Tunable Infrared Plasmonic Properties of Epitaxial Ti1-xMgxN(001) Layers

Optical transmission and reflection spectra in combination with ellipsometry and transport measurements on epitaxial rocksalt structure Ti1-xMgxN(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 h omega(e) from 2.0 to 0.8 eV and the corresponding unscreened plasma energy h omega(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 x 10(22) 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 epsilon = epsilon(1) + i epsilon(2) can be divided into a low-energy spectral region where intraband transitions result in large negative and positive epsilon(1) and epsilon(2), respectively, and a higher energy interband transition region with both epsilon(1) and epsilon(2) > 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 Ti1-xMgxN, making Ti1-xMgxN(001) layers competitive with Ti1-xScxN(001).

Automated summary

The optical transmission and reflection spectra of Ti1-xMgxN(001) layers were studied, showing a linear reduction in free carrier density with increasing Mg content x. Nitrogen vacancies in Mg-rich samples act as donors, resulting in a minimum free carrier density of 1.6 x 10(22) cm(-3) for x = 0.49. The materials have potential as refractory infrared plasmonic materials.