ESR Study of Crystallization of Hydrogenated Amorphous Silicon Thin Films
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0989-A06-13
ESR Study of Crystallization of Hydrogenated Amorphous Silicon Thin Films Tining Su1, Tong Ju2, P. Craig Taylor1, Pauls Stradins3, Yueqin Xu3, Falah Hasoon3, Qi Wang3, and Walter A. Harrison4 1 Department of Physics, Colorado School of Mines, Golden, CO, 80401 2 Department of Physics, University of Utah, Salt Lake City, UT, 84112 3 National Renewable Energy Laboratory, Golden, CO, 80401 4 Department of Applied Physics, Stanford University, Stanford, CA, 94305 ABSTRACT Electron-spin-resonance (ESR) is used to investigate the evolution of the local order surrounding the dangling bonds produced by hydrogen effusion in a-Si:H thin films prepared by both plasma-enhanced-chemical-vapor-deposition (PECVD) and hot-wire CVD (HWCVD). At 560° C, the HWCVD sample fully crystallizes after ~ 800 min, while the sample made by PECVD does not crystallize. The PECVD sample crystallizes at a higher temperature (580° C) and after a much longer annealing time (∆t = 1300 min). The ESR signal of the defects in both samples remains at about 5x1018 cm-3 as long as the sample remains amorphous during the grain nucleation period. In both HWCVD and PECVD samples, as the crystallites appear, the defect densities gradually decrease and saturate at about 3x1017 cm-3 as the crystallization is completed.
INTRODUCTION Solid-phase crystallization and the subsequent re-hydrogenation of the amorphous silicon thin films provides a low cost approach for thin-film crystalline Si:H-based photovoltaic devices [1,2]. During the hydrogen effusion, significant lattice reconstruction occurs, as hydrogen is driven out of the film. This process is accompanied by creation and migration of a large number of dangling bonds. Optical techniques, such as measurements of the reflectance and transmittance, provide in situ information of the kinetics of the crystallization [1,2]. However, these techniques do not provide information of local structural changes during crystallization. Experimentally, samples prepared by HWCVD crystallize faster and at lower temperatures, compared to samples made by PECVD [1,2]. This phenomenon is not well understood. Although most of the hydrogen is driven out of the film during effusion, there exists a rather persistent residual hydrogen concentration of about 1019 cm-3. It is not clear if this residual hydrogen plays any role during the crystallization. Another issue of interest is the effect of rehydrogenation after the film is crystallized. The local order of the hydrogen atoms is crucial in understanding the effect of hydrogen passivation of the excessive defects. We have investigated the evolution of the defects created during hydrogen effusion and the crystallization process, using magnetic resonance techniques such as nuclear magnetic resonance (NMR) and electron spin resonance (ESR). In this article, we report the results from ESR during the annealing and crystallization and present a simple model of the exchange interaction in the disordered a-Si.
EXPERIMENTAL DETAILS Amorphous silicon samples were deposited by bot
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