bluuebunny/crossref_metadata_2025 huggingface dataset:
Band-gap narrowing in novel III-V semiconductors
Authors: Jain S. C. , McGregor J. M. , Roulston D. J.
Published: July 2002

A predictive model for band-gap narrowing has been applied to several III-V semiconductors. Band-gap narrowing is expressed as ΔEg =AN1/3+BN1/4+CN1/2 ; values for A, B, and C are predicted for these materials. The commonly used N1/3 relation is shown to be valid for the p-type materials considered, but not for n-type materials.
Canonical aberration theory in electron optics
Authors: Ximen Jiye
Published: July 2002

By defining the Hamiltonian function in an electron optical system the canonical aberration theory has been developed in the up to fifth-order approximation. Therefore, the canonical position and momentum aberrations can be expressed by eikonal functions and equivalently by Poisson brackets. Consequently, all fifth-order canonical aberration coefficients for rotationally symmetrical electron optical systems have been derived at any arbitrary observation planes.
Conductivity degradation in oxygen-aged polypyrrole
Authors: Tansley T. L. , Maddison D. S.
Published: July 2002

The conductivity of doped polypyrrole, as high as 100 Scm−1 in freshly prepared samples, falls slowly with time on exposure to atmospheric oxygen. Typically, the fall in conductivity is fairly slow at room temperature, but the process can be accelerated by heating samples and measuring the decrease in conductivity as a function of time. Analysis of the kinetics of this process shows a preliminary, 120 min regime in which a surface adsorption-diffusion mechanism with an activation energy of 0.41 eV appears to mediate. Once a degree of adsorbate saturation is attained subsequent behavior is well described by standard bulk diffusion considerations and is activated at 0.48 eV. We propose a mechanism whereby oxygen adsorption onto the surfaces of the polymer fibrils and diffusion into the interior are respective rate limiting steps in the incorporation of oxygen into the closely cross-linked mesh, blocking conducting pathways.
Magnetostriction and applications of 6.5% Si steel sheet
Authors: Ninomiya H. , Tanaka Y. , Hiura A. , Takada Y.
Published: July 2002

6.5% Si steel is widely known as an excellent soft magnetic material because its magnetostriction is nearly zero. The ac magnetic properties and magnetostriction of 6.5% Si steel sheet were evaluated and compared with those of conventional 3% Si steel sheet. From these data, this sheet is considered to be one of the best core materials available for audio frequency transformers. Toroidal and laminated-type transformers were made from both 6.5% steel sheet and grain-oriented Si steel sheet and the iron losses and noise were investigated. The results indicate that it is possible to operate at high induction, which suggests that this size of a transformer can be reduced by using 6.5% Si steel sheet.
Temperature dependence of minority-carrier lifetime in iron-diffused p-type silicon wafers
Authors: Hayamizu Y. , Hamaguchi T. , Ushio S. , Abe T. , Shimura F.
Published: July 2002

Minority-carrier recombination lifetime was measured with a noncontact laser/microwave method for nondiffused and iron-diffused p-type silicon wafers in the temperature range from 28 °C to 230 °C. The lifetime increased monotonically with temperature in nondiffused silicon, while the lifetime in iron-diffused silicon showed a broad peak around 110 °C and a depression around 170 °C. The temperature dependence of the lifetime in iron-diffused silicon was analyzed based on Shockley–Read–Hall statistics. The origin of the lifetime temperature dependence was attributed to the dissociation of iron-boron pairs. Our experimental data supported that an electron trap for an iron-boron pair at Ec−0.29 eV was more effective as a recombination center than a hole trap at Ev + 0.1 eV. It was also shown that the effect of iron in concentrations as low as 1×1011 cm−3 on the lifetime can be detected with the noncontact laser/microwave method.
A digitizer based on reflections in delay lines
Authors: Hristoforou E. , Reilly R. E.
Published: July 2002

A new type of digitizer is proposed, based on the generation of an acoustic pulse reflection in a magnetostrictive delay line (MDL), at a point where a force is applied. A conductor wire C crosses an array of MDL positioned at a distance close to their end and carries a pulsed current. The measuring portion of the delay lines is under a flexible mat, so that they are not pressed at any point if no force is applied. The initial acoustic pulse in the line is detected by short coils between C and the end of each line. If a force is applied on the flexible mat above a delay line, the position and the amplitude of the applied force are respectively defined by the delay and the amplitude of the reflected pulse, with respect to the initial one. Experimental results are given for the function of the peak value of the reflected signal with respect to the applied input force under various values of pulsed current and dc bias field, for a 1-mm-wide and 150-cm-long Metglas 2605SC delay line. The resolution of this type of digitizer along the length of the delay line can be 0.1 mm, while in the other dimension it depends on the pitch of the flexible array channels (i.e., on the width of the delay lines) and can be improved by using two arrays of delay lines, orthogonal to each other.
Photoluminescence characterization of plasma exposed silicon surfaces
Authors: Henry A. , Monemar B. , Lindström J. L. , Bestwick T. D. , Oehrlein G. S.
Published: July 2002

The variation of photoluminescence (PL) spectra obtained with silicon exposed to various plasmas as a function of plasma etch treatment conditions is reported. Phosphorus- or boron-doped covering a large range of doping concentrations, Czochralski or float-zone-grown silicon crystals were investigated. The effect of various etching gases on the luminescence spectra as well as the effects of subsequent annealings are reported. Two types of recombination process are observed: (i) The first gives rise of sharp luminescence lines, such as the W (1018 meV), X (1040 meV), T (935 meV), I (965 meV), G (967 meV), C (790 meV), and P (767 meV) lines, which are known to originate from defects produced by high-energy irradiation and then manifest damage of the crystalline material. Other sharp PL lines at 1015, 1008, and 997 meV were introduced upon annealing at 400 °C. (ii) The second recombination process induces broad lines or bands in the photoluminescence spectra. The formation and nature of the defects giving rise to both recombination processes are discussed in terms of the plasma conditions and starting material.
Millimeter-wave dielectric properties of epitaxial vanadium dioxide thin films
Authors: Hood P. J. , DeNatale J. F.
Published: July 2002

The dielectric properties of grain-oriented thin-film vanadium dioxide (VO2) on single-crystal sapphire have been measured as a function of temperature across the phase transformation at 68 °C. The properties of the low- and high-temperature phases were determined by measuring the millimeter-wave transmittance and phase shift through the samples. The real part of the dielectric constant in the high-temperature phase increases with increasing thickness from less than 1000 for a 40-nm-thick film (the resolvable limit of this measurement technique) to over 90 000 for a 580-nm-thick film. The strong thickness dependence in epitaxial VO2 thin films can be attributed to a dielectric mixture phenomenon and can be described by effective-medium theory. The large real dielectric constant and its thickness dependence is likely due to bonding distortions at the grain boundaries. Analysis of the experimental data shows the VO2 thin films used in this study have no secondary grain-boundary phases, but do have interfacial stresses which affect the dielectric properties of the grain-boundary regions that lie near the VO2-sapphire interface. This study concludes that the millimeter-wave properties of VO2 thin films are strongly influenced by microstructure, crystalline orientation, grain-boundary stress, and stoichiometry.
Positron annihilation at the Si/SiO2 interface
Authors: Leung T. C. , Weinberg Z. A. , Asoka-Kumar P. , Nielsen B. , Rubloff G. W. , Lynn K. G.
Published: July 2002

Variable-energy positron annihilation depth-profiling has been applied to the study of the Si/SiO2 interface in Al-gate metal-oxide-semiconductor (MOS) structures. For both n- and p-type silicon under conditions of negative gate bias, the positron annihilation S-factor characteristic of the interface (Sint) is substantially modified. Temperature and annealing behavior, combined with known MOS physics, suggest strongly that Sint depends directly on holes at interface states or traps at the Si/SiO2 interface.
The diffusivity-mobility ratio in nonparabolic materials
Authors: Ghatak K. P. , Mondal M.
Published: July 2002

An attempt is made to study the Einstein relation for the diffusivity mobility ratio of the carriers in III–V, II–VI, IV–VI materials and Bi by formulating the respective expressions in accordance with the Kane, Hopfield, Cohen, and McClure and Choi models, respectively, which have been stated in the literature as to be the best models in describing the carrier energy spectra in these nonparabolic materials. It is found, by taking various types of aforementioned samples and Bi, that the above ratio increases with increasing electron concentration in different manners and the rates of variations are totally band structure dependent. We have also suggested an experimental method of determining the Einstein relation in degenerate materials having arbitrary dispersion laws. In addition, the corresponding well-known results for relatively wide band gap materials have also been obtained as special cases of our generalized formulations.
Application of the pressure wave propagation method to the study of interfacial effects in e-irradiated polymer films
Authors: Cals M. P. , Marque J. P. , Alquié C.
Published: July 2002

The pressure wave propagation method is applied here to the study of charging and discharging processes of polymer films of spatial interest. The irradiation of the materials is performed by a monoenergetic electron beam in the range 5–50 keV. Since the irradiated surface of the sample is free, we extended the model of the induced signals, previously developed, to take into account the total reflection of the pressure at the polymer-vacuum interface. All the experiments, irradiations and measurements, are operated in a vacuum environment. After the description of the experimental setup, typical signals showing the interfacial effects are discussed. They concern the behavior of the sample-vacuum interface and charge injection at the target-sample interface. These experiments, which were carried out both on metallized and nonmetallized samples bonded to the target electrode, allow a direct observation of charge injection at interfaces.
Radiative heat transfer in hot-filament chemical vapor deposition diamond reactors
Authors: Wolden C. , Mitra S. , Gleason K. K.
Published: July 2002

Hot-filament chemical vapor deposition is a common method employed for diamond deposition. Due to the filament-substrate proximity, large temperature variations across the substrate is often possible. Variations in substrate temperature need to be minimized in order to deposit polycrystalline diamond films of uniform thickness over large areas. Thus heat transfer calculations which consider radiation from the filament to the substrate, radiation from the substrate to the reactor walls, and finally conduction in the silicon wafer have been developed to predict substrate temperature profiles as a function of the filament shape and geometry. The calculated values are found to be in reasonable agreement with experimentally measured substrate temperatures. It was found that hydrogen atom recombination makes a significant contribution to the absolute substrate temperature, but that the normalized temperature profiles are determined primarily by the radiation flux distribution. The effects of the other deposition parameters are also discussed. Comparison with experimental results show an apparent correlation between growth rate profiles and radiation flux profiles from the filament as predicted by the calculations.
Magnetic properties of copper-magnetite nanocomposites prepared by ball milling
Authors: Pardavi-Horvath M. , Takacs L.
Published: July 2002

Small particles of Fe3O4, embedded in a Cu matrix, have been prepared both by direct milling of a mixture of Cu and Fe3O4 and by reaction milling of CuO and Fe. It is shown that the most significant changes in phase composition, magnetization, coercivity, remanence, and switching field distribution take place during the first 2 h of milling. Both processes provide small (about 25 nm) semihard magnetite particles after 1 h, with Hc up to 350 Oe and squareness of 0.3. Prolonged milling causes partial oxidation of magnetite to α-Fe2O3 and the presence of superparamagnetic magnetite particles, both reducing the magnetization of the composites. Conclusions based on magnetic measurements about the details of the technological process and microstructural changes are in very good agreement with results of x-ray diffraction measurements.
Characterization of submicrometer periodic structures produced on polymer surfaces with low-fluence ultraviolet laser radiation
Authors: Bolle Matthias , Lazare Sylvain
Published: July 2002

Periodic ripples and dot patterns with spacing smaller than 200 nm have been produced on various highly absorbing polymers [poly(ethylene terephthalate), poly(butylene terephthalate), polystyrene, polycarbonate, etc.] using a polarized beam of an ArF and KrF excimer laser. The period of the structures increases with the wavelength used and with the angle of incidence of the beam, whereas the ripple direction is parallel to the polarization direction. The fluence interval in which the pattern can be produced is far below the ablation threshold of the polymer but probably allows a local melting or photolysis of the surface. For polystyrene, a strong influence of the presence of oxygen was observed, which leads to the conclusion that photooxidative processes play a major role in the structure growing mechanisms. Due to the small size (0.1 μm) of the structure scattering techniques using visible light (0.4–0.8 μm) could not be used in this study and scanning electron microscopy and transmission electron microscopy were found adequate techniques. However, as alternative means of characterization a method called sample rotation ellipsometry has been successfully developed.
Improvement in electroabsorption and the effects of parameter variations in the three-step asymmetric coupled quantum well
Authors: Susa Nobuhiko
Published: July 2002

Exciton binding energies, radiuses, emission energies, and oscillator strengths in a three-step asymmetric coupled quantum well (QW) are numerically analyzed as a function of electric field. This analysis predicts that the change in the absorption coefficient for an n=1 heavy-hole exciton can be made larger and on-state transmission loss can be lowered by reducing oscillator strength at zero bias and increasing it at operating voltages. The Stark shifts also can be increased, thus allowing lower operating voltages. Estimation of influences of alloy composition and thickness variations on electroabsorption reveals that the thickness of epitaxial layers must be controlled to within one monoatomic layer and that variations in alloy composition must be controlled to within ±4%. An analysis of exciton peak shifts caused by one-monolayer change in the QW layer thicknesses predicts that interface roughness broadening may be small when the electric field is increased, in contrast to the case of conventional square-potential QWs. The influence of the broadening of the exciton spectra on the proposed improvements is also examined.
Mathematical model and laser-scattering temperature measurements of a direct-current plasma torch discharging into air
Authors: Murphy A. B. , Kovitya P.
Published: July 2002

The numerical plasma torch model of D. A. Scott, P. Kovitya, and G. N. Haddad [J. Appl. Phys. 66, 5232 (1989)], in which both the arc and plume regions are included in the computational domain, has been extended to treat the effect of the mixing of the plasma gas with the ambient gas. Both laminar diffusion and turbulent mixing are considered. The predictions of the model are compared with laser-scattering measurements of the temperature distribution in the plume of a plasma torch, for the case in which the plasma gas is argon and the ambient gas is air at atmospheric pressure. Good agreement is found between the measurements and the predictions of the model. The rapid decay in plume temperature away from the exit of the torch nozzle is shown to be mainly due to cooling by air entrained by turbulent mixing. The K-ε turbulence model is found to adequately approximate the turbulence phenonema involved.
Plasma shielding during picosecond laser sampling of solid materials by ablation in He versus Ar atmosphere
Authors: Mao X. L. , Chan W. T. , Shannon M. A. , Russo R. E.
Published: July 2002

The influence of plasma shielding on the coupling of laser energy to a target surface during picosecond pulsed laser–material interactions is demonstrated using a He and Ar gas atmosphere. An inductively coupled plasma-atomic emission spectrometer (ICP-AES) is used to monitor the quantity of copper material removed during picosecond and nanosecond pulsed-laser sampling. The intensity of Cu i emission from the ICP-AES was found to be 16.4 times larger with He as the gas medium compared to Ar during picosecond laser sampling. It was also observed that depth of craters in the copper targets decreased as the gas pressure was increased beyond 10 Torr in Ar and 100 Torr in He. Possible mechanisms of shock waves, multiphoton ionization, and plasma shielding to explain these observations are discussed. For plasma shielding to occur in the picosecond time regime, the existence of high-energy photoelectrons emitted from a Cu sample during the leading edge of laser pulse is postulated. These electrons form a plasma in the gas above the target via an inverse bremsstrahlung process and the plasma absorbs part of laser energy. The electron density versus pressure was calculated from a simple model and found to have similar behavior as the crater-depth data.
Empirical pseudopotential band-structure calculation for Zn1−xCdxSySe1−y quaternary alloy
Authors: Feng Y. P. , Teo K. L. , Li M. F. , Poon H. C. , Ong C. K. , Xia J. B.
Published: July 2002

The band structure of the Zn1−xCdxSySe1−y quaternary alloy is calculated using the empirical pseudopotential method and the virtual crystal approximation. The alloy is found to be a direct-gap semiconductor for all x and y composition. Polynomial approximation is obtained for the energy gap as a function of the composition x and y. Electron and hole effective masses are also calculated along various symmetry axes for different compositions and the results agree fairly well with available experimental values.
A two-dimensional fluid model for an argon rf discharge
Authors: Passchier J. D. P. , Goedheer W. J.
Published: July 2002

A fluid model for an argon rf discharge in a cylindrical discharge chamber is presented. The model contains the particle balances for electrons and ions and the electron energy balance. A nonzero autobias voltage is obtained by imposing the condition that the time-averaged current toward the powered and grounded electrode is zero. Particle densities and ionization profiles peak strongly in front of the smaller, powered electrode. There electric fields are stronger and the electron current density is higher, resulting in more ohmic heating and therefore higher ionization rates. The radial uniformity of the plasma in front of the powered electrode gives a homogeneous ion flux toward this electrode. The asymmetric character of the profiles of the cylindrical geometry is in clear contrast with the essentially one-dimensional infinite parallel-plate geometry, which is fully symmetric with respect to the center of the discharge and has a zero dc autobias voltage. A comparison with results of a one-dimensional model shows that the average ion density, the average ion flux, and the average ionization rate in the cylindrical reactor are comparable to those in a parallel-plate reactor. The numerical treatment of the time evolution of the transport equations and Poisson’s equation needs an implicit method to avoid numerical instabilities. The resulting system of discretized equations is solved by a multigrid technique. The spatial discretization uses the Sharfetter–Gummel scheme.
Piezoelectric, dielectric, and elastic properties of poly(vinylidene fluoride/trifluoroethylene)
Authors: Wang H. , Zhang Q. M. , Cross L. E. , Sykes A. O.
Published: July 2002

The symmetry-predicted nonzero components of the piezoelectric coefficient, dielectric constant, and elastic compliance matrices have been determined on poly(vinylidene fluoride/trifluoroethylene)(75/25) copolymer at room temperature and a frequency of 500 Hz. The temperature dependence of each of the complex piezoelectric coefficients and complex dielectric constants has been measured in the temperature range of −100 to 65 °C. The frequency dependence of these coefficients has also been measured at room temperature. It is found that the relaxation observed in the tensile piezoelectric coefficients of this material is different from that of the dielectric constants, whereas the relaxation of the piezoelectric shear constants shows behavior similar to that of the dielectric constants.
Measurement of interface strength by the modified laser spallation technique. III. Experimental optimization of the stress pulse
Authors: Yuan J. , Gupta V. , Pronin A.
Published: July 2002

In Part III of this series, experimental optimization of the laser spallation technique is achieved by investigating the effect of the kind and thickness of the constraining materials and the energy-absorbing film on the amplitude and profile of the generated stress pulse. An experimental setup consisting of a 0.5-μm-thick Al film as the energy absorbing film and a 5 μm layer of solid water glass as the constraining material is found to maximize the stress pulse amplitude. In addition, the effect of the substrate material on the shape of the stress pulse is also determined. Finally, it was found that for a given substrate, the pulse profile is independent of the laser fluence, indicating similar laser-material interaction phenomena over the range of fluence levels considered here.
Selective formation of an efficient Er-O luminescence center in GaAs by metalorganic chemical vapor deposition under an atmosphere containing oxygen
Authors: Takahei Kenichiro , Taguchi Akihito
Published: July 2002

To investigate the effects of oxygen codoping on Er luminescence centers in GaAs, we grew Er-doped GaAs by low-pressure metalorganic chemical vapor deposition (MOCVD) and measured the photoluminescence spectrum due to the intra-4f-shell transition of Er. The spectrum of a sample grown in a pure hydrogen atmosphere was complicated, showing many lines and bands. The spectrum of a sample grown in a hydrogen atmosphere containing 0.2 ppm oxygen, on the other hand, was simple and had few lines. The spectrum of the oxygen-codoped sample showed higher peak intensities as well as higher integrated luminescence intensity in the 1.5–1.6 μm region. Secondary-ion mass spectroscopy revealed that the oxygen-codoped sample had a higher concentration of oxygen, indicating the formation of an Er-O complex center. One kind of optically active efficient Er-O complex luminescence center can, therefore, be selectively formed under suitable MOCVD growth conditions.
Effects of strain energy on the preferred orientation of TiN thin films
Authors: Oh U. C. , Je Jung Ho
Published: July 2002

The effects of strain energy on the preferred orientation of TiN thin films were investigated. In the TiN film deposited by plasma-enhanced chemical-vapor deposition with a power of 50 W, the overall energy of the film mainly depended on the surface energy because its strain energy was relatively small. The preferred orientation of the film corresponded to the plane with the lowest surface energy, i.e., (200). However, in the TiN film deposited by rf sputtering with a power of 200 W, the overall energy of the film was largely controlled by strain energy due to its large strain energy, and its growth orientation corresponded to the plane with the lowest strain energy, i.e., (111). Furthermore, the preferred orientation of the TiN film was changed from (200) to (111) with the film thickness. It is considered that this phenomenon is due to the increase of strain energy with its thickness.
Deviatoric stress measurement under uniaxial compression by a powder x-ray diffraction method
Authors: Funamori Nobumasa , Yagi Takehiko , Uchida Takeyuki
Published: July 2002

The complete stress field in a polycrystalline sample compressed in a modified Drickamer-type apparatus was determined from x-ray diffraction data. The incident x rays, from a synchrotron source, were perpendicular to the compression axis, and the diffracted energy-dispersive signals were simultaneously determined for two directions relative to the compression axis. The two sets of d values measured by this system were analyzed by making use of a new equation derived by Singh, and the uniaxial stress component σ1−σ3 and the parameter α, which describes the stress and strain conditions across the grain boundaries of the sample, were obtained. This method was applied to NaCl and the results give the important information on the stress state and the pressure determination method under direct compression of a solid sample.
Positronium formation in SiO2 films grown on Si substrates studied by monoenergetic positron beams
Authors: Uedono A. , Wei L. , Tanigawa S. , Suzuki R. , Ohgaki H. , Mikado T. , Kawano T. , Ohji Y.
Published: July 2002

The annihilation characteristics of positrons in SiO2 films grown on Si substrates were studied by using monoenergetic positron beams. Doppler broadening profiles of the annihilation radiation and lifetime spectra of positrons were measured as a function of incident positron energy for SiO2 (166 nm)/Si specimens fabricated by thermal oxidation. From the measurements, it was found that about 90% of positrons implanted into the SiO2 film annihilate from positronium (Ps) states. This fact was due to the trapping of positrons by open-space defects and a resultant enhanced formation of Ps in such regions. For the SiO2 film grown at 650 °C, the lifetime of ortho-Ps was found to be shorter than that in the film grown at 1000 °C. This result suggests that the volume of open-space defects in the SiO2 film decreased with decreasing the growth rate of the SiO2 film.
Furnace formation of silicon oxynitride thin dielectrics in nitrous oxide (N2O): The role of nitric oxide (NO)
Authors: Tobin Philip J. , Okada Yoshio , Ajuria Sergio A. , Lakhotia Vikas , Feil William A. , Hedge Rama I.
Published: July 2002

We have studied the growth kinetics of the N2O furnace oxynitridation process demonstrating the importance of input flow rate, and therefore gas residence time, in determining the final film thickness and the nitrogen concentration. This dependence on residence time can explain the variation in the tendency to thickness saturation observed in the film growth data reported by several groups. Using published gas phase kinetic data, we have shown that, for a 950 °C oxynitridation process, N2O decomposes into N2, O2, and NO before reaching the wafer load. Again using published information, we have derived a simple equation which describes the subsequent reaction between NO and O2 to produce NO2 as the gas flows down the tube. This reaction results in loss of NO by an amount which depends on the gas residence time and therefore on the input gas flow rate and the dimensions of the system. Since it can be argued that NO2 does not contribute to nitridation, this system-dependent loss of NO can explain the variation in the reported film growth data. Combining our experimental data and model, we find that the peak nitrogen concentration in the film depends linearly on the NO gas phase concentration. Further, the oxynitride grows more slowly as the NO concentration increases supporting the idea that oxidation sites are blocked by nitrogen as oxynitridation time increases.
Circular streamline model of shaped-charge jet and slug formation with asymmetry
Authors: Curtis J. P. , Kelly R. J.
Published: July 2002

The formation of a jet as a result of the collapse of a shaped charge liner in the presence of asymmetry is considered. The development of a satisfactory analytic formation model based on the assumption of incompressible fluid flow requires the solution of the classically indeterminate problem of the collision of two unequal streams. A method of closing the problem is presented. It rests on the assumptions that there is a stagnant core region and that the flows of material from the impinging streams into the jet and slug turn by following circular streamlines with no decrease in speed. Balances of the centrifugal forces with the pressure in the stagnant core, relations derived from the flow geometry, the equation of mass conservation, and Bernoulli’s law provide the mathematical statement of the problem. These equations are manipulated to produce a reduced set of four equations in four unknowns, enabling a solution to be determined. This analytic solution predicts that both the jet and the slug are deflected by the same acute angle from the line of bisection of the angle between the impinging streams. The percentages of material in each stream which turn to form the jet are the same. The new model recovers analytically the classical Birkhoff, MacDougall, Pugh, and Taylor [J. Appl. Phys. 19, 563 (1948)] jet formation model in the symmetric case. It also yields the correct analytic result for the head-on collision of two streams of equal speeds but differing widths. More generally the model predicts an approximately linear dependence of the off-axis jet velocity component on the percentage difference in the stream speeds and a similar dependence for the widths. The predicted absolute values of the off-axis velocity are greater for a given difference in the stream speeds than for the same percentage difference in the widths. Finally, fair agreement with some previous experimental work on the collision of streams of unequal widths is demonstrated.
Magnetic properties of iron clusters prepared by the sol-gel method
Authors: Wang Jian-Ping , Luo He-Lie
Published: July 2002

Iron clusters containing from a few atoms to a few hundred atoms dispersed in a silica matrix have been prepared by a sol-gel method with very low iron concentration, which was determined by the x-ray-diffraction, electron selected-area diffraction, and magnetic measurements. Magnetic measurement shows that the magnetic moment per atom of these clusters is higher than that of bulk iron and the magnetic behavior of these clusters is well described using the theory of superparamagnetism.
Order-disorder and magnetic exchange interactions in substituted strontium hexaferrite SrAxFe12−xO19(A=Ga, In)
Authors: Thompson G. K. , Evans B. J.
Published: July 2002

The unusual magnetic behavior of the 12k site in SrGaxFe12−xO19 (0.3≤x≤6.0) and SrInxFe12−xO19 (0.3≤x≤1.4) has been investigated by means of 57Fe Mössbauer spectroscopy at 298 K. The bifurcation of the 12k Mössbauer subspectrum into 12k1 and 12k2 components of widely differing intensities has been confirmed for both Ga3+ and In3+. The bifurcation is apparent at x values as low as 0.3, and the less intense 12k2 pattern corresponds to an abrupt drop in the hyperfine magnetic field Heff, of 50 and 75 kOe for Ga3+ and In3+, respectively. Heff for the more intense 12k1 pattern is virtually unchanged from that of pure SrFe12O19 for x values as high as 1.0. Room-temperature Mössbauer spectra confirm that the Ga3+ and In3+ ions preferentially occupy the 4f2 octahedral site. There are indications, however, that the dopants also enter the 2b trigonal-bipyramidal site. The change in Heff at the 12k sites indicates that complex and competing exchange interactions are present, and that the splitting of the 12k magnetic hyperfine spectrum is not due simply to random disruptions of the exchange interactions. The intensities of the 12k1 and 12k2 subspectra have been used to model the exchange interactions at the 12k site.
Imaging spectroscopy with the atomic force microscope
Authors: Baselt David R. , Baldeschwieler John D.
Published: July 2002

Force curve imaging spectroscopy involves acquiring a force–distance curve at each pixel of an atomic force microscope image. Processing of the resulting data yields images of sample hardness and tip-sample adhesion. These images resemble Z modulation images and the sum of forward and reverse friction images, respectively, and like them exhibit a number of potentially misleading contrast mechanisms. In particular, XY tip motion has a pronounced effect on hardness images and the meniscus force on adhesion images.
Structure of LaFe9Si4 intermetallic compound
Authors: Tang Wei-hua , Liang Jing-kui , Chen Xiao-long , Rao Guang-hui
Published: July 2002

The crystal structure of LaFe9Si4 intermetallic compound equilibrated at 1173 K has been determined by the x-ray powder diffraction and refined by the Rietveld technique. The crystal structure belongs to the tetragonal symmetry with space group I4/mcm, which can be derived from the cubic NaZn13-type structure. Each unit cell contains four formula units of LaFe9Si4. The lattice parameters are a=7.932(1) Å and c=11.677(2) Å. The calculated density is Dx=6.86(4) g/cm3. In the structure there are five kinds of equivalent positions, i.e., 4a, 16l(1), 16k, 16l(2), and 4d, which are occupied by 4La, 16Fe(1), 16Fe(2), 16Si, and 4Fe(3) atoms, respectively.
Optical scattering and absorption by metal nanoclusters in GaAs
Authors: Nolte D. D.
Published: July 2002

Optical extinction by a dilute dispersion of metal nanoclusters in GaAs is calculated using the optical theorem and Maxwell–Garnet theory with complex dielectric functions for Cr, Fe, Ni, Cu, Ag, Au, Er, and As. The large dielectric function of the semiconductor host shifts the surface plasmon resonance frequencies from the ultraviolet to the near infrared. The noble metals have well-defined resonances with significant absorption and Rayleigh scattering at photon energies compatible with diode lasers and semiconductor electro-optic modulators. Interband transitions in metals such as As, Cr, Fe, Ni, and Er strongly damp the surface plasmon modes, quenching the resonant absorption by the particles, but providing significant absorption to wavelengths longer than 1.5 μm. Metal-semiconductor composites may arise during growth or processing of materials, such as GaAs:Er for fiber-optic applications, and GaAs:As in which metallic precipitates of As form in GaAs after low temperature growth of GaAs using molecular beam epitaxy.
Copper nitride thin films prepared by radio-frequency reactive sputtering
Authors: Maruyama Toshiro , Morishita Tomonori
Published: July 2002

Copper nitride thin films were obtained by the reactive sputtering method. A metallic copper target was sputtered in nitrogen gas with radio-frequency (rf) magnetron sputtering equipment. Highly [100]-oriented polycrystalline films of the cubic anti-ReO3 structure were obtained. Films with a lattice constant above 3.868 Å were conductors, while films with a lattice constant below 3.868 Å were insulators. The resistivity of conducting films was 0.5–3×10−2 Ω cm. The insulating films showed an optical energy gap of 1.3 eV, while the conducting films showed a smaller value which decreased with decreasing resistivity.
Inhomogeneous and temperature-dependent p-InGaAs/n-InP band offset modification by silicon δ doping: An internal photoemission study
Authors: Almeida J. , dell’Orto Tiziana , Coluzza C. , Fassò A. , Baldereschi A. , Margaritondo G. , Rudra A. , Buhlmann H. J. , Ilegems M.
Published: July 2002

We measured by internal photoemission the conduction-band discontinuity ΔEc in p-In0.53Ga0.47As/n-InP heterojunctions with a Si δ layer (1×1012 cm−2) inserted in InP at 10 Å from the interface. The n-type Si δ doping induced an inhomogeneous and temperature-dependent conduction-band offset reduction as revealed by two onsets in the spectral response. The first one was absent in room-temperature data and was due to the Si intralayer presence. The second correlated with the conduction-band discontinuity value for heterojunctions without δ doping and its presence served as an indication of the inhomogeneity of the Si δ layer. The measured value of the modification was 0.11±0.04 eV in good agreement with the calculated one. Current-voltage measurements confirmed that the Si δ layer modified the transport parameters of the heterojunction only at low temperature.
Normal to relaxor ferroelectric transformations in lanthanum-modified tetragonal-structured lead zirconate titanate ceramics
Authors: Dai Xunhu , Xu Z. , Viehland Dwight
Published: July 2002

Lanthanum modified lead zirconate titanate (PLZT) ceramics near a crossover between normal and relaxor ferroelectric states for Zr/Ti ratios of 40/60, 20/80, and 0/100 were investigated by dielectric spectroscopy, Sawyer–Tower polarization techniques and transmission electron microscopy. It was found possible to induce a relaxor state in each of these tetragonal PLZT compositional sequences with increasing La content. The transformation between normal and relaxor-like behaviors was found to be associated with a domain evolution from normal micron-sized domains, to tweed-like subdomain structures, and finally to polar nanodomains. A spontaneous relaxor to normal transformation was found near a critical La content in each compositional sequence. Temperature dependent polarization studies revealed a field-induced relaxor to normal ferroelectric transition at temperatures above that of the spontaneous transformation.
Electronic properties of zinc-blende GaN, AlN, and their alloys Ga1−xAlxN
Authors: Fan W. J. , Li M. F. , Chong T. C. , Xia J. B.
Published: July 2002

The electronic properties of wide-energy gap zinc-blende structure GaN, AlN, and their alloys Ga1−xAlxN are investigated using the empirical pseudopotential method. Electron and hole effective mass parameters, hydrostatic and shear deformation potential constants of the valence band at Γ and those of the conduction band at Γ and X are obtained for GaN and AlN, respectively. The energies of Γ, X, L conduction valleys of Ga1−xAlxN alloy versus Al fraction x are also calculated. The information will be useful for the design of lattice mismatched heterostructure optoelectronic devices based on these materials in the blue light range application.
Conductance modulation of submicrometer metal–oxide–semiconductor field-effect transistors by single-electron trapping
Authors: Mueller H. H. , Schulz M.
Published: July 2002

The capture and emission of electrons at single, individual interface traps is studied in sub-μm metal–oxide–semiconductor field-effect transistors (MOSFETs) by the random telegraph signals (RTSs) they induce by source-drain conductance modulations. The magnitude of the RTSs observed frequently exceeds 10% of the channel conductance and it exhibits a large scatter over two orders of magnitude. Analytical estimates and computer modeling show that the magnitude of the RTSs and the scatter cannot occur for a uniform MOSFET channel. It is concluded that fixed oxide and interface charge centers, which are present in the active device area at a high concentration, cause a percolating current distribution in the channel. The lucky trap centers located close to current paths give rise to large RTSs. The scatter in the magnitude of the RTSs is due to the random location of traps in the percolation pattern. Trapping centers causing RTSs thus act as atomic probes of the nonuniform current distribution in the channel.
Surface recombination velocity of highly doped n-type silicon
Authors: Cuevas Andrés , Basore Paul A. , Giroult-Matlakowski Gaëlle , Dubois Christiane
Published: July 2002

New experimental data for the minority-carrier surface recombination velocity of n-type silicon, Sp, are reported. The data, obtained from photoconductance decay measurements of the recombination currents corresponding to different phosphorus diffusions, include oxide-passivated, unpassivated and metal-coated surfaces. For the passivated case, Sp increases linearly with surface dopant density, ND, for dopant densities higher than 1×1018 cm−3, while for unpassivated (bare) and for metal-coated silicon Sp remains essentially constant, at about 2×105 cm/s and 3×106 cm/s, respectively. The experiments also allow for a determination of the apparent energy bandgap narrowing as a function of dopant density, ΔEgapp=14 meV [ln(ND/1.4×1017 cm−3)]. These surface recombination velocity and ΔEgapp data form, together with the dependences of minority-carrier lifetime, τp, and mobility, μp, used in the analysis, a consistent set of parameters that fully characterize highly doped n-type silicon.
Intersubband-coupling and screening effects on the electron transport in a quasi-two-dimensional δ-doped semiconductor system
Authors: Hai Guo-Qiang , Studart Nelson , Peeters François M. , Koenraad Paul M. , Wolter Joachim H.
Published: July 2002

The effects due to intersubband coupling and screening on the ionized impurity scattering are studied for a quasi-two-dimensional electron system in δ-doped semiconductors. We found that intersubband coupling plays an essential role in describing the screening properties and the effect of ionized impurity scattering on the mobility in a multisubband system. At the onset of the occupation of a higher subband, the screening due to the intersubband coupling leads to a reduction of the small angle scattering rate in the lower subband. We showed that such an effect is significant in a δ-doped quantum well and results in a pronounced increase of the quantum mobility at the onset of the occupation of a higher subband.
Slab plasmon polaritons and waveguide modes in four-layer resonant semiconductor waveguides
Authors: Stiens J. , Vounckx R. , Veretennicoff I. , Voronko A. , Shkerdin G.
Published: July 2002

This paper presents a detailed study of the waveguide and plasmon polariton properties of four-layer systems involving highly doped semiconductor material. The dispersion relations of waveguide and plasmon polariton modes are calculated for different geometrical parameters and material properties. Special attention is paid to the transition region between the latter modes, which exhibits a complex behavior. Slab plasmon polaritons at wavelengths slightly larger than the plasma wavelength, yielding a positive real part of the permittivity, have been found. Finally, applications at wavelengths near the transition region and near the plasma wavelength are proposed and discussed.
Nonlinear optical properties for TiO2 containing phosphate, borophosphate, and silicate glasses
Authors: Le Boiteux S. , Segonds P. , Canioni L. , Sarger L. , Cardinal Thierry , Duchesne Claire , Fargin E. , Le Flem G.
Published: July 2002

A theoretical and experimental analysis of the optical properties for titanium-containing glasses is reported. A bond orbital theory elaborated by M. E. Lines [Phys. Rev. B 41, 3372 (1990); 41, 3383 (1990); 43, 11 978 (1991)] for crystals has been extended to these glasses, where the influence of the Ti–O bond environment has been considered. The resulting calculated values obtained for linear and nonlinear indices have been successfully compared with absolute experimental indices independently measured using a Mach–Zehnder interferometer. This agreement involves an adjustment of a local field factor f, varying from 1 to 1.2, which emphasizes the importance of the titanium–oxygen environment particularly upon the nonlinear optical properties of the investigated materials.
Effect of an electric field on the scattering of excitons by free carriers in semiconducting quantum-well structures
Authors: Koh Tong San , Feng Yuan-ping , Spector Harold N.
Published: July 2002

We have calculated the effect of an electric field applied along the direction of carrier confinement on the scattering of excitons by free carriers in semiconducting quantum-well structures. We find that although the application of the electric field does not change the functional form of the dependence of the differential cross section on the scattering angle or the dependence of the total cross section on the incident wave vector of the free carrier, it does change the magnitude of the total cross section. For scattering of heavy hole excitons by electrons, the peak value of the total elastic cross section decreases with increasing electric field while when the exciton is scattered by heavy holes, the peak value of this cross section increases with electric field. This reduction or enhancement is negligible for narrow wells or for weak fields but there is an appreciable change in the total elastic scattering cross section for wider wells and stronger fields. We also find that the electric field also enhances the ionization cross section for the scattering of excitons by both electrons and heavy holes.
Origin of the increase in resistivity of manganese–zinc ferrite polycrystals with oxygen partial pressure
Authors: Byeon Soon Cheon , Hong Kug Sun , Park Jae Gwan , Kang Won Nam
Published: July 2002

In our present study, the origin of the increase in resistivity of polycrystalline Mn0.47Zn0.47Fe2.06O4 with increasing oxygen partial pressure was investigated by measuring thermoelectric power and electrical resistivity, and through analysis of grain size. The ferrous ion (Fe2+) concentration of the samples was estimated using the thermoelectric power data and it indicated that the increase of oxygen partial pressure accompanied only a 0.5 wt. % decrease in the concentration of Fe2+. The decrease in Fe2+ concentration failed to explain the order of magnitude increase in resistivity. Preferential oxidation of the grain boundaries did not contribute to the increase in resistivity since all the samples were cooled under the same conditions, i.e., constant oxidation potential. Impedance spectroscopy revealed that the increase in resistivity arose from the increase in resistivity of the grain boundary; this is discussed in terms of the microscopic shape factor, the brick-layer model, and the Maxwell–Wagner model. It is suggested that the increase in resistivity, with increasing oxygen partial pressure, originates from the increase in the microscopic shape factor of the grain boundary.
Photonic band gaps and holography
Authors: Berger V. , Gauthier-Lafaye O. , Costard E.
Published: July 2002

Photonic band gap materials are holograms with extremely high refractive index contrasts. The refractive index function can be approximated by a small number of plane waves, as a consequence of the photonic crystal periodicity. Photonic crystals can hence be constructed with a simple holographic recording of a very small number of optical plane waves, and appear in this regard as the simplest holograms. Various photonic band gap structures are theoretically analysed and those concepts are illustrated experimentally with the fabrication of a two-dimensional triangular lattice in GaAs. The extension of the method to the three-dimensional diamond structure is discussed.