Raman spectroscopy of long-range order in epitaxialSi0.5Ge0.5alloys

The Raman spectrum of compositionally ordered ${\mathrm{Si}}_{0.5}$ ${\mathrm{Ge}}_{0.5}$ grown at low temperatures by molecular-beam epitaxy has been measured and compared to the spectrum of the corresponding random alloy. We observe changes in the relative intensities of the different optical phonons which are consistent with the proposed structure of a microscopically strained, ordered phase where the majority of the bonds are homopolar and the minority are heteropolar. Within this model, the ordering appears to be incomplete. Resonant Raman scattering suggests that the ${\mathit{E}}_1$ gaps of the ordered layers can become identifiable at the bilayer thickness level.