Direct band gap germanium in high Q-factor cavities

Germanium as well as silicon suffers from the indirect band gap character of their electronic band structures limiting their integration as efficient source in Si-photonics. This limitation can be overcomed in germanium by tensile strain engineering. Theoretical analysis reported in the literature have predicted the indirect-to-direct transition cross-over in germanium for values of biaxial tensile strain, ranging from 1.5 to 2 %. In this work, we show that direct band gap germanium can be obtained by transferring tensile strain with nitride stressor layers. We have experimentally determined that the cross-over occurs for a biaxial tensile strain of 1.67% by using temperature-dependent photoluminescence measurements. While several methods have been employed to achieve very high tensile strain to germanium, very few works have addressed their compatibility with the design of optical resonators allowing the control of the emission for lasing operation. We have fabricated highly strained microdisks with enhanced Q-factors for Fabry-Perot (FP) quasi-radial modes thanks to a specific design of Bragg reflector pattern at the periphery. We have obtained Q-factor values of 2000 for FP modes with strained Ge disk cavities opening perspectives to develop efficient sources with this material.