Recombination of preadsorbed oxygen atoms at the surface of solids

Relaxation measurements are used to establish that for a concentration of gas-phase oxygen atoms n=10¹⁴ cm⁻³ heterogeneous recombination of oxygen atoms at the surface of copper and zinc sulfide takes place by a mechanism involving recombination of preadsorbed (precursor state) atoms. A hypothesis is put forward for the existence of a universal mechanism for the catalytic acceleration of heterogeneous chemical reactions. Pis’ma Zh. Tekh. Fiz. 25, 27–32 (August 12, 1999)     

Use of high-purity AlxGa1−x as layers in epitaxial structures for high-power microwave field-effect transistors

The fabrication of high-purity layers of Al_xGa_1−x As solid solutions in the range 0⩽x⩽0.38 by molecular beam epitaxy is reported. The low-temperature photoluminescence spectra of these layers reveal predominantly the free exciton recombination line (X). The narrow width of the X line, the high intensity ratio of this line to that of the band-acceptor transition line, and the linear dependence of the X line intensity on the excitation power density in the range between 1×10⁻⁴ and 100V·cm⁻² indicate a low concentration of background impurities in these layers. Using this material in pseudomorphic AlGaAs/InGaAs/GaAs heterostructures for high-power microwave transistors produced devices with a specific saturated output power of 0.9 W/mm at 18 GHz. Pis’ma Zh. Tekh. Fiz. 25, 8–15 (August 12, 1999)     

Laser implantation of impurities in cadmium telluride crystals

Processes of laser implantation of shallow donors (aluminum and indium) and an acceptor (antimony) in CdTe crystals (n,p∼10¹⁵ cm⁻³) are investigated. Thin dopant films vacuum deposited on the etched surface of the crystals are irradiated by ruby (λ=0.694 μm) and Nd:YAG (λ=1.06 μm) laser pulses (pulse duration 20 ns) over a wide energy interval (0.1–1.8 J/cm²). The irradiated surfaces are studied by x-ray microanalysis, Auger spectroscopy, and the thermopower method. It is it is shown that irradiation by a Nd:YAG laser produces a uniform doping of a subsurface layer of the crystal by aluminum. The implantation of indium leads to the formation of a precipitate. The concentration of implanted impurities reaches 10¹⁹–10²¹ cm⁻³. Pis’ma Zh. Tekh. Fiz. 24, 1–6 (June 12, 1998)     

Critical importance of the single-level approximation to account for the highly nonmonotonic dependences of carrier lifetimes on recombination impurity concentration

This paper is a continuation of an analysis regarding an increase in the lifetimes of nonequilibrium electrons π_n and holes πp by several orders of magnitude, observed with increasing concentration of recombination centers. It is shown that a substantial increase in πn and πp may also occur for three charge states of the recombination impurities N, and the curves πn=f(N) and π n=f(N) may each have two minima and maxima. Pis’ma Zh. Tekh. Fiz. 23, 39–45 (April 12, 1997)     

Recombination of two vortex filaments and jet noise

The recombination of two vortex filaments in a viscous incompressible fluid is analysed by the use of the vorticity equation. The analysis is confined to a local flow field, where the recombination process occurs, and is based on several assumptions, such as the conservation of the fluid impulse, spatial symmetry of the flow field etc. The flow field is expanded as polynomials of coordinates, and variations of their coefficients are obtained by the use of the vorticity equation. It is proved that the process is completed within a short time ofO (σ ²/Γ) and the viscous effect is essential;σ and Γ are the size and the circulation of the vortex filaments, respectively. This result is applied to predict the far-field noise of a circular jet by assuming that the main noise source is the recombination process in deformed vortex rings in the jet near field. The predicted noise intensity shows theU dependence and has an additional new factor (d/σ)⁶;U is the jet velocity andd is the average spacing between vortex rings.     

Spinodal decomposition in fine grained materials

We have used a phase field model to study spinodal decomposition in polycrystalline materials in which the grain size is of the same order of magnitude as the characteristic decomposition wavelength (Xsu). In the spirit of phase field models, each grain (i) in our model has an order parameter (η _i) associated with it;η _i has a value of unity inside the ith grain, decreases smoothly through the grain boundary region to zero outside the grain. For a symmetric alloy of composition,c = 0–5, our results show that microstructural evolution depends largely on the difference in the grain boundary energies, y_gb, of A-rich (a) and B-rich (β) phases. If Y _gb ^α is lower, we find that the decomposition process is initiated with an a layer being formed at the grain boundary. If the grain size is sufficiently small (about the same as λ_sd), the interior of the grain is filled with the β phase. If the grain size is large (say, about 10λ_SD or greater), the early stage microstructure exhibits an A-rich grain boundary layer followed by a B-rich layer; the grain interior exhibits a spinodally decomposed microstructure, evolving slowly. Further, grain growth is suppressed completely during the decomposition process.     

A decomposition of the class of semiconvex games

Summary The paper studies the classSC ^N of cooperative games with player setN which have the semiconvexity property.SC ^N is decomposed into an algebraic sum of convex cones of games for which generating sets are available. The union of these sets thus forms a generating set forSC ^N . Special attention is paid to one of the considered cones, in the decomposition ofSC ^N . In particular, the so called airport savings gamesW _y, ℝ ^N , defined byw _y (S)=Σ _jεS Y _i −max _jεS Y _j for 0 ≠S⋞N, are emphasized. This work is part of the Ph.D. thesis of the first author (Derks 1991).     

Polarization Effect in ESR of Co Doped CuGeO<Subscript>3</Subscript>

Electron spin resonance of a single crystal of CuGeO₃ doped with 2% of Co has been studied at f = 99 GHz in temperature range 1.8–50 K. Contributions to ESR absorption from Cu²⁺ chains and from Co²⁺ ions were derived. It is found that functions obtained for ESR integrated intensities: Curie-Weiss for Cu²⁺ (χCu ∼ C _Cu/(T + Θ), with Θ = 92 K) and Curie for Co²⁺ (χCo ∼ C _Co/T) are well consistent with temperature dependence of static magnetic susceptibility. Strong dependence of ESR absorption on polarization of oscillating magnetic field was discovered for Co²⁺ contribution. Polarization effect was studied for magnetic field applied along a, b and c directions. Values of g-factors of resonance lines are presented.     

Heating of a current filament and formation of constrictions in a pulsed vacuum discharge

It is shown that a rapid current rise in a pulsed vacuum discharge is accompanied by enhanced compression of the current filament by its self-induced magnetic field. As a result, a constriction forms at a distance L≃1 mm from the cathode and the electron temperature increases to 10²–10³ eV at currents of order 1 kA. This behavior explains the observed increase in the degree of ion charge and the appearance of x-rays as the current pulse length decreases. The criterion for a rapid rise is the condition τ<L/V≃10⁻²⁷ s, where τ is the characteristic current amplification time and V≃10⁶ is the velocity of the cathode plasma. Pis’ma Zh. Tekh. Fiz. 24, 50–56 (September 26, 1998)     

Systematic hardness measurements on mixed and doped crystals of rubidium halides

Efforts are made to improve the hardness of rubidium halide crystals by (i) solid solution hardening and (ii) impurity hardening. Systematic microhardness measurements have been made on rubidium halide mixed crystals (RbBr-RbI and KI-RbI) and rubidium halide crystals doped with Sr²⁺ ions. The composition dependence of the hardness of mixed crystals follows the law ΔH _v =K x (1− x),where ΔH _v is the enhancement in hardness,K a constant andx and (1 −x) the concentrations of the first and second component of the mixed crystals, respectively. The hardness of doped crystals increases with the concentrationC of the dopant according to the law, ΔH _v+6 =k C ^m ,wherek andm are constants. The relative efficacy of the two methods of hardening is discussed.     

Electrical and photoelectric properties of GeS layered crystals grown by different techniques

Thermally stimulated currents and anisotropic electrical conductivity are studied in GeS layered crystals prepared by the Bridgman-Stockbarger, Pizzarello, and sublimation techniques. All the crystals arep-type, regardless of the growth technique, owing to the presence of Ge vacancies. The conductivity anisotropy in the melt-grown crystals is high compared to the vapor-grown GeS. The anisotropy rises exponentially with temperature. The concentrations and ionization energies of traps in GeS crystals are determined from thermally stimulated current curves. The spectral response of the photocurrent through the crystals prepared by sublimation, whose structural perfection is higher than that of the melt-grown crystals, is governed by the spectral dependence of the absorption coefficient forad ≪ 1 (near-edge region) and by the spectral dependence of reflectivity for αd > 1 (high-α region). Regardless of the growth technique, the 293-K photocurrent spectra of GeS crystals show strongly polarized peaks at 1.65 (E ∥a) and 1.78 eV (E ∥b), which are due to the Λ ₁ ^v → Λ ₁ ^c and Δ ₂ ^v → Δ ₂ ^c optical transitions. The low-temperature photoresponse athv < 1.7 eV is due to absorption by Si impurity.     

<Emphasis Type="Italic">p</Emphasis>-<Emphasis Type="Italic">n</Emphasis> junctions in ZnO implanted with group V ions

n-Type ZnO〈Ga〉 films were implanted with 150-keV N⁺ (As⁺) ions to a dose of 7 × 10¹⁵ cm⁻² and then annealed in atomic oxygen at different temperatures. p-Type conductivity was obtained at annealing temperatures in the range 770–870 K. The parameters of the p-type layers were determined by photoluminescence spectroscopy, secondary ion mass spectrometry, and Hall effect measurements. According to the Hall data, the p-type layers had a resistivity of ∼30 Ω cm, carrier mobility of ∼2 cm²/(V s), and carrier concentration of ∼10¹⁸ cm⁻³. The electroluminescence spectra of the p-n junctions produced by ion implantation showed a band at 440 nm, due to recombination via donor-acceptor pairs.     

Recombination properties of silicon passivated with rare-earth oxide films

The recombination properties of silicon passivated with films of rare-earth oxides were investigated. The films were obtained by thermal resistive sputtering of a rare-earth metal followed by thermal oxidation of the obtained layer in air at 400 °C. It was established that the effective nonequilibrium charge-carrier lifetime measured by photoconductivity relaxation is 2–3 times higher after deposition of the rare-earth oxide film. Surface recombination rates at the silicon-rare-earth oxide interface were determined to be 290–730 cm/s for different rare-earth oxides. The combination of high optical transmittance of the experimental materials and low recombination losses in silicon coated with a rare-earth oxide film makes it possible to recommend rare-earth oxide films as optical antireflection and passivating coatings for silicon photoelectric devices. Pis’ma Zh. Tekh. Fiz. 24, 16–21 (April 12, 1998)     

Field-ion microscopy of deformation effects near the surface in ion-implanted metals (Ir)

Field-ion microscopy is used to determine the deformation due to ion implantation (E=20 keV, D=10¹⁸ ions/cm², j=300 μA/cm²) near the surface in pure iridium. The effect is manifested as a high density of various types of defects in the near-surface volume (∼50 nm from the irradiated surface) of the material. Pis’ma Zh. Tekh. Fiz. 25, 60–64 (March 26, 1999)     

Dielectric relaxation behaviour of Sr<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript>SbMnO<Subscript><Emphasis Type="Bold">6</Emphasis></Subscript> ceramics fabricated from nanocrystalline powders prepared by molten salt synthesis

Double perovskite polycrystalline single phase and dense Sr ₂ SbMnO ₆ (SSM) ceramics, fabricated using the nanocrystalline powders synthesized by molten salt method, exhibited high dielectric constant with low dielectric loss as compared to that of SSM ceramics obtained from the powders prepared by solid-state synthesis method. The dielectric data obtained over a wide frequency (100 Hz–1 MHz) and temperature (190 K–300 K) ranges exhibited distinct relaxations owing to both the grain and grain boundary. The dielectric dispersion was modeled using the Cole–Cole equation consisting of two separate relaxation terms corresponding to the grain and grain boundary. The grain and grain boundary relaxations observed in the Nyquist plots (Z ^′ and Z ^″) were modeled by an equivalent circuit consisting of two parallel RC circuits connected in series with each other. A careful analysis of both the impedance (Z ^″ vs ω) and modulus (M ^″ vs ω) behaviour corroborated the conclusions drawn from the dielectric data.     

Conversion of F⇆F + centers in anion-defective corundum crystals

The method of photostimulated electron emission is used to investigate the processes responsible for redistributing charges formed by photoionization of F centers in anion-defective corundum single crystals in an external electric field. It is shown that the observed features of the excitation spectra are due to conversion transitions between F and F ⁺ centers, whose probability in the surface layer of the crystal is much higher than in the interior because of the high excitation density and the presence of an electric field. The results obtained could be of interest for studying radiation-stimulated phenomena in dielectric materials. Pis’ma Zh. Tekh. Fiz. 25, 66–70 (October 12, 1999)     

Synthesis and nonlinear optical properties of novel Y-type polyurethanes with high thermal stability of dipole alignment

2,3-Di-(2′-hydroxyethoxy)benzylidenemalononitrile (3) was prepared and condensed with 2,4-toluenediisocyanate and 3,3′-dimethoxy-4,4′-biphenylenediisocyanate to yield novel Y-type polyurethanes 4–5 containing 2,3-dioxy benzylidenemalononitrile group as a nonlinear optical (NLO)-chromophore, which constituted parts of the polymer backbones. Polyurethanes 4–5 were soluble in common organic solvents such as acetone and N,N-dimethylformamide. They showed a thermal stability up to 270 °C in thermogravimetric analysis thermograms and the glass-transition temperatures (T _g) obtained from differential scanning calorimetry thermograms were around 116–135 °C. The second harmonic generation (SHG) coefficients (d ₃₃) of poled polymer films at 106.4 mm⁻¹ fundamental wavelength were around 9.07 × 10⁻¹⁹ C (2.72 × 10⁻⁹ esu). The dipole alignment exhibited high thermal stability up to 10 °C higher than T _g, and there was no SHG decay below 145 °C due to the partial main-chain character of the polymer structure, which was acceptable for nonlinear optical device applications.     

Photodiodes based on <Emphasis Type="Italic">n</Emphasis>-GaSb/<Emphasis Type="Italic">n</Emphasis>-GaInAsSb/<Emphasis Type="Italic">p</Emphasis>-AlGaAsSb heterostructures grown using rare-earth elements for the 1.1–2.4 μm spectral range

Photodiodes sensitive in the wavelength range of 1.1–2.4 μm have been created based on n-GaSb/n-GaInAsSb/p-AlGaAsSb heterostructures with a narrow-gap n-GaInAsSb layer (E _g ≅ 0.5 eV) grown in the presence of a rare-earth element (holmium). The electron concentration in the narrow-gap layer is n = 1 × 10¹⁶ cm⁻³, which is about one-fourth of that in an analogous structure grown without the rare-earth element. The proposed structure is characterized by increased quantum efficiency and response speed.     

Electroluminescence of quantum-well structures on type-II InAs/GaSb heterojunctions

The electroluminescent properties of quantum-well diode structures on staggered type-II heterojunctions in the InAs/GaSb system, obtained by molecular-beam epitaxy on InAs substrates, are investigated. Electroluminescence is observed in the spectral range 3–4 μm at T=77 K. It is found that emission bands due to recombination transitions involving electrons from the size-quantization levels of both the self-matched quantum wells at the InAs/GaSb type-II heterojunction and of the square quantum wells in short-period superlattices. Pis’ma Zh. Tekh. Fiz. 24, 50–56 (June 26, 1998)     

Microstructure of a vacuum diffusion layer in the joint of Fe—Al alloy with low-carbon steel

On the interface of diffusively bonded Fe—16Al and Q235, we observe the formation of a well-pronounced layer formed by FeAl, Fe₃Al, and α-Fe(Al) solid solutions. This layer can decrease the diffusion activation energy (facilitating the diffusion of elements and, hence, accelerating its growth). The formation of the diffusion layer is preceded by a latent period t ₀ and its thickness x increases with time according to the parabolic law: x ² = 4.8·10⁴ exp[−133,020/(RT)](t−t ₀). Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 5, pp. 74–78, September–October, 2005.