# Recent Advances in Photodetectors Based on Two-Dimensional Material/Si Heterojunctions
## 3 Photodetectors Based on 2D Material-Si Heterojunctions
### Graphene/Si Heterojunctions
investigated photodetectors based on graphene/Si nanotip heterojunctions and achieved a high photoresponsivity of 3 A/W under white light illumination [90]. The nanotip not only enhanced light absorption, but also created a large electric field at the tip apex, which could induce impact ionization, resulting in gain. [PERSON] et al. introduced a thin wide-bandgap AlN insulator at the graphene/Si heterojunction interface and found that the strong electrical field within the insulator caused impact ionization, which resulted in gain [91]. The Fermi level of graphene in graphene/Si Schottky diodes can be adjusted by a bias voltage due to the limited density of states of graphene near the Dirac point, resulting in a tunable Schottky potential [92]. Consequently, the barrier for electron transfer from graphene to Si can be lowered under reverse bias voltage. By exploiting this characteristic, we achieved high-performance graphene/Si heterojunction photodetectors by inserting a WS\({}_{2}\) interfacial layer between graphene and n-Si [93]. The WS\({}_{2}\) interfacial layer was directly synthesized on the Si surface through magnetron sputtering followed by sulfurization. The interfacial layer not only passivated the surface states on the Si surface but also acted as a hole-trapping layer due to its band alignment with Si. The photogenerated holes were trapped at the WS\({}_{2}\)/Si interface, attracting electrons into the Si due to the reduced barrier height under reverse bias, as shown in Figure 4d, resulting in gain similar to that in a photoconductive device. In this configuration, a responsivity of 54.5 A/W was achieved at 800 nm, as shown in Figure 4e. In addition, the responsivity was found to be dependent on the size of the graphene/Si junction area, i.e., decreasing as the junction area increased, due to an increased recombination rate. Moreover, by finely tuning the interfacial layer thickness, the responsivity could be further increased to 8.96 \(\times\) 10\({}^{4}\) A/W, as shown in Figure 4f [94]. [PERSON] et al. fabricated a gate-controlled photodetector based on a graphene/Si Schottky junction [95]. The device demonstrated a substantial ON/OFF photo-switching ratio of 10\({}^{4}\), a high photoresponsivity of 70 A/W and a low dark current on the order of \(\upmu\)A/cm\({}^{-2}\) across a broad wavelength range of 395 to 850 nm. As shown in Figure 4g, the device consisted of a graphene/Si heterojunction, an Al\({}_{2}\)O\({}_{3}\) dielectric layer and a ZnO top gate. The authors proposed a unique gain mechanism in which the injection rate of holes is amplified by the negative gate voltage, leading to an increased photocurrent. The current observed was not a pure photocurrent but rather an amplified substrate current initiated by the photocarriers, as illustrated in Figure 4h. For better comparison and understanding of the graphene/Si Schottky junction photodetectors, the performance parameters of some typical devices are listed in Table 1.  
Figure 4: Graphene/Si heterojunction characteristics concerning gain. (**a**) Photocurrent mapping of the graphene/n-Si device under different bias voltages. Reproduced with permission from Ref. [88]. Copyright 2017, American Chemical Society. (**b**) Schematic of the cross-section of the graphene/n-Si heterojunction. (**c**) Band diagram along different directions shown in (**b**). (**b,c**) Reproduced with permission from Ref. [89]. Copyright 2016, American Chemical Society. (**d**) Band diagram of the graphene/WS\({}_{2}\)/Si heterojunction under reverse bias and light illumination. (**e**) Responsivity as a function of wavelength for devices with different sizes. (**d,e**) Reproduced with permission from Ref. [93]. Copyright 2019, WILEY-VCH. (**f**) Responsivity and specific detectivity as a function of WS\({}_{2}\) film thickness. Reproduced with permission from Ref. [94]. Copyright 2021, Royal Society of Chemistry. (**g**) Schematic of the gate-tunable graphene/Si photodetector. (**h**) Schematic of the device describing photocurrent component at negative gate bias.