Time-dependent degradation of AlGaN/GaN heterostructures grown on silicon carbide

The AlGaN/GaN heterostructure field-effect transistors (HFETs) were grown on 4H-SiC substrates by metal-organic chemical-vapor deposition (MOCVD) with a range of Al compositions (30–35%) and AlGaN barrier thicknesses. Films with higher strains exhibited a time-dependent degradation of the two-dimensional electron gas (2DEG) that varied from days to weeks. Atomic force microscopy (AFM) measurements of the degraded films revealed a hexagonal cracking pattern with an increase in the medium-scale surface roughness. The localized strain relaxation of AlGaN barriers and increased roughness of the AlGaN/GaN interface and AlGaN surface result in a broad shoulder at the lower angle of the AlGaN peak and a loss of satellite fringes in the (0006) reflection x-ray diffraction (XRD) curve. This degradation raises serious questions with regard to reliability and survivability of AlGaN HFETs and may complicate device fabrication.     

Material quality characterization of CdZnTe substrates for HgCdTe epitaxy

Cd_1−xZn_xTe (CZT) substrates were studied to investigate their bulk and surface properties. Imperfections in CZT substrates affect the quality of Hg_1−xCd_xTe (MCT) epilayers deposited on them and play a role in limiting the performance of infrared (IR) focal plane arrays. CZT wafers were studied to investigate their bulk and surface properties. Transmission and surface x-ray diffraction techniques, utilizing both a conventional closed-tube x-ray source as well as a synchrotron radiation source, and IR transmission micro-spectroscopy, were used for bulk and surface investigation. Synchrotron radiation offers the capability to combine good spatial resolution and shorter exposure times than conventional x-ray sources, which allows for high-resolution mapping of relatively large areas in an acceptable amount of time. Information on the location of grain boundaries and precipitates was also obtained. The ultimate goal of this work is to understand the defects in CZT substrates and their effects on the performance and uniformity of MCT epilayers and then to apply this understanding to produce better infrared detectors.     

Use of the effective heat of formation model to determine phase formation sequences of In-Se, Ga-Se, Cu-Se, and Ga-In multilayer thin films

Multilayer thin films of indium-selenium, gallium-selenium, copper-selenium, and gallium-indium have been analyzed for the purpose of understanding thin film phase transformations that are relevant to the production of Cu(In,Ga)Se₂ photovoltaic solar cells. For example, the intermetallic phases that are present during selenization of precursor films will affect film microstructure and the resulting film properties. Throughout this research, the effective heat of formation model¹ has been used to predict phase formation sequences. The accuracy of these predictions was explored experimentally. By using differential thermal analysis, the reactions of product film formation were examined. X-ray diffraction was used to determine reactant and product phases.     

Characterization and FT-IR study of nanostructured alumina-supported V-Mo-W-O catalysts

The effect of tungsten incorporation in the surface composition and its catalytic performance is evaluated for alumina supported Mo-V-O, Mo-W-O, V-W-O and Mo-V-W-O nanostructurated-oxide catalysts. The characterization results reveal that the surface of Mo-V-W-O catalysts is further different from the binary counterparts, due to the presence of stabilized and reduced structures dispersed on the support. Such species are not present in the Mo-V-O catalysts; indicative that tungsten acts as a structural–chemical promoter. The in situ FT-IR study of these catalysts under propane + oxygen atmosphere showed that Mo-V-W-O catalyst is able to thermally activate the propane oxotransformation and ODH-products are registered in the gas-phase whereas oxygenate-compounds are detected on the surface of catalysts.     

Effect of varying blends of finished RO, surface and ground waters on solid lead surfaces

PbO, PbCO₃, and Pb₃(OH)₂(CO₃)₂ were identified as primary corrosion products on lead coupons following three months of exposure to finished RO, surface and ground water by XRD and XPS. Based on these results, a Pb₃(OH)₂(CO₃)₂ solid phase equilibrium model was used as a function of water quality, was modified using diffusion theory and accurately predicted lead release for a majority of field observations. SEM photographs revealed sulfate and chloride affected the surface structure of corrosion scales and lead release.     

Identification by photoelectrochemistry of oxide phases grown during the initial stages of thermal oxidation of AISI 441 ferritic stainless steel in air or in water vapour

Room temperature photoelectrochemistry was used to characterise oxide phases grown during the initial stages of oxidation of the ferritic stainless steel AISI441 at 650°C and 850°C in synthetic air or in water vapour. Grazing incidence X-ray diffraction and Raman spectroscopy were additionally used to discuss PEC results. Haematite Fe₂O₃ (∼2.0 eV), chromia Cr₂O₃ (3.0 and 3.5 eV) and their mutual solid solution (∼ 2.5 eV) were detected by their respective bandgap values determined from photocurrent vs. energy curves. The Cr/Fe ratio of the films increased with time/temperature and was higher in air-grown than in H₂O-grown oxides. Observation of photocurrent vs. potential curves indicated that chromia was N-type in all specimens, resulting from thermodynamic equilibrium with the metallic substrate and not with the gas phase.     

Characterization of the corrosion products of electrodeposited Zn, Zn–Co and Zn–Mn alloys coatings

The morphology, composition, phase composition and corrosion products of coatings of pure Zn (obtained from two types of electrolytic bath: an acidic bath (Zn_acid) and a cyanide-free alkaline bath (Zn_alkaline)) and of Zn–Mn and Zn–Co alloys on steel substrates were studied. To achieve this, diverse techniques were used, including polarization curves, atomic force microscopy (AFM), scanning electron microscopy (SEM), glow discharge spectroscopy (GDS), X-ray diffraction (XRD), and the salt spray test. In the salt spray test, the exposure time required for the coatings to exhibit red corrosion (associated with the oxidation of steel) decreased in the following order: Zn–Mn_(432h) > Zn–Co_(429h) > Zn_{alkaline(298h)} > Zn_acid(216h). The shorter exposure times required for corrosion of the pure Zn coatings are related to the coating composition and the crystallographic structure. Analysis of the corrosion products disclosed that Zn₅(OH)₈Cl₂·H₂O was a corrosion product of all of the coatings tested. However, the formation of oxides of manganese (MnO, Mn_0.98O₂, Mn₅O₈) in the Zn–Mn coating, and the formation of the hydroxide Zn₂Co₃(OH)₁₀·2H₂O in the Zn–Co coating, produced more compact and stable passive layers, with lower dissolution rates.     

A new method for the synthesis of molybdenum carbide

The carburization of MoO₃ using toluene as carbon source provides a new route to β-Mo₂C.     

Microstructural Characterization of Cobalt-Tungsten Coated Graphite Fibers

The present research concerns an electrodeposited cobalt-tungsten alloy coating on graphite fibers. Annealed and unannealed coated fibers were analyzed by scanning electron microscopy/energy dispersive x-ray spectroscopy (SEM/EDS), x-ray diffraction (XRD), and transmission electron microscopy (TEM). The mole fraction of tungsten in the as-deposited cobalt-tungsten coating was found to be (7.10 ± 0.82) %, and the crystalline lattice was determined to be hexagonal close-packed. Note: The uncertainties reported here are expanded uncertainties (i.e., 2 standard deviation estimates). After annealing at 1100 °C for 1.5 h, the lattice was found to be a cobalt-tungsten-carbon (Co-W-C) alloy with a face centered cubic lattice. The mole fraction of tungsten in the annealed coating was shown to be (5.30 ± 0.66) %. XRD analysis revealed that the annealed coating contained regions having three slightly different lattice parameters. The lattice parameters in the three regions of the Co-W-C alloy coating corresponded to a mole fraction of carbon of 0.66 %, 0.40 %, and 0.19 % (± 0.18 %). These compositions are discussed with respect to the Co-W-C phase diagram. Various tungsten carbide species were identified in the coating and fiber, and a network of overlapping WC crystals was found at the coating-fiber interface. This network appeared to serve as a protective barrier to extend the lifetime of the fibers at elevated temperatures.     

A simple one step method for the synthesis of hexagonal Cd1 − xZnxS (x = 0-0.75)

Attempt has been made to synthesize hexagonal Cd_1 − xZn_xS (x = 0-0.75) by a simple solution phase method. The samples were characterized by XRD, SEM and UV-Vis spectroscopy. The XRD studies confirmed the formation of hexagonal modification of the above mentioned ternary system. SEM revealed formation of an agglomerated product with grain sizes of ∼ 100 nm. The mode of growth was confirmed as Ostwald ripening from UV-Vis studies. Shift in the exciton peak on increasing the Zn²⁺ was explained on the basis of formation of the solid solution. Photoluminescence studies reveal a shift in the luminescence band with Zn signifying the formation of solid solution. Also the band arises from the surface states demonstrating high concentration of surface defect states.