From pseudo-direct hexagonal germanium to direct silicon-germanium alloys

We present ab initio calculations of the electronic and optical properties hexagonal ${\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ alloys in lonsdaleite structure. Lattice constants band structures excellent agreement with experiment are obtained using density-functional theory. Hexagonal Si has an indirect gap, whereas Ge a pseudo-direct i.e., transitions at minimum direct gap very weak. The character pure is efficiently lifted by alloying. Already for small admixture Si, symmetry reduction enhances oscillator strength lowest transitions. content below 45 %. validate group-IV to be efficient emitters, suitable integrated optoelectronic applications.