Structural determination of the low-coverage phase of Al on Si(001) surface

The atomic structure of Al layer on Si(001)-(2 x 1) surface has been studied by coaxial impact collision ion scattering spectroscopy. When 0.5 monolayer (ML) of Al atoms are adsorbed on Si(001) at room temperature, it is found that Al adatoms are dimerized and Al ad-dimers are oriented parallel to the underlying Si dimers at the position of centering T3 site with a height of 1.02 Angstroms from the first layer of Si(001). The bond length of the Al dimer is 2.67 Angstroms. With increasing Al coverage up to one ML, Al ad-dimers still occupied near T3 site and the next favorable site is near HH site.     

Single P and As dopants in the Si(001) surface

Using first-principles density functional theory, we discuss doping of the Si(001) surface by a single substitutional phosphorus or arsenic atom. We show that there are two competing atomic structures for isolated Si-P and Si-As heterodimers, and that the donor electron is delocalized over the surface. We also show that the Si atom dangling bond of one of these heterodimer structures can be progressively charged by additional electrons. It is predicted that surface charge accumulation as a result of tip-induced band bending leads to structural and electronic changes of the Si-P and Si-As heterodimers which could be observed experimentally. Scanning tunneling microscopy (STM) measurements of the Si-P heterodimer on a n-type Si(001) surface reveal structural characteristics and a bias-voltage dependent appearance, consistent with these predictions. STM measurements for the As:Si(001) system are predicted to exhibit similar behavior to P:Si(001).     

Theoretical prediction of heterogeneous molecular wires on the Si(001) surface

Using first-principles density-functional calculations, we propose a self-assembly technique for fabrication of the heterogeneous molecular wire on the dangling-bond wire generated on a H-passivated Si(001) surface. Here, we choose pyridine and borine as Lewis base and acid molecules, respectively, to demonstrate different behaviors in the chemical reactivity and selectivity on the dangling-bond wire, leading to formation of the heterogeneous pyridine-borine wire.     

Self-directed growth of benzonitrile line on H-terminated Si(001) surface

Using first-principles density-functional calculations we predict a self-directed growth of benzonitrile molecular line on a H-terminated Si(001) surface. The C[triple bond]N bond of benzonitrile reacts with a single Si dangling bond which can be generated by the removal of a H atom, forming one Si-N bond and one C radical. Subsequently, the produced C radical can be stabilized by abstracting a H atom from a neighboring Si dimer, creating another H-empty site. This H-abstraction process whose activation barrier is 0.65 eV sets off a chain reaction to grow one-dimensional benzonitrile line along the Si dimer row. Our calculated energy profile for formation of the benzonitrile line shows its relatively easier formation compared with previously reported styrene and vinylferrocene lines.     

Effects of adsorbed bismuth on Si(001) surface electronic states

It is shown that depositing Bi on an Si(001) surface fills the free broken-bond surface states, whose concentration decreases linearly as the bismuth coating 8 increases up to _st, = 0.6 monolayer. The bismuth desorption activation energy is constant < _st, (E_d = 2.77 ± 0.1 eV) and decreases for > _st.Taras Shevachenko Kiev National University, ul. Vladimirskaya 64, 252601 Kiev-17, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 3, pp. 168–171, May–June, 1996. Original article submitted September 19, 1995.     

Initial stage of Si(001) surface oxidation from first-principles calculations

A comprehensive density-functional theory (DFT) study of the atomic structure, electronic properties, and optical response of the Si(001) surface at the initial stages of oxidation is presented. The most favored adsorption position of a single O atom on top of the (4 x 2)-reconstructed Si(001) surface is found at the back-bond of the "down" Si dimer atom. There is no energy barrier for oxygen insertion into this bond. The ionization energy of the surface reaches a maximum when the oxidation of the second Si monolayer starts. Oxidation leads to an increase of the energy gap between occupied and empty surface states. The calculated reflectance anisotropy spectroscopy (RAS) data in comparison with experiment suggest a considerable amount of surface disorder already after oxidation of the first monolayer.     

Two dissociation pathways of water and ammonia on the Si(001) surface

Using first-principles density-functional calculations, we investigated two competing pathways for the dissociation of water and ammonia on a Si(001) surface. For both systems, we found that, in addition to the conventionally accepted intradimer transfer of the H atom, the interdimer transfer of the H atom can be equally probable with the same reaction mechanism. Our analysis shows that the two dissociation pathways occur through the Lewis acid-base reaction between the partially positive H ion and the electron-abundant up atom of the buckled Si dimer. The result of the interdimer H transfer not only supports a recently proposed model for C-defect on Si(001) but also corresponds to the recent scanning tunneling microscopy data of ammonia dissociation on Si(001).     

Interaction of oxygen with an Si(001) surface coated with bismuth

Ionization and Auger spectroscopy show that a submonolayer coating of bismuth on Si(001) reduces the initial attachment coefficient for molecular oxygen by comparison with a clean Si(001) surface by two orders of magnitude, while exposure to >10⁵ Langmuir of O₂ causes bismuth to stimulate the formation of surface silicon oxide having stoichiometry close to SiO₂. Taras Shevchenko Kiev University, 64 ul. Vladimirskaya, Kiev 17, Ukraine 252601. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 33, No. 2, pp. 124–127, March–April, 1997.     

A new Si tetramer structure on Si (001)

Interactions between Si ad-dimers on Si (001) have been studied by molecular dynamics simulations using the Stillinger–Weber potential. The interactions determine the formation of clusters from diffusing dimers. We show different pathways for the formation of multiple-dimer clusters and propose a new tetramer (T_CC) structure formation by two diffusing dimers interacting. This tetramer structure has been found to be energetically stable with respect to isolated ad-dimers. Moreover, their local bonding configuration is very similar to the B-type step edge which is known to be the favoured adsorption site for epitaxial growth. The proposed tetramer may play a crucial role as the nucleus of the new epitaxial layer on Si (001).     

Different adsorption structures of pyridine on Si(001) and Ge(001) surfaces

The adsorption and reaction of pyridine on the Si(001) and Ge(001) surfaces are investigated by first-principles density-functional calculations within the generalized gradient approximation. On both surfaces the N atom of pyridine initially reacts with the down atom of the dimer, forming a single bond between the N atom and the down atom. On Ge(001) such an adsorption configuration is most favorable, but on Si(001) a further reaction with a neighboring dimer occurs, resulting in formation of a bridge-type configuration. Especially we find that on Ge(001) the bridge-type configuration is less stable than the gas phase. Our results provide an explanation for a subtle difference in the adsorption structures of pyridine on Si(001) and Ge(001), which was observed from recent scanning tunneling microscopy experiments.     

First-principles study of the adsorption of cesium on Si(001)(2 x 1) surface

First-principles calculations based on density functional theory-generalized gradient approximation method have been performed on cesium adsorption on Si(001)(2 x 1) surface. The optimized geometries and adsorption energies have been obtained and the preferred binding sites have been determined for the coverage (Theta) of one monolayer and half a monolayer. At Theta = 0.5 ML the most stable adsorption site is shown to be T3 site. At Theta = 1 ML two Cs atoms are adsorbed at HH and T3 sites, respectively. It was found that the saturation coverage of Cs for the Si(001)(2 x 1)-Cs surface is one monolayer instead of half a monolayer. This finding supports the majority of experimental observations but does not support recent coaxial impact collision ion scattering spectroscopy investigations [Surf. Sci. 531, L340 (2003)] and He(+) Rutherford backscattering spectroscopy studies [Phys. Rev. B 62, 4545 (2000)]. Mulliken charge and overlap population analysis showed that the Cs-Si bond is indeed ionic rather than polarized covalent as generally assumed for alkali metal (AM) on Si(001)(2 x 1) surface. Geometrical structure analysis seems to have limitations in determining the nature of AM-substrate bond. We also found that the silicon surface is metallic and semiconducting for the coverages of 0.5 and 1 ML, respectively.     

Adsorption dynamics of ethylene on Si(001)

The dynamics of ethylene adsorption on the Si(001) surface was investigated by means of molecular beam techniques. A constant decrease of initial sticking probability s(0) was observed with increasing kinetic energy indicating a non-activated adsorption channel. With increasing surface temperature, s(0) decreases as well, pointing towards adsorption via a precursor state. Quantitative evaluation of the temperature dependence of s(0) via the Kisliuk model was possible for surface temperatures above 250 K; below that value, the temperature dependence is dominated by the adsorption dynamics into the precursor state. Maximum surface coverage was found to be reduced with increasing surface temperature, which is discussed on the basis of a long lifetime of the precursor state at low temperatures.     

Structure and binding energies of unsaturated hydrocarbons on Si(001) and Ge(001)

The adsorption of acetylene, ethylene, and benzene on the Si(001) and Ge(001) surfaces is investigated by first-principles density-functional calculations within the generalized-gradient approximation. We find that the adsorption energies of the three hydrocarbons containing a triple bond, a double bond, and a pi-conjugated aromatic ring decrease as the sequence of C2H2>C2H4>C6H6. We also find that the bondings of acetylene, ethylene, and benzene to Ge(001) are much weaker than those to Si(001). As a result, benzene is weakly bound to Ge(001) while it is chemisorbed on Si(001), consistent with temperature-programmed desorption data.     

Analysis of the cleaved topaz (001) surface

We report on the first study of the cleaved (001) topaz surface and the characterization of the chemical composition and atomic arrangement of the surface. We conclude that there is strong evidence for a hydroxyl group termination appropriate for further chemical reactions. The surface itself is easily accessible, atomically flat and suitable for potential technological applications.     

First principles analysis of the initial oxidation of Si(001) and Si(111) surfaces terminated with H and CH3

Transition state analyses have been carried out within a density functional theory setting to explain and quantify the distinctly different ways in which hydrogen and methyl terminations serve to protect silicon surfaces from the earliest onset of oxidation. We find that oxidation occurs via direct dissociative adsorption, without any energy barrier, on Si(111) and reconstructed Si(001) that have been hydrogen terminated; oxidation initiates with a barrier of only 0.05 eV on unreconstructed Si(001). The commonly measured protection afforded by hydrogen is shown to derive from a coverage-dependent dissociation rate combined with barriers to the hopping of adsorbed oxygen atoms. Methyl termination, in contrast, offers an additional level of protection because oxygen must first undergo interactions with these ligands in a three-step process with significant energy barriers: adsorption of O(2) into a C-H bond to form a C-O-O-H intermediate; decomposition of C-O-O-H into C-O-H and C=O intermediates; and, finally, hopping of oxygen atoms from ligands to the substrate.     

Ab initio surface reaction energetics of SiH4 and Si2H6 on Si(001)-(2 x 2)

First-principles pseudopotential calculations, within a simple dynamically constrained scheme, have been performed to investigate the reaction of 0.25 ML coverage of SiH4 and Si2H6 with the Si(001)-(2 x 2) surface. The silane molecule (SiH4) is adsorbed on to the surface at a number of different sites (on dimer, interrow, or intrarow) with varying barrier heights. Two distinct structures, which are similar in energy, arise from the initial dissociative reaction SiH4-->SiH3(silyl) + H, where the dissociated species are adsorbed either on the same dimer components or on adjacent dimer components. Several further decays of silyl from SiH4 are presented in two separate regimes of high and low ambient hydrogen coverages. The decomposition of silyl can form two different bridging structures: an on top or an intrarow bridging structure in both of the two hydrogen coverage regimes. The disilane molecule (Si2H6) is also adsorbed upon this surface with varying energy barriers, resulting in a dissociation reaction where two SiH3 species are adsorbed on one dimer or in an adjacent dimer configuration. Plausible energy reaction paths for the above models are presented. The stability of the SiH2 species is also discussed.     

On the stability of the W(001) surface

Recently, the present authors have suggested that some of the basic features of the unstable W(001)-(1×1) surface can be explained by the competition between the direct attractive and indirect repulsive interactions present at the surface. To understand this mechanism in more detail, number of surface structures (steps in the (0,1) and (1,1) direction, vacancy, adatoms etc.) have been investigated within a simple LCAO recursion scheme. The direct interactions (two-body term) and the indirect ones (three-body terms) are approximately additive for surface atoms and adatoms having four nearest neighbours. The unreconstructed (1×1) surface is under compressive (repulsive) stress and we suggest that, for example, steps should expand near their edges.     

Molecular beam investigation of hydrogen dissociation on Si(001) and Si(111) surfaces

The influence of molecular vibrations on the reaction dynamics of H2 on Si(001) as well as isotopic effects have been investigated by means of optical second-harmonic generation and molecular beam techniques. Enhanced dissociation of vibrationally excited H2 on Si(001)2 x 1 has been found corresponding to a reduction of the mean adsorption barrier to 390 meV and 180 meV for nu=1 and nu=2, respectively. The adsorption dynamics of the isotopes H2 and D2 show only small differences in the accessible range of beam energies between 50 meV and 350 meV. They are traced back to different degrees of vibrational excitation and do not point to an important influence of quantum tunneling in crossing the adsorption barrier. The sticking probability of H2 on the 7 x 7-reconstructed Si(111) surface was found to be activated both by H2 kinetic energy and surface temperature in a qualitatively similar fashion as H2/Si(001)2 x 1. Quantitatively, the overall sticking probabilities of H2 on the Si(111) surface are about one order of magnitude lower than on Si(001), the influence of surface temperature is generally stronger.