Formation and properties of self-organized II-VI quantum islands

The formation and optical properties of CdSe based, self-assembled quantum dot (QD) like nano-structures embedded in ZnSe have been studied. Self-assembling growth was achieved under both standard molecular beam epitaxy (MBE) and low temperature atomic layer epitaxy (230 degrees C) with a subsequent annealing step. While in the case of standard MBE the competition between the relaxation via misfit dislocations and the desired dot formation leads to a low reproducibility, the latter method allows a more controlled formation of the QDs, which is clearly indicated by reflection high energy electron diffraction. In particular, a capping of the structures with ZnSe usually recovers a 2D surface thus allowing a stacking of several sheets of QDs. Small dots with a lateral diameter of 5-6 nm, which corresponds to the bulk exciton Bohr radius, and a height of 5-6 hit could be obtained as confirmed by transmission electron microscopy. The optical and structural properties of the QDs were studied by means of time resolved, resonant photoluminescence and were compared with a series of quantum wells (QW). Because of the high bandgap difference of ZnSe and CdSe, deep potential fluctuations exist within the QWs. These are caused by local interdiffusion and interface roughness and can act like low dimensional traps. However, because of their nature, they are not necessarily laterally isolated and can interact via tunneling and phonon assisted hopping. This leads to a very typical red shift of the emission peak with time in time-resolved photoluminescence (PL). In the case of self-assembled QDs, the potentials defined by the QDs are spatially well separated, as the typical dot densities observed are in the mid 10(10)-10(11) cm(-2) range. The interaction between these potentials is thus strongly suppressed, which clearly shows in the temporal evolution of both maximum position and the half width of the emission peaks. For the First time we were also able to demonstrate that CdSe can be grown with a CdS compound and additional Se flux. Sulfur seems to act as a surfactant that leads to surface smoothing and a reduced inhomogeneous broadening of the PL emission. The results are quite promising as the layers grown can be thermally activated to reorganize to coherently strained islands. (C) 2000 Elsevier Science S.A. All rights reserved.