Trap-related photoconductivity in ZnO epilayers

A strong photoconductive response is observed for ZnO epilayers in the presence of both above bandgap and below bandgap photoexcitation. Photoexcitation for energies larger than the bandgap results in a photoconductive response with fast and slow time constants on the order of nanoseconds and larger than milliseconds, respectively. The fast and slow time constants are attributed to minority carrier recombination and slow escape of holes from traps, respectively. Photoexcitation in the visible spectral region, below the bandgap energy, results in slow rise and fall time constants on the order of minutes and hours. A model for the photoconductive response based on rate equations is presented providing an accurate fit to measured photoconductivity data. The rate equation model suggests the presence of hole trap levels in the energy range of 0.6 eV to 1.0 eV relative to the valence bandedge. The passivation of the ZnO surface with SiO2 shows significantly reduced photoconductive transient decay time constants, suggesting a significant reduction of deep surface defects on the ZnO material.