William F. Banholzer

Temperature and concentration distribution of H2 and H atoms in hot‐filament chemical‐vapor deposition of diamond

The temperature and concentration distributions of hydrogen in a hot‐filament chemical‐vapor deposition reactor of diamond have been measured simultaneously by coherent anti‐Stokes Raman scattering (CARS). The bright background from the filament was rejected by using CARS and gating on the detector as well as spatial filtering. The CARS spectra provided direct and accurate measurements of the H2 temperature and concentration distributions. The concentration distribution of atomic hydrogen was also determined by assuming a constant pressure condition and equilibrium between translational and rotational degrees of freedom in the system. These temperature and concentration distributions are essential for the understanding and modeling of the diamond growth processes. It was found that the H‐atom distribution departed substantially from the thermal equilibrium prediction except very near the filament; however, a diffusion‐controlled model predicted the slope of this distribution throughout the measured region.

A reflection-absorption infrared study of carbon monoxide and nitric oxide adsorption on platinum (100)

Abstract The adsorption of NO, CO, and NO/CO mixtures, onto Pt(100), is studied by RAIRS. CO and NO are found to adsorb into islands at 300 K, but the islands breakup upon heating to 400 K. Dosing with a mixture of NO and CO at temperatures below 325 K is found to produce a mixed NO/CO island. There is a shift in the CO peak and NO peak during mixed island formation which is attributed to a strong chemical interaction between the adsorbed NO and CO. This interaction is found to produce an increase in the desorption temperature of CO. Autocatalytic behavior is found to arise because of an enhanced reactivity when CO enters a mobile state. The autocatalytic behavior could be responsible for the “surface explosion” reported by Lesly and Schmidt.

A model for the plane to plane variations in catalytic activity seen during nitric oxide decomposition on platinum

Abstract Previous work has shown that the reactivity of platinum for nitric oxide decomposition varies significantly with the geometry of the exposed surface of the platinum. Here, the available data are compared to the predictions of a model, which is based on conservation of orbital symmetry and a simplified picture of the band structure of the surface. The model predicts that the (100) face of platinum should be more active than either the (111) or the (110). The (410) should be more active still. These predictions agree with recent experiment. The model makes several other predictions which need to be tested experimentally. These results suggest that symmetry conservation methods, similar to the ones developed by Woodward and Hoffmann for organic reactions, could yield many valuable insights into the relative activity of various catalyst structures.

Anisotropic thermal conductivity in chemical vapor deposition diamond

The thermal conductivity of thick‐film diamond prepared by chemical vapor deposition (CVD) has been measured with heat flowing in a direction perpendicular to the plane of the film. A laser flash technique with fast infrared detection has been devised for measurement of thin samples with high conductivity. The conductivity perpendicular to the plane is observed to be at least 50% greater than with heat flowing parallel to the plane. This anisotropy is attributed to low‐quality grain boundaries in the columnar microstructure. The observed dependence of the thermal conductivity on microstructure has important implications for thermal management of microelectronic devices with CVD diamond.

Nitric oxide decomposition on Pt(410)

Abstract The decomposition of nitric oxide on platinum (410) was studied by temperature-programmed desorption. The (410) surface was found to be unusually active for NO decomposition, decomposing more than 98% of the NO in the flash. Nitrogen desorption from an NO-covered Pt(410) surface was found to follow simple second-order kinetics, with an activation energy of 18 kcal/mol, and a preexponential ( v ) of 3 × 10 −3 cm 2 /sec. The saturation coverage of NO at room temperature was found to be about half of that of CO on the same surface, suggesting that NO takes up two sites. These results show that m (110) × n (100) stepped surfaces with n > m + 1 are unusually active for NO bond breaking, as predicted by the orbital symmetry conservation model of W.F. Banholzer, Y. O. Park, K. M. Mak, and R. I. Masel ( Surf. Sci. 128 , 176 (1983)).

XPS, Auger study of Cu3Si and its reaction with oxygen

Abstract The copper silicon alloy Cu 3 Si (η phase) was investigated by XPS and Auger analysis. XPS binding energies of Si2p 3 2 = 99.4 eV and Cu2p 3 2 = 932.7 eV were found, identical to their values in pure elemental Si or Cu, respectively. The X-ray induced Cu LMM Auger peak was observed at kinetic energy of 914.1 eV, again similar to that found for pure copper metal. In the electron excited N ( E ) Auger spectrum the Si LVV for η phase displays a shoulder at 92.3 eV on the main Si peak (89.0 eV). This shoulder causes the derivative spectrum to split into two negative excursions at 90.3 and 94.3 eV. Cu 3 Si was found to react readily with oxygen. The oxidation state of Cu in η phase was stable with respect to O 2 exposure. Silicon however, was preferentially oxidized to SiO 2 with a decrease in the surface Cu/Si ratio with exposure to O 2 and air. Exposure of Cu 3 Si to air for two weeks produced oxide layers 3600 A deep.

Active site formation in the direct process for methylchlorosilanes

Abstract The direct process for methylchlorosilane production was studied in a laboratory-scale fluidized bed reactor. The silicon was found to have discrete sites at which silanes were produced. The density of sites was controlled by the nature of the native oxide layer over the silicon. The active sites displayed regular geometry, indicating an anisotropic silicon reaction which preferentially leaves (111) planes exposed as the reaction proceeded. Carbon and SiO2 layers 200 A thick prevented active site formation.

Properties of diamond with varying isotopic composition

Abstract Gem-quality diamonds of varying isotopic composition have been synthesized by sequential application of chemical vapor deposition and high-pressure technologies of diamond growth. The resulting diamond crystals have a high crystal quality, a low defect density and low internal stresses. Their thermal conductivity increases with increasing isotope purity with the maximum thermal conductivities occurring with either isotopically pure 12C or 13C. The thermal conductivity is a symmetric function of isotopic composition with a minimum at 50% 13C. As the isotopic concentration is changed, the interatomic spacing decreases linearly with increasing 13C concentration. The intensity of the first-order Raman line also decreases with increasing 13C concentration. The wavenumber of the first-order Raman line is a monotonic but non-linear function of isotope concentration with the line wavenumber decreasing by 50 cm−1 from pure 12C to pure 13C. The two- and three-phonon absorption bands also move towards lower wave numbers with increasing 13C concentration.

Understanding the mechanism of CVD diamond

Abstract Recent work at GE on the chemical vapor deposition diamond process is discussed. An experimental apparatus is described which allows in situ kinetic studies of the hot filament growth of diamond. This apparatus was used to demonstrate that diamond could be formed on a graphite substrate simultaneously while etching occured. Substitution of deuterium for hydrogen decreased the deposition rate, suggesting that a C-H bond was involved in the rate-limiting step. The difficulty when using carbon filaments is discussed in terms of equilibrium analysis. Hydrogen, and especially atomic hydrogen, is critical for this process and performs several critical functions, including stabilization of the diamond surface, creation of a volatile source of carbon, formation of vacancies on the growth surface, reduction of the surface free energy, formation of carbon radicals and possibly the etching of graphite material.

Isotopic-disorder induced Raman scattering in diamond

We compare the Raman spectra of three isotopically disordered diamond samples with that of natural diamond. The isotopically disordered samples exhibit disorder induced Raman-activated features on the low energy tail of the zone center optical phonon. The strength of these additional scattering depends on the degree of disorder. The experimental spectra are compared with calculations using the coherent-potential-approximation (CPA) and also with lowest order perturbation theory. Good agreement with experiments is found especially for the CPA results.