Lateral surface leakage current ͑I s ͒ on an AlGaN / GaN heterostructure was systematically investigated by using a two-parallel gate structure with a gap distance ͑L GG ͒ of 200 nm-5 m. The surface current I s systematically increased as L GG decreased. A simple resistive layer conduction that should show 1 / L GG dependence failed to account for the drastic increase in I s when L GG was reduced to less than 1 m. However, no dependence on L GG was seen in vertical current that flows in the Schottky interface. The I s showed a clear temperature dependence proportional to exp͑−T −1/3 ͒, indicating two-dimensional variable-range hopping through high-density surface electronic states in AlGaN. A pronounced reduction in surface current of almost four orders of magnitude was observed in a sample with SiN x passivation.

We have investigated the effects of thin native oxide layers on the AlGaN surface on the DC characteristics of AlGaN/GaN high electron mobility transistors (HEMTs). After HEMT fabrication, the AlGaN surface between the electrodes was intentionally oxidized using either O 2 or N 2 O plasma. X-ray photoelectron spectroscopy (XPS) analysis showed that both methods produced a native oxide of AlGaN with a thickness of about 1 nm and that an N-O chemical bond was present in the N 2 O plasma oxide. We observed pronounced degradation in the DC characteristics and current collapse in the HEMT with O 2 plasma oxidation. In contrast, the formation of native oxide by the N 2 O plasma had no effect on the DC characteristics or current stability of the AlGaN/GaN HEMT. Possible mechanisms for device degradation were discussed in terms of stress and deep levels in the AlGaN layer induced by oxide formation or oxygen incorporation.

Asbestos exposure is considered a social burden by causing mesothelioma. Despite the use of synthetic materials, multi-walled carbon nanotubes (MWCNTs) are similar in dimension to asbestos and produce mesothelioma in animals. The role of inflammatory cells in mesothelial carcinogenesis remains unclear. Here, we evaluated the differences in inflammatory cell responses following exposure to these fibrous materials using a luminometer and L-012 (8-amino-5-chloro-7-phenylpyrido[3,4-d]pyridazine-1,4-(2H,3H) dione) to detect reactive oxygen species (ROS). Rat peripheral blood or RAW264.7 cells were used to assess the effects on neutrophils and macrophages, respectively. Crocidolite and amosite induced significant ROS generation by neutrophils with a peak at 10 min, whereas that of chrysotile was ~25% of the crocidolite/amosite response. MWCNTs with different diameters (~15, 50, 115 and 145 nm) and different carcinogenicity did not induce significant ROS in peripheral blood. However, the MWCNTs induced a comparable amount of ROS in RAW264.7 cells to that following asbestos treatment. The peaks for MWCNTs (0.5–1.5 h) were observed earlier than those for asbestos (1–5 h). Apocynin and superoxide dismutase significantly inhibited ROS generation for each fiber, suggesting an involvement of NADPH oxidase and superoxide. Thus, asbestos and MWCNTs induce different oxidative responses in inflammatory cells, indicating the importance of mesothelial cell evaluation for carcinogenesis.

No abstract

Previous results for the generation of linear, icosahedral, Jahn-Teller (JT) Hamiltonians with continuous group symmetries are extended. It is demonstrated that it is possible to define electronic generalized tensor operators on a direct sum electronic space such that a set of these operators is closed under commutation with another set of electronic generalized tensor operators which act as the generators of a continuous group. The normal modes carrying irreducible representations of the continuous group are then coupled 'equally' to produce a JT Hamiltonian which is invariant under the operations of the continuous group. The continuous groups generated on the direct sum spaces (T 1 ⊕ T 2 ), (T i ⊕ G), (T i ⊕ H ) and (G ⊕ H ) are discussed in detail. These additional continuous groups are of interest when the lowest JT states of certain icosahedral JT systems (such as some of those found in C 60 ) are modelled. The additional continuous group symmetry allows an analytic diagonalization of the linear JT matrix to be provided and thus facilitates an exact treatment of the vibronic ground state for these models.