Neville Jonathan

Temperature programmed desorption of molecular hydrogen from a Si(111) surface: Theory and experiment

New experimental temperature programmed desorption (TPD) data have been obtained under carefully controlled conditions for atomic deuterium on single crystal Si(111). A wide range of initial coverages from Θ=1.25 to 0.05 ML was used. It was found that the results could only be satisfactorily interpreted in terms of a two‐site adsorption model in which it is suggested that two formally second‐order reactions involving the monohydride (deuteride) contribute to the well‐known β1 desorption peak at ≊810 K with the relative importance of these two reactions changing with initial coverage. The pre‐exponential factors for these reactions were found to be 2 cm2 s−1 and 1 cm2 s−1 with corresponding activation energies of 57.5±2 kcal mol−1 and 56.5±2 kcal mol−1, respectively, for deuterium desorption when the energy difference between the two sites was taken to be 2.5 kcal mol−1. The other desorption channel (β2) was also found to exhibit second‐order kinetic behavior involving the dihydride (deuteride). In this ca...

The adsorption and reactions of water on Si(100)-2 × 1 and Si(111)-7 × 7 surfaces

The dissociative adsorption of D2O on single crystal silicon surfaces and the subsequent desorption of D2 and SiO have been investigated using temperature programmed desorption. Adsorption across a single dimer of the Si(100)-2 × 1 reconstruction involves a mobile and long-lived precursor at low surface temperatures and leads to first order Langmuir kinetics at higher temperatures. Occasionally, two diagonally adjacent dangling bonds are occupied and consequently the saturation coverage is (0.41 ± 0.02) ML with 0.18 ML of unreacted but isolated dangling bonds remaining. Approximately 17% of the adsorbed OD can be abstracted as D2O by reaction with atomic deuterium. The initial sticking probability for adsorption across a rest-atom-adatom dangling bond pair of the Si(111)-7 × 7 reconstruction is 0.23 ± 0.08. Although adsorption at the remaining isolated adatom sites occurs with a very much reduced probability, a saturation coverage of (0.22 ± 0.02) ML is eventually achieved. Desorption of 90% of the adsorbed deuterium on Si(100) and 85% of the adsorbed deuterium on Si(111) occurs via the β1 channel. The activation energy for desorption via the β1 channel increases with initial D2O coverage due to an extended and cumulative interaction with adsorbed oxygen. The remaining coverage independent fraction is adsorbed at sites with one or two oxygen nearest neighbours and is more directly associated with SiO desorption. The desorption of SiO from Si(100) can be simulated using first order kinetics with a pre-exponential factor of 2.5 × 1017s−1 and an activation energy that increases with initial D2O coverage from 75 kcal mol−1 at 0.05 ML mol−1 at 0.4 ML.

The desorption of molecular hydrogen from Si(100)-2×1 and Si(111)-7×7 surfaces at low coverages

The mechanisms leading to desorption of molecular hydrogen from Si(100)-2×1 and Si(111)-7×7 surfaces have been elucidated and refined by detailed examination of the thermal desorption kinetics with particular emphasis on low and very low coverages. In the case of hydrogen desorption from Si(100)-2×1, a lattice-gas model incorporating the interactions that are responsible for pairing and clustering of adsorbed hydrogen atoms has been employed to fit temperature programmed desorption (TPD) peaks resulting from initial coverages between 0.01 and 1.0 monolayer (ML). From analysis of our low coverage data, we find that the pairing and clustering energies are (3.2±0.3) kcal mol−1 and (3.4±0.5) kcal mol−1, respectively. A subtle shift of the TPD peak maximum position as the initial coverage increases from 0.2 to 1.0 ML indicates that the pre-exponential factor and activation energy are weakly coverage dependent. We discuss how this is consistent with coupling of a dihydridelike transition state to its neighbors....

A quasi-equilibrium model for the uptake kinetics of hydrogen atoms on Si(100)

Abstract A quasi-equilibrium model has been developed to describe the uptake kinetics of atomic hydrogen on Si(100) at 373 and 635 K. A new model is required because a simple consideration of only adsorption at dangling bond sites and Eley-Rideal abstraction cannot be reconciled with a saturation coverage of one monolayer (ML) at 635 K. We argue that although diffusion at low temperatures can be explained by hot precursor dynamics, slow vibrational relaxation in the chemisorption potential does not lead to an almost coverage independent sticking probability. Instead, we propose that a high sticking probability is maintained by reaction with doubly occupied dimers to give dihydride species which then migrate across the surface by an isomerization reaction. Subsequently, dihydride units either react with unoccupied dimer sites or molecular hydrogen is lost by desorption from two dihydride units (the β 2 desorption channel). At 635 K the dihydride species may be regarded as a mobile chemisorbed precursor, although only the bonding arrangement is propagated by isomerization. When the atomic hydrogen source is turned off, the small steady-state dihydride coverage is rapidly lost to leave a saturated monohydride phase. A saturation coverage of 1.5 ML is obtained when abstraction and adsorption reactions are in dynamic equilibrium at 373 K. It is shown that the rate constant for abstraction from monohydride is essentially the same as for abstraction from dihydride. The measured initial rate constant for loss of adsorbed hydrogen during exposure to atomic deuterium is (1.8 ± 0.1) times larger than that for loss of adsorbed deuterium during atomic hydrogen exposure at 635 K. At this temperature, the initial rate constant for loss of deuterium during exposure to atomic hydrogen is found to be the same regardless of initial coverage, but the rate decreases more slowly than expected for first-order kinetics as the reaction proceeds. The uptake model also implies that a small amount of deuterium is lost as D 2 and HD via the β 2 thermal desorption channel at 635 K.

High‐sensitivity photoelectron spectrometer for studying reactive transient species

The construction and operation of a high‐sensitivity photoelectron spectrometer, specifically designed for studying reactive transient species, is described. Fast data acquisition is used to minimize spectral drift problems caused by contamination resulting from species production conditions. The spectrometer is equipped with sample differential pumping to retard contamination of the electron optics and its modular design enables individual components to be readily dismantled for cleaning purposes. A microchannel‐plate (MCP)/phosphor/silicon‐intensified‐target (SIT) camera detector is used, operated under modest voltage conditions. A unique, dedicated, dual‐buffer, data interface is constructed, incorporating the use of a video window to select data. The interface can be used in a stand‐alone mode for rapid data examination, when optimum spectrometer conditions can conveniently be established using the detector two‐dimensional imaging capability and a standard TV monitor for assessment of line image quali...

Forbidden bands in the photoelectron spectra of diatomic molecules

Abstract Configuration interaction CNDO/INDO calculations have been used to interpret weak bands which have been observed in the photoelectron spectra of gaseous N 2 , CO and CS. These bands are due to photoionisation to states which are formally forbidden on the simple one-electron transition model. The more general implications of configuration interaction bands in polyatomic molecules are discussed, and some examples are suggested where it is believed that the effect may be important.

Temperature programmed desorption from the Si(100):Br and Si(100):D/Br surfaces: theory and experiment

New experimental temperature programmed desorption (TPD) data have been obtained under carefully controlled UHV conditions following the adsorption of bromine and the co-adsorption of bromine and atomic deuterium on the single crystal Si(100): 2 × 1 surface. Coverages ranged from maxima of θ = 1.5 ML (atomic deuterium alone) and 1.0 ML (atomic bromine alone) and included a wide combination of the two co-adsorbed species down to a lower limit of 0.05 ML atomic deuterium and 0.2 ML atomic bromine. When bromine alone was absorbed at surface temperatures of 380 ± 10 K, the only observed TPD product was silicon dibromide. When the total coverage of bromine and deuterium atoms was high (≳ 1 ML), the TPD data were much more complex and as well as showing the usual s1 and s2 desorption peaks of deuterium, analogous peaks for DBr were also observed together with the above-mentioned SiBr2 peak. The data have been interpreted using a kinetic lattice-gas model which describes the atomic bromine and deuterium co-adsorbed on the surface in terms of nine basic units: dideuteride (SiD2), doubly occupied dimers (DSiSiD), singly occupied dimers (SiSiD), unoccupied dimers (SiSi) together with the analogous bromine substituted and mixed deuterium-bromine species. Using a quasi-equilibrium approximation and including up to five competing desorption pathways corresponding to the product peaks observed, the TPD spectra for deuterium, deuterium bromide and silicon dibromide are determined and compared with the observed experimental data. By fitting the simulated data to the experimental curves it was shown that both s1 desorption processes and desorption of SiBr2 were consistent with first order kinetic behaviour. On the other hand both s2 processes were found to obey second order kinetics. The frequency factors for the first-order processes were determined as 1 × 1015s−1 (D2 and DBr) and 2 × 1015s−1 (SiBr2) with the corresponding activation energies being 57.5 kcal mol−1 (D2), 61 kcal mol−1 (DBr) and 61 kcal mol−1 (SiBr2). For the second-order processes, the frequency factors (expressed in first-order units) were found to be 2 × 1015s−1 (for both D2 and DBr) with the activation energies being 45 kcal mol−1 (D2) and 49 kcal mol−1 (DBr).

UHV studies of the adsorption of hydrazoic acid on Si(100)

Abstract The adsorption of hydrazoic acid, HN 3 , on Si(100)2 × 1 has been studied by temperature programmed desorption and Auger electron spectroscopy. The desorption products are molecular hydrogen and nitrogen which are interpreted as being consistent with dissociative adsorption yielding NH and N 2 as the adsorbed species. Auger data are given which show that HN 3 is a more efficient nitriding agent for silicon than ammonia or hydrazine. Additionally the potential of HN 3 as a source of “active nitrogen” for low temperature chemical vapour deposition of the nitrides of silicon and the group III metals, is clarified.

A fast sorption pump for the 10-150 mtorr region

The design, operation and performance of a large sorption pump for use in making flow system studies of transient gaseous species is described.

Response to ‘‘Comment on ‘Temperature programmed desorption of molecular hydrogen from a Si(100)‐2×1 surface: Theory and experiment’ ’’ [J. Chem. Phys. 101, 2648 (1994)]

Arguments are given for the validity of defining the Si(100):D surface in terms of four basic species and two equilibria. Differences between experimental results and theoretical calculations remain unresolved especially whether hydrogen desorption occurs directly from a dimer pair or via a site defect mechanism.