Tunable Optical Gap at a Fixed Lattice Constant in Group-IV Semiconductor Alloys

A direct absorption edge tunable between 0.8 and ~1.4 eV is demonstrated in strain-free ternary Ge_1-x-ySiₓSny alloys epitaxially grown on Ge-buffered Si. This decoupling of electronic structure and lattice parameter---unprecedented in group-IV alloys---opens up new possibilities in silicon photonics, particularly in the field of photovoltaics. The compositional dependence of the direct band gap in Ge_1-x-ySiₓSny exhibits a nonmonotonic behavior that is explained in terms of coexisting small and giant bowing parameters in the two-dimensional compositional space.