Effects of Structural Disorder on the Electronic Properties of Silicon: Tight-Binding Calculations of Grain Boundaries
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EFFECTS OF STRUCTURAL DISORDER ON THE ELECTRONIC PROPERTIES OF SILICON: TIGHT-BINDING CALCULATIONS OF GRAIN BOUNDARIES M. KOIIYAMA* and R. YAMAMOTO** * Glass and Ceramic Material Department, Government Industrial Research Institute, Osaka, 1-8-31, Midorlgaoka, Ikeda, Osaka 563, Japan. *. Institute of Industrial Science, University of Tokyo, 7-22-1, Roppongi, Minato-ku, Tokyo 153, Japan. ABSTRACT The atomic and electronic structures of tilt and twist boundaries In Si have been calculated by using the transferable semi-empirical tight-binding (SETB) method, and the relations between the local structural disorder and the electronic properties of SI have been obtained clearly. The odd-membered rings and the four-membered rings induce the changes of the shape of the The bond distortions generate the peaks at local density of states (LDOS). the band edges in the LDOS, and greatly distorted bonds induce the weak-bond states inside the band gap. The three-coordinated defect generates a deep state in the band gap, which is much localized at the three-coordinated atom. The five-coordinated defect generates both deep and shallow states. The deep state is localized in the neighboring atoms except the five-coordinated atom, although the shallow states exist among the five-coordinated atom and the neighboring atoms. Configurations of boundaries are very effective in order to clarify the effects of the local structural disorder in amorphous Si. INTRODUCTION It is of much importance to Investigate the relations between the local structural disorder and the electronic properties of amorphous Si. It has been considered that the structural disorder such as odd-membered rings, bond -length and bond-angle distortions, dihedral-angle disorder and coordination defects are connected with the observed band-tails or mid-gap states. The purposes of many studies of amorphous SI have been to understand the effects of such structural disorder or to develop methods to control them. On the theoretical side, a lot of studies have been performed so far in Many of such order to clarify the effects of such structural disorder [1-91. studies are calculations using simple model Hamiltonlan describing electronic In order to structure or using simple models of atomic configurations. obtain more realistic results, it is desirable to use more realistic Hamiltonian and to use more realistic atomic configurations obtained by relaxations or molecular dynamics through electronic structure calculations. Recently, such calculations have been performed for the supercell of However, there exists amorphous Si using ab-initlo molecular dynamics [8,9]. a problem in such calculations. It Is not easy to clarify the effects of respective structural disorder on the electronic properties in such atomic configurations of amorphous Si, because various types of structural disorder are contained densely in such configurations. Thus in this paper, we show that calculations of grain boundaries in Si are very effective in order to clarify the effects of respective structural dis
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