The first-principles electronic state calculations based on (local) density functional approach has been established for a powerful practical method which can describe and predict realistic systems. In this introductory article for the special issue, some general concepts and methods for practical calculation, as well as the first-principles molecular dynamic method are explained.

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Density Functional Calculations which have been performed in order to reveal mechanisms of semiconductor epitaxial growth are reviewed. Two topics are discussed: First, appearance of {311} facets during the epitaxial growth on Si (100) surfaces; secondly precipitation of group V atoms during surfactant-mediated epitaxial growth of Ge on Si(100). In the first topic it is shown that step bunching inherent to Si(100) leads naturally to the {311} facet. In the second it is emphasized that the first adsorption geometry is crucial to the precipitation. Capability and possibility of first-principles calculations are emphasized.

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Photo-induced desorption of molecules from metal surfaces is discussed. First, we present potential energy surfaces as a function of the distance between the adsorbate and the surface for CO and NO on Pt, which have been calculated using three-atom clusters to model the chemisorption systems, and discuss the desorption processes in terms of the Menzel-Gomer-Redhead (MGR) model. In the MGR model, the initial excitation by photons is assumed to be localized around the adsorbate, but a calculation with an extended slab to represent CO/Pt(111) suggests that states derived from the adsorbates are extended over the substrate, implying that this assumption is not always valid. A desorption mechanism via the nonadiabatic coupling is then illustrated that describes desorption processes in which the initial excitation is extended over the substrate. Finally, several important issues remaining to be solved are presented.

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GaAs(001) surfaces, on which epitaxial growth is most commonly performed, contain, in general, either missing dimer rows or excess As ad-dimers, and As coverage is not equal to 1.00. We investigated theoretically how the layer-bylayer growth is maintained on GaAs(001) surfaces by controlling the excess and deficiency of surface As atoms. Our calculations indicate that pre-adsorbed Ga atoms act as “self-surfactant atoms”; and induce the As rearrangement on the surface during GaAs epitaxial growth. We show that this effect originates from the energy stabilization due to the charge transfer on the surfaces.

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Scanning tunneling microscopy (STM) has made it possible not only to observe atomic structures and electronic states of surfaces with atomic resolutions but also to modify surface structures in atomic scale. To investigate atomic processes and electronic processes induced by the STM tip, a first-principles recursion-transfer matrix (RTM) method has been developed. In this article the RTM method is introduced, and examples of application of the method to atom manipulation and atomic contact are presented. Adiabatic potential surfaces of atom extracted from a silicon surface to an aluminum tip under applied bias voltage are calculated, and decrease of activation barrier is discussed in terms of the changes of electron density distributions around the atom. Current density and tunneling barrier distributions are also clarified in detail, and the formation of atomic contact with approach of a tip is described.

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Several kinds of super structure have been recently observed by LEED measurements in alkali-atom adsorbed metal surfaces and attributed to substitutional adsorption in addition to simple adatom one. A first-principles study on the stable structure and its stability of such alkali-adsorbed systems, Cu(001)-Li and Al(001)-Na, is reviewed. Assuming the alkali-atom coverage of 0.5 and considering simple-adatom type c(2×2) and substitutional (2×1) and c(2×2) geometry, the most stable structure of Cu(001)-Li and Al(001)-Na is found to be substitutional (2×1) and c(2×2), respectively, being consistent with experiments. Mechanism of the stability is interpreted in terms of surface-vacancy formation and chemical bond formation. In any cases, the energy difference between these adsorbed systems is quite small (the order of 0.1 eV per adatom or less) and a complicated phase diagram may be possibly expected.

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Oxidation of the Si(001) surface, one of the most important processes in the modern Si technology, was studied by using the first-principles calculation technique with spin-polarized gradient approximation. The inclusion of spin degrees of freedom greatly suppresses the reactivity of an O₂ molecule. As a result, at most sites, incoming O₂ molecules do not show spontaneous dissociative chemisorption. The activation energy needed for direct oxidation increases from ca 1.0eV to ca 2.5eV, as the chemisorbed site becomes deeper from the Si surface. We found, however, there exist narrow channels, along which O₂ molecules are chemisorbed onto the Si surface without extra energy. We have also clarified the apparent barrierless reaction mechanism of the backbond oxidation through diffusion of O atoms from the metastable site on the surface towards the backbond site.

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The adsorption and desorption behavior of NO on polycrystalline Rh was investigated by X-ray photoelectron spectroscopy (XPS) to elucidate the unique catalytic properties of Rh in the reduction of NO. After the adsorption of NO at room temperature, nitride and oxide which were the dissociatively adsorbed species of NO together with molecularly adsorbed NO were observed. A part of the nitride was comparatively easily desorbed by increasing NO pressure or by rising temperature. However, the oxide was not desorbed but diffused into the bulk of Rh with rising temperature. These XPS results implied the superiority of Rh for NO reduction in reducing atmospheres in spite of relatively low activity for direct decomposition of NO.

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The amounts of adsorbed oxygen on clean (0001) surface of natural graphite at 23°C has been determined by using the Knudsen flow capillary method. Graphite samples were the flattened (0001) single crystals (135 pieces, total weight: 0.146g, total geometrical surface area: 22.2cm² ) occur in South Africa and the scaly single crystals (36, 326 pieces, total weight 0.35g, total geometrical surface area 219cm²) occur in Guatemala. The results obtained indicate that the saturated quantities of adsorbates are 1.7×10¹⁵ (molecules/cm²) for the South Africa-graphite and 9×10¹⁴ (molecules/cm²) for the Guatemala-graphite, respectively. These data suggest that oxygen molecules are adsorbed on the basal plane of graphite surface.

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Based on the introductions on the surface superstructures and surface electrical conductions, described in the previous issue, the present paper shows our main results on the electrical conduction through surface-state bands that are inherent in the superstructures. The references and figures are cited with serial numbers followed from those in the previous article.

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A microscopic understanding of epitaxial growth is important in order to obtain atomically controlled heterostructures of semiconductors. A quantum chemical approach is useful in the investigation of elementary chemical reaction processes. We have analyzed gas-phase reactions in the metalorganic chemical vapor deposition (MOCVD) of compound semiconductors, using ab initio molecular orbital calculations. In this review, the reactions of group-III source molecules and hydrogen (both atomic and molecular) are discussed. The calculated results indicate that hydrogen plays an important role in controlling the quality of the epitaxial layers. It is well known that semiconductor surfaces exhibit many kinds of reconstruction depending on the growth conditions. The electronic state and the detailed structure of the reconstructed surfaces should be taken into account in order to understand the surface reaction mechanisms. A surface As dimer which appears in the molecular beam epitaxy (MBE) of GaAs (001) is analyzed based on a cluster model. The first quantum chemical study of GaAs (001)-(2×4)β₁ reconstruction is also described.

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