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108 Graphene – Synthesis, Characterization, Properties and Applications the order of 10 nm) pinhole-free gate dielectrics on graphene by ALD, this can cause a significant reduction of field-effect mobility of charge carriers and, correspondingly, the channel conductance of graphene-based FETs. 3.2 ALD of Al2O3 on graphene after ozone treatment Lee et al. (2008, 2010) investigated coating of exfoliated graphene and HOPG surfaces with Al2O3 layer in O3-based ALD process. First, they used freshly cleaved HOPG surfaces and found that while TMA-H2O process caused selective deposition of Al2O3 only along step edges (as in the experiments of Xuan et al. (2008)), the TMA/O3 process began to provide nucleation sites on basal planes of HOPG surface. Lee et al. (2008) proposed that the chemically inert HOPG surface was converted to hydrophilic mainly through epoxide functionalization. Later, in order to deposit a uniform Al2O3 dielectric layer on top of a graphene flake, they used an ALD seed layer (~1 nm in thickness) grown by applying 6 cycles of TMA and O3 at 25 oC. After that 155 cycles of TMA/H2O at 200 oC were applied to deposit additional Al2O3 layer (~15 nm in thickness). For their films, they obtained the dielectric constant k ~ 8, which is higher than the value typically reported for the Al2O3 films deposited on graphene flakes. The mobilities of their devices reached ~5000 cm2/Vs. Lee et al. (2010) also found out that an O3 treatment at 25 oC introduces minor amount of defects in a single layer graphene, while a substantial number of defects appear at 200 oC (Fig. 10). Fig. 10. Raman spectra of a pristine single layer graphene (bottom) and after treatment (top) (a) with O3 at 25 oC and (b) with O3 at 200 oC for 20 s. (Adapted from Lee et al., 2010). 3.3 Metal seed layer for ALD of Al2O3 on graphene In order to make graphene-based top-gated FETs, Kim et al. (2009) first annealed the chips with exfoliated graphene flakes in a hydrogen atmosphere at 200 oC and then deposited a thin layer of Al by e-beam evaporation. After oxidization this layer served as a nucleationPDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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