Sb Surface Research Articles

Surfactant mediated growth of Sb clusters on Si(1 1 1) surface

Adsorption kinetics of Sb onto the Si(1 1 1) surface has been modified by incorporation of a Si(1 1 1)–In(√3×√3) surface phase (θ In = 1 3 ML) . In contrast to Sb adsorption on the clean Si surface, the formation of Sb nano-size clusters with good crystalline quality was observed below ≈200°C. Hemispherical clusters grow up to the saturation coverage of about 3 ML and stabilize in the average diameter and height of 40 and 5 nm, respectively, occupying approximately 15% of the surface (cluster density is estimated at 1.2×10 10 clusters/cm 2). The structure of clusters is determined to be a double-domain rhombohedral with (0 0 0 1) stacking. Both structure and lattice parameters of the clusters coincide with those of bulk Sb. The inter-cluster surface is covered with a Sb(2×1) surface phase ( θ Sb=1 ML). Using reflection high-energy electron diffraction and scanning tunneling microscopy techniques it has been found that indium acts as a surfactant providing layer-by-layer growth of Sb clusters. By increasing the coverage of initial In layer, the higher cluster density can been achieved.

Characterization of the (0001) cleavage surface of antimony single crystals using scanning probe microscopy: Atomic structure, vacancies, cleavage steps, and twinned interlayers

Atomically resolved scanning tunneling microscopy images of the unreconstructed hexagonal structure of surface atoms on Sb(0001) are presented. Lateral and vertical lattice parameters have been determined. The interatomic spacing of 4.31 \AA{} $(\ifmmode\pm\else\textpm\fi{}0.05\AA{})$ on the Sb(0001) surface corresponds to the known bulk data. Cleavage has been found to occur always between adjacent double layers, yielding at least diatomic cleavage steps of 3.75 \AA{} $(\ifmmode\pm\else\textpm\fi{}0.10\AA{})$ height. Different kinds of defect structures on the cleavage plane have been imaged with atomic resolution. Point defects, caused by a single or by three missing surface atoms, have been uncovered. Stable imaging of cleavage steps, which were found to be straight along the atomic rows, has been achieved. Twinned interlayers formed upon cleavage of Sb even at room temperature have been revealed by atomic force microscopy. The mean twinning angle of $2.42\ifmmode^\circ\else\textdegree\fi{}$ $(\ifmmode\pm\else\textpm\fi{}0.20\ifmmode^\circ\else\textdegree\fi{})$ is quantitatively in accord with the value of $2.45\ifmmode^\circ\else\textdegree\fi{}$ predicted by the model of twinning in Sb crystals. The observed features are discussed with respect to other layered materials and with regard to their relevance for the use of Sb(0001) as a support of nanostructures.

Sb surface segregation during epitaxial growth of SiGe heterostructures: The effects of Ge composition and biaxial stress

Antimony is the most widely used n-type dopant for Si molecular-beam epitaxy (MBE). However, because of surface segregation during growth, the control of doping profiles remains difficult. The case of ${\mathrm{S}\mathrm{i}/\mathrm{S}\mathrm{i}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ heterostructures is complicated by the existence of stresses, which may affect both the thermodynamics and kinetics of segregation. In this study, we analyze the segregation of Sb resulting from the MBE growth of ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}/\mathrm{Si}(100)$ heterostructures using secondary ion mass spectrometry as a function of (i) growth temperature $(200\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}l~T\ifmmode^\circ\else\textdegree\fi{}l~550\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}),$ (ii) germanium content $(0l~xl~0.2),$ and (iii) stresses (compressively strained and relaxed layers). We show that Sb segregation: (i) increases with temperature, (ii) increases with Ge content in biaxially compressed layers, (iii) decreases with Ge content in relaxed layers. The temperature variation indicates that Sb surface segregation during growth is kinetically controlled. The contrasting behaviors observed as a function of Ge content in stressed and relaxed layers can thus be explained by a decrease of the segregation enthalpy induced by Ge addition and an increase of near-surface diffusion in stressed layers.

Mechanical interface conditions affect morphology and cellular activity of sclerotic bone rims forming around experimental loaded implants

A characteristic bony structure found at revision surgery for failed joint replacement, and implicated as being associated with poorer subsequent implant fixation, is a sclerotic bone rim (SB rim). This study is a histomorphometric analysis of the SB rim formed around an experimental canine micro-motion implant system under stable or unstable conditions with polyethylene (PE) particles, after 8 weeks. A point count histomorphometric analysis was performed to determine the cellular activity at the surface of the SB rim, and the morphology of the structure was determined by image analysis. A SB rim was found to form under both stable and unstable conditions, but with unstable conditions the rim was more distinct, thick, continuous, and was located near the drill hole, and had high and ongoing formative activity at both surfaces with little resorption. Under stable conditions, thinner second or third SB rims were observed. The difference in width and distance between implant and the SB rim is significant ( p<0.05), as was the difference in fraction of resorption surfaces at the SB surface facing the implant. This study observed an in vivo primary bone response to controlled stable and unstable loaded implants. Sclerosis of trabeculae in a semi-continuous SB rim can serve to isolate the implant from the marrow space. The increases in SB rim width and continuity is consistent with the previously demonstrated knowledge that increase of total bone mass and low risk for trabeculae perforation is the consequence of low resorptive and high formative activity.

LEED structure determination of the Ni( [formula omitted])(√3×√3) R30°-Sn surface

Quantitative low energy electron diffraction has been used to determine the structure of the Ni(1 1 1)(√3×√3) R30°-Sn surface phase. The results confirm that the surface layer comprises a substitutional alloy of composition Ni 2Sn as previously found by low energy ion scattering (LEIS), and also shows that there is no stacking fault at the substrate/alloy interface as has been found in (√3×√3) R30°-Sb surface alloys on Ag and Cu(1 1 1). The surface alloy layer is rumpled with the Sn atoms 0.45 ± 0.03 Å higher above the substrate than the surrounding Ni atoms. This rumpling amplitude is almost identical to that previously reported on the basis of the LEIS study. Comparison with similar results for Sn-induced surface alloy phases on Ni(1 0 0) and Ni(1 1 0) shows a clear trend to reduced rumpling with reduced surface atomic layer density, an effect which can be rationalised in terms of the different effects of valence electron charge smoothing at the surface.

Compositional Nonuniformity in Molecular Beam Epitaxy Grown InAsSb on GaAs(111)A Substrates

The compositional nonuniformity of InAsSb alloys on GaAs(111)A substrates grown by molecular beam epitaxy (MBE) has been investigated. In contrast to the three-dimensional (3D) island growth on GaAs(001) substrates, the alloy growth occurs in the two-dimensional (2D) growth mode from the initial stage when the GaAs(111)A substrate is used. The lattice constant fluctuation estimated from X-ray diffraction linewidths is much smaller in the alloys grown on (111)A substrates than in the alloys grown on (001) substrates, indicating that the InAsSb alloys grown on (111)A substrates are more homogeneous. We have also found that homogeneous InAsSb alloys are grown at lower temperatures when the (111)A substrates are used instead of the (001) substrates. Because the adhesion force of Sb atoms to the (111)A surface is much smaller than to the (001) surface, the desorption rate of Sb atoms from the (111)A surface should be much larger than that from the (001) surface. Even in the InAsSb alloys grown on (111)A substrates, however, a new inhomogeneity, which includes As-enriched stripes and As precipitates appears at low growth temperatures. This phenomenon is not associated with Sb surface segregation, but is induced by the excess As incorporation in the epilayer. The As-enriched stripes as well as the As precipitates can be avoided by optimizing the growth conditions. Thus, homogeneous InAsSb alloys are successfully grown even at temperatures as low as 370°C less than the critical temperature of 415°C which is obtained on GaAs(001) substrates.

Cr/V/Sb mixed oxides, catalysts for the ammoxidation of propane to acrylonitrile: Part II. Effect of catalyst composition on catalytic performance

Rutile-type, Cr/V/Sb mixed oxides having different atomic ratio between components were studied as catalysts for propane and propylene ammoxidation to acrylonitrile. Catalysts were more active than Cr/Sb and V/Sb mixed oxides; this was attributed to (i) the higher specific surface area of Cr/Sb/O and Cr/V/Sb/O with respect to V/Sb/O, and (ii) the preferential formation of V 4+ in Cr/V/Sb/O. The nature of the V species and the catalytic performance of Cr/V/Sb/O samples were functions of the (Cr+V)/Sb atomic ratio. When the latter was higher than ≈1, the prevailing species was V 4+; the catalysts were very active but poorly selective to acrylonitrile (selectivity lower than 20%) because of the prevailing formation of carbon oxides and propylene. This was due to the absence of sites able to transform the unsaturated intermediate to acrylonitrile. When the (Cr+V)/Sb ratio was between ≈1 and ≈0.5, catalysts reached a selectivity to acrylonitrile between 20 and 30%, and to propylene lower than 10%. In these samples, the presence of intra-crystalline Sb gradients in the rutile lattice provided a Sb surface enrichment and the development of allylic ammoxidation sites, able to transform the unsaturated intermediate to acrylonitrile. When the (Cr+V)/Sb ratio was lower than ≈0.5, i.e. in systems having excess Sb, the prevailing species was V 3+; the selectivity to acrylonitrile was higher than 30%, with low formation of carbon oxides and of propylene. In this case additional sites for allylic ammoxidation were provided by excess antimony oxide dispersed over the rutile surface.

Atomic structure and formation process of the Si(1 1 1)–Sb(√7 × √7) surface phase

During the study of Sb condensation on the Si(111)–In(√3×√3) surface phase we observed the formation of a new Sb-induced surface structure with (√7×√7) lattice below 450°C. This phase may not be prepared by direct Sb deposition on the Si(111) surface. Instead, the substitution for In atoms from T4 bonding sites by incoming Sb and formation of a γ-phase strongly determine the Sb(√7×√7) formation process. When the concentration of Sb exceeds the value of 0.2ML the irreversible γ√3→√7 phase transition occurs. Based on STM observations, the structural model for this reconstruction has been proposed.

Sb-induced(1×1)reconstruction on Si(001)

We combine low-energy electron diffraction and reflectance anisotropy spectroscopy (RAS) with ah initio calculations of the geometry, band structure, and optical anisotropy to investigate the adsorption of Sb on vicinal Si(001) (1×2). We focus, in particular, on the controversy concerning the Si(001)-(I ×1)-Sb surface. On the basis of total-energy and band-structure calculations, we find that the Sb-undimerised model is unstable and metallic, while experimentally the (1×1) Sb shows no evidence of a Fermi edge. In contrast, the dimerised (2 X 1)-Sb and c(2×2)-Sb reconstructions are found to be semiconducting with a minimal difference in total energy. Furthermore, the RAS spectra calculated for both dimerised reconstructions show strong similarities to one another, and agree well with the experimental RAS data for the Sb-induced (1×1)-Sb surface, with a dominant feature centered at 3.7 eV. We report that these findings are compatible with the (1×1)-Sb surface comprising a mixture of the Sb-dimer-terminated (2×1)-Sb and c-(2×2)-Sb structures.

Sb Pile-up at Oxide/Si Interface during Drive-in Diffusion after Predeposition Using Doped Oxide Source

The Sb pile-up phenomenon during drive-in diffusion at 1000°C in N2, dry O2 and wet O2 atmospheres after predeposition was investigated. The Sb surface concentration during the predeposition using doped oxide source was about 7×1018 cm-3 which is lower than the solid solubility. From the measured Sb depth profiles in both Si and oxide, it was found that the Sb pile-up layer is located at the oxide side of the oxide/Si interface, the Sb peak concentration of the layer is of the order of 1019 cm-3 irrespective of diffusion atmospheres, and the layer is not a single monolayer but above 4 nm in width. The Sb amount taken into oxide during drive-in diffusion increases with the increase in oxide thickness and Sb surface concentration in Si. For drive-in diffusion in N2 ambient, the Sb pile-up amount in oxide is nearly saturated within the initial 10 min, while for the diffusion in dry O2 ambient, this occurs within 30 min.

Investigation of the surface topography and double layer characteristics of variously pre-treated antimony single crystal electrodes

The geometrical roughness factor, root mean square departure of the surface from flatness, the medium lateral correlation length and other parameters have been obtained for variously pre-treated Sb(1 1 1) and Sb(0 0 1) electrodes. Atomic resolution was achieved for cleaved at liquid nitrogen temperature Sb(1 1 1) surface using UHV-STM method. The surface pits with various depths and widths on the surface of electrochemically and chemically etched Sb(1 1 1) electrodes, and deep furrows on the surface of cleaved Sb(0 0 1) have been observed. Electrical double layer characteristics of variously pre-treated Sb(1 1 1) and Sb(0 0 1) electrodes were studied in terms of the Debye length dependent roughness theory (i.e., non-linear Poisson–Boltzmann theory), recently developed by Daikhin et al. [Phys, Rev. E 53 (1996) 6192; Electrochim. Acta 42 (1997) 2853 and J. Chem. Phys. 108 (1998) 1715]. According to the experimental data and results of theoretical calculation, the surface roughness increases in the order of Sb electrodes: cleaved at the temperature of liquid nitrogen Sb(1 1 1) < electrochemically polished single crystal Sb(1 1 1) < electrochemically etched Sb(1 1 1) < chemically etched Sb(1 1 1) < cleaved at temperature of liquid nitrogen Sb(0 0 1). This order of the electrode surface roughness is in agreement with the data obtained from STM, AFM and impedance measurements.

Layer-by-layer surfactant-induced growth of Ag on Cu( [formula omitted]): an impact collision ion scattering spectroscopy and scanning tunnelling microscopy study

We have investigated the structure of Ag/Sb/Cu(1 1 1) surfaces by using time-of-flight impact collision ion scattering spectroscopy. Also, scanning tunneling microscopy measurements were used in studies of the topmost layers of thin film heteroepitaxy systems at room temperature. It was found that on a surface pre-covered with a 0.2 and 0.4 ML of deposited Sb, the (1) Ag(1 1 1)[1 1 2 ̄ ]//Cu(1 1 1)[1 1 2 ̄ ] versus (2) Ag(1 1 1) [ 1 ̄ 1 ̄ 2 ]//Cu(1 1 1)[1 1 2 ̄ ] modes of Ag layers become 50% versus 50%. These ratios are much different from deposition of Ag on clean Cu(1 1 1) surfaces. Furthermore, for 0.4 ML of Sb pre-covered Cu(1 1 1), the outermost first-layer of Ag atoms shift from fcc- to hcp-sites (only the top layer of Ag atoms occupies hcp-sites). In addition, for this case, a Ag(1 1 1)(√3×√3) R30°-Sb surface forms on top of the first Ag layer. For both 0.2 and 0.4 ML coverages, Sb atoms are transported to the next growing Ag layer during subsequent Ag deposition. Sb atoms displace up to 1/3 of the first-layer of Ag atoms and are incorporated into first-layer Ag atom sites with an outward displacement of 0.2 Å with respect to Ag atoms in the first layer. With a pre-coverage of Sb atoms, Ag(1 1 1) planes prefer to grow as type (2) mode between 300 and 633 K. This growth behavior of Ag(1 1 1) planes is different from that shown previously without Sb pre-coverage on Cu(1 1 1).

Structure analysis of the Si()--Sb surface by means of CAICISS combined with LEED–AES–RBS techniques

Abstract The composition and atomic arrangement of the Si(1 1 1)- 3 × 3 -Sb surface have been studied by means of co-axial impact collision ion scattering spectroscopy combined with low energy electron diffraction, Auger electron spectroscopy and Rutherford backscattering spectrometry techniques. It is found that the Sb/Si(1 1 1) surface forms the 3 × 3 structure at a coverage of 0.90 ML, which is determined by RBS. The azimuthal angle scan is also found to show prominent focussing peaks at ±12° around the (1 1 2) planes at a polar angle of 13°. It is determined from the azimuthal angle scan that the Si(1 1 1)- 3 × 3 -Sb surface forms the trimer centered at the T4 site and the spacing of Sb–Sb in the trimer is 2.8±0.05 A.

KPFM imaging of Si(1 1 1) [formula omitted]-Sb surface for atom distinction using NC-AFM

We investigate the possibility of distinction of individual atom species using noncontact atomic force microscopy (NC-AFM) combined with the electrostatic force (ESF) measurement. We simultaneously measured the topography and the surface potential on the Si(1 1 1) 5 3 ×5 3 -Sb surface by Kelvin probe force microscopy (KPFM). The surface potential image indicates the existence of two kinds of adatoms while the difference is not clear in topography. Atom species of each adatom are specified from other NC-AFM images with different experimental conditions and therefore spots with a lower surface potential are specified as Si adatoms. It is found that, in addition to adatoms, Si rest atoms can be specified from the surface potential image. The result indicate that KPFM has the ability to distinguish the individual atom species on intermixing surfaces.

Time-resolved reflectance difference spectroscopy study of Sb- and As-terminated InP(1 0 0) surfaces

Using time-resolved reflectance difference spectroscopy (RDS) we studied the formation of Sb and As overlayers on InP(1 0 0) under real-time metalorganic vapour-phase epitaxy conditions at 560°C. Exposure of a clean P-rich InP surface to TBAs vapour caused a (2×4)-like RDS spectrum similar to previous reports for bulk InAs. This (2×4)-like surface evolved to a (4×2)-like In-rich surface with a strong negative minimum at 2.0 eV after a 2 s H 2 purge. The RDS spectrum then recovered very rapidly to the InP(2×4)-like surface after only a few monolayers of InP growth. Exposure of the P-rich InP surface to TMSb vapour resulted in the formation of a new RDS spectrum with a strong negative peak at 1.9 eV and a positive feature at 3.7 eV which is attributed to surface–surface transitions. Sb surface layers were found to be much more stable and persistent than As layers, and RDS data indicate significant Sb after as much as 200 Å of subsequent InP growth.

A comparison of surface segregation for two semi-Heusler alloys: TiCoSb and NiMnSb

Very different types of surface segregation are found for very similar Heusler alloy materials. We observed significant manganese and antimony segregation to the surfaces and near surface regions of the semi-Heusler alloys NiMnSb and TiCoSb respectively. The Mn and Sb surface enrichment was characterized by angle resolved core level photoemission. Indications of surface disorder from low energy electron diffraction provide complimentary evidence of segregation.

THE TRANSITION TO STEP FLOW GROWTH ON THE CLEAN AND SURFACTANT COVERED Si(111) SURFACE STUDIED BY IN-SITU LEEM

The transition to step flow growth on the clean Si(111) (7×7), Si(111)-In [Formula: see text] and Si(111)-Sb (7×7) surfaces have been investigated using low energy electron microscopy (LEEM). The critical terrace width for step flow on the clean surface displays in Arrhenius behaviour, although with markedly different prefactor and activation energy above and below 750°K. The abrupt change in Arrhenius parameters was revealed by LEEM to correlate with the crossover from homogeneous to heterogeneous island nucleation behavior. The dependence of the critical terrace width upon step orientation is attributed to anisotropic step attachment kinetics. Sb and In surfactants were found to suppress and enhance step flow, respectively, in accordance with expectations. The preparations of Si(111)-Sb (7×7) and Si(111)-In [Formula: see text] surfaces that are morphologically suitable for step flow growth are also described.

Effects of Sb Adatoms on Kinetics of Electrocatalytic Oxidation of HCOOH at Sb-Modified Pt(100), Pt(111), Pt(110), Pt(320), and Pt(331) SurfacesAn Energetic Modeling and Quantitative Analysis

Electrocatalytic oxidation of formic acid on Pt(100), Pt(110), Pt(111), Pt(320), and Pt(331) surfaces modified with Sb adatoms (Pt(hkl)/Sb) was investigated using cyclic voltammetry, potential step technique, and in-situ FTIR spectroscopy. Emphasis has been placed on investigations of the effects of Sb adatoms at Pt single-crystal surfaces on the kinetics of HCOOH oxidation. Quantitative data of kinetics of HCOOH oxidation via reactive intermediates were obtained by applying a potential step technique in combination with a data processing method of integration transform of j−t transients, which has been described in our previous paper (Sun, S. G.; Yang, Y. Y. J. Electroanal. Chem. 1999, 467, 121). The current results illustrated that the kinetics of HCOOH oxidation is faster on bare surfaces of Pt(100), Pt(110), and Pt(320) than that on Sb-modified surfaces of these Pt single-crystal electrodes, but it is slower on bare surfaces of Pt(111) and Pt(331) than that on Sb-modified surfaces of the two Pt single-crystal electrodes. The peak potentials of HCOOH oxidation on Pt(hkl)/Sb were all converged in a narrow potential region between 0.20 and 0.30 V (SCE), implying that the adsorption of Sb on Pt(hkl) planes has altered the apparent activation energy of HCOOH oxidation. On the basis of quantitative results, we have proposed, for the first time, a rectifying factor of apparent activation energy (γ/kJ mol-1) that describes the alteration by Sb adatoms of the apparent activation energy of HCOOH oxidation on Pt(hkl)/Sb surfaces. The values of γ have been determined quantitatively for γPt(100) = 12.80 ± 0.317, γPt(110) = 18.74 ± 0.393, γPt(320) = 16.683 ± 0.349, γPt(111) = −7.886 ± 0.288, and γPt(331) = −11.69 ± 0.245. The transfer coefficient β of HCOOH oxidation on the five Sb-modified Pt(hkl) electrodes was determined to be within the range of 0.12 ± 0.03, which is less dependent on the orientation of a Pt single crystal when it is modified with Sb adatoms and signifies a stepwise transfer of two electrons involved in HCOOH direct oxidation.

Atomic structure of the Sb-terminated Si(111) surface: A photoelectron diffraction study

The deposition under certain conditions of antimony on a $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ surface removes the $7\ifmmode\times\else\texttimes\fi{}7$ reconstruction, producing a passivated Si(111)-$(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$-Sb (1 monolayer) surface. In this work, a quantitative determination of the atomic structure of this reconstruction using photoelectron diffraction is reported. In particular, high-energy photoelectron diffraction (forward-focusing regime) has been applied to investigate the stacking sequence of the atomic layers of the silicon substrate, and scanned-energy photoelectron diffraction (backscattering regime) has been used to determine quantitatively the atomic structure of the surface. Our results show that the formation of a $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ phase produces a bulklike-terminated $\mathrm{Si}(111)1\ifmmode\times\else\texttimes\fi{}1$ substrate free of stacking faults. Regarding the atomic structure of the interface, this study strongly favors the $T4$-site milkstool model over the $H3$ one.

Structural study of the adsorption of Sb on Ag([formula omitted]) using medium energy ion scattering

The structure of the Ag(111)(√3×√3)R30°-Sb surface phase formed by a nominal 1/3 ML of Sb has been investigated by medium energy ion scattering (MEIS) using 100 keV H+ ions. This surface is widely accepted as involving an outermost layer comprising a substitutional alloy, but some recent studies have indicated that the atoms in this layer may occupy hcp hollow sites at the surface, directly above second layer Ag atoms of the underlying substrate, creating an effective stacking fault at the surface-alloy/substrate interface. The new MEIS study shows that it is possible to form both faulted and unfaulted surface phases, probably dependent on the previous Sb dosing history of the Ag(111) sample. The faulted surface phase may also be accompanied by sub-surface stacking faults in the Ag(111) substrate. The higher Sb coverage (0.6±0.1 ML) Ag(111)(2√3×2√3)R30°-Sb was found to be accompanied by multiple stacking faults in some 20 outermost layers of the Ag(111) substrate to an essentially hcp structure. The effect is tentatively attributed to a lowering of the stacking fault energy induced by low concentrations of dissolved Sb. The detailed structural parameters of the faulted surface Ag(111)(√3×√3)R30°-Sb phase are in excellent agreement with those found in previous studies by electron and X-ray diffraction.