The effects of active layer thickness and annealing conditions on the electrical performance of ZnON thin-film transistors

Thin-film transistors (TFTs) based on ZnON semiconductors were fabricated, and their electrical characteristics were evaluated with respect to the active layer thickness and annealing conditions. At a fixed annealing temperature, the electrical performance decreases with decreasing ZnON thickness. X-ray photoelectron spectroscopy (XPS) studies on the oxygen 1s peak of ZnON suggest that the diminished charge transport properties may be attributed to the formation of an oxygen rich capping layer with fixed thickness on top of the active layer upon heat treatment. Secondary ion mass spectrometry (SIMS) analyses show that indeed the top portion of ZnON films becomes rich in oxygen as the anneal temperature in air increases. The experimental results indicate that optimum field effect mobility and switching ability are achieved at an annealing temperature of 300 °C with a 20 nm-thick ZnON active layer. XPS analyses on the nitrogen 1s peak of ZnON films suggest that the portion of stoichiometric Zn₃N₂is highest at this annealing temperature, which is anticipated to provide high conductivity paths to the free electrons. The devices were next subjected to negative bias illumination stress (NBIS), however the amount of ZnON TFT degradation does not exhibit any mobility dependence, unlike what is observed in devices incorporating conventional oxide semiconductors such as In-Ga-Zn-O (IGZO).