Time-of-flight Scattering And Recoiling Spectrometry Research Articles

Measurements of ion‐induced ferroelectric emission and surface charge dynamics on LiTaO3(0001) by time‐of‐flight scattering and recoiling spectrometry (TOF‐SARS)

AbstractWe have derived a special surface analysis method for studying surface charge dynamics on ferroelectrics from time‐of‐flight ion scattering and recoiling spectrometry (TOF‐SARS). In this method, shifts of the TOF‐SARS peaks of a ferroelectric crystal are measured as a function of sample temperature, and these spectral data are used to deduce changes in electrical potential on the sample surface. These changes are then converted to information on surface charge dynamics. In addition, the method is also applicable in extracting data regarding ion‐induced ferroelectric electron emission (FEE) from the TOF‐SARS spectra. In this work, we have tested the method with LiTaO3(0001) crystals having the nominal stoichiometry and single domain properties. Our results show that for temperature changes from 25 to ∼100 °C, the excess amount of surface charge induced by pyroelectricity, ion irradiation, and ion‐induced electron emission is drained mainly by surface conductivity. For temperature changes above 100 °C, the bulk ionic conductivity becomes an important charge drainage channel. Our measurements give an activation energy of 0.75 eV for the thermally activated ionic conductivity, and the result agrees well with the literature value previously obtained by d.c. conductivity measurements. Copyright © 2006 John Wiley & Sons, Ltd.

X-ray photoelectron spectroscopic and ion scattering study of the SrTiO 3(0 0 1) surface

The chemical and structural properties of several as-received (unspecified and TiO 2 terminated) SrTiO 3(0 0 1) surfaces have been analyzed prior to and following UHV and O 2 annealing using X-ray photoelectron spectroscopy (XPS), time-of-flight scattering and recoiling spectrometry (TOF-SARS), and low energy electron diffraction (LEED). Well resolved 1×1 LEED patterns were noted for the TiO 2 terminated surface. Simulations of the TOF-SARS azimuthal scans indicate that the O atoms are situated 0.1 Å above the Ti layer (surface plane). This TiO 2 terminated surface reconstructed following O 2 annealing resulting in the loss of the azimuthal TOF-SARS features and the LEED pattern. This disordered surface also exhibited a stronger Sr TOF-SARS signal, and a high binding energy (HBE) component in the Sr 3d, 3p and 4p XPS spectra similar to that observed in the unspecified surfaces. Cleaning with organic solvents and exposure to air further enhanced the HBE components, suggesting Sr–C formation. No HBE components were observed in the Sr spectra from the TiO 2 terminated surface. Valence band XPS spectra reveal the presence of surface states and Ti 3d sub-band states.

Kinetics and structure of O 2 chemisorption on Ni(1 1 1)

The kinetics of O 2 chemisorption at low dose on Ni(1 1 1) and the nature of the chemisorption site have been studied at 300 and 500 K using time-of-flight scattering and recoiling spectrometry (TOF-SARS), low energy electron diffraction, and scattering and recoiling imaging code (SARIC) simulations. Variations in the TOF-SARS spectra with different crystal alignments during O 2 dosing provide direct information on the location of oxygen adatoms on the Ni(1 1 1) surface at very low coverages as well as site-specific occupation rates ( S fcc and S hcp) and occupancies ( θ fcc and θ hcp). A system of equations has been developed that relate the slopes of the scattering and recoiling intensities versus O 2 exposure dose to these probabilities and occupancies. The results identify three chemisorption stages as a function of oxygen exposure, each with its own specific occupation rates and occupancies. The first- stage is observed up to θ 1=θ fcc ∼0.21 ML with θ hcp=0, constant S= S fcc∼0.18±0.01, and coverage ratio w= θ hcp/ θ fcc∼0 for both 300 and 500 K. The second-stage is observed at coverages between θ 1∼0.21 and θ 2∼0.32 ML with constant S fcc=−(0.05±0.01) and S hcp=(0.16±0.02) at 300 K and S fcc=(0.005±0.003) and S hcp=(0.007±0.003) at 500 K, and coverage ratios w= θ hcp/ θ fcc∼1 at 300 K and w= θ hcp/ θ fcc∼0.10 at 500 K. The third-stage, observed for θ>0.32 ML, involves saturation coverage of the adsorption sites. SARIC simulations were used to interpret the spectra and the influence of oxygen chemisorption and vibrational effects. A method for determining the “effective Debye temperature Θ D *” that uses the experimental TOF-SARS intensity variations as a function of temperature and the simulated SARIC signals as a function of the mean square vibrational amplitude 〈 u 2〉 has been developed. The result for this system is Θ D *=314±10 K.

Scattered and recoiled ion fractions from LiTaO3(100) surfaces with different electrical properties

Time-of-flight scattering and recoiling spectrometry (TOF-SARS) was used to investigate the scattered and recoiled ion fractions from 3 keV Ar+ ion beams on LiTaO3(100) single crystals. The TOF-SARS measurements were found to be sensitive to the electrical properties of the crystal. ac impedance measurements of the electrical conductivity showed that LiTaO3 is an insulator at room temperature and that its conductivity increases by ∼103 at temperatures in the range 100–200 °C. This increase in conductivity could be monitored in TOF-SARS by measuring the current through the crystal induced by the impinging Ar+ ions as a function of temperature. The activation energy for this transition was estimated from both the impedance and scattering measurements to be ∼1 eV. Azimuthal anisotropy of the scattered Ar+ ions from Ta atoms was observed at room temperature but not at elevated temperatures. Scattered Ar+ ion fraction measurements showed that scattered Ar+ ions are enhanced by charge buildup on the LiTaO3 surface, whereas recoiled ions are not affected. The effects of surface charging phenomena on TOF-SARS could be eliminated by either heating the LiTaO3 crystal to ∼200 °C or by application of a low energy electron beam to the crystal surface.

Ion beam deposition of 107Ag(111) films on Ni(100)

Monolayer thick silver films were grown on an Ni(100) surface using direct ion beam deposition of 107Ag + ions with 20 eV kinetic energy and the substrate at 25°C. Deposition was performed by means of a mass-selected, low-energy, ultra-high vacuum ion beam system with a well-defined ion energy. The films were analyzed in situ by time-of-flight scattering and recoiling spectrometry (TOF-SARS) and by low energy electron diffraction (LEED). Classical ion trajectory simulations were used to model the TOF-SARS spectra. Results of the TOF-SARS spectra and azimuthal scans are consistent with the growth of a monolayer Ag film with a fcc (111) structure on the Ni(100) surface. The deposited films exhibited a faint hexagonal c(2×8) LEED pattern. Molecular dynamic simulations of the deposition process are in agreement with the experimental observations. The growth of such isotopically pure metal monolayers at room temperature with controlled orientations provides the opportunity for exploring a variety of unique physical properties.

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.

Chemisorption site of methanethiol on Pt{111}

The chemisorption site of the simplest prototypical model alkanethiol compound, methanethiol [CH3SH], on a Pt{111} surface in the temperature range 298–1073 K has been investigated by means of time-of-flight scattering and recoiling spectrometry (TOF-SARS) and low-energy electron diffraction (LEED). TOF-SARS spectra of the scattered and recoiled ions plus fast neutrals were collected as a function of crystal azimuthal rotation angle δ and beam incident angle α using 4 keV Ar+ primary ions. At room temperature, the adsorption of methanethiol produces a partially disordered overlayer that gives rise to a diffuse (3×3)R30° LEED pattern and three-fold symmetry in the scattering profiles. Heating this surface layer results in the sequential dehydrogenation of the methanethiol and the formation of S–C species at elevated temperatures. By ∼373 K, hydrogen is absent from the TOF-SARS spectra and a sharp (3×3)R30° LEED pattern is observed. The model developed from the scattering data is consistent with the preservation of the adsorption site at elevated temperatures, but a change in the S–C bond angle with respect to the surface plane. For the fully dehydrogenated species, the S atoms reside ∼1.6±0.2 Å above the surface in face-centered-cubic (fcc) three-fold sites and the C atoms reside ∼1.5±0.4 Å in hexagonal-close-packed (hcp) three-fold sites. It is proposed that the remarkable stability of this SC adsorbate results from bonding of both the S and C atoms to the surrounding Pt atoms, i.e., a Pt-stabilized SC moiety.

Thermal stimulation of the surface termination of LaAlO3{100}

The surface termination, structure, and morphology of the LaAlO3{100} surface has been studied as a function of temperature by means of time-of-flight scattering and recoiling spectrometry (TOF-SARS), atomic force microscopy (AFM), low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and x-ray photoelectron spectroscopy (XPS). The results show that the surface is terminated in a Al–O layer from room temperature up to ∼150 °C and a La–O layer at temperatures above ∼250 °C. The surfaces are terminated exclusively in either Al–O or La–O layers, with mixed terminations observed only in the intermediate region of 150–250 °C. These surfaces exhibit ordered (1×1) LEED patterns and stepped AFM images with step heights of 3.9±0.5 Å and terrace widths of ∼1000 Å. A mechanism is proposed for this low-temperature surface stoichiometry change which is linked to the observation of the creation of surface oxygen deficiencies upon heating. The oxygen deficient Al–O terminating layer transforms to a La–O terminating layer, creating a near-surface vacancy in the twelve coordinate site of the perovskite structure.

Surface composition and structure of GaN epilayers on sapphire

The surface composition and structure of GaN films grown on sapphire substrates by organometallic vapor-phase epitaxy (OMVPE) have been determined through the use of time-of-flight scattering and recoiling spectrometry (TOF-SARS), classical ion trajectory simulations, and low-energy electron diffraction (LEED). TOF-SARS spectra of scattered and recoiled ions plus fast neutrals were collected using 4 keV Ar+ primary ions. The scattering results were simulated by means of the three-dimensional scattering and recoiling imaging code (SARIC). This data leads to the conclusions that both N-terminated {001̄}-(1×1) and Ga-terminated {0001}-(1×1) surfaces occur, however no evidence was obtained for mixed terminations. No relaxation or reconstruction was detected on either surface, although both surfaces exhibited two structural domains. The {0001̄} surfaces are well-ordered and contained hydrogen atoms bound to the N atoms of the outermost layer. The {0001} surfaces are highly reactive towards adsorption of carbon and oxygen from residual gases, however unlike the {0001̄} surfaces, they adsorb very little hydrogen. These Ga-terminated surfaces are stabilized and obtain more ordered structures as a result of the contamination.

Composition and structure of the Al 2O 3{0001}-(1 × 1) surface

The surface composition, termination, and structure of the Al 2O 3{0001}-(1 × 1) surface have been determined through the use of time-of-flight scattering and recoiling spectrometry (TOF-SARS), low energy electron diffraction (LEED), and classical ion trajectory simulations. TOF-SARS spectra of scattered and recoiled ions plus fast neutrals were collected as a function of crystal azimuthal rotation angle using 4 keV Ar + primary ions. The scattering and recoiling imaging code SARIC was used to simulate the azimuthal angle scans as a function of the termination layer and the first-second-layer interatomic spacing. This first-second-layer spacing was determined from the best agreement of the simulated scans with the experimental scans using a reliability, or R, factor analysis. The results lead to the conclusions that there are two domains on the surface that are rotated by 180°. It is found that the surface is terminated in an Al layer which relaxes downward so that the Al first-layer to O second-layer spacing is 0.3 ± 0.1 A ̊ . This relaxation of 63% from the bulk value can be made possible through a bond length conserving buckling of the O atoms. Hydrogen atoms were present on the surface, even at the annealing temperature of 1100°C.

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}.

Composition and structure of the GaN{0001-bar}-(1 x 1) surface.

The composition and structure of the n-type GaN{0001\ifmmode\bar\else\textasciimacron\fi{}}-(1\ifmmode\times\else\texttimes\fi{}1) surface of samples grown on sapphire by organometallic vapor-phase epitaxy (OMVPE) has been determined through the use of time-of-flight scattering and recoiling spectrometry (TOF-SARS), three-dimensional classical ion trajectory simulations, low-energy electron diffraction (LEED), and thermal decomposition mass spectrometry (MS). Elastic recoil detection was used to determine the bulk hydrogen concentration. TOF-SARS spectra of scattered and recoiled ions plus fast neutrals were collected as a function of crystal azimuthal rotation angle \ensuremath{\delta} and beam incident angle \ensuremath{\alpha} using 4 keV ${\mathrm{Ne}}^{+}$ or ${\mathrm{Ar}}^{+}$ primary ions in order to determine the surface termination layer, presence and location of impurities, and possible reconstruction or relaxation. LEED, TOF-SARS, and MS were monitored as a function sample temperature up to the point of decomposition. The totality of these data leads to the conclusions that the (1\ifmmode\times\else\texttimes\fi{}1) surface is neither reconstructed nor relaxed, that it is terminated in a N layer, that Ga comprises the second layer, that there are two domains rotated by 60\ifmmode^\circ\else\textdegree\fi{} from each other, and that there are steps on the surface. Hydrogen atoms are bound to the outerlayer N atoms and protrude outward from the surface with a coverage of \ensuremath{\sim}3/4 monolayer, facilitating autocompensation of the (1\ifmmode\times\else\texttimes\fi{}1) structure. The bulk hydrogen concentration is \ensuremath{\sim}4\ifmmode\times\else\texttimes\fi{}${10}^{19}$ atoms/${\mathrm{cm}}^{3}$. Evolution of gases commences at \ensuremath{\sim}850 \ifmmode^\circ\else\textdegree\fi{}C with the observed evolution of ${\mathrm{N}}_{2}$, ${\mathrm{NH}}_{2}$, and ${\mathrm{H}}_{2}$. These results are discussed in terms of reconstruction phenomena, autocompensation, film/substrate polarity matching, and the role of hydrogen in stabilizing the growth of GaN. \textcopyright{} 1996 The American Physical Society.

Method for determining the composition and orientation of III–V {001} semiconductor surfaces

A method for determining the composition and orientation of III–V {001} semiconductor surfaces is presented and applications are described. The information is obtained from the techniques of time-of-flight scattering and recoiling spectrometry (TOF-SARS), using the composition from azimuth-specific elemental accessibilities (CASEA) method, and low energy electron diffraction (LEED). The azimuth-specific elemental accessibilities (ASEA) are measured experimentally and calculated from the number of accessible atoms in the unit cell and from three-dimensional trajectory simulations using the SARIC program. The in situ analyses identify the 1st-layer elemental species and determine the orientation of the reconstructed surface symmetry elements with respect to the bulk crystallographic directions. This is demonstrated for the III–V {001} compound semiconductor surfaces of GaAs and InAs in the (4 × 2) and (4 × 2) phases and InP in the (4 × 2) phase. The analyses confirm the missing-row-dimer (MRD) structure for GaAs and InAs in which the missing row direction is parallel to the direction of the 1st-layer multimers (dimers) and the missing-row-trimer-dimer (MRTD) structure for InP in which the missing row direction is perpendicular to the direction of the 1st-layer multimers (trimers).

Composition and reconstruction of the GaP{100}-(4 × 2) surface

The composition and reconstruction of the GaP{100}-(4 × 2) surface has been studied 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 (polar) angle α. Compositional analysis of the surface was obtained from 4 keV Ne + scattering and recoiling spectra. Structural analysis was obtained from the azimuthal anisotropy of the δ-scans and the features of the α-scans using both 4 keV Ne + and Kr + for scattering and recoiling. The azimuthal δ-scans were simulated by means of a shadow cone focusing model and a classical ion trajectory model. The totality of this data shows that the (4 × 2) reconstruction of GaP is a Ga missing-row-trimer P dimer (MRTD) structure in which every fourth Ga〈01̄1〉 row is missing, the Ga atoms are trimerized along the 〈011〉 azimuth, and the 2nd-layer P atoms exposed in the 〈01̄1〉 troughs are dimerized. This MRTD model is consistent with all of the data, is autocompensated, and has Ga intratrimer spacings of 3.2 ± 0.2 A ̊ and P intradimer spacings of 2.7 ± 0.2 A ̊ . The results of the simulations suggest that the two end Ga atoms of the trimers are relaxed downward by a minimum of 0.2 Å. A missing-row-dimer (MRD) model, similar to that proposed for the GaAs (4 × 2) structure, in which every fourth Ga 〈011〉 row is missing and Ga dimers form along 〈011〉, was also considered. This MRD model is inconsistent with large portions of the experimental data and the simulations. The results are discussed in terms of the differences in the chemical bonds of phosphorus and arsenic.

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.

Structure of the InAs{100}−(4 × 2) epilayer surface on InP

The composition and structure of an InAs{100}−(4 × 2) epilayer grown on an InP 100 surface has been studied by time-of-flight scattering and recoiling spectrometry (TOF-SARS) and low energy electron diffraction (LEED). Time-of-flight spectra of scattered neutrals plus ions were collected as a function of the crystal azimuthal angle, δ, and the beam incident angle, α, using 4 keV Ne+ ions. The composition of the outermost layer was obtained from grazing incidence scattering. Experimental scattering images were used with the composition from azimuthal specific elemental accessibilities (CASEA) method to define the azimuthal directions with respect to the bulk crystallographic axes. Structural analysis was obtained from the azimuthal anisotropy of the δ-scans, from the features of the incident α-scans, and from the scattering images. These α- and δ-scans and images were simulated by means of classical ion trajectory calculations. The totality of this data is consistent with the InAs{100}−(4 × 2) surface being In-terminated and with the reconstruction being of the missing-row-dimer (MRD) type. The data show that the interatomic spacings are consistent with the coexistence of both one- and two-missing In rows in the surface. The 1st-layer In atoms are found to shift by 0.4 ± 0.1 Å, yielding an In-In dimer spacing of 3.5 ± 0.1 Å. Data for the InP (4 × 2) surface has been included in order to contrast the missing-row-trimerdimer (MRTD) reconstruction model of InP with that of the MRD reconstruction model of InAs.

Structural Study of the Ni{100}−(2 × 2)-C Surface by Time-of-flight Scattering and Recoiling Spectrometry

The structure of the Ni{100}−(2 × 2)-C surface has been investigated by time-of-flight scattering and recoiling spectrometry (TOF-SARS), low-energy electron diffraction (LEED), and classical scattering simulations. The surface was prepared by chemisorption of either carbon monoxide, benzene, or aniline onto a Ni{100} surface at 600 °C. The scattering flux from 4 keV of Ne+ and the recoiling flux of carbon from 4 keV of Ar+ were monitored as a function of the crystal azimuthal angle. An efficient simulation method, which is based on the binary collision approximation and shadowing and blocking effects, has been developed and applied to simulation of the TOF-SARS azimuthal scans. Quantitative values for the geometrical parameters were obtained by using a reliability (R) factor to compare the experimental and simulated azimuthal scans. The results are consistent with a surface structure which consists of 80% of a clock reconstructed (2 × 2)p4g phase and 20% of an unreconstructed (2 × 2) phase. For the recons...

Orthogonal alignment of the missing-rows and dangling bonds in {001} surfaces of different III–V semiconductors with the same periodicity

It is shown that the {001} surfaces of different III–V compound semiconductors with the same periodicity can have different reconstructed geometries with orthogonal alignments of the 1st-layer dangling bond direction and the missing row direction. This is manifest at the InAs InP {001} interface, where time-of-flight scattering and recoiling spectrometry (TOF-SARS) is used to characterize the surface elemental composition and the structural transition.

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

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 〈01̄1〉 row is missing, the In atoms are trimerized along the 〈011〉 azimuth, and the 2nd-layer P atoms exposed in the 〈01̄1〉 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 Å and P intradimer spacings of 2.95 ± 0.20 Å. The results of the simulations suggest that the two end In atoms of the trimers are relaxed downward by a minimum of 0.15 Å. Two other models were considered: (1) An In missing-row (MR) model without trimers or dimers in which every fourth In 〈01̄1〉 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 a Si epilayer on Ni{100}

Silicon epilayere have been grown on a Ni{100}-p(1 × 1) substrate at 650°C using an effusion cell. The deposited layers exhibit a c(2 × 2) low energy electron diffraction (LEED) pattern. The structure of the c(2 × 2)-Si epilayer was investigated by time-of-flight scattering and recoiling spectrometry (TOF-SARS) using 4 keV Kr + ions to recoil the Si and Ni atoms. The sharp anisotropy of Si recoil intensity as a function of crystal azimuthal angle (δ) demonstrates that the Si epilayer is ordered. The alignment of the azimuthal patterns of the Si and Ni recoils shows that the Si atoms occupy four-fold hollow sites on the Ni{100} surface. Plots of Si recoil intensity versus beam incident angle (α) for monolayer and fractional monolayer coverages exhibit high sensitivity to Si defects. Classical ion trajectory simulations that have been used to model these α-scans allow interpretation of the results in terms of direct and surface recoils. The height of the Si epilayer above the four-fold hollow Ni sites, as determined from the agreement between the trajectory simulations and the α-scans, of 1.7 ± 0.2 Å gives a SiNi bond length of 2.5 Å. TOF-SARS is demonstrated to be a viable technique for monitoring in situ epitaxial film growth.