Investigation of Hysteresis, Off-Current, and Instability in In-Ga-Zn Oxide Thin Film Transistors Under UV Light Irradiation

Abstract

Amorphous oxide-based thin film transistors (TFTs), for instance, amorphous indium gallium zinc oxide (IGZO) TFTs, are expected to meet emerging technological demands where conventional silicon-based TFTs confront with the limitation of the electrical performance such as field-effect mobility, uniformity, and process temperature. However, the variation of characteristics and the stability in IGZO TFTs under light illumination still needs to be verified for further application. In this thesis, the characteristics and reliability of IGZO TFTs under light illumination were investigated. Furthermore, the effect of mechanical bending on flexible IGZO TFTs was analyzed for flexible displays. First, the effects of light on initial characteristics of IGZO TFTs were studied. Under illuminated condition, significant hysteresis and off-current (Ioff) were observed due to the creation of donor-like interface states near conduction band energy level arising from ionized oxygen vacancy (Vo2+). From hysteresis, the response time (~10^0 s) of Vo2+ at the interface was obtained, which is important parameter for analyzing hysteresis. On the contrary to conventional mechanism of photo-current, the change in Ioff increased with increasing light intensity. The increase of Ioff occurs because Vo2+ at the interface prevents carrier depletion with Fermi-level pinning. Second, the reliability of IGZO TFTs under the conditions combined with negative gate bias stress and light illumination were investigated. Under illumination, negative shift of threshold voltage (Vth) is accelerated by the photo-induced holes and Vo2+. In TFTs featuring passivation layer, a long characteristic time (~10^2 s) for Vo2+ generation in IGZO bulk was extracted. It was also found that the charge trapping probability of single carrier did not change. Finally, the reliability of flexible IGZO TFTs was analyzed when the bending radius was 10 mm, 4 mm, and 2 mm. The device characteristics were hardly changed under mechanical strain unless the gate bias stress was applied. However, Vth shift was increased by mechanical strain under the gate bias stress due to valence band energy level shift.