Mobility edge in hydrogenated amorphous carbon

Abstract

Optical and electronic properties of plasma enhanced chemical vapor deposited diamondlike amorphous carbon (a-C:H and a-C1−xSix:H) films were studied by photothermal deflection spectroscopy and the spectral photocurrent method. Two different regimes of photoconductivity in the diamondlike carbon films have been detected. The carrier mobility due to the σ-σ* bands is found to be 1–2 orders of magnitude higher than that due to the π-π* transitions. The σ-σ* mobility edge of diamondlike hydrogenated amorphous carbon is equal to 5.3±0.1eV and appears to be closely correlated with the band gap of diamond. We have also shown that the mobility edge in diamondlike amorphous carbon is not associated with its band gap. The optical band gap of diamondlike amorphous carbon films depends on the sp2 phase content as already reported by Robertson [Phys. Rev. B 53, 16302 (1996)]. However, the mobility edge is not influenced by either the Si doping level, the film thickness, or the ion impact energy during deposition.