# Recent Advances in Photodetectors Based on Two-Dimensional Material/Si Heterojunctions
## 3 Photodetectors Based on 2D Material-Si Heterojunctions
### TMD/Si Heterojunctions
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Materials**} & **Measurement** & \multirow{2}{*}{**R/A-W\({}^{-1}\)**} & \multirow{2}{*}{**D\({}^{*}\)/Jones**} & \multirow{2}{*}{**EQE**} & **Time** & \multirow{2}{*}{**Ref.**} \\  & & & & & & **( Rise/Down)** & \\ \hline
3L Gr/Si+PCA & 885 nm/\(-\)2 V & 0.435 & & 65\% & & [47] \\ \hline \multirow{4}{*}{Gr/n-Si} & \multirow{4}{*}{890 nm/0 V} & \multirow{4}{*}{0.73} & \(4.08\times 10^{13}\) & & \multirow{4}{*}{0.32 ms/0.75 ms} & \multirow{4}{*}{[77]} \\  & & & (in air) & & & \\ \cline{1-1} \cline{5-6}  & & & 5.77 \(\times\) 10\({}^{13}\) & & & \\ \cline{1-1} \cline{5-6}  & & & (in vacuum) & & & \\ \hline Si QDs/Gr/Si & 877 nm/\(-\)1 V & 0.495 & 7.4 \(\times\) 10\({}^{9}\) & & 25 ns & [80] \\ \hline Gr/Si NTCas & 780 nm/0 V & 0.45 & & 97\% & 60 μs/105 μs & [84] \\ \hline Gr/Si & 1550 nm/10 V & 0.02 & & & & [86] \\ \hline nMag/Si & 1300 nm/\(-\)1 V & & 1.6 \(\times\) 10\({}^{11}\) & & 20 ns/200 ns & [48] \\ \hline Gr/SiO\({}_{2}\)/p-Si & 633 nm/\(-\)5 V & 1.2 & & 235\% & 40 ns/100 ns & [89] \\ \hline Gr/SiO\({}_{2}\)/n-Si & 633 nm/\(-\)5 V & 0.45 & & & 40 ns/100 ns & [89] \\ \hline Gr/Si-tip & 880 nm/\(-\)0.5 V & 3 & & 88\% & & [90] \\ \hline Gr/AIN/n-Si & 850 nm/\(-\)10 V & 3.96 & 1.13 \(\times\) 10\({}^{8}\) & & & [91] \\ \hline Gr/WS\({}_{2}\)/Si & 800 nm/\(-\)0.3 V & 54.5 & 4.1 \(\times\) 10\({}^{12}\) & & 45 μs/210 μs & [93] \\ \hline Gr/WS\({}_{2}\)/Si & 690 nm/\(-\)5 V & 8.96 \(\times\) 10\({}^{4}\) & 8.86 \(\times\) 10\({}^{11}\) & & 0.84 ms/2.1 ms & [94] \\ \hline ZnO/Gr/Si &
\begin{tabular}{c} 850 nm/ \\ Vg = \(-\)15, Vd = 0.1 V \\ \end{tabular} & 70 & \(2\times\) 10\({}^{13}\) & & [95] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of properties of graphene/silicon heterojunction devices. p-n photodiodes, as shown in Figure 5a[104]. We observed both Zener and avalanche breakdowns at low temperatures, while only Zener breakdown was observed at room temperature, which was conformed in few-layer WS\({}_{2}\) flake/p-Si heterojunction by another research group [105]. The device under reverse bias exhibited excellent photodetection performance, including good linearity of the photocurrent to light intensity, excellent stability, fast response and wide spectral response. The maximum responsivity was observed at 660 nm with a value of 5.7 A/W. The spectral response differed between both WS\({}_{2}\) and Si, indicating that photogenerated carriers were generated in both WS\({}_{2}\) and Si for short wavelengths and only in Si for wavelengths beyond the band edge of WS\({}_{2}\). During the transfer of a 2D material onto the surface of a Si substrate, contamination of the 2D material/Si interface is inevitable. This contamination often leads to a degradation of the photodetector performance of the photodetector devices. Therefore, a more advantageous approach is the direct synthesis of 2D material on Si, as it can improve the performance of the junction devices. For example, [PERSON] et al. successfully synthesized a multilayer n-MoS\({}_{2}\) film directly on an n-Si substrate through a thermolysis method [106]. The resulting heterojunction demonstrated diode-like electrical behavior in the absence of light and exhibited excellent photodetection performance under reverse bias. At a bias voltage of \(-\)2 V, the responsivity reached 11.9 A/W at 650 nm. Moreover, due to the high quality of the hetero-interface, the device showed a rapid response with a rise time of 30.5 \(\upmu\)s and a decay time of 71.6 \(\upmu\)s. Another study by [PERSON] et al. employed pulsed laser deposition (PLD) to deposit a MoTe\({}_{2}\) film directly on an n-Si substrate, forming an n-n junction [107]. To improve the carrier collection efficiency, a layer of graphene was transferred onto the surface of the MoTe\({}_{2}\) film. The resulting device demonstrated a clear diode-like behavior and exhibited excellent photodetection performance, including a high responsivity of 0.19 A/W and a large specific detectivity of 6.8 \(\times\) 10\({}^{13}\) [PERSON].