Zuo-Guang. Ye Advanced Dielectric Piezoelectric and Ferroelectric Materials: Synthesis, Characterisation and Applications

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Handbook of dielectric, piezoelectric and ferroelectric materials560

>>3 nm such effects are extrinsic, usually caused by oxygen vacancy gradients

or strain gradients near the electrode interfaces (Bratkovsky and Levanyuk,

2005). This conclusion is based on recent Belfast–Cambridge studies on

very thin (50–100 nm) single crystals, prepared via FIB. The use of FIBs to

cut nano-ferroelectrics was pioneered by Ramesh’s group in Maryland (Ganpule

et al., 1999) for ceramic films but recently extended by Gregg’s group in

Belfast to preparing single crystals of BaTiO

(Saad et al., 2004). The results

of the latter team have been to characterize single-crystal capacitors of thickness

down to ca. 65nm. This has shown unambiguously that effects of Curie

temperature shift or dielectric peak broadening are extrinsic (Bratkovsky

and Levanyuk, 2005), contrary to the previous conventional wisdom. The

availability of single crystals with thickness d < 50nm should permit clear

tests of finite-thickness theory. Cho et al. (2002) have shown that they permit

some elegant optical devices for the future, despite the high cost of such

single-crystal devices.

18.14 Summary

It has now been over 80 years since ferroelectricity was discovered (Valasek,

1921) and 62 years since oxide ferroelectrics without hydrogen bonds

(originally thought to be requisite) were found in several countries during

World War II (von Hippel, 1944). The progress in the past few years has

been quite rapid, emphasizing thin films and nano-devices. A brief review

has been given in this chapter of a dozen subtopics of current experimental

interest in modern ferroelectricity. Most relate to nano-structures, and many

suggest device development of great commercial potential. Present commercial

ferroelectric cells in 64Mb or 32Mb FRAMs (Figs 18.1 and 18.13) are as

small as 450 × 400 × 120nm

in ceramic form; single crystals as thin as

250 nm

Ir/IrO

Pb(Zr

0.4

0.6

Pt/IrO

/Ir

18.13

32 Mbit PZT FRAM (Samsung).

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Ferroelectric nanostructures for device applications 561

50–70nm and films as thin as 2nm are under study at universities, as are

high-aspect DRAM trenches (Fig. 18.14) and [3D] nanotubes for microfluidics

as well as 5nm diameter rings and 1 nm domain widths, with PZT/Pt-nanowire

arrays giving nearly 1Tb/in

nonvolatile memory arrays. Read–write speeds

are as fast as 280ps for laboratory devices and e-beam lithography (Fig.

18.15) still leads self-assembly techniques for the registration required for

commercial random access memories.

18.15 Future trends

The near-term predictions for ferroelectrics are that they will emphasize

submicrometre thin films. These are already at 140nm for commercial FRAM

memories and could go down to ca. 20nm without incurring very detrimental

leakage currents. Lateral sizes are ca. 400nm in 64Mb FRAMs, and could

decrease further to real nano-scale (<100nm) and still produce enough switched

charge for the sense amplifiers (>1000 e is a good rule of thumb). In the next

five years the competition between MRAMs and FRAMs to replace ‘flash’

EEPROMs should be a stimulus for researchers. In other non-memory areas,

the application to microfluidic transport seems appealing. Pfizer has a new

insulin inhaler on the market (‘Exubera’), and the need for this and similar

medical devices for monodisperse small droplets is paramount. There are

500 nm

18.14

Ruthenium nano-trenches in Si (Funakubo).

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Handbook of dielectric, piezoelectric and ferroelectric materials562

190 million people with diabetes in the world, and the US market for diabetes

is $25 million/year and growing at more than 10% per year as the population

ages and obesity becomes more common. Similar inroads may occur with

inkjet printers, where nano-tube piezoelectric print heads could give improved

grey scale control. Finally, the combination of carbon nanotubes with

piezoelectric/pyroelectric PZT tips seems intriguing (Jang et al., 2005;

Kawasaki et al., 2007), for both pyroelectric detectors and piezoelectric

arrays. Both nano-scale lithography (‘top-down’) and self-assembly (‘bottom-

up’) techniques will continue to share importance, with lithography continuing

to dominate industry products for another decade but self-assembly becoming

dominant thereafter. It is likely that the optoelectronic aspects of such devices

will be useful. Photonic arrays utilizing registered self-patterned ferroelectrics

are already in prototype form (Gregg and Scott, 2007). It should be noted

that GaAs and GaN are available in mesoporous form, so ferroelectric nanotubes

and nanorods need not be limited to Si or Al

. Although non-oxide

ferroelectrics, especially fluorides, will receive increased attention, commercial

devices are likely to emphasize oxide films for the foreseeable future, with

some increase in the use of polymeric ferroelectrics, such as polyvinylidene

fluoride (PVDF). The replacement of SQUIDs by much cheaper

magnetoelectric (ferroelectric) sensors for weak magnetic fields may occur

within the decade, on the basis of room temperature operation. This could

permit portable devices for geologists and oil exploration, along with solid-

state research devices.

18.15

e-beam lithography: PZT capacitor array (Alexe) lithography:

PZT capacitor array (Alexe

et al

., 2001).

100 nm

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Ferroelectric nanostructures for device applications 563

18.16 Further reading

The attention of readers is called to the Proceedings of the Rank Prize

Conference on Electrooptics, Grasmere, UK (September 11–14, 2006), which

will be published as a special issue in Ferroelectrics (2007), edited by J. M.

Gregg and J. F. Scott. This issue deals primarily with applications of ferroelectric

nanostructures to electrooptics. Other recent topical reviews on the subjects

discussed here are to be found by Miranda et al., (2002) – agile microwave

devices, Scott (2006a) – nano-ferroelectrics, Fiebig (2005), Catalan (2006),

Catalan and Scott (2007), Hill (2000) Hemberger et al., (2005), Schmid

(2001) – magnetoelectrics, Auciello et al. (1998), Auciello (2006) – electroding

and interface physics, Dawber et al. (2005a) theory and experiment of oxide

films and Eerenstein et al. (2006) – multiferroics. The Springer series ‘Topics

in Applied Physics’ provides very good texts on ferroelectric memories

(FRAMs), especially that of Ishiwara et al. (2004) and the earlier text by the

present author (Scott, 2000) [available also in Japanese (Springer Japan,

translated by M. Tanaka et al., Tokyo 2003) and in Chinese (Tsinghua Press,

translated by J. Zhu, Beijing, 2004)]. The rapid increase in ferroelectric

memory R&D in China is noticeable, and the author hopes this very inexpensive

Chinese text will help. Another book in this series, emphasizing nano-

ferroelectric memory devices and the Samsung FRAM design strategy, is

expected in 2007 by the author’s group (Jung et al., 2007).

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Ferroelectric nanostructures for device applications 565

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Handbook of dielectric, piezoelectric and ferroelectric materials566

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Ferroelectric nanostructures for device applications 567

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