Picosecond time resolved luminescence is investigated of excitons localized by alloy potential fluctuations in CdS1–xSex. Time resolved luminescence spectra, as well as time decay curves and time integrated luminescence spectra of a CdS0.36Se0.64 sample are fitted using a model in which transfer to lower energy states occurs through the tunnel effect assisted by acoustic phonons. The density of states is taken to be exponential with a characteristic energy, E0, equal to 2.2 meV. The radiative lifetime is 1.5 ns. The nonradiative lifetime due to transfer is inversely proportional to the number of available states at lower energy. Close agreement with the experimental results can also be found with a phenomenological model.



Nous etudions la luminescence resolue en temps due aux excitons localises par les fluctuations de potential d'alliage dans CdS1–xSex. Les spectres de luminescence resolue en temps, les courbes de decroissance temporelle ainsi que les spectres de luminescence integree en temps d'un echantillon de composition CdS0,36Se0,64 sont ajustes en utilisant le modele de transfert par effet tunnel assiste par phonon acoustique. La densite d'etats decroit exponentiellement avec une energie caracteristique E0 = 2,2 meV. Le temps de vie radiatif est egal a 1,5 ns. Le temps de vie non-radiatif dǔ au transfert est inversement proportionnel au nombre d'etats disponsibles a plus basse energie. Un modele phenomenologique donne egalement un bon accord avec les resultats experimentaux.

Exciton Transfer between Localized States in CdS1–xSex Alloys

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

The emission spectra of CdS1–xSex solid solutions (x ≦ 0.3) at helium temperatures are studied under monochromatic excitation in different regions of the absorption spectrum. In all samples very intense luminescence due to excitons localized by composition fluctuations is observed. High intensity of luminescence and complete absence of free exciton emission indicate a high probability of exciton localization. A model for exciton localization is proposed. It is assumed that only the hole component of the exciton is localized by the potential fluctuations. The electron is bound to this hole by Coulomb attraction. The energy migration between the localized states is discussed. Three different regions can be distinguished in the excitation spectrum of localized excitons. The first region corresponds to the excitation of free excitons which are trapped to localization sites in the process of energy migration. There is no energy migration in the third spectral region and all the emission is caused by directly photo-excited states. In the intermediate region II a limited energy transfer between the localized states takes place via phonon assisted tunneling.



Die Emissionsspektren von CdS1–xSex-Mischkristallen (x ≦ 0,3) werden bei Heliumtemperaturen unter monochromatischer Anregung in verschiedenen Bereichen des Absorptionsspektrums unter- sucht. In allen Proben wird sehr intensive Lumineszenz von Exzitonen beobachtet, die an Zusam- mensetzungsfluktuationen lokalisiert sind. Die hohe Intensitat der Lumineszenz und das vollstandige Fehlen der Emission freier Exzitonen weist auf eine hohe Wahrscheinlichkeit von Exzitonlokalisierung hin. Ein Modell fur die Exzitonlokalisierung wird vorgeschlagen. Es wird angenommen, ds nur der Lochanteil des Exzitons durch die Potentialfluktuationen lokalisiert ist. Das Elektron ist an diesem Loch durch Coulombanziehung gebunden. Die Energieubertragung zwischen den lokalisierten Zustanden wird diskutiert. Drei unterschiedliche Bereiche lassen sich im Anregungsspektrum der lokalisierten Exzitonen unterscheiden. Der erste Bereich entspricht der Anregung freier Exzitonen, die an Lokalisierungsplatzen wahrend der Energieubertragung getrapt werden. Im dritten Spektralbereich existiert keine Energieubertragung und die gesamte Emission wird durch direkt photo-angeregte Zustande verursacht. Im Zwischenbereich II findet ein begrenzter Energieubergang zwischen den lokalisierten Zustanden uber phononassistierte Tunnelung statt.

Localized Excitons in CdS1−xSex Solid Solutions

The origin of optical nonlinearities of glasses doped with CdSxSe1−x microcrystallites is studied both experimentally and theoretically. At room temperature a large shift (≅50 meV) of the absorption spectrum to higher energies is observed in a CdSχSe1−x glass with increased light intensity. This blue shift is analyzed by using a semiconductor plasma theory. Experimental and theoretical results suggest that band filling is the dominant mechanism for the observed nonlinearity. An independent measurement for the intensity-dependent dispersive changes is presented.

Optical nonlinearities of glasses doped with semiconductor microcrystallites.

Systematic investigations of the photoluminescence spectra of Cd1-xMnxTe mixed crystals were performed for 0<or=x<or=0.4 as a function of temperature and magnetic field. Two luminescence lines are distinguished in the excitonic region. The first line is caused by recombination of excitons bound to neutral acceptors. A detailed theoretical interpretation of this line is given in terms of the bound magnetic polaron. For the second line a structure on the curve of temperature dependence is observed in the vicinity of the spin-glass phase transition. That line is attributed to a magnetic localisation of excitons.

S. Verbin

Papers

Localized Excitons in CdS1−xSex Solid Solutions

Emission of Localized Excitons in Mixed CdS1−xSex Crystals