Atomic force microscopy (AFM) is typically used to measure the quantum dot shape and density formed by lattice mismatched epitaxial growth such as InAs on GaAs. However, AFM images are distorted when two dots are situated in juxtaposition with a distance less than the AFM tip width. Scanning electron Microscope (SEM) is much better in distinguishing the dot density but not the dot height. Through these measurements of the growth of In x Ga 1)x As cap layer on InAs quantum dots, it was observed that the InGaAs layer neither covered the InAs quantum dots and wetting layer uniformly nor 100% phase separates into InAs and GaAs grown on InAs quantum dots and wetting layer, respectively.

The mechanisms responsible for the shift of the photoluminescence spectrum to longer wavelength by depositing an InCaAs cap layer on InAs/GaAs quantum dots are studied in detail. It is demonstrated that the phase separation growth of InAs and GaAs rather than the stress in the lnAs quantum dots is the main reason for the wavelength shifts. Also, AFM image of single lnAs quantum dot is observed. The reason for the differences between AFM and SEM images is discussed.

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