Zone-boundary phonons in hexagonal and cubic GaN

We present results of second-order Raman-scattering experiments on hexagonal and cubic GaN covering the acoustic and the optical overtone spectral region. Based on a comparison of the experimental scattering data with the calculated phonon-dispersion curves as well as the group-theoretically derived selection rules, we were able to assign the observed structures to particular phonon branches and determined the points in the Brillouin zone from which the scattering originates. Our measurements reveal the energies of acoustic zone-boundary phonons in hexagonal GaN.