EFFECTS OF ISOTOPE DISORDER ON PHONONS IN GERMANIUM DETERMINED FROM BOUND EXCITON LUMINESCENCE

The effects of isotope disorder on the energies of the transverse acoustic (TA(L)), longitudinal acoustic (LA(L)) and transverse optical (TO(L)) phonons at the L point have been investigated using the luminescence of excitons bound to copper acceptors. In a sample with approximately equal Ge-70 and Ge-76 the TO(L) phonon is 0.14 +/- 0.03 meV higher in energy than expected for a crystal of the same mean atomic mass, while the LA(L) phonon is 0.07 +/- 0.03 meV lower in energy; the TO(L) phonon is also broadened by 0.4 meV. In this disordered sample the Cu no-phonon line is 0.05 +/- 0.02 meV higher than expected.