We report on the photoresponsivity of ZnO Schottky barrier diodes grown on (0001) GaN/Al2O3 substrates by plasma-assisted molecular-beam epitaxy. First, ZnO Schottky barrier diodes show a reverse saturation current of similar to 10(-8) A in the dark, and they present a large current buildup of similar to 10(3) A under ultraviolet light illumination, with maintaining stable diode characteristics. Second, ZnO Schottky barrier diodes have a large bandwidth of 195 nm, where the short-wavelength cutoff and the long-wavelength cutoff are 195 and 390 nm, respectively. Third, ZnO Schottky barrier diodes have a time constant of 0.36 ms. Consequently, it is suggested that the ZnO Schottky barrier diodes are very promising for ultraviolet photodetector applications. (c) 2006 American Vacuum Society.