Optical properties of variable-composition aluminum/alumina thin films, Part 2

Optical properties were measured for reactively sputtered, substoichiometric alumina/aluminum thin films using ellipsometry, transmission, and thickness measurements. The film deposited at the lowest (0%) oxygen flow ratio was nearly metallic and could be modeled with one Drude and two Tauc-Lorentz oscillators. Interband absorption was reduced compared to bulk aluminum. This reduction was thought to be an effectof volume oxides and disorderliness. Free-electron optical mass was higher and relaxation time shorter than for bulk aluminum; thus, the optical conductivity was lower as a result of residual oxygen molecules gettered to the aluminum atoms. Substoichiometric films depositedat1.5to 4%oxygen flow ratios, unlike the aluminum and previously reported alumina(5 to 9%) films, were nonintrinsic and inhomogeneous in optical properties. Effective medium approximation modeling was used to address these issues. Simply mixing intrinsic (stoichiometric) materials did not simulate the nonintrinsic properties well. Significant improvements were obtained by mixing optically or compositionally neighboring materials. Inhomogeneity was addressedbygrading the constituentsofthe effectivemedium approximation. For 1.5to3%films, a linear profile of the optical properties worked, whereas the 4% film required an exponential profile with most of changesat the bottomofthe film where aluminum particles were distributed, but not agglomerated, due to the oxygen content in the film.