Fröhlich modes in GaN columnar nanostructures

GaN columnar nanostructures fabricated by electrochemical dissolution of bulk material have been studied by micro-Raman spectroscopy. The anodization induces an increase in the intensity of Raman scattering accompanied by a breakdown of the polarization selection rules and by the appearance of a new mode at $716\hspace{0.5em}{\mathrm{cm}}^{-1},$ i.e., in the frequency gap between the transverse optical and longitudinal optical bulk phonons. We present a Raman line-shape analysis based on the effective dielectric function of a composite that brings to light the Fröhlich character of this mode.