Excitation-dependent photoluminescence, conduction mechanisms via impedance spectroscopy, and DRT investigations in Al-doped ZnO ceramics

Abstract

This study presents the synthesis and comprehensive characterization of Al-doped ZnO (AZO) ceramics, emphasizing their structural, dielectric, and electronic properties. A novel contribution of this work is the first detailed correlation of AZO impedance behavior with the distribution of relaxation times (DRT), providing deeper insights into charge carrier dynamics. AZO ceramics were prepared via a reduction method. X-ray diffraction (XRD) and Rietveld refinement confirmed the formation of a single-phase wurtzite structure. Field-emission scanning electron microscopy (FESEM) was used to investigate the ceramic morphology, revealing a maximum average grain size of 0.50 μm for AZO2 (Zn_0.98Al_0.02O/2%). Temperature-dependent dielectric analysis revealed two dielectric anomalies in all samples. AZO6 (6% Al-doped ZnO) exhibited a maximum dielectric constant of 74 400 at 5 kHz. Impedance spectroscopy and DRT analysis were used to investigate the thermal conduction mechanisms of charge carriers associated with grains and grain boundaries. Conduction in ZnO and AZO6 followed a correlated barrier hopping model, while a non-overlapping small polaron tunneling model better described conduction in AZO2 and AZO4 (4% Al-doped ZnO). AZO2 exhibited the highest saturation polarization (P_s = 27.74 μC cm⁻²) and remanent polarization (P_r = 25.56 μC cm⁻²). The bandgap of the AZO ceramics ranged from 3.000 to 3.044 eV. Excitation-dependent photoluminescence (PL) was observed, with CIE chromaticity coordinates shifting from cyan to French blue with increasing excitation wavelength for each ceramic. These findings position AZO ceramics as promising candidates for advanced electronic and optoelectronic applications.