Energy of Trapped Electrons in Ionic Solids

The calculations of Simpson, on the energy of an electron localized in a crystal at an impurity carrying a single positive charge, have been extended to impurities carrying a multiple positive charge. The thermal and optical activation energies corresponding to the transition of the electron from the $1s$-orbit to the conduction band are derived. The various contributions to the polarization energy of the lattice near the impurity are analyzed in detail, which leads to an expression differing from that used by Simpson. The results are applied to a tentative interpretation of activation energies of SiC crystals as observed by Busch.