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

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Handbook of dielectric, piezoelectric and ferroelectric materials690

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Part VI

Piezo- and ferroelectric films

693

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23.1 Introduction

In most materials of technological interest structural defects play detrimental

roles. Therefore, the heteroepitaxial growth of thin films free from structural

defects, especially of semiconductors, has received a great deal of attention

in the last decades. In mismatched epitaxial films, which grow in a two-

dimensional mode (i.e. step flow or layer-by-layer growth), biaxial stress is

built up. Usually the stress relaxation in mismatched epitaxial films leads to

the formation of misfit dislocations (MDs) at the film/substrate interface,

which are commonly accompanied by threading dislocations (TDs) that extend

across the film. Structural defects, such as MDs and TDs, are well known to

significantly influence the physical properties of semiconductor thin films

and heterostructures (Fitzgerald, 1991; Queisser and Haller, 1998). For

example, epitaxial film quality is of great importance to the success of all

III–V semiconductor devices. Dislocation densities above ≈10

–2

significantly reduce the efficiency of GaP and GaAs optoelectronic devices.

The threading dislocation density depends only marginally on the substrate

material (and hence on the misfit between the substrate and the layer) but

rather on the growth technique and conditions (Matthews, 1975; Miguel and

Kardar, 1997; Kaganer et al., 2005; Kapolnek et al., 1995).

Although the presence of TDs in functional ternary-oxide thin films,

including ferroelectric perovskite thin films and heterostructures has been

reported (Heying et al., 1999; Alpay et al. 1999; Canedy et al., 2000; Misirlioglu

et al., 2006a,b), details on their origin, role and particularly on their avoidance

(Romanov et al., 1999) are not sufficiently understood. If large single crystals

cannot be grown easily, as in case of the ferroelectric PbZr

1–x

(PZT)

perovskite, for a broad range of compositions (Clarke and Whatmore, 1976;

Clarke et al., 1976; Haun et al., 1989), the intrinsic properties can be studied

on single-crystalline films, if the quality of growth permits to achieve films

that are free from extended defects. This chapter presents very recent results

regarding heteroepitaxial ferroelectric (PbTiO

PZT) thin films synthesized

Single crystalline PZT films and the impact

of extended structural defects on the

ferroelectric properties

I VREJOIU, D HESSE and M ALEXE,

Max Planck Institute of Microstructure Physics, Germany

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by pulsed-laser deposition (PLD), emphasizing the role of extended structural

defects for their ferroelectric performance by comparing thin films with and

without defects.

23.2 Pulsed laser deposition of epitaxial

ferroelectric oxide thin films

Pulsed-laser deposition (Chrisey and Hubler, 1994; Bäuerle, 2000; Willmott

and Huber, 2000) is a thin film synthesis technique based on laser ablation,

in which the energy of, typically, ultraviolet (UV) photons produced by

excimer lasers is coupled to the bulk starting material via electronic processes.

An intense laser pulse is focused onto a target placed in a vacuum chamber,

where it is partially absorbed and results in material ejection in the form of

a luminous plume, when the power density exceeds a certain threshold. The

threshold power density depends on the properties of the target material and

its morphology, and on the laser pulse wavelength and duration. The ablated

material is then allowed to condense on a suitable substrate, which may be

heated when crystalline films are desired, and thus thin film growth occurs.

The deposition process may be performed in high vacuum or in a background

of an inert (argon) or reactive (oxygen, nitrogen, N

O, etc.) gas. The latter

may affect the ablated plume species in the gas phase or the surface reactions

on the substrate, resulting in reactive PLD.

Among the several well-established techniques for thin film fabrication,

PLD offers some important advantages: (i) the energy source (laser photons)

is outside the vacuum chamber; (ii) almost any condensed matter material

can be ablated; (iii) the pulsed nature of PLD enables flexible control over

the film growth rate; (iv) the vaporized material originates only from the

area defined by the laser focus; (v) under optimal conditions, the ratios of the

elemental components of the target and the film are the same, even for multi-

elemental systems with complex stoichiometry; and (vi) the possibility to

produce species in energetic states far from chemical equilibrium may lead

to the production of novel or metastable materials that are unattainable under

thermal equilibrium. There are also several technical and fundamental

drawbacks one has to deal with when employing PLD: (i) the possible

production of up to micrometre-sized particles and droplets during the ablation

process; (ii) impurities in the ceramic/single crystal target material; (iii) the

limited size (around 1cm

) of thin film uniform thickness (Chrisey and

Hubler, 1994; Bäuerle, 2000; Willmott and Huber, 2000), unless specific

methods are applied (Pignolet et al., 1996; Lorenz et al., 1997). Additionally,

special care should be involved, when thin films with multicomponent

stoichiometry are synthesized by PLD. The angular distribution in the ablation

plume may vary for the different chemical elements (Ma et al., 1996). Several

studies dedicated to XeCl laser ablation of PbZr

1–x

indicated that the

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spatial distribution of the PZT film constituents has two components, one

resulting from thermal evaporation and the other from the photoablative

process, and are strongly dependent on the laser fluence and the ambient

oxygen pressure. The distribution profiles of Pb, the volatile element, exhibit

a dip at the centre for fluences higher than 0.2J/cm

and it has been concluded

that the Pb deficiency in PZT thin films may be caused by resputtering with

energetic Zr atoms and ions bombarding the growing deposit (Hau et al.,

1992, 1995).

PLD has been employed successfully for the fabrication of high-quality

thin films of ferroelectrics. A broadly documented review related to the PLD

synthesis of ferroelectrics is given in Leuchtner and Grabowski (1994).

Deposition of ferroelectric films of the correct stoichiometry is fundamental

for their utility. However, the synthesis is rather complicated, because often

the ferroelectric material involves volatile components such as Pb, Li or K.

Therefore, the most critical parameters that influence the film composition

are substrate temperature, ambient gas pressure and target composition itself,

whereas the laser energy density (fluence) can affect the chemical composition

to a less important extent. Since 1995, much progress has been made in the

controlled PLD-epitaxy of ferroelectric perovskites on single crystalline

substrates, such as SrTiO

, LaAlO

, DyScO

or MgO, with various orientations.

PLD systems equipped with high-pressure reflection high-energy electron

diffraction (RHEED) became available to monitor the layer-by-layer growth

of homoepitaxial and heteroepitaxial thin films and superlattices in situ (Neave

et al., 1985; Tarsa et al., 1996; Blank et al., 1999; Lee et al., 2005). Additionally,

a novel growth method, pulsed laser interval deposition, has been developed

to enable layer-by-layer growth in a regime (temperature, pressure) where

otherwise island formation would dominate the growth (Koster et al., 1999).

X-ray reflectivity and scattering, and optical reflectivity measurements, can

also be employed for in situ monitoring the growth regime of perovskites,

shedding light on the fundamental aspects (atomic level) of the PLD growth

process (Fei et al., 2004; Wang et al. 2004).

23.3 Epitaxial ferroelectric oxide thin films and

nanostructures with extended structural

defects

Great effort has been devoted to growing epitaxial thin films on lattice-

mismatched substrates for device application. When the epilayer exceeds a

certain critical thickness, misfit dislocations are generated to reduce the

mismatch strain and are usually accompanied by threading dislocations that

extend across the thin film (Matthews, 1975). The possibilities for the generation

of threading dislocations during epitaxial growth are:

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• the extension of substrate dislocations;

• the accommodation of translational and rotational displacements between

agglomerating islands that are close to epitaxial orientation;

• the formation of dislocation loops by the aggregation of point defects;

• plastic deformation of the film, during growth and subsequent cooling.

In the case of oxide ferroelectrics, it has been demonstrated that misfit and

threading dislocations have an important impact on the ferroelectric properties

of thin films and nanostructures. The strain field surrounding dislocations is

predicted to significantly perturb the ferroelectric and dielectric properties

of the neighbouring material (Alpay et al., 2004; Nagarajan et al., 2005;

Misirlioglu et al., 2006b). In nanostructures, owing to their confined

dimensions, the influence of defects is even more pronounced, because of

the relatively enhanced volume of the defective regions. In a joint study by

quantitative high-resolution electron microscopy and piezo-force microscopy

(PFM), Chu et al. (2004) showed that epitaxial PbZr

0.52

0.48

nano-islands

exhibited edge-type misfit dislocations, whose strain fields were affecting

relatively large tubular regions of PZT with a cross-section of 4nm (height)

× 8nm (width). Therefore, PZT islands that were only ~10nm high showed

an apparent polarization instability, associated with the distorted PZT lattice

within the strain fields. The PZT lattice deviated from the expected tetragonal

structure and the long-range correlations of local polarizations might thus

break down, resulting in polarization instability.

In epitaxial thin films, although it is unanimously accepted that TDs and

MDs have an important impact, it is more difficult to correlate directly the

presence of various types of dislocations with the ferroelectric and dielectric

properties of the material (Nagarajan et al., 1999, 2005; Ryen et al., 1999;

Canedy et al., 2000; Balzar et al., 2002; Misirlioglu 2006a). Nagarajan et al,

(2005) reported a quantitative study of the thickness dependence of the

polarization and piezoelectric properties of epitaxial PZT films with two

compositions, PbZr

0.52

0.48

(PZT 52/48) and PbZr

0.2

0.8

(PZT 20/80),

grown on SrRuO

-coated (001) SrTiO

. Cross-section transmission electron

microscopy (TEM) micrographs of 20 nm thin PZT films of both compositions

showed that dislocations were present in the PZT films. However dislocations

formed in a much higher density for the PZT 52/48 film, which has a larger

in-plane lattice mismatch with the SrTiO

substrate (–3.9% for PZT 52/48,

about only –0.7% for PZT 20/80, at growth temperature). As a consequence,

a drastic decrease of both the switchable polarization ∆P and piezoelectric

coefficient d

was observed for PZT 52/48 films thinner than 100nm. The

lattice-matched PZT 20/80 films, with low density of dislocations, showed

no scaling in ∆P or d

down to 15nm. The severe drop in ∆P and d

values

in the PZT 52/48 systems occurring for much thicker layers indicated the

overriding role of dislocations in the size effects in ferroelectrics.

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Single crystalline PZT films 699

Another important aspect is the polydomain formation, which is inherent

to heteroepitaxial ferroelectric films. The formation of polydomains relaxes

the total strain energy during the paraelectric–ferroelectric phase transformation.

Ferroelectric thin films, deposited at elevated temperature, undergo this

transition, when cooled down to room temperature through the phase transition

(Roytburd 1998a,b; Alpay and Roytburd, 1998; Roytburd et al., 2001; Foster

et al., 1996a; Nagarajan et al., 1999; Li et al., 2005; Pertsev et al., 2003;

Kukhar et al., 2006). The existence of dislocations influences the ferroelectric

domain patterns and movement as well (Speck and Pompe, 1994; Speck

et al., 1994; Emelyanov and Pertsev, 2003). Hu et al. (2003) found that the

presence of interfacial dislocations in tetragonal ferroelectric thin films locally

changes the ferroelectric transition temperature and leads to preferential

formation of ferroelectric domains around misfit dislocations.

The formation of domains can be regarded as a drawback, if single-

domain thin films are desired, when studying intrinsic ferroelectric properties

of materials. On the other hand, the domains can be employed for device

fabrication, exploiting the extrinsic contribution of domain wall motion to

the enhancement of dielectric and piezoelectric response of the material

(Nagarajan et al., 2003; Le Rhun et al., 2006).

23.3.1 TEM and HRTEM analyses of the defects in

epitaxial PZT 20/80 films

Vrejoiu et al. (2006a) reported on the PLD synthesis of ferroelectric epitaxial

PZT 20/80 films and analyzed in detail the defects exhibited by the PZT

layers and their effect on the polarization backswitching. PZT 20/80 films

have a tetragonal crystal structure. Vicinal single crystalline SrTiO

(100)

(STO) substrates with a miscut angle of 0.1–0.2° (CrysTec, Berlin, Germany)

were etched in buffered hydrofluoric acid and annealed so that single-unit-

cell stepped terraces with uniform TiO

-termination were formed (Koster

et al., 2000). A single-crystalline conductive SrRuO

(SRO) layer of a thickness

of 20–75nm was first grown in a step-flow growth regime (Hong et al.,

2005) onto the SrTiO

substrate. The SRO layer is to be used as bottom

electrode for the future metal–ferroelectric–metal heterostructure. Subsequently,

PZT layers up to 350nm thick were deposited at a substrate temperature T

= 575–600°C in 150–300mtorr O

, on top of the SRO-coated STO (001). A

ceramic Pb

1.1

0.2

0.8

target purchased from SCI Engineered Materials

(Columbus, OH) was used for PLD. Atomic force microscopy (AFM) images

taken on 2µm × 2µm areas on the surface of such SRO and PZT layers are

shown in Fig. 23.1. The surface of the 250 nm thick PZT layer exhibited a

90° grid that indicated the formation of 90° twin a–c domains, typical for

tetragonal ferroelectric thin films (Foster et al., 1996b; Alpay and Roytburd,

1998; Nagarajan et al., 1999; Li et al., 2005).

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