Abstract
High-entropy alloys (HEAs) represent a class of materials that is intensively investigated for a range of possible applications. They generally show a high degree of phase stability by the high entropy while the structure is a random atom position disorder unlike other alloys which can influence physical properties differently than regular alloys. In general, there are few studies on oxidation of HEA and studies on adding oxygen during the fabrication of HEA. In this study we have sputtered thin films of CrFeCoNiCu onto insulating and optically transparent substrates in order to measure structural, electrical and optical functional properties. We have varied the oxygen pressure in the sputtering environment as well as oxidizing the samples at elevated temperatures after deposition. Optical and electrical characterization was performed on films sputter deposited on fused quartz wafers. The films were characterized by TEM, XRD and XPS. The films with no intentional oxygen had an FCC structure with a texture showing strong (111) preferred orientation as seen by XRD. TEM analysis showed columnar morphology with twins parallel to (111) planes. Samples sputtered under high oxygen content showed a simple NaCl structure(FeO). The samples were annealed in air and O2 ambient in the temperature range of 300 to 500 °C. This caused an oxide layer growing on top of the FCC structure. XPS was utilized to find the atomic compositions and chemical stated of the elements. Hall measurements and Seebeck measurements were performed on the as prepared and oxidized films from 10 K to 600K. For the FCC structure the resistivity was a factor 104 higher than the elemental metals while can be satisfactory described by electron phonon scattering by the Bloch-Grüneisen description and the low temperature negative temperature effect by the Kondo effect. The sign of the Hall coefficient was positive while the Seebeck coefficient was negative, indicating the Fermi surface containing pockets of electrons and holes and an energy dependent scattering time. A detailed comparison of the as prepared and the oxidized thin films will be discussed in terms of the structural chemical and electrical properties of the grown films. In addition the electric properties will be discussed in terms of a model considering electronic structure and scattering.
