Fabrication and Characterization of High-Quality Epitaxial Nanocolumnar Niobium Films with Abrupt Interfaces on YSZ(001)

Niobium (Nb) is a promising refractory metal with a wide variety of technological applications in nanoelectronics, optoelectronics, photonics, and energy-related technologies. However, to further advance the field of nanoelectronics and nanophotonics, very high-quality Nb films with excellent structural order are needed. While much progress has been made in understanding the heteroepitaxy of Nb on oxide substrates, the underlying fundamental mechanisms, especially to realize films showing abrupt interfaces with the substrate and without needing the expensive equipment and/or processing under extreme conditions, remain a challenge. In this context, herein, we demonstrate an approach to stabilize the epitaxial, nanocolumnar bcc Nb films with highly ordered and abrupt interfaces on YSZ(001) substrates using simple and industrially widely adopted sputter deposition. 90 nm Nb films deposited onto YSZ(001) at 500 °C exhibit a strained, epitaxial structure. As evidenced in X-ray diffraction and transmission electron microscopy analyses, the structural quality of Nb films is driven by substrate-assisted selective nanocrystallization. Coupled with granular morphology and high structural quality, the optical properties measured by spectroscopic ellipsometry and reflectivity indicate highly reflective Nb films with enhanced optical constants. Corroborated with surface/interface quality, microstructure, and optical properties, the epitaxial Nb films exhibit excellent mechanical characteristics. The hardness and elastic modulus of the Nb epitaxial films were 18 and 240 GPa, respectively, which are attributed primarily to the growth of a compact nanocolumnar Nb epitaxial film on YSZ.