Gate modulation of below-band-gap photoconductivity in ZnO nanowire field-effect-transistors

We investigated the modulation of the photoconductivity under below-band gap excitation in single ZnO nanowire field effect transistors. Light excitation at 550 nm does not induce any change in the drain-source current when the gate voltage is kept at V-gs = 0 V, but results in a current increase when it is set to V-gs = -50 V. At this negative value of the gate voltage we further investigated the photo-reaction in the below-band-gap range 400-800 nm, observing a qualitative similar profile for all the photo-current curves. These results were attributed to a local effect, suggesting that the change in conductivity is due to the release of electrons from interface states located between the ZnO nanowire active channel and the gate dielectric SiO2.