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 degrees C showed 15cm(2)/(VS) mobility and 107 on/off ratio, with +5V threshold voltage, which enables enhancement mode TFT operation. (C) 2008 The Japan Society of Applied Physics.