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

A transmission electron microscopy (TEM) study of the structure and chemical composition of 0.5 to 3.0 ML (monolayer) CdSe sheets that are buried in a ZnSe matrix is presented. The CdSe layers were grown by migration-enhanced epitaxy at a growth temperature of 280 °C. We find two-dimensional (2D) ${\mathrm{Cd}}_x{\mathrm{Zn}}_{1-x}\mathrm{Se}$ layers with a total thickness of approximately 3 nm for all samples independent of the nominal CdSe content that contain inclusions (islands) with an enlarged Cd concentration. Plan-view TEM revealed two types of islands: First, small 2D islands with a lateral size of less than 10 nm, 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 evaluation by lattice fringe analysis (CELFA) procedure allow the precise measurement of the Cd-concentration profiles of the ${\mathrm{Cd}}_x{\mathrm{Zn}}_{1-x}\mathrm{Se}$ layers. The CELFA evaluation yields a full width at half maximum value of $(10\mathrm{}\pm 1)\mathrm{ML}.$ The most probable origin of the broadening is a strong interdiffusion of Cd and Zn with an additional contribution of the segregation of the Cd atoms. The diffusion length of the Cd diffusion in ZnSe during the growth of the ZnSe cap layer is $L_D=(3.6\mathrm{}\pm 0.8)\mathrm{ML}$ and the segregation probability is estimated to be $R=(0.6\mathrm{}\pm 0.2).$ It is shown that neither objective lens aberrations nor specimen tilt are the main sources for the observed enormous broadening of the CdSe interlayers.