Origin of fast-response photocurrent in ZnO thin film

We study the origin of fast-response photocurrent in ZnO system grown by dc-unbalanced magnetron sputtering. Current-voltage (I-V) measurements show that ZnO thin film is sensitive to UV-visible wavelength and the response time is about 5 ms. This typical sensitivity is related to the contribution of native point defects in electrical transport, as indicated by a space-charge-limited current regime. I-V curves in vacuum, Ar, and O2 environments reveal that oxygen vacancies (VO) play an important role in adsorption and desorption processes. Using absorption and temperature-dependent photoluminescence spectroscopy, the presence of VO is confirmed and dominates even at low temperature. The strong contribution of VO promotes low excitonic absorption, resulting in low photocurrent and fast-response detector. We propose that fast-response ZnO photodetector is not only originated from Schottky barrier but also from VO-related defect states. This result is important to improve the functionalities of ZnO for optoelectronic applications.