Electron microscopy investigation of the defect configuration in CdSe/ZnSe quantum dot structures

In this study we report the dependence of the defect configuration observed in CdSc/ZnSe quantum dot structures on the ZnSe cap layer thickness by transmission electron microscopy (TEM) and reflection high-energy electron diffraction (RHEED). The samples were grown by molecular-beam epitaxy at 350degreesC. The nominal thickness of the CdSe layers was approximately 3 monolayers and the cap layer thickness was varied in the range from 3 to 60 nm. In all samples, RHEED showed a transition from the two-dimensional (2D) to the three-dimensional (3D) growth mode during the CdSe deposition. By TEM we found pairs of stacking faults (SFs) lying on one of the two pairs of lattice planes given by {(111)-(111)} and {((1) over bar1 (1) over bar)-(1 (1) over bar(1) over bar)}. The SFs are bound by Shockley partial dislocations. Both of the SFs forming one pair originate from the same stair-rod dislocation with Burgers vector b = 1/6 {110} lying at the CdSe-ZnSe interface inside a Cd-rich region (island). Measuring the atomic displacements in the vicinity of the SFs, we obtained that all SI's have an intrinsic nature. At the line of contact of two SFs that belong to different pairs, we observed stair-rod dislocations with b = 1/3 {100} which are inclined to the interface. The length of all dislocation lines and the sizes of the SFs increase with increasing ZnSe cap layer thickness. We find that the 2D-3D transition observed by RHEED is most probably caused by defect formation, which is discussed with respect to the relaxation of strain in the CdSe layer.