Sub-bandgap photoconductivity in ZnO epilayers and extraction of trap density spectra

Photoconductivity is observed in ZnO epilayers due to photoexcitation in the visible spectral region of 400-700 nm, below the ZnO bandgap energy of 3.4 eV. Photoconductive transients due to visible photoexcitation have time constants in the order of minutes. Treatment of the ZnO surface with SiO2 passivation layers results in a significant reduction in the photoconductive signal and photoconductive time constant. The photoconductive response is attributed to hole traps in ZnO, where a rate equation model is presented to describe the photoconductive characteristics. A method of extracting the hole trap density spectrum is presented on the basis of the rate equation model and assumptions for hole capture lifetime and carrier recombination lifetime that are validated by experimental time-resolved photoluminescence measurements of the material under study. Traps are found to be distributed near 0.75 eV and 0.9 eV from the valence band edge for SiO2 passivated and unpassivated ZnO epilayers, respectively.