Strain engineering of Ge/GeSn photonic structures

Silicon compatible light sources have been referred to as the \holy grail" for Si photonics. Such devices would give the potential for a range of applications; from optical interconnects on integrated circuits, to cheap optical gas sensing and spectroscopic devices on a Si platform. Whilst numerous heterogeneous integration schemes for integrating III-V lasers with Si wafers are being pursued, it would be far easier and cheaper to use the epitaxial tools already in complementary-metal-oxide-semiconductor (CMOS) lines, where Ge and SiGe chemical vapour deposition is used in a number of advanced technology nodes. Germanium is an efficient absorber, but a poor emitter due to a band-structure which is narrowly indirect, but by only 140 meV. Through the application of strain, or by alloying with Sn, the Ge bandstructure can be engineered to become direct bandgap, making it an effcient light emitter. In this work, silicon nitride stressor technologies, and CMOS compatible processes are used to produce levels of tensile strain in Ge optical micro-cavities where a transition to direct bandgap is predicted. The strain distribution, and the optical emission of a range of Ge optical cavities are analyzed, with an emphasis on the effect of strain distribution on the material band-structure. Peak levels of strain are reported which are higher than that reported in the literature using comparable techniques. Furthermore, these techniques are applied to GeSn epi-layers and demonstrate that highly compressive GeSn alloys grown pseudomorphically on Ge virtual substrates, can be transformed to direct bandgap materials, with emission >3 m wavelength { the longest wavelength emission demonstrated from GeSn alloys. Such emission is modeled to have a good overlap with methane absorption lines, indicating that there is huge potential for the such technologies to be used for low cost, Si compatible gas sensing in the mid-infrared