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International Summer School on Surfaces and Interfaces in Correlated Oxiides, Vancouver, 29 Aug – 01 Sep 2011<br />
Photoelectron spectroscopy of functional oxides:<br />
Heterostructures and buried interfaces<br />
Ralph Claessen (U Würzburg, Germany)<br />
• Photoelectron spectroscopy (PES)<br />
• PES theory in a nutshell<br />
• PES with hard x-rays (HAXPES)<br />
• HAXPES of oxide heterostructures<br />
FOR<br />
1346
Heterostructures of functional oxides<br />
3d transition metal oxides<br />
strong coupling between charge/orbital/spin/lattice<br />
degrees of freedom lead to:<br />
- metal-insulator transitions<br />
- charge and orbital ordering<br />
- local magnetism (ferro, antiferro,…)<br />
- high-temperature superconductivity<br />
- collossal magnetoresistance<br />
- …<br />
Epitaxial heterostructures by <strong>MB</strong>E, PLD<br />
controlled interfaces, additional functionalities:<br />
- strain engineering<br />
- interfacial 2dim electron gas (2DEG)<br />
- electrostatic doping (by polarity or field effect)<br />
- artificial multiferroics<br />
- spin injection<br />
- …
Oxide heterostructures<br />
"The interface is the device"<br />
(H. Kroemer, Nobel lecture 2000)<br />
Want information on:<br />
• chemical composition<br />
• electronic structure<br />
• vertical depth profile<br />
�photoelectron spectroscopy (PES)<br />
with soft and hard x-rays
Photoelectron spectroscopy (PES)
sample<br />
Photoelectron spectroscopy (PES)<br />
spectrum<br />
hν<br />
E kin<br />
E kin = hν – E B - Φ 0<br />
measure kinetic energy<br />
distribution of photoelectrons
sample<br />
Photoelectron spectroscopy (PES)<br />
spectrum<br />
Chemistry (core levels):<br />
→ composition<br />
→ chemical bonding<br />
→ valencies<br />
Electronic structure (valence band):<br />
→ density of states<br />
→ band structure<br />
→ Fermi surface<br />
→ spectral function A < (k,E)
Intensity [a.u.]<br />
Core level spectroscopy: ESCA<br />
Electron Spectroscopy for Chemical Analysis<br />
400<br />
•Cu 2p<br />
•CuO<br />
600<br />
Bi 2Sr 2CaCu 2O 8+δ<br />
800<br />
O 1s<br />
Bi 4d<br />
hν = 1486.6 eV [Al - Kα]<br />
1000<br />
Ca 2p<br />
C 1s<br />
1200<br />
Kinetic Energy [eV]<br />
Bi 4f<br />
Sr 3d<br />
Bi 5d<br />
1400<br />
Intensity [a.u.]<br />
Intensity [a.u.]<br />
1470<br />
Bi 4f 5/2<br />
1310<br />
1320<br />
1480<br />
Bi 4f 7/2<br />
1330<br />
Kinetic Energy [eV]<br />
Fermi level<br />
1490<br />
Kinetic Energy [eV]<br />
1340<br />
1500<br />
courtesy of A.F. Santander-Syro
Core level spectroscopy: Chemical shift and valency<br />
Example: alkali metal doping of TiOCl<br />
Na<br />
valence change:<br />
Ti 3+ (3d 1 ) � Ti 2+ (3d 2 )<br />
doping x (%)<br />
Ti 3+<br />
Ti2p 3/2 (+ Na)<br />
Ti 2+<br />
462 460 458 456 454<br />
binding energy (eV)<br />
Na1s<br />
1080 1075 1070 1065<br />
binding energy (eV)<br />
37%<br />
32%<br />
23%<br />
15%<br />
10%<br />
4%<br />
PRL 106, 056403 (2011)
Valence band spectroscopy<br />
TiOCl<br />
k-integrated spectrum<br />
O 2p / Cl 3p<br />
Ti 3d<br />
PRB 72, 125127 (2005)
Valence band spectroscopy: ARPES<br />
Angle-Resolved PhotoElectron Spectroscopy<br />
� band structure and Fermi surface<br />
emission angle (i.e. momentum)<br />
energy<br />
courtesy T. Deveraux/A. Damascelli
PES instrumentation<br />
• rare gas discharge lamp (
PES theory in a nutshell:<br />
1) Independent electron approximation
PES theory: Independent electrons<br />
Time-dependent perturbation theory<br />
Unperturbed electron system:<br />
one-electron states with energy E<br />
ψ<br />
Perturbation:<br />
classical radiation field with vector potential<br />
� Fermi´s Golden Rule<br />
for the photoinduced transition rate from initial to final states:<br />
� � � 2<br />
ik<br />
⋅r<br />
�<br />
wi→ f ∝ ψ f A0e<br />
⋅ p ψ i δ ( E f − Ei<br />
− hν<br />
)<br />
Hence, the total photoelectron current is:<br />
IPES ( ε ) ∝ ∑ wi→<br />
f δ ( ε − E f )<br />
i,<br />
f<br />
� �<br />
A(<br />
r,<br />
t)<br />
=<br />
�<br />
A<br />
0<br />
e<br />
� �<br />
i(<br />
k ⋅r<br />
−2πνt<br />
)
PES theory: Independent electrons<br />
� � �<br />
wi→ f ∝ f 0<br />
i f i<br />
2<br />
ik<br />
⋅r<br />
�<br />
ψ A e ⋅ p ψ δ ( E − E − hν<br />
)<br />
final state:<br />
inverted LEED state<br />
(eigenstate of semi-infinite crystal)<br />
energy conservation<br />
initial state:<br />
Bloch wave or core level
PES theory: Independent electrons<br />
� � �<br />
wi→ f ∝ f 0<br />
i f i<br />
2<br />
ik<br />
⋅r<br />
�<br />
ψ A e ⋅ p ψ δ ( E − E − hν<br />
)<br />
final state: high-energy Bloch state of infinite crystal,<br />
inverted LEED steps state 2 and 3 incoherently decoupled<br />
(eigenstate of semi-infinite crystal)<br />
One-step model Three-step model<br />
courtesy<br />
A. Damascelli
PES theory: Independent electrons<br />
� � �<br />
wi→ f ∝ f 0<br />
i f i<br />
2<br />
ik<br />
⋅r<br />
�<br />
ψ A e ⋅ p ψ δ ( E − E − hν<br />
)<br />
transition matrix element<br />
If the radiadion field is only weakly modulated on atomic length scales,<br />
(i.e. k >> few Å), the photon momentum can be neglected in<br />
the transition matrix element:<br />
�<br />
λ = 2π<br />
k �<br />
� � � �<br />
ik<br />
⋅r<br />
�<br />
�<br />
f A0e<br />
⋅ p i ≈ f A0<br />
⋅ p i<br />
�<br />
∝ A0<br />
⋅ f<br />
�<br />
er<br />
i<br />
Examples:<br />
hν = 20 eV � λ ≈ 600 Å<br />
hν = 2000 eV � λ ≈ 6 Å<br />
Dipole approximation
PES theory: Independent electrons<br />
Dipole approximation and k-selection rule for Bloch states<br />
momentum conservation:<br />
�<br />
k<br />
f<br />
� � �<br />
= k + G + k<br />
i<br />
� ARPES<br />
photon<br />
only"vertical"<br />
transitions
Transition metal oxides: electronic correlations<br />
oxides of the 3d transition metals: M = Ti, V, … ,Ni, Cu<br />
basic building blocks: MO 6 octahedra (or other ligand shells)<br />
electronic configuration: O 2s 2 p 6 = [Ne]<br />
TM 3d n<br />
cubic perovskites perovskite-like anatas rutile spinel<br />
O 2-<br />
TM X+<br />
quasi-atomic,<br />
strongly localized<br />
� strong intraatomic Coulomb interaction<br />
and breakdown of independent electron approx.
PES theory in a nutshell:<br />
2) Many-body picture
hν<br />
Many-body effects in photoemission<br />
N interacting electrons:<br />
E kin<br />
Photoemission process:<br />
sudden removal of an electron from<br />
N-particle system<br />
"loss" of kinetic energy due to<br />
interaction-related excitation energy<br />
stored in the remaining N-1 electron<br />
system !
Fermi´s Golden Rule for N-particle states:<br />
I( 2<br />
N 0<br />
s<br />
with<br />
Ψ<br />
Ψ<br />
ε ) ∝ ∑ Ψ ˆ<br />
f , s ∆ Ψi,<br />
0 δ ( EN<br />
, s − E , − hν<br />
)<br />
i , 0<br />
f , s<br />
i=<br />
=<br />
Reinterpretation of Fermi´s Golden Rule<br />
=<br />
N,<br />
0<br />
�<br />
k , N<br />
N �<br />
∆ˆ � �<br />
= ∑ A(<br />
ri<br />
) ⋅ pi<br />
1<br />
−1,<br />
s<br />
=<br />
M<br />
if<br />
N-electron ground state of energy E N, 0 ("initial state")<br />
N-electron excited state of energy E N, s, ("final state")<br />
consisting of N-1 electrons in the solid and<br />
a free photoelectron of momentum k and<br />
energy ε<br />
�<br />
c<br />
+<br />
f<br />
c<br />
i<br />
in second quantization with suitable oneelectron<br />
basis<br />
one-particle matrix element
Electron removal spectrum<br />
Fermi´s Golden Rule for N-particle states:<br />
I( 2<br />
N 0<br />
s<br />
ε ) ∝ ∑<br />
Ψ ˆ<br />
f , s ∆ Ψi,<br />
0 δ ( EN<br />
, s − E , − hν<br />
)<br />
I(ε<br />
)<br />
a little bit of math<br />
and a few plausible assumptions<br />
(sudden approximation)<br />
The ARPES signal is directly proportional to the<br />
< 1<br />
single-particle spectral function A ( ω) = − ImG(<br />
ω)<br />
× f ( ω)<br />
probability of removing an electron<br />
at energy ω from the system<br />
π<br />
single-particle<br />
Green´s function
TiOCl<br />
Example: PES of the Mott insulator TiOCl<br />
O 2p / Cl 3p<br />
Ti 3d 1<br />
d 1 → d 0<br />
LHB<br />
U<br />
µ<br />
d 1 → d 2<br />
UHB<br />
spectral function A < (ω) (DMFT)<br />
ω
Photoemission probing depth:<br />
soft and hard x-ray PES
courtesy<br />
A. Damascelli<br />
Inelastic scattering of the photoelectron<br />
Three-step model<br />
Step 2: photoelectron transport to the surface<br />
� inelastic scattering with other electrons<br />
(excitation of e-h-pairs, plasmons)<br />
• generation of secondary electrons<br />
("inelastic background")<br />
intensity intrinsic spectrum<br />
incl. background<br />
E kin
courtesy<br />
A. Damascelli<br />
Inelastic scattering of the photoelectron<br />
Three-step model<br />
Step 2: photoelectron transport to the surface<br />
� inelastic scattering with other electrons<br />
(excitation of e-h-pairs, plasmons)<br />
• generation of secondary electrons<br />
("inelastic background")<br />
• loss of unscattered photoelectron current<br />
⇒ inelastic mean free path λ
Photoemission probing depth<br />
λ(E kin) "universal curve"<br />
"conventional" VUV/XUV-PES:<br />
surface sensitive on<br />
atomic length scale !<br />
hν<br />
λ(E kin)<br />
E kin<br />
hard x-ray PES = HAXPES<br />
soft x-ray PES (SX-PES)<br />
� probing depth (3λ) up to >10 nm<br />
� access to bulk, buried nanostructures, and<br />
interfaces<br />
� depth profiling of thin films
Transition metal oxides: Instability of polar surfaces<br />
Transition metal (TM) oxides form lattice of ionic charges<br />
� Classification of surfaces (Tasker):<br />
- surface charge Q<br />
- electrical dipole moment µ in repeat unit<br />
�<br />
TM X+<br />
µ = 0<br />
�<br />
µ = 0<br />
�<br />
Q = 0<br />
Q ≠ 0<br />
Q<br />
≠ 0<br />
O 2-<br />
O 2-<br />
TM X+<br />
µ ≠ 0<br />
�<br />
P. W. Tasker, J. Phys. C 12, 4977 (1979)
-σ<br />
+σ<br />
-σ<br />
+σ<br />
Transition metal oxides: Instability of polar surfaces<br />
type 3 surfaces are energetically unfavorable:<br />
charge field potential<br />
O 2-<br />
TM X+<br />
"polarization catastrophe"<br />
will be avoided by<br />
atomic/ionic/electronic<br />
surface reconstruction<br />
⇒ surface ≠ bulk
Transition metal oxides: Instability of polar surfaces<br />
Example: Fe 3O 4 (magnetite)<br />
8.2 Å<br />
different reconstructions<br />
of the (111) surface (STM)<br />
PRB 76, 075412 (2007)
Transition metal oxides: Instability of polar surfaces<br />
Example: Fe 3O 4 (magnetite)<br />
VUV-PES<br />
surface-sensitive<br />
Soft X-ray PES<br />
probing depth 2x larger<br />
EPL 70, 789 (2005)
Hˆ<br />
t<br />
kinetic energy,<br />
itinerancy<br />
= −t<br />
Surface effects in Mott-Hubbard-type oxides<br />
+<br />
∑c ∑<br />
iσ<br />
c jσ<br />
+ U ni↓ni<br />
↑<br />
i,<br />
j<br />
, σ<br />
U<br />
local Coulomb energy,<br />
localization<br />
i<br />
U/t<br />
spectral function (DMFT for n=1)
Surface effects in Mott-Hubbard-type oxides<br />
Example: CaVO 3<br />
surface<br />
"bulk"<br />
lower<br />
Hubbard band<br />
A. Sekiyama et al., PRL 2004<br />
quasiparticle<br />
peak<br />
U/t<br />
spectral function (DMFT for n=1)
Surface effects in Mott-Hubbard-type oxides<br />
Example: CaVO 3<br />
surface<br />
"bulk"<br />
lower<br />
Hubbard band<br />
A. Sekiyama et al., PRL 2004<br />
quasiparticle<br />
peak<br />
reduced atomic coordination @ surface:<br />
� stronger electron localization<br />
� smaller effective bandwidth<br />
Wsurf < Wbulk � surface stronger correlated:<br />
U / Wsurf >U / Wbulk
Photoemission probing depth<br />
λ(E kin) "universal curve"<br />
"conventional" VUV/XUV-PES:<br />
surface sensitive on<br />
atomic length scale !<br />
hν<br />
λ(E kin)<br />
E kin<br />
hard x-ray PES = HAXPES<br />
soft x-ray PES (SX-PES)<br />
� probing depth (3λ) up to >10 nm<br />
� access to bulk, buried nanostructures, and<br />
interfaces<br />
� depth profiling of thin films
HAXPES: drawbacks and caveats<br />
Non-negligible photon momentum<br />
hν = 6 keV � λ ≈ 2 Å, k phot ≈ 3 Å -1
HAXPES: drawbacks and caveats<br />
Non-negligible photon momentum<br />
• suppression of direct (k-conserving) transitions<br />
Debye-Waller factor for direct transitions<br />
W<br />
ARPES of W(110) @ hν = 870 eV<br />
Plucinski et al., PRB 78, 035108 (2008)<br />
hν = 6 keV � λ ≈ 2 Å, k phot ≈ 3 Å -1<br />
dir<br />
= exp α<br />
( 2<br />
− k T M )<br />
phot<br />
atom
HAXPES: drawbacks and caveats<br />
Non-negligible photon momentum<br />
• suppression of direct (k-conserving) transitions<br />
• atomic recoil effect<br />
photon-absorbing atom takes up recoil energy<br />
2 2<br />
Ekin = � k phot 2M<br />
at the expense of<br />
photoelectron energy,<br />
depending on atom mass and lattice stiffness<br />
hν = 6 keV � λ ≈ 2 Å, k phot ≈ 3 Å -1<br />
Y. Takata et al., PRB 75, 233404 (2007)
HAXPES: drawbacks and caveats<br />
Non-negligible photon momentum<br />
• suppression of direct (k-conserving) transitions<br />
• atomic recoil effect<br />
• quadrupolar contribution to transition matrix element<br />
� � � � �<br />
ik<br />
⋅r<br />
�<br />
f A e p i f A ( �<br />
ik<br />
r ) �<br />
0<br />
⋅ ≈ 0 1+<br />
⋅ ⋅ p i<br />
hν = 6 keV � λ ≈ 2 Å, k phot ≈ 3 Å -1
HAXPES: drawbacks and caveats<br />
Non-negligible photon momentum<br />
• suppression of direct (k-conserving) transitions<br />
• atomic recoil effect<br />
• quadrupolar contribution to transition matrix element<br />
Low photoemission signal<br />
• cross section for photoemission<br />
• electron analyzer transmission<br />
� need bright x-ray source…<br />
( ) 3 −<br />
σ ∝ hν<br />
−1<br />
t<br />
∝ Ekin<br />
hν = 6 keV � λ ≈ 2 Å, k phot ≈ 3 Å -1
HAXPES set-up @ PETRA III (DESY, Hamburg)<br />
other HAXPES instruments worldwide:<br />
- Spring-8, Japan (>4)<br />
- BESSY, Germany (HIKE)<br />
- ESRF, France (ID-9)<br />
- Soleil, France (under construction)<br />
- Diamond, UK (under construction)<br />
X-rays from<br />
PETRA III<br />
"High-resolution hard x-ray<br />
photoemission for materials<br />
science" (B<strong>MB</strong>F)<br />
• joint project with C. Felser (U<br />
Mainz) and W. Drube (DESY)<br />
• photon energy: 2.5…15 keV<br />
• energy resolution: 30 meV<br />
• linearly/circularly polarized xray<br />
radiation<br />
• commissioned in 2010<br />
• user operation since 2011
HAXPES of oxide heterostructures:<br />
(1) Fe 3O 4/GaAs
Epitaxial growth of Fe 3O 4/GaAs<br />
surface<br />
Fe 3O 4<br />
GaAs<br />
semimetallic ferromagnet<br />
(100% spin polarization @ EF) resistively matched to semiconductor<br />
Datta-Das spin transistor<br />
PRB 79, 233101 (2009)<br />
semiconductor with<br />
large spin diffusion<br />
length<br />
� Fe 3O 4 (magnetite), (RE,Sr)MnO 3, CrO 2, Heusler compounds, …
Epitaxial growth of Fe 3O 4/GaAs<br />
surface<br />
Fe 3O 4<br />
GaAs<br />
PRB 79, 233101 (2009)<br />
<strong>MB</strong>E growth of thin magnetite film:<br />
• epitaxial Fe deposition @ RT<br />
• postoxidation @ 600 - 800K / p(O2) = 10-5 mbar<br />
(10-30 min)<br />
� Fe valency?<br />
� mixed-valent Fe3O4 vs. (Fe2+ )O and (Fe 3+ ) 2O3 ?<br />
� chemical depth profile ?
Valence signatures in Fe 2p spectrum<br />
Fe 2O 3<br />
Fe 3O 4<br />
FeO<br />
Fe<br />
charge transfer satellites<br />
700 705 710 715 720 725 730 735 740 745 750<br />
binding energy (eV)<br />
2p 1/2 2p 3/2<br />
Fe 3+<br />
Fe 2+ /Fe 3+<br />
Fe 2+<br />
Fe 0
Depth profiling of Fe 3O 4/GaAs<br />
surface<br />
Fe 3O 4<br />
GaAs<br />
Fe 2p spectra<br />
PRB 79, 233101 (2009)<br />
interface<br />
surface
Depth profiling of Fe 3O 4/GaAs<br />
Tuning the information depth by variation of<br />
(1) photon energy, or (2) photoelectron escape angle<br />
mean free path<br />
energy<br />
λ eff = λ IMFP cos θ<br />
θ<br />
λ eff
Depth profiling of Fe 3O 4/GaAs<br />
surface<br />
Fe 3O 4<br />
GaAs<br />
Fe 2p spectra<br />
film: mixed-valent Fe 2+/3+<br />
PRB 79, 233101 (2009)<br />
interface<br />
surface<br />
interface: divalent and metallic Fe (O-deficient)
Depth profiling of Fe 3O 4/GaAs<br />
surface<br />
Fe 3O 4<br />
interface<br />
(Fe, FeO x, GaO x, AsO x)<br />
GaAs<br />
PRB 79, 233101 (2009)<br />
Fe 2p spectra As 2p 3/2 spectra<br />
film: mixed-valent Fe 2+/3+<br />
interface: divalent and metallic Fe (O-deficient)<br />
oxidized Ga,As
Validation by electron microscopy<br />
surface<br />
Fe 3O 4<br />
interface<br />
(Fe, FeO x, GaO x, AsO x)<br />
GaAs<br />
TEM<br />
STEM-EELS<br />
J. Verbeeck, H. Tian, and G. van Tendeloo, U Antwerp
Fe 3O 4<br />
ZnO<br />
Fe 3O 4/ZnO: An all-oxide structure<br />
film grown by reactive deposition<br />
in O 2-atmosphere (∼10 -6 mbar)<br />
HAXPES TEM<br />
APL 98, 012512 2011<br />
also PLD-grown contacts: R. Gross et al.
HAXPES of oxide heterostructures:<br />
(2) Interface 2DEG in LaAlO 3/SrTiO 3
LAO/STO heterostructures in a nutshell<br />
• epitaxial growth by PLD<br />
LaAlO 3<br />
∆=5.6eV<br />
SrTiO 3<br />
∆=3.2eV<br />
A. Ohtomo et al., Nature 419, 378 (2004)<br />
S. Thiel et al., Science 313, 1942 (2006)<br />
N. Reyren et al., Science 317, 1196 (2007)
LAO/STO heterostructures in a nutshell<br />
• epitaxial growth by PLD<br />
• both oxides: wide gap insulators<br />
• if LaAlO 3 film thicker than 3 unit cells (uc) :<br />
→ formation of a high-mobility 2DEG<br />
at the interface<br />
conductivity<br />
sheet carrier density (Hall)<br />
LaAlO 3<br />
∆=5.6eV<br />
2DEG<br />
SrTiO 3<br />
∆=3.2eV<br />
A. Ohtomo et al., Nature 419, 378 (2004)<br />
S. Thiel et al., Science 313, 1942 (2006)<br />
N. Reyren et al., Science 317, 1196 (2007)
LAO/STO heterostructures in a nutshell<br />
properties of the 2DEG:<br />
• tunable conductivity by electric gate field<br />
• superconducting below 200 mK<br />
• magnetoresistance<br />
• coexistence of s.c and magnetism /<br />
electronic phase separation<br />
� origin of 2DEG, threshold behavior ?<br />
LaAlO 3<br />
∆=5.6eV<br />
2DEG<br />
SrTiO 3<br />
∆=3.2eV<br />
A. Ohtomo et al., Nature 419, 378 (2004)<br />
S. Thiel et al., Science 313, 1942 (2006)<br />
N. Reyren et al., Science 317, 1196 (2007)
charge:<br />
-1<br />
+1<br />
-1<br />
+1<br />
-1<br />
+1<br />
0<br />
0<br />
0<br />
0<br />
-1/2<br />
+1<br />
-1<br />
∆q = -1/2 +1<br />
-1<br />
+1<br />
-1/2<br />
0<br />
0<br />
0<br />
Polar catastrophe and how to avoid it<br />
AlO 2<br />
LaO<br />
AlO 2<br />
LaO<br />
AlO 2<br />
LaO<br />
TiO2 SrO<br />
TiO2 SrO<br />
AlO 2<br />
LaO<br />
AlO 2<br />
LaO<br />
AlO 2<br />
LaO<br />
TiO2 SrO<br />
TiO2 SrO<br />
Nakagawa et al., Nature Mat. 5, 204 (2006)<br />
electrostatic energy increases<br />
linearly with thickness of<br />
polar film<br />
polar catastrophe<br />
charge reconstruction<br />
electronic or ionic<br />
0.5e - per layer unit cell<br />
� n 2D = 3.5×10 14 cm -2<br />
partial Ti 3d occupation<br />
� Ti 3.5 (d 0.5 ) = Ti 3+ /Ti 4+
HAXPES of LAO/STO heterostructures<br />
2p 1/2<br />
Ti 2p spectrum<br />
Ti 4+<br />
2p 3/2<br />
undoped SrTiO 3: |3d 0 > � Ti 4+<br />
Ti 3+<br />
doped LAO/STO interface: |3d 0 > + |3d 1 > � Ti 3+ /Ti 4+<br />
LaAlO 3<br />
2DEG<br />
SrTiO 3<br />
PRL 102, 176805 (2009)
Dependence on LAO overlayer thickness<br />
Ti 4+<br />
Ti 3+<br />
� interface charge density increases with LAO overlayer thickness<br />
� non-zero Ti d 1 signal already for 2uc sample (?)<br />
Ti 3+<br />
PRL 102, 176805 (2009)
Depth profiling by angle-resolved HAXPES<br />
e -<br />
θ<br />
e -<br />
� 2DEG thickness<br />
d<br />
� sheet carrier density<br />
PRL 102, 176805 (2009)
e -<br />
Quantitative analysis: 2DEG thickness<br />
θ<br />
e -<br />
d<br />
Sample 2 uc 4 uc 5 uc 6 uc<br />
d (uc*) 3 ± 1 1 ± 0.5 6 ± 2 8 ± 2<br />
*lattice constant of STO unit cell (uc) = <strong>3.8</strong> Å<br />
� interface thickness < 3 nm<br />
consistent with<br />
- CT-AFM Basletic et al. (2008)<br />
- TEM-EELS Nakagawa et al. (2006)<br />
- density functional theory Pentcheva et al. (2009)<br />
- 2D superconductivity Reyren et al. (2007)<br />
- ellipsometry Dubroka et al. (2010)<br />
PRL 102, 176805 (2009)
Quantitative analysis: sheet carrier density<br />
Sample 2 uc 4 uc 5 uc 6 uc<br />
n 2D (10 13 cm -2 ) 2.1 3.9 8.1 11.1<br />
el. reconstruction<br />
35<br />
� n 2D > Hall effect data<br />
PRL 102, 176805 (2009)
Ti 3d<br />
photon<br />
in<br />
Ti 2p<br />
PRB 82, 241405(R) (2010)<br />
RIXS on LAO/STO<br />
Ti 3+ (3d 1 )<br />
e g<br />
t 2g<br />
photon<br />
out<br />
RIXS e g-excitation as fct. of # LAO-overlayers
PRB 82, 241405(R) (2010)<br />
Sheet carrier density: HAXPES, RIXS & Hall effect<br />
• n 2D much smaller than<br />
expected for purely electronic<br />
reconstruction (35 x 10 13 cm -2 )<br />
• n 2D higher than Hall effect data<br />
• photo-generated carriers<br />
cannot fully account for<br />
observed excess<br />
• remaining excess due to<br />
additional localized Ti 3d<br />
electrons?<br />
(cf. DFT - Popovic et al., PRL 2008)
LAO/STO: Valence band spectroscopy with HAXPES<br />
O2p-derived<br />
vb states<br />
~3 eV<br />
LaAlO 3<br />
2DEG<br />
SrTiO 3<br />
Ti 3d electrons should be here,<br />
but HAXPES cross-section too small !<br />
(theor. estimate: 10 -4 of O2p emission)
E<br />
Band situation from density-functional theory<br />
STO LAO<br />
CBM<br />
VBM<br />
2DEG<br />
core levels<br />
surface<br />
E F<br />
Yu Lin et al., arXiv 0904.1636 (2009)<br />
Pentcheva and Pickett, PRL 102, 107602 (2009)
E<br />
STO LAO<br />
CBM<br />
VBM<br />
Band situation from density-functional theory<br />
2DEG<br />
core levels<br />
e - surface<br />
E F<br />
e -<br />
holes<br />
@ LAO VBM<br />
interface<br />
electrons<br />
@ STO CBM<br />
Yu Lin et al., arXiv 0904.1636 (2009)<br />
Pentcheva and Pickett, PRL 102, 107602 (2009)
E<br />
STO LAO<br />
CBM<br />
Band situation from density-functional theory<br />
VBM<br />
2DEG<br />
core levels<br />
e - surface<br />
E<br />
E F<br />
e -<br />
holes<br />
@ LAO VBM<br />
electrons<br />
@ STO CBM<br />
Yu Lin et al., arXiv 0904.1636 (2009)<br />
Pentcheva and Pickett, PRL 102, 107602 (2009)
Results from HAXPES<br />
valence band<br />
~3 eV<br />
VBM: ~ 3 eV below E F<br />
Al 1s core level<br />
same width for all samples!
E<br />
band theory versus experiment<br />
STO LAO<br />
CBM<br />
VBM<br />
2DEG<br />
core levels<br />
e - surface<br />
E F<br />
STO LAO<br />
also observed by Segal et al.,<br />
PRB 80, 241107(R) (2009)
Valence band offsets<br />
band alignment<br />
CB<br />
VB<br />
STO LAO STO LAO<br />
type I type II<br />
• VBM LAO above VBM STO<br />
• type II interface<br />
(valence band offset: 0.35 ± 0.1eV)<br />
• confirmed by core level analysis<br />
valence band analysis<br />
0.35eV
DFT band theory:<br />
Photoemission:<br />
Band alignment: A possible scenario<br />
STO LAO<br />
localized hole states<br />
induced by surface<br />
O-vacancies<br />
interface states (itinerant and localized)
HAXPES of oxide heterostructures:<br />
(3) LaVO 3/SrTiO 3 – electrostatic doping of a<br />
Mott a insulator
LaAlO 3<br />
band ins.<br />
∆=5.6eV<br />
q2DEG<br />
SrTiO 3<br />
band ins.<br />
∆=3.2eV<br />
Electrostatic doping of a Mott insulator<br />
LAO/STO<br />
polar<br />
… …<br />
(AlO 2) -<br />
(LaO) +<br />
(TiO 2) 0<br />
(SrO) 0<br />
non-polar<br />
Idea:<br />
replace Al3+ LaVO3 Mott ins.<br />
∆≈1 eV<br />
by<br />
trivalent ??? transition metal<br />
SrTiO<br />
� LaVO3 3<br />
band ins.<br />
∆=3.2eV<br />
LVO/STO<br />
Ohtomo/Hwang, Nature 427, 423 (2004) Hotta et al., PRL 99, 236805 (2007)
LaVO 3<br />
Mott ins.<br />
∆≈1 eV<br />
???<br />
SrTiO 3<br />
band ins.<br />
∆=3.2eV<br />
Electrostatic doping of a Mott insulator<br />
LVO/STO<br />
LaVO 3: - valence configuration V 3+ (d 2 )<br />
- polar oxide<br />
- Mott insulator (∆ LVO
LVO/STO: Sample growth and characterization<br />
RHEED pattern AFM image<br />
RHEED oscillations<br />
pulsed laser deposition<br />
STEM image<br />
interface
LVO/STO: metal-insulator transition in transport<br />
� metal-insulator transition for n-type interface<br />
� p-type interface insulating<br />
� critical thickness: ∼ 9 uc LVO (Hotta et al.: 5 uc)<br />
� high carrier mobility
HAXPES of LVO/STO: V 2p depth profiles<br />
insulating conducting<br />
6 uc LVO<br />
STO<br />
homogeneous<br />
"V 3+ " profile<br />
10 uc LVO<br />
STO<br />
extra electronic<br />
charge on V<br />
near interface
HAXPES of LVO/STO: Ti 2p<br />
no Ti 3+ (d 1 ) signal<br />
possibly some bandbending<br />
on STO side of interface<br />
10 uc LVO<br />
STO<br />
extra electronic<br />
charge on V<br />
near interface
LVO/STO: electronic reconstruction picture
LaVO 3<br />
Mott ins.<br />
∆≈1 eV<br />
"q2DEG"<br />
SrTiO 3<br />
band ins.<br />
∆=3.2eV<br />
Electrostatic doping of a Mott insulator<br />
LaVO 3/SrTiO 3:<br />
• creation of 2D metal states in a<br />
correlated electron system<br />
by interface engeering<br />
• purely electrostatic doping<br />
• no disorder by chemical dopants
Summary<br />
Photoelectron spectroscopy of functional oxides:<br />
Heterostructures and buried interfaces<br />
• Photoelectron spectroscopy (PES)<br />
yields (destruction-free) information on<br />
- chemical composition, valencies, local chemistry<br />
- electronic structure (band structure, spectral function)<br />
• PES with hard x-rays (HAXPES)<br />
- enhanced probing depth giving access to bulk and buried interfaces<br />
- needs high x-ray intensity (� synchrotron radiation)<br />
- caveat: high photon momentum (ARPES difficult, recoil effects)<br />
• Future directions:<br />
- magnetic information with polarized x-rays (XMCD, XMLD) and/or spin detection<br />
- soft x-ray ARPES: band mapping of buried interfaces
Reading<br />
Photoemission:<br />
• S. Hüfner, Photoelectron Spectroscopy – Principles and Applications, 3rd ed. (Berlin,<br />
Springer, 2003)<br />
• A. Damascelli, Angle-resolved photoemission studies of the cuprate superconductors,<br />
Rev. Mod. Phys. 75, 473 (2003)<br />
HAXPES:<br />
• K. Kobayashi: Hard x-ray photoemission spectroscopy,<br />
Nucl. Instr. Meth. Phys. Res. A 601, 32 (2009)<br />
• László Kövér: X-ray photoelectron spectroscopy using hard X-rays,<br />
J. Electron Spectrosc. Rel. Phen. 178-179, 241 (2010)<br />
HAXPES of oxide heterostructures<br />
• R. Claessen et al.: Hard x-ray photoelectron specroscopy of oxide hybrid and<br />
heterostructures: a new method for the study of buried interfaces,<br />
New J. Phys. 11, 125007 (2009)
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