Transmission electron microscopy investigationsof the CdSe based quantum structures

Abstract

In this work, the structural morphology of the active region of the ZnSe laser diode:quaternary CdZnSSe quantum well or CdSe quantum dots embeddedin CdSe/ZnSSe supperlattices is investigated using Transmission Electron Microscopy. The conventional as well as high resolution imaging studies indicated that the degradationof the ZnSe laser diodes is connected with the formation of extended defectsin the optical active region leading to a local strain relaxation of the quantumwell. Furthermore the outdiffusion of Cd from the quantum well occurs predominantlywhere the defects are located.The chemical composition and orderingphenomena in CdSe/ZnSSe supperlattices were also investigated,employing a series of five-fold structures with different spacerlayer thickness and a ten-fold structure.The composition in the CdSe/ZnSSe superlattice was determinedto a certain extent using different techniques. Generally, the encountered difficultiesregarding the accuracy of the obtained values are correlated with the complexity of theinvestigated system and with the available experimental methods used.Regarding the alignement of the dots, experimental results support a strain drivenordering process, in which the strain fields from buried dots lead to heterogeneous nucleation conditions for the dots in the subsequently deposited layers.An increased ordering with subsequent stackingof the dot layers is was also found. An anisotropy of the lateral alignment of the CdSe dots was alsoobserved in two different <110> zone axes. The similar plan-view images shows that the preferential alignment of the dots doesnot follow low-index crystallographic directions. However, it is assumed thatthis is attributed to the anisotropic elastic strain distribution combined withsurface diffusion.