Physics of Solid-Phase Epitaxy of Hydrogenated Amorphous Silicon for Thin Film Si Photovoltaics
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0910-A15-05
Physics of Solid-Phase Epitaxy of Hydrogenated Amorphous Silicon for Thin Film Si Photovoltaics Paul Stradins, Yanfa Yan, Robert Reedy, David L. Young, Charles W Teplin, Eugene Iwaniczko, Yueqin Xu, Kim Jones, Glenn Teeter, A. Harv Mahan, Howard M. Branz, and Qi Wang National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO, 80401 ABSTRACT Solid state crystallization of hydrogenated amorphous silicon (a-Si:H) prepared by hot-wire CVD is studied in solid phase epitaxy mode. By using a novel optical method combined with cross-sectional TEM and SIMS, a reduction of epitaxial growth speed is observed with increase in a-Si:H film thickness. Namely, in films thinner that 0.9 micron, solid phase epitaxy velocity depends linearly on film thickness. As the film thickness increases beyond 1 micron, the average velocity of solid phase epitaxy decreases considerably with respect to that in thinner films. In this regime, its velocity becomes also time-dependent: initial slow propagation of crystallization front gets considerably accelerated after the front has traveled above 400nm. SIMS depth profiles of hydrogen shows considerably more residual hydrogen in thicker films after the start of solid phase epitaxy. In addition, prolonged pre-dehydrogenation at lower temperatures results in the increase in the average epitaxy speed in thicker films. These phenomena are likely related to delayed hydrogen outdiffusion in thicker films, which also leads to time-dependent speed of the solid-phase epitaxy front. Thus, the excess residual hydrogen in CVD films reduces the rate of solid-phase crystalline growth, similarly to earlier results on H-implantation and indiffusion.
INTRODUCTION Crystallization of amorphous Si is used for TFT and, more recently, for thin-film Si solar cell manufacturing [1, 2]. Extensive studies [3] of both random crystallization (RC) and solidphase epitaxy (SPE) have not, however, been able to relate in detail the solid-phase crystallization with film structural properties, such as Si network medium range order, residual hydrogen content, or deposition method. Electronically active dopants such as B, P, As are known to promote crystallization while O, N, C impurities suppress it [3, 4]. Role of H is less clear, since, on one hand, it assists in modifying Si network structure by inserting into weak Si-Si bonds and thus might facilitate structural rearrangements needed for crystallization. On the other, it presents an impurity for crystalline Si and will be expelled from it due to low solubility at crystallization temperature. It has been suggested that presence of H and related absence of Si dangling bond defects inhibits crystallization in SPE mode [5, 6]. Role of H needs to be understood in detail for crystallization of hydrogenated amorphous silicon (a-Si:H). In this work, we study the effects of hydrogen on the SPE in hot-wire chemical vapor deposition (HWCVD) aSi:H films. Studies of SPE allow to decouple crystalline growth from gain nucleation. In the experiment, we use our previously deve
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