Light emission from Si would revolutionize semiconductor industry since it allows integration of electronic and optical functionality in one main technology platform. This has, however, been impossible due to the indirect band gap of Si. In this talk I will discuss 2 different approaches, using unique properties of nanowires, to realize light emission from Si-based compounds.

In the first route, we focus on the fabrication of defect-free GeSn compounds. GeSn has been shown to exhibit a direct band gap at Sn concentrations above 12.5% in the infrared part of the spectrum (around 0.5 eV). In the nanowire geometry, the GeSn/Ge lattice strain can be effectively relieved in the radial direction, which is exploited to grow Ge/GeSn core shell nanowires with high (13%) Sn content. The core/shell nanowires are free of dislocations and therefore show a very high photoluminescene internal quantum yield at room temperature. In this talk the growth mechanism, the structural properties and the temperature dependent optical properties are studied.

In the second route, we concentrate on Si and Ge with the hexagonal (2H) crystal structure, which have been predicted to have a direct band gap with an energy close to the telecom wavelength. Here, we employ crystal structure transfer, in which we use wurtzite GaP as a template to epitaxially grow SiGe compounds with the hexagonal crystal structure. We show that with this method we can grow defect free hexagonal SiGe shells and branches with tunable Ge concentration. The structural and optical properties of these new crystal phases will be discussed. We show room temperature photoluminescence from hexagonal SiGe.