# Recent Advances in Photodetectors Based on Two-Dimensional Material/Si Heterojunctions
## 3 Photodetectors Based on 2D Material-Si Heterojunctions
### Graphene/Si Heterojunctions
This doping strategy led to remarkable achievements, including a record-high responsivity of 0.495 A/W, as can be seen in Figure 2f, an unprecedented short response time of 25 ns and an excellent specific detectivity of 7.4 \(\times\) 10\({}^{9}\) Jones. To address the issue of high light reflection (35%) in planar Si, which hinders efficient light absorption and results in low EQE, researchers have commonly used antireflective layers and nanostructured Si in graphene/Si heterojunction-based solar cells [81; 82; 83]. However, these approaches have rarely been explored in the context of graphene/Si heterojunction photodiodes. Recently, [PERSON] et al. proposed a design that incorporates highly anti-reflective Si nanometer truncated cone arrays (Si NTCas) in graphene/Si heterojunction photodetectors [84]. The Si NTCas effectively reduced light reflection compared to planar Si due to the light-trapping effect of the nanostructures. As a result of the enhanced light absorption, the graphene/Si NTCas heterojunction photodetectors achieved an impressive EQE of 97% at 790 nm, along with a short rise/fall time of 60/105 \(\upmu\)s.  
The detection spectral range of the graphene/Si heterojunction photodetectors is primarily determined by the Si component, as graphene typically serves as the transparent electrode. However, graphene can also act as a photosensitive material. When photoexcited carriers are generated in graphene, they can be emitted to Si across the Schottky barrier, a process known as internal photoemission, enabling the detection of light with photon energy higher than the Schottky barrier [85]. Despite graphene's broadband spectral absorption capabilities, its absorption of incident light is extremely low, resulting in a weak photoresponse to light beyond the absorption edge of Si. To overcome this limitation, [PERSON] et al. designed a resonant cavity structure to enhance the light absorption in graphene at 1550 nm, as schematically shown in Figure 3a [86]. The spectral response of the detector without the Au back mirror is shown in Figure 3b, which shows pronounced wavelength-dependent periodic photocurrent peaks that align with the resonant characteristics of a Fabry-Perot cavity. By introducing a Au back mirror, the responsivity of the photodetector was increased by a factor of 3, accompanied by a blue spectral shift of the resonance wavelengths, as shown in Figure 3c. The photoresponsivity also exhibited a dependence on the reverse bias voltage, reaching approximately 20 mA/W at a bias voltage of 10 V near 1550 nm, as shown in Figure 3d. In addition to resonant cavity structures, other forms of graphene have been used to construct broadband graphene/Si photodiodes [87]. For instance, [PERSON] et al. fabricated a highly crystalline macroscopic assembled graphene nanofilm (nMAG) and transferred it onto the surface of Si, forming an nMAG/Si heterojunction photodetector [48]. The 45 nm thick nMAG displayed an extinction coefficient of 74-81%, corresponding to approximately 40% light absorption, as shown in Figure 3e. Furthermore, nMAG exhibited a long carrier relaxation time (~20 ps), a low work function (4.52 eV) and suppressed carrier number fluctuations, all of which are advantageous for achieving high-performance nMAG/Si heterojunction photodetectors. Under these circumstances, the device operated in the wavelength range of 1.5-4.0 \(\upmu\)m at room temperature Figure 3f, demonstrating a fast response time, as shown in Figure 3g, and high detectivity under pulsed laser illumination. The spectral response of the photodetector extended beyond the intrinsic absorption of Si, indicating that nMAG played a significant role in the photoresponse. The specific photoresponse mechanism depended on the photon energy range, as illustrated in Figure 3h. When the photon energy exceeded 2.1 eV, the photogenerated electrons had sufficient energy to overcome the Schottky barrier and contribute directly to the photocurrent.