AVS 50th International Symposium
    Thin Films Thursday Sessions
       Session TF-ThM

Paper TF-ThM1
Mechanisms and Energetics of H Insertion into Si-Si Bonds in Hydrogenated Amorphous Silicon Films

Thursday, November 6, 2003, 8:20 am, Room 329

Session: Modeling & Fundamentals in Thin Film Deposition
Presenter: M.S. Valipa, University of California, Santa Barbara
Authors: S. Sriraman, University of California, Santa Barbara
M.S. Valipa, University of California, Santa Barbara
E.S. Aydil, University of California, Santa Barbara
D. Maroudas, University of Massachusetts
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Understanding the mechanism of chemically-induced disorder-to-order structural transitions is relevant to the synthesis of various technologically important materials, including nanocrystalline silicon thin films used in the fabrication of solar cells and flat panel displays. Thin hydrogenated amorphous silicon (a-Si:H) films crystallize at low temperatures when exposed to H atoms from an H@sub 2@ plasma, or when the SiH@sub 4@ feed gas is heavily diluted with H@sub 2@. Recently, we showed that H-induced crystallization is mediated by insertion of H atoms into strained Si-Si bonds as the H atoms diffuse through the a-Si:H film.@footnote 1@ This presentation focuses on analyses of the mechanisms and energetics of various pathways for H insertion into Si-Si bonds in a-Si:H. These pathways are generated through molecular-dynamics simulations of repeated H atom impingement on a-Si:H films, grown computationally on crystalline Si (c-Si) substrates. Prior to insertion, the diffusing H atom bonds to one of the Si atoms that form the original Si-Si bond; upon insertion, an intermediate bridging configuration forms where the H atom is bonded to both Si atoms. After the H atom leaves the bridging configuration, the Si-Si bond may be further strained or broken, or it may relax to restore the Si-Si bond closer to the equilibrium c-Si bond length. A diffusing H atom can also occupy the bond-centered (BC) location between two Si atoms that are not bonded together. However, after the H atom diffuses away from this BC location, the two Si atoms form a Si-Si bond. The activation energy barriers are calculated for the H insertion reactions; the resulting distribution of activation energies is correlated with the Si-Si bond strain. The role of these H-insertion reactions in the structural relaxation of the a-Si:H network that results in disorder-to-order transitions is discussed. @FootnoteText@@footnote 1@ S. Sriraman, S. Agarwal, E. S. Aydil, and D. Maroudas, Nature 418, 62-65 (2002).