Effect of annealing of NbLaO dielectric on the electrical properties of ZnO thin-film transistor

Abstract

ZnO thin-film transistors (ZnO TFTs) with high-k NbLaO as a gate dielectric were fabricated on an indium tin oxide (ITO)-coated glass substrate. The NbLaO film was prepared by the sputtering method at room temperature and then annealed in N2 at 200, 300, and 400 °C. The effect of annealing temperature on the quality of NbLaO and ZnO films, especially on the electrical properties and the bias-stress stability of the ZnO TFT, was investigated. The AFM images reveal that the NbLaO film annealed at 300 °C exhibits a relatively smooth surface morphology with a root mean square roughness of 0.31 nm. AFM and x-ray diffraction measurements confirmed that the grain size of ZnO thin films slightly decreases with the increase of the annealing temperature. Except the mobility of slightly less than that of the device annealed 200 °C, the other electrical properties (off-state current, on/off current ratio, and subthreshold slope) of the ZnO TFT annealed at 300 °C are better than those of the samples annealed at 200 and 400 °C and show excellent gate-bias stress stability, which is due to a combination effect of a smoother interface, a denser structure, and the absence of indium doping in the NbLaO gate dielectric, thus resulting in a lower interfacial trap density.