Temperature dependence of carrier mobility in undoped and gadolinium-doped <Emphasis Type="Italic">p</Emphasis>-GaSe crystals

We have studied the temperature dependence of carrier mobility (μ) in undoped and gadolinium-doped p-GaSe crystals. The results demonstrate that, in the temperature range 77–420 K, the carrier mobility in high-resistivity p-GaSe〈Gd〉 crystals increases at low temperatures as a power law. At high temperatures, μ(T) has a maximum and varies roughly as $T^{ - \tfrac{3} {2}} $T^{ - \tfrac{3} {2}} . In addition, μ is a nonmonotonic function of gadolinium content. The observed anomalies in the temperature dependence of carrier mobility in the crystals studied are due to partial disorder in these materials. The present experimental data can be interpreted in terms of a two-barrier energy-band model of partially disordered semiconductors.     

Experimental and theoretical analysis of selective hydrogenation of <Emphasis Type="Italic">p</Emphasis>-nitroanisole to <Emphasis Type="Italic">p</Emphasis>-anisidine over Pd/C: kinetics and catalyst deactivation

The selective reduction of nitro compounds to the corresponding amines in multi-functional moieties is attempted in a variety of industries using polluting processes including iron–acid and sulfide/polysulfide reduction processes. In the current work, selective hydrogenation of p-nitroanisole (PNA) to p-anisidine (PA) has been carried out using supported metal catalysts. 5% Pd/C was found to be highly effective in comparison with other metal catalysts used. Kinetic interpretation has been made by studying the important process parameters using 5% Pd/C as the catalyst. The experimental results show that conversion of PNA reaches 100% under the appropriate reaction conditions. The reaction was found to be 100% selective towards PA. It was possible to determine both the rate constant and adsorption equilibrium constant for the reaction. The activation energy of reaction and free energy of adsorption were found to be 10.25 and –2.4 kcal mol^–1, respectively, indicating that the reaction is Langmuir-Hinshelwood kinetically controlled. The catalyst was found to be deactivated due to the blocking of channels by product precipitation, which was considered as independent deactivation. A deactivation model was developed and it was found to fit the data very well. A complete theoretical and experimental analysis is presented. Electronic Publication     

Electrical characteristics of (<Emphasis Type="Italic">p</Emphasis>)3C-SiC-(<Emphasis Type="Italic">n</Emphasis>)6H-SiC heterojunctions

The electrical characteristics of (p)3C-SiC-(n)6H-SiC epitaxial heterostructures obtained by sublimation epitaxy in vacuum are studied. The band discontinuities are determined and the energy band diagram of the heterojunction is constructed. It is shown that the obtained heterostructure offers a promising material for high electron mobility transistors.     

Study of defect states in a-Se<Subscript>85</Subscript>Te<Subscript>15−<Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> thin films by space charge limited conduction mechanism

Current–Voltage (I–V) characteristics have been studied at various temperatures in vacuum evaporated thin films of a-Se₈₅Te_15−x Pb _x (x = 0, 2, 4, 6) alloys. These characteristics show that, at low electric fields, an ohmic behaviour is observed. However, at high electric fields (E ∼ 10⁴ V/cm), the current becomes superohmic. At high fields, in case of samples having 0 and 2 at% of Pb, the experimental data fits well with the theory of space charge limited conduction (SCLC) in case of uniform distribution of localized states in the mobility gap. Such type of behaviour is not observed at higher concentration of Pb in the present glassy system due to high conductivity. In these samples, joule heating due to large currents may prohibit the measurement of SCLC. Using the theory of SCLC for the uniform distribution of the traps, the density of localized defect states near Fermi level is calculated for these compositions. The results indicate that the density of defect states near Fermi level increases on addition of Pb to binary Se₈₅Te₁₅alloy. This is explained in terms of electronegativity of Pb as compared to host elements.     

Electrical characterization of low temperature deposited oxide films on ZnO/<Emphasis Type="Italic">n</Emphasis>-Si substrate

Thin films of silicon dioxide are deposited on ZnO/n-Si substrate at a low temperature using tetra-ethylorthosilicate (TEOS). The ZnO/n-Si films have been characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The border trap density (Q_bt) and fixed oxide charge density (Q_f/q) of the SiO₂/ZnO/n-Si films are found to be 3.9 × 10¹⁰ cm⁻² and 1.048 × 10¹¹ cm⁻², respectively. The trapping characteristics and stress induced leakage current (SILC) have also been studied under Fowler-Nordheim (F-N) constant current stressing.     

Critical Current Densities and Vortex Dynamics in Ba(Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript> Single Crystals

Superconducting properties are evaluated for high-quality single crystals of Ba(Fe_1−x Co _x )₂As₂ in a wide range of doping levels. The critical current density, J _c , in an optimally-doped crystal (T _c ∼24 K) shows a fishtail effect with its value over 10⁵ A/cm² even at 5 T below 10 K. Magneto-optical imaging has clarified rather homogeneous supercurrent flow in the crystal, in spite of a large amount of impurities. In the heavy-ion irradiated sample, the presence of columnar defects are confirmed and J _c has been enhanced by a factor of five at low temperatures, reaching 6×10⁶ A/cm² at 2 K under zero field. Flux creep rate in the heavy-ion irradiated sample has been reduced in accordance with the enhancement of J _c .     

Nuclear-Physical Properties of <Superscript>105, 106<Emphasis Type="Italic">m</Emphasis>, 110<Emphasis Type="Italic">m</Emphasis></Superscript>Ag

The nuclear-physical properties of the nuclides ^{105, 106m,110m} Ag, which are concomitant nuclides in reactor production of ¹⁰³Pd and ¹⁰⁹Cd used in metrology of ionizing radiations and in nuclear medicine, were studied. The following quantities were determined by semiconductor X-ray and γ-ray spectrometry: the parameter K _α/K _β of X-ray K radiation accompanying the decay of ^110m Ag and the intensities of γ-ray quanta with the energies of 280 and 345 keV (¹⁰⁵Ag); 430, 451, and 512 keV (^106m Ag); 658, 764, 885, 938, and 1384 keV (^110m Ag). The half-lives determined using the above γ-ray lines are as follows: T _1/2(¹⁰⁵Ag) = 41.2 ± 0.1 days; T _1/2(^106m Ag) = 8.30±0.07 days. The errors are given for 95% confidence level.__________Translated from Radiokhimiya, Vol. 47, No. 1, 2005, pp. 31–34.Original Russian Text Copyright © 2005 by Popov, Zakharova, Sadulin, Andreev, Pakhomov.     

Worm-like morphology of semi-rigid substituted poly(<Emphasis Type="Italic">p</Emphasis>-phenylene)

The rheological and dynamic mechanical properties of a semi-rigid substituted poly(p-phenylene) were investigated. At temperatures below the glass transition temperature of 152 °C the amorphous polymer is very stiff compared with flexible chain polymers. Although the material can be processed in the melt, the melt is unusually elastic. Atomic force and transmission electron microscopy revealed a worm-like morphology. Blends with polycarbonate yield transparent materials. Microscopy and X-ray diffraction of these blends indicated that the flexible chain polymer is located in between the worm-like structures of the semi-rigid polymer.     

Magneto-Peltier cooling of single-crystal Bi<Subscript>1-<Emphasis Type="Italic">X</Emphasis></Subscript>Sb<Subscript><Emphasis Type="Italic">X</Emphasis></Subscript> (<Emphasis Type="Italic">X </Emphasis>= 0.12 and 0.15) alloys

The cooling temperatures of rectangular parallelepiped Bi_1-X Sb _X (X = 0.12 and 0.15) single-crystals with the same thickness of t = 2 mm but different width W were measured at 113 K and 290 K as a function of electric current in the magnetic field B up to 2.17 T. The magnetic field was aligned along the thickness t of a sample and the current flows along its length L through the copper leads soldered to both end surfaces of cross section (W × t), where W, t and L are parallel to the binary, bisector and trigonal axes of the single-crystal, respectively. The thermoelement was not in contact with a heat sink. The cooling temperature of Bi_0.85Sb_0.15 at 290 K was increased with an increase of B and was almost symmetric for the reverse of the field direction, while at 113 K it exhibited a maximum at B = ±0.25 T and a strong asymmetry for the field direction. The largest maximum cooling temperature ΔT _max of Bi_0.85Sb_0.15 was achieved when a thermoelement has optimum dimensions so that heat energy is hardly generated at the cold side. When the single-crystal Bi_0.85Sb_0.15 alloy has optimum dimensions of L = 15 mm, W = 4 mm and t = 2 mm, the ΔT _max at 290 K increased from 4.2 K in B = 0 T to 9.6 K in B = +2.17 T, so that it exceeded ΔT _max values of 5.7 K obtained for a typical Bi₂Te₃ and 8.5 K measured previously for Bi single-crystal in B = +2.17 T.     

<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.     

Electrical impedance of ethylene-carbon monoxide/propylene-carbon monoxide (<Emphasis Type="Italic">EPEC</Emphasis>-<Emphasis Type="Italic">69</Emphasis>) thermoplastic polyketone

Electrical impedance technique was employed to investigate the electrical properties of ethylene-carbon monoxide/propylene-carbon monoxide terpolymer (EPEC-69). The measurements were performed in the frequency range 0.1–10⁵ Hz and in the temperature range 30–110 °C. The results reveal that the dielectric constant, loss factor, modulus, and ac conductivity are dependent of frequency and temperature. A Debye relaxation peak was detected in the plot of Z″ versus frequency where the activation energy was determined and found to be 1.26 eV. When the surface phenomenon effects were separated using the imaginary part of the complex admittance a second dielectric dispersion was observed in the low frequency region. Two models were proposed from the impedance measurements depending on temperature range.     

Asymmetric current transfer in isotype <Emphasis Type="Italic">n</Emphasis>-In<Subscript>2</Subscript>Se<Subscript>3</Subscript>/<Emphasis Type="Italic">n</Emphasis>-InSe heterocontacts

We have studied the mechanisms of current transfer and the capacitance-voltage characteristics of a radiation-resistant isotype n-In₂Se₃/n-InSe heterocontact in which the region depleted of major charge carriers is localized predominantly in a relatively low-ohmic semiconductor—the α-type In₂Se₃ with a large density of defects. The spectrum of the relative quantum efficiency of photoconversion in this structure depends on the thickness of the In₂Se₃ layer and on the geometry of illumination of the heterocontact.     

Effect of thallium doping on the parameters of localized states in <Emphasis Type="Italic">p</Emphasis>-GaSe single crystals

Experimental evidence is presented that, at temperatures from 167 to 250 K, electrical transport in undoped and thallium-doped p-GaSe single crystals is due to variable-range hopping. The main parameters of localized states in the band gap of p-GaSe〈T1〉 (0, 1, 2, and 2.5 mol % Tl) have been estimated as functions of doping level.     

Thermal decomposition of trimethylamine borane as a precursor to nanocrystalline CVD BC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> films

We have studied the kinetics of BC _x N _y chemical vapor deposition through trimethylamine borane decomposition at atmospheric pressure. The rate constant of the heterogeneous interaction between trimethylamine borane and an adsorption center has been determined to be k _s ⁰ = 2.7 × 10⁷exp(−10560/T) cm/s. The obtained kinetic parameters of the reaction fully determine the growth rate of nanocrystalline carbonitride films under kinetic control. The film thickness has been determined as a function of time, temperature, reactant concentration, and reactor dimensions.     

Injection photodiode based on <Emphasis Type="Italic">p</Emphasis>-CdTe film

Photosensitive large-block p-type cadmium telluride (p-CdTe) film with a resistivity of ρ ≈ 10⁶−10⁷ Ω cm has been used to create an Al-p-CdTe-Mo structure with a Schottky barrier, which exhibits the properties of an injection photodiode. Being switched in the forward current passage direction (with “+” on the Mo contact) at high illumination levels, the proposed structure exhibits a responsivity of S _λ ≈ 2.6 A/W at a wavelength of λ = 0.625 μm, which is about five times the spectral response of the ideal photodetector for this wavelength range.     

Autoionization resonances in outer-shell <Emphasis Type="Italic">p</Emphasis>-electron photoionization cross sections of alkali metal atoms

New results of many-electron calculations of the outer-shell p-electron photoionization cross sections for rubidium and cesium atoms are presented. The resonance structure of these cross sections, related to the excitation of the autoionization sates ns ¹ np ⁶(n + 1)s ¹ mp (m = n + 1, n + 2, n + 3, …), is analyzed. The many-electron correlations are taken into account within the framework of the random phase approximation with exchange and the many-body perturbation theory. The decisive role of two-electron processes in the formation of resonance profiles is demonstrated. Differences in the behavior of electrons with various spin moment projections are traced.     

A <Emphasis Type="Italic">p-i-n</Emphasis> Model of CdTe/CdS Heterostructures

CdTe/CdS heterostructures are analyzed in terms of a p-i-n model in which the i layer consists of a CdTe_{1 − x} S_x solid solution resulting from Te and S diffusion into CdS and CdTe, respectively. The lattice parameters of the solid solution are determined. The i layer is shown to be too thick for tunneling.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 8, 2005, pp. 915–917.Original Russian Text Copyright © 2005 by Janabergenov, Mirsagatov, Karazhanov.     

Ultrasonic treatment-induced modification of the electrical properties of InAs <Emphasis Type="Italic">p-n</Emphasis> junctions

The effect of a high-frequency (f = 5 MHz) ultrasonic treatment (UST) on the low-temperature (T = 77 K) electrical properties of smooth InAs p-n junctions has been studied. It is established that the most UST-sensitive parameter is the tunneling current component, which is probably related to the channels of increased conductivity localized near dislocations. The current component determined by the recombination of carriers in the space charge region free of dislocations is less sensitive. The electrical characteristics of p-n junctions exhibit a tendency to restoration during the long-term storage of treated samples under laboratory conditions. Possible models of the influence of UST on the electrical properties of smooth InAs p-n junctions are discussed.     

Ionic Conductivity of Ln<Subscript>2 + <Emphasis Type="Italic">x</Emphasis></Subscript>Zr<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>7 − <Emphasis Type="Italic">x</Emphasis>/2</Subscript> (Ln = Sm-Gd) Solid Solutions

The electrical conductivity of Ln_{2 + x} Zr_{2 − x} O_{7 − x/2} (Ln = Sm-Gd) solid solutions prepared from mechanically activated Ln₂O₃ and ZrO₂ is shown to correlate with their structural properties. In the three systems, the x-T regions are determined in which electrical transport is dominated by oxygen-ion conduction. In the Sm₂O₃-ZrO₂ system, ionic conductivities from 5 × 10⁻⁴ to 6 × 10⁻³ S/cm at 740°C are found in Sm_{2 + x} Zr_{2 − x} O_{7 − x/2} with 26.6, 33.3, 35.5, 37, and 40 mol % Sm₂O₃ prepared at 1450, 1530, and 1600°C. Eu_{2 + x} Zr_{2 − x} O_{7 − x/2} and Gd_{2 + x} Zr_{2 − x} O_{7 − x/2} containing 33.3 to 37 mol % Ln₂O₃ have 740°C ionic conductivities of 10⁻³ to ∼7.5 × 10⁻³ and 10⁻³ to 7 × 10⁻³ S/cm, respectively. The activation energy of conduction in Ln_{2 + x} Zr_{2 − x} O_{7 − x/2} (Ln = Sm-Gd), E _a = 0.84–1.04 eV, increases with the atomic number of Ln and x. The highest ionic conductivity is offered by the stoichiometric Ln₂Zr₂O₇ (Ln = Sm-Gd) pyrochlores prepared at 1600°C, owing to the optimal concentration of Ln_Zr + Zr_Ln antistructure pairs (∼5–22%). The grains in the ceramic samples studied range in size from 0.5 to 2 µm.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 8, 2005, pp. 975–984.Original Russian Text Copyright © 2005 by Shlyakhtina, Kolbanev, Knotko, Boguslavskii, Stefanovich, Karyagina, Shcherbakova.     

Recombination instability and double S-switching in <Emphasis Type="Italic">p</Emphasis>-Ge(Au)

We report on the experimental investigation of p-Ge(Au) samples and compare the results qualitatively to a one-dimensional theoretical model describing the recombination instability of current in gold-compensated germanium in a two-parameter (voltage-emission) space. Experiments showed the existence of three regimes of the system functioning in the parametric space. The second S-switching region was found on the current-voltage characteristic, which is probably related to a noise-induced nonequilibrium phase transition.