complex oxide tunnel junctionscomplex oxide tunnel junctions
TRANSCRIPT
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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