Hydrogen as a Cause of Abnormal Subchannel Formation Under Positive Bias Temperature Stress in a-InGaZnO Thin-Film Transistors

Abstract

This paper analyzes the abnormal degradation induced by hydrogen annealing. Although device performance is enhanced after hydrogen annealing, an abnormal hump is observed in transfer characteristics ( ${I}_{D} - {V}_{G}$ ) under positive bias temperature stress (PBTS). Threshold voltage shift ( $\Delta \text{V}_{\text {TH2}}$ ) in this hump region increases with increasing stress voltage and temperature. Additionally, $\Delta \text{V}_{\text {TH2}}$ is independent of the channel width. A novel hydrogen rupture-diffusion model is proposed to explain the degradation. COMSOL simulation and ${C} - {V}$ measurement are utilized to clarify the precise degradation position. Moreover, variable S/D spacing ( $\text{L}_{\text {SD}}$ ) devices are designed to support the mechanism. Finally, ISE-TCAD software is carried out to verify the proposed model. Our results from electrical measurement suggest that hydrogen can cause additional instability, which shares a similar conclusion for those by using material analyzation and first-principle simulation.