Polarity-dependent photoemission spectra of wurtzite-type zinc oxide

Electronic structures of the polar and nonpolar surfaces of ZnO were investigated by hard x-ray photoelectron spectroscopy (HXPES) using synchrotron radiation. The profile of HXPES spectra in the valence band region varied with the crystalline orientation (polarity) of the surface. This polarity dependence was always found in the HXPES spectra regardless of sample conditions such as surface treatment and doping. The possible origin of the polarity dependence of HXPES spectra is discussed in terms of spontaneous polarization.