Control of the threshold voltage in ZnO nanobelt field-effect transistors by using MoOx thin film

Abstract

We report on the feasible control of the threshold voltage (Vth) in ultra-thin ZnO nanobelt FETs by using substoichiometric molybdenum trioxide (MoOx, x < 3) either as a modification layer on the surface of ZnO nanobelts or as electrodes instead of the widely used Ti/Au. ZnO nanobelt FETs using Ti/Au as the electrodes usually exhibit a negative threshold voltage, indicating n-channel depletion mode behavior, whereas ZnO FETs with MoOx/Au electrodes instead of Ti/Au show a positive shift of threshold voltage, exhibiting an n-channel type enhancement mode, which can be explained by a high Schottky barrier created at the interface of MoOx and the ZnO channel. In contrast, the decoration on the surface of ZnO channel by MoOx significantly increases the zero-bias conductivity and electron carrier concentration, and then negatively shifts the threshold voltage. We propose that MoOx thin film may play a passivation effect role, much more so than the doping effect role, due to the large amount of adsorbed species on as-grown ZnO nanobelts, especially oxygen species.