(011(BAR)0) ALPHA-QUARTZ SURFACE - A LEED, XANES AND ELS STUDY

We studied the structure of the (0110BAR) alpha-quartz surface after different treatments (900-degrees-C vacuum or air heating and ion bombardment). The long-range order was investigated by low-energy electron diffraction (LEED), the mean-range order by X-ray absorption spectroscopy at the oxygen K edge (XANES) and the electronic structure of the surface band gap by low-energy electron loss spectroscopy (ELS). The (1 x 1) structure is obtained only after a slight chemical etching in HF. A 900-degrees-C air-annealed sample exhibits a (3 x 1) or (1 X 3) LEED pattern associated with reconstructed surface regions, which structure is either related to the bulk alpha --> beta transition phase occurring at 573-degrees-C, or to a new SiO2 phase present only at the surface (tridymite). Finally, a 900-degrees-C heating in vacuum provides a glassy surface, also containing oxygen vacancies. These three structures lead to different surface electronic properties: the reconstructed surface presents a larger surface band gap than the (1 X 1) (9.5 instead of 8.6 eV). The disordered surface shows an 8.8 eV wide surface band gap, containing two electronic levels associated with surface oxygen vacancies.