Complex optical properties of single crystal and atomic layer deposited alumina from the terahertz to vacuum ultraviolet spectral range

Abstract

Anisotropic optical properties of single crystal Al2O3 and atomic layer deposited (ALD) thin film alumina have been studied from 0.42 meV to 8.5 eV using spectroscopic ellipsometry. Generalized spectroscopic ellipsometry is used to obtain the complex dielectric function (ɛ = ɛ1 ± iɛ2) and complex refractive index (N = n + ik) spectra of Al2O3 single crystals for electric fields oscillating parallel (extraordinary) and perpendicular (ordinary) to the optic axis from the terahertz (THz) to vacuum ultraviolet (VUV) spectral ranges. Spectra in ɛ in the infrared (IR) spectral range show four transverse optical (TO) phonon modes at 385.5, 441.1, 568.6, and 633.1 cm−1 and four longitudinal optical (LO) phonon modes at 386, 481.5, 629, and 903.34 cm−1 along the ordinary direction. Two TO modes at 400.1 and 583.9 cm−1 and two LO phonon modes at 510.79 and 887.209 cm−1 are observed in the extraordinary direction. Birefringence is ∼0.011 in the near-IR to VUV and ∼0.34 in the THz spectral ranges. The refractive index (n) for amorphous ALD thin film alumina is smaller than that obtained for both directions of single crystal alumina, except from ∼6.8 to 7.6 eV where n for ALD alumina is greater due to an absorption onset at lower photon energies and Kramers–Kronig consistency. The single IR vibrational mode observed at 586 cm−1 for the ALD film is broader and has a smaller amplitude than that for all TO phonon modes observed for single crystal alumina along both principal directions.