Residual Stresses in TiO 2 Anatase Thin Films Deposited on Glass, Sapphire and Si Substrates
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Residual Stresses in TiO2 Anatase Thin Films Deposited on Glass, Sapphire and Si Substrates Ibrahim A. Al-Homoudi1, Linfeng Zhang2, D.G. Georgiev2, R. Naik3, V.M. Naik4, L. Rimai2, K.Y.Simon Ng5, R.J. Baird2, G.W. Auner2, G. Newaz1 1 Department of Mechanical Engineering, Wayne State University, Detroit, MI 48202. 2 Department of Electrical and Computer Engineering, Wayne State University, Detroit, MI 48202. 3 Department of Physics and Astronomy, Wayne State University, Detroit, MI 48201. 4 Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, MI 48128. 5 Department of Chemical Engineering and Material Science, Wayne State University, Detroit, MI 48202. ABSTRACT Anatase-TiO2 films (thickness 100-1000 nm) were grown on glass, sapphire, and Si(100) substrates using pulsed dc-magnetron reactive sputtering. By measuring the curvature of substrates before and after the thin film deposition, the residual stresses were determined. These results clearly show that the bi-axial stresses are compressive type and decreases with the increasing film thickness. The Raman spectra of these films were measured with two different excitation wavelengths (514 and 785 nm) and the thickness dependent shifts of Eg phonon mode were studied. The dominant 144 cm-1 Eg mode in TiO2 anatase clearly shifts to a higher value by 0.45 to 17.4 cm-1 depending on the type of substrate and the thickness of the film. Maximum shift was seen for the films on glass substrate indicating a higher bi-axial compressive stress in agreement with the curvature measurements. The excitation wavelength dependent shift of Eg mode clearly shows that the bi-axial stress increases along the film depth, being larger at the film/substrate interface. INTRODUCTION Titanium dioxide (TiO2) thin films have been used extensively in many device applications such as gas and humidity sensors, protective coatings on optical elements, solar energy converters etc. Many different fabrication techniques have been developed to prepare TiO2 thin films, however, the processing conditions have been found to strongly influence the structural properties of the resulting films. Often the films exhibit strain due to thermal stresses caused by differential thermal expansion between substrate and the film or by lattice constant mismatch between the two. It has been recognized that the residual compressive stresses may cause film delamination from the substrate whereas the tensile stresses may cause surface cracks in the films. Most of these stresses either compressive or tensile will eventually end up in severe failure problems in devices [1-6]. Most commonly used methods to measure the residual stresses in films are substrate curvature measurement, X-ray diffraction (XRD) and Raman spectroscopy. The XRD and the curvature measurements can be used directly to determine the average stress in the film. It is also known that zone center Raman phonon lines shift to higher/lower frequency under compressive/tensile stress [7-11]. However, relating the observed Raman spectra
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