Sequential Design of PEALD In–Ga–Zn–O Active Layer for Enhancing TFT Stability

The accelerating downscaling of transistors in recent years has resulted in significant attention being paid to the development of amorphous oxide semiconductors, such as indium–gallium–zinc oxide (IGZO) using atomic layer deposition (ALD). To improve electrical performance, indium (In)-rich base IGZOs have been developed, but the increased In content in these materials can lead to issues with carrier control and crystallization. In this study, a newly designed plasma-enhanced ALD (PEALD) supercycle is proposed to suppress the crystallinity and electron carriers in In-rich IGZO. By inserting Ga2O3 and ZnO into the In2O3 sublayer using carefully PEALD sequences, the crystallinity of In2O3 was effectively suppressed by the lattice mismatch between the layers. As a result, the subthreshold swing (SS) characteristics in IGZO thin-film transistors (TFTs) improved from 0.42 to 0.23 V/decade. Additionally, positioning an insulating Ga2O3 layer between the In2O3 sublayer reduced the number of carriers, leading to a shift in the threshold voltage (
${V}_{\text {th}}$
) from −4.63 to −1.83 V. Furthermore, the suppressed crystallinity and stable Ga–O bonding between the In2O3 layers improved the reliability of the IGZO TFTs, as evidenced by the decrease in the
${V}_{\text {th}}$
shift from 3.75 to 0.83 V under positive bias stress (PBS). This new approach of controlling the ALD supercycles to insert different materials into the In2O3 layer provides a groundbreaking method for suppressing crystallization and controlling carriers in In-rich IGZO thin films. It constitutes a breakthrough in addressing the tradeoff between mobility and stability.