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Hydrogen Equilibration and Metastability in Amorphous Silicon Howard M. Branz and S.B. Zhang National Renewable Energy Laboratory Golden, CO 80401 U.S.A. ABSTRACT We review evidence for the vital role of H in metastability and defect equilibration in hydrogenated amorphous silicon. H pairs act both as an H reservoir that equilibrates with DB’s and dopants and as the metastable H state of light-induced metastability. With ab initio pseudopotential theory, we calculate energies and configurations for two novel H pairing sites associated with small hydrogenated vacancies. We describe microscopic models of equilibration and metastability based on our calculations. INTRODUCTION In hydrogenated amorphous silicon (a-Si:H) thin films, H is both essential and problematic. H passivates threefold-coordinated dangling bond (DB) defects and relieves network strain, which permits doping and formation of useful electronic devices. However, H also mediates defect and dopant thermal equilibration at moderate temperature and is implicated in the Staebler-Wronski effect (SWE) [1], a deleterious metastable degradation of film properties caused by illumination and carrier injection. In recent years, the proposal of “negative U” H pair formation has been shown to explain a variety of metastability and equilibration phenomena, but without convincing microscopic models of the paired H configurations. Although it is widely believed that H microstructures, such as small vacancies, are correlated with SWE susceptibility, there have been no realistic calculations of H pair configurations near vacancies. In this paper, we review the evidence for H involvement in equilibration and metastability with emphasis on the H-based phenomenological models of these effects. These models suggest that paired configurations of H are vital to an understanding of the phenomena. After describing evidence for small hydrogenated vacancies, we use ab initio pseudopotential theory to explore the properties of two new H pair configurations at the vacancies. A unified picture of H reconfiguration during light-soaking and annealing of a-Si:H emerges. HYDROGEN IN EQUILIBRATION AND METASTABILITY In 1985, Smith and Wagner [2] suggested that the defect structure of a-Si:H reaches thermal equilibrium above a cooling-rate-dependent “freeze-in” temperature. Ast and Brodsky [3] had previously observed that cooling rate could dramatically impact the room T conductivity of doped amorphous silicon. Detailed study of this effect [4] and related bias-anneal phenomena [5] revealed that the activated dopant fraction was reaching thermal equilibrium with the electronic Fermi energy. In 1987, Street et al. [6] proposed the “H-glass model,” that above a fictive temperature, Tglass , dopant and defect equilibration in a-Si:H were mediated by diffusing hydrogen atoms. Subsequent work confirmed that the equilibration rate in a-Si:H was linearly correlated with H diffusion coefficient in a broad range of samples [7] and that the H-glass

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