bluuebunny/crossref_metadata_2025 huggingface dataset:
Metal-organic deposition of thin-film yttria-stabilized zirconia-titania
Authors: Swider Karen E. , Worrell Wayne L.
Published: March 2008

Mixed-conducting yttria-stabilized zirconia-titania (YZTi) has attractive applications in solid oxide fuel cells (SOFC's) and electrocatalysis, particularly when used as a thin film to reduce its electrical resistance. Thin films of yttria (12 mol %) stabilized zirconia-titania (8 mol %) have been prepared using metal-organic deposition (MOD) whereby metal-organic solutions of Zr-, Y-, and Ti-2-ethylhexanoates are spun onto suitable substrates. Variables affecting the film surface-morphology, chemistry, and crystal structure are examined using scanning electron microscopy (SEM), auger electron spectroscopy (AES), and x-ray diffraction (XRD), respectively. Uniform, pore-free films having a low carbon content (<1 at. %) are made by sintering on a hot plate at 530 °C. The effect of thermal cycling on the chemical compatibility and adherence is examined for YZTi films on yttria-stabilized zirconia substrates.
A parametric study of titanium silicide formation by rapid thermal processing
Authors: Amorsolo A. V. , Funkenbusch P. D. , Kadin A. M.
Published: March 2008

A parametric study of titanium silicide formation by rapid thermal processing was conducted to determine the effects of annealing temperature (650 °C, 750 °C), annealing time (30 s, 60 s), wet etching (no HF dip, with HF dip), sputter etching (no sputter etch, with sputter etch), and annealing ambient (Ar, N2) on the completeness of conversion of 60 nm Ti on (111)-Si to C54–TiSi2 based on sheet resistance and the uniformity of the sheet resistance measurements across the entire wafer. Statistical analysis of the results showed that temperature, annealing ambient, and sputter etching had the greatest influence. Increasing the temperature and using argon gas instead of nitrogen promoted conversion of the film to C54–TiSi2. On the other hand, sputter etching retarded it. The results also indicated significant interactions among these factors. The best uniformity in sheet resistance was obtained by annealing at 750 °C without sputter etching. The different sheet resistance profiles showed gradients that were consistent with expected profile behaviors, arising from temperature variations across the wafer due to the effect of a flowing cold gas and the effects of the wafer edge and flats.
Effect of zirconia doping on the electrical behavior of yttria
Authors: Wang C. C. , Patton V. D. , Akbar S. A. , Alim M. A.
Published: March 2008

The ac electrical behavior of yttria doped with a zirconia concentration ranging from 0.15 to 20 mole % is investigated in the temperature range of 800 to 1300 °C. The ac electrical data, obtained in the range from 5 Hz to 13 MHz, indicated two distinct relaxations when analyzed in the impedance plane. These relaxations are attributed to lumped grains and trapping within grain boundaries, including possible electrode/sample effects. The admittance plane analysis revealed a semicircular relaxation in the low-frequency region, indicating identical response to that of the low-frequency relaxation of the impedance plane. The incorporation of zirconia into yttria is found to lower the activation energy of conduction in the grains and enhance ionic contribution to the overall electrical conduction. The PO2 studies and transference number measurements near atmospheric region indicate that p-type conduction dominates for the lightly doped yttria. An ionic contribution to the conduction processes becomes significant in heavily doped samples at/near atmospheric PO2.
Influence of copper on the structural characteristics of carbon nanofibers produced from the cobalt-catalyzed decomposition of ethylene
Authors: Chambers A. , Rodriguez N. M. , Baker R. T. K.
Published: March 2008

We have used a combination of techniques to examine modifications in the structural characteristics of carbon nanofibers produced from the interaction of cobalt and copper-cobalt powders with ethylene at temperatures over the range 425 to 700 °C. The nanofibers generated from the interaction of cobalt with ethylene at 600 °C were found to be highly crystalline in nature. Incorporation of as little as 2% copper into the cobalt created a major modification in the conformation of the solid carbon deposit, which was composed of multiple nanofiber limbs emanating from a single catalyst particle, and in this state the carbon structures tended to be disordered. As the composition of the bimetallic was progressively changed to the point where copper became the major component, there was a significant increase in the degree of crystalline perfection of the nanofibers even though they maintained their multidirectional form. The transformation in structural characteristics of the carbon nanofibers is rationalized, according to a concept wherein the crystalline order of the deposit is related to the wetting properties of the bimetallic particles with graphite.
On the intergranular coupling in soft nanocrystalline materials
Authors: González J. M. , Murillo N. , González J. , Blanco J. M. , Echeberría J.
Published: March 2008

The magnetic softness of nanocrystalline materials prepared from amorphous precursors is attributed to the average of the local magnetocrystalline anisotropy of the individual crystallites. In the present paper we have studied the effective magnetic anisotropy of Fe-based nanocrystalline samples with different microstructures. These microstructures were produced by using different heating rates when crystallizing the precursor material by means of continuous heating treatments. From the results of our study of the magnetic properties of the samples, carried out from the measurement of the bias field dependence of the transverse susceptibility, it was possible to discern the occurrence of intergranular coupling and to evaluate the typical dimensions of the coupled units. Since these dimensions were larger than the characteristic length of the microstructure, we suggest that the enhancement of the soft properties is linked to the decrease of the microstructure-magnetization interactions originating in large units of coupled magnetic moments.
Current-voltage hysteresis in GdBa2Cu3O7−δ ceramics in air at room temperature
Authors: Kurihara Yoshiaki , Okamoto Tomoichiro , Huybrechts Ben , Takata Masasuke
Published: March 2008

We report here on the appearance of a hysteresis in the current-voltage characteristic, when a voltage is applied to a GdBa2Cu3O7−δ rod at room temperature. This hysteresis was caused by the appearance and disappearance of a hot spot. When a gradually increasing dc voltage (0–8 V) was applied to a GdBa2Cu3O7−δ rod, a hot spot appeared at a given voltage, Va. Further increasing of the voltage increased the spot size, while the current stayed constant. When a voltage of 8 V was reached, the voltage was gradually decreased. The spot did not disappear at Va, but at a voltage 0.2 to 1.3 V lower than Va. The current remained constant until the spot disappeared. By the authors' best knowledge, this is the first report of a hysteresis in the current-voltage characteristic at room temperature for a high Tc superconductor.
Growth defects in GaN films on sapphire: The probable origin of threading dislocations
Authors: Ning X. J. , Chien F. R. , Pirouz P. , Yang J. W. , Khan M. Asif
Published: March 2008

Single crystal GaN films with a wurtzite structure were grown on the basal plane of sapphire. A high density of threading dislocations parallel to the c-axis crossed the film from the interface to the film surface. They were found to have a predominantly edge character with a Burgers vector. In addition, dislocation hal-loops, elongated along the c-axis of GaN, were also found on the prism planes. These dislocations had a mostly screw character with a [0001] Burgers vector. Substrate surface steps with a height of were found to be accommodated by localized elastic bending of GaN (0001)GaN planes in the vicinity of the film/substrate interface. Observations show that the region of the film, with a thickness of ∼100 nm, adjacent to the interface is highly defective. This region is thought to correspond to the low-temperature GaN “buffer” layer which is initially grown on the sapphire substrate. Based on the experimental observations, a model for the formation of the majority threading dislocations in the film is proposed. The analysis of the results leads us to conclude that the film is under residual biaxial compression.
Low temperature structures of the second stage cesium graphitide and effect of trace impurities
Authors: Duclaux L. , Rannou I. , Béguin F. , Lelaurain M.
Published: March 2008

Stage 2 CsC24 graphite-cesium derivatives were synthesized and characterized using x-ray diffraction. Pure CsC24 specimens are single phase stage 2 in the range 77–300 K. From the 00l scans, we observed on samples slightly polluted during the transfer in the glove box, the tridimensional segregation of stage 2 in a main CsC26 structure and an additional dense CsC20 structure. The 2D Laue diffraction photographs are the same as already reported, but are interpreted as a mixture of 2 × 2 R 0° lattice commensurate and 2.54 × 2.54 R 14.5° lattice incommensurate structures. In oxidized compounds, we identified another 2.23 × 2.23 R 25.5° 2D additional incommensurate structure corresponding to the composition CsC20 already evidenced by analysis of the 00l diffractograms. We suggest that oxygen impurities are at the origin of this particular stiff and dense structure at low temperature.
The role of ordered A1-site vacancies in belt nano-domains of Pb1−xBaxNb2O6 (PBN) solid solution
Authors: Xiao Xiaoyue , Xu Yan , Zeng Zhigang , Gui Zhilun , Li Longtu , Zhang Xiaowen
Published: March 2008

The order-disorder states of the A-site vacancy of PBN solid solution affected by different thermal treatments were studied with the aid of High Resolution Electron Microscopy (HREM). PBN ceramics around the morphotropic phase boundary were prepared through two routes to control ordering degree of the A-site vacancy: (1) samples through quenching processes resulted in chaotic states of the A-site vacancies and misfit anti-phase boundary; (2) samples through slow cooling led to an ordered structure of the vacancies in the A1-site. The ordered A1-site vacancies were modulated by interchanges of the sublattices of the ordered vacancies and the Pb2+ cations in the A1-sites along both and orientations, forming a narrow discommensurate wall between two anti-phase domains. The anti-phase domains were observed as a regular belt structure with dimensions of about 45 nm × (≥120) nm. The belt nano-domain structures were a result of quasi-equilibrium thermodynamic processes.
Effective optical constants n and κ and extinction coefficient of silica aerogel
Authors: Zeng J. S. Q. , Greif R. , Stevens P. , Ayers M. , Hunt A.
Published: March 2008

In this work the normal reflectance, R, at a planar silica aerogel interface and the normal transmittance, T, of a silica aerogel slab were measured using a Fourier Transform Infrared Spectrometer. Two procedures were used to obtain the effective optical constants, i.e., the refractive index n and the absorption index κ, of silica aerogel. One procedure determined κ from the measured transmittance T and then determined n from the results for κ and from the measured reflectance R using the Kramers–Kronig relation; the other procedure determined n and κ of silica aerogel from n and κ of fully dense silica glass by using the Clausius–Mossotti equation, Maxwell Garnett formula, and Bruggeman formula. The first procedure has a relatively large error due to the inaccuracy of the transmission and reflection measurements. The second procedure, especially the Clausius–Mossotti equation, yields values of n that are consistent with experiments and may be used for the calculation of the effective optical constants and the extinction coefficient of silica aerogel.
On the optimization of a dc arcjet diamond chemical vapor deposition reactor
Authors: Reeve S. W. , Weimer W. A. , Dandy D. S.
Published: March 2008

Based on results from chemical kinetic model calculations, a method to improve diamond film growth in a dc arcjet chemical vapor deposition reactor has been developed. Introducing the carbon source gas (CH4) into an Ar/H2 plasma in close proximity to the substrate produced diamond films exhibiting simultaneous improvements in quality and mass deposition rates. These improvements result from a reduced residence time of the methane in the plasma which inhibits the hydrocarbon chemistry in the gas from proceeding significantly beyond methyl radical production prior to encountering the substrate. Improvements in growth rate were modest, increasing by only a factor of two. Optical emission actinometry measurements indicate that the flux of atomic hydrogen across the stagnation layer to the substrate is mass diffusion limited. Since diamond growth depends upon the flux of atomic H to the substrate, these results suggest that under the conditions examined here, a low atomic H flux to the substrate poses an upper limit on the attainable diamond growth rate.
Space filling by nucleation and growth in chemical vapor deposition of diamond
Authors: Stiegler J. , von Kaenel Y. , Cans M. , Blank E.
Published: March 2008

Phase transformations, including chemical vapor deposition (CVD) of diamond, taking place by nucleation and growth are commonly described by Avrami or Johnson-Mehl type models. In order to avoid the restrictions of such models with respect to assumptions concerning nucleation rates and growth velocities, the variation with time of nucleation and growth of diamond particles during the deposition of microwave plasma-assisted CVD was studied. The size distributions obtained from image analysis enabled us to trace back details of the nucleation and growth history. Three sources of particle formation were operating during deposition. A general growth law suitable for all particles did not exist. These observations limited the applicability of Avrami-type models to describe space filling. Computer simulation of surface coverage and particle growth was successful because one particular mode of particle formation and growth dominated surface coverage. Based on image analysis and the determination of the film growth rate, the evolution of the diamond volume fraction with time, starting from three-dimensional particle growth followed by a continuous transition to one-dimensional film growth, was described.
Characterization of SiC fiber (SCS-6) reinforced-reaction-formed silicon carbide matrix composites
Authors: Singh M. , Dickerson R. M.
Published: March 2008

Silicon carbide fiber (SCS-6) reinforced-reaction-formed silicon carbide matrix composites were fabricated using a reaction-forming process. Silicon-2 at. % niobium alloy was used as an infiltrant instead of pure silicon to reduce the amount of free silicon in the matrix after reaction forming. The matrix primarily consists of silicon carbide with a bimodal grain size distribution. Minority phases dispersed within the matrix are niobium disilicide (NbSi2), carbon, and silicon. Fiber pushout tests on these composites determined a debond stress of ∼67 MPa and a frictional stress of ∼60 MPa. A typical four-point flexural strength of the composite is 297 MPa (43.1 KSi). This composite shows tough behavior through fiber pullout.
Influences of stress on the measurement of mechanical properties using nanoindentation: Part I. Experimental studies in an aluminum alloy
Authors: Tsui T. Y. , Oliver W. C. , Pharr G. M.
Published: March 2008

The influence of applied stress on the measurement of hardness and elastic modulus using nanoindentation methods has been experimentally investigated using special specimens of aluminum alloy 8009 to which controlled stresses could be applied by bending. When analyzed according to standard methods, the nanoindentation data reveal changes in hardness with stress similar to those observed in conventional hardness tests. However, the same analysis shows that the elastic modulus changes with stress by as much as 10%, thus suggesting that the analysis procedure is somehow deficient. Comparison of the real indentation contact areas measured optically to those determined from the nanoindentation data shows that the apparent stress dependence of the modulus results from an underestimation of the contact area by the nanoindentation analysis procedures.
Influences of stress on the measurement of mechanical properties using nanoindentation: Part II. Finite element simulations
Authors: Bolshakov A. , Oliver W. C. , Pharr G. M.
Published: March 2008

The finite element method has been used to study the behavior of aluminum alloy 8009 during elastic-plastic indentation to establish how the indentation process is influenced by applied or residual stress. The study was motivated by the experiments of the preceding paper which show that nanoindentation data analysis procedures underestimate indentation contact areas and therefore overestimate hardness and elastic modulus in stressed specimens. The NIKE2D finite element code was used to simulate indentation contact by a rigid, conical indenter in a cylindrical specimen to which biaxial stresses were applied as boundary conditions. Indentation load-displacement curves were generated and analyzed according to standard methods for determining hardness and elastic modulus. The simulations show that the properties measured in this way are inaccurate because pileup is not accounted for in the contact area determination. When the proper contact area is used, the hardness and elastic modulus are not significantly affected by the applied stress.
Macrosegregation of Y2Ba1Cu1O5 particles in Y1Ba2Cu3O7−δ crystals grown by an undercooling method
Authors: Endo A. , Chauhan H. S. , Egi T. , Shiohara Y.
Published: March 2008

Macrosegregation of Y2Ba1Cu1O5 (Y211) particles was observed in Pt-added Y1Ba2Cu3O7−δ (Y123) crystals grown by an undercooling method. It was found that the macrosegregation of Y211 particles depended on the growth direction and the growth rate (R) as a function of undercooling (ΔT). The amount of Y211 particles in Y123 crystals grown at large R was larger than at small R. Also, the amount of Y211 in Y123 growing along the a-direction was larger than that along the c-direction. Further, it was noted that the smaller Y211 particles in size were distributed in Y123 grown at large R. These phenomena could be at least qualitatively explained by the prevalent trapping/pushing theory. In the direct observation of magnetic flux with the Faraday effect of iron garnet film, the flux pinning force was found to be in good agreement with the macrosegregation of Y211 particles.
Electromagnetic and microstructural investigations of a naturally grown 8° [001] tilt bicrystal of Bi2Sr2CaCu208 + x
Authors: Wang Jyh-Lih , Tsu I-Fei , Cai X. Y. , Kelley R. J. , Vaudin M. D. , Babcock S. E. , Larbalestier D. C.
Published: March 2008

Electromagnetic characterization and high resolution transmission electron microscopy have been conducted on the same 8° [001] symmetrical (010) tilt boundary in a naturally grown, bulk-scale bicrystal of Bi2Sr2CaCu2O8 + x (BSCCO-2212). The resistive transition showed excess resistance above and below Tc, suggesting some weak coupling at the boundary, but the inter- and intragranular voltage-current characteristics, irreversibility fields, and critical current density (Jc) values were very similar and characteristic of strongly coupled grains and grain boundary. The misorientation was accommodated by a set of partial dislocations with the Frank spacing of 1.9 nm. The dislocation cores appeared to be separated by relatively undistorted regions of crystal. The Jc, values at 25 K exceeded 103 A/cm2 in fields of several tesla, more than two orders of magnitude larger than that found earlier in [001] twist boundaries of BSCCO-2212. This result is consistent with the view that low angle [001] till boundaries play an important role for current transport in polycrystalline BSCCO tapes.
The role of transesterification in the multistep “prehydrolysis” sol/gel synthesis of aluminum-rich aluminosilicate gels
Authors: Pozarnsky G. A. , McCormick A. V.
Published: March 2008

Using a prehydrolysis technique, transparent gels with very high aluminum content can be achieved with the use of isopropanol. Here 13C, 27Al, 29Si and 17O NMR at various stages of preparation show that when the aluminum added exceeds the number that silanols can fully protect, the excess aluminum alkoxide groups readily undergo transesterification with isopropanol. The aluminum isopropoxide (Al–OPri) groups thus formed are shown to be sufficiently stable that attack by water is impeded, thus allowing the remaining silicon alkoxide groups to hydrolyze and condense to form a homogeneous gel.
Thermal wave analysis of contact damage in ceramics: Case study on alumina
Authors: Wei Lanhua , Lawn Brian R.
Published: March 2008

Thermal waves are used to image damage accumulation digitally beneath Hertzian contacts in ceramics. Alumina ceramics over a range of grain size 3–48 μm are used in a case study. The nature of the images changes with increasing alumina grain size, reflecting a transition in damage mode from well-defined cone fracture in the finer-grain materials to distributed subsurface microfracture in the coarser-grain materials. Quantitative determinations of microcrack densities are evaluated in the latter case by deconvoluting thermal diffusivities from the image data. These determinations confirm the grain-size dependence of degree of damage predicted by fracture mechanics models. Potential advantages and disadvantages of thermal waves as a route to damage characterization in ceramic systems are discussed.
Gas-phase combustion synthesis of titanium boride (TiB2) nanocrystallites
Authors: Axelbaum R. L. , DuFaux D. P. , Frey C. A. , Kelton K. F. , Lawton S. A. , Rosen L. J. , Sastry S. M. L.
Published: March 2008

Two techniques are described for synthesizing nanometer-sized TiB2particles by gas-phase combustion reactions of sodium vapor with TiCl4and BCl3: a low-pressure, low-temperature burner and a high-temperature flow reactor. Both methods produce TiB2particles that are less than 15 nm in diameter. The combustion by-product, NaCl, is efficiently removed from the TiB2by water washing or vacuum sublimation. Material collected from the low-temperature burner and annealed at 1000 °C consists of loosely agglomerated particles 20 to 100 nm in size. Washed material from the high-temperature flow reactor consists of necked agglomerates of 3 to 15 nm particles. A thermodynamic analysis of the Ti/B/Cl/Na system indicates that near 100% yields of TiB2are possible with appropriate reactant concentrations, pressures, and temperatures.
Oriented overgrowth of metals onto poly-1,4-phenylene
Authors: Torii Hidenori , Tsuji Masaki , Kawaguchi Akiyoshi
Published: March 2008

Thin, oriented films of poly-1,4-phenylene (PPP) were prepared with a specially designed reaction vessel. Various kinds of metals were deposited by vacuum evaporation on the PPP substrate, the temperature of which was able to be changed up to 400 °C. Of the studied metals, tellurium and bismuth exhibited the oriented crystallization on it. The oriented overgrowth of tellurium crystallites occurred in the temperature range of 150 °C–200 °C. There were two modes of oriented overgrowths of crystallites in the case of bismuth; one was performed by deposition around room temperature and the other at about 100 °C. The oriented overgrowths of bismuth and tellurium on PPP were examined in terms of the mismatch parameter of the crystal lattice, and it was found that the overgrowths were caused by the lattice matching at the interface between the crystallites of the metals and the PPP substrate.
Growth of CNxHy films by reactive magnetron sputtering of carbon in Ar/NH3 discharges
Authors: Sjöström H. , Lanford W. , Hjörvarson B. , Xing K. , Sundgren J-E.
Published: March 2008

Results on hydrogenated carbon nitride (CNxHy) thin films grown by reactive magnetron sputtering in a mixed Ar/NH3 discharge are reported. Depending on the growth temperature (Ts) and negative substrate bias voltage (Vs), both the composition and the microstructure were altered. Using nuclear reaction analysis and resonant backscattering spectroscopy, the maximum N and H content were both 15 at. %. Both the hydrogen and nitrogen content of the films was found to decrease with increasing growth temperature. The results also show pronounced chemical resputtering effects, resulting in no net film growth for Vs > 75–100 V. X-ray photoelectron spectroscopy showed no signs of N bound to sp3 hybridized C. Also, the microstructure of the films was found to change with Ts. For Ts < 150 °C, a structure with crystalline clusters embedded in an “fullerene-like” matrix was observed by high-resolution transmission electron microscopy. Power-spectra obtained from the clusters could be identified with the cubic diamond structure. For Ts ≥ 300 °C, no crystalline clusters were found and the films had a homogeneous “fullerene-like” microstructure with strongly bent planes and closed shell-like features resembling bucky-onions. Evaluation of nanoindentation results from the homogeneous “fullerene-like” films gave hardness values between 7 and 11 GPa and elastic recoveries of 55–60%. This should be compared with hardness and elastic recoveries of 40–60 GPa and 85–90%, respectively, previously reported for on nonhydrogenated carbon nitride CNx films grown under the same conditions, but in pure N2 discharges.
Direct observations of heteroepitaxial diamond on a silicon(110) substrate by microwave plasma chemical vapor deposition
Authors: Chen C. J. , Chang L. , Lin T. S. , Chen F. R.
Published: March 2008

Heteroepitaxial diamond has been successfully deposited on a Si(110) substrate by the microwave plasma chemical vapor deposition method. The pretreatment consisted of carburization and bias-enhanced nucleation steps. Cross-sectional transmission electron microscopy reveals that diamond can be in the cube-on-cube epitaxial relationship with the Si substrate. Various orientation relationships between diamond and Si substrates have also been observed, depending on the location where the plasma applied. Near the center of the plasma, twins were rarely observed in cube-on-cube epitaxial regions. Away from the center of the plasma ball, Σ3 twins are seen first, and then additional Σ9 and Σ27 twins occur near the edge of the plasma. In general, defect density in the epitaxial films is less than that observed in polycrystalline ones. No interlayer could be observed between diamond and silicon. In addition, 2H-type hexagonal diamond has also been found, and is in epitaxy with the Si substrate.
Undoped and doped GaN thin films deposited on high-temperature monocrystalline AlN buffer layers on vicinal and on-axis α(6H)–SiC(0001) substrates via organometallic vapor phase epitaxy
Authors: Weeks T. Warren , Bremser Michael D. , Ailey K. Shawn , Carlson Eric , Perry William G. , Piner Edwin L. , El-Masry Nadia A. , Davis Robert F.
Published: March 2008

Monocrystalline GaN(0001) thin films have been grown at 950 °C on high-temperature, ≈ 100 nm thick, monocrystalline AlN(0001) buffer layers predeposited at 1100 °C on α(6H)−SiC(0001)Si substrates via OMVPE in a cold-wall, vertical, pancake-style reactor. These films were free of low-angle grain boundaries and the associated oriented domain microstructure. The PL spectra of the GaN films deposited on both vicinal and on-axis substrates revealed strong bound excitonic emission with a FWHM value of 4 meV. The near band-edge emission from films on the vicinal substrates was shifted slightly to a lower energy, indicative of films containing residual tensile stresses. A peak attributed to free excitonic emission was also clearly observed in the on-axis spectrum. Undoped films were too resistive for accurate Hall-effect measurements. Controlled n-type, Si-doping in GaN was achieved for net carrier concentrations ranging from approximately 1 × 1017 cm−3 to 1 × 1020 cm−3. Mg-doped, p-type GaN was achieved with nA−nD ≈ 3 × 1017 cm−3, ρ ≈ 7 Ω · cm, and μ ≈ 3 cm2/V · s. Double-crystal x-ray rocking curve measurements for simultaneously deposited 1.4 μm GaN films revealed FWHM values of 58 and 151 arcsec for deposition on on-axis and off-axis 6H−SiC(0001)Si substrates, respectively. The corresponding FWHM values for the AlN buffer layers were approximately 200 and 400 arcsec, respectively.
Growth of diamond thin films by microwave plasma chemical vapor deposition process
Authors: Barshilia H. C. , Mehta B. R. , Vankar V. D.
Published: March 2008

A very high vacuum compatible microwave plasma chemical vapor deposition system has been fabricated for the growth of diamond thin films. Microcrystalline diamond thin films have been grown on silicon substrates from the CH4−H2 gas mixture. Scanning electron microscopy and x-ray diffraction have been used to study the surface morphology and the crystallographic structure of the films. Optical emission spectroscopy has been used for the detection of chemical species present in the plasma. The strong dependence of the film microstructure on the intensity of CH emission line has been observed.
Characteristics of spin-on ferroelectric SrBi2Ta2O9 thin film capacitors for ferroelectric random access memory applications
Authors: Chu Peir Y. , Jones Robert E. , Zurcher Peter , Taylor Deborah J. , Jiang Bo , Gillespie Sherry J. , Lii Y. T. , Kottke Mike , Fejes Peter , Chen Wei
Published: March 2008

We report on the properties and characterization of SrBi2Ta2O9 (SBT, or Y − 1) thin film capacitors for ferroelectric random access memory (FERAM) applications. The films were prepared by spin-coating from carboxylate precursors. The remanent polarization (Pr) was 5−6 μC/cm2 and the coercive field was ∼55 kV/cm. Excellent fatigue endurance was observed up to 1011 cycles. Auger analysis indicates bismuth diffusion through the Pt electrode after capacitor anneal which might require excess Bi in the precursor solution for stoichiometry control. No detectable amount of α emission was found from SBT films, which reduces the possibility of soft error when used in the memory devices.
Magnetic alignment in 2212 Bi-based superconducting system: Part I. Magnetic orientation of Bi2Sr2Ca1−x(RE)xCu2O8−y [(RE) = Gd, Dy, Ho, Er] powder dispersed in epoxy resin at room temperature
Authors: Stassen S. , Cloots R. , Vanderbemden Ph. , Godelaine P. A. , Bougrine H. , Rulmont A. , Ausloos M.
Published: March 2008

The magnetic anisotropy of rare-earth substituted 2212 materials (Bi2Sr2Ca0.8RE0.2Cu2Ox with RE = Gd, Dy, Ho, Er) is put into evidence. Superconducting powder dispersed in epoxy resin is oriented under an external magnetic field (4 T) in a direction that depends on the nature of the rare-earth used in the substitution. Both directions of observation (parallel or perpendicular to the field) were investigated. Splitting of (00l) peaks is neatly observed and discussed.
The relation between the undercooling and the growth rate of YBa2Cu3O6+x superconductive oxide
Authors: Nakamura Y. , Endo A. , Shiohara Y.
Published: March 2008

To clarify the effect of undercooling on the crystal growth of Y-123, the growth rate was measured with different undercoolings. The growth rate of the {100} face shows a quadratic dependence of undercooling, while that of the {001} face shows a linear dependence in the sample with nominal 123 composition. In the case with 211-rich composition, the growth rate of each face was larger than that compared with nominal 123 composition since the mass flux from 211 particle for peritectic reaction becomes large. Addition of excess 211 alters the undercooling dependence of Ra from quadratic to linear. It is considered that the entrapment of 211 particles into 123 crystals supplies step sources beside screw dislocations. The growth rate of the {001} face is larger than that of the {100} face up to 26° of undercooling.
Effect of sintering periods on the microstructure and electrical transport properties of high-Tc superconducting Bi–(Pb)–Sr–Ca–Cu–O tapes
Authors: Vo N. V. , Liu H. K. , Dou S. X.
Published: March 2008

The effect of sintering periods for monocore (Bi, Pb)2Sr2Ca2Cu3O10+x (Bi-2223) tapes on microstructure and critical current density (Jc) have been studied. The results show that long sintering periods (of duration ≥100 h) give better grain growth, but greater misalignment. Prolonged sintering also necessitates the increase in porosity of the core due to random grain growth, increasing the chance of penetrating into the silver matrix and oxide core interface. Critical currents for long sintering periods are found to be lower in comparison with those obtained for slightly shorter sintering periods. The increase in frequency of intermediate cold uniaxial pressing between sintering periods assists grain alignment. However, when the sintering period is further reduced by increasing the frequency of intermediate deformation, it is found that microcracks are unable to heal as there is not enough time for grain regrowth. The tapes made using “three-to-four-sinter-period” (each period ∼60 h) show superior electrical transport properties which are attributable to the fact that their oxide core is more dense and better aligned relative to the “two-sinter-period” (each period ≥100 h) tapes and contain less cracks relative to the “five-to-six-sinter-period” (each period ∼40 h) tapes.
Relationship between growth rate and undercooling in Pt-added Y1Ba2Cu3O7−x
Authors: Endo A. , Chauhan H. S. , Nakamura Y. , Shiohara Y.
Published: March 2008

Y1Ba2Cu307−x (Y123) crystals were grown by two different methods, the constant undercooling solidification and the continual cooling method, with top seeding by Sm123 seed crystals in order to investigate a relationship between undercooling (ΔT) and a growth rate (R). The crystals of Y123 with a sharp faceted interface, which consisted of {100} and {001} faces, grew epitaxially from the seed. It was found that the growth rates of {100} face (Ra) and that of {001} face (Rc) showed an increasing trend with increasing ΔT, and Rc was faster than Ra within these experimental conditions, ΔT < 20 K. The relation between R and ΔT follows the parabolic equation, viz. Ra ∝ ΔT1.9 and Rc ∝ ΔT1.3 for {100} and {001} faces, respectively. The simulated crystal size using the R and ΔT relations obtained from the constant undercooling method showed good agreement with experimental data by the continual cooling.
Zn doping in YBCO single crystal by the solute-rich liquid-crystal pulling method
Authors: Yao X. , Ohtsu K. , Tajima S. , Zama H. , Wang F. , Shiohara Y.
Published: March 2008

Zn-doped YBa2Cu3O6+δ (YBCO) single crystals were fabricated by the solute-rich liquid-crystal pulling (SRL-CP) method. The Zn content of x ranged from 0.0035–0.029 and almost uniformly distributed in YBa2(Cu1−xZnx)3O6+δ. There is a nearly linear relationship between the added Zn content in the liquid and the analyzed Zn content in the liquid/in the crystal. When the added Zn content Cl was 1.086 at. %, decreasing of growth temperature led to increasing growth rate. However, growth temperature had no obvious effect on the doped Zn content in the crystal Cs, the Zn content in the flux Cl, and the effective distribution ratio of k′. In addition, growth spirals on the grown crystal surface were observed by atomic force microscopy (AFM). X-ray Laue pattern and RBS measurements are indicative of good crystallinity of YBa2(Cu1−xZnx)3O6+δ. Critical temperatures of Tc changed from 89 to 57 K when the Zn content x ranged 0.0035 to 0.029 after appropriate oxygenation.
Epitaxial growth and magnetic behavior of NiFe2O4 thin films
Authors: Venzke S. , van Dover R. B. , Phillips Julia M. , Gyorgy E. M. , Siegrist T. , Chen C-H. , Werder D. , Fleming R. M. , Felder R. J. , Coleman E. , Opila R.
Published: March 2008

Thin films of NiFe2O4 were deposited on SrTiO3 (001) and Y0.15Zr0.85O2 (yttria-stabilized zirconia) (001) and (011) substrates by 90°-off-axis sputtering. Ion channeling, x-ray diffraction, and transmission electron microscopy studies reveal that films grown at 600 °C consist of ∼300 Å diameter grains separated by thin regions of highly defective or amorphous material. The development of this microstructure is attributed to the presence of rotated or displaced crystallographic domains and is comparable to that observed in other materials grown on mismatched substrates (e.g., GaAs/Si or Ba2YCu3O7/MgO). Postdeposition annealing at 1000 °C yields films that are essentially single crystal. The magnetic properties of the films are strongly affected by the structural changes; unannealed films are not magnetically saturated even in an applied field of 55 kOe, while the annealed films have properties comparable to those of bulk, single crystal NiFe2O4. Homoepitaxial films grown at 400 °C also are essentially single crystal.
Thermal stability of Nb thin films on sapphire
Authors: Wagner Thomas , Lorenz Marko , Rühle Manfred
Published: March 2008

The Nb/α−Al2O3 system has been used as a model study for investigating the stability of different MBE grown epitaxial Nb films on α−Al2O3 substrates. The films were grown at 800 °C in ultrahigh vacuum. The growth process was monitored in situ by reflection high energy electron diffraction (RHEED). After deposition the structure of the film was investigated by x-ray diffraction (XRD) and conventional transmission electron microscopy (CTEM) which encompasses also selected area diffraction (SAD). Both techniques revealed the following orientation relationship between the Nb film and the α–Al2O3 substrate: (0001)α–Al2O3‖ (111)Nb; [2110]α–Al2O3‖ [110]Nb. The stability of the niobium films was investigated by annealing the Nb-film/α–Al2O3 system to temperatures up to 1500 °C for different periods of time. Surprisingly, the orientation relationship between the Nb film and the substrate changed to (0001)α–Al2O3‖ (110)Nb; [0110]α–Al2O3‖ [001]Nb. A model will be developed which shows that above a critical film thickness the growth orientation is metastable with respect to its crystallographic orientation. Furthermore, high resolution transmission electron microscopy (HREM) was performed to investigate the defect structure of the annealed Nb/α–Al2O3 interface.
Elasticity and fracture in particulate composites with strong and degraded interfaces
Authors: Lekatou A. , Faidi S. E. , Lyon S. B. , Newman R. C.
Published: March 2008

Silane-coated glass microspheres randomly embedded in an epoxy polymer matrix have been employed as a model system to investigate the degradation of disordered composite materials by water, and to test various models of deformation and fracture. Numerous composites containing sodalime (A) glass in the range 0 to 25% by volume were tested dry and immersed in saturated NaCl at 40 °C for periods up to 70 days before testing. Enhanced osmotic water uptake due to percolating interface damage was observed for composites containing more than 15% glass. The electrical resistance of similar composites filled with conducting spheres confirmed the existence of a percolation transition, though with high resistance values implying no direct contact of the spheres. Tensile measurements conducted on dry material at a nominal strain rate of about 10−3 s−1 showed an increase in elastic modulus and a decrease in the fracture strength with increasing glass content. New detail was apparent in these curves and confirmed by statistical analyses. For wet specimens, in addition to a general embrittlement effect of water absorption, there was a distinct plateau or small peak in fracture strength in the range 9 to 12% glass, and an abrupt drop between 12 and 15%. The plateau can be related to favorable crack interaction effects between disconnected clusters of interfaces just below the percolation threshold. The steep increase in elastic modulus with glass content seen in the dry material vanished entirely in wet material, which behaved like a porous polymer above 6% glass, owing to osmotic interface damage within particle clusters.
A low-temperature technique for measuring enthalpies of formation
Authors: Weihs T. P. , Barbee T. W. , Wall M. A.
Published: March 2008

A technique to accurately measure the formation enthalpies of transition metal compounds at relatively low temperatures using thick multilayer foils and differential scanning calorimetry is demonstrated. The enthalpy of formation of Cu51Zr14 was measured using 25 μm thick, free-standing Cu–Zr multilayer foils. The multilayers were deposited onto Si substrates using a planetary, magnetron source sputtering system. They were removed from their substrates, cut into 6 mm diameter specimens, and scanned in temperature from 50 °C to 725 °C in a differential scanning calorimeter. Three distinct exothermic reactions were systematically observed. The heats from the first two reactions were summed and then analyzed using a simple model that accounts for interfacial reactions and heat losses during deposition. The enthalpy of formation for Cu51Zr14 was measured to be 14.3 ± 0.3 kJ/mol. This quantity agrees with the single value of ΔHf = 14.07 ± 1.07 kJ/mol reported in the literature for this Cu–Zr compound. The advantages of measuring formation enthalpies using thick multilayer foils and low temperature calorimetry are discussed.
Dielectric properties of TiO2–Nb2O5 crystallographic shear structures
Authors: Cava R.J. , Krajewski J. J. , Peck W.F. , Roberts G. L.
Published: March 2008

The dielectric constants (K) have been measured near ambient temperature for polycrystalline bulk ceramics of the crystallographic shear compounds TiNb24O62, Ti2Nb10O29, and TiNb2O7. These show enhanced K's over Nb2O5 and TiO2, with the 2: 10:29 phase displaying an ambient temperature K of approximately 130. We also report the effects of partial substitution of Nb by Ta. In general, the dielectric constants are enhanced for 5–10% Ta substitution, with the detailed behavior differing for the three phases.
Characteristics of titanium nitride films grown by pulsed laser deposition
Authors: Chowdhury R. , Vispute R. D. , Jagannadham K. , Narayan J.
Published: March 2008

Laser physical vapor deposition (LPVD) has been used to grow titanium nitride films on hydrogen-terminated silicon(100) substrates at deposition temperatures ranging from room temperature to 600 °C. A pulsed KrF excimer laser (λ = 248 nm, τ = 25 ns) was used with the deposition chamber maintained at a base pressure of 10−7 Torr prior to deposition. Different properties of the films were investigated by x-ray diffraction, Auger electron spectroscopy, Raman spectroscopy, optical, scanning, and high resolution transmission electron microscopy, and measurement of electrical resistivity. When the substrate temperature was low (at and below 500 °C), oxygen atoms from the residual gases were incorporated in the films. The microstructures and resistivities of TiN films were found to be strongly dependent on the temperature of the silicon substrates. The TiN films deposited at 600 °C were oxygen-free, as observed from Auger analysis, and the room temperature resistivity was found to be 14–15 μΩ-cm. Raman spectroscopy of the films showed that the nitrogen-related optical phonon peak increased with deposition temperature in comparison with the titanium-related acoustic peak. Transmission electron microscopy and x-ray diffraction analyses showed that the films were polycrystalline at low temperature with grain size ranging from 300–600 Å, depending on the temperature of the substrate. At 600 °C, the films were found to be single crystals with occasional presence of dislocation loops. The spacing of Moiré fringes in TiN/Si samples deposited at 600 °C established the nearly periodic elastic strain field extending into the TiN and Si at the interface. Although there exists a large misfit between TiN and Si (24.6%), the epitaxial growth of TiN films on Si(100) substrates was explained by means of domain-matched epitaxy with a 4-to-3 match in unit cells for TiN/Si structure, giving rise to a residual lattice misfit of only 4%.
Microstructural instability in single-crystal thin films
Authors: Seifert A. , Vojta A , Speck J. S. , Lange F. F.
Published: March 2008

Epitaxial PbTiO3 thin films were produced from a mixed Pb–Ti double-alkoxide precursor by spin-coating onto single crystal (001) SrTiO3 substrates. Heat treatment at 800 °C produces a dense and continuous, epitaxial lead titanate film through an intermediate Pb-Ti fluorite structure. A microstructural instability occurred when very thin single crystal films were fabricated; this instability caused the films to become discontinuous. Scanning electron microscopy and atomic force microscopy observations show that single crystal films with a thickness less than ∼80 nm developed holes that expose the substrate; thinner films broke up into isolated, single crystal islands. The walls of the holes were found to be (111) perovskite planes. A free energy function, which considered the anisotropic surface energies of different planes, was developed to describe the microstructural changes in the film and to understand the instability phenomenon. The function predicted that pre-existing holes greater than a critical size are necessary to initiate hole growth, and it predicted the observed morphological changes in the current system. Morphological stability diagrams that explain the stability fields for different film configurations, i.e., either completely covered, with holes, or single crystal islands, can be calculated for any film/substrate system.
Low pressure chemical vapor deposition of silicon nitride using the environmentally friendly tris(dimethylamino)silane precursor
Authors: Levy R. A. , Lin X. , Grow J. M. , Boeglin H. J. , Shalvoy R.
Published: March 2008

This study investigates the use of the environmentally benign precursor tri(dimethylamino)silane (TDMAS) with NH3 to synthesize silicon nitride films by low pressure chemical vapor deposition. The growth kinetics are investigated as a function of deposition temperature, total pressure, and NH3/TDMAS flow ratios. The deposits are found to be essentially stoichiometric and to contain ∼5 at. % carbon when appropriate NH3 concentrations are present. The films are found in all cases to be amorphous and highly tensile. For optimized processing conditions, values of the refractive index are close to those reported for Si3N4. The film density is observed to increase with higher deposition temperatures up to 800 °C and then decrease due to the onset of gas phase nucleation effects. This behavior is readily reflected in the etch rate of those films. FTIR spectra reveal the presence of hydrogen even at high deposition temperatures (900 °C). Hardness and Young's modulus of the films are seen to increase with higher deposition temperatures, reaching saturation values near 20 and 185 GPa, respectively, above 800 °C.
Surface microstructure of Zr41.25Ti13.75Cu12.5Ni10.0Be22.5, a bulk metallic glass
Authors: LaMadrid M.A. , O'Connor S.D. , Peker A. , Johnson W.L. , Baldeschwieler J.D.
Published: March 2008

The surface of Zr41.25Ti13.75Cu12.5Ni10.0Be22.5, a bulk metallic glass prepared by RF induction melting, has been imaged using atomic force microscopy. The untreated surfaces were very smooth; features were no higher than 3 nm over a 10 × 10 μm region, comparable to many polished surfaces. Two types of microstructure were also observed; periodic striations forming either a striped or a checkered structure were present, with wavelengths between 1 and 2 μm, and amplitude of approximately 2 nm; in other cases, “cracked mud”-like patterns were observed. These microstructures could be related to strain-induced surface roughening; preliminary calculations are presented that are consistent with this hypothesis.