Isotopic disorder-effects on the phonons in germanium

High-resolution (±0.02 cm^-1) Raman and infrared transmission (ir) measurements were used to study the effect of isotopic disorder on the vibrational properties of Ge crystals. Calculations using the coherent potential and the self-consistent Born approximations were performed and compared with the experiment. The linewidth of the Raman modes is predominantly determined by the anharmonic decay of the optical phonons into lower-energy phonons, with a characteristic decay time of 6.1(±0.1) ps in natural Ge. The phonon energies of the Raman active phonon and the transverse optical ir modes at the Brillouin zone boundary (L,K,W,X-critical point) are shifted by the isotopic disorder in the material. The frequency shifts and the line broadenings are in good agreement with the theoretical calculations.