Gerald Lucovsky

Defect states in the forbidden band-gap below the conduction band edge are active as electron traps in nano-grain high-) transition metal (TM) oxides with thickness >0.3 nm, e.g., ZrO₂ and HfO₂. These oxides have received considerable attention as gate-dielectrics in complementary metal oxide semiconductor (CMOS) devices, and more recently are emerging as candidates for charge storage and memory devices. To provide a theoretical basis for device functionality, ab-initio many-electron theory is combined with X-ray absorption spectroscopy (XAS) to study O K edge and TM core level transitions. These studies identify ligand field splittings (ΔLF) for defect state features,. When compared with those obtained from O-atom and TM-atom core spectroscopic transitions, this provides direct information about defect state sun-nm bonding arrangements. comparisons are made for (i) elemental TiO₂ and Ti₂O₃ with different formal ionic charges, Ti⁴⁺ and Ti³⁺ and for (ii) Magneli Phase alloys, Ti_nO_2n-1, n is an integer 9≥n>3, and (TiO₂)_x(HfO₂)_1−x alloys. The alloys display multi-valent behavior from (i) different ionic-charge states, (ii} local bond-strain, and (iii) metallic hopping transport. The intrinsic bonding defects in TM oxides are identified as pairs of singly occupied dangling bonds. For 6-fold coordinated Ti-oxides defect excited states in 2nd derivative O K pre-edge spectra are essentially the same as single Ti-atom d² transitions in Tanabe-Sugano (T-S) diagrams. O-vacated site defects in 8-fold coordinated ZrO₂ and HfO₂ are described by d⁸ T-S diagrams. T-S defect state ordering and splittings are functions of the coordination and symmetry of vacated site bordering TM atoms. ΔLF values from the analysis of T-S diagrams indicate medium range order (MRO) extending to 3rd and 4th nearest-neighbor (NN) TM-atoms. Values are different for 6-fold Ti, and 8-fold ZrO₂ and HfO₂, and scale inversely with differences in respective formal ionic radii. O-vacated site bonding defects in TM nano-grain oxides are qualitatively similar to vacant-site defects in non-crystalline SiO₂ and GeO₂ for ulta-thin films, < 0.2 nm thick, and yield similar performance in MOSCAPs on Ge substrates heralding applications in aggressively-scale CMOS devices.

Three related topics are addressed in this article: (i) X-ray spectroscopy (XAS) studies of remote plasma deposited (RPD) nc-SiO₂ and nc-GeO₂ emphasizing (a) band-edge states and (b) pre-existing bonding defects; (ii) interpretation of X-ray absorption and photoemission spectra based on many electron theory, and in particluar charge transfer multiplets (CTs); and (iii) band-edge electronic structure and intrinsic defects in nc-SiO2 and nc-GeO2 thin films and their interfaces with Si and Ge substrates. The most significant result is the identification of local atomic structure in medium range order (MRO) clusters in which the pre-existing defects are embedded. These defects are vacated O-atom sites in which O-atoms have never been resident. They are confined to 1 nm scale chemically-ordered clusters distributed non-periodically with quartz-structured 4-fold coordinated Si(Ge) and 2-fold coordinated O bonding in 12-atom regular rings. Vacated O-atom sites defects are formed during processing and annealing, and reducing macroscopic as well as local bond-strain strain. They are qualitatively different, and readily distinguished from defects introduced by electrical, and by X-ray, γ-ray or high energy electron stressing of nc-SiO₂ and nc-GeO₂.

This article addresses low defect densities in hydrogenated amorphous silicon, a-Si_1-x :H _x with approximately 10 at. % bonded H, x∼0.1. Based low defect densities at mid-gap, ∼0.5 to 1×10¹⁶ cm^-3, a-Si:H thin films have been integrated into photovoltaic (PV) devices and thin film transistors (TFT's). Amorphous Si (a-Si) thin films with no detectable bonded-H have been used as precursors for polycrystalline gate electrodes in microelectronic applications. PV and TFT alloys have been deposited by glow discharge (GD), remote plasma-enhanced chemical vapor deposition (RPECVD), and reactive magnetron sputtering (RMS) with different bonded-H content determined by deposition precursors and substrate temperatures. Two conditions are required for the lowest Si dangling bond densities: (i) a monohydride, Si–H, concentration of ∼10 at. % H, and (ii) deposition, and/or a post-deposition annealing at 240 to 300 °C.

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.

Electron spin resonance (ESR) studies on bulk-quenched, noncrystalline (nc-) silica glasses (henceforth, nc-SiO2) have distinguished between (1) pre-existing process-induced defects introduced either after growth or annealing at high temperatures and (2) x-ray or γ-ray radiation or energetic electron particle-created defects. The ESR activity in these pre-existing defects is activated by x-rays. Similar pre-exiting defects have been demonstrated for remote plasma-deposited thin films of nc-SiO2 and nc-GeO2. Concentrations of pre-existing defects increase exponentially with increasing quenching and annealing temperatures. This is always the case for so-called “dry silicas” with no detectable Si–OH bonding. Nonbonding O-hole centers or nonbonding O-associated hole centers are also detected in dry silicas but only after significant x-ray, γ-ray, or energetic electron irradiation. Pre-existing defect has also been detected by second derivative O K pre-edge x-ray absorption spectroscopy in thermally grown and remote plasma-deposited nc-SiO2 and nc-GeO2 thin films. These spectra display singlet and triplet features that can be symmetry state labeled according to Tanabe–Sugano diagrams. This is demonstrated by combining ab initio theory and experiment by identifying the pre-existing defects as vacated O-atom sites in which an O-atom has never been resident.