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

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Handbook of dielectric, piezoelectric and ferroelectric materials710

chose to grow epitaxial PbTiO

and PbZr

0.2

0.8

films onto well-defined

vicinal SrTiO

(001) (STO) substrates of uniform surface termination by

PLD. First the necessary bottom electrode consisting of SrRuO

(SRO) was

deposited on STO (001) by PLD. SRO is an excellent template for the

heteroepitaxial growth of high-quality ferroelectric perovskites. For thickness

below ~75nm (Oh and Park, 2004) (the value also depends upon the growth

technique and conditions), SRO can grow in a single crystalline manner,

with an atomically flat surface and coherently strained on vicinal STO (001)

substrates (Hong et al., 2005). Bulk tetragonal PbTiO

(PTO) has the in-

plane lattice parameter a = 3.904Å (Lichtensteiger et al., 2005), which

perfectly matches the lattice parameter of the cubic STO substrate (3.905Å).

Concerning PZT, Vrejoiu et al. focused on the PZT 20/80 composition,

because it has a room temperature bulk in-plane lattice parameter of 3.935 Å

(Li et al., 2002a), which is closely matched to that of pseudocubic SRO

(3.93Å), and has a rather small misfit to the slightly smaller lattice parameter

of the STO substrate. Nevertheless, under such circumstances, it is expected

that compressive biaxial strain would build up in the heteroepitaxially grown

SRO and PZT 20/80 layers, which may enhance the tetragonality of the PZT

20/80 thin films.

The critical thickness for misfit dislocation formation for PbTiO

epitaxial

films grown by PLD directly on SrTiO

(001) crystals was calculated (Stemmer

et al., 1995), following the equilibrium theory of MD formation proposed by

Matthews and Blakeslee (Matthews, 1975). For dislocations with a Burgers’

vector b = [010], the PTO critical thickness at room temperature (where the

in-plane lattice mismatch was +0.16%) was estimated to be 57nm. However,

at the deposition temperature (i.e. 600°C), the lattice mismatch becomes

compressive and much larger, –1.3% (the biaxial strain due to the lattice

mismatch changes from tensile to compressive at about 200°C). Therefore,

the critical thickness is much smaller, around 5–6nm at the growth temperature

and most of the misfit is thus expected to be relaxed by MD formation at the

deposition temperature for the 87nm thick PTO film investigated by Stemmer

et al. (1995). As a further example, in the case of a SrTi

0.5

epitaxial

film grown on SrTiO

(001) the misfit is –2.5% and Matthews and Blakeslee

(MB) theory predicts a critical thickness of 1.6nm (~four unit cells) (Langjahr

et al., 1998).

The misfit relaxation by dislocations for SrRuO

grown epitaxially on

SrTiO

(001) was studied by Oh and Park (2004). In this case the in-plane

lattice mismatch is –0.64% at room temperature and, based on the same MB

equilibrium theory, the authors estimate that for a 20 nm SRO film the misfit

will be shared between elastic strain and dislocations and a 75nm SRO film

approaches complete relaxation but is still somewhat elastically strained.

However, we note here that the growth of thin films by PLD is typically

a strong non-equilibrium process (Bäuerle, 2000) and therefore the MB

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Single crystalline PZT films 711

theory may not be entirely valid for this case. Thus films with thicknesses

well beyond the predicted critical thickness can be actually grown coherently

on dissimilar substrates.

Figure 23.9 shows AFM and TEM investigations performed on a PbTiO

SrRuO

/SrTiO

(001) heterostructure grown by PLD by Vrejoiu et al. The

SRO layer was deposited at 700°C in 0.14mbar O

and the PbTiO

(PTO)

layer at 575°C in 0.3mbar O

(see details about the fabrication of SRO in

Vrejoiu et al., 2006b). The AFM image in Fig. 23.9(a) shows that the PTO

layer grew in a layer-by-layer regime onto the SRO-coated STO. The TEM

cross-section micrograph of such a heterostructure (different sample, with

similar thickness of the PTO and SRO layers) (Fig. 23.9b) proved that the

~100 nm thick PTO thin film grown on top of an ~30nm thick SRO had very

low density of MDs and no threading dislocations or 90° a–c domains (compare

with the TEM micrographs shown by Foster et al., 1996b), and it was fully

c-axis oriented and almost coherently grown on STO (001).

Morita and Cho (2004) reported on a similar PbTiO

/SrRuO

/SrTiO

(001)

heterostructure, synthesized by the hydrothermal method (see also Jung et

al., 2005), in which the 100nm thick PbTiO

film was also fully c-axis

oriented, having no twins. However, a time as long as 24h was required to

grow a 100nm thick PTO layer by Morita’s method, whereas it took only

10–20min by PLD. In a more recent report, Morita and Cho (2006) mentioned

that at the interface between the 100nm thick hydrothermally synthesized

PTO film and the 50nm thick SRO bottom electrode, a 20 nm thin interface

layer that was a mixture between PTO and SRO formed, which is not the

case for the sharp interfaces of the PLD-fabricated PTO/SRO/STO (001)

heterostructures (Fig. 23.9b).

Figure 23.10 shows detailed AFM investigations performed on top of

SrRuO

/SrTiO

(001), PbZr

0.2

0.8

/SrTiO

(001) and PbZr

0.2

0.8

/SrRuO

0 200 400 600 800

800

600

400

200

0 a

2.0 nm

1.0 nm

0.0 nm

TEM

PbTiO

SrRuO

SrTiO

50 nm

23.9

(a) AFM image taken on the surface of a layer-by-layer grown

PbTiO

/SrRuO

/SrTiO

(001) heterostructure; (b) a TEM cross-section

micrograph, seen from [010] direction, of such a heterostructure,

with a 100nm thick PbTiO

layer and a 30nm thick SrRuO

layer.

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Handbook of dielectric, piezoelectric and ferroelectric materials712

3.0 nm

1.5 nm

0.0 nm

4.00

3.00

2.00

1.00

3.00

2.00

1.00

2.00

1.00

0 1.00 2.00 3.00 4.00

µm

0 1.00 2.00 3.00 4.00

µm

0 1.00 2.00

µm

0 0.25 0.50 0.75 1.00

µm

0 1.00 2.00

µm

0 1.00 2.00

µm

1.00

0.75

0.50

0.25

2.00

1.00

2.00

1.00

23.10

AFM images of (a) a vicinal STO (001) substrate with single-

unit-cell stepped terraces of uniform TiO

termination; (b) a 100nm

thick PZT 20/80 film grown directly on a vicinal STO substrate; (c) an

~50nm step-flow grown SRO film on a vicinal STO substrate; (d) an

ultrathin PZT 20/80 film (2–3nm) grown on top of a SRO-coated STO;

(e) a PZT 20/80 film (100nm) grown on top of a SRO-coated STO;

(f) a PZT 20/80 film (250nm) grown on top of a SRO-coated STO.

WPNL2204

Single crystalline PZT films 713

SrTiO

(001) heterostructures grown by PLD by Vrejoiu et al. (details about

the fabrication procedure are given in Vrejoiu et al., 2006b). Figure 23.10(a)

displays the AFM image taken on the surface of a vicinal STO (001) substrate

with single-unit-cell stepped terraces formed after etching and annealing. On

such a STO (001) substrate, a PZT 20/80 layer (100nm thick) was deposited

in a layer-by-layer regime, as shown in Fig. 23.10(b), for further XRD and

reciprocal space mapping (RSM) investigations. For metal–ferroelectric–

metal devices, a SRO layer (20–75nm thick) was deposited on thus prepared

vicinal STO (001) substrate in step-flow growth regime, as shown in Fig.

23.10(c) for an ~50nm thick SRO film. The morphology of the top surface

of PbZr

0.2

0.8

/SrRuO

/SrTiO

(001) heterostructures is presented in Fig.

23.10(d)–(f), for PZT 20/80 layers of various thickness, from 2nm up to

250nm, grown in a layer-by-layer regime. The 90° grid of traces that one can

see in the AFM image of the 250nm thick film indicates that MDs and 90°

domains were formed, which one expected for this rather thick PZT 20/80

film that must have partially relaxed via the formation of MDs and 90° a–c

domains. Such traces were not observed for PZT 20/80 films with thicknesses

of up to 100nm, however.

TEM cross-section investigations of three types of heterostructures based

on PZT 20/80 revealed that the PZT 20/80 layers are free from extended

structural defects, such as TDs and SFs (unlike the heterostructures presented

in Section 23.3.1). Figure 23.11 summarizes TEM micrographs taken on the

cross-sections of PbZr

0.2

0.8

/SrTiO

(001) (Fig. 23.11a), PbZr

0.2

0.8

SrRuO

/SrTiO

(001) (Fig. 23.11b) and SrRuO

/PbZr

0.2

0.8

/SrRuO

/SrTiO

(001) (Fig. 23.11c). The PZT 20/80 layer of the heterostructure given in Fig.

23.11(c) was sandwiched between top and bottom SRO electrodes, the top

SRO electrode being epitaxially grown at 575°C, immediately after finishing

the deposition of the PZT layer. Thus the entire SRO/PZT/SRO/STO (001)

heterostructure was cooled to room temperature in short-circuit conditions

and, although ~200 nm thick, the PZT 20/80 film is fully c-axis oriented,

with no twin 90° domains (Li et al., 2002b). Electron diffraction patterns

(Fig. 23.11d) of the three hetrostructures showed that the PZT 20/80 layers

are single-crystalline tetragonal, coherently grown with the in-plane lattice

parameter a matching that of the STO substrate and the out-of-plane lattice

parameter c undergoing an enlargement, so that a tetragonality ratio

c/a ≈ 1.07–1.075 resulted. This value of the tetragonality ratio was confirmed

by our X-ray diffraction reciprocal space mapping (XRD-RSM) measurements

(Ishida et al., 2002; Morioka et al., 2003). Figure 23.12 displays the XRD-

RSM measured for the (422) and (303) reflections of a PZT/STO (001)

heterostructure with a PZT 20/80 layer of 100–120nm thickness, whose

TEM micrograph is given in Fig. 23.11(a). These XRD-RSM patterns confirmed

that the PZT 20/80 film was fully c-axis oriented, without a–c twin domains

and only partly relaxed.

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Handbook of dielectric, piezoelectric and ferroelectric materials714

In contrast with the PZT 20/80 layers with high density of structural

defects, presented in the previous paragraphs, these PZT 20/80 layers free

from threading dislocations and stacking faults were synthesized from a

different PZT 20/80 target, purchased from PRAXAIR (Woodinville, WA).

Careful optimization of the PLD parameters and the obviously good

stoichiometry and density of this PLD target led to the outstanding improvement

of the PZT film quality.

23.5 Comparison of ferroelectric properties of PZT

20/80 films with and without extended

structural defects

In order to determine the ferroelectric and dielectric properties of PbZr

0.2

0.8

SrRuO

/SrTiO

(001) heterostructures, Vrejoiu et al. (2006a,b) deposited

top electrodes through a shadow mask. First SRO was deposited at room

temperature by PLD; subsequently Pt was sputtered to ease the contacting of

the individual capacitors. Figure 23.13 compares the polarization and switching

current loops measured on two types of PZT 20/80-based heterostructures.

We stress that these samples were prepared in the same PLD chamber,

23.11

TEM cross-section micrographs, seen from [010] direction of

heterostructures involving defect-free single-crystalline PbZr

0.2

0.8

thin films: (a) PbZr

0.2

0.8

on STO (001); (b) PbZr

0.2

0.8

SrRuO

/SrTiO

(001); (c) PbZr

0.2

0.8

film

sandwiched between

epitaxial SRO bottom and top electrodes. The electron diffraction

pattern of the heterostructure shown in (b) is given in (d) and was

correctly rotated with respect to (b).

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Single crystalline PZT films 715

10000 (rlu)

4300

4200

4100

4000

3900

3800

(422)

STO

PZT

8400 8500 8600 8700 8800 8900

10000 (rlu)

1.7

3.3

6.2

11.8

22.6

43.0

81.9

156.0

297.1

566.0

1078.0

2053.5

3911.5

7450.7

14192.3

27033.8

51494.5

98087.8

186839.6

355895.9

677917.8

1.7

3.3

6.4

12.3

23.6

45.4

87.3

167.8

322.5

619.8

1191.4

2290.2

4402.3

8462.1

16266.1

31267.0

60102.0

115529.1

222072.2

426871.3

820539.9

STO

PZT

(303)

Si holder

5600 5700 5800 5900 6000 6100 6200

10000 (rlu)

6000

5900

5800

5700

5600

5500

5400

5300

23.12

XRD-RSM measurements performed on a 100nm PbZr

0.2

0.8

film grown on STO (001): (422) reflection (left) and

(303) reflection (right).

WPNL2204

Handbook of dielectric, piezoelectric and ferroelectric materials716

employing the same PLD set-up, under similar deposition parameters. The

only difference in the synthesis procedure was the use of two different PZT

targets, purchased from two vendors, which are of nominally the same

composition, Pb

1.1

0.2

O.8

. Nevertheless, we suspect that the target quality

was significantly different, as the target from which the defective films were

produced exhibited a dark grey colour, unlike the PRAXAIR target which

was light yellow.

Figure 23.13(a) shows polarization and switching current hysteresis curves,

measured on a heterostructure whose PZT 20/80 layer of 250nm thickness

was defective (exhibiting TDs, SFs and a high density of 90° domains), and

whose TEM and PFM analyses are given in Fig. 23.5 and 23.10, respectively.

The remnant polarization of this MFM heterostructure was P

= 65 ± 10 µC/

. Figure 23.13(b) displays the hysteresis curves measured on a

heterostructure with a 215nm thick PZT 20/80 layer that was free from TDs

and had only a low fraction of 90° domains. The remnant polarization of this

MFM heterostructure is significantly higher, P

= 95 ± 10µC/cm

, which, to

the best of our knowledge, is the highest polarization value reported for PZT

20/80 thin films. We note that the formation of twin a-domains did not affect

severely the value of the remnant polarization, as both PZT films with and

without twins have P

in the range 80–100µC/cm

, thus considerably higher

than the P

of the PZT films with TDs and SFs. The shape of the polarization

hysteresis curve is more square-like in the case of the PZT 20/80 layer

without dislocations. The reduced value of the polarization that the defective

PZT 20/80 layers show correlates with the observation that the 180° domains

were pinned by the structural defects, such as the metal-rich SFs, and were

undergoing backswitching when analysed by PFM (Vrejoiu et al., 2006a).

The value of the dielectric constant calculated assuming the plane-parallel

capacitor model was also significantly different for the two types of

heterostructures, although they have a similar thickness of the ferroelectric

23.13

Polarization and switching current hysteresis loops (as

indicated by the arrows) measured on (a) a defective PZT 20/80 film

(250nm thick) and (b) a PZT 20/80 film (215nm thick) free of

extended structural defects, both with 90°

–

domains.

0.003

0.002

0.001

0.000

–0.001

–0.002

–0.003

Current (A)

–4 –2 0 2 4

Voltage (V)

100

–25

–50

–75

–100

Polarization (µC/cm

)

Current (A)

0.004

0.002

0.000

–0.002

–0.004

100

–25

–50

–75

–100

Polarization (µC/cm

)

–4 –2 0 2 4

Voltage (V)

(a)

(b)

WPNL2204

Single crystalline PZT films 717

layer and a similar electrode area. At 100 kHz, the defective layer, with high

density of TDs and 90° domains, has an effective dielectric constant of about

235. The layer without TDs and SFs, but a minor fraction of 90° domains,

has an effective dielectric constant of 170. The higher values of the dielectric

constant measured on the defective PZT layers are assumed to arise from

extrinsic contributions of extended structural defects and a–c twin domains

to the effective dielectric response of the material (Kim et al., 2005; Li et al.,

2005). For the defect-free PZT 20/80 thin films, which do not have 90°

domains (films thinner than 100nm), the dielectric permittivity ε

calculated

from the plane-parallel capacitor model decreases to ε

≈ 75 (Pintilie et al.,

2007).

Piezoresponse atomic-force microscopy (PFM) measurements were

performed either on the free surface of the epitaxial PZT 20/80 layers or on

top of the electrode pads that were employed for macroscopic measurements

(Le Rhun et al., 2006; Vrejoiu et al., 2006a,b). Figure 23.14 shows the

piezoresponse images taken on the free surface of a defective PZT 20/80

layer (250nm thick, whose TEM cross-section is displayed in Fig. 23.5(a)

and detailed PFM scans are shown in Fig. 23.8) and of a PZT 20/80 layer

without TDs and SFs (215nm thick). The hysteresis loops of the piezoelectric

coefficient d

, acquired on top of Pt/SRO contact pads (50–75 µm in diameter),

are given in Fig. 23.15. The piezo-hysteresis loop of the PZT film free from

TDs is more squared and saturated (Fig. 23.15b) in comparison with the

piezo-hysteresis of the PZT film with TDs and SFs (Fig. 23.15a).

23.6 Summary

The incongruent melting of PbZrO

and PbTiO

as well as the high volatility

of PbO at growth temperatures pose a challenge in establishing the equilibrium

200 nm

400 nm

23.14

Piezo-response images taken on the surface of a PZT 20/80 film

with threading dislocations (250nm thick) (a) and on the surface of a

PZT 20/80 film (215nm thick) without threading dislocations (b), both

with 90°

–

domains.

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Handbook of dielectric, piezoelectric and ferroelectric materials718

of bulk crystals of PZT. Therefore, large single-domain single crystals of

PZT have never been synthesized for a significant compositional range across

the solid solution phase. This leaves the intrinsic properties of single domain

PZT crystals under debate. Hence, the growth of single crystalline and defect-

free PZT thin films may enable the study of fundamental physical properties.

For instance, the spontaneous polarization P

, which is a fundamental parameter

that defines the performance of a ferroelectric material, is rarely reported

due to the lack of single crystals and perfectly polar-axis-oriented films.

Although the macroscopic polarization is the basic quantity used to describe

ferroelectrics, it has long escaped precise microscopic definition. Novel

concepts such as polarization as a Berry phase of the electronic Bloch

wavefunctions have been recently introduced (Resta, 1997; Sai et al., 2002).

Additionally, accurate investigations of the highly debated issue of size

effects in nanostructured ferroelectrics demand the growth of materials free

from extended defects, to rule out the extrinsic contribution of defects to the

disappearance/suppression of ferroelectricity below a certain critical size.

Epitaxially grown ferroelectric films often exhibit misfit and threading

dislocations, twins and anti-phase boundaries that generally have a negative

impact on their ferroelectric performance. The existence of threading

dislocations and stacking faults in epitaxial PZT 20/80 films was shown to

be correlated with the polarization backswitching.

The herein reported single crystalline defect-free PZT 20/80 thin films

grown by PLD show excellent ferroelectric performance, with remnant

polarization as high as P

≈ 100µC/cm

that clearly stems from their high-

quality microstructure.

23.15

Piezo-coefficient

hysteresis loops of a PZT 20/80 film with

threading dislocations (250nm thick) (a) and of a PZT 20/80 film

without threading dislocations (215nm thick) (b), both with 90°

–

domains.

dc voltage [V]

–4 0 42–2

[pm/V]

–80

–60

–40

–20

dc voltage [V]

–3 –2 –1 0 1 2 3

[pm/V]

–80

–60

–40

–20

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Single crystalline PZT films 719

23.7 Acknowledgments

The authors are grateful to Drs Nikolai D. Zakharov, Gwenaël Le Rhun,

Steffen Schmidt, Lucian Pintilie, Eckhard Pippel, Stephan Senz and Andreas

Schubert for their contributions to the characterization of the PZT films and

to Professor Ulrich Gösele for fruitful discussions. Partial financial support

from Volkswagen Foundation, through the ‘Nanosized ferroelectric hybrids’

project no I/80897, is gratefully acknowledged.

23.8 References

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single crystals’, Ferroelectrics 13, 497–500.

Dai Z R, Wang Z L, Duan X F and Zhang J (1996), ‘Link-up of 90° domain boundaries

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/LaAlO

’, Appl. Phys. Lett. 68, 3093–3095.

Emelyanov A Y and Pertsev N A (2003), ‘Abrupt changes and hysteretic behavior of 90

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Fei Y Y, Zhu X D, Liu L F,Lu H B, Chen Z H and Yang G Z (2004), ‘Oscillations in

oblique-incidence optical reflection from a growth surface during layer-by-layer epitaxy’,

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Fitzgerald E A (1991), ‘Dislocations in strained-layer epitaxy: theory, experiment, and

applications’, Matter. Sci. Rep 7, 87–142.

Foster C M, Pompe W, Daykin A C and Speck J S (1996a), ‘Relative coherency strain and

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