Ceria is a transparent oxide suitable for various optical and optoelectronic devices. In this work, we tailor independently the refractive index n and fundamental gap Eg of nanocrystalline Ceria films by varying the substrate temperature or using Ar+ ion beams during growth with electron beam evaporation. Spectroscopic ellipsometry and x-ray reflectivity are employed to study n and Eg and to identify the physical parameters that affect them. We correlate n (varies from 1.65 to 2.15 in the studied films) with the film density through a universal, square law. The film composition strongly affects Eg, which varies from 2.8 to ∼2.0 eV. The optical absorption below 3 eV and the Eg shift are attributed to O-defect states and not to modifications in interband transitions.
Conductive nitrides, such as TiN, are key engineering materials for electronics, photonics, and plasmonics; one of the essential issues for such applications is the ability of tuning the conduction electron density, the resistivity, and the electron scattering. While enhancing the conduction electron density and blueshifting the intraband absorption towards the UV were easily achieved previously, reducing the conduction electron density and redshifting the intraband absorption into the infrared are still an open issue. The latter is achieved in this work by alloying TiN by rare earth (RE = Sc, Y, La) or alkaline earth (AE = Mg, Ca) atoms in Ti substitutional positions. The produced TixRE1−xN and TixAE1−xN thin film samples were grown by a hybrid arc evaporation/sputtering process, and most of them are stable in the B1 cubic structure. Their optical properties were studied in an extensive spectral range by spectroscopic ellipsometry. The ellipsometric spectra were analyzed and quantified by the Drude-Lorentz model, which provided the conduction electron density, the electron mean free path, and the resistivity. The observed interband transitions are firmly assigned, and the optical and electrical properties of TixRE1−xN and TixAE1−xN are quantitatively correlated with their composition and crystal structure.
Conductive transition metal nitrides are emerging as promising alternative plasmonic materials that are refractory and CMOS-compatible. In this work, we show that ternary transition metal nitrides of the B1 structure and consisting of a combination of group-IVb transition metal, such as Ti or Zr, and group III (Sc, Y, Al) or group II (Mg, Ca) elements can have tunable plasmonic activity in the infrared range in contrast to Ta-based ternary nitrides, which exhibit plasmonic performance in the visible and UV ranges. We consider the intrinsic quality factors of surface plasmon polariton for the ternary nitrides, and we calculate the dispersion of surface plasmon polariton and the field enhancement at the vicinity of nitride/silica interfaces. Based on these calculations, it is shown that among these nitrides the most promising are TiScN and TiMgN. In particular, TiScN can have plasmonic activity in the usual telecom bands at 850, 1300, and 1550 nm. Still, these nitrides exhibit substantial electronic losses mostly due to fine crystalline grains that deteriorate the plasmonic field enhancement. This unequivocally calls for improved growth processes that would enable the fabrication of such ternary nitrides of high crystallinity.
The optical and electronic properties of rocksalt structure tungsten nitride (B1-WN) were investigated by x-ray photoelectron spectroscopy (XPS) and UV–visible-Fourier transform infrared optical reflectivity. Both 111-textured polycrystalline and epitaxial WN(111) films with [N]/[W] ratios of 1.12 and 0.87, respectively, were found to be electron conductors with partially filled W-5d conduction bands. However, their electronic behavior is dominated by high conduction electron losses, which are attributed to scattering at both anion and cation vacancies and are more pronounced for films with high nitrogen content, yielding high resistivity values of 1.4–2.8 mΩ cm. The dielectric function is well described with a Drude–Lorentz model over a large wavelength range from 0.2 to 100 μm, and exhibits an ε1 that becomes negative above a relatively high critical wavelength that increases with increasing nitrogen content from 22 to 100 μm. Compositional interpolation of XPS data provides a W4f7/2 electron binding energy for pure stoichiometric B1-WN of 31.9 eV, while increasing the N-content results in a reduction of the density of states from the W-5dt2g bands at and near the Fermi level. The overall results do not confirm the predicted promising plasmonic properties of B1-WN but instead reveal possible alternative applications for this compound as photothermal or epsilon-near-zero material.
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