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

J. Ryan Nolen, Evan L. Runnerstrom, Kyle P. Kelley, Ting S. Luk, Thomas G. Folland, Angela Cleri, Jon Paul Maria, Joshua D. Caldwell

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Abstract

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.

Original languageEnglish (US)
Article number025202
JournalPhysical Review Materials
Volume4
Issue number2
DOIs
StatePublished - Feb 28 2020

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All Science Journal Classification (ASJC) codes

  • Materials Science(all)
  • Physics and Astronomy (miscellaneous)

Cite this

Nolen, J. R., Runnerstrom, E. L., Kelley, K. P., Luk, T. S., Folland, T. G., Cleri, A., Maria, J. P., & Caldwell, J. D. (2020). Ultraviolet to far-infrared dielectric function of n -doped cadmium oxide thin films. Physical Review Materials, 4(2), [025202]. https://doi.org/10.1103/PhysRevMaterials.4.025202