# Recent Advances in Si-Compatible Nanostructured Photodetectors
## 2 Thin Film and Quantum Well Photodetectors  
A quantum well is a very thin layer of one material with narrow bandgap is situated between two materials with wider bandgaps. As a result a potential step is formed. This potential well limits the motion of carriers in one direction while they are free to move in two other directions. When the width of the potential well is small enough, the energy spectrum of carriers becomes discrete and their motion in the thin layer becomes quantized. In this review we will consider one of the simplest quantum well devices--quantum well photodetector. The quantum well photodetector operation relies on the intersubband absorption within either the conduction band (for electrons) or the valence band (for holes). The principle of operation of a quantum well photodetector can be explained by the basic lawsof quantum mechanics. The quantum well is equivalent to the classical problem of motion of a particle in a box. To obtain the energy spectrum of such particle the time independent Schrodinger equation should be solved. The SiGe/Si material system almost immediately proved to be perspective for various applications of nanostructures in electronics and optoelectronics. Then, SiGeSn material system in various combinations of constituent elements became very promising, since within their framework it is possible to control the band gap and obtain direct-gap semiconductors. These materials can be used to create photodetectors, solar cells, and light-emitting devices, successfully competing with materials traditionally for optoelectronics based on III-V and II-VI compounds [14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. GeSn techniques have drawn a lot of attention of Si-based technology in the last couple decades because the operating wavelengths range of this system spreads into the near infrared and short-wave infrared regions. Generally, scientific research on GeSn detectors has involved an increase in the past decade leading to the development of high-performance GeSn detectors. Today, there are several photodetectors based on Si-Ge. One of them is Ge-Si-Sn alloy, which is widely used in quantum well infrared photodetectors. For example, in the work [24] a photoconductor detector made of one layer of GeSn on Ge, and working in the range between 1.5-2 \(\upmu\)m, was shown (Figure 1). The photoconductor GeSn samples used in this photodetector were grown using a reduced-pressure chemical vapor deposition technology. A study has been conducted on this photoconductor using 0.9, 3.2, and 7% Sn. Electrical and optical characteristics under 300 to 77 K were measured and the results of the detectivity, dark current, and responsivity were observed and measured under equivalent conditions. It was noted that the higher the temperature the less the performance of this photodetector is, which is caused by the decrease in the number of thermally activated carriers. This leads to a higher noise currents, while the responsivity of the photoconductor shows the linear increase with applied voltage that is indicative of the photoconductive gain [24]. Another study has been made on GeSn alloy where a photodiode with one layer of Ge quantum well was investigated [25]. The structure was grown on Si by molecular beam epitaxy. Figure 2 shows a schematic cross-section of the p-i-n photodetector structure. The photodetector was fabricated with quasiplanar technology. The 1550 nm photodiode was investigated and the electrical and optical characteristics were measured under certain conditions. The dark current density of the photodiode increased by more than one order of magnitude, which was due to the high Sn concentration in this sample. The optical responsivity of the photodetector was also studied, which had shown the excellent quality of the fabricated GeSn layers. A different study had been done based on one layer GeSn on Si [26]. The GeSn samples were grown using reduced-pressure chemical vapor deposition technologies and mesa structures were fabricated by photolithography and wet chemical etch processes.