Vol 54, No 11 (2018)

MnO₂ nanolayers (~29 nm) produced on the surface of single-crystal GaAs wafers by magnetron sputtering have been shown to act as oxygen transfer agents for the thermal oxidation of the semiconductor. The presence of MnO₂ increases the oxide film growth rate by three to nine times relative to stimulator-free oxidation of GaAs. The films thus grown range in thickness from 35 to 200 nm and possess good dielectric properties (resistivity on the order of ~10¹⁰ Ω cm and dielectric strength in the range (5–8) × 10⁶ V/cm). According to X-ray diffraction data, the films are enriched in oxidized arsenic and have a regular grain structure in the surface layer, with a roughness height within 30 nm (according to atomic force microscopy data).

As a result of heterosegregation, we have obtained a gallium nitride phase, including oriented micro- and nanocrystallites, on the surface of gallium arsenide. The mechanism underlying the growth of crystallites of the surface phase has been analyzed.

We have carried out an integrated study of technological steps in the preparation of indium phosphide substrates for the epitaxial growth of heterostructures. We have investigated the surface morphology and condition of indium phosphide in (100)-oriented substrates and tested various chemical etchants for final chemical surface processing. Our results demonstrate that an optimal substrate preparation process is two-step chemical–mechanical polishing on both sides using zeolite slurries, with chemical polishing in a mixture of bromine and isopropanol as the final step.

We have studied the effect of size factors of substrates on the formation of TiO₂ layers with a well-developed acicular–fibrous structure on VT-20 titanium. The proposed technique allows a TiO₂ layer with a large surface area to be produced by etching in a KOH solution (without using hydrothermal synthesis conditions), followed by heat treatment. The layers thus produced consist of fibers 20–100 nm in diameter and up to 1 μm in length, which are strongly bonded to the substrate surface. Their phase composition is dominated by rutile. The fiber growth process is shown to be influenced by substrate etching and heat treatment conditions, weakly influenced by the curvature of the substrate, and independent of its dimensions.

This paper presents a comparative study of the catalytic properties of the ZnMn₂O₄ and ZnMnO₃ manganites for carbon oxidation. The manganites have been prepared by a sol–gel process using reaction mixtures with different ratios of the constituent oxides. The particles of the two manganites were comparable in size (200–500 nm) and specific surface area (34–42 m²/g) and ensured similar behaviors of the catalytic reaction in the temperature range 260–470°C. The catalytic activity of the perovskite phase ZnMnO₃ can be accounted for by the high absolute oxygen nonstoichiometry, δ = 0.29, as determined by iodometry. Contact interaction between spinel ZnMn₂O₄ and carbon leads to the reduction of Mn³⁺ to Mn²⁺ and is accompanied by Mn₃O₄ formation. The change in the relationship between the constituent oxides in the surface layer of the ZnMn₂O₄ particles leads to the formation of the active perovskite phase ZnMnO₃, which initiates catalytic reaction.

Using the Pechini process in the presence of citric acid and mannitol, followed by annealing in the range 800–1000°C, we have synthesized nanostructured rare-earth borate tungstates crystallizing in hexagonal symmetry (sp. gr. P6₃): undoped Gd₃BWO₉ and Gd_{3 –x – y}Yb_xEr_yBWO₉, containing Yb³⁺ and Er³⁺ active ions. We have measured upconversion luminescence spectra of the Er³⁺ ions in Gd_{3 –x – y}Yb_xEr_yBWO₉ for the ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions under excitation by an IR laser (λ = 974 nm). The spectra consist of two groups of broadened bands in the red and green spectral regions. Brighter luminescence is observed in the green spectral region, which contains two bands corresponding to the Er³⁺²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions. We have analyzed the effect of annealing temperature on the upconversion luminescence intensity of Gd_{3 –x – y}Yb_xEr_yBWO₉ and determined the energy yield of upconversion luminescence in the Gd_{3 – x – y}Yb_xEr_yBWO₉ borate tungstates. The highest value, B_en = 0.19%, has been obtained for the phosphor with Yb : Er = 7 : 1.

We have studied luminescence and electron paramagnetic resonance (EPR) spectra of manganese-activated fluorozirconate glasses. In ZrF₄–BaF₂–LaF₃–AlF₃–NaF (ZBLAN) glasses, manganese gives a green luminescence band peaking at 545 nm. Partial chlorine substitution for fluorine in the glass leads to a shift and broadening of the luminescence band. Complete BaCl₂ substitution for BaF₂ in ZBLAN〈Cl〉 fluoride chloride glass increases the peak emission wavelength to 610 nm. The observed distinctions between the EPR spectra of the Mn-doped ZBLAN and ZBLAN〈Cl〉 glasses correlate with those between their luminescence spectra. The effect of chlorine substitution for fluorine on the luminescence and EPR spectra of the fluorozirconate glass is tentatively attributed to a change in the oxidation state of the manganese ions.

A process has been proposed for the fabrication of reaction-bonded dicalcium phosphate dihydrate-based composite materials possessing rather high strength owing to reinforcement with discrete glass fiber and compaction. We have studied the effects of the heat treatment temperature of tricalcium phosphate, a precursor to dicalcium phosphate dihydrate, and filler concentration on the chemical and physicomechanical properties of the composite materials thus produced.