# Recent Advances in Photodetectors Based on Two-Dimensional Material/Si Heterojunctions
## 4 Summary and Outlook  
In summary, significant progress has been made in enhancing the photodetection performance of 2D material/Si heterojunction photodetectors. The performance of these photodetectors surpasses that of commercial Si photodiodes in certain aspects, including detection spectral range, detectivity and responsivity. However, several challenges still hinder the practical application of 2D material/Si heterojunction photodetectors, which are listed below. 1. The gain mechanism in semiconducting 2D material/Si heterojunction remains unknown. Further experimental and theoretical investigations are necessary to elucidate the unique gain mechanism in these heterojunctions. 2. Polarization-sensitive photodetectors offer more information compared to isotropic photodetectors. However, Si, being an isotropic material, lacks polarization that is sensitive to polarized light. Some 2D materials possess intrinsic anisotropy, making it possible to achieve a polarization-sensitive 2D material/Si heterojunction photodetector by employing anisotropic 2D materials. Nevertheless, the anisotropic property cannot be maintained in polycrystalline 2D material films, and synthesizing large-area single crystal 2D materials remains a significant challenge. Alternatively, utilizing optical structures to achieve polarization-sensitive photodetection is a viable approach. 3. Most graphene/Si heterojunctions rely on transferring graphene onto Si, which unavoidably introduces residues and defects at the interface, leading to performance degradation. Exploring alternative methods, such as the direct growth of graphene on Si, can alleviate this issue. 4. The crystal quality of directly grown 2D materials on Si substrates is poor. These 2D materials are polycrystalline with numerous defects. These defects can act as recombination centers, compromising the performance of the photodetectors. Therefore, it is crucial to exploring new techniques to enhance the quality of 2D materials. Additionally, for integration with COMS readout circuits, synthesis methods compatible with COMS technology are required. Figure 7: Other 2D material/Si heterojunction photodetectors. (**a**) Schematic of the Bi\({}_{2}\)Se\({}_{3}\)/Si heterojunction. Reproduced with permission from ref. [134]. Copyright 2016, American Chemical Society. (**b**) Schematic of the vertically aligned Bi\({}_{2}\)Se\({}_{3}\) nanoplates/Si heterojunction. (**c**) Scanning electron microscopy image of the vertically aligned Bi\({}_{2}\)Se\({}_{3}\) nanoplates. (**b**,**c**) Reproduced with permission from ref. [135]. Copyright 2020, American Chemical Society. (**d**) Schematic of the Ti\({}_{3}\)C\({}_{2}\)/SiO\({}_{x}\)/Si heterojunction and digital photograph of the device. (**e**) Responsivity and specific detectivity as a function of light power for the Ti\({}_{3}\)C\({}_{2}\)/SiO\({}_{x}\)/Si heterojunction photodiode. (**d**,**e**) Reproduced with permission from ref. [140]. Copyright 2021, WILEY-VCH. (**f**) Schematic of the Te/Si heterojunction. Reproduced with permission from ref. [61]. Copyright 2023, WILEY-VCH. 5. The controlled doping of 2D materials is necessary to regulate the built-in electrical field and depletion region, optimizing the performance of the heterojunction photodetectors. However, achieving stable and reliable doping in 2D materials remains a challenge due to their ultrathin thickness. 6. Most reported 2D material/Si heterojunction photodetectors exhibit exceptional key parameters in one or two aspects, which may not be suitable for practical applications. Photodetectors with well-balanced key parameters are preferable. Therefore, it is essential to comprehensively evaluate the key parameters of photodetectors. Si has well-established processing technology within the optoelectronic field, while heterojunctions fully exploits the advantages of both the 2D materials and the Si. In addition to the above six points, numerous challenges still impede the commercialization of 2D material/Si heterojunction photodetectors.