Phonons in isotopically disordered Ge

We report Raman- and neutron-scattering, infrared-transmission, and photoluminescence experiments in a series of isotopically enriched samples of Ge, including natural Ge and an alloy ⁷⁰Ge0.5 ⁷⁶Ge0.5. Emphasis is put on the study of disorder-induced effects on the phonon spectra. Results are compared to exhaustive theoretical calculations using the coherent-potential and self-consistent Born approximations. Reasonably good agreement is found between theory and experiment for the former. Data for the dependence of the energy of the E₁ interband electronic gap on isotopic mass are also presented.