Low-Temperature Epitaxy of Fe₃O₄ Thin Films on ZnO(0001) and Related Interface Studies

The heteroepitaxial growth of Fe3O4 thin films on ZnO is studied in the context of opto-spintronic applications, paying thorough attention to interface optimization. The samples are grown by pulsed laser deposition using either Fe2O3 or Fe3O4 targets. First, we establish the Fe3O4 growth windows at various oxygen partial pressures and temperatures for optimal crystalline structure and magnetic properties. The stoichiometry of iron oxide is found to be strongly affected by the partial pressure of oxygen in a growth chamber, the role of which is shown to depend on the substrate temperature markedly, varied from 200 to 600 °C. The conditions to favor the growth of pure FeO thin films are also established using an Fe3O4 target, which is less rich in oxygen than Fe2O3. The cationic interdiffusion at the Fe3O4/ZnO interface is investigated by high-resolution scanning transmission electron microscopy (HR-STEM) and secondary ion mass spectroscopy (SIMS). Our analysis reveals that the (111) planes of Fe3O4 grow epitaxially on the (0001) ZnO planes, regardless of the substrate polarity, as confirmed by reflection of high-energy electrons, X-ray diffraction, and HR-TEM analysis. SIMS depth profiling results indicate a significant diffusion limitation of Zn cations toward the film when the growth temperature is lowered to 260 °C. Well-controlled ultrathin FeO buffer layers are shown to have a negligible role on epitaxy and interdiffusion due to their oxidation into Fe3O4 during the growth process.