# Recent Advances in Photodetectors Based on Two-Dimensional Material/Si Heterojunctions
## 3 Photodetectors Based on 2D Material-Si Heterojunctions
### TMD/Si Heterojunctions
Due to the narrow bandgap of MoTe\({}_{2}\), the device was capable of detecting light with wavelengths ranging from 300 to 1800 nm, as shown in Figure 5b. Moreover, the device exhibited a fast response speed, with a 3 dB bandwidth approaching 0.12 GHz. [PERSON] et al. fabricated a MoTe\({}_{2}\)/Si heterojunction by directly growing a MoTe\({}_{2}\) film onto a Si substrate using a shadow mask-assisted CVD method [108]. The resulting device exhibited good photodetection performance, with a high photo-switching ON/OFF ratio of \(\sim\)10\({}^{4}\), a zero-bias responsivity of 0.26 A/W and a high specific detectivity of 2 \(\times\) 10\({}^{13}\) Jones at 700 nm. [PERSON] et al. fabricated a MoSe\({}_{2}\)/Si heterojunction array by directly depositing a Mo film onto a Si substrate and performing post-selenization [109]. This device demonstrated an impressive responsivity of 720.5 mA/W and a high specific detectivity of 10\({}^{13}\) Jones at zero bias. Reactive magnetron sputtering and direct magnetron sputtering with post-annealing have also been used to directly deposit TMD films directly onto Si substrates to form heterojunctions, resulting in decent photodetection performance [65,110]. The in-plane carrier transport in TMDs is exceptional due to their large in-plane carrier mobility. However, the vdW gap between the layers can hinder carrier transport. In the case of a 2D/Si heterojunction, the carrier extraction is inefficient when the 2D plane is parallel to the junction interface. To address this issue, [PERSON] et al. successfully achieved a vertical aligned MoS\({}_{2}\) 2D plane on a p-Si substrate using magnetron sputtering and subsequent annealing [111]. The resulting detector demonstrated sensitivity to a broad range of wavelengths from visible to near-IR light and exhibited a high detectivity of 10\({}^{13}\) Jones. This can be attributed to the strong built-in electrical field and high-speed path for the transport of photogenerated carriers, resulting in a fast response speed with a rise time of 3 \(\upmu\)s. Similarly, [PERSON] et al. synthesized vertically layered WS\({}_{2}\) nanosheets on a p-Si substrate through radio frequency sputtering at 400 \({}^{\circ}\)C, forming a heterojunction photodetector [112]. As expected, this device demonstrated an ultrafast photoresponse with a response time of 1.1 \(\upmu\)s. [PERSON] et al. used the CVD method to synthesize vertically oriented few-layer MoS\({}_{2}\) nanosheets, as shown in Figure 5c, which were then transferred onto a p-Si substrate to study the lateral photovoltaic effect [113]. The device exhibited a strong lateral photovoltaic effect due to the strong light absorption and fast carrier transport speed of the vertically aligned MoS\({}_{2}\) sheets, as well as the strong built-in electrical field at the interface. The intralayer and interlayer transport times in the vertically aligned MoS\({}_{2}\) nanosheets were determined to be 5 and 11 ns, respectively. Taking advantage of the superior properties of the vertically aligned MoS\({}_{2}\) nanosheets/Si heterojunction, [PERSON] et al. observed that the heterojunction device exhibited a high responsivity of 908.2 mA/W and a high specific detectivity of 1.9 \(\times\) 10\({}^{13}\) Jones at 808 nm under a bias voltage of \(-\)2 V [114]. This can be attributed to the high crystal quality of the CVD-synthesized MoS\({}_{2}\) and the MoS\({}_{2}\)/Si interface. Furthermore, the device demonstrated an unprecedented response speed with a rise and a decay times of 56 and 825 ns, respectively. Planar Si is not conducive of efficient light absorption; therefore, Si with a nanostructured surface is used to fabricate TMD/Si heterojunction photodetectors. [PERSON] et al. prepared nanostructured porous Si through the electrochemical etching method and deposited MoO\({}_{3}\) via magnetron sputtering, followed by sulfurization, to fabricate a MoS\({}_{2}\)/Si heterojunction photodetector [115].