Photodetector

Abstract

This photodetector comprises a first electroconductor layer (first metal electrode and contact layer 101), a detection layer, and a second electroconductor layer (contact layer 109 and second metal electrode) disposed in that order from a first surface side, wherein: the detection layer comprises a quantum well in which a first barrier layer (102), a first well layer (103) that is an excitation well layer, a second barrier layer (104), a second well layer (105) that is a transition well layer, and a third barrier layer (106) are provided in that order from the side of a surface that abuts the first electroconductor layer; the first well layer (103) has at least a ground stratum and an excitation stratum; the energy levels of conduction band energy of the first through third barrier layers (102, 104, 106) are higher than that of the excitation stratum, and the thicknesses of these layers are such that a tunneling current flows through; the second well layer (105) has two or more strata An (where n are integers starting from 1); the stratum A1 abuts a first surface where the second barrier layer (104) is disposed and is roughly equal to the excitation stratum; and the other strata abut a second surface where the third barrier layer (106) is disposed.

Description

Photodetector

[0001] The present disclosure relates to photodetectors.

[0002] Mid-infrared light with wavelengths of 3 to 10 μm is an important wavelength range for temperature measurement, night vision, gas measurement, and the like. HgCdTe has traditionally been widely used as an infrared detector for this wavelength range. However, due to the toxicity of Hg and Cd, alternative photodetectors are needed, and quantum well infrared detectors that utilize intersubband transitions in quantum wells are considered promising. These detectors are disclosed, for example, in Patent Documents 1 and 2 and Non-Patent Document 1.

[0003] There are two types of quantum well infrared detectors: photoconductive and photovoltaic. Of these, photovoltaic quantum well infrared detectors require delicate quantum well design, but because they have no dark current, they are considered suitable for use in camera detection elements with limited electron storage capacity, as well as high-temperature and high-speed photodetectors. Among photovoltaic quantum well infrared detectors, quantum cascade detectors in particular have attracted attention because they can achieve photovoltaic operation through the cascade structure of quantum wells, and are disclosed, for example, in Patent Document 3 and Non-Patent Documents 2 and 3.

[0004] Japanese Patent Publication No. 2021-100033 Japanese Patent Publication No. 2023-162492 Japanese Patent Publication No. 2013-41933

[0005] M. F. Hainey, Jr. ,T. Mano, T. Kasaya, Y. Jimba, H. Miyazaki, T. Ochiai, H. Osato, K. Watanabe, Y. Sugimoto, T. Kawazu, Y. Arai, A. Shigetou, and H. T. Miyazaki, “Patchwork metasurface quantum well photodetectors with broaded photoresponse”, Opt. Express, Vol. 29, No. 1, pp. 59-69 (Jan. 2021) https: / / doi. org / 10.1364 / OE. 408515L. Gendron, C. Koeniguer, V. Berger, and X. Marcadet, “High resistance narrow band quantum cascade photodetectors”, Applied Physics Letters, Vol. 86, No. 12, 121116 (2005) https: / / dx. doi. org / 10.1063 / 1.1884257C. Koeniguer, G. Dubois, A. Gomez, and V. Berger, “Electronic transport in quantum cascade structures at equilibrium”, Physical Review B, Vol. 74, No. 23, 235325 (2006) https: / / doi. org / 10.1103 / Physical RevB. 74.235325

[0006] One of the most important performance characteristics of a photodetector is the ratio of noise to sensitivity (SNR), which is the detectability. The main noise in a photovoltaic photodetector operating under unbiased conditions is Johnson noise. Since Johnson noise is inversely proportional to the 1 / 2 power of the resistance under unbiased conditions, this noise can be reduced by increasing the resistance.

[0007] However, conventional quantum cascade detectors have a binary structure, with only one conduction band energy in the barrier and well layers. This means that current leakage from the ground state to the intermediate subband cannot be ignored, making it difficult to achieve high resistance. As a result, conventional quantum cascade detectors have the problem of being unable to ensure a sufficient signal-to-noise ratio.

[0008] An object of the present disclosure is to provide a quantum well photodetector that can solve the problem of the S / N ratio of the quantum well photodetector, particularly the quantum cascade photodetector, described above, and can obtain high detection capability.

[0009] In order to solve the above-described problems, a photodetector according to one aspect of the present invention is a photodetector in which a first conductor layer, a detection layer, and a second conductor layer are sequentially arranged from a first surface side, wherein the detection layer has a quantum well in which a first barrier layer, a first well layer which is an excitation well layer, a second barrier layer, a second well layer which is a transition well layer, and a third barrier layer are sequentially arranged from a surface side in contact with the first conductor layer, wherein the first well layer has at least a ground level and an excitation level, wherein the energy levels of conduction band energies of the first barrier layer, the second barrier layer, and the third barrier layer are higher than the excitation level, and the thicknesses of the first barrier layer, the second barrier layer, and the third barrier layer are thicknesses that allow a tunnel current to flow, and wherein the second well layer has two or more levels A ｎ (n is an integer starting from 1), and one of the levels is level A １ is in contact with the first surface on which the second barrier layer is disposed, and is substantially equal to the excitation level, and another level A ｎ′ (n' is an integer of 2 or more) is in contact with the second surface on which the third barrier layer is disposed, and １ The energy levels are as follows:

[0010] In order to solve the above-described problems, a photodetector according to one aspect of the present invention is a photodetector in which a first conductor layer, a detection layer, and a second conductor layer are sequentially arranged from a first surface side, wherein the detection layer has a quantum well including, sequentially from a surface side in contact with the first conductor layer, a first barrier layer, a first well layer which is an excitation well layer, a second barrier layer, a second well layer which is a transition well layer, and a third barrier layer, wherein the first well layer has a ground subband and one or more higher-order subbands, wherein the energy levels of the conduction band energies of the first barrier layer, the second barrier layer, and the third barrier layer are higher than the maximum energy level of the higher-order subband, and wherein the thicknesses of the first barrier layer, the second barrier layer, and the third barrier layer are thicknesses that allow a tunnel current to flow, The second well layer includes two or more subbands composed of a plurality of regions of different conduction band energy, in which the energy level of the conduction band energy in the second well layer decreases stepwise and / or in a gradual manner from the second barrier layer to the third barrier layer.

[0011] In order to solve the above-described problems, a photodetector according to one aspect of the present invention is a photodetector that includes a barrier layer with high conduction band energy and a well layer with low conduction band energy in a detection layer, and generates a photocurrent by utilizing intersubband absorption occurring in the well layer, wherein an electron supply layer, a first barrier layer and each of the barrier layers and well layers from the first barrier layer and the first well layer to the mth barrier layer and the mth well layer (m is a natural number of 2 or more), an (m+1)th barrier layer, and an electron collection layer are sequentially arranged from a first surface side, wherein the first well layer includes a ground subband having the lowest energy level and one or more higher-order subbands having an energy level higher than that of the ground subband, and absorbs light to excite photoelectrons from the ground subband to an excited subband consisting of one or more of the higher-order subbands, and the second well layer to the mth well layer transport the photoelectrons toward the electron collection layer, The second well layer is composed of a plurality of regions with different conduction band energies, and has two or more subbands with a monotonically higher energy level on the first surface side and a monotonically lower energy level on the second surface side in a stepwise and / or gradient manner from the first surface side on which the second barrier layer is disposed to the second surface side on which the third barrier layer is disposed.

[0012] In order to solve the above-mentioned problems, one aspect of the present invention provides a photodetector that includes a detection layer having barrier layers with low valence band energy and well layers with high valence band energy, and generates a photocurrent by utilizing intersubband absorption occurring in the well layers, the photodetector including a hole supply layer, a first barrier layer and each of the barrier layers and well layers from the first barrier layer and the first well layer to the mth barrier layer and the mth well layer (m is a natural number of 2 or more), an (m+1)th barrier layer, and a hole collection layer, which are arranged in this order from a first surface side, the first well layer having a ground subband with the highest energy level and one or more higher-order subbands having an energy level lower than that of the ground subband, and absorbing light to excite holes from the ground subband to an excited subband consisting of one or more of the higher-order subbands, the second well layer, the third well layer, ... the mth well layer transporting holes toward the hole collection layer, The second well layer is composed of a plurality of regions with different valence band energies, and has two or more subbands that are stepped and / or gradient from the first surface side where the second barrier layer is disposed to the second surface side where the third barrier layer is disposed, with the energy level monotonically lower on the first surface side and higher on the second surface side.

[0013] According to the present disclosure, it is possible to provide a quantum well photodetector that has a high resistance, thereby reducing Johnson noise and achieving high detection capability with an excellent S / N ratio.

[0014] 1 is a cross-sectional structural diagram illustrating an overview of the structure of a photodetector according to the present disclosure; 2 is a structural diagram illustrating a cross-sectional view of the structure of a main part of a quantum well infrared detector; 3 is a band structure diagram illustrating the conduction band energy structure and subbands of a photodetector according to the present disclosure; 4 is a band structure diagram illustrating the conduction band energy structure and subbands of a conventional quantum cascade detector; 5 is a characteristic diagram obtained by calculation of the spatial and energy distance dependence of the conductance from the ground subband to an arbitrary subband; 6 is a characteristic diagram illustrating the sensitivity spectrum at no bias of a photodetector having the band structure shown in FIG. 3 and Table 1 of Example 1, compared with a conventional method; 77 K was measured at a temperature of 77 K; 8(a) is a characteristic diagram illustrating the current characteristics and resistance at no bias of a photodetector having the band structure shown in FIG. 3 and Table 1 of Example 1, and 8(b) is a characteristic diagram for the conventional method having the band structure shown in FIG. 4 and Table 2; 8(b) is a characteristic diagram illustrating the specific detectivity spectrum at no bias of a photodetector having the band structure shown in FIG. 3 and Table 1 of Example 1, compared with a conventional method; 77 K was measured at a temperature of 77 K; 1A and 1B are structural diagrams of a plasmon resonator-integrated quantum well infrared detector fabricated in Example 2, and an SEM photograph (c) showing a prototype thereof. FIG. 1C is a characteristic diagram showing the temperature dependence of sensitivity in the absence of a bias of a photodetector having the band structure shown in FIG. 3 and Table 1 of Example 1, in comparison with a conventional method. FIG. 1D is a characteristic diagram showing the resistance R ０ 3 and Table 1 of Example 1. ｊ ＊ , D ＢＧ ＊ 10 is a band structure diagram illustrating the conduction band energy structure and subbands of a photodetector according to a third embodiment of the present disclosure.

[0015] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0016] Note that A to B in the text means A and up to B, that is, greater than or equal to A and less than or equal to B.

[0017] First Embodiment In the first embodiment, a quantum well photodetector, which is a basic type, will be described.

[0018] <Concept> First, the conventional structure and its problems will be described.

[0019] Quantum cascade detectors have been actively researched in recent years as a promising photovoltaic quantum well infrared detector. Quantum cascade detectors have a binary structure in which the barrier and well layers each have a single conduction band energy, and the energy of photoexcited electrons is transported toward the electron collection layer by stepping down the energy in fixed energy increments using fast polar optical phonon scattering to prevent backflow.

[0020] The resistance of a quantum well infrared detector increases as the spatial and energy distance of the wave function from the fundamental subband to other subbands increases. Therefore, to increase the resistance, it is necessary to locate the subband that transports electrons as far away from the fundamental subband as possible in terms of both spatial and energy. However, with a simple binary barrier / well layer combination, each layer is only a few nanometers thick, and well layers closer to the initial stage, such as the second and third well layers, must be made thinner in order to achieve higher subband positions.

[0021] The distance that the spatial electron wave function travels during one energy step drop is limited to only a few nm, which is determined by the layer thickness, and is determined by the structure, and it has been difficult to freely set the spatial and energy arrangement of the subbands. For example, in Non-Patent Document 2, a 1000 Ωcm at a temperature of 77 K was measured while repeating 40 periods of the detection unit structure. ２ This means that the resistance per detection unit structure is only 25 Ωcm ２ That was all.

[0022] Next, the element structure and effects of the present disclosure (first embodiment) will be described.

[0023] In this disclosure, the structure of the second well layer, which is the first step in the transport of excited photoelectrons, is devised to have a structure in which small well layers are incorporated into a single well layer, with multiple regions of different conduction band energies. Two or more subbands arranged vertically within the second well layer are realized, enabling a rapid one-step energy drop to a spatially distant subband by polar optical phonon scattering within the second well layer alone. This prevents electron backflow compared to conventional quantum cascade detectors, while achieving an order of magnitude higher resistance and lower noise.

[0024] The structure of the element of embodiment 1 can be defined by at least one of structure 1 defined by the position of the energy level, structure 2 defined by the position of the conduction band energy, and structure 3 defined by the repeating unit of the quantum well.

[0025] <Structure 1> As shown in Fig. 1, a photodetector 1001 is provided in which a first conductor layer 1, a detection layer 2, and a second conductor layer 3 are arranged in this order from the first surface side. In Fig. 1, light 15 is depicted as being incident from the first surface side, but this is not a limitation, and light may be incident from the second surface side, which is the surface opposite to the first surface side.

[0026] Fig. 3 is an energy band diagram (energy state diagram) of the detection layer 2. As shown in Fig. 3, the detection layer 2 includes a quantum well having, in order from the surface in contact with the first conductor layer 1 (1a), a first barrier layer 102, a first well layer 103 which is an excitation well layer, a second barrier layer 104, a second well layer 105 which is a transition well layer, and a third barrier layer 106.

[0027] The first well layer 103 has at least a ground level and an excited level. The first well layer 103 is preferably formed with n-type impurities. When the well layer is n-type impurities, the effective mass of electrons is generally smaller than the effective mass of holes, resulting in high light absorption efficiency and fast response of the detection layer 2. In addition, the abundance of research examples makes it easy to design the detection layer 2 with high precision.

[0028] The conduction band energy levels of the first barrier layer, the second barrier layer 104, and the third barrier layer 106 are higher than the excitation level of the first well layer 103. The thicknesses of the first barrier layer, the second barrier layer 104, and the third barrier layer 106 are such that a tunnel current flows through them.

[0029] The second well layer 105 has two or more levels A ｎ (n is an integer starting from 1). One of the levels is level A １ is in contact with the first surface on which the second barrier layer 104 is disposed, and is substantially equal to the excitation level of the first well layer 103. Another level A of the second well layer 105 ｎ′ (n' is an integer of 2 or more) is in contact with the second surface on which the third barrier layer 106 is disposed and has a level A １ The energy levels are as follows:

[0030] Level A １ and the excitation level of the first well layer 103 is preferably 0 meV or more and 15 meV or less (more preferably 0 meV or more and 10 meV or less). When the energy difference is within this range, photoelectrons and energy are efficiently transported and backflow of photoelectrons is suppressed, thereby increasing the sensitivity and resistance as a photodetector.

[0031] The first conductor layer 1 includes a first conductive layer 1a and a first metal electrode 1b. The first conductive layer 1a preferably has the property of forming an ohmic connection between the first metal electrode 1b and the detection layer 2. From this viewpoint, the first conductive layer 1a also functions as a first contact layer. The second conductor layer 3 includes a second conductive layer 3a and a second metal electrode 3b. The second conductive layer 3a preferably has the property of forming an ohmic connection between the second metal electrode 3b and the detection layer 2. From this viewpoint, the second conductive layer 3a also functions as a second contact layer. Incident light 15 is incident through the first conductor layer 1a or the second conductor layer 3a, and the first conductor layer 1a and / or the second conductor layer 3a are transparent to the incident light 15.

[0032] The first conductor layer 1 and the second conductor layer 3 preferably comprise, as part of their structures, a mixed crystal of one or more elements selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge. In particular, it is preferable that the mixed crystal be in contact with the detection layer 2. In other words, the first conductive layer 1a and the second conductive layer 3a are preferably made of one or more materials selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge. This material configuration ensures good electrical contact between the first metal electrode 1b and the detection layer 2, and between the second metal electrode 3b and the detection layer 2.

[0033] The detection layer 2 preferably contains one or more elements selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge. The detection layer 2 is also preferably a mixed crystal consisting of one or more elements selected from this group. The detection layer 2 is also preferably composed of one or more elements selected from this group. The inclusion of materials from these groups makes it possible to control the crystallinity, thickness, and flatness at the atomic layer level, and the formation of mixed crystals from these materials allows for the required conduction band energy to be achieved. As a result, the use of materials from these groups makes it possible to design a detection layer structure suited to the requirements and realize it exactly as designed.

[0034] When light 15 is incident, excitation occurs in the first well layer 103, and the excited photoelectrons are transported by the tunneling effect to the second well layer 105. The second well layer 105 has a plurality of energy levels that decrease with increasing distance from the first well layer 103. As a result, polar optical phonon scattering in the second well layer 105 causes a rapid one-step energy drop to a spatially distant subband, causing the flow of photoelectrons to become one-way and increasing resistance.

[0035] Here, if a cascade well layer group is formed between the third barrier layer 106 and the second conductor layer 3 a in the detection layer 2, where one or more pairs of cascade well layers and barrier layers are provided, and in particular, if a cascade well layer group is formed in which the energy levels of the ground subbands in the cascade well layers monotonically decrease from the third barrier layer 106 toward the second conductor layer, this further improves the sensitivity and response speed, which is preferable.

[0036] <Structure 2> As shown in FIG. 1, a photodetector 1001 is provided in which a first conductive layer 1, a detection layer 2, and a second conductive layer 3 are arranged in this order from the light 15 incident surface side.

[0037] As shown in the energy band diagram (energy state diagram) of FIG. 3 , the detection layer 2 includes a quantum well having, in order from the surface in contact with the first conductor layer 1 (1 a), a first barrier layer, a first well layer 103 which is an excitation well layer, a second barrier layer 104, a second well layer 105 which is a transition well layer, and a third barrier layer 106.

[0038] The first well layer 103 has a ground subband and one or more higher-order subbands, the energy levels of the conduction band energy of the first barrier layer 102, the second barrier layer 104, and the third barrier layer 106 are higher than the maximum energy level of the higher-order subbands, and the thicknesses of the first barrier layer 102, the second barrier layer 104, and the third barrier layer 106 are set to be thicknesses that allow tunneling current to flow. Here, the first well layer 103 is preferably formed with n-type impurities. With n-type impurities, the effective mass of electrons is generally smaller than the effective mass of holes, which has the effect of high light absorption efficiency and fast response. In addition, a wealth of research examples makes high-precision design easy.

[0039] The second well layer 105 includes two or more subbands each composed of a plurality of regions with different conduction band energies. The energy level of the conduction band energy in the second well layer 105 decreases stepwise and / or in a gradual manner from the second barrier layer 104 to the third barrier layer 106.

[0040] Similar to structure 1, first conductor layer 1 comprises a first conductive layer 1a and a first metal electrode 1b that transmit incident light 15. It is preferable that first conductive layer 1a has the property of forming an ohmic connection between first metal electrode 1b and detection layer 2, and from this viewpoint, first conductive layer 1a also functions as a first contact layer. Second conductor layer 3 comprises a second conductive layer 3a and a second metal electrode 3b. It is preferable that second conductive layer 3a has the property of forming an ohmic connection between second metal electrode 3b and detection layer 2, and from this viewpoint, second conductive layer 3a also functions as a second contact layer.

[0041] The first conductor layer 1 and the second conductor layer 3 preferably comprise, as part of their structures, a mixed crystal of one or more elements selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge. In particular, it is preferable that the mixed crystal be in contact with the detection layer 2. In other words, the first conductive layer 1a and the second conductive layer 3a are preferably made of one or more materials selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge. This material configuration ensures good electrical contact between the first metal electrode 1b and the detection layer 2, and between the second metal electrode 3b and the detection layer 2.

[0042] The detection layer 2 preferably contains one or more elements selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge. The detection layer 2 is also preferably a mixed crystal consisting of one or more elements selected from this group. The detection layer 2 is also preferably composed of one or more elements selected from this group. The inclusion of materials from these groups enables control of crystallinity, thickness, and flatness at the atomic layer level. The required conduction band energy can be achieved by forming a mixed crystal consisting of materials from these groups. By using materials from these groups, it is possible to design a detection layer structure suited to requirements and realize it exactly as designed.

[0043] When light 15 is incident, excitation occurs in the first well layer 103, and the excited photoelectrons are transported to the second well layer 105 by the tunneling effect. The second well layer 105 is composed of a plurality of regions with different conduction band energies, and the energy level of this conduction band energy decreases stepwise and / or in a gradient from the second barrier layer 104 to the third barrier layer 106. As a result, the flow of photoelectrons in the detection layer 2 becomes one-way, and the resistance increases.

[0044] Here, if a cascade well layer group having one or more pairs of cascade well layers and barrier layers is formed between the third barrier layer 106 and the second conductor layer in the detection layer 2 (particularly, a cascade well layer group in which the energy level of the ground subband in the cascade well layer monotonically decreases from the third barrier layer 106 toward the second conductor layer), the sensitivity and response speed of the detection layer 2 are further improved, which is preferable.

[0045] 1 , a photodetector 1001 of structure 3 has a first conductor layer 1, a detection layer 2, and a second conductor layer 3 arranged in this order from the incident surface side of light 15, and includes a barrier layer with high conduction band energy and a well layer with low conduction band energy in the detection layer 2. The photodetector 1001 is a semiconductor photodetector that generates a photocurrent by utilizing intersubband absorption occurring in the well layer.

[0046] As shown in FIG. 3 , which is an energy band diagram (energy state diagram), the semiconductor device includes an electron supply layer 101, a first barrier layer 102, a first well layer 103, a second barrier layer 104, and first to m-th (m is an integer of 2 or more) pairs of barrier layers and well layers, arranged in this order from the first surface side onto which light 15 is incident, an (m+1)-th barrier layer (in FIG. 3 , m=6, and layers up to the seventh barrier layer 108 are illustrated), and an electron collection layer 109.

[0047] The first well layer 103 has a ground subband with the lowest energy level and one or more higher-order subbands with higher energy levels than the ground subband, and absorbs light to excite photoelectrons into an excited subband consisting of one or more of the ground subband to the higher-order subbands. Here, the first well layer 103 is preferably formed with n-type impurities. With n-type impurities, the effective mass of electrons is generally smaller than the effective mass of holes, resulting in high light absorption efficiency and fast response. Furthermore, a wealth of research examples facilitates high-precision design.

[0048] The second well layer 105 is composed of multiple regions with different conduction band energies. The second well layer 105 has two or more subbands with monotonically higher energy levels on the A-side and lower energy levels on the B-side in a step-like and / or gradient manner from the A-side where the second barrier layer 104 is disposed to the B-side where the third barrier layer 106 is disposed. This allows a rapid one-step energy drop in a spatially distant subband due to polar optical phonon scattering only within the second well layer 105.

[0049] The m well layer transports photoelectrons from the second well layer 105 toward the electron collection layer. In this way, a low-noise photodetector is provided that achieves high resistance while preventing backflow of electrons compared to conventional quantum cascade detectors.

[0050] Here, the position of the center of gravity of the square of the wave function of the ground subband of the second well layer 105 is preferably located 15 nm to 100 nm away from the position of the center of gravity of the square of the wave function of the ground subband of the first well layer 103. When the position of the center of gravity of the square of the wave function is in this range, electrons can be transported as far as possible while maintaining the coherency of the wave function, thereby realizing high resistance.

[0051] It is also preferable that any higher-order subband in the first well layer 103 is coupled to a higher-order subband in the second well layer 105, thereby forming a bonding band and an antibonding band originating from each subband. The formation of the bonding band and the antibonding band produces the effect of efficiently transporting electrons from the first well layer 103 to the second well layer 105 at high speed by the coherent resonant tunneling effect.

[0052] The energy difference between the bonding band and the antibonding band is preferably 0.1 meV to 15 meV, inclusive. When the energy difference between the bonding band and the antibonding band is in this range, the effect of suppressing the backflow of electrons is particularly produced.

[0053] Furthermore, it is preferable that the contribution of polar optical phonon scattering is maximized in the relaxation transition from the higher subbands of the second well layer 105 to the ground subband of the second well layer 105. When the contribution of polar optical phonon scattering is maximized, an effect is achieved in which the transition of electrons from one subband to another subband occurs efficiently and at high speed.

[0054] Furthermore, the energy difference between the higher subband of the second well layer 105 and the base subband of the second well layer 105 is preferably more than 15 meV and not more than 60 meV. When the energy difference between the subbands is in this range, electrons transition to the next level efficiently and quickly due to resonance with the energy of LO phonons (36 meV in the case of GaAs).

[0055] Furthermore, when m is an integer of 3 or more, it is preferable that the energy levels of the high-order subbands of the second well layer 105 and the ground subbands of the second well layer 105 and subsequent well layers decrease in order. By decreasing the energy levels of the ground subbands in order, efficient and high-speed electron transport toward the electron collection layer 109 is realized.

[0056] Similar to structures 1 and 2, the first conductor layer 1 comprises a first conductive layer 1a and a first metal electrode 1b that transmit incident light 15. The first conductive layer 1a preferably has the property of forming an ohmic connection between the first metal electrode 1b and the detection layer 2, and from this perspective, also functions as a first contact layer. The second conductor layer 3 comprises a second conductive layer 3a and a second metal electrode 3b. The second conductive layer 3a preferably has the property of forming an ohmic connection between the second metal electrode 3b and the detection layer 2, and from this perspective, the second conductive layer 3a also functions as a second contact layer.

[0057] Similar to structures 1 and 2, first conductor layer 1 and second conductor layer 3 preferably include, as part of the structure, a mixed crystal of one or more selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge. In particular, it is preferable that the mixed crystal be in contact with detection layer 2. In other words, the first conductive layer 1a and the second conductive layer 3a are preferably made of one or more materials selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge. This material configuration ensures good electrical contact between the first metal electrode 1b and the detection layer 2, and between the second metal electrode 3b and the detection layer 2.

[0058] The detection layer 2 preferably contains one or more elements selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge. The detection layer 2 is also preferably a mixed crystal consisting of one or more elements selected from this group. The detection layer 2 is also preferably composed of one or more elements selected from this group. The inclusion of materials from these groups makes it possible to control the crystallinity, thickness, and flatness at the atomic layer level. The mixed crystals made of materials from these groups enable the realization of the required conduction band energy. The use of materials from these groups allows the design of a detection layer structure suited to the requirements and the realization of that structure.

[0059] The detection layer 2, like the structures 1 and 2, can be fabricated by growing an epitaxial crystal on a single crystal by the MBE (Molecular Beam Epitaxy) method. ２ , WSe ２ , MoS ２ , MoSe ２ Other examples include a method of stacking layered materials such as black phosphorus, graphene, and hexagonal BN.

[0060] The detection layer 2 may have a configuration in which two or more layer structures are arranged in series. One layer structure includes m pairs of barrier layers and well layers (first barrier layer 102, first well layer 103, second barrier layer 104, second well layer 105, ..., mth barrier layer, mth well layer) (m: an integer of 2 or more). By configuring the detection layer 2 by stacking layer structures including m pairs of barrier layers and well layers, the absorption efficiency and resistance of the detector 2 can be improved.

[0061] In addition, in the structures 1, 2, and 3, any one or more of the first barrier layer 102, the first well layer 103, the second barrier layer 104, the second well layer 105, and the third well layer 107 may have a superlattice structure composed of a plurality of barrier layers and well layers. The introduction of a superlattice structure enables more arbitrary setting of subband energy, and also enables robust electron transport independent of fabrication errors because the formation of a miniband generates a width in the energy level.

[0062] Second Embodiment A photodetector according to a second embodiment is a photodetector in which a plasmon resonator is combined with the quantum well photodetector described in the first embodiment to dramatically improve performance such as sensitivity.

[0063] 9A, a quantum well photodetector (plasmon resonance quantum well infrared detector) 1004 using a plasmon resonator according to the second embodiment has a quantum function layer and a first metal electrode 11b stacked in this order on a metal layer 13c. The quantum function layer is made of a detection layer 12 sandwiched between a first contact layer 11a and a second contact layer 13a, as described in the first embodiment. In addition, island-shaped or mesh-shaped metal patches 17 are formed on the quantum function layer in the photodetection region.

[0064] A metal layer 13c, to which the second metal electrode 13b is electrically connected, is disposed below the quantum function layer, and the quantum function layer is sandwiched between the metal patch 17 and the metal layer 13c. Here, the metal layer 13c may be a metal substrate that constitutes a substrate in itself, or may be a substrate with sufficient rigidity, such as a metal layer 13c formed on a semiconductor substrate 13d made of Si or GaAs. In the photodetector 1004 shown in Figure 9(a), the metal layer 13c is formed on the surface of a semiconductor substrate 13d made of GaAs.

[0065] The material of the metal layer 13c is not particularly limited as long as it is a conductor whose real part of the dielectric constant is a negative value, but a material whose imaginary part of the dielectric constant is small is preferable. Examples of materials that can be used for the metal layer 13c include gold (Au), silver (Ag), copper (Cu), aluminum (Al), platinum (Pt), tungsten (W), and molybdenum (Mo). The material of the metal layer 13c is not necessarily limited to metal, and transparent conductors such as ITO, AZO, and GZO can also be used.

[0066] A second contact layer 13a is disposed between the detection layer 12 and the metal layer 17. The detection layer 12 is in ohmic contact with the metal layer 13c, which is preferable for increasing the sensitivity and output of the photodetector.

[0067] Here, the thickness of the quantum functional layer is preferably set to a film thickness that causes plasmon resonance. A specific range of thickness D between the metal patch 17 and the metal layer 13c (i.e., the combined thickness of the detection layer 12, the first contact layer 11a, and the second contact layer 13a) is greater than 0 and less than λ / (2n). Here, the refractive index n is the average refractive index of the detection layer 12, the first contact layer 11a, and the second contact layer 13a for the wavelength λ of the incident light. More specifically, the thickness D is a thickness that sufficiently causes plasmon resonance for incident light of a target wavelength based on Maxwell's equations.

[0068] The material of the metal patch 17 is not particularly limited as long as it is a conductor whose real part of the dielectric constant has a negative value, but a material whose imaginary part of the dielectric constant is small is preferable. Examples of materials that can be used for the metal patch 17 include gold (Au), silver (Ag), copper (Cu), aluminum (Al), platinum (Pt), tungsten (W), and molybdenum (Mo). The material of the metal patch 17 is not limited to metal, and transparent conductors such as ITO, AZO, and GZO can also be used.

[0069] The metal patches 17 are preferably arranged in an array (linear array, one-dimensional array) or a matrix (lattice array, two-dimensional array), for example. Instead of arranging the metal patches 17 in an array or matrix, a metal layer in which periodic openings are formed in a mesh pattern (also called a mesh pattern) can be used. This is to maximize the use of incident light for plasmon resonance and improve sensitivity and output.

[0070] The pattern pitch P and pattern width L of the metal patches 17 are preferably set to a size that sufficiently induces plasmon resonance for incident light of a target wavelength based on Maxwell's equation. The pattern pitch P of the metal patches 17 can be 0.5 μm or more and 200 μm or less. For example, to detect light with a wavelength of 7.0 μm, the pattern width L is preferably 0.99 μm and the pitch P is preferably 2.5 μm.

[0071] The height of the metal patch 17 is preferably thick enough to sufficiently induce plasmon resonance, but thin enough not to block light. The height of the metal patch 17 can be set to, for example, 30 nm or more and 300 nm or less.

[0072] 9B, the photodetector of the second embodiment may be configured such that the order of the first and second conductor layers is reversed, with the first conductor layer 11 being formed first and sandwiching the detection layer 12 therebetween. In this case, the detector becomes a plasmon resonance quantum well infrared detector 1005, in which, for example, the first contact layer 11a is formed in contact with the metal layer 11c, and then the detection layer 12, the second contact layer 13a, and the second metal electrode 13b are formed sequentially on the first contact layer 11a. Whether the outermost surface of the detection layer 12 is on the electron supply layer 101 side or the electron collection layer 109 side is simply a difference in the fabrication procedure, and no particular difference in performance occurs regardless of which is selected.

[0073] The photodetector 1004 of the second embodiment combines the quantum well photodetector with the dramatically improved S / N ratio described in the first embodiment and the plasmon resonator with dramatically increased sensitivity without compromising the characteristics of either, resulting in a photodetector with excellent S / N ratio and sensitivity.

[0074] Third Embodiment A photodetector according to a third embodiment is a photodetector that uses holes instead of electrons.

[0075] In the first and second embodiments, photodetection is performed using the flow of photoelectrons. By incorporating the features of the second well layer of the present disclosure, it is also possible to provide a photodetector with an inverse polarity structure. That is, by reversing the energy relationships, such as replacing electrons with holes, n-type with p-type, and the conduction band with the valence band, a photodetector with an inverse polarity structure can be provided.

[0076] The photodetection wavelength of the photodetectors of the present disclosure shown in Embodiments 1, 2, and 3 is not particularly limited due to its operating principle, and can target, for example, far ultraviolet, ultraviolet, visible, infrared, and far infrared regions with wavelengths of 100 nm to 1000 μm. Therefore, the 3 to 10 μm infrared region of conventional photodetectors using HgCdTe as described in the background can also be detected, and the present disclosure makes it possible to avoid the problem of toxic contamination by Hg and Cd.

[0077] GaAs, which is easy to stably manufacture with excellent quality in terms of crystallinity and low defects, is particularly suitable as a material for the detection layer 2 of the photodetector of the present disclosure. When GaAs is used for the detection layer, its band gap control range makes it possible to provide a photodetector that can suitably detect light in the infrared region with wavelengths of 4 μm to 30 μm.

[0078] Example 1 In Example 1, a photodetector 1002, which is a specific example of the photodetector 1001 described in Embodiment 1, was fabricated as a prototype, and its evaluation results are shown in comparison with a conventional photodetector having the band structure shown in FIG. 4 . Like the photodetector 1001, the photodetector 1002 includes a detection layer 2 having the band structure shown in FIG. 3 . Note that FIG. 2A is a structural diagram showing the structure of a main part of the photodetector 1002 in cross section. FIG. 2B is a structural diagram showing the structure of a main part of a photodetector 1003, which is a modified example of the photodetector 1002, in cross section. The photodetector 1002 used a GaAs substrate as the conductive substrate 13c (see FIG. 2A ). The photodetector 1003 used a GaAs substrate as the conductive substrate 21c (see FIG. 2B ). More specifically, n-type GaAs was used as the GaAs constituting each of the substrates 13c and 21c. The doping amount of n-type GaAs constituting substrate 13c was determined by prioritizing high electrical conductivity over high optical transmittance. On the other hand, the doping amount of n-type GaAs constituting substrate 21c was determined by prioritizing high optical transmittance over high electrical conductivity. In other words, the doping amount of n-type GaAs constituting substrate 21c is lower than the doping amount of n-type GaAs constituting substrate 13c.

[0079] 2A, the photodetector 1002 includes a first contact layer 11a, a first metal electrode 11b, a detection layer 12, a second contact layer 13a, a second metal electrode 13b, and a substrate 13c. The first contact layer 11a and the first metal electrode 11b form a first conductor layer, and the second contact layer 13a, the second metal electrode 13b, and the substrate 13c form a second conductor layer 13.

[0080] 2B, the photodetector 1003 includes a first contact layer 21a, a first metal electrode 21b, a substrate 21c, a detection layer 22, a second contact layer 23a, and a second metal electrode 23b. The first contact layer 21a, the first metal electrode 21b, the substrate 21c, the detection layer 22, the second contact layer 23a, and the second metal electrode 23b of the photodetector 1003 correspond to the first contact layer 11a, the first metal electrode 11b, the substrate 13c, the detection layer 12, the second contact layer 13a, and the second metal electrode 13b of the photodetector 1002, respectively. In the substrate 21c, the doping amount of n-type GaAs is determined so that the transmittance of the light ray 25 exceeds a predetermined transmittance. 2B, one side surface of the substrate 21c (the side surface located on the left side in FIG. 2B) is polished to form an angle of 45° with respect to the main surface. A light ray 25 enters the substrate 21c from the side surface that is at an angle of 45° with respect to the main surface, and reaches the first contact layer 21a.

[0081] <Element Structure> The band structure (conduction band energy structure and subband wave functions) of the detection layer 12 of the photodetector 1001 according to Example 1 is shown in Fig. 3, and its detailed layer structure is shown in Table 1. As a comparative example, a conventional quantum cascade detector shown in Fig. 4 and Table 2 was fabricated.

[0082] In most photovoltaic quantum well infrared detectors reported to date, the same detection unit structure is repeated several dozen times. However, in Example 1, a basic structure with only one detection unit structure will be described so that only the characteristics determined by each barrier layer and each well layer can be clearly observed and compared. In the photodetector 1002, electrons are supplied from the electron supply layer 101, and an amount of photoelectrons proportional to the brightness of the incident light 15 is excited in the first well layer 103. The photoelectrons are then transported rightward and reach the electron collection layer 109 located on the right end side, where they are detected by an external circuit.

[0083] On the other hand, to enable a fair comparison with the photodetector 1002 of Example 1, the comparative quantum cascade detector was designed with reference to Non-Patent Document 2 so that the detection wavelength and the thickness of each barrier layer and each well layer were as common as possible with those in FIG. 3 and Table 1. Specifically, in the comparative quantum cascade detector, the section from the electron supply layer 201 to the first barrier layer 202 has the same structure as the section from the electron supply layer 101 to the first barrier layer 102 of the photodetector 1002 of Example 1. Furthermore, the section from the third well layer 207 to the electron collection layer 209 has the same structure as the section from the third well layer 107 to the electron collection layer 109. However, the comparative quantum cascade detector includes the second well layer 205 to the fourth barrier layer in the portion corresponding to the second well layer 105 and the third barrier layer 106 of the photodetector 1002 of Example 1. Therefore, the latter common portion corresponds to the fourth well layer and beyond in the comparative quantum cascade detector.

[0084] A first metal electrode is disposed at the left end of Example 1 (FIG. 3) and the comparative quantum cascade detector (FIG. 4). Next to each first metal electrode is a 48 nm thick n-type GaAs contact layer that functions as the electron supply layer 101 and the electron supply layer 201. To achieve a tunneling ohmic junction with the electrode, a Si doping density configuration, which will be described later, is used. The electrode side is doped with 5×10 ... １８ cm －３ Furthermore, the doping is 3×10 １２ cm －２ Seven delta-doped layers of 2×10 are inserted (see Patent No. 7002126). The 20 nm thick regions near the first barrier layer 102 and the first barrier layer 202 have a doping density of 2×10 １８ cm －３ With this configuration, an ohmic junction can be achieved simply by depositing a metal film without alloying, and good performance can be maintained even when combined with a plasmon resonator as in Example 2 described below.

[0085] Each barrier layer is basically Al with Al composition x=0.40. ｘ Ga １－ｘElectrons are supplied from the electron supply layer 101 and the electron supply layer 201 to the first well layer 103 and the first well layer 203 by tunneling through the first barrier layer 102 and the first barrier layer 202 .

[0086] The first well layer 103 and the first well layer 203 are 1×10 １８ cm －３ The photodetector 1002 of Example 1 and the comparative quantum cascade detector have slightly different thicknesses (one atomic layer). This is based on the result of designing them so that they have peak sensitivities at approximately the same wavelength. The second barrier layer 104 and the second barrier layer 204 are common to both photodetectors, have a thickness of 5.65 nm, and are made of Al with an Al composition of x=0.40. ｘ Ga １－ｘ As.

[0087] In the comparative quantum cascade detector, the undoped GaAs well layer and the Al ｘ Ga １－ｘ As barrier layers are alternately repeated up to the eighth barrier layer 208. However, the thickness of the well layers gradually increases. This is because the energy of photoexcited electrons is sequentially transported by polar optical phonon scattering, which can rapidly relax the energy to lower levels. This configuration was determined with reference to Non-Patent Document 2, but in this example, the thickness of the barrier layers from the third barrier layer to the eighth barrier layer was set to a constant value (3.96 nm) to prevent the phenomenon from becoming complicated. In Example 1, each barrier layer was selected to be thicker than in general cases in order to increase the resistance of the photodetector as much as possible.

[0088] It should be noted here that the photodetector 1002 of Example 1 has a structure of the second well layer 105 that is significantly different from that of the comparative example, which has a conventional structure.

[0089] A region with a different Al composition x is formed in the second well layer 105. The first half of the second well layer 105 is an Al well layer with a thickness of 11.31 nm and x=0.32. ｘ Ga １－ｘ As, and the latter half is Al with a thickness of 5.65 nm and x = 0.25. ｘ Ga １－ｘThe second well layer 105 has a step-like bottom (conduction band energy). The subsequent third barrier layer 106 is made of Al, with a thickness of 5.65 nm and x=0.40, just like the second barrier layer 104. ｘ Ga １－ｘ This is thicker than the thickness of the third barrier layer 206 and subsequent layers in the comparative quantum cascade detector, which was set to 3.96 nm. ｘ Ga １－ｘ (The term "layer" refers to As.) After that, the third well layer 107 to the seventh barrier layer 108 of the photodetector 1002 of Example 1 and the fourth well layer to the eighth barrier layer 208 of the comparative quantum cascade detector have the same structure. Electrons that reach the last well layer tunnel through the last barrier layer and are transported to the electron collection layer 109 and the electron collection layer 209.

[0090] The 48-nm-thick n-type GaAs contact layer, which serves as the electron collection layer 109 and the electron collection layer 209, is formed by inverting the structure of the n-type GaAs contact layer of the electron supply layer. The highly doped region of the n-type GaAs contact layer is in contact with the second metal electrode, realizing a tunneling ohmic junction.

[0091] <Operation Principle> The reason why the photodetector 1002 of the first embodiment is designed as shown in FIG. 3 and Table 1, as opposed to the conventional quantum cascade detector shown in FIG. 4 and Table 2, is based on the following principle.

[0092] A theoretical calculation method for the resistance of a quantum cascade detector when no bias is present is given in Non-Patent Document 3, and the resistance can be described by the transition rate between subbands as shown in FIGS.

[0093] More specifically, the resistance is inversely proportional to the sum of the transition rates from the fundamental subband of the first well layer 103 and the first well layer 203 to the other subbands.

[0094] Resistance (no bias resistance: R ０ ) is normalized by the area (A) of the element, and the resistance at no bias is multiplied by the area R ０ A (unit: Ωcm ２) is generally of the form, and the sum of the transition speeds ΣG ｉｊ The relationship is expressed as follows:

[0095] Here, k Ｂ is the Boltzmann constant, T is the absolute temperature, and e is the electron charge. Since the reciprocal of resistance is called conductance, the transition rate can be thought of as representing conductance.

[0096] As a principle of transition between subbands, polar optical phonon scattering, which is the fastest and most dominant transition, is considered. This is considered to be a reasonable assumption in the temperature range mainly considered here, such as absolute temperature 77 K. Polar optical phonon scattering typically occurs with a lifetime of 1 picosecond (ps). The transition speed G ｉｊ The lower the value of for all transition paths (i→j), the higher the resistance that can be realized.

[0097] Transition speed G ｉｊ The detailed mathematical expression for the transition rate due to polar optical phonon scattering is roughly described by the overlap integral of the wave functions of both subbands and the Fermi-Dirac function representing the electron occupation state.

[0098] The overlap integral of wave functions represents the magnitude of spatial overlap of the wave functions, and in Figures 3 and 4, it represents the degree of overlap or deviation when two wave functions are viewed along the horizontal axis. For example, in Figures 3 and 4, states 2 and 3, which are reached by transition from the base subband (state 1) due to light absorption, have wave functions located directly above state 1, so there is a large spatial overlap, and the transition speed from state 2 or state 3 to state 1 is fast (easy to transition). On the other hand, state 4, which is located to the lower right of state 2 or state 3, has a center of gravity that moves rightward relative to state 1, so the overlap integral is smaller than that of state 2 or state 3, and the transition speed from state 4 to state 1 is slow (difficult to transition).

[0099] A photovoltaic quantum well infrared detector is a photodetector that attempts to exhibit photosensitivity by designing and fabricating an artificial structure in which electrons that have transitioned from state 1 to state 2 or state 3 due to photoexcitation are transported exclusively to one of states 4 to 8, rather than to state 1 or the electron supply layer 101 and the electron supply layer 201.

[0100] In conventional photodiodes based on pn junctions, electrons and holes generated by interband light absorption are automatically transported in one direction due to the shape of the conduction band energy and valence band energy formed by the pn junction, generating an output current.On the other hand, photovoltaic quantum well infrared detectors are photodetectors that artificially transport electrons generated by intersubband light absorption formed in the quantum well in one direction using a cleverly designed and fabricated quantum well array.

[0101] Since the structure discussed in this example is based on an n-type semiconductor, only electrons (holes are not excited) are excited in the intersubband transition, and unlike conventional pn junction devices, it operates solely with electrons.

[0102] If photoelectrons excited in the first well layer 103 and the first well layer 203, which act as light absorption layers, are left as they are, they will relax to state 1 directly below, and the electrons will not be transported, resulting in no optical output. However, if they are transported to the right faster than they can relax directly below, an asymmetry will occur, and there is a possibility that the electrons will ultimately be transported to the right. What is important here is to use polar optical phonon scattering, which is known for its fast relaxation, to extract the excited electrons to the right.

[0103] Conventional quantum cascade detectors have aimed to achieve high optical sensitivity by making electron transport more efficient and robust through the following: (1) strong tunnel coupling between the excited subband of the first well layer 203 and the ground subband of the second well layer 205 adjacent to it on the right, distributing the wave function of the excited subband widely to the right; and (2) extracting the electrons widely distributed to the right into the subband of the third well layer 207 at high speed using polar optical phonon scattering transitions.

[0104] The tunnel coupling (1) is a phenomenon in which the energies of the excitation subband of the first well layer 203 and the ground subband of the second well layer 205 are set to be approximately equal, causing the two wave functions to interact with each other via the second barrier layer 204 and mix to form two new levels (a bonding level and an antibonding level). This results in two new levels that are like the sum of the original wave functions of the first well layer 203 and the second well layer 205, with the same sign and the opposite sign, respectively. State 2 and State 3 in FIG. 4 are formed in this way. To achieve this, in the quantum cascade detector of FIG. 4 and Table 2, the second well layer 205 is made as thin as possible (only four atomic layers) without interfering with fabrication, so that the ground subband is positioned very high. The second barrier layer 204 is set to be not too thick, but thick enough to allow the tails of the wave functions of the first well layer 203 and the second well layer 205 to intermingle with each other. The tunnel transition between adjacent well layers is known to be an even faster transition (<1 ps) than the polar optical phonon scattering transition described above, and electrons excited from state 1 by light absorption move rapidly to states 2 and 3, which are widely distributed states.

[0105] State 4, formed as the ground subband of the third well layer 207, is set to overlap these states widely in space and have an energy interval close to 36 meV, the polar optical phonon energy of GaAs. Polar optical phonon scattering transitions occur resonantly and at high speed (0.1 to 1 ps) when the energy interval is close to the polar optical phonon energy. As a result, when electrons in state 1 absorb light, they move via resonant tunneling to states 2 and 3, and then to state 4 via polar optical phonon scattering transition at high speed. When this happens, the electrons can no longer easily return to state 1.

[0106] If states 5, 6, 7, and 8 are set so that the energy drops by 36 meV from state 4 formed by the third well layer 207, electrons are transported successively to the lower right. Since the advent of quantum cascade lasers, this type of structure, in which subbands are set in a stepped manner, has generally been called a "cascade" structure. In this way, electrons can be transported in one direction from the electron supply layer 201 to the electron collection layer 209.

[0107] However, careful analysis of this situation using the theory in Non-Patent Document 3 reveals that electron transport does not occur perfectly as described above. There is a slight spatial overlap between state 4 and state 1, and the transition rate between them is not completely zero. Furthermore, state 5, which transitions from state 4 to state 1 at approximately 36 meV below due to polar optical phonon scattering, still has a non-negligible overlap with state 1.

[0108] In addition to the overlap of the wave functions, the Fermi-Dirac function also plays an important role in the transition rate described in Non-Patent Document 3. Simply put, the farther apart the energy of the two wave functions is, the smaller the transition rate is, and the closer the energy is, the larger the transition rate is. This can be discussed in terms of how close the wave functions are when viewed along the vertical axis in Figures 3 and 4. State 5, which transitions from state 4, moves further away from state 1 in space. However, state 5 is closer to state 1 in energy than state 4. The actual resistance of the entire device is calculated from the sum of these individual element transition rates. However, in conventional quantum cascade detectors, the electrons that transition sequentially move away from each other in space but instead move closer in energy. This has been a problem, making it difficult to achieve a low transition rate and high resistance between the ground subbands of the first well layer 103 and the first well layer 203.

[0109] Figure 5 shows the results of quantitative calculations of how spatial distance and energy distance affect resistance for a typical model case. Assuming a typical wave function is the ground state of a 5 nm wide quantum well in a GaAs / AlGaAs structure with a conduction band energy difference of 0.3 V, the transition rate (unit: m) between two identical wave functions at 77 K when they are spaced apart by Δz and energy ΔE is calculated. －２ s －１ ) are represented by contour lines. This allows visual representation of how the transition speed changes between the base subband located at the origin and the destination subband located at an arbitrary position (Δz, ΔE).

[0110] As expected, the transition rate decreases and the resistance of the route increases as the wave functions become more spatially separated (to the right of the graph in Figure 5) and more energetically separated (to the top of the graph in Figure 5). Quantitatively, the transition rate decreases by an order of magnitude for every 2.1 nm spatial separation or every 15 meV energy separation.

[0111] <<Problems with Conventional Quantum Cascade Detectors>> Figure 5 plots the positions of the centers of gravity p12 to p17 of the squares of the wave functions (electron existence probability) of each subband as viewed from the base subband of the comparative quantum cascade detector in Figure 4. p12 to p17 correspond to states 2 to 7 in Figure 4, respectively. According to this, the transition speed from state 2 and state 3 to state 4 due to photoexcitation is 10 １２ m －２ s －１ The transition rates are within the above range. Since states 2 and 3 are almost directly above state 1, it is inevitable that they will exhibit large transition rates, but it can be seen that state 4, which should be spatially separated, also has a similar transition rate. Because the transition rates to these three states are equally high, the resistance of the comparative quantum cascade detector is limited to a low value.

[0112] Once the wavelength to be detected is determined, the energy difference between the ground subband and the excited subband of the first well layer 203 is determined, and the energy positions of states 2 and 3 are almost fixed. Furthermore, when attempting to maximize the polar optical phonon scattering transition from states 2 and 3, the energy position of state 4 is also almost fixed at approximately 36 meV lower than states 2 and 3. Therefore, conventional quantum cascade detectors have almost no design freedom in the energy direction of state 4. In addition, the thicknesses of the third barrier layer 206 and the third well layer 207 required to achieve such an energy position are also almost specified, so the design freedom in the spatial direction is also limited. As a result, it is inevitable that state 4 will have a high transition rate identical to states 2 and 3, which creates a bottleneck that prevents improvements in resistance. The inventors believe this is the fundamental reason why significant improvements in resistance have not been achieved in conventional quantum cascade detectors.

[0113] <<Improvements of the Present Disclosure>> What is required here is to move the spatial position of State 4 as far away as possible while keeping the energy position of State 4 the same, to reduce the transition rate to State 4, and to increase the resistance of the entire element. Therefore, in the photodetector 1002 of Example 1, the structure of the second well layer 105 was significantly modified.

[0114] First, the second well layer 105 is extremely thick and shallow compared to the other well layers. To align the energy position with the excitation subband of the first well layer 103, the conduction band energy at the bottom of the well is higher than that of the other well layers, making it raised. Furthermore, a small daughter well layer with a low conduction band energy is fabricated within the second well layer 105. To achieve this stepped conduction band energy profile, the second well layer 105 is formed from a combination of AlGaAs with different Al compositions x. This configuration allows one daughter well layer to realize State 4, and States 2, 3, and 4 coexist within the second well layer 105. Since one of States 2 and 3 originates from the excited state of the first well layer 103, the other of States 2 and 3 and State 4 are purely derived from the structure of the second well layer 105.

[0115] Furthermore, state 4 is the ground subband of the second well layer 105. State 4 is located below states 2 and 3 within one well, and there is a large spatial overlap. Furthermore, the energy difference from states 2 and 3 to state 4 is set to be close to 36 meV, which is the energy of polar optical phonons, and the polar optical phonon scattering transition from states 2 and 3 to state 4 is accelerated. Furthermore, by making the third barrier layer 106 5.65 nm thick, which is thicker than the corresponding portion of the comparative quantum cascade detector, the wave function of state 4 in the child well layer of the second well layer 105 is strongly localized, the amplitude of the wave function is increased, and the transition rate from states 2 and 3 is maximized.

[0116] The positions p22 to p26 of the center of gravity of the wave function of each subband as viewed from the fundamental subband of the photodetector 1002 of Example 1 shown in FIG. 3 are plotted in FIG. 5. p22 to p26 correspond to states 2 to 6 in FIG. 3, respectively. It can be seen that by modifying the second well layer 105, the position of the center of gravity of the wave function of state 4 was moved 9 nm farther away than in the comparative quantum cascade detector. The transition rate was reduced by more than four orders of magnitude. Once moved this far away, there was virtually no possibility of a transition from state 4 to state 1. Thus, by modifying the structure of the second well layer 105, the bottleneck that limited the resistance in the comparative quantum cascade detector was eliminated, making it possible to achieve a large resistance.

[0117] <Fabrication Method> The photovoltaic quantum well infrared detectors shown in FIGS. 3 and 4 were fabricated using a molecular beam epitaxy (MBE) apparatus (COMPACT21T, manufactured by RIBER).

[0118] First, an n-type GaAs (100) substrate was prepared, and the oxide film on its surface was removed by heating at 580°C. Then, a GaAs buffer layer was grown to a thickness of 300 nm using an MBE system to flatten the surface.

[0119] Then, Al with x=0.90 ｘ Ga １－ｘ As sacrificial layer 900 nm, Al of x = 0.55 ｘ Ga １－ｘAs sacrificial layers were formed to a thickness of 100 nm at a temperature of 580° C. These sacrificial layers were inserted for the purpose of being used later in Example 2. １７ cm －３ The silicon is doped to make it n-type, and in the first embodiment, it is used as a conductive layer.

[0120] 3 and 4 was formed in the detection layer 12. In this case, the structure shown in Table 1 or 2 was laminated in the following order from the bottom up: the contact layer to be the electron collection layer 109 or 209, the seventh barrier layer 108 or 8th barrier layer 208, ... the first well layer 103 or 203, the first barrier layer 102 or 202, and finally the contact layer to be the electron supply layer 101 or 201. The formation temperature for these layers and subsequent layers was 530°C.

[0121] It should be noted that the multi-quantum well structure actually fabricated was not exactly as designed. Crystal growth by MBE was performed according to Tables 1 and 2, but based on X-ray diffraction analysis of the grown sample and the absorption wavelength characteristics of the fabricated photodetector, it is estimated that the layer thickness was 7% larger and the Al composition x was 7% smaller. The band diagrams shown in Figures 3 and 4 were plotted taking these errors into account, and represent the raw state of the final sample.

[0122] Because an optical electric field perpendicular to the quantum well layer is essential for intersubband transitions, quantum well photodetectors are insensitive to perpendicularly incident light (which only contains an electric field parallel to the quantum well layer). Some optical configuration must be devised to allow the light to be incident obliquely onto the quantum well layer. Known configurations include Brewster angle incidence (Figure 2(a)) and incidence from the 45° polished backside (Figure 2(b)). In Example 1, the Brewster angle incidence configuration shown in Figure 2(a) was selected.

[0123] The area of ​​the detector was a rectangle of 100 μm × 400 μm or 200 μm × 800 μm, and the light was incident obliquely along the long side at a large angle of 65 degrees from the vertical. The exact Brewster angle is 73 degrees, but for the convenience of the measurement system, an incident angle of 65 degrees, which is close to that, was used. Although the area of ​​the element used varies depending on the measurement, all subsequent characteristics will be discussed using values ​​normalized by the detector area, so the area of ​​the element is irrelevant.

[0124] Although both sides of the contact layer are depicted as covered with metal electrodes in Figures 3 and 4, the front surface of the contact layer is not directly covered with a metal electrode. In Figure 2, most of the contact layer on the electron supply layer side is uncovered, serving as a light entrance window. A portion of the contact layer is covered with a first metal electrode 11b formed by vapor deposition of 3 nm thick Ti and 100 nm thick Au, and is connected to the outside of the device by wiring. The contact layer in the light entrance region 15 is maintained at an equipotential with the wiring through the highly conductive contact layer 11a. In Figure 2, the quantum well portion in the other region is entirely etched to leave the detector region in a mesa shape, and the second metal electrode 13b is formed on the etched surface near the detector region. Therefore, in this example, there is no portion where the second metal electrode is in direct contact with the contact layer on the electron collection layer (109, 209) side, as in Figures 3 and 4, but the n-type Al electrode fabricated in this portion ｘ Ga １－ｘ The As sacrificial layer acts as a conductive layer, and the electron collection layer (109, 209) is in actuality in electrical contact with the second metal electrode 13b.

[0125] <Characteristics> The sensitivity spectra of the photodetector 1002 of Example 1 and the comparative quantum cascade detector when no bias is applied are shown by the solid line and the dashed line in Figure 6, respectively. The photodetector temperature was 77 K in both cases. This sensitivity characteristic was determined by placing a photodetector sealed and cooled in a cryostat (LTS-101DL-IR-OPT-LV, manufactured by Nikki Kogei) at the sample position of a Fourier transform infrared spectrometer (FT / IR-6200, manufactured by JASCO Corporation), amplifying the signal obtained from the photodetector with a microcurrent amplifier (CA5350, manufactured by NF Corporation) and inputting it into the external input terminal of the Fourier transform infrared spectrometer, and comparing it with a sensitivity-calibrated broadband standard detector (PVI-2TE-10.6, manufactured by VIGO SYSTEM).

[0126] The sensitivity of the photodetector 1002 of Example 1 (173.5 μA / W) is inferior to that of the comparative quantum cascade detector (399.6 μA / W). This is thought to be because, compared to proven conventional quantum cascade detectors, the quantum well design of the photodetector 1002 of Example 1 has not yet been optimized, and electron transport is not as expected. However, what is ultimately important for an infrared detector is not the sensitivity itself, but the detectability (signal-to-noise ratio), which is determined as the ratio of signal intensity to noise, and therefore these results do not allow a comparison of the superiority or inferiority of the two photodetectors.

[0127] Although both detectors were designed to have peak sensitivity at the same wavelength, due to manufacturing errors and design imperfections, the peak sensitivity wavelengths could not be matched, resulting in a difference of 5% (Example 1: 6.42 μm, comparative quantum cascade detector: 6.75 μm). However, this is not a major problem when comparing the two detectors.

[0128] In this prototype, the quantum well design of the present disclosure was not sufficiently optimized, and therefore, in the characteristics without bias, the photodetector according to the present disclosure had inferior sensitivity compared to the comparative quantum cascade detector. However, the characteristics with bias applied provide an idea of ​​the performance when an optimal design is achieved. In Figure 6, the maximum sensitivity of each photodetector obtained by applying a bias is indicated by x and +. X and + represent the spectra at maximum sensitivity of the photodetector 1002 of Example 1 and the comparative quantum cascade detector, respectively.

[0129] The photodetector 1002 of Example 1 (1011 μA / W at 77 K and a bias voltage of +0.34 V) exhibited higher sensitivity than the comparative quantum cascade detector (644.9 μA / W at 77 K and a bias voltage of +0.08 V). This suggests that by adopting the quantum well structure of FIG. 3 and Table 1, the characteristic of electrons moving in one direction without backflow is improved, making it possible to realize an infrared detector with higher performance than conventional quantum cascade detectors.

[0130] 7(a) and (b) show the current density-voltage characteristics near the origin in the dark state of the photodetector 1002 of Example 1 and the comparative quantum cascade detector, respectively. The dark state was achieved by covering the front surface of the photodetector with a black-painted 29 K cold shield to prevent any radiation from entering the photodetector.

[0131] Reflecting the large asymmetry of the quantum well structure, the characteristics change drastically when the sign of the bias voltage changes. Johnson noise, which is important in photovoltaic quantum well infrared detectors, is represented by R ０ A. The higher the resistance, the lower the noise of the photodetector, and the closer the tangent line at the origin in the current density vs. voltage diagram is to a horizontal line, the better. This tangent line is shown by a dashed line in Figure 7.

[0132] It should be noted that the display range of the vertical axis in FIG. 7(b) is one digit wider than that in FIG. 7(a). The specific resistance value of the photodetector 1002 of Example 1 in FIG. 7(a) is 2.28×10 ５ Ω cm ２ , and the comparative quantum cascade detector in Fig. 7(b) is 9.34 × 10 ３ Ω cm ２ Example 1 exhibits a resistance 24.4 times higher than that of Example 1. This is a direct effect of the structure of the present disclosure.

[0133] The resistance of the comparative quantum cascade detector is also significantly higher than that of the conventional quantum cascade detector, and is 25 Ω cm, which was obtained by applying the high resistance technology at the time in Non-Patent Document 2. －２This is nearly 400 times higher than the conventional example. This is because the design was made with a strong focus on increasing resistance overall, including the use of thicker barrier layers in all structures than in the conventional example.

[0134] The most important specific detectivity for an infrared detector is D ＊ (Specific detectivity is an index that makes it easier to compare the detectivity of any photodetector) is the ratio of sensitivity R to noise current spectral density i ｎ / Δf １／２ The relationship is expressed as follows: Also, Johnson noise Therefore, the specific detectivity D of the infrared detector when Johnson noise is dominant is Ｊ ＊ is expressed as follows:

[0135] The specific detectivity spectra in the dark state of the photodetector 1002 of Example 1 and the comparative quantum cascade detector are shown by the solid line and the dashed line in Figure 8, respectively. In this region, Johnson noise is dominant as noise, and the photodetector 1002 of Example 1 has a specific detectivity in the dark state of the comparative quantum cascade detector (5.92 x 10 ８ cmHz １／２ / W) with a specific detectivity of 1.27 × 10 ９ cmHz １／２ By adopting the quantum well structure shown in FIG. 3 and Table 1, it was possible to realize an infrared detector with higher performance than conventional quantum cascade detectors.

[0136] Example 2 In Example 2, it is shown that even in a plasmon resonator type quantum well infrared detector using the same quantum well structure as in Example 1, the photodetector according to the present disclosure can achieve higher performance than a conventional quantum cascade detector.

[0137] <Structure> As mentioned above, quantum well photodetectors have the problem of being insensitive to perpendicularly incident light, because an optical electric field perpendicular to the quantum well layer is essential for intersubband transitions. Conventional quantum well photodetectors have required special arrangements and shapes to achieve oblique incidence on the quantum well layer, such as Brewster angle incidence or 45° backside incidence, as shown in Figure 2. However, recent advances in plasmonics research have led to the discovery of a configuration that fundamentally solves this problem. This is the plasmon resonator introduced in Example 2, which is described in detail in Non-Patent Document 1 and Patent Document 1.

[0138] The photodetector (plasmon resonance quantum well infrared detector) 1005 of Example 2 is an infrared detector incorporated in a plasmon resonator in which a first metal layer, a dielectric layer (including a semiconductor material), and a second metal layer are stacked, and its basic structure is shown in FIG. 9(b).

[0139] In the area that constitutes the plasmon resonator, the first metal layer is made up of metal layer 11c, the dielectric layer is made up of first contact layer 11a, detection layer 12, and second contact layer 13a, and the second metal layer is made up of metal patch 17. Here, metal layer 11c is formed on a semiconductor substrate 11d, and outside the photoelectric area irradiated with incident light 15, a first metal electrode 11b is arranged in contact with metal layer 11c, and a second metal electrode 13b is arranged in contact with second contact layer 13a. Note that semiconductor substrate 11d is made of GaAs, similar to semiconductor substrate 13d of photodetector 1004 shown in FIG. 9A.

[0140] Therefore, in the plasmon resonance type quantum well infrared detector 1005, the portion sandwiched between the continuous metal layer 13c and the square metal patch 17 with a side length of L acts as a plasmon resonator 18, which is arranged in a square lattice pattern with a period P in the irradiation area of ​​the incident light 15.

[0141] The materials for the first metal layer and the second metal layer are not particularly limited as long as they are conductors whose real part of the dielectric constant is a negative value, but those whose imaginary part of the dielectric constant is small are preferred, and gold (Au), silver (Ag), copper (Cu), aluminum (Al), platinum (Pt), tungsten (W), and molybdenum (Mo) are preferably used. Furthermore, the materials are not necessarily limited to metals, and transparent conductors such as ITO, AZO, and GZO can also be used.

[0142] The thickness of the dielectric layer sandwiched between the first metal layer and the second metal layer is preferably set to a thickness that causes plasmon resonance. As a guideline for a specific range, the average refractive index of the dielectric layer with respect to the wavelength λ of incident light is set to n ｅｆｆ When the ｅｆｆ ) or less. More specifically, it is preferable to set the thickness so that plasmon resonance is sufficiently induced for incident light of the target wavelength based on Maxwell's equations. Due to this constraint, it is inconvenient to combine the quantum well structure with a plasmon resonator using a repetition of several tens of periods, as has been done in many conventional photovoltaic quantum well infrared detectors. For this reason, in Example 2, the quantum well structure has only one period, as shown in Figures 3 and 4.

[0143] <Fabrication Method> In Example 2, the wafer grown by MBE in Example 1 was bonded to another Au substrate by wafer bonding. To this end, a 3-nm-thick Ti and 150-nm-thick Au substrate was first formed by sputtering on the contact layer on the electron supply layer side located on the top surface of the quantum well structure. A metal layer consisting of 10-nm-thick Ti and 500-nm-thick Au was also formed by sputtering on a second GaAs (100) substrate 13d, which would become the final semiconductor substrate. The substrates with the quantum wells formed on them were then inverted, the two Au surfaces were brought into contact, and the two substrates were heated at 250°C for 60 minutes in a nitrogen gas atmosphere while applying a load of 5 MPa. This resulted in the two GaAs substrates being integrated via the Au. This integration formed a metal layer 11c consisting of Au and Ti.

[0144] Next, the first GaAs substrate was mechanically polished, leaving about 100 μm of the surface, and the remaining GaAs portion and buffer layer of the first GaAs substrate were selectively removed at 38° C. using a solution of 1 g / ml citric acid solution and hydrogen peroxide solution in a 10:1 ratio to expose the AlGaAs sacrificial layer. Finally, the AlGaAs sacrificial layer was selectively removed using a hydrofluoric acid solution.

[0145] In this way, a multiple quantum well structure was fabricated that was equivalent to a structure in which the contact layer (second contact layer 13a) on the electron collection layer 109 side was exposed on the outermost surface, mounted on an Au substrate. Whether the outermost surface is the electron supply layer 101 or the electron collection layer 109 can be determined, as described above, depending on whether the multiple quantum well structure is fabricated with the first well layer 103 on the upper surface or the first well layer 103 on the lower surface. Whichever is selected, there is no particular difference in performance.

[0146] The metal patch 17 was fabricated by electron beam lithography and lift-off. An electron beam resist was applied, and a pattern was drawn within a 100 μm square area (which would become the photodetection region) using an electron beam. After that, the contact layer on the electron collection layer side was exposed by development so as to have the shape of the metal patch 17. A 3-nm-thick Ti film and a 100-nm-thick Au film were then deposited thereon as a metal patch layer.

[0147] The resist was dissolved (lift-off method) to form a metal patch 17 with a predetermined pattern. Separately, photolithography was used to form a second metal electrode 13b on the contact layer and to etch the quantum well portion other than the contact layer so as to leave the photodetector region in a mesa shape.

[0148] The exposed metal layer was used as it was as the first metal electrode 11b. Finally, the necessary areas including the photodetector region and electrode region on the second GaAs substrate were cut out by cleavage, and mounted on an 8-pin ceramic package with conductive epoxy. The electrodes were then bonded to the package pins with Au wires using Ag paste, completing the photodetector 1005.

[0149] The dimensions of the square metal patches 17 (Au patches) were L = 0.84 μm and P = 2.0 μm in the photodetector of this study, and L = 0.88 μm and P = 1.9 μm in the comparative quantum cascade detector. The dimensions of the Au patches 17 in both detectors were different because the sensitivity wavelengths of the original wafers themselves were different, as seen in Figure 8 of Example 1, and so the dimensions were adjusted to match the peak wavelengths of each. An example of a cross-sectional scanning electron microscope photograph of the completed array of square Au patches 17 is shown in Figure 9(b).

[0150] <Characteristics> The relationship between the peak sensitivity of the photodetector according to the present disclosure and the comparative quantum cascade detector in the unbiased state and the temperature is shown by the solid and dashed lines in Figure 10 . Compared to the sensitivity shown in Figure 6 , despite the same quantum well structure, the sensitivity at 77 K in Figure 10 is in the 100 mA / W range, more than 1,000 times higher. This is precisely the effect of the plasmon resonator. Also, as in Figure 6 , the sensitivity of the photodetector 1005 of Example 2 according to the present disclosure (200 mA / W at 77 K) is inferior to that of the comparative quantum cascade detector (570 mA / W at 77 K), even though both detectors use a plasmon resonator. This is because the degree of sensitivity enhancement by the plasmon resonator is almost the same. However, in the high-temperature region, the photodetector 1005 according to the present disclosure, which is inherently low in sensitivity, exhibits higher sensitivity. This indicates that the present disclosure suppresses the influence of noise, which becomes more pronounced as temperatures increase, and improves high-temperature characteristics.

[0151] As with Figure 6, the maximum sensitivity when a bias was applied was also compared. Figure 10 shows the maximum sensitivity at 77 K of each photodetector obtained by applying a bias, indicated by an x ​​and a +. The x and + symbols represent the maximum sensitivity of the photodetector 1005 of Example 2 and the comparative quantum cascade detector, respectively. The photodetector 1005 of Example 2 (949 mA / W at 77 K and a bias voltage of +0.32 V) exhibited higher sensitivity than the comparative quantum cascade detector (931 mA / W at 77 K and a bias voltage of +0.20 V). This suggests that the adoption of the quantum well structure of Figure 3 and Table 1 improves the characteristic of electrons moving in one direction without backflow, making it possible to realize an infrared detector with higher performance than conventional quantum cascade detectors.

[0152] Next, the relationship between the resistance and temperature in the dark state with no bias for the photodetector 1005 of Example 2 and the comparative quantum cascade detector is shown by the solid line and the dashed line, respectively, in Fig. 11. Although the resistance of the quantum well structure portion was slightly reduced by processing the plasmon resonator, it is clear that the photodetector according to the present disclosure consistently exhibited a higher resistance.

[0153] Finally, the relationship between the specific detectivity and temperature in the absence of bias for the photodetector 1005 of Example 2 and the comparative quantum cascade detector is shown by the solid line and the dashed line in FIG. 12, respectively.

[0154] In FIG. 12, there are two types of symbols, open and filled, which represent the relative detectability D when Johnson noise is dominant as explained in Example 1. Ｊ ＊ As mentioned above, this is the specific detectivity observed when the detector is darkened by the cold shield. This improves as the detector is cooled.

[0155] On the other hand, the black area indicates the specific detectivity D measured by removing the cold shield and exposing the photodetector to a 300K environment (viewing angle 162 degrees). ＢＧ ＊ This is saturated at low temperatures for all photodetectors. The difference in specific detectability depending on whether or not there is a cold shield is a feature that can only be seen in infrared detectors with a certain level of sensitivity. In the photodetector 1002 of Example 1, there was no difference in specific detectability regardless of whether or not there was a cold shield. This means that Johnson noise is dominant, and the specific detectability is Ｊ ＊ It could be described simply by

[0156] In the case of the plasmon resonator type photodetector 1005 of Example 2, the sensitivity is enhanced by three orders of magnitude or more, and as a result, the specific detectability reaches a new range called the background light limited (BLIP) condition.

[0157] In this region, simply exposing an infrared detector to a 300K environment and attempting to measure some incident light causes a large photocurrent to flow in the photodetector due to radiation from the 300K environment, and the shot noise from this photocurrent now becomes dominant over Johnson noise.

[0158] Both the light incident from a 300K environment and the shot noise proportional to the square root of that light are noises that are mainly environmentally-derived and determined by Planck radiation. Therefore, further cooling of the photodetector cannot reduce the noise any further, in principle, and therefore the specific detectivity cannot be improved.

[0159] Furthermore, once the sensitivity of the photodetector becomes sufficiently high, there is an absolute physical limit to the achievable specific detectivity, and it is not possible to reach any higher. The specific detectivity that can ultimately be achieved does not depend on the photodetector. The fact that this BLIP condition is satisfied indicates that the photodetector has high performance. In order to use the photodetector, it is necessary to expose it to a 300K environment, so D Ｊ ＊ The performance of a photodetector is not actually observed, and the performance of a photodetector is determined by the specific detectivity D when background light shot noise is dominant. ＢＧ ＊ It is expressed as:

[0160] Background light limiting ratio detectability D ＢＧ ＊ is mainly determined by the sensitivity, so the quantum cascade detector for comparison (5.60 × 10 at 77 K) １０ cmHz １／２ / W) is the photodetector 1005 of Example 2 (3.28×10 at 77K). １０ cmHz １／２ / W). Moreover, this value is already approaching the physical limit. However, if the quantum well design of the present disclosure is optimized to achieve more appropriate electron transport, the photodetector of the present disclosure will approach the same performance as the comparative quantum cascade detector. As with sensitivity, the temperature dependence is important. The photodetector of the present disclosure also has a higher specific detectivity in the high temperature range of 100 K or higher.

[0161] The BLIP temperature T is an index that expresses the temperature characteristics of an infrared detector in one word. ＢＬＩＰ This is the temperature at which the magnitude of Johnson noise and background light shot noise become equal, and at temperatures lower than this, the BLIP condition is reached, where performance is determined by background light shot noise. In Figure 12, this is the boundary where the white and black areas begin to separate. T ＢＬＩＰThe photodetector 1005 of Example 2 achieved a temperature of 98K, compared to 94K for the comparative quantum cascade detector, demonstrating that the present disclosure is superior. This difference will likely widen further with future improvements in quantum well design. Thus, even in the case of the plasmon resonator system, by adopting the quantum well structure of FIG. 3 and Table 1, it has been possible to realize an infrared detector with higher performance than conventional quantum cascade detectors.

[0162] In this prototype, the quantum well design of the present disclosure was not sufficiently optimized, so that the photodetector of the present disclosure was not able to surpass the comparative quantum cascade detector in terms of characteristics at no bias, other than sensitivity in the high temperature region and BLIP temperature. However, as a result of realizing a dramatic improvement in resistance, by applying an appropriate bias voltage, the photodetector of the present disclosure was able to achieve a background light limited ratio detectivity D that exceeded that of the comparative quantum cascade detector. ＢＧ ＊ The results are shown in Figure 12 by x and +. The x and + symbols respectively represent the maximum specific detectivity D that could be observed by optimizing the bias voltage when the photodetector 1005 of Example 2 and the comparative quantum cascade detector were exposed to an environment of 300K. ＢＧ ＊ is.

[0163] Since the important practical feature of photovoltaic detectors is that they can be used simply by connecting them to an ammeter, their characteristics without bias are mainly discussed. However, if high performance can be obtained in terms of the most important characteristic, the signal-to-noise ratio (specific detectivity), it is acceptable to apply a slight bias voltage. In fact, just as it is common to use photodiodes with a reverse bias voltage applied, in many practical applications, they are often used with a slight bias voltage applied.

[0164] As a result of realizing high resistance according to the present disclosure, even when a slight bias voltage is applied, the dark current is suppressed to a practically acceptable level, and a higher specific detectivity than when no bias voltage is applied is observed. １０ cmHz １／２ / W) is a quantum cascade detector for comparison (77K, bias voltage +0.04V, 6.43 × 10 １０cmHz １／２ / W). In the future, this difference will widen further as quantum well design improves. Thus, it was clearly demonstrated that even in the case of the plasmon resonator method, by adopting the quantum well structure shown in Figure 3 and Table 1, it is possible to realize an infrared detector with higher performance than conventional quantum cascade detectors.

[0165] Example 3 In Example 3, an example is reported in which the second well layer 105 having a function as a transition layer has a superlattice structure in which thin wells and barriers are repeated.

[0166] In Example 1, the second well layer 105 contains two Al wells, one with x=0.32 and the other with x=0.25, which act as a daughter well. ｘ Ga １－ｘ Although an As region is provided, the shape of the sub-well does not necessarily have to be limited to a simple single well, as long as an equivalent wave function is formed.

[0167] In a superlattice structure with repeated thin wells and barriers, the ground subbands of multiple wells couple to form a tightly packed band-like miniband. Because the miniband has a finite width, it is easier to realize a fast polar optical phonon scattering transition robustly by utilizing the miniband than by trying to precisely match the energy difference between specific states to the polar optical phonon energy of 36 meV.

[0168] The band structure of a photodetector designed using the miniband is shown in FIG. 13 as Example 3, and the details of its layers are shown in Table 3.

[0169] In this example, the Si doping concentration of the first well layer is set to 3×10 １８ cm －３ Instead, we verified whether the element resistance, which tends to decrease as a result, can be maintained high by applying the present disclosure.

[0170] In Example 3, the second well layer 305 was composed of two types of regions, x = 0.29 and x = 0.08. However, the portion corresponding to the sub-well was realized not by a single well but by repeating (twice) ultrathin wells with a thickness of 1.41 nm and x = 0.08.

[0171] Furthermore, after the third barrier layer 306, a 1.70 nm thick GaAs well layer and a 4.52 nm thick AlGaAs barrier layer with x=0.40 are repeated (three times), so that five wells are coupled and five wave functions are densely packed into a band to form a miniband. These five states function as recipients of relaxation due to polar optical phonon scattering, just like state 4.

[0172] In Example 3, the same structure is repeated up to the sixth barrier layer 308. On the other hand, the cascade structure provided in FIGS. 3 and 4 described in Example 1 is not present. However, as seen in FIG. 5, once the electrons are spatially transported to a state where the transition rate is orders of magnitude lower, the possibility of them returning to State 1 can be made extremely low. Therefore, even if no special measures are taken to speed up relaxation (such as high-speed relaxation due to polar optical phonon scattering transitions), the electrons will eventually relax to the electron recovery region through some kind of transition. This does not pose a problem unless the application requires a high-speed response on the picosecond (ps) level.

[0173] The no-bias sensitivity achieved with the structure of Example 3 was 101 μA / W at a peak wavelength of 6.32 μm. This is inferior to both the photodetector 1002 of Example 1 shown in FIG. 6 and the comparative quantum cascade detector as a comparative example. An extremely high Si doping concentration was adopted in the hope that high sensitivity would be easily obtained, but the band structure design was still insufficient, so the expected high sensitivity was not obtained. However, the no-bias resistance R ０ A is 833 Ω cm ２ This is a typical conventional quantum cascade detector (25 Ωcm) reported in Non-Patent Document 2. ２ ) was 33 times higher than that of

[0174] From the above, it has been confirmed that a photovoltaic quantum well infrared detector with high resistance and low noise can be realized by realizing the subband structure as illustrated in Figure 3 with a superlattice structure consisting of multiple thin barrier layers and well layers instead of bulk semiconductors.

[0175] (Summary) A photodetector according to a first aspect of the present disclosure is a photodetector in which a first conductor layer, a detection layer, and a second conductor layer are sequentially disposed from a first surface side, the detection layer having a quantum well in which a first barrier layer, a first well layer which is an excitation well layer, a second barrier layer, a second well layer which is a transition well layer, and a third barrier layer are sequentially disposed from a surface side in contact with the first conductor layer, the first well layer has at least a ground level and an excitation level, the energy levels of conduction band energies of the first barrier layer, the second barrier layer, and the third barrier layer are higher than the excitation level, the thicknesses of the first barrier layer, the second barrier layer, and the third barrier layer are thicknesses through which a tunnel current flows, and the second well layer has two or more levels A ｎ (n is an integer starting from 1), and one of the levels is level A １ is in contact with the first surface on which the second barrier layer is disposed, and is substantially equal to the excitation level, and another level A ｎ′ (n' is an integer of 2 or more) is in contact with the second surface on which the third barrier layer is disposed, and １ The energy levels are as follows:

[0176] A photodetector according to a second aspect of the present disclosure is the photodetector according to the first aspect, wherein photoelectrons excited from a ground state in the first well layer by received light are transported to the second well layer.

[0177] A photodetector according to a third aspect of the present disclosure is the photodetector according to the first and second aspects, wherein the level A １ The energy difference between the excited level and the excited level is 0 meV or more and 15 meV or less.

[0178] A photodetector according to a fourth aspect of the present disclosure is a photodetector in which a first conductor layer, a detection layer, and a second conductor layer are sequentially disposed from a first surface side, and the detection layer has a quantum well including, from a surface side in contact with the first conductor layer, a first barrier layer, a first well layer which is an excitation well layer, a second barrier layer, a second well layer which is a transition well layer, and a third barrier layer, in that order, and the first well layer has a ground subband and one or more higher-order subbands, and the first barrier layer, the second barrier layer, and the third barrier layer the energy level of the conduction band energy of the well layer is higher than the maximum energy level of the higher-order subbands, the thicknesses of the first barrier layer, the second barrier layer, and the third barrier layer are such that a tunneling current flows, and the second well layer includes two or more subbands composed of a plurality of regions of different conduction band energy, in which the energy level of the conduction band energy within the second well layer decreases stepwise and / or in a gradual manner from the second barrier layer to the third barrier layer.

[0179] A photodetector according to Aspect 5 of the present disclosure is the photodetector according to any one of Aspects 1 to 4, wherein a cascade well layer group including one or more pairs of cascade well layers and barrier layers is formed between the third barrier layer and the second conductor layer in the detection layer.

[0180] A photodetector according to a sixth aspect of the present disclosure is the photodetector according to the fifth aspect, wherein the energy level of the base subband in the cascade well layer monotonically decreases from the third barrier layer toward the second conductor layer.

[0181] A photodetector according to a seventh aspect of the present disclosure is the photodetector according to any one of the first to sixth aspects, wherein the first conductive layer and the second conductive layer comprise, as part of their structures, a mixed crystal of one or more selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge.

[0182] A photodetector according to an eighth aspect of the present disclosure is the photodetector according to the seventh aspect, wherein the mixed crystal is in contact with the detection layer.

[0183] A photodetector according to a ninth aspect of the present disclosure is a semiconductor photodetector that includes a barrier layer with high conduction band energy and a well layer with low conduction band energy within a detection layer, and generates a photocurrent by utilizing intersubband absorption occurring in the well layer, the photodetector including a first surface side and an electron supply layer, a first barrier layer and a first well layer to an m-th barrier layer and an m-th well layer (m is a natural number of 2 or more) (m is a natural number of 2 or more) well layer, an (m+1)-th barrier layer, and an electron collection layer, and the first well layer includes a ground subband with the lowest energy level and a well layer with an energy level lower than the ground subband. The mth well layer transports the photoelectrons toward the electron collection layer. The second well layer is composed of a plurality of regions with different conduction band energies, and has two or more subbands whose energy levels are monotonically higher on the first surface side and lower on the second surface side in a stepwise and / or gradient manner from the first surface side on which the second barrier layer is disposed to the second surface side on which the third barrier layer is disposed.

[0184] A photodetector according to aspect 10 of the present disclosure is the photodetector according to aspect 9, wherein the detection layer has a configuration in which the first barrier layer and the (m+1)th barrier layer have the same structure, and each of the barrier layers and well layers from the first barrier layer and first well layer to the mth (m is a natural number of 2 or more) barrier layer and mth well layer, and the (m+1)th barrier layer are arranged in sequence as one set, and the (m+1)th barrier layer and the first barrier layer of the next set are overlapped to form two or more sets arranged in series.

[0185] A photodetector according to aspect 11 of the present disclosure is the photodetector according to aspect 9 or 10, wherein the center of gravity of the square of the wave function of the ground subband of the second well layer is located 15 nm or more and 100 nm or less away from the center of gravity of the wave function of the ground subband of the first well layer.

[0186] A photodetector according to aspect 12 of the present disclosure is the photodetector according to aspect 9 or 10, in which any of the higher-order subbands of the first well layer is coupled to a higher-order subband of the second well layer, forming a bonding band and an antibonding band originating from each subband.

[0187] A photodetector according to a thirteenth aspect of the present disclosure is the photodetector according to the twelfth aspect, wherein the energy difference between the bonding band and the antibonding band is 0.1 meV or more and 15 meV or less.

[0188] A photodetector according to aspect 14 of the present disclosure is the photodetector according to aspect 12 or 13, wherein the contribution of polar optical phonon scattering is greatest in the relaxation transition from the higher-order subband of the second well layer to the ground subband of the second well layer.

[0189] A photodetector according to a fifteenth aspect of the present disclosure is the photodetector according to the fourteenth aspect, wherein the energy difference between the higher subband of the second well layer and the ground subband of the second well layer is greater than 15 meV and less than or equal to 60 meV.

[0190] A photodetector according to aspect 16 of the present disclosure is the photodetector according to any one of aspects 9 to 15, wherein when m is a natural number equal to or greater than 3, the energy levels of the base subbands of the second well layer and subsequent well layers decrease in order.

[0191] A photodetector according to Aspect 17 of the present disclosure is the photodetector according to any one of Aspects 1 to 16, wherein the first well layer is formed with n-type impurities.

[0192] A photodetector according to an eighteenth aspect of the present disclosure is the photodetector according to any one of aspects 1 to 17, wherein the detection layer includes one or more selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge.

[0193] A photodetector according to Aspect 19 of the present disclosure is the photodetector according to any one of Aspects 1 to 18, wherein one or more of the first barrier layer, the first well layer, the second barrier layer, the second well layer, and the third well layer have a superlattice structure composed of a plurality of barrier layers and well layers.

[0194] A photodetector according to a twentieth aspect of the present disclosure is a semiconductor photodetector that includes a barrier layer with a low valence band energy and a well layer with a high valence band energy in a detection layer, and generates a photocurrent by utilizing intersubband absorption occurring in the well layer, the photodetector including a hole supply layer, a first barrier layer and each of the barrier layers and well layers from the first barrier layer and the first well layer to the m-th barrier layer and the m-th well layer (m is a natural number of 2 or more), an (m+1)-th barrier layer, and a hole collection layer that are sequentially arranged from the first surface side, and the first well layer has a ground subband with the highest energy level and a well layer with an energy level lower than the ground subband. the mth well layer transports holes toward the hole collection layer; the second well layer is composed of a plurality of regions with different valence band energies, and has two or more subbands with monotonically lower energy levels on the first surface side and higher energy levels on the second surface side in a stepwise and / or gradient manner from the first surface side on which the second barrier layer is disposed to the second surface side on which the third barrier layer is disposed.

[0195] A photodetector according to aspect 21 of the present disclosure is a photodetector according to any one of aspects 1 to 20, in which a metal mesh is formed in contact with the first surface side or above the first surface side, and in which island-shaped metal patches or openings that induce plasmon resonance are periodically arranged.

[0196] A photodetector according to Aspect 22 of the present disclosure is the photodetector according to Aspect 21, wherein the metal patches are arranged in an array and / or a matrix.

[0197] Photodetector according to Aspect 23 of the present disclosure: In the photodetector according to Aspect 22, the arrangement pitch of the metal patches is not less than 0.5 μm and not more than 200 μm.

[0198] The present disclosure provides quantum well photodetectors that offer high detectivity with excellent signal-to-noise ratios and have a wide range of applications, including sensors and cameras for consumer, industrial, and military applications.

[0199] Therefore, we believe that this disclosure will have a significant impact on society and have a significant influence on industry.

[0200] 1: First conductor layer 1a: First contact layer, first conductive layer 1b: First metal electrode 2: Detection layer 3a: Second contact layer, second conductive layer 3b: Second metal electrode 11: First conductor layer 11a: First contact layer 11b: First metal electrode 11c: Metal layer 11d: Semiconductor substrate 12: Detection layer 13: Second conductor layer 13a: Second contact layer 13b: Second metal electrode 13c: Substrate, metal layer (conductor film) 13d: Semiconductor substrate 15: Light beam 17: Metal patch, Au patch 18: Plasmon resonator 21a: First contact layer 21b: First metal electrode 21c: Substrate 22: Detection layer 23a: Second contact layer 23b: Second metal electrode 25: Light beam 101: Electron supply layer, contact layer 102: First barrier layer 103: First well layer 104: Second barrier layer 105: Second well layer 106: Third barrier layer 107: Third well layer 108: Seventh barrier layer 109: Electron collection layer, contact layer 201: Electron supply layer, contact layer 202: First barrier layer 203: First well layer 204: Second barrier layer 205: Second well layer 206: Third barrier layer 207: Third well layer 208: Eighth barrier layer 209: Electron collection layer, contact layer 301: Electron supply layer, contact layer 302: First barrier layer 303: First well layer 304: Second barrier layer 305: Second well layer 306: Third barrier layer 307: Third well layer 308: Sixth barrier layer 309: Electron collection layer, contact layer 1001: Photodetector 1002: Photodetector 1003: Photodetector 1004: Photodetector, plasmon resonance type quantum well infrared detector 1005: Photodetector, plasmon resonance type quantum well infrared detector

Claims

1. A photodetector having a first conductor layer, a detection layer, and a second conductor layer sequentially arranged from a first surface side, wherein the detection layer has a quantum well having a first barrier layer, a first well layer which is an excitation well layer, a second barrier layer, a second well layer which is a transition well layer, and a third barrier layer sequentially arranged from a surface side in contact with the first conductor layer, wherein the first well layer has at least a ground level and an excitation level, the energy levels of the conduction band energies of the first barrier layer, the second barrier layer, and the third barrier layer are higher than the excitation level, the thicknesses of the first barrier layer, the second barrier layer, and the third barrier layer are such that a tunnel current flows, and the second well layer has two or more levels A ｎ (n is an integer starting from 1), and one of the levels is level A １ is in contact with the first surface on which the second barrier layer is disposed, and is substantially equal to the excitation level, and another level A ｎ′ (n' is an integer of 2 or more) is in contact with the second surface on which the third barrier layer is disposed, and １ The photodetector is at the following energy levels:

2. The photodetector according to claim 1, wherein photoelectrons excited from the ground state in said first well layer by received light are transported to said second well layer.

3. Level A １ 3. The photodetector according to claim 1, wherein an energy difference between the excited level and the excited level is 0 meV or more and 15 meV or less.

4. A photodetector having a first conductor layer, a detection layer, and a second conductor layer sequentially arranged from a first surface side, wherein the detection layer has a quantum well having, sequentially from a surface side in contact with the first conductor layer, a first barrier layer, a first well layer which is an excitation well layer, a second barrier layer, a second well layer which is a transition well layer, and a third barrier layer, wherein the first well layer has a ground subband and one or more higher subbands, wherein the energy levels of the conduction band energies of the first barrier layer, the second barrier layer, and the third barrier layer are higher than the maximum energy level of the higher subband, and wherein the thicknesses of the first barrier layer, the second barrier layer, and the third barrier layer are such that a tunnel current flows through them, the second well layer includes two or more subbands formed of a plurality of regions of different conduction band energy, in which the energy level of the conduction band energy in the second well layer decreases stepwise and / or in a gradual manner from the second barrier layer to the third barrier layer.

5. A photodetector according to any one of claims 1 to 4, wherein a cascade well layer group comprising one or more pairs of cascade well layers and barrier layers is formed between the third barrier layer and the second conductive layer in the detection layer.

6. The photodetector according to claim 5, wherein the energy level of the base subband in the cascade well layer monotonically decreases from the third barrier layer toward the second conductor layer.

7. The photodetector according to any one of claims 1 to 6, wherein the first conductive layer and the second conductive layer comprise, as part of their structure, a mixed crystal of one or more selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge.

8. The photodetector of claim 7, wherein the mixed crystal is in contact with the detection layer.

9. A photodetector having a barrier layer with high conduction band energy and a well layer with low conduction band energy in a detection layer, and generating a photocurrent by utilizing intersubband absorption occurring in the well layer, comprising an electron supply layer, a first barrier layer and each of the barrier layers and well layers from the first barrier layer and the first well layer to the m-th barrier layer and the m-th well layer (m is a natural number of 2 or more), an (m+1)-th barrier layer, and an electron collection layer, which are arranged in order from the first surface side, the first well layer having a ground subband with the lowest energy level and one or more higher-order subbands with an energy level higher than that of the ground subband, and absorbing light to excite photoelectrons from the ground subband to an excited subband consisting of one or more of the higher-order subbands, the second well layer to the m-th well layer transporting the photoelectrons toward the electron collection layer, the second well layer is composed of a plurality of regions with different conduction band energies, and has two or more subbands whose energy levels are monotonically higher on the first surface side and lower on the second surface side in a stepwise and / or gradient manner from the first surface side on which the second barrier layer is disposed to the second surface side on which the third barrier layer is disposed.

10. The photodetector according to claim 9, wherein the detection layer has a configuration in which the first barrier layer and the (m+1)th barrier layer have the same structure, and each of the barrier layers and well layers from the first barrier layer and first well layer to the mth (m is a natural number of 2 or more) barrier layer and mth well layer, and the (m+1)th barrier layer are arranged in sequence to form one set, and two or more sets are arranged in series by overlapping the (m+1)th barrier layer with a first barrier layer of a different set.

11. The photodetector according to claim 9 or 10, wherein the squared center of gravity of the wave function of the fundamental subband of the second well layer is located 15 nm to 100 nm away from the squared center of gravity of the wave function of the fundamental subband of the first well layer.

12. A photodetector according to claim 9 or 10, wherein any of the higher-order subbands of the first well layer is coupled to a higher-order subband of the second well layer, forming a bonding band and an antibonding band originating from each subband.

13. The photodetector according to claim 12, wherein the energy difference between the bonding band and the antibonding band is 0.1 meV or more and 15 meV or less.

14. The photodetector according to claim 12 or 13, wherein the contribution of polar optical phonon scattering is greatest in the relaxation transition from the higher subband of the second well layer to the ground subband of the second well layer.

15. The photodetector of claim 14, wherein the energy difference between the higher subband of the second well layer and the ground subband of the second well layer is greater than 15 meV and less than or equal to 60 meV.

16. A photodetector according to any one of claims 9 to 15, wherein, when m is a natural number equal to or greater than 3, the energy levels of the ground subbands of the second and subsequent well layers decrease in order.

17. A photodetector according to any one of claims 1 to 16, wherein the first well layer is formed with n-type impurities.

18. The photodetector of any one of claims 1 to 17, wherein the detection layer comprises one or more selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge.

19. The photodetector of any one of claims 1 to 18, wherein one or more of the first barrier layer, the first well layer, the second barrier layer, the second well layer, and the third well layer have a superlattice structure composed of a plurality of barrier layers and well layers.

20. A photodetector having a detection layer including barrier layers with low valence band energy and well layers with high valence band energy, and generating a photocurrent by utilizing intersubband absorption occurring in the well layers, comprising a hole supply layer, a first barrier layer and each of the first to m-th (m is a natural number of 2 or more) barrier layer and m-th (m is a natural number of 2 or more) well layer, an (m+1)-th barrier layer, and a hole collection layer, arranged in this order from the first surface side, wherein the first well layer has a ground subband with the highest energy level and one or more higher-order subbands with an energy level lower than that of the ground subband, and absorbs light to excite holes from the ground subband to an excitation subband consisting of one or more of the higher-order subbands, and the second well layer, third well layer, ..., the m-th well layer transports holes toward the hole collection layer, the second well layer is composed of a plurality of regions with different valence band energies, and has two or more subbands with monotonically lower energy levels on the first surface side and higher energy levels on the second surface side in a stepwise and / or gradient manner from the first surface side on which the second barrier layer is disposed to the second surface side on which the third barrier layer is disposed.

21. A photodetector according to any one of claims 1 to 20, wherein a metal mesh in which island-shaped metal patches or openings that induce plasmon resonance are periodically arranged is formed in contact with the first surface side or above the first surface side.

22. The photodetector according to claim 21, wherein the metal patches are arranged in an array and / or a matrix.

23. The photodetector according to claim 22, wherein the arrangement pitch of the metal patches is 0.5 μm or more and 200 μm or less.

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