Energy Levels of Iron and Aluminum in SrTiO3

An attempt is made to locate the energy levels associated with iron and aluminum in the band gap of SrTi${\mathrm{O}}_3$. A relative energy-level scheme for these impurities is derived from data in the literature, principally ESR and optical absorption. Measurements of thermally stimulated current, electrical conductivity, and thermoelectric effect are reported in this paper. These results suggest specific energy-level assignments as follows: oxygen-ion-vacancy donors 0.085 eV below the conduction band; ${\mathrm{Al}}^{3+}$ acceptor, 0.18 eV; ${\mathrm{Fe}}^{5+}$ acceptor, 0.75 eV; ${\mathrm{Fe}}^{4+}$ acceptor, 1.06 eV; and ${\mathrm{Fe}}^{3+}$-vacancy acceptor 1.35 eV above the valence band. Other levels whose identities are unknown are acceptors at 0.50 and 0.65 eV from the valence band and a donor 0.35 eV from the conduction band. The possibility is discussed that the acceptor ${\mathrm{Fe}}^{3+}$ vacancy acts to couple the bound-free vacancy equilibrium to the electron-hole equilibrium.