SiGe nano-stressors for Ge strain-engineering

The integration of photonics with electronics still lacks a CMOS compatible light source. Germanium is an appealing material because of its quasi-direct electronic band structure. By applying tensile strain to Ge, the band structure can be changed to a direct gap structure favorable for light emission. We present a top-down approach of strain engineering to deform germanium locally by control of the perimeter forces exerted by epitaxial SiGe nanostructures. SiGe grown coherently on Ge will be under tensile strain. By patterning the SiGe layer, compressive or tensile strain can be released and transferred to the Ge layer beneath. This method offers several degrees of freedom to tune the strain inside Ge. Stripe-like SiGe stressors separated by gaps of nanometer size were grown epitaxially on bulk Ge and shaped by nanolithography. The strain distribution inside Ge is investigated with micro-Raman spectroscopy. We reach strain values up to 4% uniaxial tensile, which mark the transition of Ge from an indirect band-gap semiconductor to a direct one. In addition, in order to obtain larger active areas, we realized also suspended crystalline Ge structures to implement the external stressors on thin germanium membranes of 100 nm thickness and 10 μm lateral extension. In this case, the Ge layer has a comparable thickness as compared to the SiGe stressors. This can increase the transfer of elastic energy from the stressor to the substrate. Micro-photoluminescence measurements are used to evaluate the optical properties of the material system. These thin Ge layers show a promising light emitting quality despite their low thickness and the release process.