# Integrated Photodetectors Based on Group IV and Colloidal Semiconductors: Current State of Affairs
## 4 Colloidal Semiconductor
### Metal Oxide  
Colloidal metal oxides have attracted much attention as based semiconductor material, because of their high stability [24; 113; 114]. Moreover, colloidal metal oxides showed adjustable electrical properties as resistivity, work function (WF) and dielectric constant (\(\epsilon\)), which can be tuned by controlling vacancy density, doping material or other controllable parameters [115]. Finally, their showed tunable optical properties (e.g., photoluminescence, absorption, and transparency) tuning the band gap by changing alloys and engineering of components [116; 117]. In PDs, colloidal metal oxides are largely employed because of their wide band gap, large exciton binding energy, combined with the simplicity of fabrication on a large-scale, with low cost, and tunable physical and chemical properties [118; 119; 120; 121]. Also in this case, PDs can be divided into planar PDs and vertical PDs, according to device design [122; 123]. PDs in planar configuration consist of an electrode and one only active layers (MOs), as shown in Figure 6a. Under no illumination, planar PDs have small Schottky barriers at the contacts, while when they are illuminated, photons are absorbed and electron-hole pairs are created, resulting in a photocurrent, by applying a voltage. The applied voltage reduce the carrier transit time and the photocurrent increases (Figure 6a) [124]. In vertical PDs, a n-doped semiconductor layer and a p-doped semiconductor layer are added, as shown in Figure 6b. Under illumination, the doped layers transport electrons and holes, decreasing the carrier transit time and, thus, increasing the efficiency of the device (Figure 6b) [125]. Focusing on the active layer in planar PDs, in recent years, a lot of efforts have been employed to syntetize colloidal wide band gap MOs such as TiO\({}_{2}\)[126] and ZnO [127], used in UV PDs [128; 129]. Nowadays, colloidal nanowires (NW) and nanobelts (NB) offer excellent solutions for optoelectronic applications and in particular for PD. Figure 6: **(a)** Planar configuration and its basic working operation; (**b**) vertical configuration and its basic working operation. In [130], the authors synthesize SnO\({}_{2}\) NWs with a quite uniform diameter of about 26 nm, assemble a film by dip coating and fabricate a high performance PD device. In fact, it has excellent selectivity and light stability. Moreover, in [130] the EQE has been calculated with the follow equation:  
\[EQE=\frac{t_{life}}{t_{tran}}=\frac{hc}{e\lambda}\frac{\Delta I}{PS} \tag{5}\]  
where \(t_{life}\) and \(t_{tran}\) are the lifetime of carriers and the transit time between electrodes, respectively, \(h\) is he [PERSON]'s constant, \(c\) is the velocity of light, \(e\) is the electronic charge, \(\lambda\) is the incident wavelength, \(\Delta I\) is the difference between the current and dark current, \(P\) is the light power density and \(S\) is the irradiated area of a single nanowire. For the present device, irradiated by 320 nm light at 0.91 mW cm\({}^{-2}\), EQE has been calculated and a value of \(1.32\times 10^{7}\) was obtained, four orders of magnitude larger than that of standard SnO\({}_{2}\) photodetectors. This detector is proposed not only for optoelectronic applications, but also in the visible and UV spectrum. The PD assembled in a film of In\({}_{2}\)Ge\({}_{2}\)O\({}_{7}\) nanobelt with high quality monocrystals are expected to find wide optoelectronic applications in switches, optical sensors and generally in communication systems. In [131], the developed detectors showed high optical and electrical performance, i.e., EQE of \(2.0\times 10^{8}\)%, decay times of \(\sim\)3 ms and responsivity of \(3.9\times 10^{5}\) A/W, in addiction to high stability, reproducibility, sensitivity and selectivity.