The chemisorption of ethylene (CzH4) on SiWOlXZ X 1) at 300 K has been studied by scanning tunneling microscopy @TM) and spectroscopy @I’S). l%posure of the surface to C,H, does not cause large scale rearrangement of the original Si surface atoms. The adsorption of individual molecules and changes in the local structure can, however, be observed. At low coverage, the &II, molecules prefer to adsorb on alternate dimer sites creating either a local (2 x 2) or d2 x 4) structure. The individual domains are relatively small (< 50 ii> and the change in reconstruction cannot be detected by any diffraction technique. STS recorded from regions of the surface covered by adsorbates show the removal of the relatively weak ?r-bo& formed by the dangling bonds and the participation of the dangling bond electrons in stronger sp3-like bonds to the carbon atoms of the molecule after chelation. The (2 X 1) dimer surface characteristic of clean Si@Ol) is restored upon heating the adsorbate covered surface. The pictures are consistent with a model in which the desorbed C2H4 molecules leave restored dimers behind and the molecules which are left remain undissociated on the same type of dimer sites as before.

High resolution electron microscopy (HREM) has been used to study misfit dislocations of ZnSe films grown on vicinal Si(001) substrates tilted 4° towards the [11̄0] axis. In images taken with the electron beam parallel to the [11̄0] direction, 60° dislocations were found to predominate whereas mostly Lomer dislocations or closely spaced 60° dislocations (separated by <2 nm) were observed in images taken in the orthogonal direction. A model is presented here to explain the formation of the asymmetric dislocation structure on the basis of mechanisms for propagation and formation of misfit dislocations.

We have studied the mechanisms that determine the density and structure of threading dislocations (TDs) in GaAs on Si by growing GaAs films on continuous Si substrates and on 10–34-μm-wide Si mesas that provided free-sidewall growth. The effects of a soft ZnSe interlayer and of postgrowth annealing to 850 °C were also investigated. TD densities were accurately determined using large area plan-view transmission electron microscopy. Burgers vector analysis of the TDs showed that threading segments associated with both sessile 90° misfit dislocations and glissile 60° misfit dislocations were present after growth. A difference in dislocation structure between the annealed and unannealed samples was observed. It was also found that the dislocations were unaffected by proximity to free sidewalls and by the ZnSe interlayer. The results indicate that dislocation interactions during the early stages of growth determine the structure and density of TDs in as-grown films. It was also concluded that plastic relaxation of thermal mismatch strain during cooldown from the growth temperature does not strongly affect the TD density in the films. This is in contradiction to previous studies.

Significant atomic rearrangement at the interface was found to take place after post-growth annealing treatments of epitaxial ZnSe on As-passivated Si(001) substrates which were tilted by 4 degrees towards the [1 $($) over bar$$ 10] direction. The thermal stability of the ZnSe/As:Si interface was studied by rapid thermal annealing at temperatures up to 960 degrees C after growing an epitaxial GaAs cap layer to prevent evaporation of the ZnSe during the anneals. The ZnSe/As:Si interface was examined by high-resolution electron microscopy. After an anneal at 900 degrees C the ZnSe/As:Si interface transformed from an atomically smooth interface found in the as-grown films to a facetted structure with {111}-oriented sidewalls that extended preferentially in the [1 $($) over bar$$ 10] direction. The 60 degrees dislocations that were previously observed along this direction combined into closely spaced pairs or into Lomer dislocations which were associated with the facets. We present a model for the atomic structure of the facetted interface which is consistent with the experimental data and satisfies electron-counting considerations. Total-energy calculations of the ZnSe/As:Si(001) interface were compared with those for the {111} interfaces seen after facetting.

We have investigated threading dislocation (TD) removal from GaAs films on Si by introduction of additional InGaAs graded strain layers in combination with growth on patterned substrates. The substrate patterns consisted of mesas with 10–34 μm widths. The mesa sidewalls were either overhanging (concave), leading to free sidewalls for the film on the mesas, or outward sloping (convex) sidewalls with {111} orientation. The dislocation structure was studied using transmission electron microscopy. It was found that the graded strained layers led to a reduction of dislocation density by a factor of ∼5 in films grown both on mesas with concave sidewalls and on unpatterned substrates. This reduction was due to dislocation reactions leading to annihilation of TDs. For films with graded strained layers on mesas with convex sidewalls, an additional factor of ∼3 reduction in TD density was observed in the part of the film that was grown on top of the mesas. In this case all mobile TDs (TDs associated with 60° misfit dislocations, i.e., TDs that could glide to relieve misfit stress) were removed from the film on top of the mesas to the regions above the sidewalls and only TDs associated with 90° misfit dislocations remained. We suggest that this is due to pinning of the TDs associated with 60° misfit dislocations at the mesa edges and we have presented an explanation for this pinning in terms of the stress conditions at the {111} oriented mesa edges. In addition, this leads us to suggest that in order to obtain minimum TD density it is imperative to prevent formation of 90° misfit dislocation during lattice mismatched heteroepitaxial growth.

Aside from atomic topography, three factors influence an STM image: the local density of states, the local work function or potential at an atom, and the shape of the tunnel barrier in the tip-surface gap. We present atomically resolved images of STM barrier height above the Si(113) surface. These show that measurement of the potential above individual atoms is possible, but complicated by topography in practice.

We have studied the possibilities of reducing the threading dislocation (TD) density in GaAs on Si using free side wall growth on patterned Si substrates and/or using a soft ZnSe interlayer in combination with post growth annealing procedures. TD densities were accurately determined using large area plan view TEM and were found to be unaffected by proximity to free side walls and by the ZnSe interlayer. Post growth heat treatments led to a factor of ~2 reduction in TD density and to bunching of dislocations throughout the thickness of the film.

The chemisorption of ethylene (C2H4) on Si(001)(2 × 1) at 300 K has been studied by scanning tunneling microscopy (STM) and spectroscopy (STS). Exposure of the surface to C2H4 does not cause large scale rearrangement of the original Si surface atoms. The adsorption of individual molecules and changes in the local structure can, however, be observed. At low coverage, the C2H4 molecules prefer to adsorb on alternate dimer sites creating either a local (2 × 2) or c(2 × 4) structure. The individual domains are relatively small (<50 Å) and the change in reconstruction cannot be detected by any diffraction technique. STS recorded from regions of the surface covered by adsorbates show the removal of the relatively weak π-bond formed by the dangling bonds and the participation of the dangling bond electrons in stronger sp3-like bonds to the carbon atoms of the molecule after chemisorption. The (2 × 1) dimer surface characteristic of clean Si(001) is restored upon heating the adsorbate covered surface. The pictures are consistent with a model in which the desorbed C2H4 molecules leave restored dimers behind and the molecules which are left remain undissociated on the same type of dimer sites as before.

Scanning tunnelling microscopy (STM) has been used to study Stranski-Krastanow growth (two-dimensional layer + three-dimensional island growth) of evaporated Ge on Si(113) and Si(100) substrates. The transition from purely two-dimensional layer growth to the formation of three-dimensional inelastically relaxed islands was found to proceed through a number of intermediate stages of growth. We have interpreted the growth morphology of the different stages in terms of partial elastic relaxation of the overlayer strain. The stabilizing effect of the relaxation is correlated to the stress tensor of the reconstructed surface.

The authors present some observations of Si(001)-(2*1) and GaAs(001)-(2*4) surfaces. They show what is seen when the surface is exposed to ethyne and to ethene and provide a foundation for investigating adsorbates on semiconductor surfaces.

The scanning tunnelling microscope (STM) has been used to study two quite different systems. First, the unsaturated hydrocarbons ethene and ethyne, chemisorbed on a silicon substrate, and secondly, a monolayer of 4,4'-bipyridyl disulfide (PySSPy), chemisorbed on a gold single-crystal sphere, in pure water. The images of the hydrocarbons on silicon showed, at low coverages, random clusters of adsorbates, but at higher coverages, approaching saturation the molecules prefer to be adsorbed on alternate dimer sites. The in situ STM study of the PySSPy on gold, revealed that the monolayer exhibits a marked molecular alignment with specific sites on the gold surface. The periodic unidirectional array showed a separation of 46+/-3 angstrom with a corrugation of ca. 1.5 angstrom.

There is considerable interest in imaging of molecular species with the STM, due to the potentially very high resolution available. Experience over the last few years suggests, however, that reliable and easily interpretable images are difficult to obtain. In particular, substrate condition, conduction mechanisms of the molecular species, and molecule-surface or tip-molecule interactions can give problems. In this paper we discuss how uncertainties of these types may be minimised, and hence what kinds of molecules and substrates may be used to obtain repeatable and reliable images.

We present simulations of voltage-dependent STM images for the Si(113)3x2 surface using a simple tight-binding description of surface electronic structure. Quantitative agreement with experiment is obtained. Our approach is particularly well suited for complex structures which present difficulties for more exact calculations. The local barrier for tunnelling electrons is shown to have an important effect on the interpretation of STM images.

Scanning tunnelling microscopy (STM) has been used to obtain images, in ultra-high vacuum (UHV), of molecules of eta 5-cyclopentadienyl-bis triphenylphosphine ruthenium 4-pentaoxybenzonitrile hexafluorophosphate (Ru* complex) and a monolayer of polymerized polydiacetylene (PDA) deposited on Au substrates in the form of Langmuir-Blodgett films. Tunnelling spectroscopy has been used to detect the electronic structure on the surface due to Ru* complex molecules. Ultraviolet photoelectron spectroscopy (UPS) was used to confirm this electronic structure.

The Si(113) surface is very stable despite its high index but until now its atomic structure has been uncertain. Using a scanning tunneling microscope, we have obtained images of both empty and filled states which provide strong evidence for a particular structural model with a 3 x 2 unit cell. We explain our results in terms of a general rehybridization principle which accounts for the low surface energy and the spatial distribution of empty and filled states. Our images reveal a high density of domain boundaries which introduce energy states that pin the Fermi level and explain earlier reports of a 3 x 1 reconstruction.

The (113) face of Si is highly stable due to rehybridization of dangling bonds. The authors show that adsorbed atomic hydrogen upsets this stability. Structural models are provided for the resultant mixed 2*2 and 2*3 surface.

Different Si homojunction and strained Si1-xGex/Si heterojunction diodes and bipolar transistors have been fabricated by Si-MBE. The effect of annealing on Si homojunction diodes and transistors are studied. It is found that annealing generally improves the Si device performance, such as the ideality factor and breakdown characteristics. The influence of60Co γ irradiation on the Si1-xGex/Si diode performances are investigated by studying the temperature dependence of their electrical characteristics, and the results are correlated with the quality of the MBE-films. γ irradiation causes a drop in material conductivity due to the generation of atom-displacement defects in the whole volume of the wafers and increases the defect density at hetero-interfaces. The forward I-V curves of Si1-xGex/Si devices may shift towards lower or higher voltages, depending on the film quality and the irradiation dose. The increase of defect density in strained Si1-xGex/Si films appears to occur easier for the films with lower quality. Electrical measurements and calculations show that the defect-associated tunneling process is important in current transport for these MBE grown Si homojunction and strained Si1-xGex/Si heterojunction devices, which have initially medium film quality or have been treated by irradiation.

The structure and surface morphology of In overlayers on Si(100) surfaces were investigated as a function of substrate temperature and surface coverage using low-energy and reflection high-energy electron diffraction as well as Auger electron spectroscopy. Desorption kinetics of adsorbed In was studied with modulated-beam desorption and temperature-programmed desorption spectroscopies. Indium was found to grow on Si(100) according to a Stranski-Krastanov mechanism with the initial formation of several two-dimensional phases preceding the nucleation and growth of three-dimensional In islands. Binding energies and frequency factors were extracted from the desorption measurements using a model based on first-order desorption from several interdependent surface phases. First-order and zeroth-order kinetics were observed for the total desorbing flux from coexisting surface phases.

A single-grid ultra-high-vacuum-compatible ion source was used to provide accelerated In+-dopant beams during Si(100) growth by molecular-beam epitaxy. Indium incorporation probabilities σ, determined by secondary ion mass spectrometry, in films grown at Ts=800 °C were too low to be measured for thermal In (σIn was <3×10−5 at Ts>550 °C) . However, for accelerated In+ doping, σIn+ at 800 °C ranged from 0.03 to ∼1 for In+ acceleration energies EIn+ between 50 and 400 eV. Temperature-dependent Hall-effect and resistivity measurements were carried out on In+-doped Si films grown at Ts =800 °C with EIn+=200 eV . Indium was incorporated substitutionally into electrically active sites over a concentration ranging from 2×1015−2×1018 cm−3, which extends well above reported equilibrium solid-solubility limits. The acceptor-level ionization energy was 156 meV, consistent with previously published results for In-doped bulk Si. Room-temperature hole mobilities μ were in good agreement with the best reported data for B-doped bulk Si and were higher than previously reported values for annealed In-implanted Si. Temperature-dependent (77–400 K) mobilities μ(T) were well described by theoretical calculations, with no adjustable parameters, including lattice, ionized-impurity, neutral-impurity, and hole-hole scattering. Lattice scattering was found to dominate, although ionized-impurity scattering was still significant, at temperatures above ∼150 K where μ varied approximately as T−2.2 . Neutral-impurity scattering dominated at lower temperatures. Plan-view and cross-sectional transmission electron microscopy observations showed no indications of dislocations or other extended defects. Considering the entire set of results, there was no evidence of residual ion-bombardment-induced lattice damage.