Ordered structures in SixGe1-x alloy thin films

Compositional ordering in Si0.5Ge0.5 alloy films grown epitaxially on Si(001) substrates was investigated using four-circle x-ray-diffraction measurements. Thin-film alloy samples grown by molecular-beam epitaxy, as well as a sample grown by pulsed chemical vapor deposition were studied, and half-integral reflections, indicating a doubling of the conventional cubic unit cell, were observed in all samples. Quantitative structure factors measured in absolute units were compared with proposed structural models to extract the atomic structure and degree of compositional ordering. The measurements are consistent with the predictions of a bilayer step-flow growth model in which ordering results from atomic segregation at specific kink sites. In addition to the half-integral reflections, weak integral hkl reflections, which are forbidden for the diamond structure, were also observed. These reflections are attributed to the presence of a secondary ordered structure similar to CuAu-I, with a superlattice periodicity along the in-plane [100] direction.