A. A. Klochikhin

Exciton absorption in CdS1-xSex and ZnSe1-xTex solid solutions

Absorption spectra of CdS1-xSex and ZnSe1-xTex semiconductor solid solutions have been studied at T = 2 K in the region of fundamental absorption edge for composition range (0.02 < x < 0.6). It is shown that potential fluctuations due to compositional disorder of the alloy have a strong effect on both the exciton state broadening and the band gap shift. A model for description of the exciton absorption spectra is developed. The contribution of the fluctuations to the part of the band gap shift which is nonlinear in concentration is separated from the other mechanisms.

Exciton luminescence of quasi-two-dimensional solid solutions

Absorption and luminescence of the quantum wells formed by the (Zn-Cd)Se and (Ga-In)As solid solutions are studied in the range of exciton size-quantization ground state. The spectra observed are described by a model assuming the two-dimensional character of fluctuation states in quantum wells and the presence of a percolation threshold within the absorption contour.

Disorder-induced exciton localization in 2D wide-gap semiconductor solid solutions

Exciton localization in quantum wells (QWs) formed by wide-gap solid solutions with isoelectronic substitution of ZnCdSe and InGaN is discussed. As a first step we discuss the microscopic mechanisms of exciton localization which are common for both 2D and bulk solid solutions. In most important for application solid solutions with good solubility, the exciton localization at small solution concentrations originates from the statistical clusters formed by a few atoms of the narrow-gap component. A theoretical model of absorption and photoluminescence (PL) spectra of diluted solutions is presented. The model explicitly accounts for the statistics of the substitutional atoms over the lattice sites. The parameters of the model can be determined from the analysis of the fine structure of PL spectra from the cluster states with different number of atoms. As a practical application of the model, the description of exciton spectra of diluted bulk InGaN solutions is presented. The effect of the lowering of dimensionality on exciton localization and the conditions for formation of 2D QWs from solid solutions are discussed. Experimental data on exciton localization in ZnCdSe/ZnSe QWs are presented. For diluted solutions the spectra can be described under the assumption of random distribution of Cd atoms along the QW plane. An increase of concentration due to the partial phase separation of ZnCdSe solutions leads to the formation of planar islands strongly enriched in Cd content. The results of optical study of the exciton states in the QWs with such island are reported.

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).

Phonon sidebands in luminescence spectra of alloy-trapped excitons in disordered solid solutions

Abstract We have calculated the luminescence spectra of localized excitons in disordered solid solutions. The theory takes into account two different aspects of electron-phonon interactions: phonon-assisted tunneling between tail states and phonon-assisted recombination. It has been shown that a shift of the luminescence band relative to the exciton absorption line is due to both processes. The high energy side of the luminescence band is governed by the phonon-assisted tunneling with the rate strongly dependent on the spatial size of the localized exciton wavefunction in the mobility edge vicinity. The phonon-assisted recombination was taken into account for fitting the luminescence spectra. Calculations were compared with experimental findings on CdS (1 − c ) Se c . For satisfactory fitting of CdS (1 − c ) Se c luminescence spectra in a wide composition range two models of localized excitons should be used: (I) exciton trapped as a whole and (II) trapped hole with Coulomb bound electron.

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.

Fine structure and spin relaxation of excitons localized at CdSe sub-monolayer insertions in a ZnSe matrix

Resonant spin-flip Raman scattering in tilted magnetic fields up to 14 T has been used to investigate excitons localised by the random disorder created by sub-monolayer insertions of CdSe in a ZnSe matrix. The field dependences of the Raman shifts, of the linewidths of the signals and of their intensities have been measured and fitted by a simple theoretical model and the g-factors and exchange parameters have been determined. It is concluded that spin-flip of the total exciton spin is one of the important mechanisms of spin relaxation.

Two-dimensional localization of excitons in QWs formed by II–VI solid solutions

Abstract We have studied the localization of two-dimensional exciton states in II–VI heterostructures by compositional fluctuations of solid solution, forming the quantum well. It is shown that due to the two-dimensional character of exciton motion in quantum wells, the effect of compositional fluctuation on exciton states is much stronger than in three-dimensional solutions of the same composition. The method for calculation of the density of fluctuation states below the edge of two-dimensional exciton band and the spectral density of exciton transitions is developed. The classification of states with respect to its migration properties and contribution to the luminescence processes has been carried out using the continuum percolation theory. As a result, the shape of emission band, its relative shift with respect to absorption band and the position of mobility edge are calculated. The results of calculations are in good agreement with the experimental PL and PLE spectra of ZnSe superlattices with submonolayer insertions of CdSe.

Long-time luminescence kinetics of localized excitons and conduction band edge smearing in ZnSe(1−c)Tec solid solutions

It is shown that the integrated luminescence intensity of localized excitons in solid solutions ZnSe(1−c)Tec has a component slowly decaying with time. After the excitation above the mobility threshold, the long-time intensity decreases exponentially, with a fractional exponent changing from a value corresponding to the critical index of anomalous diffusion to the index of normal diffusion as the temperature increases from 5 to 80 K. This change allows estimation of the energy scale for the fluctuation tail of the conduction band.

Spin-flip Raman scattering in submonolayer CdSe/ZnSe structures

Abstract We have used spin-flip Raman scattering in magnetic fields up to 14T to investigate g -factors and exchange constants of excitons related to a submonolayer insertion of CdSe in a ZnSe matrix. Sharp spectral signatures are obtained for excitation within the inhomogeneously broadened luminescence. The observed g factors are close to those of bulk ZnSe, a fact which is of particular relevance when considering the different models for the nature of excitons near submonolayer insertions and their microscopic structure (islands, homogeneous alloys, self-organized quantum dots). In measurements for different magnetic field directions we identify a “dark” (dipole-forbidden) exciton peak. Its exchange splitting from the dipole-allowed exciton increases linearly with the localization energy of the CdSe-related excitons.