  We present germanium microdisk optical resonators under a large biaxial
tensile strain using a CMOS-compatible fabrication process. Biaxial tensile
strain of ~0.7% is achieved by means of a stress concentration technique that
allows the strain level to be customized by carefully selecting certain
lithographic dimensions. The partial strain relaxation at the edges of a
patterned germanium microdisk is compensated by depositing compressively
stressed silicon nitride layer. Two-dimensional Raman spectroscopy measurements
along with finite-element method simulations confirm a relatively homogeneous
strain distribution within the final microdisk structure. Photoluminescence
results show clear optical resonances due to whispering gallery modes which are
in good agreement with finite-difference time-domain optical simulations. Our
bandgap-customizable microdisks present a new route towards an efficient
germanium light source for on-chip optical interconnects.
