A. V. Latyshev

Transformations on clean Si(111) stepped surface during sublimation

Abstract In situ ultrahigh vacuum reflection electron microscopy has been applied to the study of the clean Si(111) stepped surface during sublimation under heating by electric current through the crystal. Reversible transformations of the surface structure from the regular monoatomic step system to step bunching were observed. These transformations were found to depend on the temperature and the electric current direction. The conditions for step bunching were discussed in terms of the electrostatic interaction between diffusion-linked monoatomic steps and adatoms.

Reflection electron microscopy study of structural transformations on a clean silicon surface in sublimation, phase transition and homoepitaxy

Abstract Reflection electron microscopy has been applied to in situ study of silicon sublimation, phase transition and homoepitaxy. Reversible rearrangement of the stepped Si(111) surface from a regular monoatomic step system to step bunching was observed. These transformations were found to depend on the temperature and direction of the electric current heating the crystal. The redistribution of about 4 × 10 14 cm −2 atoms on the Si(111) surface during (7 × 7) (1 × 1) phase transition has been established by the displacement of monoatomic steps and a change in size of two-dimensional sublimation and growth islands under this transition. The relation between the areas of the Si(001) surface having the structures (1 × 2) and (2 X 1) has been observed to depend on the direction of electric current heating the crystal. The anisotropy of the adatom diffusion on Si(001) surface has been shown. During homoepitaxy on Si(111) the intensity oscillations of the specular electron beam have been shown to be caused by the periodic changes of the surface micromorphology due to the growth of two-dimensional islands. One oscillation period corresponds to the growth of one monolayer. The areas adjacent to monoatomic step have been observed to be free from two-dimensional islands. Contrast analysis of REM images obtained by 1 7 order reflection shows that these areas on the upper terrace of step are characterized by the (7 × 7) structure, but the structure of areas free from islands on the lower terrace is (1 × 1). Analysis of individual monoatomic step motion during sublimation, phase transition and homoepitaxy showed that the energetic barrier for the incorporation of adatoms into the monoatomic step from the lower terrace is less than that from the upper terrace.

Novel Cholesterol-Based Cationic Lipids for Gene Delivery

Gene therapy based on gene delivery is a promising strategy for the treatment of human disease. Here we present data on structure/biological activity of new biodegradable cholesterol-based cationic lipids with various heterocyclic cationic head groups and linker types. Enhanced accumulation of nucleic acids in the cells mediated by the lipids was demonstrated by fluorescent microscopy and flow cytometry. Light scattering and atomic force microscopy were used to find structure/transfection activity correlations for the lipids. We found that the ability of the lipids to stimulate intracellular accumulation of the oligodeoxyribonucleotides and plasmid DNA correlates well with their ability to form in solution lipid/NA complexes of sizes that do not exceed 100 nm. Screening of the lipids revealed the most promising transfection agents both in terms of low toxicity and efficient delivery: cholesterol-based lipids with positively charged pyridine and methyl imidazole head groups and either the ester or carbamate linker.

UHV REM study of the anti-band structure on the vicinal Si(111) surface under heating by a direct electric current

Abstract Reflection electron microscopy (REM) in ultrahigh vacuum (UHV) has been applied to the in situ study of transformations of a vicinal Si(111) surface during sublimation. Besides monatomic step bunching, an anti-band formation has been found to exist after long time annealing in the step bunching temperature range. The anti-band is located near the step band from the upper side and contains monatomic steps with opposite sign compared with that in the bands. Anti-band formation is a reversible process. It has been found the influence of the DC heating electric current on the distance between step band and anti-band which is determined by the adatoms electromigration on the Si(111) surface.

Surface enhanced Raman scattering of light by ZnO nanostructures

Raman scattering (including nonresonant, resonant, and surface enhanced scattering) of light by optical and surface phonons of ZnO nanocrystals and nanorods has been investigated. It has been found that the nonresonant and resonant Raman scattering spectra of the nanostructures exhibit typical vibrational modes, E2(high) and A1(LO), respectively, which are allowed by the selection rules. The deposition of silver nanoclusters on the surface of nanostructures leads either to an abrupt increase in the intensity (by a factor of 103) of Raman scattering of light by surface optical phonons or to the appearance of new surface modes, which indicates the observation of the phenomenon of surface enhanced Raman light scattering. It has been demonstrated that the frequencies of surface optical phonon modes of the studied nanostructures are in good agreement with the theoretical values obtained from calculations performed within the effective dielectric function model.

Electromigration and gold-induced step bunching on the Si(111) surface

Abstract Step bunching instability induced by adatom electromigration is studied on the Si(111) surface at temperatures of 830–1000°C by ultrahigh vacuum reflection electron microscopy. To highlight the role of the effective charge of adatoms in step bunching, a small amount of gold atoms was deposited onto the Si(111) surface at these temperatures. It was found that in the case of sample heating by DC in the step-up direction, regular steps on the clean surface were unstable after submonolayer gold adsorption, while step bunches on the clean surface were transformed to regular steps on the gold-adsorbed surface during heating by DC in the step-down direction. We conclude that gold adsorption changes the sign of the effective charge of silicon adatoms from positive to negative. In this temperature range, the value of the adatom effective charge was estimated to be 0.004±0.001 in units of the elementary charge.

Precise surface measurements at the nanoscale

Availability of self-assembly effects occurring at the atomically clean Si(1 1 1) surface during high temperature anneals in an ultrahigh vacuum chamber for fabrication of a precise calibrator at nanoscale measurements is discussed. These effects provide formation of ordered monatomic step arrays assembled by step bunches divided by almost singular surface areas with widely spaced monatomic steps suitable for calibration of atomic force microscopes. The monatomic step height at the Si(1 1 1) surface and its replication by the native oxide layer was attested by the high-resolution transmission electron microscopy followed by Digital Micrograph analysis and found to be equal to interplanar spacing (0.314 nm) in the volume of Si crystal with ±0.001 nm of accuracy. Excellent replication of the monatomic step height by oxide film covering the Si surface makes available precise AFM calibration at the nanoscale at ambient conditions. The averaged step height measured by AFM scanning of 1 × 1 µm2 is found to be 0.314 ± 0.003 nm (~1% of uncertainty). However, when the scan area becomes bigger than 2 × 2 µm2, the height measurement uncertainty increases sharply 15 times (0.310 ± 0.034 nm). We assume that this is due to differences between piezo element calibrations at small and large scan areas. The height measurement uncertainty for step bunches with well-defined quantity of steps (28) even at a large scan area (18 × 18 µm2) turns out to be 0.3%.

UHV-REM study of gold adsorption on the Si(111) surface

The initial stages of gold adsorption on the silicon (111) surface with different morphology have been investigated by ultra high vacuum reflection electron microscopy (UHV-REM). The main attention was paid to the sequent stages of three dimensional island nucleation of gold on the Si(111) surface at various substrate temperatures. The influence of step bunches created under DC heating of the sample on surface distribution of the three dimensional gold islands has been revealed. Contribution of the heating electric current to the migration and shape modification processes of the three dimensional gold islands was investigated. A dominant migration of the gold islands in the step-up direction independently on the direction of the heating electric current was found. The obtained results evidence that the creation of chains of gold islands without application of any lithography is possible by means of the self-organized processes on the silicon (111) surface during gold adsorption.

Nanotechnologies in semiconductor electronics

The various technologies for fabrication micro- and nanosized systems included semiconductor, metal and insulator structures are reviewed on the base of the data obtained in Novosibirsk Institute of Semiconductor Physics. Main attention is devoted to development of method of molecular beam epitaxy, silicon-on-insulator fabrication process, electron beam lithography and scanning probe nanolithography.

In situ REM study of monatomic step behaviour on Si(111) surface during sublimation

Abstract Reflection electron microscopy (REM) in ultra-high vacuum (UHV) has been applied to an in situ study of transformations of the vicinal Si(111) surface during sublimation. Besides monatomic step bunching described previously [Surf. Sci. 213 (1989) 157], an antiband formation has been found to exist after long-time annealing at step-bunching temperatures. The antibands are located near the step bands from the upper side and contain monatomic steps with opposite sign compared with that in the bands.