Gas-surface Interface Research Articles

Comparison of one-dimensional and three-dimensional models for the energy accommodation coefficient

The accommodation coefficient (AC) for the transfer of energy at a gas–surface interface is treated with a one-dimensional (1D) theoretical model for classical scattering that retains full temperature dependence of both the gas and surface. Calculations are carried out for a purely repulsive gas–surface interaction and for the case in which there is an attractive adsorption well. Comparison with 3D calculations and with data for the accommodation of the rare gases at a tungsten surface indicates that full 3D dynamics are important for calculating the AC.

Laser assisted associative desorption of N2 and CO from Ru(0001)

An experimental technique, laser assisted associative desorption (LAAD), is described for determining adiabatic barriers to activated dissociation at the gas-surface interface, as well as some aspects of the dynamics of associative desorption. The basis of this technique is to use a laser induced temperature jump (T-jump) at the surface to induce associative desorption and to measure the translational energy distribution of the desorbing molecules. The highest translational energies observed in desorption are a lower bound to the adiabatic barrier and the shapes of the translational energy distributions provide information on the dynamics. Implementation of the experimental technique is described in detail and unique advantages and possible limitations of the technique are discussed. The application of this technique to very high barrier surface processes is described; associative desorption of N2 from Ru(0001) and CO formed by C+O and C2+O on Ru(0001). N2 barriers to dissociation increases strongly with N coverage and co-adsorbed O, in good agreement with DFT calculations. No isotope effects are seen in the associative desorption, indicating that tunneling is not important. The full energy distributions suggest that very large energy loss to the lattice occurs after recombination at the high barrier and prior to N2 desorption into the gas phase. The mechanism for this remarkably large energy loss is not well understood, but is likely to be general for other high barrier associative desorption reactions. CO associatively desorbs nearly thermally from both C+O and C2+O associative reactions. It is argued that this is due to large energy loss for this system as well, followed by indirect scattering in the deep CO molecular well before final exit into the gas phase.

Formal treatment of catalytic combustion and catalytic conversion of methane

Catalytic combustion and conversion of methane is investigated numerically for transient one-dimensional flow configurations (catalytic foil and catalytic wire). The reactive flow is coupled with the processes at the gas–surface interface. The analyses include detailed reaction mechanisms in the gas phase and on the surface as well as a detailed transport model. Computational tools are applied to study the ignition of heterogeneous methane oxidation on a platinum foil and the transition from heterogeneous to homogeneous combustion in this system. Furthermore, the interaction of gas phase and surface reactions in catalytic conversion of methane is considered. Time-dependent selectivity and conversion are calculated.

Growth of diamond films on a diamond {001}(2×1):H surface by time dependent Monte Carlo simulations

Time dependent Monte Carlo (TDMC) simulations are performed on a diamond lattice to determine the effect of surface properties/conditions on the growth of diamond thin films on flat and stepped diamond {001}(2×1):H surfaces under chemical vapor deposition conditions. The gas–surface interface consists of reactions of incoming gas-phase species, such as H2 molecules and H and CH3 radicals with surface radical, π-bond and step edge sites on the diamond {001}(2×1):H surface. The rates and probabilities of adsorption, abstraction, desorption, and incorporation reactions, as well as the reverse reactions, are explicitly calculated either via molecular dynamics or transition state theory methods, or taken from experimental measurements. The TDMC method allows all these reactions to occur simultaneously, though probabilistically, at each time step. The microscopic and macroscopic characteristics of the growing film are observed as functions of time. Diamond films of 10∼100 layers are grown in the simulation and the observed growth rate (∼0.5μm/h at 1200 K) is in agreement with experimental results. The contributions to the activation energy of growth by specific processes such as H abstraction, CH3 adsorption and CH2 incorporation into the trough sites have been determined. The contributions to the activation energies by specific processes are not linearly additive, and the CH3 adsorption at step edges leads to enhanced growth at the edges.

Chemical downstream etching of silicon–nitride and polycrystalline silicon using CF4/O2/N2: Surface chemical effects of O2 and N2 additives

By comparing the etching characteristics of silicon and silicon nitride in CF4/O2/N2 microwave downstream plasmas we demonstrate clearly how low concentrations of energetic species can play a dominant role in remote plasma processing: Injection of 5% N2 to a CF4/O2 plasma increases the silicon nitride etch rate by a factor of 7, while not significantly affecting the bulk composition of the discharge. Downstream injection of N2 is ineffective. Using surface spectroscopies we directly show a dramatic enhancement of the reactivity of fluorine and oxygen atoms with silicon and silicon–nitride surfaces upon N2 injection to the discharge. Our results can be explained by the production of energetic metastable species in the discharge region which transport energy to the gas-surface interface.

Simulation and sensitivity analysis of the heterogeneous oxidation of methane on a platinum foil

The heterogeneous oxidation of methane–air mixtures in a stagnation point flow onto a platinum foil is investigated numerically and results are compared with experiments. The analysis includes detailed reaction mechanisms for the gas phase as well as the surface. Gas-phase transport and its coupling to the surface is described using detailed multicomponent models. The heterogeneous ignition, extinction, and autothermal behavior are interpreted in terms of elementary steps at the gas–surface interface. A sensitivity analysis of the surface reactions is performed to identify crucial steps in the mechanism, thus pointing to those reactions for which better kinetic and thermodynamic data are needed urgently.

Modelling and simulation of heterogeneous oxidation of methane on a platinum foil

The heterogeneous oxidation of methane-air mixtures in a stagnation point flow onto a platinum foil is investigated numerically and results are compared with experiments. The analysis includes a detailed reaction mechanism in the gas phase as well as on the surface. The heterogeneous ignition occurring around 600°C, extinction, and autothermal behaviour are interpreted in terms of elementary steps at the gas-surface interface.

ChemInform Abstract: Gas‐Phase Chemistry in the Processing of Materials for the Semiconductor Industry

Abstract At the center of rapid changes in technological processes is the ability to fabricate new materials or to prepare known materials in forms or structures not possible previously. Nowhere is this more evident than in the semiconductor industry where the demand for components with pattern definitions below 1 pm together with the pressure to develop and improve the properties of optoelectronic components have stimulated research in a wide range of fields related to materials processing. Many of the methods that are employed currently involve chemical reactions at a gas-surface interface. This includes both the etching and deposition of material under carefully controlled conditions. In many cases, the chemical identity of the species that arrive at the surface is controlled by a complex sequence of gasphase reactions. The ability to understand and control these reactions is an important element for the improvement of existing technologies.

Surface studies relevant to silicon carbide chemical vapor deposition

Experimental studies of C2 H4, C3 H8, and CH4 reactions on the Si(111) surface and C2 H4 reaction on the Si(100) surface have been performed for surface temperatures in the range of 1062–1495 K. These studies used x-ray photoelectron spectroscopy and related ultrahigh vacuum procedures to identify the reaction products, characterize the solid-state transport mechanisms, determine the surface nucleation mechanisms and growth kinetics, and assess the effects of surface orientation. The reaction product was found to be essentially carbidic throughout the course of the reaction, although the surface layer may contain partially hydrogenated C adspecies. The dominant transport process was shown to be Si out-diffusion rather than C in-diffusion. The Si adspecies produced by out-diffusion react at the gas-surface interface with C adspecies to form the SiC film via a two-dimensional nucleation and layer-by-layer growth mechanism. The reaction efficiency for C2 H4 on the Si(111) and Si(100) surfaces was shown to be ∼10−3. The reaction efficiency for C3 H8 and CH4 on the Si(111) surface was shown to be ∼10−5. For growth temperatures above 1395 K, Si diffusion limitations and sublimation from the SiC surface were found to limit the availability of Si for the SiC growth process. In the absence of Si adspecies, the adlayer formed by the reaction of C2 H4 on SiC appeared to passivate the surface with respect to further C2 H4 reaction. When combined with previously reported modeling studies of the associated gas phase chemistry, these results provide the basis for a mechanistic model of the β-SiC chemical vapor deposition process.

Gas-phase chemistry in the processing of materials for the semiconductor industry

Abstract At the center of rapid changes in technological processes is the ability to fabricate new materials or to prepare known materials in forms or structures not possible previously. Nowhere is this more evident than in the semiconductor industry where the demand for components with pattern definitions below 1 pm together with the pressure to develop and improve the properties of optoelectronic components have stimulated research in a wide range of fields related to materials processing. Many of the methods that are employed currently involve chemical reactions at a gas-surface interface. This includes both the etching and deposition of material under carefully controlled conditions. In many cases, the chemical identity of the species that arrive at the surface is controlled by a complex sequence of gasphase reactions. The ability to understand and control these reactions is an important element for the improvement of existing technologies.

Theory of laser-simulated surface processes

Theoretical techniques for describing laser-stimulated surface processes in a vacuum and at a gas-surface interface are presented. For adspecies-surface systems, the laser excitation of vibrational degrees of freedom is considered, and quantum-mechanical and classical models and also an “almost first-principles” treatment of the competition between multiphoton absorption and multiphonon relaxation are discussed. The laser excitation of electronic degrees of freedom is considered with respect to surface states of semiconductors and metals, for the predissociation of diatomic adspecies on metal substrates, for ionization, and for resonance fluorescence of a gaseous atom near a metal. In connection with gas-surface interactions, the influence of laser radiation on diffraction patterns and energy transfer in atom-surface scattering is explored. Collisional ionization and ion neutralization in the presence of laser radiation are discussed. The roles of partial pressure and surface coverage in laser-stimulated surface processes are analyzed. Finally, some ideas on surface waves and annealing are presented.

A semiclassical wave packet model for the investigation of elastic and inelastic gas–surface scattering

A semiclassical wave packet model that permits simultaneous study of surface diffraction, Debye–Waller effects, and inelastic energy transfer processes at the gas–surface interface is presented. The incident atomic beam is represented by a quantum mechanical wave packet whose momentum-space probability density corresponds to that employed in the molecular-beam experiments being simulated. A semiclassical forced-oscillator-type approximation couples the surface motion to that of the wave packet through a time-varying potential obtained from a solution of Hamilton’s equations of motion for the lattice. Explicit integration procedures are used to evolve the wave packet through this time-varying field. The average final kinetic energy of the beam, the angular distributions and diffraction patterns, Debye–Waller broadening effects, and the surface phonon spectrum are obtained from the scattered wave packet and its Fourier transform. The model has been applied to the case of an atomic hydrogen beam having a square distribution of incident momenta impinging upon a simple two-dimensional lattice. The results yield diffraction patterns that correlate with the known grating of the lattice. The average final kinetic energy of the beam is found to vary linearly with incident energy and with surface temperature in accord with the results of recent molecular beam experiments, and a surface phonon spectrum that exhibits one-, two-, and three-phonon processes is obtained.

Laser-stimulated molecular dynamics and rate processes

Abstract : A review of theoretical work by the author and collaborators and related experiments is presented for two segments of laser-induced chemistry: molecular dynamics and rate processes in the gas phase and at a solid surface and gas-surface interface. For the gas phase, the focus is on situations where the radiation interacts directly with the dynamics to enhance or create new processes. The field of laser-surface chemistry is still undergoing a procedure of definition, and progress to date is discussed. Theoretical failures as well as successes are analyzed and viewpoints are offered as to new directions for both theory and experiment. (Author)

Direct Experimental Verification of the Reciprocity Principle for Scattering at the Gas–Surface Interface

Direct Experimental Verification of the Reciprocity Principle for Scattering at the Gas–Surface Interface

508. Equilibrium cosine law and scattering symmetry at the gas-surface interface

508. Equilibrium cosine law and scattering symmetry at the gas-surface interface

Equilibrium Cosine Law and Scattering Symmetry at the Gas–Surface Interface

Earlier derivations of the equilibrium cosine scattering law at the gas–surface interface based on the second law of thermodynamics and on the principle of detailed balance are shown to be inconclusive. Presented is a derivation involving time reversal invariance, along with an assumption of a canonical distribution of states in the solid, which results in a reciprocity relationship for the surface scattering cross section that yields the cosine law at equilibrium. A symmetry property for the scattered particle flux results from the reciprocity relationship, and molecular beam data are shown to exhibit the indicated symmetry. An application of this previously unrecognized symmetry permits the prediction of the scattered particle flux distribution for an arbitrary angle of incidence from experimental data reported for a limited number of incident angles.

Studies of Molecular Scattering at the Solid Surface

Experimental studies of molecular scattering at the gas-surface interface are described in which molecular beams of N2 were scattered at surfaces of steel, aluminum, and glass. Polar surveys of the issuing molecule flux indicate the scattering to be well represented by a cosine distribution in most cases. A moderate deviation from such a distribution was observed in the case of the glass-N2 interaction. It is shown how measurements of scattering at the surface may be used to derive values of the momentum transfer coefficient, f(s), in a rarefied gas flow where s is defined as the ratio of the mass speed of the flow to the most probable thermal speed of the molecules. In order to complete the calculation where scattering information alone is available from experiment, assumptions must be made relating to the energy exchange at the surface. In the case of the present calculation, where a surface interaction model was formed in part from the glass -N2 scattering data, two extremal assumptions relating to the energy exchange lead to values of f(s) of approximately 0.97 at s=0.1.