Electric response as a function of applied voltage of Nb-doped
Bi
4
Ti
3
O
12
thin films
Jong-Ho Park
a
, Jong-Seong Bae
b
, Hui-Jin Park, Yu-Sung Kim, Byeong-Eog Jun
c
,
Byung-Chun Choi
c,
⁎
, Jung-Hyun Jeong
c
a
Department of Science Education, Chinju National University of Education, Jinju 660-756, South Korea
b
Nano Surface Technology Research Laboratory, Korea Basic Science Institute, Busan 609-735, South Korea
c
Department of Physics, Pukyong National University, Busan 608-737, South Korea
Available online 13 July 2007
Abstract
We have investigated the electrical properties of sol–gel deposited Nb-doped Bi
4
Ti
3
O
12
(NBIT) ferroelectric thin films. The obtained values of
remanent polarization (2P
r
) and coercive voltage (V
c
) were 7 μC/cm
2
and 2.5 Vof NBIT thin film, respectively. From complex dielectric spectra, we
observed the dielectric response consisting of two regions for measuring frequency; the low frequency region may be due to diffusion charge transport
caused by impurities, while the dielectric relaxation mechanism of high frequency region seems to be the modified Debye type. A model was proposed to
account for the observed phenomena, which fits very well to the dielectric dispersion relation: e
⁎
ðxÞ¼ e
l
þ
es el
1þðixsÞ
n þ i
r
e0x
. The occurrence of an
anomaly in n, σ, τ, and ε
S
- ε
∞
parameters near V
c
indicates a coupling between the charge carriers and ferroelectricity.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Thin films; Ferroelectrics; Nb–Bi
4
Ti
3
O
12
; Dielectric; Impedance spectroscopy
1. Introduction
Many researchers have studied the structures and ferroelec-
tric phase transition of Bismuth Layered Structure Ferroelectrics
(BLSFs) ceramics and thin films because of their good
applications and physical properties [1–17]. BLSFs have a
general formula of (Bi
2
O
2
)
2+
(A
m-1
B
m
O
3m+1
)
2-
, where A = Bi,
Pb, Ba, Sr, La, Ca, Na, K; B=Ti, Nb, Ta, W, Mo, Fe, Co, Cr, m
is the number of BO
6
octahedra in the pseudo-perovskite block
(m = 2,3,4,5). Among them, for Bi
4
Ti
3
O
12
(BIT), Bi and Ti ions
occupy A and B site, respectively, and n = 3 is given. However,
the BIT has a high leakage current and domain pinning due to
defects. Therefore, the electrical conduction becomes one of the
major obstacles for the practical nonvolatile ferroelectric
random access memory (NFRAM) applications. Recently, the
structural characteristics were studied and the ferroelectric
properties were improved by cation doping on A- and B-sites.
Park et al. reported on La-doped Bi
4
Ti
3
O
12
(BLT) thin films
fabricated by a pulsed laser deposition method [5]. They proposed
that the BLT thin film exhibits a fatigue-free characteristic and
large remanent polarization. However, even if the BLT thin film is
free from fatigue and imprint to simple Pt electrodes, they can also
see an interfacial (electrode/bulk) problem similar to Pb(Zr
x
Ti
1-x
)
O
3
(PZT) [18] and (Ba, Sr)TiO
3
(BST) [19–21] thin films. In order
to clarify the interfacial problem, the frequency dependence of
complex dielectric constant for some compounds in PZT and
(Bi
2
O
2
)
2+
(A
m-1
B
m
O
3m+1
)
2-
systems has been studied. A
remarkable low frequency dielectric dispersion below T
c
was
observed in thin film capacitors of PZT [18], BST [19–21],
Sr
x
Bi
y
Ta
2
O
9
(SBT) [8], Bi
2
Ti
2
O
7
[9], and Bi
4
Ti
3
O
12
[10]. It has
been shown that this phenomenon greatly affects the electrical
properties of the capacitors. The frequency dependence of the
measured dielectric constant and dielectric loss reflects an
intrinsic property of the bulk material, the effects from the
electrodes, and any internal interfacial barriers. It is generally
accepted that the presence of oxygen vacancies in these
compounds plays an important role in the dielectric properties
of oxide ceramics and thin films. The accumulation of oxygen
vacancies at the grain surface of these oxide compounds or at the
domain walls within the grains results in a huge dielectric constant
increase due to the appearance of spatial charge polarization [11].
Available online at www.sciencedirect.com
Thin Solid Films 516 (2008) 5304 – 5308
www.elsevier.com/locate/tsf
⁎
Corresponding author.
E-mail address: bcchoi@pknu.ac.kr (B.-C. Choi).
0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2007.07.041