Real Time Monitoring of the Crystallization of Hydrogenated Amorphous Silicon

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Real Time Monitoring of the Crystallization of Hydrogenated Amorphous Silicon Paul Stradins, David Young, Howard M. Branz, Matthew Page, and Qi Wang National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, Colorado 80401, USA ABSTRACT In-situ real-time optical reflectance spectroscopy is applied to investigate structural changes as hydrogenated amorphous silicon (a-Si:H) loses H and crystallizes at elevated temperature. The interference fringe spectrum (cutoff energy and amplitude) mainly characterize changes in the bulk, while the the crystal Si (c-Si) direct-transition ultra-violet reflectance signatures reveal the presence of any crystalline phase at the surface. Effusion of atomic hydrogen is monitored by a decrease of the interference fringe cutoff energy and is thermally activated with about 1.7 eV. In a-Si:H on glass, optical reflectance spectra are consistent with 2.8 eV activated homogeneous nucleation and growth of a small grain (~100 nm) polycrystalline phase. In contrast, a-Si:H on c-Si crystallizes by solid phase epitaxy with very different spectral kinetics. Our measurements reveal the temperature-time window for thermal crystallization of a-Si:H for photovoltaic device applications, and highlight the versatility of the in-situ spectral reflectance monitoring.

INTRODUCTION Recently, the search for new methods of producing inexpensive crystal Si based photovoltaic devices has intensified. Material fabrication methods will likely need to be different from conventional melt-growth and wafer sawing, and to yield Si of better quality than vapor-phase deposited hydrogenated microcrystalline silicon. One of the approaches is to create a large-grain seed layer on glass substrate and grow the rest of the device by some kind of Si epitaxy [1, 2], with solid phase thermally-induced epitaxy as a promising candidate. Alternately, whole amorphous devices can be thermally crystallized into polycrystalline silicon form, providing that the grains are sufficiently large and well passivated [3]. To evaluate and apply these approaches we must understand of solid phase crystallization processes in a-Si:H layers. A simple, robust real-time monitoring of phase transformations in situ is necessary. We have developed an in-situ optical reflectance spectroscopy technique that is capable of detecting H effusion and the growth of the crystalline phase, both in the bulk and near the film surface. Using this method, one can easily distinguish homogenous growth of randomly oriented polycrystallites from the solid phase epitaxy and can estimate the activation energies related to these different growth modes. EXPERIMENTAL Undoped a-Si:H layers were deposited by hot wire chemical vapor deposition (HWCVD) from pure SiH4 gas. About 1 µm film at a rate about 10 Å/s was grown onto Corning 1737F glass at 320°C. Films up to 2.5 µm at 100 Å/s were deposited onto c-Si substrates at 370°C. Crystalline Si (100) oriented substrates were HF-cleaned and subjected to a 60 sec HW atomic H treatment before deposition. On c-Si substrate