# Recent Advances in Si-Compatible Nanostructured Photodetectors
## 3 Quantum Dot Photodetectors
This work has demonstrated that high quantum efficiency and fast response speed can be achieved in Ge quantum dot based photodetectors simultaneously at low voltage. Also, this work highlighted the importance of the thickness dependence of response speed in Ge quantum dot photodetectors. A new example of quantum dot photodetectors was shown in the work [42]. Here a Ge quantum dot photodetector with one layer of quantum dots that has a thickness of 160 nm was fabricated. Ge and SiO\({}_{2}\) were cosputtered on the n-Ge substrate. The schematic diagram of the photodetector is shown in Figure 7.  
The current-voltage characteristics were investigated in the range of wavelengths from 400 to 1550 nm. Results on the responsivity, reflectance, and quantum efficiency of the detector were presented and it was shown that the detector had its best performance and efficiency between 500-600 nm. High detectivity was achieved at the temperature range of 100-300 K [42]. Next, a midinfrared photodetector based on Ge quantum dots in Si has been presented [43]. The multilayer Si/Ge samples were synthesized by molecular beam epitaxy on the p+-Si(001) substrate. The schematic design of the detector is shown in Figure 8.  
Figure 6: Schematic diagram of the photodetector consisting of an ITO electrode and an active layer with quantum dots on top of an n-Ge substrate. Figure 7: Schematic structure of the photodetector with Ge quantum dots in SiO\({}_{2}\). Ten layers of germanium quantum dots separated by 50 nm silicon barriers served as active layers. Each Si barrier contained a boron delta-doping layer inserted 5 nm below the Ge-wetting layer. The active region was sandwiched between the 200-nm-thick intrinsic Si buffer and the cap layers. Finally, a 200-nm-thick p\({}^{+}-\)Si top contact layer was deposited. The noise current, detectivity, and responsivity as a function of voltage and temperature were considered and the study has shown that such a device exhibited low dark current and operated until 200 K [43]. One more study with a multilayer quantum dot photodetector was introduced next where the multiple germanium quantum dot layers were grown on Si\({}_{0.82}\)Ge\({}_{0.18}\) virtual substrate [44]. Ten layers of germanium quantum dots were separated by 35-nm Si\({}_{0.82}\)Ge\({}_{0.18}\) barriers (Figure 9). In this study another photodetector sample was fabricated based on pure silicon instead of SiGe for performance comparison purpose. Both samples have shown a responsivity in the whole mid-wave infrared range. Also, there was a significant improvement in the Ge/SiGe detector sensitivity over the wavelength region from 3 to 5 \(\upmu\)m, as compared to the Ge/Si heterostructure. This is associated with the smaller hole effective mass in SiGe layers, which enables more efficient light absorption and photoexcited hole transportation. The photoconductive gain was studied and the noise characteristics were measured, and the SiGe detector has shown a larger gain, probably due to the increase of the hole mobility, and, therefore, the decrease of the hole transit time through the device. Moreover, in this work, a metallic plasmonic structure was added to the detector to achieve better performance of the quantum dot photodetector. The measured responsivity of the SiGe-based photodetector with and without the plasmonic structure have shown that plasmonic structures enhance photocurrent at the plasmon resonance frequencies compared with bare detectors [44]. The development of these works led to the emergence of the idea of using plasmonic effects and microresonators in order to enhance the properties of silicon-germanium photosensitive structures [45]. These attempts have shown a significant increase in the photodetector parameters, such as resonant responsivity, when using such plasmonic structures [46]. Figure 8: Schematic drawing of the Ge/Si photodetector with 10 layers of quantum dots. Finally, we will discuss a study that has been made on a photodetector with mixed technologies for producing nanowires and quantum dots.