Temperature-Dependent Optical Properties of Plasmonic Titanium Nitride Thin Films

Due to their exceptional plasmonic properties, noble metals such as, gold and silver, have been the materials of choice for the demonstration of various plasmonic and nanophotonic phenomena. However, noble metals' softness, lack of tailorability, and low melting point, along with melting point depression in nanostructures as well as challenges in thin film fabrication and device integration in standard semiconductor processing, have prevented the realization of practical plasmonic devices for technologically important high temperature and heat-assisted applications. In the recent years, titanium nitride (TiN) has emerged as a promising plasmonic material with good metallic and refractory (high temperature stable) properties. The refractory nature of TiN could enable practical plasmonic devices operating at elevated temperatures for energy conversion and harsh-environment industries such as gas and oil. Here we report on the temperature-dependent dielectric functions of TiN thin films of varying thicknesses in the technologically relevant visible and near-infrared wavelength range from 330 to 2000 nm for temperatures up to 900 degrees C using in situ high temperature ellipsometry. Our findings show that the complex dielectric function of TiN at elevated temperatures deviates from the optical parameters at room temperature, indicating degradation in plasmonic properties both in the real and imaginary parts of the dielectric constant. However, quite strikingly, the relative changes of the optical properties of TiN are significantly smaller compared to its noble metal counterparts. In fact, at temperatures over 400 degrees C the quality factors of localized surface plasmon resonances and propagating surface plasmons in thin TiN films become nearly the same as those in polycrystalline noble metals. Furthermore, no structural degradation was observed in any of TiN films upon heat treatment. Using simulations, we demonstrate that incorporating the temperature-induced deviations into the numerical models leads to significant differences in the optical responses of high temperature nanophotonic systems. These studies hold the key for accurate modeling of high temperature TiN-based optical elements and nanophotonic systems for energy conversion, harsh-environment sensors, and heat-assisted applications.