Structure and bandgap nonlinearity in BaSn1−xTixO3 epitaxial films

Abstract

BaSn1−xTixO3 (BSTO, x = 0, 0.25, 0.5, 0.75, 1.0) epitaxial films were deposited on LaAlO3(001) substrates by employing a pulsed laser deposition system. Their composition dependent structure and optical properties were systematically investigated using density functional theory (DFT) calculations. High resolution x-ray diffraction characterization showed that the film lattice parameters decrease linearly with the increase in Ti concentrations, following Vegard's law. Optical property measurements revealed that all films have the optical transmittance of more than 70% in the visible and infrared wavelength range. Interestingly, the bandgap nonlinearity of BSTO films was observed, and the film at Ti doped x = 0.25 has the largest bandgap value of 4.04 eV. The calculated electronic structure and the density of states of BSTO films determined by DFT suggested that the strong hybridization between Sn 5s and Ti 3d orbitals caused the initial expansion of the bandgap, whereas upon further increasing the doping content, Ti 3d orbitals dominated the conduction band, leading to the bandgap reduction.