Reliability of gamma-irradiated n-channel ZnO thin-film transistors: electronic and interface properties

Radiation-induced charge trapping and interface traps in n-channel ZnO thin film transistors are characterised as a function of total dose and irradiation bias following exposure to gamma-rays. Devices were irradiated up to ~ 60kGy(SiO2) and the electrical characteristic exhibits two distinct regimes. In the first regime, up to a total dose of 40 kGy(SiO2), the threshold voltage increases positively. However, in the second regime with irradiation greater than 40kGy(SiO2), the threshold voltage moves in the opposite direction. This reversal of threshold voltage is attributed to the influence of the radiation-induced interface and oxide- charge, in which both have opposite polarity, on the electrical performance of the transistors. In the first regime, the generation of the oxide- charge is initially greater than the density of interface traps and caused a positive shift. In the second regime, when the total doses were greater than 40 kGy(SiO2), the radiation-induced interface traps are greater than the density of oxide- charge and caused the threshold voltage to switch direction. Further, the generated interface traps contributed to the degradation of the effective channel mobility, whereas the density of traps at the grain-boundaries did not increase significantly upon irradiation. Isothermal annealing of the devices at 363K results in a reduction in the trap density and an improvement of the effective channel mobility to ~ 90% of its pre-irradiation value