Light-receiving element, optical circuit, and method for manufacturing light-receiving element

Abstract

A light-receiving element according to an embodiment of the present invention includes: a nitride semiconductor layer; and a chalcogenide layer that is disposed above the nitride semiconductor layer, includes a transition metal, and is epitaxially grown on the nitride semiconductor layer.

Description

Photodetector, optical circuit, and method for manufacturing a photodetector

[0001] The embodiments relate to a photodetector, an optical circuit, and a method for manufacturing a photodetector.

[0002] Conventionally, heterojunction type photodetectors have been disclosed. For example, Patent Document 1 discloses an n-type gallium nitride layer and a p-type Mo ｘ Re １－ｘ S ２ A light-receiving element comprising a layer is disclosed.

[0003] Chinese Patent Application Publication No. 115863488 Specification

[0004] However, there is a need for photodetectors to operate at higher speeds. The embodiment aims to provide a photodetector, an optical circuit, and a method for manufacturing a photodetector that can operate at high speed.

[0005] The light-receiving element according to this embodiment comprises a nitride semiconductor layer and a chalcogenide layer disposed on the nitride semiconductor layer, containing a transition metal, and epitaxially grown relative to the nitride semiconductor layer.

[0006] According to the embodiment, it is possible to provide a photodetector, an optical circuit, and a method for manufacturing a photodetector that can operate at high speed.

[0007] This is a plan view of the photodetector according to the embodiment. This is a cross-sectional view taken along line II-II in Figure 1. This is a model diagram showing the atomic arrangement of the second layer and the chalcogenide layer disposed thereon in the photodetector according to the embodiment. This is a schematic diagram showing the band diagram of the photodetector according to the embodiment. This is a conceptual diagram, a plan view, showing the configuration of the optical circuit according to the embodiment. This is a cross-sectional view taken along line VI-VI in Figure 5. This is a cross-sectional view showing the configuration of the optical circuit according to a modified example of the embodiment. This is a diagram illustrating the nitride semiconductor layer formation process in the manufacturing method of the photodetector according to the embodiment. This is a diagram illustrating the chalcogenide layer formation process in the manufacturing method of the photodetector according to the embodiment. This is a diagram illustrating the electron barrier layer formation process in the manufacturing method of the photodetector according to the embodiment. This is a diagram illustrating the mesa formation process in the manufacturing method of the photodetector according to the embodiment. This is a diagram illustrating the substrate removal process in the manufacturing method of the photodetector according to the embodiment. This is a diagram illustrating the conductive layer formation process in the manufacturing method of the photodetector according to the embodiment. This is a diagram illustrating the protective film formation process in the manufacturing method of the photodetector according to the embodiment. This is a diagram illustrating the electrode formation process in the manufacturing method of the photodetector according to the embodiment. This is the GaN of the second layer and the MoS of the chalcogenide layer disposed thereon in the photodetector according to the example. ２ This is a scanning transmission electron microscope image of a cross-section. This is a diagram showing the J-V characteristics of the photodetector according to the embodiment. This is a diagram showing the spectral sensitivity. This is a diagram showing the pulse response. This is a diagram showing the high-frequency response.

[0008] Embodiments of the present invention will be described below with reference to the drawings.

[0009] <Light-receiving element> Figure 1 is a plan view of the light-receiving element 100 according to the embodiment. Figure 2 is a cross-sectional view taken along line II-II of Figure 1. As shown in Figures 1 and 2, the light-receiving element 100 according to the embodiment comprises a first substrate 10, a nitride semiconductor layer 20, a chalcogenide layer 30, an electron barrier layer 40, a conductive layer 50, a protective film 60, an anode 70, and a cathode 71. The components will be described below.

[0010] (First Substrate 10) The first substrate 10 has light transmittance for light with a wavelength equal to or longer than the bandgap energy of the chalcogenide layer 30 in order to receive light such as visible light from the side of the first substrate 10 in the light receiving element 100. The first substrate 10 is preferably, for example, a sapphire substrate, an AlN substrate, or a GaN substrate.

[0011] (Nitride Semiconductor Layer 20) The nitride semiconductor layer 20 is disposed on the first substrate 10. Further, the nitride semiconductor layer 20 has light transmittance for light with a wavelength equal to or longer than the bandgap energy of the chalcogenide layer 30. The nitride semiconductor layer 20 includes a first layer 21 and a second layer 22 disposed on the first layer 21. The second layer 22 is disposed between the first layer 2I and the chalcogenide layer 30 and contacts the chalcogenide layer 30.

[0012] It is preferable that the first layer 21 and the second layer 22 contain GaN. Further, it is preferable that the first layer 21 and the second layer 22 contain Al ｘ In ｙ Ga １－（ｘ＋ｙ） N (0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ x + y ≤ 1). Thereby, the lattice constants of the nitride semiconductor layer 20 and the chalcogenide layer 30 can be made closer to each other to reduce lattice mismatch. Further, it is preferable that the first layer 21 and the second layer 22 contain Al ｘ Ga １－ｘ N (0 ≤ x ≤ 1). Thereby, it is easy to make the bandgap energy of the first layer 21 and the second layer 22 larger than the bandgap energy of the chalcogenide layer 30, so that the chalcogenide layer 30 can efficiently absorb light.

[0013] It is preferable that the first layer 21 has an n-type conductivity type. The impurity concentration of the first layer 21 is preferably higher than the impurity concentration of the second layer 22. Thereby, ohmic contact of the electrode can be achieved and the resistance can be reduced. For example, the impurity concentration of the first layer 21 is 1 × 10 １８ / cm ３ or more and 1 × 10 ２１ / cm ３ or less, preferably 5 × 10 １８ / cm ３ or more and 1 × 10 ２０ / cm ３The following applies. The conductivity type of the second layer 22 is preferably n-type or i-type, and more preferably i-type. In this specification, i-type is a layer that is intentionally not doped with impurities, and is also called an undoped layer. By making the conductivity type of the second layer 22 i-type, impurity scattering is reduced, and the electron transport speed is improved. The impurity concentration of the second layer 22 is 1 × 10⁻⁶ １３ / cm ３ The above 1 x 10 １７ / cm ３ The following, preferably 1 × 10 １４ / cm ３ The above 5 x 10 １６ / cm ３ The following is more preferable: 1 × 10 １４ / cm ３ The above 1 x 10 １６ / cm ３ The following applies. Even if a semiconductor layer is formed without intentionally doping impurities so that the conductivity type of the second layer 22 is type i, unintended impurities may still be introduced. Therefore, the impurity concentration of the second layer 22 is 1 × 10⁻⁶. １６ / cm ３ Even if the following conditions are met, this specification will refer to them as type i or undoped. Examples of impurities used include Si (silicon), Ge (germanium), Mg (magnesium), or C (carbon). The impurity concentration can be measured by secondary ion mass spectrometry (SIMS). If the region where the second layer 22 is provided is narrower than the range that can be analyzed by SIMS, the composition and impurity concentration may be analyzed using a three-dimensional atom probe.

[0014] In this specification, when the layer structure of a photodetector is examined using SIMS or a three-dimensional atom probe, if the impurity concentration of the second layer 22 is the lowest compared to other semiconductor layers, the carrier density of the second layer 22 is determined from the carrier density obtained by C-V measurement. Since the chalcogenide layer 30 is a layered compound and the number of stacked layers is relatively small, there is no practical problem in considering the carrier density obtained from C-V measurement as the carrier density of the second layer 22.

[0015] The thickness of the second layer 22 may be, for example, 0.5 μm or more and 5 μm or less, preferably 1 μm or more and 3 μm or less. This reduces the capacitance and the time constant, thereby increasing the response speed.

[0016] (Chalcogenide layer 30) The chalcogenide layer 30 is placed on the nitride semiconductor layer 20 and is in contact with the second layer 22. The conductivity type of the chalcogenide layer 30 may be p-type. The chalcogenide layer 30 is epitaxially grown relative to the nitride semiconductor layer 20. The chalcogenide layer 30 is grown aligned with the crystal plane of the nitride semiconductor layer 20. The chalcogenide layer 30 is lattice-matched with the nitride semiconductor layer 20. More specifically, the chalcogenide layer 30 is lattice-matched with the second layer 22. This can be seen by observing the atomic arrangement of a cross-section containing the nitride semiconductor layer 20 and the chalcogenide layer 30 using a scanning transmission electron microscope (STEM).

[0017] The chalcogenide layer 30 contains a transition metal. Furthermore, the chalcogenide layer 30 includes at least one selected from the group consisting of Mo (molybdenum), W (tungsten), Re (rhenium), Ti (titanium), Nb (niobium), Ta (tantalum), V (vanadium), Sb (antimony), and In (indium), and at least one selected from the group consisting of S (sulfur), Se (selenium), and Te (tellurium). Also, the chalcogenide layer 30 is a transition metal dichalcogenide. The chalcogenide layer 30 contains MoS ２ WS ２ MoSe ２ ReS ２ Or ReSe ２ It is preferable that it consists of the following: MoS ２ WS ２ MoSe ２ ReS ２ Or ReSe ２ These molecules have a two-dimensional planar structure and form a layered material. Furthermore, the chalcogenide layer 30 has semiconducting properties.

[0018] MoS ２Because it has a smaller band gap energy than GaN and possesses electron affinity equivalent to GaN, the chalcogenide layer 30 is MoS ２ It is preferable that it consists of

[0019] Furthermore, the chalcogenide layer 30 is ReS ２ Preferably consists of ReS ２ It has a direct transition band structure with a band gap at the Γ point, which is consistent with nitride semiconductors. Therefore, electron transport can occur without a change in wavenumber, which is advantageous for improving photodetection efficiency and speeding up response times.

[0020] (Electron barrier layer 40) The electron barrier layer 40 is placed on top of the chalcogenide layer 30. The electron barrier layer 40 is a layer that prevents electrons generated by the light absorption of the chalcogenide layer 30 from flowing to the anode side. The electron barrier layer 40 has a band gap energy greater than that of the chalcogenide layer 30. The electron barrier layer 40 is made of, for example, Al ｚ Ga １－ｚ It is composed of N (0 < z < 1) or hexagonal boron nitride. The electron barrier layer 40 is positioned in contact with the chalcogenide layer 30 in order to efficiently block electrons. Preferably, the electron barrier layer 40 is in contact with the entire upper surface of the chalcogenide layer 30. In this way, the contact area between the electron barrier layer 40 and the chalcogenide layer 30 is increased, so the electron barrier layer 40 can efficiently block electrons generated by the light absorption of the chalcogenide layer 30. Therefore, the amount of electrons sent from the chalcogenide layer 30 to the cathode 71 can be increased. Note that the electron barrier layer 40 may be omitted.

[0021] The electron barrier layer 40 preferably contains hexagonal boron nitride. In this way, the electron barrier layer 40 can efficiently allow holes from the nitride semiconductor layer 20 to pass through while simultaneously blocking electrons.

[0022] (Conductive layer 50) The conductive layer 50 is placed on top of the electron barrier layer 40 and between the chalcogenide layer 30 and the anode 70. If the electron barrier layer 40 is not provided, it is preferable that the conductive layer 50 is in contact with the entire upper surface of the chalcogenide layer 30. In this way, the contact area between the conductive layer 50 and the chalcogenide layer 30 is increased, and the exchange of holes between the conductive layer 50 and the chalcogenide layer 30 is increased. The conductive layer 50 is made of a metallic material such as copper, chromium, silver, or gold.

[0023] In a plan view, it is preferable that the dimensions of the second layer 22 and the chalcogenide layer 30 are the same. Furthermore, in a plan view, it is preferable that the dimensions of the second layer 22, the chalcogenide layer 30, the electron barrier layer 40, and the conductive layer 50 are the same.

[0024] (Protective film 60) The protective film 60 covers a part of the upper surface of the first substrate 10, the side surface of the nitride semiconductor layer 20, the side surface of the chalcogenide layer 30, the side surface of the electron barrier layer 40, the side surface of the conductive layer 50, and a part of the upper surface. The protective film 60 is made of SiO ２ It is composed of insulating materials such as the above.

[0025] (Anode 70) The anode 70 is electrically connected to the chalcogenide layer 30. In the example shown in Figure 2, the anode 70 is connected to the upper surface of the conductive layer 50, extends toward the first substrate 10 on the outside of the protective film 60, and further extends along the first substrate 10. That is, the anode 70 is in direct contact with the conductive layer 50 and forms an interface. If the conductive layer 50 is not provided, it is connected to the upper surface of the electron barrier layer 40, and if neither the conductive layer 50 nor the electron barrier layer 40 is provided, it is connected to the upper surface of the chalcogenide layer 30. That is, the anode 70 may be in direct contact with the electron barrier layer 40 or the chalcogenide layer 30 and form an interface. The anode 70 is made of a metallic material such as copper, chromium, silver, gold, titanium, aluminum, or platinum.

[0026] (Cathode 71) The cathode 71 is electrically connected to the nitride semiconductor layer 20. Since the first layer 21 can have a higher impurity concentration than the second layer 22, preferably the cathode 71 is electrically connected to the first layer 21 of the nitride semiconductor layer 20. In the example shown in Figure 2, the cathode 71 is connected to a part of the upper surface and the side surface of the first layer 21 of the nitride semiconductor layer 20, and further extends along the first substrate 10. The cathode 71 is made of a metallic material such as copper, chromium, silver, gold, titanium, aluminum, or platinum. Alternatively, the cathode 71 may be designated as the first cathode, the second cathode may be placed on the upper surface of the first layer 21 exposed from the second layer 22, and the first cathode may be placed on top of the second cathode. In this case, the first cathode may be used as a pad electrode.

[0027] (Epitaxial growth) The smaller the lattice mismatch between one material and the other, the more easily the other material can be epitaxially grown relative to the other. For example, GaN and MoS ２ ya WS ２ The lattice mismatch is less than 1%, and GaN and MoS ２ The lattice constants of the materials are almost identical. Therefore, MoS ２ ya WS ２ It is possible to grow epitaxially with respect to GaN. In the above example, the degree of lattice mismatch is calculated as (lattice constant of the layer formed on the underlying layer - lattice constant of the underlying layer) ÷ lattice constant of the underlying layer × 100 (%). ReS ２ It also has a relatively small degree of lattice mismatch and is capable of epitaxial growth.

[0028] Figure 3 is a model diagram showing the atomic arrangement of the second layer 22 and the chalcogenide layer 30 disposed on top of it in the photodetector 100 according to the embodiment. As shown in Figure 3, in the photodetector 100 according to the embodiment, the second layer 22 and the chalcogenide layer 30 are lattice-matched. Figure 3 is an example in which the chalcogenide layer 30 is an epitaxially grown layer relative to the second layer 22. The chalcogenide layer 30 is a layered compound, and the number of chalcogenide layers 30 may be 1 to 10, with 1 to 7 and 1 to 5 being preferred. This allows for sufficient absorbance. Furthermore, since the number of chalcogenide layers 30 is relatively small, the response speed is increased.

[0029] (Single-travel carrier photodiode) Figure 4 is a schematic diagram showing the band diagram of the photodetector according to the embodiment. The photodetector 100 according to the embodiment can be used as a single-travel carrier photodiode.

[0030] A unitraveling-carrier photodiode (UTC-PD) receives light in the chalcogenide layer 30, and the resulting carriers travel through it. The band structure of the nitride semiconductor layer 20 acts as a carrier transport layer for electrons and as a barrier layer for holes. The chalcogenide layer 30 can receive light with a peak wavelength in the visible light band and a wavelength larger than the wavelength corresponding to the band gap energy of the nitride semiconductor layer 20, and electrons are generated by photoelectric conversion. Since only electrons travel through the nitride semiconductor layer 20, the operating speed of the unitraveling-carrier photodiode, which does not depend on the carrier velocity of holes, is faster than the operating speed of a photodiode with a general PIN structure. Therefore, the unitraveling-carrier photodiode can operate at, for example, 1 GHz to 10 THz, preferably 0.1 THz to 10 THz, and more preferably 0.1 THz to 1 THz. The light-receiving element according to the embodiment operates as a UTC-PD even without providing an electron barrier layer 40. Preferably, by providing an electron barrier layer 40 in the light-receiving element according to the embodiment, electron drift can be restricted to the nitride semiconductor layer 20 side, thereby stabilizing the operation of the UTC-PD.

[0031] In general photodiodes, there is a tendency to increase the light-receiving area in order to capture more light. On the other hand, the light-receiving element 100 according to the embodiment increases the operating speed by reducing the light-receiving area. For example, in the light-receiving element 100 according to the embodiment, the maximum length of the chalcogenide layer 30 that receives light in a plan view is preferably 1 μm or more and 60 μm or less, 1 μm or more and 30 μm or less, or 1 μm or more and 20 μm or less. In a plan view, the shape of the chalcogenide layer 30 may be, for example, circular, elliptical, or polygonal. The maximum length of the chalcogenide layer 30 in a plan view is the diameter if it is circular, the major axis if it is elliptical, and the length of the diagonal if it is polygonal. The area of ​​the chalcogenide layer 30 in a plan view is, for example, π μm ２ More than 3600πμm ２ Below, πμm ２ More than 900πμm ２ The following, or πμm ２ 400πμm or more ２ The following is preferable:

[0032] The photodetector 100 according to this embodiment receives light with a wavelength longer than the wavelength corresponding to the bandgap energy of the nitride semiconductor layer 20. Therefore, instead of receiving light in the ultraviolet region, it operates by receiving light with a peak wavelength in the range of 400 nm to 800 nm, preferably 420 nm to 650 nm. The photodetector 100 according to this embodiment receives light incident from the first substrate 10 side with the chalcogenide layer 30. Therefore, it is preferable that the photodetector 100 be flip-chip mounted.

[0033] As described above, the photodetector 100 according to the embodiment comprises a nitride semiconductor layer 20 and a chalcogenide layer 30 disposed on the nitride semiconductor layer 20, containing a transition metal and epitaxially grown relative to the nitride semiconductor layer 20. This configuration enables high-speed operation of the photodetector 100.

[0034] The light-receiving element 100 according to this embodiment is used in communication devices in free space such as the atmosphere, water, or outer space, inspection devices that see through objects, or optical integrated circuits, etc.

[0035] The photodetector 100 according to the embodiment may include the following configuration. That is, the photodetector 100 comprises a nitride semiconductor layer 20 and a chalcogenide layer 30 disposed on the nitride semiconductor layer 20, containing a transition metal and epitaxially grown relative to the nitride semiconductor layer 20, wherein the chalcogenide layer 30 may be a photodetector capable of receiving optical signals with a modulation frequency of 0.1 THz to 10 THz.

[0036] By placing a chalcogenide layer 30 on top of a nitride semiconductor layer 20, a photodetector 100 can be obtained that can track modulation frequencies in the range of terahertz (0.1 THz to 10 THz).

[0037] Let's explain using GaN as an example for the nitride semiconductor layer 20. The saturation drift rate for electrons in GaN is greater than that of InP, which is the material for the carrier transport layer in a typical UTC-PD. Therefore, by using GaN as the carrier transport layer, it is possible to fabricate a UTC-PD that can follow frequencies higher than those of a typical UTC-PD.

[0038] Furthermore, by using the nitride semiconductor layer 20, the high-frequency response characteristics are less likely to deteriorate even when receiving high-power (or high-luminosity) light, and the maximum output power of the photodetector 100 can be increased at high frequencies.

[0039] The electric field strength applied when the saturation drift velocity for electrons in GaN is taken is greater than the electric field strength applied when the saturation drift velocity for electrons in InP is taken. When fabricating a photodetector capable of high-speed response, the photodetector is driven near the saturation drift velocity so that the element can be driven at a large drift velocity.

[0040] When the photodetector 100 receives light, photocarriers are generated. As the photocarriers move through the carrier transport layer, the net electric field acting on the carrier transport layer decreases due to the voltage drop caused by the space charge effect, which is a factor in reducing the electron drift velocity.

[0041] When subjected to a voltage drop due to the space charge effect, InP exhibits a more significant decrease in saturation drift velocity than GaN. That is, as a result of the voltage drop for a given amount of photoreceived current, the drift velocity decreases significantly, and the signal in the high-frequency range is attenuated. On the other hand, GaN is less affected by the voltage drop, and the decrease in drift velocity is smaller compared to InP. Therefore, when GaN is used as the carrier transport layer, photocarriers can be transported at a high drift velocity, allowing for a higher maximum output power of the photodetector at high frequencies.

[0042] <Optical Circuit> Figure 5 is a conceptual diagram showing the configuration of the optical circuit 200 according to the embodiment, and is a plan view. Figure 6 is a cross-sectional view along the line VI-VI in Figure 5. As shown in Figures 5 and 6, the optical circuit 200 according to the embodiment comprises a laser element 80, the above-mentioned photodetector 100, an optical waveguide 83, electrical wiring 87, and a second substrate 86.

[0043] A known method for generating terahertz light (0.1 THz to 10 THz) involves using two monochromatic light sources with different wavelengths (frequencies), an optical interferometer, and a photodiode. A single-mode laser is used as the light source, and by interfering it with a beam splitter, a light intensity beat corresponding to the frequency difference between the two is created. This is called a photomixer. This is then photoelectrically converted by a high-speed photodetector 100, and a high-frequency signal such as a terahertz signal is extracted by an antenna.

[0044] The laser element 80 provided in the optical circuit 200 according to the embodiment has a first laser element 81 and a second laser element 82. The first laser element 81 emits first laser light with a peak frequency of a first frequency. The second laser element 82 emits second laser light with a peak frequency of a second frequency different from the first frequency. The photodetector 100 is as described above. The first laser element 81 and the second laser element 82 can be semiconductor laser diodes, etc. The semiconductor laser diode is preferably a DFB (Distributed Feedback) laser or a DBR (Distributed Bragg Reflector) laser. This makes it possible to realize a longitudinal mode single. The oscillation frequency of a DBR laser can be varied by modulating the refractive index of the diffraction grating. Therefore, it is preferable that at least one of the laser elements 80 is a DBR laser, and it is preferable that both of the laser elements 80 are DBR lasers.

[0045] Furthermore, the materials of the first laser element 81 and the second laser element 82 are composed of the same material or a combination of different materials. The materials of the first laser element 81 and the second laser element 82 may be III-V semiconductors.

[0046] The optical waveguide 83 includes an interferometer 84. The interferometer 84 optically couples with the first laser element 81, the second laser element 82, and the photodetector 100, interfering the first laser beam and the second laser beam and guiding them to the photodetector 100. The interferometer 84 is an optical waveguide type beam splitter. The interferometer 84 is, for example, a directional coupler, a Y-shaped branch waveguide, or an X-shaped branch waveguide. The material of the interferometer 84 is an epitaxial crystalline thin film (GaN, Si, AlN, InP, etc.) or an amorphous thin film (SiN, SiON, Ta ２ O ５ NbO ２ ) are some examples.

[0047] The electrical wiring 87 is a metal wire through which signals, information, and electricity (power supply and electrical signals) are transmitted. The electrical wiring 87 is connected to the antenna 85.

[0048] The second substrate 86 is a support for the first laser element 81, the second laser element 82, the photodetector 100, the optical waveguide 83, and the electrical wiring 87. The first laser element 81, the second laser element 82, the photodetector 100, the optical waveguide 83, and the electrical wiring 87 are arranged on the same second substrate 86.

[0049] In the optical circuit 200 according to this embodiment, the output end faces 81a and 82a of the first laser element 81 and the second laser element 82 are coupled to an optical waveguide 83. When the first laser light (peak frequency f1) emitted by the first laser element 81 and the second laser light (peak frequency f2) emitted by the second laser element 82 interfere in an interferometer consisting of the optical waveguide 83, a beat signal is generated. The frequency of the generated optical beat signal is f1 - f2. By adjusting the frequency difference, a terahertz signal can be generated. The generated optical beat signal is then incident directly from the optical waveguide 83 to the photodetector 100 on the second substrate 86, and the photoelectrically converted electrical signal (terahertz wave) is extracted from the electrical wiring 87. The antenna 85 that radiates terahertz light is formed on the same second substrate 86 or placed externally and connected by the electrical wiring 87.

[0050] Figure 7 is a cross-sectional view showing the configuration of an optical circuit 201 according to a modified embodiment. The cross-section in Figure 7 corresponds to the direction along the VI-VI line in Figure 5. In the optical circuit 201 according to the modified embodiment, as shown in Figure 7, the laser element 80 and the interferometer 84 are arranged on the same second substrate 86. The light-receiving element 100a and electrical wiring 87a may be arranged on a different layer from the laser element 80 and the interferometer 84 on the second substrate 86, or the light-receiving element 100b and electrical wiring 87b may be arranged on the side of the second substrate 86 opposite to the laser element 80 and the interferometer 84, with the second substrate 86 in between. The antenna 85 may be arranged on the same layer as the electrical wiring 87a and connected to the electrical wiring 87a, or it may be placed externally and connected by the electrical wiring 87a. Similarly, the antenna 85 may be arranged on the side of the second substrate 86 opposite to the laser element 80 and the interferometer 84, with the second substrate 86 in between, and connected to the electrical wiring 87b, or it may be placed externally and connected by the electrical wiring 87b. Furthermore, the second substrate 86 may be a light-transmitting substrate.

[0051] According to the optical circuits 200 and 201 of this embodiment, a terahertz light source can be obtained with a simple configuration. The terahertz light source can be used for a variety of applications, such as high-speed communication, non-destructive testing, non-radiation testing, and imaging.

[0052] Furthermore, because the photodetector 100 contains a nitride semiconductor, it has a higher voltage resistance compared to silicon-based or GaAs-based photodetectors. Therefore, it can receive light of greater power and extract photocurrent, thus increasing the signal intensity (photocurrent). ２ The power of the terahertz waves, defined by × (the electrical resistance of the antenna), can be further increased. In addition, since the photodetector 100 can be miniaturized, a small, high-power terahertz light source can be obtained.

[0053] Furthermore, the optical circuit may use two or more semiconductor laser elements with different frequencies, and may also use three or more. The optical circuit can obtain a waveform by interfering any number of laser beams with different frequencies, and then photoelectrically converting the resulting waveform using the photodetector 100. Two or more interferometers may be used. Each semiconductor laser element may be coupled to an optical waveguide and may be coupled to any interferometer.

[0054] <Manufacturing Method for Photodetector> Figure 8 is a diagram illustrating the nitride semiconductor layer formation process in the manufacturing method of the photodetector 100 according to the embodiment. Figure 9 is a diagram illustrating the chalcogenide layer formation process in the manufacturing method of the photodetector 100 according to the embodiment. Figure 10 is a diagram illustrating the electron barrier layer formation process in the manufacturing method of the photodetector 100 according to the embodiment. Figure 11 is a diagram illustrating the mesa formation process in the manufacturing method of the photodetector 100 according to the embodiment. Figure 12 is a diagram illustrating the substrate removal process in the manufacturing method of the photodetector 100 according to the embodiment. Figure 13 is a diagram illustrating the conductive layer formation process in the manufacturing method of the photodetector 100 according to the embodiment. Figure 14 is a diagram illustrating the protective film formation process in the manufacturing method of the photodetector 100 according to the embodiment. Figure 15 is a diagram illustrating the electrode formation process in the manufacturing method of the photodetector 100 according to the embodiment.

[0055] As shown in Figures 8, 9, 10, 11, 12, 13, 14, and 15, the manufacturing method of the light-receiving element 100 according to this embodiment comprises a nitride semiconductor layer formation step, a chalcogenide layer formation step, an electron barrier layer formation step, a mesa formation step, a substrate removal step, a conductive layer formation step, a protective film formation step, and an electrode formation step.

[0056] In the nitride semiconductor layer formation process, as shown in Figure 8, a nitride semiconductor layer 20 is formed on the first substrate 10. For nitride semiconductor layer formation, for example, chemical vapor deposition (CVD) is used, and metal-organic chemical vapor deposition (MOCVD) is preferably used. The nitride semiconductor layer 20 is made of, for example, Al ｘ In ｙ Ga １－（ｘ＋ｙ） N is such that (0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ x + y ≤ 1), and the composition of the nitride semiconductor layer 20 is as described above.

[0057] In the nitride semiconductor layer formation process, for example, a first layer 21 having an n-type conductivity and a second layer 22 having either an n-type conductivity or an i-type conductivity are formed. When the second layer 22 is i-type, no impurities are intentionally added. However, the inclusion of unavoidable impurities is permitted.

[0058] In the chalcogenide layer formation process, as shown in Figure 9, a chalcogenide layer 30 is epitaxially grown on the nitride semiconductor layer 20. The chalcogenide layer formation process is carried out by the MOCVD method to epitaxially grow the chalcogenide layer 30. The materials constituting the chalcogenide layer 30 are as described above. MoS is used as the material for the chalcogenide layer 30. ２ For example, the raw materials for sulfur are, for instance, H ２ S gas can be used. Various materials can be used as raw materials for transition metals, such as metal carbonyls and chlorides. For example, Mo(CO) can be used as a Mo source. ６ It is available. Mo(CO) ６ It is a solid, and can be used as a Mo source by passing the gas generated by its vapor pressure through Ar. Other options include MoCl５ You can use this as the Mo source.

[0059] In the electron barrier layer formation step, as shown in Figure 10, an electron barrier layer 40 is formed on the chalcogenide layer 30. The electron barrier layer 40 may be formed, for example, by sputtering, CVD, or MOCVD. In the manufacturing method of the light-receiving element 100 according to this embodiment, the electron barrier layer 40 does not need to be formed.

[0060] In the mesa formation process, as shown in Figure 11, a mesa is formed that includes a portion of the first layer 21 of the nitride semiconductor layer 20 formed in the above process, the second layer 22 of the nitride semiconductor layer 20, the chalcogenide layer 30, and the electron barrier layer 40. The mesa is formed by selectively removing the electron barrier layer 40, the chalcogenide layer 30, the second layer 22, and a portion of the first layer 21 in the thickness direction in sequence. For example, the mesa is formed by etching using the RIE (Reactive Ion Etching) method. This RIE etching is stopped midway through the thickness direction of the first layer 21.

[0061] In the substrate preparation process, a portion of the first layer 21 is etched in the thickness direction to expose the surface of the first substrate 10, as shown in Figure 12.

[0062] In the conductive layer formation process, as shown in Figure 13, the conductive layer 50 is formed on the electron barrier layer 40. Note that in the manufacturing method of the light-receiving element 100 according to this embodiment, the conductive layer 50 does not need to be formed. Furthermore, in the conductive layer formation process, for example, vapor deposition (CVD) or metal-organic vapor deposition (MOCVD) can be used. At this time, all surfaces except the top surface of the electron barrier layer 40 are protected by a mask.

[0063] In the protective film formation process, as shown in Figure 14, a protective film 60 is formed on the sides of the first layer 21, the second layer 22, the chalcogenide layer 30, the electron barrier layer 40, and a portion of the sides and top surface of the conductive layer 50. The materials constituting the protective film 60 are as described above.

[0064] In the electrode formation process, as shown in Figure 15, an anode 70 and a cathode 71 are formed. The anode 70 is connected to the upper part of the conductive layer 50 and is formed extending from the upper surface of the protective film 60 along the side surface of the protective film 60 toward the first substrate 10. If the conductive layer 50 is not formed, the anode 70 is connected to the upper part of the electron barrier layer 40. If the electron barrier layer 40 and the conductive layer 50 are not formed, the anode 70 is connected to the upper part of the chalcogenide layer 30. The cathode 71 is connected to a part of the upper surface and side surface of the first layer 21 of the nitride semiconductor layer 20 and is formed extending along the first substrate 10. The materials constituting the anode 70 and cathode 71 are as described above.

[0065] According to the manufacturing method of the light-receiving element 100 as embodied in this embodiment, it is possible to provide a light-receiving element 100 that can operate at high speed.

[0066] The light-receiving element 100 according to the embodiment will be described in detail below with reference to examples. However, the light-receiving element 100 according to the embodiment is not limited to this embodiment.

[0067] (Example 1) A sapphire substrate was prepared as the first substrate. On the sapphire substrate, a nitride semiconductor layer was formed by MOCVD, consisting of a first layer of GaN with an n-type conductivity and a second layer of GaN with an n-type conductivity. At this time, silicon was used as the impurity in the nitride semiconductor layer. The impurity concentration of the first layer was set to 1 × 10⁻⁶. １９ / cm ３ The impurity concentration of the second layer is 1 × 10⁻⁶. １７ / cm ３ That's what I decided.

[0068] Next, on the second layer, as a chalcogenide layer, MoS ２ The material was formed by the MOCVD method, and a chalcogenide layer formation process was carried out.

[0069] Subsequently, mesa formation, substrate removal, protective film formation, and electrode formation processes were carried out to fabricate the photodetector according to the example. In addition, three photodetectors with a chalcogenide layer diameter of 60 μm were fabricated.

[0070] (Example 2) The light-receiving element of Example 2 was modified such that the second layer was an undoped GaN layer. Also, in the light-receiving element of Example 2, a conductive layer was disposed on the chalcogenide layer. The anode was connected to the upper surface of the conductive layer. The cathode was provided with a second cathode connected to a part of the upper surface of the first layer and a first cathode as a pad electrode connected to the second cathode.

[0071] After manufacturing the light-receiving element of Example 1, the cross-section of the MoS of the GaN in the second layer and the chalcogenide layer ２ was observed with an electron microscope. FIG. 16 is a STEM photograph of the cross-section of the MoS of the chalcogenide layer disposed on the GaN in the light-receiving element according to Example 1. The black line shown at the lower left of FIG. 16 represents a line having a length of 2 nm. Also, the broken line shown at the upper right of FIG. 16 is a line added to clearly show the positional relationship between the atomic arrangement of the GaN in the second layer and the atomic arrangement of the MoS of the chalcogenide layer. As shown in FIG. 16, the lattice intervals of GaN and MoS ２ were matched, and GaN and MoS ２ were lattice-matched. Therefore, MoS ２ was epitaxially grown on GaN. Also, as shown in FIG. 16, MoS ２ was in a layered form. ２ was epitaxially grown on GaN. Also, as shown in FIG. 16, MoS ２ was in a layered form. <9000244> Regarding the surface of the MoS of the chalcogenide layer ２ , analysis by Raman spectroscopy was performed. From the Raman spectrum, spectra derived from the E ２ mode and the A ２ｇ mode, which are vibration modes of MoS １ｇ , were obtained. The wavenumber difference between the peaks of these spectra was 23 cm －１ . The relationship between the number of layers of MoS ２ and the wavenumber difference is known from the literature C. Lee, et al., ACS Nano 2010, 4, 5, 2695-2700. Comparing this with the above, it was confirmed that the number of layers of MoS ２ in Example 1 was about 3 or 4 layers.

[0073] Figure 17 shows the J-V characteristics of the photodetector according to Example 1. It shows the current density measured when a voltage was applied to the anode and cathode. As shown in Figure 17, the photodetector exhibited diode characteristics.

[0074] Figure 18 shows the spectral sensitivity. The spectral sensitivity was measured by spectrally analyzing light from a lamp and igniting the light from the nitride semiconductor layer side. As shown in Figure 18, structures derived from GaN were detected in the ultraviolet region between 350 and 400 nm. The reason for the decrease in sensitivity above 450 nm in Figure 18 is due to the sensitivity limit. As shown in Figure 18, MoS was detected in the visible region between 400 and 450 nm. ２ The origin of the structure was detected. Therefore, as shown in Figure 18, the structure comprising a nitride semiconductor layer and a chalcogenide layer was shown to be functioning as a photodetector. In other words, the photodetector according to the embodiment can be operated by receiving light in the above wavelength range of the visible region, rather than receiving light in the ultraviolet region.

[0075] Figure 19 shows the pulse response. Figure 19 shows the response waveform of the photodetector according to Example 1 as measured by an oscilloscope when irradiated with a pulsed laser with a wavelength of 420 nm. Figure 20 shows the high-frequency response. Figure 20 shows the response speed measured at 405 nm in the visible region shown in Figure 18. Figure 20 shows the frequency response when the laser was modulated, measured with a vector network analyzer. The dashed line in Figure 20 shows the high-frequency response result when the photodetector is not receiving light and represents noise. The solid line in Figure 20 shows the high-frequency response result when the photodetector of Example 1 receives light. The dashed line in Figure 20 shows the high-frequency response result when the photodetector of Example 2 receives light.

[0076] As shown in Figure 19, a response waveform is observed in the visible region between 400 and 450 nm, indicating that the structure comprising the nitride semiconductor layer and the chalcogenide layer is functioning as a photodetector. Furthermore, as shown in Figure 20, it was found that the photodetector according to the embodiment is capable of high-speed operation. ９Measurements were also taken with a GaAs photodetector that has a large gain even in the high frequency band above GHz, and it behaved similarly. Therefore, 10 ９ The gain behavior in the frequency band above GHz is due to the reduced tracking ability of the laser light source to the modulation, and does not represent the gain of the photodetector itself in Examples 1 and 2. The gain of the photodetector in Example 2 was sufficiently large compared to the noise. Therefore, this indicates that the photodetector in Example 2 is 10 ９ The results suggested that the device possessed gain even in the high-frequency range above GHz. This was thought to be because the second layer was made of i-GaN, which broadened the depletion layer, reduced the capacitance C, and consequently lowered the time constant, enabling a response at high frequencies.

[0077] Furthermore, comparing the high-frequency response results of Example 1 and Example 2, the gain of the photodetector in Example 2 was higher. The gain of the photodetector in Example 2 was approximately 20 dB higher than the gain of the photodetector in Example 1. This was thought to be partly due to the fact that the conductive layer was the same size as the chalcogenide layer, allowing a reverse bias to be applied to the entire chalcogenide layer, and enabling efficient extraction of electrons generated by photoelectric conversion by the chalcogenide layer.

[0078] The embodiments and their modifications described above are examples that embody the present invention, and the present invention is not limited to these embodiments and modifications. For example, the present invention also includes the addition, deletion, or modification of some components or processes in the embodiments and modifications described above. Furthermore, the embodiments and modifications described above can be implemented in combination with each other.

[0079] The present invention includes the following embodiments.

[0080] (Note 1) A photodetector comprising: a nitride semiconductor layer; and a chalcogenide layer disposed on the nitride semiconductor layer, containing a transition metal, and epitaxially grown relative to the nitride semiconductor layer.

[0081] (Note 2) The photodetector according to Note 1, further comprising an electron barrier layer disposed on the chalcogenide layer.

[0082] (Supplementary Note 3) The light-receiving element according to Supplementary Note 2, wherein the electron barrier layer contains hexagonal boron nitride.

[0083] (Supplementary Note 4) The nitride semiconductor layer has a first layer with an n-type conductivity type and a second layer with an n-type or i-type conductivity type, and the second layer is disposed between the first layer and the chalcogenide layer and contacts the chalcogenide layer. The light-receiving element according to any one of Supplementary Notes 1 to 3.

[0084] (Supplementary Note 5) The light-receiving element according to Supplementary Note 4, wherein the conductivity type of the second layer is i-type. (Supplementary Note 6) The light-receiving element according to Supplementary Note 4 or 5, wherein the thickness of the second layer is 0.5 μm or more and 5 μm or less.

[0085] (Supplementary Note 7) Further comprising an anode electrically connected to the chalcogenide layer and a cathode electrically connected to the nitride semiconductor layer. The light-receiving element according to any one of Supplementary Notes 1 to 6.

[0086] (Supplementary Note 8) Further comprising a conductive layer disposed between the chalcogenide layer and the anode and contacting the entire upper surface of the chalcogenide layer. The light-receiving element according to Supplementary Note 7.

[0087] (Supplementary Note 9) The first layer and the second layer contain GaN, and the impurity concentration of the first layer is higher than the impurity concentration of the second layer. The light-receiving element according to any one of Supplementary Notes 4 to 8.

[0088] (Supplementary Note 10) The chalcogenide layer is a layered compound, and the number of layers of the chalcogenide layer is 1 or more and 10 or less. The light-receiving element according to any one of Supplementary Notes 1 to 9.

[0089] (Supplementary Note 11) The chalcogenide layer contains at least one selected from the group consisting of Mo, W, Re, Ti, Nb, Ta, V, Sb, and In and at least one selected from the group consisting of S, Se, and Te. The light-receiving element according to any one of Supplementary Notes 1 to 10.

[0090] (Supplementary Note 12) The chalcogenide layer is MoS ２ or ReS ２A light-receiving element as described in Appendix 11, including the above.

[0091] (Note 13) The nitride semiconductor layer is Al ｘ In ｙ Ga １－（ｘ＋ｙ） A light-receiving element consisting of N (0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ x + y ≤ 1), as described in any one of the appendices 1 to 12.

[0092] (Note 14) The photodetector according to any one of Notes 1 to 13, wherein the chalcogenide layer is capable of receiving light in a wavelength band whose peak wavelength is in the visible light band and is larger than the wavelength corresponding to the bandgap energy of the nitride semiconductor layer.

[0093] (Note 15) The photodetector according to any one of Notes 1 to 14, wherein the chalcogenide layer is a photodetector capable of receiving optical signals with a modulation frequency of 0.1 THz or more and 10 THz or less.

[0094] (Note 16) An optical circuit comprising: a first laser element that emits first laser light having a peak frequency of a first frequency; a second laser element that emits second laser light having a peak frequency of a second frequency different from the first frequency; a photodetector described in any one of Notes 1 to 15; and an optical waveguide including an interferometer that optically couples with the first laser element, the second laser element, and the photodetector to interfere the first laser light and the second laser light and guide them to the photodetector.

[0095] (Note 17) A method for manufacturing a photodetector, comprising: a nitride semiconductor layer formation step of forming a nitride semiconductor layer on a substrate; and a chalcogenide layer formation step of epitaxially growing a chalcogenide layer containing a transition metal on the nitride semiconductor layer by organometallic vapor phase growth.

[0096] 10 First substrate, 20 Nitride semiconductor layer, 21 First layer, 22 Second layer, 30 Chalcogenide layer, 40 Electron barrier layer, 50 Conductive layer, 60 Protective film, 70 Anode, 71 Cathode, 80 Laser element, 81 First laser element, 81a Output end face (of the first laser element), 82 Second laser element, 82a Output end face (of the second laser element), 83 Optical waveguide, 84 Interferometer, 85 Antenna, 87, 87a, 87b Electrical wiring, 86 Second substrate, 100, 100a, 100b Photodetector, 200, 201 Optical circuit

Claims

1. A photodetector comprising: a nitride semiconductor layer; and a chalcogenide layer disposed on the nitride semiconductor layer, containing a transition metal, and epitaxially grown relative to the nitride semiconductor layer.

2. The photodetector according to claim 1, further comprising an electron barrier layer disposed on the chalcogenide layer.

3. The photodetector according to claim 2, wherein the electron barrier layer contains hexagonal boron nitride.

4. The photodetector according to any one of claims 1 to 3, wherein the nitride semiconductor layer comprises a first layer having an n-type conductivity and a second layer having either an n-type conductivity or an i-type conductivity, and the second layer is disposed between the first layer and the chalcogenide layer and in contact with the chalcogenide layer.

5. The photodetector according to claim 4, wherein the conductivity type of the second layer is i-type.

6. The photodetector according to any one of claims 1 to 5, further comprising an anode electrically connected to the chalcogenide layer and a cathode electrically connected to the nitride semiconductor layer.

7. The photodetector according to claim 6, further comprising a conductive layer disposed between the chalcogenide layer and the anode, and in contact with the entire upper surface of the chalcogenide layer.

8. The photodetector according to any one of claims 4 to 7, wherein the first layer and the second layer contain GaN, and the impurity concentration of the first layer is higher than the impurity concentration of the second layer.

9. The light-receiving element according to any one of claims 1 to 8, wherein the chalcogenide layer is a layered compound, and the number of chalcogenide layers is 1 or more and 10 or less.

10. The photodetector according to any one of claims 1 to 9, wherein the chalcogenide layer comprises at least one selected from the group consisting of Mo, W, Re, Ti, Nb, Ta, V, Sb, and In, and at least one selected from the group consisting of S, Se, and Te.

11. The chalcogenide layer is MoS ２ Or ReS ２ The light-receiving element according to claim 10, including the following.

12. The photodetector according to claim 4 or 5, wherein the thickness of the second layer is 0.5 μm or more and 5 μm or less.

13. The nitride semiconductor layer is Al ｘ In ｙ Ga １－（ｘ＋ｙ） A light-receiving element according to any one of claims 1 to 12, comprising N (0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ x + y ≤ 1).

14. The photodetector according to any one of claims 1 to 13, wherein the chalcogenide layer is capable of receiving light in a wavelength band whose peak wavelength is in the visible light band and is larger than the wavelength corresponding to the bandgap energy of the nitride semiconductor layer.

15. The photodetector according to any one of claims 1 to 14, wherein the chalcogenide layer is a photodetector capable of receiving an optical signal having a modulation frequency of 0.1 THz or more and 10 THz or less.

16. An optical circuit comprising: a first laser element that emits first laser light having a peak frequency of a first frequency; a second laser element that emits second laser light having a peak frequency different from the first frequency; a photodetector according to any one of claims 1 to 3; and an optical waveguide including an interferometer that optically couples with the first laser element, the second laser element, and the photodetector to interfere the first laser light and the second laser light and guide them to the photodetector.

17. A method for manufacturing a photodetector, comprising: a nitride semiconductor layer formation step of forming a nitride semiconductor layer on a substrate; and a chalcogenide layer formation step of epitaxially growing a chalcogenide layer containing a transition metal on the nitride semiconductor layer by organometallic vapor phase growth.

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