Synthesis and characterization of volatile, fluorine-free β-ketoiminate lanthanide MOCVD precursors and their implementation in low-temperature growth of epitaxial CeO₂ buffer layers for superconducting electronics

A new class of volatile, low-melting, fluorine-free lanthanide metal-organic chemical vapor deposition (MOCVD) precursors has been developed. The neutral, monomeric Ce, Nd, Gd, and Er complexes are coordinatively saturated by a versatile, multidentate ether-functionalized β-ketoiminato ligand series, the melting point and volatility characteristics of which can be tuned by altering the alkyl substituents on the keto, imino, and ether sites of the ligand. Direct comparison with conventional lanthanide β-diketonate complexes reveals that the present precursor class is a superior choice for lanthanide oxide MOCVD. Epitaxial CeO₂ buffer layer films can be grown on (001) YSZ substrates by MOCVD at significantly lower temperatures (450-650 °C) than previously possible by using one of the newly developed cerium β-ketoiminate precursors. Films deposited at 540 °C have good out-of-plane (Δω = 0.85°) and in-plane (Δφ = 1.65°) alignment and smooth surfaces (rms roughness ∼ 4.3 Å). The film growth rate decreases and the films tend to be smoother as the deposition temperature is increased. High-quality yttrium barium copper oxide (YBCO) films grown on these CeO₂ buffer layers by pulsed organometallic molecular beam epitaxy exhibit very good electrical transport properties (T_c = 86.5 K, J_c = 1.08 x 10⁶ A/cm² at 77.4 K).