Influence of oxygen-related defects on In-Ga-Sn-O semiconductor due to plasma-enhanced atomic layer deposition of Al2O3 for low-temperature thin-film transistor in terms of electrical properties

Abstract

In-Ga-Sn-O (IGTO) thin-film transistors (TFTs) were fabricated with Al2O3 gate insulators by plasma-enhanced atomic layer deposition (PEALD) at a temperature of 150 °C. Electrical performances of IGTO TFTs were significantly affected by plasma conditions when Al2O3 was deposited on IGTO thin films by PEALD. Analyses of X-ray photoemission spectroscopy revealed that excessive oxygen species activated at high plasma power above 150 W were chemisorbed to IGTO films to create excess oxygen bonds such as oxygen interstitials (Oi). Distributions of density of states in IGTO bandgap extracted by photonic capacitance-voltage measurement matched well with positive bias stress (PBS) instability of TFTs. Split-oxygen interstitials [Oi (split)] first created on IGTO by oxygen plasma might migrate to the octahedral site by a constant high gate bias to form octahedral-oxygen interstitials [Oi2- (oct)] by capturing electrons under PBS and reside deep states. Significant changes of subthreshold slope in I-V characteristics during the recovery time after removing PBS indicated that these octahedral-oxygen interstitials such as Oi (oct) and Oi2- (oct) obviously originated from Oi (split) initially formed by plasma and that Oi (oct) defects created by a relaxation process acted as a neutral acceptor-like state above Fermi level and deteriorated electrical properties of IGTO TFT.