Effect of Cr incorporation on the structural and optoelectronic properties of TiO2:Cr deposited by means of a magnetron co-sputtering process

In this work, we report on the effect of Cr incorporation on the microstructural and optical properties of TiO2:Cr thin films deposited by the RF-magnetron sputtering method. The structural, morphological, chemical bonding and optoelectronic properties of the sputter-deposited TiO2:Cr films were systematically investigated, as a function the incorporated Cr content, by means of various techniques including X-ray diffraction (XRD), atomic force microscopy (AFM), Fourier-Transform Infra-Red (FTIR) absorption, X-ray Photoelectron Spectroscopy (XPS) and ellipsometry. The Cr incorporation into the TiO2 films was controlled by adjusting the RF power (P-Cr) on the Cr target during the co-sputtering process of TiO2 and Cr. We were thus able to demonstrate that by varying P-Cr from 8 W to 150 W, the Cr content of the TiO2:Cr films can be fairly controlled from similar to 2 at.% to similar to 18 at.% and their associated bandgap engineered from 3.3 eV to 1.5 eV. The room-temperature deposited TiO2:Cr are mainly amorphous with the presence of some TiO2 nanocrystallites, and their density increases as their Cr content is increased. The Cr inclusions were found to coexist under both metallic and oxidized forms in the films. By subjecting the TiO2:Cr films to post-annealing treatment (at 550 degrees C), their crystalline structure was found to be sensitive to their Cr content. Indeed, an anatase-to-rutile phase transformation has been pointed out to occur at a Cr content of similar to 7 at.%. Likewise, the Cr-content dependence of the bandgap of annealed TiO2:Cr films undergoes a transition around the 7 at.% of Cr. Our results demonstrate the ability to control the Cr-content of TiO2:Cr films, which leads to tune their optoelectronic properties, such as bandgap or optical absorption edge. (C) 2011 Elsevier B.V. All rights reserved.