NANOSCALE FERROELECTRICS
Nanodevices using functionality in ferroelectrics
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magnetmulti - FERRO - electr - ICS
elast ORDER PARAMETER
M, P, s
MANY ABO3 – TYPE
METAL OXIDES ARE
FERROICS
FIELD
H, E, σ
Ferroics = crystals having two or more equivalent orientation states
(domains), which can be switched from one to another by external field
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magnetmulti - FERRO - electr - ICS
elast ORDER PARAMETER
M, P, s
MANY ABO3 – TYPE
A = Ba, Sr, Pb, K, Na, ….
B = Ti, Ta, Nb, Zr, Sc, …
FIELD
H, E, σ
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ABO3 – type perovskite ferroelectrics
possess remarkable properties with huge
potential for versatile device applications,
for instance:
A
dielectric permittivity
r
P
ε ~ (1…100)×103
B
O
piezoelectric coefficient
deff ~ (1…30)×102 pm/V
pyroelectric coefficient
pi ~ -(1…50)×10-4 C/(m2K)
In modern devices, NANOSCALE FERROELECTRICS are required.
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NANOSCALE FERROELECTRICS
SUBSTRATE
SUBSTRATE
SUBSTRATE
FILMS
d ~ 10 nm
ISLANDS
∅ ~ 10 nm
SUPERLATTICES
Λ ~ 1 nm
POWDERS/
PARTICLES
FINE-GRAINED
CERAMICS/
COMPOSITES
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POLYMERBASED
COMPOSITES
SUBSTRATE
EPITAXIAL
COMPOSITES
ENGINEERED
NANO-SCALE
DOMAINS
NANO-SCALE:
POSSIBLE ORIGIN OF SIZE EFFECTS
IN FERROELECTRICS
STRAIN / CLAMPING (due to strong s-P coupling)
INTERFACES / SURFACE (due to peculiar crystal and
electronic structure)
SIZE (critical size for ferroelectric stability)
…………………………………………………………
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STRAIN in ultrathin epitaxial films
film
strain
s
(001) plane
in pseudo-cubic
perovskite cell
substrate
Pseudocubic epitaxial films can be grown on cubic substrates with match of (001) planes.
Misfit strain appears due to difference between in-plane lattice parameters.
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Ultrathin epitaxial films of SrTiO3 with in-plane strain
NANOSCALE FERROELECTRICS
SUBSTRATE
SUBSTRATE
SUBSTRATE
FILMS
d ~ 10 nm
ISLANDS
∅ ~ 10 nm
SUPERLATTICES
Λ ~ 1 nm
POWDERS/
PARTICLES
FINE-GRAINED
CERAMICS/
COMPOSITES
SUBSTRATE
EPITAXIAL
COMPOSITES
temperature
FILM (001)
PARAELECTRIC
ferroelectricity in strained
films of paraelectric
FERROELECTRIC
“-” compressive strain
0
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“+” tensile strain
POLYMERBASED
COMPOSITES
ENGINEERED
NANO-SCALE
DOMAINS
Ultrathin epitaxial films of SrTiO3 with in-plane strain
In order to study strain-induced behavior in paraelectrics, single-crystal epitaxial films of SrTiO3 with sufficiently large
in-plane strain are required.
Such ultrathin (10 – 20 nm) films were grown by in-situ pulsed laser deposition on various single-crystal substrates.
The crystal structure of the films was studied by x-ray diffraction.
DyScO3
LaAlO3
KTaO3
I (cps)
4
10
d = 130 Å
d = 147 Å
2
d = 123 Å
Kβ
10
44
46
48
44
46
48
44
46
48
2Θ (deg)
Θ-2Θ x-ray diffraction patterns of epitaxial STO films on LaAlO3, DyScO3, KTaO3, substrates in the vicinity of STO
(002) reflections. The satellites are the peaks of Laue function due to small thickness of high-quality smooth singlecrystalline films. The thickness d of STO films is determined from the positions of minima of Laue function.
Strain to 2.1 % is obtained.
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J. Phys.: Condens. Matter FTC 21, 232203 (2009); poster
Influence of strain on optical properties of SrTiO3 films
Optical properties (complex index of refraction) of ultrathin strained epitaxial films of SrTiO3 were
studied using spectral ellipsometry and compared to those of bulk single-crystal SrTiO3.
bulk
bulk
film
(n, k)
film
s = +0.85%
s = +2.1 %
bulk
bulk
film
film
2
4
2
4
E (eV)
The observed increase of energies of optical transitions (spectra are shifted to
higher E) can be ascribed to presence of the strain induced polarization.
Strain-induced onset of polarization is indicated.
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J. Phys.: Condens. Matter FTC 21, 232203 (2009)
STRAIN in ultrathin epitaxial (111) films
film
misfit
(111) plane
in pseudo-cubic
perovskite cell
substrate
Epitaxial (111) films can be grown on single-crystal substrates with match of (111) planes.
Misfit strain appears due to difference between in-plane lattice parameters.
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Computational study of (111) epitaxially strained
ferroelectric perovskites BaTiO3 and PbTiO3
Phase transitions and polarization in PbTiO3 and BaTiO3 under (111) epitaxial
strain were investigated using density functional theory calculations.
In PbTiO3, the total polarization is found to be INDEPENDENT of strain.
In BaTiO3, compressive strain can completely SUPPRESS the polarization.
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Phys. Rev. B 78, 094102 (2008)
STRAINED epitaxial (001) superlattices
NANOSCALE FERROELECTRICS
SUBSTRATE
Constituent 2, thickness
d2
Constituent 1, thicknessFILMS
d1
d ~ 10 nm
SUBSTRATE
SUBSTRATE
ISLANDS
∅ ~ 10 nm
SUPERLATTICES
Λ ~ 1 nm
SUBSTRATE
EPITAXIAL
COMPOSITES
SUBSTRATE
POWDERS/
PARTICLES
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FINE-GRAINED
CERAMICS/
COMPOSITES
POLYMERBASED
COMPOSITES
ENGINEERED
NANO-SCALE
DOMAINS
Strain and permittivity in superlattices
of Ba0.8Sr0.2TiO3 : Ba0.4Sr0.6TiO3
Superlattices were grown by in situ pulsed laser deposition.
Their crystal structure and dielectric response were experimentally analyzed.
BST80/20
900
with decreasing period,
lattice parameters and STRAIN
do NOT CHANGE!
3,95
INCREASE
of permittivity
800
ε
a (Å)
4,00
700
superlattice:
pseudo-morphic
in-plane growth
10
600
superlattices with
SIMILAR strain
BST40/60
100
Λ (u.c.)
10
100
Λ (u. c.)
NO correlation between constant strain and increasing permittivity.
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Phys. Rev. B 76, 134107 (2007)
Role of INTERFACES in superlattices
To reveal influence of interfaces, strain-free superlattices without spontaneous polarization are required.
Such superlattices containing NaNbO3 and SrTiO3 were grown and studied.
UNIT CELL VOLUME AS A FUNCTION
OF PERIOD – LATTICE EXPANSION
PERMITTIVITY AS A FUNCTION OF
NUMBER OF INTERFACES: 1/ε ∝ n
8
T = 200, 300, 400 K
-3
3
V (Å )
1/ε (10 )
61
60
4
10
Λ (u. c.)
100
Analysis of crystal structure revealed strong lattice
expansion that cannot be explained by possible
intermixing or presence of defects.
10
n
100
Fit evidences presence of interfacial
dielectric layers between NNO:STO.
NNO:STO interfaces can determine crystal structure and properties of SLs.
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Phys. Rev. B 79, 014106 (2009)
Nature of NaNbO3 : SrTiO3 interface
Structural and electronic properties of NNO/STO superlattices were studied using first-principles
LDA (local density approximation) and LSDA+U (Hubbard parameter U) methods.
TWO POSSIBLE TYPES OF INTERFACES
NbO2 : SrO (n)
NaO : TiO2 (p)
O
Ti
Nb
Due to ionic charge mismatch [nominal charges
Na(1), Sr(2), Ti(4), Nb(5)], the NaO:TiO2-type
interface has an extra hole for retaining charge
neutrality.
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Na
Sr
SUBSTRATE
SUBSTRATE
Na
Sr
O
Nb
Ti
Due to ionic charge mismatch [nominal charges
Na(1), Sr(2), Ti(4), Nb(5)], the NbO2:SrO-type
interface has an extra electron for charge
neutrality.
Phys. Rev. B (manuscript); poster
Fine-grained ferroelectrics
Films and non-planar structures of Sr-Ti-O, Ba-Sr-Ti-O, Sr-Bi-Ta-O, Bi-Fe-O
are grown by ATOMIC LAYER DEPOSITION, electrospinning, sol-gel.
NANOSCALE FERROELECTRICS
Ba-Ti-O nano-structured tubes
SUBSTRATE
SUBSTRATE
SUBSTRATE
FILMS
d ~ 10 nm
ISLANDS
∅ ~ 10 nm
SUPERLATTICES
Λ ~ 1 nm
POWDERS/
PARTICLES
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FINE-GRAINED
CERAMICS/FILMS
COMPOSITES
POLYMERBASED
COMPOSITES
SUBSTRATE
EPITAXIAL
COMPOSITES
ENGINEERED
NANO-SCALE
DOMAINS
manuscript; poster
Fine-grained ultrathin films
Thin-film capacitor heterostructures were formed by atomic layer deposition of 54 nm thick polycrystalline barium
strontium titanate film on silicon substrates using Pt top and bottom electrodes. The dielectric response of capacitors
was experimentally studied as a function of frequency, temperature, and applied field, and analyzed considering
presence of an interface capacitance.
ε = εL +
ξ=
ξ=
c
T −Θ
∂ ⎛1⎞
⎜ ⎟
∂T ⎝ ε ⎠
c
[ε L (T − θ ) + c]2
ξ (10 -5 K -1 )
In thin-film BST, a paraelectric state with low Curie temperature, small Curie constant, and small
intrinsic permittivity is detected.
1
Curie constant is
c ≈ 0.45×105 K
– smaller than in bulk
0
200
400
600
T (K)
Intrinsic dielectric properties of films differ from those of bulk.
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Integrated Ferroelectrics 102, 29 (2008)
Nano-scale ferroelectric domains
NANOSCALE FERROELECTRICS
SUBSTRATE
FE properties are determined
by
FILMS
d ~ 10 nm into
polarization, which is arranged
domains.
SUBSTRATE
SUBSTRATE
ISLANDS
∅ ~ 10 nm
SUPERLATTICES
Λ ~ 1 nm
EPITAXIAL
COMPOSITES
POLYMERBASED
COMPOSITES
ENGINEERED
NANO-SCALE
DOMAINS
SUBSTRATE
With decreasing dimensions of FEs,
domain width is known to shrink,
resulting in nano-domains.
Besides dimensions of
FEs, domain
POWDERS/
configuration can depend
on strain,
PARTICLES
clamping, internal electric field.
Nano-domains can be created by
applying local electric field.
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FINE-GRAINED
CERAMICS/
COMPOSITES
Nano-scale ferroelectric domains
Domains are visualized, created and/or modified using piezo-response force microscopy (PFM).
In the PFM phase images, dark and light areas correspond to domains with different directions of polarization.
Epitaxial (001) film of rhombohedral PbZr0.65Ti0.35O3.
400nm
Configuration of nano-domains
differs from those of large (~ μm)
domains in bulk ferroelectrics.
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PFM-assisted “domain writing”:
large domains are obtained by
applying dc electric field.
manuscript; poster
Device: printable FRAM
Printable memory solutions for sensor, ID, and media applications
(Collaborative Project, FP7, Theme 3, ICT)
NANOSCALE FERROELECTRICS
Top view
SUBSTRATE
SUBSTRATE
SUBSTRATE
FILMS
d ~ 10 nm
ISLANDS
∅ ~ 10 nm
SUPERLATTICES
Λ ~ 1 nm
topSUBSTRATE
electrode
EPITAXIAL
COMPOSITES
Side view
ferroelectric
nanoparticle
layer
POWDERS/
PARTICLES
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FINE-GRAINED
CERAMICS/
COMPOSITES
POLYMERBASED
COMPOSITES
ENGINEERED
NANO-SCALE
bottom electrode
DOMAINS
www.primebits.eu
Device: printable FRAM
Ferroelectric polarization carries
the content of the memory bit.
permittivity
New printing inks are developed based
on ferroelectric BaTiO3 nanoparticles
First samples are prepared by printing
on PET.
thickness ~ 4 µm
εr ~ 25
Pr ~ 0.1 uC/cm2
-20 V
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dc voltage
+20 V
www.primebits.eu
NANOSCALE FERROELECTRICS
STRAIN / CLAMPING can influence properties of (001) oriented
nano-FEs. However, its role in (111) oriented FEs and superlattices
can be less pronounced.
INTERFACES /SURFACES can be as IMPORTANT as strain.
Pure SIZE effect seems to be impossible to experimentally detect.
Controlling nano-scale effects is crucial for realizing DEVICES.
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OUR RESEARCH TEAM
University of Helsinki
Helsinki University of
Technology
Microelectronics/ Materials
Physics Laboratories
Laboratory of Inorganic
Chemistry
COMP/Laboratory of Physics
J. Narkilahti
Dr. M. Vehkamäki
R. Oja
M. Plekh
M. Heikkilä
Dr. J. Frantti
J. Levoska (2007-2008)
E. Santala
Dr. Y. Fujioka
Dr. M. Tyunina
Prof. M. Ritala
Dr. K. Johnston (2007-2008)
(marinat@ee.oulu.fi)
Prof. M. Leskelä
Prof. R. Nieminen
VTT Technical
Research Centre of
Finland
Information and
communication technologies
cluster
Dr. T. Mattila, Prof. H. Seppä
University of Oulu
(coordinator)
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