Photoelectric effects in magnesium aluminum spinel

The electronic and transport properties of magnesium aluminum spinel have been investigated photoelectrically. Photoemission of holes and electrons into spinel from metal contacts (Pt, Au, Ta, Mo, and Cu) has been used to determine barrier heights. From this information, as well as vacuum-uv photoconductivity data, we have obtained the following energy-band parameters: The Fermi level lies ∼3 eV below the bottom of the conduction band, which in turn is located ∼ 2 eV below the vacuum level, and the band gap is ∼ 9 eV. Photoemission was not observed from graphite electrodes which form Ohmic contacts. Electron, neutron, or gamma irradiation produced three photoconductivity bands of half-width ∼0.5 eV, with peaks at 4.5, 5.0, and 5.5 eV. The three bands, equally spaced in energy, may be associated with the same center. The optical absorption band of the $F$ center is in the same region of the spectrum; however, its shape did not correspond to the observed photoconductivity spectrum. A lower limit of ${10}^{-6\mathrm{}}$ ${\mathrm{cm}}^2$/V has been calculated for the $\mu \tau$ product of the photogenerated charge carriers.