The velocity and attenuation of 35-GHz longitudinal $c$-axis acoustic phonons have been measured as a function of temperature and free-carrier concentration by high-resolution Brillouin spectroscopy. These measurements determine both the acoustoelectric and the anharmonic contributions to the phonon damping for comparison to theoretical models.

• Received 17 May 1971

DOI:https://doi.org/10.1103/PhysRevB.5.2997