Ion-scattering Measurements Research Articles

Pb nanowire formation on Al/lead zirconate titanate surfaces in high-pressure hydrogen

Piezoelectric systems are well known to degrade in hydrogen because of various mechanisms including loss of polarization, Pb migration into the electrode, and surface blistering. Understanding damage mechanisms is crucial for potential high-pressure applications such as injectors for hydrogen-fueled vehicles. In this paper, we report on a previously unreported form of high-pressure hydrogen damage resulting from growth of surface Pb nanowires from an Al electrode on lead zirconate titanate. Wires were observed with roughly 80 nm diameter and with length that varied between 5 and 100 μm. Microscopy to characterize the nanowires and ion-scattering measurements to quantify concurrent Pb surface migration and hydrogen absorption effects are described.

Thermal Annealing Effects on the Atomic Layer Deposited LaAlO[sub 3] Thin Films on Si Substrate

The changes in film structure of amorphous atomic layer deposited thin films after thermal annealing were examined by medium-energy ion-scattering measurements and angle-resolved X-ray photoelectron spectroscopy. Thermal annealing induces Si-rich LaSiO and Al-deficient layers on a few monolayers of . Al atoms do not participate in silicate formation during annealing. Instead, they migrate toward the film surface, which induces nonhomogeneity in the films along the vertical direction. The concentrations of Al and La on the film surface increase and decrease, respectively, as a result of Si diffusion from the substrate and silicate formation.

Transition structure at the Si(100)-SiO2 interface.

We characterize the transition structure at the Si(100)-SiO2 interface by addressing the inverse ion-scattering problem. We achieve sensitivity to Si displacements at the interface by carrying out ion-scattering measurements in the channeling geometry for varying ion energies. To interpret our experimental results, we generate realistic atomic-scale models using a first-principles approach and carry out ion-scattering simulations based on classical interatomic potentials. Silicon displacements larger than 0.09 A are found to propagate for three layers into the Si substrate, ruling out a transition structure with regularly ordered O bridges, as recently proposed.

Cluster formation during annealing of ultra-low-energy boron-implanted silicon

The clustering of low-energy ion-implanted boron has been investigated. Two 1 keV boron implantations at doses of 1×1015 and 5×1015 cm−2 were annealed for 10 s between 700 and 1100 °C. The evolution of the boron concentration profiles was monitored using secondary ion mass spectrometry. Electrical activation was measured with four-point-probe measurements and spreading resistance profiling. The displaced Si concentration profiles were determined from medium-energy ion-scattering measurements.

Structure determination of Ag(111) by low-energy electron diffraction

A quantitative structure determination of the Ag(111) clean surface has been performed using low-energy electron diffraction. Optimisation of the outermost layer spacings and vibrational amplitudes was achieved using two different dynamical scattering computational methods, one using the LEEDFIT code and the second exploiting a Simulated Annealing algorithm implemented on the conventional Van Hove/Tong code. The results obtained show that a bulk-terminated structural model describes the data best (Δ d 12=0.00±0.02 Å and Δ d 23=0.00±0.03 Å), and in particular, the analysis excludes a 2.5% contraction of the outermost layer spacing, which was recently reported on the basis of ion-scattering measurements. The LEED result of no contraction is consistent with prior results for the (111) face of several other fcc metals.

Initial growth morphology in a heteroepitaxial system at low temperature: Fe on Ag(100)

The growth of ultrathin iron films on Ag(100) at 135 K has been studied by means of thermal-energy helium diffraction. Film deposition has been monitored by measuring the oscillations of the specular beam intensity during the growth. Information on the island morphology of the deposited film has been obtained by exploiting the dependence of the scattered intensity on the momentum transfer. The set of data indicates that there are definite differences between the growth mode of the first and subsequent layers. The surface of the first monolayer exhibits a two-dimensional morphology, while the estimate of the interface width $w$ [J. W. Evans, Phys. Rev. B 43, 3897 (1991)] from a kinematic analysis of data provides values of ${w}^{2}$ ranging from 0.28 at 2 ML to 0.37 at 5 ML. The obtained values are consistent with the so-called one-hop transient mobility model with adsorption at the fourfold hollow site and show that from the second layer on the morphology gradually deviates towards island growth. The analysis of rocking curves also provides the step height ${(h}^{\ensuremath{'}})$ of the growing interface. At 2, 3, 4, and 5 ML, ${h}^{\ensuremath{'}}$ assumes values that are slightly larger than the value of bulk bcc iron ${h}_{\mathrm{Fe}}=1.43$ \AA{}. This finding is assigned to the occurrence of intermixing through simple arguments based on the conservation of the atomic volumes. The Ag population at the surface of the film is found to decrease with film thickness consistently with recent ion-scattering measurements.

Surface-melting induced faceting of aluminium

Abstract Medium-energy ion scattering measurements have been used to study the temperature dependent behaviour of aluminium surfaces with surface orientations in the (110) zone. The ion-scattering measurements show that surface-melting induced faceting occurs between the melting (110) and the non-melting (111) orientations. The dry and melted facet orientations Θd and Θd have been determined up to 0.3 K from the bulk melting point. Close to the melting point these orientations are almost independent of temperature, and amount to Θd = 7 ± 2° and Θm = 29 ± 2° with respect to the (111) plane.

Difference in surface melting between indium (110) and (011)

Medium-energy ion scattering measurements and Monte Carlo computer simulations have been used to study the melting behaviour of the two open low-index surfaces of indium: In(110) and In(011). Because of the tetragonal lattice structure, the atomic density at the In(110) surface is 3.5% lower than that at the In(011) surface. The corresponding higher surface free energy of the (110) surface is expected to give rise to a slightly stronger surface melting effect at this face than at the (011) face. The ion-scattering measurements indeed show such a difference in melted-layer thickness at temperatures close to the bulk melting point, T m = 429.76 K. Surprisingly, however, the order of the onset of surface disordering is reversed. In(110) starts disordering about 25 K closer to T m than In(011). We attribute this counter-intuitive difference in disordering onset to the difference in adatom-vacancy creation energies at the two surfaces. This idea is corroborated by energy calculations and Monte Carlo computer simulations, in which a Finnis-Sinclair interaction potential between indium atoms was employed. The slightly increased nearest-neighbour distance on the more densely-packed In(011) surface leads to a decreased creation energy of adatoms and vacancies. The resulting higher densities of adatoms and vacancies make In(011) unstable at a lower temperature than In(110). These results strongly suggest that the onset of surface disordering involves a mechanical surface instability.

Atomic structure and thermal stability of two-dimensional Er silicide on Si(111).

The atomic structure of Er silicides on Si(111) in the submonolayer range has been studied by means of medium-energy ion scattering and surface x-ray diffraction. We find good agreement between the two techniques. The ion-scattering measurements indicate that the Er adsorbs in a single two-dimensional layer on top of which a completely relaxed (bulklike) Si bilayer is located. This bilayer is exclusively of {ital B} type, i.e., rotated by 180{degree} with respect to the substrate bilayers. The x-ray diffraction results show that Er is positioned on {ital T}{sub 4} sites only, i.e., above second-layer substrate atoms. For low coverages (0.23 ML), the height of the top bilayer appears to be reduced by approximately 0.4 A. We attribute this to the fact that the Er does not form a closed layer, leading to the formation of less ordered Er-Si complexes in which extra Si adatoms provide the backbonds for the Si hexagons on top of the Er. Finally, we investigate the thermal stability of the two-dimensional silicide, and find that the two-dimensional Er layer evolves into relaxed three-dimensional islands at temperatures above 800{degree}C with concomitant strain relaxation. {copyright} {ital 1996 The American Physical Society.}

Anharmonicity but absence of surface melting on Al(001).

Medium-energy ion-scattering measurements and Monte Carlo simulations have been used to study the Al(001) surface as a function of temperature. Surface melting does not occur and the surface stays well ordered up to the bulk melting point ${\mathit{T}}_{\mathit{m}}$=933.52 K. The combination of the effective-medium potential, used to simulate the interacting Al atoms, and ion-scattering calculations on the simulated Al(001) surface, reproduces the ion-scattering intensities very well. The Monte Carlo simulations show that the surface relaxations and the vibration amplitudes both display strongly anharmonic behavior which extends several layers into the crystal and increases with increasing temperature.

N-induced (2 x 3) reconstruction of Cu(110): Evidence for long-range, highly directional interaction between Cu-N-Cu bonds.

The scanning-tunneling-microscope (STM) topographies taken from a N-induced (2\ifmmode\times\else\texttimes\fi{}3) reconstructed Cu(110) surface provide unique information, which offers the basis for an explanation of the peculiar fact that every third 〈110〉 row is missing and also confirm in detail the structural model inferred from recent ion-scattering measurements. The close analysis of STM images with atomic resolution taken from both ordered (2\ifmmode\times\else\texttimes\fi{}3) domains and local defective arrangements evidences the existence of long-range, highly directional interactions between Cu-N-Cu bonds, which seems to determine ultimately the nature of the reconstruction.

Self-diffusion at a melting surface observed by He scattering.

Self-diffusion at surfaces can be studied with quasielastic scattering of low-energy He atoms. This is demonstrated for the Pb(110) surface close to the melting point, ${\mathrm{T}}_{\mathrm{m}}^{\mathrm{Pb}=600.7}$ K. From the width of the quasielastic energy distribution of scattered He atoms it is inferred that at T\ensuremath{\ge}450 K the surface atoms are anomalously mobile. At T\ensuremath{\ge}550 K, surface mobilities are found to exceed the bulk liquid value. These results complement recent ion-scattering measurements on surface disorder of Pb(110).

Direct observations of the deuterium-induced surface reconstruction of the Ni(110) plane.

The reconstruction of the Ni(110) plane induced by the adsorption of deuterium has been investigated in atomic detail with the field ion microscope. Small clusters of Ni atoms were observed to rearrange from a bulk-terminated structure to a ``missing-row'' structure upon exposure to ${10}^{\mathrm{\ensuremath{-}}8}$-Torr deuterium at 165--175 K. The observed structure is inconsistent with the ``paired-row'' structure deduced from low-energy electron diffraction and ion-scattering measurements.

Measurement of the silicon (111) surface contraction.

Distinct differences have been found in the response of LVV and KLL Auger yields from 7\ifmmode\times\else\texttimes\fi{}7 and 1\ifmmode\times\else\texttimes\fi{}1 Si(111) surfaces probed with x-ray standing-wave excitations. By the taking into account of the shorter mean free path of the LVV electrons, the differences can be explained by a contraction of the top two (111) atom planes by approximately 0.5 A\r{}. This agrees well with earlier analyses of ion-scattering measurements, but such a contraction has not been detected in other surface measurements or predicted by pseudopotential calculations. Current stacking-fault models of the 7\ifmmode\times\else\texttimes\fi{}7 reconstruction may require modification in order to be consistent with these standing-wave measurements.

The atomic geometry of Si(100)-(2×1) revisited

An R-factor analysis of published elastic low-energy electron diffraction (ELEED) intensity data from the (2×1) structure on Si(100) is presented. This analysis reveals the existence of a new asymmetric dimer structure which is compatible with ion-scattering measurements yet provides a description of the measured ELEED intensities which is improved relative to that afforded by structures proposed on the basis of prior ELEED intensity analyses. The structure is similar to but not identical with those predicted by several energy-minimization calculations. Consequently, our analysis resolves an extant incompatibility between ELEED and ion-scattering results as well as reveals the qualitative correctness of modern total-energy-minimization methods for surface-structure determinations

The atomic geometry of Si(100)-(2×1) revisited

An R-factor analysis of published elastic low-energy electron diffraction (ELEED) intensity data from the (2×1) structure on Si(100) is presented. This analysis reveals the existence of a new asymmetric dimer structure which is compatible with ion-scattering measurements yet provides a description of the measured ELEED intensities which is improved relative to that afforded by structures proposed on the basis of prior ELEED intensity analyses. The structure is similar to but not identical with those predicted by several energy-minimization calculations. Consequently, our analysis resolves an extant incompatibility between ELEED and ion-scattering results as well as reveals the qualitative correctness of modern total-energy-minimization methods for surface-structure determinations

Oxygen adsorption on (110) silver

The adsorption of oxygen on a (110)Ag surface is investigated by means of Auger electron spectroscopy, LEED and low energy helium ion scattering (IS). With LEED two ordered structures, i.e. (3×1) and (2×1) were observed at oxygen exposures of 1700 L and 7000 L respectively. The oxygen signal observed by AES and IS increases monotonically with oxygen exposure. The signals can be related to absolute coverage by comparison with Δφ measurements and by the use of the LEED data. With this calibration and with theoretical scattering cross-sections the IS measurements allow the position of the adsorbed oxygen to be estimated. The observation of a strong azimuthal anisotropy of the IS signal, e.g. a large oxygen signal if the plane of scattering is parallel to the [110] direction and a relatively small oxygen signal in the [100] direction, leads to the conclusion that the oxygen is adsorbed in a bridge position between two Ag atoms of the [110] surface channels, its centre being slightly below the centres of the Ag atoms.

Interaction Energies and Scattering Cross Sections of Hydrogen Ions in Helium

The interaction energies for the H+–He and H2+–He system have been calculated for a range of internuclear distances and, in the H2+–He case, for two different configurations. The results are used to analyze the ion-scattering measurements of Simons and co-workers. It has been found that the predicted scattering cross sections are in remarkable agreement with the measured ones, provided allowance is made for a finite width of the ion beam, as suggested by Amdur and Harkness. This agreement implies that the method of interpreting measured cross sections in terms of molecular forces is correct, and that specific apparatus effects have been correctly analyzed. The present calculations suggest, in contrast to previous assumptions, that the important forces are ones of repulsion rather than attraction, with the exception of those cases where chemical binding is clearly involved. As expected, to obtain precise force information of physical significance from such scattering measurements, it is again necessary to make allowance for the finite width of the ion beam. This has been illustrated for the scattering of H3+ ions in helium. Finally, the cross-section equations have been integrated for the case of scattering by an exponential potential and by a Lennard-Jones (8–4) potential.