Resonant Raman-active acoustic phonons in the mixed amorphous-microcrystalline phase of ion-implanted GaAs

We have observed a new, strong, low-frequency peak (at 47 ${\mathrm{cm}}^{\mathrm{-}1}$) in the Raman spectrum of ion-implanted GaAs having a mixed amorphous-microcrystalline microstructure. It is strongly resonant near 1.7 eV, just above the band gap, in contrast to the longitudinal-optic phonon line of the microcrystals (which resonates differently) and the bands of the amorphous component (which do not resonate). We tentatively interpret this peak in terms of acoustic phonons made Raman active by the presence of microcrystal-amorphous interface regions, and discuss several models.