Electronic structure of pristine and solute-incorporated SrTiO₃: III, perfect-crystal, grain-boundary geometry, and acceptor doping

Electronic structure is investigated for donor-impurity-incorporated perfect-crystal and 36.8° symmetric tilt Σ5 (310) grain-boundary geometries of SrTiO₃. The relaxed model of the atomic structure of the grain boundary used in the present investigations is the same as that used in Part II, as derived by Ravikumar et al. using lattice-statics simulations based on pair-potential calculations. As in Part II, the methodology of one-electron first-principle cluster calculations, which is discussed in Part I, is extended to clusters with single-donor impurity substitutions at the central titanium and strontium sites. The effects of niobium substitution at a titanium site and lanthanum substitution at a strontium site in the bulk and at the grain-boundary core have been investigated by determining the aspects of the electronic structure discussed for the acceptors. The influence of grain-boundary crystallography on donor impurity incorporation has been evaluated in terms of variations in densities of states, spatial charge densities, and charge populations at the grain boundary. As in Parts I and II, no additional local lattice relaxations around the impurity are considered for the impurity-incorporated clusters. Donor compensation mechanisms reported in the literature are discussed in connection with the electrical activity of the material. The calculations reveal that, in perfect-crystal geometry, although lanthanum exhibits almost the expected donor behavior at the strontium site, niobium does not exhibit very good donor behavior at the titanium site. Moreover, a decrease in the donor behavior of these impurities is observed in the grain-boundary geometry. Such variations in the electronic behavior of donors are due to the crystallographic variations at the grain boundary and are likely to decrease the grain-boundary-charge and associated space-charge effects in the presence of donors as compared with the increase of such effects in the presence of acceptors. Consequently, a decrease in space-charge-induced segregation of donors at grain boundaries and the effects thereof can be expected.