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

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

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) q PLA + D Disadvantages: “Splashing” (solid particulates and liquid droplets) Angular distribution of ablated material cosnq, n10 (small area or inhomogeneous thickness)

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

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

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: gs: substrate free surface gf: film free surface gi: substrate-film interface gf gs gi

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

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 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

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

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

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)

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 = 11.103 Å b = 18.924 Å c = 5.572 Å (A.. YOSHIASA et al., Mater. Res. Bull. 21 (1986) 175)

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

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

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) 132105

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

Conductivity measurements NdGaO3 substrates Pt electrodes and wires

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

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

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

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

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.