V. E. Roslikov

Formation mechanisms of nanocomposite layers based on multiwalled carbon nanotubes and non-stoichiometric tin oxide

Nanocomposite layers based on multiwalled carbon nanotubes (MWCNTs) and non-stoichiometric tin oxide (SnOx) have been grown by magnetron deposition and CVD methods. In the case of the CVD method, the study of the structure and phase composition of obtained nanocomposite layers has shown that a tin oxide “superlattice” is formed in the MWCNT layer volume, fixed by SnOx islands on the MWCNT surface. During magnetron deposition, the MWCNT surface is uniformly coated with tin oxide islands, which causes a change in properties of individual nanotubes. Electrical measurements have revealed the sensitivity of nanocomposite layers to (NO2)− molecule adsorption, which is qualitatively explained by a change in the conductivity of the semiconductor fraction of p-type MWCNTs.

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.

Formation of por-Si/SnOx nanocomposite by high-power ion beams of nanosecond duration

Layers of por-Si/SnOx nanocomposite formed by high-power ion beam irradiation with nanosecond duration have been studied. The results of structural and elemental analyses of these layers are presented. The high sensitivity of this nanocomposite to low NO2 concentrations at room temperature has been revealed.

Formation of two-layer composite-on-insulator structures based on porous silicon and SnO x . Study of their electrical and gas-sensing properties

The objective of this work is to fabricate and study multilayer “composite-on-insulator” sensor structures based on porous silicon and nonstoichiometric tin oxide. Two-layer structures “macroporous silicon-mesoporous silicon” on single-crystal silicon with sharp geometrical boundaries are grown. Test “composite-on-insulator” structures are fabricated. Oxide on macroporous silicon walls and a buried layer of oxidized mesoporous silicon play the role of the insulator. Nonstoichiometric tin oxide deposited onto the extended surface of oxidized macroporous silicon by chemical vapor deposition (CVD) is the sensitive layer. The gas sensitivity is studied upon exposure to NO2 and degassing in air at room temperature. The sensitivity of the por-Si/SnOx composite structures is higher than the sensitivity of tin-oxide film samples.

Electrical and gas sensing properties of por-Si/SnO x nanocomposite layers

The electrical characteristics and chemical reactant sensitivity of layers of heterogeneous nanocomposites based on porous silicon and nonstoichiometric tin oxide por-Si/SnOx, fabricated by the magnetron sputtering of tin with subsequent oxidation, are studied. It is shown that, in the nanocomposite layers, a system of distributed heterojunctions (Si/SnOx nanocrystals) forms, which determine the electrical characteristics of such structures. The sensitivity of test sensor structures based on por-Si/SnOx nanocomposites to NO2 is determined. A mechanism for the effect of the adsorption of NO2 molecules on the current-voltage characteristics of the por-Si(p)/SnOx(n) heterojunctions is suggested.

Spectroscopic ellipsometry study of porous silicon-tin oxide nanocomposite layers

The layer-by-layer distribution of components in a porous silicon-tin oxide nanocomposite produced by the following three methods is studied by spectroscopic ellipsometry: chemical vapor deposition, atomic layer deposition, and magnetron sputtering. It is shown that, in the nanocomposites fabricated by these methods, SnOx penetrates to a depth more than 400 nm and is nonuniformly distributed over the porous layer thickness. The nanocomposite prepared by magnetron sputtering followed by heat treatment has the maximum penetration depth and the maximum uniformity of layer-by-layer SnOx distribution.

Study of the interaction mechanisms between absorbed NO 2 and por -Si/SnO x nanocomposite layers

The interaction mechanisms between NO2 molecules and the surface of por-Si/SnOx nanocomposites obtained by magnetron deposition and chemical vapor deposition (CVD) are studied by infrared absorption spectroscopy and electron paramagnetic resonance methods. The observed increase in the free carrier concentration in the por-Si/SnOx nanocomposite layers is explained by a change in the charge state of Pb centers due to the formation of neutral “surface defect-adsorbed NO2 molecule” complexes with free carrier generation in the crystallite bulk. In the nanocomposite layers grown by the CVD method, the increase in the free hole concentration during NO2 adsorption is much less pronounced in comparison with the composite grown by magnetron deposition, which is caused by the competing interaction channel of NO2 molecules with electrically neutral Pb centers.

Formation and properties of the buried isolating silicon-dioxide layer in double-layer “porous silicon-on-insulator” structures

The oxidation of mesoporous silicon in a double-layer “macroporous silicon–mesoporous silicon” structure is studied. The morphology and dielectric properties of the buried insulating layer are investigated using electron microscopy, ellipsometry, and electrical measurements. Specific defects (so-called spikes) are revealed between the oxidized macropore walls in macroporous silicon and the oxidation crossing fronts in mesoporous silicon. It is found that, at an initial porosity of mesoporous silicon of 60%, three-stage thermal oxidation leads to the formation of buried silicon-dioxide layers with an electric-field breakdown strength of Ebr ~ 104–105 V/cm. Multilayered “porous silicon-on-insulator” structures are shown to be promising for integrated chemical micro- and nanosensors.

The effect of the hydrogen state in lattice on the introduction efficiency of donor centers in oxygen-containing silicon

The efficiency of the introduction of donor centers in oxygen-containing Si by heat treatments at 450°C after preliminary hydrogenation in hydrogen plasma and irradiation with 60Co γ-ray photons was studied. It is shown that the highest rate of the donor-center introduction is observed in the samples that contain atomic hydrogen. Irradiation with 60Co γ-ray photons of Si with a layer preliminarily hydrogenated in hydrogen plasma leads to the release of atomic hydrogen from bound states. This makes the rate of the introduction of donor centers higher in subsequent heat treatments at 450°C.