High spatial resolution information on the structure and composition of semiconductor quantum dots is necessary to relate microstructure to macroscopic electron-optical properties [1]. Scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy (EELS) can be used to determine the elemental composition of nanometer-sized particles. Applying these techniques to quantum dots is challenging because the dot nucleates on a very thin wetting layer of similar composition and is embedded in a matrix. Here we present a strategy to extract absolute compositional information on InGaAs dots. The method relies on modeling both the dot shape and the electron probe profile.

Samples were prepared by depositing four monolayers of In0.5Ga0.5As onto a GaAs substrate giving a nominal wetting layer thickness of 1.2 nm [2]. STEM was performed on a Vacuum Generator's HB501 equipped with a GATAN parallel electron energy-loss spectrometer. An ES Vision system was used to acquire spatially resolved electron energy-loss spectra from the wetting layer and the quantum dots.