Amorphous IGZO TFT with High Mobility of ∼70 cm2/(V s) via Vertical Dimension Control Using PEALD

Amorphous InGaZnO_x (a-IGZO) thin-film transistors (TFTs) are currently used in flat-panel displays due to their beneficial properties. However, the mobility of ∼10 cm²/(V s) for the a-IGZO TFTs used in commercial organic light-emitting diode TVs is not satisfactory for high-resolution display applications such as virtual and augmented reality applications. In general, the electrical properties of amorphous oxide semiconductors are strongly dependent on their chemical composition; the indium (In)-rich IGZO achieves a high mobility of 50 cm²/(V s). However, the In-rich IGZO TFTs possess another issue of negative threshold voltage owing to intrinsically high carrier density. Therefore, the development of an effective way of carrier density suppression in In-rich IGZO will be a key strategy to the realization of practical high-mobility a-IGZO TFTs. In this study, we report that In-rich IGZO TFTs with vertically stacked InO_x, ZnO_x, and GaO_x atomic layers exhibit excellent performances such as saturation mobilities of ∼74 cm²/(V s), threshold voltage of -1.3 V, on/off ratio of 8.9 × 10⁸, subthreshold swing of 0.26 V/decade, and hysteresis of 0.2 V, while keeping a reasonable carrier density of ∼10¹⁷ cm^-3. We found that the vertical dimension control of IGZO active layers is critical to TFT performance parameters such as mobility and threshold voltage. This study illustrates the potential advantages of atomic layer deposition processes for fabricating ultrahigh-mobility oxide TFTs.