Imaging of three-dimensional (Si,Ge) nanostructures by off-axis electron holography.

Quantitative phase mapping in transmission electron microscopy is applied to image the three-dimensional (3D) morphology of (Si,Ge) islands grown on Si substrates. The phase shift of the transmitted electrons induced by the crystal inner potential was recorded by using off-axis electron holography. The analysis of the experimental data requires the knowledge of the mean inner potential (MIP) of the (Si,Ge) solid solution. The MIP was calculated using different models of isolated or bonded atoms, which are based on the interpolation of first principle data. The results are compared with structure modeling and related MIP calculations applying classical molecular dynamics (MD) simulations. For MD simulations bond order potentials were applied, which can take into consideration both electronic effects and elastic relaxations. The calculated mean inner potential is used to transform the phase shifts into thickness mapping for the reconstruction of the 3D island morphology. Both, phase shift due to dynamical electron diffraction and structural relaxation influence the resulting 3D reconstruction.

Electron microscope characterization of CdSe/ZnSe quantum dots based on molecular dynamics structure relaxations

Molecular dynamics simulations using empirical potentials are applied to characterize the structure, the energy relaxation and the stability of pyramidal-shaped quantum dots in the CdSe/ZnSe system. The relaxed structure models are used for a reliable interpretation of electron microscope investigations to analyze the size, the shape and the strain fields of the quantum dots. Though the elastic strains modify the electron microsope image contrast by creating virtual truncations of the pyramids or additional black-white lobes, optimum imaging conditions chosen will reveal the shape and the size of the dots.

TEM characterization of self-organized CdSe/ZnSe quantum dots.

CdSe quantum dots (QDs) grown on ZnSe were investigated by various transmission electron microscopy (TEM) techniques including diffraction contrast imaging, high-resolution and analytical transmission electron microscopy both of plan-view as well as cross-section specimens. The size of the QDs ranges from about 5-50 nm, where from the contrast features in plan-view imaging two classes can be differentiated. In the features of the smaller dots there is no inner fine structure resolvable. The larger ones exhibit contrast features of fourfold symmetry as expected for pyramid-like islands. Corresponding simulations of diffraction contrast images of truncated CdSe pyramids with the edges of the basal plane orientated parallel to <100> are in relatively good agreement with this assumption. In TEM diffraction contrast imaging of cross-section samples the locations of the quantum dots are visualized by additional dark contrast features. The QDs have a distinct larger extension in growth direction compared to the almost uniformly thick CdSe wetting layer. The presence of the CdSe QDs was also confirmed by energy-dispersive X-ray spectroscopy.