Molecular Beam System Research Articles

Irradiation effects of synchrotron radiation on silicon epitaxial growth using a disilane molecular beam system

Photo-epitaxy excited by synchrotron radiation (SR) has been achieved using a disilane molecular beam. The surfaces of the as-grown films deposited at silicon substrate temperatures of 380–720 °C exhibit a 2 × 1 reconstructed reflection high energy electron diffraction pattern, indicating two-dimensional growth. No carbon and oxygen contamination by SR is detected in the epitaxial film. In the temperature range below 600 °C, the SR-excited growth rate is larger than that of thermal growth. This result suggests that SR enhances the surface reactions, such as desorption of hydrogen and surface migration of adsorbed species. Low temperature cleaning of a silicon surface using disilane gas has been carried out using photochemical reactions excited by SR. Epitaxial growth is attained on a silicon surface cleaned by this method at 700 °C. The oxygen concentration at the interface between the substrate silicon and the epitaxial film is reduced by decreasing disilane beam intensity and increasing cleaning temperature. The principal reaction is attributed to the SR-enhanced reduction of surface oxide by adsorbed silicon atoms.

NO adsorption on Rh(110)

NO adsorption on Rh(110) has been investigated using a molecular beam system and TPD. NO appears to adsorb initially dissociatively at 300 K, followed by molecular adsorption at higher exposures. The initial sticking coefficient is high (0.67) at all temperatures, declines only slowly with increasing coverage and the saturation coverage at 300 K is 1.1 ( ± 0.1) monolayers. This seems to be composed of a mix of atomic adsorption in the trough sites and molecular adsorption on on top sites. If the adsorption measurements are performed above 380 K, N 2 is seen to evolve during the experiments, with kinetics which depend on the binding strength of the N atoms to the surface. TPD shows that is dramatically weakened above a total atomic coverage of ~ 0.5 monolayers, due to the presence of coadsorbed oxygen atoms. Since the saturation atomic coverage under these circumstances can be as high as 1.5 monolayers it appears that this destabilisation is due to a structural rearrangement of the surface which displaces nitrogen atoms to low coordination sites (perhaps on top) while the oxygen remains in the more favourable sites.

Molecular beam studies of CO oxidation and noxidation on Rh(110)

The reaction of CO with O2 and NO on Rh(110) has been studied using a thermal molecular beam system. For both reactions the rate is low at high temperatures due to the formation of reconstructed, oxidized Rh.

Molecular beam studies of the dehydrogenation of alcohols on oxidised copper {110}

The dehydrogenation of a number of alcohols (methanol, ethanol, propan-1-ol and propan-2-ol) have been studied on oxygen-covered copper (110) under UHV conditions using a thermal molecular beam system. This paper will illustrate how this technique has given new information about these complex reactions and their rates, showing a strong dependence on the surface temperature, oxygen precoverage, as well as the alcohol involved. This is due to the beam technique being able to give absolute values of the sticking probability of the reactant, while simultaneously measuring product evolution. All alcohols give common products of the appropriate aldehyde (or ketone) water and hydrogen, however, the reaction shows a change in stoichiometry from 2R1R2CHOH + O(a) → 2R1R2CO + H2 + H2O at low temperatures to R1R2CHOH + O(a) → R1R2CO + H2O at higher temperature temperature depending on the alcohol involved). The preadsorbed oxygen strongly enhanced the absolute sticking probability of all the alcohols studied, but also poisons the dehydrogenation reaction at higher coverages. The oxygen coverage variation studies clearly show that the alcohol adsorption involves a precursor state. Isotopic labelling experiments have also been carried out which help to explain clearly many of the effects observed, especially pointing to the importance of the stability of the intermediate alkoxy species.

Molecular beam studies of the dehydrogenation of alcohols on oxidised copper {110}

Abstract The dehydrogenation of a number of alcohols (methanol, ethanol, propan-1-ol and propan-2-ol) have been studied on oxygen-covered copper (110) under UHV conditions using a thermal molecular beam system. This paper will illustrate how this technique has given new information about these complex reactions and their rates, showing a strong dependence on the surface temperature, oxygen precoverage, as well as the alcohol involved. This is due to the beam technique being able to give absolute values of the sticking probability of the reactant, while simultaneously measuring product evolution. All alcohols give common products of the appropriate aldehyde (or ketone) water and hydrogen, however, the reaction shows a change in stoichiometry from 2R1R2CHOH + O(a) → 2R1R2CO + H2 + H2O at low temperatures to R1R2CHOH + O(a) → R1R2CO + H2O at higher temperature temperature depending on the alcohol involved). The preadsorbed oxygen strongly enhanced the absolute sticking probability of all the alcohols studied, but also poisons the dehydrogenation reaction at higher coverages. The oxygen coverage variation studies clearly show that the alcohol adsorption involves a precursor state. Isotopic labelling experiments have also been carried out which help to explain clearly many of the effects observed, especially pointing to the importance of the stability of the intermediate alkoxy species.

Synchrotron radiation excited Si epitaxial growth using disilane gas source molecular beam system

Silicon photoepitaxy excited by synchrotron radiation (SR) has been observed for the first time. The epitaxial growth is observed even at lower than a 400 °C substrate temperature. The surface of the as-grown film exhibits a 2×1 reconstruction reflection high-energy electron diffraction pattern, indicating two-dimensional growth. At lower than 600 °C, the SR-irradiation growth rate is larger than that of thermal growth. This result suggests that SR irradiation enhances the dynamic surface reactions, such as desorption of hydrogen and surface migration of adsorbed species.

Acetate formation and explosive decomposition during ethanol oxidation on Rh

The adsorption and reaction of ethanol with the Rh(110) surface has been studied using a thermal molecular beam system and temperature programmed desorption. On the clean surface, ethanol shows a very simple dehydrogenation, producing hydrogen in the gas phase, adsorbed CO (which is desorbed by heating to 550 K) and carbon. Since in alcohol synthesis reactions it is likely that the surface will be partially oxidised, the reaction with predosed oxygen was also investigated. The reaction pathway then becomes much more complex. The main changes are (i) CH4 and H2O evolution during adsorption, and (ii)Acetate formation by oxygen insertion in the molecule. The acetate shows very unusual decomposition kinetics — a surface ‘explosion’ with a very narrow peak-yielding CO2 and H2 in the gas phase and adsorbed C. The acetate is always seen on Rh catalysts which are selective for alcohol synthesis from CO and H2, and it is proposed that oxidic promoters such as vanadia may act to stabilise this intermediate.

Synchrotron Radiation Excited Semiconductor Processes.

An outline of a synchrotron radiation (SR) excited reaction is given in connection with silicon epitaxy and related surface processes. Experimental results on SR-excited silicon epitaxial growth and surface cleaning using a disilane molecular beam system are presented. Epitaxial growth was attained at a surface temperature as low as 380°C. It is concluded that the growth rate in the low temperature region is limited by hydrogen desorption from the surface due to SR irradiation. The surface cleaning was realized at a surface temperature of 700°C mainly by an SR excited reductive reaction of the silicon surface oxide by disilane molecules.

The pyrolysis of trimethylgallium on a heated Si(100) substrate

Trimethylgallium (TMGa) is an important source gas for metalorganic chemical vapor deposition (MOCVD) and metalorganic molecular beam epitaxy (MOMBE). In the last three years, workers in three different laboratories have examined the pyrolysis of trimethylgallium (TMGa) on heated substrates. However, the mechanism of the decomposition process is still not understood. In this paper we consider the mechanism of the TMGa decomposition on a hot Si(100) substrate in a molecular beam system. It is found that at TMGa doses of 1 × 10 15 molecules/cm 2 or less, the decomposition process in the beam system is the same as that reported by Lee et al., Surface Sci. 216 (1989) 173, in studies where TMGa was dosed at low temperature and then the substrate was heated. However, at higher TMGa doses, the mechanism in the molecular beam system is different than the ones reported previously. It is possible to transiently populate a weakly bound state which we assign to TMGa adsorption in the second monolayer. If the sample is below 400 K, the TMGa in the second monolayer desorbs. However, at higher temperatures, the TMGa in the second monolayer can react to form a CH x group ( x = 3 or 4) and dimethylgallium. The dimethylgallium desorbs. However, some of the CH x groups remain bound to the substrate leading to extra carbon incorporation. There also is evidence for formation of traces of monomethylgallium above 500 K and an indication of methyl radical desorption above 650 K. Thus it seems that at high doses, the decomposition of TMGa on a hot substrate is somewhat different than the decomposition of a monolayer of TMGa which has been adsorbed at low temperatures and then heated. These results explain some of the apparent discrepancies in the literature.

A novel type of doubly charged cluster

A combined supersonic molecular beam system and mass spectrometer has been used to prepared mixed cluster ions from neutrals of the form Ar n ·X and (CO 2) n ·X, where X is either 1,3-butadiene or 1,3,5-hexatriene. In addition to singly charged clusters and fragments being present in the mass spectra, doubly charged species of the type {Ar n ·X} 2+ and {(CO 2) n ·X∝ 2+ are observed. Whereas the latter ions follow the usual pattern of exhibiting Coulomb explosion below some critical value of n (≅ 35), those identified as {Ar n ·X} 2+ are stable down to n < 16. It is proposed that these observations are due to the fact that in {Ar n ·X} 2+ clusters, both charges reside on the molecule. Such behaviour is contrary to that expected from a consideration of the relative ionization potentials.

Design and synthesis of CVD precursors to thin film ceramic materials

The application of cyclic organometallic compounds as single source precursors for the chemical vapor deposition of materials such as AIN and SiC is discussed and new results relating to the decomposition of a novel SiC CVD precursor are presented. The decomposition of the cyclic carbosilane [μ-(CH 2) 2Si(CH 3)(H)Si(CH 3)(CH 2SiH 2CH 3)], ( I) on a heated glassy carbon subst rate in an ultra-high vacuum molecular beam system has been studied by pulsing the precursor molecule onto the surface and following the mass spectrum as a function of the substrate surface temperature. The evolution of CH 3SiH 2, C 2H 5 and CH 3 was evidenced, suggesting loss of excess carbon as C 1 and C 2 species.

The elementary reaction of O( 3P) atoms with Cr(CO) 6 in the gas phase

The reaction of Cr(CO) 6 with O( 3P) atoms was studied in an isothermal discharge-flow reactor with He and Ar as a main carrier gas. O atoms were produced by discharging O 2/He mixtures. The species in the flow system were detected with a quadrupole mass spectrometer via a molecular-beam system. The rate constant of the reaction O( 3P) + Cr(CO) 6→products was measured at a pressure of p=2 mbar under pseudo-first-order conditions [O] 0⪢[Cr(CO) 6] 0 by following the [Cr(CO) 6] decay: k(298 K) = (1.2±0.3) × 10 12cm 3mol −1s −1. The reaction product CO 2 was detected directly in the pressure range 5⩽ p⩽10 mbar. Cr oxides or Cr atoms or clusters were not observed in the gas phase as products of the above reaction. Cr(CO) 5 was observed indirectly via the formation of Cr(CO) 6 in the reaction Cr(CO) 5+CO→Cr(CO) 6. For the first reaction the primary step Cr(CO) 6+O→CO 2+Cr(CO) 5 is discussed.

Activated dissociation of oxygen on Cu(110)

Oxygen adsorption on Cu(110) has been studied using a thermal molecular-beam system. The sticking probability (S) is independent of substrate temperature above 220 K but depends strongly on beam temperature, showing the adsorption to be directly activated from the gas phase by 3kj mol −1. However, at low substrate temperatures there is also evidence of the involvement of an accommodated, molecularly chemisorbed species in the dissociation process. The adsorption scales with total energy of the incoming molecule and so eitheir the potential surface appears isotropic in space, or there is a rather tortuos entrance channel to reach the barrier, which exists between unionised (physisorbed) oxygen and O− 2 at the surface.

A simple molecular beam system for surface reactivity studies

A simple design for a thermal molecular beam system is described, which can be added as a bolt-on to a conventional vacuum system. It provides a beam of well-defined spatial distribution, which can be heated or cooled, and used for a variety of applications. These include the accurate and rapid determination of sticking probabilities and their coverage dependence, the gas temperature dependence of sticking, and reactive scattering measurements. The use of the beam is exemplified for the dissociative sticking of oxygen on Cu(110) and the reaction of a beam of methanol with a predeposited patch of oxygen. The initial O2 sticking probability is determined to be 0.21 (±0.01) independent of substrate temperature (300–600 K). Methanol is dehydrogenated by preadsorbed O atoms, but by different mechanisms at high and low substrate temperatures.

Mechanisms of the reaction of unsaturated organic halides with small gas-phase vanadium clusters

Small gas-phase vanadium clusters synthesized in a molecular beam system by a laser vaporization technique are reacted with 1-bromopropene, 3-bromopropene, 3-chloropropene, and propene in a fast-flow reactor, and the analysis of the products was made by laser ionization-time-of-flight mass spectrometry. The results observed can be explained in terms of three different mechanisms for the reaction of halopropene: halogen abstraction, halogen substitution followed by molecular hydrogen evaporation, and molecular addition followed by evaporation of molecular hydrogen and hydrogen halide. The addition reaction followed by molecular hydrogen evaporation can account for the reaction involving propene. The dependence of the relative importance of the three different mechanisms on the vanadium cluster size and the structure of the halopropene is discussed.

Precursor state effects in catalysis: Oxidative dehydrogenation of propan-2-ol on Cu(110)

The oxidative dehydrogenation of propan-2-ol has been investigated using a thermal molecular beam system. The main products after predosing the surface with oxygen atoms are the dehydrogenated molecule (acetone), water and hydrogen. The effect of oxygen is to enhance the sticking of propan-2-ol from approximately 0.05 on the clean surface to 0.76 (±0.02). By varying the oxygen predose we have shown unambiguously that precursor state effects dominate the catalytic reaction, since at oxygen coverages as low as 0.008 monolayers the propanol sticking probability remains at 0.76. This is due to the long lifetime and diffusivity of the reactant on the surface enabling it to‘seek out’ the active sites (those with oxygen atoms) on the surface.

Molecular-beam studies of methanol partial oxidation on Cu(110)

A thermal molecular-beam system has been used to examine the oxidative dehydrogenation of methanol on the Cu(110) surface. The initial sticking probability for oxygen is 0.21 (±0.01) at room temperature and shows a near linear dependence of sticking coefficient on atomic coverage due to an island growth mechanism with dissociation on an oxygen dilute inter-island phase. Beam temperature variations show this adsorption to be activated. The reaction of methanol in the beam with a predeposited patch of oxygen depends strongly on surface temperature and oxygen coverage. There is a change in stoichiometry in the reaction from 2CH3OH + O(a)→ 2H2CO + H2+ H2O at 330 K to CH3OH + O(a)→ H2CO + H2O above ca. 450 K. Oxygen promotes the methanol adsorption and reaction at low coverages, but shows poisoning effects at half a monolayer [saturation, p(2 × 1) structure] where the rate of reaction is very much reduced, and there is an induction time before products are seen in the gas phase (ca. 30 min at 333 K under these conditions). This is explained by stabilisation of the methoxy species when the adjacent (110) trough sites are blocked by either adsorbed oxygen or hydroxyl groups; a kinetic model is being developed to describe these complex kinetics, based on a slow production of vacant sites in these (110) troughs. This is shown to describe the kinetics quite well in a semi-quantitative manner. Use of CD3OD in the beam shows a marked isotope effect, whereas CH3OD does not, again indicating that it is methoxy decomposition which limits the product evolution rate.

ChemInform Abstract: Reaction of Carbon Dioxide with Gaseous Niobium and Niobium Oxide Clusters.

AbstractSmall niobium oxide clusters (Nbx and NbxOy; x = 1‐13, y = 1‐4) are synthesized by laser vaporization in a supersonic molecular beam system and reacted with carbon dioxide and its oxygen isotopically labeled derivative (CO*2) to form Nb‐O and Nb‐CO bonds.

Mass-spectroscopic study of the influence of nozzle material on high-pressure SF6 arcs

The interrupting capability of a gas-blast high-voltage circuit breaker (CB) is mainly determined by the self-induced pressure rise caused by the thermal arc energy, the composition of the arc plasma and the chemical reactions occuring during and after current interruption. We have studied the nozzle materials boron nitride (BN), quartz (SiO2), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene (ETFE), polyethylene (PE) and epoxy resin (ER) with respect to their influence on these processes with the aid of a model circuit breaker (MCB). Direct measurements of the arc-induced pressure rise reveal that the portion of the arc energy available for the pressure rise varies greatly (∼20%–65%) with the properties of the nozzle material. Nozzle erosion is significantly higher for materials with high values (e.g. polymers). Therefore, the lifetime of polymer nozzles is considerably shorter than that of ceramic nozzles. We have investigated the influence of the nozzle material on the decomposition products formed in the arc discharge of our MCB by studying the composition and time dependence of these products. The MCB was directly attached to the time-of-flight mass spectrometer (TOFMS) with the aid of a molecular-beam sampling system, which allowed real-time measurements of the arced gas during and after current interruption, thus providing information on the ablation mechanism and on the reaction kinetics of vaporised nozzle material with dissociated SF6. The most abundant long-lived reaction products are SF4, SOF2, C2H2, CO, and CS2. Their formation rates have been determined as functions of the nozzle material. With respect to quantities and properties of decomposition products, ceramics are superior to polymers since they form only small concentrations of corrosive and toxic products.

Trimethylgallium Decomposition on a Heated Si(100) Substrate

AbstractTrimethylgallium (TMGa) is an important source gas for metal-organic chemical vapor deposition (MOCVD) and metal-organic molecular beam epitaxy (MOMBE). In the last three years, workers in three different laboratories have examined the pyrolysis of trimethylgallium (TMGa) on heated substrates. However, the mechanism of the decomposition process is still not understood. In this paper we consider the mechanism of the TMGa decomposition on a hot Si(100) substrate in a molecular beam system. It is found that at TMGa doses of 1×1015 molecules/cm2 or less, the decomposition process in the beam system is the same as that reported by Lee et al., Surface Sci. 216, 173 (1989) in studies where TMGa was dosed at low temperature and then the substrate was heated. However, at higher TMGa doses, the mechanism in the molecular beam system is different than the ones reported previously. It is possible to transiently populate a weakly bound state which we assign to TMGa adsorption in the second monolayer. If me sample is below 400 K, the TMGa in the second monolayer desorbs. However, at higher temperatures, the TMGa in the second monolayer can react to form a CHx group (x=3 or 4) and dimethylgallium. The dimethylgallium desorbs. However, some of the CHx groups remain bound to the substrate leading to extra carbon incorporation. There also is evidence for formation of traces of monomethylgallium above 500 K and an indication of methyl radical desorption above 650 K. Thus it seems that at high doses, the decomposition of TMGa on a hot substrate is somewhat different than the decomposition of a monolayer of TMGa which has been adsorbed at low temperatures and then heated. These results explain some of the apparent discrepancies in the literature.