Electrodeposition of copper oxide thin films and characterization of their semiconducting properties

Abstract

Copper (II) Oxide (CuO) is a p-type semiconductor with potential for applications in photo-electrochemical and optoelectronic technologies due to its narrow bandgap, ability to absorb visible light, elemental abundance, and low cost. Despite numerous studies on the material, many discrepancies remain in the reported value of its bandgap, suggesting that synthesis conditions, film morphology, and post-treatment environments may influence its electronic properties. The extent to which post-annealing treatments can tune the oxide’s bandgap without inducing phase transformation remains unresolved. In this study, numerous copper depositions occurred on fluorine-doped tin oxide (FTO) andindium-doped tin oxide (ITO) glass substrates using a three-electrode electrochemical sys- tem. Metallic copper thin films were subsequently oxidized through thermal annealing to form CuO thin films. Applied voltage, along with thermal annealing temperature and time, were systematically varied to establish optimal process windows for producing stable and uniform oxide films. Analysis of Raman spectra confirmed the successful phase conversion of copper metal films to cupric oxide following thermal oxidation. Optical properties, namely the oxide’s bandgap, were characterized using UV-Vis reflectancespectroscopy and a Tauc plot analysis. Most, if not all, CuO films exhibited direct bandgap energies in the range of approximately 1.3–1.5 eV and indirect transitions near 1.1 eV. No- tably, no meaningful bandgap variation was observed between samples annealed in air and those annealed in pure oxygen, nor following secondary post-annealing treatments in oxygen or argon atmospheres. These findings indicate that under the processing conditions studied, post-annealing atmospheric modifications do not significantly alter the intrinsic electronic band structure of CuO thin films. Photoluminescence measurements were also taken and revealed band-edge emission at around1.2 eV for CuO films. Electrical resistance measurements were performed under vacuum, dry oxygen, and humid oxygen environments, those of which demonstrated atmospheric dependent conductivity changes. However, observed color change on the film’s surface and the appearance of photoluminescence features following acidic oxygen exposure suggests that electrical variations were associated with phase or compositional transformation, rather than surface defect modulation. Overall, this work demonstrates that the bandgap of electrodeposited and annealed CuO thinfilms is largely governed by its intrinsic stoichiometry established during initial formation, and that post-atmospheric treatments alone are insufficient to achieve controlled bandgap tuning without inducing phase alteration.