ELECTRONIC-STRUCTURE OF THE (0001) AND (10(1)OVER-BAR-0) QUARTZ SURFACES AND OF THEIR DEFECTS AS OBSERVED BY REFLECTION ELECTRON-ENERGY-LOSS SPECTROSCOPY (REELS)

Reflection electron energy loss experiments were carried out on monocrystalline (1010BAR) and (0001) quartz samples. Five bulk single loss structures appear at 10.5, 12.5, 14.3, 17.9 and 21.3 eV. The 10.5 eV peak is due to the SiO2 exciton, the 21.3 eV peak to the SiO2 plasmon, and the three other transitions are interband transitions, as confirmed by the calculated JDOS. Our experiments show that two electronic transitions at 5.1 and 7.2 eV appear in the gap of alpha-quartz under several excitations: by phonon excitation (UHV heating), by atomic excitation (collision between the 7 keV ions and the nuclei of the surface and near-surface atoms), and by electronic excitation (interaction with the 350 eV electron beam). The defects responsible for these structures are located at the surface and related to the neutral oxygen vacancy that is the precursor of the E' centre.