J. A. Yarmoff

Stimulated desorption of Cl+ and the chemisorption of Cl2 on Si(111)-7×7 and Si(100)-2×1

The chemisorption of Cl2 on Si(111)-7 × 7 and Si(100)-2 × 1 and the mechanism for stimulated desorption of Cl+ from Si are studied with soft X-ray photoelectron spectroscopy (SXPS) and photon-stimulated desorption (PSD). It is shown that Cl2 interacts with Si(111)-7 × 7 at room temperature to form mono-, di- and tri-chlorides, while primarily monochlorides are formed on Si(100)-2 × 1. These differences are explained in terms of the reconstructions of each clean surface. The stimulated desorption of Cl+ ions has a threshold at ~ 20 eV that results from a direct excitation of a Cl 3s electron to an unoccupied Cl antibonding level. No direct desorption of Cl+ ions is observed at the Si 2p edge. Differences between the mechanisms for F+ and Cl+ desorption from Si are discussed.

The adsorption of I2 on Si(111)-7×7 studied by soft X-ray photoemission

The adsorption Of I2 on Si(111)-7 x 7 at room temperature is studied with soft X-ray photoelectron spectroscopy. I2 adsorbs dissociatively, forming a mixture of SiI, SiI2 and SiI3 moieties, of whic ...

Bonding geometries of F and Cl on Si(100)-2 × 1

Angle- and energy-resolved F+ and Cl+ electron-stimulated desorption distributions are collected from Si(100)-2 × 1 surfaces exposed at room temperature to XeF2 or Cl2. These distributions are fit to a model that accounts for ion-surface interactions in order to extract bond angle information. It is found that both F and Cl chemisorb on the dangling bonds of intact Si dimers, at an angle of ∼ 20° from normal along the [011] azimuth. By annealing a chlorinated surface above ∼ 400 K, some normally oriented SiCl bonds are generated. In addition to determining bonding geometries, quantitative information is obtained regarding the image-charge interaction and neutralization for F+ and Cl+ interacting with Si.

Sputtering-induced vacancy cluster formation on TiO2"110…

Defects are introduced on an atomically clean TiO{sub 2}(110) surface in a controllable manner via bombardment by 0.5 keV Ar{sup +} ions, and the resultant material is probed with scanning tunneling microscopy and x-ray photoelectron spectroscopy. It is shown that ion bombardment leads to a change of stoichiometry and the formation of vacancy clusters, while annealing after prolonged bombardment forms regular rectangular cavities with single-atomic-layer steps. It is concluded that curvature-dependent sputtering in combination with adatom and vacancy diffusion is responsible for the observed structures.

K ion scattering from Au nanoclusters on TiO2(110)

The neutralization of 3keV K+39 ions scattered from Au nanoclusters deposited on TiO2(110) was measured with time-of-flight spectroscopy. All of the K remains ionic in scattering from a Au thin film or from large clusters, but up to ∼20% is neutralized in scattering from small clusters. The results demonstrate that the K 4s ionization level couples to states associated with Au nanoclusters, such that the neutralization probability is a function of the nanocluster size. The data are compared to previous experiments with Na and Li projectiles.

Reaction of SeF6 with iron and iron oxide surfaces: a model for selenate reduction

The reaction of SeF6 with Fe and Fe oxide surfaces is investigated as a model of the reduction of aqueous selenate. Polycrystalline iron foils were reacted with gaseous SeF6 in vacuum and then characterized by X-ray photoelectron spectroscopy. A novel use of electron stimulated desorption was used to determine if any SeF bonds remain following reaction. It was found that SeF6 completely dissociates, growing a film of FeF2. The growth initially follows Mott–Cabrera kinetics, but then abruptly stops. Thin Fe oxide layers on the surface impede the adsorption of SeF6. It is suggested that the fluoride and oxide films both quench the reaction by inhibiting the transport of electrons that are needed to dissociate SeF6 at the surface.

Remote inductive effects in the Si 2p spectra of halogenated silicon

Abstract Si 2p spectra collected from fluorinated Si(111)−7 × 7 are analyzed to determine whether chemical shifts induced by remote F atoms are discernible. Although the traditional interpretation adequately accounts for the observed chemical shifts without considering remotely induced shifts, recent experimental evidence indicates that such shifts may occur for highly electronegative adsorbates. Thus, an extended scheme is outlined that is consistent with all known data, but which includes remotely induced shifts. It is found that, although they differ quantitatively, both approaches yield the same qualitative results for fluorinated Si.

Multiply charged Al recoils with impact of 2.0keV Si+ ions

Scattered Si and fast recoiled Al are both present in time-of-flight spectra collected when 2.0keV Si+ ions are incident on an Al(100) surface. Monte Carlo simulations were used to determine the origin of the spectral features, and electrostatic analysis was used to measure the ion yields and charge states as a function of the recoil and incident angles. The analysis indicates that all of the scattered Si projectiles are neutralized, while the recoiled Al is produced as neutrals and in singly, doubly, and triply charged states. The formation of multiply charged ions at this low energy is attributed to inner-shell promotion during the nearly symmetric Si–Al collision. Further experiments used Cl+ and SiCl+ projectiles for comparison. Multiply charged recoils were not observed with Cl+, while the behavior of SiCl+ could be interpreted as a linear combination of Si+ and Cl+ projectiles.

Photon Stimulated Desorption of Fluorine from Semiconductor Surfaces

Photon stimulated desorption (PSD) from XeF2-exposed Si(111) and GaAs(l00) is measured. The F+ threshold from Si is at 27.5 eV, which corresponds to the transition from F 2s to the conduction band minimum (CBM). Features at the Si 2p edge, which distinguish the oxidation state of the bonding Si, are used to determine the structure of the XeF2 etching reaction layer and to show how metal contaminants trap SiF4 on the surface. For GaAs, the PSD threshold is at ∼6 eV, which corresponds to the transition from F 2p to the CBM. Transitions from the 3d core levels of the bonding Ga and As atoms do not contribute to PSD.