Optical Surface Phonons in Zinc Oxide Detected by Slow-Electron Spectroscopy

Scattering of electrons from surfaces of ZnO single crystals has been studied in ultrahigh vacuum with a high-resolution electron-impact spectrometer (<20 meV, impact energy E₀=1-100 eV). The energy-loss spectrum in specular reflection shows a series of equally spaced loss peaks. The characteristic loss energies 68. 8 meV for the (1100) surface and 67. 3 meV for the (0001) and the (0001) surfaces agree with the energies of optical surface phonons determined by the condition Re₍ () =-1.