Anticrossing of axial and planar surface-related phonon modes in Raman spectra of self-assembled GaN nanowires

GaN columnar nanostructures usually called nanowires have been investigated by micro-Raman spectroscopy. In addition to conventional Raman scattering by confined optical phonons of a wurtzite structure (i.e., E-2h and QLO modes), an unusual two peaks band centered near 700 cm(-1) is observed and analyzed as a function of several experimental parameters (polarization, filling factor, incidence angle). The surface character of these two modes is experimentally confirmed by their high sensitivity to the dielectric constant of the as-grown nanowires surrounding medium. Calculations describing the nanowires' environment by means of an effective dielectric function are in good agreement with this two peaks structure and suggest that these two peaks result from the anticrossing of axial and planar surface-related phonons, which should be observed in all anisotropic polar semiconductors.