Reducing the Persistent Photoconductivity Effect in Zinc Oxide by Sequential Surface Ultraviolet Ozone and Annealing Treatments

We developed a method to reduce the persistent photoconductivity (PPC) effect of zinc oxide (ZnO) thin film transistors (TFTs) by sequential surface treatment, which includes preannealing/ultraviolet ozone/postannealing (PUP) treatments. Preannealing treatment improves the surface uniformity of the film and eliminates the residual solvent and contaminant molecules without affecting the zinc–oxygen bonds. The ultraviolet ozone (UVO) treatment reduces the oxygen vacancy and increases the amount of metal hydroxide groups on the surface of ZnO by converting adsorbed O2– on the ZnO and atmospheric oxygen to highly chemisorbed OH–. Postannealing treatment improves the film quality by making the film densely and inducing zinc–oxygen bonding in the ZnO film. Therefore, the negative threshold voltage shift decreased upon visible light irradiation of the PUP ZnO TFT. The treatment also improved the photoresponsivity upon the irradiation with ultraviolet (UV) light. Moreover, the photoelectric recovery time after the irradiation of the UV light was shortened, which means that the PPC effect on the PUP ZnO TFT decreased. The decrease in the PPC effect originated from the reduced oxygen vacancies and the increased surface metal hydroxide groups upon PUP treatment. Our experiment results suggest a method to reduce the PPC effect on ZnO TFTs via PUP treatment, which is useful for developing high-performance UV phototransistors that are based on ZnO.