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Nanomaterials 2015, 5(2), 792-803; doi:10.3390/nano5020792

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

1
Institute of Photonics, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland
2
Center for Advance Studies in Energy (CAS-EN), National University of Sciences and Technology (NUST), Sector H-12, 44000 Islamabad, Pakistan
*
Authors to whom correspondence should be addressed.
Academic Editor: Thomas Nann
Received: 28 February 2015 / Revised: 10 May 2015 / Accepted: 13 May 2015 / Published: 18 May 2015
(This article belongs to the Special Issue Nanophotonic Materials)
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Abstract

We investigate the optical and thermo-optical properties of amorphous TiO\(_2\)–Al\(_2\)O\(_3\) thin-film bilayers fabricated by atomic layer deposition (ALD). Seven samples of TiO\(_2\)–Al\(_2\)O\(_3\) bilayers are fabricated by growing Al\(_2\)O\(_3\) films of different thicknesses on the surface of TiO\(_2\) 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\textsuperscript{\textregistered}. 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 TiO\(_2\) films. The effects of TiO\(_2\) surface defects on the films' thermo-optic properties are reduced and modified by depositing ultra-thin ALD-Al\(_2\)O\(_3\) diffusion barrier layers. Increasing the ALD-Al\(_2\)O\(_3\) thickness from 20 nm to 30 nm results in a sign change of the thermo-optic coefficient of the ALD-TiO\(_2\). The thermo-optic coefficients of the 100 nm-thick ALD-TiO\(_2\) film and 30 nm-thick ALD-Al\(_2\)O\(_3\) film in a bilayer are (0.048 \(\pm\) 0.134) \(\times 10 ^{-4} {^\circ}\mathrm {C}^{-1}\) and (0.680 \(\pm\) 0.313) \(\times 10^{-4} {^\circ} \mathrm {C}^{-1}\), respectively, at a temperature \(T = 62 ^\circ \mathrm{C}\). View Full-Text
Keywords: atomic layer deposition (ALD); thin barrier films; thermo-optic coefficient; titanium dioxide; aluminum oxide; optical materials atomic layer deposition (ALD); thin barrier films; thermo-optic coefficient; titanium dioxide; aluminum oxide; optical materials
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Ali, R.; Saleem, M.R.; Pääkkönen, P.; Honkanen, S. Thermo-Optical Properties of Thin-Film TiO2–Al2O3 Bilayers Fabricated by Atomic Layer Deposition. Nanomaterials 2015, 5, 792-803.

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