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

Abstract

A direct absorption edge tunable between 0.8 and $\ensuremath{\sim}1.4\text{ }\text{ }\mathrm{eV}$ is demonstrated in strain-free ternary ${\mathrm{Ge}}_{1\ensuremath{-}x\ensuremath{-}y}{\mathrm{Si}}_{x}{\mathrm{Sn}}_{y}$ 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 ${\mathrm{Ge}}_{1\ensuremath{-}x\ensuremath{-}y}{\mathrm{Si}}_{x}{\mathrm{Sn}}_{y}$ exhibits a nonmonotonic behavior that is explained in terms of coexisting small and giant bowing parameters in the two-dimensional compositional space.