The effects of ultraviolet exposure on the device characteristics of atomic layer deposited-ZnO:N thin film transistors

We investigated the effects of ultraviolet (UV) light illumination on nitrogen-doped atomic layer deposited (ALD)-ZnO:N thin film transistors (TFTs). ALD ZnO:N thin films grown at 125C were used as active layers for back-gate TFT devices. As-fabricated ALD ZnO:N TFTs showed proper drain current modulation response to a gate voltage sweep with a 5.4 V threshold voltage and a clear pinch-off. However, the threshold voltage was significantly shifted in the negative direction by UV exposure due to an associated increase in carrier concentration, resulting in the loss of current modulation by gate voltage sweep. In addition, we observed a resistivity change in ALD ZnO:N thin films with time after UV exposure. The resistivity decreased by several orders of magnitude upon UV light exposure and recovered toward its original value after switching off the UV light. Accordingly, the transfer curves of TFT devices using a ZnO:N active layer also exhibited recovery characteristics. We formed a thin Al₂O₃ passivation layer on top of the TFT surface in order to suppress the recovery effect.