Size Effects in Thin Film Ferroelectric Capacitors

Condensed Matter Physics and Material ScienceDepartment of Pure and Applied PhysicsQueen’s University Belfast

Jonny McAneney,

Supervisor: Marty Gregg

Exhibit Spontaneous Polarisation Ps when T < TC (ferroelectric)

Spontaneous Polarisation Ps = 0 when T > TC (paraelectric)

Spontaneous Polarisation direction reversed upon application of E field

PS

T TC

-30

-20

-10

0

10

20

30

-1000 -500 0 500 1000

Ps

Electric Field E / kVcm-1

Ba0.5

Sr0.5

TiO3

Properties of Ferroelectrics

Large Dielectric Constant >1000

Ferroelectric materials used in

Non-Volatile Memoriese.g. SIM Cards,

Visa Cards, NVRAM

Charge Storage Devicese.g. Capacitors, DRAM etc.

IMD

ILD-3

ILD-2

ILD-1

IMD

ILD-3

W

Al-1

Al-2Al-2

Al-1

PolyPlugG-poly

TE ; Ir/IrO 2

BE ; Pt/IrO2Ohmic Barrier ; Ir

FE ; PZT

Ox.-mask

Ferroelectriccapacitor

Plate-line

Strappingline

W Bit-line

Word-line

Power line

Signal line &Landing PAD of Al-1

Signal line

Silicon Sub.

3-Metal, 1T1C 4Mb FRAM

Size Effects

0

50

100

150

200

250

0 50 100 150 200

Die

lect

ric

Con

stan

t

Thickness d / nm

Ba0.7

Sr0.3

TiO3

300 K

Dielectric constant collapses with decreasing thickness

Can be explained by the presence of an Interfacial Capacitance acting in series with the “bulk” dielectric material

Origin of Interfacial Capacitance is NOT knownBasceri et al, J. Appl Phys 82, 2197 (1997)

Capacitor Fabrication

KrF Eximer Laser = 248nm

Heated substrate

Plasma plume

Ceramic target

Pulsed Laser Deposition

Thin Film Capacitors

Au top electrodes

SrRuO3 or (La,Sr)CoO3

MgO substrate

Ba0.5Sr0.5TiO3

Dielectric Constant Measured as function of

Thickness & Microstructure – TEM

Temperature (80 – 400 K)Frequency (102 – 105 Hz)

Capacitors

21

111

CCCT

2

2

1

1

ddd

T

T

d

AC 0

Dielectric d

++ ++ ++

- - - ---C = CapacitanceA = Electrode Aread = Thickness = Dielectric Constant

21

111

CCCT

Capacitors in Series

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 200 400 600 800 1000

d//

nm

Thickness d / nmIn

vers

e ca

pac

itan

ce

d i/

i(n

m)

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 200 400 600 800 1000

d//

nm

Thickness d / nmIn

vers

e ca

pac

itan

ce

d i/

i(n

m)

Series Capacitor Model

Electrode

Electrode

Bulk

Interface

Interface

Electrode

Electrode

Bulk

Interface

Interface

K

dd

bulkeff

1

linterfaciabulkeff CCC

111

1

2

Sinnamon et al. Appl. Phys. Lett. 78, 1724 (2001)

Origin of Interfacial Capacitance?

Some suggestions include:

Schottky Barriers

C. S. Hwang et al. J. Appl. Phys. 85, 287 (1999)

Surface polarisation effects

Zhou et al. J. Appl Phys. 82, 3081 (1997)

Electric field penetration into electrodes

Dawber et al. Ferroelectrics 268, 445 (2002)

Defect Layers

Nakano et al. Jpn. J. Appl. Phys. 36, 3564 (1997)

Previous Work

TEM shows no evidence of a “dead layer” of Finite Thickness

i

di di

b

Au / Ba0.5Sr0.5TiO3 / SrRuO3

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1 10 100 1000

d/

/ nm

Thickness d / nm

1/K = 0.4 ± 0.05 nm

b = 1000 ± 200

400 K

No deviation from Series Capacitor model for d = 7.5nm

Deviation from Series Capacitor model when d < di i.e. no more bulk component

i

i

beff

ddd

Extension of Series Capacitor Model

0

200

400

600

800

1000145 nm260 nm450 nm775 nm975 nm

100 150 200 250 300 350 400

Die

lect

ric

Con

stan

t

Temperature / K

Au / Ba0.5Sr0.5TiO3 / (La,Sr)CoO3

0

50

100

150

200

250

300

350

400

0 200 400 600 800 1000 1200 1400

Die

lect

ric

Con

stan

t

Thickness d / nm

400 K

Results and Modelling

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1 10 100 1000

5nm10nm15nm

d/e

/ nm

Thickness d / nm

di di

i

b

i

i

beff

ddd

Modelled assuming

di = 5, 10, and 15 nm

0.0

1.0

2.0

3.0

4.0

0 200 400 600 800 1000 1200 1400

d/

/ nm

Thickness d /nm

1/K = 0.6 nm

b = 400

400 K

1/K = 0.6 ± 0.1 nm

b = 400 ± 30

Ultra Thin Data

0.0

1.0

2.0

3.0

4.0

1 10 100 1000

d/

/ nm

Thickness d / nm

Thickness of “dead layer” could be di ~ 10 nm

But no distinct layer of this thickness observed

di di

i

b

More Ultra Thin Data Needed

5 nm

Au

LSCO

BST

Characteristics of Interfacial Capacitance

0

500

1000

1500

2000145 nm220 nm340 nm660 nm950 nm

0.03

0.04

0.05

0.06

0.07

0.08

100 150 200 250 300 350 400

145 nm220 nm340 nm660 nm950 nm

Tan

Temperature / K

SrRuO3 / BST (La,Sr)CoO3 / BST

0.00

200.00

400.00

600.00

800.00

1000.00145 nm260 nm450 nm775 nm975 nm

0.00

0.01

0.02

0.03

0.04

0.05

100 150 200 250 300 350 400

145 nm260 nm450 nm775 nm975 nm

tan

Temperature / K

Extraction of Bulk and Interfacial Capacitance

We consider real (') and imaginary ('') components.

Use series capacitor model to extract b', b'', K' and K'' K

dd

beff

1

10 K intervals (80 – 400 K) Frequency (102 –105 Hz)

'

″

tan

Extraction of Bulk and Interfacial Capacitance

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

200 K400 K

0

10

20

30

40

0 200 400 600 800 1000

200 K400 K

d/'

' / n

m

Thickness d / nm

SRO / BST LSCO / BST

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

150 K350 K

0

50

100

150

200

250

0 200 400 600 800 1000 1200 1400

150 K350 K

d/'

' / n

m

Thickness d / nm

Extracted Bulk

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

100 150 200 250 300 350 400 450

100 Hz1 kHz 10 kHz 100 kHz

Temperature / K

0

5000

10000

15000

20000

25000

0

0.05

0.1

0.15

0.2

0.25

100 150 200 250 300 350 400 450

Ext

ract

ed B

ulk b

Ext

ract

ed T

an

Temperature / K

10 kHz

50 100 150 200 250 300 350 400 450

100 Hz1 kHz10 kHz100 kHz

0

0.0005

0.001

0.0015

0.002

0.0025

Temperarure / K

400

600

800

1000

1200

1400

1600

0

0.02

0.04

0.06

0.08

0.1

50 100 150 200 250 300 350 400 450

Ext

ract

ed B

ulk b

Ext

ract

ed T

an

Temperarure / K

10 kHz

Little Frequency Dispersion

Obeys Curie-Weiss Law above TC

'b Peak T ~ 250 K (bulk TC = 248 K)

SRO / BST LSCO / BST

C

TT C

1

Bulk Ceramic Behaviour

Extracted Real Interfacial Capacitance

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

50 100 150 200 250 300 350 400 450

100 Hz 1 kHz

10 kHz 100 kHz

100 Hz 1 kHz

10 kHz 100 kHz

Temperature / K

SRO / BST

LSCO / BST SRO / BST

Little frequency and temperature dependence

LSCO / BST

•Little temperature dependence

T < 300 K

•Large temperature dependence

•Little frequency dependence

•Large frequency dependence

T > 300 K

Extracted Imaginary Interfacial Capacitance

0.0

0.5

1.0

1.5

2.0

100 150 200 250 300 350 400 450

100 Hz1 kHz 10 kHz 100 kHz

Temperature / K

Similar behaviour to real component!!!

SRO / BST

But what is going on?

18

19

20

21

22

23

24

25

26

2 3 4 5 6 7 8

100 Hz1 kHz10 kHz100 kHz

1000/T / K-1

16

18

20

22

2.5 2.6 2.7 2.8 2.9 3.0 3.1

ln(K'')

1000/T / K -1

(I)(II)

Thermal Activation?

Tk

EKK

B

Aexp0

Region (I) – non-temperature dependent

Arrhenius Plot

Region (II) – Thermally activated

EA ~ 0.6 eV

After considering intrinsic background

Nature of Thermal Activation?

K″ is a conductive component since KAd

A 0

Thermal de-trapping of electrons from shallow level traps associated with defects

Oxygen Vacancies: EA = 0.65 eV

C. Ang et al. Phys Rev B. 62, 228 (2000)

Ti3+: EA = 0.7 eV

M. Dawber et al. arXiv:cond-mat/0212004v1

Etrap

Econd

EfermiEval

Defects

Defects form in a plane parallel to and (probably) next to the electrode interface

Since thermally activated conduction is ONLY observed in the interfacial component we can conclude from the Series Capacitor Model that

Defects must act in series with Bulk component

AND

Summary

SRO / BST

Thermal activation due to de-trapping of electrons associated with interfacial defects

LSCO / BST

Bulk properties consistent with bulk ceramic

Bulk properties consistent with bulk ceramic

Little temperature and frequency dependence of Interfacial capacitance below T = 300K

No thermal activation i.e. small concentration of interfacial defects.

Little temperature and frequency dependence of Interfacial capacitance

Conclusions

Defects contribute to interfacial capacitance but are not the fundamental origin of this interfacial capacitance.

Series Capacitor model is generally applicable for all temperatures

Defects form in a plane parallel and next to the electrode interface

Thermally activated conduction ONLY observed in interfacial component - Defects act in series with Bulk component

Fundamental origin of interfacial capacitance is probably NOT due to a dead layer of finite thickness

Structural Size Effects

Appearance of phases forbidden in bulk

Future Work – Ultra Thin BaTiO3 Capacitors

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

Vanderbilt et al, arXiv:cond-mat/0402101

Future Work – Ultra Thin BaTiO3 Capacitors

Ferroelectric Size Effects

Plus - Mapping of the critical thickness for ferroelectricity!!!

Increase of coercive field with decreasing thickness

Switching and tunnelling currents in Ultra thin films

Contreras et al, Appl Phys Lett. 83, 4595 (2003)

Pertsev et al, Appl Phys Lett. 83, 3356 (2003)

Junqura & Ghosez, Nature 422, 506 (2003)

Acknowledgements

Marty GreggRobert Bowman

Lesley Grattan (nee Sinnamon)Akeela Lookman

QUB

Cambridge University

Jim ScottMatt Dawber