A. Klochikhin

Spectroscopic Evidence for the Exciton Percolation Threshold in Low‐Dimensional ZnCdSe Solutions with Nano‐Islands

We have studied photoluminescence (PL), PL excitation (PLE), and micro-PL spectra of single quantum wells (QWs) formed by CdSe insertions in ZnSe matrix with different nominal Cd thickness (1-3 monolayers (ML)). The PL spectra are considerably red-shifted with respect to the position expected for homogeneous Cd distribution over the QW and can be attributed to the luminescence of CdSe-rich islands. It has been found that PLE spectra of different points of the PL band show a characteristic divergence at excitation below some characteristic energy E ME . This energy is identified with the percolation threshold above which the exciton is able to move over the whole lateral plane of QW whereas below the E ME only a resonant excitation of island related states is possible.

Exciton states and spin relaxation in CdSe/ZnSe self-organized quantum dots

The nature and the relaxation properties of the localized electronic states responsible for the radiative recombination in the ensemble of self-organized CdSe/ZnSe quantum dots (QDs) are studied and discussed. It has been found that due to the complicated topological relief of the localizing potential in the lateral plane of the QDs, both ground and metastable states of dots contribute to the emission at helium temperatures. The metastable states are subjected to further energy relaxation, with the rate being dependent on the temperature. The temperature dependence of the relaxation rate is responsible for the anomalous temperature shift of the luminescence band maximum. By the study of polarization properties of the emission at resonant excitation by linearly or circularly polarized light we have shown that, in the samples studied, the photoluminescence (PL) of quantum dots arises due to the radiative recombination of both excitons and trions formed in the charged dots. The exciton emission comes mostly from the metastable states responsible for the high-energy part of the PL band, whereas the trion emission corresponds to the ground states of the quantum dots and forms the low-energy wing of the PL band. The main attention is given to the theoretical and experimental study of the spin-relaxation processes of the electronic states in QDs. We have demonstrated that the broadening of localized electronic levels caused by multi-phonon processes strongly influences the spin relaxation. Such broadening removes the limitations on the energy of phonons assisting the one-phonon transitions between spin sublevels. Taking into account the multi-phonon broadening of spin sublevels we were able to describe theoretically the temperature dependence of the polarization degree of PL spectra of CdSe/ZnSe quantum dots and to restore from experimental data the characteristic function, describing the interaction of localized electronic states with phonons.

OPTICAL SPECTROSCOPY OF 2D NANOISLANDS IN QUANTUM WELLS: LATERAL ISLAND PROFILES AND NATURE OF EMITTING STATES

Results of experimental studies of photoluminescence (PL) and PL excitation (PLE) spectra of MBE grown single quantum wells (QWs) formed by insertion of few CdSe monolayers in the ZnSe matrix are reviewed. PL spectra of such quantum objects originate from the luminescence of CdSe-rich nanoislands. Two types of island emitting states, namely ground and metastable ones, contribute to the low- and high-energy parts of the PL band, respectively. An interplay between these contributions is responsible for the anomalous temperature dependence of the maximum position of the PL band. The optical orientation and optical alignment experiments at resonant excitations allow to elucidate the nature of the two types of the emitting states. PLE spectra of ground and metastable states have strongly differing characters at excitation below some characteristic energy EME which is identified as the exciton percolation threshold. A theoretical model of the absorption spectra of emitting island states is presented, and practical applications of the model for the characterization of the island lateral concentration profiles are reported.

Percolation and Localization in Disordered Solid Solutions

We present an approach to quantitative description of the states split off from the electronic bands due to disorder which exists in crystalline solid solutions. The concentration of the solution component creating the localizing potential is supposed to be below the critical one for percolation over the lattice sites. In this concentration range the clusters composed by the atoms of minor solution component, which produce the localized states for carriers and/or excitons, are isolated in space. The calculation of the number of cluster states is one of well known problems of the percolation theory over the crystal sites. We describe how the results of this theory can be used for calculation of the density of localized states, which, in turn, is used for the modelling of the spectral density (or absorption coefficient) of exciton states. Then, the continuum classical percolation theory over the overlapping spheres is used for the description of the luminescence spectrum of localized excitons. The models used describe quite well the steady-state optical spectra of diluted GaS1-cIncN solid solution and also that of ZnSe1-cTec and CdS1-cSec solid solutions in all the composition range. These models can also account for the nontrivial peculiarities of the luminescence intensity decay at long delay times after the pulse excitation.

Temperature Dependence of Exciton Spin Relaxation Rates in Semiconductor Quantum Dots

We have studied temperature dependence of the signals of optical orientation of excitons at resonant excitation of the ensemble of planar self‐assembled CdSe/ZnSe quantum dots and show that spin memory in this system survives up to 100K. To describe the experimental results we apply the model that considers explicitly the effect of multi‐phonon processes on the broadening of bright‐exciton‐phonon sublevels and demonstrate that calculated temperature dependences of spin relaxation rates are in good agreement with obtained experimental data.

Characterization of the emitting states in quantum wells with planar nano‐islands by polarization spectroscopy

he temperature dependence of photoluminescence spectra of ultrathin ZnCdSe/ZnSe quantum wells with inhomogeneous Cd distribution indicates that there are two types of emitting states (metastable and ground), which contribute to the high‐energy and low energy part of the PL band, respectively. Optical orientation and optical alignment experiments at resonant excitation show that considerable part of islands contains an extra electron. As a result, the deepest states of islands are formed mostly by the ground states of trions while the metastable states in charged island correspond to the exciton states spatially isolated from localized electron.

Exciton states of nano-islands in ZnCdSe quantum wells

The photoluminescence (PL) and PL excitation (PLE) spectra of quantum wells (QWs) formed by CdSe insertions in ZnSe matrix reveal the states of heavy and light excitons localized in CdSe-rich islands, and the energy EME which is associated with the percolation threshold over the entire lateral plane of QW. The model calculations are performed which result in evaluation of the island mean size and composition of ZnCdSe solid solution within and outside of islands.