Dark field transmission electron microscope images of III–V quantum dot structures

Multi-layer quantum dot structures are becoming increasingly common in order to improve the efficiency of quantum dot lasers. Each layer of dots may be influenced by the preceding dot layer, and the dot density can vary from layer to layer. Characterization of such structures relies on the reliable determination of the shape, size and density of dots in each layer. Dark field transmission electron microscopy (TEM) images using the 002 diffraction condition are frequently used, viewing the layers edge-on in a cross-section sample. A simple model is used to describe the contrast as a function of dot size and shape, specimen thickness, and the composition of the dot and surrounding materials. Good agreement with experimental results is obtained. It is found that the dot size is not accurately related to the bright region seen in such images. 002 images can be used to determine the size and shape of dots, a density per unit area cannot be calculated in the cross section geometry without either measuring—or assuming—the specimen thickness. In multilayer structures, plan-view TEM images show the layers as overlapping, losing the information from individual layers. By tilting a cross-section specimen to allow imaging with the dark field 113 diffraction condition, the density in individual layers can be measured. Additional information, such as wetting layer thickness variations and alignment of dots due to surface roughness or substrate offcut, can also be obtained.