Using density functional theory to engineer direct gap germanium-tin alloy

Germanium is a group IV semiconductor commonly used in Short Wave Infrared (SWIR) optical devices due to its relatively small band gap of 0.66eV. Like silicon in the period above it, the conduction band minimum of germanium does not lie at the same point in k space as the valence band maximum, making it an indirect-gap material and thus reducing its absorption efficiency. Unlike silicon however, the direct-gap of germanium is only slightly larger than its indirect-gap energy, giving it the possibility of possibly transitioning to a direct-gap material with clever band structure engineering. One such method showing promise involves alloying germanium with tin in various ratios. Using density functional theory (DFT), we can calculate the effects the alloy has on the band structure for different percentages of tin and thus predict the percentage needed to transition germanium into a direct-gap material.