Clusters as crystal fragments of silicon bulk

Silicon clusters of 13 to 43 atoms were studied with the semi-empirical method SINDO1. Crystalline structures of face-centered cubic (fcc), hexagonal close packed (hcp) and diamond type and noncrystalline structures of icosahedral type were compared. Noncrystalline structures are most stable for clusters up to 13 atoms. Clusters with 19 and more atoms of the fcc structure are preferable to the less dense diamond structure. With more than 35 Si atoms, the diamond structure is favored over the hcp structure. The binding energy of fcc and hcp structures decreases and that of the diamond structure increases with increasing cluster size. A similar trend is observed for the HOMO-LUMO energy gap which is taken as a measure of the band gap.     

Synthesis of novel liquid crystal compounds with aromatic amide mesogenic cores

The synthesis and properties of a new series of compounds having aromatic amide mesogenic cores are reported. Most of these new compounds are thermotropic mesogens. In consideration of the fact that aromatic amides form crystals of high melting point which is unfavourable for the formation of thermotropic liquid crystals, we make use of lateral substitution to decrease both the packing efficiency and the hydrogen bonding, so that the melting temperature of the aromatic amides is sufficiently depressed. The lateral substituent used in these new compounds is bromine. In order to investigate the influence on properties of the end groups, different alkoxy, alkyl and other groups are used at the two ends of the rod-like molecules. The two ends are either identical or different, with an electron-donating alkoxy as one end and the electron-accepting cyano group as the other. The results indicate that appropriate lateral and terminal substitution is essential for the aromatic amides to form thermotropic liquid crystals. The peculiar mesophase characterized by an X-ray diffraction pattern of a SmC phase, but a texture of a nematic phase is also noted.     

Raman-microanalysis of strain and crystal orientation in laser-crystallized silicon

Summary In this work we report on the use of Micro-Raman-Spectroscopy (MRS) to determine residual strain in cw-laser crystallized silicon on insulator (SOI)-thin layers and to measure slight deviations in crystal orientation which occur along the crystallization path during the crystallization process. The spatial resolution achieved is 0.8 m. Relative peak positions of the silicon phonon line have been measured with a reproducibility better than ±0.02 cm^–1. Frequency shifts in the crystalline layer were in the range of –1.5 cm^–1. This corresponds to a tensile biaxial stress of 3.8 kbar. Polarization dependent intensity monitoring has been carried out to measure the local tilt of crystal orientation in crystallized thin layers. For this we use polished cross-cut specimens which enable a change in observation direction, thus achieving an angular resolution of ±1°. At a crystallization path length of 90 m the measured backward tilt of the [001]-axis was 3°.     

Alpha-activation analysis of nitrogen in boron-doped silicon single crystal

The rapid dissolution of silicon and the substoichiometric separation of¹⁸F were studied in order to determine nitrogen in a boron-doped silicon single crystal by activation analysis via¹⁵N(,n)¹⁸F. The silicon dissolution method was developed by using a nitric acid-based solution containing a known amount of ammonium fluoride, instead of hydrofluoric acid, as the¹⁸F carrier. The silicon could be dissolved to a depth of 100 m with a 10 mm Ø in 9 minutes. The conditions for¹⁸F steam distillation and substoichiometric precipitation as lanthanum fluoride were improved. 50% of the fluorine could be separated substoichiometrically even though the carrier amount was increased from 6–12 mmol to more than 16 mmol in order to add it to the dissolving solution. This dissolution method and improved substoichiometric separation were used to determine nitrogen in a boron-doped silicon single crystal. The nitrogen concentration was found to be less than the detection limit (50 ppb).     

Determination of trace element in a silicon single crystal

Neutron activation analysis was applied to the determination of trace impurity elements in a silicon ingot. Detection limits of 36 elements were calculated semi — empirically and compared with minimum concentrations detected in a silicon single crystal. The sources of the impurities were estimated from element concentrations detected in polycrystalline silicon and a quartz crucible. Segregation coefficients were determined from the concentration curves in a single crystal and discussed by comparing with reported values.     

Determination of trace elements in a silicon single crystal

Instrumental neutron activation analysis was applied to the determination of trace impurity elements in a silicon single crystal. Impurity concentrations in polysilicon melt were compared with those in a single crystal. Impurity concentrations in artificial quartz were also compared with those in natural quartz. Segregation coefficients in Au, Ir and Sb were determined at different concentrations. The segregation coefficient of an element in a silicon single crystal is constant over a critical concentration, it becomes larger gradually under the critical one, and at last it becomes larger than 1.     

Structure of the starch granule--a curved crystal

A structure model of the molecular arrangement in native starch proposed earlier is further considered, with special regard to the lateral packing of cluster units. The amylopectin molecules are radially distributed, with branches concentrated in clusters. Within each cluster the polyglucan chains form double helices which are hexagonally packed. The clusters form spherically concentric crystalline layers with amylose in an amorphous form acting as a space-filler. A translational mechanism for the change of helical direction at boundaries between clusters is proposed which can account for variations in the curvature of the concentric layers. The model is related to X-ray diffraction data and optical birefringence, considering dissembly at gelatinization. The structure is also discussed in relation to biosynthesis. Some aspects of gelatinization, such as the recent glass-transition approach, are then considered.     

Mechanisms of interactions between atomic hydrogen and vacancies in the silicon crystal lattice

SCF-MO-LCAO calculations in the MINDO/3 approximation were used to determine some mechanisms of interactions between atomic hydrogen and silicon lattice vacancies and between interstitial silicon atoms and hydrogen-charged lattice vacancies. In a completely hydrogen-charged vacancy, hydrogen atoms are localized on the Si−Si bond of the second coordination sphere with respect to the vacancy, so that the crystal lattice is ordered around the vacancy. The capture of atomic hydrogen in any charge state by a vacancy significantly decreases the potential barriers of the annihilation of the Frenkel pairs. After an interstitial atom has been captured by a hydrogen-charged vacancy, it is energetically profitable for the hydrogen atom to transfer to a neighboring vacancy. The interaction mechanisms revealed are consistent with the model of the accelerated annealing of lattice vacancy defects by high-frequency plasma treatment. In addition, the calculation results suggest that materials with hydrogen-charged silicon should be more stable to ionizing radiation than materials with hydrogen-free silicon, since the probability that the interstitial silicon involved in the Frenkel pair will be recaptured by the lattice point is rather high. Kiev Polytechnical Institute. Translated fromZhurnal Strukturmoi Khimii, Vol. 37, No. 1, pp. 22–28, January–Feburary, 1996. Translated by I. Izvekova     

Application of instrumental neutron activation analysis in Czochralski silicon crystal growth

Contamination sources of trace elements introduced into silicon crystals grown by the Czochralski technique are investigated: the signale comparator method in neutron activation analysis is applied to the determination of impurities in a silica crucible polycrystalline silicon, grown crystals and residual melt. When particular care is taken to avoid contamination during crystal growth, it is possible to obtain high purity silicon crystals in which impurity contents are almost below the detection limits of the analytical method.     

Structure of the core of a screw dislocation in smectic A liquid crystals

The structure of the core region of a screw dislocation in smectic A liquid crystals is investigated by a Ginzburg-Landau type expansion of the smectic order parameter. The core radius and the energy of a screw dislocation are discussed.     

Patterning nanocluster polystyrene brushes grafted from initiator cores on silicon surfaces by lithography processing

In this study, we formed grafted polystyrene (PS) brushes possessing nanocluster structures through atom transfer radical polymerization from initiator cores presented on Si surfaces that had been generated using reactive ion etching (RIE). We established the surface grafting polymerization kinetics of the nanoclustered PS chains on the Si surfaces to fit their experimentally determined thickness (ellipsometry) and number-average molecular weight (M _n) of “free” PS (gel permeation chromatography). The propagation rate (k _p) and active grafting species deactivation rate (k _d) were obtained from reactions involving styrene concentrations from 0.2 to 2 M. We also used scanning electron microscopy to observe the morphologies of the PS grafted to the surfaces after various reaction times at various styrene concentrations. The PS brushes grafted onto the Si surfaces under styrene concentrations of 0.2, 0.5, 1, and 2 M exhibited clustered structures having cluster diameters of 12, 28, 42, and 45 nm, respectively; from these observations, we calculated the critical grafting density. In addition, we generated highly dense, well-defined patterns of PS on patterned Si(100) surfaces through the use of a very-large-scale integration process involving electron beam lithography and RIE. We employed the RIE system to generate a high density of reactive species at the bottom of the trenches for graft polymerization. After 21 h of grafting, AFM imaging revealed dense line patterns of nanoclustered PS.     

Structure characterization of free-standing filaments drawn in the liquid crystal state

Stable free-standing liquid filaments formed by some layered mesophases of bent-core mesogens are unique structures. Some of their physical properties have been analyzed in recent studies, but their microscopic structure and conditions for stability have still been unclear. We explore details of filament shapes and surface profiles of filaments drawn in liquid crystal phases of bent-core mesogens by AFM and SEM measurements, and we present a microscopic structure model. Conclusions on the stabilizing mechanisms are drawn. Qualitative differences in mechanical properties are found for different mesophases, even though the macroscopic appearance of the filaments is very similar.     

Structure of hydrogenated silicon clusters. Small clusters

The ground state structures of silicon hydride clusters Si_nH_m containing up to 12 silicon atoms are obtained by numerical modeling. The cluster geometry is optimized for a wide set of initial structures using the MINDO/3 approximation for Monte Carlo simulation of interatomic interactions. The energy of the cluster depending on the content of hydrogen is studied, and it is shown that the Si-H and Si-Si bond energies depend little on the cluster size.     

Structure of hydrogenated silicon clusters. Medium-sized clusters

The structures of the Si_nH_m clusters containing 10 to 70 silicon atoms and different numbers of hydrogen atoms are calculated in the MINDO/3 approximation using the Monte Carlo technique. The geometry optimization of the clusters showed the existence of several structural varieties that determine the optimal geometry of the clusters differing in size and hydrogen content. Small clusters (n < 20) with various geometrical configurations often have a hollow structure if the number of silicon atoms in the cluster is more than 12. For 20 ≤ n < 60 and the hydrogen content m ≤ n, hollow spheroidal geometry is most favorable. Staring from n ≈ 56−60, diamond structures are more favorable. The ratio c = m/n < 1, at which the spheroidal structure remains optimal, decreases with further increase in n.