On the relevance of large scale pulsed-laser deposition: Evidence of structural heterogeneities in ZnO thin films

Pulsed-laser deposition is known as a well-suited method for growing thin films of oxide compounds presenting a wide range of functional properties. A limitation of this method for industrial process is the very anisotropic expansion dynamics of the plasma plume, which induces difficulties to grow on large scale films with homogeneous thickness and composition. The specific aspect of the crystalline or orientation uniformity has not been investigated, despite its important role on oxide films properties. In this work, the crystalline parameters and the texture of zinc oxide films are studied as a function of position with respect to the central axis of the plasma plume. We demonstrate the existence of large non-uniformities in the films. The stoichiometry, the lattice parameter, and the distribution of crystallites orientations drastically depend on the position with respect to the plume axis, i.e., on the oblique incidence of the ablated species. The origin of these non-uniformities, in particular, the unexpected tilted orientation of the ZnO c-axis may be attributed to the combined effects of the oblique incidence and of the ratio between oxygen and zinc fluxes reaching the surface of the growing film

Surface composition of BaTiO3/SrTiO3(001) films grown by atomic oxygen plasma assisted molecular beam epitaxy

We have investigated the growth of BaTiO3 thin films deposited on pure and 1% Nb-doped SrTiO3(001) single crystals using atomic oxygen assisted molecular beam epitaxy (AO-MBE) and dedicated Ba and Ti Knudsen cells. Thicknesses up to 30 nm were investigated for various layer compositions. We demonstrate 2D growth and epitaxial single crystalline BaTiO3 layers up to 10 nm before additional 3D features appear; lattice parameter relaxation occurs during the first few nanometers and is completed at {\guillemotright}10 nm. The presence of a Ba oxide rich top layer that probably favors 2D growth is evidenced for well crystallized layers. We show that the Ba oxide rich top layer can be removed by chemical etching. The present work stresses the importance of stoichiometry and surface composition of BaTiO3 layers, especially in view of their integration in devices.Comment: In press in J. Appl. Phy

Jahn-Teller stabilization of a "polar" metal oxide surface: Fe3O4(001)

Using ab initio thermodynamics we compile a phase diagram for the surface of Fe3O4(001) as a function of temperature and oxygen pressures. A hitherto ignored polar termination with octahedral iron and oxygen forming a wave-like structure along the [110]-direction is identified as the lowest energy configuration over a broad range of oxygen gas-phase conditions. This novel geometry is confirmed in a x-ray diffraction analysis. The stabilization of the Fe3O4(001)-surface goes together with dramatic changes in the electronic and magnetic properties, e.g., a halfmetal-to-metal transition.Comment: 4 pages, 4 figure

First-principles study of the polar O-terminated ZnO surface in thermodynamic equilibrium with oxygen and hydrogen

Using density-functional theory in combination with a thermodynamic formalism we calculate the relative stability of various structural models of the polar O-terminated (000-1)-O surface of ZnO. Model surfaces with different concentrations of oxygen vacancies and hydrogen adatoms are considered. Assuming that the surfaces are in thermodynamic equilibrium with an O2 and H2 gas phase we determine a phase diagram of the lowest-energy surface structures. For a wide range of temperatures and pressures we find that hydrogen will be adsorbed at the surface, preferentially with a coverage of 1/2 monolayer. At high temperatures and low pressures the hydrogen can be removed and a structure with 1/4 of the surface oxygen atoms missing becomes the most stable one. The clean, defect-free surface can only exist in an oxygen-rich environment with a very low hydrogen partial pressure. However, since we find that the dissociative adsorption of molecular hydrogen and water (if also the Zn-terminated surface is present) is energetically very preferable, it is very unlikely that a clean, defect-free (000-1)-O surface can be observed in experiment.Comment: 10 pages, 4 postscript figures. Uses REVTEX and epsf macro

Determination of the cation site distribution of the spinel in multiferroic CoFe2O4 / BaTiO3 layers by X-ray photoelectron spectroscopy

International audienceThe properties of CoFe2O4/BaTiO3 artificial multiferroic multilayers strongly depend on the crystalline structure, the stoichiometry and the cation distribution between octahedral (Oh) and tetrahedral (Td) sites (inversion factor). In the present study, we have investigated epitaxial CoFe2O4 layers grown on BaTiO3, with different Co/Fe ratios. We determined the cation distribution in our samples by X-ray magnetic circular dichroism (XMCD), a well accepted method to do so, and by X-ray photoelectron spectroscopy (XPS), using a fitting method based on physical considerations. We observed that our XPS approach converged on results consistent with XMCD measurements made on the same samples. Thus, within a careful decomposition based on individual chemical environments it is shown that XPS is fully able to determine the actual inversion factor

Ferromagnetic resonance signature of metallic Co clusters in ferromagnetic ZnCoO thin films

International audienceHighly Co-doped Zn(0.7)Co(0.3)O thin films that can be reproducibly grown ferromagnetic by pulsed laser deposition have been investigated by optical absorption and ferromagnetic resonance spectroscopy (FMR). The saturation magnetization depends strongly on the O(2) content during the growth, the highest value being obtained for the lowest oxygen pressure conditions. Whereas the optical absorption spectra confirm the incorporation of Co as Co(Zn)(2+) in ZnO, the FMR spectra prove the additional presence of randomly oriented metallic Co nanoparticles. The angular variation in the FMR spectra allows us to determine in the most ferromagnetic film the effective magnetization, 4 pi M similar to 800 G at 300 K and the g factor, g=2.18, which is that of metallic Co. The fraction of the two species Co(Zn)(2+) and Co metal is estimated to be similar to 3:1 (C) 2008 American Institute of Physics

High-temperature ferromagnetism by means of oriented nanocolumns: Co clustering in (Zn,Co)O

International audienceIn order to provide insight into the magnetic properties of semiconductors with ferromagnetic inclusions, we present a thorough quantitative analysis of (Zn,Co)O films with high Co concentration, which give rise to high saturation magnetization values (similar to 110 kA m(-1)) at room temperature. From the orientation-, temperature-, and field-dependent responses of the magnetization and of the Q-band ferromagnetic resonance modes, we give evidence of two types of nanosized ferromagnetic Co clusters and derive their respective contributions. The first type are spherical with diameters about 5 nm and blocking temperatures T(B) about 100 K. The second type are elongated along the surface normal and crystallographically oriented with respect to the ZnO host. Some are nanocolumns about 4 nm wide, whose height may reach up to 60 nm, leading to T(B) >= 300 K. These results are confirmed by high-resolution transmission electron microscopy analysis. In addition to provide transition from ferromagnetism to superparamagnetism above 300 K, the nanocolumns lead to strong anisotropic magnetic properties. We believe that they could find applications in spintronic devices

Unstrained islands with interface coincidence sites versus strained islands: X-ray measurements on Ag ∕ ZnO

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