Three-dimensional measurement of composition changes in InAs/GaAs quantum dots

We present a transmission electron microscope (TEM) analysis of InAs/GaAs quantum dots (QDs) which have been subject to thermal annealing. Annealing produces a significant shift in infrared detector response to longer wavelengths, indicating changes in QD size, composition, or both. A three-dimensional reconstruction of an 'average' QD is obtained by combining compositionally-sensitive dark field 002 TEM images of many QDs, followed by fitting to a structural model assuming cylindrical symmetry. Errors in compositional measurements are estimated, and the validity of the model is assessed by TEM image simulations using 2-beam dynamical electron diffraction theory. As-grown material exhibits an increasing concentration of In towards the top of the QDs, whereas annealed material shows diffusion of indium from the upper part of the QD towards the sides and base. There is no measurable change in the position of the outer interface of the QD.