We have investigated how the density of threading dislocations in GaN affects the decay of the intensity of nano-acoustic waves. We carried out measurements using a reflection-type femtosecond pump probe, and thus, we determined the local dislocation density from the lifetime of nano-acoustic waves. We found that for a dislocation density of 10(8) cm(-2), defect scattering will surmount other scattering mechanisms and dominate the attenuation of 100GHz acoustic phonons.
