## 3 IPE-Based Silicon Photodetectors Based on Graphene Schottky Junctions
The optical cavity is able to enhance the interaction of the light with graphene in order to increase its absorption. In this work, the authors prove that the device responsivity peaks coincide with the resonances of the Fabry-Perot microcavity and that the responsivity increases with the number of light round trips of the cavity. Thanks to the multiple reflections in the cavity, the graphene absorption increases up to 8% leading to a maximum responsivity increasing with the reverse voltage applied and reaching a maximum of \(\sim\)20 mA/W at \(-\)10 V (Figure 6b); this value is the highest reported so far for free-space illuminated Si PDs at 1550 nm. In addition, the authors prove the SBH dependence on  
Figure 5: (a) Scketch of the Graphene/Si Schottky photodetectors (PD) investigated in Goykhman [36]. (b) Responsivity of Graphene/Si Schottky PDs and Metal–Si PDs (for comparison). Solid lines are the curve fitting obtained by taking into account a combined avalanche multiplication and thermo field emission effect. Reprinted with permission from ([PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON]. Integrated, Silicon–Graphene Plasmonic Schottky Photodetector with High Responsivity and Avalanche Photogain. _Nano Lett._**2016**, _16_, 3005–3013. Direct link: [[https://pubs.acs.org/doi/10.1021/acs.nanolett.5b05216](https://pubs.acs.org/doi/10.1021/acs.nanolett.5b05216)]([https://pubs.acs.org/doi/10.1021/acs.nanolett.5b05216](https://pubs.acs.org/doi/10.1021/acs.nanolett.5b05216))). Copyright (2016) American Chemical Society. Further permissions related to the material excerpted should be directed to the ACS. applied reverse voltage. Finally, the dark current at \(-10\) V has been measured as 147 \(\upmu\)A while the device bandwidth has been estimated in 120 MHz. In order to increase the low graphene absorption of the previous device (only 8%), the same author has theoretically proposed a resonant cavity-enhanced (RCE) graphene/Si PD working at 1550 nm based on IPE [39]. Device is essentially a Fabry-Perot interferometer constituted by three layers of crystalline Si, graphene and hydrogenated amorphous Si surrounded by two high-reflectivity Bragg mirrors as shown in Figure 7a. The optical field enhancement allows increasing the single-layer graphene optical absorption up to 100%. The optoelectronic transduction mechanism is based on IPE where the photoexcited carriers are emitted from graphene to Si. In this work, it has been theoretically proved that an optimized device can reach an external quantum efficiency of 35% and responsivity of 0.43 A/W, as shown in Figure 7b. Finally, device speed and noise, have been discussed. Figure 6: (**a**) Optical image of fabricated device investigated in Casalino [38]. (**b**) Responsivity vs reverse voltage. Reprinted with permission from ([PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] et al. Vertically Illuminated, Resonant Cavity Enhanced, Graphene-Silicon Schottky Photodetectors. _ACS Nano_**2017**, _11_, 10955-10963). Copyright (2017) American Chemical Society. Figure 7: (**a**) Section sketch and (**b**) theoretical responsivity and quantum efficiency of the high-finesse resonant cavity enhanced PD investigated in Casalino [39]. Reprinted with permission from ([PERSON]. Design of Resonant Cavity-Enhanced Schottky Graphene/Silicon Photodetectors at 1550 nm. _J. Lightwave Technol._**2018**, _36_, 1766-1774). Copyright (2018) IEEE. Very recently this family of devices have demonstrated the capability to work also at wavelengths longer than 1550 nm [28]. Indeed, in the [PERSON]'s work [28] vertically-illuminated silicon PD operating at 2 \(\upmu\)m has been reported. The sketch of the device is shown in Figure 8a; it is a graphene/silicon Schottky junctions whose electrical parameters are carefully extracted at various temperature increasing from 280 to 315 K. The Schottky barrier is 0.62 eV at 300 K and shows a dependence on temperature ascribed to interface defects.