Influence of cobalt doping on the crystalline structure, optical and mechanical properties of ZnO thin films

Uniform and transparent thin films of Zn_1 − xCo_xO (0 ≤ x ≤ 0.10) were fabricated by sol-gel spin coating technique. Co addition up to x = 0.075, led to refinement in structure and improvement in film quality together with average grain size reduction from 17 nm in undoped ZnO to 15 nm with x = 0.05 and 12 nm with x = 0.10 Co additions. For x ≥ 0.035, CoO (cubic) was detected as the secondary phase. Influence of Co addition on the volume fraction of grain boundaries has been interpreted. Increase in Co content in the range 0 ≤ x ≤ 0.10 led to quenching of near-band edge and blue emissions, decrease in band gap energy (E_g) from 3.36 eV to 3.26 eV, decrease in film thickness and refractive index and an increase in extinction coefficient of Zn_1 − xCo_xO thin films. The change in nature of stress from compressive to tensile with lower to higher doping of Co is corroborative with the angular peak shift of (002) plane of ZnO lattice. An overall increase in microhardness of Zn_1 − xCo_xO thin films up to x = 0.05 is attributed to change in microstructure and evolution of secondary phase and as the secondary phase separates out the overall stress is released leading to lowering of hardness after this concentration. Hall-Petch behavior is also studied and found to obey until x = 0.05, however, considerable deviation after this dopant concentration is attributed to the increase in the volume fraction of grain boundaries, which results from the secondary phase separation from this dopant concentration.