Fabrication and characterization of nanostructured Cadmium Oxide thin films doped with Indium by sol-gel spin-coating for CdO (n)/Si (p) heterojunction photodiode applications

In this study, nanostructured thin films based on cadmium oxide doped with Indium CdO: In were fabricated by sol-gel spin-coating technique on p-type monocrystalline silicon c-Si (p) for the integration in n-p heterojunction photodiode applications. A comprehensive analysis for structural, morphological, compositional, optical and electrical properties of the surface of undoped CdO and CdO:In films on silicon substrates is conducted. Upon In doping, the cubic structure of polycrystalline CdO film and the preferential (111) orientation are maintained. In the uniformly distributed nanostructures network, the average grain size of CdO/Si is decreased from 35 nm to 23 nm for CdO:In/Si. Smooth heterointerfaces with good adhesion of CdO-based thin films to the Si substrate imply suitable heterojunction quality and favorize good surface passivation and electrical transport. Based on chemical composition and electronic states analysis, In dopants uniform distribution and incorporation by In ³⁺ ion substitution in quasi-stoichiometric CdO thin films are confirmed. The anti-reflection role of undoped CdO and doped CdO: In is validated through lower optical reflectance compared to bare Si substrate, especially in visible range. The electrical current-voltage I-V characteristics in dark and under illumination conditions are employed to determine the main diode parameters of different Cd (n)/ c-Si (p) heterojunction structures. A clear rectifying diode behavior with asymmetrical and non-linear dependency is obtained for CdO/Si and CdO:In heterojunctions. Compared to undoped CdO thin films, doping of CdO with In leads to higher ideality factor and reverse saturation current, but lower potential barrier and series resistance. Higher photogenerated current at Si region with more light sensitivity is obtained CdO:In/Si diode owing to better transparency and wider bandgap than undoped CdO film.Owing to lower conduction band offset at CdO:In/Si heterojunction, the charge carrier transport is improved for CdO:In compared to CdO/Si diode. Our results demonstrate the effective integration of nanostructured undoped and In-doped CdO thin films in CdO/Si n-p heterojunction photodiode applications.