Forms of Hydrogen and Hydrogen Diffusion in Realistic Models of a-Si:H
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Forms of Hydrogen and Hydrogen Diffusion in Realistic Models of a-Si:H P. A. Fedders Department of Physics, Washington University, St. Louis, MO 63130, U.S.A.
ABSTRACT Using ab initio density functional calculations we investigate the energetics of hydrogen in a-Si:H starting from a basic supercell that contains 142 atoms and is completely free of defects. The study includes isolated H atoms bonded to dangling bonds, H atoms bonded to dangling bonds in regions of clustered H, bond centered hydrogen, and molecular hydrogen. In particular, the difference between clustered and isolated hydrogen has been largely ignored in the past. Energetics of doped as well as undoped cells are considered. The results are discussed with particular emphasis on H diffusion and the replacement of atoms in molecular hydrogen with those bonded.
INTRODUCTION Over the years there have been many supercells of a-Si:H presented and many papers have been written on the energetics of various configurations of hydrogen in the supercells [1]. In this paper we present another set of a-Si:H supercells and corresponding set of energetics calculations. Yet, we believe that these supercells and calculations are considerably more realistic and complete than previous ones. The supercells each contain about 140 atoms and the calculations are performed with SIESTA [2,3], an ab initio fully self-consistent local orbital code. Although 140 atoms is not large by the standards of those using classical methods, it is large by ab initio standards but we consider it to be about as small a cell as one would wish to consider because of wrap around effects which cause defects to interact with themselves. Further, the basic supercell contains no spectral or geometrical defects and thus has a substantial clean gap without excessive band tailing caused by strain defects. We note that even one defect in 140 atoms corresponds to a grossly unphysical defect density and several defects would be intolerable. Experiments clearly show that there are at least three types of hydrogen present in a-Si:H in substantial amounts [4-7]. This includes isolated bonded H, clustered bonded H, and molecular H. Further, bond centered (BC) H is important because of its possible relevance to H migration. The distinction between isolated and clustered H is usually ignored although this work shows that they have very different energetics. We also investigate the effects of dopants on all the forms of hydrogen and find, as expected, that for some forms this makes a great deal of difference. We argue that an understanding of all four types of hydrogen is necessary in order to form an understanding of important phenomena such as light induced defects and the migration of hydrogen. For example, consider the following apparent paradox. Experiments clearly show that dangling bonds are rarely, if ever, accompanied by nearby hydrogen [8-10]. Yet in a region of clustered hydrogen if an H atom leaves its dangling bond, one will have a dangling bond near a number of H atoms! The calculations in this pa
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