Recoiling Imaging Spectrometry Research Articles

Electron-ion/atom coincidence measurements of 3 keV He + interacting with a SiH(1 0 0)-(2 × 1) surface

An electron-scattered ion/atom coincidence technique has been developed and applied to the interaction of 3 keV He + ions with a Si(1 0 0)–(2 × 1)-H surface. The technique extends scattering and recoiling imaging spectrometry (SARIS) to include electron-scattered particle coincidence methods. The distributions of the scattered projectiles (He 0 and He +) on a position-sensitive detector in the range of large scattering angles and in coincidence with the emitted electrons have been measured. The results allow separation of the scattered atom or ion flux from different surface layers and their contributions to the total scattered flux.

Blocking map of He+ scattered from a Si(001)-(2×1) surface

Abstract Scattering and recoiling imaging spectrometry (SARIS) in the “blocking configuration” for the system of 5 keV He+ scattering from a Si(0 0 1)-(2 × 1) surface has been used to measure a two-dimensional blocking map. The map demonstrates that this real-space surface crystallographic technique is sensitive to interatomic spacings in the surface and subsurface layers of a crystal. Classical ion trajectory simulations using the scattering and recoiling imaging code (SARIC) and “two-atom” molecular dynamics (MD) simulations are used to interpret the blocking patterns and to provide quantitative interpretations of the experimental data. The “blocking configuration”, in which atomic trajectories scattered from subsurface layers are blocked by atoms in layers nearer to the surface, is demonstrated to be a viable method for simultaneous acquisition of data from the surface and individual surface layers. It is found that the blocking map is representative of the crystal geometry. The results show that the major blocking cones for 5 keV He+ on Si(0 0 1)-(2 × 1) originate from fourth to seventh-atomic layer scattering, while cones of lesser intensity are observed from as deep as the 12th-atomic layer.

Ion fraction map of He + scattered from a Si( [formula omitted])-(2×1) surface

Scattering and recoiling imaging spectrometry in the blocking configuration is used to obtain a three-dimensional ion fraction map of 5 keV He + scattered from a Si(1 0 0)-(2×1)-two domain surface. The scattered ion fractions Y are highly sensitive to the alignment of the He + beam with the crystal geometry, with Y varying from a minimum of 2% to a maximum of 10%. Scattering and recoiling imaging code simulations, including Auger-neutralization and charge exchange in the close collision, show that the highest Y values correspond to ions that collide with Si atoms in the outermost surface layers. The experimentally measured fraction of He + surviving as ions from the surface layers are as follows: Y 1st∼70%, Y 2nd∼25%, and Y 3rd∼5%. After fitting the experimental charge fraction with the two adjustable parameters v 0 (characteristic velocity) and P c (charge exchange probability in the close collision) for three blocking exit angles of 35°, 45°, and 55°, good agreement between experiment and simulation was found for v 0=(1.9±0.1)×10 7 cm/s and P c=0.34±0.09.

Universal expression for blocking cone size based on the ZBL potential

Calculations of blocking cone sizes for the “blocking geometry” of the scattering and recoiling imaging spectrometry (SARIS) technique have been performed. By fitting calculated points in the space of the parameters of the interacting atomic species, a universal formula for calculating the blocking cone size for arbitrary energies and interacting species has been derived. This provides a “blocking cross-section” and an estimate of the total scattering cross-section of the process under consideration. The results obtained from the formula are compared with experimental SARIS blocking cone data for He + and Ne + scattering from a Pt(1 1 1) surface in the energy range 3–20 keV. The blocking cones in this low-energy range are appreciably asymmetric with respect to the interatomic axis. At small interatomic distances and low-projectile energies, the difference in angular size of the upper and lower halves of the blocking cone can be as large as 15%. The results of scattering and recoiling imaging code simulations and molecular dynamics blocking cone trajectory simulations using the Ziegler–Biersack–Littmark potential are in good agreement with experimental blocking cone sizes. Comparison is also made to the results of other formulas for the critical blocking angle found in the literature.

Real-Space Surface Crystallography from Ion Scattering

Ion beams in the kiloelectronvolt energy range impinging on crystal surfaces are atomic-size probes of the nanoscale magnifying and focusing formed by the periodic structures, These lenses disperse the scattered and recoiled atom trajectories into macroscopic projections according to their velocities as a function of projectile/target atom masses and deflection angles. The resulting patterns reflect the near-surface interatomic vectors. The spatially and temporally resolved images of these patterns can now be acquired by means of new developments in ion scattering spectrometry, providing direct, element-specific. real-space projections of surface structure. The resulting scattering and recoiling imaging spectrometry (SARIS) provides unique capabilities for analyzing surface composition and structure and for investigating the interactions of ions with surfaces. A classical ion trajectory program called scattering and recoiling imaging code (SARIC) that simulates the spatial and time intensity distributions of the SARIS images has been developed. A quantitative comparison of experimental and simulated images is achieved by means of a two-dimensional reliability (R) factor. Examples of applications include experimental stereographic projections of the Ni(110) and Pt(111) surfaces, the termination layer of the CdS(0001) surface, the chemisorption of Cl-2 on Ni(110) and H-2 On Pt(111), and the atomic lenses in the Pt(111) surface.

Real-space surface crystallography: Experimental stereographic projections from ion scattering

Scattering and recoiling imaging spectrometry (SARIS) in the blocking configuration is used to obtain experimental two-dimensional stereographic projections of the Ni(110) and Pt(111) surfaces. The development of this technique as an element-specific real-space surface crystallography that is sensitive to interatomic spacings in the surface and subsurface layers of a crystal is described. This projection imaging method is based on the blocking of atomic trajectories scattered from subsurface layers by atoms in layers nearer to the surface. The resulting magnification of the blocking cones, as captured by a gated position-sensitive microchannel plate detector, is similar to 10(9). The images of the blocking patterns provide direct information on interatomic spacings and surface symmetry and structure. Classical ion trajectory simulations using the three-dimensional scattering and recoiling imaging code (SARIC) are used to simulate the stereographic projections and blocking patterns and to provide quantitative interpretations. The method is sensitive to interatomic spacings in the surface and subsurface layers. The physical properties of the blocking process are derived from analysis of the data and a simplified, approximate, two-atom model of the scattering/blocking process is developed. (C) 2000 American Institute of Physics. [S0021-9606(00)71216-5].

How do hydrogen atoms on surfaces affect the trajectories of heavier scattered atoms?

The recently developed technique of scattering and recoiling imaging spectrometry (SARIS) is used to probe the effect of hydrogen atoms on the trajectories of 5 keV Ne+ scattering from a Pt(111) surface. Classical kinematic calculations and ion trajectory simulations, using the scattering and recoiling imaging code (SARIC), are carried out in order to probe the details of the interaction and the nature of the perturbation. It is demonstrated that adsorbed hydrogen atoms are capable of deflecting these low kilo-electron-volt Ne trajectories scattering from a Pt surface. These perturbations result in spatial shifts and broadenings of the anisotropic features of the SARIS images that are readily detectable. The scattered Ne atoms lose 0–18% of their initial kinetic energy as a result of the perturbation by the H atoms. The physics of the perturbation on the trajectories can be understood from straightforward classical kinematic calculations and SARIC ion trajectory simulations.

Scattering and recoiling imaging spectrometry (SARIS) study of chlorine chemisorption on Ni(1 1 0)

The clean and chlorine chemisorbed Ni(1 1 0) surface has been investigated by scattering and recoiling imaging spectrometry (SARIS). Rare gas ion beams at 4 keV were scattered at different incident ( α) and exit ( β) angles along different azimuthal directions of the surface. The LEED pattern changed from a sharp (1 × 1) to a weak (3 × 1) after exposure to Cl 2. The He + images from the Cl/Ni surface exhibited very little differences compared to those of the clean surface. This is due to the high penetration depth of the He + ions, which results in low surface sensitivity. The images obtained from the Cl/Ni surface with the heavier ions exhibited obvious changes due to their low penetration depths, which facilitates high surface sensitivity. Scattering and Recoiling Imaging Code (SARIC) simulations were carried out in order to interpret the perturbations induced by the chlorine adatoms. Different chemisorption sites for chlorine were tested. The results of the experiments and simulations agree that Cl atoms are chemisorbed in the short-bridge sites above the [1 1 0] rows.

Temporal and spatial resolution of scattered and recoiled atoms for surface elemental and structural analysis

Developments in low-energy ion scattering over the past 10 years have led to new techniques for surface elemental and structural analyses. The fundamental physics involved in these new methods is summarized herein and some examples of the applications of the techniques are presented. Three major new developments are considered. First, time-of-flight scattering and recoiling spectrometry (ToF-SARS) takes advantage of ToF techniques to detect simultaneously both ions and fast neutrals that are scattered and recoiled from surfaces. Elemental analyses are obtained by application of binary collision theory, and structural analyses are performed by rotation of the sample in order to measure intensity changes as a function of incident and azimuthal angles. Second, scattering and recoiling imaging spectrometry (SARIS) takes advantage of a large position-sensitive microchannel plate detector, coupled with ToF techniques, to capture element-specific, time-resolved, spatial and intensity distributions of scattered and recoiled atoms from surfaces. These images combine atomic scale microscopy and spatial averaging because they are created from a macroscopic surface area but they are directly related to the atomic arrangement of the surface at the subnanoscale level; the features of the images are sensitive to changes in interatomic spacings at a level of <0.1 A. Third, a classical ion trajectory simulation program, called scattering and recoiling imaging code (SARIC), which is designed specifically for structural interpretation of ToF-SARS and SARIS data, has been developed. This program allows quantitative comparison of experimental and simulated data for surface structure determinations.

Deconstruction of theAu{110}−(1×2)surface

The deconstruction of the $\mathrm{Au}{110}\ensuremath{-}(1\ifmmode\times\else\texttimes\fi{}2)$ missing-row (MR) surface has been studied by variable temperature scattering and recoiling imaging spectrometry (SARIS). SARIS images were acquired over the range of 298--720 K and for an amorphousized surface produced by high-dose ${\mathrm{Ar}}^{+}$ sputtering. The sharp, anisotropic features observed in the images from the ordered $(1\ifmmode\times\else\texttimes\fi{}2)$ surface at 298 K change monotonically with increasing temperature into broadened, more featureless images in which the first-layer blocking arcs shift to very low exit angle values. The basic features of the $(1\ifmmode\times\else\texttimes\fi{}2)$ MR structure are still observed even near 700 K where three-dimensional roughening begins. These $(1\ifmmode\times\else\texttimes\fi{}2)$ features are completely obliterated on the sputtered amorphousized surface, depicting a surface that is dominated by three-dimensional roughening. Classical ion trajectory simulations using the scattering and recoiling imaging code are used to probe the effects of vibrational amplitude and conversion to a $(1\ifmmode\times\else\texttimes\fi{}1)$ structure. The results accentuate the remarkable stability of the $(1\ifmmode\times\else\texttimes\fi{}2)$ MR structure up to the roughening temperature and exclude early proposals of an order-disorder transition via lattice gas formation. The findings are in agreement with more recent models in which roughening induces a simultaneous deconstruction transition.

Scattering and recoiling imaging spectrometer (SARIS)

An ultrahigh vacuum spectrometer system has been designed and constructed for obtaining spatial- and time-resolved, element-specific images of atoms that are scattered and recoiled from surfaces. A pulsed noble gas ion beam in the 1–5 keV range is used to scatter and recoil atoms from a surface. A large, position-sensitive microchannel plate detector with resistive anode encoder, that is sensitive both to ions and fast neutrals records the spatial distribution patterns of the emitted atoms. The use of time-of-flight methods allows capture of these patterns in time windows as short as 10 ns. The sensitivity of these patterns to the details of surface structure provides the basis for a scattering and recoiling imaging spectrometry (SARIS). The primary ion beam current is ∼0.1 nA/cm2, supplied in 20 ns pulses at a rate of 30 kHz, resulting in ∼5×102 ions/pulse; images with adequate statistics can be obtained in several seconds with a total ion dose of &amp;lt;1010 ions/cm2. The SARIS technique can provide unique, element-specific, real-space, surface structural information at a resolution of &amp;lt;0.1 Å on a time scale of seconds for analysis of surface composition, structure, and dynamics. Examples provided for demonstrating the capabilities of the system are 4 keV He+ and Ar+ scattering and recoiling from Pt{111} and 4 keV Kr+ scattering and recoiling from CdS{0001}.

Time-resolving, position-sensitive detection system for scattering and recoiling imaging spectrometry

A time-resolving, position-sensitive, single particle detector has been developed for scattering and recoiling imaging spectrometry. A custom designed multiple stop time-to-digital converter is integrated with a commercially available position sensitive detector system utilizing microchannel plates and a resistive anode encoder. The detector system successfully captures images of scattered and recoiled ions and fast neutrals from surfaces in time frames as short as 10 ns.

Surface and subsurface distortions of the Au{110}-(1×2) structure

The reconstruction of the Au{110}-(1×2) missing-row surface has been studied by means of the new scattering and recoiling imaging spectrometry (SARIS) technique. The three-dimensional focusing patterns observed for scattering of 4 keV He +, Ne + and Ar + ions are highly sensitive to the structure of both the surface and subsurface layers. Classical ion trajectory simulations using the scattering and recoiling imaging code (SARIC) were used to simulate the scattering patterns. Using an R-factor comparison of the experimental and simulated images, it is demonstrated that SARIS is sensitive to changes of the order of 0.02 Å in the structural parameters of this Au surface. These parameters involve interlayer spacings, row pairing and row buckling in the first-through fifth atomic layers. Results for the shallow surface layers are in general agreement with the those of previous studies. The new results include structural parameters for the deeper subsurface layers and the observation of an oscillatory behavior of the layer spacings which is damped towards deeper layers.

Element-, velocity- and spatially resolved images of Kr + scattering and recoiling from a CdS surface

Developments in ion scattering spectrometry and detector technology allow measurement of element-, velocity- and spatially resolved distribution patterns of scattered and recoiled atoms, forming the basis for a scattering and recoiling imaging spectrometry (SARIS). SARIS images are architectural fingerprints of surface short-range order. They exhibit extreme sensitivity to structural details, providing a unique probe for determination of surface composition and structure. Quantitative analysis is obtained by comparison with simulated images using the scattering and recoiling imaging code (SARIC). The SARIS technique is demonstrated herein for analysis of the composition and termination of a CdS (0001) surface. Using 4 keV Kr + primary ions, it is possible to observe separate images of Kr scattering from Cd and images of recoiled H, O, S and Cd atoms. The O and H atoms are impurities which are observed on this surface, even after annealing to 700°C. It is found that the Cd-terminated surface contains Cd atoms in the first layer and S atoms in the second layer. The surface structure appears to be stabilized by bonding of the first-layer Cd atoms to O or OH moieties.

Design and construction of a triple-axis ultrahigh vacuum goniometer

A triple-axis goniometer was designed and constructed for positioning objects within an ultrahigh vacuum chamber. The goniometer is applied to move a large position-sensitive microchannel plate (MCP) detector which is used for scattering and recoiling imaging spectrometry (SARIS). The goniometer allows the MCP to be independently rotated in horizontal (φ) and vertical (θ) planes as well as translated with respect to a fixed sample position. The angle φ can be varied from 0° to 160° in order to change the scattering angle for SARIS. The angle θ can be varied from 0° to 80°, allowing the MCP to be moved out of the scattering plane. Translation allows the MCP to be moved relative to the sample position over a range of 10–60 cm. All of the movements are driven by computer-controlled stepping motors.

Focusing of He+ ions on semichannel planes in the Pt{111} surface

Focusing of scattered ions on semichannel planes has been observed for 4 keV He+ scattering from a Pt{111}-(1 × 1) surface. Atoms scattered on such semichannel planes are highly focused by the detailed structural features of the surface and subsurface layers, giving rise to intricate three-dimensional spatial patterns. These scattering patterns can be readily observed through the use of a time- and spatially-resolving microchannel plate detector using the technique of scattering and recoiling imaging spectrometry (SARIS). Such three-dimensional scattering features are not normally observed in conventional ion scattering experiments using small-area detectors. Classical ion trajectory simulations using the scattering and recoiling imaging code (SARIC) are used to simulate the scattering patterns and to provide an analysis of the surface and subsurface layers involved in the scattering, the focusing of incoming and outgoing atom trajectories, and the role of thermal vibrations on the scattering patterns. The high sensitivity of the SARIS images to the details of the surface and subsurface atomic arrangements advocate its potential as a surface structural analysis technique.

Three-dimensional focusing patterns of He + ions scattering from a Au{110} surface

Three-dimensional focusing patterns have been observed for 4 keV He + scattering from a Au{110}-(1 × 2) surface for beam incident angles higher than the Lindhard critical angle. Under these conditions, the scattered atoms are highly focused by the detailed structural features of the surface and subsurface layers into non-planar directions, giving rise to intricate symmetrical and asymmetrical spatial patterns in accord with the crystal azimuths involved. These scattering patterns can be readily observed through the use of a time- and spatially resolving microchannel plate detector using the technique of scattering and recoiling imaging spectrometry (SARIS). Such non-planar scattering features are not normally observed in conventional ion scattering experiments using small-area detectors. The features of the images can be interpreted qualitatively by means of a simple blocking-cone analysis. Classical ion trajectory simulations using the scattering and recoiling imaging code (SARIC) are used to simulate the scattering patterns, to delineate the surface and subsurface layers involved in the scattering, and to illustrate the focusing of incoming and outgoing ion trajectories involved in producing various features of the images.

Surface structure determination from ion scattering images

Spatial- and time-resolved, element-specific images which represent real-space projections of the interatomic vectors in a surface have been obtained for kiloelectron-volt scattered He and Ar and recoiled Pt atoms from Pt{111}. Spatial-dispersion results from blocking and focusing by “atomic lenses” into structure-sensitive patterns, forming the basis for a scattering and recoiling imaging spectrometry. The technique is coupled with classical ion trajectory simulations for determination of the Pt{111} surface relaxation for which previous low energy electron diffraction and ion scattering measurements have produced controversial results. Pt{111} has become a standard for surface science measurements, and accurate determination of its surface relaxation is important for use as a standard and for verifying and calibrating theories of relaxation and reconstruction.

Element-, time- and spatially-resolved images of scattered and recoiled atoms

Element-specific, time- and spatially-resolved images of keV scattered and recoiled atomic ions plus fast atoms are exploited as a new technique of scattering and recoiling imaging spectrometry (SARIS). SARIS, an outgrowth of time-of-flight scattering and recoiling spectrometry (TOF-SARS), uses a large position-sensitive microchannel plate and TOF methods to capture images of energetic atoms which are scattered and recoiled from surfaces using a pulsed keV ion beam. The atoms are dispersed according to their velocities as a function of projectile/target atom masses and deflection angles. The spatial distributions of these atoms are captured by the MCP in time-resolved frames as short as 10 ns. Classical ion trajectory simulations provide a good description of the interactions, allowing direct simulation and interpretation of the experimental images. The images combine atomic scale microscopy and spatial averaging since they are created from a macroscopic surface area but they are directly related to the atomic arrangement of the surface at the sub-nanoscale level; the accuracy for measurement of interatomic spacings is expected to be better than 0.01 Å. Example data is presented for Pt{111}and Au{110}.

Scattering and recoiling imaging code (SARIC)

A new classical ion trajectory simulation program based on the binary collision approximation has been developed in order to support the results of time-of-flight scattering and recoiling spectrometry (TOF-SARS) and scattering and recoiling imaging spectrometry (SARIS). The code was designed to provide information directly related to the TOF-SARS and SARIS measurements and to operate efficiently on small personal computers. The calculation uses the Ziegler-Biersack-Littmark (ZBL) universal screening function or the Moliére screening function to simulate the three-dimensional motion of atomic particles and includes simultaneous collisions involving several atoms. For TOF-SARS, the program calculates the energy and time-of-flight distributions of scattered and recoiled particles, polar (incident) angle α-scans, and azimuthal angle δ-scans. For SARIS, the program provides images of the scattering and recoiling intensities in polar exit angle and azimuthal angle (β, δ)-space. A two-dimensional reliability factor ( R) has been developed in order to obtain a quantitative comparison of experimental and simulated images. Examples of simulations are presented for Ni{100}, {110} and {111} surfaces and a Pt{111} surface. The R-factor is used to quantitatively compare the simulated Pt{111} image to an experimentally emulated image.