Electric Clock for NanoMagnet Logic Circuits - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 94

chemical vapor deposition, hydrothermal growth and sol-gel techniques. It has
been demonstrated that ZnO can be successfully used for the realization of pho-
todetectors, light-emitting diodes, and chemical sensors while the presence of
piezoelectricity [ 67 ] makes ZnO suitable for the realization of surface acoustic
wave resonators, nanogenerators and touch-sensitive piezoelectric sensors. How-
ever, as semiconductor ZnO has some advantages and a high electrical resistance,
high breakdown voltages and consequently suffers from high electrical losses.
Some of these problems, together with its poor piezoelectric properties, can be
solved by properly doping ZnO. Aluminum nitride (AlN) can be used in place
of ZnO, thanks to its high electrical resistance, high breakdown voltage and
low dielectric loss [ 68 - 70 ]. Despite its smaller piezoelectric constant, [ 67 ]alower
dielectric permittivity respect to the one of PZT makes AlN competitive in the
realization of MEMS devices. Similar to PZT, barium titanate belongs to the
class of perovskite-based materials. The presence of a non centro-symmetrical
tetragonal crystal structure at room temperature allows it to feature piezoelec-
tricity. BT is a high dielectric material with a piezoelectric coecient higher than
those of ZnO and AlN [ 71 ]. As in the previous cases, also BT can be synthesized
in different ways, both from physical and chemical vapor deposition methods,
and by hydrothermal synthesis. The main problem of BT is the low TC, which
is around 120 ⓦ C but can be increased by doping the material. A lot of work
has to be done in order to improve ZnO, AlN and BT electromechanical prop-
erties to become comparable with the PZT ones. Despite all the recent devel-
opments in the synthesis of lead-free, non toxic functional materials, PZT still
remains the best choice for the realization of sensors and actuators for the micro
and nanotechnologies.

Fabrication of Bulk and Thin Film Electro-Ceramics. Ferroelectricity
and piezoelectricity in ceramic materials were discovered in the early 1940s, after
the observation of high dielectric constants in ceramic barium titanate capac-
itors [ 72 ]. From then, intensive studies have been done, especially about two
compositional systems: barium titanate (BaTiO3 or BTO) and lead zirconate
titanate (Pb[ZrxTi1-x]O3 where 0 1 or PZT) leading to many applications in

medical ultrasonic, high deformation actuators, integrated circuits [ 73 - 76 ]. Some
applications utilize chemical coprecipitation [ 77 ] or hydrothermal [ 78 ] techniques;
however ferroelectric ceramics are traditionally obtained from powders formu-
lated starting from individual oxides.
In the standard method for the fabrication of piezoelectric bulk ceramics, the
key steps are calcination and sintering: during these steps there is a consistent
redistribution of atoms in order to minimize the free energy of the system: new
phases are created, the number of surfaces decreases and the grain size increases
[ 79 ]. The fabrication process starts from the preparation of a specific composition
in powder from the chosen precursors; then, the desired shape is created, before
densification and finishing. The strong research and development activity in fuel
cells, sensors, ferroelectric memories and MEMS technologies has stimulated
efforts to fabricate electroceramic films of sub-micron thickness range.

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