Synthesis and investigation of superconducting properties of heterosubstituted magnesium diborides

The properties of magnesium diborides with the composition Mg_{1 ? x} M _x B₂, where M = Al, Ga, In, and Tl, and Mg (B_{1 ? x} E _x ), where E = N, P, and Si (0.05 ≤ x ≤ 0.3), made by sintering of calculated amounts of boron, magnesium, and a substituent element or by a solid-phase exchange reaction of a corresponding halogenide of a metal with MgB₂ at 1070–1170 K are studied. Using RFA technique, it is shown that, from all employed heterosubstituents, only the atoms of aluminum, gallium, and silicon in the amount x ≤ 0.2 are incorporated into magnesium and boron sublatticies. Here, the temperature of the superconducting transition T _c = 39 ± 1K determined for pure magnesium diboride hardly changes.     

Properties of CdSe single crystals activated with selenium by ion implantation

Selenium ion implantation has been used to produce p-type layers on cadmium selenide single crystals. Their conductivity is governed by acceptor levels at E _ν + 0.05 eV and E _ν + 0.32 eV, due to native point defects in the ion-implanted layer.     

Spectral sensitivity of (Si<Subscript>2</Subscript>)<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>(CdS)<Subscript>x</Subscript> solid solutions

Epitaxial layers of n-type solid solutions of the (Si₂)_{1 ? x} (CdS)_x system (0 ≤ x ≤ 0.01) were grown by liquid phase epitaxy from a tin-based solution melt confined between two horizontal p-type single crystal silicon substrates. The photosensitivity spectra of p-Si/n-(Si₂)_{1 ? x} (CdS)_x structures have been measured. A photoresponse peak at E ≈ 2.35 eV (1.25 eV below the top of the valence band of silicon) has been observed, which is probably related to an impurity level due to CdS molecules.     

Tunable Magnetic Interaction of Co-Doped SiC Monolayer Under Electric Field: Ab Initio Study

The magnetic properties of Co-doped silicon carbide monolayer under an external electric field are investigated using a first-principles method. In the absence of the electric field, magnetism is observed for both Si (Co_Si) and C (Co_C) sites. Then, the interaction between two Co atoms has been studied in the Co_Si system. Both antiferromagnetic (AFM) and ferromagnetic (FM) states have been found. The results show that the FM behavior is originated by the p–d hybridization between Co and its neighboring C atoms. When an electric field was introduced along the c-axis, the interaction between two Co dopants switched from FM to AFM, which could be dominated by the competition between p–d exchange and superexchange. Moreover, the magnetic anisotropy (MA) prefers to parallel to the a-axis and seems not to be turned into the c-axis under the electric field.     

Role of lanthanum in thermoluminescence properties of La<Subscript>2<Emphasis Type="Italic">x</Emphasis></Subscript>Lu<Subscript>2(1−<Emphasis Type="Italic">x</Emphasis>)</Subscript>SiO<Subscript>5</Subscript>:Ce crystals

The influence of lanthanum content on thermally stimulated luminescence properties of La_2xLu_2(1?x)SiO₅:Ce (x = 0, 0.08, 0.18 and 1.50 at.%) crystals was investigated. Trapping parameters such as electron trap depth E_t and electron traps content n ₀ were fitted with general order kinetic function. According to the results of the VUV transmittance spectra and band gap calculation which based on the density functional theory with the generalized gradient approximation of Perdew–Burke–Ernzerhof (GGA–PBE), band structure of the crystals and recombination mechanisms of released electrons were further studied. Results showed that there were mainly two kinds of electron traps, namely shallow (E_t ≈ 0.2 eV) and deep (E_t ≈ 1.0 eV) traps. With the increasing of La content, the concentration of deep traps was obviously depressed, and the depth of deep traps decreased at the same time, which was explained by suggesting the downward shift of the bottom of conduction band.     

Optical and transport properties of Fe-doped Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Se (<Emphasis Type="Italic">x</Emphasis> = 0.35)

The optical and transport properties of Fe²⁺-doped Cd _x Hg_1?x Se crystals with a midgap Fe²⁺ level have been studied. The results demonstrate that Fe²⁺ ions influence both the optical and transport properties of Cd _x Hg_1?x Se〈Fe²⁺〉. The observed optical absorption bands are due to a donor Fe²⁺ level in the band gap, with a depth E _Fe = 0.21 eV, and to band-band transitions. Thermal anneals in Hg and Se vapors have different effects on the carrier concentration and mobility in the crystals. The effect of annealing on the transport properties of the Fe²⁺-doped crystals differs from that for undoped crystals and is governed by the state of point defects.     

Temperature dependence of the optical absorption spectra of InP/ZnS quantum dots

The optical-absorption spectra of InP/ZnS (core/shell) quantum dots have been studied in a broad temperature range of T = 6.5–296 K. Using the second-order derivative spectrophotometry technique, the energies of optical transitions at room temperature were found to be E ₁ = 2.60 ± 0.02 eV (for the first peak of excitonic absorption in the InP core) and E ₂ = 4.70 ± 0.02 eV (for processes in the ZnS shell). The experimental curve of E ₁(T) has been approximated for the first time in the framework of a linear model and in terms of the Fan’s formula. It is established that the temperature dependence of E ₁ is determined by the interaction of excitons and longitudinal acoustic phonons with hω = 15 meV.     

Perchlorosilanes and perchlorocarbosilanes as precursors for SiC synthesis

Homologues with the general stoichiometry a(SiCl₄): bSi: cC: d(SiC) are shown to be potential precursors for the low-temperature gas-phase synthesis of silicon carbide. Thermal decomposition of these precursors yields the chemically stable gaseous species SiCl₄ and condensed Si, C, SiC, SiC + Si, or SiC + C. We use thermodynamic modeling of the thermal decomposition of octachlorotrisilane, Si₃Cl₈, to analyze the key features of the thermolysis of perchlorosilanes with the general stoichiometry a(SiCl₄): bSi. The equilibrium compositions of reaction products in the Si₃Cl₈-CO system are determined. This reaction system enables low-temperature (400–1200 K) formation of silicon carbide.     

Deposition of luminescent a-SiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>:H Films with SiH<Subscript>4</Subscript>–N<Subscript>2</Subscript> gas mixture by VHF–PECVD using novel impedance matching method

Hydrogenated amorphous silicon nitride (a-SiN _x :H) films are produced from a SiH₄–N₂ gas mixture by plasma enhanced chemical vapor deposition (PECVD) system with a newly developed impedance matching method at frequencies 13.6–150 MHz. An increase in the rf power from 35 to 350 mW/cm² at the highest frequency of 150 MHz increases the optical bandgap (E _opt) from 2.0 to 4.5 eV. Optical emission spectroscopy (OES) of the SiH₄–N₂ plasma shows that the emission intensity of SiH* (414 nm) is almost proportional to deposition rate. Films of a-SiN _x :H deposited at 150 MHz and 210 mW/cm² has an optical bandgap of E _opt ≈ 4.1 eV and emits visible photoluminescence (PL) at room temperature (RT).     

Thermoelectric properties of tetradymite-like layered materials in the pseudoternary system Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>–GeTe–Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>

The Hall coefficient, resistivity, and thermoelectric power of quaternary tetradymite-like layered materials in the pseudoternary system Sb₂Te₃–GeTe–Bi₂Te₃ have been measured in the temperature range 100–800 K. The results demonstrate that all of the samples studied in the Sb₂Te₃–GeTe–Bi₂Te₃ system are p-type and have high hole concentration due to point defects. Plots of lnσ against 1/T in the intrinsic region were used to evaluate the band gap (ΔE) of two materials: GeSb_3.91Bi_0.03Te_6.91(ΔE = 0.22 eV) and GeSbBiTe₄ (ΔE = 0.197 eV).     

Low-temperature acoustic properties of single-crystal hafnium

Five independent adiabatic elastic tensor components c _ij of single-crystal Hf (0.3% Zr) have been precisely measured in the range 78–300 (c ₁₁ and c ₃₃) or 78–160 K (c ₁₂, c ₁₃, and c ₄₄) at 50 MHz using an ultrasonic pulse technique. The corresponding linear absorption coefficients α _ij (T) have been measured in the same temperature ranges. The c _ij data have been used to determine adiabatic compressibilities β_∥ and β_⊥, and Young’s moduli E _∥ and E _⊥ along and across the c axis, respectively. Near 140 K, we have revealed an anomaly in c ₃₃(T) due to changes in the phonon spectrum of single-crystal α-Hf. The observed anomalies in temperature-dependent α₁₁ and α₃₃ are tentatively attributed to grown-in dislocations.     

A diffusion criterion of the crystal-liquid phase transition

A diffusion criterion of the crystal-liquid phase transition (PT) is proposed according to which the PT begins when the E _d/k _b ratio reaches a threshold value of E _d(s)/k _b T _m, where E _d is the self-diffusion energy, k _B is the Boltzmann constant, T is the absolute temperature, and E _d(s) is the self-diffusion energy in the solid phase at the melting temperature T _m. It is shown that this criterion is a generalization of the Lindemann criterion and is applicable both to solids exhibiting normal melting and to those melting with a decrease in the specific volume. Based on the new criterion, it is possible to explain the relation T _N < T _m, where T _N, is the crystallization onset temperature. The results of calculations of the T _N/T _m ratio well coincide with experimental data.     

Fabrication of <Emphasis Type="Italic">p-n</Emphasis> junctions in ZnO by arsenic ion implantation followed by annealing in atomic oxygen

We report the properties of p-n junctions produced in ZnO films by As⁺ ion implantation followed by annealing in atomic oxygen. The starting films, n-ZnO〈Ga〉, were grown by magnetron sputtering on amorphous SiO₂ substrates. As shown by x-ray diffraction, the c axis of ZnO is perpendicular to the film surface. The resistivity of the p-ZnO〈As〉 layer is 30–35 Ω cm, carrier mobility 1–2.5 cm²/(V s), and hole concentration 3 × 10¹⁸ cm^?3. The arsenic and gallium concentrations are 2.5 × 10¹⁹ and 1 × 10¹⁸ cm^?3, respectively. The arsenic profile is Gaussian in shape, as shown by secondary ion mass spectrometry. Current-voltage data attest to the presence of an i-layer and to recombination in the space charge region. The barrier height evaluated from current-voltage and capacitance-voltage data is about 3.4 eV, which is comparable to the band gap of ZnO. The electro-and photoluminescence spectra of the junctions show bands at 440 and 510 nm. The 510-nm emission is typical of as-grown n-ZnO〈Ga〉 films. The 440-nm band is attributable to donor-acceptor recombination.     

Homogeneity of high-resistivity <Emphasis Type="Italic">p</Emphasis>-Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>〈Au〉 crystals

p-Si_{1 ? x} Ge _x crystals have been diffusion-doped with gold. Gold diffusion in the p-Si_{1 ? x} Ge _x 〈Au〉 samples and their electrical properties have been studied. The results demonstrate that the highest gold concentration in the crystals can be achieved in the temperature range 1000–1050°C. An expression has been derived which indicates that, all other factors being the same, compensation with Au, an amphoteric impurity, insures better homogeneity compared to codoping with acceptor and donor impurities. The hole concentration homogeneity in gold-compensated samples is at the same level as or even better than that in the uncompensated material.     

X-Ray Spectral Methods in Investigation of the UO<Subscript>2</Subscript> Electronic Structure

Fine structure of the X-ray photoelectron spectrum of UO₂ at electron binding energies from 0 to ～40 eV is primarily due to electrons of outer (0–15 eV) and inner (15–40 eV) valence molecular orbitals formed from unoccupied U5f, 6d, 7s and O2p and occupied low-energy U6p and O2s shells of the neighboring uranium and oxygen atoms, respectively. This is consistent with the results of the relativistic calculation of the electronic structure of the UO ₈ ^12? cluster with O _h symmetry, simulating the nearest surrounding of uranium in UO₂, and is confirmed by the data of X-ray spectroscopy (conversion electron, nonresonance and resonance X-ray O_4,5(U) emission, O_4,5(U) XANES, photoelectron resonance, and Auger spectroscopy of oxygen). The fine structure of the X-ray photoelectron spectra, associated with electrons from outer valence and inner valence molecular orbitals, allows estimation of the degree of participation of U6p, 5f electrons in chemical bonding, as well as the structure of the nearest surrounding of the uranium atom and the bond length in its oxides. The total contribution from electrons of inner valence molecular orbitals to the absolute value of the chemical bonding energy can be compared with the corresponding contribution of the electrons from outer valence molecular orbitals to bonding of the atoms. Inner valence molecular orbitals can be formed in compounds of any elements, and this is an important new fact in chemistry and physics of condensed state.     

Infralow-frequency oscillations in photoconductivity of chromium-doped silicon

The photoconductivity of chromium-doped silicon at T=4.2 K under constant illumination in the fundamental absorption band exhibits infralow-frequency oscillations at 10^?2 Hz, the amplitude of which sharply increase with the illumination intensity. No such oscillations are observed in samples of n-Si〈Cr〉 or in chromium-free p-Si and n-Si control samples. A model qualitatively explaining the appearance of such oscillations is proposed, which is based on the fact that chromium in silicon can occur in two states, one being electrically (and recombination) active, and the other, inactive.     

Band structure of silicon carbide nanotubes

Using the linear augmented cylindrical wave method in the muffin-tin approximation, we have calculated the band structure of (n, n) and (n, 0) silicon carbide nanotubes for n = 5–10. In the range n = 7–10, (n, n) nanotubes are semiconductors, and their band gap decreases steadily with increasing n: 0.28 eV at n = 7, 0.26 eV at n = 8, 0.19 eV at n = 9, and 0.11 eV at n = 10. Nanotubes with n = 5 and 6 are metallic. At n = 7–9, (n, 0) nanotubes are semiconductors, and their band gap increases steadily with n: 0.39 eV at n = 7, 0.46 eV at n = 8, and 0.62 eV at n = 9. Nanotubes with n = 5 and 6 have metallic conductivity according to our results.     

A model of hydrogen absorption by metals

The influence of the electron spectrum of transition metals on the hydrogen absorption process is considered. An absorption model is proposed where electrons, followed by protons, from adsorbed molecules or atoms of hydrogen transit into the metal. In terms of this model, the driving force of the absorption process ΔX is equal to the difference of the total electron energies in a hydrogen atom and in a metal at the Fermi level: ΔX = E _H ? (E _F + A _work). The equilibrium state in the absorption process corresponds to the equation 13.53 = E _F + A _work, which makes it possible to find the Fermi energy of transition metals and the valence of hydrogen absorbing metals, to define the molecular formula of hydrides, and to calculate the maximum hydrogen content in hydride phases. The hydrogen content in the composition of hydrides is shown to decrease in approaching the middle of the transition-metal rows. In groups, on the contrary, the hydrogen content grows as the atomic weight of metals increases. A good agreement of calculated and experimental compositions of hydride phases is found for the transition metals whose d band is less than half-filled with electrons.     

Chromium diffusion in GaAs in an open system

We have studied chromium diffusion from a surface layer produced by thermal evaporation into n-type GaAs in a flowing inert-reducing atmosphere. The temperature dependences of the Cr diffusivity and solubility in GaAs are well represented by Arrhenius equations with D ₀ = 1.7 × 10^?2 cm²/s and Q _D = 1.43 eV for the diffusivity and C _Cr ⁰ = 8.9 × 10²¹ cm^?3 and Q _Cr = 1.22 eV for the solubility.     

Parameters of electrode discharges in supersonic air flows

Results are given of experimental investigations of basic microscopic parameters of plasma of a longitudinal-and-transverse dc discharge in a supersonic wind tunnel with a backward-facing step. The results of measurements of electric field E, of temperatures of gas T _g and of excitation of electron levels T _e , and of reduced electric field E/N are compared with the measured or previously found parameters of transverse and longitudinal discharges in supersonic flows and in stationary air in a wide range of pressures and currents.