Interpretation of phase and strain contrast of TEM images of InxGa1-xAs/GaAs quantum dots

Transmission electron microscopy (TEM) observations were performed on capped single and vertically stacked In0.5Ga0.5As/GaAs quantum dots. Cross-sectional images were obtained both in the [001] and [011] zone axes. In the [011] zone axis the dots exhibit a lens shape, whereas in the [001] zone axis their shape is more likely to be a truncated pyramid or a truncated cone. We demonstrate that, due to the chemical sensitivity of the [001] zone axis, it is possible to distinguish, from the phase contrast features of high-resolution TEM (HRTEM) images, the regions where In is located and consequently get more reliable information about the dot shape. By performing HRTEM simulations, we discuss the experimental conditions under which the compositional signal is observable. [100] plan-view samples were investigated by conventional TEM in the multibeam zone axis condition. The contrast features of the images were correlated to the strain fields in the three-dimensional islands. We show that the different diffraction contrast observed in our samples is due to coherent superposition of the strain field of dots having different sizes along the stack.