Cd Distribution and Defects in Single and Multilayer CdSe/ZnSe Quantum Dot Structures

Conventional and high-resolution transmission electron microscopy (TEM) was applied to a study of the Cd distribution and structure of defects in single and multiple CdSe layers in a ZnSe matrix. The samples containing nominal CdSe layer thicknesses tn between 1.7 and 3.5 monolayers (ML) were grown by molecular beam and atomic layer epitaxy under different conditions. In all samples ternary CdZnSe wetting layers with Cd-rich regions with sizes of less than 10 nm (small islands: SIs) and a density of ∼1011 cm—2 are observed. The Cd concentration in the wetting layer and the SIs increase with tn. In addition, regions with sizes of 20–30 nm (large islands: LIs) and Cd concentrations >40% occur in specimens with tn > 2.5 ML. In the vicinity of the LIs stacking faults are preferably generated leading to a “coffee-bean” contrast in plan-view TEM images. The multilayer structures with tn ≈ 2.5 ML display a predominant vertical correlation of the SIs at a ZnSe spacer thickness of 12 ML.