Abstract
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.
Highlights
The continuous miniaturization of optical and photonic components with the growing inquisitiveness to explore atomic-scale phenomena has led researchers to develop innovative techniques to fulfill the requirements for atomic-level control
We study the thermo-optic coefficients of atomic layer deposition (ALD)-TiO2 films of a 100-nm thickness and the influence of the ALD-Al2 O3 thin-films of different thicknesses (10–70 nm) as barrier layers, in order to validate the surface porosity model introduced earlier [10,12]
The adsorption of water molecules either undergoes dissociations at particular defect sites or they are adsorbed molecularly. Such evaporation of water molecules can be effectively minimized by an ALD-deposited Al2 O3 diffusion barrier layer
Summary
The continuous miniaturization of optical and photonic components with the growing inquisitiveness to explore atomic-scale phenomena has led researchers to develop innovative techniques to fulfill the requirements for atomic-level control. Atomic layer deposition (ALD), initially developed for the deposition of uniform thin-films over wide areas of electroluminescent display devices [1], is a unique method enabling an atomic-level control of film thickness and reproducible growth of defect-free films. It is cost effective and, suitable for low-cost mass production. The availability of ALD reactors has paved the way for further improving the precise atomic-level thickness control, achieving self-controlled alternating surface reactions, improving the wide area uniformity and obtaining continuous pinhole or defect-free films [2]. One important feature of ALD is the excellent conformality of the deposited films over corrugated and/or zigzag surface profiles [5]
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