Time-resolved luminescence and photoconductivity of polycrystalline ZnO films

Abstract

The relative intensities of the green and blue luminescence of a ZnO film was shown to depend on the excitation regime. Time-resolved and steady-state luminescence were studied along with photoconductivity transients. Under continuous excitation the film emitted green light, while under pulsed excitation the luminescence was either blue or green, depending on the intensity of the excitation pulse. The intensity of the blue component depended linearly on the pulse intensity while the green intensity followed a sublinear power law dependence with the exponent α=1/3. The transient luminescence exhibited fast (below nanosecond) and slow (microsecond) decay components at room temperature. The fast component was ascribed to interband exciton recombination, and the slow component was attributed to an electron-hole recombination involving a donor-acceptor complex, which most likely consisted of oxygen and zinc vacancies. In this model, the complex can emit light only when it is activated, i.e., oxygen vacancy is in its singly ionized state and the acceptor (zinc vacancy) captures a hole. The density of the activated complex depends on the Fermi level position, bend bending, and thickness of the depletion layer.