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:

sliu@anl.gov. 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.

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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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