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Atomic Layer Deposition of High-k Oxides on Graphene 107 It is known that doping of graphene can influence the positions of Raman peaks (Das et al., 2008; Ni et al., 2008b) but the shift of the 2D-band in this case cannot be greater than that of G-band. In our experiments, however, the shift of the 2D-band is markedly greater. Edge effects and/or changes in the doping level can also cause changes in the peak positions but in that case narrowing of the G-peak should take place (Casiraghi et al., 2007; Ni et al., 2008b). In our case, on the contrary, the width of the G-peak increases. We also performed annealing experiments with a single layer graphene flake under conditions similar to those used during two-temperature ALD process of HfO2. We observed blueshifts of G- and 2D- peaks by 4 cm-1 and 7 cm-1, respectively. At the same time full width at half maximum (FWHM) of the 2D-peak slightly increased by a few cm-1, while FWHM of the G-peak decreased by ~4 cm-1. On the basis of these data, doping of graphene, edge effects, and influence of high-temperature treatment during the ALD process could be excluded from the list of most important reasons for changes of Raman spectra caused by deposition of HfO2. Thus, the compressive strain developed in graphene during the two-temperature ALD process is the most probable reason for the blueshifts in the Raman spectra. Using the biaxial strain coefficient of –58 cm-1/% for the Raman G-mode and –144 cm-1/% for the Raman 2D- mode (Mohiuddin et al., 2009) and assuming elastic behavior of graphene, we estimated the compressive strain to be ~0.15% in our single layer graphene flake. This strain can be well explained by the relatively large and negative thermal expansion coefficient of graphene (7 x 10-6 K-1 at room temperature; Bao et al., 2009) while the thermal expansion of HfO2 is of the same magnitude but with opposite (positive) sign (Wang et al., 1992), and it indicates strong adhesion of HfO2 to graphene. 3. ALD of high-k dielectrics on functionalized graphene 3.1 ALD of Al2O3 on graphene after treatment with NO2 and TMA In order to reduce the leakage currents through the Al2O3 gate dielectric deposited on graphene, Williams et al. (2007) adopted the method proposed by Farmer & Gordon (2006) for pretreatment of carbon nanotubes and used NO2 and TMA for functionalization of the graphene surface prior ALD of Al2O3. The exfoliated graphene flakes were cleaned with acetone and isopropyl alcohol (IPA) immediately before inserting them into the ALD reactor. Next, after the chamber was pumped down to a pressure of 0.3 Torr, non-covalent functionalization layer (NCFL) was deposited at room temperature using 50 cycles of NO2 and TMA followed by 5 cycles of H2O-TMA in order to prevent desorption of the NCFL. Finally, Al2O3 was grown at 225 oC with 300 ALD cycles, each of those consisting of a pulse of H2O vapor (1 Torr, 0.2 s) and a pulse of TMA vapor (1.5 Torr, 0.1 s), under continuous flow of N2 and with 5 s intervals between pulses. As a result, ~30 nm thick oxide layer was obtained on top of graphene consisting of NCFL and Al2O3 and having a mean dielectric constant k ~ 6. The same receipe was later also used by Lin et al. (2009), who fabricated top-gated graphene-based FETs operating at gigahertz frequencies (up to 26 GHz). They functionalized the surface of exfoliated graphene with 50 cycles of NO2-TMA and deposited after that a 12 nm thick Al2O3 layer as the gate insulator. The dielectric constant of the oxide layers was determined by C-V measurements to be ~7.5. However, in their experiments severe degradation of measured mobilities (down to ~400 cm2/Vs) was observed. Consequently, although this kind of functionalization process allows one to deposit thin (ofPDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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