Atomic-size effects on the growth of SrF sub 2 and (Ca,Sr)F sub 2 on Si(111)

Strain-induced lattice relaxation during overlayer growth is investigated by {ital in} {ital situ} monitoring of SrF{sub 2} and Ca{sub {ital x}}Sr{sub 1{minus}{ital x}}F{sub 2} growth on Si(111) with x-ray standing-wave fluorescence and low-energy electron diffracton. Submonolayer films show both intrinsic lateral disorder and variable interface stoichiometry with discommensuration parallel to the surface after completion of the first monolayer. In contrast, local commensuration is observed for Ca-rich alloy films. In the alloy, Sr and Ca occupy different sites in the first monolayer, but have similar average positions in subsequent layers.