Deposition of PZT thin films with - Advanced Materials Letters

Research Article Adv. Mat. Lett. 2012, 3(2), 102-106 ADVANCED MATERIALS Letters
film N where as for {111}-orientation, there are three 71 0
domain j and one 180 0 domain ( ) w.r.t. film normal N.
Table 2. Electrical characteristics of PZT thin films.
Orientation e r tan
{110}
{001}
{111}
Random
1081
961
915
798
0.05
0.07
0.06
0.05
Dielectric
tunability at
E= 200kV/cm
62%
48%
39%
50%
2P r
(c/cm 2 )
40
40
22
33
2E c
(kV/cm)
100
150
125
160
|e 31 * | (C/m 2 ) at
100kV/cm
Fig. 5. Fatigue characteristics of oriented PZT thin films. Inset shows the
P-E loops (before fatigue) for various orientations.
The switching of the non-180 0 domains, dictated by the
crystal symmetry, lead to nonlinearity in e31 * . In {110}-PZT
films, the vector facilitates polarization rotation [7].
This accounts for the observed high magnitude and
nonlinearity in piezo response. Polarisation rotation in such
{110}-oriented films also accounts for the high values of er
(1081), dielectric tunability (62% at 200kV/cm) and
remnant polarization, Pr (2Pr = 40C/cm 2 ). In {001} - PZT,
the domains a switch by 90 0 whereas in {111}-PZT the
domains j switch by only 71 0 leading to lower values of
e31 * when compared to that in {001}-PZT. This is also
confirmed by the differences in their dielectric
characteristics er, tan and tunability) (Table 2). In
randomly orientated films, the e31 * lies between that of
{111} and {001}-oriented film as more domains are
involved in the switching process.
As preferentially oriented films have different domain
configurations, it is of interest to study their effect on
polarization fatigue characteristics. For this purpose, thin
film capacitors were subjected to a fatigue pulse having an
amplitude of 10V at a frequency of 1MHz. Fig. 5 shows the
variation of Pr as a function of switching cycles. It is
observed that in {111}-PZT, after 10 6 cycles, there is
degradation of switchable polarization whereas in {110}-
PZT, onset of polarization fatigue is seen only after 10 7
cycles. In thin films presence of internal electric field also
7.8
6.8
5.0
6.1
strongly influences the fatigue characteristics. Higher
fatigue endurance in {110} –PZT suggest that internal
electric field developed along is lower when
compared to that developed along . The PZT film
with {001}-orientation exhibit excellent fatigue endurance
characteristics showing negligible fatigue even after 10 9
cycles. This is because the polar axis is aligned along [001]
due to the high degree,
I I I 92%
of {001}-
I ( 001)
( 00l)
( 110)
( 111)
orientation. It can also be inferred that no net internal field
develop along .
Conclusion
Dense, crack- free PZT thin films of MPB composition
with {001}, {110}, and {111}-orientations were deposited
on to Pt/Ti/SiO2/Si substrate employing a hybrid MOD and
sol- gel method using spin coating technique. All the films
exhibited columnar microstructure. The extent of extrinsic
contribution to the transverse piezoelectric coefficient e31 *
have been explained on the basis of the differences in the
domain wall contributions dictated by different
crystallographic orientations of the thin films. Because of
the high average value of e31 * (-6.6 C/m 2 ) and improved
fatigue endurance characteristics, {001}-oriented PZT films
have potential advantages in MEMS applications.
Acknowledgements
The work was supported by DST, India under Grant No.
SR/ME/041/2008. One of the authors, D.A would like to acknowledge
CSIR, India for the senior research fellowship. Also the authors would
like to acknowledge SAIF, IITM, Chennai for providing the HRSEM
facility.
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Adv. Mat. Lett. 2012, 3(2), 102-106 Copyright © 2012 VBRI Press