Extended Wavelength Responsivity of a Germanium Photodetector Integrated With a Silicon Waveguide Exploiting the Indirect Transition

Photo-detection in the wavelength range 1850 to 2000 nm using evanescently-coupled germanium detectors grown on silicon waveguides is described. Devices were fabricated at a silicon photonics foundry using a process flow associated with operation in the O, C and L bands, and as such offer a solution for extended wavelength detection which is readily available. Intrinsic sensitivity is via indirect band transitions, which is enhanced by tensile strain and we postulate that it may be further enhanced by defects which arise from the thermal processes associated with Ge on Si growth. The responsivity of p-i-n detectors is 20 mA/W at 1850 nm falling to 5 mA/W at 2000 nm, for a detector length of 50 mu m. Responsivity is suppressed by electrical doping in the germanium detector which provides parasitic absorption from free carriers. Modifications to the current design are suggested such that integrated germanium p-i-n detectors, directly grown on silicon waveguides would be suitable for high-bandwidth photo-detection up to at least a wavelength of 2000 nm. A Separate-Absorption-Charge Multiplication Avalanche Photo-Detector is fabricated exploiting the same indirect transition. This detector has a responsivity of 0.31 A/W at 1850 nm and 0.08 A/W at 1970 nm, for a detector length of only 14 mu m.