# Recent Advances in Photodetectors Based on Two-Dimensional Material/Si Heterojunctions
## 3 Photodetectors Based on 2D Material-Si Heterojunctions
### Other 2D/Si Heterojunctions
achieved high specific detectivity and responsivity in photodiodes by incorporating a chemically regrown interfacial SiO\({}_{x}\) layer Figure 7d and precisely controlling the thickness of the Ti\({}_{3}\)C\({}_{2}\) MXene [140]. These photodiodes demonstrated a high specific detectivity of 2.03 \(\times\) 10\({}^{13}\) Jones and an impressive responsivity of 402 mA/W at zero bias and 900 nm, as shown in Figure 7e. To enhance the stability of the Ti\({}_{3}\)C\({}_{2}\) MXene, a layer of Silicone was applied to the device, resulting no noticeable degradation. Additionally, [PERSON] et al. successfully reduced the dark current in photodiodes by introducing a thin layer of h-BN (sub-1 nm thick) as the interfacial layer, exploiting its dangling bond-free nature [141]. As a result, the dark current was reduced by two orders of magnitude and the specific responsivity reached 10\({}^{13}\) Jones. Monoelemental 2D materials, such as BP, antimony, bismuth and tellurium, have gained considerable attention in recent decades due to their unique electrical and optoelectronic properties [142, 143]. Tellurium, in particular, has emerged as a focal point of research due to its intriguing characteristics, including a thickness-dependent band gap ranging from 0.3 to 1.04 eV [144, 145], a high hole mobility of about 700 cm\({}^{2}\)/Vs at room temperature [146] and exceptional air stability [147]. Consequently, the integration of Te with Si can lead to the development of heterojunction photodetectors with fascinating photodetection capabilities. In a study by [PERSON] et al., Te nanosheets were synthesized by a hydrothermal method and subsequently transferred onto the surface of an n-Si substrate to create p-n photodiodes [148]. Type I heterojunction was formed between p-Te and n-Si with a built-in electrical field at the interface. The photodiodes accordingly exhibited low dark current and displayed a robust photoresponse even under zero bias condition, including an impressive responsivity of 6.49 A/W and a high specific detectivity of 7.79 \(\times\) 10\({}^{12}\) Jones at a wavelength of 808 nm. Additionally, these devices demonstrated a polarization-sensitive response to linearly polarized light, exhibiting an anisotropic ratio of 2.1 at 635 nm due to the in-plane low-symmetry atomic structure of the Te nanosheets. Nonetheless, the transfer of nanosheets for the fabrication of Te/Si heterojunctions is unsuitable for the mass production of devices. In addition, the contamination of the interface is difficult to avoid during the transfer process. To address this issue, [PERSON] et al. developed Te/Si heterojunction arrays by directly depositing Te films onto pre-patterned SiO\({}_{2}\)/Si substrates using a PLD technique [66]. Utilizing the small band gap of the Te, the resulting photodiodes exhibited an ultrabroad spectral response extending from the UV to the near-IR (370.6-2240 nm). Moreover, the substantial built-in electrical field in the interface enabled the photodiodes to achieve a high photo switching ON/OFF ratio of 10\({}^{8}\) and a rapid response speed. Furthermore, a matrix array of 35 \(\times\) 35 devices was fabricated, showcasing an average responsivity of 249 A/W, an EQE of 76,350% and a specific detectivity of 1.15 \(\times\) 10\({}^{11}\) Jones. Similarly, [PERSON] et al. fabricated Te/Si heterojunctions by magnetron sputtering a Te film onto the surface of a Si substrate, as shown in Figure 7f [61]. Although the device did not exhibit obvious rectification, it delivered excellent photodetection performance, including an ultrahigh responsivity of 437.24 A/W and a high specific detectivity of 4.86 \(\times\) 10\({}^{11}\) Jones at 1064 nm under zero bias. In addition, the device had a fast response speed, with a rise time of 920 ns and a decay time of 200 \(\upmu\)s.  
For better comparison and understanding of the other 2D material/Si heterojunction photodetectors, the performance parameters of some typical devices are listed in Table 4.