Surface optical phonon modes in GaN nanowire arrays: Dependence on nanowire density and diameter

We present a systematic study, by means of Raman scattering, of the surface optical modes of GaN nanowires (NW) as a function of the fill factor, defined as the relative concentration of GaN NWs and the surrounding air. We show that changes in the fill factor from 0.8 to 0.3 result in a shift of the surface optical mode of more than 60 cm(-1), which is explained by theoretical calculations of surface vibrations for a cylindrical nanowire based on the dielectric continuum model. Two cases are considered: the effective dielectric function approximation (Maxwell-Garnett approximation) and a model for isolated NWs with various diameters. We conclude, in agreement with the Maxwell Garnett approximation, that the dependence of the surface optical (SO) frequency with fill factor is dominated by dipolar interaction between neighboring NWs. Finally, a splitting of the SO mode is observed and ascribed to the anticrossing between surface modes with A(1) and E-1 symmetry, due to a deviation of the exciting light from the NW axis. This splitting is a consequence of the strong optical anisotropy and the polar nature of GaN, and should be present in NWs based on materials with similar characteristics.