Effects of Tantalum Doping on Electrical Characteristics of High-Mobility Zinc Oxynitride Thin-Film Transistors

Abstract

The effects of Ta doping on the electrical characteristics of high-mobility zinc oxynitride (ZnON) thin-film transistors (TFTs) are investigated. With increasing Ta content ( ${C}_{\text {Ta}}$ ) in ZnON, the threshold voltage ( ${V}_{\text {th}}$ ) of Ta-doped ZnON TFTs increases from negative to positive values, and the subthreshold swing ( SS ) becomes smaller, leading to significantly improved switching characteristics as compared with those for undoped ZnON TFTs. Furthermore, Ta doping effectively suppresses the negative ${V}_{\text {th}}$ shift with time. However, excess Ta doping is found to degrade the field-effect mobility ( $\mu _{\text {fe}}$ ). To elucidate the mechanism behind these results, the bonding states of Ta in ZnON are analyzed using X-ray photoelectron spectroscopy. Through the optimization of ${C}_{\text {Ta}}$ , a high $\mu _{\text {fe}}$ of 49 cm2/Vs is achieved, together with a less negative ${V}_{\text {th}}$ , a smaller SS , and a better long-term ${V}_{\text {th}}$ stability. In addition, the annealing temperature dependence of the properties of Ta-doped ZnON TFTs is investigated. When the annealing temperature is increased to 250 °C, a considerable decrease in the on-current is observed, which is attributed to nitrogen desorption from ZnON, as confirmed by thermal desorption spectroscopy.