Post-annealing temperature-dependent electrical properties of thin-film transistors with a ZnO channel and HfOx gate insulator deposited by atomic layer deposition

Abstract

Electrical properties of an oxide semiconductor thin-film transistor (TFT) with a ZnO channel layer and a HfOx gate insulator, both of which are deposited by atomic layer deposition (ALD), are investigated at varying post-annealing temperatures. The TFTs that are post-annealed at 250 and 300 °C show relative low on/off ratios < 102. They also have a counter-clockwise hysteresis in the transfer curves with slightly reduced threshold voltage upon repeatedly applying a positive gate voltage. However, the transfer curves of the devices post-annealed at 350 °C exhibit the increased on/off ratios > 102 and clockwise hysteresis with a little increased threshold voltage due to electron charging at the trap states in the HfOx/ZnO interface or inside the HfOx gate insulator. The threshold voltage shift, however, is negligible at the gate voltage of +20 V and as low as about 2.2 V at the highest gate voltage of +40 V, which guarantees stable operations of TFTs without significant degradation of electrical performance. The channel mobilities are around 4.0 cm2 V−1 s−1 at this annealing temperature range. The presented results report the dependence of electrical performance such as on/off ratio and electrical instability, possibly caused by electrical charging, on the post-annealing temperature, which requires post-annealing at 350 °C or higher temperatures for stable operations of TFTs with ALD-ZnO and HfOx.