JP6754355B2 - Graphene and electronic devices and their manufacturing methods - Google Patents

Description

The present invention relates to a graphene manufacturing technique, and more particularly to a technique for producing a large area and high quality single layer graphene at a low temperature and a technique for applying such graphene to an electronic device.

Graphene has various specific properties due to its ideal two-dimensional structure, for example, good conductivity and high electron / hole mobility. In addition to this, properties such as inelastic electron conductivity, spin conductivity, mechanical strength, light absorption and light emission, and heat conduction are also attracting attention, and are expected to be industrially applied to various fields. Is.

Currently, as a method for producing graphene, a method for exfoliating from a pyrolysis graphite crystal, a solid phase reaction method, a chemical vapor deposition method, and the like are known.

The first discovery of graphene was made by Novoselov et al., Which is obtained by peeling pyrolysis graphite crystals with scotch tape and transferring them to a Si substrate. The size of the graphene crystal is about several microns, but it is characterized in that a graphene film can be easily obtained with relatively good reproducibility (Non-Patent Documents 1 and 2).

In the solid-state reaction method, silicon carbide crystals are vacuum-heated at about 2000 ° C. to evaporate (silicon sublimation) the Si element on the surface, and a graphene crystal layer is formed on the surface of silicon carbide (Non-Patent Documents 3 to 7). ).

In the vapor phase growth method, a graphene film is formed on the surface of a metal crystal such as nickel or iron by growing a hydrocarbon gas as a raw material by a thermal CVD method (Non-Patent Documents 8 and 9).

Further, according to the plasma CVD method using a copper foil as a catalyst, graphene can be synthesized at a low substrate temperature of 300 to 400 ° C. (Non-Patent Document 10), but the obtained graphene is not a single layer but a multilayer. is there.

Further, according to the plasma assist method, it is said that a single layer graphene can be obtained even at a relatively low substrate temperature of about 600 ° C. (Non-Patent Document 11), but the Raman scattering spectrum of the obtained graphene is 1300 cm ^−. A very strong D peak due to a defect appearing in the vicinity of ¹ is observed. In addition, a large amount of minute graphene nuclei are generated as the substrate temperature decreases, and as a result, when the synthesized graphene film is viewed microscopically, grain boundaries exist at high density, which significantly reduces electron mobility. I will let you. In addition, there is also a problem that defects are formed on the surface and inside of the film due to the impact of plasma ions, and the electrical characteristics are deteriorated.

As described above, it is very difficult to obtain single-layer graphene at a relatively low temperature by the conventional method. For example, in the vapor phase growth method by thermal CVD, it becomes close to a graphite film in which several tens of layers are laminated. There is.

Moreover, since it is very difficult to isolate the synthesized graphene from the base material and transfer it onto an electronic device substrate such as Si, there is also a problem from the viewpoint of application to the electronic device.

When graphene is used as a part of parts such as electronic devices, sensors, and wiring, it is an important technical issue to dispose graphene at a desired position and in a desired direction. There is a problem that various technical issues cannot be effectively dealt with.

The main cause of these problems is the fact that pyrolyzed graphite crystals are peeled off with scotch tape as in the existing graphene element reports, and the graphene film accidentally retransferred is found and used for element formation. As shown by, it is very difficult to obtain a large-area graphene film. There is a large variation in the size and uniformity of the exfoliated graphene itself, and the inability to arbitrarily control the transfer position is also a major factor. Furthermore, since graphene is originally a film with the ultimate thickness of a single atomic layer composed of a six-membered carbon ring, handling in a large area involves fundamental and principle difficulties.

In addition, from the viewpoint of application to electronic devices and the like utilizing the features of graphene, a technique capable of further lowering the synthesis temperature of a large-area, high-quality single-layer graphene is desired.

Regarding the problem of synthesizing graphene at low temperature, the present inventors have proposed a method for producing a graphene film capable of producing large-area graphene without requiring high temperature (Patent Document 1).

In the method reported by the present inventors in Patent Document 1, first, an amorphous carbon film having a diameter of several millimeters to several centimeters and a thickness of several nanometers is synthesized as a raw material of the target graphene film.

Specifically, as a raw material for an amorphous carbon film, an organic film such as a collodion film or a fornvar film is formed, the organic film is scooped up with a ceramic ring such as alumina, and an organic film is put on the ceramic ring.

Next, the organic film is vacuum-heat-treated to carbonize it into an amorphous state to synthesize an amorphous carbon film. For example, when a ceramic ring covered with an organic film is vacuum-heat-treated at 500 ° C. for 30 minutes, the organic film is decomposed and denatured into carbon.

Further, the amorphous carbon film thus obtained is brought into contact with the surface of liquid gallium, transferred, and heat-treated in vacuum. Due to the extremely strong surface tension of liquid gallium, the liquid surface rises in a convex shape, and the amorphous carbon film is transferred to the convex liquid surface of the liquid gallium.

Then, the amorphous carbon film is heat-treated in a vacuum at 1000 ° C. for about 10 to 30 minutes to form graphene at the contact interface between the liquid gallium and the amorphous carbon film.

International Publication No. WO2010 / 110153

Novoselov, K.S. et al. “Electric field effect in atomically thin carbon films” Science 306 (2004) 666-669. Meyer JC, et al, “The structure of suspended graphene sheets” Nature. 446 (7131) (2007) 60-63. A. J. van Bommel, et al. “A. LEED and Auger electron observations of the SiC (0001) surface” Surf. Sci. 48 (1975) 463-472. I. Forbeaux et al. “Heteroepitaxial graphite on 6H-SiC (0001): Interface formation through conduction-band electronic structure” Phys. Rev. B 58 (1998) 16396-16406. Berger, C. et al. “Ultrathin epitaxy graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics” J. Phys. Chem. B, 108, (2004) 19912-19916. Berger, C. et al. “Electronic confinement and coherence in patterned ceramic grapheme”, Science 312, (2006) 1191-1196. Ohta, T. et al. “Controlling the electronic structure of bilayer grapheme”, Science 313 (2006) 951-954. T. A. Land, et al. "STM investigation of single layer graphite structures produced on Pt (111) by hydrocarbon decomposition", Sur. Sci. 264 (1992) 261-270. A. Nagashima et al. “Electronic states of monolayer graphite formed on TiC (111) surface”, Surf. Sci. 291 (1993) 93-98. J. Kim et al. “Low-temperature synthesis of large-area graphite-based transparent conductive films using surface wave plasma chemical vapor deposition” Appl. Phys. Lett., 98, 091502 (2011). S. Chan et al. “Low-temperature synthesis of graphite on Cu using plasma-assisted thermal chemical vapor deposition” Nanoscale Res. Lett., 8, 285 (2013).

The method of Patent Document 1 is aimed at synthesizing graphene at low temperature, but the present inventors "from the study of the reaction process between gallium and carbon, the graphitization reaction at the interface between liquid gallium and amorphous carbon". Is induced at a relatively low temperature of about 1000 ° C., and graphene having a large area can be formed. ”Therefore, it cannot be said that the temperature reduction is sufficient.

As described above, from the viewpoint of application to electronic devices and the like utilizing the features of graphene, a low-temperature synthesis technology for large-area and high-quality single-layer graphene is desired, and in particular, it is hybridized with existing silicon devices. Assuming the above, a technique capable of producing the graphene even at a low temperature of about 400 ° C. is desired.

The present invention has been made in view of the problems of the conventional method, and an object of the present invention is to produce graphene having a large area and high quality at a low temperature that cannot be achieved by the conventional method. Is to provide the technology to do.

In view of the above problems, the method for producing graphene according to the first aspect of the present invention includes a first step of forming a carbon six-membered ring-forming nucleus on the main surface of the substrate and two around the forming nucleus. A second step of dimensionally forming a carbon six-membered ring to form a graphene film is provided, and the formation of the carbon six-membered ring in the second step is carried out at a temperature of 300 ° C. or lower.

In one embodiment, in the first step, a gas containing a hydrocarbon is supplied in a state where a metal droplet having a melting point of 400 ° C. or lower is in contact with the main surface of the substrate, and the main surface of the substrate and the said. The generated nucleus is formed on the contact surface with the metal.

For example, the metal having a melting point of 400 ° C. or lower is at least one selected from the group of gallium, indium, and gallium-indium alloy.

In another aspect, in the formation of the generated nuclei in the first step, amorphous carbon nuclei previously arranged on the main surface of the substrate are formed by a photolithography method, and a liquid is formed on the main surface of the substrate. It is carried out by heat treatment with gallium in contact.

Furthermore, in another aspect, the production nucleus formed in the first step is a carbon six-membered ring formed by the exfoliation method.

Preferably, in the second step, hydrocarbons are added in a state where a metal having a catalytic action for graphene synthesis and having a melting point of 400 ° C. or lower is brought into contact with the main surface region of the substrate on which the generated nuclei are formed. The gas contained is supplied, and the carbon six-membered ring is formed from the contact interface between the main surface of the substrate and the metal to form a graphene film.

Further, preferably, a metal having a melting point lower than the formation temperature of the six-membered carbon ring is selected as the metal having a catalytic action, and the metal droplets are brought into contact with the main surface region of the substrate. Form a six-membered carbon ring.

For example, the metal having a melting point of 400 ° C. or lower is at least one selected from the group of gallium, indium, and gallium-indium alloy.

Preferably, the gas containing the hydrocarbon is converted into plasma and supplied.

The method for producing graphene according to the second aspect of the present invention comprises a first step of forming a carbon six-membered ring-forming nuclei on the main surface of the substrate, and graphene synthesis on the main surface of the substrate on which the forming nuclei are formed. A metal having a catalytic action of the above and having a melting point of 400 ° C. or lower is brought into contact with the metal, and a gas containing hydrocarbon is supplied while the metal is held at a temperature exceeding the melting point, and the metal is combined with the metal on the main surface of the substrate. It comprises a second step of growing graphene at the contact interface.

For example, the metal having a melting point of 400 ° C. or lower is at least one selected from the group of gallium, indium, and gallium-indium alloy.

Preferably, the first step is carried out by supplying a gas containing a hydrocarbon while holding the substrate at a temperature of 1000 ° C. or higher.

Further, preferably, in the first step, a gas containing the hydrocarbon in a state where a metal having a catalytic action for graphene synthesis and having a melting point of less than 1000 ° C. is brought into contact with the main surface of the substrate. Is supplied and executed.

For example, the metal having a melting point of less than 1000 ° C. is at least one selected from the group of gallium, indium, antimony and alloys thereof.

In some embodiments, the first step is performed by transferring graphite or graphene onto the main surface of the substrate by a peeling method.

In another aspect, the first step is performed by patterning graphite or graphene on the main surface of the substrate by a photolithography method.

Further, in another aspect, the second step is carried out by increasing the decomposition efficiency of the hydrocarbon gas by the plasma assist method.

Preferably, the second step selects gallium as the metal having a melting point of 400 ° C. or lower and is carried out at a temperature of 300 ° C. or lower.

Further, preferably, following the second step, a third step of separating the graphene from the substrate is provided.

For example, the substrate is a sapphire whose main surface is the (0001) plane (C plane) whose off-angle absolute value is within 30 °.

In the method for producing an electronic element according to the present invention, a metal having a catalytic action for graphene synthesis and having a melting point of 400 ° C. or lower is brought into contact with a resist pattern formed on the main surface of the substrate, and the metal exceeds the melting point. It is characterized by including a step of supplying a gas containing hydrocarbon while being held at a temperature, growing graphene at a contact interface with a metal on the main surface of the substrate, and grapheneizing the resist pattern to form a circuit. To do.

For example, the metal having a melting point of 400 ° C. or lower is at least one selected from the group of gallium, indium, and alloys thereof.

The graphene according to the present invention is composed of a plurality of domains composed of a set of six-membered carbon rings, and each of the plurality of domains has a generation nucleus of the six-membered carbon ring in the domain, and the density thereof is 1. It is characterized in that it is × 10 ⁶ cm- ² or less.

Preferably, the orientation of the six-membered carbon ring in the domain is substantially the same within a range of at least ^10-7 cm ² of the graphene membrane, and the continuous membrane has an area of 1 × ^10-7 cm ² or more. , Substantially a monolayer film.

Also preferably, the peak intensity ratio of the peak and 1590 cm ^-1 near the G bands of 2D band 2700 cm ^-1 vicinity found in Raman during scattering spectra obtained from the graphene (2D / G ratio ₌ I 2D / _{I G} ) Is 1 or more.

Still preferably, the intensity of the peak of the D band peak and 1300 cm ^-1 near the G band of 1590 cm ^-1 vicinity found in the Raman scattering spectrum obtained from graphene ratio (G / D ratio ₌ I G / _{I D} ) Is 3 or more.

The electronic device according to the present invention is an electronic device provided with the above-mentioned graphene.

In a preferred embodiment, it has a channel region composed of the graphene.

In the present invention, a carbon 6-membered ring formation nucleus is formed in advance on the substrate, and a two-dimensional carbon 6-membered ring is formed around the formation nucleus. This provides a large area and high quality single layer graphene low temperature synthesis technique.

By applying such graphene to electronic devices, its good conductivity, high electron / hole mobility, inelastic electron conductivity and spin conductivity, mechanical strength, light absorption and light emission, heat conduction, etc. High-performance electronic devices can be realized due to the superior characteristics of.

It is a figure for demonstrating an example of the method for synthesizing (manufacturing) graphene which concerns on this invention, FIG. 1 (a) is a figure which shows the structural example of a reaction system, and FIG. 1 (b) is a process temperature sequence. It is a figure which shows the example of. SEM image of the generated nuclei formed after the first step (5 minutes at 1050 ° C.) (FIG. 2 (a)), SEM image of graphene grown after the second step (6 minutes at 400 ° C.) (FIG. 2 (Fig. 2)). b)), SEM image of graphene grown after the second step (7 minutes at 400 ° C.) (FIG. 2 (c)), μ Raman scattering spectrum from sapphire substrate, generated nuclei, graphene, and graphene edges (b)). An AFM image (FIG. 2 (e)) showing how the generated nuclei shown in FIGS. 2 (d) and 2 (a) have a hexagonal spiral structure, and obtained after the second step. It is an AFM image (FIG. 2 (f)) which shows the surface state of graphene. 2D / G ratio Raman mapping (FIGS. 3 (a) to (c)) and G / D ratio Raman mapping corresponding to each sample whose SEM image is shown in FIGS. 2 (a) to 2 (c). 3 (d) to 3 (f)). It is a TEM image obtained from graphene after the completion of the second step. It is an example of the optical micrograph (FIG. 5 (a)) of the FET prototyped by the present inventors using the graphene of the present invention, and the CV curve (FIG. 5 (b)) thereof. 6 is an SEM image of graphene obtained at a synthesis temperature of 200 ° C. It is a Raman scattering spectrum from graphene obtained at a synthesis temperature of 200 ° C. It is a Raman scattering spectrum from graphene obtained at a synthesis temperature of 100 ° C.

Hereinafter, with reference to the drawings, graphene according to the present invention, a method for producing the same, and an application example to an electronic device will be described.

In the present invention, when graphene is synthesized by using a metal such as gallium as a catalyst and a hydrocarbon gas such as methane as a carbon source, graphene is formed on a substrate on which a carbon six-membered ring formation nucleus is formed in advance. Based on the new findings of the present inventors, it is possible to obtain substantially a single layer of graphene in a large area and with high quality even at an extremely low temperature of about 100 ° C., which is much lower than the temperature at which graphene is synthesized by the conventional method. It is based on.

As described in Patent Document 1 described above, from a metallurgical point of view, gallium and carbon are completely insoluble. However, X-ray absorption near edge spectrum (NEXAFS) data show that a bond between gallium and carbon atoms is formed. As a result of further studies, the present inventors have formed a carbon 6-membered ring formation nucleus on the substrate in advance, and the formation of a two-dimensional carbon 6-membered ring around this formation nucleus is 100. We have come to the conclusion that it progresses even at extremely low temperatures such as ° C.

In the following description, the metal acting as a catalyst will be described as gallium, but the present invention does not exclude metals having a catalytic action similar to that of gallium, and examples of such metals include indium and antimony. Can be exemplified. Further, an alloy of gallium, indium, or antimony may be used as one having such a catalytic action.

FIG. 1 is a diagram for explaining an example of a method for synthesizing (manufacturing) graphene according to the present invention, FIG. 1A is a diagram showing a configuration example of a reaction system, and FIG. 1B is a diagram. Is a diagram showing an example of a process temperature sequence.

In the example of the reaction system shown in this figure, as the first step, a metal having a catalytic action of graphene synthesis on the main surface of the substrate 10 inside the quartz tube 100 capable of high-frequency heating and having a melting point of 1000. A gas containing a hydrocarbon is supplied in a state where the metal 20 having a temperature lower than ° C. is in contact with the metal 20 to form a carbon six-membered ring-forming nucleus on the main surface of the substrate 10 by the CVD method.

The formation nuclei of the six-membered carbon ring are, for example, hexagonal graphite or multi-layer graphene, and the carbon that is the source of such formation nuclei is supplied to the contact interface between the substrate 10 and the metal 20 and reaches the interface region. A production nucleus will be formed.

The reason why the melting point of the metal 20 is less than 1000 ° C. is that the metal 20 needs to be in a liquid state in order to act as a catalyst for graphene synthesis, and the formation nucleus of the carbon six-membered ring is efficiently formed. This is because a temperature of 1000 ° C. or higher is required for formation.

That is, in the example shown in FIG. 1A, in the first step, a gas containing a hydrocarbon is supplied while the substrate 10 is held at a temperature of 1000 ° C. or higher, so that the nuclei produced by the CVD method are used. The formation is carried out.

Here, examples of the above-mentioned metal having a melting point of less than 1000 ° C. include gallium (melting point 29.78 ° C.), indium (melting point 156.61 ° C.), antimony (melting point 630.74 ° C.), and the like. At least one of these metals is selected. Further, instead of these metals, alloys of gallium, indium and antimony may be used. That is, at least one selected from the group of gallium, indium, antimony and alloys thereof may be selected as the above-mentioned "metal having a melting point of less than 1000 ° C.".

As will be described later, according to the method of the present invention, it is possible to form graphene at a low temperature of 300 ° C. or lower, but the premise is that the formation nucleus of the carbon six-membered ring on the main surface of the substrate is formed. It is formation.

In the example shown in FIG. 1, when forming a carbon six-membered ring-forming nucleus on the main surface of the substrate, a metal having a catalytic action for raphen synthesis and having a melting point of less than 1000 ° C. is contacted on the main surface of the substrate. In this state, a gas containing a hydrocarbon is supplied. However, in addition to such a method, a carbon six-membered ring-forming nucleus may be formed on the main surface of the substrate as follows.

For example, a gas containing a hydrocarbon is supplied in a state where a metal droplet having a melting point of 400 ° C. or less is in contact with the main surface of the substrate, and a carbon six-membered ring is formed on the contact surface between the main surface of the substrate and the metal. It can also form a nucleus.

In this case, as the metal having a melting point of 400 ° C. or lower, at least one selected from the group of gallium, indium, and gallium-indium alloy can be exemplified.

Further, by forming an amorphous carbon nucleus arranged in advance on the main surface of the substrate by a photolithography method and heat-treating the main surface of the substrate in contact with liquid gallium, the main surface of the substrate and the metal are formed. It is also possible to form a formation nucleus of a carbon six-membered ring on the contact surface with.

Further, a six-membered carbon ring may be formed on the main surface of the substrate by the peeling method and used as a production nucleus.

By providing such a formation nucleus in advance, a carbon six-membered ring is two-dimensionally formed around the formation nucleus in the graphene growth step described later to form a graphene film.

In the second step following the first step, for example, a metal 25 having a catalytic action for graphene synthesis and having a melting point of 400 ° C. or lower was brought into contact with the main surface of the substrate 10 after the formation of the nucleated nuclei. In this state, graphene is grown by supplying a gas containing a hydrocarbon as in the first step.

That is, in the second step, a gas containing a hydrocarbon in a state where a metal having a catalytic action for graphene synthesis and having a melting point of 400 ° C. or less is in contact with the main surface region of the substrate on which the formation nuclei are formed. Is supplied, and the carbon six-membered ring is formed from the contact interface between the main surface of the substrate and the metal to form a graphene film.

For example, a metal having a melting point lower than the formation temperature of the 6-membered carbon ring is selected as the metal having a catalytic action, and the 6-membered carbon ring is formed in a state where the droplets of the metal are in contact with the main surface region of the substrate. ..

For example, the metal having a melting point of 400 ° C. or lower is at least one selected from the group of gallium, indium, and gallium-indium alloy.

Further, when graphene is grown by supplying a gas containing a hydrocarbon, the gas containing the hydrocarbon may be converted into plasma and supplied.

The carbon atoms that make up graphene are supplied to the contact interface between the substrate 10 and the metal 25, and graphene grows CVD around the generated nuclei in the interface region.

The reason why the melting point of the metal 25 is set to 400 ° C. or lower is that the metal 25 needs to be in a liquid state in order to act as a catalyst for graphene synthesis, and the present invention aims to lower the graphene synthesis temperature. This is because the purpose is to synthesize graphene at a low temperature of about 400 ° C., assuming hybridization with an existing silicon device. Of course, it goes without saying that the graphene according to the present invention can be produced even if the processing temperature in the second step is set to a temperature exceeding 400 ° C.

Here, as described above, as the metal having a melting point of 400 ° C. or lower, gallium (melting point 29.78 ° C.) and indium (melting point 156.61 ° C.) can be exemplified, and alloys thereof may be used.

As described above, the first aspect of the graphene production method of the present invention is a first step of forming a carbon six-membered ring-forming nucleus on the main surface of the substrate, and two-dimensional carbon around the carbon. A second step of forming a six-membered ring to form a graphene film is provided, and the formation of a carbon six-membered ring in the second step is carried out at a temperature of 300 ° C. or lower. In this case, preferably, in the first step, a gas containing a hydrocarbon is supplied in a state where a metal droplet having a melting point of 400 ° C. or lower is in contact with the main surface of the substrate, and the main surface of the substrate is supplied. The generated nucleus is formed on the contact surface between the metal and the metal.

In addition, the second aspect of the method for producing graphene of the present invention is a first step of forming a carbon six-membered ring-forming nuclei on the main surface of the substrate, and graphene on the main surface of the substrate on which the forming nuclei are formed. A metal having a synthetic catalytic action and having a melting point of 400 ° C. or lower is brought into contact with the metal, and a gas containing hydrocarbon is supplied while the metal is held at a temperature exceeding the melting point, and the metal is combined with the metal on the main surface of the substrate. It comprises a second step of growing graphene at the contact interface of the.

The CVD conditions are appropriately optimized, including the selection of the hydrocarbon gas type as the carbon source. For example, 5% methane (CH ₄ ) gas as a source gas in the quartz tube 100 is 1 sccm and has a purity of 6 N. Argon (Ar) gas is supplied at 250 sccm to maintain the pressure during the CVD reaction at approximately 0.5 atm. Under this condition, the partial pressure of methane is approximately 10 Pa.

As the substrate 10, for example, a sapphire having a (0001) plane (C plane) having an absolute value of off angle within 30 ° as a main plane can be used. This is because the atomic arrangement in the C plane has a symmetry of 6 times, so that an advantage that it matches the symmetry of the atomic arrangement of the carbon 6-membered ring can be expected. In addition, the off-angle may be appropriately selected in consideration of the density (and terrace width) of the steps that become kink during nucleation formation. Since it is considered that a terrace width capable of connecting two benzene rings is necessary for the growth of graphene, the absolute value of the off angle is preferably within 30 °. The present inventors mainly used a C-plane sapphire substrate having an absolute off-angle value of 0.2 °.

Liquid gallium as the metal 20 (25) acting as a catalyst has low wettability with the main surface of the sapphire substrate 10. Therefore, the liquid gallium is close to a state of being "dropped" on the substrate, and the contact interface with the main surface of the substrate also maintains the property as a "surface". As a result, the contact angle exceeds 90 ° and is in contact with the main surface of the substrate in a state of being greatly raised.

When the raw material gas is supplied into the quartz tube 100, the methane gas decomposes into hydrogen (H or H ₂ ) and carbon (C) on the surface of the liquid gallium even at an extremely low temperature of, for example, about 100 ° C. As mentioned above, gallium and carbon are totally insoluble, but since gallium atoms and carbon atoms can be bonded, some of the carbon atoms generated by the decomposition of methane gas are on the surface of liquid gallium. Bond with an atom.

Since gallium is in a liquid state, atoms are easily diffused near the surface (including the contact interface with the main surface of the substrate). Therefore, the carbon atom bonded to the gallium atom on the surface easily diffuses on the surface of the liquid gallium in the form guided by the gallium atom, and the carbon is rapidly supplied to the contact interface with the main surface of the substrate. On the other hand, since carbon atoms are hardly supplied to the inside of the "droplets" of liquid gallium, the supply of carbon atoms to the contact interface with the main surface of the substrate is from the surface of liquid gallium (including the contact interface with the main surface of the substrate). Will only be done. That is, in the method of the present invention, the supply of carbon atoms to the main surface of the substrate is substantially limited to two-dimensional ones, and the supply of three-dimensional carbon atoms is suppressed, so that the supply of single-layer graphene is suppressed. Extremely advantageous for synthesis.

In the sequence example shown in FIG. 1 (b), the temperature is raised to 1050 ° C. in an Ar atmosphere, methane gas is supplied, and the first step (1st step) is executed. The formation of nucleation nuclei in the first step starts immediately after the supply of methane gas, and nucleation nuclei having a sufficient density are formed in a short time of about 5 minutes. In an example in which the present inventors conducted a nucleation formation experiment on a C-plane sapphire substrate (1 cm ² ) having an off-angle of 0.2 ° using gallium as a catalyst metal, the temperature was 1050 ° C. for 5 minutes. after the reaction, product nuclei have been obtained at a density of 10 ⁵ cm ^-2 order with hexagonal spiral structure.

In the example shown in FIG. 1B, after the first step described above, the supply of methane gas is stopped, the substrate 10 is rapidly cooled in the quartz tube 100, the temperature is raised to 400 ° C. again, and the methane gas is supplied. The second step (2nd step) is carried out to grow a carbon six-membered ring centering on the graphene-forming nucleus formed in the first step.

Graphene grows even at a temperature lower than 400 ° C. (for example, 100 ° C.), and in particular, when gallium is selected as the metal having a melting point of 400 ° C. or lower, a high-quality single layer can be used even at a temperature of 300 ° C. or lower. Graphene can be obtained in the first area. However, as the temperature decreases, the rate at which carbon atoms diffuse on the surface of liquid gallium decreases, and as a result, the rate of supply to the formation nuclei decreases, so the reaction time for obtaining single-layer graphene of a predetermined area decreases. Becomes longer. That is, the reaction time of the second step is appropriately set according to the reaction temperature and the film area of graphene to be finally obtained.

For the problem that the growth rate decreases due to the lowered graphene growth temperature, a method of increasing the decomposition efficiency of hydrocarbon gas by the plasma assist method is effective. By applying the plasma assist method, a desired growth rate can be secured even when the growth temperature is lowered.

FIG. 2 shows an SEM image of the produced nuclei formed after the first step (5 minutes at 1050 ° C.) (FIG. 2 (a)) and an SEM image of graphene grown after the second step (6 minutes at 400 ° C.). (Fig. 2 (b)), SEM image of graphene grown after the second step (7 minutes at 400 ° C.) (Fig. 2 (c)), μ from the sapphire substrate, generated nuclei, graphene, and graphene edges. The Raman scattering spectrum (FIG. 2 (d)), the AFM image (FIG. 2 (e)) showing how the generated nuclei shown in FIG. 2 (a) have a hexagonal spiral structure, and the second 6 is an AFM image (FIG. 2 (f)) showing the surface state of graphene obtained after the step.

The black contrast portion appearing in the SEM image shown in FIG. 2A is the generated nucleus formed in the first step. Each of these generated nuclei has a substantially hexagonal contour with a diameter of about 2 to 3 μm, and has a weak contrast region around it. Then, such production nuclei are formed at intervals of several μm.

Looking at FIG. 2 (b), 6 minutes after the start of the second step, graphene grows around the individual nuclei and binds to the graphene grown from the adjacent nuclei to form a single membrane. You can see how it is going. Then, as shown in FIG. 2 (c), it can be confirmed that graphene covers most of the substrate surface 7 minutes after the start of the second step.

The four μ Raman scattering spectra shown in FIG. 2 (d) are the measurement points (sapphire substrate portion) indicated by diamond marks in FIG. 2 (a) and the square marks (graphene ends) in FIG. 2 (b). , Triangular mark (graphene part), circle mark (generated nucleus part) obtained from the measurement points.

Both peaks and 2320cm ^-1 near the peak of 1550 cm ^-1 vicinity found in these spectra are those of due to the sapphire substrate.

Looking at the Raman scattering spectra obtained from nuclei, the peak of G-band peak and 1590 cm ^-1 vicinity of 2700 cm ^-1 vicinity of 2D band is observed clearly. In addition, a weak peak in the D band near 1300 cm ^{-1 is} also observed.

Since the discovery of single-walled carbon nanotubes, many research reports have been made on the Raman scattering spectrum of carbon materials. A characteristic peak derived from lattice vibration appears in the Raman scattering spectrum obtained from graphene, and the G band is a band caused by in-plane expansion and contraction vibration of the 6-membered ring structure of carbon atoms in graphite. Further, it is known that the D band is a peak appearing due to a double resonance effect accompanied by inelastic scattering due to its defect structure, and the intensity ratio (G / D ratio = _IG / _ID ) of these bands is , It is an index of the high crystallinity of graphene. The present inventors have decided to judge the height of "crystallinity" by this G / D ratio, and evaluate a graphene having a G / D ratio of 3 or more as graphene having high crystallinity.

Also, the index of evaluation for the 2D band affected by interactions in graphene sheet layers, the intensity ratio of the G band (2D / G ratio = I _{2D / I G)} is the number of layers of graphene (unilamellar) It becomes. For example, the 2D / G ratio of graphene in two layers is about 1, and the 2D / G ratio of graphene in four layers is about 0.5. Therefore, if the 2D / G ratio is 2 or more, it can be judged that the layer is almost perfect. The present inventors determine the height of "monolayer" by this 2D / G ratio, and evaluate a graphene having a 2D / G ratio of 1 or more as graphene having high monolayer.

Since the 2D / G ratio of the Raman scattering spectrum obtained from the generated nuclei shows a value close to 2, it is possible that the generated nuclei formed in the first step are already graphene having extremely high monolayers. Highly sex. Since the 2D / G ratio of the Raman scattering spectrum from the graphene grown in the second configuration also maintains a value close to 2, the graphene having extremely high monolayerity is used as the generation nucleus, and a single layer is formed around the graphene. It is considered that graphene, which has extremely high sex, is growing.

Further, in both the spectra of the generated nucleus and graphene, the intensity of the D band due to the defect structure is very low, and the G / D ratio exceeds 10. Therefore, it is considered that graphene having high crystallinity has already been formed at the stage of the formation nucleus, and graphene is growing around the highly crystalline formation nucleus while maintaining its crystallinity.

Thus, according to the present invention, the peak intensity ratio of the G band peak and 1590 cm ^-1 vicinity of 2700 cm ^-1 vicinity of 2D band (2D / G ratio ₌ I 2D / _{I G)} is 1 or more single Graphene with high stratification can be synthesized at extremely low temperatures.

Further, according to the present invention, High peak intensity ratio of D band peak and 1300 cm ^-1 vicinity of 1590 cm ^-1 near the G band (G / D ratio ₌ I G / _{I D)} is 3 or more Shitsuna Graphene can be synthesized at extremely low temperatures.

Since the area of graphene obtained depends on the area of the formation region of the generated nuclei on the substrate, a large area of high-quality single-layer graphene can be obtained by increasing the area.

Most of the above-mentioned generated nuclei have a hexagonal spiral structure as shown in the AFM image shown in FIG. 2 (e). Then, in the second step, a six-membered carbon ring grows two-dimensionally around this production nucleus and coalesces with the six-membered carbon ring that has grown around the adjacent production nucleus to form a graphene film. To. The white linear contrast observed in the AFM image shown in FIG. 2 (f) is a trace that the six-membered carbon rings grown from the adjacent generation nuclei are united to form one graphene film.

What kind of carbon product this product nuclei becomes depends on the processing temperature and time of the first step, but it can be a single-layer graphene or a multi-layer graphene, and it can be made of graphite. It can also be a production nucleus.

FIG. 3 shows Raman mapping of 2D / G ratios (FIGS. 3 (a) to (c)) and G / D ratios corresponding to each sample whose SEM image is shown in FIGS. 2 (a) to 2 (c). Raman mapping (FIGS. 3D to 3f).

The regions darkly shown in FIGS. 3 (a) to 3 (c) are regions showing a 2D / G ratio of 1.5 or more. Further, the regions darkly shown in FIGS. 3 (d) to 3 (f) are regions showing a G / D ratio of 1.5 or more. In the second step, it can be seen that graphene grows two-dimensionally around the formation nucleus and eventually becomes one graphene film.

The graphene thus obtained is composed of a plurality of domains composed of a set of six-membered carbon rings, and each of the plurality of domains has a generation nucleus of the six-membered carbon ring in the domain.

The orientation of the six-membered carbon ring within the domain is then substantially the same over the entire surface of the graphene membrane, and is substantially the same within a range of at least ^10-7 cm ² . Here, "substantially the same" means that the deviation of the orientation of the six-membered carbon ring in the region of interest is 15 ° or less.

The density of the formation nuclei of the 6-membered carbon ring is preferably 1 × 10 ⁶ pieces / cm ² or less. If the density of the generated cores is too high, it will be difficult to make the orientation of the six-membered carbon ring within the domain substantially the same over the entire surface of the graphene membrane.

As described above, the area of one continuous graphene film depends on the area of the formation region of the generated nuclei on the substrate, and is substantially 1 × ^10-7 cm ² or more (corresponding to approximately 3 μm × 3 μm). It can also be a single-layer continuous film.

FIG. 4 is a TEM image obtained from graphene after the completion of the second step. Although this graphene was grown by setting the CVD reaction temperature in the second step to 500 ° C., a similar TEM image can be obtained from graphene grown at a temperature of 400 ° C. or lower.

4 (a) is a dark field image, FIG. 4 (b) is a bright field image, and FIGS. 4 (c) and 4 (d) are high-magnification images. As a result of analyzing the lattice images shown in FIGS. 4 (c) and 4 (d), the difference in the orientation of the six-membered carbon ring in the film was less than 5%. That is, it can be confirmed that the single-layer graphene obtained in the present invention is composed of a set of carbon six-membered rings having substantially the same orientation over the entire surface of the film.

The nuclei produced in the first step do not need to be produced by the CVD method as described above. For example, graphite or graphene may be transferred to the main surface of the substrate by a peeling method.

Further, the first step may be carried out by patterning graphite or graphene on the main surface of the substrate by a photolithography method.

Further, following the second step, a step of separating graphene from the substrate (third step) may be provided.

Graphene has a unique conical band structure with smooth sides where the conduction band and the valence band are in contact at one point (Dirac point). Due to such a specific band structure, the electrons in graphene are in a kind of free state, and the carrier mobility shows an extremely large value. Taking advantage of this characteristic, if graphene is provided in the gate region and the voltage applied to the graphene is changed, the carrier density (conductivity) can be continuously controlled.

If such graphene is applied to an electronic device, its good conductivity, high electron / hole mobility, inelastic electron conductivity and spin conductivity, mechanical strength, light absorption and light emission, heat conduction, etc. Due to its superior characteristics, high-performance electronic devices can be realized. As such an electronic device, for example, a field effect transistor having a channel region made of such graphene can be exemplified.

The electronic element is, for example, a metal having a catalytic action for graphene synthesis on a resist pattern formed on the main surface of a substrate and having a melting point of 400 ° C. or lower (for example, at least one of gallium and indium, or gallium-. Indium alloy) is brought into contact with the metal, and a gas containing hydrocarbons is supplied while the metal is held at a temperature exceeding the melting point. Graphene is grown at the contact interface with the metal on the main surface of the substrate, and the resist pattern is formed into graphene. It can be manufactured by a process including the step of forming a circuit.

FIG. 5 is an example of an optical micrograph (FIG. 5 (a)) of an FET prototyped by the present inventors using the above-mentioned graphene and its CV curve (FIG. 5 (b)). The hole mobility is estimated to be about 600 cm ² / V _s .

In this embodiment, an example of graphene synthesis at a low temperature, which could not be achieved by the prior art, will be described.

[Low temperature synthesis of single membrane]
As the substrate, a sapphire substrate (1 cm × 1 cm) having an off angle of 0.2 ° was used. In order to remove impurities on the surface in advance, a cleaning treatment by oxygen plasma ashing was performed, a gallium metal was placed on the main surface of this sapphire substrate, and as shown in FIG. 1 (a), the inside of the quartz tube. Inserted in. Then, RF heating was performed while flowing Ar gas as a carrier gas to raise the substrate temperature to 850 ° C., and the temperature was maintained at this temperature to melt gallium.

Next, Ar gas was flowed at a flow rate of 200 sccm and CH ₄ gas at a flow rate of 1 sccm (0.5 atm), and the substrate was rapidly heated to 1050 ° C. and held for 5 minutes. After that, the supply of CH ₄ gas was stopped, and the mixture was cooled to room temperature in an Ar gas atmosphere. Under this condition, generated nuclei were formed at a density of 103 pieces / cm ² .

Following this, RF heating was performed while flowing Ar gas as a carrier gas to raise the substrate temperature to 200 ° C., Ar gas was flowed at a flow rate of 200 sccm and CH ₄ gas at a flow rate of 1 sccm (0.5 atm), and the substrate temperature was 200. It was held at ° C. for 10 minutes. After that, the supply of CH ₄ gas was stopped, and the mixture was cooled to room temperature in an Ar gas atmosphere.

FIG. 6 is an SEM image of graphene thus obtained. The obtained graphene is composed of a plurality of domains composed of a set of 6-membered carbon rings, and each of the plurality of domains has a production nucleus of the 6-membered carbon ring in the domain, and these domains. Are connected neatly to form a single film.

[Raman spectrum of graphene synthesized at 200 ° C]
As the substrate, a sapphire substrate (1 cm × 1 cm) having an off angle of 0.2 ° was used. In order to remove impurities on the surface in advance, a cleaning treatment by oxygen plasma ashing was performed, a gallium metal was placed on the main surface of this sapphire substrate, and as shown in FIG. 1 (a), the inside of the quartz tube. Inserted in. Then, RF heating was performed while flowing Ar gas as a carrier gas to raise the substrate temperature to 850 ° C., and the temperature was maintained at this temperature to melt gallium.

Next, Ar gas was flowed at a flow rate of 200 sccm and CH ₄ gas at a flow rate of 1 sccm (0.5 atm), and the substrate was rapidly heated to 1050 ° C. and held for 2 minutes. After that, the supply of CH ₄ gas was stopped, and the mixture was cooled to room temperature in an Ar gas atmosphere.

Following this, RF heating was performed while flowing Ar gas as a carrier gas to raise the substrate temperature to 200 ° C., Ar gas was flowed at a flow rate of 200 sccm and CH ₄ gas at a flow rate of 1 sccm (0.5 atm), and the substrate temperature was 200. It was held at ° C. for 20 minutes. After that, the supply of CH ₄ gas was stopped, and the mixture was cooled to room temperature in an Ar gas atmosphere.

FIG. 7 is a Raman scattering spectrum from graphene thus obtained. Peak intensity ratio of the G band peak and 1590 cm ^-1 vicinity of 2700 cm ^-1 vicinity of 2D band spectrum (N) from nuclei (2D / G ratio ₌ I 2D / _{I G)} is below 1, It seems to be a multi-layered graphene. On the other hand, the 2D / G ratio of the spectrum (G) from the graphene region grown around the generated nucleus exceeds 2.5, confirming that the graphene is extremely high monolayer graphene.

Further, the peak intensity ratio of D band peak and 1300 cm ^-1 near the G band of 1590 cm ^-1 vicinity spectra from grown graphene region around the product nucleus (G / D ratio ₌ I G / _{I D)} is It was about 3.

A Raman spectrum similar to this is obtained from the entire surface of the film.

[Raman spectrum of graphene synthesized at 100 ° C]
As the substrate, a sapphire substrate (1 cm × 1 cm) having an off angle of 0.2 ° was used. In order to remove impurities on the surface in advance, a cleaning treatment by oxygen plasma ashing was performed, a gallium metal was placed on the main surface of this sapphire substrate, and as shown in FIG. 1 (a), the inside of the quartz tube. Inserted in. Then, RF heating was performed while flowing Ar gas as a carrier gas to raise the substrate temperature to 850 ° C., and the temperature was maintained at this temperature to melt gallium.

Next, Ar gas was flowed at a flow rate of 200 sccm and CH ₄ gas at a flow rate of 1 sccm (0.5 atm), and the substrate was rapidly heated to 1050 ° C. and held for 5 minutes. After that, the supply of CH ₄ gas was stopped, and the mixture was cooled to room temperature in an Ar gas atmosphere.

Following this, RF heating was performed while flowing Ar gas as a carrier gas to raise the substrate temperature to 100 ° C., Ar gas was flowed at a flow rate of 200 sccm and CH ₄ gas at a flow rate of 1 sccm (0.5 atm), and the substrate temperature was 100. It was held at ° C. for 60 minutes. After that, the supply of CH ₄ gas was stopped, and the mixture was cooled to room temperature in an Ar gas atmosphere.

FIG. 8 is a Raman scattering spectrum from graphene thus obtained. Peak intensity ratio of the peak and 1590 cm ^-1 near the G band of 2700 cm ^-1 vicinity of 2D band spectrum (G) from grown graphene region around the product nucleus (2D / G ratio ₌ I 2D _{/ I} G) Is more than 1, and it is presumed that one or two layers of graphene are formed.

Further, the peak intensity ratio of D band peak and 1300 cm ^-1 near the G band of 1590 cm ^-1 vicinity spectra from grown graphene region around the product nucleus (G / D ratio ₌ I G / _{I D)} is It is about 10, and may be evaluated as a film having few defects.

A Raman spectrum similar to this is obtained from the entire surface of the film.

As described above, according to the present invention, there is provided a low temperature synthesis technique for large area and high quality single layer graphene.

Claims (8)

A first, in which a gas containing a hydrocarbon is supplied in a state where gallium droplets are in contact with the main surface of the substrate, and a carbon six-membered ring-forming nucleus is formed on the contact surface between the main surface of the substrate and the gallium. Process and
A gas containing a hydrocarbon is supplied in a state where gallium is in contact with the main surface region of the substrate on which the formation nuclei are formed, and a carbon six-membered ring is two-dimensionally formed around the formation nuclei to form a graphene film. With a second step
A method for producing graphene, wherein the formation of a carbon six-membered ring in the second step is carried out at a temperature of 300 ° C. or lower and 100 ° C. or higher. The method for producing graphene according to claim 1, wherein in the second step, the gas containing the hydrocarbon is converted into plasma and supplied. The main surface of the substrate, a first step of a gas containing carbon hydrogen in a state contacting the gallium was supplied to form a product nucleus of a main surface to a carbon six-membered ring of the substrate,
The main surface of the substrate on which the generated nuclei are formed is brought into contact with gallium having a catalytic action for graphene synthesis, and a gas containing a hydrocarbon is supplied while the gallium is held at a temperature exceeding the melting point to supply the main surface of the substrate. A second step of growing graphene at a temperature of 300 ° C or lower and 100 ° C or higher at the contact interface with gallium.
How to make graphene. The method for producing graphene according to claim 3, wherein the first step is carried out by supplying a gas containing a hydrocarbon while holding the substrate at a temperature of 1000 ° C. or higher. The method for producing graphene according to claim 3 or 4, wherein the second step is carried out by increasing the decomposition efficiency of the hydrocarbon gas by a plasma assist method. The method for producing graphene according to any one of claims 3 to 5, further comprising a third step of separating the graphene from the substrate following the second step. The method for producing graphene according to any one of claims 3 to 6, wherein the substrate is a sapphire whose main surface is a (0001) plane (C plane) having an absolute off-angle value of 30 ° or less. Gallium is brought into contact with the resist pattern formed on the main surface of the substrate, a gas containing a hydrocarbon is supplied while the gallium is held at a temperature exceeding the melting point, and graphene is provided at the contact interface with gallium on the main surface of the substrate. A method for producing an electronic element, which comprises a step of growing the resist pattern at a temperature of 300 ° C. or lower and 100 ° C. or higher to graphene the resist pattern to form a circuit.

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