Modulated Molecular Beam Research Articles

Self-oscillatory heterogeneous recombination of hydrogen atoms and nonequilibrium desorption of molecules from the surface (Teflon)

The channel of the accommodation of the energy of a heterogeneous chemical reaction (recombination of hydrogen atoms) related to vibrational V-V exchange between excited chemical reaction products and adsorption layer molecules (H2O, HDO, D2O, and H2) was studied by the method of modulated molecular beams. The chemical reaction was found to proceed in an oscillatory mode caused by the nonequilibrium character of its elementary steps. The participation of adsorbed molecules in accommodation was studied by analyzing nonequilibrium desorption of these molecules. An isotope effect was observed in nonequilibrium desorption. The kinetic mechanism of the reaction and the micromechanism of elementary reaction events, which determine the “physical” mechanism of catalysis in the system under study, are discussed.

A modulated molecular beam study of the extent of H2O dissociation on TiO2(110)

The adsorption-desorption kinetics of H2O have been studied on three well-defined surface configurations of TiO2(110) using temperature programmed desorption (TPD) and modulated molecular beam scattering in order to measure the extent of dissociative adsorption. The condensation coefficient for H2O is unity below 150 K and falls to zero near 200 K. The sticking coefficient is unity for all coverages and temperatures investigated. The adsorption-desorption kinetics are the same on the 1 × 1, 1 × 2 and argonsputtered surfaces at the high coverages probed in TPD measurements. Small differences are observed at coverages between 10−2 and 10−4 monolayers probed in the beam studies. Separate monolayer and multilayer desorption peaks are observed with TPD, but it is not possible to distinguish between dissociative and non-dissociative adsorption based on a peak shape analysis. Based on the temperature dependence of the scattered H2O waveforms and the linearity of the kinetics as revealed by a Fourier analysis of the waveforms, several models for the adsorption-desorption kinetics can be ruled out. These include first- and second-order desorption from a uniform surface, and defect-mediated first-order desorption in the limit of rapid surface diffusion prior to desorption. The data are best fit by a model which assumes first-order desorption from an immobile adsorbate on a heterogeneous surface for all three surface phases. The degree of heterogeneity deduced from the modulated beam measurements is consistent with the TPD data. A very small degree of non-linearity in the desorption kinetics is the only evidence of H2O dissociation for surface temperatures as high as 500 K. We conclude that very few of the water molecules incident on the surface undergo dissociative adsorption, even in the limit of zero coverage.

Microstructural properties of superlattices and InAsxSb1 − x ordered alloys grown by modulated molecular beam epitaxy

Modulated molecular beam epitaxy (MMBE) has been demonstrated to be useful in controlling group-V alloys such as InAs x Sb 1-x and AlAs x Sb 1-x . Further studies of the MMBE grown InAs x Sb 1-x ordered alloys and InAs/InAs x Sb 1-x superlattices by using cross-sectional scanning tunneling microscopy show well defined interfaces between InAs x Sb 1-x ordered alloys and InAs. Clear composition modulation is observed in the InAs x Sb 1-x ordered alloy layers. Transmission electron diffraction and microscopy, X-ray diffraction, and low temperature photoluminescence experiments show no obvious sign of CuPt orderings. These results suggest that MMBE provides a possible means to bypass the ordering problem of InAs x Sb 1-x random alloys.

Critical behaviour of super-heated (1900 - 2000 K) vapours

We report on the experimental observation of a critical phenomenon (phase-transition-like) in super-heated vapours inside an alumina effusive oven in the temperature range of 1900 - 2000 K. This phenomenon is studied by modulated molecular beam mass spectrometry and the main observations are dramatic discontinuous decay of the flux by a factor of within and a simultaneous dramatic rise of a signal. Nearly complete thermal reversibility is demonstrated. Under flow conditions with 2 - 10 Torr helium, and are also observed. Both exhibit the same critical behaviour as . Tentative interpretations are in terms of heterogeneous gas - surface critical kinetic phenomena or vapour condensation leading to formation of liquid-like films followed by complete disintegration and oxidation.

A liquid nitrogen cooled ionizer for a quadrupole mass spectrometer

An ultrahigh vacuum compatible cooled ionizer suitable for use with a quadruple mass spectrometer has been constructed. It allows one to monitor rapidly changing signals of condensable gases as are commonly encountered in modulated molecular beam studies of surface reaction kinetics with minimal distortions due to adsorption/desorption phenomena within the ionizer.

Atomic-scale structure of InAs/InAs1−xSbx superlattices grown by modulated molecular beam epitaxy

We present cross-sectional scanning tunneling microscopy (STM) studies of the atomic-scale structural and electronic properties of InAs/InAs1−xSbx superlattices grown by modulated molecular-beam epitaxy. In this technique the Group V composition is controlled by rapid modulation of the Group V beams rather than adjustment of the Group V flux ratios. A superlattice sample was grown at 475 °C consisting of 52 Å InAs1−xSbx alternating with 172 Å InAs for 30 periods on a GaSb (001) substrate. The InAs1−xSbx alloy layers consisted nominally of 7.8 Å InAs alternating with 5.2 Å InSb for four periods. X-ray diffraction was used to determine an average composition of InAs0.76Sb0.24 for the alloy layers. Constant-current STM images of the superlattice exhibit clear, electronically induced contrast between the InAs layers and InAs0.76Sb0.24 layers, and also reveal ordering within the InAs0.76Sb0.24 layers. Interfaces between the InAs layers and the InAs0.76Sb0.24 layers appear sharp, though there is evidence of more atomic intermixing at the InAs-on-InAs0.76Sb 0.24 interface. Significant variation in composition within individual InAs0.76Sb 0.24 alloy layers and apparent Sb incorporation from the InAs0.76Sb0.24 layers into the surrounding InAs layers are also observed.

Hydrogen and water vapor adsorption on and reaction with uranium

By combining modulated molecular beam scattering, temperature programmed desorption, and atomic force microscopy experiments, the fundamental properties of the interaction of hydrogen and water vapor with uranium at room temperature and above have been investigated. The initial sticking probability, hydride formation probabilities and desorption kinetics for dihydrogen and water vapor on clean uranium have been measured. In addition, saturation coverages for hydrogen and water vapor are determined for an initially clean uranium surface. Atomic force microscopy revealed site-specific initiation of hydride formation for slightly oxidized uranium at grain boundaries. The areal (and hence time) dependence of the hydride formation probability was also observed. The clean uranium surface is passivated significantly by oxidation and carburization. The sticking probabilities for dihydrogen and water vapor are one to two orders of magnitude smaller on oxidized and carbided surfaces. The carbided surface is efficiently produced by reaction of C60 with uranium surfaces between 600 and 800 K.

Oxidation of GaAs(110) with NO2: A molecular beam study

We describe time-resolved modulated molecular beam measurements of the adsorption of NO2 on the GaAs(110) surface. These measurements provide a quantitative description of the gas–surface interaction; thermal NO2 has a nearly unity sticking probability, dissociating to form NO (which quickly desorbs) and a surface oxide of ∼1/3 ML saturation coverage. The NO2 dissociation probability is strongly controlled by the defect density, the oxygen coverage, and the surface temperature. We have characterized the competition between the processes leading to oxygen deposition, i.e., desorption and diffusion of NO 2 to reactive defects. The barrier energies Ediff=6±2 kcal/mol and Edes=9±2 kcal/mol provide an interesting example of the transition from defect-dominated surface chemistry, typical of low Miller index metal surfaces to defect- insensitive diffusion-limited chemistry, which we argue is generic to corrugated semiconductor surfaces.

Molecular Beam Studies on the Laser‐Enhanced Surface Reaction of GaAs(100) with Chlorine

AbstractA cw supersonic Cl2 molecular beam coupled with an angle‐resolved time‐of‐flight (TOF) technique has been used to investigate the laser‐enhanced surface reaction of GaAs(100) with chlorine. The mass and velocity distributions of the major reaction products under 1064 nm laser irradiation have been measured as a function of laser fluence, detection angle, surface temperature and normal component of the translational energy of the incident chlorine molecules. It has been found that increasing both laser fluence and the translational energy of incident chlorine molecules markedly enhance mis surface reaction. The measured flux angular distributions of major reaction products can be fit satisfactorily with a bi‐cosine function. Measurements of the mass and angular distributions of reaction products by a modulated molecular beam mass spectrometry show that the surface temperature effect is obvious for the Cl2/GaAs(100) “dark” thermal reaction. A direct activated dissociative chemisorption is proposed for the mechanism of Cl2 chemisorption on the GaAs(100) surface.

C 58 production from dissociation of C 60 by scattering from silica and highly oriented pyrolytic graphite

The dissociation of C 60 to C 58 has been observed on surfaces of silica and highly oriented pyrolytic graphite by modulated molecular beam reaction spectroscopy. At 1200 K on both surfaces approximately 5% of the incident C 60 dissociates and desorbs as C 58. The activation energy for C 58 production from C 60 on the surface (≈ 2.5 eV) is considerably smaller than that required in the gas phase, possibly due to the stabilization of the C 1 or C 2 products on the surface. At surface temperatures between 1000 and 1200 K, carbon was detected on the SiO 2 surface.

Reaction of β-SiC with thermal atomic hydrogen by modulated molecular beam mass spectrometry

The reaction of polycrystalline β-SiC with thermal atomic hydrogen is investigated by modulated molecular beam mass spectrometry. The temperature and equivalent pressure on the surface are 300–1100 K and 6 × 10 −6 –2 × 10 −5 Torr, respectively. The reaction probability of atomic hydrogen is 9 × 10 −5–5 × 10 −4. Grain boundary effects are negligible and no bulk or surface diffusion is observed. The major reaction products are SiH 4, CH 4, and C 2H 2. SiH 4 and CH 4 formation reactions are linear with respect to the H-atom pressure while C 2H 2 formation is nonlinear. A precursor model is proposed for SiH 4 and CH 4. C 2H 2 appears to be produced in the defect site where SiH 4 forms and desorbs.

Chemical dry etching of GaAs(100) by HC1: products, rate, and a kinetic model

The chemical dry etching of GaAs by HC1 has been studied using molecular beam scattering techniques. Unlike Cl 2 which etches GaAs at room temperature, HCl does not etch GaAs at a measurable rate below 670 K. For an incident HCl flux of 3.6 monolayers/s and surface temperatures of 670–870 K, the major volatile reaction products are GaCl and As 2. Above 850 K, GaAs evaporation also produces Ga atoms. Quantitative analysis of the product mass spectra and angular distributions shows that the product evolution rates are consistent with stoichiometric etching. Studies of the etching kinetics by modulated molecular beam scattering (MMBS) indicate that the rate of arsenic evolution limits GaAs etching to temperatures above 670 K. A simple reaction model which utilizes kinetic parameters derived from independent studies in the literature quantitatively reproduces the etching products, rate, and kinetics under both steady state and modulation conditions with only one adjustable parameter.

Desorption kinetics of CO from epitaxial Pd films on W(110) in the low coverage limit

The adsorption-desorption kinetics of CO from ultrathin epitaxial Pd films has been studied using modulated molecular beam techniques. We find that the rate constant for desorption, k d, is nearly four orders of magnitude higher for the monolayer Pd film than that for the bilayer film. The rate decreases further by a factor of 7 between 2 and 6 monolayers at which k d attains the value seen on Pd single crystals. We conclude that although a bilayer film is similar to a bulk Pd(111) surface in binding CO, true bulk-like behavior is only reached above six atomic layers of Pd. Temperature programmed desorption at low coverage can be fit using the beam determined kinetic parameters, showing that defects play no major role in the beam experiments. Significant departures in the values of the activation energy for desorption and the preexponential factor for the monolayer film from expected values are attributed to a possible adsorbate induced surface reconstruction.

Dry etching of GaAs with Cl 2: correlation between the surface Cl coverage and the etching rate at steady state

While dry etching of GaAs with chlorine is technologically important for manufacturing semiconductor devices, little is known conclusively about the surface chemical reactivity responsible for this etching process. In this work, modulated molecular beam scattering (MMBS) has been combined with temperature-programmed reaction (TPR) and Auger electron spectroscopy (AES) to study the reaction of molecular chlorine with GaAs. The MMBS and AES results indicate that the surface coverage of chlorine during steady state etching over the temperature range of 350–650 K is in the monolayer regime. Above 700 K the surface is chlorine free. A direct correlation is observed between the number of vacant surface sites and the Cl 2 reaction probability. This result suggests a Langmuir adsorption model for the surface reaction, and it is shown that such a model combined with the product evolution kinetics determined from TPR studies successfully simulates the temperature dependence of the chlorine evolved from the surface during modulated molecular beam scattering.

Competition between desorption and diffusion at a GaAs(110) surface

We directly demosntrate for NO 2 on GaAs(110), using time-resolved modulated molecular beam techniques, that desorption is a competitive rate process with surface diffusion. Using defects as markers, we observe a transition from defect-dominated low coverage chemistry to defect insensitive adsorption/desorption near room temperature. We argue that this competition is generic to adsorbates on semiconductors, in distinct contrast to adsorbates on metal surfaces where diffusion lengths are often large ( >10 3 Å) for almost all accessible temperatures.

Thermal decomposition of ultrathin oxide layers on Si(100)

Decomposition of ultrathin oxide layers on Si(100) has been studied using isothermal desorption, temperature programmed desorption and readsorption on partially desorbed layers using isotopically labeled oxygen. We find that inhomogeneous decomposition, with void formation in which clean silicon is exposed, occurs at coverages as low as 0.3 monolayers. Whereas the activation energy for SiO(g) formation is essentially independent of coverage between 10−3 and 10 monolayers, the apparent preexponential factor decreases substantially with increasing coverage. The discrepancy between the kinetic parameters measured for SiO(g) production in modulated molecular beam experiments and those measured using temperature programmed desorption is attributed to a strong decrease in the rate constant for desorption of SiO(g) with increasing coverage. Both methods give similar results at nearly identical coverages.

New modulated molecular beam scattering methods for probing nonlinear and coverage-dependent reaction kinetics at surfaces

New modulated molecular beam scattering methods for probing nonlinear and coverage-dependent reaction kinetics at surfaces

The reaction of atomic oxygen with Si(100) and Si(111): I. Oxide decomposition, active oxidation and the transition to passive oxidation

The reactions of atomic oxygen with the (100) and (111) surfaces of silicon have been investigated by employing supersonic molecular beam techniques and X-ray photoelectron spectroscopy. The kinetics and mechanism of the active oxidation reaction, i.e., O x(g) + Si(s) → SiO(g) where x = 1 or 2, has been evaluated by employing modulated molecular beam reactive scattering (MMBRS). On both surfaces, the reaction of atomic oxygen involves the formation of a single stable surface intermediate, which reacts via first-order kinetics to produce SiO(g). The reaction of molecular oxygen, however, involves two stable surface intermediates that are formed sequentially, the second of which is identical to that formed by the reaction with atomic oxygen. We propose that the first intermediate formed in the molecular oxygen reaction is chemisorbed O 2(a), e.g., a peroxy radical or a peroxide bridge. The intermediate formed in the atomic oxygen reaction is assigned to either an isolated oxygen adatom or adsorbed SiO (a surface silanone complex). Oxide decomposition in the mono- and multi-layer regime has been examined with temperature-programmed desorption (TPD). Both increasing oxygen coverages and higher adsorption temperatures lead to higher decomposition temperatures for the oxygen adlayers formed. Above the monolayer regime, adlayers formed on Si(100) decompose at higher temperatures than those on Si(111). On Si(100), simple first-order decomposition kinetics is only observed in the monolayer regime, and in the limit of low coverage (θ < 0.3 ML). The implied rate coefficient for desorption in this regime is at least 2 orders of magnitude smaller than that measured by MMBRS at the same temperature. At higher coverages (θ ⩾ 4 ML) and on both surfaces, the decomposition reaction appears to be heterogeneous, involving void nucleation and growth. The transition between active and passive oxidation has been examined employing in situ, real-time mass spectrometric and XPS measurements. The transition involves a competition between highly activated ( E d ∼ 80 kcal mol −1) desorption of SiO, and nearly unactivated nucleation of surface oxide.

Kinetics of hydrogen oxidation to water on the Rh(111) surface using multiple source modulated molecular beam techniques

We have examined the kinetics of the oxidation of hydrogen to water on the Rh(111) surface using modulated molecular beam reactive scattering. For reactant pressures below 10−4 Torr and temperatures from 450–1250 K we observe serial steps, with apparent activation energies of 2.5 ± 1 and 10 ± 1 kcalmol. Pseudo-first-order preexponential factors are 105 and 107 s−1, respectively, varying slightly with oxygen coverage. Reaction is inhibited by excess oxygen. Maximum water production occurs around 650 K. At lower temperatures the reaction becomes nonlinear. We use a new three-molecular-beam arrangement. Two continuous, independently adjustable beams establish steady-state surface concentrations, while a weaker modulated third beam induces small concentration perturbations around the selected steady state. With this technique we varied surface oxygen coverages, used isotopic substitution in the three beams to produce H2O, D2O and HDO, and linearized the HDO reaction.

Kinetics of hydrogen oxidation to water on the Rh(111) surface using multiple source modulated molecular beam techniques

We have examined the kinetics of the oxidation of hydrogen to water on the Rh(111) surface using modulated molecular beam reactive scattering. For reactant pressures below 10 −4 Torr and temperatures from 450–1250 K we observe serial steps, with apparent activation energies of 2.5 ± 1 and 10 ± 1 kcal mol . Pseudo-first-order preexponential factors are 10 5 and 10 7 s −1, respectively, varying slightly with oxygen coverage. Reaction is inhibited by excess oxygen. Maximum water production occurs around 650 K. At lower temperatures the reaction becomes nonlinear. We use a new three-molecular-beam arrangement. Two continuous, independently adjustable beams establish steady-state surface concentrations, while a weaker modulated third beam induces small concentration perturbations around the selected steady state. With this technique we varied surface oxygen coverages, used isotopic substitution in the three beams to produce H 2O, D 2O and HDO, and linearized the HDO reaction.