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.     

Surface electronic structure of the (3 × 2) reconstruction induced by Yb on a Si(1 1 1) surface

We have investigated the electronic structure of the Yb/Si(1 1 1)-(3 × 2) surface using angle-resolved photoelectron spectroscopy. Five surface states have been identified in the gap of the bulk band projection. Among these five surface state, the dispersions of three of them agree well with those of the surface states of monovalent atom adsorbed Si(1 1 1)-(3 × 1) surfaces. The dispersions of the two other surface states agree well with those observed on the Ca/Si(1 1 1)-(3 × 2) surface, whose basic structure is the same as that of monovalent atom adsorbed Si(1 1 1)-(3 × 1) surfaces. Taking these results into account, we conclude that the five surface states observed in the band gap originate from the orbitals of Si atoms that form a honeycomb-chain-channel structure.     

Highly resolved scanning tunneling microscopy study of Si(0 0 1) surfaces flattened in aqueous environment

A surface preparation method with fine SiO₂ particles in water is developed to flatten Si(0 0 1) surfaces on the nanometer scale. The flattening performance of Si(0 0 1) surfaces after the surface preparation method is investigated by scanning tunneling microscopy. The observed surface is so flat that 95% of the view area (100 × 100 nm²) is composed of only three atomic layers, namely, one dominant layer occupying 50% of the entire area and two adjacent layers. Furthermore, a magnified image shows the outermost Si atoms regularly distributed along the 〈1 1 0〉 direction on terraces.     

Surface states and annihilation characteristics of positrons trapped at reconstructed semiconductor surfaces

Positron probes of the Si(1 0 0) surface that plays a fundamental role in modern science and technology are capable to non-destructively provide information that is both unique to the probe and complimentary to that extracted using other more standard techniques. This paper presents a theoretical study of positron “image-potential” surface states and annihilation characteristics of surface trapped positrons at the Si(1 0 0) surface. Calculations are performed for the reconstructed Si(1 0 0)-p(2 × 2) surface using the modified superimposed-atom method to account for discrete-lattice effects, and the results are compared with those obtained for the non-reconstructed and reconstructed Si(1 0 0)-(2 × 1) and Si(1 1 1)-(7 × 7) surfaces. The effect of orientation-dependent variations of the atomic and electron densities on localization and extent of the positron surface state wave function at the semiconductor surface is explored. The positron surface state wave function is found to extend into the Si lattice in the regions where atoms are displaced from their ideal terminated positions due to the p(2 × 2) reconstruction. Estimates of the positron binding energy and positron annihilation characteristics reveal their sensitivity to the specific atomic structure of the topmost layers of Si. The observed sensitivity of annihilation probabilities to crystal face indicates that positron spectroscopy techniques could serve as an important surface diagnostic tool capable of distinguishing different semiconductor surfaces and defining their state of reconstruction.     

Site selective atomic chlorine adsorption on the Si(1 1 1)7 × 7 surface

The spontaneous dissociation of trichloroethylene molecules on the Si(1 1 1)7 × 7 surface was investigated using STM. Chlorine atoms were identified by using voltage dependent imaging and by observing voltage dependent tip-induced diffusion. At low coverage, we identify one chlorine that dissociates and binds to an adatom, leaving a nearby chlorovinyl group as the other product bound to the surface. Chlorine atoms show strong site selectivity for corner adatoms and some preference for the faulted half of the unit cell. This result differs significantly from previous studies of chlorine on this surface and a site-selective mobile precursor model is used to explain this discrepancy. The observed site-selectivity is consistent with the high electronegativity value for chlorine.     

Co growth on Si(0 0 1) and Si(1 1 1) surfaces: Interfacial interaction and growth dynamics

In situ X-ray photoelectron spectroscopy (XPS) and ex situ atomic force microscopy (AFM) were used to study the growth of thin cobalt films at room temperature (RT) on both clean and H-terminated Si(0 0 1) and Si(1 1 1) surfaces. The growth proceeds by first forming an initial CoSi₂-like phase at the growth front of the Si substrate. With increasing Co coverage the interfacial layer composition becomes richer in Co and eventually a metallic Co film is formed on top. Hydrogen termination of the Si surface did not suppress the reaction of Co and Si. A pseudo-layer-by-layer growth mode is proposed to describe the growth of Co on H-terminated Si surfaces, while closed-packed small island growth occurs on clean Si surfaces. The difference in growth mode can be attributed to the increase in the surface mobility of Co adatoms in the presence of hydrogen.     

Temperature programmed desorption studies of deuterium passivated silicon nanocrystals

As grown silicon (Si) surfaces are known to reconstruct in order to reduce the number of dangling bonds. Surface reconstructions of hydride-terminated Si(1 0 0) and Si(1 1 1) surfaces have already been extensively studied using temperature programmed desorption (TPD). The surfaces of nanocrystals, are yet to be probed using TPD. Si nanocrystals less than 8 nm and ranging from 50 to 200 nm in diameter are grown on SiO₂ surfaces in an ultra high vacuum chamber and the as grown surfaces are exposed to atomic deuterium. Desorption spectra are interpreted using analogies to Si(1 0 0). TPD spectra show that that the nanocrystals surfaces are covered by a mix of monodeuteride, dideuteride and trideuteride species. Monodeuteride species can be isolated by selectively annealing away the dideuteride and trideuteride, monodeuteride and dideuteride species can be isolated by annealing away the trideuteride. The relative populations of the deuterides depend on particle size, and their manner of filling on nanoparticles differs from that for extended surfaces. Etching of the nanocrystal surface is observed during TPD, which is a confirmation of the presence of trideuteride species on the nanocrystal surface.     

Growth of oriented crystalline Ag nanoislands on air-exposed Si(0 0 1) surfaces

Growth of Ag islands under ultrahigh vacuum condition on air-exposed Si(0 0 1)-(2 × 1) surfaces has been investigated by in-situ reflection high energy electron diffraction (RHEED). A thin oxide is formed on Si via exposure of the clean Si(0 0 1)-(2 × 1) surface to air. Deposition of Ag on this oxidized surface was carried out at different substrate temperatures. Deposition at room temperature leads to the growth of randomly oriented Ag islands while well-oriented Ag islands, with (0 0 1)_Ag||(0 0 1)_Si, [1 1 0]_Ag||[1 1 0]_Si, have been found to grow at substrate temperatures of ≥350 °C in spite of the presence of the oxide layer between Ag islands and Si. The RHEED patterns show similarities with the case of Ag deposition on H-passivated Si(0 0 1) surfaces.     

The interaction of carbon with Si(1 1 1):As and Si(1 1 1):H surfaces, a theoretical study

Density functional theory calculations have been performed to determine the adsorption site of carbon at the Si(1 1 1):As and Si(1 1 1):H surfaces at different coverages. The As- and H-passivated surfaces were simulated by replacing the topmost Si layer by As or by saturating the Si dangling bonds with hydrogen atoms, respectively. Different high symmetry sites were considered. Carbon was placed successively in the fourfold (T₄) or threefold coordinated (H₃), the ontop (T₁) sites or substituted for a Si atom in the S₅ position located underneath the Si adatom in the T₄ site. We found that the preferred carbon adsorption site depends on the coverage of the passivated surfaces. At low coverages i.e. at 1/16 ML and 1/3 ML, it prefers a distorted T₄ position whereas at 1 ML, it occupies an H₃ site. This contrasts with the clean surface where the most energetically favored site is the S₅ at all coverages. Carbon adsorption induces a significant change in the structural geometry of the surface atoms, leading to a charge re-arrangement in the surface layers.     

Growth of (√3 × √3)-Ag and (1 1 1) oriented Ag islands on Ge/Si(1 1 1) surfaces

We have studied the growth of Ag on Ge/Si(1 1 1) substrates. The Ge/Si(1 1 1) substrates were prepared by depositing one monolayer (ML) of Ge on Si(1 1 1)-(7 × 7) surfaces. Following Ge deposition the reflection high energy electron diffraction (RHEED) pattern changed to a (1 × 1) pattern. Ge as well as Ag deposition was carried out at 550 °C. Ag deposition on Ge/Si(1 1 1) substrates up to 10 ML has shown a prominent (√3 × √3)-R30° RHEED pattern along with a streak structure from Ag(1 1 1) surface. Scanning electron microscopy (SEM) shows the formation of Ag islands along with a large fraction of open area, which presumably has the Ag-induced (√3 × √3)-R30° structure on the Ge/Si(1 1 1) surface. X-ray diffraction (XRD) experiments show the presence of only (1 1 1) peak of Ag indicating epitaxial growth of Ag on Ge/Si(1 1 1) surfaces. The possibility of growing a strain-tuned (tensile to compressive) Ag(1 1 1) layer on Ge/Si(1 1 1) substrates is discussed.     

The role of antiphase boundaries during ion sputtering and solid phase epitaxy of Si(0 0 1)

The Si(0 0 1) surface morphology during ion sputtering at elevated temperatures and solid phase epitaxy (SPE) following ion sputtering at room temperature has been investigated using scanning tunneling microscopy. Two types of antiphase boundaries form on Si(0 0 1) surfaces during ion sputtering and SPE. One type of antiphase boundary, the AP2 antiphase boundary, contributes to the surface roughening. AP2 antiphase boundaries are stable up to 700 °C, and ion sputtering and SPE performed at 700 °C result in atomically flat Si(0 0 1) surfaces.     

Self-assembled octadecyltrichlorosilane monolayer formation on a highly hydrated silica film

A MVD silica layer that consists of a highly hydrated surface favorable for organosilane surface reaction is investigated. The MVD silica layer lacks free surface silanol groups while supporting a more extensive adsorbed water layer as compared to oxidized Si(1 0 0). Octadecyltrichlorosilane monolayers (OTS) deposited on the MVD silica layer are found to follow the same mechanisms of growth and exhibit properties comparable to those formed on oxidized Si(1 0 0) surfaces. The growth process of octadecylsiloxane films is investigated as a function of immersion time and temperature by utilizing ATR-FTIR, ellipsometry, contact angle analysis, and AFM. The MVD silica layer is shown to support an ordered interfacial water structure that is more tightly bound due to a higher degree of hydrogen bonding associated with the hydroxylated surface. The importance of interfacial water on the OTS film formation process is highlighted and the role of free OH groups on the adsorption mechanism is diminished. It is shown that OTS films can be formed on a highly hydrated surface comparable to those formed on oxidized Si(1 0 0) surfaces.     

Comparative investigation of the c(2 × 2)-Si/Cu(0 1 1) and (√3 × √3)R30°-Cu2Si/Cu(1 1 1) surface alloys using DFT

The electronic structure of the c(2 × 2)-Si/Cu(0 1 1) surface alloy has been investigated and compared to the structures seen in the three phases of the (√3 × √3)R30°Cu₂Si/Cu(1 1 1) system, using LCAO-DFT. The weighted surface energy increase between the alloyed Cu(0 1 1) and Cu(1 1 1) surfaces is 126.7 meV/Si atom. This increase in energy for the (0 1 1) system when compared to the (1 1 1) system is assigned to the transition from a hexagonal to a rectangular local bonding environment for the Si ion cores, with the hexagonal environment being energetically more favorable. The Si 3s state is shown to interact covalently with the Cu 4s and 4p states whereas the Si 3p state, and to a lesser extent the Si 3d state, forms a mixture of covalent and metallic bonds with the Cu states. The Cu 4s and 4p states are shown to be altered by approximately the same amount by both the removal of Cu ion cores and the inclusion of Si ion cores during the alloying of the Cu(0 1 1) surface. However, the Cu 3d states in the surface and second layers of the alloy are shown to be more significantly altered during the alloying process by the removal of Cu ion cores from the surface layer rather than by the addition of Si ion cores. This is compared to the behavior of the Cu 3d states in the surface and second layers of the each phase of the (√3 × √3)R30°-Cu₂Si/Cu(1 1 1) alloy and consequently the loss of Cu-Cu periodicity during alloying of the Cu(0 1 1) surface is conjectured as the driving force for changes to the Cu 3d states. The accompanying changes to the Cu 4s and 4p states in both the c(2 × 2)-Si/Cu(0 1 1) and (√3 × √3)R30°-Cu₂Si/Cu(1 1 1) alloys are quantified and compared. The study concludes with a brief quantitative study of changes in the bond order of the Cu-Cu bonds during alloying of both Cu(0 1 1) and Cu(1 1 1) surfaces.     

Growth of silver structures on silicon surfaces observed in vivo by scanning tunneling microscopy

Early stages of growth of silver thin films on oriented silicon surfaces Si(1 0 0)2 × 1 and Si(1 1 1)7 × 7 were studied directly during deposition at room temperature by the scanning tunneling microscopy. Single Ag atoms deposited on the Si(1 0 0)2 × 1 surface diffuse too fast on the surface to be imaged by the microscope. Nucleation on C-type defects of the Si(1 0 0)2 × 1 reconstruction has been observed. During further growth, the defects represent stable terminations of silver chains. Ag nanoclusters growing on the Si(1 1 1)7 × 7 surface have been studied as a system with low diffusivity at room temperature. On this surface, presence of effective interaction between Ag clusters and individual Ag atoms in neighboring cells of the reconstruction has been identified. The interaction results in lowering the barrier for Ag atom hopping to an adjacent unit cell occupied by an Ag cluster. Unique possibilities arising from scanning the surface directly during growth are demonstrated.     

Fabrication and scanning tunneling microscopy studies of the Si(1 1 1)-(√31 × √31)-In surface

We report on the fabrication of single phase of the Si(1 1 1)-(√31 × √31)-In reconstruction surface, observed by scanning tunneling microscopy (STM) at room temperature. By depositing specific amounts of indium atoms while heating the Si(1 1 1)-(7 × 7) substrate at a critical temperature, the single phase of Si(1 1 1)-(√31 × √31)-In surfaces could be routinely obtained over the whole surface with large domains. This procedure is certified by our high-resolution STM images in the range of 5-700 nm. Besides, the high resolution STM images of the Si(1 1 1)-(√31 × √31)-In surface were also presented.     

Density functional study of ethylamine and allylamine on Si(1 0 0)-2 × 1 and Ge(1 0 0)-2 × 1 surfaces

We have investigated the interactions of ethylamine and allylamine with models of the Si(1 0 0)-2 × 1 and Ge(1 0 0)-2 × 1 semiconductor surfaces. Ab initio molecular orbital calculations, along with density functional theory (DFT), are used to examine the interaction of these amines with cluster models of the semiconductor surfaces. The transition states and final adsorption products for adsorption of the molecules are predicted. The DFT calculations show the amines form N-dative bond states with Si(1 0 0)-2 × 1 or Ge(1 0 0)-2 × 1 as the initial adsorption product. The initial dative-bond products can be further activated, resulting in N-H bond cleavage on both surfaces. The overall reaction of a given amine on Si(1 0 0) via N-H dissociation is more exothermic than on the Ge(1 0 0) surface.     

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.     

A first-principles surface phase diagram study for Si-adsorption processes on GaAs(1 1 1)A surfaces

The adsorption processes of an Si atom on GaAs(1 1 1)A surfaces under growth conditions are investigated on the basis of first-principles surface phase diagrams, in which adsorption-desorption behavior is described by comparing the calculated adsorption energy obtained by total-energy electronic-structure calculations with vapor-phase chemical potential estimated by quantum statistical mechanics. The calculated surface phase diagram as functions of temperature and As₂ pressure demonstrates that both Ga and As atoms are adsorbed on the Ga-vacancy site of GaAs(1 1 1)A-(2×2) surface under low As-pressure conditions, resulting in the formation of (2×2) surface with an As adatom. The surface phase diagrams as functions of temperature and Si pressure also reveal that an Si atom can be adsorbed on the (2×2) surface with an As adatom for temperatures less than ∼1160 K and this Si atom can occupy one of As-lattice sites after the incorporation of another As atom, leading to p-type conductivity. In contrast, the (2×2) surface with an As trimer is found to be stabilized under high As-pressure conditions. The surface phase diagram for Si incorporation clarify that an Si atom can be adsorbed at one of Ga-lattice sites of the (2×2) surface with an As trimer for temperatures less than ∼870 K. These calculated results provide one of possible explanations for the formation of p-type and n-type GaAs on GaAs(1 1 1)A surfaces under low and high As-pressure conditions, respectively.     

Electronic properties of clean unreconstructed 6H-SiC(0 0 0 1) surfaces studied by angle resolved photoelectron spectroscopy

The properties of the clean and unreconstructed 6H-SiC(0 0 0 1) and 6H-SiC surfaces were investigated by means of angle-resolved photoelectron spectroscopy (ARPES). These highly metastable surfaces were prepared by exposing hydrogen terminated surfaces to a high flux of synchrotron radiation. On both surfaces we find a band of surface states with 1 × 1 periodicity assigned to unsaturated Si and C dangling bonds located at 0.8 eV and 0.2 eV above the valence band maximum, respectively. Both states are located below the Fermi level. The dispersion of the surface bands amounts to 0.2 eV for the Si derived band and 0.7 eV for C derived band. It is suggested that the electronic properties of these surfaces are governed by strong correlation effects (Mott-Hubbard metal insulator transition). The results for the (0 0 0 1) surface are directly compared to Si-rich (√3 × √3)R30° reconstructed surface. Distinct differences in electronic structure of the (√3 × √3)R30° and 1 × 1 surfaces are observed.     

Photoemission electron microscopy study of anthracene growth on Si(1 1 1)

The thin film growth of anthracene films on Si(1 1 1) surfaces is studied by photoemission electron microscopy (PEEM). The thin film growth of anthracene on Si(1 1 1) is similar to the growth of pentacene on silicon. Initially a layer of flat lying molecules chemisorbs on the surface. Subsequent growth of fractal islands with standing up molecules proceeds on top of this flat layer.