Illumination instabilities in ZnO/HfO2 thin-film transistors and influence of grain boundary charge

The illumination instabilities of nanocrystalline ZnO thin-film transistors (TFT) with HfO2 gate dielectrics are reported via zero gate bias multiwave length illumination stress method. TFT ID–VG curves exhibit a negative threshold voltage shift together with an increase in ID off current and increase in subthreshold slope with increasing photon energy and illumination time. Analysis of transistor characteristics indicates that one component governing negative threshold voltage shifts is a decrease in grain boundary-trapped charge areal density due to illumination. This relationship can be explained by conduction based on thermionic emission over potential barriers formed at the ZnO crystallite boundaries. ID off-state current trends with photon energy in a manner consistent with exponentially decreasing absorption below the conduction band edge.