Dielectric functions and energy band gap variation studies of manganese doped Bi3.25La0.75Ti3O12 thin films using spectroscopic ellipsometry

Single phase polycrystalline Mn-modified Bi3.25La0.75Ti3O12 (BLT) thin films were prepared by chemical solution deposition method using spin coating technique on Pt/Ti/SiO2/Si (1 0 0) substrates. Raman spectroscopy of these films shows that Mn3+ is well substituted at Ti4+ site. The optical properties of BLT and Mn modified BLT thin films were investigated at room temperature by using spectroscopic ellipsometry (SE) in the energy range 0.72-6.2 eV. A double Tauc-Lorentz (DTL) dispersion relation was successfully used to model the dielectric functions of these films where a shift to the lower energy side with Mn doping is seen. The full width at half maxima (FWHM) (Г) of dielectric function is found to increase with Mn doping. This increase in FWHM may be attributed to the increase in the trap density in forbidden band which consequently decreases the value of direct optical band gap (Egd). The direct optical band gap (Egd) is found to decrease with increase in Mn content in the studied composition range. This decrease in Egd with doping may be attributed to the variation in the defect concentration present in the structure.