Schottky barrier diode

Abstract

To provide a novel Schottky barrier diode in which MXene is used.SOLUTION: There is provided a Schottky barrier diode in which a semiconductor portion and a conductive portion are bonded. The semiconductor portion comprises an oxide semiconductor, and has surface roughens of 10 nm or less on a surface bonded to the conductive portion. The conductive portion comprises particles of a layered material that includes one or more layers, and the layers each include: a layer body represented by a formula MmXn (in the formula, M is at least one kind of group 3-7 metals, X is a carbon atom, a nitrogen atom or a combination thereof, n is 1 or more and 4 or less, and m is more than n and 5 or less); and a modification or a terminal T (T is at least one kind selected from a group consisting of a hydroxyl group, a fluorine atom, a chlorine atom, an oxygen atom, and a hydrogen atom) present on a surface of the layer body. A distance between the two layers adjacent to each other in the conductive portion is 1.10 nm or more in the Schottky barrier diode.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a Schottky barrier diode. [Background technology]

[0002] In recent years, MXene has attracted attention as a novel electrically conductive material. MXene is a type of so-called two-dimensional material, and as described below, it is a layered material having the form of one or more layers. Generally, MXene has the form of particles of such layered materials (which may include powders, flakes, nanosheets, etc.).

[0003] Schottky barrier diodes generally have a structure in which a semiconductor and a metal are bonded together, and utilize a Schottky barrier formed between them. Known examples of Schottky barrier diodes that use MXene instead of a metal include a photodetector in which GaAs and MXene are bonded together (Non-Patent Document 1), a photodetector and light-emitting diode in which GaN and MXene are bonded together (Non-Patent Document 2), and a photodetector in which Si and MXene are bonded together (Non-Patent Document 3). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Kiana Montazeri, et al., "Beyond Gold: Spin-Coated Ti3C2-Based MXene Photodetectors", Advanced Materials, Volume 31, Issue 43, 1903271 [Non-patent document 2] Chujun Yi, "MXene-GaN van der Waals Heterostructures for High-Speed Self-Driven Photodetectors and Light-Emitting Diodes", Advanced Electronic Materials, 2021, Volume 7, Issue 5, 2000955 [Non-patent document 3] Weidong Song, et al., "Interface Engineering Ti3C2 MXene / Silicon Self-Powered Photodetectors with High Responsivity and Detectivity for Weak Light Applications", Small, 2021, Volume 17, Issue 23, 2100439 [Non-patent document 4] Hyunho Kim, et al., "MXetronics: Eectronic and photonic applications of MXenes", Nano Energy, 2019, Volume 60, pp. 179-197 [Non-patent document 5] SK Cheung et al., "Extraction of Schottky diode parameters from forward current-voltage characteristics", Applied Physics Letters, 1986, Volume 49, pp85-87 Summary of the Invention [Problem to be solved by the invention]

[0005] The MXene / GaAs photodetector described in Non-Patent Document 1 has the drawback that, due to the high toxicity of the As element, the GaAs containing this element is also highly toxic. The MXene / GaN photodetector described in Non-Patent Document 2 has the drawback that it is extremely difficult to produce high-purity GaN single crystals, making them expensive. The MXene / Si photodetector described in Non-Patent Document 3 has the drawback that the Si surface is very susceptible to oxidation, which can change the diode characteristics. The photodetectors described in Non-Patent Documents 1 and 2 also have the drawback that if the surfaces of the compound semiconductors GaAs and GaN are oxidized, this can change the diode characteristics.

[0006] An object of the present invention is to provide a novel Schottky barrier diode using MXene that does not have the above-mentioned drawbacks. [Means for solving the problem]

[0007] According to one aspect of the present invention, there is provided a Schottky barrier diode in which a semiconductor portion and a conductive portion are joined together, the semiconductor portion is made of an oxide semiconductor and has a surface roughness of 10 nm or less on a surface that is bonded to the conductive portion; the conductive portion is made of particles of layered material comprising one or more layers; The layer may comprise a compound having the following formula: M m X n (wherein M is at least one Group 3, 4, 5, 6, or 7 metal; X is a carbon atom, a nitrogen atom, or a combination thereof; n is between 1 and 4, m is greater than n and less than or equal to 5) and a modification or termination T (T is at least one selected from the group consisting of a hydroxyl group, a fluorine atom, a chlorine atom, an oxygen atom, and a hydrogen atom) present on the surface of the layer body, There is provided a Schottky barrier diode, wherein the distance between two adjacent layers in the conductive portion is 1.10 nm or more.

[0008] In one embodiment of the present invention, the oxide semiconductor may be an n-type oxide semiconductor having a work function smaller than that of the layered material.

[0009] For example, the n-type oxide semiconductor may include at least one selected from the group consisting of ZnO, TiO2, SnO2, CeO2, In2O3, Ga2O3, BaTiO3, and SrTiO3.

[0010] In another embodiment of the present invention, the oxide semiconductor may be a p-type oxide semiconductor having a work function greater than that of the layered material.

[0011] For example, the p-type oxide semiconductor may contain at least one selected from the group consisting of NiO, Cu2O, CuO, CoO, Mn3O4, SnO, and ZnCo2O4. [Effects of the Invention]

[0012] According to the present invention, it has been discovered that a Schottky barrier diode can be realized by adjusting the surface roughness of a semiconductor portion made of an oxide semiconductor to 10 nm or less and contacting the surface with a conductive portion containing a predetermined layered material (also referred to in this specification as "MXene") with an interlayer distance of 1.10 nm or more.The present invention provides a novel Schottky barrier diode that uses MXene, is low- or non-toxic, can be manufactured relatively inexpensively, and provides stable diode characteristics even when manufactured in an oxidizing environment. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic cross-sectional view illustrating a Schottky barrier diode according to one embodiment of the present invention. [Figure 2]1A and 1B are diagrams illustrating a Schottky barrier diode according to one embodiment of the present invention, in which (a) shows a schematic cross-sectional view of a conductive portion arranged in contact with the surface of a semiconductor portion, and (b) shows a schematic perspective view of particles of a layered material (MXene) in the conductive portion. [Figure 3] 1A and 1B are schematic cross-sectional views showing particles of a layered material (MXene) that can be used in one embodiment of the present invention, where (a) shows a single-layer MXene particle and (b) shows a multi-layer (exemplarily two-layer) MXene particle. [Figure 4] FIG. 2 is a schematic cross-sectional view illustrating samples of Examples 1 to 5 and Comparative Example 1. [Figure 5] FIG. 10 is a schematic cross-sectional view illustrating a sample of Comparative Example 2. [Figure 6] 10A and 10B are schematic cross-sectional views illustrating samples of Comparative Examples 3 and 4. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0014] A Schottky barrier diode according to one embodiment of the present invention will be described in detail below, but the present invention is not limited to this embodiment.

[0015] Referring to FIG. 1 , the Schottky barrier diode 20 of this embodiment has a configuration in which a semiconductor portion 11 and a conductive portion 13 are joined together. In the Schottky barrier diode 20, the semiconductor portion 11 and the conductive portion 13 may be arranged in contact with each other in any appropriate manner, as long as a Schottky barrier is formed between them. The Schottky barrier diode 20 may be electrically connected to the outside in any appropriate manner. While FIG. 1 shows an example in which the semiconductor portion 11 and the conductive portion 13 are connected to leads via contacts 15 a and 15 b, respectively, this embodiment is not limited to such an example.

[0016] Referring to FIGS. 1 to 2, the semiconductor portion 11 is made of an oxide semiconductor and has a surface roughness of 10 nm or less on the surface 11a joined to the conductive portion 13. The conductive portion 13 is composed of particles 10 of a predetermined layered material, and in the conductive portion 13, the distance between two adjacent layers is 1.10 nm or more.

[0017] A predetermined layered material that can be used in this embodiment is MXene, which is defined as follows: A layered material including one or more layers, wherein the layer has the following formula: M m X n (In the formula, M is at least one metal of Groups 3, 4, 5, 6, and 7, so-called early transition metals, and may include at least one selected from the group consisting of Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and Mn. X is a carbon atom, a nitrogen atom, or a combination thereof. n is 1 or more and 4 or less. m is greater than n and 5 or less.) A layered material including a layer main body represented by the formula (the layer main body may have a crystal lattice in which each X is located within an octahedral array of M) and a modification or termination T (T is at least one selected from the group consisting of a hydroxyl group, a fluorine atom, a chlorine atom, an oxygen atom, and a hydrogen atom) present on the surface of the layer main body (more specifically, at least one of the two surfaces of the layer main body facing each other). This can be understood as a layered compound, and is also represented as "M m X n T s ", where s is an arbitrary number, and conventionally, x may be used instead of s). Typically, n can be 1, 2, 3, or 4, but is not limited thereto.

[0018] In the above formula of MXene, M is preferably at least one selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and Mn, and more preferably at least one selected from the group consisting of Ti, V, Cr, and Mo.

[0019] MXene is a compound represented by the formula: M m X n However, it is known that it can be expressed as follows: Sc2C, Ti2C, Ti2N, Zr2C, Zr2N, Hf2C, Hf2N, V2C, V2N, Nb2C, Ta2C, Cr2C, Cr2N, Mo2C, Mo 1.3 C, Cr 1.3 C, (Ti,V)2C, (Ti,Nb)2C, W2C, W 1.3 C, Mo2N, Nb 1.3 C, Mo 1.3 Y 0.6 C (in the above formula, "1.3" and "0.6" mean approximately 1.3 (= 4 / 3) and approximately 0.6 (= 2 / 3), respectively), Ti3C2, Ti3N2, Ti3(CN), Zr3C2, (Ti,V)3C2, (Ti2Nb)C2, (Ti2Ta)C2, (Ti2Mn)C2, Hf3C2, (Hf2V)C2, (Hf2Mn)C2, (V2Ti)C2, (Cr2Ti)C2, (Cr2V)C 2, (Cr2Nb)C2, (Cr2Ta)C2, (Mo2Sc)C2, (Mo2Ti)C2, (Mo2Zr)C2, (Mo2Hf)C2, (Mo2V)C2, (Mo2Nb)C2, (Mo2Ta)C2, (W2Ti)C2, (W2Zr)C2, (W2Hf)C2, Ti4N3, V4C3, Nb4C3, Ta4C3, (Ti,Nb)4C3, (Nb,Zr)4C3, (Ti2Nb2)C3, (Ti2Ta2)C3, (V2Ti2)C3, (V2Nb2)C3, (V2Ta2)C3, (Nb2Ta2)C3, (Cr2Ti2)C3, (Cr2V 2)C3, (Cr2Nb2)C3, (Cr2Ta2)C3, (Mo2Ti2)C3, (Mo2Zr2)C3, (Mo2Hf2)C3, (Mo2V2)C3, (Mo2Nb2)C3, (Mo2Ta2)C3, (W2Ti2)C3, (W2Zr2)C3, (W2Hf2)C3, (Mo 2.7 V 1.3 ) C3 (In the above formula, "2.7" and "1.3" mean approximately 2.7 (= 8 / 3) and approximately 1.3 (= 4 / 3), respectively.)

[0020] Representative examples include M m Xn However, Ti2C, Ti3C2, Ti3(CN), (Cr2Ti)C2, (Mo2Ti)C2, (Mo2Ti2)C3, and (Mo 2.7 V 1.3 ) C3.

[0021] Such MXene particles (hereinafter simply referred to as "MXene particles") 10 can be synthesized by selectively etching (removing and optionally separating) A atoms (and optionally some M atoms) from the MAX phase. The MAX phase has the following formula: M m AX n (wherein M, X, n, and m are as defined above, and A is at least one Group 12, 13, 14, 15, or 16 element, usually a Group A element, typically Group IIIA or Group IVA, and more particularly may include at least one element selected from the group consisting of Al, Ga, In, Tl, Si, Ge, Sn, Pb, P, As, S, and Cd, and is preferably Al). and M m X n (each X may have a crystal lattice in which it is located in an octahedral array of M) and a layer composed of A atoms is located between them. In the MAX phase, typically when m=n+1, one layer of X atoms is arranged between each of the n+1 layers of M atoms (these are collectively referred to as "M m X n The MAX phase has a repeating unit in which a layer of A atoms (also referred to as an "A layer") is arranged as a layer next to the n+1th layer of M atoms, but is not limited to this. By selectively etching (removing and optionally separating) the A atoms (and optionally some of the M atoms) from the MAX phase, the A atom layer (and optionally some of the M atoms) is removed to expose the M atoms. m X n The surface of the layer is modified with hydroxyl groups, fluorine atoms, chlorine atoms, oxygen atoms, hydrogen atoms, etc. present in the etching solution (usually, an aqueous solution of fluorine-containing acid is used, but this is not limited to this) to terminate the surface.

[0022] The above etching may be performed by etching with acids such as HF, HCl, HBr, HI, sulfuric acid, phosphoric acid, nitric acid, etc. For example, it may be a method using a mixed solution of lithium fluoride and hydrochloric acid, a method using hydrofluoric acid, etc. Thereafter, layer separation of MXene (delamination, separating multilayer MXene into single-layer MXene) may be promoted by appropriately performing any suitable post-treatment (for example, ultrasonic treatment, hand shaking or an automatic shaker, etc.). For example, delamination treatment can be performed for a predetermined time using a mechanical shaker, a vortex mixer, a homogenizer, an ultrasonic bath, etc.

[0023] In the present invention, MXene may contain a relatively small amount of residual A atoms, for example, 10% by mass or less with respect to the original A atoms. The residual amount of A atoms may preferably be 8% by mass or less, more preferably 6% by mass or less. However, even if the residual amount of A atoms exceeds 10% by mass, there may be no problem depending on the use and usage conditions of the Schottky barrier diode.

[0024] The thus synthesized MXene particles 10 can be used as a raw material for the conductive portion 13 of the present embodiment. The MXene particles 10 are particles of a layered material containing one or more MXene layers 7a, 7b (as an example of the MXene particles 10, one-layer MXene particles 10a are shown in Fig. 3(a) and two-layer MXene particles 10b are shown in Fig. 3(b), but it is not limited to these examples). More specifically, the MXene layers 7a, 7b have a layer main body (M m X n represented by M m X n layer) 1a, 1b and modifications or terminations T 3a, 5a, 3b, 5b present on the surfaces of the layer main bodies 1a, 1b (more specifically, at least one of the two surfaces facing each other in each layer). Therefore, the MXene layers 7a, 7b are "M m X n T sIt can also be expressed as "」, and s is an arbitrary number. The MXene particles 10 can be those in which such MXene layers are individually separated and exist as a single layer (a monolayer structure shown in Fig. 3(a), so-called monolayer MXene particles 10a), a laminate in which a plurality of MXene layers are spaced apart from each other and laminated (a multilayer structure shown in Fig. 3(b), so-called multilayer MXene particles 10b), or a mixture thereof. The MXene particles 10 can be particles (which can also be referred to as powders or flakes) as an aggregate composed of monolayer MXene particles 10a and / or multilayer MXene particles 10b. In the case of multilayer MXene particles, two adjacent MXene layers (for example, 7a and 7b) do not necessarily have to be completely separated and may be in partial contact.

[0025] Although not limiting this embodiment, the thickness of each layer of MXene (corresponding to the above-mentioned MXene layers 7a, 7b) is, for example, 0.8 nm or more and 5 nm or less, particularly 0.8 nm or more and 3 nm or less (mainly depending on the number of M atomic layers contained in each layer), and the maximum dimension (which can correspond to the "in-plane dimension" of the particle) in a plane parallel to the layer (two-dimensional sheet plane) is, for example, 0.1 μm or more, particularly 1 μm or more, for example, 200 μm or less, particularly 40 μm or less.

[0026] When the MXene particles are laminate (multilayer MXene) particles, the interlayer distance (or void dimension, indicated by Δd1 in Fig. 3(b)) inside each individual laminate particle is not particularly limited in the state before forming the conductive portion 13, for example, 0.8 nm or more and less than 10 nm, particularly 0.8 nm or more and 5 nm or less, more particularly about 1 nm, and the maximum dimension (which can correspond to the "in-plane dimension" of the particle) in a plane perpendicular to the lamination direction (two-dimensional sheet plane) is, for example, 0.1 μm or more, particularly 1 μm or more, for example, 100 μm or less, particularly 20 μm or less.

[0027] The total number of layers in the MXene particles may be 1 or 2 or more, for example, 1 or more and 20 or less, and the thickness in the lamination direction (which can correspond to the "thickness" of the particle) is, for example, 0.8 nm or more and 20 nm or less.

[0028] Incidentally, each of the above dimensions can be determined as a number-average dimension (e.g., number-average of at least 40) based on a photograph taken by a scanning electron microscope (SEM), a transmission electron microscope (TEM), or an atomic force microscope (AFM), or as a distance in real space calculated from the position on the reciprocal lattice space of the (002) plane measured by the X-ray diffraction (XRD) method.

[0029] The conductive portion 13 of this embodiment can be formed by disposing such MXene particles 10 (which may be single-layer MXene particles, multi-layer MXene particles, and mixtures thereof) on the surface 11a of the semiconductor portion 11 and subjecting them to heat treatment as appropriate if necessary (see FIG. 2). The method of disposing the MXene particles 10 on the surface 11a of the semiconductor portion 11 is not particularly limited. For example, a slurry in which the MXene particles synthesized as described above are dispersed and / or suspended in an appropriate solvent is prepared, the slurry is applied to the surface 11a of the semiconductor portion 11, and the solvent is (at least partially, preferably substantially entirely) dried and removed. The application of the slurry may be carried out by spraying, spin casting, or the blade method. Drying can be natural drying and / or hot air drying, etc. When heat treatment is carried out, the heat treatment conditions can be set as appropriate. For example, it may be carried out at a maximum temperature of 30°C or higher and 500°C or lower under vacuum (a pressure lower than atmospheric pressure, for example, a pressure of 0.01 MPa (absolute pressure) or less). The heat treatment time can vary depending on the applied pressure and temperature. The oxygen partial pressure (oxygen concentration) of the ambient atmosphere during heat treatment does not need to be strictly controlled and is not particularly limited.

[0030] The work function of MXene is given by the above formula: M m X nIt can vary greatly depending on the material type of the layer body represented by and the modification / termination T. For example, as understood from Fig. 4 of Non-Patent Document 4, it is understood that the work function of MXene can be controlled within the range of about 1.6 eV or more and about 8 eV or less by adjusting the material type of the layer body and the modification / termination T. For example, when the layer body is Ti3C2 and MXene particles having at least a hydroxyl group as the modification / termination T are used as raw materials, by disposing such MXene particles on the semiconductor portion 11 and subjecting them to heat treatment, the hydroxyl group changes to an oxygen atom, and thus the work function of MXene in the resulting conductive portion 13 can be controlled.

[0031] On the other hand, the oxide semiconductor constituting the semiconductor portion 11 is appropriately selected according to the MXene constituting the conductive portion 13 and the diode characteristics desired for the Schottky barrier diode. Such an oxide semiconductor may contain impurities (intentionally added dopants and / or trace elements inevitably mixed in).

[0032] The oxide semiconductor can be selected so as not to contain highly toxic elements such as As, and thus the finally obtained Schottky barrier diode 20 can be made low-toxic, preferably non-toxic. In addition, the oxide semiconductor can be commercially available and / or manufactured at a lower cost compared to a high-purity GaN single crystal, and thus the finally obtained Schottky barrier diode 20 can be manufactured at a relatively low cost. Furthermore, since the oxide semiconductor already forms a stable oxide, even when exposed to an oxidizing environment during the manufacturing process of the Schottky barrier diode 20 (for example, when heat treatment is performed as described above), the electrical characteristics of the oxide semiconductor can be maintained, and thus stable diode characteristics can be easily obtained. In contrast, in the case of Si or a compound semiconductor, surface oxidation occurs in an oxidizing environment, and the semiconductor may be altered and its electrical characteristics may change. Therefore, in order to prevent surface oxidation, special operations such as strictly controlling the oxygen partial pressure in the ambient atmosphere during heat treatment are required.

[0033] The oxide semiconductor can be selected in consideration of the work function of the MXene constituting the conductive portion 13. Whether a diode can be formed is determined by the magnitude relationship between the work function of the oxide semiconductor and the work function of the MXene. More specifically, an n-type oxide semiconductor having a work function smaller than that of the MXene, or a p-type oxide semiconductor having a work function larger than that of the MXene can be used.

[0034] The n-type oxide semiconductor may include, for example, at least one selected from the group consisting of ZnO, TiO2, SnO2, CeO2, In2O3, Ga2O3, BaTiO3, and SrTiO3. Typically, it may be SrTiO3 doped with Nb (which may be denoted as "Nb:SrTiO3" or "NSTO", etc.). The p-type oxide semiconductor may include, for example, at least one selected from the group consisting of NiO, Cu2O, CuO, CoO, Mn3O4, SnO, and ZnCo2O4. Typically, it may be NiO, Cu2O, etc. For these exemplified oxide semiconductors, Table 1 shows the work function when undoped (when the doping amount is zero) and the work function when highly doped (when the work function hardly changes any further even if the doping amount is further increased). Referring to Table 1, it is understood that the work function of each oxide semiconductor can be controllably adjusted as desired within the range between the work function when undoped and the work function when highly doped by adjusting the doping amount. In Table 1, TiO2(R) means rutile-type TiO2, and TiO2(A) means anatase-type TiO2.

[0035]

Table 1

[0036] As described above, in the Schottky barrier diode 20 of the present embodiment, the surface roughness of the surface 11a of the semiconductor portion 11 that is joined to the conductive portion 13 is 10 nm or less, and the distance between two adjacent MXene layers in the conductive portion 13 is 1.10 nm or more. As a result, a Schottky barrier diode 20 with a small diode ideality factor and a small reverse saturation current (in other words, a high rectification ratio) is realized. The diode ideality factor (n) can be, for example, 8 or less, preferably 5 or less, and the lower limit is not particularly limited, but theoretically it is 1 or more (it is desirable that the diode ideality factor (n) is closer to 1). The reverse saturation current (I0) is, for example, 5×10 -4 A or less, preferably 2.00×10 -4 A or less, and the lower limit is not particularly limited, but theoretically it is a value greater than 0 (it is desirable that the reverse saturation current (I0) is closer to 0).

[0037] Although the present invention is not restricted by any theory, it can be considered as follows. First, since the surface roughness of the surface 11a of the semiconductor portion 11 is 10 nm or less, the coverage of the surface 11a by the MXene particles 10 in the conductive portion 13 formed in contact therewith becomes high. As a result, the ratio of the region where actual contact is formed between the semiconductor portion 11 and the MXene particles 10 to the region where contact is intended between the semiconductor portion 11 and the conductive portion 13 increases significantly, and extremely good contact is obtained between the semiconductor portion 11 and the conductive portion 13. Second, in the conductive portion 13, since the distance between two adjacent MXene layers is 1.10 nm or more, the MXene particles 10 can exist with high orientation in the conductive portion 13, and the conductivity of the conductive portion 13 composed of the MXene particles 10 increases. As a result of these combined actions, it is considered that a Schottky barrier diode 20 with a small diode ideality factor and a small reverse saturation current (in other words, a high rectification ratio) is realized.

[0038] On the other hand, when the surface roughness of the surface 11a of the semiconductor portion 11 is greater than 10 nm, the coverage of the surface 11a by the MXene particles 10 becomes low, and good contact cannot be obtained between the semiconductor portion 11 and the conductive portion 13, resulting in an ohmic contact. As a result, even when attempting to extract diode characteristics by fitting the current-voltage curve with an exponential function, the current-voltage curve becomes linear and cannot be fitted with an exponential function, failing to function as a diode. Also, in the conductive portion 13, when the distance between two adjacent MXene layers is less than 1.10 nm, the orientation of the MXene particles 10 in the conductive portion 13 becomes low, the conductivity of the conductive portion 13 composed of the MXene particles 10 decreases, and the coverage of the surface 11a by the MXene particles 10 also decreases. As a result, even when attempting to extract diode characteristics by fitting the current-voltage curve with an exponential function, the current-voltage curve becomes linear and cannot be fitted with an exponential function, failing to function as a diode.

[0039] The surface roughness of the surface 11a of the semiconductor portion 11 is determined as follows. The cross-section of the semiconductor portion 11 (the conductive portion 13 may or may not be present on the semiconductor portion 11) is exposed, and this cross-section is observed with a field emission scanning electron microscope (FE-SEM). The thickness of the semiconductor portion 11 is measured at ten points, and the root mean square height Rq (the square root of the average of the squares of the deviations from the average line to the measurement curve) is obtained. This Rq is taken as the surface roughness.

[0040] In the conductive portion 13, the distance between two adjacent MXene layers (also simply referred to as the "interlayer distance of MXene" in this specification) means the distance between any two adjacent MXene layers, and the two MXene layers may belong to different MXene particles or the same MXene particle. When the two MXene layers belong to different MXene particles, the interlayer distance corresponds to the distance Δd2 between a layer of one MXene particle 10 (which may be either a single-layer MXene particle or a multi-layer MXene particle, and in the case of a multi-layer MXene particle, the outermost layer) and a layer of another MXene particle 10 adjacent to this layer (as above) as shown in Fig. 2 (in Fig. 2, only single-layer MXene particles are shown for convenience, but the MXene particles 10 may be either single-layer MXene particles or multi-layer MXene particles, or a mixture thereof). When the two MXene layers belong to the same MXene particle, the interlayer distance corresponds to the interlayer distance Δd1 inside the multi-layer MXene particle 10b as shown in Fig. 3. The "interlayer distance of MXene" is an inclusive value obtained by combining these cases.

[0041] The interlayer distance of MXene is determined as follows. The conductive portion 13 is measured by the X-ray diffraction (XRD) method to obtain the XRD profile of the θ-axis direction scan (the vertical axis is intensity and the horizontal axis is 2θ, generally referred to as the "XRD profile"), and based on the peak derived from the (002) plane of MXene, the interplanar spacing of the (002) plane is obtained from Bragg's equation. This interplanar spacing of the (002) plane is taken as the interlayer distance of MXene.

[0042] The orientation of the MXene particles 10 in the conductive portion 13 can be understood based on the full width at half maximum of the peak derived from the (002) plane of MXene obtained by X-ray diffraction measurement of the conductive portion 13. The smaller this full width at half maximum, the higher the orientation of the MXene particles 10. The full width at half maximum of the peak derived from the (002) plane of MXene is obtained from the above XRD profile.

[0043] The smaller interlayer distance of MXene means that there are fewer impurities such as moisture, which is preferable in that it can increase the conductivity of the conductive portion 13. For example, by performing the heat treatment as described above, moisture that may be present between the layers can be appropriately removed, and the interlayer distance of MXene can be made smaller. As a result, when the conductivity of the conductive portion 13 is improved, the series resistance component (Rs) of the equivalent circuit of the Schottky barrier diode 20 can be reduced. However, if the heat treatment is carried out excessively, although the interlayer distance of MXene becomes smaller, traces of removed moisture remain in the conductive portion **********13, and the orientation of the MXene particles 10 becomes lower. As a result, even when trying to extract diode characteristics by fitting the current-voltage curve with an exponential function, the current-voltage curve becomes linear and cannot be fitted with an exponential function, and it does not function as a diode. From this perspective, the interlayer distance of MXene is set to 1.10 nm or more to ensure high orientation. The interlayer distance of MXene may be 1.10 nm or more, but can be, for example, 2.00 nm or less, particularly 1.50 nm or less, and can be 1.25 nm or less, particularly 1.20 nm or less when a particularly low series resistance component is desired.

[0044] Furthermore, when a Schottky barrier diode with excellent high-frequency characteristics is desired, it is preferable that the turn-on voltage is low. According to the present embodiment, the turn-on voltage can also be lowered, and thus, the Schottky barrier diode 20 with excellent high-frequency characteristics is realized. The dominant factor of the turn-on voltage of the Schottky barrier diode is the difference between the work function of MXene constituting the conductive portion 13 and the work function of the oxide semiconductor constituting the semiconductor portion 11, and the smaller such work function difference is, the smaller the turn-on voltage can be. As described above, the work function of MXene can be controlled by adjusting the material type of the layer main body and the modification / termination T, and the work function of the oxide semiconductor can also vary. Therefore, by selecting an appropriate oxide semiconductor and controlling the work function of MXene, a Schottky barrier diode 20 with a low turn-on voltage and excellent high-frequency characteristics can be obtained.

[0045] The Schottky barrier diode in one embodiment of the present invention has been described in detail above, but the present invention can be variously modified. It should be noted that the Schottky barrier diode of the present invention may be manufactured by a method different from the manufacturing method in the above-described embodiment.

Example

[0046] <Example 1> (Operation 1-1) As the semiconductor part 11, a substrate made of NSTO single crystal (manufactured by Shin-Etsu Chemical Co., Ltd., SrTiO3 substrate doped with Nb: Nb doping amount 0.5 wt% (about 1.0 at%), planar dimensions 15 mm × 15 mm and thickness 0.5 mm, single-sided polishing, plane orientation (001)) was commercially obtained and heat-treated in an oxygen atmosphere at 900 °C for 10 minutes using an infrared high-speed heating device (manufactured by Advance Riko Co., Ltd., QH-E68VHT).

[0047] (Operation 1-2) The semiconductor part 11 that had undergone the above Operation 1-1 was cut to expose the cross-section, and the surface roughness Rq was obtained by measuring the surface (polished surface) of the semiconductor part using FE-SEM (manufactured by Hitachi High-Technologies Corporation, S-5000). The results are shown in Table 2.

[0048] (Operation 1-3) TiC powder, Ti powder, and Al powder (all manufactured by High Purity Chemical Laboratories, Inc.) were put into a ball mill containing zirconia balls in a molar ratio of 2:1:1 and mixed for 24 hours. The obtained mixed powder was fired at 1350 °C for 2 hours under an Ar atmosphere. The fired body (block) thus obtained was pulverized with an end mill to a maximum size of 40 μm or less. Thereby, Ti3AlC2 particles (powder) were obtained as MAX particles.

[0049] The Ti3AlC2 particles (powder) obtained above were added to 9 mol / L hydrochloric acid together with LiF (1 g of LiF and 10 mL of 9 mol / L hydrochloric acid per 1 g of Ti3AlC2 particles), and stirred with a stirrer at 35 °C for 24 hours to obtain a solid component (Ti3C2T) derived from the Ti3AlC2 particless A solid-liquid mixture (suspension) containing particles was obtained. In contrast, operations of washing with pure water and separating and removing the supernatant by decantation using a centrifuge (washing the remaining sediment again) were repeatedly performed about 10 times. Then, a mixture obtained by adding pure water to the sediment was stirred with an automatic shaker for 15 minutes. As a result, a roughly purified MXene (Ti3C2T s ) slurry was obtained. This roughly purified MXene slurry was subjected to a centrifugation operation using a centrifuge, and unnecessary portions were separated and removed as appropriate to obtain an MXene (Ti3C2T s )-aqueous dispersion.

[0050] 1 g of the MXene-aqueous dispersion obtained above was diluted with pure water so that the solid content concentration became 2 mg / mL. The diluted MXene-aqueous dispersion was ultrasonically treated in an ice bath with an ultrasonic cleaner (manufactured by SND Co., Ltd., US-4) for 15 minutes.

[0051] (Operation 1-4) A shadow mask having an opening with a size of 2 mm × 2 mm was placed on the surface (polished surface) of the semiconductor portion 11 that had undergone the above Operation 1-1. With respect to the opening, the MXene-aqueous dispersion obtained in the above Operation 1-3 was sprayed at an air pressure of 0.40 MPa (absolute pressure) using a commercially available airbrush (manufactured by Tamiya Co., Ltd., Spray Work HG Airbrush Wide (Trigger Type), Airbrush System No. 53 Spray Work Power Compressor 74553). As a result, a precursor layer containing MXene (Ti3C2T s ) particles was formed on the region corresponding to the opening on the surface of the semiconductor portion 11. Warm air was blown onto the precursor layer on the semiconductor portion 11 using a hand dryer (manufactured by Panasonic Corporation, EH5206P-A) to dry the precursor layer.

[0052] (Operation 1-5) The spraying and drying operations in the above Operation 1-4 were repeated. Then, the shadow mask was removed. As a result, MXene (Ti3C2T sA conductive portion 13 made of .DELTA.) particles was formed on the semiconductor portion 11.

[0053] (Operation 1-6) The bonded structure of the semiconductor portion 11 and the conductive portion 13 obtained in the above steps 1-5 was measured for the XRD profile of the conductive portion 13 using an XRD device (MiniFlex, manufactured by Rigaku Corporation) (characteristic X-rays: CuKα=1.54 Å). s Based on the peak originating from the (002) plane (peak appearing at 2θ = approximately 4 to 10°), the interlayer distance of MXene was calculated as the interplanar spacing of the (002) plane using the Bragg equation. Furthermore, the half-width of the peak originating from the (002) plane was calculated. These results are shown in Table 2.

[0054] (Operation 1-7) Referring to FIG. 4, a substrate 30 having a silicon oxide (SiO) layer 32 and a Pt layer 33 on a silicon (Si) substrate 31 was formed with an InGa layer as a metal layer 34 on a predetermined region of the Pt layer 33. Furthermore, a junction structure of the semiconductor portion 11 (NSTO single crystal in Example 1) and the conductive portion 13 obtained in steps 1-5 above was placed on top of the InGa layer. This resulted in a Schottky barrier diode sample of Example 1 (FIG. 4). Two wires (output and ground) of a probe potentiometer (2400 series, manufactured by Keithley Instruments, Inc.) were connected to two metal probes on a sample stage (UMC-2, manufactured by MMR Technologies, Inc.). The sample was placed on the sample stage, and an optical microscope (SZ40, manufactured by Olympus Corporation) was used to bring the metal probe (output) into contact with the semiconductor portion 13 and the other metal probe (ground) into contact with the Pt layer 33, electrically connecting the metal layer 34 to ground. The potentiometer was used to measure the current while sweeping the linear voltage to obtain a current-voltage curve.

[0055] (Operations 1-8) Using the Cheung method (the method described in Non-Patent Document 5), the diode ideality factor n (-), series resistance Rs (Ω), reverse saturation current I0 (A), and Schottky barrier Φ0 (eV) were derived from the current-voltage curve (I-V characteristics) obtained in the above operations 1-7. Subsequently, the work function MX-WF (eV) of MXene was derived using the Schottky-Mott equation. When determining the work function MX-WF of MXene, the electron affinity of NSTO, 3.9 eV, was applied. Also, in the current-voltage curve (I-V characteristics) obtained in the above operations 1-7, the voltage when the current reached 0.1 mA was defined as the turn-on voltage (eV). These results are shown in Table 2.

[0056] <Example 2> (Operation 2-1) The same operations as operations 1-1 to 1-5 described above in Example 1 were performed.

[0057] (Operation 2-2) The joined structure of the semiconductor part 11 and the conductive part 13 obtained in the above operation 2-1 was vacuum heat-treated by heating it to 200 °C at 100 Pa (absolute pressure) using a vacuum isothermal dryer (manufactured by Advantec, DRV320DC), and immediately cooling it (i.e., with a holding time of 0 hours) after reaching 200 °C. Then, after natural cooling until the temperature inside the vacuum isothermal dryer became 40 °C or lower, the joined structure was taken out of the vacuum isothermal dryer.

[0058] (Operation 2-3) The same operations as operations 1-6 to 1-8 described above in Example 1 were performed.

[0059] <Example 3> (Operation 3-1) The same operations as operations 1-1 to 1-5 described above in Example 1 were performed.

[0060] (Operation 3-2) The same operation as operation 2-2 described above in Example 2 was performed, except that the holding time was set to 2 hours.

[0061] (Operation 3-3) The same operations as those in Steps 1-6 to 1-8 described above in Example 1 were carried out.

[0062] Example 4 (Operation 4-1) As the semiconductor portion 11, a substrate made of NiO single crystal (manufactured by Crystal Base Co., Ltd., planar dimensions 10 mm × 10 mm, thickness 0.5 mm, one side polished, surface orientation (001)) was commercially available and heat-treated in an oxygen atmosphere at 900°C for 10 minutes using an infrared high-speed heating device (manufactured by Advance Riko Co., Ltd., QH-E68VHT).

[0063] (Step 4-2) The same operations as operations 1-2 to 1-5 described above in Example 1 were carried out, then the same operation as operation 2-2 described above in Example 2 was carried out, and then the same operation as operation 1-6 described above in Example 1 was carried out.

[0064] (Step 4-3) 4, for the bonded structure of semiconductor portion 11 (NiO single crystal in Example 4) and conductive portion 13 obtained in step 4-2 above, an Au layer was vapor-deposited as a metal layer 34 on the surface of semiconductor portion 11 opposite conductive portion 13, and the resulting structure was placed on substrate 30, which had a silicon oxide (SiO2) layer 32 and a Pt layer 33 on a silicon (Si) substrate 31, as shown in the figure. In this way, a Schottky barrier diode sample of Example 4 was obtained (FIG. 4). Two wires (output side and ground side) of a probe potentiometer (2400 series, manufactured by Keithley Instruments, Inc.) were connected to two metal probes of a sample stage (UMC-2, manufactured by MMR Technologies, Inc.), respectively. The sample was placed on a sample stage, and using an optical microscope (Olympus Corporation, SZ40), a metal probe (output side) was brought into contact with Pt layer 33 to electrically connect it to metal layer 34, and another metal probe (ground side) was brought into contact with semiconductor portion 13 to electrically connect it to ground. Using the potentiometer, the current was measured while linearly sweeping the voltage, and a current-voltage curve was obtained.

[0065] (Step 4-4) The same operations as in Operation 1-8 described above in Example 1 were carried out. Note that, when determining the work function MX-WF of MXene, the ionization potential of NiO, 5.1 eV, was applied.

[0066] <Example 5> (Operation 5-1) The same operations as in Steps 4-1 to 4-4 described above in Example 4 were performed, except that a substrate made of CuO single crystal (manufactured by Crystal Base Co., Ltd., planar dimensions 10 mm × 10 mm, thickness 0.5 mm, one side polished, surface orientation (001)) was used as the semiconductor portion 11. Note that the ionization potential of CuO, 5.3 eV, was used to determine the work function MX-WF of MXene.

[0067] <Comparative Example 1> (Operation 6-1) The same operations as those in Steps 1-1 to 1-5 described above in Example 1 were carried out.

[0068] (Operation 6-2) The same procedure as in Operation 2-2 described above in Example 2 was carried out, except that the holding time was changed to 8 hours.

[0069] (Operation 6-3) The same operations as in Steps 1-6 to 1-8 described in Example 1 were performed. In Comparative Example 1, the current-voltage curve (IV characteristics) was linear and could not be fitted with an exponential function, and various diode characteristics (diode ideality factor n, series resistance Rs, reverse saturation current I0, Schottky barrier Φ0, and turn-on voltage) could not be derived, and the work function MX-WF (eV) of MXene could not be derived. Therefore, in Table 2, these are indicated by "N / A."

[0070] <Comparative Example 2> (Operation 7-1) Referring to FIG. 5, quartz glass substrate 30' was subjected to UV ozone plasma treatment using an excimer lamp (H0011, manufactured by Ushio Inc.) to clean the surface of substrate 30'.

[0071] (Operation 7-2) An aqueous solution containing Sr and Ti was prepared by adding anhydrous strontium nitrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), titanium oxide (Degussa P25), and 1M aqueous sodium hydroxide solution to pure water. Niobium chloride was added to this aqueous solution so that the Nb content in the synthesized NSTO was 0.5 wt%. An aqueous solution containing Sr, Ti, and Nb was obtained.

[0072] (Operation 7-3) The substrate 30' obtained in step 7-1 above was placed on a spin coater (Mikasa Corporation, 1H360S), and the aqueous solution containing Sr, Ti, and Nb obtained in step 7-2 above was applied to the substrate 30' through a filter (Whatman plc, polypropylene-coated polytetrafluoroethylene, pore size 0.2 μm) to form a liquid film of the aqueous solution. The spin coating conditions were 600 rpm for less than 2 seconds from the start of application, and then increased to 2000 rpm after 2 seconds, for a total of 6 seconds. The substrate 30' on which the liquid film had been formed was placed on a hot plate at 120°C for 30 seconds to evaporate the solvent, and then heated at 120°C for an additional 1 minute while still on the hot plate.

[0073] (Operation 7-4) The substrate 30' with the coating film obtained in the above operation 7-3 was heat-treated in an oxygen atmosphere at 600°C for 10 minutes using an infrared high-speed heating device (QH-E68VHT, manufactured by Advance Riko Co., Ltd.), thereby crystallizing the coating film.

[0074] (Operation 7-5) The substrate 30' with the crystallized film obtained in step 7-4 above was heat-treated in a nitrogen atmosphere at 800°C for 10 minutes using an infrared high-speed heating device (QH-E68VHT, manufactured by Advance Riko Co., Ltd.). As a result, a film made of NSTO polycrystal (Nb-doped SrTiO: Nb doping amount 0.5 wt% (approximately 1.0 atomic %), thickness 500 nm) was formed as the semiconductor portion 61 on the quartz glass substrate 30'.

[0075] (Operation 7-6) The same operations as those in the steps 1-2 to 1-6 described above in Example 1 were carried out, except that the semiconductor portion 61 was used instead of the semiconductor portion 11.

[0076] (Operation 7-7) Referring to FIG. 5, an InGa layer was formed as metal layer 35 on a predetermined region of semiconductor portion 61 (NSTO polycrystalline film) obtained in step 7-6 above. This resulted in a Schottky barrier diode sample of Comparative Example 2 (FIG. 5). Two wires (output side and ground side) of a probe potentiometer (2400 series, manufactured by Keithley Instruments, Inc.) were connected to two metal probes on a sample stage (UMC-2, manufactured by MMR Technologies, Inc.), respectively. The sample was placed on the sample stage, and an optical microscope (SZ40, manufactured by Olympus Corporation) was used to bring the metal probe (output side) into contact with semiconductor portion 13, and the other metal probe (ground side) into contact with metal layer 35, electrically connected to ground. Using the potentiometer, a current was measured while linearly sweeping the voltage, and a current-voltage curve was obtained.

[0077] (Operation 7-8) The same procedures as in Procedures 1-8 described above in Example 1 were carried out.

[0078] <Comparative Example 3> (Operation 8-1) Referring to FIG. 6, a substrate 30 having a silicon oxide (SiO2) layer 32 and a Pt layer 33 on a silicon (Si) substrate 31 was subjected to UV ozone plasma treatment using an excimer lamp (manufactured by USHIO INC., H0011) to clean the surface of the substrate.

[0079] (Operation 8-2) Nickel acetate tetrahydrate (manufactured by FUJIFILM Wako Pure Chemical Corporation) was used as the metal salt, 2-methoxyethanol (manufactured by Nacalai Tesque, Inc.) was used as the solvent, and diethanolamine (manufactured by Nacalai Tesque, Inc.) was used as the complexing agent to prepare a 0.5 M Ni complex solution. At this time, diethanolamine was added so that the molar ratio to Ni was 3.

[0080] (Operation 8-3) An operation similar to Operation 7-3 described above in Comparative Example 2 was performed except that the substrate 30 obtained in the above Operation 8-1 and the Ni complex solution obtained in the above Operation 8-2 were used.

[0081] (Operation 8-4) An operation similar to Operation 7-4 described above in Comparative Example 2 was performed. As a result, a film (thickness: 500 nm) made of NiO polycrystals was formed as the semiconductor portion 61 on the substrate 30.

[0082] (Operation 8-5) An operation similar to Operation 4-2 described above in Example 4 was performed. As a result, a sample of the Schottky barrier diode of Comparative Example 3 was obtained (FIG. 6).

[0083] (Operation 8-6) Two wires (output side and ground side) of a probe potentiometer (manufactured by Keithley Instruments, Inc., Series 2400) were respectively connected to two metal probes of a sample stage (manufactured by MMR Technologies, Inc., UMC-2). The above sample was placed on the sample stage, and using an optical microscope (manufactured by Olympus Corporation, SZ40), the metal probe (output side) was brought into contact with the Pt layer 33 for electrical connection, and the other metal probe (ground side) was brought into contact with the semiconductor portion 13 for electrical ground connection. Using the above potentiometer, the current was measured while linearly sweeping the voltage to obtain a current-voltage curve.

[0084] (Operation 8-7) The same operation as Operation 4-4 described above in Example 4 was performed.

[0085] <Comparative Example 4> (Operation 9-1) The same operation as Operation 8-1 described above in Comparative Example 3 was performed.

[0086] (Operation 9-2) The same operation as Operation 8-2 described above in Comparative Example 3 was performed, except that copper(II) acetate tetrahydrate (manufactured by FUJIFILM Wako Pure Chemical Corporation) was used as the metal salt. Note that diethanolamine was added so that the molar ratio to Cu was 3.

[0087] (Operation 9-3) The same operation as Operation 8-3 described above in Comparative Example 3 was performed, except that the Cu complex solution obtained in the above Operation 9-2 was used.

[0088] (Operation 9-4) The same operation as Operation 8-4 described above in Comparative Example 3 was performed. As a result, a film (thickness: 500 nm) composed of Cu2O polycrystals was formed as the semiconductor portion 61 on the substrate 30. Thereafter, the same operations as Operations 8-5 to 8-6 described above in Comparative Example 3 were performed.

[0089] (Operation 9-5) The same operations as operations 1-8 described in Example 1 were performed. When determining the work function MX-WF of MXene, the ionization potential of Cu2O, 5.3 eV, was applied.

[0090]

Table 2

[0091] Referring to Table 2, in the samples of the Schottky barrier diodes fabricated in Examples 1-5, the interlayer distance of MXene was 1.10 nm or more, and the surface roughness of the semiconductor portion was 10 nm or less. A Schottky barrier was formed between the semiconductor portion and the conductive portion composed of MXene particles, and it was confirmed that the samples functioned as diodes. The samples of Examples 1-5 had a small diode ideality factor, a small reverse saturation current, and a low turn-on voltage. Among them, the sample of Example 1 had an extremely small diode ideality factor. The samples of Examples 2-5 had an extremely small series resistance.

[0092] On the other hand, in the sample of Comparative Example 1, the interlayer distance of MXene was less than 1.10 nm. In the samples of Comparative Examples 2-4, the surface roughness of the semiconductor portion exceeded 10 nm. In the samples of Comparative Examples 1-4, the current-voltage curve was linear and could not be fitted with an exponential function, and they did not function as diodes. This is presumably because in the samples of Comparative Examples 1-4, the contact between the semiconductor portion and the conductive portion composed of MXene particles was ohmic contact.

Industrial Applicability

[0093] The Schottky barrier diode of the present invention can be used in any suitable application as an electronic component utilizing a Schottky junction.

Explanation of Reference Numerals

[0094] 1a, 1b Layer body (M m X n Layer) 3a, 5a, 3b, 5b Modified or terminal T 7a, 7b MXene layers 10, 10a, 10b MXene (layered material) particles 11 Semiconductor part 11a surface 13 Conductive part (composed of MXene particles) 15a, 15b Contact 20 Schottky barrier diode 30 substrates (Pt / SiO2 / Si substrates) 30' substrate (quartz glass) 31 Silicon substrate 32 silicon oxide layer 33 Pt layer 34 Metal layer 35 Metal parts 61 Semiconductor part (with rough surface)

Claims

**Claim 1** A Schottky barrier diode in which a semiconductor portion and a conductive portion are joined, wherein the semiconductor portion is made of an oxide semiconductor and has a surface roughness of 10 nm or less on the surface joined to the conductive portion, the conductive portion is made of particles of a layered material including one or more layers, the layer has the following formula: M ｍ X ｎ (wherein M is at least one Group 3, 4, 5, 6, or 7 metal, X is a carbon atom, a nitrogen atom, or a combination thereof, n is 1 or more and 4 or less, m is greater than n and 5 or less) and includes a layer body represented by the formula and a modification or termination T (T is at least one selected from the group consisting of a hydroxyl group, a fluorine atom, a chlorine atom, an oxygen atom, and a hydrogen atom) present on the surface of the layer body, and in the conductive portion, the distance between two adjacent layers is 1.10 nm or more, the Schottky barrier diode. **Claim 2** The Schottky barrier diode according to claim 1, wherein the oxide semiconductor is an n-type oxide semiconductor having a work function smaller than the work function of the layered material. **Claim 3** The n-type oxide semiconductor is ZnO, TiO ２ , SnO ２ , CeO ２ , In ２ O ３ , Ga ２ O ３ , BaTiO ３ , and SrTiO ３ The Schottky barrier diode according to claim 2, comprising at least one selected from the group consisting of **Claim 4** The Schottky barrier diode according to claim 1, wherein the oxide semiconductor is a p-type oxide semiconductor having a work function larger than the work function of the layered material. **Claim 5** The p-type oxide semiconductor is NiO, Cu ２ O, CuO, CoO, Mn ３ O ４ , SnO and ZnCo ２ O ４ The Schottky barrier diode according to claim 4, comprising at least one selected from the group consisting of.

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