Surface morphology smoothing of Ultra-Thin Nb films via MoCx dispersion

Metallic thin-films with atomic-level smoothness are critical to enhance the performance of advanced functional devices, particularly in the ultra-short-wave optics, superconductors, and plasmonics. Among these materials, niobium (Nb) stands out due to its exceptional physical and chemical properties, however, conventional Nb films suffer from optical and electrical performance degradation due to crystallization and islands-coalescence-induced surface coarsening. In this study, we build a novel growth-modification strategy employing molybdenum carbide (MoCx) as a dopant to engineer ultrathin Nb films with a root-mean-square (RMS) roughness (σRMS) as low as 0.128 nm. Investigation on the thickness-dependent morphological evolution during the film growth process (1–40 nm) demonstrates that the MoCx incorporation significantly refines Nb grains and flattens the surface morphology, which is crucial for lowering rough-induced performance loss. Furthermore, by periodically stacking Nb-MoCx/Si dual-layer units, we fabricate multilayer structures that maintain ultra-smooth surfaces (σRMS ∼ 0.2 nm), comparable to bare Si substrates (0.163 nm) and Nb-MoCx monolayers. This work provides an effective approach of regulating film growth and identifies a promising metallic-thin-film candidate for high-performance, low-loss ultra-short wave optical devices.