Interface properties and built-in potential profile of a $\phantom{\rule{4pt}{0ex}}\mathrm{LaCr}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ superlattice determined by standing-wave excited photoemission spectroscopy

Interface properties and built-in potential profile of a $LaCr O_{3} / SrTi O_{3}$ superlattice determined by standing-wave excited photoemission spectroscopy

S.-C. Lin, C.-T. Kuo, R. B. Comes, J. E. Rault, J.-P. Rueff, S. Nemšák, A. Taleb, J. B. Kortright, J. Meyer-Ilse, E. Gullikson, P. V. Sushko, S. R. Spurgeon, M. Gehlmann, M. E. Bowden, L. Plucinski, S. A. Chambers, and C. S. Fadley

Phys. Rev. B 98, 165124 – Published 15 October 2018

Abstract

$LaCr O_{3} (LCO) / SrTi O_{3} (STO)$ heterojunctions are intriguing due to a polar discontinuity along $[001]$ , exhibiting two distinct and controllable charged interface structures [ ${(LaO)}^{+} / {(Ti O_{2})}^{0}$ and ${(SrO)}^{0} / {(Cr O_{2})}^{-}$ ] with induced polarization, and a resulting depth-dependent potential. In this study, we have used soft- and hard-x-ray standing-wave excited photoemission spectroscopy (SW-XPS) to quantitatively determine the elemental depth profile, interface properties, and depth distribution of the polarization-induced built-in potentials. We observe an alternating charged interface configuration: a positively charged ${(LaO)}^{+} / {(Ti O_{2})}^{0}$ intermediate layer at the $LC O_{top} / ST O_{bottom}$ interface and a negatively charged ${(SrO)}^{0} / {(Cr O_{2})}^{-}$ intermediate layer at the $ST O_{top} / LC O_{bottom}$ interface. Using core-level SW data, we have determined the depth distribution of species, including through the interfaces, and these results are in excellent agreement with scanning transmission electron microscopy and electron energy-loss spectroscopy mapping of local structure and composition. SW-XPS also enabled deconvolution of the LCO and STO contributions to the valence-band (VB) spectra. Using a two-step analytical approach involving first SW-induced core-level binding-energy shifts and then VB modeling, the variation in potential across the complete superlattice is determined in detail. This potential is in excellent agreement with density functional theory models, confirming this method as a generally useful tool for interface studies.

Received 5 July 2018

DOI:https://doi.org/10.1103/PhysRevB.98.165124

Physics Subject Headings (PhySH)

Electronic structure

Interfaces Superlattices

Photoemission spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

S.-C. Lin^1,2,*, C.-T. Kuo^1,2, R. B. Comes^3,4, J. E. Rault⁵, J.-P. Rueff⁵, S. Nemšák^6,7, A. Taleb⁵, J. B. Kortright², J. Meyer-Ilse², E. Gullikson², P. V. Sushko³, S. R. Spurgeon^3,8, M. Gehlmann^2,6, M. E. Bowden⁹, L. Plucinski⁶, S. A. Chambers³, and C. S. Fadley^1,2,†

¹Department of Physics, University of California Davis, Davis, California 95616, USA
²Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
³Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, USA
⁴Department of Physics, Auburn University, Auburn, Alabama 36849, USA
⁵Synchrotron SOLEIL, L'Orme des Merisiers, Saint-Aubin-BP48, 91192 Gif-sur-Yvette, France
⁶Peter Grünberg Institut PGI-6, Research Center Jülich, 52425 Jülich, Germany
⁷Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁸Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, USA
⁹Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, USA

^*shclin@ucdavis.edu
^†fadley@physics.ucdavis.edu

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Vol. 98, Iss. 16 — 15 October 2018

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Figure 1
(a) Schematic of the superlattice made up of 10 bilayers of LCO and STO, consisting of 5 unit cells of LCO, 17.6 Å thick, and 10 unit cells of STO, 39.2 Å thick, grown epitaxially on a Nb-doped STO(001) substrate. The two sources of standing-wave structure in the rocking curves are indicated: Bragg reflection from the multilayer with period $d_{M L}$ and Kiessig fringes associated with the full thickness of the multilayer stack $D_{M L}$ . (b) The x-ray absorption coefficient over the $La M_{5}$ edge. (c) The real (delta) and imaginary (beta) parts of the index of refraction, as derived by Kramers-Kronig analysis. To enhance the reflectivity and thus the strength of the standing-wave effect, two photon energies were chosen, below and above the $La M_{5}$ absorption maximum. The electric-field strength distribution derived from x-ray optics calculations at these two energies, (d) 829.7 eV and (e) 831.5 eV, as a function of sample depth and incidence angle. Note the significant shift in position between the two energies. The corresponding calculated photoemission yields with depth, (f) and (g), plotted on ${log}_{10}$ scales.
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Figure 2
(a) Experimental spectra and fitted components of the strongest core level for each atomic species in the LCO/STO superlattice at a photon energy of 829.7 eV. In several cases in (b) and (c), the intensities used are the sums of blue and green components in (a). Experimental (open circles) and yxro simulated (solid) rocking curves of representative elemental states at photon energies of (b) 829.7 eV and (c) 831.5 eV. The dashed vertical lines indicate the phase difference of the rocking curves. (d) As in(b) and (c) but for experimental and simulated rocking curves at a photon energy of 3.5 keV. Note that in the case of 3.5 keV, clear Bragg peaks and Kiessig fringes are visible in both experiment and theory.
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Figure 3
(a) The sample structure determined via fitting yxro simulations of both the hard- and soft-x-ray SW rocking curves in Figs. 2, (c) and 3. The models used for interface interdiffusion at the ${LCO}_{top} / {STO}_{bottom}$ and ${STO}_{bottom} / {LCO}_{top}$ interfaces and surface contamination layer are indicated below the main panel. (b) The separate depth profiles of major atomic species in the LCO/STO superlattice derived from yxro. (c) Corresponding principal component analysis-filtered STEM-EELS composition maps and a representative STEM-HAADF image of the LCO/STO superlattice. The color codes of EELS are yellow, red, blue, and green for the $Sr L_{23}, Ti L_{23}, Cr L_{23}$ , and $La M_{45}$ absorption edges, respectively. The HAADF is shown in grayscale.
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Figure 4
Experimental RCs for the superlattice valence-band spectra at photon energies of (a) 829.7 eV and (b) 831.5 eV. (c) Angle-integrated spectra for (a) and (b) (black curves) and corresponding decomposed LCO-like (red curves) and STO-like (blue curves), representing matrix-element-weighted densities of states. (d) Reference XPS valence-band spectra of bulk STO (single-crystal substrate) and thick-film LCO acquired with Al Kα (1486.6 eV).
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Figure 5
Experimental and simulated relative peak shifts for $Sr 3 d$ and $La 4 d$ core levels versus incidence angle at photon energies of (a) 829.7 eV and (b) 831.5 eV. Experimental valence-band decompositions, showing the contributions from the STO and LCO layers, and corresponding simulations using XPS reference spectra from bulk STO and thick-film LCO, at photon energies of (c) 829.7 eV and (d) 831.5 eV. (e), (f) SW-XPS-derived (turquoise curves) and DFT-calculated (PBEsol) depth-resolved valence-band maximum (black curves) for the top three layers of the LCO/STO superlattice. This SW-XPS-derived depth profile is determined by optimizing the simulations in (a)–(d). The DFT theoretical profile is calculated in (e) with $U_{eff} (Ti) 8.0 eV$ and $U_{eff} (Cr) = 3.0 eV$ to match the bulk band gaps in STO and LCO, and in (f) with $U_{eff} (Ti) 4.0 eV$ and $U_{eff} (Cr) = 1.5 eV$ , which yields the best fit to experiment.
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Physical Review B

covering condensed matter and materials physics

Interface properties and built-in potential profile of a $LaCr O_{3} / SrTi O_{3}$ superlattice determined by standing-wave excited photoemission spectroscopy

S.-C. Lin, C.-T. Kuo, R. B. Comes, J. E. Rault, J.-P. Rueff, S. Nemšák, A. Taleb, J. B. Kortright, J. Meyer-Ilse, E. Gullikson, P. V. Sushko, S. R. Spurgeon, M. Gehlmann, M. E. Bowden, L. Plucinski, S. A. Chambers, and C. S. Fadley

Phys. Rev. B 98, 165124 – Published 15 October 2018

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Physical Review B

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Interface properties and built-in potential profile of a LaCrO3/SrTiO3 superlattice determined by standing-wave excited photoemission spectroscopy

S.-C. Lin, C.-T. Kuo, R. B. Comes, J. E. Rault, J.-P. Rueff, S. Nemšák, A. Taleb, J. B. Kortright, J. Meyer-Ilse, E. Gullikson, P. V. Sushko, S. R. Spurgeon, M. Gehlmann, M. E. Bowden, L. Plucinski, S. A. Chambers, and C. S. Fadley

Phys. Rev. B 98, 165124 – Published 15 October 2018

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Interface properties and built-in potential profile of a $LaCr O_{3} / SrTi O_{3}$ superlattice determined by standing-wave excited photoemission spectroscopy