Dynamic and static photoresponse of ultraviolet-detecting thin-film transistors based on transparent NiO_x electrodes and an n-ZnO channel

We report on the fabrication of an ultraviolet (UV)-detecting thin-film transistor (TFT) using NiO_x as source/drain electrodes and n-ZnO as its channel layer deposited on a SiO₂/p-Si substrate. Rapid thermal annealing of the TFT was carried out in an O₂ ambient at 350 °C for 1 min to increase the transparency of NiO_x. In an accumulation mode with a gate bias of 40 V, a drain current of only 2 μA was obtained in the dark. However, under an illumination of UV light with wavelength 325 nm, the drain current dramatically increased up to 13 μA. Under UV photons with wavelength 254 nm or energy of 4.9 eV, much higher than 4.1 eV, the energy gap of NiO_x, the photocurrent slightly decreased to ∼10 μA due to the absorption by NiO_x. These photoelectric effects were more pronounced under a gate-bias condition for a depletion (off) mode. The UV-dynamic behavior of our TFTs was also investigated, yielding the UV response time of ∼300 ms and UV-on/off ratio of about 10.