bluuebunny/crossref_metadata_2025_split huggingface dataset:
Diffusion via native defects, and the appropriate choice of independent thermodynamic variables in both quasi-equilibrium and nonequilibrium experimental designs
Authors: Cohen R. M. 
Published: April 2011

ABSTRACTIt is often incorrectly assumed that temperature alone defines the equilibrium state of a crystal. However, the Gibbs phase rule shows that the number of independent thermodynamic variables required to define the equilibrium state depends upon the experimental design. In most practical cases, this means that temperature and components of the external phase or phases in proximate contact with the sample will determine the equilibrium state, e.g., the equilibrium native defect concentrations, impurity solubility, etc. If native defect concentrations approach their equilibrium values in a time which is short compared to the time of an experiment, then impurity diffusion can be described well by a thermodynamic model and impurity diffusion is analogous to classic tracer diffusion. If native defect concentrations require a long time to approach their equilibrium values, then diffusivities will exhibit significant time dependence and simple models do not apply. However, multi-phase, multi-component systems generally have several possible equilibrium regions within their phase diagram. Relating the phase diagram to a given experimental design allows one to qualitatively understand how the native defect concentrations change as a crystal in a nonequilibrium state relaxes toward one (of several possible) well-defined equilibrium states. Examples will focus largely on the diffusion of impurities in single crystal GaAs. Evidence will be presented that native defect concentrations can rapidly approach equilibrium in a limited group of experimental designs. We shall show why a description of diffusivity in terms of carrier concentration, a dependent thermodynamic variable, sometimes succeeds but often fails. Examples of commonly used experimental designs, in which there is inadequate control over the independent variables, will demonstrate some large variations in measured diffusivity. An enormous range of reported diffusivities, in GaAs covered by SiO2 or Si3N4 encapsulant layers, will demonstrate the difficulty and the futility of measuring diffusion when the solid is cut off from the external phases which define the equilibrium state.
Electric-Field Effects on Reactions Between Oxides
Authors: Johnson Matthew T. , Gilliss Shelley R. , Carter C. Barry 
Published: April 2011

ABSTRACTThin films of In2O3 and Fe2O3 have been deposited on (001) MgO using pulsed-laser deposition (PLD). These thin-film diffusion couples were then reacted in an applied electric field at elevated temperatures. In this type of solid-state reaction, both the reaction rate and the interfacial stability are affected by the transport properties of the reacting ions. The electric field provides a very large external driving force that influences the diffusion of the cations in the constitutive layers. This induced ionic current causes changes in the reaction rates, interfacial stability and distribution of the phases. Through the use of electron microscopy techniques the reaction kinetics and interface morphology have been investigated in these spinel-forming systems, to gain a better understanding of the influence of an electric field on solid-state reactions.
Crystallography of Hydride Formation in Zr-Nb Alloys
Authors: Srivastava D. , Dey G. K. , Banerjee S. , Ranganathan S. 
Published: April 2011

ABSTRACTThe crystallographic and morphological aspect associated with the formation of γ hydride phase (fct) from the β phase in β abilized Zr-20%Nb alloy has been reported. In this paper the βto γ transformation has been considered in the terms of the phenomenological theory of martensitic crystallography in order to predict the crystallographic features of the γ hydride in the β to γ transformation. The prediction made in the present analysis has been found to match very closely to the experimentally observed habit plane. The possibility of the α to γ transition through the formation of a transient β configuration has been examined.
Effects of Aging and Thermal Cycling Treatments on the Damping Capacity of Cu-Mn Alloys
Authors: Chung T. S. , Jun J. H. , Lee Y K. 
Published: April 2011

ABSTRACTThe effects of aging and thermal cycling treatments on the damping capacity and microstructure of Cu-(47, 55, 65)%Mn alloys and Cu-65%Mn-4.5%Ni alloy have been studied using an optical microscope, a transmission electron microscope (TEM), and a cantilever type damping measuring apparatus. The maximum damping capacity appeared after aging at 400°C for 18 hours in Cu-47%Mn, 8 hours in Cu-55%Mn, 4 hours in Cu-65%Mn, and 36 hours in Cu-65%Mn-4.5%Ni alloy, respectively. Storage at 100°C slightly reduced damping capacity of these alloys, and the higher the Mn content, the smaller the amount of the degradation. This result might be ascribed to α-Mn precipitation inside microtwins in Cu-65%Mn alloy, and to the microstructural change from tweed structure to mottled structure in the other alloys. The thermal cycling treatment between room temperature (20°C) and 250°C led to an increase in damping capacity, which is probably due to the refinement of microstructure(tweed or twin).
Amorphization Of Zr-Al Under Mechanical Alloying At Different Temperatures: A Re-Entrant Melting Phenomenon
Authors: Sheng H. W. , Lu K. , Ma E. 
Published: April 2011

ABSTRACTZr100−xAlx powder blends have been subjected to ball milling at different temperatures to investigate the amorphization process. At low temperatures the Zr-Al solid solutions amorphized under the polymorphous constraints, whereas at higher temperatures there was an obvious twophase coexistence region. The Al concentration for the complete amorphization of Zr-Al increased with increasing temperature, suggesting a re-entrant melting behavior. Both of the temperature- and composition-dependent amorphization mechanisms are analyzed in terms of the thermodynamic properties of the phases involved, as well as the dynamic effects brought in by the non-equilibrium milling process.
Microstructure of Ni2B Laser-Induced Surface-Alloyed α-Fe
Authors: Dehm G. , Scheu C. , Bamberger M. 
Published: April 2011

ABSTRACTSubstrate surfaces of α-Fe were laser alloyed with a CW-CO2laser using Ni2B powder. Single laser scans and laser scans with a 50% overlap between successive passes were carried out to obtain hard surface coatings on the α-Fe substrate. High hardness values in the range of 520 - 600 Hv1 were achieved by laser boriding of α-Fe, compared to a substrate hardness of ∼ 110 Hv1. The improvement in hardness is caused by an off-eutectic microstructure, which consists of Fe dendrites and Fe3B and Fe lamellae. Both phases, Fe3B and Fe, contain Ni in solid solution. For samples treated with a single laser scan the Fe phase solidifies in the bcc structure. In contrast, a 50 % overlap between successive laser passes causes the iron to crystallize in the fcc γ-phase. The difference in crystallographic structure is caused by the amount of Ni dissolved in Fe during laser alloying: only ∼ 9 at.% Ni is contained in the α-Fe phase after a single laser scan, while up to ∼ 40 at.% of Ni is dissolved in the γ-Fe phase after a 50 % overlap between each laser scan due to the nearly complete melting of all injected Ni2B particles. However, the Fe3B phase solidifies during laser alloying independent of the amount of Ni in the orthorhombic Fe3C structure, which is metastable for Fe contents exceeding 45 at.%.
Atomic Scale Study Of Precipitate / Matrix Interfaces in a Metallic Alloy
Authors: Rozdilsky I. , Cerezo A. , Smith G. D. W. , Watson A. 
Published: April 2011

ABSTRACTThe study of phase transformations involves making assumptions about interfaces within a material in order to apply mathematical models. An example of this is the Gibbs classical theory of nucleation and growth which is based upon the assumption that the precipitate / matrix interface is sharp. Recent developments in atom probe microscopy have made it possible for the first time to characterize complex three-dimensional internal interfaces within materials to sub-nanometre accuracy. We have used the OPoSAP (Optical Position Sensitive Atom Probe) to characterize the precipitate / matrix interface in the dilute Cu-Co system which is a model alloy for the study of homogeneous nucleation and growth. Interface widths derived from radial composition profiles were measured to be 0.9 ± 0.2 nm in size. The effects of thermal energy, positioning inaccuracies, and statistical limitations on the measurement of interface widths are examined through the use of mathematical models and computer simulations. Similar widths are measured for precipitates in the coarsening regime which shows the interface velocity does not affect the interface width.
Interfacial Reaction in Aluminum/Carbon Multilayer Thin Films
Authors: Landry K. , Sieber H. , Sui M. , Perepezko J. H. 
Published: April 2011

ABSTRACTThe reaction at the interface between Al and amorphous C in Al/C multilayer thin films with modulation wavelengths of about 25nm and 125nm has been investigated by differential scanning calorimetry, X-ray diffraction, transmission electron microscopy/selected area electron diffraction and high resolution transmission electron microscopy. The reaction was found to take place in two steps. The first step onsets at about 300°C, and was identified as the diffusion of C into Al. The second step starts above 400°C, at a temperature strongly dependent on the modulation wavelength of the film, and is the formation of A14C3. The carbide has been observed to nucleate and grow inside the Al layers. The multilayer structure is preserved in the samples up to at least 550°C, and Al grains start to grow at or below 300°C.
In-Situ UHV Tem Investigations of the Initial Oxidation Stage of Copper Thin Films
Authors: Yang J. C. , Yeadon M. , Kolasa B. , Gibson J. M. 
Published: April 2011

ABSTRACTThe nucleation and growth of Cu2O due to oxidation of Cu(001) films were monitored at various temperatures and oxygen partial pressures. For all examined temperatures and pressures, Cu2O islands were observed to form epitaxially with respect to the copper film. The nucleation of these oxide islands was homogeneous –no clear evidence was observed for either steps or dislocations being preferential nucleation sites. Based on this data, we have developed a semiquantitative model of the initial oxidation stage where the dominant mechanism for transport, nucleation and growth of oxide islands is oxygen diffusion on the Cu surface. We are presently comparing our experimental data with nucleation rate theory, where the predictions qualitatively describe our observations, but not quantitatively.
The Formation of Cu3Si from Cu/a-Si Multilayers
Authors: Chromik R. R. , Neils W. K. , Cotts E. J. 
Published: April 2011

ABSTRACTThe kinetics of the formation of Cu3Si in Cu/a-Si diffusion couples have been investigated by means of differential scanning calorimetry and x-ray diffraction. Multilayered composites of average stoichiometry Cu3Si were prepared by sputter deposition with individual layer thicknesses varying in different samples between 2 and 100 nm. We observed diffusion limited growth of Cu3 Si upon annealing these diffusion couples below 500 K. Reaction constants were measured for a temperature range of 455 to 495 K for thicknesses of growing Cu3Si between 2.6 and 80 nm. The temperature dependence of the reaction constant, k2, was characterized as k2 = k0 exp(− Ea/kbT) with activation energy, Ea = 1.0 eV/atom and pre-factor, k0 = 1.9×10−3 cm2/s.
Metastable Phase Formation in Mechanically Alloyed Sm-Co-Fe
Authors: Smith P. A. I. , Ding J. , McCormick P. G. , Street R. 
Published: April 2011

ABSTRACTA detailed phase analysis of mechanically alloyed (Sm0.18Co0.82)100-xFex powders has been performed using X-ray diffraction and Mössbauer spectroscopy. A two-phase structure develops as the Fe content is increased, with an increasing proportion of bcc Fe-Co in addition to amorphous Sm-Co-Fe. Both phases become richer in Fe, but Fe is concentrated in the bcc phase, due to a limited ability of Fe to substitute in amorphous Sm-Co. Changes in phase formation with increasing Fe content can be correlated with changes in the calculated free energy of mixing of amorphous Sm-Co-Fe.
The Solidification Microstructure of Ai-Cu-Si Alloys Metal Matrix Composites
Authors: Garbellini O. , Palacio H. , Biloni H. 
Published: April 2011

ABSTRACTThe relationship between solidification microstructure and fluidity in MMC was studied. The composites were fabricated by infiltration of liquid metal into a alumina SAFFIL fibers preform under a gas pressure, using alloys of the AlCuSi system as matrices. The fluidity was measured in terms of classic foundry practice (i.e., the distance of flow liquid metal into the preform, while solidifying). The characterization of solidification microstructure in the cast composite was analysed and correlated with the results of fluidity. The attention was particularly focused on such effects as the presence or absence of selective nucleation, the refinement of certain solidifying phases in the presence of fibers and their influence on microstructure formation and segregation of certain elements present in the liquid at the fiber matrix interface. By comparing reinforced and non reinforced zones, it was shown that the presence of fibers resulted in a refinement of the dendritic arm spacing of the α-Al phase, with nucleation of Si on the fibers and without nucleation of primary Al dendrites. The results were discussed and compared with the microstructures and fluidity test of the unreinforced Al-Cu-Si alloys.
Formation of Ni/Pt/Au Ohmic Contacts to p-GaN
Authors: Jang Ja-Soon , Kim Hyo-Gun , Park Kyung-Hyun , Um Chang-Sub , Han Il-Ki , Kim Sun-Ho , Jang Heong-Kyu , Park Seong-Ju 
Published: April 2011

AbstractWe report a new Ni/Pt/Au (20/30/80 nm) metallization scheme to achieve a low ohmic contacts to p-type GaN with a carrier concentration of 9.4 × 1016 cm-3. A Mg-doped GaN layer (0.5 μm) was grown on (0001) sapphire substrate by metalorganic chemical vapor deposition (MOCVD). All metal thin films were deposited on the p-GaN layer in an electron-beam evaporation system. Samples were annealed by a rapid thermal annealing (RTA) process at a range of temperatures from 300 °C to 850 °C under a flowing Ar atmosphere. A circulartransmission line model (c-TLM) was employed to calculate the specific contact resistance, and current-voltage (I-V) data were measured with HP4155A. The Ni/Pt/Au contacts without the annealing process showed nearly rectifying characteristics. The ohmic contacts were formed on the samples annealed at 500 °C for 30 sec and the I-V data showed a linear behavior. The specific contact resistance was 2.1 × 10-2 Ωcm2. However with increasing the annealing temperature above 600 °C, ohmic contacts were again degraded. Auger electron spectroscopy (AES) depth profiles were used to investigate the interfacial reactions between the trilayer and GaN. AES results suggested that Pt plays a significant role in forming ohmic contact as an acceptor at the interface. Atomic force microscope (AFM) also showed that the samples with good ohmic contact have very smooth surface.
Electrical and Optical Characterisation of Homojjunction Gallium Nitride Light Emitting Diodes
Authors: Laws G M , Morgan J , Ren G B , Harrison I , Larkins E C , Orton J W , Hoopert S E , Chengt T , Foxont C T 
Published: April 2011

AbstractWe report on the fabrication and characterisation gallium nitride light emitting diodes (LEDs) grown by molecular beam epitaxy on (0001) oriented sapphire and (111)B GaAs substrates. The current voltage characteristics of the devices grown on sapphire show turn on voltages of between 4 and 5V with large on-series resistance of 600Ω; for corresponding devices grown on GaAs these parameters are between 6 and 7V and 150 Ω, respectively.Room temperature electroluminescence (EL) spectra from the GaN LEDs ,grown on sapphire substrates, show a dominant emission at 3.2 eV (397nm) with a full width half maximum (FWHM) of 335 meV which is attributed to free electron to acceptor transitions (e, A−Mg). A broad low intensity deep level emission is also observed centred at 2.4 eV (506nm). The peak of the EL from the devices grown on GaAs is at 3.1eV rather than 3.2eV. The differences between the two sets of devices are probably caused by the different device geometry.Preliminary results show that an “annealing” effect caused by electrical stressing resulted in an improvement of the EL spectra. The stressed samples show an increase in the near band edge emission intensity, a 20meV reduction in the FWHM and a significant reduction in the intensity of the deep level emission. The devices have a large 1/f noise contribution which does not appear to change after electrical stressing.
Microstructural Evaluation of Zno Thin Films Deposited by Mocvd
Authors: Gorla C. R. , Liang S. , Emanetoglu N. , Mayo W. E. , Lu Y. 
Published: April 2011

AbstractZnO films are being used in many emerging technologies such as high frequency RF filters, substrate/buffer layer for GaN deposition, and as active layers in blue-UV LEDs and lasers. These applications require epitaxial films with high structural quality. In the present paper, results on the structure of ZnO films deposited by MOCVD on c-plane and r-plane sapphire substrates are presented. These films have been investigated by XRD, SEM and TEM. X-ray phi-scans show that the films have a single epitaxial relationship with each substrate orientation. (11.0) ZnO grows on (01.2) sapphire (r-plane), while (00.1) oriented films grow on (00.1) sapphire (c-plane). Smooth films are obtained on r-sapphire, while films with a columnar morphology are obtained on c-sapphire. Based on the morphology of the columnar grains grown on c-sapphire, it is expected that the films are oxygen terminated. The interface between ZnO and sapphire is atomically sharp as observed by HRTEM. Misfit dislocations at the interface between ZnO and r-sapphire have been observed. Grain boundaries between coalescing islands during film growth are the main type of defects. Under certain conditions, very long whiskers were observed to grow by the VLS mechanism. By modifying our system, we have been able to prevent the growth of these whiskers.
Growth Mechanism and Structure of Ain Films Grown on Sapphire by MOCVD
Authors: Li Yun-Xin , Salamanca-Riba L. , Wongchotigul K. , Zhou P. , Spencer M. G. , Jones V. K. 
Published: April 2011

AbstractAIN films grown on sapphire by MOCVD with different V/IAR ratios were investigated by XRD, TEM and AFM. The AIN films show single crystalline character as well as columnar structure. The growth of AIN has three zones: (1) high-density nucleation layer (2) fine columnar growth and (3) grain merging and lateral growth. The films grown at intermediate V/III ratio have the maximum value of [0002] FWHM. When the V/HI ratio increases, the thickness of the nucleation zone and the film misorientation increase, but when the ratio is increased further, the nucleation zone decreases and the AIN film has a more highly oriented growth. These results suggest that the AIN films with optimum crystalline quality can be obtained by varying the V/III ratio during growth.
GaN Crystals: Growth and Doping Under Pressure
Authors: Grzegory I. , Bockowski M. , Lucznik B. , Wroblewski M. , Krukowski S. , Weyher J. , Nowak G. , Suski T. , Leszczynski M. , Teisseyre H. , Suski T. , Litwin-Staszewska E. , Porowski S. 
Published: April 2011

AbstractThe recent progress in high pressure crystallization of GaN is reported. The results of the growth from the solutions of atomic nitrogen in pure Ga and in its alloys with Mg, Ca and Zn are discussed. It is shown that the growth mechanisms and the physical properties of the crystals depend on the type of dopant added into the solution. In particular, high resistivity (104-106Ωcm) GaN crystals of improved structural quality can be grown from solutions containing Mg. It was also observed that the addition of Mg, Ca and Zn suppress yellow photoluminescence commonly observed in GaN crystals grown without an intentional doping. The preparation of surfaces of GaN substrates for homoepitaxy by mechanical and mechanochemical polishing is discussed. It is shown that atomically flat, thermally stable surfaces are possible to obtain by the applied procedures.Some most interesting results concerning homoepitaxial growth by MOCVD and MBE is shortly reviewed. In particular, it is shown that perfectly matched (strain free) GaN layers can be deposited on the highly resistive GaN:Mg substrates.
Drastic Change in the Gan Film Quality by In-Situ Controlling Surface Reconstructions In Gsmbe
Authors: Shen X. Q. , Tanaka S. , Iwai S. , Aoyagi Y. 
Published: April 2011

AbstractGaN growth was performed on 6H-SiC (0001) substrates by gas-source molecular beam epitaxy (GSMBE), using ammonia (NH3) as a nitrogen source. Two kinds of reflection high-energy electron diffraction (RHEED) patterns, named (1×1) and (2×2), were observed during the GaN growth depending on the growth conditions. By careful RHEED study, it was verified that the (1×1) pattern was corresponded to a H2-related nitrogen-rich surface, while (2×2) pattern was resulted from a Ga-rich surface. By x-ray diffraction (XRD), photoluminescence (PL) and atomic force microscopy (AFM) characterizations, it was found that the GaN quality changed drastically grown under different RHEED patterns. GaN film grown under the (1×1) RHEED pattern showed much better qualities than that grown under the (2×2) one.
Growth of GaN on Lithium Gallate Substrates for Development of a GaN Thin Compliant Substrate
Authors: Doolittle W. A. , Kropewnicki T. , Carter-Coman C. , Stock S. , Kohl P. , Jokerst N. M. , Metzger R. A. , Kang S. , Lee K. , May G. , Brown A. S. 
Published: April 2011

AbstractThe GaN on LGO system is the near perfect template (due to extremely high etch selectivity) for developing a viable thin film/compliant GaN substrate. Herein, we report on our efforts to grow GaN on LGO, including improvement of the microscopic surface morphology using pre-growthpretreatments. We also report on the first transferred thin film GaN substrate grown on LGO, transferred off of LGO, and mounted on GaAs. With this approach, (InAl)GaN alloys can be grown on thin GaN films, implementing a truly “compliant” substrate for the nitride alloy system. In addition, the flexibility of bonding to low cost Si, metal or standard ceramic IC packages is an attractive alternative to SiC and HVPE GaN substrates for optimizing cost verses thermal conductivity concerns. We have demonstratedhigh quality growth of GaN on LGO. X-Ray rocking curves of 145 arc-seconds are obtained with only a 0.28 μm thick film. We present data on the out of plane crystalline quality of GaN/LGO material. Likewise, we show 2 orders of magnitude improvement in residual doping concentration and factors of 4 improvement in electron mobility as compared to the only previously reported electrical data. We show substantial vendor to vendor and intra-vendor LGO material quality variations. We have also quantified the desorption of Ga and Li from the surface of LGO at typical growth temperatures using in situ desorption mass spectroscopy and XPS.
Atomic Structure Of Grain Boundaries And Interfaces In Iiinitrides Epitaxial Systems
Authors: Ruvimov S. , Liliental-Weber Z. , Washburn J. , Amano H. , Akasaki I. , Koike M. 
Published: April 2011

AbstractHigh resolution electron microscopy (HREM) was applied to study atomic structure of stacking faults, grain boundaries and interfaces in III-nitrides epitaxial layers grown by MOVPE on sapphire. Defects formed in GaN epitaxial layers grown by MOVPE were reviewed in comparison with those in MBE grown materials