Comparison of OMVPE Grown GaAs/AlGaAs and GaAs/InGaP HEMT and PHEMT structures

Symmetric δ-doped InGaP and AlGaAs PHEMT structures have been grown by organometallic vapor phase epitaxy with properties that approach those of MBE grown AlGaAs structures. The 300 and 77K carrier concentrations for the InGaP PHEMT were 2.72 and 2.56 × 10¹² cm^{2 −2} and the mobilities were 5,920 and 22,000 cm^{2 2}/V.s. These excellent values suggest that problems associated with switching the anion at the channel heterojunction have been overcome. The corresponding values for the AlGaAs PHEMT were 2.51 and 2.19 × 10¹² cm^{2 −2} and 6,500 and 20,400 cm²/V.s. The uniformity in the indium concentration in the InGaAs layer as determined by photoluminescence, photoreflection, double crystal x-ray diffraction, and Rutherford backscattering was found to be good, but the percent In in the AlGaAs pseudo-morphic high electron mobility transistor (PHEMT) was less than that in the InGaP PHEMT even though the programmed values were the same. The uniformity in the doping distribution as determined by secondary ion mass spectroscopy and electrochemical capacitance-voltage measurements was found to be good, but it decreased with distance from the center of the susceptor. Also, most of the dopants in the δ-doped InGaP and AlGaAs layers were activated.     

A case-control study of menopausal estrogen therapy and breast cancer

The association between estrogen replacement therapy and female breast cancer was studied in two Los Angeles area retirement communities. The 138 study cases of breast cancer occurring in residents younger than 75 years were compared with age- and race-matched community control subjects. The risk ratio for a total cumulative dose in excess of 1,500 mg was estimated to be 2.5 in women with intact ovaries. This increase was present using various independent sources of drug usage information but was inconsistent at low dose and undetectable in oophorectomized women. No important sources of confounding could be identified, and no risk modifiers could be identified except for a history of surgically confirmed benign breast disease. In such women with intact ovaries, the risk ratio for a high cumulative dose rose to 5.7 relative to nonusers with normal breasts.     

Tetraalkynylated and Tetraalkenylated Benzenes and Pyridines: Synthesis and Photophysical Properties

Tetra(arylalkynyl)pyridines and tetra(arylalkenyl)pyridines and their benzene analogues were prepared by palladium‐catalyzed cross‐coupling reactions from the commercially available chlorinated substrates in good to excellent yields. The photophysical properties of selected compounds were investigated and compared to each other. Very good fluorescence quantum yields were observed, especially for the pyridine derivatives.     

Alanine Scan of the Peptide Antibiotic Feglymycin: Assessment of Amino Acid Side Chains Contributing to Antimicrobial Activity

The antibiotic feglymycin is a linear 13‐mer peptide synthesized by the bacterium Streptomyces sp. DSM 11171. It mainly consists of the nonproteinogenic amino acids 4‐hydroxyphenylglycine and 3,5‐dihydroxyphenylglycine. An alanine scan of feglymycin was performed by solution‐phase peptide synthesis in order to assess the significance of individual amino acid side chains for biological activity. Hence, 13 peptides were synthesized from di‐ and tripeptide building blocks, and subsequently tested for antibacterial activity against Staphylococcus aureus strains. Furthermore we tested the inhibition of peptidoglycan biosynthesis enzymes MurA and MurC, which are inhibited by feglymycin. Whereas the antibacterial activity is significantly based on the three amino acids D ‐Hpg1, L ‐Hpg5, and L ‐Phe12, the inhibitory activity against MurA and MurC depends mainly on L ‐Asp13. The difference in the position dependence for antibacterial activity and enzyme inhibition suggests multiple molecular targets in the modes of action of feglymycin.     

A parametric study of a horizontal rotating fluidized bed using slotted and sintered metal cylindrical gas distributors

The pressure drop in a horizontal rotating fluidized bed was measured using slotted and sintered metal cylindrical gas distributors as a function of rotating speed, gas velocity and bed thickness. Experiments were conducted using polydisperse alumina particles and nearly monodisperse glass beads. The pressure drop for the slotted distributor exhibited a much larger pressure overshoot at incipient fluidization than the sintered metal distributor. This behavior was also studied using high-resolution photography. Physically consistent explanations are presented for the observed phenomena. The experimental results are compared to theoretical models available in the literature.     

Development of a multifunctional coating system for laser-induced material transport

The aim of our research is to develop a novel surface coating for the use in laser microdissection and laser pressure catapulting (LMPC). LMPC is a contact- and contamination-free technique to separate histologic material and living cells for further proteomic and genomic analysis. Several physico-chemical functions must be included within the optimum coating system designed for this purpose, like optical absorption at the laser wavelength, combined with optical transparency in the visible region, a control of the laser ablation process, mechanical stability and biocompability for the adhesion of the histologic material.To achieve the optimum system the combination of several layers is required. The optical absorbance to capture the radiation energy from a frequency-tripled Nd:YAG laser (λ = 355 nm) is reached by a thin layer of zinc oxide (ZnO), deposited by hollow cathode gas flow sputtering. The laser ablation process is controlled by a polyelectrolyte multilayer, consisting of poly(diallyldimethylammonium chloride) (PDADMAC) and poly(sodium 4-styrenesulfonate) (PSS). The evaporation of chemisorbed water from the film is used to promote the catapulting process. For the mechanically stable, laser-dissectible layer organic coatings, like photoresists or lacquers, are suitable. Silica-containing polyacrylate nanocomposites were employed for this purpose.The investigation of the coating system included LMPC experiments with varying compositions of the layer system. The best results were obtained using a system consisting of ZnO, a polyelectrolyte multilayer deposited from 0.1 M Na₂SO₄ containing polymer solutions, and a 1.5-μm thick layer of the polyacrylate nanocomposite.To check the quality of the developed system, experiments with the commonly used poly(ethylene naphthalate) (PEN) foil were performed simultaneously. In addition to the determination of the parameters required for LMPC, quantitative real-time Polymerase Chain Reaction (rt-PCR) of the dissected material verified the benefit of the new system.