  A study of the transverse acoustic phonons on nano-structured AlN films has
been carried out by using high-resolution micro-Brillouin spectroscopy. Dense
films have been deposited by radio frequency (r.f.) magnetron sputtering under
ultra high vacuum at room temperature. Films with different morphologies were
prepared and investigated by transmission electron microscopy and Brillouin
Spectroscopy (equiaxed nano-sized, nano-columnar grains and amorphous phase).
Results show a dependence of the transverse modes on the nano-structure. The
nano-columnar film exhibits two transversal modes as expected for the AlN
w{\"u}rtzite while the equiaxed nano-sized and the amorphous films only exhibit
one isotropic transverse mode as expected in amorphous materials. One important
result is that the sound propagation velocity in the AlN amorphous phase is
higher than the one in the non-textured nano-crystalline phase. This phenomenon
has, however, already been observed in ferroelectric ceramics.
