Max Planck Institute for Solid State Research
Stuttgart (Germany)
WORLD OF HIGH QUALITY COMPLEX OXIDE
HETEROSTUCTURE:
OXIDE MOLECULAR BEAM EPITAXY – TOUR DE FORCE
Gennady Logvenov
Scientific Facility Technology
INTERNATIONAL SCHOOL AND WORKSHOP ON ELECTRONIC CRYSTALS ECRYS-2017
August 21-September 2, 2017
Institut d’Etudes Scientifiques de Cargèse, Corse, France.
Max Planck Institute for Solid State Research, Stuttgart, Germany
Prof. Dr. J.Maier
Dr. G. Gregori
Dr. F. Baiutti
Prof. Dr. P. A. van Aken
Dr. Yi Wang
Dr. W. Sigle
Dr. A. F. Mark
Y. E. Suyolcu
G. Christiani
P. Specht
B. Stuhlhofer
Y. Stuhlhofer
S. Schmid
B. Lemke
AcknowledgementsAcknowledgements
Prof. Dr. B. Keimer
Dr. A. Boris
Dr. E. Benckiser
Dr. F. Wrobel
D. Putzky
G. Kim
Dr. A. Suter
Dr. T. Prokscha,
Dr. Z. Salman,
Dr. E. Stilp
OutlineOutline
� Motivations
� Our method –Oxide MBE
� Pros and Cons of the method
� Demonstrations (defect chemistry)
� Conclusions and outlook
MotivationsMotivations
New playground – transition metal oxides
„Often, it may be said that the interface is the device.”
A variety of functional oxides Complex phase diagrams Different mechanismsInterfaces
Ozone assisted Atomic-Layer-by-Layer MBEOzone assisted Atomic-Layer-by-Layer MBE
TOUR DE FORCE
Great flexibility
Ozone delivery systemOzone delivery system
Wide range of oxidation conditions
Configuration of ALL-MBEConfiguration of ALL-MBE
1 - Load Lock
2 - Storage
3 - Distribution
4 - Growth
Max Planck Institute for Solid State Research, Technology Facility
ProsPros
0 0.1 1 10 100
Energy [eV]
MBE
Sputtering
PLD
Substrate damage
Lowest energy of impinging atoms !!!
Evaporators – Effusion (Knudsen) cells
Typical RGA spectra for cuprate HTSTypical RGA spectra for cuprate HTS
It needs to have high oxidation conditions: Cu → CuO
O
O2
O3
Wide range of oxidation conditions in MBE regime
Different deposition schemes (sequence control) Different deposition schemes (sequence control)
1. Co-deposition
2. Layer-by-layer
Pros (Summary)Pros (Summary)
� Lowest energy of impinging atoms !!!
� Wide range of oxidation conditions in MBE regime
� Sequence control of atomic layers
� Ultra pure environmental conditions (UHV+pure metals)
� High quality thin epitaxial films – interface engineering
� Wide range of complex oxides
� Platform for device engineering
Cons: stoichiometry control !!!Cons: stoichiometry control !!!
� Quartz Crystal Oscillator (QCM) ∼ 3-10%
� Atomic Adsorption Spectroscopy (AAS) ∼ 1-10%
� XRD (Kiessig fringes/final thickness oscillations)
∼ 10%
� Rutherford Backscattering Spectrometry (RBS)
∼ 1-10%
� In-situ RHEED control (RHEED) – Real time feedback
In-situ RHEED feedbackIn-situ RHEED feedback
WITHOUT
FEEDBACK
A
result
WITH
FEEDBACKresult
Goal Artificial superlattice: ABAB….
B
…
the error propagates
It’s not enough to know the absolute rates -- one needs feedback
and real-time diagnostic and corrections
Comparative study of LNO/LAO heterostructures:
PLD vs MBE
Comparative study of LNO/LAO heterostructures:
PLD vs MBE
F. Wrobel, et al., Appl. Phys. Lett. 110, 041606 (2017)
ADF images (thanks to A.F. Mark)
MBE PLD
XRD analysis
Transport: 25 u.c. LNO (MBE&PLD) ) ρ(300K)/ρ(2K)=RRR
MBE: RRR=18.4 better compared to previously reported data
PLD: RRR=4.9
3D-RP faults compensate non-stoichiometry A/B sites
[4xLNO+4xLAO]x3
Prof. P. Van Aken
Bozovic paradox (scaling Tcwith N
s0)Bozovic paradox (scaling T
cwith N
s0)
Bozovic, et al. Nature 536 (2016)
Anomalous disappearance of
superconducting carrier density in the
over-doped regime
For Ns0
> 0.03, Tc
= T0
+ α Ns0
where T0
≈ 7 K
For Ns0
< 0.03, Tc
= γ √Ns0
Defects are not included
Lee-Hone, et al., arXiv:1704.04803 (2017)
Defects are included
Non stoichiometry is compensated by defectsNon stoichiometry is compensated by defects
In extreme case 3D precipitates are formed
AFM image of La2Cuo
4: Cu rich – CuO precipitates
Non stoichiometry is compensated by defectsNon stoichiometry is compensated by defects
Metal excess defects:
extra anion occupying interstitial site
Metal deficiency defects:
anionic (oxygen) vacancies
Defect chemistry
The MBE growth of complex oxides becomes
less sensitive to non-stoichiometry
J. Maier et al., Adv. Mater. 3 (1991)
E.J. Opila et al., J. Am. Ceram. Soc. 76 (1993)
Native La2CuO4
Antiferromagnetic insulator
Defect chemistry of A2BO4 systems
••
++→++OLaOLa
VSrOLaOLaSrO/
32222
•
++→++ hSrOLaOLaSrOLaLa
222122/
322
•
+→+ hOVOii
221//
2
][2][][2][ /// •••
+=+OiLa
VhOSr
Substitution La by Sr or oxygen interstitials induced
combinations of effects: oxygen vacancies , hole carriers and
associated defects
Defect chemistry of (La,Sr)2CuO
4Defect chemistry of (La,Sr)
2CuO
4
Formation of oxygen vacancies in the overdoped
regime
Formation of oxygen vacancies in the overdoped
regime
J. Maier et al., Adv. Mater. 3 (1991)
E.J. Opila et al., J. Am. Ceram. Soc. 76 (1993)
Experiment have been done by Gideok Kim
Instead of hole doping we could form oxygen vacancies in the overdoped regime
••
++→++OLaOLa
VSrOLaOLaSrO/
32222
•
++→++ hSrOLaOLaSrOLaLa
222122/
322
Selective oxygen manipulation (Gideok Kim)Selective oxygen manipulation (Gideok Kim)
As grown
Vacuum annealed
O3 annealed
→ Oxygen vacancies → where? Superconductivity was gone
Crystal structure was almost the same
Raman spectroscopy combined
with confocal microscope
Raman spectroscopy combined
with confocal microscope
M. Hepting, et al., PRL 113, 227206 (2014)
Steps of 1µm with an accuracy 0.5 µm
12 nm PrNiO3
film on a LSAT substrate at 10KTemperature range 10K-300K
Case study: selective oxygen manipulationCase study: selective oxygen manipulation
100-400 cm-1 400-1000 cm-1
∼ 220cm-1 – A1g phonon mode motion La/Sr atoms bonded to apical oxygen O(2)
∼ 600cm-1 – A1g* vibration of apical oxygens O(2)
selective sensitive markers to judge the local modifications of crystal structure
Case study: Doping dependence of 600 cm-1
mode
Case study: Doping dependence of 600 cm-1
mode
To clarify the symmetry and origin of the peak we measure Raman spectra with zz-polarization
Anisotropic A1g* mode we can assign to the vibration of the apical oxygens.
The phonon mode at ∼ 600cm-1 increases with Sr doping concentration
(more oxygen vacancies)
Case study: selective oxygen manipulationCase study: selective oxygen manipulation
Importance of formation of oxygen vacancies
→ Defect chemistry
Conclusion Nr. 1: Oxygen vacancies are formed in
the overdoped regime
Conclusion Nr. 1: Oxygen vacancies are formed in
the overdoped regime
Sr
J. Maier et al., Adv. Mater. 3 (1991)
E.J. Opila et al., J. Am. Ceram. Soc. 76 (1993)
Conclusion Nr. 1: Oxygen vacancies are formed in
the overdoped regime
Conclusion Nr. 1: Oxygen vacancies are formed in
the overdoped regime
Sr
J. Maier et al., Adv. Mater. 3 (1991)
E.J. Opila et al., J. Am. Ceram. Soc. 76 (1993)
Case study: dopant size effect
(HTS interface)
Case study: dopant size effect
(HTS interface)
A. Gozar et al. Nature 455, 782-785 (2008)
Case study: dopant size effect
(HTS interface)
Case study: dopant size effect
(HTS interface)
V. Y. Butko et al. Adv. Mater., 21, 3644-3648 (2009)
Increase Tc by tuning c-axis lattice parameter
&
Revealing dopant size effect
See Poster by Y. Eren Suyolcu
Tc
increases linearly with c-lattice constant
Case study: dopant size effect
(HTS interface)
Case study: dopant size effect
(HTS interface)
See Poster by Y. Eren Suyolcu
MBE synthesis of atomically smooth oxide heterostuctures
Y. E. Suyolcu, et al. Scientific Reports 7:453 (2017)
Case study: dopant size effect (Ups)Case study: dopant size effect (Ups)
Y. E. Suyolcu, et al. Scientific Reports 7:453 (2017)
c-axis lattice parameter increases linearly, Tc - not
Conclusion Nr. 2Conclusion Nr. 2
� In spite off the c-lattice constant is linearly
depends on
the dopant size
� Tc
& c-lattice constant is NOT linear.
� Each dopant shows different distribution (size
effect)
Homogeneous vs. Heterogeneous dopingHomogeneous vs. Heterogeneous doping
Artistic rendering by Ursus Wehrli
F. Baiutti et al., Nat. Commun. 6, 8585 (2015)
Sr-δ-doped La2CuO
4(concept)Sr-δ-doped La
2CuO
4(concept)
R x (SrO-LaO-CuO2
+ N x LaO-LaO-CuO2)
Total thickness ~ 40 nm
ca. 100 samples
Sr-dopant
Nx R
superlattices
2D doping
F. Baiutti et al., Nat. Commun. 6, 8585 (2015)
Sr-δ-doped La2CuO
4Sr-δ-doped La
2CuO
4
F. Baiutti et al., Nat. Commun. 6, 8585 (2015)
Incre
asi
ng
N
R x (SrO-LaO-CuO2
+ N x LaO-LaO-CuO2)
Max Tc ≈ 35 K for 5 < N < 9
Local HTSC arises across each charged layer
Dr. F. Baiutti
HAADF-STEMHAADF-STEMY. Wang et al., ACS Appl. Mater. Interfaces 2016, 8, 6763
Prof. P. Van Aken
Sr-δ-doped La2CuO
4Sr-δ-doped La
2CuO
4
F. Baiutti et al., Nat. Commun. 6, 8585 (2015)
Local structural properties (TEM*) *TEM analysis by StEM Group – MPI Stuttgart
Asymmetric dopant profile:
o Backward side is abrupt: 0.9±0.1 nm (< 1 u.c.)
o Upward side is smeared: 2.3±0.4 nm (2.5-3 u.c.)
STEM – EELS analysis STEM – EELS analysis
O-K edge analysis
Upward and downward side have similar Tc
At the upward side, several planes are SC
Y. Wang et al., ACS Appl. Mater. Interfaces, 8, 6763 (2016)F. Baiutti et al., Nat. Commun. 6, 8585 (2015)
Local oxygen octhahedral distortions (J-T):
Anomalous expansion (+15pm) of O-O out
of plane at the downward interface side
Overlay of simultaneously
acquired HAADF and ABF images
Y. Wang et al., ACS Appl. Mater. Interfaces, 8, 6763 (2016)
Conclusions Nr. 3Conclusions Nr. 3
Homogeneous (0-D) dopingHeterogeneous (2-D) doping
Upward side:
• No hole/Sr decoupling
• HTSC is distributed over several u.c.
• Cationic profile is a consequence of MBE kinetics
Downward side:
• HTSC is induced by space charge effects
and confined in a single atomic plane
• Lattice distortions are present
RHEED-assisted growth of δ-LCON: N=7RHEED-assisted growth of δ-LCO
N: N=7
Asymmetry is caused by growth kinetics and dopant size
Y. Wang et al., ACS Appl. Mater. Interfaces, 8, 6763 (2016)
Backward side is 0.9±0.1 nm
(< 1 u.c.)
Upward side is 2.3±0.4 nm
(2.5-3 u.c.)
Mechanisms of Sr distributionMechanisms of Sr distribution
Two dimensional (δ) Sr doping
EE
LS i
nte
nsi
ty (
arb
. u
.)
Downward Upward
De Souza et al., J. Solid State Ionics 176, 2005
Lee et al., J. Am. Ceram. Soc. 135, 2013
Suyolcu et al., Scientific Reports 7, 2017
ε
Ionic size mismatch
+ + + + +
|La
Sr-
••
OV
Surface polarity
Substrate temperatureDecoupling between ionic and electronic charge
Space-charge effects inducing HTSC
F. Baiutti, et al., ACS Appl. Mater. Interfaces, 8, 27368 (2016)
Conclusions Nr.4Conclusions Nr.4
F. Baiutti, et al., ACS Appl. Mater. Interfaces, 8, 27368 (2016)
Downward side:
thermal diffusion
surface roughness
Ekin
incoming particles
incomplete bonds
Upward side:
Growth kinetic
Segregation
Good agreement with typical diff. coef.
„Muraki“ model
(for semiconductors)
Summary & OutlookSummary & Outlook
� ALL-Oxide MBE enables to fabricate “high quality” complex oxides films and
heterostructures
� Great versatility and flexibility in the choice of the oxide compounds
� Advantage of Atomic layer-by-layer deposition is
Lowest energy of impinging atoms (sharp interfaces)
� Disadvantage is stoichiometry control (defect chemistry)
� Evidence of the presence of oxygen vacancies in the overdoped regime
� Dopant size effect on the HTS interface superconductivity
� New concept: precise defect control (importance of defect chemistry)
� Oxide MBE is the experimental laboratory
Combination of extraordinary methodsCombination of extraordinary methods
JEOL ARM200FOXIDE MBE
…and many others….
Thank you very much!!!Thank you very much!!!
