# Recent Advances in Photodetectors Based on Two-Dimensional Material/Si Heterojunctions
## 3 Photodetectors Based on 2D Material-Si Heterojunctions
### Other 2D/Si Heterojunctions  
In addition to graphene, TMDs and NMDs, there are various other types of 2D materials that can form vdW heterojunctions with Si and demonstrate remarkable photodetection capabilities. However, due to the rapid advancements in this field, we will focus on discussing only a few of these materials. Topological insulators (TIs) are materials characterized by an insulating bulk with spin-momentum-lock Dirac cones on their surfaces. [PERSON] et al. have predicted that the Tl Dirac-like surface state exhibited strong optical absorption, making them promising candidates for high-performance broadband photodetectors operating from the IR to the terahertz range [132]. Some TIs, such as Bi\({}_{2}\)Te\({}_{3}\) and Bi\({}_{2}\)Se\({}_{3}\), are layered materials capable of forming vdW heterojunctions with bulk materials, thereby enabling the development of intriguing optoelectronic devices. [PERSON] et al. employed pulsed laser deposition to deposit a Bi\({}_{2}\)Te\({}_{3}\) film on an n-Si, forming a Bi\({}_{2}\)Te\({}_{3}\)/Si heterojunction photodetector [133]. This heterojunction demonstrated pronounced rectification behavior, with a rectification ratio exceeding \(5\times 10^{3}\). Moreover, it exhibited ultrabroad band photodetection capabilities spanning from the UV to the terahertz range. The photoresponse for photon energies greater than the band gap of Si primarily originated from the photogenerated carriers in the Si. Photon energies below the bulk Bi\({}_{2}\)Te\({}_{3}\) band edge were attributed to the surface state of the Bi\({}_{2}\)Te\({}_{3}\), while photon energies between the Si band edge and the bulk Bi\({}_{2}\)Te\({}_{3}\) band edge were attributed to the bulk state of the Bi\({}_{2}\)Te\({}_{3}\). In another study, [PERSON] et al. fabricated a Bi\({}_{2}\)Se\({}_{3}\)/Si heterojunction photodiode, as shown in Figure 7a, by depositing a Bi\({}_{2}\)Se\({}_{3}\) film on the surface of an n-Si substrate by physical vapor deposition [134]. The strong built-in electrical field at the Bi\({}_{2}\)Se\({}_{3}\)/Si interface greatly facilitated the separation and transport of photogenerated carriers. The photodiode consequently exhibited a high responsivity of 24.28 A/W and a high detectivity of 4.39 \(\times\) 10\({}^{12}\) Jones at a wavelength of 808 nm under a bias voltage of \(-\)1 V. More recently, [PERSON] et al. prepared a Bi\({}_{2}\)Se\({}_{3}\)/Si heterojunction with vertically oriented Bi\({}_{2}\)Se\({}_{3}\) nanoplates on a p-Si substrate, as shown in Figure 7b,c [135]. This unique structure enhanced light absorption and provided more efficient paths for photocarrier transport, resulting in improved photodetection performance. [PERSON] et al. utilized a Si substrate with pyramid microstructures to enhance light absorption and directly deposited Bi\({}_{2}\)Se\({}_{3}\) on the pyramids using physical vapor deposition [136]. This device achieved a wide spectra response ranging from 635 to 2700 nm, a high specific detectivity of 1.24 \(\times\) 10\({}^{11}\) Jones and a fast response speed in the microsecond range. 2D transition metal carbides, known as MXenes, are a promising class of 2D materials that exhibited intriguing electronic and optoelectronic properties [137; 138]. [PERSON] et al. conducted a study on the optoelectronic properties of Ti\({}_{3}\)C\({}_{2}\)T\({}_{x}\)/Si heterojunction photodetectors [139]. They discovered that the Ti\({}_{3}\)C\({}_{2}\)T\({}_{x}\) not only served as a transparent electrode but also facilitated the separation and transportation of photogenerated carriers. Although the photodetectors exhibited fast response speeds and a wide dynamic range, their responsivity and detectivity were relatively low. This can be attributed to the presence of a high density of surface states between the MXene and the Si, leading to carrier recombination and the Fermi-level pinning of Si. To address this issue, [PERSON] et al.