JP2013144621A - Graphene film, method for manufacturing graphene film, graphene device, and method for manufacturing graphene device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve such a problem that an index to control the uniformity of domains of a graphene film is not present since the uniformity of domains of the graphene film needs to be controlled in order to improve the performance of a device using the graphene although individual results have been acquired weather the domains of the graphene formed on a catalyst metal surface using various transition metals are uniform or have domain boundaries.SOLUTION: A graphene film has a substrate 3, a catalyst metal 2 disposed on the substrate 3, and a graphene 1 disposed on the catalyst metal 2, wherein the catalyst metal 2 is osmium.

Description

  The present invention relates to a graphene film and a graphene device.

  In recent years, research and development for applying graphene, which is a single layer of graphite, as an active element has been actively conducted. Graphene has different characteristics from existing semiconductor materials. In particular, due to the unique band structure of graphene, the mass of electrons moving through the graphene is almost zero. For this reason, the electron mobility of graphene reaches about 100 times that of silicon (Si), which is a conventional semiconductor material.

  As a method of forming graphene, there is a micromechanical method. In this method, graphene is obtained by attaching a tape to a graphite sample and transferring it onto a substrate. However, with this method, it is difficult to control the size and position of graphene.

  In order to control the size and formation position of graphene, a method for forming graphene on a catalytic metal surface has been researched and developed. In this method, graphene is formed on the surface by chemical vapor deposition (CVD) or the like, and then transferred onto a substrate having a gate insulating film such as a SiO2 / Si substrate.

  As a catalyst metal material, as disclosed in Non-Patent Documents 1-8, transition metals such as Co, Ni, Cu, Ru, Rh, Pd, Ir, and Pt are used. The uniformity of the graphene domains formed on the catalytic metal surface depends on the catalytic metal material. For example, the domains of graphene formed on the surfaces of Co (Non-Patent Document 1), Ni (Non-Patent Document 2), Ru (Non-Patent Document 4), and Rh (Non-Patent Document 5) are uniform, while Cu ( Non-patent literature 3), Pd (non-patent literature 6), Ir (non-patent literature 7), and graphene formed on the surface of Pt (non-patent literature 8) have been reported to have domain boundaries.

  As disclosed in Non-Patent Documents 9-10, a graphene device can be manufactured by forming an electrode over graphene transferred onto a substrate having a gate insulating film. It is reported that Ni is used in Non-Patent Document 9 and Pd is used in Non-Patent Document 10 as electrode materials. The contact resistance of the electrode was 500 Ωμm for Ni and 100 Ωμm for Pd.

ACS Nano 4, 2010, 7407-7414 The Journal of Physical Chemistry Letters No. 1, 2010, 3101-3107 Nano Letters No. 10, 2010, 3512-3516 Physical Review B 76, 2010, 075429 ACS Nano 4, 2010, 5773-5782 Nano Letters 9th, 2009, 3985-3990 New Journal of Physics No.11, 2009, 023006 Physical Review B 80, 2009, 245411 Applied Physics Letters 96, 2010, 013512 Nature Nanotechnology No.6, 2011, 179-184

  Individual results have been obtained on whether the domains of graphene formed on the catalytic metal surface using various transition metals are uniform or have domain boundaries. In order to improve the performance of graphene devices, it is necessary to control the uniformity of graphene domains. However, there is a problem that there is no guideline for controlling the uniformity of graphene domains.

  In order to improve the performance of the graphene device, it is necessary to reduce the contact resistance of the electrode of the graphene device. However, when Ni or Pd is used, there is a problem that the contact resistance of the electrode of the graphene device is high.

  SUMMARY OF THE INVENTION The present invention solves the above-described conventional problems and aims to provide a catalyst metal that can form graphene more uniformly by finding a guideline for controlling the uniformity of graphene. It is another object of the present invention to provide an electrode material that can reduce the contact resistance of an electrode of a graphene device.

  In order to solve the problem of uniformity of the graphene, the graphene film on the metal of the present invention is composed of graphene, a catalyst metal, and a substrate, the graphene is on the catalyst metal, and the catalyst metal is on the substrate. And the catalytic metal is osmium.

  In order to solve the problem of the contact resistance, an electrode of the graphene device according to the present invention includes a graphene, an electrode, an insulating film, and a substrate. The electrode is on the graphene, and the graphene is on the insulating film. In addition, the insulating film is disposed on the substrate, and the electrode material is osmium.

  According to the graphene film of the present invention, the uniformity of graphene can be further improved by using osmium as a catalyst metal. According to the electrode of the graphene device of the present invention, the contact resistance of the electrode of the graphene device can be reduced by using osmium as the electrode material.

FIG. 7 illustrates the graphene film of Embodiment 1. Diagram showing the definition of barrier height Diagram explaining that the graphene domain size varies depending on the barrier height The figure showing the graphene device of Embodiment 2 Sectional view enlarging the electrode-graphene interface

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1)
FIG. 1 is a diagram showing Embodiment 1 of the present invention.

  The graphene film illustrated in FIG. 1 includes graphene 1, a catalyst metal 2, and a substrate 3. The uniformity of the graphene 1 varies depending on the type of material of the catalytic metal 2. The inventor has found a guideline for controlling the uniformity of the graphene 1 by simulation using first-principles electronic state calculation based on density functional theory.

  FIG. 2 shows the barrier height that is an important parameter in the guideline for controlling the uniformity of the graphene 1. As shown in FIG. 2A, the graphene 1 is a sheet in which carbon atoms are periodically bonded according to the lattice vector of the surface unit cell 10. The distribution of potential energy when graphene 1 moves from an adsorption site on the surface of catalyst metal 2 by the amount of the arrow shown in the figure (shown as the graphene movement distance) is shown in FIG. It is shown. At this time, the difference between the maximum value and the minimum value of potential energy is defined as the barrier height. The barrier height represents energy required when the graphene 1 moves on the surface of the catalytic metal 2. That is, if the barrier height is large, it means that the graphene 1 is difficult to move. The potential energy is a value per one carbon atom constituting the graphene.

  FIG. 3 is a diagram showing the relationship between the barrier height and the size of the graphene domain. In the process of graphene growing on the catalytic metal surface, clusters of nanometer-sized carbon atoms migrate on the surface and bonds together (New Journal of Physics No. 10, 2008, page 093026). As the cluster grows, the energy for moving the cluster increases, so the mobility decreases. Hereinafter, the description will be divided into the size of the barrier height of the catalyst metal.

<Catalyst metal with low barrier height>
Since the clusters are easy to move, the clusters tend to be joined and become large. Since the cluster is easy to move, the cluster rotates with respect to the catalytic metal surface and is adsorbed at various sites. For this reason, when the cluster becomes large, a domain boundary 11 is generated as shown in FIG.

<Catalyst metal with large barrier height>
Because clusters are difficult to move, they are difficult to grow. However, since the cluster strongly adsorbs on the surface of the catalyst metal, it does not rotate with respect to the catalyst metal and adsorbs on a certain site. For this reason, as shown in FIG. 3B, the graphene grows uniformly.

  As described above, the present inventor has hypothesized that the parameter for controlling the uniformity of graphene domains is the barrier height. Therefore, the first-principles electronic state calculation was used to calculate the barrier height of various metal materials and to examine the correlation with the homogeneity of the graphene domain. First, the results of the barrier height are shown in Table 1-3.

  The nature of the transition metal is determined by the arrangement of d electrons and the energy level. Since 3d, 4d, and 5d have different energy levels, comparison is made for each of the 3d, 4d, and 5d transition metals.

<3d transition metal>
From Table 1, Co and Ni are about 0.1 eV or more, which is relatively high. Cu is low at about 0.01eV.
The domain of graphene is uniform in Co (Non-patent document 1) and Ni (Non-patent document 2). In Cu (Non-patent Document 3), domain boundaries having different rotation angles are generated.

<4d transition metal>
From Table 2, Ru and Rh are about 0.4 eV, which is relatively high. Pd is a little low at about 0.3eV.
The domain of graphene is uniform in Ru (Non-Patent Document 4) and Rh (Non-Patent Document 5). In Pd (Non-Patent Document 6), domain boundaries with different periodicities occur.

<5d transition metal>
From Table 3, Os is about 0.5 eV, which is relatively high. Ir and Pt are slightly low at about 0.34eV.
In the graphene domain, Ir (non-patent document 6) and Pt (non-patent document 7) have domain boundaries with different rotation angles. As for Os, no graphene film formation experiments have been reported.

  These results indicate that the homogeneity of graphene domains is determined by the barrier height in graphene migration. Therefore, since the barrier height of Os is sufficiently large in the 5d transition metal, it is assumed that the graphene domain is uniform.

  In the present embodiment, by using osmium as the catalyst metal 2, the graphene domains can be made uniform.

  A method for forming graphene of this embodiment will be described. It consists of the following two steps.

(A) Formation process of catalyst metal 2 A thin film of the catalyst metal 2 is formed on the substrate 3.

(B) Graphene film forming step Graphene 1 composed of carbon atoms is formed.

  When osmium is used as the catalyst metal 2, it is possible to form an osmium thin film by the film forming method disclosed in Japanese Patent No. 3644472. An example of the substrate 3 is a sapphire substrate.

  In the present embodiment, osmium is used as the catalyst metal 2. When osmium is heated, it is easily oxidized to produce osmium tetroxide. Since osmium tetroxide is easily sublimated, osmium can be easily removed simply by heating the osmium. That is, after the step (b), the catalyst metal 2 can be removed by annealing in an oxidizing atmosphere.

(Embodiment 2)
FIG. 4 is a diagram showing the second embodiment of the present invention.

  The graphene device shown in FIG. 4 includes a graphene 1, a substrate 3, an insulating film 4, and an electrode 5. The contact resistance of the electrode 5 of the graphene device varies depending on the material of the electrode 5. The present inventor has found that osmium is desirable as a material with low contact resistance.

  FIG. 5 is an enlarged cross-sectional view of the interface between the electrode 5 and the graphene 1. In FIG. 5, carbon atoms constituting graphene are represented by circles. The carbon atoms located in the cross-sectional view are large, and the carbon atoms located on the far side from the cross-sectional view are small. The graphene 1 has pi electrons 12 having directivity in the direction perpendicular to the graphene surface. In order to reduce the contact resistance of the electrode, the electronic connection between the electrode and graphene must be strengthened. For that purpose, the energy level of the d band of the electrode is close to the energy level of the Dirac point of graphene, and the pi electrons 12 of graphene are arranged immediately above the metal atoms of the electrode 5, The overlap of electron clouds must be increased. It is necessary to investigate energy level matching and selectivity of graphene adsorption sites.

<Energy level alignment>
The energy level matching between the electrode and graphene will be described. As an index representing the matching of these energy levels, the reciprocal (εd−εD) −1 of the difference between the energy level εd at the center of the d band of the metal and the energy level εD at the Dirac point of graphene was used. Table 4-6 shows the values of (εd−εD) −1 calculated using the calculation of the first principle electronic state. Similar to the barrier height, compare each 3d, 4d, and 5d transition metal.

    As can be seen from Table 4-6, in each of the 3d, 4d, and 5d transition metals, it can be seen that (εd−εD) −1 tends to decrease as the atomic number increases.

<Selection of adsorption site>
The selectivity of the adsorption site of graphene is determined by the barrier height shown in Embodiment Mode 1. The larger the barrier height, the stronger the selectivity of the adsorption site.

  Here, Ni and Pd will be described.

  From Table 4, the value of (εd-εD) -1 of Ni is about 0.40 eV-1. In the 3d transition metal, the value is relatively large, and it can be considered that the energy levels are matched. Also, from Table 1, the barrier height of Ni is large in the 3d transition metal. Furthermore, the inventor found that in the state where graphene is most stably adsorbed on the metal surface, the pi electrons of one carbon atom in the surface unit cell of graphene are arranged immediately above the metal atom (FIG. 2 (a)). ) Is confirmed. This means that the contact resistance is reduced because the electron cloud overlaps between the graphene and the electrode.

  From Table 5, the value of (εd−εD) -1 of Pd is about 0.36 eV-1. In the 4d transition metal, the value is small, and it can be considered that the energy level is not matched. Also, from Table 2, the barrier height of Pd is not so large in the 4d transition metal. Furthermore, the inventor has confirmed that in the state in which graphene is most stably adsorbed on the metal, the pi electrons of either carbon atom in the surface unit cell of graphene are not arranged immediately above the metal atom. This means that the contact resistance is not greatly reduced because the electron cloud overlap between the graphene and the electrode is small.

  From the above results, it is considered that Ni is better in electronic connection than Pd. However, in the experimental report, the contact resistance is 500 Ωμm for Ni (Non-Patent Document 9) and 100 Ωμm for Pd (Non-Patent Document 10), and Pd is better than Ni. This is presumably because, in the case of Ni, the contact resistance deteriorates due to the formation of an oxide film.

  Next, Os is described. From Table 6, Os has a value of (εd−εD) −1 of about 0.36 eV−1. In the 5d transition metal, the value is large and it can be considered that the energy levels are matched. Further, from Table 3, the barrier height of Os is large in the 5d transition metal. Furthermore, the inventor has confirmed that the graphene on the metal surface has the same arrangement as that of Ni. Unlike Ni, Os oxide is known to be conductive.

  As described above, osmium is presumed to be the most suitable electrode material for graphene devices from the viewpoint of energy level matching between electrode and graphene, selectivity and arrangement of graphene adsorption sites, and oxide conductivity of electrode material .

  In this embodiment, by using osmium as the electrode 5, it is possible to reduce the contact resistance of the graphene device and improve the performance.

  A method for forming graphene of this embodiment will be described. It consists of the following three steps.

(A) Graphene Arrangement Step The graphene 1 produced in the first embodiment is arranged on the substrate 3 provided with the insulating film 4 on the top thereof.

(B) Patterned electrode formation step After applying a resist, lithography is performed using a mask having a desired pattern. Next, a metal film of an electrode material is formed, and the resist is removed to lift off the metal on the resist to form a patterned electrode 5.

  When osmium is used as the electrode 5, an osmium thin film can be formed by the film forming method disclosed in Japanese Patent No. 3644472 as in the first embodiment. Examples of the substrate 3 and the insulating film 4 include a Si substrate and SiO2.

  The formed graphene according to the present invention is useful as an electronic device for low power consumption.

DESCRIPTION OF SYMBOLS 1 Graphene 2 Catalytic metal 3 Substrate 4 Insulating film 5 Electrode 10 Surface unit cell 11 Domain boundary 12 Pi electron

Claims (4)

A substrate,
A catalytic metal disposed on the substrate;
Comprising graphene disposed on the catalytic metal,
A graphene film, wherein the catalytic metal is osmium. A first step of forming catalytic metal on the substrate;
Forming a graphene on the catalytic metal,
A method for producing a graphene film, wherein the catalytic metal is osmium. A substrate,
An insulating film disposed on the substrate;
Graphene disposed on the insulating film;
An electrode disposed on the graphene,
A graphene device, wherein the material of the electrode is osmium. A first step of disposing graphene on the insulating film disposed on the substrate;
Forming a patterned electrode on the graphene, and
The manufacturing method of the graphene device whose material of the said electrode is osmium.

Images