bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical Characterization of Erbium Doped III-Nitrides Prepared by Metalorganic Molecular Beam Epitaxy
Authors: Hommerich U. , Seo J. T. , Thaik Myo , MacKenzie J. D. , Abernathy C. R. , Pearton S.J. , Wilsont R.G. , Zavadat J. M.
Published: April 2011

We are currently engaged in a systematic study of the optical properties of Er doped III-nitrides prepared by metalorganic molecular beam epitaxy (MOMBE). Under below-gap excitation it was observed that GaN: Er samples with [O]∼1020 cm-3 and [C]∼1021 cm-3 luminesce at 1540 nm with an intensity of more than two orders of magnitude greater than samples with low oxygen and carbon concentrations (< 1019 cm-3). Associated with the different oxygen and carbon concentrations were different thermal quenching behaviors and below-gap absorption bands. Interestingly, for above-gap excitation only small differences in absolute Er3+ PL intensity and quenching behavior were observed for samples of varying 0 and C content. Initial lifetime studies were performed and showed a rather unusual short decay time of ∼100 μts at room temperature. In order to gain more insight in the Er3+ PL, a comparison of the integrated PL intensity and lifetime was performed for the temperature range 15-500K. The result reveals that the Er3+ PL quenches above room temperature due to the onset of non-radiative decay and the reduction in excitation efficiency. All samples were also investigated for visible luminescence. Red luminescence was observed from GaN: Er on sapphire substrates under below-gap excitation.
Growth of Oriented Thick Films of Gallium Nitride From the Melt
Authors: Dyck Jeffrey S. , Kash Kathleen , Grossner Michael T. , Hayman Cliff C. , Argoitia Alberto , Yang Nan , Hong Moon-Hi , Kordesch Martin E. , Angus John C.
Published: April 2011

While significant strides have been made in the optimization of GaN-based devices on foreign substrates, a more attractive alternative would be homoepitaxy on GaN substrates. The primary motivation of this work is to explore the growth of thick films of GaN from the melt for the ultimate use as substrate material. We have previously demonstrated the synthesis of polycrystalline, wurtzitic gallium nitride and indium nitride by saturating gallium metal and indium metal with atomic nitrogen from a microwave plasma source. Plasma synthesis avoids the high equilibrium pressures required when molecular nitrogen is used as the nitrogen source. Here we report the growth of thick oriented GaN layers using the same technique by the introduction of (0001) sapphire into the melt to serve as a substrate. The mechanism of this growth is not established, but may involve transport of the metal as a liquid film onto the sapphire and subsequent reaction with atomic nitrogen. The films were characterized by x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. X-ray diffraction showed that the GaN films were oriented with their c-axes parallel to the sapphire c-axis. The TEM analysis confirmed the orientation and revealed a dislocation density of approximately 1010 cm-2. The E2 Raman active phonon modes were observed in the GaN films.
Piezoelectric Effects in GaN/AlGaN Multiple Quantum Wells Probed by Picosecond Time-Resolved Photoluminescence
Authors: Kim H. S. , Lin Y. , Jiang H. X. , Chow W. W. , Botchkarev A. , Morkoç H.
Published: April 2011

Piezoelectric effects in GaN/AlGaN multiple quantum wells (MQWs) have been directly probed by picosecond time-resolved photoluminescence (PL) spectroscopy. The time-resolved PL spectra of the 40 Å well MQWs reveal that the PL transition peak position is in fact blueshifted at early delay times due to the collective effects of quantum confinement of carriers, piezoelectric field, and Coulomb screening. However, the spectral peak position shifts toward lower energies as the delay time increases and becomes redshifted at longer delay times. By comparing experimental and calculation results, we have obtained a low limit of the piezoelectric field strength to be about 560 kV/cm in the 40 Å well GaN/Al0.15Ga0.85N MQWs.
Effects of Surface Preparation on Epitaxial GaN on 6H-SIC Deposited Via Mocvd
Authors: Xie Z. Y. , Wei C. H. , Li L. Y. , Edgar J. H. , Chaudhuri J. , Ignatiev C.
Published: April 2011

A comparison was made of 6H-SiC surfaces etched with H2, C2H4/H2, and HCl/H2, and the resulting crystal quality of epitaxial GaN films deposited on these substrates. To remove the many fine scratches and to smooth the rough surfaces typical of commercial SiC substrates, the Si-face 6H-SiC substrates were etched in H2, C2H4/H2, and HCl/H2 at 1450°C. GaN was subsequently deposited on these etched surfaces after first depositing a low temperature GaN buffer layer via metalorganic chemical vapor deposition (MOCVD). The surface morphologies after etching and after GaN deposition were characterized by atomic force microscopy and Normaski differential interference contrast microscopy, while the crystal quality of the GaN films was assessed by double crystal x-ray rocking curves and x-ray topography. 6H-SiC substrate surfaces were improved in terms of the removal of scratches and the reduction of surface roughness, and both surface morphology and crystal quality of the subsequently deposited GaN films were enhanced. However, the dislocation density was not decreased by the surface etching. The best GaN film was produced by etching the substrate in pure H2 for 40 minutes before growth. Recommendations for the optimum substrate treatment are made.
Influence of the First Preparation Steps on the Properties of GaN Layers Grown on 6H-SIC by Mbe
Authors: Lantier R. , Rizzi A. , Guggi D. , Lüth H. , Neubauer B. , Gerthsen D. , Frabboni S. , Colì G. , Cingolani R.
Published: April 2011

The Gan heteroepitaxy on 6H-SiC is affected by the bad morphology of the substrate surface. We performed a hydrogen etching at 1550°C on the 6H-SiC(0001) substrates to obtain atomically flat terraces. An improvement of the structural properties of GaN grown by MBE on such substrates after deposition of a LT-AIN buffer layer is observed. A value of less than 220 arcsec of the FWHM of the XRD rocking curve, showing a reduced screw dislocations density, is comparable with the best results reported until now for thick GaN samples. Photoluminescence showed a structured near band edge emission spectrum with evidence of the A, B and C free exciton recombinations.
Temperature Effect on the Quality of Ain Thin Films
Authors: Thompson Margarita P. , Drews Andrew R. , Huang Changhe , Auner Gregory W.
Published: April 2011

AIN thin films were deposited at various substrate temperatures via Plasma Source Molecular Beam Epitaxy. The films were grown on 6H-SiC (0001) substrates. Reflection High Energy Electron Diffraction and Atomic Force Microscopy showed a dramatic change in the surface morphology of the film grown at 640°C. This is attributed to a change in the growth mechanism from pseudomorphic at lower temperatures to three-dimensional at higher than 640°C temperatures. Photoreflectance measurements showed an absorption shift toward 200 nm as the deposition temperature increases which is attributed to the change in the growth mechanism at higher temperatures. X-Ray Diffraction was unable to conclusively determine the AIN (0002) peak due to a significant diffuse intensity from the SiC (0002) peak. A MIS structure was created by depositing Pt contacts on the film grown at 500°C. I-V measurements showed that the Pt/AIN contact is Schottky.
Piezoelectric Field Effect on Optical Properties of GaN/GaInN/AlGaN Quantum Wells
Authors: Im Jin Seo , Kollmer H. , Gfrörer O. , Off J. , Scholz F. , Hangleiter A.
Published: April 2011

We designed and studied two sample groups: first, GaInN/AlGaN/GaN quantum wells with asymmetric barrier structure and secondly, GaInN/GaN quantum wells with asymmetrically doped barriers. Time-resolved measurements on the asymmetric structure reveal an enhanced oscillator strength when the AlGaN barrier is on top of the GaInN quantum well, indicating a better carrier confinement in such a structure. The photoluminescence emission energy of the GaInN/GaN quantum well with doped GaN barriers shifts towards higher energy than that of undoped samples due to screening, but only when the GaN barrier layer below the quantum well is doped. In contrast, the sample where only a GaN cap layer above the quantum well is doped, shows no blue-shift. These results, showing asymmetries in GaInN/GaN quantum wells, provide confirming evidence of the piezoelectric field effect and allow us to determine the sign of the piezoelectric field, which points towards the substrate in a compressively strained quantum well. Furthermore, we performed model calculations of the global band bending and the screening effect, which consistently explain our experimental findings.
Rapid Thermal Processing of Implanted GaN Up to 1500°C
Authors: Cao X. A. , Pearton S. J. , Singh R. K. , Abernathy C. R. , Han J. , Shul R. J. , Rieger D. J. , Zolper J. C. , Wilson R. G. , Fu M. , Sekhar J. A. , Guo H. J. , Pennycook S. J.
Published: April 2011

GaN implanted with donor(Si, S, Se, Te) or acceptor (Be, Mg, C) species was annealed at 900-1500°C using AIN encapsulation. No redistribution was measured by SIMS for any of the dopants and effective diffusion coefficients are ≤2×10-13 cm2 s-1 at 1400°C, except Be, which displays damage-enhanced diffusion at 900°C and is immobile once the point defect concentration is removed. Activation efficiency of ∼90% is obtained for Si at 1400°C. TEM of the implanted material shows a strong reduction in lattice disorder at 1400-1500°C compared to previous results at 1100°C. There is minimal interaction of the sputtered AIN with GaN under our conditions, and it is readily removed selectively with KOH.
Photoelectrochemical Etching of InxGal-xN
Authors: Cho Hyun , Donovan S.M. , Abernathy C.R. , Pearton S.J. , Ren F. , Han J. , Shul R.J.
Published: April 2011

A comparison of KOH, NaOH and AZ400K solutions for UV photo-assisted etching of undoped and n+ GaN is discussed. The etching is diffusion-limited (Ea < 6kCal-mol-1) under all conditions and is significantly faster with bias applied to the sample during light exposure. No etching of InN was observed, due to the very high n-type background doping (> 1020cm-3) in the material.
Modeling of a GaN Based Static Induction Transistor
Authors: Bunea Gabriela E. , Dunham S.T. , Moustakas T.D.
Published: April 2011

Static induction transistors (SITs) are short channel FET structures which are suitable for high power, high frequency and high temperature applications. GaN has particularly favorable properties for SIT operation. However, such a device has not yet been fabricated. In this paper we report simulation studies on GaN static induction transistors over a range of device structures and operating conditions. The transistor was modeled with coupled drift-diffusion and heat-flow equations. We found that the performance of the device depends sensitively on the thermal boundary conditions, as self-heating effects limit the maximum voltage swing.
Ensemble Monte Carlo Study of Electron Transport in Bulk Indium Nitride
Authors: Bellotti E. , Doshi B. , Brennan K. F. , Ruden P. P.
Published: April 2011

Ensemble Monte Carlo calculations of electron transport at high applied electric field strengths in bulk, wurtzite phase InN are presented. The calculations are performed using a full band Monte Carlo simulation that includes a pseudopotential band structure, all of the relevant phonon scattering agents, and numerically derived impact ionization transition rates. The full details of the first five conduction bands, which extend in energy to about 8 eV above the conduction band minimum, are included in the simulation. The electron initiated impact ionization coefficients and quantum yield are calculated using the full band Monte Carlo model. Comparison is made to previous calculations for bulk GaN and ZnS. It is found that owing to the narrower band gap in InN, a lower breakdown field exists than in either GaN or ZnS.
Monte Carlo Simulation of Hall Effect in n-Type GaN
Authors: Albrecht J.D. , Ruden P.P. , Bellotti E. , Brennan K.F.
Published: April 2011

Results of Monte Carlo simulations of electron transport for wurtzite phase GaN in crossed, weak electric and magnetic fields are presented. It is found that the Hall factor, τH = μHall/μdrift, decreases monotonically as the temperature increases from 77K to 400K. The low temperature value of the Hall factor increases significantly with increasing doping concentration. The Monte Carlo simulations take into account the electron-lattice interaction through polar optical phonon scattering, deformation potential acoustic phonon scattering (treated as an inelastic process), and piezoelectric acoustic phonon scattering. Impurity scattering due to ionized and neutral donors is also included, with the latter found to be important at low temperature due to the relatively large donor binding energy which implies considerable carrier freeze-out already at liquid nitrogen temperature. The temperature dependences of the electron concentration, drift mobility, and Hall factor are calculated for donor concentrations equal to 5 × 1016 cm-3, 1017 cm-3, and 5 × 1017 cm-3. The Monte Carlo simulations are compared to classical analytical results obtained using the relaxation-time approximation, which is found to be adequate at low temperatures and sufficiently low carrier concentrations so that inelastic scattering effects due to optical phonons and degeneracy effects are negligible. The influence of dislocations on the Hall factor is discussed briefly.
Phonon Dynamics and Lifetimes of AIN and GAN Crystallites
Authors: Bergman Leah , Alexson Dimitri , Nemanich Robert J. , Dutta Mitra , Stroscio Michael A. , Balkas Cengiz , Davis Robert F.
Published: April 2011

The quasi-LO and quasi-TO modes of AIN crystallite were investigated. The analysis indicates that the Raman mode behavior concurs with Loudons’ model of mode-mixing in wurtzite (WZ) structure crystals which is due to the long-range electrostatic field. Phonon-lifetimes of GaN and AIN crystallites were studied via Raman lineshape. It was found that the low energy E2 mode lifetime is about an order of magnitude longer than that of the other modes, and that impurities impact significantly the phonon-lifetimes.
Transport, Growth Mechanisms, and Material Quality in GaN Epitaxial Lateral Overgrowth
Authors: Coltrin Michael E. , Willan Christine C. , Bartram Michael E. , Han Jung , Missert Nancy , Crawford Mary H. , Baca Albert G.
Published: April 2011

Growth kinetics, mechanisms, and material quality in GaN epitaxial lateral over-growth (ELO) were examined using a single mask of systematically varied patterns. A 2-D gas phase reaction/diffusion model describes how transport of the Ga precursor to the growth surface enhances the lateral rate in the early stages of growth. In agreement with SEM studies of truncated growth runs, the model also predicts the dramatic decrease in the lateral rate that occurs as GaN over-growth reduces the exposed area of the mask. At the point of convergence, a step-flow coalescence mechanism is observed to fill in the area between lateral growth-fronts. This alternative growth mode in which a secondary growth of GaN is nucleated along a single convergence line, may be responsible for producing smooth films observed to have uniform cathodoluminescence (CL) when using 1 μm nucleation zones. Although emission is comprised of both UV (∼365nm) and yellow (∼550nm) components, the spectra suggest these films have reduced concentrations of threading dislocations normally associated with non-radiative recombination centers and defects known to accompany growth-front convergence lines.
Fabrication of Self-Assembling AlGaN Quantum Dot on AlGaN Surfaces Using Anti-Surfactant
Authors: Hirayama H. , Aoyagi Y , Tanaka S.
Published: April 2011

We report on the first artificial fabrication of self-assembling AlGaN quantum dots (QDs) on AIGaN surfaces using metal organic chemical vapor deposition (MOCVD). The AlGaN QDs are fabricated using a growth mode change from 2-dimensional step-flow growth to 3-dimensional island formation by modifying the AlGaN surface energy with Si anti-surfactant. The average lateral size and the thickness of fabricated AlGaN QDs, as determined by AFM, are approximately 20 nm and 6nm, respectively. The dot density was found to be controlled from 5×1010cm-2 down to 2×109 cm-2 by increasing the dose of Si anti-surfactant. We obtained the photoluminescence (PL) from AlGaN QDs embedded with Al0.38Ga0.62N capping layers. The Al incorporation in AlGaN QDs was controllable within the range of 1-5 %.
Ni/Si-Based Contacts to GaN: Thermally Activated Structural Transformations Leading to Ohmic Behavior
Authors: Kaminska E. , Piotrowska A. , Jasinski J. , Kozubowski J. , Barcz A. , Golaszewska K. , Thomson D.B. , Davis R.F. , Bremser M.D.
Published: April 2011

Structural transformations in Ni/Si-based contacts to GaN occurring under heat treatment have been studied using transmission electron microscopy and secondary ion mass spectrometry. Transition from non-ohmic to ohmic behavior correlates with reaction between Ni and Si, and decomposition of the initially formed interfacial Ni:Ga:N layer. Transport of dopant atoms from metallization into GaN testifies in favour of the SPR process of ohmic contact formation
Epitaxial and Conductive RuO2 Thin Films Grown on MgO and LaAlO3 by MOCVD
Authors: Lu P. , He S. , Li F. X. , Jia Q. X.
Published: April 2011

Conductive RuO2 thin films have been grown epitaxially on (100) MgO and (100) LaAlO3 substrates by metal-organic chemical vapor deposition(MOCVD) at different temperatures. The microstructural properties of the RuO2 films have been studied using x-ray diffraction and scanning electron microscopy. Different growth and microstructure properties were observed for the films deposited on the two substrates. The films on MgO are epitaxial at deposition temperatures as low as 350°C, and consist of two variants with an orientation relationship given by (110) RuO2 /(100) MgO and [001] RuO2//[011]MgO. The films on LaAlO3, on the other hand, are epitaxial only at deposition temperatures of 600°C and above, and contain four variants with an orientation relationship given by (200)RuO2//(100)LaAlO3 and [011] RuO2//[011] LaAlO3. The observed microstructures of epitaxially grown films can be explained based on geometric considerations for the films and substrates.
Dependence of Ferroelectric Properties of SBT Based Capacitors on the Electrode Material
Authors: Tirumala S. , Ryu S. O. , Lee K. B. , Vedula R. , Desu S. B.
Published: April 2011

The effect of various electrode materials on the ferroelectric properties of SrBi2Ta2O9 (SBT) thin films has been investigated for non-volatile memory applications. Two sets of electrode structures, viz., Pt-Ir based and Pt-Rh based, were sputter deposited in-situ on Si substrates. SBT thin films were deposited on these electrodes using a metal-organic solution deposition technique followed by a post-deposition anneal at 750 °C in oxygen. Structural characterization revealed a polycrystalline nature with predominant perovskite phase in SBT thin films. Ferroelectric properties were studied in capacitor mode by depositing top electrodes, where the top electrode material is identical to that of the bottom electrode. Extensive analysis of the ferroelectric properties signify the important role played by the electrode material in establishing the device applicability is reported in this work.
All Epitaxial Babi4Ti4O15 - LaNiO3 Heterostructures
Authors: Satyalakshmi K. M. , Pignolet A. , Alexe M. , Zakharov N. D. , Harnagea C. , Senz S. , Reichelt S. , Hesse D. , Gösele U.
Published: April 2011

Bismuth-based layer-structured ferroelectric oxides are gaining much attention for ferroelectric thin film applications due to their low fatigue. Epitaxial thin films of these layered ferroelectric oxides grown on epitaxial perovskite-type conducting oxide electrodes such as LaNiO3 are known to further improve the fatigue resistance. In this paper the ferroelectric properties of BaBi4Ti4O15films grown by pulsed laser deposition on epitaxial LaNiO3/SrTiO3(100) and LaNiO3/YSZ/Si(100) substrates are presented. BaBi4Ti4O15thin films with mixed a - and c - orientation exhibit ferroelectric hysteresis loops with a remanent polarization Pr of 2 μC/cm2and a coercive field Ec of about 75 kV/cm. The effect of the deposition parameters on thin film orientation, morphology and the ferroelectric properties of BBiT are discussed.
Chemical Vapor Deposition Technology of (Ba,Sr)TiO3 Thin Films for Gbit-Scale Dynamic Random Access Memories
Authors: Horikawa Tsuyoshi , Tarutani Masayoshi , Kawahara Takaaki , Yamamuka Mikio , Hirano Noriko , Sato Takehiko , Matsuno Shigeru , Shibano Teruo , Uchikawa Fusaoki , Ono Kouichi , Oomori Tatsuo
Published: April 2011

The current status of (Ba,Sr)TiO3 [BST] capacitor technology using a liquid source chemical vapor deposition (CVD) method is reviewed, focusing on the CVD techniques and the physical, electrical and process-integration-related properties of Ru/BST/Ru capacitors. The use of a new titanium metalorganic (MO) source, titanium bis(tert-butoxy) bis(dipivaloylmethanato) [Ti(tertBuO)2 (DPM)2] dissolved in tetrahydrofuran (THF) turned out to enable highly conformal deposition of BST films with a coverage ratio of ∼ 70 % for a trench with an aspect ratio of ∼ 5. Electrical properties of a 24-nm-thick BST film, deposited on a Pt substrate at a low substrate temperature of 480 °C, were also confirmed to be equivalent SiO2 thickness (teq) of ∼ 0.5 nm and leakage current of ∼ 1 ×10-7 A/cm2 at 1 V. As for the Ru/BST/Ru capacitors, no deteriorations of Ru electrode and BST/Ru interface were observed after 750 °C post-annealing experiment, showing good thermal stability of Ru as a practical electrode material. Although current leak through Ru/BST/Ru capacitors slightly increased after the H2 annealing, such degradation in the leakage properties was restored by post-annealing in N2 ambience. Integrated Ru/BST/Ru capacitors with a 30-nm-thick CVD-BST film were fabricated by 0.5 μm ULSI technology, and low leakage current was confirmed for the stacked capacitors. Regarding the reproducibility of BST deposition by the liquid source CVD method, the deviation ratio of ∼ ± 2.3 % in film thickness was obtained for ∼ 100 successive depositions, thickness uniformity across the wafers was ∼ ± 1.1 %. The above results imply the potential applicability of BST capacitor technology using a liquid source CVD method for Gbit-scale DRAMs.
Schottky Barrier Heights of Tantalum Oxide, Barium Strontium Titanate, Lead Zirconate Titanate and Strontium Bismuth Tantalate
Authors: Robertson J , Chen C W
Published: April 2011

Schottky barrier heights of various metals on tantalum pentoxide, barium strontium titanate, lead zirconate-titanate and strontium bismuth tantalate have been calculated as a function of metal work function. These oxides have a dimensionless Schottky barrier pinning factor, S, of 0.28 – 0.4 and not close to 1, because S is controlled by the Ti-O type bonds not Sr-O type bonds, as assumed previously. Band offsets on silicon are asymmetric with much smaller offset at the conduction band, so that Ta2O5 and barium strontium titanate (BST) are relatively poor barriers to electrons on Si.
In-Situ TEM Study of Domain Propagation in Ferroelectric Barium Titanate, and Its Role in Fatigue
Authors: Krishnan A. , Bisher M.E. , Treacy M.M.J.
Published: April 2011

We have conducted in-situ transmission electron microscopy (TEM) experiments on thinned single crystal barium titanate in order to study the effects of applied electric field, temperature, electron beam irradiation and UV irradiation on domain nucleation and propagation. We observe two basic modes of domain wall motion; (i) a lateral motion which uniformly widens or narrows the total domain width; (ii) a “zipping” motion in which one end of a domain narrows to a point, which then propagates lengthwise, widening (or narrowing) the domain behind it. Both domain creation and destruction can occur by this latter process. When cooling from above Tc, domain growth usually occurs by the “zipping” motion. We believe that both the lateral and “zipping” modes of motion are related. The “zipping” mode tends to occur in the presence of inhomogeneous long-range strain fields, or when trapped charges are present locally. In some instances, the trapped charge is strong enough to show significant image contrast in bright-field. Domain motion, initiated by heat, electric fields or UV irradiation, moves such charges. A model of domain motion is presented which shows how displacement charge can be injected into the ferroelectric, and which may contribute to the fatigue of these materials.
Strain in Epitaxial BaTiO3 Thin Films Prepared by MOCVD
Authors: Chattopadhyay S. , Teren A. , Wessels B.W.
Published: April 2011

The microstrain in epitaxial BaTiO3 thin films has been investigated using x-ray diffraction. The full width half maximum of the (001) diffraction peaks ranged from 0.12 to 0.49 deg. From the analysis of the angular dependence of the diffraction peak broadening, it is concluded that the broadening is due predominantly to strain. The magnitude of the microstrain decreases sharply with increasing film thickness.
MOCVD Process of Ferroelectric Lead Germanate Thin Films and Bottom Electrode Effects
Authors: Zhang Fengyan , Li Tingkai , Nguyen Tue , Hsu Sheng Teng
Published: April 2011

Pb5Ge3O11 is a new promising candidate for nonvolatile memory such as one-transistor memory applications because of its moderate polarization and relative low dielectric constant. But this material is a low symmetry ferroelectric material and it is said the spontaneous polarization exists only along the c-axis. Furthermore, in the PbO- GeO 2 system, the stability range for the Pb5Ge3O11 phase is very limited and that even a relatively small deviation in composition or in growth temperature could lead to the formation of other lead germanate compounds. Many methods have been used to grow Pb5Ge3O11 film. This is the first time the Pb5Ge3O1 thin film has been successfully grown by advanced MOCVD technique with a liquid delivery system. This paper will discuss the MOCVD process for growing Pb5Ge3O11 thin films on different electrodes (Pt, Ir and Pt/Ir). The properties of these electrodes, the microstructures of the Pb5Ge3O11 thin films on these electrodes and their ferroelectric properties will be discussed. The advanced two-step deposition technique has been used to improve the uniformity of the Pb5Ge3O11 film. Pure c-axis oriented Pb5Ge3O11 thin film and polycrystalline Pb5Ge3O11 thin film with preferred c-axis orientation and some Pb3GeO5 secondary phase have been successfully grown on these electrodes with good ferroelectric properties.
A High Stability Electrode Technology for Stacked SrBi2Ta2O2 Capacitors
Authors: Kudo J. , Nagata M. , Ogata N. , Yamazaki S. , Urashima H. , Okutoh A. , Miyoshi T. , Ishihara K.
Published: April 2011

A high stability electrode technology with TaSiN as a barrier metal was used to fabricate a stacked SBT capacitor array on polySi plugs, and hysteresis characteristics and contact properties of the stacked capacitor were studied.
Piezoelectric Measurements with Atomic Force Microscopy
Authors: Christman J. A. , Maiwa H. , Kim S.-H. , Kingon A. I. , Nemanich R. J.
Published: April 2011

An atomic force microscope (AFM) is used to measure the magnitude of the effective longitudinal piezoelectric constant (d33) of thin films. Measurements are performed with a conducting diamond AFM tip in contact with a top electrode which is driven by an externally applied voltage. The interaction between the tip and electric field present is a potentially large source of error that is eliminated through the use of this configuration and the conducting diamond tips. Measurements yielded reasonable piezoelectric constants of X-cut single crystal quartz, thin film ZnO, Pb(Zr,Ti)O3 (Zr/Ti = 30/70), and nonpiezoelectric amorphous SiO2 thin films. The system was also used to measure d33 hysteresis loops for Pb(Zrx,Ti1−x)O3 thin films.
Analysis of Undoped and Ag-doped High-Tc YBCO Superconducting Bolometers Fabricated Using a Novel Anti-Reflective Coating and Photolithographic Technique
Authors: Moxey D. E. , Sharma A. , Narayan J. , Lee C. B. , Oktyabrsky S. , Muth J.
Published: April 2011

Superconducting transition edge bolometers have been fabricated on Ag-doped and undoped YBCO layers on Si(100) substrates using a YSZ buffer layer. We have also fabricated bolometer devices using a stacked-structure of undoped and Ag-doped YBCO material. These thin films were deposited using pulsed laser deposition (PLD). These devices were fabricated using a novel photolithographic/anti-reflective coating (ARC) process. The use of the anti-reflective coating shows no degradation in the overall quality of the superconducting film, but rather it serves as a barrier to moisture, and improves the overall optical collection efficiency of the bolometer device. At a bias of ImA, and chopping frequency of 100Hz, we have measured photoresponse as a function of device temperature, calculated responsivity, and estimated noise equivalent power (NEP) for each of the three device structures. Peak photoresponse, responsivity, and estimated NEP for the undoped bolometer device are 12μV, 0.6mV(W/cm2), and 2.03×103 W/cmHz1/2 respectively. Peak photoresponse, responsivity, and estimated NEP for the Ag-doped bolometer device are 50μV, 2.5mV(W/cm2), and 9.40×10−8 W/cmHz1/2 respectively. Peak photoresponse, responsivity, and estimated NEP for the stacked structure bolometer device are 70μV, 3.5mV(W/cm2), and 1.02× 10−7 W/cmHz1/2 respectively. These results are the first for Ag-doped YBCO bolometers, as well as for superconducting bolometers fabricated using our novel photoresist/anti-reflective coating method.
Humidity Effects and Aging Behavior in Granular Media
Authors: Restagno F. , Gayvallet H. , Bocquet L. , Charlaix E.
Published: April 2011

We present a study of humidity effects on the maximum stability angle in granular media. We show that a granular medium of small glass beads exhibits aging properties: the first avalanche angle increases logarithmically with the resting time of the pile. This aging behavior is found to depend on the relative humidity of the surrounding atmosphere. A short interpretation of this effect, based on a model of activated capillary condensation, is proposed.
Experimental Evidence for the Dominance of vuv Photons in Remote Hydrogen Plasma Treatments
Authors: Wilken R. , Hollander A. , Behnisch J.
Published: April 2011

The flux of vacuum ultraviolet (VUV) and ultraviolet (UV) photons from a hydrogen plasma was detected by fluorescence measurements of sodium salicylate sheets with and without a MgF2 window. In order to expose the samples to the same photon flux in all experiments, the power input to the microwave plasma source was adjusted. Polyethylene (PE), polypropylene (PP), and polystyrene (PS) were treated with VUV radiation or with a remote hydrogen plasma. The mass loss and the CH-absorption loss were recorded by in situ quartz crystal micro balance and by in situ IR reflection absorption spectroscopy, respectively. The VUV irradiation and plasma treatment caused similar effects in the case of PE, PP showed an increased polymer ablation in plasma treatments, and for PS a negligible loss in mass and CH absorption was detected in plasma and VUV treatments.
Arf Excimer Laser and Xe2· Excimer Lamp Induced Photochemical Fluorination of Polyimide Film
Authors: Ikegame T. , Murahara M.
Published: April 2011

Only one weak point of all aromatic group polyimide surface was changed to water repellency. Polyimide surface was photo-chemically modified to be fluorinate with ArF excimer laser and Xe2 · excimer lamp irradiation. To promote the photo- chemical reaction, Xe2· excimer lamp was employed to produce CFn. radical from CF4 gas. Simultaneously, ArF excimer laser was irradiated on the polyimide surface to dissociate C-H bond. Dangling bond of C was reacted with CF, radical and produced C-CF, on the polyimide surface. By this modification, polyimide surface was changed to water repellency. As a result, polyimide surface was photo-chemically modified to fluorinate with CF4 gas pressure of 100Torr, the Xe2 excimer lamp of 7mW/cm2, and the ArF excimer laser of 30mJ/cm2, 10Hz and 3000shots, the contact angle with water was 134 degrees. And chemical composition of the photo-modified polyimide surface was inspected by ATR-FTIR spectra measurement.
Effect of ion Bombardment in Polymer Surface Modification: Comparison of Pulsed High Frequency Plasma and ion Beam
Authors: Zabeida O. , Klemberg-Sapieha J. E. , Martinu L. , Morton D.
Published: April 2011

The energy and the flux of impinging ions are important factors which determine the properties of deposited films and of exposed surfaces (microstructure, density, hardness, roughness, stress, chemical structure, adhesion etc.). In the present work, we use a multigrid retarding field analyzer to study ion bombardment characteristics in two different systems: a pulsed microwave plasma reactor, and a cold cathode ion source. We have found that the ion energy distribution functions (IEDF) possess specific features for each mode of operation: we evaluate the shape and the maximum and the mean ion energies of the IEDF for different gases such as Ar and N2. These ion characteristics are correlated with surface restructuring of differently treated polymers (polycarbonate and polyethylene terephthalate), analyzed by XPS.
Thermophysical Properties of Fluids Measured Under Microgravity Conditions
Authors: Fecht H.-J. , Wunderlich R.K.
Published: April 2011

The analysis of nucleation and growth processes relies mostly on circular arguments since basic thermophysical properties necessary, such as the Gibbs free energy (enthalpy of crystallization, specific heat), the density, emissivity, thermal conductivity (diffusivity), diffusion coefficients, surface tension, viscosity, interfacial crystal / liquid tension, etc. are generally unknown with sufficient precision and therefore often deduced from insufficient linear interpolations from the elements. The paucity of thermophysical property data for commercial materials as well as research materials is mostly a result of the experimental difficulties arising from the unwanted convection and reactions of melts with containers at high temperatures. An overview will be given on the results of thermophysical property measurements during several different space flights using containerless processing methods. Furthermore, a perspective on a future measurement program of thermophysical properties supported by the European Space Agency is described. In this regard, the International Space Station is considered as the ideal laboratory for high precision measurements of thermophysical properties of fluids which help to improve manufacturing processes for a number of key industries.
Microgravity Processing of Biopolymer/Metal Composites for NLO Applications
Authors: Cuttle C.M. , Cattaneo M.V. , Gresser J.D. , Wise D.L. , Frazier D.O. , Aranda F. , Trantolo D.J.
Published: April 2011

The overall objective of this project is the development of NLO-active materials with optical clarity and mechanical strength. These materials are intended for laser eye protection. By combining χ2 and χ3 optical properties, the intensity of incident laser radiation may be efficiently reduced. Using an in-plane poling technique, aligned films of liquid crystal poly(benzyl-Lglutamate), PBLG, were made which showed higher second harmonic generation (SHG) values compared to quartz. Silver sols in the 10-90 nm diameter size range were complexed with tricyanovinyl aniline, TCVA, resulting in composite PBLG/Ag Sol films with higher than at least an order of magnitude of χ3 values materials such as polydiacetylenes and nitroanilines. These polymeric NLO materials offer definite advantages in terms of easy processability into films for the manufacture of the optical elements necessary for laser eye protection.
AC Modulation Calorimetry of Undercooled Liquid Ti34Zr11Cu47Ni8 and Zr57Nb5Ni12.6Al10Cu15.4: An MSL-1 Experiment Using TEMPUS
Authors: Glade S.C. , Lee D.S. , Wunderlich R. , Johnson W.L.
Published: April 2011

An AC modulation calorimetry (ACMC) experiment was performed on two bulk metallic glass forming alloys, Ti34Zr11CU47Ni8 (VIT 101) and Zr57Nb5Ni12.6Al10Cu15.4(VIT 106) on the recent MSL-1 shuttle flight using the TEMPUS hardware. VIT 106 exhibited a maximum undercooling of 140 K in free radiative cooling, while VIT 101 exhibited a smaller undercooling of 50 K. Specific heat measurements were done in both the stable and undercooled liquid regions. These results will be combined with further ground-based measurements in an electrostatic levitator for modeling the nucleation kinetics of these alloys and for calculation of the free energies and entropies of the undercooled liquids in these systems.
Materials for Shielding Astronauts from the Hazards of Space Radiations
Authors: Wilson J. W. , Cucinotta F. A. , Miller J. , Shinn J. L. , Thibeault S. A. , Singleterry R. C. , Simonsen L. C. , Kim M. H.
Published: April 2011

One major obstacle to human space exploration is the possible limitations imposed by the adverse effects of long-term exposure to the space environment. Even before human spaceflight began, the potentially brief exposure of astronauts to the very intense random solar energetic particle (SEP) events was of great concern. A new challenge appears in deep space exploration from exposure to the low-intensity heavy-ion flux of the galactic cosmic rays (GCR) since the missions are of long duration and the accumulated exposures can be high. Because cancer induction rates increase behind low to rather large thickness of aluminum shielding according to available biological data on mammalian exposures to GCR like ions, the shield requirements for a Mars mission are prohibitively expensive in terms of mission launch costs. Preliminary studies indicate that materials with high hydrogen content and low atomic number constituents are most efficient in protecting the astronauts. This occurs for two reasons: the hydrogen is efficient in breaking up the heavy GCR ions into smaller less damaging fragments and the light constituents produce few secondary radiations (especially few biologically damaging neutrons). An overview of the materials related issues and their impact on human space exploration will be given.
Impurity Characterization of Solar Wind Collectors for the Genesis Discovery Mission by Resonance Ionization Mass Spectrometry
Authors: Calaway W. F. , McCann M. P. , Pellin M. J.
Published: April 2011

NASA's Genesis Discovery Mission is designed to collect solar matter and return it to earth for analysis. The mission consists of launching a spacecraft that carries high purity collector materials, inserting the spacecraft into a halo orbit about the L1 sun-earth libration point, exposing the collectors to the solar wind for two years, and then returning the collectors to earth. The collectors will then be made available for analysis by various methods to determine the elemental and isotopic abundance of the solar wind. In preparation for this mission, potential collector materials are being characterized to determine baseline impurity levels and to assess detection limits for various analysis techniques. As part of the effort, potential solar wind collector materials have been analyzed using resonance ionization mass spectrometry (RIMS). RIMS is a particularly sensitivity variation of secondary neutral mass spectrometry that employs resonantly enhanced multiphoton ionization (REMPI) to selectively postionize an element of interest, and thus discriminates between low levels of that element and the bulk material. The high sensitivity and selectivity of RIMS allow detection of very low concentrations while consuming only small amounts of sample. Thus, RIMS is well suited for detection of many heavy elements in the solar wind, since metals heavier than Fe are expected to range in concentrations from 1 ppm to 0.2 ppt. In addition, RIMS will be able to determine concentration profiles as a function of depth for these implanted solar wind elements effectively separating them from terrestrial contaminants. RIMS analyses to determine Ti concentrations in Si and Ge samples have been measured. Results indicate that the detection limit for RIMS analysis of Ti is below 100 ppt for 106 averages. Background analyses of the mass spectra indicate that detection limits for heavier elements will be similar. Furthermore, detection limits near 1 ppt are possible with higher repetition rate lasers where it will be possible to increase signal averaging to 108 laser shots.
Orientation Dependent Strength and Cross-Slip Structure of Ordinary Dislocations in Single Crystal γ-Ti-56A1
Authors: Feng Q. , H. S.
Published: April 2011

The temperature as well as orientation dependence in anomalous hardening occurs in single crystal Ti-56AI between 673K and 1073K under single slip of ordinary dislocations. The ordinary dislocations (1/2<110]) are gliding not only on (111) plane but also on (110) plane in the temperature range where the anomalous hardening occurs in single crystal Ti-56A1. The TEM study shows that the (110) cross-slip of ordinary dislocations is a double cross-slip in nature in which first, the dislocations cross-slip from the primary (111) slip plane to (110) plane followed by cross-slipping again onto another primary slip plane. This double cross-slip leaves a pair of edge segments 'superjogs' in (110) planes. It appears that these superjogs are immobile in the forward direction and act as pinning points. Furthermore, these pinning points would act as a Frank-Read source for the double cross-slipped dislocations, which generate dislocation loops as well as dislocation dipoles. The pinning structure, multiplane dislocation loops, and dipoles of double cross-slip origin all contribute to anomalous hardening at high temperatures in this material.
Size Scaling in the Self-Immortalization of Superdislocations in the L12 Compounds Displaying the Yield Strength Anomaly
Authors: Chrzan D. C. , Uchic M. D. , Nix W. D.
Published: April 2011

A simple statistical analysis of the distribution of superkinks along a single superdislocation is presented. This analysis is then used to estimate the rate of self-immobilization of superdislocations. The self-immobilization rate decreases exponentially with increasing superdislocation length. The implications for experiment are discussed.
Processing and Characterization of Al2Ti/Al 3Ti Two-Phase Alloys
Authors: Lukitsch M. J. , Benci J. E.
Published: April 2011

Several buttons of each of three binary Ti-Al alloys containing nominally 70, 71 and 72 at.% Al were prepared from elemental Ti and Al by plasma arc-melting. One button of each composition was then either solution heat-treated, subjected to a two-step heat treatment, or hot forged. Each composition and material condition was then characterized using SEM/EDS and microhardness testing. The results show that while all three compositions have a predominantly two-phase, coarse microstructure in the as-cast condition, it is possible to produce an essentially single phase material through an appropriate solution heat treatment for the two lower Al content alloys. A fine two-phase microstructure can be achieved through an additional, lower temperature heat treatment step. The microhardness results show that the solution heat treatment reduced the hardness for all three compositions compared to the as-cast condition while hot forging as-cast samples increased hardness. The 70% Al alloy has the highest hardness for all four material conditions studied while the 71% Al alloy has the lowest hardness in three of the four material conditions.
Role of the Interfaces on the Activation of Slip Systems in The PST TiAl Alloy
Authors: Zghal Slim , Inui Haruyuki , Yamaguchi Masaharu , Couret Alain
Published: April 2011

The operative slip systems in the so-called PST alloy are determined by transmission electron microscopy. Both post-mortem analyses and in situ observations are presented. It is shown that glide of ordinary dislocations and twinning are the most easily deformation modes activated at yield. The role of the interfaces on the activation of these operative slip systems is then examined and discussed.
Ab initio Simulation of Vacancy Processes in Ni3Al
Authors: Schweiger H. , Moroni E. , Wolf W. , Püischl W. , Pfeiler W. , Podloucky R.
Published: April 2011

Properties of point defects such as antisites and vacancies in Ni3A1 are studied by means of ab initio calculations for supercells. Temperature dependent quantitities such as defect formation energies are derived by means of a grandcanonical ensemble. Stimulated by experiments of residual resistivities suggesting an outstandingly large activation energy of 4.6 eV due to Al vacancies, several models for point like defects are treated in combination with calculated migration barriers for nearest neighbor jumps and also the six-jump model.
Progress in adapting the Investment Casting of NiAl
Authors: Scheppe F. , Rothe H. , Sahm P. R.
Published: April 2011

NiAl alloys offer significant advantages as structral materials in gas turbine applications due to a unique range of physical and mechanical properties. The high melting point and low RT plasticity of NiAl alloys present special problems for mold materials. Typical shell molds for advanced superalloys are very close to there application limits. On the one hand it is necessary to keep the SiO2 level as low as possible, but on the other hand the mold has to be strong enough for the cast process and weak enough to allow for the contraction of the metal during the solidification. The result of this research is the application of a SiO2 shell mold for a heat shield. To scale up these results to production and quantities will be the further work.
Deformation Behavior of Polycrystalline MoSi2 Between 495°C and 1250°C
Authors: Junker L. , Bartsch M. , Messerschmidt U.
Published: April 2011

In order to study the deformation processes of reaction-sintered and hot pressed MoSi2, compression tests have been performed and the microstructure of the deformed samples has been investigated by optical, scanning and transmission electron microscopy. At a low strain rate of 2.5× 10−7 S−1 permanent deformation has been achieved at 1250°C. The deformation behaviour is controlled by different microprocesses within different temperature ranges. Below 1000°C, hardening occurs beyond the upper and lower yield points. Inside the grains, dislocations glide at sufficiently high stresses in a homogeneous way as well as in localized slip bands. A plateau in the dependence of the yield stress on temperaturecorresponds to the flow stress anomaly of easy slip systems in single crystals. However, since there are not enough independent slip systems to satisfy the van Mises criterion, the material does not deform homogeneously in all grains. Intergranular and intragranular cracks, which are formed after the yield point, carry an increasing part of strain at decreasing temperatures. Above 1000°C, softening occurs after the yield point. A grain boundary phase flows viscously and carries most of the deformation. Since the grains do not deform themselves, intergranular cracks develop also at high temperatures.