Tungsten disulfide-containing film and method for producing the same

Abstract

To provide a group 6 transition metal dichalcogenide-containing film showing a 1T' phase and a method for making the same.SOLUTION: This making method includes heating a precursor including tungsten, hydrogen sulfide, and inert gas in the presence of alkali metal salt at 450°C or higher and 550°C or lower. Here, the volume ratio of hydrogen sulfide to the precursor is higher than 4.05×103. The precursor can be selected from tungsten hexafluoride and tungsten hexacarbonyl. The alkali metal salt can be selected from halide, hydroxide, oxide, sulfide, carbonate, nitrate, sulfate, phosphate, and carboxylate.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] One embodiment of the present invention relates to a Group 6 transition metal dichalcogenide-containing film exhibiting a metastable phase and a method for producing the same. For example, one embodiment of the present invention relates to a tungsten disulfide-containing film exhibiting a metastable phase and a method for producing the same.

Background Art

[0002] Group 6 transition metal dichalcogenides such as tungsten disulfide (WS2), molybdenum disulfide (MoS2), tungsten diselenide (WSe2), molybdenum diselenide (MoSe2), tungsten ditelluride (WTe2), and molybdenum ditelluride (MoTe2) generally take a trigonal prism structure (1H phase) which is the most stable structure, an octahedral structure (1T phase) which is a metastable structure, or a distorted octahedral structure (1T' phase) in which the chalcogen is displaced from the octahedral structure. Thin films of Group 6 transition metal dichalcogenides exhibiting the 1T' phase have attracted great attention in recent years because they have semimetallic properties, function as a low-resistance ohmic contact material for the 1H phase, exhibit a topological state, exhibit superconductivity, function as a hydrogen generation catalyst, etc. (see Non-Patent Documents 1 to 5).

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0004] One of the embodiments of the present invention is to provide a Group 6 transition metal dichalcogenide-containing film having a 1T' phase and a method for producing the same. Alternatively, one of the embodiments of the present invention is to provide an atomic layer film of a Group 6 transition metal dichalcogenide having a 1T' phase and a method for producing the same.

Means for Solving the Problems

[0005] One of the embodiments of the present invention is a method for producing a tungsten sulfide film. This production method includes heating a precursor containing tungsten, hydrogen sulfide, and an inert gas at a temperature of 450°C or higher and 550°C or lower in the presence of a salt of an alkali metal. Here, the volume ratio of hydrogen sulfide to the precursor is 4.05×10 3 Higher.

[0006] One of the embodiments of the present invention is a tungsten sulfide film. In this tungsten sulfide film, the full width at half maximum of the peak with a Raman shift of 130 cm -1 Is 10 cm -1 Or less.

Effects of the Invention

[0007] According to one of the embodiments of the present invention, a single-atom layer film, a two-atom layer film, or a few-atom layer film of a Group 6 transition metal dichalcogenide having a 1T' phase, and a method for producing the same are provided.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Mode for Carrying Out the Invention

[0009] Hereinafter, embodiments of the invention disclosed in the present application will be described. The present invention can be implemented in various modes without departing from the gist thereof, and is not to be construed as being limited to the description of the embodiments exemplified below. Even if the effects brought about by the following embodiments are different, those that are obvious from the description of this specification or can be easily predicted by those skilled in the art are naturally understood to be brought about by the present invention.

[0010] For the sake of clearer explanation, the drawings may schematically represent the width, thickness, shape, etc. of each part compared to the actual aspect, but this is merely an example and does not limit the interpretation of the present invention. In this specification and each figure, elements having the same functions as those described with respect to the previously shown figures may be denoted by the same reference numerals, and duplicate explanations may be omitted.

[0011] 1. Method for Producing a Group 6 Transition Metal Dichalcogenide-Containing Film A Group 6 transition metal dichalcogenide-containing film (hereinafter, also simply referred to as a dichalcogenide-containing film) according to one embodiment of the present invention is produced using a gas-phase reaction. Specifically, as schematically shown in FIG. 1(A), a device 100 including a chamber 102 to which a source 104 of a precursor containing a Group 6 transition metal as a raw material (hereinafter, simply referred to as a precursor), a source 106 of an inert gas for dilution (hereinafter, referred to as an inert gas source), and a chalcogen gas source 108 for supplying a chalcogen-containing gas acting as a raw material and a reducing agent are connected is used to produce the dichalcogenide-containing film. Although not shown, a flow meter for monitoring the flow rates of the precursor, the inert gas, and the chalcogen-containing gas is provided in the device 100. A gaseous precursor, an inert gas, and a chalcogen-containing gas are supplied from the precursor source 104, the inert gas source 106, and the chalcogen gas source 108 to the chamber 102, respectively. Further, a gas discharge port, a back pressure valve, and the like are provided in the device 100.

[0012] A substrate for growing the dichalcogenide-containing film is disposed in the chamber 102. The precursor, the inert gas, and the chalcogen-containing gas are heated and reacted in the chamber 102, and a dichalcogenide-containing film is formed on the substrate. As the substrate, a mica substrate, a silicon substrate, a germanium substrate, a gallium arsenide substrate, a gallium nitride substrate, a sapphire substrate, a quartz substrate, a graphite substrate, a hexagonal boron nitride substrate, a glass substrate, a gold substrate, a strontium titanate (SrTiO3) single crystal substrate, or the like is used. The mica substrate may be a natural mica substrate or an artificial mica substrate such as a fluorophlogopite substrate. When a substrate containing a semiconductor such as silicon, germanium, or a compound semiconductor is used, an oxide may be formed on its surface.

[0013] As the precursor, a gaseous compound is used at a temperature near room temperature (for example, 20 °C or higher and 40 °C or lower) and at normal pressure (for example, 1 atmosphere, that is, 101.325 kPa). For example, tungsten hexafluoride (WF6), molybdenum hexafluoride (MoF6), etc. can be used. Alternatively, even if it is a solid or liquid at room temperature and normal pressure, a precursor having a vapor pressure of 1 Pa or more at room temperature may be used. Examples of such precursors include molybdenum hexacarbonyl (Mo(CO)6) and tungsten hexacarbonyl (W(CO)6). Examples of the chalcogen-containing gas include hydrogen sulfide (H2S), hydrogen selenide (H2Se), hydrogen telluride (H2Te), etc. As the inert gas, a noble gas such as argon or helium or nitrogen can be used. The precursor is supplied from the precursor supply source 104 to the chamber 102 in a neat state or in a state diluted with an inert gas, and is further diluted with the inert gas supplied from the inert gas supply source 106 in the chamber 102. The ratio of the precursor in the mixed gas is, for example, 0.1% by volume or more and 5% by volume or less, typically 1% by volume. By using the gaseous precursor and the chalcogen-containing gas, it becomes possible to strictly control the composition ratio and concentration of the precursor and the chalcogen-containing gas. Therefore, as will be described later, the structure of the obtained dichalcogenide-containing film is controlled, the formation of the 1H phase, which is the most stable structure, is suppressed, and a high-quality and highly crystalline 1T´-phase single atomic layer or several atomic layer dichalcogenide-containing film can be obtained.

[0014] When producing the dichalcogenide-containing film, the amount (partial pressure) of the precursor in the chamber 102 is set relatively low. For example, the amount of the precursor in the chamber 102 can be set to be 0.1 ppm or more and 500 ppm or less, 1.0 ppm or more and 10 ppm or less, or 1.0 ppm or more and 5.0 ppm or less with respect to the total amount of the precursor, the chalcogen-containing gas, and the inert gas. The ratio of the chalcogen-containing gas to the precursor is set high, and the volume ratio of the chalcogen-containing gas to the volume of the precursor is 1.00×10 1 or more and 1.00×10 5 or less, 4.00×10 3 or more and 8.00×104 Hereinafter, 4.05×10 3 or more and 8.11×10 4 , 4.05×10 3 greater than 2.03×10 4 or less, or 2.03×10 4 or more and 8.11×10 4 It is preferable to carry out the reaction so as to be below. By using an extremely large excess of the chalcogen-containing gas with respect to the precursor in this way, the resulting dichalcogenide-containing film is highly electron-doped by the chalcogen-containing gas that functions as a reducing agent. As a result, the phase stability of the dichalcogenide-containing film is improved, the transition to the more stable 1H phase is suppressed, and it becomes possible to selectively form the 1T' phase. It is known that the crystal field splitting received by metal atoms is different between the 1T' phase and the 1H phase, and the 1T' phase is stabilized by doping the 1H phase with excess electrons.

[0015] The heating of the precursor, the inert gas, and the chalcogen-containing gas in the chamber 102 is carried out in the presence of a salt of an alkali metal. The alkali metal is selected from lithium, potassium, sodium, and cesium. There is no restriction on the anion constituting the salt. For example, the salt can be selected from halides, oxides, hydroxides, sulfides, carbonates, bicarbonates, nitrates, sulfates, phosphates, carboxylates such as citric acid, acetic acid, and ethylenediaminetetraacetic acid, and the like. Preferably, a salt having a melting point higher than the heating temperature is used, and typically sodium chloride or potassium chloride can be used. The salt of the alkali metal may be filled as a powder in a container such as quartz and placed in the chamber 102. Alternatively, the salt of the alkali metal may be adhered to the substrate on which the dichalcogenide-containing film is grown or another substrate, and this substrate may be placed in the chamber 102. By using the salt of the alkali metal, ions of the alkali metal are supplied onto the substrate, and as a result, the reaction between the precursor and the chalcogen-containing gas is promoted. Also, as described above, the dichalcogenide-containing film is obtained in a highly electron-doped state, but alkali metal ions are adsorbed on the crystal surface during growth and stabilize this highly electron-doped state as counter cations.

[0016] The heating temperatures of the precursor, the inert gas, and the chalcogen-containing gas are relatively low and are set in the range of 450°C or higher and 550°C or lower. At high temperatures, a phase change to a stable phase occurs, so heating at a low temperature is preferred. The total pressure in the chamber 102 is 0.1 kPa or higher and 10 kPa or lower, 0.5 kPa or higher and 5 kPa or lower, or 0.5 kPa or higher and 2 kPa or lower at the heating temperature, and for example, the reaction can be carried out at 1 kPa. The heating time may be appropriately adjusted according to the partial pressures and heating temperature of the precursor, the inert gas, and the chalcogen-containing gas, and for example, it may be appropriately selected within the range of 1 minute or longer and 30 hours or shorter, 1 hour or longer and 12 hours or shorter, or 1 hour or longer and 6 hours or shorter.

[0017] 2. Structure and properties of the group 6 transition metal dichalcogenide-containing film The dichalcogenide-containing film according to one embodiment of the present invention obtained by the above-described production method is a substantially single atomic layer, bilayer, or few atomic layer dichalcogenide that selectively takes a 1T′ phase structure. Due to the low heating temperature, the dichalcogenide-containing film does not contain the 1H phase, which is the most stable structure. Further, the dichalcogenide-containing film can be obtained as a single crystal of dichalcogenide on the substrate or as an aggregate of a plurality of dichalcogenide crystal grains.

[0018] As described above, in this production method, the precursor, the chalcogen-containing gas, and the inert gas are heated in the presence of an alkali metal salt, and the alkali metal salt stabilizes the highly doped state of the dichalcogenide-containing film. Therefore, the dichalcogenide-containing film may contain alkali metal ions. For example, the alkali metal ions may be adsorbed on the dichalcogenide-containing film at a concentration of 1 ppm or higher and 100 ppm or lower, or 10 ppm or higher and 100 ppm or lower. The confirmation and quantification of the alkali metal ions may be performed using, for example, X-ray photoelectron spectroscopy.

[0019] Here, generally, in a 1T'-phase dichalcogenide, as shown in FIGS. 1(B) and 1(C), Group 6 transition metal atoms (M) are arranged in a zigzag pattern within substantially the same layer, and the layer formed by these Group 6 transition metal atoms (metal layer) is sandwiched by the layer formed by chalcogen atoms (chalcogen layer). In bulk 1T'-phase dichalcogenide, a plurality of laminates composed of chalcogen layer / metal layer / chalcogen layer are stacked. Each of these laminates corresponds to a single atomic layer of Group 6 transition metal dichalcogenide. On the other hand, the dichalcogenide-containing film according to one embodiment of the present invention exists as a substantially single atomic layer to a few atomic layer film, typically a single atomic layer film or a bilayer film. That is, the number of metal layers in each dichalcogenide crystal grain is substantially 1 to 2, and at most 2, 3, 4, or 5. That is, most of each dichalcogenide crystal grain is composed of a single laminate composed of chalcogen layer / metal layer / chalcogen layer or two laminates overlapping each other. When there are a plurality of laminates, alkali metal ions may be present between the vertically adjacent laminates.

[0020] Also, by applying the above-described manufacturing method, a large dichalcogenide-containing film that cannot be achieved by conventional manufacturing methods can be obtained. For example, the maximum length of the dichalcogenide-containing film is observed in the range of 100 μm or more and 500 μm or less. Here, the maximum length of the dichalcogenide-containing film refers to the maximum length in the direction parallel to the upper surface of the film (that is, the direction perpendicular to the thickness direction).

[0021] Furthermore, the dichalcogenide-containing film according to one embodiment of the present invention has high crystallinity and an extremely small crystal defect density. Due to this, the dichalcogenide-containing film gives a specific Raman spectrum. For example, when the dichalcogenide is tungsten disulfide, the full width at half maximum of the peak with a Raman shift of 130 cm -1 is 2 cm -1 or more and 10 cm -1 or less, or 2 cm -1 or more and 5 cm -1 or less. Also, with a Raman shift of 213 cm -1The Raman shift with respect to the peak intensity is 130 cm -1 The ratio of the peak intensity is 5 or more and 40 or less, 6 or more and 40 or less, or 15 or more and 25 or less, and is, for example, 10 or more. Further, for the peak intensity at a Raman shift of 213 cm -1 the ratio of the peak intensity at a Raman shift of 408 cm -1 is 5 or more and 30 or less, at 10 or more and 30 or less, or 10 or more and 20 or less.

[0022] As described above, by applying the production method according to one embodiment of the present invention, a single-atom layer or two-atom layer dichalcogenide-containing film having a large area and having a 1T′ phase structure can be formed on a substrate. Further, as shown in the examples, the dichalcogenide-containing film is characterized by an extremely small crystal defect density and includes large-area and high-quality 1T′ phase dichalcogenide crystal grains. This is in contrast to the fact that a high-quality large-area dichalcogenide-containing film could not be produced by the conventional method using solid raw materials. Also, in the cleavage of bulk 1T′ phase dichalcogenide, a single-atom layer or two-atom layer of 1T′ phase dichalcogenide cannot be selectively obtained (see Non-Patent Document 5). Therefore, it can be said that the production method according to one embodiment of the present invention is an extremely effective tool for creating an electronic device including a single-atom layer or two-atom layer of 1T′ phase dichalcogenide having specific physical properties.

Examples

[0023] 1. Example 1 In this example, an example of producing a dichalcogenide-containing film containing tungsten disulfide as a dichalcogenide using WF6, H2S, and argon as a precursor, a chalcogen-containing gas, and an inert gas, respectively, will be described.

[0024] As the precursor source, a cylinder filled with a mixed gas of WF6 and argon containing 1% by volume of WF6 was used. As the chalcogen gas source and the inert gas source, cylinders filled with H2S and argon were used respectively. 200 mg of sodium chloride powder, a single crystal silicon substrate (1 cm × 1 cm) with a surface oxide film adhered with sodium chloride, and a cleaved mica substrate (1 cm × 1 cm, manufactured by Crystal Base Co., Ltd., synthetic mica single crystal substrate or natural mica substrate) as a substrate for forming a dicalcogenide-containing film were placed in a chamber to which these sources were connected. The single crystal silicon substrate with sodium chloride adhered was prepared by applying a 0.1 M aqueous solution of sodium chloride to the single crystal silicon substrate and evaporating the water. Argon, H2S, and the mixed gas of the precursor and argon were supplied to the chamber at flow rates of 84, 16, and 0.025 sccm respectively, and the inside of the chamber was heated at 450 to 550 °C. The heating time was 2 hours, and the pressure inside the chamber during heating was maintained at 1 kPa. After the heating was completed, the inside of the chamber was cooled to room temperature, and a tungsten disulfide-containing film (Sample 1) formed on the cleaved mica substrate was obtained.

[0025] The optical microscope image of Sample 1 is shown in Figure 2. In Figure 2, the region observed as bright spots with high luminance is bulk tungsten disulfide. On the other hand, the region with a shape close to a triangle is a single-atomic-layer film or a two-atomic-layer film of tungsten disulfide, and these can be distinguished from each other by the difference in contrast. From Figure 2, it is understood that a single-atomic-layer film or a two-atomic-layer film of tungsten disulfide is selectively obtained, and the generation of bulk tungsten disulfide is limited to a very small part. In addition, from the Raman spectrum, since the Raman spectrum intensity of a relatively bright region excluding the bright spots is about twice that of a relatively dark region, it was confirmed that the former is a two-atomic-layer 1T´ phase tungsten disulfide and the latter is a single-atomic-layer 1T´ phase tungsten disulfide.

[0026] The optical microscope image of a single atomic layer tungsten disulfide film is shown in Fig. 3. As shown in Fig. 3, it was found that a tungsten disulfide-containing film having a generally triangular shape was obtained. Almost no spots were observed in the optical microscope images of each tungsten disulfide-containing film (see Fig. 2), suggesting that each tungsten disulfide-containing film has a uniform layer structure. From Fig. 3, it is understood that the maximum length of Sample 1 exceeds 200 μm. From this, it was confirmed that a large-area chalcogenide-containing film can be obtained by the manufacturing method according to one of the embodiments of the present invention.

[0027] The scanning transmission electron microscope (STEM) images of the single atomic layer tungsten disulfide film of Sample 1 are shown in Figs. 4(A) and 4(B). The STEM images were acquired using a scanning transmission electron microscope (JEM-ARM200F modified) manufactured by JEOL Ltd. As shown in Fig. 4(A), chains in which tungsten atoms are arranged in a zigzag pattern are recognized in the STEM image. From this, it is confirmed that the tungsten disulfide of Sample 1 takes the 1T' phase. Also, from Fig. 4(B), it was confirmed that these zigzag-arranged chains are continuous over a long distance. This suggests that the defect density of the 1T' phase is low and each crystal grain has a large size. Since the sodium of sodium chloride used during fabrication is ionized, it is not easily volatilized and may remain in the fabricated film, but it does not appear in the STEM image due to the difference in atomic number.

[0028] Fig. 5(A) shows the Raman spectrum of the single atomic layer tungsten disulfide film of Sample 1. Fig. 5(A) also shows the Raman spectra of a cleaved mica substrate and 1H-phase tungsten disulfide as references. In the Raman spectrum of 1H-phase tungsten disulfide, strong peaks are observed in the region where the Raman shift is from 300 cm -1 to 400 cm -1 , and relatively sharp peaks are also observed at around 420 cm -1 and 175 cm -1 . On the other hand, these peaks are not seen in the Raman spectrum of Sample 1. Instead, peaks at 130 cm -1 and 408 cm -1Characteristic peaks are confirmed. These results indicate that the tungsten disulfide of Sample 1 does not take the 1H phase but selectively takes the 1T' phase.

[0029] Figure 5(B) shows the Raman spectrum of Sample 1 immediately after fabrication. As can be seen from Figure 5(B), peaks considered to be crystal defects of 1T'-phase tungsten disulfide are observed at 213 cm -1 in the Raman spectrum (see the arrow in the figure), but their intensity is very small. In this Sample 1, the ratio of the peak intensity at a Raman shift of 130 cm -1 to the peak intensity at a Raman shift of 213 cm -1 and the peak at 408 cm -1 was 24 and 12, respectively. These values are much larger compared to those of monolayer 1T'-phase tungsten disulfide crystals grown from solid raw materials (see Non-Patent Document 1). Also, the full width at half maximum of the peak at a Raman shift of 213 cm -1 was 4.8 cm -1 . From this, it is understood that the production method according to one embodiment of the present invention gives a 1T'-phase chalcogenide-containing film having high crystallinity.

[0030] 2. Example 2 In this example, the results of examining the influence of the amount of the precursor on the phase structure of the chalcogenide-containing film are shown.

[0031] Similar to Example 1, WF6, H2S, and argon were used as the precursor, the chalcogen-containing gas, and the inert gas, respectively, and a dichalcogenide-containing film containing tungsten disulfide as a dichalcogenide was fabricated by changing the ratio of WF6 to H2S. The optical microscope image of the obtained tungsten disulfide-containing film is shown in Figure 6. In samples (a) to (c) of Figure 6, it was confirmed from the Raman spectrum that the regions with high brightness are the 1H phase and the regions with low brightness are the single-atom-layer 1T' phase. In sample (d), the bright spots are due to bulk tungsten disulfide. As shown in Figure 6, the volume ratio of H2S to WF6 is 4.05×10 3In this case, although the 1H phase can be confirmed in the central part, it can be seen that the 1T' phase is generated (a). Also, when the volume ratio of H2S to WF6 increases, the generation of the 1H phase is significantly suppressed, and it was confirmed that almost the entire tungsten disulfide-containing film selectively takes the 1T' phase ((b) to (e)). Also, the generation of bulk tungsten disulfide was negligible. These results indicate that it is also possible to selectively obtain a single-atomic-layer 1T'-phase tungsten disulfide-containing film by using a large excess of H2S with respect to WF6.

Explanation of symbols

[0032] 100: apparatus, 102: chamber, 104: precursor source, 106: inert gas source, 108: chalcogen gas source

Claims

1. including heating a precursor containing tungsten, hydrogen sulfide, and an inert gas at a temperature of 450°C or higher and 550°C or lower in the presence of a salt of an alkali metal, wherein a volume ratio of the hydrogen sulfide to the precursor is 2.03×10^4 or more, a method for producing a tungsten sulfide film.

2. The method according to claim 1, wherein the precursor is selected from tungsten hexafluoride and tungsten hexacarbonyl.

3. The method according to claim 1, wherein the salt is selected from halides, hydroxides, oxides, sulfides, carbonates, nitrates, sulfates, phosphates, and carboxylates.

4. The method according to claim 1, wherein the heating is performed on a substrate selected from a mica substrate, a silicon substrate, a quartz substrate, a sapphire substrate, a graphite substrate, a gold substrate, a strontium titanate single crystal substrate, and a boron nitride substrate.

5. The full width at half maximum of the peak with a Raman shift of 130 cm －１ is 10 cm －１ or less, and a tungsten disulfide-containing film having a 1T′ phase.

6. The ratio of the peak intensity at a Raman shift of 130 cm －１ to the peak intensity at a Raman shift of 213 cm －１ is 10 or more, and the tungsten disulfide-containing film according to claim 5.

7. The ratio of the peak intensity at a Raman shift of 408 cm －１ to the peak intensity at a Raman shift of 213 cm －１ is 5 or more, and the tungsten disulfide-containing film according to claim 5.

8. A tungsten disulfide-containing film according to claim 5, wherein a maximum number of tungsten layers in one crystal grain is 2.

9. A tungsten disulfide-containing film according to claim 5, wherein a maximum length is 1 μm or more and 500 μm or less.

10. A tungsten disulfide-containing film according to claim 5, further containing an alkali metal.

11. A tungsten disulfide-containing film according to claim 5, located on a substrate selected from a mica substrate, a silicon substrate, a quartz substrate, a sapphire substrate, a graphite substrate, a gold substrate, a strontium titanate single crystal substrate, and a boron nitride substrate.

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