Figure 1. a) Two freestanding FE layers. The initial polarization in layers 1 and 2 are P0,1 and P0,2, respectively. b) A bilayer constructed by joining the layers in a), sandwiched between metallic top and bottom electrodes, c) a heteroepitaxial bilayer made up of the bilayer in b) on a thick substrate. Due to interlayer coupling the polarization in layer 1 (P1) is less than its uncoupled value P0,1 and the polarization in layer 2 (P2) is larger than P0,2, d) theoretical polarization in an unconstrained and heteroepitaxial BaTiO3-SrTiO3 (BT-ST) bilayer, and e) anomalous dielectric response of an unconstrained and heteroepitaxial BT-ST bilayer. This behavior has the characteristics of a “phase transformation.” The relative thicknesses of the layers can be used as a design parameter for bilayer, multilayers, and superlattices with exceptional dielectric properties. Another important practical conclusion is that a heterostructure consisting of a ferroelectric sandwiched between two dielectrics and then subsequently sandwiched between electrodes would have a polarization and dielectric response superior to a ferroelectric between electrodes and this configuration would also eliminate leakage losses.

CAREER: Ferroelectric Multilayers, Superlattices, and Compositionally Graded Films DMR-0132918

PI: S. Pamir Alpay Department of Materials Science and Engineering, University of Connecticut

h1

Layer 2

Layer 1 P0,1

P0,2Layer 2

Layer 1 P0,1

P0,2h2 Layer 2

Layer 1 P0,1

P0,2

h

h2

h1 P1<P0,1

P2>P0,2Layer 2

Layer 1 P1<P0,1

P2>P0,2Layer 2

Layer 1 P1 P0,1

P2> P0,2

Electrode

Electrode

Layer 2

Layer 1 <

0 20 40 60 80 1000

1

2

3

4

5

6

Rel

ativ

e m

ean

die

lect

ric

con

stan

t (x

103 )

Unconstrained Heteroepitaxial

Volume fraction of ST, ST

(%)

0 20 40 60 80 1000

10

20

30

40

Po

lari

zati

on

in B

T la

yer

PB

T (C

/cm

2 )

Volume fraction of ST, ST

(%)

Unconstrained Heteroepitaxial

h

h2

h1 P1<P0,1

P2>P0,2Layer 2

Layer 1 P1<P0,1

P2>P0,2Layer 2

Layer 1 P1 P0,1

P2> P0,2

Electrode

Electrode

Layer 2

Layer 1 <

Substrate

(c)

(a)

(e)

(b)

(d)

h1

Layer 2

Layer 1 P0,1

P0,2Layer 2

Layer 1 P0,1

P0,2h2 Layer 2

Layer 1 P0,1

P0,2

h

h2

h1 P1<P0,1

P2>P0,2Layer 2

Layer 1 P1<P0,1

P2>P0,2Layer 2

Layer 1 P1 P0,1

P2> P0,2

Electrode

Electrode

Layer 2

Layer 1 <

0 20 40 60 80 1000

1

2

3

4

5

6

Rel

ativ

e m

ean

die

lect

ric

con

stan

t (x

103 )

Unconstrained Heteroepitaxial

Volume fraction of ST, ST

(%)

0 20 40 60 80 1000

10

20

30

40

Po

lari

zati

on

in B

T la

yer

PB

T (C

/cm

2 )

Volume fraction of ST, ST

(%)

Unconstrained Heteroepitaxial

h

h2

h1 P1<P0,1

P2>P0,2Layer 2

Layer 1 P1<P0,1

P2>P0,2Layer 2

Layer 1 P1 P0,1

P2> P0,2

Electrode

Electrode

Layer 2

Layer 1 <

Substrate

(c)

(a)

(e)

(b)

(d)

When a ferroelectric (FE) and a paraelectric (PE) layer are brought together, there exists a critical PE layer thickness at which the bilayer has a gigantic dielectric response. Determination of this critical layer fraction allows us to design heterostructures that can store large amounts of charge. This capability is particularly useful in computer memories and other electronic applications where a large dielectric constant is required.

Education:During the past three years, 8 undergraduate students (Mr. Dean Halter, Mr. Scott Virkler, Mr. Michael White, Ms. Sofia Iddir, Ms. Elisabeth Jordan, Mr. Robin Young, Mr. Kevin Rankin, and Ms. Zoe Weber) and 5 graduate students (Mr. G. Akcay, Dr. (Mr.) Z.-G. Ban, A. Sharma, Mr. I.B. Misirlioglu, and Mr. S. Zhong) contributed to this program.

Dr. Ban received his Ph.D. in 2003 and was awarded the Outstanding Graduate Student Award of the School of Engineering in 2004. He is currently employed by KennaMetal Inc., PA as a research scientist after a brief stay at Argonne National Labs. Mr. Sharma graduated with an M.S. degree in April 2004 and is pursuing a career in business management. Mr. Zhong graduated with an M.S. degree in July 2005 and is continuing his work towards a PhD degree.

Outreach:A popular science web site on the science and technology of thin film materials was developed and uploaded thanks to the efforts of Mr. Halter, Mr. White, Mr. Virkler, and Ms. Weber. They have worked not only on the design of the web pages but on the content as well.

CAREER: Ferroelectric Multilayers, Superlattices, and Compositionally Graded Films DMR-0132918

PI: S. Pamir Alpay Department of Materials Science and Engineering, University of Connecticut