiconductor light-receiving element according to the second embodiment, is made of Ti and Au. A voltage is applied to the APD in the reverse direction, and the operating voltage is 10 V to 100 V. The electric field of the i-type InAlAs multiplication layer 13 when the APD is operating is 500 kV / cm to 900 kV / cm. The electric field of the i-type InAs / GaAs digital alloy structure light absorption layer 5 is set to 300 kV / cm or less. The multiplication factor is used at 3 to 30, but when operating in Geiger mode, it is 100 or more.

[0100] <Function of the semiconductor photodetector (APD) according to the second embodiment> The following describes the function of the semiconductor photodetector (APD) according to the second embodiment. As with the PD according to the first embodiment, the APD according to the second embodiment uses an InAs / GaAs digital alloy structure light absorption layer as the light absorption layer of the APD, thereby enabling the APD to have high sensitivity and a wide bandwidth. Therefore, it is possible to obtain an APD with sufficient receiving sensitivity even for applications with a wide response bandwidth of 25 Gbps or more.

[0101] <Effects of Second Embodiment> As described above, the semiconductor photodetector according to the second embodiment has an InAs / GaAs digital alloy structure light absorption layer, which provides an effect of providing a semiconductor photodetector that operates over a wide response band and has high reception sensitivity.

[0102] Variation of the Second Embodiment <Features of the Semiconductor Photodetector (APD) According to the Variation of the Second Embodiment> Before describing the specific structure of the semiconductor photodetector according to the variation of the second embodiment, the digital alloy structure multiplication layer, which is a structural feature of the semiconductor photodetector according to the variation of the second embodiment, will be described below.

[0103] 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).

[0104] 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 (3) to (5). Multiplication time TM = multiplication factor M / GB product (3) GB product = 1 / (2πNkτav) (4) In other words, the multiplication time TM = 2πNkMτav (5)

[0105] 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.

[0106] 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 has been difficult to make the ionization rate ratio k zero unless the structure is optimized. The digital alloy structure is described in Non-Patent Document 1.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] The following describes a front-illuminated APD, which is one example of a semiconductor photodetector according to a modification of embodiment 2, and an edge-illuminated APD, which is another example. The semiconductor photodetector according to the modification of embodiment 2 is structurally different from the semiconductor photodetector according to embodiment 2 in that the i-type InAlAs multiplication layer 13 of the semiconductor photodetector according to embodiment 2, i.e., the i-type InAlAs multiplication layer with a random alloy structure, is replaced with an i-type InAs / AlAs digital alloy structure multiplication layer.

[0111] An example of the structure of the i-type InAs / AlAs digital alloy structure multiplication layer 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.

[0112] 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 in layer thickness during fabrication of the semiconductor light receiving element 100 of 20%, 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.

[0113] <Function of Semiconductor Photodetector (APD) According to Modification of Second Embodiment> The function of an APD, which is an example of a semiconductor photodetector according to a modification of the second embodiment, will be described below. The inventors have discovered that using a digital alloy structure multiplication layer, as in the APD according to the modification of the second embodiment, enhances the dead space effect, i.e., the effect of reducing the ionization rate ratio k. FIG. 8 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 of FIG. 8, 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.

[0114] 9A to 9C are graphs showing the ionization rates of electrons and holes, respectively. FIG. 9A shows the case of electron ionization, FIG. 9B shows the case of hole ionization, and FIG. 9C shows the ionization rate when the multiplication layer is thinned. In a conventional InAlAs random alloy multiplication layer, as shown in the graph of FIG. 8, 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 increased noise, making it difficult to obtain an APD with good receiver sensitivity.

[0115] On the other hand, in the InAs / AlAs digital alloy structure multiplication layer of the APD according to the modification of the second embodiment, as shown in the graph of FIG. 8, the reciprocal of the applied electric field is 1.47×10 -6 In the case of a dc voltage of 1.0 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 APD according to the modification of the second embodiment.

[0116] 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. 8. 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.

[0117] Fig. 10 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. 10 together with the measurement results of References 1 and 2 described in Fig. 10. References 1 and 2 in Fig. 10 are as follows:

[0118] (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) Reference 2 Wenyang Wang, et al “Characteristics of thin InAlAs digital alloy avalanche photodiodes” pp. 3841, Vol. 46, No. 16 / 15 August 2021 / Optics Letters

[0119] As shown in Figure 10, 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.

[0120] 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 Figure 10. 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.

[0121] As shown in the graph of FIG. 10 , 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.

[0122] 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 -6In the case of a thickness of 170 nm, the thickness is approximately twice the length of the dead space. Therefore, considering that the length of the dead space is 85 nm as shown in FIG. 10, 170 nm, which is twice the length of the dead space, is a suitable upper limit for the thickness of the InAs / AlAs digital alloy structure multiplication layer.

[0123] 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 8. 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.

[0124] Furthermore, as shown in Figure 8, 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, a ratio of the length of the dead space to the thickness of the multiplication layer of 29% or more but less than 100% is preferable. 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).

[0125] 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.

[0126] 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 (6) and (7). Note that equation (6) represents the condition for the difference in dead space length, and equation (7) represents the condition for the tunnel current. Dhe=Dh-De>0 (6) Dh>Tmin (7)

[0127] 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 Figure 9C, when the thickness of the multiplication layer is equal to or smaller than the dead space length of holes, holes are no longer multiplied and the ionization rate ratio k = 0, so the condition for the difference in dead space length is set as shown in Equation (7) above.

[0128] 8 and 10, 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 of about 40 nm and Dh of 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.

[0129] 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.

[0130] 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.

[0131] 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.

[0132] 11A to 11D are diagrams showing the ionization rates in the multiplication layer and the electric field relaxation layer, with Fig. 11A being for an InAlAs random alloy structure multiplication layer, Fig. 11B being for an InAs / AlAs digital alloy structure multiplication layer, Fig. 11C being for a partially disordered InAs / AlAs digital alloy structure multiplication layer, and Fig. 11D being for a combination of a thick electric field relaxation layer and an InAs / AlAs digital alloy structure multiplication layer. Compared to the InAlAs random alloy structure multiplication layer shown in Fig. 11A, the InAs / AlAs digital alloy structure multiplication layer shown in Fig. 11B 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. 11C.

[0133] In order to avoid the influence of disordering 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 (8): dN / dt=D(d 2 N / d 2 x)-F (8)

[0134] In equation (8), 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 (8).

[0135] 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:

[0136] The electric field relaxation amount ΔE is expressed by the following equation (9): ΔE=W q N / ε (9) When the electric field relaxation amount ΔE is constant, if the carrier concentration of the electric field relaxation layer is increased, the thickness of the electric field relaxation layer must be reduced in inverse proportion to the carrier concentration, where 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.

[0137] The carrier concentration N of the field relaxation layer is 5×10 18 cm -3In 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.

[0138] On the other hand, as shown in Figure 11D, if the thickness of the electric field buffer layer is 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 6, in the random alloy structure, the dead space length is 45 nm or less, so the layer thickness of the random alloy structure electric field buffer layer 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 digital alloy electric field buffer layer must be 130 nm or less.

[0139] The lengths of the dead spaces shown in FIGS. 11A to 11D have the following relationship: dead space (FIG. 11A)<dead space (FIG. 11D)<dead space (FIG. 11C)<dead space (FIG. 11B).

[0140] <Effects of the semiconductor photodetector (APD) according to the modification of the second embodiment> First, a first effect of the semiconductor photodetector according to the modification of the second embodiment will be quantitatively described below. The 3 dB bandwidth fc of a conventional APD is expressed by the following equation (10), where frc is the bandwidth limitation due to the RC time constant, ftr is the bandwidth limited by the carrier transit time, and fm is the bandwidth limitation due to the multiplication time. fc_APD=1 / ((1 / frc) 2 +(1 / ftr) 2 +(1 / fm) 2 ) 0.5 (10)

[0141] On the other hand, the 3 dB bandwidth fc of the APD having an InAs / AlAs digital alloy structure multiplication layer according to the modification of the second embodiment is limited only by the RC time constant and the carrier transit time according to equations (3), (4), and (5) because the ionization rate ratio k is close to zero, and therefore can be expressed by the following equation (11): fc_APD=1 / ((1 / frc) 2+(1 / ftr) 2 ) 0.5 (11) In equation (11), the transit time ftr of the carriers includes the transit time in the light absorption layer plus the transit time in the multiplication layer.

[0142] 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 (11) is expressed as the following Equation (12): fc = ftr / √2 (12)

[0143] The 3 dB bandwidth ftr determined by the transit time is expressed by the following equation (13): ftr=3.5Vav / (2πWt) (13)

[0144] In equation (13), 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.

[0145] Vav=5.35×10 6 Substituting cm / s and Wt = 500 nm into equation (13) yields ftr = 59.6 GHz. Furthermore, substituting the calculated ftr into equation (12) yields 42.2 GHz as the 3 dB bandwidth of the APD having the InAs / AlAs digital alloy structure multiplication layer according to the modification of the second embodiment. Therefore, the above considerations reveal that the APD having the InAs / AlAs digital alloy structure multiplication layer according to the modification of the second embodiment can meet the 37.5 GHz bandwidth required for 50G-PON systems. In the following descriptions of devices, systems, etc., an APD having an InAs / GaAs digital alloy structure light absorption layer according to the present disclosure, or an APD having an InAs / GaAs digital alloy structure light absorption layer and an InAs / AlAs digital alloy structure multiplication layer, will be referred to as a DA-APD according to the present disclosure.

[0146] <Effects of the Variation of Second Embodiment> As described above, the semiconductor photodetector according to the variation of the second embodiment 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 over a wider response band, has high reception sensitivity, and is highly reliable.

[0147] Embodiment 3 Fig. 12 is a cross-sectional view showing the element structure of a front-illuminated PD, which is an example of a semiconductor light-receiving element 120 according to embodiment 3. Fig. 13 is a cross-sectional view showing the element structure of a front-illuminated PD, which is an example of a semiconductor light-receiving element 120a according to embodiment 3.

[0148] <Device Structure of Semiconductor Photodetector (PD) According to Third Embodiment> A semiconductor photodetector 120 according to the third embodiment shown in FIG. 12 includes an n-type InP substrate 1 and a PD having a carrier concentration of 1 to 5×10 18 cm -3 and an n-type InP buffer layer 2 having a thickness of 0.1 to 1.0 μm and a carrier concentration of 5×10 17 cm -3 an i-type InP electron transit layer 3 having a thickness of 0.1 to 1.0 μm and a carrier concentration of 5×10 17 cm -3 an i-type InAlGaAs graded layer 4 having a thickness of 5 to 50 nm and a carrier concentration of 1×10 17 cm -3 an i-type InAs / GaAs digital alloy structure light absorption layer 5 having a thickness of 50 to 2000 nm, in which i-type InAs layers (for example, a layer thickness of two atomic layers, about 0.6 nm) and i-type GaAs layers (for example, a layer thickness of two atomic layers, about 0.6 nm) are alternately stacked multiple times; and a carrier concentration of 5×10 17 cm -3 an i-type InAlGaAs / InAlAs graded layer 6 having a layer thickness of 5 to 50 nm and a carrier concentration of 5×10 17 cm -3The n-type InP buffer layer 2 is made up of an n-type InP window layer 11 having a thickness of 0.1 to 3.0 μm or less, a p-type diffusion region 15 provided in the n-type InP window layer 11, a p-type InGaAs contact layer 8 provided on the p-type diffusion region 15, an n-type electrode 31 formed on the back surface side of the n-type InP substrate 1, and a p-type electrode 32 formed on the p-type InGaAs contact layer 8. The n-type InP buffer layer 2 is also called an n-type semiconductor layer.

[0149] 13 has the same configuration as the semiconductor light-receiving element 120 according to the third embodiment, except that an Fe-doped semi-insulating InP substrate 1a is used as the substrate and an n-type electrode 31a is formed on the front side of an n-type InP conductive layer 2b. The back side electrode 31b is not intended to pass current but is necessary for fixing the semiconductor light-receiving element with solder.

[0150] The semiconductor photodetectors 120 and 120a according to the third embodiment differ from the semiconductor photodetectors 100 and 100a according to the first embodiment in that the n-type InP constituting the n-type InP window layer 11 is undoped (i-type) or has a low carrier concentration, and in that a p-type diffusion region 15 is provided in the n-type InP window layer 11. The thickness of the n-type InP window layer 11 of the semiconductor photodetectors 120 and 120a is 0.1 μm or more and 3 μm or less, and the carrier concentration is 5×10 17 cm -3 The n-type InP window layer 11 may be made of InAlAs instead of InP, or may have a laminated structure of InP and InAlAs.

[0151] The p-type diffusion region 15 is formed by partially selectively diffusing a p-type dopant such as Zn in a solid or vapor phase. The carrier concentration of the p-type diffusion region 15 is 5×10 17 cm -3That is all. The tip of the p-type diffusion region 15 may be located at a depth reaching partway into the n-type InP window layer 11, at a depth reaching the i-type InAlGaAs / InAlAs graded layer 6, or at a depth reaching the i-type InAs / GaAs digital alloy structure light absorption layer 5. In the device structures shown in FIGS. 12 and 13, the p-type diffusion region 15 has a depth reaching the i-type InAs / GaAs digital alloy structure light absorption layer 5. A p-type InGaAs contact layer 8 is provided on the p-type diffusion region 15.

[0152] By diffusing Zn halfway through the i-type InAs / GaAs digital alloy structure light absorption layer 5, for example to a depth of about 0.2 μm, it is also possible to convert a part (on the p-type electrode side) of the i-type InAs / GaAs digital alloy structure light absorption layer 5 into a p-type layer. In the part that has been p-type layered by Zn diffusion, the InAs / GaAs digital alloy structure may become disordered, resulting in a random alloy structure, but the p-type layered part becomes a uni-traveling carrier (UTC) structure, so the response speed does not deteriorate. This is because, in the UTC structure, of the electron-hole pairs generated by light absorption in the p-type layer (non-depleted p-type InGaAs) in the i-type InAs / GaAs digital alloy structure light absorption layer 5, only electrons that can move at high speed are supplied to the depletion layer (the InAs / GaAs digital alloy structure light absorption layer that remains i-type), enabling high-speed response of the PD. In other words, the non-depleted p-type light absorption layer portion does not need to have a digital alloy structure; as long as the depleted i-type light absorption layer portion has a digital alloy structure, high-speed response will not be degraded. However, if the thickness of the p-type layer in the i-type InAs / GaAs digital alloy structure light absorption layer 5 is increased, electrons generated in the p-type layer will recombine, resulting in reduced efficiency. Note that the p-type InGaAs portion in the i-type InAs / GaAs digital alloy structure light absorption layer 5 may be formed by doping during epitaxial crystal growth rather than by Zn diffusion as described above.

[0153] In a front-illuminated PD, which is an example of a semiconductor photodetector 120 shown in FIG. 12, an n-type electrode 31 is provided on the back surface. On the other hand, in a front-illuminated PD, which is an example of a semiconductor photodetector 120a shown in FIG. 13, an n-type electrode 31a is provided on the front surface in addition to an electrode 31b on the back surface. That is, in the semiconductor photodetector 120a, an n-type InP conductive layer 2b is provided on an Fe-doped semi-insulating InP substrate 1a. After crystal growth, the semiconductor layers above the n-type InP conductive layer 2b are partially removed, and then the n-type electrode 31a is formed on the n-type InP conductive layer 2b. The semiconductor photodetector 120a may use a p-type InP substrate or an n-type InP substrate instead of the Fe-doped semi-insulating InP substrate 1a.

[0154] <Function of the Semiconductor Photodetector (PD) According to the Third Embodiment> In a mesa structure such as the front-illuminated PD, which is an example of the semiconductor photodetector 100 according to the first embodiment shown in Figure 4, the side portion of the multiplication layer to which an electric field is applied is exposed to the outside and is therefore prone to deterioration. In particular, when an InAs / GaAs digital alloy structure is used as the light absorption layer, the strain in each layer constituting the i-type InAs / GaAs digital alloy structure light absorption layer 5 is high, so dislocation defects and disorder are likely to occur from the exposed portion toward the inside, which may result in the problem of deterioration of the semiconductor photodetector.

[0155] As a result, in the case of a light absorption layer in which an InAs / GaAs digital alloy structure is thinned, as in the semiconductor photodetectors 100 and 100a according to the first embodiment, the dark current generated in the side portions may shorten the life of the semiconductor photodetector compared to the conventional InGaAs random alloy structure.

[0156] 12 and 13 , when a p-type diffusion region 15 is provided, the portion of the i-type InAs / GaAs digital alloy structure light absorbing layer 5 to which an electric field is applied, i.e., the portion directly below the p-type diffusion region 15, is not exposed to the outside of the crystal layer, so that the occurrence of deterioration and disorder can be prevented in the i-type InAs / GaAs digital alloy structure light absorbing layer 5, in which the strain in each layer is high. As a result, it is possible to maintain a high absorption coefficient as a light absorbing layer.

[0157] <Effects of Third Embodiment> As described above, according to the semiconductor photodetector of the third embodiment, the non-depleted P-type layer portion in the i-type InAs / GaAs digital alloy structure light absorption layer 5 has a random alloy structure, but the response speed does not deteriorate due to the above-mentioned UTC effect. On the other hand, the remaining depleted i-type portion of the InAs / GaAs digital alloy structure light absorption layer 5, i.e., the portion directly below the p-type diffusion region 15, is not exposed to the outside of the crystal layer and therefore does not become disordered, so that high-speed response does not deteriorate. Therefore, even though Zn diffusion is performed to form a p-type diffusion region during device structure formation, disordering of the i-type InAs / GaAs digital alloy structure light absorption layer 5 can be prevented, thereby achieving the effect of providing a semiconductor photodetector that operates over a wide response band, has high reception sensitivity, and is highly reliable.

[0158] Fourth Embodiment Fig. 14 is a cross-sectional view showing the device structure of a front-illuminated APD, which is an example of a semiconductor light-receiving element 130 according to the fourth embodiment. Fig. 15 is a cross-sectional view showing the device structure of a front-illuminated APD, which is an example of a semiconductor light-receiving element 130a according to the fourth embodiment.

[0159] <Element Structure of Semiconductor Photodetector (APD) According to Fourth Embodiment> The semiconductor photodetector 130 according to the fourth embodiment has an n-type InP substrate 1 and a semiconductor layer having a carrier concentration of 1 to 5×10 18 cm -3 and an n-type InAlAs buffer layer 2a having a thickness of 0.1 to 1.0 μm and a carrier concentration of 1×10 17 cm -3 an i-type InAlAs multiplication layer 13 having a layer thickness of 50 to 500 nm and a carrier concentration of 1×10 16 ~5 x 10 18 cm -3 and a p-type InP field relaxation layer 14 having a layer thickness of 10 to 70 nm, and a carrier concentration of 1×10 17 cm -3 an i-type InAs / GaAs digital alloy structure light absorption layer 5 having a thickness of 50 to 2000 nm, in which i-type InAs layers (for example, a layer thickness of two atomic layers, about 0.6 nm) and i-type GaAs layers (for example, a layer thickness of two atomic layers, about 0.6 nm) are alternately stacked multiple times; and a carrier concentration of 5×1017 cm -3 an i-type InAlGaAs / InAlAs graded layer 6 having a layer thickness of 5 to 50 nm and a carrier concentration of 5×10 17 cm -3 The n-type InAlAs buffer layer 2a is made up of an n-type InP window layer 11 having a thickness of 0.1 to 3.0 μm or less, a p-type diffusion region 15 provided in the n-type InP window layer 11, a p-type InGaAs contact layer 8 provided on the p-type diffusion region 15, an n-type electrode 31 formed on the back surface side of the n-type InP substrate 1, and a p-type electrode 32 formed on the p-type InGaAs contact layer 8. The n-type InAlAs buffer layer 2a is also called an n-type semiconductor layer.

[0160] 15 has the same configuration as the semiconductor photodetector 130 according to the fourth embodiment, except that an Fe-doped semi-insulating InP substrate 1a is used as the substrate and an n-type electrode 31a is formed on the front side of an n-type InP conductive layer 2b. The back side electrode 31b is not intended to pass current but is necessary for fixing the semiconductor photodetector with solder.

[0161] The semiconductor photodetectors 130 and 130a according to the fourth embodiment differ from the semiconductor photodetectors 110 and 110a according to the second embodiment in that the n-type InP constituting the n-type InP window layer 11 is undoped (i-type) or has a low carrier concentration, and in that a p-type diffusion region 15 is provided in the n-type InP window layer 11. The thickness of the n-type InP window layer 11 of the semiconductor photodetectors 120 and 130 is 0.1 μm or more and 3 μm or less, and the carrier concentration is 5×10 17 cm -3 The n-type InP window layer 11 may be made of InAlAs instead of InP, or may have a laminated structure of InP and InAlAs.

[0162] The p-type diffusion region 15 is formed by partially selectively diffusing a p-type dopant such as Zn in a solid or vapor phase. The carrier concentration of the p-type diffusion region 15 is 5×10 17 cm -3That is all. The tip of the p-type diffusion region 15 may be located at a depth reaching partway into the n-type InP window layer 11, at a depth reaching the i-type InAlGaAs / InAlAs graded layer 6, or at a depth reaching the i-type InAs / GaAs digital alloy structure light absorption layer 5. In the device structures shown in FIGS. 14 and 15 , the p-type diffusion region 15 has a depth reaching the i-type InAlGaAs / InAlAs graded layer 6. A p-type InGaAs contact layer 8 is provided on the p-type diffusion region 15.

[0163] In a front-illuminated APD, which is an example of a semiconductor photodetector 130 shown in FIG. 14, an n-type electrode 31 is provided on the back surface. On the other hand, in a front-illuminated APD, which is an example of a semiconductor photodetector 130a shown in FIG. 15, an n-type electrode 31a is provided on the front surface in addition to an electrode 31b on the back surface. That is, in the semiconductor photodetector 130a, an n-type InP conductive layer 2b is provided on an Fe-doped semi-insulating InP substrate 1a. After crystal growth, the semiconductor layers above the n-type InP conductive layer 2b are partially removed, and then the n-type electrode 31a is formed on the n-type InP conductive layer 2b. In the semiconductor photodetector 130a, a p-type InP substrate or an n-type InP substrate may be used instead of the Fe-doped semi-insulating InP substrate 1a.

[0164] <Function of the Semiconductor Photodetector (APD) According to the Fourth Embodiment> In a mesa structure such as the front-illuminated APD, which is an example of the semiconductor photodetector 110 according to the second embodiment shown in Figure 6, the side portion of the multiplication layer to which an electric field is applied is exposed to the outside and is therefore prone to deterioration. In particular, when an InAs / GaAs digital alloy structure is used as the light absorption layer, the strain in each layer of the i-type InAs / GaAs digital alloy structure light absorption layer 5 is high, so dislocation defects and disorder are likely to occur from the exposed portion toward the interior, which may result in the problem of deterioration of the semiconductor photodetector.

[0165] As a result, in the case of a light absorption layer in which an InAs / GaAs digital alloy structure is thinned, as in the semiconductor photodetectors 110 and 110a according to the second embodiment, the dark current generated in the side surface portion may shorten the life of the semiconductor photodetector compared to the conventional InGaAs random alloy structure.

[0166] 14 and 15 , when a p-type diffusion region 15 is provided, the portion of the i-type InAs / GaAs digital alloy structure light absorbing layer 5 to which an electric field is applied, i.e., the portion directly below the p-type diffusion region 15, is not exposed to the outside of the crystal layer, so that the occurrence of deterioration and disorder can be prevented in the i-type InAs / GaAs digital alloy structure light absorbing layer 5, in which the strain in each layer is high. As a result, it is possible to maintain a high absorption coefficient as a light absorbing layer.

[0167] Effect of Fourth Embodiment As described above, according to the semiconductor photodetector of the fourth embodiment, even though Zn diffusion is performed to form a p-type diffusion region when forming the device structure, disordering of the InAs / GaAs digital alloy structure light absorption layer and the InAs / GaAs digital alloy structure light absorption layer can be prevented, and therefore, an effect is achieved in which a semiconductor photodetector that operates over a wide response band, has high reception sensitivity, and is highly reliable can be obtained.

[0168] Modification of the Fourth Embodiment A front-illuminated APD, which is an example of a semiconductor light-receiving element according to a modification of the fourth embodiment, will be described below.

[0169] The semiconductor photodetector according to the modification of the fourth embodiment is structurally different in that the i-type InAlAs multiplication layer 13 in the front-illuminated APD, which is an example of the semiconductor photodetector 130 and 130a according to the fourth embodiment, i.e., the i-type InAlAs multiplication layer with a random alloy structure, is replaced with an i-type InAs / AlAs digital alloy structure multiplication layer.

[0170] The layer structure and layer thickness of the i-type InAs / AlAs digital alloy structure multiplication layer are the same as those of the i-type InAs / AlAs digital alloy structure multiplication layer of the semiconductor light-receiving element according to the modified example of the second embodiment, and therefore, description thereof will be omitted.

[0171] <Function of Semiconductor Photodetector (APD) According to Modification of Fourth Embodiment> In the semiconductor photodetector according to the modification of the fourth embodiment, even if a high-temperature heat treatment is performed to diffuse Zn in the diffusion step of forming the p-type diffusion region 15, disordering of the i-type InAs / AlAs digital alloy structure multiplication layer can be prevented, and therefore, the dead space length does not shorten as shown in Fig. 11C, and it is possible to maintain a long dead space length as shown in Fig. 11B. As a result, it is possible to maintain the ionization rate ratio k at approximately zero despite the Zn diffusion performed in the manufacturing process.

[0172] <Effects of the Variation of the Fourth Embodiment> As described above, the semiconductor photodetector according to the variation of the fourth embodiment 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 over a wider response band, has high reception sensitivity, and is highly reliable.

[0173] Fifth Embodiment Fig. 16 is a cross-sectional view showing the device structure of a front-illuminated APD, which is an example of a semiconductor light-receiving device 140 according to a fifth embodiment.

[0174] <Element Structure of Semiconductor Photodetector (APD) According to Fifth Embodiment> A front-illuminated APD, which is an example of semiconductor photodetector 140 according to the fifth embodiment, is characterized in that, in addition to the element structure of the front-illuminated APD, which is an example of semiconductor photodetector 130 according to the fourth embodiment, isolation groove 17 is provided along the outer periphery of p-type diffusion region 15 formed in n-type InP window layer 11.

[0175] The depth of the separation groove 17 is preferably in the range of 2 μm to 5 μm. The opening width of the separation groove 17 is preferably in the range of 0.5 μm to 100 μm. The bottom of the separation groove 17 reaches at least the i-type InAs / GaAs digital alloy structure light absorption layer 5. Note that FIG. 16 shows an example in which the bottom of the separation groove 17 reaches partway through the n-type InAlAs buffer layer 2a. The separation groove 17 may be formed by either dry etching or wet etching. However, a method in which wet etching is added after dry etching, which has excellent depth control, to remove damaged layers caused by the dry etching is preferred.

[0176] The inside of the isolation groove 17 and the surface of the n-type InP window layer 11 are protected by a surface protective film 18, which is an insulating film made of an oxide such as SiN or SiO. The surface protective film 18 also serves as an anti-reflective coating for the light receiving section. The thickness of the surface protective film 18 is preferably in the range of 50 nm to 5000 nm. Alternatively, the surface protective film 18 may be an organic film such as benzocyclobutene (BCB).

[0177] <Function of the Semiconductor Photodetector (APD) According to the Fifth Embodiment> When the light absorption layer has an InAs / GaAs digital alloy structure, the strain in each layer of the InAs / GaAs digital alloy structure is high, and the application of external stress easily causes disordering of the InAs / GaAs digital alloy structure. Therefore, by providing a separation groove 17 along the outer periphery of the p-type diffusion region 15, as in the semiconductor photodetector 140 according to the fifth embodiment, stress across the entire wafer during the manufacturing process can be alleviated. Furthermore, even when the semiconductor photodetector 140 is in the individual state, the presence of the separation groove 17 alleviates stress, thereby alleviating stress concentration in the central light-receiving portion of the semiconductor photodetector 140. Furthermore, because the i-type InAs / GaAs digital alloy structure light absorption layer 5 is exposed in the separation groove 17, it is desirable that the surface of the separation groove 17 be covered with the above-described surface protection film 18. Furthermore, because a high electric field is not applied to the separation groove 17, it does not become a starting point for degradation.

[0178] When the p-type diffusion region 15 is provided as in the semiconductor light-receiving element 140 shown in FIG. 16 , the portion of the i-type InAs / GaAs digital alloy structure light-absorbing layer 5 to which an electric field is applied, i.e., the portion directly below the p-type diffusion region 15, is not exposed to the outside of the crystal layer, which has the effect of preventing deterioration in the i-type InAs / GaAs digital alloy structure light-absorbing layer 5 in which the strain in each layer is high.

[0179] Furthermore, since stress is alleviated by the separation groove 17 even during operation of the semiconductor light-receiving element 140, no disorder occurs even when the semiconductor light-receiving element 140 is used for a long period of time. In other words, the semiconductor light-receiving element 140 of the fifth embodiment can achieve high reliability, such as being able to operate over a wide band for a long period of time and maintaining low noise.

[0180] Effect of Fifth Embodiment As described above, according to the semiconductor photodetector of the fifth embodiment, even though Zn diffusion is performed to form the p-type diffusion region 15 during device structure formation, disordering of the InAs / GaAs digital alloy structure light absorption layer can be prevented, and furthermore, the presence of the separation groove can alleviate stress, thereby achieving the effect of providing a semiconductor photodetector that operates over a wide response band, has high reception sensitivity, and is more reliable.

[0181] Modification of the Fifth Embodiment A front-illuminated APD, which is an example of a semiconductor light-receiving element according to a modification of the fifth embodiment, will be described below.

[0182] The semiconductor photodetector according to the modified example of the fifth embodiment is structurally different in that the i-type InAlAs multiplication layer 13 in the front-illuminated APD, which is an example of the semiconductor photodetector 140 according to the fifth embodiment, i.e., the i-type InAlAs multiplication layer with a random alloy structure, is replaced with an i-type InAs / AlAs digital alloy structure multiplication layer.

[0183] The layer structure and layer thickness of the i-type InAs / AlAs digital alloy structure multiplication layer are the same as those of the i-type InAs / AlAs digital alloy structure multiplication layer of the semiconductor light-receiving element according to the modified example of the second embodiment, and therefore, description thereof will be omitted.

[0184] <Function of Semiconductor Photodetector (APD) According to Modification of Fifth Embodiment> In the semiconductor photodetector according to the modification of the fifth embodiment, even if a high-temperature heat treatment is performed to diffuse Zn in the diffusion step of forming the p-type diffusion region 15, disordering of the InAs / AlAs digital alloy structure multiplication layer can be prevented, and therefore, the dead space length does not shorten as shown in Fig. 11C, and it is possible to maintain a long dead space length as shown in Fig. 11B. As a result, it is possible to maintain the ionization rate ratio k at approximately zero despite the Zn diffusion performed in the manufacturing process.

[0185] Effect of Modification of Fifth Embodiment As described above, the semiconductor photodetector according to the modification of the fifth embodiment has a digital alloy structure multiplication layer whose layer thickness is controlled within a preset range, and although Zn diffusion is performed to form a p-type diffusion region when the device structure is formed, disordering of the InAs / GaAs digital alloy structure light absorption layer and the InAs / AlAs digital alloy structure multiplication layer can be prevented, and furthermore, stress can be alleviated by the presence of the separation groove, so that an effect is achieved in that a semiconductor photodetector which operates over a wide response band, has high reception sensitivity, and is more reliable can be obtained.

[0186] Sixth Embodiment Fig. 17 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor light-receiving device 140a according to a sixth embodiment.

[0187] <Element Structure of Semiconductor Photodetector (APD) According to Sixth Embodiment> While a front-illuminated APD, which is an example of the semiconductor photodetector 140 according to the fifth embodiment, receives light from the front side, a back-illuminated APD, which is an example of the semiconductor photodetector 140a according to the sixth embodiment, is characterized in that it has an element structure in which a portion of the n-type electrode 31c on the back side is removed to provide an opening 33, and light is incident on the n-type InP substrate 1 through an anti-reflective coating film 35 formed to cover the opening 33. That is, the opening 33, which is covered with the anti-reflective coating film 35 and serves as an incident region for incident light 90, is provided on the back side of the n-type InP substrate 1 opposite the p-type electrode 32. In addition, a central portion of the p-type InGaAs contact layer 8 is partially removed, and a surface protective film 18 made of an insulating film made of an oxide film such as SiN or SiO is formed on the exposed p-type diffusion region 15, and this is further covered with the p-type electrode 32, thereby increasing the reflectance of light from the p-type electrode 32.

[0188] Furthermore, because the area of ​​the p-type diffusion region 15 can be formed smaller in a back-illuminated APD such as the semiconductor photodetector 140a than in a front-illuminated APD such as the semiconductor photodetector 140, the stress generated during p-type diffusion can be further reduced, further preventing disordering of the i-type InAs / GaAs digital alloy structure light-absorbing layer 5. As a result, disordering of the i-type InAs / GaAs digital alloy structure light-absorbing layer 5 can be prevented even though Zn diffusion is performed during device structure formation. As a result, the light-absorbing layer can maintain a high absorption coefficient. Furthermore, because the separation grooves 17 can reduce stress, a semiconductor photodetector can be obtained that is highly reliable, operates over a wide response band, and has excellent low-noise characteristics.

[0189] Effect of Sixth Embodiment As described above, the semiconductor photodetector according to the sixth embodiment has a back-illuminated APD structure, which allows the area of ​​the p-type diffusion region to be smaller than that of a front-illuminated APD, and the isolation trenches further reduce stress, resulting in a semiconductor photodetector that is highly reliable, operates over a wide response band, and has excellent low-noise characteristics. Furthermore, as described above, the reflectivity of light from the p-type electrode 32 is increased, so that light that is not absorbed by the i-type InAs / GaAs digital alloy structure light absorption layer 5 and that transmits through the i-type InAs / GaAs digital alloy structure light absorption layer 5 is reflected by the p-type electrode 32 and returns to the i-type InAs / GaAs digital alloy structure light absorption layer 5, thereby increasing the photosensitivity. As a result, the i-type InAs / GaAs digital alloy structure light absorption layer 5 can be made thinner, which shortens the transit times of electrons and holes. This, combined with the i-type InAs / GaAs digital alloy structure light absorption layer 5, provides a semiconductor photodetector that can further widen the response band.

[0190] Modification of Sixth Embodiment A back-illuminated APD, which is an example of a semiconductor light-receiving element according to a modification of the sixth embodiment, will be described below.

[0191] The semiconductor photodetector according to the modified example of the sixth embodiment is structurally different in that the i-type InAlAs multiplication layer 13 in the back-illuminated APD, which is an example of the semiconductor photodetector 140 according to the sixth embodiment, i.e., the i-type InAlAs multiplication layer with a random alloy structure, is replaced with an i-type InAs / AlAs digital alloy structure multiplication layer.

[0192] The layer structure and layer thickness of the i-type InAs / AlAs digital alloy structure multiplication layer are the same as those of the i-type InAs / AlAs digital alloy structure multiplication layer of the semiconductor light-receiving element according to the modified example of the second embodiment, and therefore, description thereof will be omitted.

[0193] <Function of Semiconductor Photodetector (APD) According to Modification of Sixth Embodiment> Similar to the semiconductor photodetector according to the modification of the fifth embodiment, the back-illuminated APD, which is an example of the semiconductor photodetector according to the modification of the sixth embodiment, can prevent disordering of the InAs / AlAs digital alloy structure multiplication layer even when a high-temperature heat treatment is performed to diffuse Zn in the diffusion step for forming the p-type diffusion region 15. Therefore, the dead space length does not shorten as shown in FIG. 11C , and it is possible to maintain a long dead space length as shown in FIG. 11B .

[0194] Effect of Modification of Sixth Embodiment As described above, according to the semiconductor photodetector according to the modification of the sixth embodiment, since the device structure is a back-illuminated APD, the area of ​​the p-type diffusion region can be made smaller than that of a front-illuminated APD, and the device has a digital alloy structure multiplication layer whose layer thickness is controlled within a predetermined range. Despite the Zn diffusion performed to form the p-type diffusion region during device structure formation, disordering of the InAs / GaAs digital alloy structure light absorption layer and the InAs / AlAs digital alloy structure multiplication layer can be prevented, and furthermore, the presence of the separation grooves can alleviate stress, resulting in an effect of providing a semiconductor photodetector that operates over a wide response band, has high reception sensitivity, and is more reliable.

[0195] Seventh Embodiment Fig. 18 is a cross-sectional view showing the element structure of a front-illuminated PD, which is an example of a semiconductor light-receiving element 150 according to a seventh embodiment. Also, Fig. 19 is a cross-sectional view showing the element structure of a front-illuminated PD, which is an example of a semiconductor light-receiving element 150a according to the seventh embodiment.

[0196] <Element Structure of Semiconductor Photodetector (PD) According to Seventh Embodiment> The semiconductor photodetector 150 according to the seventh embodiment has the same structure as the semiconductor photodetector 100 according to the first embodiment, from the n-type InP substrate 1 to the i-type InAlGaAs / InAlAs graded layer 6, and therefore a description thereof will be omitted.

[0197] The semiconductor photodetector 150 according to the seventh embodiment shown in FIG. 18 includes a structure ranging from an n-type InP substrate 1 to an i-type InAlGaAs / InAlAs graded layer 6, an n-type InP window layer 11 having a thickness of 0.1 to 3.0 μm, a p-type InAlAs conductive layer 25, a p-type InGaAs contact layer 8, an n-type electrode 31 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 8.

[0198] 19 has the same configuration as the semiconductor photodetector 150 according to the seventh embodiment, except that an Fe-doped semi-insulating InP substrate 1a is used as the substrate and an n-type electrode 31a is formed on the front side of an n-type InP conductive layer 2b. The back side electrode 31b is not intended to pass current but is necessary for fixing the semiconductor photodetector with solder.

[0199] The semiconductor photodetectors 150 and 150a according to the seventh embodiment differ from the semiconductor photodetector 100 according to the first embodiment in that the p-type InAlAs conductive layer 25 formed on the n-type InP window layer 11 is formed in a mesa shape, and a p-type InGaAs contact layer 8 and a p-type electrode 32 are provided on the p-type InAlAs conductive layer 25. The n-type InP window layer 11 may have a thickness of 50 nm or more, but a thickness of 200 nm or less is preferable in order to prevent the carrier transit time from becoming long. The conductivity type of the n-type InP window layer 11 may be undoped instead of n-type. In the case of n-type, the carrier concentration is 5.0×10 17 cm -3 The following is desirable:

[0200] The method for manufacturing the semiconductor light-receiving elements 150 and 150a according to the seventh embodiment is characterized in that the p-type InAlAs conductive layer 25 is crystal-grown on the n-type InP window layer 11 by MOVPE, MBE, or the like, and then the p-type InGaAs contact layer 8 is crystal-grown, and thereafter the p-type InAlAs conductive layer 25 is removed while leaving the light-receiving portion.

[0201] The thickness of the p-type InAlAs conductive layer 25 is preferably 100 nm or more and 3000 nm or less. The carrier concentration of the p-type InAlAs conductive layer 25 is preferably high, that is, 5.0×10 17 cm -3 The p-type InAlAs conductive layer 25 may be a laminated structure of p-type InP, p-type InGaAs, p-type InGaAsP, or p-type InAlGaAs instead of p-type InAlAs.

[0202] 18 has an n-type electrode 31 on the back surface side, whereas the semiconductor light-receiving element 150a shown in FIG. 19 has an n-type electrode 31a on the front surface side in addition to the back surface side electrode 31b. That is, in the semiconductor light-receiving element 150a, an n-type InP conductive layer 2b is provided on an Fe-doped semi-insulating InP substrate 1a, and after crystal growth, the layers above the n-type InP conductive layer 2b are partially removed, and then the n-type electrode 31a is formed on the n-type InP conductive layer 2b. A p-type InP substrate or an n-type InP substrate may be used instead of the Fe-doped semi-insulating InP substrate 1a.

[0203] <Operation of the Semiconductor Photodetector (PD) According to Seventh Embodiment> The operation of the semiconductor photodetector 150, 150a according to the seventh embodiment will be described below. When forming the p-type InAlAs conductive layer 25 to which a voltage is applied, the p-type diffusion process involving a heat treatment at a high temperature of 400°C or higher is not performed, as in the semiconductor photodetector 120 according to the third embodiment shown in FIG. 12 . This prevents disordering of the i-type InAs / GaAs digital alloy structure light absorbing layer 5. This makes it possible to maintain a high absorption coefficient within the i-type InAs / GaAs digital alloy structure light absorbing layer 5. Meanwhile, as in the third embodiment, the light absorbing layer directly below the light receiving portion to which a high electric field is applied is located away from the side of the device, thereby achieving high reliability as a semiconductor photodetector. Therefore, it is possible to achieve both high photosensitivity and high reliability for the semiconductor photodetector. In other words, a highly reliable semiconductor photodetector, i.e., a PD, is obtained.

[0204] <Effects of Seventh Embodiment> As described above, according to the semiconductor photodetector of the seventh embodiment, similar to the semiconductor photodetector of the third embodiment, the light absorption layer is located away from the side surface of the element, and therefore a semiconductor photodetector that is highly reliable and operates over a wide response band can be obtained.

[0205] Eighth Embodiment Fig. 20 is a cross-sectional view showing the element structure of a front-illuminated APD, which is an example of a semiconductor light-receiving element 160 according to the eighth embodiment. Also, Fig. 21 is a cross-sectional view showing the element structure of a front-illuminated APD, which is an example of a semiconductor light-receiving element 160a according to the eighth embodiment.

[0206] <Element Structure of Semiconductor Photodetector (APD) According to Eighth Embodiment> The semiconductor photodetector 160 according to the eighth embodiment has the same structure as the semiconductor photodetector 110 according to the second embodiment, from the n-type InP substrate 1 to the i-type InAlGaAs / InAlAs graded layer 6, and therefore the description thereof will be omitted.

[0207] The semiconductor photodetector 160 according to the eighth embodiment shown in FIG. 20 includes a structure ranging from an n-type InP substrate 1 to an i-type InAlGaAs / InAlAs graded layer 6, an n-type InP window layer 11 having a thickness of 0.1 to 3.0 μm, a p-type InAlAs conductive layer 25, a p-type InGaAs contact layer 8, an n-type electrode 31 formed on the back surface side of the n-type InP substrate 1, and a p-type electrode 32 formed on the p-type InGaAs contact layer 8.

[0208] 21 has the same configuration as the semiconductor photodetector 160 according to the eighth embodiment, except that an Fe-doped semi-insulating InP substrate 1a is used as the substrate and an n-type electrode 31a is formed on the front side of an n-type InP conductive layer 2b. The back side electrode 31b is not intended to pass current but is necessary for fixing the semiconductor photodetector with solder.

[0209] The semiconductor photodetectors 160 and 160a according to the eighth embodiment differ from the semiconductor photodetector 110 according to the second embodiment in that the p-type InAlAs conductive layer 25 formed on the n-type InP window layer 11 is formed in a mesa shape in the semiconductor photodetectors 160 and 160a, and a p-type InGaAs contact layer 8 and a p-type electrode 32 are provided on the p-type InAlAs conductive layer 25. The n-type InP window layer 11 may have a thickness of 50 nm or more, but a thickness of 200 nm or less is preferable in order to prevent the carrier transit time from becoming long. The conductivity type of the n-type InP window layer 11 may be undoped instead of n-type. In the case of n-type, the carrier concentration is 5.0×10 17 cm -3 The following is desirable:

[0210] The method for manufacturing the semiconductor light-receiving elements 160 and 160a according to the eighth embodiment is characterized in that the p-type InAlAs conductive layer 25 is crystal-grown on the n-type InP window layer 11 by MOVPE, MBE, or the like, and then the p-type InGaAs contact layer 8 is crystal-grown, and thereafter the p-type InAlAs conductive layer 25 is removed while leaving the light-receiving portion.

[0211] The thickness of the p-type InAlAs conductive layer 25 is preferably 100 nm or more and 3000 nm or less. The carrier concentration of the p-type InAlAs conductive layer 25 is preferably high, that is, 5.0×10 17 cm -3 The p-type InAlAs conductive layer 25 may be a laminated structure of p-type InP, p-type InGaAs, p-type InGaAsP, or p-type InAlGaAs instead of p-type InAlAs.

[0212] 20 has an n-type electrode 31 on the back surface side, whereas the semiconductor light-receiving element 160a shown in Fig. 21 has an n-type electrode 31a on the front surface side in addition to the back surface side electrode 31b. That is, in the semiconductor light-receiving element 160a, an n-type InP conductive layer 2b is provided on an Fe-doped semi-insulating InP substrate 1a, and after crystal growth, the layers above the n-type InP conductive layer 2b are partially removed, and then the n-type electrode 31a is formed on the n-type InP conductive layer 2b. A p-type InP substrate or an n-type InP substrate may be used instead of the Fe-doped semi-insulating InP substrate 1a.

[0213] <Operation of the Semiconductor Photodetector (APD) According to the Eighth Embodiment> The operation of the semiconductor photodetector 160, 160a according to the eighth embodiment will be described below. When forming the p-type InAlAs conductive layer 25 to which a voltage is applied, the p-type diffusion process involving a heat treatment at a high temperature of 400°C or higher is not performed, as in the semiconductor photodetector 130 according to the fourth embodiment shown in FIG. 14 . This prevents disordering of the i-type InAs / GaAs digital alloy structure light absorbing layer 5. This makes it possible to maintain a high absorption coefficient within the i-type InAs / GaAs digital alloy structure light absorbing layer 5. Meanwhile, as in the fourth embodiment, the light absorbing layer directly below the light receiving portion to which a high electric field is applied is located away from the side of the device, thereby achieving high reliability as a semiconductor photodetector. Therefore, a semiconductor photodetector, i.e., an APD, can be obtained that achieves both high photosensitivity and high reliability.

[0214] <Effects of Eighth Embodiment> As described above, according to the semiconductor photodetector of the eighth embodiment, similarly to the semiconductor photodetector of the fourth embodiment, the light absorption layer is located away from the side surface of the element, and therefore, an effect is achieved in that a semiconductor photodetector that operates in a wide response band, has high receiving sensitivity, and is more reliable can be obtained.

[0215] Modification of Eighth Embodiment A front-illuminated APD, which is an example of a semiconductor light-receiving element according to a modification of the eighth embodiment, will be described below.

[0216] The semiconductor photodetector according to the modified example of the eighth embodiment is structurally different in that the i-type InAlAs multiplication layer 13 in the front-illuminated APD, which is an example of the semiconductor photodetector 160 or 160a according to the eighth embodiment, i.e., the i-type InAlAs multiplication layer with a random alloy structure, is replaced with an i-type InAs / AlAs digital alloy structure multiplication layer.

[0217] The layer structure and layer thickness of the i-type InAs / AlAs digital alloy structure multiplication layer are the same as those of the i-type InAs / AlAs digital alloy structure multiplication layer of the semiconductor light-receiving element according to the modified example of the second embodiment, and therefore, description thereof will be omitted.

[0218] <Function of Semiconductor Photodetector (APD) According to the Modification of Embodiment 8> The front-illuminated APD, which is an example of a semiconductor photodetector according to the modification of embodiment 8, can maintain a long dead space length as shown in FIG. 11B, for example, without shortening the dead space length as shown in FIG. 11C, similar to the semiconductor photodetector according to the modification of embodiment 2.

[0219] <Effects of the Variation of the Eighth Embodiment> As described above, the semiconductor photodetector according to the variation of the eighth embodiment has 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 is more reliable, operates over a wide response band, and has high reception sensitivity.

[0220] Ninth Embodiment Fig. 22 is a cross-sectional view showing the device structure of a front-illuminated APD, which is an example of a semiconductor light-receiving device 170 according to a ninth embodiment.

[0221] <Element Structure of Semiconductor Photodetector (APD) According to Ninth Embodiment> A front-illuminated APD, which is an example of the semiconductor photodetector 170 according to the ninth embodiment, is characterized in that a separation groove 17 is further provided along the outer periphery of the p-type InAlAs conductive layer 25 in addition to the element structure of the front-illuminated APD, which is an example of the semiconductor photodetector 160 according to the eighth embodiment.

[0222] The depth of the separation groove 17 is preferably in the range of 2 μm to 5 μm. The opening width of the separation groove 17 is preferably in the range of 0.5 μm to 100 μm. The bottom of the separation groove 17 reaches at least the i-type InAs / GaAs digital alloy structure light absorption layer 5. Note that FIG. 22 shows an example in which the bottom of the separation groove 17 reaches partway through the n-type InAlAs buffer layer 2a. The separation groove 17 may be formed by either dry etching or wet etching. However, a method in which wet etching is added after dry etching, which has excellent depth control, to remove damaged layers caused by the dry etching is preferred.

[0223] The inside of the isolation groove 17 and the surface of the n-type InP window layer 11 are protected by a surface protective film 18, which is an insulating film made of an oxide such as SiN or SiO. The surface protective film 18 also serves as an anti-reflective coating for the light receiving section. The thickness of the surface protective film 18 is preferably in the range of 50 nm to 5000 nm. Alternatively, the surface protective film 18 may be an organic film such as BCB.

[0224] <Function of the Semiconductor Photodetector (APD) According to Ninth Embodiment> When an InAs / GaAs digital alloy structure is used as the light absorption layer, the strain in each layer of the InAs / GaAs digital alloy structure is high, and therefore, when external stress is applied, disordering is likely to occur. Therefore, by providing a separation groove 17 along the periphery of the p-type InAlAs conductive layer 25, as in the semiconductor photodetector 170 according to the ninth embodiment, stress across the entire wafer during the manufacturing process can be alleviated. Furthermore, even when the semiconductor photodetector 170 is in the individual state, the presence of the separation groove 17 alleviates stress, thereby alleviating stress concentration in the central light-receiving portion of the semiconductor photodetector 170. As a result, disordering of the digital alloy structure light absorption layer can be prevented, thereby enabling the absorption coefficient to be maintained at a high level. Furthermore, because the i-type InAs / GaAs digital alloy structure light absorption layer 5 is exposed in the separation groove 17, it is desirable for the surface of the separation groove 17 to be covered with the above-mentioned surface protection film 18. Since a high electric field is not applied to the separation groove 17, it does not become a starting point of degradation.

[0225] <Effects of Embodiment 9> As described above, according to the semiconductor photodetector of embodiment 9, the separation groove can relieve stress due to heat treatment and the like in the manufacturing process, and therefore disordering of the digital alloy structure light absorption layer can be prevented, thereby achieving the effect of maintaining a high absorption coefficient, and of obtaining a semiconductor photodetector that operates in a wide response band, has high receiving sensitivity, and is more reliable.

[0226] Modification of the Ninth Embodiment A front-illuminated APD, which is an example of a semiconductor light-receiving element according to a modification of the ninth embodiment, will be described below.

[0227] The semiconductor photodetector according to the modified example of the ninth embodiment is structurally different in that the i-type InAlAs multiplication layer 13 in the front-illuminated APD, which is an example of the semiconductor photodetector 170 according to the ninth embodiment, i.e., the i-type InAlAs multiplication layer with a random alloy structure, is replaced with an i-type InAs / AlAs digital alloy structure multiplication layer.

[0228] The layer structure and layer thickness of the i-type InAs / AlAs digital alloy structure multiplication layer are the same as those of the i-type InAs / AlAs digital alloy structure multiplication layer of the semiconductor light-receiving element according to the modified example of the second embodiment, and therefore, description thereof will be omitted.

[0229] <Function of Semiconductor Photodetector (APD) According to Modification of Ninth Embodiment> In the semiconductor photodetector according to the modification of the ninth embodiment, it is not necessary to form the p-type diffusion region 15. In other words, the high-temperature heat treatment required for Zn diffusion to form the p-type diffusion region 15 is not required. This prevents disordering of the InAs / AlAs digital alloy structure multiplication layer. Therefore, it is possible to maintain a long dead space length as shown in Fig. 11B without shortening the dead space length as shown in Fig. 11C. As a result, it is possible to maintain the ionization rate ratio k at approximately zero.

[0230] Effect of the Variation of Ninth Embodiment As described above, the semiconductor photodetector according to the variation of the ninth embodiment has a digital alloy structure multiplication layer whose layer thickness is controlled within a preset range, and does not require high-temperature heat treatment, so that disordering of the InAs / GaAs digital alloy structure light absorption layer and the InAs / AlAs digital alloy structure multiplication layer can be prevented, and furthermore, the presence of the separation grooves can relieve stress, so that an effect is achieved in that a semiconductor photodetector having higher reliability, operation over a wide response band, and high reception sensitivity can be obtained.

[0231] Tenth Embodiment Fig. 23 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor light-receiving device 170a according to a tenth embodiment.

[0232] <Device Structure of Semiconductor Photodetector (APD) According to Tenth Embodiment> While a front-illuminated APD, which is an example of the semiconductor photodetector 170 according to the ninth embodiment, receives light from the front side as shown in Fig. 22 , a back-illuminated APD, which is an example of the semiconductor photodetector 170a according to the tenth embodiment, is characterized in that it has a device structure in which a portion of the n-type electrode 31c on the back side is removed to provide an opening 33, and light is incident on the n-type InP substrate 1 through an anti-reflective coating film 35 formed to cover the opening 33, as shown in Fig. 23 . That is, the opening 33, which is covered with the anti-reflective coating film 35 and serves as an incidence region for incident light 90, is provided on the back side of the n-type InP substrate 1 opposite the p-type electrode 32. In addition, a central portion of the p-type InGaAs contact layer 8 is partially removed, and a surface protective film 18 made of an insulating film made of an oxide such as SiN or SiO is formed on the exposed p-type InAlAs conductive layer 25, and this is further covered with the p-type electrode 32, thereby increasing the reflectance of light from the p-type electrode 32.

[0233] The back-illuminated APD, which is an example of the semiconductor photodetector 170a according to the tenth embodiment, can reduce stress due to heat treatment and the like in the manufacturing process by using the separation grooves, similar to the semiconductor photodetector 170 according to the ninth embodiment, and therefore can prevent disordering of the i-type InAs / GaAs digital alloy structure light absorption layer 5.

[0234] Furthermore, since the area of ​​the p-type InAlAs conductive layer 25 can be formed smaller in a back-illuminated APD such as the semiconductor photodetector 170a than in a front-illuminated APD, stress due to heat treatment and the like in the manufacturing process can be further reduced, thereby further preventing disordering of the i-type InAs / GaAs digital alloy structure light absorption layer 5. As a result, even though a heat treatment process is performed during device structure formation, disordering of the i-type InAs / GaAs digital alloy structure light absorption layer 5 can be prevented, resulting in the effect of providing a semiconductor photodetector that is highly reliable, operates over a wide bandwidth, and has excellent low-noise characteristics.

[0235] Furthermore, as described above, the reflectivity of light from the p-type electrode 32 is increased, so that light that is not absorbed by the i-type InAs / GaAs digital alloy structure light absorption layer 5 and that is transmitted through it is reflected by the p-type electrode 32 and returns to the i-type InAs / GaAs digital alloy structure light absorption layer 5, thereby increasing sensitivity. As a result, the i-type InAs / GaAs digital alloy structure light absorption layer 5 can be made thinner, which shortens the transit time of electrons and holes, thereby providing the effect of obtaining a semiconductor photodetector that can further widen the response band.

[0236] Furthermore, in a back-illuminated APD such as the semiconductor photodetector 170a, the area of ​​the mesa-shaped p-type InAlAs conductive layer 25 can be reduced compared to that of a front-illuminated APD. This reduces the effect of stress from the mesa portion of the p-type InAlAs conductive layer 25, preventing disordering of the i-type InAs / GaAs digital alloy structure light-absorbing layer 5. Furthermore, since stress is relaxed even during operation of the semiconductor photodetector 170a, the i-type InAs / GaAs digital alloy structure light-absorbing layer 5 does not become disordered even over long periods of time. As a result, the light-absorbing layer can maintain a high absorption coefficient. In other words, the semiconductor photodetector 170a according to the tenth embodiment can operate over a wide response band for a long period of time and maintain high light-receiving sensitivity.

[0237] Effect of Tenth Embodiment As described above, according to the semiconductor photodetector of the tenth embodiment, the area of ​​the p-type conductive layer can be made small, and therefore it is possible to further reduce the stress generated in the heat treatment process, thereby further preventing disordering of the i-type InAs / GaAs digital alloy structure light absorption layer, and the separation grooves can further reduce the stress, thereby providing an effect of providing a semiconductor photodetector that operates in a wide response band, has high reception sensitivity, and is highly reliable.

[0238] Modification of Tenth Embodiment A front-illuminated APD, which is an example of a semiconductor light-receiving element according to a modification of the tenth embodiment, will be described below.

[0239] The semiconductor photodetector according to the modified example of the tenth embodiment is structurally different in that the i-type InAlAs multiplication layer 13 in the front-illuminated APD, which is an example of the semiconductor photodetector 170a according to the tenth embodiment, i.e., the i-type InAlAs multiplication layer with a random alloy structure, is replaced with an i-type InAs / AlAs digital alloy structure multiplication layer.

[0240] The layer structure and layer thickness of the i-type InAs / AlAs digital alloy structure multiplication layer are the same as those of the i-type InAs / AlAs digital alloy structure multiplication layer of the semiconductor light-receiving element according to the modified example of the second embodiment, and therefore, description thereof will be omitted.

[0241] <Function of Semiconductor Photodetector (APD) According to Modification of Tenth Embodiment> In the semiconductor photodetector according to the modification of the tenth embodiment, it is not necessary to form the p-type diffusion region 15. In other words, the high-temperature heat treatment required for Zn diffusion to form the p-type diffusion region 15 is not required. Therefore, disordering of the InAs / AlAs digital alloy structure multiplication layer can be prevented. For example, the dead space length does not shorten as shown in Fig. 9C, but rather it is possible to maintain a long dead space length as shown in Fig. 9B. As a result, it is possible to maintain the ionization rate ratio k at approximately zero.

[0242] Effect of Modification of Tenth Embodiment As described above, the semiconductor photodetector according to the modification of the tenth embodiment has a digital alloy structure multiplication layer whose layer thickness is controlled within a preset range, and does not require high-temperature heat treatment, thereby preventing disordering of the InAs / GaAs digital alloy structure light absorption layer and the InAs / AlAs digital alloy structure multiplication layer, and furthermore, the presence of the separation grooves enables stress to be alleviated, thereby providing an effect of providing a semiconductor photodetector that operates over a wide response band, has high reception sensitivity, and is more reliable.

[0243] Eleventh Embodiment Fig. 24 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor light-receiving device 180 according to an eleventh embodiment.

[0244] <Element Structure of Semiconductor Photodetector (APD) According to Eleventh Embodiment> A semiconductor photodetector 180 according to the eleventh embodiment includes an Fe-doped semi-insulating InP substrate 1a and an Fe-doped semi-insulating InP layer having a carrier concentration of 1 to 5×10 18 cm -3 and a p-type InAlGaAs contact layer 40 having a thickness of 0.1 to 1 μm and a carrier concentration of 1 to 5×10 18 cm -3 and a p-type InP conductive layer 41 having a layer thickness of 0.1 to 1 μm, and a p-type or low carrier concentration (5×10 17 cm -3 hereinafter) and an n-type or i-type InAlGaAs / InAlAs graded layer 42 having a carrier concentration of 1×10 17 cm -3 an i-type InAs / GaAs digital alloy structure light absorption layer 43 having a digital alloy structure in which i-type InAs layers (for example, a layer thickness of two atomic layers, about 0.6 nm) and i-type GaAs layers (for example, a layer thickness of two atomic layers, about 0.6 nm) are alternately stacked multiple times and having a layer thickness of 50 to 2000 nm; and a carrier concentration of 1×10 17 cm -3 ~5 x 10 18 cm -3 and a p-type InP field relaxation layer 44 having a thickness of 10 to 100 nm, and a carrier concentration of 1×10 17 cm -3 an i-type InAlAs multiplication layer 45 having a thickness of 50 to 500 nm and being equal to or less than 100 nm; an n-type InAlAs field adjustment layer 46 having a thickness of 10 to 50 nm; an n-type InP window layer 47 having a thickness of 0.1 to 2 μm; and an n-type InP window layer 48 having a carrier concentration of 5×10 17 ~8 x 10 18 cm -3 and an n-type InAlAs conductive layer 49 having a thickness of 0.1 to 2 μm, and a carrier concentration of 5×10 17 ~8 x 10 18 cm -3The semiconductor device is composed of an n-type InGaAs contact layer 50 having a thickness of 0.1 to 2 μm, an n-type electrode 51 formed on the n-type InGaAs contact layer 50, a p-type electrode 52 formed on the p-type InAlGaAs contact layer 40, a metal film 53 formed on the back surface side of the Fe-doped semi-insulating InP substrate 1 a, and an anti-reflective coating film 35 provided in an opening 33 in the metal film 53.

[0245] The p-type InAlGaAs contact layer 40 and the p-type InP conductive layer 41 are also called p-type semiconductor layers. The central portion of the n-type InGaAs contact layer 50 is partially removed, and a surface protection film 18 made of an insulating film made of an oxide such as SiN or SiO is formed on the exposed n-type InAlAs conductive layer 49. This is then covered with an n-type electrode 51, thereby increasing the reflectance of light from the n-type electrode 51.

[0246] <Method for Manufacturing a Semiconductor Photodetector (APD) According to Eleventh Embodiment> A method for manufacturing a semiconductor photodetector 180 according to the eleventh embodiment will be described below. An Fe-doped semi-insulating InP substrate 1a is doped with an InP layer having a carrier concentration of 1 to 5×10 by MOVPE or MBE. 18 cm -3 The p-type InAlGaAs contact layer 40 is crystal-grown to a thickness of 0.1 to 1 μm. Here, an n-type InP substrate may be used instead of the Fe-doped semi-insulating InP substrate 1a. Furthermore, the p-type InAlGaAs contact layer 40 may be made of p-type InP, p-type InGaAsP, or p-type InGaAs instead of p-type InAlGaAs.

[0247] On the p-type InAlGaAs contact layer 40, a carrier concentration of 1 to 5×10 18 cm -3 The p-type InP conductive layer 41 is grown to a thickness of 0.1 to 1 μm. The p-type InP conductive layer 41 may be made of p-type InGaAsP or p-type InAlGaAs instead of p-type InP.

[0248] Next, an i-type, n-type, or p-type InAlAs layer having a small barrier to holes may be formed on the p-type InP conductive layer 41. Furthermore, a p-type or low carrier concentration (5×10 17 cm -3After growing the n-type or i-type InAlGaAs / InAlAs graded layer 42 (below), the i-type InGaAs light absorption layer 43 is grown by crystal growth to a layer thickness of 0.1 to 2 μm. The i-type InAs / GaAs digital alloy structure light absorption layer 43 is made of a low carrier concentration (5×10 17 cm -3 Here, either the p-type InP conductive layer 41 or the n-type InAlGaAs / InAlAs graded layer 42, or both, are not necessarily required.

[0249] Next, the carrier concentration is 1×10 17 ~5 x 10 18 cm -3 The p-type InP field buffer layer 44 is crystal-grown to a thickness of 10 to 100 nm. Examples of p-type dopants for the p-type InP field buffer layer 44 include Be, Zn, and C. The p-type InP field buffer layer 44 does not necessarily have to be p-type InP, and may also have a p-type InAlAs or p-type InAs / AlAs digital alloy structure.

[0250] Furthermore, an InAlGaAs / InAlAs graded layer having a thickness of 10 to 100 nm and an intermediate band gap value, such as InAlGaAs or InGaAsP, may be provided between the i-type InAs / GaAs digital alloy structure light absorption layer 43 and the p-type InP electric field relaxation layer 44.

[0251] An i-type InAlAs multiplication layer 45 and an n-type InAlAs field adjustment layer 46 are formed as multiplication layers by crystal growth on the p-type InP field relaxation layer 44. The n-type InAlAs field adjustment layer 46 is provided to prevent an excessive electric field from being applied to the outermost n-type InP window layer 47, which would increase dark current and reduce reliability. The n-type InP window layer 47 is formed by crystal growth on the n-type InAlAs field adjustment layer 46.

[0252] The i-type InAs / GaAs digital alloy structure light absorption layer 43 is composed of semiconductor layers in which an InAs layer (having a thickness of two atomic layers, approximately 0.6 nm) and a GaAs layer (having a thickness of two atomic layers, approximately 0.6 nm) are alternately stacked in this order from the side of the Fe-doped semi-insulating InP substrate 1a. The i-type InAs / GaAs digital alloy structure light absorption layer 43 may be formed in the order of a GaAs layer and an InAs layer.

[0253] The number of atomic layers in each layer of the i-type InAs / GaAs digital alloy structure light absorption layer 43 is preferably 2 to 4, and is optimally 2. The reason for this is that the thinner the atomic layer thickness of each layer, the greater the effect of the digital alloy structure in reducing the ionization rate ratio k.

[0254] The thickness of the i-type InAs / GaAs digital alloy structure light absorption layer 43 is in the range of 50 nm to 2 μm. For example, if the thickness of the i-type InAs / GaAs digital alloy structure light absorption layer 43 is 500 nm, the number of repetitions of the InAs layer (two atomic layers) / GaAs layer (two atomic layers) is 417. Figure 8 shows the case of the multiplication layer, where the reciprocal of the applied electric field is 1.47 × 10 -6 At a 1000 Ω / cm / V, the dead space is about 80 nm, but the applied electric field in the light absorption layer of the PD or APD is one order of magnitude smaller. In other words, the reciprocal of the applied electric field is 1.47 × 10 -5 Considering that the dead space effect is approximately cm / V, the dead space effect becomes significantly apparent at a layer thickness of approximately 800 nm or less, and therefore a range of 800 nm or less is most suitable as the layer thickness of the i-type InAs / GaAs digital alloy structure light absorption layer 43. Furthermore, if the layer thickness is 1600 nm or less, the electron traveling speed is fast at a distance of 800 nm, which is 50% or more of the layer thickness, and therefore a large dead space effect can be obtained even in the layer thickness range of 1600 nm or less.

[0255] The conductivity type of the i-type InAs / GaAs digital alloy structure light absorption layer 43 is i-type, and the carrier concentration is 1×10 17 cm -3 However, the carrier concentration is 5×10 18 cm -3 It may be p-type or n-type as follows.

[0256] An n-type InAlAs conductive layer 49 is grown as an n-type conductive layer on the n-type InP window layer 47, and an n-type InGaAs contact layer 50 is grown as an n-type contact layer. The thickness of the n-type InAlAs conductive layer 49 and the n-type InGaAs contact layer 50 are each 0.1 to 2 μm, and the carrier concentration is 5×10 17 cm -3 ~8 x 10 18 cm -3 is.

[0257] After crystal growth of the n-type InGaAs contact layer 50, the n-type InAlAs conductive layer 49 and the n-type InGaAs contact layer 50 are etched into a mesa shape to form a first mesa. Then, a second mesa is formed by etching the outer side of the first mesa to include the first mesa and reach the p-type InAlGaAs contact layer 40. The second mesa does not need to reach the p-type InAlGaAs contact layer 40 as long as it can electrically isolate the i-type InAs / GaAs digital alloy structure light absorption layer 43. The distance between the first and second mesas is preferably 1 μm or more. Alternatively, the first mesa may be formed after the second mesa is formed.

[0258] The p-type electrode 52 is formed on the p-type InAlGaAs contact layer 40, and the n-type electrode 51 is formed on the n-type InGaAs contact layer 50. Note that, in the case of an n-type semiconductor, the ohmic resistance is one order of magnitude smaller than that of a p-type semiconductor, so it is not necessarily necessary to use the n-type InGaAs contact layer 50, which has a small band gap; n-type InP, n-type InAlGaAs, or n-type InGaAsP may also be used. Alternatively, a direct contact may be made with the n-type InAlAs conductive layer 49. Through the above steps, the semiconductor light-receiving element 180 according to the eleventh embodiment is completed.

[0259] <Functions and Effects of Semiconductor Photodetector (APD) According to Eleventh Embodiment> The semiconductor photodetector 180 according to the eleventh embodiment is characterized in that the conductivity types of the semiconductor photodetector 170a according to the tenth embodiment are inverted from n-type to p-type and from p-type to n-type, respectively, and the conductivity type of the upper surface side is made n-type.

[0260] The first function and effect of the semiconductor photodetector 180 according to the eleventh embodiment will be described below. Because epitaxial crystal growth is maintained at high temperatures for a long time, diffusion of p-type dopants from the p-type InP field buffer layer 44 into the i-type InAs / GaAs digital alloy structure light-absorbing layer 43 may result in disorder. In the semiconductor photodetector 180 according to the eleventh embodiment, the total thickness of the semiconductor layers above the p-type InP field buffer layer 44 is approximately one-third that of the semiconductor photodetector 170a according to the tenth embodiment. That is, in the semiconductor photodetector 180 according to the eleventh embodiment, the crystal growth time required for epitaxially growing the remaining semiconductor layers after the p-type InP field buffer layer 44 is approximately half that of the semiconductor photodetector 130a according to the fourth embodiment, making it difficult for disorder to occur in the i-type InAs / GaAs digital alloy structure light-absorbing layer 43.

[0261] A second function and effect of the semiconductor light-receiving element 180 according to the eleventh embodiment will be described below. The upper electrode of the semiconductor light-receiving element, i.e., the electrode on the front surface side, is a p-type electrode 32 in the back-illuminated APD shown in FIG. 23 and an n-type electrode 51 in the back-illuminated APD shown in FIG. 24. To increase the speed, it is necessary to further reduce the electrode area of ​​the front surface side electrode to reduce the capacitance. However, reducing the electrode area of ​​the upper electrode increases the contact resistance between the electrode and the semiconductor layer, which causes a problem of an increased RC time constant and a narrower response band.

[0262] In the semiconductor light-receiving element 180 according to the eleventh embodiment, the upper electrode, i.e., the n-type electrode 51, is in contact with the n-type semiconductor, and therefore the ohmic resistance is reduced to one-tenth of that of a contact between a p-type electrode and a p-type semiconductor. This allows the area of ​​the n-type electrode 51 to be reduced, thereby reducing stress from the electrode and making it less likely for disorder to occur in the i-type InAs / GaAs digital alloy structure light-absorbing layer 43. While the above description has been given of an APD as an example of the semiconductor light-receiving element 180 according to the eleventh embodiment, a PD may also have an element structure in which the conductivity types of the semiconductor light-receiving element 150 according to the seventh embodiment are inverted, for example, from n-type to p-type and vice versa.

[0263] <Effects of Eleventh Embodiment> As described above, according to the semiconductor photodetector of the eleventh embodiment, the crystal growth time required for epitaxially growing the remaining semiconductor layers after crystal growth of the p-type InP field relaxation layer is about half that of the semiconductor photodetector of the tenth embodiment, and therefore disordering of the InAs / GaAs digital alloy structure light absorption layer is even less likely to occur, thereby providing an effect of providing a semiconductor photodetector that is highly reliable, operates over a wide bandwidth, and has excellent low-noise characteristics. Furthermore, as described above, since the reflectivity of light from the n-type electrode 51 is increased, light that is transmitted without being absorbed by the i-type InAs / GaAs digital alloy structure light absorption layer 43 is reflected by the n-type electrode 51 and returns to the i-type InAs / GaAs digital alloy structure light absorption layer 43, thereby increasing the light receiving sensitivity. As a result, the i-type InAs / GaAs digital alloy structure light absorption layer 43 can be made thinner, which reduces the transit time of electrons and holes and provides a semiconductor light receiving element capable of further widening the response band.

[0264] Modification of Eleventh Embodiment A front-illuminated APD and a back-illuminated APD, which are examples of semiconductor light-receiving elements according to a modification of the eleventh embodiment, will be described below.

[0265] The semiconductor photodetector according to the modified example of the eleventh embodiment is structurally different in that the i-type InAlAs multiplication layer 45 in the front-illuminated APD and back-illuminated APD, which are examples of the semiconductor photodetector 180 according to the eleventh embodiment, i.e., the i-type InAlAs multiplication layer with a random alloy structure, is replaced with an i-type InAs / AlAs digital alloy structure multiplication layer.

[0266] The layer structure and layer thickness of the i-type InAs / AlAs digital alloy structure multiplication layer are the same as those of the i-type InAs / AlAs digital alloy structure multiplication layer of the semiconductor light-receiving element according to the modified example of the second embodiment, and therefore, description thereof will be omitted.

[0267] <Function of Semiconductor Photodetector According to Modification of Eleventh Embodiment> In the semiconductor photodetector (APD) according to the modification of the eleventh embodiment, the crystal growth time required for epitaxially growing the remaining semiconductor layers after crystal growth of the i-type InAs / AlAs digital alloy structure multiplication layer is about one-third that of the semiconductor photodetector 150 according to the seventh embodiment, and therefore disordering of the i-type InAs / AlAs digital alloy structure multiplication layer is unlikely to occur.

[0268] Effect of Modification of Eleventh Embodiment As described above, the semiconductor photodetector according to the modification of the eleventh embodiment has a digital alloy structure multiplication layer whose layer thickness is controlled within a preset range, and the crystal growth time required for epitaxially growing the remaining semiconductor layers after crystal growth of the i-type InAs / AlAs digital alloy structure multiplication layer is shorter than that of an element structure having an opposite conductivity type. Therefore, disordering of the InAs / GaAs digital alloy structure light absorption layer and the InAs / AlAs digital alloy structure multiplication layer can be prevented, thereby providing an effect of providing a semiconductor photodetector which is more reliable, operates over a wide response band, and has excellent low-noise characteristics.

[0269] 25 is a configuration diagram showing an optical line terminal (OLT) 260 of a 50G-PON system according to a twelfth embodiment. 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. Note that the DA-PD of the present disclosure may be applied instead of the DA-APD 268 of the present disclosure.

[0270] The DA-APD of the present disclosure refers to an APD described in each of the above embodiments in which the light absorption layer has an InAs / GaAs digital alloy structure, or an APD in which the light absorption layer has an InAs / GaAs digital alloy structure and the multiplication layer has an InAs / AlAs digital alloy structure. Furthermore, the DA-PD of the present disclosure refers to a PD described in each of the above embodiments in which the light absorption layer has an InAs / GaAs digital alloy structure.

[0271] 26 is a configuration diagram illustrating an optical network unit (ONU) of a 50G-PON system according to embodiment 12. 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.

[0272] 27 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.

[0273] As shown in the optical line terminal 250a of a 50G-PON system as a comparative example in Figure 27, the 50G-PON system as a comparative example required digital bandwidth compensation, i.e., a DSP 255. On the other hand, a 50G-PON system using the DA-APD of the present disclosure does not require digital bandwidth compensation. That is, as shown in the optical line terminal (ONU) of a 50G-PON system according to embodiment 12 in Figure 26, by using the DA-APD of the present disclosure, i.e., an APD having at least an InAs / GaAs digital alloy structure light absorption layer, a wide response band and high receiving sensitivity are possible, thereby enabling simplification and power saving of the DSP circuit and reduction of the output power of the SOA.

[0274] 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 (14): S / N ratio = Iph 2 ・M 2 / (2q(Iph+Id)M 2 ・F・B+4Kb・T・Ft・B / Rt) (14)

[0275] In equation (14), 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.

[0276] To simplify equation (14), 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 (15): S / N ratio=Iph / (4q F B) (15)

[0277] The excess noise factor F is given by the following equation (16): F = M(1-(1-k) ((M-1) 2 / M 2 )) (16)

[0278] In the case of a conventional InAlAs random alloy multiplication layer, as mentioned above, it is difficult to thin the layer (to approximately 70 nm) enough to cause the dead space effect due to the influence of tunnel current, and therefore there are no examples of its application to APDs. 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 12, the excess noise factor F = 3.9.

[0279] On the other hand, in the 50G-PON system according to the twelfth 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, when the DA-APD of the present disclosure is applied, the S / N ratio is improved by 3 dB.

[0280] 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.

[0281] 28 is a diagram illustrating the configuration of an optical line terminal (OLT) of a 50G-PON system according to embodiment 12. 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. Note that the DA-PD of the present disclosure may be applied instead of the DA-APD 268 of the present disclosure.

[0282] 29 is a diagram illustrating a configuration of an optical network unit (ONU) of a 50G-PON system according to embodiment 12. 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.

[0283] The effects of the semiconductor photodetector according to the present disclosure will now be further explained. Among the DA-APDs disclosed herein, APDs having an InAs / AlAs digital alloy structure multiplication layer can achieve a multiplication time of approximately zero in equation (5) by controlling the layer thickness of the multiplication layer within a preset range and setting the ionization rate ratio k to zero. As a result, the response bandwidth of the APD does not deteriorate even when the multiplication factor is increased. In other words, in the DA-APD disclosed herein, the response bandwidth is limited only by the RC time constant and 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.

[0284] Furthermore, when the ionization rate ratio k approaches zero, excess noise that deteriorates receiver sensitivity is suppressed, eliminating the need for SOA amplification of the optical signal. 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.

[0285] <Effects of Embodiment 12> As described above, according to the optical line terminal of embodiment 12, the DA-APD or DA-PD disclosed herein 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.

[0286] Embodiment 13 Fig. 30 is a diagram showing the configuration of a multi-level intensity modulation transmitting / receiving apparatus 300 according to embodiment 13. Also, Fig. 31A and Fig. 31B are diagrams showing received waveforms of the multi-level intensity modulation transmitting / receiving apparatus 300 according to embodiment 13.

[0287] 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.

[0288] Meanwhile, in the receiving section, the light passes through the optical fiber cable 310 and the optical system and enters the DA-APD 305, which is the semiconductor light receiving element of the present disclosure, where the light signal is converted into a current and multiplied, and then amplified by the Linear-TIA 306, after which it is converted into a digital signal by the ADC 302b, and signal processing is performed by the DSP 301. Note that the DA-PD of the present disclosure may be used instead of the DA-APD 305 of the present disclosure.

[0289] <Functions and Effects of the Multilevel Intensity Modulation Transmitter / Receiver According to Embodiment 13> 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 FIG. 31A. To determine whether the received waveform in PAM4 is good or bad, an index called TDECQ (Transmitter Dispersion and Eye Closure Quaternary) is used. TDECQ is calculated using the following equation (17): TDECQ(dB)=10・log(OMA / (6・Qt・R)) (17)

[0290] In equation (17), 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.

[0291] 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.

[0292] 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 received waveform in Figure 31B, if the linearity and dynamic range deteriorate, the eye opening formed between level 2 and level 3 deteriorates.

[0293] In the case of PDs and APDs, the linearity deteriorates when the photocurrent increases with increasing 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.

[0294] The inventors have studied a model for the degradation of APD linearity. Figures 32A and 32B are diagrams for explaining the operation of a PD when a high optical input is applied. As shown in Figure 32A, when the optical input increases and the photocurrent increases, a space charge effect occurs, as if a voltage drop occurs due to series resistance, preventing voltage from being 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 Figure 32B. The series resistance Rli that degrades the linearity of an APD is expressed by the following equation (18): Rli = Rsc + Rd + Rlo (18)

[0295] In equation (18), 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.

[0296] The inventors have found that Rsc can be expressed by the following equation (19), 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) (19) In equation (19), 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, if the traveling speed of electrons and holes is increased to reduce Td, it is possible to reduce the resistance Rsc.

[0297] In the PDs and APDs having at least the InAs / GaAs digital alloy structure optical absorption layer of the present disclosure, the optical absorption layer has a high absorption coefficient, which allows for a thinner optical absorption layer, which in turn reduces the resistance Rsc. As a result, the eye opening becomes uniform, and the TDECQ satisfies the specified value. Furthermore, it becomes possible to increase the transmission distance and reduce the drive current of the transmitting laser.

[0298] Among the DA-APDs disclosed herein, the following describes the case where an APD in which both the light absorption layer and the multiplication layer have a digital alloy structure is used. First, the operation of an APD at high light input will be described. FIG. 33 is a 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 APD's multiplication layer 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 APD's linearity 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 (20): Residence time Tdm = Multiplication time = 2πNkMτav (20)

[0299] In equation (20), 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.

[0300] Figure 34 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.

[0301] As a result, while the eye opening of PAM4 was non-uniform with a conventional APD as shown in Fig. 31B, the eye opening is uniform with the DA-APD of the present disclosure as shown in Fig. 31A, 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.

[0302] <Effects of Embodiment 13> As described above, the multi-level intensity modulation transceiver according to the thirteenth embodiment uses the DA-APD or DA-PD disclosed herein as the semiconductor photodetector, 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.

[0303] Fourteenth Embodiment Fig. 35 is a schematic diagram showing the configuration of a radio-on-fiber (RoF) system 400 according to a fourteenth embodiment. Fig. 36 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. Note that the DA-PD according to the present disclosure may be used instead of the DA-APD 403 according to the present disclosure.

[0304] In a radio-on-fiber system 400 according to the fourteenth 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 path 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.

[0305] The radio-on-fiber system 400 according to the fourteenth embodiment can efficiently supply signals to an antenna 404 that is located at a distance from an electric 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.

[0306] <Functions and Effects of the Radio-on-Fiber System According to the Fourteenth Embodiment> In the radio-on-fiber system 450 of the comparative example shown in FIG. 36, 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 signal cannot be emitted from the antenna.

[0307] Furthermore, when a conventional APD is used, as shown in Figures 32A and 32B, 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.

[0308] On the other hand, in the PD or APD used in the radio-on-fiber system 400 according to the fourteenth embodiment, which has at least the InAs / GaAs digital alloy structure optical absorption layer of the present disclosure, the optical absorption layer has a high absorption coefficient, making it possible to thin the optical absorption layer, which in turn reduces the resistance Rsc. 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-PD or DA-APD of the present disclosure, making it possible to output a strong radio wave signal with good linearity even over a long optical transmission distance.

[0309] Furthermore, in the DA-APD 403 disclosed herein, which has an optical absorption layer formed of an InAs / GaAs digital alloy structure and a multiplication layer formed of an InAs / AlAs digital alloy structure, the residence time Tdm of electrons and holes in the multiplication layer is short, as shown in FIG. 34 , and therefore changes in the electric field distribution in the multiplication layer are suppressed. As a result, a response with excellent linearity can be obtained over a wide dynamic range. In other words, because the DA-APD 403 disclosed herein multiplies a signal, the original signal can be reproduced and a large current amplitude can be obtained.

[0310] 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 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 not 100% but about 80%, it is optimal to use a multiplication factor of 2 to 3 to compensate for such loss.

[0311] Effect of Fourteenth Embodiment As described above, according to the fourteenth embodiment, the radio-on-fiber system is configured using the DA-APD or DA-PD of the present disclosure, and therefore it is possible to obtain an effect of a radio-on-fiber system that can output a strong radio wave signal even if the optical transmission distance is long.

[0312] 37 is a schematic diagram showing the configuration of a digital coherent receiving apparatus 500 according to embodiment 15. The digital coherent receiving apparatus 500 according to embodiment 15 is characterized in that it uses a DA-APD 505a according to the present disclosure. Note that the DA-PD according to the present disclosure may be used instead of the DA-APD 505a according to the present disclosure.

[0313] 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.

[0314] The signal light and laser light are multiplexed, and the signal light is further separated into orthogonal components (I, Q) and output. Four balanced detectors 505 are arranged inside the 90-degree hybrids 503a and 503b, each of which has two DA-APDs 505a of the present disclosure connected in series as a pair. A total of four optical signals consisting of orthogonal I and Q components for each polarization are incident on each of the four balanced detectors 505. The electrical signals output from the balanced detectors 505 are input to the DSP 506. The digital coherent receiving device 500 according to the fifteenth embodiment has the above configuration.

[0315] <Operation of the Digital Coherent Receiving Apparatus According to the Fifteenth Embodiment> FIG. 38A is a diagram showing waveforms of a digital coherent receiving apparatus as a comparative example, and FIG. 38B is a diagram showing waveforms of a digital coherent receiving apparatus according to the fifteenth embodiment.

[0316] 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 frequency to be reduced. Furthermore, when a conventional APD is used, as shown in FIG. 33 , 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 FIG. 38A , 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.

[0317] On the other hand, in the PD or APD that has at least the InAs / GaAs digital alloy structure light absorption layer of the present disclosure that is used in the digital coherent receiving device 500 according to the fifteenth embodiment, the light absorption layer has a high absorption coefficient, and therefore the light absorption layer can be made thinner. The thinner light absorption layer makes it less susceptible to the influence of resistance Rsc due to the space charge effect, and therefore a constellation waveform with excellent linearity can be obtained over a wide dynamic range.

[0318] Furthermore, in the DA-APD 505a disclosed herein, which has an optical absorption layer formed of an InAs / GaAs digital alloy structure and a multiplication layer formed of an InAs / AlAs digital alloy structure, the residence time Tdm of electrons and holes in the multiplication layer is short, as shown in Figure 34, and therefore changes in the electric field distribution in the multiplication layer are suppressed. As a result, as shown in Figure 38B, when the DA-APD 505a disclosed herein 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 when a signal is multiplied by an APD, the original signal can be reproduced, and a large current amplitude can be obtained.

[0319] 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.

[0320] <Effects of Embodiment 15> As described above, according to the digital coherent receiving device of embodiment 15, the DA-APD or DA-PD disclosed herein is used as the 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.

[0321] 39 is a schematic diagram showing the configuration of a SPAD sensor system according to embodiment 16. 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.

[0322] 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.

[0323] 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.

[0324] Furthermore, when the SPAD sensor system 600 uses a DA-APD 505a having a light absorption layer made of an InAs / GaAs digital alloy structure and a multiplication layer made of an InAs / AlAs digital alloy structure according to the present disclosure, photons incident on the SPAD sensor system 600 are absorbed in the light absorption layer made of an InAs / GaAs digital alloy structure of the SPAD sensor 602 made of the DA-APD according to the present disclosure, generating electron-hole pairs, and the electrons flow into the multiplication layer. An electric field about 10% higher than the avalanche breakdown electric field is applied to the multiplication layer.

[0325] This state is called the Geiger mode. In the Geiger mode, electrons are 6 The generated electrons flow as a current and are passed through the photoelectron measurement circuit 601. If the current generated by one photon is known in advance, it is possible to count the number of photons incident on the SPAD sensor system 600.

[0326] FIG. 40A is a diagram showing the multiplication characteristics of a SAPD sensor system according to a comparative example, and FIG. 40B is a diagram showing the multiplication characteristics of a SAPD sensor system according to embodiment 16. 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 FIGS. 40A and 40B , 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.

[0327] 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.

[0328] <Functions and Effects of the SPAD Sensor System According to the Sixteenth Embodiment> The SPAD sensor system 600 according to the sixteenth embodiment can be used not only to count the number of photons but also as a highly sensitive semiconductor light-receiving element. 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.

[0329] 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.

[0330] In an APD having at least the InAs / GaAs digital alloy structure light absorption layer of the present disclosure, the light absorption layer has a high absorption coefficient, making it possible to thin the light absorption layer. Thinning the light absorption layer reduces the resistance Rsc, thereby reducing the breakdown voltage. Using the DA-APD of the present disclosure in a SPAD reduces the difference between the quenching voltage and the Geiger mode voltage, i.e., the applied voltage difference, making it possible to improve the response bandwidth and simplify the quenching circuit and reduce power consumption.

[0331] 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 FIGS. 9A and 9B , 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 breakdown voltage is defined as the voltage at which the dark current exceeds 10 μA, 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.

[0332] 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.

[0333] 41 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 electric field is 120 kV / cm lower than that of an InAs / AlAs digital alloy structure multiplication layer having a superlattice structure similar to the InAs / AlAs digital alloy structure multiplication layer of the present disclosure but a layer thickness of 200 nm or more that has not been thinned.

[0334] <Effects of Sixteenth Embodiment> As described above, according to the SPAD sensor system of the sixteenth 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.

[0335] Embodiment 17. Fig. 42 is a diagram showing the configuration of a LIDAR (Light Detection and Ranging: LiDAR) device according to embodiment 17. Fig. 43A is a diagram showing the received waveform of the APD of a LIDAR device that is a comparative example, and Fig. 43B is a diagram showing the received waveform of the APD of a LIDAR device 700 according to embodiment 19.

[0336] A LIDAR device 700 according to the seventeenth embodiment includes a light source 701, a DA-APD 702 according to the present disclosure, a TIA 703, and a ranging circuit 704. Note that a DA-PD according to the present disclosure may be used instead of the DA-APD 702 according to the present disclosure. The light source 701 emits pulsed light (hereinafter referred to as pulsed light) or frequency-modulated light.

[0337] In the LIDAR device 700 according to the seventeenth 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 sensitivity of the semiconductor light-receiving element needs to be increased. Therefore, in the LIDAR device 700 according to the seventeenth embodiment, the DA-APD 702 according to the present disclosure is used as the semiconductor light-receiving element with high amplification.

[0338] 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 the time when the intensity of the pulse signal exceeds a preset discrimination line is determined to be the arrival time, as shown in Figures 43A and 43B. 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.

[0339] <Functions and Effects of the LIDAR Device According to Embodiment 17> 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 FIG. 43A , 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.

[0340] On the other hand, in the PD and APD used in the LIDAR device 700 according to embodiment 17, which have at least the InAs / GaAs digital alloy structure light absorption layer of the present disclosure, even when the light receiving sensitivity of a conventional APD is insufficient, the APD using the InAs / GaAs digital alloy structure for the light absorption layer can achieve high sensitivity. 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 achieving power saving and increasing safety for the eyes.

[0341] Furthermore, in the DA-APD 505a disclosed herein, which has an optical absorption layer made of an InAs / GaAs digital alloy structure and a multiplication layer made of an InAs / AlAs digital alloy structure, even at high multiplication rates of 20 times or more, as described in the explanation of the operation of embodiment 1, the tunnel current does not increase, making it easy to identify weak light. Furthermore, as shown in FIG. 34 , the residence time in the multiplication layer is short, and as shown in FIG. 43B , 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.

[0342] Effect of Seventeenth Embodiment As described above, according to the LIDAR device of the seventeenth embodiment, the reflected light from an object is received by the DA-APD or DA-PD of the present disclosure, which has the effect of making it possible to measure the distance to a distant object, reducing the power consumption of the light source, and providing a LIDAR device that is also highly safe for the eyes.

[0343] Although the present disclosure describes various exemplary embodiments and examples, 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.

[0344] 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.

[0345] 1 n-type InP substrate, 1a Fe-doped semi-insulating InP substrate, 2 n-type InP buffer layer, 2a n-type InAlAs buffer layer, 2b n-type InP conductive layer, 3 i-type InP electron transport layer, 4 i-type InAlGaAs graded layer, 5, 43 i-type InAs / GaAs digital alloy structure light absorption layer, 6 i-type InAlGaAs / InAlAs graded layer, 7 p-type InP window layer, 8 p-type InGaAs contact layer, 11, 47 n-type InP window layer, 13, 45 i-type InAlAs multiplication layer, 15 p-type diffusion region, 17 isolation groove, 18 surface protective film, 20 Fe-doped semi-insulating InP buried layer, 25 p-type InAlAs conductive layer, 31, 31a, 31c, 51 n-type electrode, 31b backside electrode, 32, 52 p-type electrode, 33 opening, 35 anti-reflection coating film, 40 p-type InAlGaAs contact layer, 41 p-type InP conductive layer, 42 i-type InAlGaAs / InAlAs graded layer, 14, 44 p-type InP field relaxation layer, 46 n-type InAlAs field adjustment layer, 49 n-type InAlAs conductive layer, 50 n-type InGaAs contact layer, 53 metal film, 90 incident light, 100, 100a, 110, 110a, 120, 120a, 130, 130a, 140, 140a, 150, 150a, 160, 160a, 170, 170a, 180 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-over-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 Ranging circuit, 705 Object,

Claims

1. A semiconductor light receiving element comprising an InP substrate, an n-type semiconductor layer formed on the InP substrate, a multiplication layer formed on the n-type semiconductor layer, a p-type electric field relaxation layer formed on the multiplication layer, and a light absorption layer formed on the p-type electric field relaxation layer and having a digital alloy structure.

2. A semiconductor light receiving element comprising an InP substrate, an n-type semiconductor layer formed on the InP substrate, an electron traveling layer formed on the n-type semiconductor layer, and a light absorption layer formed on the electron traveling layer and having a digital alloy structure.

3. The semiconductor light receiving element according to claim 1 or 2, wherein the digital alloy structure is characterized in that two types of semiconductor layers each composed of different semiconductor materials are alternately laminated at a period of 2 atomic layers to 6 atomic layers.

4. The semiconductor light receiving element according to claim 3, wherein the two types of semiconductor layers are any combination of an InAs layer and a GaAs layer, an InAlAs layer and an InGaAs layer, or InAlGaAs layers having different composition ratios from each other.

5. The semiconductor light receiving element according to claim 1, wherein the multiplication layer has an InAs / AlAs digital alloy structure, and the layer thickness of the multiplication layer is 60 nm or more and 130 nm or less.

6. The semiconductor light receiving element according to any one of claims 1 to 5, wherein the n-type semiconductor layer is an n-type conductive layer, and an n-type electrode is provided at a portion where the n-type conductive layer formed on the InP substrate is partially exposed.

7. The semiconductor light receiving element according to any one of claims 1 to 6, wherein an i-type or n-type window layer is formed on the light absorption layer, a p-type impurity diffusion region is formed at least inside the window layer, and a p-type electrode is provided above the p-type impurity diffusion region.

8. The semiconductor light receiving element according to claim 7, wherein a separation groove reaching the n-type semiconductor layer is provided at an outer peripheral portion of the p-type impurity diffusion region.

9. The semiconductor light receiving element according to claim 7, wherein a light incident region is provided on a back surface of the InP substrate opposite to the p-type electrode.

10. The semiconductor light receiving element according to claim 1 or 2, wherein a mesa-type p-type conductive layer is formed on the light absorption layer, and a p-type electrode is formed on the p-type conductive layer.

11. The semiconductor light receiving element according to claim 10, wherein a separation groove is provided on the outer peripheral portion of the mesa-type p-type conductive layer, and the bottom of the separation groove reaches at least the n-type semiconductor layer.

12. A semiconductor light receiving element comprising: an InP substrate; a p-type semiconductor layer formed on the InP substrate; an optical absorption layer formed on the p-type semiconductor layer and having a digital alloy structure; a p-type electric field relaxation layer formed on the optical absorption layer; a multiplication layer formed on the p-type electric field relaxation layer; and an n-type electric field relaxation layer formed on the multiplication layer.

13. A semiconductor light receiving element comprising: an InP substrate; a p-type semiconductor layer formed on the InP substrate; an optical absorption layer formed on the p-type semiconductor layer and having a digital alloy structure; and an electron traveling layer formed on the optical absorption layer.

14. The semiconductor light receiving element according to claim 12 or 13, wherein the digital alloy structure is characterized in that two types of semiconductor layers each composed of different semiconductor materials are alternately laminated at a period of 2 to 6 atomic layers.

15. The semiconductor light receiving element according to claim 14, wherein the two types of semiconductor layers are any combination of an InAs layer and a GaAs layer, an InAlAs layer and an InGaAs layer, or InAlGaAs layers having different composition ratios.

16. The semiconductor light receiving element according to claim 12, wherein the multiplication layer has an InAs / AlAs digital alloy structure, and the layer thickness of the multiplication layer is 60 nm or more and 130 nm or less.

17. The semiconductor light receiving element according to any one of claims 1 to 16, wherein a part of the optical absorption layer has a random alloy structure.

18. The semiconductor light receiving element according to any one of claims 1 to 17, wherein a part of the optical absorption layer is doped with an n-type or p-type impurity.

19. An optical line termination device comprising: the semiconductor light receiving element according to any one of claims 1 to 18; an optical multiplexer / demultiplexer for incident light signals on the semiconductor light receiving element; an amplifier circuit for amplifying an electrical signal output from the semiconductor light receiving element; a clock / data reproduction circuit connected to the amplifier circuit for reproducing clock / data from the amplified electrical signal; and a forward error correction circuit connected to the clock / data reproduction circuit for correcting an error in the clock / data.

20. An optical line termination device comprising: the semiconductor light receiving element according to any one of claims 1 to 18; an optical multiplexer / demultiplexer that makes an optical signal enter the semiconductor light receiving element; an amplifier circuit that amplifies the electrical signal output from the semiconductor light receiving element; an analog / digital conversion circuit that is connected to the amplifier circuit and converts the amplified electrical signal into a digital signal; a digital signal processing circuit that is connected to the analog / digital conversion circuit and processes the digital signal; and a forward error correction circuit that is connected to the digital signal processing circuit and corrects errors in the digital signal.

21. A multi-value intensity modulation transceiver device comprising: the semiconductor light receiving element according to any one of claims 1 to 18 that receives a multi-value intensity modulated optical signal; an amplifier circuit that amplifies the electrical signal output from the semiconductor light receiving element; an analog / 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 / digital conversion circuit and processes the digital signal.

22. An optical fiber radio system comprising: a light source that emits an analog modulated optical signal; the semiconductor light receiving element according to any one of claims 1 to 18 that receives the analog modulated optical signal; a transmission line that transmits the analog electrical signal output from the semiconductor light receiving element to an antenna; and an antenna that is connected to the transmission line and radiates the analog electrical signal as a radio wave signal.

23. A digital coherent receiver device comprising: the semiconductor light receiving element according to any one of claims 1 to 18; a polarization separator that separates the polarization of a polarization multiplexed optical signal whose intensity and phase are modulated; a 90-degree hybrid that demultiplexes and multiplexes the optical signal output from the polarization separator; and a digital signal processing circuit that is connected to the 90-degree hybrid and processes a digital signal.

24. A SPAD sensor system comprising: a SPAD sensor constituted by the semiconductor light receiving element according to any one of claims 1, 5, 12, and 16; a quenching circuit that repeatedly applies a voltage equal to or higher than the breakdown voltage and a voltage lower than the breakdown voltage to the SPAD sensor; and a photoelectron measurement circuit that measures the electrical signal output from the SPAD sensor.

25. A lidar device comprising: a light source that emits pulsed light or frequency-modulated light; the semiconductor light-receiving element according to any one of claims 1 to 18 that receives the light reflected from an object and returned after being emitted from the light source; an amplifier circuit that amplifies the electrical signal output from the semiconductor light-receiving element; and a distance measurement circuit that calculates a distance based on the electrical signal amplified by the amplifier circuit.

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