Electrical characterization of Ba(Zr0.1Ti0.9)O3 thin films grown bypulsed laser ablation technique

Sandip Halder, Sudipta Bhattacharyya, S.B. Krupanidhi *

Materials Research Centre, Indian Institute of Science, Bangalore 560012, India

Received 7 January 2002; received in revised form 11 April 2002; accepted 23 April 2002

Abstract

In situ annealed thin films of ferroelectric Ba(Zr0.1Ti0.9)O3 were deposited on platinum substrates by pulsed laser ablation

technique. The as grown films were polycrystalline in nature without the evidence of any secondary phases. The polarization

hysteresis loop confirmed the ferroelectricity, which was also cross-checked with the capacitance�/voltage characteristics. The

remnant polarization was about 5.9 mC cm�2 at room temperature and the coercive field was 45 kV. There was a slight asymmetry

in the hysteresis for different polarities, which was thought to be due to the work function differences of different electrodes. The

dielectric constant was about 452 and was found to exhibit low frequency dispersion that increased with frequency. This was related

to the space-charge polarization. The complex impedance was plotted and this exhibited a semicircular trace, and indicated an

equivalent parallel R �/C circuit within the sample. This was attributed to the grain response. The DC leakage current�/voltage plot

was consistent with the space-charge limited conduction theory, but showed some deviation, which was explained by assuming a

Poole�/Frenkel type conduction to be superimposed on to the usual space-charge controlled current. # 2002 Elsevier Science B.V.

All rights reserved.

Keywords: Ferroelectric; Thin films; Barium zirconium Titanate; Laser ablation

1. Introduction

One of the most important properties of dielectric thin

film materials is their relatively high dielectric constant,

which is not only indispensable for the application of

thin films in integrated capacitors but also fundamental

for future applications of thin film capacitors in

dynamic random access memories (DRAMs). At pre-

sent, ferroelectric materials are well recognized to be

excellent capacitor materials for DRAMs in ultra large-

scale integration [1�/4]. The most attractive advantage of

ferroelectric materials over conventional nitride-oxide is

that the former has a very high dielectric constant,

especially for memory densities above 64 MB and above

[5�/7].

Solid solution of BaTiO3 and BaZrO3 (Ba(Zrx -

Ti1�x)O3 or BZT) is very important for multilayer

ceramic capacitors. With increase in Zr percentage

three-phase transitions (as in pure BaTiO3) move closer

together and finally coalesce at x�/0.1 [8].

There had been a few reports of the synthesis of single

crystals and ceramic samples of BZT [8,9]. However, to

the best of our knowledge, there had been no attempts

of growing this material in a thin film form. Ba(Zr0.1-Ti0.9)O3 is a promising material candidate for DRAMs

because of its high dielectric constant in the paraelectric

phase. In this article we report about the growth of BZT

thin film by pulsed laser ablation. The samples were also

characterized electrically in the paraelectric phase. Their

DC electrical properties were analyzed by assuming a

distribution of trap states in the band gap. The AC

electrical properties were also studied to compare theresults obtained from the DC measurements.

2. Experimental

For the present work a KrF excimer laser operating at

5 Hz was used. The beam was focused to the desired

energy by an ultraviolet (W) grade plano convex lens of* Corresponding author

E-mail address: sbk@mrc.iisc.ernet.in (S. Halder).

Materials Science and Engineering B95 (2002) 124�/130

www.elsevier.com/locate/mseb

0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 2 2 2 - 2

50 cm focal length and was coupled into the vacuum

chamber through a quartz port. The angle of incidence

of the beam with the target was 458 with an energy

density (fluence) of 4 J cm�2. The deposition pressurewas controlled with an MKS operated in conjunction

with a mass flow controller.

A dense ceramic target of Ba(Zr0.1Ti0.9)O3 was pre-

pared via the conventional solid-state reaction. The

starting materials (BaCO3, TiO2 and ZrO2 all of purity

99.99%) were ball-milled for 4 h in acetone and then

calcined at 1300 8C for 4 h. The calcined powder was

then pressed into 18 mm targets. The targets weresintered at 1360 8C for 6 h. A freshly polished surface

was used for each deposition, and the target was

mounted on a rotating carousel to ensure uniform

ablation. Substrates were placed in parallel with the

target at a distance of 3 cm. The substrates used for the

present deposition were (111)-oriented Pt on

TiO2½SiO2½Si. The films were grown in situ at different

temperatures (610�/670 8C). Prior to deposition thebase pressure of the chamber was brought down to

2.5�/10�5 Torr. During deposition the oxygen pressure

was maintained at 50 mTorr.

The Ba(Zr0.1Ti0.9)O3 films were characterized structu-

rally by X-ray diffraction (XRD). Compositional uni-

formity was determined at various points over the film

using energy dispersive X-ray analysis (EDAX). The

film thickness was determined by an optical spectro-meter (Filmetrics F20). The film thickness varied

between 400 and 700 nm. With the help of a shadow

mask, Au dots of 1.96�/10�3 were deposited by thermal

evaporation.

3. Results and discussion

3.1. Structural characterization

3.1.1. Effect of oxygen pressure

Fig. 1(a) shows the XRD patterns for the films

deposited at different oxygen pressures between 50 and

100 mTorr, while keeping the temperature constant at

670 8C. All the films are polycrystalline in nature. At

low oxygen pressures, the films tend to show bettercrystallinity. With increase in oxygen pressure the

intensity of the perovskite peaks decreases. It was seen

that when the films are deposited at a pressure of 100

mTorr, (100) and (200) peaks are absent and the

intensity of (110) peak has reduced. With increase in

pressure, the energy of the deposited species is reduced,

hence affecting the crystallinity which accounts for the

decrease in peak heights at higher pressures. Thecrystallite size analysis using the Scherrer equation

reveals it to be approximately 60 nm for films deposited

at all pressures.

3.1.2. Effect of temperature

From the XRD pattern, Fig. 1(b), for the films

deposited at different substrate temperatures, it has

been found that the crystallinity increases with the

deposition temperature. It has been observed from

crystallite size analysis that there was a significant

increase in grain size with an increase in substrate

temperature. For the films deposited at 610 8C the

crystallite size was around 60 nm, while the grain size is

around 130 nm at 670 8C. As observed from earlier

reports [10] the variation can be attributed to the

increase in mobility of the deposited species at higher

substrate temperatures.

Fig. 1. (a) XRD graph of BZT films deposited at different pressures.

(b) XRD graph of BZT films deposited at different temperatures.

S. Halder et al. / Materials Science and Engineering B95 (2002) 124�/130 125

3.2. Polarization hysteresis

The ferroelectric nature of the Ba(Zr0.1Ti0.9)O3 thin

films was confirmed from the polarization hysteresis

measurements, as shown in Fig. 2. The measured values

of spontaneous and remnant polarization were 13.4 and

5.9 mC cm�2, respectively, with a coercive field of 45 kV

cm�1. The asymmetric behavior that occurred in the

films can be induced by various factors, such as defect

charges present in the ferroelectric material or due to

different work functions of the top and bottom electro-

des [11].

3.3. Dielectric properties

Fig. 3(a) shows the change of o?r with temperature for

different frequencies. The value of o?r at lower frequen-

cies (100 Hz) rises sharply with increase in temperature,

while at higher frequencies (100 KHz) it is found to

decrease. This suggests the contribution of the space

charge to the complex dielectric constant at lower

frequencies [12�/15]. At higher frequencies, the merging

of the dielectric curves indicates that the space-charge

effect diminishes. The influence of the space charge is

also reflected in the imaginary part of the dielectric

constant (/o??r); as shown in Fig. 3(b) where we observed

similar frequency dispersion at higher temperatures. The

dielectric phase transition is shown in Fig. 4. It is evident

from the o �/T curve that the phase transition was

diffused in nature, which occurred between 270 and

330 K. The diffused nature of the phase transition could

be due to the fine grained structure of the films.

3.4. I �/V characteristics

The DC leakage behavior of a typical Ba(Zr0.1Ti0.9)O3

thin film is shown in Fig. 5. It was seen that the current�/

voltage characteristics were highly non-linear in nature.

The plot has been given in log�/log scale. A power law

relation could represent the current with voltage as

I �V a:

The exponent ‘a ’ is a parameter that characterizes the

type of conduction. In the log�/log plot, the slope of the

I �/V curve would give the measure of the parameter ‘a ’.Fig. 2. Polarization hysteresis curve of a typical in situ deposited BZT

thin film.

Fig. 3. (a) Real part of dielectric constant as a function of frequency at

various temperatures. (b) Imaginary part of dielectric constant versus

frequency at different temperatures.

S. Halder et al. / Materials Science and Engineering B95 (2002) 124�/130126

It was seen that, in the observed I �/V plot, the low-

field region began with a linear dependence on the

voltage, as evident from the unit slope of the plot.

Hence, we might conclude that the bulk is dominating

the low-field conduction through the thin film, and there

is no interface barrier (Schottky) near to the electrodes.

If there were a barrier in the electrode, then, particularly

at the low fields, the major part of the voltage drop

would be across the interface (since the resistance of a

Schottky barrier at low field would be very high). In

such a case, the true bulk-like nature would not be

observed. The Arrhenius plot of the current at a low

voltage (0.5 V) is also shown in Fig. 6. The correspond-

ing activation energy was found to be 0.80 eV. This

could be attributed to the movement of oxygen vacan-

cies [16].

Beyond a voltage of 5 V, there was a non-linearity

observed. The slope of I �/V plot at that region was

around 8.14 at room temperature. At higher tempera-

tures, the slope was slightly reduced. For example, the

slope in the non-linear region was 6.08 at a temperature

of 150 8C. This trend was observed in the intermediate

temperatures also. Since the film was deposited at high

temperature, it could be expected to consist of a

columnar microstructure, i.e. column-shaped grains

extending from one side to the other side of the film

along the thickness. It is a reported fact that the in situ

deposited films tend to show a fibrous grain structure

[17]. Scott reported that, in columnar grains, the current

would be governed by the space-charge-limited conduc-

tion mechanism [18]. In space-charge controlled current,

the current should follow a square-law dependence on

voltage. However, in the presence of traps in the sample,

the traps first would get filled with the injected charges,

and then the current would sharply rise to the trap-filled

limit [19]. At this region, for the traps with distributed

energy levels, the value of ‘a ’would be of the order of 7�/

10. At low temperature, the electron distribution among

the various energy levels would be a sharply decreasing

function near the Fermi energy. But as the temperature

increases, the distribution function would start getting

rounded and the change in the electron density would be

a relatively slowly changing function with energy. The

rise in the space-charge current in the trap-filled region

is basically a consequence of this behavior of the Fermi�/

Fig. 4. Dielectric constant versus temperature.

Fig. 5. Current�/voltage characteristics of thin BZT sample.

Fig. 6. Arrhenius plot of the DC conductivity.

S. Halder et al. / Materials Science and Engineering B95 (2002) 124�/130 127

Dirac distribution function; at the trap-filled voltage,

the Fermi energy basically passes through the trap

energy levels. However, since the electron population

in the trap levels becomes more homogeneous at high

temperature, the rise in the current is expected to be

slower than that at lower temperature. Beyond the

region where the current shows a very rapid increase

with voltage, there exists another region of the I �/V

curve, in which current varies relatively slowly. This

region is known as the trap-free space-charge regime,

since all the trap levels would be filled by this time, and

any further charge carriers would directly be injected in

the conduction band. In such a case, the sample would

behave as if there are no traps in it, and the current

would vary as the square of the voltage. The high-field

I �/V curve is shown in Fig. 7 and the slope was found to

be 1.96. This was also another proof that the space-

charge contribution was dominant in the high-field

region of the I �/V plot. The low-field conduction was

governed by the bulk-generated charge carriers, and was

ohmic in nature. This was evident from the unit slope of

the low-field current�/voltage plot in the log�/log scale.

The low-field region, the trap-filled limit and the high-

field trap-free square-law regime formed a triangle

called the Lampert’s triangle [20].

The high-field region of the I �/V curve, which

exhibited a slope of nearly ‘2’ in the log�/log scale, was

also a function of temperature. This could be explained

on the basis of the existence of shallow traps in the

sample. Since the value of the Fermi�/Dirac probability

function is always lesser than unity above the Fermi

level, and the location of the shallow traps is also above

Fermi level, it could be assumed that the shallow traps

would always remain partially unfilled. Therefore, the

role of the shallow traps would still be seen even beyond

the (deep) trap-filled limit. It was shown by Lampert [19]

that, in the presence of shallow traps, the space-charge

current would still show a square-law dependence on thevoltage.

The trap-filled limit was defined by Lampert [19] as

the onset of non-linearity in a trap-controlled space-

charge conduction phenomenon. In our case, the trap-

filled limit was at 5 V at room temperature, and showed

an increasing trend with temperature. This trend was

observed up to a temperature of 100 8C. Above

100 8C, the trap-filled limit was seen to decrease withvoltage. The initial trend was consistent with the space-

charge controlled current theory, in which the number

of unfilled traps just below the Fermi level (deep traps)

would increase at elevated temperatures. Hence, more

amount of charges would have to be injected (by

applying a higher voltage) into the sample to fill those

empty trap levels. This would finally result in a net

upward shift of the trap-filled limit with temperature.Even though there would not be a single trap level

present in the sample, rather a superposition of several

trap levels differing in energy would exist, but since trap-

filled limits for every individual trap level are going to

increase, the net trap-filled limit would also increase.

However, above 100 8C, it was seen that the trap-filled

limit showed a decreasing trend. This trend continued

till 240 8C and above. It was also observed by ourgroup that, in case of barium bismuth niobate thin films,

the same phenomenon is noticed, but at a relatively

higher temperature [20]. The reason was explained on

the basis of a dynamic equilibrium between the trapping

and de-trapping of injected electrons. However, in the

study of space-charge current conduction, the trapping

is assumed to be a temperature-limited phenomenon.

However, any physical traps would exhibit someamount of electric field dependence on their trapping

and de-trapping processes. In fact, it is an established

fact that space-charge current could be strongly mod-

ified by field-assisted Poole�/Frenkel de-trapping pro-

cess [21]. In such a case, the application of higher field

would lead to the release of trapped electron rather than

injecting more number of charges to fill the already

empty traps. And this process would also be acceleratedif more thermal energy were supplied. The field-assisted

de-trapping would then reduce the effective trap-filled

limit. This might be the reason for the reduction of VTFL

at high temperatures.

3.5. Complex impedance analysis

To understand the role of microstructure, the complex

impedance of the sample was measured at varioustemperature over the frequency range of 100�/100 kHz.

Complex impedance spectroscopy had been a well-

recognized method to gain an insight into the internalFig. 7. The Lampert’s triangle.

S. Halder et al. / Materials Science and Engineering B95 (2002) 124�/130128

structure of the sample in terms of equivalent circuits

[22]. Fig. 8 shows the complex impedance of the sample

for different frequencies with variable temperature. It

was seen that the complex impedance plot had the shape

of a semicircle that passed through the origin. The

nature of this plot indicated that there was a single

internal circuit equivalent to a parallel combination of a

capacitor and a resistor. It can be seen that, for aparallel R �/C combination, the total impedance follows

a frequency dependence of the form [23]

Z��R

1 � jvCR: (1)

If the impedance is represented in the complex plane,

then the tip of the complex vector Z* would trace asemicircle which would pass through the origin. In this

case, it was assumed that both the resistance and the

capacitance were frequency independent. The peak of

the semicircle would correspond to the frequency

vp�1

2pRC: (2)

In the present case, the peak frequency was found to be

at 300 Hz at 275 8C. The peak frequency shifted to

higher values as the temperature of the sample was

increased. For example, the peak frequency was in-

creased to 6 kHz at a temperature of 350 8C. The above

expression for the peak frequency could explain this

phenomenon. With temperature, the resistance of the

sample would decrease nearly exponentially, and, sincethe sample was in paraelectric state at the mentioned

temperatures, the capacitance also would have fallen

down at elevated temperatures. It was seen that the

temperature dependence of the peak frequency vp was

slightly faster than an exponential increase with tem-

perature, which was just a consequence of the previous

statement. It is known that the resistance falls of

exponentially with the temperature [13]. Since the

capacitance also decreased with temperature, the net

inverse of their product would be a faster increasing

function. However, it was not possible to associate any

activation energy with this type of temperature depen-

dence. From the intersection of the extrapolated semi-

circle with the real axis of the impedance plot, the DC

resistance was calculated. This followed an Arrhenius-

type temperature dependence, and the activation energy

was found to be about 0.84 eV (Fig. 9), which was

comparable to that obtained from the actual DC

measurements. This resistance was recognized as the

grain resistance. The nearly equal values of Ea suggested

that both DC and AC conduction properties of the

sample were governed by the grain properties. Since

there was a single semicircle in the complex impedance

plot, it was considered that there was only one parallel

RC element in the circuit, at least in the measured

frequency window. The interfacial capacitance, if at all

present, would be of a very high value, and this would

result in a high time constant. Hence, the interfacial

effects would appear at a very low frequency to be

detected. However, the energies obtained from both the

observed and extrapolated results indicated that there

was a dominant role of the oxygen vacancies in the

conduction behavior of the thin film sample.

Fig. 8. Complex impedance plot at various temperatures.

Fig. 9. Arrhenius plot of the grain conductivity.

S. Halder et al. / Materials Science and Engineering B95 (2002) 124�/130 129

4. Conclusions

The in situ-deposited polycrystalline films were suc-

cessfully deposited which were ferroelectric in nature.This was confirmed from the polarization hysteresis.

The asymmetry in the polarization hysteresis was

explained on the basis of the different electrode materi-

als having different work functions. At higher tempera-

tures the frequency dispersion in the low frequency

regime was much more than high-frequency regime.

This was attributed to space-charge effects. The complex

impedance plots traced a single semicircle, whichindicated a single parallel R �/C circuit within the

sample. The current�/voltage plot indicated a predomi-

nant space-charge conduction mechanism. However,

there was some deviation, which was explained on the

basis of the Poole�/Frenkel effect superimposing itself

on the space-charge conduction mechanism at higher

electric fields.

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