A comparison of zinc oxide thin-film transistors on silicon oxide and silicon nitride gate dielectrics.

We compared the properties of ZnO thin-film transistors in the inverted coplanar geometry on thermally grown silicon oxide gate dielectric to devices on silicon nitride, grown by plasma-enhanced chemical vapor deposition at 150 °C or magnetron sputtering at room temperature. The ZnO semiconductor was sputtered without substrate heating at oxygen partial pressures in the range of 10^-5-10^-4 Torr. At the lowest oxygen partial pressure, transistor characteristics were similar for all dielectrics. The field-effect mobility approached ∼5 cm²/V s and devices generally operated in depletion mode. With increasing oxygen partial pressure, the mobility decreased by 1000× on SiO₂, whereas the decrease on silicon nitride was considerably smaller. On SiO₂ the threshold voltage was >30 V but <5 V on silicon nitride. Devices on SiO₂ operated in enhancement mode, whereas they operated in depletion mode on PECVD silicon nitride. Photoluminescence of ZnO revealed that deep-level emission depended on the specific dielectric on which ZnO was grown. Green emission for ZnO on chemically vapor-deposited silicon nitride was consistent with the presence of oxygen vacancies in ZnO. Yellow emission for ZnO on silicon oxide was associated with electron acceptor defects, such as oxygen adsorbed in grain boundaries or interstitially in ZnO. We concluded that differences in transistor properties were attributable to modification of ZnO defect chemistry, mediated by growth on a specific dielectric. [ABSTRACT FROM AUTHOR]