Information Technology Reference
In-Depth Information
response of the material and its ferroelectricity are enhanced [
57
,
58
]. Differ-
ent works show that the piezoelectric constant d33, as well as permittivity and
ferroelectricity, can reach their maximum values near the MPB condition [
59
]. In
particular, it is reported that the (100)-oriented rhombohedral phase is expected
to have excellent piezoelectric properties [
58
]. The transition between a centro-
symmetrical paraelectric cubic phase and a non-centro-symmetrical ferroelectric
tetragonal one is also related to temperature, occurring at a particular value, the
so called Curie temperature (TC). For PZT TC is around 300
ⓦ
C[
60
]. Piezoce-
ramic bulk materials for the realization of MEMS/NEMS based technology must
be used in the form of thin films. Generally, sputtering and sol-gel are the most
diffused techniques for the deposition of PZT thin films. Independently of the
deposition technique, the selection of the substrate material is quite deleterious
for both electrical and piezoelectric properties of PZT. Generally polycrystalline
PZT films can be simply grown using non-epitaxial Si substrates, in order to
get the monolithic integration of PZT thin films in the Si-based technology. It
is also reported that the epitaxial growth on substrates like MgO or SrTiO3 has
to be preferred to enhance the piezoelectric response of the material [
59
].
However, large residual stress, either compressive or tensile, in the order
of some MPa can arise, affecting the physical properties of the material itself
[
59
,
61
,
62
]. Suciently thick and crack-free PZT films have been fabricated
and their applications in MEMS devices have been fully demonstrated [
63
,
64
].
Ferroelectric PZT thin films are also widely used in the study and fabrication
of nonvolatile ferroelectric random access memories (FeRAM) while the strong
piezoelectric response of PZT, in accordance with the presence of high electro-
mechanical coupling coecients, is exploited in the realization of sensors and
actuators, especially for acoustic applications, where large piezoelectric coef-
ficients and electromechanical coupling factors are preferred. Especially in the
field of ultrasonic imaging, where transducers operating at frequencies comprised
between 20 and 50 MHz are required, PZT films have been recently used and
transducers operating at 20 MHz have been demonstrated [
65
,
66
]. Furthermore,
good electrical properties, i.e., a high dielectric constant and a strong depen-
dence of the dielectric constant on the DC bias field, in accordance with ferro-
electricity, makes PZT a good candidate for the realization of microwave devices
applications, like voltage-controlled oscillators, varactors, delay lines and phase
shifters [
57
].
Even though PZT is the most performing piezoceramic, thanks to its high
piezoelectric constant, recently lots of efforts have been devoted to the study and
development of innovative, non toxic, lead-free piezoelectric materials. Among
them zinc oxide (ZnO), barium titanate (BT) and aluminum nitride (AlN) are
by far the most studied. Zinc oxide is a lead-free, pyroelectric, piezoelectric,
wide band-gap semiconductor (3.37 eV) with a large exciton binding energy
(60 meV) at room temperature and a wurtzite hexagonal structure. It can be
synthesized both in the form of dense thin films and different kinds of nanos-
tructures (nanowires, nanobelts, nanorods, nanocombs) by physical vapor depo-
sition techniques like sputtering, pulsed laser deposition, atomic layer deposition,
Search WWH ::
Custom Search