Structural and chemical analysis of CdSe/ZnSe nanostructures by transmission electron microscopy

A transmission electron microscopy (TEM) study of the structure acid chemic al composition of 0.5 to 3.0 ML (monolayer) CdSe sheers that are buried in a ZnSe matrix is presented. The CdSe layers were grown by migration-enhance d epitaxy at a growth temperature of 280 degrees C. We find two-dimensional (2D) CdxZn1-xSe layers with a total thickness of approximately 3 nm for al l samples independent of the nominal CdSe content that contain inclusions ( islands) with an enlarged Cd concentration. Plan-view TEM revealed two type s of islands: First, small 2D islands with a lateral size of less than 10 n m, and second, large 2D islands with a lateral size between 30 and 130 nm. The combination of two-beam dark-field imaging and the new composition eval uation by lattice fringe analysis (CELFA) procedure allow the precise measu rement of the Cd-concentration profiles of the CdxZn1-xSe layers. The CELFA evaluation yields a full width at half maximum value of (10 +/- 1) ML. The most probable origin of the broadening is a strong interdiffusion of Cd an d Zn with an additional contribution of the segregation of the Cd atoms. Th e diffusion length of the Cd diffusion in ZnSe during the growth of the ZnS e cap layer is L-D = (3.6 +/- 0.8) ML and the segregation probability is es timated to be R =(0.6 +/- 0.2). It is shown that neither objective lens abe rrations nor specimen tilt are the main sources for the observed enormous b roadening of the CdSe interlayers.