We investigated nucleation and growth characteristics of atomic layer deposition (ALD) HfO 2 on exfoliated and chemical vapor deposition (CVD) graphene by using two Hf precursors, tetrakis(dimethylamino)hafnium (TDMAH) and hafnium tetrachloride (HfCl 4 ). Experimental results and theoretical calculations indicate that HfO 2 nucleation is more favorable on CVD graphene than on exfoliated graphene due to the existence of defect sites. Also, the TDMAH precursor showed much more unfavorable nucleation and growth than HfCl 4 due to different initial adsorption mechanisms, affecting lower leakage currents and breakdown electric field. ALD growth characteristics of HfO 2 will be fundamentally and practically significant for realizing the fabrication of graphenebased electronic devices.
■ INTRODUCTIONGraphene has attracted a great deal of attention for potential applications in electronic devices due to its novel electrical properties, such as high electron and hole mobility above 100,000 cm 2 /(V s). 1 In order to realize graphene-based electronic devices, high quality high-k dielectric thin films are required. 2 However, since conventional techniques of thin film deposition employ energetic radicals in the plasma state for sputtering or chemical reactions of precursors for chemical vapor deposition (CVD), the physical properties of graphene which is composed of an ideal single atomic layer are easily affected by deposition environments. 3 Thus, the damage-free deposition method is needed to form high-k dielectrics on graphene. Compared to other deposition techniques, atomic layer deposition (ALD) produces dense and pinhole-free since ALD films are formed through a layer-by-layer growth manner based on the surface self-saturated reaction of precursors. In addition, damages on an original surface in ALD are less significant than those in CVD and physical vapor deposition (PVD). 4 Therefore, ALD has been one of the essential fabrication methods for graphene-based devices and has been widely used.In earlier studies, ALD dielectrics such as Al 2 O 3 and HfO 2 were attempted on graphite surfaces which have chemically identical surface properties to those of graphene. 5−10 Selective growth of ALD dielectrics along the step edge sites of highly ordered pyrolytic graphite (HOPG) was observed since the step edge sites are chemically more reactive than the basal planes. 5−7 In the following studies, dielectric deposition by ALD on graphene prepared from the exfoliation of HOPG shows similar results to previous reports about ALD on HOPG since there was no chemically available adsorption site on graphene surfaces. 8−10 In other research, interestingly, on graphene synthesized by CVD, there was no selectivity in the growth of ALD dielectrics rather island growth over all the surface. 11−13 Different growth behaviors of ALD dielectrics on graphene surfaces are probably affected by nonideal surface properties of graphene originating from different synthesis methods and preparation processes. Although exfoliated graphene from HOPG and ...