bluuebunny/crossref_metadata_2025 huggingface dataset:
Production of Nano-Crystalline Zirconia-Based Powders Using a Flow-Through Hydrothermal Process
Authors: Darab John G.
Published: April 2011

Nano-crystalline, zirconium oxide and oxyhydroxide particulates were produced using a novel, continuous, flow-through hydrothermal technology which was previously developed at the Pacific Northwest National Laboratory. Homogeneous aqueous solutions containing 0. LM ZrO(NO3)2, 0.2-0.5M urea, and in some cases 0.025-0.10M NaF were pumped at pressures of 6000-8000 psi through a heated stainless steel reaction tube at 340-350°C. Flow at pressure was maintained using a nozzle at the down-stream end of the reaction tube. In this process, termed Rapid Thermal Decomposition of precursors in Solution (RTDS), the formation of zirconium oxide and oxyhydroxide nano-crystals (diameters of generally less than 15-nm) occurred during the solution's brief residence time (<15 seconds) in the reaction tube, yielding aqueous slurries of agglomerated particulates upon exiting the nozzle. Powders were separated from these slurries, washed, then dried. RTDS powder prepared from 0.1M ZrO(NO3)2and 0.5M urea had a surface area of 210 m2/g and was determined to be a mixture of predominantly nano-crystalline (<10 nm crystallite diameter) cubic ZrO2 with the balance being monoclinic ZrO2. This powder was subsequently calcined to 500°C, pressed into a pellet at 20,000 psi and sintered at 500°C for one hour to yield a mechanically stable porous support with a measured surface area of 125 m2/g. In contrast, RTDS powder prepared from 0.1M ZrO(NO3)2, 0.5M urea, and 0.1M NaF was determined to be pure monoclinic ZrO2with a surface area of 58 m2/g.
Rheology of Colloidal Suspensions and its Implication for Tank Waste Processing
Authors: Hill Kimberly M. , Martin James E. , Adolf Douglas A.
Published: April 2011

The rheology of colloidal suspensions can vary enormously depending on the contents of the suspensions and the details of how they were produced. Certain interactions between particles within colloidal suspensions can greatly affect their rheology which can be a serious impediment to their processing. For example, at DOE's Hanford site, the radioactive tank wastes are very concentrated and have formed a sludge that is too viscous to process. We are interested in understanding the rheology of this system in order to develop an efficient method for processing the waste. Towards this end we have developed a model silica system for the tank wastes whose particles can interact through chemical bonding, Keesom interactions, and Coulombic interactions. Through systematic variation of the solvent and the particle concentration we can alter the rheological behavior from shear thickening to shear thinning to Newtonian. We present preliminary results for this system as it applies to the processing of the tank waste at Hanford.
Aluminum Foam, “Alporas”: The Production Process, Properties and Applications
Authors: Miyoshi T. , Itoh M. , Akiyama S. , Kitahara A.
Published: April 2011

The production of foamed aluminum has long been considered difficult to realize because of such problems as the low foamability of molten metal, the varying size of cellular structures, solidification shrinkage and so on. Recently these problems have been solved by a number of researches and some manufacturers produce foamed aluminum by their own methods. We have been employing a batch casting process and manufacturing foamed aluminum under the tradename ALPORAS® since 1986. This paper presents the manufacturing process, physical properties and some typical applications of ALPORAS.
Eddy Current Characterization of Metal Foams
Authors: Dharmasena K. P. , Wadley H. N. G.
Published: April 2011

Cellular materials are characterized by their relative density, pore shape and orientation, the average cell size, and the degree of pore interconnectivity which all depend upon the method and conditions used for processing. This has created an interest in non-invasive sensor techniques to characterize the foam structure. Multifrequency electrical impedance measurements were performed using an eddy current technique on open cell aluminum foam with systematically varied relative densities and pore sizes. The impedance was dominated at all frequencies by the amount of metal contained within a probed volume of foam and the tortuosity of the current path. At low frequency, the impedance data were found to be relatively insensitive to pore size variations enabling an independent measure of the relative density. At high frequency, the data indicated a strong dependence on the cell size.
In-Situ Sensing of the Expansion of Low Density Core (LDC) Ti-6Al-4V Sandwich Structures
Authors: Queheillalt D. T. , Choi B. W. , Wadley H. N. G. , Schwartz D. S.
Published: April 2011

A combination multifrequency eddy current and laser ultrasonic sensors have been used to measure the pore expansion kinetics and elastic moduli evolution during the annealing of low density core (LDC) Ti-6Al-4V sandwich structures. The LDC samples were heated to 920°C and held there for up to 12 hr. The eddy current sensor measured the sample thickness (i.e. relative density) and revealed that the samples began to expand early during heating and was nearly complete after 4 hr at 920°C. The laser ultrasonic sensor measurements indicated a concomitant decrease in the elastic moduli with the reduction in relative density. The combination of an eddy current and laser ultrasonic sensor is therefore able to measure both the density and the elastic moduli independently during the annealing stage of LDC Ti-6Al-4V sandwich structure processing providing a simple method for directly controlling the parameters most critical to aerospace applications of these new materials.
Polycrystalline Sil-x-yGex.Cy for Suppression of Boron Penetration in PMOS Structures
Authors: Chang C. L. , Sturm J. C.
Published: April 2011

We have fabricated polycrystalline Sil-x-yGexCy by Rapid Thermal Chemical Vapor Deposition and used it as part of a polycrystalline gate structure for PMOS devices. The results showed that the use of carbon in polycrystalline Sil-x-yGexCy suppressed boron penetration across the gate oxide. No effects of gate depletion with the use of poly-Sil-x-yGexCy were observed. Our work suggests that the addition of carbon reduced the chemical potential of boron in Sil-x-yGexCy, which deterred boron from diffusing across the underlying gate oxide.
Characterisation of Low Energy Boron Implantation and Fast Ramp-Up Rapid Thermal Annealing
Authors: Collart E. J. H. , de Cock G. , Murrell A. J. , Foad M. A.
Published: April 2011

The effects of ramp-up rate during rapid thermal processing of ultra-shallow boron implants have been investigated. Ramp-up rates were varied between 25 °C and 200 °C for two types of anneals: soak anneals and spike anneals. It was found that the ramp-up rate had very little influence on junction depth or electrical activation for both types of anneals. Spike anneals did produce shallower profiles than soak anneal for a comparable electrical activation and may be an option for future processes.
Process Design & Integration of Salicide and Source/Drain Process Modules for Improved Device Performance
Authors: Apte Pushkar P. , Saxena Sharad , Rao Suraj , Vasanth Karthik , Prinslow Douglas A. , Kittl Jorge A. , Breedijk Terence , Pollack Gordon
Published: April 2011

In integrated circuit (IC) fabrication, understanding and optimizing process interactions and variability is critical for swift process integration and performance enhancement, especially at dimensions ≤0.25μm. We present here an approach to address this challenge, and we apply it to improve the process design for two critical modules in a typical CMOS IC process—salicide and source/drain. Together, these modules impact the silicide-to-diffusion contact resistance (Rc), and the gate sheet resistance (Rs); which, in turn, significantly affect transistor series resistance and circuit delays respectively. In our approach, we have investigated a process domain consisting of both silicide and source/drain process variables; and we have developed a quantitative framework for analysis and optimization, along with qualitative insight into underlying the physical mechanisms. We demonstrate that the transistor drive current (Id) improves by ≈5‥, and circuit performance, as measured by the figure-of-merit (FOM), by ≈4‥. This improvement is significant, and an added benefit is that other transistor characteristics such as effective channel length, off-current, substrate current etc. are affected minimally. Finally, we use this approach to optimize trade-offs such as Rc vs Rs and performance vs manufacturability; thus enabling manufacturable processes that meet the requirements for high performance.
Bandgap Shifting of an Ultra-Thin InGaAs/InP Quantum Well Infrared Photodetector via Rapid Thermal Annealing
Authors: Sengupta D. K. , Kim S. , Kuo H. C. , Curtis A. P. , Hsieh K. C. , Bishop S. G. , Feng M. , Stillman G. E. , Gunapala S. D. , Bandara S. V. , Chang Y. C. , Liu H. C.
Published: April 2011

We demonstrate that SiO2 cap rapid thermal annealing in ultra-thin p-type InGaAs/InP quantum wells can be used to produce large blue shifts of the band edge. A substantial bandgap blue shift, as much as 292.5 meV at 900°C has been measured and the value of the bandgap shift can be controlled by the anneal time. Theoretical modeling of the intermixing effect on the energy levels is performed based on the effective bond-orbital method, and we obtain a very good fit to the photoluminescence data. Compared to the as-grown detector, the peak spectral response of the annealed detector was shifted to longer wavelength without any major degradation in the responsivity characteristics.
On the Entrance Effects and the Influence of Buoyancy Forces on the Fluid Flow In Rtp Reactors
Authors: Rainova Yu. P. , Antonenko K. I. , Pezoldt J. , Schenk A.
Published: April 2011

Flow and heat transfer effects play a critical role in chemical vapour deposition (CVD) reactors. Holographic interferometry offers the possibility to study the flow dynamics of an entire region in real time under process conditions with negligible disturbances of the investigated process. The advantage of the method is the possibility of accurate measurements of flow patterns and temperature profiles in a fast dynamic process under actual experimental conditions. Mixed and forced flow conditions were studied for two gases with distinct different thermophysical properties. The influence of the gas inlet system and buoyancy forces on the resulting fluid flow are highlighted. A special attention was drawn on the visualization of edge effects.
In-Line Ambient Impurity Measurement on a Rapid Thermal Process Chamber by Atmospheric Pressure Ionisation Mass Spectrometry
Authors: Kondoh Eiichi , Vereecke Guy , Heyns Marc M. , Maex Karen , Gutt Thomas , Nwnyeil Zsolt
Published: April 2011

Gaseous impurities in the chamber of a SHS2800ε rapid thermal processor were quantitatively measured by using atmospheric pressure ionisation mass spectrometry (APIMS). APIMS is a very sensitive technique to detect trace impurities in a bulk (1 atm) gas. A wide dynamic range (0.1 ppb - 10 ppm) measurement was successfully performed, which allowed in-situ monitoring of impurities during RTP. This work reports the fundamental behaviour of ambient impurities originating from different sources. The sources discussed in this paper are threefold: system background, wafer loading, and the wafer itself. Ambient management requires a better understanding of the independent contribution of each source on processing.
Ripening Mechanisms in Ultrathin Metal Films
Authors: Rosenfeld Georg , Essera Marcus , Morgensternat Karina , Comsa George
Published: April 2011

Results of recent experimental model studies on ripening of submonolayer films via Ostwald ripening and dynamic coalescence are described. The experiments have been performed on ensembles of Ag-adatom or vacancy islands on a Ag(111) surface using Scanning Tunneling Microscopy. For Ostwald ripening of adatom islands, deviations from the classical mean-field ripening behaviour are observed which show up as pronounced local correlations in island decay and growth rates. For ripening via dynamic coalescence which is studied for ensembles of vacancy islands, it is found that the increase of the average island size with time in the late-stage regime is correctly described by the classical binary collision model.
A Continuum Approach to Two-Component Thin Film Growth
Authors: Yu Yi-Kug , Wille Luc T.
Published: April 2011

Theoretical effort so far in understanding epitaxial growth has focused mainly on the one-component growth, i.e. growth that can be fully characterized by a surface (or height) profile. The predictions are also quite limited to the height-height correlation functions as a function of substrate size and the amount of deposition. In this paper, we consider the case of a two-component growth which is quite common in metallic thin films. Instead of using large-scale simulation, we first write down the appropriate two-component growth equations in continuum form. These equations are carefully designed such that in the limit of one-component growth the corresponding equation is recovered. Analytical and numerical analysis of the proposed equations allow us to study the long-range physics associated with these growth processes. Comparison with computer growth experiments is also mentioned.
Direct Method of Microstructure Modeling
Authors: Aivazov A A. , Bodyagin N.V. , Vikhrov S.P.
Published: April 2011

There discussed some possibilities basic concept of the nonlinear system theory and methods of modeling the microstructure of solids. It is demonstrated, that the substance distribution on the surface and in the volume of some materials possesses a determined chaotic character.
Bending of an Optical Glass Fiber “Pigtail” in a Splice Box Structure
Authors: Suhir E.
Published: April 2011

We examine a glass fiber “pigtail” in a gain equalization filter (GEF) splice box (SB) design. The fiber experiences bending on the surface of the SB cylinder. The objective of the analysis is to evaluate the “natural” configuration of the fiber, and to suggest, its optimized configuration for minimum curvatures and lower bending stresses. We show that the effect of such an optimization is the largest (about 20%) for cylindrical surfaces of large radii. The obtained solutions can be helpful in the analysis of optical glass fibers experiencing bending on cylindrical surfaces.
B-value and Optical Fiber Lifetime
Authors: Bubnov M. M. , Semjonov S. L.
Published: April 2011

In this work we analyze the existing literature relating to the B-value and the inert strength measurements. The weak flaw statistic model for fiber lifetime estimation is considered with regard for the two-region crack growth.
Creep Resistant Solders for Packaging in Optical Communication Devices
Authors: Jin S. , Mavoori H.
Published: April 2011

Very fine oxide dispersoids have been incorporated into solder alloys to create a new, improved solder structure with an ultrafine grain size of ˜200 -500 nm and significantly enhanced mechanical properties. The microstructure is seen to be quite stable upon high temperature exposure (e.g.. 120°C), which is attributed to the presence of very fine dispersoid particles which impede grain boundary sliding and dislocation movement. As much as three orders of magnitude reduction in the steady state creep rate has been achieved. The new solders also exhibit improved (4–5 times higher) tensile strength at low strain rates and improved ductility under high strain rate deformation. It is demonstrated that with a dispersion of TiO2 particles, the Pb-Sn eutectic solder with a low melting point of 183°C can be made more creep-resistant than the 80Au-2OSn eutectic solder with a much higher melting point of 278°C. The new creep-resistant solders can be useful for optical or optoelectronic packaging in which the dimensional stability of the assembled structure is essential.
RBS Lattice Site Location and Damage Recovery Studies In GaN
Authors: Alves E. , DaSilva M.F. , Soares J.C. , Bartels J. , Vianden R. , Abernathy C.R. , Pearton S.J.
Published: April 2011

Erbium was implanted with 160 keV at doses between 5x1014 and 5x1015 at/cm2 into (0001) epitaxial GaN on sapphire and annealed at various temperatures between 6000° and 10000° C. The RBS/Channeling technique was used to analyze the damage recovery during different annealing steps and to determine the lattice location of the implanted Er. For a sample implanted with 5x1014 and annealed for 30 min at 600° C a complete overlap of the Er and Ga angular scans across the <0001> axis was observed, indicating that 100% of Er occupies substitutional sites. Measurements along the <101> channel show that Er is located on Ga sites. The damage recovery was slightly better for the samples co-implanted with the same dose of Oxygen in an overlapping profile (E=25 keV). However, a complete recovery of the damage caused by the implantation was not achieved. Samples implanted with higher Er and 0 doses (5x 1015 at/cm2) and at the same energies as above were annealed at 600° for 30 min and at 900°, 1000° C for 120 s using a proximity cap. The higher dose caused an almost complete amorphisation of the surface layer. After annealing indications of epitaxial regrowth were observed, however, the substitutional fraction remains substantially lower and the damage recovery is less complete.
Optical and Structural Properties of AlGaN/GaN Quantum Wells Grown by Molecular Beam Epitaxy
Authors: Grandjean Nicolas , Massies Jean , Leroux Mathieu , Latigt Marguerite , Lefebvre Pierre , Gil Bernard , Allègre Jacques , Bigenwald Pierre
Published: April 2011

AIGaN/GaN quantum well (QWs) were grown on (0001) sapphire substrates by molecular beam epitaxy (MBE) using ammonia as nitrogen precursor. The Al composition in the barriers was varied between 8 and 27 % and the well thickness from 4 to 17 monolayers (MLs, 1ML = 2.59Å). X-ray diffraction (XRD) experiments are used to investigate the strain state of both the well and the barriers. The QW transition energy are measured by low temperature photoluminescence (PL). A large quantum confined Stark effect is observed leading to QW luminescence much lower than the emission line of the GaN buffer layer for well width above a certain critical thickness. The built-in electric field responsible for such a phenomenon is deduced from fit of the PL data. Its magnitude is of several hundred kV/cm and increases linearly with the Al composition.
Defect Luminescence in Heavily Mg Doped GaN
Authors: Reshchikov M. A. , Yi G.-C. , Wessels B. W.
Published: April 2011

Behavior of the photoluminescence band at about 2.8 eV in heavily Mg doped GaN has been studied at different temperatures and excitation intensities. The 2.8 eV band is attributed to donor-acceptor transitions involving a Mg acceptor. The large blue shift of the band with increasing excitation intensity is explained by variation in the contribution of close and distant pairs to the luminescence. The red shift of the band with increasing temperature under high excitation intensity conditions results from thermal release of carriers from close pairs. The thermal activation energy of the deep donor, about 0.4 eV, is determined from the quenching of the 2.8 eV luminescence band at high temperatures.
Mechanisms of Optical Gain in Cubic GaN and Ingan
Authors: Holst J. , Hoffmann A. , Broser I. , Frey T. , Schöttker B. , As D.J. , Schikora D. , Lischka K.
Published: April 2011

The epitaxial growth of zinc-blende (cubic) GaN and InGaN on GaAs with a common cleavage plane and readily high-quality, low-cost wafers may be considered as an alternative approach for the future realization of cleaved laser cavities. To obtain detailed information about the potential of cubic GaN and InGaN for device applications we performed optical gain spectroscopy accompanied by time-integrated and time-dependent photoluminescence measurements at 2 K and 300 K. From intensity-dependent gain measurements, the identification of the gain processes was possible. For moderate excitation levels, the biexciton decay is likely to be responsible for a gain structure at 3.265 eV in cubic GaN [10]. For the highest pump intensities, the electron- hole-plasma is the dominant gain process, providing gain values up to 200 cm-1. Furthermore cubic GaN samples with different cavity lengths from 250 to 600 mim were cleaved to investigate the influence of the sample geometry on the gain mechanisms. In these samples increased gain values up to 150 cm-1 as well as lower threshold excitation densities were observed, indicating the potential of cubic GaN for device applications. The results of GaN will be compared with intensity-dependent gain measurements on InGaN samples, grown on GaAs with varying In-content. The observed gain mechanisms in cubic InGaN will be discussed in detail.
Spectroscopic Studies in InGaN Quantum Wells
Authors: Chichibu S. E , Sota T. , Wada K. , DenBaars S. P , Nakamura S.
Published: April 2011

Fundamental electronic modulations in strained wurtzite If-nitride, in particular InxGa1-xN, quantum wells (QWs) were treated to explore the reason why practical InGaN devices emit bright luminescences in spite of the large threading dislocation (TD) density. The emission mechanisms were shown to vary depending on the well thickness L and InN molar fraction x. The electric field across the QW plane, F, which is a sum of the fields due to spontaneous and piezoelectric polarization and the pn junction field, causes the redshift of the ground state resonance energy through the quantum confined Stark effect (QCSE). The absorption spectrum is modulated by QCSE, quantum-confined Franz-Keldysh effect (QCFK), and Franz-Keldysh (FK) effect from the barrires when, for the first approximation, potential drop across the well (F/L) exceeds the valence band discontinuity, δEv. Under large F/L, holes are confined in the triangular potential well formed at one side of the well. This produces apparent Stokes-like shift in addition to the in-plane net Stokes shift on the absorption spectrum. The QCFK and FK further modulate the electronic structure of the wells with L greater than the three dimensional (3D) free exciton (FE) Bohr radius, aB. When F/L exceeds ΔEc, both electron (e) and hole (h) confined levels drop into the triangular potential wells at opposite sides of the wells, which reduces the wavefunction overlap. Doping of Si in the barriers partially screens the F resulting in a smaller Stokes-like shift, shorter recombination decay time, and higher emission efficiency. Finally, the use of InGaN was found to overcome the field-induced oscillator strength lowering due to the spontaneous and piezoelectric polarization. Effective in-plane localization of the QW excitons (confined excitons, or quantized excitons) in quantum disk (Q-disk) size potential minima, which are produced by nonrandom alloy potential fluctuation enhanced by the large bowing parameter and F, produces confined e-h pairs whose wavefunctions are still overlapped when L<aB. Their Coulomb interaction is more pronounced for F L<ΔEv.
Ranges of Deposition Temperatures Applicable for Metalorganic Vapor Phase Epitaxy of GaN Films Via the Technique of Pendeo-Epitaxy
Authors: Thomson D. B. , Gehrke T. , Linthicum K. J. , Rajagopal P. , Davis R. F.
Published: April 2011

Pendeo-epitaxy is a type of selective growth of thin films from the sidewalls of etched forms. The resulting films are suspended from the sidewalls and do not interface with the substrate. This process route has advantages over conventional lateral epitaxial overgrowth (LEO) techniques. In this research, pendeo-epitaxial growth of GaN films has been achieved on elongated GaN seed columns. The seed columns were etched from GaN grown on 6H-SiC (0001) substrates via metalorganic vapor phase epitaxy (MOVPE). Silicon nitride mask layers atop the GaN seed columns forced growth from the sidewalls. Pendeo-epitaxial growth of GaN was investigated using several growth temperatures. Higher growth temperatures resulted in improved coalescence due to greater lateral to vertical growth ratios.
High Quality Hydrothermal ZnO Crystals
Authors: Suscavage M. , Harris M. , Bliss D. , Yip P. , Wang S.-Q. , Schwall D. , Bouthillette L. , Bailey J. , Callahan M. , Look D. C. , Reynolds D. C. , Jones R. L. , Litton C. W.
Published: April 2011

Zinc Oxide crystals have historically been grown in hydrothermal autoclaves with a basic mineralizer; however, doubts have been raised about the quality of such crystals because they have often exhibited large x-ray rocking curve widths and low photoluminescence (PL) yield with large linewidths. Several ZnO crystals were grown hydrothermally and sliced parallel to the c-plane. This resulted in opposite surfaces (the C+ and C-) exhibiting pronounced chemical and mechanical differences. Different surface treatments were investigated and compared by PL both at room temperature and liquid helium temperatures, and by double axis X-ray rocking curve measurements. The high quality of hydrothermally-grown ZnO is substantiated by the narrow rocking curve widths and sharp PL peaks obtained. A critical factor in obtaining these results was found to be surface preparation.
Si Delta Doped GaN Grown by Low-Pressure Metalorganic Chemical Vapor Deposition
Authors: Kim Jong-Hee , Yang Gye Mo , Choi Sung Chul , Choi Ji Youn , Cho Hyun Kyung , Lim Kee Young , Lee Hyung Jae
Published: April 2011

Si delta-doping in the GaN layer has been successfully demonstrated by low-pressure metalorganic chemical vapor deposition at a growth temperature of 1040°C. Si delta-doping concentration increases and then decreases with an increase in delta-doping time. This indicates that delta-doping concentration is limited by the desorption process owing to much higher thermal decomposition efficiency of silane at high growth temperatures of GaN. In addition, it was observed that the use of a post-purge step in the ammonia ambient reduces Si delta-doping concentration. From capacitance-voltage measurement, a sharp carrier concentration profile with a full-width at half maximum of 4.1 nm has been achieved with a high peak concentration of 9.8 x 1018 cm-3
Influence of Doping on the Lattice Dynamics of Gallium Nitride
Authors: Kaschner A. , Siegle H. , Hoffmann A. , Thomsen C. , Birkle U. , Einfeldt S. , Hommel D.
Published: April 2011

We present results of Raman-scattering experiments on GaN doped with Si, C, and Mg, respectively, grown by molecular beam epitaxy (MBE). The influence of the different dopants on strain and free-carrier concentration was investigated. Furthermore, we report on several local vibrational modes (LVM) around 2200 cm-1 in Raman spectra of highly Mg-doped GaN. A possible explanation of these high-energy modes in terms of hydrogen-related vibrations is given. We also found a variety of new structures in the range of the GaN host lattice phonons. Secondary ion mass spectroscopy (SIMS) was applied to determine the concentration of magnesium and unintentionally incorporated hydrogen.
Photoluminescence Excitation Spectroscopy of Carbon-Doped Gallium Nitride
Authors: Reuter E. E. , Zhang R. , Kuech T. F. , Bishop S. G.
Published: April 2011

We have done a comparative study of carbon-doped GaN and undoped GaN utilizing photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies in order to investigate deep levels involved in yellow luminescence (YL) and red luminescence (RL). When the GaN was excited by above-bandgap light, red luminescence (RL) centered at 1.82 eV was the dominant below-gap PL from undoped GaN, but carbon-doped GaN below-gap PL was dominated by yellow luminescence (YL) centered at 2.2 eV. When exciting PL below the band-gap with 2.4 eV light, undoped GaN had a RL peak centered at 1.5 eV and carbon-doped GaN had a RL peak centered at 1.65 eV. PLE spectra of carbon-doped GaN, detecting at 1.56 eV, exhibited a strong, broad excitation band extending from about 2.1 to 2.8 eV with an unusual shape that may be due to two or more overlapping excitation bands. This RL PLE band was not observed in undoped GaN. We also demonstrate that PL spectra excited by below gap light in GaN films on sapphire substrates are readily contaminated by 1.6-1.8 eV and 2.1-2.5 eV chromium-related emission from the substrate. A complete characterization of the Cr emission and excitation bands for sapphire substrates enables the determination of the excitation and detection wavelengths required to obtain GaN PL and PLE spectra that are free of contributions from substrate emission.
Selective Area Growth (SAG) and Epitaxial Lateral Overgrowth (Elo) of GaN Using Tungsten Mask
Authors: Kawaguchi Yasutoshi , Nambu Shingo , Sone Hiroki , Yamaguchi Masahito , Miyake Hideto , Hiramatsu Kazumasa , Sawaki Nobuhiko , Iyechika Yasushi , Maeda Takayoshi
Published: April 2011

Selective area growth (SAG) and epitaxial lateral overgrowth (ELO) of GaN using tungsten (W) mask by metalorganic vapor phase epitaxy (MOVPE) and hydride vapor phase epitaxy (HVPE) have been studied. The selectivity of the GaN growth on the W mask as well as the SiO2 mask is excellent for both MOVPE and HVPE. The ELO-GaN layers are successfully obtained by HVPE on the stripe patterns along the <1100> crystal axis with the W mask as well as the SiO2 mask. There are no voids between the SiO2 mask and the overgrown GaN layer, while there are triangular voids between the W mask and the overgrown layer. The surface of the ELO-GaN layer is quite uniform for both mask materials. In the case of MOVPE, the structures of ELO layers on the W mask are the same as those on the SiO2 mask for the <1120> and <1100> stripe patterns. No voids are observed between the W or SiO2 mask and the overgrown GaN layer by using MOVPE.
Epitaxial Lateral Overgrowth of GaN on SiC and Sapphire Substrates
Authors: Yu Zhonghai , Johnson M.A.L. , Brown J.D. , El-Masry N.A. , Muth J. F. , Cook J.W. , Schetzina J.F. , Haberern K.W. , Kong H.S. , Edmond J.A.
Published: April 2011

The epitaxial lateral overgrowth (ELO) process for GaN has been studied using SiC and sapphire substrates. Both MBE and MOVPE growth processes were employed in the study. The use of SiO2 versus SiNx insulator stripes was investigated using window/stripe widths ranging from 2 μm/4 μm to 3 μm/15 μm. GaN film depositions were completed at temperatures ranging from 800°C to 1120°C. Characterization experiments included RHEED, TEM, SEM and cathodolumenescence studies. The MBE growth experiments produced polycrystalline GaN over the insulator stripes even at deposition temperatures as high as 990°C. In contrast, MOVPE growth produced single-crystal GaN stripes with no observable threading dislocations.
Photoluminescence of Fs-GaN Treated in Alcoholic Sulfide Solutions
Authors: Zhilyaev Y.V. , Kompan M.E. , Konenkova E.V. , Raevskii S.D.
Published: April 2011

Results are presented on the photoluminescence of n-GaN (T=300 K) after surface treatment with sulfide (Na2S and (NH4)2S) solutions in water or isopropyl alcohol.

It has been shown that the intensity of the n-GaN photoluminescence band is enhanced as a result of the surface treatment with alcoholic sulfide solutions, this enhancement being greater for a strongly basic Na2S solution than for a weakly basic (NH4)2S solution.
Xps Study of Oxygen Adsorption on (3X3) Reconstructed MBE Grown GaN Surfaces
Authors: Beach R. A. , Piquette E. C. , McGill T. C. , Watson T. J.
Published: April 2011

The incorporation of oxygen onto the (3x3) reconstructed surface of GaN(0001) has been studied using X-ray Photoelectron Spectroscopy (XPS). It was found that the (3x3) reconstruction corresponds to a fractional Ga adlayer atop a Ga terminated GaN surface. Our measurements indicate a surface coverage of 1.15 ± 0.2 monolayers of relaxed Ga on the surface. The binding energy separation between the relaxed surface Ga3d core level and bulk Ga3d level was measured to be 1.1 ± 0.1 eV. A metallic component extending from the bulk GaN valence band maximum out to 0 eV was also present in the XPS spectrum. The separation between the bulk valence band maximum and the Fermi level of the metallic component was found to be 2.1 ± 0. 1 eV. The relaxation of the surface Ga was found to decrease with oxygen exposure indicating Ga-O bonding, with oxygen adsorption terminating at 1.3 ± 0.2 monolayers. The Ols core level was found to have a FWHM of 2.0 ± 0.1 eV.
Extrinsic Performance Limitations of AlGaN/GaN Heterostructure Field Effect Transistors
Authors: Ruden P.P. , Albrecht J.D. , Sutandi A. , Binaril S.C. , Ikossi-Anastasiou K. , Ancona M.G. , Henry R.L. , Kolesket D.D. , Wickendent A.E.
Published: April 2011

Extrinsic effects on the DC output characteristics of AlGaN/GaN HFETs with lllm gate lengths are examined. The devices investigated were fabricated on MOCVD-grown AlGaN/GaN heterostructures on sapphire substrates. An analytical model that takes into account parasitic resistances and thermal effects is constructed, and its results are compared with experimental data. With parameters determined from characterization experiments on the same wafer and from independent theoretical results, the agreement between the data and the model predictions is found to be very good. The model is then applied to performance predictions for devices with improved series resistances and heat sinking.
Temperature Dependence of Bound Exciton Emissions in GaN
Authors: Chtchekine D. G. , Gilliland G. D. , Feng Z. C. , Chua S. J. , Wolford D. J. , Ralph S. E. , Schurman M. J. , Ferguson I.
Published: April 2011

The dissociation channels of two prominent bound exciton complexes in wurtzite GaN thin films are determined via an extensive temperature dependent photoluminescence study. The shallow donor bound exciton dissociation at low temperatures (T ≤ 50 K) is found to be dominated by the release of a free exciton with thermal activation energy consistent with the exciton localization energy. At higher temperatures a second dissociation channel with activation energy EA = 28 ± 2 meV is observed. The dissociation of a bound exciton complex with exciton localization energy Exloc = 11.7 meV is also dominated by the release of a free exciton. In contrast to previous studies evidence is presented against the hypothesis of this emission being due to the exciton bound to an ionized donor. We find that it originates most likely from an exciton bound to a neutral acceptor.
Physics-Based Intrinsic Model for AlGaN/GaN HEMTs
Authors: Wu Shangli , Webster Richard T. , Anwar A. F. M.
Published: April 2011

DC and intrinsic small signal parameters are reported for AlGaN/GaN high electron mobility transistors. The calculations are based upon a self-consistent solution of Schrödinger and Poisson's equation to model the quantum well formed in GaN. Transport parameters are obtained from an ensemble Monte Carlo simulation.
Theoretical Studies of ZnO and Related MgxZn1-xO Alloy Band Structures
Authors: Lambrecht Walter R. L. , Limpijumnong Sukit , Segall B.
Published: April 2011

First principles calculations are carried out for ZnO, MgO and ZnMgO2 in various crystal structures. The nature of the valence band ordering in ZnO is shown to depend strongly on the Zn3d band position. MgO in the wurtzitic form is found to gave an unusual 5-fold coordiated structure. The band gap dependence in the alloy system is found to be in fair agreement with experimental data and the band-offset is predicted to be type I.
Uniformity and Performance Characterization of GaN P-I-N Photodetectors Fabricated From 3-Inch Epitaxy
Authors: Hickman R. , Klaassen J. J. , Hove J. M. Van , Wowchak A. M. , Polley C. , Rosamond M. F. , Chow P. P.
Published: April 2011

Gallium nitride wafer epitaxy on large diameter substrates is critical for the future fabrication of large area UV linear or 2D imaging arrays, as well as for the economical production of other GaN-based devices. Typical group III-nitride deposition is now performed on 2-inch diameter or smaller sapphire substrates. Reported here are visible blind, UV GaN p-in photodetectors which have been fabricated on 3-inch diameter (0001) sapphire substrates by RF atomic nitrogen plasma MBE. The uniformity across the wafer of spectral responsivity and shunt resistance (R0) for the p-i-n photodetectors has been characterized. Spectral responsivity and 1/f noise as a function of temperature exceeding 250°C will be presented for the GaN p-i-n photodetectors. Spectral response with >0.17 A/W at peak wavelength and having 4-6 orders of magnitude visible rejection has been achieved. 1/f noise typically less than 10-14 A/Hz1/2 at room temperature also has been achieved with GaN p-i-n photodiodes. The results have been correlated with proposed models for dark current and 1/f noise in GaN diodes.
Analysis of Compression Behavior of a [011] TA Single Crystal with Orientation Imaging Microscopy and Crystal Plasticity
Authors: Schwartz A.J. , Stölken J.S. , King W.E. , Campbell G.H. , Lassila D.H. , Shu J.Y. , Sun S. , Adams B.L.
Published: April 2011

High-purity tantalum single crystal cylinders oriented with [011] parallel to the cylinder axis were deformed 10, 20, and 30 percent in compression. The engineering stress-strain curve exhibited an up-turn at strains greater than ∼20% while the samples took on an ellipsoidal shape during testing, elongated along the [100] direction with almost no dimensional change along [011]. Two orthogonal planes were selected for characterization using Orientation Imaging Microscopy (OIM): one plane containing [100] and [011] (longitudinal) and the other in the plane containing [011] and [011] (transverse). OIM revealed patterns of alternating crystal rotations that develop as a function of strain and exhibit evolving length scales. The spacing and magnitude of these alternating misorientations increases in number density and decreases in spacing with increasing strain. Classical crystal plasticity calculations were performed to simulate the effects of compression deformation with and without the presence of friction. The calculated stressstrain response, local lattice reorientations, and specimen shape are compared with experiment.
Metallurgical Control of the Ductile-Brittle Transition in High-Strength Structural Steels
Authors: Morris J.W.
Published: April 2011

The models that have been successfully used to control the ductile-brittle transition in high strength structural steels are qualitative in nature, and address the microstructural control of the mechanisms of brittle fracture. The basic idea is incorporated in the “Yoffee diagram”, which dates from the 1920's and attributes the ductile-brittle transition to the competition between deformation and fracture; the more difficult brittle fracture becomes, the lower the temperature at which ductile processes dominate. There are two important brittle fracture modes: intergranular separation and transgranular cleavage. The intergranular mode is usually due to chemical contamination, and is addressed by eliminating or gettering the contaminating species. There are also examples of brittle fracture that is due to inherent grain boundary weakness. In this case the failure mode is overcome by adding beneficial species (glue) to the grain boundary. Transgranular cleavage is made more difficult by refining the effective grain size. In high strength steel this is done by refining the prior austenite grain size, by interspersing islands of metastable austenite that transform martensitically under plastic strain, or by disrupting the crystallographic alignment of ferrite grains or martensite laths. The latter mechanism offers intriguing possibilities for future steels with exceptional toughness.
In Situ Examination of moving Crack Tips in Ordered Intermetallics
Authors: Heuer J.K. , Lam N.Q. , Okamoto P.R. , Stubbins J.F.
Published: April 2011

Recent studies have shown that high stress concentrations at moving crack tips in the intermetallic compound NiTi can induce a crystalline-to-amorphous (C-A) transformation of the crack tip region. This stress-induced C-A transformation has a temperature dependence and crystallization behavior similar to those of ion irradiation-induced C-A transformation of NiTi. The present study examines if these similarities between stress- and irradiation-induced amorphization hold true for two other intermetallic compounds, CuTi and Ni3Ti. In situ straining was performed in an intermediate-voltage transmission electron microscope. The presence or absence of an amorphous phase was determined by dark field imaging and selected area diffraction of crack tip regions. Crack tips in both CuTi and Ni3Ti were found to remain crystalline upon fracture. The observed absence of stress-induced amorphization in Ni3Ti is consistent with its known absence during irradiation, but the absence in CuTi differs from its known irradiation-induced amorphization behavior. Reasons for the similarity and difference are discussed.
Mode-III Fracture Propagation in a Two-Dimensional Continuum Model with Frictional Dissipation
Authors: Ching Emily S.C. , Nakanishi Hiizu
Published: April 2011

Mode-III fracture propagation in a two-dimensional continuum model is studied the- oretically. The material is assumed to be isotropic and linearly elastic with a frictional dissipation, and the crack-tip r-gion is modeled by a cohesive zone. As the externally ap- plied load increases, the steady-state crack speed increases to a maximum value which can be substantially smaller than the sound speed when the dissipation is large. The fracture energy also becomes dependent on the velocity.
Making, Breaking and Sliding of Nanometer-Scale Contacts
Authors: Carpick R.W. , Enachescu M. , Ogletree D.F. , Salmeron M.
Published: April 2011

The contact between an atomic force microscope tip and a sample surface can form an ideal single asperity of nanometer dimensions, where the interaction forces can be measured with sub- nanoNewton force resolution. Studies of contact, adhesion and friction for these nano-asperities have been carried out for a variety of tips and single crystal sample surfaces. The major result is the observation of proportionality between friction and true contact area for a variety of systems, and an impressive agreement with continuum mechanics models for contact area even at the nanometer scale. The relevant continuum models can in fact be understood in the framework of fracture mechanics.
Chemical Solution Processing of Strontium Bismuth Tantalate Films
Authors: Lakeman C.D.E. , Boyle T. J.
Published: April 2011

We describe Chemical Solution Deposition (CSD) processes by which Strontium Bismuth Tantalate (SBT) thin films can be prepared at temperatures as low as 550°C. In this paper, we will present strategies used to optimize the properties of the films including solution chemistry, film composition, the nature of the substrate (or bottom electrode) used, and the thermal processing cycle. Under suitable conditions, ∼1700 Å films can be prepared which have a large switchable polarization (2Pr >10μC/cm2), and an operating voltage, defined as the voltage at which 0.80 × 2Prmax is switched, 2.OV. We also describe an all-alkoxide route to SBT films from which SBT can be crystallized at 550°C.
Effect of Characteristics of SrRuO3 Buffer Layer on the Ferroelectric Properties of (Pb0.97La0.30)(Zr0.66Ti0.34)O3 Thin Films
Authors: Chen Yu-Jen , Jamn Gwo , Liu Kuo-Shung , Lina I-Nan
Published: April 2011

A two-step pulsed laser deposition (PLD) process, including PLD at a substrate temperature lower than 150°C and rapid-thermal-annealing (RTA) at around 550°C (30 s), has been successfully applied for growing (Pb0.97La0.03)(Zr0.66Ti0.034)0.9875O3, PLZT, thin films. Interdiffusion between layers is pronouncedly suppressed due to the presence of the SrRuO3 layer, which markedly improves the electrical properties of PLZT films. The PLZT films thus obtained exhibit large remanent polarization P,=19 µC/cm2 (with coercive force Ec=78 kV/cm), low leakage current density J11≤8 × 1O−6 A/cm2 (up to 400 kV/cm) and fatigue free characteristics.
The Role of Processing Parameters in Variation of Microstruture of Sol-Gel Derived Lead Zirconate Titanate Thin Films
Authors: Kim Chang Jung , Kim Tae-Young , Chung Ilsub , Yoo In Kyung
Published: April 2011

The PZT thin films were fabricated to investigate the effect of sol-gel processing parameters on the physical and the electrical properties. The films were made with different amount of excess Pb precursors and drying temperatures, and then annealed in various ambients. The physical properties of the films such as crystallinity and microstructure were evaluated using x-ray diffraction, scanning electron microscopy and atomic force microscopy. The ferroelectric properties and current density characteristics of the films were investigated using a standarized feiroelectric test system and pA meter, respectively. It is found that the drying temperature was playing a key role in the formation of the secondary phase on the PZT thin films. In addition, it turned out that the use of nitrogen as an annealing ambient promoted overall ferroelectric properties, when compared to oxygen ambients.
MBE Growth of Ferroelectric YMnO3 Thin Films on Si(111) Using Y2O3 Buffer Layers
Authors: Imada Shogo , Shouriki Shigeto , Tokumitsu Eisuke , Ishiwara Hiroshi
Published: April 2011

A ferroelectric YMnO3 thin films are grown on Si (111) substrates using Y2O3 buffer layers by molecular beam epitaxy (MBE). In-situ reflection high-energy electron diffraction (RHEED) analyses show that both Y2O3 and YMnO3 films are epitaxially grown on Si substrates. X-ray rocking curve measurements also show that the best FWHM (full width at half maximum) values for Y2O3 and YMnO3 films are 0.40° and 0.8°, respectively. C-V characteristics of Al/YMnO3/Y2O3/Si structures indicate the ferroelectric properties of YMnO3 films with a memory window of 0.7V.
Dielectric Properties of Pulsed Laser Deposited SrTiO3 Thin Films
Authors: Clark A. M. , Hao Jianhua , Si Weidong , Xi X. X.
Published: April 2011

Low frequency dielectric loss and nonlinearity in pulsed laser deposited SrTiO3 thin films were studied. A low loss tangent in the order of 10−4, close to the level found in SrTiO3 single crystals, was observed. Combined with a large tunability, this resulted in a figure of merit for frequency and phase agile materials that can rival that observed in single crystals. The SrTiO3 thin films with thickness ranging from 25 nm to 2.5μ1 were grown on SrRuO3 electrode layers. The loss depends strongly on the thickness, but differently above and below T ∼ 80 K. Our result suggests that, in the high temperature regime, the interfacial dead layer effect dominates while, in the low temperature regime, the losses related to the structural phase transition and quantum fluctuations are important. The effect of interfacial potential was studied by using different electrode materials that result in different Schottky barriers.
Modelling Intercalation Kinetics of Polymer Silicate Nanocomposites
Authors: Lee J. Y. , Baljon A. R. C. , Loring R. F. , Panagiotopoulos A. Z.
Published: April 2011

Polymer layered silicates form an important class of nanocomposite materials. During their formation by melt intercalation, polymer molecules from a bulk fluid move into the galleries of layered silicates. An essential feature of this process is the flow of macromolecules from a bulk fluid to a confined environment. To model this phenomenon, we have performed molecular dynamics simulations of the flow of polymer molecules from a bulk melt into a rectangular slit. The simulations are consistent with a diffusive description of the transport, and show qualitative agreement with time-dependent x-ray diffraction measurements of intercalation kinetics in layered nanocomposites.
On the Glass Transition in Polymer Films: Recent Monte Carlo Results
Authors: Baschnagel J. , Binder K.
Published: April 2011

This paper reports results of a Monte Carlo simulation for a simplified lattice modelof a supercooled polymer film. The film geometry is realized by two opposite hard walls.The distance between the walls is varied. The chains exhibit a strong tendency to orientparallel to the walls and are flattened when being very close to them. This deviation of thepolymer structure with respect to the bulk is accompanied by an acceleration of local densityfluctuations. On the other hand, the diffusion coefficient of a chain remains unaffected.