bluuebunny/crossref_metadata_2025 huggingface dataset:
Picosecond Raman Studies of Electron-Phonon Interactions in the Wide Bandgap Semiconductor GaN
Authors: Tsen K.T. , Ferry D.K. , Botchkarev A. , Sverdlov B. , Salvador A. , Morkoc H.
Published: April 2011

Picosecond Raman spectroscopy has been employed to study electron-phonon interactions in the wide bandgap semiconductor GaN. An ultraviolet picosecond laser with photon energy ћω= 4.36eV was used to excite electron-hole pairs in an undoped bulk GaN. The relaxation of these high energy electrons and holes were used to interrogate electron-phonon interactions. We have found that electrons thermalize toward the bottom of the conduction band by emitting primarily longitudinal optical phonons. Our work demonstrates that the Fröhlich interaction is much stronger than the deformation potential interaction in wurtzite GaN.
Photoluminescence Related to the 2-Dimensional Electron Gas in Modulation Doped GaN/AlGaN Structures
Authors: Bergman J.P. , Lundström T. , Monemar B. , Amano H. , Akasaki I.
Published: April 2011

We report low temperature photoluminescence (PL) spectra related to a two-dimensional electron gas confined at a GaN/AlGaN heterointerface. The recombination between electrons confined in the bottom of the interface potential and photoexcited holes causes a broad PL emission about 50 meV below the bulk GaN exciton emission. A second emission, attributed to the recombination of electrons in the first excited level at the interface, is also observed close to the excitonic band gap in GaN. The data agrees with a self consistent calculation of the energy levels and the electron concentration at the interface. Similar PL data from a modulation doped AlGaN/GaN quantum well exhibit three PL emissions related to the 2D electron gas.
Gallium Nitride Epitaxy on Silicon: Importance of Substrate Preparation
Authors: Martin G. A. , Sverdlov B. N. , Botchkarev A. , Morkoç H. , Smith D. J. , Tsen S.-C. Y. , Thompson W. H. , Nayfeh M. H.
Published: April 2011

Hexagonal GaN films grown on non-isomorphic substrates are usually characterized by numerous threading defects which are essentially boundaries between wurtzite GaN domains where the stacking sequences do not align. One origin of these defects is irregularities on the substrate surface such as surface steps. Using Si <111> substrates and a substrate preparation procedure that makes wide atomically flat terraces, we demonstrate that reduction of these irregularities greatly improves the crystalline and luminescent quality of GaN films grown by plasma-enhanced molecular beam epitaxy. X-ray rocking curve width decreases from over 1 degree to less than 20 minutes, while PL halfwidth decreases from over 15 meV to less than 10 meV.
Hydrogen in GaN
Authors: Johnson N. M. , Götz W. , Neugebauer J. , Van de walle C. G.
Published: April 2011

Hydrogen is implicated in the low doping efficiency of acceptors (e.g., Mg) in MOCVD-grown GaN by the following observations: the well-established role of hydrogen in the same phenomenon in other semiconductors; re-passivation of thermally activated Mg in GaN upon exposure to NH3 or H at temperatures > 400°C; correlation between Mg and H concentrations; and computational studies on the diffusivities of charged H in GaN and on the configuration and stability of the Mg-H complex in GaN. Strong experimental evidence for the Mg-H complex comes from variable-temperature Hall effect measurements which reveal that after hydrogenation of activated GaN:Mg the hole mobility increases as the hole concentration decreases, which is consistent with removal of ionized scattering centers and not with the introduction of separate compensating defects. However, experimental evidence is also accumulating to suggest that the Mg activation process involves the generation or transformation of electrically active defects.
Growth of GaN on (100)Si Using a New C-H and N-H Free Single-Source Precursor
Authors: Kouvetakis John , McMurran Jeffrey , Beach David B. , Smith David J.
Published: April 2011

We have demonstrated growth of crystalline GaN on Si substrates by using, for the first time, a novel inorganic precursor Cl2GaN3 and ultra-high-vacuum chemical vapor deposition techniques. Cross-sectional electron microscopy of the highly conformal films showed columnar growth of wurtzite GaN while Auger and RBS oxygen- and carbon-resonance spectroscopies showed that the films were pure and highly homogeneous. In addition to the high growth rates of 70–500 Å per minute, the low deposition temperature of 550–700 °C, and the nearly perfect GaN stoichiometry that we obtain, another notable advantage of our method is that it provides a carbon-free growth environment which is compatible with p-doping processes.
Thermal Stability of Ohmic Contacts to n-InxGa1−xN
Authors: Duibha A. , Pearton S. J. , Abernathy C. R. , Lee J. W. , Holloway P. H. , Ren F.
Published: April 2011

The microstructural properties and interdiffusion reactions of Au/Ge/Ni, Ti/Pt/Au, WSix and AuBe contacts on GaN and In0.5Ga0.5N have been examined using Scanning Electron Microscopy and Auger Electron Spectroscopy. The WSix contacts possess excellent thermal stability and retained good structural properties at annealing temperatures as high as 800°C on GaN. The electrical characteristics of WSix contacts on In0.5Ga0.5N had a specific contact resistivity of 1.48×10−5Ωcm2 and an excellent surface morphology following annealing at 700°C. The increase in contact resistance observed at higher temperatures was attributed to intermixing of metal and semiconductor. In contrast the Ti/Pt/Au and Au/Ge/Ni contacts were stable only to ≤ 500°C. AuBe contacts had the poorest thermal stability, with substantial reaction with GaN occurring even at 400°C. The WSix contact appears to be an excellent choice for high temperature GaN electronics applications.
Optically Pumped GaN-AlGaN Double-Heterostructure Lasers Grown by ECR-GSMBE and HVPE
Authors: Maki P. A. , Molnar R. J. , Aggarwal R. L. , Liau Z-L. , Melngailis I.
Published: April 2011

We have observed laser emission with well-defined cavity modes in optically pumped GaN-Al0.1Ga0.9N double-heterostructure (DH) lasers. The laser structures were grown using an electron-cyclotron-resonance nitrogen-discharge source and gas-source molecular beam epitaxy (ECR-GSMBE) on thick (≥10 µm) GaN buffers grown by hydride vapor-phase epitaxy (HVPE) on c-plane sapphire. Transversely pumped cavities using a 337.1 nra nitrogen laser pump source exhibit a threshold pump fluence ranging from 0.15 to 0.3 mJ/cm2 at 77 K, a linear light output above threshold, a lasing wavelength of 358 nm, and an estimated differential quantum efficiency of 1%. The room-temperature threshold is about 1.7 times higher. Longitudinal mode structure has been resolved in a shorter-cavity (23 µn) device at 77 K. The measured mode spacing of 0.56 nm corresponds to a group index of 5.0. Far-field measurements in a plane perpendicular to the plane of the heterostructure indicate a double-lobed pattern for a 1000 Ǻ thick GaN active region, and a single lobe with a FWHM of 60° for a 4000 Ǻ active region. The thick HVPE GaN buffer layer provides for a lattice-matched growth and results in improved nucleation in MBE, as indicated by a high-quality reflection-electron-diffraction pattern of the as-loaded wafers. The surface morphology of the MBE layers on the HVPE buffer shows improved optical smoothness as compared to layers grown directly on sapphire using a low-temperature, MBE-grown GaN buffer. Laser facets were formed either by saw cutting or cleaving of the GaN buffer and epilayer along crystal planes. Details of the material development and laser performance are described.
Thin Films of Ferroelectrics Made by Pulsed Laser Deposition for Optoelectronic Applications
Authors: Chrisey D.B. , Knauss L.A. , Horwitz J.S. , Auyeung R.C.Y. , Knies D. , Grabowski K.S. , Donovan E.P. , Mccallion K. , Johnstone W.
Published: April 2011

We have deposited ferroelectric thin films by pulsed laser deposition to be used as an inline optoelectronic modulator utilizing evanescent field coupling to a side-polished optical fiber. In order to satisfy waveguiding conditions, amorphous quartz substrates (n= 1.447) were necessary. Ferroelectric films of (Pb0.91La0.90)(Zr0.65,Ti0.35)O3, Pb0.80La0.20TiO3 (PLZT), and Sr0.5Ba0.5TiO3 were deposited on quartz substrates and thin Bi4Ti3O12 buffer layers were explored as a chemical barrier and to improve crystallographic texturing. The morphology of thick films (˜3 µm) of PLZT was not acceptable for optical applications, whereas the Sr0.5Ba0.5TiO3 films were very smooth, but had some cracking. The Sr0.5Ba0.5TiO3 films were optically determined to be uniform and of sufficient quality for use in fabricating in-line fiber optic modulators.
Application of Laser Microetching in Formation of Air-Bearing Surface for Magnetic Head Sliders
Authors: LU Y.F. , YE K.D.
Published: April 2011

Laser-induced etching of polycrystalline Al2O3TiC material by a tightly-focused CW Ar ion laser has been investigated in a KOH solution with different concentrations. It is found that the KOH concentration can strongly affect the etching quality where low KOH concentration can result in rough and irregular patterns. The etching effect is also related to laser power and scanning speed. Laser-induced etching of polycrystalline AI2O3TiC in a KOH solution is found to be a photothermal reaction in which a threshold laser power exists. With an appropriate set of etching parameters, well defined grooves can be obtained with clean side walls and with an etching rate up to several hundred micrometers per second. It is also found that the grains in the polycrystalline Al2O3TiC material play an important role in the etching dynamics and etching quality. This etching process is believed to be applicable to the formation of a slider surface of magnetic heads in the future.
Laser Dry Cleaning of Zro2 Particles from Air Bearing Surface of Magnetic Head Sliders
Authors: Lu Y.F. , Song W.D. , Hong M.H. , Chong T.C. , Low T.S.
Published: April 2011

Laser dry cleaning of ZrO2 particles from air bearing surface (ABS) of magnetic head sliders has been investigated. The experimental results show that the cleaning threshold of removing ZrO2 particles from ABS is about 100 mJ/cm2. For laser fluence larger than this threshold, cleaning efficiency increases with increasing laser fluence and pulse number, but does not depend on repetition rate up to 30 Hz. The mechanisms of this laser dry cleaning are laser-induced ABS vibration, particle thermal expansion and particle vibration, which produce forces strong enough to detach ZrO2 particles from ABS. With increasing laser fluence and pulse number, the average acceleration and vibration number of particles induced by laser increase respectively, so that it is easier to remove particles which corresponds to higher cleaning efficiency. For fixed laser fluence and pulse number, changing the repetition rate does not change the average acceleration or the vibration number of particles, therefore laser cleaning efficiency is almost the same for different repetition rates.
Amide to Active Amine Conversion of Nylon by Deep UV Irradiation
Authors: Kelley Michael J. , Cohen Jeffrey D. , Erkenbrecher Carl , Haynie Sharon L. , Kobsa Henry , Roe A. Nicholas , Scholla Michael H.
Published: April 2011

Ultraviolet light having sufficiently short wavelength can drive photochemical processes at polymer surfaces. We find that irradiation at 193 nm, but not at 248 nm, results in conversion of amide groups at the nylon surface to amines, still bound in the polymer chain. These amines show anti-microbial activity [1].
Mechanical Properties of High Strength Aluminum Alloys Formed by Pulsed Laser Deposition
Authors: Knapp J. A. , Follstaedt D. M.
Published: April 2011

Very high-strength alloys of Al(O) have been formed using a pulsed laser deposition (PLD) system to deposit from alternating targets of Al and Al2O3. Ion beam analysis and transmission electron microscopy show that the deposited material is uniform in composition with up to 33 at.% O and has a highly refined microstructure consisting of a fine, uniform dispersion of ∼1 nm diameter γ-Al2O3 precipitates. Ultra-low-load indentation testing combined with finite-element modeling is used to determine the mechanical properties of the layers. Yield stresses as high as 5.1 GPa have been measured in these materials, greatly exceeding the strengths of aerospace Al alloys (∼0.5 GPa) and even high strength steels. The key to the properties of these materials is the dispersion of small, hard precipitates spaced only a few Burgers vectors apart; dislocations are apparently unable to cut through and must bow around them.
Structural Relaxation and De-Relaxation Phenomena in Amorphous Ge Films upon Irradiation with Short and Ultrashort Laser Pulses
Authors: Solis J. , Siegel J. , Afonso C.N.
Published: April 2011

The state of relaxation of amorphous Ge films has been modified upon melting and rapid solification induced by irradiation with ps laser pulses. The material obtained upon irradiation with fluences sligthly above the melting threshold is less relaxed than the as-deposited one (de-relaxation process). For larger fluences the results obtained suggest that the degree of relaxation is related to the supercooling achieved prior solidification.
Laser Assisted Deposition of Amorphous Hydrogen Containing Carbon at Room Temperature
Authors: Stenberg G. , Piglmayer K. , Boman M. , Carlsson J.-O.
Published: April 2011

Hydrogen-containing amorphous carbon films were produced from CH,I, by photolytical laser assisted chemical vapour deposition at room temperture. An Ar+ laser operated in the ultraviolet spectral region (351 and 364 nm) was used as the excitation source. The beam was focused in parallel to the Si(lOO) substrate. The deposition process was studied for different partial pressures of CH,I2 and for different heights of the laser beam above the substate surface. The mass thickness of the film was determined by energy dispersive X-ray spectrometry. The hydrogen content of the carbon films was determined by nuclear reaction analysis and the iodine content in the films was obtained by Auger electron spectroscopy and energy dispersive X-ray spectrometry. Carbon hybridization was investigated by Raman and infrared spectroscopy while morphology was examined by atomic force microscopy.
Open Air Fabrication Process of Cu Thin Films on Teflon Surface Using Arf Excimer Laser Irradiation
Authors: Okoshi M. , Toyoda K. , Murahara M.
Published: April 2011

Copper thin films were selectively fabricated on a Teflon surface by using an ArF excimer laser and a Cu electroless plating solution in open air. Photo-excited C-F bonds of the surface were effectively defluorinated with hydrogen atoms which were photodissociated from the Cu solution. The dangling bonds of the defluorinated carbon atoms were combined with the oxgen and the copper atoms which were also photodissociated. Thus, C-O-Cu bonds were formed on the surface and functioned as Cu nuclei for an electroless plating. The Cu thin films were successfully deposited in electroless plating solution only on the Cu nuclei.
Orientation and Velocity Dependence of the Nonequilibrium Partition Coefficient
Authors: Beatty K.M. , Jackson K.A.
Published: April 2011

Monte Carlo simulations based on a Spin-1 Ising Model for binary alloys have been used to investigate the non-equilibrium partition coefficient (kneq ) as a function of solid-liquid interface velocity and orientation. In simulations of Si with a second component kneq is greater in the [111] direction than the [100] direction in agreement with experimental results reported by Aziz et al. The simulated partition coefficient scales with the square of the step velocity divided by the diffusion coefficient of the secondary component in the liquid.
Bubble Formation in Zr Alloys Under Heavy Ion Implantation
Authors: Pagano Luciano , Motta Arthur T. , Birtcher Robert C.
Published: April 2011

We report here the results of a study conducted to examine the effect of Kr ion irradiation on bubble formation in Zr alloys. We used the HVEM/Tandem facility at Argonne National Laboratory to irradiate several Zr alloys, including Zircaloy-2 and Zircaloy-4, at temperatures from 300 to 800 C and to doses up to 2 × 1016ion.cm−2. Both in-situ irradiation of thin foils as well as irradiation of bulk samples with an ion implanter were used in this study. For the thin foil irradiations, a distribution of small bubbles in the range of 30-100 Å was found, at all temperatures with the exception of the Cr-rich Valloy where bubbles of 130 Å were found. The irradiation of bulk samples at high temperature (700–800 C) produced large faceted bubbles (up to 300 Å) after irradiation to 2 × 1016ion.cm−2. The results are examined in the context of existing models for bubble formation and growth in other metals.
On the Roles of Temperature and Interfaces in Irradiation and Thermally Induced Solid State Amorphization
Authors: Wang L.M. , Gong W.L. , Ewing R.C. , Webert W.J.
Published: April 2011

The roles of irradiation temperature and interfaces (free surfaces and grain boundaries) in irradiation- and thermally- induced amorphization of ceramics (coesite, apatite, olivines and spinels) have been studied by transmission electron microscopy (TEM). The irradiations were performed with 1.5 MeV Kr+, 200 keV and 1 MeV electrons over a wide temperature range (20-700 K). The critical amorphization dose at which amorphization is complete, Dc, increased with increasing irradiation temperature for most materials except coesite (a high pressure polymorph of Si02) which showed a decreasing Dc with increasing temperature under 1 MeV electron irradiation. Although amorphization may occur directly within a displacement cascade or by cascade overlap, this study shows that free surfaces and grain boundaries are favorable sites for nucleation of amorphous volumes. Once the amorphous volume is formed at interfaces, it may grow rapidly under continued irradiation. Coesite which has a glass transition temperature higher than its melting temperature underwent spontaneous amorphization during thermal annealing at 1200 K. This thermally-induced amorphization also started at free surface and grain boundaries and propagated into the interior of the crystal. The interface-mediated amorphization is analogous to the process of thermodynamic melting.
A Tem Study of Inverse Melting in Nb45Cr55
Authors: Sinkler W. , Michaelsen C. , Bormann R.
Published: April 2011

Inverse melting of bcc Nb4sCr55 is investigated using transmission electron microscopy, high-resolution TEM and electron diffraction. It is shown that the transformation to the amorphous phase initiates at the bcc grain boundaries. The transformation results in an increase in incoherence, evidenced by a loss of bend contours. Some anisotropy is found in the amorphous phase produced by inverse melting, which is associated in HRTEM with preferentially oriented but discontinuous and distorted fringes. The results are consistent with the production of an amorphous phase by inverse melting.
Thermodynamic Properties of Coherent Interphase Boundaries in Substitutional Fcc Alloys
Authors: Asta Mark
Published: April 2011

Thermodynamic and structural properties of coherent interphase boundaries (IPBs) in the Al-Li and Ag-Al alloy systems are studied using the cluster variation method (CVM) and low-temperature expansion (LTE) techniques. The energy parameters required as input for the CVM and LTE calculations were obtained by fitting to phase diagram data in the case of Al-Li alloys and from a fit to the results of first-principles total energy calculations for Ag-Al. Interphase energies are calculated as a function of temperature and composition profiles across coherent IPBs are computed for high-symmetry crystallographic orientations. It is demonstrated that the calculated temperature dependence of interphase energies and the “widths” associated with compositionally diffuse IPBs can be appreciable even at temperatures well away from critical points.
Kinetic Competition in Undercooled Liquid Alloys
Authors: Perepezko J. H. , Allen D. R.
Published: April 2011

Most alloy systems can develop more than one solid phase for a given composition during solidification. For metastable undercooled liquids a thermodynamic analysis can provide insight into the range of structural and compositional options that are possible. The numerous examples of metastable phase formation during solidification of undercooled melts demonstrates that the actual phase selection is dominated by kinetics. Indeed, the observation of undercooling is linked to kinetic control. Different forms of kinetic control can be identified and include suppression of heterogeneous nucleation during slow cooling and constrained growth during rapid quenching. In both cases the kinetic competition involved in phase selection is mediated by the thermal history. Nucleation controlled kinetics can be examined in fine powders of undercooled liquids where crystallization rates and metastable phase diagrams provide a basis for analysis. Similarly, constrained growth can be examined during controlled cooling where microstructures and kinetic models can be used for analysis. The existing kinetics models appear to be adequate but the processing conditions are often dynamic and in nonsteady state conditions so that critical tests are difficult unless specific kinetic transitions are available.
Kinetics of Structural Relaxation in Metal Glasses
Authors: Nakonechna O.I. , Zakharenko N.I. , Kopan V.S. , Dashevsky N.N. , Babich N.G.
Published: April 2011

Relaxation processes in different amorphous transition metal-metalloid (TM-M)-type alloys have been studied using thermopower measurements. The migration of two types of atomic complexes has been considered. For Ti-based amorphous alloys the migration of TM atoms and TM-M complexes diffusion are at least two processes that define the structural relaxation. For Ni and Fe-based alloys the relaxation processes are determined first of all by isolated TM atom migration.
Phase transition in non-crystalline solids viewed from heating
Authors: Nakayama K. , Kojima K. , Takahashi N. , Masaki Y. , Kitagawa A. , Suzuki M.
Published: April 2011

The heating-rate dependence of crystallization temperature, Tc, and the glass transition temperature, Tg, is studied from the view points of nucleation and fragmentation processes in disordered structures. Tc and Tg are expected to increase monotonically with heating rate. Such behaviors of Tc and Tg are classified into four characteristic regions with respect to the heating rate. Results are summarized in the Transient Phase Diagram where Tc and Tg are given as a function of heating rate. The scaling rule in the Transient Phase Diagram is given.
Thermodynamics of the System Ag2se-Ag2s: A Molecular Dynamics Study
Authors: Smith A. , Ravelo R. , Pingitore N.
Published: April 2011

We report on Molecular Dynamics (MD) studies of free energy and vibrational entropy differences of the binary system Ag2Se-Ag2S as function of temperature. We have employed the methods of adiabatic switching and temperature integration over a reversible path to obtain the phase diagram. We discuss the nature of the entropy difference in terms of ion mobility and previous MD calculations of diffusion rates in these systems.
An Investigation of Grain Refinement in Deeply Undercooled Alloy Droplets Using Texture Analysis
Authors: Norman A.F. , Gärtner F. , Eckler K. , Zambon A. , Greer A.L. , Ramous E. , Herlach D.M.
Published: April 2011

For Cu - 30 at.% Ni droplets processed by containerless solidification under lévitation, the crystallographic texture correlates strongly with the microstructure seen in optical metallography and with the undercooling at which solidification started. The variation in texture is useful in understanding the development of the microstructure, in particular at the extremes of the undercooling range where grain-refined microstructures are observed.
Morphological Evolution and Properties of LPE Grown GaSb, AlGaSb and AlGaAsSb
Authors: Dutta P.S. , Bhat H.L. , Kumar Vikram , Dieguez E.
Published: April 2011

The nucleation morphologies of LPE grown GaSb, AlGaSb and AlGaAsSb layers on GaSb substrates are presented. The morphology of the GaSb layers grown from Sb-rich melts showed facets on highly terraced surface, whereas those grown from Ga-rich melts exhibited fine terraces without facets. An optimum temperature in the range of 500 – 550°C was found to be suitable for the growth of mirror smooth layers from Ga-melts. The surface morphology of the AlxGa1-xSb layers degrades drastically with increase in Al content beyond x = 0.5. The surface morphology of AlGaAsSb epilayers has been found to depend strongly on the pre-growth melt dissolution sequence.
Self-Limiting Growth Kinetics of 3D Coherent Islands
Authors: Chen K. M. , Jesson D. E. , Pennycook S. J. , Thundat T. , Warmack R. J.
Published: April 2011

Large 3D coherent islands are found to kinetically resist their further growth during post-deposition annealing of metastable 2D strained films. We reveal that a kinetic energy barrier exists to successive facet-layer growth of a 3D strained island at the expense of its surrounding 2D structure. The barrier increases with further growth of the island, which defines a self-limiting behavior of island growth. This self-limiting effect naturally explains a number of relevant features which have been observed experimentally including narrow island size distributions.
Equilibrium Shapes of Small Strained Islands
Authors: Spencer Brian J. , Tersoff J.
Published: April 2011

We calculate the equilibrium morphology of a strained layer, for the case where it wets the substrate (Stranski-Krastonow growth). Assuming isotropic surface energy and equal elastic constants in the film and substrate, we are able to calculate two-dimensional equilibrium shapes as a function of the island size and spacing. We present asymptotic results for the equilibrium shape of a thin island where the island height is much smaller than the island width. We also present numerical results of the full equations to describe the island shape when the islands are widely separated. From these solutions we are able to determine the chemical potential of the island as a function of island volume and the strain energy density along the surface of the island for small to medium-sized islands.
X-Ray Scattering and Absorption Studies of MnAs Thin Films Grown by MBE on GaAs (001) Substrates
Authors: Huang S. , Ming Z. H. , Soo Y. L. , Kao Y H. , Tanaka M. , Munekata H.
Published: April 2011

MnAs thin films grown on GaAs (001) substrates by molecular beam epitaxy have been studied by grazing incidence x-ray scattering (GIXS), x-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS). Microstructures in films prepared with different first-layer growth conditions (template effects) are compared in terms of the interfacial roughness, lattice constants, epilayer thickness, local environment surrounding the Mn atoms, coordination number, and local disorder. The template effects are found to cause significant differences in the local structures and crystallinity of MnAs epitaxial layers.
Void and Dislocation Microstructure Development in Heteroepitaxial Film Growth
Authors: Smith Richard W. , Srolovitz David J.
Published: April 2011

Two dimensional, non-equilibrium molecular dynamics simulations have been performed to examine the microstructures of both homoepitaxial and heteroepitaxial thin films grown on single crystal substrates. The principal microstructural features to develop within these films are small voids and edge dislocations. Voids form near the surface of the growing film as surface depressions between microcolumns pinch off to become closed volumes. These voids often form in such a way as to introduce dislocations into the crystal with their cores positioned within the voids. Dislocations are also formed during heteroepitaxy at the interface between the substrate and film. These dislocations tend to be mobile. When voids are present in the film and when the lattice misfit is low, dislocations tend to be trapped in the voids or pulled toward them due to dislocation image interactions. Once attached to voids, dislocations are effectively pinned there. When voids are absent or when the misfit is high, dislocations are restricted to the film-substrate interface. In the case of heteroepitaxy, dislocations are found to relieve either tensile or compressive misfit stresses. Misfit stresses may also be accommodated, to some extent, merely by the free volume of the voids themselves.
Deformation Behavior of Strained Layer Heterostructures
Authors: Fischer A. , KüHNE H.
Published: April 2011

We present a new Volterra dislocation approach in equilibrium theory for strain relaxation in heteroepi-taxial semiconductor structures, one which includes surface relaxation effects and elastic interactions between straight misfit dislocations. The free-surface boundary conditions are satisfied by placing an image dislocation outside the crystal in such a manner that its stress field cancels that of the real interface misfit dislocation at the surface. The effect of the Airy stress function that removes the fictitious shear and normal stresses at the surface are discussed. This image method provides an equilibrium theory which correctly predicts critical strained layer thicknesses and completely describes the elastic and plastic strain relief as well as the phenomenon of work hardening in lattice mismatched epilayers.
Equilibrium Configuration of Epitaxially Strained Thin Film Surfaces
Authors: Jagannadham K. , Narayan J. , Hirth J.P.
Published: April 2011

The formation of convex and concave regions on the surface of a strained thin epitaxial film on a thick substrate is analyzed by minimization of energy associated with the configuration. The strain energy change resulting from the formation of undulations is modelled with the strain in the film represented by a continuous distribution of dislocations along the perturbed surface and the interface. A discrete dislocation model is also used when periodic undulations are formed. Results of energy minimization for germanium or germanium-silicon alloy films on silicon substrate illustrate that convex regions tend to grow. On the other hand, convex regions formed to conserve mass in shape changes associated with concave regions become stable with minimum energy under quasi-equilibrium when the mobility of adatoms is low. We have determined the size and radius of curvature of the undulations at minimum energy and conclude that it is favorable to form atomic steps on the surfaces from which dislocation generation and strain relaxation takes place.
Growth of GaAS and InAlAs on High Quality, Epitaxial, NiAl Metal Film
Authors: Hung C.-Y. , Weckwerth M.V. , Visokay M.R. , Pao Y.C. , Harris J.S.
Published: April 2011

Success in incorporating single crystal, epitaxial metal layers into semiconductors will have a significant impact on electronic and optical devices. The growth of GaAs on (001) pseudomorphic NiAl/GaAs by molecular beam epitaxy has been studied using transmission electron microscopy. The island growth of GaAs on pseudomorphic (lattice matched) NiAl is the origin of the formation of stacking faults and microtwins in the GaAs overlayer. The GaAs islands are considered to be due to heterogeneous nucleation, which occurs more easily at the corner of a step on a strained metal film. Growth of almost twin free In0.25Al0.75As on thick, lattice relaxed, epitaxial NiAl films has been achieved, which supplying useful information for the investigation of the possible factors causing poor quality of GaAs overlayers.
A New Method for Fast Simulation of Adsorbate Dynamics
Authors: Kumar P.V. , Warakomski Steven J. , Fichthorn Kristen A.
Published: April 2011

We present a dynamical version of the Smart Monte Carlo method to model the diffusion dynamics of a metal atom on a metal surface. This method, in conjunction with umbrella sampling, can be used to simulate the dynamics of metal thin film growth, including all relevant atomic-scale phenomenon, over long time scales, characteristic of experimental studies. To demonstrate the accuracy of this method we simulate the dynamics of Rh on Rh(111) and Cu on Cu(100). Interatomic forces are modeled with Lennard-Jones and Corrective-Effective-Medium potentials for the Rh and Cu systems, respectively. We show that this new simulation method correctly reproduces the diffusion dynamics and, with some modification, allows us to reach experimental time scales.
Fabrication of Fine Grain Size Shape Memory Alloys Through Crystallization of Amorphous Ribbon
Authors: Shi J. D. , Ma J. L. , Gao Y. , Pu Z. J. , Wu K. H.
Published: April 2011

The objective of this work is to fabricate fine grain size NiTi-based shape memory alloys through crystallization of amorphous materials. An amorphous material with a composition of Ti50Ni25Cu25 was used as a sample for this study. First, a systematic investigation was conducted to understand the kinetics of the crystallization process of amorphous materials under an isothermal mode and under a continuous heating mode, respectively. The characteristic parameters associated with the crystallization process, such as the starting and finishing times for the isothermal crystallization process, the peak temperature for the continuous heating crystallization process, and the activation energy of the material were determined. Based on the results of the kinetic study, a series of isothermal annealing experiments were conducted to characterize the microstructure of the fully crystallized materials. The TEM micrographs indicate that the mean grain size of the fully crystallized Ti50Ni25Cu25 alloy is independ of isothermal temperature. The mean grain size is around 400-500nm, which is 1/10 of the grain size of usual NiTi based shape memory alloys. In addition, the crystallization micro-mechanism of the amorphous material is discussed based on TEM observations.
Solidification Analyses for the Fabrication of Bulky Multicomponent Metallic Glasses and their Properties
Authors: Inoue Akihisa , Shinohara Yoshiyuki , Yokoyama Yoshihiko
Published: April 2011

The liquid-crystalline transformation during continuous cooling was examined for a Zr60Al10Ni10Cu15Pd5 alloy which was ejected into a wedge-shape cavity in a copper mold or solidified on a copper hearth by unidirectional arc melting. The structure consists of a glassy phase in the wedge angles smaller than 10 degrees and changes to glassy and crystalline phases in the higher angle range. The start (Cs) and termination (Ct) points for the transformation were determined and the continuous-cooling-transformation (C. C. T. ) curves were constructed. The nose temperature (Tn) and the time (tn) up to the nose point in the C. C. T. curves were 1018 K and 0.93 s. The critical cooling rate for glass formation defined by (Tm-Tn)/tn is 110 K/s. Furthermore, with the aim of clarifying a solidification condition for formation of a bulk glassy alloy by unidirectional arc melting, the relation between the formation of a glassy phase and the solidification parameters of moving velocity of liquid/solid interface (V), temperature gradient (G) and cooling rale (R) was examined. The glassy phase was obtained in the condition of V>3 mm/s, G>4 K/mm and R>40 K/s. The supercooling reaches 385 K at 40 K/s. The large supercooling ability is presumably due to the formation of a highly dense randomly packed structure where the nucleation of a crystalline phase and the atomic rearrangement for growth reaction are difficult.
Mechanical Properties of Porous Nanophase Materials Measured by Instrumental Indentation
Authors: Van Swygenhoven H. , Wagner W. , Löffler J.
Published: April 2011

Mechanical properties of nanostructured intermetallic Ni3Al synthesized by the inert-gas condensation technique are studied by means of instrumental indentation using the ICT-CSEMEX indenter. This instrument is a microindenter which continously measures load and displacement. Load-displacement curves are performed as function of grain size, consolidation- and annealing temperature. The mean grain size of the samples are studied by means of x-ray diffraction and small-angle neutron scattering.
Surface Nucleation and Template Growth of Ti Silicides
Authors: Tung R. T.
Published: April 2011

The direct nucleation and growth of Ti silicide on the surfaces of Si(100) and amorphous Si were studied. Silicide phase formation depended on the temperature and the stoichiometry of deposition and the crystallinity of the substrate. A very low temperature, − 500°C, for the nucleation of the low-resistivity C54-TiSi2 phase was observed on amorphous Si. Stoichiometric and uniform TiSi2 layers were grown with the depositions of pure Ti. On crystalline Si, uniform TiSi2 layers were also grown at ∼ 500°C with a co-deposited template layer. The much reduced C54 formation temperature is discussed in terms of a possible circumvention of precursor amorphous silicide phases during surface nucleation.
Substrate Influence on the Formation of Titanium Silicide on Polycrystalline Silicon
Authors: da Silva Ana Neilde Rodrigues , Furlan Rogerio , Santiagoaviles J. J.
Published: April 2011

In this work we investigated the influence of the polycrystalline silicon substrate on the titanium silicide formation process. The results are compared to those obtained when single crystal silicon wafers are used. We observed that the polycrystalline substrate affects both the kinetics of formation of the phase TiSi2-C49 and the temperature of transition from TiSi2-C49 to TiSi2-C54. The temperature of this phase transition is also influenced by the presence of phosphorous in the polycrystalline silicon substrate. In order to prevent degradation of the silicide film formed on polycrystalline silicon, the maximum temperature of RTP has to be lower than 900°C. Due to the high chemical reactivity between titanium and oxygen present in the ambient, we also investigated the use of a protective cap. For this purpose, a thin amorphous silicon layer was sputter-deposited sequentially on Ti films without breaking the vacuum.
Formation of Titanium Silicide Strap Lines by the Deposition of a Double Layer of Amorphous Silicon - Titanium
Authors: da Silva Ana Neilde Rodrigues , Furlan Rogerio , Santiagoaviles J. J.
Published: April 2011

The reaction between an amorphous silicon film, with a titanium film, yields the formation of a titanium silicide film useful in the fabrication of local interconnection lines (straps) for integrated circuits. In this work we look at the influence of the films thickness ratios on the silicide formation on silicon dioxide. For structures with 70 or 80 nm of amorphous silicon deposited on 40 nm of titanium, we found that after a low temperature treatment of 650 °C, nominally utilized in the first processing step for the formation of self-aligned silicides, the formation of the metastable TiSi2 (C 49) and Ti5Si3 phases occurs. For silicon rich structures i.e.. 90 nm of amorphous silicon on 40 nm of titanium, besides the stable TiSi2 (C54) the metastable phases mentioned above are also detected. In this case strong morphological changes, and silicon precipitates are observed.
In Situ Analysis of the Formation of thin TISI2, (>50 nm) Contacts in Submicron Cmos Structures during Rapid Thermal Annealing
Authors: Clevenger L. A. , Cabral C. , Roy R. A. , Lavoie C. , Viswanathan R. , Saenger K. L. , Jordan-Sweet J. , Morales G. , Ludwig K. L. , Stephenson G. B.
Published: April 2011

A detailed in situ study of silicide reactions during rapid thermal annealing of patterned structures was performed to determine the effects of linewidth (0.2 to 1.1 μm), dopants (arsenic, boron or phosphorus) and silicon substrate type (poly-Si or <100>-Si) on the C49 to C54-TiSi2 transformation. A synchrotron x-ray source and a high speed position sensitive detector were used to collect x-ray diffraction patterns of the reacting phases on a millisecond time scale, in situ, during annealing. We demonstrate that most patterned C49-TiSi2 structures (0.2 to 1.1 μm in width, 2 to 4 μm2 in area) will incompletely transform into C54-TiSi2 during rapid thermal annealing. The C49 to C54 transformation ends at about 900°C and further annealing to higher temperatures does not force the remaining C49 to transform into C54. We also observed that the C54 formation temperature increases as the linewidth of the silicide structure decreases. These results are explained by a low density of C54 nuclei in C49 which leads to a one-dimensional growth of C54 grains along the length of the patterned lines. Finally the incorporation of a Mo implant into either poly-Si or <100>-Si before the deposition of titanium is shown to increase the percentage of C49 that transforms into C54 and also to lower the C54 formation temperature.
Selective Titanium Silicide for Industrial Applications
Authors: Maury D. , Regolini J. L. , Gayet P.
Published: April 2011

The possibilities of using TiSi2 for contacts and interconnects in 0.25 μm and below design rule ICs are at present being investigated using the standard solid phase reaction (salicide) technique. Some problems still remain, such as phase transformation or dewettability over narrow lines. Over the last few years we have developed a TiSi2 selective CVD technique which is being used for 200 mm wafers in an industrial integrated cluster reactor. The system consists of two reaction chambers: one for Si epitaxy (and related alloys) and the other for TiSi2 deposition. The process sequence involves three main steps: (i) a wet chemical wafer cleaning, (ii) a selective silicon epitaxy for elevated source/drains to obtain minimum substrate consumption and (iii) the deposition of TiSi2 using the H2/SiH4 (or DCS)/TiCl4 gas system in the 650–750°C temperature range. The whole process time per wafer is shorter than 4 min. Selectivity, stoichiometric composition and resistivity of around 16 μmΩcm are obtained. The growth rate, uniformity, grain size and doping effects have been studied on single, polycrystalline, doped and undoped silicon. Device wafers have been used to compare the two processes, standard and CVD, obtaining electrical results such as lower series resistances, and a linewidth independent sheet resistance of down to 0.20 μm.
Stability Hierarchy of New Epitaxial Phases in CoSi2
Authors: Miglio Leo , Tavazza Francesca
Published: April 2011

In this paper we report the cohesion energy curves for different CoSi2 structures calculated by a semiempirical tight binding scheme. We set up a stability hierarchy among them and provide a kinetic model which could explain very recent and apparently contradictory Molecular Beam Epitaxy findings concerning the stability of a new pseudomorphic phase, i.e. the defected CsCl.
Structural Properties of β-FeSi2 Bulk Crystal Grown by Horizontal Gradient Freeze Method
Authors: Kakemoto H. , Tsaic Y. , Beye A. C. , Katsumata H. , Sakuragi S. , Makita Y. , Obaraa A. , Kobayashi N. , Shibata H. , Uekusa S. , Tsukamoto T. , Tsunoda T. , Imaif Y.
Published: April 2011

We report on the synthesis of β-FeSi2 bulk materials using Horizontal Gradient Freeze (HGF) method. Chunk and powder FeSi2 or high-purity Fe (4N) and Si (9N) were used as starting materials. Three values (1:2, 2:5 and 1:3) of the Fe:Si ratio were selected in the very narrow α and β ranges of the equilibrium phase diagram. Samples were melted between 1300°C and 1500°C in high purity graphite crucibles covered with boron nitride. After cooling, the samples were kept at 800°C and 900°C during 66 to 100 hours, leading to transformation from α to β phase. Cooling rate and annealing time were taken as the two main parameters to optimize the growth conditions. Principal structural characterization was made by X-ray diffraction (XRD). Correlation with stoichiometry was achieved using the results of Rutherford Backscattering Spectroscopy (RBS) and Electron Probe X-Ray Micro Analysis (EPXMA). The samples obtained from 1:2 ratio exhibited mainly β phase while the 2:5 ratio specimens revealed almost α phase structure. The samples prepared with 1:3 ratio was found as a mixture of α and β phases under Si-rich conditions.
Initial Stages of Reactions between Monolayer Fe and Si(001) Surfaces
Authors: Hasegawa M. , Kobayashi N. , Hayashi N.
Published: April 2011

Reactions between 1.5 monolayer(ML) Fe deposited on Si(001)-2 × 1 and -dihydride surfaces were studied in situ by reflection high-energy electron diffraction and time-of-flight ion scattering spectrometry with the use of 25 keV H ions. The reactions between Fe and Si which were successively deposited on Si(001)-dihydride surface were also studied. After the room temperature deposition Fe reacted with Si(001)-2 × 1 substrate resulting in the formation of polycrystalline Fe5Si3. By annealing to 560∼650°C composite heteroepitaxial layer of both type A and type B β -FeSi2 was formed. On the dihydride surface polycrystalline Fe was observed after 1.5ML Fe deposition at room temperature, and reaction between Fe and Si(001)-dihydride surface is not likely at room temperature. We observed 3D rough surface when we deposited only Fe layer on the dihydride surface and annealed above 700°C. The hydrogen termination of Si(001) surface prevents the deposited Fe from diffusing into the substrate below 500°C, however the annealing above 710°C leads to the diffusion. We obtained 2D ordered surface, which showed 3 × 3 RHEED pattern as referenced to the primitive unreconstructed Si(O01) surface net, when we deposited 2.5ML Fe and 5.8ML Si successively onto Si(001)-dihydride surface and annealed to 470°C.
Role of the Surface Steps on the Growth of CrSi2 on {111} Silicon.
Authors: Rocher André M. , Oustry André , David Marie José , Caumont Michel
Published: April 2011

CrSi2 layers grown by solid phase epitaxy on a nominal (111) Si surface exhibit in the same proportion two different orientation relationships, named A and B. When CrSi2 is deposited on a 8° vicinal (111) Si surface, B-type orientation is favoured with respect to the A type. This result can be explained by the fact that both the step width introduced by the miscut and the planar coincidence between {1100}Crsi2 and {111}Si are nearly equal to 23Å. Edge type misfit dislocations are observed at the interface with the same spacing. Their Burgers vector component along [111] is almost compensated by the atomic steps along the <110> directions. The role of the steps is discussed in term of elastic energy. Steps introduce misfit dislocations which make possible coherent growth of the B type orientation.
Microstructure and Stress in Mo Films Sputttr-Deposited on Glass Substrates
Authors: Lee Tsang-Shaw , Gu Gin-Lern , Tseng Bae-Heng
Published: April 2011

Mo films are deposited on various glass substrates using the sputtering technique. The film stress and texture are greatly affected by the substrate materials. For the films grown on borosilicate glass both DC and RF sputtering may produce films with desirable properties. However, the films with compressive stress can be deposited on sodalime glass substrates only by RF sputtering. Preferred orientations are (211) and (110) for the film grown on borosilicate glass substrate and the film grown on sodalime glass substrate, respectively.
Grain Rotation in Thin Films of Gold
Authors: King Alexander H. , Harris Karen E.
Published: April 2011

Transmission electron microscope observations of microstructural development in thin films of gold are presented. In a sequential annealing experiment, we find that individual grains are constantly rotating and that this contributes to the rearrangement of grain boundaries by allowing their individual energies to vary.