Determination of thermo-optic properties of atomic layer deposited thin TiO2 films for athermal resonant waveguide gratings by spectroscopic ellipsometry

We report on variation in the refractive index of amorphous and isotropic TiO2 thin films grown by Atomic Layer Deposition (ALD) in nano optical devices. ALD-TiO2 films of thicknesses ≤ 200 nm exhibiting negative thermo-optic coefficient (TOC) due to decrease in refractive index with temperature, owing to inherent hydrophilic nature. While ALD-TiO2 films with thicknesses > 200 nm show positive TOC due to the predominance of TiO2 thickness over the very thin surface porosity region. The negative TOC of thin TiO2 films was controlled by depositing thin ALD-Al2O3 diffusion barrier films that showed impermeable behavior to block the evaporation of adsorbed water molecules on TiO2 surfaces in thermal environments. This approach turns negative sign of TOC of TiO2 thin films to positive one which is necessary to stabilize the central resonance peak of a guided mode resonance filter (GMRF). The ALD-TiO2 and ALDAl2O3 bi-layer stack was modeled by VASE analysis of spectroscopic ellipsometry using Cauchy Model to extract refractive indices at various temperatures, measured at two different angle of incidence (65° and 75°), covering a wide spectral range 380 ≤ λ ≤ 1800. The temperature dependent index and density of TiO2 films were calculated from ellipsometric measured data using Lorentz-Lorenz relation.