Epitaxial Growth and Characterization of Ruddlesden-Popper Nickelates (La_n+1Ni_nO_3n+1)

The rare-earth nickelates, as potential analogs to cuprates, have been extensively studied for their promise in achieving high-temperature superconductivity. Here, we report on the crystalline quality of a series of Ruddlesden–Popper nickelates (La_n+1Ni_nO_3n+1), with n values up to 7, grown epitaxially using oxide molecular beam epitaxy. Utilizing a shutter-controlled growth technique, we achieve layer-by-layer growth with precise control, resulting in high-quality RP members with high n values. High resolution X-ray diffraction (XRD) measurement confirms the crystalline quality of epitaxial c-axis oriented nickelate films, revealing a linear relationship between the c-axis lattice parameter and the n value of nickelates. All films exhibit (00L) diffraction peaks for all superlattice peaks, even for thin films with 5 formula units. Our XRD and rocking curve analysis further indicate that growth temperature and annealing conditions play a crucial role in determining the crystalline quality. Atomic force microscopy revealed smooth film surfaces with RMS roughness of around 3-5 Å. We also investigated transport properties, finding a clear dependence on the n value, with conductivity increasing as n increases. Additionally, our results highlight the strong influence of crystalline quality on the transport properties. Future studies will focus on optimizing growth conditions to further enhance the crystalline quality and investigate high Tc superconductivity phenomena, potentially advancing the realization of high-temperature superconductivity in layered nickelate-based materials.