Two dimensional electron gas
at the SrTiO3 surface
and interfaces
U. Scotti di UccioS. Amoruso, C. Aruta, R. Bruzzese, G. M. De Luca, R. Di Capua, E. Di Gennaro, D. Maccariello, P. Perna, M. Radovic, Muhammad Riaz, Z. Ristic, M. Salluzzo, A. Sambri, R. Vaglio, X. Wang, I. Maggio-Aprile*, and F. Miletto Granozio
CNR-SPIN & University FEDERICO II, Napoli (Italy)
C. Cantoni, J. Gazquez, A. R. Lupini, M. P. Oxley, S. J. Pennycook, N. Plumb, M. Varela
Oak Ridge National Laboratory
*Dép. de Physique de la Matière Condensée, University of Geneva
1
This talk regards two dimensional electron gases that are formed at the STO surface and
interfaces. I will actually present a few data regarding different experiments and
involving several people at CNR SPIN and University of Napoli with the cooperation of a
group at Oak Ridge National Laboratory, mainly involved in TEM measurements.
donors
Polar-non polar interfaces
Where do electrons come from?
Cation
intermixing
Oxygen
vacancies
Donor defects
Electronic reconstruction
LAO
0 0 +1 -1 +1 -1
Po = −−−− ½ e/S
σb = ½ e/S
+
+
+
+
STO
VB
CB
VB
e-
+++++++
-------STO LAO STO LAO
2
This talk:focus on oxygen vacancies
2
A 2DEG is basically characterized by a quantum well where electrons
coming from some donor state are trapped. One of the still debated
issues regarding STO/LAO interfaces is just the nature of the donor
states. In the electronic reconstruction models, electrons come from
the valence band of the polar layer, that is lifted above the Fermi level
by the built-in potential due to the polarization. But one may also
consider defects as donors. These can be due to disorder, but in this
talk I will focus on the effect of oxygen vacancies.
2DEG at STO interfaces
Polar-non polar heterostructures
10-6 mbar Oxygen vacancies dominate!
A. Brinkman, et al., Nature Mat. 2007
3
Reversing the argument:
Can we demonstrate the formation of a 2DEG in
STO as exclusively due to oxygen vacancies?
Can we present interfaces where the role of
oxygen vacancies appears as negligible?
Oxygen vacancies can actually dominate the transport properties of samples that are
fabricated at too low oxygen partial pressure. In that case we can even get bulk
conductivity.
2DEG at STO surfaces and interfaces
STO surface
ARPESon cleaved crystals
Deliberately introduce oxygen vacanciesInvestigate STO surface
Nature 469,189
(2010)
In order to verify whether oxygen vacancies can determine 2DEG formation, we
performed an experiment regarding the free surface of STO. The motivation stems from
a recent work of ARPES on cleaved STO crystals, which indicated that a robust 2DEG is
formed with properties independent on bulk carrier density. I will compare the results
with data concerning oxygen vacancies at the interfaces.
Single crystal (100) SrTiO3 with TiO2 termination planeSelective ethcing procedureUn. Twente, APL 73, 2920 (1998)
AFM
1 µµµµm ×××× 1 µµµµm
STM
2DEG at STO surfaces
after low temperature UHV treatments
5R. 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
We started with TiO2 terminated STO crystals…
(-10)
(01)
(0-1)
(10)
(-10) (10)
(01)
(0-1)
Thermal treatment
Annealing in oxygen at 850°C
Highly insulatingSurface charging: no LEED pattern
Annealing in UHV at 250-350°C
Bulk insulating (transparent)1×1 reconstruction
Annealing in UHV at 900°CBulk conducting (shiny black)2×1 reconstruction
Sample A
Sample B
6Po = 5×10-11 mbar
…and performed a series of thermal treatments.
Scanning Tunneling
Spectroscopy
Regulation points
V = -1.5 V, I = 0.1 nA
V = -1.5 V, I = 0.7 nA
Average on grid of points
7
tip STO
tip STO
STO surface
STM tip
UHV
Agreement with STS Surf. Sci. 542, 177 (2003)
Agreement with ARPES Nature 469, 189 (2010)
Here is the main result. The dI/dV curves of the different samples show a remarkable
difference. I will stay at the phenomenological level, by saying that the red curve is
consistent with the idea that a high temperature treatment determines conducting bulk
and insulating surfaces, consistently with several previous reports. The blue curve is
instead consistent with the idea that the surface itself is conducting, that is, it possesses
a finite density of occupied states at the Fermi level.
XPS: Ti 3p doublet
Results on 2DEGSpectroscopic signature of 2DEG: Ti 3+ states
Confinement: nm scale
2DEG at STO surfaces
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Photoelectrons mfp≈nmShallow angle emission: surface probe
XPS
250 °C 900 °C
The photoemission from the Ti3p triplet essentially confirms the existence of occupied
Ti3+ states, that constitute the conduction band, and since the experiment is surface-
sensitive, it also demonstrate the surface confinement within the nm scale.
Since we have no other contamination, the evolution of oxygen is the only possible
source of doping.
VB
Left side of dI/dV:
VB of STOi.e. defect states
9
The investigation of the VB by photoemission also clarifies the transport mechanism.
The left part of the dI/dV plots mainly carry information on the occupied DOS of STO. It
is suggestive to compare between XPS spectra and dI/dV of the different samples; the
data indicate that the surface defect states (blue curve) are source of enhanced junction
conductance.
Out of equilibrium:
flux of vacancies through
the surface into the bulk
Equilibrium:
cs > cb
Oxygen Vacancies at STO surfaces
10
Message
Local doping by Oxygen vacancies
CAN
determine a 2DEG at STO surface
STO environment
Gib
bs
en
erg
y o
f va
ca
nc
ies
First-principles calculationsPHYSICAL REVIEW B 73, 064106 (2006)
The results are consistent with a picture where the Gibbs energy of oxygen vacancies at
the surface is lower, as also deduced by computation. When we raise the chemical
potential of the environment, the vacancies are introduced and at the equilibrium they
reach a higher surface concentration. Finally, if we choose the right conditions, we can
actually form a 2DEG due to local doping.
LAO
STO
LGO
STO
2DEG at STO interfaces
Polar-non polar heterostructures
P. Perna, et al, APL 2010
11Lanthanum Gallate ≈ Lanthanum Aluminate
Now we turn to the issue of 2DEG formation at interfaces STO interfaces. I will consider
on the same foot both STO/LAO and STO/LGO, that in our experience share most
relevant physical properties.
Spectroscopy with atomic-scale resolution
EELS in the aberration corrected microscope
Conducting sampleAbrupt interface
C. Cantoni, et al. ADV. MAT. 2012 12
If we want to investigate the oxygen content at interfaces we need a tool that is
sensitive to chemical properties of samples. We resorted to EELS.
Oxygen vacancy
stoichiometry
δδδδ =0 @1×10-3 mbarbut
2% maximum error 0.12 e- /cell
Integrated O-K
O-K ELNES
C. Cantoni, et al. ADV. MAT. 2012
13
Simulation
Inte
ns
ity (
a.u
.)
experiment
Here are some results. In the upright panel, the integrated O K scan profile confirms the
sensitivity of EELS to determine oxygen stoichiometry with spatial resolution at the
atomic level. However, to get the quantitative information it is necessary to consider
with due care the O-K ELNES spectra, since not all the components are directly
connected to oxygen stoichiometry. Here I show the depth profile of the “c” line
intensity, which is constant across the sampled region within STO.
By quantitatively analyzing the data, we get the best estimation of vacancies
stoichiometry d = 0. This fact is a support to the idea that in a conductive sample grown
at 1x10-3 mbar the 2DEG is not populated by electrons provided by oxygen vacancies.
However, the experimental maximum error is 2%. This means that the required
information is very close to the experimental sensitivity.
We then need further confirmation to conclude that the 2DEG at interfaces can be
populated even in absence of oxygen vacancies.
Further limiting the role of oxygen vacancies
Deposition at high oxygen pressure
DrawbackThe plasma plume is stopped by the buffer gas
The energy of impinging species is reduced
The surface mobility of adatoms drops
growth conditions may be bad!
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
14
To this aim we considered growth at very high oxygen pressure, in spite of the possible
drawbacks.
LGO/STO
Growth at 10-1 mbar
AFM
A. Aruta, et al, APL 2010
Plume analysis and film growth @10-1 mbar
Insulating samples!
Time Resolved Emission Spectroscopy
15
2D growth
Plume fast photography
In our previous work we demonstrated that 2D growth of LAO and of LGO can be
achieved even at 10-1 mbar, but samples were insulating.
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 µµµµ(O2)
16
We decided to analyze the growth dynamics by resorting to plume diagnostics such as
fast photography and time resolved spectroscopy. Here are some results.
At 10-1 mbar the plume propagates in a shock wave regime and it halts after travelling
about 35 mm away from the target. Effects of the buffer gas are: the species loose
energy by collisions and reach the substrate with low kinetic energy; instead, they
possess high internal energy; moreover, the oxygen supersaturation is increased by the
strong compression of the gas on the plume front edge. However, care must be paid to
the target-substrate distance.
More on growth @10-1 mbar
R (
Ω
Ω
Ω
Ω /
)
17
By optimizing the distance, conductive samples can be realized. At the given conditions,
it is generally agreed that the content of oxygen vacancies in samples is marginal.
Conclusions
2. STO interfaces
10-3 mbar : V(O) = 0 (EELS)
10-1 mbar: conductive interfaces
1. Oxygen vacancies can determine the formation of a 2DEG at theSTO free surface
Local V(O) 2DEG
18
2DEG Local V(O)
In conclusion, our measurement demonstrate that oxygen vacancies can be collected at
the free surface of STO crystals and there determine the formation of a 2DEG. In the
case of interfaces, on one hand we know that oxygen vacancies can affect the
conductivity; but on the others, we proved that at standard deposition conditions they
don’t accumulate at the interface; and that it is possible to grow conducting interfaces
in extremely oxidizing conditions. In other words, observation of a 2DEG doesn’t seem
to imply the presence of oxygen vacancies.
Conclusions
2. STO interfaces
10-3 mbar : V(O) = 0 (EELS)
10-1 mbar: conductive interfaces
1. Oxygen vacancies can determine the formation of a 2DEG at theSTO free surface
Local V(O) 2DEG
19
2DEG Local V(O)
“different forms of electron confinement at the
surface of SrTiO3 lead to essentially the same 2DEG”
Our measurement demonstrate that oxygen vacancies can be collected at the free
surface of STO crystals and there determine the formation of a 2DEG. In the case of
interfaces, on one hand we know that oxygen vacancies can affect the conductivity; but
on the others, we proved that at standard deposition conditions they don’t accumulate
at the interface; and that it is possible to grow conducting interfaces in extremely
oxidizing conditions. In other words, observation of a 2DEG doesn’t seem to imply the
presence of oxygen vacancies.
In conclusion, it is apparent that different forms of electron confinement (even in terms
of different donor states) at the surface of SrTiO3 lead to essentially the same 2DEG.
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