Mid-wave to near-IR optoelectronic properties and epsilon-near-zero behavior in indium-doped cadmium oxide

Indium-doped cadmium oxide (In:CdO) thin films exhibit tunable epsilon-near-zero (ENZ) modal frequencies across a wide spectral range, bridging the mid-wave and near-infrared (IR). In:CdO thin films are prepared by reactive cosputtering from metallic Cd and In targets using high-power impulse magnetron sputtering (HiPIMS) and radio frequency sputtering, respectively. Using this approach, CdO thin films with carrier concentrations ranging from $2.3\ifmmode\times\else\texttimes\fi{}{10}^{19}$ to $4.0\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}3}$ and mobilities ranging from 300 to $400\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{V}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$ are readily achieved. UV-VIS absorption spectra are used to measure optical bandgap, revealing a Burstein-Moss shift of 0.58 eV across the doping range investigated. Optical measurements demonstrate the tunability of near-perfect plasmonic ENZ absorption across the mid-wave and into the near-IR spectral ranges by controlling the carrier concentration through doping, while tuning the film thickness for impedance matching. In comparison to other dopants that can be introduced to HiPIMS-deposited CdO, In offers the largest range of carrier concentrations while maintaining high mobility, thus allowing for the widest accessibility of the IR spectrum of a single plasmonic material grown by sputtering.