Character of the Cd distribution in ultrathin CdSe layers in a ZnSe matrix

The character of Cd distribution and the morphology of CdSe layers with nominal thicknesses between 0.7 and 3.6 ML in a ZnSe matrix were studied by conventional transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) using plan-view and cross-section TEM samples. The Cd distribution was determined on an atomic scale by two different techniques. The first method is based on the measurement of local lattice parameters from zone-axis HRTEM images. The second technique relies on the evaluation of Fourier amplitudes derived from off-axis lattice fringe images. Continuous ${\mathrm{Cd}}_x{\mathrm{Zn}}_{1-x}\mathrm{Se}$ layers are observed, which are significantly broadened compared to the nominal thicknesses. The layers contain Cd-rich inclusions (small islands) with a size of less than 10 nm, and regions with a lower Cd concentration. With increasing nominal CdSe thickness, the Cd concentration and the island density increase. In addition, larger islands with a density two orders of magnitude below the small island density and a shape asymmetry are found in the 3.6-ML CdSe layer. The results are discussed with respect to the impact of the averaging effect caused by the finite TEM foil thickness on the measured Cd content in the Cd-rich islands and the surrounding region.