Fabrication and Characterization of Germanium Photodetectors

Abstract

Si photonics has become one of the most intensive research domains in the world since it holds great promise for maintaining the performance roadmap known as Moore?s Law. First, the recent progresses in the development and integration of Ge-photodetectors on Si-based photonics is comprehensively reviewed, along with remaining technological issues to overcome and future research trend. Second, the impact of selective-epitaxial-germanium is discussed, specifically its local strain effects, on high-performance PIN photodetector for near-infrared applications. Then Si-waveguide-integrated lateral Ge-PIN photodetectors using novel Si/SiGe buffer and two-step Ge-process are demonstrated. Comparative analysis between lateral Ge PIN and vertical p-Si/i-Ge/n-Ge PIN are made. Furthermore, device performance of scaled thin-film-Ge lateral PIN photodetectors integrated on Si-waveguide is presented. The photodetectors are with closely spaced p+/n+ regions (0.8 ?m) on Ge region with short length (5-20 ?m) and narrow width (2.4 ?m). Though with thin Ge-layer (~220 nm including bottom SiGe buffer), light is evanescent-coupled from Si waveguide effectively to the overlying Ge detector. The device exhibits f3dB bandwidth of 18 GHz with external responsivity of 0.13 A/W for 1550 nm at -1V. Considering the coupling loss and waveguide loss, the internal responsivity is as high as 0.65 A/W. It is shown that with increasing detector length, device?s internal quantum efficiency can be improved to ~90% and by suppressing parasitic effects, speed can be boosted further towards several tens of GHz. To address the photodiodes? scalability issue, this work demonstrates a scalable (with gate length of 1 ?m) Ge-photodetector based on junction field-effect-transistor (JFET) structure with high sensitivity and improved response time. To overcome the low detection efficiency issue of typical JFET photodetectors, a high quality Ge epi-layer as the gate of JFET was achieved using a novel epi-growth technique. By laser surface-illumination of 3 mW on the Ge gate, an Ion/Ioff ratio up to 185 was achieved at wavelength of 1550 nm for the first time. Moreover, SOI wafers are utilized to improve the Ge JFET detector?s 3dB bandwidth. The results on high-speed silicon-waveguided Ge JFET-based photodetector are reported. While the Ge layer?s footprint on wafer is as small as 2 ?m?2 ?m, low stand-by current (0.5 ?A@1 V), high responsivity (642 mA/W) and high speed (8 GHz) are achieved. The reported Ge JFET is a promising candidate for the further scale-downed photodetector in the next-generation Si photonics.