Metallization of Silicon upon Potassium Adsorption

We report novel features of potassium deposition on a Si(111)-(2×1) surface as a function of coverage. The binding is ionic even at the saturation coverage without any overlayer metallization. Up to a threshold coverage, the alkali-metal electrons are donated to the empty surface state resulting in a 1D metallic chain. Above this coverage, the conduction-band states are occupied, so that the surface electrons become itinerant leading to the metallization of the substrate and onset of enhanced conductivity. © 1987 The American Physical Society

Adsorption site of alkali metal overlayers on Si(001) 2 × 1

The alkali metal semiconductor interfaces are currently being investigated by a variety of tools. Most studies to date at half a monolayer coverage have shown preference for either a quasi-hexagonal (H) site or a long-bridge (B) site. At this coverage one-dimensional chain structure for K on Si(001) 2 × 1 have now been confirmed by scanning tunneling microscopy (STM). The data, however, is consistent with either of the two sites. STM investigations at low coverages suggested that alkali metals like K and Cs occupy a novel site, Y, which is a bridge site between two Si atoms belonging to different dimers along the dimer row [110] direction. The total energy calculations for this new Y site, discovered by STM, have shown that it is indeed a site of (local) energy minimum. The ability of the surface silicon atoms, which are not adjacent to the alkali metal atom, to buckle makes the Y site a competitive adsorption site. We deduce the nature of bonding between alkali metals and Si using the STM data. It is concluded that the bond is substantially ionic in nature. © 1992

Surface metallization of silicon by potassium adsorption on Si(001)-(2×1)

We present the detailed results of self-consistent and geometry-optimized total-energy, band-structure, and charge-density calculations for a potassium-covered Si(001)-(2×1) surface, and for an unsupported potassium monolayer. We found that the (2×1) reconstruction and the dimer bonds of the Si surface continue to be stable after the adsorption of alkali-metal atoms. At the monolayer coverage the charge from the adsorbed potassium atoms is transferred into the empty, antibonding dangling-bond surface states, resulting in the metallization of the Si(001) substrate surface. The bonding between the overlayer and the substrate surface is ionic, and the Fermi level is pinned by the partially filled silicon surface states. Our theory for the metallization and the surface collective excitations is different from previous ones developed for an alkali-metal overlayer on a metal substrate which suggest that the system undergoes a Mott transition, and can successfully account for recent experimental observations. The presence of the active dangling-bond states prevents the alkali-metal monolayer from metallization, and thus provides the crucial difference between metal and semiconductor substrates. © 1988 The American Physical Society

Long-range order and segregation in semiconductor superlattices

Results of self-consistent energy-minimization calculations provide strong evidence that the ordered phases in epitaxially grown Ga1-xAlxAs and strained Si1-xGex alloys are metastable, in the sense that segregation into constituents is favored. We show that the long-range order in intermediate metastable structures leads to significant changes in the electronic properties of semiconductor superlattices. Segregation gives rise to micro-quantum-wells with staggered band lineup and multiple confined states in the potential barrier. © 1987 The American Physical Society

Scanning-tunneling microscopy at small tip-to-surface distances

The scanning-tunneling microscopy (STM) of graphite at small tip-to-surface distances is investigated using the self-consistent-field pseudopotential method. We have calculated potential, charge density in the region between the tip and surface, and the force corrugation. Our results reveal that the tip at the close proximity to the surface disturbs the states near the Fermi level, and induces localized states. The STM images, which are usually related to the local density of states at the Fermi level of the clean surface, are affected by these localized states. The tunneling barrier is shown to collapse at small distances and a new mechanism for current is postulated. Some experimental evidence for this effect is presented. © 1987 The American Physical Society

Quantum effects in electrical and thermal transport through nanowires

Nanowires, point contacts and metallic single-wall carbon nanotubes are one-dimensional nanostructures which display important size-dependent quantum effects. Quantization due to the transverse confinement and resultant finite level spacing of electronic and phononic states are responsible for some novel effects. Many studies have revealed fundamental and technologically important properties, which are being explored for fabricating future nanodevices. Various simulation studies based on the classical molecular dynamics method and combined force and current measurements have shown the relationship between atomic structure and transport properties. The atomic, electronic and transport properties of these nanostructures have been an area of active research. This brief review presents some quantum effects in the electronic and phononic transport through nanowires

Effect of tip profile on atomic-force microscope images: A model study

Adopting the empirical silicon interatomic potential of Stillinger and Weber, we investigate the effect of the tip profile on the atomic-force microscope images for a prototype system, Si(001)-(2×1), and conclude that the tip profile has a profound effect on the observations. We also study relaxation of the surface under the influence of the tip using a many-body energy minimization procedure and find that the force exerted by the tip should be less than 10-9 N for the atomic-force microscope to be a nondestructive tool. © 1988 The American Physical Society

Molecular-dynamics study of self-interstitials in silicon

Results of a molecular-dynamics computer simulation are presented for atomic relaxations and relaxation energies for self-interstitials in a silicon crystal. The Stillinger-Weber model potential containing two- and three-body terms is used and is expected to be more realistic than a simple Keating potential. The host crystal is represented by a cluster of 800 atoms, and the additional silicon atom was embedded in various interstitial sites near the center. The whole assembly was then periodically continued to fill the entire space. It is found that significant atomic relaxations occur in a shell of a radius 11 a.u. and decay exponentially. In fact the relaxation is oscillatory in nature and also nonuniform within some shells. The calculated formation energies of vacancy and self-interstitials at equilibrium show trends which are in agreement with the self-consistent field total-energy calculations. These energy values are also in agreement with the known self-diffusion activation energy. From calculated formation energy values, we are able to draw the conclusion that the tetrahedral-site interstitial can be most readily formed. The hexagonal-site interstitial, on the other hand, is most repulsive. The migration from tetrahedral to dumbbell interstitial site appears to be most favorable. © 1987 The American Physical Society

Theory of Schottky barrier and metallization

The formation of the rectifying Schottky barrier on metal-semiconductor interfaces is one of the longest standing problems of solid-state physics. We present a review of the models and theories for Schottky barrier. Two important examples of metal-semiconductor interfaces, namely those containing simple and alkali metals, are analyzed in order to evaluate these models and theories in the light of ab-initio calculations. © 1991

Absence of metallicity in Cs-GaAs(110): A Hubbard-model study

Using an approximate solution of the Hubbard-model Hamiltonian, we are able to establish that the Cs-GaAs(110) system becomes a Mott insulator at submonolayer Cs coverages. We also provide a consistent interpretation of electron-energy-loss and scanning-tunneling-spectroscopies data. The correlation effects are important for this system with an estimated correlation energy of 0.4 eV. © 1993 The American Physical Society