Low-energy-electron loss spectroscopy of ZnTe (110) and (111) surfaces. I. Bulk and surface losses

The present paper reports on electron-energy-loss spectroscopy of ZnTe (110) and (111) surfaces, cleaned by ${\mathrm{Ar}}^{+}$ bombardment and annealed under ultrahigh-vacuum conditions, at primary electron energies from 75 to 250 eV. Several surface losses involving surface states have been measured in addition to bulk losses due to bulk interband transitions, bulk and surface plasmon excitations, and the core-level transitions from the Zn $3d$ and Te $4d$ levels to conduction-band states. The surface losses change in magnitude between the (110) and (111) surfaces, or change with primary-electron energies. They are sensitive to the state of the surface. On the basis of these observations we distinguish the surface losses from the bulk losses and try to identify the surface states involved in the surface losses. From the transition energy of the surface loss involving the Zn $3d$ core level, the empty Zn surface state is located at 2.2 eV above the valence-band maximum in the one-electron picture. The energy-loss spectrum of the clean and well-ordered surface shows that there are no surface states in the band-gap region. Upon oxidation or ${\mathrm{Ar}}^{+}$ bombardment a surface state appears in the band-gap region, which is ascribed to the surface Te atoms.