S. A. Permogorov

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.

Localisation of excitons at small Te clusters in diluted ZnSe1-xTex solid solutions

The low-temperature recombination luminescence of ZnSe1-xTex solid solutions is studied in the Te concentration range 1%<or=x<or=2%. The dependence of the spectra on Te content and on excitation photon energy allows the authors to discriminate three different emission components. They are ascribed to recombination of excitons localised at clusters very probably consisting of two and more than two Te atoms. Selective excitation reveals substantial inhomogeneous broadening of states due to fluctuations in the local environment of the clusters. The phonon side-band structure concomitantly observed gives strong support to their model in which the large-radius exciton is tightly bound through the hole. The excitation spectra of the LO phonon replica intensity and of its polarisation degree provide information on the energy distribution of the localised states and their dynamic properties. The strong modification of the localised state emission spectrum found at higher Te concentration can be qualitatively understood by the increasing cluster interaction.

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.

Resonant Brillouin scattering in biaxially strained ZnSe

Abstract Resonant Brillouin scattering experiments have been performed in the 1S exciton region of ZnSe layers grown on GaAs substrate. Information about the biaxial strain in the ZnSe epilayer, the exchange splitting δ, and the longitudinal-transverse splitting Δ LT can be obtained by comparing the experimental Stokes shifts with those calculated from the dispersion of the 1S exciton-polariton in the strained crystal. For these calculations we used a Hamiltonian which contains the kinetic energy of the exciton center-of-mass motion, the electron-hole exchange interaction and the strain-induced effects. The polariton dispersion is calculated using a multi-component oscillator model. We find a quantitative agreement between the calculated and experimental Stokes shifts for a biaxial strain of +0.03% and exchange parameters δ=0.4 meV and Δ LT =1.2 meV.

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).

Resonant brillouin scattering from polaritons in ZnSe/GaAs epitaxial layers

Abstract The first observation of resonant Brillouin scattering in an epitaxially grown semiconductor layer is reported. The material is ZnSe grown by metalorganic vapor phase epitaxy on a GaAs substrate. We have studied the spectra and kinetics of the emission occurring under resonant optical excitation in the exciton-polariton region by a tunable picosecond laser. The Brillouin scattering data are used to analyze in detail the effect of strain in the sample and determine the exchange splitting of the exciton states, which is found in quantitative agreement with the reflectivity.

Temperature dependence of photoluminescence intensity of self-assembled CdTe quantum dots in the ZnTe matrix under different excitation conditions

The temperature dependence of the integrated photoluminescence intensity of nanometer-sized ZnTe/CdTe/ZnTe quantum wells has been investigated under different excitation conditions. It has been shown that the character of thermal decay of the luminescence intensity depends on the frequency of the exciting light and, under the above-barrier excitation, strongly depends on the optical excitation power density. It has been found that an increase in the excitation intensity leads to a saturation of thermal quenching of the luminescence in the low-temperature range. The conclusion has been drawn that this behavior reflects the saturation of nonradiative recombination centers with photoexcited carriers.

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.