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Atomic Layer Deposition of High-k Oxides on Graphene 111 In their later experiments, Meric et al. (2011) have used PVA (poly(vinyl)alcohol) to provide a surface to seed ALD growth on exfoliated graphene. PVA is known to have a relatively high dielectric constant (k ~ 6). After the contact deposition they first cleaned the graphene surface by annealing at 330 oC for 3 h in forming gas and then dipped into an 1% aqueous solution of PVA for 12 h. This procedure resulted in ~2.5 nm thick layer of PVA on the graphene surface. After that a brief UV/ozone treatment was employed to activate the –OH groups before the ALD growth of HfO2 at 150 oC with Hf(NMe2)4 and H2O for 50 cycles, yielding a 5-nm thick HfO2 film (see Fig. 12b). As a result, the doping and the mobility of the top-gated graphene-based FET stayed relatively unchanged (see Fig. 12c). Meric et al. (2011) also found that without PVA, ALD growth proceeds only on the SiO2 area (see Fig. 12a) and the occasional patches of oxide on graphene are formed most likely by surface contamination. 4. Conclusion As integration of graphene with high-quality ultrathin dielectrics is very important in development of graphene-based nanoelectronic devices, significant efforts have been concentrated on ALD of dielectric films on the graphene surface. An expected result of these studies is that at substrate temperatures most frequently used for ALD, i.e. at 200–400 oC, the deposition of uniform dielectric layers on clean surface of graphene is not possible due to the chemical inertness of this surface. However, using low-temperature ALD, several research groups have succeeded to grow dielectrics (e.g. Al2O3 and HfO2) even on this kind of surfaces. Unfortunately, the quality of these films is usually not very high and/or the deposition process has a significant negative effect on properties of graphene. Thus, in order to cover graphene with uniform high-quality dielectric layers, different approaches to initiate the film growth have been investigated. It has been demonstrated that a seed layer can be grown by ALD on graphene using highly reactive precursors and very low deposition temperatures close to room temperature. Another way is to functionalize the graphene surface by deposition of metal or polymer buffer layers. It has to be noted, however, that functionalization, for instance with a metal seed layer can lead to degradation of the electronic properties of graphene. On the other hand, in the case of polymeric buffer layers, even when the electronic properties of graphene are not affected much, the total thickness of polymer/high-k dielectric layer might be too big for some kind of applications. Thus, the processes for deposition of dielectrics on graphene definitely need further optimization. It should also be pointed out that only a limited number of ALD experiments have been performed on CVD graphene, which has a great potential as a material for future nanoelectronics. 5. Acknowledgment We would like to thank Pertti Hakonen for supporting the initiation of graphene studies at the Institute of Physics of the University of Tartu, Kaupo Kukli for useful discussions and Aleks Aidla and Alma-Asta Kiisler for technical assistance in experiments. This work was supported by Estonian Science Foundation (Grants No. 6651, 6999, 7845 and 8666), Estonian Ministry of Education and Research (targeted project SF0180046s07) and European Social Fund (Grant MTT1).PDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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