A modeling and convolution method to measure compositional variations in strained alloy quantum dots

We have developed a method to quantitatively measure the absolute composition of nanometer sized capped quantum dots in semiconductor alloys. The method uses spatially resolved electron energy-loss spectroscopy in a scanning transmission electron microscope to measure compositional profiles across the center of the quantum dot and the adjacent nanometer wide wetting layer. The measurements from the wetting layer are used to derive a spatial broadening function which includes the effects of probe size, instabilities and beam spreading in the sample. This broadening function is employed to simulate compositional profiles from the quantum dots. Information on the dimensions of dots is extracted from annular dark-field images. The method is applied to In_yGa_1-yAs (y=0.5) quantum dots grown on a GaAs substrate. In this system, a simple truncated cone model is found to give an adequate description of the compositional variations across the dot. We find a substantial enrichment in In at the center of the dots, in agreement with theoretical predictions.