# Recent Advances in Si-Compatible Nanostructured Photodetectors
## 4 Future Directions
Works on the development of new designs of nanostructures [72; 73], using graphene-like 2D materials [74; 75; 76; 77; 78] and exploiting the resonant cavity [79; 80; 81] and plasmonic effects [82; 83] are also constantly being carried out. One more complex way for the integration of photodetectors on a silicon chip is the fabrication of III-V epitaxial structures [84] or colloidal quantum dots [85; 86; 87; 88] on a silicon substrate. Elements of other groups may also be used as a dopant in silicon or germanium [89; 90]. Moreover, germanium photodiodes manufactured on Ge-on-insulator substrate, already showing very high performance, also fit the requirements of silicon-based technology [91]. Search for exotic nongroup IV elements combined with silicon is another route for the evolution of Si-compatible detectors [92]. In pace with the development of silicon-based photodetectors, a huge amount of work on the creation of group IV avalanche detectors (including single-photon avalanche diodes, SPAD) is carried out [93; 94; 95]. They are vital for a wide range of employment, from medical and LIDAR applications to quantum communication technologies [96; 97; 98]. Over the past decade, the academic and industrial community has achieved tremendous results in the improvement of the performance of SPADs [99; 100; 101]. Another very promising direction of further development in the field of silicon-based integration of electronic and photonic devices [102] is the creation of group IV light-emitting structures [103; 104; 105]. In recent years significant progress was achieved in the realization of GeSn [106; 107; 108] and diamond [109] laser-active media. The use of the GeSiSn ternary compound to improve the light-emitting characteristics of such structures also appears to be very promising [110]. Quantum dots are also considered important light-emitting nanostructures and have been intensively studied for several decades from the point of view of experimental aspects of the synthesis [111; 112; 113; 114; 115; 116; 117; 118; 119; 120], their optical properties [121], and applications [122; 123; 124; 125]. Finally, efficient group-IV single photon sources and semiconductor qubits [126; 127; 128; 129; 130] were recently shown, supporting the development of on-chip quantum information processing. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Model** & **Wavelength** & **Responsivity** & **Dark Current Density** & **Detectivity** \\ \hline  & **nm** & **A/W** & **mA/cm2** & **cm**-Hz**1/2.**W\({}^{-1}\) \\ \hline One layer GeSn/Ge & 1550– & 0.18 & – & 1.0–10\({}^{9}\) \\ photoconductor [24] & 2000 & 0.22 & 70 & – \\ \hline One layer Ge/GeSn/Ge p-i-n photodiode [25] & 1550– & 0.30 & 10 & 4.0–10\({}^{9}\) \\ \hline Black silicon Ge-on-Si p-i-n photodiode [27] & 1550 & 0.34 & 150 & – \\ \hline Multiple quantum wells GeSn/Ge & 1550– & 0.11 & 40 & 2.1\(\cdot\)10\({}^{8}\) \\ photodetector [28] & 2200 & 0.31 & 16 & 1.4\(\cdot\)10\({}^{10}\) \\ \hline Multiple quantum wells GeSn/Ge & 1550– & 0.31 & 16 & 1.4\(\cdot\)10\({}^{10}\) \\ photodetector [48] & 2200 & 0.47 & – & – \\ \hline One layer SiO2: Ge QDs/Ge & 640– & 0.47 & – & – \\ photodetector [41] & 1550 & 1.12 & – & 2.0\(\cdot\)10\({}^{10}\) \\ \hline Multiple layer Ge QDs/Si & up to & 0.43 & 10\({}^{-6}\) & 6.2\(\cdot\)10\({}^{10}\) \\ photodetector [43] & 4000 & 0.43 & – & 1.4\(\cdot\)10\({}^{11}\) \\ \hline Multiple layer Ge QDs/SiGe & up to & 40 & – & 1.4\(\cdot\)10\({}^{11}\) \\ photodetector [44] & 6000 & & – & \\ \hline Si nanowire: Ge QDs/SiO2/Si & 1200– & 1700 & 5.50 & – & 9.3\(\cdot\)10\({}^{11}\) \\ phototransistor [47] & 1700 & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of the performance characteristics of various types of photodetectors.