# Recent Advances in Photodetectors Based on Two-Dimensional Material/Si Heterojunctions
## 3 Photodetectors Based on 2D Material-Si Heterojunctions
### Graphene/Si Heterojunctions  
Graphene, renowned for its remarkable electrical and optical properties, has emerged as the most extensively studied 2D material. In particular, graphene exhibits ultra-high carrier mobility, reaching 2 \(\times\) 10\({}^{5}\) cm\({}^{2}\)/Vs at low temperature [69], which greatly facilitates carrier transport. Moreover, monolayer graphene absorbs only 2.3% of the incident light within the 400-800 nm wavelength range [70]. In addition, graphene functions as a semimetal with high conductivity [71, 72]. This unique combination of low light absorption and high conductivity positions graphene as a potentially transparent electrode [73, 74]. Consequently, graphene can be used to fabricate Schottky diodes, which can serve as solar cells and photodetectors. In this review, we will focus only on the advancements in graphene/Si heterojunction photodetectors. Typically, graphene on substrates exhibits unintentional p-doping [1, 75, 76]. Consequently, n-type Si is usually used to form a heterojunction with graphene. Given the semimetal nature of graphene, the graphene/Si heterojunction functions as a Schottky diode. [PERSON] et al. performed a transfer of chemical vapor deposition (CVD)-synthesized graphene onto lightly n-doped Si (\(\rho\) = 1-10 \(\Omega\cdot\)cm) to create a Schottky photodiode, as shown in Figure 2a [47]. Their findings revealed a substantial photovoltage responsivity exceeding 10\({}^{7}\) V/W and a low NEP of approximately 1 \(\text{pW}/\text{Hz}^{1/2}\) when operating in photovoltaic mode. The photodiode demonstrated a current responsivity of 435 mA/W and an expansive LDR of 60 dB, as shown in Figure 2b. Furthermore, the device exhibited fast response times in the millisecond range, and the photo-to-dark current ON/OFF ratios exceeded 10\({}^{4}\). However, the photodiodes displayed a small short-circuit current under light illumination, which was attributed to the limited density of states of graphene near the Dirac point. The small short-circuit current, however, was attributed to the poor interface between graphene and Si by [PERSON] et al. [55], resulting in the fast recombination of photogenerated carriers. Nevertheless, the straightforward fabrication method of these photodiodes has sparked considerable enthusiasm for further investigation of graphene/Si photodiodes. In general, un-passivated Si surfaces possess numerous surface states that can function as recombination centers for photogenerated carriers, leading to significant leakage current. Coating the surface with a layer of graphene alone does not effectively passivate the underlying Si due to its saturated surface. Therefore, the incorporation of a passivation layer is necessary to improve the performance of graphene/Si heterojunction-based photodiodes. [PERSON] et al. introduced a thermally grown SiO\({}_{2}\) layer between Si and graphene as an interfacial layer to passivate the dangling bonds in the Si surface [77]. The introduction of this interfacial layer resulted in a significant reduction in the dark current, while the photoresponsivity and response speed remained unaffected, as shown in Figure 2c. As a result, the NEP, specific detectivity and the photo-to-dark current ON/OFF ratio showed substantial improvements, as can be seen in Figure 2d. The internal built-in electrical field at the graphene/Si interface is crucial for achieving high-performance photodetectors. Previous studies on graphene/Si solar cells have demonstrated that p-doping on graphene can increase both the Schottky barrier height at the interface and enhance the conductance in graphene, resulting in improved device performance [78; 79]. [PERSON] et al. successfully employed Si quantum dots (QDs) to achieve p-doping in graphene, and the resulting photodetectors based on the doped graphene-Si heterojunctions exhibited significantly enhanced performance, as shown in Figure 2e [80]. The introduction of Si QDs on top of graphene increased the built-in potential of the graphene/Si junction.