Based on some papers, Germanium can be converted to direct gap by applying tensile, it is because Gamma conduction energy can be lowered faster than its L conduction energy. Why gamma energy can be lowered faster?
The simple answer would be that the deformation potential of the Gamma point (around 10 eV) is about twice of that for the L point. This is generally true for nearly all group IV, III-V, and II-VI semiconductors.
How about the Sn alloy effect? I also found that by mixing with Sn, the gamma point energy can be lowered more and produce direct gap. Can we explain it with tensile effect?
Usually strain is a small effect in alloying. The more significant effect is the chemical difference between the atoms. I am sure someone has done the electronic structure calculation for GeSn, but I don't have personal experience with this alloy system.
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