Microscopic origin of the mobility enhancement at a spinel/perovskite oxide heterointerface revealed by photoemission spectroscopy

Rapid Communication

Microscopic origin of the mobility enhancement at a spinel/perovskite oxide heterointerface revealed by photoemission spectroscopy

P. Schütz, D. V. Christensen, V. Borisov, F. Pfaff, P. Scheiderer, L. Dudy, M. Zapf, J. Gabel, Y. Z. Chen, N. Pryds, V. A. Rogalev, V. N. Strocov, C. Schlueter, T.-L. Lee, H. O. Jeschke, R. Valentí, M. Sing, and R. Claessen

Phys. Rev. B 96, 161409(R) – Published 27 October 2017

Abstract

The spinel/perovskite heterointerface $γ - {Al}_{2} O_{3} / {SrTiO}_{3}$ hosts a two-dimensional electron system (2DES) with electron mobilities exceeding those in its all-perovskite counterpart ${LaAlO}_{3} / {SrTiO}_{3}$ by more than an order of magnitude, despite the abundance of oxygen vacancies which act as electron donors as well as scattering sites. By means of resonant soft x-ray photoemission spectroscopy and ab initio calculations, we reveal the presence of a sharply localized type of oxygen vacancies at the very interface due to the local breaking of the perovskite symmetry. We explain the extraordinarily high mobilities by reduced scattering resulting from the preferential formation of interfacial oxygen vacancies and spatial separation of the resulting 2DES in deeper ${SrTiO}_{3}$ layers. Our findings comply with transport studies and pave the way towards defect engineering at interfaces of oxides with different crystal structures.

Received 5 April 2017
Revised 11 August 2017

DOI:https://doi.org/10.1103/PhysRevB.96.161409

Physics Subject Headings (PhySH)

Dopants Electrical conductivity Impurities Irradiation effects

Solid-solid interfaces Two-dimensional electron gas

Ab initio calculations Photoemission spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

P. Schütz¹, D. V. Christensen², V. Borisov³, F. Pfaff¹, P. Scheiderer¹, L. Dudy¹, M. Zapf¹, J. Gabel¹, Y. Z. Chen², N. Pryds², V. A. Rogalev^1,4, V. N. Strocov⁴, C. Schlueter⁵, T.-L. Lee⁵, H. O. Jeschke³, R. Valentí³, M. Sing¹, and R. Claessen¹

¹Physikalisches Institut and Röntgen Center for Complex Material Systems (RCCM), Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
²Department of Energy Conversion and Storage, Technical University of Denmark, DK-4000 Roskilde, Denmark
³Institute of Theoretical Physics, Goethe University Frankfurt am Main, D-60438 Frankfurt am Main, Germany
⁴Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen, Switzerland
⁵Diamond Light Source, Harwell Sciene and Innovation Campus, Oxfordshire OX11 0DE, United Kingdom

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 96, Iss. 16 — 15 October 2017

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
(a) Sheet resistivity ( $R_{S}$ ), carrier density ( $n_{S}$ ), and mobility ( $μ$ ) as a function of temperature $T$ of a $γ - {Al}_{2} O_{3} / {SrTiO}_{3}$ heterostructure before and after 6 months of storage at room temperature in a vacuum desiccator. (b) Photograph, electronic structure, and corresponding schematic photoemission spectrum of stoichiometric ${SrTiO}_{3}$ and (c) oxygen-deficient ${SrTiO}_{3 - δ}$ .
Reuse & Permissions
Figure 2
(a) Photoemission spectra of a $γ - {Al}_{2} O_{3} / {SrTiO}_{3}$ heterostructure upon tuning the photon energy across the Ti $L$ absorption edge. Two in-gap features, IGA and IGB, are distinguished by their shifted resonance photon energies. Inset: Comparison between the systems ${LaAlO}_{3} / {SrTiO}_{3 - δ}$ and $γ - {Al}_{2} O_{3} / {SrTiO}_{3 - δ}$ ( $h ν = 459.4 eV$ ). (b) Resonant Ti $3 d$ spectral weight upon oxygen dosing (schematically depicted in the inset). While IGB becomes quenched entirely, a finite IGA weight remains at maximum dosing. (c) Depth-resolved photoemission spectroscopy of the Ti $3 d$ in-gap states and the second-order light-induced Ti $2 p$ core level ( $h ν = 459.8 eV$ ). The spectra are normalized to the spectral weight between $E = - 0.5$ and $- 3.5 eV$ . Inset: Relative spectral weight as a function of photoemission angle. The strong IGA-to-IGB angle dependence evidences the sharp vertical separation of IGA and IGB.
Reuse & Permissions
Figure 3
(a) $GGA + U$ calculated Ti $3 d$ -projected density of states (PDOS) and real-space electron density of bulk ${SrTiO}_{3}$ with an isolated oxygen vacancy. The real-space charge map is shown for a (100) layer cutting the oxygen vacancy and obtained through energy integration of the IGA state. The experimentally observed peak is depicted as guide-to-the-eye curve [full width at half maximum $(FWHM) = 500 meV$ ]. (b) Local ionic coordination of bulklike (type A) and interfacial (type B) oxygen vacancies in an idealized, unrelaxed spinel/perovskite heterointerface and calculated (relative) formation enthalpy $Δ H$ for oxygen vacancies at and away from the interface. (c) Ti $3 d$ -PDOS and real-space electron density of the $γ - {Al}_{2} O_{3} / {SrTiO}_{3}$ interface with one oxygen vacancy in the ${TiO}_{2}$ layer. The real-space charge density is shown for the interfacial ${TiO}_{2}$ layer in direct proximity to the spinel film and obtained through energy integration of the resulting ${IGB}_{1}$ and ${IGB}_{2}$ state, respectively.
Reuse & Permissions
Figure 4
(a) Schematic comparison of oxygen vacancy distribution and energetics in ${LaAlO}_{3} / {SrTiO}_{3}$ and low-mobility $γ - {Al}_{2} O_{3} / {SrTiO}_{3}$ . Possible intrinsic doping mechanisms associated with the polar discontinuity have been omitted here. (b) Proposed mechanism for high-mobility conductivity in $γ - {Al}_{2} O_{3} / {SrTiO}_{3}$ heterostructures. The interfacial oxygen vacancies act as the dopant layer, which is spatially separated from the 2DES in the deeper-lying layers of ${SrTiO}_{3}$ , where ionized donor scattering becomes minimized. Gradual oxygen vacancy diffusion towards the interface during annealing or room-temperature storage changes the oxygen vacancy density from the depicted low- $μ$ into a high- $μ$ density distribution.
Reuse & Permissions

Physical Review B

covering condensed matter and materials physics