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 cm2/(V s) mobility and 107 on/off ratio, with +5 V threshold voltage, which enables enhancement mode TFT operation.

Polarization-dependent x-ray absorption measurements were performed on a crystalline ZnO three-dimensional array consisting of highly oriented microrods as well as on particulate thin film consisting of monodisperse spherical nanoparticles. Strong anisotropic effects have been observed for the highly oriented ZnO rods, unlike for the isotropic spherical ones. Full-potential calculations of orbital-resolved x-ray absorption of a ZnO wurtzite periodic crystal, including the Zn 3d as part of the valence states, shows a very good agreement with the experimental findings. Comprehensive fundamental knowledge of the electronic structure of ZnO is obtained by probing and demonstrating the orbital symmetry of oxygen and its contribution to the conduction band of this important II-VI semiconductor.

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