# Integrated Photodetectors Based on Group IV and Colloidal Semiconductors: Current State of Affairs
## 3 Group IV Semiconductors
### Ge  
Germanium has a direct bandgap energy of 0.8 eV, high optical absorption over the 1.3-1.55 \(\upmu\)m wavelength range used for fiber-optic communications [61] and a perfect compatibility with the conventional silicon processing technology. For these characteristics, Ge-based photodetectors have been widely studied since the end of 1990s [62; 63; 64; 65] and, currently, Ge PDs performance are comparable to that of III-V materials. For nanophotonics applications, Ge photodetectors are often integrated at the end of optical waveguides, indeed in this configurations light absorption takes place along the optical mode propagation direction and perpendicularly to the carrier collection path. Moreover, both Ge homo-junction [62; 63; 64] and Si-Ge-Si hetero-junction [65; 66; 67] photodetector architectures, which are made with a p-i-n junction where light absorption occurs in the intrinsic regions, have been experimentally demonstrated. A lateral hetero-structured silicon-Ge-silicon (Si-Ge-Si) junction was realized to obtain a p-i-n waveguide photodetectors working under low reverse bias at 1.55 \(\upmu\)m [67]. Such hetero-structured Si-Ge-Si photodetector allows a superior signal detection of high-speed data traffic, having efficiency-bandwidth products of \(\sim\)9 GHz at \(-\)1 V and \(\sim\)30 GHz at \(-\)3 V, with a leakage dark current as low as \(\sim\)150 nA. For conventional 10 Gbps, 20 Gbps, and 25 Gbps data rates, a bit-error rate of \(10^{-9}\) has been obtained, leading to optical power sensitivities of \(-\)13.85 dBm, \(-\)12.70 dBm, and \(-\)11.25 dBm, respectively. An interesting solution for the Ge-on-Si PD was proposed by [PERSON] et al. [68]; the authors developed arrayed germanium-on-silicon waveguide photodetectors for high-power analog applications. Arrays of photodetector with 2, 4, and 8 elements allow to have output powers of \(-\)0.4 dBm, 10 dBm, and 14.3 dBm at 18 GHz, 12 GHz, and 5 GHz, respectively. The 4-photodiode array shows a 5-dB improvement over a single PD. An integrated 20 GHz receiver based on a couple of balanced PDs and a Mach-Zehnder delay line interferometer has been also demonstrated in an optical phase-modulated link. Recently, several solution-processed has been proposed in order to obtain good performance of PDs based on Ge. For example, [PERSON] et al. successfully fabricated a perovskite/germanium heterojunction photodetector with excellent photo-response properties [69]. The heterostructure device was realized by a uniform and pinhole-free perovskite film deposited on top of a single-crystal germanium layer, as illustrated in Figure 3a. Results demonstrated a broad spectrum compared with the single-material-based device, as showed in Figure 3b where the photon response properties are characterized from the visible to near-infrared spectrum. In particular, at optical fiber communication wavelength of 1550 nm this device presents the highest responsivity of 1.4 A/W, while at a visible light wavelength of 680 nm, it exhibits considerable detectivity and responsivity of \(1.6\times 10^{10}\) Jones and 228 A/W, respectively. The authors have been also estimated the photoconductive gain of the realized device. The gain is defined as [69]  
\[G=\frac{I_{ph}}{I_{Pl}}=\frac{\tau}{t_{p}}=\frac{\tau\xi(\mu_{e}+\mu_{h})}{L} \tag{4}\]  
where \(I_{Pl}\) is the primary photocurrent, \(\tau\) is the carrier lifetime, \(t_{p}\) is the carrier transit time through the electrodes, \(\xi\) is the applied electric field, \(L\) is the width of the channel, \(\mu_{h}\) and \(\mu_{e}\) are the hole and electron mobility, respectively. Some of the photogenerated electrons are moved from the perovskite layer to the germanium layer when visible light excitation are considered. Respect to the perovskite, the long carrier lifetime (\(\approx\)200 \(\upmu\)s) and ultra-high electron mobility (\(\approx\)3800 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\)) in the germanium layer lead to an enhanced gain of about \(10^{4}\) in this heterojunction photodetector.