Epitaxial titanium nitride on sapphire: Effects of substrate temperature on microstructure and optical properties

Titanium Nitride (TiN) is a mechanically-robust, high-temperature stable, metallic material receiving considerable attention for resilient plasmonics. In this work, we fabricated six hetero-epitaxial TiN films on sapphire using controllably unbalanced reactive magnetron sputtering. We examined the effect of substrate growth temperature on the plasmonic and crystalline quality of the film. Optical properties of all films were obtained from spectroscopic ellipsometry; plasmonic quality factors were determined from the real and imaginary parts of the dielectric function. We determined crystallinity using X-ray diffraction and surface morphology using atomic force microscopy. X-ray diffraction showed (111) TiN peaks with Pendellösung fringes indicating consistent hetero-epitaxy. Atomic force microscopy showed smooth surfaces with RMS surface roughness ranging from 0.2-2.6 nm. Based on this characterization, we determined that the substrate deposition temperature of 550˚C yielded (111)-oriented hetero-epitaxial TiN with minimal surface roughness. We found that 550˚C also gave highest plasmonic quality factors for all wavelengths, approaching the values of today’s best plasmonic materials (such as Au and Ag). Further, the Q-factors at wavelength 1550 nm inversely correlated with calculated lattice constants indicating stoichiometric tuning of plasmonic properties. Our results indicate that the plasmonic response of TiN is directly linked with structural quality and stoichiometry of the film