Tight-binding calculation of optical gain in tensile strained [001]-Ge/SiGe quantumwells

Abstract

It is known that under a tensile strain of about 2% of the latticeconstant, the energy of the bottom conduction state of bulk Ge at theΓ point falls below the minimum at the L point, leading to a direct gapmaterial. In this paper we investigate how the same condition is realizedin tensile strained Ge quantum wells. By means of a tight-bindingsp3d5s* model, we studytensile strained Ge/Si0.2Ge0.8 multiple quantum well (MQW) heterostructures grown on a relaxed SiGeSn alloy bufferalong the [001] direction. We focus on values of the strain fields at the crossover betweenthe indirect and direct gap regime of the MQWs, and calculate band edge alignments,electronic band structures, and density of states. We also provide a numericalevaluation of the MQW material gain spectra for TE and TM polarization underrealistic carrier injection levels, taking into account the leakages related to theoccupation of the electronic states at the L point. The analysis of the differentorbital contributions to the near-gap states of the complete structure allows usto give a clear interpretation of the numerical results for the strain-dependentTM/TE gain ratio. Our calculations demonstrate the effectiveness of the structures underconsideration for light amplification.