Cross-sectional scanning-tunneling microscopy

of stacked InAs quantum dots

M. Dähne,* H. Eisele, O. Flebbe, T. Kalka, F. Heinrichsdorff, A. Krost, and D. Bimberg

Institut für Festkörperphysik, Technische Universität Berlin, Germany

Cross-sectional scanning-tunneling microscopy (XSTM) is an ideal technique for the investigation of the structural and local electronic properties of semiconductor nanostructures with atomic resolution. Recently, several XSTM investigations of self-assembled InAs quantum dots on GaAs(001) were reported. Here we present an XSTM study of three-fold stacked InAs quantum dots in a GaAs matrix, which were grown by metal-organic chemical-vapor deposition. As shown in figure 1, the dots are vertically aligned and show a layer-dependent size, which is related to preferential growth on top of the overgrown dots. The wetting layer is found to be inhomogeneous, but no In diffusion into the overgrowing GaAs layer is observed. From detailed images with atomic resolution, like the one shown in figure 2, it is concluded that the dots have a flat prismatic shape with a (001) top face as well as {110} and {111} side faces. The homogeneous appearance of the dots indicates that they consist of pure InAs. Furthermore it is found that the image contrast is mainly caused by a cleavage-induced outward relaxation of the strained quantum dots, while electronic effects play a minor role. This is demonstrated quantitatively by computer simulations of strain relaxation.

This work is supported by the Sfb 296, project A4.