Amorphous In-Ga-Zn-O Thin Film Transistor for Future Optoelectronics.

Abstract

After initial report of its potential use for flexible/large area electronics, amorphous In-Ga-Zn-O (a-IGZO) is now emerging worldwide as a new semiconductor for next gen-eration thin-film transistor (TFT) flat panel displays and imagers. This dissertation work examines in detail the basic properties and physics of the a-IGZO TFTs, including the photofield-effect, numerical simulations, electrical instability and noise characteristics. Our a-IGZO TFTs have following electrical performance: field-effect mobility (μeff) of 7~12.3 cm2V-1s-1, threshold voltage of 1~3V, subthreshold swing of 130~420mV/decade and on/off current ratio over 108. Aluminum and titanium are both suitable for source/drain (S/D) electrodes with the contact resistivity (rC) lower than 10-3Ω-cm2. The active layer thickness was also found to have a major impact on S/D series resistance. To accurately model the TFT current-voltage (I/V) properties, a gate-to-source voltage dependent μeff model is proposed. Light wavelength and intensity dependent photo-responses were studied. The a-IGZO TFT is stable under visible light illumination (460~660nm). TFT off-state drain current starts to increase when the photon energy is higher than its band-gap (~3.05eV); and we observed a high UV-photocurrent conversion efficiency. In addition, the a-IGZO mid-gap density-of-states (DOS) was extracted and is more than an order of magnitude lower than the values of hydrogenated amorphous silicon (a-Si:H). The DOS model for a-IGZO was then developed. In this model, the donor-like states are proposed to be associated with oxygen vacancy in a-IGZO. We showed through numerical simulation that the a-IGZO TFT has a very sharp conduction band-tail slope (Ea=13meV). The impacts of rC and DOS on TFT electrical properties were also studied. Bias-temperature-stress (BTS) induced electrical instability was investigated. Our re-sults suggest that the observed shifts in TFT I/V curves are primarily due to channel charge injection/trapping. The validity of using stretched-exponential model in simulating the time, voltage and temperature dependences of BTS data was demonstrated for a-IGZO TFTs. Finally, the TFT low frequency noise properties were examined. The 1/f noise is the dominant source in a-IGZO TFT and can be modeled by Hooge mobility fluctuation theory. The a-IGZO has a lower Hooge’s parameter than a-Si:H and may be better used in imaging applications.