bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal Activation of Beryllium in GaN Grown by RF-Plasma Molecular Beam Epitaxy
Authors: VanMil B. L. , Lee Kyoungnae , Wang Lijun , Giles N. C. , Myers T. H.
Published: April 2011

Differences in the optical activity of Be in GaN epilayers grown on different surface polarities by rf-plasma molecular beam epitaxy are investigated. Nitrogen-polar GaN doped with Be exhibits a significantly higher intensity of donor-acceptor pair (DAP) photoluminescence (PL) than similarly doped Ga-polar GaN, indicating the Be is incorporating at microscopically different sites, or possibly is forming different compensating complexes. Highly Be-doped Ga-polar GaN apparently forms isolated polarity-inverted regions which then incorporate Be via the N-polar mechanism resulting in the DAP PL. High temperature annealing of the Ga-polar layers both under nitrogen/hydrogen mixtures and under pure nitrogen atmospheres activates the DAP PL.
Effects of growth interruption on the structural and optical properties of GaN self-assembled quantum dots
Authors: Hoshino K. , Kako S. , Arakawa Y.
Published: April 2011

We report the effects of thermal annealing and growth interruption on the formation of GaN self-assembled quantum dots (QDs) grown by low-pressure metalorganic chemical vapor deposition (MOCVD). For the uncapped QD structures, atomic force microscopy (AFM) analysis shows that the density of the QDs rapidly increases and then decreases with increasing the annealing time. This is due to the combined effect of the QD formation process and the ripening process. The size of the QDs increases with increasing the annealing time. We have also investigated the photoluminescence (PL) property of the AlN capped QD structures with different growth interruption times. The PL from the QDs showed a red-shift with increasing the growth interruption time. In contrast, the PL from the wetting layer (WL) shows a blue-shift. These results indicate that the formation of the QDs is proceeding at the expense of the WL, which is consistent with the AFM analysis of the thermal annealing effect. These results show that the growth interruption plays an important role in the formation of the QDs.
Intersubband Absorptions in Doped and Undoped GaN/AlN Quantum Wells at Telecommunication Wavelengths Grown on Sapphire and 6H-SiC Substrates
Authors: Helman A. , Tchernycheva M. , Lusson A. , Warde E. , Julien F. H. , Monroy E. , Fossard F. , Si Dang Le , Daudin B.
Published: April 2011

In this paper we present experimental and theoretical study of intersubband transitions at telecommunication wavelengths in GaN/AlN hexagonal-phase quantum wells grown by molecular beam epitaxy on sapphire substrates. Crossed structural and photoluminescence experiments show that strong in-plane carrier localization occurs due to thickness fluctuations at GaN/AlN interfaces. Fourier transform infrared spectroscopy and photo-induced absorption spectroscopy performed on doped and undoped samples reveal a systematic blue-shift of the e1-e2 transitions with doping due to many body interactions. A good agreement is achieved between experiments and self-consistent Schrödinger-Poisson calculations.
TEM investigation of defect reduction and etch pit formation in GaN
Authors: Vennemann Angelika , Dennemarck Jens , Kröger Roland , Böttcher Tim , Hommel Detlef , Ryder Peter
Published: April 2011

GaN samples of this study were chemically wet etched to gain easier access to the dislocation sturcture. The scanning electron microscopy and transmission electron microscopy investigations revealed four different types of etch pits. After brief etching, several dislocations with screw component showed large etch pits, which may be correlated with the core of the screw dislocation. By means of SiNx micromasking the dislocation density could be reduced by more than one order of magnitude. The reduction of threading dislocations in the SiNx region in GaN grown on 〈0001〉 sapphire is due to bending of the threading dislocations into the {0001} plane, such that they form dislocation loops if they meet dislocations with opposite Burgers vectors. Accordingly, the achievable reduction of the dislocation density is limited by the probability that these dislocations interact. Edge dislocations bend more easily on account of their low line tension. This results in a preferential bending and reduction of dislocations with edge character.
Strain evolution and phonons in AlN/GaN superlattices
Authors: Darakchieva V. , Paskov P. P. , Schubert M. , Valcheva E. , Paskova T. , Arwin H. , Monemar B. , Amano H. , Akasaki I.
Published: April 2011

AlN/GaN superlattices (SLs) with different periods grown on GaN buffer layers were studied by infrared spectroscopic ellipsometry (IRSE), Raman scattering (RS) and highresolution reciprocal space mapping (RSM). The lattice parameters and the degree of strain in the GaN buffer and the SL constituents were determined. Phonon modes originating from the buffer layer and the SL sublayers were identified and their frequency shifts were correlated with the strain state of the films.
MOCVD Growth of InAlAsSb Layer for High-Breakdown Voltage HEMT Applications
Authors: Yokoyama Haruki , Sugiyama Hiroki , Oda Yasuhiro , Sato Michio , Watanabe Noriyuki , Kobayashi Takashi
Published: April 2011

This paper studies the decomposition characteristic of group-III sources during InAlAsSb growth on InP substrates by metalorganic chemical vapor deposition (MOCVD) using trimethylindium (TMI), trimethylaluminum (TMA), trimethylantimony (TMSb) and arsine (AsH3). A composition analysis of InAlAsSb layers shows that the group-III compositions in the InAlAsSb layer change remarkably when the flow rate of the group-V source is varied. To clarify the reason for this phenomenon, the growth rates of InAsSb and AlAsSb component are examined. Their changes indicate that TMSb suppresses the decomposition of TMA while AsH3 enhances it. Moreover, the HEMT structure with InP/InAlAsSb Schottky barrier layer, whose InP layer acts as a recess-etch-stop layer, is fabricated for the first time. The I-V characteristics of a fabricated Schottky barrier diode indicate that the reverse leakage current of InP/InAlAsSb is about one order of magnitude smaller than that of commonly used InP/InAlAs.
Evidence for localization effects in GaAsSb/InP heterostructures from optical spectroscopy
Authors: Chouaib Houssam , Bru-Chevallier Catherine , Benyattou Taha , Lahreche Hacene , Bove Philippe
Published: April 2011

GaAsSb is a promising material for the base of a new generation of Heterojunction Bipolar Transistors (HBT) on InP, as it is expected to allow the elaboration of high-speed digital circuits (80 to 100 Gbits/s). Emitter-base interfaces between GaAsSb and InP need to be well controlled to ensure good performance of the HBT, and this requires a careful analysis of both material and interface quality. Photoluminescence (PL) experiments as a function of temperature and of power excitation density, as well as photoreflectance (PR) measurements are performed on GaAsSb/InP heterostructures in order to get information about ordering and segregation effects in antimonide alloys.

From the PL recombination energy across the type II interface and at the GaAsSb band-edge, the band offset ΔEC between InP and GaAsSb is calculated. The evolution of the band to band PL recombination is studied as a function of temperature: the energy and intensity of the type I PL transition are shown to exhibit specific behaviors, which are typical of carrier localization effects in semiconductor alloys. At low temperature, the shape of the PR spectrum results in an atypical steplike background, which is analyzed as a band filling effect in consequence of the carrier localization on potential fluctuations.
CdZnSe/Zn(Be)Se Quantum Dot Structures: Size, Chemical Composition and Phonons
Authors: Gu Y. , Kuskovsky Igor L. , Fung J. , Robinson R. , Herman I. P. , Neumark G. F. , Zhou X. , Guo S. P. , Tamargo M. C.
Published: April 2011

The size and chemical composition of optically active CdZnSe/ZnSe and CdZnSe/Zn0.97Be0.03Se quantum dots (QDs) are determined using photoluminescence, photoluminescence excitation and polarized Raman scattering spectroscopies. We show that the addition of Be into the barrier enhances the Cd composition and the quantum size effect of optically active QDs. Additionally, surface phonons from QDs are observed in CdZnSe/ZnBeSe nanostructures.
Characterization of Uranium Particles Produced via Pulsed Laser Deposition
Authors: Glade S. C. , Trelenberg T. W. , Tobin J. G. , Hamza A. V.
Published: April 2011

We have constructed an experimental apparatus for the synthesis (via pulsed laser deposition) and analysis of nanoparticles and thin films of plutonium and other actinides. In-situ analysis techniques include x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS). Also, the oxidation kinetics and the reaction kinetics of actinides with other gaseous species can be studied with this experimental apparatus. Preliminary results on depleted uranium are presented.
Quasicrystal approximants with novel compositions and structures
Authors: Mihalkovič M. , Widom M.
Published: April 2011

We identify several new quasicrystal approximants in alloy systems in which quasicrystals have not been previously reported. Some occur in alloys with large size contrast between the constituent elements, either containing small Boron atoms, or large Ca/Eu atoms, leading to quasicrystal structures quite different from currently known systems where the size contrast is smaller. Another group of the approximants are layered Frank-Kasper structures, demonstrating competition between decagonal and dodecagonal ordering within this family of structures.
Phononic Quasicrystals
Authors: Sutter Daniel , Krauss Günter , Steurer Walter
Published: April 2011

Phononic crystals are to sound what photonic crystals are to light and crystals are to electrons. If the wavelength is of the same order of magnitude as the typical distances between the objects in their ordered arrangement, then the interaction is dominated by diffraction besides refraction. Depending on the spectral properties of the material, band gaps appear for certain frequencies. In special cases, these gaps are omni-directional. A huge amount of application-driven research has been performed on photonic crystals, much less on phononic crystals, and almost nothing on phononic quasicrystals. We present here the first experimental studies on two-dimensional, quasiperiodic, phononic crystals. Our experiments have been performed with ultrasound on phononic crystals and quasicrystals consisting of steel cylinders in water. The pros and cons of phononic quasicrystals are discussed.
Quasicrystal Formation in a Zr-based Bulk Metallic Glass
Authors: Liu L. , Chan K. C. , Pang G. K. H.
Published: April 2011

Zr65Ni10Cu7.5Al7.5Ag10 bulk metallic glass with a diameter of 2 mm and a length of 50 mm was prepared by copper-mould casting. The metallic glass obtained follows two-step crystallization reactions with the initial formation of metastable quasicrystals from amorphous phase, followed by the formation of a stable intermetallic Zr2Cu from the post-formed quasicrystals. In this paper, the microprocess of the amorphous-to-quasicrystalline transformation of Zr65Ni10Cu7.5Al7.5Ag10 bulk metallic glass was studied in detail by TEM and high resolution TEM coupled with nanobeam EDX. It was found that the amorphous-to-quasicrystalline transformation in the present system does not follow the traditional nucleation/growth mechanism. Instead, it undergoes a series of inter-processes and follows the sequence of phase transformation of amorphous → FCC Zr2Ni → Tetragonal Zr2Ni → Quasicrystals. Nanobeam EDX revealed that atomic diffusion was involved in all inter-processes of the phase transformations, suggesting that the amorphous-to-quasicrystalline transformation in the present bulk metallic glass is a non-polymorphous reaction.
Mechanical Spectroscopy of the Formation of Quasicrystalline Order in Rapidly Quenched Ti- and Zr-based Alloys
Authors: Sinning Hans-Rainer , Golovin Igor S. , Jianu Adrian
Published: April 2011

The development of quasicrystalline (qc) order from different initial states (amorphous, nano- or poly-qc) was studied in rapidly quenched Ti-Zr-Ni(-Ag), Ti-Zr-Fe, and Zr-Cu-Ni-Al alloys by means of mechanical spectroscopy (vibrating-reed technique), including both the irreversible changes of Young's modulus and the Snoek-type relaxation peak of absorbed hydrogen as a probe, which are obviously sensitive to quite different types of ordering processes. The strongest annealing effects on Young's modulus were found for extreme nano-qc material at the amorphous limit, whereas order-induced narrowing of the hydrogen peak occurs only at comparatively larger grain sizes. More important differences in the properties of the hydrogen peak are found between quasicrystals of different compositions. The results are briefly discussed with respect to their implications for the amorphous-to-quasicrystalline transition and for the experimental distinction between different types of icosahedral quasicrystalline order.
Oxidation of amorphous Zr70Pd30 and coarse crystalline Zr2Pd
Authors: Jastrow Lioba , Köster Uwe
Published: April 2011

Zr-based bulk metallic glasses are of increasing interest due to their excellent properties, e.g. high elastic limit or catalytic activity. For some applications (golf clubs, hydrogen storage) a good oxidation resistance is necessary; in other cases (catalysis) fast oxidation is required.

Zr-Pd melt-spun glasses are known to exhibit “catastrophic” oxidation even at temperatures far below their glass transition. However, the surface of as-cast amorphous Zr70Pd30 was found to be protected by a thin native ZrO2 layer, thus allowing only localized nucleation of the oxidation reaction at preferred nucleation sites activated by destruction of the native oxide layer.

Detailed cross-sectional SEM and TEM revealed a rather complicated microstructure of the oxide islands consisting of a lamellar structure of tetragonal and moniclinic ZrO2 with different Pd contents and embedded nanocrystalline Pd. There is also evidence for a Pd enriched reaction zone between the oxide cone and the glassy matrix.

In order to understand the influence of the structure on the oxidation, the behavior of the metallic glass was compared with that of a crystallized alloy or even coarse crystalline Zr2Pd. Coarse crystalline Zr2Pd was found to be rather stable against oxidation, but exhibit very fast oxygen diffusion along the grain boundaries followed by oxide formation deep inside the material. Grain boundaries in the crystallized Zr70Pd30 does not exhibit such a behavior.
Shear thinning of the bulk metallic glass forming alloy Zr41.2 Ti13.8 Cu12.5 Ni10.0 Be22.5 close to the liquidus temperature.
Authors: Shaw Tyler , Way Christopher , Busch Ralf
Published: April 2011

The viscous properties of the multi component Zr41.2 Ti13.8 Cu12.5 Ni10.0 Be22.5 bulk metallic glass forming alloy was studied above the liquidus temperature. A shear thinning behavior for the molten alloy has been discovered. The shear thinning behavior can be characterized as a power law fluid with a power-law index of -0.5. Additionally, at low shear rates and temperatures near the liquidus temperature a visco-elastic type response in the flow behavior is observed.
SiGe HBT/BiCMOS Technologies and their Applications to Communication ICs/LSIs
Authors: Oda Katsuya , Washio Katsuyoshi , Hashimoto Takashi
Published: April 2011

Self-aligned ultra-high-speed SiGe HBTs were developed by using selective epitaxial growth (SEG) technology. The use of HCl-free SEG, incorporation of C, and optimization of doping profiles significantly improves the performance of the HBT, producing a transistor with a high cutoff frequency of 170 GHz and a maximum oscillation frequency of 204 GHz, for a minimum ECL gate delay time of 4.8 ps. This is applied in a 16:1 MUX with a maximum clock rate of 57 GHz. A 0.13-μm SiGe BiCMOS technology is also realized without any degradation of CMOS due to the high stability of SiGe HBTs. Furthermore, the structure of SiGe HBT is optimized for an emitter scaled down towards 100 nm, mainly through the use of a funnel-shaped emitter electrode to reduce both emitter and base resistances. High-speed operation of a static frequency divider demonstrates the advantage of SiGe HBTs for ultra-high-speed communications systems.
Heteroepitaxial growth and characterization of Ge and SiXGe1−X films on patterned silicon structures
Authors: Vanamu Ganesh , Datye Abhaya K. , Zaidi Saleem H.
Published: April 2011

We describe novel 2-D structures that facilitate strain relief and allow us to obtain Ge epilayers that are free of defects. These structures can potentially absorb thermal expansion and lattice expansion mismatch as well as enable liftoff of heteroepitaxial layers for subsequent wafer reuse. Conventional lithography techniques were combined with reactive ion and wet-chemical etching to fabricate 2-D patterns of silicon posts. The dimensions of the posts were varied keeping the pitch (center to center distance) constant. Heteroepitaxial growth of Ge/SixGe1−x on these micrometer-scale structures was investigated. While, keeping the growth parameters constant, the geometry of the structures was varied to determine the optimum configuration for the highest quality heteroepitaxial growth. The quality of the Si1−xGex buffer system was investigated using high-resolution x-ray diffraction. Transmission electron microscopy (TEM) was used to analyze the epilayer cross-sections. Surface morphology was analyzed using scanning electron microscopy (SEM), atomic force microscopy (AFM) and optical microscopy. Our results show that the quality of the heteroepitaxial layers improves as the width of the posts in the 2-D pattern was decreased.
Arsenic diffusion in Si and Si0.9Ge0.1 alloys: Effect of defect injection
Authors: Uppal Suresh , Bonar J. M. , Zhang Jing , Willoughby A. F. W.
Published: April 2011

Results of intrinsic As diffusion in Si as well as in strained and relaxed Si0.9Ge0.1 layers are presented. Using Molecular Beam Epitaxy in-situ As doped epitaxial Si and compressively strained and relaxed Si-Ge layers were grown on Si substrates. The samples were annealed using Rapid Thermal Annealing (RTA) at 1000 °C. Arsenic diffusion is seen to be enhanced in SiGe than in Si. The enhancement factor is calculated to be 2.3 and 1.3 for relaxed and strained Si0.9Ge0.1, respectively. Also, using RTA in oxygen atmosphere, interstitial and vacancies were selectively injected in to the sample structures. Diffusion enhancement is also recorded in Si and Si-Ge structures with interstitial as well as vacancy injections over inert anneal. The results suggest that both interstitial and vacancy defects contribute to As diffusion in Si and Si0.9Ge0.1.
Ion Implantation and 1 MeV Electron Irradiation of 4H-SiC---Comparison Studies
Authors: Evwaraye A. O. , Smith S. R. , Mitchel W. C. , Farlow G. C. , Capano M. A.
Published: April 2011

Argon ions (Ar+) were implanted into n-type 4H-SiC epitaxial layers at 600 °C. The energy of the ions was 160 keV and at a dose of 2 × 1016 cm−2. After post-implantation annealing at 1600 °C, Schottky diodes were fabricated on the ion implanted samples. Bulk n-type 4H-SiC samples were irradiated at room temperature with 1 MeV electrons at doses of 1 × 1016 to 5.1 × 1017 el/cm2. The current density of the beam was 0.91 μA/cm2. Deep Level Transient Spectroscopy (DLTS) was used to characterize the induced defects. DLTS studies of Ar+ implanted samples showed six defect levels at EC – 0.18 eV, EC – 0.23eV, EC – 0.31eV, EC – 0.38 eV, EC – 0.72 eV, and EC – 0.81 eV. Z1/Z2 defect is the dominant defect in the electron irradiated sample and anneals out completely after 10 minutes at 1000 °C. However, Z1/Z2 defect in Ar+ implanted samples was stable up to 1600 °C. It is suggested that the annealing behavior of Z1/Z2 depends on the source of its formation.
Laser Direct Write and Gas Immersion Laser Doping Fabrication of SiC Diodes
Authors: Tian Z. , Quick N.R. , Kar A.
Published: April 2011

Laboratory prototype SiC diodes are fabricated using a combination of gas immersion laser doping (GILD) and laser direct write (LDW) in situ metallization in a commercial SiC wafer. Trimethylaluminum (TMA) and nitrogen are the precursors used to produce p-type and n-type SiC, respectively. Using these techniques, a 150 nm p-type doped junction is fabricated in semiinsulating 6H-SiC and n-doped 4H-SiC wafers. Ohmic contacts are created by laser direct metallization producing carbon rich conductive phases in these doped materials. Alternatively an excimer laser can be used to create silicon rich Schottky contacts. The geometry of the diodes can be vertical or planar to the wafer surface and the laser processes are thought to reduce defect densities in the irradiated areas. These laser-processed diodes are intended for use in high temperature, high voltage and high frequency switching and sensing applications.
Sic Power Diodes Improvement by Fine Surface Polishing
Authors: Godignon P. , Perez R. , Tournier D. , Mestres N. , Mank H. , Turover D.
Published: April 2011

Surface treatment is a key technological parameter in the microelectronics technology and especially for SiC devices since high temperatures must be used for implanted impurities annealing and crystal damage recovery. In this work we take profit of a novel fine polishing process developed by NOVASIC to improve the electrical characteristics of Boron and Aluminium implanted Schottky diodes, which are surface quality highly sensitive devices. The mentioned fine polishing process allows to remove a layer thickness of 100nm to 3000nm on the surface of a processed SiC wafer, reducing the surface roughness to RMS of 1Å. The impact of this process on the electrical properties of the samples shows a general improvement of characteristics reproducibility, reduction of leakage current and improvement of breakdown of Boron implanted diodes.
Deep Level Defects in He-implanted n-6H-SiC Studied by Deep Level Transient Spectroscopy
Authors: Chen X. D. , Ling C. C. , Fung S. , Beling C. D. , Wu H. S. , Brauer G. , Anwand W. , Skorupa W.
Published: April 2011

Deep level transient spectroscopy (DLTS) was used to study deep level defects in He-implanted n-type 6H-SiC samples. Low dose He-implantation (fluence ∼2×1011 ions/cm2) has been employed to keep the as-implanted sample conductive so that studying the introduction and the thermal evolution of the defects becomes feasible. A strong broad DLTS peak at 275K-375K (called signal B) and another deep level at EC-0.50eV were observed in the as-implanted sample. The intensity of the peak B was observed to linearly proportional to the logarithm of the filling pulse width, which is a signature for electron capture into a defect related to dislocation. After annealing at 500°C, the intensity of peak was significantly reduced and the remained signal has properties identical to the well known Z1/Z2 deep defects, although it is uncertain whether the Z1/Z2 exist in the as-implanted sample or it is the annealing product of the dislocation-related defect. The E1/E2 defect (EC-0.3/0.4eV) was not presence in the as-implanted sample, but was observed after the 300°C annealing.
Modified Abrasives based on fumed SiO2 and Al2O3 for the Cu CMP Process
Authors: Zeidler D. , Bartha J.W. , ortz W.L , Brandes R.
Published: April 2011

New abrasive particles based on SiO2 and Al2O3 were produced with different coating and doping. Seven specifically designed particles were dispersed to prepare slurries for Cu CMP. Glycin was used as complexing agent and hydrogenperoxid as oxidizer. The experimentally obtained removal rate, selectivity, surface quality and planarisation ability, demonstrate a significant impact of the different abrasives tested. SiO2 particles covered with Al2O3 increased the removal rate for Cu. In comparison to this behavior, a low rate for TaN proved a high selectivity copper removal required by the Cu CMP process. A new method for the planarisation length monitoring (step polish response) shows also significant differences in planarisation length (PL) by the polish of copper with slurries composed of these new particles.
Effects of Annealing Atmosphere on the Characteristics and Optical Properties of SiON Films Prepared Plasma Enhanced Chemical Vapor Deposition
Authors: Yun Ki-Jun , Jung Dong-Ryeol , Hong Sung-Kil , Moon Jong-Ha , Kim Jin-Hyeok
Published: April 2011

SiON thin films were deposited by plasma-enhanced chemical vapor deposition method at 350 °C using N2O/SiH4 gas mixtures as precursors. As-deposited SiON films were annealed in different gas atmospheres (air, N2, and O2) and at different annealing temperatures (800 oC ∼ 1100 oC). Effects of annealing atmosphere on the Si-O, Si-N, Si-H, and N-H bonding characteristics in SiON films and their structural and optical properties have been investigated. Cross-sectional and planar microstructures were characterized by scanning electron microscopy and atomic force microscopy, and crystallinity was investigated by X-ray diffraction. Chemical bonding characteristics and optical properties SiON films were studied using fourier transform infrared spectroscopy and prism coupler. Xray diffractions showed no evidence of any crystals in all SiON films. The deposition rate strongly depended on the processing parameters such as radio frequency (rf) power, N2O/SiH4 flow ratio, and SiH4 flow rate. Deposition rate increased as N2O/SiH4 flow ratio increased and SiH4 flow rate increased. It was possible to obtain SiON films with surface roughness of about 1 nm and a high deposition rate of about 4 μm/h when the processing parameters were optimized as rf power of 200 W, N2O/SiH4 flow ratio of 3, SiH4 flow rate of 100 sccm. It was observed that the intensity and the shift of the Si-O stretch and Si-N peaks depended on the annealing atmosphere as well as the annealing temperature. The intensity of Si-O peaks increased in the samples annealed in oxygen atmosphere, but it decreased in the samples annealed in nitrogen atmosphere. The intensity of Si-N peak decreased in the samples annealed in oxygen atmosphere, but it increased in the samples annealed in nitrogen atmosphere. The position of Si-O peaks shifted from 1030 nm to 1140 nm in the samples annealed both in oxygen and in nitrogen atmosphere. It was also observed that the intensities of Si-H (∼2250 cm−1) and N-H (∼3550 cm−1) peaks decreased apparently as the annealing temperature increased in all annealed samples.
Structure analysis of terbium aluminosilicate glass
Authors: Qi Xiaoyuan , Sung Sang-Yeob , Mondal Samir K. , Stadler Bethanie J. H
Published: April 2011

Glasses rich in rare-earth ions have attracted a lot of interest due to their applications in optical isolators and optical amplifiers. Integrating optical isolators with various optoelectronic devices allows sources to be integrated with lower costs, easier alignment and longer lifetimes. Glasses rich in rare-earth ions have large Verdet constants, so large Faraday rotations. Among the rare-earth ions used in paramagnetic glasses, Tb3+ ions have largest Faraday rotation per ion and the glasses are transparent down to 1.6um. These glasses can also avoid the temperature incompatibility and lattice match problems which are encountered when magneto-optical garnets mare used for integration. The Tb3+doping is also widely used in optical amplifiers. In this paper we have explored the metastable phases present in the sputtered Tb-Al-Si-O system in order to fabricate paramagnetic films with the highest possible Faraday rotations, lowest optical losses and that are easily integrated with semiconductors. A broad peak was observed in the microdiffraction pattern around 2θ= 30deg. This peak corresponded with the close-packed Tb-O plane spacing (111 for FCC Tb4O7 or 002 for HCP Tb2O3), but it was an “amorphous” peak with a 5 deg FWHM. Amorphous films were obtained even when the Tb concentrations were very high. Since high concentrations of Tb are known to devitrify glasses, the discovery of a high-Tb concentrated glass is exciting.
Eu-doped Yttria and Lutetia Thin Films Grown on Sapphire by PLD
Authors: Bär S. , Scheife H. , Huber G. , Gonzalo J. , Perea A. , Font A. Climent , Paszti F. , Munz M.
Published: April 2011

This paper focuses on the preparation and characterization of crystalline thin films of rare-earth-doped sesquioxides (Y2O3 and Lu2O3) grown by pulsed laser deposition on single-crystal (0001) sapphire substrates. The crystal structure of the films (thicknesses between 1 nm and 500 nm) was determined by X-ray diffraction and surface X-ray diffraction analysis. These measurements show that the films were highly textured along the <111> direction. Using Rutherford backscattering analysis the correct stoichiometric composition of the films could be proved. The surface morphology of the thin films has been studied using atomic force microscopy. Crystalline films show a triangular surface morphology, which is attributed to the <111> growth direction. The emission and excitation spectra of the Eu-doped films down to a thickness of 100 nm look similar to those of the corresponding crystalline bulk material, whereas films with a thickness ≤ 20 nm show a completely different emission behaviour.
Self-Organization of Nanoscopic Building Blocks into Ordered Assemblies
Authors: Horsch Mark A. , Iacovella Christopher R. , Zhang Zhenli , Glotzer Sharon C.
Published: April 2011

We studied the self-assembly of nanoscopic building blocks comprised of polymer-tethered nanoparticles using computer simulation and predict that these building blocks can assemble into mono- and multi-layer sheets and shells. The simulations further demonstrate that for some nanoparticle geometries and tethered nanoparticle topologies, ideas from block copolymers, surfactants and liquid crystals can be used to predict the ordered morphologies attained via self- assembly and that for specific cases the morphologies are consistent with Israelachvili packing rules.
Confinement of Nanocrystals and Possible Charge Storage Mechanism for MIS Memory Devices with Ge Nanocrystals Embedded in SiO2
Authors: Ho V. , Choi W.K. , Chim W.K. , Teo L.W. , Du A. Y. , Tung C. H.
Published: April 2011

Memory effect in a metal-insulator-silicon (MIS) structure with the insulator layer consisting of a sputtered capping SiO2 / Ge nanocrystals embedded in SiO2/ rapid thermal oxide structure has been observed. For the devices with a co-sputtered SiO2+Ge middle layer, larger nanocrystals were formed devices with a higher Ge concentration. It was noted that for such devices, the RTO and the capping oxide layers were able to confine the nanocrystals within the middle layer to some extent. However, in devices with pure sputtered Ge middle layer, the RTO and capping oxide layers were not as effective in confining the Ge nanocrystals. In addition, we have consistently observed memory effect from devices annealed in Ar and the absence of memory effect from devices annealed in forming gas. However, Ge nanocrystals were found in devices annealed in both ambient. This implies that having nanocrystals does not necessarily imply the presence of charge storage or memory effect.
Role of Deformable Fine Spinel Particles in High-Strain-Rate Superplastic Flow of Tetragonal ZrO2
Authors: Morita Koji , Hiraga Keijiro , Kim Byung-Nam , Sakka Yoshio
Published: April 2011

The role of MgAl2O4 spinel particle dispersion in high-strain-rate superplasticity (HSRS) of tetragonal ZrO2 was examined by characterizing microstructural changes during deformation. The dispersed spinel particles elongate with strain along tensile direction and the elongation tends to be pronounced with increasing strain rate. In the elongated spinel particles, intragranular dislocations lying along the elongated direction were observed, suggesting that the elongation relates to the dislocation motion. The flow behavior characterized by a stress exponent of ≈ 2.0 suggests that grain boundary sliding (GBS) is the predominant flow mechanism. The dislocation-induced plasticity in the spinel particles may assist the relaxation of stress concentrations exerted by GBS, leading to HSRS in tetragonal ZrO2.
Optimized screen-printing and SEM-FIB characterization of YSZ thin films for Solid Oxide Fuel Cells and gas sensors devices
Authors: Morata A. , Tarancón A. , Dezanneau G. , Peiró F. , Morante J. R.
Published: April 2011

In the present work, the screen printing technique has been used to deposit thick films of Zr0.84Y016O1.92 (8YSZ). In order to control the final porosity in view of a specific application (SOFCs or gas sensor), an experimental design based on analysis of variances (ANOVA) has been carried out. From this, we were able to determine the influence of several technological parameters on films porosity and grain size. The films obtained have been analysed with both Scanning Electron Microscopy (SEM) and Focused Ion Beam (FIB) combined with SEM. We show that only the combination of experimental design and advanced observation technique such as Focused Ion Beam allowed us to extract significant information for the improvement of the deposition process.
The Influence of Environmental Conditions and Passive Film Properties on the MIC of Engineered Barriers in the Yucca Mountain Repository
Authors: Lloyd A.C. , Schuler R.J. , Noël J.J. , Shoesmith D.W. , King F.
Published: April 2011

A combination of gamma radiation fields, the absence of moisture, and the high temperatures on the drip shield (DS) and waste package (WP) should combine to suspend microbial activity on the DS/WP surfaces for many tens of thousands of years. This lack of microbial activity, coupled with the corrosion resistance of the titanium Grade7 (Ti-7 drip shield) and the Alloy-22 (waste package) materials make microbially induced corrosion (MIC) of these engineered barrier materials extremely unlikely.
Material Analysis and Radioisotope Studies of Spent Nuclear Fuel found in the Marine Environment
Authors: Baikie I.D. , Dennis Frank , Crawford Ron , Scirea Massimo
Published: April 2011

Sand-sized particles of spent nuclear fuel have been found in the environment at Dounreay, north Scotland. These particles, believed to have been discharged during early stages of site operations in the 1960s, have been recovered from areas within the Site, coastal foreshore adjacent to the Site and offshore (marine) environments. As part of the Dounreay Site Restoration Plan, a significant program of work is being undertaken to establish the fate of the Dounreay particles in the marine environment. This program includes materials analysis of particles and particle analogues

The diverse depositional environments, coupled to relatively long fuel-particle residence times, allow insight to be gained into the behaviour of the particles in the natural environment. This includes the effects of physical, chemical and mechanical processes such as particle erosion and corrosion, particle abrasion and particle-saltwater interaction. Using materials analysis techniques including Scanning Electron Microscopy, Energy Dispersive X-Ray Analysis, Electron Probe Microanalysis, together with 137Cs activity measurement we review the fuel-particle mechanical and chemical stability and current radio-isotope composition. These studies allows us to identify the fuel provenance, fuel treatment history and draw some general conclusions of the ultimate fuel particle residence time. We demonstrate that radioisotope modelling strongly indicates that particle break-up is limited by the chemical effects of the marine environment coupled with the native oxide films present on the metallic fuel. Particle transport and distribution are governed by environmental effects on the sediment and sediment transport mechanisms such as tidal currents and storm events.

This study is unique in that it involves the potential effects of long-term saltwater and abrasion interactions with fuel matrixes containing both U-Al and U-Mo of which there is little literature available.
Application of A Monte Carlo Approach to Surface Complexation Modelling
Authors: Askarieh M.M. , Heath T.G. , Tearle W.M.
Published: April 2011

A Monte Carlo-based approach has been adopted for development of a chemical thermodynamic model to describe the goethite surface in contact with sodium nitrate solutions. The technique involves the calculation of the goethite surface properties for the chemical conditions corresponding to each experimental data point. The representation of the surface was based on a set of model parameters, each of which was either fixed or was randomly sampled from a specified range of values. Thousands of such model representations were generated for different selected sets of parameter values with the use of the standard geochemical speciation computer program, HARPHRQ. The method allowed many combinations of parameter values to be sampled that might not be achieved with a simple least-squares fitting approach. It also allowed the dependence of the quality of fit on each parameter to be analysed. The Monte Carlo approach is most appropriate in the development of complex models involving the fitting of several datasets with several fitting parameters.

Introduction of selenate surface complexes allowed the model to be extended to represent selenate ion sorption, selenium being an important radioelement in evaluation of the long-term safety of ILW disposal. The sorption model gave good agreement with a wide range of experimental sorption datasets for selenate.
Estimated Effects of Temperature-Relative Humidity Variations on the Composition of In-Drift Water in the Potential Nuclear Waste Repository at Yucca Mountain, Nevada
Authors: Browning Lauren , Fedors Randall , Yang Lietai , Pensado Osvaldo , Pabalan Roberto , Manepally Chandrika , Leslie Bret
Published: April 2011

We define four distinct thermohydrochemical environments for drip shield and waste package corrosion in the potential nuclear waste repository, referred to here as the Dry, Seepage + Evaporation, Seepage + Condensation + Evaporation, and the Seepage + Condensation environments. These environments are bounded by temperature and relative humidity conditions at drift wall and drip shield/waste package surfaces judged most likely to initiate fundamental changes in the quantity and/or chemistry of in-drift waters. The duration in which different environments might exist is evaluated by comparing simulated, time-dependent temperature and relative humidity curves for three different locations within repository drift 25. In-drift conditions and processes postulated to cause drip shield/waste package corrosion are evaluated within the context of the thermohydrochemical environments by various means, including analytical calculations and geochemical simulations. Of the four environments considered here, the Seepage + Evaporation environment presents the most significant potential for aqueous corrosion of drip shield and waste package materials, and may persist for approximately 500 years in center drift locations. The likelihood for corrosion in other thermohydrochemical environments is significantly lower, but may increase with the acquisition of new data or the demonstration of extenuating circumstances.
Corrosion Study of Actinide Waste Forms with Garnet-Type Structure
Authors: Yudintsev Sergey V. , Osherova Anna A. , Dubinin Alexander V. , Zotov Alexander V. , Stefanovsky Sergey V.
Published: April 2011

Garnet-based ceramics doped with Gd, Th, and Ce as waste surrogates and compositions: (Ca1.5GdTh0.5)ZrFeFe3O12 (G3) and Ca2.5Ce0.5Zr2Fe3O12 (G21) were prepared at 1300 °C in air. Corrosion rates in deionized water measured using an MCC-2 procedure were found to be (in g × m-2 × day-1): 10-6 for Ce (G21), 3×10-5 for Gd and 10-6 for Th (G3). Corrosion rates of Ce, Gd, and Th in an acid solution (0.01 M HCl) were 2 × 10-1 (G21), 4 × 10-3 and 10-3 (G3), respectively. Chemical durability of the garnet ceramics is comparable to durability of titanate zirconolite and pyrochlore samples under similar conditions. Scanning electron microscopy has revealed no alteration of the sample surfaces after their contact with water, and the presence of newly formed iron-enriched phases of 1 to 3 micrometers in size, after interaction of the G21 sample with acid solution for 30 days. High leaching of the elements from the Ce-loaded garnet may be due to reduction of Ce in acid solution to Ce(III) or may be explained by lower stability of the structure of garnets (Ca2.5An0.5)Zr2Fe3O12 compared to (Ca1.5GdAn0.5)(ZrFe)Fe3O12, An = Ce4+ and Th4+. It was found that the (Ca2.5Ce0.5)Zr2Fe3O12 and (Ca2.5Th0.5)Zr2Fe3O12 garnets have distorted the elementary unit cell with decreasing lattice symmetry to tetragonal.
Experimental and Numerical Studies on Colloid-Enhanced Radionuclide Transport - The Effect of Kinetic Radionuclide Sorption onto Colloidal Particles -
Authors: Kurosawa Susumu , Ibaraki Motomu , Yui Mikazu , Ueta Shinzo , Yoshikawa Hideki
Published: April 2011

Numerous studies have shown that colloidal particles in groundwater can facilitate radionuclide transport in subsurface environments. A series of laboratory experiments was conducted to investigate the effects of radionuclide sorption onto colloids and the surfaces of rock fractures. This research especially focused on the kinetic behavior of the sorption process. A mixed solution of Cs and clay colloids was injected into a single artificial fracture in a granite column. Simulations were also performed to analyze the experimental results using a numerical code, COLFRAC, which describes colloid-facilitated solute transport in discretely-fractured media. The code allows for either equilibrium or kinetic sorption onto the colloidal particles. The experimental and analytical results indicate that transport of Cs is facilitated by the colloidal particles, which can sorb Cs and transport through the fracture. The analyses also illustrate the importance of evaluating parameters that describe kinetic sorption onto colloids. Furthermore, radionuclide transport is likely to be retarded as colloidal particles that sorb radionuclides are filtered on the fracture surface.
Corrosion/Electrochemistry of Uranium Dioxide in Slightly Alkaline Hydrogen Peroxide Solutions
Authors: Goldik Jon S. , Noël James J. , Shoesmith David W.
Published: April 2011

The kinetics of H2O2 reduction have been studied in slightly alkaline (pH 9.7 NaCl) solution on 1.5 at.% SIMFUEL. Using cyclic voltammetric techniques, we have shown that the cathodic reduction of H2O2 is kinetically facile on UIVUVO2+x surfaces. Carbonate ions are found to have a significant effect on the kinetics of H2O2 reduction. Suppression of the reaction rate is observed at very cathodic potentials, and has been attributed to a competition between carbonate and hydrogen peroxide for catalytic sites on the electrode surface. A small enhancement of the reduction current occurs between –200 and +100 mV vs. SCE, which appears to be due to a surface adsorbed carbonate complex. The large values of the Tafel slopes for H2O2 reduction have been interpreted in terms of a chemical-electrochemical mechanism involving surface UV species. The results are discussed in terms of a mixed potential model for the prediction of nuclear fuel dissolution rates under permanent disposal conditions.
An Impact Study of the Implementation of a Materials Science and Engineering Module at the Fifth Grade Level
Authors: Zekri Souheil , Clayton LaNetra , Ferguson Emily , Okogbaa Geoffrey , Kumar Ashok , Das Tapas , Centeno Grisselle , Martin-Vega Louis
Published: April 2011

An impact study of the implementation of a newly developed Materials Science and Engineering Module was conducted as part of a National Science Foundation funded GK-12 project at the University of South Florida. The objective of GK-12 STARS (Students, Teachers and Resources in the Sciences) program is to foster systemic change in elementary by enriching math and science curricula and encouraging long-term professional development for teachers in the K-5 band. The program also aims to decrease the current educational gap in science and math curricula prevalent among certain schools within the same school district, which is reflected in the outcome of the Florida Comprehensive Assessment Test (FCAT). The module was developed for the purpose of enhancing existing textbook driven science instruction and creating a fundamentally sound scientific exposure in elementary school students. As part of this activity, students from three different schools (one private, one suburban, and one urban) were introduced to basic concepts in materials science and engineering through hands-on experiments, presentations, and field trips to the university's material research related laboratories (i.e. polymer chemistry, microelectronics, nanotechnology, geotechnics, corrosion, etc.) The developed module offered information ranging from basic definitions to newly discovered cutting edge phenomenon in the field of nanotechnology. Subsequently, pre and post test instruments were administered to assess student performance. Results from the pretest showed that students from all participating schools performed within the standard deviation. The post assessment test showed that the experimental group had twice as many correct answers, as the control group from each school.
Backside Storage Non-Volatile Memories: Ultra-Thin Silicon Layer on a Complex Thin Film Structure
Authors: Silva H. , Tiwari S.
Published: April 2011

Backside storage memories present an alternative to the conventional front-floating gate geometries by storing charge in defects on the back of a thin depleted silicon channel. This paper focuses on the fabrication of these devices using a modified Smart-Cut™ substrate preparation process followed by standard CMOS processing. The substrate is a complex silicon-on-insulator (SOI) substrate where instead of the buried oxide alone a charge trapping multi-layer stack of oxide-nitride-oxide (ONO) is used as the buried insulator. We demonstrate here the operation of these device structures at ultra-short length scales and summarize the characteristics of their operation.
Gold Langmuir-Blodgett deposited nanoparticles for non-volatile memories
Authors: Kolliopoulou S. , Tsoukalas D. , Dimitrakis P. , Normand P. , Paul S. , Pearson C. , Molloy A. , Petty M. C.
Published: April 2011

In this work, we demonstrate a MISFET memory device that incorporates a monolayer of Langmuir-Blodgett (LB) deposited gold nanoparticles as floating gate charge storage elements. The FET device is fabricated on a SOI substrate using conventional silicon processing. The nanoparticle layer is separated from the channel area of the FET with a 5 nm thermal SiO2 layer and is isolated from Al gate contact with a LB-deposited organic insulator layer. The memory effect is tested using voltage pulses on the gate of the device and monitored through drain current measurements. The nanocrystals can be charged either from the channel through the thermal oxide layer by applying pulses smaller than 5 V or from the gate through the organic insulator for higher voltage depending on the pulse duration.