Displacive phase transition in $SrTiO3$ thin films grown on Si(001)

Polarization dependent near and extended x-ray absorption fine-structure (XANES and EXAFS), in combination with x-ray diffraction, has been used to study the structure of $SrTiO3$ (STO) ultra thin films grown on Si(001). For the in-plane direction (200), the x-ray diffraction data indicate that all films (from 40 to 200 Å) are equally expanded. This is in contradiction to previous reports claiming that the growth is pseudomorphic and epitaxial (coherent), which would predict an inplane contraction. Even the thinnest films (40 Å) grow in a relaxed mode (not coherent) at the deposition temperature (700 °C). As the system is brought to room temperature, the films (now anchored to the substrate) are not allowed to compress as much as bulk STO. The residual film expansion is quantitatively explained by the differential thermal expansion of Si and STO. For the out-of-plane direction (002), the x-ray diffraction data indicate that STO films are expanded for the thinnest films, and relaxed for a thickness of 200 Å. The in-plane and out-of-plane EXAFS and XANES data show that the perpendicular expansion of the thinner films is accompanied by a displacive phase transition of $SrTiO3$ where the Ti atom moves toward the (002) direction. This ferroelectric-type behavior of the thinner films implies important potential applications in electronics.