Center for Functional Nanomaterials Seminar

The integration of Ge in Si technology provides a great potential for future applications due to the higher charge carrier mobility and smaller band gap of Ge. Even the drawback of the indirect band gap in Si and Ge can be overcome in Ge nanostructures [Liu et al., Opt. Ex. 15, 11272 (2007)]. It has, e.g., recently been shown that it is possible to fabricate an efficient Ge laser on Si working at room temperature by proper band gap engineering [Liu et al., Opt. Let. 35, 679 (2010)]. One approach to overcome the problems caused by the lattice mismatch of 4.2% is the surfactant-mediated epitaxy (SME) of Ge structures on Si. Due to the modulation of surface properties, suppressed or enhanced 3D growth as well as less defects in the Ge structures and a reduced intermixing between Ge and the Si substrate can be observed [J. Falta et al., Phys. Rev. B 51, 7598 (1995)].
Hereby, I will show that Ag is an extremely versatile tool for these purposes. Depending on the substrate orientation and the Ag-induced reconstruction and morphology, either a suppressed or an enhanced 3D growth of the Ge structures can be generated. On Si(111), huge flat islands are observed with in-situ LEEM. For a growth temperature of about 500°C, the LEEM data indicate that these islands coalesce and form a continuous film at a Ge coverage of about 20 nm. XPEEM experiments give evidence for the suppression of intermixing at 500°C. The formation of a periodic misfit dislocation network and a twin formation are observed, using GIXRD and TEM.
A completely different behavior is found for the Si(112) and Si(113) surfaces. In these cases, Ag induces the formation of a regular pattern of nanofacets along the [1-10] direction. Subsequently grown Ge forms small anisotropic nanoislands with a wire-like geometry at an aspect ratio of about 4:1. These nanowires are aligned along the Ag-induced facets.