Optimizing the Electronic Structure of In₂O₃ through Mg Doping for NiO/In₂O₃ p–n Heterojunction Diodes

In₂O₃ is a wide bandgap oxide semiconductor, which has the potential to be used as an active material for transparent flexible electronics and UV photodetectors. However, the high concentration of unintentional background electrons existing in In₂O₃ makes it hard to be modulated by the electric field or form p–n heterojunctions with a sufficient band-bending width at the interface. In this work, we report the reduction of the background electrons in In₂O₃ by Mg doping (Mg–In₂O₃) and thereby improve the device performance of p–n diodes based on the NiO/Mg–In₂O₃ heterojunction. In particular, Mg doping compensates the free electrons in In₂O₃ and reduces the electron concentration from 1.7 × 10¹⁹ cm^–3 without doping to 1.8 × 10¹⁷ cm^–3 with 5% Mg doping. Transparent p–n heterojunction diodes were fabricated based on p-type NiO and n-type Mg–In₂O₃. The device performance was considerably enhanced by Mg doping with a high rectification ratio of 3 × 10⁴ and a remarkable high breakdown voltage of >20 V. High-resolution X-ray photoelectron spectroscopy was used to investigate the interfacial electronic structure between NiO and Mg–In₂O₃, revealing a type II band alignment with a valence band offset of 1.35 eV and a conduction band offset of 2.15 eV. A large built-in potential of 0.98 eV was found for the undoped In₂O₃ but decreased to 0.51 eV for 5% Mg doping of In₂O₃. The NiO/Mg–In₂O₃ diodes with an improved rectification ratio and wider depletion region provide the possibility of achieving photodetectors with rapid photoresponse.