electrostatic charging and redox effects in oxide...
TRANSCRIPT
Electrostatic charging and redox effects in oxide heterostructures
Peter Littlewood1,2,3 Nick Bristowe3 & Emilio Artacho3,6
Miguel Pruneda4 and Massimiliano Stengel5
1Argonne National Laboratory 2University of Chicago
3University of Cambridge 4Centre d'Investigacion en Nanociencia i Nanotecnologia, Barcelona
5Institut de Ciencia de Materials, Barcelona 6CIC Nanogune, and DIPC, San Sebastian
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Bristowe et al., PRB 80, 45425 (2009); 83,20545 (2011); 85, 021406 (2012); JPCM 23, 081001 (2011); PRL 108, 166802 (2012)
• Transition metal oxides are responsible for some interesting and useful physics and materials science: magnetism, superconductivity, ferroelectricity, ...
• We use oxides to make batteries, capacitors, photovoltaics, ... • Physically, these are systems where the carriers are “small” (strongly
correlated). Strong correlations means high energy density and therefore “useful”
• Control of doping by chemistry is the usual route, and it’s challenging • In semiconductors, we have learned to modulation dope, and this has
been responsible for much modern semiconductor technology and essentially ALL of modern semiconductor physics
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Why are heterostructure oxides interesting?
• LaAlO3/SrTiO3 (polar-insulator) • Introduction: interface 2DEG, “polar catastrophe” argument • Net interface charges – formal argument • Model test – n-p superlattices • Origin of 2DEG in reality – surface redox?
• BaTiO3/La0.7Sr0.3MnO3 (ferroelectric-half metal) • Introduction: ferroelectric screening • Redox calculations • Magnetoelectric and tunneling electroresistance
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Outline
An example: LaAlO3 / SrTiO3
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Whence the carriers? Two (at least) points of view
Ideal interface is charged (polar catastrophe) produces electrical potential attracts (mobile?) carriers Charged interfaces may destabilise growth produces (neutralising) charged defects also generates carriers
A Ohtomo & H Hwang, Nature 427, 423 (2004) Interface between two band insulators is (sometimes) conducting
LaAlO3 / SrTiO3
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Sheet resistance of n-type SrTiO3/LaAlO3 interfaces. Temperature dependence of the sheet resistance for SrTiO3/LaAlO3 conducting interfaces, grown at various partial oxygen pressures A. Brinkman et al Nat.Mater. 2007, 6, 493
.
Ohtomo and Hwang Nature 427, 423 2004
Surface polar adsorbates • Carrier density and conductivity depends on details of
growth and also (reversibly) on surface adsorbates • AFM used to modulate buried interface layer conduction
e.g. Cen et al. Nature Mater. 7, 298-302 (2008).
• Carrier density at buried interface in LAO/STO is modulated by surface adsorption of polar molecules
Xie et al Arxiv1105:3891
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Counting charge and polarisation
• How to count charge at an interface? – Remember: dopants in a semiconductor, eg P (group V) in Si (group IV) – P adds 5 valence electrons and +5 charge to core – Compared with Si +1 impurity and 1 extra electron – in this case
weakly bound – In compound semiconductors, it is possible to separate the charge
from the impurity and capture it in a quantum well – modulation doping
– Is this the correct picture in oxides ?
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Modulation doping
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(a) Two semiconductors not in electrical contact
(b) In electrical contact, chemical potentials balance
(c) Donors added to wide gap material ionise and populate
interfacial electron layer
Capacitor Plate Model for STO/LAO
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σc is the chemical charge density – formal ionic charges
12x 12 superlattice of LaAlO3 / SrTiO3
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Bristowe et al PRB 80, 045425 (2009)
• Linear shift of potential with distance from interface • Quantitative agreement of DFT results with capacitor
plate model – including lattice relaxation – including a non-linear dielectric response P as function of electric
field and strain – NB contribution to P from ferroelectricity of strained SrTiO3
• But not with experiment .... see later
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.... science fiction interlude ....
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Self- modulation doping: Mobile charges in coupled valence and conduction bands
Once E > Egap /d , mobile (?) carriers appear at the two interfaces.
Beyond a critical spacing dc ~ 5 unit cells the charge transferred compensates the formal ionic charge
The potential difference between the interfaces is pinned approximately at the gap (owing to large DoS).
Carrier density grows with d but is generally quite small, for example
n12/12 = 0.073 e/unit cell In LAO/STO strong screening -> large effective
Bohr radius Potentially interesting Coulomb correlations –
Mott transition for excitons; superfluids and solids etc.
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e/cell
Ultracapacitor / Photovoltaic • Excitons: add electrons
and holes in pairs • Energy cost = Energy gap –
binding energy + interaction energy – Tuned by external bias ~ 0
• Capacitance is theoretically very large – Store one exciton/Bohr
radius (Mott density)
• Intrinsic photovoltaic – Enormous internal field ~
V/nm
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Energy per pair (in exciton Rydberg) as a function of density
Zhu et al, PRB 54, 13575 (1996)
... back to reality ....
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Not particularly consistent with experiments
Problems: • Strong dependence of carrier density on growth conditions
(particularly oxygen pressure) – more metallic when grown in lower oxygen pressures
• Model critical thickness (>6 u.c.) > observed ( <4 u.c.) • No mobile holes seen at surface • XPS measures no internal field • HAXPS see electrons at interface before onset of conduction Resolution? • Defects (bulk/surface) contribute to carriers ....
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Resolution ? • Several suggested mechanisms: (Review: Schlom & Mannhart 2010)
• STO oxygen vacancies • Ionic intermixing (La-Sr) • La doping • Hard to explain MIT of 4 u.c. consistently measured by
several groups using various growth techniques • None aid LAO screening • Surface redox reactions stabilised under growth conditions
by generating screening charge
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Bristowe et al, arXiv:1008:1951; Stephenson and Highland, arXiv:1101.0298
Surface redox reactions generated under growth conditions
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E.g. Oxygen vacancy – creates (double) donor Electrons transfer to interface Thermodynamically stable for a thick enough layer – transferred charges lower capacitive energy that grows with thickness Energy cost to create vacancy balanced against Coulomb
Surface donor states stabilised by Coulomb?
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Free energy cost to create vacancy balanced against Coulomb
Critical separation
n = defect density d = film thickness Defect creation energy Defect interactions
Electrostatic energy
Intrinsic charge density
Reconstructed charge density
Field energy if interface partially screened
Surface redox reactions and 2DEGs
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Bristowe et al. PRB 83, 205405 (2011)
• LaAlO3/SrTiO3 (polar-insulator) • Introduction: interface 2DEG, “polar catastrophe” argument • Net interface charges – formal argument • Model test – n-p superlattices • Origin of 2DEG in reality – surface redox?
• BaTiO3/La0.7Sr0.3MnO3 (ferroelectric-half metal) • Introduction: ferroelectric screening • Redox calculations • Magnetoelectric and tunneling electroresistance
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Outline
Surprising stability of domains in thin BTO films
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Source of screening • Conventional view : charged species and metallic
screening -“accidental” charged species • Redox reaction freeing charge to the interface ?
thermodynamic equilibrium established in writing/growth
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In context of ferroelectrics, see e.g. Stephenson and Highland, arXiv:1101.0298
Methods
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Bristowe et al PRB 85, 024106 (2012)
Atomic displacements
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• No polarisation in pristine film • Bulk P (up/down) stabilised by compensating charges
from (adatom/vacancy) [~1/2 per unit cell] – Not surprising since it slightly overscreens the bulk polarization
• Needs further work to establish equilibrium density • In addition
– Large magneto-electric effect – Large tunnelling electroresistance – See also Burton and Tsymbal Phys. Rev. B 82, 161407 (2010);
Phys. Rev. Lett. 106, 157203 (2011) – mechanism slightly different but principles the same
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Substantial magnetoelectric effect
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Electroresistance • Perfect screening of surface redox • Imperfect screening in LSMO • Interface dipole shifts with polarisation state
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Tunnelling electroresistance
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There are other mechanisms for TER and ME effects involving changed magnetic ground states of the LSMO (Tsymbal 2010/2011)
Electrochemistry?
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Spectroscopy of adatom/vacancy in manganites
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Bryant et al, Nat. Commun. 2:212 (2011)
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Bryant et al, Nat. Commun. 2:212 (2011)
Electrochemical Strain Microscopy on LAO/STO
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1V
30V
Apply a tip bias, and measure both the topographic changes and the elastic response – ramp up voltage in a sequence
Conclusions • Centrosymmetric materials can have a (non-switchable)
polarization e.g. LAO – The polarization is precisely half a quantum
• Heterostructures require polarization screening – Either by internal charge transfer “self modulation doping” – Or by charged defects
• Surface oxygen/vacancies plausible candidates? • Can we control surface electrochemistry with
nanoelectrodes? • Is O vacancy a double donor? – correlation effects?
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Collaborators: Nick Bristowe, Emilio Artacho, Miguel Pruneda, Massimiliano
Stengel, Sergei Kalinin
Discussions: Mineral Sciences group, G Catalan, J Scott, N Mathur, X
Moya, V Garcia, M Bibes, J Junquera, J Iniguez, P Ordejon, H Hwang, W Pickett, D Vanderbilt, F Schoofs, T Fix, M Blamire
Funding:
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