Effects of Annealing on the Radio Frequency Sputtered CuO/ZnO Thin Film Heterostructure for Optoelectronic Applications

ZnO and CuO thin films were deposited separately using the radio frequency (RF) sputtering technique, and the effect of annealing in nitrogen and oxygen ambient environments was investigated. In this article, structural, optical, vibrational, and electrical characterizations were sequentially performed using techniques such as X-ray diffraction (XRD), UV-visible spectroscopy (UV-vis), Raman spectroscopy, photoluminescence (PL) spectroscopy, and current-voltage measurements using a DC four-probe station. XRD confirmed a high-crystallinity and wurtzite structure for ZnO, with the preferred orientation being along the c-axis (0001), and a monoclinic structure for CuO, with preferential orientation along the (002) axis. The absorption edges of the ZnO and CuO thin films were determined to be 3.24 eV and 2.89 eV, respectively. However, Urbach tails were observed only in the ZnO thin films, confirming the presence of localized Zn interstitials and oxygen vacancies. The absorption of CuO showed weak Urbach tails, suggesting that the defects were not localized. Raman spectroscopy performed on the ZnO and CuO thin films showed the appearance of weak E₂(high) and prominent A_g/B_2g modes, confirming the presence of ZnO and CuO bonding states, respectively. PL studies revealed room temperature emission for both the CuO and ZnO thin films, which is crucial for thin film solar cells and photodetectors. Two thin film heterostructures were fabricated with and without MoS₂ (a hole transport layer) on FTO substrates. The Al/FTO/CuO/ZnO/Al heterostructure revealed a rectifying behavior with a photo current of 2 mA in the dark, whereas light-induced characteristics resulted in a photocurrent of 5 mA. The Al/FTO/MoS₂/CuO/ZnO/Al heterostructure exhibited a similar rectifying behavior, with improved photo currents of 5 mA in the dark and 9 mA in the light.