bluuebunny/crossref_metadata_2025_split huggingface dataset:
Atomic structure of pyramidal defects in GaN:Mg: Influence of annealing
Authors: Liliental‐Weber Z. , Tomaszewicz T. , Zakharov D. , O'Keefe M. , Hautakangas S. , Saarinen K. , Freitas J. A. , Henry R. L. 
Published: May 2006

AbstractThe atomic structure of the characteristic defects (Mg‐rich hexagonal pyramids) in p‐doped bulk and MOCVD GaN:Mg thin films grown with Ga polarity was determined at atomic resolution by direct reconstruction of the scattered electron wave in a transmission electron microscope. Small cavities were present inside the defects, confirmed also with positron annihilation. The inside walls of the cavities were covered by GaN of reverse polarity compared to the matrix. Annealing of the MOCVD layers lead to slight increase of the defect size and an increase of the room temperature photoluminescence intensity. Positron annihilation confirms presence of vacancy clusters of different sizes triggered by the Mg doping in as‐grown samples and decrease of their concentration upon annealing at 900 and 1000 °C. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optimization of AlGaN/GaN HEMTs for high frequency operation
Authors: Palacios T. , Dora Y. , Chakraborty A. , Sanabria C. , Keller S. , DenBaars S. P. , Mishra U. K. 
Published: May 2006

AbstractIn the article [1] featured at Editor's Choice, the structure and processing of AlGaN/GaN high electron mobility transistors (HEMTs) have been optimized for maximum small signal gain at high frequencies. The cover picture combines the sample structure – shown schematically and in a scanning electron microscopy image – with the band diagram of the sample with an InGaN back‐barrier used to increase the electron confinement in comparison to a standard HEMT.The first author, Tomás Palacios, is currently a Project Scientist at UCSB. His research interest focuses on the search of novel GaN‐based transistors for mm‐wave applications and biological sensors. He is one of the winners of the physica status solidi Young Researcher Awards for his outstanding presentation at the 6th International Conference on Nitride Semiconductors held in Bremen, Germany, in 2005. Further articles from ICNS‐6 will also be published in phys. stat. sol. (b) 243, No. 7 (2006) and phys. stat. sol. (c) 3, No. 6 (2006).The present issue of phys. stat. sol. (a) as well as phys. stat. sol. (c) 3, No. 5 (2006) also contain papers presented at the International Conference on Nanoscale Magnetism (ICNM‐2005) in Gebze, Turkey.
InN: Fermi level stabilization by low‐energy ion bombardment
Authors: Piper L. F. J. , Veal T. D. , McConville C. F. , Lu H. , Schaff W. J. 
Published: May 2006

AbstractThe near‐surface electronic properties of InN have been investigated with high‐resolution electron‐energyloss spectroscopy. Low‐energy (∼400 eV) nitrogen ion bombardment followed by low temperature annealing (<300 °C) was found to dramatically increase the n ‐type conductivity of InN, close to the surface. This is explained in terms of the formation of amphoteric defects from the ion bombardment and annealing combined with the band structure of InN. Low‐energy ion bombardment and annealing is shown to result in a damage‐induced, donor‐like defect‐profile instead of the expected electron accumulation for InN. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Analysis of buffer‐trapping effects on current collapse of GaN FETs
Authors: Horio K. , Takayanagi H. , Nakano H. 
Published: May 2006

AbstractTwo‐dimensional transient simulation of GaN MESFETs is performed in which a deep donor and a deep acceptor in a semi‐insulating buffer layer are considered. Quasi‐pulsed I‐V curves are derived from the transient characteristics, and are compared with the steady‐state I‐V curves. It is shown that the current collapse is more pronounced when the deep‐acceptor density in the buffer layer is higher and when an off‐state drain voltage is higher, because the trapping effects become more significant. It is suggested that to minimize current collapse in GaN‐based FETs, an acceptor density in a semi‐insulating GaN layer should be made low, although the current cutoff behaviour may be degraded. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Electronic states in nitride semiconductor quantum dots: A tight‐binding approach
Authors: Schulz S. , Czycholl G. 
Published: May 2006

AbstractWe investigate the electronic properties of a lens‐like InN/GaN QD with a wurtzite structure within a tightbinding (TB) model. The TB approach provides a theoretical description of these structures on a microscopic level. An sp 3‐TB model is used which well‐reproduces the band structure of the bulk material. The resulting parameters are used as an input to model the low dimensional heterostructure. Wurtzite nitrides exhibit a (spontaneous) polarization P which leads to the appearance of an electrostatic potential Vp . We incorporate the potential Vp in our TB model to examine the influence on the energy levels and the single particle wave functions for the investigated QD structure. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
An AlGaN/GaN two‐color photodetector based on an AlGaN/GaN/SiC HEMT layer structure
Authors: Marso M. , Fox A. , Heidelberger G. , Bernát J. , Lüth H. 
Published: May 2006

AbstractThe optoelectronic properties of MSM diodes that are based on an AlGaN/GaN HEMT layer system are investigated as function of the applied bias. For low voltages, the two‐dimensional electron gas acts as a barrier for the depletion layer. Therefore, only the upper AlGaN barrier layer contributes to the photocurrent. In the high voltage regime the depletion region penetrates the GaN buffer that adds the spectral responsivity of GaN to the pure AlGaN behaviour in the low voltage regime. The ratio of the responsivities at 350 nm and 300 nm wavelength can be switched from 0.01 at 2.5 V to 0.8 at 4 V bias. This property makes the MSM‐2DEG a candidate for use as two‐color photodetector. Device fabrication uses standard HEMT processing steps, allowing integration in HEMT circuits without the need of sophisticated growth or etching techniques. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Ion implantation doping for AlGaN/GaN HEMTs
Authors: Suita Muneyoshi , Nanjo Takuma , Oishi Toshiyuki , Abe Yuji , Tokuda Yasunori 
Published: May 2006

AbstractSelective doping using ion implantation has been developed for the source/drain contacts of AlGaN/GaN high electron mobility transistors (HEMTs). As the annealing temperature of the electrical activation increased, the gate leakage current increased, although the specific contact resistance decreased. The characteristics of ion implanted HEMTs were improved using the optimized annealing temperature of 1150 °C. A saturation drain current of 0.68 A/mm and a maximum transconductance of 0.17 S/mm were obtained without degrading the off‐state breakdown voltage. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Extremely smooth surface morphology of GaN‐based layers on misoriented GaN substrates for high‐power blue‐violet lasers
Authors: Tachibana Koichi , Nago Hajime , Nunoue Shin‐ya 
Published: May 2006

AbstractWe have successfully realized a very smooth surface morphology of GaN‐based epitaxial layers on n‐GaN (0001) substrates with the misorientation angle of approximately 0.2° only toward [1¹100] direction, which is confirmed using Nomarski microscopy and atomic force microscopy. In x‐ray diffraction spectra, the narrow width of the satellite peak of superlattices also indicated the smooth surface morphology and abrupt interfaces of the epitaxial layers on the GaN (0001) substrate with the misorientation angle only toward [1$ \bar 1 $00] direction. The realization of smooth surface morphology of the layers grown on GaN (0001) substrates misoriented only towards [1$ \bar 1 $00] direction is considered to be closely related to the anisotropic inplane growth rate. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
High temperature characteristics of doped channel AlGaN/GaN MIS‐HFETs with thin AlGaN barrier layer
Authors: Wang C. X. , Maeda N. , Hiroki M. , Yokoyama Y. , Makimoto T. , Kobayashi T. , Enoki T. 
Published: May 2006

AbstractThe device performance of AlGaN/GaN‐based metal‐insulator‐semiconductor heterostructure field effect transistors (MIS‐HFETs) with a very thin AlGaN barrier and a heavily doped GaN channel for high 2DEG density has been investigated at elevated temperatures up to 250 °C. The devices exhibited ultra‐low gate current leakages under the reverse gate bias with very reasonable transconductance characteristics at both RT and high temperatures. MIS‐HFETs with doped channel showed much higher saturation drain current and weaker current collapse than the conventional devices at RT and high temperatures. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Towards a better understanding of nano‐islands formed during atmospheric pressure MOVPE
Authors: van der Laak N. K. , Oliver R. A. , Barnard J. S. , Cherns P. D. , Kappers M. J. , Humphreys C. J. 
Published: May 2006

AbstractWe report on transmission electron microscopy (TEM) characterisation of nano‐islands formed during atmospheric pressure growth of InGaN on GaN by metal‐organic vapour phase epitaxy (MOVPE). Although initial results seemed to indicate that three‐dimensional InGaN nano‐islands had formed, detailed TEM characterisation revealed that the islands do not have a hexagonal crystal structure, and electron energy loss spectroscopy suggests that the observed structures are metallic indium droplets formed during growth, which may be partially nitrided, rather than the expected strain‐induced epitaxial InGaN nano‐islands. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Hydrogen generation from aqueous water using n‐GaN by photoassisted electrolysis
Authors: Fujii Katsushi , Ohkawa Kazuhiro 
Published: May 2006

AbstractWe propose a new GaN application for water splitting using photoelectrolysis. Hydrogen gas generation from a counterelectrode was clearly observed using light‐illuminated n‐type GaN as a working photoelectrode in an electrolyte. The hydrogen generation in KOH was the highest. Corrosion of GaN in NaCl and HCl was, however, quite less than that in KOH during photoelectrolysis. A combination of GaN and HCl is promising in stable photo‐electrolysis for H2 generation. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
High quality tin zinc oxide/Ag ohmic contacts for UV flip‐chip light‐emitting diodes
Authors: Hong Hyun‐Gi , Ban K.‐Y. , Song June‐O , Cho J. , Park Y. , Kwak J. S. , Ferguson Ian T. , Seong Tae‐Yeon 
Published: May 2006

AbstractHigh quality SnZnO (TZO)/Ag ohmic contacts to p ‐GaN (Na = 4×1017/cm3) for UV flip‐chip light‐emitting diodes (LEDs) were investigated. The TZO (2.5 nm)/Ag (250 nm) contacts produce contact resistivity of 1.58 × 10‐4 Ωcm2 and reflectance of 76% at 405 nm when annealed at a temperature of 530 °C for 1min in air. Near‐UV LEDs made with the annealed TZO (2.5 nm)/Ag (250 nm) contacts give reverse leakage current of ∼10‐8 A at reverse voltage of –10 V. Based on X‐ray photoelectron spectroscopy, possible ohmic formation mechanisms are discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A comparative study on MOVPE InN grown on Ga‐ and N‐polarity bulk GaN
Authors: Wang W. J. , Miwa H. , Hashimoto A. , Yamamoto A. 
Published: May 2006

AbstractThe influence of substrate polarity on the growth of InN film by MOVPE was investigated using bulk GaN as a substrate. Single‐crystalline In‐ and N‐polarity InN films were obtained on Ga‐ and N‐polarity GaN substrate, respectively. Significant difference of the morphologies between the In‐ and N‐polarity InN films was found. For the In‐polarity InN film, the morphology was similar to that grown on sapphire substrate. The film surface was consisted of grains with small facets. In contrast, for the N‐polarity InN film, the surface was consisted of large hexagonal shape crystal grains with flat surface. The grain size was about 2 μm in diameter on the average, and two‐dimensional growth was enhanced obviously for each crystal grain. The influence of the growth temperature on the morphology, polarity, and optical property was also investigated. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Investigation of AlN nucleation layers for AlGaN/GaN heterostructures on 4H‐SiC
Authors: Boeykens S. , Leys M. R. , Germain M. , Cheng K. , Derluyn J. , Van Daele B. , Van Tendeloo G. , Belmans R. , Borghs G. 
Published: May 2006

AbstractGrowth of GaN on AlN nucleation layers (NL) by LP‐MOVPE on 4H‐SiC substrates is investigated in order to obtain high quality GaN layers for AlGaN/GaN HEMTs. The relationship between structural quality, surface morphology and electrical properties has been investigated using HR‐XRD, AFM, TEM, Van der Pauw‐Hall and TLM measurements. It is shown that there is a correlation between AlN NL strain and HEMT 2DEG properties. Optimum layers show a Hall sheet resistance of 250 Ω/sq. Finally, HEMT transistors have been processed and measured, showing a saturated drain current of 1.2 A/mm for a gate length of 1.5 μm. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Membrane structures containing InGaN/GaN quantum wells fabricated by wet etching of sacrificial silicon substrates
Authors: Park S.‐H. , Liu C. , Gu E. , Dawson M. D. , Watson I. M. , Bejtka K. , Edwards P. R. , Martin R. W. 
Published: May 2006

AbstractInGaN/GaN multiple quantum wells (MQWs) emitting at 410‐505 nm, with either 3 or 16 repeat periods, were grown on commercial GaN‐on‐silicon templates using metal organic vapour phase epitaxy. Spectroscopic and structural studies confirmed these MQWs are of similar quality to analogues grown on sapphire substrates. Wet etching of the silicon (111) substrates in HF‐based solutions allowed the MQW structures to be converted into membranes up to 2 mm in diameter, suspended above macroscopic via holes. Such a fabrication step is attractive for the production of microcavities, and other forms of surface emitting laser. Several MQWs have been compared by photoluminescence and cathodoluminescence spectroscopy before and after conversion to membranes. These measurements indicated a consistent increase in luminescence intensity after substrate removal, accompanied by small redshifts in peak position. We have further demonstrated plasma etching of membrane structures from the underside, as will be used to optimise structures for optical pumping, and used atomic force microscopy to monitor associated changes in surface roughness. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Photocapacitance characteristics of (In,Ga)N/GaN MQW structures
Authors: Rivera C. , Pau J. L. , Muñoz E. , Ive T. , Brandt O. 
Published: May 2006

AbstractPhotocapacitance techniques have been used to study the electronic properties of (In,Ga)N/GaN quantum wells (QWs). Negative differential capacitance (NDC) features were observed at room temperature for the first time in capacitance‐voltage (C‐V) experiments under optical excitation. A detailed analysis of the results seems to indicate that charge accumulation in quantum dot‐like structures embedded in the QWs could be responsible of such behaviour. Polarization field effects present in these structures are detected through C‐V hysteresis, allowing to estimate built‐in electric fields as high as 1.9 MV/cm in 14% In QWs. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Evaluation of wavelength shifts of green and blue light emitting diodes with InN/GaN multiple quantum wells
Authors: Kim Je Won , Lee Kyu Han , Hong Sangsu 
Published: May 2006

AbstractInN/GaN multiple quantum wells were grown by metalorganic chemical vapor deposition. The structural and electrical properties were characterized by transmission electron microscopy and electroluminescence measurements. From the transmission electron micrographs, it was shown that the well layer was grown like a quantum dot. The well layer is expected to be the nano‐size structures in the InN multiple quantum well layers. The multi‐photon confocal laser scanning microscopy was used to investigate the optical properties of the light emitting diode structures with InN active layers. It was found that the two‐photon excitation was possible in InN system. The pit density was measured by using the far‐field optical technique. In the varied current conditions, the blue light emitting diode with the InN multiple quantum well structures did not have the wavelength shift. With this result, we can expect that the white light emitting diodes with the InN multiple quantum well structures do not show the color temperature changes with the variations of applied currents. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Structural analysis of pyramidal defects in Mg‐doped GaN
Authors: Pretorius A. , Schowalter M. , Daneu N. , Kröger R. , Rečnik A. , Rosenauer A. 
Published: May 2006

AbstractPyramidal defects in GaN:Mg grown by metalorganic vapour phase epitaxy are studied by high resolution transmission electron microscopy, energy dispersive X‐ray analysis and scanning transmission electron microscopy. High resolution transmission electron microscopy images were simulated using the multislice approach in order to analyse the basal plane of the pyramidal defects. The simulated images were compared quantitatively with the experimental images. Two structural models for the basal plane inversion domain boundary containing antibixbyite‐type layers are presented which show the best observed agreement with the experimentally found inversion domain boundary. Nevertheless, both models still do not match the experimental images satisfactorily, indicating that some other structure than antibixbyite is present at the boundary. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The initial growth stage in PVT growth of aluminum nitride
Authors: Heimann P. , Epelbaum B. M. , Bickermann M. , Nagata S. , Winnacker A. 
Published: May 2006

AbstractThe main issue in homoepitaxial growth of aluminum nitride (AlN) on native seed substrates is the formation of an aluminum oxynitride (AlON) layer at temperatures between 1850‐1950 °C leading to polycrystalline growth. On the contrary, heteroepitaxial growth of AlN on silicon carbide (SiC) is relatively easy to achieve due to natural formation of a thin molten layer of (Al2OCx) on the seed surface and consequent growth of AlN via the molten buffer layer. Optimization of the seeding process can be achieved by use of ultra‐pure starting material. Another critical issue of AlN growth on SiC is cracking of the grown layer upon cooling as a result of different thermal expansion coefficients. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Carrier dynamics of optical emission from two‐dimensional electron gas in undoped AlGaN/GaN single heterojunctions
Authors: Kwack H. S. , Cho Y. H. , Kim G. H. , Park M. R. , Youn D. H. , Bae S. B. , Lee K.‐S. , Lee J. H. , Lee J. H. 
Published: May 2006

AbstractThe strucutral and optical properties of undoped AlGaN/GaN single heterojunctions (HJs) were studied by means of high‐resolution x‐ray diffraction, photoluminescence (PL), cathodoluminescence (CL), and time‐resolved PL spectroscopy. An additional two‐dimensional electron gas (2DEG)‐related PL and CL emission appeared at about 40 meV below the GaN band‐edge emission energy and persisted up to about 100 K, while this peak disappeared when the top AlGaN layer was removed by reactive ion etching. Depth‐resolved CL spectra reveal the presence of a 2DEG at the heterointerface. The additional PL and CL emission below the GaN band‐edge emission is attributed to the recombination between photogenerated holes and electrons confined at 2DEG states in the triangular‐shaped interface potential. For the 2DEG emission, we observed an about 50‐ps delayed rise time than the GaN and AlGaN emissions by using time‐resolved PL, indicating effective carrier transfer from the GaN flatband and AlGaN regions to the heterointerface. From the results, we explained the optical properties and carrier recombination dynamics of 2DEG, GaN, and AlGaN emissions in undoped AlGaN/GaN single HJs. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Localization of non‐equilibrium carriers in deep InGaN quantum dots and its impact on the device performance
Authors: Sizov D. S. , Sizov V. S. , Lundin V. V. , Zavarin E. E. , Tsatsul'nikov A. F. , Vlasov A. S. , Musikhin Yu. G. , Ledentsov N. N. , Mintairov A.M. , Sun K. , Merz J. 
Published: May 2006

AbstractStructural and optical properties of MOCVD grown InGaN quantum dot (QD) layers have been studied using transmission electron microscopy, X‐ray diffraction, near‐field spectroscopy and selectively excited photoluminescence. We have shown that the presence of Indium in the matrix strongly increases the activation energy of QD layers, due to stimulated phase separation. The increase of the activation energy is accompanied by an increase of the activation time, which was revealed from photocurrent spectroscopy. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Active‐controlled, non‐inferiority trials in oncology: arbitrary limits, infeasible sample sizes and uninformative data analysis. Is there another way?
Authors: Carroll Kevin J. 
Published: May 2006

AbstractIn oncology, it may not always be possible to evaluate the efficacy of new medicines in placebo‐controlled trials. Furthermore, while some newer, biologically targeted anti‐cancer treatments may be expected to deliver therapeutic benefit in terms of better tolerability or improved symptom control, they may not always be expected to provide increased efficacy relative to existing therapies. This naturally leads to the use of active‐control, non‐inferiority trials to evaluate such treatments. In recent evaluations of anti‐cancer treatments, the non‐inferiority margin has often been defined in terms of demonstrating that at least 50% of the active control effect has been retained by the new drug using methods such as those described by Rothmann et al., Statistics in Medicine 2003; 22:239–264 and Wang and Hung Controlled Clinical Trials 2003; 24:147–155. However, this approach can lead to prohibitively large clinical trials and results in a tendency to dichotomize trial outcome as either ‘success’ or ‘failure’ and thus oversimplifies interpretation. With relatively modest modification, these methods can be used to define a stepwise approach to design and analysis. In the first design step, the trial is sized to show indirectly that the new drug would have beaten placebo; in the second analysis step, the probability that the new drug is superior to placebo is assessed and, if sufficiently high in the third and final step, the relative efficacy of the new drug to control is assessed on a continuum of effect retention via an ‘effect retention likelihood plot’. This stepwise approach is likely to provide a more complete assessment of relative efficacy so that the value of new treatments can be better judged. Copyright © 2006 John Wiley & Sons, Ltd.
Pharmacological activity of three commercial Hypericum perforatum preparations in mice
Authors: Jean D. , Pouligon M. , Henriot A. C. 
Published: May 2006

AbstractSt John's Wort (Hypericum perforatum) is the main herbal species used to treat depression. The products available on the pharmaceutical and dietary supplement markets are obtained by a variety of preparation processes and their pharmacological effects may differ significantly. The purpose of this study therefore was to investigate the effect of different St John's Wort commercial preparations available on the French market. Only one preparation gave significant results in the forced swimming test. Copyright © 2006 John Wiley & Sons, Ltd.