I. Ferroelectric materials for piezoelectric, pyroelectric, and memory applications II. Nanoscale ferroelectrics

ISIF-2005 Shanghai, April 17, 2005

Marin AlexeMax Planck Institute of Microstructure PhysicsHalle – Germany

Outline

Introduction Basics of FerroelectricityFerroelectric materials for– Piezoelectric applications– Non-volatile memories– Pyroelectric applications

Multi-ferroicsNanoscale ferroelectrics

Outline

Textbook– Ferroelectric crystals, F. Jona and G. Shirane, Pergamon, Oxford 1962– Principles and Applications of Ferroelectric Crystals …, M.E. Lines and A.M. Glass,

Clarendon, Oxford 1977– Ferroelectric memories, J. F. Scott, Berlin, Springer, 2000– Ferroelectric devices, K. Uchino, Dekker, 2000– Nanoelectronics and Information Technology, R. Waser (ed). Wiley-VCH, 2003

Edited books– Thin film ferroelectric materials and devices, R. Ramesh (ed.), Kluwer, 1997– Ferroelectric thin films : synthesis and basic properties, Paz de Araujo, Scott, and Taylor,

Gordon and Breach, 1996– Nanoscale phenomena in ferroelectric thin films, S. Hong (ed), Kluwer, 2004– Nanoscale characterisation of ferroelectric materials : scanning probe microscopy

approach, Alexe and Gruverman (eds), Springer, 2004Databases

– Landolt-Börnstein, vol. 16, Ferroelectric and related substances, ed. E. Nakamura, Springer, 1981

Review papers– The physics of ferroelectric ceramic thin films for memory applications, J.F. Scott,

Ferroelectrics Review 1, 1 (1998)– Ferroelectric, dielectric and piezoelectric properties of ferroelectric thin films and

ceramics, D. Damianovici, Rep. Prog. Phys. (61)1267 (1998)– Physics of this-film ferroelectric oxides, M. Dawber et al., Rev Mod. Phys., in press– M. Fiebig, Revival of the magnetoelectric effect, J. Phys. D, 38 (2005) R123 – etc.

Introduction

Mark Twain

"There are three kinds of lies: lies, damned lies and statistics.“

Introduction

02000400060008000

10000120001400016000

1970-1975

1981-1985

1991-1995

2001-2005

Papers onFerroelectrics

INSPEC database

020000400006000080000

100000120000140000160000

1970-1975

1981-1985

1991-1995

2001-2005

Papers onSemiconductors

0

0,02

0,04

0,06

0,08

0,1

0,12

1970-1975

1981-1985

1991-1995

2001-2005

Ferro/Semicon

02468

101214161820

1970-1975

1981-1985

1991-1995

2001-2005

Nature & Science

Ferroelectrics

Basics of ferroelectricity

Piezolectrics:– Charge generation by mechanical fields

Pyroelectrics:– Charge generation by thermal fields

Ferroelectrics:– Charge generation by electrical fields

What are they good for?

And converse !

Basics of …

What are they good for?Piezoelectrics– Mechanical strain (stress) ⇔ Electric field (charge)

E

Basics of …

What are they good for?Piezoelectrics– Mechanical strain (stress) ⇔ Electric field (charge)

Stress X

+Q

-Q

P

D= deffX

Di= dijkXjk

D – charge densitydeff – effective piezoelectric coeff.X – stress

Basics of …

What are they good for?Pyroelectrics– Thermal variation ⇔ Charge generation

∆T

+Q

-Q

P

∆Q=p∆T

Ferroelectrics– Switching by electrical field ⇔ Charge generation

Basics of …

P

E -6 -4 -2 0 2 4 6

-60

-40

-20

0

20

40

60 P

V

P-E Characteristics (hysteresis loop)

What are they good for?

∆Q=2Pr

Applications

The physics of ferroelectric memories, Auciello O, Scott JF, Ramesh R., Physics Today 51, p22, July 1998

Ferroelectricity – symmetry-based phenomenon

Electrostrictive32 classes

No symmetry centre21 classes

Piezoelectric20 classes

Non-piezoelectric1 class

Pyroelectric10 classes

Non-Pyroelectric10 class

symmetry centre11 classes

Ferroelectric

Ion shift in the perovkite cell

Ferroelectric Materials

Main ferroelectric oxides

Pb-based materials - Pb(Zr,Ti)O3

Layered perovkites – SrBi2Ta2O9, Bi4Ti3O12

BaTiO3-based materials – (Ba,Sr)TiO3

There are >500 ferroelectric compounds (without solid-solutions)– Landolt-Bornstein, Ferro- and Antiferroelectric Substances, Springer, 1975

For most demanding applications only oxides are seriously consideredChoosing the optimum material is an application-dependent problem

Ferroelectric Materials

Ferroelectric materials for piezoelectric applications

Ferroelectric materials for piezoelectrics

Direct piezoelectric effect:

D= deffX

Converse piezoelectric effect

x= deffE

D – charge densitydeff – effective piezoelectric coeff.X – stressx - strain

Material property: piezoelectric coefficient (third-rank tensor, dijk)

Electrostrictive effect – quadratic effect, present for all materials

xij=QijklPkPl

Di= dijk Xjk

Ferroelectric materials for piezoelectrics

Relation between piezoelectric coefficient and polarization:

dim=εikQmikPk

For particular case of tetragonal symmetry:

d33=2ε33Q33P3d31=2ε33Q13P3d15=2ε11Q44P3

ε - dielectric permittivitym –index in the matrix notation

Relationship between dzz an Pz

PZTtetragonal

BaTiO3

Relationship between dzz an Pz

PZTrhombohedral

Ferroelectric materials for piezoelectrics

Figures of merit:

x=dEE=gX

d – actuator figure of meritg – sensor figure of merit

For polycrystalline materials depends on the sample symmetry:

Pd33

d31

Electromechanical coupling factor k

k2=Stored mechanical (electrical) energy/Stored electrical (mechanical) energy

g=d/ε

k2=d2/(εs) s – elastic stiffness

Ferroelectric materials for piezoelectric appl. Maximum d in systems with morphotropic phase boundary (MPB)

Pb(Zr,Ti)O3

Du et al., APL 72, 2421(1998)Single-crystal d-values

Damianovici,Rep.Prog.Phys.Ceramic d-values

6 possible orientations8 possible orientations

Ferroelectric materials for piezoelectric appl. Monoclinic phase at MPB

Noheda et al, Phys. Rev. B, 63 (2000) 014103

- not a sharp boundary between tetragonal and rhombohedral phases

-An additional monoclinic phase might exists

-There are three different phases with similar free energies and the polarizationcan rotate easily among different directions*

* Fu and Cohen, Nature 403 (2000) 281

Ferroelectric materials for piezoelectric appl. MPB are present in many systems:

Pb(Zn1/3Nb2/3)O3-PbTiO3 – PZN-PT Pb(Mg1/3Nb2/3)O3-PbTiO3 – PMN-PT

Ferroelectric materials for piezoelectric appl. Lead-free materials (K0.5Na0.5)1-xLix)(Nb1-yTay)O3

Saito et al., Nature 432 (2004) 84

Pyroelectrics

by Paul Muralt, Swiss Federal Institute of TechnologyLausanne

PbTiO3 based thin films deposited on silicon substrates

Relative dielectric constant

Pyro

elec

tric

coef

ficie

nt (

Cm-

2 K-1

)

300

100

200

400

500

600

0100 200 300 400 500 6000

PT

15/85 25/75 30/70

10

PZT

15

PLT

20

10

10

PCT

Substitutions by:

Zr (PZT)La (PLT)Ca (PCT)

Various literature data

20/80

Figure of merit

∝ p/ ε

p

ε

Intruder alarm

absorption layer black Pt

top electrode Cr/Au

pyroelectric film PbTiO 3 (PT)

membrane Si3N4/SiO2

SiO 2

Design layout (large geometry

Si

exaggerated vertical scale1 mm

1 mm

Pyroelectric thin film detectorpackaged into a TO-39 housing

30 m max. Bell 1994, Kohli 1997

IR MicrosystemsSwitzerland

Linear pyroelectric arraysApplication in infrared gas spectroscopy

Willing, Kohli, Muralt et. al. 1995-1999Willing. Kohli, SeifertSince 2000

MINAST

grating

lamp

Working thin film focal plane array for thermal imaging

ROIC

PYROEL.

mirror

semi-transparent top electrodes

transparent bottom electrode

metal plug

Cortesy Raytheon Infrared Imaging SystemsNETD about 100 mK in 2004.

Hansen and Beratan et al., 2000-2004

SignalContact

BottomElectrode

Ferroelectric

TopElectrode

CommonContact

ROIC

(Cross S

(Raytheon)

Ferroelectric Materials

Materials for ferroelectric memories

Ferroelectric Materials

Useful property: charge by switching

Ferroelectric Materials

Intrinsic requirements– Polarization ↑– Switching speed ↑– Coercive Field ↓– Retention ↓– Fatigue ↓– Imprint ↓

Extrinsic requirements– Processing temperature ↓– CMOS compatibility ↑– Availability ↑– Cost ↓– ETC…..

Ferroelectric Materials

Basic Phenomena in ferroelectrics – Switching

Switching → nucleation-driven

D.J. Jung et al. Integrated Ferroelectrics 48 (2002) 59

Switching models

Ishibashi-Orihara based on Kolmogov-Avrami*

*Orihara et al, J. Phys. Soc. Jpn. 63 (1994) 1031

- Inhomogeneous nucleation with fixed rate; it occurs at fixed nucleation sites (usually at the interfaces)

kD

C fE =

Ec – coecive fieldf – frequencyD – dimensionality factork – waveform factor

Switching models

nucleation-limited-switching Tagantsev model*

*Tagantsev et al., Phys Rev. B 66 (2002) 214109

i. The film is presented as an ensemble of elementary region.ii. The switching of an elementary region occurs once a domain of reversed

polarization is nucleated in the region.iii. Time needed for switching of an elementary region is equal to the waiting time for

the first nucleation, i.e., the time needed for filling the region with the expanding domain is neglected compared to the waiting time.

iv. The distribution of the waiting times for the ensemble of the elementary regions is smooth and exponentially broad, i.e., covering many decades.

Switching models

Du-Chen model*

*X. Du and I. W. Chen, MRS Proc. 493 (1998) 311D.J. Jung et al., Integrated Feroelectrics 48 (2002) 59

-nucleation process is connected to defects (pinning) -nucleation is thermally activated

2011lnln

CEG

kTff ∆−=

Ec – coecive fieldf – frequency∆G – critical energy

to form a nuclei

⎟⎟⎠

⎞⎜⎜⎝

⎛∆= 20

11expCE

GkT

ττ

Ferroelectric Materials

Fatigue and imprint

Both are defect-driven effects:Switching is hindered by internal fields generated by defects

= domain pinning

virgin

> 106 Cycles

Imprint

virgin

Unipolar Cycles

Ferroelectric Materials

Simple perovskites – ABO3

– Pb(Zr,Ti)O3 – PZT – BaTiO3

Layered perovskites

– SrBi2Ta2O9 – SBT – Bi4Ti3O12 – BiT

A

B

O

TiO6

O

Ti

Bi

TaO6

O

Ta

Bi

Sr

Multiferroics

Multiferroics

Elastic Magnetic

Electric

Ferro ≡ two or more states exists and can be shifted by field

ParaFerroAntiferro

Magneto-elastic

Ferro-elastic Magneto-electric

Multiferroics

Lines and Glass, Principles and applications of ferroelectrics…, Clarendon Press, Oxford, 1977

Multiferroics

Lines and Glass, Principles and applications of ferroelectrics…, Clarendon Press, Oxford, 1977

2. Coupled elastic and magnetic properties – Magnetoelastic:

K2NiF4 : PPP → PFcAc

3. Coupled electric and magnetic properties – Magnetoelectric:

1. Coupled elastic and electric properties – Ferroelastic:

BaTiO3: PPP→FcFcP →FcFcP →FcFcP

RbFeF4 : PPP → PFP → PFA

BaCoF4 : PPP → FPP → FFA

Examples

Multiferroics

*Wang et al., Epitaxial BiFeO3 multiferroic thin film heterostructures, Science (2003) 1719** Erenstein et al Comment on “Epitaxial BiFeO3 Multiferroic Thin Film Heterostructures”, Nature

Example 1 – Ferroelectric and Ferromagnetic*: BiFeO3 – thin epitaxial films

dE/dHMagnetoelectric coefficient

Multiferroics

*Zheng et al., Multiferroic BaTiO3-CoFe2O4 Nanostructures, Science (2004) 661

Extrinsic Magnetoelectric materials*: BaTiO3-CoFe2O4 Nanostructures BaTiO3 Matrix

Multiferroics

*Hur et al., Electric polarization reversal in a multiferroic material induced by magnetic field, Nature, 392 (2004)

Example 2: Intrinsic magneto-electric* TbMn2O5

Switching of polarizationvia magnetic field

II. Nanoscale ferroelectrics

Fabrication of nanosize ferroelectrics

Fabrication of nanosize ferroelectrics

Nanosize Nanosize ferroelectric ferroelectric structuresstructures

Lithography Lithography methodsmethods

d>50 nmd>50 nm

SelfSelf--assembly assembly methodsmethods

d<50 nmd<50 nm

Vapor Vapor depositiondeposition

CSDCSD

MOCVDMOCVD

ee--beambeam

imprintimprint

??

Lithography

Maskless patterning methods

Ion-beam milling

Fabrication of nanosize ferroelectrics

(a)

(b)

(c) (d)

(e) (f)

Ion milling Ion milling -- Ramesh et al. Univ. of MarylandRamesh et al. Univ. of Maryland

Fabrication of nanosize ferroelectricsIon beam millingIon beam milling

NagarajanNagarajan et al.et al.Nature Mat. 2, 43 (2003)Nature Mat. 2, 43 (2003)

1 µm

with awith a--domainsdomains no ano a--domainsdomains

Ferroelectric characterizationSize effects Size effects -- ion milled PZT structuresion milled PZT structures

Nagarajan et al., Nature Materials 2, pp43, 2003

E-beam lithography

Fabrication: e-beam direct writing

1. Metalorganic layer 1. Metalorganic layer depositiondeposition

2. E2. E--beam exposurebeam exposure

4. Crystallization4. Crystallization3. Developing 3. Developing

Fabrication: e-beam direct writing

Patterned test structure

Fabrication: e-beam direct writing

Annealed 650°CAnnealed 650°C

NonNon--annealedannealed

Fabrication: e-beam direct writing

“Large area” uniform patterning

Ferroelectric characterizationSize effects and polarization imprintSize effects and polarization imprint

0 200 400 600 800 1000-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

Offs

etCell size (nm)

ββ δδ

β=(34±3.7) nmδ=(7.5±4.5) nm

12121

12121

2

2

+⎟⎠⎞

⎜⎝⎛ −⎟

⎠⎞

⎜⎝⎛ −

−⎟⎠⎞

⎜⎝⎛ −⎟

⎠⎞

⎜⎝⎛ −

=Ω

hd

hdδβ

δβ

Imprint lithography

Fabrication methodsFerroelectric nanostructures by imprint lithography

Harnagea et al. APL, Sept. 1 2003

PZT structures on PZT structures on SrTiOSrTiO33:Nb:Nb

Array of 300 nm PZT structuresArray of 300 nm PZT structures6 x 6 µm6 x 6 µm

Imprint lithography

Soft lithography

Soft lithographyMicro Contact Printing Micro Molding

George Whitesides at Harvard

Nanosize ferroelectrics fabricated by self-assembly methods

Latex monolayer as mask

Ma et al., Appl. Phys. Lett. 83, 3770 (2003)Ma et al., Appl. Phys. Lett. 83, 3770 (2003)

Self-assembly methods

Island growth mode of MOCVDIsland growth mode of MOCVDM. Shimizu et al.M. Shimizu et al.

Tune the growth conditions to Tune the growth conditions to achieve island growth mode, achieve island growth mode, which allows fabrication of which allows fabrication of nanoscale islands nanoscale islands

Towards single FE grains on the nano scale

T. Schneller, A. Roelofs, RWTH Aachen, ISIF 2000

ApproachSeparation of grains bynon-continuous CSD films

FE film: PbTiO3Route: APP, 0.3 m, 4 coatings, 750 C

R. Waser, et al. Integrated Ferroelectrics, Vol. 36, pp. 3-20 (2001).

A. Seifert, A. Vojta, J.S. Speck, F.F. Lange, J. Mater. Res. 11 (1996) 1470.

Ferroelectric nano-crystals by self-assembly

Microstructural instability in epitaxial ultraMicrostructural instability in epitaxial ultra--thin CSDthin CSD--deposited films deposited films

Nanosize ferroelectrics by self-assembly

selfself--assembled PZT structures obtained by CSDassembled PZT structures obtained by CSD

X-ray diffraction pattern

10 20 30 40 50 60 70 80

In

tens

ity (a

. u.)

2θ (deg.)

100

STO

300

STO20

0S

TO

300

PZT

200

PZT

100

PZT

200

STO

, Kβ

300

STO

, Kβ

Structural investigations - XRD

Annealing temperature

Nanosize ferroelectrics by self-patterning

800oC 950oC 1100oC

1- D ferroelectric systems

Nanowires

Ferroelectric nanowires

H. Park, Harvard UniversityNanoletters 2, 447 (2002)

Ferroelectric nanowires

H. Park, Harvard UniversityH. Park, Harvard UniversityNanolettersNanoletters 2, 447 (2002)2, 447 (2002)

Nano-shell tubes

Piezoelectric nano-shell tubesHigh aspect ratio

Si templateLuo et al. APL 83, 440 (2003)

Ferroelectric nano-tubes

•• Material: PZT & BTOMaterial: PZT & BTO•• Diameter 0.5 to 2 Diameter 0.5 to 2 µµmm•• Wall thickness 20Wall thickness 20--30 nm30 nm•• Length 100Length 100--200 200 µµmm

WaferWafer--scale tube arraysscale tube arrays

Piezoelectric nano-shell tubes

-10 -8 -6 -4 -2 0 2 4 6 8 10-100

-80-60-40-20

020406080

100

2nd measurement 1st measurement

d 33(p

m/V

)

Voltage(V)

Ferroelectric nano-tubes

Ferroelectric nano-tubes

100 1000 100000.1

1

10

0.1

1

10

Si absorbtion

Wav

elen

gth

(µm

)

lattice constant (nm)

Si Al2O3

Ferroelectric nano-tubes4’’ wafer4’’ wafer

Summary

Mark Twain

"It is easier to stay out than get out.“