Role of grain boundaries in ZnO nanowire field-effect transistors

ZnO nanowires have attracted strong interest for potential nanoelectronics, optoelectronics, and nanosensor applications. The role of grain boundaries (GBs) in ZnO nanowire transistors is examined by solving a two-dimensional Schrodinger equation in the nanowire cross section, coupled to a drift-diffusion equation along the nanowire. We show that a GB results in a potential barrier with the thickness determined by the gate insulator thickness and the height determined by the number of the trap states at the GB. The GB leads to a decrease of the source-drain current because the voltage drop at the GB reduces the electric field at other channel positions. The on current depends on the nanowire diameter nonmonotonically due to two competing mechanisms. Increasing the number of GBs in the channel decreases both the on current and off current. When the total number of GBs is small, its effect on the I-V characteristics can be phenomenologically viewed as an increase of the threshold voltage. When the total number of GBs is larger, it must be viewed as a combined effect of the increase of the threshold voltage and the decrease of the channel effective mobility. (c) 2007 American Institute of Physics.