bluuebunny/crossref_metadata_2025 huggingface dataset:
Mobility and the Glass Transition in Mixed Polymer-Diluent Glasses by NMR
Authors: Jones Alan A. , Inglefield Paul T. , Liu Yu , Cauley Bonnie , Roy Ajoy K. , Kambour Roger
Published: March 2011

Phosphorous and proton line shape studies of a trioctyl phosphate diluent in polycarbonate and in a fifty-fifty blend of polystyrene with polyphenylene oxide have been performed to monitor diluent dynamics. Both the phosphorous and proton line shapes are superficially bimodal with a narrow, mobile component of the line shape appearing at temperatures well below the thermal glass transition temperatures. Plots of the h.ction of mobile material versus temperature show two step increases separated by a plateau region. This behavior is analyzed in terms of a distribution of activation energies which results in a bimodal description. The bimodal activation energy distribution corresponds to a bimodal correlation time distribution. The more mobile material is identified with diluent molecules in contact with other diluent molecules in a lattice description which assumes a single phase system is assumed with a purely random distribution of diluent and polymer on the lattice.
AntiPlasticization in the Glassy States of Poly(Vinyl Butyral) and Poly(2,6-Dimethyl Phenylene Oxide)
Authors: Parker Anthony A. , Shieh Yang T. , Ritchey William M.
Published: March 2011

Solid state NMR and dynamic mechanical analysis (DMA) techniques have been used to study the “antiplasticization” effects of dihexyladipate (DHA) and tricresylphosphate (TCP) on poly(vinyl butyral-co-vinyl alcohol) (PVB) and poly(2,6-dimethyl phenylene oxide) (PPO) respectively. The DHA and TCP diluents decrease the primary viscoelastic transition temperatures (ax processes) along with their apparent activation energies [1,2] However, glassy state mechanical transitions (P processes) are characterized by increased activation energies. Similarly, carbon T1, T1p, and inversion recovery cross polarization (IRCP) experiments generally show that DHA and TCP suppress motion in the MHz, KHz, and near static frequency regimes. These results collectively suggest that the extent of antiplasticization is dependent on both the frequency regime of observation, and the morphology of the polymers.
Morphology of Polyamideliquid Crystalline Polymer Blend
Authors: Nishii K. , Usui M. , Muraya T. , Kimura K.
Published: March 2011

Polymer blend technology is attractive from the standpoint of both science and industry, and many combinations have been studied. Recently, the polymer blends, including liquid crystalline polymer, have been especially worthy of notice, [1,2,3]. In order to obtain materials with a high mechanical strength and moldability for use in thin molded items, we chose polyamide (PA)-liquid crystalline polymer (LCP) blends. In this study, we first measured the mechanical properties, then studied the features of the polymer structure. We also examined the relationship between morphology and mechanical properties. As a result, we found that the mechanical properties of the blends depended largely on blend morphology, and that mechanical strength increased as blend compatibility increased. On the other hand, we also found that the blends showed compatible and microheterogeneous dispersion at less than 25 wt% LCP, while at more than 30 wt% LCP, blends tended to show twophase separation.
HgCdTe/CdTe Multiple Quantum Wells: Growth, Stability, and Optical Properties
Authors: Feldman R.D. , Austin R.F. , Cesar C.L. , Islam M.N. , Soccolich C.E. , Kim Y. , Ourmazd A.
Published: March 2011

We have grown HgCdTe/CdTe multiple quantum wells by molecular beam epitaxy which show room temperature photoluminescence and sharp absorption steps at mid-infrared wavelengths. Quantitative chemical mapping, performed by transmission electron microscopy, indicates minimal interdiffusion during growth. Annealing experiments performed at higher temperatures show that the interdiffusion coefficient is a strong function of the depth of the interface below the surface. Absorption spectra have been accurately modeled with a square well/envelope function approach. The films have been used to passively mode lock color center lasers and produce pulses as short as 120 fsec near 2.7 μm.
Heteroepitaxial HgCdTe/CdZnTe/GaAs/Si Materials for Infrared Focal Plane Arrays
Authors: Johnson S.M. , James J.B. , Ahlgren W.L. , Hamilton W.J. , Ray M. , TOMPA G.S.
Published: March 2011

The structural properties of LPE-grown HgCdTe on heteroepitaxial MOCVD-grown CdZnTe/GaAs/Si substrates were evaluated using high-resolution x-ray diffraction techniques and TEM. Large tilts {up to 4°} between CdZnTe layers and GaAs/Si substrates are a general characteristic of this heteroepitaxial system and are are attributed to the interaction of closely spaced misfit dislocations that arrange to form a tilt boundary. Either {112}CdTe or {552}CdTe can be grown on {112}GaAs/Si; the {552} was shown to result from a first-order twinning operation of {112}. Lamnella {111} microtwins in {111}CdZnTe/{100}GaAs/Si substrates, measured by x-ray techniques, are not readily propagated into the LPE-grown HgCdTe layer. The x-ray FWHM of the LPE HgCdTe is typically at least a factor of two lower than that of the Si-based substrate from annealing and due to the increased thickness of the layer; both mechanisms promote dislocation interaction and annihilation. High performance MWIR and LWIR HgCdTe 128×128 hybrid focal plane arrays were fabricated on these Si-based substrates. An array average of ROAj = 17.8 ohmcm2 for a cutoff wavelength of 10.8 μm at 78K was demonstrated.
New Class of the Semiconductor Materials for Infrared Photodetection
Authors: Akimov Boris A. , Brandt Nikolai B. , Chesnokov Sergei N. , Khokhlov Dmitriy R.
Published: March 2011

We Present the new class of infrared Photodetectors based on the lead-tin tellurides doped with group III impurities. The Persistent photoconductivity effect appearing in these materials provides the opportunity of internal signal integration resulting in the considerable increase in signalto- noise ratio. The integration characteristic time may be changed by means of the operating temperature or alloy composition variation. Even if the integration time is higher than the operation time required there exists an opportunity to quench quickly (∼10−5s) the Persistent photoconductivity. In some regime of quenching the effect of giant quantum efficiency stimulation has been observed. Both the bulk crystal and thin film technologies of the Photodetector Production are developed.
HgTe-CdTe Superlaltice and HgCdTe Epilayer Device Structures Grown by Photon-Assisted Molecular Beam Epitaxy
Authors: Myers T.H. , Yanka R.W. , Mohnkern L.M. , Harris K.A. , Dietz D.W. , Dudoff G.K. , Girouard K.M. , Wang S.C.H.
Published: March 2011

HgCdTe grown by photon-assisted molecular beam epitaxy is now suitable for use in high performance detector fabrication. These are the preliminary results for infrared detectors which have been fabricated in HgCdTe grown using this technique at GE. The detectors were fabricated using a modified Hg-diffused diode process. In addition, the first high quantum efficiency infrared detectors based on the HgTe-CdTe superlattice material system is presented as an example of the sophisticated structures obtainable with photon-assisted molecular beam epitaxy. The superlattice detectors exhibited quantum efficiencies as large as 66% (at 140K) at the peak wavelength of 4.9μm and an average quantum efficiency over the 3–5μm waveband of 55%.
The Preparation of InAsSb/InSb SLS and InSb Photodiodes by MOCVD
Authors: Biefeld R. M. , Cunningham B. T. , Kurtz S. R. , Wendt J. R.
Published: March 2011

Infrared absorption and photoluminescence have been demonstrated for InAs1-xSbx/InSb strained-layer superlattices (SLS's) in the 8-15 μm region for As content less than 20%. This extended infrared activity is due to the type II heterojunction band offset in these SLS's. The preparation of the first MOCVD grown p-n junction diode was achieved by using dimethyltellurium as an n-type dopant. Several factors, such as background doping and dopant profiles affect the performance of this device. InSb diodes have been prepared using tetraethyltin. The resulting current-voltage characteristics are improved over those of diodes grown previously using dimethyltellurium. Doping levels of 8x1015 to 5x1018cm−3 and mobilities of 6.7x104 to 1.1x104 cm2/Vs have been measured for Sn doped InSb. SLS diode structures have been prepared using Sn and Cd as the dopants. Structures prepared with p-type buffer layers are more reproducible.
Semiconductor Devices and Materials for Optical Communication at 2-4 μM Wavelengths Range
Authors: Fuxi Gan , Hailong Wang
Published: March 2011

In this paper we reported most attractive semiconductor materials for optical sources and detectors with wavelength In the range of 2-4 μm, such as GaInAsSb/GaAlAsSb, InAsPSb/InAs heterostructure and PbCdSSe HgCdTe.The Lasers and detectors have been made using these materials. The performance of the devices were discussed.
MBE Growth of GaInAsSb/A1GaAsSb Double Heterostructures for Diode Lasers Emitting Beyond 2 μm
Authors: Eglash S.J. , Choi H.K. , Turner G.W. , Finn M.C.
Published: March 2011

Molecular beam epitaxy has been used to grow GaInAsSb/AlGaAsSb double heterostructures, lattice matched to GaSb substrates, for diode laser emission at 2.3 μm. Doublecrystal x-ray diffraction measurements were used to determine alloy lattice constants, and photoluminescence and infrared absorption spectroscopies were used to determine the bandgaps of the GaInAsSb layers. Alloy compositions measured by Auger electron spectroscopy were consistent with measured lattice constants and bandgaps. Diode lasers fabricated from the double heterostructures were operated in the pulsed mode at room temperature with threshold current densities as low as 1.5 kA cm−2, differential quantum efficiencies as high as 50 percent, and output power as high as 900 mW per facet.
Low Temperature p- and n- Type Doping of InSb Grown on GaAs Using Molecular Beam Epitaxy.
Authors: Thompson Phillip E. , Davis John L. , Simons David S.
Published: March 2011

30 nm thick layers of Be-doped InSb were grown by MBE, sandwiched between undoped layers of InSb on semi-insulating GaAs substrates. Using secondary ion mass spectroscopy (SIMS), it has been demonstrated that the p-type dopant, Be, undergoes an anomalous migration towards the top surface for substrate growth temperatures in excess of 340°C. A thermal diffusion process can be eliminated as an explanation since there was not an associated diffusion towards the substrate. At a substrate growth temperature of 340°C, the Be-doped layer had backside and frontside doping concentration gradients of 0.067 and 0.147 decade/nm. respectively. A sample grown at 420°C had a comparable backside doping concentration gradient, but the frontside gradient was essentially zero, i. e., uniform doping tothe surface. Electrical activation of the Be dopant, when grown at 340°C, compared to activation in GaAs grown at 580°C. was 38% and 47% for growth rates of 1.0 and 0.5 μm/h. While the Si dopant does not undergo a migration comparable to that of Be, it has been demonstrated that n-type doping of InSb with Si must be done at growth temperatures less than or equal to 340°C to obtain high carrier concentrations. The maximum n-type carrier concentration obtained was 6.2x1018/cm3, for a 0.5 μm/h growth rate at 340°C.
Dry Etching Techniques and Chemistries for III-V Semiconductors
Authors: Pearton S. J.
Published: March 2011

Dry etching of HI-V materials using both Cl-based (CCl2F2, SiCl4, BCl3, Cl2) and CH4/H2 discharges will be reviewed. The etch rates using chlorine-based mixtures are generally faster than those utilizing CH4/H2, but the latter gives smoother surface morphologies for In-containing compounds. The use of microwave (2.45 GHz) electron cyclotron resonance (ECR) discharges minimizes the depth of lattice disorder resulting from dry etching, relative to conventional RF (13.56 MHz) discharges. Recent results on the systematics of ECR plasma etching of both In- and Ga-based III-V semiconductors using CCl2F2/O2 and CH4/H2 mixtures will be discussed, including the determination of the maximum self-biases allowable which do not induce near-surface damage to the semiconductor. A further key issue is the prevention of changes in the surface stoichiometry of materials such as InP, where the lattice constituents may have considerably different volatilities in the particular discharge.
Organometallic Epitaxy of Extrinsic N-Type HgCdTe Using Trimethylindium
Authors: Taskar N.R. , Parat K.K. , Bhat I.B. , Chandhi S.K.
Published: March 2011

Indium doping of mercury cadmium telluride, grown by organometallic epitaxy, has been accomplished using trimethylindium (TMIn) as the dopant source. Layers, grown by the Direct Alloy Growth (DAG) process, exhibited a linear doping vs TMIn partial pressure characteristic over the 5 × 1016 to 3 × 1018 cm−3 range. A maximum doping concentration of 5 × 1018 cm−3 was obtained in these layers.

The optical band edge in these layers was observed to move to higher energy with increased doping. It is shown that this is caused by the Burstein-Moss shift of the bandedge with doping, and that the Cd fraction is independent of the doping concentration.
Hg1-xCdxTe: Defect Structure Overview
Authors: Berding M.A. , Sher A. , Chen A.-B.
Published: March 2011

Native point defects play an important role in HgCdTe. Here we discuss some of the relevant mass action equations, and use recently calculated defect formation energies to discuss relative defect concentrations. In agreement with experiment, the Hg vacancy is found to be the dominant native defect to accommodate excess tellurium. Preliminary estimates find the Hg antisite and the Hg interstitial to be of comparable densities. Our calculated defect formation energies are also consistent with measured diffusion activation energies, assuming the interstitial and vacancy migration energies are small.
Materials Issues and Device-Degradation in the Ingaas(P)/Inp System
Authors: Ueda O.
Published: March 2011

This paper reviews the current status of material issues in the InGaAs(P)/InP system and our understanding of degradation in InGaAsP/inP double-heterostructure lasers and LED's.

Among the materials issues for this system are the generation of defects and the thermal stability of the material. Crystal growth-induced defects can be classified into interface and bulk defects.; the former group includes misfit dislocations and inclusions and the latter structural and non-structural precipitates. Thermal stability can lead to structural imperfections stuch as quasi-periodic modulated structure due to spinodal decomposition of the crystal either at the liquid/solid interface or growth surface, and atomic ordering which also occurs on the growth surface through migration and reconstruction of deposited atoms. Diffusion processes in InP often lead to the generation of microdefects in this material. Finally, in metallization procedures, alloy reactions between the semiconductor and electrode occasionally take place forming non-planar/non-uniform interfaces.

Three major degradation modes, rapid degradation, gradual degradation, and catastrophic failure in InGaAsP/InP double-heterostructure lasers and LED's, are discussed. For rapid degradation, recombination-enhanced dislocation climb and glide, which are responsible for degradation, do not occur easily InGaAsP/JnP system. Differences in the ease with which these phenomena occur in InGaAsP/InP and other systems are presented. Based on the results, dominant parameters involved in the phenomena are discussed. Gradual degradation takes place presumably due to recombination enhanced point defect reaction in GaAlAs/GaAs-based optical devices. However, we do not observe this mode in InGaAsP/InP-based optical devices. Catastrophic failure due to catastrophic optical damage at a mirror or at a defect in GaAlAs/GaAs DH lasers, is not found in InGaAsP/lnP DH lasers. Degradation of InGaAsP/InP double-heterostructure LED's with large current application is also presented. These results indicate that InGaAsP/InP optical devices ensure very high reliability in longwavelength communication systems.
Photoreflectance and Double Crystal X-Ray Study of Strained InGaAsP Layers on InP Substrates
Authors: Flemish J. R. , Shen H. , Dutta M. , Jones K.A. , Ban V.S.
Published: March 2011

Determining the composition of quaternary epitaxial films requires accurate measurements of both the lattice parameter and the bandgap energy. Complications arise in lattice-mismatched material, because the mismatch produces tetragonal distortion of the epi-layer and splitting of the valence band energies in a manner which depends on the film composition. We present studies on strained InGaAsP grown on (100) InP. Using room temperature photoreflectance (PR) we observe shifting of the band gap and splitting of the valence band energies, and using the (115) and (004) reflections from double crystal x-ray diffraction (DXRD) we determine the values of the parallel and perpendicular lattice constants. By combining the lattice parameter measurements with band splitting data, we accurately determine the quaternary composition from a self-consistent model using an iterative procedure. By linear interpolation of the elastic-stiffness constants, C11 and C12. as well as the shear and hydrostatic deformation potentials for the four binary compounds in the InGaAsP system, we relate the state of biaxial stress to the induced shifts in the valence band energies.
Low Temperature Infrared Measurements and Photo-Induced Persistent Changes of Intersubband Transitions in GaAs/AlGaAs Multiple Quantum Wells.
Authors: Dischler B. , Ralston J.D. , Koidl P. , Hiesinger P. , Ramsteiner M. , Maier M.
Published: March 2011

Utilizing infrared absorption and Hall measurements, a detailed study is presented of the temperature dependence (10 K < T < 300 K) of the intersubband transition and related doping behaviour in GaAs/Al0.32Ga0.68As multiple quantum well structures with Si doping concentrations between 1×1018 and 8×1018 cm−3 in the quantum wells. The intersubband transition frequency increases with decreasing temperature, which can be adequately modelled by considering the different effective masses and the thermal populations of the two electron subbands. This model also accounts for the decrease in linewidth of the infrared absorption resonance upon cooling. In Hall measurements a persistent photo-induced decrease in the free electron concentration within the GaAs quantum wells is observed for Si doping levels exceeding 5×1018 cm−3. Corresponding decreases are observed both in the integrated absorption and the absolute frequency of the intersubband transition, the latter arising from a reduction in the depolarization shift. The spectral dependence of the photo-induced changes over the energy range 1.2 eV < hν < 2.5 eV, along with thermal regeneration over the temperature range 70 K < T < 150 K, have been studied.
Properties of Graphite Interconnect Circuit Boards with Anisotropic Thermal Expansion
Authors: Malamas J. , Bambha R.P. , Ramsey J.B. , Garrett W.C. , Kelso E.G. , Hahn T.A.
Published: March 2011

We report the investigation of an interconnect circuit board (ICB) with anisotropic thermal expansion for use with bump bonded, indirect hybrid, scanning focal plane arrays. This ICB is designed to reduce significantly the thermal stresses on the indium bump bonds during thermal cycling. Highly oriented pyrolitic graphite (HOPG) was chosen because its anisotropic thermal expansion meets the criteria for forming an indirect hybrid ICB using silicon processor circuits and mecury cadmium telluride detectors. Properties of HOPG influencing its performance as an ICB have been investigated including thermal expansion, electrical conductivity, durability, and adherence of electrically insulating thin films.
Long-Wavelength Germanium Photodetectors by Ion Implantation
Authors: Wu I.C. , Beeman J.W. , Luke P.N. , Hansen W.L. , Haller E.E.
Published: March 2011

Extrinsic far-infrared photoconductivity in thin high-purity germanium wafers implanted with multiple-energy boron ions has been investigated. Initial results from Fourier transform spectrometer(FTS) measurements have demonstrated that photodetectors fabricated from this material have an extended longwavelength threshold near 192μm. Due to the high-purity substrate, the ability to block the hopping conduction in the implanted IR-active layer yields dark currents of less than 100 electrons/sec at temperatures below 1.3K under an operating bias of up to 70mV. Optimum peak responsivity and noise equivalent power(NEP) for these sensitive detectors are 0.9A/W and 5×10−16 W/Hz1/2 at 99μm, respectively. The dependence of the performance of devices on the residual donor concentration in the implanted layer will be discussed.
II-VI / III-V Strained-Layer Superlattice Materials and Devices
Authors: Fuxi Gan , Hailong Wang , Jie Cui
Published: March 2011

The optical and spectroscopy properties of Il-VI wide gap semiconductor SLSs have been studied. We report the first observation of the pulse compression in ZnS-ZnSe MQW and the folded LA phonon modes in ZnS-ZnSe SLS;The confined LOn phonon modes in ZnSe-ZnTe SLS are observed and the critical thickness of ZnSe-ZnTe SLS are determined,for the first time, by Raman scattering.
Reflection High-Energy-Electron Diffraction Study of Inp and InAs (100) In Gas-Source Molecular Beam Epitaxy
Authors: Chin T. P. , Liang B. W. , Hou H. Q. , Tu C. W.
Published: March 2011

InP and InAs (100) were grown by gas-source molecular-beam epitaxy (GSMBE) with arsine, phosphine, and elemental indium. Reflection high-energy-electron diffraction (RHEED) was used to monitor surface reconstructions and growth rates. (2×4) to (2×1) transition was observed on InP (100) as phosphine flow rate increased. (4×2) and (2×4) patterns were observed for In-stabilized and As-stabilized InAs surfaces, respectively. Both group-V and group-rn-induced RHEED oscillations were observed. The group-V surface desorption activation energy were measured to be 0.61 eV for InP and 0.19 eV for InAs. By this growth rate study, we are able to establish a precise control of V/HII atomic ratios in GSMBE of InP and InAs.
Infra-Red Photodiodes in Hg1-xCdxTe Grown by OMVPE
Authors: Parat K.K. , Taskar N.R. , Ehsani H. , Bhat I.B. , Ghandri S.K.
Published: March 2011

Hg1-xCdxTe layers, grown by the organometallic vapor phase epitaxy (OMVPE), are p-type with carrier concentrations around 4 × 1016/cm3 due to the Group II vacancies in them. Following a Hg saturated anneal at 220°C, these layers become n-type with carrier concentrations around 4 × 1014/cm3. In order to fabricate p-n junction diodes, Hg1-xCdxTe layers were grown with a 0.5–0.8 μm thick CdTe cap. By opening windows in this CdTe cap, the underlying Hg1-xCdxTe Te layer was annealed in a selective manner, thus forming planar p-n junctions. The CdTe cap, which is used as the diffusion barrier for Hg during the selective anneal, also served as the junction passivant for the photodiodes. Details of device fabrication and characterization are presented in this paper.
Cone-Beam 3-D Reconstruction with Double Circular Trajectory
Authors: Rizo Ph. , Grangeat P. , Sire P. , Lemasson P. , Delageniere S.
Published: March 2011

In X-ray cone-beam tomography, the only planar source trajectory that does not produce incomplete data is the infinite line. Such a source trajectory is not experimentally possible. To ensure complete data acquisition with cone-beam radiographs, a set of nonplanar trajectories has been studied. Among the trajectories proposed in the literature, a simple one is a set of two circular trajectories with intersection of the two trajectory axes. The angle between the two axes is related to the maximum aperture of the cone beam. We propose here an exact method for performing this reconstruction using the 3-D Radon transform of the object. The modulation transfer function of this algorithm remains identical to that for the central slice of reconstruction in a single circular trajectory. The relative mean square error for density stays within 2% for an aperture of ±30°. With a single circular trajectory, the relative mean square error may reach 20% at the same aperture. With a double circular trajectory, horizontal artifacts are nearly suppressed.
Ultrasonic Computed Tomography for Imaging Bones and Adjoining Soft Tissues
Authors: Sehgal C. M. , Greenleaf J. F.
Published: March 2011

The skeletal system of bones and cartilage forms a framework that supports and protects soft tissues. It also provides surfaces to which muscles, ligaments and tendons attach and coordinate movement of bones. Since such musculoskeletal system serves as a “structural” element of the body it is easy to see that its functional capabilities are closely related to it mechanical strength. Consequently, there have been numerous attempts to characterize the properties of bones. Currently, several sophisticated diagnostic procedures and radiographic imaging techniques are available for quantitative purposes. Virtually all the available methods are based on the measurement of mineral content of the bone. It is well known that it is the combination of the organic and the inorganic components that determine the strength of the bones. Thus, in principle, the traditional methods can provide only part of the information about the bone tissues. With this shortcoming in view an effective case can be made for the development of a technique that measures both the components. Many researchers have looked at ultrasonic energy to fulfill this role. The rationale for the choice of this energy is that ultrasound is a “mechanical radiation” and its propagating properties provide a direct measure of mechanical strength or related property of the medium.
Three Dimensional (3D) Electron Paramagnetic Resonance Imaging Technique For Mapping Porosity in Ceramics
Authors: Kordas G. , Kang Y.H.
Published: March 2011

A three dimensional (3D) Electron Paramagnetic Resonance Imaging (EPRI) method has been developed to probe the structure and size of pores in ceramic materials. A computer control current source and magnetic field gradient coil assembly were added on the conventional EPR instrument as imaging devices. This added-on facility was tested using a phantom sample having cavities filled with DPPH (2,2-diphenyl-1-picrylhydrazyl) particles. Pumice was then used to demonstrate the feasibility of the technique. Porous pumice stone was immersed in a 0.5mM 15N-PDT-water solution to introduce the spin labels into the open volume of the sample. A two-dimensional image was reconstructed from a set of 1-D projections using a filtered back-projection technique. A three-dimensional image was derived from 22 2-D images each constructed by 22 1-D projections. At present, the facility allows a resolution of 69 and 46 μm for 2-D and 3-D imaging.
Fast Radial NMR Imaging of Transport Processes
Authors: Majors Paul D. , Smith Douglas M. , Caprihan Arvind
Published: March 2011

Fast NMR imaging techniques for the study of objects with circular or spherical symmetry are introduced. Quantitative, radially-resolved information for an object with circular symmetry is obtained by Abel inversion of a single one-dimensional (1D) NMR image or equivalently by Hankel transformation of the 1D time domain NMR signal. With adequate sensitivity, the entire image information is obtained in a single experimental iteration, providing snapshot temporal resolution. Thus these techniques are useful for the study of transport phenomena. Spherical objects can also be radially resolved from a 1D projection image by applying two sequential Abel inversions, or in one simple differentiation step. These radial imaging techniques are easy to implement and the computational and data storage requirements are nominal. These procedures are also applicable to microscopic and solids NMR imaging, as well as to X-ray CAT and other projective imaging techniques. Noise effects, and the detection and handling of distortions from circular symmetry are also considered.
Nondestructive Imaging of Materials Microstructures Using X-Ray Tomographic Microscopy
Authors: Kinney J.H. , Nichols M.C. , Bonse U. , Stock S.R. , Breunig T.M. , Guvenilir A. , Saroyan R.A.
Published: March 2011

A technique for nondestructively imaging microstructures of materials in situ, especially a technique capable of delineating the time evolution of chemical changes or damage, will greatly benefit studies of materials processing and failure. X-ray tomographic microscopy (XTM) is a high resolution, three-dimensional inspection method which is capable of imaging composite materials microstructures with a resolution of a few micrometers. Because XTM is nondestructive, it will be possible to examine materials under load or during processing, and obtain three-dimensional images of fiber positions, microcracks, and pores. This will allow direct imaging of microstructural evolution, and will provide time-dependent data for comparison to fracture mechanics and processing models.
Improved Light Soaking Stability of R.F. Sputter Deposited Amorphous Silicon
Authors: Wynveen A. , Fan J. , Kakalios J. , Shinar J.
Published: March 2011

Studies of r.f. sputter deposited hydrogenated amorphous silicon (a-Si:H) find that the light induced decrease in the dark conductivity and photoconductivity (the Staebler-Wronski effect) is reduced when the r.f. power used during deposition is increased. The slower Staebler-Wronski effect is not due to an increase in the initial defect density in the high r.f. power samples, but may result from either the lower hydrogen content or the smaller optical gap found in these films.
Thermal Annealing of Light-Induced K Centers in Hydrogenated Amorphous Silicon Nitride
Authors: Tober E. D. , Sigari E. , Kanicki J. , Crowder M. S.
Published: March 2011

The thermally-induced decay of light-induced, paramagnetic neutral silicon dan-gling bonds(K centers ) in hydrogenated amorphous silicon nitride thin films is monitored using electron spin resonance. The nitride films are of gate-quality and nitrogen-rich and are deposited at two different temperatures (250 and 400 °C). The kinetics for isothermal annealing of the light-induced K° states is dependent upon sample deposition temperature and is observed to follow a stretched exponential dependence, exp {— (t/τ)β}, upon annealing time (t). The stretched exponential factor, β, shows a non-linear dependence upon annealing temperature including temperature independent regimes. Thermal annealing is thermally activated with an apparent activation energy of ∼ 0.4 eV and is independent of deposition temperature. These results indicate that annealing is a dispersive process which involves hopping and multiple trapping or trap controlled hopping in the thermal annealing of light induced K centers in amorphous SiN1.6:H.
Radiation Detection with Evaporated Csl(TI) Coupled to a-Si:H Photodiode Layers
Authors: Fujieda I. , Cho G. , Drewery J. , Gee T. , Jing T. , Kaplan S. N. , Perez-Mendez V. , Wildermuth D.
Published: March 2011

Csl(TI) layers 100–1000 μm thick were evaporated on glass substrates from a crystal Csl(TI). When they were exposed to calibrated X-ray pulses, their scintillation properties were found to be comparable to those of a crystal Csl(TI). Single p particles from radioisotopes were successfully detected by these layers coupled to a crystalline Si photodiode. The light spread inside evaporated Csl(TI) was measured by an amorphous Si (a-Si:H) photodiode array coupled to evaporated Csl(TI) layers. Monolithic X-ray detectors were fabricated by evaporating Csl(TI) on a-Si:H photodiodes directly. The signal yield and noise of this prototype were 1.5×10+4 electrons/MeV and 3×10+4 electrons FWHM, respectively. Larger signal size and lower noise are expected by optimizing the photodiode design.
Ferroelectric Liquid Crystal Light Valve Using SiO2 / A-Si:H Photodiode
Authors: Horikawa T. , Tahata S. , Kaho S. , Masumi T. , Mikami N. , Takahashi K. , Nunoshita M. , Nakajima H. , Nishi K.
Published: March 2011

A new type of ferroelectric liquid crystal light valve (FLCLV) is presented. The design of the FLCLV is based upon the linear equivalent circuit analyses. A photosensor in the FLCLV consists of a metal-insulator-semiconductor (MIS) photodiode. A-Si:H doped with boron and nitrogen [a-Si:(:N:B)] is used in the MIS diode. The a-Si:H(:B:N) film has a dark-conductivity of less than 1×1012 S/cm and a high photosensitivity.

Consequently, the writing characteristics of the FLCLV for a two dimensional (2D) image are evaluated. Using writing light of 630 nm and 1 mW/cm2, a high resolution capability of 120∼140 1p/mm is obtained.
Influence of Deposition Conditions on the optical and Electronic Properties of a-Ge:H
Authors: Paul W. , Jones S. J. , Marques F. C. , Pang D. , Turner W. A. , Wetsel A. E. , Wickboldt P. , Chen J. H.
Published: March 2011

We have measured the optical absorption edge spectra for absorption coefficients between 1 and 105 cm-1, the photoluminescence spectra at 77 K, the conductivity-temperature relations, and the photoconductivity magnitude and spectral dependence for a series of r.f. glow discharge films of a-Ge:H. The films were deposited at the powered cathode and unpowered anode of a diode capacitive reactor having very different electric fields and plasma conditions, while substrate temperature, H2 dilution of a GeH4 plasma and applied r.f. power were varied. The structure of the films are radically different, with the anode-deposited films displaying a microstructure of low density material (voids) and the cathode-deposited films displaying homogeneity similar to that of device-quality a-Si:H. A self consistent explanation of the differences in measured optical and electronic properties is given, taking full account of the structural observations. From this analysis the conditions required for the production of a-Ge:H of good photoelectronic quality may be inferred. Preliminary structural, optical and photoelectronic data for a-Si1.x Gex :H of large x, prepared under conditions extrapolated from the a-Ge study, indicate significant improvements from current data in the literature.
1 / f Noise in Hydrogenated Amorphous Silicon
Authors: Parman C. , Kakalios J.
Published: March 2011

Measurements of co-planar current fluctuations in n-type doped hydrogenated amorphous silicon (a-Si:H) find that the spectral density of the noise accurately obeys a 1/f frequency dependence over the frequency range of 1 Hz to 1 kHz for temperatures ranging from room temperature to 450K. The noise displays a power law dependence on the d.c. curent passing through the sample, with a temperature dependent power law exponent. In addition, the resistance of the a-Si:H as a function of time displays switching phenomena; a surprising result given the effective volume ( ∼10-6 cm3) of the sample.
The Effect of Hydrogen Dilution on the Deposition of Sige Alloys and the Device Stability
Authors: Yang L. , Chen L. , Catalano A.
Published: March 2011

We have studied the structural and transport properties of a-Si1−x Gex:H alloys made by DC glow discharge of SiH4 and GeH4 with substantial amount of H2 dilution. The dilution ratio, i.e. H2/(SiH4 + GeH4), was varied up to 40:1. IR spectroscopy revealed that H2 dilution greatly suppresses the tendency of forming microvoids due to Ge incorporation. Photoconductivity and ambipolar diffusion length, which represent transport properties of electrons and holes respectively, are also significantly improved by using H2 dilution. At a dilution ratio of 20:1, the electron υτ product reached 10-6 cm2/V at a bandgap 1.55 eV which is even larger than that of a device quality a-Si:H. Solar cells using alloy i-layers made with H2 dilution were fabricated. The stability of the devices under light exposure was found to be much improved with increasing H2 dilution, suggesting a strong correlation between the stability and the density of microvoids.
Temperature Dependence of the Growth and Saturation of Light-Induced Defects in a-Si:H
Authors: Isomura M. , Hata N. , Wagner S.
Published: March 2011

We report new experimental data on the light-soaking of a-Si:H with a Kr-ion laser at an optical generation rate G of at least 4×1021 to 3×1022 cm-3s1. We studied the temperature and intensity dependence of the saturation of the defect density and found that the saturation value of light-induced defects (Nsat) is insensitive to temperature and light intensity below about 90°C. Above 90°C Nsat drops with increasing temperature. This behavior can be explained within the defect pool model by a limited number of defect sites coupled with an annealing process.
Amorphous Silicon Thin-Film Transistors Modified by Doping and Plasma Treatment of the Nitride
Authors: Nickel Norbert , Saleh Rosari , Fuhs Walther , Mell Helmut
Published: March 2011

Thin—film transistors (TFTs) were prepared by the glow—discharge deposition of amorphous silicon nitride (a—SiNx:H) and amorphous silicon (a—Si:H). The properties of these TFTs were varied in two ways: a) doping of the amorphous silicon film with phosphine or diborane and b) exposure of the a—SiNx:H film to an oxygen plasma prior to the deposition of the a—Si:H layer. The TFTs are characterized by measurements of the transfer characteristic, ISD(VG) and of the effective density of interface states, Ni(E), using a transient current spectroscopy (TCS). The dependence of Ni(E) on the Fermi—level position in the a—Si:H film suggests that for EC—EF > 0.6eV this quantity is mainly determined by interface related defect states whereas for Ec,—EF <0.6eV it is determined by doping—induced defect states. The exposure to the oxygen plasma results in a reduction of Ni in both the upper and lower half of the gap and in an improvement of the characteristic, in particular in p—channel TFTs. These changes are discussed in terms of the chemical-equilibrium or defect—pool concept.
The Application of Hydrogenation to Amorphous Silicon Thin Film Transistors for the Decrease of the off Current
Authors: Kobayashi K. , Murai H. , Hayama M. , Yamazaki T.
Published: March 2011

The influence of hydrogenation on OFF current of TFTs with a bottom gate staggered structure has been investigated. The hydrogenation is done by exposing the surface of the a-Si:H channel layer to H2 plasma. The hydrogenation decreases the OFF current by more than one order of magnitude. The decrease in the OFF current is attributed to the increase in the density of states at the interface between the a-Si:H channel layer and the SiN passivating layer.
Reduction of Threshold Voltage Shift by Bias Annealing of Hydrogenated Amorphous Silicon Thin Film Transistors
Authors: Deane S. C. , Milne W. I. , Powell M. J.
Published: March 2011

We show that bias annealing modifies the rate of threshold voltage shift under bias stress in the regime where state creation dominates. Previously we have shown that the deep state distribution near the interface of a-Si:H TFTs can be altered by bias annealing. In particular the energy position of the defects can be made high or low in the energy gap, consistent with the defect pool model.

In this paper, we relate the kinetics of new state creation at room temperature to a particular equilibrated defect distribution. In particular we show that the rate of state creation can be reduced by prior bias annealing.
Amorphous Silicon Thin—Film Transistors with Non—Uniform Distribution of Gap States
Authors: Saleh Rosari , Nickel Norbert , Fuhs Walther , Mell Helmut
Published: March 2011

A theoretical study of the properties of a-Si:H thin-film transistors (TFTs) has been performed using uniform and non-uniform distributions of gap states in the a-Si:H layer. The potential profile in the semiconductor is calculated in a straightforward way by solving Poisson's equation numerically. From the band bending the source-drain current ISD(VG) of the TFT is predicted. In addition we calculate the transient discharge current I(t) generated by switching the gate voltage from positive to negative values. In experimental studies such transients are analyzed to obtain an effective density of interface states, Ni(E). We find that N;(E) depends little on the spatial variation of gap states, N(E,x), but sensitively on the energy distribution of N(E,x). It is shown that experimental N;(E) curves and ISD(VG) characteristics from the same sample can be fitted by the same N(E,x) which however cannot be unequivocally determined.
The Study of the Optoelectronic Properties of a-SiGe:H Photodiodes
Authors: Kuo L. C. , Lee C. C. , Chen K. H. , Fang Y. K. , Yu C. H.
Published: March 2011

We have studied the spectrum of various types of a-SiGe:H alloys for pin and Schottky barrier photodlodes, in which the band gaps of the a-SiGe:H vary between 1.75 eV and 1.35 eV. It has been found that the spectral re-ponse of the Schottky barrier diode shifts significantly to longer wavelength and the quantum efficiency decreases with an increase in i layer thickness. However, for the pin diode, an increase in the i layer thickness can hardly shift the spectrum to longer wavelength. For both pin and Schottky barrier diodes, the quantum efficiency can be increased by increasing the reverse bias. Therefore, an enhanced spectrum with a maximum at 800nm and a tail to lun can be achieved for a reverse-biased a-SiGe:H Schottky barrier diode. The results indicate that a-SiGe:H has a great potential for a low cost infrared photodetector.
Novel Application of Amorphous Silicon in Electrostatic Loudspeakers
Authors: Schropp R. E. I. , Smits M. , Meiling H. , van Sark W.G.J.H.M. , Boone M. M. , Bezemer J. , van der Weg W. F.
Published: March 2011

A thin film of amorphous silicon (a-Si:H) has been developed for application as the charge retention layer in electrostatic loudspeakers. The vibrating diaphragm in this type of loudspeakers is usually coated with a thin film in order to assure homogeneous charge distribution across the surface. This thin film should be capable of maintaining a high surface charge density and possess a high lateral resistivity to prevent charge displacement. Furthermore, it should be mechanically stable, capable of accomodating mechanical deformation, and resistant against humidity. Conventionally, a graphite layer is used in these applications. However, the conventional layers are frequently unstable and suffer from charge displacement effects eventually leading to electric breakdown. Further requirements are that the film can be deposited continuously and homogeneously over large areas in excess of 1 m2 and that the deposition technique is compatible with the properties of the thin membrane. We investigated small-area prototypes of electrostatic loudspeakers with a-Si:H thin films deposited on polyimide substrates in the Utrecht deposition system, ASTER. Plasma-deposited amorphous silicon films fabricated under certain conditions are shown to meet all of the above requirements.
Light-Response Characteristics of Amorphous Silicon Arrays for Megavoltage and Diagnostic Imaging
Authors: Antonuk L. E. , Yorkston J. , Kim C. W. , Huang W. , Morton E. J. , Longo M. J. , Street R. A.
Published: March 2011

Light-sensitive hydrogenated amorphous silicon pixel arrays are now under development for real-time megavoltage and diagnostic fluoroscopic imaging. Such applications place stringent demands upon a variety of array properties including the uniformity of the light-response function of the pixels. It is desirable that the design and fabrication of these imaging arrays maximize such uniformity. The implications of uniformity for imaging are reviewed, and data obtained from small arrays are presented and discussed.
Defect Structure and Network Disorder in Boron Doped Amorphous Silicon
Authors: Schubert M. B. , Schumm G. , Lotter E. , Eberhardt K. , Bauer G. H.
Published: March 2011

A series of boron doped a-Si:H films have been characterized by PDS, FTIR, Raman, and SIMS in order to evaluate the effects of boron incorporation on structural properties and hydrogen bonding. Doping by B2H6 or B(CH3)3 does not significantly enhance the overall disorder of the silicon network showing up in the TO-like Raman halfwidth whereas remarkable changes in local, defect related structures are evident from PDS. An analysis of the data suggests two bands of defects in the pseudogap at low boron concentration and only one band for higher concentration. To account for Fermi level positions, shifts of the hole transport path well into the valence band tail upon doping must be invoked.
Effect of the Local Disorder in a-Si on the Electronic Density of States at the Band Edges.
Authors: Davidson B. N. , Lucovsky G. , Bernholc J.
Published: March 2011

We have systematically investigated the formation of electronic states in the region of the conduction and valence band edges of a Si as functions of variations in the bond angle distributions. Local Density of States (LDOS) for Si atoms in disordered environments have been calculated using the cluster Bethe lattice method with a tight-binding Hamiltonian containing both first and second nearest neighbor interaction terms. LDOS for atoms with bond angle dis ortions in the nearest neighbor and second neighbor shells are compared and contrasted, both showing an influence on the LDOS near the gap. We also consider the role of the second neighbor term in the Hamiltonian by comparing the DOS for a distoned infinite Bethe lattice using Hamiltonians with and without the second neighbor interactions. It is found that in this case the second neighbor interaction terms cause greater conduction band tailing than using the nearest neighbor interaction terms alone.
The Distribution of Occupied Deep Levels in a-Si:H Determined from CPM Spectra
Authors: Hata Nobuhiro , Wagner Sigurd
Published: March 2011

We report the determination of the distribution of occupied defect states in hydrogenated amorphous silicon from the deconvolution of constant photocurrent measurement (CPM) spectra, and the modelling of the distribution with the defect pool.

The CPM spectra were taken on undoped a-Si:H samples either in their as-grown state, in the annealed state, after quenching from high temperature, or after light-soaking. The spectra were deconvoluted to account for transitions from deep levels and from valence band tail states to a conduction band assumed to have a sharp edge. As-grown or annealed-state samples show a peak at 1.0 eV (0.2 eV FWHM) below the conduction band edge. Assuming a mobility gap of 1.9 eV, this peak lies 0.9 eV above the valence band edge. We ascribe this peak to the Do/+ transition. CPM spectra of light-soaked and thermally quenched samples show shifts in the peak position and increases in die peak height in accordance with the defect pool model. The model calculations agree with the CPM results, so that the applicability of CPM spectral analysis to obtaining detailed values of defect pool parameters is demonstrated.
Charge-Defect Equilibrium Description of the Staebler-Wronski Defect Concentration and Formation Energy
Authors: Fortmann C. M. , Dawson R. M. , Wronski C. R.
Published: March 2011

An equilibrium framework for the high temperature behavior of dangling bond defects in amorphous materials is developed. With this framework it is possible to relate the thermal formation of defects directly with those created by high temperature illumination or current injection. It is found that the free energy change associated with dangling bond formation is negative. The negative free energy change means that if one considered only the structural changes, the lowest energy state for the system is with the weak bonds split into neutral and charged dangling bonds!