S. D. Baranovskii

On the conduction mechanism in ionic glasses

Conduction mechanism in ionic glasses is still considered one of the great challenges in physics and chemistry of glasses [A. Bunde, K. Funke, and M. Ingram, Solid State Ionics 105, 1 (1998)]. We show that consequent application of the routine percolation theory leads to the consistent description of most puzzling conduction effects for both direct current (dc) and alternating current (ac) conductivity. Moreover, comparison of the theoretical results with experimental data reveals the well-known random-energy model suggested a few decades ago for ionic transport in glasses as a very plausible model. The results provide a general basis for the study of transport phenomena in ionic glasses.

The applicability of the transport-energy concept to various disordered materials

It is known that in disordered semiconductors with purely exponential energy distribution of localized band-tail states, as in amorphous semiconductors, all transport phenomena at low temperatures are determined by hopping of electrons in the vicinity of a particular energy level, called the transport energy. We analyse whether such a transport level exists also in materials with densities of localized states (DOSs) different from the purely exponential one. We consider two DOS functions with , typical for polymers, heavily doped semiconductors, and, probably, liquid semiconductors and , typical for mixed crystals. It is shown that in both cases the transport energy exists, implying that it also exists for all intermediate forms of the DOS. Special attention is paid to the dependences of the transport level and of its width on the DOS parameters and temperature.

Quantitative description of disorder parameters in (GaIn)(NAs) quantum wells from the temperature-dependent photoluminescence spectroscopy

Photoluminescence in (GaIn)(NAs) quantum wells designed for laser emission was studied experimentally and theoretically. The observed temperature dependences of the luminescence Stokes shift and of the spectral linewidth evidence the essential role of disorder in the dynamics of the recombining excitations. The spatial and energy disorders can cause a localization of photocreated excitations supposedly in the form of excitons. Theoretical study of the exciton dynamics is performed via kinetic Monte Carlo simulations of exciton hopping and recombination in the manifold of localized states. Direct comparison between experimental spectra and theoretical calculations provides quantitative information on the energy scale of the potential fluctuations in (GaIn)(NAs) quantum wells. The results enable one to quantify the impact of annealing on the concentration of localized states and/or on the localization length of excitons in (GaIn)(NAs) quantum wells.

Lucky drift impact ionization in amorphous semiconductors

The review of avalanche multiplication experiments clearly confirms the existence of the impact ionization effect in this class of semiconductors. The semilogarithmic plot of the impact ionization coefficient (α) versus the reciprocal field (1∕F) for holes in a-Se and electrons in a-Se and a-Si:H places the avalanche multiplication phenomena in amorphous semiconductors at much higher fields than those typically reported for crystalline semiconductors with comparable bandgaps. Furthermore, in contrast to well established concepts for crystalline semiconductors, the impact ionization coefficient in a-Se increases with increasing temperature. The McKenzie and Burt [S. McKenzie and M. G. Burt, J. Phys. C 19, 1959 (1986)] version of Ridley’s lucky drift (LD) model [B. K. Ridley, J. Phys. C 16, 3373 (1988)] has been applied to impact ionization coefficient versus field data for holes and electrons in a-Se and electrons in a-Si:H. We have extracted the electron impact ionization coefficient versus field (αe vs F...

Percolation Approach to Hopping Transport in Organic Disordered Solids

Percolation approach is used to study the dc hopping conductivity and thermopower in systems with a Gaussian density of localized states typical for disordered organic materials. It is shown that the theoretical methods developed earlier for the description of hopping transport in disordered inorganic solids, such as amorphous semiconductors, can also be successfully applied to description of hopping transport in organic disordered solids, such as conjugated or molecularly doped polymers. Calculations within the percolation approach give results in excellent agreement with those obtained by using a more transparent, though less rigorous approach based on the concept of the transport energy.

Avalanche multiplication phenomenon in amorphous semiconductors : Amorphous selenium versus hydrogenated amorphous silicon

Although the effect of the impact ionization and the consequent avalanche multiplication in amorphous selenium (a-Se) was established long ago and has led to the development and commercialization of ultrasensitive video tubes, the underlying physics of these phenomena in amorphous semiconductors has not yet been fully understood. In particular, it is puzzling why this effect has been evidenced at practical electric fields only in a-Se among all amorphous materials. For instance, impact ionization seems much more feasible in hydrogenated amorphous silicon (a-Si:H) since the charge carrier mobility in a-Si:H is much higher than that in a-Se and also the amount of energy needed for ionization of secondary carriers in a-Si:H is lower than that in a-Se. Using the description of the avalanche effect based on the lucky-drift model recently developed for amorphous semiconductors we show how this intriguing question can be answered. It is the higher phonon energy in a-Si:H than that in a-Se, which is responsible f...

The concept of transport energy and its application to steady-state photoconductivity in amorphous silicon

Abstract A particular energy level in the band tail of a disordered semiconductor, called the ‘transport energy’, plays a crucial role in hopping transport of carriers in both equilibrium and non-equilibrium conditions and for both steady-state and transient phenomena. The transport energy is re-derived in a way that explains the universality of this energy level for different physical phenomena. It is shown that the transport energy determines each hopping event of an electron in the band tail and hence determines all phenomena dominated by the hopping. The concept of the transport energy is illustrated by comparing two recently suggested approaches to the description of the steady-state photoconductivity in amorphous semiconductors.

Recombination and photoconductivity in amorphous semiconductors at low temperatures

Abstract The problem of simultaneous hopping diffusion and recombination of electron-hole pairs, photoexcited in non-crystalline semiconductors at low temperatures, is reduced to a universal geometrical problem whose solution does not depend on the density of states function. The distribution of radiative recombination times after pulse excitation is found. A theory of steady state carrier concentration and of the dc and ac photoconductivity is presented.

Columnar [001]-oriented nitrogen order in Ga(NAs) and (GaIn)(NAs) alloys

By calculations in the framework of the valence force field method, we show that nitrogen atoms in diluted GaAs1−xNx tend to align along the [001] direction. In quaternary alloys Ga1−yInyAs1−xNx this tendency is observed only in “as-grown” samples, while in the annealed samples nitrogen atoms build more energetically favorable bonds with indium. Experimentally observed inhomogeneous strain profiles in these material systems, as well as their dissolution upon annealing, agree qualitatively with results of the calculations.

On the efficiency of exciton dissociation at the interface between a conjugated polymer and an electron acceptor

It is a matter of controversy why excitons can efficiently dissociate into free carriers at an intrinsic polymer/fullerene interface, despite the strong Coulomb interaction between the charges provided by the very low dielectric constant in organic materials. The effect has been ascribed to the presence of intrinsic dipoles on the polymer/fullerene interface, though assuming an unrealistically small carrier effective mass necessary for exciton dissociation. We improve the model showing that it allows realistic carrier effective masses. The dissociation probability is calculated as a function of electric field acting on the dissociating electron-hole pairs.