WO2023157968A1

WO2023157968A1 - Method for producing chalcogenide-based atomic layer film

Info

Publication number: WO2023157968A1
Application number: PCT/JP2023/006033
Authority: WO
Inventors: 博幸山川; 慎太朗日向; 敏宏島田
Original assignee: 株式会社アイシン; 国立大学法人北海道大学
Priority date: 2022-02-21
Filing date: 2023-02-20
Publication date: 2023-08-24
Also published as: JP2023121236A

Abstract

This method for producing a chalcogenide-based atomic layer film involves: a base material forming step for forming a metal oxide single crystal base material having a metal oxide atomic layer; and a synthesizing step for generating a chalcogenide-based atomic layer by bringing the metal oxide single crystal base material into contact with a reducible chalcogen gas under a condition at a temperature lower than the melting point of the metal oxide single crystal base material to cause part or whole of the metal oxide atomic layer to undergo topotactic reaction.

Description

Method for producing chalcogenide atomic layer film

The present disclosure relates to a method for manufacturing a chalcogenide atomic layer film.

A chalcogen compound having a chalcogenide atomic layer on its surface (hereinafter referred to as a chalcogenide atomic layer material) is attracting attention as a topological insulator. For example, chalcogenide-based atomic layer materials are attracting attention for application to terahertz detection devices using Dirac electrons of topological insulators. Among them, a film-like chalcogenide-based atomic layer material, that is, a chalcogenide-based atomic layer film can be used, for example, in a high-sensitivity image sensor using terahertz waves. Therefore, there is a demand for a manufacturing method that can easily obtain a chalcogenide-based atomic layer film.

Patent Document 1 discloses that a multi-layered metal oxide single crystal base material is treated with a chalcogen gas under temperature conditions below the lower temperature of the base material melting point and the base material sublimation point and in the range of 500 to 1000 ° C. Disclosed is a method of synthesizing chalcogenide-based multi-atomic layer films by contact with a gas mixture with an inert carrier gas.

Patent Document 2 discloses a method for producing nanoparticles of bismuth telluride as a chalcogenide-based atomic layer material. According to the method disclosed in Patent Document 2, bismuth telluride nanoparticles are prepared by mixing a solution of bismuth chloride and a protective material dissolved in a solvent with an aqueous alkali metal hydroxide solution to prepare a mixed solution, and the mixed solution is added with tellurium. and a reducing agent are placed in a pressure vessel, and the reduction reaction is allowed to occur while stirring the inside of the pressure vessel in a sealed state.

Non-Patent Document 1 discloses a method of synthesizing a maize-like chalcogenide-based multilayer atomic layer material from bismuth halide via a halide-based metal oxide (metal oxyhalide).

JP 2021-161015 A JP-A-2005-343782

J. Zhang et. al., Chinese Science Bulletin, 59, 1787, (2014) DOI: 10.1007/s11434-014-0292-8

(Problems to be solved by the invention)
According to the method described in Patent Document 1, a chalcogenide-based atomic layer film can be produced, but the reaction temperature (synthesis temperature) is as high as 500° C. or higher, so there is still room for improvement. In addition, the method described in Patent Document 2 and the method described in Non-Patent Document 1 cannot produce a chalcogenide-based atomic layer film.

An object of the present invention is to provide a manufacturing method for obtaining a chalcogenide-based atomic layer film at a relatively low temperature.

The present disclosure provides a base material forming step of forming a metal oxide single crystal base material having a metal oxide atomic layer, and a temperature condition lower than the melting point of the metal oxide single crystal base material. a synthesis step of subjecting part or all of the metal oxide atomic layer to a topotactic reaction to produce a chalcogenide atomic layer by contacting it with a reducing chalcogen gas below. A method for manufacturing a membrane is provided.

　The above synthesis process can be configured to be performed in a closed container

In the above synthesis step, the metal oxide single crystal base material may be brought into contact with a reducing chalcogen gas having a concentration of 5 vol % or less. In this case, the concentration of the reducing chalcogen gas is preferably less than 1 vol %.

It is also possible to configure such that the synthesis step is carried out under temperature conditions of 100°C or more and less than 500°C.

The above synthesis step can also be configured to be performed under pressurized conditions. In this case, the synthesis step can also be configured to be carried out under pressure conditions of more than 1 atm and less than or equal to 20 atm, preferably more than 1 atm and less than or equal to 10 atm.

The reducing chalcogen gas may be a hydrogen chalcogenide gas or a mixed gas of chalcogen gas and hydrogen gas. In this case, the reducing chalcogen gas includes hydrogen sulfide gas, hydrogen selenide gas, hydrogen telluride gas, mixed gas of sulfur gas and hydrogen gas, mixed gas of selenium gas and hydrogen gas, and tellurium gas and hydrogen gas. and at least one selected from the group consisting of a mixed gas of

In the above synthesis step, a metal oxide single crystal base material and an aqueous solution containing chalcogen as a source of reducing chalcogen gas are enclosed in a sealed container, and the temperature is lower than the melting point of the metal oxide single crystal base material. It can also be configured to be a step of contacting a metal oxide single crystal base material with a reducing chalcogen gas generated from a chalcogen-containing aqueous solution under a low temperature condition of 100° C. or more and less than 500° C.

The metal oxide single crystal base material may be a bismuth-based metal oxide, and the chalcogenide-based atomic layer film may be a chalcogenide bismuth thin film. In this case, the chalcogenide-based atomic layer film may be one selected from the group consisting of a bismuth sulfide thin film, a bismuth selenide thin film, and a bismuth telluride thin film.

The base material forming step is a step of crystal-growing a metal oxide single crystal base material on a single crystal substrate having an in-plane lattice constant substantially matching the in-plane lattice constant of the metal oxide single crystal base material. It can be configured as follows. Here, "substantially coincident" means that the metal oxide single crystal grows so that the crystal orientation of the single crystal substrate and the crystal orientation of the metal oxide single crystal base material are aligned in the base material forming step. It means that the in-plane lattice constants of both are the same. In this case, the single crystal substrate may be an STO single crystal substrate, and the metal oxide single crystal base material may be bismuth chloride oxide.

According to the present invention, a chalcogenide-based atomic layer film can be produced at a relatively low temperature of, for example, less than 500°C.

FIG. 1 shows a schematic configuration of a mist CVD apparatus that can be used in the base material forming process. FIG. 2 shows a schematic diagram of a reactor that can be used in the synthesis process. FIG. 3 shows the X-ray diffraction pole measurement result of the bismuth chloride oxide thin film formed in Example 1. 4 shows the X-ray diffraction pole measurement result of the STO single crystal substrate used in Example 1. FIG. 5 shows the X-ray diffraction results of the sample produced in Example 2. FIG. FIG. 6 shows the Raman shift of the sample produced in Example 2. FIG. FIG. 7 shows the photocurrent measurement results of the sample produced in Example 2. FIG. FIG. 8 shows the X-ray diffraction results for each sample prepared in Example 2 by changing the concentration of the selenourea aqueous solution.

The chalcogenide-based atomic layer film manufactured by the manufacturing method according to the present embodiment is a film-like body having an atomic layer (chalcogenide-based atomic layer) composed of a compound of chalcogen and metal. Here, the chalcogenide-based atomic layer refers to a layer having a thickness of one or several atoms forming a crystal structure composed of metal and chalcogen, and may be a single layer or multiple layers. good. Each element in one chalcogenide-based atomic layer is strongly bonded by a covalent bond, an ionic bond, or the like. Further, when the chalcogenide-based atomic layers are multi-layered, each atomic layer adjacent in the stacking direction is bonded by a weak force such as van der Waals force. The thickness of one chalcogenide atomic layer is estimated to be approximately 1 nanometer. Therefore, for example, when 6 to 10 chalcogenide atomic layers are laminated, the total thickness of the atomic layers is estimated to be approximately 6 to 10 nanometers.

In addition, the chalcogenide-based atomic layer film manufactured by the manufacturing method according to the present embodiment may be configured in the form of a film by the chalcogenide-based atomic layer alone, or may be configured in the form of a film by the chalcogenide-based atomic layer and other substances. It's okay to be. For example, when a chalcogenide-based atomic layer is formed on the surface of a film-like base material, the chalcogenide-based atomic layer film is composed of the film-like base material and the chalcogenide-based atomic layer formed on the surface of the base material.

The method for manufacturing a chalcogenide-based atomic layer film according to this embodiment includes a base material forming step and a synthesizing step.

In the base material forming step, a metal oxide single crystal base material is formed. This metal oxide single crystal base material has an atomic layer (metal oxide atomic layer) made of metal oxide on the surface. In addition, the metal oxide single crystal base material contains the metal (A), and the metal (A) can be used as the center metal of the crystal lattice of the chalcogenide atomic layer of the chalcogenide atomic layer film, which is the target product. . Molybdenum, tungsten, vanadium, niobium, tantalum, titanium, zirconium, hafnium, bismuth, antimony and germanium as metal (A). Examples include at least one metal selected from the group consisting of tin, gallium, indium, platinum, iron, and iridium.

The crystal structure of the metal oxide single crystal base material is not particularly limited as long as it can form a metal oxide atomic layer. The metal oxide single crystal has a tetragonal structure, a fluorite structure, a corundum structure, a spinel structure, a perovskite structure, an ilmenite structure, a scheelite structure, a _K2NiF structure, a pyrochlore structure, and a magneto It may have a crystal structure selected from the group consisting of plumbite-type structures.

In addition, as such a base material, it is desirable to use a material having a crystal plane with a specific plane orientation on its surface (exposed surface). Furthermore, the surface (exposed surface) of the base material is a plane in which the arrangement of the metal (A) in the specific plane orientation is the same as the arrangement of the metal (A) in the plane perpendicular to the direction of the desired chalcogenide atomic layer. is desirable. As the base material having a crystal plane of a specific plane orientation on the surface (exposed plane), for example, planes with plane orientations of (001), (100), (101), and their inclined planes are selected. It is also possible to use a base material having a surface (exposed surface) of one type of crystal plane that has a low energy surface (exposed surface) or a base material that has a low-energy crystal surface (exposed surface).

The metal oxide single crystal base material may be a simple metal oxide, for example, bismuth oxide for a bismuth-based metal oxide, tungsten oxide for a tungsten-based metal oxide, or other metals. It may be a composite metal oxide with or a halogenated metal oxide with halogen (chlorine, bromine, iodine). When the metal oxide single crystal base material is a composite metal oxide with metal (A) and other metals, the other metals are metals other than metal (A) and calcium, silicon, lithium, beryllium , sodium, magnesium, potassium, rubidium, strontium, cesium, barium, germanium, titanium, aluminum, gallium, and indium. Further, examples of metal halide oxides include bismuth chloride oxide (bismuth oxychloride). The other metals or halogens mentioned above must be removed by a topotactic reaction in the synthesis process described later. Therefore, a halogenated metal oxide using a halogen that can be easily removed in the synthesis process is preferably used as the metal oxide single crystal base material.

　The metal oxide single crystal base material is preferably in the form of a thin film. A thin-film metal oxide single crystal base material can be formed by depositing it on a substrate by, for example, a CVD method.

FIG. 1 shows a schematic configuration of a mist CVD apparatus that can be used in the base material forming process. As shown in FIG. 1, this mist CVD apparatus 10 includes a mist generator 11, a heating furnace 12, a trap container 13, a thinning gas pipe 14, a carrier gas pipe 15, a mist pipe 16, and an outlet pipe 17. , an exhaust pipe 18 and an ultrasonic oscillator 19 .

Nitrogen gas flows through the thinning gas pipe 14 as a thinning gas whose flow rate is controlled by a mass flow controller. Nitrogen gas flows through the carrier gas pipe 15 as a carrier gas whose flow rate is controlled by a mass flow controller.

The mist generator 11 is configured in a container shape, and is filled with a precursor solution PS, which is a solution containing constituents of the metal oxide single crystal base material. The downstream end of the carrier gas pipe 15 is connected to the mist generator 11 so as to open to the upper space of the mist generator 11 . Also, an ultrasonic oscillator 19 is attached to the bottom of the mist generator 11 .

One end of the mist pipe 16 is connected to the mist generator 11 so as to open to the upper space of the mist generator 11 . The downstream end of the thinning gas pipe 14 and the other end of the mist pipe 16 are connected to the inlet port 12in of the heating furnace 12 . A single crystal substrate SB is placed at a predetermined position in the heating furnace 12 . One end of an outlet pipe 17 is connected to the outlet port 12out of the heating furnace 12 . The other end of outlet pipe 17 is connected to trap container 13 . The trap container 13 is filled with water up to a predetermined liquid level, and the other end of the outlet pipe 17 is connected to the trap container 13 so as to open into the water inside the trap container 13 . One end of the waste gas pipe 18 is connected to the trap container 13 so as to open to the gas portion in the trap container 13, and the other end of the waste gas pipe 18 is open to the atmosphere.

By oscillating the ultrasonic oscillator 19 in the mist CVD apparatus 10, the precursor solution PS in the mist generator 11 is misted. The misted precursor solution (hereinafter referred to as precursor solution mist) flows through the mist pipe 16 together with the carrier gas (nitrogen gas). The precursor solution mist flowing through the mist pipe 16 is further merged with the thinning gas (nitrogen gas) and diluted before entering the heating furnace 12 . The temperature in the heating furnace 12 is set in advance to such a temperature that the precursor solution mist can react (decompose) in the heating furnace 12 to deposit the desired metal oxide on the single crystal substrate. there is Therefore, the precursor solution mist that has entered the heating furnace 12 is decomposed in the heating furnace 12 and deposited on the single crystal substrate as metal oxide. Thereby, a thin-film metal oxide single crystal base material is formed. In addition, the gas that has reacted in the heating furnace 12 and the unreacted gas flow through the outlet pipe 17 and are introduced into the trap container 13, purified by the water filled in the trap container 13, and then discharged. It is discharged to the atmosphere through the air pipe 18 .

It is preferable to epitaxially grow the thin film on the substrate in order to obtain a metal oxide single crystal intended for the thin film deposited on the single crystal substrate. In this case, the single-crystal substrate is such that the lattice constant (in-plane lattice constant) of the crystal orientation exposed on the surface of the single-crystal substrate substantially matches the in-plane lattice constant of the target metal oxide single-crystal thin film. is better to choose. For example, when forming bismuth chloride oxide with an in-plane lattice constant a of 3.89 Å as a metal oxide single crystal, strontium titanate (hereinafter referred to as STO ) single crystal substrates can be used. According to this, a metal oxide single crystal base material having the same crystal orientation as that of the single crystal substrate can be crystal-grown on the single crystal substrate.

In the synthesis process, a chalcogenide atomic layer is generated using the metal oxide single crystal base material formed in the base material formation process. At this time, the metal oxide single crystal base material is brought into contact with a reducing chalcogen gas under a predetermined temperature condition lower than the melting point of the metal oxide single crystal base material formed in the base material forming step. Then, a topotactic reaction occurs between the metal oxide single crystal base material and the chalcogen gas, and while the crystals of the metal oxide single crystal base material and the crystals of the product maintain a three-dimensional orientation relationship, the base material Oxygen, other metals and halogens contained in the material are replaced with chalcogen. This produces a single crystal layer of a compound of chalcogen and metal. That is, in the synthesis step, a chalcogenide-based atomic layer is produced by subjecting part or all of the metal oxide atomic layer of the metal oxide single crystal matrix to a topotactic reaction with a reducing chalcogen gas.

The reducing chalcogen gas that comes into contact with the metal oxide single crystal base material in the synthesis process is a gas that contains chalcogen and has a reducing action. The reducing chalcogen gas may be a compound gas of a component having a reducing action and chalcogen, or a mixed gas of a gas having a reducing action and a chalcogen gas. Further, the compound gas is preferably a chalcogenide gas. Furthermore, the mixed gas is preferably a mixed gas of chalcogen gas and hydrogen gas. Examples of the hydrogen chalcogenide gas include hydrogen sulfide gas, hydrogen selenide gas, and hydrogen telluride gas. Examples of the mixed gas include a mixed gas of sulfur gas and hydrogen gas, a mixed gas of selenium gas and hydrogen gas, and a mixed gas of tellurium gas and hydrogen gas. Therefore, the reducing chalcogen gas is preferably at least one selected from the group consisting of the gases exemplified above.

The synthesis process should be carried out in a closed container. By carrying out the synthesis step in a closed container, the amount of highly toxic chalcogen gas released to the outside can be reduced. In addition, by performing the synthesis process in a closed container, the chalcogenide-based atomic layer film can be produced using the minimum required amount of chalcogen gas, and wasteful consumption of chalcogen gas can be prevented.

The synthesis step is preferably performed at a temperature lower than the melting point of the metal oxide single crystal base material and at a temperature of 100°C or higher and lower than 500°C. According to this, a chalcogenide-based atomic layer film can be produced under relatively low temperature conditions of less than 500.degree.

The synthesis process is performed under pressurized conditions. The pressurization condition is at least a pressure condition higher than the atmospheric pressure, that is, a condition that a pressure exceeding 1 atm is applied. Therefore, in principle, a reactor that releases the gas inside the reactor to the outside during the synthesis process cannot satisfy the pressurization condition. conditions can be met. In this case, the metal oxide single crystal base material formed in the base material forming step and the chalcogen gas are enclosed in a closed container, and the temperature is lower than the melting point of the metal oxide single crystal base material and is 100° C. or more and less than 500° C. It is preferable to carry out the synthesis step by raising the temperature in the closed vessel to the reaction temperature of and maintaining the reaction temperature for a predetermined time. According to this, the gas in the sealed container is heated and expanded to satisfy the pressurization condition. In this way, it is not necessary to positively apply pressure to the inside of the closed container, and the pressure condition can be satisfied by adjusting the internal pressure of the container according to the temperature inside the closed container or the concentration of chalcogen gas generated inside the closed container. . Moreover, the pressurization condition is a pressure higher than 1 atm as described above. Further, when the pressure exceeds 20 atm, the cost of the closed container increases in order to ensure sufficient pressure resistance performance of the closed container. Therefore, the pressurization condition is preferably greater than 1 atm and 20 atm or less, preferably greater than 1 atm and 10 atm or less.

The reducing chalcogen gas used in the synthesis process may be introduced from a cylinder or the like into the reaction vessel (for example, a closed vessel) where the synthesis process is performed, but hydrogen telluride is unstable due to its instability. cannot be sealed in cylinders. Also, since reducing chalcogen gas is highly toxic, it is desirable to reduce the amount used. Also, from the point of view of post-use processing and cleaning costs of reaction vessels and the like, reducing the amount of reducing chalcogen gas used is highly effective in terms of industrial utilization. Therefore, the amount of chalcogen gas discharged to the outside by generating the minimum amount of reducing chalcogen gas in a closed container and performing the synthesis process in the closed container using the gas thus generated. can be reduced. In this case, in the synthesis step, a reducing chalcogen gas can be generated in a sealed container using an aqueous solution containing chalcogen. For example, in the synthesis step, a metal oxide single crystal base material and an aqueous solution containing chalcogen are enclosed in a sealed container, and the temperature is lower than the melting point of the metal oxide single crystal base material and is 100° C. or more and less than 500° C. and the step of contacting the metal oxide single crystal base material with a reducing chalcogen gas generated from an aqueous solution containing chalcogen. According to such a synthesis process, by generating highly toxic chalcogen gas in the closed container, it is possible to improve safety compared to the case of introducing the chalcogen gas from the outside into the reaction container.

Figure 2 shows a schematic diagram of a reactor that can be used in the synthesis process. As shown in FIG. 2, the reaction device 20 includes a closed container 21 and a heater 22 . The sealed container 21 is a pressure-resistant container having a pressure-resistant performance that does not break even when a predetermined pressure of 20 atm or less is applied. The heater 22 is arranged around the closed container 21, and when driven (for example, energized), the temperature inside the closed container 21 is raised to a predetermined reaction temperature, and the raised reaction temperature is maintained for a certain period of time. configured to allow

In the reaction apparatus 20 shown in FIG. 2, the metal oxide single crystal base material formed in the base material forming step and the aqueous solution containing chalcogen are enclosed in the sealed container 21 . At this time, for example, a partition plate or the like may be provided in the closed container 21 so that the metal oxide single crystal base material and the aqueous solution containing chalcogen do not come into direct contact within the closed container 21 . Next, the heater 22 is driven and controlled so that the temperature inside the sealed container 21 is lower than the melting point of the metal oxide single crystal matrix and reaches a predetermined reaction temperature of 100° C. or more and less than 500° C. When the reaction temperature is maintained for a predetermined period of time, the aqueous solution containing chalcogen in the sealed container 21 is heated to generate vapor of reducing chalcogen gas. Further, when the chalcogen gas thus generated and having a reducing property contacts the metal oxide single crystal base material in the closed container 21, a topotactic reaction occurs and a chalcogenide-based atomic layer is generated on the base material surface.

In the synthesis process, as described above, the metal oxide single crystal base material is brought into contact with the reducing chalcogen gas. At this time, the desorption of oxygen atoms in the metal oxide single crystal base material is promoted by the reducing action of the chalcogen gas. This promotes topotactic reactions. Therefore, even if the synthesis step is performed under a relatively low temperature condition, for example, a temperature condition of 100° C. or more and less than 500° C., the topotactic reaction proceeds and a chalcogenide atomic layer film can be obtained. . For example, as will be described later, it has been confirmed that a bismuth selenide thin film as a chalcogenide atomic layer can be synthesized even when the synthesis process is performed at a temperature of 170°C. In addition, even when a low-concentration reducing chalcogen gas is used, the topotactic reaction proceeds sufficiently to obtain a chalcogenide-based atomic layer film.

In addition, the synthesis process is performed in a closed vessel, and the pressure in the closed vessel is set to a pressurized state of greater than 1 atm and 20 atm or less by adjusting the temperature conditions, the concentration conditions of the reducing chalcogen gas, and the like. be able to. By carrying out the synthesis step under such pressurized conditions, the topotactic reaction is further promoted. Thereby, a chalcogenide-based atomic layer film can be obtained at a lower temperature. Furthermore, by bringing a reducing chalcogen gas into contact with a metal oxide single crystal base material under pressurized conditions, the topography can be achieved only by bringing a non-reducing chalcogen gas into contact with the metal oxide single crystal base material under normal pressure. A chalcogenide-based atomic layer film can be obtained by causing a topotactic reaction even for a material that did not undergo a tactical reaction.

The concentration of the reducing chalcogen gas generated in the sealed container 21 is preferably 5 vol% or less, more preferably less than 1 vol%, and even more preferably less than 0.1 vol%. Safety can be further improved by using such a low-concentration chalcogen gas. Furthermore, in the present embodiment, the topotactic reaction can proceed even when such a low-concentration chalcogen gas is used.

The chalcogen-containing aqueous solution enclosed in the sealed container 21 is preferably a chalcogen compound aqueous solution capable of generating reducing chalcogen gas. As such a chalcogen compound aqueous solution, a thioacetamide aqueous solution, a thiourea aqueous solution, and a selenourea aqueous solution can be exemplified.

An organic method or a reducing agent method can be exemplified as a method of generating chalcogenide hydrogen gas as a reducing chalcogen gas in a closed container. Further, as a method of generating hydrogen sulfide gas by an organic method, a method of heating a thioacetamide aqueous solution or a thiourea aqueous solution, which are chalcogen compound aqueous solutions, can be exemplified. Furthermore, as a method of generating hydrogen selenide gas by an organic method, a method of heating an aqueous solution of selenourea, which is an aqueous solution of a chalcogen compound, can be exemplified.

In the reducing agent method, a reducing agent that generates hydrogen by thermal decomposition or the like is mixed with chalcogen (sulfur, selenium, tellurium, etc.), and the mixture is heated to the melting point of chalcogen (sulfur: 115°C, selenium: 221°C, tellurium: 450°C). ° C.) and below the boiling point (sulfur: 455° C., selenium: 685° C., tellurium: 988° C.) to generate a mixed gas of chalcogen gas and hydrogen gas. Examples of the reducing agent used at this time include sodium borohydride (melting point: 400° C., decomposes at 500° C.), potassium borohydride (decomposes at 500° C.), and the like.

A chalcogenide-based atomic layer film can be used for various purposes. Among them, bismuth-based atomic layer films are expected to be applied to topological materials. A topological insulator can be exemplified as one of the topological materials. A topological insulator is a material whose interior is an insulator and whose surface is a conductor. In such a topological insulator, it is very important to control the number of chalcogenide atomic layers on the surface. It is possible to obtain semiconductor properties different from those of Therefore, when a chalcogenide-based atomic layer film is used as a topological insulator, the number of chalcogenide-based atomic layers must be controlled to be less than 20 layers.

In particular, it is said that the optimum number of layers for infrared absorption using Dirac electrons is 6 or more and 10 or less. By controlling the number of layers within the above range, the chalcogenide atomic layer film can be used as an image sensor that absorbs infrared rays. Further, according to this embodiment, a chalcogenide-based atomic layer with a large area can be obtained on a thin-film metal oxide single crystal base material, so it is expected to be used as an infrared absorption type image sensor with higher sensitivity. be done.

The number of chalcogenide atomic layers affects the reaction time, the pressure during the reaction, and the reaction gas concentration (concentration of reducing chalcogen gas). Therefore, it is considered that the number of layers can be controlled by controlling these.

The number of chalcogenide-based atomic layers can be determined directly from a transmission electron microscope (TEM) image, or indirectly from a measurement using an XRR (X-ray reflectometer) and a Raman shift of crystal lattice vibration. can also judge. When the number of layers is determined indirectly from the Raman shift of crystal lattice vibration, the number of layers may be identified from the difference in the frequencies of two Raman-active vibration modes.

In addition, the topotactic reaction that occurs in the synthesis process proceeds from the outside (surface) of the metal oxide single crystal base material toward the inside (inside), and depending on time, the metal oxide of the base material It has been experimentally confirmed that the atomic layer becomes a chalcogenide atomic layer. Therefore, by adjusting the reaction time and the like, a part (from the outermost surface to a predetermined thickness) or all of the metal oxide atomic layer of the base material can be caused to undergo a topotactic reaction. Further, when the progress of the topotactic reaction is stopped at a certain time, all the atomic layers from the outermost surface to the predetermined number of layers are chalcogenide atomic layers, and all atomic layers after that are metal oxide atomic layers. layer. In this way, since the topotactic reaction proceeds from the outside, when the synthesis step is performed using the multi-layered metal oxide single crystal base material and the topotactic reaction is allowed to occur for a predetermined time, the metal oxide single crystal A chalcogenide atomic layer film having a single or several chalcogenide atomic layers can be formed on a crystal matrix. In this case, if the metal oxide single crystal base material is an insulator, a semiconductor (chalcogenide atomic layer) can be obtained on the insulator. film) from the base material.

[Example 1] Synthesis of bismuth sulfide thin film using hydrogen sulfide gas (1) Preparation of bismuth chloride oxide single crystal base material thin film1. Preparation of Substrate In preparing the bismuth chloride oxide single crystal base material thin film as the metal oxide single crystal base material, it is necessary to epitaxially grow the bismuth chloride oxide single crystal thin film on the substrate. Here, the in-plane lattice constant a of strontium titanate (hereinafter referred to as STO) is 3.905 Å compared to the in-plane lattice constant a of bismuth chloride oxide of 3.89 Å, and the lattice mismatch is as small as 0.4%. Therefore, an STO single crystal substrate (manufactured by Shinko Co., dimensions: 10 mm×10 mm×0.5 mm (thickness), plane orientation (001)) was used. Here, the plane orientation (001) means that the crystal plane appearing on the substrate surface is the (001) plane.

2. Formation of bismuth chloride oxide single crystal base material thin film (base material forming process)
The bismuth chloride oxide single crystal base material thin film was formed by a mist CVD method using a mist CVD apparatus 10 whose schematic configuration is shown in FIG. In the mist CVD method, as described above, the precursor solution PS is misted by ultrasonic waves, the precursor solution mist is transported into the heating furnace 12 by an inert carrier gas, and the mist is placed at a predetermined position in the heating furnace 12. This is a method of forming a highly oriented film on an STO single crystal substrate. In this example, the precursor solution was prepared by dissolving 5 mM bismuth chloride powder in n,n-dimethylformamide (hereinafter referred to as DMF) as a solvent. This precursor solution contains bismuth, chlorine, and oxygen as constituents of the metal oxide single crystal matrix. A 2.4 MHz ultrasonic atomization unit HMC-2400 manufactured by Honda Electronics Co., Ltd. was installed as an ultrasonic element including an ultrasonic oscillator 19 on the bottom surface of the mist generator 11 . Also, high-purity nitrogen gas was used as a carrier gas. The heating furnace 12 was composed of a quartz tube with a diameter of 25 mm and an electric furnace (heater) provided around the quartz tube. A prepared STO single crystal substrate was placed at a predetermined position in the heating furnace 12 (quartz tube). Then, the temperature of the heating furnace 12 was set to 400° C., the ultrasonic element was started in a sufficiently stable state, and the flow rate of the carrier gas was set to 0.5 L/min. , the flow rate of the thinning gas is set to 3.5 L/min. , and conveyed into the heating furnace 12 together with the precursor solution mist.

The precursor solution mist transported into the heating furnace 12 is decomposed in the heating furnace 12, thereby depositing bismuth chloride oxide on the STO single crystal substrate. As a result, a bismuth chloride oxide thin film is formed on the STO single crystal substrate. In this example, the film formation time is 60 min. A thin film having a thickness of 30 nm was formed on the STO single crystal substrate. When the film-formed sample was confirmed by X-ray diffraction, a (001)-oriented bismuth chloride oxide peak was confirmed. From this result, it can be seen that a bismuth chloride oxide thin film was formed on the STO single crystal substrate.

3. Confirmation of Single Crystal Formation of Bismuth Chloride Oxide Thin Film X-ray diffraction pole measurement was performed to confirm the single crystal of the formed bismuth chloride oxide thin film. The measurement results are shown in FIG. As shown in FIG. 3, when pole measurement was performed at the diffraction angle (2θ = 34.833°) of the (111) crystal plane of bismuth chloride oxide, four-point diffraction spots (No. 1 , 2, 3, 4) were confirmed. Similarly, the STO single crystal substrate was subjected to X-ray diffraction pole measurement. The measurement results are shown in FIG. As shown in FIG. 4, when pole measurement was performed at the diffraction angle (2θ = 39.954°) of the (111) crystal plane of STO, four-point diffraction spots (Nos. 1 and 2 , 3, 4) were confirmed.

The in-plane rotation angles β at which the four diffraction spots of the bismuth chloride oxide thin film and the STO single crystal substrate were confirmed were 46°, 136°, 226°, and 316°, which completely coincided with each other. It was confirmed that the material thin film was epitaxially grown on the STO single crystal substrate and was a single crystal.

(2) Synthesis of bismuth sulfide thin film by topotactic reaction using thioacetamide (synthesis process)
The bismuth chloride oxide base material thin film prepared as described above and 1 mL of an aqueous thioacetamide solution having a concentration of 60 mM were placed in a glass pressure-resistant container (MicrowaveVial, manufactured by Biotage) having an internal volume of about 32 mL as a sealed container, and these were placed in a glass container. sealed in a pressure-resistant container. Then, the temperature inside the glass pressure vessel was raised to 150° C., and the temperature was maintained at 150° C. for 1 hour. As a result, hydrogen sulfide gas is generated from the thioacetamide aqueous solution in the glass pressure vessel, and the generated hydrogen sulfide gas is brought into contact with the bismuth chloride oxide single crystal base material thin film to perform a sulfurization treatment by a topotactic reaction. Ta. At this time, the pressure in the glass pressure vessel is about 20 atm, which is the pressure resistance of the vessel, and the concentration of the hydrogen sulfide gas generated in the glass pressure vessel is about 2 vol %.

After the synthesis process was completed, the sample (bismuth chloride oxide single crystal base material thin film in contact with hydrogen sulfide gas) was taken out from the glass pressure vessel, and the crystal structure was evaluated by X-ray diffraction. As a result, a peak of STO, a peak of bismuth chloride oxide, and a peak of bismuth sulfide were confirmed. From this result, it was confirmed that a bismuth sulfide thin film was formed on the bismuth chloride oxide single crystal base material thin film.

[Comparative Example 1] Synthesis of Bismuth Sulfide Thin Film Using Sulfur Gas An alumina boat containing 3 g of sulfur powder was placed in a SUS tubular furnace as a heating furnace for generating sulfur gas and heated to 200°C. This generated sulfur gas in the SUS tubular furnace. Also, argon gas as a carrier gas was supplied at a flow rate of 0.5 L/min. to a SUS tubular furnace, whereby the sulfur gas was introduced into the reactor together with the carrier gas. The reactor was an electric furnace using a quartz tube with a diameter of 25 mm. A bismuth chloride oxide single crystal base material thin film prepared in the same manner as in Example 1 was placed at a predetermined position in the quartz tube. In addition, before introducing sulfur gas into the reactor (quartz tube), a vacuum pump is used to evacuate the quartz tube to a pressure of less than 10 Pa, and then a bypass line is used to remove argon gas from the quartz tube. While introducing into the tube, the temperature inside the quartz tube was set to the reaction temperature. When the temperature was sufficiently stabilized, the valve of the sulfur gas passage was opened to introduce the sulfur gas into the reactor (quartz tube) together with the carrier gas for the reaction time. As a result, the bismuth chloride oxide base material single crystal thin film was brought into contact with the sulfur gas in the reactor. The reaction temperature was 320° C., and the reaction time was 30 minutes and 90 minutes. After the reaction time had elapsed, the sample (bismuth chloride oxide single crystal base material thin film in contact with sulfur gas) was taken out from the reactor.

When the crystal structure of the extracted sample was evaluated by X-ray diffraction, a slight shift of the diffraction line peak relative to the bismuth chloride oxide single crystal base material could be confirmed, but no peak due to bismuth sulfide could be confirmed. Therefore, it is considered that the bismuth sulfide thin film is not formed on the bismuth chloride oxide single crystal base material thin film.

The temperature in the synthesis process of Example 1 is 150°C, and the temperature in the synthesis of Comparative Example 1 is 320°C, both of which are temperatures of 100°C or more and less than 500°C. Further, in the synthesis process of Example 1, the bismuth chloride oxide single crystal base material thin film is brought into contact with a reducing chalcogen gas (hydrogen sulfide gas). On the other hand, in the synthesis of Comparative Example 1, the bismuth chloride oxide single crystal base material thin film is brought into contact with non-reducing chalcogen gas (sulfur gas). From this, it is possible to generate a chalcogenide-based atomic layer (bismuth sulfide thin film) under a temperature condition of 100° C. or more and less than 500° C. by bringing a reducing chalcogen gas into contact with the bismuth chloride oxide single crystal base material thin film. Recognize.

[Example 2] Preparation of bismuth selenide thin film using hydrogen selenide gas (1) Preparation of bismuth chloride oxide base material thin film1. Preparation of Substrate An STO single crystal substrate similar to that of Example 1 was prepared.
2. Formation of bismuth chloride oxide base material thin film (base material formation process)
By the same procedure as in Example 1, a bismuth chloride oxide single crystal base material thin film was formed on an STO single crystal substrate.
(2) Synthesis of bismuth selenide thin film by topotactic reaction using selenourea (synthesis process)
The bismuth chloride oxide single crystal base material thin film prepared as described above and 150 μL of an aqueous solution of selenourea having a concentration of 60 mM were placed in the same glass pressure vessel as used in Example 1, and these were sealed in the glass pressure vessel. did. Then, the temperature inside the glass pressure vessel was raised to 170°C, and the temperature was maintained at 170°C for 40 min. maintained. As a result, vapor of hydrogen selenide gas is generated from the selenourea aqueous solution in the glass pressure-resistant container, and by bringing the generated hydrogen selenide gas vapor into contact with the bismuth chloride oxide single crystal base material thin film, topotactic A selenization treatment was performed by reaction. At this time, the water in which selenourea is dissolved is adjusted to pH 4.8 with acetic acid having a concentration of about 0.03 mM. In order to prevent the bismuth chloride oxide single crystal base material thin film from coming into contact with the liquid selenourea aqueous solution, a quartz plate was placed in a pressure-resistant glass container to separate the bismuth chloride oxide single crystal base material thin film from the selenourea aqueous solution. ing. The pressure in the glass pressure vessel during the synthesis process was about 6 atm, and the concentration of hydrogen selenide gas generated in the glass pressure vessel was about 0.07 vol %. After completion of the synthesis process, a sample (bismuth chloride oxide single crystal base material thin film in contact with hydrogen selenide gas) was taken out from the glass pressure-resistant container.

(3) Confirmation of crystals by analytical instrument 1. Confirmation of crystal structure by X-ray diffraction The crystal structure of the sample was confirmed by out-of-plane X-ray diffraction. FIG. 5 is a graph showing X-ray diffraction results. In FIG. 5, graph A1 is a graph showing the X-ray diffraction results of a commercially available bismuth selenide (Bi ₂ Se ₃ ) bulk, and graph B1 is the X-ray diffraction results of a bismuth chloride oxide single crystal base material thin film. Fig. 3 is a graph, and graph C1 is a graph showing the X-ray diffraction results of a sample produced by a synthesis process; As can be seen from the graph B1, in the bismuth chloride oxide single crystal base material thin film, 2θ=12, 24, 36, 49, 63, 78 deg. A peak (the peak indicated by ◯ in graph B1) was observed in the vicinity. These angles 2.theta. correspond to diffraction angles caused by the (001) plane of the bismuth chloride oxide bulk and its higher-order planes. From this result, it was confirmed that a single crystal of bismuth chloride oxide was certainly formed on the STO single crystal substrate, and that the (001) plane and its higher-order planes were oriented parallel to the substrate surface. It turns out that there is

Also, according to graph B1, 2θ=22, 46, 73 deg. A high-intensity peak (the peak indicated by □ in graph B1) is also seen in the vicinity. These peaks originate from the (001) plane of the STO single crystal substrate and its higher-order planes. On the other hand, as can be seen from graph C, in the sample produced by the synthesis process, the peaks due to bismuth chloride oxide disappeared, and instead, 2θ = 9, 18, 28, 37, 57 deg. A peak (the peak indicated by ● in graph C1) appeared in the vicinity. These angles 2θ correspond to diffraction angles due to the (003) plane of the bismuth selenide bulk and its higher-order planes, as also shown in graph A1. From this result, it was found that the produced sample was a single crystal of bismuth selenide, and it was (003) oriented. Moreover, since the single-crystal thin film is formed from the single-crystal base material, it is considered that the bismuth selenide single-crystal thin film was produced by a topotactic reaction.

2. Evaluation of Crystalline State by Raman Spectroscopy The crystalline state of commercially available bismuth selenide bulk, bismuth chloride oxide single crystal base material thin film, and synthesized sample (bismuth selenide thin film) was evaluated by Raman spectroscopy. A Renishaw inVia Reflex AP-100079 was used for Raman spectroscopic measurements. This apparatus mainly uses a green laser of 532 nm as the incident light. Further, only Stokes scattering is detected, and a wavelength band in which the amount of Raman shift of Stokes scattering is 100 cm ⁻¹ or less is blocked by a filter on the light receiving side, thereby greatly reducing the intensity. In bismuth selenide, it is known that two vibration modes, E ² _g (in-plane) and A ² _1g (out-of-plane), are mainly generated by light irradiation, and appear at 130 and 175 cm ⁻¹ as Raman shifts, respectively. (Ref: S. Jerng, et al., Journal of Nanoscale, 5, 21 (2013)).

FIG. 6 is a graph showing Raman shifts measured by the above apparatus. In FIG. 6, graph A2 is a graph showing the Raman shift of a commercially available bismuth selenide (Bi ₂ Se ₃ ) single crystal bulk, and graph B2 is a graph showing the Raman shift of a bismuth chloride oxide single crystal base material thin film. Graph C2 is a graph showing the Raman shift of the sample produced by the synthesis process. As shown in graph B2, a peak was observed at 143 cm ⁻¹ in the bismuth chloride oxide single crystal base material thin film, whereas as shown in graph C2, in the sample after synthesis (bismuth selenide thin film), 130, Two peaks were observed at 174 cm ⁻¹ . Moreover, as shown in graph A2, two peaks were observed in the commercially available bismuth selenide single crystal bulk as in graph C2. From this result, it can be seen that the bismuth selenide thin film produced by this example has the same vibration mode of the Se—Bi—Se—Bi—Se unit layer (unit layer) as the commercially available bulk bismuth selenide single crystal, and optically responds. I was able to confirm that. Therefore, it was proved that the commercially available bismuth selenide single crystal bulk and the bismuth selenide thin film of this example have similar unit layers.

3. Measurement of photocurrent In order to confirm that the prepared sample (bismuth selenide thin film) is a semiconductor, an electrode was placed on the surface of the sample, and photocurrent was measured by laser light irradiation. A silver paste was used for the electrodes, and the distance between the electrodes was set to about 100 μm. A tungsten probe was applied to the electrode, and this tungsten probe was connected to a semiconductor analyzer (B1500) manufactured by Keysight. Then, the space between the electrodes was intermittently irradiated with a laser beam from a laser device, and the photocurrent flowing at that time was measured. The laser equipment used is 670 nm 4.5 mW manufactured by Thorlabs.

FIG. 7 is a graph showing the measurement results of the photocurrent. As shown in FIG. 7, when the laser irradiation was turned on and off repeatedly, it was confirmed that the photocurrent flowed in accordance with the turning on. From this result, it was confirmed that the sample surface was a semiconductor.

4. Control of the number of layers of the bismuth selenide thin film In this example, the bismuth selenide thin film was prepared by using an aqueous solution of selenourea and replacing oxygen in the base material with chalcogen (selenium) by a topotactic reaction. there is At this time, in order to confirm that the number of atomic layers of the bismuth selenide thin film can be controlled by changing the concentration of the selenourea aqueous solution, the concentration of selenourea was changed to 10 mM, 20 mM, 40 mM, 60 mN, 80 mM, and 120 mM. 150 μL of selenourea aqueous solution was prepared. Then, in the synthesis step, each aqueous solution was sealed in the glass pressure vessel together with the bismuth chloride oxide single crystal base material thin film, the temperature inside the glass pressure vessel was raised to 170 ° C., and the temperature was 170 ° C. 40 min. maintained. As a result, the bismuth chloride oxide single crystal base material thin film was brought into contact with the vapor of the selenourea aqueous solution (hydrogen selenide gas) to perform selenization treatment by topotactic reaction. After that, each of the prepared samples (bismuth chloride oxide single crystal base material thin film in contact with hydrogen selenide gas) was taken out from the pressure-resistant glass container.

The crystal structure of each sample taken out was confirmed by X-ray diffraction. FIG. 8 is a graph showing X-ray diffraction results. In FIG. 8, graph A3 is a graph showing the X-ray diffraction results of a commercially available bulk bismuth selenide, and graph B3 is a graph showing the X-ray diffraction results of a bismuth chloride oxide single crystal base material thin film. In FIG. 8, graph C3x (x=10, 20, 40, 60, 80, 120) is a graph showing the X-ray diffraction results of a sample prepared using an aqueous selenourea solution with a concentration of x mM in the synthesis step. be.

As shown in FIG. 8, the graphs (C310, C320, C340) of the X-ray diffraction results of the samples prepared using the selenourea aqueous solution having a concentration of 40 mM or less show the sixth peak of the (003) plane of bismuth selenide. 57 deg. Almost no peak is seen in the vicinity (Bi ₂ Se ₃ (0018)). On the other hand, the graphs (C360, C380, C3120) of the X-ray diffraction results of the samples prepared using the selenourea aqueous solution with a concentration of 60 mM or more show 57 deg. It can be confirmed that peaks can be seen in the vicinity. From this, it can be estimated that the sixth atomic layer of bismuth selenide is formed in the sample prepared using the selenourea aqueous solution having a concentration of 60 mM or more. Moreover, when the concentration is 60 mM or more, the higher the concentration, the 57 deg. It was confirmed that the peaks in the vicinity became higher. From this, it was confirmed that the number of atomic layers of bismuth selenide can be easily controlled by the concentration of the selenourea aqueous solution.

Although the embodiments of the present invention have been described above, the present invention should not be limited to the above embodiments. For example, in the above examples, the method for producing a bismuth sulfide thin film and a bismuth selenide thin film as chalcogenide atomic layer films was illustrated, but the present invention can also be applied to other chalcogenide atomic layer films. In the above examples, an example was shown in which a chalcogenated hydrogen gas was generated by heating an aqueous solution containing chalcogen (aqueous thioacetamide solution, aqueous selenourea solution) sealed in a sealed container. Hydrogen gas may be generated. In this manner, the present invention can be modified without departing from its gist.

Claims

a base material forming step of forming a metal oxide single crystal base material having a metal oxide atomic layer;
By contacting the metal oxide single crystal base material with a reducing chalcogen gas under a temperature condition lower than the melting point of the metal oxide single crystal base material, a part or all of the metal oxide atomic layer is topotactic reaction to form a chalcogenide atomic layer;
A method for producing a chalcogenide-based atomic layer film, comprising:
A method for producing a chalcogenide-based atomic layer film according to claim 1,
A method for producing a chalcogenide-based atomic layer film, wherein the synthesis step is performed in a closed container.
A method for producing a chalcogenide-based atomic layer film according to claim 1 or 2,
A method for producing a chalcogenide atomic layer film, wherein in the synthesis step, the metal oxide single crystal base material is brought into contact with a reducing chalcogen gas having a concentration of 5 vol % or less.
A method for producing a chalcogenide-based atomic layer film according to claim 3,
A method for producing a chalcogenide-based atomic layer film, wherein the concentration of the reducing chalcogen gas is less than 1 vol %.
A method for producing a chalcogenide-based atomic layer film according to any one of claims 1 to 4,
A method for producing a chalcogenide-based atomic layer film, wherein the synthesis step is performed under a temperature condition of 100°C or higher and lower than 500°C.
A method for producing a chalcogenide-based atomic layer film according to any one of claims 1 to 5,
A method for producing a chalcogenide-based atomic layer film, wherein the synthesis step is performed under pressure conditions higher than 1 atm and 20 atm or less.
A method for producing a chalcogenide-based atomic layer film according to any one of claims 1 to 6,
The method for producing a chalcogenide-based atomic layer film, wherein the reducing chalcogen gas is chalcogenide hydrogen gas or a mixed gas of chalcogen gas and hydrogen gas.
A method for producing a chalcogenide-based atomic layer film according to claim 7,
The reducing chalcogen gas includes hydrogen sulfide gas, hydrogen selenide gas, hydrogen telluride gas, a mixed gas of sulfur gas and hydrogen gas, a mixed gas of selenium gas and hydrogen gas, and a mixed gas of tellurium gas and hydrogen gas. A method for producing a chalcogenide-based atomic layer film, which is at least one selected from the group consisting of
A method for producing a chalcogenide-based atomic layer film according to claim 2,
In the synthesis step, the metal oxide single crystal base material and an aqueous solution containing chalcogen as a source of the reducing chalcogen gas are enclosed in the sealed container, and the metal oxide single crystal base material is A chalcogenide-based atom, which is a step of contacting the metal oxide single crystal base material with a reducing chalcogen gas generated from the chalcogen-containing aqueous solution under a temperature condition of 100 ° C. or more and less than 500 ° C. lower than the melting point of A method for manufacturing a layered film.
A method for producing a chalcogenide-based atomic layer film according to any one of claims 1 to 9,
The metal oxide single crystal base material is a bismuth-based metal oxide,
A method for producing a chalcogenide atomic layer film, wherein the chalcogenide atomic layer film is a chalcogenide bismuth thin film.
A method for producing a chalcogenide-based atomic layer film according to any one of claims 1 to 10,
The base material forming step is a step of crystal-growing the metal oxide single crystal base material on a single crystal substrate having an in-plane lattice constant substantially matching the in-plane lattice constant of the metal oxide single crystal base material. A method for producing a chalcogenide-based atomic layer film.
A method for producing a chalcogenide-based atomic layer film according to claim 11,
The single crystal substrate is an STO single crystal substrate,
The metal oxide single crystal base material is bismuth chloride oxide,
A method for producing a chalcogenide-based atomic layer film.