bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of the Permeability of Carbon Aerogels by Gas Flow Measurements
Authors: Kong F-M. , Hulsey S.S. , Alviso C.T. , Pekala R.W.
Published: March 2011

Carbon aerogels are synthesized via the polycondensation of resorcinol and formaldehyde, followed by supercritical drying and pyrolysis at 1050 °C in nitrogen. Because of their interconnected porosity, ultrafine cell structure and high surface area, carbon aerogels have many potential applications, such as in supercapacitors, battery electrodes, catalyst supports, and gas filters. The performance of carbon aerogels in the latter two applications depends on the permeability or gas flow conductance in these materials. By measuring the pressure differential across a thin specimen and the nitrogen gas flow rate in the viscous regime, we calculated the permeability of carbon aerogels from equations based upon Darcy's law. Our measurements show that carbon aerogels have apparent permeabilities on the order of 10−12 to 10−10 cm2 for densities ranging from 0.44 to 0.05 g/cm3. Like their mechanical properties, the permeability of carbon aerogels follows a power law relationship with density and average pore size. Such findings help us to estimate the average pore sizes of carbon aerogels once their densities are known. This paper reveals the relationships among permeability, pore size and density in carbon aerogels.
C60 Transformations at High Pressures
Authors: Yoo C. S. , Nellis W. J. , Sattler M. L. , Musket R. G. , Hinsey N. , Brocious W.
Published: March 2011

C60 molecules have been studied at both shock and static high pressures. Under shock compressions C60 fullerenes are stable into the 13-17 GPa pressure range. The onset of a fast (∼0.5 μs) reconstructive transformation to graphite occurs near 17 GPa. The graphite recovered from 27 GPa and about 900 K is relatively well ordered with La = 100 Å. Above 50 GPa a continuous transformationto an amorphous state is observed in recovered specimens. A transparent, metastable carbon phase was recovered from thin films of C60, shocked to 69 GPa and 2200 K and then rapidly quenched to 1000 K. The selected area diffraction patterns indicate thatthe metastable carbon contains an amorphous diamond and n-diamond. Under hydrostatic compressions C60 molecules transform reversibly to a semi-transparent phase in the pressure range of 15-25 GPa with a large pressure hysteresis. The high pressure phaseconsists of interconnected strongly interacting C60 agglomerates, or networksof fullerenes, whose stability continuously increases with increase of pressure. Above 27 GPa the transition becomes irreversible, and the material recovered from high pressureis metastable and diamond-like at ambient conditions. These pressure-induced transitions are explained in terms of nr-electron rehybridization between C60 molecules, which occurs at substantially decreased intermolecular distances.
Rotations, Vibrations and Structure in Solid C60: Investigations by Neutron Scattering
Authors: Kamitakahara W. A. , Copley J. R. D. , Cappelletti R. L. , Rush J. J. , Neumann D. A. , Fischer J. E. , Mccauley J. P. , Smit A. B.
Published: March 2011

In a series of experiments on solid C60, the low-energy rotational dynamics of the molecules, the higher-energy vibrational spectra, and aspects of disorder in the static structure, have been studied. The nearly spherical shape of the C60 molecule, and the occurrence of an orientational phase transition at Tc=256 K, make the C60 solid an excellent system for investigations of orientational dynamics. Coherent quasi-elastic neutron scattering above Tc is described quantitatively by a rotational diffusion model. Below Tc, inelastic scattering froma soft librational mode is observed between 2 and 3 meV. Models of the strong, temperaturedependent diffuse scattering due to orientational disorder are discussed. Finally, measurements of the density of states for the intramolecular ( E ≥ 33 meV) vibrational modes are also presented.
Raman Scattering and Superconductivity of Alkali-Fullerene Materials
Authors: Lannin J. S. , Mitch M. G. , Bacsa W. , Chase S. J.
Published: March 2011

Raman scattering measurements in alkali—fullerene alloys in ultrathin and thin films provide evidence for variations in electron—phonon coupling. For x — 3similar behavior of Rb3 C60 films of different thickness support substantial electron—phonon induced damping of specific Hg(i) modes derived from intramolecular modes of C60. In 400A thick films a reduction of induced scattering from Raman inactive C60 modes substantiates the importance ofHg(2), but not Hg(3) modes for phonon—mediated superconductivity. In contrast to RbxC60 and KxC60 ultrathin film solid solutions, similar Raman spectra for NaxC60 indicate substantially reduced coupling consistent with the absence of superconductivity in this system.
A Model Calculation for the Vibrational Modes in C60
Authors: Jishi R.A. , Mirje R.M. , Dresselhaus M.S.
Published: March 2011

A force constant model for the vibrational modes in C60, based on bond-stretching and anglebending interactions, is presented. The results of this model are compared with the experimental data available from Raman, infrared, and high resolution electron energy loss spectroscopies, as well as neutron inelastic scattering measurements. Excellent agreement is obtained between the calculated and experimental mode frequencies. The pressure dependence of the Ramanand infrared-active mode frequencies is calculated within a simple model and is compared to available experimental data.
Dodecahedron- and Bowl-Shaped Structures C20
Authors: Slanina Zdenék , Adamowicz Ludwik
Published: March 2011

Purely carbonaceous aggregates C20have been studied by the AM1 quantumchemical method. In addition to one dodecahedron-shaped structure possessingC1symmetry another three-dimensional species is revealed, viz. a bowl-shaped structureofC5vsymmetry (and also one two-dimensional and two one-dimensional species). Temperature dependence of the relative stabilities of both three-dimensional structures is evaluated, showing that in the relevant temperature region the fullerenic species is prevailing. However, in a very high temperature region a relative-stability interchange has been predicted.
Study of Linear and Nonlinear Microscopic Polarizabilities of C60
Authors: Westin Erik , RosÉn Arne
Published: March 2011

The low energy part of the linear optical response spectrum and lowest order hyperpolarizability γ(3) of C60 are calculated by a sum over state approach using single particle wavefunctions and. energy levels determined from local density calculations. Lorentz local field factors, as well as a RPA correction are introducedto facilitate comparison with the dielectric function ε(ω) determined fromfilms of C60. γ( 2 ) for a centrosymmetric molecule such asC60 is zero and the lowest non-zero contribution to the polarizability is γ( 3 ). Reasonable agreement is found with linear optical response experiments if a RPA screening is used. However, SHG and THG experiments on C60 in solid or solution form, yields values closer to the unscreened results.
Photoemission Study of Single Crystal C60
Authors: Wu J. , Shen Z.-X. , Dessau D. S. , Cao R. , Marshall D. S. , Pianetfa P. , Lindau I. , Yang X. , Terry J. , King D. M. , Wells B. O.
Published: March 2011

We report angle-resolved photoemission data from single crystals of C60 cleaved in UHV. Unlike the other forms of pure carbon, the valence band spectrum of C60 consists of many sharp features that can be essentially accounted for by the quantum chemical calculations describing individual molecules. This suggests that the electronic structure of solid C60 is mainly determined by the bonding interactions within the individual molecules. We also observe remarkable intensity modulations of the photoemission features as a function of photon energy, suggesting strong final state effects. Finally, we address the issue of the band width of the HOMO state of C60. We assert that the width of the photoemission peak of C60 does not reflectthe intrinsic band width.
Fullerene Endohedrals of Praseodymium
Authors: Ruoff R. S. , Malhotra R. , Moro L. , Becker C. H. , Lorents D. C.
Published: March 2011

Endohedral fullerenes were first observed in molecular beams produced in laser vaporizations sources.[1] Since the discovery of the arc method [2] of producing large quantities of fullerenes several laboratories have been searching for methods to produce and isolate sufficient quantities of endohedral fullerenes to permit the study of their unique material properties.
An Nmr Study of the Occupation of C60 Interstitial Sites by Oxygen Molecules
Authors: Assink Roger A. , Loy Douglas A. , Schirber James E. , Morosin Bruno
Published: March 2011

The 13C NMR of FCC C60 under magic angle spinning (MAS) conditionsyields linewidths on the order of 1 Hz at fields of 4.7 T. The spectrum consists of a primary resonance at 143.7 ppm and a minor peak shifted 0.7 ppm downfield. The intensity of the minor resonance relative to the primary resonance was found to vary from 0.6 to 5.5 % depending on the sample history. The downfield shift obeys Curie's law and isattributed to the Fermi contact coupling interaction between paramagnetic oxygen molecules and all 60 carbon atoms of rapidly rotating adjacent C60 molecules. Exposure of the sample to 1 kbar oxygen for 1 3/4 hours resulted in a spectrum of 7 evenly spaced resonances corresponding to 0 to 6 of the adjacent octahedral interstitial sites being filled with oxygen molecules. At ambient pressure, the oxygen diffused out of the lattice on time scales ranging from hours to days.
Theory for New Carbon-Based Materials
Authors: Brenner D.W. , Mowrey R.C. , Mintmire J. W. , Harrison J.A. , Robertson D.H. , Lyons M. , Dunlap B.I. , White C.T.
Published: March 2011

We review results of our local-density-functional-based cluster calculations and molecular dynamics simulations of fullerenes and related structures. These include predictions of cohesive energies, electronic structures, and photoelectron spectra for a number of pure and chemically substituted fullerenes, studies of the resilience of C60 under severe compression and during surface collisions, simulations of the trapping of Hein the interior of C60, predictions of the strain energy, electronic and elastic properties of graphitic tubules, and simulations of the folding and curling of graphitic ribbons.
Novel Hybrid Composites Based on Carbon Foams
Authors: Hager Joseph W. , Lake Max L.
Published: March 2011

The extraordinary mechanical properties of commercial carbon fiber are due to the unique graphitic morphology of the spun filaments. Contemporary advanced structural composites exploit these properties by creating a disconnected network of graphitic filaments held together by an appropriate matrix. Carbon foam derived from a blown mesophase pitch precursor can be considered to be an interconnected network of graphitic ligaments. As such interconnected networks, they represent a potential alternative reinforcing phase for structural composite materials. Based on this notion, consideration is given to novel forms of graphitic carbon and the processing routes to create hybrid composites, such as net shape fabrication and fiber placement processes.
Polarons and Triplet Excitons in C60: An Optically Detected Magnetic Resonance (Odmr) and Light-Induced Esr (Lesr) Study
Authors: Lane P. A. , Swanson L. S. , Ni Q.-X. , Shinar J. , Engel J , Barton T. J. , Wheelock J. , Jones L.
Published: March 2011

The photoluminescence (PL), X-band ODMR. and LESR of C60 films and C60 isolated in a toluene/polystyrene glass matrix (C60:T/PS) is described. The delocalized triplet LESR and ODMR of C60:T/PS are similar to previously reported LESR. In films, however, the ODMR indicates that the delocalized triplet is larger and distorted by neighboring molecules. In addition, another, localized triplet, and a narrow PL-enhancing line at g = 2.0017 ± 0.0005 are observed. These features are similar to those observed in several π-conjugated polymers. The former is attributed to a localized triplet of size similar to a 5- or 6-membered ring, tentatively on a face adjacent to another C60 molecule; the latter is believed to result from intermolecular polaron recombination.
Early Nucleation of Diamond in a Combustion Flame
Authors: McClure Michael T. , von Windheim Jesko A. , Glass Jeffrey T. , Prater John T.
Published: March 2011

The early stages of a gas phase pretreatment for diamond nucleation in an oxy-acetylene flame were investigated. The pretreatment involved a low oxygen-to-acetylene ratio (Rf = 0.93) performed at 15 mm from the torch on a scratched Si substrate. The nature of the carbon species deposited was analyzed using X-ray photoelectron spectroscopy, Auger electron spectroscopy, and scanning electron microscopy. The pretreatment process produced diamond crystals after 30 seconds and a complete film in the center of the deposition area after 180 seconds. Furthermore, the pretreatment process kept the initial Siob2 layer less than the thickness generated by the control conditions, (Rf = 0.97, d = 10mm). The success of the pretreatment was attributed to changes in the flame structure and chemistry from the control conditions.
In-Vacuo Surface Analytical Study of Diamond Nucleation on Copper Vs. Silicon
Authors: Wolter S. D. , Stoner B. R. , Ma G.-H. M. , Glass J. T.
Published: March 2011

A study was performed on polycrystalline copper versus that of Si(100) utilizing a negative substrate bias to enhance diamond nucleation. The biasing pretreatment and subsequent growth of the diamond were performed via microwave plasma chemical vapor deposition and the initial stages of nucleation were characterized by in-vacuo surface analysis. The biasing pretreatment step proved to have a tremendous influence on the nucleation density pertaining to Si(100) substrates, however, the nucleation density on polycrystalline copper was only increased slightly. A highly graphitic surface coverage of roughly 10A was evident on the copper substrates prior to the detection of diamond and was quite stable in thickness in as early as 15 minutes of biasing. The Si(100) substrates, however, were characterized by the formation of a carbide with some form of nondiamond carbon present on the surface throughout the biasing pretreatment.
Defect Structure of Diamond Film at the Interface with Amorphous Carbon
Authors: Chang Li
Published: March 2011

Diamond films prepared by microwave plasma chemical vapor deposition, using a gas mixture of methane and hydrogen with ethanol, were formed on silicon substrates. Highresolution transmission electron microscopy was employed to characterize the microstructure at interface regions. It was found that the diamond crystals were grown on an amorphous carbon layer. Twins and stacking faults were observed at the regions interfaced with the amorphous carbon layer, suggesting that the defects may already exist in the nucleation stage and at the very first stage of growth. Also, some diamond nuclei embedded in the amorphous layer were observed.
Theoretical Investigation of Fluorinated and Hydrocenated Diamond <100> Filks
Authors: Pederson Mark R. , Pickett Warren E.
Published: March 2011

To investigate some of the fundamental differences between halogen and hydrogen assisted diamond film growth we have performed several calculations related to the <100> diamond surface. The models used in these investigations include ten-layer periodic slabs of free standing fluorinated diamond films as well as isolated clusters [C21F6H20]. For purposes of comparison, we have also performed calculations on models of the hydrogenated <100> surface. The calculations are performed within the density-functional framework using LCAO and LAPW computational methods. We have considered two geometries of a monofluoride surface. The first surface, best described as an ideal l×l surface with a monolayer of ionically bonded fluorines, exhibits a metallic density of states in contrast to a 2×l reconstructed surface with chemically bonded fluorines that is found to be insulating. We compare theoretical carbon core level shifts with experimental values and discuss growth models based on these surface calculations.
Antimony on Diamond: A Comparison to Sb/Si and Sb/Ge
Authors: Wu J. , Richter M. , Cao R. , Terry J. , Pianetita P. , Lindau I.
Published: March 2011

Diamond is an important semiconductor which has great potential in high temperature, high power device applications. In the fabrication process of diamond electronic device, doping ofdiamond and understanding of diamond/metal interfaces are important. As a Column V element, Sb is a possible dopant for diamond. Early work reported that Sb is incorporated into diamond by ion implantaion [1]. In addition, Sb plays an important role in Si and Ge heteroepitaxial growth. On the Si or Ge surface one ordered monolayer of Sb occupies the epitaxial sites and saturates the surface dangling bonds, which leads to uniform epitaxial growth. While diamond has the same crystal structure as both silicon and germanium, it has a drastically smaller lattice and much stronger bond. This makes it very difficult to extrapolate antimony's behavior on diamond from its behavior on either silicon or germanium. In this work, we have studied the electronic and geometric structure of Sb on diamond surfaces using photoelectron spectroscopy and low energy electron diffraction. While the exact adsorption sites could not be determined, we find that antimony strongly bonds to the diamond surface. Further, antimony behaves very differently on the diamond(100) face as compared to the diamond(111) face. We also find that neither Sb/diamond system behaves like antimony on either silicon or germanium. We attribute these results to the drastically smaller diamond lattice and the stronger C-C bond.
Formation and Characterization of Ohmic Contacts on Diamond
Authors: Venkatesan V. , Malta D.M. , Das K.
Published: March 2011

Low resistance ohmic contacts have been fabricated on a naturally occurring lib diamond crystal and on polycrystalline diamond films by B ion-implantation and subsequent Ti/Au bilayer metallization. A high B concentration was obtained at the surface by ion implantation, a postimplant anneal and a subsequent chemical removal of the graphite layer. A bilayer metallization of Ti followed by Au, annealed at 850°C, yielded specific contact resistance values (as measured using a standard transmission line model (TLM) pattern) of the order of 10-5Ω cm2for chemical vapor deposition (CVD) grown polycrystalline films and the natural lib crystal. Specific contact resistance values have also been determined from circular TLM measurements on CVD films and the values compared to those from standard TLM measurements. These contacts were stable to a measurement temperature of ∼400°C and no degradation due to temperature cycling was observed.
Effect of Mechanical Stress on Polycrystalline Diamond Schottky Diode I-V Characteristics
Authors: Zhao G. , Charlson E. M. , Charlson E. J. , Stacy T. , Meese J. , Popovici G. , Prelas M.
Published: March 2011

Schottky diodes to be used for mechanical stress effect studies were fabricated using aluminum contacts to polycrystalline diamond thin films grown by a hot-filament assisted chemical vapor deposition process. Compressive stress was found to have a large effect on the forward biased current-voltage characteristics of the diode. At selected values of constant forward biased current, a linear relationship between voltage and stress, for stress less than 10 N was observed. The stress sensitivity of the diode was as high as 0.74 V/N at 1 mA forward bias. This study shows that polycrystalline diamond Schottky diodes are stress sensitive devices and have potential as mechanical sensors.
Electrical and Optical Properties of Heterostructures Made from Diamond-Like Carbon Layers on Crystalline Silicon
Authors: Hammerschmidt Albert , Mandel Th. , Helbig R. , Birkle S. , Kammermaier J.
Published: March 2011

Thin films of amorphous, diamond-like carbon (DLC) were deposited on crystalline silicon substrates in a capacitively coupled RF-plasma discharge. The investigations concentrated on I-V and C-V measurements, on photoconductivity and on optical transmission between wavelengths of 2 and 50 microns of the heterostructure DLC/Si. The photoconductivity of the heterostructure is investigated for the first time; it depends on the band gap of the film, the thickness and the doping of the substrate. The DLC/Si heterostructure works like a MIS-photoelectric cell.
The Effect of Pyrolysis Temperature and Formulation on Pore Size Distribution and Surfacearea of Carbon Aerogels
Authors: Hulsey S.S. , Alviso C.T. , Kong F.M. , Pekala R.W.
Published: March 2011

Recently we reported the chemistry-structure-property relationships of organic aerogels, which are synthesized by the polycondensation of resorcinol and formaldehyde in a slightly basic medium, followed by supercritical drying. These materials can be pyrolyzed in an inert atmosphere to form vitreous carbon aerogels. As measured by gas adsorption techniques, the BET surface area and pore size distributions of micro and meso pores of the carbon aerogels are affected both by the pyrolysis temperature and the formulation. Definite trends are observed in our preliminary measurements; for example, the surface area decreases with increasing pyrolysis temperature until a plateau is reached at about 900°C. This paper explores the effects of pyrolysis temperature and aerogel density on the BET surface area and pore size distributions.
The Effect of Thermal Processing in Vacuum on the Water Adsorption Characteristics of Carbon Black
Authors: Schachner Kurt D. , Thoma Paul E.
Published: March 2011

Commercially available carbon black contains oxygen complexes on its surface that affect the surface properties of the carbon. Water adsorption on the surface of carbon black is influenced by the amount and type of oxygen complexes present. When carbon black is heated in vacuum at sufficiently high temperatures, removal of the oxygen complexes occurs and the surface of the carbon particles is modified. The amount of water adsorbed by the carbon is dependent on the vacuum heat treatment temperature. As the heat treatment temperature increases, water adsorption on the carbon decreases.

Commercially available electrically conductive carbon black adsorbs from 1.25% to 2.50% water when exposed to 50% relative humidity for 24 hours at 25°C. This variation in water adsorption is due to a difference in the amount of oxygen complexes on the surface of the carbon. The carbon with more oxygen complexes adsorbs more water. However, when this carbon black is heat treated at 1200°C for 4 hours in a vacuum of 1 × 10−5 torr or better, the water adsorbed by the carbon is 0.4% when exposed to 50% relative humidity. Data showing the dependence of water adsorption on vacuum thermal processing are presented and discussed.
Surface Chemical Modification and Analysis of Carbon Fibers - The Key to Novel Composites
Authors: Sherwood Peter M.A.
Published: March 2011

Carbon fibers are finding increasing application in making various composites with special properties. It is found that such composites have properties that can be markedly effected by the carbon fiber surface. The effects of surface modification by electrochemical and plasma oxidation is discussed and the surface chemical changes described. The interaction of the matrix with the fiber surface is a subtle mixture of physical and chemical effects. The paper discusses work in the author's laboratory using surface studies with core and valence band X-ray photoelectron spectroscopy (XPS) and other techniques.
The Growth and Characterization of Germanium-Carbon Alloy Thin Films
Authors: Yuan Haojie , Williams R. Stanley
Published: March 2011

Thin films of pure germanium-carbon alloys (GexC1−x with x ≈ 0.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) have been grown on Si(100) and A12O3 (0001) substrates by pulsed laser ablation in a high vacuum chamber. The films were analyzed by x-ray θ-2θ diffraction (XRD), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), conductivity measurements and optical absorption spectroscopy. The analyses of these new materials showed that films of all compositions were amorphous, free of contamination and uniform in composition. By changing the film composition, the optical band gap of these semiconducting films was varied from 0.00eV to 0.85eV for x = 0.0 to 1.0 respectively. According to the AES results, the carbon atoms in the Ge-C alloy thin film samples has a bonding configuration that is a mixture of sp2 and sp3 hybridizations.
X-RAY Diagnostics of Large-Period Lattice-Matched InGaAs/InP Superlattices
Authors: Vandenberg J. M. , Rttter D. , Hamm R. A. , Chu S. N. G. , Panish M. B.
Published: March 2011

The interfacial structure of InGaAs/InP superlattices grown on (100) InP by metalorganic molecular beam epitaxy has been studied by fully dynamical simulations of high-resolution x-ray diffraction curves of the (200) and (400) reflectjon. The superlattice under investigation is lattice-matched and has a long period of ∼630Å. This kind of structure creates a very symmetrical x-ray pattern enveloping a large number of closely spaced satellite intensities with pronounced maxima and minima. It appears in the dynamical analysis that the position and shape of these maxima and minima is extremely sensitive to the number N of monolayers and atomic spacing d of the InGaAs and InP layer, as well as the presence of interfacial layers and impurities.
Xafs Study on Interfaces in III-V Semiconductor Heterostructures
Authors: Ogata Kiyoshi , Kazufumisuenaga  , Nakano Asao , Mozume Teruo
Published: March 2011

Interfaces in InGaAs/InP heterostructures were investigated by XAFS analysis using synchrotron radiation. Two types of InGaAs/InP heterostructures were made by MEE using Ga, In, AsH3 and PH3 as sources. These InGaAs layers were terminated by (a) InGa and (b) As. The local structures around P were investigated by fluorescent XAFS analysis. It shows that the nearest neighbor around P is Ga and In in the InGa (a) sample and mainly In in the As (b) sample.
The Influence of Lattice Mismatch on Indium Phosphide Based High Electron Mobility Transistor (HEMT) Structures Observed in High Resolution Monochromatic Synchrotron X-Radiation Diffraction Imaging
Authors: Steiner Bruce , Comas James , Tseng Wen , Laor Uri
Published: March 2011

The formation of mismatch dislocations in layered semiconductor structures was found recently in high resolution monochromatic synchrotron x-radiation diffraction images to be correlated with characteristics of the substrate as well as with the layer thickness and degree of lattice mismatch of non pseudomorphic layers.1,2We have now extended these studies to examine the accommodation to strain as a function of lattice mismatch in a series of high electron mobility transistor (HEMT) structures grown by molecular beam epitaxy (MBE) on indium phosphide substrates.

Five distinct types of irregularity are observed: 1) lattice warping, 2) the formation of a nonpseudomorphic layer, 3) the formation of extended arrays of linear mismatch dislocations at the interface between the substrate and a nonpseudomorphic layer, 4) the formation of oval regions of tweed-like local lattice variation imbedded among these arrays, and 5) extended tweed-like local lattice variation over large peripheral areas in which the formation of straight mismatch dislocation arrays is not observed.

Warping of the lattice is found in nearly all layered structures. A distinct layer with a different lattice parameter but without visible misfit dislocations is formed with a mismatch of 0.27 %. With increase of the mismatch to 0.5 %, the other three forms of accommodation appear in distinct regions of the structure: arrays of <011> mismatch dislocations; oval regions of tweed-like irregularity, oriented in the [011] direction; and peripheral regions of extended tweed-like local lattice variation.
The Use of Strain to Optimize Quantum Well Device Performance
Authors: Koteles Emil S.
Published: March 2011

Recently, conventional wisdom has reversed its view on the disadvantages of strain in active semiconductor devices. Now lattice-mismatch-induced biaxial strain is being actively studied as a technique for optimizing bulk and quantum well (QW) device performance. It has been realized that, in addition to permitting once inaccessible energy ranges to be reached, strain has the potential of altering the properties of energy bands in a precise and advantageous manner provided that care is taken that the strain remain pseudomorphic (i.e., that the critical thickness, beyond which the layer relaxes with detrimental effects on quality, not be exceeded).

This paper reviews progress in this field including techniques for determining the magnitude of pseudomorphic strain in single QWs, and recent theory and experiments detailing the advantages which strain, both compressive and tensile, can bring to QW laser diodes. The use of strain to balance the polarization anisotropy inherent in QW structures is also discussed. Finally, a novel use of biaxial tensile strain in the barriers of QW devices to minimize polarization sensitivity is introduced. The idea is to create a structure in which the light and heavy hole valence bands are degenerate, as in bulk material, but which still retains the advantages of QWs (higher quantum and differential quantum efficiency, etc.).
Interfacial Quality of Strained-Layer InGaAs/GaAs Quantum Well Lasers Grown by Gas-Source Molecular Beam Epitaxy
Authors: Zhang G. , Ovtchinnikov A. , Pessa M.
Published: March 2011

We report a study of interfacial quality of strained-layer InGaAs/GaAs quantum well lasers grown by gas-source molecular beam epitaxy. It was found that the growth temperature (Tgr) of the InGaAs layer plays an important role in the interfacial quality. For Tgr < 515 °C, a large amount of non-radiative recombination centers is likely to exist in the InGaAs/GaAs quantum well, which can be attributed to the presence of vacancies and atom clusters and lattice misfit defects. For Tgr > 515 °C, the InGaAs/GaAs interfaces show significant roughness due to In segregation. Rapid thermal annealing grades the InGaAs/GaAs interface because of interdiffusion of group-III atoms at the interface, and removes most of the non-radiative recombination centers from the low Tgr (<515 °C) samples. In addition, we observed that the interfacial quality of the InGaAs/GaAs quantum well shows no strong dependence on (100) vicinal orientations of GaAs substrate.
Characterization of AlGaAs/InGaAs/GaAs Heteroepitaxial Layers by Transmission Electron Microscopy and Energy Dispersive Spectroscopy
Authors: Rai R. S. , Tartaglia J. M. , Quinn W. E. , Martel D. C.
Published: March 2011

AlGaAs/InGaAs/GaAs heteroepitaxial layers grown by molecular beam epitaxy were studied by cross-sectional transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The presence of waviness/roughness, fine periodic striation contrast due to Al composition oscillations, and defects were observed by TEM in selected samples. EDS on the TEM was of limited utility in determining the composition of thin epitaxial layers and in comparing the composition near and away from a defect. Arguments are presented to rationalize these results.
The Influence of the Real Structure on the Deformations in Layer Systems
Authors: Jenichen B. , Kohler R.
Published: March 2011

The relaxation of thermal stresses during the Czochralsky growth leads to frozen in deformations in the boules. These deformations lead to a nonintentional inhomogeneous miscut of the wafers. From the X-ray double crystal measurement of frozen in deformations in GaAs bulk crystals a nonintentional miscut of about 10−4 may be estimated. The measurement of the misorientation distribution over a LEC GaAs wafer yielded even higher inhomogeneities which are believed to be due to local curvature of the lcm2 samples. The relaxation of GaAs/AlAs Braggreflectors of 3μm overall thickness depends on the density of threading dislocations i.e. of the type of substrate (LEC or HB). The inhomogeneous strain field of misfit dislocations in InGaAs/GaAs multilayer systems and their inhomogeneous distribution leads to deformations in epitaxial layers limiting the dimensions of coherently scattering domains. The resulting mosaic spread in the epitaxial layer systems has been obtained using triple crystal measurements. The experimentally determined mosaic spread has to be taken into account to simulate the the diffraction pattern more correctly.
Manifestation of the DX Centre in Heavily δ-Doped GaAs(Si)
Authors: Arscott S. , Missous M. , Dobaczewski L. , Harness P. C. , Maude D. K. , Portal J. C.
Published: March 2011

Shubnikov-de Haas and Hall measurements have been performed on singly delta doped GaAs(Si) structures, grown by molecular beam epitaxy, enabling us to study the effects of illumination and temperature upon bulk and individual subband, mobilities and carrier concentrations. In a highly doped sample, where the peak 3D electron concentration approaches 2×1019cm−3, we have observed novel changes in subband transport characteristics, not observed in the lower doped samples, which we attribute to the presence of DX centre phenomena. This paper explains the variations in individual subband transport properties due to a possible shift of the electronic wave functions contained in the potential well. This shift occurs due to a recombination-autoionization(R-A) process involving filled DX centres and free holes upon sample illumination at low temperatures.
Electronic Structure of GaAs/GaInP Strained Layer Quantum Wells
Authors: Rerbal K. , Zitouni K. , Kadri A.
Published: March 2011

This work reports on the calculations of the electronic structure of GaAs/GaxIn1−x P strained layer quantum wells. The calculations are based on the multiband envelope function approach, considering the valence band offset value that has been recently proposed. The effect of strain due to the lattice-mismatch is studied. The calculated results are compared with the data obtained from photoreflectance measurements. Satisfactory agreement is obtained.
Graded Buffer Layers for Molecular Beam Epitaxial Growth of High in Content InGaAs on GaAs for Optoelectronics
Authors: Lord S. M. , Pezeshki B. , Marshall A. F. , Harris J. S. , Fernandez R. , Harwit A.
Published: March 2011

Linearly-graded buffer layers are used to achieve exciton resonances in InGaAs/GaAs at the 1.3 μm dispersion minimum of optical fibers. We investigate the effect of various gradients in the linearly-graded InGaAs buffer layer of a p-i-n structure designed as an optical modulator. Samples were grown by molecular beam epitaxy on GaAs substrates at 450°C. Each consisted of an n-type graded buffer region followed by thirty In0.5Ga0.5 As quantum wells with Alo.35Ga0.65As barriers, and 5000 Å of ρ-type InGaAs. Gradients of 7.5%, 15%, and 30% In/μm, or 0.5%, 1%, and 2% misfit/μm were used to grade the In concentration from 0% to 35%. Room temperature absorption measurements reveal well-defined zero-field excitonic features near 1.3 μm for all samples. The exciton resonance becomes sharper as the gradient decreases. Analyses of these samples by TEM and double crystal X-ray diffraction confirm the trend that a slower grading yields better quality material.
Materials and Structures for Advanced III-HBTs
Authors: Asbeck P. M. , Tu C. W. , Ho M. C. , Fu S. L. , Gee R. C. , Chin T. P.
Published: March 2011

This paper reviews the present status of heterojunction bipolar transistor (HBT) technology based on GaAlAs/GaAs and InP/InGaAs materials, and discusses a variety of approaches for device improvement. Among the possibilities presented are novel structures to reduce base-collector capacitance, and novel materials to increase breakdown voltage and reduce base-emitter turn-on voltage.
Delta Doping for Deep Level Analysis in Semiconductors
Authors: Piprek J. , Krispin P. , Kostial H. , BÖer K. W.
Published: March 2011

The occupation of deep-level defects in semiconductors is investigated by delta-doping such impurities at a specified distance from the metallurgical boundary within Schottky diodes. Capacitance-voltage characteristics are analyzed using ID device simulation software. These characteristics change significantly depending on the deep-level energy and the sheet position. This new approach to deep-level analysis is applied to Schottky diodes on MBE-grown n-GaAs with a planar titanium doped sheet. At moderate Ti concentrations the well-known Ti acceptor level near Ec-0.2 eV governs the electrical properties. In addition, two other types of Ti defects are found.
Materials and Device Characteristics of InAlAs/InGaAs HEMTs
Authors: Ho Pin , Kao M. Y. , Chao P. C. , Duh K. H. G. , Smith P. M. , Martin P. A. , Liu S. M. J. , Hwang K. C. , Ballingall J. M. , Yu T. , Swanson A. W.
Published: March 2011

High electron mobility transistors (HEMTs) based on the InAlAs/InGaAs heterostructure have been grown on InP by molecular beam epitaxy. At room temperature, typical sheet charge densities of 2.1–3.0×1012 cm−2 and Hall electron mobilities over 10000 cm2 /V-s are obtained. An electron mobility as high as 13000 cm2 /V-s is achieved with a pseudomorphic Iny Ga1−y As channel and a y value of 0.70.

HEMTs with a T- or Γ-shaped gate and with gate lengths ranging from 0.1–0.25 urn have been fabricated. A record low noise figure of 0.7 dB with an associated gain of 8.6 dB at 62 GHz has been achieved with 0.1 μm Γ-gate devices, while T-gate devices exhibit a minimum noise figure of 1.2 dB with 7.2 dB associated gain at 94 GHz. Separately, a record fmax value of 455 GHz was determined by extrapolating at -6 dB/octave from the measured gain of 13.6 dB at 95 GHz.

Power HEMTs using a double heterojunction structure exhibit a record peak power-added efficiency (P.A.E.) of 49% with 8.6 dB power gain and 0.30 W/mm power density measured at 60 GHz. When biased and tuned for maximum output power, our best 60 GHz output power density to date is 0.52 W/mm with 33% P.A.E. and 5.9 dB power gain using a single heterojunction HEMT scheme with pseudomorphic channel. A similar device also yields peak P.A.E. of 26% with 0.20 W/mm power density and 4.9 dB gain at 94 GHz. These results represent the highest P.A.E.S and power gains ever reported for any transistor at these frequencies.
Characteristics and Growth of Strained-Layer InGaAs/GaInAsP/GaInP Quantum Well Lasers
Authors: Zhang G. , Ovtchinnikov A. , Näppi J. , Asonen H.
Published: March 2011

Strained-layer InGaAs/GalnAsP/GalnP separate-confinement-heterostructure quantum well lasers emitting at 980 nm have been developed. The lowest threshold current densities obtained for the single-quantum-well and three-quantum-well lasers are 72 and 150 A/cm2, respectively. The internal quantum efficiency is as high as 94 %, and the internal waveguide loss 5.4 cm−1. The transparency current density and gain coefficient are 33 A/cm2 per well and 0.091 μm A−1, respectively. High characteristic temperatures ranging from 220 to 280 K was obtained. The vertical and lateral full-width at half-maximum of the far-field pattern of the ridge waveguide laser are 47° and 13°, respectively. The results are comparable with the best values reported for the InGaAs/AlGaAs lasers.
Multilayer AlxGa1−xAs Heterostructures for Second-Harmonic Generation
Authors: Janz S. , Chatenoud F. , Dai H. , Vilks E. , Buchanan M. , Normandin R. , Springthorpe A. J.
Published: March 2011

Quasi-phase matching in AlxGa1−xAs heterostructure optical waveguides can be used for efficient second-harmonic (SH) generation. The efficiency of these devices depends on the linear and nonlinear optical properties of the component materials. We present measurements of the SH susceptibility variation in AlxGa1−xAs with Al concentration, for fundamental light at λ = 1.06 μ. The measured SH susceptibility decreases by an order of magnitude as the Al concentration is varied from x = 0 to x = 0.97. These measurements are used to evaluate the SH generation efficiency of AlxGa1−xAs heterostructures as the structural and material parameters are varied.
Vertical-Cavity Optoelectronic Structures: CAD, Growth, and Structural Characterization
Authors: Christensen D. H. , Crochiere S. M. , Pellegrino J. G. , Rai R. S. , Parsons C. A. , Tseng W. F. , Hickernell R. K.
Published: March 2011

Simulations of reflectance spectra and electric field distributions for vertical-cavity structures were used in the computer aided design of epitaxial mirrors and lasers. The binary GaAs/AlAs superlattice alloys and AlxGa1−xAs random alloys that compose these structures were grown by molecular beam epitaxy. Photoluminescence, photoreflectance, reflectance spectroscopy, scanning electron microscopy, transmission electron microscopy, and double crystal x-ray diffractometry were applied to characterize cavity and Bragg mirror layer thicknesses and alloy composition.
Optical Channel Waveguides in AlGaAs Multiple Quantum Well Structures Formed by Focused Ion Beam Induced Compositional Mixing
Authors: Kumar Mukesh , De Brabander Gregory N. , Chen Peter , Boyd Joseph T. , Steckl Andrew J. , Choo Ahn Goo , Jackson Howard E. , Burnham Robert D. , Smith Stephen C.
Published: March 2011

Optical channel waveguiding in AlGaAs multiple quantum well structures formed by compositional mixing implemented by focused ion beam (FIB) implantation is demonstrated. To achieve selective mixing, Si is FIB implanted with a dose of 5×1014 cm−2 followed by RTA at 950°C for 10 s. Raman microprobe spectra are used to characterize the lateral variation of mixing. Propagation loss in a channel waveguide is measured. Measurement of the waveguide mode field distribution allows for the determination of changes in refractive index due to mixing and an approximate mixing depth.
Enhanced photoluminescence from AlGaAs/GaAs superlattice gratings fabricated by Si FIB implantation
Authors: Steckl A. J. , Chen P. , Choo A. G. , Jackson H. E. , Boyd J. T. , Ezis A. , Pronko P. P. , Novak S. W. , Kolbas R. M.
Published: March 2011

Results are presented on the fabrication of optical gratings on an Al0.3Ga0.7As/GaAs superlattice (SL) with equal 3.5 nm barrier and well widths, by using locally FIB-enhanced mixing. As the first step, the mechanism of the mixing was studied. Si++ was accelerated to 50 kV and lOOkV and implanted at doses ranging from 1013 to 1015/cm2. A rapid thermal anneal of 10 s at 950°C was utilized. The average Al inter-diffusion coefficient and length were calculated as a function of FIB dose from SIMS depth profiling. The mixing was significantly enhanced by the FIB implantation. The ion dose as low as l×1014/cm2 followed by RTA yields a mixing parameter of ∼90% and results in a two-order of magnitude increase in the diffusion coefficient, to a value of 4.5×10−14cm2/sec, in contrast to 1.3×10−16cm2/sec from RTA-only. The maximum mixing occurred in the region where neither Si ions nor vacancies have their maximum concentration. Instead, it coincides with the location of the positive maximum of the second derivative of the vacancy concentration profile. This fact suggests that in the time frame of RTA and with low dose, the diffusion of nonequilibrium point defects plays a major role in the process of enhancing Al-Ga interdiffusion. DBR optical gratings, consisting of thousands of spacing lines with 350nm period, were fabricated with a lOOkV FIB dose of 2×1013 andl×1014/cm2. Photoluminescence (PL) spectra were taken from the grating region as well as the unimplanted superlattice region. The PL intensity from cavity region of the DBR was about 16 times higher than that from the original SL. This PL enhancement was verified to occur in the cavity region only by spatially scanning over the entire sample. A possible mechanism for this PL enhancement is optical feedback provided by the gratings.
Defect Reduction in GaAs Grown on Si by Using Saw-Tooth-Patterned Substrates
Authors: Zhu Jane G. , Al-Jassim M. M. , Karam N. H. , Jones K. M.
Published: March 2011

Epitaxial GaAs layers have been grown on saw-tooth-patterned (STP) Si substrates by metal-organic chemical vapor deposition and analyzed by transmission electron microscopy. The utilization of this special interface feature is effective in suppressing the formation of antiphase boundaries and reducing the threading dislocation density. The growth of GaAs has been studied with the epilayer thicknesses ranging from several hundred angstroms to several microns. Very flat growth front on (100) plane above the STP region is observed. The dislocation density decreases very rapidly in the area farther away from the interface. The dislocation configuration at this STP interface is very different from that at the extensively studied two-dimensional planar interface.
TEM Study of the Growth Modes in ALMBE GaAs Layers on Si
Authors: Vilà A. , Comet A. , Morante J. R. , González L. , González Y. , Briones F. , Ruterana P.
Published: March 2011

In this work, we have compared, by using Transmission Electron Microscopy (TEM) techniques, the initial stages of epitaxial growth of GaAs on Si (100) by conventional MBE and ALMBE, trying to find the conditions necessary to achieve 2D growth at the earliest stage of deposition. Our results show that flat layers with a good surface coverage can be obtained by reducing the GaAs ALMBE deposition temperature down to 200°C.
Crystal Quality and Surface Morphology Improvement of Movpe-Grown GaAs-on-Si Using Tertiarybutylarsine
Authors: MIYAGAKI S. , Ohkubo S. , Takai K. , Takagi N. , Kimura M. , Kikuchi Y. , Eshita T. , Takasaki K.
Published: March 2011

We developed GaAs heteroepitaxy on a Si substrate by metalorganic vapor phase epitaxy (MOVPE) using tertiarybutylarsine (TBAs). In buffer layer growth at 450°C, the surface morphology and crystal quality of TBAs-grown films were slightly inferior to those of AsH3-grown films. At buffer layer growth below 400°C, the quality of TBAs-grown films improved. The GaAs films we grew using TBAs had a better quality than those grown using AsH2.
Growth of InSb/GaAs Layers on YIG-Coated GGG Substrate
Authors: Jelen C. , Charrière S. , Razeghi M. , Leppert V. J.
Published: March 2011

We report the first growth of InSb and GaAs epilayers upon a garnet (YIG = Y3Fe5O12) epilayer. The YIG was deposited using liquid phase epitaxy on a garnet (GGG = Gd3Ga5O12) substrate oriented in the [111] direction. The growth of the GaAs was carried out using laser ablation and no superlattice was used to buffer the lattice mismatch between YIG and GaAs. The growth of InSb was done by low-pressure metalorganic chemical vapor deposition. From x-ray diffraction analysis it was found that the GaAs and InSb were both (110) monocrystalline epitaxial layers.
SiGe Heterojunctions Transistors and Optoelectronic Devices
Authors: Arienzo Maurizio , Comfort James H. , Crabbe Emmanuel F. , Harame David L. , Iyer Subramanian S. , Kesan Vijay P. , Meyerson Bernard S. , Patton Gary L. , Stork Johannes M.C. , Sun Yuan-Chen
Published: March 2011

SiGe alloys have been successfully applied to a number of semiconductor devices, including bipolar heterojunction transistors, field effect transistors (FET's), and optoelectronic devices and structures. This review paper will first summarize the results obtained to-date in bipolar transistors, highlighting the design flexibility and the trade-offs offered by SiGe heterojunction technology and bandgap engineering, like junction field/capacitance control, liquid nitrogen operation and complementary processes. The leverage of this technology in high speed circuits will be discussed, including the record 75 GHz fr and 60 GHz fmax heterojunction bipolar transistors, the achievement of sub-25 ps ECL ring oscillator delay, and the doubling of the mobility in p-MODFETs. The applications of this technology to optoelectronic devices, including detectors and waveguides, will also be reviewed, to extend the use of silicon technology to long wavelength communication technology and infrared imaging.