Quantitative analysis of defect states in amorphous InGaZnO thin-film transistors using photoinduced current transient spectroscopy

The device and defect characteristics of amorphous indium-gallium-zinc oxide (In:Ga:Zn = 1:1:1 at.%) thin-film transistors (TFTs) as a function of the oxygen partial pressure were investigated. It was found that as the oxygen partial pressure increased, the field effect mobility decreased, the threshold voltage saw a positive shift, and this shift of threshold voltage increased under a negative gate bias stress. From our qualitative analysis of defect states below the conduction band, it was found that as the oxygen partial pressure increased, defect states in the shallow levels decreased, while defect states in the deep levels increased. A quantitative analysis of the defect states in the TFT structures was conducted using photoinduced current transient spectroscopy. It was found that as the oxygen partial pressure used during fabrication of the TFTs increased from 0% to 10% to 60%, the defect states in the shallow levels decreased from 2.74 × 10¹⁸ to 2.93 × 10¹⁷ to 3.55 × 10¹⁶ cm⁻³, while the defect states in the deep levels increased from non-availability to 1.86 × 10¹⁶ to 3.25 × 10¹⁶ cm⁻³. As the oxygen partial pressure increased, the decrease in shallow level defect density is strongly related to a decrease in carrier concentration; the increase in deep level defect density affects the mobility and causes device instability.