Morphology of CdSe/ZnSe quantum dots grown by MBE

The features of CdSe/ZnSe quantum dots (QDs) grown by molecular beam epitaxy at 280 degrees C and 240 degrees C are studied by transmission electron microscopy (TEM), atomic force microscopy (AFM) and photoluminescence (PL). The epitaxial structures consist of a single layer of CdSe islands sandwiched between two 40 nm thick ZnSe layers grown on a [100]-oriented GaAs substrate. UHV-AFM shows that islands are formed as a result of strain relaxation in 3 ML of CdSe pseudomorphically deposited on ZnSe. After encapsulation at 240 degrees C, there is no change in the QDs' morphology. On the other hand, at higher temperature, interdiffusion and segregation phenomena take place. Flat islands originating from a wetting layer and pointing to the substrate are observed, with a Cd concentration much lower in their center than at the outskirts. Both size and composition are consistent with the similar to 2.35 eV PL energy that we measured. Moreover the unexpected QD morphology with Cd-rich outskirts can explain why the PL decay time depends on the samples because the confinement of holes in the QD edges makes the electron-hole overlap very sensitive to QD size.