Hard x-ray photoemission and density functional theory study of the internal electric field in SrTiO_3/LaAlO_3 oxide heterostructures

摘要

A combined experimental and theoretical investigation of the electronic structure of the archetypal oxide heterointerface system LaAlO_3 on SrTiO_3 is presented. High-resolution, hard x-ray photoemission is used to uncover the occupation of Ti 3d states and the relative energetic alignment-and hence internal electric fields-within the LaAlO_3 layer. First, the Ti 2p core-level spectra clearly show occupation of Ti 3d states already for two unit cells of LaAlO_3. Second, the LaAlO_3 core levels were seen to shift to lower binding energy as the LaAlO_3 overlayer thickness, n, was increased, agreeing with the expectations from the canonical electron transfer model for the emergence of conductivity at the interface. However, not only is the energy offset of only ～300 meV between n = 2 (insulating interface) and n = 6 (metallic interface) an order of magnitude smaller than the simple expectation, but it is also clearly not the sum of a series of unit-cell-by-unit-cell shifts within the LaAlO_3 block. Both of these facts argue against the simple charge-transfer picture involving a cumulative shift of the LaAlO_3 valence bands above the SrTiO_3 conduction bands, resulting in charge transfer only for n≥ 4. We discuss effects which could frustrate this elegant and simple charge-transfer model, concluding that although it cannot be ruled out, photodoping by the x-ray beam is unlikely to be the cause of the observed behavior. Turning to the theoretical data, our density functional simulations show that the presence of oxygen vacancies at the LaAlO_3 surface at the 25% level reverses the direction of the internal field in the LaAlO_3. Therefore, taking the experimental and theoretical results together, a consistent picture emerges for real-life samples in which nature does not wait until n = 4 and already for n = 2 mechanisms other than internal-electric-field-driven electron transfer from idealized LaAlO_3 to near-interfacial states in the SrTiO_3 substrate are active in heading off the incipient polarization catastrophe that drives the physics in these systems.