# Recent Advances in Si-Compatible Nanostructured Photodetectors
## 2 Thin Film and Quantum Well Photodetectors
Figure 3 shows a schematic cross-sectional view of the photodetector. Figure 1: Cross-section view of the one-layer GeSn on Ge photoconductor. There were two samples with 7% and 10% tin. Electrical and optical characteristics were investigated for both samples. The reverse current density increased with the temperature as a result larger number of thermally activated carriers. The 10% Sn device showed higher current densities compared to that of the 7% Sn device at equal conditions due to the narrower bandgap, which also resulted in the larger number of thermally excited carriers. Other parameters were also studied such as detectivity for both samples. They have shown very close values under the same conditions [26]. This study has shown good performance and detectivity compared with the detectors dominating the market. The next study with one layer structure used Ge on the Si system [27]. This paper considered a photodiode working at 1550 nm and consisting of a 300 nm thick Ge layer, which was fabricated using molecular beam epitaxy and standard etching and lithography techniques. The Ge p-i-n diode was grown on the Si substrate and covered by doped silicon capping layer. After the fabrication of p-i-n photodiode, the back side of the substrate was covered by black silicon. Optical power was induced from the rear side. The diameter of the mesa-structure was 400 \(\upmu\)m, which is shown in Figure 4.  
It was noted that the black silicon nanostructure did not spoil the I-V characteristics and acted similar to waveguide structures. This article established that the application of black silicon light-trapping structures is a possible technology to increase the responsivity of Ge-on-Si photodiodes. Besides that, rather good combination of bandwidth and responsivity was achieved, making these structures viable for ultra-fast applications for communication [27]. To enhance the absorption efficiency of the detector multiple layers with quantum wells may be used. One example with a GeSn/Ge multiple quantum wells (MQW) detector was shown in the recent work by [PERSON] et al. [28]. The multilayer p-i-n structure was grown on a 300-mm Ge-buffered Si substrate by reduced pressure chemical vapor deposition. Figure 5 shows a schematic diagram of the photodetector with a photon-trapping microstructure. Figure 3: Schematic cross-sectional view of the GeSn on Ge p-i-n photodetector. Figure 2: Schematic cross-section of the GeSn on Ge p-i-n photodiode structure. Characteristics investigations were made which have shown a flat photo response curve under a reverse bias voltage that indicates an efficient collection of photon-generated carriers and the capability of the photodetector regarding low-energy consumption. The dark current was investigated with and without a photon-trapping structure at room temperature. A low dark current density of 31.5 mA/cm\({}^{2}\) was achieved which is higher if compared with other photodetectors, and is also considered to be among the lowest for group-IV photodetectors. The dark current density increased to 45.2 mA/cm\({}^{2}\) at \(-1\) V when the photon-trapping microstructure was incorporated. It was investigated and revealed that the increased dark current density mainly results from the increased surface leakage current introduced by the photon-trapping microstructure [28]. To sum up, these works enable group-IV photodetectors to be very perspective for various infrared systems. It is also desirable to monolithically integrate these detectors with lasers, transistors, waveguides, modulators and other devices on one silicon platform. Figure 4: Scheme of the black silicon Ge-on-Si photodiode. Figure 5: Schematic diagram of the multiple quantum wells GeSn/Ge photodetector with photon-trapping microstructure. A lattice matched SiGeSn multiple quantum well structures should be developed to reduce the noise currents of this type of photodetectors in the future [28].