Hole Transport in a-Si:H(F) Prepared by Hydrogen-Radical-Assisted Chemical Vapor Deposition

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Abstract
The modes of hole transport in both non-doped and BF3-doped a-Si:H(F) films prepared by hydrogen-radical-assisted chemical vapor deposition (HRCVD) were investigated by the time-of-flight method. Non-dispersive transport was observed at temperatures above approximately 300 K, and dispersive transport below this temperature. The drift mobility conforms with a multiple-trapping model which assumes that the valence-band tail is distributed exponentially in terms of energy. The characteristic temperatures provided by this model, 310 K for non-doped a-Si:H(F) and 286 K for a-Si:H(F) doped using 30 ppm of BF3, are lower than the corresponding value for a-Si:H films prepared by an R.F. glow discharge of SiH4. This result shows that the distribution of valence-band tail states is narrower in the case of HRCVD. Sub-bandgap profiles based on the wavelength dependence of photoconductivity also reveal a decrease in the density of mid-gap states.