Comparison of the phonon spectra of Ge70 and natural Ge crystals: Effects of isotopic disorder

The difference in phonon energies and broadenings between an isotopically enriched crystal of Ge70 (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 EL=2.41 eV and EL=2.183 eV, respectively) and compared to coherent-potential-approximation and lowest-order perturbation-theory calculations. The energy of the first-order O() phonon shifts up by 4.90.1 cm-1 in the isotopically pure crystal, in good agreement with the calculated value of 4.30.4 cm-1. No broadening due to isotopic disorder was observed for optical phonons at the point [full width at half maximum (FWHM) of 0.990.03 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 6.50.5 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.