Titanium Nitride as a Plasmonic Material from Near-Ultraviolet to Very-Long-Wavelength Infrared Range

Abstract

Titanium nitride is a well-known conductive ceramic material that has recently experienced resumed attention because of its plasmonic properties comparable to metallic gold and silver. Thus, TiN is an attractive alternative for modern and future photonic applications that require compatibility with the Complementary Metal-Oxide-Semiconductor (CMOS) technology or improved resistance to temperatures or radiation. This work demonstrates that polycrystalline TiN_x films sputtered on silicon at room temperature can exhibit plasmonic properties continuously from 400 nm up to 30 μm. The films' composition, expressed as nitrogen to titanium ratio x and determined in the Secondary Ion Mass Spectroscopy (SIMS) experiment to be in the range of 0.84 to 1.21, is essential for optimizing the plasmonic properties. In the visible range, the dielectric function renders the interband optical transitions. For wavelengths longer than 800 nm, the optical properties of TiN_x are well described by the Drude model modified by an additional Lorentz term, which has to be included for part of the samples. The ab initio calculations support the experimental results both in the visible and infra-red ranges; particularly, the existence of a very low energy optical transition is predicted. Some other minor features in the dielectric function observed for the longest wavelengths are suspected to be of phonon origin.

Keywords: infrared range; photonics; plasmonics; titanium nitride.

Figure 1

(a) Picture of one of the three series of samples; (b) SEM image of a cross-section of the stoichiometric sample; (c) composition of TiN_x samples determined from SIMS experiment as a function of the wavelength, at which the real part of the dielectric function crosses zero; (d) GIXRD patterns proving that the crystal structure $$\mathrm{Fm}\overline{3}\mathrm{m}$$ is preserved despite changes in composition; (e) EDS spectra proving that there are no other elements than Ti and N; the Si line comes from the substrate.

Figure 2

Extracted values of the real (a,c) and imaginary (b,d) part of the dielectric function in the 193 nm to 1.69 μm (a,b) or 1.7 μm to 30 μm (c,d) spectral range. Arrows in Figure (b) point to the optical transitions. Insets in Figures (b,d) show the loss function (minus the imaginary part of the reciprocal dielectric function) as a function of energy.

Figure 3

(a) Band structure and (b) imaginary part of the dielectric function.

Figure 4

(a) Experimental Raman spectra of all samples and (b) phonon band structure with a corresponding density of states.