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