Morphology of CdSe/ZnSe quantum dots grown by MBE

Abstract

The features of CdSe/ZnSe quantum dots (QDs) grown by molecular beam epitaxy at 280 °C and 240 °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 °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 ∼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. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)