Temperature dependence of DC conductivity of as-deposited and annealed selenium films

The linear DC dark- and photo-current-voltage characteristics observed for as-deposited (T-s = 25 degrees C) and annealed (T-a = 70 degrees C) thermally-evaporated selenium films in the temperature range 150-300 K were used to deduce the respective temperature dependencies of the dark-conductivity sigma(dark) and photo-conductivity sigma(ph) of these samples. Both XRD and SEM studies confirmed the amorphous state of the as-deposited films and the hexagonal-like polycrystalline structure of the annealed films. The ln sigma versus 1/T plots of the highly resistive amorphous selenium (a-Se) samples revealed only one thermally-activated process in the region 250-300 K for both sigma(dark) and sigma(ph) with an activation energy E-sigma = 0.73 eV, and 0.49 eV, respectively. This has been interpreted considering the conduction models based on the existence of the inherent charged defect centres (dangling bonds D-, D+) in the band tails. The conductivity of the annealed specimens was at least three orders of magnitude larger than that of the as-deposited ones and the temperature-dependence of their sigma(dark) and sigma(ph) exhibited two different conduction mechanisms; one being operative in the high-temperature region (230-300 K) and the other being dominant at low temperatures (220-150 K). As these annealed samples were polycrystalline, the former activated process has been analysed on the basis of grain-boundary-limited conduction with E sigma = 0.17 eV (sigma(dark)) and 0.15 eV (sigma(ph)). The low-temperature conductivity of these sample has been described in terms of variable range hopping conduction, of Mott's type, which appears to be applicable in polycrystalline solids at relatively low temperatures. The observed reduction in the grain-boundary potential barriers and in the degree of disorder on illuminating the annealed selenium films has been understood by the phenomenon of neutralization of the native dangling bond states present at the interface of the grain boundaries of these samples by the photo-generated charge carriers.