Due to its transparency in the telecommunications C-band near 1550 nm, silicon is inefficient in converting light into an electrical signal. Up to now, the mainstream of integrated photodetectors in silicon photonics for the communication wavelengths includes employing germanium (Ge) or monolayer graphene as photodetection material [53,54]. Ge-Si hybrid photodetectors have been widely studied and reached significant maturity thanks to their ability to absorb light in the telecommunications band near 1550 nm. High responsivity larger than 1 A/W has been achieved and an impressive bandwidth of 265 GHz has been demonstrated recently [53,55]. Compared to Ge/Si photodetectors, graphene/Si photodetectors hold great potential in reaching ultra large operation bandwidth thanks to the ultrahigh carrier mobility of graphene as well as its absorption ability in a broader wavelength range. For graphene/silicon photodetectors, there are three different mechanisms for the photodetection, which are photovoltaic effect (PV), photo-thermoelectric effect (PTE) and photo-bolometric effect (PB). The graphene photodetectors based on these three mechanisms are discussed in detail below.

The PV effect relies on the separation of photoexcited electrons and holes by an applied electric field to generate photocurrent, which can be utilized by graphene/silicon photodetector structures with normal light incidence [53,54]. As the representative work of early endeavors in graphene-based photodetectors, Xia et al. demonstrated that the graphene/Si photodetectors can reach 40 GHz [55]. Moreover, the intrinsic response time of graphene photodetectors is experimentally demonstrated to be 2.1 ps, indicating a high bandwidth of 262 GHz (Figure 1a) [56].

However, the responsivities of graphene-based photodetectors with normal light incidence are normally low. To increase the light-graphene interaction and obtain better performances, waveguide-based photodetectors have been extensively researched [18,46,57,58,59]. A typical waveguide integrated structure is shown in Figure 1b [46]. In their design, the graphene lies on top of the silicon slot waveguide as the absorption layer. Here, the graphene monolayer absorbs the light within the waveguide mode’s evanescent field, and a responsivity of 0.273 A/W is measured.

Meanwhile, hybrid integration of the silicon waveguide and graphene could also be fabricated at the wafer-scale, thanks to its low fabrication complexity [60]. The proposed photodetector holds a bandwidth of 41 GHz and the maximum responsivity is 46 mA/W. To further optimize the performance of the photodetector, Ding et al. proposed a waveguide-coupled integrated graphene plasmonic photodetector (Figure 1c) [57]. The plasmonic slot waveguide is formed by two different metallic slabs with a gap of 120 nm. This structure induces subwavelength light confinement within the surface of the graphene, dramatically increasing light absorption within the graphene. Meanwhile, different types of metal cause different doping levels in the graphene, enhancing the internal electrical field in the gap and more effectively separating the photogenerated carriers, leading to a high responsivity. Owing to these two points, the device features responsivity up to 0.36 A/W and an operation bandwidth larger than 110 GHz.

When the plasmonic gap becomes even narrower (less than 50 nm), there exist a competition between the PV and PB effect within the photodetection process, as Ma et al. thoroughly investigated [61]. An impressively high responsivity of 0.7 A/W at 1310 nm is achieved based on their work with PB effect. The PB effect refers to the modification of the channel resistance by either a change in the number of carriers or a change of the temperature-dependent carrier mobility. Thus, the PB effect leads to a negative photocurrent due to the increased channel resistance caused by the smaller mean free path induced by a temperature change due to photon absorption. A typical work based on PB effect is reported by Ma Ping employing a bowtie structure to reach a high responsivity of 0.5 A/W and a bandwidth of 110 GHz (Figure 2a) [62]. Wang et al. demonstrated a coherent, plasmonic-structure-based graphene optical receiver (Figure 2b) which is capable of the reception of both a 200 Gbit/s quadrature phase-shift keying (QPSK) signal and a 240 Gbit/s 16 quadrature amplitude modulation (16 QAM) signal on a single-polarization carrier, thanks to the ultrahigh operation bandwidth of the graphene photodetector [58]. Finally, a plasmonic/silicon hybrid graphene photodetector covering 1.55 μm and 2 μm is reported by Guo et al. where the responsivity is 0.07 A/W and 0.4 A/W at 1.55 μm and 2 μm respectively [63].

From the above-mentioned works on plasmonic graphene photodetectors, we can conclude that plasmonic structures can dramatically optimize both the responsivity and the bandwidth of graphene-based receivers. Metallic absorption remains an obstacle, acting to reduce responsivity, since its characteristic absorption does not contribute to the photocurrent. To address this issue, Yan et al. proposed and demonstrated a double slot structure consisting of both the silicon slot waveguide and the plasmonic slot waveguide (Figure 2c) [59]. By optimizing the structural parameters, the metallic absorption is reduced to 0.2 dB/μm and the responsivity increased to 0.6 A/W.

Besides the PV and PB effect, the PTE effect has also recently been employed in silicon/graphene photodetectors. The PTE effect uses on the photon-induced electron temperature difference between two different graphene doping regions, which is normally achieved by external gating. Thus, the photocurrent is generated by an optically induced temperature gradient, which is proportional to the Seebeck coefficient. By optimizing the gate and source voltages simultaneously, a maximum responsivity of 0.36 A/W and a 3-dB operation bandwidth of 42 GHz is reached [64].

Moreover, photodetectors based on PTE effect can directly convert the optical signal to voltage signal, removing the need for a transimpedance amplifier. Various structures including photonic crystal waveguides [65], mircro-ring resonators [66] and double layer graphene [67] have been employed to enhance the light-matter interaction of the PTE effect, reaching impressive performances, such as responsivity higher than 90 V/W and operation bandwidths larger than 65 GHz. Although the external gating function adds complexity to the fabrication process compared to the photodetectors based on PV and PB effect, the PTE graphene photodetector is appealing to industry since it supports the direct connection between the photodetector and the read-out electric circuit. The performances of a typical graphene/silicon photodetector and state-of-the-art photodetectors based on conventional bulk materials is compared in Table 1. Graphene/silicon photodetector can achieve high ultra-high bandwidth more than 110 GHz, thanks to the ultrahigh carrier mobility of graphene. The responsivity of graphene photodetector is comparable with the other material platforms and can be further improved with further optimization by enhancing the light-graphene overlap and interaction.