Electrochemical Li insertion into boron-doped multiwall carbon nanotubes (B-MWNTs) was investigated in a nonaqueous medium. Transmission electron microscopy observations showed that the walls of the tubes consisted of highly aligned ca. 35-45 graphene layers with good 3D ordering feature. Raman studies revealed that boron doping in multiwall carbon nanotubes (MWNTs) destroyed the local hexagonal symmetry. X-ray photoelectron spectra of B-MWNTs further supported the results of Raman spectra and confirmed the presence of BC3 nanodomains. N2 adsorption measurements indicated that the Brunauer-Emmett-Teller (BET) surface areas of undoped and doped nanotubes were 10 and 12 cm2/g, respectively, with almost similar mesopore volumes. Galvanostatic discharge-charge measurements revealed that the reversible capacity was 156 mAh/g for undoped and 180 mAh/g for B-doped nanotubes in the first cycle with almost equal coulomb efficiencies of 55-58%. The coulomb efficiency increased to more than 90% after the second cycle. Cyclic voltammetry (CV) showed that highly reversible intercalation/deintercalation of Li occurred with some undesirable reduction processes in the initial discharge process. The cycle lives of both undoped and doped samples were quite satisfactory. Slow-scan CV confirmed that the intercalation of lithium in these nanotubes occurred through staging transition, usually observed in Li graphite intercalation compounds. © 2001 The Electrochemical Society. All rights reserved.

The formation and evolution of defects in 4H-SiC Schottky barrier diode high-energy particle detectors have been investigated and correlated with the detectors' properties. Low temperature annealing at 300 °C is found to significantly recover the charge collection efficiency as degraded by 1 MeV electron irradiation. At higher temperatures, an anneal-induced degradation in the detector's performance is observed. Current-voltage, capacitance-voltage, and deep level transient spectroscopy (DLTS) measurements are used to ascertain the effect of defects on the detector performance. The latter reveals that the DLTS defect levels, EH1 and EH3, are related to the initial recovery of the charge collection efficiency.

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Fast and thick 4H-SiC epitaxial growth is demonstrated in a vertical-type reactor under a low system pressure within the range 13-40 mbar. A very fast growth rate of up to 250 m/h is obtained. The material quality of the epilayers grown in the reactor is evaluated by low-temperature photoluminescence, deep level transient spectroscopy, microwave photoconductive decay, synchrotron topography and room temperature PL imaging. The carrier lifetime of thick epilayers with or without the application of the C+-implantation/annealing method and extended defects in the epilayers grown on 8º and 4º off substrates are discussed.

We investigated the structures and expansion behavior of double-Shockley stacking faults (DSFs) formed in heavily nitrogen-doped 4H-SiC during annealing. Heavily doped epilayers prepared as specimens were successively annealed. Various types of DSFs showing different shapes and dislocation contrasts were found in photoluminescence and synchrotron X-ray topography images. Taking account of every possible stacking sequence forming DSFs, the structures of various types of DSFs were determined from observations by plan-view transmission electron microscopy (TEM) and cross-sectional high-angle annular dark-field scanning TEM. We found that a bounding dislocation enclosing a DSF splits into two partial dislocations (PDs), and their Burgers vectors are identical, while the distance of the two PDs depended on their core structures (30° Si-, 30° C- or 90° C-core). We also discussed the contrast rule for the dislocation consisting of two PDs in the synchrotron X-ray topography images and the mobile PDs for the DSF expansion in the epilayers with different nitrogen concentrations.

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We performed fast growth of a 4H-SiC crystal using the gas-source method and investigated the crystal to reveal changes in dislocation densities along the growth direction. The remarkable reduction in densities of threading and basal plane dislocations to be less than 1/10 and 1/20, respectively, was confirmed in the crystal grown at ∼3 mm h−1. The change in radial distribution of threading dislocations indicates enhanced reduction in dislocation densities within an area containing high density dislocations. We discussed possible mechanisms which could explain declining densities of threading dislocations according to the results obtained by classifying threading dislocations into each Burgers vector.