# Recent Advances in Si-Compatible Nanostructured Photodetectors
## 3 Quantum Dot Photodetectors  
A quantum dot is a nanometer-sized semiconductor particle. The concept of \"artificial atoms\", or quantum dots, has emerged since the implement of molecular beam epitaxy and the first work on the reduced dimensionality of semiconductors. Quantum dots are widely used for their fully discrete energy spectrum and unique optical properties due to quantum size effects. They absorb and emit light of specific wavelengths if their energy spectra are matched. These wavelengths can be accurately controlled by changing size, shape or material composition of the particle. Hence, quantum dots are exploited in active layers for many widely-spread devices, for example, quantum dot displays and photon detectors. Thus, an alternative to photodetectors based on quantum wells can be photodetectors with quantum dots. Such heterostructures are promising for photovoltaic applications and as receiving modules in fiber optic communication lines. Quantum dots have been successfully integrated in existing types of photodetectors, significantly enhancing their performance. To realize all the potential advantages of photodetectors with quantum dots for the near-infrared range (increased sensitivity to normally incident radiation, high photoelectric multiplication factor, low thermal generation rate, narrow sensitivity range), the photosensitive region of the detector must contain an array of quantum dots with high density (on the order of 10\({}^{11}\)-10\({}^{12}\) cm\({}^{-2}\)) [7; 29; 30; 31]. The principles of operation of a photodetector with quantum dots are similar to those with quantum wells. The only difference is that in a quantum dot, the carrier is limited in its movement in all three directions, hence, zero-dimensional systems of carriers are realized [32]. It is expected that due to this limitation, quantum dot photodetectors can provide a better performance, namely higher operating temperatures (due to longer carrier lifetimes), low dark current, and high photoelectric gain [33], which ultimately translates into high sensitivity and detectability [34]. In addition, due to different selection rules for the absorption of light in photodetectors with quantum dots, it becomes possible to absorb incident radiation polarized along the normal to the layers of quantum dots. In order to achieve this regime, self-assembled islands are embedded into layers of semiconductors with a larger energy gap and appropriate band discontinuities. Due to their properties, quantum dots are of high interest to modern technologies. Silicon-based semiconductor materials with nanosized germanium inclusions have been actively studied since the early 1990s [4; 5]. Such nanoheterostructures with self-organizing germanium quantum dots on silicon, grown by molecular beam epitaxy, exhibit a number of new nanoscale effects (associated with low-dimensional quantization effects), which are promising from the point of view of their application in optoelectronic devices [35; 36; 37; 38; 39; 40]. Next, we will introduce some of the recent scientific studies that contained a quantum dot infrared photodetector with different silicon-related materials and a different number of quantum dot layers. Starting with one layer of quantum dots, the following study shows an ultrathin layer photodetector with Ge quantum dots [41]. A 13 nm active layer of Ge quantum dots in SiO\({}_{2}\) matrix was fabricated on n-Ge substrate with (100) orientation. Quantum dots diameters varied from approximately 3 to 8 nm. The schematic diagram of the photodetector is shown in Figure 6.  
The current-voltage characteristics of the photodetector were studied at various temperatures which varied from 77 K to 300 K in the dark and under illumination with the 640 and 1550 nm light. Reducing the active layer thickness to an ultrathin regime produced a high dark current at ambient conditions. Rise and fall times below 10 ns and 25 ns were achieved at \(-1\) V in the visible and near-infrared ranges which is very promising [41].