Enhancement-Mode ZnO Thin-Film Transistor Grown by Metalorganic Chemical Vapor Deposition

We developed a method to control threshold voltage and on/off ratio of ZnO thin-film transistor (TFT) grown by metalorganic chemical vapor deposition (MOCVD). ZnO usually shows oxygen deficiency, which shows up as n-type defects of zinc interstitial or oxygen vacancy. In order to reduce these defects, we allowed sufficient oxidation time during growth. Instead of one long oxidation step, we repeated thin-layer growth and oxidation, until desired thickness is achieved. By using this method, we could obtain high quality ZnO TFT by MOCVD. Our ZnO TFT grown at 450 °C showed 15 cm 2 /(V s) mobility and 10 7 on/off ratio, with +5 V threshold voltage, which enables enhancement mode TFT operation.