| Germanium-on-Silicon (Ge-on-Si) photodiodes are critical for low-cost Si-based optical-electronic-integrated-circuits (OEIC). However, it is challenging to integrate Ge on Si due to Ge/Si lattice mismatch. In addition to the process and material engineering, the structure innovation of photodetector is important as well.
To facilitate full monolithic integration of photodetector with other photonics devices (such as waveguides, filters and modulators), waveguided photodetector configuration is considered as one of the key solutions, instead of surface illuminated devices. Waveguide configuration allows the simultaneous boosting of photon-absorption efficiency and detecting speed since the photon absorption is in perpendicular direction to the electrical carriers collection. This configuration relaxes the Ge-thickness requirement since the photon absorption is achieved by the length of the Si-waveguide evanescently coupled with overlaying Ge. Therefore, the realisation of Ge photodetector integrated on Si-photonic platform would signify a major step towards full monolithic integration for cost-effective OEIC.
IME's recent realisation of evanescent-coupled waveguided lateral Ge PIN photodetectors (LPD) and its comparative analysis with vertical Ge PIN (VPD) demonstrate these excellent features when Ge is monolithically integrated on the Si-photonics platform.
Using a novel low-temperature SEG-Ge deposition on Si waveguide, IME researchers have demonstrated waveguide photodetectors in Si and Ge with excellent device performances in their low dark current (less than 1uA at reverse bias at -1V) and high responsivity up to 1.12A/W and 3dB bandwidth of 3.4GHz from lateral Ge PIN detectors. In comparison, vertical p-Si/i-Ge/n-Ge PIN detectors show much lower responsivity at 0.29A/W though with higher speed (5.5GHz).
From our analysis, the speed of the devices could be further boosted by optimising the devices layout, such as electrode spacing. These devices can be readily integrated with existing Si-CMOS platform for applications such as optical transceivers. In the future, this will enable the industry to design multi-channel data chip without incurring extra expenses for assembly, as in the case of external photodetectors.
SEM pictures of lateral Ge and vertical p-Si/i-Ge/n-Ge PIN photodetectors
TEM images of the selective epi grown (SEG) Ge on Si/SiGe buffer on Si |
