Growth and spectroscopy of II-VI CdSe quantum dots

In this paper we review the recent progress in the growth and spectroscopy of CdSe quantum dots. In particular, atomic layer epitaxy (ALE) has been used to grow ZnSe/CdSe and ZnSe/CdSe:Mn magnetic quantum dots. For samples grown without a ZnSe capping layer, dot densities of the order of 10⁹ cm^-2 were measured by atomic force microscopy (AFM). In the capped samples, the ensemble dot photoluminescence (PL) was observed over a range of energies between 2.1 and 2.5 eV and in the high Mn concentration samples a spectrally broad emission at 2.15 eV from the internal Mn²⁺ transition. Single dot spectroscopy was carried out by confocal microscopy and the PL linewidth was measured as a function of Mn concentration. Also, CdSe/MgS quantum dots have been grown successfully by molecular beam epitaxy using a thermally activated reorganization process that occurs during growth interruption. Unlike the ZnSe/CdSe dots the PL measurements show emission from both QDs and the wetting layer, with emission energies ranging between (2.3 and 3.8 eV). AFM topography and μm-PL measurements also show evidence of quantum dot structures and power dependent PL measurements carried out on the dots give a value of 30 meV for the bi-exciton binding energy at 77 K.