P. L. Trevor

In Situ Scanning Tunneling Microscopy of Corrosion of Silver-Gold Alloys

An in situ scanning tunneling microscope (STM) was used to observe the morphological changes accompanying the selective dissolution of Ag from low-Ag content Ag-Au alloys in dilute perchloric acid. This study was undertaken to explore the role of surface diffusion in alloy corrosion processes. These results are interpreted within the framework of the kink-ledge-terrace model of a crystal surface and a recent model of alloy corrosion based on a variant of percolation theory. The corrosion process leads to roughening of the surface by dissolution of Ag atoms from terrace sites. Annealing or smoothening of the surface occurs by vacancy migration through clusters and the subsequent annihilation of clusters at terrace ledges.

Atom-surface scattering dynamics at hyperthermal energies

We report molecular beam scattering of hyperthermal Xe atoms over an energy range 1<Ei(eV)<10 from single crystal surfaces of GaAs(110), Ag(100), and Ge(100). The angular distributions from the corrugated surfaces show sharp backscattered rainbow maxima related to the topography of the crystal surface. In contrast the smooth surfaces yield quasispecular lobes suggestive of structure scattering. The large energy loss for all surfaces scales on average with the energy of local normal motion. A simple binary interaction model is developed which accounts for many of the phenomena observed from corrugated surfaces. With the aid of a comparison classical trajectory study, these results provide some understanding of the mechanism by and extent to which a solid can dissipate the energy of a hyperthermal collision.

Trajectory studies of hyperthermal Xe scattering from GaAs(110)

We present the results of stochastic classical trajectory simulations of the scattering of a nonreactive gas, xenon, from a semiconductor surface of known structure and electronic and vibrational properties, GaAs(110). The range of incident energies considered is 1 to 8 eV, in order to make direct comparison with results of molecular beam experiments reported in the accompanying paper. We employed a 48‐atom three‐layer slab of GaAs, with periodic boundary conditions in two dimensions and frictional and stochastic forces in the third (surface normal) dimension. Pairwise additive Lennard‐Jones potentials describe the gas–surface interaction. The calculations reproduce the large energy exchange and surprising structural sensitivity observed experimentally. Energy transfer is dominated by an initial binary interaction of the Xe with a single Ga or As atom. The repetitive collision nature of this binary encounter produces angular scattering patterns very similar to those obtained for scattering of light atoms,...

Hyperthermal gas–surface scattering

We summarize our recent results of scattering hyperthermal (1–15 eV) rare‐gas atoms from single‐crystal semiconductors. These include collisional excitation of carriers in the semiconductor, the ejection of charged and neutral surface particles, and the projectile scattering distributions characteristic of the strongly inelastic collisions. The dependence of the electronic excitation of the semiconductor upon the incident energy, angle, and atom are consistent with the creation of a local thermal hot spot with which there is fast electronic equilibration. In contrast, for the ejection of ions from the surface, the incident and ejected particle energy and angle distributions suggest direct, large momentum‐transfer processes. The scattered rare‐gas atom distributions exhibit large energy loss, yet are strongly structured and surprisingly sensitive to surface topography.

Electron-hole pair creation at a Ge(100) surface by ground-state neutral Xe atoms

We have directly measured the excitation of electron‐hole pairs at a crystal surface by ground‐state neutral atoms. Utilizing seeded molecular beam techniques we have scattered hyperthermal (1–6 eV) Xe atoms from the (100) face of a Ge p‐i‐n diode and recorded the current transient induced due to the scattering process. We find the product of the excitation and collection probability to be ∼10−4 over a range of kinetic energies 2<EXe(eV)<6. The excitation of electron‐hole pairs constitutes a small portion of the massive energy loss (ΔE/E∼70%) of the Xe atom to the crystal.

Inelastic helium scattering from the Ag(001) and Ag(001)c(2×2)Cl surfaces

Abstract Inelastic He atom scattering has been used to investigate the surface phonon spectra of the Ag(001) surface and the change in the vibrational spectra when an ordered adsorbate, c(2 × 2)Cl, is present. The lowest energy transversely polarized surface mode is resolved and found to lie above the lower edge of the bulk band for both Ag(001) and Ag(001)c(2 × 2)Cl. Higher energy ( >10 meV) adsorbate-substrate vibrations were not observed. Although these results are not comprehensive, they provide an opportunity to evaluate the application of this technique to the study of high energy surface vibrations. Some quantitative assessments are described.

Carrier excitation by atomic collision on GaAs(110)

Abstract We have measured the excitation yield for carriers produced at the outermost atomic layer of a GaAs(110) surface by impinging hyperthermal Xe and Kr neutral atoms. The excitation yield roughly scales as exponential in the inverse of the mean energy deposited in the collisions. This is consistent with previous results for InP, which indicated a rapid electronic equilibration with the local lattice kinetic energy dissipated in the vicinity of the atomic impact. However, for GaAs(110) the maximum in the excitation yield, as a function of incident angle, is not normal to the surface. Instead, the maximum yield is found for polar angles nearly perpendicular to the tilted GaAs surface atom bonds. In addition the yield is two orders of magnitude greater than that estimated from equilibrium arguments. These results are in apparent disagreement with the simplest ideas of local equilibration, and may provide the first insights into the collisional excitation mechanism of electrons by neutral atoms.

Gas–surface interactions on InP monitored by changes in substrate electronic properties

The electronic properties of compensated InP crystals can be used to sensitively monitor gas–surface interactions. When a gas is adsorbed on these low carrier density semiconductors both the conductance and minority carrier lifetime exhibit large changes which we interpret in terms of band bending. Changes of greater than 50% in the bulk‐averaged conductance of Fe‐compensated semi‐insulating InP crystals have been measured for adsorption of ∼0.5 monolayers of Cl2. Using modulated NO2 molecular beams the conductance changes are demonstrated to be fast (<1 ms to steady state) so as to be capable of yielding quantitative rate information about the gas–surface interaction.

Inelastic helium scattering from Ag(001) and Ag(001)c(2×2)Cl

Inelastic helium atom scattering has been used to investigate Ag(001) surface phonon modes and the influence of an adsorbed chlorine monolayer. For the Ag(001) surface we observe the preferential interaction of the He beam with shear‐vertically (SV) polarized surface phonons. The scattered intensity per steradian from the Ag(001)c(2×2)Cl surface is reduced approximately two orders of magnitude compared to that for the clean surface. Our results for the chlorinated surface indicate that a SV polarized surface mode does not appear significantly below the lower bulk band edge in contrast to oxygen adsorbed on Ni(001). Although we did not observe high energy surface vibrations, based on these experiments and recent theoretical developments, we indicate conditions which would improve the observability of high energy surface vibrations by means of inelastic He scattering.

Carrier Excitation Yield from Atomic Collisions at Semiconductor Surfaces

We have furthered our understanding of the collisional excitation of carriers at semiconductor surfaces by hyperthermal neutral rare gas atoms. We have introduced optical carrier excitation in order to characterize the samples, to determine carrier recombination rates, and to calibrate the collisional e−h+ excitation probability for Xe on the InP(110) surface.