Dielectric properties of lead lanthanum zirconate titanate thin films with andwithout ZrO2 insertion layersShanshan Liu, Beihai Ma, Manoj Narayanan, Sheng Tong, Rachel E. Koritala et al. Citation: J. Appl. Phys. 113, 174107 (2013); doi: 10.1063/1.4804170 View online: http://dx.doi.org/10.1063/1.4804170 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v113/i17 Published by the American Institute of Physics. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors
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Dielectric properties of lead lanthanum zirconate titanate thin filmswith and without ZrO2 insertion layers
Shanshan Liu,1,a) Beihai Ma,1 Manoj Narayanan,1 Sheng Tong,2 Rachel E. Koritala,2
Zhongqiang Hu,1 and Uthamalingam Balachandran1
1Energy Systems Division, Argonne National Laboratory, Argonne, Illinois 60439, USA2Nanoscience and Technology Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
(Received 7 March 2013; accepted 22 April 2013; published online 7 May 2013)
The dielectric properties of lead lanthanum zirconate titanate (PLZT) thin films on platinized
silicon (Pt/Si) with and without ZrO2 insertion layers were investigated in the temperature range
from 20 �C to 300 �C. Permittivity, dielectric loss tangent, and tunability were reduced for the
samples with ZrO2 insertion layers compared to those without the layers. Additionally, the
permittivity was less dependent on frequency over the broad temperature range studied
(20–300 �C). The leakage current behavior of the PLZT films with and without ZrO2 insertion
layers was also investigated, and on the basis of those results, a probable conduction mechanism
has been suggested. The improved electrical properties in the PLZT with ZrO2 layers are attributed
to the ZrO2 layer blocking the mobile ionic defects and reducing free charge carriers to transport.VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4804170]
I. INTRODUCTION
Because of high dielectric permittivity, large polariza-
tion, and reasonable dielectric loss, ferroelectric thin films,
such as (Pb,La)(Zr,Ti)O3 (PLZT) and BaxSr(1�x)TiO3 (BST),
are of special interest for their potential applications in the
devices of ferroelectric memories, tunable microwave devi-
ces, and infrared sensors.1–3 Good temperature stability is
also required for high performance and long-term reliability
in these devices.4 The dielectric permittivity in ceramic thin
films is strongly dependent on temperature, so decreasing the
temperature coefficient capacitance will be beneficial for
many device applications. At present, much work has been
done to optimize the properties and improve the temperature
stability of the ferroelectric materials and reliability of the
devices. Xia et al.5 investigated the substitution of Pb for Ba
in the BST system and reported that adding A-site doping of
Pb led to a diffuse phase transition with respect to tempera-
ture, improving the thermal stability of the dielectric pro-
perties. Cole et al.,6 Lu et al.,7 and Zhu et al.8 studied
compositionally graded BST thin films. They all observed a
broad and flat profile in the permittivity-versus-temperature
curve for the graded films, indicating that it will be possible
to build devices with almost negligible temperature depend-
ence on the capacitance. Furthermore, the multilayer struc-
tured films were reported to improve the dielectric properties
and lower the temperature dependence by combining ferro-
electric thin films with dielectric materials that have low
dielectric loss. Qin et al.9 reported reduced dielectric loss in
Ba(Zr,Ti)O3/BST/Ba(Zr,Ti)O3 (BZT/BST/BZT) multilayer
thin films, though the dielectric permittivity and tunability of
multilayer thin films were slightly reduced. Singh et al.,10
Fang et al.,11 and Sahoo12 also noted that interposing of one
or more ZrO2 layers between the perovskite-related materials
such as CaCu3Ti4O12 (CCTO) or BST considerably reduced
the leakage current in the multilayer structure. We are not
aware of any report on dielectric spectroscopic studies of
PLZT/ZrO2 thin films over a broad temperature range. The
Pb(ZrxTi1�x)O3 (PZT) compositions exhibit exceptional
dielectric and ferroelectric properties around the morpho-
tropic phase boundary with Zr/Ti ratio of 52/48 because of
the enhanced possibility of polarization alignments.13
Furthermore, the La donor dopants loosen the unit cells for
ease of domain reorientation/rotation and decrease the oxy-
gen vacancies, which result in a higher relative permittivity
(er) and spontaneous polarization (Ps) and lower leakage cur-
rent.14 In this work, we studied the dielectric relaxation as a
function of temperature and frequency for (Pb0.92La0.08)
(Zr0.52Ti0.48)O3 (PLZT) thin films on platinized silicon
(Pt/Si) with and without ZrO2 insertion layers and investi-
gated the effect of the ZrO2 layers on the electrical properties
of PLZT thin films devices.
II. EXPERIMENTAL
(Pb0.92La0.08)(Zr0.52Ti0.48)O3 (PLZT) thin films were de-
posited on platinized silicon (Pt/Si) by the sol-gel method. A
0.5 molar PLZT solution was prepared by dissolving appro-
priate amounts of lead acetate trihydrate, titanium isopropox-
ide, zirconium propoxide, and lanthanum nitrate hexahydrate
in 2-methoxyethanol. We used 20 mol. % excess lead in the
starting solution to compensate for the lead loss during the
high-temperature crystallization. The details of the solution
synthesis are reported elsewhere.15 The ZrO2 films were de-
posited using RF-magnetron sputtering (ORION-8, AJA
International, Inc.). The ZrO2 target had a 50-mm diameter
and 3.175-mm thickness. The sputtering system base pres-
sure was maintained at � 9� 10�8 Torr before introduction
of the sputtering gas. The platinized silicon substrates were
cleaned ultrasonically in acetone and methanol for 10 min
prior to deposition. During the ZrO2 deposition, the substrate
a)Author to whom correspondence should be addressed. Electronic mail:
[email protected]. Tel.: þ1 630 252 3632.
0021-8979/2013/113(17)/174107/6/$30.00 VC 2013 AIP Publishing LLC113, 174107-1
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temperature was maintained at room temperature (RT), and
argon and oxygen (95:5 ratio mixture) with pressure of 3 mT
were introduced as the sputtering gas. The RF power was
fixed at 100 W, corresponding to 5 W/cm2. The thickness of
the ZrO2 films was kept at 6 nm.
For sample fabrication, PLZT stock solution was spin
coated onto platinized silicon substrates at 3000 rpm for 30 s.
Films were then pyrolyzed at 450 �C for 10 min and crystal-
lized at 650 �C for 5 min, with a final annealing of 650 �C for
20 min. The final thickness of the PLZT films was measured
to be 850 nm. The two layers of ZrO2 films were evenly
inserted into the PLZT films on Pt/Si substrates, as shown in
the cross-sectional scanning electron microscopy (SEM)
image of Fig. 1. This sample microstructure was characterized
by SEM using a Hitachi S4700 field-emission microscope.
After film deposition, platinum top electrodes (250-lm diam-
eter and 100-nm thickness) were deposited through a shadow
mask by electron-beam evaporation. Dielectric measurements
were made as a function of temperature with an Agilent
E4980A LCR meter using an oscillator level of 0.1 V in con-
junction with a Signatone QuieTempVR
probe station with hot
stage (Lucas Signatone Corp., Gilroy, CA). To study the
dielectric permittivity of the fabricated structures, we per-
formed low-signal (0.1 V) impedance measurements in
parallel circuit mode over a range of frequencies. A Keithley
237 high-voltage source meter and Radiant Technologies
Precision Premier II Tester were used to measure the leakage
current.
III. RESULTS AND DISCUSSION
Cross-sectional SEM images of PLZT films with and
without ZrO2 layers are shown in Figs. 1(a) and 1(b), respec-
tively. The SEM graphs demonstrated that both films exhib-
ited dense and well-crystallized microstructures. The pure
PLZT film shows a columnar growth throughout the film
thickness of 850 nm. In contrast, PLZT/ZrO2 multilayered
films reveal the presence of visible internal interfaces, which
halt the continuous growth of PLZT film. These films do not
contain any obvious defects or other secondary phases,
which is also confirmed by x-ray diffraction pattern. The
dielectric permittivity response as function of temperature
and frequency for pure PLZT film and PLZT/ZrO2 multilay-
ered films is shown in Fig. 2. For both samples, the permit-
tivity increased with temperature until the Curie point and
decreased past that point at all frequencies. The pure PLZT
films on Pt-Si substrates exhibit a diffused phase transition
for all frequencies. The corresponding temperature (Tm) of
maximum permittivity, which might correspond to the phase
transition, is �170 �C for PLZT/Pt/Si. This value is close to
the calculated value of 180 �C for bulk PLZT (8/52/48).16
When compared to the pure PLZT thin films, the PLZT film
with ZrO2 insertion layers possesses a broader, flatter, more
diffuse dielectric response as a function of temperature from
RT to 300 �C.
The temperature coefficient of capacitance (TCC)
(Ref. 4) was calculated by using the plots of permittivity (er)
versus temperature from RT to Tm and the following
equation
TCC ¼ De=ðeRTDTÞ; (1)
where De is the change in permittivity with respect to eRT at
RT, and DT is the change in Tm relative to RT. Our calcula-
tions showed that at 10 kHz, the TCC value of PLZT films
with ZrO2 insertion layers (�8.4� 10�4/ �C) is lowered by
45% in comparison to pure PLZT films (�15.4� 10�4/ �C)
in the temperature range of RT to Tm. Thus, the PLZT/ZrO2
thin film had reduced temperature variation compared with
pure PLZT film.
Figure 3 shows the frequency dependence of the permit-
tivity and dielectric loss tangent for PLZT thin films with
and without ZrO2 insertion layers measured at RT. At
10 kHz, the permittivity for the Pt/PLZT/Pt and Pt/PLZT/
ZrO2/PLZT/ZrO2/PLZT/Pt capacitors are around 1370 and
820, respectively; the corresponding values for dielectric
loss are 0.033 and 0.026. The lower permittivity and dielec-
tric loss for the PLZT/ZrO2 films are possibly due to the
introduction of the low-permittivity ZrO2 insertion layers.
The effective permittivity of multilayered films can be deter-
mined by series combination of capacitors, while assuming
that the interdiffusion between PLZT and ZrO2 is negligible.
The effective permittivity eef f can then be calculated via the
series capacitor model
ttotal
eef f¼ tplzt
eplztþ tZrO2
eZrO2
; (2)FIG. 1. Scanning electron microscopy cross-sectional images of (a) pure
PLZT film and (b) PLZT/ZrO2 film on Pt-Si substrates.
FIG. 2. Temperature-dependent dielectric permittivity for pure PLZT film
and PLZT/ZrO2 film on Pt-Si substrates measured at 1 kHz, 10 kHz,
100 kHz, and 1 MHz.
174107-2 Liu et al. J. Appl. Phys. 113, 174107 (2013)
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where ttotal is the total thickness of the PLZT/ZrO2 multilay-
ered films, and tplzt and tZrO2are the thickness of PLZT and
ZrO2, respectively. Also, eef f is the effective permittivity of
multilayered films, whereas eplzt is the permittivity for PLZT
films, which is about 1370 at 10 kHz in this study, and eZrO2
is the permittivity of ZrO2, which is near 25.17 The calcu-
lated eef f from Eq. (2) is 780 for PLZT thin films with two
layer of ZrO2, which is very close to the experimental result.
Over the frequency range 0.1–1000 kHz, there is broad
agreement with the calculated permittivity values. This find-
ing indicates no significant interdiffusion between the PLZT
and ZrO2 layers, as similarly reported by Singh et al.10 The
inset of Fig. 3 shows the RT tunability as a function of
applied bias field measured at 10 kHz for pure PLZT thin
films and PLZT/ZrO2 multilayered films. The tighter tunabil-
ity curve for the PLZT film shows the higher tuning at the
same applied field with respect to the PLZT film with ZrO2
insertion layers. For example, at 40 V (236 kV/cm), the tuna-
bility is �48% and �63% for PLZT films on Pt/Si substrates
with and without ZrO2 insertion layers, respectively. The
tunability of PLZT/ZrO2 thin films is reduced by 24% com-
pared with pure PLZT thin films.
The leakage current densities as a function of electric
field for the PLZT thin films with and without ZrO2 insertion
layers were also measured with both polarities of the applied
electric field on the top electrode. The leakage current density
was not affected by reversing the electric field since the PLZT
samples were completely symmetric with or without ZrO2
insertion layers. At given electric field, the leakage current
density was reduced by introducing the 6-nm ZrO2 layers. For
example, the leakage current density at 20 V (�118 kV/cm)
for multilayered films of PLZT/ZrO2 was around 1.73� 10�7
A/cm2, compared with 2.24� 10�7 A/cm2 for pure PLZT
films. This difference might arise from different conduction
mechanisms. In order to understand the charge transport
mechanism that is responsible to this behavior, we measured
the dielectric properties at different bias voltages as a function
of frequency for pure PLZT thin films and PLZT/ZrO2 films.
Figures 4(a) and 4(b) show the dc bias voltage depend-
ence of the permittivity and loss tangent measured at differ-
ent frequencies for samples with and without ZrO2 insertion
layers. The permittivity and dielectric loss tangent of pure
PLZT films in Fig. 4(a) increase with increase in bias volt-
age. This indicates that there are charge carriers in the pure
PLZT films. However, when we inserted two 6-nm layers of
ZrO2 into the pure PLZT films, this phenomenon disap-
peared, as shown in Fig. 4(b). Within the measured fre-
quency range, neither the permittivity nor the dielectric loss
was affected by applying dc bias voltages of 0–0.8 V; this
behavior is associated with a decrease of the charge carriers
in the dielectric materials. The PLZT/ZrO2 samples exhib-
ited less dependence on bias voltage, indicating more diffi-
culty for the transport of free charge carriers. This suggests
that the ZrO2 layer acts as an insulating barrier through
which the charge carriers are unable to pass. Though the
interdiffusion at the interfaces of perovskite and insulator
might overlap through the insulating layers, we observed no
such behavior in our samples, indicating that the diffusion
length for free charge carriers is considerably smaller than
6 nm under the experimental conditions used.10,11
FIG. 3. Frequency dependence of dielectric properties measured at room
temperature for pure PLZT and PLZT/ZrO2 on Pt-Si substrates. Inset shows
the RT tunability as a function of applied field measured at 10 kHz for pure
PLZT and PLZT/ZrO2 on Pt-Si substrates.
FIG. 4. DC bias voltage dependence of
dielectric properties as various frequen-
cies measured at room temperature for
(a) pure PLZT and (b) PLZT/ZrO2 on
Pt-Si substrates.
174107-3 Liu et al. J. Appl. Phys. 113, 174107 (2013)
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The electrical properties of dielectric thin films strongly
depend on the free charge carriers, such as impurities or
defects.18,19 The dielectric loss in the PLZT perovskite thin
films arises mainly from conductive loss and the relaxation
loss due to the dipole.20 In the conductive loss mechanism,
the energy is consumed by the free charge carriers in the thin
films, while the dielectric loss due to the relaxation mecha-
nism is correlated to high values of permittivity, since
enhanced polarization increases the energy dissipation dur-
ing relaxation. For the pure PLZT samples, the enhanced
dielectric permittivity and loss reveal a strong dependence
on bias voltage, indicating a conduction mechanism caused
by free charge carriers. Figures 5(a) and 5(b) show the leak-
age current densities as a function of applied field for PLZT
thin films with and without ZrO2 layers measured at room
temperature. As shown in these figures, the leakage current
density plot could be segmented into different regions.
Initially for the PLZT/ZrO2 thin films in Fig. 5(b), the leak-
age current density increases linearly with external electric
field with a slope close to 1 in the region of low electric field,
which suggests ohmic conduction. After that, the leakage
current density versus electric field is constant, consistent
with similar behavior reported by Cross et al.21 This region
might indicate electron trapping by defects and formation of
a depletion region near the interface with a negative charge,
resulting in a decrease in the effective electric field and cur-
rent injection. As the applied field increases, the leakage cur-
rent density sharply increases, indicating strong injection of
charge carriers in the bulk of the film. In the region with the
highest slope, injected electrons are trapped in the insulator
and form the space charge. Scott et al. proposed a space-
charge-limited-current (SCLC) mechanism for the leakage
current density in PZT22
J ¼ CVn
dm; (3)
where C, n, m are arbitrary constants, V is the applied volt-
age, and d is the film thickness. Rose23 has shown that n has
values larger than 3 when the energy distribution of the traps
in the energy gap is continuous. In our samples, the conduc-
tion mechanism observed closely follows SCLC behavior.
As shown in Fig. 5(a), the pure PLZT thin film shows
ohmic behavior at electric field below 80 kV/cm, while the
leakage current density shows a slow, steady increase
between 80 and 310 kV/cm; by contrast, the leakage current
density for the sample with ZrO2 insertion layers remained
flat between 120 and 320 kV/cm. This difference indicates
that incorporating ZrO2 layers into PLZT thin film increases
the role of the traps in ZrO2 and interfaces and induces the
leakage current response.
In general, for PLZT films, the oxygen vacancies are eas-
ily formed during fabrication under high sintering tempera-
ture, and the activation energy of oxygen vacancies falls
between 0.7 and 1.1 eV for perovskite materials.24,25 Figures
6(a) and 6(b) show the leakage current density as a function
of time (E � 60 kV/cm) measured at different temperatures
for the PLZT film with and without ZrO2 insertion layers.
The leakage current density of PLZT/ZrO2 multilayered films
is lower than that of pure PLZT films at all temperatures.
FIG. 5. Leakage current density (J) vs.
applied field (E) for (a) pure PLZT and
(b) PLZT/ZrO2 on Pt-Si substrates. The
slope values are shown.
FIG. 6. Leakage current density at dif-
ferent temperatures for (a) pure PLZT
and (b) PLZT/ZrO2 on Pt-Si substrates.
174107-4 Liu et al. J. Appl. Phys. 113, 174107 (2013)
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Upon increasing the temperature from RT to 300 �C, the leak-
age current density increased for both samples. For example,
over this temperature range at 100 s, the leakage current
density increased from �4.3� 10�8 A/cm2 to �1.3� 10�5
A/cm2 for PLZT/ZrO2 films and �6.0� 10�8 A/cm2 to
�2.1� 10�5 A/cm2 for pure PLZT thin films. The higher
temperature enhances the mobility of charge carriers in the
films and induces the higher leakage current. In this field
region, the leakage current density obeys the Arrhenius rela-
tionship. The activation energy for the samples was calcu-
lated by plotting the dc leakage current density at 100 s
versus reciprocal temperature in Fig. 7 and applying the
Arrhenius law in the following equation:
Jdc ¼ A exp�Ea
kBT
� �; (4)
where Jdc is the leakage current density, A is the constant, Ea
is the activation energy, kB is Boltzmann’s constant, and T is
the temperature in Kelvin. The least square fittings of the data
clearly show two different activation energies (or barrier
heights) for each sample. In the lower temperature region, the
slope for pure PLZT film and PLZT/ZrO2 film in Fig. 7 is
very close, and the calculated activation energies fall into the
range of 0.05–0.07 eV, which indicates that conduction is
probably not fully thermally activated. This measured value
is near to activation energy of 0.1 eV, which is the first-
ionization energy of oxygen vacancies.26 These free elec-
trons, which result from the first-ionization oxygen vacancies,
accumulate at the interfaces and contribute to the dielectric
polarization. Furthermore, direct measurement of the activa-
tion energy in the high temperature region of Fig. 7 yields
activation energies of 0.59 and 0.64 eV for pure PLZT film
and PLZT/ZrO2 film, respectively. These values agree well
with the previously reported activation energies for diffusion
of doubly ionized oxygen vacancies (Vo€).27,28 This finding
probably suggests that the oxygen vacancies are the main
charge carriers dominating the dc leakage current. The effect
of Vo€ on resistance degradation in the PLZT thin films has
been attributed to the migration and subsequent accumulation
of Vo€ in front of the cathode, where the accumulation of Vo€at the cathode leads to a significant increase in electron con-
centration. The resistance degradation would be more pro-
nounced in systems that have higher Vo€values. The insertion
of ZrO2 layers will block the mobile ionic defects, such as
Vo€, and the slightly increased trap level with the incorpora-
tion of the ZrO2 layer could be responsible for the observed
reduction in the leakage current in PLZT/ZrO2 thin film
devices.
IV. CONCLUSIONS
We deposited PLZT films on Pt/Si substrates with and
without ZrO2 insertion layers to study the temperature de-
pendence of the dielectric properties and leakage current.
Compared with the pure PLZT films, the dielectric loss and
leakage current density were reduced in the PLZT films with
ZrO2 insertion layers, accompanied by a reduction of the per-
mittivity. Dielectric measurements as a function of tempera-
ture and frequency were also less dependent over a broad
temperature range (20–300 �C). The leakage current behav-
ior demonstrated that both samples exhibited the dominant
conduction mechanisms: ohmic conduction at low field and
the SCLC mechanism at high field. Introducing ZrO2 layers
into the devices increased the activation energy slightly,
from 0.59 to 0.64 eV; this indicated that the conduction
charge carriers are from doubly ionized oxygen vacancies.
Such less temperature-dependent permittivity and reduced
leakage current are useful in designing ferroelectric memo-
ries and other applications.
ACKNOWLEDGMENTS
This work was supported by the U.S. Department of
Energy, Vehicle Technologies Program, under Contract No.
DE-AC02-06CH11357. The electron microscopy was
accomplished at the Electron Microscopy Center at Argonne
National Laboratory, a U.S. Department of Energy Office of
Science Laboratory operated under Contract No. DE-AC02-
06CH11357 by UChicago Argonne, LLC.
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