GeSn optical gain and lasing characteristics modelling

Abstract

With recent demonstrations of lasing, germanium-tin (GeSn) stands out as a promising candidate for the integration of a low threshold, room temperature monolithic laser source in silicon (Si) photonics. The impact of physical properties, such as energy band structure, crystal quality, and cavity loss on lasing performances (i.e., lasing threshold and maximal lasing temperature) has to be better understood, however. In this work, we calculate the theoretical band-to-band net gain for relaxed, [100] uniaxial, and (100) biaxial tensile-strained GeSn. We show that the band-to-band net gain depends not only on the interband gain introduced by the transition between valence bands and conduction bands, but also on the intervalence band absorption between the valence bands. Both approaches, GeSn with high Sn concentration and/or with high tensile strain, can yield a band-to-band net gain at room temperature. Then, the integration of another source of absorption---free carrier absorption---will be discussed. Finally, from the simulation of GeSn lasing characteristics, we show the important role the crystal quality has on the lasing threshold. Based on the results, we suggest using GeSn with low/medium Sn concentrations, under very high [100] uniaxial or (100) biaxial tensile strain, to obtain a low threshold, room temperature lasers.