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 Ge-70(0.5) Ge-76(0.5). 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.