COMPARISON OF THE PHONON-SPECTRA OF GE-70 AND NATURAL GE CRYSTALS - EFFECTS OF ISOTOPIC DISORDER

The difference in phonon energies and broadenings between an isotopically enriched crystal of Ge-70 (95.9%) and a germanium crystal of natural isotopic composition was measured with high accuracy in first- and second-order Raman spectra (with laser excitation energies of E(L) = 2.41 eV and E(L) = 2.183 eV, respectively) and compared to coherent-potential-approximation and lowest-order perturbation-theory calculations. The energy of the first-order O (GAMMA) phonon shifts up by 4.9 +/- 0.1 cm-1 in the isotopically pure crystal, in good agreement with the calculated value of 4.3 +/- 0.4 cm-1. No broadening due to isotopic disorder was observed for optical phonons at the GAMMA-point [full width at half maximum (FWHM) of 0.99 +/- 0.03 cm-1 at 80 K for both crystals]. However, additional broadening at the W and L point was found in the second-order Raman spectra (FWHM broadening of 6.5 +/- 0.5 cm-1 at the L point). This broadening and the shifts of the acoustic- and optical-phonon peaks in the second-order spectra agree well with calculations of the phonon overtone density of states.