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

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
_Ta
) in ZnON, the threshold voltage (V
_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 Vth shift with time. However, excess Ta doping is found to degrade the field-effect mobility (μ
_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
_Ta
, a high μ
_fe
of 49 cm
²
/Vs is achieved, together with a less negative V
_th
, a smaller SS, and a better long-term V
_th
stability. In addition, the annealing temperature dependenceof 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.