その場放射光電子分光による...

SR RHEEDMonitoring

Pulsed LaserDeposition

Moving EdgeMask Pattern

Ceramic Targets

物構研シンポジウム’09放射光・中性子・ミュオンを用いた表面・界面科学の最前線2009年11月17-18日（17日（金））@つくば国際会議場エポカル

その場放射光電子分光による

強相関ヘテロ界面の電子状態の研究

その場放射光電子分光による

強相関ヘテロ界面の電子状態の研究

東京大学大学院工学系研究科JST-さきがけ

東大放射光連携研究機構

組頭広志

SR

COLLABORATORSCOLLABORATORS

東京大学大学院工学系研究科応用化学専攻吉松公平、安原隆太郎、簑原誠人

近松彰*、堀場弘司#,+、尾嶋正治#,+

東京大学大学院理学研究科物理学専攻和達大樹、滝沢優、藤森淳

高エネルギー加速器研究機構久保田正人、小野寛太#

東京大学物性研究所Mikk Lippmaa

東北大学金属材料研究所川崎雅司#

東京大学大学院新領域研究科鯉沼秀臣

*東大院理

#JST-CREST+東大放射光連携研究機構

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はじめにはじめに

1. 成膜技術（レーザーMBE法など）の発展により、原

子レベルで構造を制御した酸化物薄膜・超格子の

作製が可能となった。

2. 強相関酸化物においても、今後、物質設計の自由

度が高く、物性を自由自在に制御可能な「超構

造」を用いた研究が主流になる。

3. 放射光を用いた分光法は、酸化物薄膜・超格子研

究の強力な牽引力になる。

Heterojunctions based onPerovskite Oxides

INTRODUCTIONINTRODUCTION

A’B’O3

ABO3

Controlling physical properties using interface effects

Ferroelectric FET

T. Kanki et al., Appl. Phys. Lett. 83, 4860 (2003).

Resistance RAM

T. Fujii et al., Appl. Phys. Lett. 86, 012107 (2005).

Spin

Charge

Orbital

e-

Heterointerface

Epitaxial StrainSpin Exchange

Charge Transfer

High TcCMRMI transition

SR analysis for oxide heterointerfaceSR analysis for oxide heterointerface

Charge

Charge Transfer, ValenceBand diagram

Orbital

XAS(LD)

PES(DOS, Core level)

MCDA’B’O3

ABO3?Spin

SRe-

Epitaxial StrainSpin Exchange

界面数nmの領域（少ないボリューム）における電子・スピン・軌道状態を元素選択的に測定可能

☆Non-destructive☆Surface (Interface) Sensitive（5〜30Å）☆Direct Determination

of Electronic States

High Directionality

High Brilliance

High Resolution

Tunable Photon Energy

Advantage of SR-PESAdvantage of SR-PES

Chemical ShiftDOSBand Diagram

SRAnalyzerhν

e-High-throughput

High-resolution

SpaceEnergyAngle

100μm

yx

Hard-XrayPES@SPring-8 (〜1000 A)

SR Combinatorial Laser MBE Chamber

PhotoemissionChamber

LEED & Auger

Prep. Cham.

Sample Entry

In-situ PES + Laser MBE systemIn-situ PES + Laser MBE system

K. Horiba, HK et al., Rev. Sci. Instrum. 74, 3406 (2003).

Inte

nsity

(arb

. uni

ts)

Binding Energy (eV)

La0.6Sr0.4MnO3 thin film hν = 600 eV

T = R.T.

EF510

Combinatorial Laser MBE ApparatusCombinatorial Laser MBE ApparatusLaser Molecular Beam Epitaxy

Pulsed Nd: YAGLaser for Ablation

RHEED

CW Diode LaserFor Sample Heating

Stepping MotorFor Mask

& Sample

Stepping Motorfor Target

RHEED Oscillation

Plume

CeramicTarget

Substrate

O2inlet

PulsedLaser

100 nm

Atomically-flat surface of La1-xSrxMnO3 thin films

0.4nm

&Well-ordered Surface

Combinatorial Laser MBE ApparatusCombinatorial Laser MBE ApparatusLaser Molecular Beam Epitaxy

Pulsed Nd: YAGLaser for Ablation

RHEED

CW Diode LaserFor Sample Heating

Stepping MotorFor Mask

& Sample

Stepping Motorfor Target

RHEED Oscillation

Plume

CeramicTarget

Substrate

O2inlet

PulsedLaser

[001

]

m

ABO3

ABO3

ABO3

A’B’O3

A’B’O3

・・・・

・・・・

Resonant PES of LSMO (x=0.4)

LSMO

LSFO

I = exp(-d/λ)

IPES

d

LSMO

LSFO

InterfaceInterface

Probing Electronic Structureat the Interface (Mn 3d PDOS)

hν = 644 eV

Resonant PES ApproachResonant PES Approach

Intensity (arb. units)

660 660

655 655

650 650

645 645

640 640Ph

oton

Ene

rgy

(eV

)

Inte

nsity

(arb

. uni

ts)

Binding Energy (eV)10 515 EF

La0.6Sr0.4MnO3 thin filmT = R. T.

On

Off

RHEED Pattern & AFM ImagesRHEED Pattern & AFM Images

1 x 1 μm

LSFO x=0.4

Nb-STO substrate

LSMOx=0.55 LSMO x=0.4 (20 ML)

1 ML2 ML

A B C D

A B C D

Inte

nsity

(arb

. uni

ts)

Binding Energy (eV)

A B C D

T = R.T.

Mn 2p-3d Resonancehν = 640 eV

EF24 135 -1

STO substrate

LSMO x=0.55

LSMO x=0.4

LSFO x=0.4

1ML2ML

(20ML)

A B C D

LSMO

LSFOCharge

Transfer

Spectral Evidence of Charge Transfer at LSMO/LSFO Interface

Mn 2p-3d Resonant PES

Resonant PES of LSMO at InterfaceResonant PES of LSMO at Interface

t2geg

K. Horiba, H.K. et al., Phys. Rev. B 71, 155420 (‘05)

H. Kumigashira et al., Appl. Phys. Lett. 84, 5353 (2004).

LSMO (20ML)

LSFO

LSMO (3ML)

Inte

nsity

(arb

. uni

ts)

730725720715710705700

Photon Energy (eV)

Fe 2p XAS

LSMO/LSFO(1ML)/LSMO

LSMO/LSFO(2ML)/LSMO

Fe-2p XAS spectra

XAS Spectra of Interfacial LSFO LayerXAS Spectra of Interfacial LSFO Layer

Comparison of Fe-2p XAS spectrabetween Interfacial LSFO layer and LSFO films

Comparison of Fe-2p XAS spectrabetween Interfacial LSFO layer and LSFO films

Inte

nsity

(arb

. uni

ts)

730725720715710705

Photon Energy (eV)

La1-xSrxFeO3 film

x = 0.67

x = 0.4

x = 0.2

x = 0

Fe 2pthe spin-orbit splitting of the Fe 2p core hole

Evidence of Charge Transferfrom LSMO to LSFO

Fe-2p XAS spectraIn

tens

ity (a

rb. u

nits

)

725720715710705700

Photon Energy (eV)

Fe 2p XAS

LSMO /LSFO (1ML) /LSMO

LSFO film (x = 0.2)

LSFO film (x = 0.4)

LSMO /LSFO (2ML) /LSMO

H. Wadati, H.K. et al., Phys. Rev. B 71, 035108 (05).

Robust Ti4+ states

Ti3+

Ti4+

K. Morikawa et al., Phys. Rev. B 54, 5446 (1996).

C.T.

Mn3+→Mn4+

Ti4+→Ti3+TiO2

TiO2SrO

SrOMnO20.6 0.4

MnO2La Sr O0.6 0.4

La Sr O

Charge Transfer

Ti 2p Core Level Spectra at InterfacesTi 2p Core Level Spectra at Interfaces

H. Kumigashira et al., Appl. Phys. Lett. 88, 192504 (‘06).

1.5 eV

TiO 2

TiO 2SrO

SrO

MnO 2La Sr O0.6 0.4

MnO 2La Sr O0.6 0.4

STO

LSMO

Mn3+→Mn4+

Ti4+TiO 2TiO 2SrO

SrOMnO 2

La Sr O0.6 0.4MnO 2

La Sr O0.6 0.4

STO/LSMO

Difference of 3d levels among transition metalsFeO 2

FeO 2

MnO 2La Sr O0.6 0.4

MnO 2La Sr O0.6 0.4

La Sr O0.6 0.4

La Sr O0.6 0.4

Mn3+→Mn4+

Fe4+→Fe3+

LSFO/LSMO

Origin of Charge TransferOrigin of Charge Transfer

Mn 3d

t2g

t2g3eg1.6t2g3eg0.6t2g0

Bin

ding

Ene

rgy

Ti 3d Fe 3d

t2gt2g

egeg

e-

Electron-donor layer Charge redistribution at interface

Interfacial Electronic Structure ofLaAlO3/ SrTiO3 Heterojunctions

n-type (Metallic) p-type (lnsulating)

LaAlO3 /TiO2-SrTiO3 LaAlO3 /SrO-SrTiO3

A. Ohtomo and H. Y. Hwang, Nature 427, 423 (‘04).

Origin of the Metallic Interface

DOS at EF is different between the two scenarios.

2. Oxygen Vacancies

SrTiOSrTiO33--δδ

LaAlOLaAlO33

W. Simons et al.,Phys. Rev. Lett. 98,196802 (2007).

n

z

λIPES

O 2p

EF

1. Charge Transfer

2Ti4+(d 0 ) + e- → Ti3+( d 1 )+Ti4+

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

0.5 e-

0.5 e-

(Ti(Ti4+4+OO2222--))00(Sr(Sr2+2+OO22--))00

(Ti(Ti3.5+3.5+OO2222--))0.50.5--(La(La3+3+OO22--))++

λ

n

zIPES

O 2p Ti 3d

EF

(Al(Al3+3+OO2222--))--

Detection Limit of PES Mearuements2. Oxygen Vacancies

(wide distribution over STO)

SrTiOSrTiO33--dd

n

IPES

1. Charge Transfer(Confinement@interface region)

n

dIPES = exp(-d/λ)

O 2p Ti 3d

EF

SrTiOSrTiO33--dd

LaAlOLaAlO33LaAlOLaAlO33λ λ

O 2p

EF

Ti 3dTi 3d

EF

PES detection limit

DOS at EF is different between the two scenarios.

d

Inte

nsity

(arb

. uni

ts)

Binding Energy (eV)EF

0 ML(SrTiO3)

1 ML

2 ML

3 ML

4 ML

5 ML

6 ML

LaAlO3 / TiO2-SrTiO3Valence Band hν = 600 eV

246810Binding Energy (eV)

EF

0 ML (SrTiO3)

1 ML

2 ML

3 ML

4 ML

5 ML

6 ML

LaAlO3 / TiO2-SrTiO3

Near EF

123

No detectable Ti 3d DOS at EF

Valence Band Spectra of n-type InterfacesPES spectra of LSTO

T. Yoshida et al., Europhys. Lett. 59, 258 (‘02)

K. Yoshimatsu et al., Phys. Rev. Lett. 101, 026802 (08).

Numerical Simulations

The 3d electrons only exist at TiO2 layer adjacent to (LaO)+ layer.

0.5 e-

(Sr2+O2-)0(Ti3.5+O22-)0.5-

(Al3+O22-)-

(La3+O2-)+

(Ti4+O22-)0z

N

Inte

nsity

(arb

. uni

ts)

2.0 1.0 0.0 -1.0Binding Energy (eV)

1 ML

0 ML (SrTiO3)


EF

Near EF

2 ML

3 ML

4 ML

5 ML

6 ML SimulationsConfinement at the interface(δ function)

Inte

nsity

(arb

. uni

ts)

3 2 1 0 -1Binding Energy (eV)


6 ML

5 ML

4 ML

On - Resonance Off - Resonance

EF

La0.6Sr0.4TiO3On-resonanceOff-resonance

Near EF

EF

Ti 3d

123

Chemical stabilities of Ti4+ states in TiO2irrespective of the neighboring donor LaO layer

Ti 2p → 3d Resonant Spectra

Charge transfer doesn’t occur at the LAO/STO interface.

Inte

nsity

(arb

. uni

ts)

Relative Binding Energy (eV)1 0 -1 -2

LaAlO3 / TiO2-SrTiO3hν = 800 eV

Ti 2p

0 ML(TiO2-SrTiO3)

6 ML

2

4+3+

EF

VBM

CBM

3.2 eV 2.9 – 3.0 eV

SrTiO3

5.6 eVIn

terf

ace

LaAlO3

Band Diagram of the n-type Interface

Inten

sity (

arb.

units

)

Binding Energy (eV)EF

0 ML(SrTiO3)

1 ML

2 ML

3 ML

4 ML

5 ML

6 ML

LaAlO3 / TiO2-SrTiO3Valence Band hν = 600 eV

246810Binding Energy (eV)

EF

0 ML (SrTiO3)

1 ML

2 ML

3 ML

4 ML

5 ML

6 ML

LaAlO3 / TiO2-SrTiO3 Near EF

123

1. VBM of STO is located at 2.9-3.0 eV below EF.2. VBM is nearly continuous between STO and LAO.

(The leading edge of valence band structures is located at almost constant energy position.)

Eg(STO) = 3.2 eVEg(LAO) = 5.6 eV

Valence band spectra

CBM is located at 0.2 - 0.3 eV above EF

A Notched Structure

0.4

0.3

0.2

0.1

0.0

-0.1

Rel

ativ

e B

indi

ng E

nerg

y (e

V)

6543210 LAO film thickness (ML)


EF

VBM

CBM

3.2 eV 2.9 – 3.0 eV

SrTiO3

5.6 eVIn

terf

ace

LaAlO3

Band Diagram of the n-type Interface

CBM in STO is nearly attained at EF

Ti 2p core level spectra

461 460 459 458 457


hν = 800 eV Ti 2p 0 ML

(TiO2-SrTiO3)

6 ML

Binding Energy (eV)

In te n

s i ty (

a rb .

u nits

)

n-type (metallic)

n-type

Band bending of 0.2-0.3 eV to higher binding energy from STO to the metallic interface.

0.2-0.3 eV

EF

VBM

CBM

3.2 eV 2.9 – 3.0 eV

SrTiO3

5.6 eVIn

terf

ace

LaAlO3

Band Diagram of LAO/STO InterfaceTi 2p core level spectra

461 460 459 458 457



(TiO2-SrTiO3)

6 ML

Binding Energy (eV)

Inten

si ty (

a rb .

u nits

)

461 460 459 458 457

LaAlO3 / SrO-SrTiO3


(SrO-SrTiO3)

6 ML

Binding Energy (eV)

0.4

0.3

0.2

0.1

0.0

-0.1

Ener

gy S

hift o

f Ti 2

p (eV

)

6543210LaAlO3 film thickness (ML)


LaAlO3 / SrO-SrTiO3

n-type (metallic) p-type (insulating)

n-type

p-type

No peak shift at the p-type interfaceBand bending on the STO side is responsible for the metallic states.

EF

VBM

CBM

3.2 eV 2.9 – 3.0 eV

SrTiO3

5.6 eV

Inte

rfac

e

LaAlO3

EF

0.25 ± 0.07 eV

e-e-e-

TiO2-terminated SrTiO3

++++++

------

+ - + -

Interface

(LaO

)+

(AlO

2)-

(LaO

)+

(AlO

2)-

TiO

2

SrO

TiO

2

SrO

LaAlO3

Origin of Metallic Conductivity in LAO/STO

The metallic conductivity originate from the accumulation of carriers on the notched structure formed at the interface.

A Notched Structure


EF

0.25 ± 0.07 eV

SrO-terminated SrTiO3

++++++

------

Interface

+

(LaO

)+

-

(AlO

2)-

+

(LaO

)+

-

(AlO

2)-

TiO

2

SrO

TiO

2

SrO

LaAlO3

Origin of M-I Transition by Inserting SrO

The notched structure disappears by inserting SrO atomic layer between LAO and STO.

EF

VBM

CBM

3.2 eV 2.9 – 3.0 eV

SrTiO3

5.6 eV

Inte

rfac

e

LaAlO3

Insulating


Summary in LAO/STO InterfaceWe determined the band diagrams of LAO/STO heterojunctionsby using in situ photoemission spectroscopy.

1. There is no detectable Ti 3d DOS at EF expected from charge transfer through the interface.

2. Owing to the band discontinuity of LAO/STO, a notched structure is formed at the metallic interface.

We have found

3. The structure, however, is absent at the insulating interface.

The metallic states at the interface between band insulators LAO/STO originate not from charge transfer through the interface on a short-range scale but from the accumulation of carrier on a long-range scale.

SR

ConclusionConclusion

1. レーザーMBE法（PLD法）の発展により、原子レベ

ルで構造を制御した酸化物薄膜・超格子の作製が

可能となった。

2. 強相関酸化物においても、今後、物質設計の自由

度が高く、物性を自由自在に制御可能な「超構

造」を用いた研究が主流になる。

3. 放射光を用いた分光法は、酸化物薄膜・超格子研

究の強力な牽引力になる。

いま

、ここ

その場放射光電子分光による...

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