Mapping the Phase-Change Parameter Space of Hot-Wire CVD Si:H Films Using In-Situ Real Time Spectroscopic Ellipsometry
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Mapping the Phase-Change Parameter Space of Hot-Wire CVD Si:H Films Using In-Situ Real Time Spectroscopic Ellipsometry Dean H. Levi, Brent P. Nelson, and John D. Perkins National Center for Photovoltaics, National Renewable Energy Laboratory 1617 Cole Blvd., Golden, CO 80401, U.S.A. ABSTRACT In-situ real-time spectroscopic ellipsometry (RTSE) provides detailed information on the evolution of the structural and optical properties of Si:H films during film growth. We have used in-situ RTSE to characterize the film morphology and crystallinity of hot-wire CVD (HWCVD) Si:H films as a function of hydrogen dilution R=[H]/[H+SiH4], substrate temperature Ts, and film thickness db. Transitions from one mode of film growth to another are indicated by abrupt changes in the magnitude of the surface roughness during film growth. The degree of crystallinity of the film can be determined from the bulk dielectric function. We have studied the growth parameter space consisting of R from 0 to 12, Ts from 150oC to 550oC, and db from 0 to 1 um. For each set of R and Ts values, the structural evolution of the film can be characterized by the shape of the surface roughness thickness ds versus bulk thickness db curve. In contrast to studies done by Collins et al on PECVD growth of Si:H films, our studies of HWCVD growth find no conditions where ds remains constant after coalescence of the initial nucleation centers. Most of the films grown within the range of parameters studied exhibit a secondary nucleation and coalescence signature. The transition between a-Si:H and uc-Si:H growth is near the R=3 to R=4 dividing line. Initial coalescence of purely uc-Si:H material doesn’t occur until R>8. We have verified the RTSE crystallinity classification using ex-situ Raman scattering. INTRODUCTION Hot-wire CVD of a-Si:H and uc-Si:H holds great promise for production of large-area thinfilm silicon devices because of the high deposition rates achievable with this technique [1]. In order to realize this potential, it is imperative that characterization techniques be developed to provide the necessary feedback to optimize the deposition conditions to produce optimum quality films. In-situ real-time spectroscopic ellipsometry (RTSE) is an ideal tool for such optimization because it can provide detailed information on the structural, electronic, and optical properties of the film as a function of time, or equivalently, thickness of the film. It is capable of doing this remotely and non-invasively in harsh environments such as that encountered in HWCVD growth chambers. These properties are in contrast to ex-situ characterization techniques that only provide bulk averages of the film properties, or rely upon time consuming and destructive sample preparation and depth profiling. RTSE is unique in its ability to accurately determine the film properties on a monolayer scale as the film is growing. The utility of RTSE for process optimization of thin-film a-Si:H and uc-Si:H has been demonstrated by Collins and co-workers in numerous studies [2]. Their studies
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