SiGe nanostructures inducing tensile strain in suspended germanium membranes.

Integration of photonics and electronics still lacks a light source that is monolithically integrable into existing CMOS technology. Germanium is investigated as a promising candidate for the active gain medium for a laser source, since it is CMOS compatible and because of its quasi-direct band-structure. Tensile strain in Ge decreases the energy barrier between the indirect and direct conduction band valleys to facilitate carrier injection into the direct gap in Γ. We propose a top-down method to strain Ge on Si layers by nano-structured SiGe layers. The method consists of the coherent growth of SiGe on Ge. The tensile SiGe top-layer can relax when trenches are carved, thereby pulling apart the Ge in between. FEM strain simulations show that the effect of SiGe nanostructures can be stronger if their perimetral forces are exerted on a Ge membrane rather than on bulk. The enhancement of elastic energy transfer from nanostructure to the membranes can result in the fabrication of larger strained regions of Ge. We present our first µRaman results for SiGe stressors on 100 nm thick Ge suspended membranes. While the strain is still below the induction of a direct-gap transition, we can show that SiGe nanostructures fabricated on a membrane are about twice as effective in creating strain as compared to the same nanostructures created on bulk Ge.