Germanium-based photodetector is a key component in silicon based photonics because of its unique properties of response at telecommunication band and compatibility with CMOS techniques. However, the limitations of low quantum efficiency and high surface recombination in ultrathin germanium film, especially in the near-infrared range, put huge obstructions on the road toward applications. Nowadays, practical applications require more nanoscale devices with lower power consumption as well as higher responsivity and response speed. In this work, we first demonstrate a germanium-graphene hybrid structure photodetector that consists of an ultrathin 20 nm germanium layer and a monolayer graphene. The photodetector can achieve a broadband detection from ultraviolet to near-infrared range. A conductive gain of 155 and a responsivity of 66.2 A W-1 are achieved, which is about 3 orders of magnitude higher than pure graphene photodetectors and about 4 times larger than pure germanium photodetectors. Such enhancement owes to effective generation, separation and transfer of photogenerated carriers at material interface. The photodetector based on germanium--graphene hybrid structure presents a new paradigm for the realization of small but high performance device in the process of integration in silicon-based optical chips. And it offers new opportunities for imaging, sensing, and other optoelectronic field applications.
Keywords: broadband; germanium; graphene; hybrid structure; photodetector.