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Electrical properties and deep trap spectra are reported for MgZnO(P) films grown by pulsed laser deposition on undoped n-ZnO substrates. The as-grown films are n type with a “bulk” donor concentration of ∼1018 cm-3 and have a compensated high resistivity layer near the surface. Deep trap spectra in these films are dominated by electron traps with an activation energy of 0.3 eV, hole traps with an activation energy of 0.14 eV, and some unidentified electron traps with a barrier for capture of electrons. After annealing in oxygen at 800 °C the MgZnO(P) becomes p type, with the dominant hole traps having an activation energy of 0.2 eV. The space charge region of the formed p-n junction is mainly located in the n-ZnO substrate. The main hole traps in this part of the heterojunction have activation energies of 0.14 and 0.84 eV, while the main electron traps have activation energies of 0.15 and 0.3 eV. [ABSTRACT FROM AUTHOR]

Provides information on a study investigating low-frequency infrared (IR) spectra of fullerene and metastable forms of C60 obtained at high pressures and temperatures. Methodology used to conduct the study; Information on the presence of the fullerene bands in the range from 42 to 44 Cm-1; Findings of the study.

The results of studies of nanodimensional silicon (NS) samples, which do not degrade under the intense laser radiation exposure, are presented. It was shown that a significant increase in the intensity of the photoluminescence signal from the NS can be attributed to both the special features of their structural configuration and the available thin SiO 2 layer on the surface of the nanocrystals. [ABSTRACT FROM AUTHOR]

Methods used for designing and controlling parameters of prototypes of ferromagnetic semiconducting materials are described. Examples of applications of the above methods are given. The up-to-date ferromagnetic semiconductor design level for spin applications is briefly considered. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors (HEMTs) with similar active layers structures were grown on SiC or sapphire substrates using different buffer layer structures, including GaN of different thickness (1 or 2 μm) or composite AlGaN/GaN buffers. The highest density of hole traps was observed in the buffer on sapphire, while the lowest density of hole traps was obtained in the thick (2 μm) GaN buffer on SiC. The reverse leakage currents in HEMTs were lower in the devices grown on SiC substrates and the on-off ratios improved by two orders of magnitude for thicker GaN buffers or composite AlGaN/GaN buffers compared to a standard 1 μm GaN buffer. The maximum drain-source currents and tranconductances were all larger for the devices on SiC compared to sapphire. [ABSTRACT FROM AUTHOR]

AlGaN/GaN structures grown on Si substrates by metal organic chemical vapor deposition have been processed into high power transistors with maximum input power density of 12 W/mm. The transistor showed low concentration of bulk electron traps in the GaN buffer and of interfacial traps at the AlGaN/GaN interface. A new version of contactless thermography method based on microcathodoluminescence spectra for transistors under operation was shown to produce similar results to more established techniques based on measurements of the E2(high) phonon frequency shift in the GaN buffer and of the TO phonons in the Si substrate. The results of temperature measurements are in good agreement with modeling results. [ABSTRACT FROM AUTHOR]

Nanoscale titanium dioxide modifications with anatase and η-TiO structures have been obtained by the sulfate method. Bands characteristic of the η-TiO phase are selected in the Raman and IR spectra of the samples containing this phase. A study of the properties of synthesized samples applying a complex of physicochemical methods revealed that nanocrystals in the samples containing phases of different modifications differ in size, structure, and surface composition. It is shown that the abovementioned nanoscale modifications are characterized by the efficient sorption of As (V), Bi (III), and V (V) from aqueous media; the degree of sorption depends on the size and structural form of nanocrystals. [ABSTRACT FROM AUTHOR]

Growth of GaN on polycrystalline chemical vapor deposition (CVD) diamond prepared on Si was achieved by hydride vapor phase epitaxy (HVPE). If the polycrystalline CVD diamond is separated from the Si substrate and the side turned to Si is covered by either TiN or anodic Al oxide, this is found to facilitate deposition of good crystalline quality GaN films. Porous TiN in these experiments was formed by Ti evaporation and subsequent in situ nitridation in the HVPE reactor during pregrowth heating. The films showed double crystal x-ray (0002) rocking curve half width of 245 in. and a strong bandedge luminescence. Thick films self-separated from the substrate. When growing GaN on the Si side of the diamond substrate is covered with porous Al anodic oxide, the authors observed the formation of well defined (0001) GaN texture with the maximum angle between the c-axes of various grains lower than 6.5°, a very low (120 in.) half-width of the (0002) rocking curves of individual grains and an intense bandedge luminescence. [ABSTRACT FROM AUTHOR]

Single-walled carbon nanotubes have been filled with cobalt bromide. The microstructure, optical properties, and effect of the introduced CoBr2 compound on the electronic structure of the nanotubes have been studied. It has been shown that the electron density in the resulting nanocomposites is transferred from the walls of the nanotubes to the nanocrystals of cobalt bromide, which is an electron acceptor. [ABSTRACT FROM AUTHOR]

The electrical characteristics and persistent-conductivity effects in MgZnO:P alloys grown by the method of pulsed laser deposition on undoped n-ZnO substrates are systematically investigated. It is shown that the pronounced persistent conductivity and the persistent photocapacitance related to the presence of high-barrier electron traps for electron capture are observed in initial unannealed layers. These traps are located in the lower half of the band gap and have the optical ionization threshold of 2.8 eV and the electron-capture barrier height of ∼0.4 eV. Alongside such centers, the hole traps with the ionization energy of 0.14 eV are also observed. The annealing at 850°C transforms the material into that of p-type conductivity with the depth of dominant phosphorus-related acceptors close to 0.2 eV. The conductivity compensation and the formation of hole traps with the activation energy of 0.14 and 0.84 eV in the n-ZnO substrate are also observed, and these traps are associated with the acceptor-defect diffusion into the substrate at annealing. [ABSTRACT FROM AUTHOR]

Capacitance-voltage (C-V), capacitance-frequency (C-f), admittance spectroscopy, deep trap spectra, and far infrared reflectance measurements were performed on undoped and N-doped ZnO films deposited on sapphire by molecular beam epitaxy. The results show existence of a heavily doped n+ layer near the interface with the substrate. The presence of these layers explains the large difference between the electron concentrations measured in the films by Hall effect and C-V profiling or calculated from the plasma minimum frequency in reflectance. C-V data obtained at low temperatures show a prominent persistent photocapacitance in the films. Admittance spectra were dominated by electron traps with ionization level EC-0.3 eV commonly observed in ZnO crystals grown by all techniques. [ABSTRACT FROM AUTHOR]

We have experimentally and theoretically studied IR-active optical phonons, which are spatially confined in the volume of semiconducting CdS nanocrystals of various shapes synthesized in a dielectric matrix (porous aluminum oxide). Within an approach admitting the mixing of all expected types of vibrations, the complete sets of phonon modes are determined for a spherical quantum dot (QD) and a cylindrical quantum wire (QW) in this matrix. Based on these results, the polarizability spectra of QDs and QWs, as well as the effective dielectric function of a composite material containing such nanoparticles, are calculated for the far-IR wavelength range. It is established that the spectrum of the dielectric function exhibits specific features in the region between the transverse and longitudinal optical phonon frequencies of the massive semiconductor material. These features explain the rather unusual structure of the IR spectra of the composite samples studied. [ABSTRACT FROM AUTHOR]

A complex spectroscopic investigation of oil-in-water emulsions containing silicon nanoparticles synthesized by plasma chemical vapor deposition has been performed for the first time. It is established by electron microscopy and Raman and IR spectroscopy that nanoparticles synthesized by this method have a crystalline structure, sizes of about 10–15 nm, and an outer shell whose chemical composition depends on the powder synthesis atmosphere. Comparative measurements of the transmission spectra of silicon-containing emulsions showed that their transmission, taking into account scattering, decreases with a decrease in wavelength in the range below 450 nm. The wavelength dependences for particles with an oxynitride outer shell and particles having an oxide shell are significantly different. This result indicates a contribution of the outer shell of silicon nanoparticles to the transmission spectra of emulsions. This factor must be taken into account in design of UV protectors based on silicon powder. In addition, calculations performed for transparent media containing silicon nanoparticles predict the possibility of enhancement of the protective properties of such emulsions in the UV range with increasing sizes of particles above 10 nm. [ABSTRACT FROM AUTHOR]

Optical properties of the CdxHg1 − x − y ZnyTe semiconductor alloy are examined in ultraviolet, visible, and infrared spectral regions. The relations are obtained, which make it possible to estimate the x and y composition of this material by the position of singular points E 0 and E 1 in the optical spectra. Theoretical and experimental results in the composition regions 0.09 < x < 0.22 and 0.02 < y < 0.17 agree well. [ABSTRACT FROM AUTHOR]

The structure of the particles of nanocrystalline silicon synthesized in argon plasma with added oxygen is studied. An amorphous shell composed of silicon oxide is formed on the surface of silicon nanoparticles. The particles form clusters with a fractal structure. The adsorption of nitrogen on a powder of nanocrystalline silicon at 77 K is studied, and adsorption isotherms obtained for nanocrystalline silicon and nonporous silica adsorbents with identical specific surface areas are compared. The values of surface fractal dimension of powdered nanocrystalline silicon are calculated using the Frenkel-Halsey-Hill equation for multilayer adsorption under the dominant contribution of van der Waals or capillary forces. It is shown that surface fractal dimension is a structure-sensitive parameter characterizing both the morphology of clusters and the structure (roughness) of the surface of particles and their aggregates. [ABSTRACT FROM AUTHOR]

The optical properties of porous GaAs layers obtained by electrochemical etching of single-crystal n- and p-GaAs(100) wafers are studied. It is shown that the shape of the nanocrystals, their mean diameter, and their surface states depend on the conductivity type of the initial crystal. A shift of the peaks corresponding to the main optical phonons to lower frequencies in the Raman spectra is observed. Surface-phonon frequencies determined from the Raman spectra coincide with those determined from the reflection spectra in the infrared spectral region. The forms of the spectral dependences of the reflection coefficient in the phonon-resonance region in bulk GaAs differ from those in porous GaAs. This is caused by the appearance of additional oscillators related to spatially confined lattice vibrations in GaAs nanocrystals. Atomic-force microscopy is used to study the surface morphology of porous GaAs samples formed on the n-GaAs substrates, and a nanosized surface profile is observed. Estimations made for the mean diameter of GaAs nanocrystals based on data from the Raman scattering, infrared spectroscopy, photoluminescence, and atomic-force microscopy yield results that are in good agreement with each other. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

Far-infrared reflectance (20-700 cm[sup -1]) and Raman spectra of porous PbTe were studied. The spectral dependences found significantly differ from those observed in the case of bulk single-crystal PbTe. Aside from the transverse (TO) and longitudinal (LO) optical phonon modes, additional vibrational modes were detected in the spectra, whose frequencies are close to those in a bulk material. Their number and spectral positions are in satisfactory agreement with the results of calculations carried out to determine the vibrational mode frequencies for nanocrystallites taking into account boundary conditions at their surface. The macroscopic continuum model allows calculation of the frequencies of the mixed phonon modes in quantum dots by the simultaneous consideration of atomic displacements and the accompanying change in the electrostatic potential. It was found experimentally that the detected modes are mixed (LO-TO), as was predicted by theory. The data of atomic-force microscopy allowed us to estimate the average radius of quantum dots, which form the nanorelief of the porous PbTe layer. [ABSTRACT FROM AUTHOR]

The problem of stabilizing the properties of porous silicon films was studied. A thermodynamical analysis of electrochemical processes occurring in the course of anodic Si dissolution is performed. A new description of electrode reaction of silicon interaction with hydrofluoric acid is suggested. It is shown that hydrogen-induced passivation of the Si surface governs the equilibrium dissolution potential of the silicon electrode. Thermodynamic calculations indicate that it is possible to substitute the chemically and thermally unstable surface groups (of the SiH[sub x] type) for more stable silicon-oxygen compounds directly in the course of formation of porous Si in electrolytes with the addition of strong hydrohalic acids. The results obtained made it possible to explain thermodynamically the stabilizing effect of an HCl additive in electrolytes used for formation of porous Si on its chemical and physical properties. [ABSTRACT FROM AUTHOR]

Frequency dependences of the transmittance T(ω) of Cd[sub 1-x]Zn[sub x]Te (0 < x < 0.1) single crystals grown by a modified Bridgman method were studied using long-wavelength optical spectroscopy in the temperature range of 5-300 K. A sharp increase in absorption for energies smaller than the band gap was observed for unannealed samples of p-type conductivity. Moreover, with the temperature variation of the sample from 5 to 300 K, the dependences T(ω) intersect almost at the same wavelength. The theoretical frequency dependences T(ω) with allowance for substantial contribution of the intersubband transition of charge carriers in the valence band were calculated. The Fermi level position was evaluated for the samples at T = 77.3 and 295 K. A nonmonotonic dependence of the Fermi level position on the composition was observed for x = 0.040-0.047. The possible influence of tellurium precipitates on the transmittance decrease in Cd[sub 1-x]Zn[sub x]Te in the wave number range of 3000-400 cm[sup -1] is discussed. [ABSTRACT FROM AUTHOR]

The structure of hydrogenated microcrystralline silicon (mc-Si: H) films deposited by hot-wire chemical vapour deposition was investigated by means of X-ray diffraction (XRD) and infrared (IR) spectroscopy. Roughly random orientation of crystallites grained preferentially by (111) planes was established by XRD. The behaviour of IR absorption lines were analysed with respect to the variation in mean grain sizes and high-temperature treatments. As was established, the characteristic features of IR absorption in mc-Si: H come from the bonding of hydrogen on grain boundaries in the form of monohydrides and polyhydride groups and the splitting of absorption bands. The latter was interpreted in terms of absorption of optically anisotropic SiH monolayers covering the internal surfaces in mc-Si: H. [ABSTRACT FROM AUTHOR]

The infrared transmission spectra of single-crystal samples of n-type CdTe and n-type Cd[sub 1 - x]Zn[sub x]Te at 295 and 77 K were investigated. The experimental data are analyzed under the assumption that the observed spectral dependences of the transmission coefficient are due to absorption of infrared radiation by free charge carriers. It is shown that the computational results agree satisfactorily with the experimental data. The electron densities and mobilities calculated from the transmission spectra are close to their values obtained from electric measurements. [ABSTRACT FROM AUTHOR]

The properties of porous silicon samples prepared by adding hydrochloric acid to the usual hydrofluoric-acid electrolyte have been studied. These samples exhibit an intense photoluminescence that does not degrade over time and is unaffected by exposure to intense laser illumination. The peak photoluminescence from these layers of porous silicon occurs at photon energies of 1.85-1.9 eV. The photoluminescence signal from samples prepared in the standard way under the same initial conditions but without HCl in the composition of the electrolyte is two orders of magnitude less intense. Studies of the degradation of these porous silicon samples with time and exposure to various power levels of laser illumination revealed that the samples with the maximum content of HCl in the electrolyte composition emitted photoluminescence that was unaffected by laser illumination. In this work, the infrared spectra of the samples was measured in order to monitor the chemical state of their surfaces. It was found that the abrupt 100-fold increase in the intensity of the photoluminescence signal from samples made according to the method proposed here is associated both with distinctive features of the structure of porous silicon layers and with the presence of a thin crystalline layer of SiO[sub 2] on the surface of nanocrystallites. [ABSTRACT FROM AUTHOR]

Selective photoconductivity at frequency ω=155 cm-1 was discovered in PbTe(Ga) narrow-gap semiconductors at liquid-nitrogen temperatures. The corresponding energy is much lower than all characteristic energies of the electronic spectrum of the semiconductor. The effect is attributed to optical excitation of a local vibrational mode of an impurity center leading to delocalization of the electrons. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Vibrational modes were studied in n-type and p-type ZnMgO films doped with P (Mg composition of 7 at. %) grown by pulsed laser deposition on sapphire. The characteristic phonon frequencies were deduced from the analysis of IR reflectance measured by Fourier-transform spectroscopy. From comparison with similarly grown ZnO (P) films, Mg incorporation reduced the frequency of TO phonons by 14.5 cm-1 and introduced two Mg related modes near 530 and 969 cm-1. The first is likely to belong to the local vibrational mode of substitutional Mg, and the second is tentatively attributed to strongly lattice relaxed off-center Mg atoms. In addition, it was found that Mg incorporation triggers the formation of two phonon bands with characteristic frequencies of 501 and 634 cm-1 that most likely belong to lattice defects. [ABSTRACT FROM AUTHOR]

Current transport mechanisms and persistent photoconductivity effects were studied in nitrogen-doped ZnO films grown by molecular beam epitaxy having p-type or n-type conductivity at 25 °C. In both types of samples the current flow is determined by the n-type channels surrounded by higher resistivity regions. The persistent photoconductivity wave form is reasonably described by the stretched-exponents-type expression, with only a slight temperature dependence of the characteristic decay time. The persistent photocurrent decay process is greatly accelerated by infrared illumination (threshold energy of the photons ∼1.4 eV). The results suggest that the Fermi level in the higher resistivity regions is pinned near Ev+1.9±0.1 eV and the height of the potential barrier for electrons in the n-type channels is around 1.4±0.1 eV. [ABSTRACT FROM AUTHOR]

Based on the results of experimental studies, it is shown that, when the effect of size quantization on the vibratory properties of CdS nanocrystals is insignificant, the absorption spectra of nanocomposites containing CdS quantum wires and quantum dots exhibit not only the peak associated with the transverse optical phonons but also two other pronounced peaks associated with electrostatic vibration modes. When the size of nanocrystals is so small that the effect of size quantization cannot be neglected, the structure of the reflection and transmission spectra becomes much more complicated because of the contribution of the mixed longitudinal-transverse phonon modes. [ABSTRACT FROM AUTHOR]

Specific features of the electrical properties of p-type Cd[sub x]Hg[sub 1-x]Te/CdZnTe heterostructures have been studied at liquid-nitrogen temperatures. The adopted theoretical model satisfactorily describes the obtained experimental data. Errors in determining the concentration and mobility of heavy holes—arising when contributions to the electrical parameters from electrons and light holes are disregarded—are evaluated. [ABSTRACT FROM AUTHOR]

A new model is proposed for DX-like impurity centers, which are responsible for the Fermi-level stabilization and long-term relaxation effects in IV-VI semiconductors doped with group III elements. The model is based on the idea of a variable valence of the impurity, whereas the nature of the long-term effects at low temperatures is associated with the formation of an effective barrier caused by a change of two units in the impurity valence upon photoexcitation. The model is applied to an analysis of the photoconductivity spectra in PbTe(Ga). The model can also be applied to the classical DX centers in III-V semiconductors. [ABSTRACT FROM AUTHOR]

Reflectivity spectra of Cd[sub 3-x]Zn[sub x]As[sub 2] solid solutions with compositions x=1.2, 1.8, 2.25, and 2.55 were measured at room temperature within the range of 50-700 cm-1. Analysis based upon Kramers-Kronig relation has helped to calculate the basic optical functions (real and imaginary parts of dielectric constant, reflection, and absorption idecis) in the measured energy range. Dispersion analysis has been applied to find the parameters of phonons within the range of 50-300 cm-1. As a result a curve has been made to characterize the dependency of the most strong oscillator upon their composition features. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]