Anisotropic Optical Spectra of α-Al2O3 Single Crystals in the Vacuum Ultraviolet Region. I. Spectra of Absorption Tail and Reflectivity

The spectra of absorption constant in the intrinsic absorption tail region of α-Al 2 O 3 single crystals at 297 K, 78 K and 10 K are presented for the electric fields of incident light parallel to the optic axis of α-Al 2 O 3 and normal to it. The absorption tail spectra obey the Urbach rule for each polarization, indicating that absorptions in the tail region result from the direct interband transitions. The optical anisotropy characterized by the Urbach rule is interpreted in terms of the bond length O 2- –Al 3+ participating in the absorption processes. The spectra of reflectivity are presented in the range of photon energies from 4.6 eV to 120 eV for electric fields parallel to and normal to it. Spectral positions of main maxima in the reflectivity spectra for each polarization are determined at 297 K and 10 K; the spin-orbit doublet of the Al 3+ L 2,3 is revealed at 78.67 eV and 79.12 eV for the first time in the case of α-Al 2 O 3 .