Origins of the Gap States in Polycrystalline Silicon: Tight-Binding Calculations of Twist Boundaries
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ORIGINS OF THE GAP STATES IN POLYCRYSTALLINE SILICON: TIGHT-BINDING CALCULATIONS OF TWIST BOUNDARIES M. KOHYAMA*, S. KOSE* and R. YAMAMOTO** * Glass and Ceramic Material Department, Government Industrial Research Institute, Osaka, 1-8-31, Midorigaoka, Ikeda, Osaka 563, Japan. ** Research Center of Advanced Science and Technology. University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153, Japan. ABSTRACT The atomic and electronic structures of the twist boundaries (Z=3(011), E=7(l11) and E=5(O01)) In Si have been calculated by using the transferable SETB method coupled with the supercell technique. The twist boundaries In Si contain larger structural disorder or more defects and larger interfacial Several kinds of structural disorder or grain boundaries. energies than tilt defects have been found to generate characteristic electronic states inside the gap. The present structural disorder or defects and the gap states are the candidates of the origins of the observed band-tails or mid-gap states In polycrystalline Si as well as those In amorphous Si. INTRODUCTION It is of much importance to investigate the grain boundaries in Si for the application of polycrystalline Si to various electronic devices [1,2]. grain Significant advances have been made In the understanding of the tilt Such boundaries consist of arrangement of structural boundaries In SI [3-6]. units without any dangling bonds and contain no states Inside the minimum gap. As the next step, it Is of much Interest to Investigate twist Recently, the atomic and electronic structures of the Z=5(O01) boundaries. This boundary twist boundary In Ge have been calculated theoretically [7]. has been found to contain very complex structure, large interfacial energy, This indicates that twist and the boundary states inside the minimum gap. boundaries should have larger structural disorder or more defects generally boundaries. And such structural disorder or defects as compared with tilt should be common to general boundaries and may be the origins of the observed band-tails or mid-gap states in polycrystalline Si [8,9]. In this paper, we calculate theoretically the atomic and electronic structures of twist boundaries in SI. We deal with the E=3(Oll), E=7(lll) and E=5(O01) boundaries, which contain different rotation axes and twodimensional periodicities, respectively. We will show that all these boundaries contain large structural disorder or many defects and boundary states Inside the minimum gap. We analyze the relation between the structural disorder or defects and the electronic structure. METHOD OF CALCULATIONS We have used the transferable semi-empirical tight-binding (SETB) method [10,111. The SETB method has been shown to be quite useful for calculations boundaries in S [3-61. of the atomic and electronic structures of tilt However, the SETB method Is less reliable for largely disordered systems such as twist boundaries, because the trasferability for structures other than The transferable SETB method has four-coordinated ones is not guaranteed. been developed i
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