Björgvin Hjörvarsson

Hydrogen uptake in alumina thin films synthesized from an aluminum plasma stream in an oxygen ambient

We describe the hydrogen uptake during the synthesis of alumina films from H2O present in the high vacuum gas background. The hydrogen concentration in the films was determined by the 1H(15N,αγ)12C nuclear resonance reaction. Furthermore, we show the presence of hydrogen ions in the plasma stream by time-of-flight mass spectrometry. The hydrogen content increased in both the film and the plasma stream, as the oxygen partial pressure was increased. On the basis of these measurements and thermodynamic considerations, we suggest that an aluminum oxide hydroxide compound is formed, both on the cathode surface as well as in the film. The large scatter in the data reported in the literature for refractive index and chemical stability of alumina thin films can be explained on the basis of the suggested aluminum oxide hydroxide formation.

GROWTH OF EPITAXIAL ALN(0001) ON SI(111) BY REACTIVE MAGNETRON SPUTTER-DEPOSITION

2H–AlN(0001) layers have been grown on Si(111) by reactive magnetron sputtering from an Al target in Ar+N2 gas mixtures at temperatures Ts=400–900u2009°C. Variations in reactive gas consumption, target voltage, and current–voltage characteristics versus nitrogen partial pressure were used to determine deposition parameters required to yield stoichiometric AlN with growth rates ≥2 μm h−1. High‐resolution cross‐sectional transmission electron microscopy (XTEM) analyses of films grown at 900u2009°C showed that the initial 6–8 monolayers were (111)‐oriented cubic 3C before transforming to the (0001)‐oriented 2H polytype. The epitaxial relationship was found by XTEM and x‐ray diffraction (XRD) to be 2H–AlN(0001)//3C–AlN(111)//Si(111) with 2H–AlN[1210]//3C–AlN[110]//Si[110]. High‐resolution XRD ω−2Θ and ω rocking curve widths for films grown at Ts=900u2009°C were 70 and 500 arcu2009sec, respectively, the lowest values yet reported.2H–AlN(0001) layers have been grown on Si(111) by reactive magnetron sputtering from an Al target in Ar+N2 gas mixtures at temperatures Ts=400–900u2009°C. Variations in reactive gas consumption, target voltage, and current–voltage characteristics versus nitrogen partial pressure were used to determine deposition parameters required to yield stoichiometric AlN with growth rates ≥2 μm h−1. High‐resolution cross‐sectional transmission electron microscopy (XTEM) analyses of films grown at 900u2009°C showed that the initial 6–8 monolayers were (111)‐oriented cubic 3C before transforming to the (0001)‐oriented 2H polytype. The epitaxial relationship was found by XTEM and x‐ray diffraction (XRD) to be 2H–AlN(0001)//3C–AlN(111)//Si(111) with 2H–AlN[1210]//3C–AlN[110]//Si[110]. High‐resolution XRD ω−2Θ and ω rocking curve widths for films grown at Ts=900u2009°C were 70 and 500 arcu2009sec, respectively, the lowest values yet reported.

Role of hydrogen for the elastic properties of alumina thin films

We describe the role of the presence of hydrogen on the elastic properties of AlxOyHz (0.32less than or equal toxless than or equal to0.4, 0.54less than or equal toyless than or equal to0.6, 3x10(- ...

Composition, structure, and dielectric tunability of epitaxial SrTiO3 thin films grown by radio frequency magnetron sputtering

Epitaxial (001) oriented SrTiO3 films have been deposited on LaAlO3(001) substrates by off-axis radio frequency magnetron sputtering in Ar:O2 gas mixtures at substrate temperatures ranging from 650 to 850u200a°C. For the deposition conditions used, stoichiometric targets yielded 20% Sr-deficient films, whereas Sr-enriched targets (Sr1.1Ti0.9O3.0) resulted in stoichiometric films. The Sr-deficient films had a mosaic structure and a larger lattice parameter in comparison to bulk SrTiO3. The stoichiometric films on the other hand had a much higher crystalline quality in the as-deposited condition. The mosaicity of the latter films was primarily limited by the crystalline quality of the LaAlO3 substrates. The lattice parameters of the stoichiometric films were also smaller than the Sr-deficient ones and closer to the bulk value. The dielectric properties of the stoichiometric films were superior to the Sr-deficient films. For films with a thickness of ∼300 nm, the typical dielectric constants as measured at ∼77 K...

Hydrogen in carbon nanostructures

The non-dissociative hydrogen uptake for two types of carbon-based materials was explored. The maximum measured differential hydrogen uptake capacities at hydrogen pressures of 10 MPa were experimentally determined with the volumetric Sieverts method to 2.9 (300 K), and 4.1 wt% (77 K) for MWNB (multi-walled nanobarrels) and 2.6 (300 K), and 3.3 (77 K) wt% for ANPC (amorphous nanoporous carbon). The total hydrogen uptake was calculated to be 3.2 (300 K) and 6.2 (77 K) wt% for MWNB at a hydrogen pressure of 10 MPa, and 2.8 (300 K) and 4.2 (77 K), respectively, for ANPC. The adsorption energies were determined to be 7.2 and 4.2 (KJ/mol) for ANPC and MWNB, respectively, at the lowest coverage. At higher coverage (concentration), a multi-site model is required to describe the coverage dependence, consistent with large heterogeneity of the adsorption sites.

Hydrogen desorption from sand-blasted and acid-etched titanium surfaces after glow-discharge treatment

Hydrogen desorption from argon plasma-treated titanium implants with a high surface roughness was studied. Implants with a high surface roughness have shown an increase in mechanical stability in bone tissue and a different behavior of osteoblasts in vitro. High surface roughness was produced by grit blasting and acid etching, resulting in an increase of the sub-surface hydrogen concentration and the formation of a titanium hydride. After an argon plasma treatment the surface oxide, which always covers titanum surfaces exposed to an oxygen-containing environment, and some of the hydrogen were sputtered away, decreasing the hydrogen concentration in the sub-surface region. Nuclear reaction analysis was used to determine the hydrogen concentration as a function of depth. The total amount of sub-surface (down to a depth of < or = 2 microm) hydrogen remaining after plasma treatment decreased with increasing plasma intensity to below the levels observed in non-acid-etched samples (approximately 1-2%). Thermal desorption spectroscopy was used for desorption studies and investigation of H(2) desorption activation energies. With a surface oxide present, the onset of hydrogen desorption is at ca 400 degrees C, which is the oxide decomposition temperature in vacuum, with an activation energy of ca 2 eV/molecule of H(2). After plasma treatment, that is, without surface oxide present, the onset of desorption was observed at ca 300 degrees C and with an activation energy of ca 0.8 eV/molecule of H(2), indicating a bulk diffusion-limited desorption.

Hydrogen induced changes of the interlayer coupling in Fe(3)/V(x) superlattices (x=11–16)

Abstract The effect of hydrogen on the magnetic exchange coupling between iron layers through vanadium spacer layers has been studied with magneto-optical Kerr effect experiments in Fe (3) /V ( x ) superlattices. Here x refers to the number of V monolayers varying from 11 to 16 and the Fe layer thickness is fixed at three monolayers. Without hydrogen the superlattice is antiferromagnetic (AFM) for x between 12 and 14 and ferromagnetic (FM) in all other cases. With hydrogen loading the coupling can be switched from AFM to FM and vice versa. As previously observed with neutron reflectivity measurements (Hjorvarsson et al., Phys. Rev. Lett. 79 (1997) 901) the change of the interlayer coupling upon hydrogen uptake is not simply due to the expansion of the non-magnetic vanadium spacer layer but more likely to the distortion of the Fermi surface. Bilinear and biquadratic exchange couplings can be recognized by the magnetic hysteresis loops and their coupling energies have been extracted by fits to the curves. For all samples the easy axis of the magnetization is in the plane without any preferred in-plane direction. Hydrogen loading does not affect the magnetic anisotropy of these samples.

Temporal development of the plasma composition of a pulsed aluminum plasma stream in the presence of oxygen

We describe the temporal development of the plasma composition of pulsed aluminum plasma streams at various oxygen pressures. The plasma was formed with a vacuum arc plasma source and the time resolved plasma composition was measured with time-of-flight charge-to-mass spectrometry. The temporal development of the plasma composition as well as the Al average ion charge state was found to be a strong function of the oxygen pressure. Oxygen and hydrogen concentrations of up to 0.36 and 0.32, respectively, were found in the first 50 μs of the pulse at oxygen pressures of ⩾5×10−5 Torr. The average charge state of aluminum ions was found to vary from +1.2 to +2.5 depending on the oxygen pressure and the time elapsed after ignition of the arc. These results are of fundamental importance for the understanding of the evolution of the composition (through the plasma composition) and microstructure (through the Al ion flux energy) of alumina thin films produced by pulsed, reactive aluminum plasmas.

Hydrogen induced change of the atomic magnetic moments in Fe/V-superlattices

Abstract We have studied the change of the magnetic saturation of (Fe n /V m ) 30 superlattices (30 periods with n monolayers of Fe and m monolayers of V) upon loading with hydrogen using a highly sensitive Faraday balance and in situ loading with hydrogen. We find that the measured magnetic saturation moment for all samples increases with the hydrogen. The measured magnetic saturation moment for all samples increases with the hydrogen concentration. For the superlattice (Fe 3 /V 11 ) 30 we find the maximum increase, corresponding to a change of the atomic magnetic moments of +0.35xa0μ B /Fe atom. We attribute this remarkable effect to a change of the Fe and V magnetic moments at the interfaces caused by the charge transfer from the hydrogen atoms to the vanadium d bands.

Hydride Formation in Mg/Ni-Sandwiches Studied by Hydrogen Profiling and Volumetric Measurements*

The hydrogen uptake of magnesium films with nickel as a catalytic overlayer has been studied for different exposure times and pressures. The total uptake was measured with volumetric measurements and the hydride distribution in the film studied utilizing the 1H(15N,ay)12C-nuclear reaction. A complete hydride formation is seen for depths up to 1000 Â. The uptake decreases for reduced overlayer thicknesses. For the pressures used, the uptake process is found to be kinetically limited.