Metal-optic cavity for a high efficiency sub-fF germanium photodiode on a silicon waveguide.

We propose two designs of nanoscale sub-fF germanium photodiodes which are efficiently integrated with silicon waveguides. The metal-optic cavities are simulated with the finite difference time domain method and optimized using critical coupling concepts. One design is for a metal semiconductor metal photodiode with <200 aF capacitance, 39% external quantum efficiency, and 0.588 (λ/n)³ cavity volume at 1.5 µm wavelength. The second design is for a vertical p-i-n photodiode with <100 aF capacitance, 51% external quantum efficiency, and 0.804 (λ/n)³ cavity volume. Both designs make use of CMOS compatible materials germanium and aluminum metal for potential future monolithic integration with silicon photonics.