Exciton states and energy relaxation in ZnCdSe nano‐islands

Abstract

We suggest a new concept of exciton states in nano-islands based on the study of optical spectra, namely the temperature dependence of cw photoluminescence (PL) and PL excitation (PLE), as well as PL spectra and kinetics under ps pulsed excitation. The scheme of the exciton states in nano-islands is as follows: (I) deep spatially isolated ground states, (II) excited meta-stable states the energy relaxation rate of which strongly depends on the temperature, and (III) the states extending over the whole island size. We show that a redistribution of the population between meta-stable and ground states is responsible for the anomalous temperature shift of the PL band in QWs with islands. Steady-state PL and PLE spectra were excited via a monochromator with a Xe-lamp. PL spectra were excited also by the 2.807 eV and 3.814 eV He-Cd laser lines. For the study of the time-resolved PL a frequency-doubled Ti : sapphire laser at 2.9 eV photon energy with a pulse length of about 150 fs and a repetition rate of 76 MHz was used. The overall time resolution of the detection setup including a synchroscan streak camera was better than 5 ps.In this report we present results for two samples: sample 1 has been prepared by technology (i) and sample 2 -by technology (ii). The nominal thickness of CdSe deposited in ZnSe matrix is about 2 monolayers. As it has been established by transmission electron microscopy the average thickness of the CdSe sheet in these samples is 2.5-3 nm. Both samples have a bimodal distribution of lateral sizes of 2D islands (15-25 nm and less than 10 nm for the sample 1 and about 10 nm and less than 5 nm for the sample 2).

Investigation of the unusual temperature dependence of InGaN/GaN quantum well photoluminescence over a range of emission energies

“…We note that a similar 'S-shape' behaviour in II-VI ternary solid solutions has been observed in ZnSeTe [14] and ZnMgSe [15] nanostructures. In the case of ZnCdSe nanostructures it has been explained in terms of two sets of states: localized ground states and higher metastable states [8]. The latter have a temperature dependent lifetime controlled by exciton-phonon relaxation.…”

“…The PL peak emission energy shows a decrease-increase-decrease pattern and hence does not follow the usual behaviour for semiconductor materials [3]. Alternative explanations for this unusual behaviour have been given earlier [4][5][6][7][8]. The occurrence of the 'S-shape' could be explained by considering different localization centres randomly distributed and separated by potential barriers [4].…”

Temperature dependence of photoluminescence bands inZn1−xCdxSKlochikhin1, Reznitsky2, Don3 et al. 2004Phys. Rev. BSelf Cite

“…We show that the redshift of the PL maximum indicates that an appreciable part of the emitting states of excitons in nanoislands at the lowest temperatures are metastable states 46 which can relax to deeper states as the temperature increases. We consider some simple models of island ensembles which describe the appearance of the metastable states.…”

Temperature dependent excitonic relaxation in CdSe/ZnSe quantum islands: experiment and computer simulation

“…In these systems, the shift has been interpreted in terms of an inter-island relaxation [7]. Interpretation of optical spectra has, however, indicated the fractal nature of the island shape [8,9]. In the resulting potential, intra-island hopping relaxation of excitons would be possible.…”