There is a need for simple rapid tests for evaluating the carcinogenic potential of the thousands of chemical compounds that are developed each year. The DNA-damaging effects of 83 National Toxicology Program (NTP) chemicals, including noncarcinogens and carcinogens, were examined in the umu test by measuring the expression of the umuDC-lacZ genes in Salmonella typhimurium TA1535/pSK1002. Salmonella were exposed to individual NTP chemicals at 37 degrees C for 2 hr both with and without a rat liver S9 mix; the treated cells were then diluted and incubated for a further 2 hr (posttreatment assay). O-nitrophenyl-beta-D-galactoside was added to the cultures and the beta-galactosidase activity driven by the Salmonella umuDC-lacZ genes was determined by measurement of the OD(420 nm) and OD(550 nm) of the cultures. Salmonella cell number was simultaneously determined by measurement of OD(600 nm). The overall concordance between genotoxicity in the umu test and carcinogenicity was 67%, which was similar to the concordance between Ames' test results and carcinogenicity (63%) using the same 83 NTP chemicals. The results of this study indicate that the umu test with a single Salmonella strain is a simple rapid system, with accuracy comparable to existing, more time-consuming assays.

This paper attempts to form AlGaN/GaN quantum wire (QWR) network structures on patterned GaN (0001) substrates by selective MBE growth. Substrate patterns were prepared along <112

The feasibility of the selective molecular beam epitaxy (MBE) growth of AlGaN/GaN quantum wire (QWR) structures on prepatterned substrates is investigated. The detailed studies on growth features have revealed that size-reducing selective growth is possible on mesa patterns having the <11-20>-orientation, but not on those having the <1-100>-orientations. The behavior reflects complex growth kinetics on high-index crystalline facets. The lateral wire width of QWR structures formed selectively on a top mesa can be controlled by adjusting the growth thickness and the initial size of mesa patterns. From cathodoluminescence (CL) measurements, emission from the embedded AlGaN/GaN QWR structure has been clearly identified. KEYWORDS: selective growth, molecular beam epitaxy (MBE), patterned substrates, GaN, AlGaN IntroductionRecently, the AlGaN/GaN system has been opened up and has led to the establishment of blue/UV photonic and high-power electronic device application areas. This material system, however, is potentially suitable for not only these areas but also for high-operating-temperature quantum devices such as quantum-wire transistors (QWR-Trs) and single-electron transistors (SETs). This is due to wide energy gaps with a large E c and to the availability of high-density two dimensional electron gases (2DEGs) even under non-doped conditions which avoids the doping fluctuation problem commonly observed in nanometer-scale devices. Here, a key question is how to form the AlGaN/GaN-based nanostructures in a size-and position-controlled manner. Recent studies on the fabrication of GaN-based quantum structures have been mostly focused on the self-assembled formation of quantum dots (QDs) using the Stranski-Krastanov mode [1,2] or anti-surfactant methods [3,4]. In these methods, control of size and position seems to be very difficult. On the other hand, we have shown that selective molecular beam epitaxy (MBE) growth on a prepatterned substrate is a very powerful approach for the formation of high-density quantum nanostructures for with tight control of both position and size.The purpose of this study is to investigate the basic behavior and feasibility of the selective MBE growth of AlGaN/GaN quantum wire (QWR) structures on prepatterned substrates for the first time. For this purpose, GaN and AlGaN layers were grown by rf-radical-assisted MBE growth. Through growth on planar GaN (0001) substrates, the basic growth conditions were optimized using in situ reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD) analyses. Then, selective growth experiments were

Fundamental growth properties were investigated for the size-controlled selective MBE growth of AlGaN / GaN nanowires on the GaN ͑0001͒ prepatterned substrates both experimentally and theoretically. The lateral size of the present GaN nanowire was determined by two facet boundaries formed within AlGaN barrier layers. From the series of wire growth experiments, the growth selectivity and the measured angle of the facet boundary strongly depended on the Al composition and the initial crystalline facets of the mesa patterned templates. The experimental evolution of the cross-sectional structures was well reproduced by a computer simulation based on the phenomenological growth model where the slope angle dependence of lifetime of adatoms was taken into account. The lateral width of present nanowires could be kinetically controlled by the growth conditions and the supply thickness of AlGaN layers.

No abstract