Modeling optical absorption in GeSn alloys for photonic device applications

Germanium tin alloys are an attractive material system because of their compatibility with silicon and their optical properties in the infrared range. These properties give GeSn alloys potential to be used for low-cost, high-efficiency photonic devices like photodetectors, LEDS, and infrared lasers that can be integrated with silicon electronics. The optical properties of GeSn, namely the absorption coefficient and spontaneous emission levels, are dependent on the material’s composition, strain, and the density of unoccupied states in the conduction band. Pure germanium is an indirect band gap material but can transition to a direct gap material with high levels of strain. Sn, a direct gap material, is introduced into the lattice allowing a transition to a direct gap material. A direct gap material is favorable because emission is possible without phonon assistance, which is required in indirect gap materials, thereby increasing optical absorption and emission. We use modeling to predict the absorption coefficient of GeSn alloys near the direct band gap with various amounts of Sn and strain. The model includes the effects of strain on the band structure as well as the occupation of the conduction band states, which can dramatically affect the absorption and emission properties.