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Materials Science and Engineering B 153 (2008) 36–46

Contents lists available at ScienceDirect

Materials Science and Engineering B

journa l homepage: www.e lsev ier .com/ locate /mseb

tructural, optical and ac conduction properties of Bi2V1−xNbxO5.5 (0 ≤ x ≤ 0.4)hin films fabricated by pulsed laser deposition technique

eelam Kumari, K.B.R. Varma, S.B. Krupanidhi ∗

aterials Research Centre, Indian Institute of science, Bangalore 560012, India

r t i c l e i n f o

rticle history:eceived 18 May 2008eceived in revised form 21 July 2008ccepted 9 September 2008

eywords:ismuth vanadatehin filmc conductivity

a b s t r a c t

Bi2V1−xNbxO5.5 {(x = 0, 0.1, 0.2, 0.3, 0.4), (BVN)} thin films were grown by pulsed laser deposition on(1 1 1) Pt/TiO2/SiO2/Si and Corning glass substrates and investigated systematically for their microstruc-tural, optical and ac conduction properties. The undoped bismuth vanadate, Bi2VO5.5 (BVO) thin filmswere highly textured and have been associated with the c-axis oriented grains of the layered perovskitestructure, while the Nb doped films consisted of randomly oriented crystallites. The scanning electronmicroscopy of the films indicates that the grain size increases with increase in Nb content. The opticaltransmission studies carried out on the samples deposited on Corning glass substrates revealed that thesefilms were nearly 80% transparent in the 400–900 nm range and the band gap of Nb doped BVO thin films

ptical transmissionniversal dielectric response

was slightly higher (3.13 eV for x = 0.4) than that of the undoped (2.91 eV) films. The dielectric constant ofthe Nb doped films increased with increase in Nb content, while the dielectric loss decreased especiallyin the 3–100 kHz frequency range. At a particular frequency, the conductivity decreased with increasein Nb content in the BVN thin films. In the higher temperature range, the activation energy varied from0.61 eV (x = 0.1) to 0.76 eV (x = 0.4) measured at 100 Hz. The frequency analysis of the dielectric and acconduction properties of these films suggests the conduction process in these films to be via oxygen ionvacancy motion through various defect sites.

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. Introduction

The ferroelectric materials belonging to the family of Auriv-llius phases have been of particular interest for a variety ofntegrated device applications such as non-volatile memories,ptical memories, and piezoelectric and electro-optic devices1–7]. Bismuth vanadate {Bi2VO5.5 (BVO)} is one such inter-sting ferroelectric compound identified in the large group oferroelectrics with layer-type structure with the general formulaBi2O2)2+(An−1BnO3n+1)2− [8–11]. This compound has attractedhe attention of many researchers from the point of view of itsielectric, ferroelectric and high ionic conductivity (at elevatedemperatures) in bulk and thin film configurations [12–15]. Fromhe structural point of view, bismuth vanadate (Bi2VO5.5) could

e formulated as (Bi2O2)2+(VO3.5�0.5)2−, where � represents oxide

on vacancies. BVO could hence be considered to be analogous to-Bi2WO6, an n = 1 member of Aurivillius family of oxides with

ntrinsic oxygen vacancies in the perovskite layer [8–10]. It crys-

∗ Corresponding author. Tel.: +91 80 22932601; fax: +91 80 2360 7316.E-mail addresses: neelamkandwal@gmail.com (N. Kumari), sbk@mrc.iisc.

rnet.in (S.B. Krupanidhi).

eepwssddc

921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2008.09.021

© 2008 Elsevier B.V. All rights reserved.

allizes in a non-centrosymmetric, polar orthorhombic class ands ferroelectric at room temperature. It exhibits three main poly-

orphs: a non-centrosymmetric �-phase at room temperature,ransforming to a centrosymmetric �-phase at 730 K and a cen-rosymmetric �-phase which is stable beyond 835 K and finally

elts at 1153 K [13]. The high temperature �-phase of BVO con-ists of (Bi2O2)2+ sheets interleaved with perovskite like layers ofVO3.5�0.5)2−. The high ionic conductivity associated with the �-hase 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 conductiv-

ty.Extensive work has been carried out on the effect of partial sub-

titution of vanadium by various ions (Nb5+, Ta5+, Ti4+, Zr4+, Cu2+,tc.) on the ionic conductivity of BVO in bulk form [12,16–18]. How-ver, the effect of these ions on the structural, dielectric and opticalroperties of thin films has not been investigated in detail. Recently,e conducted detailed studies on BVO thin films deposited on Si

ubstrates in a MOS configuration and found to exhibit excellenttability and lattice match on Si [19]. Also the ferroelectric andielectric properties of BVO thin films deposited by pulsed lasereposition were investigated in detail in metal–insulator–metalonfiguration [20]. The optical properties of BVO films on platinized

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N. Kumari et al. / Materials Scienc

ilicon substrate have been studied in detail by spectroscopic ellip-ometry operated in refection mode [21].

The present work is aimed at the visualization of the effect ofhe partial substitution of Nb5+ for V5+ on the structural, opticalnd electrical properties of BVO thin films. It is well known thathe properties of ferroelectric materials could be modified by theddition of dopants, in particular, the ferroelectric ion such as Nb5+.n BVO the vanadium is coordinated to oxygen in polyhedral (likeetrahedral and octahedral) environment as a consequence of itsxistence in more than one oxidation state depending upon thenergy supplied to remove the electrons to stabilize a particularalance state. In BVO, vanadium could exist mostly in two stabletates, i.e. V4+ and V5+. This leads to the large amount of oxygen ionacancy formation. Nb was found to remain in Nb5+ state and is not

asily reduced, when it was substituted for vanadium in BVO. Thisn turn is expected to give rise to reduced leakage current and betterhysical properties. In order to visualize the effect of Nb doping atifferent levels on the physical properties of BVO films, the presentork has been taken up. The Nb substituted BVO thin films were

hdhfd

Fig. 1. X-ray diffraction pattern of Nb dop

Engineering B 153 (2008) 36–46 37

abricated by pulsed laser ablation technique because this methods more suitable for depositing multicomponent oxides and has thedvantage of maintaining excellent stoichiometry. The influence ofb doping on the crystal structure of BVO thin films for differentompositions was studied. Also, the microstructural, optical andlectrical properties of Nb substituted BVO thin films were inves-igated. The details of the above experimental results are reportedn this article.

. Experiment

The polycrystalline Bi2V1−xNbxO5.5 (x = 0.1, 0.2, 0.3, 0.4) (BVN)argets for the pulsed laser deposition technique were preparedy the conventional solid-state reaction route. Fine powders of

igh-purity (Aldrich Chemicals 99.9%) Bi2O3, V2O5 and Nb2O5 withifferent molar ratios were weighed and ball milled for 5 h to obtainomogeneous mixtures. The powders were then calcined at 800 ◦C

or 5 h. The calcined powders were cold pressed (80 kN in an 18 mmie) and sintered at 825 ◦C for 8 h to form a single-phase dense tar-

ed BVO thin films grown at 650 ◦C.

3 e and Engineering B 153 (2008) 36–46

gt

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taacctipthere is a structural change. This structural change is further sup-ported by the change in c-axis lattice constant of BVN thin filmswith increasing Nb content.

8 N. Kumari et al. / Materials Scienc

et. The resultant high-density targets were used for deposition ofhe films.

BVN thin films were deposited on platinized siliconPt(1 1 1)/Ti/SiO2/Si} and Corning glass (Corning 7059) usingKrF excimer laser whose emission was at 248 nm wavelength.

he deposition was carried out at a laser pulse repetition rate ofHz, with an energy density of 160 mJ/pulse. The base vacuumas maintained at 1 × 10−6 Torr. The optimization of the oxygenartial pressure and the deposition temperature was carried outnd the films were deposited at the oxygen ambient pressure of00 mTorr and the deposition temperature was fixed at 650 ◦C (onlatinized Si) and 550 ◦C (on Corning glass) after optimization.

The X-ray powder diffraction studies were carried on BVN filmssing Cu K� (1.541 Å) radiation (Scintag XR2000 Diffractometer).he surface morphology of the films was monitored using scanninglectron microscope {SEM, (Sirion 200)} operated in secondarylectron emission mode. A semi-quantitative analysis of the film’somposition was carried out using energy dispersive X-ray anal-sis (EDAX). The thickness of the films was confirmed by theross-sectional SEM of the sample. The measured thickness of thelms was about 325 nm. The surface features of the samples used

or optical studies were examined by means of an ex citu con-act mode atomic force microscope (AFM) (Veeco CP-II). The AFMmages were obtained in the repulsive force regime with a forceonstant of 1.5 nN between the AFM tip and the sample surface.he optical properties of the BVN films were studied in trans-ission mode using spectrophotometer (Hitachi U3000) in the

00–900 nm wavelength range. The optical transmission spectraere obtained for the films deposited on the Corning 7059 glass

ubstrates. The optical properties of BVN1 prepared on platinizedilicon were studied by Spectroscopic ellipsometry at room temper-ture in the 0.75–4.0 eV (ultraviolet–visible) photon energy rangesing a computer controlled variable angle of incidence spectro-copic ellipsometer (SENTECH 850, SENTECH Instruments). Theptical constants of the BVO films were determined by fitting theodel function to the measured data using SpectraRay2-5656b

oftware package (SETTECH Instruments).For the dielectric constant measurements, gold dots of

.96 × 10−3 cm2 area were deposited on the top surface of the filmshrough a shadow mask and thermal evaporation technique. Thelectrode dots were annealed at 250 ◦C for 30 min. The Pt coatingas used as the bottom electrode for all the electrical measure-ents. The dielectric constant and the alternating current (ac)

lectrical property measurements were performed as a function ofrequency (100 Hz–100 kHz) in the temperature range of 25–250 ◦C,sing a Keithley 3330 LCZ meter with oscillation amplitude of 0.5 V.

. Results and discussions

.1. Crystal structure and surface morphology

BVN (x = 0.1, 0.2, 0.3, and 0.4) thin films of thickness ∼325 nmere deposited on platinized silicon substrates at various substrate

emperatures and pressures. These samples are henceforth abbre-iated as BVN1, BVN2, BVN3 and BVN4, respectively. The X-rayiffraction (XRD) patterns obtained for Nb (x = 0–0.4) substitutedVO thin films are shown in Fig. 1. The diffraction peaks corre-ponding to x = 0, could be indexed on the basis of an orthorhombicVO [JCPDS Data No. 42-0135] and those for the rest of the compo-

itions could be indexed on the basis of tetragonal cell of BVO [13].t is well known that the Nb substitution in BVO causes the phasehange from orthorhombic to tetragonal at x = 0.1. Higher intensi-ies of the Bragg reflections corresponding to the (0 0 l) planes ofVO are observed in undoped films, indicating that the films are

FC

ig. 2. Variation of c-axis lattice constant of BVN thin films deposited on platinizedilicon substrate as a function of Nb content.

extured having preferred orientation along the orthorhombic c-xis (normal to the substrate). It has been observed that there isstrong effect of Nb content on the texturing of the films. As the

omposition of Nb changes from 0 to 0.4, the texture of the filmshanges from (0 0 l) to randomness. For example, for the composi-ion corresponding to x = 0.4, the (1 1 3) Bragg reflection is of higherntensity than the rest of the peaks. Also there is a slight shift in theeak position of the (0 0 l) towards lower 2� values indicating that

ig. 3. X-ray diffraction pattern of Nb doped BVO thin films grown at 550 ◦C onorning glass substrate.

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N. Kumari et al. / Materials Scienc

Fig. 2 shows the variation of c-axis lattice constant of BVN thinlms deposited on platinized silicon substrate as a function of Nbontent. The c-axis lattice parameter increases with increasing Nbontent in BVO. There is a gradual increase in ‘c’ up to the composi-ion corresponding to x = 0.3 and subsequently the increase is rapid.he substitution of relatively large ionic radii Nb5+ ions (0.69 Å) for(0.59 Å) in BVO layered ferroelectric structure is expected to result

n an increase in the c-axis lattice constant and is in agreement withhe present result (Fig. 2). Similar trend of increase in lattice param-ter with an increase in Nb concentration in BVN bulk samples haseen reported earlier by Varma and Prasad [22]. It is important toote that the value of c-axis lattice parameter for the as depositedVN films is slightly smaller than that of the unstressed bulk value.he small difference in the value of lattice parameters between theulk and thin films may be attributed to the incorporation of defects

n the form of interstitial oxygens during deposition and also dueo the lattice mismatch between the substrate and the film. As it isbserved in Fig. 1, the peak intensity of the (0 0 l) planes in BVN thinlms decreases with increase in Nb composition. This could be dueo the two major factors; (i) as we go from x = 0 to x = 0.4, there iscrystallographic structural change from orthorhombic to tetrag-nal which may favor the random orientation and (ii) increase in

isorder in the BVN films with varying x due to larger Nb5+ ions.

The X-ray diffraction pattern obtained for BVN thin filmseposited on Corning glass substrate at 550 ◦C under 100 mTorrxygen partial pressure is depicted in Fig. 3. Similar to the case

tn

C

ig. 4. Scanning electron micrograph (SEM) of Nb doped BVO thin films deposited at 650d) x = 0.3, (e) x = 0.4 samples and (f) cross-sectional features of x = 0.1 sample.

Engineering B 153 (2008) 36–46 39

f BVN films deposited on platinized silicon substrate, the (0 0 l)lanes of BVO are observed in undoped films, indicating thathe films are textured having preferred orientation along therthorhombic c-axis whereas the samples with higher content ofb doping showed randomness. Also for the samples BNN2 toVN4, the (1 1 3) peak shifted towards left indicating the structuralhange.

Surface characteristics of the Nb doped BVO thin films depositedn platinized silicon surface were obtained using the scanning elec-ron microscope. Fig. 4(a–d) shows the surface SEM micrographsf the BVN thin films for different compositions of Nb along withhe undoped BVO sample image. The micrograph for undoped filmonsists of mixture of spherical and platelet grains of uneven sizehere as the BVN films consist of closely packed rod-shaped grains

round 0.3–0.35 �m in size. The grain size is found to increase withncrease in the Nb content which may be attributed to the moreandom growth of the films that will enhance the coalesce process.ig. 4(e) shows the cross-sectional SEM of the x = 0.1 BVN film. Its observed that the film has dense microstructure and the thick-ess of the film estimated from the cross-sectional SEM is around25 nm. Similar features were found for the other films with variousoping levels. The energy dispersive X-ray analysis carried out on

he well crystalline films of BVN has confirmed the stoichiometricature of the films.

The surface microstructure of the BVN films deposited onorning glass substrate has been studied with an atomic force

◦C and 100 mTorr showing surface microstructures of (a) x = 0, (b) x = 0.1, (c) x = 0.2,

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0 N. Kumari et al. / Materials Scienc

icroscope and the resulting topography are shown in Fig. 5a–e; for BVO, BVN1, BVN2, BVN3 and BVN4) along with thehree-dimensional image of BVN3 (Fig. 5(f)). The homogeneous dis-ribution of grains was observed for all the samples and the grain

ize was found to decrease with increase of Nb content. This trendas just opposite to the trend observed in BVN films depositedn platinized silicon substrate. This difference might be due tohe difference in the nature of substrate as the growth kineticsill be different on glass and crystalline substrate. The root mean

sic

ig. 5. The 5 �m × 5 �m AFM micrographs of Nb doped BVO thin films deposited on Cornd) x = 0.3, (e) x = 0.4 samples and (f) Three-dimensional topography of a x = 0.3 sample.

Engineering B 153 (2008) 36–46

quare (rms) roughness of the BVN films has been calculated for a�m × 5 �m area and was in the range of 6–3 nm.

.2. Optical properties

The optical properties of the BVN films were studied using apectrophotometer in transmission mode. For making optical stud-es, the films were deposited on Corning glass substrates. Therystalline nature of these films was established by the XRD studies

ing glass substrate showing surface morphologies of (a) x = 0, (b) x = 0.1, (c) x = 0.2,

N. Kumari et al. / Materials Science and Engineering B 153 (2008) 36–46 41

Fs

ps3etgatwwfacshvtimid

st

˛

wddTtmpc2id

tt

Fcd

uoai

n

a

M

where s is the refractive index of the substrate. It is found thatthe refractive index of all the samples showed dispersion with thewavelength, i.e. the refractive index of the x = 0.1 BVN thin filmdecreased from 3.8 at 450 nm to 2.6 at 850 nm. Also the values

ig. 6. Optical transmission spectra of BVN films deposited on Corning glass sub-trates. The inset shows the zoomed part of transmission near cutoff wavelength.

rior to the optical property measurements. Optical transmissionpectra of BVN films for different concentrations recorded in the00–900 nm wavelength range is shown in Fig. 6. All the samplesxhibited interference fringe patterns in the visible region of theransmission spectra with an average transmission >80% indicatingood optical quality of the deposited films with low scattering orbsorption losses. A careful observation of the graphs suggests thathe transparency window widens towards the short wavelengthith the increase in Nb content. The increase in the transmissionindow with the increase in Nb content is associated with two

actors: (i) a decrease in the number of oxygen ion vacancies withn increase in Nb content which in turn will reduce the scatteringenters. Upon the substitution of vanadium with niobium, the +5tate becomes more stable as the reduction of Nb5+ to Nb4+ requiresigher energy, thereby decreasing the possibility of the oxygen ionacancy formation. (ii) The decrease in scattering centers due tohe presence of less number of grain boundaries with an increasen Nb content, which was established from the scanning electron

icrographs of films of different Nb content. Since the grain sizencreases with increase of Nb content, the grain boundary volumeecreases.

The optical absorption coefficient, ˛, of an indirect band gapemiconductor near the band edge, for photon energy h� greaterhan the band gap energy Eg of the semiconductor, is given by [23]

h� = constant(

h� − Eg)2

(1)

here h is the Planck’s constant and � is the frequency of the inci-ent photon. The Tauc plots of (˛h�)1/2 vs. energy h� for all theeposited BVN films for various Nb content are shown in Fig. 7.he band gap energy is obtained by extrapolating the linear part ofhe Tauc plots to the energy axis (i.e. where (˛h�)1/2 = 0). The esti-

ated band gap energy values for all the samples under study arelotted in the inset of Fig. 7 as a function of the Nb content. It islear from the figure that the estimated band gap increases from.91 to 3.13 eV as the Nb content increases from x = 0 to x = 0.4. This

s attributed to the reduced number of oxygen ion vacancies in Nboped BVO films as explained earlier.

The refractive index of BVN films are computed from theransmission spectra. The interference effects observed in theransmission spectra reveal that the thicknesses of the films are

Fl

ig. 7. Tauc plot of (˛h�)2 as a function of h� for as deposited films for various Nbontent. The inset shows the variation of band gap of Bi2V1−xNbxO5.5 thin films withifferent Nb content.

niform. These fringes were used to calculate the refractive indexf the thin film using the Swanepoel method [24] and the resultsre shown in Fig. 8 for four different compositions. The refractivendex n was calculated using the formula

=[

M +(

M2 − s2)1/2

]1/2(2)

nd

= 2s

Tm− s2 + 1

2(3)

ig. 8. The variation of the refractive index of Bi2V1−xNbxO5.5 thin films with wave-ength for different Nb content.

42 N. Kumari et al. / Materials Science and Engineering B 153 (2008) 36–46

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3

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Ffi

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ig. 9. The refractive index (n) and the extinction coefficient (k) for Bi2V0.9Nb0.1O5.5

hin films deposited on platinized silicon substrate.

f refractive index increase with the increase in Nb content. Thiss attributed to an increase in dielectric constant for Nb dopedlms.

To study the optical properties of BVN samples prepared on pla-inized silicon substrate spectroscopic ellipsometry was employed21]. Fig. 9(a and b) shows the refractive index, n and extinc-ion coefficient, k of the BVN1 films obtained from the best-fit

odel. The details regarding the other samples will be commu-icated separately. The refractive index initially increases sharplynd shows a maximum around 340 nm and then decreases rapidlyith increasing wavelength. On the other hand, the extinction

oefficient decreases rapidly with increasing wavelength up to00 nm and subsequently the decrease is small. In the visibleange (650 nm), the refractive index of the BVN1 film is about.22, with an extinction coefficient of 0.052. The optical constantsf BVN1 films obtained from ellipsometric measurement analysisre in close agreement with those obtained from the transmis-ion studies of the BVN1 thin films deposited on the Corning glassubstrate.

.3. Dielectric studies

Fig. 10(a and b) depicts the dielectric response of BVN thinlms with varying dopant concentrations of Nb. The dielectric

onstant shows frequency dependence especially in the low fre-uency range (100 Hz–1 kHz) for the samples corresponding to= 0.1–0.3, while this dispersion tend to reduce with increasing Nb

raction. For instance, for the composition corresponding to x = 0.4,he low frequency dispersion of the dielectric constant is negligi-

�

wds

ig. 10. (a) Frequency dispersion of the dielectric constants of Bi2V1−xNbxO5.5 thinlms. (b) Frequency dispersion of the dielectric loss of Bi2V1−xNbxO5.5 thin films.

le in the entire range of frequencies studied. At low frequencies,he dispersion in dielectric response is mainly due to space chargeccumulation, while at higher frequencies, the grain–grain bound-ry capacitance interference becomes mostly responsible. It maylso be realized from Fig. 10(a) that the dielectric constant increasesith Nb content. At 100 kHz the values of dielectric constant mea-

ured at room temperature increase from 78 (x = 0.1) to 132 for= 0.4. Such a behavior may be attributed to the following: prac-

ically the dielectric relaxation of any system is strongly influencedy the complexity of the microstructure and the presence of anypace charge carrier. The increase in the dielectric constant of theresent films is ascribed to their microstructures. The variation ofhe dielectric loss as a function of frequency at different dopantoncentrations is shown in Fig. 10(b). It may be realized that theielectric loss decreases with increasing Nb content, which may bessociated with the reduced oxygen vacancies and concomitant V5+

tabilization.

.4. ac conduction properties

To visualize the ac conduction response and the nature ofharge carriers in BVN films, the ac conductivity studies werearried out. The variation of the ac conductivity of BVN thinlms with frequency measured at various temperatures has beenhown in Fig. 11 for different Nb contents. The ac conductivityncreases with increasing frequency in the measured frequencyange (100 Hz–100 kHz) for all the four different compositions. Itas been observed that at a particular frequency and temperature,he conductivity decreased with increase in Nb content in the BVNhin films. The ac conductivity in general is related to the dielectricoss through the relation

ac = ωεoε′r tan ı (4)

here εo is the permittivity of the free space, ε′r is the relative

ielectric constant, ω is the angular frequency and tan ı is the dis-ipation factor. In Nb doped BVO films, the loss is found to decrease

N. Kumari et al. / Materials Science and Engineering B 153 (2008) 36–46 43

y resp

wt

B

�

w

Fig. 11. ac conductivity vs. frequenc

ith the increase in Nb content due to the stabilization of V5+ state

hereby decreasing the ac conductivity.

The ac conductivity plotted (Fig. 11) for all the four samples ofVN thin films was found to obey the power law relation [25]

T (ω) = �dc + An(T)ωn(T) (0 < n < 1) (5)

mssat

onse in Bi2V1−xNbxO5.5 thin films.

here n(T) is the temperature dependent exponent, which deter-

ines the strength of the coupling between different charged

pecies (small value of n(T) corresponds to strongly interactingystems) and An(T) determines the strength of the polarisibilityrising from the universal mechanism and �dc is the dc conduc-ivity. While fitting the observed data for ac conductivity to the

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4 N. Kumari et al. / Materials Scienc

bove equation (Eq. (5)), we found the deviation at lower frequencyide for the data at lower temperature. While the low temperatureata show a relatively conventional power law relation, the nearlyrequency independent conductivity becomes evident with risingemperature. It is clear from Fig. 11 that at low frequencies, thec conductivity shows almost a frequency independent response,hich is due to the dominance of the dc leakage current through-

ut the sample. However the saturation of �ac is never completeor all samples as we are not approaching the zero frequency lim-ts. With increase in temperature, the ac conductivity increases.lso the ac conductivity slightly increases on high frequency side atonstant temperature. This suggests that the conductivity at higheremperature is mainly activated by increasing the temperature andhe dc conductivity dominates at higher temperature. It has beenbserved that the parameter ‘n’ in the frequency dependence of theonductivity is related to doping and stoichiometry of the thin filmsnder consideration. The exponent ‘n’ is estimated from log �(ω) vs.

og ω plot and found that it decreases with the increase in tempera-ure as shown in Fig. 12. It may be noted that the varied nature of thearameter ‘n’ with different Nb concentrations in BVO films indi-ates either a broad distribution of relaxation times [26] or multipleite polaron hopping [27]. The values of ‘n’ less than unity are asso-iated with charge carriers or with ‘extrinsic’ dipoles arising fromhe presence of defects or impurities. The value of ‘n’ has shown aecreasing trend with rising temperature, indicating that the inter-ction between different charged species decreases as the latticecales down.

The overall trend of ac conductivity variation with frequencyFig. 11) obeys a ‘universal’ power law (Eq. (5)), which is basedn rigorous many body interactions. The difference in macroscopicroperties between films mainly originates from the difference inhe microscopic nature of the charge carriers in each film. It mainlyriginates from the outermost free-charge carriers and/or trappedharge carriers. The similarity of the response corresponding to dif-erent Nb concentrations could be due to the interactions among

any bodies, such as charged species, regardless of whether theyre electrons, ions, dipoles, or defects. In hopping conduction, local-zed charge carriers at each atomic site jump to another site by

hermal energy from time to time. In this case, each jump of aharge carrier is considered to occur completely at random, with-ut any correlation between one jump and the next. Each pairf potential well corresponds to a certain time constant (proba-

ig. 12. Variation of exponent ‘n’ with temperature in Bi2V1−xNbxO5.5 thin films.

stbbTa

O

wI0asftoaitatsltvt

Engineering B 153 (2008) 36–46

ility) of transition from one site to another. Naturally, the timeonstant for transition across a higher energy barrier is large asompared to that across the smaller ones. Therefore the traps withhe high activation energy can respond only at low frequencies.lso one would anticipate that the number of traps having lowernergies (i.e. defect sites involving lower distortion of the lattices)re more than those having higher energy values (assuming a nearaxwell–Boltzmann distribution); the ac conductivity is expected

o be more. At higher temperatures the energy distribution of theraps becomes more uniform and the variation of ac conductiv-ty with frequency becomes less. In our study on BVN films, the

agnitude of the slope decreased from 0.83 to 0.31. As the temper-ture is high enough for the band-to-band transition to dominatehe conduction process, ‘n’ approaches zero since electronic con-uction is frequency independent. This results in ac conductivityin log �(ω) vs. log ω plots for different temperatures) showing

converging trend at higher frequency indicating that at highrequencies, the ac conductivity becomes almost independent ofemperature.

Fig. 13 displays the Arrhenius plots of the total conductivity ofVN thin films with four different contents of Nb, where the ac con-uctivity is plotted as a function of 1000/T for different frequencies.

t is observed that the plots for all the samples consist of two dif-erent regions corresponding to two different activation energies.hese two regions could be distinctly seen in Fig. 13 for all the sam-les. It has been observed that the respective activation energy for= 0.4 film has shown the highest activation energy among all theompositions studied here. As it is clear from the figure that in theow temperature region, for all the films, the conductivity dependsignificantly on frequency. With increase in temperature, a remark-ble dielectric relaxation takes place and this reduces the frequencyependence of �ac. A linear relation between ln �ac and 1/T in thisegion suggests the validity of the relation

ac = �0 exp(

−Econd

kT

)(6)

here �0 is a constant and Econd is the activation energy for conduc-ion. The slope of the straight line fit to �ac vs. 1000/T on semi-logcale gives the activation energy for the conduction process. Theemperature dependent ac conductivity is divided into two regionselow and above 100 ◦C. Below 100 ◦C the conduction process mighte a trap controlled space charge current conduction in the sample.he conduction electrons could be created from the donor states asconsequence of ionized oxygen vacancies [28]

o = V ••o + 2e− + 1

2O2(7)

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

n the high temperature region, the activation energy varied (from.51 to 0.78 eV for x = 0.4 composition film) at different frequencies,nd the activation energy at the lowest frequency (100 Hz) undertudy is 0.78 eV. This frequency dependent activation energy resultsrom the multiple site hopping conduction mechanism and thisrend of decrease in activation energy with increase in frequency isbserved in entire temperature range although the change is smallt lower temperature. When the temperature is increased the local-zed charge carriers at each atomic site jump to another site byhermal energy on complete random base. Each site behaves aspotential well and has its own time constant of transition and

he time constant (inverse of frequency of response) for the tran-

ition across a higher energy barrier (higher activation energy) isarge as compared to that across the smaller ones. Therefore theraps with high activation energy respond at lower frequency andice versa. This gives the frequency dependent activation energy inhese samples. From the calculated values of activation energy it is

N. Kumari et al. / Materials Science and Engineering B 153 (2008) 36–46 45

ductiv

soe(vpt

n

Fig. 13. Arrhenius plots of ac con

uggested that the conduction at higher temperatures is due to thexygen vacancies. In BVN system, oxygen vacancies could be gen-

rated due to the presence of reduced valance state of vanadiumV4+) ions. One oxygen ion vacancy will form with two tetravalentanadium ions entering into the crystal structure in place of twoentavalent vanadium ions, in order to maintain the electrical neu-rality. The over all reaction could be described using Kroger–Vink

2

wTm

ity in Bi2V1−xNbxO5.5 thin films.

otation:

1

V5+ + Ox

o → 2V4+ + V ••o +

2O2(g) (8)

here oxo is the oxygen ion in oxygen site with zero effective charge.

he doubly charged oxygen vacancies V ••o are considered to be the

ost mobile charges in perovskite ferroelectrics. Above 100 ◦C, the

4 e and

oai

4

hgifiedNwitrwTtioo

R

[[[

[[[

[[

[

[

[[[[

[

6 N. Kumari et al. / Materials Scienc

xygen vacancies at the boundary of film/electrode are thermallyctivated, leading to a low frequency dispersion which is observedn the present dielectric constant measurements.

. Conclusions

In conclusion, the Bi2V1−xNbxO5.5 (x = 0.1, 0.2, 0.3, 0.4) thin filmsave been successfully fabricated on Pt/TiO2/SiO2/Si and Corninglass substrates using pulsed laser deposition technique and stud-ed their structural, optical and dielectric properties. All the abovelms exhibited layered perovskite structure with no preferred ori-ntation unlike in the case of undoped BVO. The films exhibitedense microstructure and the grain size increased with increasingb content. The band gap of Nb doped BVO films increased slightlyith the increase in Nb content and was attributed to a decrease

n number of oxygen ion vacancies. The refractive index of BVNhin films was found to increase with increasing Nb content. Theoom temperature dielectric constant of BVN thin films increasedhereas the dielectric loss decreased with increasing Nb content.

his is an important property to be exploited for device applica-ions. The films with higher content of Nb exhibited frequencyndependent characteristics. The ac conductivity studies carriedut on BVN films suggested the hopping of charge carries such asxygen ion vacancies as the predominant conduction mechanism.

eferences

[1] B.H. Park, B.S. Kang, S.D. Bu, T.W. Noh, J. Lee, M. Jo, Nature (London) 401 (1999)682.

[[

[

[

Engineering B 153 (2008) 36–46

[2] H.N. Lee, D. Hesse, N. Zakharov, U. Gosele, Science 296 (2002) 2006.[3] J.F. Scott, Paz de Aranjo, Science 246 (1989) 1400.[4] R. Watton, M.A. Todd, Ferroelectrics 118 (1991) 279.[5] Y. Nemirovsky, A. Nemirovsky, P. Muralt, N. Setter, Sens. Actuators A 56 (1996)

239.[6] D.L. Polla, Microelectron. Eng. 29 (1995) 51.[7] C.E. Land, P.D. Thacher, G.H. Haertling, Appl. Solid State Sci. 4 (1974) 137.[8] A.A. Bush, Yu.N. Venevtsev, Russ. J. Inorg. Chem. 31 (5) (1986) 769.[9] V.G. Osipian, L.M. Savchenk, V.L. Elbakyan, P.B. Avakyan, Inorg. Mat. 23 (1987)

467.10] K.V.R. Prasad, K.B.R. Varma, Mater. Chem. Phys. 38 (1994) 406.11] B. Aurivillius, Nature (London) 2 (1950) 519.12] F. Abraham, J.C. Biovin, G. Mairesse, G. Nowogrocki, Solid State Ionics 40–41

(1990) 934.13] K.V.R. Prasad, K.B.R. Varma, J. Mater. Sci. 30 (1995) 6345.14] C.K. Lee, D.C. Sinclair, A.R. West, Solid State Ionics 62 (1993) 193.15] K.V.R. Prasad, K.B.R. Varma, A.R. Raju, K.M. Satyalakshmi, R.M. Mallya, M.S.

Hegde, Appl. Phys. Lett. 63 (1993) 1898.16] R. Essalim, B. Tanouti, J.P. Bonnet, J.M. Reau, Mater. Lett. 13 (1992) 382.17] V. Sharma, A.K. Shukala, J. Gopalakrishnan, Solid State Ionics 58 (1992)

359.18] J.B. Goodenough, A. Manthiram, M. Paranthaman, Y.S. Zhen, Mater. Sci. Eng. B

12 (1992) 357.19] N. Kumari, J. Parui, K.B.R. Varma, S.B. Krupanidhi, Solid State Commun. 137

(2006) 566.20] N. Kumari, S.B. Krupanidhi, K.B.R. Varma, Mater. Sci. Eng. B 138 (2007) 22.21] N. Kumari, S.B. Krupanidhi, K.B.R. Varma, Appl. Phys. A 91 (2008) 693–699.22] K.B.R. Varma, K.V.R. Prasad, J. Mater. Res. 30 (1995) 6345.23] J.C. Tauc, Optical Properties of Solids, North-Holland, Amsterdam, 1972,

p. 372.24] R. Swanepoel, J. Phys. E 17 (1984) 896.

25] A.K. Joncher, Dielectric Relaxation in Solids, Chelsea, London, 1983.26] S. Bhattacharyya, S.S.N. Bharadwaja, S.B. Krupanidhi, J. Appl. Phys. 88 (2000)

4294.27] S.S.N. Bharadwaja, P. Victor, P. Venkateswarulu, S.B. Krupanidhi, Phys. Rev. B 65

(2002) 174106.28] D.M. Smyth, Annu. Rev. Mater. Sci. 15 (1985) 329.