Ordinary dielectric function of corundumlike α -Ga 2 O 3 from 40 meV to 20 eV

The linear optical response of metastable $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ is investigated by spectroscopic ellipsometry. We determine the ordinary dielectric function from lattice vibrations up to the vacuum ultraviolet spectral range at room temperature for a sample with a $(0001)$ surface. Three out of four ${E}_{\mathrm{u}}$ infrared-active phonon modes are unambiguously determined, and their frequencies are in good agreement with density functional theory calculations. The dispersion of the refractive index in the visible and ultraviolet part of the spectrum is determined. High-energy interband transitions are characterized up to $20\phantom{\rule{0.222222em}{0ex}}\mathrm{eV}$. By comparison with the optical response of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ and with theoretical results, a tentative assignment of interband transitions is proposed.