Graphene shows a glaring ambipolar electric field effect such that charge carriers can be tuned continuously between electrons to holes. Single layer graphene atop a thermally grown SiO2 layer on a highly doped Si substrate may serve as a prototype of a field effect transistor. Under this configuration SiO2 serves as an insulating layer, so a back-gate voltage can be applied to vary carrier concentration (figure 11b).
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Early graphene FET devices demonstrated by Novoselov exhibited dopant concentrations as high as 1013 cm–2and achieved a mobility that could exceed 10,000 cm2 /Vs (Novoselov, Geim et al. 2004).This translates into ballistic transport on submicron scales. The room-temperature mobilityis limited by impurities or corrugations of the graphene surface, which means that it can still be improved significantly up to the order of 105 cm2 /Vs (Bolotin, Sikes et al. 2008; Du,Skachko et al. 2008).
Electrons in graphene behave like mass-less relativistic particles, which govern most of its electronic properties. One of the most important consequences of such unusual dispersion relation is the observation of half-integer Quantum Hall Effect and the absence of localization, which can be very important for graphene-based field effect transistors(Geim and Novoselov 2007). Mechanical exfoliation of highly ordered pyrolitic graphite(HOPG) or high purity graphite flakes can lead to obtain graphene crystals with very few defects, which in turn exhibit high mobility of the charge carriers. Figure 12 shows scanning electron microscopy (SEM) and atomic force microscopy (AFM) of the grapheme based device reported in the literature as having the highest electron mobility to date (Bolotin, Sikes et al. 2008). The graphene film was obtained by mechanical exfoliation of graphite on Si/SiO2 substrate in which the oxide layer underneath the grapheme was etched in order to obtain a free-standing graphene flake connecting the metal electrodes.
Electrical measurements of resistivity vs. gate voltage show the intrinsic ambipolar behavior of graphene. It was also established that the transfer characteristics of the device is greatly improved after undergoing a high-current annealing process to remove contaminants from the graphene surface. The mobility ? for this device reaches the outstanding value of 230,000 cm2/Vs measured at the highest carrier density n = 2x1011cm-2. Such high mobility would in principle favor high frequency performance. Furthermore, graphene devices pursuing high frequency have demonstrated encouraging characteristics, exhibiting a cutoff frequency fT of 26 GHz, which is the frequency at which the current gain becomes unity and signifies the highest frequency at which signals are propagated (Lin, Jenkins et al. 2008). Only recently, P. Avouris and collaborators reported the fabrication of graphene FETs on SiC substrates with cutoff frequency of 100 GHz for adevice of gate length of 240 nm and using a source-drain voltage of 2.5 V (Lin, Dimitrakopoulos et al.). This fT exceeds those previously reported for graphene FETs as well as those for Si metal-oxide semiconductor FETs for the same gate length (~40 GHz at240 nm) (Meric, Baklitskaya et al. 2008; Moon, Curtis et al. 2009).
CVD graphene for macroelectronics: Transparent conductive films
Another intrinsic property of graphene is its transparency. A single sheet of grapheme absorbs only 2.3 % of the incident light. Such combination of high conductivity and lowlight absorption makes this material an ideal candidate as a transparent conductive film. It isvery tempting to use the unique properties of graphene for technology applications even beyond graphene FET applications. Composite materials, photo-detectors, support for biological samples in TEM, mode-lockers for ultrafast lasers and many more areas wouldgain strongly from using graphene for non-FET purposes.
Solar energy harvesting using organic photovoltaic (OPV) cells has been proposed as a means to achieve low-cost energy due to their ease of manufacture, light weight and compatibility with flexible substrates. A critical aspect of this type of optoelectronic device is the transparent conductive electrode through which light couples into the device. Conventional OPVs typically use transparent indium tin oxide (ITO) or fluorine doped tin oxide (FTO) as such electrodes (Peumans, Yakimov et al. 2003). However, the scarcity of indium reserves, intensive processing requirements, and highly brittle nature of metal oxides impose serious limitations on the use of these materials for applications where cost, physical conformation, and mechanical flexibility are important.
Graphene monolayer has a transparency of 97-98 percent and the sheet resistance of undoped graphene is of the order of ~6k?; for which graphene films are suitable for applications as transparent conductive electrodes where low sheet resistance and high optical transparency are essential (Gomez De Arco, Zhang et al.). Conventional methods to obtain graphene thin films such as epitaxial growth, micromechanical exfoliation of graphite and exfoliation of chemically oxidized graphite are either expensive, unscalable or yield graphene with limited conductivity due to a high defect density. However, chemical vapor deposition has surged as an important method to obtain high quality graphene films. In particular, films with sheet resistance of 280 ?/sq (80% transparent) and 770 ?/sq (90%transparent) have been reported in the literature for graphene synthesized on Ni films,while sheet resistance of 350 ?/sq (90% transparent) has been reported for CVD grapheme on Cu films, which represents a good advance in the use of graphene as transparent conductive films. Another advantage of CVD is its scalability; we have reported wafer-scalesynthesis and transfer of single- and few-layer graphene for transistor and photovoltaic device fabrication (Gomez De Arco, Zhang et al.; Gomez, Zhang et al. 2009).
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