University of Illinois

Non-linear Electrodynamic Responseof Dielectric Materials

microwave applications (radar, etc)phase shifterstuned filtersvoltage controlled oscillators

optical applications (wdm, etc)amplitude modulatorsphase modulatorsfrequency shifters

these things are cm longbetter to be microns long!

PowerPoint Presentation by: Professor Jim EcksteinDepartment of Electrical and Computer Engineering

University of Illinois

What gives rise to non-linear response?

Saturation of ionic and electronic polarization

In materials with inversion symmetry (2) is zeroamorphous materialsvaporssimple cubic crystals

In materials lacking inversion symmetry (2) is nonzeroferroelectrics

poled organic films (with non-centrosymmetric molecules)

....)( )3()2()1( lkjijklkjijkjijoi EEEEEEP

P

E

Dielectric polarization of a ferroelectric, which has broken inversion symmetry.

02

2)2(

Ee dE

Pd

Easy to see that this is not zero in materials with broken symmetric response.

But, two solutions or branches.(2) diverges as T Tc

University of Illinois

P0

P

E

For non-linear optical and other field tuning applications would prefer a characteristic like this.

Stable, single solutioncan’t de-pole

Permanently polarized dielectric with big (2)

Use molecular nanostructuring to make such a material (MBE)(once you figure out what matters!)

University of Illinois

What you would like for non-linear modulators, etc…

Artificial structuresusing ALL-MBE to synthesize materials and heterostructures not found in nature

• broken symmetry throughout film favors one polarization• permits stable operation nearer to Curie temperature• obtain larger response at zero bias

Grow crystal using ferroelectric and related phases:stack with structurally broken c-axis inversion symmetry

lattice property, e.g. strain, Tc

favo

red

po

lariz

atio

n

unfa

vore

d po

lariz

atio

n

sup

erc

ell

Controlling material properties via epitaxial strain

Producing new “materials” by modulated heterostructure growth

Tensile strain-induced magnetic anisotropy in magnetic oxide

-20

-10

0

10

20

X

-20

-10

0

10

20

Y

-5

0

5

Z

-20

-10

0

10

20

X

-5

0

5

Z

anisotropy energy surface(La0.7Ca0.3MnO3 on SrTiO3 substrate)

University of Illinois

University of Illinois

Introducing the actors , perovskite titanate phases

BaTiO3

ferroelectric (order-disorder, weakly 1st order)

SrTiO3

non ferroelectric, but would like to bewell known substrate

CaTiO3

ferroelectricity, what’s that??

4.0 A

3.9 A

3.8 A

Combine these in single crystal heterostructures to investigate the effects of compositional (strain) symmetry breaking

Inversion symmetry?CTOSTOBTO

222+ NO

STOBTO

CTO222- NO

CTO STO

BTO1212 YES

CTOSTO

BTO422+ NO

CTOSTO

BTO622+ NO

supe

rcel

l nan

ostr

uctu

re (e

ach

rect

angl

e is

one

mon

olay

er)

Atomic Layer-by-Layer Molecular Beam Epitaxy

OzonegeneratorO

xyg

en

Pump

Ca

Sr

Ba

Al

La

Y

Ti

Mn

Cu

Bi

rotating substrate positioner

quartzcrystalmonitor

RHEED

electrongun

hollowcathode

lamp

photomultipliertube quadrupole

massspectrometer

turbopump

loadlock

shutters

hollowcathode

lamp

photomultipliertube

substrateholder

• atomic absorption spectroscopy for feedback control

• ozone oxidation

• in-situ RHEED with digital video

• We have control over the source fluxes to better than 1% accuracy (AA, RHEED)

ozonestill

RHEED reveals surface crystal structure

1.0 34.0 67.0 100.0 133.0 time (s)

Intensity

a

a

aa a a

RHEED images at different points of the

super cell growth

Start of Super Cell

CTO Surface

End of Super Cell

CTO SurfaceAfter 1 ML BTO After 1 ML STO

After 0.5 ML STO

Specular SpotOscillation from

1 Super Cell

1 ML BTO

2 ML BTO

1 ML STO

2 ML STO

1 ML CTO

2 ML CTO

Growth and Processing of Capacitor Devices

SubstrateSTO or NGO

Dielectric superlattice

LSMO base electrode

LSMO top electrodeIn-situ Au

In-situ AuTop LSMO

Ex-situ Au

Dielectric superlattice

Base LSMO

SiO2

Few 100 m

University of Illinois

400

500

600

700

800

900

1000

-100 -50 0 50 100

1765 (1212) Device A13' vs E Field (kV/cm)

' at 25K' at 100K' at 200K' at 300K' at 340K

'

E Field (kV/cm)

600

700

800

900

1000

1100

1200

1300

-100 -50 0 50 100

1737 (222+) Device B12' vs E Field (kV/cm)

' at 100K

' at 200K

' at 280K

' at 340K

'

E Field (kV/cm)

500

600

700

800

900

1000

-100 -50 0 50 100

1763 (222-) Device C1 ' vs E Field (kV/cm)

' at 100 K' at 160 K' at 280 K' at 340 K

'

E Field (kV/cm)

0

200

400

600

800

1000

1200

1400

-600 -400 -200 0 200 400 600

1737 (222+) Device B12' vs E Field (kV/cm)

' at 100K

' at 200K

' at 280K

' at 340K

'

E Field (kV/cm)

(E;T) for 222+

0

200

400

600

800

1000

-600 -400 -200 0 200 400 600

1765 (1212) Device A13' vs E Field (kV/cm)

' at 25K' at 100K' at 200K' at 300K' at 340K

'

E Field (kV/cm)

(E;T) for 1212

0

200

400

600

800

1000

-600 -400 -200 0 200 400 600

1763 (222-) Device C1 ' vs E Field (kV/cm)

' at 100 K' at 160 K' at 280 K' at 340 K

'

E Field (kV/cm)

(E;T) for 222-

University of Illinois

Now, add more BaTiO3

this has large at higher T in bulk

0

100

200

300

400

500

600

700

-600 -400 -200 0 200 400 600

1755 (422+) Device E8' vs E Field (kV/cm)

' at 20K' at 120K' at 180K' at 260K' at 370K

'

E Field (kV/cm)

422+ shows large range of linear response at zero bias

from peak in estimate Pno hysteresis, single solution

200

300

400

500

600

700

800

900

1000

-600 -400 -200 0 200 400 600

1766 (622+) Device B14 ' vs E Field (kV/cm)

' at 100K

' at 180K

' at 280K

' at 360K

'

E Field (kV/cm)

622+ shows larger linear response at zero bias

from peak in estimate Pno hysteresis, single solution

Both show temperature indepen-dent effect from 180 to 360 K

P

E

P0

Measure

Obtain P0 by integrating dE

Psat should be the same size at + and – infinite bias field

EdE

dPer 11 )1(

EdEEdEP rr 0

0

0 2

1

20 11.0m

CP

University of Illinois

Obtain permanently polarized material with tunable dielectric constant