Theoretical estimates are presented in this paper for the composition-dependent direct bandgap of bulk unstrained CSiGeSn. Interpolation between the corresponding bandgaps of elemental C, Si, Ge and Sn at their Γ, L, Δ, and X conduction band minima showed that each of the binary alloys CSn, SiSn and GeSn had a range of direct-gap compositions. Those ranges were plotted on a quaternary composition chart in order to define the boundaries of CSiGeSn composition space within which the alloy is “truly direct.” With the CSiGeSn cubic lattice parameter in the range of 0.576 nm to 0.649 nm, the predicted direct gaps ranged from 1.50 eV down to −0.41 eV. The boundaries of this space shift towards lower Sn content when bowing of the direct and indirect gaps is taken into account.
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Richard A. Soref (2014) studied this question.
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