M.J. Dresser

Adsorption and thermal behavior of ethylene on Si(100)-(2 × 1)

The adsorption of ethylene on Si(100)-(2 × 1) has been studied in ultrahigh vacuum. Chemisorption was found to occur via a mobile precursor mechanism. The activation energy difference for desorption and chemisorption from the precursor (Ed − Er), was found to be 2.9 kcal mol−1. The saturation coverage of ethylene is 1 C2H4/Si2 dimer. Hydrogen-site blocking and thermochemical arguments suggest that C2H4 bonds as a di-σ surface complex to dimer sites; upon chemisorption of C2H4 the SiSi dimer is cleaved. Chemisorbed ethylene desorbs unimolecularlfrom Si(100) at ∼ 550 K, with approximately 2% of the monolayer undergoing dissociation. The activation energy of C2H4 desorption is 38 kcal mol−1, and for the di-σ C2H4Si2 complex, each SiC bond has a strength of ∼ 73 kcal mol−1. The low desorption activation energy allows C2H4 to desorb prior to signifi dissociation, preventing the formation of significant coverages of surfaces carbon and hydrogen.

The adsorption and decomposition of NH3 ON Si(100)-detection of the NH2(a) species

The dissociative adsorption of NH3 on Si(100)-(2 × 1) has been studied using accurate surface coverage measurements, temperature programmed desorption. Auger spectroscopy and digital ESDIAD/LEED methods. It has been found that NH2 surface species (amino species) are produced to a saturation coverage of 1 NH2/Si dimer at 120 K. This is accompanied by the production of a Si-H surface species. Digital ESDIAD measurements of the H+ angular distribution from NH2(a) species indicate that torsional oscillations about the Si-NH2 bond are responsible for the characteristic elliptical H+ pattern whose long axis is perpendicular to the Si-Si dimer bond direction. It has been shown that NH, dissociatively adsorbs with unity sticking probability at 120 K up to 86% of full coverage, indicative of a mobile precursor adsorption mechanism. The preadsorption of atomic H onto the Si dangling bond sites reduces the adsorptive capacity of the Si(100) surface, and 1 H/Si completely passivates the surface for NH3 chemisorption. The NH2(a) species, produced by adsorption at 120 K are stable up to about 600 K, where decomposition occurs to produce N(a) and H(a). A minor reaction channel involving NH2(a)+ H(a) to produce recombined NH3(g) is observed in the temperature range 600–700 K. Above 700 K, surface N(a), produced from NH2(a) decomposition, enters into the Si(100) lattice.

Enhancement of the ESDIAD method for imaging bond directionality in chemisorbed species

Abstract The ESDIAD method for imaging adsorbate bond directions by photographic observation of positive ion angular distributions during electron stimulated desorption suffers from inherent low contrast due to background effects. The use of a digital acquisition system designed to overcome this difficulty in ESDIAD measurement is presented. Measurements on a Ni(110) single crystal substrate show the presence of a significant background signal due to soft X-ray generation by electron impact. By subtraction of the background signal, a significant enhancement of positive ion signal-to-noise ratio is achieved in ESDIAD, converting the ESDIAD method into a high contrast, high resolution surface measurement technique. Quantitative studies of the soft X-ray background have shown it to be linearly dependent on electron current density and electron energy, with no change in angular shape. These properties permit an accurate background subtraction procedure to be employed to significant;y enhance the capability of the ESDIAD method.

Interaction between NH3 and CO on the Ni(111) and (110) surfaces: A study by ESDIAD

The interaction between adsorbed NH3 and adsorbed CO molecules on two Ni single crystal planes has been investigated using ESDIAD and temperature programmed desorption (TPD). Interactions have been observed on both surfaces which influence the ESDIAD patterns of both adsorbed species. Evidence for long distance azimuthal orientation interactions of NH3 with CO on Ni(110) is observed, whereas shorter distance interactions are observed on Ni(111). In the case of the short distance CO…NH3 interactions on Ni(111), a tipping of the C3v axis of NH3 away from the normal is seen. The role of the substrate crystal structure is shown to be important in determining the character of the intermolecular interactions on the two surfaces.

Si-F bond directions on Si(100) — A study by ESDIAD

Abstract We report the first observation of ion angular distributions originating from electron stimulated desorption of an adsorbed atomic species on a semiconductor surface (ESDIAD). F+ is emitted from Si(100)-(2×1) along 4 azimuths corresponding to the principal crystal axes. The most probable F+ energy is 2.4 eV. The F+ emission angle, α ≈ 36° ± 5° to the surface normal, corresponds closely to the SiF surface bond direction. This F+ angular distribution is consistent with F bonding to Si dimers which are in two orthogonal reconstruction on Si(100)-(2×1). The threshold electron energy, VcT = 27.5 ± 1 eV for F+ production from the SiF surface species.

The dissociative adsorption of ammonia on Si(100)

Abstract The nature of the NH 3 adsorption process on Si(100) at 120 K was studied by isotopic mixing with adsorbed atomic deuterium and thermal desorption spectroscopy. NH 3 was found to dissociatively adsorb onto Si(100) dimer sites at 120 K as NH 2 (a) and H(a). The NH 2 (a) species persist up to about 700 K where two reaction channels become available. The major reaction channel leads to the decomposition of NH 2 (a) to N(a) and H(a). The minor channel is a recombination reaction that leads to the desorption of ammonia. This recombination reaction exhibits a deuterium kinetic isotope effect.

Alkali sensitization of H+ electron stimulated desorption from H adsorbed on Ni(111)

It is shown that alkali adatoms coadsorbed in the presence of adsorbed hydrogen on Ni(111) can cause a large increase in the cross section for H+ emission during electron stimulated desorption. This phenomenon was investigated using the digital ESDIAD (electron stimulated desorption ion angular distribution) technique as well as by temperature programmed desorption (TPD). H+ ions produced by electron impact on alkali–hydrogen complexes are ejected in sharp normally oriented ion angular distributions. The sensitization of the H(a)→e−H+ process occurs with Li, Na, and K but not as strongly with Rb and Cs. Attractive interactions exist in the adsorbed layer between the hydrogen and the alkali adatoms. A model is discussed involving the formation of ‘‘alkali–hydride‐like’’ surface complexes, with Hδ− located on top of alkali atom sites.

The epitaxial formation of adsorbed multilayers as studied by ESDIAD: NH3 adsorption on top of chemisorbed CO on nickel crystal surfaces

The epitaxial growth of an adsorbed layer of NH 3 on top of chemisorbed CO on Ni(111) and Ni(110) surfaces was studied using ESDIAD. A strong interaction yielding an activation energy for NH 3 desorption of ∼12 kcal/mol was observed. This interaction, possibly involving hydrogen bonding, between the adsorbed NH 3 and adsorbed CO causes a tilting of the NH 3 molecules on the CO-covered Ni surfaces. For the NH 3 /CO/Ni(110) system, the two-fold symmetry of the underlying Ni substrate is transmitted through the CO spacer layer to the NH 3 overlayer. This symmetry transfer was not observed for the NH 3 /CO/Ni(111) system at the current resolution of our ESDIAD detection system.

Ion angular distribution of species desorbed from single crystal surfaces by electron impact

Abstract The measurement of the angular distribution of desorbing positive ions produced by electron impact desorption (ESDIAD) is of fundamental importance in understanding molecular structure in the chemisorbed layer. In this short review, two applications of ESDIAD to structural problems in the adsorbed layer will be described. Examples of strong chemisorption and weaker physical adsorption effects will be discussed. In addition, interactions between adsorbed species, leading to changes in bonding geometry will be described. The apparatus used for this work allows digitized acquisition of ion angular distributions in the absence of background effects due to soft X-ray emission stimulated by electron impact.

Background effects in electron stimulated desorption ion angular distribution (ESDIAD) measurements on Si(111)‐(7×7)

The background effect in electron stimulated desorption ion angular distribution (ESDIAD) measurements due to soft x‐ray production on Si(111)‐(7×7) is investigated. We find that the background intensity from a Si(111)‐(7×7) surface varies linearly with incident electron beam energy and current density. It is also found that the elimination of the background effect (by subtraction) plays a crucial role in both quantitative and qualitative interpretations of digital ESDIAD measurements on silicon, as well as to similar measurements on other surfaces.