Thermal and Bias Stability of InAlZnO-Based Amorphous Thin-Film Transistors

Abstract

Abstract Body: Amorphous oxide semiconductor (AOS) thin film transistors (TFTs) based on In2O3 have attracted much interest for use as pixel switching elements in next generation active-matrix liquid crystal and active-matrix organic light emitting diode displays. The high field effect mobility of In2O3-based AOS devices (10–25 cm2/Vsec) offers significant performance improvements over present-day a-Si TFTs ( 1E4 times compared to that of unannealed IAZO after low temperature annealing at 200 °C in air. Photoelectron spectroscopic studies reveal that the typical intrinsic (vacancy-based native defect) or extrinsic (cation substitution) doping mechanisms are not the primary cause of the channel carrier increase. From high pressure oxidation with much enhanced reactivity of reaction gases, it is identified that the equilibrium carrier density of IAZO is much higher than those used in typical TFT channel application. The low channel carrier density tends to increase and reach the higher equilibrium carrier density in the absence of kinetic constraints. In addition, to investigate the bias stability of IAZO TFTs, the transfer characteristics were measured over bias stress time at a negative gate bias of -30 V and a positive gate bias of 30 V. During the negative bias stress (NBS) tests, no significant changes were observed in transfer characteristics, which is attributed to the depletion of channel free carriers due to the application of negative bias, regardless of stress time. However, clear positive shifts of the TFT transfer characteristics are detected during the positive bias stress (PBS) measurements as shown in the figure above. TFT parameters such as field effect mobility, sub-threshold swing, VTh and interfacial trap density were carefully analyzed during PBS measurements. (Financial support: NSF Award No. ECCS-1931088)