Al–Ca co-doped ZnO nanostructures for enhanced electrical and dielectric properties

In this work, Al (1%) and Ca (5%) co-doped ZnO (ACZO) nanopowders were successfully synthesized using a modified sol-gel method combined with supercritical ethanol drying followed by thermal annealing. X-ray diffraction confirmed that all samples crystallize in the hexagonal wurtzite structure, with a reduction of the crystallite size from 52 nm for pure ZnO to 29 nm for ACZO due to lattice distortion induced by co-doping. SEM and EDS analyses revealed a porous morphology and a homogeneous distribution of Al and Ca within the ZnO matrix. Electrical measurements showed a clear thermally activated semiconducting behaviour, with conductivity following Jonscher's universal power law and improved charge transport in the co-doped sample compared to undoped ZnO. The activation energy extracted from DC conductivity (112 ± 5 meV) and from modulus relaxation (96 ± 5 meV) are very close, confirming that both conduction and relaxation arise from the same hopping mechanism of localized charge carriers. Impedance, Nyquist, dielectric and modulus analyses all demonstrate that Al-Ca co-doping reduces grain boundary resistance, enhances carrier mobility, and improves dielectric response. These results highlight the effectiveness of dual doping in tuning the structural and electrical properties of ZnO for potential applications in electronic, sensing, and energy devices.