Wigbert J. Siekhaus

Reactions of Modulated Molecular Beams with Pyrolytic Graphite. II Oxidation of the Prism Plane

The reaction of a modulated beam of molecular oxygen with the prism plane of pyrolytic graphite was investigated. Diffusional processes in the bulk dominated the response of the emission rates of CO and CO2. The phase lags of these products relative to the impinging reactant beam indicated that the surface reactions were strongly affected by diffusion of oxygen in the grain boundaries and then into the grains of the pyrolytic graphite structure. This double diffusion process so strongly demodulated the product signals that the apparent reactivity of the prism plane was less than that of the basal plane. This reactivity inversion is peculiar to the ac modulated beam method and would not occur in dc (steady state) experiments. The reactivity of graphite which had been annealed to 3000°C was found to be an order of magnitude larger than that of the as‐received material. This increase in reactivity was due to reduction of the demodulation effect which resulted from closing off diffusional paths in the bulk by...

Molecular Beam Sources Fabricated from Multichannel Arrays. IV. Speed Distribution in the Centerline Beam

The collision model utilized by Giordmaine and Wang to describe the centerline beam intensity from long tubes has been extended to account for molecular‐speed effects. The modification was accomplished by utilizing the velocity‐dependent mean‐free path instead of the Maxwellian averaged value. The analysis shows that the centerline beam intensity (or peaking factor) is practically unaffected by explicitly accounting for the distribution of molecular speeds. The speed distribution on the axis of the beam, however, is depleted of slow‐speed molecules. The perturbation of the Maxwellian spectrum begins at a Knudsen number based on tube length of ∼10. For Knudsen numbers less than ∼0.1, the spectrum is no longer dependent on the Knudsen number. The average translational energy of the centerline beam in the low Knudsen number limit is 5.5% greater than that of a Maxwellian beam.

Molecular Beam Sources Fabricated from Multichannel Arrays. V. Measurement of the Speed Distribution

A technique for measuring the speed distribution of molecular beams which uses symmetric modulation and phase‐sensitive detection is described. Corrections for system errors on experiments of this type are developed and verified. In addition to the expected 1/v dependence of the ionizer, the extraction efficiency of the quadrupole mass spectrometer detector is shown to depend on the velocity of the molecules before ionization. The prediction of 5.5% energy enhancement of beams from channel sources operated at pressures well above the free‐molecule flow limit has been verified.A technique for measuring the speed distribution of molecular beams which uses symmetric modulation and phase‐sensitive detection is described. Corrections for system errors on experiments of this type are developed and verified. In addition to the expected 1/v dependence of the ionizer, the extraction efficiency of the quadrupole mass spectrometer detector is shown to depend on the velocity of the molecules before ionization. The prediction of 5.5% energy enhancement of beams from channel sources operated at pressures well above the free‐molecule flow limit has been verified.

Studies of acetylene and oxygen adsorption on silicon surfaces by low energy electron loss spectroscopy

Abstract Acetylene and oxygen adsorption on disordered and ordered silicon crystal surfaces of (111) orientation were studied by low-energy electron loss spectroscopy (ELS). There are marked structure-dependent differences in the ELS spectra in the presence of these adsorbates. A model for the acetylene molecule orientation on the silicon (111) ordered surface is proposed to explain the experimental results. ELS data suggest that the electrons in the silicon-oxygen chemisorption bond are more tightly bound on the disordered than on the silicon (111) surface with a (7 X 7) surface structure.

A modulated molecular beam study of the energy of simple gases scattered from pyrolytic graphite

Abstract The energy of rare gases and several diatomic gases after scattering from pyrolytic graphite was determined by molecular beam-phase sensitive detection methods. The angle of incidence of the molecular beam and the angle of sampling of the reflected molecules were both fixed at 45°. The phase shifts induced by changing either the temperature of the incident gas or the solid were utilized to determine an apparent temperature of the scattered molecules. The results showed that the apparent temperature of the reflected molecules was independent of the temperature of the incident gas. With increasing solid temperature, the reflected gas temperature increased up to a limiting value which was dependent on the gas and the surface only. Both the basal and prismatic faces of graphite were investigated. In addition, a prism plane specimen which had not been subjected to high temperature heat treatment was studied. The apparent temperature of the reflected beam did not depend upon the crystallographic face of the graphite but was markedly reduced by high temperature annealing.

Enhanced crystallinity of silicon film deposited at low temperature

Silicon thin films were vacuum deposited onto fused quartz at 600 °C with a prior coating of ultrathin aluminum layer (∼ 500 A). Significant increase in crystallinity was observed from both x−ray diffraction and transmission electron microscopy studies. The problem of handling aluminum film at this temperature in a vacuum of 1×10−6 Torr is discussed, and distribution of aluminum in the silicon film is studied using Auger spectroscopy.

Auger analysis of silicon thin films deposited on carbon at high temperatures

Silicon films (2000–3000 A thick) were vacuum deposited onto pyrolytic graphite, extruded graphite, and glassy carbon at 1150–1200 °C to test the possibility of using carbon as a substrate in thin‐film silicon solar cells. The distribution of silicon and carbon in the carbon‐silicon interface was studied using Auger spectroscopic depth profiling. The results showed that, for all substrates, carbon diffused deeply into silicon films, and for all substrates except glassy carbon, silicon also diffused deeply into carbon. In both cases, silicon carbide is formed. Several methods to build diffusion barriers were tested. A diffusion mechanism is proposed according to which silicon atoms first form a carbide layer with the surface carbon atoms, whereafter cracking of the carbon lattice and diffusion of silicon through the carbide layers into the carbon substrates follow. This mechanism is shown to explain very well the dependence of the measured silicon and carbon contents on the structure of the carbon substrat...

ENHANCED CRYSTALLINITY OF LOW TEMPERATURE DEPOSITED SILICON FILMS ON GRAPHITE SUBSTRATES

Silicon thin films (2000–3000 A) were vacuum deposited onto graphite substrates held at 600 °C with a prior coating of an ultrathin Si‐Al‐Si (∼100‐500‐100 A) sandwich layer. X‐ray diffraction showed a considerable increase in silicon crystallinity over those directly deposited onto graphite at the same substrate temperature. The increase observed is comparable to that using quartz substrates. The distribution of aluminum in the silicon films thus deposited is determined using Auger spectroscopic depth profiling.