Stress tuning of the fundamental absorption edge of pure germanium waveguides.

One-dimensional stress dependence on the fundamental absorption edge of pure germanium (Ge) waveguide has theoretically and experimentally been studied, considering built-in two-dimensional stress-tensile Ge grown on Si. Based on the results, we have designed Ge Franz-Keldysh (FK) electroabsorption (EA) modulators to work at 1550 nm. Application of one-dimensional [110] compressive stress above -350 MPa on a pure Ge [-110] waveguide should allow 1550 nm light transmission, unless otherwise a pure Ge modulator can only operate at 1600 nm or longer due to the built-in two-dimensional tensile stress in Ge. The prediction has experimentally been verified using a SiNx stressor film. This concludes that the presented stress-tuning approach of the pure Ge waveguides should expand the operation wavelength of Ge FK-EA modulators to C band. Since stress tuning can be locally done in the back-end-of-line of complementary metal oxide semiconductor (CMOS) process, the presented stress-tuning method should enable "field-programable" control of the operation wavelengths of the monolithically integrated Ge modulators in Si photonics platform.