Analysis of Material Property and Reliability Testing of Thin-Film Transistor of Amorphous Indium-Gallium-Zinc-Oxide

In the past years, there has been increasing interest in using amorphous indium gallium zinc oxide(a-IGZO) to the channel material of the thin-film transistors (TFTs) for Active Matrix Organic Light Emitting Diode (AMOLED). As compared to hydrogenated amorphous silicon (a-Si) TFTs, the a-IGZO TFTs have high on/off current ratio and high carrier mobility. In process, it is possible to deposit the channel material (a-IGZO) by RF sputter at the room temperature and control the uniformity. Therefore, a-IGZO is a major channel material candidate for the next generation TFTs.
The structure and optical properties of a-IGZO thin films depend on the preparation methods, growth condition, and subsequent annealing treatment. The optical properties of the films were investigated by the use of photoluminescence (PL). PL spectra were measured by He–Cd laser with an excitation wavelength of 325 nm at room temperature. The strong shallow donor level to valence band emission was clearly observed at a wavelength of 450 nm (~2.78eV) accompanied with a weak broad deep emission peaking which is donor to acceptor pair emission (DAP) at around 850 nm (~1.45eV) for 100nm thick samples deposited by sputtering. At the low temperature measurement, the peak at 575nm (~2.15 eV) is the shallow trap level to valence band transition. It was found that the excitation related emission of a-IGZO thin films depends on annealing gas. The shallow donor level to valence band emission increases with the increase of annealing temperature and annealing time. To optimize the process conditions, PL is the most important analytical techniques to reveal the structure and sub-gap state.
The reliability issues have become the most rising topic. However, the reliability problem of a-IGZO TFTs is even more complicated as compared to single crystalline MOSFETs and rarely investigated. The complete understanding of device degradation mechanisms is attractive and noteworthy. For n-channel a-IGZO TFTs, the negative and positive bias temperature instability for a-IGZO TFTs will be performed. For the practical application, the dynamic bias temperature stress will also be studied. The stress-induced sub-gap states will be extracted directly from the temperature-dependent field effect measurement (M-N rule). The 2-D simulation of a-IGZO TFTs based on proposed density of states (DOSs) model will be used to explain the physical mechanism.