Vibrational excitations ina-Si1−xGex:F,a-Si1−xGex:H, andGa1−xAlxAs alloys

We report here the results of a first calculation of the vibrational excitations of hydrogenated (or fluorinated) a-${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ge}}_{\mathrm{x}}$:H (a-${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ge}}_{\mathrm{x}}$:F) and the unhydrogenated ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Al}}_{\mathrm{x}}$As alloys using a cluster Bethe-lattice method. The study has been made for two types of distributions of constituent atoms of the alloy: a random sequence and a chemically ordered one. The Raman data of a-${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ge}}_{\mathrm{x}}$ alloys can be well understood if one assumes the occurrence of a chemically ordered sequence in which every minority atom has majority atoms located at all nearest-neighboring sites. The vibrational modes induced by H (F) atoms outside the bulk phonon region of an a-${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ge}}_{\mathrm{x}}$ alloy remain undisturbed by the presence of the kind of atoms lying on the next-nearest-neighboring sites of the H (F) atom. On the other hand, the frequencies of modes induced by F atoms lying in the bulk phonon region change regularly due to the different concentrations of the Si and Ge atoms in the alloy. The calculated results are in good agreement with the available infrared and Raman data. However, more careful and detailed experimental investigations are very much needed to detect the predicted extra structure especially in the bulk phonon-frequency region. For ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Al}}_{\mathrm{x}}$As alloys, the two-mode behavior is very clearly evident in the calculated local phonon density of states. The available experimental data in the optical-phonon region is in excellent agreement with the computed phonon density for ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Al}}_{\mathrm{x}}$As alloys, under assumption of a random distribution of Ga and Al atoms.