# Integrated Photodetectors Based on Group IV and Colloidal Semiconductors: Current State of Affairs
## 4 Colloidal Semiconductor
### Metal Oxide
PD based on NB of Nb\({}_{2}\)O\({}_{5}\) showed high photosensitivity, selectivity to light and excellent photocurrent stability for \(>\)2500 s. The responsivity and EQE are determined to reach 15.2 A/W and 6070% respectively, demonstrating the potential of NB Nb\({}_{2}\)O\({}_{5}\) in next-generation photosensors and PDs into the UV spectrum [132]. Zn\({}_{2}\)SnO\({}_{4}\) (ZTO) has aroused great interest as a base material for PDs for its photodetection performance. In particular, ZTO is used as a PD in association with various polymers that are used to decorate the surface of ZTO, improving its ZTO properties. In [133], it has been shown that by combining ZTO's NWs with polydimethylsiloxane (PDMS) it is possible to exploit the transparency and flexibility of PDMS with its NWs photodetection functionality. Furthermore, the photoactive materials are chemically bound in the PDMS matrix and less exposed to air and therefore to oxidation. Compared to the NWs completely immersed in the polymer matrix, the decorated NWs have a significant improvement in the PD in terms of speed of response (less than 0.8) with a recovery time of around 3 s.  
Another type of decoration of NWs has been presented by [PERSON] et al. in [134]. The authors manufactured a PD with heterojunction structure with high performance in the UV spectrum, in which the active material is made up of ZTO NWs decorated with quantum dots (QDs) in ZnO. the NWs decorated with ZnO are summarized in two steps. In Figure 7, a sketch of carriers separation in ZnO QD decorated ZTO NW PD on lighting is shown. Figure 7: Sketch of carrier separation in the cross section of ZTO nanowires (NWs) decorated with ZnO quantum dots (QDs) upon illumination. Compared to simple ZTO NWs, decorated QD NWs has 10 times higher photocurrent and response speed, with a light-dark current ratio up to \(6.8\times 10^{4}\) and a photoconductive gain up to \(1.1\times 10^{7}\), in addition to excellent stability. Decoration of ZnO nanoparticles (NPs) has been also used to broader the absorption band and improve the photosensitivity. As example, in ZnO NPs the excess of Zn\({}^{2+}\) ions toghedar with oxygen lack cause a great green photoluminescence [135; 136], but coating the ZnO NPs with polyvinyl-alcohol, as done in [137], the ratio of photocurrent on dark current ranges from \(3.8\times 10^{6}\) to \(1.34\times 10^{8}\). In [138] simply changing the solution method of ZnO NPs into a coating spray, the authors shows that it is possible to tune the percentage of vacuum and zinc interstitial spaces. In this way, a significant broadening in visible light photo-detection is reached. Finally, in [139], [PERSON] et al. synthesized gold decorated ZnO NPs colloidal spin coating solution. The fabricated film shows strong plasmon assisted SSA and scattering, leading to an increase of photoresponse by 80 times over the control simple ZnO NPs. In the end of this paragraph, we would underline how colloidal NPs can be also easily integrated in a Si based heterojunction by drop casting; as done in [140], where authors synthesized In\({}_{2}\)O\({}_{3}\) NPs and casted them on an n-type single-crystal Si wafer, showed an high photoresponse in visible and near-infrared regions. Infact in [140], in order to fabricate a heterojunction photodetector, thin films In\({}_{2}\)O\({}_{3}\) NPs is deposited on a (100)-oriented 10 \(\Omega\) cm n-type silicon by drop-casting and dried at 80 \({}^{\circ}\)C for 15 min under vacuum with multiple layers deposition and condensation steps. The final thickness of the In\({}_{2}\)O\({}_{3}\) layer was 150 nm. Aluminum electrodes have been fabricated by means of standard lithographic process to make the ohmic contacts on the In\({}_{2}\)O\({}_{3}\) nanoparticles layer and the back side of the Si wafer