Graphene-based photodetectors

Ultrafast photodetectors have many potential applications, ranging from environment monitoring to telecommunications, sensing, and basic research. To date, commercialized photodetectors are dominated by IV and III-V semiconductors, e.g., Si, Ge, GaAs, and GaN. However, the intrinsic bandgap of standard semiconductors restricts the photoresponsivity of the detectors to specific light bandwidths. Novel materials with broadband absorption are, therefore, required for wideband, tunable detection. Unlike conventional semiconductor materials, graphene, a 2D carbon nanosheet, shows attractive mechanical and photoelectrical properties, particularly wavelength- independent light absorption, outstanding high carrier mobility as well as ease of fabrication process [6]. Therefore, graphene has been 66the foremost 2D atomic crystal investigated for photodetectors that show substantial advances in broadband detection, fast response, high cutoff frequency, and strain tolerance. The first reported graphene photodetector, which is a phototransistor, was reported in 2009. T. Mueller and F. Xia studied the photoelectric response of graphene field effect transistor. For a common metal-graphene- metal structure, carrier transfer from the metal contacts to graphene enables a bandgap bending and creates a built-in internal electric field at the interface. Driven by the electric field, photo-exited electron-hole pairs in graphene will be separated and then extracted under bias voltages, generating an electric output [22]. Using nearfield scanning imaging, they showed that the strongest photocurrent spots occur where the metal contacts with graphene due to the existence of local electric field near the interface. The photocurrent gradually decreases as the distance increases to hundreds of nanometers away from the contacts, as shown in Fig. 5.1a. Later in the same year, they realized ultrafast photodetection using graphene phototransistor. The bandwidth of the device reached 40 GHz, and the maximum operation frequency is predicted to be as high as 500 GHz in theory [31]. This pioneer work is the first step toward exploring the full potential of graphene in broadband and ultrafast photodetection. In 2010, by using asymmetric electrodes in an interdigitated metal-graphene-metal photodetector, the mirror symmetry of built-in electric field in the vicinity of two metal contacts was broken, enhancing the separation of the photocarriers; thus, performances of the graphene photodetector were improved with a responsivity of 6.1 mAW - 1 in 10 Gbit· s - 1 optical data link [23], as shown in Figs. 5.1b, c, which approved that graphene photodetector is a promising candidate for applications in high-speed optical communication.