Electronic surface and bulk transitions on clean ZnO surfaces studied by electron energy-loss spectroscopy

Clean cleaved ($1\overline{1}00$), (0001), and ($000\overline{1}$) surfaces of ZnO have been studied by electron energy-loss spectroscopy with primary energies $E_0$ between 35 and 100 eV. Differences between the nonpolar ($1\overline{1}00$) and the polar (0001) surfaces can be explained by the optical anisotropy of ZnO. The different chemical composition of the two polar surfaces studied simultaneously by Auger-electron spectroscopy explains the different loss spectra on these faces. A strong increase of surface sensitivity for $E_0\le 40$ eV allows the observation of surface excitations near 7.5 and 11 eV. The interpretation of the 7.5-eV transition as due to oxygen and zinc "dangling-bond" surface states is supported by uv photoemission measurements on the clean and oxygen-covered polar (0001) Zn face.