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174 Graphene – Synthesis, Characterization, Properties and Applications Fig. 13. (a) 4-inch wafer with back-gated few-layer graphene devices; insets show SEM and AFM images of a typical device and device channel, respectively. (b) IDS-VDS measurements for different gate voltages, VG = 2.5 V, 1.5 V and -1.5 V for the black, red and blue curves, respectively and (c) IDS-VG curve of one of the FET devices for VDS = 0.01 V. Four-probe measurements performed on the FLG films revealed a sheet resistance of ~68 kΩ/sq. IDS-VDS characteristics depicted in figure 13b shows that the drain current increases with the increase of negative gate voltage, indicating a weak p-type behavior in the films. Figure 13c shows the transfer characteristics for a device with channel width of 20 μm and channel length of 4 μm. Most devices were highly conductive and exhibited a weak modulation of the drain current by the gate bias, which is consistent with a 2D semimetal. Compared to carbon nanotubes, graphene FETs typically exhibit low current on/off ratios, which can be improved significantly by patterning graphene into nanoribbons (Han, Ouml et al. 2007). Single graphene layer is a zero-gap semiconductor, but interlayer interactions bring in a semimetal behavior in FLG. Therefore, the transfer characteristics observed in Figure 2.5c can be attributed to a screened gating effect due to irregularities of the film and the presence of more than two graphene layers in the films. A striking difference is observed in the transfer characteristics of graphene FETs with single or bilayer graphene as the device channel, as can be observed in Figure 14 where typically 3-4 inch diameter CVD graphene wwere transferred to Si/SiO2 wafers for graphene FET fabrication at wafer scale. Figure 14 shows representative I-Vg characteristics of single and bilayer graphene FTEs synthesized on Cu and Ni substrates. The nearly-symmetric ambipolar transfer characteristics exhibited by these devices are typical from single or bilayer graphene FETs with high quality and low doping concentration. Graphene FETs offer the advantage of gate voltage dependent hole or electron conduction.PDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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