In-gap electronic structure of LaAlO${}_{3}$-SrTiO${}_{3}$ heterointerfaces investigated by soft x-ray spectroscopy

In-gap electronic structure of LaAlO $_{3}$ -SrTiO $_{3}$ heterointerfaces investigated by soft x-ray spectroscopy

A. Koitzsch, J. Ocker, M. Knupfer, M. C. Dekker, K. Dörr, B. Büchner, and P. Hoffmann

Phys. Rev. B 84, 245121 – Published 16 December 2011

Abstract

We investigated LaAlO $_{3}$ -SrTiO $_{3}$ heterointerfaces grown either in oxygen-rich or -poor atmosphere by soft x-ray spectroscopy. Resonant photoemission across the Ti L $_{2, 3}$ absorption edge of the valence band and Ti 2 $p$ core-level spectroscopy directly monitor the impact of oxygen treatment upon the electronic structure. Two types of Ti $^{3 +}$ related charge carriers are identified. One is located at the Fermi energy and related to the filling of the SrTiO $_{3}$ conduction band. It appears for low oxygen pressure only. The other one is centered at $E_{B} \approx 1$ eV and independent of the oxygen pressure during growth. It is probably due to defects. The magnitude of both excitations is comparable. It is shown that low oxygen pressure is detrimental for the Ti-O bonding. Our results shed light on the nature of the charge carriers in the vicinity of the LaAlO $_{3}$ -SrTiO $_{3}$ interface.

Received 28 March 2011

DOI:https://doi.org/10.1103/PhysRevB.84.245121

Authors & Affiliations

A. Koitzsch¹, J. Ocker¹, M. Knupfer¹, M. C. Dekker¹, K. Dörr^1,*, B. Büchner¹, and P. Hoffmann²

¹Institute for Solid State Research, IFW-Dresden, P.O. Box 270116, DE-01171 Dresden, Germany
²Helmholtz-Zentrum Berlin, BESSY, Albert-Einstein-Str. 15, DE-12489 Berlin, Germany

^*Present address: Institute for Physics, MLU Halle-Wittenberg, Von-Danckelmann-Platz 3, 06120 Halle, Germany.

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Vol. 84, Iss. 24 — 15 December 2011

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Images

Figure 1
Sheet resistance of sample OP (grown in oxygen poor atmosphere).Reuse & Permissions
Figure 2
Valence band photoemission of LaAlO $_{3}$ -SrTiO $_{3}$ (sample OP, grown in oxygen poor atmosphere). (a) Ti 2 $p \to 3 d$ resonant photoemission valence band obtained by varying the photon energy through the Ti L $_{2, 3}$ absorption threshold. The grey area shows the enhancement of Ti 3 $d$ derived states by comparison with the off-resonant spectrum at $h ν = 440$ eV. (b) Difference spectra near $E_{F}$ obtained by subtracting the off resonant spectrum ( $h ν = 440$ eV) from the on-resonant spectra. Thick lines are obtained from the original data by moderate smoothing. (c) Ti L $_{2, 3}$ absorption edge taken in total electron yield mode compared to a multiplet calculation.[25] Color bars at the top indicate the photon energy of the valence band spectra. Inset shows a comparison of calculated L $_{2, 3}$ Ti $^{4 +}$ and Ti $^{3 +}$ absorption edges.Reuse & Permissions
Figure 3
Valence band photoemission of LaAlO $_{3}$ -SrTiO $_{3}$ (sample OR, grown in oxygen rich atmosphere). (a) Ti 2 $p \to 3 d$ resonant photoemission valence band obtained by varying the photon energy through the Ti L $_{2, 3}$ absorption threshold. The grey area shows the enhancement of Ti 3 $d$ derived states by comparison with off-resonant spectra at 440 eV. (b) Expanded view on the spectra shown in (a) near $E_{F}$ . Grey line corresponds to the off-resonant measurement. (c) Ti L $_{2, 3}$ absorption edges of samples OR and OP taken in total electron yield mode. Color bars indicate the photon energy for the valence band spectra. Red asterisks correspond to the integrated valence band between $E_{B} = 3$ and 9 eV for different photon energies (constant initial state scan, CIS). Inset: expanded view on the first two peaks.Reuse & Permissions
Figure 4
(a) Resonance enhancement of the valence band for $h ν = 458.3$ eV ( $t_{2 g}$ ) and 460.6 eV ( $e_{g}$ ) for samples OP and OR. The calculated density of Ti 3 $d$ states is reproduced from Fujii et al.[29] Shown is the difference between on and off resonant. (b) Upper part (blue spectra): comparison of low-energy spectra (uppermost curves) and resonance enhancement (difference on, off resonant) for $h ν = 458.3$ eV excitation energy ( $t_{2 g}$ ). Lower part (orange/red): the same for $h ν = 460.6$ eV ( $e_{g}$ ).Reuse & Permissions
Figure 5
Ti 2 $p$ spectra measured in normal emission geometry fitted with Voigt functions (see text). Ti $^{4 +}$ peaks are shown in green, the smaller Ti $^{3 +}$ peaks in blue. The overall fit is red. Background has been removed before fitting. Vertical dashed lines highlight the positions of the low-energy components.Reuse & Permissions
Figure 6
O 1 $s$ spectra measured in normal emission geometry with $h ν = 1200$ eV.Reuse & Permissions

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