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HYDROGEN DIFFUSION MECHANISM IN AMORPHOUS SILICON FROM D TRACER DIFFUSION: THEORY AND EXPERIMENT HOWARD M. BRANZ, SALLY ASHER, BRENT P. NELSON AND MATHIEU KEMP* National Renewable Energy Laboratory, Golden, CO 80401 *University of North Carolina, Chapel Hill, NC 27599 ABSTRACT We compare experimental diffusion studies to the results of a theoretical study of diffusion controlled by a single deep trap level. Analytic solutions for the D profiles after annealing depend on the characteristic H release time, c, from the deep trap. At times much shorter than -r, the D profile develops exponential wings whose decay length is the mean D displacement between trapping events. The long-time D profile is a solution to the ideal diffusion equation, but with an effective diffusion coefficient that can be calculated from features of the early-time profiles. New experimental data establish the validity of the model at a range of anneal times and temperatures. We also find that the mean displacement of free H before retrapping decreases with both increased illumination and increasing anneal temperature. INTRODUCTION Device-quality amorphous silicon requires hydrogen to reduce the density of gap-state electronic defect levels. However, many workers suspect H of enabling the deleterious metastable effects caused by illumination and carrier-injection [1] that limit commercial applications of the material. Improvements in hydrogenated amorphous silicon (a-Si:H) requires a greater understanding of hydrogen bonding and transport mechanisms. In recent studies of D tracer diffusion in a-Si:H/a-Si:H:D/a-Si:H (HDH) sandwich structures, we demonstrated that H diffusion in a-Si:H is trap-controlled [2]. H diffusion in a-Si:H proceeds through a minority of the H in a transport level that permits rapid diffusion. The observed H diffusion rate is roughly ten orders of magnitude lower than the free H diffusion coefficient (in the transport level), because diffusion is limited by deep-trapping. The trap is a deep Si-H level that contains nearly all the H in a-Si:H. In this paper, we compare experimental diffusion studies to the results of a theoretical study of diffusion controlled by a single trap level [3] . The theoretical results include predictions of D profiles after diffusions for times shorter and longer than the characteristic H release time from the deep trap. Our new experimental data establishes the validity of the model at a range of times and temperatures. We also find that the mean displacement of free H before retrapping decreases with both increased illumination and increasing anneal temperature. THEORY In Kemp and Branz [3] , we solve for the D profiles observed after annealing an HDH sandwich structure, assuming a trap-controlled H diffusion model. We model the a-Si:H matrix as comprising two types of H sites: numerous transport (free) sites and far fewer deep trap sites. We show a schematic diagram of this model in Figure 1. The transport sites are either interstitial or Si-Si bond-centered sites. H diffusion is the result of r