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The objective of this research program was to design and develop direct band gap solar cells which would have high below band gap energy (infra-red (IR)) reflectivity for use in high efficiency thermophotovoltaic (TPV) energy conversion systems. Two types of customized GaAs cells were grown on intrinsic or semi-insulating GaAs substrates with either p-type GaAs or Al(.9) Ga (.1) As stop-etch layers. Holes were drilled through the substrate using a laser photochemical etch technique. Cells with high IR reflectivity and low series resistance were fabricated by drilling a hole which stopped at the contact layer of the cell with the Al(.9) Ga(.1) As stop-etch layer and then depositing gold in the hole and on the back surface. Measurements of these customized cells indicate that IR reflectivities in excess of 90%, corresponding to TPV efficiencies in excess of 35%, are feasible. Keywords include: Thermophotovoltaic energy conversion; GaAs solar cells; Photochemical etching; GaAs; Free carrier absorption; and Infra-Red reflectivity.

Contents: Nature and topography of solid surfaces and the study of Van Der Waals' forces in their immediate vicinity; Properties of ordered physisorbed phases observed with LEED; Structure and properties of metal surfaces; Configuration and energy of defects on the (100) surface of a molecular crystal; Surface configurations of the (111) plane of UO2; LEED study of FCC metal surfaces; Wave interference mechanism for fractional order peaks in LEED intensity curves; Field emission studies of epitaxy and nucleation of copper deposited on tungsten and oxidation of the copper; Reaction of oxygen with thin crystals of copper; Physical adsorption; Chemisorption; Interaction of gases with surfaces of solid catalysts; Adsorption of rare gases on a rare gas surface; Isotope separation in physical adsorption; Adsorption of cadmium on tungsten; O, H, and N adsorption on (110), (100), (111), and (113) oriented tungsten single crystal surfaces; Isotopic mixing in the oxidation of CO on Pl; Oxygen-18 tracer studies on the C-O reaction; Oxidation of a thin film of a carbonaceous char at low pressures; Thermal accommodation of gases on solids; Molecular beam scattering from solid surfaces; Theory of gas-surface collisions; Simple classical model for the scattering of gas atoms from a solid surface; Scattering of particles from rough surfaces; Velocity of molecular beam molecules scattered by Pt surfaces; Time of flight measurements in an argon beam deflected by a heated Pt target; Scattering of He from the cleavage plane of LiF; Thermal accommodation of the rare gases on clean metal surfaces.

The report outlines experimental research concerned with several areas of non-reactive atom-surface interactions, performed with modulated molecular-beam techniques. Included are general scattering studies from Mg(0001), Ag(001), Si(111) and macroscopically rough surfaces; atom diffraction from LiF(001); 'rainbow' scattering from LiF(001); symmetry in gas-surface interactions; and determination of surface Debye temperatures using atom-beam scattering as a probe. (Author), See also report dated 30 Sep 69, AD-697 083.

The heterogeneous oxidation of CO on epitaxially grown Pt(111) surfaces has been investigated using modulated molecular beam techniques. The overall reaction is apparently first order. The observance of higher order kinetics in these experiments is argued to be an artiface of the beam technique used here and suggests a high surface diffusion rate for adsorbed CO. The measured angular dependence of the CO2 desorption rate from the Pt(111) surfaces does not follow the Knudsen law of random (cosine) desorption, but is much more sharply peaked about the surface normal. The adsorption of CO is also shown to have an angular dependence and is not activated on Pt(111). The results of this study strongly suggest that a Langmuir-Hinshelwood mechanism dominates the reaction, with CO being the principal diffusing species. (Modified author abstract)

A molecular beam study of the reaction of C2H4 with O2 on Ag(111) was attempted and supplemented with data obtained using an Isotropic Source (IS) configuration. In the beam configuration, the possible product signals at 18 amu (H2O) or 44 amu (C2H4O, CO2) could not be discerned. From C2H4 beam scattering data, coupled with the experimental ion spectrum obtained from C2H4 by electron impact, it was possible to estimate that roughly about 15% of the incident C2H4 fragmented upon collision with the surface; 10% going to C2H3 and 5% to C2H2. From the surface temperature dependence in the IS configuration, an activation energy was obtained for the mass 44 product.

The results of experimental studies of molecular beam-surface scattering studies are summarized. These studies include the scattering distributions of H2, D2, HD, He, Ne, Ar, and Xe at thermal energies less than 0.3 eV from Au, Ag, Ni, LiF, and mica. Continuous evaporative deposition was used to provide clean, epitaxially grown single crystals in the case of the metal scattering surfaces. A complete list of reports, publications, and formal tables resulting from this research is also included. (Author)

The scattering of He, Ne, Ar, and Xe from Ni(111) is examined as a function of beam temperature and angle of incidence. The results are consistent within themselves when considered in light of earlier data and the predictions from the simplified 'cube' models. However, a careful comparison of the present data with earlier Au(111) and Ag(111) data shows trends that may be related to the lattice properties of the solid and which demonstrate that a simple description of the solid in terms of mass and heat of physical adsorption is inadequate. (Author)

Experimental studies of the scattering of atomic and molecular beams from the surfaces of freshly deposited Ag single crystal films are described. The results of two specific experiments are summarized: (1) the scattering of velocity-filtered beams of A and Xe in the velocity range 23200 < or = sub zero < or = 53300 cm/sec and (2) the scattering of HD, H2, D2, He3, and He4. From the first experiments it is demonstrated that, contrary to previous assumptions, the details of the scattering process are not obscured by the use of a beam with a large (Maxwellian) distribution of velocities, and the result of the second series of experiments supports an earlier inference from experimental data, i.e., rotational state interactions in the heavier diatomic molecules affect the scattering distribution of these molecules. (Author), See also AD-643 013.

The ultimate goal of the research program is to provide data concerning the transfer of energy and momentum between atoms and molecules and a solid surface in free-molecule flow, and to investigate other processes that may occur in the gas-surface interaction. The work during the reporting period was largely confined to two areas: light gas (He, H2, and D2) scattering from (111) Ag, and construction and testing of a compact slotted-disc velocity selector. In addition, some measurements were made of Ar scattering out of the plane of incidence., See also AD-628 066.

The design of a molecular beam experiment to study chemical reactions on clean metal surfaces is described. The motivation for the particular approach to the attainment of clean surfaces and surfaces with a controlled concentration of surface adsorbate is discussed. Finally, the results of the first 12 months effort on this program are summarized, and certain experimental problem areas are discussed. (Author)

Through the use of modulated atomic beam techniques, the reflection and dissociation of H2 at a tungsten surface above 2500 K was studied. Two experimental configurations were employed. In the first configuration, a collimated beam of molecular hydrogen was directed at the tungsten surface, and the angular distribution of particles evaporating from the surface was examined mass-spectrometrically. In the second configuration, the tungsten test surface was located in the first of three differentially pumped vacuum chambers. H2 was admitted into this chamber thereby providing an isotropic source of incident particles. Particles evaporating from the surface were collimated and examined mass-spectrometrically in the third chamber. From these experiments it is observed that at high temperatures the angular distribution of atomic hydrogen evaporating from the target surface obeys the cosine law. However, the angular distribution of reflected H2 displays a shift toward the specular ray. Above 2500 K the sticking probability of H2 on tungsten approaches a limiting value of 0.3. (Author)

The report reviews experimental studies of gas-surface scattering reported in the period from approximately 1969 through the first half of 1972. It emphasizes rare gas scattering from a variety of metal single crystals at incident energies from about 0.025 eV to nearly 20 eV, and includes a discussion of the various scattering regimes in that energy range. In addition, light gas scattering from LiF and certain highly structured metal surfaces is surveyed. The observation of single-phonon inelastic events are described. Recent studies of desorption phenomena are reviewed. A correlation of the various results is made and areas in which questions remain are indicated. Apart from parametric correlations, no attempt is made to assess the validity of the various theoretical models that have been developed to describe the scattering process. (Author)