Fundamental spectra of optical functions for indium bromide in the energy range of 2–30 eV at 4.2 K

The spectra of sets of optical fundamental functions are determined for an indium-bromide crystal in the range of 0–30 eV at 4.2 K for the polarizations E ∥ a and E ∥ c. The calculations are carried out using experimental reflection spectra R(E) and several software packages. Their basic features are established.     

Fine Structure of the long-wavelength edge of exciton-phonon absorption and hyperbolic excitons in silicon carbide of 6<Emphasis Type="Italic">H</Emphasis> polytype

The fine structure of the long-wavelength edge of the polarization spectra of exciton-phonon absorption in moderate-purity n-type 6H-SiC crystals with a concentration of uncompensated donors N_D?N_A=(1.7–2.0)×10¹⁶ cm^?3 at T=1.7 K was studied. The analysis of new special features found at the absorption edge and the reliable detection of the onset of exciton-phonon steps related to the emission of phonons from acoustical and optical branches allowed highly accurate determination of a number of important parameters such as the band gap, the exciton band gap, the exciton binding energy, and the energies of spin-orbit and crystal-field splitting of an exciton. For the first time, transitions with the emission of LA phonons to the 1S exciton state with an M₁-type dispersion law were detected in E ∥ Z(C) polarization (the electric-field vector is parallel to the optical axis of the crystal). This observation supports the previously predicted “two-well” structure of the conduction band minimum in 6H-SiC.     

Specific features of the charge carrier mobility in nanowires in transverse electric and magnetic fields

The effect of an electric field E orthogonal to the quantum-wire axis and a magnetic field H (H ⊥ E, H ‖ E) on conductivity is studied within the context of the parabolic potential model. It is shown that, if the interaction of charge carriers with the rough surface of the nanostructure is taken into account, the charge-carrier mobility μ as a function of increasing E is described by an unsteady oscillating curve. A physical interpretation of such behavior of μ with E is proposed. The specific features of mobility in a transverse magnetic field are discussed.     

On the photopleochroism coefficient and its temperature dynamics in native oxide-p-InSe heterojunctions

The temperature dependence of the photopleochroism coefficient for a native oxide-p-InSe heterojunction is studied. Different temperature dependences of the shift of the photocurrent long-wavelength edge are recorded for two polarization orientations E ‖ C and E ⊥ C.     

Fundamental spectra of optical functions of ferroelectric sodium nitrite

Spectra of optical fundamental functions of ferroelectric sodium nitrite were determined in the range 4–24 eV at 77 K for the three polarizations: E ∥ a, E ∥ b, and E ∥ c. The calculations were based on the experimental R(E) reflection spectra and integral Kramers-Kronig relations. Using the method of Argand diagrams, the permittivity and bulk characteristic electron loss spectra were decomposed into the elementary transverse and longitudinal components. Their main parameters were determined. The obtained data were compared with the theoretical calculations of the permittivity spectra performed using the FPLAPW method. The main features of the permittivity spectra, the parameters of the transitions, and their theoretical nature were established.     

Effect of nonstoichiometry and doping on the photoconductivity spectra of GeSe layered crystals

The polarization photoconductivity spectra of Bi-doped nonstoichiometric GeSe layered crystals grown by static sublimation were investigated. Two strongly polarized maxima at the photon energies hν_max = 1.35 eV (E ∥ a) and 1.44 eV (E∥ b) due to the V ₁ ^V → V ₁ ^c and Δ ₂ ^v → Δ ₁ ^c optical transitions, respectively, were found in the spectra of nominally undoped GeSe crystals near the intrinsic absorption edge at 293 K. In the low-temperature region, an exciton photoconductivity band peaked at hν_max=1.32 eV, which is due to exciton dissociation at the cation vacancies, was revealed. With an increase in excess Se in crystals, a sharp increase in the intensity of the exciton peak in the photoconductivity spectra was observed. It is shown that doping of GeSe crystals with donor Bi impurity is an effective tool of the control of their electrical and photoelectric properties. Although introduction of Bi into germanium monoselenide does not lead to the conductivity conversion from the p to n type, a sharp increase in the resistivity is observed, the crystals become photosensitive, and a strong impurity band peaked at 1.11 eV arises in the photoconductivity spectra.     

Nonmonotonic behavior of magnetophotoconductivity in p-CdHgTe

Magnetophotoconductivity of p-Cd_xHg_1?x Te, measured in the Hall configuration with illumination along the magnetic field (k ∥ B ⊥ E), exhibits nonmonotonic behavior with magnetic field in the temperature range in which the material has a mixed conduction. The effect manifests itself as a peak at B ≠ 0; it is caused by a significant magnetoresistance associated with equilibrium charge carriers. The peak appears if the conductivity of equilibrium electrons is greater than or equal to half the conductivity of equilibrium heavy holes; in Cd _x Hg_1?x Te with x = 0.22, this condition is satisfied at temperatures of 140–150 K.     

Spectroscopy in propyne and ethyl-Chloride by means a tunable CO2 RF laser

Some lines in ν5 band of propyne (CH₃CCH) and many strongest lines in the ν9 band of ethyl- Chloride (C₂H₅Cl³⁵ A-type) are measured and identified, giving a new set of spectroscopic constants. In particular the ethyl-Chloride set allows to assign all the FIR transitions reported in literature.     

Donor–Acceptor–Donor Modular Small Organic Molecules Based on the Naphthalene Diimide Acceptor Unit for Solution-Processable Photovoltaic Devices

Two novel solution-processable small organic molecules, 4,9-bis(4-(diphenylamino)phenyl)-2,7-dioctylbenzo[3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (S6) and 4,9-bis(benzo[b]thiophen-2-yl)-2,7-dioctylbenzo[3,8]phenanthroline-1,3,6,8 (2H,7H)-tetraone (S7), have been successfully designed, synthesized, characterized, and applied in solution-processable photovoltaic devices. S6 and S7 contain a common electron-accepting moiety, naphthalene diimide (NDI), with different electron-donating moieties, triphenylamine (S6) and benzothiophene (S7), and are based on a donor–acceptor–donor structure. S7 was isolated as black, rod-shaped crystals. Its triclinic structure was determined by single crystal x-ray diffraction (XRD): space group \(P\bar{1}\) , Z = 2, a = 9.434(5) Å, b = 14.460(7) Å, c = 15.359(8) Å, α = 67.256(9) degrees, β = 80.356(11) degrees, γ = 76.618(10) degrees, at 150 Kelvin (K), R = 0.073. Ultraviolet–visible absorption spectra revealed that use of triphenylamine donor functionality with the NDI acceptor unit resulted in an enhanced intramolecular charge transfer (ICT) transition and reduction of the optical band gap compared with the benzothiophene analogue. Solution-processable inverted bulk heterojunction devices with the structure indium tin oxide/zinc oxide (30 nm)/active layer/molybdenum trioxide (10 nm)/silver (100 nm) were fabricated with S6 and S7 as donors and (6,6)-phenyl C₇₀-butyric acid methyl ester (PC₇₀BM) as acceptor. Power conversion efficiencies of 0.22% for S6/PC₇₀BM and 0.10% for S7/PC₇₀BM were achieved for the preliminary photovoltaic devices under simulated AM 1.5 illumination (100 mW cm^?2). This paper reports donor–acceptor–donor modular small organic molecules, with NDI as central accepting unit, that have been screened for use in solution-processable inverted photovoltaic devices.     

Permittivity spectra and characteristic energy losses of electrons in ZnO at 100 K

For the first time, full sets of fundamental optical functions have been obtained for zinc oxide in the range 0–30 eV at 100 K for E ⊥ c and E ‖ c polarizations. Spectra of the transverse and longitudinal components of transitions and their basic parameters (peak energies E _i , half-widths H _i of transition bands, band areas S _i , and oscillator strengths f _i ) have also been determined for the first time. The calculations are performed using synchrotron experimental reflectance spectra. The main features of spectra of the optical functions and components of transitions are established. These features are compared to the results of known theoretical calculations of the bands and spectra of optical functions.     

Indirect interband transitions in graphite with a wide quasigap

Sets of fundamental optical functions in the energy range of 0–40 eV for longitudinal indirect interband transitions in graphite with a quasi-gap up to 2.6 eV have been determined for the first time. Their structure and parameters depend strongly on the electron wave vector transfer q. The nature of the peaks and shoulders of these functions is explained within the model of theoretical shifted bands for four electron wave vector transfers q in a volume characteristic electron-energy losses for the Γ-P direction in the Brillouin zone. The calculations were performed on the basis of the known experimental volume characteristic electron-energy loss spectra of graphite in the range of 0–40 eV for |q| = 0.00, 0.375, 0.625, and 1.00 Å^?1 using a special software package.     

Diffraction of an electromagnetic wave from an infinitely long slit in an infinite metallic sheet

The diffraction of anE-polarized and anH-polarized wave by an imperfectly conducting slit (on which impedance boundary conditions are imposed) in an infinite metallic plane is investigated. The two independent problems are solved by using integral transforms, the Wiener-Hopf technique and asymptotic approximations. It is found that the diffracted field consists of the sum of fields produced by the two edges of the planes formed by the slit and a field due to the interaction of the two edges.     

3<Emphasis Type="Italic">C</Emphasis>-SiC <Emphasis Type="Italic">p-n</Emphasis> structures grown by sublimation on 6<Emphasis Type="Italic">H</Emphasis>-SiC substrates

Sublimation epitaxy in a vacuum has been employed to grow n-and p-type 3C-SiC layers on 6H-SiC substrates. Diodes have been fabricated on the basis of the p-n structure obtained, and their parameters have been studied by measuring their current-voltage and capacitance-voltage characteristics and by applying the DLTS and electroluminescence methods. It is shown that the characteristics of the diodes studied are close to those of diodes based on bulk 3C-SiC. A conclusion is made that sublimation epitaxy can be used to fabricate 3C-SiC p-n structures on substrates of other silicon carbide polytypes.     

On the Far Infra Red Excitation of LO Vibrations in Thick Single Crystals

New experiments show that the solution of the complicated problem of Far Infrared (FIR) absorption by crystals of very low symmetry, illuminated at quasi-normal incidence, might have a simple solution, which has been suggested now that a number of spectroscopic studies on monoclinic Triglycine Sulphate (TGS) are available: i) in a first classical approximation where only the strongest bands are considered, the absorption spectrum depends only on the direction of the electric field E of the IR radiation, and Transverse Optic (TO) vibrations are directly excited with electric dipole variations dp parallel to E. ii) in a second approximation looking only at the weakest absorption bands, it depends only on the direction of the wave vector k and Longitudinal Optic (LO) vibrations are excited with dp parallel to k and extinction coefficients 3 orders of magnitude smaller.     

Ionospheric wave spectrum measurements

A significant quantity to be measured in the ionosphere is the local spectrum, S(k,ω), of either potential or electron density fluctuations. One can determine from S(k,ω) characteristics of the macroscopic plasma such as the local density and temperature, transport coefficients, and drift current. Determination of S(k,ω) may be carried out by measurement of the cross-power spectrum. Signals from two probes are passed through a cross-power spectrum analyzer, and the measurement is repeated for a series of probe separations. The resulting cross-power spectra, H(r,ω), must be Fourier-transformed to obtain S(k,ω). A number of questions pertinent to the use of this method in space are discussed. These include the expected signal-to-noise ratio, and the frequency and wave-number resolution attainable.     

Effect of a transverse electric field on charge carrier mobility in nanowires

For nanowires with a parabolic potential, the effect of an electric field E orthogonal to the system??s axis on the mobility of charge carriers ?? is investigated. With consideration for the interaction of charge carriers with a rough surface, it is shown that, for a nondegenerate electron (hole) gas, the mobility ?? decreases with increasing E. For a degenerate electron (hole) gas, the dependence of ?? on E is described by a nonmonotonic oscillating function. A physical interpretation of the predicted effect is proposed.     

The model of self-compensation and pinning of the Fermi level in irradiated semiconductors

A model is developed to analyze numerically the electrical properties and the steady-state (limiting) position of the Fermi level (F _lim) in tetrahedral semiconductors irradiated with high-energy particles. It is shown that an irradiated semiconductor represents a highly compensated material, in which F _lim is identical to 〈E _G〉/2, where 〈E _G〉 is the average energy gap between the conduction band and valence band within the entire Brillouin zone of the crystal. The experimental values of F _lim, the calculated values of 〈E _G〉/2, and the data on the electrical properties of irradiated semiconductors are presented. The chemical trends controlling the variation in the quantity F _lim in groups of semiconductors with the similar types of chemical bonding are analyzed.     

Optical properties and electronic structure of mercury iodide

The spectra of sets of optical functions of a HgI₂ crystal for the polarization E ⊥ c are determined in the energy range of 0–20 eV. The ?₂ and ?Im?^?1 spectra are decomposed into elementary components with establishment of their principal parameters. The calculations are carried out based on the experimental reflection spectrum at 100 K for E ⊥ c and computer programs composed with the help of integral Kramers-Kronig relations, analytical formulas, and the method of combined Argand diagrams. The principal features of the ?₂ and ?Im?^?1 spectra of optical functions and parameters of components of the decomposition bands are established.     

Nature of the diamagnetic maximum in the temperature dependences of the magnetic susceptibility of crystals of (Bi2 ? x Sb x )Te3 alloys (0 < x < 1)

The temperature dependences of the magnetic susceptibility ?? of crystals of (Bi_{2 ? x} Sb _x )Te₃ alloys (0 < x < 1) are studied using a SQUID magnetometer in the temperature range from 2 to 400 K with the parallel and perpendicular orientations of the vector of magnetic field strength H relative to the trigonal axis of the crystal C ₃ (H ?? C ₃ and H ?? C ₃). It is found that the diamagnetic susceptibility of the samples with x = 0.2 (Bi_1.8Sb_0.2Te₃) and x = 0.5 (Bi_1.5Sb_0.5Te₃) increases in the range from 50 K to temperatures preceding the emergence of intrinsic conductivity (250 K). It is found that the diamagnetic maximum manifests itself in the same temperature range, in which an anomalous increase in the Hall coefficient is observed. It is shown that the nature of the diamagnetic maximum is associated with the nonparabolicity of the energy spectrum of light diamagnetic holes, a decrease in whose concentration is accompanied by a decrease in their effective masses, which provides an increase in the diamagnetic susceptibility with increasing temperature. These results are confirmed by the dynamics of the temperature variation in the resonance frequency of plasma oscillations of free charge carriers.