Papers by Author: Efstathios I. Meletis

Abstract: Tungsten (W) reinforced diamond-like carbon (DLC) nanocomposite thin films were deposited on silicon substrates by magnetron sputtering in a CH₄/Ar discharge. The W content of the films was varied by varying the W target power (20, 40, 60, 80, and 100 W). The evolution of the W-DLC nanocomposites was studied by high-resolution transmission electron microscopy, X-ray energy dispersive spectroscopy, X-ray photoelectron spectroscopy, 3D optical profilometry and Raman spectroscopy. Increasing the W target power resulted in an almost liner increase in the W content, reduced the hardness and the sp³/sp² ratio in the films, while it increased the surface roughness and promoted formation of WC nanoparticles. Tribological properties were studied by conducting sliding reciprocating testing. Wear tracks were analyzed with Raman spectroscopy and 3D optical profilometry. Increasing the W content in the films (increasing target power) resulted in a reduction of both, the friction coefficient and wear rate. The film deposited at 80 W target power (~8 at. % W) exhibited the lowest friction coefficient (0.15) and wear rate (6x10^-7 mm³N^-1m^-1). The observed low friction and wear rate were attributed to the particular nanocomposite structure of the films involving a fine distribution of WC nanoparticles surrounded by a sp² dominant carbon network. The present W-DLC nanocomposite films offer a highly desirable combination of low friction and low wear rate.

Abstract: Nickel films are grown by radio frequency magnetron sputtering on Corning glass, polyimide foils and on the native oxide of Si (100) wafers at low (1x10^-3 mbar) and relatively high (2.5x10^-2 mbar) Argon pressure at 100 °C. The base pressure of the high vacuum chamber is 1x10^-7 mbar. X-ray diffraction experiments are performed to reveal the different texture of various Ni films. Magneto-optical Kerr effect hysteresis loops and magnetic force microscopy images show correlation between Argon pressure, texture and magnetic anisotropy of the films. The results are discussed with respect to relevant measurements of Ni/NiO magnetic multilayers prepared under similar experimental conditions.

Abstract: Cu and Ni from CuNi metallic targets (composition 20-80 and 46-54 at.%) are deposited on Corning glass, quartz and the native oxide of Si (100) wafers by direct current magnetron sputtering in a high vacuum chamber (base pressure 5 x 10^-5 mbar). The CuNi films, with thickness 40 200 nm, are post annealed at temperatures 400 - 500 °C in a furnace under atmospheric air in order to be fully oxidized. The structure of the films is studied by x-ray diffraction experiments. Phase separation of the oxides is evident. The optical properties are studied via ultraviolet-visible light absorption spectroscopy. The spectra of CuNi-oxide films are compared with the spectra of the pure CuO and NiO films. Features originating from both CuO and NiO are detected in the spectra of the CuNi-oxide thin films.

Abstract: . In this work, we present a simple method to fabricate high quality Ni/NiO multilayers with the use of a single magnetron sputtering head. Namely, at the end of the deposition of each single Ni layer, air is let to flow into the vacuum chamber through a leak valve. Then, a very thin NiO layer (~ 1nm) is formed by natural oxidation. The process is reproducible and the result is the formation of a multilayer with excellent layering. Magnetization hysteresis loops recorded at 5 K and room temperature reveal a tendency for perpendicular magnetic anisotropy as the thickness of the individual Ni layers decreases. It is shown that the Ni/NiO interface has sizeable positive surface/interface anisotropy, i.e. it favors the development of perpendicular magnetic anisotropy. This is rather unusual for a Ni-based multilayered system and may render Ni/NiO multilayers useful for magneto-optical recording applications.

Abstract: Thin Cu films of thickness 0.4 – 150 nm were deposited via radio frequency magnetron sputtering on Si(100) wafers, corning glass and quartz. Subsequently the Cu films were oxidized in ambient air at 230oC and 425oC in order to produce single-phase Cu2O and CuO, respectively. Selected samples were measured in the transmission geometry with the help of an ultraviolet – visible spectrophotometer. From the absorption spectra of the films, it was found that the gap EB for the dipole allowed transitions showed blue shifts of about 1.2 eV for the Cu2O thinnest film (0.75 nm), whereas the Edirect for the direct gap transitions showed blue shifts of about 0.16 eV for the CuO thinnest film (0.7 nm). The blue shift of the energy gap in the copper-oxide semiconductors is an indication of the presence of strong quantum confinement effects.

Abstract: Synthesis of high quality epitaxial LaMnO3 and (La,Sr)MnO3 films on large areas is highly desirable. Recently, we have deposited LaMnO3 and (La,Sr)MnO3 films on the MgO (001) and LaAlO3 (001) substrates using RF magnetron sputtering. Highly epitaxial quality thin films have been successfully obtained at 750 °C by manipulating processing parameters as characterized by X-ray diffraction, electron diffraction and HRTEM. The epitaxial LaMnO3 and (La,Sr)MnO3 thin films have either a tetragonal or orthorhombic crystal structure depending on the film (target) composition and substrate type. The (La,Sr)MnO3 films were found to have an orthorhombic crystal structure when deposited on LaAlO3 substrate and a tetragonal structure when deposited on MgO substrate; whereas LaMnO3 films have a tetragonal structure when deposited on LaAlO3 substrate and an orthorhombic crystal structure when deposited on MgO substrate. The orthorhombic structures of the (La,Sr)MnO3 film on LaAlO3 and LaMnO3 on MgO are oriented with their c-axis on the film plane. Magnetic studies show that the epitaxial films have higher phase transition temperature than the corresponding bulk material and to those obtained using pulse laser deposition. Successful synthesis of highly epitaxial quality films by RF magnetron sputtering over a larger area can result in reduced cost for fabricating and processing epitaxial thin films.

Abstract: Two-dimensional in-plane interface structures of highly epitaxial perovskite (La,Ca)MnO3 (LCMO) and (Pb,Sr)TiO3 (PSTO) thin films on salt-rock type MgO substrate were studied using Transmission Electron Microscopy (TEM). Cross-section TEM studies revealed that both LCMO and PSTO films are good single crystal quality and have atomic sharp interface with respect to the MgO substrate with -6.4% and -6.2% lattice mismatch, respectively. Electron Diffraction Patterns (EDPs) of plan-view LCMO/MgO and PSTO/MgO interfaces exhibit double diffraction spots. An analytical approach was employed using double diffraction to study the two-dimensional in-plane interfaces of perovskite structure films on the salt-rock type substrate. The lattice mismatch near the interface regions was determined using EDPs of the plan-view interfaces and found to be -8.0% for LCMO/MgO and -7.14% for PSTO. Both latter values are larger than those obtained using cross-section TEM. Studies of the sharpness of double diffraction spots and plan-view high resolution (HR) TEM brought a conclusion that the PSTO film is well commensurate with the MgO substrate over large areas, whereas LCMO film is only locally commensurate with the substrate. These studies provide additional evidence to our previous observations that plan-view TEM of the interface is able to provide critical and valuable information that is lacking from the cross-section TEM analysis.

Abstract: We report the fabrication of the orientation preferred structures in BaTiO3 thin films on Ni substrates using pulsed laser deposition. Transmission electron microscopy studies showed that the films consist of crystalline structures of tetragonal BaTiO3. More than 60% of BaTiO3 grains in the films exhibit nearly the same crystallographic orientation with their a-axis lying in the film plane and the [011] direction parallel to the growth direction. Such orientation preferred structures were grown on a Ni nanocrystalline buffer layer. This result demonstrated the possibility of approximating an oriented single crystalline ceramic oxide structures on metallic substrates.