Survey of acceptor dopants in SrTiO3: Factors limiting room temperature hole concentration

Unintentional impurities often found in strontium titanate (doped or undoped) have hindered efforts to study individual impurities experimentally. To fill this gap, a computational survey of acceptor-type point defects of common intentional or unintentional impurities (Al, Cu, Fe, K, Mg, Mn, N, Na, Ni, and Zn) is presented. Utilizing defect formation energies from density functional theory using hybrid exchange correlation functionals in a grand canonical model of the defect chemistry, the equilibrium Fermi level (mu(e)) was calculated as a function of processing conditions for pure SrTiO3, SrTiO3 individually doped with each impurity, and SrTiO3 co-doped with Al and N. Above a certain concentration, each impurity reduced the maximum predicted hole concentration relative to the intrinsic case. Al, Mg, Zn, K, and Na exhibited similar trends and behaved more like ideal acceptors while N, Ni, Fe, Mn, and Cu were all unique and pinned mu(e) near or above the mid-gap in most conditions. Al/N:SrTiO3 also exhibited similar trends at 800 degrees C for all Al/N ratios, but more variation at 25 degrees C. Additionally, the behavior of Al:SrTiO3 was not recovered until Al/N = 10(4). This suggests that to achieve SrTiO3 with free holes at room temperature, the concentration of most impurities must be controlled.