Splitting of the Surface Phonon Modes in Wurtzite Nanowires

We analyze the surface optical modes of GaN nanowires (NW) and perform a comparative study with the characteristics expected for other polar NWs. The theoretical analysis of the modes is performed within the context of the effective medium theory that takes into account the dipolar interaction between neighboring NWs (Maxwell-Garnett approximation). It is shown that deviations of the exciting light from the NWs axis, which coincides with the wurtzite c-axis, result in the anticrossing of two distinct surface phonon branches, leading to their splitting in axial and planar components and the appearance of two peaks in the Raman spectra. Additional calculations are performed that determine the relevant filling factor at which this anticrossing takes place for NWs based on AlN, InN, ZnO, as well as other material systems that may crystallize in the wurtzite structure when grown in the form of NWs, such as GaP and InAs. We analyze the splitting of the surface modes as a function of the material's ionicity.