Forschungszentrum JülichInstitut für FestkörperforschungJülich, Germany

Complex Oxide Tunnel JunctionsComplex Oxide Tunnel Junctions

Hermann Kohlstedt

Christian-Albrechts-University KielChristian-Albrechts-University KielTechnical FacultyNanoelectronics

GermanyGermany

1

EMRS Strasbourg June 2009

Layer Sequence of a Tunnel Junction

Top electrode(50 nm)Tunnel Barrier

(0.5 nm – 3 nm)

Substrate

B ttBottom Electrode(50 nm)

2

Electron Tunneling

real realRe

φxikxeB=ΨBΨ k k xkreal

imaginary

realB eBΨB xk xk xk

xikC

xeC=Ψ

x

Re AΨ

xikA

xeA −=Ψ Transmission coefficient

⎪⎫⎪⎧ t2⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧−= ∫ dxxmCT

t

0

)(22exp φh

3Y. Frenkel, Phys. Rev. 36 (1930). A. Sommerfeld and H. Bethe, Handbuch der Physik Springer 1933, XXIV, p.450R. Holm and W. Meissner, Z. Phys. 74, 715 (1932).

AIMActive Tunnel Barrier = New Functionalities

Tunnel barrier:

Tunneling electrons...

vacuum, amouphous dielectrics,epitaxial non-polar dielectrics

Metal Metal

+

1 3

...and a ferroelectric barrier.

1nm – 3nm

4Kohlstedt, Pertsev, Waser, Ferroelectric Thin Films X, Vol. 688 (Material Research Society) 2002, p. 161.

Outline

MaterialsSi

Encouraging Results - good News - Size

Effects- good News -

StrainApplicationsandP tiPerspectives

Electron-Ionic ScreeningTerminationMagnetoelectricMagnetoelectric

Interface EffectsElectron tunneling

5

Tunnel barriersAndelectrodesMaterials

PVDFBaTiO3

Polyvinylidene fluoride: [C2H2F2] nBa

FC+PTi

H

H+P -P

O

CH

-Pc

FOther barriers: BiFeO3. PbZrxTi1-xO3, PbVO3 etc.

Electrodes: SrRuO3, LaxSr1-xMnO3, Pt, Ir, Au, Fe, Co,... Ferromagnets, Superconductors, etc.

Ferroelectric HysteresispolarizationMaterials

“1” Pr

BaTiO

Metal

P zatio

n

BaTiO3

Metal

P

E

Pol

ariz

Electric Field

Ec“0”

Electric FieldComparison:

1 l t / f t

Remanent polarization:

Pr = 10 – 100 µC/cm21 electron/per surface atom:1015/cm2 x 1.6 x 10-19 C =160 µC/cm2

7strong interface effects expected!!

Size EffectsScaling the ferroelectric

Metal2 nm – 3 nm

Metal

Ferroelectric P

Metal

Pr

Pr

x

8

Size effectsScaling the ferroelectric

Ferroelectricity is a spontaneous electric polarization

(stable)

M Lines & A Glass (1979)

Ferroelectricity is a spontaneous electric polarization of a material that can be reversed by the application of an external electric field.

M. Lines & A. Glass (1979). Principles and applications of ferroelectrics and related materials. Clarendon Press, Oxford.

Not ferroelectricNot ferroelectric

State not useful for applications

9J. F. ScottNanoferroelectrics: statics and dynamics, J. Phys. Condens. Matter 18 (2006).

StrainScaling the ferroelectric

N.A. Pertsev, et al., Phys. Rev. Lett. 80, 1988 (1998)

Film

Substrate:side view E h f P iblside view Enhancement of P possible

Sm = (b – a0)/bc

out-of-plane

Sm (b a0)/b

b = Substrate lattice parametera0 = Equiv. cubic cell constant of

ab

in plane10

free film, Prototypic cell

Sm: Misfit strain

in-plane

StrainScaling the ferroelectric

In-plane lattice constants: Substrates and Ferroelectrics

BaTiO3

SrTiO3

Darrell Schlom, Annu. Rev. Mater. Res. 2007. 37:589–626

Hysteresis P-V loopsStrain

E (KV/cm) E (KV/cm)30 nm BTO 56 nm BTOSrTiO3/SrRuO3/BaTiO3/SrRuO3

40

60-395 -263 -132 0 132 263 395

801 KHz@ 300 K

40

-268 -179 -89 0 89 179 268

20

20

0

20

0

40

I (µA

)

P ( μ

C/c

m2 )

0

20

0

P (μ

C/c

m2 )

I (μ

A)

-60

-40

-20

-80

-40ssb060427 -40

-20

-40

-20

P

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.260 -80

U (V) -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

U (V)

Quasi ideal ferroelectric P-V loops,p ,low leakage current

Large value of the spontaneous polarization (40 µC/cm2),

12Bulk Single Crystal: 26µC/cm2

Strong Imprint A. Petraru J. Appl. Phys. 2007

Scaling the ferroelectricElectron and Ionic Screening

Exclusively by electrons

+ + + + + + + + + + + + + + + + + + + + +

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _Metal

0⇒DE0=⋅∫ dsE+ + + + + + + + + + + + + + + + + + + + +

EDP

(only for perfect screening!!)

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

φMetal

+ + + + + + + + + + + + + + + + + + + + + φ

tt

13P. Würfel and I. P. Batra , Ferroelectrics 12, 55 (1976).J. Juncquera and Ph. Ghosez, Nature 422, 506 (2003).

….but there is more!

Scaling the ferroelectricElectron and Ionic Screening

Fong, et al., Phys. Rev. B 71, 144112 (2005).

14Theoretically confirmed: G. Gerra et al., PRL (2006).

Scaling the ferroelectricElectron and Ionic Screening

metal ferroelectric Long rang and short rangeinteractions

Thomas-Fermi screening andKretschmer-Binder effect

CTF CKB

Bond charge compensation by freecarriers in the ferroelectric

Extension of the ionic polarizationExtension of the ionic polarizationinto the metal; Ionic distortion also in the metal

Sketch taken from G Gerra et al

15

Sketch taken from G. Gerra et al.,PRL 96. 107603 (2006). Fig.1

A. K. Tagantsev et al., PRB B77 (2008)

Unit-cell Ccale Mapping of FerroelectricityScaling the ferroelectric

Kretschmer-Binder Effect

Toda ξ and λ can be meas red!Nature Mat. 6, 64 (2007)C. Jia et al.

Today ξ and λ can be measured!(HRTEM)

Wedgelike BaTiO3 Films to Study Size Effects

SrRuO3 top electrodes(50 nm)( )

Tunnel BarrierTunnel Barrier(0 nm – 5 nm)

Substrate SrTiO3

Electrode SrRuO3

BaTiO3

Substrate SrTiO3

Base Electrode SrRuO3

BaTiO3

Substrate SrTiO3Substrate SrTiO3

10 mm

Wedge film approach: Borrowed from spintronics

17

A. Petraru et al. APL 93, 072902 (2008)

Polarization versus Thickness from I-V MeasurementsUltrathin BaTiO3 films 3

44

46

T = 77 K

38

40

42

m2 )

34

36

38

P r (μC

/cm

30

32

34P

3,5 4,0 4,5 5,0 5,5

28

18Film Thickness (nm)

Ultrathin films: IV loops measured at 77 K

t = 5.5 nm BTO

6x10-4

30 KHz 1 Hz 4x10-4

After leackage subtraction

0

2x10-4

4x10-4

rent

(A)

0

2x10-4

rent

(A)

-6x10-4

-4x10-4

-2x10-4

Cur

-4x10-4

-2x10-4Cur

r

-1,5 -1,0 -0,5 0,0 0,5 1,0 1,5-8x10-4

Voltage (V)

-1,5 -1,0 -0,5 0,0 0,5 1,0 1,5

4x10

Voltage (V)

Voltage (V)

19

Scaling the ferroelectricElectron and Ionic Screening

ype

rmitt

ivity

Inve

rse

p

20Enhancement of ferroelectricity at metal-oxide interfacesM. Stengel et al., Nature Materials 8, 392 (2009).

Magnetoelectric Interface Effect

Fe/BaTiOFe/BaTiO3

Interface between a ferromagnet and a ferroelectric

PPg

Top interface

DO

SMinority-spin charge density

Bottom interface

21C.-G. Duan, S.S. Jaswal and E. Y. Tsymbal,PRL 97, 047201 (2006).

EF

Electron TunnelingTunneling andferroelectrics

Top electrode(50 nm)Tunnel Barrier

(0.5 nm – 3 nm)

Substrate

B ttBottom Electrode(50 nm)

22

Electron TunnelingTunneling andferroelectrics

Dielectric Barrier Density of states effects

Superconductor Superconductor Magnet Magnet

[ ]dEEfeVEfEneVEnETAeVI )()()()()(2)( 21 −−⋅−= ∫∞

h

π 2∫∞−h

Metal Metal

Ferroelectric tunnel junction:

Cooperative phenomenon

23Ferroelectric Barrier

located in the barrier !

A Tunnel Junction is an Interface Device!

Fig 1Fig. 1

J. P. Velev et al., PRL 98 137201 (2007)

24

PRL 98, 137201 (2007).

Applications and PerspectivesSensors

Stress dependence of the normalized electro resistancenormalized electro resistance

Y. Zheng and C. H. Woo

25

gGiant piezoelectric resistance inferroelectric tunnel junctionsNanotechnology 20 (2009) 075401.

Stress Sensor (Theory)

Applications and Perspectives RTD

M. K. Li, et al., Phys. Rev. B 75, 212106 2007

26Ferroelectric Resonant Tunneling Diode (Theory)

Electron TunnelingTunneling andferroelectrics

27M. Y. Zhuralev et al., Phys. Rev. Lett. 96, 107603 (2005).H. Kohlstedt et al., Phys. Rev. B 72, 125341 (2005).E. Y. Tsymbal and H. Kohlstedt, Science 313, 181 (2006).

Multiferroic Tunnel Junctions

La: BiMnO

4 bit Memory:

2 from Ferroelectricity2 from Magnetism

More about Multiferroics:N. A. Spaldin and M. Fiebig,

W. Eerenstein, N. D. Mathur, J. F. Scott

p g,Science 309, 391 (2005).

Nature, 442, 759 (2006).

28M. Gajek et al., Tunnel junctions with multiferroic barriersNature Materials 2007

Encouraging Results - Good News - ResistiveMemory

Giant tunnel electroresistance for non-destructivereadout of ferroelectric statesV. Garcia et al., Nature Letters – online (2009).

No topographic changes after writing!!

V. Garcia et al., Nature Letters online (2009).NdGaO3/LSMO/BaTiO3

Giant resistive switching effects observed

BaTiO31nm – 3nm

NdGaO

LSMO

1nm 3nm

NdGaO3

SrTiO3/LSMO/SrTiO3 (2.4 nm)

29SupplementaryInformation:

3 3 ( )Almost no change in the junctionresistance(STO non-ferroelectric)

Encouraging Results - Good News -

Giant tunnel electroresistance for non-destructive readout of ferroelectric statesV. Garcia et al., Nature Letters – online (2009).

> 16 Gbit/inch2

V. Garcia et al., Nature Letters online (2009).

Conductive AFM:writing and reading

BaTiO3

NdGaO3

LSMO

2nm - 3nm Ferroelectric Tunnel JunctionFTJ

Similar to the milliped concept:P Vettiger et al In

FTJ

30

P. Vettiger et al. InNanoelectronics and Information Technology,Chap. 28, p. 687AFM based Mass Storage – The Millipede Concept

Current Developments: Ferroelectric - and Multiferroic Tunnel JunctionsFerroelectric and Multiferroic Tunnel Junctions

Unit cell mappingHRETM XRDMagnetoelectric interface effect

(Surface magnetization)HRETM - XRD

Reconstruction d t l i tiT i ti ff t due to polarizationTermination effectsSpin dependent screening

(J. M. Rondinelle et al.Nat. Nanotechnology2008)

SrRuO3

Pt

LaxSr1-xMnO3

Fe

BaTiO3BiFeO3

2008)

TransportElectronicand ionic screening

Size effect of the

New Devices:Resistive MemoriesTMR TER MemoriesAb i iti

and ionic screening

Size effect of the ferroelectric

TMR-TER MemoriesStrain sensors

Resonant Tunnel Diodes

Ab-initio

Landau

31

Multiferroic Tunnel Junctions

Ferroelectric

PyroelectricPiezoelectricDi l t i

Paramagnet(Anti)-Ferromagnet

+ + +

- - -Multiferroic

Anti-ferroelectric Dielectric

P MSuperconductor

+ + +

- - -

(Insulator)(Tunnel Barrier)

Magnetic

P, M

MagneticAnti-ferromagnetic

(Do not forget ferroelectric polymers!)

P. A. Dowben et al.,Appl. Surf. Sci. 254 (2008).J Choi et al Chem Phys Lett 410 (2005)

32

J. Choi et al., Chem. Phys. Lett. 410 (2005).A. V. Bune et al., Nature 391 (1998).

What`s new?

Multiferroic Tunnel Junction

Tunable interfacial properties via ferroelectric polarizationby an external electric field.

33

A few Milestones in Electron Tunneling

2010 Experiments and theory on ferroelectric and multiferroictunnel junctionss

1990 Superconducting High - Tc tunnel junctions First all-oxide magnetic tunnel junctiontunnel junctions

Oxi

des

Magnetic Tunnel Junctions

Supe co duct g g c tu e ju ct o s

1970

Superconducting Tunnel Junctions

Magnetic Tunnel Junctions

Magnetic/Superconducting Hybrids, Spin Polarization,

s

1950

Superconducting Tunnel Junctions (Low Tc superconductors)

Met

als

1950

341930 Theory and first experiments: Metal/Barrier/Metal

Acknowledgement

A. Petraru, M. Disch, U. Poppe and R. Waser, , ppInstitut für Festkörperforschung,Forschungszentrum JülichJülich, Germany

N.A. PertsevA. F. Ioffe Physico-Technical InstituteySt. Petersburg, Russia

A. Solbach, U. Klemradt,II. Physikalisches Institut RWTH Aachen Aachen, Germany

M. Hambe, V. NagarajanUniversity of New South WalesDepartment of Material Science

35

pSydney, Australia

Sponsors

DAADDAAD: University of New South WalesV Nagarajan SydneyV. Nagarajan, Sydney

DFG:Material Science NetworkUC B k l R R hUC Berkeley, R. Ramesh

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