Optical absorption properties of Mg-doped GaN nanocolumns

Optical properties of GaN nanocolumnar films with and without Mg doping are characterized in the visible and ultraviolet regions. Strong uniaxial anisotropy of dielectric constants is observed by ellipsometry. The complex dielectric functions determined from the reflectance and transmittance spectra showed that the ε 2 value is found to be reduced by approximately 50% of that of the epitaxial-GaN film in the energy range above the band gap regardless of Mg doping. This anisotropy and reduction in dielectric constants are due to polarization fields of nanocolumnar crystallites and their interactions. The absorption in undoped GaN nanocolumnar film extends below the band gap of epitaxial GaN, probably due to defects in the nanocolumnar film. Further extension of the absorption tail by Mg doping can be attributed to the transition from a Mg-acceptor level detected in the cathodoluminescence spectra from Mg-doped samples.