Dr. JOS

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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

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Pulsed Laser Deposition (PLD)
High-vacuum chamber
Substrate on substate heater
O2 pressure control
Rotating target (sintered ceramic)
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Pulsed Laser Deposition (PLD)

• Advantages
• Stoichiometric transfer of material (Complex
  oxides YBa2Cu3O7-d)
• Direct relation number of pulses- thickness (?
  0.1-0.3 Å/pulse)
• Few experimental parameters (T, PO2)

• Disadvantages
• Splashing (solid particulates and liquid
  droplets)
• Angular distribution of ablated material ?cosnq,
  n?10 (small area or inhomogeneous thickness)

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Pulsed laser-matter interaction
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
S
Optical absorptivity Thermal diffusivity Other
properties...
Roughly I ? 104 - 105 W/cm2 heating I ? 105
107 W/cm2 melting I ? 107 1010 W/cm2
vaporization and plasma formation
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PL-matterinteraction
D. BÄUERLE Laser Processing and Chemistry.
Springer (2000)
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Thin film nucleation and growth
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Models for epitaxial growth
Free-energy gs substrate free surface gf film
free surface gi substrate-film interface
gf
gs
gi
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Models for epitaxial growth
Frank-Van der Merwe (2-D layer-by-layer) gs gt gf
gi Volmer-Weber (3-D islands) gs lt gf
gi Stranski-Krastanov
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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!
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Epitaxial strain
Deformation of film lattice to match the
substrate lattice
Lattice mismatch
Commensurate epitaxy Coherent strain
Strain e 1 Hookes law s E e s F /
Ao stress, e Dl / lo strain, E Young
modulus Oxides E 1011 Pa ?
mctc constant
Epitaxial stress s 1 GPa

• Substrate choice
• Compressive (afgtas) or tensile (afltas) strain
• Modulation of strain by substrate lattice
  parameter
• Modulation of the film properties

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La1.9Sr0.1CuO4 superconductors
PLD
Tc values Bulk LSCO 25 K LSCO/SrTiO3 (c) 10
K LSCO/SrLaAlO4 (t) 49.1 K !!!
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Multilayers of ionic conductors
l
Space charge region l 2LD
MBE
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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)

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Sr4Fe6O13?
Parent member of the mixed conducting family
Sr4Fe6-xCoxO13
x 2 very high oxygen conductivity
s sel si
Intergrowth structure
c
a
Fe-O double layer
Perovskite-type layer Sr-Fe-O
b
Orthorhombic Iba2 a 11.103 Å b 18.924 Å
c 5.572 Å (A.. YOSHIASA et al., Mater.
Res.
Bull. 21 (1986) 175)
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Sr4Fe6O13/SrTiO3(100) films
b-oriented. Cube-on-cube epitaxy
J. A. PARDO et al., Journal of Crystal Growth 262
(2004) 334
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Lattice parameters vs. thickness
Sr4Fe6O13/SrTiO3 Thickness range t 15 300 nm
t lt 30 nm fully strained films
t gt 170 nm relaxed films
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Epitaxial strain vs. thickness
Sr4Fe6O13/SrTiO3(100)
? t -1 for misfit dislocation-mediated plastic
deformation
J. SANTISO et al., Applied Physics Letters 86
(2005) 132105
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Oxygen content vs. thickness
Sr4Fe6O13?/SrTiO3 films deposited under the same
O2 pressure Oxygen superstructure with
modulation vector q aam 13-d 122a
M. D. ROSSELL et al., Chem. Mater. 16 (2004) 2478
Strain relaxation through change in oxygen
superstructure
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Conductivity measurements
NdGaO3 substrates
Pt electrodes and wires
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Impedance spectroscopy
Furnace up to 800 ºC Controlled atmosphere O2,
Ar
Impedance analyzer HP-4192A (5 Hz - 13 MHz)
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Sr4Fe6O13/NdGaO3(100) films
b-oriented films. Cube-on-cube epitaxy
Plane matrix of Sr4Fe6O13? Needle-like
precipitates of SrFeO3-z
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Conductivity of SFO/NGO in O2
J. A. PARDO et al. Solid State Ionics (submitted)
Strong dependence conductivity-thickness
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Effect of stress on conductivity
Small polaron hopping s(T) (A/T) exp(-Ea/kT)
Conductivity increases under compressive
epitaxial stress
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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.