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 $2.3\times {10}^{19}$ to $4.0\times {10}^{20}\mathrm{c}{\mathrm{m}}^{\text{–}3}$ and mobilities ranging from 300 to $400\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$ 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.

Figure 1

Electronic properties as a function of power density in In:CdO thin films (a) as-deposited and (b) after a 30 minanneal at 635 °C in static oxygen.

Figure 2

(a) 2θ-ω XRD scans for heteroepitaxial post-anneal In:CdO grown on r-plane ${\mathrm{Al}}_2{\mathrm{O}}_3$ with varying In content. (b) ω rocking curves for the same set of films. FWHM values are annotated for each rocking curve.

Figure 3

AFM height retrace images of post-anneal In:CdO thin films with carrier concentrations of (a) $2.7\times {10}^{19}\mathrm{c}{\mathrm{m}}^{\text{–}3}$, (b) $1.2\times {10}^{20}\mathrm{c}{\mathrm{m}}^{\text{–}3}$, and (c) $3.9\times {10}^{20}\mathrm{c}{\mathrm{m}}^{\text{–}3}$.

Figure 4

Residual resistivity ratio (RRR, ${\mathit{\rho }}_{300\mathit{K}}/{\mathit{\rho }}_{3\mathit{K}}$), out-of-plane thermal conductivity (${\mathit{\kappa }}_{\mathrm{total}}$), and electronic contribution of thermal conductivity (${\mathit{\kappa }}_{\mathit{e}}$), for annealed In:CdO films.

Figure 5

Tauc plots from UV-VIS transmission data for annealed In:CdO films, showing a shift in the optical absorption threshold with increased doping. The sharp feature around 2.9 eV occurs due to a filter switch in the instrument.

Figure 6

(a) Simulated and (b) experimental reflectometry maps of ENZ modes in 29 nm In:CdO sample with $n=2.7\times {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$ and $\mu =281\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$.

Figure 7

Normalized experimental ATR-IR curves for In:CdO at various doping levels at a 50 ° incident angle.