bluuebunny/crossref_metadata_2025_split huggingface dataset:
Optical Characteristics of Amorphous III-V Nitride Thin Films
Authors: Khoshman Jebreel M. , Kordesch Martin E. 
Published: April 2011

ABSTRACTThe optical constants and polarized optical properties of amorphous III-V nitride thin films, a-(Al, Ga, In) N, deposited by RF magnetron sputtering onto crystalline silicon, c-Si, (111) and glass substrates have been investigated over the wavelength range 300 – 1400 nm. The optical constants of a-AlN were obtained by analysis of the measured ellipsometric spectra through the Cauchy–Urbach model while the optical constants of a-(In, Ga) N were determined using the Tauc-Lorentz model. Analysis of the absorption coefficient of a-AlN (in the range 200 – 1400 nm) and a-GaN (in the range 300 – 1400 nm) show the optical bandgap to be 5.9 ± 0.05 and 3.44 ± 0.05 eV. The absorption coefficient of a-InN (in the range 300 – 1400 nm) as a function of photon energy shows the absorption edge to be about 1.74 ± 0.05 eV. From the angle dependence of the p-polarized reflectivity we deduced Brewster and principal angles of these films. Measurement of the polarized optical properties revealed a high transmissivity (70 % – 95 %) and low absorptivity (< 18 %) for all three thin films in the visible and near infrared regions.
On the dynamics of InGaN dot formation by RF-MBE growth
Authors: Yamaguchi Tomohiro , Einfeldt Sven , Figge Stephan , Kruse Carsten , Roder Claudia , Hommel Detlef 
Published: April 2011

ABSTRACTThe growth of InGaN on GaN (0001) by plasma-assisted molecular beam epitaxy was investigated with special focus on the dynamics of the formation of the dots. A metastable 2D growth regime, where the surface changes from smooth to rough by thermal treatment during growth interruption, existed previous to the 2D-3D transition. Both small regular-shaped dots and large irregular-shaped islands were observed. The large islands were suppressed by choosing correct growth conditions. The critical thickness for the transition from 2D to 3D growth also depended on the growth conditions. The growth of GaN capping layer to cover InGaN dot-structure was also attempted.
Unusual Properties of the Red and Green Luminescence Bands in Ga-rich GaN
Authors: Reshchikov M. A. , Morkoç H. 
Published: April 2011

ABSTRACTWe studied photoluminescence (PL) from deep-level defects in GaN grown under Garich conditions at relatively low temperatures (700–800°C) by molecular-beam epitaxy (MBE). The dominant features of PL spectrum are red and green bands peaking respectively at ∼1.8 and ∼2.35 eV. Both PL bands decay exponentially at low temperatures (15 – 100 K) after pulsed excitation. The characteristic lifetime for the red band decreases by almost two orders of magnitude from 110 to 2 μs with increasing temperature from 15 to 100 K, while its integrated intensity after each pulse remains nearly unchanged in the same temperature range due to an increase in the peak intensity in the time-resolved PL curve. The lifetime of the green band remained unchanged in this temperature range. We suggest that these PL bands are caused by transitions between excited and ground states of some deep defects rather than transitions involving a shallow donor, conduction or valence bands.
Efficient Luminescence from {11.2} InGaN/GaN Quantum Wells
Authors: Funato Mitsuru , Nishizuka Koji , Kawakami Yoichi , Narukawa Yukio , Mukai Takashi 
Published: April 2011

ABSTRACTInGaN/GaN multiple quantum wells (MQWs) with [0001], <11.2>, and <11.0> orientations have been fabricated by means of the re-growth technique on patterned GaN templates with striped geometry, normal planes of which are (0001) and {11.0}, on sapphire (0001) substrates. It was found that photoluminescence intensity of the {11.2} QW is the strongest among the three QWs, and its internal quantum efficiency was estimated to be as large as about 40% at room temperature. The radiative recombination lifetime of the {11.2} QW was about 0.39 ns at 14 K, which was 3.8 times shorter than that of conventional c-oriented QWs emitting at a similar wavelength. These findings are well explained by the high internal quantum efficiency in the {11.2} QW owing to the suppression of piezoelectric fields.
AlGaN/GaN HFETs and Insulated Gate HFETs DC and RF stability
Authors: Koudymov Alexei , Saygi Salih , Tipirneni Naveen , Simin Grigory , Adivarahan Vinod , Yang Jinwei , Asif Khan M. 
Published: April 2011

ABSTRACTThe comparative study of the DC parameters and RF power stability of nitride-based conventional HFETs and Metal-Oxide-Semiconductor HFETs (MOSHFETs) is presented. The average lifetime under DC stress is estimated to be as high as 2.5 years at room temperature. Under the large-signal RF stress, the gate leakage current of conventional HFETs increases significantly with time showing faster degradation as compared to DC stressing. MOSHFETs demonstrate superior RF performance stability, which perfectly correlates with the DC stability data. It is shown that in conventional HFETs, the combination of the self-heating and positive dynamic gate bias leads to the defect accumulation in the AlGaN barrier under the gate. In MOSHFETs, the absence of the gate leakage is a key to stable RF performance.
SURFACE STABILIZATION FOR HIGHER PERFORMANCE ALGAN/GAN HEMT WITH IN-SITU MOVPE SIN
Authors: Germain M. , Leys M. , Derluyn J. , Boeykens S. , Degroote S. , Ruythooren W. , Das J. , Vandersmissen R. , Xiao D.P. , Wang W. , Borghs G. 
Published: April 2011

ABSTRACTPassivation of AlGaN/GaN HEMT by SiN has been shown by several authors to improve device performance. In this paper we demonstrate a new method of passivating the top AlGaN layer, that leads to a drastic improvement of the device characteristics. On top of the AlGaN/GaN HEMT layer structures, a thin SiN layer is grown at high temperature in-situ, i.e. in the MOCVD reactor prior to unloading the HEMT epiwafers. The quality of the SiN is shown to be very high. As the AlGaN surface is protected against any air contamination, ohmic contacts deposited directly on the SiN layer exhibit better properties than on uncapped AlGaN layers. Transistor performance increases impressively: the source-drain dc current, for a positive gate voltage of 2 V, increases from 0.5 A/mm to 1.2 A/mm for similar AlGaN/GaN HEMT structures by depositing SiN in-situ. RF characteristics also show some improvement. The positive impact of in-situ SiN layer has different origins, some being intrinsically linked: surface protection against oxide formation, better ohmic contact formation, passivation of the surface states, reduction of AlGaN relaxation. All this leads to an increase of the 2DEG carrier concentration.
Self-Organized GaN/AlN Superlattice Nanocolumn Crystals Grown by RF-MBE
Authors: Yamano Kouji , Kikuchi Akihiko , Kishino Katsumi 
Published: April 2011

ABSTRACTGaN nanocolumns including a GaN/AlN superlattice (SL) region were grown by rf-plasma assisted molecular beam epitaxy. The photoluminescence (PL) peak intensity of the GaN/AlN SL nanocolumns was 300∼500 times stronger than that of conventional GaN continuous films with a dislocation density of 3∼5×109 cm−2 and thickness of 3.75 μm grown by metalorganic chemical vapor deposition (MOCVD). The peak wavelengths of the GaN (10.2 ML)/AlN (15.2 ML) SL and GaN (7.7 ML)/AlN (12.4 ML) SL were 420 and 380 nm, respectively. The theoretically calculated transition wavelength agreed well with experimental values, suggesting that GaN/AlN SL nanocolumns involve a large built-in electrostatic field of about 5.8 MV/cm. The effect of the surface morphology of nanocolumns on the PL intensity was studied using GaN/AlN SL nanocolumns with different surface morphologies but with the same nanocolumn structure. Integrated PL intensity was increased by a factor of 2.2 upon changing the surface morphology from continuous to columnar.
Origin of the multi-exponential decay dynamics in light-emitting silicon nanocrystals
Authors: Reynaud Cécile , Guillois Olivier , Herlin-Boime Nathalie , Ledoux Gilles , Huisken Friedrich 
Published: April 2011

ABSTRACTLight-emitting silicon nanocrystals (nc-Si) have attracted much interest due to their importance for optoelectronic devices. Electron hole recombination in a quantum confined system is generally considered as the theoretical frame explaining the photoluminescence (PL) origin. However, there is still a living debate, in particular regarding the PL decay dynamics. The decay is not single exponential and decay curves described by a stretched exponential law were systematically reported for all types of nanocrystalline silicon. The origin of this multi-exponential decay is often attributed to migration effects of the excitons between nanocrystals. In contrast to these approaches, the absence of carrier hopping has been demonstrated experimentally in porous silicon. In order to elucidate this question, specific samples were prepared, consisting in deposits made from gas phase grown silicon nanocrystals with different particle density. The nanoparticles were synthesized by laser pyrolysis of silane in a gas flow reactor, extracted as a supersonic beam, size-selected, and deposited downstream as films of variable densities by changing the deposition time. The nanoparticle number densities were determined by atomic force microscopy. Time-resolved photoluminescence measurements on these films were carried out as a function of the film density and at different PL wavelengths. The reported results showed photoluminescence properties independent of the film density. Even in the very low density film (∼4*109 particles/cm2) where nanoparticles are completely isolated from each other, the decay kinetics corresponds to a multi-exponential law. This means that exciton migration alone cannot explain the stretched exponential decay. Its origin must be linked to an intrinsic characteristic of the nc-Si particle. In this paper, the experimental results are described in more details and compared to the theoretical predictions available in the frame of the quantum confinement model. Then, the possible origins of the multi-exponential character of the decay dynamics is discussed, and the particular properties of the PL in indirect band-gap semiconductors emphasized.
Nanorings, Nanopillars and Nanospikes on Si(111) by Modified Nanosphere Lithography: Fabrication and Application
Authors: Rose Jefferson , Baugh Delroy 
Published: April 2011

ABSTRACTUniform arrays of nanopillars, nanospikes, nanorings and rings atop nanopillars were fabricated using modified nanosphere lithography on the Si(111) surface and observed using AFM and SEM. A self-assembled monolayer mask which utilized 450±10nm polystyrene spheres were used as an etch mask during a fabrication process of physical Ar+ ion bombardment followed by SF6RIE to produce nanopillars with flat tops that could be as large as 210nm in height and 175nm in diameter. Nanospikes of approximately the same height were fabricated made with a FWHM of 250nm. The initial Ar+ion bombardment step was varied to achieve up to 4 times faster etch rates and narrowed widths in the nanospikes. Both types nanostructures systems were “self-wired” which is an intrinsic result of this fabrication process. Self-wired nanorings and rings atop nanopillar structures were also fabricated using a two step etching process along with sonication. The nanorings had inner and outer diameters of 225±5nm and 175±5 nm, respectively and were up to 20nm in height and their nanowire connectivity could be made optional by a judicious choice of solvent for sonication. The self-wired Si nanostructures were also “dressed” with self-assembled Ge quantum dots. These Ge/Si nano-architectured structures possess the basic framework for fabricating novel nanoelectronic and nano-optoelectronic devices in the foreseeable future.
Characterization of SiC nanowires grown by APCVD using single precursors
Authors: Rho Dae-Ho , Kim Jae-Soo , Byun Dong-Jin , Yang Jae-Woong , Lee Jae-Hoon , Kim Na-Ri 
Published: April 2011

ABSTRACTSiC nanowire was grown by APCVD using single precursors. Grown SiC nanowires had 10∼60nm diameters and lengths of several micrometers. Nanowire's diameters and lengths were varied with kind of catalysts. Nanowire's growth scheme was divided by two regions with diameter of nanowire. At first region, nanowire was grown by VLS (vapor-liquid-solid) mechanism, but at the second region, nanowire growth was made by VS (vapor-solid) reaction. These differences were made from limitations of growth rate and deactivation effects. Growth temperature, time and flow rates of source gases were affected nanowire's diametesr and its lengths. And kind of catalysts, coating methods and precursors were affected growth direction and microstructures too.
Hard PVD Coatings for Austenitic Stainless Steel Machining: New Developments
Authors: Endrino J. L. , Wachter A. , Kuhnt E. , Mettler T. , Neuhaus J. , Gey C. 
Published: April 2011

ABSTRACTThe machinability of austenitic stainless steels is, in general, considered to be a difficult process. This is due in great part to the high plasticity and tendency to work-harden of the workpiece, which normally results in extreme conditions imposed on cutting edges. Additionally, austenitic stainless steels have much lower thermal conductivities in comparison to plain carbon and tool steels; this inflicts high thermal loads within the chip-tool contact zone, which can significantly increase the wear rate. The complex machining of austenitic stainless steels can be, in part, relieved by use of a hard coating with low thermal conductivity and an adequate coating surface quality. This can lead to a low friction coefficient between the parts and an improved chip evacuation process. In this study, stainless steel plates were machined using a high speed finishing process and cemented carbide tools coated with four high aluminum containing coatings namely, AlCrN, AlCrNbN, fine grained (fg) AlTiN, and nanocrystalline (nc) AlTiN. Both AlTiN and AlCrN-based coatings are known for their high oxidation resistances and the formation of aluminum oxide surface layers during oxidation [1]. The coating surface textures before and after post-deposition treatment were analyzed by means of the Abbot-Firestone ratio curves in order to study the influence of surface configurations leading to reduced tool wear. A maximum tool life of 150 m was observed for the tool coated with the nc-AlTiN coating.
Effect of Surface Geometry on Stress Generation in Thermal Barrier Coatings During Plasma Spray Deposition
Authors: Ye Guosheng , Basu Soumendra 
Published: April 2011

ABSTRACTA fully coupled thermo-mechanical finite element model was used to study the buildup of stresses during splat solidification, and to understand the effect of deposition conditions on crack formation during plasma spray deposition. Through the simulation, the locations and magnitudes of maximum stresses were identified, where crack formation would presumably initiate. The model showed that the stresses scaled with the temperature difference between the superheated splat and the substrate. The simulation further showed that the stresses scale with the three geometric parameters, and two independent geometric ratios were defined; ζ (defined as t/λ) and ψ (defined as A/λ). 2D maps of maximum S11 and S22 under different combinations of ζ and ψ were constructed. The mappings showed that only roughness features on the scale of splat thickness were important in providing locations of maximum stress concentration.
Fabricating Neural and Cardiomyogenic Stem Cell Structures by a Novel Rapid Prototyping—the Inkjet Printing Method
Authors: Xu Tao , Gregory Cassie , Molnar Peter , Boland Thomas 
Published: April 2011

ABSTRACTDirect printing of living cardiomyogenic stem cells and embryonic cortical neurons to generate complex cellular patterns and structures of such cells was demonstrated in the study. Furthermore, the immunostaining analyses and the whole-cell patch clamp recordings showed the cortical neurons grown in the printed cellular patterns and structures maintained their basic cellular functions, including neuronal phenotypes and electrophysiological properties. These results and findings may greatly prompt the inkjet printing method evolving into a viable and cost-effective approach for engineering human neural and cardiac tissues or even organs.
Development of Novel Biodegradable Amino Acid Ester Based Polyphosphazene– Hydroxyapatite Composites for Bone Tissue Engineering
Authors: Sethuraman Swaminathan , Nair Lakshmi S. , Singh Anurima , Bender Jared D , Greish Yaser E. , Brown Paul W. , Allcock Harry R. , Laurencin Cato T. 
Published: April 2011

ASTRACT:Hydroxyapatite formed from low temperature setting calcium phosphate cements (CPC) are currently been used for various orthopaedic applications. CPCs are attractive candidates for the development of scaffolds for bone tissue engineering, since they are moldable, resorbable, set at physiological temperature without the use of toxic chemicals, and can be processed in an operating room setting. However they may have mechanical disadvantages which seriously limit them to non-load bearing orthopaedic applications. The aim of the present study was to develop composites from polyphosphazenes and calcium deficient hydroxyapatite precursors to form poorly crystalline hydroxyapatite-polymer composites. Composites were formed from calcium deficient hydroxyapatite precursors (Ca/P – 1.5, 1.6) and biodegradable polyphosphazenes, poly[bis(ethyl alanato)phosphazene] (PNEA) and poly[(50%ethyl alanato) (50%methyl phenoxy)phosphazene] (PNEA50mPh50) at physiological temperature. The results demonstrated that poorly crystalline hydroxyapatite that resembled the mineral component of bone was formed in the presence of biodegradable polyphosphazenes. The surface morphology of all the four composites was identical with a porous microstructure. The composites supported the adhesion and proliferation of osteoblast like MC3T3-E1 cells making them potential candidates for bone tissue engineering.
Enhanced Osteoblast Functions on Nanophase Titania in Poly-lactic-co-glycolic Acid (PLGA) Composites
Authors: Liu Huinan , Slamovich Elliott B. , Webster Thomas J. 
Published: April 2011

ABSTRACTMuch work is needed in the design of more effective bone tissue engineering materials to induce the growth of normal bone tissue. Nanotechnology offers exciting alternatives to traditional bone implants since bone itself is a nanostructured material composed of nanofibered hydroxyapatite well-dispersed in a mostly collagen matrix. For this purpose, poly-lactic-co-glycolic acid (PLGA) was dissolved in chloroform and nanometer grain size titania was dispersed by various sonication powers from 0 W to 166 W. Previous results demonstrated that the dispersion of titania in PLGA was enhanced by increasing the intensity of sonication and that greater osteoblast (bone-forming cells) adhesion correlated with improved nanophase titania dispersion in PLGA. However, adhesion of osteoblasts to material surfaces, alone, is not adequate to determine long-term functions of implant materials. For this reason, and as a next step to determine the efficacy of nanocomposites in bone applications, subsequent functions of osteoblasts on nanophase titania/PLGA composites were investigated in vitro in this study. For the first time, results correlated better osteoblast long-term functions, specifically the deposition of calcium-containing mineral, with improved nanophase titania dispersions in PLGA.In this manner, the present study demonstrated that PLGA composites with well-dispersed nanophase titania can improve osteoblast functions necessary for the further investigation of these materials in orthopedic applications.
Properties and Applications of Novel DNA-Based Surfactants
Authors: Paunov Vesselin N. , Xu Chun , Taylor Pietro , Ersoz Mustafa , Fletcher Paul D.I. 
Published: April 2011

ABSTRACTWe have designed a novel class of functional surfactants based on DNA and have explored their properties in relation to several possible applications. The DNA-surfactants consist of short chain DNA oligonucleotides covalently bound to a large hydrophobic group, which makes the DNA molecules amphiphilic. We demonstrate that these materials behave like common detergents and are surface-active at various fluid surfaces, e.g. air-water, oil-water interfaces, and lipid bilayers. We also show that once adsorbed the DNA-surfactants remain on the liquid surface upon hybridisation with a complementary DNA chain. We use complementary DNA-surfactants to functionalise fluid surfaces and to program the interactions between them based on Watson-Crick pairing. By selecting the appropriate DNA base sequences, the interaction between the fluid surfaces functionalised with DNA-surfactants can be programmed with a very high specificity. We have also developed a novel procedure for micro-patterning of solid surfaces with DNA by a microcontact printing with aqueous inks of DNA-surfactants which can be utilised for rapid fabrication of DNA assays and genetic biochips.
Alkoxide Based Sol-Gel Processing of CMR Manganites
Authors: Pohl Annika , Westin Gunnar 
Published: April 2011

ABSTRACTWe have prepared a soluble manganese alkoxide, and used in the first all-alkoxide synthesis of Ca, Sr, and Ba substituted lanthanum manganites. The phase development from gel to target oxide was monitored by TGA, DSC, powder-XRD, FT–IR spectroscopy, and TEM-EDS. Here we present the effect of dopant on the phase development. It was found that the dopants influence the temperature of complete oxide formation (≥650°C). The main difference in phase evolution was seen in the 300–600°C range, where the formation of crystalline phases in trace quantities seemed to temporarily reduce the elemental homogeneity.
Magnetic Structure of K2NiF4-type Iron(III) Oxide Halides
Authors: Hector Andrew L. 
Published: April 2011

ABSTRACTMaterials with the K2NiF4 structure and a magnetic atom on the B site are examples of a pseudo-2D square lattice. The nearest neighbour interactions are very strong and, with iron(III), usually lead to xy antiferromagnetic coupling. We have used powder neutron diffraction at 2 K to study the magnetic structures of A2FeO3X where A is Ca or Sr and X is Cl or Br. There are two common magnetic structures for K2NiF4-type xy antiferromagnets, those of La2NiO4 and La2CuO4, here we find the latter. In Sr2FeO3F we find a more complex situation. The reflections corresponding to the La2CuO4 magnetic structure are present from 2 K to ambient temperature, but a further set of peaks decays from 2 K to around 60 K. These correspond to a magnetic structure with a doubled c-axis. This is due to a magnetic structure in which La2NiO4-and La2CuO4-type stacking alternate. To our knowledge this is the only example of this magnetic structure type.
The Rôle of Defects in Photographic Latent Image Formation
Authors: Wilson D. J. , Sokol A. A. , French S. A. , Catlow C. R. A. 
Published: April 2011

ABSTRACTWe report a density functional theory study of the structure and energies of point defects in silver chloride. The localisation of photoelectrons and corresponding holes at these defects has been studied, along with the corresponding relaxation of the lattice. We have applied state-of-the-art periodic and QM/MM embedding methods, to provide new insights into the long-standing problem of the fundamental processes controlling latent image formation.
Ultrafast Laser Processing for Lab-on-a-Chip Device Manufacture
Authors: Sugioka Koji , Cheng Ya , Midorikawa Katsumi 
Published: April 2011

ABSTRACT3D microstructuring of photosensitive glass is demonstrated by femtosecond (fs) laser for lab-on-a-chip manufacture. True 3D hollow microstructures embedded in the glass are fabricated by the fs laser direct write followed by heat treatment and successive wet etching. A variety of microcomponents for a lab-on-a-chip device like a microfluidics, a microvalve, a microoptics, a microlaser, etc. are fabricated by using this technique. The fs laser direct write process is also applied for selective metallization of internal walls of the hollow microstructures embedded in the glass for electric control of movement of the micromechanical components in the lab-on-a-chip device.