Effect of Cr thickness on optical and electronic properties of the (Cr/TiO₂) x10 multilayer

In the present work, we have studied the influence of Cr thickness on the optical and its electronic properties of the (Cr/TiO₂) x 10 multilayer. Pure TiO₂ and (Cr/TiO₂) x 10 multilayer deposited on Si (100) substrate by pulsed laser deposition system using third harmonic (X=355nm) Nd-YAG laser. From the X-ray reflectivity measurement, the estimated thickness of Cr layer varying from t_Cr = 0.5 Å to 14.0 Å, while the thickness of an individual TiO₂ layer is 50.0 Å. When Cr-thickness ranging from 0Cr < 1.8 Å, the XRR pattern looks like a single layer. When t_Cr > 1.8 Å, appearance of Bragg peak has been observed, it suggests that a sharp bilayer of Cr and TiO₂ with a stake of 10 bilayer has been formed. Pure TiO₂ film exhibits amorphous in nature, while the presence of Cr stabilizes the formation of rutile TiO₂ phase. Crystalline size of TiO₂ rutile phase increases as a function of Cr thickness. It indicates that the effect of Cr worked as nucleation for the growing of rutile phase formation. The optical band gap energy (Eg) measured for pure TiO₂ film is found to be 3.2eV. Initially, the Cr-doping has also increased the bandgap energy and after that, it has been reduced with increasing the doping of Cr into TiO₂. XPS and XAS spectra reveal oxygen vacancy present for thinner film. As the thickness of Cr increases, Ti⁺⁴ and Cr⁺³ state dominated which is the responsible reduction of band gap energy at higher thickness. Valence band spectra of these films indicate that it mainly comprises of Ti 3d derived state for lower thickness films, however, Cr derived state has also been observed in Cr doped films and CIS intensity is increasing as a function of Cr thickness. (author)