Analysis of Quantum Dot Formation and Exciton Localisation in the (Zn, Cd) (S, Se) System

Abstract

The formation process and exciton localisation properties of CdSe/ZnSe quantum dot structures were studied. Pseudomorphically strained structures were exposed to a stationary sulphur vapour after the formation process of three-dimensional (3D) islands. Temperature dependent photoluminescence investigations confirm the increasing 2D-like character with increasing sulphur exposure times. The sulphur exposure had no impact on the island formation in partially relaxed structures.Introduction The growth of self-assembled quantum dots (QDs) using molecular beam epitaxy (MBE) or metal-organic chemical vapour deposition (MOCVD) is of considerable interest as it offers the possibility for low-threshold-current opto-electronic devices for the green spectral range. Furthermore, such structures have the potential to study basic phenomena of quantum information processing. However, reliable data necessary for a controlled formation of well ordered arrays of CdSe QDs are difficult to obtain and not fully available presently.As reported earlier [1], two distinctly different types of CdSe islands are formed in succession during the deposition of about 3 monolayers (ML) CdSe on ZnSe (001). From kinetic analysis we have concluded that above a critical threshold of about 2.1 ML the islands are formed by the Stranski-Krastanov (SK) growth process, which is thermodynamically controlled. To proof that, in this paper, we demonstrate for the first time the reversible nature of this particular growth mode transition. A similar approach has been reported earlier for the behaviour of InAs islands on GaAs [2]. The presence of deep localisation sites as provided by e.g. SK grown islands lead to a pronounced redshift of the emission wavelength with increasing temperature. As it was shown by temperature dependent and time delayed photoluminescence (PL) spectroscopy [3,4], this redshift is caused by a lateral redistribution of excitons to deeper localisation sites.

Relaxation dynamics of bimodally distributed CdSe quantum dots

“…21 and 45-48͒ only a few investigations have been performed in bimodal QD distributions 28,31,[49][50][51][52] or quasibimodal QD distribution without a transfer from high to low energy states. 18,53 For simplicity, in what follows, the high energy and the low energy emissions are referred to as high energy dots ͑QDs1͒ and low energy dots ͑QDs2͒, respectively.…”