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

摘要

We investigate the optical and thermo-optical properties of amorphous TiO₂–Al₂O₃ thin-film bilayers fabricated by atomic layer deposition (ALD). Seven samples of TiO₂–Al₂O₃ bilayers are fabricated by growing Al₂O₃ films of different thicknesses on the surface of TiO₂ 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 TiO₂ films. The effects of TiO₂ surface defects on the films’ thermo-optic properties are reduced and modified by depositing ultra-thin ALD-Al₂O₃ diffusion barrier layers. Increasing the ALD-Al₂O₃ thickness from 20 nm to 30 nm results in a sign change of the thermo-optic coefficient of the ALD-TiO₂. The thermo-optic coefficients of the 100 nm-thick ALD-TiO₂ film and 30 nm-thick ALD-Al₂O₃ 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.