Recombination Of Hydrogen Atoms Research Articles

Cylindrically symmetric diamond parts by hot-filament CVD

In the absence of forced substrate cooling, the substrate temperature in hot-filament CVD diamond deposition is controlled by the heat coming from the filaments in the form of radiation, gas conduction and atomic hydrogen generation and recombination. Atomic hydrogen heat transport is the dominant process of substrate heating. In many industrial applications, forced substrate cooling cannot be used because of the geometry of the CVD diamond part or the impracticality of cooling connections. Overheating of an uncooled substrate can only be avoided by using as few filaments as possible because of the high heat flux emanating from each filament. In general, utilization of only a few filaments will lead to nonuniform and irregular diamond deposits. However, if the ratio of the substrate diameter to the filament-substrate spacing is kept within certain limits determined by theory and confirmed by experiments, cylindrically symmetric diamond parts can be successfully deposited on a stationary uncooled substrate with only one or two filaments.

In situ mass spectrometry during diamond chemical vapor deposition using a low pressure flat flame

A combination of experiments and detailed kinetic modeling was used to investigate diamond deposition chemistry in low pressure combustion synthesis. Microprobe sampling was employed to provide in situ, quantitative measurements of the stable gas-phase species impinging the growth surface. The reactant gas ratio was found to be the most critical experimental variable. A detailed kinetic model was developed for the stagnation flow system. Comparison of experimental measurements showed very good agreement with model predictions. The model was then used to predict the concentration of radical species and analyze the sensitivity of predictions to γH, the probability of atomic hydrogen recombination on the surface. It was shown that γH dramatically affects the distribution of radical species near the diamond surface. The analysis also indicates that atomic carbon may be an important gas-phase precursor in this system. Comparison of mole fraction measurements and observations of film morphology were used to draw conclusions on the growth mechanism.

Growth Rate of Diamond on Polycrystalline 〈110〉 Diamond Substrates from Carbon Disulfide in Hydrogen by Hot‐Filament‐Assisted Chemical Vapor Deposition

The growth rates for the deposition of diamond by hot‐filament‐assisted chemical vapor deposition using carbon disulfide (CS2) in hydrogen and methane (CH4) in hydrogen were investigated on polished, 〈110〉 textured, polycrystalline diamond substrates. The carbon concentration in each system was varied from 1% to 2.5%. The deposition rate increased proportionately with carbon concentration for both systems. The CH4‐H2 system gave higher growth rates with an essentially constant difference in rate between CS2 and CH4. Possible explanations for this are discussed with the most likely either a catalytic effect of sulfur or sulfur‐containing species on gas‐phase homogeneous atomic hydrogen recombination, or the surface chemisorption of relatively less labile sulfur‐containing species at the diamond surface.

Kinetics of heterogeneous recombination of hydrogen atoms on solid surfaces

Heterogeneous recombination rates of hydrogen atoms on solid surfaces were measured for the first time. An anomalous form of these curves testifies that the reaction on surfaces of metals, semiconductors and dielectrics follows a universal mechanism with participation of preadsorbed atoms.

Effect of surface species concentrations and temperature on diamond film morphology in inductively coupled r.f. plasma CVD

Diamond films were deposited with an r.f. plasma operating at atmospheric pressure. The gas composition above the growth surface was measured with a gas chromatograph. Experiments were conducted over a range of methane flow rates and substrate temperatures. Films were characterized by SEM and X-ray diffraction texture analysis. Increases in either input methane-hydrogen ratio or substrate temperature caused film morphology to shift from {111} toward {100} faceting, while the measured ratio of CH4 to C2H2 decreased. We speculate that the temperature effect on species concentrations and film morphology is due to the temperature dependence of surface-catalyzed atomic hydrogen recombination.

Participation of preadsorbed atoms in heterogeneous recombination of hydrogen atoms on semiconductor surfaces

Atoms trapped from the vapor phase in the preadsorbed state were found to provide the catalytic activity of semiconductors in the heterogeneous recombination of hydrogen atoms.

Components of overvoltage of hydrogen electrode reaction on nickel

For the case of hydrogen evolution reaction (HER) in alkaline solutions on nickel, the overvoltage of the electron transfer step (the first step of HER) was separated from that of the overall reaction on the basis of the results obtained by the galvanostatic transient method, which has been developed by us earlier. The difference of the overall-reaction overvoltage and electron-transfer overvoltage, which is considered as the overvoltage of the step of an adsorbed hydrogen atom recombination (the last step of HER), was also determined. It is known that on nickel, during cathodic polarization in alkaline solutions, the intermetallic compound M(I) of an alkali metal with nickel is formed as practically the main intermediate. In this case, the Tafel line, corresponding to the last step, shows a slope of 70–80 mV, and the observed overvoltage can be explained in terms of the decrease of the work function of the surface component, caused by the M(I) formation. On the other hand, on a cobalt-deposit-modified nickel, the Tafel line shows a slope of 30 mV, even in alkaline solutions. The nickel surface may be completely covered with the cobalt deposit. Therefore, no intermetallic compound seems to be formed on the surface, and the main intermediate is likely to be an adsorbed hydrogen atom, as in the case of HER on platinum in acid solutions.

Direct simulation Monte Carlo study of H/H2 and H/H2/CO mixtures for diamond chemical vapor deposition

One-dimensional direct simulation Monte Carlo calculations have been carried out on H/H2 and H/H2/CO mixtures under operating conditions typical of diffusion-dominated diamond chemical vapor deposition processes. Mechanisms have been included in the model for the adsorption and recombination of hydrogen atoms on the diamond surface and the dissociation of molecular hydrogen at the interior of the reactor. Hydrogen atom fluxes and recombinative and conductive heat fluxes to the diamond surface are calculated as a function of pressure, gas composition, hydrogen dissociation and surface reaction probabilities, reactor temperature, and distance between the activating source and substrate. The numerical calculations are shown to be in excellent agreement with analytical results in the limiting regimes of free-streaming particles at low pressures and continuum hydrodynamics at high pressures.

Analysis of cracking efficiency of an atomic hydrogen source, and its effect on desorption of AlxGa1−xAs native oxides

The absolute cracking efficiency of a commercial atomic hydrogen source is found to be as great as 30%. Negligible recombination of atomic hydrogen into molecular hydrogen occurs in the molecular beam epitaxy chamber. The cracking process can be accurately modeled as an equilibrium process. An atomic hydrogen flux impinging on the surface of GaAs lowers the oxide desorption temperature by ∼100 °C. At temperatures up to 700 °C, atomic hydrogen does not enable the thermal desorption of AlxGa1−xAs oxides for x≳10%–20%.

Dissociation and atom recombination of H2 and D2 on metallic surfaces: A theoretical survey

Abstract

Spin Catalysis of Chemical Reactions

The spin catalysis of chemical reactions is a physical phenomenon of spin transformation of chemically reactive species induced by the exchange interaction of these species with external spin carriers. It manifests itself in the radical pair recombination, cis−trans isomerization of molecules with double bonds, recombination of spin-polarized and spin-aligned hydrogen atoms at cryogenic temperatures, etc. The theory and quantitative kinetics of spin catalysis in three- and four-spin systems are considered. Quantitative hierarchy of spin carriers with respect to their spin catalytic efficiency is discussed.

Investigations in the field of nontraditional electrochemistry and corrosion of materials carried out by the school of I. N. Frantsevich

The electrochemical theory of high-temperature corrosion resistance developed by I. N. Frantsevich is briefly reviewed, and a comparative study is made of the electrochemical parameters of both the high-temperature oxidation of tungsten and its oxidation in 1 N H2SO4. Consideration is given to the principal results of experimental and theoretical studies of the limiting stages of hydrogen cathodic evolution on the surface of metals, semiconductors, and certain binary refractory compounds. Topics included are the catalytic recombination of hydrogen atoms, their electrochemical desorption, the interaction of hydrogen atoms with electrons as disclosed by calorimetric, cyclotron resonance, and EPR methods of investigation. Particular attention is paid to the corrosion of copper and silver coatings on copper parts used in sealed systems containing the decomposition products of polymeric insulation. Weak-current corrosion of contacts, and the electrochemical protection of steel structures in aggressive soils and sea water are also reviewed.

Determination of atomic hydrogen density with catalytic probes

The determination of atomic hydrogen density with specially designed catalytic probes is described. The probe is a small disc made of a metal with a high recombination coefficient, connected to a pair of thin thermocouple wires. The temperature of a probe placed in a mixture of atomic and molecular hydrogen rises substantially over the temperature of the surrounding gas because of the energy dissipated on the probe surface due to the recombination of hydrogen atoms. The density of atomic hydrogen in the vicinity of the probe is calculated from the measured data on the energy dissipated on the probe surface in a unit time. The density was measured at different total pressures in a vacuum system between 6 × 10 −2 mbar and 2 × 10 1 mbar. The atomic hydrogen source was a low pressure inductively coupled RF hydrogen plasma with the density of the order of 10 16 m −3 and the electron temperature of 6–8 eV. It was found that the atomic hydrogen density increased with increasing pressure reaching the maximum value of 5 × 10 21 m −3 at the total pressure of 8 mbar. At higher pressures, however, the density was found to decrease with increasing total pressure.

Hydride formation in MgZrFe 1.4Cr 0.6 composite material

In the present work, we report on hydride formation in an Mg-ZrFe 1.4Cr 0.6 composite. The thermodynamic properties, the hydrogen absorption and desorption kinetics at different temperatures, the cycling properties, and the resistance to oxidation were examined. The two phases of the composite, i.e. Mg and the alloy, retained their original thermodynamic properties upon repeated cycling. The hydrogen absorption and desorption in the composite were rapid, even at moderate temperatures (150 °C). The Mg particles were found to be partially covered by the alloy. The alloy islands acted as a catalyser for the dissociation and recombination of hydrogen atoms. Furthermore, the alloy overlayer protected the Mg from oxidation during sorption cycles and exposure to the atmosphere. The hydrogen absorption and desorption were determined by standard volumetric methods. Structural and chemical analyses were performed by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy and selected-area diffraction.

Hydrogen atom yield in RF and microwave hydrogen discharges

The hydrogen atom yield in pure-H2 RF and microwave-sustained discharges is investigated both theoretically and experimentally. A particle balance model is developed that provides the concentrations of the H, H2, H+, H 2 + , and H 3 + species. It is also shown that an approximate solution of this model is adequate for calculating the concentration of H atoms (required, for instance, in diamond film deposition) in the 0.1–10 torr range. Next, the validity of the actinometry technique applied to the determination of the H-atom density in pure-H2 discharges is examined. Using this diagnostic, it is observed that the H-atom concentration decreases when the vessel wall temperature increases, owing to the increased efficiency of atomic hydrogen recombination on the wall. To overcome this effect, the discharge tube wall is cooled off with dimethyl polysiloxane, a low-loss dielectric liquid. It improves significantly the H-atom concentration at 2450 MHz provided the pressure is typically below a few torr and the power density is not too high.

The effects of gamma irradiation on low-pressure chemical-vapor-deposited silicon dioxide metal-oxide-silicon structures

The effects of gamma irradiation on as-deposited, oxygen-annealed, and dual-dielectric gate (undoped polysilicon/oxide) low-pressure chemical-vapor-deposited (LPCVD) silicon dioxide (SiO 2) metal-oxide-silicon (MOS) structures were investigated. As-deposited LPCVD SiO 2 MOS structures exhibit the largest shift in flatband voltage with gamma irradiation. This is most likely due to the large number of bulk oxide traps resulting from the nonstochiometric nature of as-deposited LPCVD SiO 2. Dual-dielectric (undoped polysilicon/annealed LPCVD SiO 2) MOS structures exhibit the smallest shift in flatband voltage and increase in interface state density compared to as-deposited and oxygen-annealed LPCVD SiO 2 MOS structures. The interface state density of dual-dielectric MOS structures increases from 5 × 10 10 eV cm −2 to 2–3 × 10 11 eV cm −2 after irradiation to a gamma total dose level of 1 Mrads(Si). This result suggests that the recombination of atomic hydrogen atoms with silicon dangling bonds, either along grain boundaries or in crystallites of the undoped polysilicon layer in dual-dielectric (undoped polysilicon/annealed LPCVD SiO 2) MOS structures, probably reduces the number of atomic hydrogen atoms reaching the Si/SiO 2 interface to generate interface states.

Quasi-equilibria of gaseous species in the C [sbnd]H system

The influence of a constant concentration of atomic hydrogen on the steady state gas-phase composition (quasi-equilibrium) and its development in hydrocarbon-hydrogen mixtures was studied by means of computational modeling. In the thermodynamic equilibrium, methane is virtually the only stable gas species at temperatures below 1000 °C, whereas acetylene prevails at higher temperatures. The steady state of the gas phase in the presence of atomic hydrogen is also divided between these two stability regimes. The transition temperature between them, however, is considerably lowered even by small H concentrations. Concentrations of other species, especially CH 3, are increased by several orders of magnitude over their equilibrium concentrations, having peak values in the vicinity of this transition temperature. Cyclic reaction sequences, consisting of H-consuming reactions, result in a catalytic effect on atomic hydrogen recombination. Generally, atomic hydrogen accelerates the development of a steady state, but not necessarily with a time constant decreasing with increasing temperature.

Transport phenomena in the scale-up of hot filament-assisted chemical vapor deposition of diamond

Abstract Heat transfer and fluid flow in a diamond deposition reactor were examined to identify parameters important to reactor design and scale-up. Physical and mathematical modeling of hot filament-assisted diamond deposition reactors indicated that both ordinary and thermal diffusion are equally important in the transport of nutrient species to the substrate surface. The atomic hydrogen concentration field in the reactor is established mainly by diffusive mixing of the atomic hydrogen with molecular hydrogen and other species in the gas phase. Homogeneous chemical reactions in the gas phase do not significantly affect the atomic hydrogen concentration profiles. In hot filament reactors, in addition to convection, conduction and radiation, filament-to-substrate heat transfer takes place by dissociation of molecular hydrogen at or near the filament and recombination of atomic hydrogen at the substrate surface Computer simulation of the heat transfer and fluid flow in a typical hot filament reactor for diamond deposition indicated that system geometry, filament temperature and pressure are important parameters in the design of reactors for coating large areas. The investigation clearly indicates considerable promise of science-based design and scale-up of hot filament reactors for diamond deposition.

Hydrogen atom recombination on tungsten and diamond in hot filament assisted deposition of diamond

A hot-filament chemical vapor deposition reactor in which the substrate is a heated wire was used to analyze the energy transport to the substrate during diamond deposition. In this reactor, radiation and convection contribute negligibly to substrate heating. Atomic hydrogen recombination to molecular hydrogen on the substrate surface accounts for ∼90% of the energy reaching the substrate. The atomic hydrogen recombination rate and the thermal accommodation coefficient for the energy released by atomic hydrogen recombination were estimated from in situ measurements for tungsten and diamond surfaces. The atomic hydrogen concentration gradient was estimated for simple geometries. The experimental conditions under which atomic hydrogen transport to the substrate is diffusion controlled were found. Addition of methane reduced the atomic hydrogen recombination rate on the substrate. An optimum pressure was found for the transport of atomic hydrogen to the substrate.

Effects of H2O on Atomic Hydrogen Generation in Hydrogen Plasma

The effects of water vapor added to hydrogen plasma were investigated by measuring the relative concentrations of hydrogen atoms in the plasma and in the downstream. The added water vapor increased the concentration of hydrogen atoms in the downstream to an amount 80 times greater than that without addition of water vapor. The relative concentration of hydrogen atoms generated from water-vapor-added hydrogen gas was only slightly decreased in the downstream. To clarify the role of water vapor, we examined the behavior of oxygen atoms and OH radicals. As a result, it was revealed that water vapor passivated the quartz surface and the recombination of hydrogen atoms was prevented.