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

Abstract

Highlights: What are the main findings? ZnO and CuO thin films prepared by RF sputtering were optimized under varying annealing temperatures and ambient conditions, leading to improved structural quality. Annealed ZnO films showed increased grain size and strong sub-band absorption, with a PL band-edge at 3.27 eV. Annealed CuO films exhibited direct band-to-band absorption at 2.89 eV due to the disappearance of defect-related sub-bands. Raman spectroscopy and XRD analysis confirmed reduced defects and enhanced crystallinity in annealed CuO films. Al/FTO/MoS2/CuO/ZnO/Al heterostructure demonstrated improved photocurrents, achieving ~5 mA in the dark and ~9 mA under illumination. What are the implications of the main findings? The ZnO/CuO heterostructure device shows improved photocurrent response with the insertion of MoS2 as a hole transport layer, suggesting a viable pathway for high-performance heterostructure devices. 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 Ag/B2g 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.