Surface Reordering During Layer-by-Layer Growth on SrTiO₃

With the development of layer-by-layer growth techniques such as molecular beam epitaxy (MBE), it is now possible to construct materials with atomic-level precision, with the layer sequence primarily determined by the growth recipe. However, materials can still restructure in the high-temperature growth environment with surface thermodynamics strongly influencing growth behavior. Here, we demonstrate that growth on ${\mathrm{SrTiO}}_3$ (001), the principal platform for oxide electronics, does not occur in a layer-by-layer fashion but follows a more complex process in which a ${\mathrm{TiO}}_2$ plane continually diffuses toward the growth surface. Employing in situ synchrotron X-ray scattering combined with ab initio thermodynamic calculations, we discover the existence of a stable ${\mathrm{TiO}}_2$ double-layer structure on the pristine substrate and the occurrence of dynamic layer rearrangement during homoepitaxial growth by oxide MBE. Our findings suggest that the current methods used to precisely control ${\mathrm{SrTiO}}_3$ surfaces are limited, as well as our ability to dictate the composition of ultrathin films, resulting in important ramifications regarding the surface reactivity of perovskite materials grown on ${\mathrm{SrTiO}}_3$ .