Low-Temperature\nEpitaxy of Fe<sub>3</sub>O<sub>4</sub> Thin Films on ZnO(0001) and\nRelated Interface Studies

The heteroepitaxial\ngrowth of Fe<sub>3</sub>O<sub>4</sub> thin\nfilms on ZnO is studied in the context of opto-spintronic applications,\npaying thorough attention to interface optimization. The samples are\ngrown by pulsed laser deposition using either Fe<sub>2</sub>O<sub>3</sub> or Fe<sub>3</sub>O<sub>4</sub> targets. First, we establish\nthe Fe<sub>3</sub>O<sub>4</sub> growth windows at various oxygen partial\npressures and temperatures for optimal crystalline structure and magnetic\nproperties. The stoichiometry of iron oxide is found to be strongly\naffected by the partial pressure of oxygen in a growth chamber, the\nrole of which is shown to depend on the substrate temperature markedly,\nvaried from 200 to 600 °C. The conditions to favor the growth\nof pure FeO thin films are also established using an Fe<sub>3</sub>O<sub>4</sub> target, which is less rich in oxygen than Fe<sub>2</sub>O<sub>3</sub>. The cationic interdiffusion at the Fe<sub>3</sub>O<sub>4</sub>/ZnO interface is investigated by high-resolution scanning\ntransmission electron microscopy (HR-STEM) and secondary ion mass\nspectroscopy (SIMS). Our analysis reveals that the (111) planes of\nFe<sub>3</sub>O<sub>4</sub> grow epitaxially on the (0001) ZnO planes,\nregardless of the substrate polarity, as confirmed by reflection of\nhigh-energy electrons, X-ray diffraction, and HR-TEM analysis. SIMS\ndepth profiling results indicate a significant diffusion limitation\nof Zn cations toward the film when the growth temperature is lowered\nto 260 °C. Well-controlled ultrathin FeO buffer layers are shown\nto have a negligible role on epitaxy and interdiffusion due to their\noxidation into Fe<sub>3</sub>O<sub>4</sub> during the growth process.