Amorphous Silicon Films and Solar Cells Deposited by HWCVD at Ultra-High Deposition Rates

The deposition conditions for hydrogenated amorphous silicon, deposited by hot wire chemical vapor deposition, are linked to the film structure as we increase deposition rates (R_d) to 100 Å/s. At low R_d (< 20 Å/s), films with optimal properties are deposited under low silane depletion conditions, and all measures of structure (X-ray diffraction, Raman spectroscopy, H evolution, small-angle X-ray scattering (SAXS)) indicate a compact material. At high R_d (100 Å/s), optimum films are deposited under silane depletion conditions as high as 75-80%, and all structural properties except for the SAXS results once again indicate a compact material. We relate changes in the film electronic structure (Urbach edge) with increasing R_d to the increase in the SAXS signals, and note the invariance of the saturated defect density versus R_d, discussing reasons why these microvoids do not play a role in the Staebler-Wronski effect for these films. Finally, we present device results over the whole range of R_d that we have studied and suggest why, at high R_d, device quality films can be deposited at such high silane depletions.