Scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray (EDX) spectroscopy, and x-ray photoelectron (XPS) spectroscopy were used to study the microstructure and surface chemistry of NiFeCrCo-on-Si(001) thin films grown by magnetron sputtering at 100 °C using a near-equiatomic NiFeCrCo target. It was revealed that the high entropy alloy (HEA) thin films predominantly exhibit 111-oriented face-centered-cubic submicrometer columns. The diameter of the columnar grains increased progressively along the growth direction. Two types of structural defects were identified with regular orientations. One type formed between adjacent columnar grains, while the other originated within individual columns, consisting of nanotwins and stacking faults, inclined to the columnar growth direction. Their densities increased in the regions near the film surface. XPS and EDX indicated a surface oxidation tendency of Cr Fe ∼ Co Ni, despite compositional variations (±2.0 at. %) induced by varied sputtering power, which correlated well with the segregation-induced surficial elemental distributions. These findings provide new insights into the growth behavior of HEA thin films produced by magnetron sputtering.
Ng et al. (Fri,) studied this question.