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Results for “substrate amorphization”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Digital image processing of nanometer-size metal particles on amorphous substrates

The task of differentiating very small metal aggregates supported on amorphous films from the phase contrast image features inherently stemming from the support is extremely difficult in the nanometer particle size range. Digital image processing was employed to overcome some of the ambiguities in evaluating such micrographs. It was demonstrated that such processing allowed positive particle detection and a limited degree of statistical size analysis even for micrographs where by bare eye examination the distribution between particles and erroneous substrate features would seem highly ambiguous. The smallest size class detected for Pd/C samples peaks at 0.8 nm. This size class was found in various samples prepared under different evaporation conditions and it is concluded that these particles consist of 'a magic number' of 13 atoms and have cubooctahedral or icosahedral crystal structure.

Soria, F.↗
NASA NTRS1982

The effect of grooves in amorphous substrates on the orientation of metal deposits. I - Carbon substrates

The graphoepitaxial alignment of vapor-deposited discrete metal crystallites is investigated in the nucleation and growth stages and during annealing by in situ UHV/TEM techniques. Various stages of nucleation, growth and coalescence of vapor deposits of Au, Ag, Pb, Sn, and Bi on amorphous, topographically structured C substrates are analyzed by advanced dark-field techniques to detect preferred local orientations. It is found that the topography-induced orientation of metal crystallites depends strongly on their mobility and their respective tendency to develop pronounced crystallographic shapes. Lowering of the average surface free energies and increasing the crystallographic surface energy anisotropies cause generally improved graphoepitaxial alignments.

Anton, R.↗
NASA NTRS1987

Interactions of amorphous Ta(x)Cu(1-x) (x = 0.93 and 0.80) alloy films with Au overlayers and GaAs substrates

Amorphous Ta(93)Cu(7) and Ta(80)Cu(20) alloy films are prepared by cosputtering of pure Ta and pure Cu targets with a rotating sample holder table. To investigate the possible application of these materials as diffusion barriers for the Au-GaAs system, vacuum annealings are made in the temperature range from 200 to 800 C. Resistivity change, X-ray diffraction, and Auger electron spectroscopy measurements are performed to find the chemical and metallurgical stabilities of these materials in this system. The reaction temperature for Ta(x)Cu(1-x) in contact with GaAs lies between 500 and 700 C. For Au in contact with Ta(x)Cu(1-x) the reaction occurs at about 600 C. Amorphous Ta(93)Cu(7) shows different interdiffusion characteristics with surrounding elements than does Ta(80)Cu(20).

Oh, Jae E.↗
NASA NTRS1987

Structural characterization and gas reactions of small metal particles by high resolution in-situ TEM (Transmission Electron Microscopy) and TED (Transmission Electron Diffraction)

The detection and size analysis of small metal particles supported on amorphous substrates becomes increasingly difficult when the particle size approaches that of the phase contrast background structures of the support. An approach of digital image analysis, involving Fourier transformation of the original image, filtering, and image reconstruction was studied with respect to the likelihood of unambiguously detecting particles of less than 1 nm diameter on amorphous substrates from a single electron micrograph.

Heinemann, K.↗
NASA NTRS1975

Direct observation of small cluster mobility and ripening

Direct evidence is reported for the simultaneous occurrence of Ostwald ripening and short-distance cluster mobility during annealing of discontinuous metal films on clean amorphous substrates. The annealing characteristics of very thin particulate deposits of silver on amorphized clean surfaces of single crystalline thin graphite substrates were studied by in-situ transmission electron microscopy (TEM) under controlled environmental conditions (residual gas pressure of 10 to the minus 9th power torr) in the temperature range from 25 to 450 C. Sputter cleaning of the substrate surface, metal deposition, and annealing were monitored by TEM observation. Pseudostereographic presentation of micrographs in different annealing stages, the observation of the annealing behavior at cast shadow edges, and measurements with an electronic image analyzing system were employed to aid the visual perception and the analysis of changes in deposit structure recorded during annealing. Slow Ostwald ripening was found to occur in the entire temperature range, but the overriding surface transport mechanism was short-distance cluster mobility.

Heinemann, K.↗
NASA NTRS1976

Direct observation of small cluster mobility and ripening

Direct evidence is reported for the simultaneous occurrence of Ostwald ripening and short-distance cluster mobility during annealing of discontinuous metal films on clean amorphous substrates. The annealing characteristics of very thin particulate deposits of silver on amorphized clean surfaces of single-crystalline thin graphite substrates have been studied by in situ transmission electron microscopy (TEM) under controlled environmental conditions in the temperature range from 25 to 450 C. It was possible to monitor all stages of the experiments by TEM observation of the same specimen area. Slow Ostwald ripening was found to occur over the entire temperature range, but the overriding surface transport mechanism was short-distance cluster mobility. This was concluded from in situ observations of individual particles during annealing and from measurements of cluster size distributions, cluster number densities, area coverages, and mean cluster diameters.

Heinemann, K.↗
NASA NTRS1973

Development of large diameter carbon monofilament

A process for preparing large diameter carbon-boron monofilament was developed. The process involves chemical vapor depositing a carbon-boron alloy monofilament from a BCl3, CH4, and H2 gas mixture onto a carbon substrate. Amorphous alloys were formed when gaseous mixtures containing greater than 20 percent methane (80 percent BCl3) were used. The longest uninterrupted lengths of carbon-boron monofilament were produced using a CH4/BCl3 gas ratio of 2.34. It was found that the properties of the carbon-boron alloy monofilament improved when the carbon substrate was precleaned in chlorine. The highest strength monofilament was attained when a CH4/BCl3 gas volume ratio of 0.44 was 28 million N/sq cm (40 million psi). While the highest strengths were attained in this run, the 0.44 gas ratio and other CH4/BCl3 ratios less than 2.34 would not yield long runs. Runs using these ratios were usually terminated because of a break in the monofilament within the reactor. It is felt better process control could probably be achieved by varying the amount of hydrogen; the BCl3/H2 ratio was kept constant in these studies.

Jacob, B.↗
NASA NTRS1982

Chemisorption and reactions on clusters of nickel atoms

The nucleation and growth of metallic clusters on a substantially amorphous substrate are discussed with emphasis on the geometrical and electronic structure of the clusters. Several clusters of different symmetry containing five to nine nickel atoms were studied. It was found that the energy range of primary d-like states is not significantly different from the width of the d-band states in nickel metal, as long as the interatomic distance is comparable to that in the bulk metal. The approach of one or more molecules to the cluster is examined using at the hydrogenation of acetylene and the dehydrogenation of ethylene as examples.

Waber, J. T.↗
NASA NTRS1989

Making Submicron CoSi2 Structures On Silicon Substrates

Experimetnal fabrication process makes submicron-sized structures of single-crystal metallic CoSi2 on silicon substrates. Amorphous Co:Si(1:2) crystallized by electron beam becoming single-crystal CoSi2. Remaining amorphous Co:Si then preferentially etched away. When fully developed, process used to make fine wires or dots exhibiting quantum confinement of charge carriers.

Nieh, Simon K. W.↗
NASA NTRS1980

Direct /TEM/ observation of the catalytic oxidation of amorphous carbon by Pd particles

The catalytic oxidation of amorphous carbon substrates by Pd particles is observed by in situ transmission electron microscopy. Various modes of selective attack of the carbon substrate in the immediate neighborhood of Pd particles are observed, which can be correlated with different degrees of particle mobility. Using amorphous substrates we have been able to demonstrate that the particle-substrate interaction is influenced by the structure of the particle. This has not previously been noted.

Moorhead, R. D.↗
NASA NTRS1981

Cluster adsorption on amorphous and crystalline surfaces - A molecular dynamics study of model Pt on Cu and model Pd on Pt

Molecular dynamics was used to study the structure, dispersion and short-time behavior of ten-atom clusters adsorbed onto amorphous and crystalline substrates, in which the cluster atoms differed from the substrate atoms. Two adatom-substrate model systems were chosen; one, in which the interaction energy between adatom pairs was greater than that between substrate pairs, and the other, in which the reverse was true. At relatively low temperature ranges, increased dispersion of cluster atoms occurred: (a) on the amorphous substrate as compared to the FCC(100) surface, (b) with increasing reduced temperature, and (c) with adatom-substrate interaction energy stronger than adatom-adatom interaction. Two-dimensional clusters (rafts) on the FCC(100) surface displayed migration of edge atoms only, indicating a mechanism for the cluster rotation and shape changes found in experimental studies.

Garofalini, S. H.↗
NASA NTRS1994

Amorphous to Amorphous Form Transitions of Water Ice and Astrophysical Implications

We have combined Selected Area Electron Diffraction (SAED) and cryogenic techniques in an instrumental configuration that allows observing the structure of vapor deposited ice as it evolves during warmup. The ice is deposited in-situ inside an Hitachi H-500 H transmission electron microscope at a base pressure of 1-5 x 10(exp -7) torr on a thin amorphous carbon substrate at 15K or 86K and warmed up at a rate of 1-2 K/min. We find a progression of amorphous forms and well defined amorphous to amorphous transitions. Apart from the well known low-density form of ice, we confirm the presence of a high-density form and find a third amorphous form that coexists with cubic ice. We will report too on the amorphous to crystalline transition and the implications of these results for radical diffusion and gas retention observed in laboratory analog studies of interstellar and cometary ices.

Jenniskens, Peter↗
NASA NTRS1975

Interface effects in the dissolution of silicon into thin gold films

The dissolution of crystalline Si and amorphous Si substrates into thin films of evaporated Au was studied with an electron microprobe and scanning electron microscopy. The dissolution pattern was found to be nonuniform along the plane of the surface and dependent on the crystalline orientation of the Si substrate. The dissolution is greatly facilitated when a very thin layer of Pd is evaporated between the Si substrate and the Au film.

Sankur, H.↗
NASA NTRS1985

Chemical structure of interfaces

The interfacial structure of silicon/dielectric and silicon/metal systems is particularly amenable to analysis using a combination of surface spectroscopies together with a variety of chemical structures of Si/SiO2, Si/SiO2Si3N4, Si/Si2N2O, Si/SiO2/Al, and Si/Native Oxide interfaces using high resolution (0.350 eV FWHM) X ray photoelectron spectroscopy. The general structure of these dielectric interfaces entails a monolayer chemical transition layer at the Si/dielectric boundary. Amorphous Si substrates show a wide variety of hydrogenated Si and Si(OH) sub x states that are not observed in thermal oxidation of single crystal material. Extended SiO2 layers greater than 8 A in thickness are shown to be stoichiometric SiO2, but to exhibit a wide variety of local network structures. In the nitrogen containing systems, an approach to stoichiometric oxynitride compounds with interesting impurity and electron trapping properties are seen. In native oxides, substantial topographical nonuniformity in oxide thickness and composition are found. Analysis of metal/oxide interfacial layers is accomplished by analytical removal of the Si substrate by UHV XeF2 dry etching methods.

Grunthaner, F. J.↗
NASA NTRS1985

Palladium silicide formation under the influence of nitrogen and oxygen impurities

The effect of impurities on the growth of the Pd2Si layer upon thermal annealing of a Pd film on 100 line-type and amorphous Si substrates is investigated. Nitrogen and oxygen impurities are introduced into either Pd or Si which are subsequently annealed to form Pd2Si. The complementary techniques of Rutherford backscattering spectrometry, and N-15(p, alpha)C-12 or O-18(p, alpha)N-15 nuclear reaction, are used to investigate the behavior of nitrogen or oxygen and the alterations each creates during silicide formation. Both nitrogen and oxygen retard the silicide growth rate if initially present in Si. When they are initially in Pd, there is no significant retardation; instead, an interesting snow-plowing effect of N or O by the reaction interface of Pd2Si is observed. By using N implanted into Si as a marker, Pd and Si appear to trade roles as the moving species when the silicide front reaches the nitrogen-rich region.

Ho, K. T.↗
NASA NTRS1985

Modelling of Laser-Enhanced Chemical Vapor Deposition

Research is directed at development of a detailed model of mass and heat transfer and chemical reaction in the pyrolysis of silane for the growth of thin amorphous silicon substrates incorporating laser heating of the gas phase above the film. The model will be the basis for evaluation of the relative importances of the decomposition of SiH4 in the vapor phase, mass transfer of the intermediate species, e.g., SiH2, and the evolution of hydrogen gas. Plans are also underway for developing a model for homogeneous nucleation of Si in the vapor phase to model the rate limitations observed at high gas-phase temperatures and high partial pressures of silane. Work was concentrated on an almost one-dimensional model for the coupling of the CO2 laser beam for heat transfer of the vapor phase with simple kinetic models for SiH4 decomposition and subsequent absorption of Si vapor on the substrate. Mass transfer in the vapor phase is assumed to be solely by diffusion. The role of convection in the vapor phase caused by the large changes in density in and around the center of the laser beam will be analyzed to evaluate the potential of microgravity experiments for increasing the uniformity of the film and the deposition rate.

Brown, R. A.↗