Determination of polarity of ZnO single crystal by contactless electroreflectance and photoreflectance

Directions of polarization fields are different on Zn-face and O-face of c-plane ZnO wurtzite single crystal due to the influence of the spontaneous polarization. Photoreflectance (PR) and contactless electroreflectance (CER) spectra have been used to determine the polarity of c-plane GaN films. In this work we determined the internal field directions of both faces of the 0.5mm thick ZnO single crystal by comparing phases of its PR and CER spectra. Its carrier concentration is in the range of 10¹⁶ cm^-3. It was found that the PR and CER spectrum of the Zn-face has inversion phase line-shape, which means its internal field is directed outward. However, O-face has the same phase line-shape, which means its internal field is directed inward. This is consistent with the direction of polarization, which is dominant in the surface region in this range of carrier concentration. Hence, the polarity of c-plane ZnO crystal can be determined by comparing phases of its PR and CER spectra.