Spectroscopic ellipsometry and Fourier transform infrared spectroscopy were applied to extract the ultraviolet to far-infrared $(150–33333\mathrm{c}{\mathrm{m}}^{-1})$ complex dielectric functions of high-quality, sputtered indium-doped cadmium oxide (In:CdO) thin crystalline films on MgO substrates possessing carrier densities $\left(N_d\right)$ ranging from $1.1\times {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$ to $4.1\times {10}^{20}\mathrm{c}{\mathrm{m}}^{-3}$. A multiple oscillator fit model was used to identify and analyze the three major contributors to the dielectric function and their dependence on doping density: interband transitions in the visible, free-carrier excitations (Drude response) in the near- to far-infrared, and IR-active optic phonons in the far-infrared. More specifically, values pertinent to the complex dielectric function such as the optical band gap $\left(E_g\right)$, are shown here to be dependent upon carrier density, increasing from approximately 2.5–3 eV, while the high-frequency permittivity (${\varepsilon }_{\infty }$) decreases from 5.6 to 5.1 with increasing carrier density. The plasma frequency (${\omega }_p$) scales as $\sqrt{N_d}$, resulting in ${\omega }_p$ values occurring within the mid- to near-IR, and the effective mass ($m^{*}$) was also observed to exhibit doping density-dependent changes, reaching a minimum of $0.11m_o$ in unintentionally doped films ($1.1\times {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$). Good quantitative agreement with prior work on polycrystalline, higher-doped CdO films is also demonstrated, illustrating the generality of the results. The analysis presented here will aid in predictive calculations for CdO-based next-generation nanophotonic and optoelectronic devices, while also providing an underlying physical description of the key properties dictating the dielectric response in this atypical semiconductor system.

• Received 26 July 2019
• Accepted 5 February 2020
• Corrected 13 August 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.025202