Investigation of LAO/STO heterointerfaces using light-ion ......Image: E. Olsson & N.Ljustina, Chalmers Nanoscience (MCC026), 2017-11-23 The LAO/STO interface A. Ohtomo & H. Hwang,

Nanoscience (MCC026), 2017-11-23

Nanoxide electronics

Alexey Kalabukhov

Quantum Device Physics Laboratory

MC2, room D515 [email protected]

mailto:[email protected]


Playing Lego with oxide materials:

G. Rijnders, D.H.A. Blank, Nature 433, 369 (2005)


Materials: Perovskite oxides

LaxSr1-xMnO3

Ferromagnetic

PbZryTi1-yO3

Ferroelectric

High-Tc superconductors

All materials belong to one

structural group –

Perovskites, ABO3

ReBa2Cu3O7-x

SrRuO3

Metallic

LaAlO3

Insulating


Emerging phenomena

Charge: Metal-

insulator

transitions

(FET, memory,

adaptive

electronics)

Strain:

Induced

ferroelectric

polarization,

1D domain

walls Spin:

Induced magnetic

order, magnetic

interactions at

interfaces

FET, memory

P. Zubko et al., Annu. Rev. Condens. Matter Phys. 2:141–65 (2011)

Spintronics

Multiferroics


Outline:

I: 2DEG between wide-

band gap insulators

II: Multiferroic materials


Structural properties

Perovskite: CaTiO3 (L. Perovsky, 1839), face-centered cubic lattice

A-site: Alkali metals, structural properties

B-site: transition metals, electro-magnetic properties

1)(2

75.0

OB

OAf

rr

rrt

Tolerance factor : A – O

B – O2

A – O Densest crystal

lattice!


Electronic properties

Oxygen atoms in octahedra act as point charges

d-orbitals in the B-site split into two groups: eg and t2g

B-O6 octahedron: B-O6 d-orbitals:

eg

t2g

t2g eg


“The value of seeing nothing”, J. Mannhart and D. G. Schlom, Science 430, p.620 (2004)

Example: SrTiO3

SrTiO3

Annealing in

vacuum at 900 C

SrTiO3-δ

transparent, wide

band gap insulator

(EG=3.2 eV)

Metallic,

superconducting

Tc ~ 0.3 K


Electrical doping in SrTiO3

Doping by oxygen vacancies: SrTiO3 -> SrTiO3-δ+ δVO2++ δ2e-

H.P.R.Frederikse, W.R.Hosler, Phys.Rev. 161, 822 (1967)

5-10% of oxygen

vacancies!


Oxide-MBE@ BNL PLD@ Chalmers Oxide-Sputter@

UUpsala

Pulsed Laser Deposition:

+ preserves stoichiometry

+ high degree of flexibility

+ high dynamic range

- Many control parameters

- Low purity

Molecular Beam Epitaxy:

+ ultimate control of

composition

+ High purity and crystal quality

- Expensive, not very flexible

- Oxygen stoichiometry

Magnetron sputtering:

+ low growth rate

+ smoother films

- Stoichiometry is not

preserved in general

- Limited to a narrow

range of oxygen

pressures

Thin film growth


PLD vs MBE

M.P. Warusawithana et al., Nature Comms. 4:2351 (2013)

Molecular Beam Epitaxy:

LaAlO3 on SrTiO3 interface

is metallic only when La/Al

ratio is below 0.97!

Pulsed Laser Deposition:

Films are not

stoichiometric, La/Al ratio

below 0.9

Smallest change in

composition results

in metal-insulator

transition!


Atomic layer-by-layer growth

G.J.H.M. Rijnders et al. Materials Science and Engineering B 56 (1998)

Pulsed Laser Deposition with atomic control: Reflection High Energy Electron

Diffraction (RHEED) intensity

oscillations:

atomic layer-by-layer growth


5 µ

m

2 µ

m

as-received BHF-etched Annealed in O2

flow, 950° C

Control of SrTiO3 surface

M.Kawasaki et al., Science 266, 1540(1994); G.Koster et al., Mat. Sci. Eng. B 56, 209(1998)

Mixed: TiO2+SrO TiO2 non-reconstructed TiO2 reconstructed


Surface termination and film growth

Non-terminated

TiO2-terminated

M.Kawasaki et al., Science 266, 1540 (1994)

Atomic Force

Microscope image

RHEED during

growth

No oscillations in

the beginning!

Layer-by-layer

growth


”Engineered” oxygen vacancies

D.A. Muller et al., Nature 430 (2004)

PLD-RHEED of oxygen

SrTiO3/SrTiO3-x superlattice:

High-resolution TEM: sharp

boundaries between layers

Oxygen vacancies are

unstable: clustering,

diffusion, etc…


Part I: Electrostatic carrier doping in oxide

interfaces.

2DEG at the LAO/STO interface


Electrostatic Carrier Doping

e

E

eS

Qn

ESdEd

SQ

d

VE

d

SC

VCQ

GD

GG

GG

G

02

00

0

Field effect experiment: Simple electrostatics:

Sheet carrier

concentration

Limited by breakdown

field: EBG

SiO2: ε = 3.9, EBG = 7.5 MV/cm,

n2D ≈ 16x1012 cm-2

SrTiO3: ε = 240, EBG = 1.5 MV/cm,

n2D ≈ 200x1012 cm-2

Sample

Gate

Insulator

Drain Source

VG

d


Thomas-Fermi screening

C. H. Ahn et al., Nature 424, 1015 (2003)

Thomas-Fermi model:

screening is about inter-atomic

distance in metals!

ne

aTF

2

0

4

In metals, carriers are doped only in few atomic layers close to the interface.

Oxides: low carrier densities -> higher tunability


Vg = 5.6 eV a = 3.87 Å

LaAlO3: SrTiO3:

Vg = 3.2 eV a = 3.91 Å

Both are wide band-gap insulators

High dielectric constant in STO (ε ~ 240 at 300 K)

Good lattice match – can probably make good interface!

SrTiO3 and LaAlO3


The LAO/STO interface

Pulsed Laser Deposition of ultra-

thin LaAlO3 film on SrTiO3

substrate:

A. Ohtomo & H. Hwang, Nature 427

(2004)

High crystalline quality

confirmed by electron

microscopy:

LaAlO3

SrTiO3

Image: E. Olsson & N.Ljustina,

Chalmers


The LAO/STO interface

A. Ohtomo & H. Hwang,

Nature 427 (2004) N. Reyren et al., Science 317 (2007) J. A. Bert et al., Nature Physics 7 (2011)

High electrical

conductivity and mobility:

Two-dimensional

superconductivity: Ferromagnetism:


Thickness of the electron gas

G. Herranz et al., Nature Materials 7 621 (2008)

Atomic force microscope (AFM) images with conducting tip: C-AFM

Experiment: C-AFM in

the perpendicular

direction:

ST

O

LA

O

C-AFM cross-section: thickness about 5 nm


QHE in the LaAlO3/SrTiO3 ”2DEG”

F. Trier et al. (2016)

http://arxiv.org/pdf/1603.02850

Unconventional Quantum Hall Effect:

∆𝑅𝑥𝑦−1 = 10 𝑒2 ℎ for B < 6T, ∆𝑅𝑥𝑦

−1 = 20 𝑒2 ℎ for B > 6 T.

Possible explanation: single quantum well with parallel subbands

Gariglio, Fête and Triscone, J. Phys.: Condens. Matter 27 (2015)

http://arxiv.org/pdf/1603.02850


LAO/STO and GaAs 2DEG’s

J. Mannhart and D. G. Schlom, Science 327, 1607 (2010)

GaAs: single quantum well generated by band bending

LAO/STO: multiple quantum wells due to electron correlations of the TiO6

orbitals


Lifshitz transition

Smink et al., PRL 118 2017; Joshua et al., Nature Comms. 2012

• Universal critical density nc ~ (2-3)x1013 cm-2, change in Fermi surface topology

(Lifshitz transition)

• ntot < nc : dxy band occupied, low mobility, magnetism

• ntot > nc : dyz,xz bands occupied, high mobility, superconductivity


N. Nakagawa et al., Nature Mat. 5, 204 (2006)

”p-type”: SrO-AlO2 ”n-type”: (TiO2-LaO)

Origin of the 2DEG in the LAO/STO

Stacked sequence of AB-BO2 layers in (001) direction

Non-polar in STO (SrO0-TiO20), polar in LAO (LaO1+-AlO2

1-)

The interface is polar


S. Thiel et al., Science 313, 1942 (2006)

Critical thickness effect

Abrupt insulator – metal transition at 4 unit cells of LAO film:

Insulating

Metallic


Defect mechanism

Ti-Al antisite defects:

compensate polar

discontinuity

Oxygen vacancies:

donate electrons and

create 2DEG at the

interface

L. Yu & Alex Zunger, Nature Communications 5, 2014

t < 4 uc: Ti<->Al

antisite defects

t > 4 uc: oxygen vacancies

on the LAO surface


Giant electric field effect

S. Thiel et al., Science 313, 1942 (2006)

Insulator – metal transition using electric field in 3 uc thick film

On/Off ratio about 107

Slow response – oxygen vacancies rather electrons!


C. Chen et al., Nature Materials 7, 298 (2008)

AFM Nano-lithography

Writing

Erasing

Insulating 3 uc

LAO

Conducting after

applying field

Linewidth 3 nm!

AFM tip is used to create conducting paths in insulating LAO/STO interface

LAO film thickness is slightly below 4 uc - close to MIT


AFM Nano-lithography

C. Chen et al., Science 323, 1026 (2009)

Tunneling junctions with 2 nm gap, FET three-terminal devices


Summary LAO/STO

• Novel 2DEG with unconventional electronic

properties

• Prototype of future oxide nano-electronics: devices

“beyond Moor’s law”

• Challenges: to increase electron mobility, fabrication on

other substrates, e.g. silicon.

• Superconductivity and magnetism: unconventional

pairing, topological superconductivity and quantum

computing


Part II: Magnetoelectric coupling in oxide

interfaces


Multiferroic materials

N. A. Spaldin, M. Fiebig, Science 309, 391 (2005)

Multiferroics: a combination of two (or more) “ferroic” properties:

Important case: magneto-electric coupling (hard drives, spintronics, tunable

electronic components).

Co-existence of ferroelectric and ferromagnetic properties in a single phase is

contradictory (FE are good insulators, FM are half-filled metals).

Ferroelectricity

Ferromagnetism Ferroelasticity


Ferroelectrics

Similar to ferromagnetics, second-order phase transition at T < TC

Double-well potential

energy as a function of

cation position

Important condition for

ferroelectric behavior:


Perovskite ferroelectrics

FE polarization stems from B-O displacements

Short range repulsions (electron clouds) – non-FE symmetric structure

Bonding considerations: B-O hybridization, controlled by A-cation size

B cation usually has d0 state - insulating

Conducting electrons screen electric fields – FE must be insulating


Ferromagnets

Second-order phase transition at T=Tc:

T < Tc T > Tc

Hysteresis loop

Nicola A. Hill J. Phys. Chem. B, Vol. 104, No. 29, 2000


Origin of ferromagnetism

• Magnetic dipoles moments of atoms line up below TC

• First five d electrons have parallel spins (minimizes exchange energy)

• Maximum moment for d5

Nicola A. Hill J. Phys. Chem. B, Vol. 104, No. 29, 2000

Perovskites: B-cation usually has d5 state - metallic


Multiferroic ME

Ferroelectricity: d0 B-cations, insulating

Ferromagnetism: d5 B-cations, half metallic

Only few magnetic ferroelectrics known!

Solutions: chemical doping, strain, interface interactions


Magnetoelectric effect

...),(

...),(

...2

1

2

1),(

0

0

000

iijjij

S

ii

jijjij

S

ii

jiijjiijjiiji

S

ii

S

i

EHMHEM

HEPHEP

HEMMEEHMEPFHEF

Landau theory, expansion of the free energy:

Magneto-electric coupling:

electric field to re-orient the magnetic polarization and vise versa

PHME

,

Linear ME effect

L. D. Landau and E. M. Lifshitz, “Electrodynamics of continuous media“ Pergamon, Oxford (1960).


Magnetoelectric effect

Upper bound for the linear ME coupling:

M

jj

E

ii ij

ME effect is strong only in materials with high electrical and magnetic susceptibility,

e.g. in multiferroics

Multiferroic ME

”Direct” coupling:

Ferroelectric <-> Ferromagnetic

Composite ME

”Indirect” coupling:

Ferroelectric

<-Piezoelectric->

Ferromagnetic

W.F. Brown R. M. Hornreich S. Shtrikman Phys. Rev. 168, 574–577 (1968)


Example I: strained EuTiO3, theory

C.J. Fennie, K.M. Rabe, PRL 97, 267602 (2006)

EuTiO3: Anti-ferromagnet TC ~ 5 K, paraelectric

When bi-axially strained: can be ferromagnetic and ferroelectric

(theory), see strain phase diagram:


Example I: strained EuTiO3,experiment

H. Lee et al., Nature 466 (2010)

Substrate consideration: Growth by Molecular Beam Epitaxy:

No available substrates to induce compressive strain

DyScO3: at the boundary of AFM-FM transition with tensile strain


Ferroelectic

below 250 K

Very weak

ferromagnet

below 5 K

Example I: strained EuTiO3,experiment Ferroelectric loops:

Magnetization v.s. temperature:

H. Lee et al., Nature 466 (2010)


Example II: BiFeO3/BiCrO3 superlattice

N. Ichikawa et al., Appl.Phys.Express 1, 101302 (2008)

BiFeO3: The only multiferroic material at room temperature, but it is anti-ferromagnetic

Can be ferromagnetic if Fe replaced by 50 % Cr (ordered!)

PLD of BiFeO3/BiCrO3

superlattice High-resolution TEM image


Example II: BiFeO3/BiCrO3 superlattice

N. Ichikawa et al., Appl.Phys.Express 1, 101302 (2008)

Weak ferromagnetic behavior: Piezoelectric properties by AFM:

Co-existence of weak ferromagnetism and weak ferroelectricity

Low break-down voltages, doping due to oxygen vacancies

Deviation from ordered periodicity in Cr/Fe sites


Example III: Interface coupling

R. Ramesh and N. Spaldin, Nature Materials 6, 21 (2007)

Magneto-electric device: coupling at the interface

Ferroelectric anti-ferromagnetic coupling: BiFeO3

Magnetization in FM layer is coupled to AFM by exchange bias mechanism

Application of electric field results in FM switching


Example III: Interface coupling

J.T. Heron et al., PRL 107, 217202 (2011)

Soft FM layer on the top of

BiFeO3 film:

Resistace v.s. magnetic field

in-plane direction:

Magnetoresistance reversal by 180° under applied electric field

Direct proof of magneto-electric coupling


Example IV: LuFeO3/LuFe2O4

LuFe2O4 – Ferrimagnetic, not ferroelectric

LuFeO3 – Ferroelectric, not ferromagnetic


LuFeO3/LuFe2O4

LuFe2O4 / LuFeO3 superlattice is

ferroelectric and ferromagnetic above

room temperature:

Only indirect evidence of magneto-electric

coupling:

How to measure ME

coupling directly?


Summary

Investigation of LAO/STO heterointerfaces using light-ion ......Image: E. Olsson & N.Ljustina, Chalmers Nanoscience (MCC026), 2017-11-23 The LAO/STO interface A. Ohtomo & H. Hwang,

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