Role of environmental and annealing conditions on the passivation-free in-Ga-Zn-O TFT

We examined the characteristics of passivation-free amorphous In-Ga-Zn-O thin film transistor (a-IGZO TFT) devices under different thermal annealing atmospheres. With annealing at higher temperature, the device performed better at the above-threshold operation region, which indicated the film quality was improved with the decrease of defects in the a-IGZO active region. The mobility, threshold voltage and subthreshold swing of a-IGZO TFT annealed at 450 °C was 7.53 cm²/V s, 0.71 V and 0.18 V/decade, respectively. It was also observed that the a-IGZO was conductive after thermal annealing in the vacuum, due to the ease of oxygen out-diffusion from the a-IGZO back channel. The oxygen deficiency resultantly appeared, and provided leaky paths causing electrical unreliability when TFT was turned off. In contrast, the annealing atmosphere full of O₂ or N₂ would suppress the oxygen diffusion out of the a-IGZO back channel. The worst V_th degradation of a-IGZO TFT after positive gate bias stress and negative gate bias stress (NGBS) was about 2 V and - 2 V, respectively. However, the V_th shift in the NGBS testing could be suppressed to - 0.5 V in vacuum chamber. Material analysis methods including X-ray photoelectron spectroscopy and scanning electron microscopy were used to investigate the change of a-IGZO film after different thermal annealing treatments. The variation of O 1s spectra with different annealing atmospheres showed the consistence with our proposed models.