bluuebunny/crossref_metadata_2025 huggingface dataset:
Linear and nonlinear optical properties of sol-gel-derived Au nanometer-particle-doped alumina
Authors: Hosoya Yousuke , Suga Taiji , Yanagawa Tsutomu , Kurokawa Youichi
Published: July 2002

The precursor Au-doped alumina gel film was prepared by a sol-gel method using HAuCl4⋅4H2O and alumina sol derived from AlCl3⋅6H2O. The precursor gel was heat treated in a H2 gas flow at 300–800 °C. Average particle diameters of the Au particle of doped films were in the range of 4.6–12.7 nm. In the absorption spectra of the film, plasmon resonance absorption was strongly dependent on the heat treatment temperature of the alumina matrix, showing more sharp spectra and significant red shift with decreasing particle size. It was interpreted by a modified Mie–Drude equation by taking into consideration of a spilling out of electrons from the Au particle to the alumina matrix. The measured values of χm(3) of the doped films are in the range of 9.2×10−6–4.0×10−5 esu in the neighborhood of plasmon resonance wavelength of 530–570 nm. These are larger than those reported for the other doped systems. A dominant nonlinear response on the 1–3 ps time scale was obtained, reflecting higher thermoconductivity of alumina compared to that of conventional matrix such as SiO2.
Analysis of n+p silicon junctions with varying substrate doping concentrations made under ultraclean processing technology
Authors: Aharoni Herzl , Ohmi Tadahiro , Oka Mauricio Massazumi , Nakada Akira , Tamai Yukio
Published: July 2002

Using highly controlled ultraclean processing technology, marked improvements in n+p Si junction quality are achieved presenting a theoretical significance. Boron-doped substrates with various boron doping concentrations Ns were As+ implanted, forming the n+ junction sides. The diffusion (Id) and generation (Igen) currents, as well as the ideality and the generation factors, are significantly reduced, and bulk generation lifetimes are prolonged. Using Shockley–Read–Hall theory it is found that a deviation of the trapping centers energy (Et) from the midband-gap energy (Ei) is responsible for the improvements. The experimental results show that |Et−Ei| is a function of Ns, and that the Igen/Id ratio is significantly low. Accordingly, it is proposed that Igen/Id ratio should be regarded, under certain conditions, as a figure of merit for junction quality. It is concluded that the |Et−Ei| deviation is related to the ultraclean processing technology used, due to the formation of new energy levels far from Ei and the suppression of introduction of new energy levels near Ei. Surface generation currents were found experimentally to be significant, and thus not negligible. Surface effects in general demonstrated similar trends to the bulk generation effects.
Magnetoresistance in ferromagnet-insulator-ferromagnet tunnel junctions with half-metallic ferromagnet NiMnSb compound
Authors: Tanaka C. T. , Nowak J. , Moodera J. S.
Published: July 2002

Ferromagnet-insulator-ferromagnet tunnel junctions with one NiMnSb electrode were studied to test the 100% conduction electron spin polarization predicted from band structure calculations performed on this compound. This phenomenon, known as half-metallic ferromagnetism (HMF), should result in significantly larger junction magnetoresistance (JMR) than in junctions using only conventional ferromagnetic materials such as Ni, Co, and Fe alloys which show JMR of up to 32%. Analysis by x-ray diffraction, Rutherford back scattering, SQUID, AFM, and STM confirm that the NiMnSb has the desired physical properties. A maximum JMR of 8.1% was observed in NiMnSb/Al2O3/Ni0.8Fe0.2 junctions at 77 K and 5.7% for NiMnSb/Al2O3/Co0.5Fe0.5. Maximum JMR for these two types of junctions at room temperature was 2.4% and 3.7%, respectively. The JMR observed is much lower than that expected for an HMF-I-FM junction. This could be due to scattering of the spins at the FM-I interfaces resulting from surface degradation of the NiMnSb, since the film growth requires deposition at elevated temperatures of >400 °C.
Novel absolute linear displacement sensor utilizing giant magnetoresistance elements
Authors: Miller M. M. , Prinz G. A. , Lubitz P. , Hoines L. , Krebs J. J. , Cheng S. F. , Parsons F. G.
Published: July 2002

A novel concept for an absolute linear displacement sensor utilizing giant magnetoresistance elements is introduced. This device is based on the division of a magnetically soft layer of a spin-valve device into two antiparallel magnetic domains. The total resistance of the spin valve varies linearly with displacement of the domain wall because of different resistances of the aligned and antialigned magnetic states of the spin valve. Domain-wall translation is achieved by the use of a magnetic wall “trap” generated by permanent magnets. Displacement is thus related directly to the device resistance. Position resolution to date is on the order of 10 μm over ranges of approximately 2 cm and on the order of a micron over reduced ranges. Magnetic hysteresis yields backlash on the order of tens of microns.
Photothermal study of subsurface cylindrical structures. II. Experimental results
Authors: Ocariz A. , Sanchez-Lavega A. , Salazar A.
Published: July 2002

We present an extensive experimental study of the photothermal signal produced by subsurface cylinders immersed in opaque solids. The results provided by three different modulated photothermal techniques (thermoreflectance, infrared radiometry, and mirage) for a variety of sample and cylinder thermal properties show very good agreement with the theoretical models presented in Part I of this work. A single subsurface cylinder can be quantitatively characterized by means of photothermal measurements (i.e., the determination of its size, depth, orientation, and thermal properties). We also present experimental data on a series of buried cylinders parallel to the surface that show multiple-scattering effects on the incident thermal wave.
Quantum confinement in nanostructured CdNiTe composite thin films
Authors: Alvarez-Fregoso O. , Mendoza-Alvarez J. G. , Zelaya-Angel O.
Published: July 2002

Cd 1−x Ni x Te nanostructured thin films have been deposited using the radio frequency sputtering technique. Films are formed by sphericallike microcrystallites with a broad grain size distribution. The measured nanocrystallite average diameters were 35, 30, and 26 nm corresponding to the films with Ni concentrations of 5%, 10%, and 15%, respectively. The film structure was zincblende, resembling the crystalline structure for bulk CdTe. Particle-size effects were observed in the optical absorption spectra. As the Ni content in the films increases, the grain size diminishes, and the optical band gap (Eg) energy shows a blue shift at a rate of about 7.5 meV/at. %, for Ni concentrations up to 15 at. %. Alloying effects additional to quantum confinement effects are discussed. The Eg shifts due to quantum confinement have been found to fall in the intermediate regime between strong and weak confinement. The quantum yield for the photoluminescence peak increases as the particle size decreases, probably as a result of exciton recombination enhancement due to better nanocrystallites quality.
Infrared spectroscopy of intraband transitions in self-organized InAs/GaAs quantum dots
Authors: Sauvage S. , Boucaud P. , Julien F. H. , Gérard J.-M. , Marzin J.-Y.
Published: July 2002

We have investigated the midinfrared absorption between confined levels of undoped InAs/GaAs quantum dots obtained by self-organized growth. The infrared absorption is measured by a photoinduced infrared spectroscopy. Quantum dots with different sizes are analyzed as a function of temperature, interband pump photon energy, intensity, and infrared polarization. We show that in the 90–250 meV energy range the quantum dots exhibit intraband absorption between confined levels, which are polarized along the growth axis as for usual conduction intersubband transitions in quantum wells. Intraband absorption is observed for either selective excitation of the dots or excitation via absorption in the wetting and GaAs layers. Based on the energy position and the temperature dependence, the infrared resonances are attributed to intraband transitions between confined holes and to bound-to-continuum transitions of electrons, which, respectively, shift to high and low energy as the dot size is decreased. The reported features are found in qualitative agreement with the theoretical predictions of Grundmann et al. [Phys. Rev. B 52, 11 969 (1995)].
Molecular dynamics simulation of reactive ion etching of Si by energetic Cl ions
Authors: Hanson D. E. , Voter A. F. , Kress J. D.
Published: July 2002

We report results from molecular dynamics simulations of the etching of a Si surface by energetic Cl atoms (15 eV⩽E⩽200 eV). We find that the energy dependence of the Si yield (number of Si atoms desorbed per incident Cl ion) is in reasonable agreement with recent experiments and with previous simulations performed up to 50 eV. We also investigate the variation of the Si yield with the impact angle of incidence, the stoichiometry of the desorbed material, and the effect of a thermal background Cl flux to the surface in the presence of an ion flux at 50 eV. Surface roughening due to etching was observed and the calculated rms roughness is in reasonable agreement with experiments.
Strain effects in CdTe/Si heterostructures
Authors: Han M. S. , Kang T. W. , Leem J. H. , Lee M. H. , Kim K. J. , Kim T. W.
Published: July 2002

Photoluminescence (PL) and spectroscopic ellipsometry measurements on CdTe/Si strained heterostructures grown by molecular beam epitaxy were carried out to investigate the effect of the strain and the dependence of the strain on the Si tilted substrates. The results of the PL spectra showed that the relative intensity ratio between the peak at 1.452 eV and the bound-exciton peak for the CdTe epilayer grown on the Si (100) 1° tilted substrate had a minimum value and that the strain for the CdTe epilayer grown on the Si (100) 8° tilted substrate had a minimum value. When rapid thermal annealing (RTA) was performed at 55 °C, the PL spectra showed that the relative intensity ratio between the peak at 1.452 eV and that at 1.574 eV for the CdTe epilayer grown on the Si (100) 8° tilted substrate had a minimum value and that the strain for the CdTe epilayer grown on the Si (100) 1° tilted substrate had a minimum value. Spectroscopic ellipsometry measurements showed that the spectrum of the dielectric constant of the CdTe epilayer grown on the Si (100) 8° tilted substrate is similar to that of the CdTe bulk. These results indicate that the strains in the CdTe layers grown on Si substrates are strongly dependent on the Si substrate orientation and that the crystallinity of the CdTe epitaxial layer grown on the Si substrate is remarkably improved by RTA.
Photothermal radiometric and spectroscopic measurements on silicon nitride thin films
Authors: Nestoros M. , Gutierrez-Llorente A. , Othonos A. , Christofides C. , Martinez-Duart J. M.
Published: July 2002

Thin films of silicon nitride of various thicknesses, deposited by radio frequency magnetron sputtering on silicon quartz substrate, have been characterized by laser-induced and frequency scanned photothermal radiometry. Fourier transform infrared spectroscopy was also used to provide a qualitative description of the behavior of the films in the infrared range which shows favourable properties of these coatings to be used in passive cooling applications.
Permittivity of lossy composite materials
Authors: Boudida A. , Beroual A. , Brosseau C.
Published: July 2002

An ab initio numerical simulation model has been used to compute the complex effective dielectric constant of a two-component lossy composite material, in the quasistatic limit. A computational algorithm with a conventional finite element formulation solves Laplace’s equation for a spatially heterogeneous medium, using the field calculation package FLUX3D. In this way, different three-dimensional topological arrangements of the components were considered. The composite material consists of dense spheres of uniform size that are arranged in simple, body-centered, and face-centered cubic lattices. The accuracy of the method is checked by comparing with results previously presented in the literature. Detailed predictions provide a comparison with percolation theory when the imaginary part of the relative permittivity of the spheres is very large. A comparison with McLachlan’s generalized effective medium equation [D. S. McLachlan, J. Phys. C 20, 865 (1987)] is further provided over a wide range of conditions. From these calculations one can conclude that there are significant discrepancies between the ab initio evaluated values of the effective permittivity and those obtained on the basis of McLachlan’s analysis. On the one hand, the numerical method demonstrated here shows that the real part of the effective permittivity, obtained from ab initio results, can be significantly different from that predicted on the basis of McLachlan’s equation when the imaginary part of the permittivity of the inclusion is very large compared to its real part. On the other hand, these computational results capture the trends in the percolation threshold variation with cubic lattice packing. We measured the exponents s and t which determine how the real and imaginary parts of the permittivity scales with the distance from the percolation threshold. This behavior is most probably due to the drastic differences in the basic assumptions existing between McLachlan’s modeling and our numerical approach. In particular, this analysis makes it clear that any approach based only on the dipole approximation must fail to correctly describe the complex effective dielectric constant, over the entire range of volume fraction of spherical inclusions.
Real-time depth profile reconstruction of the thermal conductivity of inhomogeneous solids
Authors: Kolarov R. , Velinov T.
Published: July 2002

A new method for deriving the one-dimensional temperature distribution in a highly absorbing solid with continuously varying parameters is proposed. This method is based on a Riccati first-order ordinary differential equation and assumes sinusoidally modulated surface heating. Numerical solutions for several thermal conductivity profiles are compared with data obtained from models that approximate the original profiles with steplike variation of the thermal conductivity. A fast real-time inversion procedure is proposed to reconstruct the thermal conductivity profile from surface temperature data. It is based on the step-by-step reconstruction of this profile by sweeping the modulation frequency of the heating beam. The algorithm uses data from previous, higher frequencies to find the next optimum frequency. The measurements and the profile reconstruction are performed simultaneously. This procedure is tested for different profiles, including some with complex behavior and is found to be stable and reliable. Some limitations concerning the profile of the thermal conductivity are discussed.
In situ transmission electron microscopy of AlN growth by nitridation of (0001) α-Al2O3
Authors: Yeadon Mark , Marshall Michael T. , Hamdani Fayçal , Pekin Senol , Morkoç Hadis , Gibson J. Murray
Published: July 2002

Using a novel ultrahigh vacuum transmission electron microscope (TEM) with in situ reactive molecular beam epitaxy system, we report the successful synthesis of epitaxial AlN on the (0001) sapphire surface upon exposure to ammonia. The substrate, an electron transparent sapphire foil, was annealed in oxygen at high temperature to eliminate damage induced during preparation and to create large atomically flat regions. The sample was then held at 950 °C in flowing ammonia inside the microscope for 2 h during which periodic observations of microstructural development were made. We report direct observation of the formation of epitaxial AlN with the orientation relationship (0001)AlN//(0001)sub, [1̄010]AlN//[1̄1̄20]sub and present both TEM and atomic force microscope images of the sample before and after nitridation. The results are consistent with a diffusion-limited reaction model involving transport of oxygen and nitrogen ions through the growing AlN epilayer between the free surface and the unreacted α-Al2O3.
Combination of thermal and electromechanical breakdown mechanisms to analyze the dielectric breakdown in polyethylene terephthalate
Authors: Zebouchi N. , Malec D.
Published: July 2002

As an extension of the previous work on numerical analysis of electrical breakdown in polyethylene terephthalate films using electromechanical and thermal breakdown theories separately, an attempt is now made to combine these two models by introducing into the heat balance equation, variations in sample thickness which depend on Young’s modulus. Results of calculations have shown that although this model has improved the explanation of the observed negative temperature dependence and the positive pressure dependence of the experimental electrical breakdown field investigated at temperatures of 35, 70, and 100 °C under hydrostatic pressure ranging from 50 to 500 bar, a difference still remained between experimental and theoretical results. This could be attributed to other effects such as the space charge, which had to be neglected in this analysis to avoid complications.
Structural and magnetic properties of “expanded” Mn
Authors: Grigorov Ilya L. , Walker J. C. , Hawley M. E. , Brown G. W. , Lütt M. , Fitzsimmons M. R.
Published: July 2002

Structural and magnetic properties of “expanded” Mn deposited on (111) oriented fcc noble metals were studied with single-crystal x-ray diffraction and exchange bias measurements. A single peak corresponding to this phase was found at momentum transfer q=2.86 Å−1 along six equivalent [112̄] directions of the noble metal substrate. Magnetic hysteresis of the field cooled Fe/Mn bilayers exhibited a characteristic shift along the field axis, indicating antiferromagnetic order in the expanded Mn with TN⩾20 K. The magnetic and structural data are consistent with understanding the expanded phase as trigonally distorted α-Mn.
Physical mechanisms in double-carrier trap-charge limited transport processes in organic electroluminescent devices: A numerical study
Authors: Shen Jun , Yang Jie
Published: July 2002

We have developed a numerical model to simulate the double-carrier injection trap-charge limited (TCL) transport processes in organic electroluminescent devices (OEDs). Current–voltage characteristics, energy and charge profiles are obtained and analyzed to understand the mechanisms governing the OED performance under TCL conditions. Good agreement with general experimental trends and previous analytical predictions are achieved. Dependencies on several important parameters (carrier injection level, device thickness, recombination constant, temperature, etc.) are also studied.
Electrical relaxation mechanism in unconventional bismuth cuprate glasses
Authors: Hazra S. , Ghosh A.
Published: July 2002

The frequency dependent dielectric constant and loss of the unconventional bismuth cuprate glasses have been presented in wide temperature and frequency ranges. Alternating current dielectric loss peak has been observed in the measurable frequency range at higher temperatures, where the measured ac conductivity approaches the dc conductivity. The temperature range where ac loss peak has been observed, varies systematically with glass composition. The dielectric data at these temperature ranges have been analyzed in terms of different theoretical models to find out the possible relaxation mechanism. It has been observed that the dipolar relaxation model with a distribution of relaxation times can best explain the experimental data. The dipolar relaxation occurs due to the hopping of charge carriers between different localized valence states of copper ions within a range of energies near the mobility edge. High value of the dielectric constant observed in these glasses can be attributed to the influence of high polarizability of the Bi3+ ions of the unconventional network former Bi2O3 to the ac response. The higher distribution of the relaxation times in the present glasses compared to the unconventional lead cuprate glasses indicates less homogeneity of the bismuth cuprate glass system.
Resonant interband tunneling current in InAs/AlSb/GaSb/AlSb/InAs double barrier diodes
Authors: Kitabayashi Hiroto , Waho Takao , Yamamoto Masafumi
Published: July 2002

We have investigated the resonant interband tunneling currents (IRIT) in InAs/AlSb/GaSb/AlSb/InAs double-barrier resonant interband tunneling diodes. We have prepared samples with various thicknesses of the GaSb well (Lw) ranging from 5 to 47 monolayers (ML) and various thicknesses of the AlSb barrier (Lb) ranging from 0 to 5 ML, and analyzed the dependence of IRIT on these thicknesses. As Lw increases, IRIT increases if Lw is smaller than a certain boundary value of Lw (Lwb), but IRIT decreases if Lw is larger than Lwb. We have found that this boundary value Lwb depends on Lb, that is, Lwb decreases from 23 to 10 ML when Lb changes from 0.5 to 5 ML. Furthermore, we have also found that, for constant Lw, IRIT increases as Lb increases to a certain boundary value of Lb, and IRIT decreases for further increase in Lb. These marked behaviors of Lwb and IRIT on Lb are characteristic of the type II material systems and cannot be explained by the conventional model applied to type I resonant tunneling diodes. To explain these characteristic behaviors, we propose a model where the resonance level in the valence band of the GaSb well moves up toward the valence-band edge with an increase in Lb.
Electronic characteristics of the SrBi2Ta2O9–Pt junction
Authors: Watanabe K. , Hartmann A. J. , Lamb R. N. , Scott J. F.
Published: July 2002

The voltage and film composition dependence of leakage currents of ferroelectric SrBi2Ta2O9 thin films, sandwiched between Pt has been studied. Schottky emission dominated the leakage current at voltages above the ohmic conduction regime, while space charge limited currents (SCLC), for which the observed temperature dependence is correctly predicted in Rose’s theory, appeared to dominate the leakage current in high conductivity SrBi2Ta2O9 thin films including bismuth-excess samples. A consequence of the latter was the observation of negative differential resistivity in high conductivity SrBi2Ta2O9 thin films. X-ray photoemission spectroscopy depth profiling indicated that Bi has diffused into the ferroelectric–metal interface and also influenced the electronic conduction mechanism of the ferroelectric capacitors. Confirmation of this was found through the current–voltage dependence of Pt/SrBi2Ta2O9/Bi, which is compatible with space charge limited currents. The theory of Rose was successfully applied to the temperature and voltage dependencies of leakage currents in Bi-excess SBT; this is the first application of such theories to ferroelectric films and results in a generalization of the Child’s Law approximation for space-charge-limited currents. A single fitting parameter T*=315±20 K satisfied data at the voltages 3<V<4 and temperatures 300 K<T<400 K most useful for engineering device applications.
Atomistic simulations of the vapor deposition of Ni/Cu/Ni multilayers: The effects of adatom incident energy
Authors: Zhou X. W. , Wadley H. N. G.
Published: July 2002

Vapor deposited multilayers consisting of a low electrical resistivity conductor sandwiched between ferromagnetic metals such as cobalt or nickel-iron alloys sometimes exhibit giant magnetoresistance (GMR). The GMR properties of these films are a sensitive function of structure and defects in the films and therefore depend upon the processing conditions used for their synthesis. A three-dimensional molecular dynamics method has been developed to simulate the [111] growth of model Ni/Cu/Ni multilayers and was used to investigate the role of vapor atom impact energy upon the film structure and defects. High incident atom energies were found to lower interfacial roughness but promoted intermixing by an atomic exchange mechanism. Low incident energies reduced intermixing, but resulted in films with rough, defective interfaces. The simulations identified an intermediate incident energy between 1 and 2 eV that resulted in both low roughness and intermixing, and an anticipated large GMR effect. The simulation methodology was extended to explore the benefits of a modulated incident atom energy deposition strategy. When a thermal energy was used to deposit the first few monolayers of each new metal layer, intermixing by the exchange mechanism during subsequent hyperthermal energy deposition could be eliminated, and films with almost no interfacial roughness or intermixing could be grown. The modulated energy deposition method can be simply implemented using ion beam deposition.
Deep levels in virtually unstrained InGaAs layers deposited on GaAs
Authors: Pal D. , Gombia E. , Mosca R. , Bosacchi A. , Franchi S.
Published: July 2002

The dislocation-related deep levels in InxGa1−xAs layers grown by molecular beam epitaxy on GaAs substrates have been investigated. Virtually unstrained InGaAs layers with mole fraction x of 0.10, 0.20, and 0.30 have been obtained by properly designing the In composition of linearly graded InxGa1−xAs buffers. Two electron traps, labeled as E2 and E3, whose activation energy scales well with the energy gap, have been found. Unlike E2, E3 shows: (i) a logarithmic dependence of the deep level transient spectroscopy amplitude on the filling pulse width and (ii) an increase of concentration as the buffer/InGaAs interface is approached. These findings, together with the observation that, in compressively strained In0.2Ga0.8As, the E3-related concentration is definitely higher than that of virtually unstrained In0.2Ga0.8As, indicate that this trap is likely originated by extended defects like threading dislocations.
Magnetic-domain structure of Nd2Fe14B permanent magnets
Authors: Zhu Yimei , McCartney M. R.
Published: July 2002

We report our studies of magnetic-domain structure in Nd2Fe14B permanent magnets using advanced transmission and scanning electron microscopy, including Lorentz imaging and electron holography. Domain-wall spacing D was measured as a function of thickness t for thin crystals, and also wall width of 90° and 180° domains; the domain-wall energies were calculated. Orientation and field dependence of the domain configuration and domain-wall behavior were examined in situ under different temperatures and applied fields. Characteristic magnetic structural features unique to corresponding bulk samples, such as reversed spike-domains and undulating domain boundaries, were also investigated.
Thermal donor formation and annihilation at temperatures above 500 °C in Czochralski-grown Si
Authors: Götz W. , Pensl G. , Zulehner W. , Newman R. C. , McQuaid S. A.
Published: July 2002

Thermal donors (TDs) are generated in Czochralski (CZ)-grown silicon by heat treatments around 450 °C. They form several individual effective-mass-like donors with slightly different ionization energies and act as double donors (TDx0,TDx+). Heat treatments at elevated temperatures (e.g., >500 °C) lead to a competition of the formation and the annihilation of TDs. We studied the formation and the annihilation of TDs in the temperature range between 520 and 700 °C. CZ-grown Si samples with an initial total TD concentration of ∼5×1015 cm−3 were employed to study the annihilation of TDs. The number of interstitial oxygen atoms generated per annihilated TD center depends on the temperature and ranges from 4 to 24. For the temperature range investigated the activation energy for thermal annihilation of TDs was determined to be 2.5±0.4 eV. The same CZ-Si material but with an initial TD concentration of ∼2×1013 cm−3 was used to study the formation of TDs. During annealing, the concentrations of individual TDs reach equilibrium concentrations, which depend on the temperature of the final annealing step and the total oxygen concentration. We demonstrate that a model in which the individual TDx centers are represented by oxygen clusters of different sizes consistently explains our experimental data.
Some effects of conduction band nonparabolicity on electron reflection spectrum of multiquantum barriers
Authors: Irikawa Michinori , Ishikawa Takuya , Sasaki Yoshitaka , Iwasawa Katsumi , Suemune Ikuo , Iga Kenichi
Published: July 2002

The effect of nonparabolicity of conduction band on the electron reflection spectrum of multiquantum barriers (MQB) has been examined. Drastic reduction in the effective barrier height is expected by adopting the nonparabolic model on the MQB which had been designed using the parabolic model for 1.5 μm semiconductor lasers (LDs). The predicted enhancement in barrier height by the MQB is over 600 meV under parabolic model. However, it decreases to 40 meV under nonparabolic model with the same structure. On the other hand, the experimental enhancement in barrier height by the MQB is estimated to be around 30 meV on 1.5 μm LDs, close to the value calculated by nonparabolic model. Those results suggest that a much higher effective barrier height can be realized by optimizing the MQB design taking the nonparabolicity into account. The conduction band nonparabolicity was incorporated by the k⋅p perturbation method.
Quasiballistic transport in GaAs-based heterojunction and homojunction bipolar transistors
Authors: St. Denis A. R. , Pulfrey D. L.
Published: July 2002

An iterative approach to solving an implicit integral relation for the electron distribution function in the base of a bipolar transistor is exploited to achieve a solution to the field-free Boltzmann transport equation. The method, which is based on one previously applied to Si homojunction transistors, is extended here to hetero- and homojunction transistors in the GaAs material system. This involves incorporating tunneling and reflection into the boundary condition for the injected flux at the emitter end of the quasineutral base, and considering anisotropic and inelastic scattering mechanisms. The ballistic, scattered, and reflected portions of the distribution are examined as the base width is reduced to values where quasiballistic transport is evident. Numerical results are presented for the carrier concentration and velocity profiles, and for the base transit time.
Infrared studies of transition layers at SiO2/Si interface
Authors: Ono Haruhiko , Ikarashi Taeko , Ando Koichi , Kitano Tomohisa
Published: July 2002

We investigated transition layers at the interface of the thin SiO2 film successively etched back by diluted HF, using infrared reflection-absorption spectroscopy. The etching rate of the oxide film reveals that there is a Si-rich transition layer within 0.6 nm of the interface. However, frequency shift in the longitudinal optical phonon due to Si-O-Si asymmetric stretching toward lower wave numbers takes place less than 1.5 nm from the interface. We propose a model in which the transition layer is assumed to be Si-rich suboxide layers caused by the compositional roughness of the SiO2/Si interface. Through estimating the phonon frequencies which depend on the composition of the suboxide structure in this model, we found that the phonon frequency apparently starts to shift at around 1.5 nm from the interface, even if there are suboxide-rich layers within 0.6 nm, which can be caused by 1–2 monolayers of roughness.
Fabrication and radio frequency characterization of high dielectric loss tubule-based composites near percolation
Authors: Browning S. L. , Lodge J. , Price R. R. , Schelleng J. , Schoen P. E. , Zabetakis D.
Published: July 2002

Microscopic lipid tubules with an average aspect ratio (length/diameter) of approximately 12 were metallized electrolessly with copper or nickel-over-copper, and mixed with vinyl to make foot-square composite dielectric panels. As loadings increased the metal tubule composites displayed an onset of electrical percolation with accompanying sharp increases in real and imaginary permittivities. Gravity-induced settling of the tubules while the vinyl was drying increased true loading density at percolation threshold for nickel/copper tubules to ∼12 vol %. This threshold was at a significantly lower loading density than that previously measured for percolation by composites containing spherical conducting particles. Qualitatively, the shape of the composite permittivity versus loading density curves followed predictions by the effective-mean field theory for conducting stick composites. Changes in permittivity of the vinyl panels were observed for several days after fabrication, and were apparently associated with solvent evaporation from the matrix.
An improved permeameter for thin film measurements up to 6 GHz
Authors: Pain D. , Ledieu M. , Acher O. , Adenot A. L. , Duverger F.
Published: July 2002

We report a broad band technique allowing the measurement of the permeability of thin ferromagnetic films up to 6 GHz. The permeameter is based on a single coil technique. The input impedance of the loop is measured with and without the sample. The permeability is deduced from the impedance measurement using two approaches: the first one uses an equivalent electrical circuit model and the second a transmission line theory model. This leads to significant corrections compared to previous models. Calibration of the apparatus is presented using a known sample. Our two approaches are compared to the theoretical spectral permeability. The validity of the measurements is proven by showing a full spectral gyromagnetic response of a saturated magnetic sample at different fields.
Simulation of dislocations on the mesoscopic scale. II. Application to strained-layer relaxation
Authors: Schwarz K. W.
Published: July 2002

Numerical simulations are used to study various dislocation interactions expected to occur when a strained epitaxial layer relaxes. Frank–Read sources located on parallel glide planes are shown to produce elaborate network patterns of dislocations in the unstrained substrate of the layer. These are strikingly similar to experimentally observed patterns in high-quality relaxed SiGe layers grown by low-temperature ultrahigh vacuum chemical vapor deposition. Frank–Read sources on intersecting glide planes, on the other hand, produce characteristic corner-pileup structures which have previously been attributed to a “modified” Frank–Read source mechanism. The blocking of threading arms by misfit dislocations is found to occur only very near the critical strain. A much more important mechanism preventing ideal layer relaxation appears to be the formation of immobile pairs of threading dislocations on parallel glide planes.
Piezoresistive response induced by piezoelectric charges in n-type GaAs mesa resistors for application in stress transducers
Authors: Hsu Y. W. , Lu S. S. , Chang P. Z.
Published: July 2002

The resistance change due to piezoelectric charge densities in n-GaAs mesa resistors has been studied as a function of surface stress. Those changes are verified by measuring some realized stress transducers with such resistors fabricated on the surface of micromachined thin GaAs membranes or GaInP/GaAs cantilever beams. It is shown here that the surface stresses induced by the deformation of cantilever beams can cause nonuniform stress distributions within the resistors, and the stress gradients consequently yield considerable piezoelectric charge densities that lead to an appreciable resistance change. In addition, this effect of piezoelectric charges is examined from several types of resistors that have a different doping, direction, thickness h, and width-to-thickness ratio L/h. According to those results, optimization of this mechanism is related to the doping and geometrical design of GaAs mesa resistors. In addition to the effect of piezoelectric charges, mobility change can affect the resistance as well. The resistance changes observed from the low doped resistors exhibit an opposite sign for the resistors oriented along [011] and [011̄] directions. These directional-dependent characteristics confirm that the effect of piezoelectric charges indeed exists in mesa resistors. Fitting the width-dependent sensitivities measured in experiments allows us to estimate each contribution of these two effects. For the [011̄] oriented resistors with L/h of 10, and ns of 4.8×1011, the sensitivity of relative resistance change as high as 92.7%/GPa is obtained. Results in this study demonstrate that the resistance change in n-GaAs mesa resistors is attributed to the effects of piezoelectric charges and mobility change. Moreover, GaInP/GaAs material system with its piezoresistive response originated from piezoelectric charges is highly promising in III–V compound semiconductor stress transducers.
InGaAsSb thermophotovoltaic diode: Physics evaluation
Authors: Charache G. W. , Baldasaro P. F. , Danielson L. R. , DePoy D. M. , Freeman M. J. , Wang C. A. , Choi H. K. , Garbuzov D. Z. , Martinelli R. U. , Khalfin V. , Saroop S. , Borrego J. M. , Gutmann R. J.
Published: July 2002

The hotside operating temperatures for many projected thermophotovoltaic (TPV) conversion system applications are approximately 1000 °C, which sets an upper limit on the TPV diode band gap of 0.6 eV from efficiency and power density considerations. This band gap requirement has necessitated the development of new diode material systems never previously considered for energy generation. To date, InGaAsSb quaternary diodes grown lattice matched on GaSb substrates have achieved the highest performance. In this article we relate observed diode performance to electro-optical properties such as minority carrier lifetime, diffusion length, and mobility and provide initial links to microstructural properties. This analysis has bounded potential diode performance improvements. For the 0.53 eV InGaAsSb diodes used in this analysis (active layer doping is 2×1017 cm−3) the dark current density measured is 2×10−5 A/cm2 versus a potential Auger and/or a radiative limit of 2×10−6 A/cm2 (no photon recycling), and an absolute thermodynamic limit of 1.4×10−7 A/cm2. These dark current limits are equivalent to open circuit voltage gains of 60 (20%) and 140 mV (45%), respectively.
Mo/SiO
2
multilayers for soft x-ray optical applications
Authors: Wang Feng-ping , Wang Pei-xuan , Lu Kun-quan , Fang Zheng-zhi , Gao Min , Duan Xiao-feng , Cui Ming-qi , Ma Hong-ji , Jiang Xiao-ming
Published: July 2002

The optimal reflectivity of soft-x-ray multilayers made of new pairs of materials was theoretically calculated in the wavelength range of 2.0–4.5 nm. Molybdenum and silicon dioxide were then selected for “high index” and “low index” layer, respectively, in the multilayer system. The microstructures and composition profiles of multilayers of molybdenum and silicon dioxide were investigated by means of low-angle x-ray diffraction, cross-sectional high resolution electron microscopy, an energy-filtering transmission electron microscope elemental mapping, and Auger electron spectrometry. The results show that no diffusion of Si into Mo layers has occurred, and only slight diffusion of O into Mo layers are seen. Sharp and relatively smooth interfaces have formed. With increasing number of layers, the interfacial roughness was propagated through the multilayer stack and low-frequency roughness increases while the high-frequency roughness decreases.
Interlayer coupling of Fe/Si/Fe trilayers with very thin boundary layers
Authors: Endo Y. , Kitakami O. , Shimada Y.
Published: July 2002

The interlayer magnetic coupling of a Fe/Si/Fe trilayer shows an analogous feature to that of Fe/Si superlattices. With an increase in Si layer thickness, it oscillates as ferromagnetic (first F), antiferromagnetic (AF), ferromagnetic (second F), and finally reaches a noncoupling (N) state. We have investigated interlayer coupling of Fe/Si/Fe trilayers inserting very thin (1 or 2 ML thick) boundary layers X (X=Ag, Ge, Fe–Si, Ta, etc.). They are expected to suppress interatomic diffusion between Fe and Si layers. Interlayer coupling of Fe/X/Si/X/Fe with negligible interdiffusion is simply F and changes to N as the Si layer thickness increases. Furthermore, Fe/Fe–Si/Fe trilayers which show coupling of first F, AF but not second F, reproduce second F when a Si layer is inserted in the Fe–Si spacer. These results imply that an amorphous Si spacer mediates ferromagnetic coupling between neighboring Fe layers while the first F and the strong AF coupling usually observed in Fe/Si superlattices are caused by diffused crystalline Fe–Si.
Numerical analysis of shroud gas effects on air entrainment into thermal plasma jet in ambient atmosphere of normal pressure
Authors: Kang Kyoung Doo , Hong Sang Hee
Published: July 2002

A numerical analysis of the influence of air entrainment into the plasma jet on the thermal plasma characteristics is performed to provide a design basis for nontransferred plasma torches operated in an ambient air of atmospheric pressure along with shroud gas injection. The assumption of steady-state, axisymmetric, local thermodynamic equilibrium, and optically thin plasma is adopted in a two-dimensional modeling of thermal plasma flow with an annular shroud gas shell. A control volume method and a modified semi-implicit pressure linked equations revised algorithm (known as SIMPLER) are used for solving the governing equations, i.e., the conservation equations of mass, momentum, and energy along with the equations describing the so-called K–ε model for flow turbulent kinetic energy (K) and its dissipation rate (ε), and the mass fraction equations for gas mixing. The two-dimensional distributions of temperature and flow velocity of the thermal plasma jet as well as the air mole fraction mixed with the plasma are found in an exterior jet expanding region outside the torch, and they are compared for the two cases with and without shroud gas injection. As a result of calculations, the flow rate of the injected shroud gas and the location of its injector turn out to be major parameters for controlling ambient air entrainment. The calculations also reveal that the annular injection of shroud gas surrounding the plasma jet reduces air entrainment into the plasma jet remarkably while it does not significantly affect the plasma temperature and velocity. The present numerical modeling suggests the optimum design and operating values of an argon shroud gas injector for minimizing air entrainment into the thermal plasma flame ejected from the nontransferred plasma torch operated at normal pressure in the ambient atmosphere.
Oxygenation and air-annealing effects on the electronic properties of Cu(In,Ga)Se2 films and devices
Authors: Rau U. , Braunger D. , Herberholz R. , Schock H. W. , Guillemoles J.-F. , Kronik L. , Cahen David
Published: July 2002

Post-deposition air-annealing effects of Cu(In,Ga)Se2 based thin films and heterojunction solar cell devices are studied by photoelectron spectroscopy and admittance spectroscopy. Ultraviolet photoelectron spectroscopy reveals type inversion at the surface of the as-prepared films, which is eliminated after exposure of several minutes to air due to the passivation of surface Se deficiencies. X-ray photoelectron spectroscopy demonstrates that air annealing at 200 °C leads to a decreased Cu concentration at the film surface. Admittance spectroscopy of complete ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells shows that the Cu(In,Ga)Se2 surface type inversion is restored by the chemical bath used for CdS deposition. Air annealing of the finished devices at 200 °C reduces the type inversion again due to defect passivation. Our results also show that oxygenation leads to a charge redistribution and to a significant compensation of the effective acceptor density in the bulk of the absorber. This is consistent with the release of Cu from the absorber surface and its redistribution in the bulk.
Alternative pathway for the formation of C54 TiSi2
Authors: Mouroux Aliette , Zhang Shi-Li
Published: July 2002

The influence of interfacial Mo on the formation of TiSi2 is studied using 120 nm Ti layers deposited on Si (100) substrates. After annealing at 450 °C, C54 TiSi2 and C40 (Ti,Mo)Si2 are found in the samples initially having an interposed layer of Mo 1.6–2 nm thick. In the absence of Mo, only C49 TiSi2 is obtained. The pathway for the formation of C54 TiSi2 is altered from the usual C49–C54 phase transformation to the epitaxial growth of C54 TiSi2 on C40 (Ti,Mo)Si2. The resistivity of the TiSi2 layers formed is about 14 and 61 μΩ cm for the C54 and C49 phase, respectively. However, for equal annealing time, the thickness of the C49 TiSi2 formed is about ten times that of the C54 TiSi2 grown on C40 (Ti,Mo)Si2, because of the barrier effects on Mo or C40 (Ti,Mo)Si2 on Si diffusion. The experimental results are discussed on the basis of energetic arguments to account for the suppressed formation of C49 TiSi2 and the enhanced formation of C54 TiSi2 at 450 °C.
Changes in chemical structure and optical properties of polyimide films induced by ion bombardment
Authors: Zhang Lulu , Yasui Toshiaki , Tahara Hirokazu , Yoshikawa Takao
Published: July 2002

The effect of ion bombardment on the chemical structure and optical properties of polyimide films was investigated. Oxygen and nitrogen ion beams with energies in the range of 0.5–1.0 keV and doses between 1.7×1015 and 8.2×1016 ions/cm2 were irradiated on the films, and in situ x-ray photoelectron spectroscopy (XPS) and spectral transmittance analyses were carried out. The in situ XPS analyses showed that the addition reaction of oxygen and nitrogen atoms and desorption of components caused chemical structural changes in ion-bombarded polyimides. The in situ spectral analyses showed that the spectral transmittances of films were decreased by ion bombardment.
Variable range hopping conduction in bulk samples composed of single-walled carbon nanotubes
Authors: Yosida Y. , Oguro I.
Published: July 2002

An experimental study of the electrical transport phenomena in magnetic fields H up to 15 T in pressed samples of a porous system composed of single-walled carbon nanotubes in the temperature range 1.39 K <T<300 K is reported. Below 60 K, the exp[1/T1/4] temperature dependence of the direct current electrical resistance R(T,0) is found. Low-field transverse magnetoresistance (MR), [R(T,H)−R(T,0)]/R(T,0), below 60 K is negative and is discussed in terms of a quantum interference effect on a three-dimensional variable range hopping (3D–VRH) mechanism. Based on the results of R(T,0) and MR, we propose 3D–VRH conduction within the nanotube bundles (intrabundle hopping) or between the nanotube bundles (interbundle hopping) through their contacts.
Langmuir probe in low temperature, magnetized plasmas: Theory and experimental verification
Authors: Koo Bon-Woong , Hershkowitz Noah , Sarfaty Moshe
Published: July 2002

Langmuir probe theory, adapted to magnetized, partially ionized, low temperature processing plasmas with radial diffusion dominated by electron-neutral collisions, was verified in electron cyclotron resonance (ECR) plasmas. Plasma parameters such as plasma potential, electron temperature, plasma density, and the ratio of electron saturation current to ion saturation current (Ie*/Ii*) were measured by single-sided planar probe in various ECR plasmas (H2, He, N2, O2, Ar, and CF4). The neutral pressure was varied between 0.5 and 8.5 mTorr and the microwave power between 170 and 1250 W with good matching conditions; the reflected power was kept at less than 3% of the input power. The measured ratios of Ie*/Ii*, and other plasma parameters were consistent with the probe theory for pressures greater than 2.0 mTorr for various plasmas of Ar, He, H2, and N2. These results indicate that the electron-neutral collisional probe theory works well for magnetized ECR plasmas (magnetic flux densities of 0.8–1.0 kG).