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)
LaAlO3 / TiO2-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)
LaAlO3 / TiO2-SrTiO3
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)
LaAlO3 / TiO2-SrTiO3
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
LaAlO3 / TiO2-SrTiO3
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
LaAlO3 / TiO2-SrTiO3
hν = 800 eV Ti 2p 0 ML
(TiO2-SrTiO3)
6 ML
Binding Energy (eV)
Inten
si ty (
a rb .
u nits
)
461 460 459 458 457
LaAlO3 / SrO-SrTiO3
hν = 800 eV Ti 2p 0 ML
(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 / TiO2-SrTiO3
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
K. Yoshimatsu et al., Phys. Rev. Lett. 101, 026802 (08).
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
K. Yoshimatsu et al., Phys. Rev. Lett. 101, 026802 (08).
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