bluuebunny/crossref_metadata_2025 huggingface dataset:
Spin-polarized tunneling in discontinuous CoFe/HfO2 multilayers
Authors: Sankar S. , Dieny B. , Berkowitz A. E.
Published: July 2002

We have investigated the structural, magnetic, and transport properties of discontinuous CoFe/HfO2 multilayers consisting of layers of ferromagnetic particles dispersed in an insulating matrix. Negative magnetoresistance due to spin-polarized tunneling has been observed with the current both in the plane and perpendicular to the plane of the film. The properties of these films are intermediate between tunnel junctions and cermet films. They exhibit high magnetoresistance sensitivity at low fields, are easy to prepare, are very robust due to the protective oxide matrix, and are resistant to electrical breakdown. These properties can be optimized with annealing.
Subpicosecond surface-restricted carrier and thermal dynamics by transient reflectivity measurements
Authors: Tanaka Takayuki , Harata Akira , Sawada Tsuguo
Published: July 2002

Ultrafast measurements of photoexcited carrier dynamics within a 60 nm subsurface of a crystalline silicon wafer were carried out using subpicosecond transient reflectivity. A uv pump light was employed to restrict carrier generation to occur within the subsurface by direct interband transitions. Carrier diffusion was found to be suppressed in the subsurface region of the intrinsic silicon wafer. For ion-implanted silicon wafers, heat was generated within a few picoseconds after the laser irradiation. By scanning a partially ion-implanted silicon wafer, the two-dimensional image was obtained, which showed that time-resolved imaging can separately map photoexcited carrier density and transient temperature rise. The possibility of three-dimensional process monitoring was considered as well.
Wall energy and wall thickness of exchange-coupled rare-earth transition-metal triple layer stacks
Authors: Raasch D. , Mathieu C.
Published: July 2002

The room-temperature wall energy σw=4.0×10−3 J/m2 of an exchange-coupled Tb19.6Fe74.7Co5.7/Dy28.5Fe43.2Co28.3 double layer stack can be reduced by introducing a soft magnetic intermediate layer in between both layers exhibiting a significantly smaller anisotropy compared to Tb–FeCo and Dy–FeCo. σw will decrease linearly with increasing intermediate layer thickness, dIL, until the wall is completely located within the intermediate layer for dIL⩾dw, where dw denotes the wall thickness. Thus, dw can be obtained from the plot σw versus dIL. We determined σw and dw on Gd–FeCo intermediate layers with different anisotropy behavior (perpendicular and in-plane easy axis) and compared the results with data obtained from Brillouin light-scattering measurements, where exchange stiffness, A, and uniaxial anisotropy, Ku, could be determined. With the knowledge of A and Ku, wall energy and thickness were calculated and showed an excellent agreement with the magnetic measurements. A ten times smaller perpendicular anisotropy of Gd28.1Fe71.9 in comparison to Tb–FeCo and Dy–FeCo resulted in a much smaller σw=1.1×10−3 J/m2 and dw=24 nm at 300 K. A Gd34.1Fe61.4Co4.5 with in-plane anisotropy at room temperature showed a further reduced σw=0.3×10−3 J/m2 and dw=17 nm. The smaller wall energy was a result of a different wall structure compared to perpendicular layers.
Effect of ultraviolet light irradiation on amorphous carbon nitride films
Authors: Zhang Mei , Nakayama Yoshikazu
Published: July 2002

The amorphous carbon nitride films were produced using electron cyclotron resonance nitrogen plasma with various mixtures of N2 and CH4 gases. The dependence of film structures on the nitrogen incorporation and the structural modifications of the film due to ultraviolet (UV) light irradiation were investigated using infrared and UV-VIS spectroscopy. It is found that UV irradiation results in the decrease of CH bonding, increase of CC and CN double bonding in the film and increase of the optical band gap of the film. It appears that both bond removal and reordering have taken place as a result of UV irradiation. The structural modifications due to nitrogen incorporation and UV light irradiation are explained by a cluster model.
Device model for single carrier organic diodes
Authors: Davids P. S. , Campbell I. H. , Smith D. L.
Published: July 2002

We present a unified device model for single layer organic light emitting diodes (LEDs) which includes charge injection, transport, and space charge effects in the organic material. The model can describe both injection limited and space charge limited current flow and the transition between them. We specifically considered cases in which the energy barrier to injection of electrons is much larger than that for holes so that holes dominate the current flow in the device. Charge injection into the organic material occurs by thermionic emission and by tunneling. For Schottky energy barriers less than about 0.3–0.4 eV, for typical organic LED device parameters, the current flow is space charge limited and the electric field in the structure is highly nonuniform. For larger energy barriers the current flow is injection limited. In the injection limited regime, the net injected charge is relatively small, the electric field is nearly uniform, and space charge effects are not important. At smaller bias in the injection limited regime, thermionic emission is the dominant injection mechanism. For this case the thermionic emission injection current and a backward flowing interface recombination current, which is the time reversed process of thermionic emission, combine to establish a quasi-equilibrium carrier density. The quasi-equilibrium density is bias dependent because of image force lowering of the injection barrier. The net device current is determined by the drift of these carriers in the nearly constant electric field. The net device current is much smaller than either the thermionic emission or interface recombination current which nearly cancel. At higher bias, injection is dominated by tunneling. The bias at which tunneling exceeds thermionic emission depends on the size of the Schottky energy barrier. When tunneling is the dominant injection mechanism, a combination of tunneling injection current and the backflowing interface recombination current combine to establish the carrier density. We compare the model results with experimental measurements on devices fabricated using the electroluminescent conjugated polymer poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] which by changing the contacts can show either injection limited behavior or space charge limited behavior.
A quantitative analysis of surface deformation by stick/slip atomic force microscopy
Authors: Kerssemakers J. , De Hosson J. Th. M.
Published: July 2002

This article presents a quantitative determination of static deformation at a nanometer scale of a surface caused by the tip of an atomic force microscope. An analysis of cantilever displacements while in contact with the surface leads to a directly measurable dimensionless parameter which is well sensitive to surface deformation. The method is specifically aimed at stick/slip friction measurements like on layered compounds, like TiS2 or on a relatively rigid surface of an ionic crystal, in this study NaCl [100]. Stick/slip friction images offer a possibility to investigate details of strain-dependent deformation. The observed deformation in TiS2could play an important role in the occurrence of strong stick/slip friction in this and other layered materials.
Reconstructed (881) Si surface structure observed by scanning tunneling microscopy
Authors: Kawamura Tsutomu , Kojima Shiro , Kanzawa Tomohide
Published: July 2002

The high-index (881) surface has been studied by ultra-high-vacuum scanning tunneling microscopy (STM). Reconstruction of the (881) Si surface was found to occur, as evident from the appearance of a number of different characteristic patterns in the STM image, including various kinds of terraces, step walls, and kinks. A typical terrace had, on average, a (7×2) reconstruction. The step walls consisted of several split lattices. Their multiple reciprocal networks and their relationship to the step walls has been analyzed. The kinks were found to consist of one-dimensional long-period superlattices. The reason for the disorder is discussed in terms of the relationship between the effective dangling-bond density and the periodic bond chain vector approximation.
Conduction mechanisms in some graphite–polymer composites: Effects of temperature and hydrostatic pressure
Authors: Celzard A. , McRae E. , Marêché J. F. , Furdin G. , Sundqvist B.
Published: July 2002

This article is devoted to the conduction mechanisms involved in some highly electrically anisotropic resin–graphite particle composites. These materials are known to show a percolation phenomenon as the filler content is varied; they are epoxy or polyurethane based, the conducting particles are oriented single-crystal platelets, and samples are in the form of thick films. Because of their strong anisotropy, two types of measurements were made, i.e., parallel to and perpendicular to the plane of the films. Study of the resistivity variations of samples containing various concentrations in conducting particles was carried out first as a function of temperature from 4.2 to 300 K at ambient pressure, and second as a function of hydrostatic pressure up to 1.2 GPa, at room temperature. As the temperature is varied, the changes in resistivity of all the samples studied (i.e., above percolation threshold) are weak; analysis leads to the conclusion that thermally activated tunneling plays a dominant role above but close to the percolation threshold φc. As a function of pressure, more samples were studied: when the filler content is above φc, resistivity changes are quantitatively in agreement with what is expected from both percolation theory and tunneling; below threshold, the observed behavior is partially attributable to an ionic conduction mechanism throughout the polymer.
High-field transport and electroluminescence in ZnS phosphor layers
Authors: Dür Manfred , Goodnick Stephen M. , Pennathur Shankar S. , Wager John F. , Reigrotzki Martin , Redmer Ronald
Published: July 2002

A full-band Monte Carlo simulation of the high-field electron transport in the ZnS phosphor layer of an alternating-current thin-film electroluminescent device is performed. The simulation includes a nonlocal empirical pseudopotential band structure for ZnS and the relevant scattering mechanisms for electrons in the first four conduction bands, including band-to-band impact ionization and impact excitation of Mn2+ luminescent centers. The steady-state electron energy distribution in the ZnS layer is computed for phosphor fields from 1 to 2 MV/cm. The simulation reveals a substantial fraction of electrons with energies in excess of the Mn2+ impact excitation threshold. The computed impact excitation yield for carriers transiting the phosphor layer exhibits an approximately linear increase with increasing phosphor field above threshold. The onset of Mn2+ impact excitation coincides with the onset of band-to-band impact ionization of electron-hole pairs which prevents electron runaway at high electric fields.
Nitrogen ion beam-assisted pulsed laser deposition of boron nitride films
Authors: Angleraud B. , Cahoreau M. , Jauberteau I. , Aubreton J. , Catherinot A.
Published: July 2002

This work deals with the study of boron nitride (BN) thin films deposited onto silicon substrates using the ion beam-assisted pulsed laser deposition method. Influence of experimental deposition parameters on the percentage of sp3 chemical bonds (representative of the cubic BN phase) contained in the deposited film and on chemical composition is presented. The best percentages of sp3 bonds, up to 80%, are obtained under bombardment by a pure nitrogen ion beam, and we do not observe any significant contribution of argon ions in the bombarding ion beam to the presence of sp3 bonds in the deposited films. X-ray photoelectron spectroscopy and atomic force microscopy performed on various BN films reveal important changes with the type of chemical bonds (sp2 and sp3) contained within these films.
Ion beam induced amorphization in α quartz
Authors: Harbsmeier F. , Bolse W.
Published: July 2002

The amorphization of α quartz induced by light and medium mass ion bombardment has been investigated by means of Rutherford backscattering spectrometry in channeling geometry, resonant nuclear reaction analysis and mechanical surface profiling. The single crystalline samples were irradiated at 77 K with H+, N+, Ne+ and Na+ ions at energies of 10–100 keV and fluences up to 5×1016 ions/cm2. Disordering of α quartz by ion bombardment was found to occur by three-dimensional nucleation and growth of defect agglomerates and, possibly, small, spatially separated amorphous zones in the still crystalline surrounding. The nucleation rate scales with the energy density FD in elastic collisions. Above a critical energy density EC=1.92(7)eV/at deposited by nuclear collisions (corresponding to a displacement rate of 0.04 dpa) a coherent amorphous layer forms at the depth of maximum energy deposition, which then grows towards the surface and into larger depths during continued ion bombardment. Mechanical surface profiling reveals large compressive stresses (∼1.5 GPa), built up at low fluences, which are released at higher fluences after the amorphous layer extends to the surface. The stress release is accompanied by a significant reduction of the atomic density by about 19%.
Shift in the magnetic percolation threshold of phase separated Co-rich CoAg very thin films due to reduced dimensionality
Authors: Butera A. , Klemmer T. J. , Barnard J. A.
Published: July 2002

An investigation of the structural and magnetic properties of phase separated Co-rich CoAg very thin films (5 nm–50 nm) as a function of film thickness and Co concentration is presented. In the as-deposited state the films are very fine grained and the coercive field is relatively low (Hc<20 Oe). However, after annealing at 420 °C for half an hour in high vacuum, grain growth is promoted and a drastic change in the magnetic properties is observed. The coercivity now has a strong thickness dependence with a maximum at approximately 15 nm. The largest room temperature value of coercivity, Hc=850 Oe, was found for a CoAg 70:30 volume percent alloy. The concentration of maximum coercivity is usually associated with the percolation threshold, xp, of the ferromagnetic element and is much higher than the ∼50 vol % value normally observed in thick film and bulk granular ferromagnets. Values of xp≲0.65 were found for 30 nm thick films increasing up to xp∼0.75 for 10 nm films. This behavior is explained as a shift in the percolation limit to higher values associated with the reduced dimensionality of very thin films. This shift implies that the volume fraction of the ferromagnetic component should be larger than its bulk counterpart if granular ferromagnets are to be used as thin film media for magnetic recording. The temperature dependence of Hc was also studied and coercivities exceeding 1800 Oe have been found at 50 K. The presence of a thin antiferromagnetic oxide layer is manifested as a shift in the low temperature hysteresis loops due to the anisotropic exchange coupling interaction with ferromagnetic Co.
The development of high performance Nd–Fe–Co–Ga–B die upset magnets
Authors: Saito T. , Fujita M. , Kuji T. , Fukuoka K. , Syono Y.
Published: July 2002

The magnetic properties and the microstructure of Nd–Fe–B and Nd–Fe–Co–Ga–B die upset magnets produced from amorphous materials were studied. The Nd–Fe–B die upset magnets had fine polygonal Nd2Fe14B grains and showed magnetic anisotropy. The compositional modification and optimization of the die upset condition led to the increase in the remanence and coercivity values of the Nd–Fe–B die upset magnets. The optimally deformed Nd–Fe–Co–Ga–B die upset magnets showed the maximum energy product of 54.4 MGOe.
Nanostructured NdFeB films processed by rapid thermal annealing
Authors: Yu M. , Liu Y. , Liou S. H. , Sellmyer D. J.
Published: July 2002

Nanostructured NdFeB films were prepared by magnetron sputtering followed by rapid thermal annealing at a ramp rate of 200 °C/s. Isotropically oriented Nd2Fe14B crystallites were formed in the films and coercivities up to 20 kOe and remanence ratios up to 0.8 were obtained. Transmission electron microscopy analysis shows that the majority phase of the magnetic layer in the high coercivity films consists of Nd2Fe14B nanocrystallites with an average size of about 50 nm. These nanocrystallites are believed to be single-domain particles, which are responsible for the high coercivities. MFM measurements show that the domain size is about 500 nm. It is thus indicated that the 50 nm Nd2Fe14B nanocrystallites are strongly exchange-coupled into the 500 nm domains and the high remanence ratio originates from this exchange coupling.
Evolution in the charge injection efficiency of evaporated Au contacts on a molecularly doped polymer
Authors: Ioannidis Andronique , Facci John S. , Abkowitz Martin A.
Published: July 2002

Injection efficiency from evaporated Au contacts on a molecularly doped polymer (MDP) system has been previously observed to evolve from blocking to ohmic over time. In the present article this contact forming phenomenon is analyzed in detail. The initially blocking nature of the Au contact is in contrast with that expected from the relative workfunctions of Au and of the polymer which suggest Au should inject holes efficiently. It is also in apparent contrast to a differently prepared interface of the same materials. The phenomenon is not unique to this interface, having been confirmed also for evaporated Ag and mechanically made liquid Hg contacts on the same MDP. The MDP is a disordered solid state solution of electroactive triarylamine hole transporting TPD molecules in a polycarbonate matrix. The trap-free hole-transport MDP provides a model system for the study of metal/polymer interfaces by enabling the use of a recently developed technique that gives a quantitative measure of contact injection efficiency. The technique combines field-dependent steady state injection current measurements at a contact under test with time-of-flight (TOF) mobility measurements made on the same sample. In the present case, MDP films were prepared with two top vapor-deposited contacts, one of Au (test contact) and one of Al (for TOF), and a bottom carbon-loaded polymer electrode which is known to be ohmic for hole injection. The samples were aged at various temperatures below the glass transition of the MDP (85 °C) and the evolution of current versus field and capacitance versus frequency behaviors are followed in detail over time and analyzed. Control measurements ensure that the evolution of the electrical properties is due to the Au/polymer interface behavior and not the bulk. All evaporated Au contacts eventually achieved ohmic injection. The evaporated Au/MDP interface was also investigated by transmission electron microscopy as a function of time and showed no evidence of Au interdiffusion in the MDP layer, remaining abrupt to within ∼10 Å over the course of the evolution in injection efficiency. Mechanisms related to Au penetration into the MDP are therefore unlikely. Rapid sequence data acquisition enabled the detection of two main processes in the injection evolution. The evolving injection efficiency is very well fit by two exponentials, enabling the characterization of time and temperature dependence of the evolution processes.
Arsenic deactivation enhanced diffusion: A time, temperature, and concentration study
Authors: Rousseau P. M. , Griffin P. B. , Fang W. T. , Plummer J. D.
Published: July 2002

The electrical deactivation of arsenic in silicon has been studied with regard to its effect on enhanced diffusion. Experimental structures consist of a buried boron layer as an interstitial detector, and a fully activated arsenic doped laser annealed surface layer. As these structures are annealed at temperatures between 500 and 750 °C, arsenic in the surface layer deactivates and we observe enhanced diffusion of the buried boron layer. A study with time reveals that the enhanced diffusion transient and the deactivation transient are similar, indicating a strong correlation between both phenomena. The dependence on concentration shows a maximum enhanced diffusion for concentrations between 3 and 4×1020 cm−3 of initially active arsenic. Above these concentrations, the large supersaturation of interstitials nucleates dislocation loops and lowers the overall enhancement measured in the buried boron layer. Temperature data show that even for temperatures as low as 500 °C, enhanced diffusion is observed. These data are convincing evidence that the enhanced diffusion observed is due to the deactivation of arsenic and provides important insights into the mechanisms of deactivation. We propose that arsenic deactivation forms small clusters of various sizes around a vacancy with the injection of an associated interstitial into the bulk.
Structure, hyperfine interactions, and magnetic behavior of amorphous and nanocrystalline Fe80M7B12Cu1 (M=Mo, Nb, Ti) alloys
Authors: Miglierini M. , Kopcewicz M. , Idzikowski B. , Horváth Z. E. , Grabias A. , Škorvánek I. , Duźewski P. , Daróczi Cs. S.
Published: July 2002

Fe 57 Mössbauer spectrometry is used to elucidate the structural arrangement and nature of hyperfine interactions in amorphous and nanocrystalline forms of Fe80M7B12Cu1 (M=Mo, Nb, and Ti) alloys. Paramagnetic at room temperature as-quenched M=Mo amorphous alloy shows a distribution of quadrupole splitting (TC=265 K). After partial crystallization, a wide range of hyperfine interactions is observed in the residual amorphous matrix of all samples. The hyperfine field distributions corresponding to the amorphous phase and interfacial zone provide a quantitative basis for a design of topography of hyperfine interactions. Closer inspection of magnetic interactions is made by the help of unconventional radio frequency (rf) Mössbauer technique. The rf Mössbauer results show that the nanocrystalline Fe phase has a larger anisotropy than that of the parent amorphous phase. Conventional magnetic measurements, differential scanning calorimetry, x-ray diffraction, transmission electron microscopy, and scanning tunneling microscopy experiments are employed to provide complementary information.
Some considerations on the secondary electron emission, δ, from e− irradiated insulators
Authors: Cazaux Jacques
Published: July 2002

The so-called “total yield” approach often fails to explain the measured sign of the surface potential, VS, and the shift of the nominal critical energy EC2∘ (where δ°+η°=1) of electron irradiated insulators. Here, a simple modification of this approach consists in including some extra interactions of the secondary and backscattered electrons with the electron traps generated previously by the irradiation itself. The trends in the evolution of the total yield, δ+η, and of VS as a function of the irradiation time (from their initial values up to their steady values) are then deduced for a wide primary beam energy range (1–50 keV) and for different external collector (or specimen holder) bias. New mechanisms are suggested for the contrasts observed in insulators investigated in scanning electron microscopy (SEM). The present analysis applies for a wide variety of electron beam techniques (SEM, Auger electron spectroscopy, and electron probe microanalysis) operated on a wide variety of insulating specimens and this analysis can be easily extended to any device based on the electron emission from insulators.
Silicide formation in the Ta/Ti/Si system by reaction of codeposited Ta and Ti with Si (100) and Si (111) substrates
Authors: Pelleg Joshua , Goldshleger N.
Published: July 2002

Codeposited Ta and Ti on Si (111) and Si (100) substrates were investigated at 1023, 1123, and 1223 K by x-ray diffraction and sheet resistance measurements. Rapid thermal annealing at 1223 K resulted in a complete solid solution of (Ti,Ta)Si2. A tendency for its formation was also observed at the lower temperatures of 1023 and 1123 K on Si (111) during the time interval up to 200–300 s used in this work. Complete solid solution is expected to form at sufficiently high temperatures when annealing time is long enough. The reaction rate of (Ti,Ta)Si2 formation on Si (100) is appreciably lower than on Si (111).
Nonvolatile switchable Josephson junctions
Authors: Clinton T. W. , Johnson Mark
Published: July 2002

In a simple bilayer geometry, a thin, microstructured ferromagnetic film spans a superconducting strip. Locally strong magnetic fringe fields at the edge of the ferromagnet quench the superconductivity, creating a weak link. The strength of the weak link can be varied between on and off by controlling the orientation of the in-plane magnetization of the ferromagnet. The observation of Shapiro steps demonstrates the ac Josephson effect in the “quenched” state. In this switchable Josephson junction, energy is required only to change states, which are thereafter maintained in thermodynamic equilibrium.
Thermal deterioration mechanism of CoFeB/PdPtMn spin valves
Authors: Aoshima K. , Kanai H. , Kane J. , Miyajima T.
Published: July 2002

We clarified the mechanism of thermal deterioration that occurs in the CoFeB/PdPtMn spin-valve films at temperatures above 310 °C. Two kinds of CoFeB spin-valve film, Ta/NiFe/CoFeB/Cu/CoFeB/PdPtMn/Ta (B:0–3 at. %) and Ta/NiFe/CoFeB/Cu/CoFeB/FeMn/Ta (B:2 at. %), were prepared and annealed in a magnetic field. For the PdPtMn spin-valve samples, the magnetoresistance (MR) output decreased after annealing at a temperature above 330 °C and there was an increase in the interlayer coupling field of free and pin layers (Hin). There was no large change in the sheet resistance for annealing below 330 °C. For CoFe/PdPtMn(no B) spin valves, the MR output decreased during annealing at more than 310 °C and there was an increase of Hin. An addition of only 1 at. % of boron into the free and pinned layers is sufficient to obtain thermal stability. For the CoFeB/FeMn spin-valve samples, the MR output decreased for annealing above 280 °, the sheet resistance increased, and the Hin value did not change for annealing below 300 °C. The interface properties of PdPtMn spin valves were examined using high resolution scanning transmission electron microscopy with energy dispersive spectroscopy. We observed the connection between the free and the pinned layers through Cu pinholes and reduced Mn inter diffusion for the deteriorated sample. The increase of Hin can be explained by the pinholes of the Cu layer which result in a connection between the free and pinned layers. Boron in the free and pinned layer has the effect of blocking diffusion, thereby preventing a connection of the free and pinned layers.
Surface roughness effects on magnetoresistive and magnetic properties of NiFe thin films
Authors: Choe G. , Steinback M.
Published: July 2002

The effect of surface roughness on magnetoresistance (MR) and magnetic properties of Ta/NiFe films is investigated as a function of surface roughness and NiFe thickness. The surface roughness effects on the MR ratio, Δρ/ρ, of Ta/NiFe films can be separated into two different regimes. In regime I, Δρ/ρ of NiFe films linearly decreases with increasing roughness mostly due to the increase in resistivity of NiFe, while Δρ remains constant. In regime II at increased roughness, domain wall pinning is induced by surface roughness causing a degradation of the uniaxial anisotropy of NiFe and leading to the decrease in Δρ, resulting in the steeper MR ratio decrease. The change in MR properties of NiFe films with varying thicknesses can be correlated with the change in surface resistivity, ρs. Strongly textured NiFe(111) grown onto smooth substrate yields higher anisotropy and lower coercivity than those deposited onto rough substrates over a wide range of NiFe thicknesses.
Temperature-dependent magnetism in transition metal films observed by magnetic force microscopy and classical magnetometry
Authors: Soh Yeong-Ah , Aeppli G.
Published: July 2002

Pure Co and Pd–Co alloy films were prepared using electron beam evaporation and sputtering. They were characterized by superconducting quantum interference device magnetometry and magnetic force microscopy (MFM). The magnetic hysteresis curves for the Co film show that the magnetization direction is in the plane of the film. The magnetization data reveal a Curie temperature for the alloy of 370 K, and a preference for the moments to orient perpendicular to the film. MFM images of magnetic domains were obtained for 298 and 323 K. In the Pd–Co film, the phase contrast between the oppositely magnetized domains decreases by 35% as the temperature of the film is raised, even though the dominant length scale of the domains (about 0.25 μm) remains unchanged. A very different result was obtained for the pure Co film: Over the same temperature range the phase contrast in the magnetic images in the Co film decreased only by 10%. This indicates that the observed behavior in the Pd–Co film is mainly due to the much more rapid change in magnetic properties as a function of temperature. We also observe that, while less temperature dependent, the contrast of the magnetic domain images of the Co film is smaller than that of the Pd–Co, a result most likely due to the reorientation of the moments on going from the pure Co film to the Pd–Co film.
Stress reduction and bond stability during thermal annealing of tetrahedral amorphous carbon
Authors: Ferrari A. C. , Kleinsorge B. , Morrison N. A. , Hart A. , Stolojan V. , Robertson J.
Published: July 2002

A comprehensive study of the stress release and structural changes caused by postdeposition thermal annealing of tetrahedral amorphous carbon (ta-C) on Si has been carried out. Complete stress relief occurs at 600–700 °C and is accompanied by minimal structural modifications, as indicated by electron energy loss spectroscopy, Raman spectroscopy, and optical gap measurements. Further annealing in vacuum converts sp3 sites to sp2 with a drastic change occurring after 1100 °C. The field emitting behavior is substantially retained up to the complete stress relief, confirming that ta-C is a robust emitting material.
Dependence of the fundamental band gap of AlxGa1−xN on alloy composition and pressure
Authors: Shan W. , Ager J. W. , Yu K. M. , Walukiewicz W. , Haller E. E. , Martin M. C. , McKinney W. R. , Yang W.
Published: July 2002

Optical absorption studies were performed to investigate the dependence of the fundamental band gap of AlxGa1−xN epitaxial films on Al content and applied hydrostatic pressure. The results of absorption measurements performed at atmospheric pressure yielded the variation of the band-gap energy E(x)=3.43+1.44x+1.33x2 eV for the AlxGa1−xN system. Optical absorption edge associated with the direct Γ band gap shifts linearly towards higher energy under applied pressure. By examining the pressure dependence of the absorption edge in samples with different AlN mole fractions and taking into account the difference of compressibility between the epitaxial films and sapphire substrate, the pressure coefficients for the direct Γ band gaps of AlxGa1−xN were determined.
Investigation of nitric oxide and Ar annealed SiO2/SiC interfaces by x-ray photoelectron spectroscopy
Authors: Li Hui-feng , Dimitrijev Sima , Sweatman Denis , Harrison H. Barry , Tanner Philip , Feil Bill
Published: July 2002

Silicon dioxide (SiO2)/silicon carbide (SiC) structures annealed in nitric oxide (NO) and argon gas ambiences were investigated using x-ray photoelectron spectroscopy (XPS). The XPS depth profile analysis shows a nitrogen pileup of 1.6 at. % close to the NO annealed SiO2/SiC interface. The results of Si 2p, C 1s, O 1s, and N 1s core-level spectra are presented in detail to demonstrate significant differences between NO and Ar annealed samples. A SiO2/SiC interface with complex intermediate oxide/carbon states is found in the case of the Ar annealed sample, while the NO annealed SiO2/SiC interface is free of these compounds. The Si 2p spectrum of the Ar annealed sample is much broader than that of the NO annealed sample and can be fitted with three peaks compared with the two peaks in the NO annealed sample, indicating a more complex interface in the Ar annealed sample. Also the O 1s spectrum of the NO annealed samples is narrow and symmetrical and can be fitted with only one peak whereas that of the Ar annealed sample is broad and asymmetrical and is fitted with two peaks. It is evident that the Ar annealed sample contains some structural defects at the interface, which have been removed from the interface by NO annealing as shown by O 1s spectra. The C 1s spectra at the interface reveal the subtle difference between NO and Ar annealed samples. An additional peak representing the interface oxide/carbon species is observed in the Ar annealed sample. At the interface, the N 1s spectrum is symmetrical and can be fitted with one peak, representing the strong Si≡N bond. However, the N 1s and C 1s XPS spectra acquired in the bulk of the dielectric showed not only the Si≡N bond but also a trace amount of the N–C bond.
Residual stress, microstructure, and structure of tungsten thin films deposited by magnetron sputtering
Authors: Shen Y. G. , Mai Y. W. , Zhang Q. C. , McKenzie D. R. , McFall W. D. , McBride W. E.
Published: July 2002

The residual stress and structural properties of tungsten thin films prepared by magnetron sputtering as a function of sputtering-gas pressure are reported. The films were analyzed in situ by a cantilever beam technique, and ex situ by x-ray diffraction, cross-sectional transmission electron microscopy (TEM), x-ray photoelectron spectroscopy, electron energy-loss spectrometry, and energy-filtered electron diffraction. It is found that the residual stress, microstructure, and surface morphology are clearly correlated. The film stresses, determined in real time during the film formation, depend strongly on the argon pressure and change from highly compressive to highly tensile in a relatively narrow pressure range of 12–26 mTorr. For pressures exceeding ∼60 mTorr, the stress in the film is nearly zero. It is also found that the nonequilibrium A15 W structure is responsible for the observed tensile stress, whereas the stable bcc W or a mixture of bcc W and A15 W are in compression. Cross-sectional TEM evidence indicates that the compressively stressed films contain a dense microstructure without any columns, while the films having tensile stress have a very columnar microstructure. High sputtering-gas pressure conditions yield dendritic-like film growth, resulting in complete relaxation of the residual tensile stresses. Structural details of the A15 W and amorphous W phases were also investigated at the atomic level using energy-filtered electron diffraction with reduced radial distribution function G(r) analysis. By comparing the experimental and simulated G(r) distributions, the A15 W structure is determined to be composed of ordered and stacking faulted W3W structures and the amorphous W has a disordered structure of W3O. The effect of oxygen in stabilizing the A15 phase found is explained on the basis of structural and thermodynamic stability.
Electrodeposition of soft, high moment Co–Fe–Ni thin films
Authors: Liu Xiaomin , Zangari Giovanni , Shen Liyong
Published: July 2002

The electrodeposition of Co–Fe–Ni films from electrolytes without additives and their magnetic and structural properties in the as-deposited and vacuum annealed states are reported. Co52Fe26Ni22 alloy films exhibit a saturation magnetization Bs of 20 kG, coercivity Hc around 1 Oe, resistivity of 22 μΩ cm and saturation magnetostriction of about 10−6. Crystalline structure from bcc to fcc changes with increasing Ni content; films with best soft magnetic properties are obtained when bcc and fcc structures coexist. Varying electrolyte formulation or using pulse plating, the deposition parameters which yield optimum soft magnetic properties vary, but alloy composition and crystalline structure of optimum films are unchanged. dc plated films exhibit Ni enrichment at the interface in a region of about 1 μm thickness. Easy axis annealing in a vacuum of 10−7 Torr at 200–280 °C for 30 min decreases Hc and slightly decreases the anisotropy field. Annealing, however, does not change Bs, crystalline structure or lattice parameters, demonstrating stability of the film structure when subjected to typical device processing temperature cycles.
Length in statistical thermodynamics
Authors: Salamon Peter , Nulton James D. , Berry R. Stephen
Published: July 2002

A theorem is proved establishing the equality of ‘‘lengths’’ calculated with the entropy-based metric ∂2S/∂Xi∂Xj used by Ruppeiner and with a metric based on the information–theoretic entropy max(−Σpj ln pj).
Excited electronic states and optical spectra of ZnS and CdS crystallites in the ≊15 to 50 Å size range: Evolution from molecular to bulk semiconducting properties
Authors: Rossetti R. , Hull R. , Gibson J. M. , Brus L. E.
Published: July 2002

Very small ZnS and CdS crystallites are made and stabilized in aqueous and methanolic media without organic surfactants. Low temperature (−77 °C) synthesis in methanol produces the smallest crystallites, ≈30 Å diameter cubic CdS and <20 Å diameter cubic ZnS. The crystallites are characterized by transmission electron microscopy and in situ optical spectroscopy (λ≳200 nm). The crystallites are too small to exhibit bulk band gaps in their optical spectra. In the band gap region, the small crystallites show a higher energy absorption threshold with a resolved spectral feature (quantum size exciton peak), not present in the spectra of larger crystals. The far ultraviolet spectra are unaffected by size at present resolution. These results can be understood in terms of the crystallite molecular orbitals, and an elementary confined electron and hole model.
Density expansion (DEX) mixing rules: Thermodynamic modeling of supercritical extraction
Authors: Mansoori G. Ali , Ely James F.
Published: July 2002

Conformal solution theory and the density expansion expression of the radial distribution function of fluids are used to derive a set of mixing rules. The new mixing rules are composition, density, and temperature dependent. To test the new mixing rules they are used for thermodynamic modeling of supercritical extraction. Comparison of the result of calculation by the mixing rules with the van der Waals mixing rules indicates a profound improvement over the latter in prediction of properties of mixtures consisting of species with large molecular size and shape differences.
A network thermodynamic approach to Hill and King–Altman reaction–diffusion kinetics
Authors: Peusner Leonardo , Mikulecky D. C. , Bunow Barry , Caplan S. Roy
Published: July 2002

The major graphical approaches to kinetics—Hill diagrams and the King–Altman rules—can be considerably simplified using network techniques. An algorithmic procedure is developed and applied to several concrete examples which permits systems described in terms of first order transitions to be represented as connected networks. The utility of this representation is: (1) it permits powerful results from network thermodynamics and network theory to be applied to complex kinetic networks such as those representing biological dynamic systems. (2) The intrinsic role played by the thermodynamic principle of microscopic reversibility in networks containing cycles is emphasized: microscopic reversibility implies Onsager reciprocity for perturbed equilibrium systems. It now becomes clear that Onsager reciprocity is a topological property of systems to which thermodynamics is applicable. Reciprocity, or its lack can be rigorously established for macroscopic systems far outside the domain of Onsager’s original demonstration. These ideas are applied to produce a complete analysis of a complex active transport system previously studied by Hill diagrams.
Generalized moment expansion for observables of stochastic processes in dimensions d>1: Application to Mössbauer spectra of proteins
Authors: Nadler Walter , Schulten Klaus
Published: July 2002

The generalized moment expansion provides an effective algorithm for the approximation of the time dependence of observables that monitor stochastic processes. Up to now this method had been applied mainly to one-variable birth–death processes or to one-dimensional Fokker–Planck systems since in these cases analytical and numerical methods for the evaluation of the generalized moments were available. Here we demonstrate that numerical sparse matrix methods can be used to extend the range of application of the generalized moment expansion to higher dimensions. For this purpose we introduce a simple but general discretization scheme for Fokker–Planck operators of Smoluchowski type which is, for these special operators, superior to common numerical discretization schemes for differential operators. As an application we determine the Mössbauer absorption spectrum of a Brownian particle in certain two- and three-dimensional potentials. This serves as a model for the motion of the heme group in myoglobin.
The ionization potentials of F2: A comparison of multiconfigurational electron propagator (MCEP) with other large scale methods using the same basis set
Authors: Golab Joseph T. , Thies Brenda S. , Yeager Danny L. , Nichols Jeffrey A.
Published: July 2002

The multiconfigurational electron propagator technique (MCEP) gives reliable vertical ionization potentials (I.P.s) and electron affinities (E.A.s) for atoms and molecules, including open-shell and highly correlated systems. Shake-up and inner-valence I.P.s can be accurately obtained and characterized. In contrast, perturbative-type Green’s function (PTGF) approaches are useful for closed-shell systems with relatively little correlation. Perturbative-type Green’s functions cannot consistently reliably predict shake-up and inner-valence I.P.s. We have applied the MCEP method to F2 at 2.68 a.u. using 〈4s3p1d〉 and 〈5s4p2d〉 basis sets. In F2, the complete active space of all valence orbitals is small. Hence, reliable MCEP results should be obtained for all valence ionization processes. In addition, comparison calculations are given using other large scale techniques, i.e., ΔSCF, ΔMCSCF, PTGF, and Δ multireference CI using the same basis sets. The photoelectron spectrum of F2 below 30 eV is not well characterized experimentally except for the three lowest principal peaks, and apparently no reliable spectra have been reported for higher energies. We report and characterize several I.P.s in F2 0–60 eV, including several I.P.s that have not as yet been observed experimentally.
Orientation, alignment, and hyperfine effects on dissociation of diatomic molecules to open shell atoms
Authors: Band Yehuda B. , Freed Karl F. , Singer Sherwin J.
Published: July 2002

We demonstrate how magic angle detection of the circular polarization ratio can be used to probe the orientation of open shell atomic photofragments produced in photodissociation with circularly polarized light. The orientation provides new information concerning the dynamics of photodissociation which supplements that available from the alignment or from angular distribution measurements. The general theory of alignment and orientation is provided for photodissociation of diatomic molecules to open shell atoms and incorporates the asymptotic nonadiabatic interactions. Hyperfine depolarization factors are computed and tabulated for the first excited 2P3/2,1/2 states of a number of alkali atoms. They are used to transform between theoretical electronic orientation and alignment and experimentally observed values in the limit where the nuclear spins are unpolarized prior to photodissociation and where hyperfine transitions are not separated in the detection process. Orientation and alignment parameters are calculated for a variety of photodissociations in the high energy axial recoil limit.
Branching ratios for electronically excited oxygen atoms formed in the reaction of N+ with O2 at 300 K
Authors: Langford Andrew O. , Bierbaum Veronica M. , Leone Stephen R.
Published: July 2002

Absolute branching ratios for production of O(3P), O(1D), and O(1S) in the reaction of N+ with O2 are measured using the flowing afterglow/visible chemiluminescence technique. The O(1S) product is monitored by the O(1S)–O(1D) emission at 557.7 nm. The O(1D) product is monitored via sensitized fluorescence at 760 nm from O2(b 1Σ+g) formed by energy transfer from O(1D) to O2(X 3Σ−g). Absolute O(1D) and O(1S) yields of 70±30% and ≤0.1%, respectively, of the total atomic oxygen product are inferred by comparison to the known O(1S) and O2(1Σ+g) emission intensities from the reaction of Ar(3P) with O2. The low O(1S) yield is also obtained directly from the relative O(1S) and O2(1Σ+g) emission intensities from the title reaction. A qualitative reaction mechanism consistent with these observations is presented.
Relativistic and correlation effects in pseudopotential calculations for Br, I, HBr, HI, Br2, and I2
Authors: Schwerdtfeger P. , Szentpály L. v. , Vogel K. , Silberbach H. , Stoll H. , Preuss H.
Published: July 2002

Pseudopotentials are used for investigating the effects of relativity and correlation on atomic properties of Br and I, and on bond lengths and dissociation energies of the molecules HBr, HI, Br2, and I2. The pseudopotentials are adjusted to energies of Br6+ and I6+ on different levels of approximation: (i) to Hartree–Fock data for the nonrelativistic case, (ii) to Dirac–Fock data for the relativistic case, and (iii) to experimental energies for the relativistic case including correlation. At the highest level of approximation, atomic ionization energies, electron affinities, molecular dissociation energies, and bond lengths are obtained in good agreement with the experimental results.
The local approach: Electronic correlations in small hydrocarbon molecules
Authors: Stollhoff G. , Vasilopoulos P.
Published: July 2002

The local approach for the computation of electronic correlation energies is applied to ab initio calculations for C2H2, C2H4, and C2H6. Specific correlation patterns arise from the different kinds of bonds. They are investigated and explained. The accuracy of the local approach is improved so that experimental energies can be predicted with high precision. Finally we discuss the range of applicability of this calculational scheme.