Thin films of oxygen-deficient perovskite phases by pulsed-laser ablation of strontium titanate

An oxygen-deficient phase has been formed by pulsed-laser ablation of strontium titanate under vacuum conditions $\left({10}^{-7}\mathrm{mbar}\right)$. By the complementary use of Rutherford backscattering spectrometry and x-ray diffraction, the composition and structure of films were determined. A large oxygen deficiency is revealed in the films as the corresponding formula is $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{2.5}$. This understoichiometry means that ${\mathrm{Ti}}^{3+}$ are present in such films, while only ${\mathrm{Ti}}^{4+}$ is observed in stoichiometric compound $\left(\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3\right)$. The presence of ${\mathrm{Ti}}^{3+}$ species in the $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{2.5}$ films was checked by x-ray absorption spectroscopy. The $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{2.5}$ films were found to crystallize in the cubic perovskite structure with a lattice parameter of $0.40\mathrm{nm}$, considerably higher than that of the stoichiometric compound $\left(0.39\mathrm{nm}\right)$. The $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{2.5}$ films were epitaxially grown on MgO single-crystal substrates, with a “cube-on-cube” in-plane orientation with the substrate. In contrary to stoichiometric $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ films which are perfectly insulating, the $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{2.5}$ films were found to be conductor or semiconductor at room temperature, depending on the growth temperature. Resistivities at room temperature in the ${10}^{-3}–{10}^{-2}\Omega \mathrm{cm}$ range are thus obtained.