Mid-infrared emission from direct Ge<inf>0.922</inf>Sn<inf>0.078</inf> alloy in strained Ge/Ge<inf>0.922</inf>Sn<inf>0.078</inf>/Ge LEDs

Abstract

Density functional theory with SQS-32 (Special Quasi-random Structures with 32-atom super-cell) configuration is applied to study the energy-gap properties of strained GeSn alloys. The calculation results show that the direct-indirect crossover occurs at the mole fraction of ~ 6%. The energies of direct and indirect gaps at x ~ 0.075 are well agreement to recent experiments. The calculated emitting powers for these gaps reveal that the lowest peak energy in electroluminescence experiment of strained Ge/Ge 0.922 Sn 0.078 /Ge LED mainly originates in the direct gap rather than indirect. It is in contrast to the recent experimental report. Our results show that the strained Ge 0.922 Sn 0.078 is direct gap.