Structural characterization and surface polarity determination of polar ZnO films prepared by MBE

O-polar and Zn-polar ZnO films were grown by molecular beam epitaxy (MBE) on sapphire substrates. The growth process was in-situ monitored by reflection high-energy electron diffraction (RHEED). The structure, surface morphology, wettability and chemical etching properties of the polar films were characterized and compared to the commercial ZnO wafers with definite polarity. It was found that there is no significant difference in X-ray diffraction patterns and Raman spectra between Zn-polar and O-polar samples. However, the X-ray photoelectron spectroscopy valence band spectra are highly dependent on the crystal polarity and can be used as a simple and nondestructive method for determining the crystalline polarity of ZnO. Meanwhile, the polarity of the film was further confirmed by chemical etching method which shows rough small hill and hexagonal pit patterns for O-polar and Zn-polar ZnO after etching in HCl solution, respectively. The etching mechanism is discussed and the difference in morphology is attributed to the surface activity of polar ZnO. This also leads to the difference in wettability between water and ZnO determined by contact angle measurement. The controlled growth of polar ZnO films and development of easy and precise polarity determination method will be greatly beneficial for the application of polar ZnO in devices.