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High mobility conduction at (110) and (111) LaAlO3/SrTiO3 interfaces
Gervasi Herranz, Florencio Sánchez, Nico Dix, Mateusz Scigaj, Josep Fontcuberta
TL;DR
Only a limited set of SrTiO3 interfaces had been shown to host high-mobility two-dimensional electron liquids, leaving the phenomenon’s microscopic basis unresolved. This paper examines epitaxial and amorphous oxide interfaces on SrTiO3 and shows that conducting, high-mobility interfaces can also arise on multiple SrTiO3 surfaces and with distinct amorphous oxides.
Problem
2DEL behavior had been reported for only a limited set of SrTiO3 interfaces, leaving the broader interface conditions and microscopic origin open questions.
Method
The study investigates epitaxial and amorphous oxide layers deposited on SrTiO3 surfaces, including (110) interfaces and distinct amorphous oxides.
Results
Amorphous (110) LaAlO3/SrTiO3, SrTiO3/SrTiO3, and YSZ/SrTiO3 interfaces were conductive, while SrTiO3 surfaces supported metallic conductivity with low-temperature high carrier mobility.
Takeaways & Limitations
Conducting, high-mobility interfaces can be induced across a variety of SrTiO3 surfaces and oxide overlayers, broadening the scope for materials research and carrier-doping studies.
Takeaways & Limitations
A crude polar-discontinuity scenario is insufficient to account for the observed interface behavior.
Abstract
from arXiv · showhide
In recent years, striking discoveries have revealed that two-dimensional electron liquids (2DEL) confined at the interface between oxide band-insulators can be engineered to display a high mobility transport. The recognition that only few interfaces appear to suit hosting 2DEL is intriguing and challenges the understanding of these emerging properties not existing in bulk. Indeed, only the neutral TiO2 surface of (001)SrTiO3 has been shown to sustain 2DEL. We show that this restriction can be surpassed: (110) and (111) surfaces of SrTiO3 interfaced with epitaxial LaAlO3 layers, above a critical thickness, display 2DEL transport with mobilities similar to those of (001)SrTiO3. Moreover we show that epitaxial interfaces are not a prerequisite: conducting (110) interfaces with amorphous LaAlO3 and other oxides can also be prepared. These findings open a new perspective both for materials research and for elucidating the ultimate microscopic mechanism of carrier doping.
Introduction
LAO/STO interfaces are a rare platform for highly mobile two-dimensional electron liquids, historically associated mainly with (001) STO. This work shows that conductive, high-mobility interfaces also arise on (110) and (111) STO, including with amorphous oxide overlayers.
- Introduction: LAO/STO 2DELs are notable for confinement within a few unit cells and for reported superconducting and incipient magnetic behavior.The interface conductance was reported with mobilities up to a few thousands of cm2/Vs and sheet carrier densities slightly above 10^13 cm–2.
Results
LAO/STO interfaces on (110) and (111) SrTiO3 become metallic above orientation-dependent thickness thresholds, with room-temperature conductances comparable across crystallographic orientations. Transport then varies strongly with LAO thickness, including a bell-shaped mobility dependence for (110) interfaces.
- Electrical transport: At 7 (110) and 9 (111) LAO monolayers, interfaces abruptly become metallic, reaching Gxx ≈ 6.5 × 10–5 Ω–1 and Gxx ≈ 2 × 10–5 Ω–1, respectively, with above-threshold conductances similar across orientations.Below the thresholds, resistance exceeds 10 MΩ; the corresponding (001) threshold is about 4 monolayers.
- Carrier density and mobility: Above the (110) insulator-to-metal transition, the low-temperature sheet carrier density is nsheet (5K) ≈ 1013 – 1014 cm–2, comparable to (001)-oriented interfaces.The sheet carrier density increases slightly with temperature, similarly to (001)-oriented interfaces.
- Carrier density and mobility: μ5K rises from ≈680 cm2/Vs at 8 MLs to ≈2500 cm2/Vs at 10 MLs, then falls to ≈880 cm2/Vs at 14 MLs and ≈30 cm2/Vs at 18 MLs.The resulting mobility-versus-thickness dependence is bell-shaped.
Discussion
The results show that conducting, low-temperature high-mobility interfaces arise across SrTiO3 surfaces with different symmetry and polarity, using either crystalline or amorphous oxides. These findings challenge polar-discontinuity explanations and identify Ti 3d0 orbitals as common electron acceptors.
- Discussion: Metallicity in epitaxial and amorphous (110) STO interfaces cannot be explained by pure electrostatic effects from ideal polar atomic stacking.The findings instead point toward cation intermixing or oxygen depletion during growth, potentially related to energetic deposition or adatom oxygen affinity.
- Discussion: Conducting interfaces with low-temperature high mobility form on SrTiO3 surfaces of distinct symmetry and polarity, regardless of whether the adjoining oxide is crystalline or amorphous.The reported behavior spans polar and non-polar oxides and multiple interface structures.
- Discussion: Across the reported interfaces, Ti 3d0 empty orbitals act as fundamental electron acceptors through electronic or chemical effects at oxide interfaces or with vacuum.This common role provides the paper’s proposed unifying feature for the observed electron gases.
Methods
The study prepared (110)- and (111)-oriented STO samples with epitaxial or amorphous oxide films using controlled deposition and oxygen-cooling protocols. Surface morphology and temperature-dependent Hall measurements characterized the samples, revealing n-type conduction in conductive samples.
- Sample preparation: LAO films were grown by pulsed laser deposition at 850 ºC under PO2 = 10^-4 mbar, with (110)- and (111)-oriented STO substrates placed simultaneously.Growth was monitored by high-pressure RHEED, using a 248 nm laser at 1 Hz and approximately 26 mJ pulse energy.
- Sample preparation: The study prepared epitaxial LAO thicknesses spanning approximately 5–100 Å and 2.5–44 nominal monolayers for (110) and (111) interfaces, respectively.Ultrathin amorphous LAO, STO, and YSZ films were also deposited at 10^-4 mbar oxygen with nominal thicknesses of 4.8, 7.8, and 4.0 nm.
Additional information
The supplementary information documents interface structure, growth characterization, transport measurements, and amorphous-film morphology, while reporting no competing financial interests.
- Supplementary measurements: Supplementary transport and growth figures cover conductive-interface thickness thresholds, sheet resistance, carrier density, mobility, and RHEED/AFM characterization.The figures include critical-thickness conductance, temperature-dependent resistance, carrier density and mobility, and structural characterization for multiple interface orientations and overlayer materials.
- Additional disclosures: The supplementary materials include Supplementary Table S1 and declare that the authors have no competing financial interests.The paper also states that supplementary information accompanies the article online.
- Supplementary characterization: Supplementary characterization confirms high-quality (110) and (111) SrTiO3 surfaces and verifies the thickness of a 36-ML (110) LaAlO3 film.Supplementary Figure S1 shows Bragg spots and Kikuchi lines in RHEED patterns; Supplementary Figure S2 matches the measured film thickness to 36 MLs.
- Supplementary film structures: Supplementary data characterize (111) LaAlO3/SrTiO3 and amorphous LaAlO3, YSZ, and SrTiO3 films grown on (110) SrTiO3.Supplementary Figures S3 and S5 provide AFM/RHEED images and identify amorphous films through the absence of Bragg spots and the presence of a diffusion halo.