Transport in microcrystalline silicon thin films deposited at low temperature by hot-wire chemical vapor deposition

This work is focused on the determination of the variation of local mobility of charge carriers with thickness (< 1 mu m) for undoped microcrystalline silicon layers deposited by the hot-wire chemical vapor deposition technique. We observed that the temperature of the layers T-s evolves with the deposition time, once the tungsten filament has been heated from room temperature to a fixed definite value. Thus, experiments have been realized by fixing the gas pressure (41 mTorr), the dilution of silane in hydrogen (50%), by setting the filament temperature (1600 degrees C) and letting the time run. An average substrate temperature T-s,T-av has been defined, whose value depends on deposition time. As a result, the local mobility deduced from time-resolved microwave conductivity increases almost linearly with T-s,T-av up to 193 degrees C, i.e. with thickness tip to 400 nm corresponding approximately to the amorphous-microcrystalline transition and then increases sublinearly up to T-s,T-av = 221 degrees C, i.e. a 900-nm-thick layer. These results, compatible with the highest AM 1.5 efficiency (> 9%) reported so far for p-i-n mu c-Si:H solar cells realized at T-s = 185 degrees C [S. Klein, F. Finger, R. Carius, T. Dylla, B. Rech, M. Grimm, L. Houben, M. Stutzmann, Thin Solid Films 430 (2003) 202], suggest that in the range of T-s,T-av from 190 degrees C to 220 degrees C, hydrogen plays a dominant role in the HWCVD growth of mu c-Si:H films. (c) 2005 Elsevier B.V. All rights reserved.