Comments on the origin of the
two dimensional electron gas
at the polar/non polar oxide
interfaces
U. Scotti di Uccio
S. Amoruso, C. Aruta, R. Bruzzese, E. Di Gennaro, A. Gadaleta, D. Maccariello, D. Marrè, L.
Marrucci, F. Miletto Granozio, I. Pallecchi, D. Paparo, P. Perna, M. Radovic, Mohamed
Riaz, A. Rubano, A. Sambri, X. Wang
R. Di Capua, Z. Ristic, M. Salluzzo, R. Vaglio, I. Maggio-Aprile*
CNR-SPIN & University FEDERICO II, Napoli (Italy)
C. Cantoni, S. J. Pennycook, M. Varela
Oak Ridge National Laboratory
*Dép. de Physique de la Matière Condensée, University of Geneva
1
Polar-non polar interfaces
Where do electrons come from?
Electronic reconstruction
Po = − ½ e/S
0 0
donors
+1 -1 +1 -1
+
STO
Donor defects
LAO
+
+
+
Cation
intermixing
σb = ½ e/S
Oxygen
vacancies
eCB
VB
VB
STO
LAO
STO
----
LAO
+
+
+
+
+
+
+
This talk:
focus on oxygen vacancies
2
2DEG at STO interfaces
Polar-non polar heterostructures
A. Brinkman, et al., Nature Mat. 2007
Fabrication conditions:
10-6 mbar Oxygen vacancies dominate!
MODEL
A quantum well at STO surface/interfaces is formed
Different types of donors can provide mobile carriers
Questions:
Can we demonstrate a system where the formation of a
2DEG in STO as exclusively due to oxygen vacancies?
Can we present 2DEG in STO where the role of oxygen
vacancies appears as negligible?
3
2DEG at STO surfaces
Deliberately introducing oxygen vacancies
Investigating the electronic properties of the STO surface
1 µm × 1 µm
AFM
STM
R. Di Capua, M. Radovic, G. M. De Luca, I. Maggio-Aprile, F. Miletto Granozio, N. C. Plumb, Z. Ristic, U. Scotti di Uccio, R. Vaglio, M. Salluzzo
Submitted to Phys. Rev. B
Thermal treatment
Annealing in oxygen at 850°C
Highly insulating
Surface charging: no LEED pattern
(01)
Sample A
(-10)
(10)
(0-1)
Annealing in UHV at 250-350°C
Bulk insulating (transparent)
1×1 reconstruction
(01)
Sample B
(-10)
(10)
(0-1)
Annealing in UHV at 900°C
Bulk conducting (shiny black)
2×1 reconstruction
Po = 5×10-11 mbar5
Scanning Tunneling
Spectroscopy
tip STO
STM tip
UHV
tip STO
STO surface
Average on grid of points
Regulation points
V = -1.5 V, I = 0.1 nA
V = -1.5 V, I = 0.7 nA
Agreement with STS Surf. Sci. 542, 177 (2003)
Agreement with ARPES Nature 469, 189 (2010)
6
2DEG at STO surfaces
XPS
Photoelectrons mfp≈nm
Shallow angle emission: surface probe
250 °C
XPS: Ti 3p doublet
900 °C
Results on 2DEG
Spectroscopic signature of 2DEG: Ti 3+ states
Confinement: nm scale
7
2DEG at STO interfaces
Polar-non polar heterostructures
P. Perna, et al, APL 2010
LAO
LGO
STO
STO
Lanthanum Gallate ≈ Lanthanum Aluminate
8
EELS in the aberration corrected microscope
Spectroscopy with atomic-scale resolution
Conducting sample
Abrupt interface
C. Cantoni, et al. ADV. MAT. 2012
9
Oxygen vacancy
stoichiometry
Integrated O-K
Simulation
δ =0 @1×10-3 mbar
but
2% maximum error 0.12 e- /cell
experiment
C. Cantoni, et al. ADV. MAT. 2012
Intensity (a.u.)
O-K ELNES
10
Further limiting the role of oxygen vacancies
Deposition at high oxygen pressure
Drawback
The plasma plume is stopped by the buffer gas
The energy of impinging species is reduced
The surface mobility of adatoms drops
Energetic particles can favor 2D growth of
oxide thin films through an island break-up
mechanism with prompt insertion, at very low
coverage, and enhanced surface diffusion,
above 50% monolayer coverage
growth conditions may be bad!
11
Plume analysis and film growth @10-1 mbar
Plume fast photography
Time Resolved Emission Spectroscopy
Growth at 10-1 mbar
AFM
2D growth
LGO/STO
A. Aruta, et al, APL 2010
Insulating samples!
12
More on growth @10-1 mbar
• Shock wave regime
• Halting @35 mm
• Internal energy drops @30 mm
Interesting regime
Low K (≈eV)
High U
High µ(O2)
13
R (Ω
Ω / )
More on growth @10-1 mbar
14
Conclusions
1. Oxygen vacancies can determine the formation of a 2DEG at the
STO free surface
Local V(O) 2DEG
2. STO interfaces
10-3 mbar : V(O) = 0 (EELS)
10-1 mbar:
conductive interfaces
2DEG Local V(O)
15
Conclusions
1. Oxygen vacancies can determine the formation of a 2DEG at the
STO free surface
Local V(O) 2DEG
2. STO interfaces
10-3 mbar : V(O) = 0 (EELS)
10-1 mbar:
conductive interfaces
2DEG Local V(O)
Message:
Different donor states lead to essentially the same 2DEG
at the surface and interfaces of SrTiO3
16
MODA LAB: an integrated laboratory
for fabrication of oxide films and for surface analyses
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