Lattice Vibration Spectra of Germanium-Silicon Alloys

Abstract

The infrared lattice absorption spectra of germanium-silicon alloys were determined in the wavelength region from 8-48 μ and have been analyzed to obtain information on the lattice vibrational spectra of the disordered crystals. The evolution of the spectra was followed from pure germanium through the alloys to pure silicon and the major two-phonon combination bands were identified. The optical and acoustical frequencies near the edge of the reduced zone were determined throughout the alloy system by identifying equivalent bands in the alloys with those of pure germanium and silicon which were assigned by using neutron scattering data. The gross features of the spectra revealed a continuous variation of the positions, shapes, and the intensities of the absorption bands as a function of composition; however, the strongest germanium-like summation band ($TA+TO$) approaches but does not merge with the equivalent silicon-like band. It is possible to observe both of these bands simultaneously on samples near the central range of composition. The appearance of new bands, not present in either pure germanium or pure silicon, due to the addition of small amounts of silicon to germanium or vice versa, were identified as vibrational impurity bands. The relative insensitivity of the phonon energies with composition are in agreement with a previous determination of equivalent phonons involved in the indirect transitions of the intrinsic absorption edge. The results of the analyses of the lattice absorption spectra of germanium-silicon alloys are compared with various theoretical models that have been proposed for the vibrational spectra of disordered crystals.