Chemistry and structure of homoepitaxial SrTiO₃ films and their influence on oxide-heterostructure interfaces

The properties of single-crystal SrTiO₃ substrates and homoepitaxial SrTiO₃ films grown by pulsed laser deposition have been compared, in order to understand the loss of interfacial conductivity when more than a critical thickness of nominally homoepitaxial SrTiO₃ is inserted between a LaAlO₃ film and a SrTiO₃ substrate. In particular, the chemical composition and the structure of homoepitaxial SrTiO₃ investigated by low-energy ion-scattering and surface X-ray diffraction show that for insulating heterointerfaces, a Sr-excess is present between the LaAlO₃ and homoepitaxial SrTiO₃. Furthermore, an increase in the out-of-plane lattice constant is observed in LaAlO ₃, indicating that the conductivity both with and without insertion of the SrTiO₃ thin film originates from a Zener breakdown associated with the polar catastrophe. When more than a critical thickness of homoepitaxial SrTiO₃ is inserted between LaAlO₃ and SrTiO₃, the electrons transferred by the electronic reconstruction are trapped by the formation of a Sr-rich secondary phase and Sr-vacancies. The migration of Sr towards the surface of homoepitaxial SrTiO₃ and accompanying loss of interfacial conductivity can be delayed by reducing the Sr-content in the PLD target.