bluuebunny/crossref_metadata_2025 huggingface dataset:
Controlled radical polymerization of N‐vinylcaprolactam mediated by xanthate or dithiocarbamate
Authors: Wan Decheng , Zhou Qing , Pu Hongting , Yang Genjin
Published: April 2008

Controlled radical polymerization of N‐vinylcaprolactam (NVCL) via reversible addition‐fragmentation chain transfer (RAFT) polymerization or macromolecular design via the interchange of xanthate (MADIX) was described, employing 2‐diphenylthiocarbamoylsulfanyl‐2‐methyl‐propionic acid (CTA1), ((O‐ethylxanthyl)methyl)benzene (CTA2) and (1‐(O‐ethylxanthyl)ethyl)benzene (CTA3) as chain transfer agents (CTA). It was found that all the CTAs led to controlled radical polymerization of NVCL, with the molecular weight increased along with the conversion of monomer and a relatively narrow molecular weight distribution could be obtained, as determined with matrix‐assisted laser desorption and ionization time‐of‐flight (MALDI‐TOF) and gel permeation chromatography (GPC), the polydispersity indices, as determined by MALDI‐TOF, were typically on the order of 1.24, but the polymerization did not proceed in a strictly living manner. The chain transfer ability of these CTAs was in the following order: CTA1 ≈ CTA2 < CTA3. MALTI‐TOF measurement showed that the major population of polymer retained the chain‐end functional group, but minor population deactivated by radical coupling. In preparation of the block copolymer of NVCL and vinyl acetate (VAc) by sequential polymerization, the sequence of monomer addition was important. Using VAc as the first monomer could lead to a block copolymer presenting a unimodal GPC trace and a narrow PDI index, and if NVCL was used as the first monomer, the polymerization was less well controlled. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3756–3765, 2008
Slow motion in [ring‐fluoro]polycarbonate by CODEX
Authors: Weldeghiorghis Thomas K. , Stueber Dirk , Schaefer Jacob
Published: April 2008

Chain dynamics in [ring‐fluoro]polycarbonate (an A‐B alternating copolymer that has a single fluorine substituent on every fourth main chain ring) have been characterized by centerband only detection of exchange (CODEX) and rotating‐frame 13C spin‐lattice relaxation. The slow motions detected by CODEX are facilitated by a mechanically active lattice reorganization that permits a flip of the fluorinated ring about its C2 axis. Nonfluorinated rings undergo small‐amplitude reorientations and C2 flips, both of which are fast and not CODEX active. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1062–1066, 2008
A Novel Device for Single Particle Light Scattering Size Analysis and Concentration Measurement at High Pressures and Temperatures
Authors: Umhauer Heinz , Meyer Jörg , Schiel Annette
Published: April 2008

Based on the findings of previous work, a novel instrument was developed for the size analysis and concentration measurement of particles dispersed in gases at high temperatures (600 °C) and pressures (16 bar). The main motivation for the construction of this device was a measurement requirement at the conditions of a pressurized pulverized coal combustion (PPCC) test installation in Dorsten, Germany. The development of a high efficiency (> 50 %), coal based, combined cycle process, and specifically, the development of efficient gas cleaning technology for gas combustion under demanding conditions (1400 °C and 16 bar) was the main target. A suitable measurement technique was required for the determination of particle size and concentration downstream of the gas cleaning equipment, which is able to operate close to the given conditions. The performance of the novel device was tested in several measurement series with various monodisperse aerosols at ambient conditions as well as in high pressure, high temperature situations with very satisfactory results, i.e., the lower detection limit (50 % counting efficiency at ca. 0.3 μm) and resolution of the novel device are comparable to state of the art instruments (of the same principle) intended for room temperature operation.
Generation of Monodisperse Micron‐Sized Droplets using Free Adjustable Signals
Authors: Riefler Norbert , Wriedt Thomas
Published: April 2008

This paper describes the generation of small water droplets in the size range down to a few microns. Commercially available inkjet printing devices are not suitable for producing such droplets since they produce satellite droplets. Furthermore, standard drop‐on‐demand devices are normally restricted to the generation of droplets with the same size as the orifice diameter. Using a new and more sophisticated computer‐based signal generation system, smaller‐sized droplets can be generated from the same orifice. A key feature of the design is the generation of freely definable pulses. This enables the generation of acoustic modes within the fluid of the droplet generator, which leads to the generation of droplets without satellites. Only very few pulse forms enable the generation of suitable acoustic modes. Therefore, it is necessary to look for the specific pulse corresponding to the chosen droplet generator. Flexible pulse form generation appears to be more suitable than simple pulse forms for the generation of such droplets.
Use of Video Enhanced Microscopy for Characterization of Solid‐Liquid‐Liquid Mixtures
Authors: Morales Carlos , Riebel Ulrich , Padrón Alida , Zuñiga Percy , Sorrentino José
Published: April 2008

Although some particle sizing devices have gained a lot of popularity, none of them is able to distinguish between drops and solids and few are able to handle viscous oily samples. This paper discusses the use of the optical microscopy, enhanced by the use of digital video capabilities and image analysis software, to characterize oil production wastes. Several thousands of particles were counted and their projected areas were measured. A discussion is given about the different corrections required in order to extract the most reliable information from the image analysis, overcoming some of the drawbacks of this kind of measurements. The technique developed here offers unique features like the determination of the fraction of droplets that exist in the form of non‐coalesced agglomerates. When cross‐linked with the results obtained by standard ASTM procedures, it allows to infer the amount of disperse phase that exists in the form of submicron droplets or large free water drops that are usually not sampled for microscope slides.
A Clustering Approach for the Separation of Touching Edges in Particle Images
Authors: Korath Jose M. , Abbas Ali , Romagnoli Jose A.
Published: April 2008

The occurrence of touching objects in images of particulate systems is very common especially in the absence of dispersion methods during image acquisition. The separation of these touching particles is essential before accurate estimation of particle size and shape can be achieved from these images. In the current work, clustering approaches based on the fuzzy C‐means algorithm are employed to identify the touching particle regions. Firstly, clustering in the multidimensional space of image features, e.g., standard deviation, gradient and range calculated in a certain neighborhood of each pixel, is performed to trap the touching regions. Then, in a novel proposed method, the clustering of pixel intensity itself into two fuzzy clusters is performed and a feature, referred to as the ‘Fuzzy Range', is calculated for each pixel from its membership values in both clusters and is presented as a distinguishing feature of the touching regions. Both approaches are compared and the superiority of the latter method in terms of the non‐necessity of neighborhood based calculations and minimum disfiguration is elucidated. The separation methods presented herein do not make any assumption about the shape of the particle as is undertaken in many methods reported elsewhere. The technique is proven to minimize greatly the deleterious effects of over‐segmentation, as is the case with traditional watershed segmentation techniques, and consequently, it results in a superior performance.
High intensity red emission from Eu doped GaN powders
Authors: Shi Junxia , Chandrashekhar M. V. S. , Reiherzer Jesse , Schaff William , Lu Jie , Disalvo Francis , Spencer Michael
Published: April 2008

Eu doped GaN powders have been produced by a low‐cost and high‐yield method. The effect of temperature on the optical properties and structure was investigated by photoluminescence, X‐ray diffraction and Raman spectroscopy. The effective Eu incorporation was extracted by a non‐destructive strain analysis correlating the Raman with the XRD data. Luminescence intensity was closely related to the effective Eu incorporation. The maximum Eu incorporation was determined to be 0.5 at% for the sample grown at 1000 °C, which showed the strongest red photoluminescence with the highest crystallinity. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
MOVPE growth of blue InxGa1–xN/GaN LEDs on 150 mm Si(001)
Authors: Schulze F. , Dadgar A. , Krtschil A. , Hums C. , Reissmann L. , Diez A. , Christen J. , Krost A.
Published: April 2008

We report on the implementation of GaN‐based, blue light emitting diodes on 150 mm Si(001) substrates grown by metalorganic vapor phase epitaxy. The 2.8 µm thick samples are completely crack‐free and the optically active layers consist of fivefold InxGa1–xN/GaN multiple quantum wells. The homogeneity of the light emission across the whole sample is determined by photoluminescence wafer mapping exhibiting an averaged peak wavelength of 456 nm at room temperature and a standard deviation of less than 1%. A bright blue light emission is obtained by an electrical excitation with a turn‐on voltage of below 2.8 V. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Semipolar GaN grown on m‐plane sapphire using MOVPE
Authors: Wernicke Tim , Netzel Carsten , Weyers Markus , Kneissl Michael
Published: April 2008

We have investigated the MOVPE growth of semipolar gallium nitride (GaN) films on (10$ \bar 1 $0) m‐plane sapphire substrates. Specular GaN films with a RMS roughness (10 × 10 μm2) of 15.2 nm were obtained and an arrowhead like structure aligned along [$ \bar 2 $113] is prevailing. The orientation relationship was determined by XRD and yielded (2$ \bar 1 $$ \bar 1 $2)GaN ‖ (10$ \bar 1 $0)sapphire and [$ \bar 2 $113]GaN ‖ [0001]sapphire as well as [$ \bar 2 $113]GaN ‖ [000$ \bar 1 $]sapphire.

PL spectra exhibited near band edge emission accompanied by a strong basal plane stacking fault emission. In addition lower energy peaks attributed to prismatic plane stacking faults and donor acceptor pair emission appeared in the spectrum. With similar growth conditions also (10$ \bar 1 $$ \bar 3 $) GaN films on m‐plane sapphire were obtained. In the later case we found that the layer was twinned, crystallites with different c‐axis orientation were present. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Low on‐resistance AlGaN/GaN HEMTs by reducing gate length and source‐gate length
Authors: Ide T. , Shimizu M. , Yagi S. , Inada M. , Piao G. , Yano Y. , Akutsu N. , Okumura H. , Arai K.
Published: April 2008

To reduce the on‐resistance of AlGaN/GaN HEMTs, the gate length (LG) and gate‐source (LSG) length were reduced to submicron order. The reduction of LG and LSG effectively decreased the drain‐source spacing (LSD), which led to the reduction of the on‐resistance without degrading the breakdown voltage. Moreover, the reduction of LG and LSG enhanced the drain current and transconductance. The specific on‐resistance of 0.1 mΩ‐cm2 was achieved at a LG of 0.10 μm and a LSG of 0.5 μm. The breakdown voltage of 56 V was obtained at a specific on‐resistance of 0.12 mΩ‐cm2. This high breakdown voltage was achieved by extending the drain‐gate spacing due to the short LG and LSG. These characteristics of the fabricated device are equivalent to the trade‐off characteristics of the numerical calculation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
New results on HVPE growth of AlN, GaN, InN and their alloys
Authors: Usikov A. , Soukhoveev V. , Shapovalova L. , Syrkin A. , Kovalenkov O. , Volkova A. , Sizov V. , Ivantsov V. , Dmitriev V.
Published: April 2008

A variety of new results on HVPE growth of group III nitride layers and heterostructures are described. Crack free from 10 to 30 μm thick AlN layers were grown on 100‐mm SiC substrates. Bowing of the AlN/SiC wafers was controlled below 10 microns. Low growth rate HVPE with deposition rate below 10 nm/min was demonstrated for GaN, AlN, and AlGaN. This process resulted in the first HVPE grown multi‐quantum wells (MQWs) and superlattices. P‐type as‐grown GaN layers with acceptor concentrations up to 3×1019 cm–3 were demonstrated. InGaN layers from 0.002 to 2 μm thick were grown by HVPE for the whole composition range using c‐plane GaN/sapphire template substrates. Non‐polar InGaN layers were grown on a‐GaN/r‐sapphire templates and m‐plane GaN bulk substrates. Blue and green all‐HVPE LEDs were made utilizing InGaN MQWs grown on p‐GaN/sapphire templates. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Effects of sapphire substrate misorientation on the GaN‐based light emitting diode grown by metalorganic vapour phase epitaxy
Authors: Nakamura A. , Yanagita N. , Murata T. , Hoshino K. , Tadatomo K.
Published: April 2008

We have systematically investigated the influence of sapphire substrate misorientation on the structural and optical properties of GaN‐based LEDs grown by MOVPE. The LED properties were found to be very sensitive to the misorientation. We grew five samples of LEDs on sapphire substrates with various misorientation angles (0.02∼1.0°). The EL intensity increased when misorientation angles increased to 0.25°. The intensity, however, decreased dramatically when misorientation angles exceeded 0.25°. We also grew the samples of each layer of the LEDs, viz., GaN, InGaN, InGaN/GaN MQWs, and p‐GaN layer. CL and PL analyses showed that the crystalline quality of InGaN (MQWs) layer became lower while that of GaN layer improved as misorientation angles increased. In addition, the increased misorientation caused the decreased indium composition and the indium compositional fluctuation. Moreover, the hole (carriers) concentration of p‐GaN layer decreased with increasing misorientation angles. As these results, the misorientation angle of around 0.25° is suitable for the LEDs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Effects of resonant LO phonon assisted excitation on the photoluminescence spectra of InGaN/GaN quantum wells
Authors: Dawson P. , Hylton N. P. , Kappers M. J. , McAleese C. , Humphreys C. J.
Published: April 2008

We have observed oscillations in the low temperature (T < 50 K) photoluminescence excitation spectra of an InGaN/GaN single quantum well at photon energies above the GaN bandgap. We attribute the features in the spectra to excitation of electrons at multiples of the LO phonon energy above the GaN conduction band edge. The rapid cooling of these electrons to the GaN conduction band edge and their subsequent capture leads to a shift, in the photoluminescence spectrum and hence the oscillations in photoluminescence. We interpret the shift in the spectrum as being due to a modification of the occupation of the distribution of localization centres by electrons. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Comparison of GaInN laser structures grown on different substrates
Authors: Dräger A. , Fuhrmann D. , Netzel C. , Rossow U. , Schenk H. P. D. , Hangleiter A.
Published: April 2008

In order to get a better understanding of the influence of different substrates on GaInN laser structures we performed optical gain spectroscopy using the Variable Stripe Length Technique on laser samples grown by MOVPE. These samples were grown on SiC, GaN template on sapphire and on freestanding GaN substrates. Using the measurements on these samples the influence of the substrate on the gain performance and on the recombination processes in the samples is determined. The main focus of the studies are the measurements made on samples grown on different substrates during the same growth‐process and in addition to reach an emitting wavelength longer than 450 nm. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Electrical and optical properties of thick highly doped p‐type GaN layers grown by HVPE
Authors: Usikov A. , Kovalenkov O. , Soukhoveev V. , Ivantsov V. , Syrkin A. , Dmitriev V. , Nikiforov A. Yu. , Sundaresan S. G. , Jeliazkov S. J. , Davydov A. V.
Published: April 2008

In this paper we report 3‐7 μm thick p‐GaN growth by hydride vapor phase epitaxy (HVPE) on sapphire substrates. Mg impurity was used for doping. As‐grown GaN layers had p‐type conductivity with concentration NA‐ND up to 3×1019 cm–3. Mg atom concentration was varied from 1017 to 1020 cm–3. Hydrogen concentration was about 10 times less than that for Mg, which may explain effective p‐type doping for as‐grown GaN layers. Micro‐cathodoluminescence revealed a columnar‐like structure of the GaN layers with a non‐uniform distribution of material regions having dominant 362 nm or 430 nm luminescence. Use of these thick p‐GaN layers to grow InGaN‐based blue and green LEDs by the HVPE is demonstrated. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
In‐situ passivation combined with GaN buffer optimization for extremely low current dispersion and low gate leakage in Si3N4/AlGaN/GaN HEMT devices on Si (111)
Authors: Germain M. , Cheng K. , Derluyn J. , Degroote S. , Das J. , Lorenz A. , Marcon D. , Hove M. Van , Leys M. , Borghs G.
Published: April 2008

Using Si substrates is the sole route towards III‐nitrides process on large wafer diameter: flat, crack‐free HEMT epiwafers have been grown by MOVPE with diameter up to 150 mm, exhibiting very low sheet resistivity (260 Ω/sq.) and high uniformity (<2%), thanks to in‐situ Si3N4 capping (<5 nm). We show here that simultaneous optimization of GaN growth and of device passivation results in a drastic reduction of buffer and surface traps. This has impact onto device performance, assessed by pulse IV measurements. Drain lag is correlated with the appearance of deep defect‐related peaks in photoluminescence spectra (6 K), and completely eliminated by tuning growth temperature. Gate formation (Ni/Au) onto MOVPE‐grown Si3N4, followed by cleaning and PECVD ex‐situ passivation, leads to very low gate current dispersion, associated to gate leakage current in μA/mm range. Both dc and pulsed characteristics show a current density in the order of 0.8‐1 A/mm, whereas maximal transconductance is 250 mS/mm (Lg = 1 μm). (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Influence of the Mn compositional distribution on the magnetic order in diluted GaMnN layers
Authors: Mai D. , Niermann T. , Zenneck J. , Roever M. , Pinto A. Bedoya , Malindretos J. , Seibt M. , Rizzi A.
Published: April 2008

GaMnN is a prototypical GaN‐based dilute magnetic semiconductor (DMS) and its potential as a room temperature ferromagnetic material is still a controversial question to be answered. Diluted GaMnN layers with Mn concentrations up to several percent have been grown by plasma‐assisted MBE on MOVPE‐GaN templates either on p‐Si(111) or on Al2O3(0001) substrates. None of the samples reported here shows second phase precipitates in XRD or in the TEM analysis. However different composition distributions have been evidenced by EDX during the scanning TEM analysis.

GaMnN grown at a substrate temperature of TS = 650 °C exhibits extended structural defects and strong compositional inhomogeneity. This could be indicative of decomposition due to local strain. Samples grown at TS = 775 °C with a Mn concentration of about 3% show a homogeneous Mn distribution in the layer. The magnetic properties are found to vary from spin glass to ferromagnetic behaviour at room temperature among different samples. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
High breakdown voltage AlGaN/GaN heterojunction field effect transistors on sapphire
Authors: Shi Junxia , Choi Y. C. , Pophristic M. , Spencer M. G. , Eastman L. F.
Published: April 2008

AlGaN/GaN heterojunction field effect transistors (HFETs) on sapphire substrates for high‐power switching applications were fabricated using a self‐align process. Without any additional field plate design, the fabricated devices with gate‐drain spacing (Lgd) of 16 µm exhibited a high breakdown voltage (BV) of 1580 V with a specific on‐resistance (ARon) of 4.0 mΩ‐cm2. This result approaches the SiC theoretical limit. The BV‐ARon performance showed a linear trend with variations of Lgd, indicating a predictable performance. In addition, the effects of the source and drain contact lengths Ls and Ld on ARon have also been investigated and optimized. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Intensity‐dependent photoluminescence studies of the electric field in N‐face and In‐face InN/InGaN multiple quantum wells
Authors: Chern‐Metcalfe G. D. , Readinger E. D. , Shen H. , Wraback M. , Koblmüller G. , Gallinat C. S. , Speck J. S.
Published: April 2008

We probe the electric field in In‐face and N‐face InN/InGaN multiple quantum wells (MQWs) using low temperature intensity‐dependent photoluminescence (PL) under continuous‐wave laser excitation at 900 nm. The In‐face structure consists of 30 periods of 2.5 nm thick InN wells and 24 nm thick In0.92Ga0.08N barriers. The N‐face sample consists of 25 periods of 1.5 nm InN wells and 15 nm thick In0.88Ga0.12N barriers. At low excitation power, the PL peak energy from both In‐face and N‐face MQWs is redshifted relative to that from bulk InN. As excitation power increases, we observe a 50 and 44 meV blueshift of the PL peak energy from the In‐face and N‐face MQWs, respectively. The blueshift is a result of the optically induced screening of the built‐in electric field. From the measured blueshift, we calculate a minimum change of the electric field to be ∼0.4 MV/cm and ∼0.6 MV/cm in the In‐face and N‐face well regions, respectively. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Fabrication and characterization of m‐plane (1‐100) GaN based metal‐oxide‐semiconductor capacitors
Authors: Tilak V. , Matocha K. , Dunne G.
Published: April 2008

M‐plane non‐polar, Gallium Nitride(GaN) based metal‐oxide‐semiconductor (MOS) capacitors were fabricated by depositing 150 nm thick low‐pressure chemical vapor deposition silicon dioxide at 900 °C. No positive shift in the flat band voltage of m‐plane GaN MOS capacitors was observed versus temperature, confirming the absence of pyroelectric effects. In contrast, c‐plane (0001) polar, GaN MOS capacitors, show a positive flat band voltage shift versus temperature and the pyroelectric coefficient of –3×109 q/cm2‐K was measured. A small negative shift in flat band voltage was observed in m‐plane GaN consistent with changes in the bulk potential and corresponding decrease in the occupancy of interface traps. Time dependent dielectric(TDDB) measurements were performed at 200 °C and a mean time to failure (MTTF) of 40 hours was measured at an electric field of 5 MV/cm. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Strain and compositional analyses of Al‐rich Al1–xInxN films grown by MOVPE: impact on the applicability of Vegard's rule
Authors: Darakchieva V. , Beckers M. , Hultman L. , Xie M. , Monemar B. , Carlin J.‐F. , Grandjean N.
Published: April 2008

We have studied composition and strain in Al1–x Inx N films with 0.128≤ x≤ 0.22 grown on GaN‐buffered sapphire substrates by metalorganic vapor phase epitaxy. A good agreement between the In contents determined by Rutherford backscattering spectrometry (RBS) and Xray diffraction (XRD) is found for x≤ 18, suggesting applicability of Vegard's rule in the narrow compositional range around the lattice matching to GaN. The increase of the In content up to x = 0.22 leads to a formation of sub‐layers with a higher composition, accompanied by deviations from Vegard's rule. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Double heterostructure ultraviolet light emitting diodes with nanometer scale compositionally inhomogeneous active regions
Authors: Sampath A. V. , Reed M. L , Chua C. , Garrett G. A. , Dang G. , Readinger E. D. , Shen H. , Usikov A. , Kovalenkov O. , Shapovalova L. , Dmitriev V. A. , Johnson N. M. , Wraback M.
Published: April 2008

We report on the fabrication and evaluation of flip‐chipped double heterostructure UVLEDs operating at 320 nm that employ a bulk (80nm) AlGaN active region containing nanoscale compositional inhomogeneities (NCI), deposited by plasma‐assisted molecular beam epitaxy. The devices were deposited on AlGaN templates grown by hydride vapor phase epitaxy and contain electron and hole injection layers grown by metalorganic chemical vapor deposition. A packaged, 300 μm x 300 μm device with 3% internal quantum efficiency has a peak output power of 0.56 mW at 90 mA DC current that corresponds to an external quantum efficiency of 0.15%. Nearly constant external quantum efficiency is observed for DC current density up to 100 A/cm2, suggesting that the NCI regions effectively suppress the deleterious effects of polarization fields at low injection currents that should inhibit radiative recombination in double heterostructure devices. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Electron transport properties of low sheet‐resistance two‐dimensional electron gases in ultrathin AlN/GaN heterojunctions grown by MBE
Authors: Cao Yu , Wang Kejia , Jena Debdeep
Published: April 2008

A study of the transport properties of polarizationinduced 2DEGs at MBE‐grown single AlN/GaN heterostructures with different growth rates is reported. It is observed that faster growth rates lead to high mobilities, approaching ∼ 1600 cm2/Vs at 300 K and ∼ 6000 cm2/Vs at low temperatures for ultrathin (2.3 nm AlN/GaN) heterojunctions. By using a theoretical model in conjunction with experimentally measured transport properties, it is concluded that the 300 K sheet resistances of very high density 2DEGs at AlN/GaN heterojunctions are currently limited (∼ 170 Ω /□) by interface roughness scattering, and can be further reduced by improving the growth conditions. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Integration of InGaN quantum dots into nitride‐based microcavities
Authors: Kruse C. , Figge S. , Dartsch H. , Tessarek C. , Hommel D. , Lohmeyer H. , Kalden J. , Sebald K. , Gutowski J.
Published: April 2008

We report on the successful integration of a single InGaN quantum dot sheet into a resonant half cavity structure con‐sisting of an AlGaN/GaN distributed Bragg reflector and a GaN λ‐cavity layer. For reference, small‐diameter pillar mi‐crocavites fabricated by focused‐ion beam etching from an empty planar cavity are studied. These structures exhibit discrete transversal modes and Q factors of 260. The quantum dot active region has been grown by metal‐organic vapour‐phase epitaxy using a two‐step deposition technique. Optical spectra measured by micro‐photoluminescence reveal distinct spectrally sharp emission lines around 2.73 eV which can be attributed to the emission of single InGaN quantum dots. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Enlargement of bulk non‐polar GaN substrates by HVPE regrowth
Authors: Lai K. Y. , Wheeler V. D. , Grenko J. A. , Johnson M. A. L. , Hanser A. D. , Preble E. A. , Liu L. , Paskova T. , Evans K. R.
Published: April 2008

M‐plane non‐polar bulk GaN substrates were regrown by hydride vapour phase epitaxy (HVPE), aiming to enlarge their lateral size. The non‐polar GaN substrates were synthesized by growing thick GaN boules by HVPE along the c‐axis, and wafering transversely to expose non‐polar (m‐ or a‐plane) surfaces. Non‐polar GaN substrates obtained in this manner contains lower defect density comparing to those produced by heteroepitaxial growth. After the regrowth, substantial lateral expansion in [0001], [1120] and [1120] was observed. Threading dislocation densities were estimated by cathodoluminescence measured at the cross‐section areas. The dislocation densities in the substrate and the regrown areas were approximately 5×106 cm–2 and 1×106 cm–2, respectively, indicating a comparable structural quality achieved by the regrowth. Non‐polar GaN substrates prepared by this process provide a promising method to fabricate devices for next generation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
GaN MMICs for RF power applications in the 50 GHz to 110 GHz frequency range
Authors: Micovic M. , Kurdoghlian A. , Hashimoto P. , Hu M. , Antcliffe M. , Willadsen P. J. , Wong W. S. , Bowen R. , Milosavljevic I. , Yoon Y. , Schmitz A. , Wetzel M. , Chow D. H.
Published: April 2008

At 2006 IEDM outstanding potential of GaN MMIC technology for RF power applications at frequencies exceeding 80 GHz was reported for the first time. The first reported W‐band GaN power MMIC had over 16 dB of saturated power gain in a frequency range between 78 GHz and 88 GHz and produced almost the same output power at this frequency band as the state of the art InP HEMT MMIC with 10 times larger output device periphery. In this presentation we will describe improvements in GaN MMIC process that led to these breakthrough results and discuss the impact of GaN technology on the development of high frequency millimeter‐wave power modules. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A one‐sided MEWMA chart for health surveillance
Authors: Joner Michael D. , Woodall William H. , Reynolds Marion R. , Fricker Ronald D.
Published: April 2008

It is often important to rapidly detect an increase in the incidence rate of a given disease or other medical condition. It has been shown that when disease counts are sequentially available from a single region, a univariate control chart designed to detect rate increases, such as a one‐sided cumulative sum chart, is very effective. When disease counts are available from several regions at corresponding times, the most efficient monitoring method is not readily apparent. Multivariate monitoring methods have been suggested for dealing with this detection problem. Some of these approaches have shortcomings that have been recently demonstrated in the quality control literature. We discuss these limitations and suggest an alternative multivariate exponentially weighted moving average chart. We compare the average run‐length performance of this chart with that of competing methods. We also evaluate the statistical performance of these charts when the actual increase in the disease count rate is different from the one that the chart was optimized to detect quickly. Copyright © 2008 John Wiley & Sons, Ltd.
Use of carbon isotope analysis to understand semi‐arid erosion dynamics and long‐term semi‐arid land degradation
Authors: Turnbull Laura , Brazier Richard E. , Wainwright John , Dixon Liz , Bol Roland
Published: April 2008

Many semi‐arid areas worldwide are becoming degraded, in the form of C4 grasslands being replaced by C3 shrublands, which causes an increase in surface runoff and erosion, and altered nutrient cycling, which may affect global biogeochemical cycling. The prevention or control of vegetation transitions is hindered by a lack of understanding of their temporal and spatial dynamics, particularly in terms of interactions between biotic and abiotic processes. This research investigates (1) the effects of soil erosion on the δ13C values of soil organic matter (SOM) throughout the soil profile and its implications for reconstructing vegetation change using carbon‐isotope analysis and (2) the spatial properties of erosion over a grass‐shrub transition to increase understanding of biotic‐abiotic interactions by using δ13C signals of eroded material as a sediment tracer. Results demonstrate that the soils over grass‐shrub transitions are not in steady state. A complex interplay of factors determines the input of SOM to the surface horizon of the soil and its subsequent retention and turnover through the soil profile. A positive correlation between event runoff and δ13C signatures of eroded sediment was found in all plots. This indicates that the δ13C signatures of eroded sediment may provide a means of distinguishing between changes in erosion dynamics over runoff events of different magnitudes and over different vegetation types. The development of this technique using δ13C signatures of eroded sediment provides a new means of furthering existing understanding of erosion dynamics over vegetation transitions. This is critical in terms of understanding biotic‐abiotic feedbacks and the evolution of areas subject to vegetation change in semi‐arid environments. Copyright © 2008 John Wiley & Sons, Ltd.
Long‐term influence of manure and mineral nitrogen applications on plant and soil 15N and 13C values from the Broadbalk Wheat Experiment
Authors: Senbayram Mehmet , Dixon Liz , Goulding Keith W. T. , Bol Roland
Published: April 2008

The Broadbalk Wheat Experiment at Rothamsted Research in the UK provides a unique opportunity to investigate the long‐term impacts of environmental change and agronomic practices on plants and soils. We examined the influence of manure and mineral fertiliser applications on temporal trends in the stable N (15N) and C (13C) isotopes of wheat collected during 1968–1979 and 1996–2005, and of soil collected in 1966 and 2000. The soil δ15N values in 1966 and 2000 were higher in manure than the mineral N supplied soil; the latter had similar or higher δ15N values than non‐fertilised soil. The straw δ15N values significantly decreased in all N treatments during 1968 to 1979, but not for 1996–2005. The straw δ15N values decreased under the highest mineral N supply (192 kg N ha−1 year−1) by 3‰ from 1968 to 1979. Mineral N supply significantly increased to straw δ13C values in dry years, but not in wet years. Significant correlations existed between wheat straw δ13C values with cumulative rainfall (March to June). The cultivar Hereward (grown 1996–2005) was less affected by changes in environmental conditions (i.e. water stress and fertiliser regime) than Cappelle Desprez (1968–1979). We conclude that, in addition to fertiliser type and application rates, water stress and, importantly, plant variety influenced plant δ13C and δ15N values. Hence, water stress and differential variety response should be considered in plant studies using plant δ13C and δ15N trends to delineate past or recent environmental or agronomic changes. Copyright © 2008 John Wiley & Sons, Ltd.
Robust trajectory tracking for a reversing tractor trailer
Authors: Pradalier Cédric , Usher Kane
Published: April 2008

Tractor‐trailer reversing is a classical nonlinear control problem in which many of the solutions proposed in the literature perform poorly in the presence of real‐world constraints such as steering angle, rate limits, and lags. In this paper we describe a new method in which an inner loop controls the hitch angle of the trailer, creating a virtual articulated vehicle to which existing control techniques can be applied. We provide an analysis of the stability and convergence properties of this control approach, as well as experimental results that illustrate the robustness of this approach to model estimation errors, low‐level control loop dynamics, and other disturbances introduced by, for example, state estimation errors. © 2008 Wiley Periodicals, Inc.
Long‐term simulations of the dynamics of trout populations on river reaches bypassed by hydroelectric installations—analysis of the impact of different hydrological scenarios
Authors: Gouraud V. , Capra H. , Sabaton C. , Tissot L. , Lim P. , Vandewalle F. , Fahrner G. , Souchon Y.
Published: April 2008

Changes in a brown trout (Salmo trutta L.) population result from interaction among various mechanisms which are dependent on environmental conditions and biotic processes. In reaches influenced by the presence of dams, the instream flow in the bypassed section is not the only parameter which affects the population. Flood episodes, the general connectivity of the bypassed section, and the characteristics of the substrate which define the availability and quality of spawning grounds may also have a crucial impact. The design and fine‐tuning of tools which take environmental parameters into account can improve our understanding of the dynamics of such influenced populations. In this perspective, a deterministic model (MODYPOP) has been developed in an attempt to integrate all these factors and to test the effect of different long‐term scenarios of influenced flow regimes on the structure of trout populations. MODYPOP was applied to three populations and three reaches (on the Roizonne, Neste d'Aure and Lignon du Forez rivers in France). For each stream, experiments were carried out on a bypassed section downstream of a hydropower station, before and after an increase in the minimum instream flow due to relicensing. These experiments allowed integrating into MODYPOP local phenomena (impact of flood episodes, impact of flushing, impact of downstream migration of juveniles and adults) affecting the populations during the study period and then calibrating them. To estimate the change in the population due to the increase in minimum instream flow, different long‐term simulations were run, selecting discharge patterns at random. These scenarios help to evaluate the time required for the population to return to a range close to habitat saturation after an improvement in the hydraulic habitat or following a flood event. These applications have enabled determining the relative importance of changes in population density due to different types of events. Copyright © 2008 John Wiley & Sons, Ltd.
Fabrication of Suspended Silicon Nanowire Arrays
Authors: Lee Kook‐Nyung , Jung Suk‐Won , Shin Kyu‐Sik , Kim Won‐Hyo , Lee Min‐Ho , Seong Woo‐Kyeong
Published: April 2008

A method to fabricate suspended silicon nanowires that are applicable to electronic and electromechanical nanowire devices is reported. The method allows for the wafer‐level production of suspended silicon nanowires using anisotropic etching and thermal oxidation of single‐crystal silicon. The deviation in width of the silicon nanowire bridges produced using the proposed method is evaluated. The NW field‐effect transistor (FET) properties of the device obtained by transferring suspended nanowires are shown to be practical for useful functions.
Prenatal diagnosis of posterior mediastinal lymphangioma by two‐ and three‐dimensional ultrasonography
Authors: Ruano R. , Takashi E. , Schultz R. , Zugaib M.
Published: April 2008

Lymphangioma is a rare benign tumor characterized by proliferating lymph vessels and composed of large cyst spaces with endothelium‐lined channels of varying dimensions. The incidence of lymphangioma is approximately one in 6000 pregnancies. Less than 1% of lymphangiomas are purely mediastinal. The great majority of cases are of cystic lymphangioma, but very rarely there is a mixed lesion consisting of multiple cysts of dilated capillary and lymph vessels. We report a case of posterior mediastinal lymphangioma diagnosed at 28 weeks' gestation, in which three‐dimensional ultrasonography was helpful in determining the precise location of the tumor. A Cesarean section was performed at 39 weeks and the tumor was resected on the 5th day postdelivery; histological examination revealed a mixed cystic lymphangioma. Copyright © 2008 ISUOG. Published by John Wiley & Sons, Ltd.
Adductor myotomy in cerebral palsy: Uni or bilateral
Authors: Fernandez-Palazzi Federico , Carpio Annie
Published: April 2008

Introduction

In patients with cerebral palsy, the prevention of hip dislocation should be the “primum momens” of early surgery. The surgery consists of a myotomy of the adductor medium, mayor and gracilis, and, in non-ambulatory cases, a neurectomy of the second branch of the obturator nerve. The purpose of this study was to examine whether the adductor myotomy should be performed on the contralateral side at one sitting.

Materials and methods

In a study performed by our team on 1,474 patients, 792 patients (53.7%) had a unilateral procedure and 682 a bilateral procedure.

Results

The non-myotomized spastic adductors alter the movement of forces on the femoral head and, thus, 78.20% required a secondary contralateral procedure. The dislocation of the hip in patients whose operations were performed at two different sittings occurred in 20% of cases, and in those with a bilateral procedure at one sitting only in 1%. The increase in the cervicodiaphyseal angle occurred in 23.1% of patients with surgeries performed at two sittings and only in 10% of patients with bilateral procedures at one sitting.

Conclusions

Our results suggest that the bilateral surgical procedure at one sitting should be recommended to equalize the movements of force in spastic hips.
Activation of kappa opioid receptors decreases synaptic transmission and inhibits long‐term potentiation in the basolateral amygdala of the mouse
Authors: Huge Volker , Rammes Gerhard , Beyer Antje , Zieglgänsberger Walter , Azad Shahnaz C.
Published: April 2008

Background: The amygdala plays an important role in the processing of chronic pain and pain memory formation. Particularly, it is involved in the emotional and affective components of the pain circuitry. The role of kappa opioid receptors in these pain conditions is only partly known. The present study investigates the effect of kappa receptor activation on synaptic transmission and synaptic plasticity in the amygdala.

Methods: Electrophysiological in vitro experiments were carried out in brain slices of male C57BL/6JOlaHsd mice. The effect of the kappa opioid receptor agonist U50,488H (5μM) and the selective kappa opioid receptor antagonist nor‐BNI (3μM) on field potential (FP) amplitude and the induction of long‐term potentiation (LTP) in the basolateral amygdala (BLA) was examined.

Results: High frequency stimulation (HFS) of afferents in the lateral amygdala with two trains of 100 pulses at 50Hz increased the FP amplitudes to 119±2% (mean±SEM; n=6) in the BLA. U50,488H decreased synaptic transmission (baseline: 100±0.5%; U50,488H: 86.3±2.4%; n=6) and blocked the induction of LTP (U50,488H: 100±4.1%; HFS: 102.6±7%; n=6). The effect on synaptic transmission and on LTP was completely reversed or prevented by application of nor‐BNI, which itself had no effect on synaptic transmission or the induction of LTP.

Conclusion: Kappa opioid receptor activation decreases synaptic transmission and inhibits the induction of LTP in the BLA of the mouse. These findings may be associated with the effects of kappa opioid agonists in chronic pain and pain memory formation.
Fibre-induced drag reduction
Authors: GILLISSEN J. J. J. , BOERSMA B. J. , MORTENSEN P. H. , ANDERSSON H. I.
Published: April 2008

We use direct numerical simulation to study turbulent drag reduction by rigid polymer additives, referred to as fibres. The simulations agree with experimental data from the literature in terms of friction factor dependence on Reynolds number and fibre concentration. An expression for drag reduction is derived by adopting the concept of the elastic layer.
Lichenothelia renobalesiana sp. nov. (Lichenotheliaceae), for a lichenicolous ascomycete confused with Polycoccum opulentum (Dacampiaceae)
Authors: ATIENZA Violeta , HAWKSWORTH David L.
Published: April 2008

The name Polycoccum opulentum proves to have been wrongly applied to a lichenicolous fungus belonging to the genus Lichenothelia, and which differs from the type material of P. opulentum in having non-ostiolate broadly stipitate ascomata and narrow ascospores with a conspicuous gelatinous sheath. The Lichenothelia represents an hitherto undescribed species, named here as L. renobalesiana, sp. nov. which is associated with verrucarialean lichens on hard limestones in temperate regions. The true P. opulentum is known only from the original collection on Polyblastia hyperborea from Sweden.
A new species of Lecanora s. lat., growing on Lasallia pustulata
Authors: PÉREZ-ORTEGA Sergio , ETAYO Javier
Published: April 2008

The new species Lecanora lasalliae Pérez-Ortega & Etayo is described from Spain. It is included provisionally in Lecanora s. lat as characters such as Lecanora-type ascus, exciple composed of thick radiating hyphae and the usual presence of algal cells in the excipulum or its lichenicolous habitus on Lasallia pustulata, do not fit well within any known genus of lichenicolous or lichenized fungi. Its taxonomic affinities with several taxa are discussed, including the parasitic Lecanora gyrophorina.
Aspicilia berntii, a new name for a poorly known species
Authors: NORDIN Anders , TIBELL Leif , OWE-LARSSON Björn
Published: April 2008

The new name Aspicilia berntii is proposed to accommodate Lecanora mastoidea Lynge in Aspicilia and a lectotype is designated. Additional characters are added to the species description and a comparison is made with similar species. The species is reported as new to Scandinavia with localities in northern Norway. An assessement of the phylogenetic relationships, based on a split network analysis of ITS sequences, places A. berntii close to A. verrucigera in the A. cinerea group.
Lichen diversity of coarse woody habitats in a Pinus-Larix stand in the Italian Alps
Authors: NASCIMBENE Juri , MARINI Lorenzo , MOTTA Renzo , NIMIS Pier Luigi
Published: April 2008

In the European Alps, subalpine forests are important for lichen diversity conservation at multiple levels. However, the potential role of coarse woody debris (CWD) has been only marginally considered in the previous literature. The aims of this case study are (i) to provide a first evaluation of the effects of different types of CWD (stumps, logs, and snags) and wood decay on lichen diversity in Pinus-Larix forests in the Italian Alps, and (ii) to increase the knowledge of the lichen biota on CWD in Italy. Seventy-eight species were found, including 12 nationally rare and 10 calicioid species; Placynthiella hyporhoda is new to Italy. Different types of CWD in different decay stages had significantly different lichen communities. Snags seem to be particularly effective for conservation, since they host the highest number of nationally rare species. Our survey suggests that CWD could be an important substratum for several nationally rare species, indicating that further surveys at a regional scale are urgently required in order to evaluate the role of CWD for lichen diversity conservation in the Alps.
David Hume and international political theory: a reappraisal
Authors: VAN DE HAAR EDWIN
Published: April 2008

David Hume’s ideas on international relations are different than most international relations academics suppose. Close scrutiny of Hume’s views on the nation, international society, war, balance of power, empire and trade reveals the need to reassess his place within international political theory. Taking an English School perspective, the analysis also shows the possible benefits for IR theorists within this tradition to focus on Scottish Enlightenment philosophy, which will also strengthen the position of the pluralist perspective within international society.
Biopolitics of security in the 21st century: an introduction
Authors: DILLON MICHAEL , LOBO-GUERRERO LUIS
Published: April 2008

This essay addresses two questions. It first asks what happens to security practices when they take species life as their referent object. It then asks what happens to security practices which take species life as their referent object when the very understanding of species life undergoes transformation and change. In the process of addressing these two questions the essay provides an exegesis of Michel Foucault’s analytic of biopolitics as a dispositif de sécurité and contrasts this account of security with that given by traditional geopolitical security discourses. The essay also theorises beyond Foucault when it interrogates the impact in the twentieth century of the compression of morbidity on populations and the molecular revolution on what we now understand life to be. It concludes that ‘population’, which was the empirical referent of early biopolitics, is being superseded by ‘heterogenesis’. This serves as the empirical referent for the recombinant biopolitics of security in the molecular age.