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Pulsed laser deposition of oxide epitaxial thin films

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Presentation on theme: "Pulsed laser deposition of oxide epitaxial thin films"— Presentation transcript:

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. thickness
10 100 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.

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