  Germanium and silicon-germanium alloys have found entry into Si technology
thanks to their compatibility with Si processing and their ability to tailor
electronic properties by strain and band-gap engineering. Germanium's potential
to extend Si functionalities, as exemplified by lasing action of strained-Ge on
Si substrates, has brought the material back to attention. Yet despite these
advances, non-radiative transitions, induced by crystal defects originating
from the Ge/Si interface, continue to be a serious bottleneck. Here we
demonstrate the drastic emission enhancement achieved via control and
mitigation over the parasitic activity of defects in micronscale Ge/Si
crystals. We unravel how defects affect interband luminescence and minimize
their influence by controlling carrier diffusion with band-gap-engineered
reflectors. We finally extended this approach designing efficient quantum well
emitters. Our results pave the way for the large-scale implementation of
advanced electronic and photonic structures unaffected by the ubiquitous
presence of defects developed at epitaxial interfaces.
