Compositional ordering in SiGe alloy thin films

We have performed surface x-ray diffraction experiments on Si{sub 0.5}Ge{sub 0.5} films grown on Ge(001) substrates. The results for our thinnest film (eight monolayers) show the compositional order at the initial stages of growth. This ordering is observed underneath the surface (2{times}1) dimer reconstruction, and was previously predicted by calculations on the equilibrium alloy surface. This initial surface compositional ordering is also consistent with the ultimate structure for the bulk of these films. Measurements from thicker alloy films show an increase in the average order parameter with increasing thickness, but then a decrease with the thickest film of 1000 {Angstrom}. {copyright} {ital 1998} {ital The American Physical Society}