Correlating the structural, optical and surface electronic properties of sputtered Co ₃ O ₄ films

Abstract Cobalt tetraoxide (Co 3 O 4 ) films were grown on indium tin oxide (ITO) substrates using a reactive direct current (DC) magnetron sputtering system. The oxygen flow rate was systematically varied from 5.0 to 7.0 standard cubic centimeter per minute (sccm) to investigate its influence on the structural, optical, and surface electronic properties of the films. X-ray diffraction (XRD) patterns and Raman spectroscopy confirmed the formation of the cubic spinel Co 3 O 4 phase, exhibiting a decrease in lattice strain and an increase in crystallinity with the increase of the oxygen flow rate. A non-monotonic trend was observed in the intensity of the optical absorption band at 1.67 eV, which was found to be related to the non-linear changes in the Co 3+ /Co 2+ ratio. Furthermore, Kelvin probe force microscopy (KPFM) measurements revealed that the oxygen flow rate enables the tuning of the film’s surface potential and work function. To quantitatively evaluate the relation among all measured variables, a statistical correlation analysis was conducted, revealing a strong negative correlation from the oxygen flow rate and lattice strain (r = −0.89), as well as between the Co 3+ /Co 2+ ratio and the absorption coefficient at 1.67 eV (r = −0.82). These findings demonstrate the complex interplay of the reactive sputtering oxygen flow rate with the resulting physical properties of the Co 3 O 4 films, providing valuable insights into how a single deposition parameter can be strategically exploited to tailor the material’s fundamental characteristics.