bluuebunny/crossref_metadata_2025 huggingface dataset:
Time-resolved spectroscopy of Zn- and Cd-doped GaN
Authors: Bergman P. , Ying Gao , Monemar B. , Holtz P. O.
Published: July 2002

Photoluminescence transients and time-resolved luminescence spectra are reported for the violet-blue emissions from epitaxial layers of Zn- and Cd-doped GaN. A typical decay time of 300 ns is reported for the blue GaN:Zn emission, peaking at about 2.89 eV. For GaN:Cd a somewhat longer decay time of τ≊1 μs dominates for the broad peak centered at ≊2.72 eV. In both cases it is concluded that the simple process involving a free-to-bound transition of an electron to a hole bound at the ZnGa, respectively, the CdGa acceptor is the dominating recombination mechanism corresponding to these characteristic decay rates.
Formation and properties of NdFeB prepared by mechanical alloying and solid-state reaction
Authors: Schultz L. , Wecker J. , Hellstern E.
Published: July 2002

Magnetically isotropic Nd15Fe77B8 powder is prepared by mechanical alloying and a solid-state reaction from the elemental powders. Due to the extremely short reaction time (for example, 10 min at 700 °C) the magnetically isotropic particles have a very fine microstructure comparable to rapidly quenched samples, show a pinning-type behavior and possess excellent magnetic values such as IHC up to 13 kOe and BHmax up to 12.8 MGOe. Details on the development of the microstructure during milling, the reaction kinetics, and the magnetic properties are reported.
Frustrated antiferromagnetism at heterointerfaces in a semiconductor superlattice: MnSe/ZnSe
Authors: Chang S.-K. , Lee D. , Nakata H. , Nurmikko A. V. , Kolodziejski L. A. , Gunshor R. L.
Published: July 2002

The role of interfaces in influencing behavior of antiferromagnetic semiconductors has been studied in the new strained layer MnSe/ZnSe superlattice system, grown by molecular-beam epitaxy, with individual MnSe layers approaching the monolayer limit. Large paramagneticlike contributions to overall magnetization are observed at low temperatures. Such anomalous characteristics are interpreted in terms of frustration against antiferromagnetic ordering by microstructure effects at the heterointerfaces.
Knudsen flow through a random bed of unidirectional fibers
Authors: Faley Timothy L. , Strieder William
Published: July 2002

A rigorous variational upper bound for the permeability of a porous medium made up of long fibers oriented with axes mutually parallel, but perpendicular to the direction of flow, are formulated and expressed in terms of the appropriate statistical properties of the random fiber bed. Explicit calculations for the two-dimensional model pore structure made up of overlapping fibers give a permeability coefficient 2π2/(16+π2). The result is quite different from the well-known value (12)/(13) obtained by Derjaguin in three dimensions.
Spin dynamics in Zn1−xMnxTe
Authors: Giebultowicz T. M. , Rhyne J. J. , Ching W. Y. , Huber D. L. , Furdyna J. K.
Published: July 2002

First results of inelastic neutron scattering studies of quasiharmonic magnons in Zn1−xMnxTe are reported. Measurements carried out on two single-crystal samples with x=0.65 and x=0.41 at T=4.2–5.0 K revealed broad excitation modes, which were in excellent agreement with the results of theoretical modelling. Results for x=0.65 yield JNN=(7.9±0.2) K which is the first determination of the nearest-neighbor exchange constant in Zn1−xMnxTe in this x region. Data taken at higher temperatures showed only weak renormalization of the energy spectra.
Nonlocal transport models of the self-consistent potential distribution in a plasma sheath with charge transfer collisions
Authors: Jurgensen Charles W. , Shaqfeh Eric S. G.
Published: July 2002

Plasma sheaths are often assumed to be collision free; however, high-voltage cathode sheaths are typically thicker than the mean free path for charge transfer collisions at pressures encountered in glow discharge processing equipment (greater than 10 mTorr). In this paper, the potential distribution in a plasma sheath is determined by solving Poisson’s equation self-consistently using a kinetic theory nonlocal ion transport model for charge transfer collisions. The relationship between the potential distribution, ion flux, and thickness of a plasma sheath is presented for arbitrary values of the sheath thickness relative to the mean free path for charge transfer. The results may be used to estimate the ion flux from measurements of the sheath thickness and potential drop across the sheath. Ion energy distribution functions and a one-parameter approximation to the numerically determined potential distribution are also presented. These results apply to rf discharges in a time-averaged sense when the ion sheath transit time is much longer than the rf cycle time, and they apply to high-voltage cathode sheaths in ‘‘abnormal’’ dc and low-frequency rf discharges. The present model is compared to earlier self-consistent sheath models, including the collision-free approximation, the local mobility model, and a nonlocal fluid approximation known as the viscous drag model.
New approaches to epitaxy of transition metals and rare earths: Heteroepitaxy on lattice-matched buffer films on semiconductors (invited)
Authors: Farrow R. F. C. , Parkin S. S. P. , Speriosu V. S.
Published: July 2002

Examples of new approaches to epitaxy of magnetic transition metals and rare earths are described. Epitaxy of bcc Fe on lattice-matched InxGa1−xAs films is accompanied by an interfacial reaction. This is indicative of a more general problem of instability of interfaces between Ni, Fe, Co, and compound semiconductors. Such reactions can be avoided if an intermediate epitaxial film of Ag is interposed between the semiconductor and the Fe. However, this results in long-range coherency strain and tilted epitaxy of Fe. Epitaxial growth of the rare-earth metal Dy onto epitaxial films of LaF3 on GaAs(1̄1̄1̄) is described and the magnetic properties of epitaxial sandwich structures of LaF3/Dy/LaF3 summarized.
Thermoelectric power and electrical resistivity of crystalline antimony telluride (Sb2Te3) thin films: Temperature and size effects
Authors: Das V. Damodara , Soundararajan N.
Published: July 2002

Crystalline Sb2Te3 thin films of different thicknesses have been prepared by subsequent annealing (at 500 K) of vacuum deposited, as-grown, amorphous thin films of Sb2Te3 prepared on glass substrates at room temperature. Thermoelectric power and electrical resistivity of these annealed (crystalline) films have been determined as a function of temperature. The size dependence of thermoelectric power and electrical resistivity have been analyzed by the effective mean free path model of size effect. It is found that both the thermoelectric power and the electrical resistivity are linear functions of the reciprocal of thickness of the films. The data from the analyses of thermoelectric power and electrical resistivity have been combined to evaluate important material parameters such as carrier concentration, their mean free path, Fermi energy, and effective mass. The values of some of these are compared with the previous available values from literature.
Preparation of epitaxial Pb(ZrxTi1−x)O3 thin films and their crystallographic, pyroelectric, and ferroelectric properties
Authors: Takayama Ryoichi , Tomita Yoshihiro
Published: July 2002

Lead zirconate-titanate (PZT) thin films with tetragonal and rhombohedral structures have been successfully grown with good epitaxy on MgO single crystals and on [100]-oriented platinum thin films by rf-magnetron sputtering. The PZT films were obtained by using sputtering targets with compositions of Pb(ZrxTi1−x)O3, where x=0.2–0.8. The tetragonal PZT thin films have a strong [001] orientation, and the rhombohedral ones are (100) oriented. Asymmetric D-E hysteresis loops were observed on the tetragonal films. Nearly symmetric loops were observed on the rhombohedral films and the rhombohedral loops around the morphotropic phase boundary had slim loops. The tetragonal thin films of x=0.45 have a spontaneous polarization of 45 μC/cm2, a remanent polarization of 40 μC/cm2, and a coercive field of 150 kV/cm. It was found that significant pyroelectric currents are detected on both [001]-oriented tetragonal and (100)-oriented rhombohedral PZT films even without a poling treatment, and the direction is the same in all specimens. The relative dielectric constant εr achieves a maximum at the morphotropic phase boundary, as occurs for PZT ceramics. The pyroelectric coefficient γ increases with increase of Zr content and reaches a peak near the phase boundary in the tetragonal thin films. On the other hand, for the rhombohedral films, γ drops around the phase boundary and increases upon increasing Zr content. Thin films of x=0.45 have εr of 400 and γ of 4.2×10−8 (C/cm2 K). These films will be suitable for use in piezoelectric transducers and pyroelectric infrared detectors.
Analytical base-collector heterojunction capacitance model including collector current effects
Authors: Liou J. J.
Published: July 2002

The base-collector heterojunction space-charge-region capacitance of double heterojunction bipolar transistors is treated. This capacitance cannot be modeled as the conventional heterojunction depletion capacitance because it is heavily influenced by the collector current, which is neglected in the depletion capacitance model. The analysis includes heterojunction effects as well as high current phenomena such as base pushout and space-charge-limited flow. Large discrepancies are found when comparing the present model with the depletion model at high current densities. Comparison of the base-collector junction capacitances calculated from a single heterojunction bipolar transistor and a double heterojunction bipolar transistor is also included.
Three-dimensional theory of pulsed photothermal deformation
Authors: Li B. C.
Published: July 2002

In this paper, the three-dimensional theory of pulsed photothermal deformation is presented; by taking two appropriate assumptions, the rigorous solutions of three-dimensional thermal conduction and thermoelastic equations are given for the first time. The theoretical shape of the signal of two different samples is calculated, and the effects of pulse width, radius of pump beam, and sample thickness are discussed. As an example, the potential application of pulsed photothermal deformation for thin-film thickness measurement is analyzed.
Simulation studies of silicon electro-optic waveguide devices
Authors: Giguere Stephen R. , Friedman Lionel , Soref Richard A. , Lorenzo Joseph P.
Published: July 2002

Several silicon-on-insulator guided-wave structures have been analyzed as potential electro-optic waveguide modulators using the pisces-ii two-dimensional (2D) device simulation program. From the electron and hole concentration data, the 2D refractive index profile is obtained. The profile is then spatially averaged with respect to a 2D cosine representation of the guided-wave E field to obtain the effective modal index changes at 1.3 and 1.55 μm. The channel-waveguide devices studied include the metal-oxide-semiconductor (MOS) diode, and the one or two-gate metal-oxide-semiconductor field-effect transistor (MOSFET) with single-or double-transverse injection. The single-gate double-injection MOSFET modulator offers the most promise with 10−3 refractive index changes possible, changes comparable in size to the Pockels effect in LiNbO3 or GaAs.
Excitonic photoluminescence in CuGaS2 crystals
Authors: Shirakata S. , Saiki K. , Isomura S.
Published: July 2002

The low-temperature photoluminescence (PL) spectra of CuGaS2 are investigated in this work. At 4.2 K, high-quality single crystals grown by both the iodine-transport and the melt-growth methods showed many sharp PL lines. Free-exciton (2.504 eV) and bound-exciton (2.501, 2.495, and 2.493 eV) lines are studied in detail with respect to transition energy, temperature, and excitation intensity. Phonon-assisted transitions have been observed in the low-energy regions of zero-phonon lines at 2.493, 2.404, and 2.397 eV. Most of them have been well interpreted on the basis of the phonon energies previously obtained by Raman measurements.
Free energy and emittance growth in nonstationary charged particle beams
Authors: Reiser Martin
Published: July 2002

Charged particle beams that are not perfectly matched or centered when injected into a focusing channel (or accelerator) have a higher total transverse energy than the equivalent ideal stationary beam. The excess energy ΔE represents free energy that can cause emittance growth. A general relationship between the possible emittance growth and ΔE is derived, and analytical results are presented for nonuniform, mismatched, and off-centered beams.
Measurements of negative ion densities in 13.56-MHz rf plasmas of CF4, C2F6, CHF3, and C3F8 using microwave resonance and the photodetachment effect
Authors: Haverlag M. , Kono A. , Passchier D. , Kroesen G. M. W. , Goedheer W. J. , de Hoog F. J.
Published: July 2002

The high-power density of a frequency quadrupled pulsed Nd-YAG laser has been used to photodetach electrons from negative ions in rf plasmas generated within a microwave cavity. Negative ion densities have been determined by measuring the frequency shift of the resonance transmission, the shift being caused by the photoelectrons created by irradiating the plasma with the laser pulse. By measurement of the shape of the resonance curve as a function of time and of microwave frequency, and consecutive fitting of a parabola to the top of the resonance curve, the negative ion density has been determined as a function of gas pressure, rf power, and position in the plasma. Measurements were performed in plasmas of CF4, C2F6, CHF3, and C3F8. The results indicate that the negative ion densities are about one order of magnitude larger than the electron density, which is in good agreement with a fluid model calculation. The pressure and power dependence of the electron density and of the negative ion density gives insight in the relation between the electron temperature and the macroscopic plasma parameters. Measurements as a function of the laser wavelength, using a pulsed dye laser, show that in CF4 the negative ions mainly consist of F−, whereas in C2F6 significant densities of other negative ions may occur.
Phosphorus and nitrogen doping into polycrystalline SiC films prepared by plasma-enhanced chemical vapor deposition at 700 °C
Authors: Hasegawa S. , Furuta N. , Takeshita T. , Inokuma T. , Kurata Y.
Published: July 2002

Doped polycrystalline SiC films were deposited from a SiH4-CH4-H2-(PH3 or N2) mixture by plasma-enhanced chemical vapor deposition at 700 °C. The best crystallinity was obtained at x∼0.53 in Si1−xCx for both undoped and doped films. The crystallinity was enhanced by both P and N doping, but deteriorated again under high doping conditions. Also, better crystallinity was obtained by doping with P rather than N. Intrinsic tensile and compressive stresses were observed for P- and N-doped films, respectively. The resistivity and dangling-bond density decreased in correspondence to the enhancement in crystallinity. Origins of the dangling bonds and of a change in the crystallinity were discussed.
Evolution of electrical magnitudes in gradual p-n junctions with deep levels during the emission of majority carriers
Authors: Jimenez-Tejada J. A. , Lopez-Villanueva J. A. , Cartujo P. , Vicente J. , Carceller J. E.
Published: July 2002

A real (nonabrupt) p-n junction has been theoretically analyzed. It consists of a graded profile of shallow dopant atoms and a uniform profile of deep impurities, their relative concentrations varying along the structure (i.e., there are regions where deep impurity concentration is highest and others where dopant concentration dominates). This type of structure was excited by majority-carrier pulses, which allowed us to describe and explain new components in the charge distribution through the junction. The change in the interpretation of results from the application of capacitance techniques to these samples is quite remarkable. The validity of the theory is verified by comparison with experimental results obtained for silicon p-n junctions highly doped with platinum. The detailed analysis of the electrical model of a gradual junction with two deep levels, located in both halves of the band gap has allowed us to explain the following: (a) the disappearance of peaks in deep level transient spectroscopy (DLTS), (b) the existence of both positive and negative signals in a majority-DLTS spectrum, and (c) decreasing capacitance and voltage transients due to the emission of majority carriers or transients in which rising and falling sections are combined. The last two points cannot be explained by using the extended model of p+-n or n+-p junctions even if the deep-level concentration NT is assumed to be of the same order as the free carrier concentration.
Ignition and extinguishing characteristics of Cs-Ba tacitron
Authors: Murray Chris , Wernsman Bernard , El-Genk Mohamed S. , Kaibyshev Vladimir
Published: July 2002

The operation characteristics of the Cs-Ba tacitron as a switch are investigated experimentally in three modes: (a) breakdown mode; (b) I-V mode; and (c) current modulation mode. The switching frequency, grid potentials for ignition and extinguishing of discharge, and the Cs pressure and emission conditions (Ba pressure and emitter temperature) for stable current modulation are determined. The experimental data is also used to determine the off time required for successful ignition, and the effects of the aforementioned operation parameters on the ignition duty cycle threshold for stable modulation. Operation parameters measured include switching frequency up to 8 kHz, hold-off voltage up to 180 V, current densities in excess of 15 A/cm2, switch power density of 1 kW/cm2, and a switching efficiency in excess of 90% at collector voltages greater than 30 V. The voltage drop strongly depends on the Cs pressure and to a lesser extent on the emission conditions. Increasing the Cs pressure and/or the emission current lowers the voltage drop, however, for the same initial Cs pressure and emission conditions, the voltage drop in the I-V mode is usually lower than that during current modulation. As long as the discharge current is kept lower than the emission current, the voltage drop during stable current modulation could be as low as 3 V.
Effect of hydrogen dilution on the properties and bonding in plasma-deposited silicon nitride
Authors: Rocheleau Richard E. , Zhang Zhe , Niles David W. , Mason Alice
Published: July 2002

The effects of hydrogen dilution on the properties and structure of silicon nitride films deposited by plasma-enhanced chemical vapor deposition from NH3/SiH4 mixtures were studied. The addition of relatively small amounts of hydrogen at a fixed NH3/SiH4 ratio increased the excess Si in the film with a corresponding increase in the Si—H/N—H bonding ratio. At higher dilution [H2/(NH3+SiH4)] the films became more stoichiometric with significant changes in the hydrogen bonding. Decreases in the etch rate and refractive index with increasing hydrogen flow are discussed in terms of the changes in bonding structure and were found to be well correlated to changes in the Si—N bond density.
FeAlN/SiO2 and FeAlN/Al2O3 multilayers for thin-film recording heads (invited)
Authors: Kryder M. H. , Wang S. , Rook K.
Published: July 2002

FeAlN films and FeAlN/SiO2 and FeAlN/Al2O3 multilayer films have been studied as potential high moment head materials. The magnetic properties of FeAlN films on glass are similar to those of FeN, but the Al decreases the saturation magnetostriction constant and shifts the minimum coercivity point to a higher N content. Compared with the single layers, FeAlN/SiO2 and FeAlN/Al2O3 multilayers have reduced coercivity, and their permeance spectra have increased roll-off frequencies. In the multilayers, a 4πMs of 18–22 kG, a coercivity of 0.3 Oe, and a saturation magnetostriction constant smaller than 2×10−6 are obtained. These magnetic properties are very stable to annealing up to 360 °C, due to the presence of the Al. FeAlN and FeN single and multilayers show localized corrosion at defect sites when subjected to extended periods of a high-humidity/high-temperature test, but the corrosion is slow. Films with comparable properties to those on glass have been obtained on n-Si substrates; films on alumina-TiC substrates have also been studied.
Analysis and design of microwave amplifiers employing field-emitter arrays
Authors: Calame J. P. , Gray H. F. , Shaw J. L.
Published: July 2002

An analysis of the microwave ( f≳1 GHz) properties of field-emitter arrays (FEAs) and several representative medium power (10–100 W) microwave amplifiers employing FEAs is presented. The FEA analysis is limited to parallel-plate structures having discrete pointlike vertical emitter tips and gate apertures aligned to each tip. A transmission line analysis of wave propagation in this structure is presented and used to evaluate the geometries and materials needed for microwave operation. This analysis is used to investigate the performance capabilities and emitter requirements of both modulated-emission linear beam tubes and microdevices based on FEAs. Specific microtriode designs are used to investigate practical problems such as space charge and thermal effects. Competitive performance should be achievable in gated-emission linear beam tubes by using FEAs that perform at levels previously reported by several laboratories. Existing FEA technology (currents of 10 μA per emitter, transconductances of 1 μS per emitter, 1 μm oxide thickness, and 3 μm emitter spacing) is suitable for use in cavity klystrodes(r) at frequencies through 10 GHz, and in moderately bunched beam (bunch width of 180°), octave-bandwidth traveling-wave-tube applications through 3 GHz. Extending the operating frequency and/or reducing the bunch width will require a larger ratio of transconductance to current. Microtriodes operating at 10 GHz will benefit from a modified FEA structure and improved emitter performance.

An extra acceleration electrode must be added above the gate aperture to alleviate problems due to space charge between the gate and collector, and the gate oxide thickness must be increased to at least 2 μm. A FEA incorporating these features and capable of producing 5 μS and 100 μA per emitter could generate 130 W from a 5-mm-wide device with 8.6 dB gain, 7% bandwidth, and 36% power added efficiency. To allow higher gain and wideband operation, the transconductance at a given current must be increased. A FEA capable of producing 5 μS at only 10 μA per emitter would result in a microtriode with more than 1 octave bandwidth, 45 W output power, 10 dB gain, and 34% power added efficiency. Anode-to-case temperature differences of less than 100 °C appear possible in this device if BeO is used as the dielectric.
A single magnetic transition in a-Fe91Sc9
Authors: Ren Hong , Ryan D. H.
Published: July 2002

The magnetic ordering behavior of Fe91Sc9 has been studied by Mössbauer spectroscopy and magnetization measurements. Mössbauer spectra in an external field parallel to the γ beam clearly shows that the noncollinearity develops as soon as magnetic order appears, indicating a single transition to an asperomagnetic state, in contrast to the mean-field theory and Monte Carlo simulations which predict a transition from paramagnet to spin glass for strongly frustrated alloys. A possible origin for this discrepancy is discussed.
Measurements of polycapillary x-ray optics
Authors: Ullrich J. B. , Kovantsev V. , MacDonald C. A.
Published: July 2002

Assemblies of hollow capillary tubes, termed ‘‘Kumakhov optics,’’ can be used to control x rays for a large variety of applications. Measurements of x-ray transmission in polycapillary glass tubes were performed for the purpose of understanding their behavior in such devices. X-ray transmission was measured for straight, uniformly curved, and nonuniformly bent fibers. The data agree well with a computer simulation based on classical ray tracing.
Channel propagation in water and gelatin by a free-running erbium laser
Authors: Forrer Martin , Frenz Martin , Romano Valerio , Weber Heinz P. , Silenok Alexander , Konov Vitaly I.
Published: July 2002

Precise cutting of biological tissue is possible with the Er:YAG laser because of the strong absorption of radiation exhibited by water containing media at a wavelength of 2.94 μm. To achieve control of the depth of drilled channels a thorough knowledge of the channel propagation mechanism is required. The channel propagation process of pulsed erbium laser radiation in liquid water, and in gelatin with a high water content, as substitutes for biological tissue is investigated experimentally and modeled theoretically. We explain the propagation process with a hydrodynamic model, which describes the channel propagation process in terms of energy, mass, and momentum balance equations, which influence the evaporation pressure at the phase boundary. As the key feature, the theory takes into account the deformability of cold material below the zone of absorption. We show that by modeling this hydrodynamic effect with the Bernoulli equation we can explain the channel propagation velocity in water and gelatin as a function of laser intensity over three orders of magnitude with appreciable accuracy. The comparison with the experimental data suggests that the channel propagation velocity for intensities below 0.1 MW/cm2, and the threshold intensity of 12 kW/cm2 for channel propagation, are dependent on the surface tension and the liquid viscosity. At intensities above 0.1 MW/cm2, we can even predict a small difference between the propagation velocities found in these materials by considering the effect of the different elastic properties on the pressure in the propagating channel.
Pressure and temperature effects on the kinetics and quality of diamond films
Authors: Harris Stephen J. , Weiner Anita M.
Published: July 2002

Quantitative measurements of the effects of pressure on the kinetics and quality of diamond films grown with hot-filament chemical-vapor deposition are reported. Pressure affects growth kinetics largely because it affects transport of precursors to the growing surface. H and CH3 concentrations at the growth surfaces are determined with a recombination enthalpy technique combined with appropriate transport analyses. The growth rate rises and then falls with increasing pressure, although the concentrations of CH3 and atomic hydrogen at the surface are nearly constant. Both the rise and the fall in growth rate at higher pressure are explained with a chemical kinetics model as due in large part to an increase in substrate temperature at higher pressures. The fall at higher pressure (temperature) is due to the rate of thermal desorption of the CH3 precursor increasing more rapidly with temperature than the competing rate of its incorporation: Once these rates become comparable, higher substrate temperatures lower the incorporation rates, and the growth rate decreases. Previously measured Arrhenius plots for diamond-growth kinetics are explained quantitatively. The quality of the diamond, as determined using Raman and scanning electron micrograph data, falls with increasing pressure and substrate temperature. For the first time, this decline in quality is correlated with experimental temperature, H:CH3 ratio, and C2H2 concentration measurements.
Piezoelectric properties of sol-gel-derived ferroelectric and antiferroelectric thin layers
Authors: Li Jie-Fang , Viehland D. D. , Tani T. , Lakeman C. D. E. , Payne D. A.
Published: July 2002

The piezoelectric properties of ferroelectric and antiferroelectric thin layers were investigated by interferometry as a function of frequency and dc electric bias. Materials included: a ferroelectric lead zirconate titanate (PZT) with a Zr/Ti ratio of 53/47; a relaxor ferroelectric La-modified PZT with a Zr/Ti ratio of 70/30 and a La content of 7.8 at. %; and antiferroelectric lead zirconate. The hysteretic behavior of the piezoelectric coefficient due to polarization reversal was also studied. The layers were prepared by a sol-gel method, and were found to have properties similar to bulk ceramics. Significant strain levels could be achieved in the materials due to the relatively high electric strengths supported in thin-layer form. Polarization was found to be completely switchable between forward and reverse directions; however, the strain levels generated by switching were found to be inequivalent. In addition, pronounced piezoelectric relaxations and strong piezoelectric losses were observed.
On the theory of thermal stresses in a thin film on a ceramic substrate
Authors: Cherepanov Genady P.
Published: July 2002

It is generally recognized that thermal stresses in thin films are primarily responsible for morphological changes in thin films, including hillocks, whiskers, and void or pit formations, which present serious problems of reliability in microelectronics. In the present paper, the analytical theory of thermal stresses is constructed under some natural simplifying assumptions. The latter include that of the thermoelastic or thermoelastic-plastic behavior of the materials of a film and substrate after depositing the film and cooling the composite system to an operational temperature. The closed system of governing partial differential equations for thermal stresses in a thin film is derived for any in-plane shape of the film. Some particular problems are solved in an explicit form and the implication of the solutions for the prediction of hillocks is discussed. Numerical and physical experiments are planned to verify the theory.
Heat generation in semiconductor devices
Authors: Lindefelt U.
Published: July 2002

A general and practical model for heat generation that can be used in the heat conduction equation for nonisothermal semiconductor device simulations is presented. The model is developed for cubic semiconductors with position-dependent, multivalley, and multiband band structures, Fermi-Dirac statistics and an accurate treatment of electron-hole scattering. Starting from the Boltzmann transport equations for electrons, holes, and phonons, an internal energy balance equation, consistent with basic thermodynamic principles, is derived. By applying the linear phenomenological equations of irreversible thermodynamics and Onsager’s relations, the energy balance equation is reformulated into a heat conduction equation, and a heat generation source term is identified. The Peltier coefficients appearing in the model are analyzed, and thermal boundary conditions are given. The influence from optical effects, in particular photon reabsorption, is also discussed. Finally, physical insight into the mechanisms governing heat generation in semiconductor devices is obtained by an interpretation of the various contributions to the heat generation source term.
Temperature distributions in a heat dissipation system using a cylindrical diamond heat spreader on a copper heat sink
Authors: Hui P. , Tan H. S.
Published: July 2002

A rigorous analytical series solution is presented for the problem of temperature distributions in a heat dissipation system consisting of a cylindrical heat spreader on a semi-infinite heat sink. It is believed that this solution has not been published in the literature before. The method uses the Dini series and the Hankel transform and results in two intrinsic sets of infinite integrals, which are independent of geometric dimensions and thermal properties of the heat dissipation system. By transforming the infinite integrals into definite integrals, an efficient way to evaluate these integrals for the determination of the temperature distributions has been formulated. With diamond as the heat spreader and copper as the heat sink, the method using 40 expansion terms has been checked against the surface element method with ten matching nodes given by Beck et al. [J. Heat Transfer 115, 51 (1993)]. The calculations of radial distributions of temperature and the average temperature over the heating area for various values of normalized thickness of the heat spreader show the existence of a thickness of the heat spreader to achieve a minimum temperature. The dependence of the average temperature on the ratio of the radius of the heat spreaders to the radius of the heating aperture clearly shows that increasing the ratio beyond 20 will not reduce the average temperature significantly, indicating the existence of an effective radius for the heat spreader. Finally, a sensitivity study of the average temperature as a function of the spreader thickness, the radius, and its thermal conductivity reveals that the radius of the heat spreader is the most effective design parameter for lowering the surface temperature. This analytical study provides both some physical insights into the thermal behavior and a mathematical basis for optimal design of such a heat dissipation system.
Atomic ordering and domain structures in metal organic chemical vapor deposition grown InGaAs (001) layers
Authors: Seong Tae-Yeon , Norman A. G. , Booker G. R. , Cullis A. G.
Published: July 2002

Transmission electron microscope, transmission electron diffraction, and high resolution electron microscope studies have been made of metal organic chemical vapor deposition In0.53Ga0.47As layers grown on (001) InP or GaAs substrates to investigate the CuPt-type atomic ordering and associated microstructures present. The amount of ordering, the geometry of the (1̄11) and (11̄1) ordered domains, and the occurrence of anti-phase boundaries (APBs) were determined as a function of the layer growth temperature and rate. The results are interpreted in terms of mechanisms involving ordering at the layer surface and disordering in a transition region below the surface. From a consideration of the former it is concluded that atomic steps associated with surface undulations have a major influence on the domain geometry and APBs. The different structures that occur, their dependence on growth conditions and their possible effects on the electrical and optical properties are discussed.
Electron-energy-loss spectroscopy of mass-selected ion-beam-deposited diamondlike carbon
Authors: Kulik J. , Lifshitz Y. , Lempert G. D. , Rabalais J. W. , Marton D.
Published: July 2002

Amorphous diamondlike carbon (DLC) films grown by low-energy mass-selected ion-beam deposition have been examined by electron-energy-loss spectroscopy (EELS). Films grown using deposition energies of 50, 120, and 300 eV have been studied. For these deposition energies, all films exhibit similar EELS characteristics indicating a very high degree of sp3 bonding. The bulk plasmon resonance is intermediate between that of graphite and that of diamond; however, the properties of the low-energy-loss spectra of the DLC films are more similar to those of diamond. The near-K-edge carbon EELS data from the films exhibit a π* feature which is much smaller than that of graphite or evaporated carbon. The use of previously proposed computational methods on the near-K-edge EELS data indicates that over 80% of the carbon atoms are sp3 bonded. The size of the π* feature is larger for smaller plasmon energies, as expected. The present data are in accord with other analyses of similar films that indicate a broad (∼30–300 eV) energy window for diamondlike film formation.
Magnetic properties of a novel Sm3(Fe,Ti)29Ny nitride
Authors: Yang Fu-Ming , Nasunjilegal B. , Wang Jian-Li , Pan Hua-Yong , Qing Wei-Dong , Zhao Ru-Wen , Hu Bo-Ping , Wang Yi-Zhong , Liu Gui-Chuan , Li Hong-Shuo , Cadogan J. M.
Published: July 2002

A new interstitial nitride of Sm3(Fe0.933Ti0.067)29Ny (y=5) has been synthesized. Its x-ray pattern can be indexed in Nd3(Fe,Ti)29-type monoclinic symmetry with the lattice parameters of a=1.098 nm, b=0.882 nm, c=0.985 nm, and β=97.50°. The Sm3(Fe0.933Ti0.067)29Ny nitride exhibits ferromagnetic ordering with a Curie temperature Tc=750 K. The saturation magnetization Ms is 160 A m2/kg at 4.2 K and 140 A m2/kg at 293 K. The anisotropy field is 18.1 T at 4.2 K and 12.8 T at 293 K. The coercivity of μ0iHc=0.83 T at 293 K on this new nitride has been developed.
Nonequilibrium and bolometric photoresponse in patterned YBa2Cu3O7−δ thin films
Authors: Danerud M. , Winkler D. , Lindgren M. , Zorin M. , Trifonov V. , Karasik B. S. , Gol’tsman G. N. , Gershenzon E. M.
Published: July 2002

Epitaxial laser deposited YBa2Cu3O7−δ films of ∼50 nm thickness were patterned into detectors consisting of ten parallel 1 μm wide strips in order to study nonequilibrium and bolometric effects. Typically, the patterned samples had critical temperatures around 86 K, transition widths around 2 K and critical current densities above 1×106A/cm2 at 77 K. Pulsed laser measurements at 0.8 μm wavelength (17 ps full width at half maximum) showed a ∼30 ps response, attributed to electron heating, followed by a slower bolometric decay. Amplitude modulation in the band fmod=100 kHz–10 GHz of a laser with wavelength λ=0.8 μm showed two different thermal relaxations in the photoresponse. Phonon escape from the film (∼3 ns) is the limiting process, followed by heat diffusion in the substrate. Similar relaxations were also seen for λ=10.6 μm. The photoresponse measurements were made with the film in the resistive state and extended into the normal state. These states were created by supercritical bias currents. Measurements between 75 and 95 K (i.e., from below to above Tc) showed that the photoresponse was proportional to dR/dT for fmod=1 MHz and 4 GHz. The fast response is limited by the electron-phonon scattering time, estimated to 1.8 ps from experimental data. The responsivity both at 0.8 and 10.6 μm wavelength was ∼1.2 V/W at fmod=1 GHz and the noise equivalent power was calculated to 1.5×10−9 WHz−1/2 for the fast response.
The potential generated by current sources located in an insulated rectangular volume conductor
Authors: Ferguson A. Stewart , Stroink G.
Published: July 2002

An analytic solution is derived for the potential generated by a current dipole source located in a three-dimensional insulated volume conductor with rectangular cross section. The solution is first obtained from an eigenfunction expansion for a point current source in the volume with Neumann boundary conditions. A distributed volume current sink is introduced to maintain continuity of charge within the volume. The dipole potential is obtained directly from the point source potential and is shown to be reducible to a doubly infinite summation. An algorithm is presented for modifying the summation to achieve the most rapid convergence dependent on the specific geometry of the problem.
Detailed analysis of carrier transport in InAs0.3Sb0.7 layers grown on GaAs substrates by metalorganic chemical-vapor deposition
Authors: Besikci C. , Choi Y. H. , Labeyrie G. , Bigan E. , Razeghi M. , Cohen J. B. , Carsello J. , Dravid V. P.
Published: July 2002

InAs0.3Sb0.7 layers with mirrorlike morphology have been grown on GaAs substrates by low-pressure metalorganic chemical vapor deposition. A room-temperature electron Hall mobility of 2×104 cm2/V s has been obtained for a 2-μm-thick layer. Low-temperature resistivity of the layers depended on TMIn flow rate and layer thickness. Hall mobility decreased monotonically with decreasing temperature below 300 K. A 77 K conductivity profile has shown an anomalous increase in the sample conductivity with decreasing thickness except in the near vicinity of the heterointerface. In order to interpret the experimental data, the effects of different scattering mechanisms on carrier mobility have been calculated, and the influences of the lattice mismatch and surface conduction on the Hall measurements have been investigated by applying a three-layer Hall-effect model. Experimental and theoretical results suggest that the combined effects of the dislocations generated by the large lattice mismatch and strong surface inversion may lead to deceptive Hall measurements by reflecting typical n-type behavior for a p-type sample, and the measured carrier concentration may considerably be affected by the surface conduction up to near room temperature. A quantitative analysis of dislocation scattering has shown significant degradation in electron mobility for dislocation densities above 107 cm−2. The effects of dislocation scattering on hole mobility have been found to be less severe. It has also been observed that there is a critical epilayer thickness (∼1 μm) below which the surface electron mobility is limited by dislocation scattering.
Critical-state magnetization of type-II superconductors in rectangular slab and cylinder geometries
Authors: Johansen T. H. , Bratsberg H.
Published: July 2002

A scheme is described for analytical calculation of critical-state magnetization M of superconductors in the geometry of long rectangular slabs and cylindrical specimens in a parallel magnetic field. The simplicity of the general scheme is demonstrated by deriving compact expressions for the ascending and descending field branches of M in the exponential model Jc=jc0 exp(−B/B0) and in the Kim, Hempstead, and Strnad model [Phys. Rev. 129, 528 (1963)], Jc=jc0/(1+B/B0). The analyses focus on the vertical width ΔM of large field magnetization hysteresis loops. While Bean’s result [Phys. Rev. Lett. 8, 250 (1962)], Jc∝ΔM, today is used extensively to infer the critical current, it is well known that the method lacks consistency when a field dependence is seen in ΔM. For the two models it is shown explicitly that in the expansion of the functional relation ΔM(Jc), Bean’s result corresponds to the lowest-order term. Also to the next order in the functional expansion we find a unifying form of expressing the model behaviors. This term contains the second derivative of J2c(B) with a prefactor that depends on the sample geometry. A model-independent proof for the two first terms in the expansion of ΔM(Jc) is also given, which allows the significance of size and shape, i.e., thickness and aspect ratio, to be discussed on a general basis. New methods to extract Jc from ΔM data, one of them without having to invoke specific critical-state models, are indicated.
Phase transitions involving vacancy ordering in two metal mercuric iodides, Ag2HgI4 and Cu2HgI4
Authors: Lumsden Mark , Steinitz Michael , McAlduff E. J.
Published: July 2002

We have investigated the thermochromic order-disorder phase transitions in two superionic conducting compounds, Ag2HgI4 and Cu2HgI4, using capacitance dilatometry to determine the relative volume change, ΔV/V, at the transitions. We find an average ΔV/V of 9.6×10−3 for Ag2HgI4 at a transition temperature of 48.75 °C and an average ΔV/V of 4.0×10−3 for Cu2HgI4 at a transition temperature of 63 °C.
Effect of rapid thermal annealing on the photoluminescence properties of SiGe/Si heterostructures
Authors: Souifi A. , Benyattou T. , Guillot G. , Brémond G. , Dutartre D. , Warren P.
Published: July 2002

The effect of growth temperature and thermal treatments on the luminescence properties of SiGe/Si heterostructures grown by rapid thermal chemical-vapor deposition is reported. While the excitonic luminescence of the strained Si1−xGex layer is observed in the samples grown above 700 °C, the signal completely disappears for deposition temperatures lower than 650 °C. After rapid thermal annealing, we show that a drastic improvement of the luminescence efficiency of the layers deposited at low temperatures is obtained. A spectral blue shift of the excitonic luminescence can also be observed and is interpreted in terms of interdiffusion of Si and Ge atoms during the heating process. The photoluminescence spectra after a rapid thermal annealing at 1050 °C have been used for the first time to perform an accurate study of the thermal stability of strained Si0.85Ge0.15 alloys. It is shown that when the layers are in a metastable state before annealing, the relaxation phenomenon leads to a photoluminescence signal which consists of both band-edge and dislocation-related recombinations. In this case, the strain relaxation is mainly attributed to the formation of misfit dislocations at the SiGe/Si heterointerface. In very thin SiGe layers, only the band-edge luminescence can be observed, but it is shifted to the high-energy side as expected by the interdiffusion model. Using a simple theoretical approach, this shift can be used to calculate the interdiffusion coefficient in good agreement with the literature data.
Photoreflectance temperature dependence of graded InAlAs/InGaAs heterojunction bipolar transistor layers
Authors: Chen K. L. , Lin H. H. , Jan G. J. , Chen Y. H. , Tseng P. K.
Published: July 2002

Photoreflectance(PR) spectra of the graded InAlAs/InGaAs heterojunction bipolar transistor layers were investigated at various temperatures between 8 K and 300 K. The energy features of the PR spectra were fitted and identified as band-to-band transitions in the graded layers which were grown by pulsed molecular beam epitaxy (pulsed-MBE) and InGaAs as well as InAlAs layers. The temperature variation of energy gaps can be described by the Varshni and Bose-Einstein equations. A linear variation relationship of band gaps with Al composition (z) was observed and approximated to be E0(z)=0.809+0.769z eV at T=0 K. However, the parameters aB and ΘB derived from the Bose-Einstein expression do not change meaningfully in the whole range of Al composition. From the observed Franz-Keldysh oscillations (FKOs) we have evaluated the built-in dc electric fields in the i-InGaAs collector, i-InGaAs spacer and n-InAlAs emitter regions. The electric fields are in good agreement with the continuity condition of electric displacements in the interfaces between emitter and base.
Layer sequence and silicide formation of a Co/(refractory metal) bilayer on (100)Si substrate
Authors: Byun Jeong Soo , Kim Hyeong Joon
Published: July 2002

The solid state reactions of bilayer systems, such as Co/Cr and Co/V, with a silicon substrate have been investigated. The layer sequence could be explained in terms of competitive relations between the diffusion of cobalt atoms toward the substrate and the surface reaction (i.e., silicide formation at the silicon substrate). It was also found that the intermixing between the cobalt and the refractory beneath it is related to the solid solubility between them, and the layer reversal phenomenon critically depends on the silicide formation temperature of the interlayer refractory metal. For example, in the Co/Cr bilayer, where the silicide formation temperature of the chromium is not higher than that of the cobalt, only a partial layer reversal occurs. However, in the Co/V bilayer, where vanadium has a higher silicide formation temperature than cobalt, a complete layer reversal occurs.
Electric and magnetic fields in solenoidal coils from statics to MHz frequencies
Authors: Henjes Katja
Published: July 2002

The time-varying electric and magnetic fields inside a solenoidal coil have recently been calculated by Harmon. He modeled the coil by a cylindrical current sheet, used a physical approach starting from the Maxwell equations, and found results very different from what was believed from circuit theory since the 1920’s. Harmon’s derivation neglects the electric field-current relation in the wire, i.e., it violates Ohm’s law. In this article, the fields are derived from the Maxwell equations with this omission amended. Instead of the current, the terminal voltage of the coil is taken as the independent input. The relation between the current and the electric field in the coil wire constitutes an additional boundary condition. It allows as additional variable an energy influx from the power source, whereas Harmon considered radiation from the coil outward only. With these modifications, the old circuit theory results are recovered again. Implications are outlined for the physical legitimacy of other models involving a given current sheet.