A. Reznitsky

Effect of disorder on the optical spectra of wide-gap II–VI semiconductor solid solutions

Abstract Experimental results on the optical properties of wide-gap II–VI semiconductor solid solutions are reviewed. The main attention is given to the effect of solution disorder on the density of states and dynamical properties of electronic excitations. The disorder of compositional, positional or structural origin causes scattering of excitons or carriers as well as exciton or carrier localization, leading to a strong modification of the density of states and dynamical properties of excitations near the band edges in solid solutions. As it is evidenced by the studies of absorption and reflection spectra the disorder at alloying produces qualitatively similar broadening of fundamental edge in solutions with substitution in anion or cation sublattices. However, the studies of recombination processes through the luminescence spectra reveal that dynamical properties of excitons are essentially different in these two classes of solutions. Whereas in solutions with anion substitution an efficient localization of excitons or holes has been observed at low temperatures the energy migration in solutions with substitution in cation sublattice is not so strongly affected by alloying. The dynamical properties and the energy spectra of localized states in II–VI solutions with anion substitution are widely discussed.

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.

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.

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.

Measurements of polarization memory of localized states in CdS1−xSex mixed crystals

We performed measurements of the degree of polarization (PD) in thick samples of CdS1−xSex. At temperatures between 2 and 30 K a partly polarized luminescence at resonant excitation of the localized excitons was observed for k ∥ c. This finding is due to the local anisotropy of the potential wells localizing the particles. The spectral shape of PD changes with increasing temperature and vanishes above 30 K. By changing the excitation intensity, we can conclude that there are only weak interaction mechanisms between the localized excitons. The PD-data are strongly related to the dephasing of these states. We compare the measurements with photon echo experiments made in this material by another group and find a very similar behaviour for the density dependence. It follows that the localization process leads to a strong decrease of the interaction between the excitons.

Exciton and pair recombination through alloy-trapped states in CdS1 − xSex and ZnSe1 − xTex solid solutions

Abstract An additional pair-recombination channel in the form of tunnel recombination between an alloy-trapped hole and a donor bound electron is proposed to explain the features of time-resolved and steady-state luminescence spectra of A2B6 anionic ternary solid solutions, which do not fit into the framework of the localized exciton recombination model.

Selective excitation of localised excitons in ZnSe1-xTex solid solutions

Employing selective monochromatic laser excitation, the photoluminescence in ZnSe1-xTe/sub /x alloys with Te concentrations 1%<or=x<or=2.3% is investigated. The structured emission band is interpreted as being due to recombination of excitons localised at different types of small Te clusters. The energy spectrum of these excitons is inhomogeneously broadened. The details of exciton-lattice interaction revealed under resonant excitation of these exciton states strongly favour the model of large-radii excitons tightly bound through the hole. For excitation above the exciton gap, resonant Raman scattering by longitudinal optical (LO) phonons is observed. The measured degree of polarisation is used to discriminate between this process and the narrow-line LO resonance luminescence found under selective excitation of the localised states. The smooth transformation from Raman scattering to resonance fluorescence indicates that, owing to strong interaction between the cluster and band states, no energy gap exists between the free and localised exciton states.

Exciton–phonon interaction and energy relaxation in ZnSe-based low-dimensional heterostructures

Using a tunable laser excitation we have studied with a high resolution the PL and PLE spectra of localized excitons in ZnCdSe/ZnSe MQWs and ZnSe superlattices (SML) with submonolayer insertions of CdSe. Detailed study of fine structure of PLE spectra and the PL spectra at resonant monochromatic excitation indicates a strong coupling of localized excitons to the phonons all over the Brillouin band, suggesting a tight localization of exciton states. We believe that such a localization is produced by composition fluctuations within QWs or SML, the role of which is strongly enhanced by the low-dimensional character of exciton states.

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.