The influence of multiphonon transitions on interband optical absorption in semiconductors with a Coulomb-type random field

The temperature dependence of the optical absorption coefficient in the tail region is studied in semiconductors with a random Coulomb-type field. The results of numerical calculation are compared with data for chalcogenide glasses.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 63–67, September, 1981.The author is grateful to V. L. Bonch-Bruevich for his attention to the work.     

Interband absorption in layered semiconductors with a smooth random field

The effect of a smooth random field on interband absorption is studied in highly anisotropic semiconductors. Such fields can arise because of semimacroscopic defects induced during production and randomly distributed throughout the sample. The calculations are performed using a semiclassical approximation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 45–47, September, 1981.We wish to thank V. L. Bonch-Bruevich for discussions of these results and his constant attention to the work reported here.     

Interzone light absorption with consideration of multiphonon transitions in semiconductors with a Coulomb-like random field

The method of calculating the interzone light absorption coefficient in a semiconductor with Coulomb-like random field developed by Bonch-Bruevich is generalized by inclusion of multiphonon processes. A formula is obtained for the frequency dependence of the absorption coefficient in the optical tail range in the Condon approximation with neglect of frequency effect.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 64–67, July, 1981.In conclusion, the author expresses his gratitude to V. L. Bonch-Bruevich for his interest in the study.     

Scattering mechanism in doped semiconductors with a smooth random field

The scattering mechanism of charge carriers in doped compensated semiconductors with a smooth random field is considered in the interval of temperatures 4–100°K and of impurity concentrations 10¹⁴–10¹⁸ cm^–3. It is shown that in the conditions mentioned above at the lowest temperatures scattering in the Coulomb field associated with charged impurity dominates, while at elevated temperatures scattering in the smooth random field is important. Scattering by phonons and by resonant and virtual levels associated with the random potential is insignificant in the present conditions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 23–27, February, 1984.     

Theory of hopping conductivity in disordered semiconductors in a strong magnetic field

The percolation approach is used to study the vanishing of the linear section in the temperature dependence of the hopping conductivity of disordered semiconductors in a strong magnetic field.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 62–69, February, 1976.I Thank V. L. Bonch-Bruevich for discussion and constant interest in the work.     

Interband optical absorption in disordered semiconductors including multiphonon effects. II

We examine the effect of multiphonon transitions on the frequency dependence of the absorption coefficient in the optical tail region in a semiconductor with a smooth Gaussian random field for the cases of high and low heat liberation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 39–45, March, 1980.I thank V. L. Bonch-Bruevich for suggesting this topic and constant interest in this work and B. Esser for valuable discussion.     

Exciton absorption of light in a smooth random field

A study is made of interband absorption of light in a smooth random field that varies slowly in space. When the exciton energy in the random field is much less than the exciton binding energy, the influence of the random field on the discrete spectrum of the exciton is expressed through the appearance of an exponential “tail” to every exciton line. But if the exciton energy in the random field is much greater than the exciton binding energy, the exciton effects are unimportant because the random field breaks up the excitons.     

Theory of interband optical absorption in a quasiclassical random field

Green's functions are used to study interband transitions in a quasiclassical (i.e. slowly varying in space) irregular field. It is assumed that the irregular field is random, i.e., the mean square of the fluctuation of the potential energy of an electron in such a field is much less than the width of the forbidden band. Explicit expressions are obtained for the coefficient of interband optical absorption for the case of an anisotropic irregular field and for the case when the irregular fields that act on electrons in the conduction and the valence band are different. As examples, we consider a fluctuation random field in ferroelectrics and a random field produced by long-wave acoustic lattice vibrations. The behavior of the tail of the optical absorption coefficient in ferroelectrics in the region of a phase transition is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 121–130, April, 1972.I am most grateful to V. L. Bonch-Bruevich for suggesting the subject and for his constant interest.     

Effect of a magnetic field on the phonon-assisted two-photon absorption in semiconductors

The theory of the phonon-assisted two-photon transitions in a magnetic field is developed by means of perturbation theory. Three different band models have been introduced. The electronic transitions are defined by (i) interband-interband indirect transitions in the four-band model, (ii) interband-intraband indirect transitions in the three-band model, and (iii) intraband-intraband indirect transitions in the two-band model. These models differ from each other by the number of bands, the type of the photon matrix elements and the Landau selection rules. The result shows that by increasing the value of the magnetic field, the absorption co-efficient adopting the two-band model is strongly increased, but for a fixed value of the field the four-band model gives the largest contribution.     

Exciton effects in disordered semiconductors

Experimental data on optical and photoelectric properties of molecular (polymeric) disordered semiconductors of inorganic and organic nature are analyzed. It is shown that anomalies in optical properties and in the structure of the internal photoeffect quantum-yield spectrum can be understood by assuming the existence of exciton states in disordered semiconductors.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 86–101, February, 1976.The author is grateful to Prof. S. G. Grenishin for his interest in this work.     

Multiphonon hopping in disordered semiconductors

Equations of motion for the nonequilibrium single-frequency density matrix are used to obtain a balance equation describing the kinetics of electron hopping between localized states in disordered semiconductors. Conclusions are drawn about the applicability of the theory of multiphonon processes to systems with a small nonadiabaticity. The decoupling used is a generalization of one used earlier to the multiphonon case. The balance equation found in the Markov approximation for the average number of sites occupied in an electric field contains the field dependence of the multiphonon-transition probability, tending to the well-known weak-field limit.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No.2, pp.35–42, February, 1976.In conclusion the author is grateful to V. L. Bonch-Bruevich and A. G. Mironov for discussing this work.     

Electron transport in disordered semiconductors

The density matrix of an electron in an amorphous semiconductor, written as a double path integral, is averaged over random disorder. For small disorder the golden rule result is recovered. When the mean disorder energy approaches k_BT, the electronic mobility goes to zero signalling disorder induced localization.     

On the mean field instability of a random model for disordered superconductors

A phenomenological model for a strongly disordered superconductor is considered. This is a modification of the Bardeen-Cooper-Schrieffer approach for a system with random phonon-electron interaction. We show that the instability of its mean field theory cannot be fitted by a power law with a positive exponent, in contrast to a recent result based on perturbation theory.Supported in part by the Deutsche Forschungsgemeinschaft     

Trap-assisted recombination in disordered organic semiconductors

The trap-assisted recombination of electrons and holes in organic semiconductors is investigated. The extracted capture coefficients of the trap-assisted recombination process are thermally activated with an identical activation energy as measured for the hole mobility μ(p). We demonstrate that the rate limiting step for this mechanism is the diffusion of free holes towards trapped electrons in their mutual Coulomb field, with the capture coefficient given by (q/ε)μ(p). As a result, both the bimolecular and trap-assisted recombination processes in organic semiconductors are governed by the charge carrier mobilities, allowing predictive modeling of organic light-emitting diodes.     

Light spectra in the near field of random media

The spectrum of light in close proximity to a highly scattering, randomly inhomogeneous medium is investigated. We find differences between the far- and near-field spectra that are due to evanescent wave contributions. We show that these spectral modifications depend on the coherence properties of an equivalent planar source, which, in turn, relates to the statistical characteristics of the interface.     

Theory of hopping conductivity in disordered semiconductors

The bond percolation method is used to investigate the conditions under which a linear (and not exponential) temperature dependence of hopping conductivity can be observed.Translated from Izvestiya Vysshikh Uchbenykh Zavedenii, Fizika, No.2, pp. 52–62, February, 1976.