Journal of Crystal Growth 337 (2011) 7–12

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Journal of Crystal Growth

0022-02

doi:10.1

n Corr

Univers

Tel.: þ4

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journal homepage: www.elsevier.com/locate/jcrysgro

Structural and dielectric behavior of pulsed laser ablated Sr0.6Ca0.4TiO3 thinfilm and asymmetric multilayer of SrTiO3 and CaTiO3

Pradip Chakraborty n, Palash Roy Choudhury, S.B. Krupanidhi

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

a r t i c l e i n f o

Article history:

Received 13 April 2011

Received in revised form

8 September 2011

Accepted 12 September 2011

Communicated by A. Ohtomotransition. Moreover, the Curie–Weiss temperature, determined from the e0(T) data above the transition

Available online 29 September 2011

Keywords:

A1. Curie–Weiss

A1. Polarization

B2. Antiferroelectric

B2. Paraelectric

48/$ - see front matter & 2011 Elsevier B.V. A

016/j.jcrysgro.2011.09.030

esponding author. Current address: Depar

ite de Geneve, 30, quai Ernest Ansermet, 1

1 22 37 96106; fax: þ41 22 37 96103.

ail address: pradip.chakraborty@unige.ch (P.

a b s t r a c t

Homogeneous thin films of Sr0.6Ca0.4TiO3 (SCT40) and asymmetric multilayer of SrTiO3 (STO) and

CaTiO3 (CTO) were fabricated on Pt/Ti/SiO2/Si substrates by using pulsed laser deposition technique.

The electrical behavior of films was observed within a temperature range of 153 K–373 K. A feeble

dielectric peak of SCT40 thin film at 273 K is justified as paraelectric to antiferroelectric phase

temperature is found to be negative. Using Landau theory, the negative Curie–Weiss temperature is

interpreted in terms of an antiferroelectric transition. The asymmetric multilayer exhibits a broad

dielectric peak at 273 K, and is attributed to interdiffusion at several interfaces of multilayer. The

average dielectric constants for homogeneous Sr0.6Ca0.4TiO3 films (�650) and asymmetric multilayered

films (�350) at room temperature are recognized as a consequence of grain size effect. Small frequency

dispersion in the real part of the dielectric constants and relatively low dielectric losses for both cases

ensure high quality of the films applicable for next generation integrated devices.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Ferroelectric thin films are known to be potential candidatesfor integrated circuits in electronic devices. Among the variousferroelectric compositions, thin films of SrTiO3 (STO) are wellknown for their incipient ferroelectricity and quantum paraelec-tricity [1]. Paraelectric films with a high dielectric constant, suchas STO, are of great interest for a variety of integrated devicessuch as integrated capacitors [2,3], strong capacitors in long-scaledynamics random access memories (DRAMs) [2–4], monolithicmicrowave integrated circuits [4], and pyroelectric IR sensors andpiezoelectric microactuators undertaken with dc bias or pulsed dcelectrical load [2]. However, it has been reported that the proper-ties of STO thin films, which are necessary for device applications,are not as good as those of bulk single crystals. Their tunability islower and dielectric losses are high [5,6]. It has also been shownthat introducing buffer layer like SrRuO3 (SRO) on LaAlO3 (LAO)substrate can significantly improve the dielectric behavior [7]. Inparticular Urban et al. reported the high-resolution transmissionelectron microscopy study of STO/SRO bilayer films on LAOsubstrates. A very careful analysis of the misfit relaxation andmisfit dislocation generation mechanism at two interfaces has

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tment de chimie physique,

211 Geneve 4, Switzerland.

Chakraborty).

been discussed within the context of dielectric improvement[8–11]. Study of the effects of A-site doping of ABO3 typeperovskite oxides is in demand for frequency agile materials formicrowave electronics. Among them, BaxSr1�xTiO3 (BST) is themost extensively studied material [12,13,24,25]. In recent years,A-site doping of STO by Ca2þ has received substantial attention inan effort to understand the structural as well as dielectric proper-ties [14,15]. From the earlier studies on this system, with max-imum Ca2þdoping to an extent of 20%, the anomalies in e(T) wererelated to an unknown type of transition into a nearly cubicstructure [16]. Later the dielectric anomalies were explained aftera detailed structural analysis by Ranjan et al. [17,18] and thephase transition was established as a transition from Pbnm to

Pbcm symmetry. From the polarization point of view, the phasetransition was supposed to be of paraelectric to antiferroelectricin nature and it has been seen well behaved in bulk Ca2þ dopedSTO [15]. The theme of the present investigation is to study thestructural and dielectric comparison between the effect of 40%Ca2þ substitution in STO in its thin film form (according to thetheoretical calculations in the phase diagram [15] of Sr1�xCaxTiO3

for x¼0.4) and the artificially engineered asymmetric multilayerof STO and CTO grown by pulsed laser ablation technique.

In recent years, it has been realized that the formation ofmultilayer of different materials in a constitutional mode canexhibit newer and better properties [8–11,19–21]. It is possiblefor such structures to show properties much different from theirparent members. From various investigations [22,23], it has been

Fig. 1. Schematics of thin films fabrication normal to the substrate surface

(a) STO/CTO multilayer and (b) SCT40 film.

P. Chakraborty et al. / Journal of Crystal Growth 337 (2011) 7–128

observed that the different properties are mainly due to theformation of artificial superlattices, interlayer electrostatic cou-pling, individual layer thickness effect, etc. Herein, we havefabricated asymmetric multilayer thin films of STO and CTO toimitate artificially a periodic inhomogeneous system equivalentto the composition of SCT40. In this configuration the thicknessesof STO and CTO are 12 nm and 8 nm, respectively, while in SCT40each of the individual layer thickness is approximately 20 nm.Fig. 1 shows the schematics of the fabrication normal to thesubstrate surface. In this paper, we have made an attempt toexplain the observed dielectric behavior in terms of interfaceeffect and grains size effect as neither of STO or CTO has the phasetransition at the temperature region of our investigation[1,26,27].

Fig. 2. XRD data of SCT40 (a) bulk (b) thin film and (c) STO/CTO multilayer at

room temperature.

2. Experiment

Sr1�xCaxTiO3 (with x¼0.4), STO and CTO targets for pulsedlaser deposition were prepared by conventional solid-state synth-esis. Stoichiometric amounts of SrCO3 (purity of 99.9%), CaCO3

(99.9% purity) and TiO2 (99.5% purity) powder were ball milled for4 h in zirconia jar using zirconia balls in acetone medium. Theball-milled powder was then calcined at 1100 1C for 6 h. Con-firming the crystalline phase formation of SCT40 by X-ray powderdiffraction, the calcined powder was again ball milled for 4 h tobreak the agglomerates. The fine SCT powder thus obtained wasthen pelletized (1 cm diameter) at a load of 90 kN with 4%polyvinyl acrylate (PVA) solution as a binder and sintered at1300 1C for 6 h. The heating rate was kept at 5 1C/min from roomtemperature to 650 1C and kept at 650 1C for 2 h in order toremove the PVA binder and finally heated up to 1300 1C atthe same rate. The sintered pellet was cooled at the same rate(5 1C/min) to room temperature [28]. Same solid-state synthesisroute, by means of heat treatment and ball milling, was followedfor both STO and CTO.

SCT40 thin films and STO/CTO multilayers were fabricated bypulsed laser deposition (PLD) technique on Pt coated siliconsubstrate Pt(200)/TiO2/SiO2/Si(100). All substrates were thor-oughly cleaned in 2-propanol (IPA) and distilled water priorto film deposition. All depositions were done by using PulsedLaser Deposition (KrF excimer laser of 248 nm, Lambda Physik,Compex Pro) in oxygen pressure of 50 mTorr and of temperature700 1C and at a frequency of 5 Hz and fluence of 3.5 J/cm2. Thetarget-substrate distance was kept fixed at 35 mm for all thedepositions. In case of STO/CTO multilayered thin films, STO wasfirst grown on Pt (200) followed by CTO because of higher latticemismatch between Pt (200) and CTO (3.835 A) [23] andSTO/bottom electrode has excellent resistance during multilayer

film growth [29]. Thickness of the bilayers was kept fixed atnearly 20 nm, with individual layer thicknesses of 12 nm for theSTO layer and 8 nm for the CTO layer (Fig. 1). The crystal structurewas investigated by y-2y X-ray diffraction (XRD). All the electricalmeasurements were done using metal–insulator–metal (MIM)configuration. Real (e0) and imaginary (e00) parts of the dielectricconstant were measured by Keithley 3330 LCZ meter and Keithleymodel 230 programmable voltage source interfaced to a compu-ter using TestPoint software. Top electrodes of Au (circular dots ofarea 1.96�10�3 cm2) were deposited by thermal evaporationand shadow masking technique.

3. Results and discussion

3.1. X-ray diffraction analysis

Fig. 2 shows the XRD patterns for both homogeneous SCT40thin films and STO/CTO multilayer (12 nm/8 nm). The growthmechanism of thin films strongly depends on the substratematerial and crystallographic orientation in addition to a host ofother factors such as growth temperature, oxygen partial pres-sure, etc [30]. In our case, both types of films are expected to growalong [004]. As our chosen substrate is Pt (200), accordinglysmaller lattice mismatch and higher symmetry of STO withrespect to Pt (200) results a strong peak at [004] in the case ofmultilayer. Whereas in case of SCT40 films, relatively higherlattice mismatch results lowering in symmetry along the growthdirection (00l) alternatively favored in plane (220) and (222)peaks. For further confirmation, from the XRD pattern of SCT40thin film (Fig. 2b), a splitted peak (absent in bulk SCT40, Fig. 2a) at2y¼33.31 were observed, and can be attributed to the lowering insymmetry in its thin film form as compared to bulk. Duringgrowth of SCT40 thin film on Pt (200), there is a high probabilityof misfit strain originating from the lattice mismatch and thus weconclude that the SCT40 films were strained on Pt (200).To confirm a nearly cubic structure of SCT40, detailed crystalstructure refinement for SCT40 bulk ceramic was reported in theliterature [31]. It was shown from Rietveld refinement for bulkSr1�xCaxTiO3 with x¼0.40 that the nearly equal values of theelementary pseudocubic perovskite cell parameters ap¼3.877 A,

P. Chakraborty et al. / Journal of Crystal Growth 337 (2011) 7–12 9

bp¼3.8775 A and cp¼3.8744 A, derived from A0, B0 and C0 valuesusing the relationship A0E(2)½ap, B0E(2)½bp and C0E4cp.However it has been also clearly shown that the structure of SCTfor x¼0.40 belongs to the ‘‘nearly cubic’’ type (i.e. apEbpEcp) but itsspace group is Pbcm not Pbnm or Cmcm as believed earlier [31,32].

3.2. Film morphology

The surface morphology and thickness of the films are shownin Fig. 3a and b. From the Scanning Electron Micrograph (SEM), ithas been observed for SCT40 film, the average grain size is largeras compared to the smaller uniform grains in the multilayerfabrication. The typical film thicknesses as observed from cross-sectional scanning electron micrograph are 300 nm for SCT40films and 360 nm for STO/CTO multilayered films.

3.3. Electrical properties

Fig. 4a and b show the variation of dielectric constant withtemperature within the frequency range 100 Hz–100 kHz. It has

Fig. 3. SEM micrograph showing surface morphology and cros

Fig. 4. Temperature dependence of dielectric constant of (a) STO/CTO mult

been observed that SCT40 film shows a feeble dielectric peak at273 K, whereas STO/CTO multilayer shows a broad peak at thesame temperature region. Moreover, low frequency dispersion isobserved in e0 and e00 as a function of frequency shown in Fig. 5aand b. Relatively higher values of e0 and low values of e00 withinthe abovementioned frequency range ensure nominal effects ofextraneous factors useful for high frequency microwave andcapacitor application.

Sr0.6Ca0.4TiO3 (SCT40) can be compositionally described as abulk material because of its different crystal structures thatdepend upon temperature and pressure; thus in our study wehave described it as 40% SCT solid solution. Eventually in presenceof 40% Ca2þ in STO matrix, the interaction between ions willvary from unit cell to unit cell. Consequently, the dipolar orienta-tion and the coupling between several dipoles in the unit cellchanges, therefore structural change from cubic to orthorhombic(specifically ‘‘nearly cubic’’) has been observed above somecritical concentration of Ca2þ ion at room temperature [18]. Butin pure STO and CTO coupling can be represented by the netaverage dipole moment of the lattice. In thin films, additional

s section of (a) STO/CTO multilayer and of (b) SCT40 film.

ilayer and of (b) SCT40 film at different frequencies (100 Hz–100 kHz).

Fig. 5. e0-e00 vs. Frequency at different temperatures (153 K–373 K) of (a) STO/CTO multilayer and of (b) SCT40 film.

Fig. 6. Curie–Weiss fit to e0(T) data for SCT40 thin film showing negative value of

Curie–Weiss temperature (Tc).

P. Chakraborty et al. / Journal of Crystal Growth 337 (2011) 7–1210

factors such as interfacial inhomogeneity between substrate andfilm plays an important role in which interfacial strain is expectedto modify internally buildup lattice strain. Consequently, thesharp but small change of dielectric constant is attributed tostructural changes (cubic to ‘‘nearly cubic’’). The shift of the peakin the dielectric constant from room temperature to 273 K can beattributed to strain relaxation at the substrate/film interface.Thus, the observed results are due to a change of SCT40 fromthe paraelectric phase to an antiferroelectric phase at 273 K.Moreover it can be justified from e0(T) vs. T data that the dielectricanomaly at that composition x¼0.40 can be attributed to athermodynamic phase transition in thin film [17].

From Curie–Weiss behavior it is known that antiferroelectrictransition, e0(T) above Tm

0 follows:

1

e0ðTÞ ¼

T�Tc

Cð1Þ

where Tc is the Curie–Weiss temperature and C is the Curieconstant. For a second order phase transition, Tc is same as the

thermodynamic phase transition temperature Tm0 , obtained by

dielectric measurement. For a first order phase transition, Tc isknown to be less than Tm

0 [33]. From Fig. 6, the Curie–Weisstemperature turns out to be negative (�250 K). We can explainthe significance of negative Curie–Weiss temperature, found inthe Curie–Weiss fit to the observed e0(T) data in the paraelectricphase within the framework of Kittel’s phenomenological theoryof antiferroelectric transition [34]. An antiferroelectric crystal iscomposed of two collinear sublattices with equal and oppositepolarizations. In general, one may have two independent sub-lattice polarizations Pi and Pj. The Landau free energy density canbe expanded in powers of Pi and Pj as:

GðT ,Pi,PjÞ ¼ G0ðTÞþ f ðP2i þP2

j ÞþgPiPjþhðP4i þP4

j Þþ jðP6i þP6

i Þþ � � �

ð2Þ

where g is the coupling strength of the sublattice polarization inthe antiferroelectric phase. If h40, this expression leads to asecond order phase transition, whereas for ho0 and j40, a firstorder phase transition occurs.

For g40 and 2f-g showing the following type of temperaturedependence near the transition temperature

f ¼ ð1=2ÞgþlðT�T0Þ ð3Þ

In this phenomenological model of antiferroelectricity, thedielectric constant above the transition temperature is given by

e0ðTÞ ¼ 1=½gþlðT�T0Þ� ð4Þ

This expression can recast to the Curie–Weiss form

e0ðTÞ ¼ C=½T�Tc� ð5Þ

By choosing C¼1/l and Tc¼T0�g/l. Obviously, for g/l4T0,Tc will be negative. This is exactly what we have observed inSCT40 thin film (Fig. 6).

In case of STO/CTO multilayers, very high compositionalfluctuation, strain relaxation and interdiffusion at several inter-faces during film growth rule out the sharp change of dielectricconstant as a function of temperature [21]. As a result a broaddielectric peak has been observed around 273 K. The effect ofvarious stacking and annealing sequence of multilayered filmsrelated to structural and dielectric properties have been discussedelsewhere [35]. However from SEM microstructure (Fig. 3), thereis a clear difference in surface morphologies between two films.The transition shape is dependent on the grain size of the sample.

Fig. 7. Polarization hysteresis behavior of (a) STO/CTO multilayer and of (b) SCT40 film at 273 K.

P. Chakraborty et al. / Journal of Crystal Growth 337 (2011) 7–12 11

If the grain size is smaller the diffusiveness of the transition isincreased [35]. In our system, the small uniform grains in multi-layered film (Fig. 3a) account the possibilities of interdiffusionbetween each layer, which is supposed to be the primary causesto show diffuse phase transition. On the other hand, within thesimilar frame of argument the feeble dielectric peak for SCT40film can also be explained by the larger grain size as revealedfrom SEM (Fig. 3b).

From Fig. 5a and b, the relatively lower dielectric constant(�350) at room temperature for multilayer films can be attrib-uted to the smaller grain size responsible for the suppression inthe spontaneous polarization [36]. In the low frequency region,the increase of the dielectric loss for the SCT40 films is due to thegrain boundary leakage and electrode/sample interface effect[37]. However, opposite effect has been observed in multilayerfilms related to the interfacial polarization of space charges [38].

Moreover, from further study, polarization vs. electric fieldhysteresis (P–E loop) at 273 K for both types of films shows thatin case of SCT40 thin film there is linear polarization up to a biaselectric field of 63 kV/cm, whereas STO/CTO asymmetric multi-layer shows nonlinear hysteresis loop (typical paraelectric beha-vior) with 2Pr of �2 mC/cm2 and Ec of 20 kV/cm (Fig. 7a and b). Bycombining the context of negative Tc and linearity in P–E loop forSCT40 thin film refer to the fact that the dielectric anomaly can beattributed to the antiferroelectric phase transition. The observeddifference between two films accounts for the possibility ofantiferroelectric ordering in the SCT40 thin film and a paraelectricbehavior in the STO/CTO multilayer fabrication.

4. Conclusion

In summary thin films of Sr0.6Ca0.4TiO3 and multilayer ofSrTiO3 and CaTiO3 were grown on Pt/Si substrates by pulsed laserablation. A feeble dielectric peak of SCT40 thin film at 273 K isjustified as paraelectric to antiferroelectric transition from nega-tive Curie–Weiss temperature and linear polarization. However,the multilayer films of STO/CTO exhibit a broad dielectric peak at273 K. The broad dielectric peak of multilayer films has beenattributed to the high compositional fluctuation and interdiffu-sion at several interfaces and small uniform grain morphology asobserved from microstructure. Relatively lower dielectric losses ofthe films indicate their potential application for high frequencymicrowave devices, capacitor application and miniaturization of

electronic devices. Future investigations involve the effect ofepitaxial growth of STO/CTO multilayer on SRO buffered LAOsubstrate with different individual layer thickness on microstruc-ture and dielectric properties.

Acknowledgments

The authors gratefully acknowledge Dr. J. Parui, MaterialResearch Center, Indian Institute of Science, Bangalore, India fordiscussion and valuable suggestions.

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