Thermo-Optical Properties of Thin-Film TiO(2)–Al(2)O(3) 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®. 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 ± 0.134) × 10(−4) °C(−1) and (0.680 ± 0.313) × 10(−4) °C(−1), respectively, at a temperature T = 62 °C.