Epsilon-near-Zero Modes and Surface Plasmon Resonance in Fluorine-Doped Cadmium Oxide Thin Films

In this report we demonstrate fluorine-doped CdO as a model infrared plasmonic material by virtue of its tunable carrier density, high mobility, and intense extreme-subwavelength plasmon-polariton coupling. Carrier concentrations ranging from 10¹⁹ to 10²⁰ cm^-3, with electron mobility values as high as 473 cm²/V·s, are readily achieved in epitaxial CdO films over a thickness range spanning 50 to 500 nm. Carrier concentration is achieved by reactive sputtering in an Ar/O₂ atmosphere with trace quantities of CF₄. Infrared reflectometry measurements demonstrate the possibility of near-perfect plasmonic absorption through the entire mid-IR spectral range. A companion set of reflectivity simulations are used to predict, understand, and optimize the epsilon-near-zero plasmonic modes. In the context of other transparent conductors, CdO exhibits substantially higher electron mobility values and thus sharp and tunable absorption features. This highlights the utility of high-mobility transparent conducting oxides as a materials system for supporting strong, designed light-matter interactions.