bluuebunny/crossref_metadata_2025 huggingface dataset:
Energy-specific equilibrium in nanowires for efficient thermoelectric power generation
Authors: Linke Heiner , Humphrey Tammy , O'Dwyer Mark
Published: April 2011

There is great scientific, economic and environmental interest in the development of thermoelectric materials capable of direct thermal-to-electric energy conversion with high efficiency. Recent theory predicts that in materials with a fine-tuned electronic density of states, electrons can be placed in energy-specific equilibrium, and the efficiency of thermoelectric power generation can approach the fundamental Carnot limit. Here we review the relevant theory of energy-specific equilibrium. We describe a concept for a proof-of principle demonstration of near-Carnot efficient power conversion involving a single, ballistic nanowire at low temperatures, and we discuss the potential for room-temperature applications in diffusive materials.
Substitutions in the Homologous Family CsPbmBi3Te5+m and Preliminary Thermoelectric Results
Authors: Guéguen Aurélie , Quarez Eric , Kanatzidis Mercouri G.
Published: April 2011

We report analogs of the homologous family CsPbmBi3Te5+m, namely Cs0.74K0.76Bi3.5Te6, CsNa0.98Bi4.01Te7, Cs0.69Ca0.65Bi3.34Te6, Rb0.82Pb0.82Bi3.18Te6, Rb0.19K1.31Bi3.50Te6, RbSnBi3Te6, Rb0.94Ca0.94Bi3.06Te6 and RbYbBi3Te6. They all crystallize in the orthorhombic space group Cmcm and are isostructural to CsPbBi3Te6 the first member of the family except for CsNa0.98Bi4.01Te7 which is isostructural to CsPb2Bi3Te7, the second member of the family. Preliminary thermoelectric measurements on Cs0.76K0.74Bi3.5Te6 show electrical conductivity values in the range of 600-1000 S/cm. The thermopower shows n-type charge transport with room temperature values of ∼ -35 µV/K. By comparison, the electrical conductivity at room temperature of Rb0.82Pb0.82Bi3.18Te6 was ∼1400S/cm, while the thermopower was ∼ -20 µV/K.
Transport Properties of Type II Sodium-Silicon Clathrates
Authors: Beekman Matt , Grkyo Jan , Nolas George S.
Published: April 2011

We have synthesized the type II silicon clathrates Na1Si136 and Na8Si136, and report on the electrical and thermal transport in these materials. The crystal structure consists of a covalently bonded silicon framework in which sodium guest atoms are encapsulated inside the silicon host framework. Differential scanning calorimetry measurements show the compounds decompose above 600°C to diamond-structure silicon. Temperature dependant electrical resistivity measurements show the specimens to have an insulating character, with magnitudes that decrease with increasing sodium content. For the first time, thermal conductivity measurements on type II sodium-silicon clathrates are presented. The thermal conductivity is very low for both specimens, and for Na8Si136 exhibits a clear dip in the range from 50 to 70 K. These data suggest that the “rattling” behavior observed in type I clathrates may also be present in type II clathrates.
Thermoelectric properties of polycrystalline Si1-xGex grown by die-casting vertical Bridgman growth technique
Authors: Baba Takashi , Iida Tsutomu , Hirahara Hisashi , Itoh Takashi , Akasaka Masayasu , Takanashi Yoshifumi
Published: April 2011

The die-casting growth process combined with an advanced version of the Bridgman method was employed for manufacturing the multicrystalline bulk crystal of Si1−xGex. This process provides a form of phase transformation which is completely different from that predicted by the Si-Ge phase diagram. By combining this growth with subsequent heat treatment of the precipitated sample, the variation in the germanium content obtained was within ± 4 % for Si0.65Ge0.35 sample with a carrier concentration in the mid-1018 cm−3. The power factor obtained exhibited a quite flat characteristic over the temperature range of room temperature to 800 K. However, there was a drop in the Seebeck coefficient at about 800 K, which corresponded to a rise in the electrical conductivity. The value of the thermal conductivity was about 0.04 W/cmK at temperatures ranging from 600 to 900 K. The maximum value of the figure of merit obtained for the grown Si0.65Ge0.35 sample was 0.19 at 773 K.
400 element ErAs:InGaAs/InGaAlAs superlattice power generator
Authors: Zeng Gehong , Bahk Je-Hyeong , Bowers John E. , Zide Joshua M. O. , Gossard Arthur C. , Zhang Yan , Singh Rajeev , Bian Zhixi , Shakouri Ali , Kim Woochul , Singer Suzanne , Majumdar Arun
Published: April 2011

We report the fabrication and characterization of thin film power generators composed 400 p- and n-type ErAs:InGaAs/InGaAlAs superlattice thermoelectric elements. The thermoelectric elements incorporate erbium arsenide metallic nanoparticles into the semiconductor superlattice structure to provide charge carriers and create scattering centers for phonons. 10 µm p- and n-type InGaAs/InGaAlAs superlattices with embedded ErAs nano-particles were grown on InP substrates using molecular beam epitaxy. Thermal conductivity values were measured using the 3ω method and cross-plane Seebeck coefficients were determined using Seebeck device test patterns. 400 element ErAs:InGaAs/InGaAlAs thin film power generators were fabricated from superlattice elements 10 µm thick and 200 µm × 200 µm in area. The output power was 4.7 milliwatts for an external electrical load resistor of 150 Ω at about 80 K temperature difference drop across the generator. We discuss the limitations to the generator's performance and provide suggestions for further improvement.
Surface Segregation in Multicomponent Clusters
Authors: Dowben Peter A. , Wu Ning , Palina Natalie , Modrow Hartwig , Müller Robert , Hormes Josef , Losovyj Yaroslav B.
Published: April 2011

Nanostructured materials are not immune from surface segregation, as can be shown for solid samples made from nanosized BaFe12−2xCoxTixO19 barium ferrite particles and a variety of free clusters. Both theory and experiment provide ample demonstration that very limited dimensions of very small clusters does not necessarily impart stability against surface and grain boundary segregation. In fact, with the larger surface to volume ratio in small clusters and lower average atomic coordination, we anticipate that compositional instabilities in small clusters will readily occur.
Photo-oxidation of Multiwalled Carbon Nanotubes
Authors: Parekh B. , Debies Thomas , Knight Pam , Santhanam K. S. V. , Takacs Gerald A.
Published: April 2011

MWNTs were photo-oxidized in quartz boats using low-pressure Hg lamps and emission downstream from Ar microwave plasmas which are primarily atomic line sources of 253.7 and 184.9 nm UV, and 106.7 and 104.8 nm vacuum UV (VUV) radiation, respectively. X-ray photoelectron spectroscopy (XPS) showed rapid oxidation during the first hour of UV treatment and then an increase that was directly proportional to the time of treatment up to 4 h where the oxygen concentration was 7.5 at%. VUV photo-oxidation resulted in an oxygen concentration up to 9.5 at% with exposure time for the initial 2 h of treatment. Beyond 2 h, the oxygen concentration decreased with VUV exposure due to a larger rate of de-oxidation than oxidation at the surface. Curve fitting of the XPS C1s spectra revealed mainly the C-O-C functional group with the presence of C=O, O-C=O and O=C-O-C=O moieties. SEM micrographs showed no apparent effect on the structure or appearance of the MWNTs as expected for surface modification.
Degradation Phenomenon in Semiconductive Nanoamorphous Structures from Molybdenum Oxide
Authors: Markosyan A. , Kamanchadgyan Sergo , Malkhasyan Razmik
Published: April 2011

The condition of degradation of electrical properties of samples prepared from powder of nanoamorphous semiconductive oxide in the course of time is discovered and studied when investigating electro-physical properties of the given materials for the first time synthesized at Scientific Production Enterprise “ATOM”. It has been shown that degradation phenomenon is also present in amorphous materials, which relies on the configuration of the pores of the sample.

The resistance of sample in the open air changes over a period of several months. However, it decreases and is comparatively fast stabilized when placed in vacuum. The absorption and de-absorption of different gases have diffusion nature and highly depend on environment temperature. Water steam in atmosphere has been determined to be the main cause of degradation.
Superconductors in confined geometries
Authors: Xiao Zhili , Hor Yew-San , Welp Ulrich , Ito Yasuo , Patel Umesh , Hua Jiong , Mitchell John , Kwok Wai-Kwong , Crabtree George W.
Published: April 2011

The synthesis of nanoscale superconductors with controlled geometries is extremely challenging. In this paper we present results on synthesis and characterization of one-dimensional (1D) NbSe2 superconducting nanowires/nanoribbons. Our synthesis approach includes the synthesis of 1D NbSe3 nanostructure precursors followed by nondestructive and controlled adjustment of the Se composition to formulate NbSe2. The morphology, composition and crystallinity of the synthesized 1D NbSe2 nanostructures were analyzed with scanning electron microscopy, x-ray diffraction and transmission electron microscopy. Transport measurements were carried out to explore the electronic properties of these confined superconducting nanostructures.
Simulation of Nanostructure Formation in Thin Polymer Films
Authors: Kim Dongchoul , Lu Wei
Published: April 2011

A thin polymer film subjected to an electrostatic field may lose stability at the polymer-air interface, leading to uniform self-organized pillars emerging out of the film surface. This paper presents a three dimensional dynamic model to account for the behavior. The coupled diffusion, viscous flow, and dielectric effect are incorporated into a phase field framework. Numerical simulations reveal rich dynamics of the pattern formation process and the substrate effect. The pillar size is insensitive to the film thickness, but the distance between pillars and the growth rate are significantly affected. The study suggests an approach to control structural formation in thin films with a designed electric field.
Modification on Hydrophilic property for Harsh Environment
Authors: Tsai Hsin-Chang , Chang Yu-Ru , Shing Tai-Kang
Published: April 2011

In this research, a 1∼2 um NiCo film coated by electroplating technique as protection film in high temperature environment was studied. The improvements of hydrophilic property and high temperature stability by coating NiCo film upon pure Cu plate have been characterized through experiments. In thermal stability characterization, the specimens were put into a temperature-controlled oven and applied a thermal load at 100°C, 200°C and 300°C for annealing. After thermal annealing, the hydrophilic property was characterized to confirm that NiCo film was stable in high temperature environment. The hydrophilic property was represented by the contact angle between the DI water and the surface of NiCo film. The outgasing effect of contaminated polymer was also study. The electroplated NiCo film has advantages of good hydrophilic property, high temperature stability and easy fabrication. Therefore, the NiCo film could help the development of thermal related products.
Anti–scatter coating on slab laser head for preventing evanescent wave by photo–oxidation of silicone oil
Authors: Funatsu Takayuki , Sato Nobuhiro , Sato Yuji , Okamoto Takayuki , Murahara Masataka
Published: April 2011

A low refractive index, hard SiO2 film was photochemically coated on the surface of slab laser head at room temperature by using silicone oil and a Xe2 excimer lamp. Nd+:glass slab laser produces high power laser because the light is amplified with a repeated total reflection in the laser medium. However, an evanescent wave arises on the total reflection interface, which causes a loss of the light energy. A low refractive index film 2 μm thick is, therefore, needed to prevent this problem. The vacuum vapor deposition method as a dry process and the spin-coating method as a wet process are generally used for making optical thin films. The former method can laminate a hard thin film, but requires temperatures above 500°C, and the thermal denaturation of the optical substrate is unavoidable. On the other hand, the latter method can form a low refractive index thin film, but the produced thin film has a poor adhesiveness and a low hardness. Besides, all these films are inferior in water resistance. We, therefore, formed a water-resistant, hard, and low refractive index protective coating directly on the laser glass surface at room temperature with photochemical reaction by the Xe2 excimer lamp.
An Analysis of Surface Temperature Rise at Small Scale Sliding Contacts
Authors: Ray Sudipto , Chowdhury S. K. Roy
Published: April 2011

A great deal of work has been carried out in recent years on temperature rise at the contact between sliding bodies with engineering scale roughness. However, as surfaces become smoother and loading decreases, in applications such as MEMS and NEMS devices, the analysis of surface temperature rise must consider the small-scale asperity height distributions and the surface forces that may be operating at small separations. The paper attempts to predict surface temperature rise at sliding contacts with small-scale roughness considering the influence of relevant parameters. The important observation here is that in addition to the dependence on load, speed and material parameters the contact temperature steadily increases with surface adhesion.
Electrical Conduction in Thermally Sprayed Thin Metallic Coatings
Authors: Sharma Atin , Gambino Richard J. , Sampath Sanjay
Published: April 2011

The in-plane electrical conductivity and the apparent density of air plasma sprayed (APS) molybdenum coatings having thicknesses in the range 30 um to 450 um were measured as a function of coating thickness. Both conductivity and the apparent density were found to increase with thickness until a saturation density (∼ 80% of the bulk density of molybdenum metal) was achieved. Attributing this increase in the apparent density to the increased volume fraction of the metallic component (or decreased porosity volume) in the coatings allows for the treatment of the problem in a framework of percolation theories. In order to eliminate the contribution of surface roughness the results were further analyzed using a two-layer parallel resistor model of the coating. In this model the top layer, which was composed of coating roughness, was assumed to have fixed thickness and conductivity whereas the conductivity of the bottom layer was assumed to vary with thickness (density). A fit to the data obtained from the above model showed that the conductivity of the bottom layer obeys a power law relationship of the type σ α (V –Vc)n throughout the composition range (with n=1.72 and Vc=0.09). These results show that that the coatings behave as a three-dimensional percolation system [1] and also indicate the asymmetric shape of the metallic and insulator regions in the coatings [2].
Tunneling Injection Quantum-Dot Lasers
Authors: Chuang Shun Lien , Kim Jungho , Kondratko Peter K. , Walter Gabriel , Holonyak Nick , Heller Richard B. , Zhang Xuebing B. , Dupuis Russell D.
Published: April 2011

We investigate a tunneling injection InP quantum-dot (QD) laser theoretically and experimentally. The device consists of a single compressively strained InP QD layer coupled closely to two tensile strained InGaP quantum wells (QWs). While most tensile strained QW lasers in this wavelength (red) range lase in the transverse-magnetic (TM) polarization, our QD laser lases in the transverse-electric (TE) polarization from the first excited state of the compressively strained QDs, which is coupled to the ground state of the tensile-strained InGaP QWs. When we measure TE and TM modal gain spectra, a typical QW gain evolution behavior is observed at low injection currents, which can be theoretically explained by the quasi-equilibrium of carrier distribution. When the injection current is increased near threshold, a TE gain narrowing and a simultaneous TM gain pinning are observed in the measured modal gain spectra, which cannot be explained via the quasi-equilibrium model. We propose a polarization-dependent photon-mediated carrier re-distribution in the QD-coupled-QW structure to explain this TE and TM gain evolution behavior. When the injection current is just below threshold, the strong carrier depletion via stimulated emission due to coupling between the InP QD and InGaP QW states plays an important role in carrier re-distribution, which depends on the optical transition energy and polarization. This polarization-dependent photon-mediated carrier re-distribution explains the TE gain narrowing and TM gain pinning behavior. To quantitatively demonstrate the photon-mediated carrier re-distribution near the threshold current, a set of coupled rate equations are solved taking into account the polarization-dependent stimulated emission processes. The calculated polarization power ratio based on the coupled rate equations explains the experimental observations.
Structure, Composition and Optoelectronic Properties of Small Pyramidal Semiconductor Quantum Dots of Ga and In Atoms With As
Authors: Pozhar Liudmila A. , Yeates Alan T. , Szmulowicz Frank , Mitchel William C.
Published: April 2011

Electronic properties of small pyramidal clusters of Ga, In and As atoms are investigated using a multi-configuration self-consistent field approximation (MCSCF). The structure and composition of some of these (pre-designed) clusters have been derived from those of the symmetry elements of the corresponding bulk fcc lattices, to reflect the cluster formation in confinement or on surfaces. The counterpart (vacuum) clusters obtained without any spatial constraints applied to the atomic positions have also being synthesized computationally. It has been shown that manipulations with the cluster structure, composition and spatial constraints permit to engineer virtual (i.e., fundamental theory-based, computational) clusters with pre-designed electronic properties that can serve as prototypes for nanoscale heterostructure (NHS) units. Computational data so obtained are in agreement with existing experimental evidence concerning such pyramidal clusters.
InGaAs/InGaAsP Quantum-Well Engineering for Multiple Regrowth MOVPE Process
Authors: Lohe Hans-Jeorg , Gini Emilio , Scollo Riccardo , Robin Franck , Jaeckel Heinz
Published: April 2011

For the heterogeneous integration of several layer structures for absorber, gain and passive waveguide sections in a monolithically-integrated mode locked laser diode, the bandgap of the absorber section has to be matched to the emission wavelength of the gain section. Because of the use of a multiple regrowth process for optical butt-coupling, the first grown multiple quantum-well gain material undergoes a quantum-well intermixing process, resulting in a blue shift of the emitting optical wavelength. Experimental results show, that the blue shift is dependent on the process details and cannot be investigated by simple thermal cycling of unprocessed quantum well-structures. With the introduction of an effective quantum-well width computed from the emission wavelength we found a linear relationship between the effective quantum well width shrinkage and the cumulated regrowth heating time of 8.3Å/h at a growth temperature of 630°C. Therefore knowing the cumulated regrowth time for a laser fabrication, we could successfully design the initial quantum well thickness that yields the targeted emitting wavelength and excellent matching to the absorber bandedge.
Nano-patterned Growth of Ge Quantum Dots for Infrared Detector Applications
Authors: Chen Christopher , Cha Dongho , Lee Joo-young , Kim Hyung-jun , Liu Fei , Tong Song , Wang Kang L. , Wang Jia-Yu , Russell Thomas P.
Published: April 2011

Quantum dot infrared photodetectors (QDIPs) have been studied widely for normal-incidence infrared detection. The 3D confinement provided by quantum dots allows for the elimination of gratings that are typically required for normal-incidence detection in quantum well infrared photodetectors (QWIPs). Furthermore, the growth of Ge dots on Si substrates offers the potential for integration with existing CMOS platforms. To date, however, Ge QDIPs have typically been grown epitaxially by Stranski-Krastonov growth – producing pancake-like dots with base dimensions of 50-100 nm, heights of 7-10 nm, and an aerial dot density of 109–1010 cm−2. Such dots have poor lateral confinement, causing them to have non-ideal normal-incidence absorption characteristics, similar to quantum wells. In this work, we demonstrate infrared absorption in Ge dots with base dimensions of approximately 15 nm. These dots are epitaxially grown on pre-patterned Si substrates, with an aerial dot density of approximately 1011 cm−2. The substrates are prepared by using diblock copolymers to create a nano-pattern on the substrate surface which is transferred to the substrate by dry etching. The size of this pattern determines the base dimensions of the Ge dots. After growth, these dots are then tested for their infrared absorption properties using Fourier Transform Infrared (FTIR) Spectroscopy. The normal-incidence absorption of the dots can be studied with FTIR by varying the polarization angle of the infrared light. We present FTIR absorption spectra for samples grown with various conditions (e.g., different dot doping levels, numbers of layers, and dot base dimensions) and investigate the effects of different growth conditions on infrared absorption properties. We also report on the normal-incidence absorption characteristics of these dots by presenting absorption spectra for various polarization angles of infrared light.
A Novel InSb Photodiode Infrared Sensor Operating at Room Temperature
Authors: Ueno Koichiro , Camargo Edson Gomes , Kawakami Yoshifumi , Moriyasu Yoshitaka , Nagase Kazuhiro , Kuze Naohiro
Published: April 2011

A microchip-sized InSb photodiode based infrared sensor (InSb PDS) that operates at room temperature was developed. The InSb PDS consists of 700 photodiodes connected in series and consumes no power, because it works in photovoltaic mode to output an open-circuit voltage. The InSb PDS has a typical responsivity of 1,900 V/W and an output noise of 0.15 μV/Hz1/2. A detectivity of 2.8×108 cmHz1/2/W was obtained at 300 K. The InSb PDS has performance high enough for applications such as mobile electronic equipment, personal computers, and consumer electronics
Effect of Substrate Variation on Electrical and Magnetic Properties of Mn Doped ZnO of Dilute Magnetic Semiconductors
Authors: Manchiraju Srikanth , Mundada Govind , Kehl Ted , Vera Craig , Patel Rishi , Kahol Pawan , Ghosh Kartik
Published: April 2011

In this paper, the effect of substrate on the domain structure growth and electrical and magnetic properties of epitaxial Mn-doped Zn0.8Mn0.15O (ZnMnO) thin films has been investigated. Epitaxial thin films of ZnMnO dilute magnetic semiconductors (DMS) were grown on various substrates such as single crystal sapphire, single crystal silicon, and quartz substrates using Pulsed Laser Deposition (PLD) technique . Structural, surface, magnetic, and optical properties have been observed on these films using X-Ray diffraction (XRD), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), and Raman spectroscopy. X-Ray Diffraction shows that films are highly epitaxial and c-axis oriented with some induced strain. AFM images show that film surface is smooth with RMS roughness of the order of 1-2 nm over 5*5sq.micron. Magnetic characteristic properties such as carrier concentration, mobility, and temperature dependent resistivity were also investigated. Carrier concentration decreases and mobility increases for both the films on silicon and quartz substrates when compared to film on sapphire.
Improved Photoconductivity of ZnO by Ion Beam Bombardment
Authors: Wellenius I. P. , Dhawan Anuj , Muth J. F. , Guardala Noel A. , Price Jack L.
Published: April 2011

In this study, single crystal ZnO films are grown by pulsed laser deposition and ion beam processing is used alter the resistivity of the films. A 3He ion beam was chosen with a specific energy to transmutate oxygen into nitrogen. Analytical ion beam techniques were used to monitor the transmutation process, and changes in film characteristics were monitored by making resistance, photoconductivity and luminescence measurements before and after ion beam processing. The amount of nitrogen produced by this method was estimated to be ∼ 1014 cm−3, and was too low to be observable as a p-type dopant due to compensation by the naturally n-type material. However, the ion beam processed films improved dramatically in resistivity, defect luminescence was reduced and photoconductivity increases consistent with the improvements with resistivity were observed. These improvements were attributed to ion beam annealing of the crystal resulting in a reduction of point defects. In some films, blistering of the surface occurred and was attributed to the formation of gas bubbles which causes delamination of the film from the substrate.
Changes in Optical Properties of GaAsN During Annealing
Authors: Liu Ting , Chandril Sandeep , Schires Eric D. , Wu Nianqiang , Chen Xinqi , Korakakis Dimitris , Myers Thomas H.
Published: April 2011

GaAs1−xNx layers and quantum dot-like structures were grown on (100) GaAs substrates by molecular beam epitaxy. The dependence of photoluminescence emission spectra on annealing temperature is consistent with literature at lower temperatures but after annealing at 750 °C a net red-shift is consistently observed. X-ray photoelectron spectroscopy measurements indicate that for different annealing times and temperatures, the nitrogen and arsenic surface concentrations changed compared to that of as-grown samples, specifically arsenic is lost from the material. Raman measurements are consistent with the trends in photoluminescence and also suggest the loss of arsenic occurs at higher annealing temperatures in both samples capped with GaAs and uncapped samples.
In-Situ Investigation of Surface Stoichiometry During InGaN and GaN Growth by Plasma-Assisted Molecular Beam Epitaxy Using RHEED-TRAXS
Authors: Tompkins Randy Preston , VanMil Brenda L. , Lee Kyoungnae , Schires Eric D. , Chye Yewhee , Lederman David , Myers Thomas H.
Published: April 2011

Reflection high-energy electron diffraction total-reflection-angle x-ray spectroscopy (RHEED-TRAXS) uses high-energy electrons from RHEED to excite x-ray fluorescence. Monitoring characteristic x-rays of selected elements thus allows study of surface coverage of materials. In this study, surface coverage of Ga and In during growth of GaN and InGaN was probed using this technique. Evolution of the surface layer of Ga on GaN during growth and deposition of Ga on static GaN at room temperature were studied. RHEED-TRAXS measurements were performed during growth of InGaN by measuring the ratio of the In Lα to Ga Kα intensity. A significant surface coverage of In was observed at all temperatures investigated regardless of actual In incorporation.
Modification of InN Properties by Interactions with Hydrogen and Nitrogen
Authors: Losurdo Maria , Giangregorio Maria Michela , Bruno Giovanni , Kim Tong-Ho , Wu Pae , Choi Soojeong , Morse Mike , Brown April , Masia Francesco , Polimeni Antonio , Capizzi Mario
Published: April 2011

The interaction of InN epitaxial films grown by r.f. plasma assisted molecular beam epitaxy with atomic hydrogen and nitrogen, produced by remote r.f. H2 and N2 plasmas, is investigated. InN strongly reacts with both atomic hydrogen and nitrogen yielding depletion of nitrogen and concurrent formation of In clusters. The impact of hydrogen treatments on the optical properties of InN is assessed using photoluminescence (PL). It is found that hydrogen suppresses the intense PL band peaked at approximately 0.7eV for the as-grown InN epitaxial layers, and results in the appearance of a new PL band whose peak energy and intensity increase with H-dose. The effect of exposure to atomic hydrogen and nitrogen on electrical properties of InN is investigated using Hall effect measurements. Atomic force microscopy is also used for studying the morphological changes of InN upon interaction with atomic hydrogen and nitrogen.
Growth, processing and characterization of GaN/AlGaN/SiC vertical n-p diodes
Authors: Boeykens Steven , Leys Maarten , Germain Marianne , Poortmans Jef , Van Daele Benny , Van Tendeloo Staf , Belmans Ronnie , Borghs Gustaaf
Published: April 2011

Application of SiC substrates instead of the most commonly used sapphire for the heteroepitaxial growth of III-Nitrides offers advantages as better lattice matching, higher thermal conductivity, and electrical conductivity. This namely offers interesting perspectives for the development of vertical III-Nitride devices for switching purposes. For example, an AlGaN/SiC heterojunction could improve the performance of SiC bipolar transistors. In this work, n-type GaN layers have been grown by MOVPE on p-type 4H-SiC substrates using Si doped Al0.08Ga0.92N or Al0.3Ga0.7N nucleation layers. They have been characterized with temperature dependent current-voltage (I-V-T), capacitance-voltage (C-V) techniques and transmission electron microscopy (TEM).
Fabrication and electrical characteristics of Ti/Al ohmic contact to Si+ implanted GaN
Authors: Ito Nobuyuki , Suzuki Akira , Kawamura Mitsunori , Nomoto Kazuki , Kasai Takeshi , Mishima Tomoyoshi , Inada Taroh , Nakamura Tohru , Satoh Masataka
Published: April 2011

Ti/Al ohmic contact with an extremely low specific contact resistance has been formed by the deposition of Ti and Al films on Si+ lanted GaN. The ohmic contact formed by annealing at 600 o C of Ti film with a thickness of 50 nm and Al film with a thickness of 200 nm reveals the good smooth surface and uniform structure as compare to those of contacts formed above 700 °C, which is correlated to whether the Al-Ti alloy is melted during the annealing of ohmic contact or not. The specific contact resistance of 2 × 10-6Ω-cm2 is obtained for Si+ implanted GaN with a dose of 5 × 1013 cm-2. As Si ion dose increases to 5 × 1014 /cm2, the specific contact resistance is reduced to 2 × 10-8 Ω-cm2. It is revealed that the selective doping at high impurity concentration in the surface region by Si+ implantation is useful to reduce the contact resistance for Ti/Al contact to GaN.
Low Temperature Selected Area Re-Growth of Ohmic Contacts for III-N FETs
Authors: Saripalli Yoganand , Zeng Chang , Jin Yawei , Long Joseph P , Grenko Judith A , Dandu Krishnanshu , Johnson Mark A.L , Barlage Doug W
Published: April 2011

GaN has a wide band gap energy, high electron mobility, high saturation velocity, and excellent thermal properties making it a promising material for high power and high frequency electronic devices. The development of enhancement mode GaN metal oxide semiconductor (MOS) transistors has been elusive due to the non-availability of a good insulating gate dielectric and the difficulty in forming of ohmic source/drain regions. Ion-implantation of dopants causes severe lattice damage requiring a high temperature post-implant anneal and has not been a successful method to obtain acceptable low-resistance source/drain regions. At the same time, gate dielectrics for most compound semiconductors, in addition to difficulties in minimizing the density of interface states which pin the Fermi level by inducing trap levels in the midgap, are degraded by instabilities as a result of high temperature annealing. The paper presents the development ohmic source/drain contacts for GaN MOSFETs by selected area epitaxial regrowth. Re-growth of GaN on patterned substrates by metal-organic chemical vapor deposition (MOCVD) employs a growth regime to decrease the enhanced growth rates and island formation that result from the diffusion of precursors to the selected area. The enhanced growth rate is 4.5μm/hr compared to 0.5μm/hr of the as-grown GaN on the unpatterned substrate. The enhanced growth rate also results in heavily porous GaN. Selected area growth, device processing, the material and device characterization results will be presented. In particular the selected area growth of doped contacts in the 800°C temperature range leads to superior morphology and contact resistance as compared to similar contacts grown at 1060°C. The contact resistivity of the n+ re-growth region measured was ∼2×10-4 Ω-m and the morphology of the re-grown region was comparable to the as-grown GaN with an RMS roughness ∼1.4nm. The success of fabricating low temperature contacts for GaN enhancement mode MOS transistors is a critical step in fabricating these devices opening new applications.
Plasma-Assisted MOCVD Growth of ZnO Thin Films
Authors: Losurdo Maria , Giangregorio Maria Michela , Capezzuto Pio , Bruno Giovanni , Malandrino Graziella , Blandino Manuela , Fragalà Ignazio
Published: April 2011

ZnO thin films have been grown by metalorganic chemical vapor deposition (MOCVD) also plasma assisted (PA-MOCVD) on c-axis oriented sapphire (0001) and Si(001) substrates using the alternative Zn(TTA)2·tmed (HTTA=2-thenoyltrifluoroacetone,TMED=N,N,N’,N’-tetramethylethylendiamine) precursor. The structural, morphological and optical properties of ZnO films have been investigated. The results show that the O2 plasma assisted growth results in an improvement of the structure, in smoother morphologies and in a better optical quality with a sharp and intense exciton of ZnO films.
High quality GaN layers grown on slightly miscut sapphire wafers
Authors: Brüeckner Peter , Feneberg Martin , Thonke Klaus , Habel Frank , Scholz Ferdinand
Published: April 2011

HVPE grown layers typically show a high density of pyramidal structures on the surface. We found that a slight off-orientation of the substrate totally suppresses the development of these structures. Further we found that a misorientation toward the m-plane of GaN features a smoother surface morphology, compared to an off-orientation towards the a-plane. After the improvement of the surface morphology and other properties of the HVPE grown layers, we studied self-separation processes. Our approaches to remove the thick GaN-layer from the substrate were a low-temperature interlayer and a structured dielectric mask.
Polarized Photoluminescence Study on AlGaN of AlGaN/GaN Heterostructure
Authors: Kitagawa Sachio , kosaka Kenichi , Tuchiya Tadayoshi , Suzuki Akira , Araki Tsutomu , Nanishi Yasushi
Published: April 2011

Optical properties of tensile strained AlxGa1-xN films of AlxGa1-xN/GaN heterostructures grown on sapphire were investigated by using polarization-resolved photoluminescence spectroscopy. Emissions from AlxGa1-xN with polarization of E//c and E⊥c were obtained at different peak energies. The energy separation of these emissions with polarization was increased linearly with the increase in Al mole fraction of the strained AlxGa1-xN, indicating that the energy separation was due to biaxial strain in the tensile strained AlxGa1-xN.
Refractive Indices of A-plane GaN Thin Films on R-plane Sapphire
Authors: Cai Ailing , Wellenius Ian Patrick , Gerhold Mike , Muth John , Osinsky Andrei , Xie J. Q. , Dong J. W.
Published: April 2011

A-plane GaN grown on r-plane sapphire is a promising material for optoelectronic applications such as laser diodes and LEDs. The absence of a built-in electrostatic field in the nonpolar a-plane GaN-based quantum wells can limit the piezoelectric polarization effect and result in higher emission efficiency. The knowledge of the planar anisotropic nature of nonpolar a-plane GaN is important to understand the growth mechanism of a-plane GaN is fundamental for optoelectronic device design. Two a-plane GaN samples were grown on r-plane sapphire, one being grown on intentionally miscut 4 degrees from r-plane sapphire. The films exhibited different surface morphologies and were characterized using imaging cathodoluminescence and SEM. The ordinary and extraordinary indices of a-plane GaN thin film were obtained using a prism-coupling technique. It was found that one sample the c-axis lay in the plane of the thin film as expected, and in the sample grown on miscut sapphire the c-axis was out of the plane of the film.
High quantum efficiency of photoluminescence in GaN and ZnO
Authors: Reshchikov Michael A. , Gu Xing , Nemeth Bill , Nause Jeff , Morkoç Hadis
Published: April 2011

The quantum efficiency (QE) of photoluminescence (PL) has been estimated in GaN and ZnO samples. A Si-doped GaN layer grown by molecular beam epitaxy (MBE) exhibited the highest QE of about 90% at low temperatures. Recombination via the shallow donor-acceptor pair transitions dominated in this sample. In contrast, a bulk ZnO crystal with the QE of PL of about 85% contained almost no defect- or impurity-related PL signatures besides the emission attributed to free and bound excitons. The sources of radiative and nonradiative recombination in GaN and ZnO are discussed.
Influence of the Annealing Ambient on Structural and Optical Properties of Rare Earth Implanted GaN
Authors: Lorenz Katharina , Nogales E. , Nédélec R. , Penner J. , Vianden R. , Alves E. , Martin R. W. , O`Donnell K. P.
Published: April 2011

GaN films were implanted with Er and Eu ions and rapid thermal annealing was performed at 1000, 1100 and 1200 °C in vacuum, in flowing nitrogen gas or a mixture of NH3 and N2. Rutherford backscattering spectrometry in the channeling mode was used to study the evolution of damage introduction and recovery in the Ga sublattice and to monitor the rare earth profiles after annealing. The surface morphology of the samples was analyzed by scanning electron microscopy and the optical properties by room temperature cathodoluminescence (CL). Samples annealed in vacuum and N2 already show the first signs of surface dissociation at 1000 °C. At higher temperature, Ga droplets form at the surface. However, samples annealed in NH3 + N2 exhibit a very good recovery of the lattice along with a smooth surface. These samples also show the strongest CL intensity for the rare earth related emissions in the green (for Er) and red (for Eu). After annealing at 1200 °C in NH3+N2 the Eu implanted sample reveals the channeling qualities of an unimplanted sample and a strong increase of CL intensity is observed.
Characterization of lattice mosaic ofa-plane GaN grown onr-plane sapphire by metalorganic vapor-phase epitaxy
Authors: Kusakabe Kazuhide , Ando Shizutoshi , Ohkawa Kazuhiro
Published: April 2011

Nonpolara-plane GaN films were grown on r-plane sapphire substrates by atmospheric metalorganic vapor-phase epitaxy. The as-grown layers were studied by high-resolution x-ray diffraction. Thea-plane GaN lattice mosaic is orientation dependent as determined by x-ray rocking curve (XRC) measurements. The tilt mosaic measured with the c-axis within the scattering plane (c-mosaic) was greater than the mosaic measured with the m-axis within the scattering plane (m-mosaic). The mosaic along both azimuths decreased and thec-mosaic/m-mosaic ratio was increased with increase of growth temperature from 1050 °C to 1080 °C. The morphological transition was correlated to change in thea-plane GaN tilt mosaic measured by XRC.
Energy Use and Greenhouse Gas Emissions in the Life Cycle of CdTe Photovoltaics
Authors: Fthenakis Vasilis , Kim Hyung Chul
Published: April 2011

The life cycle of the thin film CdTe PV modules in the U.S. have been investigated based on materials and energy inventories for a commercial 25 MW/yr production plant. The energy payback times (EPBT) of these modules are 0.75 years and the GHG emissions are 18 gCO2-eq/kWh for average U.S. solar irradiation conditions. Adding the impact of an optimized ground-level balance of system (BOS), result in a total EPBT of 1.2 years and total life-cycle GHG emissions of 24 gCO2-eq/kWh.
Suggested Strategies for the Ecotoxicology Testing of New Nanomaterials
Authors: Stone Vicki , Fernandes Teresa , Ford Alex , Christofi Nick
Published: April 2011

Nanotechnology is a rapidly expanding and advancing field of research that has already yielded a variety of commercially available products including cosmetics, suntan lotions, paints, self cleaning windows and stain resistant clothing. The Royal Society and the Royal Academy of Engineering in their recent report ‘Nanotechnology and nanoscience: opportunities and uncertainties’ (http://www.nanotec.org.uk/finalReport.htm) concluded that nanotechnology is likely to have ‘huge potential’. While this report indicated that ‘many applications of nanotechnology pose no new health or safety risks’, it also recognised that the health, safety and environmental hazards of nanoparticles (diameter less than 100nm) and nanotubes requires investigation. A significant body of data exists regarding the toxicological effects of nanoparticles (also termed ultrafine particles) in mammalian systems, particularly with respect to the lungs and cardiovascular system. Such studies suggest that smaller particles, with a larger surface area per unit mass, are more potent at inducing oxidative stress and inflammation leading to adverse health effects. However, very few papers have been published regarding the effects of nanoparticles on other phyla such as micro-organisms, invertebrates and vertebrates from terrestrial and aquatic habitats. Since nanoparticles from both domestic and industrial products will be released into the environment, eg. wastewater, it is essential to investigate the impact on such species and the ecosystem. This presentation will aim to discuss how existing knowledge regarding the mammalian toxicology of nanoparticles could be used to generate an effective, efficient and focused strategy for testing the ecotoxicolgy of nanoparticles.
Polymer-Coated Ultra-Fine Particles
Authors: Brousseau Patrick , Dubois Charles
Published: April 2011

Ultra-fine metallic particles have demonstrated recently their potential in tailoring the performance of energetic materials. DRDC Valcartier has explored methods to create controllable nanometric coatings on metallic particles and has opted to use polymers to treat the particles. Those coatings can have multiple positive effects. For example, in the case of aluminium, small particles are very reactive and tend to cause interations with the surrounding media. One example is the ageing of aluminium nanoparticles in the presence of air and humidity. Ultra-fine particles age much faster than micron-size particles. The long-term stability of energetic material mixes containing ultra-fine particles will be affected by this reactivity, and coatings would help to solve this problem. Another example is the interaction of aluminium nanoparticles with nitramines that causes gassing. Three coating methods will be presented: by thermoplastics using a Ziegler-Natta reaction, by thermosets through a polycondensation reaction initiated at the surface of the particles, and in-situ coating of particles by on-line polymerization during the plasma production of powders. The results of coating experiments using those methods will be presented. It will be shown that, for aluminium particles, adequate dispersion is a challenge and affects the results of the coating experiments. To assess the performance of the coating methods, ageing tests were carried out on coated and uncoated nanoparticles. The results of ageing tests with those methods will be presented and compared. It will be shown that the polymer coatings reduce significantly the loss of active metal content during accelerated ageing tests. Since the purpose of the powders is to be used in energetic materials, a study on the rheological effects of the coated particles in polymeric solutions will be presented as well. Coated particles increase the relative viscosity of HTPB-Al solutions by a factor of 100 at low shear rates, but much less with PPG.
Inorganic Nanoparticles for Gun Propellants
Authors: Baschung Barbara
Published: April 2011

The possibility of increasing the burning rate of solid rocket propellants by adding nanoparticles of aluminum into the propellant formulation has already been well-known for many years. This paper deals with micron- and nanoparticles embedded in gun propellants. The objective is to increase the gun performance. The burning behavior of solid propellants based on ultra-fine aluminum powder was investigated in a high pressure range which is reached in a gun tube. The burning rate of such a propellant is much higher (nearly two orders of magnitude) than for the similar propellant with the micron-sized aluminum. This paper presents a review of burning experiments with propellants based on the nano- and micron-sized particles of aluminum. The burning behavior of NENA solid propellants based on nano-scale aluminum was studied as a function of the portion of aluminum in the mixture. The burning of these propellants follows Vieille's burning law. The burning rate increases by augmenting the aluminum portion in the propellant. Theoretical models are reviewed in order to understand these experimental burning results. An advanced propellant coated with appropriate nanoparticles is presented in the conclusion. With this propellant and a special ignition by microwaves it should be possible to ignite solid propellants by using high loading densities (> 1.2 g/cm3).
Hot Spots from Dislocation Pile-up Avalanches
Authors: Grisé William
Published: April 2011

The model of localized adiabatic heating associated with release of a dislocation pile-up avalanche is described and re-evaluated. The model supplies a fundamental explanation of shear banding behavior in metal and non-metal systems. Now, a dislocation dynamics description is provided for more realistic assessment of the hot spot heating, for both straight dislocation pile-ups and circular loop pile-ups. Such a localized heating effect was overestimated in the earlier work, in part, to show the dramatic enhancement of the work rate, and the corresponding temperature build-up, potentially occurring in the initial pile-up release, say, at achievement of the critical dislocation mechanics-based stress intensity for cleavage. Proposed applications are to potentially brittle metal, ionic, and energetic material systems.
A Study of Shock-Wave Propagation in Single Crystallinefcc-Al and α-Fe2O3and an Interface between Two Such Phases Using MD Simulations
Authors: Tomar Vikas , Zhou Min
Published: April 2011

This research focuses on shock wave propagation in <100>, <110>, and <111> oriented single crystalline Al, <0001> oriented single crystalline Fe2O3, and an interface between the {100} surface of Al and the {0001} surface of Fe2O3. An interatomic potential is developed to describe interatomic interactions in the Fe+Al+Fe2O3+Al2O3material system. The potential is capable of describing the behaviors of Fe, Al, Fe2O3, and Al2O3as well as of composites consisting of any combination of these components. The analyses in single crystals focus on the relationships between the shock wave velocity (US) and the particle velocity (UP) as a function of single crystalline orientation. Calculations also focus on the formation and propagation of defects. The analyses reveal that the US-UPrelationship as well as the type and the extent of deformation strongly depend on the crystallographic orientation. The interfacial shock wave propagation analyses focus on the change in atomic structure near the interface as a function of UP. Calculations show that there is a threshold UPvalue above which the atomic structure on both sides of the interface undergoes significant changes which are accompanies by enhanced increase in temperature and pressure relative to what is seen in both phases when they are deformed alone. This observation suggests a possibility that this threshold may represent the onset of a shock-induced reactive structural change.