Scanning tunnelling microscope images of layered materials in a non-uhv environment exhibit various anomalous phenomena, including enhanced corrugation heights, periodicity over large areas and a marked absence of point defects. We have modified a precision indentation device to allow STM rastering of a tip across a surface, while simultaneously monitoring mechanical contact. Images we have obtained from this apparatus on an HOPG sample exhibit atomic scale resolution with contact areas much larger than a single atom. Contrast in the image results from periodic conductance fluctuations as the layers of the sample undergo shear in the region of the tip. We provide a model for this process, which explains a variety of curious, and otherwise unrelated phenomena occurring during STM imaging of these materials.

In this paper, we show that atomic resolution in scanning tunneling microscopy (STM) originates from pz or dz2 states on the tip. Consequently, only a limited selection of tip materials can provide atomic resolution: d-band metals, for example, Pt, Ir, Pd, Rh, W, Mo; semiconductors that tend to form p-like dangling bonds, for example, Si.

Scanning Tunnelling Microscopy was used to examine the surface structure of gold films and supported gold particles. It was found that freshly evaporated gold films have a relatively smooth surface morphology. However, after 1 hr of annealing, a series of dendrites were clearly visible on the surface of the films. With additional annealing, the dendrites got larger, until the film broke into particles. STM images of annealed gold particles showed that annealed gold particles larger than 150 Å have a polyhederal shape and a smooth surface morphology. However, the surfaces of the particles smaller than 60 Å remained highly corrugated even after a long anneal. We believe that the difference in morphology of large and small particles may have some profound implications in supported metal catalysis.

Surface superstructures (reconstructed structures) have been observed by many authors. However, it is not easy to confirm that a superstructure does exist at an interface between two solid layers. The present paper reports a direct observation, by a grazing incidence x-ray diffraction technique with use of synchrotron radiation, of superstructures at the interface. Firstly, the boron-induced R30° reconstruction at the Si interface has been investigated. At the a Si/Si(111) interface, boron atoms at 1/3 ML are substituted for silicon atoms, thus forming a R30° lattice. Even at the interface between a solid phase epitaxial Si(111) layer and a Si(111) substrate, the boron-induced R30° reconstruction has been also observed. Secondly, SiO2/Si(100)-2×l interfacial superstructures have been investigated. Interfacial superstructures have been only observed in the samples of which SiO2 layers have been deposited with a molecular beam deposition method. Finally, the interfaces of MOCVD-grown AIN/GaAs(100) have been shown to have 1×4 and 1×6 superstructures.

Direct imaging of a 7×7 superstructure buried at the interface between amorphous-Si and a Si(111) substrate was demonstrated by cross-sectional high resolution transmission electron microscopy (HRTEM). The electron diffraction pattern of the interface region in cross-section geometry showed diffuse streaks of fractional order which suggested the existence of the superstructure at the interface. The <110> cross-sectional HRTEM image showed the atomic configuration of the projected interface superstructure, which had a periodicity of 23 Å along the interface. Such periodicity corresponds to 7 times the 1/3{224} periodicity which is 3.3 Å at the interface. The interface atomic model of the buried 7×7 superstructure was also proposed by using the image matching technique between the experimental and computer simulated images.

Line defects at the interfaces of epitaxial silicide layers grown at room temperature on Si(111) are found to correspond to steps on the original surface. This has enabled the examination, by transmission electron microscopy, of the topography of large areas of the Si surface after various treatments. Methods for removal of surface oxide and carbide are compared. Silicon molecular beam epitaxy (MBE) is shown to occur via step-flow mechanism at high temperatures, and through nucleation and growth of islands on terraces at low temperatures.

We have investigated the electronic and atomic properties of the interface between amorphous hydrogenated carbon (a-C:H) films and the semiconductor materials Si, Ge and GaAs with photoelectron spectroscopy, high resolution transmission electron microscopy and ion channeling technique. The different properties of the interfacial layers are summarized and compared to the adhesion quality of a-C:H films on these semiconductor materials.

The ultra-rapid melting and subsequent resolidification of Embedded Atom Method models of the fcc metals copper and gold are followed using a Non-Equilibrium Molecular Dynamics computer simulation method. Results for the resolidification of an exposed (100) face of copper at room temperature are in good agreement with recent experiments using a picosecond laser. At T = 0.5 Tm, the morphology of the solid/liquid interface is shown to be similar to a Lennard-Jones model. The morphology of the crystal-vapor interface at 92% of Tm shows a significant disordering of the topmost layers. Difficulties with the EAM model for gold are observed. Comparison of the Baskes et al. and Oh and Johnson embedding functions are discussed.

X-ray diffraction, inelastic light scattering, photoluminescence and photoluminescence excitation spectroscopies have been employed to study the effects of growth conditions on the interfacial sharpness of GaAs/AlAs superlattices. In addition to describing which changes in the MBE growth procedures result in the sharpest interfaces, this study addresses the relative strengths and weaknesses of the experimental probes employed in the examination of interfacial irregularities.

A transmission electron microscope weak beam imaging technique has been developed for the characterization of interfacial roughness in lattice strained (InGa)As/GaAs multiple layered structures. In this technique, the heterointerfaces of (100) type strained layers are imaged in an inclined projection with a g311 diffracted reflection at off-Bragg conditions which gives an enhanced contrast from variations in strained layer thickness. A calculation based on the kinematic theory of contrast was made in order to gain a better understanding of the contrast. The calculation suggests that the observed contrast is due to monolayer scale variations in thickness of the strained layers.

Using chemical lattice imaging in combination with vector pattern recognition, we obtain quantitative profiles of the chemical change across single interfaces with atomic plane resolution. We thus study interdiffusiuon across single GaAs/AlGaAs interfaces as a function of temperature, depth of interface beneath the surface, and doping. Since our technique is sensitive to interdiffusion coefficients as small as 10−20 cm2/s, we can study atomic level changes at a single interface at the low temperatures used for many device processing steps (∼700C). Our results show interdiffusion, and hence the layer stability depend not only on temperature and doping, but also on the distance of the interface from the surface. The implications of these results for the stability of multilayered structures are discussed.

Vibrational Raman scattering has been used in conjunction with luminescence, luminescence excitation spectroscopy, and x-ray diffraction to study the interfaces of GaAs/AlAs superlattices grown by MBE with and without growth interruptions at the interfaces. The confined LO phonon spectra clearly indicate that in the sample grown with growth interrupts at least one of the interfaces in each GaAs layer is truly smooth. In addition it is shown that the intensity of the interface phonons probed via Raman scattering is sensitive to the state of the interface.

A systematic study of the structure of tilt grain boundaries in aluminum has been initiated. High resolution transmission electron microscopy is being used to examine the interface structure of several bicrystals with <110> tilt axes. In this paper, we report the structure determination of a grain boundary close to the Σ9 (221) symmetric orientation. The grain boundary plane, which appears wavy at lower magnification, is actually composed of atomically flat microfacets. Two distinct, symmetric structures with (221) boundary planes have been identified within individual microfacets. These observations have been compared with structures calculated using the Embedded Atom Method. The semi-quantitative comparison between the observed and predicted grain boundary structures is accomplished using multislice image simulations based on the calculated structures. The results of these comparisons and the evaluation of the relative energies of the microfacets are discussed.

The study of boundaries in small particles is of great interest in areas such as catalysis, metallurgy and surface science. A particularly interesting case is that of metallic particles containing one or several metals. In the present work we present a number of examples of different mono and bimetallic systems.

The structure of inversion domain boundaries in β-SiC (i.e. boundaries between domains with inverted Si and C positions) is investigated by means of a Keating model. For the (110) boundary, the relaxation of the C-C and Si-Si bonds towards the ideal bond lengths which occur in the diamond structure can basically be achieved by a rotation of neighboring Si-C bonds. For the (001) boundary, it is achieved by varying the spacing between the domains. The electronic properties and total energy of formation of the relaxed (110) boundary are studied by means of linear muffin-tin orbital calculations. The interface localized states in the semiconducting gap are mainly due to the Si-Si bonds and lead to a semimetallic situation near the interface.

Nanophase silicon was investigated using atomistic simulation. The simulations employed a modified Stillinger-Weber potential appropriate for crystalline and amorphous silicon. Computer “samples” of nanophase material were formed by compressing together three grains of several hundred atoms each, using Fletcher-Powell minimization and external pressures of 0.5 to 13.5 GPa. Relative densities obtained in the samples ranged from 65% to 98% as compared to the perfect crystal. The nanophase materials maintained crystalline order up to the interfaces and no highly disordered interfaces were observed. Calculated bulk moduli exhibited a linear dependence with respect to density, with no significant dependence on structure. The calculated thermal expansion coefficients were up to twice as large as that in the perfect crystal and were structure dependent.

Recent reports have correlated the mechanical properties of multilayers with a structural expansion of the constituents. Whether this expansion is an interface or bulk effect remains in dispute. We apply the technique of glancing angle EXAFS to obtain information about the local structure and multilayer composition and use these results to shed light on certain aspects of this controversy. The experiments were performed on platinum-nickel multilayers of varying bilayer thickness. Measurements were made by fluorescence detection of EXAFS beyond both the Pt LIII and the Ni K edges. Considerable interlayer mixing is evident, as is the presence of order within the interface. We additionally find that the bulk metallic character is retained within the layers.

We present first-principles calculations of the densities of states (DOS's) of unrelaxed and relaxed twist and tilt grain boundaries (GB's) in Cu. The relaxed configurations were obtained through the use of the Embedded Atom Method (EAM), while the DOS's were calculated using the real-space multiplescattering theory (RSMST) approach recently introduced in the literature. The DOS's of GB's are compared against those of bulk materials as well as against one another. Although the RSMST calculations are still not self-consistent, these comparisons allow us to verify certain expected trends in the DOS's, and to verify the usefulness and reliability of our method.

Based an the local equilibrium among vacancies, solute atans and vacancy-solute atom complexes, a mechanism for solute segregation to grain boundaries is suggested for a alloy system with binding energy of complex S≫kT. A set of dynamic equations for grain boundary segregation is ived, which can describe both equilibrium and nonequilibrium segregations, as the effect of the equilibrium segregation is taken into account in the bonndary condition. Theoretical calculation is made by computer for boron segregation at austenite grain boundaries as functions of isothermal holding time, cooling rate and quenching teaperature, which agree well with experimental results.

The oxidation of iron-rich olivine to produce magnetite is a model system for the study of phase transitions involving mass transport. High-resolution lattice images of have been obtained from magnetite precipitates in naturally modified iron-rich olivines. The magnetite/olivine interface is shown to be extremely sharp. Steps and misfit dislocations are present at the interface.