Thermo-Optical Properties of Thin-Film TiO2–Al2O3 Bilayers Fabricated by Atomic Layer Deposition

We investigate the optical and thermo-optical properties of amorphous TiO\(_2\)–Al\(_2\)O\(_3\) thin-film bilayers fabricated by atomic layer deposition (ALD). Seven samples of TiO\(_2\)–Al\(_2\)O\(_3\) bilayers are fabricated by growing Al\(_2\)O\(_3\) films of different thicknesses on the surface of TiO\(_2\) films of constant thickness (100 nm). Temperature-induced changes in the optical refractive indices of these thin-film bilayers are measured by a variable angle spectroscopic ellipsometer VASE\textsuperscript{\textregistered}. The optical data and the thermo-optic coefficients of the films are retrieved and calculated by applying the Cauchy model and the linear fitting regression algorithm, in order to evaluate the surface porosity model of TiO\(_2\) films. The effects of TiO\(_2\) surface defects on the films' thermo-optic properties are reduced and modified by depositing ultra-thin ALD-Al\(_2\)O\(_3\) diffusion barrier layers. Increasing the ALD-Al\(_2\)O\(_3\) thickness from 20 nm to 30 nm results in a sign change of the thermo-optic coefficient of the ALD-TiO\(_2\). The thermo-optic coefficients of the 100 nm-thick ALD-TiO\(_2\) film and 30 nm-thick ALD-Al\(_2\)O\(_3\) film in a bilayer are (0.048 \(\pm\) 0.134) \(\times 10 ^{-4} {^\circ}\mathrm {C}^{-1}\) and (0.680 \(\pm\) 0.313) \(\times 10^{-4} {^\circ} \mathrm {C}^{-1}\), respectively, at a temperature \(T = 62 ^\circ \mathrm{C}\).