Growth and characterization of epitaxial graphene films by Molecular Beam Epitaxy

Growing macroscopic graphene films with the aim of making graphene commerically viable is being researched a lot recently. Although graphene isolated by exfoliation of Highly Oriented Pyrolytic Graphite (HOPG) crystals has been in place for sometime now, its micro sample size has triggered the research to produce wafer-scale graphene films. Chemical Vapor Deposition (CVD) of graphene on metallic substrates and thermal decomposition of SiC are two such efforts in the direction of producing wafer-scale graphene films but none of these techniques are full-proof. While CVD graphene needs to be transferred from a metallic substrate to an insulating one for device applications, graphene synthesized through thermal decomposition relies so much on the rate of Silicon (Si) sublimation that getting a uniform graphene coverage remains a challenge. In this dissertation, I attempt to grow epitaxial graphene by Molecular Beam Epitaxy (MBE) by depositing Carbon (C) from a high purity solid graphite source where the growth rate of graphene, the rate of deposition and the substrate temperature can be controlled independently. In this research work, I studied the growth of graphene on two substrates with hexagonal symmetry: c-plane sapphire and 4H-SiC (000¯ 1). Both these substrates are decently lattice matched to graphene. The dynamics of the growth process which is dependent on the substrate used is studied in detail. It will be reported that in both the substrates, the growth starts in an epitaxial manner and progresses to being polycrystalline with increase of thickness. The MBE grown films are systematically analyzed with in-situ RHEED, ex-situ XPS, AFM, Raman Spectroscopy and Electrical Transport. Clear evidence of tensile stress is seen in the AFM and Raman studies in the graphene films grown on c-plane sapphire. Whereas, Raman studies confirm the presence of compressive stress in the graphene films grown on 4H-SiC (000¯ 1) where the mismatch of the coefficient of thermal expansion (CTE) plays an important role. Raman studies show a clear evidence of the defect peak (D) in all the films grown on c-plane sapphire no matter how smooth the morphology is. However, the D peak is absent in very thin epitaxial graphene grown on 4H-SiC (000¯ 1) substrates. The symmetrical nature of the 2D peak in the Raman studies of multi-layered graphene films grown on both c-plane sapphire and 4H-SiC (000¯ 1) indicate the presence of random stacking order. Electrical transport in both the classes of graphene films shows a non-metallic behavior: power law behavior in the high temperature regime and a generalized Variable Range Hopping (VRH) type behavior at low temperatures. The low temperature transport of graphene grown on c-plane sapphire will be shown to be an interplay of both 2D and 3D Mott VRH. Whereas, Efros Shklovskii VRH plays a dominant role in the low temperature transport of graphene grown on 4H-SiC (000¯ 1). With all these findings in mind, some potential solutions are proposed which would take this research forward