Content uploaded by Alexei Gruverman
Author content
All content in this area was uploaded by Alexei Gruverman on Jan 01, 2016
Content may be subject to copyright.
Available via license: CC BY 3.0
Content may be subject to copyright.
Tunnel electroresistance in junctions with ultrathin ferroelectric Pb(Zr0.2Ti0.8)O3
barriers
Daniel Pantel, Haidong Lu, Silvana Goetze, Peter Werner, Dong Jik Kim, Alexei Gruverman, Dietrich Hesse, and
Marin Alexe
Citation: Applied Physics Letters 100, 232902 (2012); doi: 10.1063/1.4726120
View online: http://dx.doi.org/10.1063/1.4726120
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/100/23?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Enhanced magnetoelectric effect in La0.67Sr0.33MnO3/PbZr0.52Ti0.48O3 multiferroic nanocomposite films with
a SrRuO3 buffer layer
J. Appl. Phys. 113, 164106 (2013); 10.1063/1.4803057
Role of dual-laser ablation in controlling the Pb depletion in epitaxial growth of Pb(Zr0.52Ti0.48)O3 thin films with
enhanced surface quality and ferroelectric properties
J. Appl. Phys. 111, 064102 (2012); 10.1063/1.3694035
Coexistence of tunneling magnetoresistance and electroresistance at room temperature in La0.7Sr0.3MnO3/(Ba,
Sr)TiO3/La0.7Sr0.3MnO3 multiferroic tunnel junctions
J. Appl. Phys. 109, 07D915 (2011); 10.1063/1.3564970
Characteristics of metal-ferroelectric-insulator-silicon structures with ferroelectric ( Pb 0.8 Ba 0.2 ) Zr O 3 thin
films and ( Ba 0.5 Sr 0.5 ) Ti O 3 buffer layer
Appl. Phys. Lett. 88, 122901 (2006); 10.1063/1.2186103
Effect of oxygen stoichiometry on the ferroelectric property of epitaxial all-oxide La 0.7 Sr 0.3 MnO 3 /Pb(Zr 0.52
Ti 0.48 )O 3 /La 0.7 Sr 0.3 MnO 3 thin-film capacitors
J. Vac. Sci. Technol. A 18, 2412 (2000); 10.1116/1.1288195
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
204.62.122.194 On: Thu, 24 Dec 2015 00:14:49
Tunnel electroresistance in junctions with ultrathin ferroelectric
Pb(Zr
0.2
Ti
0.8
)O
3
barriers
Daniel Pantel,
1
Haidong Lu,
2
Silvana Goetze,
1
Peter Werner,
1
Dong Jik Kim,
2
Alexei Gruverman,
2
Dietrich Hesse,
1
and Marin Alexe
1,a)
1
Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany
2
University of Nebraska, Lincoln, Nebraska 68588, USA
(Received 11 April 2012; accepted 19 May 2012; published online 5 June 2012)
In ferroelectric tunnel junctions, the ferroelectric polarization state of the barrier influences the
quantum-mechanical tunneling through the junction, resulting in tunnel electroresistance (TER).
Here, we investigate tunnel electroresistance in Co/PbZr
0.2
Ti
0.8
O
3
/La
0.7
Sr
0.3
MnO
3
tunnel
junctions. The ferroelectric polarization in tunnel junctions with 1.2-1.6 nm (three to four unit
cells) PbZr
0.2
Ti
0.8
O
3
thickness and an area of 0.04 lm
2
can be switched by about 1 V yielding a
resistive ON/OFF-ratio of about 300 at 0.4 V. Combined piezoresponse force microscopy and
electronic transport investigations of these junctions reveal that the transport mechanism is
quantum tunneling and the resistive switching in these junctions is due only to ferroelectric
switching. V
C2012 American Institute of Physics.[http://dx.doi.org/10.1063/1.4726120]
The ferroelectric polarization state in a metal/ferroelec-
tric/metal heterostructure is usually read by detecting the
switched charge generated by a voltage pulse applied to the
heterostructures.
1
This approach is currently used in ferro-
electric random access memories with the obvious disad-
vantage of a destructive read-out process. Thus, reading the
ferroelectric state in a non-destructive manner using a
polarization-coupled physical property, for instance, the
electric resistance,
2
would be advantageous. The change in
the resistance upon the ferroelectric polarization reversal is
called ferroelectric electroresistance effect. Since the polar-
ization can be switched by a short voltage pulse, this might
be regarded as a genuine electronic resistive switching
effect.
Resistive switching in ferroelectrics has already been
reported several times.
3–12
However, there are mechanisms
other than ferroelectric polarization switching, which might
contribute to or generate a resistive switching effect, such as,
for instance, surface electrochemical reactions.
4,13
Espe-
cially, experiments performed in ambient conditions by
using a conductive atomic force microscope (AFM) tip in
contact with a ferroelectric surface are prone to such
effects.
14
Up to now, most of the measurements performed
on ferroelectric barriers with thicknesses small enough to
allow direct quantum-mechanical tunneling have been per-
formed by such a method.
5,6,10
In order to mitigate the spurious effects of the ambient
environment and make the experimental results more reli-
able, capacitor-like heterostructures with top electrodes
should be used instead of AFM tips as top electrodes. Resis-
tive switching of such capacitors with relatively thick
Pb(Zr,Ti)O
3
(PZT) barriers (ca. 9 nm) has already been per-
formed.
7
In these studies, Schottky emission was suggested
as the dominating transport mechanism. For smaller barrier
thickness, direct tunneling should be the dominant transport
mechanism, as predicted by theoretical simulations.
15
Because of this feature, these structures are typically referred
to as ferroelectric tunnel junctions (FTJ).
16
Although electro-
resistance is lower for direct tunneling as compared to
Schottky emission, the current density is higher,
15
which is
an important parameter for prospective applications. Further-
more, FTJs allow a combination of tunnel electroresistance
(TER) effect with the tunnel magnetoresistance (TMR)
effect, which can lead to interesting magnetoelectric effects
and memory devices with four states.
17–21
One of the chal-
lenges in realization of functional FTJs is a necessity to
directly correlate the change in resistance with polarization
reversal,
4,11
which would provide an unambiguous proof of
the genuine electronic nature of resistive switching in the fer-
roelectric barrier.
Here, we combine piezoresponse force microscopy
measurements (PFM) and electronic transport investigations
by means of conductive AFM on Co/PbZr
0.2
Ti
0.8
O
3
/
La
0.7
Sr
0.3
MnO
3
(Co/PZT/LSMO) tunnel junctions with the
PZT barrier thickness of three to four unit cells. We present
the evidence of the ferroelectric (electronic) origin of TER in
these heterostructures, which is significantly larger as com-
pared to BaTiO (BTO)-based FTJs.
22
PZT/LSMO oxide heterostructures were epitaxially and
fully coherently grown by pulsed laser deposition on (100)-
oriented SrTiO
3
substrates at elevated temperatures. Poly-
crystalline Au/Co top electrodes were deposited ex-situ by
thermal evaporation at room temperature. Nanosphere li-
thography was used to fabricate triangular shaped tunnel
junctions with an area of 0.04 lm
2
.
7
A transmission electron
microscope (TEM) image of a tunnel junction is shown in
Fig. 1. Details of the growth and fabrication process can be
found elsewhere.
7
In particular, structural and electrical
quality of the LSMO and PZT layers used in the present
study are similar to those used in Ref. 7. The LSMO layers
show good metallic conduction behavior at room tempera-
ture yielding an estimated serial resistance of the bottom
electrode of about 800 X, which is far below the typical
a)
Author to whom correspondence should be addressed. Electronic mail:
malexe@mpi-halle.de.
0003-6951/2012/100(23)/232902/4/$30.00 V
C2012 American Institute of Physics100, 232902-1
APPLIED PHYSICS LETTERS 100, 232902 (2012)
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
204.62.122.194 On: Thu, 24 Dec 2015 00:14:49
resistance of a 0.04 lm
2
tunnel junction. Thicker PZT layers
grown at the same growth conditions show a spontaneous
polarization of about 110 lC/cm
2
and an excellent retention,
7
as expected for PZT films free from extended defects.
23
Due to the low PFM-signal of the ultrathin PZT barriers,
PFM was performed in the dual ac resonance tracking
mode
24
using a MFP-3D
TM
AFM microscope (Asylum
Research) and platinum coated tips (DPER18, MikroMasch).
Ferroelectric properties of the PZT layers at room tempera-
ture were measured by obtaining PFM hysteresis loops with
an ac modulation voltage amplitude of 0.3 V. IV-curves have
been recorded with a conductive diamond coated tip (CDT-
NCHR, NanoWorld) and setting the current limit of the
current amplifier to 20 nA. The voltage was applied to the
bottom electrode and contact was made through the conduc-
tive tip to the top electrode for both measurement methods
as depicted in the inset of Fig. 2(a).
Figure 2(a) shows the typical PFM hysteresis loops
measured in Co/PZT/LSMO tunnel junctions. The coercive
voltages vary from junction to junction and are on average
þ0.8 60.5 V and 1.4 60.5 V. This alone is an interesting
result, as it demonstrates switchable ferroelectric polariza-
tion at room temperature in the PZT layers of only three to
four unit cell thickness, which is close to the theoretical
value of the critical thickness for ferroelectric PbTiO
3
(1.2 nm (Ref. 25)), the parent compound of present PZT. The
average coercive field E
c
¼690 kV/mm is about three times
larger than that of the 9 nm thick PZT layer,
7
which is in
agreement with the known thickness dependence of E
c
in fer-
roelectric films.
26
The resistive state of the FTJs can be switched from
high conducting (ON-state) to low conducting (OFF-state)
and vice versa by sweeping the voltage from negative to pos-
itive and back, respectively (Fig. 2(a)). The OFF/ON resist-
ance ratio R
OFF
/R
ON
is approximately 300 at 0.4 V. Resistive
switching occurs here on average at þ0.7 60.2 V from ON
to OFF and at 1.7 60.2 V from OFF to ON. Thus, the
switching voltages of ferroelectric switching and resistive
switching coincide within the experimental errors confirming
that the resistive switching is only due to polarization switch-
ing. The junction is in ON- or OFF-state for polarization
pointing towards LSMO or Co, respectively. The resistive
switching in ferroelectric tunnel junctions can be explained
by assuming a simple electrostatic model of a ferroelectric
tunnel barrier that considers the effective screening
lengths,
15,21
which is larger for LSMO than for the Co
electrode.
27,28
The resistance area product (RA) of the junctions is
about 6 MXlm
2
in the ON-state (measured at 100 mV),
which is in the typical range for tunnel junctions.
29
More
insight into the transport mechanism can be gained from the
tunnel conductance G¼dI/dVcalculated from the IV-charac-
teristics. The G(V)-curve is parabola-like (Fig. 2(b))as
expected for transport-dominating direct tunneling.
30
It can
be fit to the Brinkman model
30
for a fixed PZT thickness d
yielding reasonable fit parameters for barrier heights U
i
(i ¼1,2) and effective electron mass m
e,ox
as shown in
FIG. 1. (a) TEM image of a Co/PZT(3 to 4 unit
cells)/LSMO tunnel junction grown on STO. The
Co is capped by a gold-layer to prevent oxidation.
(b) High resolution TEM image showing the epitax-
ial PZT/LSMO interface of the same sample in a
region without Co top electrode.
FIG. 2. (a) Current (-n-), PFM phase (-~-), and PFM amplitude (->-) sig-
nals measured on a Co/PZT(3 to 4 unit cells)/LSMO tunnel junction with an
area of 0.04 lm
2
as a function of an applied voltage. The direction of the
voltage sweeps is given by the arrows. The measurement geometry is
depicted in the inset. (b) Tunnel conductance as a function of voltage in the
ON- (->-) and OFF-state (-n-). The grey lines in (a) are to mark the coercive
voltages and in (b) are fits to the Brinkman model.
TABLE I. Fit parameter to the Brinkman model obtained from the solid
lines in Fig. 1(b).
State d (nm) m
e,ox
[m
e
]U
1
(eV) U
2
(eV)
ON 1.6 3 0.65 0.7
OFF 1.6 6 0.44 0.59
232902-2 Pantel et al. Appl. Phys. Lett. 100, 232902 (2012)
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
204.62.122.194 On: Thu, 24 Dec 2015 00:14:49
Table I. The change in m
e,ox
on polarization reversal can be
explained on the basis of lattice strains
31
or a polarization de-
pendent complex band structure.
12
The present results are a
direct experimental evidence of the influence of the ferro-
electric polarization on the direct tunneling in ultra-thin fer-
roelectric barriers. This implicitly suggests a genuine
electronic mechanism of resistive switching and TER in a
ferroelectric tunnel junction.
Unfortunately, spontaneous backswitching from resis-
tive ON- to OFF-state occurs rather fast, i.e., within several
tens of seconds, as observed from the current relaxation (not
shown here). This effect is likely due to polarization back-
switching which in turn could be a result of large imprint of
about 250 kV/mm revealed by the PFM-hysteresis (Fig.
2(a)). Moreover, not all junctions show resistive switching.
Some junctions remain in the ON-state even after application
of a large positive bias (Fig. 3(a)). Such a non-switchable
ON-state can be explained by a pinned polarization state
with polarization pointing towards LSMO. Similarly, a
pinned polarization state may also lead to a non-switchable
OFF-state for polarization pinned towards Co. To gain
insight in this behavior, we have performed PFM imaging of
tunnel junctions to reveal the domain structure underneath
the top electrodes. There are basically three different types
of junctions (Figs. 3(b)–3(g)). Two types, which exhibit high
PFM amplitude signal, have polarization pointing towards
either Co or LSMO, as concluded from the PFM phase con-
trast in Figs. 3(c) and 3(d). The polarization states in these
junctions cannot be permanently switched by voltage pulses
(Figs. 3(d) and 3(e)) because of the large imprint and corre-
sponding backswitching of polarization during the PFM scan
(several minutes).
In an extreme case, some junctions show low or zero
PFM-amplitude and a noisy PFM-phase (Figs. 3(f) and 3(g)).
On those devices, the ferroelectric behavior is suppressed
most probably by defects in the PZT layer.
32
In this case, the
current would be high because a paraelectric barrier yields a
higher conductivity than a ferroelectric barrier.
33
This high
non-uniformity can be charged to the growth of the top Co
electrodes which is an ex situ process. Different Co/PZT-
interface quality across the sample might result in different
preferred polarization states or even inferior Co/PZT-inter-
face quality. Additionally, a small difference in PZT thick-
ness might induce as quite a strong difference in behavior.
This is especially severe near the critical ferroelectric thick-
ness as it is in the present case.
In summary, Co/PZT/LSMO ferroelectric tunnel junc-
tions with PZT layers of only 3 to 4 unit cells thickness have
been fabricated. Ferroelectric switching in the PZT layers
has been demonstrated by PFM. The tunnel junctions show
TER at room temperature with an ON/OFF ratio of about
300. The TER effect takes place only upon ferroelectric
switching, and the ferroelectric coercive voltage and the
voltage at which the resistance switches correlate within an
experimental error. This provides direct experimental evi-
dence of the effect of the ferroelectric polarization on direct
tunneling in FTJs. Fitting of the experimental data by the
Brinkman model yielded reasonable values for energy bar-
rier height. The interface quality seems to play a crucial role
in electronic processes in ferroelectric tunnel junctions and
an in-situ structure monitoring of both metal-ferroelectric
interfaces will be advantageous for optimizing the FTJ
performance.
This work has been partly supported by DFG via SFB
762 and by EU project IFOX. D.P., A.G., and M.A. would
like to acknowledge the support of the Leverhulme Trust
for international network funding (F/00 203/V). D.H. and
M.A. thank Norbert Schammelt and Mareike Herrmann for
technical support. Work at the University of Nebraska was
supported by the National Science Foundation (NSF)
through Materials Research Science and Engineering Cen-
ter (NSF Grant No. 0820521) and by the U.S. Department
of Energy, Office of Basic Energy Sciences, Division of
Materials Sciences and Engineering (DOE Grant DE-
SC0004876).
1
J. F. Scott, Ferroelectric Memories (Springer, Berlin, 2000).
2
L. Esaki, R. B. Laibowitz, and P. J. Stiles, IBM Tech. Discl. Bull. 13,
2161 (1971).
3
J. R. Contreras, H. Kohlstedt, U. Poppe, R. Waser, C. Buchal, and N. A.
Pertsev, Appl. Phys. Lett. 83, 4595 (2003).
4
H. Kohlstedt, A. Petraru, K. Szot, A. Ru¨diger, P. Meuffels, H. Haselier, R.
Waser, and V. Nagarajan, Appl. Phys. Lett. 92, 062907 (2008).
5
V. Garcia, S. Fusil, K. Bouzehouane, S. Enouz-Vedrenne, N. D. Mathur,
A. Barthe´le´my, and M. Bibes, Nature (London) 460, 81 (2009).
FIG. 3. (a) Current versus applied voltage in a switchable (->-) and a non-switchable (-n-) Co/PZT(3 to 4 unit cells)/LSMO tunnel junction with an area of
0.04 lm
2
each. The inset shows an AFM topography image of a typical junction. (b) PFM amplitude and (c) PFM phase images of the junction presented in the
inset of (a). The low signal at the edges can be attributed to a bad mechanical and/or electrical contact between tip and electrode surface. (d) and (e) PFM phase
images of another junction with a high PFM amplitude signal after applying (d) a 3 V and (e) a þ3 V pulse to the Co electrode. Yellow (violet) depicts areas
with polarization towards LSMO (Co). (f) PFM amplitude and (g) PFM phase images of a third type of a junction showing weak PFM response. All scale bars
are 200 nm.
232902-3 Pantel et al. Appl. Phys. Lett. 100, 232902 (2012)
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
204.62.122.194 On: Thu, 24 Dec 2015 00:14:49
6
A. Crassous, V. Garcia, K. Bouzehouane, S. Fusil, A. H. G. Vlooswijk, G.
Rispens, B. Noheda, M. Bibes, and A. Barthe´le´my, Appl. Phys. Lett. 96,
042901 (2010).
7
D. Pantel, S. Goetze, D. Hesse, and M. Alexe, ACS Nano 5, 6032
(2011).
8
A. Q. Jiang, C. Wang, K. J. Jin, X. B. Liu, J. F. Scott, C. S. Hwang, T. A.
Tang, H. B. Liu, and G. Z. Yang, Adv. Mater. 23, 1277 (2011).
9
P. Maksymovych, S. Jesse, P. Yu, R. Ramesh, A. P. Baddorf, and S. V.
Kalinin, Science 324, 1421 (2009).
10
A. Gruverman, D. Wu, H. Lu, Y. Wang, H. W. Jang, C. M. Folkman, M.
Y. Zhuravlev, D. Felker, M. Rzchowski, C. B. Eom, and E. Y. Tsymbal,
Nano Lett. 9, 3539 (2009).
11
A. Bune, S. Ducharme, V. Fridkin, L. Blinov, S. Palto, N. Petukhova, and
S. Yudin, Appl. Phys. Lett. 67, 3975 (1995).
12
A. Chanthbouala, A. Crassous, V. Garcia, K. Bouzehouane, S. Fusil, X.
Moya, J. Allibe, B. Dlubak, J. Grollier, S. Xavier, C. Deranlot, A. Moshar,
R. Proksch, N. D. Mathur, M. Bibes, and A. Barthe´le´my, Nat. Nanotech-
nol. 7, 101 (2011).
13
R. Waser, R. Dittmann, G. Staikov, and K. Szot, Adv. Mater. 21, 2632
(2009).
14
S. Kalinin, S. Jesse, A. Tselev, A. P. Baddorf, and N. Balke, ACS Nano 5,
5683 (2011).
15
D. Pantel and M. Alexe, Phys. Rev. B 82, 134105 (2010).
16
E. Y. Tsymbal and H. Kohlstedt, Science 313, 181–183 (2006).
17
V. Garcia, M. Bibes, L. Bocher, S. Valencia, F. Kronast, A. Crassous, X.
Moya, S. Enouz-Vedrenne, A. Gloter, D. Imhoff, C. Deranlot, N. D.
Mathur, S. Fusil, K. Bouzehouane, and A. Barthe´le´my, Science 327, 1106
(2010).
18
M. Hambe, A. Petraru, N. A. Pertsev, P. Munroe, V. Nagarajan, and H.
Kohlstedt, Adv. Funct. Mater. 20, 2436 (2010).
19
J. P. Velev, C.-G. Duan, J. D. Burton, A. Smogunov, M. K. Niranjan, E.
Tosatti, S. S. Jaswal, and E. Y. Tsymbal, Nano Lett. 9, 427 (2009).
20
D. Pantel, S. Goetze, D. Hesse, and M. Alexe, Nat. Mater. 11, 289–293
(2012).
21
M. Y. Zhuravlev, S. S. Jaswal, E. Y. Tsymbal, and R. F. Sabirianov, Appl.
Phys. Lett. 87, 222114 (2005).
22
M. Y. Zhuravlev, R. F. Sabirianov, S. S. Jaswal, and E. Y. Tsymbal, Phys.
Rev. Lett. 94, 246802 (2005).
23
I. Vrejoiu, G. Le Rhun, L. Pintilie, D. Hesse, M. Alexe, and U. Go¨ sele,
Adv. Mater. 18, 1657 (2006).
24
B. J. Rodriguez, C. Callahan, S. V. Kalinin, and R. Proksch, Nanotechnol-
ogy 18, 475504 (2007).
25
P. Ghosez and K. M. Rabe, Appl. Phys. Lett. 76, 2767 (2000).
26
H. F. Kay and J. W. Dunn, Philos. Mag. 7, 2027 (1962).
27
X. Hong, A. Posadas, and C. H. Ahn, Appl. Phys. Lett. 86, 142501 (2005).
28
J. Ko¨ tzler and W. Gil, Phys. Rev. B 72, 060412 (2005).
29
J. J. A
˚kerman, J. M. Slaughter, R. W. Dave, and I. K. Schuller, Appl.
Phys. Lett. 79, 3104 (2001).
30
W. F. Brinkman, R. C. Dynes, and J. M. Rowell, J. Appl. Phys. 41, 1915
(1970).
31
H. Kohlstedt, N. A. Pertsev, J. R. Contreras, and R. Waser, Phys. Rev. B
72, 125341 (2005).
32
M.-W. Chu, I. Szafraniak, R. Scholz, C. Harnagea, D. Hesse, M. Alexe,
and U. Go¨ sele, Nat. Mater. 3, 87 (2004).
33
J. P. Velev, C.-G. Duan, K. D. Belashchenko, S. S. Jaswal, and E. Y.
Tsymbal, Phys. Rev. Lett. 98, 137201 (2007).
232902-4 Pantel et al. Appl. Phys. Lett. 100, 232902 (2012)
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
204.62.122.194 On: Thu, 24 Dec 2015 00:14:49
