Initial growth stages of CaF on Si 111/ investigated by scanning 2 tunneling microscopy

The initial stages of calcium fluoride CaF2.growth on Si 111.- 7=7.have been studied using ultrahigh-vacuum UHV. scanning tunneling microscopy STM., low-energy electron diffraction LEED., and X-ray photoelectron spectroscopy XPS.from the submonolayer range up to two monolayers ML: 1 ML CaF2s7.84=1014 cmy2 .. The STM images directly indicate that the initial growth mode changes from an island formation at around 6408C to the growth of a wetting row-like region above about 7508C. At a substrate temperature of around 6408C, the islands of characteristic shapes, with steps arranged in thew110xdirections of the substrate, are formed initially both at steps and on the flats of Si terraces. The island is a CaF layer that has a 1=1.periodicity. At a higher temperature of around 7508C, the region that has a well-ordered row structure along thew110xdirections are observed only at Si step edges. The LEED pattern indicates that the row-like region has a 3=1.periodicity, and XPS measurements show that the row-like region still has a Ca:F stoichiometry of 1:1. Based on the coverage of the deposited CaF2 molecules, the row-like region is a reconstructed layer induced by CaF adsorption. Furthermore, in situ STM measurements of the initial growth stages of CaF2 have been performed at 7008C and 8008C. The results clearly show that the first row-like region and the second layer grown on the first region have different growth modes. q2000 Elsevier Science B.V. All rights reserved