Nano-scale chemistry of self-assembled nanostructures in epitaxial SiGe growth

Epitaxial growth of SiGe alloy films under certain kinetically limiting conditions has previously been shown to lead to the self-assembly of nanostructures called Quantum Dot Molecules, QDMs. These QDMs consist of an assembly of pyramidal pits and elongated pyramidal islands, which exist at the edges of the pits. We investigate the nano-scale chemistry of QDMs in Si _0.7Ge_0.3/Si (100) using Auger electron spectroscopy (AES). First, our AES analysis indicates that compressively strained QDM pit bases are most Ge rich regions in QDMs, consistent with previously reported observations. Second, our Auger analysis of the QDMs shows that Ge composition continuously increases from outside edges of the islands to the pit cusps. This results in asymmetric distribution of Ge about the apex of the islands. The segregation of Ge to the pit cusps is unexpected on the basis of strain energy minimization, and we propose that it is due to attachment of Ge to steps on the interiors of the QDMs. The continually varying Ge composition towards the pit cusps is also inconsistent with previous observations of islands in GeSi growth, where the composition is reported to be symmetric about the apex of islands.