Anomalous Lattice Expansion Of Coherently Strained Srtio3 Thin Films Grown On Si(001) By Kinetically Controlled Sequential Deposition

X-ray diffraction has been used to study the epitaxy and lattice expansion of SrTiO3 thin films grown coherently on Si(001) by kinetically controlled sequential deposition. The coherent growth of SrTiO3 on Si produces films with greater in-plane compressive strain than previously attained, -1.66%. The measured expansion of the out-of-plane lattice constant exceeds the prediction of the bulk elastic constants of SrTiO3 by nearly 100%. This expansion agrees with recent theoretical predictions and experimental findings of room-temperature ferroelectricity in SrTiO3 films under epitaxial mismatch strain. Our first principles density functional calculations determine an energetically favorable interfacial-defect and surface-charge structure that allows the ferroelectric polarization in these ultrathin films.