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WO2025210764A1 - Élément de réception de lumière à semi-conducteur, dispositif de terminaison de ligne optique, dispositif d'émission/réception à modulation d'intensité multiniveau, dispositif de réception numérique cohérente, système radio sur fibre, système de capteur spad et dispositif lidar - Google Patents

Élément de réception de lumière à semi-conducteur, dispositif de terminaison de ligne optique, dispositif d'émission/réception à modulation d'intensité multiniveau, dispositif de réception numérique cohérente, système radio sur fibre, système de capteur spad et dispositif lidar

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Publication number
WO2025210764A1
WO2025210764A1 PCT/JP2024/013696 JP2024013696W WO2025210764A1 WO 2025210764 A1 WO2025210764 A1 WO 2025210764A1 JP 2024013696 W JP2024013696 W JP 2024013696W WO 2025210764 A1 WO2025210764 A1 WO 2025210764A1
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WIPO (PCT)
Prior art keywords
layer
type
semiconductor
light
multiplication
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Pending
Application number
PCT/JP2024/013696
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English (en)
Japanese (ja)
Inventor
栄太郎 石村
晴央 山口
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Mitsubishi Electric Corp
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Mitsubishi Electric Corp
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Priority to JP2024547059A priority Critical patent/JP7661633B1/ja
Priority to PCT/JP2024/013696 priority patent/WO2025210764A1/fr
Publication of WO2025210764A1 publication Critical patent/WO2025210764A1/fr
Pending legal-status Critical Current
Anticipated expiration legal-status Critical

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Classifications

    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F30/00—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors
    • H10F30/20—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors the devices having potential barriers, e.g. phototransistors
    • H10F30/21—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors the devices having potential barriers, e.g. phototransistors the devices being sensitive to infrared, visible or ultraviolet radiation
    • H10F30/22—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors the devices having potential barriers, e.g. phototransistors the devices being sensitive to infrared, visible or ultraviolet radiation the devices having only one potential barrier, e.g. photodiodes
    • H10F30/225—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors the devices having potential barriers, e.g. phototransistors the devices being sensitive to infrared, visible or ultraviolet radiation the devices having only one potential barrier, e.g. photodiodes the potential barrier working in avalanche mode, e.g. avalanche photodiodes
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F77/00—Constructional details of devices covered by this subclass
    • H10F77/10—Semiconductor bodies
    • H10F77/12—Active materials
    • H10F77/124—Active materials comprising only Group III-V materials, e.g. GaAs
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F77/00—Constructional details of devices covered by this subclass
    • H10F77/30—Coatings

Definitions

  • PDs photodiodes
  • APDs avalanche photodiodes
  • PONs Passive Optical Networks
  • G(E)-PON systems that transmit signals at 1-2 Gbps
  • 10G-EPON systems that transmit signals at 10 Gbps
  • Jinwen Song, et al. “High-efficiency and high-speed germanium photodetector enabled by mu ltiresonant photonic crystal” De Gruyter Nanophotonics 20200455 (2020) Jiyuan Zheng, et al. , “Digital Alloy InAlAs Avalanche Photodiodes”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 36, NO. 17, SEPTEMBER 1, pp. 3580-3585, 2018
  • PDs and APDs used in high-speed optical communications use InGaAs as the material for their light absorption layers, which has a high absorption coefficient in the 1.3 ⁇ m and 1.55 ⁇ m wavelength bands, which are the wavelength bands for optical communications. For example, in the 1.3 ⁇ m band, a high absorption coefficient of 10,000/cm or more can be achieved.
  • the quantum efficiency ⁇ is 63%.
  • Vav is the average saturated transit velocity of electrons and holes.
  • the light absorption layer is made of InGaAs
  • Vav 5.35 ⁇ 10 6 cm/s
  • W 1 ⁇ m
  • the layer thickness W of the light absorption layer is made thicker than 1 ⁇ m, the quantum efficiency ⁇ will be higher than 63%, but the response bandwidth will be lower than 29.8 GHz. To improve this trade-off between the response bandwidth and quantum efficiency, it is necessary to increase the absorption amount of the light absorption layer.
  • a photonic crystal layer is also known as a two-dimensional periodic structure.
  • Patent Documents 2 to 8 utilize resonance by a photonic crystal layer, so as shown in Figure 3 of Patent Document 6, for example, the photonic crystal layer acts like a filter only within a specific narrow wavelength range. Therefore, to achieve high reception sensitivity at the desired wavelength, high precision is required in crystal growth and processing. Furthermore, because semiconductor photodetectors are not temperature-controlled during actual use, there is a risk that changes in the ambient temperature will cause the resonance wavelength to change by 10 nm or more, resulting in a significant change in reception sensitivity.
  • Non-Patent Document 1 reports an element structure in which the optical confinements in the vertical direction of the optical absorption layer are strengthened, causing multiple resonance modes to be generated, and the multiple resonance modes resonate in combination, thereby achieving high reception sensitivity over a wide wavelength range.
  • the semiconductor photodetector described in Non-Patent Document 1 relates to a PD with a germanium (Ge) absorption layer formed on a SiO2 layer on a silicon (Si) substrate. It is difficult to apply the device structure described in Non-Patent Document 1 to an APD with high reception sensitivity that uses an InGaAs absorption layer on an InP substrate. This is because, in the semiconductor photodetector described in Non-Patent Document 1, holes are provided in the Ge absorption layer to form a PD with a photonic crystal. However, since a high electric field is applied to an APD, providing holes in, for example, an InGaAs absorption layer would result in an increase in dark current.
  • the InGaAs layer that makes up the optical absorption layer has a small band gap, carriers generated by light absorption recombine on the surface of the holes, resulting in reduced reception sensitivity. Furthermore, in order to generate multiple resonance modes, it is necessary to strengthen the light confinement in the optical absorption layer, and a structure in which the optical absorption layer is sandwiched between low-refractive-index layers is used. However, in APDs, it is difficult to lower the refractive index of the multiplication layer adjacent to the optical absorption layer. For these reasons, there is a strong demand for an optimal device structure for APDs that uses InGaAs on an InP substrate as an absorption layer.
  • 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.
  • the ionization rate of electrons is expressed as ⁇
  • the ionization rate of holes as ⁇ .
  • the greater the ratio of the ionization rates of electrons and holes the smaller the excess noise generated during multiplication and the higher the receiving sensitivity. Furthermore, the greater the ratio of the ionization rates of electrons and holes, the shorter the multiplication time in the multiplication layer, resulting in a wider response band.
  • k ⁇ / ⁇ .
  • Compound semiconductor materials such as InAlAs or InP are used for the multiplication layer of APDs for optical communications.
  • InAlAs When InAlAs is selected as the material for the multiplication layer, the difference in 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, with the hole ionization rate being approximately twice the electron ionization rate. On the other hand, when InAlAs is selected as the material for the multiplication layer, the electron ionization rate is greater than the hole ionization rate, with the electron ionization rate being approximately five times the hole ionization rate. Therefore, since using InAlAs for the multiplication layer results in higher reception sensitivity, InAlAs is more suitable than InP as the material for the multiplication layer of an APD.
  • APDs are semiconductor light-receiving elements, to have a wide response bandwidth and high receiving sensitivity.
  • 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 bandwidth at high multiplication factors.
  • APDs with a multiplication layer made of InAlAs a material used in optical communications, have a wider response bandwidth than APDs made of other semiconductor materials, but at a multiplication factor of 6 or more, the response bandwidth remains at approximately 20 GHz. In other words, the wide response bandwidth of 37.5 GHz or more required for 50G-PON systems is difficult to achieve when using conventional APDs.
  • Patent Document 1 describes an APD that uses a superlattice as a multiplication layer, but because each layer of the superlattice is 5 to 10 nm thick, it acts as a quantum well that reflects the band gap of each layer. When the thickness of each stacked layer exceeds a few nm, energy unevenness that reflects the band gap of each layer is created, hindering the movement of carriers and reducing their travel speed.
  • the response bandwidth of semiconductor light-emitting elements and semiconductor light-receiving elements, as well as the optical output of semiconductor light-emitting elements and the receiving sensitivity of semiconductor light-receiving elements, are insufficient. For this reason, it is being considered to install 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.
  • DSP digital signal processor
  • the optical line terminal i.e., the receiving device on the central office side
  • SOA semiconductor optical amplifier
  • EML electro-absorption modulated laser diode
  • DSPs and SOAs consume a lot of power, which increases costs, and there are concerns that this will hinder progress in replacing existing PON systems with 50G-PON systems.
  • transceivers are designed that incorporate expensive, power-hungry DSPs and SOAs into the ONU and OLT, but this results in problems such as increased power consumption and costs.
  • This disclosure has been made to resolve the above-mentioned problems, and aims to provide a semiconductor photodetector that has high receiving sensitivity over a wide wavelength range and operates over a wide response band.
  • the semiconductor light receiving element comprises: A substrate; an n-type semiconductor layer formed on the substrate; a multiplication layer formed on the n-type semiconductor layer; a p-type electric field buffer layer formed on the multiplication layer; a light absorbing layer formed on the p-type electric field buffer layer; a first window layer formed on the light absorbing layer; a second window layer formed on the first window layer and having a two-dimensional periodic structure in which a plurality of holes or remnants are two-dimensionally arranged at a constant period; a surface protective insulating film formed at least on the two-dimensional periodic structure; a surface electrode formed on the surface protective insulating film; Equipped with.
  • a digital coherent receiving device includes: The semiconductor light receiving element described above; a polarization separator that separates the polarizations of the intensity- and phase-modulated polarization multiplexed optical signal; a 90-degree hybrid that splits and combines the optical signals output from the polarization splitter; and a digital signal processing circuit connected to the 90-degree hybrid device for processing a digital signal.
  • the SPAD (Single Photon Avalanche Diode) sensor system comprises: a SPAD sensor configured by the 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 an optoelectronic measurement circuit that measures the electrical signal output from the SPAD sensor.
  • optical line terminal, multilevel intensity modulation transceiver, digital coherent receiver, radio-on-fiber system, SPAD sensor system, and LIDAR device disclosed herein use the semiconductor photodetector element disclosed herein as the semiconductor photodetector element, thereby achieving the effect of obtaining devices and systems with excellent performance.
  • Figures 30A to 30D are conceptual diagrams showing the ionization rate in the multiplication layer and electric field relaxation layer, where Figure 30A is a conceptual diagram showing the ionization rate in the case of a multiplication layer having a random alloy structure, Figure 30B is a conceptual diagram showing the ionization rate in the case of a multiplication layer having a digital alloy structure, Figure 30C is a conceptual diagram showing the ionization rate in the case of a multiplication layer having a partially disordered digital alloy structure, and Figure 30D is a conceptual diagram showing the ionization rate in the case of a combination of a thick electric field relaxation layer and a multiplication layer having a digital alloy structure.
  • FIG. 13 is a cross-sectional view illustrating the element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to a third modification of the third embodiment.
  • FIG. 11 is a cross-sectional view illustrating the element structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element according to a fourth modification of the third embodiment.
  • FIG. 1 is a configuration diagram showing an optical line terminal (OLT) of a 50G-PON system as a comparative example.
  • FIG. 10 is a diagram illustrating the configuration of a multilevel intensity modulation transmitting/receiving device according to a fifth embodiment.
  • 42A and 42B are conceptual diagrams showing received waveforms of a multilevel intensity modulation transmitting/receiving apparatus according to the fifth embodiment.
  • 43A and 43B are conceptual diagrams illustrating the operation of a PD when a high optical input is applied.
  • FIG. 1 is a conceptual diagram illustrating the operation of an APD when a high light input is applied.
  • FIG. 51A is a conceptual diagram showing the multiplication characteristics of a SAPD sensor system as a comparative example
  • FIG. 51B is a conceptual diagram showing the multiplication characteristics of a SAPD sensor system according to embodiment 8.
  • FIG. 10 is a diagram showing the calculated difference between the quenching electric field and the Geiger mode electric field for each multiplication layer configuration.
  • FIG. 13 is a diagram illustrating the configuration of a LIDAR device according to a ninth embodiment.
  • Figure 54A is a conceptual diagram showing the received waveform of the APD of a LIDAR device as a comparative example
  • Figure 54B is a conceptual diagram showing the received waveform of the APD of the LIDAR device according to embodiment 9.
  • Figure 2 shows that the InAs/AlAs digital alloy structure has peaks in the absorption coefficient near 650 nm and 800 nm, with the absorption coefficient improving by 1.5 times.
  • a decrease in the absorption coefficient is observed near 720 nm. This is thought to be because, even with roughly the same overall composition ratio, the band structure changes due to the periodicity of the InAs and AlAs layers, resulting in periodicity in the wavelength dependence of the optical absorption coefficient.
  • the absorption coefficient of the InAs/AlAs digital alloy structure drops sharply. This phenomenon is thought to explain the smaller refractive index in the wavelength range of 1200 nm to 1600 nm.
  • PDs that use a two-dimensional periodic structure it is possible to increase reception sensitivity by confining light in the light absorption layer.
  • PDs that increase the proportion of light confined in the light absorption layer are called Photo-Trapping-Enhanced PDs.
  • a multiplication layer with a digital alloy structure has a smaller refractive index than a multiplication layer with a random alloy structure. Therefore, a multiplication layer with a digital alloy structure confines more light to the light absorption layer than a multiplication layer with a random alloy structure, and furthermore, the complex resonance of light generated by the application of a two-dimensional periodic structure has a synergistic effect of improving reception sensitivity over a wide wavelength range.
  • ⁇ Element structure of semiconductor light receiving element according to first embodiment> 3 and 4A are a cross-sectional view and a top view showing the device structure of a back-illuminated APD, which is an example of the semiconductor light-receiving device 100 according to embodiment 1.
  • the upper side refers to the direction in which the semiconductor layers are stacked from the surface of the semiconductor substrate, that is, the stacking direction
  • the lower side refers to the direction opposite to the stacking direction.
  • the back-illuminated APD which is an example of the semiconductor light-receiving element 100 according to the first embodiment, includes an n-type InP substrate 1, an n-type InP buffer layer 2 having a carrier concentration of 1 to 5 ⁇ 10 cm and a layer thickness of 0.1 to 1.0 ⁇ m, which are sequentially formed on the n-type InP substrate 1, an InAs/AlAs multiplication layer 3 (hereinafter referred to as the i-type InAs/AlAs digital alloy structure multiplication layer 3) having a digital alloy structure in which an i-type AlAs layer (for example, a layer thickness of two atomic layers, about 0.6 nm) and an i-type InAs layer (for example, a layer thickness of two atomic layers, about 0.6 nm) are alternately stacked multiple times, and an n-type InAs/AlAs multiplication layer 3 having a carrier concentration of 0.1 to 50 ⁇ 10 cm a p-type InP field relaxation layer 4 having
  • the semiconductor device is composed of a surface protective insulating film 10, a Zn-diffused p-type region 12 formed in a part of the i-type InAlGaAs/InAlAs graded layer 6 and in the i-type InP first window layer 7, a plurality of holes 11 arranged two-dimensionally in the p-type InAlAs second window layer 8, an n-type electrode 31 and an anti-reflection film 40 formed on the back surface of the n-type InP substrate 1, and a p-type electrode 32 formed on the p-type InGaAs contact layer 9 and on the SiN surface protective insulating film 10 surrounded by the p-type InGaAs contact layer 9.
  • the p-type electrode 32 is omitted in Figures 4A and 4B to make it easier to understand the structure when viewed from above.
  • the p-type InAlAs second window layer 8 may be constructed using InAlGaAs, InGaAsP, or InP instead of InAlAs. Furthermore, the p-type InAlAs second window layer 8 may be constructed from multiple semiconductor layers instead of a single InAlAs layer.
  • the p-type InP electric field buffer layer 4 may be a p-type InAlAs electric field buffer layer having a random alloy structure, or a p-type electric field buffer layer having an InAs/AlAs digital alloy structure.
  • the hole 11 is set deep enough that its bottom reaches the i-type InP first window layer 7. That is, the i-type InP first window layer 7, in which the Zn-diffused p-type region 12 is provided, is exposed at the bottom of the hole 11. However, the bottom of the hole 11 does not have to reach the i-type InGaAs light absorption layer 5; it may be located, for example, inside the i-type InP first window layer 7 or inside the p-type InAlAs second window layer 8.
  • the p-type InAlAs second window layer 8 exposed on the outer periphery of the ring-shaped p-type InGaAs contact layer 9 is etched away.
  • the cross-sectional view of Figure 8A and the top view of Figure 8B show the state after the p-type InAlAs second window layer 8 has been etched away.
  • the reason for etching the p-type InAlAs second window layer 8 exposed on the outer periphery of the p-type InGaAs contact layer 9 is that a high electric field is applied to the boundary 13 of the Zn-diffused p-type region 12, and if the InAlAs layer, which is easily oxidized, is placed on the outermost surface, dark current is likely to increase.
  • the first and second window layers can both be made of i-type InP layers, which are resistant to oxidation. In this case, there is no need to etch the InP second window layer around the periphery of the p-type InGaAs contact layer 9.
  • the first window layer can be made of an InAlAs layer and the second window layer of InP. The reason for using different materials for the first and second window layers is that this improves control over the depth of the hole 11 (or the height if the hole is left as the remaining portion), i.e., the etching depth. Control over the etching depth can also be improved by providing an etching stopper layer between the first and second window layers.
  • a SiN surface protective insulating film 10 is formed on the entire surface of the front side as a surface protective film, and then the SiN surface protective insulating film 10 on the ring-shaped p-type InGaAs contact layer 9 is removed to form an ohmic contact portion.
  • the cross-sectional view in Figure 9A and the top view in Figure 9B show the state after the ohmic contact portion has been formed.
  • the SiN surface protective insulating film 10 is one example of an insulating film.
  • the surface protective film may be an insulating film of other film types, such as an SiO2 film, an SiON film, or an insulating film made of an organic material. Furthermore, a film configuration in which an SiN film and an SiO2 film are stacked may also be used.
  • a p-type electrode 32 is formed on the ring-shaped etched p-type InGaAs contact layer 9 and on the SiN surface protective insulating film 10 on the inner circumference of the ring.
  • the cross-sectional view in Figure 10A and the top view in Figure 10B show the state after the p-type electrode 32 has been formed.
  • the p-type electrode 32 uses Ti and Au, or Pt, as its metallic material.
  • the p-type electrode 32 also functions as a metal reflective film. Specifically, it is composed of a multilayer film such as, from the semiconductor layer side, Ti/Au, Ti/Pt/Au, Ti/Au/Pt/Au, Ti/Au/Ti/Pt/Au, or Pt/Ti/Au/Ti/Pt/Au.
  • an n-type electrode 31 and an anti-reflection film 40 are formed on the rear surface of the n-type InP substrate 1.
  • the n-type electrode material is deposited over the entire rear surface of the n-type InP substrate 1, and the n-type electrode material deposited on the rear surface opposite the portion of the n-type InP substrate 1 surrounded by the p-type InGaAs contact layer 9 on the front surface is removed to form the n-type electrode 31.
  • an anti-reflection film 40 made of a SiN film or the like is formed on the back surface of the n-type InP substrate 1, in the area from which the n-type electrode material has been removed, that is, the area facing the area surrounded by the p-type InGaAs contact layer 9 on the front surface side of the n-type InP substrate 1. This completes the method for manufacturing the semiconductor photodetector according to the first embodiment.
  • Incoming light 90 from the anti-reflection film 40 formed on the back side of the n-type InP substrate 1 is incident perpendicularly on the i-type InGaAs light absorption layer 5 provided on the front side of the n-type InP substrate 1.
  • the light-receiving region surrounded by the p-type InGaAs contact layer 9 is circular, its diameter is in the range of 5 ⁇ m to 1 mm, and when the light-receiving region is rectangular, its long side is in the range of 5 ⁇ m to 1 mm.
  • Incident light 90 incident on the back side of the semiconductor photodetector 100 passes through the anti-reflection film 40 and enters the interior of the semiconductor photodetector 100.
  • the incident light 90 passes through the n-type InP substrate 1, and after a portion of the incident light 90 is absorbed in the i-type InGaAs light absorption layer 5, it reaches the i-type InP first window layer 7 and the p-type InAlAs second window layer 8.
  • optical resonance 91 also occurs between the i-type InGaAs light absorption layer 5 and the i-type InP first window layer 7.
  • the light receiving sensitivity of the semiconductor light receiving element is high at wavelengths where optical resonance 91 occurs, but complex resonance increases the light receiving sensitivity over an even wider wavelength range.
  • the reason for this is that the equivalent refractive index differs at the resonance points from position A to position D, causing optical resonance 91 to occur at different wavelengths.
  • position A which is the resonance point
  • the center of the resonance point is in the first window layer, which has a low refractive index
  • the resonance wavelength is shorter than at position D, where the center of the resonance point is in the light absorption layer, which has a high refractive index.
  • complex resonance can be generated in the semiconductor light receiving element 100 according to embodiment 1, it is possible to obtain a semiconductor light receiving element with high reception sensitivity over a wide wavelength range of several tens of nanometers or more.
  • the equivalent refractive index varies depending on the layer structure and the aperture ratio of the holes 11, and falls within the range of 2.8 to 3.6.
  • the diameter of the hole 11 is a definition that applies when the opening of the hole 11 is circular. If the opening of the hole 11 has a shape other than circular, such as a rectangle, it should be defined as the length of the diagonal, and if it is a triangle, it should be defined as the length of the long side.
  • the period in the x direction and the period in the y direction be approximately the same value.
  • the reason for this is that if the period in the x direction and the period in the y direction are different, polarization dependency of the reception sensitivity occurs. From the perspective of reducing the polarization dependency of the reception sensitivity, it is desirable that the shape of the opening of the holes 11 be close to circular. However, even if the shape of the opening of the holes 11 is a polygon such as a triangle or a rectangle, or an ellipse, it can also be applied as a two-dimensional periodic structure 70. Furthermore, it is also possible to arrange multiple patterns closely together, such as arranging holes with triangular or rectangular shapes closely together, and have them function as a single hole.
  • the advantages of the semiconductor photodetector 100 according to embodiment 1 over the semiconductor photodetector described in Non-Patent Document 1 are described below.
  • the semiconductor photodetector described in Non-Patent Document 1 has holes periodically arranged in a lattice pattern in a light absorption layer made of germanium (Ge). This causes electron-hole pairs generated by light absorption to recombine on the surface of the holes, resulting in a problem of reduced reception sensitivity.
  • germanium germanium
  • the Ge layer is exposed at the bottom of the holes, there is also the problem of increased dark current and reduced reliability.
  • an APD which is an example of a semiconductor photodetector 100 according to embodiment 1
  • holes 11 are formed in the p-type InAlAs second window layer 8, which has a larger band gap than the i-type InGaAs light absorption layer 5. This prevents electrons and holes generated in the i-type InGaAs light absorption layer 5 from diffusing to the holes 11.
  • the semiconductor photodetector 100 according to embodiment 1 makes it possible to realize a semiconductor photodetector with even higher reception sensitivity, lower dark current, and higher reliability than the conventional semiconductor photodetector described in Non-Patent Document 1.
  • Non-Patent Document 1 Furthermore, in the semiconductor photodetector described in Non-Patent Document 1, light incident from the surface that cannot be absorbed by the Ge light absorption layer, which is a photonic crystal, is transmitted to the substrate side.
  • the APD which is an example of the semiconductor photodetector 100 according to embodiment 1, a SiN surface protective insulating film 10 and a p-type electrode 32 are formed on top of the p-type InAlAs second window layer 8. Therefore, light that has transmitted through the p-type InAlAs second window layer 8 is reflected by the SiN surface protective insulating film 10 and p-type electrode 32 and returned to the i-type InGaAs light absorption layer 5, contributing to optical resonance and thereby improving reception sensitivity.
  • the i-type InAs/AlAs digital alloy structure multiplication layer 3 is expected to achieve low noise and a wide bandwidth due to its small ionization rate ratio k, but high reception sensitivity can also be achieved by applying an InAs/AlAs digital alloy structure to the multiplication layer of the semiconductor photodetector 100 according to embodiment 1.
  • high reception sensitivity is possible will be explained below.
  • To increase reception sensitivity it is necessary to confine light as much as possible in the light absorption layer. As mentioned above, high reception sensitivity can be achieved when the resonance point exists in the light absorption layer. In other words, since the light absorption layer has the highest refractive index, most light components are confined in the light absorption layer.
  • Figure 12 shows the refractive index distribution between points P and Q at position A in Figure 11.
  • the dotted line in Figure 12 shows the refractive index of the InAlAs random alloy structure multiplication layer
  • the solid line shows the refractive index of the InAs/AlAs digital alloy structure multiplication layer.
  • the multiplication layer is constructed with an InAlAs random alloy structure, as shown in Figure 1, the high refractive index of the InAlAs random alloy structure multiplication layer causes the resonance point of the light absorption layer to be biased toward the semiconductor substrate, resulting in light dissipation.
  • the optical confinement rate in the light absorption layer decreases.
  • Variation 1 of embodiment 1. ⁇ Features of the semiconductor photodetector (APD) according to the first modification of the first embodiment> 13 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor light-receiving device 110 according to Modification 1 of Embodiment 1.
  • the semiconductor light-receiving device 110 according to Modification 1 of Embodiment 1 is characterized in that the first window layer is composed of multiple layers with different refractive indices.
  • the configuration of each layer from the n-type InP substrate 1 to the i-type InAlGaAs/InAlAs graded layer 6 is the same as that of the semiconductor photodetector 100 according to Embodiment 1, and therefore a description thereof will be omitted.
  • an i-type InP first window layer 7a having a thickness of 0.1 to 3.0 ⁇ m, an i-type InAlAs first window layer 7b having a thickness of 0.1 to 3.0 ⁇ m, an i-type InP second window layer 8a having a thickness of 0.1 to 3.0 ⁇ m, and a p-type InGaAs contact layer 9 are formed in this order on top of the i-type InAlGaAs/InAlAs graded layer 6.
  • the i-type InAlAs first window layer 7b may alternatively be formed of an i-type InAlGaAs layer.
  • the carrier concentration of the i-type InP first window layer 7a, the i-type InAlAs first window layer 7b, and the i-type InP second window layer 8a is each 5 ⁇ 10 17 cm -3 or less.
  • the composition wavelength ⁇ g of the InAlGaAs layer is 50 to 100 nm shorter than the wavelength of light incident on the APD, so no light absorption occurs in the first window layer 7b.
  • the p-type InGaAs contact layer 9, SiN surface protective insulating film 10, n-type electrode 31 formed on the back surface of the n-type InP substrate 1, and p-type electrode 32 formed on the SiN surface protective insulating film 10 surrounded by the p-type InGaAs contact layer 9 have the same configuration as the semiconductor photodetector 100 according to embodiment 1 shown in FIG. 3.
  • the two-dimensional periodic structure 70 also has the same configuration as the semiconductor photodetector 100 according to embodiment 1.
  • the Zn-diffused p-type region 12 is formed in the same region as in the semiconductor photodetector 100 according to the first embodiment, i.e., the region shown in the cross-sectional view of FIG. 13. Meanwhile, in the cross-sectional view of FIG. 6A and the top view of FIG. 6B, which show the manufacturing method of the semiconductor photodetector 100 according to the first embodiment, the p-type InAlAs second window layer 8 outside the outer periphery of the ring-shaped p-type InGaAs contact layer 9 is removed. However, in the semiconductor photodetector 110 according to the first variation of the first embodiment, the i-type InP second window layer 8a is composed of an InP layer, so this removal is not necessary.
  • the semiconductor light-receiving element 110 according to the first modification of the first embodiment also generates complex resonance at the resonance points as shown in Fig. 14.
  • the i-type InAlAs first window layer 7b has a higher refractive index than the i-type InP first window layer 7a and the i-type InP second window layer 8a, but a lower refractive index than the i-type InGaAs light absorption layer 5.
  • the refractive index of the light absorption layer is highest, the proportion of light intensity at positions C and D in Fig. 14 is high as the light resonance point. In order to obtain high receiving sensitivity over a wide wavelength range by complex resonance, it is also necessary to ensure optical resonance at positions A and B in Fig. 14.
  • the refractive index of the i-type InAlAs first window layer 7b is higher than in the refractive index profile shown in Figure 12, so complex resonance is likely to occur even at positions A and B in Figure 14.
  • the first window layer is configured as a two-layer structure consisting of an i-type InP first window layer 7a and an i-type InAlAs first window layer 7b, which results in a refractive index difference of 0.05 to 0.25, making it easier for complex resonance to occur.
  • the digital alloy structure multiplication layer solid line
  • the random alloy structure multiplication layer dotted line
  • the semiconductor photodetector according to the first modification of the first embodiment has a first window layer made up of multiple layers to increase the refractive index of the upper layer, a two-dimensional periodic structure in the second window layer, and a SiN surface protective insulating film and a p-type electrode covering the semiconductor layer to reflect light internally. Furthermore, the use of an InAs/AlAs digital alloy structure in the multiplication layer increases the amount of light confined in the light absorption layer, thereby achieving both improved light confinement in the light absorption layer and complex resonance.
  • the semiconductor photodetector 120 is characterized in that it does not include a second window layer, but only has an i-type InP window layer 7c, and has a plurality of holes 11a formed in a SiN surface protective insulating film 10a, thereby forming a lattice-like two-dimensional periodic structure 70a.
  • the thickness of the SiN surface protective insulating film 10a is in the range of 50 nm to 500 nm.
  • a SiO2 film may be used instead of the SiN surface protective insulating film 10a.
  • the shape of the opening of the hole 11a may be circular, rectangular, triangular, or the like, but may also be other shapes.
  • the ratio of the diameter of the holes 11a to the fixed period is preferably in the range of 10% to 80%.
  • the depth of the holes 11a (or height if the holes are considered the remainder) is preferably in the range of 100 nm to 1000 nm.
  • a p-type electrode 32 is disposed on the ring-shaped p-type InGaAs contact layer 9 and on the SiN surface protective insulating film 10a within the ring.
  • the semiconductor light receiving element 120 according to the second modification of the first embodiment similarly to the semiconductor light receiving element 100 according to the first embodiment, a complex resonance occurs corresponding to the period of the plurality of holes 11 a arranged in a lattice pattern, and the light intensity in the light absorption layer increases over a wide wavelength range, resulting in an effect of obtaining high receiving sensitivity.
  • the advantage of the semiconductor light receiving element 120 over the semiconductor light receiving element 100 according to the first embodiment is that the semiconductor light receiving element 120 does not have a second window layer, and therefore the overall layer thickness of the epitaxial crystal growth layer can be made thinner.
  • the holes 11a are generally formed in an insulating film such as a SiN film or a SiO2 film by dry etching or wet etching. Because selective etching of the insulating film and the semiconductor layer is easy, the bottoms of the holes 11a can be accurately stopped at the surface of the semiconductor layer. This provides the advantage of making the depths of the multiple holes 11a that make up the two-dimensional periodic structure 70a uniform (or the heights, if the holes are left as remaining portions).
  • the hole 11a is filled with a p-type electrode 32.
  • a metal film made of Ti/Au is used as the p-type electrode 32, the difference in refractive index between the p-type electrode 32 and the SiN surface protective insulating film 10a becomes large, which also has the effect of making it easier to obtain the resonance effect.
  • a lattice-like two-dimensional periodic structure is provided in the SiN surface protective insulating film on the i-type InP window layer.
  • Light is incident from the semiconductor substrate side, and the light is internally reflected by covering the semiconductor layer with the SiN surface protective insulating film and p-type electrode.
  • This allows for complex resonance, thereby enabling the semiconductor photodetector to have high reception sensitivity over a wide wavelength range. It also provides a semiconductor photodetector that can achieve low noise and a wide bandwidth.
  • the depth (or height, if the hole is considered the remaining portion) of the holes provided in the SiN surface protective insulating film is uniform, and the refractive index difference can be increased, which facilitates optical resonance and improves light absorption efficiency.
  • a semiconductor photodetector that can achieve high reception sensitivity, low noise, and a wide bandwidth can be achieved. Therefore, an APD with sufficient reception sensitivity can be obtained even for applications requiring a response bandwidth of 25 Gbps or more.
  • Modification 3 of embodiment 1. ⁇ Features of Semiconductor Photodetector (APD) According to Modification 3 of First Embodiment> 17 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor photodetector 130 according to a third modification of the first embodiment.
  • the semiconductor photodetector 100 according to the first embodiment shown in FIGS. 3 and 4A is provided with a p-type InAlAs second window layer 8 having a plurality of holes 11 arranged in a lattice pattern.
  • the semiconductor photodetector 130 is characterized in that it does not include a second window layer, but instead includes only an i-type InP window layer 7c, and has holes 11b in the p-type electrode 32a on the SiN surface protective insulating film 10, thereby forming a lattice-like two-dimensional periodic structure 70b.
  • the thickness of the p-type electrode 32a is in the range of 50 nm to 500 nm.
  • the constituent material of the p-type electrode 32a may be Ti/Pt/Au or other materials in addition to Ti/Au.
  • a two-dimensional periodic structure 70b is a configuration in which holes 11b are arranged in a lattice pattern in the p-type electrode 32a.
  • the opening shapes of the holes 11b may be circular, rectangular, triangular, etc., but may also be other shapes.
  • the ratio of the diameter of the holes 11b to the fixed period is preferably in the range of 10% to 80%.
  • the depth of the holes 11b (or height if the holes are considered the remainder) is preferably in the range of 100 nm to 1000 nm.
  • a p-type electrode 32a is provided on the ring-shaped p-type InGaAs contact layer 9, forming an ohmic contact portion.
  • Dry etching or wet etching is generally used to form the holes 11b in the p-type electrode 32a, which is made of a metal film such as Ti/Au. Because selective etching of the SiN surface protective insulating film 10 below the p-type electrode 32a is easy, the bottom of the holes 11b can be accurately stopped at the surface of the SiN surface protective insulating film 10. This has the effect of making the depths (heights, if the holes are left as remaining parts) of the multiple holes 11b that make up the two-dimensional periodic structure 70b uniform.
  • the hole 11b in the p-type electrode 32a it is also possible to form the hole 11b in the p-type electrode 32a using lift-off. Furthermore, since the difference in refractive index between the p-type electrode 32a and the hole 11b (i.e., air) becomes larger, it also has the effect of making it easier to obtain the resonance effect.
  • a lattice-like two-dimensional periodic structure is provided on the p-type electrode on the SiN surface protective insulating film, and light is incident from the semiconductor substrate side.
  • This provides the advantage of enabling high reception sensitivity of the semiconductor photodetector over a wide wavelength range through complex resonance.
  • the depth (or height, if the hole is considered the remaining portion) of the hole provided in the p-type electrode is uniform, and the refractive index difference can be increased, making it easier for optical resonance to occur, thereby improving light absorption efficiency.
  • a semiconductor photodetector with high reception sensitivity, low noise, and a wide bandwidth can be realized. Therefore, an APD with sufficient reception sensitivity can be obtained even for applications requiring a response bandwidth of 25 Gbps or more.
  • Variation 4 of embodiment 1. is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor photodetector 140 according to the fourth modification of the first embodiment.
  • the semiconductor photodetector 100 according to the first embodiment shown in FIGS. 3 and 4A is provided with a p-type InAlAs second window layer 8 having a plurality of holes 11 arranged in a lattice pattern.
  • the semiconductor photodetector 140 according to the fourth modification of the first embodiment is characterized in that it does not include a second window layer, but instead includes only an i-type InP window layer 7c, and has a plurality of holes 11c formed in the p-type InGaAs contact layer 9a on the i-type InP window layer 7c, thereby forming a lattice-like two-dimensional periodic structure 70c.
  • the thickness of the p-type InGaAs contact layer 9a is in the range of 50 nm to 500 nm.
  • a two-dimensional periodic structure 70c is a structure in which holes 11c are arranged in a lattice pattern in a p-type InGaAs contact layer 9a.
  • the openings of the holes 11c may have a circular, rectangular, triangular, or other shape, but may also have other shapes.
  • the ratio of the diameter of the holes 11c to the fixed period is preferably in the range of 10% to 80%.
  • the depth of the holes 11b is preferably in the range of 100 nm to 1000 nm.
  • An ohmic contact is formed by providing a p-type electrode 32 on the p-type InGaAs contact layer 9a. Note that the SiN surface protective insulating film 10b does not need to be removed from the portion of the p-type InGaAs contact layer 9a where the hole 11c is not provided.
  • the holes 11c are generally formed in the p-type InGaAs contact layer 9a using dry etching or wet etching. This has the advantage that the holes 11c can be formed simultaneously in the process of forming the ring-shaped ohmic contact, eliminating the need for a separate process for forming the holes 11c.
  • a p-type electrode 32 is provided in the portion of the p-type InGaAs contact layer 9a where the hole 11c is not formed, forming an ohmic contact with the p-type InGaAs contact layer 9a. This increases the contact area between the p-type InGaAs contact layer 9a and the p-type electrode 32, thereby reducing the ohmic resistance of the semiconductor light-receiving element 140.
  • a two-dimensional lattice-like periodic structure is provided in the p-type InGaAs contact layer on the i-type InP window layer, and light is incident from the semiconductor substrate side.
  • This allows for complex resonance, thereby enabling the semiconductor photodetector to have high reception sensitivity over a wide wavelength range, as well as low noise and a wide bandwidth.
  • the depth (or height, if the hole is considered the remaining portion) of the holes provided in the p-type InGaAs contact layer is uniform, which facilitates optical resonance and improves light absorption efficiency.
  • a semiconductor photodetector with high reception sensitivity, low noise, and a wide bandwidth can be realized. Therefore, an APD with sufficient reception sensitivity can be obtained even for applications requiring a response bandwidth of 25 Gbps or more.
  • FIG. 19 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor photodetector 150 according to a fifth modification of the first embodiment.
  • the p-type region is formed by selective diffusion of Zn.
  • the semiconductor photodetector 150 according to the fifth modification of the first embodiment as shown in FIG.
  • the p-type InP first window layer 7d, the p-type InAlAs second window layer 8b, and the p-type InGaAs contact layer 9b are p-doped during epitaxial crystal growth, resulting in a mesa APD having a mesa structure (hereinafter simply referred to as a mesa).
  • p-type dopants include Be, Zn, Mg, and C.
  • the doping concentration of each layer is 5 ⁇ 10 17 cm ⁇ 3 or more, preferably about 1 ⁇ 10 18 cm ⁇ 3 .
  • the generated carriers diffuse slowly and return to the Zn-diffused p-type region 12, where they are extracted as a current signal. Because it takes longer for carriers to diffuse in regions where no electric field is applied than in regions where an electric field is applied, the frequency response deteriorates. As a result, there is a risk of a problem in which trailing edges occur in the APD pulse response waveform.
  • the element has a mesa structure, so an electric field is applied across the entire area inside the mesa structure, and therefore no tailing occurs in the pulse response waveform, and the frequency response does not deteriorate.
  • the semiconductor photodetector according to the fifth modification of the first embodiment has a mesa-type device structure, and an electric field is applied across the entire region inside the mesa structure. This prevents the pulse response waveform from trailing, and therefore the frequency response does not deteriorate.
  • This provides the advantage of realizing a semiconductor photodetector that has high reception sensitivity, low noise, and a wide bandwidth. Therefore, even in applications requiring a response bandwidth of 25 Gbps or more, a pulse response waveform with an excellent S/N ratio can be obtained, and an APD with sufficient reception sensitivity can be obtained.
  • FIG. 20 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor light-receiving device 160 according to the sixth modification of the first embodiment.
  • the back-illuminated APD which is an example of the semiconductor light-receiving element 160 according to the sixth modification of the first embodiment, includes an Fe-doped semi-insulating InP substrate 1a, an n-type InP buffer layer 2a having a carrier concentration of 1 to 5 ⁇ 10 cm and a thickness of 0.1 to 1.0 ⁇ m, which are sequentially formed on the Fe-doped semi-insulating InP substrate 1a, an i-type InAs/AlAs digital alloy structure multiplication layer 3 formed by alternately stacking an i-type AlAs layer (for example, a layer thickness of two atomic layers, about 0.6 nm) and an i-type InAs layer (for example, a layer thickness of two atomic layers, about 0.6 nm) multiple times, and an n-type InAs/AlAs digital alloy structure multiplication layer 3 having a
  • n-type InP buffer layer 2a having a carrier concentration of 1 to 5 ⁇ 10 cm ⁇ 3 and a thickness of 0.1 to 1.0 ⁇ m is grown on an Fe-doped InP substrate 1a.
  • the n-type InP buffer layer 2a functions as both a conductive layer and a contact layer.
  • an i-type InAs/AlAs digital alloy structure multiplication layer 3 a p-type InP field relaxation layer 4, an i-type InGaAs light absorption layer 5, and an i-type InAlGaAs/InAlAs graded layer 6 are sequentially grown by crystal growth on the n-type InP buffer layer 2a.
  • a p-type InP first window layer 7d is grown on the i-type InAlGaAs/InAlAs graded layer 6, as in the semiconductor photodetector 150 according to Variation 5 of Embodiment 1. Furthermore, as in the semiconductor photodetector 100 according to Embodiment 1, a p-type InAlAs second window layer 8 and a p-type InGaAs contact layer 9 are grown sequentially by crystal growth.
  • the p-type InGaAs contact layer 9 is processed into a ring shape, and multiple holes 11 are formed in the p-type InAlAs second window layer 8 so that they are arranged in a lattice pattern, thereby providing a two-dimensional periodic structure 70 in the p-type InAlAs second window layer 8.
  • the outer periphery of the ring-shaped p-type InGaAs contact layer 9 is etched away until the bottom surface reaches the n-type InP buffer layer 2a.
  • the entire surface is covered with a SiN insulating film, and the SiN insulating film is removed from the area where the ohmic contact will be formed, forming the p-type electrode 32 and n-type electrode 31a.
  • the p-type electrode 32 is formed on the ring-shaped p-type InGaAs contact layer 9 and on the SiN surface protective insulating film 10a surrounded by the p-type InGaAs contact layer 9.
  • the n-type electrode 31a is formed to cover the side of the mesa, which is formed by etching down to the n-type InP buffer layer 2a, via the SiN surface protective insulating film 10a provided on the side of each semiconductor layer.
  • the electrode covering the side of the mesa may not be connected to the n-type electrode 31a formed on the n-type InP buffer layer 2a, but may be an electrode independent of each other.
  • the electrode on the side of the mesa may be formed simultaneously with the p-type electrode 32.
  • the semiconductor photodetector 160 uses an Fe-doped semi-insulating InP substrate 1a, which has lower optical absorption loss than the n-type InP substrate 1, thereby further improving reception sensitivity.
  • a reflective mirror made of a SiN surface protective insulating film and an n-type electrode is provided on the side surface of the mesa, which enables light traveling horizontally to be returned to the interior, thereby improving the receiving sensitivity of the semiconductor photodetector. Therefore, it is possible to obtain an APD with sufficient receiving sensitivity even in applications requiring a response bandwidth of 25 Gbps or more.
  • Variation 7 of embodiment 1. 21 is a cross-sectional view showing the device structure of an edge-illuminated APD, which is an example of a semiconductor light-receiving device 170 according to the seventh modification of the first embodiment.
  • the semiconductor light-receiving device 170 according to the seventh modification of the first embodiment shown in FIG. 21 is characterized in that an anti-reflection film 41 is provided on the entire end face of the side portion of the mesa, and incident light 90 is incident from the end face.
  • ⁇ Effects of Modification 7 of First Embodiment> In the semiconductor photodetector according to the seventh modification of the first embodiment, a two-dimensional periodic structure is provided in the second window layer, and an InAs/AlAs digital alloy structure is applied to the multiplication layer, thereby increasing the amount of light confined in the light absorption layer, thereby enabling high reception sensitivity over a wide wavelength range through complex resonance.
  • an anti-reflection film is provided on the entire facet, even higher reception sensitivity is possible, resulting in an edge-illuminated semiconductor photodetector that is capable of achieving lower noise and a wider bandwidth. Therefore, an APD with sufficient reception sensitivity can be obtained even in applications requiring a response bandwidth of 25 Gbps or more.
  • Embodiment 2 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor photodetector 180 according to the second embodiment.
  • the semiconductor photodetector 180 according to the second embodiment is characterized in that an n-type DBR layer 14 (Distributed Bragg Reflector: DBR) is provided between an n-type InP buffer layer 2 and an i-type InAs/AlAs digital alloy structure multiplication layer 3, and the n-type InAlGaAs layers and n-type InP layers, each having a carrier concentration of 1 to 5 ⁇ 10 cm ⁇ 3, are alternately stacked in this order.
  • the configuration other than the n-type DBR layer 14 is substantially the same as that of the semiconductor photodetector 100 according to the first embodiment.
  • the n-type DBR layer 14 also functions as a buffer layer. Therefore, the n-type InP buffer layer 2 is not necessary.
  • the n-type DBR layer 14 is formed by a crystal growth method such as MOCVD or MBE.
  • the n-type DBR layer 14 is also called an n-type semiconductor layer.
  • an i-type InP first window layer may be used instead of the i-type InP first window layer 7.
  • an i-type InAlAs first window layer may be used.
  • a p-type InP second window layer may be used instead of the p-type InAlAs second window layer 8.
  • each layer constituting the n-type DBR layer 14 may be an odd multiple of the layer thickness t, i.e., 1t, 3t, 5t, etc.
  • the reflectivity of the n-type DBR layer 14 can be changed by varying the composition of the InAlGaAs layers and the number of InAlGaAs/InP layer pairs.
  • the refractive index n is 3.38, and the thickness of the InAlGaAs layers that make up one pair of n-type DBR layers 14 is 96.2 nm.
  • the refractive index n of InP is 3.20, so the thickness of the InP layers that make up one pair of n-type DBR layers 14 is 101.6 nm. Therefore, the total thickness of one pair of n-type DBR layers 14 is 197.8 nm.
  • the number of InAlGaAs/InP pairs in the n-type DBR layer 14 is preferably 2 to 20, and the overall thickness of the n-type DBR layer 14 is preferably 0.4 ⁇ m to 4 ⁇ m.
  • the number of pairs depends on the refractive index of each layer stacked, and the smaller the difference in refractive index between the two layers stacked, the more pairs are required.
  • the n-type DBR layer 14 may also be any of the following pairs: InGaAs/InP, InGaAsP/InP, InGaAsP/InAlGaAs, InAlGaAs/InAlAs, and InGaAs/InAlAs.
  • the semiconductor photodetector 180 according to the second embodiment has the same functions and effects as the semiconductor photodetector 100 according to the first embodiment, and also has the function and effect of further improving the receiving sensitivity.
  • the function of the n-type DBR layer 14 will be described below with reference to FIG.
  • incident light 90 entering through the anti-reflection film 40 reaches the n-type DBR layer 14.
  • the n-type DBR layer 14 reflects light, but transmits wavelengths that satisfy the resonance conditions of each layer located above the n-type DBR layer 14. As a result, a vertical cavity is formed between the n-type DBR layer 14 and the p-type electrode 32, causing optical resonance.
  • the vertically resonant light confined by the n-type DBR layer 14 and p-type electrode 32 enhances the optical resonance caused by the two-dimensional periodic structure 70 formed by the multiple holes 11 arranged in a lattice pattern in the p-type InAlAs second window layer 8, i.e., the resonant light shown in Figure 22.
  • the receiving sensitivity of the semiconductor photodetector 180 is improved.
  • Figure 23 is a graph showing the dependency of the reflectivity and quantum efficiency of the DBR layer on the number of pairs in a back-illuminated APD, which is an example of a semiconductor photodetector 180 according to embodiment 2.
  • the wavelength of the incident light is 1300 nm
  • the refractive index n of the InAlGaAs layer is 3.38
  • the layer thickness is 96.2 nm
  • the refractive index n of the InP layer is 3.20
  • the layer thickness is 101.6 nm.
  • the refractive index of the i-type InGaAs light absorption layer 5 is 3.595
  • the layer thickness is 542 nm
  • the absorption coefficient is 12716/cm.
  • the reflectivity of the two-dimensional periodic structure 70 and p-type electrode 32 is 80%. In other words, the two-dimensional periodic structure 70 and p-type electrode 32 function as a mirror with a reflectivity of 80%.
  • the efficiency (quantum efficiency) of absorption due to cavity resonance was calculated using a model in which an InGaAs light absorption layer, an InAs/AlAs digital alloy structure multiplication layer, and an InP window layer are placed between the n-type DBR layer 14 and a mirror with a reflectivity of 80%.
  • the vertical axis represents the reflectivity of the n-type DBR layer 14 and the quantum efficiency of light absorbed in the InGaAs light absorption layer
  • the horizontal axis represents the number of pairs in the n-type DBR layer 14.
  • the reflectivity of the n-type DBR layer 14 increases monotonically as the number of pairs in the n-type DBR layer 14 increases. Meanwhile, the quantum efficiency increases up to eight pairs, but decreases once the number of pairs exceeds eight. This is because, when the number of pairs constituting the n-type DBR layer 14 exceeds eight pairs, the amount of light that transmits through the n-type DBR layer 14 decreases, and therefore the amount of light that resonates in the cavity formed by the n-type DBR layer 14, the two-dimensional periodic structure 70, and the p-type electrode 32 decreases.
  • the reflectivity of the n-type DBR layer 14 is preferably 1% to 40%. Furthermore, the number of pairs constituting the n-type DBR layer 14 is preferably 2 to 14 pairs. To achieve a higher quantum efficiency of 85% or more, the reflectivity of the n-type DBR layer 14 is preferably 5% to 33%, and the number of pairs constituting the n-type DBR layer 14 is even more preferably 4 to 12 pairs.
  • Patent Document 2 discloses a semiconductor photodetector that uses a photonic crystal and a DBR layer, but the DBR layer only functions to return light that is incident on the surface and is not absorbed by the light absorption layer but is transmitted through. Therefore, the semiconductor photodetector described in Patent Document 2 does not have a vertical cavity formed by the two-dimensional periodic structure 70 and p-type electrode 32 and n-type DBR layer 14 as disclosed herein, and therefore does not achieve improved reception sensitivity due to optical resonance in the vertical direction.
  • the semiconductor photodetector according to the second embodiment has an n-type DBR layer below the multiplication layer, a two-dimensional periodic structure in the second window layer, and a vertical cavity formed by covering the two-dimensional periodic structure with a SiN surface protective insulating film and a p-type electrode. This allows light to be incident from the semiconductor substrate side, resulting in a semiconductor photodetector with high reception sensitivity.
  • an InAs/AlAs digital alloy structure increases the amount of light confined in the light absorption layer, resulting in complex resonance that enables high reception sensitivity over a wide wavelength range, and further reduces noise and enables a broadband semiconductor photodetector. Therefore, an APD with sufficient reception sensitivity can be obtained even for applications requiring a response bandwidth of 25 Gbps or more.
  • Variation 1 of embodiment 2. 24 is a cross-sectional view showing the device structure of a back-illuminated PD, which is an example of a semiconductor photodetector 190 according to a first modification of the second embodiment.
  • the configuration in which the n-type DBR layer 14 is provided below the multiplication layer 3 is also possible in the first to sixth modifications of the first embodiment, and the same functions and effects can be obtained.
  • the semiconductor photodetector 190 according to the first modification of the second embodiment is characterized in that the i-type InAs/AlAs digital alloy structure multiplication layer 3 and the p-type InP field relaxation layer 4 are removed from the semiconductor photodetector 180 according to the second embodiment to form a PD.
  • the semiconductor photodetector 190 according to the first modification of the second embodiment also achieves substantially the same effects as the semiconductor photodetector 180 according to the second embodiment.
  • Variation 2 of embodiment 2. 25 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor photodetector 200 according to Modification 2 of Embodiment 2.
  • the semiconductor photodetector 200 according to Modification 2 of Embodiment 2 has a configuration in which the n-type InP buffer layer 2a in the semiconductor photodetector 160 according to Modification 6 of Embodiment 1 shown in FIG. 20 is replaced with an n-type DBR layer 14a. Higher receiving sensitivity can be obtained by confining light in both the horizontal and vertical directions.
  • Variation 3 of embodiment 2 26 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor light-receiving element 210 according to Modification 3 of Embodiment 2.
  • the semiconductor light-receiving element 210 according to Modification 3 of Embodiment 2 is characterized in that the conductivity types of the semiconductor light-receiving element 210 according to Modification 3 of Embodiment 2 are reversed, with the substrate side being p-type and the front surface side being n-type, compared to the semiconductor light-receiving element 200 according to Modification 2 of Embodiment 2.
  • the back-illuminated APD which is an example of a semiconductor light-receiving element 210 according to the third modification of the second embodiment, includes an Fe-doped semi-insulating InP substrate 1a, a p-type InP buffer layer 2b having a thickness of 0.1 to 1.0 ⁇ m, which are sequentially formed on the Fe-doped semi-insulating InP substrate 1a, a p-type DBR layer 14b having a carrier concentration of 1 to 5 ⁇ 10 18 cm ⁇ 3 and formed by alternately stacking p-type InAlGaAs layers and p-type InP layers, an i-type InAlGaAs/InAlAs graded layer 6, an i-type InGaAs light absorption layer 5 having a thickness of 0.1 to 2.0 ⁇ m, and an i-type InAlGaAs/InAlA
  • a p-type InP field relaxation layer 4 having a thickness of 10 to 70 nm and a thickness of 3; an i-type InAs/AlAs digital alloy structure multiplication layer 3 formed by alternately stacking multiple times an i-type AlAs layer (for example, a layer thickness of two atomic layers, about 0.6 nm) and an i-type InAs layer (for example, a layer thickness of two atomic layers, about 0.6 nm); an n-type InP first window layer 7e having a thickness of 0.1 to 3.0 ⁇ m; an n-type InAlAs second window layer 8c having a thickness of 0.1 to 3.0 ⁇ m and a two-dimensional periodic structure 70d; an n-type InGaAs contact layer 9c having a ring shape; and a plurality of n-type InAlAs contact layers 9c arranged two-dimensionally in the n-type InAlAs second window layer 8c.
  • a SiN surface protective insulating film 10 a formed on the surfaces of the n-type InP first window layer 7 e and the n-type InAlAs second window layer 8 c exposed at the bottom of the hole 11 d and on the side surfaces of each layer; a p-type electrode 32 b formed on the exposed p-type DBR layer 14 b on the surface and on the SiN surface protective insulating film 10 a formed on the side surfaces; an n-type electrode 31 b formed on the ring-shaped n-type InGaAs contact layer 9 c and on the SiN surface protective insulating film 10 a surrounded by the n-type InGaAs contact layer 9 c; and an antireflection film 40 formed on the back surface side of the Fe-doped semi-insulating InP substrate 1 a.
  • a p-type InP buffer layer 2b is grown by crystal growth to a thickness of 0.1 to 1 ⁇ m on an Fe-doped semi-insulating InP substrate 1a.
  • a p-type DBR layer 14b is grown by crystal growth, consisting of p-type InAlGaAs layers and p-type InP layers alternately stacked in this order, each having a carrier concentration of 1 to 5 ⁇ 10 cm ⁇ 3.
  • the p-type DBR layer 14b differs only in conductivity type from the n-type DBR layer 14 of the semiconductor photodetector 200 according to embodiment 2; the thickness and constituent materials of each layer of the pair are the same. Furthermore, the n-type InP first window layer 7e, n-type InAlAs second window layer 8c, and n-type InGaAs contact layer 9c differ in conductivity type from the semiconductor photodetector 150 according to variation 5 of embodiment 1, but the thickness and carrier concentration of each layer are the same.
  • etching After etching, the entire surface is covered with a SiN film, and the SiN film is removed from the area where the ohmic contact will be formed, forming a p-type electrode 32a and an n-type electrode 31b.
  • the n-type electrode 31b is formed on the ring-shaped n-type InGaAs contact layer 9c.
  • the p-type electrode 32a is arranged to cover the side of the mesa, which is formed by etching down to the p-type DBR layer 14b, i.e., the side of each semiconductor layer, via the SiN surface protective insulating film 10a also provided on the side of each semiconductor layer.
  • the electrode covering the side of the mesa may be an independent electrode that is not connected to the p-type electrode 32a.
  • the electrode on the side of the mesa may be formed simultaneously with the n-type electrode 31b.
  • n-type semiconductor has less optical absorption loss than a p-type semiconductor.
  • the conductivity type is upside down compared to semiconductor photodetector 200 according to Modification 2 of Embodiment 2. Therefore, the first window layer in which the resonance point is formed is n-type instead of p-type, which has the excellent effect of reducing optical absorption loss in n-type InP first window layer 7 e.
  • the multiplication layer has been described as having an i-type InAs/AlAs digital alloy structure.
  • the digital alloy structure multiplication layer which is one of the structural features of each of the above-mentioned semiconductor light-receiving elements.
  • a next-generation high-speed PON system can be realized without using a DSP or an SOA.
  • the response band is (1) RC time constant (R is element resistance, C is element capacitance) (2) Carrier transit time (the time it takes for electrons or holes to transit through the depletion layer)
  • R element resistance
  • C element capacitance
  • Carrier transit time the time it takes for electrons or holes to transit through the depletion layer
  • it is further limited by (3) It is also limited by the multiplication time (the time it takes for electrons and holes to multiply in a chain reaction in the multiplication layer, which increases in proportion to the multiplication factor).
  • the dead space length is longer for holes than for electrons
  • the holes cannot be ionized, and the ionization rate ratio k decreases.
  • a higher electric field must be applied to the multiplication layer to achieve the desired multiplication rate, which creates a new problem: increased leakage currents such as tunnel currents. In other words, an increase in tunnel current increases the noise generated in the APD.
  • the inventors have demonstrated that the ionization rate ratio k of 0, which was impossible to achieve with a multiplication layer made of a random alloy structure or a multiplication layer made of a thick digital alloy structure, can be achieved by applying a multiplication layer made of the digital alloy structure disclosed herein.
  • thinning the multiplication layer of an APD with a digital alloy structure multiplication layer is more effective at reducing the ionization rate ratio k than thinning the multiplication layer of an APD made of conventional materials.
  • APD which is an example of a semiconductor photodetector according to embodiments 1 and 2.
  • One of the features of the semiconductor photodetector according to embodiments 1 and 2 is that the multiplication layer is constructed using a digital alloy structure.
  • i-type InAs/AlAs digital alloy structure is a digital alloy structure in which i-type AlAs layers (for example, two atomic layers, approximately 0.6 nm thick) and i-type InAs layers (for example, two atomic layers, approximately 0.6 nm thick) are alternately stacked multiple times.
  • i-type AlAs layers for example, two atomic layers, approximately 0.6 nm thick
  • i-type InAs layers for example, two atomic layers, approximately 0.6 nm thick
  • FIG. 27 is a graph showing the electric field dependence of the electron dead space in an InAlAs multiplication layer.
  • the inventors analyzed the electron multiplication characteristics of the digital alloy structure multiplication layer and found that, as shown in the graph in Figure 27, the InAs/AlAs digital alloy structure multiplication layer of the present disclosure has a longer dead space than the conventional InAlAs random alloy structure multiplication layer.
  • Figures 28A to 28C are conceptual diagrams showing the ionization rates of electrons and holes, respectively.
  • Figure 28A shows the case of electron ionization
  • Figure 28B shows the case of hole ionization
  • Figure 28C shows the ionization rate when the multiplication layer is thinned.
  • the dead space length is approximately 45 nm, so the thickness of the multiplication layer needs to be thinned to approximately 1.5 times the dead space (approximately 70 nm).
  • thinning the multiplication layer to 70 nm increases the electric field in the multiplication layer, causing a sudden increase in tunneling current and increasing noise.
  • the dead space length is approximately 85 nm. Therefore, even if the thickness of the multiplication layer is approximately 1.5 times the dead space (approximately 130 nm), the ionization rate ratio k can be made to approach zero. Therefore, the effect of the tunnel current is small in the APDs according to the first and second embodiments.
  • the dead space is highly dependent on the applied electric field.
  • the reciprocal of the applied electric field is 1.27 ⁇ 10 ⁇ 6 cm/V
  • the dead space length is approximately 50 nm, as shown in the graph in Figure 27, so the multiplication layer must be thinned to 75 nm.
  • the thickness of the InAs/AlAs digital alloy structure multiplication layer can be made thicker than that of the InAlAs random alloy structure multiplication layer.
  • Figure 29 shows the dependence of the ionization rate ratio and tunnel current on the thickness of the multiplication layer.
  • the inventors fabricated APDs with an InAs/AlAs digital alloy structure multiplication layer and an InAlAs random alloy structure multiplication layer, measured the ionization rate ratio k, and plotted the results in Figure 29 together with the measurement results from References 1 and 2, which are listed in Figure 29.
  • References 1 and 2 in Figure 29 are as follows:
  • the InAs/AlAs digital alloy structure multiplication layer disclosed herein has a large dead space, and as shown in Figure 29, as the multiplication layer is made thinner, the ionization rate ratio k begins to decrease to 0.1 or less at a layer thickness of 170 nm.
  • 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.
  • the layer thickness of the InAs/AlAs digital alloy structure multiplication layer is more than twice as thick as that of the InAlAs random alloy structure multiplication layer.
  • the minimum thickness of the multiplication layer for which the tunneling current is 1 ⁇ A is 40 nm in an APD with a pn junction diameter of 20 ⁇ m, then a layer thickness in the range of 40 nm to 170 nm is the optimal range for an InAs/AlAs digital alloy structure multiplication layer, and layer thicknesses within this range can be fabricated with sufficient reproducibility.
  • the thickness of the InAs/AlAs digital alloy structure multiplication layer at which the dead space effect is sufficient to reduce the ionization rate ratio k is considered to be approximately twice the dead space length when the reciprocal of the applied electric field is 1.47 ⁇ 10 -6 cm/V. Considering that the dead space length is 85 nm as shown in Figure 29, 170 nm, which is twice the dead space length, is a suitable upper limit for the thickness of the InAs/AlAs digital alloy structure multiplication layer.
  • the graph in Figure 29 shows that a multiplication layer thickness of 150 nm or less is preferable.
  • the optimal multiplication layer thickness is in the range of 60 nm to 130 nm. If a margin of 10 nm is allowed when fabricating an APD, the multiplication layer thickness should preferably be set in the range of 70 nm to 120 nm.
  • Figure 27 shows that the length of the dead space is preferably between 50 nm and 90 nm.
  • equation (7) represents the condition for the difference in dead space length
  • equation (8) represents the condition for the tunnel current.
  • Tmin 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.
  • the ionization rate ratio k begins to decrease as the multiplication layer is made thinner at values De of approximately 80 nm and Dh of approximately 170 nm.
  • the minimum layer thickness Tmin is the same for InAs/AlAs digital alloy structure multiplication layers and InAlAs random alloy structure multiplication layers, as the band gaps of both are the same.
  • De is approximately 40 nm and Dh is approximately 80 nm.
  • Dh is approximately 170 nm.
  • dopants in the electric field buffer layer i.e., impurities, may diffuse into the InAs/AlAs digital alloy structure multiplication layer during the fabrication process, potentially causing disorder within the multiplication layer.
  • Figures 30A to 30D are conceptual diagrams showing the ionization rates in the multiplication layer and electric field buffer layer.
  • Figure 30A shows the ionization rate for an InAlAs random alloy structure multiplication layer
  • Figure 30B shows the ionization rate for an InAs/AlAs digital alloy structure multiplication layer
  • Figure 30C shows the ionization rate for a partially disordered InAs/AlAs digital alloy structure multiplication layer
  • Figure 30D shows the ionization rate for a combination of a thick electric field buffer layer and an InAs/AlAs digital alloy structure multiplication layer.
  • the dead space length of the InAs/AlAs digital alloy structure multiplication layer shown in Figure 30B is longer, but due to dopant diffusion from the electric field buffer layer, the dead space length is shorter in the partially disordered InAs/AlAs digital alloy structure multiplication layer, as shown in Figure 30C.
  • N is the impurity concentration
  • t is time
  • D is the diffusion constant
  • x is position
  • F is the external force acting on the diffusion.
  • materials for the field buffer layer include InP, an InAlAs random alloy structure, and an InAs/AlAs digital alloy structure.
  • 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 with the InAs/AlAs digital alloy structure multiplication layer. The potential barrier corresponds to F in equation (9).
  • the carrier concentration of the electric field buffer layer is preferably 2 ⁇ 10 18 cm ⁇ 3 or less so that the variation in the amount of electric field buffering, that is, the product of the layer thickness and the carrier concentration, does not increase.
  • InAlAs is used as the constituent material of the electric field buffer layer, Zn doping is optimal, and the carrier concentration is optimally 2 ⁇ 10 18 cm ⁇ 3 or less. Note that if the impurity concentration is higher than 2 ⁇ 10 18 cm ⁇ 3 , the amount of inactive impurities increases and diffusion becomes more likely, so the carrier concentration must be 5 ⁇ 10 18 cm ⁇ 3 or less.
  • W is the thickness of the electric field relaxation layer
  • q is the elementary charge
  • N is the carrier concentration of the electric field relaxation layer
  • ⁇ is the dielectric constant.
  • the thickness of the electric field buffer layer is about 10 nm.
  • the carrier concentration of the electric field buffer layer is controlled to be 5 ⁇ 10 cm ⁇ 3 or less.
  • the electric field buffer layer must have a thickness of 10 nm or more.
  • the dead space length is 45 nm or less, so the thickness of the electric field buffer layer made of a random alloy structure must be 70 nm or less.
  • the dead space length is 85 nm or less, so the thickness of the electric field buffer layer made of a digital alloy structure must be 130 nm or less.
  • dead space lengths shown in Figures 30A to 30D have the following relationship: dead space (Figure 30A) ⁇ dead space ( Figure 30D) ⁇ dead space ( Figure 30C) ⁇ dead space ( Figure 30B).
  • the transit time ftr of the carriers includes the time it takes to transit through the light absorption layer plus the time it takes to transit through the multiplication layer.
  • Vav is the average saturated transit velocity of electrons and holes
  • Wt is the sum of the thicknesses of the light absorption layer and the multiplication layer.
  • Figure 31 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor photodetector 220 according to embodiment 3.
  • the semiconductor photodetector 220 according to embodiment 3 differs from the semiconductor photodetector 100 according to embodiment 1 only in that the i-type InAs/AlAs digital alloy structure multiplication layer 3 of the semiconductor photodetector 100 according to embodiment 1 is an i-type InAlAs random alloy structure multiplication layer 3a.
  • the multiplication layer is constructed with an InAlAs random alloy structure
  • a two-dimensional periodic structure 70 is provided in the p-type InAlAs second window layer 8, and light is incident from the semiconductor substrate side. The light is reflected internally by covering the semiconductor layer with the SiN surface protective insulating film 10 and the p-type electrode 32. This increases the amount of light confined in the i-type InGaAs light absorption layer 5, enabling high reception sensitivity over a wide wavelength range due to complex resonance, and achieving the effect of obtaining a semiconductor photodetector that also enables low noise and a wide bandwidth.
  • Variation 1 of embodiment 3. 32 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor photodetector 230 according to Modification 1 of Embodiment 3.
  • the semiconductor photodetector 230 according to Modification 1 of Embodiment 3 differs from the semiconductor photodetector 120 according to Modification 2 of Embodiment 1 only in that the i-type InAs/AlAs digital alloy structure multiplication layer 3 is an i-type InAlAs random alloy structure multiplication layer 3a.
  • the multiplication layer is constructed with an InAlAs random alloy structure
  • a lattice-like two-dimensional periodic structure 70a is provided in the SiN surface protective insulating film 10a on the i-type InP window layer 7c.
  • Light is incident from the semiconductor substrate side, and the light is reflected internally by covering the semiconductor layer with the SiN surface protective insulating film 10a and p-type electrode 32. This enables the semiconductor photodetector to have high reception sensitivity over a wide wavelength range through complex resonance, as well as low noise and a wide bandwidth.
  • the depth (height, if the hole is considered the remaining portion) of the holes 11a provided in the SiN surface protective insulating film 10a is more uniform than in the semiconductor photodetector 220 according to Embodiment 3, and the refractive index difference can be made larger, making it easier for optical resonance to occur, thereby improving light absorption efficiency.
  • a semiconductor photodetector can be realized that offers high reception sensitivity, low noise, and a wide bandwidth.
  • Variation 2 of embodiment 3. 33 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor photodetector 240 according to Modification 2 of Embodiment 3.
  • the semiconductor photodetector 240 according to Modification 2 of Embodiment 3 differs from the semiconductor photodetector 130 according to Modification 3 of Embodiment 1 only in that the i-type InAs/AlAs digital alloy structure multiplication layer 3 is an i-type InAlAs random alloy structure multiplication layer 3a.
  • the multiplication layer is constructed with an InAlAs random alloy structure
  • a lattice-like two-dimensional periodic structure 70b is provided on the p-type electrode 32a on the SiN surface protective insulating film 10, and light is incident from the semiconductor substrate side. This improves the reception sensitivity of the semiconductor photodetector over a wide wavelength range due to complex resonance.
  • Variation 4 of embodiment 3 35 is a cross-sectional view showing the device structure of a back-illuminated APD, which is an example of a semiconductor photodetector 247 according to Modification 4 of Embodiment 3.
  • the semiconductor photodetector 247 according to Modification 4 of Embodiment 3 differs from the semiconductor photodetector 180 according to Embodiment 2 only in that the i-type InAs/AlAs digital alloy structure multiplication layer 3 is an i-type InAlAs random alloy structure multiplication layer 3a.
  • an APD with an InAs/AlAs digital alloy structure multiplication layer i.e., the DA-APD disclosed herein
  • FIG 39 is a diagram showing the configuration of an optical line terminal (OLT) in a 50G-PON system according to embodiment 4.
  • the optical line terminal 260a in 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 according to the present disclosure.
  • Figure 40 is a diagram showing the configuration of an optical network unit (ONU) in a 50G-PON system according to embodiment 4.
  • the optical network unit 260b in 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 according to the present disclosure.
  • the thickness of the multiplication layer is controlled within a preset range, setting the ionization rate ratio k to zero, thereby reducing the multiplication time in equation (6) to nearly zero.
  • the response bandwidth of the APD does not deteriorate even when the multiplication factor is increased.
  • the response bandwidth is limited only by the RC time constant and carrier transit time, just like with conventional PDs. This makes it possible to widen the response bandwidth required for 50G-PON systems, enabling reception without digital bandwidth compensation using a DSP.
  • the DA-APD of the present disclosure is used as the semiconductor photodetector, and therefore an optical line terminal that can increase the transmission distance of optical signals and reduce power consumption is obtained.
  • FIG. 41 is a diagram showing the configuration of a multi-level intensity modulation transmitting/receiving apparatus 300 according to embodiment 5.
  • Fig. 42A and Fig. 42B are conceptual diagrams showing received waveforms of the multi-level intensity modulation transmitting/receiving apparatus 300 according to embodiment 5.
  • the multilevel intensity modulation transmitter/receiver 300 is a multilevel intensity modulation transmitter/receiver using PAM (Pulse Amplitude Modulation), a multilevel intensity modulation method.
  • PAM Pulse Amplitude Modulation
  • the digital signal generated by the DSP 301 is converted to analog by the DAC 302a, amplified by the driver amplifier 303, and drives the light source 304, which consists of a DFB laser or EML, to emit an optical signal onto the optical fiber cable 310.
  • TDECQ Transmitter Dispersion and Eye Closure Quaternary
  • the optical modulation amplitude 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)
  • R is the additional noise value required to achieve the SER value.
  • TDECQ (dB) is defined as, for example, 3 dB or less. To reduce TDECQ (dB), (1) The eye opening at each level must be uniform. (2) The noise at each level must be low.
  • the semiconductor photodetector In order for the eye opening at each of the four levels, which have different optical signal intensities, to be uniform, the semiconductor photodetector must have excellent linearity.
  • 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 linearity can be said to be good if Iph/Pin is constant.
  • PAM needs to receive signals with a wide range of intensities, from low to high, so it needs to have a good dynamic range.
  • the dynamic range can be said to be good if the decrease in Iph/Pin is small.
  • the eye opening formed between level 2 and level 3 will deteriorate.
  • one cause of linearity degradation is that when the photocurrent increases with increasing optical input, the number of holes and electrons traveling through the multiplication layer and light absorption layer increases, causing a change in the electric field distribution in the multiplication layer and light absorption layer. This phenomenon is called the space charge effect.
  • Figures 43A and 43B are conceptual diagrams illustrating the operation of a PD at high optical input.
  • 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 the conceptual diagram of Figure 43B.
  • Rsc is the resistance due to the space charge effect
  • Rd is the element resistance
  • Rlo is the load resistance.
  • Rd and the load resistance are usually several tens of ohms, but Rsc can be several hundred ohms or more.
  • W is the thickness of the depletion layer
  • ⁇ is the dielectric constant
  • 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, the resistance Rsc can be reduced.
  • the optical absorption layer has a high absorption capacity due to optical resonance, making it possible to thin the optical absorption layer, which in turn reduces the resistance Rsc.
  • the eye opening becomes uniform, and TDECQ satisfies the specified value. Furthermore, it becomes possible to increase the transmission distance and reduce the drive current of the transmitting laser.
  • FIG. 44 is a conceptual diagram illustrating the operation of an APD at high light input.
  • the electric field in the multiplication layer of the APD changes, that is, the so-called space charge effect occurs.
  • the occurrence of this space charge effect reduces the multiplication factor of the APD and deteriorates its linearity.
  • the deterioration of the linearity of an APD is caused by the series resistance Rsc, so it is necessary to reduce the residence time Tdm of electrons and holes in the depletion layer.
  • N is the Emmons coefficient (which depends gently on the ionization rate ratio k)
  • M is the multiplication factor
  • ⁇ av 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.
  • Figure 45 shows the residence time Tdm of electrons and holes for each material that makes up the multiplication layer.
  • the residence time Tdm within the multiplication layer is dramatically reduced.
  • electrons and holes are quickly discharged from the multiplication layer, suppressing the space charge effect in the multiplication layer, resulting in improved linearity and dynamic range in an InAs/AlAs digital alloy structure multiplication layer.
  • a PD was used as the semiconductor light-receiving element that receives the signal light.
  • the DA-APD 505a disclosed herein can multiply the signal, making it possible to minimize the local light.
  • a conventional APD is used, as shown in the conceptual diagram of Figure 44, 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 insufficient electrical signal amplitude, but also leads to distortion of the analog signal.
  • the interval between waveforms A1 and B1 becomes narrower, distorting the intensity signal of the constellation waveform, making it difficult to apply an APD.
  • the light absorption layer has a high absorption capacity, making it possible to make the light absorption layer thinner. This makes the light absorption layer less susceptible to the influence of resistance Rsc due to the space charge effect, resulting in a constellation waveform with excellent linearity over a wide dynamic range.
  • the residence time Tdm of electrons and holes in the multiplication layer is short, thereby suppressing changes in the electric field distribution within the multiplication layer.
  • the interval between waveform A and waveform B is widened, resulting in a constellation waveform with excellent linearity across a wide dynamic range. 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.
  • the DA-APD 505a disclosed herein can be used with a gain in the range of 1.2 to 10x. However, as the signal becomes distorted as the gain increases, it is preferable to use a gain in the range of 1.2 to 5x.
  • Embodiment 8. 50 is a schematic diagram showing the configuration of a SPAD sensor system according to embodiment 8.
  • a SPAD sensor system 600 includes a photoelectron measurement circuit 601, a SPAD sensor 602 including a DA-APD according to the present disclosure, and a quenching circuit 603.
  • SPADs can be used not only to count photons but also as highly sensitive light-receiving elements. However, they require constant cycling from A: Quenching voltage (described below) to B: Geiger mode voltage (described below). The cycling cycle is on the order of nanoseconds to microseconds. If the cycling cycle between A: Quenching voltage and B: Geiger mode voltage can be shortened, the response speed of the SPAD can be increased.
  • This state is called the Geiger mode.
  • the electrons are multiplied to a level of 10 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.
  • Figure 51A is a conceptual diagram showing the multiplication characteristics of a SAPD sensor system as a comparative example
  • Figure 51B is a conceptual diagram showing the multiplication characteristics of a SAPD sensor system according to embodiment 8.
  • the voltage is reduced from B: Geiger mode voltage to A: quenching voltage to stop chain multiplication, and then the voltage is increased again from A: quenching voltage to B: Geiger mode voltage, thereby enabling high-sensitivity reception of incident photons.
  • the quenching circuit 603 that controls the voltage is available in passive and active circuits.
  • a passive circuit when a current flows due to photons incident on the SPAD sensor 602, a voltage drop occurs across the resistor connected in series to the SPAD sensor 602, resulting in a decrease in the voltage applied to the SPAD sensor 602.
  • the quenching circuit 603 operates by repeatedly applying a voltage above the breakdown voltage and a voltage below the breakdown voltage to the SPAD sensor 602.
  • the SPAD sensor system 600 according to the eighth embodiment can be used not only for counting the number of photons but also as a semiconductor light-receiving element with high reception sensitivity. However, it is necessary to constantly repeat the process from the Geiger-mode voltage B to the quenching voltage A. The repetition period is on the order of nanoseconds to microseconds. If the difference between the quenching voltage A and the Geiger-mode voltage B can be reduced, the repetition period can be shortened, and the response speed of the SPAD sensor system 600 can be increased.
  • 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, reduce power consumption, and also widen the response band.
  • the light absorption layer has a high absorption capacity, making it possible to make the light absorption layer thinner. Thinning the light absorption layer reduces the resistance Rsc, which also reduces the breakdown voltage.
  • Using the DA-APD disclosed herein 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.
  • the dead space length is long, so multiplication does not occur at low electric fields. However, as the electric field is increased, the dead space length shortens, causing a rapid increase in the multiplication factor, leading to breakdown.
  • the voltage at which the dark current exceeds 10 ⁇ A is defined as the breakdown voltage
  • the multiplication factor at 90% of the breakdown voltage exceeds 10x.
  • the multiplication factor at 90% of the breakdown voltage is 10x or less.
  • 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, the effect will be verified here 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.
  • Figure 52 shows 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 for each multiplication layer is 170 kV/cm, which is exceptionally low.
  • the InAs/AlAs digital alloy structure multiplication layer (thinned DA) of the present disclosure has a superlattice structure similar to the InAs/AlAs digital alloy structure multiplication layer of the present disclosure, but has an electric field 120 kV/cm lower than that of an unthinned InAs/AlAs digital alloy structure multiplication layer with a layer thickness of 200 nm or more.
  • the LIDAR device 700 includes a light source 701, a DA-APD 702 according to the present disclosure, a TIA 703, and a ranging circuit 704.
  • the light source 701 emits pulsed light (hereinafter referred to as pulsed light) or frequency-modulated light.
  • the distance to the object 705 is calculated by measuring the time it takes for pulsed light emitted from the light source to hit the object 705 and return to the semiconductor light-receiving element.
  • An LD or the like is used as the light source 701.
  • the light output of the LD must be increased, but there is an upper limit on the amount of light emitted from the LD for eye safety reasons. Therefore, the receiving sensitivity of the semiconductor light-receiving element must be increased. Therefore, in the LIDAR device 700 according to the ninth embodiment, the DA-APD 702 disclosed herein is used as the semiconductor light-receiving element with a high multiplication factor.
  • the detected light pulse is multiplied by the DA-APD 702 of the present disclosure and converted into a current pulse. It is then amplified by the TIA 703 and input to the distance measurement circuit 704, and as shown in the conceptual diagrams of Figures 54A and 54B, the time when the intensity of the pulse signal exceeds a preset discrimination line is determined to be the arrival time.
  • the timing at which the light 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 the two 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.
  • the DA-APD that utilizes the optical resonance of the present disclosure and is used in the LIDAR device 700 according to embodiment 9 can achieve high reception sensitivity even in cases where conventional APDs lack the reception sensitivity.
  • the optical output of the light source 701 can be reduced, thereby saving power and improving safety for the eyes.
  • the tunnel current does not increase as described in the explanation of Figure 29, making it easy to identify weak light.
  • the residence time in the multiplication layer is short, and as shown in the conceptual diagram of Figure 54B, a current pulse with a high peak intensity is obtained, resulting in high identification sensitivity.
  • the optical output of the light source can be reduced, thereby saving power and providing the added benefit of increased safety for the eyes.
  • the reflected light from an object is received by the DA-APD of the present disclosure, which makes it possible to measure the distance to a distant object, reduces the power consumption of the light source, and provides a LIDAR device that is also highly safe for the eyes.

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Abstract

Un élément de réception de lumière à semi-conducteur (100) selon la présente divulgation comprend : un substrat (1) ; une couche semi-conductrice de type n (2) formée sur le substrat (1) ; une couche de multiplicateur (3) formée sur la couche semi-conductrice de type n (2) ; une couche de relaxation de champ électrique de type p (4) formée sur la couche de multiplicateur (3) ; une couche d'absorption de lumière (5) formée sur la couche de relaxation de champ électrique de type p (4) ; une première couche de fenêtre (7) formée sur la couche d'absorption de lumière (5) ; une seconde couche de fenêtre (8) formée sur la première couche de fenêtre (7) et ayant une structure périodique bidimensionnelle (70) dans laquelle une pluralité de trous (11) ou de résidus (8s) sont agencés de manière bidimensionnelle selon une certaine périodicité ; un film isolant de protection de surface (10) formé au moins sur la structure périodique bidimensionnelle (70) ; et une électrode de surface (32) formée au moins sur le film isolant de protection de surface (10).
PCT/JP2024/013696 2024-04-03 2024-04-03 Élément de réception de lumière à semi-conducteur, dispositif de terminaison de ligne optique, dispositif d'émission/réception à modulation d'intensité multiniveau, dispositif de réception numérique cohérente, système radio sur fibre, système de capteur spad et dispositif lidar Pending WO2025210764A1 (fr)

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PCT/JP2024/013696 WO2025210764A1 (fr) 2024-04-03 2024-04-03 Élément de réception de lumière à semi-conducteur, dispositif de terminaison de ligne optique, dispositif d'émission/réception à modulation d'intensité multiniveau, dispositif de réception numérique cohérente, système radio sur fibre, système de capteur spad et dispositif lidar

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