Development of CuO–ZnO-based rectifying junctions for advanced electronic applications

Abstract

Current rectification is crucial for both classical and emerging quantum devices, where charge transport and interfacial control critically determine device performance. Here, we report a facile ethylene glycol (EG)-assisted hydrothermal synthesis of CuO and ZnO nanostructures for p–n rectifying junctions. SEM observations show a reduction in the crystallite size for CuO nano-starfishes (∼90 nm) and ZnO nanorods (∼130 nm). Raman and XRD analyses collectively reveal that increasing the EG concentration decreases the CuO/Cu₂O area ratio (7.87 → 3.73), confirming the partial reduction of Cu²⁺ to Cu⁺. Furthermore, XPS analysis depicts a reduction of CuO (Cu⁺/Cu²⁺: 0.63 → 1.39) and increased surface hydroxylation (OH/CO: 2.99 → 12.30) indicating the partial transformation of CuO → Cu₂O. In ZnO, a decrease in O²⁻/CO and an increase in O_V/O²⁻ ratios suggest Zn-rich, oxygen-deficient (O_V) surfaces. The coexistence of CuO, Cu₂O, and Cu(OH)₂ phases and Zn-rich oxygen vacancies, optimizes interfacial band alignment and facilitates charge transport across the CuO–ZnO p–n interface. The optimized p–n junction CZ4 device exhibits a moderate knee voltage (1.08 V), low leakage current (224 µA at −1 V), and a high ideality factor (17.95), indicating non-ideal diode behavior dominated by series resistance and barrier height. Temperature I–V measurements reveal thermally activated conduction, with the activation energy decreasing from 0.57 eV for CZ0 to 0.44 eV for CZ4, confirming reduced barrier height and improved carrier transport in the EG-modified device. Fowler–Nordheim (FN) analysis reveals a maximum tunneling current near 1.4 V, with a linear FN regime emerging from ∼0.9 V at 275 K, indicating the onset of field-assisted tunneling at relatively low bias. A second sharp linear transition near 3.3 V at 275 K suggests the activation of additional tunneling channels at higher electric fields. With temperature variation, the transition voltages for CZ4 shift to approximately 3.0 and 2.1 V at 253 and 294 K, respectively, demonstrating a strong temperature dependence of tunneling onset and transport mechanisms. This study investigates the correlations between morphology, defect chemistry, and rectification performance. EG-assisted defect and surface engineering is demonstrated as a scalable and effective strategy for tailoring metal-oxide heterojunctions, providing a promising route toward versatile advanced electronic applications.