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 ($\Delta E<\mathrm{}20\mathrm{}$ meV, impact energy $E_0=1-100\mathrm{}$ 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 ($1\overline{1}00$) surface and 67.3 meV for the (0001) and the ($000\overline{1}$) surfaces] agree with the energies of optical surface phonons determined by the condition $\mathrm{Re}{\epsilon }_n\left(\omega \right)=-1\mathrm{}$.