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.     

The impurity optical absorption and conduction band structure in 6H-SiC

Absorption spectra of nitrogen-doped n-type 6H-SiC crystals were studied for two orientations of the light wave electric field (E) relative to the optical axis (C), E ∥ C and E ⊥ C, within the range from the near-infrared region to the fundamental band. For E ∥ C, a weak absorption band peak at 2.85 eV was investigated for the first time. All the absorption bands observed are attributed to donor (nitrogen) photoionization, as a result of which electrons are transferred to the conduction-band upper minima that correspond to different critical points of the Brillouin zone. A combined analysis of the new data obtained in this study, the previous experimental results concerning photoionization of nitrogen, and theoretical data on 6H-SiC conduction band structure made it possible to refine the arrangement and symmetry of the additional conduction-band extrema in the Brillouin zone.     

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.     

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 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.     

Structurally complex two-hole and two-electron slow traps with bikinetic properties in <Emphasis Type="Italic">p</Emphasis>-ZnTe and <Emphasis Type="Italic">n</Emphasis>-ZnS crystals

Thermal-activation and photoactivation methods were used to ascertain the existence of two-hole traps in p-ZnTe crystals and two-electron traps in n-ZnS. It was found that these traps have a large number of energy states that are grouped in two series of levels: E_V+(0.46–0.66) eV and E_V+(0.06–0.26) eV in p-ZnTe and E_C?(0.6–0.65) eV and E_C?(0.14–0.18) eV in n-ZnS. Both the hole and the electron traps belong to the class of slow traps with bikinetic properties. These traps feature normal kinetic properties in the state with a single trapped charge carrier and feature anomalous kinetic properties in the state with two charge carriers. Multiple-parameter models allowing for a relation of traps in p-ZnTe and n-ZnS to the vacancy-impurity pairs distributed according to their interatomic distances and localized in the region of microinhomogeneities with collective electric fields that repel the majority charge carriers are suggested. The main special features of behavior of electron and hole traps are explained consistently using the above models.     

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.     

Carbazole and benzimidazole/oxadiazole hybrids as bipolar host materials for sky blue,green, and red PhOLEDs

Two novel bipolar host materials (CBzIm and COxaPh) comprising of a hole-transport (HT) carbazole core functionalized with electron-transport (ET) moieties (benzimidazole/oxadiazole) at C3 and C6 positions have been synthesized. Their thermal, photophysical, electrochemical properties, and carrier mobilities were characterized. Theoretical calculations revealed that the HOMO orbitals were generally delocalized over the hole- and electron-transport moieties for both CBzIm and COxaPh, whereas the LUMO orbitals distribution only involved one benzimidazole moiety in CBzIm instead of fully delocalization over the whole polar moieties for COxaPh, which is consistent with the observation of good hole mobilities for both hosts and better electron mobility for COxaPh over CBzIm. CBzIm with high E_T (2.76 eV) is suitable to serve as a blue phosphor host, where a sky blue phosphor (DFPPM)₂Irpic exhibiting superior properties than those of popular blue emitter FIrpic was used to give highly efficient phosphorescent OLEDs, achieving a maximum external quantum efficiency (η_ext) of 15.7%. The better π-delocalization of COxaPh led to a lower triplet energy (E_T = 2.65 eV), which can be used to accommodate green and red phosphors, providing excellent device performance with η_ext as high as 17.7% for green [(ppy)₂Ir(acac)] and 20.6% for red [Os(bpftz)₂(PPh₂Me)₂], respectively.     

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.     

Traveling wave amplifier using “VP×B” effect

The “v _p ×B” effect is introduced into a traveling wave amplifier trying to increase its output power and efficiency. The numerical calculating results show that by introducing an tapered static magnetic field into a traditional traveling wave amplifier, its output power and efficiency can be increased obviously comparing with an ordinary traveling wave amplifier, because of the effective interactive duration of electron bunches and a rf field is extended by the “v _p ×B” effect.     

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.     

Numerical analysis of open circular longitudinally magnetized p-Ge and n-InSb waveguides

By means of “coupled” modes, there have been studied the dispersion dependencies of the main and three higher modes of open circular $\hat \varepsilon $ -gyrotropic waveguides made of semiconductor materials having electron or hole conductivity. Dispersion dependencies of such waveguides have been studied at various biasing magnetizing field inductions B and concentrations N of charge carriers. Cut-off frequency dependence on concentration N has been computed.     

Photovoltaic properties of low band gap copolymers based on phenylenevinylene donor and cyanovinylene 4-nitrophenyl acceptor units

Two novel copolymers P1 and P2 having phenylenevinylene donor and cyanovinylene 4-nitrophenyl acceptor units, were synthesized by heck coupling and employed as electron donor along with PCBM or modified PCBM (F) as electron acceptor for the fabrication of bulk heterojunction (BHJ) photovoltaic devices. These copolymers P1 and P2 showed broad band absorption around 640-700 nm and optical band gap of 1.60 eV and 1.72 eV, respectively. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) estimated from cyclic voltammetry measurement reveals that these values are well suitable for the use of these copolymers as electron donor along with PCBM derivatives as electron acceptor for BHJ active layer. The suitable LUMO off set allows efficient photo-induced charge transfer at the donor/acceptor interfaces in the BHJ photovoltaic device and resulting power conversion efficiency (PCE) of 2.8% and 3.29% for P1 and P2, respectively, when PCBM is used as acceptor. This value has been improved up to 3.52% and 4.36% for the devices based on P1 and P2 when F is used as electron acceptor, instead of PCBM. We have also investigated the effect of solvent annealing on the photovoltaic performance of device based on P1: F and P2: F blends and found that the over all PCE of the devices is 4.36% and 4.88%, respectively. The increase in PCE is mainly due to the improvement in the J_sc, which is due to the increased charge transport in the annealed device as compared to as cast device.     

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.     

Charge carrier mobility through vacuum–sublimed glassy films of s-triazine- and carbazole-based bipolar hybrid and unipolar compounds

Instead of physical mixtures as bipolar charge transport media for organic electronics, chemical hybrids comprising non-conjugated spacers are explored to strive for miscibility and morphological stability. Bipolar TRZ-3Cz(MP)2 and TRZ-1Cz(MP)2 as well as unipolar C3-2TRZ(2tBu), C2-2TRZ(2tBu) and C3-2Cz(MP)2 were synthesized with propylene or ethylene spacers serving to decouple the Cz(MP)2 and TRZ moieties. Glassy films were prepared by vacuum sublimation for the characterization of transport properties using the photocurrent time-of-flight technique. The results indicate that both the TRZ:Cz(MP)2 ratio and the spacer length enable charge-carrier mobility to be modulated across orders of magnitude. The C2-2TRZ(2tBu) film exhibits the highest electron mobility of all the unipolar TRZ-based glassy films reported to date.     

The effect of thickness fluctuations on the static electrical conductivity of a semiconductor quantum wire

Expressions for the relaxation time, electron mobility, and static electrical conductivity along a semiconductor quantum wire are derived in relation to a random field of Gaussian fluctuations in the wire’s thickness. In the case of nondegenerate statistics for charge carriers at relatively low temperatures (T), electron mobility is given by u _n ∝ T^1/2. In the limiting case of a strong magnetic field H directed along the wire, the factor H^?1/2 appears in the mobility expression. It is shown that the considered mechanism of charge-carrier relaxation is important for the electrical conductivity of a fairly thin and pure quantum wire at low temperatures.     

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.     

Direct exchange between silicon nanocrystals and tunnel oxide traps under illumination on single electron photodetector

In this paper we present the trapping of photogenerated charge carriers for 300 s resulted by their direct exchange under illumination between a few silicon nanocrystals (ncs-Si) embedded in an oxide tunnel layer (SiO_x = 1.5) and the tunnel oxide traps levels for a single electron photodetector (photo-SET or nanopixel). At first place, the presence of a photocurrent limited in the inversion zone under illumination in the I–V curves confirms the creation of a pair electron/hole (e–h) at high energy. This photogenerated charge carriers can be trapped in the oxide. Using the capacitance-voltage under illumination (the photo-CV measurements) we show a hysteresis chargement limited in the inversion area, indicating that the photo-generated charge carriers are stored at traps levels at the interface and within ncs-Si. The direct exchange of the photogenerated charge carriers between the interface traps levels and the ncs-Si contributed on the photomemory effect for 300 s for our nanopixel at room temperature.     

Phenoxazine-based organic dyes with different chromophores for dye-sensitized solar cells

A series of organic dyes (POZ-2, POZ-3, POZ-4 and POZ-5) involving phenoxazine were synthesized as sensitizers for application in dye-sensitized solar cells (DSSCs). For comparison, three different electron donors namely 10-phenyl-10H-phe-nothiazine, 10-phenyl-10H-phenoxazine and triphenylamine were separately appended onto the 7-position of the model dye (POZ-2). The obtained four dyes exhibit considerably high values of conversion efficiencies of 6.6%, 7.8%, 7.1% and 6.4%, respectively, under the simulated AM1.5G conditions. The geometries of the dyes were optimized to gain insight into the molecular structure and electron distribution, and then the charge extraction and transient photovoltage decay measurements were further performed to understand the influence of electron donors on the photovoltaic behaviors.