A. N. Reznitskii

Exciton luminescence from fluctuation-induced tails in the density of states of disordered solid solutions

The shape of the luminescence spectra of excitons localized by composition fluctuations in a disordered solid solution, is calculated by a theoretical model that takes into account two different aspects of the electron-phonon interaction: 1) the lifetimes of localized states are limited because of transitions (tunneling) between states of the tail with emission of phonons. This implies that only a relatively small fraction of the states in the tail-those which have no access to such transitions-are populated long enough to emit radiation; 2) the luminescence spectra from these long-lived radiating states is also caused by the simultaneous emission of phonons. It is shown that both these aspects are important in explaining the observed shift in the maximum of the luminescence band relative to the maximum of the exciton absorption line. The shape of the short-wavelength edge of the luminescence band is determined primarily by the dependence of the number of clusters of minimum size on the localization energy, in particular its rapid decrease in the neighborhood of the mobility edge, whereas the spectrum of recombination with emission of phonons determines the shape of the long-wavelength tail of the primary emission band. The calculated shape of the emission spectrum is compared with spectra obtained experimentally for luminescence from the solid solution CdS(1−c)Sec. It turns out that a satisfactory description of the experimental spectra of CdS(1−c)Sec over a wide range of compositions requires two models of the localized exciton: localization of the exciton as a whole (model I) or localization of the hole with the electron bound to it by the Coulomb interaction (model II).

Fluctuation states and optical spectra of solid solutions with a strong isoelectronic perturbation

We propose an approach to describing the density of fluctuation states in a disordered solid solution with a strong perturbation introduced by isoelectronic substitution in the range of attraction-center concentrations below the threshold of percolation along the sites of a disordered sublattice. To estimate the number of localized states we use the results of lattice percolation theory. We describe a method for distinguishing, within the continuum percolation theory, among the various “radiating” states of the fluctuation-induced tail, states that form the luminescence band at weak excitation. We also establish the position of the band of radiating states in relation to the absorption band of the excitonic ground state and the mobility edge of the system. The approach is used to describe the optical spectra of the solid solution ZnSe1−cTec, which at low Te concentrations can be interpreted as a system with strong scattering. We take into account the exciton-phonon interaction and show that the calculated and observed luminescence spectra of localized excitons are in good agreement with each other.

Exciton dynamics in ZnCdSe/ZnSe quantum-well structures

Exciton dynamics in ZnCdSe/ZnSe quantum-well structures have been studied from luminescence spectra obtained at T=2 K. The energy and phase relaxation times of localized exciton states have been determined from a study of the destruction of exciton optical alignment by an external magnetic field and direct measurements of the polarized-radiation decay kinetics in the picosecond range. The exciton polarization lifetimes measured by two independent techniques are found to be in a good agreement.

Diffusion of excitons in CdS-Se and ZnSe-Te solid solutions at high excitation levels

It is shown that, with strong pulsed excitation, the intensity of the exciton recombination band in the fluctuation tail of the density of states in the limit of large times in the presence of traps is described by the asymptote of a solution to the diffusion equation. The critical diffusion index corresponds to a “normal” process in the CdS-Se solid solution and to “anomalous” diffusion in the case of ZnSe-Te.

Optical and thermal orientation of localized excitons in solid solutions under resonant excitation in a longitudinal magnetic field

A theoretical and experimental study of the effect of a longitudinal magnetic field on optical orientation and magneto-circular polarization of the luminescence of localized excitons in semiconducting solid solutions is reported. It is shown that recombination takes place through two types of emitting states differing substantially in the degree of anisotropy, g factor, and spin relaxation time. Estimates are made of the g factors, anisotropic and exchange splittings, lifetime, and spin relaxation time of localized states in a CdS0.96Se0.04/GaAs solidsolution epitaxial layer.

Optical spectra and a tail in the density of states of the disordered solid solution ZnSe1−c Tec

A calculational scheme is presented to determine the density of states in the fluctuation tail for the disordered solid solution ZnSe1−cTec at concentrations below the threshold for percolation over sublattice sites. Zero-phonon absorption and luminescence bands in the region of the exciton ground state are found using an approach developed earlier. Phonon coupling is taken into account, and vibronic absorption and luminescence bands are obtained. Experimental data are shown to be in a good agreement with the calculations.