Jaume Roqueta

Competing misfit relaxation mechanisms in epitaxial correlated oxides

Strain engineering of functional properties in epitaxial thin films of strongly correlated oxides exhibiting octahedral-framework structures is hindered by the lack of adequate misfit relaxation models. Here we present unreported experimental evidence of a four-stage hierarchical development of octahedral-framework perturbations resulting from a progressive imbalance between electronic, elastic, and octahedral tilting energies in La(0.7)Sr(0.3)MnO(3) epitaxial thin films grown on SrTiO(3) substrates. Electronic softening of the Mn-O bonds near the substrate leads to the formation of an interfacial layer clamped to the substrate with strongly degraded magnetotransport properties, i.e., the so-called dead layer, while rigid octahedral tilts become relevant at advanced growth stages without significant effects on charge transport and magnetic ordering.

Effects of pulsed laser radiation on epitaxial self-assembled Ge quantum dots grown on Si substrates

Laser irradiation of Ge quantum dots (QDs) grown on Si(100) substrates by solid-source molecular beam epitaxy has been performed using a Nd:YAG laser (532 nm wavelength, 5 ns pulse duration) in a vacuum. The evolution of the Ge QD morphology, strain and composition with the number of laser pulses incident on the same part of the surface, have been studied using atomic force microscopy, scanning electron microscopy and Raman spectroscopy. The observed changes in the topographical and structural properties of the QDs are discussed in terms of Ge-Si diffusion processes. Numerical simulations have been developed for the investigation of the temperature evolution of the QDs during laser irradiation. The obtained results indicate that the thermal behaviour and structural variation of the nanostructures differ from conventional thermal annealing treatments and can be controlled by the laser parameters. Moreover, an unusual island motion has been observed under the action of subsequent laser pulses.

Self-Arranged Misfit Dislocation Network Formation upon Strain Release in La0.7Sr0.3MnO3/LaAlO3(100) Epitaxial Films under Compressive Strain.

Lattice-mismatched epitaxial films of La0.7Sr0.3MnO3 (LSMO) on LaAlO3 (001) substrates develop a crossed pattern of misfit dislocations above a critical thickness of 2.5 nm. Upon film thickness increases, the dislocation density progressively increases, and the dislocation spacing distribution becomes narrower. At a film thickness of 7.0 nm, the misfit dislocation density is close to the saturation for full relaxation. The misfit dislocation arrangement produces a 2D lateral periodic structure modulation (Λ ≈ 16 nm) alternating two differentiated phases: one phase fully coherent with the substrate and a fully relaxed phase. This modulation is confined to the interface region between film and substrate. This phase separation is clearly identified by X-ray diffraction and further proven in the macroscopic resistivity measurements as a combination of two transition temperatures (with low and high Tc). Films thicker than 7.0 nm show progressive relaxation, and their macroscopic resistivity becomes similar than that of the bulk material. Therefore, this study identifies the growth conditions and thickness ranges that facilitate the formation of laterally modulated nanocomposites with functional properties notably different from those of fully coherent or fully relaxed material.

Thickness evolution of the twin structure and shear strain in LSMO films

X-ray diffraction analysis and orientation contrast scanning electron microscopy imaging of La0.7Sr0.3MnO3 epitaxial layers grown on (001)-SrTiO3 substrates have been used to track the shear strain and twin domain period as a function of the thickness of the films, t. To this end, the diffraction by a periodically modulated twinned structure is analyzed in detail. In contrast with current equilibrium models, here we demonstrate the occurrence of a critical thickness, tτ ∼ 2.0–2.5 nm, for twin formation in rhombohedral perovskite films. The absence of twinning below tτ is explained by the formation of a monoclinic interfacial phase presumably driven by electronic interactions between film and substrate not taken into account in theoretical models. Above tτ, twin domains develop concomitantly with the build-up of misfit shear strains associated with the formation of the rhombohedral structure. At a thickness ∼10 nm, the in-plane and out-of-plane shear strain components exhibit similar values, as imposed by the rhombohedral symmetry. However, upon increasing the film thickness, both strain components are found to follow divergent trajectories indicating a progressive perturbation of the octahedral framework which allows the in-plane lattice parameters to remain fully strained within the explored thickness range (up to 475 nm). Despite these structural perturbations, the twin size follows a t1/2 dependence as predicted for homogeneous films by equilibrium models.

Magnetic, structural properties and B-site order of two epitaxial La2CoMnO6 films with perpendicular out-of-plane orientation

We report on the magnetic and structural properties of two epitaxial La2MnCoO6thin films grown on (001)-oriented SrTiO3 (STO) substrates, prepared in order to achieve opposite characteristics: one showing perfect Co/Mn cation ordering and a single domain structure; and the second presenting strong B-site disorder. These two films (115 nm) were grown with different orientations. The ordered film with LMCO [110]//STO [001], and the second (disordered) with LMCO [001]//STO [001]. The former consistent with P21/n symmetry and the latter with Pbnm structure. Their different ferromagnetic properties are discussed.

Fabrication and characterization of yttria-stabilized zirconia membranes for micro solid oxide fuel cells

The present study is devoted to analyze the compatibility of yttria-stabilized zirconia thin films prepared by pulsed laser deposition technique for developing new silicon-based micro devices for micro solid oxide fuel cells applications. Yttriastabilized zirconia free-standing membranes with thicknesses from 60 to 240 nm and surface areas between 50x50 μm2 and 820x820 μm2 were fabricated on micromachined Si/SiO2/Si3N4 substrates. Deposition process was optimized for deposition temperatures from 200ºC to 800ºC. A complete mechanical study comprising thermomechanical stability, residual stress of the membranes and annealing treatment as well as a preliminary electrical characterization of ionic conductivity was performed in order to evaluate the best processing parameters for the yttria-stabilized zirconia membranes.

Processing and immobilization of chondroitin-4-sulphate by UV laser radiation.

Chondroitin-4-sulphate (ChS A) was immobilized by matrix assisted pulsed laser evaporation (MAPLE) with the aid of a UV KrF* excimer laser source. Distilled water was used as solvent for the preparation of the frozen composite MAPLE targets. The surface morphology, chemical structure and functional properties of laser transferred ChS A were investigated as a function of laser processing conditions. The results indicate that the amount of laser immobilized material, structure, and functional properties can be controlled by the laser fluence value used for the irradiation of the MAPLE targets. Under selected irradiation conditions besides the molecular structure, the functional properties of the laser processed ChS A molecules can be maintained.

Formation of Self-Organized Mn3O4 Nanoinclusions in LaMnO3 Films

We present a single-step route to generate ordered nanocomposite thin films of secondary phase inclusions (Mn3O4) in a pristine perovskite matrix (LaMnO3) by taking advantage of the complex phase diagram of manganese oxides. We observed that in samples grown under vacuum growth conditions from a single LaMnO3 stoichiometric target by Pulsed Laser Deposition, the most favourable mechanism to accommodate Mn2+ cations is the spontaneous segregation of self-assembled wedge-like Mn3O4 ferrimagnetic inclusions inside a LaMnO3 matrix that still preserves its orthorhombic structure and its antiferromagnetic bulk-like behaviour. A detailed analysis on the formation of the self-assembled nanocomposite films evidences that Mn3O4 inclusions exhibit an epitaxial relationship with the surrounding matrix that it may be explained in terms of a distorted cubic spinel with slight (~9o) c-axis tilting. Furthermore, a Ruddlesden-Popper La2MnO4 phase, helping to the stoichiometry balance, has been identified close to the interface with the substrate. We show that ferrimagnetic Mn3O4 columns influence the magnetic and transport properties of the nanocomposite by increasing its coercive field and by creating local areas with enhanced conductivity in the vicinity of the inclusions.