Polar Dielectrics and Their Applications

By Jack C. Burfoot and George W. Taylor

This title is part of UC Press's Voices Revived program, which commemorates University of California Press’s mission to seek out and cultivate the brightest minds and give them voice, reach, and impact. Drawing on a backlist dating to 1893, Voices Revived makes high-quality, peer-reviewed scholarship accessible once again using print-on-demand technology. This title was originally published in 1979.

• Language: English
• Publisher: University of California Press
• Release date: Jul 28, 2023
• ISBN: 9780520315334

Jack C. Burfoot

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POLAR DIELECTRICS AND THEIR APPLICATIONS

POLAR DIELECTRICS AND THEIR APPLICATIONS

JACK C. BURFOOT and GEORGE W. TAYLOR

University of California Press Berkeley and Los Angeles

University of California Press Berkeley and Los Angeles, California

ISBN: 0-520-03749-9

Library of Congress Catalog Card Number: 78-62835 Copyright © 1979 by Jack C. Burfoot and George W. Taylor Printed in Great Britain

Contents 1

Contents 1

1 Introduction

Part I

2 Preparation of Polar Materials

2.1 Growth of Single Crystals

2.2 Ceramic Fabrication

2.3 Thin Film Fabrication

2.4 Fabrication of Polar Glasses

2.5 Post-Fabrication Procedures

3 Electrical and Related Properties

3.1 Small Signal Electrical Properties

3.2 Large Signal Electrical Properties

3.3 Conductivity Effects

4 Optical and Related Properties

4.1 Refractive Index and Birefringence

4.2 Optical Dispersion

4.3 Thermo-Optic Behaviour

4.4 Elasto-Optic Effect

4.5 Electro-Optic Characteristics

4.6 Non-Linear Optical Effects

4.7 Photo-Refractive Effect

4.8 Light Scattering Effects

4.9 Absorption

4.10 Photoluminescence, Electroluminescence and Luminescence

5 Mechanical Properties 5.1 Introduction

5.2 Elasticities and piezoelectric coefficients

5.3 Measurement

5.4 Mechanisms of mechanical loss

5.5 Effects of applying mechanical stress

5.6 Mechanical properties of improper and ferroelastic materials

5.7 Surface waves

5.8 Effects of the composition variable

6 Thermal Properties 6.1 Introduction

6.2 Thermal parameters (table 6.1, at A)

6.3 Thermal-electrical parameters (Table 6.1, at B)

6.4 Thermal-mechanical parameters (Table 6.1, at C)

7 The Transition 7.1 Anomalies and the classical model

7.2 Transitions

7.3 The lattice vibration viewpoint

7.4 Antiferroelectrics

7.5 Nomenclature

7.6 Non-linearity

8 Symmetry 8.1 Symmetry groups

8.2 Symmetry of materials

8.3 Symmetry of sites

8.4 Magnetic symmetries

9 Ferroelectric Materials 9.1 Selection of data

9.2 Transition and space groups

10 Spectroscopy 10.1 Introduction

10.2 Characterisation of Spectra

10.3 Spectroscopy

10.4 Interpretation of Spectra

10.5 Fluctuations

10.6 Central Peak and Other Aspects of Fluctuations

10.7 Appendix: Relations between Spectroscopic, Optical and Dielectric Parameters

11 Modes of Lettice Vibration 11.1 Dielectric parameters

11.2 Lattice vibrations

11.3 Anharmonicity

11.4 Improper ferroelectrics

11.5 Order-disorder and pseudospin mechanisms

12 Domains and Switching 12.1 Introduction

12.2 Methods of observing domains

12.3 The depolarisation problem

12.4 Depolarisation in static domains

12.5 Static domain wall structure

12.6 Static domain configurations

12.7 Dynamics of domain movements

12.8 Ideal insulator characteristics

Wall velocity

12.10 Physical properties and wall effects

12.11 Compensation

12.12 Surface effects

Part II

13 Applications—Some Basic Considerations 13.1 Introduction

13.2 Memories and Displays

13.3 Methods of Addressing

13.4 Basic Limitations of Polar Materials

14

14.1 Matrix Addressed Memory

14.2 Shift Register

14.3 Transcharger

14.4 Devices Based on the Self Reversal Effect

15.1 Ferroelectric-Piezoelectric Devices

15.2 Ferroelectric-optic Devices

15.3 TANDEL

16 Applications Involving Switching—Complex Structure

16.1 Ferroelectric-electroluminescent Devices

16.2 Ferroelectric-photoconductor Devices

16.3 Ferroelectric-semiconductor Devices

17.1 Capacitors

17.2 PTC Thermistors

17.3 Artificial Gems

17.4 Electro-optic Applications

17.5 Non-linear Optical Applications

17.6 Photo-refractive Applications

17.7 Pyroelectric Applications

17.8 Piezoelectric Applications

17.9 Elasto-optic Applications

18

18.1 Materials

18.2 Electro-optic Effects

18.3 Electro-optic Applications

18.4 Thermo-optic Effects and Applications

Bibliography1

Author Index*

Subject Index

1 Introduction

Dielectric materials can be divided into 32 crystal classes or point groups. In all of these classes a polarisation or dipole moment can be induced by an applied electric field. Twenty of the 32 classes are piezoelectric and they have the property that a polarisation can also be induced by an applied mechanical stress. Half of the piezoelectric classes of materials, i.e. ten of the original dielectric classes, exhibit the very important property that a finite and permanent value of polarisation, known as spontaneous polarisation, exists in the absence of an applied electric field or stress. Such dielectrics are termed polar materials and are the principal subject matter of this book.

The spontaneous polarisation of a polar material results from an inherent asymmetry within the basic crystal cell. This asymmetry gives rise to ionic and/or electronic forces that create elemental dipole moments. Because of cooperative effects the dipole moments add to give a finite and permanent polarisation. The spontaneous polarisation of a polar material cannot be measured directly with an electrometer, since charge compensation rapidly occurs within the crystal. For the same reason, the short circuiting together of electrodes on opposite surfaces of a polar material does not destroy the spontaneous polarisation. By comparison, in non-polar dielectric materials, the induced polarisation can be measured with an electrometer and can be destroyed by shorting the surface electrodes.

A classical method of detecting spontaneous polarisation is to subject the polar material to a change in temperature. An increase or decrease in temperature alters the ionic and electronic forces within the basic crystal cell, and the extent of thermal disorganisation, which results in a change in the values of the dipole moments of the polar material. If the change in temperature is fast enough, then there is not time for charge compensation of the dipoles to occur. The net result is that a detectable current, termed the pyroelectric current, will flow out of electrodes placed on opposite surfaces of a polar material. Since all polar materials can, in theory, exhibit pyroelectricity, the adjectives polar and pyroelectric have been used synonymously by some authors. In practice, pyroelectric effects have only been measured in about a hundred out of a multitude of polar materials. The probable reason for this is the difficulties of measurement together with the lack of interest, until recently, in the pyroelectric effect. The pyroelectric coefficients of the materials that have been measured vary greatly in magnitude¹ . They range from 17000/¿Cm² K¹ for barium titanate, BaTi03, near its transition temperature down to 0 002 /iC m—² K"¹ for animal bone.

As we have seen, a polar material is a dielectric material which possesses a spontaneous polarisation. In certain polar materials, the direction of the spontaneous polarisation can be changed by a suitably applied electric field, subsequently removed. In most of these materials the change is a 180° reversal of the direction of the polar axis, but in some materials the polar axis is reoriented by less than 180°.²

Polar materials whose direction of spontaneous polarisation can be changed by an applied electric field are known as ferroelectrics or occasionally as Seignette-electrics. The term ferroelectric is derived from the analogy with ferromagnetic materials in that both types of materials possess domains, exhibit hysteresis loops and show Curie-Weiss behaviour near their phase transition temperatures. The hysteresis loop of a ferroelectric is obtained by plotting polarisation (P) against applied electric field (E), while in a ferromagnetic it is achieved by plotting magnetisation (B) against the applied magnetic field (//). The term Seignette-electric is derived from the name of the chemist who originally prepared Rochelle salt in the 17th century. This material was subsequently identified by Valasek³ in 1921 as possessing a field reversible spontaneous polarisation.

The study of the general classification of polar materials stretches back into antiquity¹ with the detailed qualitative work commencing in the 18th Century⁴ . While the study of the ferroelectric subgroup of polar materials began just over 50 years ago with Valasek’s work on Rochelle salt, it has since then grown at an exponential rate. This is shown in figure 1.1, which gives the number of papers on ferroelectricity published each year. Other manifestations of this growth are the holding every three or four years of an international

t Strictly speaking, a ferroelectric hysteresis loop is a plot of dielectric displacement (D) versus P where D = P + e0E. In practice, for most ferroelectric materials, the product of e0, the permittivite of free space, and £ is very much smaller than P; hence D is essentially equal to P.

Figure 1.1 Number of research papers, N, on ferroelectrics and antiferroelectrics published in each year. The dashed line corresponds to

N = exp(0.14 (/-1921))

Y is the year. Books on ferroelectrics published in each year are also shown. The one marked with asterisks represent a version translated into another language (from Ref. 11, Vol. 9)

meeting⁵, a regional European meeting⁶, an IEEE Applications Conference⁷ and national meetings in the USSR⁸ and other countries⁹. There is an international journal Ferroelectrics¹⁰ devoted to the subject, the publication of which has increased from, initially one volume per year in 1970, to three volumes per year in 1976. Almost 700 ferroelectric pure compounds and solid solutions have been identified at this date¹¹. The reversible spontaneous polarisations in these materials range from 5xl0⁵/iCm—² for lithium tantalate, LiTa03, down to 1.7 x 10³/iCm-2 for gadolinium molybdate, Gd2(Mo04)3.

Major sources of bibliographic information on polar materials include the Landolt-Bornstein series¹¹, Lang’s Source Book of Pyroelectricity¹ and his annual literature Guide to Pyroelectricity¹², Toyoda’s continuing Bibliography of Ferroelectrics¹³, an annually published Digest of Literature on Dielectrics¹⁴ and two O.R.N.L. Ferroelectric Literature Indexes¹⁵.

Though ferroelectrics are only a sub-group of polar materials, they include the most significant polar materials, both from a basic viewpoint and from an applications viewpoint. Many of the ferroelectrics possess some of the most interesting piezoelectric, thermal, optical and electrical properties of all the dielectric materials.

Polar materials, in particular the ferroelectrics, can exhibit a large degree of non-linearity in their electrical, optical and piezoelectric properties. Through out this book the three adjectives polar, ferroelectric and non-linear are used somewhat interchangeably. However, as much as possible, we have striven to use the most suitable term in each instance.

Like life itself, the writing of this book has been both pleasure and pain. The pleasure has stemmed from the satisfaction of covering, in an age of scientific specialisation, a subject as broad as polar materials. The chemist, the crystallographer, the ceramicist, the physicist and the mechanical and electrical engineer all have a vital interest in polar materials. The interest covers the gamut from theory through experiment to application. The pain has been the challenge of writing within a reasonable number of pages a meaningful book for a diverse readership.

In a rapidly growing field there is always the necessity to be quite selective about what to include in a book. This problem is compounded, in the present instance, because of the interdisciplinary nature of study of polar materials. Hopefully the future will reveal that our use of the hatchet for selection has been more perceptive than arbitrary.

Wherever feasible, principles have been emphasised so as to make the book most valuable to students and newcomers to the field. The examples of single crystal and ceramic materials used to illustrate the principles involved have been selected, as much as possible, from recent publications. Thus the book should be useful for experienced scientists working with or familiar with polar materials. The examples of devices have been selected on the basis of their currentness, as well as the scientific and economic significance-criteria which are most meaningful to engineers utilising or wishing to utilise polar materials.

We begin Part 1,⁴Basic Properties’, with chapter 2 giving outlines of the methods by which the materials of interest may be prepared, either for specific applications or for study and understanding of the physical properties of the materials, with a view to their attempted exploitation. Often the material preparation will consist of crystal growing or ceramic manufacture; ceramic materials are polycrystalline structures. For study of the properties, crystals will usually be preferred as being likely to give results more easily interpretable in terms of reasonably simple models. If the crystals contain imperfections or impurities this may be unavoidable or it may have resulted from deliberate attempts to modify the properties in some way—as for example in bringing the transition temperature, or Curie point, Tc, into some more convenient temperature range. For applications, the more complicated structures of ceramic materials may well be the price we pay for their ease of fabrication and relative cheapness. It is also a test of our ability to understand these materials if we can satisfactorily extend our models to include ceramic structures. Very interesting and prominent examples are the PZT ceramics, which have replaced crystal materials in many piezoelectric devices; more recently it became possible to grow PZT compositions as crystals. In this chapter we discuss also the technique of annealing, and the processes of poling, cutting, electroding, and several methods of making thin films.

Chapters 3-6 give introductions to various aspects of the physical properties—electrical, optical, mechanical and thermal. In each case a prominent effect is the anomaly (or peak) which occurs in nearly every physical property in the neighbourhood of Tc. In chapter 3 we describe the dielectric anomaly and the non-linearities, reversal of the spontaneous polarisation Ps by means of electric fields (switching), and the techniques for observing it, the relationship of switching behaviour to the parameters of the driving impulses, and various ways in which these effects are observed to decay or ‘age’. In principle the perfect ferroelectric is an insulator, but some rather interesting possibilities arise when doping has added semiconduction to its properties. Brief reference is also made to some photoconductive properties.

In chapter 4 we consider the optical properties, and their variation with frequency (mostly within the visible region), with temperature, with applied mechanical and electrical stress, and with incident light. Many of these properties form the basis of applications which are to be discussed. The variation of refractive index, or of birefringence, with electric field (electrooptics) is of particular interest in these materials, where the field involved may well not be an exterior applied field, but may follow the development of internal fields when the material undergoes the transition into its polar state. Non-linear optical effects are also important. An important application of this effect is second harmonic generation. Some absorption-edge, photo- luminescent, electroluminescent and luminescent effects are described.

Chapter 5 includes not only the elastic anomalies, but also the piezoelectric coefficients, since these link the mechanical and electrical parameters. Mechanical loss effects are considered, just as were the electrical loss effects in the chapter 4. Above jTc, in a ferroelectric-to-non-ferroelectric transition, Psis zero, and there may or may not be ‘ordinary’ piezoelectricity, unrelated to Ps of course. Nevertheless interest does attach to the differences between those materials which do or do not possess that ‘ordinary’ piezoelectricity. The quadratic effect, conventionally called électrostriction, is also discussed. The effect of applied stress on various properties is considered in section 5.5, and improper and ferroelastic materials are covered in section 5.6. Surface acoustic waves are also briefly mentioned.

In discussing the thermal properties in chapter 6 the well-known and very useful Devonshire form of thermodynamic function is introduced. Its purpose is to summarise and interrelate a vast range of experimental information. The thermal-electrical and the thermal-mechanical properties are listed in table 6.1, which includes also the interesting anomalies. The properties discussed here include the pyroelectric and electrocaloric effects, as well as the anomalous behaviours in specific heat and in thermal conductivity.

Chapter 7 links the classical description of the co-operative transition, and the anomalies which occur there, to a more recent viewpoint available from studies of lattice vibrations; the soft-mode concept is in the first place only a change of terminology, though later it will be seen to be capable of many new extensions. The antiferroelectric transition is considered in section 7.4. In section 7.5 is given a brief indication of some recent attempts to clarify the classification of the increasingly complicated phenomena under study. Anhar- monic effects require more elaborate mathematical treatment than we are able to give in this book, but an introduction to some of those studies is included. The materials in this book are distinguished experimentally by their extreme non-linearities and by the anomalies. Conceptually, the application of the principles of symmetry to them is very significant, and this is dealt with in chapter 8, not in detail, but in a manner which we hope will enable interested readers to follow up in more detailed studies elsewhere.

In Chapter 9 we give, for reference, a brief summary of aspects of the transition in some of the materials which have been used for illustration elsewhere in the book. There are hundreds of materials in our sphere of interest, and reference is made to places where more complete listings are available.

Chapters 10 and 11 carry some of the previous ideas into the region of spectroscopy, and show how the dispersion phenomena may be used to elucidate the underlying microscopic mechanisms involved in these polar transitions. Attention is paid to the importance of fluctuation phenomena when the transition is approached, and to the central peak which has in recent years become distinguishable from the soft mode. Section 11.5 deals with KDP-type materials and the ‘pseudospin’ descriptions. An appendix to chapter 10 provides a convenient summary of the related optical and dielectric parameters.

Almost all the properties of polar materials are modified considerably by the presence and mobility of domains. The methods of studying them, and the models which seem to be successful, are described in chapter 12.

Part II of this book, chapter 13-18, ‘Applications’, is concerned with devices and systems that have been or can be built with polar and ferroelectric materials. The applications covered range from those that already have a well established commercial market to those that are still at a developmental stage. Such a detailed treatment of the applications of polar materials is well overdue, since earlier books on polar and ferroelectric materials have only given a cursory treatment of this economically very significant subject.

Chapter 13 details some fundamental aspects of memories and displays, two of the major components in computers and communication systems. The basic parameters of memories and displays, including addressing techniques, are analysed. This analysis is then applied to the properties of polar materials. As is described in the succeeding chapters, polar materials are being used or actively considered for use in a variety of memory and display applications.

The next three chapters, 14-16, contain a description of a wide range of devices that utilise the reversible spontaneous polarisation property, i.e. the switching, of ferroelectric, polar materials. For convenience, this material is divided between three chapters. Chapter 14 deals with devices that involve only the reversal of the spontaneous polarisation of the polar material. Chapter 15 is concerned with polar material devices in which use is made of changes in the piezoelectric, optical and thermal properties that are associated with the reversal of the spontaneous polarisation. In chapter 16 the devices have a complex structure in that the reversible polar material is combined with another electrically active material, such as an electroluminescent material, a photoconductor or a semiconductor material.

Chapter 17 describes a large number of polar material devices in which the common factor is that no large signal switching, or reversal, of spontaneous polarisation is involved. Instead, the devices utilise the anomalously high values of dielectric permittivity, linear and non-linear optic and electro-optic coefficients or photo-refractive, pyroelectric, piezoelectric and elasto-optic constants of polar materials. Some of the devices in this chapter, such as capacitois and piezoelectric transducers, are of great commercial importance. Others, such as optical holographic storage media and colour projection TV systems, are still at a developmental stage but may prove to be economically very significant in the future.

This book concludes with a short treatment in chapter 18 of the basic physics and the electro-optic and thermo-optic applications of an important group of liquid materials having polar molecules. These materials are generally referred to as liquid crystals, since they simultaneously exhibit the physical properties of liquids and the electrical and optical characteristics of an ordered, polar crystalline structure.

References

1. S. B. Lang, Source Book of Pyroelectricity, Vol. 2 in book series Ferroelectricity and Related Phenomena (ed. I. Lefkowitz and G. W. Taylor), Gordon and Breach, London (1974)

2. L. A. Shuvalov, J. phys. Soc. Japan, 28, Supplement, 38 (1970)

3. J. Valasek, Phys. Rev., 17, 475 (1921)

4. F. U. T. Aepinus, Histoire de rAcademie Royale des Sciences et Belles Lettres, Berlin, 12, 105 (1756)

5. First International Meeting on Ferroelectricity, Prague, Czechoslovakia, June-July, 1966. Proceedings published by Institute of Physics of the Czechoslovakian Academy of Sciences (1966)

Second International Meeting on Ferroelectricity, Kyoto, Japan, Sept., 1969. Published as Supplement to J. phys. Soc. Japan, 28 (1970)

Third International Meeting on Ferroelectricity, Edinburgh, Scotland, Sept., 1973. Published in Ferroelectrics, 7 and 8, Nos. 1 and 2 (1974)

6. First European Meeting on Ferroelectricity, Saarbrücken, 1969. Stuttgart, Wissenschaftliche, Verlagsgessellschaft mbH (1970)

Second European Meeting on Ferroelectricity, Dijon, 1971. Published in J. Physique, 33, Supplement C2 (1972)

Third European Meeting on Ferroelectricity, Zurich, Sept., 1975. Pub lished in Ferroelectrics, 12, 13 and 14, Nos. 1 and 2 (1976)

7. 1969 IEEE Symposium on the Applications of Ferroelectrics, Washington DC, June, 1968. Published in IEEE Trans. Electron Devices, ED16, No. 6

(1969)

1971 IEEE Symposium on the Applications of Ferroelectrics, York town Heights, New York, June, 1971. Published in Ferroelectrics, 3, Nos. 2-4 (1972)

1975 IEEE Symposium on the Applications of Ferroelectrics, Albuquerque, New Mexico, June, 1975. Published in Ferroelectrics, 10 and 11 Nos.

1 and 2 (1976)

8. Transactions of the First All Union Conference on Ferroelectricity, Leningrad. Izv. Akad. Nauk SSSR, Ser. Fiz., 21, Nos. 2 and 3 (1957) Transactions of the Second All Union Conference on Ferroelectricity, Rostov on Don. Izv Akad. Nauk SSSR, Ser. Fiz., 22, No. 12 (1958) Transactions of the Third All Union Conference on Ferroelectricity, Moscow. Izv. Akad. Nauk SSSR, Ser. Fiz., 24, Nos. 10 and 11 (1960) Transactions of the Fourth All Union Conference on Ferroelectricity, Rostov on Don. Izv. Akad. Nauk SSSR, Ser. Fiz., 29, Nos. 6 and 11 (1965) Transactions of the Fifth All Union Conference on Ferroelectricity and Physics of Inorganic Dielectrics, Dnepropetrovsk. Izv. Akad. Nauk SSSR. Ser. Fiz., 31, Nos. 7 and 11 (1967)

Transactions of the Sixth All Union Conference on Ferroelectricity, Riga Izv. Akad. Nauk SSSRy Ser. Fiz., 33, Nos. 2 and 7 (1969)

Transactions of the Seventh All Union Conference on Ferroelectricity. Voronezh. Izv. Akad. Nauk SSSR, Ser. Fiz., 34, No. 12 (1970) and 35, Nos

9 and 12 (1971)

Transactions of the Eighth All Union Conference on Ferroelectricity. Uzhgovod. Izv. Akad. Nauk SSSR, Ser. Fiz., 39, Nos. 4, 5 and 6 (1975) Symposium on Semiconductors and Ferroelectrics, Rostov on Don. Ferroelectrics, 6, Nos. 1 and 2 (1973)

9. Ferroelectricity—Proceedings of Symposium held at Warren, Michigan. 1966. Elsevier, Amsterdam (1967).

Symposium Proceedings on Practical Applications of Ferroelectrics, Tokyo, Sept. (1969). Oyo Buturi, 39 (1970)

10. Ferroelectrics (eds. I. Lefkowitz and G. W. Taylor) published by Gordon and Breach, New York, Vol. 1 (1970), to present

11. Landolt-Bornstein New Series, Group III, Volume 3. Ferroelectric and Antiferroelectric Substances. Springer-Verlag, Berlin (1969). Landolt-Bornstein New Series, Group III, Volume 9. Ferroelectric and Antiferroelectric Substances. Supplement and extension to III/3, Springer-Verlag, Berlin (1975)

12. S. B. Lang. Literature Guide to Pyroelctricity, published annually in Ferroelectrics beginning with Ferroelectrics, 5, 125 (1973)

13. Koichi Toyoda. Bibliography of Ferroelectries, published in most issues of Ferroelectrics beginning with Ferroelectrics, 1, 43 (1970)

14. Digest of Literature on Dielectrics, published by U.S. National Research Council of National Academy of Sciences, Washington D.C., beginning in 1954

15. Oak Ridge National Laboratory Literature Guides, Vol. 1. Ferroelectric Materials and Ferroelectricity. Authors T. F. Connolly and E. Turner, IFI/Plenum, New York (1970)

Ibid.y Vol. 6. Ferroelectric Materials and Ferroelectricity. Authors T. F. Connolly and D. T. Hawkins, IFI/Plenum, New York (1974)

Part I

Basic Properties

2 Preparation of

Polar Materials

This chapter summarises the techniques used for fabricating polar materials in single crystal, ceramic, thin film and glass forms. It then goes on to discuss the various post-fabrication procedures such as annealing, poling, cutting, thinning, polishing and electroding which are usually needed before the polar materials can be used for either experiments or applications.

For basic studies, where the polar material should be as near perfect as possible, it is desirable to use single crystals. For applications, where dimensions, reproducibility and cost are crucial factors then the polar material is usually fabricated in ceramic, thin film or glass form. This division is not precise. For example, some materials have not yet been grown as single crystals and hence basic studies must be made in the ceramic form. In other cases, the larger dielectric, piezoelectric, pyroelectric, electro-optic, etc., coefficients obtainable with particular single crystal materials can make this fabrication form mandatory for certain applications. And yet again thin films offer an important advantage for studying surface layer physics.

The two major problems in fabricating large samples of high quality polar materials are the maintenance of the correct stoichiometry and the avoidance of strains. Chemical, thermal and optical methods, as well as sophisticated techniques such as X-ray, electron, neutron and proton diffraction scattering are used to determine the quality of the fabricated material. By comparison, the fabrication techniques themselves are still much of an art, as evidenced by the variety of ‘recipes’ reported for particular materials. This however may be changing. For example, it is now customary to make a careful determination of the phase diagram of the material so as to choose the optimum fabrication conditions. These diagrams can be quite complex, as can be seen from figures

2.1 and 2.2, the phase diagrams for BaTi03 and Pb(ZrSnTiNb)03. Note that in figure 2.2 because of the ternary nature of the material, (a) the material is best considered a Nb doped PbZr03-PbSn03-PbTi03 solid solution, (b) a different phase diagram is needed for each temperature. Another example of the scientific approach is the use of fluid dynamic principles to try to understand circulation currents in the melt during growth³.

Figure 2.1 Barium titanate. Phase diagram (Rase and Roy¹)

Many measurements can be made to determine the degree to which a ceramic, thin film or glass material approaches the properties of its single crystal counterpart. The hysteresis loop is a simple yet powerful criterion for making this determination. The squarer the loop and the larger its polarisation, then the better the orientation of the crystallites and the smaller the

Figure 2.2 Part of the phase diagram of Pb0.99(ZrSnTi)0 98Nb002O3 at 25° C (poled at 25°) F = Ferroelectric, A = antiferroelectric, T = tetragonal R = rhombohedral, LT = low temperature phase, HT = high temperature phase (Raider and Cook²)

percentage of non-polar material and voids present. In general, doctor-bladed ceramics and epitaxial thin films are the fabrication techniques producing materials closest to the single crystal form, whilst the glasses and conventionally sintered ceramics are the furthest away.

2.1 Growth of Single Crystals

The two major methods that have been used to date for fabricating single crystals have been the solution (or flux) growth technique and the melt growth technique.

2.1a Solution Grown Crystals

This method has been successfully used for both water soluble materials and those that are soluble in other liquids or fluxes.

The first stage in growing a water soluble crystal is to prepare an aqueous saturated solution of the polar material. Crystals can be grown by either keeping the solution at a constant temperature and allowing a gradual evaporation of the solvent or by slowly lowering the temperature while keeping the solution saturated. Slow growth, taking a thousand hours or more, usually produces the best and largest crystals. Other factors which affect the crystal

growth are the purity of the materials, the solubility-temperature characteristics of the solution, the fineness of the temperature control, the use of stirring to prevent temperature and concentration gradients from developing in the solution, and the use of seed crystals suspended in the solution to enhance growth.

Table 2.1 contains a fairly comprehensive listing of the polar materials which have been successfully grown from aqueous solution. Figure 2.3 shows a large solution-grown crystal of triglycine sulphate, TGS. The original seed crystal is visible in the centre of the photograph. The organic polar material thiourea can be grown from an aqueous solution; however, better results are obtained from an alcoholic solution.

Polar materials that are not soluble in water or alcohol can often be dissolved at high temperature in other materials, usually referred to as fluxes. For example, 26 different fluxes have been reported⁵ for barium titanate, BaTi03. In some cases, an excess of one of the constituents can act as a flux. For example, additional Bi203 will serve as a flux in growing crystals of bismuth titanate, Bi4Ti30,2.⁶

Besides the particular flux used there are many other variables in flux growth. They include the purity and particle size of the component materials, the time-temperature cycle used for forming the molten flux solution, the crucible material and its shape and size, the method of heating (both resistive and r.f. induction heating are used), the time-temperature cycle used for cooling, the thermal gradients established in the furnace (both vertical and horizontal gradients have been used), and the atmosphere maintained in the furnace.

Some of the crystals that have been grown by flux techniques are shown in table 2.1. Space does not permit detailing the growth conditions used for each material. To give some idea, however, it is worth summarising the successful technique developed by Remeika⁷ for the flux growth of BaTiOa crystals. A platinum crucible containing 30% BaTi03 powder, 70% KF (the flux) and a small trace of Fe203 is heated for 8 h at 1175°C. The Fe203 compensates for the loss of oxygen at high temperature. The crucible is then cooled slowly to 875°C, at which stage the excess liquid flux is poured off. The crystals thus formed are then cooled slowly to room temperature. Any residual flux is removed by acid etching. The crystals have a plate-like morphology and some typical examples are shown in figure 2.4a.

2.1b Melt Growth

If a polar material melts congruently, that is, if stoichiometry is maintained, then the crystal can be grown directly from the melt. As the crystal grows, either by spontaneous nucléation on to a chemically inert platinum or iridium wire or onto a seed crystal, it is gradually withdrawn from the molten liquid. In the Stockbarger method, this is done by withdrawal of the crucible containing the melt. In the Czochralski method (figure 2.5), the crystal is gradually ‘pulled’ out of the melt, and it is usual to rotate the crystal while pulling, to minimise thermal and stress gradients. Also suitable optics are provided for viewing the crystal during growth. Figure 2.5 shows a Czochralski crystal grower used for growing lithium niobate, LiNb03. The apparatus is also designed to pole the crystal during the growth process. The Czochralski method is usually the best for polar materials, in that it produces less strains and less twinning in the crystal. A large number of variations in the technique are possible including variation of the pulling and rotation rates, the method and amount of after-heating used as the crystal emerges, and the atmosphere used. Figure 2.6 is a photograph of a Gd2 (Mo04)3 crystal pulled by Kumada at 70mm/h using a rotational speed of lOOrev/min in an oxygen rich atmosphere¹⁰. Most Czochralski grown crystals have the form of figure 2.6.

Belruss et a/.¹¹ have developed a modified Czochralski technique, sometimes referred to as ‘top seeding’, in which the temperature of the melt is gradually dropped (0.2°C/h) as pulling proceeds. The crystal of figure 2.4b was grown by this technique, using a pulling rate of 0.7 mm/h. Pulled BaTi03 crystals have a polyhedral morphology and a transition temperature, T# of 132°C. By comparison, the Remeika⁷ flux grown BaTi03 crystals, shown in figure 2.4a have a plate-like structure and a Tc of 120°C. The lower Tc is due to the substitution of K atoms at some Ba sites and Fe at some Ti sites in the BaTi03 crystal lattice; the K and Fe impurities originate from the flux used.

The polar material single crystals that have been successfully grown from the melt by Czochralski type techniques are listed in table 2.1.

2.1c Other Techniques

Single crystals of both BaTi03 and antimony sulpho-iodide,SbSI, have been grown by vapour transport¹². Hydrothermal methods, which involve a combination of high pressure and temperature have been used to grow single crystals of several types of polar materials¹³ ¹⁵-see also table 2.1.

2.2 Ceramic Fabrication

The classical technique for forming ceramics is sintering at atmospheric pressure. Recent variations on this process are doctor-blading and hot pressing.

2.2a Sintering

The constituents (or their oxides) of the polar material are mixed in the correct proportions with an organic binder and then pressed at room temperature into a structure having the desired shape and dimensions. In most cases this is a cylinder, although for some applications, for example sonar, more complex shapes are used. The pressed structure is sintered or fired at an appropriate temperature in an appropriate atmosphere. This causes the organic binder to be burnt out and the pressed materials to react chemically and form the desired polar material. Table 2.2 contains a list of some of the polar materials which have been fabricated as sintered ceramics.

2.2b Doctor-Blading

Doctor-Blading¹⁶ is particularly suited for forming large area, thin sheets of ceramic. The constituents of the polar materials are mixed in a liquid together with a suitable plasticiser and the resultant slurry is poured onto flat glass. A stainless steel blade, accurately positioned a small distance, S, above the substrate is then drawn through the slurry. The resulting sheet is allowed to dry after which it can be peeled off the glass. At this stage the material is termed ‘green’ because it can be easily cut or punched into any two dimensional shape. The sintering is done in a two step process, viz. a lower temperature firing to burn out the plasticiser and then a higher temperature firing in a controlled atmosphere to form the polar material.

Whenever a ceramic is sintered there is a large amount of shrinkage. For a doctor-bladed material, the shrinkage is particularly evident as a decrease in thickness. Figure 2.7 shows the relationship between the thickness of ‘green’ and fired materials for a PbNb(Zr, Sn, Ti)03 ceramic. A fired and electroded doctor-bladed ceramic strip is shown in figure 2.14.

2.2c Hot Pressing

In the hot-pressing process the ceramic is sintered under pressure, typically developed in a hydraulic press. Hot pressing can result in ceramic densities

Figure 2.7 Tickness of ‘green’ and fired materials as a function of blade setting, S, for a doctor-bladed Pb(ZrSnTiNb)03 ceramic (Wentworth and Taylor¹⁶)

even higher than 99.9% of the theoretical maximum. As a resut, such materials have properties approaching those of a single crystal. For example, the hot-pressed ceramic can have a high value of remanent polarisation, a low value of coercive field and, as shown in figure 2.8, can be transparent.

Figure 2.8 Photograph of polished transparent Pb(ZrTiLa)03 hot-pressed ceramic samples. (Haertling and Land¹⁸) Haertling¹⁷,¹⁸ has extensively studied how the parameters of time, pressure, temperature, chemical purity and firing atmosphere affect the properties of hot-pressed Nb, Sn, Bi and La doped and Sn, Ba, and La modified Pb(Zr, Ti)03 ceramics. Figure 2.9 is typical of such results. In this case the effects of time, pressure, and temperature, on grain size of a PbNb(Zr, Sn, Ti)03 ceramic are shown. As discussed in section 15.2a the grain size is important in determining the electro-optic properties of certain ceramic compositions. It also affects many other properties of the ceramic including the permittivity. Other ceramics that have been hot pressed are listed in table 2.2.

Figure 2.9 Effect of hot pressing time, temperature and pressure on average grain diameter of a hot pressed PbNb(ZrSnTi)03 ceramic (Haertling¹⁷)

2.3 Thin Film Fabrication

There has recently been much general interest in thin films of polar materials (i.e. less than about 10 /¿m thick). In particular, for device applications, thin films have important advantages which include (a) formation of large capacitances, (b) low switching voltages, (c) the possibility of forming the film directly on the integrated semiconductor ‘driving’ circuits.

The various techniques used for making thin films are described below. The materials that have been made by these techniques are summarised in table 2.3. With the exception of r.f. sputtering, the fabrication techniques generally

As described in sections 15.2 and 17.4, the properties of the Pb(Zr, Ti)03 ceramics vary greatly depending on whether the amount of additive is less or greater than about 5 atom per cent. As a result it is convenient to use the term doped Pb(ZrTi)03 when the additive is less than 5 atom per cent and the term modified Pb(ZrTi)03 when the additive is greater than 5 atom per cent.

produce polycrystalline films, which have properties more similar to ceramics than to single crystals.

2.3a Solution Deposition

Three types of solution deposition have been used for forming thin films. They are casting, hydrolysis and electrophoresis.

Casting

Beerman¹⁹ has made thin films of TGS (a water soluble polar material) by spraying an aqueous solution of TGS onto a suitable substrate. Chapman²⁰ has formed thin films of the complex ferroelectric Pb(BiLaFeNbZr)03 by first making a colloidal suspension, or slurry, of the basic oxides of the composition. The suspension was then centrifuged onto a metallic substrate and sintered at 900°C to form the ferroelectric thin film.

Hydrolysis

Lure et al.²¹ have deposited a mixture of Pb, Zr, Sn and Ti oxides on a metallic substrate by hydrolysing a solution of Pb, Zr, Sn and Ti tetrachlorides. The oxides were then sintered to form a ferroelectric film of Pb(ZrSnTi)03.

Electrophoresis

Lamb et al.²² placed two noble metal electrodes into a suspension of BaTi03 particles in ether. The application of about 200 V cm —¹ between the electrodes caused a film to be formed on the anode. Subsequent sintering in an atmosphere of 98 % helium and 2 % oxygen at 1350°C created a stable BaTi03 film.

2.3b Melting

Nolta et al²³ have shown that a thin layer of potassium nitrate, KN03, can be easily formed by melting KN03 powder onto a metal substrate. However, KN03 has the disadvantage that it is only ferroelectric at room temperature and atmospheric pressure for a short time, before reverting to a non- ferroelectric phase²⁴. Sodium nitrite, NaN02, and barium titanate, BaTi03 films have also been prepared by melting.

2.3c Vacuum Deposition

Evaporation and sputtering techniques have been used for the vacuum deposition of thin films.

Evaporation

In his early work Feldman²⁵ evaporated BaTiOa from a coated tungsten filament onto a metallic substrate in a vacuum of less than 5 x 10'⁵ mm Hg. Due to the difference in volatility of the constituent oxides, the resultant film consisted of separated layers of BaO and Ti02. To combine the oxides the film had to be subsequently heated in air at 1100°C.

Flash evaporation onto a heated substrate is a technique developed by Muller et al²⁶ and Burfoot et al.²¹ which overcomes the dissociation problem. The polar material is evaporated in small-thickness increments, typically corresponding to a few crystal lattice spacings, by dropping the source material, a grain at a time, onto a filament heated to a temperature of about 2000°C. One variation which has proved successful in improving the quality of the evaporated films has been to leak a small amount of oxygen into the vacuum chamber. This improved stoichiometry by overcoming oxygen deficiencies. Another successful technique has been the use of multiple evaporation sources. For example, in forming BaTi03, a source of BaO and a source of Ti02 are used. The multiple sources can reduce the amount of filament material in the film.

In addition to BaTi03, thin films of lead titanate, PbTi03, and bismuth titanate, Bi4Ti3012, have been evaporated. Films evaporated onto metal substrates such as platinum are polycrystalline in nature. An epitaxial film of BaTi03 with its c-axis uniformly aligned perpendicular to the substrate can be evaporated if the substrate is a freshly cleaved alkali halide crystal such as LiF or NaF. Burfoot²⁷ has developed a technique for removing the film from the insulating substrate, so that an electrode can be placed on the thin film for electrical measurements.

Sputtering

Francombe²⁸ has suggested that sputtering offers several advantages over evaporation, namely (a) better control of stoichiometry, especially for the more complex oxide materials, (b) better thickness control and (c) freedom from material contamination. Most sputtering has been r.f. sputtering from ceramic targets. However, diode, triode and tetrode sputtering has also been used. The polar materials which have been sputtered are listed in table 2.3. Two of these materials, BaTi03 and Bi4Ti3012, have also been sputtered epitaxially.

It is instructive to summarise the sputtering technique developed for monoclinic Bi4Ti3012, since the problems which occur are rather typical, and the results obtained have been the most impressive. Takei et al.²⁹ used a

10 cm² ceramic target of bismuth titanate mounted on a water-cooled metallic base and positioned 4 cm from a heated substrate. Using a 4 mm atmosphere of 02 and Ar they were able to sputter Bi4Ti3012 films at a rate of about 1 A s " ¹ using a power level of 1 W cm ‘² and a self bias of 700 V.

The problem in vacuum deposition, as with single crystal and ceramic fabrication, is to develop the correct stoichiometry so that only the required phase is formed. In the case of sputtering Bi4Ti3012, this is done by using a

Figure 2.10 Dependence of composition on substrate temperature for films r.f. sputtered from a target of composition 80% Bi4Ti3012, 20% Bi12Ti020. (Francombe²⁸)

ceramic target of 0.8 Bi4Ti3012,0.2 Bij 2TiO20 and a substrate temperature of 650°C. The bismuth enriched target compensates for the high volatility of that oxide, while as can be seen from figure 2.10, the chosen substrate temperature favours Bi4Ti3012 over the other phases.

The Bi4Ti3012 film can be sputtered epitaxially and aligned along any preferred direction by choosing a substrate having a suitable crystal lattice. Substrates which have been used are Pt, single crystal Bi4Ti3012 and MgO and MgAl204. Except for a higher coercive field, the electrical and optical properties of the epitaxial films closely match those of the bulk (single crystal) material; see, for example, figure 2.11. The most dramatic evidence of the quality of the sputtered films of Bi4Ti3012 is their ability, as shown in figure 2.12, to duplicate the complex electro-optic light valve switching properties of the single crystal material described in section 15.2a.

2.4 Fabrication of Polar Glasses

Borrelli, Herczog and colleagues³¹,³² at Corning have succeeded in crystallising NaNb03 and (NaK)Nb03 in glass matrices of Si02, and BaTi03 and SrTi03 in a matrix of BaAl2Si2Og. Also, Isard and his colleagues³³,³⁴ at Sheffield have made glasses of PbTi03 and (PbBa) Ti03 in B203 matrices and (K Ta)Nb03 in a Si02 matrix. These polar glasses are made by the rapid quench cooling of the appropriate molten oxides, followed by suitable annealing treatment to crystallise the polar material in the low permittivity,

Figure 2.11 Birefringence values and extinction angle of bismuth titanate as a function of temperature measured at wavelength 5890 A. The solid lines are from single crystal, and the dashed lines from film. The subscripts a, b, and c indicate the monoclinic axis along which light was directed for each measurement. (Wu et

al.³⁰)

Figure 2.12 A sputtered bismuth titanate film viewed between crossed polarisers in (a) off’ condition (b) ‘on’ condition (Wu et al.³⁰). Microphotographs of an interdigital electrode array.

high resistivity glass