Fatigue in epitaxial lead zirconate titanate films
V. Ya. Shur, N. Yu. Ponomarev, N. A. Tonkacheva, S. D. Makarov, E. V. Nikolaeva,
and E. I. Shishkin
Scientific-Research Institute of Physics and Applied Mathematics, Ural State University,
620083 Ekaterinburg, Russia
L. A. Suslov, N. N. Salashchenko, and E. B. Klyuenkov
Institute of the Physics of Microstructures, Russian Academy of Sciences, 603000 Nizhni Novgorod, Russia
Submitted November 15, 1996
Fiz. Tverd. Tela St. Petersburg 39, 694–696 April 1997
S1063-78349702904-3
The potential application of ferroelectric thin films in
energy-independent memory devices has stimulated the in-
vestigation of the mechanisms which underlie the phenom-
enon of fatigue, i.e., a decrease in the charge switched as a
result of prolonged cycling by sign-alternating pulses.
1
Epitaxial PbZr
0.52
Ti
0.48
O
3
/YBa
2
Cu
3
O
7 -x
heterostruc-
tures were obtained by laser deposition on oriented 001
SrTiO
3
single-crystal films. The upper electrode with an area
of about 4 10
-4
mm
2
was fabricated by magnetron sputter-
ing of nickel. The parameters of the structures and the tech-
nologies used to obtain them were described in Refs. 2 and 3.
It is known that the complex evolution of the domain
structure during ‘‘ultrafast’’ submicrosecond polarization
reversal in ferroelectric thin films cannot be investigated by
direct methods cannot be visualized. Indirect methods in-
volving measurements of integral characteristics, such as the
switching current, must be used. A sequence of paired bipo-
lar pulses was applied to a ferroelectric thin-film capacitor
the pulse duration was 20 s, the repetition rate was 1 kHz,
and the rise time did not exceed 10 ns, and the voltage drop
on a 1.5 resistance connected in series was recorded with a
time resolution to 5 ns. To obtain the actual switching cur-
rent, the capacitive component of the current obtained in the
second pulse was numerically subtracted from the results of
the measurements made during the first pulse.
4,5
We previously showed
6,7
that a mathematical treatment
of the switching current which takes into account the topo-
logical transformations in a confined volume permits deter-
mination of the parameters that describe the evolution of the
domain structure, as well as the geometry and dimensions of
the domains in the original domain structure. By definition,
the switching current has the form
j t =2 P
s
dq t / dt , 1
where P
s
is the spontaneous polarization, and q ( t ) is the
fraction of the volume occupied by domains that have not
switched.
It was also shown
6–8
that under the ‘‘anisotropic con-
finement’’ caused by the anisotropy of the shape of the vol-
ume being switched, the switching process breaks up into
stages with different dimensions. In PZT thin films the con-
finement anisotropy is associated with the features of the
original domain structure, which consists of narrow alternat-
ing regions occupied by a and c domains see the diagram in
Fig. 1.
9–11
It is known that the density and dimensions of the
a domains in thin films depend on the mismatch of the lattice
parameters and the difference between the coefficients of
thermal expansion of the substrates and the films.
12
Under
fast switching the 90° domain walls are practically station-
ary, and 180° switching takes place independently in narrow
‘‘anisotropic’’ strips.
To analyze the currents under incomplete switching we
take into account the residual domains. Let us consider the
process
6,13,14
taking place during the two-dimensional
growth of domains in a volume of rectangular shape with an
area S =AL
2
, where A is the anisotropy of the volume the
ratio between the lengths of the sides of the rectangular
strip. Then
q t =
exp - t / t
01
2
1 -t / t
m
, t t
cat
,
exp - t / t
02
, t
cat
t ,
2
where t
cat
=L / is the catastrophe geometric transformation
time, L is the width of the volume being switched the mean
FIG. 1. Fatigue in an epitaxial PZT/YBCO heterostructure. Inset – diagram
of the alteration of the domain structure as a result of fatigue.
609 609 Phys. Solid State 39 (4), April 1997 1063-7834/97/040609-02$10.00 © 1997 American Institute of Physics