Si Adatoms Research Articles

Desorption of chlorine atoms on Si (1 1 1)-(7 × 7) surfaces induced by hole injection from scanning tunneling microscope tips

We investigated desorption of chlorine atoms on Si (1 1 1)-(7 × 7) surfaces induced by hole injection from scanning tunneling microscope tips. The hole-induced desorption of chlorine atoms had a threshold bias voltage corresponding to the energy position of the S3 surface band originated in Si backbonds. The chlorine atom desorption rate was almost proportional to the square of the tunneling current. We have discussed possible mechanisms that two holes injected into Si surface states get localized at the backbonds of chlorinated Si adatoms, which induces the rupture of Cl–Si bonds to result in chlorine atom desorption.

High-resolution photoemission spectroscopy study of the single-domainSi(110)−16×2surface

We have investigated the valence band structure and $\mathrm{Si}\phantom{\rule{0.3em}{0ex}}2p$ photoemission spectra of the single-domain $\mathrm{Si}(110)\text{\ensuremath{-}}16\ifmmode\times\else\texttimes\fi{}2$ surface with higher resolution than previous studies. We found that the highest occupied surface state, reported to be dispersive in the previous studies, is resolved into the two surface states with flat energy dispersions. This reveals that the two surface states are not produced by Si $\ensuremath{\pi}$-bonded chains, as suggested in the previous studies, but originate from the building blocks with spatially localized electronic structures such as a Si tetramer and a Si adatom. $\mathrm{Si}\phantom{\rule{0.3em}{0ex}}2p$ line shapes show directly five surface components without any curve fitting. Various atomic structure models, especially the adatom-tetramer-interstitial model, of the $\mathrm{Si}(110)\text{\ensuremath{-}}16\ifmmode\times\else\texttimes\fi{}2$ surface are considered to figure out the atomistic origins of the surface components and states.

Mechanism for Room-Temperature Single-Atom Lateral Manipulations on Semiconductors using Dynamic Force Microscopy

Vacancy-mediated lateral manipulations of intrinsic adatoms of the Si(111)-(7x7) surface at room temperature are reported. The topographic signal during the manipulation combined with force spectroscopy measurements reveals that these manipulations can be ascribed to the so-called pulling mode, and that the Si adatoms were manipulated in the attractive tip-surface interaction regime at the relatively low short-range force value associated to the manipulation set point. First-principles calculations reveal that the presence of the tip induces structural relaxations that weaken the adatom surface bonds and manifests in a considerable local reduction of the natural diffusion barriers to adjacent adsorption positions. Close to the short-range forces measured in the experiments, these barriers are lowered near the limit that enables a thermally activated hopping at room temperature.

The behavior of Co atoms on Si (111)-7 × 7 surfaces at low temperatures

The behavior of Co atoms on Si (111)-7 × 7 surfaces at low temperatures was studied by using a variable-temperature scanning tunneling microscopy (VT-STM). Co atoms deposited on Si (111)-7 × 7 surfaces are randomly adsorbed at 100 K. Co atoms start to react with adatoms of Si (111)-7 × 7 surfaces at temperatures between 126 K and 130 K. The reaction transfers the bright dots of Co atoms to dark dots under the STM observation of negative bias. Analysis of the reaction occurrence sites and comparing with the results of room temperature deposition shows that the Co atoms tend to diffuse and react with the adatoms of Si (111)-7 × 7 surfaces at the center sites of unfaulted half unit cell (UHUC) at higher temperatures.

DFT study of NH 3 dissociation on Si(1 1 1)-7 × 7. The role of intermolecular interactions

The adsorption of NH 3 molecule on the Si(1 1 1)-7 × 7 surface modelled with a cluster has been studied using density functional theory (DFT). The results indicate the existence of a precursor state for the non-dissociative chemisorption. The active site for the molecular chemisorption is the adatom; while the NH 3 molecule adsorbs on the Si restatom via this preadsorbed state, the adsorption on the Si adatom is produced practically without an energy barrier. The ammonia adsorption on the adatom induces an electron transfer from the dangling bond of this atom to the dangling bond of the adjacent Si restatom, hindering this site for the adsorption of a second NH 3 incoming molecule. However, this second molecule links strongly by means of two H-bonds. The dissociative chemisorption process was studied considering one and two ammonia molecules. For the dissociation of a lonely NH 3 molecule an energy barrier of ∼0.3 eV was calculated, yielding NH 2 on the adatom and H on the restatom. When two molecules are adsorbed, the NH 3–NH 3 interaction yields the weakening of a N–H bond of the ammonia molecule adsorbed closer the Si surface. As a consequence, the dissociation barrier practically disappears. Thus, the presence of a second NH 3 molecule at the adatom–restatom pair of the Si(1 1 1)-7 × 7 surface makes the dissociative reaction self-assisted, the total adsorption process elapsing with a negligible activation barrier (less than 0.01 eV).

Toward Atom-by-Atom Assembly of Compound Semiconductor Nanostructures: Mechanical Atomic Discrimination and Atomic Manipulation at Room Temperature

An atomic force microscope (AFM) under noncontact and nearcontact regions operated at room-temperature (RT) in ultrahigh vacuum, is used as a tool for topography-based atomic discrimination and lateral atomic manipulations of two intermixed atomic species on semiconductor surfaces. Noncontact AFM topography can give height difference between two intermixed atomic species. Therefore, noncontact AFM topography is a kind of atom selective image and hence can serve for atomic discrimination in simple cases. Besides, site-specific force curves provide the chemical covalent bonding forces between the tip apex and the atoms at the surface. Here, we introduced both examples related to topography-based atomic discrimination using selected Sn and Si adatoms in Sn/Si(111)-(√3×√3) surface. Recently, under nearcontact region, we found a lateral atom-interchange manipulation phenomenon at RT in Sn/Ge(111)-c(2×8) intermixed sample. This phenomenon can interchange an embedded Sn atom with a neighbor Ge atom at RT. Using the vector scan method under nearcontact region, we constructed “Atom Inlay”, that is, atom letters “Sn” consisted of 19 Sn atoms embedded in Ge(111)-c(2×8) substrate. Using these methods, now we can assemble compound semiconductor nanostructures atom-by-atom. Keywords: Atom-by-atom assembly, Atomic discrimination, Atomic force microscopy, Site-specific force spectroscopy, Lateral atom manipulation, Atom-interchange manipulation

Atomic structure of Ge clusters onSi(111)-(7 × 7) by non-contact AFM

We present non-contact (NC) AFM results of Ge clusters on aSi(111)-(7 × 7) reconstructed surface. The low temperature NC-AFM allows us to directly observe the atomicstructure of the Ge clusters on the Si(111). The Ge clusters reside in the middle of a half unit cell in the(7 × 7) reconstructionsurface and they are ∼1.4 Å higher than the Si adatoms. By direct NC-AFM observation, the features of the Geclusters on the Si(111) became clear. First, the Ge atoms reside in spaces between Siadatoms and on the Si/Ge atoms on the Si(111). Second, the Si adatoms shiftfrom their original position through Ge adsorption, and interact with the Geatoms accompanied by surface relaxation and a change in their spatial heights. Inaddition, the interatomic distance between the Ge atoms inside the clusters isapproximately 4.0 Å, which is larger than that between the Ge atoms in the bulk (2.4 Å). OurNC-AFM results of Ge clusters provide valuable information for the basic study ofclusters on semiconductor surfaces and may be useful for the manipulation andassembly of clusters for the realization of diverse nanostructures at the atomic level.

複素ナノ構造体を組み立てる―原子間力顕微鏡で原子を識別・操作・組立―

An atomic force microscope (AFM) under noncontact and nearcontact regions operated at room-temperature (RT) in ultrahigh vacuum, is used as a tool for topography-based atomic discrimination and atomic-interchange manipulations of two intermixed atomic species on semiconductor surfaces. Noncontact AFM topography based site-specific force curves provide the chemical covalent bonding forces between the tip apex and the atoms at the surface. Here, we introduced an example related to topography-based atomic discrimination using selected Sn and Si adatoms in Sn/Si(111)-(√3 ×√3 ) surface. Recently, under nearcontact region, we found a lateral atom-interchange manipulation phenomenon at RT in Sn/Ge(111)-c(2×8) intermixed sample. This phenomenon can interchange an embedded Sn atom with a neighbor Ge atom at RT. Using the vector scan method under nearcontact region, we constructed “Atom Inlay”, that is, atom letters “Sn” consisted of 19 Sn atoms embedded in Ge(111)-c(2×8) substrate. Using these methods, now we can assemble compound semiconductor nanostructures atom-by-atom.

Specific features of molecules’ pyrolysis on the epitaxial surface in the case of growth of the Si1 − x Ge x layers from hydrides in vacuum

Specific features of molecules’ pyrolysis in the case of growth of Si1 − x Ge x films from hydrides in vacuum are considered in the context of kinetic approximation on the basis of a growth experiment. The type of solution characteristic of the kinetic problem was studied in relation to the values of the system’s parameters for a scheme of decomposition of monohydrides of Si and Ge with a dominant role of SiH2 and GeH2 radicals in the pyrolysis process. It is shown for the first time that, in the system under consideration, there can exist at least two types of solutions that differ radically from each other in the rate of incorporations of atoms into the growing layer and in the degree of filling of the surface states with products of molecules’ pyrolysis. The type of solution characteristic of the actual experiment is determined from the condition for the segregation-related accumulation of Ge atoms at the growth surface of the Si1 − x Ge x film. Numerical analysis performed in relation to the growth temperature made it possible to estimate the rate of the monohydrides’ decomposition and the values of the factor of incorporation of Si and Ge adatoms into the crystal. The estimate of the characteristic decomposition time for monosilane yields the value of 3–4 s at the growth temperatures of 450–700°C; for monogermane, this value is about 2 s.

Epitaxy of Mn on Si(001): Adsorption, surface diffusion, and magnetic properties studied by density-functional theory

Ultrathin films of manganese silicides on silicon are of relevance as a possible material system for building spintronics devices with silicon technology. In order to achieve insight into epitaxial growth of such films on Si001, total-energy calculations are presented using density-functional theory and the full-potential augmented plane wave plus local orbital method. For adsorption of a single Mn atom on Si001, we find that binding at the subsurface sites below the Si surface dimers is 0.9 eV stronger than on-surface adsorption. There is an energy barrier of only 0.3 eV for adsorbed Mn to go subsurface, and an energy barrier of 1.2 eV for the reverse process. From the calculated potential-energy surface for the Mn adatom, we conclude that the most stable site on the surface corresponds to the hollow site where Mn is placed between two Si surface dimers. For on-surface diffusion, both along and perpendicular to the Si dimer rows, the Mn atoms have to overcome energy barriers of 0.65 eV. For deposition of 0.5 monolayers ML or more, we find that the Si dimers of the Si001 surface are broken up, and a mixed MnSi layer becomes the energetically most favorable structure. For coverages above 1 ML, the lowest-energy structure changes to a full Mn subsurface layer, capped by a layer of Si adatoms. We identify this transition with the onset of Mn-silicide formation in an epitaxially stabilized CsCl-like crystal structure. Such MnSi films are found to have sizable magnetic moments at the Mn atoms near the surface and interface, and ferromagnetic coupling of the Mn magnetic moments within the layers. Layer-resolved electronic densities of state are presented that show a high degree of spin polarization at the Fermi level, up to 45% and 27% for films with two or three Si-Mn layers, respectively.

Initial stages of the adsorption of Ge atoms on theSi(111)−(7×7)surface

Using scanning tunneling microscopy and first-principles density functional calculations, we have investigated systematically the initial stages of single Ge atoms adsorbed on a $\mathrm{Si}(111)\text{\ensuremath{-}}(7\ifmmode\times\else\texttimes\fi{}7)$ surface. When the deposition is at an elevated temperature of $420\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, single Ge atoms are found to substitute for the Si adatoms randomly. When the deposition is at room temperature, single Ge atoms do not replace the Si adatoms but move frequently within half unit cells. When the room temperature prepared sample is imaged at $78\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the Ge atoms are observed to either adsorb at the stable high coordination sites near the corner Si adatoms or hop among the high coordination sites near the center Si adatoms. The adsorption sites of Ge atoms at high coordination positions have been determined by first-principles calculations and comparisons with measured scanning tunneling microscopic images.

Ordering of Si atoms on the ten-fold surface of the decagonal Al 72Ni 11Co 17 quasicrystal

The deposition of Si at room temperature on the ten-fold quasicrystalline surface of d-Al–Ni–Co has been investigated by scanning tunnelling microscopy. At a coverage of 0.30 ML, Si pentagons in two orientations related by inversion symmetry are observed on the same substrate terrace. The side-length of the pentagons is 4.2 ± 0.2 Å. At this coverage, the Si adlayer displays quasiperiodic order. Depressions related to pentagonal features observed in STM images of the clean d-Al–Ni–Co substrate are proposed as plausible adsorption sites for the Si adatoms. As the Si coverage is increased, the well-defined structures observed are no longer distinguishable. At coverages above the monolayer, the Si overlayer follows a rough three-dimensional growth mode.

On the formation of Si nanowires by molecular beam epitaxy

Abstract Silicon nanowires can be successfully grown by applying the vapor–liquid–solid process. In the case of the commonly used chemical vapor deposition technique, a Si containing gas/precursor is cracked at Au droplets acting as seeds. Si adatoms are subsequently dissolved in the liquid metal. Due to a supersaturation within this droplet, Si precipitates predominantly at the liquid–solid interface – a nanowire grows. A different situation occurs if nanowires are grown by molecular beam epitaxy via the vapor–liquid–solid mechanism. The difference consists, for example, of the role of the metal seed, the morphology of the nanowires and their aspect ratio. In particular, surface diffusion including the metal used as well as Si, strongly influences the growth process. This article describes molecular beam epitaxy growth experiments of Si nanowires under ultra-high vacuum conditions and compares the results with other growth techniques.

Cycloaddition Reaction of 1,3-Butadiene with a Symmetric Si Adatom Pair on the Si(111)7×7 Surface

We first found experimentally a cycloaddition reaction of a molecule on a symmetry Si pair, 1,3-butadiene on the Si adatom pair of Si(111)7x7, while up to now only asymmetric Si pairs were reported to be involved in cycloaddition reactions on Si surfaces. As the symmetry of a Si pair is expected to influence significantly a cycloaddition product and a reaction pathway, the [4+2]-like cycloaddition product of 1,3-butadiene on the Si adatom pair is suggested to form through a concerted reaction pathway in comparison to a stepwise reaction pathway, which is favorable in the formation of the [4+2]-like cycloaddition product on the asymmetric Si pair (the Si adatom-restatom pair).

Diffusion of adatom in the selective epitaxial growth of Si(100): A molecular dynamics study

Tight-binding molecular dynamics simulation is performed for a Si adatom on the surface containing the {311} facet and the Si(100) surface to reproduce and explain the behavior of the {311} facet in the selective epitaxial growth of Si(100). In the simulation the adatom is observed to diffuse from the {311} facet to the Si(100) surface along the pathway of a trough between the dimer rows on Si(100) surface. By the tight-binding total energy calculation, it is also clarified that adatoms easily diffuse to the Si(100) surface because, after the adatom surmounts the first energy barrier to diffuse to the step edge near the {311} facet, there is no additional Schwoebel barrier and the other energy barriers gradually decrease to the top of the dimer row of Si(100). The pathway of the trough between dimer rows on the Si(100) surface is found to be energetically more favorable than the direct pathway to the dimer row from the tight-binding total energy calculation. Our tight-binding molecular dynamics simulation elucidates the mechanism responsible for the behavior of the {311} facet in the selective epitaxial growth of Si(100).

Real topography, atomic relaxations, and short-range chemical interactions in atomic force microscopy: The case of theα−Sn∕Si(111)−(3×3)R30°surface

We have investigated the phases of the $\mathrm{Sn}∕\mathrm{Si}(111)\text{\ensuremath{-}}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ surface below $\frac{1}{3}$ ML coverage at room temperature by means of atomic force microscopy (AFM) and density functional theory based first-principles calculations. By tuning the Sn concentration at the surface we have been able to discriminate between Sn and Si adatoms, and to assure that the AFM topography for the different phases resembles the one reported using scanning tunneling microscopy. In the mosaic and the intermediate phases, a dependence of the topographic height of the Si adatoms on the number of surrounding Sn adatoms has been identified. In the pure phase, however, variations in the measured height difference between the Sn adatoms and the substitutional Si defects, which are intrinsic to the AFM observation, are reported. Reliable room-temperature force spectroscopic measurements using the atom-tracking technique and first-principles calculations provide an explanation for these striking induced height variations on the pure phase in terms of both the different strength of the short-range chemical interaction and tip-induced atomic relaxations. Our results suggest that the corrugation measured with true atomic resolution AFM operated at low interaction forces and close to the onset of significant short-range chemical interactions provides direct access to the real structure of heterogeneous semiconductor surfaces.

Density functional theory study of the formation of Si nanostructures on a Si(1 1 1) unreconstructed surface

Density functional theory (DFT) calculations have been used to characterize the bonding geometry in Si nanostructures, such as trimers (Si 3), tetramers (Si 4) and hexamers (Si 6) formed on a Si(1 1 1) surface. Trimers and tetramers were simulated by placing three respectively four Si adatoms in high symmetry adsorption sites on the Si(1 1 1) surface. For Si 6 clusters, a combination of these adsorption sites were used. We found that only trimer and tetramer formed by adsorbing Si adatoms in ontop position can be considered as clusters because of bonding interaction among their constituents. For hexamers, bonds are also formed between Si adatoms suggesting their existence as stable entity.

Adsorption site preference of Br onSi(111)−7×7

The site preference of the bromine (Br) atom adsorption process on $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ at room temperature was investigated. The density of the reacted adatom bonds of dimer-adatom-stacking fault structure and the total coverage were determined by means of surface differential reflectivity and thermal desorption spectroscopy, respectively. Partial coverage on the rest atoms was then obtained. At the initial stage below 0.1 ML, Br atoms are adsorbed selectively on dangling bonds at the Si adatoms, but not on those at the rest atoms. Above 0.1 ML, dibromide species are formed at the adatoms even when half of the adatom dangling bonds remain intact. The observed site preference of Br atoms is quite different from that of Cl atoms, which are adsorbed randomly on the dangling bonds at both the adatoms and the rest atoms, and form dichloride only after about 80% of the adatoms and the rest atoms are covered. This study provides direct evidence of the adsorption site preference and suggests pattern formations of Br adsorbates on Si(111).

Formation of copper clusters on a thiophene mediated Si(111)-(7×7) surface via molecular anchors

Thiophene selectively binds to the pair of adjacent Si adatom and Si rest atom on the Si(111)-(7×7) surface, leading to the covalent attachment of C–S–C linkages onto the surface. Cu atoms are found to preferentially adsorb onto the S atoms of the functional C–S–C groups in the formation of copper nanoclusters on the thiophene-mediated Si(111)-(7×7) surface.

Di–σ and Dative Binding of Benzene and Pyridine on a Si(5,5,12)-2 ×1

We investigated the adsorption of benzene and pyridine on Si(5,5,12)-(2×1) at 80 K by using variable-low temperature scanning tunneling microscopy and density functional theory calculations. The benzene molecule most strongly binds to two adatoms on the D3 and D2 units in a tilted butterfly configuration, which consists of di–σ bonds between C atoms and Si adatoms and two C=C double bonds in the benzene molecule. Pyridine molecules interact with adatom(s) on the D2 and D3 units through both Si–N dative bonding and di–σ bonds. The dative bonding through the lone pair electrons of N atom produces a vertical configuration (pyridine-like), which is more stable than di–σ bonds. Di–σ bonds can be formed either through Si–N1 and Si–C4 or Si–C2 and Si–C5.