Mid-infrared emission from direct Ge0.922Sn0.078 alloy in strained Ge/Ge0.922Sn0.078/Ge LEDs

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_0.922Sn_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_0.922Sn_0.078 is direct gap.