Structural, magnetic, and electrical property modulation in ZnO and V- and Cr-doped ZnO films via defect engineering

This study presents a comprehensive investigation of the structural, microstructural, morphological, magnetic, and electrical transport properties of undoped ZnO and transition metal (TM)-doped ZnO films incorporating vanadium (V) and chromium (Cr) at doping levels of 0.2, 0.4, and 0.6 at.%. The films were deposited on glass substrates using RF/DC magnetron co-sputtering technique and thoroughly characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), vibrating sample magnetometry (VSM), and Hall effect measurements. XRD analysis confirmed the formation of single-phase wurtzite ZnO with no secondary phases. The lattice parameter $a$ decreased from 3.255 Å (undoped) to 3.234 Å (0.6 at.% V) and 3.238 Å (0.6 at.% Cr), indicating substitutional incorporation of smaller V${}^{3+}$ and Cr^3＋ ions. Crystallite size decreased from 17.149 nm (undoped) to 11.251 nm (0.6 at.% V) and 12.037 nm (0.6 at.% Cr), accompanied by increased strain and dislocation density. SEM and AFM studies revealed significant grain refinement and surface roughness evolution, with RMS roughness increasing to 14.3 $\pm$ 2.3 nm for V-doped films and 14.9 $\pm$ 1.5 nm for Cr-doped films, compared to 12.1 $\pm$ 1.1 nm for undoped ZnO. Magnetic measurements confirmed room-temperature ferromagnetism, with maximum saturation magnetization ($M_s$) values of 5.41 emu/cm${}^3$ for 0.6 at.% V and 7.10 emu/cm${}^3$ for 0.4 at.% Cr. Remanent magnetization ($M_r$) followed a similar trend, consistent with the formation of bound magnetic polarons (BMPs) mediated by oxygen vacancies. Hall effect measurements revealed $n$-type conductivity in all samples, with carrier concentration decreasing from $1.0\times 10^{15}$ cm⁻³ (undoped) to $2.1\times 10^{13}$ cm⁻³ (0.6 at.% V) and $1.2\times 10^{14}$ cm⁻³ (0.6 at.% Cr). A corresponding increase in resistivity was also observed upon doping, confirming the role of V and Cr as acceptor-like defects. Transport measurements as a function of temperature and magnetic field further confirmed the presence of a defect-induced conduction mechanism, highlighting the interaction between structural disorder and magnetic/electrical behavior. These results provide valuable insights into defect engineering strategies for tailoring the multifunctional properties of ZnO-based diluted magnetic semiconductors, with promising implications for spintronic devices, magnetoresistive sensors, and charge transport-based electronic applications.