Atomic and electronic surface structures of dopants in oxides: STM and XPS of Nb- and La-doped SrTiO${}_{3}$(001)

Atomic and electronic surface structures of dopants in oxides: STM and XPS of Nb- and La-doped SrTiO $_{3}$ (001)

Matthew S. J. Marshall, David T. Newell, David J. Payne, Russell G. Egdell, and Martin R. Castell

Phys. Rev. B 83, 035410 – Published 20 January 2011

Abstract

High resolution X-ray photoelectron spectroscopy (XPS) was performed on the (001) surfaces of Nb-doped and La-doped single crystals of SrTiO $_{3}$ . For the Nb-doped samples the XPS results demonstrate that the fraction of Ti $^{3 +}$ ions increases with increasing Nb dopant concentration. The Nb dopants are shown to be in a $5 +$ ionization state. Atomic resolution scanning tunneling microscopy (STM) images of the 0.7 at.% Nb-doped SrTiO $_{3}$ (001) surface show that Nb-dopant atoms can be imaged on the surface as bright four-point square clusters. No evidence of Nb segregation to the (001) surface was found under ultrahigh vacuum (UHV) annealing. However, UHV annealing of the 0.7 at.% La-doped SrTiO $_{3}$ (001) samples at 1400 $^{°} C$ for 1 h resulted in La surface segregation as determined by Auger electron spectroscopy. STM on these samples shows that the segregated La results in La-rich linear surface structures.

Received 9 September 2010

DOI:https://doi.org/10.1103/PhysRevB.83.035410

Authors & Affiliations

Matthew S. J. Marshall^1,*, David T. Newell¹, David J. Payne², Russell G. Egdell², and Martin R. Castell^1,†

¹Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom
²Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom

^*Present address: CRISP, Department of Applied Physics, Yale University, New Haven, CT.
^†martin.castell@materials.ox.ac.uk

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Vol. 83, Iss. 3 — 1 January 2011

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Images

Figure 1
An Al $K α$ core level XPS survey is shown for a 0.7 at.% La-doped SrTiO $_{3}$ sample in (a) with all the major peaks indicated. The Ti 2 $p$ peaks for 0.7 at.% Nb-doped, 6.0 at.% Nb-doped, and 0.7 at.% La-doped SrTiO $_{3}$ are shown in (b)–(d). The Ti $^{3 +}$ structure is shown in (b)–(d) expanded by a factor of 10 after subtraction of the Ti $^{4 +}$ peak. The Nb 3 $d$ peaks are shown in (e) and (f), and the La 3 $d$ peak is shown in (g). For each spectrum the doping level is indicated in the top left-hand corner of the box, and the electronic state that the peaks correspond to is indicated in the top right. The thin lines (colored red online) correspond to the fitted peaks over a Shirley background.Reuse & Permissions
Figure 2
STM images of 0.7 at.% Nb-doped SrTiO $_{3}$ covered with an SrO overlayer. (a) STM image showing two flat terraces separated by a step edge of SrTiO $_{3}$ (001) unit cell height (approximately 0.4 nm). Bright spots, thought to be due to the Nb dopants, are uniformly distributed across both terraces ( $69.5 \times 39.4$ nm $^{2}$ , $V_{s} =$ 0.8 V, $I_{t} =$ 0.1 nA). (b) An STM image in which the bright spots are visible as four-point clusters ( $23.1 \times 13$ nm $^{2}$ , $V_{s} =$ 1.0 V, $I_{t} =$ 0.3 nA). (c) STM image showing bright four-point clusters, where the $(1 \times 1)$ periodicity of the surrounding surface can be resolved ( $23.6 \times 13.5$ nm $^{2}$ , $V_{s} =$ 1.0 V, $I_{t} =$ 1.0 nA). Inset in (c) is an enlarged image of the top left corner of (c), in which the square unit cell of an ordered $(1 \times 1)$ domain is shown (4.3 $\times$ 4.3 nm $^{2}$ , $V_{s} =$ 1.0 V, $I_{t} =$ 1.0 nA).Reuse & Permissions
Figure 3
(a) STM image taken from a 0.7 at.% La-doped SrTiO $_{3}$ (001) sample showing a cluster of step edges and bright white nanoline features oriented along the [010] direction (170 $\times$ 170 nm $^{2}$ , $V_{s} =$ 1.0 V, $I_{t} =$ 0.3 nA). (b) A high resolution STM image showing the detail of a La nanoline (70 $\times$ 35 nm $^{2}$ , $V_{s} =$ 0.4 V, $I_{t} =$ 0.3 nA).Reuse & Permissions
Figure 4
Auger spectra of SrTiO $_{3}$ (001) obtained following differing surface preparation procedures. In (a) broad-range spectra are shown of UHV cleaved (spotted line), UHV annealed La-doped sample at 1300 $^{°} C$ (thin line), and UHV annealed La-doped sample at 1400 $^{°} C$ (thick line) decorated with the type of nanolines show in Fig. 3. The peaks are labeled relating to Ti (LMM) at 380 and 413 eV, O (KLL) at 492 and 511 eV, La (MNN) 625 eV, and Sr (LMM) at 1644 and 1711 eV. All spectra were normalized such that the value of each spectrum at 1100 eV in the featureless background region was set to the same value. Then, the spectra were offset from each other to facilitate viewing. A magnified region of the AES spectra around the La peak is shown in (b).Reuse & Permissions

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