Materials Science and Engineering B 138 (2007) 22–30
Dielectric, impedance and ferroelectric characteristics of c-oriented
bismuth 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
Abstract
Ferroelectric bismuth vandante, Bi
2
VO
5.5
(BVO) thin films with layered perovskite structure were deposited by pulsed excimer laser ablation
technique on (1 1 1) Pt/TiO
2
/SiO
2
/Si substrates. The polarization hysteresis (P versus E) studies on the BVO thin films at 300 K confirmed the remnant
polarization (P
r
) and coercive field (E
c
) to be 5.6 C/cm
2
and 113kV/cm, respectively. The same was corroborated via the capacitance–voltage
measurements. The dielectric response and conduction mechanism of BVO thin films under small ac fields were analyzed using impendence
spectroscopy. A strong low frequency dielectric dispersion (LFDD) was found to exist in these films, which was ascribed to the presence of the
ionized space charge carriers such as oxygen ion vacancies and interfacial polarization. The room temperature dielectric constant and the loss
(D) at 100kHz were 233 and 0.07, respectively. The thermal activation energy for the relaxation process of the ionized space charge carriers was
0.85eV. The frequency characteristics of BVO thin films under study showed universal dynamic response that was proposed by Jonscher for the
systems associated with quasi-free charges.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Bismuth vanadate; Ferroelectric thin film; Low frequency dielectric dispersion; Universal dielectric response
1. Introduction
The studies concerning the ferroelectric thin films have drawn
the attention of many researchers not only from the growth mech-
anism viewpoint but also from a variety of device applications
that include non-volatile memory [1], infrared detectors [2],
microelectromechanical systems [3,4]; electro-optical switches
[5], etc. Extensive work was done on lead zirconate titanate
(PZT) [6,7] for the development of non-volatile random
access memories (NVRAMs) because of its non-volatility,
large remnant polarization, and fast switching speed and radi-
ation hardness. However, one of the most serious problems
associated with Pt/PZT/Pt ferroelectric-based capacitor is the
degradation of the polarization hysterises characteristics. In
this context, bismuth-layered compounds were considered to
be superior from their better fatigue properties point of view
[8,9]. These layered perovskites belong to the Aurivillius fam-
ily [10], with the general formula (Bi
2
O
2
)
2+
(A
n-1
B
n
O
3n+1
)
2-
.
Among these materials, Bi
2
Sr
2
Ta
2
O
9
and SrBi
2
Nb
2
O
9
and
∗
Corresponding author. Tel.: +91 80 22932601; fax: +91 80 23607316.
E-mail address: kbrvarma@mrc.iisc.ernet.in (K.B.R. Varma).
other similar compounds were widely studied [11–13]. The
crystallization temperatures of these bismuth layer-structured
ferroelectrics are relatively high and also possess higher
dielectric constant at room temperature. In these Bi based
layer-structured ferroelectrics, the dielectric constant decreases
with decreasing the number of ‘n’ perovskite layers in the
unit cell.
From the structural point of view, bismuth vanadate can be
formulated as (Bi
2
O
2
)
2+
(VO
3.5
0.5
)
2-
, where represents
oxide ion vacancies. Bi
2
VO
5.5
(BVO) can hence be considered
to be analogous to -Bi
2
WO
6
, the n = 1 member of Aurivillius
family of oxides with intrinsic oxygen vacancies in the per-
ovskite layer [14–17]. It crystallizes in a non-centrosymetric,
polar orthorhombic class and is ferroelectric at room tempera-
ture. It exhibits three main polymorphs: a non-centrosymetric
-phase at room temperature, its transformation to a cen-
trosymetric -phase at 730 K and a centrosymmetric -phase
is stable beyond 835 K; and BVO finally melts at 1153 K.
Both the -phase, which has a superstructure characterized
by a tripling of the lattice parameter a and the -phase, that
has a superstructure with doubling of a, has an orthorhombic
symmetry [19]. The -phase is described by a tetragonal sym-
metry. Because the distortions of crystal cell are small, these
0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.mseb.2006.12.010