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 perfec t-crystal and 36.8 degrees symmetric tilt Sigma 5 (310) grain-boundary geom etries of SrTiO3. The relaxed model of the atomic structure of the grain bo undary 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-elect ron first-principle cluster calculations, which is discussed in Part I, is extended to clusters with single-donor impurity substitutions at the centra l titanium and strontium sites. The effects of niobium substitution at a ti tanium site and lanthanum substitution at a strontium site in the bulk and at the grain-boundary core have been investigated by determining the aspect s of the electronic Structure discussed for the accepters. The influence of grain-boundary crystallography on donor impurity incorporation has been ev aluated in terms of variations in densities of states, spatial charge densi ties, and charge populations at the grain boundary, As in Parts I and II, n o additional local lattice relaxations around the impurity are considered f or the impurity-incorporated clusters. Donor compensation mechanisms report ed in the literature are discussed in connection with the electrical activi ty of the material,; The calculations reveal that, in perfect-crystal geome try, although lanthanum exhibits almost the expected donor behavior at the strontium site, niobium does not exhibit very good donor behavior at the ti tanium site. Moreover, a decrease in the donor behavior of these impurities is observed in the grain-boundary geometry. Such variations in the electro nic behavior of donors are due to the crystallographic variations at the gr ain boundary and are likely to decrease the grain-boundary-charge and assoc iated space-charge effects in the presence of donors as compared with the i ncrease of such effects in the presence of accepters. Consequently, a decre ase in space-charge-induced segregation of donors at grain boundaries and t he effects thereof can be expected.