1. Pulsed laser deposition of oxide epitaxial thin films
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

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

3. Pulsed Laser Deposition (PLD)
Advantages:
Stoichiometric transfer of material (Complex oxides: YBa2Cu3O7-d)
Direct relation number of pulses- thickness ( Å/pulse)
Few experimental parameters (T, PO2) q
PLA + D
Disadvantages:
“Splashing” (solid particulates and liquid droplets)
Angular distribution of ablated material cosnq, n10 (small area or inhomogeneous thickness)

4. 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  W/cm2: heating
I  105 – 107 W/cm2: melting
I  107 – 1010 W/cm2: vaporization and plasma formation

5. PL-matter interaction
PLA-PLD:
t  10 ns
F  10 J/cm2
I  1 GW/cm2
Congruent ablation
Single target
F > Fthreshold
No target degradation
D. BÄUERLE: “Laser Processing and Chemistry”. Springer (2000)
UV excimer
Q-switched Nd:YAG

6. Thin film nucleation and growth
Deposited atom
(adatom)
Hot atom
Diffusion to cluster
Dimer
2D-island
Atom reevaporation
Dissociation from cluster
3D-island
Cluster

7. Models for epitaxial growth
Free-energy:
gs: substrate free surface
gf: film free surface
gi: substrate-film interface gf gs gi

8. 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

9. 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!

10. Epitaxial strain Epitaxial stress: s ≈ 1 GPa Substrate choice:
Deformation of film lattice to match the substrate lattice
Lattice mismatch:
Commensurate epitaxy
Coherent strain
Strain: e ≈ 1%
Hooke´s law: s = E e
s = F / Ao: stress, e = Dl / lo: strain, E: Young modulus
Oxides: E ≈ 1011 Pa →
mc·tc ≈ constant
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

11. La1.9Sr0.1CuO4 superconductors
PLD
Tc values:
Bulk LSCO: 25 K
LSCO/SrTiO3 (c): 10 K
LSCO/SrLaAlO4 (t): 49.1 K !!!

12. Multilayers of ionic conductors
Space charge region
l ≈ 2LD
MBE

13. 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)

14. x = 2: very high oxygen conductivity
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 = Å
b = Å
c = Å (A.. YOSHIASA et al., Mater.
Res. Bull. 21 (1986) 175)

15. Sr4Fe6O13/SrTiO3(100) films b-oriented. Cube-on-cube epitaxy
J. A. PARDO et al., Journal of Crystal Growth 262 (2004) 334

16. Lattice parameters vs. thickness
1,895
1,900
1,905
1,910
1,915
1,920
Out-of-plane parameter (nm) b o
SFO
out-of-plane 50
100
150
200
250
300
350
0,390
0,391
0,392
0,393
0,394
Thickness (nm)
in-plane a
STO d
(201)SFO
In-plane parameter (nm)
Sr4Fe6O13/SrTiO3
Thickness range:
t ≈ 15 – 300 nm
t < 30 nm
fully strained films
t > 170 nm
relaxed films

17. Epitaxial strain vs. thickness10
100
0,1 1
out-of-plane
in-plane
Strain e (%)
Thikckness (t)
 ~ t -0.6 tc
Fully
strained
Relaxed
Sr4Fe6O13/SrTiO3(100)
 ~ t -1
for misfit dislocation-mediated plastic deformation
J. SANTISO et al., Applied Physics
Letters 86 (2005)

18. Oxygen content vs. thickness
Strained
( -0.8%)
Relaxed
( < -0.2%)
1,100
1,105
1,110
1,115
0,40
0,41
0,42
0,43
0,44
0,45
12.82
12.86
12.84 a
Parameter a (nm)
12.88
Oxygen content 13-d
(  -0.8%)
Sr4Fe6O13±/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

19. Conductivity measurements
NdGaO3
substrates
Pt electrodes and wires

20. Impedance spectroscopy
Furnace up to 800 ºC
Controlled atmosphere: O2, Ar…
Impedance analyzer
HP-4192A (5 Hz - 13 MHz)

21. b-oriented films. Cube-on-cube epitaxy
Sr4Fe6O13/NdGaO3(100) films
b-oriented films. Cube-on-cube epitaxy
Plane matrix of Sr4Fe6O13±
Needle-like precipitates of SrFeO3-z

22. Conductivity of SFO/NGO in O2
J. A. PARDO et al. Solid State Ionics
(submitted)
Strong dependence conductivity-thickness

23. Effect of stress on conductivity
Small polaron hopping: s(T) = (A/T) exp(-Ea/kT)
SrTiO3
NdGaO3
Conductivity increases under compressive epitaxial stress

24. 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.