Polarization coupling in epitaxial ZnO / BaTiO3 thin film heterostructures on SrTiO$_3$ (100) substrates

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Polarization coupling in epitaxial ZnO / BaTiO3 thin film
heterostructures on SrTiO3 (100) substrates

Michael Lorenz*a, Matthias Brandta, Jürgen Schubertb, Holger Hochmutha, Holger von Wencksterna,
Mathias Schubertc, Marius Grundmanna
aUniversität Leipzig, Institut für Experimentelle Physik II, Linnéstr. 5, D-04103 Leipzig, Germany;
bForschungszentrum Jülich GmbH, IBN 1-IT, D-52425 Jülich, Germany;
cUniversity of Nebraska-Lincoln, Dep. of Electrical Engineering, Lincoln, NE 68588-0511, U.S.A.
ABSTRACT
Strong polarization coupling is expected by combining ferroelectric materials with switchable polarization and wurtzite
layers exhibiting a permanent spontaneous polarization. To demonstrate these charge coupling effects, current-voltage,
conductivity-frequency and capacitance-frequency (admittance) characteristics have been measured on epitaxial
heterostructures grown of ferroelectric BaTiO3 (001) films on conducting SrRuO3 layers on SrTiO3 (100) substrates with
oxide SrRuOx, metallic Pt and semiconducting ZnO top electrodes. The electrical measurements show clear indications
for polarization coupling of the ferroelectric perovskite BaTiO3 and the piezoelectric wurtzite ZnO thin films.
Keywords: polarization coupling, ferroelectric switching, wurtzite structure, oxide heterostructures, pulsed laser
deposition.

1. INTRODUCTION
Whereas the electrical polarization in wurtzite materials as ZnO cannot be reversed, the polarization direction in
ferroelectric perovskites as BaTiO3 can be switched by external electric fields. Innovative functionalities in electronic
storage and sensing devices are expected from a combination of wurtzite and perovskite materials [1-4]. ZnO thin film
transistors with polycrystalline high-k (Ba,Sr)TiO3 gate insulators were successfully demonstrated [4]. Considerable
research efforts were devoted to other combinations of functional oxide materials recently [5-7]. Large dielectric
constant enhancement and strong structural dependence of ferroelectric properties induced by long-range spontaneous
polarization coupling in Pb(Mg0.33Nb0.66)O3-PbTiO3 rhombohedral/tetragonal superlattices was reported recently by Lu
[5]. Oxide semiconductor p-n heterojunctions as for example Pr0.7Ca0.3MnO3 / SrTi0.9998Nb0.0002O3 interfaces show highly
rectifying current density - voltage characteristics without an apparent breakdown in the reverse up to 100 V [6].
Pr0.7Ca0.3MnO3 shows reversible resistive switching effects induced by short voltage pulses, which is interesting for
memory applications [6]. Spontaneous and piezoelectric polarization effects on electronic and optical properties of
ZnO/MgZnO quantum well structures were investigated by a gain-model with many-body effects [7]. The spontaneous
polarization constants for MgO and ZnO were determined to -0.070 C/m2 and -0.050 C/m2, respectively [7].
Recently, we have reported on rectifying semiconductor-ferroelectric polarization loops and offsets in Pt-BaTiO3-ZnO-Pt
thin film structures grown on Si(100) [1, 2] and on temperature-dependent dielectric and electro-optic properties of a
ZnO-BaTiO3-ZnO heterostructure on c-plane sapphire [3]. The observed coupling effects within ZnO wurtzite – BaTiO3
perovskite heterostructures by spectroscopic electro-optic ellipsometry birefringence measurements manifest themselves
as a "pinning" of the ferroelectric polarization in the BaTiO3 layer by the cladding ZnO layers [1, 3]. Temperature-
controlled electro-optic Raman measurements assign the electro-optic birefringence results to a temperature-driven
ferroelectric phase transition resulting from the leakage current through the BaTiO3 layer. In the previous
heterostructures grown on silicon or sapphire, the BaTiO3 did not show any preferential crystalline orientation, as tested
by X-ray diffraction (XRD) [1-3]. Therefore, in this work we have choosen SrTiO3(100) as structure matched substrate
material and use electrically conducting SrRuO3 thin films as back electrode to the BaTiO3 thin films. Because of the all-
perovskite BaTiO3 / SrRuO3 thin films on perovskite SrTiO3 substrate, now a textured or even epitaxial structure of the
BaTiO3 films is expected. This structural improvement will increase the ferroelectric response of the BaTiO3 –based
structures considerably.
*mlorenz@physik.uni-leipzig.de; phone +49 341 97 32661 fax +49 341 97 39286 www.uni-leipzig.de/~hlp/
Zinc Oxide Materials and Devices II, edited by Ferechteh Hosseini Teherani, Cole W. Litton,
Proc. of SPIE Vol. 6474, 64741S, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.715217
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OVERVIEW ON INVESTIGATED OXIDE
HETEROSTRUCTURES
Sample No. Thin film structure

J 6918 SrTiO3(100)-SrRuO3/BaTiO3/ZnO/Au
J 6919 SrTiO3(100)-SrRuO3/BaTiO3/Pt
J 6920 SrTiO3(100)-SrRuO3/BaTiO3/SrRuOx
J 6921 SrTiO3(100)-SrRuO3/BaTiO3/ZnO
2. EXPERIMENTAL
At first were grown 70 nm thin electrically conducting SrRuO3 and about 800 nm thin BaTiO3 films on 10 x 10 mm²
SrTiO3 (100) substrates by pulsed laser deposition (PLD) [8]. After this, several 2 mm diameter top electrodes were
grown ex-situ using a shadow mask, either ZnO or SrRuOx by PLD [9-10], or alternatively Pt by DC sputtering. To
provide ohmic contacts with low contact resistivity to the top ZnO electrodes, about 1 mm diameter Au films were DC
sputtered on sample No. J 6918. An overview on the investigated oxide heterostructure samples is given in Table 1.
PLD was done using an KrF excimer laser in oxygen background gas and at elevated substrate temperatures around
600°C. For more details of PLD growth of the oxide heterostructures see Refs. [8-10]. The crystalline structure of the
completed heterostructures was examined by wide-angle XRD using a Philips X’Pert with Cu Kα radiation and a
conventional slit optics. A typical 2Θ−ω scan of sample No. J 6918 (Table 1) is shown in Fig. 2. Obvious in Fig. 2 is the
very good (001) and (002) orientation of the PLD grown BaTiO3 and ZnO thin films, respectively. The widths of the
XRD rocking curves of selected thin film peaks are only slightly increased compared to the single-crystalline SrTiO3
substrate, as shown in Table 2. Even the DC-sputtered top Au contact pads show a (111) orientation perpendicular to the
substrate surface, however, with increased rocking curve width.
Table 1. Overview on investigated thin film heterostructures on SrTiO3 (100) substrates. The SrRuO3, BaTiO3 and ZnO
films were pulsed laser deposited, and the Au top contacts on ZnO were DC-sputtered.







20 406080 100
1x10
1
1x10
2
1x10
3
1x10
4
1x10
5
1x10
6
Kβ
Au (111)
STO (400)
BTO (004)
BTO (003)
STO (200)
STO (100)
J 6918: STO-SRO/BTO/ZnO/Au
Kβ
Kβ
BTO (002)
STO (300)
ZnO (002)
BTO (001)


intensity [cps]
2Θ [°]

Fig. 1. XRD 2Θ−ω scan of sample No. J 6918, the SrTiO3(STO)(100) / SrRuO3 (SRO) / BaTiO3 (BTO) / ZnO / Au
heterostructure. Obvious is the preferential structural orientation of all films perpendicular to the substrate surface:
BaTiO3(001), ZnO(002) and Au(111). For the peak widths of selected peaks see Table 2.
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XRD PEAK WIDTHS J 6918

FWHM (2Θ) FWHM (ω)
SrTiO3 (200) 0.06° 0.48°
BaTiO3 (002) 0.34° 0.51°
ZnO (002) 0.15° 0.72°
Au (111) 0.25° 2.34°
Table 2. XRD full widths of half maximum (FWHM) of the indicated peaks of the SrTiO3 single crystalline substrate, and
the PLD-grown BaTiO3 and ZnO films and the DC-sputtered Au top contacts of sample No. J 6918 (see Table 1), for
the 2Θ−ω scan (see Fig. 1), and for the corresponding ω-scan (Rocking curve), measured by a wide-angle
diffractometer with slit optics.










Current-voltage (I-U), conductivity-frequency (G(f)) and capacitance-frequency (C(f)) curves have been measured for
the BaTiO3-based heterostructures (see Table 1), using an Agilent Precision Semiconductor Parameter Analyzer, and an
Admittance spectrometer. Sawyer-Tower-Circuit measurements have been carried out using a TiePie Handyscope 3
computer controlled oscilloscope with integrated function generator. Electrical contacts were attached to the bottom
SrRuO3 film and the top ZnO(Au), or SrRuOx, or Pt electrodes, respectively. All electrical measurements were done at
room temperature.

3. RESULTS AND DISCUSSION

3.1 Current – voltage measurements
Current-voltage (I-U) measurements have been carried out on all samples described in Table 1. Fig. 2 shows the I-U-
curve of a SrRuO3-BaTiO3-SrRuOx structure on SrTiO3 (100) (sample J 6920), showing almost linear, ohmic behavior.
The resistance of this sample was about 650 Ω, measured between the top and bottom SrRuOx films, which corresponds
to a resistivity of 2.5 x 101 Ωcm, assuming the voltage drops only across the BaTiO3 layer. Most probably, conductive
shunts in the columnar BaTiO3 layer are responsible for this low resistivity value. Therefore, the ohmic currents will
mask the ferroelectric switching behavior of the BaTiO3 film of this particular sample, because the polarization current is
orders of magnitude lower than the ohmic currents. By chemical and structural improvement of the top SrRuOx electrode
we expect a considerable reduction of the resistivity of the SrRuO3/BaTiO3/SrRuOx heterostructures [8].
Fig. 3 shows the current-voltage curves of the SrRuO3-BaTiO3-Pt structure (sample J 6919), which represents an
asymmetric behavior as well as a current hysteresis. In comparison with sample J 6920 in Fig. 2, here in Fig. 3 the
maximum current is of the order of µA (not mA as in Fig. 2), being four orders of magnitude below the current observed
for sample J 6920. Furthermore, the I-U curves show a dependence on increasing integration time of the Semiconductor
Parameter Analyzer, resulting in decreased currents in backward direction, as well as for small forward voltages, and in
increased currents for larger forward voltages. The slightly decreased current with increasing integration time can be
understood with the limited necessary charge to change the polarization, resulting in lower switching-currents if a longer
timescale is considered. The increased current at large forward voltages as well as the asymmetry of the hysteresis points
to a strongly rectifying contribution to the nonideal ohmic contact behavior to the BaTiO3 film, probably to a Schottky-
like junction as discussed in [2, 11].


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-10,0
-5,0
0,0
5,0
10,0
I (mA)
-7
-6
-5
-4
-3
-2
-1
0
log
I (A)
10
-6,0 -4,0 -2,00,0
U(V)
2,04,06,0

Fig. 2. Current-voltage I-U (solid line, left I-scale) and log(I)-U (dashed line, right log(I) scale) curves of sample No. J
6920, the SrRuO3-BaTiO3-SrRuOx structure, showing linear behavior, and no indication for hysteresis effects.


-0,2
0,0
0,2
0,4
0,6
0,8
1,0
1,2
I (µA)
-12
-11
-10
-9
-8
-7
-6
-5
log
I (A)
10
-3-2 -10123
U(V)
t=2ms
i
ti=20ms
ti=200ms

Fig. 3. Current-voltage I-U (solid lines, left I-scale) and log(I)-U (dashed lines, right log(I) scale) curves of sample No. J
6919, the SrRuO3-BaTiO3-Pt structure, with asymmetric behavior and current hysteresis. The dependence of the signals
on the integration time of the Semiconductor Parameter Analyzer points to dynamic effects in the SrRuO3-BaTiO3-Pt
interfaces. The sweep durations for one loop were 4, 40 and 400 sec.. Obviously, the interfaces show in addition to the
ohmic some rectifying behaviour.
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Finally, Figs. 4 and 5 demonstrate the I-U characteristics of the ferroelectric-semiconductor heterostructure SrRuO3-
BaTiO3-ZnO with (J 6918) and without (J 6921) additional ohmic gold contacts on top of the ZnO, respectively. Different
voltage sweeps were applied to study the effect of polarization saturation on the I-U characteristics. Asymmetric I-U
behaviors are visible in Figs. 4 and 5, with increasing voltage positions of the current minimum peaks, with increasing
voltage sweep. We attribute this behavior to the increasing number of ferroelectric domains which are polarized in the
voltage sweep loop. In contradiction to the smaller sweeps in Fig. 4, for ± 6 V the light grey I-U curve in forward and
backward direction is nearly a closed loop, indicating the saturation of the BaTiO3 polarization. A comparison of Figs. 4
and 5 with Fig. 3 shows clearly the effect of the polarized ZnO top electrode.
Figure 5 shows a more pronounced resistive switching hysteresis of the I-U curves for positive bias voltages between 1
and about 3 V for the two sweep directions in comparison to Fig. 4. Such a hysteresis was already observed for the
earlier polycrystalline Pt-ZnO-BaTiO3-Pt heterostructures and was assigned to ferroelectric switching in BaTiO3
influenced by the coupling of the spontaneous wurtzite and the perovskite polarization [2]. The ferroelectric domains of
the BaTiO3 layer are partly self-pinned by the presence of the ZnO layer, and the remaining domains can be switched
and cause the asymmetric ferroelectric hysteresis in the I-U curves [2].

-14
-13
-12
-11
-10
-9
-8
-7
-6
log
I (A)
10
-6 -5 -4 -3 -2 -10123456
U(V)


Fig. 4. Current-voltage log(I)-U curves of sample No. J 6918, the SrRuO3-BaTiO3-ZnO-Au structure, for three different
voltage sweeps of ± 1 V (black curve), ± 2 V (grey), and ± 6 V (light grey). Asymmetric current hysteresis is observed,
with increasing voltage position of the current minima peaks, with increasing voltage sweep. In contradiction to the
smaller sweeps, for ± 6 V the light grey I-U curve is nearly closed, indicating the saturation of the BaTiO3 polarization.
Sweep duration for one loop was 40 sec.

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-14
-13
-12
-11
-10
-9
-8
-7
-6
log
I (A)
10
-7 -6 -5 -4 -3 -2 -1 0 1 23456 7
U(V)

Fig. 5. Current-voltage log(I)-U curves of sample No. J 6921, the SrRuO3-BaTiO3-ZnO structure, for two different voltage
sweeps from -5 to 7 V and back (black curve), and ± 7 V (light grey). For positive bias voltages, the curves show a
clear resistive hysteresis in dependence on sweep direction. Sweep duration for one loop was 40 sec.

3.2 Sawyer-Tower circuit measurements
Sawyer-Tower circuit measurements were already done for the earlier polycrystalline Pt-ZnO-BaTiO3-Pt
heterostructures in dependence on the voltage sweep up to 5 V and for different frequencies from 0.05 kHz up to 1.5
kHz, as reported in Ref. [2]. The schematic of the Sawyer-Tower circuit to measure the resulting electrical polarization
in dependence on the applied electric field is given in Fig. 2 of Ref. [2].
Now, in order to observe the saturation of the BaTiO3 polarization, we performed voltage sweeps with an amplitude of ±
10 V at a frequency of 1 kHz, as shown in Figs. 6 and 7 for the SrRuO3-BaTiO3-Pt and the SrRuO3-BaTiO3-ZnO
structures J 6919 and J 6918, respectively. For the ferroelectric film sample J 6919 in Fig. 6, a fully symmetric hysteresis
curve is observed. However, Fig. 7 shows the effect of the replacement of the Pt top electrode by the piezoelectric ZnO
film and an asymmetric polarization-field behavior is observed now. We attribute this to a coupling between the
spontaneous polarization of the ZnO film and the switchable polarization of the underlying BaTiO3 film. As mentioned
above, this behavior is similar to that reported on polycrystalline Pt-ZnO-BaTiO3 structures [2]. The strong quenching of
the hysteretic behavior for positive sweep voltage found for the earlier structures [2] could not reproduced with the
epitaxial BaTiO3-ZnO structures in Fig. 7, indicating an improved ferroelectric response of the epitaxial structure. An
extension of the simple electrical circuitry model [2] for the asymmetric polarization behavior will be provided in a
separate publication.

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-60
-40
-20
0
20
40
60
V (mV)
o
-10-505 10
V (V)
i

Fig. 6. Sawyer-Tower-Circuit measurement (1 kHz) of sample No. J 6919, the ferroelectric SrRuO3-BaTiO3-Pt structure,
with symmetric hysteresis indicating normal ferroelectric behavior.

-12
-10
-8
-6
-4
-2
0
2
4
6
V (mV)
o
-10-50510
V (V)
i

Fig. 7. Sawyer-Tower-Circuit measurement (1 kHz) of sample No. J 6918, the SrRuO3-BaTiO3-ZnO structure. The
asymmetric behavior is attributed to the coupling of the spontaneous polarization of ZnO and the switchable
polarization in BaTiO3. Note the reduced polarization expressed by V0 of this structure compared to Fig. 6.

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3.3 Conductance measurements

10-7
10-6
10-5
10-4
10-3
G(S)
102
103
104
105
106
107
108
f(Hz)
G(J6918)
G(J6919)
G(J6920)
G(J6921)

Fig. 8. Conductance G measured in Siemens (1/Ω) versus frequency for the four different oxide heterostructures, as
explained in Table 1.

10-11
10-10
10-9
C(F)
102
103
104
105
106
107
108
f(Hz)
C(J6918)
C(J6919)
C(J6920)
C(J6921)

Fig. 9. Capacitance versus frequency for the four different oxide heterostructures, as explained in Table 1.

Impedance measurements have been performed on the four oxide heterostructures, as described in Table 1. The
frequency dependent conductance and capacitance are depicted in Figs. 8 and 9, respectively. As already shown by the I-
U data (Fig. 3), the conductance of sample J 6920 is very high and almost independent of the frequency, due to the low
parasitic ohmic resistance of this particular BaTiO3 layer, probably caused by the not appropriate SrRuOx layer. For the
SrRuO3-BaTiO3-Pt sample J 6919, a large drop of the capacitance above 10KHz is observed in Fig. 9, which corresponds
to a slower increase of the conductance in Fig. 8.
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Both the conductivity (Fig. 8) and the capacitance (Fig. 9) of the BaTiO3-ZnO samples J 6921 and J 6918 show a two-
step increasing and decreasing behavior, respectively. The first step corresponds to a similar behavior of the SrRuO3-
BaTiO3-Pt sample J 6919. This leads to the conclusion, that the first decrease of capacitance is due to ionic recharging
processes which can not follow beyond the frequency of about 10 kHz, while the second step above 1 MHz is due to
recharging of impurities of the ZnO as an electronic process.
4. SUMMARY AND CONCLUSIONS
We have shown a clearly different current-voltage and polarization-electrical field (Sawyer-Tower) behavior of all-oxide
epitaxial BaTiO3 films grown on conducting SrRuO3 contact films on single crystalline SrTiO3 (100) substrates, for Pt,
SrRuOx and ZnO top electrodes. The pronounced differences of electrical behavior of the heterostructures with metallic
Pt and piezoelectric ZnO top electrode point to a coupling of the switchable ferroelectric polarization with the fixed
spontaneous polarization of the wurtzite ZnO. The frequency dependence of conductivity and capacity shows a two-step
behavior of the structures with ZnO electrode, due to recharging of impurities in the ZnO. In comparison to the earlier
polycrystalline BaTiO3-films grown on Pt/Si(100), the epitaxial structures show a more pronounced ferroelectric
switching and a stronger pinning of the ferroelectric domains by the polarized ZnO top layer.
ACKNOWLEDGMENTS
This work was supported by the German DFG within Research Group FOR 404 under Grant No. SCHU 1338/4-1.
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