Norman Braslau

Effect of Aerosols on the Transfer of Solar Energy Through Realistic Model Atmospheres. Part I: Non-Absorbing Aerosols

Abstract Extensive calculations aimed at determining the effect of aerosols on the solar energy absorbed, reflected and transmitted by cloudless, nonhomogeneous, plane-parallel atmospheric models were recently carried out with the object of treating the radiation transfer in as comprehensive a manner as possible consistent with reasonable computing time. The concentration of aerosol (spherical particles with size distribution and refractive index independent of height), ozone and water vapor were specified for 160 layers of varying thickness from the surface to 45 km. The solar spectrum (0.285–2.5 μm) was divided into 83 intervals with appropriate functions representing the scattering and absorption of gases and aerosol assigned to each, the index of refraction of the aerosol taken to be wavelength-independent. Upward and downward fluxes for each spectral interval at each level were computed taking into account all orders of scattering. Results will be presented for four model atmospheres to show the abso...

Ohmic contacts to GaAs

Abstract The present capability of obtaining ohmic contacts to GaAs over a range of doping levels is reviewed. Possible models of transport across the metal-semiconductor interface are discussed and contact techniques are described. The widely used AuGe alloyed contact is seen to have a spatially inhomogeneous interface which appears to control its contact resistance. The most satisfactory process at this time is to alloy into a previously fabricated heavily doped layer.

Barrier heights and electrical properties of intimate metal‐AlGaAs junctions

The dependence of the Schottky barrier height of Mo‐n:AlGaAs junctions, fabricated in situ by molecular beam epitaxy, on the Al mole fraction (x) was determined by internal photoemission measurements and by activation energy plots of the current versus voltage dependence on temperature. Both techniques yielded similar values. The difference in barrier height of Mo‐AlGaAs as a function of x, compared to that of Mo‐GaAs, was found to be equal to the conduction band discontinuity in AlGaAs‐GaAs heterojunctions for Al concentrations in the range 0≤x≤0.4. For x>0.4, values of the barrier heights were somewhat lower than values of the band discontinuity; however, both dependencies on x were quite similar. The temperature dependence of the current‐voltage characteristics showed that thermionic emission was the dominant transport mechanism at forward bias for temperatures higher than 250 K. At lower temperatures, current transport was governed by thermionic field emission.

Thermally stable ohmic contacts to n‐type GaAs. II. MoGeInW contact metal

It was previously found that a small amount of In impurity was able to convert MoGeW contacts from Schottky to ohmic behavior yielding thermally stable, low‐resistance ohmic contacts n‐type GaAs. In the present experiment transport measurements and materials studies were carried out for MoGeInW contacts in which a thin layer of In was directly added to the MoGeW contacts during deposition. The transition from Schottky to ohmic behavior was observed by adding an In layer as thin as ∼1 nm to the MoGeW. Contact resistances were found to be very sensitive to the deposition sequence, the annealing method, the annealing temperature, and the In layer thickness. Low resistances of ∼0.5 Ω mm were obtained in the MoGeInW contacts with 2‐nm‐thick In layers, annealed by the heat‐pulse method at temperatures in the range of 880–960 °C for 2 s. Contact resistances were stable during subsequent annealing at 400 °C for 100 h. Evidence of formation of the parallel diode areas with various barrier heights was obtained for ...

Effect of Cloudiness on the Transfer of Solar Energy Through Realistic Model Atmospheres

Abstract Extensive calculations of the effect of cloudiness on the solar energy absorbed, reflected and transmitted by nonhomogeneous plane-parallel atmospheric models have been carried out with the object of treating the radiation transfer in as comprehensive a manner as possible. The concentration of aerosol (spherical particles with size distribution and refractive index independent of height), ozone and water vapor were specified for 50 basic layers of equal geometric thickness from the surface to 50 km. A stratus cloud layer with a liquid water content of 0.0128 g m–3 was introduced between the 3 and 4 km levels of the models. The solar spectrum (0.285–2.5μm) was divided into 83 intervals with appropriate functions representing the scattering and absorption of gases, and the aerosol and liquid water drops assigned to each, the refractive indices of the aerosol and water drops taken to be wavelength-independent. For accurate computations of the upward and downward fluxes for a given model at a given w...

Sea-level solar radiation in the biologically active spectrum.

Calculations show a significant depletion of ultraviolet and visible radiation due to absorption and scattering by particulates and cloud drops for a fixed amount of ozone.

Effect of interface states on the electrical properties of W, WSix, and WAlx Schottky contacts on GaAs

Barrier heights of refractory W, WSix, and WAlx Schottky contacts deposited on n‐GaAs with different surface treatments have been electrically characterized by current‐voltage, capacitance‐voltage, and internal photoemission measurements. Internal photoemission measurements indicate that the Fermi level pins approximately at midgap (Φn ≂0.7–0.8 eV). The current‐voltage barrier heights were consistent with internal photoemission for diodes with little oxide at the GaAs/metal interface. For an oxide layer of about 2.5 nm, current‐voltage barrier heights as high as 0.9 eV were observed. Capacitance‐voltage barrier heights were found to be 0.9–1 eV with a weak dependence on interface oxide. A theoretical model was developed to explain these results. A large density of states (5×1013–1014/cm2) at the GaAs/metal interface which exchanges charge mainly with the metal appears to explain well our experimental capacitance‐voltage and current‐voltage data.

Importance of the diffuse sky radiation in evaluation of the performance of a solar cell

Abstract Representative results of the numerical simulation of responses (viz. photocurrent and optimum power output as well as efficiency) of the conventional and violet Si cells and of the Ga1−xAlxAsGaAs and GaAs cells are presented as a function of the solar zenith angle for seven different models of the terrestrial atmosphere. The atmospheric models used vary from an aerosol-free and cloud-free model with gaseous absorption to several models with moderately thick stratus cloud layer and high concentrations of aerosols. This study, restricted to horizontally situated solar cells, illustrates the manner in which characteristics are significantly affected by position of the sun, turbidity and cloudiness of the atmosphere, as well as reflectivity of the underlying surface.

Atmospheric Heating Rates Due to Solar Radiation for Several Aerosol-Laden Cloudy and Cloud-Free Models

Abstract From calculations of solar flux by direct numerical solution of the spherical harmonics approximation of the equation of transfer, heating rate profiles have been obtained for model atmospheres ranging from one with gaseous absorption only to one with additional heavy concentration of partly absorbing aersols with a tropospheric cloud of water drops. These calculations show that aersols can have a significant effect on the vertical distribution of absorbed solar energy, which is sensitive to the value of the ground reflectivity and solar zenith angle.

Spectral Characteristics of Exploding Wires for Optical Maser Excitation

As pulsed light sources, exploding wires can be used to provide intense narrow spectral lines as well as the characteristic continuum of very high temperature blackbodies. The surface brightness depends on the wire material and conditions under which the wire is exploded. Optimum conditions for 100–500‐μsec long light pulses from wires exploded in air have been found for tungsten, tantalum, and molybdenum wires 0.005 to 0.010 in. in diameter for energy inputs of 3000 J or more. Wires exploded in small diameter vacuum tubes (1–20 μ of Hg) radiate as blackbodies, but narrow line spectra are produced in enclosures of larger dimensions. In the visible and ultraviolet regions of the spectrum the spectral radiance of air‐exploded wires is one to three orders of magnitude greater than that of conventional flash lamps. Vacuum‐exploded wires have spectral radiance another factor of eight greater. Excitation of various optical masers with exploding wires is discussed.