Ultraviolet photoresponse of ZnO nanowire thin-film transistors

Thin-film transistors (TFTs) based on ZnO nanowire (NW) film are fabricated and used as ultraviolet (UV) photodetectors. The decay time constants and the ratio of photocurrent to dark current do not reach their own optimum value under the same conditions, and so a compromise between them should be decided with appropriate values of gate voltages and drain-source voltages. A negative gate voltage applied to the UV photodetectors causes the energy band tilting toward the interface of thin film/air, which make the free electrons in NWs migrate to the interface of thin film/air along the potential gradient and so promotes the depletion of free electrons by adsorption of oxygen molecules. A positive gate voltage applied before the normal time-resolved measurement of photoresponse can make additional oxygen molecules remain in close proximity to the surface of ZnO NWs, which leads to the decrease of the decay time constant. The decay time constant can be greatly decreased by applying a negative gate voltage to the device in the process of time-resolved measurement of photoresponse and a positive gate voltage in advance.