VIBRATIONAL BAND MODES IN GERMANIUM - ISOTOPIC DISORDER-INDUCED RAMAN-SCATTERING

We present Raman spectra of natural Ge, Ge-70, and (Ge0.5Ge0.5)-Ge-70-Ge-76 in the TO phonon region. The spectrum of the isotopically pure Ge-70 shows only the Raman-active phonon with k over arrow pointing right = 0 (GAMMA), as required by k over arrow pointing right conservation. The two isotopically disordered crystals, however, show spectra at the low energy side of the GAMMA phonon which correspond to isotopic disorder-induced scattering by TO phonons with k over arrow pointing right not-equal 0 (band modes). The line shape and the strength of the observed spectra agree with calculations based on the coherent potential approximation and an ansatz for the Raman polarizability. The novel (Ge0.5Ge0.5)-Ge-70-Ge-76 thus proves to be an ideal crystal to investigate mass disorder effects.