Transport through singlet states in resistive memory materials: Magneli-phase, TinO2n−1 for 9 ≥ n > 3, and TiO2-HfO2 alloys

Abstract

Energy states in the forbidden band-gap below the conduction band edge are active as electron traps in nanograin high dielectric constant (κ) transition metal (TM) oxides, e.g., ZrO2 and HfO2. These TM oxides have received considerable attention for at least 10 yr as gate-dielectrics in complementary metal oxide semiconductor devices. More recently, they are emerging as candidates for charge storage and memory devices. To provide a theoretical basis for device applications, this article combines ab initio many-electron theory and x-ray absorption spectroscopy to study O K edge and TM core level transitions. These studies allow the extraction of ligand field splittings (ΔLF) for defect state features, which can then be compared with those obtained from O and TM core spectroscopic transitions, thereby providing an increased understanding of intrinsic defect bonding arrangements. These comparisons have been made for (i) elemental Ti-oxides TiO2 and Ti2O3 with different formal charge state, Ti4+ and Ti3+, respectively, and for (ii) Ti Magneli-phase alloys, TinO2n−1, n is an integer 9 ≥ n > 3, (TiO2)x(HfO2)1−x alloys. The alloys display multivalent behavior, with additional valence states associated with bond-strain, and metallic hopping transport of electrons through singlet exited states immediately below the conduction band edge. Three significant new results have been highlighted in this article. First based on comparisons with noncrystalline SiO2 and GeO2, the intrinsic defects in TM oxides have been identified as pairs of singly occupied dangling bonds in vacated (empty) O-atom bonding sites. Second, the ordering and symmetries of two-electron features identified in second derivative O K pre-edge spectra have been compared with d2 transitions described by Tanabe–Sugano diagrams. These splitting are dependent on bonding coordination and symmetry of the bordering TM atoms, sixfold octahedral arrangements for Ti, and eightfold arrangements for ZrO2 and HfO2 in cubic and tetragonal phases. ΔLF values obtained from these studies are the core level spectroscopies and defects. For the defect states, there is medium range order that extends to third and fourth nearest-neighbor TM metal–atom correlations. Finally, and equally important, these results establish that bonding defects in TM nanograin oxides and noncrystalline SiO2 and GeO2 are qualitatively similar, each indicative of different values of ΔLF and indicating different levels of partially ionic bonding.