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:
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
2
/ SiO
2
/ Si
BTO
BVO
Pt
Au
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)
2

(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
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1077
102103104105106
240
260
280
300
320
340
(a)
Dielectric constant (r')
Frequency (Hz)
25oC (a)
25℃
10
2
10
3
10
4
10
5
10
6
Frequency (Hz)
Dielectric constant ()
340
320
300
280
260
240
102103104105106
0.0
0.1
0.2
0.3
(b)
Fre
q
uenc
y
(
Hz
)
Di
ss
i
pa
ti
on
f
ac
t
or
(D)
25oC
10
2
10
3
10
4
10
5
10
6
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
0
100
200
300
400
500
Dielectric constant(r')
Frequency (Hz)
BVO
BTO
BVBT BL
BVO
BTO
BVBT BL
10
2
10
3
10
4
10
5
Frequency (Hz)
500
400
300
200
100
0
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.
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