Thermal properties of thin Al2O3 films and their barrier layer effect on thermo-optic properties of TiO2 films grown by atomic layer deposition

We investigate the evaporation of water molecules from the surface of high index, amorphous thin TiO2 films of various thicknesses tt, grown by atomic layer deposition (ALD). The desorption of water molecules is impeded by depositing thin ALD-Al2O3 barrier layers of various thicknesses on the TiO2 thin films. Growing ALD-Al2O3 diffusion barrier layers with different thicknesses ta allows us to evaluate the water vapor evaporation rate in terms of the change in the thermo-optic coefficient (TOC) of TiO2 films over a wide spectral range 380 ≤ λ ≤ 1800 nm. An average reduction of 33% in TOC is found at a barrier layer thickness of ~ 36 nm. Furthermore, the temperature dependent index (dn/dT) and density (dρ/dT) of the ALD-Al2O3 films of various thicknesses ta are also presented. The Cauchy model is applied to all the ellipsometric measurement data to retrieve the optical constants, and subsequent modeling by the Lorentz-Lorenz relation provides the material density of Al2O3 films. The room temperature values of the thermal coefficients for an ALD-Al2O3 film of thickness ta ≈ 60 nm at wavelength λ = 640 nm are found to be dn/dT = 4.66 × 10− 5°C− 1 and dρ/dT = 4.66 × 10− 4g cm− 3C− 1.