Dr. JOSÉ A. PARDO
Department of Materials Science and Technology,
& Aragón Institute of Nanoscience
University of Zaragoza
Pulsed laser deposition of oxide epitaxial thin films.
Recent results on Sr4Fe6O13
Pulsed Laser Deposition (PLD)
High-vacuum chamber
Substrate on substate heater
Rotating target (sintered ceramic)
O2 pressure control
Pulsed Laser Deposition (PLD)Advantages:
• Stoichiometric transfer of material (Complex oxides: YBa2Cu3O7-)
• Direct relation number of pulses- thickness ( 0.1-0.3 Å/pulse)
• Few experimental parameters (T, PO2)
PLA + D
Disadvantages:
• “Splashing” (solid particulates and liquid droplets)
• Angular distribution of ablated material cosn, n10 (small area or inhomogeneous thickness)
Pulsed laser-matter interaction
Roughly: I 104 - 105 W/cm2: heating I 105 – 107 W/cm2: melting I 107 – 1010 W/cm2: vaporization and plasma formation
Wavelength Pulse duration Energy per pulse EFocused on area S
Fluence = E/S
Peak power Pp = E/
Intensity I = Pp/S
S
Optical absorptivityThermal diffusivityOther properties...
PL-matterinteraction
Congruent ablation
Single target
> threshold
No target degradation
PLA-PLD: 10 ns 10 J/cm2
I 1 GW/cm2
UV excimerQ-switched Nd:YAG
D. B
ÄU
ER
LE
: “Laser P
rocessing and Chem
istry”. Springer (2000)
Thin film nucleation and growth
Deposited atom (adatom)
Hot atom
Diffusion to clusterDimer
2D-island
Atom reevaporation
Dissociation from cluster
3D-island
Cluster
Models for epitaxial growth
Free-energy:
s: substrate free surfacef: film free surfacei: substrate-film interface
f
s i
Models for epitaxial growth
Frank-Van der Merwe(2-D layer-by-layer)
s > f + i
Volmer-Weber(3-D islands)s < f + i
Stranski-Krastanov
Features of (epitaxial) thin films• “Single crytals”:
- Anisotropy- Very low density of high-angle grain boundaries
• High surface-to-volume ratio (surface effects)
• Some particualr growth-induced defects (stacking faults, misfit dislocations, buffer layers...)
• Epitaxial strain
• Influence of substrate (diffusion, chemical reactions at substrate/film interface...)
• Miniaturization (nanotechnology, sensors...)
• Alternated thin films: Multilayers and heterostructures (planar technology devices, magnetic tunnel junctions…)
MATERIALS WITH NEW PROPERTIES!
Epitaxial strain
Deformation of film lattice to match the substrate lattice
Strain: ≈ 1%
Hooke´s law: = E = F / Ao: stress, = l / lo: strain, Young modulus
Oxides: E ≈ 1011 Pa → Epitaxial stress: ≈ 1 GPaSubstrate choice:
• Compressive (af>as) or tensile (af<as) strain• Modulation of strain by substrate lattice parameter• Modulation of the film properties
Commensurate epitaxyCoherent strains
fs
aaa
mLattice mismatch:
mc·tc ≈ constant
La1.9Sr0.1CuO4 superconductors
PLDTc values:
Bulk LSCO: 25 K
LSCO/SrTiO3 (c): 10 K
LSCO/SrLaAlO4 (t): 49.1 K !!!
Multilayers of ionic conductors
MBE
Space charge region ≈ 2LD
PLD of Sr4Fe6O13 epitaxial films
PEOPLE INVOLVED:
• Barcelona - ICMAB: J. A. Pardo, J. Santiso, C. Solís, G. Garcia, M. Burriel, A. Figueras (PLD, CVD, XRD, XRR, SEM, Impedance) • Antwerp - EMAT: G. Van Tendeloo & M. D. Rossell
(TEM, HREM and ED)• Sacavém - ITN: J. C. Waerenborgh (Mössbauer)• Barcelona - ICMAB: X. Torrellas (Synchrotron) • Lisbon - FCUL: M. Godinho (Magnetism)
Sr4Fe6O13±
Parent member of the mixed conducting family Sr4Fe6-xCoxO13
Perovskite-type layer Sr-Fe-O
Fe-O double layer
a
b
c Intergrowth structure
Orthorhombic Iba2
a = 11.103 Å b = 18.924 Å c = 5.572 Å (A.. YOSHIASA et al., Mater.
Res. Bull. 21 (1986) 175)
x = 2: very high oxygen conductivity = el + i
Sr4Fe6O13/SrTiO3(100) filmsb-oriented. Cube-on-cube epitaxy
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160
XR
D in
tens
ity
(cps
)
100
101
102
103
104
105
106
2 (degrees)
(0
2 0)
SrT
iO3 (
0 0
1)
SrT
iO3 (
0 0
3)
SrT
iO3 (
0 0
2)
(0
4 0) (
0 6
0)
(0
8 0)
(0
10 0
)
(0
12 0
)
(0
14 0
)
(0
16 0
)
(0
18 0
)
(0
20 0
)
(0
22 0
) (0
24 0
)
SrT
iO3 (
0 0
4)
J. A. PARDO et al., Journal of Crystal Growth 262 (2004) 334
13 13.5 14 14.5 15XR
D in
tens
ity
(a.u
.)
(degrees)
0.3º
Lattice parameters vs. thickness
t < 30 nmfully strained films
t > 170 nmrelaxed films
Sr4Fe6O13/SrTiO3
Thickness range:t ≈ 15 – 300 nm
1,895
1,900
1,905
1,910
1,915
1,920
Ou
t-of
-pla
ne
par
amet
er (
nm
)b
o
SFO
out-of-plane
0 50 100 150 200 250 300 3500,390
0,391
0,392
0,393
0,394
Thickness (nm)
in-planeaSTO
do
(201)SFO
In-p
lan
e p
aram
eter
(n
m)
Epitaxial strain vs. thickness
~ t -1
for misfit dislocation-mediated plastic deformation
Sr4Fe6O13/SrTiO3(100)
10 1000,1
1
out-of-plane in-plane
Str
ain
(
)
Thikckness (t)
~ t -0.6
tc
Fullystrained
Relaxed
J. SANTISO et al., Applied PhysicsLetters 86 (2005) 132105
Oxygen content vs. thickness
Strained ( -0.8%)
Relaxed ( < -0.2%)
1,100 1,105 1,110 1,1150,40
0,41
0,42
0,43
0,44
0,45
12.82
12.86
12.84
Parameter a (nm)
12.88
Oxygen
conten
t 13-
Strained ( -0.8%)
Relaxed ( < -0.2%)
Sr4Fe6O13±/SrTiO3
films deposited underthe same O2 pressure
Oxygen superstructure with modulation vector
q = am*
13- = 12+2
Strain relaxation through change in oxygen superstructure
M. D. ROSSELL et al., Chem.Mater. 16 (2004) 2478
Conductivity measurements
10-7
10-6
10-5
10-4
10-3
10-2
0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
1000/T (K-1)
SrTiO3
NdGaO3
LaAlO3
(S
/cm
)
NdGaO3
substrates
Pt electrodes and wires
Impedance spectroscopyFurnace up to 800 ºCControlled atmosphere: O2, Ar…
Impedance analyzerHP-4192A (5 Hz - 13 MHz)
Sr4Fe6O13/NdGaO3(100) films
Plane matrix of Sr4Fe6O13±
Needle-like precipitates of SrFeO3-z
b-oriented films. Cube-on-cube epitaxy
1
10
102
103
104
105
0 20 40 60 80 100
Inte
nsit
y (c
.p.s
.)
2 (degrees)
(0 2
0)
(0 4
0)
(0 8
0)
(0 1
0 0)
(0 1
2 0)
(0 1
4 0)
(0 1
6 0)
(0 1
8 0)
(0 6
0)
Conductivity of SFO/NGO in O2
2
4
6
8
10
12
1.2 1.6 2 2.4
10 nm56 nm156 nm313 nmCeramic
ln
T (
-1 c
m-1
K)
1000/T (K-1)
O2
Strong dependence conductivity-thickness
J. A
. PA
RD
O e
t al.
Soli
d St
ate
Ioni
cs(s
ubm
itte
d)
Effect of stress on conductivity
Small polaron hopping: (T) = (A/T) exp(-Ea/kT)
0.1
1
10 100
A (
106
-1 c
m-1
K)
Thickness (nm)
Conductivity increases under compressive epitaxial stress
0.1
1
10 100
In-p
lan
e st
rain
xx
(%
)Thickness (nm)
SrTiO3
NdGaO3
Summary
• PLD is a versatile technique for the deposition of high-quality epitaxial thin films of oxides.
• The conductivity of epitaxial thin films of Sr4Fe6O13/NdGaO3(100) strongly depends on the film thickness.
• This dependence is most probably due to the effect of compressive epitaxial stress.