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Perovskite materials are promising for future oxide electronics, e.g., in gate insulators, ferroelectric, magnetic, superconducting, or electrode films. The interfaces to substrates or within multilayers are of great importance. Strontium titanate, SrTiO3, is a widely used substrate material for oxide thin film devices. It has recently received great interest when combined with a similar oxide material, LaAlO3. It has been suggested that a novel two-dimensional electron gas is formed at the non-polar-to-polar interface between SrTiO3 and LaAlO3, resulting in high electrical conductivity and mobility. It is even possible to control the charge density and the conductance in a field effect transistor. However, other works indicate that the large conductivity is three dimensional and due to the doping by oxygen vacancies. In this report we demonstrate that the transport properties in the oxide heterostructures are very sensitive to the deposition parameters during thin film growth. Using cathode- and photo-luminescence studies in conjunction with measurements of electrical transport properties and microstructure we show that the electronic properties observed at a LaAlO3/SrTiO3 interface may be explained by oxygen reduced SrTiO3. Oxygen can be pushed in and out of the sample but the re-oxygenation of an initially oxygen depleted LaAlO3/SrTiO3 heterostructure is partly prevented by the presence of the cap film. Strains and dislocations at the interface may be seen by transmission electron microscopy. Our findings suggest that extrinsic doping due to oxygen vacancies in SrTiO3 may be responsible for the electrical conductivity at the LaAlO3/SrTiO3 interfaces prepared in wide range of oxygen pressure. |