$\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}R{45}^{\ensuremath{\circ}}$ surface reconstruction and electronic structure of ${\mathrm{BaSnO}}_{3}$ film

$\sqrt{2} \times \sqrt{2} R 45^{\circ}$ surface reconstruction and electronic structure of ${BaSnO}_{3}$ film

Shoresh Soltani, Sungyun Hong, Bongju Kim, Donghan Kim, Jong Keun Jung, Byungmin Sohn, Tae Won Noh, Kookrin Char, and Changyoung Kim

Phys. Rev. Materials 4, 055003 – Published 15 May 2020

Abstract

We studied the surface and electronic structures of barium stannate ( ${BaSnO}_{3}$ ) thin films by low-energy electron diffraction (LEED) and angle-resolved photoemission spectroscopy (ARPES) techniques. ${BaSnO}_{3} / {Ba}_{0.96} {La}_{0.04} {SnO}_{3} / {SrTiO}_{3}$ (10 nm/100 nm/0.5 mm) samples were grown using the pulsed-laser deposition (PLD) method and were ex situ transferred from the PLD chamber to ultrahigh vacuum (UHV) chambers for annealing, LEED, and ARPES studies. UHV annealing starting from $300^{\circ} C$ up to $550^{\circ} C$ , followed by LEED and ARPES measurements, show $1 \times 1$ surfaces with nondispersive energy-momentum bands. The $1 \times 1$ surface reconstructs into a $\sqrt{2} \times \sqrt{2} R 45^{\circ}$ one at an annealing temperature of $700^{\circ} C$ where the ARPES data show clear dispersive bands with a valence band maximum located around 3.3 eV below the Fermi level. While the $\sqrt{2} \times \sqrt{2} R 45^{\circ}$ surface reconstruction is stable under further UHV annealing, it is reversed to a $1 \times 1$ surface by annealing the sample in 400 mTorr oxygen at $600^{\circ} C$ . Another UHV annealing at $600^{\circ} C$ , followed by LEED and ARPES measurements, suggests that LEED $\sqrt{2} \times \sqrt{2} R 45^{\circ}$ surface reconstruction and ARPES dispersive bands are reproduced. Our results provide a better picture of the electronic structure of ${BaSnO}_{3}$ surfaces and are suggestive of the role of oxygen vacancies in reversible $\sqrt{2} \times \sqrt{2} R 45^{\circ}$ surface reconstruction.

Received 2 December 2019
Revised 22 March 2020
Accepted 27 April 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.055003

Physics Subject Headings (PhySH)

Electronic structure

3-dimensional systems Perovskites

Angle-resolved photoemission spectroscopy Annealing Crystal growth Laser ablation Low-energy electron diffraction X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Shoresh Soltani ^1,2,*, Sungyun Hong³, Bongju Kim², Donghan Kim ^2,3, Jong Keun Jung^2,3, Byungmin Sohn^2,3, Tae Won Noh^2,3, Kookrin Char³, and Changyoung Kim ^2,3,†

¹Institute of Physics and Applied Physics, Yonsei University, Seoul 03722, Republic of Korea
²Center for Correlated Electron Systems, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea
³Department of Physics and Astronomy, Seoul National University (SNU), Seoul 08826, Republic of Korea

^*Present address: SUPA, School of Physics and Astronomy, University of St. Andrews, St. Andrews KY16 9SS, United Kingdom.
^†changyoung@snu.ac.kr

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Vol. 4, Iss. 5 — May 2020

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Figure 1
(a) Perovskite cubic crystal structure of ${BaSnO}_{3}$ at room temperature. (b) Schematic of ${BaSnO}_{3} / {Ba}_{0.96} {La}_{0.04} {SnO}_{3} / {SrTiO}_{3}$ (10 nm/100 nm/0.5 mm) thin film grown by the PLD method (figure is not drawn to scale). (c) Reciprocal space mapping data of (103) peaks. The red dashed line shows the symmetry axis of the cubic crystal lattice. (d) An x-ray diffraction $2 θ$ scan shows the corresponding peaks for a BSO thin film (red triangles) and STO substrate (green squares). The inset shows the $ω$ -rocking curve data for the thin film (red) and the substrate (green). FWHM for the film is 0.06 while that for the substrate is 0.035. (e) XRD data in the range $42^{\circ} - 45 . 5^{\circ}$ clearly shows a fringe pattern for the film. $Δ θ$ is the angular distance between the fringe pattern peaks from which we estimate the thickness of the top BSO and BLSO buffer layers as 11.1 and 98 nm.
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Figure 2
90-eV LEED patterns of a ${BaSnO}_{3}$ thin film after annealing at (a) $400^{\circ} C$ and 1 atm oxygen with a $1 \times 1$ surface structure as shown by the dashed yellow square. (b)–(d) 300, 400, and $550^{\circ} C$ in the UHV condition, respectively, and (e) $700^{\circ} C$ ; the $1 \times 1$ surface reconstructs into a $\sqrt{2} \times \sqrt{2} R 45^{\circ}$ surface shown by the yellow square. (f) After annealing at $600^{\circ} C$ in 400 mTorr oxygen, $\sqrt{2} \times \sqrt{2} R 45^{\circ}$ disappears. (g)–(i) 600, 700, and $750^{\circ} C$ in the UHV condition. The $\sqrt{2} \times \sqrt{2} R 45^{\circ}$ reconstruction is reproduced as a result of a second UHV annealing. The sample was not exposed to air after the ex situ step in (f) (refer to the caption of Fig. 3). All UHV annealing data shown in (b)–(e) and (g)–(i) were taken without exposure to air.
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Figure 3
BSO film grown in a PLD chamber is ex situ transferred to a UHV chamber for surface studies (LEED and ARPES). (a)–(c) ARPES data before annealing, after UHV annealing at 300 and $400^{\circ} C$ . Afterwards, the sample is ex situ transferred to a tube furnace to be annealed at $400^{\circ} C$ and 1 atm flowing oxygen. (d) ARPES data taken after the sample is ex situ transferred back from the tube furnace to the ARPES chamber. (e)–(i) Sample annealed in the UHV condition starting from $300^{\circ} C$ up to $700^{\circ} C$ . Then, the sample is ex situ transferred to the PLD chamber to be annealed at $600^{\circ} C$ in 400 mTorr $O_{2}$ . (j) ARPES data taken after the sample was in situ transferred back from the PLD chamber to the ARPES chamber. (k) ARPES after annealing at $700^{\circ} C$ in UHV. All ARPES data in (a)–(k) were taken with 21.2 eV photons at low temperature (7–12 K) along $M - Γ - M$ . Electronic structure of the ${BaSnO}_{3}$ thin film taken along (l) $X - Γ - X$ and (m) $M - Γ - M$ shown with an inverse color scale of those presented in (k). (n), (o) Second derivative of the data presented in (l) and (m), respectively. (p) Momentum integrated energy distribution curves (EDCs) for $X - Γ - X$ (red) and $M - Γ - M$ (blue) directions. The inset shows EDCs just below the Fermi level with no sign of in-gap states. A Linear fit of leading edge shown by dashed line. (q) Extracted ARPES bands from momentum distribution curve (MDC) peak fitting. (r) DFT results for bulk reproduced from Ref. [24] with permission from ©2013 American Physical Society. (s) Probed location in the $k_{z}$ direction in the Brillouin zone (BZ). (t) Cubic BZ of ${BaSnO}_{3}$ . Red and blue lines show the high-symmetry cuts which our ARPES data were taken along. Samples were annealed in the preparation chamber at $750^{\circ} C$ for 1 h before an in situ transfer to the ARPES chamber. ARPES data were taken in a measurement temperature of 12 K and a photon energy of 21.2 eV.
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Physical Review Materials

2×2R45∘ surface reconstruction and electronic structure of BaSnO3 film

Shoresh Soltani, Sungyun Hong, Bongju Kim, Donghan Kim, Jong Keun Jung, Byungmin Sohn, Tae Won Noh, Kookrin Char, and Changyoung Kim

Phys. Rev. Materials 4, 055003 – Published 15 May 2020

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$\sqrt{2} \times \sqrt{2} R 45^{\circ}$ surface reconstruction and electronic structure of ${BaSnO}_{3}$ film