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 A) 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 A) 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 A....