bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth Evolution of Gallium Nitride Films on Stepped and Step-Free SiC Surfaces
Authors: Eddy Charles R. , Culbertson James C. , Bassim Nabil D. , Twigg Mark E. , Holm Ronald T. , Stahlbush Robert E. , Henry Richard L. , Neudeck Philip G. , Trunek Andrew J. , Anthony Powell J.
Published: April 2011

Silicon carbide (SiC) is rapidly becoming the substrate of choice for the development of high frequency and high power electronic devices employing the III-V nitride family of materials. This heteroepitaxial growth system continues to receive considerable attention, as materials issues remain the fundamental limiters to device performance. The heteroepitaxial growth of gallium nitride (GaN) thin films on stepped and step-free 4H SiC surfaces is reported. Step-free SiC surfaces are created by mesa patterning of a SiC wafer and subsequent epitaxial growth in a process described previously. This process results in a collection of both step-free and stepped surfaces on a given sample. We have employed an established metalorganic chemical vapor deposition process to grow first a thin (1200Å) aluminum nitride (AlN) nucleation layer and then a 2 μm thick GaN thin film. We have interrupted growth at various stages of AlN and GaN growth to evaluate the growth evolution using atomic force microscopy (AFM). The results show marked differences in the manner in which the initial AlN layer deposits. Nucleation is random with elongated grains on step-free SiC surfaces, while stepped surfaces have round nuclei of uniform dimensions and a high degree of spatial correlation with the nuclei arranged in rows. These differences diminish as the AlN layer approaches the desired thickness. Growth of the GaN epilayer is also markedly different on the two types of surfaces with step-free surfaces leading to random and low density nucleation of crystallites that remain as single grains for long growth times, whereas the stepped surfaces have large numbers nuclei that rapidly grow laterally. Cross-sectional transmission electron microscopy (TEM) reveals that grain sizes are 2–3X larger on step-free surfaces.
Structural and Optical Properties of GaN Quantum Dots
Authors: Daudin B. , Gogneau N. , Adelmann C. , Sarigiannidou E. , Andreev T. , Enjalbert F. , Monroy E. , Fossard F. , Rouvière J. L. , Hori Y. , Biquard X. , Jalabert D. , Si Dang Le , Tanaka M. , Oda O.
Published: April 2011

Growth conditions, structural and optical properties of GaN quantum dots (QDs) grown by plasma-assisted molecular beam epitaxy will be examined. It will be shown that, depending on the Ga/N ratio value and on growth temperature, the growth mode of GaN deposited on AlN can be either of the Stranski-Krastanow or of the Frank-Van der Merwe type. It will be shown that vertical correlation results in a red shift and in a narrowing of the photoluminescence spectra.

Growth of Eu-doped GaN quantum dots embedded in AlN will be described. Intense photoluminescence associated with Eu has been measured, with no GaN band-edge emission, as an evidence that carrier recombination mostly occurs through rare earth ion excitation. Persistent photoluminescence of Eu-doped GaN quantum dots as a function of temperature has been put in evidence, as a further confirmation of the recombination of confined carriers through Eu ion excitation.
InN Nanostructures: Strain and Morphology
Authors: Demangeot Francois , Frandon Jean , Pinquier Claire , Caumont Michel , Briot Olivier , Maleyre Benedicte , Clur-Ruffenach Sandra , Gil Bernard
Published: April 2011

We presented an experimental work on InN nanostructures grown on a GaN buffer layer deposited on sapphire (0001) by Metal Organic Vapor Phase Epitaxy. InN islands of controlled sizes were fabricated by using specific growth conditions and taking advantage of self-organization that results from Stranski-Krastanov growth mode. Then nanometric islands as small as 25 nm were characterized by using atomic force microscopy (AFM) and micro-Raman spectroscopy. AFM measurements revealed that the current shape of islands correspond to truncated hexagons. In-plane residual strain field was deduced from the E2 phonon frequency shift in micro-Raman spectra recorded on islands of various sizes. Careful analysis of these data made clear that the key parameter in determining the strain magnitude was the height of the islands: this was not surprising, keeping in mind that the shape was roughly independent of the size. Nevertheless, the dislocation density was believed to increase as function of the island thickness, leading to various degrees of strained relaxation, as probed by the present micro-Raman study. This conclusion was reinforced by the strain variation on the facets of single islands with respect to its value at the centre.
Extended X-ray Absorption Fine Structure Studies of GaN Epilayers Dopedin situwith Er and Eu During Molecular Beam Epitaxy
Authors: Katchkanov V. , Mosselmans J. F. W. , Dalmasso S. , O'Donnell K. P. , Martin R. W. , Briot O. , Rousseau N. , Halambalakis G.
Published: April 2011

The local structure around Er and Eu atoms introduced into GaN epilayers was studied by means of Extended X-ray Absorption Fine Structure above the appropriate rare-earth X-ray absorption edge. The samples were dopedin situduring growth by Molecular Beam Epitaxy. The formation of ErN clusters was found in samples with high average Er concentrations of 32±6% and 12.4±0.8%, estimated by Wavelength Dispersive X-ray analysis. When the average Er concentration is decreased to 6.0±0.2%, 1.6±0.2% and 0.17±0.02%, Er is found in localised clusters of ErGaN phase with high local Er content. Similar behaviour is observed for Eu-doped samples. For an average Eu concentration of 30.5±0.5% clusters of pure EuN occur. Decreasing the Eu concentration to 10.4±0.5% leads to EuGaN clusters with high local Eu content. However, for a sample with an Eu concentration of 14.2±0.5% clustering of Eu was not observed.
Effect of Impurities on Raman and Photoluminescence Spectra of AlN Bulk Crystals
Authors: Sarua A. , Rajasingam S. , Kuball M. , Garro N. , Sancho O. , Cros A. , Cantarero A. , Olguin D. , Liu B. , Zhuang D. , Edgar J. H.
Published: April 2011

Raman scattering and photoluminescence (PL) spectroscopy with sub-bandgap excitation has been applied to explore tracing of common impurities (in particular of oxygen) in AlN. Bulk AlN crystals grown by the high temperature sublimation method were studied. PL bands have been observed at around 375 nm and at 560–660 nm and have been attributed to oxygen and to nitrogen vacancy/aluminium excess defects, respectively. The 375 nm UV PL band was found to shift with oxygen concentration. Micro-Raman spectra of the bulk AlN samples were measured in different polarisations. Besides normal Raman modes of AlN the presence of additional vibrational modes was detected. The modes were discussed and tentatively attributed to oxygen and silicon local vibrational modes (LVMs) in AlN.
Platelet Inversion Domains Induced by Mg-doping in ELOG AlGaN Films
Authors: Liu R. , Ponce F. A. , Cherns D. , Wills H. H. , Amano H. , Akasaki I.
Published: April 2011

We have studied the microstructure of heavily Mg-doped Al0.03Ga0.97N films grown by metal-organic vapor phase epitaxy in the lateral overgrowth mode (ELOG). A new type of defects with a platelet shape has been observed. According to TEM analysis, these defects are embedded in the overgrowth regions. The platelet is normal to the ELOG stripe direction [1100]AIGaN, forming trapezoidal trenches on the film surface. The thickness of the platelet is about 100nm. We have identified these defects as inversion domains using convergent beam electron diffraction and HR-TEM. Mg segregation at the coalescence boundaries between ELOG islands is believed to result in the formation of the defects.
Structural Study of V-like Columnar Inversion Domains in AlN Grown on Sapphire
Authors: Jasinski J. , Tomaszewicz T. , Liliental-Weber Z. , Paduano Q. S. , Weyburne D. W.
Published: April 2011

V-like columnar inversion domains with a divergence angle of about 4.5° ± 1° grown in AlN films with N-polarity were studied using transmission electron microscopy (TEM) and atomic force microscopy. Such domains emerge at the surface forming a small islands in form of hexagonal, truncated pyramids. A model of such pyramid was proposed. TEM studies indicate a displacement of c/2 along the [0001] direction at the inversion domain boundary. A boundary itself is composed of long segments located on the {1100} planes, which are alternated by short segments on some inclined planes.
The microstructure of GaN nucleation layers grown by MOCVD on (1120) sapphire versus pressure and temperature
Authors: Wojtowicz T. , Ruterana P. , Twigg M. E. , Henry R. L. , Koleske D. D. , Wickenden A. E.
Published: April 2011

In this work, the evolution of morphology and defect structure in GaN nucleation layers on the a-plane of sapphire are investigated using TEM. The growth temperature and pressure were varied from 560 to 1100°C and from 20 to 600 torr, respectively. Whereas the highest growth temperature leads to a continuous layer, a 2D growth mode is not attained when the chamber pressure is varied from 20 to 600 torr at 1028°C. At the highest pressures (>300 t orr), a large distribution is obtained for the island sizes.
Anomalous Composition Dependence of Optical Energies of MBE-grown InGaN
Authors: Fernandez-Torrente I. , Amabile D. , Martin R. W. , O'Donnell K. P. , Mosselmans J.F.W. , Calleja E. , Naranjo F. B.
Published: April 2011

A direct comparison of the optical energies of MBE- and MOVPE-grown InxGa1-xN epilayers of similar InN content is performed for the first time. The InN fraction in the 7 MBE samples examined ranged from x ∼ 0.11 to x ∼ 0.35 while the range in available MOVPE epilayers is [0, 0.4]. Wavelength Dispersive X-ray (WDX) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopies were used to measure composition and local structure (alloy character) of the samples. Cathodoluminescence (CL) spectroscopy in situ, ex situ photoluminescence (PL) mapping and large-area optical absorption spectroscopy were used to measure various optical energies. The composition dependence of the optical energies is determined by the growth method. The absorption bandgap and luminescence peak energies vary linearly with x for both growth methods, suggesting a near-zero value of the bowing parameter. But the energy intercept at zero InN content in MOVPE samples is close to the wurtzite-GaN bandgap of 3.4 eV at room temperature, as expected, while the equivalent for MBE samples falls near 3.2 eV.
Optical and Microstructural Properties of N- and Ga-Polarity GaN
Authors: Bell A. , Smit J. L. , Liu R. , Mei J. , Ponce F. A. , Ng H. M. , Chowdhury A. , Weimann N. G.
Published: April 2011

The effect of polarity on the optical and microstructural properties of GaN is presented. A sample with adjacent domains of Ga- and N-polarity material was grown by varying the nucleation layer. This allows a unique opportunity to study the two different polarities under controlled conditions. We found that the N-polarity material has a much lower dislocation density than the Ga-polarity material. The N-polarity material contains voids that are not present in the Ga-polarity region. The surface roughness of the N-polarity material appears to be caused by Ga-polarity inversion domains which lead the growth. A cathodoluminescence study showed that the N-polarity material is much brighter than the Ga-polarity material, suggesting a higher donor concentration, probably due to increased impurity incorporation in the [0001] growth direction or possibly due to an increase in intrinsic point defects. There is also evidence that the N-polarity region contains two types of material, one is flat and can be etched, the other has an inclined facet and does not etch.
Advances in the Realisation of GaN-Based Microcavities: Towards Strong Coupling at Room Temperature
Authors: Semond F. , Byrne D. , Natali F. , Leroux M. , Massies J. , Antoine-Vincent N. , Vasson A. , Disseix P. , Leymarie J.
Published: April 2011

In a recent paper [Phys. Rev. B 68, 153313 (2003)], we reported the first experimental observation of the strong coupling regime in a GaN-based microcavity. The λ/2 GaN optical cavity was grown by molecular beam epitaxy on a Si(111) substrate. The upper mirror is a SiO2/Si3N4 dielectric mirror and the silicon substrate acts as the bottom mirror. With such a relatively simple and low-finesse microcavity, a Rabi splitting of 31 meV was measured at 5K. On the basis of this very encouraging result, approaches to fabricate high-finesse GaN-based cavities exhibiting strong coupling with stable polaritons at room temperature are discussed.
Characterization of III-nitride Based Schottky UV Detectors with Wide Detectable Wavelength Range (360–10 nm) using Synchrotron Radiation
Authors: Motogaito Atsushi , Hiramatsu Kazumasa , Shibata Yasuhiro , Watanabe Hironobu , Miyake Hideto , Fukui Kazutoshi , Ohuchi Youichiro , Tadatomo Kazuyuki , Hamamura Yutaka
Published: April 2011

Characterizations of transparent Schottky barrier GaN and AlGaN UV detectors in the vacuum UV (VUV) and soft X-ray (SX) region using synchrotron radiation are described. In the GaN UV detectors, the responsivity achieved about 0.05 A/W at 95 eV (13 nm). Thus, their device performance is shown between 3.4 and 100 eV (10 and 360 nm). Furthermore, the high responsivity spectra were realized by using AlGaN Schottky UV detectors consisting of Al0.5Ga0.5N on AlN epitaxial layer.
Mg Doped GaN Using a Valved, Thermally Energetic Source: Enhanced Incorporation, Control and Quantitative Optimization
Authors: Burnham Shawn D. , Alan Doolittle W. , Namkoong Gon , Henderson Walter
Published: April 2011

In this study, a thermally-energetic Mg source with an independent, valved-flux control was used to study the behavior of Mg incorporation into GaN. To observe effects of the thermal energy of the Mg flux on Mg incorporation, two Mg flux temperatures were investigated: one (900°C) well above the melting point of Mg and one (625°C) slightly below the melting point of Mg. Alternating Mg-doped and undoped GaN layers were grown at steps of increasing Mg flux, retaining a constant thermal energy, from below the saturation limit, to above the saturation limit. Results were analyzed and compared using secondary ion mass spectroscopy (SIMS). For a constant measured Mg flux, the incorporated Mg increased by more than an order of magnitude when the Mg thermal source temperature was raised from 625°C to 900°C. During SIMS analysis, the energy spectra of sputtered Ga atoms were fairly constant for a Mg flux above the saturation limit, and shifts for a Mg flux slightly below the critical flux for saturation, indicating a conductivity change, and possibly providing a quantitative means of optimizing p-type conduction. Furthermore, Mg incorporation into GaN strongly depends on the III-V flux ratio. During this study it was also observed that Mg incorporation into GaN was enhanced on a rough growth-layer surface under N-rich conditions, while a smoother growth-layer surface resulted in lower Mg incorporation, even under N-rich conditions1.
Epitaxy of highly optical efficient GaN on O and Zn face ZnO
Authors: Gu Xing , Reshchikov Michael A. , He Lei , Teke Ali , Yun Feng , Johnstone Daniel K. , Nemeth Bill , Nause Jeff , Morkoç Hadis
Published: April 2011

ZnO is a highly efficient photon emitter, has optical and piezoelectric properties that are attractive for a variety of applications. Due to its stacking order and close lattice to GaN, it is also considered as a substrate material for GaN epitaxy. In the past the poor preparation of ZnO surface has been a major handicap to GaN epitaxy. However, proper treatment we developed recently can make both faces of ZnO smooth with atomic level terraces. Epitaxy of GaN on O-face and Zn-face ZnO by reactive molecular beam epitaxy was performed. We used low-temperature RF growth of GaN buffer layer on ZnO surface to protect it from both ammonia and Ga. No Ga2ZnO4, an oxide with the spinel structure formed due to reaction of ZnO with Ga, was found, in contrast to earlier reports. The low-temperature photoluminescence (PL) indicates that both faces of ZnO can provide a high quality GaN with high radiative efficiency. In previous research it has been reported that O-face ZnO is slightly better for GaN epitaxy. Our new finding demonstrates that high-quality GaN epilayers can be grown on Zn face of ZnO.
Deep Levels in Multilayer Structures of Si/Si0.8Ge0.2 Grown by Low-Pressure Chemical Vapor Deposition
Authors: Tokuda Yutaka , Shirai Kenichi
Published: April 2011

Deep levels in multilayer structures of ten periods Si/Si0.8Ge0.2 (16/5 nm) grown by low-pressure chemical vapor deposition have been characterized by deep level transient spectroscopy (DLTS). DLTS measurements reveal one dominant peak (E1) at around 130 K with a minor peak (E2) at around 240 K. The dominant trap E1 (Ec – 0.19 eV) is ascribed to the dislocation-related defect. The increase of the E1 concentration by a factor of 2 to 3 and the change of its energy level to Ec – 0.22 eV are observed with annealing up to 120°C. It is speculated that hydrogen incorporated during growth associates with E1 and the behavior of E1 upon annealing is caused by the release of hydrogen from E1.
Beam Induced Lateral Epitaxy: a new way to Lateral Growth in Molecular Beam Epitaxy
Authors: Naritsuka Shigeya , Saitoh Koji , Suzuki Takashi , Maruyama Takahiro
Published: April 2011

We developed a new technique of epitaxial lateral growth without using oxide masks called beam induced lateral epitaxy (BILE). In this technique, molecular beams are directed at a nearly glancing angle with respect to a substrate that has pre-fabricated truncated ridges. By using BILE we grew GaAs laterally from the side of ridges on a GaAs substrate. The growth behavior of BILE strongly depended on both incident angle of the Ga beam and the crystal orientation of the truncated ridges. The formation of facets on the lateral growth front controlled the grown shape of the layers. By using a (111) B substrate with BILE, we grew a smooth, flat (111) B facet on the top of the layer.
Pulsed Laser Deposition of Bi- And Sb-based Solid Solutions and Multilayer Structures
Authors: Alexanian Arik G. , Avetisyan Hovsep N. , Avjyan Karapet E. , Aramyan Nikolay S. , Aleksanyan Garegin A. , Grigoryan Romen P. , Khachatryan Ashot M. , Yeremyan Arsham S.
Published: April 2011

Thin films of Bi, Sb, solid solutions Bi1-xSbx, as well as multilayer structures Bi-Sb-Bi-Sb--from elementary sources were produced by pulsed laser deposition for optoelectronic applications. KBr crystals were used as substrates. The solid solutions Bi1-xSbx were obtained by co-evaporation of single targets of Bi and Sb. Structural investigations show that the performance of produced films depends on both the amount of material deposited per pulse of laser energy and the ratio of this amount for bismuth and antimony. Based on this the technological regimes of growth temperature and laser intensity ranges were determined in which single-crystalline growth of films with certain x is possible. Single-crystalline films of Bi1-xSbx were obtained in the range of x (0.12—0.48), which corresponds to semiconductor state of this solution. The method of sequential deposition is used for fabrication of multilayer structures Bi/Sb with quantum-confined layers of bismuth. The growth regime with practically excluded interdiffusion of materials is found. Results of spectral investigations are shown to be in agreement with the theoretically predicted semimetal-to-semiconductor transition of bismuth as a result of quantum confinement.
Observation of Retarded Recombination in ChargeSeparation Structures
Authors: Ahrenkiel R. K. , Friedman D. , Metzger W. K. , Page M. , Dashdorj J.
Published: April 2011

Measurement of recombination and minority-carrier lifetimes has become a very common activity in current semiconductor technology. The two primary measurement techniques are based on photoconductive decay (PCD) and time-resolved photoluminescence (TRPL). The measurement of the “true” lifetime depends on the carriers being confined to a given spatial region of a diagnostic device. When internal electric fields exist that separate the charges, the measured value does not represent the real minority-carrier lifetime. In these cases, the measured quantity is a function of the true lifetime and the measurement technique.
Pump Wavelength Tuning of Optical Pumping Injection Cavity Lasers for Enhancing Mid-Infrared Operation
Authors: McAlpine Todd C. , Greene Katherine R. , Santilli Michael R. , Olafsen Linda J. , Bewley William W. , Felix Christopher L. , Vurgaftman Igor , Meyer Jerry R. , Yang M. J. , Lee Hao , Martinelli Ramon U.
Published: April 2011

Recent efforts to improve the performance of mid-infrared antimonide-based semiconductor lasers have focused on enhancing the absorption of the pump beam to maximize power conversion efficiencies and minimize threshold intensities. One successful approach has been the optical pumping injection cavity (OPIC) laser, in which a type-II W active region is enclosed between distributed Bragg reflector (DBR) mirrors in order to achieve multiple passes of the pump beam and thereby to enhance absorption.

Previously, fixed wavelength sources have been used for optical pumping of OPIC laser structures, with limited tuning available by adjusting the incident angle. By tuning the pump wavelength using an optical parametric oscillator, we demonstrate minimum threshold intensities and maximum slope efficiencies at the resonance of the DBR cavity surrounding the active region, further demonstrating the potential of OPIC lasers. A 3.2 μm OPIC laser operated at 350 K in pulsed mode (at the highest operating temperature of the dewar), with a characteristic temperature of 50 K. The power conversion efficiency for a single facet at 300 K was the highest ever observed in the mid-IR, at approximately 4%.

Results are presented for two OPIC samples (emitting at ∼3.2 μm and 4.3 μm at high temperature), one of which was designed with a broadened cavity resonance, suitable for pumping with a multi-mode source. Threshold intensities and slope efficiencies measured as a function of pump wavelength demonstrate the strong resonance effect, and that the “broadened OPIC” does in fact manifest a much wider resonance than the non-broadened resonance cavity design.
Elevated Temperature Characteristics of Carbon-Doped GaInP/GaAs Heterojunction Bipolar Transistor Grown by Solid Source Molecular Beam Epitaxy
Authors: Rong Zhang , Fatt Yoon Soon , Kianhua Tan , Zhongzhe Sun , Qingfeng Huang
Published: April 2011

This paper reports the characteristics of GaInP/GaAs heterojunction bipolar transistor (HBT) with carbon-doped GaAs base layer grown by solid source molecular beam epitaxy (SSMBE) using carbon tetrabromide (CBr4) as p-type dopant precursor. Hydrofluoric acid (HF) was used to passivate the GaInP/GaAs HBTs. At base bias voltages below 0.8V in the Gummel plot, the base current of large-area devices after HF treatment was greatly reduced. This indicates that the extrinsic base surface recombination current was greatly reduced. After HF treatment, detailed DC characterization of the device performance from 300K to 380K was carried out and the carrier transport properties were investigated. The base current and collector current ideality factors at 300K were 1.12 and 1.01, respectively. This indicates that the space- charge region recombination current in the base is insignificant. From the temperature- dependent Gummel plot, the activation energies of collector current and base current were obtained. For the collector current, the activation energy is 1.4eV, which is close to the bandgap of the GaAs base. This indicates that the collector current is determined by the drift-diffusion process, in which an energy barrier of the same magnitude as the base bandgap is to be overcome by electrons before they reach the collector. For the base current, the activation energy is also 1.4eV, which is close to the bandgap of GaAs, indicating that band-to-band recombination plays a dominant role in the base current. No trap-related recombination was observed for the base and collector currents, which further indicates the high quality carbon-doped GaAs base material for the HBT structures.
Analysis of Photoluminescence Efficiency of Annealed GaInNAs Quantum Well Grown by Solid Source Molecular Beam Epitaxy
Authors: Khee Ng Tien , Fatt Yoon Soon , Weijun Fan
Published: April 2011

The carrier dynamics of a 7.5 nm GaInNAs quantum well (QW) are studied by photoluminescence (PL) at a low temperature regime of 4 K to 150 K. The PL emission efficiency of the QW is initially evaluated to examine the recombination mechanisms in the QW. A dual-activation-energy model is later found to fit the integrated PL intensity vs. temperature curve better than a single-activation-energy model. The two states that correspond to the above activation energies could have resulted in a much faster PL intensity quenching in the GaInNAs QW as compared to that of a reference GaInAs QW. One of the states is identified as a localized state that traps carriers at a low temperature range of less than ∼100 K. The other state has a larger quenching effect at temperatures higher than 100 K and this state is not studied in this paper. By fitting the original PL spectra with two Gaussian functions, the temperature dependent PL integrated intensity of both Gaussian functions was also studied to further characterize the GaInNAs QW. The analysis gives evidence of the localization behaviour in this QW.
Electrostatic Force Microscopy and Secondary Electron Imaging of Double Stacking Faults in Heavilyn-type 4H-SiC after Oxidation
Authors: Mikhov M. K. , Skromme B. J. , Wang R. , Li C. , Bhat I.
Published: April 2011

Thermal processing or oxidation of 4H-SiC withn-type doping above about 3×1019cm-3is known to produce double Shockley stacking faults spontaneously. The resulting region of 3C stacking order acts like a quantum well in the 4H matrix and becomes negatively charged due to modulation doping. Some of these quantum well regions penetrate into the lightly-doped epilayers and intersect the wafer surface as straight lines, due to the 8° misorientation of the wafer from thec-axis. These intersections appear as bright lines in secondary electron images, which we tentatively attribute to increased secondary electron yield due to repulsion of secondary electrons from the negative charge in the quantum wells. Electrostatic force microscopy (EFM) and scanning Kelvin probe microscopy (SKPM) also produce clear images of the quantum well intersections, independent of surface topography. These images are similar to the SEM images.
1.5 micron InAs quantum dot lasers based on metamorphic InGaAs/GaAsheterostructures.
Authors: Ustinov V. M. , Zhukov A. E. , Kovsh A. R. , Maleev N. A. , Mikhrin S. S. , Vasil'ev A. P. , Nikitina E. V. , Semenova E. S. , Kryzhanovskaya N. V. , Musikhin Yu. G. , Shernyakov Yu. M. , Maximov M. V. , Ledentsov N. N. , Bimberg D. , Alferov Zh.I.
Published: April 2011

1.5 micron range emission has been realized using the InAs quantum dots embedded into the metamorphic InGaAs layer containing 20% of InAs grown by MBE on a GaAs substrate. Growth regimes were optimized to reduce significantly the density of dislocations propagating into the active layer from the lattice mismatched interface. 2 mm long InGaAs/InGaAlAs lasers with 10 planes of quantum dots in the active region showed threshold current density about 1.4 kA/cm2 with the external differential efficiency as high as 38%. Lasing wavelength depends on the optical loss being in the 1.44–1.49 micron range at room temperature. On increasing the temperature the wavelength reaches 1.515 micron at 85C while the threshold current characteristic temperature of 55–60K was estimated. High internal quantum efficiency (η>60%) and low internal losses (α=3–4 cm-1 ) were realized. Maximum room temperature output power in pulsed regime as high as 5.5 W for 100 micron wide stripe was demonstrated. Using the same concept 1.3 micron InGaAs/InGaAlAs quantum well lasers were fabricated. The active region contained quantum wells with high (∼40%) indium content which was possible due to the intermediate InGaAs strain relaxation layer. 1 mm stripe lasers showed room temperature threshold current densities about 3.3 kA/cm2 (λ=1.29 micron) and 400 A/cm2 at 85K. Thus, the use of metamorphic InGaAs layers on GaAs substrate is a very promising approach for increasing the emission wavelength of GaAs based lasers.
Characterization of bulk crystals of transition metal doped ZnO for spintronic applications
Authors: Kane M. H. , Varatharajan R. , Feng Z. C. , Kandoor S. , Nause J. , Summers C. , Ferguson I. T.
Published: April 2011

In this work, we report on the material properties of ZnO doped with Mn, Co, and Fe grown by a modified melt growth technique. X-ray diffraction measurements show that transition metals can be incorporated on Zn sites; an increase in the lattice parameter is apparent with increasing doping level. UV-visible transmission and reflectance measurements have also been performed. Absorption bands in the visible regime are distinctive to the individual transition metal dopants. A noticeable shift in the optical band edge has been observed from these Mn/Co/Fe-doped ZnO crystals in comparison with the undoped material. ZnO may also provide a suitable platform for the incorporation of transition metal elements through high temperature near equilibrium growth processes; however, further work is required in order to employ these materials for spintronic applications.
Synthesis and Characterization of Nanocrystalline Oxidizer/Monopropellant Formulations
Authors: Brill Thomas B. , Tappan Bryce C. , Li Jun
Published: April 2011

The objective of this work is to try to create new behaviors of energetic materials by using sol-gel processing and freeze drying to incorporate the energetic material into the fuel matrix at the nano particle size scale. Hydrazinium diperchlorate ([N2H6][ClO4]2) and resorcinol-formaldehyde were chosen in one example, and CL-20 and nitrocellulose were chosen in another. High solids loadings were achieved by the cryogel method. Characterization was carried out by elemental analysis, SEM, TEM, AFM, T-jump/FTIR spectroscopy, DSC, and drop-weight impact testing. The nanoscale formulations do indeed exhibit several different behaviors, such as enhanced burning characteristics and unusual morphologies, and appear to be a promising direction to pursue.
Surface Passivation of Bare Aluminum Nanoparticles using Perfluoroalkyl Carboxylic Acids
Authors: Jouet R. Jason , Warren Andrea D. , Rosenberg David M. , Bellitto Victor J.
Published: April 2011

Surface passivation of unpassivated Al nanoparticles has been realized using self assembled monolayers (SAMs). Nanoscale Al particles were prepared in solution by catalytic decomposition of H3Al•NMe3 or H3Al•N(Me)Pyr by Ti(OiPr)4 and coated in situ using a perfluoroalkyl carboxylic acid SAM. Because the Al particles are prepared using wet chemistry techniques and coated in solution, they are free of oxygen passivation. This SAM coating passivates the aluminum and seems to prevent the oxidation of the particles in air and renders the composite material, to some extent, soluble in polar organic solvents such as diethyl ether. Characterization data including SEM, TEM, TGA, and ATR-FTIR of prepared materials is presented.
Preparation of Energetic Metastable Nano-Composite Materials by Arrested Reactive Milling
Authors: Schoenitz M. , Ward T. , Dreizin E. L.
Published: April 2011

Highly metastable, nano-scale energetic materials were prepared by Arrested Reactive Milling (ARM). When reactive milling is carried out with materials systems suitable for Self-Propagating High Temperature Synthesis (SHS), reaction between the components occurs spontaneously and violently after a certain period of milling. In this research, metastable nanocom-posites with high energy density, were prepared by arresting the milling process prior to the spontaneous reaction. Products thus obtained are powders with particle sizes in the 10–50 μm range. Individual particles are intimate mixtures of reactive components, comparable to Metast-able Intermolecular Composites (MIC), with near theoretical maximum density. The time of arrest determines the degree of grain refinement and therefore the sensitivity to mechanical, electrical, or thermal initiation. Particle sizes of the product powders can be adjusted by appropriate choice of milling parameters. This paper describes the application of ARM to the material systems Al-Fe2O3 and Al-MoO3. After empirical determination of optimum milling parameters, the reactive composites are structurally characterized by electron microscopy and x-ray diffraction. First results of combustion tests are presented.
Ultrafast spectroscopy of laser-initiated nanoenergetic materials
Authors: Yang Yanqiang , Sun Zhaoyong , Wang Shufeng , Hambir Selezion A. , Yu Hyunung , Dlott Dana D.
Published: April 2011

A picosecond laser flash-heating technique is combined with ultrafast spectroscopic probe diagnostics to investigate the fundamental mechanisms of nanoenergetic material performance. The systems studied include Al nanoparticle aggregates in nitrocellulose (NC) oxidizer, size-selected Al nanoparticles in NC and in Teflon oxidizers, and nanoparticle thermites consisting of 30 nm Al and nanometric MoO3. The time-dependence of reactions between Al and the oxidizer on the picosecond to nanosecond time scales are studied using coherent anti-Stokes Raman scattering (CARS) to monitor oxidizer consumption. The time-dependence of energy release is measured using fast optical spectroscopy. The space-dependence of chemical reaction propagation over 100 to 1500 nm distances is studied using the average distance between nanoparticles as a ruler. The distance of reaction propagation from a flash-heated Al nanoparticle increases linearly with energy, which is explained by a hydrodynamic model.
The Shock Initiation and High Strain Rate Mechanical Characterization of Ultrafine Energetic Powders and Compositions
Authors: Field J. E. , Walley S. M. , Proud W. G. , Balzer J. E. , Gifford M. J. , Grantham S. G. , Greenaway M. W. , Siviour C. R.
Published: April 2011

This paper reviews the techniques that have been developed at the Cavendish Laboratory for the study of the mechanical and ignition properties of energetic materials.
Hydrogen Diffusion in Zr-Cu-Ni-Al Metallic Glasses
Authors: Apih Tomaz , Jastrow Lioba , Lyubenova Lyudmila , Dolinšek Janez , Zander Daniela , Köster Uwe
Published: April 2011

Zr-based metallic glasses are known to absorb high amounts of hydrogen, but exhibiting less severe embrittlement than their crystalline counterparts; therefore, they might be useful for hydrogen storage application. In order to understand kinetics of hydrogen absorption and desorption in more detail, data on hydrogen diffusion are necessary. The aim of this paper is to present hydrogen diffusivities in melt-spun amorphous Zr69.5Cu12Ni11Al7.5 alloys.

Hydrogen charging was performed electrochemically in a 2:1 glycerin-phosphoric acid electrolyte. Hydrogen contents were measured by a microbalance with accuracy of ±1 μg as well as by LECO. Diffusivities of hydrogen atoms were measured at different temperatures by the technique of Nuclear Magnetic Resonance (NMR) diffusion in a static fringe field of a superconducting magnet. The diffusion of hydrogen nuclei in an inhomogeneous magnetic field is accompanied by the change of its NMR resonance frequency, which produces motional destruction of the spin echo signal. The analysis of the echo damping allows a model-independent determination of the hydrogen self-diffusion constant, with the low limit of sensitivity D>10−10 cm2/s.

Within the temperature interval between room temperature and 420 K diffusivities in the range between 4×10−8 and 1.2×10−9 cm2/s were observed. The Arrhenius-type temperature dependence indicate a simple classical over-barrier-hopping hydrogen diffusion; the activation energy increases slightly with the hydrogen content. Whereas in a number of metallic glasses hydrogen diffusion is known to increase with the hydrogen content, in Zr69.5Ni12Cu11Al7.5 the opposite effect was observed, at least for hydrogen contents between H/M = 0.2 and 1.2. The results will be compared with other measurements known for Zr-based metallic glasses (e.g., Zr-Ni) and discussed in detail, e.g. in regard to the density of the glass.
Old Technique for New Materials: Characterization of Solid State Hydrogen Storage Materials by Temperature Programmed Reaction
Authors: Rufael Tecle S. , Williams Cabral M.
Published: April 2011

The most common diagnostic tools in studying reversible solid-state hydrogen storage materials involve volumetric or gravimetric approaches. In this study, a mature gas solid analysis technique called Temperature Programmed Reaction (TPR) is presented as an alternative and complementary approach in investigating solid-state hydrogen storage materials. TPR is fast, simple and offers valuable information from a wide range of solid materials. This technique is routinely used to characterize the reduction behavior of metal oxide and sulfide catalysts. In a typical TPR experiment, a sample powder is heated at a constant rate under a non steady-state environment of constant pressure and flow of a hydrogen/inert gas mixture, while monitoring the rate of hydrogen absorption and desorption by a thermal conductivity detector. The principle behind TPR is the high thermal conductivity coefficient of hydrogen when compared to other gases. The detector is sensitive enough that a slight change in hydrogen concentration is reflected by significant changes in the thermal conductivity signal. Hydrogen storage materials exhibit fingerprint type TPR spectra, unique to their composition and structure. Specific rates of hydrogen intake, heats of absorption and desorption as well as extent of activation can be extracted from TPR curves. In addition, multiple absorption and desorption states can be identified from the number of absorption or desorption peaks. Material stability and potential structural or phase changes can also be inferred indirectly. The simplicity and the highly sensitive thermal conductivity measurement provide a number of advantages over the traditional volumetric or gravimetric methods. Metal hydride alloys of AB5-type are used to demonstrate the effectiveness of TPR technique in characterizing hydrogen storage materials.
Using nanoporous carbon membranes in fuel cells
Authors: Ramnarayanan Ramanathan , Rajagopalan Ramakrishnan , Mallouk Thomas E. , Foley Henry C.
Published: April 2011

We have synthesized nanoporous carbon membranes that have monodisperse pores of 4–5 Å. These membranes have excellent size and shape selectivity that makes them an ideal candidate for use as separators in fuel cells. The selectivity of these membranes to gases such as N2, O2 and water gas [carbon monoxide and hydrogen] were measured using a permeation testing unit. These membranes were then tested as separators in fuel cells.
Ab initio study of the hindered rotation of H2 over benzene
Authors: Hamel Sébastien , Côté Michel
Published: April 2011

In the ongoing search for promising compounds for hydrogen storage, novel porous metal-organic frameworks (MOF) have been discovered recently[1]. Well defined binding sites were deduced from inelastic neutron scattering (INS) spectroscopy of the rotational transitions of the adsorbed molecular hydrogen. In light of this discovery we performed ab initio density functional theory (DFT) calculations of the adsorption of molecular hydrogen on this class of microporous MOF to compare different adsorption sites. As a first step, we study the case of H2 adsorbed on benzene. The DFT code used, Abinit, is based on plane-waves and pseudopotentials. Different approximations for the exchange-correlation potentials were accessed for a set of relevant properties (binding energy, energetically favored configuration, distance between the adsorbents and adsorbates). In particular, theoretical rotational spectra of the adsorbed H2 were obtained that could be compared to the experimental INS spectra.
The Influence of Surface Reconstruction and C-impurities on Photocatalytic Water Dissociation by TiO2
Authors: Nie Xiliang , Sohlberg Karl
Published: April 2011

TiO2 is well known as a prototype photocatalyst for water dissociation. To understand the mechanism of its photocatalytic water dissociation we performed first-principles calculations. We find that the surface of the catalytically favorable (TiO) termination is very different from the physically favorable (oxygen) termination. The calculated surface energy of the catalytically favorable (TiO) termination is about 10 times larger than that of the physically favorable (oxygen) termination. Analysis of the surface band structure suggests that while O-vacancies are intrinsic active sites for water dissociation into H2 and O2 gas, they are not essential for photocatalytic water dissociation. We also find that carbon impurities decrease the band-gap of TiO2, in agreement with previously reported experimental results. Moreover, we identify the origin of the arcane “double band gap” in carbon doped TiO2. The two onsets seen in the photoabsorption spectrum result from excitations from two of three C p-states within the band gap, not from domains of different composition.
Low-Temperature Aging and Phase Stabiliy of U6Nb
Authors: Hsiung L. M. , Briant C. L. , Chasse K. R.
Published: April 2011

Aging behavior and phase stability of a water-quenched U-6wt%Nb (U-14at%Nb) alloy artificially aged at 200°C and naturally aged at ambient temperature for 15 years have been investigated using Vickers hardness test, X-ray diffraction analysis, and transmission electron microscopy techniques. Age hardening/softening phenomenon is observed from the artificially aged samples according to microhardness measurement. The age hardening can be rationalized by the occurrence of spinodal decomposition, or fine scale of Nb segregation, which results in the formation of a nano-scale modulated structure within the artificially aged samples. Coarsening of the modulated structure after prolonged aging leads to the age softening. The occurrence of chemical ordering (disorder-order transformation) is found in the naturally aged sample based upon the observations of antiphase domain boundaries (APB's) and superlattice diffraction patterns. A possible superlattice structure is accordingly proposed for the chemically ordered phase observed in the naturally aged alloy sample.
Probing Phonons in Plutonium
Authors: Wong Joe , Krisch Michael , Farber Daniel L. , Occelli Florent , Schwartz Adam J. , Chiang Tai-C. , Wall Mark , Boro Carl , Xu Ruqg
Published: April 2011

Plutonium (Pu) is well known to have complex and unique physico-chemical properties [1]. Notably, the pure metal exhibits six solid-state phase transformations with large volume expansions and contractions along the way to the liquid state: α → β → γ → δ → δ' → ε→ liquid. Unalloyed Pu melts at a relatively low temperature ∼640 °C to yield a higher density liquid than that of the solid from which it melts, (Figure 1). Detailed understanding of the properties of plutonium and plutonium-based alloys is critical for the safe handling, utilization, and long-term storage of these important, but highly toxic materials. However, both technical and and safety issues have made experimental observations extremely difficult.
f-Element Influence on the Size of Nanophase Phosphate Inclusions in Silica
Authors: Beitz James V. , Skanthakumar S. , Seifert S. , Thiyagarajan P.
Published: April 2011

Insight into the factors that control the formation and size of heavy metal phosphate nanophases in vitreous silica has been gained by combining conventional and anomalous small angle x-ray scattering studies with powder x-ray diffraction and laser-induced fluorescence investigations. Europium, thorium, and uranyl ions were sorbed from aqueous solutions into a chemically functionalized porous silica (termed Diphosil). Aliquots of those samples were heated to a series of temperatures that spanned the pore collapse point. Loading with trivalent europium ions resulted in production of nanophases whose size corresponded to the average number of metal ions per pore. Thorium or uranyl ions resulted in retention of porosity to higher temperature with eventual pore collapse that evidently resulted in formation of nanophases whose size exceeded that determinable under the experimental conditions.
Combinatorial Search for Low-Temperature Combustion Catalysts
Authors: Wolter Tina , Maier Wilhelm F.
Published: April 2011

A sol-gel process was used to synthesize potential combustion catalysts based on mixed oxides on several libraries. As mixed oxides a variety of doped Zr-Al-oxides and Ce-oxides were prepared. The catalytic activity of the materials was tested with the combustion of i-octane, n-butane and methane in synthetic air. Several Pt- and Rh-containing sol-gel materials showed effective methane combustion already at 60 °C under rich conditions and at 200 °C under lean conditions. Several noble metal free doped CeO2 catalysts combusted methane already at 300 °C under lean conditions. Combustion of i-octane was catalysed already at 125 °C. The most active catalysts were synthesized conventionally, characterized and catalytic activity was confirmed in a regular gas phase flow reactor.
Multilayer Transparent Conducting Oxide Thin Film Produced by Linear Combinatorial Synthesis
Authors: Li Xiaonan
Published: April 2011

Multilayer transparent conducting oxide (TCO) structures consisting of alternating layers of tin oxide (SnO2) and cadmium oxide (CdO) pairs was studied. A low-pressure metalorganic chemical vapor deposition with a linear combinatorial approach was used to fabricate the libraries. Three libraries were studied with the pair thickness range covered from 500 nm to 4 nm. For pairs thicker than 150 nm, the material properties of the multilayer simply show a combination of CdO and SnO2 characteristics. When pair thinner than 70 nm, the interface effect started emerging. As pairs became thinner than 20 nm, the new compound phase was formed.
Application of Combinatorial Electropolymerization to the Development of Chemical Sensors
Authors: Mirsky Vladimir M. , Kulikov Valentin , Hao Qingli , Wolfbeis Otto S.
Published: April 2011

Combinatorial electropolymerization with electrical addressing was realized on the array of 96 electrode groups, each from four electrodes. The polymer synthesis was combined with subsequent high-throughput investigation of analytical properties of synthesized polymers. Two-and four-point techniques were applied simultaneously to measure electrical properties of synthesized polymers and contact resistances between electrodes and polymers and modifications of these parameters on addition of analyte. The system was used for development of sensitive materials for detection of gaseous hydrogen chloride.
Applications of Discrete and Gradient Compositions in Polymer Research
Authors: Wroczynski Ronald J. , Potyrailo Radislav A. , Pickett James E. , Rubinsztajn Malgorzata
Published: April 2011

A new general approach for rapid performance screening of polymer compositions is provided. Multiple compositions are generated as one-dimensional libraries in a micro-extruder with step or gradient composition changes in < 1 min. To accelerate testing, an environmental stress is applied to only local regions, followed by high-sensitivity spatially resolved characterization. Applying the methodology to weathering of arrays of polymeric compositions provided a 20X faster ranking of polymer/UV absorber compositions that was equivalent to traditional weathering methods.