V. V. Bolotov

Fabrication of por-Si/SnOx nanocomposite layers for gas microsensors and nanosensors

Two-phase nanocomposite layers based on porous silicon and nonstoichiometric tin oxide were fabricated by various methods. The structure, as well as elemental and phase composition, of the obtained nanocomposites were studied using transmission and scanning electron microscopy, Raman spectroscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. The results obtained confirm the formation of nanocomposite layers with a thickness as large as 2 μm thick and SnOx stoichiometry coefficients x = 1.0–2.0. Significant tin diffusion into the porous silicon matrix with Deff ≈ 10−14 cm2 s−1 was observed upon annealing at 770 K. Test sensor structures based on por-Si/SnOx nanocomposite layers grown by magnetron deposition showed fairly high stability of properties and sensitivity to NO2.

Effect of ethanol on optical and electrical parameters of porous silicon

The effect of ethanol vapor adsorption on the properties of porous silicon-based structures was studied by Raman scattering, infrared spectroscopy, and I–V characteristics. A decrease in the resistance of porous silicon layers and a simultaneous increase in the intensity of the band of infrared absorption caused by the presence of (OH)−…x(OH)− (x = 1, 2, …) groups upon exposure to ethanol vapor and vice versa in the case of degassing were detected. The observed effect is attributed to a change in the depletion region in por-Si skeleton elements due to the electrostatic interaction of (OH)− groups with positively charged surface defects. The effect of hydrogen-bonded Si-OH…OH-C2H5 centers on the increase in the silicon conductivity is discussed.

Transverse optical phonon splitting in GaAs/AlAs superlattices grown on the GaAs(311) surface studied by the method of Raman light scattering

GaAsn/AlAsm superlattices grown on the GaAs (311)A and (311)B surfaces by molecular-beam epitaxy were studied by Raman light scattering. The form of the Raman scattering tensor allowed the TOy and TOx modes to be separately observed using various scattering geometries (the y and x axes correspond to atomic displacements along and across facets formed on the (311)A surface, respectively). The TO1y and TO1x modes exhibited splitting in superlattices grown on a faceted GaAs(311)A surface. The degree of splitting increased for superlattices with an average GaAs layer thickness of 6 monoatomic layers and less. No splitting was observed for superlattices grown under the same conditions on the (311)B surface, which indicates that the splitting effect is probably due to the formation of GaAs quantum wires on the faceted (311)A surface.

Formation of silicon nanocrystals with preferred (100) orientation in amorphous Si:H films grown on glass substrates and exposed to nanosecond pulses of ultraviolet radiation

Using Raman scattering, it was ascertained that silicon nanocrystals with sizes exceeding 2 nm are formed in amorphous silicon films exposed to nanosecond ultraviolet laser radiation with energy densities ranging from 75 to 150 mJ/cm2; it is shown that these nanocrystals have sizes no smaller than 2 nm and have preferred (100) orientation along the normal to the film surface. In a system of mutually oriented Si nanocrystals, anisotropic behavior of the Raman scattering intensity was experimentally detected in various polarization configurations, which made it possible to determine the volume fraction of oriented nanocrystals. The orientational effect is presumably caused by both the macroscopic fields of elastic stresses in the film and the local fields of elastic stresses around the nanocrystals.

Effect of the catalyst on structural and electrophysical characteristics of the layers of nitrogen-containing carbon nanotubes obtained by gas phase synthesis

The morphological, structural, and electrophysical properties of the layers of nitrogen-containing carbon nanotubes synthesized by pyrolysis of acetonitrile on a silicon oxide-silicon substrate with a Ni layer catalyst, Fe volumetric catalyst obtained by the thermal decomposition of ferrocene ((C5H5)2Fe), and their combination were studied. It was found that the type of catalyst affects the morphology, thickness, homogeneity, structure, and defect content of the obtained films and individual nanotubes. The electrophysical characteristics of the carbon nanotubes are similar and have an activation dependence on temperature.

Effect of halogens on the formation and properties of the porous silicon layers

The method of atomic-force microscopy is used to study the morphology of the surface of porous silicon layers formed on the p-Si substrate and obtained by anodic etching in an electrolyte with addition of free halogens (bromine, iodine) and potassium halogenides (KCl, KI). It is established that the presence of halogens in the electrolyte is conducive to formation of large pores with the diameter as large as 150 nm. The mechanism of an increase in the pore sizes with involvement of halogens is related to an increase in the concentration of free holes due to formation of donor-acceptor pairs in the case of adsorption of halogens on the silicon surface.

Adsorption of hemoglobin molecules on porous silicon

It is established that hemoglobin can be immobilized in pores and on the surface of porous silicon. This is confirmed by the data of atomic force microscopy and Fourier-transform IR spectroscopy.

IR luminescence in thermally treated silicon

Near-edge IR luminescence with peak emission at E = 1.084 eV has been studied in n- and p-type silicon samples with different contents of interstitial oxygen under excitation with a Nd: YAG laser diode. Thermal treatments of the samples demonstrated that the luminescence nearly completely disappears upon thermal treatments at T = 1050°C and is partly restored in two stages in subsequent thermal treatments in the temperature range 550–800°C. The temperature intervals of luminescence quenching and restoration (500–600 and 700–800°C) correlate with the temperature ranges of dissolution for shallow oxygen precipitates (1000°C) and the generation of oxygen-containing thermal defects, the so-called thermal donors of types I and II. The data obtained suggest that the electronic states related to thermal donors are traps for nonequilibrium carriers and the emptying of these traps contributes to the near-edge emission.

The effect of NO2 adsorption on optical and electrical properties of porous silicon layers

The Raman spectra and current-voltage characteristics of porous silicon layers are studied before and after exposure to NO2. It is shown that spherical nanocrystallites with the diameter of approximately 6–8 nm are present in the samples’ structure. The effect of NO2 brings about a decrease in the resistance of porous Si by two-three orders of magnitude. An increase in the conductance of the structures at gas concentrations as high as 2000 ppm and a drastic decrease in this conductance if the concentration exceeds the above value are observed. This effect is explained in the context of the model that implies the formation of additional defects of the type of dangling silicon bonds at the Si/SiO2 interface as a result of oxidation of the porous silicon surface. These defects are traps for holes and reduce the increase in the hole concentration.

Formation of poly-Si films on glass substrates using excimer laser treatments

Solid phase crystallization process in thin amorphous silicon films on glass substrates was studied with application of excimer laser annealing (ELA) and rapid thermal annealing (RTA) for stimulation of nucleation. Using of ELA allowed to create homogeneous polycrystalline silicon films on glass with grain sizes up to 3 micrometers at temperatures below 550 degree(s)C. Using of RTA reduced the incubation time of nucleation from 100 to 6 hrs. The textured silicon films on glass with predominant orientation (110) and sizes of textured areas up to 30 micrometers were manufactured using the excimer laser stimulation of nucleation. The mechanism of mechanical stresses influence on grain orientation was suggested as well as it was theoretically shown, that internal stresses retard the nucleation process. Deformation addition to chemical potential difference were estimated for nucleation in amorphous silicon as 11.4 meV per nucleated atom. Retardation of crystallization after Ge implantation was observed and it was proposed to be explained within deformation mechanisms.