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

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Handbook of dielectric, piezoelectric and ferroelectric materials650

of the new needle-like domains at the Z+ surface along the electrode edges

due to the field singularities caused by the fringe effect (Fig. 21.15a). During

the second stage, these growing domains merge and propagate through the

wafer, forming the lamellar domains with the initial orientation of P

along

the electrode edges (Fig. 21.15b).

The backswitched poling also allows the undesirable effect of domain

broadening out of the electrode area to be overcome. This effect is a cause

of essential difference between the lithographically defined electrode pattern

and the periodical domain structure produced. For short domain periods the

broadening effect limits the production of the short-pitch periodical domain

patterns due to domain merging. For backswitched poling, the domains enlarged

during the poling stage under application of the field pulse shrink by the

backward wall motion.

The application of this improved poling method to LN demonstrates the

spatial frequency multiplication of the domain patterns as compared with the

spatial frequency of the electrodes and self-maintained formation of the

oriented domain arrays consisting of individual nanoscale domains (Shur et

al., 2000a, 2001). The mechanism of frequency multiplication is based on

domain formation along the electrode edges during backswitching. For

‘frequency tripling’ (Fig. 21.16c), the subsequent growth and merging of the

domains lead to the formation of a couple of strictly oriented sub-micrometer-

width domain stripes with depth about 20–50µm under the edges of wide

electrodes (Fig. 21.16d). For narrow electrodes only the ‘frequency doubling’

can be obtained (Fig. 21.16a) with the depth of the backswitched domain

stripes about 50–100µm (Fig. 21.16b).

Let us consider the stages of backswitched domain evolution for wide

(about 3µm-width) electrodes (Fig. 21.17). The patterns were observed in

different partially backswitched samples. The initial nucleation along electrode

(a)

(b)

(c)

2µm

21.15

The stages of domain kinetics along the electrode edges during

backswitching in CLN. Strip electrode oriented along

direction

covers the area between the black lines. SEM patterns of domains

revealed by etching.

+ view.

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Nano- and micro-domain engineering 651

edges (Fig. 21.17a) and subsequent merging led to the formation of a couple

of strictly oriented sub-micrometer-width domain stripes under each electrode

(Fig. 21.17b). This process can be considered as ‘frequency tripling’. The

width of the stripes can be controlled by duration of backswitching stage.

The depth of the backswitched domains is about 20–50µm.

For the longer backswitching stage a new couple of nano-domain

arrays appears out of electroded area along the electrode edges (Fig. 21.17c).

The ‘frequency pentaplication’ effect was observed in this way. It has been

shown that the distance between secondary and initial stripes (structure

period) is determined and can be controlled by the thickness of the artificial

insulating layer.

The backswitching method has been applied to 2.6µm periodical poling

of the LN 0.5mm-thick wafers. It is seen that the domain patterns on Z

–

prepared in fragments of 3-inch-diameter (75mm) wafer are practically ideal

(Fig. 21.18a).

The continuous-wave (CW) single-pass second harmonic generation (SHG)

of blue light in 4µm-period, 0.5mm-thick, 50mm-length backswitched-poled

(a) (b)

(e) (f)

10µm

21.16

Spatial frequency multiplication of the periodical domain

patterns by backswitch poling in CLN. (a), (c), (e) ‘frequency

doubling’; (b), (d), (f) ‘frequency tripling’. (a), (b)

+ view; (c), (d), (e),

(f)

cross-sections. Optical images of domains revealed by etching.

(e), (f) schematic drawing. Dark gray – switched domains, medium

gray – backswitched, light gray – non-switched.

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Handbook of dielectric, piezoelectric and ferroelectric materials652

LN devices was obtained. Using the CW Ti:sapphire pump laser 6.1%/W

efficient first-order generation of 61mW at λ = 460nm was achieved, indicating

an effective nonlinearity about one half of the nominal value. It has been

shown that the sample phase matches over the full 50mm length.

Moreover, laser-diode-pumped SHG was performed with InGaAs single-

stripe diode master oscillator. The oscillator was single frequency and tunable

from 874 to 936 nm, with maximum output of 20mW at 930nm. The pump

power after the amplifier and two pairs of optical isolators was above 2W.

With 1.5W of laser-diode power, 60mW of SHG at 465nm was produced,

given a normalized conversion efficiency of 2.8%/W. An improved efficiency

of 3.4%/W was obtained by expansion and spatial filtering of the diode beam

before it was focused into the periodically poled LN.

The optimized method for 2.6µm has been applied periodical poling of

the LT 0.3mm-thick wafers. It is seen that the domain patterns prepared in

fragments of 2-inch-diameter (50mm) wafer on Z+ are practically ideal (Fig.

21.18b).

21.7.2 Formation of oriented quasi-regular domain rays by

pulse UV irradiation

It has been shown recently that illumination of the polar face of CLN crystal

by pulsed ultraviolet laser leads to the formation of the surface domain

structure with the depth about a few micrometers (Valdivia et al., 2005). Our

detailed study of the domain kinetics under these highly non-equilibrium

conditions reveals that arising of self-assembled structures is also achieved

through the discrete switching.

5µm

(a)

(b) (c)

5µm

21.17

Stages of the domain evolution during the domain frequency

multiplication process in LN.

+ view. Strip electrode oriented along

direction covers the area between the black lines. Optical images

of domains revealed by etching.

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Nano- and micro-domain engineering 653

The studied 0.5 mm-thick single-domain wafers of congruent LN were

cut normally to the polar axis. Both polar surfaces (Z+ and Z–) were UV-

irradiated using eximer gas pulsed laser LGE-4 (wavelength 308nm, pulse

duration 25ns) produced by the laboratory of Quantum Electronics, Institute

of Electrophysics, Ural Branch of Russian Academy of Science. The diameter

of irradiated area was about 3mm. The applied energy density ranged from

0.5 to 1.5 J/cm

The optical microscopy and AFM in contact mode were used for visualization

of the domain patterns revealed by shallow etching. Additionally, the surface

domain structure was visualized with high spatial resolution without etching

by piezoelectric force microscopy (PFM). It has been shown that the obtained

domain patterns and their dependence on the energy density are qualitatively

different for Z+ and Z– polar surfaces.

In all the used range of energy density the irradiated area at the Z-surface

is covered by isolated nano-domains (‘dots’). In contrast, the types of domain

patterns appeared at Z+ surface depend essentially on the energy density.

Systematic analysis of domain images allows us to distinguish three types of

the surface domain structures at the Z+ surface: (1) ‘dots’ – isolated domain

patterns appearing in the narrow region along the boundary of the irradiated

area (Fig. 21.19a); (2) ‘lines’ – quasi-periodic domain patterns, consisting of

parallel rays, forming inside the irradiated area (Fig. 21.20); (3) ‘fractals’ –

self-similar domain patterns appearing inside the irradiated area (Fig. 21.21).

Dotted patterns represent the quasi-regular structure. The average distance

between the neighboring isolated domains is about 2.4µm while the shortest

distance between neighboring domains is 0.6µm. The obtained correlated

spatial distribution is caused by above-discussed effect of correlated nucleation

(Shur, 2005). The dotted pattern can be essentially changed by second pulse

irradiation using higher energy density. The anisotropic growth of isolated

domains leads to the formation of dash-like domains strictly oriented along

(a) (b)

10µm

21.18

Domain patterns with 2.6µm period (a)

- view in CLN and (b)

+ view in CLT revealed by etching and visualized by optical

microscopy.

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Handbook of dielectric, piezoelectric and ferroelectric materials654

Y crystallographic directions (Fig. 21.19b). The anisotropic growth is similar

to the above-discussed formation of the finger structure and can be attributed

also to the ineffective screening.

Line patterns consisting of long domain rays strictly oriented along Y+

directions are the typical domain patterns inside the irradiated area. The

average period of quasi-periodic structures is about 3.9±0.8µm, while the

ray thickness is about 200nm (Fig. 21.20a). The rays start to grow from the

isolated domains situated along the boundary of the irradiated area. During

propagation from the boundary to the center, the averaged period remains

almost constant, while the dispersion essentially decreases (Fig. 21.20b).

Fractal patterns appear as a result of ray interaction. The rays start to grow

along all three allowed directions from the isolated domains situated in the

center of the area and form the ‘three-ray stars’ (Fig. 21.22a). The rays never

cross each other and their interaction leads to ‘reflection’ (Fig. 21.22b). The

ray growing in one Y direction changes the growth direction by 120°, when

the ray tip approaches the already existing ray oriented in another Y direction.

(a) (b)

10µm10µm

21.19

Isolated domain patterns formed in CLN under pulsed UV laser

irradiation on

+ face: (a) dots, (b) dashes. Optical images of

domains revealed by etching.

(a)

10µm

σ, µm

1.05

1.00

0.95

0.90

20 30 40 50 60

∆

, µm

(b)

21.20

(a) Quasi-periodic lines pattern formed in CLN under pulsed UV

laser irradiation on

+ face. (b) Dependence of the period dispersion

on the distance from boundary of irradiated area. Optical images of

domains revealed by etching.

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Nano- and micro-domain engineering 655

The direction of further growth corresponds to the third Y direction (Fig.

21.22b). The multiple ray reflections result in the formation of ‘triangles’

and ‘zigzags’ (Fig. 21.21).

Denser fractal patterns appear for higher radiation energy due to the

branching effect (Fig. 21.21b) representing the growth of rays of next generation

from already existing ones. The branches fill the empty space inside the

large triangles of the previous generations. The fractal analysis of the domain

structures shows their self-similarity in wide size range with the fractal

dimension about 1.8 calculated by the box-counting method (Feder, 1988).

While according to optical microscopy observations the domain rays are

continuous or dash-like, AFM images reveal that in many cases these rays

represent the chains/arrays of nano-domains with average size about 200nm

(Fig. 21.23). The average period of nano-domains in a chain is about 400nm.

(a)

20µm

(b)

20µm

21.21

Typical fractal domain patterns formed in CLN under pulsed UV

laser irradiation on

+ face: (a) without branching, (b) with

branching. Optical images of domains revealed by etching.

(a)

2µm

(b)

1µm

21.22

Elementary fragments of domain patterns formed in CLN under

pulsed UV laser irradiation on

+ face: (a) ‘three-ray star’, (b)

interaction of growing domain rays – ‘reflection’. Optical images of

domains revealed by etching.

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Handbook of dielectric, piezoelectric and ferroelectric materials656

It is necessary to reveal the origin of the electric field that is the driving

force of the polarization reversal under the action of the pulse laser irradiation.

Two models can be proposed. First, the switching field is caused by the laser

ablation, which partially removes the screening charges compensating the

depolarization field in stable conditions. The uncompensated depolarization

field that arises becomes high enough for polarization reversal. The effect is

similar to the recently observed spontaneous backswitching induced by the

chemical etching (Shur et al., 2005c). Second, the switching field of pyroelectric

nature appears during cooling. The polarization reversal under the action of

the pyroelectric field is well known (Miyazawa, 1979; Nakamura and Tourlog,

1993; Zhu et al., 1994).

The next issue to be addressed is why the ‘discrete switching’ is observed.

According to previous experiments, such anomalous switching is caused by

highly non-equilibrium switching conditions due to ineffective screening of

the depolarization field (Shur, 2005). In contrast to switching under application

of external electric field the surface in these experiments remains free of

electrodes. It is clear that in such conditions the external screening is ineffective

and the residual depolarization field is abnormally high.

The formation of the nanoscale domain chains and other effects of self-

assembling is due to electrostatic domain–domain interaction, which is

abnormally strong due to ineffective screening. The reflection effect can be

1µm

21.23

Fragment of nano-domain array formed in CLN under pulsed

UV laser irradiation on

+ face. AFM images of domains revealed by

etching.

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Nano- and micro-domain engineering 657

understood as a result of suppression of the nucleation process in initial

direction due to the electrostatic domain–domain interaction. I believe that

the tailored self-similar nanoscale structures produced by pulse laser

illumination can be used for the production of the nano-domain structures

with sub-micron meter periods for various applications.

21.8 Polarization reversal in relaxors

The heterophase state exists in relaxor ferroelectrics over a wide temperature

interval. It has been shown by high-resolution electron microscopy that the

heterophase structure just below the transition from non-polar to relaxor

phase consists of polar nanoregions embedded in a non-polar medium (matrix)

(Cross, 1994; Bokov and Ye, 2006b). The polar microregions with a complicated

structure of nanoscale domains appear during subsequent cooling (Dai et al.,

1994; Egami et al., 1997; Lehnen et al., 2001; Terabe et al., 2002). The

random orientation of spontaneous polarization P

in individual polar

nanoregions leads to zero value of P

averaged over microregions. The

irreversible switching to the state with electrical field-induced macroscopic

polarization is observed in such canonical relaxors as PMN over a wide

temperature range below the temperature of the dielectric permittivity maximum

(Cross, 1994; Bokov and Ye, 2006b).

In PLZT ceramics the irreversible switching is observed below the so-

called freezing temperature T

. The unusual reversible switching behavior

was discovered and studied in PLZT ceramics in the temperature range just

above the freezing temperature (Shur et al., 1999b, 2004a, 2005a,b). In this

case, the application of a strong electric field induces averaged P

, but in

contrast with the normal ferroelectrics, this state is unstable without electric

field. The restoration of the initial state (complete spontaneous ‘backswitching

effect’) is observed after external field switch-off (Shur et al., 1999b, 2004a,

2005a,b). Special attention has been paid to this phenomenon because it

demonstrates the original kinetics of nanoscale domain structure and can be

discussed within general approach presented above.

According to the classical concepts of relaxor ferroelectrics (Isupov, 1983;

Cross, 1994; Kleemann and Lindner, 1997; Marssi et al., 1998), the temperature

increase leads to transition from the homogeneous ferroelectric state to the

heterogeneous (heterophase) one. The heterophase structure just above this

transition represents a ferroelectric multidomain matrix with isolated nanoscale

inclusions of the non-polar phase. The depolarization fields produced by

bound charges (the polarization jumps at the interfaces) prevent formation of

macro-domains and stimulate appearance of the nanoscale domain structure.

There exists the essential difference between screening of the depolarization

fields produced by bound charges situated at the surface (in normal

ferroelectrics) and at the interfaces in the bulk (in relaxors). The above-

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Handbook of dielectric, piezoelectric and ferroelectric materials658

discussed surface E

dep

(see Section 21.4) is compensated for substantially by

fast external screening (charge redistribution at the electrodes), while the

bulk E

dep

in relaxors can be compensated for by slow bulk screening only

(Shur et al., 1984a, 1997; Shur, 1998). This feature explains the peculiarities

of the domain kinetics in relaxors.

The nanoscale domains have been observed recently in several relaxors

using high-resolution microscopy (Dai et al., 1994; Lehnen et al., 2001;

Terabe et al., 2002; Abplanalp et al., 2002; Bdikin et al., 2003; Shvartsman

and Kholkin, 2004; Bai et al., 2004). It has been shown in PLZT by high-

resolution transmission electron microscopy that the relaxor state in some

temperature range is the heterophase structure consisting of polar nanoregions

with diameters down to 10nm embedded in a non-polar matrix (Dai et al.,

1994). The complicated nanoscale domain structure was observed in pure

and doped single crystals of strontium-barium niobate (Sr,Ba)Nb

by PFM

(Lehnen et al., 2001; Terabe et al., 2002). The ‘fingerprint’ sub-microscale

domain patterns have also been observed in single-crystalline Pb(Mg

1/3

2/3

–PbTiO

(PMN–PT) and Pb(Zn

1/3

2/3

–PbTiO

(PZN–PT) using

PFM (Abplanalp et al., 2002; Bdikin et al., 2003; Shvartsman and Kholkin,

2004; Bai et al., 2004). The domain patterns of similar geometry have been

revealed recently in PLZT ceramics by scanning force microscopy in acoustic

mode (Yin et al., 2004) and PFM (Shvartsman et al., 2005).

It is necessary to stress that all performed experiments allow us to visualize

the domain patterns only at the surface. Thus, the geometry of the nanoscale

structure in the bulk remains unknown. It has been shown that the information

about the geometry of the initial domain structure and its evolution during

switching can be extracted by analysis of the switching current shape proposed

in Shur et al. (1998). This analysis is valid for characterization of nanoscale

structures, because it provides unique scale-free information about

dimensionality of the domain growth.

In this section we present the results of experimental investigation of the

geometry of nanoscale domain structures using analysis of the switching

current shape and visualization of the surface domain patterns by PFM in

PLZT ceramics chosen as a model relaxor ferroelectric.

21.8.1 Hysteresis loops

The temperature evolution of the hysteresis loops in PLZT has been measured

during quasi-static bipolar switching. It has been shown that the shape of

loop changes qualitatively during heating/cooling in the vicinity of proper

temperature T

(Fig. 21.24). The classical ferroelectric loop observed at low

temperatures transforms during heating to the ‘double loop’.

The temperature evolution was studied by analyzing of the field dependence

of the derivative of the switched charge on field dQ/dE, which is proportional

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Nano- and micro-domain engineering 659

to the switching current for polarization reversal in linear increasing field.

Two peaks of dQ(E)/dE observed in the ferroelectric phase are caused by

classical polarization reversal typical for normal ferroelectrics. The maxima

of dQ(E)/dE are observed at the fields, which are close to the coercive fields

determined by conventional methods. For the double loop obtained in the

relaxor state (for T > T

) both peaks demonstrate splitting (Fig. 21.24d). The

positions of the four peak maxima E

(Fig. 21.25a) are very sensitive to

temperature variation.

The special feature of the switching in relaxor state can be discussed in

the framework of the above approach underlining the crucial role of bulk

screening. In a strong enough applied negative field the ferroelectric matrix

, nCN*cm

, µC/cm

–20

–40

PLZT 8/65/35

= 33°C

–6 –4 –2 0 2 4 6 8

E, kV/cm

, nCN*cm

, µC/cm

–20

–40

PLZT 8/65/35

= 44°C

–6 –4 –2 0 2 4 6 8

E, kV/cm

(a)

(b)

(c)

(d)

21.24

(a), (c) Hysteresis loops and (b), (d) field dependences of the

derivative of the switched charge on field d

measured in

PLZT 8/65/35 ceramics at different temperatures. (a), (b)

, (c),

(d)

, kV/cm

–2

–4

PLZT 8/65/35

30 40 50 60 70

Temperature, °C

(a)

dep

, kV/cm

PLZT 8/65/35

30 40 50 60 70

Temperature, °C

(b)

21.25

Temperature dependences of averaged coercive fields for PLZT

8/65/35 ceramics.

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