bluuebunny/crossref_metadata_2025 huggingface dataset:
Crack Deflection and Interfacial Fracture Energies in Alumina/SiC and Alumina/TiN Nanocomposites
Authors: Jiao S. , Jenkins M. L.
Published: April 2011

Crack/particle interactions in Al2O3/SiC and Al2O3/TiN nanocomposites have been observed by TEM on samples containing cracks produced by Vickers indentations. No significant crack deflection by intragranular SiC particles or microcracking around nanoparticles was found. Intergranular cracks were observed to be deflected into the matrix grains by SiC particles on grain boundaries inclined to the direction of crack propagation. TiN particles were not effective in this way. These features are briefly discussed within the framework of the interfacial fracture energies. These were calculated from interfacial energies, which were determined by the measurement of grain boundary-interface dihedral angles.
Fire Retardant Polyetherimide Nanocomposites
Authors: Lee Jongdoo , Takekoshi Tohru , Giannelis Emmanuel P.
Published: April 2011

Polyetherimide-layered silicates nanocomposites with increased char yield and fire retardancy are described. The use of nanocomposites is a new, environmentally-benign approach to improve fire resistance of polymers.
Transmission Electron Microscopy and Electron Holography of Nanophase TiO2 Generated in a Flame Burner System
Authors: Turner S. , Bonevich J. E. , Maslar J. E. , Aquino M. I. , Zachariah M. R.
Published: April 2011

Nanophase TiO2 (n-TiO2) particles were generated in a flame burner system under three experimental conditions. Selected individual nanoparticles were identified and characterized using selected area electron diffraction, bright-field and, in some cases, dark-field imaging to determine morphology and microstructural features. Previously unknown TiO2 particles with unusual central features were identified as rutile. Electron holography was used to characterize the central features which were found to be consistent with voids. More extensive characterization of individual particles may lead to improved understanding of n-TiO2 nucleation and growth.
Microstructure, Mechanical Properties, and High-Temperature Oxidation Resistance of Boronized γ - TiAl(Mn)
Authors: Kim S. , Yoon Y. , Kim H. , Park K.
Published: April 2011

To improve the wear and high-temperature oxidation resistances of reactively sintered γ -TiAl(Mn) intermetallic compounds, they were boronized in the temperature range of 900 to 1100 °C for 5 to 11 hours with powder mixtures of B4C and Na2B4O7. It was found that the coating layer consisted of three sublayers, i.e., outer, middle, and inner sublayers. The outer, middle, and inner sublayers were identified as a mixture of TiB2 and TiO2, a mixture of Al2O3 and TiO2, and Ti-rich TiAl based compounds, respectively. The coating layer significantly improved the surface hardness and the wear and high-temperature oxidation resistances. The highest surface hardness (Hv ≈ 2720) was obtained at 900 °C for 11 hours. This surface hardness is much higher than that of TiAl(Mn) (Hv ≈ 400). In addition, the specimens boronized at both 1000 and 1050 °C for 9 hours showed an excellent high-temperature oxidation resistance. It is believed that the boronizing on the TiAl(Mn) intermetallics is very effective to improve the wear and high-temperature oxidation resistances.
Tensile Properties and Deformation Mechanisms in Two-Phase Titanium Aluminide Sheet Material
Authors: Appel F. , Clemens H. , Glatz W. , Wagner R.
Published: April 2011

The mechanical properties of two-phase TiAl sheets with different compositions and microstructures were investigated over the temperature range 25–1000 °C. The microprocesses of plasticity were characterized by electron microscope observations. Particular emphasis has been paid to the mechanisms governing the deformation behavior at elevated temperatures which are relevant for the fabrication and engineering applications of structural components.
Microstructure of Massively Transformed γ-TiAl Phase Studied by High-Resolution Electron Microscopy
Authors: Abe E. , Kumagai T. , Kajiwara S. , Nakamura M.
Published: April 2011

A microstructure of the massively transformed γ-TiAl (γm) phase in a Ti-48at.%Al alloy, which was heat treated in the high-temperature α-Ti (disordered hep) single phase field (1683K), followed by ice water quenching, has been examined using high-resolution electron microscopy. The characteristic features of the microstructure originated from the α→γ massive transformation have been clarified in detail, which are as follows. (1) Extremely thin hep plates (about 0.8–2nm in thickness), which are considered to be a retained α phase, are found to exist in the γm phase. (2) Twin boundaries are found to be not flat interfaces, that is, twin interfaces are not on the exact (111) mirror plane. This situation is attributed to the existence of a number of partial dislocations at the twin boundaries. (3) Antiphase relationship between the regions either side of the thin rotated domain wall [1] is confirmed. The validity of this situation is explained by assuming that the thin rotated domain wall has been grown from a simple antiphase domain boundary. On the basis of these facts, mechanism of the α→γ massive transformation has been discussed.
Yield Stress Anomaly in B2 FeAl
Authors: Yoshimi K. , Hanada S. , Yoo M. H.
Published: April 2011

Our studies on yield stress anomaly of B2 FeAI single crystals are reviewed in this paper. A positive temperature dependence of yield stress, so-called “yield stress anomaly”, is observed in B2 FeAI in which excess vacancies are fully annealed out. Associated with the anomaly, characteristic asymmetry is found between tension and compression. While the strain-rate sensitivity is almost zero in the temperature range of the yield stress anomaly, the stress relaxation becomes significant with increasing temperature, indicating that a recovery process is thermally activated. It is ascertained by the two-surface trace analysis that slip transition from <111> direction at intermediate temperature to <100> at high temperature occurs around the peak temperature. Even at the peak temperature, in addition, operative slip vector for yielding is confirmed to be predominantly <111> by TEM. Also, it is observed that <111>-type superdislocations are frequently climb-dissociated in the temperature range of the anomaly. APB formation on {111} plane is energetically favorable, which is in agreement with the Flinn's calculation for the B2 superlattice that APB energy on {111} plane is lower than that on {110} plane. Such an anisotropy of APB energy would offer specific driving force for the climb dissociation on <111> superdislocations. On the basis of the observed results, the anomalous strengthening behavior of B2 FeAI single crystals is discussed.
Microstructure and Strengthening of Creep-Tested Cryomilled NiAl-AlN
Authors: Garg Anita , Daniel Whittenberger J. , Luton Michael J.
Published: April 2011

The mechanical grinding of prealloyed NiAl powder in liquid nitrogen (cryomilling) results in an intermetallic matrix composite where micron sized particle free aluminide cores (grains) are surrounded by thin mantles comprised of nanometer sized A1N particles and NiAl grains. Under high temperature, slow strain rate conditions both compressive and tensile creep testing have shown that the mechanical strength of hot extruded cryomilled NiAl approaches the levels exhibited by advanced NiAl-based single crystals and simple Ni-based superalloys. Transmission electron microscopy of cryomilled materials tested between 1100 and 1300 K revealed little, if any, dislocation structure within the mantle regions, while the NiAl cores contained subgrains and dislocation networks after testing at all strain rates between 10-4 and 10-8 s-1. These and other microstructural observations suggest that creep strength is the result of a fine NiAl grain/subgrain size, the inability of dislocations to move through the mantle and stabilization of the microstracture by the A1N particles.
An Explanation of the Small Strain-Rate Sensitivity of Ni3Al
Authors: Sun Y. Q.
Published: April 2011

Based on the superkink model, an explanation is given in this paper for the small strain-rate sensitivity of the anomalous yield stress of Ni3Al. The yield stress is proposed to be the stress needed to destabilise the average superkinks and it varies inversely with superkink height. The yield stress is insensitive to the strain-rate because the superkink height is controlled by the rate at which the superkink widens and is not directly related with the superkink propagation speed.
Dislocation Structures in Ni3(Al, Hf)
Authors: Kruml T. , Viguier B. , Bonneville J. , Spätig P. , Martin J. L.
Published: April 2011

Single crystalline specimens of Ni74.8Al21.9Hf3.3 were subjected to compression tests at different temperatures. Thin foils for transmission electron microscopy observations were prepared from several specimens deformed within and above the yield stress anomaly domain. The dislocation microstructure was studied. The weak beam imaging and image simulation techniques followed by anisotropie elasticity calculations were used for the determination of antiphase boundary energies in both cube and octahedral planes, resulting in values of 237 mJm-2 and 252 mJm-2 respectively. The comparison of the present results with data taken from literature shows the influence of Hf on mechanical properties, dislocation microstructures and APB energies.
Stress response by the strain-rate change in binary, stoichiometric Ni3AI single crystal
Authors: Demura M. , Hirano T.
Published: April 2011

The strain-rate dependence of flow stress in single crystals of binary, stoichiometric Ni3Al was studied in the temperature region of the yield stress anomaly. Below 400 K, the steady-state flow stress was found to be independent of strain rate, though it changed temporarily when the strain rate was changed. The strain-rate insensitivity can be explained by assuming that the flow stress is controlled by the multiplication/immobilization of mobile dislocations.
The Mechanical Behavior and Deformation Mechanisms of Nb-Al-V Alloys
Authors: Tappin D. K. , Horspool D. N. , Smith L. S. , Aindow M.
Published: April 2011

A series of Nb-Al-V alloys containing 20–40 at. % V and 10–25 at. % Al have been investigated. The phase distributions in the alloys indicate that Al promotes the formation of the A15 phase whilst V stabilises a B2 phase. Room temperature compression testing revealed that the B2 is inherently ductile such that for the alloys with less than 40% by volume of the A15 phase, strains of over 50% were obtained easily. The 2% offset yield stresses of these alloys did not vary significantly with composition, being 1.2±0.1GPa in each case. TEM studies were used to show that the deformation in the B2 phase occurs predominantly by the glide of screw-type super-partial dislocations with b=l/2<111> on {110} and {112}. In some alloys this dislocation activity was preceded by the formation of pseudo-twins, via a martensitic shear transformation.
Laves Phase-Based Materials: Microstructure, Deformation Modes and Properties
Authors: Kumar K. S.
Published: April 2011

Intermetallic compounds with the AB2 stoichiometry can exist with the C14, C15 or C36 structures, otherwise called Laves phases. In the past, single-phase Laves compounds, particularly Mg-containing binary compounds, have been examined to obtain an understanding of the dislocation structures as well as their mobility; alternately, effort has also been expended in examining the polytypic transformations that occur in such Laves phases. More recently however, Laves phases and polyphase alloys containing the Laves phase have been examined with the intent of developing structural alloys for elevated temperature applications. Specifically, alloys based on the Cr-Group IV(Ti, Zr, Hf) and Group V (Nb, Ta) elements as well as the ternary V-Hf-Nb system have been the focus of several investigations. Physical and mechanical properties have been measured, and microstructure has been characterized as a function of heat treatment. Deformation modes have been re-examined in detail. New ideas based on synchroshear have been put forth to explain the polytypic transformations that occur in these systems, and have been extended to also explain twinning and slip. Earlier observations are first summarized and then these recent advances are reviewed and the potential for these materials is assessed.
Y2O3 Morphology in an Oxide Dispersion Strengthened FeAl Alloy Prepared by Mechanical Alloying
Authors: Inkson B. J. , Threadgill P. L.
Published: April 2011

The microstructure of an oxide dispersion strengthened FeAl (Zr,B) alloy, manufactured by mechanical alloying then extrusion, has been examined by HREM. Y2O3 is dispersed throughout the FeAl matrix as particles, ranging in size from 5nm upwards, which are effective in pinning the bulk dislocations. Although in the main the observed oxide particles are irregular in morphology, a significant minority of particles exhibit faceted surfaces. In particular, the facets of the Y2O3 particles are observed to coincide with {100}B2, {110}B2 and {112}B2 planes of the surrounding bulk FeAl matrix. In addition, HREM imaging reveals uncoupled 1/2<111>FeAl superpartial dislocations lying a few nanometres from some of the FeAl - Y2O3 interfaces.
Solution and Precipitation Hardening in Carbon-Doped Two-Phase γ-Titanium Aluminides
Authors: Appel F. , Christoph U. , Wagner R.
Published: April 2011

A two-phase titanium aluminide alloy was systematically doped with carbon to improve its high temperature strength. Solid solutions and precipitates of carbon were formed by different thermal treatments. A fine dispersion of perovskite precipitates was found to be very effective for improving the high temperature strength and creep resistance of the material. The strengthening mechanisms were characterized by flow stresses and activation parameters. The investigations were accompanied by electron microscope observation of the defect structure which was generated during deformation. Special attention was paid on the interaction mechanisms of perfect and twinning dislocations with the carbide precipitates.
Void Morphology in Polyethylene/Carbon Black Composites
Authors: Marr D.W.M. , Wartenberg M. , Schwartz KB. , Agamalian M. M. , Wignall G. D.
Published: April 2011

A combination of small angle neutron scattering (SANS) and contrast matching techniques is used to determine the size and quantity of voids incorporated during fabrication of polyethylene/carbon black composites. The analysis used to extract void morphology from SANS data is based on the three-phase model of microcrack determination via small angle x-ray scattering (SAXS) developed by W. Wu12 and applied to particulate reinforced composites.
Compatibilizers made from Copolymers that Exhibit Strong Interactions
Authors: Bauer Barry J. , Liu Da-Wei
Published: April 2011

Random copolymers of poly(methyl methacrylate-d8) (PMMA) and poly(methacrylic acid) (PMAA) were synthesized and blended with polyethylene oxide (PEO), and SANS was used to estimate the strength of interaction between the various polymer pairs. The addition of PMAA greatly reduced the scattering of the blend, giving a large negative Flory-Huggins interaction parameter. Copolymers of the type PMAA-g-PS, PMAA-r-PS, PMMA-g-PS, and PMMA-r-PS (g = graft, r = random) were used as compatibilizers for the melt blending of mixtures containing 80% PS and 20% PEO. Random copolymers containing half PMMA and half PS or half PMAA and half PS were ineffective compatibilizers while graft copolymers of these polymers produced a much finer dispersion of PEO in PS when blended at 190°C. Graft copolymers made with 90% PS were different, however, with PMMA grafts having only a small effect, while PMAA grafts having a large effect. Random and graft copolymers of PS and PMMA were also used to compatibilize blends of PS and PMMA. The results were similar to the same compatibilizers with blends of PS and PEO, with the graft copolymer causing a finer dispersion.
Melt-Processable Block Copolyimides
Authors: Hunt Craig L. , VanKirk Terre L. , Kumar Sai R.
Published: April 2011

We present here our probing research with segmented block copolyimdes derived on the basis of certain design principles to optimize performance and processability. We have shown that incorporation of relatively short blocks of rigid mesogenic segments along the backbone of flexible polyimide chains produce thermoprocessable polymeric systems which exhibit self-reinforcing characteristics.
In-Situ High Resolution Transmission Electron Microscopy of Solid-Liquid Interface of Alumina
Authors: Sasaki Katsuhiro , Saka Hiroyasu
Published: April 2011

The atomic structure and dynamic behavior of the solid-liquid interface of alumina were observed in a transmission electron microscope (TEM). The in-situ heating experiment of the alumina powder was performed using the newly developed heating holder. The surface of alumina was partially melted at about 2000K. Molten alumina on the surface migrated along the surface and formed liquid droplets. Such droplets existed stably on the surface during observation. The solid-liquid interface was observed by high resolution electron microscopy. The layer-by-layer growth of solid was observed on the solid-liquid interface. The nucleation of the monolayer island and the progress of the atomic step were observed directly. The velocity of the step progress at the solid-liquid interface was directly measured.
Rapid Migration of Defects in Ion-Implanted Silicon
Authors: Lalita J. , Pellegrino P. , Hallén A. , Svensson B. G. , Keskitalo N. , Fatima S. , Jagadish C.
Published: April 2011

The temperature dependence of the so-called reverse dose rate effect for generation of vacancy-type defects in silicon has been investigated using samples implanted with 1.3 MeV protons at temperatures between 70 and 300 K. The effect is found to involve a thermally controlled process which exhibits an activation energy of ∼0.065 eV, possibly associated with rapid migration of Si self-interstitials (I). Further, using a concept of dual Si ion-implants long range migration of I:s at room temperature has been studied. Annihilation of vacancy-type defects at a depth of ∼3 μm is obtained by injection of I:s from a shallow implant with sufficiently high dose.
Suppression of Boron Transient Enhanced Diffusion in SiGe HBTs by Carbon Incorporation
Authors: Lanzerotti L. D. , Sturm J. C. , Stach E. , Hull R. , Buyuklimanli T. , Magee C.
Published: April 2011

In this paper we demonstrate, using both SIMS and transistor electrical characteristics, that substitutional carbon fractions of 0.5% in heavily doped Si0.8Ge0.2 base heterojunction bipolar transistors (HBTs) reduce both thermal diffusion and transient enhanced diffusion (TED) of boron. Furthermore we show that carbon suppresses TED of boron in carbon-free regions that surround the carbon layers.
Interface Effects on the Adhesion of Thin Aluminum Films
Authors: Schneider J.A. , Guthrie S. , Moody N.R.
Published: April 2011

Differences in the adhesion and fracture toughness of aluminum films on sapphire due to the presence of controlled contaminants are being investigated. Adhesion is evaluated by use of nanoindentation and continuous scratch tests. A comparison was made of the properties of textured thin films of aluminum (178 to 1890 nm) that were vapor deposited onto (0001) oriented sapphire substrates. A very thin (1 nm) layer of carbon was deposited at the interface of selected samples prior to the vapor deposition of the aluminum. Spalling was observed during continuous scratch testing in specimens with carbon at the interface but not in specimens without carbon at the interface.
Texture of Gold-Palladium Couples
Authors: Chung Young S. , Evans Keenan , Glaunsinger William
Published: April 2011

The crystal textures of polycrystalline films of gold-palladium couples on an oxidized silicon (100) substrates were investigated via x-ray diffraction (XRD) pole figures. Studies were performed on both as-deposited and thermally annealed films. Scanning electron microscopy (SEM) was used to examine the microstructures of the seed layer thin films as deposited. The {111} texture formation of gold-palladium thin film couples displayed a strong dependence on the nature of the underlying seed layer. Gold films deposited on a palladium seed layer revealed much less degree of {111} texture, than gold films deposited directly on a silicon dioxide surface. In contrast, palladium films deposited on polycrystalline gold films showed a higher degree of {111} texture, compared to palladium films deposited directly on silicon dioxide. The {111} texture of annealed gold-palladium alloy thin films was greater for palladium on gold than for gold on palladium. These results are interpreted in terms of the gold-palladium diffusion mechanism and the interaction of the condensing metals with the oxygens of the SiO2 substrate surface.
RCA and IMEC/SC2 Clean: Metallic Immunity and Gate Oxide Integrity
Authors: Chen Weidong , Dhayagude Tushar , Chaparala Prasad , Demirlioglu Esin , Shenasa Mohsen , Bearda Twan , Arnauts Sophia , Meuris Marc
Published: April 2011

Dilute HF/RCA and IEMC/SC2 cleans have been evaluated on two process lines with different metallic contamination levels. VPD-DSE-TXRF and SPV techniques were used to monitor the metallic contamination. Gate oxide integrity(GOI) tests were performed on several structures. Both HF/RCA and IMEC/SC2 cleans have shown good Qbd and Ebd results for the clean process line. Lower Qbd and Ebd values were obtained for both cleans in the relatively contaminated process line. These results suggest that poor GOI is related to the metallic contamination in the oxide or at the SiO2/Si interface.
Effects of Wafer Cleaning Reduction on Metals Removal and
Ultrathin Gate Oxide Quality
Authors: D'Emic Christopher P. , Cohen Stephan , Zaitz Mary Ann
Published: April 2011

The drive to reduce chemical consumption for cost savings is high in the
semiconductor industry. Recently, studies have shown that the traditional
1:1:5 ratios of the SC1, SC2 cleans can be reduced and still
maintain good cleaning. The optimization of process parameters must be done
in accordance with the type of cleaning tool. Processes in wet decks may be
optimized differently from single chamber tools, especially if extended bath
lifetimes are expected. In this study, temperature reduction and dilution of
the SC1 and SC2 cleans in a wet deck are examined for their effects on
metals removal efficiency from silicon surfaces as measured by TRXRF. For an
HF, SC1, SC2 sequence, good metal removal can be maintained by dropping the
SC1 temperature down to 35°C and chemical ratio to 1:1:40. At too low an SC1
dilution and temperature, the Cu removal efficiency drops. In SC2, good
metals removal remains without peroxide and a lower limit for HC1 is
determined. It is also found that peroxide must be reduced if HCl is reduced
in SC2. otherwise metals plating out from SC1 cannot be removed effectively.
To understand the impact of these reduced cleans on gate oxide integrity,
the electrical properties of 30 A gate oxides grown using these experimental
pre-gate cleans is discussed.
Effects of the underlayer surface state on the interconnecting
aluminum film properties
Authors: Kim Sam-Dong , Shin Chan-Soo , Kwak Noh-Jung , Lee Kyeong-Bock , Shin Cheol-Ho , Kwon Oh-Jung , Kim Chung-Tae , Yang Hong-Seon
Published: April 2011

A gradual change in thermal oxide surface state from hydrophilic to
hydrophobic was observed with time -delay in a clean room environment.
Surface quality and reflectivity for the Al/Ti metal layers showed a strong
dependency on the oxide surface state. From the hydrophilic oxide substrate,
a lower (002) Ti preferred orientation was obtained than from hydrophobic
ones. This resulted in a degraded (111) Al preferred orientation and rough
metal surface. The RF-etch process increased the smoothness and hydrophobic
surface property for the inter -metal dielectric (IMD) oxides, and therefore
greatly improved Al/Ti surface quality. When conventional CMOS double layer
metal interconnection process is performed, metal inter-line bridge yield
was strongly affected by the surface state of substrate oxides.
AFM analysis of HF Vapor Cleaned SiO2
Surfaces
Authors: Carter R. J. , Bergman E. J. , Lee D. R. , Owyang J. , Nemanich R. J.
Published: April 2011

Si(100) surfaces were cleaned using HF/IPA vapor chemistries at ambient
pressure and temperature with nitrogen as the carrier gas. Three distinct
cases for oxide removal were studied: vapor etching of native oxides, RCA
chemical oxides, and thermal oxides. Atomic Force Microscopy (AFM) was used
to characterize the surface morphology after the HF vapor etching process.
The AFM indicated exaggerated peaks in random places on the surface, These
peaks were identified as residue remaining after the vapor etching process.
The average lateral width of the peaks were ∼ 50 nm. The average height of
the peaks for native and chemical oxide etched surfaces was relatively the
same, approximately 8 nm. The average height of the peaks after thermal
oxide removal was significantly smaller, approximately 1–2 nm. Peak density
for native oxide etched surfaces was significantly greater than chemical or
thermal oxide etched surfaces. We suggest that impurities in the oxide
contribute to residue formation on the surface.
Thermoelectric BehaVior of Conducting Polymers: On the Possibility of “Off-Diagonal” Thermoelectricity
Authors: Mateeva N. , Niculescu H. , Schlenoff J. , Testardi L.
Published: April 2011

Non-cubic materials, when structurally aligned, possess sufficient anisotropy to exhibit thermoelectric effects where the electrical and thermal currents are orthogonal (“off-diagonal” thermoelectricity). We discuss the benefits of this form of thermoelectricity for devices and describe a search for suitable properties in the air-stable conducting polymers polyaniline and polypyrrole. We find the simple and general correlation that the logarithm of the electrical conductivity scales linearly with the Seebeck coefficient on doping but with proportionality in excess of the conventional prediction for thermoelectricity. The correlation is unexpected in its universality and unfavorable for thermoelectric applications. A simple model suggests that mobile charges of both signs exist in these polymers, and this leads to reduced thermoelectric efficiency. We also briefly discuss non air-stable polyacetylene, where “ambipolar” transport does not appear to occur, and where properties seem more favorable for thermoelectricity.
Modeling of Mid-Infrared Multi-Quantum Well Lasers
Authors: Andreev A. D.
Published: April 2011

Threshold characteristics of mid-infrared MQW lasers have been studied theoretically. Auger recombination rates in strained quantum wells have been calculated in the framework of 8×8 Kane model taking account of spin-orbit interaction. It is demonstrated that the Auger coefficients of both CHCC and CHHS processes essentially depends on the QW parameters (strain, QW width, barrier bandgap), but have relatively weak (non-exponential) temperature dependence. It is shown that the laser characteristics can be improved for optimized laser structures, when the Auger rate is decreased.
MID-IR Photodetectors Based on InAs/InGaSb Type-II Quantum Wells
Authors: Brown G. J. , Ahoujja M. , Szmulowicz F. , Mitchel W. C. , Lin C. H.
Published: April 2011

We report on the growth and characterization of InAs/InxGal..Sb strained-layer superlattices (SLS) designed with a photoresponse cut-off wavelength of IOlim. The structural parameters, layer thicknesses and compositions, were chosen to optimize the infrared absorption for a superlattice with an energy band gap of 120 meV. The energy band structure and optimized absorption coefficient were determined with an 8×8 envelope function approximation model. The superlattices were grown by molecular beam epitaxy and were comprised of 100 periods of 43.6Å InAs and 17.2Å In.23Ga.77Sb lattice-matched to the GaSb substrates. In order to reduce the background carrier concentrations in this material, superlattices grown with different substrate temperatures were compared before and after annealing. This set of superlattice materials was characterized using x-ray diffraction, photoresponse and Hall measurements. The measured photoresponse cut-off energies of 116 ± 6 meV is in good agreement with the predicted energy band gap for the superlattice as designed. The intensity of the measured mid-infrared photoresponse was found to improve by an order of magnitude for the superlattice grown at the lower substrate temperature and then annealed at 520 °C for 10 minutes. However, the x-ray diffraction spectra were very similar before and after annealing. The temperature dependent Hall measurements at low temperatures (<25K) were dominated by holes with quasi two-dimensional behavior. An admirably low background carrier concentration of 1×1012 cm−2 was measured at low temperature.
Monolithically Integrated Dual-Band Quantum Well Infrared Photodetector
Authors: Sengupta D. K. , Gunapala S. D. , Bandara S. V. , Pool F. , Liu J. K. , McKelvey M. , Luong E. , Torezan J. , Mumulo J. , Hong W. , Gill J. , Stillman G. E. , Curtis A. P. , Kim S. , Chou L. J. , Mares P. J. , Feng M. , Hseih K. C. , Chuang S. L. , Bishop S. G. , Chang Y. C. , Liu H. C. , Wang W. I.
Published: April 2011

A monolithic quantum well infrared photodetector (QWIP) structure has been presented that is suitable for dual bands in the two atmospheric transmission windows of 3 – 5.3 μm and 7.5 – 14μm, respectively. The proposed structure employs dual stacked, strain InGaAs/AlGaAs and latticematched GaAs/AlGaAs quantum well infrared photodetector for mid wavelength and long wavelength detection. The response peak of the strain InGaAs/AlGaAs quantum well is at 4.9 μm and the lattice-matched GaAs/AlGaAs is at 10.5μm; their peak sensitivities are in the spectral regions of 3 – 5.3mu;m and 7.5 – 14μm. The peak responsivity when the dual-band QWIP is biased at 5 Volts is ∼0.065A/W at 4.9μm and ∼0.006A/W at 10.5μm; at this voltage the dual-band QWIP is more sensitive at the shorter wavelengths due to its larger impedance thus exhibiting wavelength tunability characteristics with bias. Additionally, single colored 4.9 and 10.5μm QWIPs were fabricated from the dual-band QWIP structure to study the bias-dependent behavior and also to understand the effects of growing the strain layer InGaAs/AlGaAs QWIP on top of the lattice-matched GaAs/AlGaAs QWIP. In summary, two stack dual-band QWIPs using GaAs/AlGaAs and strained InGaAs/AlGaAs multiquantum wells have been demonstrated with peak spectral sensitivities in the spectral region of 3 – 5.3μm and 7.5 – 14μm. Also, the voltage tunable dual-band detection have been realized for this kind of QWIP structure.
Mapping of Large Area Cadmium Zinc Telluride (CZT) Wafers: Apparatus and Methods
Authors: Brunett B. A. , Van Scyoc J. M. , Yoon H. , Gilbert T. S. , Schlesinger T. E. , Lund J. C. , James R. B.
Published: April 2011

Cadmium Zinc Telluride (CZT) shows great promise as a semiconductor radiation detector material. CZT possesses advantageous material properties over other radiation detector materials in use today, such as a high intrinsic resistivity and a high cross-section for x and γ-rays. However, presently available CZT is not without limitations. The hole transport properties severely limit the performance of these detectors, and the yield of material possessing adequate electron transport properties is currently much lower than desired. The result of these material deficiencies is a lack of inexpensive CZT crystals of large volume for several radiation detector applications. One approach to help alleviate this problem is to measure the spatial distribution (or map) the electrical properties of large area CZT wafers prior to device fabrication. This mapping can accomplish two goals: identify regions of the wafers suitable for detector fabrication and correlate the distribution of crystalline defects with the detector performance. The results of this characterization can then be used by the crystal manufacturers to optimize their growth processes. In this work, we discuss the design and performance of apparatus for measuring the electrical characteristics of entire CZT wafers (up to 10 cm × 10 cm). The data acquisition and manipulation will be discussed and some typical data will be presented.
Low Optical Loss Wafer Bonded GaAs Structures for Quasi-Phase-Matched Second Harmonic Generation
Authors: Wu YewChung Sermon , Feigelson Robert S. , Route Roger K. , Zheng Dong , Gordon Leslie A. , Fejer Martin M. , Byer Robert L.
Published: April 2011

A periodic GaAs wafer-bonded structure has been proposed for quasi-phase-matched (QPM) second harmonic generation (SHG). However, current bonding processes often lead to unacceptable optical losses and poor device performance. In this study, three sources of optical losses in wafer-bonded structures were investigated, (1) interfacial defects between the wafers, (2) bulk defects within the wafers, and (3) decomposition at the exposed outer surfaces. Surface losses due to incongruent evaporation were easily eliminated by repolishing the outer surfaces. However, to minimize the losses from interfacial and bulk defects, it was necessary to investigate the relationship between these defects and the bonding parameters. It was found that an increase in bonding temperature and/or time led to a decrease in interfacial defects, but an increase in bulk and surface defects. Through a trade-off process, optimized processing conditions were developed which permitted the preparation of bonded stacks containing over 50 (100)-oriented GaAs wafers, and about 40 layers of (110))-oriented GaAs wafers. Optical losses as low as 0.1 - 0.3% /interface (at 5.3 μm and 10.6 μm) were achieved.
Characterization of CdGeAs2 Using Capacitance Methods
Authors: Smith S. R. , Evwaraye A. O. , Ohmer M. C.
Published: April 2011

Thermal Admittance Spectroscopy(TAS) has been used to detect energy levels in the bandgap of CdGeAs2 specimens. Capacitance-Voltage(CV) measurements were used to determine the net free carrier concentration of the specimens as well as the conductivity type. All specimens were found to be p-type. CV measurements determined that the free carrier densities ranged from 1.2 × 1017 cm−3 to 8 × 1018 cm−3. Usually one peak (but in some cases two) was observed in the thermal admittance spectra. One peak present in two samples indicates an acceptor with a thermal activation energy of Ev+(0.10–0.13) eV which corresponds closely to the value of 0.10–0.12 eV found from Hall effect measurements on these specimens. The additional peak observed could correspond to a second deeper acceptor at Ev+0.346 eV, however, the energy could not be accurately determined because the peak was not fully resolved. Evidence for the existence of two native acceptors from electron paramagnetic resonance has recently been reported which tends to support a two acceptor model.
Photo-Assisted Resonant Tunneling Through Localized States in AlAs/GaAs Double-Barrier Structure With Undoped Spacer Layers
Authors: Chu H. Y. , Lee K. -S. , Park H. -H. , Lee E. -H.
Published: April 2011

We report on the evidence of photo-assisted resonant tunneling through localized states in AlAs/GaAs double-barrier structures (DBSs) with undoped GaAs spacers. In the photocurrent measurement, additional peaks were observed at voltages lower than that of resonance and were enhanced with the laser power. This behavior was more pronounced as the thickness of spacer layers in the DBS increased. These results are attributed to the resonant tunneling of electrons through the localized states, they are induced in the neighboring barriers, by the photoexcited carriers in spacers. We discuss the localization effect of photoexcited carriers on the resonant tunneling.
Dislocations and Traps in MBE Grown Lattice Mismatched p- InGaAs/GaAs Layers on GaAs Substrates
Authors: Du A. Y. , Li M. F. , Chong T. C. , Zhang Z.
Published: April 2011

Dislocations and traps in MBE grown p-InGaAs/GaAs lattice-mismatched heterostructures are investigated by Cross-section Transmission Electron Microscopy (XTEM), Deep Level Transient Spectroscopy (DLTS) and Photo-luminescence (PL). The misfit dislocations and the threading dislocations observed by XTEM in different samples with different In mole fractions and different InGaAs layer thickness generally satisfy the Dodson-Tsao's plastic flow critical layer thickness curve. The threading dislocations in bulk layers introduce three hole trap levels HI, H2 and H5 with DLTS activation energies of 0.32 eV, 0.40 eV, 0.88 eV, respectively, and one electron trap El with DLTS activation energy of 0.54 eV. The misfit dislocations in relaxed InGaAs/GaAs interface induce a hole trap level H4 with DLTS activation energy between the range of 0.67–0.73 eV. All dislocation induced traps are nonradiative recombination centers which greatly degrade the optical property of the InGaAs/GaAs layers.
Chemical Etching and Post-Annealing for High Performance CdZnTe Strip Detectors
Authors: Shi Z. Q. , Stahle C. M. , Shu P.
Published: April 2011

One of the critical issues in CdZnTe detector fabrication is the surface treatment. This will not only affect the electrical properties, such as leakage current, but also influence the physical properties, such as smoothness and adhesion between the metal and the semiconductor. The latter will determine the wire bonding yield. Historically, there has been a problem in achieving both low leakage current and excellent wire bonding yield. In this paper, we report our new approach to fabricate high performance doubled sided CdZnTe strip detectors. The new surface treatments involve chemical etching and post-annealing. The leakage current, interstrip resistance and energy resolution were studied as a function of different etchants/time and post-annealing temperature. It was found that a chemical etch with bromine in ethylene glycol (Br/EG) is suitable for the double sided strip detector process. Keeping a relatively smooth surface is critical for achieving a high yield of good strips. To improve the adhesion of the metal to CdZnTe for wire bonding, the detectors were annealed from 100 to 175°C for 10 hours. It has been observed that after annealing, not only has the strip leakage current decreased, but the interstrip resistance is increased for a temperature less than 150°C.
Portable P-I-N CdZnte Gamma-Ray Spectroscopy System
Authors: Sudharsanan R. , Stenstrom C. , Vakerlis G. , Pantazis J. , Huber A. , Redus R. , Jordanov V. T.
Published: April 2011

A portable Gamma-Ray Spectroscopy system has been designed and tested for identification of nuclear materials in the field. A P-I-N CdZnTe detector was developed to minimize the leakage current and to improve energy resolution. The detector has a sensitive volume of 200 mm3 and has been constructed with novel heterojunction contacts. Thermoelectric cooling is used to reduce leakage currents in the detector element and the input thermal noise to the FET. Risetime discrimination is used to offset the negative effects of incomplete charge collection. The detector probe is connected to a single unit comprised of detector electronics, an MCA and a portable computer, all operated from battery power. Results from testing with laboratory radioisotopic sources are presented.
Mapping Detector Response Over the Area of a CdZnTe-Multiple-Electrode Detector
Authors: Lingren C. L. , Apotovsky B. , Butler J. F. , Doty F. P. , Friesenhahn S. J. , Oganesyan A. , Pi B. , Zhao S.
Published: April 2011

Semiconductor multiple-electrode detectors have been developed for the purpose of reducing effects of hole trapping in room-temperature radiation detectors. Some reported geometries maintain a nearly-uniform electric field inside the detector, but others generate an electric field that is very non-uniform and highly-concentrated at the anode. This paper reports the results of mapping such a detector (having a non-uniform electric field) with a finely collimated gamma-ray beam to determine the detector response as a function of position.
A CdTe Position Sensitive Detector for a Hard X-and Gamma-Ray Wide Field Camera
Authors: Caroli E. , Bertuccio G. , Cesare G. De , Donati A. , Dusi W. , Landini G. , Perotti F. , Sampietro M. , Stephen J. B.
Published: April 2011

An important region of the electromagnetic spectrum for astrophysics is the hard X- and gamma ray band between 10 keV and a few MeV, where several processes occur in a wide variety of objects and with different spatial distribution and time scales. In order to fulfill the observational requirements in this energy range and taking into account the opportunities given by small/medium size missions (e.g. on the ISS), we have proposed a compact, wide field camera based on a thick (1 cm) position sensitive CdTe detector (PSD). The detector is made of an array of 128×96 CdTe microspectrometers with a pixel size of 2×2 mm2. The basic element of the PSD is the linear module that is an independent detection unit with 32 CdTe crystals and monolithic front-electronics (ASIC) supported by a thin (300 μm) ceramic layer. The expected performance of the PSD over the operative energy range and some of the required ASIC functionality are presented and discussed.
Neutron Detectors Made From Chemically Vapour Deposited Semiconductors
Authors: Foulon F. , Bergonzo P. , Brambilla A. , Jany C. , Guizard B. , Marshall R. D
Published: April 2011

In this paper, we present the results of investigations on the use of semiconductors deposited by chemical vapour deposition (CVD) for the fabrication of neutron detectors. For this purpose, 20 μm thick hydrogenated amorphous silicon (a-Si:H) pin diodes and 100 μm thick polycrystalline diamond resistive detectors were fabricated. The detectors were coupled to a neutron-charged particle converter : a layer of either gadolinium or boron (isotope 10 enriched) deposited by evaporation. We have demonstrated the capability of such neutron detectors to operate at neutron fluxes ranging from 101 to 106 neutrons/cm2.s. The fabrication of large area detectors for neutron counting or cartography through the use of multichannel reading circuits is discussed. The advantages of these detectors include the ability to produce large area detectors at low cost, radiation hardness (∼ 4 Mrad for a-Si:H and ∼ 100 Mrad for diamond), and for diamond, operation at temperatures up to 500°C. These properties enable the use of these devices for neutron detection in harsh environments. Thermal neutron detection efficiency up to 22 % and 3 % are expected by coupling a-Si:H diodes and diamond detectors to 3 μm thick gadolinium (isotope 157) and 2 μm thick boron layers, respectively.
Internal Field Distribution in Organic Light Emitting Diodes with Double Layer Structure
Authors: Hochfilzer C. , Jost T. , Niko A. , Graupner W. , Leising G. , Tang C. W. , Forsythe E. , Gao Y.
Published: April 2011

Double layer organic light emitting devices (OLED) are constructed by evaporating tris(8 -hydroxy) quinoline aluminum (Alq3) on a spin cast thin film of a methyl substituted ladder type poly -para -phenylene (m -LPPP). A thick layer of Mg:Ag is used as the cathode material. These organic materials are very suitable for application in OLEDs both, as transporting materials as well as active layers. Alq3 predominantly transports electrons while m - LPPP is a conjugated polymer having higher hole mobilities. Due to these transport properties the formation and radiative recombination of the excitons in ITO/m -LPPP/Alq3/Mg:Ag devices occur close to the m -LPPP/Alq3 interface. We compare the device performance of OLEDs with varying Alq3 layer thickness (0, 50, 150, 300, 500Å) and constant m -LPPP layer thickness (900Å). A difference in the device parameters and performance as a function of the Alq3 layer thickness is observed. We analyze these results with respect to the internal electric field distribution of the double layer devices derived from electroabsorption measurements.
Transient Electroluminescence from PPV under Strong Voltage Pulses
Authors: Savvate'ev V. , Pogreb R. , Davidov D. , Chayet H. , Neumann R. , Avny Y.
Published: April 2011

We have studied the transient electroluminescence (EL) from poly(phenylenevinylene), PPV, as a function of electric field under strong electric pulses up to fields of E≈ 109 V/m with emphasis on (a) the time delay, τ, between the electric pulse and the onset of the EL emission pulse and (b) the EL intensity as a function of the field. A monotonic decrease of τ with increasing E is explained by an increase of the carriers mobility according to Frenkel- Poole model. The EL intensity at high fields is proportional to E3 suggesting that the contacts at the polymer-metal electrode interfaces are practically ohmic. We demonstrate significantly improved brightness, peak power and lifetimes for polymer-based light emitting diodes working under such a pulsed mode.
Passive Polarization Rotators for Nonlinear Optical Devices
Authors: Kowalczyk T. C. , Liu L-Y , Lackritz H. S.
Published: April 2011

Initial experiments of a passive polymeric TE-to-TM polarization converter that utilizes laterally offset upper and lower poling electrodes to induce a 45° optical axis in the core of a nonlinear optical polymer waveguide are reported. By optimizing the device length, we have demonstrated polarization conversion efficiencies greater than 95% in devices as short as 240 um with negligible insertion loss.
Thermoreversible Gels of Polyaniline: Viscoelastic and Electrical Properties
Authors: Ikkala O. T. , Vikki T. , Ruokolainen J. , Hiekkataipale P. , Passiniemi P. , Mäkelä T. , Isotalo H.
Published: April 2011

We demonstrate that polyaniline (PANI) dissolved in dodecyl benzene sulphonic acid (DBSA) shows thermoreversible gelation. The dissolution has been performed in formic acid which allows particle-free complexes according to optical microscopy. Below the gelation temperature the materials are rubber-elastic in compression experiments, the storage modulus G' does not essentially depend on frequency, and the samples are electronically conductive. Above the gelation temperature, G' indicates flow-like behavior and drastically lower ionic conductivity is observed. These results suggest reversible, i.e. fusible, network formation. The properties are compared with gels consisting of camphor sulphonic acid (CSA) doped PANI dissolved in m-cresol which are poorly thermoreversible.
Electro-Optic Effects in Nanophase Polymer Dispersed Liquid-Crystal Systems
Authors: Blacker R. S. , Lewis K. L. , Mason I. , Sage I. , Webb K.
Published: April 2011

Research into electro-optic effects in nanophase polymer dispersed liquid crystal (PDLC) materials has highlighted their potential as materials for a new class of tuneable filters. The structures, based on UV cured phase separated composites, contain liquid crystal both as discrete nano-scale droplets, and as material dissolved in the polymeric host. The essential difference between these materials and more conventional PDLC's is the scale of the refractive index inhomogeneity which is considerably smaller than the wavelength of visible light. Based upon effective medium approximations, the composite thus acts as a single isotropic medium, whose average refractive index is dependant on the level of applied electric field. Tuneable filters have been fabricated using the composite material for use in the visible spectral band.
Functionalized Tetrathiafulvalene Derivatives and their Radical Cation Salts: Synthesis and X-Ray Crystal Structures
Authors: Bryce Martin R. , Moore Adrian J. , Batsanov Andrei S. , Chesney Antony , Wood Clare L. , Howard Judith A. K.
Published: April 2011

We report our investigations into tetrathiafulvalene (TTF) derivatives bearingN-methylthiocarbamoyl and halogen substituents which engage in intermolecular interactions in the solid state. The synthesis of new donors is presented, along with the X-ray crystal structures of some of these derivatives and their radical ion salts.