Design of an on-chip germanium cavity for room-temperature infrared lasing

Germanium (Ge) is one of the most promising material platforms to enable the realization of monolithically integrated laser on silicon because it is a group-IV material with a pseudo-direct-band structure that can be converted into direct-bandgap either through the application of tensile strain or via the tin (Sn) incorporation in Ge. The bandgap modification enhances the light emission efficiency of Ge, where lasing can also be observed if a suitable cavity preserving the strain can be realized. In fact, several different research groups have reported lasing from strained Ge and GeSn optical cavities, however they all report lasing at low temperatures and room-temperature lasing, which is the ultimate goal required for a fully integrated laser, has not been demonstrated yet. In this work, we design an on-chip germanium cavity that has all the ingredients combined to make the room-temperature lasing possible. The design includes a 4.6% uniaxially tensile strained Ge gain medium embedded in a Fabry-Perot like cavity composed of two distributed Bragg reflectors. 3-dimensional (3D) Finite Element Method (FEM) based strain simulations together with a proposed fabrication methodology provides a guideline for the realization of the structure. Furthermore, 3D Finite Difference Time Domain (FDTD) simulations demonstrate that the designed structure is suitable for the room-temperature lasing in a wavelength range of 2410-2570 nm. 3D FEM-based heat transfer simulations performed for the designed cavity verifies the eligibility of the room-temperature operation paving the way for a possible demonstration of on-chip laser that could take part in the fully integrated infrared systems for a variety of applications including biological and chemical sensing, as well as security such as alarm systems and free-space optical communications.