# Integrated Photodetectors Based on Group IV and Colloidal Semiconductors: Current State of Affairs
## 3 Group IV Semiconductors
### Si
A 50 Gb/s pure silicon waveguide photodetector at 1310 nm datacom wavelength has been also developed [47]. Its working principle is based on a combination of TPA, SSA, and photon assisted tunneling (PAT) effects in a Si PN junction; responsivity was found to be 0.6 A/W, dark current was of 850 nA at 5.84 V reverse bias and eye SNR was of 7.3. Nanostructured photodetectors have attracted a lot of interests in the recent past, due to their large surface-to-volume ratios and the reduced dimension, that lead to superior performances in terms of responsivity and photoresponse gain with respect to bulk crystals. Indeed, the large surface areas of nanostructures gives rise to a prolonged photocarrier lifetime leading to higher sensitivity and responsivity. Moreover, nanostructured photodetectors show a fast response speed owing the low dimensionality which shortens the transit time during photoelectric processes [48; 49; 50], and good compatibility with semiconductor technology. On this line of research, 1D inorganic nano-wires (NWs) PD have demonstrated high responsivity, fast response, specific detectivity, high spectral selection, good flexibility, and low energy consumption [51]. The first avalanche photodetectors (APD)-based nanoscale p-i-n junction Si nanowire (SiNW) was demonstrated in 2006 and this device showed an avalanche breakdown mechanism with large reverse bias [52]. After that, several studies were performed on Schottky junction NIR photodiodes and nano-heterojunction NIR-PDs based on SiNW [53] combined with metal films (e.g., Cu [54], Ag [55] and Au/Cr [56]), graphene oxide [57], and other nanostructured semiconductor comprising carbon quantum dots [58]. With the aim to improve the excitation efficiency, [PERSON] et al. [59] investigated optical near-field excitation that is confined in a nano-scale. Here, due to the uncertainty principle, the interband transitions between different wave numbers are excited; therefore, optical near-field can directly excite the carrier in the Si indirect bandgap. The authors developed a lateral Si p-n junction with Au nanoparticles as sources to generate the field confinement and they demonstrated that the photo-sensitivity rate increases of 47.0%. Moreover, the far-field excitation is eliminated when the thin lateral p-n junction is used, confirming a 42.3% increase in the photosensitivity rate [59]. Even though the Si-based PD limitation can be partially solved by femtosecond laser processing, the surface defects and carrier activation rates produces a large dark current and narrow spectral response, which are drawbacks. To overcome this, [PERSON] et al. [60] have recently proposed a rapid thermal annealing and hydrogenated surface passivation demonstrating, at optimal conditions, a sub-bandgap responsivity of 0.80 A W\({}^{-1}\) for 1550 nm at 20 V at room temperature; these results are comparable with commercial Ge based PDs and higher than previously reported Si photodiodes. This Si-based photodetector shows a spectral range of 400 \(\div\) 1600 nm and a competitive detectivity (1.22 \(\times\) 10\({}^{14}\) Jones at \(-\)5 V). Finally, its responsivity was 1097.60 A W\({}^{-1}\) for 1080 nm at 20 V, which is the highest reported to date in Si photodetectors, thus opening good prospects for black silicon in IR light detection, night vision imaging, and fiber-optic communication [60].