bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic bonding of buried interfaces determined by soft x-ray emission spectroscopy
Authors: Perera R. C. C. , Zhang C. H. , Callcott T. A. , Ederer D. L.
Published: July 2002

A nondestructive technique to study the partial density of states of atoms at buried interfaces is presented. A high density of interface atoms has been mimicked by using a periodic multilayer structure. The silicon-carbon interface was used as a test case. Fifty alternating silicon-carbon layers were laid down, in which silicon layers nominally between 3 and 30 Å in thickness alternate with carbon layer pairs approximately 30 Å thick made by magnetron sputtering. The silicon L2,3 emission spectra was excited by monochromatized synchrotron radiation, and the s-like partial density of states of the silicon valence band was obtained. The spectrum of the thinnest silicon layer is similar to that of silicon as an impurity. When there are approximately two layers of silicon, the spectrum resembles that of silicon carbide, and as the layers become thicker, the spectrum is similar to that of amorphous silicon.
Electrical and optical properties of hydrogenated amorphous silicon-germanium (a-Si1−xGex:H) films prepared by reactive ion beam sputtering
Authors: Bhan Mohan Krishan , Malhotra L. K. , Kashyap Subhash C.
Published: July 2002

Thin films of hydrogenated amorphous silicon-germanium (a-Si1−xGex:H) alloys have been prepared by reactive ion beam sputtering of a composite target of silicon and germanium. The dependence of the deposition rate, conductivity-temperature variation, optical absorption coefficient, refractive index, imaginary part of the dielectric constant, hydrogen content, and infrared (IR) absorption spectra on germanium content (x) are reported and analyzed. For a typical composition—a-Si28Ge72:H (x=0.72), the effect of beam voltage, H2:Ar flow ratio, and substrate temperature on the material properties have also been investigated. For the films prepared with increasing x, the expected behavior of a decrease in both hydrogen content and band gap and an increase in the electrical conductivity have been observed. The films prepared at x>0.80 are found to be more homogeneous than the films deposited at 0.0<x<0.80, due to the compositional disorder introduced by the random mixing of Si and Ge atoms in the a-Si1−xGex:H network in the latter case. The a-Si28Ge72:H films exhibiting minimum conductivity (1.7×10−7 Ω−1 cm−1) have been obtained for an H2:Ar flow ratio of 10:1 and a beam voltage and substrate temperature of 1500 V and 300 °C, respectively. These films contain a hydrogen concentration of 10.2 at. % and show an optical band gap of 1.25 eV. The IR studies have shown that a-Si28Ge72:H films prepared both at low beam voltages and at low substrate temperatures show the unusual preferential attachment of hydrogen to Ge rather than to Si.
Theory of multistage intense ion-beam acceleration
Authors: Slutz S. A. , Desjarlais M. P.
Published: July 2002

We present an analytic theory for magnetically insulated, multistage acceleration of high-intensity ion beams, where the diamagnetic effect due to electron flow is important. Our theory is an extension of the single-stage diode theory developed by Desjarlais [Phys. Rev. Lett. 59, 2295 (1987)], based on a self-consistent calculation of the virtual cathode position, which has been successful in modeling Applied-B ion diode experiments on several accelerators. The new theory incorporates a finite injection energy qW for the beam ions. We have found a critical voltage V1(W) that corresponds to V* of the single-stage theory. As the voltage approaches V1, unlimited beam-current density can penetrate the gap without the formation of a virtual anode because the dynamic gap goes to zero. At voltages lower than V1, a sufficiently large injection current will cause the formation of a virtual anode in response to the large beam space charge. Furthermore, we have found that unlimited beam current can penetrate an accelerating gap operated above a second critical voltage V2(W). At voltages below V2, there is a maximum steady-state current that can be transmitted through the gap. The critical voltage V2 is smaller than V1 and is unique to the multistage theory. If fluctuations allow electron transport across magnetic field lines so that any virtual anode is neutralized, V2 goes to zero for all beam injection energies. This effect can be used to test the importance of field fluctuations on the electron dynamics in magnetically insulated ion acceleration gaps.
The effective thermal conductivity of composites reinforced by coated cylindrically orthotropic fibers
Authors: Benveniste Y. , Chen T. , Dvorak G. J.
Published: July 2002

The present paper is concerned with coated-fiber composites in which the fibers possess cylindrical orthotropy and may have an arbitrary orientation distribution. A micromechanics model is developed which predicts the effective thermal conductivity and estimates the local fields of such composites which may be subjected to uniform heat fluxes on its boundary. The micromechanics model is based on the Mori–Tanaka mean-field concept [T. Mori and K. Tanaka, Acta Metall. 21, 571 (1973)] and provides explicit expressions for the effective conductivity of the considered composite aggregate which is highly complicated. The analysis shows that special care is needed in formulating an effective theory of composites with constituents possessing curvilinear anisotropy.
Antenna radiation in the presence of a chiral sphere
Authors: Engheta Nader , Kowarz Marek W.
Published: July 2002

The radiation emitted by electromagnetic sources placed both inside and outside of a homogeneous sphere of chiral media is studied using an exact formulation. For both cases, dyadic Green’s functions are found in terms of spherical vector wave functions. The radiated fields and radiation resistance are examined for a dipole located at the center of the chiral sphere. For this case, it is shown that by choosing the sphere’s size and material parameters properly, purely circular polarization can be achieved in the far field. It is also demonstrated that the radiation resistance of the dipole can be increased.
Consequences of simultaneous exposure of inorganic solids to excimer laser light and an electron beam
Authors: Dickinson J. T. , Langford S. C. , Jensen L. C. , Eschbach P. A. , Pederson L. R. , Baer D. R.
Published: July 2002

In previous work we examined the changes in surface topography of sodium trisilicate glass (Na2O⋅3SiO2) with exposure to pulsed 248-nm excimer laser light at fluences of 2.6–5 J/cm2 (per laser pulse), as well as the character of the products emitted from the glass surface (e.g., ± ions, electrons, ground state and excited neutral atoms and molecules). At fluences above a threshold, ablative etching occurs only after a number of preliminary laser pulses. In this work, we show a dramatic synergism in the ablation process by simultaneous bombardment of the glass surface with 0.5–2 keV electrons and laser pulses. Extensive etching can be initiated immediately and sustained indefinitely at subthreshold fluences. Reflection electron energy loss spectroscopy performed on electron-bombarded surfaces shows band gap states growing with exposure. We propose that surface and near-surface defects produced by inelastic scattering of the electron beam provide single-photon absorption centers, facilitating etching at subthreshold fluences. The potential for single-photon driven etching/ablation of other wide band gap dielectric materials is also discussed. These studies also support the hypothesis that etching and damage of such surfaces can occur after prolonged exposure to laser irradiation alone due to an accumulation of absorption centers.
Laser-induced-fluorescence detection of SO and SO2 in SF6/O2 plasma-etching discharges
Authors: Greenberg K. E. , Hargis P. J.
Published: July 2002

Laser-induced-fluorescence spectroscopy was used to detect SO and SO2 molecules in the active volume of 13.56-MHz discharges through mixtures of sulfur hexafluoride and oxygen. A KrF laser was used to excite the SO B3Σ state and the SO2B̃1B1 and C̃1B2 states. 235-nm light was used to directly monitor the ground-state densities of these molecules. For a 100-W discharge through a 12.5% oxygen mixture (42 sccm SF6, 6 sccm O2, 340-mTorr total pressure), the SO and SO2 densities were determined to be ∼2×1014/cm3 and ∼4×1013/cm3, respectively. For the same discharge conditions, the SO vibrational temperature was ∼880 K. The SO vibrational temperature was independent of oxygen content in our experiments.
Analysis of characteristic thermal transit times for time-resolved infrared radiometry studies of multilayered coatings
Authors: Aamodt L. C. , Spicer J. W. Maclachlan , Murphy J. C.
Published: July 2002

Time-resolved infrared radiometry (TRIR) has been shown to be a valuable thermal characterization technique for the nondestructive evaluation of layered coatings. In this method an external step heating pulse is applied to the coating system and the resulting change in surface temperature is monitored as a function of time. Characteristic times and amplitudes can be ascribed to internal thermal reflections that return heat to the coating surface. A full description of the time dependence of the coating surface temperature can be obtained from these quantities. This paper describes a method for determining these characteristic times and amplitudes and shows their relationship to internal heat-flow patterns. Uses and limitations of the method are discussed, and comparison of the theory with experimental TRIR results is presented.
Cell gap measurement of filled twisted nematic liquid crystal displays by a phase compensation method
Authors: Lien A. , Takano H.
Published: July 2002

Two configurations of the phase compensation method are presented to measure the cell gap of a filled twisted nematic liquid crystal display (TN-LCD). The twist angle of the TN-LCD can be any value between 0° and 360° and the pretilt angle can be any practical value. The cell gap measured by this method is the true TN cell gap, namely the thickness of the LC layer which is primarily responsible for the TN optics. Due to this reason, the phase compensation method can also be applied to measure the cell gap of a color TN cell. The measurement uncertainty of this method is within ±1%.
Sheath voltage ratio for asymmetric rf discharges
Authors: Alves M. V. , Lieberman M. A. , Vahedi V. , Birdsall C. K.
Published: July 2002

Spherical and cylindrical many-particle models are used to simulate rf (radio frequency) discharges in which the rf powered and the grounded electrodes have different areas. This asymmetry determines the magnitude of the average plasma-to-electrode voltage Va (the ion bombarding energy) at the smaller powered electrode, which is a critical process parameter. A collisionless uniform ionization discharge model predicts that the voltage ratio Va/Vb scales as the fourth power of the electrode area ratio Ab/Aa, where Vb is the potential drop at the other electrode. However, measurements indicate a much weaker dependence of Va/Vb on the area ratio, which is also observed in our simulations. Over a limited range of area ratios it was found that the power dependence was close to one, in agreement with a local ionization discharge model. The simulation codes used are PDC1 (plasma device cylindrical one-dimensional) and PDS1 (plasma device spherical one-dimensional).
Plasma-generator-induced effects on the dynamics of a negative-ion surface conversion source
Authors: van Os C. F. A. , Leung K. N. , Kunkel W. B.
Published: July 2002

The production of H− ions at a barium, magnesium, strontium, molybdenum, and lanthanum-hexaboride converter immersed in a hydrogen plasma have been investigated for pulsed and dc discharges. The scaling of the negative-ion output with incident positive-ion current on the converter depends greatly on the actual geometry and type of plasma generator. A less than linear scaling is observed in a multicusp system using filaments or rf to generate the discharge. A more than linear scaling is obtained in a configuration where the filament and the converter are separated by a permanent-magnet filter. For a rf plasma generator, there is a pronounced optimum in the relative position of antenna and converter.
Magnetic properties and microstructures of the Nd-Fe-B magnet powder produced by hydrogen treatment
Authors: Nakayama R. , Takeshita T. , Itakura M. , Kuwano N. , Oki K.
Published: July 2002

The hydrogen treatment of the Nd-Fe-B alloy ingots was found to produce magnet powders with good magnetic properties. Typical magnetic properties of these powders are as follows; 4πIs = 9.5 kG, Br = 7.7 kG, iHc = 9.4 kOe, and (BH)max = 12.2 MGOe. Microstructural studies of these powders showed that they are made of fine crystalline grains of ∼0.3 μm diameter and that these crystalline grains in the individual magnet powder are not necessarily enclosed with boundary phase(s), which is quite different from previously known Nd-Fe-B magnets, i.e., the sintered magnet (the nucleation type magnet) or the amorphous ribbon magnet (the pinning type magnet). It is also noted that the size of these crystalline grains is comparable to that of the single magnetic domain of the tetragonal Nd2Fe14B intermetallic compound and the coercive force of these powders appears to be related to their fine crystalline grain size.
Deduction of central plasma parameters from line-of-sight averaged spectroscopic observations
Authors: Zastrow K.-D. , Morsi H. W. , Danielsson M. , von Hellermann M. G. , Källne E. , König R. , Mandl W. , Summers H. P.
Published: July 2002

A method is presented to deduce central ion temperature and toroidal rotation velocity from line-of-sight averaged x-ray spectra from hot plasmas. The analysis is based on atomic data for the processes that give rise to x-ray spectral lines. Combined with measured electron temperature and density profiles a synthetic spectrum is calculated. The fit of this synthetic spectrum to the observed one gives a new level of accuracy to line-of-sight integrated observations in a considerably extended range of ion temperatures and toroidal rotation velocities. The choice of model for radial profiles for ion temperature and toroidal rotation velocity is shown not to be critical. The concentration of the emitting impurity is deduced from the total line intensity, making use of the absolute calibration of the detector sensitivity. The effective plasma charge Zeff is derived from the absolute level of the continuum radiation. These measurements are based on atomic data for x-ray line and continuum radiation and measured electron temperature and density profiles. The results for ion temperature and toroidal rotation velocity obtained by this analysis are compared with those from visible charge exchange spectroscopy. The observed visible lines are shifted in wavelength, and their width is reduced, due to the velocity dependence of the cross section for the charge transfer from the neutral beam particles to the observed impurities. The theoretically predicted magnitude of these effects is verified. When the results from visible charge exchange spectroscopy are corrected for the cross-section effects, excellent agreement of central ion temperature and rotation velocity with the results of this new analysis is obtained.
Shock-induced martensitic transformation of highly oriented graphite to diamond
Authors: Erskine David J. , Nellis William J.
Published: July 2002

Shock-wave profiles of highly ordered pyrolytic graphite shocked normal to the basal plane of the graphite crystal structure have been measured. For graphite with sufficient orientational order a martensitic transformation to a diamond-like phase is observed with a transition onset pressure 19.6±0.7 GPa, the stability limit of the graphite structure under shock compression. The minimum overpressure required for the transformation is not more than 6 GPa and the two-wave structure of the transition is overdriven to a single wave above 40 GPa.
Deep levels in γ-ray irradiated n- and p-type hydrogen-grown float-zoned silicon
Authors: Yuan M. H. , Peng D. C. , Peng Q. Z. , Zhang Y. H. , Li J. Q. , Qin G. G.
Published: July 2002

The generation rates and annealing behavior of the irradiation defects in n- and p-type hydrogen-grown float-zoned silicon (irradiated with γ rays from 60Co) have been studied by the deep-level transient spectroscopy technique and compared with those of irradiated argon-grown float-zoned silicon. Assuming the generation rate of the irradiation defects created by γ rays in argon-grown float-zoned silicon is 1, then the generation rates of the A center, divacancy, and phosphorus vacancy in n-type hydrogen-grown float-zoned silicon are 0.23, 0.78, and 0.19, respectively, while the generation rates of the divacancy and H(0.37 eV) in p-type hydrogen-grown silicon are 0.79 and 0.10, respectively. Due to the existence of hydrogen, the generation rate reduction of the major irradiation defects in γ-ray irradiated silicon is more pronounced than that in 1-MeV electron irradiated silicon. Three hydrogen-related defects, H(0.10 eV), H(0.29 eV), and H(0.56 eV), were seen in γ-ray irradiated hydrogen-grown float-zoned silicon, among which H(0.10 eV) and H(0.56 eV) were reported by us to exist in electron irradiated hydrogen-grown float-zoned silicon, while H(0.29 eV) is reported for the first time. The convergence effect of annealing temperatures for the irradiation defects was observed. That is, the annealing temperatures at which the irradiation defects diminish are almost the same for most irradiation defects, similar to that in the case of electron irradiation, was observed, showing that this effect is characteristic of the hydrogen behavior in silicon, and irrelevent to the type of irradiation.
Nucleation and deposition of W on GaAs by excimer laser-assisted chemical vapor deposition
Authors: van Maaren A. J. P. , Sinke W. C. , Flicstein J.
Published: July 2002

We have investigated laser-assisted chemical vapor deposition of W on GaAs to get insight into the deposition mechanism involved. Depositions were performed with an ArF excimer laser aligned parallel to the substrate and with WF6 and H2 as precursor and reducing agents, respectively. Samples were analyzed by Rutherford backscattering spectrometry and the sheet resistance was measured with a four-point probe. We investigated the temperature dependence of the deposition rate up to 400 °C for both laser-assisted and thermal deposition. At 350 °C and a laser repetition frequency of 20 Hz, we observed a deposition rate for laser-assisted deposition of 5.3 Å/s, whereas without laser irradiation no deposition occurred. At this temperature, the film resistivity was 11 μΩ cm. For deposition temperatures above 350 °C, anomalous behavior was observed in both deposition rate and film resistivity. Experiments in which W coverage was measured as a function of laser pulse repetition frequency and as a function of time revealed that laser-assisted deposition at 300 °C consists of a two-step process. The first phase consists of exclusively laser-induced deposition at a rate of 3.3×1015 atoms/cm2/min/Hz, until a W coverage of approximately 15×1015 atoms/cm2 is reached. Laser-independent growth was not observed probably through insufficient nucleation. The second phase consists of combined laser-induced and laser-independent deposition on the initial W layer. The laser-induced growth rate was 3.2×1015 atoms/cm2/min/Hz for repetition frequencies in excess of 5 Hz. At 20 pulses per second, laser-induced deposition formed 47% of the total deposition rate. Below 5 Hz, the laser-induced rate was higher than above 5 Hz but nonlinear with pulse frequency. The growth rate by laser-independent deposition was 72×1015 atoms/cm2/min at 300 °C, which corresponds well with theory for thermally activated deposition of W on W.
Measurement and modeling of boron diffusion in Si and strained Si1−xGex epitaxial layers during rapid thermal annealing
Authors: Loechelt G. H. , Tam G. , Steele J. W. , Knoch L. K. , Klein K. M. , Watanabe J. K. , Christiansen J. W.
Published: July 2002

Boron concentration profiles in rapid thermally annealed Si and strained Si1−xGex in situ doped, epitaxial layers were measured using secondary-ion-mass spectroscopy. Comparison of the Si1−xGex samples to the Si samples after rapid thermal annealing revealed a retarded B diffusivity inside the strained Si1−xGex layers. A simple empirical expression for the B retardation, which depended linearly on the Ge concentration, was developed and incorporated into a diffusion model for dopants in heterostructures. This model accurately simulated the measured B concentration profiles over a wide range of Ge fractions (0%–10%), B peak concentrations (2×1018–3×1019cm−3), and rapid thermal annealing conditions (900–1025 °C for 20–30 s).
An optical method for evaluation of the net acceptor concentration in p-type ZnSe
Authors: Hu B. , Yin A. , Karczewski G. , Luo H. , Short S. W. , Samarth N. , Dobrowolska M. , Furdyna J. K.
Published: July 2002

We report photoluminescence (PL) results obtained on p-type ZnSe epilayers grown by molecular beam epitaxy. As an acceptor dopant, we used an active nitrogen beam produced by a free radical nitrogen source. On the basis of a detailed analysis of PL data we propose a simple semiquantitative method for a quick and contactless evaluation of the net acceptor concentration in p-type ZnSe. In particular, we show that the intensity ratio of the donor–acceptor pair (DAP) emission to the acceptor-bound exciton (I1) emission strongly depends on both the excitation power and the quality of the sample, and because of that it cannot by itself be regarded as a good measure of the net acceptor concentration. On the other hand, the intensity of the DAP emission under saturation excitation shows a simple direct proportionality to the net acceptor concentration, thus providing a reliable tool for determining the relative doping level in p-type ZnSe films.
Schottky barrier heights of Pt and Ir silicides formed on Si/SiGe measured by internal photoemission
Authors: Jimenez J. R. , Xiao X. , Sturm J. C. , Pellegrini P. W. , Weeks M. M.
Published: July 2002

Lowered-barrier-height silicide Schottky diodes are desirable for obtaining longer cutoff-wave- length Si-based infrared detectors. Silicide Schottky diodes have been fabricated by the reaction of evaporated Pt and Ir films on p-Si1−xGex alloys with a thin Si capping layer. The onset of metal-SiGe reactions was controlled by the deposited metal thickness. Internal photoemission measurements were made and the barrier heights were obtained from these. Pt-SiGe and Ir-SiGe reacted diodes have barrier heights of ∼0.27 and ∼0.31 eV, respectively, higher than typical values of 0.22 and 0.12 eV for the corresponding silicide/p-Si diodes. Their emission constants are also lower and more voltage dependent than silicide/Si diodes. PtSi/Si/SiGe diodes, on the other hand, have lower barrier heights (∼0.15 eV) than the PtSi/Si barrier height. The barrier height shifts in such silicide/Si/SiGe diodes are interpreted by accounting for tunneling through the unconsumed Si layer. This is done analytically using a simple model based on the Cohen, Vilms, and Archer (unpublished) modification to the Fowler equation, and leads to an extracted barrier height, that is, the Si barrier height reduced by the Si/SiGe band offset.
Annealing effects on the electrical properties and microscopic structure of semi-insulating polycrystalline silicon films
Authors: Cho Wonju , Takeuchi Yuji , Kuwano Hiroshi
Published: July 2002

The dependence of the electrical properties and microscopic structure of semi-insulating polycrystalline silicon (SIPOS) films prepared by low-pressure chemical-vapor deposition on annealing temperature has been investigated, and the electrical conduction mechanisms have been clarified. As-deposited SIPOS films were crystallized from an amorphous matrix by annealing, which changes the structure to polycrystalline films at annealing temperatures above 1100 °C. The electrical conduction mechanisms in films annealed at temperatures above 1000 °C can be explained by thermionic emission theory over a wide range of temperatures from high temperatures to those below room temperature, and by hopping conduction through the trap states at the grain boundaries at sufficiently low temperatures. For films annealed below 1000 °C, the electrical conduction mechanisms can be explained by conduction in the extended states over the high-temperature range, and by hopping conduction through the localized states below room temperature. Undoped and lightly doped SIPOS films exhibit p-type properties for annealing temperature above 1000 °C, but display an n-type property when annealed at temperatures below 1000 °C. The dependence of the density of localized states and the band-gap width on annealing temperature were also investigated.
Ba-ferrite thin-film media for high-density longitudinal recording (invited)
Authors: Hylton T. L. , Parker M. A. , Ullah M. , Coffey K. R. , Umphress R. , Howard J. K.
Published: July 2002

Ba-ferrite is an attractive candidate for future high-density recording media because of its large coercivity, corrosion resistance, high hardness, and durability. In contrast to most recent work on Ba-ferrite thin film media, polycrystalline films with large in-plane remanence for longitudinal recording are emphasized. Films are prepared on a variety of substrates by on-axis sputtering at low temperatures from stoichiometric targets followed by a postdeposition anneal in a rapid thermal processor at ∼850 °C to induce crystallization. Structural and chemical properties have been investigated by force microscopy, Rutherford backscattering, and other means. Stoichiometric films have large 5000 Å grains that are unsuitable for high-density recording. However, grain sizes as small as 200 Å have been produced by doping with small amounts of Cr2O3 and other additives. Compositional changes also influence crystallite orientation. Exceptional durability has been observed on disks without overcoats, presumably due to a fine-scale texturing that occurs during crystallization. Coercivities are greater than 4000 Oe even in small grain films. The effect of grain size on coercivity and signal-to-noise ratio of several Ba-ferrite disks is discussed. It is argued that the grains are magnetically decoupled based on the magnetic properties of these films.
Variations of the local anisotropy versus Tb content in amorphous CoZrTb thin films
Authors: Suran G. , Ouahmane H. , Iglesias I. , Rivas M. , Corrales J. A. , Contreras M. C.
Published: July 2002

The magnetic properties of rf sputtered amorphous (Co93Zr7)100−xTbx thin films were investigated as a function of Tb content for 0≤x≤2 by transverse biased initial susceptibility measurements. The deposition was performed in a magnetic field. This films exhibit a very well-defined in-plane anisotropy with negligible long-range fluctuations and a small coercive field Hc, in accordance with the experimental data. The overall variations of the transverse susceptibility are related to the ripple, and the variations of the structure factor S with composition was measured. The local anisotropy Kloc, deduced from S, increases with the increasing amount of Tb in the layers. The results are discussed in terms of the single-ion anisotropy of the rare earth.
Raman spectroscopic study of order–disorder in lead magnesium niobate
Authors: Idink Heike , White William B.
Published: July 2002

Precision Raman spectra have been measured on lead magnesium niobate (PMN)–lead titanate (PT) solid solution at various temperatures. The multiband Raman spectra are inconsistent with Pm3m symmetry but can be accommodated within a model of ordered nanodomains of rhombohedral symmetry. Raman band widths are large and nearly independent of temperature indicating a static structural disorder rather than a dynamic dipolar disorder. Abrupt changes in wave number and widths of certain bending modes at 200 K are interpreted as evidence for a reversible phase transition at this temperature. All of the spectroscopic properties are largely independent of the titanium concentration in the PMN–PT solid solutions.
The magnetization of bulk α″Fe16N2 (invited)
Authors: Coey J. M. D.
Published: July 2002

Evidence for giant magnetic moments in the metastable nitride α″Fe16N2 is reviewed, with reference to the related stable nitride γ′Fe4N and αFe. It is concluded from a survey of electronic structure calculations, Mössbauer spectra, and magnetization measurements on bulk multiphase samples that the average iron moment in α″Fe16N2 at T∼0 lies in the range 2.3–2.6 μB. A significantly larger moment of about 2.9 μB appears on the 4d sites which have no nitrogen neighbors.
New magnetic material based on SmCo4B
Authors: Ido Hideaki , Nashima Osamu , Takahashi Takehiro , Oda Kiwamu , Sugiyama Kiyohiro
Published: July 2002

The compound SmCo4B is a hard magnetic material; however, the saturation moment and the Curie temperature need to be increased for the compound to be a candidate for a permanent magnetic material. For this purpose, Fe substitution for the Co and Pr substitution for the Sm of SmCo4B have been attempted. In addition, the nitrogenation of SmCo2Fe2 is also attempted. The magnetic characteristics of SmCo2Fe2B have been investigated in a pulsed high magnetic field up to 350 kOe, and the data are compared with those of other related compounds investigated in this work. The compound with more Fe content, SmCo1.7Fe2.3B, has been successfully made, and TC=794 K and saturation moment Ms=81.4 emu/g at 300 K have been obtained. It is found that the Fe substitution does not decrease the uniaxial magnetic anisotropy. Heat treatments in nitrogen gas at 500 °C of SmCo2Fe2B have brought about significant increases of magnetization; however, the increase of TC is only about 20 K and the crystal volume is unchanged to within experimental uncertainty.
Output characteristics of short-channel polycrystalline silicon thin-film transistors
Authors: Dimitriadis C. A. , Tassis D. H.
Published: July 2002

An analytical model for the turn-on current-voltage characteristics of polycrystalline silicon thin-film transistors is presented. The model is based on the drift diffusion-thermionic emission conduction mechanisms and on a continuous distribution in the energy gap of the traps localized at the grain boundaries. The trap distribution and the device parameters involved in the model are determined by fitting the calculated on-state current versus gate voltage curve to the measured one in the linear region. At large drain voltage, the barrier height at the grain boundary becomes asymmetric and the injection of carriers from the lowered barrier side of the boundary is increased resulting in an exponential increase of the drain current with the drain voltage. Using the parameters obtained from the data in the linear region, the output characteristics are calculated. The good agreement between calculated results and experimental data at room temperature and at higher temperatures demonstrates the validity of the proposed current-voltage model.
Effect of substrate preparation on smectic liquid-crystal alignment. III. The significance of thermal history
Authors: Smela Elisabeth , Martínez-Miranda Luz J.
Published: July 2002

X-ray-diffraction measurements were done on octylcyanobiphenyl (8CB) films on a glass grating with a 9 μm period. The films were heated from the smectic to the isotropic phase using three different methods. It was found that the cooling rate, which determines the time the molecules are able to respond to the influence of the air and substrate interfaces, plays a critical role in the alignment of the liquid-crystal molecules. In all cases, the surface tension was strong enough, and the energy cost of an interface with a second alignment was high enough, to produce a single homeotropic alignment in films as thick as 15 μm.
Photoluminescence studies in CuAlSe2 epilayers grown by low-pressure metalorganic chemical-vapor deposition
Authors: Chichibu S. , Shirakata S. , Isomura S. , Harada Y. , Uchida M. , Matsumoto S. , Higuchi H.
Published: July 2002

Low-temperature photoluminescence (PL) spectra were investigated for CuAlSe2 epilayers grown on GaAs(001) substrates by means of low-pressure metalorganic chemical-vapor deposition. PL properties were studied with relation to metalorganic precursors used for the growth. High-quality undoped epilayers exhibited PL peaks related to a free exciton (2.739 eV) and a bound exciton (2.677 eV). The other undoped epilayers exhibited PL bands at 2.3, 2.4, and 2.5 eV originating from donor-acceptor (D-A) pair recombinations. Some of them were found to have a common activation energy for the thermal quenching of 50±10 meV. The PL spectrum changed drastically by impurity doping. Intense green emissions at 2.51 and 2.43 eV were observed in Zn and Mg-doped epilayers, respectively, which were interpreted as D-A pair recombinations based on the dependencies of the PL spectra on excitation intensity, decay time, and temperature. The donor and the acceptor activation energies (ED and EA) were estimated to be 110 and 230 meV, respectively, for the Zn-related D-A pair emission at 2.51 eV. Similarly, ED and EA for CuAlSe2:Mg were estimated to be 140±10 and 270±10 meV, respectively. Furthermore, D-A pair recombinations between 2.3 and 2.5 eV for CuAlSe2:I were studied. CuAlSe2 was proven to be a promising material for short-wavelength visible-light-emitting devices.
The effects of thermal nitridation and reoxidation on the interfacial stress and structure of silicon dioxide gate dielectrics
Authors: Yount J. T. , Lenahan P. M. , Wyatt P. W.
Published: July 2002

Electron spin resonance spectroscopy is used to study the effects of thermal NH3 nitridation and subsequent reoxidation on the structure of the dielectric/silicon interface. This is accomplished by study of the Pb center, an interfacial point defect. The values of g⊥ in Pb spectra observed in the nitrided oxide and reoxidized nitrided oxide systems differ from Si/SiO2 systems, suggesting that the average value of the tensile stress in the silicon substrate increases upon nitridation while reoxidation acts to return the interfacial stress to prenitridation levels. The implications of these structural changes upon device performance are discussed.
Phosphorescence from tungsten clusters during laser-assisted chemical-vapor deposition of tungsten
Authors: Heszler P. , Mogyorósi P. , Carlsson J. O.
Published: July 2002

Phosphorescent light emission was investigated upon irradiation of WF6/H2/noble gas (Ar, Kr, Ne, Xe) mixtures by ArF excimer laser. The continuous, broadband phosphorescence emission originates from excited tungsten clusters. The phosphorescence ceased with increased Xe and H2 concentrations. Xe inhibits the formation of tungsten clusters and H2 quenches the phosphorescence. The H2 quenching rate and the unperturbed phosphorescence lifetime were determined to be 2.8×104 mbar−1 s−1 and 23 μs, respectively. The intensity dependence of phosphorescence on the WF6 partial pressure shows a rising part, then a declining part, and finally an increasing part again. It was shown that the declining part can be interpreted by scattering of the emitted phosphorescence. The phosphorescence intensity was dependent on the noble gas/H2 concentration ratio. This effect was explained in terms of the thermal conductivity of the WF6/H2/noble gas mixture, which influences the lifetime of the activated subfluorides and, thus, the cluster formation and growth rate. The intensity of the laser beam had a nonlinear effect on the phosphorescent light intensity due to the nonlinearly coupled photolytical steps of WF6 and of the subfluorides.
Two-dimensional particle simulation of plasma expansion between plane parallel electrodes
Authors: Patel Kartik , Mago V. K.
Published: July 2002

We simulate in two dimensions the expansion of a plasma between biased plane parallel electrodes using the particle-in-cell method. Such a plasma is frequently created in many experiments by the interaction of a pulsed laser with atomic vapor or gas stream. We describe the motion of the electrons and ions and reproduce the experimentally observed bulk drift of the plasma toward the high potential electrode. This is explained in terms of the retrograde movement of the ion sheath boundary on one side of the plasma accompanied by ambipolar diffusion on the other. We estimate the reduced ion density in the plasma by observing oscillations in the space-charge-limited current. By calculating the plasma decay time constant and ambipolar diffusion coefficient, we note that computer simulation can generate data which is difficult to measure experimentally, and not possible to calculate analytically.
Manipulation of nucleation by growth rate modulation
Authors: Larsson Mats I. , Ni Wei-Xin , Hansson Göran V.
Published: July 2002

We report on manipulation of nucleation by means of periodic modulation of the growth rate R, both during molecular beam epitaxial growth of Si on Si(111) and using a simulated bilayer structure. We have found that the bilayer-by-bilayer growth can be improved and that the surface roughness can be reduced by applying the R modulation synchronized with the deposition of a multiple of full bilayers. For nonsynchronized growth the phase of reflection high-energy electron diffraction (RHEED) intensity oscillations could be shifted relative to the bilayer deposition period and a beating phenomenon of the RHEED oscillations was observed, which can be attributed to the superimposed effects of the R modulation and the regular bilayer-by-bilayer growth. We have used the phenomenological correspondence between the surface step density and the RHEED oscillations as a basis to discuss the growth. By employing a kinetic solid-on-solid Monte Carlo model without vacancies and overhangs all significant experimental features could be simulated.
Growth of InAsxP1−x/InP multi-quantum well structures by solid source molecular beam epitaxy
Authors: David J. P. R. , Hopkinson M. , Stavrinou P. N. , Haywood S. K.
Published: July 2002

We report on the growth of high quality pseudomorphic InAsxP1−x/InP single and multi-quantum well (MQW) structures by solid source molecular beam epitaxy, using valved cracker sources to accurately control the group V ratio. A series of MQW’s grown in a p-i-n configuration covering the 1.0–1.30 μm wavelength showed intense narrow room temperature photoluminescence (PL) and extremely narrow 10 K PL linewidths (≤8 meV). Devices fabricated from these structures exhibited low reverse dark currents (<10 nA at 25 V) which increased as the strain within the structure increased. These results confirm the suitability of InAsxP1−x/InP for optoelectronic devices in this wavelength range.
Predominant parameters in the shock-induced transition from graphite to diamond
Authors: Hirai Hisako , Kukino Satoru , Kondo Ken-ichi
Published: July 2002

Predominant parameters in the shock-induced transition from graphite materials to diamond were examined in the present study by using quenching and powder methods under pressures of 50–60 GPa and 80–90 GPa, respectively, in the temperature range from 750 to 3500 K. Effects of the material parameters of the starting graphite—i.e., crystallite size and crystallinity—were distinguished from effects of the experimental parameters by standardizing the shock conditions for the materials examined. In addition, only a few graphite materials possessing wholly identical parameters throughout were selected as starting materials. Detailed characterization by transmission electron microscopy and electron energy-loss spectroscopy revealed that the transition ratios of diamond and the degrees of graphitization varied with those parameters. The various changes observed were plotted in terms of pressure, temperature, and material-parameter axes to create a tentative pressure-temperature-material diagram representing the behavior of the graphite materials under shock compression. The material parameters of the initial graphite structure primarily affected the diamond transition: The lower the crystallinity and crystallite size, the more easily the diamond transition occurred in the case of a reconstructive mechanism. Smaller crystallite size and lower crystallinity elevated the initial energy states of the graphite materials because of surface energy and strain energy, making it relatively easier to transcend the activation-energy barrier to diamond transition. Temperature was fairly effective and pressure ineffective in regard to the diamond transition, a result consistent with the belief that the transition is a diffusion-controlled process. Moreover, differentiation of the transition pathways, the diamond transition, and graphitization fit a concept of alternative metastable behavior; graphitization was more favored kinetically than diamond transition under the shock conditions examined.
Investigation of electrical properties and stability of Schottky contacts on (NH4)2Sx-treated n- and p-type In0.5Ga0.5P
Authors: Kwon S. D. , Kwon Ho Ki , Choe Byung-Doo , Lim H. , Lee J. Y.
Published: July 2002

A study on the interface properties of Schottky contacts on  (NH4)2Sx-treated n- and p-type In0.5Ga0.5P is carried out. The effects of sulfur (S) treatment on Schottky barrier height are investigated by employing capacitance-voltage and current-voltage (I-V) measurements. It is also demonstrated that the passivation effects of S treatments on the interface traps can be monitored by deep level transient spectroscopy (DLTS) measurements. It is observed that the S treatment increases the dependence of Schottky barrier height on the metal work function. The interface traps in the Schottky contact formed by the heat treatment are found to give their energy state above midgap. It is found that the S treatment can passivate these interface traps as well as suppress their generation under the heat treatment. For both n- and p-In0.5Ga0.5P, contact-related majority carrier traps, which are different from the thermally generated interface traps, are observed at the Al-In0.5Ga0.5P interface and they can be annealed out by a heat treatment at 350 oC. It is also found that the I-V characteristics of Au/InGaP diodes formed on the S-treated surface degrade more rapidly than those formed on the untreated surface. Through cross-sectional transmission electron microscope observation, this poorer electrical reliability of Au contact on S-treated surfaces is attributed to the enhanced intermixing of group III elements with Au.
Mechanisms of valley currents in InAs/AlSb/GaSb resonant interband tunneling diodes
Authors: Shen Jun
Published: July 2002

Several possible contributions to the valley currents in InAs/AlSb/GaSb resonant interband tunneling diodes are analyzed. A theoretical model proposes that the field-assisted thermionic hole emission is the main valley leakage mechanism at high temperature and the Fowler–Nordheim hole tunneling is the dominant leakage path at low temperature and high bias.
Determination of elastic constants of thin films from phase velocity dispersion of different surface acoustic wave modes
Authors: Makarov S. , Chilla E. , Fröhlich H.-J.
Published: July 2002

We have carried out a systematic investigation of the achievable accuracy in the determination of the independent elastic constants c11 and c12 and the related constant c44 of thin isotropic films from the phase velocity dispersion of surface acoustic waves (SAWs). As a model system SiO2 and Au films deposited on Y-cut LiNbO3 were considered. The phase velocity dispersion was calculated for real elastic constants in dependence on the film thickness. These input data were used to determine the least-squares fits of the input dispersion and the phase velocity obtained by modifying c11 and c12. The minimum of this error field describes the solution of the inverse wave propagation problem. Error fields of the least-squares fits were calculated with ±5% variation for c11 and ±30% for c12. Different functional behavior and various magnitudes of the dispersion were compared. When simulating a small measuring uncertainty for the phase velocity the solution of the inverse problem becomes unstable which results in an insufficient accuracy of the interesting elastic constants. By superposition of two SAW modes the inaccuracy was significantly reduced. For the model system each independent SAW mode offers the ability to determine one elastic constant or one relation between different elastic parameters.
Defects generated by Fowler–Nordheim injection in silicon dioxide films produced by plasma-enhanced chemical-vapour deposition with nitrous oxide and silane
Authors: Landheer D. , Tao Y. , Xu D.-X. , Sproule G. I. , Buchanan D. A.
Published: July 2002

High quality silicon dioxide films have been produced using a direct plasma-enhanced chemical vapour deposition process with silane, nitrous oxide and helium that leaves a nitrided layer at the Si-SiO2 interface. X-ray photoelectron spectroscopy coupled with etch-back of the films has shown that the interface nitrogen is incorporated by nitridation of the silicon surface. Fowler–Nordheim injection measurements on thin films annealed after deposition for 1 minute at 950 °C show that the neutral-trap generation and interface state generation rates are comparable to that of thermal oxide if a proper deposition power is chosen. The data is consistent with an interpretation in which fast donor states, not Pb centres, account for almost all of the increase in the charge trapped at the interface. Too high deposition powers lead to excessive nitrogen and higher interface state generation rates. It is proposed that improved performance under hot-electron stress could be obtained by using an optimal deposition power to obtain an optimal nitrogen concentration followed by annealing in oxygen.
A Monte Carlo study of the kickout mechanism of boron diffusion in silicon
Authors: de Souza M. M. , Amaratunga G. A. J.
Published: July 2002

In this article, the results of a study of the diffusion mechanism of boron in silicon based on the Monte Carlo method are presented. The kickout mechanism has been examined for the case of a delta function impurity profile under both inert and oxidation conditions. It is shown that the initial conditions play a significant role in obtaining the mean migration path lengths of the atoms. The kickout mechanism in the case of an initial delta function interstitial impurity profile has been analytically examined. The atomic level computational experiments carried out in this article validate Cowern’s results of ‘‘intermittent diffusion’’ of boron in silicon and yield, the values for λ0, the prefactor for the mean migration path length, which are found to lie between 0.024 and 0.035 nm.
Structural and magnetic properties of rapidly quenched (R,Zr)(Fe,Co)10Nx (R=Nd,Sm)
Authors: Sakurada S. , Tsutai A. , Hirai T. , Yanagida Y. , Sahashi M. , Abe S. , Kaneko T.
Published: July 2002

The structural and magnetic properties of (R,Zr)(Fe,Co)10(R=Nd,Sm) prepared by the rapid-quenching method have been investigated. The presence of zirconium makes it possible to realize a TbCu7-type crystal structure with a high c/a ratio of more than 0.87, while the same crystal structure has a c/a ratio of just 0.84–0.85 in the zirconium-free case. The saturation magnetization of crystals having such a high c/a ratio as 0.87 exceeds 1.7 T at room temperature. These materials can be nitrogenated, and remarkable enhancement of magnetocrystalline anisotropy by nitrogenation can be seen in the case of Sm. The magnetic hardening of these nitrogenated materials can be achieved by annealing before nitrogenation. The highest value of (BH)max obtained so far for the nitrogenated materials is 144 kJ/m3 (18.0 MGOe). The iHc and Br values for this sample are 503 kA/m (6.3 kOe) and 1.08 T, respectively.