Dimer Vacancy Lines Research Articles

Self-limited SiH 2Cl 2 gas source molecular beam epitaxy on Si(1 0 0)

The interaction of dichloromonosilane (SiH 2Cl 2) with Si(1 0 0) -(2×1) was investigated with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy under ultrahigh vacuum conditions. Photoelectron spectroscopy measurements after saturation exposure at temperatures between 300 and 1070 K show only monochlorides on the surface, with the Cl coverage passing through a maximum with increasing adsorption temperature at a given SiH 2Cl 2 flux. STM shows that at least one pathway for decomposition of SiH 2Cl 2 proceeds via the formation of silylene groups. At an adsorption temperature of 690 K the molecular fragments can completely decompose into their atomic constituents. H ad and Cl ad adsorbates are mobile on the surface at 690 K and higher temperatures and form small islands. Si adatoms form epitaxial dimer strings without adsorbate termination. Preferential adsorbate-termination of substrate S B steps leads to an increased island density as compared to molecular beam epitaxy. Annealing saturated surfaces to 1000 K leads to c(4×4) structures and vacancy line defects. The latter consist mainly of single dimer vacancies. Mobility and lateral interaction of dimer vacancies leads to formation of the dimer vacancy lines which release the anisotropic surface stress. The metastable surface structures support a mechanism for desorption of SiCl 2 under formation of dimer vacancies.

Ordering of dimer vacancies on the Si(100) surface

We have investigated the ordering of dimer vacancies, in particular the formation of dimer-vacancy line defects, on the Si(100) surface using a tight-binding total-energy approach. We find that the dimer-vacancy line formed perpendicular to the direction of the surface dimer row is energetically favorable at low vacancy concentrations, whereas at higher vacancy concentrations the dimer-vacancy line is aligned parallel to the direction of the surface dimer row. The energetics and geometries of various dimer-vacancy configurations and the possible pathways to the line-defect formation through the diffusion of dimer vacancies are discussed. The calculated results are in good agreement with experiments and provide an explanation for the observed structural transition resulted from a temperature-driven random-ordered dimer-vacancy redistribution.

Self-organized growth of Fe nanowire array on H2O/Si(100)(2×n)

By evaporating Fe on to a water-terminated Si(100)(2×n) surface, we formed an Fe wire array reflecting the 2×n surface reconstruction. The average wire width was 2 nm and the period was 3 nm. The formation was caused by the deposited Fe atoms diffusing over the water-terminated flat area and being trapped at dimer vacancy lines. This array is applicable to magnetic devices.

Large area dimer vacancy array on the Si(100) surface studied by scanning tunneling microscope

Large area dimer vacancy arrays can be formed on the Si(100) surface when Si atoms are deposited on the Si(100)-2×1 surface followed by quenching from 1200°C. The dimer vacancy lines (DVLs) of the dimer vacancy array run perpendicular to the dimer rows and the basic building cells of the DVLs are: (i) a cluster of two missing dimers; (ii) a complex of one missing dimer and a cluster of two missing dimers. Small isolated islands with DVLs like that of the Si(100) surface are observed on the surface. The precursors of the islands are investigated. The formation mechanism of the island is that the interaction between the stress field of the dimers on the Si(100) surface and that of the island causes the dimer vacancies in the dimer rows of the island and the attraction between the dimer vacancies in adjacent dimer rows aligns the dimer vacancies.

Ge/Si(001)-2×N:Surface stress and interaction of dimer vacancy lines

We have studied surface-stress anisotropy as a function of the thickness of molecular-beam-epitaxy-grown Ge overlayers on the Si(001) substrate. The surface-stress anisotropy changes sign at a dimer-vacancy-line (DVL) periodicity of $\ensuremath{\sim}15.$ By varying the separation between two neighboring DVL's we have also investigated the DVL-DVL interaction. Based on a linear-dipole-force model for the step-step interaction and the theory of long-range elastic relaxation of orientationally inequivalent domains, we have estimated the DVL formation energy of $\ensuremath{-}0.003\mathrm{eV}/\AA{}.$

Optimum widths of dimer vacancy lines on Si(100)-(2×1)

We have used the ab initio multicenter molecular-dynamics method to calculate the surface energies of dimer vacancy lines (DVL's) formed on the Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) surface as a result of sputtering or etching. Our results show that the optimum widths of the DVL's perpendicular to the dimer rows are two and three missing dimers. Our results also show that at higher vacancy concentrations, DVL's parallel to the dimer rows (sometimes also known as ``vacancy islands'') with widths greater than three missing dimers are favored over the perpendicular DVL's. Both of these results are in agreement with experimental observations.

Growth process of Ge on Si(100)-(2×1)in atomic-layer epitaxy fromGe2H6

This study investigates the growth process of Ge on Si(100) during atomic-layer epitaxy (ALE) utilizing digermane. The surface ordering, morphology, and stoichiometry of the digermane saturated Si(100) surface at temperatures between 300 and 900 K, as well as the grown films are examined both by scanning tunneling microscopy (STM) and high-resolution core-level photoemission spectroscopy employing synchrotron radiation. An exposure of more than 15 L (1 L=${10}^{\ensuremath{-}6}$ T s) of digermane on the Si(100)-$(2\ifmmode\times\else\texttimes\fi{}1)$ surface at room-temperature results in a saturated and disordered surface. When the digermane saturated surface is heated to 725 K for 60 s, diluted Ge dimer chains are created and surrounded by SiH species. Further annealing to 810 K completely desorbs hydrogen from the surface, leaving large two-dimensional islands covering $\ensuremath{\sim}41%$ of its area. The surface recovers its smooth $2\ifmmode\times\else\texttimes\fi{}1$ structure, but is interspersed with poorly ordered short dimer vacancy lines at 900 K with no observable contrast on the atomic terraces, indicating displacive adsorption of Ge on the terraces. Significant differences of the surface diffusion phenomena between the gas-phase and solid-phase molecular-beam epitaxy (MBE) are observed. Multilayer Si deposition is performed by ALE, i.e., cyclic digermane adsorption at near room temperature, followed by thermal annealing at 900 K. STM images reveal the formation of $2\ifmmode\times\else\texttimes\fi{}n$ structures and increasing roughening of the surface as the growth cycle increases, similar to what occurs during MBE. Issues related to the atomic origins of surface core-level shifts and the chemical composition of the surface layer resulting from the formation of mixed Ge-Si or Ge-Ge during the submonolayer adsorption of Ge on Si(100) are also discussed.