High Index Si Surface Research Articles

Au-induced atomic wires on stepped Ge(hhk) surfaces

Au adsorption on high-index Si surfaces is known to form quasi-one-dimensional surface reconstructions with interesting physical properties such as Rashba-split bands and ordered arrays of local magnetic moments. Here we report on a novel family of Au-induced chain systems, hosted on Ge(hhk) substrates. Our study includes Ge(553), Ge(557), and Ge(335) for both bare and Au-adsorbed surfaces, respectively. We employ scanning tunneling microscopy and low-energy electron diffraction to characterize the topography and the occurrence of superstructures. Stable bare surfaces with regularly distributed steps are found for the (553) and (335) orientations. For nominal Ge(557) substrates a refaceting to Ge(223) is observed. Addition of Au tends to promote a change in the surface index which creates Ge(221)-Au, Ge(557)-Au, and Ge(335)-Au surfaces. Their STM images show elements that are reminiscent of the well-understood Si(hhk)-Au surfaces but also reveal important differences.

Binding Structures of Pyrrole on Si(5 5 12)–2 × 1 Surfaces

In an effort to understand the reaction mechanisms involved in the adsorption of organic aromatic molecules on high-index Si surfaces, the reactions of pyrrole molecules adsorbed onto Si(5 5 12)–2 × 1 surfaces were studied using scanning tunneling microscopy and first principle calculations. The dissociation of one or two H atom(s) bonded to N (or N and C) from the pyrrole molecules was favored, and adsorption at adatom, tetramer, dimer, or honeycomb Si(5 5 12)–2 × 1 sites occurred to produce several distinct configurations. Pyrrole was most reactive toward the dimer site, yielding two dissociated hydrogen atoms and a vertical configuration. Pyrrole also adsorbed onto the tetramer and honeycomb sites, yielding two dissociated hydrogen atoms. On the adatom row, however, pyrrole bound to an adatom via a σ bond between the adatom and N to yield one dissociated H atom adsorbed onto a nearby adatom. No other hydrogen dissociation reactions were observed. In all configurations, the aromaticity of pyrrole was retained.

The preserved aromaticity of aniline molecules adsorbed on a Si(5 5 12)−2×1 surface

We present a scanning tunneling microscopy and first-principles calculations study of the adsorption structures of aniline on a Si(5 5 12)-2x1 surface. Dissociation from the aniline molecules of one or two H atom(s) bonded to N is favored, and then adsorption onto adatom, tetramer, and dimer rows of Si(5 5 12)-2x1 occurs in several distinct configurations. On the adatom row, aniline binds to an adatom in a tilted configuration, which is formed via a sigma bond between the adatom and N, with one dissociated H atom adsorbed on a nearby adatom. No further hydrogen dissociation occurs. On the tetramer and dimer rows, the structures with two dissociated hydrogens and upright configurations are the most stable. Aniline does not adsorb onto the honeycomb chains; this adsorption configuration has a low adsorption energy. In all the adsorption configurations of aniline on this surface, the molecule's aromaticity is preserved. Thus Si-N bonding of aromatic amine molecules provides a strategy for the homogeneous aromatic functionalization of high index Si surfaces.

Adsorption structures of benzene on a Si(5512)-2×1 surface: A combined scanning tunneling microscopy and theoretical study

In the first ever attempt to study the adsorption of organic molecules on high-index Si surfaces, we investigated the adsorption of benzene on Si(5 5 12)-(2x1) by using variable-low-temperature scanning tunneling microscopy and density-functional theory (DFT) calculations. Several distinct adsorption structures of the benzene molecule were found. In one structure, the benzene molecule binds to two adatoms between the dimers of D3 and D2 units in a tilted butterfly configuration. This structure is produced by the formation of di-sigma bonds with the substrate and of two C[Double Bond]C double bonds in the benzene molecule. In another structure, the molecule adsorbs on honeycomb chains with a low adsorption energy because of strain effects. Our DFT calculations predict that the adsorption energies of benzene are 1.03-1.20 eV on the adatoms and 0.22 eV on the honeycomb chains.

Formation of antimony 1D-nanostructures on Si (5 5 12) surface

AbstractOf late, high index Si surfaces like, (5 5 12) are being explored for the formation of 1D nanostructures in the form of nanowires or chains. The surface topography of Si (5 5 12) presents an unique template for the growth of 1D nanostructures. In this paper we report the adsorption-desorption studies of Sb onto (2×1) reconstructed surface of Si (5 5 12). Motivated by our earlier studies of adsorption/desorption of Sb on low index surfaces (001) and (111), studying the interaction on high index surface like (5 5 12) is found to be interesting. The experiments have been performed in UHV with in-situ growth and probed by using AES, LEED and EELS techniques. The uptake curve shows initially a simultaneous multilayer growth of Sb up to 4 ML and then at 20 ML the entire Si surface is covered by Sb. The LEED patterns show a disorder during growth at RT above 1 ML. EELS studies at 250 eV, primary beam energy show that as the adsorption proceeds the features of bulk Sb start appearing above 4ML coverage. However, the bulk Sb features are not complete until a high enough coverage of Sb (˜20ML) which suggests the existence of the primary rows up to a higher coverage. Annealing studies have also been performed on the RT adsorbed system, to study the residual thermal desorption characteristics. Annealing the system to about 7900C results in Si (337) facets at a low coverage of 0.2 ML. The anisotropy in the LEED spots suggests an ordered Sb adsorption along (1 10) and a local disorder in the (665 ) direction. Thus we provide evidence for the formation of a zig-zag 1D-nanowire of Sb grown on the (5 5 12). The new phase is also accompanied by forming a dangling bond state at 3 eV in EELS.

Reconstruction of silicon surfaces: A stochastic optimization problem

Over the last two decades, scanning tunnelling microscopy (STM) has become one of the most important ways to investigate the structure of crystal surfaces. STM has helped achieve remarkable successes in surface science such as finding the atomic structure of Si(111) and Si(001). For high-index Si surfaces the information about the local density of states obtained by scanning does not translate directly into knowledge about the positions of atoms at the surface. A commonly accepted strategy for identifying the atomic structure is to propose several possible models and analyze their corresponding {\em simulated} STM images for a match with the experimental ones. However, the number of good candidates for the lowest-energy structure is very large for high-index surfaces, and heuristic approaches are not likely to cover all the relevant structural models. In this article, we take the view that finding the atomic structure of a surface is a problem of stochastic optimization, and we address it as such. We design a general technique for predicting the reconstruction of silicon surfaces with arbitrary orientation, which is based on parallel-tempering Monte Carlo simulations combined with an exponential cooling. The advantages of the method are illustrated using the Si(105) surface as example, with two main results: (a) the correct single-step rebonded structure [e.g., Fujikawa {\em et al.}, Phys. Rev. Lett. 88, 176101 (2002)] is obtained even when starting from the paired-dimer model [Mo {\em et al.}, Phys. Rev. Lett. 65, 1020 (1990)] that was assumed to be correct for many years, and (b) we have found several double-step reconstructions that have lower surface energies than any previously proposed double-step models.

Reconstructed (12, 2, 7) Si surface structure observed by scanning tunneling microscopy

A series of reconstructed high-index Si surfaces, with angle β from the [111] to [11̄0] direction varying between 10° and 35° (in increments of 5°) were systematically studied by ultrahigh vacuum scanning tunneling microscopy. The reconstruction of the (12, 2, 7) surface with β=30° as a typical example of the series was described in detail. Fourier transforms revealed a (2×2) structure of the terrace, the splitting lattice on the step wall, and diffuse streaks from the kinks. The results were compared with other high-index surfaces of the β series. The reconstructed terrace structures were separated into two groups by the boundary between β=20° and 25°. With regard to the reconstructed step wall structures, a remarkable characteristic pattern of these high-index surfaces was clarified, namely that they are composed of several splitting lattices. The degree of splitting proved to be closely proportional to the effective dangling bond density. The diffuse streaks from the kinks were also observed as a common phenomenon of this series.

Quasiperiodic nanoscale faceting of high-index Si surfaces.

Scanning tunneling microscopy reveals that Si(112) reconstructs into quasiperiodic, nanometer-scale facets. Each sawtoothlike facet consists of a single unit cell wide reconstructed (111) terrace (7\ifmmode\times\else\texttimes\fi{}7 or 5\ifmmode\times\else\texttimes\fi{}5) opposed by a 60 to 110 \AA{} wide (337) terrace. Nanofacets with a similar structure are also observed on Si(335), indicating that they are a general phenomenon for some range of vicinality towards [$\overline{1}\overline{1}2$] The dimensions of these nanofacets suggest that Si(112) and Si(335) would be interesting substrates for the growth of corrugated superlattices.

The high-index surface—a superlattice for two-dimensional electrons

A review is given of low-temperature studies of the properties of the two-dimensional electron gas in inversion layers on high-index Si surfaces. The long crystallographic periods associated with such surfaces produce the superlattice effect in the spectrum of the quasi-two-dimensional electron gas. The kinetic coefficients are found to exhibit singularities when the Fermi level and minigaps cross. This was established by measuring the static and the high-frequency conductivities, the Shubnikov-de Haas oscillations, the photoresistivity, the emission of thermal electrons, and the cyclotron resonance in n-type inversion layers near (001) for electron concentrations in the range 1012 – 1014 cm−2. The minigap width varies from 1 to 20 meV, depending on the electron concentration. The position of the minigaps in K-space (but not their size) is in quantitative agreement with theory.