# Recent Advances in Si-Compatible Nanostructured Photodetectors
## 4 Future Directions  
Further development of the works on the fabrication of GeSn/Ge multiple-quantum-well p-i-n photodiodes has resulted in the simultaneous realization of low dark current and high detectivity in the structures similar to those shown in Figure 5. An ultralow dark current density of 16.3 mA/cm\({}^{2}\) was achieved in the work [48] due to the low threading dislocation density in the pseudomorphic GeSn layer [49], along with the high responsivity of 0.307 A/W and specific detectivity of 1.37 \(\times\) 10\({}^{10}\) cm\(\cdot\)Hz\({}^{1/2}\cdot\)W\({}^{-1}\) at 1550 nm. Even higher values of performance characteristics are theoretically predicted for the GeSn multiple quantum well photodetector with alternating layers of GeSn with different compositions [50]. The recently proposed approach suggests the use of a GeSn-on-insulator platform for the creation of metal-semiconductor-metal photodetectors with a sensitivity in the spectral range of up to 2.2 \(\upmu\)m [51]. The works on using of GeSiSn solid solution layers have also confirmed that this material family is very promising for the fabrication of photodetectors in terms of extending their range of operating wavelengths and enhancing sensitivity [52; 53; 54; 55; 56]. The main problems of the application of nanostructures in photodetectors are connected with the low quality of the obtained structures. The improvement of technologies of synthesis should significantly increase the performance of quantum well and quantum dot photodetectors. The necessity to decrease the threading dislocation density, the number of different phase boundaries on the surface, and micro-cracks (caused by lattice mismatch) to suppress dark currents was stated repeatedly in the literature. On the other hand, tensile-strained layers may be used for the directed strain engineering of the energy spectra. Where quantum dots are concerned, they have lower dark currents than quantum well structures due to their higher activation energy of thermionic emission (because of strong quantum confinement of carriers) and reduced phonon scattering, resulting in a longer carrier lifetime. However, it is very important to achieve the highest possible homogeneity of the ensemble of nanoislands in terms of their shape and size to reduce dark current densities. Special growth modes or prepattering of the surface for the creation of sites of preferable formation of nanoislands (selective growth) are used to improve the homogeneity. Another method of suppressing noise currents is the use of hybrid designs such as quantum dots in a quantum well and quantum dots in a quantum wire, which allows the researchers to achieve lower dark current densities and higher operating temperatures by proper control and optimization of the thickness of a well or nanowire. The performance characteristics of the photodetectors reviewed in the previous sections are summarized in Table 1. It may be concluded that in terms of their performance photodetectors based on group-IV elements approach their III-V and II-VI competitors, showing values of detectivities of an order of 10\({}^{12}\) cm\(\cdot\)Hz\({}^{1/2}\cdot\)W\({}^{-1}\)[57; 58]. The use of various photon-trapping structures [59; 60] and the creation of artificial roughness on one side of the detector is a typical route to increase the absorption coefficient and quantum efficiency of the device. For example, alternating materials with different refractive indices by heteroepitaxy results in a significant reflection at the interfaces. This is used to create optical Fabry-Perot resonators that reflect light back to the optical absorbing layer, thus effectively increasing the width of the physically thin absorbing layer. These techniques may also include exploiting distributed Bragg reflectors [61], capping the photosensitive structure with antireflection coatings [62], or using waveguide structures [63; 64; 65; 66; 67; 68; 69; 70; 71].