Role of oxygen in amorphous In-Ga-Zn-O thin film transistor for ambient stability

The role of oxygen in amorphous InGaZnO thin film transistor (a-IGZO TFT) is studied for the device ambient stability. The threshold voltage (Vth) value of 350°C annealed a-IGZO TFT decreased apparently with the staying duration, and the average value shifted from 10.2 V to 5.8 V after a 9.day staying at the atmosphere. After raising the annealing temperature to 450°C, the electrical stability issue was improved significantly with superior electrical parameters, including low threshold voltage (Vth), low subthreshold swing, high carrier mobility and a small Vth variation of ±0.5 V. It can be attributed to the enhancement of bonding energy of oxygen in the thermally-annealed a-IGZO film with the increase of thermal annealing temperatures. Besides, the stronger oxygen bonding could also suppress the absorption/desorption and UV-induced migration at the back surface, causing better electrical reliability and immunity against UV radiation, respectively. All these results showed the ambient stability is greatly related to the oxygen in a-IGZO film, and the desired electrical characteristic can be achieved via the optimization of thermal annealing process.