Forschungszentrum Jülich Institut für Festkörperforschung Jülich, Germany

Complex Oxide Tunnel Junctions

Hermann Kohlstedt

Christian-Albrechts-University Kiel Technical Faculty Nanoelectronics Germany

1

EMRS Strasbourg June 2009 Layer Sequence of a Tunnel Junction

Top electrode Tunnel Barrier (50 nm) (0.5 nm – 3 nm)

Substrate

BttBottom Electrode (50 nm)

2 Electron Tunneling φ Re ik x Ψ x k real B ΨB =Be kx real kx x ikx x imaginary ΨC =C e

x

Re ΨA

−ikx x ΨA = Ae Transmission coefficient ⎪⎧ 2 t ⎪⎫ T =C exp ⎨− 2m∫ φ(x) dx ⎬ ⎪ ⎪ ⎩ h 0 ⎭ Y. Frenkel, Phys. Rev. 36 (1930). A. Sommerfeld and H. Bethe, Handbuch der Physik Springer 1933, XXIV, p.450 3 R. Holm and W. Meissner, Z. Phys. 74, 715 (1932). AIM Active Tunnel Barrier = New Functionalities

Tunnel barrier: vacuum, amouphous dielectrics, Tunneling electrons... epitaxial non-polar dielectrics

Metal Metal

+

1nm – 3nm

...and a ferroelectric barrier.

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

Encouraging Results Materials - good News - Size Effects

Applications and Strain PtiPerspectives

Electron-Ionic Screening Magnetoelectric Termination Interface Effects Electron tunneling

5 Tunnel barriers And Materials electrodes

BaTiO3 PVDF

Ba Polyvinylidene fluoride: [C2H2F2] n

F C Ti +P +P -P H O H c C -P

F

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

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

“1” Pr

Metal ation

P zz BaTiO3

Metal Polari “0” Ec

Electric Field

Remanent polarization: Comparison:

1l1 elect ron /per surf ace at om: 2 15 2 -19 Pr = 10 – 100 µC/cm 10 /cm x 1.6 x 10 C = 160 µC/cm2

strong interface 7 effects expected!! Scaling the Size Effects ferroelectric

2 nm – 3 nm Metal

Ferroelectric Pr

Metal

Pr

x

8 Scaling the Size effects ferroelectric

(stable) 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 ferroelectric

State not useful for applications

J. F. Scott 9 Nanoferroelectrics: statics and dynamics, J. Phys. Condens. Matter 18 (2006). Scaling the Strain ferroelectric

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

Film

Substrate: side view EhEnhancement of fP P poss ible out-of-plane c Sm = (b – a0)/b b b = Substrate lattice parameter a a0 = Equiv. cubic cell constant of free film, Prototypic cell in-plane 10 Sm: Misfit strain Scaling the Strain ferroelectric

In-plane lattice constants: Substrates and Ferroelectrics

BaTiO3

SrTiO3

Darrell Schlom, Annu. Rev. Mater. Res. 2007. 37:589–626 Strain Hysteresis P-V loops

SrTiO3/SrRuO3/BaTiO3/SrRuO3 E (KV/cm) 30 nm BTO E (KV/cm) 56 nm BTO -395 -263 -132 0 132 263 395 -268 -179 -89 0 89 179 268 60 1 KHz@ 300 K 80 40 40 20 ) ) 2 20 40 2 20 A)

0 μ C/cm C/cm

μ 0

μ 0 I ( I (µA)

0 ( P ( -20 PP -20 -20 -40 -40 ssb060427 -40 -40 -60 -80 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 U (V) U (V) Quasi ideal ferroelectric P-V loopp,s, low leakage current

Large value of the spontaneous polarization (40 µC/cm2), Bulk Single Crystal: 26µC/cm2 12 Strong Imprint A. Petraru J. Appl. Phys. 2007 Scaling the Electron and Ionic Screening ferroelectric

Exclusively by electrons

Metal ______⋅ = ⇒ ∫ E ds 0 ED 0 ++++++++++++ + + + + + + + + + + +++++++++++ + + + + + + + + + (only for

P

ED perfect screening!!)

______

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + φ Metal t

P. Würfel and I. P. Batra , Ferroelectrics 12, 55 (1976). ….but there is more! 13 J. Juncquera and Ph. Ghosez, Nature 422, 506 (2003). Scaling the Electron and Ionic Screening ferroelectric

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

14 Theoretically confirmed: G. Gerra et al., PRL (2006). Scaling the Electron and Ionic Screening ferroelectric metal ferroelectric Long rang and short range interactions

Thomas-Fermi screening and Kretschmer-Binder effect

CTF CKB Bond charge compensation by free carriers in the ferroelectric

Extension of the ionic polarization into the metal; Ionic distortion also in the metal Sketch taken from G. Gerra et al., PRL 96. 107603 (2006). Fig.1 15 A. K. Tagantsev et al., PRB B77 (2008) Scaling the Unit-cell Ccale Mapping of Ferroelectricity ferroelectric

Kretschmer-Binder Effect

Today ξ and λ can be measu red! Nature Mat. 6, 64 (2007) (HRTEM) C. Jia et al. Wedgelike BaTiO3 Films to Study Size Effects

SrRuO3 top electrodes (()50 nm)

Tunnel Barrier (0 nm – 5 nm)

BaTiOBaTiO33 BaseElectrode Electrode SrRuO SrRuO3 3

Substrate SrTiO 3

10 mm

Wedge film approach: Borrowed from spintronics

17 A. Petraru et al. APL 93, 072902 (2008) Polarization versus Thickness from I-V Measurements

Ultrathin BaTiO3 films

46 44 T = 77 K 42

) 40 2

mm 38 C/c 36 μ ( r 34 PP 32 30 28 3,5 4,0 4,5 5,0 5,5 Film Thickness (nm) 18 Ultrathin films: IV loops measured at 77 K

t = 5.5 nm BTO

After leackage subtraction

-4 6x10 30 KHz 4x10-4 1 Hz 4x10-4 2x10-4 2x10-4

0 0 ent (A) rent (A)

rr -2x10-4 Cur Cur -2x10-4 -4x10-4

-4 -6x10 -4x10-4

-8x10-4 -1,5 -1,0 -0,5 0,0 0,5 1,0 1,5 -1,5 -1,0 -0,5 0,0 0,5 1,0 1,5 Voltage (V) Voltage (V) Voltage (V)

19 Scaling the Electron and Ionic Screening ferroelectric yy ermittivit pp Inverse

Enhancement of ferroelectricity at metal-oxide interfaces 20 M. Stengel et al., Nature Materials 8, 392 (2009). Magnetoelectric Interface Effect

Fe/BaTiO3

P P Interface between a ferromagnet and a ferroelectric

Top interface

Minority-spin charge density DOS

Bottom interface

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

Top electrode Tunnel Barrier (50 nm) (0.5 nm – 3 nm)

Substrate

BttBottom Electrode (50 nm)

22 Tunneling and ferroelectrics Electron Tunneling

Dielectric Barrier Density of states effects

Superconductor Superconductor Magnet Magnet

∞ 2π e A 2 = − ⋅ − − I(V ) ∫ T(E) n1(E eV ) n2 (E )[]f (E eV ) f (E) dE h −∞

Ferroelectric tunnel junction:

Metal Metal Cooperative phenomenon located in the barrier ! 23 Ferroelectric Barrier A Tunnel Junction is an Interface Device!

Fig. 1

J. P. Velev et al., PRL 98, 137201 (2007). 24 Sensors Applications and Perspectives

Stress dependence of the normalized electro resistance

Y. Zheng and C. H. Woo Giant piezoelectric resistance in Stress Sensor (Theory) ferroelectric tunnel junctions 25 Nanotechnology 20 (2009) 075401. RTD Applications and Perspectives

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

Ferroelectric Resonant Tunneling Diode (Theory) 26 Tunneling and ferroelectrics Electron Tunneling

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

La: BiMnO

4 bit Memory:

2 from Ferroelectricity 2 from Magnetism

More about Multiferroics: N. A. Sppg,aldin and M. Fiebig, Science 309, 391 (2005). W. Eerenstein, N. D. Mathur, J. F. Scott Nature, 442, 759 (2006).

M. Gajek et al., Tunnel junctions with multiferroic barriers Nature Materials 2007 28 Resistive Encouraging Results - Good News - Memory

Giant tunnel electroresistance for No topographic changes after writing!! non-destructivereadout of ferroelectric states V. Garcia et al., Nature Letters – online (2009).

NdGaO3/LSMO/BaTiO3

Giant resistive switching effects observed

BaTiO3 1nm – 3nm

LSMO

NdGaO3

SrTiO3/LSMO/SrTiO3 (()2.4 nm) Supplementary Almost no change in the junction Information: resistance 29 (STO non-ferroelectric) Encouraging Results - Good News -

Giant tunnel electroresistance for > 16 Gbit/inch2 non-destructive readout of ferroelectric states V. Garcia et al., Nature Letters – online (2009). Conductive AFM: writing and reading

BaTiO3 LSMO

NdGaO3

2nm - 3nm Ferroelectric Tunnel Junction FTJ

Similar to the milliped concept: P. Vettiger et al. In Nanoelectronics and Information Technology, Chap. 28, p. 687 30 AFM based Mass Storage – The Millipede Concept Current Developments: Ferroelectric - and Multiferroic Tunnel Junctions

Unit cell mapping Magnetoelectric interface effect HRETM - XRD (Surface magnetization) Reconstruction Spin dependent screening TitifftTermination effects dtdue to pol litiarization (J. M. Rondinelle et al. Nat. Nanotechnology 2008) SrRuO La Sr MnO 3 BaTiO3 x 1-x 3 Transport Electronic Pt BiFeO3 Fe and ionic screening

New Devices: Resistive Memories Size effect of the Ab-iitiinitio TMR-TER Memories ferroelectric Strain sensors Landau Resonant Tunnel Diodes

31 Multiferroic Tunnel Junctions

Pyroelectric Ferroelectric Piezoelectric Die lec tr ic

- - - Anti-ferroelectric Paramagnet + + + (Anti)-FerromagnetPMP, M Multiferroic Superconductor (Insulator) - - - (Tunnel Barrier) + + + Magnetic Anti-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). A. V. Bune et al., Nature 391 (1998). 32 What`s new?

Multiferroic Tunnel Junction

Tunable interfacial properties via ferroelectric polarization by an external electric field.

33 A few Milestones in Electron Tunneling

2010 Experiments and theory on ferroelectric and multiferroic

ss tunnel junctions First all-oxide magnetic tunnel junction

Oxide 1990 Superco nduct in g Hi gh - Tc tunne l ju nct io ns

Magnetic Tunnel Junctions 1970 Magnetic/Superconducting Hybrids, Spin Polarization,

ss Superconducting Tunnel Junctions

(Low Tc superconductors)

Metal 1950

1930 Theory and first experiments: Metal/Barrier/Metal 34 Acknowledgement

A. Petraru,,,pp M. Disch, U. Poppe and R. Waser Institut für Festkörperforschung, Forschungszentrum Jülich Jülich, Germany

N.A. Pertsev A. F. Ioffe Physico-Technical Institute St. Petersburg, Russia

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

M. Hambe, V. Nagarajan University of New South Wales Department of Material Science Sydney, Australia 35 Sponsors

DAAD: University of New South Wales V. Nagarajan , Sydney

DFG: Material Science Network UC Ber ke ley, R. Rames h

36