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

Spectroscopic ellipsometry and Fourier transform infrared spectroscopy were applied to extract the ultraviolet to far-infrared (150-33333cm-1) complex dielectric functions of high-quality, sputtered indium-doped cadmium oxide (In:CdO) thin crystalline films on MgO substrates possessing carrier densities (Nd) ranging from 1.1×1019cm-3 to 4.1×1020cm-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 (Eg), are shown here to be dependent upon carrier density, increasing from approximately 2.5-3 eV, while the high-frequency permittivity (ϵ∞) decreases from 5.6 to 5.1 with increasing carrier density. The plasma frequency (ωp) scales as Nd, resulting in ωp 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 0.11mo in unintentionally doped films (1.1×1019cm-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.