ALBERT

Notizen: The structure of hard and adherent surface layers formed by cw laser irradiation in air of zirconium disks is examined by electron microscopy and x-ray diffraction. A model is proposed for the evolution of the interaction process in this case.

Notizen: Explosive-driven hemispherical-implosions were used to create high dynamic pressures (up to 13 GPa) and temperatures (up to 450 K) in an exploratory investigation of chemical reactions and diffusion processes in different materials subjected to extreme conditions. Diffusion coefficients as high as 200 cm2/s and chemical reaction rates as fast as 1000 mol/s (in a volume of 0.2 cm3) were inferred from shock experiments with carbontetrabromide (CBr4) and copper (Cu) powder mixtures. An outdiffusion of Ga from a single crystal GaAs substrate into a polycrystalline indium thin film deposited on it was also observed and confirmed by scanning electron microscopy and energy dispersive x-ray microanalysis.

Notizen: If the major axes of the helical and vertical field coils of a torsatron plasma confinement device are not aligned to within ∼1% of their major radii, the resulting error field can break up closed magnetic flux surfaces and reduce the effective plasma confinement volume. A novel technique for accurately locating the magnetic symmetry axes of torsatron helical and vertical coil sets to within ∼1 mm using magnetic field measurements near the device major axis (i.e., away from the confinement volume) is described and applied to the URAGAN-3 torsatron. The axis of the vertical field coil set was found to be shifted by ∼1 cm relative to the axis of the helical coil set; this shift could account for the reduced confinement volume observed in previous experiments. The magnetic measurements were repeated after coil repositioning to verify correct alignment.

Notizen: The possibility to investigate optical activity of uniaxial media by measuring the difference between the intensities of reflected electromagnetic waves (emw) of different circular polarization (reflection optical activity—ROA) is discussed. The components of the gyration tensor and the polarization of the normal emw in uniaxial media are found. The reflection of emw is studied in two cases: (a) optical axis normal to the boundary; (b) optical axis normal to the plane of incidence. Three modifications of differential spectroscopy are proposed: (1) left and right circularly polarized waves—usual reflection; (2) total reflection–circular polarization (TROA); (3) total reflection of linearly polarized waves with different azimuths of vibration. The specific features of the proposed methods (ROA and TROA) are discussed as well as the possibility of studying the optical activity induced by nondegenerate and degenerate excitations (vibrations).

Notizen: The product vibrational state distribution for the reaction N(4S)+O2→NO(2Π,v)+O has been measured using saturated multiphoton ionization spectroscopy to determine NO electronic ground-state distributions. The fraction of the reaction exothermicity appearing in product vibration is 〈 f v〉=0.34; however, the even vibrational levels (v=0,2,4,6) are relatively overpopulated with respect to the odd vibrational levels (v=1,3,5). It is not possible to obtain a good linear-surprisal fit to all the data, but the even and odd subsets fit quite well to individual surprisal plots. The results are compared with previous measurements, including a reanalysis of laser-excited fluorescence data corrected for electronic transition moment and Franck–Condon factor variations. Collisional relaxation is observed at high O2 pressures for NO levels v=4 through 7, and is fit with a phenomenological relaxation model.

Notizen: Localized epitaxial TiSi2 was grown on (111)Si by rapid thermal annealing (RTA) in Ar ambient. The best epitaxy was obtained in samples annealed at 1100 °C for 20 s. Almost full coverage of TiSi2 (epitaxial and nonepitaxial) on silicon surface was found. The epitaxial regions, about 20 μm in average size, were observed to cover 70% of the surface area. Some of the epitaxial regions were observed to be as large as 40 μm in size. Dominant mode and average size of TiSi2 epitaxy in RTA samples were found to be different from those in vacuum furnace annealed specimens. Ambient gas induced silicide surface and/or silicide/Si interface energy changes are suggested to promote the growth of differently oriented grains. The main advantages of RTA in inducing TiSi2 epitaxy appear to be better control of the annealing ambient, temperature, and time for short-time anneals in the small RTA apparatus than in a furnace.

Notizen: The response characteristics of palladium-gate metal-insulator-silicon field-effect transistor (MISFET) structures to hydrogen are shown to be logarithmically related to the partial pressure of the gas. It is proposed that these palladium-gate MIS structures should be considered as electrochemical devices where the metal/insulator structure behaves as a gas electrode. The results presented were obtained for hydrogen in air and therefore relate directly to the use of these devices as sensors for use in the ambient environment.

Notizen: A novel device, the "hollow oval magnetron,'' was designed and characterized. The device has a hollow electrode with an oval cross section. It sustains a large-area uniform plasma with low-energy ions under low pressure and moderate rf power (Ni ∼1011 cm−3, Γi ∼0.5 mA/cm2, Ei ∼30 eV, and P∼5 mTorr). The energy distribution of ion current onto the hollow electrode has a broad spread ΔEi /E¯i ∼1 in the rf mode, and a narrow peak ΔEi/ E¯i ∼0.1 with a low-energy tail in the dc mode. The device is a good candidate for low-energy high-rate plasma-material processing.

Notizen: Historically GaAlAs alloys grown by molecular-beam epitaxy have high deep level concentrations (〉1016 cm−3), low luminescent efficiency with broad peaks, and are difficult to dope controllably below mid 1016 cm−3. We report the effect that the low incident antimony molecule fluxes during growth appear to help reduce the density of deep levels below 1014 cm−3, which then allows reproducible doping control in the low 1015 cm−3 range. Exciton-dominated photoluminescence with half widths ∼4.0 meV are now routinely achieved using this technique. Further evidence for the quality improvement of AlGaAs by heavy atom (Sb) doping comes from high 4 K two-dimensional electron gas mobilities for low sheet electron densities, e.g., 1.3×106 cm2/V s for n=2×1011 cm−2. SIMS profiles of antimony content are presented and estimates of incorporated concentrations are given.

Notizen: We review work on In2O3:Sn films prepared by reactive e-beam evaporation of In2O3 with up to 9 mol % SnO2 onto heated glass. These films have excellent spectrally selective properties when the deposition rate is ∼0.2 nm/s, the substrate temperature is (approximately-greater-than)150 °C, and the oxygen pressure is ∼5×10−4 Torr. Optimized coatings have crystallite dimensions (approximately-greater-than)50 nm and a C-type rare-earth oxide structure. We cover electromagnetic properties as recorded by spectrophotometry in the 0.2–50-μm range, by X-band microwave reflectance, and by dc electrical measurements. Hall-effect data are included. An increase of the Sn content is shown to have several important effects: the semiconductor band gap is shifted towards the ultraviolet, the luminous transmittance remains high, the infrared reflectance increases to a high value beyond a certain wavelength which shifts towards the visible, phonon-induced infrared absorption bands vanish, the microwave reflectance goes up, and the dc resisitivity drops to ∼2×10−4 Ω cm. The corresponding mobility is ∼30 cm2/V s. The complex dielectric function ε is reported.These data were obtained from carefully selected combinations of spectrophotometric transmittance and reflectance data. It is found that ε can be reconciled with the Drude theory only by assuming a strongly frequency-dependent relaxation energy between the plasma energy and the band gap. We review a recently formulated quantitative theoretical model for the optical properties which explicitly includes the additive contributions to ε from valence electrons, free electrons, and phonons. The theory embodies an effective-mass model for n-doped semiconductors well above the Mott critical density. Because of the high doping, the Sn impurities are singly ionized and the associated electrons occupy the bottom of the conduction band in the form of an electron gas. The Sn ions behave approximately as point scatterers, which is consistent with pseudopotential arguments. Screening of the ions is described by the random phase approximation. This latter theory works well as a consequence of the small effective electron radii. Exchange and correlation in the electron gas are represented by the Hubbard and Singwi–Sjölander schemes. Phonon effects are included by three empirically determined damped Lorentz oscillators. Free-electron properties are found to govern the optical performance in the main spectral range. An analysis of the complex dynamic resistivity (directly related to ε) shows unambiguously that Sn ions are the most important scatterers, although grain-boundary scattering can play some role in the midvisible range.As a result of this analysis one concludes that the optical properties of the best films approach the theoretical limit. Band-gap shifts can be understood as the net result of two competing mechanisms: a widening due to the Burstein–Moss effect, and a narrowing due to electron-electron and electron-ion scattering. The transition width—including an Urbach tail—seems to be consistent with these notions. Window applications are treated theoretically from detailed computations of integrated luminous, solar, and thermal properties. It is found that In2O3:Sn films on glass can yield∼78% normal solar transmittance and ∼20% hemispherical thermal emittance. Substrate emission is found to be insignificant. Antireflection with evaporated MgF2 or high-rate sputtered aluminum oxyfluoride can give ∼95% normal luminous transmittance, ∼5% normal luminous reflectance, little perceived color and little increase in emittance. A color purity 〈1% in normal transmission and 〈10% in normal reflection is achievable for a daylight illuminant within extended ranges of film thickness.