High temperature semiconducting characteristics of magnesium-doped α-Al2O3 single crystals

In the temperature interval 300–773 K, ac and dc electrical measurements show a phenomenally large enhancement of the electrical conductivity of otherwise excellent insulator ${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$ crystals when doped with magnesium impurities. The conductivity increases linearly with the concentration of hole-trapped $[\mathrm{Mg}{]}^0$ centers and is four times higher in the direction parallel to the crystallographic c axis than in the perpendicular direction. The conductivity activation energy is 0.68 eV, independent of both $[\mathrm{Mg}{]}^0$ content and crystallographic orientation. Electroluminescence experiments indicate that holes are the majority carriers. These results favor the small-polaron-motion mechanism.