# Integrated Photodetectors Based on Group IV and Colloidal Semiconductors: Current State of Affairs
## 3 Group IV Semiconductors
### Carbon
#### 3.3.1 Carbon Nanotube  
CNTs are tubes made of carbon with diameters in the order of nanometers. When diameters are in the range of a nanometer, carbon nanotubes are called single-wall carbon nanotubes (SWCNTs). Respect to devices made with other semiconductors materials, CNT based photodetectors show a fine IR detection together with low fabrication cost, ease of production and scalability, actually making CNTs highly promising nanomaterials for multiwavelength, room-temperature IR detection applications [80]. Up to now, several IR photodetectors based on both individual CNT and CNTs films [81; 82] have been successfully proven. Moreover, few years ago, a prototype of infrared camera was realized based on single CNT photodetectors [83]. In order to develop an efficient CNT photodiodes, based on photovoltaic mechanism, an efficient separation electron-hole pairs into free carriers should be obtained as well as an increase of the absorption cross-section of the device. In particular, the first studies on the realization of CNT photodiodes demonstrated that electron-hole pairs separation and photocurrent generation can be obtained by the built-in field in a CNT Schottky and p-n diode [84; 85; 86]. Moreover, even if CNT photodiode attracts a lot of research interest to study the mechanisms of photocurrent generation  
Figure 3: (**a**) Schematic illustration of the germanium/perovskite heterojunction device fabrication process. From left to right: a cleaned SiO\({}_{2}\)/Si substrate, growth of a germanium layer by molecular beam epitaxy (MBE), Au electrode deposition on the substrate, perovskite layer construction by the vapor-solution method. (**b**) I\({}_{\text{ph}}\) and R values under an illumination wavelength of 1550 nm, 980 nm, 680 nm and 405 nm, from left to right, respectively (adapted from [69]). along with many other optoelectronic properties [86], the extremely small absorption cross-section of individual CNT devices leads to a low sensitivity [87]. Nevertheless, significant improvements have been reached in the recent past, allowing to increase the responsivity from \(\sim\)pA/W to \(\sim\)A/W [85; 88]. Since the pioneering work proposed by [PERSON] et al. in 2005 [85], where the first realization of individual semiconducting SWCNT p-i-n diode was demonstrated, several other studies were performed on these kind of devices. In detail, the device realized by [PERSON] et al. consisted in a SWCNT where two split gates where formed in adjacent parts by using electrostatic doping to realize n-region and p region, respectively. The photocurrent generated under IR illumination with a power density \(\sim\)20 W/cm\({}^{2}\) was of \(\sim\)5 pA, whereas the fill factor (FF) and the efficiency \(\eta\) were of 0.33-0.52 and \(\sim\)0.2%, respectively. In 2011 a photodetector based on SWCNT/C(60) in heterojunction photovoltaic was demonstrated with an EQE of 12% and a detectivity approaching \(\sim\)10\({}^{12}\) cm Hz\({}^{1/2}\) giving a proposal for the development of the next-generation high-performance solar cells based on CNTs [89]. Thin films of highly purified semiconducting CNTs were used as NIR optical absorbers in heterojunction photodetector and photovoltaic devices with the electron acceptor C(60), leading to a 10-fold enhanced gain in zero-bias quantum efficiency and significant gains in power conversion efficiency respect to the implementations of more electrically heterogeneous CNT/C(60) devices. In particular, the exciton migration along an effective length scale in the nanotube films has been related to the device efficiency, validating the high IQE through photoluminescence quenching, and demonstrating that for diameters \(<\)1.0 nm the driving force for exciton dissociation at the fullerene-fullerene heterointerface is optimized [89]. [PERSON] et al. demonstrated a \(\sim\)6-fold higher optical absorption in a single-tube diode photodetector monolithically integrated with a Fabry-Perot microcavity, due to the confined effect of the designed optical mode [90].