JP5666984B2 - Preparation method of carbon thin film - Google Patents

Description

  The present invention relates to a method for producing a carbon thin film such as graphene and graphite.

  In recent years, graphene, which is a monolayer sheet composed of carbon atoms, has attracted attention as a future electronic material. Graphene is obtained by extracting one atomic layer of graphite. Graphene has a large carrier mobility and is also chemically stable. For this reason, application as wiring elements, electronic elements such as field effect transistors, and transparent electrode materials is expected.

  Graphene or graphite thin films having the above-described characteristics are mainly synthesized by the following three methods.

  First, there is a peeling method (see Non-Patent Document 1). This is a method in which graphite is peeled many times with an adhesive tape and thinned, and then transferred onto a substrate. A suitable graphene piece is found by microscopic observation or the like to form a device. With this method, graphene with the best mobility can be obtained, but there is no prospect that the area can be increased at present.

  Second, there is a method of thermally decomposing SiC (see Non-Patent Document 2). In this method, silicon is sublimated by heating a SiC substrate at a high temperature to form a graphene or graphite thin film on the substrate surface. The main disadvantage of this method in industrial application is that it may require a high-temperature furnace that can be heated to a high temperature of 1000 ° C. or higher in a vacuum or in an inert gas atmosphere. In addition, a SiC substrate having a large area is not manufactured, and the cost per unit area of the SiC substrate is relatively high, which increases the cost.

  Third, there is a thermal chemical vapor deposition (thermal CVD) method (see Non-Patent Document 3). In this method, a substrate such as a transition metal is heated in an inert gas atmosphere, and a raw material gas containing carbon is supplied to synthesize graphene or a graphite thin film on the substrate surface. This method is relatively low cost among the three methods, and is expected as a method capable of synthesizing a large-area graphene or graphite thin film. However, in general, it is necessary to use a flammable / explosive gas such as methane as a raw material gas, and it takes a considerable cost to ensure safety. In addition, in the thermal CVD method, carbon in the decomposed source gas has an effect of forming a film by adhering to the surface of the already synthesized graphene or graphite thin film, but by such adhesion that does not receive interaction with the metal substrate. Film formation reduces crystallinity.

A.K.Geim and K.S.Novoselov, "The rise of grapheme", Nature Materials, vol.6, pp.183-191, 2007. H. Hibino, H. Kageshima and M. Nagase, "Epitaxial few-layer graphene: towards single crystal growth", Journal of Physics D: Applied Physics, vol.43, 374005, 2010. H.J.Park, J.Meyer, S.Roth, V.Skakalova, "Growth and properties of few-layer graphene prepared by chemical vapor deposition", Carbon, vol.48, pp.1088-1094, 2010.

  As described above, the conventional method for forming graphene or a graphite layer in which graphene is laminated by a peeling method / pyrolysis method has a problem that it cannot be formed in a large area at low cost. As described above, according to the thermal CVD method, graphene can be formed at a relatively low cost, but with this technology, a flammable / explosive gas that is not easy to handle is used, and manufacturing is not easy. There is also a problem in terms of quality.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to enable high-quality graphene to be easily produced in a large area at a low cost.

The method for producing a carbon thin film according to the present invention includes a first step of preparing a metal layer composed of a metal in which carbon dissolves, and an amorphous carbon on the metal layer by a deposition method selected from a sputtering method and a vapor deposition method. A second step of forming a carbon layer by depositing, a third step of dissolving the carbon layer in the metal layer by heating, and the carbon dissolved in the metal layer by lowering the temperature of the heating to the metal layer And a fourth step of forming graphene on the surface of the metal layer.

  In the carbon thin film manufacturing method, the metal may be nickel. In this case, the metal layer is formed in a layer thickness of 280 to 320 nm, the carbon layer is formed in a layer thickness of 16 to 20 nm, the heating temperature is in the range of 880 to 920 ° C., and the heating holding time is May be in the range of 28 to 32 minutes.

  As described above, according to the present invention, since the carbon layer is dissolved in the metal layer and then deposited, the high-quality graphene can be easily manufactured in a large area at a low cost. An effect is obtained.

FIG. 1A is a cross-sectional view schematically showing a state in each step for explaining a method for producing a carbon thin film in an embodiment of the present invention. FIG. 1B is a cross-sectional view schematically showing a state in each step for explaining a method for producing a carbon thin film in the embodiment of the present invention. FIG. 1C is a cross-sectional view schematically showing a state in each step for explaining a method for producing a carbon thin film in the embodiment of the present invention. FIG. 1D is a cross-sectional view schematically showing a state in each step for explaining the method for producing the carbon thin film in the embodiment of the present invention. FIG. 2 is a characteristic diagram showing a Raman spectrum of a carbon thin film.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1A to 1D are cross-sectional views schematically showing states in respective steps for explaining a method for producing a carbon thin film in an embodiment of the present invention.

  First, as shown in FIG. 1A, a metal layer 102 made of a metal in which carbon is dissolved is prepared (first step). For example, a polycrystalline metal layer 102 made of nickel (Ni) may be formed on a substrate 101 made of silicon on which a silicon oxide layer is formed by a sputtering method, a vacuum evaporation method, or the like. By forming the metal layer 102 through the silicon oxide layer, the reaction between silicon and metal can be prevented in the heating step described later. The silicon oxide layer may have a thickness of about 100 nm. The polycrystalline metal layer 102 made of nickel may have a thickness of about 300 nm.

  Next, as shown in FIG. 1B, a carbon layer 103 is formed on the metal layer 102 (second step). Here, the carbon layer 103 is formed by depositing carbon on the metal layer 102 by a deposition method selected from a sputtering method and an evaporation method. Examples of the vapor deposition method include a vacuum vapor deposition method and an electron beam vapor deposition method. The carbon layer 103 thus formed is in an amorphous state. For example, the carbon layer 103 may be formed to a thickness of about 18 nm by a sputtering method.

Next, the carbon layer 103 is dissolved in the metal layer 102 by heating (third step). By dissolving (solid solution) the carbon layer 103 in the metal layer 102, a solid solution layer 102a is formed on the substrate 101 as shown in FIG. 1C. For example, the substrate temperature is raised to 900 ° C. at 90 ° C./min in an electric furnace in which argon gas is supplied at 1000 sccm to make 66661 Pa (500 Torr), and this state is maintained for 30 minutes. Note that sccm is a unit of flow rate, and indicates that a fluid at 0 ° C. and 1013 hPa flows 1 cm ³ per minute.

  By heating to 900 ° C., amorphous carbon dissolves in nickel. Further, by setting the holding time at 900 ° C. to about 30 minutes, all of the carbon layer 103 having a layer thickness of about 18 nm is dissolved in the metal layer 102. Thus, in the solid solution layer 102a in which carbon is dissolved, it is considered that the carbon forms a concentration distribution in the layer thickness direction. For example, immediately after all the carbon is dissolved, the carbon concentration is higher in the vicinity of the surface of the solid solution layer 102a than in the interior. Thereafter, with the passage of time, carbon diffuses in the layer thickness direction.

  Next, by reducing the heating temperature, as shown in FIG. 1D, carbon dissolved in the metal layer 102 is deposited on the surface of the metal layer 102 to form graphene 104 on the surface of the metal layer 102 ( (4th process). For example, as described above, after holding at 900 ° C. for 30 minutes, the temperature may be decreased to room temperature (about 20 ° C.) at 20 ° C. per minute. As the temperature decreases, carbon that cannot be completely dissolved in the vicinity of the surface of the solid solution layer 102a is deposited on the surface as graphene (or a graphite layer). If a plurality of graphene is deposited, it becomes a graphite layer.

  The thickness of the graphene or graphite layer deposited on the metal layer 102 as described above depends on the amount of carbon dissolved in the vicinity of the surface of the solid solution layer 102a. All are crystallized from the interface of the metal layer 102 to form a graphene or graphite layer. In the deposited layer, there is no amorphous carbon layer.

  Here, in the formation of the graphite layer by precipitation as described above, it is important to control the layer thickness, heating temperature, and heating holding time of the metal layer 102 and the carbon layer 103. When the carbon concentration in the vicinity of the surface of the solid solution layer 102a in which carbon is dissolved is below a certain limit, even if the temperature is returned to a room temperature from a high temperature state, for example, the graphene or graphite layer no longer precipitates, It remains dissolved. Under the conditions where the metal layer 102 is too thick, the carbon layer 103 is too thin, the heating temperature is too high, or the holding time at a high temperature is too long, as described above, carbon deposition does not occur, and graphene Or a graphite layer is not obtained.

  On the other hand, if the metal layer 102 is too thin, the carbon layer 103 is too thick, the heating temperature is too low, or the holding time at high temperature is too short, all the carbon layers 103 are dissolved in the metal layer 102. You can't do it and it will remain. On the other hand, if the metal layer 102 is too thin, holes are formed in the metal layer 102 when heated, or they are aggregated discretely.

  The conditions for forming the graphene or the graphite layer by precipitation are not limited to the above-described conditions. It is important to adjust so that all the carbon layers 103 are dissolved in the metal layer 102 and the carbon concentration in the vicinity of the surface of the solid solution layer 102a becomes an appropriate value. For example, when the metal layer 102 is made of nickel, the metal layer 102 may have a layer thickness in the range of 280 to 320 nm. Moreover, the carbon layer 103 should just be the range of 16-20 nm in layer thickness. The heating temperature may be in the range of 880 to 920 ° C., and the heating holding time may be in the range of 28 to 32 minutes.

  Next, the evaluation results of the quality of the graphene or graphite layer (carbon thin film) produced according to this embodiment will be described with reference to FIG. FIG. 2 is a characteristic diagram showing a Raman spectrum of a carbon thin film. The excitation light wavelength was 532 nm. For comparison, a spectrum of graphene or graphite formed on a nickel layer by a thermal CVD method using methane as a raw material and a spectrum of an amorphous carbon thin film deposited by sputtering are also shown. In FIG. 2, (a) is the Raman spectrum of the carbon thin film in the present embodiment, (b) is the Raman spectrum of graphene or graphite formed by the CVD method, and (c) is the Raman spectrum of the amorphous carbon thin film. It is a spectrum.

  As shown in FIG. 2A, the carbon thin film in the present embodiment has no evidence of remaining amorphous carbon layer, and the obtained thin film is a graphene or graphite layered from the surface to the interface of the metal layer. It is thought that it is composed of In general, the ratio between the G band and the D band is used as an index representing the crystallinity of graphite. The larger the G / D ratio, the higher the crystallinity.

  The G / D ratio of the graphene or graphite thin film synthesized by the thermal CVD method shown in FIG. 2B is about 22, whereas the G / D ratio of the carbon thin film in this embodiment is about 60. This value of “60” is equal to or higher than the G / D ratio of graphene or graphite thin film obtained by a general thermal CVD method.

  Even in the thermal CVD method, carbon atoms once dissolved in the metal layer are considered to have the effect of precipitating from the metal as graphene or graphite thin film, but in addition to this, the source gas molecules decompose and directly carbon without passing through the substrate It is considered that there is an effect of depositing on the surface. The latter effect by the CVD method is particularly important when forming a thick graphite film. However, graphene or graphite thin films deposited from metal on the substrate surface are generally very regular in atomic arrangement due to the interaction with the metal on the substrate surface, whereas carbon is directly attached to the graphene or graphite thin film surface. Defects are easily generated in the film formation by, and the quality of the generated graphene or graphite thin film is deteriorated. Since the carbon thin film in this Embodiment is formed only by precipitation from a metal, it is thought that crystallinity is high compared with a thermal CVD method.

  According to the present invention described above, a good-quality crystalline carbon thin film that does not contain amorphous carbon can be easily obtained by using a generally used film forming apparatus and heating apparatus. For this reason, a graphene or a graphite layer can be formed safely at low cost, in a large area.

  The graphene or graphite layer thus produced can be used as a wiring material or a transparent electrode material. Further, if a graphene or graphite layer is transferred to an insulating substrate, it can also be used as an electronic element material.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious. For example, in the above description, the metal layer is made of nickel. However, the present invention is not limited to this, and the metal layer may be made of cobalt. The metal layer should just be comprised from the metal which carbon melt | dissolves.

  DESCRIPTION OF SYMBOLS 101 ... Substrate, 102 ... Metal layer, 102a ... Solid solution layer, 103 ... Carbon layer, 104 ... Graphene.

Claims (3)

A first step of preparing a metal layer composed of a metal in which carbon dissolves;
A second step of forming a carbon layer by depositing amorphous carbon on the metal layer by a deposition method selected from a sputtering method and an evaporation method;
A third step of dissolving the carbon layer in the metal layer by heating;
And a fourth step of forming graphene on the surface of the metal layer by precipitating carbon dissolved in the metal layer by lowering the heating temperature to form graphene on the surface of the metal layer. Thin film manufacturing method. In the manufacturing method of the carbon thin film of Claim 1,
A method for producing a carbon thin film, wherein the metal is nickel. In the manufacturing method of the carbon thin film of Claim 2,
The metal layer is formed in a layer thickness range of 280 to 320 nm,
The carbon layer is formed in a layer thickness of 16 to 20 nm,
The heating temperature is in the range of 880 to 920 ° C.
The heating holding time is in the range of 28 to 32 minutes.

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