Monolithic integration of up to 40 GHz Ge photodetectors in 3μm SOI

This paper presents our recent progress on fast germanium photodetector (PD) development for our 3μm silicon-on - insulator (SOI) platform. We have fabricated a horizontal PIN photodiode, which has a 3dB cutoff frequency of 40GHz and responsivity of 1.0 A/W at -1V bias for operation wavelength of 1.55μm. The high bandwidth indicates that the detector speed is limited by the transit time of the carriers over the i-region rather than the junction capacitance. The electric field in the i-region at -1V is high enough to maintain the carrier drift speed close to the maximum velocity of carriers in the Ge. The device is realized using selectively grown germanium with very low amount of stress induced crystal defects. The detector area and the Si waveguides were patterned with a common hard mask, which enables accurate lateral alignment between them. The n- and p-contacts were directly made on the Ge using Ti/Al metallization. The vertical sidewalls of the detector area were implanted in order to create the horizontal PIN structure. The subsequent dopant diffusion was estimated to secure the i-region and the junctions by controlling the thermal budget, as the two dopants have different diffusion mechanism in Ge. One of the advantages of our micron scale waveguides is that due to the high confinement of the optical mode within the Si waveguide they allow light coupling into a short detector. The junction capacitances are therefore small as the detector area is only 1x9μm. In addition, the electrical output pulse shape is not distorted by the slow diffusion current of electrons and holes as the incoming light do not overlap the doped n- and n-regions.