Graphene Adsorbed on Corundum Surface: Clean Interface and Band Gap Opening
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Graphene Adsorbed on Corundum Surface: Clean Interface and Band Gap Opening Bing Huang, Qiang Xu, and Su-Huai Wei National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401, U.S.A. ABSTRACT Using advanced first-principles calculations, we have studied the structural and electronic properties of graphene/α-Al2O3 interfaces and show that α -Al2O3 is an ideal gate dielectric material for graphene transistors. Clean interface exists between graphene and Al-terminated (or hydroxylated) Al2O3 and the valence band offsets for these systems are large enough to create injection barrier. Remarkably, a band gap of ~180 meV can be induced in graphene layer adsorbed on Al-terminated surface, which is significantly larger than graphene on other popular substrates. INTRODUCTION In order to utilize graphene, which has a zero band gap when it is pure, for electronic devices such as the field-effect transistors (FETs), it is essential to open up a band gap in graphene to realize the ON/OFF switch function of the FETs. One way to open the band gap in graphene is utilizing the quantum confinement effect, e.g., etching graphene into one-dimensional nanoribbons [1], but a large scale production of such narrow graphene nanoribbons (GNRs) is still quite challenging. Another way to open a band gap in graphene is breaking the inversion symmetry of the A, B sublattices, e.g., by placing graphene onto some special substrate. In this case, the band structure near the Dirac point or the carrier mobility of the graphene is better preserved. This approach has obvious technological advantages over the etching of graphene. However, a simple guideline on how to search an ideal substrate that could induce a sufficiently large band gap in graphene is still unclear, especially for substrate which can be integrated directly into the current FET technology [2]. Theoretically, it is known that the larger the potential difference of the A, B sublattices, ΔAB, the bigger the band gap of graphene. In this paper, using physical insights and advanced first-principles calculations we suggest that corundum (α-Al2O3) is possibly the best high-k-oxide substrate for graphene FETs. This is because the ionicity of Al2O3 is much larger than that of other popular substrates such as SiO2, HfO2, and BN, so it can generate large ΔAB in graphene. Remarkably, a large band gap of ~180 meV at the Dirac point appears in graphene layer adsorbed on Al-terminated surface, which may realize large ON/OFF ratio and high carrier mobility in graphene FETs. THEORY All the density-functional-theory (DFT) calculations are performed by using the VASP Code [3]. Projector augmented wave (PAW) potentials are used to describe the core electrons, and generalized gradient approximation (GGA) with the PBE functional [4] is selected in our calculations. The effect of vdW interactions is taken into account by using the empirical
correction scheme of Grimme (DFT + D/PBE) [5]. It is well-known that GGA type calculations usually underestimate the band gaps of semiconductors, thus we
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