L. W. Anderson

Spectroscopic determination of carbon dimer densities in and plasmas

In contrast to conventional methods of diamond chemical vapour deposition (CVD), nanocrystalline diamond CVD takes place with only a small fraction of feed gas hydrogen. Minimal amounts of , believed critical in hydrogen-rich CVD, are expected to be produced in hydrogen-deficient systems and alternative mechanisms for diamond growth must be considered. The carbon dimer, , is believed to be an important species in these growth environments. We have experimentally determined the density of gas phase in and microwave plasmas used to deposit nanocrystalline diamond. The density is monitored using high-sensitivity absorption spectroscopy of the (0, 0) band as chamber pressure, microwave power, substrate temperature and feed gas mixtures are varied for these two chemical systems. The absolute density of is most sensitive to the total chamber pressure and fraction of carbon in all molecular species in the feed gas in discharges and to the total chamber pressure and substrate temperature in plasmas. We discuss possible production channels in both chemical systems. The efficiency of production from fullerene precursors is over an order of magnitude greater than that from hydrocarbon precursors.

Ultraviolet spectroscopy of gaseous species in a hot filament diamond deposition system when C2H2 and H2 are the input gases

The methyl radical density, acetylene mole fraction, filament properties, and diamond growth rate and film quality are measured in a hot filament chemical vapor deposition system when C2H2 and H2 are used as the input gases. The methyl radical density and acetylene mole fraction depend greatly on the degree of filament surface poisoning. This poisoning prevents diamond growth due to a lack of hydrogen atoms and/or methyl radicals. Understanding the large influence of the filament surface catalytic characteristics is important for developing a gas phase model of this system. The results obtained with C2H2 and H2 as the input gases are compared to those obtained with CH4 and H2 as the input gases. Under conditions when the filament surface is not poisoned, the methyl radical concentrations are similar when either C2H2 and H2 are the input gases or when CH4 and H2 are the input gases.

Detection of CH3 during CVD growth of diamond by optical absorption

Abstract : The absolute concentration of CH3 is measured during diamond growth for a range of input CH4 concentrations using multi-element optical absorption spectroscopy at wavelengths 4 near 216 nm. A detection limit of 2xl0 to the 11th power CH3, radicals/cu cm has been achieved. We also detect C2H2 during diamond growth using the same technique.

Measurement of absolute hydrogen dissociation in a diamond deposition system

Abstract The ratio of atomic to molecular hydrogen as a function of position in a hot filament diamond chemical vapor deposition system is determined from spatially resolved measurements of CH 3 and CH densities. High sensitivity white light absorption spectroscopy is used to obtain the CH and CH 3 densities. The H atom mole fractions reported here are significantly higher than those reported in the most directly comparable previous experiments.

Spin exchange collisions and their consequences for spin polarized gas targets of hydrogen and deuterium

Abstract The effects of spin-exchange collisions on the polarization of dense spin-polarized samples of hydrogen and deuterium are analyzed. It is shown that even in large magnetic fields spin-exchange collisions transfer angular momentum between the electrons and the nuclei. This effect has important implications for the operation of densed spin-polarized targets and sources of hydrogen and deuterium. For tensor polarized targets care will be required to obtain a high tensor polarization, especially at low fields. For the specific case of sources that are spin-polarized by spin-exchange collisions with optically pumped alkali atoms, spin-exchange not only polarizes the hydrogen and deuterium electron spins, but polarizes the nuclear spins as well. For high-density vector-polarized targets this may eliminate the need for rf transitions to polarize the nuclei.

Measurement of potassium-potassium spin relaxation cross sections

The K-K spin relaxation rate has been measured in an optically pumped K vapor cell. The K-K spin relaxation cross section is determined to be 1.0 x lo-” cm’, which is about a factor of two less than a previous measurement.

Study of diamond growth from a variety of input gases

The gas phase densities of CH 3 and CH and the hydrogen dissociation fraction are measured in a hot filament diamond deposition system for each of several different hydrocarbon input gases. The crystal growth rate and the appearance of the diamond grown from the different input gases are also examined. A comparison of the measurements indicates that the nature of the input hydrocarbon is relatively unimportant because fast gas phase reactions completely scramble the identities of the input carbon atoms. The addition of oxygen greatly alters the gas phase densities and other experimental factors such as the filament surface condition. Small concentrations of atomic impurities in the gas phase are also detected using high sensitivity absorption spectroscopy.

Electron Collision Cross-Sections Measured with the Use of a Magneto-Optical Trap

We present the first application of optically trapped atoms as targets for a scattering experiment. We report absolute total electron scattering cross-sections for ground-state Rb atoms in the range of 7-500 eV.

Evaluation of a substrate pretreatment for hot filament CVD of diamond

The absolute concentration of methyl radicals (CH 3 ) and the mole fraction of acetylene (C 2 H 2 ) are measured in a hot filament chemical vapor deposition (CVD) system both during and after an initial pretreatment that has been used successfully in microwave plasma and oxyacetylene torch CVD systems to produce more uniform and higher density crystal nucleation. The pretreatment technique, which consists of deposition for a relatively short time with a high input concentration of hydrocarbon in the feed gas, was studied for both methane (CH 4 ) and C 2 H 2 as the input hydrocarbon diluted in H 2 . Scanning electron micrographs of diamond films deposited under the conditions studied indicate that the pretreatment using CH 4 is not effective in increasing the crystal nucleation density, but is moderately effective in increasing the crystal size. The C 2 H 2 pretreatment has no apparent effect upon either the crystal size or nucleation density. The spectroscopie measurements suggest that the surface condition of the filament is the prominent factor affecting the gas phase chemistry both during and after the pretreatment stage.

The electron excitation cross section for the metastable 1s3 level in Ne

Abstract The apparent and direct electron excitation cross sections of the metastable 1s 3 level of ne are measured using laser-induced fluorescence. The direct electron excitation cross section a has peak value of 2.8 x 10 -19 cm 2 at 20 eV. The effect of the 16.9 eV negative ion resonance is observed in the cross sections.