High entropy alloy CrFeNiCoCu sputter deposited films: Structure, electrical properties, and oxidation

Abstract

High entropy alloy (HEA) films of CrFeCoNiCu were prepared by sputtering. Their structure was characterized and their electric transport properties studied by temperature dependent Hall and Seebeck measurements. The HEA films show a solid solution with fcc structure. The residual electrical resistivity of the films is around 130 μΩcm which is higher than the Mott limit for a metal while the temperature dependence of the resistivity above 30 K is metal-like but with a small temperature coefficient of resistivity (2 ppm/K). The dominant scattering mechanism of charge carriers is alloy scattering due to chemical disorder in the HEA. The Hall coefficient is positive while the Seebeck coefficient is negative. This is interpreted as arising from an electronic structure where the Fermi level passes through band states having both holes and electrons as indicated by the band structure calculations. Below 30 K the conduction is discussed in terms of weak localization-and Kondo-effects. The HEA structure appears stable for annealing in vacuum, while annealing in an oxygen containing atmosphere causes the surface to oxidize and grow a Cr-rich oxide on the surface. This is then accompanied by demixing of the HEA solid solution and a decrease in the effective resistance of the film.

Controlling the Electrical Properties of Reactively Sputtered High Entropy Alloy CrFeNiCoCu Films

“…The resistivity at RT (860 μΩ cm) is high compared to Cu (1.6 μΩ cm) and also high compared to the CrFeCoNiCu-HEA (140 μΩ cm). 44 It is seen from Fig. 2b that the temperature coefficient of resistivity, TCR = ∂ln(ρ)/∂T, is always negative, with a relatively small absolute value.…”

“…Here r 0 contains resistance due to temperature-independent alloy scattering and the constant associated with weak localization. The negative coefficient r wl gives the quantum mechanical correction for weak localization, 44 and dominates the temperature dependence in Fig. 2.…”

“…2b that the temperature coefficient of resistivity, TCR = ∂ln(ρ)/∂T, is always negative, with a relatively small absolute value. So, the absolute value of the resistivity is higher than that of the Mott-Ioffe-Regel limit for metallic conductance (≈ 100 μΩ cm), 44 and given the negative TCR, the conductance in the film is not metallic. We can model the temperature dependence of the resistivity very well as seen from the solid line in Fig.…”

“…This HEA has been reported on in the literature. 1,12,25,29,[32][33][34][35][36][37][38][39][40][41][42][43][44] The crystal structure has been reported to be FCC, either as a solid solution phase with uniform composition, or as two solid solution phases with slightly different compositions. We have observed that sputtered CrFeCoNiCu has a single FCC phase.…”

“…44 With the addition of oxygen, either during deposition or by annealing in oxygen, Cr is preferentially oxidized, which alters the stability of the remaining alloy, which then decomposes into two different FCC structures, and for extended annealing time, also into a BCC structure. 44 In this work, we report on the electrical properties of reactively sputtered CrFeCoNiCu in oxygen-containing plasma, resulting in a composite film. We report on tuning properties of the composite material by varying the morphology.…”