Raman-scattering and electron-microscopy study of composition-dependent ordering in metastable (${\mathit{A}}^{\mathrm{III}}$${\mathit{B}}^{\mathrm{V}}$${)}_{1\mathrm{-}\mathit{x}}$(${\mathrm{C}}_2^{\mathrm{III}}$${)}_{\mathit{x}}$ alloys

We present Raman-scattering and electron-diffraction results in a study of the structural ordering of single-crystal metastable (${\mathit{A}}^{\mathrm{III}}$ ${\mathrm{B}}^{\mathrm{V}}$ ${)}_{1\mathrm{-}\mathit{x}}$(${\mathrm{C}}_2^{\mathrm{IV}}$ ${)}_{\mathit{x}}$ alloys. Amorphous-like Raman linewidths are observed, indicative of the severe substitutional disorder in these alloys. Increased violation of the normal zinc-blende-polarization selection rules, relative to that observed in III-V ternary alloys, is attributed to the enhanced electronic disorder in the III-IV-V system. Anomalies in the composition dependence of the Raman line shapes at x≊0.30 and x≊0.70 imply qualitative changes in the ordering at these compositions. Transmission electron microscopy results are presented which give insight into the nature of these changes. Our results are discussed in the context of competing theoretical models proposed for this system.