Localized Excitons in CdS_1−xSe_x Solid Solutions

Abstract

The emission spectra of CdSl -%SeZ solid solutions (z 5 0.3) a t helium temperatures are studied under monochromatic excitation in different regions of the absorption spectrum. I n 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. I n the intermediate region I1 a limited energy transfer between the localized states takes place via phonon assisted tunneling.Die Emissionsspektren von CdSl -.Se,-Mischkristallen (z 5 0,3) werden bei Heliumtemperaturen unter monochromatischer Anregung in verschiedenen Bereichen des Absorptionsspektrums untersucht. I n allen Proben wird sehr intensive Lumineszenz von Exzitonen beobachtet, die an Zusammensetzungsfluktuationen lokalisiert sind. Die hohe Intensitat der Lumineszenz und das vollstandige Fehlen der Emission freier Exzitonen weist auf eine hohe Wahrscheinlichkeit von Exzitonlokalisierung hin. Ein Model1 fur die Exzitonlokalisierung wird vorgeschlagen. Es wird angenommen, daO 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. I m dritten Spektralbereich existiert keine Energieubertragung und die gesamte Emission wird durch direkt photo-angeregte Zustande verursacht. I m Zwischenbereich I1 findet ein begrenzter Energieubergang zwischen den lokalisierten Zustanden uber phononassistierte Tunnelung statt. 2,We are grateful to Dr. V. Lysenko from the Institute for Solid State Physics, Academy of of Sciences of the USSR for the help with these measurements.

Exciton Transfer between Localized States in CdS1–xSex Alloys

“…This broadened exciton-phonon coupling was also used by Permogorov et al t o fit time integrated luminescence spectra [3]. We shall comment on this in the next section.…”

“…3 b and 4b). I n [2] and [3] only piezoelectric coupling is taken into account and TIL spectra are fitted in the energy region where the energy dependent transfer time becomes negligible compared to the radiative lifetime. One can then easily notice that the characteristic energy E, describing the density of localized states has no effect as soon as it becomes larger than the characteristic energy for piezoelectric coupling.…”

“…Among 11-VI semiconductor compounds, CdS1-,Sex has proved to be a good model system for the study of exciton localization due to alloy potential fluctuations [I]. Time integrated luminescence (TIL) was thoroughly investigated by Cohen and Sturge [2] and Permogorov et al [3]. Experimental time resolved luminescence (TRL) results were reported by several groups including ourselves in the nanosecond and picosecond time range [4 to 81.…”

Time‐Resolved Luminescence of Localized Excitons in CdS1—xSex under High Excitation

“…The localized states lie in an exponential tail of the density of states. This effect has been theoretically predicted [l, 21 and experimentally proved [3,4]. The luminescence spectra of mixed crystals are determined by localized excitons, leading to the typical broadening of excitonic lines in these materials.…”

Recombination radiation and electroreflection of excitons in ZnxCd1−xSe solid solutions

“…The low-temperature SR spectra of pseudobinary CdS,Sel-, compounds [6,7] a t bandgap or resonant excitation yield information on the distribution function of localized excitonic states, the effective mobility edge, the resonant and phonon-assisted tunnelling transfer of excitons between different states, etc. According to [5J, a small rise in temperature leads to the exciton delocalization and the appearance of the free-exciton line in the SR spectra of Zn,Cdl -,S crystals.…”