スライド 1

LaVOx（x=3-4）薄膜の内殻光電子分光
発表者
堀田育志（Hwang研）
共同研究者の皆様
Ｈｗａｎｇ研
椋木康滋、須崎友文、 Harold Y. Hwang
藤森研
和達大樹、藤森淳
科研費基盤研究A
「単結晶薄膜化により物性を制御した強相関系遷移金属酸化物の電子構造の研究」研究会
２００５年６月１７日（金）１３時～１８時
東京大学柏キャンパス基盤棟２階複雑理工講義室
Crystal structure and energy diagram of LaVO3
Crystal structure
Energy diagram
Orthorhombic unit cell
La
V
O
dn+1
V 3d
dn-1
～4 eV
c
d002
O 2p
d110
b
a
d1-10
Perovskite unit cell
Mott gap Ueff : ～1 eV
⇒λ : ～1240nm
(Near-IR)
Direct observation of Mott
dnL gap by optical spectroscopy using near-IR
JPSJ 64, (1995) 2488.
Structural phase transition : 141K
Orthorhombic
a=5.555Å
b=5.553Å
c=7.849Å
→
Monoclinic
a=5.594Å
b=7.760Å
c=5.565Å
γ=90.125°
P. Bordet, et. al., J. Solid State Chem. 106, 253 (1993)
1/18
Motivation of this study
Energy diagram of interface between
Mott and band insulator
○
×
LaVO3/LaAlO3 superlattice
hν
e-
La 5d
e-
Ti 3d
3.2 eV
V 3d
5.6 eV
LaAlO3
SrTiO3
LaVO3
LaAlO3
Band
insulator
Mott
insulator
Band
insulator
Absorption intensity (arb. units)
O 2p
2e3.2 eV
2e-
LaVO3
5.6 eV
Thick LVO
?
Thin LVO
1
1.05
1.1
Energy (eV)
1.15
The QWs have sub band structures?
Quantum confinement?
2/18
SrTiO3/LaTiO3 superlattice
A. Ohtomo et al., Nature 419 (2002) 378
3/18
Purpose of XPS measurement
To confirm valence state of vanadium in LaVO3 layer
X-ray
ee-
La3+ Al3+ O2-3
La3+
V3+
O2-3
Interface vanadium
La3+ Al3+ O2-3
SrTiO3
LaVO3 layer embedded in aAlO3 layers maintains Mott gap?
Valence state of vanadium ion is modulated at the interface?
4/18
Experimental condition
Growth condition
Tg : 500～900℃
PO2 : 5×10-8～1×10-2 Torr
Target : LaVO4 polycrystals
Growth rate : ～1.5 nm/min
Thickness : 40 nm (100 u.c.)
Fluence : ～3 J/cm2 (4 Hz)
Pulsed Laser Deposition system
Lamp Heater
RHEED
Substrate
CCD
Camera
e-
Characterization
RHEED
XRD
AFM
XPS
Pulse Laser
Lens
Target
O2
TMP
RP
5/18
RHEED, XRD, and AFM measurement
0
5
10
15
20
30
▲ LaVO 3
STO(002)
(-222)
(-213)
(040)
(031)
STO(002)
40
<10-5 Torr
10-6 Torr
□
□
20
60
▲
▲
10
10-6 Torr
10-4 Torr
50
(040)
20
40
50
60
50
60
STO(002)
15
□
(220)
10
▲ LaVO 3
□ LaVO 4
30
□
(031)
5
20
□
>10-5 Torr
□
(220)
10
10-5 Torr
0
10-3 Torr
20
STO(001)
15
(110)
10
(110)
5
Torr
Intensity (arb. unit)
Intensity (arb. units)
0
□ LaVO 4
STO(001)
10-4
STO(001)
Sample structure : LaVO3(100ML)/SrTiO3 (100)
Growth temperature : Tg = 600℃
Oxygen partial pressure : PO2
▲
▲
10-7 Torr
0
5
10
15
Number of unit cells
20
10
20
30
40
2θ(deg.)
5μm
6/18
Growth phase diagram of LaVOx on SrTiO3
PO2 >
Torr → LaVO4 (x=4)
PO2 = 10-5 ～ 10-4 Torr
→ Competing phase
PO2 < 10-6 Torr → LaVO3 (x=3)
10-3
No structural phases corresponding to
V4+ (x=3.5)
The optimal condition for atomically flat
epitaxial LaVO3 films were obtained.
10-1
Oxygen partial pressure (Torr)
We mapped out the growth phase
diagram of LaVOx (x=3-4) film.
Phase diagram of LaVOx
10-2
LaVO4 (V5+ phase)
10-3
10-4
Competing phase
-5
10
10-6
LaVO3 (V3+ phase)
-7
10
10-8
500
600
700
800
900
Growth temperature (℃)
Optimal condition for 2D growth
7/18
Sample structure
Thick LaVO3 with cap
Quantum-well (QW) structure
LaAlO3 (3ML)
LaAlO3 (3ML)
LaVO3 (3ML?)
LaAlO3 (3ML)
LaVO3 (1ML?)
LaAlO3 (30ML)
LaAlO3 (30ML)
SrTiO3 (5ML)
SrTiO3 (5ML)
SrTiO3 (5ML)
SrTiO3 (100)
SrTiO3 (100)
SrTiO3 (100)
LaVO3 (50ML)
RHEED oscillation data
LVO
LAO
LAO
STO
LVO
LAO
STO
0
50
100
100
200
300
400
500
600
Deposition time (sec.)
700
800
900
1000
LAO
LVO
LAO
RHEED intensity (arb. units)
RHEED intensity (arb. units)
RHEED intensity (arb. units)
STO
RHEED oscillation data
0
100
100
200
300
400
Deposition time (sec.)
500
100
200
300
0
50
100
100
200
300
400
50
100
150
Deposition time (sec.)
8/18
Wide scan and core level spectra
LaAlO3(3 ML)/LaVO 3(50 ML)/SrTiO 3
h = 1253.6 eV
La 3d
T = R. T.
O 1s
O(KVV)
860
La(MNN)
800
600
850
840
830
Binding
Energy (eV)
LaAlO3(3 ML)/LaVO
3(50 ML)/SrTiO 3
La 4s
La 4d
C 1s La 4p
Al 2p
V 2p
1000
La 3d5/2
La 3d3/2
400
Binding Energy (eV)
200
0
Intensity (arb. units)
Intensity (arb. units)
Intensity (arb. units)
LaAlO3(3 ML)/LaVO 3(50 ML)/SrTiO 3
Al 2p
80
78
76
74
72
Binding Energy (eV)
70
The La 3d spectrum in LaAlO3 layer showed a good agreement with its in La2O3.
Ref.) Chem. Phys. 253 (2000) 27
The energy position of Al 2p peak was assigned to valence state of 3+.
9/18
Quantum-well structure
LaAlO
ML)/LaVO (x ML)/SrTiO (100)
[LaAlO
3(3
3]3ML/ [LaVO33]xML/ [LaAlO3]330ML/SrTiO3(100)
Intensity (arb. units)
50 ML Thick LVO
3 ML
1 ML Well structure
LaVO3 (bulk)
Bulk : PRB 53, 1161 (1996)
V5+ V4+
520
518
516
V3+
514
512
510
Escape depth (Å)
V 2p3/2
Escape depth for electron
This study
Energy of electrons (eV)
Binding Energy (eV)
The curve shape of the V 2p3/2 spectrum in the 3ML-LVO-QW sample showed
good agreement with the result for LaVO3 bulk treated by filling in vacuum.
10/18
Core level fitting
Core level fittings of the XPS spectra were performed to estimate the component
of V3+ state in the capped LaVO3 samples.
Fitting parameter
Binding energy (B-E)
Peak intensity (INT)
Lorentzian width (L-W)
Gaussian width (G-W)
Some parameters should be fixed to obtain
a objective fitting result for the spectra.
Peak fitting of V5+ spectra
LaVO 4/SrTiO3(100)
The V 2p3/2 peak from LaVO4 is assigned
as single component of V5+ state.
Intensity (arb. units)
experiment
fitting
V5+
523
522
521
520
519
518
Binding Energy (eV)
517
516
L-W
0.60 [Ref.]
G-W
1.23
Ref. J. Phys. Chem. Ref. Data, Vol.8, 329 (1979)
11/18
3+
V
Core level fitting
4+
V
experiment
fitting
Intensity (arb. units)
Intensity (arb. units)
experiment
fitting
4+
V
3+
V
5+
QW structure (３ML)
Thick
LaVO5163 film514（５０ML)
520
518
512
3+
V
4+
V
4+
V
518
3+
V
4+
V
3+
V
5+
516
514
520
512
5+
Intensity (arb. units)
Binding
Energy(eV)
(eV)
LaAlO3(3 ML)/LaVO
Binding
energy
3(50 ML)/SrTiO
3(100)
experiment
fitting
4+
3+
V
5+
V3+
V4+
B-E
515.13
516.68
518
516
514
512
INT
1.96
0.93
Binding
Energy
(eV)
LaVO3(50
ML)/SrTiO
(100)
3
L-W
0.60
0.60
experiment
fitting
G-W
1.87
1.87
AREA V4+ 0.46
0.22
520
516
514
512
520
518
V5+
517.53520
0.00
0.60
1.23
0
516
514
512
Binding Energy (eV)
Binding energy (eV)
experiment
fitting
4+
V
3+
V
(LAO)3/(LVO)3/(LAO)50/STO
V
Intensity (arb. units)
518
Binding energy (eV)
Table : Fitting parameter
V
V
Binding
Energy (eV)
LaVO3(50 ML)/SrTiO
3(100)
Intensity (arb. units)
518
512
experiment
fitting
V
520
No
cap514
516
Binding
Energy (eV)
LaVO
ML)/SrTiO
(100)
LaVO3(50
3(50ML)/SrTiO33(100)
experiment
fitting
Intensity (arb. units)
experiment
fitting
Binding
Energy (eV)
LaAlO3(3 ML)/LaVO
3(50 ML)/SrTiO3(100)
520
Intensity (arb. units)
LaAlO3(3 ML)/LaVO3(3 ML)/LaAlO3(50 ML)/SrTiO3(100)
Intensity (arb. units)
V
(LAO)3/(LVO)50/STO
V3+
5+
V
V4+
515.13
516.68
518
516
514
Binding
Energy
(eV)
1.77
1.23
0.60
0.60
1.87
1.87
0.36
0.25
(LVO)50/STO
V5+
V3+
V4+
V5+
517.53
512
0.36
0.60
1.23
0.07
515.13
0.98
0.60
1.87
0.28
516.68
1.82
0.60
1.87
0.53
517.53
0.11
0.60
1.23
0.03
3+
V
12/18
Vanadium core level spectra
QW structure (３ML)
1.0
V
V
3+
4+
+V
5+
0.8
AREA
V3+
V4+
V5+
0.68
0.32
0
Area ratio
0.32
0.6
Thick LaVO3 film (50ML)
0.4
AREA
V3+
V4+
V5+
0.53
0.37
0.10
0.47
0.2
No cap
0
3 ML
50 ML
no cap
The role of capping layer to preserve
V3+ is obviously shown.
AREA
V3+
V4+
V5+
0.33
0.63
0.04
0.67
The vanadium ion could access valence state of 3+ in the structure.
13/18
LaVO3 layer-thickness dependence
To change LVO thickness : [LaAlO3]3ML/ [LaVO3]xML/ [LaAlO3]30ML/SrTiO3(100)
RHEED intensity data
Surface morphology
5ML
0
1
0°
X-ray
4ML
e30°
0
1
e-
55°
3ML
70°
0
1
0
200
Deposition time (sec.)
300
90°
400
LA
O3
LV M L
Ox
ML
Sample structure
ep
100
ed
0
th
:1
0Å
1ML
e-
Es
ca
p
Normalized RHEED intensity
1
LA
O1
0M
L
STO
(100)
LAO
30ML
LVO
5ML
LAO LVO
10ML 5ML
LAO LVO LAO
10ML xML 3ML
14/18
ML dependence
LaAlO3(3 ML)/LaVO3(x ML)/LaAlO3
O1s
Intensity (arb. units)
A systematic variation was shown.
The V2p peak intensity increases with
increasing the LVO layer thickness
1 ML
2 ML
3 ML
4 ML
5 ML
The spectral shapes from 1M to 5ML show
a good agreement with each other.
LaAlO3(3 ML)/LaVO 3(50 ML)/SrTiO 3
V2p3/2
V2p1/2
540
535
530
525
520
515
/SrTiO3(100)
510
505
The 50ML sample shows more broad peak,
indicating the existence of V4+ and V5+.
Binding Energy (eV)
Normalized by the O1s peaks
LaAlO3(3 ML)/LaVO3(x ML)/LaAlO3
1 ML
2 ML
3 ML
4 ML
5 ML
1 ML
2 ML
3 ML
4 ML
5 ML
Intensity (arb. units)
Intensity (arb. units)
LaAlO3(3 ML)/LaVO3(x ML)/LaAlO3
Normalized by the spectral area
LaAlO3(3 ML)/LaVO 3(50 ML)/SrTiO 3
LaVO3(50ML)/
SrTiO3 (100)
V5+ V4+
518
V3+
516
514
Binding Energy (eV)
LaAlO3(3 ML)/LaVO 3(50 ML)/SrT
/SrTiO3(100)
V5+ V4+
512
510
518
516
V3+
Satellite (Kα5)
514
Binding Energy (eV)
512
510
15/18
Angular dependence
LaAlO3(3 ML)/LaVO3(3 ML)/LaAlO3
LaAlO3(3 ML)/LaVO3(5 ML)/LaAlO3
Intensity (arb. units)
55
70
518
516
514
o
o
Intensity (arb. units)
30
o
512
510
70
Binding Energy (eV)
516
514
512
o
o
o
512
30
Intensity (arb. units)
Intensity (arb. units)
55
514
70
o
510
LaAlO3(3 ML)/LaVO
Binding
Energy (eV)
3(1 ML)/LaAlO
3
30
516
55
518
Binding
Energy (eV)
LaAlO3(3 ML)/LaVO
3(2 ML)/LaAlO3
518
30
o
o
510
55
70
518
516
514
Binding Energy (eV)
512
o
o
o
510
16/18
Angular dependence
VO2
V2O5
LaVO4
XPS results indicate that it comes from
the capping layer region.
5 0
When LaVO3 growth was interrupted,
high-oxidized V phases could appear at
the surface.
→ However, no evidence!
Å
Ex.) TEM image of LVO/STO superlattice
STO
LaVO3
LVO
STO
Growth direction
VO2LaO+
AlO2-
Where did V4+ (or V5+) come from?
Observed by L. Fitting and D. A. Muller in Cornell Univ.
17/18
Conclusion
LaVO3の成膜条件の最適化
２次元結晶成長条件 : Tg = 600℃, PO2 = 10-6 Torr
LaAlO3/LaVO3（５～１ML)/LaAlO3 超格子のXPS測定
LaVO3 の膜厚に対してV 2p3/2ピーク形状の変化なし
→ LaVO3 １MLの場合でもV3+が主な価数
角度分解XPS測定
LaVO3 ５～１MLの試料で同様の傾向
→ 上部の界面近傍により価数の高いバナジウムイオンが存在
→ LaVO3 １MLの場合にも角度依存性あり
※上部の界面でより酸化されたバナジウム相が混在？
18/18

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