Wayne A. Bryden

High quality self‐nucleated AlxGa1−x N layers on (00.1) sapphire by low‐pressure metalorganic chemical vapor deposition

High quality AlxGa1−xN alloy films with x<0.4 have been prepared on self‐nucleated (00.1) sapphire substrates by low‐pressure metalorganic chemical vapor deposition. It has been shown that the lattice constant of the films varies linearly with alloy composition x (Vegard’s law is obeyed) and that homogeneous and inhomogeneous strain and alloy clustering are minimized in these self‐nucleated AlxGa1−xN films. Consistent with their reduced strain and chemical uniformity, the derived optical band gaps of these AlxGa1−xN films also show a linear dependence on alloy composition x, yielding a bowing parameter b≊0 eV.

The coexistence of magnetic ordering and superconductivity in YBa2(Cu0.94Fe0.06)3O9−σ

Abstract We have made superconducting samples of the Y-Ba-Cu-O system with 6% Fe substituted for the Cu. The sample has been verified by X-ray diffraction to be a single-phased 1-2-3 compound. The 6% Fe substitution reduces T c from 94 to 80 K. The room temperature Mossbauer spectrum shows two pairs of doublets, indicating there are two distinct Cu sites in the sample. The most surprising result is that magnetic ordering of the Fe coexists with superconductivity of the material at 4.2 K.

Characterization of rf‐sputtered InN films and AlN/InN bilayers on (0001) sapphire by the x‐ray precession method

Thin films of InN and AlN/InN bilayers have been deposited on (0001) sapphire at a variety of substrate temperatures by reactive rf‐magnetron sputtering. For all films, the c axis of the metal nitride film parallels the c axis of the sapphire substrate, and the transmission x‐ray precession method has been employed to study the nature and azimuthal coherence of the in‐plane heteroepitaxy. For the InN films, a mixture of textured and epitaxial grains persists up to ∼200 °C and solely epitaxial grains are observed at higher substrate temperatures, to a maximum adherence temperature of 525 °C. In contrast, the AlN/InN bilayers are epitaxial at substrate temperatures of 50 °C and above, and a uniform deposition can be retained up to 650 °C. These fundamental differences in adhesion, structure and morphology of the InN films and the AlN/InN bilayers on (0001) sapphire are also clearly reflected, for example, in the variation in electrical mobility with substrate deposition temperature.

CORONA PLASMA DISCHARGE FOR RAPID ANALYSIS OF MICROORGANISMS BY MASS SPECTROMETRY

Corona plasma discharge provides a rapid and reliable tool for release of biomarkers from gram negative and positive bacteria, spores and viruses for characterization by mass spectrometry.

Structure of an organic charge‐transfer salt derived from dibenzotetrathiafulvalene and tetrafluorotetracyanoquinodimethane (DBTTF–TCNQF4). Observation of a high‐temperature phase transition

The crystal structure of the organic charge‐transfer salt DBTTF–TCNQF4 is reported. The structure has been determined by single‐crystal x‐ray diffraction techniques and has been shown to consist of segregated stacks of donor and acceptor molecules. Within both the donor and acceptor stacks, significant dimerization is observed at room temperature. From the geometries of the DBTTF and TCNQF4 molecules, it is concluded that the charge transfer is complete. The electrostatic contribution to the crystal cohesion for DBTTF–TCNQF4 has been calculated and compares well to those for similar salts of unit charge transfer. It is suggested that the room‐temperature structure may well be representative of the low‐temperature phase of a system affected by a Peierls instability. Based on diffraction data, a phase transition near 390 K is reported. The driving force for the transition is likely a spin–phonon instability. Crystal data for DBTTF–TCNQF4 are triclinic, space group P1; a = 13.159(3) A, b = 13.703(4) A, c = ...

Structural and electrical properties of reactively sputtered InN thin films on AlN‐buffered (00.1) sapphire substrates: Dependence on buffer and film growth temperatures and thicknesses

An extensive investigation of InN overlayers on AlN‐buffered (00.1) sapphire by reactive magnetron sputtering has been undertaken and the dependencies of several basic materials properties (film thickness, development and quality of heteroepitaxy, film morphology, and electrical transport) on such key deposition parameters such as the growth temperatures of the insulating AlN buffer layer and the InN overlayer and their thicknesses have been determined. Three prominent effects of the AlN buffer layer are (1) the stabilization of heteroepitaxial growth over a broad range of film and buffer layer growth temperatures; (2) the attainment of a higher Hall mobility (up to 60 cm2/V s) over much of the same range; and, (3) the retention of heteroepitaxial growth, higher Hall mobility, and pseudo‐two‐dimensional growth even in the limit of an InN layer of ∼40 A. In the context of a structure‐zone model, the AlN buffer layer is projected to effectively raise the growth temperature of the InN thin film. The increase...

Overgrowth of indium nitride thin films on aluminum nitride nucleated (00.1) sapphire by reactive magnetron sputtering

The growth and properties of thin films of InN on (00.1) sapphire and (00.1) sapphire nucleated by a 400 A layer of AlN have been found to be radically different. The unnucleated InN films exhibit a mixed morphology (largely textured with lesser amounts of epitaxial grains), a monofunctional thickness dependence on sputtering time, uniformly low mobility and carrier concentration, and high resistivity. In contrast, the AlN‐nucleated overlayers show only heteroepitaxial grains, a bifunctional dependence for the film thickness and surface roughness on sputtering time, higher mobility and carrier concentration, and lower resistivity−even in the limit of an InN overlayer on the order of 20–40 A.

EPR PERSISTENCE MEASUREMENTS OF UV-INDUCED MELANIN FREE RADICALS IN WHOLE SKIN

Electron paramagnetic resonance is used to detect the formation of free radicals caused by exposure to ultraviolet radiation in chemically untreated rabbit skin. A fast jump in EPR signal level, occurring over a few seconds, is observed immediately after a skin sample is exposed to UV. This is followed by a slower increase toward an elevated steady‐state signal over a period of hours as the skin is continuously exposed to a UV light source. Upon cessation of UV light exposure, EPR signal levels undergo an abrupt drop followed by a slower decay toward natural levels. Elevated free radical concentrations following UV exposure are found to persist for several hours in whole skin. These results are consistent with time‐resolved EPR measurements of photoinduced radicals in various natural melanins.

Magnetic ordering and superconducting in Fe‐doped 1‐2‐3 compounds

We have doped Fe into the ground YBa2Cu3O7 and have discovered that the Fe goes primarily into one of the two Cu sites. At least 95% of the Fe substitutes into the Cu(1) site in the Cu‐O layers between Ba layers. A high valence state of iron, Fe4+, is found to exist inside this compound. For even small amounts of Fe we see a spin‐glass type of magnetic ordering of the Fe spins which we feel is associated with a tendency for antiferromagnetic couplings of the Cu(1) ions. We conclude that the Cu(2)‐O2 plane is more important than the Cu(1)‐O chain for the superconductivity in the 1‐2‐3 compounds.

Initial studies on a new tellurium containing organic metal: Tetratellurafulvalene-tetracyanoquinodimethane (TTeF-TCNQ)

Abstract Experimental conditions for the growth of acicular crystals of the new organic conductor tetratellurafulvalene-tetracyanoquinodimethane (TTeF-TCNQ) are reported. The room temperature conductivity along the needle axis was found to be 1800 ± 300 (omega-cm)−1. dc Conductivity studies down to 77 K displayed metallic behavior and microwave conductivity studies showed the compound to still be highly conducting at 5 K. EPR studies down to 8 K were performed but no measurable resonances were recorded. Preliminary x-ray scattering studies indicate a segregated stack structure with a stacking distance of 3.95 ± 0.01 A. The charge transfer was found to be 0.71 ± 0.03 e- from infrared spectroscopy data. In most respects the physical properties of TTeF-TCNQ are similar to those of HMTSF-TCNQ.