P. D. Quinn

Surface and sub-surface segregation at the Pt25Rh75(111) surface: a medium energy ion scattering study

Using 100 keV incident He+ ions, medium energy ion scattering (MEIS) has been used to determine the layer-by-layer composition of the outermost three atomic layers of a Pt25Rh75(1 1 1) crystal after annealing to temperatures in the range 900–1300 K. Highly layer-specific compositional information was obtained using three different double-alignment scattering geometries, although full analysis of the data included simulation of blocking curves recorded around the double alignment geometry. The results provide independent confirmation of the conclusions of an earlier quantitative low energy electron diffraction (LEED) investigation of the same crystal, notably strong Pt segregation to the outermost layer, enhanced average Pt segregation with increasing temperature, and Pt depletion of the second layer. MEIS also confirms the presence of a weak rumpling of the outermost atomic layer, with Pt atoms lying at slightly larger layer spacing than the surrounding Rh atoms. The quantitative agreement is significantly better than in the only previous comparison of LEED and MEIS for the determination of layer-dependent alloy surface segregation. Possible reasons for anomalously large surface vibrational amplitudes are discussed.

Composition profiles of InAs–GaAs quantum dots determined by medium-energy ion scattering

The composition profile along the [001] growth direction of low-growth-rate InAs–GaAs quantum dots (QDs) has been determined using medium-energy ion scattering (MEIS). A linear profile of In concentration from 100% In at the top of the QDs to 20% at their base provides the best fit to MEIS energy spectra.

Tensor low energy electron diffraction and medium energy ion scattering determination of the Ni(110)c(2×2)-Sn surface structure

The Ni(110)c(2×2)-Sn surface phase has been investigated by the combination of quantitative low energy electron diffraction (LEED) with the aid of tensor LEED multiple scattering simulations, and medium energy ion scattering using 100 keV H+ incident ions. The structure is found to involve substitution of half of the outermost Ni atoms of the clean surface by Sn atoms, but the resulting single layer NiSn alloy is corrugated, with the Sn atoms being 0.40±0.03 A higher above the underlying Ni substrate than the outermost Ni atoms. The results are discussed in the context of previous structural studies of similar surface alloy phases; a weak trend for the amplitude of the corrugation in Ni/Sn surface alloys to become smaller as the surface layer packing density reduces may be consistent with previous ideas of the role of the depletion of valence electron density in the surface layer and the associated surface tensile stress.

Structural study of the adsorption of Sb on Ag(111) using medium energy ion scattering

The structure of the Ag(1 1 1)(√3×√3)R30°-Sb surface phase formed by a nominal 1/3 ML of Sb has been investigated by medium energy ion scattering (MEIS) using 100 keV H+ ions. This surface is widely accepted as involving an outermost layer comprising a substitutional alloy, but some recent studies have indicated that the atoms in this layer may occupy hcp hollow sites at the surface, directly above second layer Ag atoms of the underlying substrate, creating an effective stacking fault at the surface-alloy/substrate interface. The new MEIS study shows that it is possible to form both faulted and unfaulted surface phases, probably dependent on the previous Sb dosing history of the Ag(1 1 1) sample. The faulted surface phase may also be accompanied by sub-surface stacking faults in the Ag(1 1 1) substrate. The higher Sb coverage (0.6 ± 0.1 ML) Ag(1 1 1)(2√3×2√3)R30°-Sb was found to be accompanied by multiple stacking faults in some 20 outermost layers of the Ag(1 1 1) substrate to an essentially hcp structure. The effect is tentatively attributed to a lowering of the stacking fault energy induced by low concentrations of dissolved Sb. The detailed structural parameters of the faulted surface Ag(1 1 1)(√3×√3)R30°-Sb phase are in excellent agreement with those found in previous studies by electron and X-ray diffraction.

Structural analysis of the Ru(0001)(1×1)-O and Ru(0001)(2×1)-O structures by medium energy in scattering

Medium energy ion scattering using 100 keV H+ ions, has been used to investigate the structure of the (2×1) and (1×1) chemisorbed phases of atomic oxygen on Ru(0 0 0 1), aided by a new automated structural search routine. In both phases the O is found to occupy hcp hollow sites directly above second layer Ru atoms, while in the (1×1)-O surface the oxygen chemisorption induces an expansion of the outermost Ru layer spacing. More subtle subsurface structural modifications, and near-surface lateral and rumpling distortions of the Ru surface in the (2×1) phase were also investigated, but the amplitude of these distortions are not formally significant relative to the estimated precision. A 1.6 ML O coverage (1×1) surface phase showed clear evidence for subsurface oxygen and small changes in Ru subsurface layer spacings.

Aspects of layer-by-layer composition analysis using MEIS

The shadowing/blocking characteristics of medium energy ion scattering allow a precise deduction of the layer-by-layer composition of alloy crystal surfaces. The technique relies on an accurate alignment of the ion beam with known crystallographic directions to restrict the incoming beam to illuminate just the topmost one, two or three or more layers. Further layer specificity is bestowed by utilising outgoing geometries that block ions from all but the top one, two, etc., layers respectively. The size of shadow cones associated with medium energy hydrogen and helium ions is ideal for this application. A plot of the averaged experimental yields against simulated yields for several different systems implies an inconsistency in the technique. Possible explanations for this are discussed and a clarifying experiment is suggested.