High-density two-dimensional electron system induced by oxygen vacancies in ZnO

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High-density two-dimensional electron system induced by oxygen vacancies in ZnO

T. C. Rödel, J. Dai, F. Fortuna, E. Frantzeskakis, P. Le Fèvre, F. Bertran, M. Kobayashi, R. Yukawa, T. Mitsuhashi, M. Kitamura, K. Horiba, H. Kumigashira, and A. F. Santander-Syro

Phys. Rev. Materials 2, 051601(R) – Published 14 May 2018

Abstract

We realize a two-dimensional electron system (2DES) in ZnO by simply depositing pure aluminum on its surface in ultrahigh vacuum and characterize its electronic structure by using angle-resolved photoemission spectroscopy. The aluminum oxidizes into alumina by creating oxygen vacancies that dope the bulk conduction band of ZnO and confine the electrons near its surface. The electron density of the 2DES is up to two orders of magnitude higher than those obtained in ZnO heterostructures. The 2DES shows two $s$ -type subbands, that we compare with the $d$ -like 2DESs in titanates, with clear signatures of many-body interactions that we analyze through a self-consistent extraction of the system self-energy and a modeling as a coupling of a two-dimensional Fermi liquid with a Debye distribution of phonons.

Received 18 February 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.2.051601

Physics Subject Headings (PhySH)

Electronic structure Semiconductors

Functional materials Surfaces Two-dimensional electron gas

Angle-resolved photoemission spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

T. C. Rödel^1,2,3, J. Dai¹, F. Fortuna¹, E. Frantzeskakis¹, P. Le Fèvre², F. Bertran², M. Kobayashi⁴, R. Yukawa⁴, T. Mitsuhashi⁴, M. Kitamura⁴, K. Horiba⁴, H. Kumigashira⁴, and A. F. Santander-Syro^1,*

¹CSNSM, Univ. Paris-Sud, CNRS/IN2P3, Université Paris-Saclay, 91405 Orsay Cedex, France
²Synchrotron SOLEIL, L'Orme des Merisiers, Saint-Aubin-BP48, 91192 Gif-sur-Yvette, France
³Laboratory for Photovoltaics, Physics and Material Science Research Unit, University of Luxembourg, L-4422 Belvaux, Luxembourg
⁴Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba 305-0801, Japan

^*andres.santander@csnsm.in2p3.fr

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Vol. 2, Iss. 5 — May 2018

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Figure 1
(a) Schematics of the mechanism of formation of a highly doped 2DES in ZnO. Pure aluminum is thermally evaporated at room temperature in UHV at the surface of ZnO. The Al becomes alumina by pumping oxygen from the ZnO, hereby producing a 2DES at the ${AlO}_{x} / ZnO$ interface. (b) Al $2 p$ core level, measured right after deposition on ZnO. Its binding energy corresponds to completely oxidized aluminum $({AlO}_{x})$ , while no peak of pure aluminum is observed. The inset shows the ${AlO}_{x}$ /ZnO interface and the 2DES obtained after deposition of Al on ZnO. (c) Comparison of the valence bands and near- $E_{F}$ spectra of bare ZnO and Al-capped ZnO (red and blue curves, respectively). The surface redox reaction after Al deposition modifies the O $2 p$ valence band and produces an intense quasiparticle peak at $E_{F}$ , corresponding to the 2DES. As no Fermi level was detected on the spectrum at the bare ZnO surface, its binding energies were calibrated with respect to the Zn $3 d$ peak of the ${AlO}_{x}$ /ZnO interface. Photoemission data in this and all other figures of this paper were measured at $T = 7$ K.
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Figure 2
(a) ARPES Fermi-surface map of the Al(2 Å)/ZnO interface in the O-terminated $[000 \bar{1}]$ plane, measured at $h ν = 88$ eV with linear-horizontal light polarization. Red lines indicate the edges of the in-plane Brillouin zone. (b) Energy-momentum ARPES intensity map around the bulk $Γ_{002}$ point along the in-plane $k_{〈 11 \bar{2} 0 〉}$ direction, measured at $h ν = 25$ eV with linear-horizontal light polarization. The red curve is the momentum distribution curve over $E_{F} \pm 5$ meV. The black vertical bars show the Fermi momenta $k_{F}^{i}$ and $k_{F}^{o}$ of the inner and outer subbands, respectively. The Fermi liquid is coupled to phonons with a characteristic Debye energy $ω_{D}$ shown by the horizontal, dashed black line. Photoemission data in this and all other figures of this paper were measured at $T = 7$ K.
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Figure 3
(a) EDCs of the ARPES dispersion map, Fig. 2, respectively over $Γ \pm 0.05 Å^{- 1}$ (upper curve) and $\pm k_{F}^{o} \pm 0.05 Å^{- 1}$ (mid and lower curve). The peaks near $E_{F}$ for both the inner and outer subbands show a peak-dip-hump structure, with the dip at an energy $ω_{D} = - 70$ meV. (b) Maxima of the EDC (blue circles) and MDC (orange circles) peaks for the outer subband of the 2DES the Al(2 Å)/ $ZnO (000 \bar{1})$ interface. Only data for the right branch $(k > 0)$ are shown. The continuous green curve is a cosine fit to the data representing the noninteracting electron dispersion of this subband. (c), (d) Experimental real and imaginary parts of the electron self-energy (red circles), and their Kramers–Kronig transforms (black crosses), for the right branch of the outer subband. The dark blue curves are simultaneous fits to $Σ_{1}$ and $Σ_{2}$ using a 2D Fermi-liquid + Debye model. The 2D Fermi-liquid and 2D Debye components of the fit are shown by the filled light blue and gray curves. Data were symmetrized with respect to $E_{F}$ , as required by Kramers–Kronig. Similar results are obtained by an analysis of the left branch of the outer subband (see supplementary material [45]). The inset in panel (b) shows the Eliashberg coupling function resulting from the 2D Debye model used.
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Figure 4
(a) Zoom over the energy-momentum ARPES map of Fig. 2. The continuous red curve is the cosine fit to the bare outer subband. The dashed red curve is the same fit up-shifted by 377 meV, until it matches the Fermi momenta of the inner subband, representing thus the bare inner subband. The black curve is the renormalized inner subband, obtained by adding the experimental $Σ_{1}$ extracted from the outer subband, Fig. 3, to the bare inner subband. (b) Simulation of the ARPES data, using the self-energy of the 2D Fermi-liquid + Debye model fit to the data plus a constant electronic scattering rate of approximately 160 meV, corresponding to $Σ_{2}$ at $E_{F}$ from Fig. 3. The calculated spectral function was multiplied by the Fermi–Dirac distribution and then convoluted with a Gaussian resolution function of $FWHM = 20$ meV.
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