# Recent Advances in Photodetectors Based on Two-Dimensional Material/Si Heterojunctions
## 3 Photodetectors Based on 2D Material-Si Heterojunctions
### Graphene/Si Heterojunctions
Conversely, for photon energies below 2.1 eV, the photogenerated electrons thermalized into a Fermi-Dirac distribution, and only those electrons with energy above the Schottky barrier height could contribute to the photocurrent. Unlike photoconductive detectors, which always exhibit a gain, diode-like photodetectors do not exhibit gain until ionization impacts occurs, leading to carrier multiplication. However, researchers have discovered that the EQE of the graphene/Si photodetectors consistently exceeds 100%. Initially, the photosensitive area of the graphene/Si junction was thought to be the sole contributor to the photocurrent. However, it has been proven that the area beneath the SiO\({}_{2}\) layer also contributes to the photocurrent. [PERSON] et al. employed scanning photocurrent measurements to investigate the spatial distribution of the photocurrent in the graphene/Si heterojunction [88]. They observed that the Si region away from the graphene/Si junction also contributed to the photocurrent due to the diffusion of photogenerated carriers towards the junction area. Additionally, the photocurrent generated at the graphene/SiO\({}_{2}\)/Si region exhibited a strong reverse bias voltage-dependent behavior, as shown in Figure 4a. At low bias voltages, the photogenerated holes in the Si underneath the graphene/SiO\({}_{2}\) had to diffuse towards the graphene/Si junction. The presence of a large number of interfacial defects near the SiO\({}_{2}\)/Si interface acted as recombination centers, resulting in a lower photocurrent. Conversely, when the bias voltage exceeded the inversion voltage of the Si underneath the SiO\({}_{2}\) layer, the recombination rates are significantly reduced due to the passivation of the interfacial states by the inversion layer, resulting in a higher photocurrent. Therefore, it is crucial to accurately estimate the  
Figure 2: Photodetection performance of graphene/n-Si photodiodes. (**a**) Schematic of the graphene/n-Si heterojunction device and digital photograph of the device. (**b**) Photocurrent as a function of light power under different bias voltage. Inset: natural logarithm plot of the photocurrent vs. light power with bias voltage of \(-\)2 V. (**a,b**) Reproduced with permission from Ref. [47]. Copyright 2013, American Chemical Society. (**c**) The current-voltage curves of graphene/n-Si and the heterojunction with 2 nm-thick interfacial modification. (**d**) Oxide thickness dependent response at 890 nm. (**c,d**) Reproduced with permission from Ref. [77]. Copyright 2016, WILEY-VCH. (**e**) Schematic of the Si QDs doped graphene/n-Si heterojunction device. (**f**) Responsivity as a function of wavelength for devices with different structures. (**e,f**) Reproduced with permission from Ref. [80]. Copyright 2016, WILEY-VCH. actual photosensitive area when evaluating the performance of the graphene/Si heterojunction photodetectors. [PERSON] et al. investigated the photodetection performance of graphene/Si heterojunctions with a graphene/SiO\({}_{2}\)/Si step [89]. The devices exhibited a large photoresponsivity of 1.2 A/W for p-Si and 0.45 A/W for n-Si, corresponding to EQEs of 235% and 88%, respectively. The multiplication gain was attributed to the presence of the graphene/SiO\({}_{2}\)/Si step with a nanoscale gap, as illustrated in Figure 4b, which resulted in the formation of a graphene/air/Si interface acting as a nanoscale vacuum electronic structure. Impact ionization, initiated by photoinduced carrier injection into the self-induced localized electric field (reaching 10\({}^{6}\) V/cm) distributed in a 2D electron gas, as shown in Figure 4c, which was more pronounced when the graphene/Si junction area is small compared with the nanoscale gap. Additionally, p-Si exhibited a higher quantum gain than n-Si due to the higher impact ionization rate, and the gain was also dependent on the intensity of the incident light power. [PERSON] et al.