Theoretical Study of Si-Rich Transition-Metal Silicides with Double-Graphene-Like Structures
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Theoretical Study of Si-Rich Transition-Metal Silicides with Double-Graphene-Like Structures Takehide Miyazaki1 and Toshihiko Kanayama2 1 Research Institute for Computational Sciences, National Institute for Advanced Industrial Science and Technology, AIST Tsukuba Central 2, Umezono 1-1-1, Tsukuba, 305-8568, Japan 2 Advanced Semiconductor Research Center, National Institute for Advanced Industrial Science and Technology, AIST Tsukuba West 7, Onogawa 16-1, Tsukuba, 305-8569, Japan
ABSTRACT We propose a novel form of graphene-like Si nanostructure (MSi12)n with M being transition metal atom, based on ab initio total-energy calculation and geometry optimization. It has a three-layer structure, where two layers of Si atoms in graphene-like positions sandwich another layer of transition metal atoms. The electronic structure may become semiconducting for a suitable choice of M. This hypothetical material can be regarded as a Si-rich phase of transition metal silicide. A potential impact of our finding in forthcoming ultra-scaled Si technology is also discussed. INTRODUCTION Recently, there has been an enthusiasm for synthesis of graphene[1,2]. For example, graphene with the thickness of a few atomic layers shows the outstanding transport properties[3,4]. Regarding this excitement, it would be very intriguing to obtain the silicon (Si) counterpart of graphene. A reason for this from a technological point of view is that realization of atomically thin layers of Si might be important to constructing channels in ultra-scaled transistors. However, it has been argued that it is very difficult to construct Si atoms in a single stable graphene sheet[5-8] without mixing elements other than Si such as C[9,10]. In this study, we propose a completely novel form of an ultra-thin layered crystal (MSi12)n with M being a transition metal atom, Mo, W and Zr, based on ab initio total-energy calculation and geometry optimization. It has a three-layer structure, where two layers of Si atoms are positioned in graphene-like geometries that sandwich another layer of transition metal atoms. It is possible to tune the electronic structure of this layered material from metal to semiconductor by changing the element of the transition metal atoms. Our new material can be regarded as a Si- rich phase of transition metal silicide with a large Si-to-M composition ratio, 12, which may be suitable for a material to smoothly connect the interfaces between electrodes made of conventional transition metal disilicide and Si substrates in transistors.
METHOD OF CALCULATION We explore the atomic structure of the ultra-thin silicide films with an ab initio electronic structure calculation method[11]. The electronic structures are calculated based on density functional theory[12, 13]) within generalized gradient approximation[14]) to the exchangecorrelation energies. The film structures are modeled in supercell geometries. In the x- and ydirections in the film extension, either a hexagonal or rectangular unit cell (see Fig.1 for definition of the
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