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
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
SRRHEED
Monitoring

Moving Edge

Ceramic Targets

Mask PatternPulsed Laser
Deposition

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#,+*東大院理
東京大学大学院理学研究科物理学専攻#JST-CREST
和達大樹、滝沢優、藤森淳+東大放射光連携研究機構
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

SR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。
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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。

INTRODUCTIONINTRODUCTION

Heterojunctions based on Controlling physical properties
Perovskite Oxides
using interface effects
eCharge

ABO3

Charge Transfer

Heterointerface
Spin

A’B’O3Ferroelectric FET

OrbitalSpin Exchange

Epitaxial Strain

Resistance RAMHigh Tc
CMRMI transition
T. Kanki et al., Appl. Phys.
Lett. 83, 4860 (2003).

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

SR
SR analysis
analysis forfor oxide
oxide heterointerface
heterointerface

PES(DOS, Core level)

MCD

Charge Transfer, ValenceBand diagram?
Spin

ChargeABO3
A’B’O3Spin Exchangee-SR

XAS(LD)

OrbitalEpitaxial Strain

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

Advantage
Advantage ofof SR-PES
SR-PES
Hard-XrayPES@SPring-8 (〜1000 A)☆Non-destructive☆Surface (Interface) Sensitive（5〜30Å）
Chemical Shift☆Direct Determination
DOSof Electronic States Band DiagramSRhνxAnalyzer

yHigh Directionality
High BrillianceHigh Resolution
Tunable Photon Energy

e100μmHigh-throughput
High-resolution

SpaceEnergyAngle

In-situ
In-situ PES
PES ++ Laser
Laser MBE
MBE systemsystem
Combinatorial
Laser MBE ChamberSR
Intensity (arb. units)

La0.6Sr0.4MnO3 thin filmhν = 600 eV
T = R.T.

105Binding Energy (eV)EF

Photoemission
Chamber
Prep. Cham.
LEED & Auger

Sample Entry

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

Combinatorial
Combinatorial Laser
Laser MBE
MBE Apparatus
ApparatusLaser Molecular Beam Epitaxy

CW Diode Laser
For Sample Heating

Substrate

O2
inlet

PulsedLaser
Plume

Ceramic
TargetRHEED Oscillation

Stepping Motor
For Mask& Sample

RHEED

Stepping Motor
for Target

Pulsed Nd: YAG
Laser for Ablation

Atomically-flat surface of La1-xSrxMnO3 thin films

0.4nm&Well-ordered Surface

100 nm・・・・

Combinatorial
Combinatorial Laser
Laser MBE
MBE Apparatus
Apparatus
CW Diode Laser
For Sample Heating

Laser Molecular Beam Epitaxy[001]

Substrate

O2
inlet

PulsedLaser
Plume

Ceramic
TargetStepping Motor
A’B’O3
For Mask& Sample

ABO3

ABO3RHEEDm
ABO3

RHEED Oscillation

Stepping Motor
for Target

・・・・

A’B’O3

Pulsed Nd: YAG
Laser for Ablation

ResonantResonant PES
PES Approach
ApproachResonant PES of LSMO (x=0.4)
La0.6Sr0.4MnO3 thin film
T = R. T.

660

IPES

660

LSFO
650

655

650

On
645

640

OffEF
15
10
5Binding Energy (eV)

645

640

Photon Energy (eV)Intensity (arb. units)

655

LSMOd
I = exp(-d/λ)hν = 644 eVOFSLInterface
Interface

LSMO
Intensity (arb. units)

Probing Electronic Structure
at the Interface (Mn 3d PDOS)RHEEDRHEED Pattern
Pattern && AFM
AFM Images
ImagesACB

AB

DC
1 MLLSMO
x=0.55D2 ML

LSMO x=0.4 (20 ML)
Nb-STO substrate

1 x 1 μm

LSFO x=0.4

ResonantResonant PES
PES ofof LSMO
LSMO atat Interface
InterfaceMn 2p-3d Resonant PES

Intensity (arb. units)t2g

Mn 2p-3d Resonancehν = 640 eVeg T = R.T.

A

B

CAB
C
D

D

LSFO x=0.4
2ML
1ML
LSMO
x=0.55

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

LSMO
x=0.4 (20ML)

LSFO

STO substrate
5432

1Binding Energy (eV)

EF-1

LSMO

ChargeTransfer

Spectral Evidence of Charge Transfer at LSMO/LSFO Interface
H. Kumigashira et al., Appl. Phys. Lett. 84, 5353 (2004).

XAS
XAS Spectra
Spectra ofof Interfacial
Interfacial LSFO
LSFO Layer
Layer
Fe-2p XAS spectra

LSMO (3ML)
LSFOLSMO (20ML)

Intensity (arb. units)

Fe 2p XAS
LSMO/LSFO(1ML)/LSMO
LSMO/LSFO(2ML)/LSMO

700705710715

720

Photon Energy (eV)725730Comparison
ComparisonofofFe-2p
Fe-2pXASXASspectra
spectrabetweenbetweenInterfacial
InterfacialLSFOLSFOlayer
layerandandLSFOLSFOfilms
films
Fe-2p XAS spectra

Intensity (arb. units)

LSFO film (x = 0.4)

LSMO /LSFO (2ML) /LSMO

Intensity (arb. units)

La1-xSrxFeO3 film

Fe 2p XAS

Fe 2p

the spin-orbit splittingof the Fe 2p core holex = 0.67
x = 0.4
x = 0.2
x=0

LSFO film (x = 0.2)

705

710

715

720

725

730

Photon Energy (eV)LSMO /LSFO (1ML) /LSMO
700

705

710

715

Photon Energy (eV)

720

725

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

Evidence of Charge Transferfrom LSMO to LSFO

Ti
Ti 2p
2p CoreCore Level
Level Spectra
Spectra atat Interfaces
Interfaces
Charge Transfer
STO

LSMO

TiO 2
SrO
TiO 2
SrO
MnO2
La0.6Sr0.4 O
MnO2
La0.6Sr0.4 O

Ti4+→Ti3+
C.T.
Mn3+→Mn4+

MnO 2
La 0.6 Sr
0.4
MnO 2
La 0.6 Sr
0.4
TiO 2
SrO
TiO 2
SrO

OO

Ti3+

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

Ti4+
K. Morikawa et al., Phys. Rev. B 54, 5446 (1996).

Robust Ti4+ states

Origin
Origin ofof Charge
Charge Transfer
Transfer
Difference of 3d levels
among transition metals

LSFO/LSMO
Fe4+→Fe3+O

Mn3+→Mn4+

OO

STO/LSMO
TiO 2
SrO
TiO 2
SrO
MnO 2
La 0.6 Sr
0.4
MnO 2
La 0.6 Sr
0.4

Ti 3dOTi4+
OMn 3dFe 3deg

et2g

Binding Energy

FeO 2
La 0.6 Sr
0.4
FeO 2
La 0.6 Sr
0.4
MnO 2
La 0.6 Sr
0.4
MnO 2
La 0.6 Sr
0.4

t2gt2gMn3+→Mn4+O

Electron-donor layer

eg

t2g0

t2g3eg0.6

t2g3eg1.6

Charge redistribution at interface

Interfacial Electronic Structure ofLaAlO3/ SrTiO3 Heterojunctions
LaAlO3 /TiO2-SrTiO3

LaAlO3 /SrO-SrTiO3

n-type (Metallic)p-type (lnsulating)
A. Ohtomo and H. Y. Hwang, Nature 427, 423 (‘04).

Origin of the Metallic Interface
2. Oxygen Vacancies

1. Charge Transfer
2Ti4+(d 0 ) + e- → Ti3+( d 1 )+Ti4+

0.5 e0.5 e-(Al3+O22-)(La3+O2-)+(Ti3.5+O22-)0.5(Sr2+O2-)0(Ti4+O22-)0N. Nakagawa et al.,Nature Mater. 5, 204(2006).

nλIPESz

O 2p Ti 3d
EF

LaAlO3nSrTiO3-δIPESλz

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

O 2p
EF

DOS at EF is different between the two scenarios.

Detection Limit of PES Mearuements
1. Charge Transfer(Confinement@interface region)
LaAlO3(wide distribution over STO)
LaAlO3nλSrTiO3-d

2. Oxygen Vacancies

nIPES λSrTiO3-d

IPES = exp(-d/λ)PES detection limitdO 2pdTi 3d
EFO 2pTi 3d
EFTi 3dEF

DOS at EF is different between the two scenarios.

Valence Band Spectra of n-type Interfaces
LaAlO3 / TiO2-SrTiO3

LaAlO3 / TiO2-SrTiO3

Valence Band

Near EFhν = 600 eV
6 ML

Intensity (arb. units)

6 ML
5 ML

5 ML

4 ML

4 ML

3 ML

3 ML

2 ML

2 ML

1 ML

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

1 ML

0 ML

(SrTiO3)

10PES spectra of LSTO

0 ML (SrTiO3)

864

2Binding Energy (eV)EFK. Yoshimatsu et al., Phys. Rev. Lett. 101, 026802 (08).

321EF

Binding Energy (eV)No detectable
Ti 3d DOS at EF

Numerical Simulations

LaAlO3 / TiO2-SrTiO3
Near EFConfinement at the interface(δ function)

Nz

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

Intensity (arb. units)

3+O 2-)(Al20.5 e(La3+O2-)+(Ti3.5+O22-)0.5(Sr2+O2-)0(Ti4+O22-)0

Simulations

6 ML

5 ML

4 ML

3 ML

2 ML

1 ML
0 ML (SrTiO3)

2.0

1.0EF
0.0-1.0

Binding Energy (eV)

La0.6Sr0.4TiO3 Near EF
On-resonance
Off-resonanceChemical stabilities of Ti4+ states in TiO2
irrespective of the neighboring donor LaO layer

Ti 3d
1EFLaAlO3 / TiO2-SrTiO3Intensity (arb. units)

Intensity (arb. units)23

Ti 2p → 3dResonant Spectra

On - Resonance
Off - Resonance

6 ML

LaAlO3 / TiO2-SrTiO3
6 ML4+

Ti 2p0 MLhν = 800 eV(TiO2-SrTiO3)

3+5 ML24 ML
32

1

E0FBinding Energy (eV)-110-1

Relative Binding Energy (eV)

-2Charge transfer doesn’t occur
at the LAO/STO interface.

Band Diagram of the n-type Interface
Valence band spectra

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

LaAlO3 / TiO2-SrTiO3

Valence Bandhν = 600 eV

CBM is located at
0.2 - 0.3 eV above EF

2.9 – 3.0 eV

SrTiO3

Interface

VBM

Intensity (arb. units)

5.6 eV

3.2 eV

Near EF
6 ML

6 ML

CBM
EF

LaAlO3 / TiO2-SrTiO3

5 ML

5 ML

4 ML

4 ML

3 ML

3 ML

2 ML

2 ML

1 ML

1 ML

0 ML

LaAlO3(SrTiO3)

100 ML (SrTiO3)
8642Binding Energy (eV)EF321EF

Binding Energy (eV)

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.)

Band Diagram of the n-type Interface
Ti 2p core level spectra
A Notched Structure

CBMEF

5.6 eV

3.2 eV

2.9 – 3.0 eVIntensity (arb. units)

n-type (metallic)
LaAlO3 / TiO2-SrTiO3
Ti 2p0 MLhν = 800 eV(TiO2-SrTiO3)

6 ML461460459458

Band bending of 0.20.3 eV to higher binding
energy from STO to themetallic interface.

457

Binding Energy (eV)VBM

LaAlO3

CBM in STO is nearly attained at EF

Relative Binding Energy (eV)

SrTiO3

Interface

-0.1
0.0

0.2-0.3 eV

0.1
0.2

n-type0.3
0.4

LaAlO3 / TiO2-SrTiO301234

5

LAO film thickness (ML)6

Band Diagram of LAO/STO Interface
Ti 2p core level spectra

CBMEF

5.6 eV

3.2 eV

Intensity (arb. units)

n-type (metallic)

2.9 – 3.0 eV

LaAlO3 / TiO2-SrTiO3p-type (insulating)
LaAlO3 / SrO-SrTiO3hν = 800 eVhν = 800 eVTi 2p(TiO2-SrTiO3)

461

460

459

458

Energy Shift of Ti 2p (eV)Interface

Band bending on the STO side is
responsible for the metallic states.

(SrO-SrTiO3)

461

457

460

459

458

Binding Energy (eV)457

LaAlO3 / SrO-SrTiO3-0.1

No peak shift at the p-type interface

0 ML

6 ML

Binding Energy (eV)

LaAlO3Ti 2p

6 MLVBM

SrTiO30 MLp-type

0.0
0.1
0.2n-type0.30.4LaAlO3 / TiO2-SrTiO3012345

LaAlO3 film thickness (ML)6Origin of Metallic Conductivity in LAO/STO
A Notched Structure

0.25 ± 0.07 eV

CBMEF

5.6 eV
2.9 – 3.0 eVEF

TiO2-terminated SrTiO3

LaAlO3

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

(AlO2)-(LaO)+(AlO2)-(LaO)+TiO2

LaAlO3SrO

Interface

SrTiO3TiO2Interface

VBM

SrO

3.2 eV-+e-- +e- +e ++++ - +-The metallic conductivity originate from the accumulation ofcarriers on the notched structure formed at the interface.

Origin of M-I Transition by Inserting SrOInsulating
0.25 ± 0.07 eVCBMEF

5.6 eV
2.9 – 3.0 eVEF

SrO-terminated SrTiO3 LaAlO3

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

(AlO2)(LaO)+(LaO)+(AlO2)-SrO

LaAlO3TiO2Interface

SrTiO3SrO

Interface

VBM

TiO2

3.2 eV-++++++- +- +The notched structure disappears by inserting SrO atomic
layer between LAO and STO.

Summary in LAO/STO Interface
We determined the band diagrams of LAO/STO heterojunctions
by using in situ photoemission spectroscopy.
We have found1. There is no detectable Ti 3d DOS at EF expected fromcharge transfer through the interface.
2. Owing to the band discontinuity of LAO/STO, a notched
structure is formed at the metallic interface.
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 theinterface on a short-range scale but from the accumulation ofcarrier on a long-range scale.

Conclusion
ConclusionSR

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「超構こ造」を用いた研究が主流になる。
、こまい

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。