Electrical characterization for ZnO layers grown on GaN templates by molecular-beam epitaxy

We have extensively studied electrical properties for ZnO layers grown on GaN templates by molecular-beam epitaxy. First, the Schottky characteristics of Au contacts onto ZnO:N layers have been investigated by current-voltage measurements. Barrier heights and ideality factors for Au/ZnO:N Schottky contacts are systematically varied by controlling the growth temperatures and crystal-polar directions of ZnO:N layers. Second, the capacitance-voltage (C-V) characteristics of ZnO/GaN heterostructures has been investigated. Large plateau regions are observed in C-V characteristics, which are ascribed to the confined charges caused by band offset at the ZnO/GaN heterointerface. Finally, electron-trap centers in ZnO layers have been investigated by capacitance-temperature measurements. ZnO layers exhibit two electron-trap centers ET1 and ET2, whose thermal activation energies are estimated to be 33 and 0.15 eV, respectively.