F. Weiss

Pulsed injection MOCVD of functional electronic oxides

A new source has been developed in LMGP and in Vilnius University for the generation of active gases for chemical vapour deposition of layers. The principle is a sequential injection of micro amounts (few milligrams) of a solution of organic precursors into an evaporator, where a flash volatilization occurs. The system is very different from the other liquid sources currently used for the MOCVD process, the main advantages are: For a fixed injection parameter, the thickness of the layer is only determined by the number of drops injected (‘digital growth’). The system uses a flash evaporation: only one source containing a solution of the different precursors in an organic solvent is required for the synthesis of complex compounds, but, of course, it is possible to use one source for each element. Multilayers or superlattices of complex compounds are obtained with several injection sources connected to the same reactor and working alternately. A wide variety of compounds and multilayers have already been grown using this new CVD source. In the field of high-temperature superconductors, high-quality YBa2Cu3O7 epitaxial layers have been recently obtained on 3 inch LaAlO3 single-crystal substrate (Tc=92.1 K, δTc=0.15 K, Jc(77K)=4–6 MA/cm2 over the whole surface). This demonstrates that the large amount of carbon existing in the gas phase does not affect the properties of the superconductor. Copyright

High emissivity of a rough Pr2NiO4 coating

A rough and thick film of doped praseodymium nickel oxide (Pr2NiO4+δ) has been obtained by combining the spray pyrolysis deposition technique with rapid thermal annealing process. The precursors are nitrate solutions. Spectral emissivity measurements from the far infrared up to the midinfrared region at T=1000 K show the strong black body character of the film. This feature is well suited to the thermal efficiency of an industrial infrared ceramic radiant plate.

Investigation of thickness-dependent stress in PbTiO3 thin films

X-ray diffraction (XRD) and Raman spectroscopy were used to investigate stress dependence on thickness in PbTiO3 (PTO) films grown by pulsed liquid injection metalorganic chemical vapor deposition on a LaAlO3 (001) substrate (LAO). Films on sapphire substrate (R plane) were used as the polycrystalline film reference. Epitaxial PTO films with a dominant c domain structure are grown on LAO substrate, whereas the films on sapphire are polycrystalline. A detailed investigation of the PTO/LAO film microstructure by XRD gives evidence of PTO twinning. Both techniques reveal that PTO films are under tensile in-plane stresses. The study of the film thickness dependence of microstrains, grain size, volume fraction of a domains, as well as surface morphology of PTO/LAO films indicates that these parameters are clearly correlated. A change in the relaxation mechanism between 65 and 125 nm of film thickness has been evidenced. A c parameter gradient occurs throughout the film depth; it originates in stress relaxation...

Injection MOCVD : ferroelectric thin films and functional oxide superlattices

Abstract MOCVD is a suitable technique for the synthesis of functional metal oxide thin films. In the present paper, injection-MOCVD was proved to be an efficient method, not only to grow single layers, but also heterostructures and superlattices of functional oxide materials. We report on the deposition of Ba x Sr 1− x TiO 3 (BST) films and BaTiO 3 /SrTiO 3 superlattices [(BTO/STO) n ] obtained by injection MOCVD. In a first step, BST films were deposited on various substrates such as LaAlO 3 (LAO)(012), SrTiO 3 :Nb(STO:Nb)(001) and Al 2 O 3 (0001), resulting in epitaxial films in the case of STO or LAO substrates and highly textured films in the case of Al 2 O 3 substrates. The good crystallinity of the films obtained was evidenced using X-ray analysis. Concerning the dielectric properties of BST films on STO:Nb, the T c values were found to be the same as those for bulk materials. In a next step the feasibility of (BTO/STO) n superlattices was studied. The multilayers obtained were epitaxially grown on LAO and STO:Nb substrates. XPS was used to check in detail the pattern repetition of the superlattice and proved the low interface roughness of 1 nm as well as the absence of carbon contamination in the film. The measured periodicity of these superlattices were found to be in good agreement with simulation experiments. Further superlattices have been obtained by combining ferroelectric, magnetic (CMR) and superconducting (HTS) oxides. Typical results of these superlattices will be reported.

In-situ heteroepitaxial growth of CeO2/YBa 2Cu3O7 films on sapphire by injection CVD

Abstract A new principle for stable generation of precursors from volatile vapours in a CVD reactor was developed for in-situ deposition of bilayers CeO 2 /YBa 2 Cu 3 O 7 on sapphire (11¯02) substrates. This principle is based on computer controlled injection of precise microdoses of liquid precursors or solutions of solid precursors in an organic solvent in an evaporator of CVD reactor, “flash evaporation” and rapid vapour transport to the deposition zone. Application of several injections sources permits in-situ deposition of multilayered oxide structures. Heteroepitaxially grown CeO 2 (≈ 0.2 μm)/YBa 2 Cu 3 O 7 (0.5–1 μm)/Al 2 O 3 structures exhibited a sharp superconducting transition at about 90 K (Δ T = 0.6-1 K) and had a critical current density of about 10 5 A/cm 2

Raman scattering study of La0.7Sr0.3MnO3/SrTiO3 multilayers

We report room-temperature Raman scattering results on perovskite-type [(La0.7Sr0.3MnO3)m/(SrTiO3)n]15 multilayers, m and n being the thicknesses of layers alternated 15 times. Distinct changes in the Raman spectra for different n and m have been analysed in the light of structural modifications and point to a tensile-strain-induced rhombohedral-to-orthorhombic phase transition in the La0.7Sr0.3MnO3 (LSMO) layers. As a matter of fact, the presented results provide a new structural basis for the earlier proposed magnetic phase separation model. The present work validates tensile strain as an important variable in thin films, which not only allows us to distort the perovskite structure within a crystallographic space group but can readily induce a transition towards another space group and its related physical properties. As shown in this study, the fabrication of superlattice materials then constitutes a powerful method to stabilize such artificial crystal structures (here artificial orthorhombic LSMO at 300 K) over a greater overall thickness than would be possible in single films.

Pulsed liquid-injection metalorganic chemical vapor deposition of (La0.7Sr0.3MnO3/SrTiO3)15 superlattices

(La0.7Sr0.3MnO3/SrTiO3)15 superlattices have been grown by pulsed liquid-injection metalorganic chemical vapor deposition on monocrystalline substrates such as LaAlO3, SrTiO3, and MgO. The pulsed-injection technique allows one to control precisely the amount of precursors delivered to the deposition chamber and thus the thickness of each individual layer. The period of the superlattices depends indeed linearly on the number of injected droplets. In our deposition conditions, the average growth rates are ∼0.130 nm/injection for La0.7Sr0.3MnO3 and 0.042 nm/injection for SrTiO3, with no significant difference as regard to the substrate used. The strain’s state of the superlattices depends on the relative thicknesses of the La0.7Sr0.3MnO3 and SrTiO3 layers and also on the substrate used. Finally, the deposition of superlattices with ultrathin interlayers of few unit cells has been demonstrated.

Growth of SrTiO3 thin epitaxial films by aerosol MOCVD

SrTiO3 thin films were prepared by aerosol metal-organic chemical vapour deposition on (001) MgO, R-plane Al2O3 and (001) Si single-crystal substrates. Strontium tetramethyl heptadionate and titanium n-butoxide dissolved in diethyleneglycol dimethyl ether were used as precursors. The structure of the films was investigated by X-ray diffraction and transmission electron microscopy. Epitaxial films with [001] and [111] orientation perpendicular to the substrate surface were obtained on MgO and Al2O3, respectively. The epitaxial films on the MgO substrate were found to be in a relaxed state with lattice parameters corresponding to the bulk values. SrTiO3 films on the Si substrate were grown as highly textured in the [011]direction and randomly oriented in the plane parrallel to the substrate surface.

High-resolution transmission electron microscopy observations of La2Zr2O7 thin layers on LaAlO3 obtained by chemical methods

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. HRTEM observations of La2Zr2O7 thin layers on LaAlO3 obtained by chemical methods L. Rapenne, Carmen Jiménez, Tristan Caroff, C. Millon, Stéphanie Morlens, Pascale Bayle-Guillemaud, François Weiss

Continuous YBCO deposition onto moved tapes in liquid single source MOCVD systems

Abstract In a small scale MOCVD system YBCO films with j c(77 K ) > 1 MA were deposited onto substrates, which were continuously moved through the deposition chamber. Two routes for the buffer layer deposition onto textured Ni tapes were chosen: a) CeO 2 , LaMnO 3 and LaCrO 3 films were deposited in hydrogen atmosphere by MOCVD. On LaAlO 3 and R-Al 2 O 3 crystals epitaxial films were obtained b) textured NiO layers were grown by oxidation of textured Ni tapes. Based on the small scale system an enlarged MOCVD reactor was constructed.