Transcript Diapositiva 1 - AGH University of Science and Technology

Pulsed laser deposition of
oxide epitaxial thin films.
Recent results on Sr4Fe6O13
Dr. JOSÉ A. PARDO
Department of Materials Science and Technology,
& Aragón Institute of Nanoscience
University of Zaragoza
Pulsed Laser Deposition (PLD)
High-vacuum chamber
Substrate on
substate heater
O2 pressure control
Rotating target (sintered ceramic)
Pulsed Laser Deposition (PLD)
Advantages:
• Stoichiometric transfer of material
(Complex oxides: YBa2Cu3O7-d)
q
• Direct relation number of pulsesthickness ( 0.1-0.3 Å/pulse)
• Few experimental parameters (T, PO2)
Disadvantages:
• “Splashing” (solid particulates and
liquid droplets)
PLA + D
• Angular distribution of ablated
material cosnq, n10 (small area or
inhomogeneous thickness)
Pulsed laser-matter interaction
S
Optical absorptivity
Thermal diffusivity
Other properties...
Wavelength l
Pulse duration t
Energy per pulse E
Focused on area S
Fluence F = E/S
Peak power Pp = E/t
Intensity I = Pp/S
Roughly:
I  104 - 105 W/cm2: heating
I  105 – 107 W/cm2: melting
I  107 – 1010 W/cm2: vaporization and plasma formation
PL-matter
interaction
F > Fthreshold
Congruent ablation
Single target
No target degradation
UV excimer
Q-switched Nd:YAG
D. BÄUERLE: “Laser Processing and Chemistry”. Springer (2000)
PLA-PLD:
t  10 ns
F  10 J/cm2
I  1 GW/cm2
Thin film nucleation and growth
Cluster
Hot atom
Atom reevaporation
Diffusion to cluster
Dimer
Deposited atom
(adatom)
2D-island
Dissociation from cluster
3D-island
Models for epitaxial growth
Free-energy:
gs: substrate free surface
gf: film free surface
gi: substrate-film interface
gf
gs
gi
Models for epitaxial growth
Frank-Van der Merwe
(2-D layer-by-layer)
gs > gf + gi
Volmer-Weber
(3-D islands)
gs < gf + gi
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
Lattice mismatch:
Strain: e ≈ 1%
Hooke´s law: s = E e
as  af
m
as
Commensurate epitaxy
Coherent strain
mc·tc ≈ constant
s = F / Ao: stress, e = Dl / lo: strain, E: Young modulus
Oxides: E ≈ 1011 Pa
→
Epitaxial stress: s ≈ 1 GPa
Substrate choice:
• Compressive (af>as) or tensile (af<as) strain
• Modulation of strain by substrate lattice parameter
• Modulation of the film properties
La1.9Sr0.1CuO4 superconductors
Tc values:
PLD
Bulk LSCO: 25 K
LSCO/SrTiO3 (c): 10 K
LSCO/SrLaAlO4 (t): 49.1 K !!!
Multilayers of ionic conductors
l
Space
charge
region
l ≈ 2LD
MBE
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±d
Parent member of the mixed conducting family Sr4Fe6-xCoxO13
x = 2: very high oxygen conductivity
c
a
s = sel + si
Intergrowth structure
Fe-O double layer
b
Perovskite-type layer Sr-Fe-O
Orthorhombic Iba2
a = 11.103 Å
b = 18.924 Å
c = 5.572 Å (A.. YOSHIASA et al., Mater.
Res. Bull. 21 (1986) 175)
Sr4Fe6O13/SrTiO3(100) films
10
20
40
50
60
3
3
SrTiO (0 0 4)
SrTiO (0 0 3)
(0 10 0)
3
30
70
80
90
100
110
120
130
140
150
0.3º
13
13.5
14
14.5
 (degrees)
(0 24 0)
0
(0 22 0)
10
(0 20 0)
1
(0 18 0)
10
(0 16 0)
2
(0 14 0)
10
(0 12 0)
3
(0 8 0)
10
3
4
(0 6 0)
10
SrTiO (0 0 1)
5
(0 4 0)
10
SrTiO (0 0 2)
6
(0 2 0)
XRD intensity (cps)
10
XRD intensity (a.u.)
b-oriented. Cube-on-cube epitaxy
160
2q (degrees)
J. A. PARDO et al., Journal of Crystal Growth 262 (2004) 334
15
Sr4Fe6O13/SrTiO3
In-plane parameter (nm)
Thickness range:
t ≈ 15 – 300 nm
Out-of-plane parameter (nm)
Lattice parameters vs. thickness
1,920
out-of-plane
1,915
1,910
1,905
1,900
o
bSFO
1,895
o
0,394
d(201)SFO
0,393
0,392
0,391
0,390
0
in-plane
a STO
50
100
150
200
250
300
350
Thickness (nm)
t < 30 nm
fully strained films
t > 170 nm
relaxed films
Epitaxial strain vs. thickness
Sr4Fe6O13/SrTiO3(100)
out-of-plane
in-plane
Strain e (%)
1
tc
Fully
strained
e ~ t -1
for misfit
dislocation-mediated
plastic deformation
e ~ t -0.6
Relaxed
0,1
10
100
Thikckness (t)
J. SANTISO et al., Applied Physics
Letters 86 (2005) 132105
Oxygen content vs. thickness
0,45
Relaxed
(e < -0.2%)
12.88
12.86
0,42
12.84
a
0,43
0,41
0,40
1,100
Strained
Strained
(e -0.8%)
(e  -0.8%)
1,105
1,110
Parameter a (nm)
12.82
1,115
Oxygen content 13-d
0,44
Relaxed
(e < -0.2%)
Sr4Fe6O13±d/SrTiO3
films deposited under
the same O2 pressure
Oxygen superstructure
with modulation vector
q = aam*
13-d = 12+2a
M. D. ROSSELL et al., Chem.
Mater. 16 (2004) 2478
Strain relaxation through change in oxygen superstructure
Conductivity measurements
NdGaO3
substrates
-2
10
-3
s (S/cm)
10
SrTiO
LaAlO
-4
10
3
3
-5
10
-6
10
NdGaO
3
-7
10
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
-1
1000/T (K )
Pt electrodes and wires
Impedance spectroscopy
Furnace up to 800 ºC
Controlled atmosphere: O2, Ar…
Impedance analyzer
HP-4192A (5 Hz - 13 MHz)
Sr4Fe6O13/NdGaO3(100) films
b-oriented films. Cube-on-cube epitaxy
5
(0 18 0)
60
(0 16 0)
40
(0 14 0)
(0 10 0)
(0 6 0)
(0 12 0)
3
10
(0 8 0)
Intensity (c.p.s.)
4
10
(0 4 0)
(0 2 0)
10
2
10
10
1
0
20
80
100
2q (degrees)
Plane matrix of Sr4Fe6O13±d
Needle-like precipitates of SrFeO3-z
Conductivity of SFO/NGO in O2
2
10
8
-1
-1
ln sT ( cm K)
O
6
4
2
10 nm
56 nm
156 nm
313 nm
Ceramic
1.2
1.6
2
J. A. PARDO et al. Solid State Ionics
(submitted)
12
2.4
-1
1000/T (K )
Strong dependence conductivity-thickness
Effect of stress on conductivity
6
0.1
10
1
xx
In-plane strain e (%)
1
-1
-1
A (10  cm K)
Small polaron hopping: s(T) = (A/T) exp(-Ea/kT)
100
Thickness (nm)
SrTiO3
NdGaO3
0.1
10
100
Thickness (nm)
Conductivity increases under compressive epitaxial stress
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.