Materials physics of GeSn-based semiconductor lasers

Optically pumped Ge1-ySny lasers operating at low temperature have been recently demonstrated. This article reviews the challenges to efficient lasing that arise from the intrinsic properties of alloys in the Si-Ge-Sn system. The first such challenge is the possibility that even in nominally direct gap Ge1-ySny materials, a significant fraction of pumped carriers reside in the valleys associated with the indirect gap, and therefore are not available for lasing. We find theoretically that this is indeed the case at room temperature, possibly explaining the strong temperature dependence of lasing. A second challenge is finding appropriate barrier materials to reduce the lasing threshold power via spatial confinement. We show that strained layer Ge1-ySny/Ge1-y'Sny' multilayers are unlikely to produce the required direct gap Type-I alignment, and that even the introduction of ternary Ge1-x-ySixSny layers may not lead to suitable lasing structures due to giant bowing parameters in the compositional dependences of their interband transition energies.