Phonon Spectra of Ge-Si Alloys

The energies of the optical and acoustical zone-boundary phonons have been measured in single-crystal alloys of germanium and silicon as functions of alloy composition. The data were obtained by observing the effect of phonon emission upon tunneling currents at low temperatures in narrow $p-n$ junctions made in these alloys. The heavily doped alloys were prepared by solvent evaporation and thermal-gradient crystal-growth techniques. Comparison of the composition dependence of the phonon energies with calculations of Braunstein, Moore, and Herman indicates that the Ge-Si alloys form in an ordered diamond-type lattice where the minority atoms occur only as next-nearest neighbors rather than in clusters of similar atoms.