Hard x-ray photoemission and density functional theory study of the internal electric field in SrTiO3/LaAlO3 oxide heterostructures

E. Slooten, Zhicheng Zhong, Hajo Molegraaf, P.D. Eerkes, S. de Jong, F. Massee, E. van Heumen, Michelle Kruize, Sander Wenderich, J.E. Kleibeuker, M. Gorgoi, Johannes W.M. Hilgenkamp, Alexander Brinkman, Mark Huijben, Augustinus J.H.M. Rijnders, David H.A. Blank, Gertjan Koster, Paul J. Kelly, M.S. Golden

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Abstract

A combined experimental and theoretical investigation of the electronic structure of the archetypal oxide heterointerface system LaAlO3 on SrTiO3 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 LaAlO3 layer. First, the Ti 2p core-level spectra clearly show occupation of Ti 3d states already for two unit cells of LaAlO3. Second, the LaAlO3 core levels were seen to shift to lower binding energy as the LaAlO3 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 LaAlO3 block. Both of these facts argue against the simple charge-transfer picture involving a cumulative shift of the LaAlO3 valence bands above the SrTiO3 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 LaAlO3 surface at the 25% level reverses the direction of the internal field in the LaAlO3. 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 LaAlO3 to near-interfacial states in the SrTiO3 substrate are active in heading off the incipient polarization catastrophe that drives the physics in these systems.
Original languageUndefined
Pages (from-to)085128/1-085128/11
Number of pages11
JournalPhysical review B: Condensed matter and materials physics
Volume87
Issue number8
DOIs
Publication statusPublished - 2013

Keywords

  • IR-86894
  • METIS-295996

Cite this

Slooten, E. ; Zhong, Zhicheng ; Molegraaf, Hajo ; Eerkes, P.D. ; de Jong, S. ; Massee, F. ; van Heumen, E. ; Kruize, Michelle ; Wenderich, Sander ; Kleibeuker, J.E. ; Gorgoi, M. ; Hilgenkamp, Johannes W.M. ; Brinkman, Alexander ; Huijben, Mark ; Rijnders, Augustinus J.H.M. ; Blank, David H.A. ; Koster, Gertjan ; Kelly, Paul J. ; Golden, M.S. / Hard x-ray photoemission and density functional theory study of the internal electric field in SrTiO3/LaAlO3 oxide heterostructures. In: Physical review B: Condensed matter and materials physics. 2013 ; Vol. 87, No. 8. pp. 085128/1-085128/11.
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title = "Hard x-ray photoemission and density functional theory study of the internal electric field in SrTiO3/LaAlO3 oxide heterostructures",
abstract = "A combined experimental and theoretical investigation of the electronic structure of the archetypal oxide heterointerface system LaAlO3 on SrTiO3 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 LaAlO3 layer. First, the Ti 2p core-level spectra clearly show occupation of Ti 3d states already for two unit cells of LaAlO3. Second, the LaAlO3 core levels were seen to shift to lower binding energy as the LaAlO3 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 LaAlO3 block. Both of these facts argue against the simple charge-transfer picture involving a cumulative shift of the LaAlO3 valence bands above the SrTiO3 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 LaAlO3 surface at the 25{\%} level reverses the direction of the internal field in the LaAlO3. 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 LaAlO3 to near-interfacial states in the SrTiO3 substrate are active in heading off the incipient polarization catastrophe that drives the physics in these systems.",
keywords = "IR-86894, METIS-295996",
author = "E. Slooten and Zhicheng Zhong and Hajo Molegraaf and P.D. Eerkes and {de Jong}, S. and F. Massee and {van Heumen}, E. and Michelle Kruize and Sander Wenderich and J.E. Kleibeuker and M. Gorgoi and Hilgenkamp, {Johannes W.M.} and Alexander Brinkman and Mark Huijben and Rijnders, {Augustinus J.H.M.} and Blank, {David H.A.} and Gertjan Koster and Kelly, {Paul J.} and M.S. Golden",
year = "2013",
doi = "10.1103/PhysRevB.87.085128",
language = "Undefined",
volume = "87",
pages = "085128/1--085128/11",
journal = "Physical review B: Condensed matter and materials physics",
issn = "1098-0121",
publisher = "American Physical Society",
number = "8",

}

Hard x-ray photoemission and density functional theory study of the internal electric field in SrTiO3/LaAlO3 oxide heterostructures. / Slooten, E.; Zhong, Zhicheng; Molegraaf, Hajo; Eerkes, P.D.; de Jong, S.; Massee, F.; van Heumen, E.; Kruize, Michelle; Wenderich, Sander; Kleibeuker, J.E.; Gorgoi, M.; Hilgenkamp, Johannes W.M.; Brinkman, Alexander; Huijben, Mark; Rijnders, Augustinus J.H.M.; Blank, David H.A.; Koster, Gertjan; Kelly, Paul J.; Golden, M.S.

In: Physical review B: Condensed matter and materials physics, Vol. 87, No. 8, 2013, p. 085128/1-085128/11.

Research output: Contribution to journalArticleAcademicpeer-review

TY - JOUR

T1 - Hard x-ray photoemission and density functional theory study of the internal electric field in SrTiO3/LaAlO3 oxide heterostructures

AU - Slooten, E.

AU - Zhong, Zhicheng

AU - Molegraaf, Hajo

AU - Eerkes, P.D.

AU - de Jong, S.

AU - Massee, F.

AU - van Heumen, E.

AU - Kruize, Michelle

AU - Wenderich, Sander

AU - Kleibeuker, J.E.

AU - Gorgoi, M.

AU - Hilgenkamp, Johannes W.M.

AU - Brinkman, Alexander

AU - Huijben, Mark

AU - Rijnders, Augustinus J.H.M.

AU - Blank, David H.A.

AU - Koster, Gertjan

AU - Kelly, Paul J.

AU - Golden, M.S.

PY - 2013

Y1 - 2013

N2 - A combined experimental and theoretical investigation of the electronic structure of the archetypal oxide heterointerface system LaAlO3 on SrTiO3 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 LaAlO3 layer. First, the Ti 2p core-level spectra clearly show occupation of Ti 3d states already for two unit cells of LaAlO3. Second, the LaAlO3 core levels were seen to shift to lower binding energy as the LaAlO3 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 LaAlO3 block. Both of these facts argue against the simple charge-transfer picture involving a cumulative shift of the LaAlO3 valence bands above the SrTiO3 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 LaAlO3 surface at the 25% level reverses the direction of the internal field in the LaAlO3. 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 LaAlO3 to near-interfacial states in the SrTiO3 substrate are active in heading off the incipient polarization catastrophe that drives the physics in these systems.

AB - A combined experimental and theoretical investigation of the electronic structure of the archetypal oxide heterointerface system LaAlO3 on SrTiO3 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 LaAlO3 layer. First, the Ti 2p core-level spectra clearly show occupation of Ti 3d states already for two unit cells of LaAlO3. Second, the LaAlO3 core levels were seen to shift to lower binding energy as the LaAlO3 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 LaAlO3 block. Both of these facts argue against the simple charge-transfer picture involving a cumulative shift of the LaAlO3 valence bands above the SrTiO3 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 LaAlO3 surface at the 25% level reverses the direction of the internal field in the LaAlO3. 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 LaAlO3 to near-interfacial states in the SrTiO3 substrate are active in heading off the incipient polarization catastrophe that drives the physics in these systems.

KW - IR-86894

KW - METIS-295996

U2 - 10.1103/PhysRevB.87.085128

DO - 10.1103/PhysRevB.87.085128

M3 - Article

VL - 87

SP - 085128/1-085128/11

JO - Physical review B: Condensed matter and materials physics

JF - Physical review B: Condensed matter and materials physics

SN - 1098-0121

IS - 8

ER -