The refractory metal disilicides TiSi2 and TaSi2 were investigated for their usefulness as dopant diffusion sources. During furnace annealing and rapid thermal processing, strong decomposition reactions occur between the dopants D (B or As) and the respective silicide (MSi2) to form MxDy compounds. With the help of special sample preparation methods and various analytical techniques, the compound phases TiB2, TiAs, TaB2, and TaAs were unambiguously detected. The fraction of freely diffusing B in TaSi2 is determined to be below 5% of the total dose; by far, the major part of the dopant is bound within the TaB2 phase detected. Careful sample preparation and analysis of secondary-ion-mass spectrometry profiles is necessary to avoid artifacts caused by these compound particles. The MxDy-compound formation has detrimental consequences: The solubility of arsenic and even more of boron in TiSi2 and TaSi2 is limited to rather low-concentration levels (e.g., B in TaSi2: 4 × 1018 B/cm3 < CB(900 °C) < 1.6 × 1019 B/cm3) and the outdiffusion into poly- or monocrystalline silicon is strongly retarded. Also, the low interface dopant concentrations achievable result in unacceptably high values of contact resistance. The observations on metal-dopant- (M-D-) compound formation are demonstrated to agree well with the predictions from thermodynamic calculations on the respective M-Si-D system. The effects on junction formation are compared to the case of WSi2 and CoSi2, which, from a parallel study, are known not to form compounds. In all cases these comparisons support our statements on the tremendous impact of M-D-compound formation, because much improved data on diffusion and junction formation were obtained for CoSi2 and WSi2. The same holds for a comparison on contact resistances for silicide diffused junctions, which was performed for TiSi2 and CoSi2.
The redistribution of B and As ions implanted into thin layers of WSi2 and CoSi2 on poly- or monocrystalline Si and the outdiffusion into the Si substrate during furnace annealing (FA) and rapid thermal processing (RTP) were investigated by several analytical techniques. Shallow junctions (depth xj < 100 nm) with interface concentrations Cint close to the solid solubility of the respective dopant in Si (Cint≳3×1020 cm−3 for As; (Cint ≳ 8 × 1019 cm−3 for B) were obtained with RTP. For FA above 800 °C, the diffusion of B from CoSi2 into Si results in a drop of Cint < 2 × 1019 cm−3 because of strong B segregation and probably reactive loss at the SiO2/CoSi2 interface. No evidence on metal-dopant-compound formation could be found. The dopant redistribution is demonstrated to be a superposition of lattice and grain-boundary diffusion, solubility limits, layer inhomogeneities, dopant segregation at the interface and grain boundaries, and probably phase transformation of the dopants segregated at the SiO2/silicide interface. Electrical results such as, e.g., CoSi2 diode leakage currents (≊1 nA/ cm2) and contact resistances ( 2–5 × 10−7 Ω cm2 for RTP) clearly show that the formation of shallow silicided junctions by diffusion from an implanted silicide is a highly useful technological approach.
New aspects for the interpretation of the zero field splitting parameter D of the lowest excitet triplet state of polycyclic hydrocarbons are presented. A model connecting the dipole-dipole interaction between the two triplet electrons with Clar’s concept of electronic sextets is employed. It follows that even in highly condensed aromatic molecules the probability for the two triplet electrons to be localized on one CThis concept is outlined taking the experimental D-values of a series of pyrene derivatives as an example
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.