Improving the p–type conductivity of Cu₂O thin films by Ni doping and their heterojunction with n–ZnO

Cu₂O is one of the few transition metal oxides which can exhibit p–type conductivity due to native acceptor defects. Further improvements in the p–type conductivity of Cu₂O can be achieved via extrinsic doping and post–growth processing. In this work, we investigate the effects of Ni doping in Cu₂O with Ni content up to ∼11%. A variety of analytical techniques were utilized to investigate the structural, optical and electrical properties of these alloy thin films. We find that the incorporation of Ni improves the p–type conductivity of the films without altering the Cu₂O crystallographic structure and maintains a wide bandgap of ∼2.5 eV. X–ray photoelectron spectroscopy (XPS) measurements reveal that the Fermi level moves closer to the valence band with increasing x, in agreement with the increasing hole concentration. Significant improvement in the crystallinity as well as the p–type conductivity of the alloy films can be observed after rapid thermal annealing (RTA). In particular, alloys with x≥0.005 exhibit a high hole mobility µ∼12–22 cm²/V-s with low resistivity of ρ∼30–60 Ω-cm and free hole concentration N∼1×10¹⁶ cm^-3 after RTA in air–N₂ ambient at 700°C. Such improvement can be attributed to the formation of Cu vacancies which promotes Ni substitution in the presence of small amount O₂ in the air–N₂ annealing environment. Finally, we fabricated a transparent p–(Ni_xCu_1-x)₂O/n–ZnO heterojunction and evaluated its junction performance. XPS measurements reveal a type II band offset of this heterojunction with valence band and conduction band offsets of 2.81 eV and 2.07 eV, respectively. The heterojunction exhibits both rectification and photovoltaic response, demonstrating its potential for p–n junction and photodetection devices.