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Pulsed laser deposition of oxide epitaxial thin films. Recent results on Sr 4 Fe 6 O 13

Pulsed laser deposition of oxide epitaxial thin films. Recent results on Sr 4 Fe 6 O 13. 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

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Pulsed laser deposition of oxide epitaxial thin films. Recent results on Sr 4 Fe 6 O 13

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  1. 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. PLA + D q 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)

  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  104 - 105 W/cm2: heating I  105 – 107 W/cm2: melting I  107 – 1010 W/cm2: vaporization and plasma formation

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

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

  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 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 l 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. 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)

  15. Sr4Fe6O13/SrTiO3(100) films b-oriented. Cube-on-cube epitaxy J. A. PARDO et al., Journal of Crystal Growth262 (2004) 334

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

  17. out-of-plane in-plane 1 tc Strain e (%)  ~t -0.6 Fully strained Relaxed 0,1 10 100 Thikckness (t) 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

  18. 0,45 Relaxed (< -0.2%) Relaxed (< -0.2%) 0,44 12.88 0,43 12.86 a Oxygen content13-d 0,42 12.84 Strained ( -0.8%) Strained (  -0.8%) 0,41 12.82 0,40 1,100 1,105 1,110 1,115 Parameter a (nm) Oxygen content vs. thickness 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. 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. SrTiO3 NdGaO3 Effect of stress on conductivity Small polaron hopping:s(T) = (A/T) exp(-Ea/kT) 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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