Plasma-electrolytic formation, composition and catalytic activity of manganese oxide containing structures on titanium

In the present work, we report the data about formation of TiO₂-rutile or TiO₂ and Mn₂O₃, Mn₃O₄ containing oxide structures on titanium in aqueous electrolytes by means of plasma-electrolytic deposition. The layers formed are characterized by X-ray diffraction, electron probe microanalysis and scanning electron microscopy methods. The PEO coatings on titanium formed in sodium tetraborate solution contain the TiO₂ stabile rutile modification that is important when utilizing such a structure as a catalyst carrier. Manganese acetate adding into the electrolyte leads to formation of layers that contain Mn₂O₃, Mn₃O₄ and TiO₂-rutile in outer region. The manganese content in the surface layer depends on the formation conditions as well as on manganese acetate concentration in the electrolyte. Catalytic activity of the layers in CO → CO₂ reaction is studied in the static and flow conditions. The manganese-containing layers obtained possess the catalytic activity in CO → CO₂ oxidation reaction at the temperature range of 250-350 °C. The catalytic activity depends on the concentration and surface distribution of manganese as well as on the layers morphology.     

Defects in the GaAs and InGaAs layers grown by low-temperature molecular-beam epitaxy

The problem of defect formation in the GaAs and InGaAs layers grown by low-temperature molecular-beam epitaxy is discussed. The effect of growth conditions (temperature and flux ratio between the elements of groups III and V) on the morphology of growth surface, internal structure, type, and concentration of electrically-and optically active defects is analyzed. A comparison is made between the defect formation processes occuring during the epitaxial growth and post-growth annealing of the layers. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 63–72, December, 2006.     

Surface defects in semiconductor lasers studied with cross-sectional scanning tunneling microscopy

Cross-sectional scanning tunneling microscopy is used to study defects on the surface of semiconductor laser devices. Step defects across the active region caused by the cleave process are identified. Curved blocking layers used in buried heterostructure lasers are shown to induce strain in the layers above them. Devices are also studied whilst powered to look at how the devices change during operation, with a numerical model that confirms the observed behavior. Whilst powered, low-doped blocking layers adjacent to the active region are found to change in real time, with dopant diffusion and the formation of surface states. A tunneling model which allows the inclusion of surface states and tip-induced band bending is applied to analyze the effects on the tunneling current, confirming that the doping concentration is reducing and defect surface states are being formed.     

High-voltage photovoltaic effect in layers of silicon evaporated in vacuum

Silicon layers which generate a photovoltaic voltage of up to 150 V/cm at room temperature are obtained by evaporation in a vacuum onto an insulating substrate. The spectral characteristics of the voltage, the effect of thermal processing on the value of the voltage and on the resistance of the layers, and the dependence of the voltage on the direction of the illumination and on the structure of the photosensitive layers are investigated. It is concluded that the photosensitive layers of silicon possess a microcrystalline structure with an ordered arrangement of the small crystals, separated by high-resistance oxide-type layers, and are sources of elementary photovoltaic voltages which are added along the layer. It is suggested that the reason for the formation of the elementary photovoltaic voltages is the separation of electron-hole pairs, produced by the light, by the field of the barriers which exist on the surfaces of the individual crystals of the layer and in the gaps between the crystals.The authors thank M. A. Rumsha for help with the electron diffraction investigations and for useful discussions, and also M. I. Rudenok for making the electron microscope investigations.     

Magnetic aftereffect in Co-P trilayer films

This paper reports on the results of experimental investigations of the magnetic properties of a trilayer system that consists of high-coercive polycrystalline and low-coercive amorphous magnetic Co-P layers and a nonmagnetic amorphous Ni-P interlayer. It has been established that the coercive force and the bias field of the hysteresis loop of the low-coercive layer undergo anomalous changes in the range of small thicknesses of the hard magnetic layer. In the same range of thicknesses, a magnetic aftereffect is found to manifest itself, which is unusual for this type of magnetic structures. The observed features are associated primarily with the kinetics of formation of a crystal structure of the high-coercive layer.     

Optical characterization of porous silicon films and multilayer filters

Porous silicon (PS) has a great potential in optical applications due to the tunability of its refractive index. However, the electrochemical formation parameters of porous silicon have a great influence both on porosity and pore morphology and, hence, on the optical properties of the PS layers. In the present work, the optical constants of PS layers are determined in the visible-wavelength range for different electrolyte compositions and for a wide range of formation-current densities. Thus, the interval of refractive indices that can be achieved for each electrolyte composition is studied, for the further development of interference filters. In particular, it is demonstrated that a higher ethanol concentration in the electrolyte leads to a considerably higher tunability of the refractive index of PS while reducing absorption losses. In addition, the performance of PS-based multilayer interference filters is shown to improve when formed with an electrolyte of higher ethanol concentration, especially in the blue region of the visible spectrum. PACS 78.20.Ci; 78.40.-q; 78.55.Mb     

A Structural and Ellipsometric Investigation of Thin Gallium Oxide Layers Deposited on Si by Means of Laser Deposition

The morphology of Ga₂O₃ layers deposited on a silicon substrate by pulse laser deposition is studied using scanning electron microscopy in the thickness range of 30–200 nm. Using spectroscopic ellipsometry in the wavelength range from 250 to 900 nm, the thickness of layers and dispersion of their optical parameters (complex pseudodielectric function ε, refractive and absorptive indices) have been determined and the fraction of voids has been estimated in the scope of the Bruggeman effective medium approximation. In the spectrum of the imaginary part ε₂, there is observed an increase in values in the region of energy E = 4.8 eV corresponding to the direct interband transition in the Ga₂O₃ structure. X-ray structure analysis showed the presence of peaks typical for the β-Ga₂O₃ structure. The results of the investigations can be used for the formation of wide-bandgap gallium oxide layers in developing devices of power electronics and in creating optoelectronic devices for the UV range.     

Visible and near ultraviolet photocurrent generation in carbon nanotubes

Multiwall carbon nanotubes are found to generate photocurrent in the visible and near ultra violet spectral range using a photoelectrochemical technique. Peaks in the photocurrent are observed at excitation energies in the visible region. Their electron energy loss spectra exhibit the π plasmon feature, typical of graphite layers, and a peak at lower energy. Features at energies between 0 and 4 eV have been already observed for single wall carbon nanotubes and ascribed to interband electronic transitions due to the reduced dimensionality of these systems. The present measurements suggest that the usual identification of multiwall carbon nanotubes electronic density of states with that of graphite layers is not sufficient and more theoretical investigations are necessary to shed light on this point.     

Boron implantations in silicon: A comparison of charge carrier and boron concentration profiles

The concentration profiles of boron implanted in silicon were measured using secondary ion mass spectrometry. The accompanying charge carrier profiles were determined by Hall-effect sheet-resistivity measurements combined with layer removal by anodic oxidation and etching. From a mutual comparison of these profiles an electrically inactive boron fraction was found to exist in the region of maximum boron concentration. This fraction can be correlated with boron precipitates. In high dose implantations the precipitates still exist after annealing at 1000°C. In the tail of the profile a small electrically inactive boron fraction was observed. This fraction was correlated with fast diffusing non-substitutional boron. Near the surface a charge carrier peak was found that can be correlated with the damage caused by implantation. The interpretation of the observed electrical effects was facilitated by investigations on boron concentration profiles of layers implanted with different doses and annealed in accordance with different time-temperature schedules.     

Physics of Solid-State Dielectrics: Superstrong Electric Fields

Electric strengthening with decreasing dielectric thickness allows superstrong electric fields, whose strength exceeds the breakdown one for thick dielectrics, to be created in thin layers of solid-state dielectrics without an electric breakdown. Such fields are called superstrong. In thin dielectric layers of micron thickness, the processes can be investigated which cannot be observed in thicker layers due to the onset of the breakdown. In the present paper, the results of experimental investigations of processes and phenomena taking place in thin monocrystal layers of alkali-halide crystals (AHC) in superstrong electric fields are generalized. Among these processes and phenomena are: electric currents and luminescence (electroluminescence) of AHC layers, impact excitation and electronic ionization of luminescence centers and ions of the host crystal lattice, emission of electrons, accelerated in the layer by the electric field, in vacuum, formation of point and linear defects in AHC under the action of strong and superstrong electric fields, etc. All these phenomena form a new scientific direction – physics of solid-state dielectrics: superstrong electric fields. The results of investigations of superstrong electric fields allow new approaches to the understanding of mechanisms of dielectric breakdown to be realized.     

Early stages of test formation in larva of Ascidia malaca (Tunicata, Ascidiacea): ultrastructural and cytochemical investigations

The swimming larvae of ascidians are entirely covered by a hyalin coat called tunic, or test. This covering consists of two cuticular layers, C1 and C2, which surround an inner compartment composed of an amorphous hyalin matrix with numerous fibrils dispersed inside. Data from the literature agree on the key role played by the cells of the larval ectodermic layer in the synthesis and secretion of larval test components. In the present article are reported ultrastructural and cytochemical investigations made during test formation in the swimming larva of Ascidia malaca. Besides confirming the role played by ectodermic cells during the early stages of test formation, the investigations highlight the way in which the fibrillar component of the test is synthetized and secreted. At the ultrastructural level it has been evidenced that the C1 and C2 cuticular layers originate from the tight packing of fibrils. Based on the data reported in the present study, it is hypothesized that while a relevant part of the fibrils, once secreted, remains dispersed inside the matrix of the inner compartment of the test, quite likely in order to increase its consistency, packing of the remaining fibrils leads to the formation of the C1 and C2 cuticular layers. Packing of the fibrils in C1 and C2 could be favoured by their chemically adhesive nature. This hypothesis is strongly supported by the herewith reported results of the cytochemical investigations carried out on the test of the swimming larva of A. malaca. The cytochemical PA-TCH-SP reaction has in fact evidenced that both fibril types, i.e. those dispersed inside the inner compartment and those packed in the C1 and C2 cuticular layers, are constituted by glycoproteins and/or proteoglycans substances whose adhesive properties are well documented in the literature.     

Monolithic white LEDs: Approaches, technology, design

The results of investigations of monolithic white InGaAlN LEDs with an active region containing several thin InGaN layers, emitting in the range from blue to yellow-green, and separated by short-period InGaN/GaN superlattices, are presented. The influence of the growth conditions and layer sequence in the active region on the optical properties of monolithic white LEDs was studied with the aim of controlling their color parameters.     

X-ray rocking curves on inhomogeneous surface layers on Si single crystals

Two cases of inhomogeneous surface layers are considered — diffusion layers as well as implanted layers. The parameters of the layers are investigated by means of an X-ray rocking curve analysis. In this paper the concentration profile of a diffusion layer in Si is determined from X-ray rocking curves, the rocking curves on implanted layers will be discussed in the next paper. A graph is constructed for determining approximate values of surface concentrationC ₀ and diffusion lengthL from subsidiary maxima on the X-ray rocking curve. The dependence of the shape of the rocking curves on the type of the concentration profile and on the values ofC ₀ andL was shown. The influence of crystal thickness and curvature is studied theoretically and experimentally. As an example of using this method a rocking curve of a crystal with a boron diffusion layer is measured and the parameters of the concentration profile are determined. The parameters found are proved by multiplied measurement of rocking curves after anodic oxidation and by comparing these rocking curves with theoretical ones.     

Formation and manipulation of regular metallic nanoparticle arrays on bacterial surface layers: an advanced TEM study

The template-directed formation of regular nanoparticle arrays on two-dimensional crystalline protein layers after their treatment with metal salt complexes was studied by transmission electron microscopy. For these investigations, bacterial surface layers (S layers), recrystallized in vitro into sheets and tube-shaped protein crystals with typical dimensions in the micrometer range, were used as the template. As identified by electron holography and scanning force microscopy, the S-layer tubes form alternating double layers when deposited onto a solid substrate surface. Two distinct pathways for the metal particle formation at the templates have been found: the site-specific growth of metal clusters by chemical reduction of the metal salt complexes, and the electron-beam induced growth of nanoparticles in the transmission electron microscope. Both mechanisms lead to regular arrays with particle densities > 6×10¹¹ cm ^-2. Nanoparticle formation by electron exposure takes exclusively place in the flat-lying double-layered protein tubes, where a sufficient amount of metal complexes can be accumulated during sample preparation. Received 6 December 2000     

The influence of low-temperature silver-ion exchange on the spectral-luminescent properties of fluorophosphate glasses doped with PbSe

Changes in the absorption and luminescence spectra of fluorophosphate glasses doped with PbSe caused by low-temperature Ag⁺–Na⁺ ion exchange are considered. It is found that the silver distribution gradient in a near-surface layer about 16 μm thick leads to two different processes of interaction between metal and semiconductor nanoparticles. PbSe molecular clusters and quantum dots more efficiently grow in deep layers with a low silver concentration. The near-surface glass layers with a high silver concentration exhibit formation of Ag metal nanoparticles, on the surface of which interaction with PbSe molecular clusters leads to the formation of Ag–Se–Pb bonds, which transform into Ag₂Se layers in the process of heat treatment. The appearance of the new phase is confirmed by X-ray diffraction.     

Probabilistic model for fluences and peak fluxes of solar energetic particles.

The model is intended for calculating the probability for solar energetic particles (SEP), i.e., protons and Z=2-28 ions, to have an effect on hardware and on biological and other objects in the space. The model describes the probability for the > or = 10 MeV/nucleon SEP fluences and peak fluxes to occur in the near-Earth space beyond the Earth magnetosphere under varying solar activity. The physical prerequisites of the model are as follows. 1. The occurrence of SEP is a probabilistic process. 2. The mean SEP occurrence frequency is a power-law function of solar activity (sunspot number). 3. The SEP size (taken to be the > or = 30 MeV proton fluence size) distribution is a power-law function within a 10(5)-10(11) proton/cm2 range. 4. The SEP event particle energy spectra are described by a common function whose parameters are distributed log-normally. 5. The SEP mean composition is energy-dependent and suffers fluctuations described by log-normal functions in separate events.     

Phase behavior of dispersions of hard spherical particles

A series of experiments on concentrated dispersions of hard colloidal spheres is discussed. The observed phase behavior is analogous to that of simple atomic systems: colloidal fluid, crystal and glass phases are found. The structure of the crystals, revealed by light diffraction, is a strongly faulted stacking of hexagonally-packed layers of particles. Dynamic light scattering confirms that the concentration of the metastable fluid phase for which long-ranged particle diffusion ceases coincides with the concentration where the glass transition is observed macroscopically. In studies of a binary mixture of colloidal spheres with a size ratio 0.61 eutectics, glass formation and the AB₁₃ type alloy structure have been identified.     

Ferromagnetic resonance, magnetic properties, and resistivity of (CoFeZr)x(Al2O3)1 ? x/Si multilayer nanostructures

Comparative investigations of static magnetic properties, magnetoresistance, and ferromagnetic resonance data of multilayer nanostructures consisting of CoFeZr-Al₂O₃ composite magnetic layers and amorphous silicon semiconductor spacers were performed in a layer thickness range below 5 nm. The influence of layer dimension parameters and chemical peculiarities of silicon on the inner structure and type of magnetic interactions in nanostructures with 35 and 46 at % magnetic phase composite layers is discussed.     

Electrical properties of semiconductor surfaces

The formation of space charge layers by the action of surface states and the influence of more than one bulk impurity level are treated. Special attention is given to compound semiconductors. Recent experimental results are reported of subband spectroscopy, work function measurements, and metal semiconductor junction investigations. Requirements for simultaneous surface conductivity and LEED measurements are given. The contribution of various scattering processes to the dc conductivity is discussed. Finally the relations between surface reactions and space charge layers are considered.     

Transmission electron microscopy of heteroepitaxial layer structures

The characterization of heterostructural layers by transmission electron microscopy using cleaved wedge specimens proves to be a fast analysis method. Examples are given for GaAlAs/GaAs and strained GaInAs/GaAs layer systems. It is demonstrated that the (200) dark-field contrast of GaInAs/GaAs layers reverses at an In concentration of x ≈ 0.47. Experimental high-resolution electron images of the edges of cleaved 90° wedges compare very well with computer-simulated images. Characteristic image features in GaAs and AlAs are explained by non-linear beam interactions and are shown to be sensitive to electron beam misalignment. These investigations are important to make a more precise assessment of interfaces possible.