Optical phonons in isotopic Ge studied by Raman scattering

The effects of isotopic substitution on the $\Gamma$-point phonon in isotopically enriched ${(}^{70}\mathrm{Ge}$, ${}^{73}\mathrm{Ge}$, ${}^{74}\mathrm{Ge}$, and ${}^{76}\mathrm{Ge}$) and disordered (natural Ge and ${}^{70/76}\mathrm{Ge}$) germanium samples have been measured with Raman spectroscopy. We believe, in contrast to earlier work, that intrinsic bulk Raman phonons are observed only when surface oxides are removed by chemical etching, and a laser line in the red is used to greatly enhance the penetration depth of the light. In high-resolution experiments at 10 K, performed under these conditions, we obtain more precise phonon frequencies and find significantly reduced phonon linewidths than reported before. Our observations improve on previous results, and are in better agreement with predictions of coherent-potential-approximation and supercell calculations.