Sunday, January 31, 2010

Fabrication Process for Large-Area Single- and Few-Layer Graphene on Arbitrary Substrates for Electronic Components Shown by MIT Team, Patterning by Lithographic Techniques


Massachusetts Institute of Technology (Cambridge, MA) Associate Professor of Electrical Engineering Jing Kong, Alfonso Reina Cecco and Mildred S. Dresselhaus created processes for the synthesis and fabrication of large-area graphene films on arbitrary substrates (i.e., any of a wide variety of substrates) and the fabrication of patterned graphene structures based on the synthesis method.

The resulting graphene film can be used in a variety of applications. For example, the graphene film can be used as a transparent electrode, as an ultrathin conducting electrode, as an electrode for a battery, as a transistor device (both for low and high frequency).  Graphene film may also be used as a sensor to detect a chemical or biological agent, as an optical detector, as an interconnect for an integrated circuit, as an on-chip capacitor for an integrated circuit, as an on-chip inductor for an integrated circuit, as a hetero-junction device (metal-semiconductor) including graphene nanoribbons with different crystal orientations.

 Further uses include as an in-plane thermal conductor to spread heat dissipation, as a quantum device or spintronic device, as a graphene-nanotube heterostructure (together with nanotubes), as a p-n junction diode or bi-polar junction transistor, as a device with an adjustable bandgap, and as an interface for different materials (e.g., for interfacing with Si, GaN and/or GaAs), according to U.S. Patent Application 20100021708.

A film of single-layer to few-layer graphene is formed by depositing a graphene film via chemical vapor deposition on a surface of a growth substrate. The surface on which the graphene is deposited can be a polycrystalline nickel film, which is deposited by evaporation on a SiO2/Si substrate. A protective support layer is then coated on the graphene film to provide support for the graphene film and to maintain its integrity when it is removed from the growth substrate. The surface of the growth substrate is then etched to release the graphene film and the protective support layer from the growth substrate, wherein the protective support layer maintains the integrity of the graphene film during and after its release from the growth substrate. After being released from the growth substrate, the graphene film and protective support layer can be applied onto an arbitrary target substrate for evaluation or use in any of a wide variety of applications.

In recent years, research on single- or few-layer graphene (SLG or FLG) has attracted much attention. "Graphene" refers to a single layer of hexagonal carbon structure. Single-layer-graphene and few-layer-graphene structures have been predicted and demonstrated to have many remarkable properties, such as high electron and hole mobilities with a symmetrical electron and hole band structure, high current-carrying capacity, high in-plane thermal conductivity, high tensile strength and high mechanical stability. When graphene is cut into narrow strips, it bears attributes very similar to those of carbon nanotubes, which have been investigated thoroughly.

However, there are many hurdles for the application of nanotubes due to the challenges of controlling the nanotube structures, whereas graphene strips or other structures can be patterned by conventional top-down lithography methods, which can be advantageous. The observation of an unconventional quantum Hall effect in graphene has also been reported and can be seen even at room temperature. The linear E(k) relationship in the electronic band structure of graphene gives rise to an unusual massless Dirac fermion behavior of the electrons. The electrical conductance of graphene is also sensitive to the absorption or desorption of even a single gas molecule. Graphene sheets, accordingly, show great potential as another materials option for electronic applications (e.g., for electronic devices, sensors or composite materials).

Though single- and few-layer graphene offer such significant advantages, the current methods for achieving single- and few-layer graphene are very limited. Existing methods include high-temperature vacuum annealing of SiC single-crystal substrates, hydrocarbon decomposition on single crystal metal substrates under ultra high vacuum (UHV) conditions, or manually cleaving highly oriented pyrolytic graphite (HOPG) using adhesive tape on SiO2 substrates. These methods are not well suited for large-scale manufacturing.   The MIT team has developed a process that lends itself to mass production.
 

After the transfer, the graphene film can be patterned via a lithography process, such as photolithography, electron-beam lithography and interference lithography. The metal film can be patterned by lithography before formation of the graphene film. Part of the metal surface can be protected with a covering to prevent graphene growth thereon so that a graphene pattern can be directly obtained. 

FIG. 1 illustrates the evaporation and deposition of a nickel film on a thermally grown oxide layer on a silicon (Si) substrate. FIG. 2 illustrates the subsequent production of a graphene film as the carbon-doped nickel film is cooled down.  FIG. 3 illustrates a full graphene coating on the nickel film.

As shown in FIG. 1, an example of this fabrication procedure commences with the evaporation and deposition of a sub-1 micron (e.g., a 100-500 nm) metal film on a silicon (Si) substrate 12 with a thermally grown oxide (SiO2) layer, which will serve as the growth substrate for the graphene film. In an alternative embodiment, a quartz (SiO2) substrate is used in place of the Si/SiO2 substrate as the insulating substrate.

The metal film can be formed of any transition metal that either catalyzes the dehydrogenation of hydrocarbons or has certain carbon solubility under elevated temperature, such as nickel, platinum, ruthenium and copper; in this embodiment, the metal is nickel (Ni). Nickel has been widely used for the synthesis of carbon nanotubes. Furthermore, nickel has a face-centered cubic structure and its face forms a triangular network of nickel atoms with lattice parameters similar to those of graphene. A protective oxide layer (e.g., nickel oxide) can be formed on the surface of the metal film, and the oxide layer can be removed before graphene formation by contacting it with hydrofluoric acid (HF), potassium hydroxide (KOH), or sodium hydroxide (NaOH).

FIG. 4 is an image of an as-deposited nickel film. FIG. 5 is an image of that nickel film after annealing is provided. FIG. 6 is an image of the annealed nickel film coated with a graphene layer.

FIGS. 7-10 illustrate the transfer of a few-layer graphene film from the nickel-coated substrate to a target substrate.

FIGS. 11-16 are optical images and atomic-force-microscopy characterizations of the few-layer graphene film on a Si/SiO2 substrate.

FIGS. 19-22 are high-resolution transmission-electron-microscope images illustrating the synthesized graphene films with various numbers of layers.

FIG. 31 shows optical images of graphene films after being transferred to SiO2/Si substrate

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