Characteristics of the surface layer of barium strontium titanate thin films deposited by laser ablation

Abstract

${\mathrm{Ba}}_{\mathrm{0.5}}{\mathrm{Sr}}_{\mathrm{0.5}}{\mathrm{TiO}}_{\mathrm{3}}$ (BST) thin films grown on Si by an in situ ultraviolet-assisted pulsed laser deposition (UVPLD) technique exhibited significantly higher dielectric constant and refractive index values and lower leakage current densities than films grown by conventional PLD under similar conditions. X-ray photoelectron spectroscopy (XPS) investigations have shown that the surface layer of the grown films contained, besides the usual BST perovskite phase, an additional phase with Ba atoms in a different chemical state. PLD grown films always exhibited larger amounts of this phase, which was homogeneously mixed with the BST phase up to several nm depth, while UVPLD grown films exhibited a much thinner (∼1 nm) and continuous layer. The relative fraction of this phase was not correlated with the amount of C atoms present on the surface. Fourier transform infrared spectroscopy did not find any ${\mathrm{BaCO}}_{\mathrm{3}}$ contamination layer, which was believed to be related to this new phase. X-ray diffraction measurement showed that although PLD grown films contained less oxygen atoms, the lattice parameter was closer to the bulk value than that of UVPLD grown films. After 4 keV Ar ion sputtering for 6 min, XPS analysis revealed a small suboxide Ba peak for the PLD grown films. This finding indicates that the average Ba–O bonds are weaker in these films, likely due to the presence of oxygen vacancies. It is suggested here that this new Ba phase corresponds to a relaxed BST surface layer.