Missing Dimer Row Research Articles

Self-organized Bi lines on the Si(001) surface: A theoretical study

We have performed an ab initio theoretical study of the stability, atomic geometry, and electronic structure of the self-organized Bi lines on the Si(001) surface. We have examined the two currently proposed models and two new hybrid models for the structure of Bi lines. Our results confirm the model proposed by Miki et al., in which the Bi lines are formed by Bi dimers parallel to the surrounding Si dimers, with a missing dimer row between the Bi dimers. However, in contrast to the proposal of symmetrically disposed surface Si dimers (i.e., with no buckling) by Miki et al., our total-energy calculations indicate that the buckling of the Si dimers is an exothermic process, reducing the surface total energy by 0.11 eV/dimer. Our theoretically simulated scanning tunneling microscopy results suggest a low density of states close to the valence-band maximum, localized on the Bi lines, supporting the recently proposed model of quantum antiwire systems for Bi lines on the Si(001) surface.

Ab initio study of the self-organised Bi-lines on the Si( [formula omitted]) surface

We have perfomed an ab initio theoretical study of the stability, atomic geometry and electronic structure of the self-organised Bi-lines on the Si(0 0 1) surface. Our results show that the Bi-lines are formed by Bi-dimers parallel to the surrounding Si-dimers, with a missing dimer row between the Bi-dimers. In contrast to a recently proposed model of symmetrically disposed surface Si-dimers (i.e. with no buckling), our total energy calculations indicate that the buckling of the Si-dimers is an exothermic process, reducing the surface total energy by 0.11 eV/dimer. Our theoretically simulated STM results suggest a low density of states close to the valence band maximum, localized on the Bi-lines, supporting a recently proposed model of quantum antiwire systems for Bi-lines on the Si(0 0 1) surface.

Dynamics of Ga Adatoms on GaAs(001) Surface Observed by in-situ Scanning Tunneling Microscopy.

Dynamical observation of Ga adatoms on GaAs(001) surfaces was successfully made by a system using scanning tunneling microscopy(STM) and molecular beam epitaxy(MBE), which were not separated into each chamber but equipped as an incorporate unit. It is found that Ga adatoms were self-organized to a unit cell far from the B-step edge and on the missing dimer row of (2×4) β 2 As-adsorbed surface. Moreover, the three Ga adatoms formed a trimer cluster which changed to a tetramer cluster with addition of one Ga atom. Ga heptamers were also observed. The dangling bonds of a Ga dimer were completely empty, satisfying electron counting heuristics. No clear observation by the filled state imaging indicates that the adatoms are crystalline. However, the Ga trimer clearly observed is metal cluster. The Ga trimer and heptamer have 9 and 21 electrons in the molecular orbital states. The numbers are closed to the magic numbers of spherical jellium, 8 and 20, respectively. This excess electron might become the support between the cluster and the substrate surface, forming 2D magic clusters, the Ga trimer and heptamer, on GaAs(001) surface.

Ordering dynamics of anisotropic missing dimer clusters on Si(001) surfaces

Two types of anisotropic missing dimer cluster have been observed on (2×1) Si(001) surfaces: a missing dimer line parallel to the dimer direction and a missing dimer row parallel to the dimer row. The appearance condition of each type of cluster and their formation dynamics were studied using kinetic Monte Carlo simulations of the lattice gas model of missing dimers with interaction calculated by the Stillinger–Weber potential. A missing dimer row cluster exerts on a single missing dimer in the same dimer row an attractive interaction in the non-rebonded side and a repulsive interaction in the rebonded side. The dominant types of cluster formed by annealing of random missing dimers changes with increasing missing dimer concentrations from missing dimer line clusters to missing dimer row clusters. As the annealing temperature increases the average size of missing dimer line clusters reduces, while that of missing dimer row clusters increases.

Stability of anisotropic missing dimer clusters on Si(001) surfaces

Two kinds of anisotropic structures of missing dimer clusters have been observed on (2×1) Si(001) surfaces: one is elongated along the dimer direction (missing dimer line) and the other along the dimer row direction (missing dimer row). The cohesive energy of missing dimer clusters was calculated as a function of the number, m, of missing dimers in a cluster, using Stillinger–Weber inter-atomic potential. The cohesive energy became positive in the range of m≥2 for missing dimer line (MDL) clusters, while that of missing dimer row (MDR) clusters became positive in the range of m≥3 but exceeded the cohesive energy of MDL clusters. MDL clusters are stabilized by alignment along the dimer direction of rebonded bonds, and the cohesive energy increases linearly with m. MDR clusters are stabilized by formation of alternate rebonded bonds along the dimer row direction, relaxing the strain energy caused by rebonded bonds. The cohesive energy of MDR clusters increases oscillating with m and the cohesive energy of a cluster in a size of 2 n became smaller than that of (2 n−1). The growth of odd-numbered MDR clusters is endothermic but the growth of even-numbered clusters is exothermic.

Oxygen-induced missing dimer row formation on the Ge(100) surface

The initial stage of O 2 etching of the Ge(100) surface was studied by scanning tunneling microscopy. The etched surface showed two types of missing dimer rows with widths of half a dimer (S) and one dimer (2S). Two different internal structures were identified for the 2S missing dimer rows. The migration of adsorbed oxygen plays a major role in the formation of the 2S missing dimer row, whereas the S missing dimer row is formed by a 2S missing dimer row splitting under the anisotropic stress on the surface dimers.

Theoretical Investigations of Adsorption Behavior on GaAs(001) Surfaces

Adsorption behavior on GaAs(001)-(2×4)β2 and -c(4×4) surfaces is systematically investigated by the calculation of migration potentials for Ga adatoms and Monte Carlo simulation. In the calculation procedure, we use an energy formalism based on the empirical interatomic potentials and the electron counting model in order to incorporate the strain and electronic energy contributions. The calculated migration potentials for Ga adatoms imply that Ga adatoms preferentially reside in missing dimer sites on both (2×4)β2 and c(4×4) surfaces. On the (2×4)β2 surface, lattice sites in the missing dimer row near As-dimer kinks and B-type step edges are stable for Ga adatoms, whereas no preferential adsorption site is found near A-type step edges. Opposite qualitative trends are found in the migration potentials near step edges on the c(4×4) surface. The calculated results are consistent with experimental results and are discussed in terms of atomic configurations and the number of electrons remaining in Ga dangling bonds. Based on the energy formalism, an electron counting Monte Carlo (ECMC) simulation was performed to investigate the adsorption or desorption sequences on GaAs(001)-(2×4)β2 and -c(4×4) surfaces in MBE growth. The results imply that Ga adatoms impinging on the GaAs(001) surfaces play an important role in the incorporation or desorption of As to restore the electron counting model.

First principles study of arsenic incorporation on a GaAs(001) surface during MBE growth

Arsenic atom incorporation is a crucial process in epitaxial growth processes. The problem of how surface missing dimer rows on GaAs(001) surfaces are occupied by As atoms, is one of the most fundamental problems in GaAs MBE processes. This paper theoretically investigates As incorporation into the missing dimer row on a reconstructed GaAs(001)-(2 × 4) surface. To examine the coverage dependence of As incorporation, we estimated the As 2 adsorption energy at several Ga coverages by the ab initio calculation. The calculated results show that As adsorption energy drastically increases as the Ga coverage increases. We conclude from these results that missing dimer rows are occupied by As atoms after Ga atoms have been adsorbed on the surface. We also studied the structural change from a (2 × 4) β2 structure with two As dimers and found that a (2 × 4) β2 structure changed to a (2 × 4) β1 structure via a (2 × 4) α structure.

A Monte Carlo simulation study on the structural change of the GaAs(001) surface during MBE growth

A newly developed Monte Carlo (MC) simulation based on the electron counting model, called electron counting Monte Carlo (ECMC) simulation, is used to investigate the structural change of the As-stabilized GaAs(001) surface during MBE growth. The calculated results imply that the GaAs(001) surface changes its structure from initial (2 × 4) β2 to (2 × 4) α with an increase in Ga adatoms and show that subsequent As adsorption produces the (2 × 4) β1 structure. Ga adatoms on the GaAs(001)-(2 × 4) β1 structure occupy the lattice sites on the As-dimers at the low Ga adatom coverage of θ Ga ≤ 2 16 . As the coverage increases in the range of θ Ga ≥ 3 16 , Ga adatoms tend to reside in the lattice sites in the missing dimer region. These results are qualitatively consistent with those obtained by ab initio calculations. Further As incorporation onto the missing dimer row is also simulated by considering the dependence of As adsorption energy on Ga adatom coverage.

Scanning Tunneling Microscope Study of (001)InP Surface Prepared by Gas Source Molecular Beam Epitaxy

(001)InP surface prepared by gas source molecular beam epitaxy (GSMBE) using tertiarybutylphosphine (TBP) as P source was studied by ultrahigh-vacuum scanning tunneling microscope (UHV-STM), reflection high-energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS). It was found that the P-stabilized (2×4) reconstructed (001)InP surfaces can be obtained by annealing the (2×1) reconstructed surface which contains excess phosphorus. (2×4) reconstructed surfaces were found to possess the missing-dimer arrangements with at least two microscopic structures, α phase with two missing-dimer rows and β phase with one missing-dimer row, depending on the annealing temperature of the (2×1) surface. The (2×4)α phase is more stable with a wider range of annealing temperature, whereas the (2×4)β phase was obtained by annealing only near 330°C. The (2×4)α phase was observed even on a partially oxidized surface having many oxide patches, showing its remarkable stability.

Coverage dependence of migration potential of cation adatoms on GaAs(001)-(2 × 4) surface

We theoretically investigate the migration potential of cation adatoms on the reconstructed As-rich GaAs (001)-(2 × 4) surface by ab initio calculation. By increasing the number of cation adatoms, we also study how migration potential depends on coverage. The calculated results for Ga adatoms suggest that the long-bridge sites are energetically the most favorable at the initial stage of crystal growth. However, as the Ga coverage increases, the missing dimer row sites become the most favorable. Migration potentials strongly depend on the adatom coverage. Similar results were obtained for A1 adatom migration. Furthermore, based on these migration potentials, we demonstrate the dynamical behavior of cation adatoms on GaAs(001) surface by performing Monte-Carlo simulations at finite temperatures.

Cs adsorption on n- and p-type GaAs(001)(2 × 4) surfaces

A study of room temperature Cs adsorption on both n- and p-type GaAs(001)(2 × 4) surfaces is presented. We find that initially the work function decreases linearly with Cs exposure saturating 3 eV below the clean surface value. Unlike GaAs(110) which forms an ordered overlayer, Cs deposition on (001) is amorphous. Adsorption steadily degrades the initial c(2 × 8) LEED pattern, and no additional superstructure is observed. Preliminary STM images obtained at low coverage indicate that at room temperature the Cs does not adsorb at any particular atomic site. Cs atoms adsorb both on top of the arsenic dimers and above the Ga in the missing dimer row. Tunneling spectroscopic measurements obtained at a range of coverages show that the surface remains semiconducting up to saturation coverage.

Observation of GaAs (001) surfaces at high temperatures by scanning tunneling microscopy

The structure of a GaAs (001) surface is studied by scanning tunneling microscopy at high sample temperatures up to 500°C. Below 460°C, a stable missing dimer row structure is clearly observed. At 480°C, however, the observed structures become disordered and change shape from image to image. This indicates that the thermal motion causes the migration of surface atoms as well as the desorption from the surface at this sample temperature.