Determination of thermo-optic properties of atomic layer deposited thin TiO_2 films for athermal resonant waveguide gratings by spectroscopic ellipsometry

摘要

We report on variation in the refractive index of amorphous and isotropic TiO_2 thin films grown by Atomic Layer Deposition (ALD) in nano optical devices. ALD-TiO_2 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-TiO_2 films with thicknesses ＞ 200 nm show positive TOC due to the predominance of TiO_2 thickness over the very thin surface porosity region. The negative TOC of thin TiO_2 films was controlled by depositing thin ALD-Al_2O_3 diffusion barrier films that showed impermeable behavior to block the evaporation of adsorbed water molecules on TiO_2 surfaces in thermal environments. This approach turns negative sign of TOC of TiO_2 thin films to positive one which is necessary to stabilize the central resonance peak of a guided mode resonance filter (GMRF). The ALD-TiO_2 and ALD-Al_2O_3 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 TiO_2 films were calculated from ellipsometric measured data using Lorentz-Lorenz relation.