Semi-conductors with partially and with completely filled ₃d-lattice bands

Attention is drawn to a class of semi-conductors or insulators with incompletely filled ₃d bands. Their lack of conductivity, if the number of electrons per atom is an integer, is explained by the circumstance that a moving electron will have a large probability of being withdrawn to the initial atom, if only the potential barriers to be penetrated are sufficiently high to reduce the frequency of transition below a certain limit. This inhibiting factor disappears, if for ions of equal electronic levels the number of electrons per atom differs from an integer. In the case of NiO this condition is fulfilled if an electron is brought by thermal excitation from the lattice ₃d band into the somewhat raised and less occupied levels a, a' (figure 1); these levels belong to Ni ions adjacent to a vacant Ni lattice point introduced by the deviations from stoichiometry, and two of these Ni ions are at the absolute zero Ni³⁺ ions. An analogous conduction mechanism holds for non-stoichiometric Cu₂O, with a completely filled ₃d band (figure 2).

Photoconductivity is generally observed with substances with completely filled zones and never with substances of the NiO type. A tentative explanation is given for this fact on the basis of the model of figure 1 and figure 2.

In non-stoechiometric ZnO vacant oxygen lattice points are assumed (figure 3); in that case the calculation of the lattice levels shows that at the lattice holes one Zn²⁺ is converted into Zn, whereas in the lattice the additional electrons form Zn⁺ ions, as will be the case after thermal transitions of the electrons belonging to these Zn atoms into the lattice 4s band.