Tunable Infrared Plasmonic Properties of Epitaxial Ti_1-MgN(001) Layers

Optical transmission and reflection spectra in combination with ellipsometry and transport measurements on epitaxial rocksalt structure Ti_1-MgN(001) layers with 0.00 ≤ x ≤ 0.49 are employed to explore their potential as refractory infrared plasmonic materials. A red shift in the reflection edge ℏω_e from 2.0 to 0.8 eV and the corresponding unscreened plasma energy ℏω_pu from 7.6 to 4.7 eV indicate a linear reduction in the free carrier density N with increasing x. However, nitrogen vacancies in Mg-rich samples act as donors, resulting in a minimum N = 1.6 × 10²² cm^-3 for x = 0.49. Photoelectron valence band spectra confirm the diminishing conduction band density of states and indicate a 0.9 eV decrease in the Fermi level as x increases from 0 to 0.49. The dielectric function ε = ε₁ + iε₂ can be divided into a low-energy spectral region where intraband transitions result in large negative and positive ε₁ and ε₂, respectively, and a higher energy interband transition region with both ε₁ and ε₂ > 0. The screened plasma energy E_ps that separates these two regions red-shifts from 2.6 to 1.3 eV for x = 0-0.39, indicating a tunable plasmonic activity that extends from the visible to the infrared (470-930 nm). Electron transport measurements indicate a metallic temperature coefficient of resistivity (TCR) for TiN-rich alloys with x ≤ 0.26 but weak carrier localization and a negative TCR <60 K for x = 0.39 and <300 K for x = 0.49, attributed to Mg alloying-induced disorder. The plasmonic quality factor Q is approximately an order of magnitude larger than what was previously reported for polycrystalline Ti_1-MgN, making Ti_1-MgN(001) layers competitive with Ti_1-ScN(001).