Presentation on theme: "Dr. JOSÉ A. PARDO Department of Materials Science and Technology, & Aragón Institute of Nanoscience University of Zaragoza Pulsed laser deposition of oxide."— Presentation transcript:
Dr. JOSÉ A. PARDO Department of Materials Science and Technology, & Aragón Institute of Nanoscience University of Zaragoza Pulsed laser deposition of oxide epitaxial thin films. Recent results on Sr 4 Fe 6 O 13
Pulsed Laser Deposition (PLD) High-vacuum chamber Substrate on substate heater Rotating target (sintered ceramic) O 2 pressure control
Pulsed Laser Deposition (PLD) Advantages: Stoichiometric transfer of material (Complex oxides: YBa 2 Cu 3 O 7- ) Direct relation number of pulses- thickness ( Å/pulse) Few experimental parameters (T, PO 2 ) PLA + D Disadvantages: “Splashing” (solid particulates and liquid droplets) Angular distribution of ablated material cos n , n 10 (small area or inhomogeneous thickness)
Pulsed laser-matter interaction Roughly: I W/cm 2 : heating I 10 5 – 10 7 W/cm 2 : melting I 10 7 – W/cm 2 : vaporization and plasma formation Wavelength Pulse duration Energy per pulse E Focused on area S Fluence = E/S Peak power P p = E/ Intensity I = P p /S S Optical absorptivity Thermal diffusivity Other properties...
PL-matter interaction Congruent ablation Single target > threshold No target degradation PLA-PLD: 10 ns 10 J/cm 2 I 1 GW/cm 2 UV excimer Q-switched Nd:YAG D. BÄUERLE: “Laser Processing and Chemistry”. Springer (2000)
Thin film nucleation and growth Deposited atom (adatom) Hot atom Diffusion to cluster Dimer 2D-island Atom reevaporation Dissociation from cluster 3D-island Cluster
Models for epitaxial growth Free-energy: s : substrate free surface f : film free surface i : substrate-film interface ff ss ii
Models for epitaxial growth Frank-Van der Merwe (2-D layer-by-layer) s > f + i Volmer-Weber (3-D islands) s < f + i 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 Strain: ≈ 1% Hooke´s law: = E = F / A o : stress, = l / l o : strain, Young modulus Oxides: E ≈ Pa → Epitaxial stress: ≈ 1 GPa Substrate choice: Compressive (a f >a s ) or tensile (a f
La 1.9 Sr 0.1 CuO 4 superconductors PLD T c values: Bulk LSCO: 25 K LSCO/SrTiO 3 (c): 10 K LSCO/SrLaAlO 4 (t): 49.1 K !!!
Multilayers of ionic conductors MBE Space charge region ≈ 2L D
PLD of Sr 4 Fe 6 O 13 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)
Sr 4 Fe 6 O 13± Parent member of the mixed conducting family Sr 4 Fe 6-x Co x O 13 Perovskite-type layer Sr-Fe-O Fe-O double layer a b c Intergrowth structure Orthorhombic Iba2 a = Å b = Å c = Å ( A.. YOSHIASA et al., Mater. Res. Bull. 21 (1986) 175 ) x = 2: very high oxygen conductivity = el + i
Sr 4 Fe 6 O 13 /SrTiO 3 (100) films b-oriented. Cube-on-cube epitaxy J. A. PARDO et al., Journal of Crystal Growth 262 (2004) 334
Lattice parameters vs. thickness t < 30 nm fully strained films t > 170 nm relaxed films Sr 4 Fe 6 O 13 /SrTiO 3 Thickness range: t ≈ 15 – 300 nm
Epitaxial strain vs. thickness ~ t -1 for misfit dislocation-mediated plastic deformation Sr 4 Fe 6 O 13 /SrTiO 3 (100) J. SANTISO et al., Applied Physics Letters 86 (2005)
Oxygen content vs. thickness Sr 4 Fe 6 O 13± /SrTiO 3 films deposited under the same O 2 pressure Oxygen superstructure with modulation vector q = a m * 13- = 12+2 Strain relaxation through change in oxygen superstructure M. D. ROSSELL et al., Chem. Mater. 16 (2004) 2478
Conductivity measurements NdGaO 3 substrates Pt electrodes and wires
Impedance spectroscopy Furnace up to 800 ºC Controlled atmosphere: O 2, Ar… Impedance analyzer HP-4192A (5 Hz - 13 MHz)
Sr 4 Fe 6 O 13 /NdGaO 3 (100) films Plane matrix of Sr 4 Fe 6 O 13± Needle-like precipitates of SrFeO 3-z b-oriented films. Cube-on-cube epitaxy
Conductivity of SFO/NGO in O 2 Strong dependence conductivity-thickness J. A. PARDO et al. Solid State Ionics (submitted)
Effect of stress on conductivity Small polaron hopping: (T) = (A/T) exp(-E a /kT) Conductivity increases under compressive epitaxial stress SrTiO 3 NdGaO 3
Summary PLD is a versatile technique for the deposition of high-quality epitaxial thin films of oxides. The conductivity of epitaxial thin films of Sr 4 Fe 6 O 13 /NdGaO 3 (100) strongly depends on the film thickness. This dependence is most probably due to the effect of compressive epitaxial stress.