Phonons in Ge1-xSix bulk crystals

We compare infrared absorption and Raman spectra of high-quality Ge1-xSix b ulk crystals (0 <x < 0.4). We use this comparison together with lattice-dyn amical calculations to report on the behavior of the Ge-Ge phonon at 300 cm (-1), to identify Ge-Si and Si-Si modes, and to clarify contradictory assig nments of certain Ge-Si and Si-Si modes. The Ge-Ge optical phonon observed in Raman spectra at 300 cm(-1) shifts continuously to lower energies with i ncreasing Si content, also for x<0.03. This is in good agreement with theor etical calculations, but in contradiction to previously reported experiment al data. The Si local-vibrational mode at about 390 cm(-1) is infrared and Raman active. We resolve the local-vibrational modes of the three Si isotop es. Two solely Raman-active phonon modes at about 400 and 455 cm(-1) are un ambiguously identified as modes caused by Si-Si pairs. We are able to resol ve the two corresponding infrared active modes of this Si-Si pair at 310 an d 370 cm(-1). In the range of about 460 cm(-1), we can distinguish further modes caused by three and four Si nearest-neighbor atoms in infrared and Ra man experiments. Absorption lines at 675, 780, and 845 cm(-1), not reported hitherto, are identified as combination modes of phonons with the Si local -vibrational mode. [S0163-1829(99)01816-0].