The effect of titanium-zi̇rconium doping on some physical properties of CuO thin films

In this study, X-ray diffraction (XRD) analyses were conducted to investigate the effect of dopant incorporation on the structural properties of copper oxide thin films. Undoped, Zr-doped, and Ti-doped CuO thin films were annealed at 500 °C to promote the formation of crystalline structures, and their crystal properties were analyzed in detail using a Rigaku Ultima III XRD system. All samples exhibited a monoclinic structure with a polycrystalline nature. The presence of dopants did not alter the main phase structure; the characteristic diffraction planes of the CuO phase were preserved. While diffraction peaks corresponding to the (110), (−111), and (200) planes were observed in undoped CuO films, in doped films the peaks associated with the (−111) and (200) planes became more pronounced, whereas the intensity of the (110) peak decreased and, in some cases, disappeared entirely. This indicates that dopant elements influence the crystal orientation and modify the preferential growth behavior. The dislocation density and microstrain values corresponding to the (−111) and (200) crystal planes revealed that internal stresses and crystalline defects within the material varied depending on the type of dopant. Based on the obtained data, an increase in microstrain was found to result in a reduction in crystallite size, suggesting that internal stresses accumulated in the crystal structure limit crystal growth, thereby leading to the formation of smaller crystallites. Consequently, an inverse relationship between crystallite size and microstrain was established. In this research, the optical properties of undoped, titanium (Ti)-doped, zirconium (Zr)-doped, and Ti/Zr co-doped CuO thin films were systematically investigated. The thin films were deposited onto glass substrates using the widely adopted spin coating method, which ensures uniform film formation. The inclusion of Ti, Zr, and their combination reduced the absorption and absorption coefficients of CuO films, thereby increasing light transmittance and potentially enhancing their suitability for photovoltaic applications by facilitating charge carrier collection. Fundamental optical parameters such as refractive index, high-frequency dielectric constant, and static dielectric constant were calculated using established theoretical models including the Moss relation, Herve and Vandamme model, and Ravindra equation. The consistency of results obtained from these different theoretical approaches demonstrated the robustness and reliability of the optical characterizations. Among all evaluated samples, the undoped CuO thin film exhibited the highest absorption coefficient in the visible spectrum, indicating superior light harvesting capability, which a key factor for improving solar energy conversion efficiency. Additionally, the undoped CuO also demonstrated the highest optical conductivity in the wide band gap region, which is particularly advantageous for both solar cells and photodetectors. For undoped, 1% Zr, 1% Ti, 1% Zr + 1% Ti, and 1% Zr + 2% Ti doped CuO thin films, the relationship between absorption coefficient and photon energy (hν) was evaluated using the Tauc equation. Optical band gap values were determined from 〖(αhν)〗^2–hν plots, and an increase from 1.64 eV to 2.10 eV was observed depending on the doping concentrations. This band gap widening indicates the successful incorporation of dopant elements into the CuO crystal lattice, resulting in a reduction in carrier concentration. It is also considered that structural defects, modifications at grain boundaries, and improvements in crystal quality influenced the energy band gap. This effect requires higher energy photons to be absorbed due to an increase in free carrier concentration, thereby expanding the optical band gap. In addition to the band gap, dopants had significant effects on the optical transmittance properties of CuO films. Notably, the CuO thin film doped with 1% Zr + 2% Ti exhibited a transmittance of approximately 47% in the visible region, offering a substantial advantage for photovoltaic applications. This increased transmittance is believed to stem from improvements in crystal structure and increased surface smoothness, which reduce light scattering and enhance the optical performance of the film. CuO thin films with undoped and doped with various concentrations of Zr and Ti were deposited onto n-type Si substrates to form heterojunction diode structures, and their electrical properties were thoroughly examined. The inclusion of 1% Zr in the CuO layer was found to improve the rectification ratio of the diodes. Electrical characterization was conducted within the framework of Thermionic Emission (TE) theory, and parameters such as ideality factor, barrier height, and leakage current were calculated. According to the obtained results, the diode with 1% Zr doping exhibited an ideality factor of 2.128, a barrier height of 0.689 eV, and a leakage current of 2.1 × 10-9 A. These values support the role of doping in regulating carrier transport and facilitating potential barrier formation at the heterojunction. Although electrical improvements were observed for other doping types and concentrations as well, ideality factors greater than 1 in all samples indicate deviations from ideal Schottky behavior.