Effect of Al2O3 passivation layer and optimization of atomic-layer-deposited Zn–Sn–O and Al–Zn–Sn–O thin-film transistors

Abstract

This study investigated the effects of aluminum oxide (Al₂O₃) passivation layers on amorphous zinc tin oxide (a-ZTO) and amorphous aluminum zinc tin oxide (a-AZTO) thin films, along with their corresponding thin-film transistor (TFT) performances. Unoptimized passivation layers facilitated hydrogen incorporation in the a-ZTO channel, inducing a significant threshold voltage shift (ΔV_th) in the negative bias direction of −0.50 V to −6.63 V. The optimized passivation layer mitigated hydrogen inflow into the channel but caused oxygen deprivation within the a-ZTO channel, still causing a negative ΔV_th of −0.50 V to −3.89 V. In contrast, the presence of Al–O bonds in the a-AZTO TFT's channel suppressed oxygen deprivation, leading to a negligible ΔV_th of −0.40 V to −0.29 V. Nonetheless, Al diffusion from the passivation layer into the channel increased the channel resistance per unit length and the source/drain contact resistance, leading to decreased saturation mobility (μ_sat). Also, hydrogen diffused into the HfO₂ gate insulator induced an abnormal hump under positive gate bias stress tests. These issues were effectively addressed through post-deposition annealing of the a-AZTO channel and adopting the Al₂O₃ gate insulator, eliminating the observed degradation and anomaly. The optimized Al₂O₃ passivated a-AZTO TFT with a 10-nm-thick Al₂O₃/30-nm-thick SiO₂ gate insulator demonstrated a V_th of 0.16 V, a μ_sat of 9.11 cm² V⁻¹ s⁻¹, a subthreshold swing of 113 mV per decade, and minimal ΔV_ths of −0.054 V and 0.11 V during 1000 seconds of negative and positive gate bias stress tests, respectively.