Electrodynamic convection of free charge carriers in semiconductors

The possibility is demonstrated of convective motion of free charge carriers in a thin semiconducting layer with a transverse electric field applied to it. The conditions for the development of this kind of electrodynamic convection and the criteria for stability of the convection cells are studied. Fiz. Tverd. Tela (St. Petersburg) 39, 280–283 (February 1997)     

Internal waves of free charge carriers in semiconductors

We show that internal waves that are similar to internal gravity waves in the ocean can propagate in semiconductors with a gradient of free charge carrier concentration. Unlike internal gravity waves in the ocean, which are observed at infrasonic frequencies, the waves under consideration are excited in the microwave range, which opens up interesting possibilities for creating new types of electron devices. In the nonlinear rigime such waves assume the form of solitons described by the KdV equation. State Technical University, Ul’yanovsk, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 3, pp. 370–377, March, 1997.     

Photogeneration of charge carriers in amorphous selenium

A model is proposed to explain the outstanding features of the free-carrier photogeneration process in amorphous selenium. The excitation of an electron-hole pair and the subsequent separation or recombination are considered as a function of photon energy, applied electric field and temperature. The results of the model are compared in detail with new experimental data and also with published results from a number of sources.     

Universal distribution of residual carriers in tetrahedrally coordinated amorphous semiconductors

An uncommon electron spin resonance technique is used to show that a universal distribution of residual carriers exists in tetrahedrally coordinated amorphous semiconductors following optical excitation at low temperatures. This universal behavior at long decay times results because statistical fluctuations in the electron and hole densities cannot occur and therefore do not affect the kinetics. This behavior is confirmed for carrier densities between 10(16) and 10(17) cm (-3) and decay times as long as 10(4) s.     

Multi-phonon capture of charge carriers by dislocation kinks in semiconductors

In view of the problem of recombination-enhanced motion of dislocations in semiconductors, we studied the thermal capture of an electron by a smooth dislocation kink. Multi-phonon capture becomes possible due to localization of the carrier on the kink. The localized state on the smooth kink is studied in the deformation potential approximation. In this case the potential created by the kink is described by Poschl-Teller function, which enables to find the analytical expressions for the eigenstates and the corresponding wave functions. With the use of the ground state wave function we find the multi-phonon capture cross-section for two limiting temperature cases, corresponding to the thermally activated and quantum transitions between vibronic terms.     

Magnetic-field-induced retardation of the recombination of nonequilibrium charge carriers in semiconductors

The retardation of the recombination of electrons and holes in semiconductors in an applied uniform magnetic field has been predicted. It has been shown that the recombination time in germanium in the temperature range of T = 1–10 K at charge carrier densities of n _e = 10¹⁰−10¹⁴ cm⁻³ in magnetic fields of B = 3 × 10²−3 × 10⁴ G can be more than two orders of magnitude larger than that at zero magnetic field. This means that, after creation of nonequilibrium charge carriers by their injection at the p-n junction owing to some radiation sources or fast electron irradiation, the semiconductor retains its conductivity for a much longer time at nonzero applied magnetic field. The effect under study can be used, for example, to detect radiation sources.     

In-crystal and surface charge transport of electric-field-induced carriers in organic single-crystal semiconductors

Gate-voltage dependence of carrier mobility is measured in high-performance field-effect transistors of rubrene single crystals by simultaneous detection of the longitudinal conductivity sigma(square) and Hall coefficient R(H). The Hall mobility mu(H) (identical with sigma(square)R(H)) reaches nearly 10 cm(2)/V s when relatively low-density carriers (<10(11) cm(-2)) distribute into the crystal. mu(H) rapidly decreases with higher-density carriers as they are essentially confined to the surface and are subjected to randomness of the amorphous gate insulators. The mechanism to realize high carrier mobility in the organic transistor devices involves intrinsic-semiconductor character of the high-purity organic crystals and diffusive bandlike carrier transport in the bulk.     

Relaxation and dynamics of charge carriers in organic polymer semiconductors: Polyacetylene (review)

The results of investigations of cis-polyacetylene and trans-polyacetylene by EPR spectroscopy in the 2-mm wave range are systematized and generalized. The nature and dynamic properties of nonlinear soliton-type carriers are discussed, along with the charge transfer mechanism in neutral polyacetylene. Fiz. Tverd. Tela (St. Petersburg) 39, 3–17 (January 1997)     

Effect of electron-electron interaction on spin relaxation of charge carriers in semiconductors

An analysis of spin dynamics is presented for semiconductor systems without inversion symmetry that exhibit spin splitting. It is shown that electron-electron interaction reduces the rate of the Dyakonov-Perel (precession) mechanism of spin relaxation both via spin mixing in the momentum space and via the Hartree-Fock exchange interaction in spin-polarized electron gas. The change in the Hartree-Fock contribution with increasing nonequilibrium spin polarization is analyzed. Theoretical predictions are compared with experimental results on spin dynamics in GaAs/AlGaAs-based quantum-well structures. The effect of electron-electron collisions is examined not only for two-dimensional electron gas in a quantum well, but also for electron gas in a bulk semiconductor and a quantum wire.     

Intraband photoconductivity in variable-gap semiconductors produced by photon drag of charge carriers

The special features resulting from the photon drag of intraband photoconductivity carriers in variable-gap semiconductors with intrinsic type conductivity and a forbidden-gap width which varies linearly with distance are considered theoretically. An analysis is made of the dependence of the photoconductivity on the gradient of the forbidden-gap width for different surface recombination rates and sample thicknesses. It is shown that a spatial inhomogeneity of the band structure in semiconductors enables their photoconductivity to be substantially increased, especially in the case of thick samples.Institute of Applied Physics at the Ivan Franko State University, L'vov. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 48–52, September, 1993.     

Kinetics of thermalization of electric charge carriers in organic molecular semiconductors in strong electric fields

The kinetics of photogeneration of electric charge carriers in organic molecular semiconductors in strong electric fields is investigated within a theoretical model that includes the diffusion-drift mechanism of motion of electric charge carriers. In the framework of the proposed model, it is established that the time dependence of the probability of dissociation of a geminal pair exhibits an exponential behavior. The time constant strongly depends on the electric charge carrier hopping distance and the external electric field strength in accordance with the experimental data.     

Adsorption on amorphous semiconductors: A modified Haldane-Anderson model

The Haldane-Anderson model previously used for describing the adsorption on a crystalline substrate is generalized to the case of an amorphous substrate. It is demonstrated that the main difference between the occupation numbers of the adatom is observed in the case where the atomic level overlaps with the band gap (for a crystalline semiconductor) and the mobility gap (for an amorphous semiconductor). The adatom charge on the amorphous substrate is smaller than that on the crystalline substrate.     

Thermalization and recombination in amorphous semiconductors

The fate of carriers excited in an amorphous semiconductor by a pulse of light is described. Two processes are discussed: thermalization of carriers within a distribution of localized states, and carrier recombination.     

Vibrational investigation of 1-cyclopentylpiperazine: A combined experimental and theoretical study

FT-IR and Raman spectra of 1-cyclopentylpiperazine(1cppp)have been experimentally examined in the region of 4000–200cm-1.The optimized geometric parameters,conformational equilibria,normal mode frequencies and corresponding vibrational assignments of 1cppp(C9H18N2)are theoretically examined by means of B3LYP hybrid density functional theory(DFT)method together with 6-31++G(d,p)basis set.On the basis of potential energy distribution(PED)reliable vibrational assignments have been made and the thermodynamics functions,highest occupied and lowest unoccupied molecular orbitals(HOMO and LUMO)of 1cppp have been predicted.Calculations are employed for four different conformations in C1 and Cs point groups of 1cppp in gas phase.Comparison between the experimental and theoretical results indicates that B3LYP method is able to provide satisfactory results for predicting vibrational frequencies and the structural parameters,vibrational frequencies and assignments.Furthermore,C1(equatorial-axial)point group has been found as the most stable conformer of 1cppp.     

A combined theoretical and experimental study on molecular photonics

A detailed account of quantum chemical procedures for estimating the rate constants of the photophysical processes at work in polyatomic organic molecules is given. The results obtained from combined experimental and theoretical research into the spectral-luminescent properties of acridine, 9-aminoacridine, 2,7-dimethyl-9-diphenylaminoacridine, and of their protonated forms are reported. The electronic absorption and fluorescence spectra of acridine have been investigated at room temperature in ethanol solution of varying pH and in other solvents of different chemical nature and polarity. The energy of excited states, the rate constants of the deactivation of the excited states, and the dipole moments obtained by quantum chemical methods for the examined compounds are presented. The findings of the investigations are discussed. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 93–107, October, 2008.