Nickel layers on indium arsenide

We report here on the preparation and characterization of InAs substrates for in situ deposition of ferromagnetic contacts, a necessary precursor for semiconductor devices based on spin injection. InAs has been grown on InAs(111)A and (100) substrates by molecular-beam epitaxy and then metalized in situ in order to better understand the mechanisms that inhibit spin injection into a semiconductor. Initial x-ray characterization of the samples indicate the presence of nickel arsenides and indium–nickel compounds forming during deposition at temperatures above room temperature. Several temperature ranges have been investigated in order to determine the effect on nickel-arsenide formation. The presence of such compounds at the interface could greatly reduce the spin-injection efficiency and help elucidate previous unsuccessful attempts at measuring spin injection into InAs

Effect of Buffer Layer and III/V Ratio on the Surface Morphology of GaN Grown by MBE

The surface morphology of GaN is observed by atomic force microscopy for growth on GaN and AlN buffer layers and as a function of III/V flux ratio. Films are grown on sapphire substrates by molecular beam epitaxy using a radio frequency nitrogen plasma source. Growth using GaN buffer layers leads to N-polar films, with surfaces strongly dependent on the flux conditions used. Flat surfaces can be obtained by growing as Ga-rich as possible, although Ga droplets tend to form. Ga-polar films can be grown on AlN buffer layers, with the surface morphology determined by the conditions of buffer layer deposition as well as the III/V ratio for growth of the GaN layer. Near-stoichiometric buffer layer growth conditions appear to support the flattest surfaces in this case. Three defect types are typically observed in GaN films on AlN buffers, including large and small pits and "loop" defects. It is possible to produce surfaces free from large pit defects by growing thicker films under more Ga-rich conditions. In such cases the surface roughness can be reduced to less than 1 nm RMS

X-ray photoelectron spectroscopy investigation of the mixed anion GaSb/InAs heterointerface

X-ray photoelectron spectroscopy has been used to measure levels of anion cross-incorporation and to study interface formation for the mixed anion GaSb/lnAs heterojunction. Anion cross-incorporation was measured in 20 Å thick GaSb layers grown on lnAs, and 20 Å thick InAs layers grown on GaSb for cracked and uncracked sources. It was found that significantly less anion cross-incorporation occurs in structures grown with cracked sources. Interface formation was investigated by studying Sb soaks of InAs surfaces and As soaks of GaSb surfaces as a function of cracker power and soak time. Exchange of the group V surface atoms was found to be an increasing function of both cracker power and soak time. We find that further optimization of current growth parameters may be possible by modifying the soak time used at interfaces

Study of interface asymmetry in InAs–GaSb heterojunctions

We present reflection high energy electron diffraction, secondary ion mass spectroscopy, scanning tunneling microscopy and x‐ray photoelectron spectroscopy studies of the abruptness of InAs–GaSb interfaces. We find that the interface abruptness depends on growth order: InAs grown on GaSb is extended, while GaSb grown on InAs is more abrupt. We first present observations of the interfacial asymmetry, including measurements of band alignments as a function of growth order. We then examine more detailed studies of the InAs–GaSb interface to determine the mechanisms causing the extended interface. Our results show that Sb incorporation into the InAs overlayer and As exchange for Sb in the GaSb underlayer are the most likely causes of the interfacial asymmetry

Highly electronegative metallic contacts to semiconductors using polymeric sulfur nitride

The Schottky barriers formed on n‐ZnS and n‐ZnSe by polymeric sulfur nitride have been compared to barriers formed by Au. Barrier energies as determined by photoresponse, current‐voltage, and capacitance‐voltage methods show that (SN)_x is approximately 1.0 eV higher than Au on n‐ZnS and 0.3–0.4 eV higher than Au on n‐ZnSe. We believe that this is the first report of any metallic contact more electronegative than Au

Feeding and foraging ecology of Trindade petrels Pterodroma arminjoniana during the breeding period in the South Atlantic Ocean

Seabirds breeding in tropical environments experience high energetic demands, when foraging in an oligotrophic environment. The globally threatened Trindade petrel Pterodroma arminjoniana has its largest colony in Trindade Island (20°30′S–29°19′W) inside the oligotrophic South Atlantic Subtropical Gyre. Diet sampling methods, geolocator tracking and stable isotope analysis were used to describe its diet, compare foraging trips and distributions, and assess temporal variations in the trophic niche throughout the breeding period. Diet consisted mainly of squid and fish. The high species diversity and wide range of prey sizes consumed suggests the use of multiple foraging techniques. Stable isotope mixing models confirm that Trindade petrels rely mainly on squid throughout the breeding period. Its broad isotopic niche seems to reflect both a diverse diet and foraging range, since birds can reach up to 3335 km from the colony. Isotopic niche showed limited variation even in an 8-year interval, apparently due to oceanographic stability, although changes in the isotopic niche have demonstrated an adjustment to different conditions in different seasons. Petrels change foraging areas and prey during the breeding period: pre-incubating birds use more productive areas west of Trindade Island and obtain low trophic position prey; incubating petrels perform longer trips southward to consume prey of high trophic position; and chick-rearing petrels use areas around the island. These results demonstrate that to deal with high demand breeding in a colony surrounded by oligotrophic waters, Trindade petrels need to explore wide foraging areas and utilize a diverse diet, besides adjusting trophic niche according to breeding stage

Investigation of Polymer–Plasticizer Blends as SH-SAW Sensor Coatings for Detection of Benzene in Water with High Sensitivity and Long-Term Stability

We report the first-ever direct detection of benzene in water at concentrations below 100 ppb (parts per billion) using acoustic wave (specifically, shear-horizontal surface acoustic wave, SH-SAW) sensors with plasticized polymer coatings. Two polymers and two plasticizers were studied as materials for sensor coatings. For each polymer–plasticizer combination, the influence of the mixing ratio of the blend on the sensitivity to benzene was measured and compared to commercially available polymers that were used for BTEX (benzene, toluene, ethylbenzene, and xylene) detection in previous work. After optimizing the coating parameters, the highest sensitivity and lowest detection limit for benzene were found for a 1.25 μm thick sensor coating of 17.5%-by-weight diisooctyl azelate-polystyrene on the tested acoustic wave device. The calculated detection limit was 45 ppb, with actual sensor responses to concentrations down to 65 ppb measured directly. Among the sensor coatings that showed good sensitivity to benzene, the best long-term stability was found for a 1.0 μm thick coating of 23% diisononyl cyclohexane-1,2-dicarboxylate-polystyrene, which was studied here because it is known to show no detectable leaching in water. The present work demonstrates that, by varying type of plasticizer, mixing ratio, and coating thickness, the mechanical and chemical properties of the coatings can be conveniently tailored to maximize analyte sorption and partial chemical selectivity for a given class of analytes as well as to minimize acoustic-wave attenuation in contact with an aqueous phase at the operating frequency of the sensor device