Statistical strained-tetrahedron model of local ternary zinc blende crystal structures

The statistical strained-tetrahedron model was developed to overcome two common assumptions of previous models: 1) rigid undistorted ion sublattice of regular tetrahedra throughout all five configurations and 2) random ion distribution. These simplifying assumptions restrict the range of applicability of the models to a narrow subset of ternary alloys for which the constituent binaries have their lattice constants and standard molar enthalpies of formation (∆fH₀) equal or quasi-equal. Beyond these limits predictions of such models become unreliable, in particular, when the ternary exhibits site occupation preferences. The strained-tetrahedron model, free from rigidity and stochastic limitations, was developed to better describe and understand the local structure of ternary zinc blende crystals, and interpret experimental EXAFS and far-IR spectra. It considers five tetrahedron configurations with the shape and size distortions characteristic of ternary zinc blende alloys, allows nonrandom distributions and, hence, site occupation preferences, conserves coordination numbers, respects stoichiometry, and assumes that next-neighbor values determine preferences beyond next-neighbor. The configuration probabilities have three degrees of freedom. The nineteen inter-ion crystal distances are constrained by tetrahedron structures; to avoid destructive stresses, we assume that the average tetrahedron volumes of both sublattices relax to equal values. The number of distance free-parameters ≤ 7. Model estimates, compared to published EXAFS results, validate the model. Knowing the configuration probabilities, one writes the dielectric function for far-infrared absorption or reflection spectra. Constraining assumptions restrict the number of degrees of freedom. Deconvolution of the experimental spectra yields site-occupation- preference coefficient values and/or specific oscillator strengths. Validation again confirms the model

Magnetospectroscopy of symmetric and anti-symmetric states in double quantum wells

The experimental results obtained for the magneto-transport in the InGaAs/InAlAs double quantum wells (DQW) structures of two different shapes of wells are reported. The beating-effect occurred in the Shubnikov-de Haas (SdH) oscillations was observed for both types of the structures at low temperatures in the parallel transport when magnetic field was perpendicular to the layers. An approach to the calculation of the Landau levels energies for DQW structures was developed and then applied to the analysis and interpretation of the experimental data related to the beating-effect. We also argue that in order to account for the observed magneto-transport phenomena (SdH and Integer Quantum Hall effect), one should introduce two different quasi-Fermi levels characterizing two electron sub-systems regarding symmetry properties of their states, symmetric and anti-symmetric ones which are not mixed by electron-electron interaction.Comment: 20 pages, 20 figure

Parallel magnetotransport in multiple quantum well structures

The results of investigations of parallel magnetotransport in AlGaAs/GaAs and InGaAs/InAlAs/InP multiple quantum wells structures (MQW’s) are presented in this paper. The MQW’s were obtained by metalorganic vapour phase epitaxy with different shapes of QW, numbers of QW and levels of doping. The magnetotransport measurements were performed in wide region of temperatures (0.5–300 K) and at high magnetic fields up to 30 T (B is perpendicular and current is parallel to the plane of the QW). Three types of observed effects are analyzed: quantum Hall effect and Shubnikov—de Haas oscillations at low temperatures (0.5–6 K) as well as magnetophonon resonance at higher temperatures (77–300 K)

Statistical model analysis of local structure of quaternary sphalerite crystals

At the 2004 Ural International Winter School, we introduced the statistical strained tetrahedron model and discussed ternary tetrahedron structured crystals. The model allows one to interpret x-ray absorption fine structure (EXAFS) data and extract quantitative information on ion site occupation preferences and on the size and shape of each elemental constituent of the configuration tetrahedra. Here we extend the model to cover quaternary sphalerite crystal structures. We discuss the two topologically different quaternary sphalerite systems: the pseudo balanced A₁₋xBxYyZ₁₋y (2:2 cation:anion ratio), and the unbalanced AxBx C₁₋x₋x Z or AXyYy Z₁₋y₋y (3:1 or 1:3 cation:anion ratios) truly quaternary alloy systems. These structural differences cause preference values in pseudo quaternaries to vary with the relative contents, but to remain constant in truly quaternary compounds. We give equations to determine preference coefficient values from EXAFS or phonon spectra and to extract nearest-neighbour inter-ion distances by EXAFS spectroscopy. The procedure is illustrated and tested on CdMnSeTe, GaInAsSb, and ZnCdHgTe quaternary alloys

Synthesis of dye-sensitized solar cells. Efficiency cells as a thickness of titanium dioxide

Defying the influence of the thickness of TiO2 efficiency of dye-sensitized solar cell. It was confirmed that the compatibility of printed layers with the parameters closely related with the DSSC. It was found that the increase in thickness of the titanium dioxide layer, increases the distance between the electrodes, determined by the thickness of the Surlyn foil. With the rise of thickness of dyed layer of TiO2 established decrease in the value of its transmittance. Greatest transparency and aesthetic value obtained for photovoltaic modules with a single layer of titanium dioxide. The improved performance efficiency and preferred yields maximum power were noticed and exhibited by the cells covered with three layers of TiO2. It was established that the behaviour of economic efficiency in the production process, provides a range of cells with two layers of oxide, showing a similar performance and greater transparency