Photoconduction and trapping in sputtered tantalum oxide films

Abstract

Electron trapping in tantalum oxide films, prepared by reactive sputtering onto polished, fused-silica substrates, has been studied by measurements of photocurrents and thermallystimulated-currents. The results indicate the presence of a single set of traps situated about 0.25 eV from the nearest band edge. The initial trap concentration depends on the tantalum sputtering rate and the oxygen partial pressure during film preparation. Heating in a vacuum increases the trap concentration; heating in air or several days exposure to air at room temperature reduces it. Increasing the trap concentration increases both the steady-state current during optical excitation, and the time to rise to and decay from the steady state on initiation and removal of excitation, respectively. A model is proposed in which the empty traps are oxygen vacancies occupied by one electron. Once filled by optical excitation, the traps are proposed to behave like donors in quasi-thermal equilibrium with the conduction band. The rate of conductivity decay, on removal of excitation, is thus determined by the relative rates of retrapping and recombination which, in turn, depend on the relative concentrations and cross-sections of traps and recombination centers.