Franz-Keldysh effect in GaN nanowires

Surface photovoltage spectroscopy and spectral photoconductivity measurements have been carried out in the UV spectral region on GaN nanowires to analyze the near band-edge region. The results reveal the presence of tails in the band-band absorption spectra. Surface Photovoltage spectra performed on the as-grown nanowire ensamble show a long band tail of about 0.1 eV. Spectral photoconductivity on singly contacted nanowires shows that the band tail width strictly depends on the wire diameter. These results are explained by the Franz-Keldysh effect due to the internal electric field induced by Fermi level pinning at the nanowire surface. The experimental values of the absorption tail are well in agreement with the results obtained by simulating the electric field in a cylindrical model