Vol 50, No 4 (2016)

The results of calculation of the electronic structure of Si-based Pt-substituted clathrates are reported. Calculation is carried out by the linearized-augmented-plane-wave method. The effect of the number of substitutions and their crystallographic position in the unit cell on the electron-energy spectrum and the electronic properties of Pt-substituted clathrates is analyzed.

Analytical solutions for the transient current in the СELIV (Charge Extraction by Linearly Increasing Voltage) experiment are found in both the quasi-equilibrium (taking into account field-enhanced diffusion) and dispersive transport modes. The ratio of the time of current passage through the maximum to the transit time significantly decreases with increasing disorder. Simple analytical expressions for this ratio, containing only one dimensionless parameter characterizing the given material in each case, are obtained. The conditions of applicability of the known CELIV theory (without diffusion) are determined. The result allows calculations of the transit time in the case where the transient-current signal is sharply asymmetric and slowly decreases over long times.

For n- and p-type Si_0.85Ge_0.15 alloys, the thermoelectric properties (thermopower, resistivity, thermal conductivity, and thermoelectric efficiency) are determined in the range from room temperature to 1200°C. The measurements are carried out with an upgraded device [1] by absolute and steady-state methods with thermal screens. The device is upgraded to extend the working-temperature range to ~1500 K. On the basis of these data, the energy-related capabilities of the alloy are estimated; the thermal band-gap width is calculated in the temperature range of ~1300–1400 K.

It is shown that modulation of the superluminescence intensity on a characteristic time scale on the order of a few picoseconds may occur in a semiconductor because of relaxation oscillations that result from the “healing” of local perturbations of the quasi-Fermi distribution of nonequilibrium electrons in energy space. The conditions for observing this effect in GaAs are estimated.

The electronic properties of the Ba/3C-SiC(111) nanointerface are for the first time studied by photoelectron spectroscopy with the use of synchrotron radiation in the energy range 80–450 eV. The experiments are performed in situ in ultrahigh vacuum for ultrathin Ba coatings on 3C-SiC(111) samples grown by a new method of substituting substrate atoms. It is found that the adsorption of Ba brings about the appearance of induced surface states with the binding energies 1.9, 6.2, and 7.5 eV. Evolution of the surface states and the spectra of the Si 2p and C 1s core levels shows that the Ba/3C-SiC(111) interface is formed due to charge transfer from Ba adatoms to surface Si atoms and underlying C atoms.

Experimental results on the effects of changes in the impact-ionization current in silicon diffusion p⁺–n–n⁺ and n⁺–p–p⁺ structures upon nonuniform heating are presented. It is shown that the revealed effects are associated with the transformation of band energy levels caused by thermoelastic stresses of the structures.

Externally applied electric field and effective radius effects are investigated on the lowest excitedstate shallow-donor binding energy in (In,Ga)N-GaN parabolic wire within the framework of single band effective-mass approximation. The calculations are performed using the finite-difference method within the quasi-one-dimensional effective potential model. Our results reveal that: (i) the probability density is the largest on a circularity whose radius is the effective radius, (ii) the lowest excited-state binding energy is the largest for impurity located on this circularity while it starts to decrease when the impurity is away from the circularity and (iii) the binding energy is strongly-dependent on the complex interplay of spatial confinement, coulomb interaction and applied electric field.

The optical and photoelectric properties of present-day photosensitive polymers are of particular interest due to their prospects for use in various photoelectric applications. Here the absorption edge is studied by the constant photocurrent method which is widely used for studies of inorganic materials. For the objects to be studied, PCDTBT and PTB7 polymers and their mixtures with the PC₇₁BM fullerene derivative are chosen. The spectral dependences of the absorption coefficient for the pure polymers and mixtures are approximated on the assumption of the Gaussian distribution of the density of states in the bands of the components, which provides a means for estimating the electrical band gaps. In addition, such processing of the spectra makes it possible to confirm some important inferences about the states responsible for optical transitions in the low-energy absorption region.

On the basis of both published data and our own experimental data we consider the main aspects of the problem related to ensuring polytype stability for ingots of grown 4H and 6H silicon-carbide compounds.

An experimental technique for increasing the yield of carbon-nanotube nanotori using the modified arc synthesis method is proposed. New physical knowledge on the systematic features of the interrelation between the properties of nanotori and atomic-network topology are theoretically established for the first time. The experiments are performed based on new technology for synthesizing nanotori on nickel-catalyst particles by a high-voltage pulsed discharge in ethanol vapor and using atomic-force microscopy. Stability is predicted using an original procedure for calculating local atomic stresses. Simulation shows that the zigzag chirality corresponds to the most stable topology of nanotori. Using the tight binding method, it is shown that, depending on the chirality type, nanotori are divided into two classes, i.e., those with metal and semiconductor conductivity.

Optical and recombination losses in a Cu(In,Ga)Se₂ thin-film solar cell with a band gap of 1.36–1.38 eV are theoretically analyzed. The optical transmittance of the ZnO and CdS layers through which the radiation penetrates into the absorbing layer is determined. Using optical constants, the optical loss caused by reflection at the interfaces (7.5%) and absorption in the ZnO and CdS layers (10.2%) are found. To calculate the recombination loss, the spectral distribution of the quantum efficiency of CdS/CuIn_1–xGa_xSe₂ is investigated. It is demonstrated that, taking the drift and diffusion components of recombination at the front and rear surfaces of the absorber into account, the quantum efficiency spectra of the investigated solar cell can be analytically described in detail. The real parameters of the solar cell are determined by comparing the calculated results and experimental data. In addition, the losses caused by the recombination of photogenerated carriers at the front and rear surfaces of the absorbing layer (1.8% and <0.1%, respectively), at its neutral part (7.6%), and in the space-charge region of the p–n heterojunction (1.0%) are determined. A correction to the parameters of Cu(In,Ga)Se₂ is proposed, which enhances the charge-accumulation efficiency.

The temperature dependences of the efficiency η of high-efficiency solar cells based on silicon are calculated. It is shown that the temperature coefficient of decreasing η with increasing temperature decreases as the surface recombination rate decreases. The photoconversion efficiency of high-efficiency silicon-based solar cells operating under natural (field) conditions is simulated. Their operating temperature is determined self-consistently by simultaneously solving the photocurrent, photovoltage, and energy-balance equations. Radiative and convective cooling mechanisms are taken into account. It is shown that the operating temperature of solar cells is higher than the ambient temperature even at very high convection coefficients (~300 W/m² K). Accordingly, the photoconversion efficiency in this case is lower than when the temperature of the solar cells is equal to the ambient temperature. The calculated dependences for the open-circuit voltage and the photoconversion efficiency of high-quality silicon solar cells under concentrated illumination are discussed taking into account the actual temperature of the solar cells.

The main problem of the epitaxial growth of thick AlN layers on a Si substrate consists in the formation of cracks, which complicates the application of structures of this kind in the fabrication of semiconductor devices. The possibility of obtaining crack-free AlN layers with a thickness exceeding 1 μm and a mirror- smooth surface by hydride vapor-phase epitaxy is demonstrated. The properties of the layers are studied by X-diffraction analysis, optical and scanning electron microscopy, and Raman spectroscopy.

The structural and optical properties of Cu₂ZnSnS₄ thin-film layers formed by reactive pulsed laser deposition in a H₂S atmosphere at room temperature with the use of a Cu metal target and a Zn–Sn alloy target are studied in relation to the parameters of annealing in a N₂ atmosphere.

This study is devoted to the search for new possibilities of characterizing crystal-structure features using high-resolution X-ray diffraction. The emphasis is on the scanning mode across the diffraction vector (ω-scanning), since researchers usually pay little attention to this mode, and its capabilities have not yet been completely revealed. For the [011] and [01\(\bar 1\)] directions, the ω-peak half-width and the average tilt angle of the sample surface profile are compared. The diagnostic capabilities of X-ray scattering mapping are also studied. The objects of study are semiconductor nanoheterostructures with an InAlAs/InGaAs/InAlAs quantum well and an In_xAl_1–xAs metamorphic buffer grown by molecular-beam epitaxy on InP and GaAs substrates. Such nanoheterostructures are used to fabricate microwave transistors and monolithic integrated circuits. The objects under study are more completely characterized using the Hall effect, atomic-force microscopy, and low-temperature photoluminescence spectroscopy at 79 K.