Control of polarity of ZnO films grown by plasma-assisted molecular-beam epitaxy: Zn- and O-polar ZnO films on Ga-polar GaN templates

We report on the growth of polarity-controlled ZnO films by plasma-assisted molecular-beam epitaxy. Different polar (Zn- and O-polar) ZnO films on unipolar (Ga-polar) GaN epilayers are selectively grown. Polarity of ZnO films is evaluated by coaxial impact collision ion scattering spectroscopy. Zn preexposure prior to ZnO growth results in Zn-polar ZnO films (Zn face), while O-plasma preexposure leads to the growth of O-polar ZnO films (O face). High-resolution transmission electron microscopy reveals the formation of an interface layer between ZnO and GaN epilayers in O-plasma preexposed samples, while no interface layer is observed in Zn preexposed samples. The interface layer is identified as single crystalline, monoclinic ${\mathrm{Ga}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}.$ We propose models for interface configurations at ZnO/GaN heterointerfaces, which can successfully explain the different polarities of the ZnO films.