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Atomic Layer Deposition of High-k Oxides on Graphene 101 On the basis of the Raman spectra, one can conclude that electron-beam evaporation and, particularly, sputtering and PLD processes may cause significant damages to graphene. Due to that influence the mobility values of the top-gated graphene-based devices prepared using these methods are typically by an order of magnitude smaller than those of back- gated devices (Lemme et al., 2007). For this reason, alternative methods for deposition of dielectrics on graphene have been extensively investigated. In the case of traditional nanoelectronic devices, a very suitable method for controlled deposition of ultrathin homogeneous films is Atomic Layer Deposition (ALD). However, ALD of thin films on graphene is not easy because there are no dangling bonds on the defect-free graphene surface, which are needed for chemical surface reactions the conventional ALD processes are based on. Nevertheless, several groups have been able, using ALD technique, to deposit thin and continuous HfO2 layers on pristine as-cleaved graphene (Meric et al., 2008; Zou et al., 2010; Alles et al., 2011). It has also been reported that after ALD of relatively thin Al2O3 layers directly on graphene (Moon et al., 2009; Nayfeh, 2011), top-gated devices with good performance can be obtained. In order to better nucleate the growth of dielectric on graphene and obtain smooth uniform films, the graphene surface has been pretreated prior ALD (Williams et al., 2007), e.g. by using receipies successfully tested on carbon nanotubes (Farmer & Gordon, 2006). Later, other approaches for pretreatment of graphene surface have been applied as well. For instance, a thin (~1 nm thick) metal seed layer of Al that was oxydized before ALD (Kim et al., 2009) or polymer films (Wang et al., 2008; Farmer et al., 2009; Meric et al., 2011) have been deposited on graphene in order to initiate ALD of high-k dielectrics. In addition, pretreatment of the graphene surface with ozone prior ALD of Al2O3 has been investigated (Lee et al., 2008, 2010). In this work, we compare ALD experiments in which graphene has been covered by high-k dielectrics (Al2O3 and HfO2) either directly or after surface functionalization. Particular attention is focussed on attempts to grow dielectric films with higher density (and dielectric constant) using higher substrate temperatures and two-temperature ALD processes that start with formation of a thin seed layer at low temperature and proceed with depositing the rest of the dielectric layer at high temperature. 2. ALD of high-k dielectrics directly on graphene 2.1 ALD of amorphous Al2O3 and HfO2 directly on graphene Wang et al. (2008) have tried to deposit a thin Al2O3 layer, ~2 nm thick, on mechanically exfoliated graphene sheets, which were carefully cleaned by annealing at 600 oC in Ar atmosphere at a pressure of 1 Torr. The deposition of Al2O3 on graphene at 100 oC using vapors of trimethylaluminum (TMA; Al(CH3)3) and water (H2O) as precursors was unsuccessful – the Al2O3 film was preferentially formed on graphene edges and defect sites. On the basis of these results, it was concluded that ALD of metal oxides gives no direct deposition on defect-free pristine graphene. Similar results were obtained by Xuan et al. (2008) who tried to deposit Al2O3 and HfO2 films on Highly Ordered Pyrolytic Graphite (HOPG) surfaces. Fresh HOPG surfaces, obtained using Scotch tape, were transferred into ALD reactor immediately after cleaving and 1-35 nm thick Al2O3 films were deposited at the temperature of 200–300 oC by alternating pulses of TMA and H2O as precursors. For ALD of HfO2, HfCl4 and H2O were used as precursors. As a result, Xuan et al. obtained large number of Al2O3 and HfO2PDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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