R. Ramesh

Departments of Materials Science & Engineering and Physics University of California, Berkeley

Materials Science Division, Lawrence Berkeley National Laboratory

Background

Epitaxial heterostructures as model systems Role of defects

Reliability and Yield

A huge range of oxide crystals : pyrochlores, layered structures,

spinels, rock salt, …

Complex Oxides : Many Possibilities

A-site (La) Oxygen

B-site

(Mn)

eg

t2g

charge

spin

orbital

lattice

• Superconductors (YBCO)

• Ferroelectrics (BaTiO3 )

• Colossal Magnetoresistance ((La,Sr)MnO3)

• Multiferroics (BiFeO3)

• Topological Insulators (Y2Ir2O7)

• Thermoelectrics (doped SrTiO3)

• Ferromagnets (SrRuO3)

• Photovoltaics (copper oxides)

Energy Conversion/

Transduction

Field Tunable Photonic

Bandgap Structures

Information Storage

Radiation Sensing

Energy Storage

I. Interface-mediated

functionality

spin charge

lattice orbital

II. Functional interfaces

BiFeO3

Rhombohedral, R3c

ahex = 5.58 Å; chex = 13.86 Å

a = 3.96Å, ar = 0.6°

(a) (b)

(c)

MFe2

[111]

MFe1

M

(d)

G-type

Antiferromagnetic

Order

Weak

Ferromagnetic

Moment

Bismuth Ferrite, BiFeO3: Model Multiferroic

Mn3+ -- O2- -- Mn4+

--- what’s the coupling mechanism at the interface ?

--- Role of Orbital physics ?

--- can we use an electric field to control this coupling ?

Fe3+ -- O2- -- Mn3+

Fe3+ - -- O2- -- Fe3+

Fe3+ -- O2- -- Mn4+

Bi Bi O

O O Fe

O

Fe

Mn Mn O O

La Sr O O

???

Interfaces III: Artificial Interfaces

Creating and Understanding Interfaces

MBE

e.g., Schlom Group

• Highly controlled growth

• Extremely high structural

quality

Oxygen

gas

Laser-in

RHEED

screen

Target

carousel

Substrate

holder &

manipulator

Ion

source

MSRI

reflectron

DRS RHEED, TOF-ISARS

• Highly controlled growth

• Interface chemistry

Laser-MBE

• Highly controlled

growth

• Controlled interfaces

Phosphor screen

+ CCD camera

Heater with substrate

Target holder

Laserbeam

Electron gun

2-stage differentially pumped

to main turbo pump

Atomic Control of Oxide Heterostructures

J. Huijben,…, D. Blank, Univ. of Twente

2 μm

AFM:SrTiO3

Termination Engineering: B site termination

Sr O O O Sr Sr O Sr

Ti O O O Ti Ti O Ti

Ti O O O Ti Ti O Ti

La O O O Sr La O Sr

Mn O O O Mn Mn O Mn

La O O O Sr La O Sr

Mn O O O Mn Mn O Mn

All Oxide Interfaces : BFO/LSMO

Termination Engineering: A site termination

Sr O O O Sr Sr O Sr

Ti O O O Ti Ti O Ti

Ti O O O Ti Ti O Ti

Sr

O

Sr

O

Ru

O

O

Ru

O

O

Ru

O

Sr

O

Sr

Sr

O

Sr

O

Ru

O

Sr

O

Sr

O

Sr

Ru

O

Ru

O

Ru

O

O

Sr

O

Sr

Ru

O

O

Sr

La O O O Sr La O Sr

Mn O O O Mn Mn O Mn

La O O O Sr La O Sr

Mn O O O Mn Mn O Mn

Time Of Flight-Mass Spectroscopy of

Recoil Ion (TOF-MSRI)

Controlling Surface Termination

MSRI results for difference terminations Probing Surface Termination

BiO Interface La0.7Sr0.3O Interface

Atomic Structure of interfaces

SRO

La0.7Sr0.3O Interface BiO Interface

Structure and Composition of Interfaces (STEM-EELS)

Interface Control of Bulk Ferroelectric Polarization

MnO2 interface

La0.7Sr0.3O interface

Out of plane PFM image

P. Yu et al., under review(2011)

+0.15 e

-0.15 e

Interface Termination Controls Bulk properties

BiO interface La0.7Sr0.3O interface

Experimental Probe of Interface Induced Potential Step.

Internal field: shift of piezoresponse hysteresis loop;

Interface induced electrostatic potential step ~ difference

between internal fields ~ 1.2 Volts.

Vint=0.66 V Vint=-0.55 V

Interface Termination Controls Bulk properties

0

20

40

60

0 2 4 6 8 10 12 14 16

-1.0

-0.5

0.0

0.5

1.0

P(

C/c

m 2)

Po

ten

tia

l (V

)

Supercell Index

P Right

P Left

Average

LSMO LSMO BFO

MnO2 LSO

First-Principle Calculations

Collaboration with Dr. Luo, W. D., Prof. Pennycook, S. J. and Prof. Pantelides, S.T. at ORNL.

Interface induced electrostatic

potential step ～1.3V

Interface Termination Controls Bulk properties

Exchange bias comparison of different interfaces

-800 -400 0 400 800-600

-400

-200

0

200

400

600

Mag

ne

tization

(em

u/c

c)

Magnetic Field (Oe)

5 nm LSMO + 75 nm BFO

FC 1T

FC -1T

5 nm LSMO

FC 1T

FC -1T

@10K

Bias Field ~ 40 Oe BiO Interface

O Fe O Fe

Bi Bi O O

O Mn O Mn

-1500 -1000 -500 0 500 1000 1500-600

-400

-200

0

200

400

600

Ma

gn

etiza

tio

n (

em

u/c

c)

Magnetic Field (Oe)

5 nm LSMO + 75nm BFO

FC 1T

FC -1T

5 nm LSMO

FC 1T

FC -1T

@10K

Bias Field ~ 200 Oe La0.7Sr0.3O Interface

O Fe O Fe

La Sr O O

O Mn O Mn

Exchange coupling changes with Interface termination!!

Probing Exchange Coupling with XMCD

Psat

PR

P

-VC VC V

Psat = Saturation polarization

PR = Remanent polarization

VC = Coercive voltage

Memory Parameter DRAM FeRAM

Min Max Min Max

Supply Voltage 3.15 V 3.45 V 3.15 V 3.45 V

Low Power Standby - - - 10 μW

Operating Active Current - 25 mA - 25 mA

Operating Temperature 0°C 70°C 0°C 70°C

Storage Temperature -55°C 125°C -55°C 125°C

Non-Volatile Data Storage - - 0°C 70°C

Read Cycle Time 50 ns - 50 ns -

Address Access Time - 26 ns - 26 ns

Read Cycles Per Byte >1015 - >1013 -

Write Cycle Time 50 ns - 50 ns -

Non-Volatile Data Retention - - - 10 yrs.

Write Cycles Per Byte >1015 - >1013 -

FeRAMs: Solving Technology Challenges through Science

Scott & Araujo, Science 246, 1400 (1989)

Imprint

Fatigue

New Approach –

LSCO/PNZT/LSCO

Old Process –

Fatigue is an issue

100 102 104 106 108 1010 1012

Fatigue Cycles

150

100

50

0

-50

-100

-150

Re

mn

an

t P

ola

rizati

on

μC

/cm

2

100 102 104 106 108 1010 1012

Fatigue Cycles

150

100

50

0

-50

-100

-150

Re

mn

an

t P

ola

rizati

on

μC

/cm

2

Binary Metallic Oxides

• IrO2

• RuO2

• PdO2

• OsO2

• ReO3

Metallic Perovskites

• (La,Sr)CoO3

• SrRuO3

• (La,Sr)MnO3

• YBa2Cu3O7

• Bi2Sr2CaCu2O8

• LaNiO3

LSCO

LSCO

PNZT/PLZT

100 102 104 106 108 1010 1012

30

20

10

0

-10

-20

-30

Epitaxial

Epitaxial

Oriented

Polycrystalline

Fatigue Cycles

Po

lari

zati

on

– Δ

P (

μC

/cm

2)

Oriented

Polycrystalline

Ramesh, et al., Science 252, 944 (1991)

Ramesh, et al., APL 61, 1537 (1992)

Eom, et al., Science 258, 1766 (1992)

Ramesh, et al., APL 63, 3592 (1993)

Kingon, et al., JMR 9, 2968 (1994)

Basic Science Solves Applied Problems

University of California, Berkeley Turnbull Lecture November 27, 2007 21

-15 -10 -5 0 5 10 15

40

30

20

10

0

-10

-20

-30

-40

Voltage (V)

Po

lari

zati

on

(μ

C/c

m2)

-12 -8 -4 0 4 8 12

30

20

10

0

-10

-20

-30

Voltage (V) P

ola

rizati

on

(μ

C/c

m2)

Pt/PZT/Pt LSCO/PZT/LSCO

Oxide Electrodes Solve the Imprint (Internal Field) Problem

Oxide Electrodes: Eliminate Imprint

Pike, et al., APL 66, 484 (1995) & Warren, et al., APL 67, 866 (1995)

60 70 80 9030

35

40

45

50

55

60

65

70

75

Substrate temperature : 650 oC

Vaporizer temperature : 190 oC

Bi/(B

i+F

e)

ato

mic

rati

o in

film

(%

)

Bi/(Bi+Fe) solution mixing ratio (%)

(I)

20 25 30 35 40 45 5010

0

101

102

103

104

105

STO

SRO

BFO

Inte

nsit

y (

arb

. u

nit

)

2 (degree)

(I)

(II)

20 25 30 35 40 45 5010

0

101

102

103

104

105

Fe2O

3

Inte

nsit

y (

arb

. u

nit

)

2 (degree)

(II)

(III)

20 25 30 35 40 45 5010

0

101

102

103

104

105

Bi2O

3

Inte

nsit

y (

arb

. u

nit

)

2 (degree)

(III)

Processing Issues in CVD: Role of Composition

Composition effect

Topography (33 m2)

Out-of-plane piezoresponse

Fe-rich Bi-rich

Need for careful composition control!!

SRO

BFO

High quality epitaxial BFO films

20 30 40 50 60

100

1k

10k

100kS

TO

(001)

BF

O (

001)

ST

O (

002)

BF

O (

002)

Inte

ns

ity

(c

ps

)

2 (deg.)

BFO

Si

STO

SRO

Possible conduction mechanisms

Schottky emission

Space charge limited conduction

Poole-Frenkel emission

Fowler-Nordheim

tunneling BFO

BFO

R E

E

0 1 2 3 4 5 6 7 8 9 101E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

Le

ak

ag

e [

A]

Voltage [V]

Positive

Negative

Leakage

Poole-Frenkel Emission : Fe3+ Fe2+ + h+

Schottky Emission

Electron Hopping : From Fe2+ to Fe3+

Space-Charge-Limited Conduction

0 2 4 6 8 10

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

d=240nm

A=8E-6cm2

Le

ak

ag

e C

urr

en

t [A

]

Applied Voltage [V]

20C (Positive) 20C(Negative)

40C 40C

60C 60C

80C 80C

100C 100C

Critical Issue #2 How to reduce leakage ?

Understand leakage mechanisms

Chemical doping

Area: 8e-6cm2

Identifying the leakage mechanism

0.5 1.0 1.5 2.0 2.5 3.0 3.51E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

V1/2

Positive

Negative

Co

nd

uc

tiv

ity

[

-1c

m-1

]

K=13

n=3.6

Poole Frenkel Plot

0.5 1.0 1.5 2.0 2.5 3.0 3.5

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.01 Positive

Negative

J/T

2 [

A/m

2K

2]

V1/2

K=1.6

K=4.3

n=2.07

Schottky Plot

The extracted dielectric constant is too high, about twice that of the expected 6.25-6.5

Poole-Frenkel

The extracted dielectric constant is too low for the negative direction, but not far off for higher fields

Schottky

Log-log plots (not shown) do not follow the expected trend

Space Charge Limited

Need more thickness, T and E dependent measurements