Electronic transport study of high deposition rate HWCVD a-Si:H by the microwavephotomixing technique
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Electronic transport study of high deposition rate HWCVD a-Si:H by the microwave photomixing technique S.R. Sheng*, R. Braunstein*, B.P. Nelson**, and Y. Xu** *Department of Physics and Astronomy, University of California, Los Angeles, California 90024, USA; **National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, USA.
ABSTRACT The electronic transport properties of high deposition rate a-Si:H films prepared by HWCVD have been investigated in detail by employing the microwave photomixing technique. The high deposition rates (up to 1 µm/min.) were achieved by adding a second filament, increasing deposition pressure, silane flow rate, and decreasing filament-to-substrate distance. The effect of the deposition rate on the resultant film properties with respect to the substrate temperature, deposition pressure and silane flow rate was studied. It was found that the film transport properties do not change monotonically with increasing deposition rate. The photoconductivity peaks at ~70-90 Å/s, where both the drift mobility and lifetime peak, consistent with the deposition rate dependence of the range and depth of the potential fluctuations. High quality, such as a photoconductivity-to-dark-conductivity ratio of ~105 and nearly constant low charged defect density, can be maintained at deposition rates up to ~150 Å/s, beyond which the film properties deteriorate rapidly as a result of an enhanced effect of the long-range potential fluctuations due to a considerable increase in the concentration of the charged defects. Our present results indicate that medium silane flow rate, low pressure, and higher substrate temperature are generally required to maintain high quality films at high deposition rates. INTRODUCTION The deployment of amorphous silicon solar cells for large scale power generation is contingent not only upon the production of photovoltaic devices of stable high efficiency but for industrial production, high deposition rate is desirable to improve the throughput for a given machine, and also to reduce the costs. Various techniques have been attempted to improve the deposition rate of hydrogenated amorphous silicon (a-Si:H) films; among which, plasma enhanced chemical vapor deposition (PECVD) usually yields materials with poor initial performance and poor light stability when the deposition rate is increased, which has been correlated with the increased density of microvoids [1]. It has been suggested that with increasing deposition rates, higher silane related radicals, short-lifetime radicals, increased ionbombardment energies, and hence more resultant microvoids and defects, are potential causes for deterioration of the performance and stability of a-Si:H. At present, it is generally believed that the performance of a-Si:H will deteriorate monotonically with increasing deposition rates, but the high deposition rates achieved by PECVD are relatively low, less than 20 Å/s. Recently, we employed the hot-wire chemical vapor deposition (HWCVD) technique to successfully grow device-quali
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