TEMPERATURE AND THICKNESS DEPENDENCE OF PHOTOCONDUCTIVITY AND DENSITY-OF-STATES DISTRIBUTION FOR BI DOPED GE20S80

Differential thermal analysis data showed that the addition of 5 at% Bi to Ge20S80 caused T(g) and T(c) to shift towards higher temperatures but had no significant influence on the structural units responsible for the formation of the glass network. Bi addition and temperature both increased the electrical conductivity of Ge20S80. Dark electrical conductivity increased for Ge20S75Bi5 films with decreasing thickness. Photoconductivity started with values higher than those of dark conductivities and both gradually decreased to saturation with time. For any thickness, increased durations of light soaking yielded temperature dependence of dark conductivity, characterized with decreasing conductivities and increasing conduction activation energies. The density of states (DOS) distribution obtained from light-induced effects on dark conductivity showed a peak around 0.4-0.47 eV below E(c), which is the midgap. The results were explained on the basis of the additional localized states in the gap, the positive and negative defects centred at Bi and S, increased carrier mobilities with temperature, heterogeneous film structure and induced electron-hole traps,