Ultraviolet to far-infrared dielectric function of n -doped cadmium oxide thin films

Abstract

Spectroscopic ellipsometry and Fourier transform infrared spectroscopy were applied to extract the ultraviolet to far-infrared $(150--33333\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1})$ complex dielectric functions of high-quality, sputtered indium-doped cadmium oxide (In:CdO) thin crystalline films on MgO substrates possessing carrier densities $({N}_{d})$ ranging from $1.1\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ to $4.1\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. 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 $({E}_{g})$, are shown here to be dependent upon carrier density, increasing from approximately 2.5--3 eV, while the high-frequency permittivity (${\ensuremath{\varepsilon}}_{\ensuremath{\infty}}$) decreases from 5.6 to 5.1 with increasing carrier density. The plasma frequency (${\ensuremath{\omega}}_{p}$) scales as $\sqrt{{N}_{d}}$, resulting in ${\ensuremath{\omega}}_{p}$ 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 $0.11{m}_{o}$ in unintentionally doped films ($1.1\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{4pt}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$). 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.