Abstract
Entropy stabilization has garnered significant attention as a new approach to designing novel materials. Much of the work in this area has focused on bulk ceramic processing, leaving entropy-stabilized thin films relatively under-explored. Following an extensive multi-variable investigation of polycrystalline (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O thin films deposited via pulsed laser deposition (PLD), it is shown here that substrate temperature and deposition pressure have strong and repeatable effects on film texture and lattice parameter. Further analysis shows that films deposited at lower temperatures and under lower oxygen chamber pressure are ∼40× less electrically resistive than otherwise identical films grown at higher temperature and pressure. Annealing these films in an oxygen-rich environment increases their electrical resistivity to match that of the films grown at higher temperatures and pressures. Because of this, the electric conductivity is hypothesized to be the result of polaron hopping mediated by transition metal valence changes which compensate for oxygen off-stoichiometry.
Highlights
Stoichiometric (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O targets were fabricated by mixing equimolar amounts of MgO [Alfa Aesar, 99.99%], CoO [Alfa Aesar, 99.7%], NiO [Sigma Aldrich, 99.995%], CuO [Alfa Aesar, 99.995%], and ZnO [Alfa Aesar, 99.99%]
Arrays of xray fluorescence tool (XRF) measurements (Table 1) confirm that cations are homogeneously distributed in these thin films, at least on a mesoscopic scale
Thin film samples grown on FTO with a lattice constant of 4.22 Å and either (111) or (002) texturing (HTHP or HTLP, respectively) demonstrate a nonlinear electrical resistance between 1 and 3 MΩcm, while (111) textured films with a lattice constant of 4.19 Å (LTLP) are significantly more conductive, with resistivity values on the order of 50 kΩcm
Summary
Stoichiometric (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O targets were fabricated by mixing equimolar amounts of MgO [Alfa Aesar, 99.99%], CoO [Alfa Aesar, 99.7%], NiO [Sigma Aldrich, 99.995%], CuO [Alfa Aesar, 99.995%], and ZnO [Alfa Aesar, 99.99%]. These were roller milled with Y-stabilized ZrO2 milling media for 6 hours. After the 8 hour hold, each pellet was removed from the 1100◦C furnace and air quenched to room temperature This sintering profile resulted in sufficiently dense (≈87%) and uniform targets that were confirmed by x-ray diffraction to be polycrystalline single phase rock salt without preferential orientation (Fig. 2). Substrates used in this study were Borosilicate [Eagle2000] glass (EXG) and Pilkington NSG TEC 15 Glass with approximately 340nm of fluorinated tin oxide coating (FTO, 13-15 Ω/sq)
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