Synergistic ultraviolet photoresponse of a nanostructured ZnO film with gate bias and ultraviolet illumination

Abstract

We report large enhancement of photoresponse in a nanostructured ZnO film using an electrolyte as a gate dielectric with gate bias and ultraviolet (UV) illumination. The gate bias used for observing the large response is low and ≤5 V. The resultant current enhancement by the combined effect of two effects (field effect and illumination) is much larger than the simple addition of two effects when they act individually. The polymeric electrolyte used as gate dielectric forms an electric double layer at the interface between the nanostructured film and gate dielectric, which enables accumulation of large charge carrier density (≈1013–1014) at the interface. The field effect mobility, μFE, of the carriers is enhanced under modest gate bias in the presence of UV illumination due to the reduction of barrier EB in between the nano crystallites because of large surface charge induced by field effect. It is proposed that the gate induced carriers also passivate the charged defect states in the ZnO, thereby reducing carrier scattering and enhancing the photocurrent. The mechanism proposed has been validated by observation of gate controlled Photoluminescence, where the passivation of charged defect states by gate induced carriers led to suppression of visible photoluminescence, which arises from charged oxygen vacancy defect states.