bluuebunny/crossref_metadata_2025 huggingface dataset:
Fracture of Laminated and In Situ Niobium Silicide-Niobium Composites
Authors: Rigney J. D.
Published: April 2011

The mechanisms contributing to the fracture resistance of refractory metal intermetallic composites containing a BCC metallic phase (niobium) were investigated using model Nb-Si laminates and in situ composites. The controlling influence of ductile phase yield strength and fracture behavior were investigated by varying laminate processing parameters, and/or altering temperatures and applied strain rates during fracture experiments on all materials. The fracture behavior of “ductile” constituents were found to be influenced by phase grain size, solid solution content, constraint (as influenced by interfacial bond strengths), and the testing condition (high strain rates and low temperatures). The measured fracture resistance, when compared to theoretical models, was shown to be controlled by the “toughness” of the “ductile” phase and independent of the fracture behavior promoted (cleavage and ductile). The loss in ductility due to cleavage by high constraint, high strain rates and/or low temperatures was compensated by high yield and cleavage fracture stresses in order to provide a level of toughening similar to that contributed by ligaments which failed with lower yield stresses and greater strains.
Fatigue Crack Growth in Aluminum Laminate Composites
Authors: Hoffman P. B. , Carpenter R. D. , Gibeling J. C.
Published: April 2011

Fatigue crack growth has been measured in a laminated metal composite (LMC) consisting of alternating layers of AA6090/SiC/25p metal matrix composite (MMC) and AA5182 alloy. This material was tested in both as-pressed (F temper) and aged (T6 temper) conditions. Corresponding crack growth measurements were made in self-laminates of both the MMC and AA5182 materials to examine the role of the interfaces.

The LMC-T6 material has a significantly higher fracture toughness than its MMC constituent but exhibits only modest improvements in nominal fatigue crack growth resistance and a lower nominal threshold. The LMC-T6 shows high levels of crack closure which reveal poor intrinsic crack growth resistance. Self-laminated AA6090/SiC/25p MMC in the F temper has lower microhardness, lower strength and superior intrinsic crack growth resistance compared to this material in the T6 condition. The fatigue fracture surface of T6 temper MMC remains macroscopically smooth despite the development of a large amount of crack closure. The selflaminated AA5182 exhibits a significant improvement in fatigue crack growth resistance after heat treatment and a corresponding change in crack morphology from smooth to very rough. This change leads to a large increase in roughness-induced closure. The effects of heat treatment are attributed to differences in interfacial strength.
Evaluation of Impact Damage Resistance in Laminated Composites Using Resins with Different Crosslink Densities
Authors: Lesser A. J.
Published: April 2011

It is generally recognized that fiber-reinforced laminated composites are susceptible to damage resulting from low-velocity impacts. Over recent years, many strategies have been devised to increase the fracture toughness of resin matrix materials with the aim of improving the composites overall resistance to impact damage. One popular strategy for enhancing the fracture toughness of thermosets involves increasing its molecular weight between crosslinks which, in turn, enhances the resins ductility. In this paper, we investigate the efficiency of this toughening approach with regard to resisting damage in composite laminates subjected to lowvelocity impacts. Generic damage characteristics and mechanisms are reviewed and it is shown that two different events occur during the impact process. First, the laminate experiences a local failure which resembles a Hertzian fracture process followed by subsequent delamination between the plies. Results are presented illustrating the effects that systematically increasing the molecular weight between crosslinks of the resin has on each of these mechanisms. Also, the residual compressive strength (Compression After Impact) of the laminates made with these resins is presented.
Brick Structure Improved by Using Cement Mortar Containing Short Carbon Fibers
Authors: Zhu Mingguang , Chung D. D. L.
Published: April 2011

The addition of short carbon fibers in the optimum amount of 0.5% of the cement weight to mortar increased the brick-to-mortar bond strength by 150% under tension and 110% under shear when the gap between the adjoining bricks was fixed, and by 50% under tension and 44% under shear when the gap between the adjoining bricks was allowed to freely decrease due to the weight of the brick above the joint. This effect is attributed to the decrease of the drying shrinkage by the fiber addition. The drying shrinkage decrease was particularly large at 2 to 24 h of curing. At 24 h, the shrinkage was decreased by 50% by the addition of fibers in the amount of 0.5% of the cement weight. Fibers in excess of the optimum amount gave less bond strengthening due to increased porosity in the mortar.
Young's Modulus and Density of thin TiO2 Films Produced by Different Methods
Authors: Ottermann C. R. , Kuschnereit R. , Anderson O. , Hess P. , Bange K.
Published: April 2011

The Young's modulus and density are analyzed for 280 and 500 nm thick TiO2 layers deposited by reactive evaporation (RE) and ion plating (IP) by means of surface acoustic wave spectroscopy (SAWS) and grazing incidence X-ray reflection spectroscopy (GIXR). The layers are amorphous or polycrystalline, and have densities between 2.9 g/cm3 and 3.8 g/cm3, depending on the deposition conditions. The measured Young's moduli vary between 65 GPa for RE films and 147 GPa for IP layers. They are independent of film thickness, but correlate with the density. A change of the Young's modulus due to the phase transition from amorphous to anatase is described, which occurs at temperatures above 210°C.
Tribological Properties of Nitrogen Implanted and Boron Implanted Steels
Authors: Kern K. T. , Walter K. C. , Fayeulle S. , Griffin A. J. , Kung H. , Lu Y. , Nastasi M. , Tesmer J. R.
Published: April 2011

Samples of steel with high chrome content were implanted separately with 75 keV nitrogen ions and with 75 keV boron ions. Implanted doses of each ion species were 2-,4- and 8 × 1017 /cm2. Retained doses were measured using resonant non-Rutherford Backscattering Spectrometry. Tribological properties were determined using a pin-ondisk test with a 6-mm diameter ruby pin with a velocity of 0.94 m/min. Testing was done at 10% humidity with a load of 377g. Wear rate and coefficient of friction were determined from these tests. While reduction in the wear rate for nitrogen implanted materials was observed, greater reduction (more than an order of magnitude) was observed for boron implanted materials. In addition, reduction in the coefficient of friction for high-dose boron implanted materials was observed. Nano-indentation revealed a hardened layer near the surface of the material. Results from grazing incidence x-ray diffraction suggest the formation of Fe2N and Fe3N in the nitrogen implanted materials and Fe3B in the boron implanted materials. Results from transmission electron microscopy will be presented.
Copper Grown Diamond Films
Authors: Pereira E. , Fan Qi Hua , Gracio J.
Published: April 2011

Diamond has been synthesised on copper using Microwave Plasma Chemical Vapour Deposition (MPCVD). The effect of substrate pre-treatment including different polishing and cleaning on diamond nucleation was investigated. It has been found that the residues from the polishing process played an important role in the diamond nucleation. Diamond films synthesised under optimised conditions presented a narrow 1332cm-1 Raman peak. During the ramp down procedure, diamond films usually cracked due to the large thermal mismatch between copper and diamond. To avoid this problem, the effect of growth temperature and deposition time was investigated and a two-step-growth method for stress relief is proposed.
Reversible Force-Resistivity Behavior of Thin Films of the TTF-Tcnq Family
Authors: Vollmann W. , Sonntag H.-U.
Published: April 2011

The electrical properties of vacuum sublimed thin films of TTF-TCNQ and its derivatives mainly are determined by electron barriers at grain boundaries. The electrical conductivity is thermal activated and exhibits a significant dependence on a force acting perpenticularly to the film plane. The sample resistance R decreases continiously with increasing force F. TCNQ thin films on steel show a similar R-F relation. The effect has been observed already at forces of 1 N, but also up to about 60 kN. An explanation of these phenomena is given by a grain boundary limited hopping mechanism with pressure dependent potential barrier width and height. Morphology investigations by SEM support the model.
The Nucleation of Crystalline TiSi2 Phases from Amorphous TiSix Layers and Interfacial Layers
Authors: Tung R. T. , Fujii K. , Kikuta K. , Chikaki S. , Kikkawa T.
Published: April 2011

Silicide reaction between co-deposited TiSix layers and Si was studied. It was shown that an amorphous TiSix layer, with x − 0.5–1, deposited between amorphous Si and the Ti film led to a significant reduction in the observed C49→C54 TiSis transformation temperature. Amorphous TiSix layers generated by the pre-deposition of Ti and pre-annealing also enhanced the nucleation of the C54-TiSi2 phase. The efficacy of the interfacial TiSix layer was demonstrated on undoped and heavily doped n- and p-type α-Si, but was found to be absent on single crystal Si. Possible mechanisms of these phenomena are discussed.
Structural Transformations Due to Intermixing During Deposition oF Fe/Pt(001) Epitaxial Multilayers
Authors: Hufnagel T. C. , Kautzky M. C. , Ramaswamy V. , Leming A. , Clemens B. M.
Published: April 2011

Previous studies of the structure of epitaxial Fe/Pt(001) multilayers have shown that the structure of the Fe layers depends on the Fe layer thickness. For small thicknesses (< 12 Å) the Fe is fcc, while for greater thicknesses (> 22 Å) the Fe has a bcc structure. We speculated that this transition of the Fe structure is a result of intermixing with subsequently deposited Pt. We explore this transition further by examining superlattice satellite peaks around the (111) fundamental reflection, and by using grazing incidence x-ray scattering (GIXS) to examine the structure of epitaxial Fe/Pt(001) multilayers during sputter deposition. The GIXS observations show that the initial growth of Fe is in the bcc structure; thin bcc Fe layers are partially transformed to fcc by subsequent deposition of Pt. The transformation is thermodynamically favorable because the intermixed fcc phase has lower epitaxial mismatch and bulk chemical free energy than the bcc phase.
A Study on the Growth of a Ferroelectric Thin Film Using Ionized Cluster Beam Epitaxy Technique and the Application for Ulsi Fabrication
Authors: Lee Hyun Seok , Sung Man Young
Published: April 2011

The dielectric properties of Ba0.65Sr0.35TiO3(BST) films deposited by an electron beam assisted ionized cluster beam epitaxy (ICBE) technique were investigated. Highly (110) oriented BST films having a thickness up to 1, um have been successfully grown on a Si (100) substrate at 400°C using the electron beam assisted ICBE system with a plasma O2 source. It was found that the dielectric constant increases from 475 to 1191 with different process conditions. A BST film with a thickness of 500Å deposited at a substrate temperature of 400°C has a dielectric constant of 1191 and a leakage current of about 2.37×10-9 A/cm2. This shows the BST film can be applied to dielectrics of ULSI capacitors.
Grafted Metallized Particle Interconnect (G. M. P. I.)
Authors: Crotzer David R. , Wolf Thomas A. , IPD Augat
Published: April 2011

An electrical contact surface is described which combines the flexibility of a low resistance elastic interface with bonded conductive particles for penetration during compressive mating to metallic oxide layers. The processes for applying both technologies is described including the types of interfacial bonds provided. Both technologies permit high conductivity across the layers. Such technologies offer advantages for connectors requiring low normal forces, superior oxidative resistance to the environment, elastic conductors with the same resistance as bare copper, and high volume processing capabilities.

The first layer of conductive coating includes grafting an elastomeric bridge from the primary surface to the outer layer of metal particles and flakes. The secondary outer coating involves deposition of sharp, hard, conductive particles onto the grafted surface. Each coating bonds to the other forming several layers outward from the primary elastomeric substrate.

Product applications include a conductive elastic interface for interconnecting a BGA to a PCB via a socketed solution. The elastomeric interface overcomes co‐planarity issues associated with mating the BGA chip to a PCB. Compressive forces are reduced as compared to metal connectors providing lower total force for high I/O counts. Environmental constraints are reduced due to the durability of the elastomeric system.
Investigation of the Flow Behavior of Participate Filled Fluids
Authors: Driscoll T. E. , Li P. C. , Lehmann G. L. , Cotts E. J.
Published: April 2011

Underfill encapsulants, used in direct‐chip‐attachment (DCA) packaging of electronics, consist of an epoxy resin in which a high concentration of solid particles are suspended. As a fluid mixture key features of these encapsulants are their relatively large particle sizes and large particle‐to‐liquid density ratios (ρs/ρ0 ?2.4). Experiments have been conducted to characterize the flow behavior of model mixtures of negatively buoyant, spherical particles suspended in Newtonian liquids. Capillary flow in a parallel surface channel is used to simulate the underfill flow process. The effects of varying the channel spacing, particle size and liquid carrier are reported here. The flow behavior is contrasted with a linear fluid, effective viscosity model. Particle settling appears to be linked to the more complex behavior observed in both our model suspensions and measurements using an actual commercial encapsulant.
MOVPE GaN Gas-Phase Chemistry for Reactor Design and Optimization
Authors: Safvi S. A. , Redwing J. M. , Thon A. , Flynn J. S. , Tischler M. A. , Kuech T. F.
Published: April 2011

The results of gas phase decomposition studies are used to construct a chemistry model which is compared to data obtained from an experimental MOVPE reactor. A flow tube reactor is used to study gas phase reactions between trimethylgallium (TMG) and ammonia at high temperatures, characteristic to the metalorganic vapor phase epitaxy (MOVPE) of GaN. Experiments were performed to determine the effect of the mixing of the Group III precursors and Group V precursors on the growth rate, growth uniformity and film properties. Growth rates are predicted for simple reaction mechanisms and compared to those obtained experimentally. Quantification of the loss of reacting species due to oligmerization is made based on experimentally observed growth rates. The model is used to obtain trends in growth rate and uniformity with the purpose of moving towards better operating conditions.
Reactive ION Beam Etching of GaN Grown By Movpe
Authors: Saotome K. , Matsutani A. , Shirasawa T. , Mori M. , Honda T. , Sakaguchi T. , Koyama F. , Iga K.
Published: April 2011

A dry etch technique using Cl2 based reactive ion beam etching (RIBE) has been developd for GaN-based semiconductor lasers. The etching rate of 350 − 1000 Å/min was obtained. This is applicable for micro fabrication of GaN based materials in the same way as used for other III-V group semiconductors. Furthermore, it is found that the surface damage of GaN layers induced by the RIBE-etch can be removed using ultra-violet assisted wet-etching using alkali solution. The PL intensity of damaged GaN layers is increased after the post-process wet-etching.
Comparison of Ni/Au, Pd/Au, and Cr/Au Metallizations for Ohmic Contacts to p-GaN
Authors: Trexler J. T. , Pearton S. J. , Holloway P. H. , Mier M. G. , Evans K. R. , Karlicek R. F.
Published: April 2011

Reactions between electron beam evaporated thin films of Ni/Au, Pd/Au, and Cr/Au on p-GaN with a carrier concentration of 9.8 × 1016 cm−3 were investigated in terms of their structural and electronic properties both as-deposited and following heat treatments up to 600°C (furnace anneals) and 900°C (RTA) in a flowing N2 ambient. Auger electron spectroscopy (AES) depth profiles were used to study the interfacial reactions between the contact metals and the p-GaN. The electrical properties were studied using room temperature current-voltage (1-V) measurements and the predominant conduction mechanisms in each contact scheme were determined from temperature dependent I-V measurements. The metallization schemes consisted of a 500 Å interfacial layer of Ni, Pd, or Cr followed by a 1000 Å capping layer of Au. All schemes were shown to be rectifying as-deposited with increased ohmic character upon heat treatment. The Cr/Au contacts became ohmic upon heating to 900°C for 15 seconds while the other schemes remained rectifying with lower breakdown voltages following heat treatment. The specific contact resistance of the Cr/Au contact was measured to be 4.3×10−1 Ωcm2. Both Ni and Cr have been shown to react with the underlying GaN above 400 °C while no evidence of a Pd:GaN reaction was seen. Pd forms a solid solution with the Au capping layer while both Ni and Cr tend to diffuse through the capping layer to the surface. All contacts were shown to have a combination of thermionic emission and thermionic field emission as their dominant conduction mechanism, depending on the magnitude of the applied reverse bias.
Evolution of Ti Schottky Barrier Heights on n-Type GaN with Annealing
Authors: Hirsch Michèle T. , Duxstad Kristin J. , Haller E. E.
Published: April 2011

We report the effect of mild annealing on Ti Schottky diodes on n-type GaN. The Ti films were deposited by electron beam evaporation on n-type GaN grown by metal organic vapor deposition. We determine the effective barrier height Ф60 by current-voltage measurements as a function of temperature. The as-deposited Ti contacts show rectifying behavior with low barrier heights Ф60 ≤ 200meV. At annealing temperatures as low as 60°C we observe an increase of the barrier height to values of 250meV. After annealing at 230°C and above a stable barrier height of 450meV is measured. The increase in barrier height is not due to any macroscopic interfacial reaction. The origin of the observed changes are discussed in terms of the Schottky-Mott model and possible microscopic interfacial reactions.
MBE Growth of (In)GaN for LED Applications
Authors: Riechert H. , Averbeck R. , Graber A. , Schienle M. , Strauβ U. , Tews H.
Published: April 2011

We report on essential aspects of the growth of InGaN / GaN p-n junctions by gas-source molecular beam epitaxy (MBE) and present the first blue and green electroluminescence from such structures grown entirely by MBE.

A study of the growth conditions for a GaN nucleation layer on sapphire and for the subsequent growth of undoped GaN points out the necessity for Ga-stabilized growth. Unintentionally doped GaN grown at 1 μm/h shows background doping levels below 1017 cm-3 and mobilities up to 320 cm2/Vsec (at 300K). Narrow photoluminescence with very low intensity in the yellow spectral range is observed. N- and p-type doping of GaN with Si and Mg yields layers with high mobilities (220 and 10 cm2/Vsec, respectively at 300K) at carrier densities typical for devices.

Although incorporation of Indium is strongly temperature-dependent, InGaN-layers with In-contents of over 40% are obtained routinely. The optical properties of our InGaN layers typically exhibit the commonly observed, broad deep level luminescence. Finally, we present electroluminescence in the visible spectral range up to 540 nm from InGaN / GaN double-heterojunctions.
MBE Growth of III-V Nitride Films and Quantum-Well Structures Using Multiple RF Plasma Sources
Authors: Johnson M. A. L. , Yu Zhonghai , Boney C. , Rowland W. H. , Hughes W. C. , Cook J. W. , Schetzina J. F. , El-Masry N. A. , Leonard M. T. , Kong H. S. , Edmond J. A.
Published: April 2011

MBE growth of III-V nitrides is being studied at NCSU using MOVPE grown GaN buffer layers on SiC as substrates. Rf plasma sources are being used for the generation of active nitrogen during MBE deposition. Through the use of multiple rf plasma sources, sufficient active nitrogen is generated in order to examine the properties of III-V nitride layers grown at higher substrate temperatures and growth rates. The resulting MBE-grown GaN films exhibit remarkably intense photoluminescence (PL) dominated by a sharp band-edge peak at 3.409 eV having a FWHM of 36 meV at 300K. No deep level emission is observed. AlGaN and InGaN films and quantum well structures have also been prepared using multiple sources. A modulated beam MBE approach is used in conjunction with the multiple rf plasma sources to grow InGaN. RHEED and TEM studies reveal flat 2D InGaN quantum well structures. Depending on the indium content, GaN/InGaN single-quantum-well structures exhibit electroluminescence at 300K peaked in the blue-violet to the green spectral region.
Growth of (0001) ZnO Thin Films on Sapphire
Authors: Drehman A. J. , Yip P. W.
Published: April 2011

Using reactive rf sputtering, we have grown (0001) oriented ZnO films in situ on heated c-axis sapphire substrates, that are promising, particularly in terms of surface roughness, as buffer layers for the subsequent epitaxial growth of III-V nitride films. We compare the effects of on-axis and off-axis sputter geometries on the film epitaxy and smoothness. We also examined the effect of substrate temperature on the growth and smoothness and quality of the film. X-ray diffraction was used to verify the hexagonal ZnO phase, its c-axis orientation and, qualitatively, the degree of its epitaxy. Atomic Force Microscopy (AFM) was used to determine the ZnO growth morphology and roughness. Our best films, obtained by off-axis sputter deposition, have a surface roughness of less than 1 nm.
Structural and Optical Properties of Homoepitaxial GaN Layers
Authors: Baranowski J. M. , Liliental-Weber Z. , Korona K. , Pakuła K. , Stępniewski R. , Wysmołek A. , Grzegory I. , Nowak G. , Porowski S. , Monemar B. , Bergman P.
Published: April 2011

The review of structural and optical properties of homoepitaxial layers grown by MOVCD on single crystals GaN substrates is presented. The TEM technique is used to characterise the structural properties of epi-layers. It is found that the structural properties of GaN homoepitaxial layers are determined by the polarity of the substrate surface on which the growth takes place. It is shown that threading dislocations are present only in the layers grown on the [0001] “smooth” surface. On the other hand the layers grown on the [0001] “rough” surface are free from vertical defects. The characteristic feature of the growth on the “rough” surface are pinholes. The optical properties of homoepitaxial layers are predominantly determined by the growth polarity as well. It is shown also that the reflectivity measurement is the most precise way to determine the exciton energies and that emissions due to free excitons are strongly affected by polariton effects.
Determination of the Percentage of the Cubic and Hexagonal Phases in Gan with Nexafs
Authors: Katsikini M. , Paloura E. C. , Moustakas T. D. , Holub-Krappe E. , Antonopoulos J.
Published: April 2011

Angle resolved near edge X-ray absorption measurements (NEXAFS) are used to access the existence and concentration of the two allotropic phases, cubic (β-GaN) and hexagonal (α-GaN), that coexist in a mixed-phase GaN sample grown by ECR-MBE. The resonance intensities in the NEXAFS spectra from a pure cubic GaN sample are independent of the angle of incidence ϑ, while they have a characteristic angular dependence on cos2ϑ for the hexagonal material, i.e. I=A±Bcos2ϑ. From the values of A and B the bond orientations with respect to the surface normal are calculated. Furthermore, the NEXAFS resonances in a pure α- or β-GaN sample appear at characteristic energies which are independent of the angle of incidence ϑ. Contrary to that the NEXAFS resonances in the spectra from a mixed-phase GaN show a characteristic ϑ-dependent shift. On the basis of this finding, a method is proposed and applied for the determination of the fractions of the co-existing polytypes in a mixed-phase sample.
Synthesis of Bulk, Polycrystalline Gallium Nitride at Low Pressures
Authors: Argoitia Alberto , Angus John C. , Hayman Cliff C. , Wang Long , Dyck Jeffrey S. , Kash Kathleen
Published: April 2011

Bulk, polycrystalline gallium nitride was crystallized from gallium saturated with nitrogen obtained from a microwave electron cyclotron resonance source. The polycrystalline samples are wurtzitic and n-type. Well-faceted crystals give near-band-edge and yellow band photoluminescence at both 10K and 300K. The results show that atomic nitrogen is an attractive alternative to high pressure molecular nitrogen for saturation of gallium with nitrogen for synthesis of bulk gallium nitride.
Effect of Sapphire Nitridation on GaN by MOCVD
Authors: Byun Dongjin , Jeong Jaesik , Lee Jae-Inn , Kim Byongho , Yoo Ji-Beom , Kum Dong-Wha
Published: April 2011

Efficiency and lifetime of light emitting diodes and laser diodes inversely depend on defect density of the crystal. Reduction of defect density is accomplished by proper choice of the substrate or deliberate modification of substrate surface. Roughness of substrate surface for GaN deposition can be controlled by buffer growth and/or nitridation. Buffer layers or nitrided layers promote lateral growth of films due to decrease in interfacial free energy between the film and substrate. Optimum conditions for nitridation and GaN-buffer growth on Al2O3(0001) were determined by means of atomic force microscopy (AFM). AFM analysis of nitridated sapphire surfaces was also carried out to find the optimum condition for nitridation of sapphire substrate before GaN-buffer layer deposition. Nitridation of sapphires was performed only with nitrogen. Based on the fact that GaN deposited on modified surface exhibited the better crystal quality and optical property, use of AFM roughness as a reliable criterion is suggested for process optimization of GaN film growth by metallorganic chemical vapor deposition.
Electrical Characterization of Al-AlN(PSMBE Grown)-Si MIS Structures
Authors: Krupitskaya Regina Y , Auner Gregory W , Daley Tom E
Published: April 2011

AIN films were grown by plasma source Molecular Beam Epitaxy (PSMBE) on n-type Si(111) substrates under various growth parameters. I-V and C-V-f dependencies of AI-AIN-Si(111) MIS structures were measured. Electrical characterization of MIS structures with PSMBE grown textured AIN thin films as insulator shows that low current leakage can be achieved, although some samples have rectifying I-V dependencies. C-V-f characteristics reveal evidence of the presence of different types of traps. Additional investigations are needed in order to determine the nature of these traps.
Defect Transitions in GaN Between 3.0 and 3.4 eV
Authors: Rieger W. , Ambacher O. , Rohrer E. , Angerer H. , Stutzmann M.
Published: April 2011

We have studied optical transitions (absorption and luminescence) in nominally undoped and Mg-doped GaN deposited by MOCVD and MBE. In the range between 3.0 and 3.4 eV, a variety of well known low-intensity luminescence lines are observed, whose origin is discussed. In particular, by comparing excitation with subgap versus above-gap laser lines as well as by combining optical subgap absorption with spectrally resolved photoconductivity, we identify localized optical transitions occuring in isolated cubic inclusions in the otherwise hexagonal GaN epitaxial layers. Implications of these strucural defects for photocurrent transients are also presented.
Characterization of Near-Edge-Optical Transitions in Undoped and Doped GaN/Sapphire Grown by MOVPE, HVPE, and GSMBE
Authors: Leroux M. , Beaumont B. , Grandjean N. , Massies J. , Gibart P.
Published: April 2011

We report a comparative study by luminescence and reflectivity of GaN grown on sapphire by MOVPE, HVPE and GSMBE. Whatever the growth technique, undoped GaN shows well resolved reflectivity spectra, allowing hetero-epitaxial strain to be taken into account in the interpretation of luminescence spectra. MOVPE provides the highest purity samples so far. From the luminescence study of n- and p-type doped samples, activation energies of 265 ± 10 meV and 34 ± 2 meV are deduced for Mg acceptors and Si donors respectively. From the luminescence spectra of HVPE grown samples, the presence of a perturbed interfacial n+ layer is evidenced.
Effects of Strain Fields on Excitons and Phonons in Wurtzite GaN Epilayers
Authors: Gil B. , Briot O. , Aulombard R. L. , Demangeot J. F. , Frandon J. , Renucci M.
Published: April 2011

The electronic structure of a set of several tens of a few micrometer-thick GaN epilayers grown by MOVPE on (000l)-oriented sapphire is investigated by means of photoluminescence and reflectance spectroscopy. The strain-fields effects are quantitatively interpreted using a group theory analysis which predicts seven pertinent parameters: the crystal field splitting, two spin-orbit interaction parameters and four deformation potentials. From a theoretical point of view, we also examine the exciton problem. We establish the selection rules which control the strength of optical transitions for Г5 and Г1 excitons, and estimate the electron hole exchange interaction and the longitudinal transverse splitting of A, B, C excitons. Raman spectroscopy measurements are performed at various temperatures to observe the strain-induced shift of Raman frequencies. We determine four phonon deformation potentials in w-GaN.
Theory of Point Defects and Interfaces
Authors: Van de Walle Chris G , Neugebauer Jörg
Published: April 2011

First-principles theoretical results can predict and explain a variety of materials properties of the nitride semiconductors. For n-type GaN, we summarize the current understanding about incorporation of unintentional donor impurities, as opposed to nitrogen vacancies. For p-type GaN, we discuss the cause of the limited doping levels, and the role of hydrogen. We describe the role of gallium vacancies in the yellow luminescence, and the interaction between these vacancies and donor impurities. Finally, we discuss our first-principles investigations of the atomic and electronic structure of heterojunction interfaces between the III-nitrides, and provide values for natural band lineups.
Incomplete Solubility in Nitride Alloys
Authors: Ho I. H. , Stringfellow G.B.
Published: April 2011

A model based on the valence-force-field (VFF) model has been developed specifically for the calculation of the irascibility gaps in III-V nitride alloys. In the dilute limit, this model allows the relaxation of the atoms on both sublattices. It was found that the energy due to bond stretching and bond bending was lowered and the solubility limit was increased substantially when both sublattices were allowed to relax to distances as large as the sixth nearest neighbor positions. Using this model, the equilibrium mole fraction of N in GaP was calculated to be 6×l0−7 at 700°C. This is slightly higher than the calculated results from the semi-empirical delta lattice parameter (DLP) model. Both the temperature dependence and the absolute values of the calculated solubility agree closely with the experimental data. The solubility is more than three orders of magnitude larger than the result obtained using the VFF model with the group V atom positions given by the virtual crystal approximation, i.e., with relaxation of only the first neighbor bonds. Other nitride systems, such as GaAsN, AlPN, AlAsN, InPN, and InAsN were investigated as well. The equilibrium mole fractions of nitrogen in InP and InAs are the highest, which agrees well with recent experimental data where high N concentrations have been produced in InAsN alloys. Calculations were also performed for the alloy systems with mixing on the group III sublattice that are so important for device applications. Allowing relaxation to the 3rd nearest neighbor gives an In solubility in GaN at 800°C of less than 6%. Again, this is in agreement with the results of the DLP model calculation. This result may partially explain the difficulties experienced with the growth of these alloys. Indeed, evidence of solid immiscibility has recently been reported. A significant miscibility gap was also calculated for the AlInN system, but the AlGaN system is completely miscible.
Energetics of AlN Epitaxial Wetting Layers on SiC (0001)
Authors: Di Felice R. , Northrup J. E. , Neugebauer J.
Published: April 2011

We present a first-principles characterization of the initial stages of formation of AlN films on c-plane SiC substrates. Studying the competition between two-dimensional films and three-dimensional islands as a function of Al and N abundances, we find that a two-dimensional film can wet the surface in N-rich conditions. Ordered layer-by-layer growth can proceed to some extent on this wetting layer, and is improved by the formation of an atomically mixed interface which eliminates interface charge accumulation. Our results indicate that the stable AlN films grow in the (0001) orientation on the Si-terminated SiC(0001) substrate.
Band Structure and Cation Ordering in LiGaO2
Authors: Limpijumnong Sukit , Lambrecht Walter R.L. , Segall Benjamin , Kim Kwiseon
Published: April 2011

Full-potential linear muffin-tin orbital calculations were performed for LiGaO2 in different crystal structures in order to investigate the nature and origin of the cation ordering, structural relaxation and their effects on the band structure. It is found that the most important factor for the bonding is the exclusive occurrence of Li2Ga2 tetrahedra surrounding oxygen. Structures including LiGa3 and Li3Ga tetrahedra have significantly higher total energies and smaller bandgaps. The band-offset between GaN and LiGaO2 is estimated using the dielectric midgap approach.
A First Model of Amorphous GaN from Ab Initio Molecular Dynamics
Authors: Drabold D. A. , Stumm Petra
Published: April 2011

A probable byproduct of growth of crystalline GaN is an amorphous phase of the material. In this paper, we propose a structural model of amorphous GaN obtained from ah initio molecular dynamics. The radial distribution function, local bonding, electronic density of states and vibrational spectra are described. The network we obtain is highly disordered but exhibits a large state-free optical gap, and has no wrong (homopolar) bonds. These predictions are intended to elucidate the experimental signatures of amorphous GaN.
First Microscopic Observation of Cadmium-Hydrogen Pairs in GaN
Authors: Burchard A. , Deicher M. , Forkel-Wirth D. , Haller E. E. , Magerle R. , Prospero A. , Stötzler R. ,   The Isolde-Collaboration
Published: April 2011

The formation and properties of acceptor-hydrogen pairs in GaN have been studied using radioactive 111mCd acceptors and the perturbed γγ angular correlation spectroscopy (PAC). After H-loading by low energy implantation (100 eV) at temperatures between 295 K and 473 K, the formation of two Cd-H complexes involving about 30% of the Cd-acceptors is observed. The complexes have been identified as single hydrogen atoms bound to the Cd acceptor in two different configurations. The dissociation enthalpies of these configurations have been determined as 1.1(1) eV and 1.8(1) eV, respectively.
Characterization and Metrology of Low Pressure Chemical Vapor Deposited (LPCVD) Polysilicon
Authors: Asinovsky Leo , Schroth Michael , Shen Fei , Sweeney John
Published: April 2011

Polycrystalline silicon (polySi) is widely used in the semiconductor industry as a gate electrode, interconnect material and for various other applications. Small variations in deposition conditions can significantly affect this material's properties, cause errors in metrology control of the film thicknesses and, ultimately, in device performance. In this work polySi was LPCVD deposited in a vertical reactor with multiple gas inlets and a flat temperature distribution. Deposition conditions were controllably changed to create a matrix of wafers. Characterization is done using production dual wavelength, multiple-angle-of-incidence and research grade spectroscopie ellipsometers. PolySi thickness and composition uniformity are analyzed.
Optical Properties of Nano Crystalline InP in Opal 3-Dimensional Gratings
Authors: Johnson N. P. , Romanov S. G. , Butko V. , Yates H. , Pemble M. E. , Sotomayor Torres C. M.
Published: April 2011

Opal consists of a three dimensional array of silica balls with diameter in the range 150 -350 nm and acts as a 3-D grating. Within the microcrystals (a few hundred microns) of opal there is typically 5% variation in ball diameter arranged in a fee structure. Using a specially modified MOCVD process InP can be grown within the voids between the touching silica balls. These interconnecting voids are approximately 1/5 and 2/5 the silica sphere diameter, Raman spectroscopy confirms the crystallinity of the InP for two samples loaded with 2.5 volume % of InP. The LO phonon is shifted to the red by 10 cm−l from the bulk value of 345 cm−1, while the TO phonon is shifted 22 cm−l to the blue from the bulk value of 303.7 cm−l. This shift is attributed to strain in the nanocry stals.

The half width of the reflectance is dependent upon InP loading with the 2.5 vol.% samples giving 0.19 eV and the lower loaded 0.4 vol. % giving 0.17 eV for samples with the same ball diameter. Photoluminescence emission near 1.7 eV is attributed to confined InP compared with a bulk value of 1.4 eV. The energy of the emission is determined by the combined contribution of the laterally confined InP and its interaction with the opal matrix.
Preparation, Characterization and Optical Properties of Zinc Oxide Nanoparticles
Authors: Li Shoutian , Silvers Stuart J. , Samy El-Shall M.
Published: April 2011

ZnO nanoparticles are produced by the laser vaporization-controlled condensation technique. These particles are connected in a web-like agglomeration. Their properties are compared to those of ZnO nanoparticles produced by solution sol-gel and reverse micelle techniques. All particles have the bulk crystal structure and show quantum size effects in absorption and emission. They show emissions that consist of a blue bandgap feature with a sub-nanosecond lifetime and a green feature with multiexponential lifetime decays. Emission from the stearate coated particles produced by the reversed micelle method is particularly intense.
Studies of Photoredox Reactions on Nanosize Semiconductors
Authors: Wilcoxon Jess P. , Parsapour F. , Kelley D. F.
Published: April 2011

Light induced electron transfer (ET) from nanosize semiconductors of MoS2to organic electron acceptors such as 2,2′-bipyridine (bpy) and methyl substituted 4,4′,5,5′-tetramethyl-2,2′-bipyridine (tmb) was studied by static and time resolved photoluminescence spectroscopy. The kinetics of ET were varied by changing the nanocluster size (the band gap), the electron acceptor, and the polarity of the solvent. MoS2is an especially interesting semiconductor material as it is an indirect semiconductor in bulk form, and has a layered covalent bonding arrangement which is highly resistant to photocorrosion.
Theoretical Thermal Conductivity of Porous Silicon: Nonlinear Behavior
Authors: Lugo J. E. , de Río J. A. , Tagüeña-Martínez J.
Published: April 2011

The use of porous silicon (PS) in fabricating optoelectronic devices is in progress. However, the performance of such applications still needs to be improved. In particular, in solar cells heat must be dissipated to avoid a decay in their efficiency and one of the properties of PS that must be evaluated is the effective thermal conductivity. It is well known that the thermal conductivity of silicon is temperature dependent. Thus we cannot use a standard effective medium approach to obtain its effective thermal response. In this work, we extend the averaging volume and surface methods [1] to consider nonlinear effects in the effective transport coefficients. We model PS as composed of c-Si cylindrical columns covered by different overlayers (i.e. a-Si or SiO2) immersed in another medium. In our model the effective thermal conductivity has an explicit dependence on the temperature gradient. We present a parametric analysis of the model, compare it with the c-Si behavior and evaluate the importance of the nonlinear contribution.
Properties of Amorphous Silicon Thin Film Transistors with Phosphorous-Doped Hydrogenated Microcrystalline Silicon
Authors: Choi J. H. , Kim C. W. , Yang H. G. , Souk J. H.
Published: April 2011

Phosphorous (P) doped hydrogenated microcrystallme silicon (n+μ c-Si:H) films have been prepared by using the hydrogen-diluted plasma enhanced chemical vapor deposition (PECVD) method. The crystallinity of films deposited over the range of SiH4/H2 flow ratios and RF-power is studied by Raman spectroscopy. For a 900 Å thick film deposited at 250°C, a conductivity of 71Ω−1cm−1 and an average crystallinity of 49% is obtained. n+ μ c-Si:H films as well as n+ a-Si:H films are used for both etch stopper and back channel etch type TFTs and the I4-V8 characteristics are compared. For the etch stopper type TFT, the field effect mobility of 0.85 cm2/V.sec, threshold voltages of 2 – 3 V and Ion/Ioff ratio of ∼107 are obtained.
Influence of Nanocrystallinity on Properties of Photodiode and TFT Image Sensor Structure
Authors: Kolodziej A. , Nowak S. , Krewniak P.
Published: April 2011

This paper reports technological experiments which have strongly affected the properties of the multilayer image sensor structure. The high deposition rate and large hydrogen content during the reactive magnetron deposition process causes nanocrystallization of prepared silicon films. We also report experiments with laser crystallization. The effect of microstructure in a-Si:H films on TFT and pin diode characteristics has been investigated. Crystallinity was confirmed by Raman scattering, small angle X-ray diffraction (SAXD) and transmission electron microscopy (TEM). The parameters of the photodiodes such as quantum efficiency, dark and light currents, etc. have been tested using the measurements of samples in the configuration of a linear image sensor consisting of two rows, 30 to 160 pixels per row, with dimensions 0.1mm2-1mm2.
The Composition Effect on the Nanocrystallization of Finemet Amorphous Alloys
Authors: Zhu J. , Pradell T. , Clavaguera N. , Clavaguera-Mora M. T.
Published: April 2011

Differential Scanning Calorimetry (DSC), X-ray Diffraction (XRD), Neutron Diffraction (ND) and Mössbauer Spectroscopy (MS) were used to study the nanocrystallization process of Fe73.5Cu1Nb3Si22.5–xBx (x=5, 7, 8, 9 and 12) amorphous alloys. Both the temperature range and the activation energy of Fe(Si) phase precipitation from the amorphous martrix increase with the initial B composition. The initial Si composition influences the mechanism of the nanocrystallization: for the Si rich samples, the beginning of nucleation and growth processes is interface controlled, for the B rich samples it is diffusion controlled. Secondary crystallization from the remaining amorphous is mainly Fe3B and Fe2B, the ratio of Fe3B/Fe2B being dependent on the initial composition too.
Microstructure, Mechanical Properties and Wear Resistance of WC/Co Nanocomposites
Authors: Jia Kang , Fischer Traugott E.
Published: April 2011

The microstructure, mechanical properties, abrasion and wear resistance of WC-Co nanocomposites synthesized by the spray conversion technique by McCandlish, Kear and Kim have been investigated. The binder phase of WC-Co nanocomposites is enriched in W and C, compared to conventional cermets. Small amorphous regions exist in the binder despite the slow cooling after liquid phase sintering. Few dislocations are found in the WC grains. The increased WC content and the amorphous regions modify (i.e. strengthen) the binder phase of the composites. Vickers indentation measurements show a hardness of the nanocomposites reaching 2310 kg/mm2. While the toughness of conventional cermets decreases with increasing hardness, the toughness does not decrease further as the WC grain size decreases from 0.7 to 0.07 μm. but remains constant at 8 MPam1/2. Scratches caused by a diamond indenter are small, commensurate with their hardness. These scratches are ductile, devoid of the grain fracture that is observed with conventional materials. The abrasions resistance of nanocomposites is about double that of conventional materials, although their hardness is larger by 23% only. This is due to the lack of WC grain fragmentation and removal which takes place in conventional cermets. Sliding wear resistance of WC/Co is proportional to their hardness; no additional benefit of nanostructure is obtained. This results from the very small size of adhesive wear events in even large WC grains.
Processing, X-Ray, and TEM Studies of QS87 Series 56 KΩ/Square Thick Film Resistors
Authors: Crosbie Gary M. , Johnson Frank , Donlon William T.
Published: April 2011

Thick film resistors are glass/metal oxide nanocomposites used in hybrid microcircuits. These components have a small temperature coefficient of resistance that is useful in systems that experience a wide range of service temperatures. Test samples were produced by printing, drying, and firing resistor pastes in a laboratory process that simulated production conditions. The process parameters of peak firing temperature, time at peak temperature, and probe current were factors in a 23 factorial experiment that measured in-situ resistance (resistance during processing), as-fired resistance, and the temperature coefficients of resistance. As-fired resistance is shown to increase with firing time and temperature. In-situ resistance exhibited a small decrease with increasing firing temperature due to thermally-activated glass conduction at firing temperatures. The temperature coefficient of resistance measurements show that R[T] curve flattens with increasing firing time and temperature. X-ray diffraction revealed Pb-ruthenate, alumina, and Zr-silicate phases to be dispersed in the glass. Transmission electron microscopy in conjunction with energy dispersive x-ray spectroscopy revealed that the conductive phases, Pb- and CuBi-ruthenate particles, increased in size with increasing firing time and temperature. Lattice parameter measurements revealed only a small increase in the ruthenate structure. Resistance changes are attributed to increased separation of the conductive ruthenate particles by coarsening.
The Growth and Properties of Thin-Film Nano-Composites
Authors: Lewis Keith L , Pitt A M , Culus A G
Published: April 2011

Nano-composites allow a materials engineering approach to be exploited to realise specific characteristics in optical thin film ensembles. For example, films can be produced with refractive indices determined by their average composition on the basis of effective medium approximations, so freeing optical designers from the constraints imposed in pure materials. A summary is presented of the progress made in a fundamental study of films based on diverse materials such as fluorides and sulphides, fabricated using molecular beam deposition techniques. The film properties (eg refractive index, surface morphology, environmental stability) are correlated with microstructure (as determined by cross-sectional TEM techniques). The enhanced properties of the films are discussed in relation to the realisation of periodically modulated graded-index structures of the type required for optical filter applications.