No abstract

Aluminum nitride possesses a unique ability to accommodate oxygen via lattice dissolution to levels exceeding 4 at.%. The mechanism for this large accommodation of oxygen is of technological and scientific interest due to the established deleterious effects of oxygen on the thermal conductivity in this material. When doped with oxygen, AIN exhibits an intense, very broad (FWHM > 1 eV), luminescence peak in the near-UV (-375 nm). Though there is little doubt that this transition is associated with oxygen incorporation in the lattice, both the anomalous width of this feature and the specific complex from which it is derived have been a matter of debate. This paper reviews past studies of the luminescence of oxygen-related defects in AIN and presents recent detailed photoluminescence experiments which delineate changes in the luminescence as a function of oxygen content. These data are utilized in conjunction with other measurements to elucidate the nature of the oxygen-related defect and its evolution as a function of oxygen concentration. A defect-cluster model is presented which accounts for a transition in the luminescent properties of AIN and is found to be in accord with measurements on thermal conduction and unit-cell volume changes in AIN. This understanding of the oxygen-related defect in AIN from the photoluminescence studies is then utilized in cathodoluminescence studies via cathodoluminescence imaging in a scanning electron microscope and a transmission electron microscope. Such techniques are extremely useful in elucidating the distribution and interaction of oxygen in the microstructure of sintered AIN ceramics, which has been heretofore an extremely difficult problem in microstructural and microchemical analysis of such sintered ceramics. [

The extent of genetic variation found in drug metabolism genes and its contribution to interindividual variation in response to medication remains incompletely understood. To better determine the identity and frequency of variation in 11 phase I drug metabolism genes, the exons and flanking intronic regions of the cytochrome P450 (CYP) isoenzyme genes CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4 and CYP3A5 were amplified from genomic DNA and sequenced. A total of 60 kb of bi-directional sequence was generated from each of 93 human DNAs, which included Caucasian, African-American and Asian samples. There were 388 different polymorphisms identified. These included 269 non-coding, 45 synonymous and 74 non-synonymous polymorphisms. Of these, 54% were novel and included 176 non-coding, 14 synonymous and 21 non-synonymous polymorphisms. Of the novel variants observed, 85 were represented by single occurrences of the minor allele in the sample set. Much of the variation observed was from low-frequency alleles. Comparatively, these genes are variation-rich. Calculations measuring genetic diversity revealed that while the values for the individual genes are widely variable, the overall nucleotide diversity of 7.7 x 10(-4) and polymorphism parameter of 11.5 x 10(-4) are higher than those previously reported for other gene sets. Several independent measurements indicate that these genes are under selective pressure, particularly for polymorphisms corresponding to non-synonymous amino acid changes. There is relatively little difference in measurements of diversity among the ethnic groups, but there are large differences among the genes and gene subfamilies themselves. Of the three CYP subfamilies involved in phase I drug metabolism (1, 2, and 3), subfamily 2 displays the highest levels of genetic diversity.

The oxygen-related defect in an aluminum nitride (A1N) single crystal and in polycrystalline ceramics is investigated utilizing photoluminescence spectroscopy, thermal conductivity measurements, x-ray diffraction lattice parameter measurements, and transmission electron microscopy. The results of these measurements indicate that at oxygen concentrations near 0.75 at. %, a transition in the oxygen accommodating defect occurs. On both sides of this transition, simple structural models for the oxygen defect are proposed and shown to be in good agreement with the thermal conductivity and lattice parameter measurements, and to be consistent with the formation of various extended defects (e.g., inversion domain boundaries) at higher oxygen concentrations.