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Materials Science and Engineering B 138 (2007) 22–30

Dielectric, impedance and ferroelectric characteristics of c-orientedbismuth vanadate films grown by pulsed laser deposition

Neelam Kumari, S.B. Krupanidhi, K.B.R. Varma ∗Materials Research Centre, Indian Institute of Science, Bangalore 560012, India

Received 10 August 2006; received in revised form 29 November 2006; accepted 9 December 2006

bstract

Ferroelectric bismuth vandante, Bi2VO5.5 (BVO) thin films with layered perovskite structure were deposited by pulsed excimer laser ablationechnique on (1 1 1) Pt/TiO2/SiO2/Si substrates. The polarization hysteresis (P versus E) studies on the BVO thin films at 300 K confirmed the remnantolarization (Pr) and coercive field (Ec) to be 5.6 �C/cm2 and 113 kV/cm, respectively. The same was corroborated via the capacitance–voltageeasurements. The dielectric response and conduction mechanism of BVO thin films under small ac fields were analyzed using impendence

pectroscopy. A strong low frequency dielectric dispersion (LFDD) was found to exist in these films, which was ascribed to the presence of theonized space charge carriers such as oxygen ion vacancies and interfacial polarization. The room temperature dielectric constant and the loss

D) at 100 kHz were 233 and 0.07, respectively. The thermal activation energy for the relaxation process of the ionized space charge carriers was.85 eV. The frequency characteristics of BVO thin films under study showed universal dynamic response that was proposed by Jonscher for theystems associated with quasi-free charges. 2006 Elsevier B.V. All rights reserved.

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eywords: Bismuth vanadate; Ferroelectric thin film; Low frequency dielectric

. Introduction

The studies concerning the ferroelectric thin films have drawnhe attention of many researchers not only from the growth mech-nism viewpoint but also from a variety of device applicationshat include non-volatile memory [1], infrared detectors [2],

icroelectromechanical systems [3,4]; electro-optical switches5], etc. Extensive work was done on lead zirconate titanatePZT) [6,7] for the development of non-volatile randomccess memories (NVRAMs) because of its non-volatility,arge remnant polarization, and fast switching speed and radi-tion hardness. However, one of the most serious problemsssociated with Pt/PZT/Pt ferroelectric-based capacitor is theegradation of the polarization hysterises characteristics. Inhis context, bismuth-layered compounds were considered toe superior from their better fatigue properties point of view

8,9]. These layered perovskites belong to the Aurivillius fam-ly [10], with the general formula (Bi2O2)2+(An−1BnO3n+1)2−.mong these materials, Bi2Sr2Ta2O9 and SrBi2Nb2O9 and

∗ Corresponding author. Tel.: +91 80 22932601; fax: +91 80 23607316.E-mail address: kbrvarma@mrc.iisc.ernet.in (K.B.R. Varma).

tiBbhsm

921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2006.12.010

rsion; Universal dielectric response

ther similar compounds were widely studied [11–13]. Therystallization temperatures of these bismuth layer-structurederroelectrics are relatively high and also possess higherielectric constant at room temperature. In these Bi basedayer-structured ferroelectrics, the dielectric constant decreasesith decreasing the number of ‘n’ perovskite layers in thenit cell.

From the structural point of view, bismuth vanadate can beormulated as (Bi2O2)2+(VO3.5�0.5)2−, where � representsxide ion vacancies. Bi2VO5.5 (BVO) can hence be consideredo be analogous to �-Bi2WO6, the n = 1 member of Aurivilliusamily of oxides with intrinsic oxygen vacancies in the per-vskite layer [14–17]. It crystallizes in a non-centrosymetric,olar orthorhombic class and is ferroelectric at room tempera-ure. It exhibits three main polymorphs: a non-centrosymetric-phase at room temperature, its transformation to a cen-

rosymetric �-phase at 730 K and a centrosymmetric �-phases stable beyond 835 K; and BVO finally melts at 1153 K.oth the �-phase, which has a superstructure characterized

y a tripling of the lattice parameter a and the �-phase, thatas a superstructure with doubling of a, has an orthorhombicymmetry [19]. The �-phase is described by a tetragonal sym-etry. Because the distortions of crystal cell are small, these

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N. Kumari et al. / Materials Scienc

hases can be described as mean orthorhombic cell am = 5.53,m = 5.61, cm = 15.26 A. So the polymorphs of BVO can beriefly summarized as: Non-centrosymmetric orthorhombic

mm2)∼730 K−→ centrosymmetric orthorhombic (mmm)

∼835−→ cen-rosymmetric tetragonal (4/mmm). The lattice type of �-BVO isrthorhombic (space group B2cb) and the lattice parameters are= 5.543 A, b = 5.615 A, and c = 15.321 A (JCPDS 42-0135).ooryanarayana et al. [20] reported a space group, Aba2 with

he lattice parameters a = 5.602(2) A, b = 15.269(3) A, and= 5.5250(8) A using single crystal data. BVO thin films haveeen one of the promising for ferroelectric random accessemory device applications because of their low dielectric

onstant. BVO in both bulk and thin film is known to exhibithe ferroelectric behavior along the c-axis apart from the a-axisthough a-axis was reported to be the polar axis) [17,18,21].imilar observations have been made in the case of Bi4Ti3O12y Cummins and Cross [22,23]. They have interpreted thisind of behavior to the electrical switching of Ps by 90o underufficiently high fields. BVO thin films have been fabricatedsing different deposition techniques including MOCVD24], coating pyrolysis [25], chemical solution decompositionCSD)[26], and pulsed laser ablation [21] on a variety ofubstrates. The pulsed laser technique is more suitable forepositing multicomponent oxides and has the advantage ofaintaining excellent stoichiometry. Recently, we conducted

etailed study on CSD deposited BVO thin films on Si substratesn a MOS configuration and found to exhibit excellent stabilitynd lattice match on Si [26]. Since the number of pseudo-erovskite layers between the Bi2O2 layers is odd, i.e. remnantolarization Pr along the c-axis is expected to be preserved.hus, fabrication of c-axis oriented BVO films is very importantot only from the practical point of view, but also for elucidat-ng the origin of ferroelectricity in bismuth-layered structureerroelectrics.

To the best of the knowledge of the authors no reports existsn the literature on the growth and characterization of BVOhin films on platinum coated silicon substrates. Further, thenderstandings of the physical properties of BVO thin films areot conclusive. The high temperature �-phase of BVO consistsf (Bi2O2)2+ sheets interleaved with perovskite like layers ofVO3.5�0.5)2−. The high ionic conductivity associated with the-phase is attributed to the presence of oxide ion vacancies in theerovskite layer. The oxygen vacancies are disordered in � and-phases of BVO and give rise to non-negligible ionic conduc-

ivity. It is known in the literature that the existence of electricalonductivity in ferroelectric materials is an undesirable features it may influence the ferroelectric characteristics: it is difficulto pole when leakage current is high; pyroelectric measurement

ay give a space charge induced current that overshadows thepontaneous polarization current; dielectric constant measure-ent no longer reflects the real contribution to the intrinsic

olarization when space charge polarization becomes prevail-

ng. In the present article we report the results and analyses of therowth and ac conduction properties of BVO thin films depositedn a Pt coated Si substrate in metal insulator metal (MIM)onfiguration.

tlho

Engineering B 138 (2007) 22–30 23

. Experimental details

A pulsed laser technique was used for the deposition of theismuth vanadate (Bi2VO5.5) thin films from a stoichiometricrystalline target. A dense ceramic target of BVO was preparedia the conventional ceramic processing route. It was preparedy heating a stoichiometric mixture of Bi2O3 and V2O5 (bothf purity 99.99% from Aldrich Chemicals) at 500 ◦C for 12 hnd then at 800 ◦C for 24 h with two intermediate grindings.he calcined powder was then pressed into 18 mm target. The

arget was sintered at 800 ◦C for 5 h. A freshly polished surfaceas used each time for deposition. The target was mounted onrotating carousel to ensure uniform ablation. A KrF pulsed

xcimer laser (wavelength 248 nm) operating at a repetition ratef 5 Hz and 2.5 J/cm2 energy fluence was incident on the rotat-ng target. The deposition chamber was initially evacuated topressure of 1 × 10−6 mbar and then flushed with high purityxygen to the required pressure in the range of 20–200 mTorr.he substrate used was platinum-coated silicon having a con-guration (1 1 1) Pt/TiO2/SiO2/Si. The substrate temperatureuring deposition was varied from 600 to 700 ◦C to achieveood crystallization. The phase of the deposited film was con-rmed via X-ray powder diffraction (Cu K� ∼ 1.541 A) (scintagR2000 diffractometer) studies. The surface microstructure of

he films was examined using contact mode atomic force micro-cope (AFM) (Veeco CP-II). The thickness of the film wasonfirmed to be 350 ± 30 nm from the cross-sectional scanninglectron micrograph. For electrical measurements, gold dots of.96 × 10−3 cm2 area were deposited on the top surface of thelms through a shadow mask and thermal evaporation tech-ique. The electrode dots were annealed at 250 ◦C for 30 min.he Pt surface was used as the bottom electrode for capacitanceeasurements. A HP4294A impedance analyzer was used for

he conventional dielectric and C–V measurement. The dielec-ric constant and ac electrical conductivity measurements wereerformed as a function of frequency (100 Hz–1 MHz) at vari-us temperatures (25–220 ◦C), at signal strength of 0.5 V. Theolarization–electric field (P–E) hysteresis and pulsed polar-zation (PUND) phenomenon was studied at room temperaturesing a Precision Workstation (Radiant Technologies Inc.) fer-oelectric test system in virtual ground mode.

. Results and discussion

.1. Structural determination and surface morphology

Fig. 1 shows the X-ray diffraction pattern obtained for BVOhin films deposited on a Pt/TiO2/SiO2/Si substrate at 650 ◦Cnder 100 mTorr oxygen partial pressure. This diffraction pat-ern has been indexed on the basis of an orthorhombic structure17]. The lattice parameters obtained from this pattern are ingreement with those reported for the bulk BVO. The selec-

ive strong and sharp Bragg peaks (0 0 l) indicate that the pulsedaser ablation-grown films were highly textured and possessedigh degree of crystallinity. The c-axis orientation in the grainriented films was calculated using Lotgering’s method [27],

24 N. Kumari et al. / Materials Science and Engineering B 138 (2007) 22–30

F6

w

f

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3

fimaiw

F(

rtdestf(o1wscnscdtttprrhbt

ig. 1. X-ray diffraction pattern of BVO/Pt/TiO2/SiO2/Si thin film grown at50 ◦C.

here the degree of grain orientation f is given by:

= p − po

1 − po(1)

here p = ∑I0 0 l/

∑Ih k l for the given oriented sample, and

o = ∑I0 0 l/

∑Ih k l for the non-oriented sample which in our

tudy was calculated from the BVO bulk X-ray diffraction stud-es. Here Ih k l I0 0 l are the intensities of the (h k l) and (0 0 l)eaks, respectively. The films were highly oriented with therientation factor of 0.9 as calculated by the above formula.he preferential orientation of the films could be due to therystal anisotropy of the system. The orthorhombic distortionb/a) of the film was lower than the reported values for the bulk.he free energy for the growth along the c-axis could be min-

mum by which the maximum growth is along this axis. Weave observed a dense columnar growth in the cross-sectionalEM micrograph indicating an anisotropic growth mechanismhich is concomitant with its structure. The surface morphology

nd the microstructure of BVO thin films have been studied byFM and the results obtained are shown in Fig. 2(a and b) as

wo-dimensional and three-dimensional micrographs. The AFMicrograph in Fig. 2(a) shows the surface topography of a BVO

hin film. It indicates the homogeneous distribution of grains ini2VO5.5 thin film. The grain-size that is determined from theicture lies in the range of 0.3–0.4 �m. The AFM micrographn Fig. 2(b) shows the three-dimensional image. The root meanquare of the surface roughness (Ra) lies between 9 and 11 nm.

.2. Polarization hysteresis behavior and C–V analysis

The ferroelectric nature of the Bi2VO5.5 thin films was con-rmed at room temperature based on the polarization hysteresis

easurement and the results are depicted in Fig. 3(a). At the

pplied voltage of 10 V, the measured values of remnant polar-zation (Pr) and coercive field (Ec) for 350 ± 30 nm thick filmere 5.6 �C/cm2 and 113 kV/cm, respectively. The asymmet-

Fbib

ig. 2. (a) AFM micrograph showing surface morphology of Bi2VO5.5 thin film.b) Three-dimensional topography of a Bi2VO5.5 thin film.

ic nature of the loop is attributed to various factors such ashe defect charges present in the ferroelectric material or theifferent work functions associated with the top and bottomlectrodes. The value of Pr and Ec appears to be on higheride for c-axis oriented BVO thin films. This could be due tohe ionic conduction associated with BVO films. To verify thisact that we have performed the pulsed polarization experimentPUND) on the samples. The PUND measurements were carriedut using a 5 V, 10 kHz pulsed waveform with a pulse width ofms, and the test capacitor was fatigued with 5 V, 10 kHz pulsedaveform. The corresponding results are shown in Fig. 4. The

witchable polarization (P*) is essentially constant through 105

ycles, after which it starts increasing. This increase in P* isot reflecting the true behavior as polarization decreases afterubjecting the capacitor to stressing voltage for a number ofycles. So this kind of behavior could be due to the ionic con-uction in BVO thin films. Further, The difference betweenhe switchable polarization (P*) and non-switchable polariza-ion (Pˆ) is 0.022 �C/cm2 for first cycle of measurement. Sincehe non-switchable polarization (Pˆ) represents the non remnantolarization, the difference between two represents the actualemnant polarization of the sample. Hence, the true value ofemnant polarization is very less. Therefore, we propose that theigher values of Pr observed in hysteresis measurement coulde due to ionic conduction in c-oriented BVO films. The capaci-ance voltage (C–V) characteristics of a BVO thin film is shownig. 3(b) at 1 kHz. The C–V measurements were carried out

y applying a small ac signal of 0.5 V amplitude, with a vary-ng dc electric field. The dc voltage was swept from negativeias (−10 V) to positive (10 V) in steps of 0.1 V with a sweep

N. Kumari et al. / Materials Science and Engineering B 138 (2007) 22–30 25

Fv

rctl

FP

ig. 3. (a) The P–E hysteresis loop for Bi2VO5.5 thin film. (b) Capacitance–oltage characteristics of a Bi2VO5.5 thin film.

ate of 0.1 V/s and back again. The butterfly shape of the curveonfirmed the ferroelectric nature of the film and the capaci-ance shows strong voltage dependence. The two maxima of theoop correspond to the domain switching voltage in forward and

ig. 4. Polarization vs. number of 5 V pulsed voltage cycles applied to Au/BVO/t thin film capacitor.

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3

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ig. 5. The frequency dependence of: (a) ε′r and (b) ε′′

r on a log–log scale.

everse directions where the polarization reversal takes place.he asymmetry that is observed in C–V curve suggests that thelectrodes are asymmetric and the film contains mobile ions orharges accumulated at the interface between the film and thelectrode. In addition, there is a difference between the capaci-ance values of the two peaks, which may be due to some defectnergy levels in the film.

.3. Dielectric studies

Fig. 5(a and b) represents the frequency dispersion curves ofhe real and imaginary parts of the dielectric constant, ε′

r and ε′′r ,

espectively, in the 25–220 ◦C temperature range for BVO thinlms. Both ε′

r and ε′′r show strong dispersion at low frequencies,

nd is strongly effected by the temperature change. Such strongispersion in both the components of dielectric constant appears

o be a common feature in the ferroelectric associated with goodonic conductivity and is referred to as low frequency dielectricispersion (LFDD) [28,29]. This is in complete contrast with theffect due to dc conductivity where the real part remains constant

2 ce an

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dsft

ε

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ε

ε

wascrserIpbc

dqbsilpciotrcefoa

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fdmf(ttashown in Fig. 6(a and b). The excellent fitting obtained throughthis technique suggests the applicability of the ‘Universal model’in analyzing the dielectric response of the BVO thin films. Theparameters n(T) and A(T) [A(T) = a(T)S/L], where S is the elec-

6 N. Kumari et al. / Materials Scien

nd the imaginary part is proportional to 1/ω [28]. Jonscher, Hillnd Pickup [30,31] have dealt with LFDD phenomena in detail.he dispersion in the imaginary part of the dielectric constantε′′

r ) is stronger than that in the real part (Fig. 5(b)). This maye attributed to the influence of dc conductivity on ε′′

r . The lowrequency slope of the curve log ε′′

r versus log f is close to −1ndicating the predominance of dc conduction in this frequencyegion, which overshadows the true behavior of ε′′

r .In the high frequency region the slope between 0 and −1,

epending on the temperature is observed. According to Jon-cher’s universal law, the complex dielectric constant as aunction of radian frequency ω, is given by the following rela-ion:

∗ = ε′r − iε′′

r = ε∞ + σ

iεoω+ a(T )

εo

(iωn(T )−1) (2)

here ε∞ is the ‘high frequency’ value of the dielectric constant,(T) the temperature dependent exponent, which determines thetrength of the ion–ion coupling (small value of n(T) correspondso strongly interacting systems) and a(T) determines the strengthf the polarizibility arising from the universal mechanism. Theeal and imaginary parts of the dielectric constant from Eq. (2)re given as:

′r = ε∞ + (sin(n(T )π/2)ωn(T )−1a(T )/εo) (3)

′′r = σ

εoω+ (cos(n(T )π/2)ωn(T )−1a(T )/εo) (4)

here the first term in Eq. (3) characterizes the lattice responsend that in Eq. (4) reflects the dc conduction part, while theecond term in both the equations refers to the charge carrierontributions to the observed dielectric constants. Dielectricelaxation in layered perovskite ferroelectric materials repre-ents the change in polarization according to a time-variantlectric field. It depends upon frequency as various mechanismsesponsible for polarization response at different frequencies.n general, in the samples of the present kind, the dielectricroperties are strongly influenced by the complexity of grainoundaries, grain-size and orientation, and ionic space chargearriers.

The temperature and frequency dependence of the real part ofielectric constant (ε′

r) can be explained using Eq. (3). At low fre-uencies, the charge carrier term (sin(n(T )π/2)ωn(T )−1a(T )/εo)ecomes dominant and ε∞ is negligible; therefore, for a con-tant n, Eq. (2) gives a straight line with a slope equal to n − 1n the log–log plot of ε′

r and frequency. Therefore, this strongow frequency dependence dielectric constant at elevated tem-erature can be related to the contributions of the ionic spaceharge such as oxygen ion vacancies, the defect generated dur-ng film growth, interface polarization located at grain boundaryr the interface between the film and the electrode and the elec-rode polarization. At high frequencies the charge carriers fail toespond to the external field, therefore the measured dielectriconstant is mainly from intrinsic polarization. This explains the

xistence of a linear decrease in the low frequency region and arequency independent plateau at high frequencies. The behaviorf ε′′

r could be rationalized by using Eq. (4). At low frequenciesnd high temperatures, the dc conductivity term dominates and

d Engineering B 138 (2007) 22–30

ives a slope of −1 which indeed is consistent with the datahown in Fig. 5(b). It is well known that the electric propertiesf the SBT, BST and PZT thin films strongly depend on thetate of the surface, and impurities or defects which may act asrap or donors [32–34]. Waser [35] suggested that the oxygenon vacancies play an important in the resistance degradationnd dielectric degradation of polycrystalline (BST) thin films.xygen vacancies induced polarization becomes more predom-

nant at higher temperature and low frequency due to increasedacancy concentration.

The verification of the Jonscher’s model is done by success-ully fitting the experimental dielectric data to the dielectricispersion relations given in Eqs. (3) and (4). The present treat-ent helps in separating the intrinsic (lattice) dielectric constant

rom the one which is due to the charge carrier contributionextrinsic). Experimental data points from the ε′

r and ε′′r are fitted

o theoretical relations with an average relative error not morehan 5%. The experimental data obtained at 160 ◦C for ε′

r and ε′′r

re fitted to theoretical Eqs. (3) and (4) and the resultant data are

Fig. 6. The curve fitting obtained for: (a) ε′r and (b) ε′′

r .

N. Kumari et al. / Materials Science and

Ft

ttatistaTcw

3

prttrfirb

aipum

Z

wtrde

Z

wtbritnlHswd

Ztttmagnitude of imaginary component of the impedance at the peakfrequency was also a strongly varying function of the temper-ature, indicating Arrhenius type temperature dependence. Thisresult can be seen in the Arrhenius plot of the peak frequency

ig. 7. (a) The temperature dependence of the critical exponent n(T). (b) Theemperature dependence of the prefactor A(T).

rode area and L is the thickness of the sample calculated fromhe theoretical fitting have been plotted as a function of temper-ture and given in Fig. 7(a and b). With increase in temperaturehe thermal energy of the system increases thereby increas-ng the disorder. As the prefactor A(T), which determines thetrength of polarizibility, increases with increasing temperature,he charge carrier term becomes more prominent at high temper-ture, thereby increasing the low frequency dielectric dispersion.he exponent n(T) decreases with increase of temperature, indi-ating that the coupling between the charged species increasesith the increase in temperature.

.4. Impedance analysis

The impedance study is more informative, in terms of decou-ling grain and grain boundary effects, and gives the true grainesistance. Impedance analysis has been well known to resolvehe contribution from various microscopic elements such ashe grain, grain boundary, and electrodes to the total dielectric

esponse. Any real bulk ceramic material or polycrystalline thinlm can be thought of as consisting of mainly two dissimilaregions, i.e. the grain and the grain boundaries. Each region cane realistically described by a parallel combination of a capacitor

Engineering B 138 (2007) 22–30 27

nd a resistor. The classical model to describe impedence behav-or is that of Debye; in which a parallel equivalent circuit of aure resistance R and a pure capacitance C describe the materialnder investigation electrically. The complex impedance of thisodel could be written as

∗ = R

1 + iω(RC)(5)

here τ = RC is the time constant of the circuit. In a real case,here would be a distribution in the relaxation time, and also theesistance and capacitance would be slightly frequency depen-ent. The modified expression for Z in such a case would bequivalent to the Cole–Davidson relation

∗ = R

1 + (iωRC)1−α(6)

here α is the temperature dependent exponent which charac-erizes the distribution of the relaxation time and its value isetween 0 and 1. The values of R in a correlated charge car-ier system are frequency dependent. In perovskite oxides orn many other ionic solid materials, the major mode of chargeransport is a multiple hopping process. The hopping processormally takes place across the potential barriers set up by theattice structure and the local environment of other atoms/ions.owever, due to irregularities in the lattice structure near defect

ites, the potential barriers will have different magnitudes, asell as varied widths. This is thought to be responsible for theistribution in the relaxation times.

Fig. 8 shows the variation of imaginary part of impedance′′ as a function of frequency at different temperatures for BVO

hin film. Peak maxima were observed to shift systematicallyowards higher frequencies with increase in temperature andhe relaxation occurs for the several decades of frequencies. The

Fig. 8. Imaginary impedance with frequency at different temperature.

28 N. Kumari et al. / Materials Science an

Fo

itiwattftpi1blc

iIbcvlsidlt

�

Titptirttghial

cdsqtemperature. It is clear from Fig. 11 that at low frequencies, the

ig. 9. Arhennius plot of the peak frequencies obtained from the imaginary partf the impedance.

n the impedance plot (Fig. 9) which was fairly linear at highemperature. The corresponding activation energy is 0.85 eV;ndicating an oxygen vacancy based controlled conduction. Theidth of the peak was more than 1.14 decade suggesting a devi-

tion from the ideal Debye behavior. Also it is clear from Fig. 8hat all the curves merge at higher frequencies, irrespective ofhe temperature at which the measurements are made. At higherrequencies, the contribution from the grains dominates due tohe absence of the space charge effect. Fig. 10 shows the com-lex plots between the real and imaginary components of thempedance for BVO thin films at various temperatures in the

kHz–1 Mz frequency range. As expected, all the semicirclesecome smaller with increase in temperature and shifts towardsower Z′ values indicating a reduction in the resistance. At aomparatively lower temperature, a single semicircle is observed

Fig. 10. Complex impedance plot of Bi2VO5.5 thin film.

atp

d Engineering B 138 (2007) 22–30

ndicating the response from the higher resistance region (grain).n the entire temperature range, the center of the semicircle lieselow the real axis again suggesting a non-Debye response andonfirming the Jonscher’s universal frequency dependence. Thealue of the angle of deviation (from the real axis) was calcu-ated by drawing a tangent to the high frequency end of the firstemicircle, which represented the grain. The angle between themaginary axis and the tangent is same as the angle between theiameter of the circle and the real axis. According to Jonscher’saw, the deviation angle is directly related to the exponent nhrough the equation

θ = (1 − n)π/2 (7)

he inclination of the semicircle with respect to the real axiss increased with temperature indicating that departure fromhe Debye nature is partly thermally assisted. At higher tem-erature (above 146 ◦C), there is also another deviation fromhe circular nature at the low frequency side of the complexmpedance plot. A small kink appears in the low frequencyegion (shown as the inset of Fig. 10) which might be as a result ofhe response from the grain boundaries at higher temperature, ashe grain resistance decreases with increase in temperature andrain boundaries may start responding at low frequencies. Theigh frequency side originated from the grains fits fairly wellnto a semicircle. The high temperature and low frequency sidere not fully evolved because of the frequency and temperatureimitations that we had in our experimentation.

The power law dependence of the resistance and also theapacitance of BVO films were directly reflected in the ac con-uctivity plot shown in Fig. 11. While the low temperature datahow a relatively conventional power law relation, the nearly fre-uency independent conductivity becomes evident with rising

c conductivity is almost independent of frequency, which is dueo the dominance of the dc leakage current throughout the sam-le. With increase in temperature, the ac conductivity increases.

Fig. 11. Plot of the ac conductivity at different temperatures.

N. Kumari et al. / Materials Science and

Ff

Astttcpiciatt

σ

wc1TrptfcrdfTbcoti

c

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4

hppipahavtrtpn

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ig. 12. Relation between ac conductivity and 1000/T as a function of frequencyor Bi2VO5.5 film.

lso the ac conductivity slightly increases on high frequencyide at constant temperature. This suggests that the conduc-ivity at higher temperature is mainly activated by increasinghe temperature and the dc conductivity dominates at higheremperature. Fig. 12 shows the temperature dependence of aconductivity of BVO thin film, where the ac conductivity islotted as a function of 1000/T for different frequencies. Ast is clear from the figure that in the low temperature region, theonductivity depends significantly on frequency. With increasen temperature, a remarkable dielectric relaxation takes placend this reduces the frequency dependence of σac. A linear rela-ion between ln σac and 1/T in this region suggest the validity ofhe relation

ac = σo exp

(−Econd

kT

)(8)

here σo is a constant and Econd is the activation energy foronduction. The slope of the straight line fit to ln σac versus000/T gives the activation energy for the conduction process.he temperature dependent ac conductivity is divided into two

egions below and above 130 ◦C. Below 130 ◦C the conductionrocess might be a trap controlled space charge current conduc-ion in the sample. The conduction electrons could be createdrom the donor states as a consequence of ionized oxygen vacan-ies (Oo = Vo

•• + 2e− + 1/2O2) [36]. In the high temperatureegion, the activation energy varied from 0.51 to 0.84 eV atifferent frequencies, and the activation energy at the lowestrequency (100 Hz) under study was calculated to be 0.84 eV.his suggests that the conduction at higher temperatures mighte due to oxygen vacancies. In BVO system, oxygen vacanciesould be generated due to the presence of reduced valance state

f vanadium ions (V4+). One oxygen ion vacancy will form withwo tetravalent vanadium ions entering into the crystal structure,n order to maintain the electrical neutrality. The over all reaction

[[[

Engineering B 138 (2007) 22–30 29

ould be described using Kroger–Vink notation:

V+5 + Oxo → 2V+4 + Vo

•• + 1

2O2(g) (9)

here Vo•• is the oxygen vacancy with two effective positive

harges and oxo is the oxygen ion in oxygen site with zero effec-

ive charge. The doubly charged oxygen vacancies Vo•• were

onsidered to be the most mobile charges in perovskite ferro-lectrics. Above 130 ◦C, the oxygen vacancies at the boundary oflm/electrode are thermally activated; leading to a low frequencyispersion that is observed in the present dielectric constanteasurements.

. Conclusions

In conclusion, c-oriented (90%) Bi2VO5.5 thin filmsave been successfully fabricated on Pt/TiO2/SiO2/Si usingulsed laser deposition technique. The films exhibitedolarization–electric field hysteresis loops. The pulsed polar-zation measurements suggested that the actual remnantolarization values are rather low. These films exhibited remark-ble dielectric dispersion at low frequencies especially in theigh temperature regime. The observed dielectric relaxation isscribed to the thermally activated motion of ionized oxygenacancies and the trap controlled space charge current conduc-ion. From the ac conductivity plot, it is observed that the acesistance showed universal power law dependence accordingo the Jonscher’s model. The frequency plots of imaginary com-onents of impedance of BVO films suggest the relaxation to beon-Debye type.

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