Single phase, epitaxial high-entropy (Mg,Ca,Co,Ni,Zn)O thin films by pulsed laser deposition

We present a detailed study of properties of a multi-component pulsed laser deposition (PLD) target containing MgO, CaO, CoO, NiO, and ZnO obtained from solid state synthesis and of PLD high-entropy oxide (HEO) (Mg0.15Ca0.18Co0.25Ni0.26Zn0.16)O thin films obtained from the same target. While x-ray diffraction (XRD) θ-2θ diffractograms and scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDX) measurements demonstrate phase separation for the equilibrium synthesis route of the bulk ceramic target, HEO synthesis on crystalline substrates as single phase, epitaxial thin films succeeds by being deposited with the far from equilibrium PLD process. For film growth at 450 and 600 °C on MgO (001), a fully relaxed, epitaxial single crystalline rock salt [001] oriented HEO thin film is obtained. Two rotational domains are observed for [111] oriented films grown on α-Al2O3 (0001). HEO thin films grown on amorphous SiO2 are x-ray amorphous independent of the substrate temperature during growth but suffer from mechanical instability for high thermal energy growth conditions. The results obtained in this work show that solubility limits in bulk are exceeded in thin films through nonequilibrium deposition conditions and that mechanical stability and structural properties of HEO films heavily depend on the substrate-film interface.