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Focused ion beam (FIB) milling and lift-out was used to prepare atom probe tomography (APT) specimens from GaN-InGaN multi-quantum well (MQW) nanowire arrays. Modifications to the conventional FIB lift-out technique enable the site-specific analysis of different regions of a single nanowire heterostructure. Three dimensional (3-D) reconstructions of quantum wells generated by analysis along different directions with respect to the heterointerface demonstrate both the capabilities and limitations of atom probe tomographic analysis. The methods described herein can be applied to the site-specific analysis of other complex 3-D heterostructures. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN nanowires oriented along the nonpolar a-axis were analyzed using pulsed laser atom probe tomography (APT). Stoichiometric mass spectra were achieved by optimizing the temperature, applied dc voltage, and laser pulse energy. Local variations in the measured stoichiometry were observed and correlated with facet polarity using scanning electron microscopy. Fewer N atoms were detected from nonpolar and Ga-polar surfaces due to uncorrelated evaporation of N(2) ions following N adatom diffusion. The observed differences in Ga and N ion evaporation behaviors are considered in detail to understand the influence of intrinsic materials characteristics on the reliability of atom probe tomography analysis. We find that while reliable analysis of III-N alloys is possible, the standard APT procedure of empirically adjusting analysis conditions to obtain stoichiometric detection of Ga and N is not necessarily the best approach for this materials system.

Atom probe tomography (APT) was used to quantify inhomogeneities in the distribution of Mn and Co in doped epitaxial Ge thin films for which X-ray diffraction (XRD) studies indicate single phase material. The segregation of dopants into Co and Mn-rich regions with characteristic sizes was evident upon visual inspection of the APT reconstruction and a frequency distribution analysis of the concentration of Co, Mn, and Ge atoms verified that the composition fluctuations exceeded those of a random alloy. Isoconcentration surfaces were generated to establish the connectedness of regions enriched in Mn that have been proposed to enhance the Curie temperature in dilute magnetic semiconductors. The analysis demonstrates important contributions that APT can make to the understanding of magnetism in these materials.

The aquaculture program of the Environmental Research Laboratory is designed to investigate the potential of culturing the brown shrimp of the Gulf of California, Penaeus californiensis. A basic research program has been established to identify nutritional and environmental requirements. Studies in feed composition, feeding rates, and stocking rates are being carried out. In addition, a comparative growth study of P. aztecus and P. californiensis is being conducted in an environmentally-controlled aquacell. The University of Arizona has for a number of years conducted cooperative research programs with the University of Sonora at Puerto Penasco, Sonora, Mexico. Puerto Penasco is a shrimp port on the Gulf of California. The marine laboratory has been improved, and a hatchery and aquacells for grow-out studies have been added. The aquacells were converted from inflated greenhouses and are now used to provide a controlled environment for optimum shrimp growth. The seawater for the Puerto Penasco research site is provided from a seawater well which produces seawater at 35 ppt with an average temperature of 24.5 C + 2.5 C. The results of these experimental programs are presented.

Correlated atom probe tomography, cross-sectional scanning transmission electron microscopy, and cathodoluminescence spectroscopy are used to analyze InGaN/GaN multiquantum wells (QWs) in nanowire array light-emitting diodes (LEDs). Tomographic analysis of the In distribution, interface morphology, and dopant clustering reveals material quality comparable to that of planar LED QWs. The position-dependent CL emission wavelength of the nonpolar side-facet QWs and semipolar top QWs is correlated with In composition.

Atom probe tomography (APT) is used to characterize the influence of hydrogen dosing during GaN barrier growth on the indium distribution of InxGa1−xN quantum wells, and correlated micro-photoluminescence is used to measure changes in the emission spectrum and efficiency. Relative to the control growth, hydrogen dosing leads to a 50% increase in emission intensity arising from discontinuous quantum wells that are narrower, of lower indium content, and with more abrupt interfaces. Simulations of carrier distributions based on APT composition profiles indicate that the greater carrier confinement leads to an increased radiative recombination rate. Furthermore, APT analysis of quantum well profiles enables refinement of x-ray diffraction analysis for more accurate nondestructive measurements of composition.

Surface crystallography and polarity are shown to influence the detection probability of In, Ga, and N ions during atom probe tomography analysis of InxGa1−xN m-plane, c-plane, and (202¯1¯) quantum wells. A N deficit is observed in regions of the reconstruction generated from Ga-polar surfaces, and the probability of detecting group-III atoms is lower in InxGa1−xN quantum wells than in GaN barrier layers. Despite these artifacts, the detected In mole fraction is consistent throughout a given quantum well regardless of the crystal orientation of the quantum well or the evaporation surface from which the reconstruction was generated.

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Cantilever magnetometry experiments were performed on arrays of Bi nanowires of various diameters both on the semimetal and semiconductor side of the semimetal-to-semiconductor transition (SMSC) that is predicted to occur for diameters of approximately 50 nm. Our 270 nm nanowire samples, which are on the semimetal side of the SMSC transition, exhibit strong dHvA oscillations that can be interpreted by considering crystalline orientation and size effects and a large magnetization. Arrays of wires with diameters near or below the SMSC (60 and 36 nm) have dramatically reduced magnetizations per volume, and do not exhibit dHvA oscillations, consistent with a reduction in free carriers due to the SMSC.

Food samples were collected directly from serving lines in cafes and cafeterias in the Twin City metropolitan area. The samples were subjected to a battery of tests chosen to evaluate the microbiological quality of the foods. Bacterial and yeast counts and culture results are reported. Salmonellae, shigellae, and Clostridium perfringens were not found. Numbers of microorganisms found varied considerably with the type of food. The ranges found are probably indicative of values which could be expected in restaurant-cafeteria foods. The laboratory procedures used appeared to be satisfactory for a relatively small bacteriology laboratory and provided sufficient information with which to evaluate the microbiological quality of the food. Hopefully, the data will contribute to efforts being made to establish microbiological standards for restaurant-cafeteria foods.