Optical Properties of Anatase, Rutile and Amorphous Phases of TiO 2 Thin Films Grown at Room Temperature by RF Magnetron
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Optical Properties of Anatase, Rutile and Amorphous Phases of TiO2 Thin Films Grown at Room Temperature by RF Magnetron Sputtering V. M. Naik, D. Haddad1, R. Naik1, J. Benci2 and G. W. Auner2 Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, MI 1 Dept. of Physics, Wayne State University, Detroit, MI 2 Dept. of Electrical and Computer Engineering, Wayne State University, Detroit, MI
ABSTRACT Anatase (A), rutile (R) and amorphous phase TiO2 thin films have been prepared by RF magnetron sputtering on unheated glass substrates by controlling the total pressure of sputtering gases (Ar + O2) and the substrate bias. The crystal structures of the films were confirmed by xray diffraction and Raman scattering. The analysis of optical absorption data for A- TiO2 film shows an energy bandgap (Eg) of 3.2 eV (indirect extrapolation) and ~ 3.5 eV (direct extrapolation). On the other hand, R-TiO2 film shows Eg ~ 2.9 eV (indirect) and 3.2 eV (direct). The latter film also shows the presence of amorphous regions with Eg ~ 3.0 eV (indirect) and 3.8 eV (direct). The bandgap of both the films, obtained using indirect extrapolation, has a value range consistent with the previous measurements.
INTRODUCTION Recently, there has been a growing interest in preparing thin films of TiO2 because of its importance in several technological applications such as photocatalysis, sensors, solar cells and memory devices.1-5 Nanocrystalline TiO2 films have also been used in bio-analytical devices.6 TiO2 occurs in nature in three crystalline forms: anatase (A), rutile (R), which have a tetragonal structure, and brookite with orthorhombic structure. The R-TiO2 is found to be superior in optical properties and thermodynamically more stable than A-TiO2. However, the latter has attracted much attention recently as the most promising photocatalytic material. In general, the formation of R-phase of TiO2 films requires a higher substrate temperature (> 300 oC), whereas a lower substrate temperature results in either A-phase or an amorphous structure. Hence, preparing thin films of rutile and anatase forms of TiO2 on substrates held at low-temperature has been of considerable interest. Recently, a selective growth of either anatase TiO2 or rutile TiO2 films on unheated glass substrates has been demonstrated using RF magnetron sputtering.7-9 However, there have been no optical band gap measurements reported on these films. In the present work, we have prepared A- and R-TiO2 films (nominal thickness ~ 1 µm) on unheated glass substrates by rf magnetron sputtering by controlling the total pressure of sputtering gases (Ar + O2). The crystal structures of the films have been confirmed by x-ray diffraction (XRD) and Raman scattering. Optical bandgap energies have been determined by transmission measurements in the UV-Visible region (175 nm- 3000 nm).
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EXPERIMENT The TiO2 samples were deposited on unheated glass substrates using RF magnetron sputtering with a TiO2 target (2”) and target to substrate spacing of 2 to 3 inc
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