Paper EM-TuA2
Band Edge Engineering of Barium Strontium Titanate Thin Films by Ni-doping Driven Changes in Bonding Symmetry of Ti and O

Tuesday, October 21, 2008, 2:00 pm, Room 210

Suppression of the transition from direct to trap-assisted Fowler-Nordheim tunneling in metal-insulator-metal (MIM) capacitors is reported when approximately one percent Ni-doped barium strontium titanate (BST) thin film dielectrics are substituted for undoped BST. A significant leakage current reduction and improved breakdown resistance are observed for Ni-doped BST compared to undoped BST. The origin of such a large reliability enhancement of Ni-doped BST was investigated by spectroscopic studies includng spectroscopic ellipsometry (SE) and synchrotron X-ray absorption spectroscopy (XAS) measurements. For Ni-doping at the one percent level, the spectral dependence of the imaginary part of the complex dielectric constant,ε₂, obtained from SE shows significant differences in the band edge trap depth and density relative to undoped BST revealing defect states 0.2 eV shallower in energy and seven-fold reduced in density. This change in the defect state energy in Ni-doped BST is accompanied by a change in symmetry of the Ti atom empty t_2g states from either monoclinic/orthorhombic to tetragonal: the two t_2g states higher lying in the undoped BST O K1 edge and spectroscopic ellipsomtery ε₂ spectra are merged into to a single state in Ni-doped BST, while preserving the average t_2g d-state energy. The physical origin of these changes in the band edge defects is the substitution of divalent Ni²⁺ for tetravalent Ti⁴⁺ in band edge divacancy defects. Electrical measurements of J-V traces for Ni-doped BST show a symmetric direct tunneling process while undoped BST revealed asymmetric trap assistant tunneling/Fowler-Nordheim conduction process responsible for a rapid current rise. The results on improved Ni-doped BST suggest an approach for BST MIM capacitor in gigabit dynamic random access memory as well as identify a novel band edge state engineering approach based on transition metal doping which should be applied to other oxides with perovskite structures, e.g., PbTiO₃ and PbZrO₃.