On the origin of the "coffee-bean" contrast in transmission electron microscopy images of CdSe/ZnSe quantum dot structures

The origin of the "coffee-bean" strain contrast is studied, that is observe d in the plan-view transmission electron microscopy (TEM) images of CdSe/Zn Se quantum dot structures. The samples were grown by two different methods: standard molecular-beam epitaxy at 350 degreesC and atomic layer epitaxy a t 230 degreesC with annealing at 340 degreesC after the CdSe deposition. Th e nominal CdSe thickness was above 3 ML. In situ reflection high energy ele ctron diffraction during the growth or during the annealing shows the trans ition from the two- (2D) into the three-dimensional (3D) surface morphology for both samples. The coffee-bean contrast is usually assigned to three-di mensional islands which are generated after the morphological 2D/3D transit ion. It is found that the coffee-bean contrast in plan-view TEM images is a lternatively associated with pairs of stacking faults on {111} lattice plan es which are inclined against each other. The stacking faults, which are bo und by Shockley partial dislocations, are preferably generated in the vicin ity of the Cd-rich regions (large islands) of the CdZnSe layer where Cd con centrations of more than 40% are found. (C) 2001 American Institute of Phys ics.