Synthesis and comprehensive characterization of neodymium-doped indium zinc oxide thin films via RF-DC magnetron co-sputtering for enhanced transparent conductive applications

Neodymium (Nd)-doped indium zinc oxide (IZO) thin films were grown by RF-DC magnetron co-sputtering to study the enhancement of structural, optical, and electrical properties, in addition to simple linear doping behavior. The research shows a non-monotonic multi-stage doping mechanism that is controlled by competing physical processes. X-ray diffraction results revealed that the crystallite and grain sizes increased with the power of Nd up to 30 W, with maximum sizes of 37.87 and 41.67 nm, respectively, and then decreased at 40 W, suggesting substitutional saturation. A similar behavior is observed for the lattice parameters, which increase to 9.16 Å at 30 W and then decrease to 9.07 Å at 40 W. The optical transparency improves of IZO from 88% in the undoped film to 92% at 20 W; however, further increases in dopant power reduce the transparency to 75% at 30 W, followed by a partial increased to 85% at 40 W. The optical bandgap decreases from 3.58 eV (undoped) to 3.40 eV at 30 W, then increases again to reach 3.53 eV at 40 W, owing to the Burstein-Moss effect. Electrically, the carrier concentration continued to increase with Nd doping, and the mobility was maximum at 30 W (85.63 cm²/Vs), with a decrease at higher power because of increased ionized impurity and grain-boundary scattering. The minimum resistivity (2.64 × 10⁻⁴ Ω·cm) was observed at 30 W, as compared to the maximum doping. The photoluminescence intensity increased continuously up to 40 W, suggesting a divergence between optical emission and transport optimization. Overall, 30 W Nd doping is the critical point of balance, where the structural quality, carrier mobility, and conductivity are maximized simultaneously. This work highlights the importance of considering competing mechanisms and solubility limitations when optimizing transparent conducting oxides for optoelectronic applications.