Developing Subthreshold-Swing Limit of PEALD In–Sn–Ga–O Transistor via Atomic-Scaled Sn Control

Abstract

The In–Sn–Ga–O (ITGO) thin-film transistor (TFT) is promising in that it possesses enhanced electrical characteristics and stability because the tin (Sn) has large spherical s orbitals and a high binding energy with oxygen (O). Recently, there have been several reports of ITGO material fabricated via sputtering. Therefore, studies that control Sn composition to achieve unique characteristics and obtain conformal films have been limited. For these reasons, we evaluated plasma-enhanced atomic layer deposition (PEALD)-derived ITGO materials with varied Sn composition. The electrical characteristics are enhanced with the Sn subcycle ratio from 1 to 5 in the ITGO films (carrier concentration: 6.4 × 1020 to 2.2 × 1021 cm–3, resistivity: 8.6 × 10–4 to 6.1 × 10–4 Ω cm). These results are due to carrier generation by the Sn, a decrease in oxygen-related defects (22.9% to 19.5%), and suppressed indium-oxide (In2O3) crystallinity. To demonstrate their applicability to practical devices, the ITGO films were applied to TFTs as active channel layers. The ITGO TFTs with increasing Sn content show a decrease then increase of subthreshold swing (S.S.) characteristics, while the field-effect mobility (μFE) slightly increases. In addition, the ITGO TFTs show improved negative bias illumination stress (NBIS) stability with increasing Sn content, while the turnaround point is observed in the Sn 5 subcycle. As a result, the ITGO TFT with the Sn 3 subcycle shows an extremely low S.S. of 64.8 ± 1.9 mV/decade and an NBIS result of −0.5 V. The outstanding performance of the ITGO TFT with Sn 3 subcycle is attributed to its low number of oxygen-related defects, amorphous structure, and low valence band maximum (VBM) level. Therefore, the ITGO TFTs with optimized Sn content have the advantages of low power consumption and resistance to an environment with illumination.