Surface Structure Determination Research Articles

Low energy ion scattering and recoiling

Ion scattering spectrometry was developed as a surface elemental analysis technique in the late 1960's. Further developments during the 1970's and 80's revealed the ability to obtain surface structural information. The recent use of time-of-flight (TOF) methods has led to a surface crystallography that is sensitive to all elements, including hydrogen, and the ability to directly detect hydrogen adsorption sites. TOF detection of both neutrals and ions provides the high sensitivity necessary for non-destructive analysis. Detection of atoms scattered and recoiled from surfaces in simple collision sequences, together with calculations of shadowing and blocking cones, can now be used to make direct measurements of interatomic spacings and adsorption sites within an accuracy of ≲ 0.1 Å. Structures are determined by monitoring the angular anisotropies in the scattered primary and recoiled target atom flux. Applications of such surface structure and adsorption site determinations are in the fields of catalysis, thin film growth, and interfaces. This article provides a short historical account of these developments along with some examples of the most recent capabilities of the technique.

Thirty years of atomic and electronic structure determination of surfaces of tetrahedrally coordinated compound semiconductors

Surface Science has evolved considerably since the early 1960's. Experimental and theoretical techniques pioneered twenty to thirty years ago have been developed into sophisticated tools which today allow systematic and almost routine determinations of atomic geometries, electronic structures, chemical compositions and surface energies of semiconductor surfaces. In this article, I review the part of the history of this work devoted to tetrahedrally coordinated compound semiconductor surfaces.

テンソルＬＥＥＤによる構造決定

Precise knowledge about the atomic geometries of surfaces is the basis for understanding surface properties such as electronic structure and adsorption processes of molecules.The dynamic analysis of intensity vs. energy (IV) curves of low-energy electron diffraction is the most powerful method for determining the surface geometries of crystal surfaces.The disadvantage of this method is that it requires a rather long CPU time to calculate IV curves. When we treat complex surface structures with many atoms in a surface unit cell, we must carry out calculations using many possible surface structural models while changing their structural parameters, and compare the calculated spectra with experimental ones.The recently developed tensor LEED method drastically reduces these timeconsuming calculations. This paper briefly reports on the characteristic features and advantages of tensor LEED compared to the canventional method.Some examples of the application of this mathod to the determination of surface structures are also reported.

Scanning tunneling spectroscopy

The development of the field of spectroscopic measurement with the scanning tunneling microscope (STM) is discussed. A historical review of early experimental results in this field is presented, with emphasis on the techniques for data acquisition and interpretation. The applicability of STM spectroscopic measurement to surface structural determination is addressed. The role of geometric versus electronic contributions to STM images is discussed, with reference to studies of Si(111)7 × 7, Si(111)2 × 1, and Ge(111)c(2 × 8) surfaces. It is concluded that, for semiconductor surfaces, the observed corrugations are dominated by electronic effects. Issues of dynamic range in spectroscopic measurement, and interpretation of spectroscopic images, are examined.

LEED and clean metal surfaces: personal reminiscences and opinions

The early history of surface structure determination of clean metals from LEED intensities at IBM Research and in collaboration with SUNY at Stony Brook (1964–1975) is sketched. The reasons for the success of LEED intensity analysis in establishing the structure of clean metal surfaces at that time are discussed. A selective overview of the rich phenomenology of metal surface structure as we know it today is given. Personal comments are made on important current and future developments in LEED theory.

Interaction of electrons and positrons with solids: from bulk to surface in thirty years

In 1963 electrons and positrons were regarded as probes of the bulk properties of solids via, e.g., energy-band theory, fast electron energy loss spectroscopy, and positronium decay. During the late 1960s it was recognized that at energies in the range 10 ⩽ E ⩽ 1000 eV,the inelastic collision mean free paths, λ, of electrons and positrons become comparable to atomic dimensions, i.e., λ ≈ 10 Å, so that the elastic reflection or emission of these particles from solids probes the properties of the uppermost few atomic layers. This recognition led to the development of core level spectroscopies (e.g., Auger electron spectroscopy, X-ray photoelectron spectroscopy) for surface compositional analysis in the 1960s, to the application of low-energy electron diffraction for quantitative surface structure determination in the 1970s, and to that of low-energy positron diffraction for surface structure analysis in the 1980s. In addition, in the 1960s it was recognized that resonant field emission through adsorbed species offers a quantitative spectroscopy of the single-electron excitation spectrum of these species: a discovery which presaged the use of photoemission spectroscopy for this purpose in the 1970s and the development of scanning tunneling spectroscopy in the 1980s. This article is an exposition of how the evolution of the understanding of the nature of electron (positron) solid interactions together with advances in ultrahigh vacuum instrumentation and semiconductor microelectronics led from the dawn of the surface science revolution in the 1950s and 1960s to the quantitative electron (positron) surface spectroscopies which are routinely deployed in the 1990s.

Energy-minimization approach to the atomic geometry of semiconductor surfaces

A brief review of the main results obtained from applications of a quantum mechanical total-energy-minimization approach to surface structural determination of tetrahedral semiconductors is given. The tight-binding based method has proved quite reliable in providing information on the atomic and electronic structure of a variety of surfaces including the (110), (111), and (100) surfaces of zincblende and elemental semiconductors. For the Si(100) surface, a breaking of the mirror symmetry at the surface was predicted and this led to a natural explanation for the c-4 × 2 unit cells observed at this surface. The first quantitative estimates for the anisotropic monolayer and bilayer step formation energies on Si(100) surfaces were derived.

Quasikinematic low-energy electron-diffraction surface crystallography.

Based on the idea of constant-momentum-transfer averaging (CMTA) of Lagally et al. and facing the problem of CMTA pointed out by Pendry, in the present work we propose the use of the quasikinematic low-energy electron-diffraction (QKLEED) calculations in comparison with the experimental CMTA curves in surface structure determinations. In QKLEED, the influences of the zero-angle scattering on the CMTA curves are treated so that they have essentially no influence on correctly determining the structural parameters, and the mean complex atomic scattering factors are introduced as the effective scattering factors. In addition, an experimental design is proposed for obtaining CMTA curves containing the least influence of multiple scattering. The whole QKLEED/CMTA method has been carefully and systematically tested, and its capability of determining surface structures is demonstrated on the Cu(001)(1\ifmmode\times\else\texttimes\fi{}1), Si(111)(\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{}-Al, and Si(111)(\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{}-Ag surfaces.

Automated determination of complex surface structures by LEED

Conventional surface crystallography by low-energy electron diffraction (LEED) employs a trial-and-error search controlled at each step by human effort. This trial-and-error approach becomes very cumbersome and unreliable to solve complex surfaces with a large number of unknown structural parameters. We discuss automatic optimization procedures for LEED, which combine numerical search algorithms with efficient methods of determining the diffracted intensities for varying structures. Such approaches can reduce the computer time required for an entire structure determination by many orders of magnitude, while fitting many times more unknown structural parameters. Thereby, relatively complex structures, with typically 10 adjustable atoms (or 30 adjustable coordinates), can be readily determined on today's workstations. These include non-symmetrically relaxed structures, surface reconstructions and adsorbate-induced substrate distortions. We also address the theoretical and experimental requirements for an accurate structural determination.

Direct determination of crystal and surface structures in THEED

Tensor theory of transmission high-energy electron diffraction (THEED) has been formulated based on a standard first-order perturbation theory. The theory has been applied to both a thin crystal film with perfect bulk structure and a crystal slab having its surface layer reconstructed, and has been combined with a direct scheme which reduces the complicated problem of crystal and surface structure determination to a linear least-squares problem. The crystal or surface structure can be obtained either by directly inverting and n × n matrix of the linear least-squares problem, where n is the number of parameters to be determined, or, in the case when the problem is under-determined, using a singular value decomposition method. The scheme allows the variances associated with the estimate of the unknown parameters to be readily obtained and thus assessment of the reliability of the determined parameters, and can be applied to many other cases where a perturbation treatment applies.

Chemical-shift low-energy photoelectron diffraction: A determination of the InP(110) clean surface structural relaxation.

We establish chemical-shift low-energy photoelectron diffraction as a novel and powerful method for the determination of clean surface structures. Combined with a new theoretical approach based on full multiple scattering theory with complex potential, the method is applied to the case of the InP(110) clean surface relaxation. The extreme sensitivity of this technique to structural parameters allows us to measure with good accuracy both the first layer and the second layer relaxation angle (respectively 23\ifmmode^\circ\else\textdegree\fi{} and -5\ifmmode^\circ\else\textdegree\fi{}).

Surface structure investigations using plasma desorption mass spectrometry and coincidence counting

Coincidence counting techniques were used to study the plasma desorption of ions from a polycrystalline sample of αZirconium bis(monohydrogen orthophosphate)monohydrate (αZrP). Using coincidence counting in conjunction with time-of-flight (TOF) mass analysis, it was possible to obtain secondary ion spectra with a signal to background ratio a factor of ten better conventional TOF alone. It also was possible to produce a spectrum of SI that were desorbed in coincidence with PO − 3 (characteristic of the αZrP). Cluster ions observed in this spectrum are related to the αZrP crystal structure. This supports a direct emission process for cluster ion production and demonstrates the utility of particle induced desorption in the determination of surface and crystal structure.

Surface structure determination of an oxide film grown on a foreign substrate: Fe3O4 multilayer on Pt(111) identified by low energy electron diffraction.

For the first time a detailed surface structure determination is reported for an oxide film grown on a foreign metal substrate. Well-ordered iron oxide films were grown onto Pt(111) substrates and were identified by a dynamical low energy electron diffraction analysis to be magnetite, Fe[sub 3]O[sub 4]. They form an unreconstructed polar (111) surface termination that exposes 1/4 monolayer of Fe ions over a distorted hexagonal close-packed oxygen layer and minimizes the number of dangling bonds. The surface also exhibits large relaxations that may be driven by electrostatic forces.

Surface structure determination of X-ray diffraction

Surface structure determination of X-ray diffraction

Chemical substitution of surface atoms in structure determination by tensor low-energy electron diffraction.

We present a chemical perturbation method for surface-structure determination by low-energy electron diffraction (LEED). It is based on tensor LEED theory and is able to substitute selected surface atoms by other chemical species making full dynamical calculations for the structure unnecessary. The potential of the approach is demonstrated for the determination of the surface stoichiometry of substitutionally disordered alloys, for the switching from one adsorbate species to another, and for the generation of superstructures by substitution of surface atoms by vacancies.

Quantification of the Influence of Surface Structure on C—H Bond Activation by Iridium and Platinum

The trapping-mediated dissociative chemisorption of ethane on the closest packed Ir(111) surface has been investigated, and the activation energy and preexponential factor of the surface reaction rate coefficient have been measured. These results are compared to those of ethane activation on Pt(111) and on the missing row reconstructed Ir(110)-(1x2) and Pt(110)-(1x2) surfaces, allowing a quantitative determination of the effect surface structure has on the catalytic activation of C-H bonds. In the order Pt(111), Pt(110)-(1x2), Ir(111), and Ir(110)-(1x2), the activation energies for the dissociative chemisorption of ethane are 16.6, 10.5, 10.3, and 5.5 kilocalories per mole, demonstrating that the electronic and geometric effects are of approximately equal importance for ethane activation on these catalysts.

Twenty years of semiconductor surface and interface structure determination and prediction: The role of the annual conferences on the physics and chemistry of semiconductor interfaces

During the two decades 1974–93, semiconductor surface structure determination has evolved into a routine tool. This rapid evolution resulted from the confluence of three factors: the development of new techniques (e.g., scanning tunneling microscopy) and the improvement of old ones (e.g., low-energy electron diffraction), greatly increased reliability of vacuum equipment and electronics, and orders of magnitude enhancement in the speed and cost effectiveness of digital computers. A critical role of the annual conferences on the physics and chemistry of semiconductor interfaces (PCSI) in this evolution was to focus the attention of researchers on surface and interface structure as the key measurable intermediary between electronic materials processing and the performance of the resulting devices. Studies of several important systems, e.g., the (110) cleavage faces and (100) molecular-beam epitaxy growth faces of zinc blende structure compound semiconductors, were driven by events reported at PCSI conferences. These conferences were instrumental in stimulating the development of theoretical models which could extrapolate between vacuum surfaces, for which structural data have become readily available, and semiconductor heterostructures, for which comparable data are only now beginning to appear. Because of this development, the insight developed for vacuum surfaces is currently being applied to the prediction of interface specific electronic properties of semiconductor heterostructures (e.g., band offsets in heterojunctions and Schottky barrier heights in epitaxical metal–semiconductor junctions). In this way the PCSI conferences have forged a link between the surface science of semiconductors and the fabrication of microelectronic devices so that both evolve faster than either one would have otherwise. This article documents some specific examples of the nature and consequences of this process.

Fitting dozens of coordinates by LEED: automated determination of complex surface structures

Fitting dozens of coordinates by LEED: automated determination of complex surface structures

Fitting dozens of coordinates by LEED: automated determination of complex surface structures

We present the first comprehensive discussion of automatic optimization procedures for surface-structure determination by LEED. These procedures combine numerical search algorithms with efficient methods of determining the diffracted intensities for varying structures. Such approaches can reduce the computer time required for an entire structure determination by many orders of magnitude, while fitting many times more unknown structural parameters. Thereby, relatively complex structures, with typically 10 adjustable atoms (or 30 adjustable coordinates), can be readily determined on today's workstations. We list over two dozen structures so determined, many as yet unpublished.

Study of solid surfaces using micro-electron-beams

New surface-analysis apparatus and techniques have been developed which are based on a micro-electron beam. The spatial resolution of the ultra-high vacuum scanning electron microscope is about 100 Å and micro-probe reflection high-energy electron diffraction patterns can be obtained. Diffraction patterns of either Auger electrons or quasi-elastically back-scattered electrons from the probed micro-domain can be measured in combination with a specially developed “constant take-off angle electron energy analyzer”. These diffraction techniques can be used for the determination of surface structures of a region as small as 0.1 × 1 μm 2. These techniques are called μ-probe Auger electron diffraction and μ-probe grazing-incidence back-scattering medium energy electron diffraction. Study of In/Si(111) surfaces by these techniques is introduced.