Industrial production of GaN and InGaN-light emitting diodes on SiC-substrates

Blue light emitting diodes (LEDs) based on GaN or InGaN have a large market, e.g. for communication, industry and automotive applications. Since 1998, OSRAM Opto Semiconductors has been producing blue LEDs on a large scale, concentrating on the automotive market. LEDs in the wavelength range 450–480 nm are grown by metalorganic vapour phase epitaxy (MOVPE) on SiC. This substrate material offers many advantages from the epitaxial and device processing points of view. To fulfil the strong consumer recommendations not only do the epitaxial processes have to be developed, but also improvements in chip technology and package design help to stabilise electrical and optical properties on these high level demands. For example, low forward voltages, high light output powers at 20 mA, low reverse currents, long term stability and high electrostatic discharge robustness have to be guaranteed in a temperature range of −55–+85°C and at a maximum humidity of 85%. Electrical and optical parameters were tested on every produced chip not only to remove LEDs before packaging, which do not meet the specifications, but to control and optimise the applied technological steps. As an example we demonstrate a testing method for controlling the process and optimising the p-contact.Optimisation of n- and p-contact as well as the improvement of epitaxy helped to increase the light output power from 1 mW in 1998 to more than 6 mW at present when mounted in a 5 mm radial lamp. All electrical parameters of these high brightness LEDs could also be improved or at least kept in the demanded specification range.     

Signal enhancement in supercritical fluid chromatography‐diode‐array detection with multiple injection

To circumvent the detrimental effects of large‐volume injection with fixed‐loop injector in modern supercritical fluid chromatography, the feasibility of performing multiple injection was investigated. By accumulating analytes from a certain number of continual small‐volume injections, compounds can be concentrated on the column head, and this leads to signal enhancement compared with a single injection. The signal to noise enhancement of different compounds appeared to be associated with their retention on different stationary phases and with type of sample diluent. The diethylamine column gave the best signal to noise enhancement when acetonitrile was used as sample diluent and the 2‐picolylamine column showed the best overall performance with water as the sample diluent. The advantage of multiple injection over one‐time large‐volume injection was proven with sulfanilamide, with both acetonitrile and water as sample diluents. The multiple injection approach exhibited comparable within‐ and between‐day precision of retention time and peak area with those of single injections. The potential of the multiple injection approach was demonstrated in the analysis of sulfanilamide‐spiked honey extract and diclofenac‐spiked ground water sample. The limitations of this approach were also discussed.     

1,2-Stereoinduction in Radical Reactions: Stereoselective Synthesis of 2-Alkyl-3-hydroxybutanoates

Radical addition to 2-cyclohexyl-5-methylidene-6-methyl-1,3-dioxan-4-one ( 2 ) affords stereoselectively 5,6-trans-products trans- 3 . The size of the radical has no influence on the selectivity. These trans-acetals are converted into threo-3-hydroxy-butanoates 4 . Methyl 2-methylidene-3-[(tert-butyl)diphenylsilyloxy]butanoate ( 5 ), treated under the same conditions, leads mainly to the erythro-isomer of 4 after deprotection. An influence of the steric bulkyness of the radical is observed. The stereochemical course of the reactions is discussed.     

Room-temperature ordered photon emission from multiexciton states in single CdSe core-shell nanocrystals

We report room-temperature ordered multiphoton emission from multiexciton states of single CdSe(CdZnS) core(-shell) colloidal nanocrystals (NCs) that are synthesized by wet chemical methods. Spectrally and temporally resolved measurements of biexciton and triexciton emission from single NCs are also presented. A simple four level system models the results accurately and provides estimates for biexciton and triexciton radiative lifetimes and quantum yields.     

Selection of a cyclic nonapeptide inhibitor to α-chymotrypsin using a phage display peptide library

A cyclic nonapeptide library displayed on filamentous bacteriophages was selected 6 times against α-chymotrypsin (EC 3.4.21.1) at three different pH conditions (6.5, 7.0, and 7.5). Phage peptide clones from the sixth selection, at all three pH conditions, interacted more strongly with α-chymotrypsin than the original library and a wild-type phage did. DNA sequencing of the selected phage peptide clones showed that different cyclic nonapeptide sequences had been selected at the different pH conditions. The oxidized form of the synthetic peptide, Cys-Cys-Phe-Ser-Trp-Arg-Cys-Arg-Cys, selected at pH 7.5, could completely inhibit the enzymatic activity of α-chymotrypsin. The structurally related enzymes trypsin (bovine) and elastase (porcine) were only marginally inhibited by the same peptide under the same conditions. The inhibition constant for α-chymotrypsin was estimated to be 10^-6 M. Phage clones expressing this peptide had a lower affinity for phenylmethylsulfonylfluoride-modified α-chymotrypsin than for natural α-chymotrypsin as determined by an enzyme immunosorbent assay. This peptide phage clone was also competitively prevented from binding to α-chymotrypsin by the corresponding synthetic oxidized peptide. Collectively, the results suggest that the oxidized form of the selected peptide Cys-Cys-Phe-Ser-Trp-Arg-Cys-Arg-Cys interacts with the active site of α-chymotrypsin and acts as a specific inhibitor to the enzyme. To our knowledge, the selected sequence Cys-Cys-Phe-Ser-Trp-Arg-Cys-Arg-Cys has not been found in nature. This revised version was published online in August 2006 with corrections to the Cover Date.     

Elasticity of solid polymers as a result of thermal motions

ABSTRACT The elastic constants C_ik of solids are related to the amplitude of the strain fluctuations which can be simulated atomistically, thus offering a possibility of determining the C_ik values of the solid of interest based on a proposed atomistic arrangement and a given interaction between the atoms. We show that the approach is practical by calculating the elastic constants of crystalline (orthorhombic) polyethylene, crystalline (monoclinic) isotactic polypropylene, and glassy bisphenol-A-polycarbonate.