JP5763597B2 - Method for producing graphene - Google Patents

Description

  The present invention relates to a graphene production method for forming strip (nanoribbon) graphene on a silicon carbide substrate.

  Graphene has a two-dimensional structure in which carbon atoms are arranged in a honeycomb shape, and has high charge mobility and current density tolerance, so it can be applied to high-speed and high-frequency transistors, low-power transparent flexible electronic circuits, and small power devices. Is expected. Graphene itself has a semi-metallic band structure, but a band gap is required for application to a transistor channel. As a method of inducing a band gap in graphene, a method of processing into an elongated nanoribbon structure is known. As the width of the nanoribbon becomes narrower, a larger band gap is induced. In addition, the crystal orientation of the nanoribbon edge called armchair type or zigzag type affects the band structure.

  As a conventional method for producing graphene nanoribbons, graphene is first obtained by methods such as exfoliation from graphite, chemical vapor deposition on a metal substrate, or thermal decomposition of the surface of a silicon carbide (SiC) substrate. There is a technique of processing into a ribbon shape by a lithography technique (see Non-Patent Document 1). In addition, there is a technique of cutting carbon nanotubes in the axial direction and processing them into ribbons (see Non-Patent Document 2).

M. Y. Han et al., "Energy Band-Gap Engineering of Graphene Nanoribbons", PHYSICAL REVIEW LETTERS, vol.98, 206805, 2007. L. Jiao et al., "Narrow graphene nanoribbons from carbon nanotubes", Nature, vol. 458, pp.877-880, 2009.

  In order for a transistor manufactured using a graphene nanoribbon to perform ON / OFF operation at room temperature, the ribbon width needs to be on the nm level. However, when electron beam lithography is used to process graphene into a ribbon shape, first, processes such as mask fabrication and electron beam drawing are required, the processing accuracy is limited to several tens of nanometers, and the electron beam is processed into a processing region. There is a problem in that the production efficiency is poor because it must be irradiated over a wide area.

  As is well known, since graphene has a structure in which carbon atoms are bonded in a hexagonal shape, there are two typical types of ends called armchair ends and zigzag ends. For this reason, the edge of the nanoribbon made of graphene in a ribbon shape (edge) is in a non-uniform state that randomly includes the armchair end and the zigzag end, and the graphene is damaged and the impurities adhere as the mask is made and removed Problems such as doing.

  On the other hand, when a nanoribbon is formed by opening a carbon nanotube, it is difficult to control the arrangement, width, and mass production of the nanoribbon. For this reason, this technology is difficult to apply to power devices that handle circuit integration and large currents.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to allow ribbon-shaped graphene to be formed more efficiently and with good controllability.

In the method for producing graphene according to the present invention, a SiC substrate is heated, and the surface of the SiC substrate is composed of only carbon atoms and has the same structure as graphene, and a part thereof is covalently bonded to silicon atoms of the SiC substrate. A first step of forming a buffer layer composed of a monoatomic layer of carbon, and introducing hydrogen between the buffer layer and the SiC substrate surface to form a covalent bond between the buffer layer and the silicon atoms of the SiC substrate. A second step of making the buffer layer graphene by bringing hydrogen into a state of being bonded to silicon atoms generated by the cutting of the covalent bond, and heating the SiC substrate to partially convert the hydrogen bonded to the silicon atoms The graphene is partially evaporated in the plane along the crystal orientation of the SiC substrate surface by partially evaporating and returning the graphene partially to the buffer layer state. Comprising a third step of a state in which graphene nanoribbons are formed, in a third step, that controls the state of the sequence subjected to heat while applying a stress to the substrate planar direction with respect to the SiC substrate.

In the manufacturing method of the graphene in the third step, the addition to Rukoto to control the state of the sequence subjected to heat while applying a stress to the substrate planar direction, to control the heating temperature and heating time with respect to the SiC substrate Thus, the width of the graphene nanoribbon and the interval between the adjacent graphene nanoribbons via the buffer layer may be controlled. In the third step, the width of the graphene nanoribbon and the interval between adjacent graphene nanoribbons via the buffer layer may be controlled by controlling the heating temperature and the heating time.

  As described above, according to the present invention, it is possible to obtain an excellent effect that ribbon-shaped graphene can be formed more efficiently and with good controllability.

FIG. 1A is a perspective view showing a state in each step for explaining a graphene production method according to an embodiment of the present invention. FIG. 1B is a perspective view showing a state in each step for explaining the graphene production method according to the embodiment of the present invention. FIG. 1C is a perspective view showing a state in each step for explaining the graphene production method according to the embodiment of the present invention. FIG. 1D is a perspective view showing a state in each step for explaining the graphene production method in the embodiment of the present invention. FIG. 2 is a photograph showing a result of scanning tunneling microscope observation of graphene obtained by the manufacturing method according to the embodiment of the present invention. FIG. 3 is a characteristic diagram showing the relationship between the coverage of the buffer layer 102 and the distance between the graphene nanoribbons 105 to be formed.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1A to 1D are perspective views showing states in respective steps for explaining a graphene production method according to an embodiment of the present invention.

  First, as shown in FIG. 1A, the buffer layer 102 is formed on the surface of the SiC substrate 101 by heating the SiC substrate 101. The buffer layer 102 is composed of only carbon atoms, has the same structure as graphene, and is composed of a single atomic layer of carbon. In addition, one (part) of the bonds constituting the buffer layer 102 is covalently bonded to the silicon atom of the SiC substrate 101. Further, as described above, since the buffer layer 102 is covalently bonded to the SiC substrate 101, the buffer layer 102 does not have graphene-specific electrical properties and does not exhibit conductivity.

For example, the SiC substrate 101 whose main surface is the (0001) plane is cleaned with hydrofluoric acid, and this is carried into a processing chamber of a heating furnace that can be evacuated and cleaned at 1500 ° C. in a hydrogen atmosphere for cleaning. The substrate surface is etched by heating for 5 minutes. Next, after evacuating the hydrogen in the treatment chamber, heat treatment may be performed at 1700 ° C. for 5 minutes in an atmosphere in which the pressure in the treatment chamber is set to about 79993.2 Pa and argon is introduced at a flow rate of 400 sccm. By this process, the buffer layer 102 can be formed on the SiC substrate 101. 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.

  Next, as shown in FIG. 1B, hydrogen 103 is introduced between the buffer layer 102 and the surface of the SiC substrate 101, and the covalent bond between the buffer layer 102 and the silicon atoms of the SiC substrate 101 is cut, so that the hydrogen 103 is shared. The buffer layer 102 is changed to graphene 104 by being bonded to silicon atoms generated by the bond breaking. For example, if heat treatment is performed at 1000 ° C. for 30 minutes in an atmosphere in which the pressure is 13332.2 Pa and hydrogen is introduced at a flow rate of 46 sccm in the processing chamber of the above-described heating furnace, the hydrogen 103 forms the buffer layer 102 and the SiC substrate 101. And the covalent bond between them can be cut off, and graphene 104 can be formed.

Next, the SiC substrate 101 is heated to partially evaporate the hydrogen 103 bonded to the silicon atoms, thereby partially returning the graphene 104 to the state of the buffer layer 102, as shown in FIG. 1C. It is assumed that the nanosized strip-like graphene nanoribbons 105 that are partially arranged in the direction along the crystal orientation of the surface of the substrate 101 are formed. For example, the hydrogen 103 may be partially evaporated by heating in a processing chamber of the above-described heating furnace in a vacuum of 133.322 × 10 ⁻¹⁰ Pa under conditions of 670 ° C. and 2 hours.

  As shown in the scanning tunneling microscope image (photograph) in FIG. 2, a region where hydrogen partially evaporated and returned to the buffer layer and a region where graphene remained were observed. The area that remained as graphene formed a nanoribbon network in the form of a hexagonal lattice. The width of this graphene nanoribbon was several nm and the average interval was 20 nm. The interval is an interval between adjacent graphene nanoribbons through the buffer layer portion, and corresponds to “λ” in FIG. 1C. The graphene nanoribbons formed in this way are arranged in the direction along the [11-20] crystal orientation of the SiC substrate and have armchair edges.

  It has been theoretically found that when two phases having different structures exist on the crystal surface, the arrangement is such that the sum of the stress energy and the boundary energy of the two phases is minimized. When two phases having different structures are present on the crystal surface, the interval between the periodic structures formed by the two phases follows the following formulas (1) and (2) with respect to the coverage of one phase.

In the above formulas (1) and (2), λ is the interval of the periodic structure, θ is the coverage, E _wall is the boundary energy between two different phases, and σ ₁ -σ ₂ is between two different phases , A is a lattice constant, ν is a Poisson's ratio, μ is a Young's modulus, and π is a circumference.

As shown in FIG. 3, the coverage θ of the buffer layer 102 and the interval λ (see FIG. 1C) between the graphene nanoribbons 105 adjacent to each other through the buffer layer 102 in the substrate plane direction in the above-described embodiment are expressed by the formula ( It can be seen that the results agree well with the calculation results of 1) and Equation (2). In FIG. 3, the solid line indicates the calculation result, and the black square indicates the result of the graphene nanoribbon actually formed. The calculation results indicated by the solid line include the boundary energy E _wall = 3.8 meV / a of the two phases, the difference of stress tensors σ ₁ −σ ₂ = 2.6 eV / a ^{2 of the} _two phases, and the lattice constant of graphene a = 0.246 nm, Young's modulus of graphene μ = 1 TPa, Poisson's ratio ν = 0.16 of graphene, and π = 3.14 are used.

  From the equation (1), by controlling the coverage ratio of the buffer layer 102 and the stress applied to the graphene nanoribbon 105 and the buffer layer 102, the width of the graphene nanoribbon 105 and the interval between the adjacent graphene nanoribbons 105 via the buffer layer 102 are determined. It can be seen that can be controlled. For example, if the width of the graphene nanoribbon 105 can be controlled, this band gap can be controlled.

  For example, the coverage of the buffer layer 102, that is, the evaporation amount of the hydrogen 103 can be controlled by the heating time and the heating temperature in vacuum. For example, if the heating time is increased, the coverage of the buffer layer 102 is increased, the width of the formed graphene nanoribbon 105 can be reduced, and the interval λ can be further increased. Further, the stress applied to the graphene nanoribbon 104 and the buffer layer 102 can be controlled by applying external stress to the entire substrate.

  Moreover, the anisotropy of the nanoribbon network can be controlled by applying anisotropic external stress. For example, as shown in FIG. 1D, the above-described heat treatment may be performed in a state where external stress is applied to the SiC substrate 101 only in one direction in the substrate plane direction. By applying such stress, a hexagonal lattice network as shown in FIGS. 1C and 2 is extended in one direction, and a plurality of strip-like graphene extending in one direction as shown in FIG. 1D. A nanoribbon array using the nanoribbons 106 can be manufactured.

  The above-mentioned method for producing graphene in the form of nanoribbon (graphene nanoribbon) is the first knowledge obtained as a result of the inventors' extensive research. Control of temperature and time under heating conditions for evaporating hydrogen once introduced, and substrate By applying stress to the graphene, the width and interval of the graphene nanoribbons can be controlled in the range of 1-10 nm and 10-100 nm.

  Thus, the band gap can be controlled by controlling the width of the graphene nanoribbon. Graphene nanoribbons are arranged in a direction along the crystal orientation of SiC and have a uniform armchair edge. Moreover, according to the manufacturing method described above, the graphene nanoribbons are formed in a self-organized manner on the entire surface of the SiC substrate, so that the area can be easily increased and integrated.

  Further, if a plurality of graphene nanoribbons arranged in parallel by applying stress are collectively connected to an electrode, a large current can be withstood. In addition, since the process from the cleaning of the substrate surface to the production of the graphene nanoribbon can be performed in an evacuated environment in the same processing apparatus, damage to graphene and adhesion of impurities can be suppressed. Moreover, according to the manufacturing method mentioned above, since an electron beam drawing apparatus is not required, manufacturing cost can be suppressed.

  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.

  DESCRIPTION OF SYMBOLS 101 ... SiC substrate, 102 ... Buffer layer, 103 ... Hydrogen, 104 ... Graphene, 105 ... Graphene nanoribbon, 106 ... Graphene nanoribbon

Claims (3)

One atom of carbon that is composed of only carbon atoms and has the same structure as graphene on the surface of the SiC substrate by heating the SiC substrate, and part of which is covalently bonded to silicon atoms of the SiC substrate A first step of forming a buffer layer composed of layers;
Hydrogen is introduced between the buffer layer and the SiC substrate surface, the covalent bond between the buffer layer and the silicon atom of the SiC substrate is cut, and the hydrogen is bonded to the silicon atom generated by the cutting of the covalent bond A second step of making the buffer layer into graphene by setting
By heating the SiC substrate to partially evaporate hydrogen bonded to the silicon atoms and partially returning the graphene to the state of the buffer layer, in a direction along the crystal orientation of the surface of the SiC substrate. And at least a third step of forming a graphene nanoribbon made of graphene having a rectangular shape in plan view partially arranged ,
Wherein in the third step, the manufacturing method of the graphene, characterized in that while applying stress to the substrate plane direction with respect to the SiC substrate that controls the state of the sequence performs the heating. The method for producing graphene according to claim 1 ,
In the third step, the width of the graphene nanoribbon and the interval between the adjacent graphene nanoribbons via the buffer layer are controlled by controlling the heating temperature and the heating time. One atom of carbon that is composed of only carbon atoms and has the same structure as graphene on the surface of the SiC substrate by heating the SiC substrate, and part of which is covalently bonded to silicon atoms of the SiC substrate A first step of forming a buffer layer composed of layers;
  Hydrogen is introduced between the buffer layer and the SiC substrate surface, the covalent bond between the buffer layer and the silicon atom of the SiC substrate is cut, and the hydrogen is bonded to the silicon atom generated by the cutting of the covalent bond A second step of making the buffer layer into graphene by setting
By heating the SiC substrate to partially evaporate hydrogen bonded to the silicon atoms and partially returning the graphene to the state of the buffer layer, in a direction along the crystal orientation of the surface of the SiC substrate. A third step of forming a graphene nanoribbon made of graphene having a rectangular shape in plan view partially arranged;
Comprising at least
In the third step, the width of the graphene nanoribbon and the interval between the adjacent graphene nanoribbons via the buffer layer are controlled by controlling the heating temperature and the heating time.