Low-Power Sputtered Cu₂O Films with ∼70% Transmittance as a Possible Route toward Copper-Based p-n Junctions

Developing stable copper-based transparent oxide semiconductors (TOSs) remains a scientific challenge, particularly via scalable processes that enable straightforward tuning of optical absorption, sheet resistance, and carrier type (p- vs n-type). Here we present a simple and reproducible methodology combining direct current (DC) sputtering and a thermal treatment to fabricate n-type copper-oxide TOS thin films. Ultrathin Cu layers deposited at 20 W in air and grown for 2-6 s on fused silica were converted by short-time annealing at 400 °C to single-phase Cu₂O, enabling systematic modulation of thickness (39-105 nm) and average transmittance (69-45%, 400-700 nm). XPS confirmed a Cu- and O-dominated surface chemistry with no detectable nitrogen incorporation. The Cu 2p and Auger responses indicated a Cu-(I)-dominated chemical state, while the persistent high-binding-energy O 1s contribution after surface erosion was consistent with oxygen-related defective environments. Under our best-performing condition, the resulting Cu₂O TOS exhibited an average transmittance of ∼ 70%, a resistivity of 34.2 Ω·cm, a sheet resistance of 8.76 × 10⁶ Ω/□, and a high-resolution Haacke figure of merit of Φ _H-HR = 0.1396, together with a bandgap of 2.2 eV and an electron concentration of 2.3 × 10¹⁶ cm^-3. Building on our previous report of p-type copper-oxide transparent films (Cryst. Growth Des. 2022), these n-type Cu₂O TOSs provide the complementary material platform toward copper-based p-n junction architectures using straightforward processing. Beyond junction-enabled concepts, the results offer a valuable proof of concept for stable n-type Cu₂O, with potential applications in defect studies, sensors, and optoelectronic prototypes where moderate transparency and conductivity are acceptable.