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

Abstract

An oxygen-deficient phase has been formed by pulsed-laser ablation of strontium titanate under vacuum conditions $({10}^{\ensuremath{-}7}\phantom{\rule{0.3em}{0ex}}\mathrm{mbar})$. 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 $(\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3})$. 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 $0.40\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, considerably higher than that of the stoichiometric compound $(0.39\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$. 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 ${10}^{\ensuremath{-}3}--{10}^{\ensuremath{-}2}\phantom{\rule{0.3em}{0ex}}\ensuremath{\Omega}\phantom{\rule{0.3em}{0ex}}\mathrm{cm}$ range are thus obtained.