Hydrogen-related n-type conductivity in hydrothermally grown epitaxial ZnO films

Epitaxial ZnO films were grown on single crystal spinel substrates at 90 °C in water. Hall measurements showed that the n-type carrier concentration and conductivity of the ZnO films decreased as postgrowth annealing temperature increases or pH of the growth solution decreases. At the same time, x-ray photoelectron spectroscopy, room temperature and low-temperature photoluminescence, and secondary ion mass spectroscopy observations revealed that H incorporation was enhanced at a higher pH value but reduced with increasing annealing temperature. All these observations show that the unintentionally-incorporated hydrogen in solution grown ZnO acts as a shallow donor and enhances the n-type carrier density.