Optical Devices

Abstract

This optical device 1 is provided with: a silicon substrate 10 wherein the crystal plane of the main surface has the (111) plane orientation and the main surface is provided with a relief structure 11; and a conductor 20 which forms a Schottky junction with the silicon substrate 10. The conductor 20 is directly joined to the (111) plane of at least one of a projected part 11a or a recessed part 11b of the relief structure 11.

Description

[Technical field]

[0001] The present disclosure relates to optical devices. [Background technology]

[0002] Conventionally, photoelectric conversion technologies using a Schottky junction, which is a junction between a metal and a semiconductor, have been known. Among them, photoelectric conversion technologies utilizing surface plasmon resonance in a Schottky junction have attracted attention.

[0003] As one of the optical devices using such photoelectric conversion technology, a Schottky type photodetector that utilizes surface plasmon resonance in a Schottky junction has been proposed (for example, Patent Document 1).

[0004] In this type of photodetector, when light is absorbed, the free electrons in the metal temporarily become high-energy due to surface plasmon resonance. Electrons temporarily in a high-energy state due to surface plasmon resonance are called hot electrons (hot carriers), and the hot electrons generated in the metal by the absorption of light are charge-separated by crossing the Schottky barrier between the metal and the semiconductor, and flow as a photocurrent to the semiconductor side. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2019-47016 A Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present disclosure is to provide an optical device that can reduce dark current with a simple structure while achieving increased light absorption rate and controllability of absorption wavelength due to surface plasmon resonance. [Means for solving the problem]

[0007] One aspect of an optical device according to the present disclosure comprises a silicon substrate having a principal surface with a crystal plane orientation of (111) and a concave-convex structure provided on the principal surface, and a conductor forming a Schottky junction with the silicon substrate, wherein the conductor is directly bonded to the (111) surface of at least one of the convex portions and concave portions in the concave-convex structure. Effect of the Invention

[0008] According to the present disclosure, it is possible to obtain an optical device that can reduce dark current with a simple structure while realizing an increase in light absorptance and controllability of the absorption wavelength due to surface plasmon resonance. [Brief description of the drawings]

[0009] [Figure 1] 1 is a perspective view illustrating a schematic configuration of a portion of an optical device according to an embodiment. [Figure 2A] 1 is a plan view illustrating a configuration of a portion of an optical device according to an embodiment. [Figure 2B] 2B is a cross-sectional view of the optical device according to the embodiment taken along line IIB-IIB in FIG. 2A. [Diagram 3] 3A and 3B are diagrams illustrating the surface state of the concavo-convex structure in the optical device according to the embodiment. [Figure 4] 1 is a diagram showing the surface state of protrusions of a concave-convex structure of a silicon substrate when a silicon oxide film is formed on the surface of the concave-convex structure and then the silicon oxide film is removed with hydrofluoric acid; [Diagram 5] FIG. 1 is a diagram showing the surface state of the convex portions of the concave-convex structure when a silicon oxide film is formed on the surface of the concave-convex structure of a silicon substrate, the silicon oxide film is removed with hydrofluoric acid, and then a cleaning process is performed with an aqueous ammonium fluoride solution. [Figure 6] FIG. 11 is a cross-sectional view illustrating a schematic configuration of an optical device according to another embodiment of the present invention. [Figure 7] 1 is a cross-sectional view showing a configuration of a portion of an optical device according to a first modified example. [Figure 8]11 is a plan view showing a configuration of a portion of an optical device according to Modification 2. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] (How one aspect of the present disclosure was achieved) First, before specifically describing the embodiments of the present disclosure, the process by which one aspect of the present disclosure was achieved will be described.

[0011] Semiconductor photodetectors are known as photodetectors that use semiconductors. Currently popular semiconductor photodetectors use photoelectric conversion based on optical absorption due to band-to-band transition, and therefore cannot detect light having an energy lower than the band gap energy of the semiconductor.

[0012] Therefore, there is a demand for a photodetector that enables photoelectric conversion in a wider wavelength range than before. For example, if light in the near-infrared region (hereinafter, referred to as "near-infrared light") can be detected, highly sensitive imaging can be achieved regardless of day or night, so there is a demand for a photodetector that can detect near-infrared light with high sensitivity. Furthermore, since near-infrared light is highly safe for the eyes, it is expected that a photodetector that detects near-infrared light will be used as a sensor for automatic driving of automobiles.

[0013] Regarding light in the visible region (hereinafter referred to as "visible light"), photodetectors using silicon (Si) are relatively inexpensive and widely used. However, near-infrared light has lower energy than visible light, so it cannot be detected unless a semiconductor with a smaller band gap energy is used.

[0014] On the other hand, a photodetector has been proposed that utilizes surface plasmon resonance in a Schottky junction, which is a junction between a metal and a semiconductor. This type of photodetector can detect light in a wide wavelength range, including long-wavelength light in the near-infrared region, which has been difficult to use until now.

[0015] In a photodetector using a Schottky junction, there is a demand for improving the photodetection sensitivity due to surface plasmon resonance, and therefore techniques for increasing the light absorption rate due to surface plasmon resonance have been investigated.

[0016] Furthermore, in a photodetector using a Schottky junction, there is a demand for controllability of the light absorption wavelength, i.e., a photodetector having a structure that allows adjustment of the wavelength absorption region is desired.

[0017] In order to achieve both an increase in light absorption rate due to surface plasmon resonance and controllability of the absorption wavelength, a photodetector element having an uneven structure in which convex portions or concave portions are formed periodically and repeatedly has been proposed. For example, as disclosed in the above Patent Document 1, a photodetector element has been proposed that includes a semiconductor layer having a periodic uneven structure in which convex portions and concave portions are formed periodically and repeatedly, and a metal film provided on one side of the semiconductor layer in correspondence with the periodic uneven structure.

[0018] On the other hand, in a photodetector using a Schottky junction, it is desired to reduce the dark current caused by the defect level at the Schottky interface. Therefore, in the photodetector disclosed in Patent Document 1, a cap structure using an insulating layer is adopted to reduce the dark current. Specifically, in order to reduce the dark current, in the photodetector disclosed in Patent Document 1, an insulating layer is formed between the upper surface of the convex portion in the periodic uneven structure of the semiconductor layer and the metal film, and a portion where the metal film and the convex portion of the semiconductor layer do not form a Schottky junction is provided.

[0019] However, the photodetector disclosed in Patent Document 1 has a complicated device structure and requires a step of forming an insulating layer and a step of etching both the insulating layer and the semiconductor layer, which complicates the manufacturing process. In addition, with the structure of the photodetector disclosed in Patent Document 1, it is difficult to make all Schottky interfaces defect-free, and there is a limit to reducing the dark current.

[0020] Thus, with conventional photodetectors, it has been difficult to reduce dark current with a simple structure while realizing increased light absorption rate and controllability of absorption wavelength due to surface plasmon resonance. In other words, (i) increased light absorption rate due to surface plasmon resonance, (ii) controllability of absorption wavelength, and (iii) reduced dark current are important elements required for a highly sensitive photodetector, but with conventional technology, it has been difficult to realize an optical device that satisfies all of these elements with a simple structure.

[0021] Therefore, the inventors of the present application conducted intensive research into these problems and discovered an optical device that can reduce dark current with a simple structure while achieving an increase in light absorption rate and controllability of the absorption wavelength due to surface plasmon resonance.

[0022] Specifically, one aspect of the optical device according to the present disclosure comprises a silicon substrate having a principal surface with a crystal plane orientation of (111) and a concave-convex structure provided on the principal surface, and a conductor forming a Schottky junction with the silicon substrate, and the conductor is directly bonded to the (111) surface of at least one of the convex and concave portions in the concave-convex structure.

[0023] According to one aspect of the present disclosure, since the main surface of the silicon substrate is provided with a concave-convex structure, it is possible to achieve both an increase in the light absorption rate due to surface plasmon resonance and controllability of the absorption wavelength. Moreover, since the crystal plane of the main surface of the silicon substrate is oriented in the (111) plane, it is possible to easily increase the flatness of the surface of the concave-convex structure of the silicon substrate. This makes it possible to reduce dark current caused by defect levels at the Schottky interface between the concave-convex structure of the silicon substrate and the conductor. Therefore, it is possible to reduce dark current with a simple structure without using a cap structure of an insulating layer as in Patent Document 1.

[0024] Thus, according to one aspect of the optical device of the present disclosure, it is possible to reduce dark current with a simple structure while realizing an increase in light absorptance and controllability of absorption wavelength due to surface plasmon resonance, and therefore it is possible to realize an optical device with a simple structure having excellent controllability of absorption wavelength and excellent photoelectric conversion efficiency.

[0025] In an aspect of the optical device according to the present disclosure, the (111) plane of the protrusion may be an upper surface of the protrusion, and the (111) plane of the recess may be a bottom surface of the recess.

[0026] With this configuration, the top surfaces of the convex portions or the bottom surfaces of the concave portions are flat, so that it is easy to achieve both high photoelectric conversion efficiency and reduced dark current.

[0027] In addition, in one aspect of the optical device according to the present disclosure, the conductor may be directly bonded to both an upper surface of the protrusion and a bottom surface of the recess.

[0028] With this configuration, since the conductor is joined to both the top surface of the convex portion and the bottom surface of the concave portion, it is possible to achieve both higher photoelectric conversion efficiency and reduced dark current.

[0029] In one aspect of the optical device according to the present disclosure, a side surface of the protrusion or a side surface of the recess may have a portion that is not in contact with the conductor.

[0030] When the conductor is in contact with the side of the convex portion and / or the side of the concave portion, the area of ​​the Schottky interface increases, but the light absorption rate due to surface plasmon resonance decreases, compared to when the conductor is not in contact with the side of the convex portion and / or the side of the concave portion. Moreover, since the crystal plane orientation of the side of the convex portion and the side of the concave portion is not the (111) plane, the side of the convex portion and the side of the concave portion are inferior in flatness to the top surface of the convex portion and the bottom surface of the concave portion. Therefore, when the side of the convex portion and / or the side of the concave portion are in contact with the conductor, the reduction of the dark current caused by the defect level at the Schottky interface is rather hindered. Therefore, by having a portion of the side of the convex portion or the side of the concave portion that is not in contact with the conductor, the decrease in the light absorption rate can be suppressed and the photoelectric conversion efficiency can be improved, and the defect level at the Schottky interface can be suppressed and the dark current can be reduced, compared to when the conductor is in contact with the entire side of the convex portion or the entire side of the concave portion. Note that, from the viewpoint of improving the photoelectric conversion efficiency and reducing the dark current, it is preferable that the conductor is not in contact with the entire side of the convex portion and the entire side of the concave portion.

[0031] In addition, in one aspect of the optical device according to the present disclosure, the uneven structure may have a property of absorbing electromagnetic waves by surface plasmon resonance.

[0032] With this configuration, surface plasmon resonance can be induced when electromagnetic waves such as light are irradiated.

[0033] In addition, in one aspect of the optical device according to the present disclosure, the conductor may be a single metal selected from the group consisting of gold, silver, copper, palladium, and aluminum, or an alloy containing at least one of these metals.

[0034] By forming a conductor from any of the above metals having excellent plasmonic properties, hot electrons can be generated with high efficiency. In other words, by using these metals, surface plasmon resonance is easily induced. This can improve the photoelectric conversion efficiency. In addition, since the work function of the alloy layer of these metals can be adjusted by the alloy composition, by making the conductor into an alloy layer, a low Schottky barrier that does not form an ohmic contact with silicon can be realized, and the photocurrent extraction efficiency can be improved. This can further improve the photoelectric conversion efficiency.

[0035] In addition, in one aspect of the optical device according to the present disclosure, the conductor may be an oxide containing at least one selected from the group consisting of indium, tin, zinc, and cadmium.

[0036] In this case, the conductor is at least one conductive oxide selected from, for example, tin-doped indium oxide (ITO), aluminum-doped zinc oxide (AZO), gallium-doped zinc oxide (GZO), and cadmium oxide (CdO). By using a conductor made of such a material, the efficiency of plasmon absorption and the transport efficiency of hot electrons in longer wavelength regions such as the near-infrared region are increased. This can further improve the photoelectric conversion efficiency.

[0037] In addition, in one aspect of the optical device according to the present disclosure, the conductor may be a nitride containing at least one selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, and tungsten.

[0038] In this case, the conductor is at least one metal nitride selected from titanium nitride (TiN), zirconium nitride (ZrN), tantalum nitride (TaN), hafnium nitride (HfN), vanadium nitride (VN), niobium nitride (NbN), molybdenum nitride (MoN), and tungsten nitride (WN). By using a conductor made of such a material, the efficiency of plasmon absorption can be increased in the wavelength range from the long wavelength range in the visible range to the near infrared range. This can further improve the photoelectric conversion efficiency.

[0039] In addition, in one aspect of the optical device according to the present disclosure, the conductor may be a layered structure consisting of at least two selected from the metals or alloys described above, the oxides described above, and the nitrides described above.

[0040] This configuration facilitates light absorption, so that the light absorption rate can be further increased and the Schottky barrier can be reduced, thereby enabling a further improvement in photoelectric conversion efficiency.

[0041] In addition, in one aspect of the optical device according to the present disclosure, an electrode layer may be further provided that covers the protrusions, the recesses, and the conductor.

[0042] With this configuration, a current generated by photoelectric conversion upon receiving light can be easily extracted by the electrode layer.

[0043] Hereinafter, specific embodiments of the present disclosure will be described with reference to the drawings. Note that the embodiments described below all show a comprehensive or specific example of the present disclosure. Therefore, the numerical values, shapes, materials, components, the arrangement and connection forms of the components, as well as the steps and the order of the steps shown in the following embodiments are merely examples and are not intended to limit the present disclosure. Therefore, among the components in the following embodiments, components that are not described in the independent claims will be described as optional components.

[0044] In addition, each drawing is a schematic diagram and is not necessarily a precise illustration. Therefore, the scale and the like are not necessarily the same in each drawing. In addition, in each drawing, the same reference numerals are given to substantially the same configurations, and duplicated explanations are omitted or simplified.

[0045] (Embodiment) First, the configuration of an optical device 1 according to an embodiment will be described with reference to Fig. 1, Fig. 2A, and Fig. 2B. Fig. 1 is a perspective view that shows a schematic configuration of a portion of an optical device 1 according to an embodiment. Fig. 2A is a plan view that shows a configuration of a portion of the optical device 1. Fig. 2B is a cross-sectional view of the optical device 1 taken along line IIB-IIB in Fig. 2A.

[0046] As shown in FIGS. 1, 2A, and 2B, the optical device 1 includes a silicon substrate 10 and a conductor 20. The conductor 20 is a conductive material.

[0047] The silicon substrate 10 is, for example, an n-type or p-type semiconductor substrate, and functions as a semiconductor in the optical device 1. In this embodiment, the silicon substrate 10 is an n-type semiconductor substrate. The silicon substrate 10 is a Si(111) substrate in which the crystal plane of the main surface is oriented in the (111) plane.

[0048] 1, a concave-convex structure 11 is provided on the main surface of a silicon substrate 10. In other words, the concave-convex structure 11 is a part of the silicon substrate 10. The concave-convex structure 11 can be formed by processing the (111) plane, which is the main surface of the silicon substrate 10, into an concave-convex shape.

[0049] The concave-convex structure 11 is a microstructure having a configuration in which at least one of nanometer-order convex portions and concave portions is periodically arranged. The convex portions and / or concave portions constituting the concave-convex structure 11 are formed on the main surface (i.e., the (111) surface) of the silicon substrate 10. The convex portions and / or concave portions constituting the concave-convex structure 11 are repeatedly formed along a direction parallel to the main surface of the silicon substrate 10.

[0050] As shown in FIG. 1, FIG. 2A, and FIG. 2B, the uneven structure 11 in this embodiment is composed of a plurality of convex portions 11a. Specifically, the uneven structure 11 is composed of a plurality of convex portions 11a periodically formed along each of two axial directions parallel to the main surface of the silicon substrate 10 and perpendicular to each other. That is, as shown in FIG. 2A, the plurality of convex portions 11a are arranged in a matrix shape in a top view. The uneven structure 11 composed of such a plurality of convex portions 11a can be fabricated by forming a concave portion 11b as a trench on the main surface of the silicon substrate 10 by etching or the like. Therefore, the uneven structure 11 is composed of a plurality of convex portions 11a periodically arranged and a concave portion 11b which is a portion between two adjacent convex portions 11a. In this case, as shown in FIG. 1 and FIG. 2A, the concave portion 11b in this embodiment is continuous without being separated, but the concave portion 11b may be separated into a plurality of portions. Since the multiple protrusions 11a are arranged periodically, two adjacent recesses 11b are also arranged periodically in the cross section of the optical device 1, as shown in FIG. 2B.

[0051] Thus, one main surface of the silicon substrate 10, which is a Si(111) substrate, is provided with the uneven structure 11 as a trench structure or texture structure having the protrusions 11a and recesses 11b.

[0052] Each of the plurality of protrusions 11a is a columnar body (Si pillar) made of silicon. For example, each of the plurality of protrusions 11a has a shape of a quadrangular prism. As an example, each of the protrusions 11a has a shape of a square prism when viewed from above. In the present embodiment, all of the protrusions 11a have the same shape and the same size, but this is not limited thereto.

[0053] Each of the protrusions 11a has a height and width of, for example, 10 nm or more and less than 1000 nm, and an aspect ratio (height / width) of about 0.5 to 2. In other words, each of the recesses 11b has a depth and width of, for example, 10 nm or more and less than 1000 nm, and an aspect ratio (depth / width) of about 0.5 to 2. That is, the recesses 11b have a shallow trench structure, not a deep trench structure. The aspect ratio of the protrusions 11a and the recesses 11b is preferably less than 1.0. That is, the protrusions 11a and the recesses 11b are preferably shaped such that the width is longer than the height. In addition, the center-to-center distance between two adjacent protrusions 11a or two adjacent recesses 11b is preferably 10 nm or more and less than 1000 nm. The sizes of the protrusions 11a and the recesses 11b are not limited to the above numerical ranges.

[0054] Since the uneven structure 11 is formed on the (111) surface which is the main surface of the silicon substrate 10, the convex portions 11a and the concave portions 11b are also formed on the (111) surface of the silicon substrate 10. Specifically, the top surfaces of the convex portions 11a and the bottom surfaces of the concave portions 11b become the (111) surface of the silicon substrate 10.

[0055] The conductor 20 is in Schottky junction with the silicon substrate 10. It is sufficient that the conductor 20 is in Schottky junction with at least a part of the silicon substrate 10. Specifically, the conductor 20 is in Schottky junction with a concave-convex structure 11 provided on the silicon substrate 10. The concave-convex structure 11 in Schottky junction with the conductor 20 has a property of absorbing electromagnetic waves by surface plasmon resonance. For example, when light is irradiated as electromagnetic waves, the concave-convex structure 11 induces surface plasmon resonance and absorbs the light.

[0056] The conductor 20 is directly bonded to at least one (111) surface of the convex portions 11a and the concave portions 11b in the concave-convex structure 11. In this embodiment, the conductor 20 is directly bonded to both the (111) surface of the convex portions 11a and the (111) surface of the concave portions 11b. Specifically, the (111) surface of the convex portions 11a is the upper surface of the convex portions 11a, and the (111) surface of the concave portions 11b is the bottom surface of the concave portions 11b, so that the conductor 20 is directly bonded to both the upper surface of the convex portions 11a and the bottom surface of the concave portions 11b.

[0057] The conductor 20 is formed on the upper surface of each of the plurality of protrusions 11a. That is, the conductor 20 formed on the upper surface of the protrusion 11a is separated into a plurality of pieces for each protrusion 11a. In each protrusion 11a, the conductor 20 is formed on the entire upper surface of the protrusion 11a. That is, the upper surface of each protrusion 11a is covered with the conductor 20. In addition, the conductor 20 formed on the bottom surface of the recess 11b is formed over the entire bottom surface of the recess 11b in the uneven structure 11.

[0058] In this embodiment, there is a portion of the side surface of the protrusion 11a that is not in contact with the conductor 20. Specifically, the conductor 20 is not formed on the entire side surface of the protrusion 11a and the entire side surface of the recess 11b, and the entire side surface of the protrusion 11a and the entire side surface of the recess 11b are exposed. The side surface of the protrusion 11a is an outer wall surface located in the lateral direction of the protrusion 11a, and the side surface of the recess 11b is an inner wall surface located in the lateral direction of the recess 11b. In this embodiment, the side surface of the protrusion 11a and the side surface of the recess 11b are the same. Therefore, there is also a portion of the side surface of the recess 11b that is not in contact with the conductor 20.

[0059] The conductor 20 is, for example, a thin conductive film having a thickness of nano-order size. The thickness of the conductor 20 is smaller than the height of the convex portion 11a or the depth of the concave portion 11b. The film thickness of the conductor 20 is, for example, 100 nm or less, and may be 50 nm or less, or even 20 nm or less. The film thickness of the conductor 20 is constant over the entire optical device 1, but may be partially different.

[0060] In order to induce surface plasmon resonance with high efficiency, an optimum material is selected for the conductor 20 according to the wavelength of the incident electromagnetic wave, the device structure, etc. As the material for the conductor 20, for example, a metal can be used.

[0061] Specifically, the conductor 20 is a single metal selected from gold (Au), silver (Ag), copper (Cu), palladium (Pd) and aluminum (Al), or an alloy containing at least one of these metals. In this embodiment, the conductor 20 is an alloy film made of Au and Ag (AuAg alloy film).

[0062] The conductor 20 may be an intermetallic compound or a solid solution alloy containing at least two kinds of metals. An "intermetallic compound" is a compound in which two or more kinds of metals are bonded in a simple integer ratio, and refers to an alloy in which atoms are regularly arranged in an orderly manner over a relatively long distance (e.g., 1 nm or more). A "solid solution alloy" is a single-phase alloy in which multiple metal elements are uniformly and disorderly distributed within the crystal, and which has a structure in which one metal is inserted or substituted while retaining the structure of the other metal.

[0063] Whether or not a certain substance is an alloy can be confirmed by, for example, element mapping using a scanning transmission electron microscope (STEM). If the substance is not separated into phases of the multiple metal elements that are its components, it can be determined to be an alloy. More specifically, if the following conditions (1) and (2) are satisfied, for example, the conductor 20 can be said to be an alloy of a first metal and a second metal. (1) When elemental mapping measurement is performed with a resolution of 1 nm×1 nm using a STEM, the first metal and the second metal are detected in 80% or more of the entire area occupied by the conductor 20 . (2) Energy dispersive X-ray analysis (EDX) and line analysis revealed that the first and second metals were detected in the cross section of the layer at ratios that reflected the composition ratio.

[0064] Whether or not a certain substance is a solid solution alloy can be confirmed based on a diffraction pattern obtained by, for example, X-ray diffraction. If a peak shift from the peak positions of the first metal and the second metal, reflecting the composition ratio based on Vegard's law, is observed in the diffraction pattern, it can be determined that the conductor 20 is a solid solution alloy of the first metal and the second metal.

[0065] On the other hand, whether the conductor 20 is an intermetallic compound or not can be confirmed by, for example, analysis using electron beam diffraction or X-ray diffraction. If the diffraction pattern obtained by electron beam diffraction or X-ray diffraction matches the diffraction pattern of an intermetallic compound of the first metal and the second metal disclosed in a literature such as a technical book, Conductor 20 It can be determined that the intermetallic compound is the first metal and the second metal.

[0066] When the composition ratio of the intermetallic compound is different from the composition ratio disclosed in the literature, the spacing of the diffraction spots (peaks in the case of X-ray diffraction) may be slightly shifted according to the difference in the interplanar spacing. In that case, the lattice spacing may be calculated from a lattice image obtained by structural analysis of the particle using STEM, and whether or not an intermetallic compound is contained may be determined based on whether or not the peak position calculated from the lattice spacing matches the peak position disclosed in the literature. Alternatively, the composition ratio of the particle may be determined by EDX, the lattice spacing may be calculated by Vegard's law, and whether or not an intermetallic compound is contained may be determined based on whether or not the peak position calculated from the lattice spacing matches the peak position disclosed in the literature.

[0067] The conductor 20 may be an oxide containing at least one selected from the group consisting of indium (In), tin (Sn), zinc (Zn) and cadmium (Cd).

[0068] The conductor 20 may also be a nitride containing at least one selected from titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), molybdenum (Mo), and tungsten (W).

[0069] Alternatively, the conductor 20 may be a laminated structure made of at least two selected from (i) a single metal selected from gold, silver, copper, palladium, and aluminum, or an alloy containing at least one of these metals, (ii) an oxide containing at least one selected from indium, tin, zinc, and cadmium, and (iii) a nitride containing at least one selected from titanium, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, and tungsten. In other words, the conductor 20 may be a laminated film made of multiple layers.

[0070] The optical device 1 configured as above can be used as a photodetector element that performs photoelectric conversion when irradiated with light. In the optical device 1 according to the present embodiment, the photoelectric conversion function is realized by extracting hot electrons induced by surface plasmon resonance when irradiated with light as a current. For example, when light is incident on the optical device 1, the light is absorbed by the conductor 20 with high efficiency by surface plasmon resonance, and hot electrons are induced in the conductor 20. The hot electrons induced in the conductor 20 are photoelectrically converted by the Schottky junction between the conductor 20 and the silicon substrate 10. Specifically, the hot electrons induced in the conductor 20 are charge-separated by crossing the Schottky barrier between the conductor 20 and the convex portion 11a and the concave portion 11b in the concave-convex structure 11 of the silicon substrate 10, and flow as a photocurrent to the silicon substrate 10 side.

[0071] 1, the optical device 1 has a high optical absorptance due to surface plasmon resonance caused by the nanoantenna structure, and therefore the photoelectric conversion efficiency can be improved. For example, the optical device 1 of this embodiment can improve the optical absorptance of near-infrared light to about 90%.

[0072] In addition, since the optical device 1 in this embodiment has the concave-convex structure 11, it is possible to control the wavelength region of the electromagnetic wave that induces the surface plasmon resonance. For example, by adjusting the shape and size of the convex portion 11a and the concave portion 11b in the concave-convex structure 11, it is possible to control the wavelength region of the light incident on the optical device 1. Specifically, it is possible to shift the absorption peak wavelength of the light incident on the optical device 1 by adjusting the width or height of the convex portion 11a (Si pillar), the period of the convex portion 11a, and the film thickness of the conductor 20, or by adjusting the width or depth of the concave portion 11b, the period of the concave portion 11b, and the film thickness of the conductor 20. In this case, it is possible to shift the absorption peak wavelength to the long wavelength side by increasing the width of the convex portion 11a and the concave portion 11b. As an example, when an AuAg alloy film (for example, 15 nm thick) is used as the conductor 20, it is possible to set the absorption peak wavelength of the optical device 1 to 1310 nm or 1550 nm by adjusting the width, height, and period of the convex portion 11a. In this way, according to the optical device 1 of the present embodiment, it is possible to uniquely design a structure that is optimal for surface plasmon resonance, thereby achieving both improved light absorption and improved sensitivity due to a lower Schottky barrier.

[0073] Next, a method for manufacturing the optical device 1 according to the present embodiment will be described.

[0074] First, a silicon substrate 10 is fabricated having a concave-convex structure 11. The silicon substrate 10 having the concave-convex structure 11 can be fabricated using a patterning technique such as electron beam lithography.

[0075] For example, a Si(111) substrate with a crystal plane of the main surface oriented in the (111) direction is prepared as the silicon substrate 10, a positive resist is applied to the main surface of the silicon substrate 10 by spin coating or the like, an electron beam is irradiated at desired locations, and then a development process is performed to remove the resist at the locations irradiated with the electron beam. By subjecting this silicon substrate 10 to reactive ion etching or the like using an etching gas such as SF6 gas, the main surface of the silicon substrate 10 is etched only at the locations where the resist has been removed. As a result, a concave-convex structure 11 composed of convex portions 11a and concave portions 11b can be formed on the main surface of the silicon substrate 10.

[0076] Next, the surface (exposed surface) of the concave-convex structure 11 of the silicon substrate 10 exposed by etching is cleaned, and a process for reducing defects on the surface of the silicon substrate 10 is performed.

[0077] For example, for a silicon substrate 10 having a concave-convex structure 11 formed thereon, contamination on the surface of the silicon substrate 10 is removed using, for example, a piranha solution, and then the surface of the concave-convex structure 11 of the silicon substrate 10 is cleaned using an ammonium fluoride aqueous solution (NH4F aqueous solution).

[0078] As a result, the surface of the uneven structure 11 can be made into a step-terrace structure with a height of angstrom size (i.e., atomic level) as shown in Fig. 3. As a result, the (111) surface of the silicon substrate 10 in the uneven structure 11 can be flattened at the atomic level. Fig. 3 is a schematic diagram showing a cross section of a part of the upper surface of the convex portion 11a or the bottom surface of the concave portion 11b of the uneven structure 11 as viewed at the atomic level. As shown in Fig. 3, by carrying out the cleaning process, the surface of the uneven structure 11 of the silicon substrate 10 becomes a step-terrace structure consisting of terraces, which are completely flat surfaces at the atomic level, and steps at the stepped portions.

[0079] The ammonium fluoride aqueous solution used in the cleaning process should preferably be deoxygenated. The treatment agent used in the cleaning process is not limited to the ammonium fluoride aqueous solution. Instead of the ammonium fluoride aqueous solution, the cleaning process may be performed using deoxygenated ultrapure water.

[0080] In this way, by using a Si(111) substrate as the silicon substrate 10, it is possible to form a step-terrace structure on the surface of the uneven structure 11 of the silicon substrate 10. Specifically, the top surfaces of the protrusions 11a, which are the (111) surface of the uneven structure 11, and the bottom surfaces of the recesses 11b can be formed into a step-terrace structure to be flat at the atomic level.

[0081] Moreover, by performing a cleaning process on the surface of the uneven structure 11 of the silicon substrate 10, hydrogen can be terminated at the dangling bonds of silicon on the terrace surfaces of the uneven structure 11, which has become a step-and-terrace structure (i.e., the upper surfaces of the protrusions 11a and the bottom surfaces of the recesses 11b), as shown in Figure 3. Although atoms other than hydrogen, such as oxygen, may be adsorbed on the steps of the step-and-terrace structure, it is preferable that hydrogen is also terminated on the steps.

[0082] Therefore, by performing a cleaning process on the surface of the silicon substrate 10, which is a Si(111) substrate on which the uneven structure 11 is formed, the upper surfaces of the convex portions 11a and the bottom surfaces of the concave portions 11b in the uneven structure 11 can be made into flat surfaces that are planarized at the atomic level and hydrogen-terminated.

[0083] In this embodiment, after the uneven structure 11 is formed on the silicon substrate 10, a cleaning process using an ammonium fluoride solution is performed. However, before the cleaning process, a process of forming an oxide film on the surface of the silicon substrate 10 and a process of removing the oxide film may be performed. Specifically, after the uneven structure 11 is formed on the silicon substrate 10, the surface of the silicon substrate 10 is oxidized by an annealing process or a strong acid process to intentionally form a silicon oxide film, and then the silicon oxide film is removed by hydrofluoric acid (HF), and then a cleaning process is performed by an ammonium fluoride solution. As a result, a silicon oxide film with a uniform thickness is formed on the surface of the uneven structure 11, and by removing the silicon oxide film, the entire surface of the uneven structure 11 can be made into a uniform step-terrace structure. Therefore, defects on the surface of the uneven structure 11 can be further reduced. When removing the silicon oxide film using hydrofluoric acid (HF), it is preferable to intentionally overetch the silicon oxide film and remove the silicon surface layer under the silicon oxide film. This allows the entire surface of the exposed (111) plane of the uneven structure 11 to have a more uniform step-terrace structure.

[0084] Next, the silicon substrate 10 is cleaned, and then the conductor 20 is formed. For example, the conductor 20 can be formed by sputtering or vacuum deposition. This makes it possible to manufacture the optical device 1 having the structure shown in Figures 1, 2A, and 2B. When forming the conductor 20, it may be formed on part of the side surface of the protrusion 11a (or the side surface of the recess 11b).

[0085] In the optical device 1 manufactured in this manner, the silicon substrate 10 is cleaned, so that the upper surfaces of the convex portions 11a and the bottom surfaces of the concave portions 11b in the concave-convex structure 11 can be flattened at the atomic level and hydrogen-terminated. This can significantly reduce defects on the surface of the concave-convex structure 11 of the silicon substrate 10. This can prevent an oxide layer from being formed at the interface between the silicon of the concave-convex structure 11 and the conductor 20, and the concave-convex structure 11 and the conductor 20 can be directly bonded without defects.

[0086] Here, the effect of the cleaning treatment of the surface of the uneven structure 11 will be described with reference to Figs. 4 and 5. Fig. 4 shows the surface state of the convex parts 11a of the uneven structure 11 when a silicon oxide film is formed on the surface of the uneven structure 11 and the silicon oxide film is removed by hydrofluoric acid. Fig. 5 shows the surface state of the convex parts 11a of the uneven structure 11 when a silicon oxide film is formed on the surface of the uneven structure 11 and the silicon oxide film is removed by hydrofluoric acid, and then a cleaning treatment is performed by an ammonium fluoride aqueous solution. That is, Fig. 4 shows the case where the uneven structure 11 is not cleaned by an ammonium fluoride aqueous solution, and Fig. 5 shows the case where the uneven structure 11 is cleaned by an ammonium fluoride aqueous solution. Note that Figs. 4 and 5 are atomic force microscope images, and in Figs. 4 and 5, (a) shows a part of the upper surface of the convex parts 11a secondarily, and (b) shows a part of the upper surface of the convex parts 11a tertiarily.

[0087] As shown in Fig. 4, when the uneven surface structure 11 is not cleaned with an ammonium fluoride solution, the surface of the uneven surface structure 11 is planarized but not at the atomic level. As shown in Fig. 4, when the cleaning process is not performed, no step-terrace structure is observed.

[0088] On the other hand, as shown in Figure 5, when the uneven structure 11 is cleaned with an ammonium fluoride solution, a step-and-terrace structure is observed on the surface of the uneven structure 11, and it is found that the silicon surface of the uneven structure 11 is almost flat at the atomic level. In this state, the surface of the uneven structure 11 is defect-free. It was also found that the surface of the uneven structure 11 that has become a step-and-terrace structure is hydrogen-terminated.

[0089] In the optical device 1 of the present embodiment, it is sufficient that defects on the upper surface a of the convex portion 11a and the bottom surface of the concave portion 11b in the concave-convex structure 11 can be reduced, and it is not necessarily necessary that the step-terrace structure as shown in FIG. 5 can be clearly confirmed. That is, even if cleaning treatment is not performed as shown in FIG. 4, by using a Si(111) substrate as the silicon substrate 10, the surface of the concave-convex structure 11 can be made flatter than when a Si(100) substrate or a Si(110) substrate is used as the silicon substrate 10. In addition, whether the surface of the silicon substrate 10 is hydrogen-terminated can be determined by, for example, Fourier transform infrared spectroscopy (FT-IR). Furthermore, by analyzing infrared light absorption peaks caused by Si-H bonds or Si-H2 bonds using this FT-IR, the Si-H bond state or the Si-H2 bond state can be identified, and can be used as an index of the flatness or defect state of the surface of the silicon substrate 10.

[0090] As described above, according to the optical device 1 of this embodiment, the uneven structure 11 is provided on the main surface of the silicon substrate 10, so that it is possible to achieve both an increase in the light absorption rate due to surface plasmon resonance and controllability of the absorption wavelength.

[0091] Moreover, since the crystal plane of the main surface of the silicon substrate 10 is oriented in the (111) plane, the flatness of the surface of the concave-convex structure 11 of the silicon substrate 10 can be easily improved. Specifically, the upper surfaces of the convex portions 11a and the bottom surfaces of the concave portions 11b of the concave-convex structure 11 can be made flat at the atomic level. As a result, defects on the upper surfaces of the convex portions 11a and the bottom surfaces of the concave portions 11b can be reduced. This allows the convex portions 11a and the concave portions 11b to be directly bonded to the conductor 20 with fewer defects. Therefore, the dark current caused by defect levels at the Schottky interface between the concave-convex structure 11 of the silicon substrate 10 and the conductor 20 can be reduced.

[0092] In this way, the optical device 1 according to the present embodiment can reduce dark current with a simple structure while realizing an increase in light absorptance and controllability of absorption wavelength due to surface plasmon resonance. Therefore, it is possible to realize an optical device 1 with a simple structure that has excellent controllability of absorption wavelength and excellent photoelectric conversion efficiency.

[0093] The current obtained by photoelectric conversion in the optical device 1 can be extracted by using a first electrode layer 31 and a second electrode layer 32, as in the optical device 1A shown in Fig. 6. The optical device 1A shown in Fig. 6 is configured such that the first electrode layer 31 and the second electrode layer 32 are further provided in addition to the above-mentioned optical device 1. The first electrode layer 31 and the second electrode layer 32 are electrically connected by an electric wire 40.

[0094] The first electrode layer 31 covers the convex portions 11a, the concave portions 11b, and the conductors 20. Specifically, the first electrode layer 31 covers the conductors 20 and the entire exposed surface of the concave-convex structure 11. Therefore, the first electrode layer 31 is formed so as to straddle the conductors 20 that are separated into multiple pieces by being formed on the upper surfaces of the convex portions 11a. In other words, the first electrode layer 31 is in contact with each of the multiple conductors 20 that are separated from one another by the concave-convex structure 11.

[0095] The first electrode layer 31 is an ohmic electrode layer. Therefore, the portion where the first electrode layer 31 contacts the conductor 20 is in ohmic contact. The portion where the first electrode layer 31 contacts the silicon substrate 10 is also in ohmic contact. Specifically, the portion where the first electrode layer 31 contacts the side surface of the protrusion 11a (or the side surface of the recess 11b) is in ohmic contact.

[0096] The first electrode layer 31 may be made of any material as long as it has ohmic properties with respect to the conductor 20. However, in the case where the first electrode layer 31 is formed on the electromagnetic wave incident side of the optical device 1A in Fig. 6, it is desirable for the first electrode layer 31 to have a transmissive property with respect to the wavelength of the electromagnetic wave. For example, in the case where the optical device 1A absorbs visible light or infrared light, the first electrode layer 31 may be made of a transparent conductive film made of indium tin oxide (ITO) or the like.

[0097] The second electrode layer 32 is formed on the main surface of the silicon substrate 10 opposite to the main surface on which the concave-convex structure 11 is formed. Specifically, assuming that the main surface of the silicon substrate 10 on which the concave-convex structure 11 is formed is a first main surface, the second electrode layer 32 is formed on a second main surface facing away from the first main surface. For example, the second electrode layer 32 may be formed on the entire second main surface of the silicon substrate 10, or may be formed on a part of the second main surface.

[0098] The second electrode layer 32 is an ohmic electrode layer. Therefore, the portion where the second electrode layer 32 contacts the silicon substrate 10 is in ohmic contact. The second electrode layer 32 may be made of any material as long as the material has ohmic properties with respect to the silicon substrate 10.

[0099] In the optical device 1A thus configured, when the optical device 1A is irradiated with light emitted from the light source 2, hot electrons induced by surface plasmon resonance are photoelectrically converted by the Schottky junction between the conductor 20 and the silicon substrate 10, and a current flows, similarly to the above-described optical device 1. The current generated in the optical device 1A can be extracted to the outside of the optical device 1A by the first electrode layer 31 and the second electrode layer 32 connected by the electric wire 40. In other words, when the optical device 1A is irradiated with light from the light source 2, a current flows through the optical device 1A via the electric wire 40.

[0100] In FIG. 6, the light source 2 is disposed on the side (first main surface side) of the silicon substrate 10 on which the conductor 20 is formed. Specifically, the light source 2 is disposed on the first electrode layer 31 side of the optical device 1. Therefore, the light source 2 irradiates light toward the first electrode layer 31 of the optical device 1A. The light source 2 is, for example, a solid-state light-emitting light source using a semiconductor light-emitting element such as a laser diode or a light-emitting diode, or a xenon lamp, a mercury lamp, a halogen lamp, or the like. The light source 2 may emit light including a relatively wide wavelength range as long as the light includes the absorption peak wavelength of the optical device 1A. The wavelength range of the light source 2 is determined so as to include the wavelength of the surface plasmon resonance of the optical device 1A. The light source 2 may be a component of the optical device 1A or an external element of the optical device 1A.

[0101] (Modification) Although the optical device according to the present disclosure has been described based on the embodiments, the present disclosure is not limited to the above-described embodiments.

[0102] For example, in the above embodiment, the conductor 20 is formed on the upper surface of the convex portion 11a and the bottom surface of the concave portion 11b in the concave-convex structure 11, and the conductor 20 is not formed on the side surface of the convex portion 11a (i.e., the side surface of the concave portion 11b), but this is not limited thereto. Specifically, as in the optical device 1B shown in FIG. 7, the conductor 20B may be formed not only on the upper surface of the convex portion 11a and the bottom surface of the concave portion 11b, but also on the side surface of the convex portion 11a (i.e., the side surface of the concave-convex structure 11b). In FIG. 7, the conductor 20B covers the entire surface of the concave-convex structure 11 along the concave-convex shape of the concave-convex structure 11 without exposing the surface of the concave-convex structure 11. According to the optical device 1B shown in FIG. 7, it is possible to extract a current generated in the optical device 1B without requiring a first electrode layer 31 as in the optical device 1A shown in FIG. 6. That is, in the optical device 1B shown in FIG. 7, the conductor 20B not only functions as a structure that induces surface plasmon resonance, but also functions as an electrode for extracting the induced hot electrons as a current.

[0103] In the optical device 1 according to the above embodiment, the convex portions 11a in the concave-convex structure 11 are arranged in a matrix in top view, but the present invention is not limited thereto. For example, as in the optical device 1C shown in FIG. 8, the concave-convex structure 11C may be formed in a comb shape in top view. Specifically, the concave-convex structure 11C has, as convex portions, a plurality of first convex portions 11a1 in a stripe shape corresponding to the teeth of a comb, and a second convex portion 11a2 connecting the ends of the plurality of first convex portions 11a1. The plurality of first convex portions 11a1 extend in one direction and are parallel to each other. The portion between two adjacent first convex portions 11a1 is a concave portion 11b.

[0104] In the above embodiment, the conductor 20 is directly bonded to both the upper surface of the protrusion 11a and the bottom surface of the recess 11b, but this is not limited thereto. For example, the conductor 20 may be directly bonded to only one of the upper surface of the protrusion 11a and the bottom surface of the recess 11b. In other words, the conductor 20 may be Schottky bonded to only one of the upper surface of the protrusion 11a and the bottom surface of the recess 11b.

[0105] In addition, in the above embodiment, the concave-convex structure 11 is configured by a plurality of periodically formed convex portions 11a, but this is not limited thereto. The concave-convex structure 11 may be configured by a plurality of periodically formed concave portions 11b. That is, in the above optical device 1, the concave-convex structure 11 may have a shape in which the convex portions 11a and the concave portions 11b are in an inverse relationship. Note that the concave-convex structure 11 may be configured such that each of the convex portions 11a and the concave portions 11b is periodically formed in a plurality of portions.

[0106] In the above embodiment, the uneven structure 11 is composed of the convex portions 11a and the concave portions 11b of a single shape, but this is not limited thereto. The uneven structure 11 may be composed of the convex portions 11a and the concave portions 11b having a plurality of widths, heights, and depths, and the cross-sectional shapes of the convex portions 11a and the concave portions 11b may be trapezoidal as well as rectangular. In the above embodiment, the convex portions 11a (Si pillars) are composed of quadrangular prisms having a square shape in top view, but this is not limited thereto, and the uneven structure 11 may be rectangular or polygonal, or may be a cylinder. In the above embodiment, the uneven structure 11 is composed of an array with a single period, but this is not limited thereto, and the uneven structure 11 may be an array with multiple periods or a random array. In the case of such a configuration, a broad absorption spectrum associated with surface plasmon resonance at multiple wavelengths is exhibited, which is useful for widening the detection wavelength band.

[0107] In the above embodiment, the light source 2 is disposed on the side (first main surface side) of the silicon substrate 10 on which the conductor 20 is formed, but may be disposed on the second main surface side opposite to the first main surface side. In this case, the light source 2 has near-infrared light that is transparent to the silicon substrate 10, or electromagnetic waves with a longer wavelength than the near-infrared light, and it is necessary to select a material (such as indium tin oxide (ITO)) that is transparent to the electromagnetic waves from the light source 2 for the second electrode layer 32. With this configuration, surface plasmon resonance at the Schottky interface between the silicon substrate 10 and the conductor 20 can be excited with high efficiency, and the loss of induced hot electrons when propagating to the Schottky interface can be reduced, which is useful for improving photoelectric conversion efficiency.

[0108] In addition, the present disclosure also includes forms obtained by applying various modifications to the above-described embodiments that a person skilled in the art may think of, and forms realized by arbitrarily combining the components and functions of each embodiment within the scope that does not deviate from the spirit of the present disclosure. [Industrial Applicability]

[0109] The technology of the present disclosure can be used in any application in which photoelectric conversion is performed. For example, an optical device according to the present disclosure can be used as a light detection element of an image sensor or the like. [Explanation of symbols]

[0110] 1, 1A, 1B, 1C Optical Devices 2 light source 10 Silicon substrate 11, 11C uneven structure 11a Convex part 11a1 First protrusion 11a2 Second protrusion 11b Recess 20, 20B conductor 31 1st electrode layer 32 Second electrode layer 40 Electric wire

Claims

1. a silicon substrate having a main surface with a crystal plane orientation of a (111) plane and a concave-convex structure provided on the main surface; a conductor that forms a Schottky junction with the silicon substrate; the conductor is directly bonded to at least one (111) surface of the protrusions and recesses in the concave-convex structure; the (111) plane of the protrusion is the upper surface of the protrusion, the (111) plane of the recess is a bottom surface of the recess, the conductor is directly bonded to both an upper surface of the protrusion and a bottom surface of the recess; Optical devices.

2. A silicon substrate having a crystal plane of a main surface oriented in a (111) plane and having an uneven structure on the main surface; a conductor that forms a Schottky junction with the silicon substrate; the conductor is directly bonded to at least one (111) surface of the protrusions and recesses in the concave-convex structure; a side surface of the protrusion or a side surface of the recess has a portion that is not in contact with the conductor; Optical devices.

3. A silicon substrate having a crystal plane of a main surface oriented in a (111) plane and having an uneven structure on the main surface; a conductor that forms a Schottky junction with the silicon substrate; the conductor is directly bonded to at least one (111) surface of the protrusions and recesses in the concave-convex structure; The uneven structure has a property of absorbing electromagnetic waves by surface plasmon resonance. Optical devices.

4. A silicon substrate having a crystal plane of a main surface oriented in a (111) plane and having a concave-convex structure on the main surface; a conductor that forms a Schottky junction with the silicon substrate; the conductor is directly bonded to at least one (111) surface of the protrusions and recesses in the concave-convex structure; The conductor is a single metal selected from the group consisting of gold, silver, copper, palladium and aluminum, or an alloy containing at least one of these metals. Optical devices.

5. A silicon substrate having a crystal plane of a main surface oriented in a (111) plane and having a concave-convex structure on the main surface; a conductor that forms a Schottky junction with the silicon substrate; the conductor is directly bonded to at least one (111) surface of the protrusions and recesses in the concave-convex structure; The conductor is an oxide containing at least one selected from the group consisting of indium, tin, zinc, and cadmium. Optical devices.

6. A silicon substrate having a crystal plane of a main surface oriented in a (111) plane and having a concave-convex structure on the main surface; a conductor that forms a Schottky junction with the silicon substrate; the conductor is directly bonded to at least one (111) surface of the protrusions and recesses in the concave-convex structure; The conductor is a nitride containing at least one selected from titanium, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, and tungsten. Optical devices.

7. The conductor has a laminated structure made of at least two selected from the group consisting of the metal or alloy described in claim 4, the oxide described in claim 5, and the nitride described in claim 6. The optical device according to any one of claims 1 to 6.

8. Further, an electrode layer is provided to cover the protrusions, the recesses, and the conductor. The optical device according to any one of claims 1 to 7.

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