Threefold-coordinated Si Atoms Research Articles

First-principles theory of Si(110)-(16 × 2) surface reconstruction for unveiling origin of pentagonal scanning tunneling microscopy images

The origin of the scanning tunneling microscopy (STM) zigzag chain structures composed of pairs of pentagons on the Si(110)-(16 × 2) surface is unveiled through the first-principles calculation method. Stable Si(110) surface structures, on both flat and stepped surfaces, have been discovered. The energy gain of the stable step structure is larger than those of previously proposed models by 5.0 eV/(16 × 2) cell or more. The structure consists of buckled tetramers, heptagonal rings, tetragonal rings, and threefold-coordinated Si atoms, but no pentagonal rings. It reproduces the experimental STM images only when frequent flip-floppings of the buckled tetramers at room temperature are considered.

A combined first principles and Mohr-Coulomb criterion for the determination of the nanohardness of amorphous silicon

AbstractAlthough, the evaluation of the nanohardness of amorphous silicon (a-Si) has been the subject of a few experimental works but, to date, it has not been addressed theoretically yet. In this work, first principles Kohn-Sham density functional theory (DFT)-based molecular dynamics (MD) in combination with Mohr-Coulomb criterion is employed to calculate the ideal shear strength of the damped MD annealed a-Si sample containing dangling and floating bonds which are pertinent to the threefold- and fivefold-coordinated defects, respectively, as well as distorted tetrahedral bonds. The stress state beneath the nanoindenter is triaxial, and is accounted for properly. The calculated values of nanohardness are in reasonable agreement with those values measured experimentally. Consideration of the electronic charge distribution under the state of triaxial tension test reveals that the yield phenomenon in a-Si is accompanied by the transformation of a threefold-coordinated Si atom to a fourfold-coordinated.

Hydrogen-induced rupture of strained Si─O bonds in amorphous silicon dioxide.

Using ab initio modeling we demonstrate that H atoms can break strained Si─O bonds in continuous amorphous silicon dioxide (a-SiO(2)) networks, resulting in a new defect consisting of a threefold-coordinated Si atom with an unpaired electron facing a hydroxyl group, adding to the density of dangling bond defects, such as E' centers. The energy barriers to form this defect from interstitial H atoms range between 0.5 and 1.3 eV. This discovery of unexpected reactivity of atomic hydrogen may have significant implications for our understanding of processes in silica glass and nanoscaled silica, e.g., in porous low-permittivity insulators, and strained variants of a-SiO(2).

Antiferromagnetic ordering of dangling-bond electrons at the stepped Si(001) surface

Using first-principles density-functional calculations, we explore the possibility of magnetic order at the rebonded DB step of the Si(001) surface. The rebonded DB step containing threefold coordinated Si atoms can be treated as a one-dimensional dangling-bond (DB) wire along the step edge. We find that Si atoms composing the step edge are displaced up and down alternatively due to Jahn-Teller-like distortion, but, if Si dimers on the terrace are passivated by H atoms, the antiferromagnetic (AFM) order can be stabilized at the step edge with a suppression of Jahn-Teller-like distortion. We also find that the energy preference of AFM order over Jahn-Teller-like distortion is enhanced in an oscillatory way as the length of DB wires decreases, showing the so-called quantum size effects.

Magnetism induced by boron impurities in amorphous silicon

With first-principles calculations, magnetism is found in amorphous silicondoped with B impurities. The maximum magnetic moment per impurity atom is predicted to be ∼1.0 μ B which originates mostly from unsaturated bond around three-fold coordinated Si atoms. Stoner criterion is employed to account for the magnetism induced by p-type impurities. The obtained spin polarized energies are around 50 meV, indicating that the magnetism found in amorphous silicon is able to survive even at room temperature.

Ab initiomolecular-dynamics study of structural and electronic properties of liquid MgSiO3under pressure

The structural and electronic properties of liquid MgSiO3 under pressure are investigated by ab initio molecular-dynamics simulations. At ambient pressure, most of Si atoms have the same coordination even in the liquid state as in the crystalline phase, i.e., each Si atom is bonded to two bridging oxygen (BO), i.e., O atoms twofold-coordinated to Si, and two nonbridging oxygen (NBO), i.e., O atoms onefold-coodinated to Si. It is, however, found that the structural defects, such as fivefold- or threefold-coordinated Si atoms, are always formed with the rearrangement of Si-O covalent bonds in the atomic diffusion processes. The population analysis clarifies that maximum of the diffusivity in the pressure dependence results from the increasing of the number of defects under compression.

Local bond rupture of Si atoms on Si(1 1 1)-(2 × 1) induced by the surface π–π∗ excitation

A scanning tunneling microscopy study has revealed that threefold-coordinated Si atoms at intrinsic sites of reconstructed (2 × 1) structure on the Si(1 1 1) surface are removed to form a surface monovacancy by an electronic mechanism under surface-specific optical transitions at 0.45 eV. This result provides direct evidence for the relaxation of excited surface electronic states as the origin of excitation-induced structural instability on semiconductor surfaces.

Electronic bond rupture of Si atoms onSi(111)−(2×1)induced by valence excitation

Scanning tunneling microscopy study reveals the electronic local bond rupture of threefold-coordinated Si atoms at intrinsic sites on $\mathrm{Si}(111)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$ under nanosecond-laser excitation at $1064\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The rate of bond rupture leading to monovacancy formation on the surface depends superlinearly on the excitation intensity. This primary step of surface-structural change is followed by efficient formation of vacancy clusters with two distinctive morphologies: vacancy strings aligned along the Si-atom chain in one dimension and vacancy islands developed across the chains in two dimensions. Quantitative analysis of the vacancy clustering process shows that the bond-rupture rate at sites nearest to pre-existing surface defects is enhanced more than a factor of 1000 relative to perfect sites. We also studied effects of different excitation wavelength and of Fermi-level positions on the bond-rupture process. The laser-induced surface bond-rupture mechanism is discussed in terms of two-hole localization of optically generated nonequilibrated valence holes on the surface sites.

Bond rupture of threefold-coordinated Si atoms at intrinsic sites on the Si( [formula omitted])-(2×1) induced by 1.16-eV photon excitation

A scanning tunneling microscopy study reveals the electronic bond rupture of Si atoms threefold coordinated at intrinsic sites on the Si(1 1 1)-(2 × 1) surface under 1064-nm excitation resulting in the lowest valence excitation of Si. The bond rupture generates monovacancies at surface Si sites with a rate dependent superlinearly on excitation intensity, and preferential bond rupture near the vacancies leads to the formation of vacancy clusters progressively. The mechanism of the electronic process is discussed in terms of the two-hole localization on this surface.

First-principles modeling of paramagnetic Si dangling-bond defects in amorphousSiO2

We modeled paramagnetic Si dangling-bond defects in amorphous SiO2 using a generalized-gradient density-functional approach. By creating single oxygen vacancies in a periodic model of amorphous SiO2, we first generated several model structures in which the core of the defect consists of a threefold coordinated Si atom carrying a dangling bond. These model structures were then fully relaxed and the hyperfine parameters calculated. We found that the hyperfine parameters of such model defects, in both the neutral and positive charge states, reproduced those characteristic of the E', in accord with experimental observations for amorphous SiO2. By eliminating a second O atom in the nearest-neighbor shell of these defect centers, we then generated model defects in which the Si atom carrying the dangling bond forms bonds with two O atoms and one Si atom. In this defect, the spin density is found to delocalize over the Si-Si dimer bond, giving rise to two important hyperfine interactions. These properties match the characteristics of the hyperfine spectrum measured for the S center. Our results are complemented by the calculation of hyperfine interactions for small cluster models which serve the threefold purpose of comparing different electronic-structure schemes for the calculation of hyperfine interactions, estimating the size of core-polarization effects, and determining the reliability of cluster approximations used in the literature.

Molecular-dynamics study on atomistic structures of amorphoussilicon

Structural characteristics of amorphous silicon (a-Si) have been examined bymolecular-dynamics calculations using the Tersoff interatomic potential. Itwas confirmed that the computer-generated atomic configurations reproduce wellthe structural and dynamical properties of a-Si obtained experimentally. Thea-Si networks contained two types of structural defect: threefold coordinatedSi atoms (dangling bonds) and fivefold coordinated ones (floating bonds). Theaverage bond length increased with the coordination number. Bond angles weredistributed around 120° for the threefold coordinations, suggestingthe existence of the atomic clusters constructed by sp2 bonding. Onthe other hand, they had peaks at ~60° and 90° for thefivefold coordinated atoms. Partial radial distribution functions revealedthat the floating bonds have a tendency to cluster in the a-Si network.

Cluster-model calculations of hyperfine parameters and g values for defects in a-Si:H

We have calculated the 29Si hyperfine parameters using the density-functional theory and g values using an iterative extended Huckel theory for defects in Si clusters. We confirm from these calculations that the ESR signal with g=2.0055 and the g=2.0043- and 2.012-components of the light-induced ESR signal in a-Si:H originate from neutral threefold-coordinated Si atoms and negatively and positively charged weak Si–Si bonds, respectively.

Cluster-model calculations of hyperfine coupling constants of dangling bond and weak bond in a-Si : H

We have calculated the 29Si hyperfine coupling constants (hfcc's) for neutral Si dangling bond (D0), that is, neutral threefold-coordinated Si atom and charged weak SiSi bonds (W− and W+) using the density-functional theory. The hfcc's calculated for D0 and W− are in fairly good agreement with the results estimated from the hyperfine structure of the g = 2.0055-line in the ESR signal and that of the g = 2.0043-component of the light-induced ESR (LESR) signal, respectively, in a-Si : H. In addition, the magnitude of the hfcc calculated for W+ is less than the upper bound of the hfcc estimated for the g = 2.01-component of the LESR signal.

Early Stages of Sintering of Si3N4 Nanoclusters Via Parallel Molecular Dynamics

AbstractWe investigate early stages of sintering of silicon nitride (Si3N4) nanoclusters by molecular-dynamics (MD) simulations on parallel computers. Within 100 pico seconds, an asymmetric neck is formed between nanocrystals at 2,000K. In the neck region, there are more four-fold than three-fold coordinated Si atoms. In contrast, amorphous nanoclusters develop a symmetric neck, which has nearly the same number of three-fold and four-fold coordinated Si atoms. In the case of sintering among three nanoclusters, a chain-like structure forms in 200 pico seconds. The present study shows that sintering is driven by rapid diffusion of surface atoms and cluster rearrangement.

AB Initio Calculations for Defects in Silicon-Based Amorphous Semiconductors

ABSTRACTWe have calclulated the ESR hyperfine parameters of threefold-coordinated Si atoms and twofold-coordinated P and N atoms in Si-based amorphous semiconductors using the density functional theory with a local-spin-density approximation. These calculated results have been compared with the observed ESR results.

Theoretical study of the Cox-Symons model for normal muonium in silicon

A theoretical study of the Cox-Symons model for normal muonium in Si is presented. The calculations are performed using polarized basis set ab-initio Hartree Fock calculations followed by corrections for electron correlation. It is shown that, if lattice relaxations are included, the antibonding site becomes a minimum of the potential energy surface (PES) for neutral interstitial hydrogen. The energy at this minimum is lower than that at the undistorted tetrahedral interstitial site. The spin density changes from being almost entirely on the muon (for Mu at the T site) to being almost entirely on a three-fold coordinated Si atom (for Mu in the AB configuration). The mechanism required to explain the isotropy and magnitude of the observed hyperfine tensor of Mu in c-Si is complicated. Large displacements of some host atoms are needed, and the system must be dynamic. However, this model is the first able to produce a minimum of the PES together with an isotropic hyperfine interaction and a delocalized spin density.

Origin of the ESR signal with g=2.0055 in amorphous silicon.

Defect-state wave functions for threefold- and fivefold-coordinated Si atoms in amorphous silicon clusters have been calculated with use of a first-principles linear combination of the atomic orbitals method in order to clarify the origin of the ESR signal with g=2.0055 in amorphous silicon. The wave function of the defect state originating from the threefold-coordinated Si atom is strongly localized on this atom. On the other hand, that for the fivefold-coordinated Si atom is extended on this atom and its nearest neighbors. By comparing these results with the observed hyperfine structure of the ESR signal, we conclude that the origin of this ESR signal is the threefold-coordinated Si atoms.

Pressure dependence of the Pb center at the Si/SiO2 interface.

We report the first measurement of the pressure dependence of the electronic gap states at the Si/Si${\mathrm{O}}_{2}$ interface that are associated with the paramagnetic defect termed the ${P}_{b}$ center. The energy level of this amphoteric defect for the transition from double to single occupancy by holes is found by deep-level transient spectroscopy to shift towards the valence-band edge with increasing pressure. Our results are in agreement with the predictions of a cluster-model calculation in which the ${P}_{b}$ center is identified with the dangling orbital of an interfacial threefold-coordinated Si atom.

Quantitative analysis of EPR and electron-nuclear double resonance spectra of D centers in amorphous silicon: Dangling versus floating bonds.

We report a quantitative analysis of the EPR and electron-nuclear double resonance (ENDOR) data associated with the D center in amorphous Si and show that they support the recent suggestion that D centers are not dangling bonds (threefold-coordinated Si atoms) as commonly believed but instead are ``floating bonds'' (fivefold-coordinated Si atoms). The localization properties of the D-center wave function are shown to be significantly different from those of the ${P}_{b}$ center at the Si-${\mathrm{SiO}}_{2}$ interface and from model calculations of dangling bonds. They are, on the other hand, consistent with the predicted properties of floating bonds. We conclude that floating bonds are a stronger candidate for the D center. It is suggested that ENDOR data in $^{29}\mathrm{enriched}$ material may provide a further test of this conclusion.

Local electronic structure and surface geometry of Ag on Si(111)

Scanning tunneling microscopy has been used to evaluate the overall morphology and growth conditions for Ag ordered overlayer and island films on Si(111), and to determine the nature of the √3×√3 Ag/Si(111) structure. Current image tunneling spectroscopy (CITS) is used to examine the energy-dependent tunneling and evaluate possible model structures for the √3×√3 structure. Support is found for a √3×√3 structure composed of Ag trimers embedded below the top layer of threefold coordinated Si atoms. The surface electronic structure for these √3 structures shows a 1.2 eV gap, and exhibits very few intrinsic states near EF. The few defects observed in the √3 areas of this surface introduce deep levels while the most dominant tunneling features near EF appear to be associated with Ag islands that coexist with √3 areas. The implications of the surface perfection and local electronic structure to Schottky barrier formation are discussed.