He+ induced changes in the surface structure and optical properties of RF-sputtered amorphous alumina thin films

• He+ irradiation of amorphous alumina thin films does not induce crystallinity in the film.
• Surface roughness of alumina film is increased by increase in He+ irradiation.
• The optical constants of alumina films can be tailored by ion irradiation.
• Variation in film's optical properties after irradiation was attributed to electronic excitation.

High quality 50 nm thick stoichiometric amorphous aluminium oxide films were reactively sputtered on microscope glass slide substrates. The films were exposed to energetic (2.20 MeV) He+ at different ion fluences of 6 × 1012 ion/cm2; 1 × 1013 ion/cm2; 2 × 1013 ion/cm2; 3 × 1013 ion/cm2; and 4 × 1013 ion/cm2. The effect of the ion irradiation on the optical, structural phase and surface properties of the alumina films was investigated via UV–VIS–NIR spectroscopy, X-ray diffraction analysis and the atomic force microscopy respectively. The transmission and absorption spectra of the irradiated films showed variation that depended on ion fluence. The refractive index, extinction coefficient, optical conductivity, dielectric constant and energy loss functions of the films were also affected by He+ irradiation. Optical band gap and films' structural phase were however not altered by the ion irradiation. The variation in optical constants induced by radiation was attributed to electronic excitation and increase in surface roughness of the films.