The effects of isotopic substitution on the T-point phonon in isotopic
ally enriched (Ge-70, Ce-73, Ge-74, and Ce-76) and disordered (natural
Ge and Ge-70/76) germanium samples have been measured with Ranan spec
troscopy. We believe. in contrast to earlier work: that intrinsic bulk
Raman phonons are observed only when surface oxides are removed by ch
emical 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 rep
orted before. Our observations improve on previous results. and are in
better agreement with predictions of coherent-potential-approximation
and supercell calculations.