Near-ultraviolet optical absorption behavior of TiO2–Al2O3 multilayer films

Abstract

The fundamental optical absorption edge of sputter-deposited titania-alumina (TiO2–Al2O3) multilayers on fused SiO2 substrates is studied by near ultraviolet-visible spectrophotometry. We examine a family of films with bilayer architecture Λ=9–72nm TiO2∕7nm Al2O3 (TiO2 volume fraction from 0.56 to 0.91). Neither the TiO2 or Al2O3 layers have long-range crystallographic order. The absorption coefficient α in the region of optical density from 2 to 7 is determined as a function of energy E. The first significant finding is that the onset of absorption for these multilayers is determined by their TiO2 component, independent of TiO2 layer thickness. The nondirect optical gap Eo is determined by extrapolating linear α1∕2 vs E curves to α=0. Eo=2.95 (±0.02)eV for all multilayers, identical to Eo for a single layer TiO2 film with rutile short-range atomic order. The second significant finding is that the strength of α in the multilayers is diminished compared to a Vegard’s rule analysis based on the weighted sum of α(TiO2) and α(Al2O3). A modified Vegard’s rule model is proposed to account for the possible electronic effect of Al2O3 on TiO2 at an interface.