First Principles Molecular Dynamics Studies of a-Si and a-Si:H
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FIRST PRINCIPLES MOLECULAR DYNAMICS STUDIES OF a-Si AND a-Si:H
P. A. Fedders
Department of Physics, Washington University, St. Louis, MO 63130
ABSTRACT
We give a brief description of the various classes of molecular dynamics simulations and then describe what we have learned from our simulations recently. This includes information on the nature of defects in a-Si:H including positions in the energy gap and localization, relaxation and rearrangement effects, light induced defects, and the motion of H atoms and Si dangling bonds. INTRODUCTION The aim of this paper is to report on some of the results of a number of molecular dynamics simulations performed over the past several years. These simulations were performed with the first principles ab initio (local density band structure) code developed by Sankey and generalized to alloys by Sankey and Drabold.1- 2 Simulations have been performed on periodic supercells containing from 63 to 216 Si atoms and varying numbers of H atoms. The 216 atom sample was obtained by applying the ab initio code to the 216 atom WWW3 sample while the rest of the supercells were developed from liquid supercells that were quenched.4 Details of these supercell samples are contained in the literature. We shall start with a non technical discussion of the various classes of molecular dynamics (MD) codes. This discussion includes relative accuracies, strengths and weaknesses. Next we shall discuss what we have learned concerning defects in a-Si and a-Si:H, including the distinction between spectral defects (a localized state in the energy gap) and geometrical defects (Si atoms that are not 4-fold coordinated). We then discuss aspects of the relaxation and fluctuations that appear to be very prominent in a-Si but not in c-Si. Finally we discuss some very recent (preliminary) simulations on the motion of H and of dangling bonds in a-Si:H. Our philosophy in these simulations has been that we would study supercell samples with defect fractions that at least approach that of decent lab grown material. As will be shown, this is vital in order to obtain realistic results about the nature of defects in the supercell samples. However, other groups have obtained very interesting results concerning other aspects on a-Si from simulations on supercell samples containing much greater fractions of defects. MD SIMULATIONS There are many possible uses of MD (molecular dynamics) computer codes. For example, (i) one can create realistic supercell samples for further theoretical investigation, (ii) investigate the positions and relative energies of H and dopants in a-Si, (iii) perform studies on time dependent fluctuations in a-Si, the time evolution of a-Si, or study the diffusion of H or other atoms in a-Si, (iv) investigate the changes of the sample when the charge state of a defect changes (which is what happens when light causes an electron to be removed from a defect state), or (v) study the surface chemistry and growth of samples. Because of the large and unpredictable relaxation effects in a-Si, one esse
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