# Recent Advances in Photodetectors Based on Two-Dimensional Material/Si Heterojunctions
## 3 Photodetectors Based on 2D Material-Si Heterojunctions
### TMD/Si Heterojunctions
The device demonstrated a maximum responsivity of 9 A/W, a high specific detectivity of 10\({}^{14}\) Jones and a fast response time of 9 \(\upmu\)s. It exhibited improved photodetection performance compared with a MoS\({}_{2}\)/planar Si device, which can be attributed to the higher interfacial barrier height, superior light trapping and larger junction area in the MoS\({}_{2}\)/porous Si junction. Typically, TMDs synthesized using the thermolysis method exhibit poor crystal quality. To address this issue, [PERSON] et al. synthesized a reduced graphene oxide (RGO)-MoS\({}_{2}\) composite film by the thermolysis of a (NH\({}_{4}\))\({}_{2}\)MoS\({}_{4}\)-graphene oxide (GO) mixture film, followed by annealing in a reducing atmosphere [58]. The RGO-MoS\({}_{2}\) film was directly synthesized on a pyramid-Si substrate, and a trilayer graphene film was transferred onto the heterojunction as the transparent top electrode, as shown in Figure 5d. The device exhibited excellent performance, including a large responsivity of 21.8 A/W, an extremely high specific detectivity of 3.8 \(\times\) 10\({}^{15}\) Jones and an ultrabroad spectral response ranging from 0.35 to 4.3 \(\upmu\)m. The broadband spectral response was attributed to the narrowed band gap of MoS\({}_{2}\) resulting from imperfect crystallinity. [PERSON] et al. also reported ultrabroad band photodiodes based on defective WS\({}_{2}\) film/pyramid Si heterojunctions [116]. In order to achieve broadband photodetection, [PERSON] et al. employed a type II Weyl semimetal 1T'-MoTe\({}_{2}\) as the 2D material to form a vdW heterojunction with Si as the photodetector [49]. The device exhibited an ultrabroad band detection spectrum ranging from UV light to mid-IR light (10.6 \(\upmu\)m), as shown in Figure 5e,f. The long-wavelength photoresponse relied on the photogenerated carriers in the 1T'-MoTe\({}_{2}\), which could transfer to the Si side through thermionic emission or tunneling through an ultrathin insulator layer. In a recent study, [PERSON] et al. made an important discovery regarding heterojunctions with the stacking sequence of WS\({}_{2}\)/WSe\({}_{2}\)/Si, which can form a unipolar barrier heterojunction. In this configuration, WS\({}_{2}\) served as the photon absorber, WSe\({}_{2}\) acted as the unipolar barrier, and the p-Si functioned as the collector of photogenerated carrier [117]. The inclusion of the WSe\({}_{2}\) layer in the middle not only mitigated the detrimental effects of the substrate, but also established a high-conduction band barrier that effectively filtered out various dark current components while allowing the photocurrent to flow unimpeded. As a result, the device exhibited an impressive photo switching ON/OFF ratio of more than 10\({}^{5}\), along with a high specific detectivity of 2.39 \(\times\) 10\({}^{12}\) Jones and a fast response time of 8.47 ms.  
By integrating an ultrathin Si (u-Si) substrate with a 2D material, it becomes possible to fabricate a flexible 2D material/Si heterojunction photodiode. [PERSON] et al. successfully demonstrated an ultra-flexible photodiode based on a 2D-MoS\({}_{2}\)/Si heterojunction, which exhibited outstanding photodetection performance and remarkable flexibility [118]. In their study, a 15 \(\upmu\)m thick u-Si substrate was used. A layer of MoO\({}_{3}\) (3 nm) was deposited on the u-Si, followed by sulfurization to convert it into MoS\({}_{2}\) using an air pressure plasma-enhanced CVD technique. The resulting flexible photodiode displayed a commendable photoresponse, with a responsivity of 10.07 mA/W and a specific detectivity of 4.53 \(\times\) 10\({}^{10}\) Jones. Furthermore, the flexible photodetector exhibited excellent stability when exposed to external pulsed light, regardless of its bending state. In addition, even after 1000 consecutive bending cycles, the ON and OFF currents showed no significant changes. These results underscore the immense potential of flexible photodiodes in the field of wearable photodetectors.