Structural and electrical characterization of Bi2VO5.5 / Bi4Ti3O12 bilayer thin films deposited by pulsed laser ablation technique

doi:10.4236/ns.2010.210133

Paper Menu >>

Journal Menu >>

Vol.2, No.10, 1073-1078 (2010) Natural Science

http://dx.doi.org/10.4236/ns.2010.210133

Structural and electrical characterization of Bi2VO5.5 /

Bi4Ti3O12 bilayer thin films deposited by pulsed laser

ablation technique

Neelam Kumari, Saluru Baba Krupanidhi, Kalidhindi Balakrishna Raju Varma*

Materials Research Centre, Indian Institute of science, Bangalore, India; *Corresponding Author: [email protected].

Received 10 June 2010; revised 13 July 2010; accepted 18 July 2010.

ABSTRACT

The pulsed laser ablation technique has been

employed to fabricate bilayer thin films con-

sisting of layered structure ferroelectric bis-

muth vanadate (Bi2VO5.5) and bismuth titanate

(Bi4Ti3O12) on platinized silicon substrate. The

phase formation of these films was confirmed

by X-ray diffraction (XRD) studies and the crys-

tallites in these bilayers were randomly oriented

as indicated by diffraction pattern consisting of

the peaks corresponding to both the materials.

The homogeneous distribution of grains (~300

nm) in these films was confirmed by atomic

force microscopy. The cross-sectional scanning

electron microscopy indicated the thickness of

these films to be around 350 nm. The film ex-

hibited P-E hysteresis loops with Pr ~ 11 C/cm2

and Ec ~ 115 kV/cm at room temperature. The

dielectric constant of the bilayer was ~ 225 at

100 kHz which was higher than that of homo-

geneous Bi2VO5.5 film.

Keywords: Thin Films; Ferroelectric; Dielectric;

Laser ablation

1. INTRODUCTION

Fabrication and stabilization of materials that do not

occur naturally has been the subject of great interest of

current materials research [1]. Recently the investigations

of ferroelectric multilayer and superlattices have received

considerable attention due to the fact that these kinds of

engineered materials have been identified as possessing

functional properties in a sense superior to their single

phase constituent films [2-4]. The control of properties

could be achieved by tailoring the lattices [5], e.g. by la-

ttice mismatch induced strain at the interface-strain en-

gineering, polarization mismatch enhancing polarization,

chemical heterogeneity, which in turn may enhance the

physical properties or in many cases may give rise to new

properties which were not exhibited by the starting ma-

terials. The Aurivillius family of layered bismuth oxides

is a class of ferroelectrics whose properties have been

widely studied [6]. More recently, there is a renewed in-

terest because of the discovery of fatigue-free behavior

in thin films for nonvolatile memory applications [7].

More importantly, Bismuth Titanate [Bi4Ti3O12 (BTO)],

which is an n = 3 member of this family has been re-

ported to be a very good ferroelectric and electro-optic

with small amount of the substitution of impurities, such

as La, Sm and Nd for Bi and V, W and Nb for Ti in the

pseudoperovskite (Bi2Ti3O10)2− layers of BTO to im-

prove the remnant polarization and fatigue endurance.

[8-11]. Bismuth Vanadate [Bi2VO5.5 (BVO)] is a vana-

dium analog of the n = 1 member of the Aurivillius fam-

ily which has a Curie temperature of 720 K [12-14]. It

has been reported in the literature that the composite of

BVO and BTO solid solution possesses better physical

properties and low leakage current than that of BVO

[15]. The single phase BVO thin films have been pre-

pared on platinum coated Si substrates and studied their

ferroelectric and dielectric properties [16]. It has been

found that these films possess non-negligible ionic con-

ductivity attributed to the presence of oxide ion vacan-

cies in the perovskite layer. Also the contribution of

oxygen ion vacancies to the ferroelectric properties was

quite high as established through fatigue characteristics.

In this article we report the structural and electrical

properties of bilayer stacking of Bi2VO5.5 (BVO) and

Bi4Ti3O12 (BTO). We have fabricated bilayer thin films

consisting of alternating BVO and BTO layers hence-

forth mentioned as BVBT. The presence of a BTO layer

along with the BVO layer effectively suppressed the

high electrical conductivity of BVO which is commonly

observed in the laser ablated BVO thin films deposited

on Pt / TiO2 / SiO2 / Si substrates. The BVBT bilayer thin

films showed a fair increase in remnant polarization (Pr),

and more interestingly a significant reduction in coercive

field (Ec), as compared to the homogeneous BVO films

N. Kumari et al. / Natural Science 2 (2010) 1073-1078

1074

of the same thickness. The details pertaining to the struc-

tural, dielectric and ferroelectric properties of BV BT bi-

layers fabricated on platinized silicon in metal insulator

metal (MIM) configuration are illustrated in the follow-

ing sections.

2. EXPERIMENTAL

The bilayer structures consisting of BVO and BTO

were fabricated by a multitarget-pulsed laser deposition

(PLD) technique on platinized silicon substrate in the

configuration Au / BVO / BTO / Pt (111) / TiO2 / SiO2 /

Si (100). A 248 nm excimer laser (Lambda Physik Com-

pex 201) operated at 5 Hz was alternately focused onto

the well-sintered freshly polished BVO and BTO rotat-

ing targets with an energy density of 2 Jcm-2 at an angle

of 45˚ by a UV lens. The substrates were placed parallel

to the target at a distance of 3.5 cm and heated to 650℃

by a resistance heater. The chamber was first pumped

down to 1  10-6 m bar, and then high purity oxygen was

introduced using a mass flow controller to get oxygen

partial pressure of 100 m Torr. After deposition of both

the layers, the samples were cooled down to room tem-

perature under an oxygen pressure of 1 mbar to minim-

ize the oxygen ion vacancies. In these cases, the bilayer

thin films were prepared with BTO as the first layer and

BVO as the final layer with equal layer thickness.

The X-ray diffraction (XRD) studies were carried out

to characterize the phase and crystallographic structure

of the bilayer films using Cu Kα ~ 1.541 Å radiation

(Scintag XR 2000 Diffractometer). Scanning electron

microscope (SEM) (Sirion 200) and atomic force micro-

scope (AFM) (Veeco CP II) were employed to monitor

the microstructure of the films.

For electrical measurements, gold dots of 1.96 × 10-3

cm2 area were deposited on the top surface of the films

through a shadow mask using thermal evaporation tech-

nique. The electrode dots were annealed at 250℃ for 30

min. The Pt surface was used as the bottom electrode for

capacitance measurements. The dielectric constant and

C-V measurements were performed at a signal strength

of 0.5 V using impedance analyzer (HP4294A). The

polarization-electric field (P-E) hysteresis was recorded

using a Precision Workstation (Radiant Technologies,

Inc.) ferroelectric test system in virtual ground mode.

3. RESULTS AND DISCUSSIONS

Bilayered thin films of BVO and BTO were fabricated

on platinized silicon substrates by pulsed laser ablation

using the optimized deposition conditions for BVO and

BTO layers. A schematic diagram of the bilayer thin film

grown in this work is shown in Figure 1. Equal thick-

TiO

/ SiO

/ Si

BTO

BVO

Figure 1. Schematic diagram of a BVBT bilayer thin film.

ness (~ 175 nm) of the individual layers was maintained

in bilayer films. The BTO layer was grown first on pla-

tinized silicon substrate and then immediately followed

by the growth of BVO layer without any delay in order

to maintain the sharp interface between the two layers.

The sequence of the layers was also reversed, but not

much difference in terms of physical properties was ob-

served, hence the one bilayer structure i.e. BVBT bilayer

is discussed as a symbolic representative one.

Figure 2(a) shows the representative XRD pattern of

BVBT bilayer film deposited by PLD. The XRD diffrac-

tion peaks corresponding to both the BVO and BTO

phases were observed and these were indexed on the ba-

sis of orthorhombic structure of BVO [JCPDS 42-0135]

and BTO [JCPDS 72-1019]. It is observed that the film

is polycrystalline in nature and main diffraction peaks of

both BVO (113) and BTO (117) appear with higher inten-

sities along with the other low intensity peaks. Also, the

(002) reflection of BVO along with (004) of BTO appear

so close that they overlap giving only one visible peak.

The bilayer film possessed an interface between the two

individual layers and this interface does influence the

physical properties due to the lattice strain at the inter-

face. Figure 2(b) shows the (002) diffraction peak cor-

responding to BVO of the bilayer thin film. The c-axis

lattice parameter calculated for BVO from the (002) peak

was found to increase from 15.42 Å for BVO layer [17]

to 15.50 Å in BVBT bilayer. This increase in out-of-plane

lattice parameter might be due to the in plane compres-

sive stress on BVO in BVBT bilayer in the present case

in order to keep the cell volume unchanged. Therefore, it

is required to grow these films epitaxially and measure

the in-plane lattice parameters along with the residual

stress in these bilayer films in order to achieve the lattice

driven effects in these films.

The surface morphology of the BVBT BL thin film

was investigated by contact mode atomic force micro-

scope (AFM). Figure 3 shows the surface morphology

of BVBT bilayer thin film over a 5 m x 5 m scan area

as two-dimensional (a) and three-dimensional (b) mi-

N. Kumari et al. / Natural Science 2 (2010) 1073-1078

107

1075

10.5 11.0 11.5 12.0

(b)

Intensity (a.u.)

(002)



(Degree)

BVBT

BVO

10.5 11.0 11.5

12.0

2θ (Degree)

Intensity(a.u.)

BVBT

BVO

(b)

(002)

c= 15.50Å

c = 15.42 Å

Figure 2. (a) X-ray diffraction pattern of BVBT bilayer thin film. (b) Comparison of BVO and BVBT bi-

layer diffraction pattern along (002) plane of BVO.

(b)

(c)

(a)

Figure 3. (a) AFM micrograph showing surface topography, (b) 3D image of BVBT bilayer thin film and (c)

line profile.

crographs with the roughness profile mapped using line

scan depicted in (c) The BVBT bilayer films exhibited

dense surface morphology consisting of distinct grains.

Further the homogeneous distribution of grains was ob-

served with an average grain size of 0.3 m. The root

mean square of the surface roughness (Ra) was around 7

nm as observed from the roughness profile. It indicated a

good quality of the deposited bilayer films.

The cross sectional microstructure of BVBT bilayer

was studied by SEM and is depicted in Figure 4 which

indicated dense bilayer growth with sharp interface with

Pt. A columnar like structure was observed for this sam-

ple and the thickness of the sample bilayer film was

around 300 nm. It is noteworthy here that we could not

distinguish between the top BVO and bottom BTO layer

in this bilayer structure. This might be due to the very

little difference in the atoms constituting the BVO and

BTO layers which scatter the electrons almost with the

N. Kumari et al. / Natural Science 2 (2010) 1073-1078

1076

Figure 4. Cross sectional scanning electron micrograph of a

representative BVBT bilayer film.

same intensity during and as a result the contrast bet-

ween the two layers is poor.

In the present study we have further focused on the

ferroelectric (FE) properties of these BVBT bilayer thin

films deposited on platinized silicon substrate. Polariza-

tion measurements were carried out using a Precision

Workstation operating in the virtual ground mode as ex-

plained earlier. A simple triangular pulse of voltage is

applied across the electrodes of the sample which was

fabricated in Metal-Insulator-Metal (MIM) configuration

and the polarization response of the sample is observed

under an integrator circuit. Figure 5(a) shows a typical

P-E loop obtained for a BVBT bilayer thin film at room

temperature. At the applied voltage of 12 V, the meas-

ured values of remnant polarization (Pr) and coercive

field (Ec) for ~ 350 nm thick BVBT bilayer film were

around 11 C/cm2 and 115 kV/cm, respectively. The non

zero switchable polarization observed at zero applied

field is a general characteristic of a FE material [18-19].

It showed two key characteristics of a ferroelectric

which are, polarization is reversible by an application of

an electric field and polarization remains at a finite value

even after the removal of the electric field [20]. The

value of remnant polarization observed for these bilayer

films is higher than that of the homogeneous BVO thin

films of similar thickness [16]. Further the asymmetric

nature of the P-E loops with respect to electric field axis

indicates interface dominated behavior.

The capacitance voltage (C-V) characteristics of a B-

VBT bilayer thin film measured at 100 kHz as probing

frequency is shown Figure 5(b). The C-V measurements

were carried out by applying a small ac signal of 0.5 V

amplitude, with a varying dc electric field. The dc volt-

age was swept from negative bias (–8 V) to positive (8 V)

in steps of 0.1 V with a sweep rate of 0.1 V/s and back

again. The butterfly shape of the curve confirmed the

ferroelectric nature of the BVBT BL film and the ca-

pacitance shows strong voltage dependence. The two ma-

xima of the loop correspond to the domain switching

voltage in forward and reverse directions where the po-

larization reversal takes place. The asymmetry that is

observed in C-V curve suggests that the electrodes are

asymmetric and the film contains mobile ions or charges

accumulated at the interface between the film and the

electrode. In addition there is a difference between the

capacitance values of the two peaks, which may be due

to some defect energy levels in the film.

The dielectric dispersion studies were carried out on

BVBT bilayer thin film at room temperature in the fre-

quency range of 1 kHz to 1 MHz. Figure 6 shows the

variation of dielectric constant (r

ε) and the dissipation

factor (D) as a function of frequency measured at room

temperature. The dielectric constant as well as dissipa-

tion factor was found to decrease abruptly in the low

frequency region. The variation in dielectric constant is

not significant at higher frequencies. Similar trend has

been observed at lower frequencies for BVO films

grown on platinized silicon substrate [16]. The small

Figure 5. (a) The P-E hysteresis loop and (b) Capacitance- Voltage characteristics of a BVBT BL thin film

measured at room temperature.

N. Kumari et al. / Natural Science 2 (2010) 1073-1078

107

1077

102103104105106

240

260

280

300

320

340

(a)

Dielectric constant (r')

Frequency (Hz)

25oC

(a)

25℃

Frequency (Hz)

Dielectric constant ()

340

320

300

280

260

240

102103104105106

0.0

0.1

0.2

0.3

(b)

Fre

uenc

(

)

(D)

25oC

Frequency (Hz)

Dissipation factor (D)

0.3

0.2

0.1

0.0

(b) 25℃

Figure 6. (a) Variation of dielectric constant and (b) dissipation factor of a BVBT BL thin film as a function

of frequency.

dispersion observed at higher frequency has its contribu-

tion from the response of the grains, while at lower fre-

quencies grain boundaries, free charges etc. would con-

tribute significantly. However the dielectric loss for B-

VBT bilayer thin film shows an increasing trend subse-

quent to 100 kHz. The dissipation factor has the mini-

mum value (~ 0.03) around 100 kHz, where the dielec-

tric constant is ~ 252.

The comparison of the observed dielectric constant of

BVBT bilayer with that of the single layer BVO and

BTO films is shown in Figure 7. The dielectric constant

of BVBT bilayer film shows less dispersion as compared

to that of homogeneous BVO film. This might be due to

the reduced number of defect states in bilayer films

due to the presence of BTO layer. Further the observed

102103104105

100

200

300

400

500

Dielectric constant(r')

Frequency (Hz)

BVO

BTO

BVBT BL

BVO

BTO

BVBT BL

Frequency (Hz)

500

400

300

200

100

Dielectric constant ()

Figure 7. Comparison of dielectric constant of BTO, BTO and

BVBT bilayer films.

dielectric constant of bilayer film is higher(e.g. ~ 225 at

1 M Hz) than that of single layer BVO film.

4. CONCLUSIONS

Bilayer thin film structures consisting of ferroelectric

Bismuth vanadate (Bi2VO5.5) and Bismuth titanate

(Bi4Ti3O12) individual layers were fabricated on plati-

nized silicon substrate (Pt(111) / Ti / SiO2 / Si) using

pulsed laser ablation technique and investigated system-

atically their structural and electrical properties. The

X-ray diffraction (XRD) studies indicated that bilayer is

randomly oriented and the diffraction pattern consists of

diffraction peaks from both starting materials. The Ato-

mic force microscopy of the films indicates that there is

a homogeneous distribution of grains in these films. The

cross-sectional scanning electron microscopy established

the dense nature of these films. The polarization hystere-

sis and C-V studies established the ferroelectric nature of

these films. The observed dielectric constant of these

bilayer films was higher than that of single layer BVO

films.

REFERENCES

[1] Rijnders, G. and Blank, D.H.A. (2005) Materials science:

Build your own superlattice. Nature, 433, 369-370.

[2] Shimuta, T., Nakagawara, O., Makino, T., Arai, S., Ta-

bata, H. and Kawai, T. (2002) Enhancement of remanent

polarization in epitaxial BaTiO3 / SrTiO3 superlattices

with “asymmetric” structure. Journal of Applied Physics,

91, 2290-2294.

[3] Pontes, F.M., Longo, E., Leite, E.R. and Varela, J.A.

(2004) Improvement of the dielectric and ferroelectric

properties in superlattice structure of Pb(Zr,Ti)O3 thin

films grown by a chemical solution route. Applied Physic

N. Kumari et al. / Natural Science 2 (2010) 1073-1078

1078

Letters, 84, 5470-5472.

[4] Lee, H.N., Christen, H.M., Chisholm, M.F., Rouleau, C.

M. and Lowndes, D.H., (2005) Strong polarization en-

hancement in asymmetric three-component ferroelectric

superlattices. Nature, 433, 395-399.

[5] Neaton, J.B. and Rabe, K.M. (2003) Theory of polariza-

tion enhancement in epitaxial BaTiO3 / SrTiO3 superlat-

tices. Applied Physic Letters, 82, 1586-1588.

[6] Aurivillius, B. (1949) Mixed bismuth oxides with layer

lattices. II. Structure of Bi4Ti3O12. Arkiv for Kemi, 1(54),

463-480.

[7] de Araujo, C.A.P., Cuchiaro, J.D., McMillan, L.D., Scott,

M.C. and Scott, J.F. (1995) Fatigue-free ferroelectric ca-

pacitors with platinum electrodes. Nature, 374, 627-629.

[8] Park, B.H., Kang, B.S., Bu, S.D., Noh, T.W., Lee, J. and

Jo, W. (1999) Lanthanum-substituted bismuth titanate for

use in non-volatile memories. Nature, 401, 682-684.

[9] Maiwa, H., Lizawa, N., Togawa, D., Hayashi, T., Saka-

moto, W., Yamada, M. and Hirano, S. (2003) Electrome-

chanical properties of Nd-doped Bi4Ti3O12 films: A

candidate for lead-free thin-film piezoelectrics. Applied

Physic Letters, 82, 1760-1762.

[10] Watanabe, T., Funakubo, H., Osada, M., Noguchiand, Y.

and Miyayama, M. (2002) Preparation and characteriza-

tion of a- and b-axis-oriented epitaxially grown Bi4Ti

3O12-based thin films with long-range lattice matching.

Applied Physic Letters, 81, 1660-1662.

[11] Rae, A.D., Thompson, J.G., Withers, R.L. and Willis, A.

C. (1990) Structure refinement of commensurately mo-

dulated bismuth titanate, Bi4Ti3O12. Acta Crystallogr, 46,

474-487.

[12] Borisov, V.N., Poplavko, Y.M, Avakyan, P.B. and Osip-

yan, V.G. (1988) Phase transition in bismuth vanadate,

Soviet Physics - Solid State, 30, 904-905.

[13] Osipian, V.G., Savchenk, L.M., Elbakyan, V.L. and Avak-

yan, P.B. (1987) Layered boismuth vanadate ferroelctrics,

Inorganic Materials, 23, 467-469.

[14] Prasad, K.V.R. and Varma, K.B.R. (1994) Dielectric,

thermal and pyroelectric properties of ferroelectric bis-

muth vanadate single crystals. Materials Chemistry and

Physics, 38, 406-410.

[15] Prasad, K.V.R. (1994) Investigations into the structural,

dielectric and ferroelectric properties of ceramics and

single crystal of parent and substituted bismuth vanadate.

Ph. D. Thesis, Indian Institute of Science, Bangalore.

[16] Kumari, N., Krupanidhi, S.B. and Varma, K.B.R. (2007)

Dielectric, impedance and ferroelectric characteristics of

c-oriented Bismuth vanadate films grown by pulsed laser

deposition. Materials Science and Engineering B, 138,

22-30.

[17] Kumari, N. (2009) Structural, optical and electrical stud-

ies on Aurivillius oxide thin films. Ph. D. Thesis, Indian

Institute of Science, Bangalore.

[18] Lines, M.E. and Glass, A.M. (1979) Principles and ap-

plications of ferroelectrics and related materials. Clare-

don Press, Oxford.

[19] Dawber, M., Rabe, K.M. and Scott, J.F. (2005) Physics

of thin-film ferroelectric oxides. Reviews of Modern Phy-

sics, 77, 1083-1130.

[20] Richerson, D.W. (1992) Modern ceramic engineering:

Properties, processing and use in design. Marcel Dekker

Inc., New York.