B O L E T Í N DE LA S O C I E D A D E S P A Ñ O L A DE

Cerámica y Vidrio A R T I C U L O

• • •

Ferroelectric bismuth layer oxide ceramic materials and thin films. Processing, characterization and properties

J.P. MERCURIO

Laboratoire de Matériaux Céramiques et Traitements de Surface, ESA 601 5, Université de Limoges, Faculté des Sciences 123 Avenue Albert-Thomas, 87060 Limoges Cedex, France

An up to date review of processing, characterization and main properties of tLie ferroelectric complex bismuth oxides belonging to the so-called Aurivillius phases jfamily is given. Processing aspects involved powder preparation by solid state reaction of oxi­des, in molten salts synthesis and sol-gel route. Bulk ceramic materials could be fabricated either by conventional sintering or more specifically by hot forging. The sol-gel technique, which allows to easily prepare thin films with controlled homogeneity, thickness and composition, is briefly described and applied to these materials. An overview of the principal dielectric properties is presented and focussed on the ferroelectric and piezoelectric characteristics of bulk ceramics. Some properties of ferroelectric thin films, which are very good candidates for the fabrication of ferroelectric memories, are given. Future applications of these materials are set out.

Key words : Ferroelectric bistmuth layer oxides, thin films, ferroelectric properties

Materiales cerámicos y láminas delgadas f erroeléctricas de óxidos de bismuto laminados. Procesamiento, caracterización y pro­piedades

Se revisa el procesamiento, caracterización y propiedades principales de los materiales ferroeléctricos derivados de óxidos de bis­muto complejos pertenecientes a la familia denoniinada Aurivillius. Los aspectos de procesado incluyen la preparación de polvos vía reacción en estado sólido, síntesis de sales fundidas y ruta sol-gel. Los materiales cerámicos volumétricos pueden fabricarse bien por sinterización convencional o bien de forma más específica por forjado en caliente. La técnica de sol-gel, que permite la pre­paración de láminas delgadas de homogeneidad, espesor y composición controladas, se describe brevemente y se aplica a estos materiales. Se presenta una revisión de las principales propiedades dieléctricas enfocada sobre las características ferroeléctricas y piezoeléctricas de los materiales cerámicos volumétricos. Se presentan también algunas propiedades de las cerámicas delgadas ferroeléctricas que son buenas candidatas para la fabricación de memorias dieléctricas.

Palabras clave : Óxidos de bismuto laminados ferroeléctricos, láminas delgadas, propiedades ferroeléctricas.

1. INTRODUCTION

The family of the so-called Aurivillius phases (1-3) is generally formulated as 612 111-1 m^3m+3^ ^^ more conve­niently (BÍ202)(AJ^_^B^03J^^-^) since the phases are built up by the regular intergrowth of (BÍ202) "^ layers and perovski-te (Aj^.^B^Og^,^^)^" slabs where A is a combination of cations adequate for 1 ¿-coordinated interstices such as Na+, K+, Ca^+, Sr^+, Pb^+, Ln^" , Bi^+, etc., B is a combination of cations well suited to octahedral coordination, like Fe^" , Cr^+, Ti ^ , Zr^+, Nb^+, Ta^^, Mo^+, W^+, etc., and m is an integer which corresponds to the number of two-dimensional sheets of corner-sharing octahedra forming the perovskite-like slabs. Many compounds belonging to this family were synthesized by Smolenskii et al. (4) and Subbarao (5, 6). To date, more than 80 compounds have been reported, including a consi­derable number of ferroelectrics. They correspond to m values ranging from 1 to 5. At room temperature, their sym­metry is mostly orthorhombic (pseudo tetragonal a ~ b ~

V2a ^ 0.54 nm; c == 0.826(m+l) nm. The mostly investiga-

1), ted compounds of this group are BÍ2WO^ (m SrBÍ2Nb209 (m - 2) and BÍ4TÍ30^2 ^^ = ^•

The frequent occurrence of microsyntactic (disordered) intergrowths suggests the possibility of formation, in selected systems submitted to an appropriate thermal treatment, of recurrent (ordered) intergrowths. The structure of the associa­ted compounds known as mixed Aurivillius phases is built by a regular intergrowth of one half the unit cell of an m member superstructure and one half the unit cell of an (m+1) member superstructure (7). They have the formula

iBip,)(A^_,BJD,^^,) + (Bip^XA. BÍ4A„,H-m'

m'-l Bm'Osm-.l)

or, with only m and m+1 intergrowths and given n = (m+m'-l)/2

^^4^2n-1^2n+1^6n+9

136 Bol. Soc. Esp. Cerám. Vidrio, 37 [2-3] 136-142 (1998)

FERROELECTRIC BISMUTH LAYER OXIDE CERAMIC MATERIALS AND THIN FILMS. PROCESSING, CHARACTERIZATION AND PROPERTIES

Typical examples of these mixed compounds are BÍ7TÍ4Nb022 (m = 2, m' = 3, n = 2) and Bi^M^^JiyO^y (m = 3, m^ = 4, n = 3).

The aim of this paper is to give an up to date review of pro­cessing, characterization and selected properties of these fasci­nating compounds as ceramic and thin films materials.

2. POWDER PREPARATION

2.1. Solid state reactions

Polycrystalline compounds are generally prepared by solid state reactions at high temperature of the corresponding oxi­des or carbonates. The conventional processes include calcina­tion at 800-900°C and dwelling at 1100-1150°C for 5-lOh.

2.2. Molten salts synthesis

This method is based on the solubility of oxides in low mel­ting media such as alkali chlorides, sulfates , nitrates,etc.

In general, stoichiometric mixtures of oxides (or carbonates) are mixed with e.g. NaCl/KCl eutectic composition, heated up to 650°C and then soaked at 800-850°C for 1 h in platinum cru­cibles and finally cooled down to room temperature. After washing to eliminate salt residues, strongly anisotropic platelet-like powders are obtained. As shown in Fig. 1 for SrBÍ2Nb209, they develop large faces with areas in the range 1-1 Opm^ and thickness between 100 and 500 nm depending on temperature and soaking time. X-ray diffraction patterns con­firmed that the direction perpendicular to the main faces correspond to the stacking direction of the BÍ2O2 sheets.

We will see later that such anisotropic powders are particu­larly able to prepare strongly oriented ceramic materials.

750°C/30miii thickness : 120 nm

2 fim

800^C/30iiiin thickness : 150 nm

5 fim

850^C/30min thickness : 430 nm

Fig. 1. SrBz2^^2^9 powder prepared by molten salt synthesis

0.2 M Bi(OC3H40CH3)3 0.15 M Ti(OC2H40CH3)4

0.15 M Sr(C7HisCOO)2 2 ethylhexanoic acid, 120°C

CH3OCH2CH2OH 0.3 M Bi(C7Hi5COO)3

H2O / CH3OCH2CH2OH 0.3 M Nb(OEt)5 in E t O H

B¡4TÍ30t2 o r SrBÍ2Nb209 p recu r so r solutions

Drying

Crystallization

BÍ4TÍ3O12 o r SrBÍ2Nb209 powder

Fig. 2. Flow chart of the sol-gel process

Dried xerogel 100°C

Calcined xerogel 100°C

Solvent: 2-methoxyethanol

Solvent: 2-methoxyethanol/water

Fig. 3. Bi¿rifi-^2 xsrogel powders

2.3. Sol-gel synthesis

The chemical routes are more and more used to prepare homogeneous fine grain materials at relatively low tempera­ture. For the Aurivillius phases. Fig. 2 shows a typical flow chart of the sol-gel process leading to Bi^Yi^O-^2 ^^^ SrBÍ2Nb209 powders. The morphologies of the powders strongly depend on several experimental parameters (dilu­tion, nature of solvent, hydrolysis,etc.) As an example. Fig. 3 shows how a xerogel powder of BÍ4TÍ30- 2 changes according

Boletín de la Sociedad Española de Cerámica y Vidrio. Vol. 37 Núms. 2-3 Marzo-Junio 1998 137

J-P. MERCURIO

to the nature of the solvent. When pure 2-methoxyethanol is used, the powders obtained after calcination of the xerogel at 700°C for 2h are made of small individual spheres (100-300 nm in diameter), whereas the use of 2-methoxyethanol/water mixture leads larger, often coalesced spheres {l-7]im) of agglo­merates grains fo 200-400 nm (8).

phase BÍ7TÍ4Nb022 and its hysteresis loop at room temperatu­re. The high temperature maximum of the permittivity corres­pond to the ferro-para transition (Curie point) whereas the secondary maximum (at lower temperature) was assigned to a ferro-ferro transition by change of the space group (9).

3. CERAMIC FABRICATION AND PROPERTIES

3.1. Conventional sintering

This is the mostly used technique of fabrication of ceramic materials. For AuriviUius phases 95% dense materials are currently obtained by firing uniaxially or isostatically pressed pellets at 1150-1120°C for 1-3 h in air. Such ceramics are almost isotropic so that the dielectric properties (permittivity, hyste­resis loop) can be measured in any direction without signifi­cant change. As an example Fig. 4 shows the thermal varia­tions of the dielectric permittivity of the mixed AuriviUius

2500

00

B

2000 -

rO CD SO

11 ? o 100 kHz

rO CD SO

11 ? a 500 kHz 8 ^

1500 - A

X

1 MHz

2MHz

3MHz

-4MHz

1000 - _5MHz

500 -

0 - H 1 \ 1

200 400 600 800 1000

Temperature (°C )

-18 -12 -6 0 6 12

Applied Field ( kV/mm )

18

Fig. 4. Thermal variations of the dielectric permittivity and hysteresis loop at room temperature of the mixed AuriviUius phase BiyTi^Nb022

3.2. Hot-forging

The hot-forging technique consists in a uniaxial pressing at high temperature of precompacted powders without dye follo­wing a schedule as indicated in Fig. 5 where pressure, tempe­rature and time are average values used for Aurivülius com­pounds. After cooling, strongly oriented almost fully dense materials are obtained, especially when platelet-like powders (prepared by molten salt synthesis) are used. Fig. 6 shows X-ray diffraction patterns of BÍ7TÍ4Nb02^ recorded along direc­tions perpendicular (a) and parallel (b) to the forging axis com­pared to a powder pattern. In (a) only the (00/) lines are present whereas in (c), they have completely disappeared so that the orientation factor reached 97-98% for 99% dense materials.

The anisotropy of hot-forged ceramics is easily observed by scanning electron microscopy and permittivity measurements along and normal to the forging direction as shown in Fig. 7. The SEM micrograph reveals preferentially oriented grains (0.5vimx5vim) and the dielectric permittivity presents the two anomalies observed before on isotropic materials. Nevertheless, the double anomaly is drastically reduced for paraUel measurements. The reason is that the polarization vec­tor is located in the planes (a,b) perpendicular to the stacking

Mineral wo(

Boron nitride

Alumina

Fig. 5. Hot-forging : schematic device and typical thermal and pressure sche­dule

138 Boletín de la Sociedad Española de Cerámica y Vidrio. Vol. 37 Núms. 2-3 Marzo-Junio 1998

FERROELECTRIC BISMUTH LAYER OXIDE CERAMIC MATERIALS AND THIN FILMS. PROCESSING, CHARACTERIZATION AND PROPERTIES

a. O

8000 X

4 0 0 0

0 H-IC

CM a : Hot-forged -*-

1 '' O

U3

C3

\

es *— en 5 o

° o ° < 1*. il i

o

o

ce en

« r- on

X = en a _ ^ ^ U, .

20 3 0 4 0 50 60 70

O. O

1 b : Powder | 1200 - .,

8 0 0 I D

C3 CO

-^.ta t —

O es " ^ 4 0 0 -

CD O es 1 j

o CoJ

C ^ T—

C3

Od CM

to , -en '"•

0 - I ^ " / w v ^ LwVy,^.,..,.,^]

10 20 3 0 4 0 50 60 70

to

O

162

122

8 2

4 2

c ; Hot-forged / / I

iiTfWn'nf«''

rr> to t— en cvj •*>. »N«. C O

If^A/^^ 10 20 30 40 50 60

2- T h e t a

Fig. 6. X-ray diffraction pattern of hot-forged BiyTi^Nb022

1600.

J^

£(1)

J^ — 1 — J^

1200. BiyTijNbOai 1

— 1 — J^

SCO.

4 0 0 .

— 1 — J^ 0 . 1 ^ — 1 — J^ 200 .-• 400 600 .800

Tenperati« CC) , '

i I Fig. 7. Dielectric permittivity of hot-forged BiyTiJSib02i

lOUU •

g Ä £(-L)

t 1 -1

"oj 1 2 0 0 -CaBÍ8TÍ7027

g Ä £(-L)

t 1 -1

1 800 ' B

Pi 400-

g Ä £(-L)

t 1 -1 0 - I H , — H -

g Ä £(-L)

t 1 -1

2000

. 1600 +

I 1200

1 800 +

400 +

1500

200 400 600

Temperature (*»C)

800

200 400 600

Temperature ( C)

800

SrBÍ8TÍ7027

200 400 600 800

Température {^C)

1000

13UU •

^ ^ E(//)

— \ 1

^^ 1000-PbB¡aT¡7027

^ ^ E(//)

— \ 1

i

t 500-

0 -

H 1

^ ^ E(//)

— \ 1 1000

1000

400 600 800 1000

Temperature (*C)

Fig. 8. Dielectric permittivity of hot-forged M^^BigTiyO2 j

direction of the crystal structure and there would be no com­ponent along the c-axis as previously shown by single crystal studies (10). The residues observed in the figure would indi­cate that the material is not fully oriented.

Similar results were obtained for M^^BigTÍ7027hot-forged materials (Fig. 8).

Boletín de la Sociedad Española de Cerámica y V idr io . Vo l . 37 Núms. 2-3 Marzo-Junio 1998 139

1000 T

u 800 +

5 600 a v <U P^

fí (D 400 H <D

•a 3 u 200

••—MBi2Nb209

-ir~Bi4Ti3012

-3K—M2BÍ4TÍ5018

H—Bi3TiTa09

MBÍ8TÍ7027

-MBi2Ta209

-MBÍ4TÍ4015

-Bi3TiNb09

-Bi7Ti4Nb021

+ + + + 0 1 2 3 4 5

M: l - C a , 2 = Sr ,3 -Pb ,4 = Ba

Fig. 9. Curie temperatures of some Aurivillius phases (After G.A. Smolenskii, Ferroelectrics and related materials, Gordon & Breach Science Pub., New York, 1984)

3.3. Aurivillius phases as high Tc ferroelectrics and piezoe­lectric materials

TABLE I

DIELECTRIC AND ELECTROMECHANICAL CHARACTERISTICS OF SOME

AURIVILLIUS CERAMICS

Material Sr

(IkHz) 10-3 Tan ô

(IkHz) Te ¿33

(pCN-1) (%)

BÍ4TÍ3O12 [ref, 11]

130 3 675 18 3

BÍ7TÍ4Nb02i [réf. 11]

150 5 830 12 3

CaBÍ8TÍ7027 [réf. 11]

140 5 720 8 3

SrBÍ8TÍ7027 [réf. 11]

130 5 580 8 3

PbBÍ8TÍ7027 [réf. 11]

180 5 630 7 2

BaBÍ8TÍ7027 [réf. 11]

160 5 500 8 3

Pz45™ [réf. 12]

165 4 500 16 4

Pz46™ [réf. 12]

120 4 650 18 3

Like perovskites, as already shown in the introduction, the Aurivillius phases form a very large family, the properties of each individual compound depending on the composition. So, the Curie temperature can be easily tailored by adjusting the values of m and the nature of the involved cations. Figure 9 shows the Curie temperatures of some selected ceramics belonging to this family. In contrast with the PZTs, they range between 100 and 930°C, and many of them have Tes higher than 600°C. This is the case for CaBÍ2Ta209, CaBÍ2Nb209,

BÍ4Ti0^2' CaBigTÍ7027, CaBÍ4TÍ40^5, BÍ7TÍ4Nb02p BÍ3TiTa09 and BÍ3TiNb09.

As a consequence, such materials can be used as high wor­king temperature piezoelectric materials. Table I gives dielec­tric and electromechanical characteristics of some Aurivillius ceramics. Up to now, these materials are the only ones to be used at temperatures close to 500°C (11,12).

4. THIN FILM FABRICATION AND PROPERTIES

In the past 30 years, ferroelectric thin films have been deeply investigated for several electrical and optical applications (capacitors, transducers, surface acoustic wave devices, infra­red detectors, e tc . ) , due to the trend toward miniaturization of electronic components, the replacement of expensive single crystals, and finally new areas of applications like non volati­le random access memories. Many techniques of thin film fabrication have been used including sputtering, evaporation, chemical vapor deposition and more recently pulsed laser ablation and sol-gel dip- or spin-coating. They were carried out on a variety of substrates : silicon wafers, sapphire, mag­nesia, spinel and some perovskite single crystals.

In the following, only sol-gel spin coating of bismuth laye­red materials onto Pt-coated Si and (001) SrTiOg single crystals will be presented.

0.2 M Bi(OC3H40CH3)3 0.15 M Ti(OC2H40CH3)4

CH3OCH2CH2OH

H2O / CH3OCH2CH2OH

0.15 M SríCyHisCOO)^ 2 ethylhexanoic acid, 120°C

0.3 M Bi(C7Hi5COO)3

0.3 M Nb(OEt)5 in E t O H

B¡4TÍ30,2 or SrB¡2Nb209 precursor solutions

Spin-coating : a = 400 rpm/s, v = 4000rpm, t = 30s

Drying : 300°C - 10 min

Crystallization : 700°C - 2 h - air

BÍ4TÍ3O12 or SrBÍ2Nb209 thin film

Fig. 10. Flow chart of the sol-gel spin-coating process

4.1. Spin-on coating of Aurivillius phases

The preparation of the precursor solutions is identical to those used for powders. The flow chart of a typical thin film process is given in Fig. 10 for BÍ4TiO- 2 and SrBÍ2Nb209. These average deposition parameters - which apply to every Aurivillius compounds - lead to dense, homogeneous crack-free thin films (13).

140 Boletín de la Sociedad Española de Cerámica y Vidr io. Vo l . 37 Núms. 2-3 Marzo-Junio 1998

FERROELECTRIC BISMUTH LAYER OXIDE CERAMIC MATERIALS AND THIN FILMS. PROCESSING, CHARACTERIZATION AND PROPERTIES

O O

o

o o o

a.

O

o

20 30 40 50 29 0

60 70

30 290"^

Fig. 11. Crystallization of sol-gel derived SrBÍ2Nb20g thin films

4.2. Crystallization and morphology

Crystallization of pure SrBÍ2Nb209 occurs at about 550°C (Fig. 11a). Fig. l i b shows that the films deposited on Si/Si02/Ti02/Pt substrates are generally ramdomly oriented whereas those deposited on (001) SrTiOg are strongly oriented along the (001) planes. Atomic force microscopy observations clearly show that the grain size increases from 50-80 nm for one coating to 150-170 nm for 10 successive coatings.

4.3. Electrical characteristics

Thin films of Aurivillius compounds are potentially good candidates for ferroelectric memories due to their low fatigue against switching, in particular when in contact with conducti­ve oxide electrodes such as RUO2 or SrRu03. As an example. Fig. 12 shows the hysteresis loop and variation of the remanent polarization up to 10^^ switching cycles of a SrBÍ2Nb209 sol-gel spin coated capacitors designed with Pt bottom electrode and gold top electrode. The remanent polarization (2.5 pC cm'^) remains unchanged up to 10^ cycles. This behaviour is now currently enhanced up to 10^^ cycles.

5| 4 3 2I 1 01

-1 -21 -3 -4 -5 I

-6

1 « 1 « 1 " T

h Polarization (^C cm" 1 1 1 1 1 n

F H

[ 1 1 1 1 -

Electric Field (V) j

. 1 . 1 1

-4 -2

-2

BJíarizátíai iiC^)

-0-00-0

-p-00-0'

mé\,\—I I I mill—I I mud—LJ-UlJli— ul I I IIIUll l l l l l l l¿ i » m J i n m J • • • • • • • • I • •

1tf 1(f 10P 1(f 10'° l^tttifercf cydes

Fig. 12. Hysteresis loop and fatigue behaviour of a sol-gel derived SrBi^NhjDg thin film

5. CONCLUSIONS

The Aurivillius phases are currently used for the fabrication of high working temperature ferroelectric ceramics for piezo­electric devices. They are now under investigation as thin films for ferroelectric random access memories.

Would be there a short or mid-term future for these com­pounds? Sure enough!

(1) Crystal structures: they are generally well resolved for both single and mixed layered phases, but the cation ordering in modified compounds would have to be refined.

(2) Ferroelectric ceramic materials: the quality of materials with highest Curie temperatures needs improvements.

(3) Thin films: further investigations are in progress con­cerning the dielectric-electrode {e.g. conductive oxide) inter­faces.

(4) Non-stoichiometric compounds : ionic conductive mate­rials can be prepared for applications either as electrolytes for fuel cells or as catalysts by possible reversible oxygen loss and uptake (14,15).

Boletín de la Sociedad Española de Cerámica y Vidrio. Vol. 37 Núms. 2-3 Marzo-Junio 1998 141

J.P. MERCURIO

ACKNOWLEDGEMENTS

Thanks are due to Dr. R. Maalal, Dr. F. Soares-de Carvalho, Dr. M. Manier for experiments and to Dr. P. Thomas and Prof. D. Mercurio for valuable discussions as well as to Dr. W.W. Wolny (Ferroperm A/S) for giving materials characteristics. •

REFERENCES

1. B. Aurivillius, «Mixed Bi oxides with layer lattices. I. Structure of CaCb2BÍ209», Ark. Kemi., 1, 463-80 (1949).

2. B. Aurivillius, «Mixed Bi oxides with layer lattices. II. Structure of BÍ4TÍ30^2^^ Ark. Kemi., 1, 499-512 (1949).

3. B. Aurivillius, Mixed Bi oxides with layer lattices. III. Structure of BaBiJ ip^^ , Ark. Kemi., 2, 519-27 (1950).

4 . G.A. Smolenskii, V.A. Isupov and A.I. Agranovskaya, «Seignettoelectrics of the octahedral type with a layer structure», Fiz. Tverd. Tela, Leningrad, 3, (1961)895-901.

5. E.G. Subbarao, «Crystal chemistry of mixed bismuth oxides with layer type structure», J. Am. Ceram. Soc, 45,166-9 (1962).

6. E.G. Subbarao, «A family of ferroelectric bismuth compounds», J. Phys. Ghem. Solids, 23, 665-76 (1962).

7. T. Kikuchi, A. Watanabe and K. Uchida, «A family of mixed-layer type bis­muth compounds». Mat. Res. Bull., 12, 299-306 (1977).

8. F. Soares-de Garvalho, «Elaboration de couches minces ferroélectriques de tita-nate de bismuth 614X130 2 ^ ^^^ Thesis, Université de Limoges, France, (1996).

9. D. Mercurio, R. Maalal, G. TroUiard and J.P. Mercurio, «Grystal structure of the ferroelectric mixed Aurivillius phase BÍ7TÍ4Nb022», Proc. Electroceramics V, Aveiro, 1, 589-94 (1996).

10. R. Maalal, R. Von Der Mühll, G. TroUiard and J.P. Mercurio, «Electrical and optical properties of mixed Aurivillius phase BÍ7TÍ4Nb02| single crystal», J. Phys. Ghem. Solids, 57(12), 1957-62 (1996).

11. R. Maalal, «Propriétés diélectriques et structurales de la phase d'AuriviUius mixte BÍ7TÍ4Nb02i», PhD Thesis, Université de Limoges, France, (1995) .

12. W.W. Wolny, private communication. 13. J.H. Yi, P. Thomas, M. Manier, J.P. Mercurio, I. Jauberteau and B. Frit,

«SrBÍ2Nb209 ferroelectric thin films prepared by sol-gel spin-coating». Key Engineering Materials, 132-36,1135-38 (1997).

14. A. Gastro, P. Millan, M.J. Martinez-Lope and J.B. Torrance, «Substitution for Bi3+ into (BÍ202)^+ layers of the Aurivillius (BÍ202)(A n-lB^03^^;^) oxides», Sohd State Ionics, 63-65 (1-4) 897-901 (1993).

15. K.R. KendaU, J.K. Thomas and H.-G. zur Loye, Synthesis and ionic conductivity of a new series of modified Aurivillius phases, Ghem. Mater., 7 (1), 50-57 (1995).

• • •

142 Boletín de la Sociedad Española de Cerámica y Vidrio. Vol. 37 Núms. 2-3 Marzo-Junio 1998