H. Bu

Time‐of‐flight scattering and recoiling spectrometer (TOF‐SARS) for surface analysis

Low energy (< 10 keV) ion scattering spectrometry [10.1] is becoming increasingly important as a surface analysis technique in three specific areas, i.e., surface elemental analysis [10.2–4], probing surface structure [10.5–16], and studying electronic transition probabilities [10.7,7–19] between ions or atoms and surfaces. This is largely due to the following recent advances: (i) impact collision ion scattering spectrometry [10.6] (ICISS) in which the scattering angle is close to 180°, thus simplifying the scattering geometry and allowing experimental determination of the shadow cone radii, (ii) the use of alkali primary ions [10.9, 10] which have low neutralization probabilities, leading to higher scattered ion fluxes, (iii) time-of-flight (TOF) techniques [10.20–23] with detection of both neutrals and ions in a multichannel mode in order to enhance sensitivity, (iv) scattered ion fractions [10.7,17] to probe the spatial distributions of electrons, and (v) the use of recoiling [10.24, 25] to determine the structure of light adsorbates on surfaces.

On the defect structure due to low energy ion bombardment of graphite

Abstract Graphite surfaces cleaved perpendicular to the c axis have been irradiated with low doses of Ar + ions at 50 eV kinetic energy and perpendicular incidence. Scanning tunneling micrographs (STM) of these irradiated surfaces exhibited dome-like features as well as point defects. These dome-like features retain undisturbed graphite periodicity. This finding is attributed to the stopping of ions between the first and second graphite sheets. The possibility of doping semiconductors at extremely shallow depths is raised.

Composition and structure of the InP{100}- (1 × 1) and -(4 × 2) surfaces

Abstract The composition and structure of an InP{100} surface in both the (1 × 1) and the reconstructed (4 × 2) phases, prepared by ion bombardment and annealing, have been examined by time-of-flight scattering and recoiling spectrometry (TOF-SARS) and low-energy electron diffraction (LEED) . Time-of-flight spectra of scattered and recoiled neutrals plus ions were collected as a function of crystal azimuthal rotation angle δ and primary beam incident angle α. Compositional analyses of the surfaces were obtained from 4 keV Ar+ scattering and recoiling spectra. Structural analyses of the phases were obtained from the azimuthal anisotropy of the δ-scans and the features of the polar incident α-scans using 4 keV Ne+ scattering from In atoms and 4 keV Kr+ for recoiling of P atoms. These azimuthal δ-scans and incident a-scans were simulated by means of a shadow cone focusing model and a modified version of the MARLOWE code, respectively. The totality of this data leads indubitably to a model in which every fourth In 〈011〉 row is missing, the In atoms are trimerized along the 〈011〉 azimuth, and the 2nd-layer P atoms exposed in the 〈011〉 troughs are dimerized. This In missing-row-trimer P dimer model (MRTD) is consistent with all of the data, is autocompensated, and has In intratrimer spacings of 3.65 ± 0.20 A and P intradimer spacings of 2.95 ± 0.20 A. The results of the simulations suggest that the two end In atoms of the trimers are relaxed downward by a minimum of 0.15 A. Two other models were considered: (1) An In missing-row (MR) model without trimers or dimers in which every fourth In 〈011〉 row is missing. (2) An In missing-row-dimer (MRD) model, similar to that proposed for the GaAs (4 × 2) structure, in which every fourth In 〈011〉 row is missing and In dimers form along 〈011〉. These MR and MRD models are inconsistent with large portions of the experimental data and the simulations.

Structure of the hydrogen-induced Ni{110}-p(1 × 2)-H reconstructed surface

Abstract The structure of the Ni{110}-p(1 x 2)H surface resulting from saturation exposure to H2 at ∼ 350 K has been investigated by time-of-flight scattering and recoiling spectrometry (TOF-SARS). The scattered neutral plus ion flux resulting from a 4 keV Ne+ beam incident on the surface is monitored as a function of crystal azimuthal angle and beam incidence angle along selected low index azimuths. Classical trajectory simulations of the scattering events are used for comparison to the experimental data. The results for the Ni{110}-p(1 × 2)H hydrogen-saturated surface are contrasted to the results for the clean Ni{110}-p(1 × 1) surface and the Ni{110}-p(2 × 1)O oxygen-covered surface. Several different (1 × 2) reconstruction models are tested on the scattering results. Only the missing-row model, in which every other 〈1 1 0〉 first-layer row is missing, is consistent with all of the experimental data.

Structure analysis of O2 and H2O chemisorption on a Si{100} surface

The technique of time-of-flight scattering and recoiling spectrometry (TOF-SARS) has been applied to structural analysis of O2 and H2O chemisorption on a Si{100} surface near room temperature. The H, O, and Si recoil intensities were monitored as a function of the beam incident angles and crystal azimuthal angles. Structural information was directly elucidated from the angular anisotropies of these recoil intensities. The results show that the Si(2 × 1) dimer structure is gradually destroyed due to oxygen adsorption in the initial oxidation stage; the resulting structure lacks long-range or short-range order with a 1 ML oxygen coverage. Chemisorption of N2O was also carried out in order to prove the dissociative nature of O2 chemisorption. At all coverages, no short-range order for oxygen adsorbates was observed. A totally different behavior was observed upon H2O adsorption; the (2 × 1) structure was maintained even at saturation coverage. Concerning the adsorption sites, O atoms adsorb randomly at sites that are > 0.5 A above the 1st-layer Si atoms at low coverage ( < 0.5 ML) and begin to adsorb at lower positions at higher coverages; the data are consistent with this lower position being the bridge site between 1st- and 2nd-Si layers. Water is dissociatively adsorbed on the Si{100} surface, resulting in H(ad) and OH(ad) species. The data is consistent with both of the H and OH species being bound at the on-top site on through the Si dangling bonds.

Structure of benzene and phenol chemisorbed on Ni{110}

The chemisorption of benzene and phenol on a clean Ni{110}-(1 x 1) surface and an oxygen predosed Ni{110}-(3 X 1)-O surface near room temperature has been investigated by time-of-flight scattering and recoiling spectrometry accompanied by shadow cone calculations. The Ne scattering and H, C, and O recoiling fluxes exhibited strong angular anisotropies as a function of beam incident (alpha) and crystal azimuthal (delta) angles. These anisotropies are due to C and O atoms shadowing their neighboring atoms within the benzene molecules and resulting phenoxide species, demonstrating that scattering and recoiling spectrometry is capable of providing structural information on polyatomic molecular systems. The results show that both benzene and phenoxide are chemisorbed as molecules which have very good short-range order despite the absence of long-range order observable by low energy electron diffraction. Both benzene and phenoxide are oriented nearly parallel to the surface, with a maximum inclination angle of 15-degrees. The C atoms in the para positions of benzene and the C-O bond in phenoxide are oriented along the [001] azimuth. The C-H bond is bent out of the plane of the hexagonal ring so that the H atoms are above the C atom plane. Chemisorption on the oxygen predosed surface results in a reaction in which a H atom is abstracted from both benzene and phenol with the formation of surface hydroxide groups; the molecules remain well ordered on this surface also.

Reconstruction of the Ir(110) surface: A mixed faceted (1 × 3) and (1 × 1) structure

Abstract Time-of-flight scattering and recoiling spectrometry (TOF-SARS) has been used to show that the reconstructed Ir(110) surface, following annealing to 1400 ° C, consists primarily of domains of faceted (1 × 3) structures (with two missing first-layer rows and one missing second-layer row); the data are consistent with secondary domains of (1 × 1) structures (with no missing rows). This structure is determined from scans of (i) backscattering (BS) versus incidence angle α, (ii) forwardscattering (FS) versus α, and (iii) FS versus scattering angle Θ.

Coexisting (1 × 3) and (1 × 1) structures in the O2 induced phase change of Ir(110)

The O2 induced phase change of Ir(110) from a (1 × 3) reconstructed surface to a (1 × 1) bulk-like structure has been investigated by time-of-flight scattering and recoiling spectrometry (TOF-SARS) and low energy electron diffraction (LEED). A pulsed 4 keV Ar+ beam was used in the backscattering (BS), forwardscattering (FS), and recoiling (R) modes. Monitoring specific peaks in the BS intensity I(BS) versus incident angle α scans by TOF-SARS provides a measure of the relative amounts of (1 × 3) and (1 × 1) domains on the surface as a function of O2 dose and surface annealing temperature. The amount of surface oxygen was monitored by means of the oxygen R intensity I(R) in the FS mode. The results show that the clean surface is dominated by (1 × 3) domains and that the O2 exposed surface is dominated by (1 × 1) domains, however both types of structural units coexist under all conditions studied. The fraction of the surface covered by either type of structure is estimated from the data. The LEED results are qualitatively consistent with the TOF-SARS data. The temperature studies reveal that there is an activation barrier for the phase change, even in the presence of excess oxygen.

Oxygen induced added‐row reconstruction of the Ni{110} surface

The oxygen induced reconstructed phases of the Ni{110} surface have been studied by time‐of‐flight scattering and recoiling spectrometry (TOF–SARS). The substrate structures are determined from experimental measurements of azimuthal angle (δ) and polar incident angle (α) anisotropies in the scattered Ne intensities coupled with classical trajectory simulations for shadow cone analysis. By monitoring features in the TOF–SARS scans that are unique to specific phases, it is possible to follow the migration of the first‐layer Ni atoms as a function of O2 exposure. The results show that upon increasing exposures of the clean Ni{110}–(1×1) surface to O2, a series of LEED patterns [initial p(3×1), p(2×1), and final p(3×1)] is produced corresponding to three surface phases which differ only in the density of the first‐layer Ni 〈001〉 rows. These nascent ‘‘added rows’’ are stabilized by bonding to oxygen atoms which reside in the long‐bridge positions along the 〈001〉 rows. Structural models for the three phases are...

Hydrogen adsorption site on the Ni{110}-p(1 × 2)-H surface from time-of-flight scattering and recoiling spectrometry (TOF-SARS)

The adsorption site of hydrogen on the Ni{110}-p(1 × 2)-H surface resulting from saturation exposure to H2 at ∼ 310–350 K has been investigated by time-of-flight scattering and recoiling spectrometry (TOF-SARS). The recoiled neutral plus ion hydrogen atom flux resulting from 2–5 keV Ar+ or Ne+ pulsed ion beams incident on the surface was monitored as a function of crystal azimuthal angle and beam incidence angle. From classical trajectory calculations and shadowing and blocking analyses, it is concluded that hydrogen atoms are localized at the pseudo-three-fold sites on the (1 × 2) missing-row (MR) reconstructed Ni{110} surface; the (1 × 2) MR reconstruction is induced by hydrogen adsorption shown elsewhere [Surf. Sci. 259 (1991) 253]. Only the pseudo-three-fold site is consistent with all of the experimental data. The coordinates of the hydrogen adsorption site with respect to the nickel lattice were determined. The lateral distance of hydrogen from the 1st-layer Ni 〈110〉 rows is 1.56 ± 0.12 A and the vertical distance above the substrate is 0.21 ± 0.12 A, providing NiH bond lengths of 2.0 A to the two-layer Ni atoms and 1.5 A to the 2nd-layer Ni atom.