Films of a-Si have been prepared by vacuum deposition from crucibles of BeO and BN in a hydrogen ambient. The electrical conductivity of these impure films was modified by the addition of aluminum that was coevaporated during the film deposition. The changes observed are consistent with the assumption that the aluminum doping results in a p-type film for low conconcentrations (<2.3 at.%) whereas high concentrations introduce band structure changes as well. Thickness dependent sample resistivities are observed. This result is interpreted using a layer model to approximate two surface layers with resistivities ρs = 108 Ω∙cm and a bulk layer with ρB = 8 × 103 Ω∙cm. Each high-resistance layer, which is approximately 0.12 μm thick, seems to be caused by the increased hydrogen concentration at both surfaces, which is observed in the hydrogen profiling study. It is suggested that these high-resistance surface layers are due to a wider band gap material with the Fermi level near midgap. The bulk material has a smaller band gap with the Fermi level closer to one of the band edges.
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