bluuebunny/crossref_metadata_2025 huggingface dataset:
On the Microstructure of the Epoxy/Adherend Interphase
Authors: Libera M. , Zukas W. , Wentworth S. , Patel A.
Published: April 2011

It is now recognized that there is a region at the epoxy/adherend interface known as the interphase whose chemistry and structure are different from those of bulk epoxy. There is, however, no adequate understanding of its microstructure. This paper describes differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) studies of the interphase between an aromatic amine cured epoxy and alumina/oxidized-aluminum surfaces. DSC results show dramatic differences in resin-cure behavior in the presence of particulate alumina. TEM results on microtomed cross-sectional specimens of anodized aluminum wire embedded in epoxy show regions of incomplete epoxy infiltration and interfacial stress. RuO4 staining combined with high-angle annular-dark-field STEM imaging indicates that there are structural variations within the bulk epoxy at lengths of ∼10–30nm. The magnitude of these fluctuations decreases in the near the adherend interface, and there is a simultaneous variation in the average epoxy structure. A plausible interpretation of these observations is that the interphase region suffers a variation in curing-agent concentration of unknown magnitude and there is a higher concentration of homopolymerized epoxy there relative to the bulk.
Post-CMP Cleaning of W and SiO2: A Model Study
Authors: Malik Igor J. , Zhang Jackie , Jensen Alan J. , Farber Jeffrey J. , Krusell Wilbur C. , Raghavan Srini , Rajhunath Chilkunda
Published: April 2011

Chemical-Mechanical Planarization (CMP) of SiO2 is performed using alkaline silica slurries while CMP of tungsten (W) utilizes acidic slurries with alumina as the abrasive. Proposed mechanisms for the two CMP processes, with more emphasis on SiO2-CMP, have been discussed in literature. However, much less is known about the removal mechanism of residual slurry particles from the planarized surfaces - a crucial step for subsequent device processing. We discuss the chemical and physical basis of post-CMP cleaning by double-side scrubbing using polyvinyl alcohol (PVA) brushes and show how the interactions between the wafer surface, slurry, and the brush material affect the overall cleaning efficiency. Using the zeta potential concept the common features for cleaning surfaces after SiO2-CMP and W-CMP are established and the differences between these two systems are highlighted. We present surface particle levels for two model systems as a function of cleaning chemistries and discuss their influence on post-CMP surface metal levels.
Effect of SC-1 Process Parameters on Particle Removal and Surface Metallic Contamination
Authors: Hall R. Mark , Rosato John J. , Jarvis Taura , Parry Thad , Lindquist Paul G.
Published: April 2011

The effect of bath temperature, megasonic power, and NH4-OH:H2O ratio are studied for particle removal efficiency, surface roughness, and surface Fe concentration in SC-1 cleaning solutions. Experimental results are presented which show removal efficiencies better than 97% on bare silicon wafers for optimized process conditions. These results are related to the etch rate of thermal oxides and a model is developed for reducing surface roughness and minimizing Fe contamination levels while maximizing particle removal efficiency.
Adsorption and Desorption of Contaminant Metals on Si Wafer Surface in SC1 Solution
Authors: Maeda G. , Takahashi I. , Kondo H. , Ryuta J. , Shingyouji T.
Published: April 2011

Variation in the surface concentration of Fe, Ni, Cu and Zn on Si wafers due to treatment in NH4OH/H2aO2/H2O solution called SC1 is investigated. The metal concentration on the wafer surface depends on the initial surface concentration, concentration in the solution, adsorption probability, desorption rate constant and the treatment time. The surface metal concentration behavior is explained by taking into account the effects of these parameters. The variation in the desorption rate constant with the metal species, the concentration in the solution, treatment temperature and mixing ratio of SC1 is discussed.
Reduced-Pressure UV Photo-Oxidation of Organic Contaminants on Si Surfaces
Authors: Bedge Satish , Lamb H. Henry
Published: April 2011

A reduced-pressure ultraviolet (UV) photo-oxidation process is described for the removal of organic contamination from Si surfaces with concomitant growth of an ultra-thin oxide passivation layer. As in situ photo-generation of ozone (O3) from dioxygen (O2) is impractical at millitorr pressures, a 2% O3/O2 mixture from an ozone generator is fed to the surface conditioning chamber. UV/O3 surface conditioning employing simultaneous 254/185 nm UV irradiation results in essentially complete removal of carbon contamination in 180 s at 100 mTorr and 100–200°C. Kinetics studies employing cyclohexane-contaminated Si(100) surfaces suggest that carbon removal occurs via two consecutive first-order processes: initial fast photo-oxidation of adsorbed cyclohexane to readily desorbed products and slower photo-oxidation of adsorbed carbonyl intermediates to CO2 and H2O. The activation energies for both processes are 2–3 kcal/mol, consistent with the involvement of photo-generated atomic oxygen species. Self-limiting growth of an ultra-thin oxide passivation layer occurs concomitantly with carbon removal. At saturation, the oxide layer is only 3–4 Å thick, and the growth kinetics are described by a first-order Langmuirian adsorption model.
Silicon Surface Chemistry By IR Spectroscopy in the Mid- To Far-IR Region: H2O And Ethanol On Si(100)
Authors: Struck L. M. , Eng J. , Bent B. E. , Chabal Y. J. , Williams G. P. , White A. E. , Christman S. , Chaban E. E. , Raghavachari K. , Flynn G. W. , Radermacher K. , Mantl S.
Published: April 2011

The technique of external reflection infrared (IR) spectroscopy is used to study silicon surface chemistry. External reflection is enhanced by implanting a buried cobalt silicide layer in silicon to act as an infrared reflector. The preparation of clean well-ordered surfaces from the ion implanted substrates is demonstrated. The reactions of water and ethanol with Si(100) are investigated.
Vapor Breakdown During ablation by Nanosecond Laser Pulses
Authors: Liu C. L. , Leboeuf J. N. , Wood R. F. , Geohegan D. B. , Donato J. M. , Chen K. R. , Puretzky A. A.
Published: April 2011

Plasma generation through vapor breakdown during ablation of a Si target by nanosecond KrF laser pulses is modeled using 0-dimensional rate equations. although there is some previous work on vapor breakdown by microsecond laser pulses, there have been no successful attempts reported on the same subject by nanosecond laser pulses. This work intends to fill the gap. a kinetic model is developed considering the following factors: (1) temperatures of both electrons and heavy-body particles (ions, neutrals, and excited states of neutrals), (2) absorption mechanisms of the laser energy such as inverse bremstrahlung (IB) processes and photoionization of excited states, (3) ionization acceleration mechanisms that include electron-impact excitation of ground state neutrals, electron-impact ionization of excited states of neutrals, photoionization of excited states of neutrals, and all necessary reverse processes. the rates of various processes considered are calculated using a second order predictor-corrector numerical scheme. the rate equations are solved for five quantities, namely, densities of electrons, neutrals, and excited states of neutrals, and the temperatures of electrons and heavy-body particles. the total breakdown times (sum of evaporation time and vapor breakdown time) at different energy fluences are then calculated. the results are compared with experimental observations of Si target ablation using a KrF laser.
Substrate Heating Measurements in Pulsed Ion Beam Film Deposition
Authors: Olson J. C. , Thompson M. O. , Davis H. A. , Rej D. J. , Waganaar W. J. , Tallant D. R.
Published: April 2011

Diamond-like Carbon(DLC) films have been deposited at Los alamos National Laboratory by pulsed ion beam ablation of graphite targets. the targets are illuminated by an intense beam of hydrogen, carbon, and oxygen ions at a fluence of 15-45 J/cm2. Ion energies are on the order of 350 keV, with beam current rising to 35 kA over a 400 ns ion current pulse.

Raman spectra of the deposited films indicate an increasing ratio of sp3 to sp2 bonding as the substrate is moved further away from the target and further off the target normal. Using a thin film platinum resistor at various positions, we have measured the heating of the substrate surface due to the kinetic energy and heat of condensation of the ablated material. Plume power density and energy input are inferred from the temperature measurements. This information is used to determine if substrate heating is responsible for the lack of DLC in positions close to the target and near the target normal.
Laser-Solid Interaction and Dynamics of the Laser-Ablated Materials
Authors: Chen K. R. , Leboeuf J. N. , Geohegan D. B. , Wood R. F. , Donato J. M. , Liu C. L. , Puretzky A. A.
Published: April 2011

Rapid transformations through the liquid and vapor phases induced by laser-solid interactions are described by our thermal model with the Clausius-Clapeyron equation to determine the vaporization temperature under different surface pressure condition. Hydrodynamic behavior of the vapor during and after ablation is described by gas dynamic equations. these two models are coupled. Modeling results show that lower background pressure results lower laser energy density threshold for vaporization. the ablation rate and the amount of materials removed are proportional to the laser energy density above its threshold. We also demonstrate a dynamic source effect that accelerates the unsteady expansion of laser-ablated material in the direction perpendicular to the solid. a dynamic partial ionization effect is studied as well. a self-similar theory shows that the maximum expansion velocity is proportional to cs/α, where 1 – α is the slope of the velocity profile. Numerical hydrodynamic modeling is in good agreement with the theory. With these effects, α. is reduced. therefore, the expansion front velocity is significantly higher than that from conventional models. the results are consistent with experiments. We further study the plume propagates in high background gas condition. Under appropriate conditions, the plume is slowed down, separates with the background, is backward moving, and hits the solid surface. then, it splits to be two parts when it rebounds from the surface. the results from the modeling will be compared with experimental observations where possible.
Properties of Low-Resistivity Undoped indium-Oxide Films Grown by Reactive Ion Plating and Electrochromic Tungsten-Oxide Films Grown by Electron-Beam Evaporation
Authors: Lee Y. P. , Jeong J. I. , Moon J. H. , Hong J. H. , Kang J. S.
Published: April 2011

The use of gaseous discharge for ion plating and related techniques have been well known to improve coating properties in several ways. IN the arc-induced ion plating (AIIP), the ionization efficiency for the evaporants is so enhanced without any introduction of inert gases that the bias voltage for, and the temperature of the substrate are reduced in the preparation of the coatings. Highly transparent (> 90% transmission in the visible range) and highly conductive (resistivity ≅ 1.5 x 10-4 Ω cm) in-oxide films were deposited at a rate of 500 - 900 Å/min by aIIP of pure in in an O2 atmosphere of 10-4 Torr. Hall-effect measurement revealed that the observed low resistivity is due primarily to the excellent electron mobilty (≥ 70 cm2 / V sec) with carrier density up to 7 х 1020/cm3. Electrochromic WO3 films were also prepared and characterized.
In Situ Monitoring and Model Simulation of BaTiO3 Pulsed Laser Thin Film Deposition
Authors: Paul Albert J. , Schenck Peter K. , Bonnell David W. , Hastie John W.
Published: April 2011

We have applied intensified Charge Coupled Detector (ICCD) imaging to real-time, in situ monitoring of excited vapor species emission during the pulsed excimer (248 nm wavelength) laser deposition (PLD) of BaTiO3 ferroelectric thin films. Molecular beam mass spectrometry (MS) was used for species identification and velocity distribution analysis. Gasdynamic model simulations of the plume formation and transport process are tested by comparison with the ICCD and MS results. Particular attention is given to the time scale during and immediately following the laser pulse (0 - 460 ns), and also to the effects of added 02. atomic oxygen was found to be the predominant species present at the leading edge of the vapor plume. Other, slower species found included neutral Ti, TiO, Ba, and BaO.
Dielectric Susceptibility and Strain in Sr1-XBaXTio3 Ferroelectric Thin Films Grown by Pulsed Laser Deposition
Authors: Knauss L.A. , Horwitz J.S. , Chrisey D.B. , Pond J.M. , Grabowski K.S. , Qadri S.B. , Donovan E.P. , Mueller C.H.
Published: April 2011

Thin films of Srl-xBaxTiO3 (SBT) with x = 0 to 0.80 have been grown in situ by pulsed laser deposition onto single crystals of (001) LaAlO3. the films were grown to thicknesses of 0.6 μ.m and found to be single phase, highly oriented, and characterized by x-ray ω scan widths of ≤ 0.5°. the temperature dependence of the dielectric constant and the relative dissipation factor were measured at 1 kHz using au interdigital capacitors deposited on top of the ferroelectric films. the capacitance measurements indicate that the temperature dependence of the dielectric constant of the film is broad and the maximum is shifted relative to the bulk material. the differences between thin film and bulk properties are attributed to strain in the film resulting from film -substrate lattice mismatch. Thick films (∼7 μm) gave direct evidence for strain through cracking and delamination. X-ray diffraction measurements have been made to determine the non-uniform strain in the thin films which was approximately 0.1%.
Analysis of the Effects of Interior Aperture Angle on Print Quality in Solder Paste Stenciling
Authors: Monaghan K. , Head L. , Sahay C. , Constable J.
Published: April 2011

Stencil apertures of various interior angle, i.e. angle between two sides of a polygon, are analyzed for print performance. Aperture size and sidewall taper are also examined for their individual and combined effects on printing. The results show no effect of interior angle within the bounds of the experiment. Sidewall taper becomes relevant when aperture geometry is nearly square.
Indium-Copper Multilayer Composite Solder for Fluxless Bonding
Authors: Lee Chin C. , Chen Yi-Chia
Published: April 2011

A 200°C fluxless process is developed to produce In-Cu joints. The fluxless feature is achieved by the prevention of indium oxidation during the solder fabrication and the bonding process. Indium and copper are deposited on an object in high vacuum to inhibit indium oxidation. Copper interacts with indium to form CuIn compound that further protects the inner indium from oxidation. For a specific design, the resulting joints consist of mainly CuIn intermetallic grains surrounded by a small amount of pure indium as revealed by SEM with EDX. Scanning acoustic microscope examination indicates that the joints are nearly void-free. This technology enables versatile control of the alloy composition, thus leading to several remelting temperatures and various physical properties.
New Synthetic Approaches to Self-Assembled Chromophoric Multilayers as Second-Order Nonlinear Optical Materials
Authors: Lin Wenbin , Marks Tobin J. , Yitzchaik Shlomo , Lin Weiping , Wong George K.
Published: April 2011

This contribution describes the synthesis and properties of NLO-active self-assembled chromophoric multilayers. The stilbazolium self-assembled multilayers were prepared by new topotactic approaches based on siloxane self-assembly technology. X-ray photoelectron spectroscopy (XPS), advancing aqueous contact angle (θa) measurements, transmission optical spectroscopy, polarized second harmonic generation (SHG), and specular X-ray rellectivity (XRR) show that the resulting self-assembled chromophoric superlattices have very high structural regularity and very large second-order nonlinear optical susceptibility.
Spatially Reversible Electric Field-Induced Permeation in Pure and Mixed Langmuir-Blodgett Layers of Hemicyanine Dyes and Octadecanoic Acid on Poly(2-Vinylpyridine)
Authors: Lu W. , Song Q. , Bohn P.W.
Published: April 2011

Langmuir-Blodgett (LB) films of the hemicyanine dyes 4-(4-dihexadecylaminostyryl)- N-methylpyridinium iodide and 4-(4-dioctylaminostyryl)-N-propylsulfonate pyridinium mixed with octadecanoic acid have been deposited onto H2O-swollen poly(2-vinylpyridine) (P2VP) as mass transport barriers. The transport properties of these structures have been studied both with and without the application of an electric field in the range 0 ≤ E ≤ 100 V/cm, by observing the rate of permeation of both anionic (fluorescein) and zwitterionic (rhodamine B) fluorophores. The apparent diffusion coefficient at E = 0 decreases exponentially with the number of monolayers through which the probe molecule must permeate, in agreement with a hole-mediated permeation model. Application of an external electric field is observed to control the rate and direction of the mass transport of the probe molecule across the barrier layer. The electric field effect is both spatially reversible, i.e. the probe can be transported in either direction across the barrier layer, and temporally reversible, i.e. the permeability cycles with the field between large values when the field is on and small values when it is not applied. The results are consistent with an interpretation in which electroporation is the dominant mechanism for transport in the field-on state.
The Synthesis and Characterization of a Novel Block Copolymer Consisting of Poly(Propylene Fumarate) and Poly(Ethylene Oxide)
Authors: Suggs Laura J. , Payne Richard G. , Kao Edmund Y. , Alemany Lawrence B. , Yaszemski Michael J. , Wu Kenneth K. , Mikos Antonios G.
Published: April 2011

A block copolymer was developed consisting of poly(propylene fumarate) (PPF) and poly(ethylene oxide) (PEO) blocks. The former component allows the copolymer to be crosslinked, while the latter component allows the physical properties to be controlled. The compositions and segmental lengths of the two components were varied, and the copolymer was subsequently characterized.
Using Isostatic Pressing Techniques to Mold Uhmwpe
Authors: Higgins Joel , Schroeder David
Published: April 2011

Ultra High Molecular Weight Polyethylene (UHMWPE) has been and is currently the standard for bearing material used in the orthopedic industry. The components are produced using a variety of manufacturing methods, many of which can have an effect on the longevity and performance of the device. Recently there has been extensive research into the causes of loosening of orthopedic devices. One area that has been targeted as a cause for loosening is reactions to particulate debris from the bearing surfaces of these appliances. As biological reactions to particulate become better understood, there has been an increased emphasis on the quality of the UHMWPE forms used for orthopedic bearing surfaces. Due to this increased awareness, various manufacturing and quality control improvements have been made throughout the industry.
β-Glycosylamide Glycopolymers: Synthesis, Physical Properties, Chemical and Enzymatic Stability
Authors: Spevak Wayne , Tropper Frangois D.
Published: April 2011

A method is described for the preparation β-glycosylamide monomers from reducing carbohydrates. The glycosylamide monomers were copolymerized with acrylamide to formhigh molecular weight, water soluble polymers. The chemical and enzymatic stability of the β-N-glycosidic linkage was investigated. In addition, the glycopolymers were characterized by their interactions with lectins.
NH3 as Nitrogen Source in MBE growth of GaN
Authors: Kamp M. , Mayer M. , Pelzmann A. , Thies A. , Chung H. Y. , Sternschulte H. , Marti O. , Ebeling K. J.
Published: April 2011

We report on the growth of GaN in GSMBE using NH3 as nitrogen source. Special focus will be on the NH3 cracking, where we applied an On Surface Cracking technique (OSC). Using OSC we achieve photoluminescence linewidths as narrow as 5.5meV (5K) and mobilities of 220 cm2/Vs at room temperature.
Growth and Properties of Bulk Single Crystals of GaN
Authors: Suski T. , Perlin P. , Leszczyński M. , Teisseyre H. , Grzegory I. , Jun J. , Boćkowski M. , Porowski S. , Pakuła K. , Wysmołek A. , Baranowski J.M.
Published: April 2011

In this paper we review recent developments in the growth of bulk GaN crystals by a high-pressure, high-temperature method. We also provide information on various physical properties of bulk GaN material. Then, some preliminary results on the homoepitaxial growth of GaN are given. In the second part of this paper we discuss the following problems: the possible origin of the large free electron concentration in undoped GaN material, the parasitic effect of yellow luminescence and the nature of Zn- and Mg-acceptors.
Defect Microstructure of Thin Wurtzite GaN Films Grown by MBE
Authors: Sverdlov B.N. , Botchkarev A. , Martin G.A. , Salvador A. , MorkoÇ H. , Tsen S.-C.Y. , Smith David J.
Published: April 2011

Thin films of wurtzite GaN have been grown by molecular beam epitaxy on 6H SiC (basal plane), Si {111} and sapphire (c-plane) substrates with and without various buffer layers. The defect microstructure of the films and the substrate/buffer/GaN interfacial quality have been characterized by cross-sectional transmission electron microscopy. The morphology was dominated by threading defects that originated at the substrate/buffer and/or buffer/film interfaces. Typical defect densities dropped rapidly with distance from the substrate but remained ∼108–109/cm2, depending on the particular substrate, for film thicknesses approaching one micron or more. The best quality films were grown at 770°C on sapphire with A1N buffer layers, and had X-ray rocking curve full-width at half-maximum values of ∼ 55arc-sec.
High-Quality III-V Nitrides Grown by Metalorganic Chemical Vapor Deposition
Authors: Dupuis R.D. , Holmes A.L. , Grudowski P.A , Fertitta K.G. , Ponce F.A.
Published: April 2011

We report the growth of high-quality III-V nitride heteroepitaxial films on (0001) sapphire substrates by low-pressure metalorganic chemical vapor deposition (MOCVD). These films have exhibited narrow X-ray diffraction rocking curves with full-width-at-half-maximum values as low as ΔΘ∼37 arc sec. Photoluminescence and transmission electron microscopy analysis further indicate the samples to be of high quality.
MOVPE Growth of High Electron Mobility AlGaN/GaN Heterostructures
Authors: Redwing J.M. , Flynn J.S. , Tischler M.A. , Mitchel W. , Saxler A.
Published: April 2011

We have fabricated AlxGa1−xN/GaN heterostructures with high two-dimensional electron gas (2DEG) mobilities and high sheet carrier densities by metalorganic vapor phase epitaxy (MOVPE). The 2DEG sheet density and mobility exhibit a compositional dependence on the Al fraction of the electron donor layer. The highest mobility (5750 cm2/Vs at 16K) was measured in a sample with x=0.15 that had a sheet carrier density of 8.5×1012 cm−2. The undoped AlxGa1−xN layers have low background carrier concentrations and can be intentionally doped n-type using SiH4. The effect of intentional n-type doping of the AlxGa1−xN donor layer on the electrical properties of the 2DEG was studied in structures that included an undoped AlxGa1−xN spacer layer of varying thickness. Higher 2DEG mobilities were obtained when a 100Å thick undoped layer was included in the structure due to spatial separation of the 2DEG from ionized impurities in the doped AlxGa1−xN. These initial results demonstrate that the electrical properties of AlxGa1−xN/GaN heterostructures can be controlled by intentional doping and appropriate layer design.
Growth of Cubic GAN on (001) GAAS
Authors: Brandt O. , Yang H. , Trampert A. , Ploog K. H.
Published: April 2011

We present a study of the growth of cubic GaN films on (001) GaAs by molecular beam epitaxy. Our investigations focus on the nucleation stage as well as on the subsequent growth of GaN. The phenomenon of epitaxial growth at this extreme mismatch (20%) is demonstrated to arise from a coincidence lattice between GaAs and GaN. The presence of a high-density of stacking faults in the GaN layer is explained within this understanding as being a natural consequence of the coalescence of perfectly relaxed nuclei. We furthermore analyze the growth kinetics of GaN via the surface reconstruction transitions observed upon an impinging Ga flux, from which we obtain both the desorption rate of Ga as well as the diffusion coefficient of Ga adatoms on the Ga-stabilized GaN surface. The diffusivity of Ga is found to be very low at the growth temperatures commonly used during molecular beam epitaxy, which provides an explanation for the microscopic surface roughness observed on our samples.
Issues and Examples Regarding Growth of AlN, GaN and AlxGa1−xN Thin Films via OMVPE and Gas Source MBE
Authors: Davis Robert F. , Weeks T. W. , Bremser M. D. , Tanaka S. , Kern R. S. , Sitar Z. , Ailey K. S. , Perry W. G. , Wang C.
Published: April 2011

Organometallic vapor phase epitaxy (OMVPE) and molecular beam epitaxy (MBE) are the most common methods for the growth of thin films of A1N and GaN. Sapphire is the most common substrate; however, a host of materials have been used with varying degrees of success. Both growth techniques have been employed by the authors to grow AIN, GaN and AlxGa1−xN thin films primarily on 6H-SiC(0001). The mismatch in atomic layer stacking sequences along the growth direction produces double positioning boundaries in A1N and the alloys at the SiC steps; this sequence problem appears to discourage the two-dimensional nucleation of GaN. Films of these materials grown by MBE at 650°C are textured; monocrystalline films are achieved between 850°C (pure GaN) and 1050°C (pure A1N) by this technique and OMVPE. Donor and acceptor doping of GaN has been achieved via MBE without post growth annealing. Acceptor doping in CVD material requires annealing to displace the H from the Mg and eventually remove it from the material. High brightness light emitting diodes are commercially available; however, numerous concerns regarding metal and nitrogen sources, heteroepitaxial nucleation, the role of buffer layers, surface migration rates as a function of temperature, substantial defect densities and their effect on film and device properties, ohmic and rectifying contacts, wet and dry etching and suitable gate and field insulators must and are being addressed. Selected issues surrounding the growth of these materials with particular examples drawn from the authors' research are presented herein.
Alternative Routes to the MOVPE Growth of GaN and Aln
Authors: Roberts V. , Roberts J. S. , Jones A. C. , Rushworth S.
Published: April 2011

Alternative precursors to group-in nitrides have been studied based on two schemes: 1) The reaction Me3M (M=Al,Ga) with t-BuNH2 and 2) the decomposition of NH3 based adducts. Polycrystalline growth of AIN has been demonstrated by both routes. Decomposition of the adduct Me3Al:NH3 has been used to prepare epitaxial AIN on (0001) sapphire with an X-ray FWHM (full width at half maximum) of 16 arcrnin at a growth temperature of 1050C. Similar growth using analogous gallium precursors always resulted in gallium droplets. We have attributed this difference in chemical reactivities to the lower electronegativity of gallium compounds, thus leading to dissociation rather than sequential methane loss to form the nitride.
Understanding the Pyramidal Growth of GaN
Authors: Rouviere J.-L. , Arlery M. , Bourret A. , Niebuhr R. , Bachem K.-H.
Published: April 2011

By a combination of conventional, HREM and CBED TEM experiments we have studied wurtzite GaN layers grown by Metal-Organic Chemical Vapour Deposition (MOCVD) on (0001)Al2O3. We experimentally determine the structure of the macroscopic hexagonal pyramids that are visible at the surface of the layers when no optimised buffer is introduced. These pyramids look like hexagonal volcanoes with one hexagonal microscopic chimney (up to 75nm wide) at their core. The crystal inside the chimney is a pure GaN crystal with a polarity opposed to the one of the neighbouring material : the GaN layers grown on (0001)Al2O3 are everywhere Ga-terminated except in the chimneys where they are N-terminated. Some of the N-terminated chimneys grow faster and form macroscopic hexagonal pyramids. Chimneys bounded by Inversion Domains Boundaries (IDBs) originate from steps at the surface of the substrate and may be suppressed by an adapted buffer layer.
Electronic and Optical Properties of the Group-III Nitrides, their Heterostructures and Alloys
Authors: Lambrecht Walter R. L. , Kim Kwiseon , Rashkeev Sergey N. , Segall B.
Published: April 2011

Various aspects of the electronic structure of the group III nitrides are discussed. The relation between band structures and optical response in the vacuum ultraviolet is analyzed for zincblende and wurtzite GaN and for wurtzite A1N and compared with available experimental data obtained from reflectivity and spectroscopic ellipsometry. The spin-orbit and crystal field splittings of the valence band edges and their relations to exciton fine structure are discussed including substrate induced biaxial strain effects. The band-offsets between the III-nitrides and some relevant semiconductor substrates obtained within the dielectric midgap energy model are presented and strain effects which may alter these values are discussed. The importance of lattice mismatch in bandgap bowing is exemplified by comparing AlxGa1−xN and InxGa1−xN.
Structural and Electronic Properties of AlN, GaN And InN, and Band Offsets at AlN/GaN (1010) and (0001) Interfaces
Authors: Satta A. , Fiorentini Vincenzo , Bosin Andrea , Meloni F. , Vanderbilt David
Published: April 2011

Ab initio local-density-functional calculations are presented for bulk A1N, GaN, and InN in the wurtzite, zincblende, and rocksalt structures. Structural transition pressures and deformation potentials of electronic gaps are investigated. In addition, we study the band offset at the polar (0001) and non-polar (1010) AIN/GaN interfaces. Within AIN-on-GaN epitaxial conditions, we obtain valence-band offset values close to 0.7 eV for both interfaces. From the macroscopic field appearing along the growth direction of the polar interface (tentatively attributed to AIN macroscopic polarization), an estimate of the macroscopic dielectric constant of GaN is extracted. All calculations employed conjugate-gradient total-energy minimizations, ultrasoft pseudopotentials, and plane waves at 25 Ryd cutoff.
Epitaxial Growth of GaN on Lattice-Matched Hafnium Substrates
Authors: Beresford R. , Stevens K.S. , Briant C. , Bai R. , Paine D.C.
Published: April 2011

A method of producing epitaxial GaN on single-crystal Hf has been developed. The metal substrate is formed by a strain-anneal process yielding macroscopic (5-mm) grain sizes, followed by polishing, chemical etching, and Ar ion sputtering at elevated temperature in ultrahigh vacuum. The growth is conducted by plasma-assisted molecular beam epitaxy using an initial passivation layer deposited at low temperature and subsequent growth at 700 °C. The resulting films are in registry with the hep substrate lattice as observed by reflection high-energy electron diffraction during growth and verified by plan-view transmission electron microscopy. High-resolution x-ray rocking curve linewidths of the GaN and Hf [1012] peaks are as narrow as 900 and 180 arc seconds, respectively. The [0002] peak separation confirms the approximately 2.7% mismatch in the c axis spacing. Initial photo]uminescence observations at 20 K of a (donor-bound exciton) peak at 3.467 eV are consistent with the assumption of a nearly strain-free film resulting from the exact basal-plane lattice match and close thermal coefficient match between GaN and Hf.
Near-Bandgap Photoluminescence Decay Time in GaN Epitaxial Layers Grown on Sapphire
Authors: Hangleiter A. , Im J. S. , Forner T. , Härle V. , Scholz F.
Published: April 2011

Using picosecond time-resolved photoluminescence we have studied the decay time of excess carriers in GaN epitaxial layers over a wide range of temperatures from 4 K up to 400 K. At low temperature, a thermal dissociation of donor-bound excitons is observed. At higher temperatures up to room temperature, the luminescence decay at moderate excitation is governed by trapping of photogenerated electrons in ionized shallow donor levels. Using measured luminescence intensities to determine the quantum efficiency, we obtain the radiative lifetime of free excitons from low temperature up to room temperature. We use these data to determine the radiative recombination coefficient and the interband momentum matrix element.
Selective Dynamical Study of Luminescences Near the Surface and the Interface of Epitaxial GaN
Authors: Eckey L. , Hoffmann A. , Heitz R. , Broser I. , Meyer B.K. , Detchprohm T. , Hiramatsu K. , Amano H. , Akasaki I.
Published: April 2011

Recombination processes and their dynamics are selectively studied (a) near the interface betweeen a 400μm thick GaN epilayer and its Al2O3 substrate and (b) in the structurally relaxed regions near the surface of this sample. Strong radiative excitonic recombination is observed in the relaxed regions. However, the recombination dynamics of free and shallow-bound excitons here are strongly influenced by shallow and deep defects. Near the substrate the presence of dislocations suppresses radiative recombination of free and shallow bound excitons. Deeper emissions appear which we attribute to excitons deeply bound to dislocation-related defects. They exhibit ps-recombination dynamics effected by strong nonradiative contributions.
On Compensation and Impurities in State-of-the-Art GaN Epilayers Grown on Sapphire
Authors: Wickenden A.E. , Gaskill D.K. , Koleske D.D. , Doverspike K. , Simons D.S. , Chi P.H.
Published: April 2011

A comparison between 300 K electron transport data for state-of-the-art wurtzite GaN grown on sapphire substrates and corresponding theoretical calculations shows a large difference, with experimental mobility less than the predicted mobility for a given carrier concentration. The comparison seems to imply that GaN films are greatly compensated, but the discrepancy may also be due to the poorly known values of the materials parameters used in the calculations. In this work, recent analysis of transport and SIMS measurements on silicon-doped GaN films are shown to imply that the compensation, NA/ND, is less than 0.3. In addition, the determination of an activation energy of 34 meV in a GaN film doped to a level of 6×1016 cm−3 suggests either that a second, native donor exists in the doped films at a level of between 6×1017 cm−3 and 1×1017 cm−3, or that the activation energy of Si in GaN is dependent on the concentration, being influenced by impurity banding or some other physical effect. GaN films grown without silicon doping are highly resistive.
A New Single Source Precursor Approach to Gallium and Aluminum Nitride
Authors: Neumayer Deborah A. , Carmalt C.J. , Arendt M.F. , White J.M. , Cowley A.H. , Jones R.A. , Ekerdt J.G.
Published: April 2011

Single source precursors which contain preformed gallium-nitrogen and aluminum-nitrogen bonds are being considered for the growth of gallium and aluminum nitride because of their potential for overcoming problems associated with conventional precursors. Presented is the evaluation of dimethylgallium azide, Me2GaN3(1), bisdimethylamidogallium azide, (Me2N)2GaN3(2), and bisdimethylamidoaluminum azide, (Me2N)2AlN3(3) as potential precursors for A1N and GaN film growth. The compounds were evaluated for stability, ease of transport, temperature of decomposition and quality of film deposited. Amorphous thin films of GaN with a band gap of 3.4 eV were deposited with 2 at 250 °C. Increasing the substrate temperature to 580 °C resulted in the deposition of epitaxial GaN films. Polycrystalline A1N films were grown with 3 at 600 °C.
Ohmic contacts to Si-implanted and un-implanted n-type GaN
Authors: Brown J. , Ramer J , ZHeng K. , Lester L.F. , Hersee S.D. , Zolper J
Published: April 2011

We report on ohmic contacts to Si-implanted and un-implanted n-type GaN on sapphire. A ring shaped contact design avoids the need to isolate the contact structures by additional implantation or etching. Metal layers of Al and Ti/Al were investigated. On un-implanted GaN, post metalization annealing was performed in an RTA for 30 seconds in N2 at temperatures of 700, 800, and 900°C, A minimum specific contact resistance (rc) of 1.4×10−5 Ω-cm2 was measured for Ti/Al at an annealing temperature of 800°C. Although these values are reasonably low, variations of 95% in specific contact resistance were measured within a 500 µm distance on the wafer. These results are most likely caused by the presence of compensating hydrogen. Specific contact resistance variation was reduced from 95% to 10% by annealing at 900°C prior to metalization. On Si-implanted GaN, un-annealed ohmic contacts were formed with Ti/Al metalization. The implant activation anneal of 1120°C generates nitrogen vacancies that leave the surface heavily n-type, which makes un-annealed ohmic contacts with low contact resistivity possible.
Ohmic Contact Formation to Doped GaN
Authors: Smith L. L. , Bremser M. D. , Carlson E. P. , Weeks T. W. , Huang Y. , Kim M. J. , Carpenter R.W. , Davis R. F.
Published: April 2011

Ohmic contact strategies for n- and p-type GaN have been investigated electrically, chemically, and microstructurally using transmission line measurements, high-resolution EELS and cross-sectional TEM, respectively. The contributions to contact performance from work function differences, carrier concentrations, annealing treatments, and interface metallurgy have been examined. The contact materials of Ti, TiN, Au, and Au/Mg were deposited via electron beam evaporation; Al was deposited via thermal evaporation. As-deposited Al and TiN contacts to highly doped n-GaN were ohmic, with room-temperature specific contact resistivities of 8.6×10−5 Ω cm2 and 2.5×10−5 Ωcm2 respectively. The Ti contacts developed low-resistivity ohmic behavior as a result of annealing; TiN contacts also improved with further heat treatment. For p-GaN, Au became ohmic with annealing, while Au/Mg contacts were ohmic in the as-deposited condition. The performance, structure, and composition of different contact schemes varied widely from system to system. An integrated analysis of the results of this study is presented below and coupled with a discussion of the most appropriate contact systems for both n- and p-type GaN.
Fabrication and Properties of AlGaN/GaInN Double Heterostructure Grown on 6H-SiC(0001)Si
Authors: Amano Hiroshi , Sota Shigetoshi , Nishikawa Masaki , Yoshida Masato , Kawaguchi Makoto , Ohta Masahiro , Sakai Hiromitsu , Akasaki Isamu
Published: April 2011

AlGaN/GalnN double heterostructures (DH) were fabricated by metalorganic vapor phase epitaxy on the (0001)Si 6H-SiC substrate. A cleaved edge shows a very flat surface with roughness on the order of one monolayer. Stimulated emission and laser action from the UV to blue region was observed by optical pumping at room temperature (RT). The threshold power density was 27KW/cm2 which is smaller than that of the same structure grown on a sapphire (0001) substrate by a factor of four. A AlGaN/GalnN DH UV light emitting diode, using undoped GalnN is fabricated. The power efficiency and spectra width of this LED is comparable or superior to that of an LED having the same structure but grown on sapphire.
Analysis of Wurtzite GaN/AlGaN Quantum Well Lasers from First-Principles Calculations
Authors: Uenoyama T. , Suzuki M.
Published: April 2011

Optical gain of wurtzite GaN/AlGaN quantum wells has been studied from a first-principles calculation using the k • p method. Most of the parameters in the k • p method were determined by fitting the band structures by the first-principles calculation. Owing to the small spin-orbit splitting energies of the wurtzite GaN and AIN, the optical gain has been calculated using the 6×6 Hamiltonian for the valence band. It is found that the large hole effective masses and the small spin-orbit splitting cause the higher threshold current density of wurtzite GaN/AlGaN quantum well lasers.
Non-Stoichiometric Transfer of Complex Oxides by Pulsed Laser Deposition at Low Fluences
Authors: Dam B. , Rector J.H. , Johansson J. , DE Groot DG. , Griessen R.
Published: April 2011

We provide evidence that non-stoichiometric ablation of YBa2Cu3O7δ at low fluences is due to a phase separation of the target surface. On the other hand, in SrTiO3 we find at low fluences evidence for preferential ablation which is assisted by volume-diffusion. As a result, the Sr/Ti ratio of the ablated films can be tuned by choosing the appropriate fluence.
KrF Laser-Induced Nanometer Si Crystallites Formation and Tem Observation
Authors: Huang Xinfan , Li Zhifeng , Wu Wei , Chen Xiaoyuan , Li Wei , Hong Ling , Li Qi , Chen Kunji
Published: April 2011

We report the preparation of the nanometer Si crystallites constrained between a-SiNx:H barrier layers by means of KrF excimer laser induced crystallization. X-ray diffraction (XRD) and Raman scattering spectra demonstrate that ultra thin a-Si:H well layers have been crystallized. The average grain size of Si crystallites is on the order of magnitude of a nanometer. Small-angle XRD spectrum indicates multilayer structures have not undergone laser damage. The results of TEM are presented to show smooth and abrupt interfaces of layered structures and nanometer Si crystallites arrayed one-by-one in the Si layer. The thermodynamics of KrF laser-induced crystallization of ultra-thin Si layer has been discussed.
Single shot excimer laser annealing of amorphous silicon: effect of hydrogen on the properties of the poly silicon
Authors: Boher P. , Stehle M. , Godard B. , Stehle J.L.
Published: April 2011

PECVD amorphous silicon films deposited at different temperatures on low cost glass substrates have been treated by a Single Shot Excimer Laser Annealing (SSELA) at various energy densities. The influence of a thermal treatment at medium temperature (400°C) prior to the SSELA treatment was also investigated. Spectroscopie ellipsometry and Raman characterizations show that hydrogen contamination produces an important roughness increase with very little polycrystalline grains (650nm) after laser treatment. The thermal treatment prior laser annealing improves drastically the structural quality of the films. Structural results are correlated with the electrical performances of the TFT produced on these films.
Surface modifications of fullerite induced by pulsed laser irradiation
Authors: Milani P. , Manfredini M.
Published: April 2011

The formation of periodic patterns on fullerite thin film surfaces is observed after irradiation with a pulsed Nd:Yag laser (1064 nm). C60 disruption and subsequent coalescence of the fragments take place, resulting in the formation of carbon nanotubes and polyhedral hollow graphitic particles. The effects of irradiation have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman scattering.
Nitriding of Zirconia by Irradiation with NdrYAG Laser Beam
Authors: Fujitsu S. , Sawai M. , Kawamura K. , Hosono H.
Published: April 2011

The surface of partially stabilized zirconia ceramics was irradiated by a Nd:YAG laser in various atmospheres. Zirconia strongly absorbed YAG's laser beam and changed its chemistry and microstructure. The change of color of zirconia into gold was due to the formation of zirconium nitride (ZrN) observed on the irradiated surface in air, nitrogen or ammonia, which was confirmed by X-ray diffraction and secondary ion mass spectroscopy. The observed ZrN phase was 10-20 um in thickness at the irradiated area by the direct beam. The adhesion between formed ZrN and YSZ substrate was very weak.
Instabilities and Structure Formation in Laser Processing
Authors: Bàuerle D. , Arenholz E. , Arnold N. , Heitz J. , Kargl P.B.
Published: April 2011

This paper gives an overview on different types of instabilities and structure formation in various fields of laser processing. Among the examples discussed in detail are non-coherent structures observed in laser-induced chemical vapor deposition (LCVD), in laser-induced surface modifications, and in laser ablation of polymers.
Area Selective Laser Chemical Vapor Deposition of Diamond
Authors: Lindstam M. , Boman M. , Larsson K. , Stenberg G. , Carlsson J.-O.
Published: April 2011

High quality diamond spots were deposited on silicon substrates by a hot filament process combined with laser heating. A mixture of CH4(1.8 vol%) and H2was passed over a tantalum filament having a temperature of about 2200 °C. The substrate temperature was varied by small adjustments of the filament power. A focused laser beam was used to locally raise the temperature on the substrate surface. By a proper choice of filament temperature, background substrate temperature and laser induced temperature, isolated islands of polycrystalline diamond could be deposited on the silicon substrate. The deposited diamond spots were characterized by micro-Raman spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and scanning force microscopy.
Dendrite Tip-Shape Characteristics
Authors: LaCombe J.C. , Koss M.B. , Bushnell L.T. , de Jager K.D. , Glicksman M.E.
Published: April 2011

The assumption that dendrite tips are parabolic bodies of revolution pervades many of the theories and experiments addressing dendritic growth. This assumption, while reasonable, is known to become less valid as regions of interest further from the tip of the dendrite are considered. Experimental measurements were made on pure succinonitrile dendrites at several super coolings. The equation that describes the dendrite tip profile is extended from a second order polynomial (paraboloidal) form to one that includes higher-order terms. The deviation of a dendrite tip from a parabolic body of revolution can be characterized by a parameter obtained from the coefficient of the fourth-order term describing the profile. This dimensionless parameter, Q, is found to be a function of the profile orientation only, independent of supercooling.
Nucleation and Dendritic Growth in Undercooled Melts
Authors: Herlach Dieter M.
Published: April 2011

Techniques of containerless processing are applied to undercool and solidify metals and alloys. These techniques allow direct measurements of both the undercooling and the crystal growth velocity. Experimental results are presented for studies of nucleation of metastable crystalline phases and quasicrystals. Measurements of the dendrite growth velocity as a function of undercooling are exemplified for dilute Ni-based alloys and intermetallics. The results are analysed within current theories of rapid crystal growth. Their consequences on the formation of grain refined microstructures are highlighted. In addition, recent experiments on the undercooling of magnetic alloys are discussed revealing the existence of long-range magnetic ordering in an undercooled melt.
The Early Stages of Solid-State Reactions in Ti/Al Multilayer Films
Authors: Michaelsen C. , WÖHlert S. , Bormann R. , Barmak K.
Published: April 2011

We have investigated the solid-state reaction of Ti/Al multilayer films by x-ray diffraction (XRD) and differential scanning calorimetry (DSC), with focus on the early stages of the reaction provided by samples with pair thicknesses in the range 5 - 40 nm. This reaction, which results in formation of TiAl3 with metastable Ll2 structure, can be modeled by a nucleation and growth process on the basis of the Johnson-Mehl-Avrami theory, with a reaction-order parameter n ≈1. These observations indicate the significance of nucleation barriers even at early stages of solid-state reactions, and suggests that the phenomena of phase selection and formation of metastable phases can result from the presence of nucleation barriers.