Hard x-ray photoemission and density functional theory study of the internal electric field in SrTiO3/LaAlO3oxide heterostructures

Abstract

Hard x-ray photoemission and density functional theory study of the internal electric field in SrTiO3/LaAlO3 oxide heterostructures Slooten, E.; Zhong, Z.; Molegraaf, H.J.A.; Eerkes, P.D.; de Jong, S.; Massee, F.; van Heumen, E.; Kruize, M.K.; Wenderich, S.; Kleibeuker, J.E.; Gorgoi, M.; Hilgenkamp, H.; Brinkman, A.; Huijben, M; Rijnders, G.; Blank, D.H.A.; Koster, G.; Kelly, P.J.; Golden, M.S. Published in:Physical Review B DOI:10.1103/PhysRevB.87.085128 Link to publicationCitation for published version (APA): Slooten, E., Zhong, Z., Molegraaf, H. J. A., Eerkes, P. D., de Jong, S., Massee, F., ... Golden, M. (2013). Hard xray photoemission and density functional theory study of the internal electric field in SrTiO3/LaAlO3 oxide heterostructures. Physical Review B, 87(8), 085128. https://doi.org/10.1103/PhysRevB.87.085128 General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. 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 o...

Band alignment in LaAlO3/SrTiO3oxide heterostructures inferred from hard x-ray photoelectron spectroscopy

“…These results show that photon intensity reduction by three orders of magnitude does not lead to a detectable restoration of the internal LAO band bending nor to a notable decrease of charge carriers at the interface. In agreement with the conclusions of another recent HAXPES study, 22 our findings therefore seem to indicate that the effect of photogenerated carriers on band bending is small. However, already the weakest intensities used may be too close to saturation.…”

“…An alternative explanation for the apparent flat band situation has been provided by recent theoretical studies which allow for the presence of oxygen vacancies in the LAO film 22,26,[39][40][41] (not to be confused with the role of vacancies in the STO substrate). In these calculations, it is found that the cost of energy associated with the generation of these defects, i.e., their enthalpy of formation, assumes a minimum at the LAO surface and that each (positively charged) oxygen vacancy leaves two electrons which are transferred to the interface.…”

Observation of strontium segregation in LaAlO3/SrTiO3 and NdGaO3/SrTiO3 oxide heterostructures by X-ray photoemission spectroscopy

“…34 This hypothesis, if confirmed, could be consistent with the observation that BE shifts of electrostatic nature are hardly found in LaAlO 3 /SrTiO 3 . 30,[36][37][38] At the end we address the general importance of the observed Sr-segregation for the physics of the oxide heterostructures. Any attempt to directly link this phenomenon to interface conductivity would be highly speculative at this stage.…”

Influence of oxygen vacancies on two-dimensional electron systems at SrTiO3-based interfaces and surfaces

“…The electronic reconstruction scenario, however, was also seriously cast into doubt by X-ray photoemission measurements providing a fairly sensitive probe for binding energy shifts -e.g., due to an internal electric field in the LaAlO 3 film -but being unable to reveal such traces of a finite potential gradient in the data [83][84][85]. Likewise, angle-resolved photoemission measurements using soft X-rays failed to observe O 2p-derived hole pockets due to the LaAlO 3 valence band Fermi level crossing as expected and predicted in an electronic reconstruction scenario [86].…”