Browsing by Author "D'Evelyn, Mark P."

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    Chemistry of hydrogen and oxygen on the diamond (100) surface
    (1994) Struck, Lisa Marie; D'Evelyn, Mark P.
    The first investigation of the adsorption of water on diamond (100) by infrared multiple-internal-reflection spectroscopy using a natural type IIa diamond internal reflection element is reported. Following exposure to water at elevated temperatures, infrared absorption features are observed at 1250 and 1200 cm$\sp{-1}$; 1125 and 1080 cm$\sp{-1}$; and 720 cm$\sp{-1}$, and assigned to ether (C-O-C), hydroxyl (C-OH), and carbonyl ($>$C=O) modes, respectively. It is thought that water adsorbs dissociatively and at higher temperatures the adsorbed hydroxyl species further decompose to ether, carbonyl, and hydride surface species. The substantial observed reactivity of diamond with modest exposures to water indicates a potentially important role for surface hydroxyl and oxide species in the surface chemistry of diamond films grown by chemical vapor deposition. Direct evidence for the diamond surface hydride structures, obtained by deuterium substitution, is also presented. Infrared evidence is seen for both monodeuteride and dideuteride surface structures, with one or two deuterium atoms per surface carbon atom ($\delta\sb{\rm CD}$ mode at 901 cm$\sp{-1}$ and $\delta\sb{\rm CD\sb2}$ mode at 1125 cm$\sp{-1}$, respectively). The thermal stability of oxides on diamond (100) was investigated by annealing experiments. Infrared modes assigned to hydroxyl and carbonyl species disappear, presumably via desorption, upon heating the oxidized diamond surface above 1000$\sp\circ$C. Finally, surface reactions induced by exposure to a hot acid solution used to etch graphite from diamond were also investigated by infrared spectroscopy. The primary absorption band at 1020-1120 cm$\sp{-1}$ is assigned to hydroxyl groups and infrared absorptions near 1250 cm$\sp{-1}$ are assigned to ether groups. We conclude that the acid solution oxidizes the diamond surface.
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    Interaction of hydrogen with group IV semiconductor surfaces
    (1992) Yang, Yuemei L.; D'Evelyn, Mark P.
    A preliminary temperature-programmed desorption (TPD) study of hydrogen desorbing from diamond (100) concludes that the previously-reported non-saturation behavior of the surface is an experimental artifact resulting from desorption from the Ta heater. Critical conditions for conducting an accurate TPD measurement are identified. In its first application to the diamond surface study in which lateral periodic boundary conditions are incorporated, the molecular mechanics method (MM3) has been shown to be a useful tool in determining surface structures and energetics, employing modest-sized clusters. Atomic structures and energetics of the diamond (100)-(2$\times$1), (100)-(2$\times$1):H, (100)-(1$\times$1):2H, and (100)-(3$\times$1):1.33H surfaces have been calculated. Pairs of surface carbon atoms form symmetric dimers on the reconstructed diamond (100)-(2$\times$1), (100)-(2$\times$1):H, and (100)-(3$\times$1):1.33H surfaces, with dimer bond lengths of 1.46 A, 1.63 A, and 1.59 A, respectively, corresponding to strained double or single bonds. The full (100)-(1$\times$1):2H dihydride is highly strained due to H-H repulsions, causing a reduction of the H--C--H bond angle and twisting about the surface normal, and is predicted to be thermodynamically unstable with respect to dehydrogenation to the monohydride. Some important gas-surface reactions involving hydrogen and the diamond (100) and (100)-(2$\times$1):H are discussed in light of the derived energetics. We have noted that preferential pairing of chemisorbed hydrogen on Si(100) is a direct result of the partial $\pi$-bond existing on the surface dimers. A lattice-gas model has been developed based on this concept, and predicts that, with a modest pairing energy, hydrogen desorption adopts near-first-order kinetics at high coverages but deviates from first-order kinetics at low coverages. We calculated the pairing energy of adsorbed H to be about 7.5 kcal/mol, based on a comparison of the predictions of the model with experiment. We conclude that preferential pairing on dimers is a general feature of hydrogen adsorption on the (100) surfaces of group IV semiconductors.
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    Kinetics and morphology of homoepitaxial diamond growth by chemical vapor deposition
    (1996) Rawles, Robin E.; D'Evelyn, Mark P.
    To address the relative lack of understanding concerning growth mechanisms for diamond chemical vapor deposition (CVD), this work explores kinetics of diamond epitaxy--growth on single-crystal substrates--and morphology of diamond surfaces--CVD films and natural and synthetic diamond after hydrogen plasma treatment. Optical Fizeau interferometry was implemented for in situ growth-rate measurements, providing a facile and sensitive means of systematically studying diamond growth kinetics. Fizeau interferometry was also used for non-contact temperature measurement of diamond single crystals, and the index of refraction variation with temperature was determined. Kinetics of diamond (100) homoepitaxy was investigated--in particular, the effects of oxygen on epitaxial growth and etching was studied, in conjunction with the nanometer-scale film morphology. Addition of oxygen to the hot-filament reactor had similar effects for growth conditions containing 0.5% or 1% methane in hydrogen. Growth rates at lower temperatures increased relative to samples grown without oxygen, proceeded through a maximum, and then decreased until etching was observed at high temperatures. Increased etching of diamond and non-diamond carbon was also observed for hydrogen with oxygen than for hydrogen alone. Oxygen addition improved the crystallinity of CVD diamond films deposited on silicon, as characterized by scanning electron microscopy (SEM). Without oxygen such films had polycrystalline features with relatively smooth (100) and rather rough (111) facets--with oxygen these films had both smooth (100) and (111) facets. Growth rates for diamond (100) and (111), with and without oxygen, were measured simultaneously using in situ Fizeau interferometry. The ratio of the two growth rates was then correlated with the nanometer-scale morphology, characterized by atomic force microscopy (AFM). Oxygen addition affected the (100) growth rate only; (100) epilayers were smooth for growth without oxygen but contained defects and penetration twins for growth with oxygen. The (111) epilayers from growth with and without oxygen were highly defective. Single-crystal diamonds, with and without CVD epilayers, and natural and synthetic diamond powders were treated in hydrogen plasmas, and examined by AFM and SEM. Evidence for a possible role of diffusion of surface species during diamond growth was observed.