Coexistence of planar and three-dimensional quantum dots in CdSe/ZnSe structures

Two well distinguishable classes of nanoscale islands were identified in CdSe/ZnSe quantum dot structures by optical spectroscopy and transmission electron microscopy. For 2.1 to 3.1 monolayer CdSe deposition, coherent three-dimensional (3D) islands, formed in the Stranski–Krastanow (SK) mode, are found with typical diameters of ∼16 nm and a coverage-dependent density of up to ${\text{3×10}}^{10} {\mathrm{cm}}^{\mathrm{-2}}.$ Simultaneously, small islands with lateral extensions below 10 nm and a density of ${\text{∼5×10}}^{11} {\mathrm{cm}}^{\mathrm{-2}}$ are formed by strain-modified island growth. Whereas the 3D SK islands dominate the emission properties at room temperature, the latter smaller islands determine the optical properties at temperatures below 120 K.