Temperature-dependent photoluminescence of vertically stacked self-assembled CdSe quantum dots in ZnSe
Introduction
Self-assembled quantum dots (QD) formation is now well established in groups-IV and III–V semiconductor families both by molecular beam epitaxy (MBE) and in metal-organic chemical vapor deposition (MOCVD). In recent years there has been intense interest in the fabrication of II–VI QDs by the process of self-assembly [1], [2], [3], [4]. However, in contrast to the extensive studies of vertical correlations between successive dot layers carried on III–V QDs (e.g. Goldstein et al. [5], Xie et al. [6] and Sugiyama et al. [7] for MBE growth, and Heinrichsdorff et al. [8] for MOVCD growth), considerably less is known about the growth and the physical properties of vertically stacked II–VI QDs. At the same time, the interest in II–VI QDs is intense, because of their device potential in the short-wavelength visible range of the electromagnetic spectrum. In order to explore how the strain produced by QDs in one layer affects the self-assembly of QDs in successive layers in II–VI materials, we have fabricated multiple layers of self-assembled CdSe dots embedded in ZnSe, and studied their optical properties.
Section snippets
Experiments
The samples used in the present study were grown by the MBE technique, using a Riber 32 R&D machine equipped with element sources. A ZnSe buffer was first grown at 300 °C on (1 0 0) GaAs substrates to a thickness of approximately 1 μm. After the growth of ZnSe buffer, the substrate temperature was increased to 350 °C for QD growth. The QDs were formed on the ZnSe surface by depositing 2.4 monolayers (ML) of CdSe at the rate of 0.17 ML/s, followed by a growth interruption for about 2 s in Se flux.
Results and discussion
Optical emission spectra obtained at 1.5 K under 10 mW/cm2 excitation power from all our samples are shown in Fig. 1. The PL line emitted by the single-layer QD reference sample (marked QD) is centered around 2.24 eV, which is a typical result for CdSe islands embedded in a ZnSe matrix [1], [9]. The vertically stacked QD structures show a well-defined trend in the observed PL emission. As the spacers are introduced, the PL line is first seen to move toward higher energy, the blue shift for 30 ML
Acknowledgements
This work was supported by Korea Research Foundation Grant (KRF-2004-015-C00155), by NSF Grants DMR02-45227, and by KOSEF through QSRC at Dongguk University.
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