Direct Band Gap Germanium

We show that direct band gap germanium can be obtained with external silicon nitride stressor layers. The cross-over from indirect to direct band gap is found to occur for an equivalent 1.67% biaxial tensile strain. Both whispering gallery modes and quasi radial modes are observed with tensile-strained Ge microdisks. Quality factors up to 7100 have been measured around 2 p.m wavelength. We demonstrate that circular Bragg reflectors can significantly enhance the quality factors of quasi-radial modes in strained germanium microdisks.