bluuebunny/crossref_metadata_2025 huggingface dataset:
Use of real-time photoluminescence and low-power electron cyclotron resonance hydrogen plasma for passivation of SiN–InP interfaces
Authors: Malhotra V. , Kapila A.
Published: July 2002

Passivation of InP surfaces have been achieved using low-power (∼50 W) electron cyclotron resonance (ECR) hydrogen plasmas and SiNx overlayers. The nitride layers are deposited at room temperature using a ECR plasma-enhanced chemical vapor deposition technique. In situ, real-time photoluminescence measurement is used to monitor the InP surface during H plasma treatment. It is observed that a 30 s “flash” ECR H plasma treatment of InP surface helps fabrication of passivated SiNx–InP interfaces with a low density of interfacial defects.
Steady-state photoconductivity of gallium- and indium-doped hydrogenated amorphous germanium thin films
Authors: Reis F. T. , Comedi D. , Chambouleyron I.
Published: July 2002

The effects of gallium and indium p-type doping on the photoconductivity of hydrogenated amorphous germanium (a-Ge:H) thin films deposited by the rf-sputtering method are reported. The quantum efficiency-mobility-lifetime (ημτ) product was determined at room temperature as a function of the dark Fermi energy EF on samples with a relative dopant concentration range between ≈3×10−5 and ≈10−2. A decrease of ημτ is observed with the increase of the Ga concentration until a minimum is reached for compensated samples (EF close to midgap level), where ημτ is about 16 times lower than the value obtained for intrinsic samples. This behavior is followed by an ημτ increase as EF crosses the midgap level. Then, for higher Ga doping levels, ημτ decreases again. For In-doped samples, on the other hand, a monotonic decrease of ημτ is measured for all the impurity concentration range. These results are consistent with a model which assumes that the dangling bond is the main recombination path, and give independent evidence for the lack of correlation between the defect density and EF in p-type doped a-Ge:H.
Spin polarization and enhancement of the effective g-factor in one-dimensional electron systems with an in-plane magnetic field
Authors: Majumdar Amlan
Published: July 2002

We investigated the energy subbands of ideal one-dimensional electron systems (1DESs) at T=0 K in the presence of an in-plane magnetic field parallel to the channel using a numerical self-consistent solution of the Schrödinger equation and the Poisson equation. We show that the ideal 1DES is spin polarized at low electron densities and that the effective g-factor of these 1D electrons is enhanced not only at low electron densities, but also at higher densities whenever the Fermi level rises above a 1D energy subband. We find that the effective g-factor goes down when the 1D confining potential weakens and we approach the 2D limit. Neglecting the Hartree potential, we analytically calculate the effective g-factor when only one energy subband is occupied. We show that these analytical values are in good agreement with the results of the numerical, self-consistent procedure.
Vertically stacked coupler and serially grafted waveguide: Hybrid waveguide structures formed using an electro-optic polymer
Authors: Watanabe Toshio , Hikita Makoto , Amano Michiyuki , Shuto Yoshito , Tomaru Satoru
Published: July 2002

Two hybrid waveguide structures formed using an electro-optic polymer are described. They are a vertically stacked coupler and a serially grafted waveguide. These structures were fabricated by spin-coating, photolithographic, and reactive ion etching processes. Directional coupling was successfully demonstrated between the vertically stacked waveguides, and the crosstalk was −15 dB. This waveguide structure was applied to an optical intensity modulator. The serially grafted waveguide allows butt coupling between the functional waveguide and passive waveguides, and the connection loss at a grafting point was as low as 0.005 dB. This waveguide structure was applied to a Mach–Zehnder optical intensity modulator and a quasi-phase-matched second-harmonic generator.
Electro-optic and electro-absorptive modulations of AlGaAs/GaAs quantum well using surface acoustic wave
Authors: Choy Wallace C. H. , Li E. Herbert , Weiss Bernard L.
Published: July 2002

The surface acoustic wave produced electron absorptive and electro-optic modulation in AlGaAs/GaAs quantum well structures are theoretically analyzed. The quantum well structures are optimized by maximizing the optical confinement of the modal field in the active region and the piezoelectric effect of surface acoustic wave on the quantum wells. The effect of penetration depth of the surface acoustic wave on the number (1–25 periods) of quantum wells, serving as the active region, is being studied. For 1–5 period structures, the quantum wells are designed on the top surface so that a strong piezoelectric effect can be obtained. For the 25-period structure, the quantum wells locate at a depth of two-thirds the acoustic-wave wavelength in order to obtain a uniform surface acoustic-wave-induced electric field. The results show that the single and five quantum well devices are suitable for absorptive modulation and optical modulation, respectively, while a general advantage of the 25-period quantum well modulator can shorten the modulation interaction length and increase the modulation bandwidth. The effective index change of these devices are at least ten times larger than the conventional surface acoustic wave devices. These results make the surface acoustic wave quantum well modulators more attractive for the development of acousto-optic device applications.
Giant magnetoresistance properties in granular type (Co–Ag)/Ag multilayers
Authors: Zhou Shi-Ming , Wang Yu , Zhu Wei-Rong , Zhang Rong-Jun , Zheng Yu-Xiang , Jin Qing-Yuan , Chen Liang-Yao , You Biao , Ji Wu , Hu An , Zhai Hongru , Shen Honglie
Published: July 2002

The giant magnetoresistance of sputtered granular type (Co–Ag)/Ag multilayers with different Co content and sublayer thickness’ was studied. With an increasing Ag layer thickness, the magnetoresistance ratio ΔR/R first increases to reach a maximum and then decreases. With an increasing Co content in the magnetic layers, the maximum shifts towards a large Ag layer thickness. These phenomena can be attributed to the antiferromagnetic coupling between the Co clusters in neighboring Co–Ag layers. With an increasing Co–Ag layer thickness, ΔR/R increases monotonically for a small Co content in the magnetic layers, while for a large Co content, a maximum appears at a small magnetic layer thickness. It is suggested that the interplay of spin dependent scattering and the shunting effect in the Co–Ag layers results in the above thickness dependence.
Melt-quenched KCl:Eu2+ films for photostimulated luminescence applications
Authors: Jagannathan R. , Watanabe M. , Murase N. , Karthikeyani A. , Kanematsu Y. , Kushida T. , Yazawa T. , Kitamura N.
Published: July 2002

Using a conventional melt-quenching method, Eu2+-doped KCl films have been prepared. These films yield intense photostimulated luminescence (PSL) with the stimulated excitation maximum shifting to longer wavelengths as compared to the reports on the single-crystal analog. This will be advantageous for image reading by a He–Ne laser or red-emitting diode lasers when used in the PSL applications. Several types of centers have been found. Further, in the electron paramagnetic resonance spectrum, the hyperfine structure due to the Eu2+ centers has been observed to disappear at low temperatures (∼5 K). This can be attributed to the formation of complexes by the trapping of defect centers at the Eu2+ site(s). Crystallization effects and paramagnetic centers as well as optical properties of these films are discussed.
Scanning cathodoluminescence as a probe of surface recombination in phosphors excited at low electron energies
Authors: Seager C. H. , Missert N. A. , Tallant D. R. , Warren W. L.
Published: July 2002

Scanning cathodoluminescence (SCL) is a technique that has been widely used to categorize local minority carrier lifetime nonuniformities in bulk and thin film semiconductors. In the case of phosphor powders it has been less popular, partly because the collection and scattering of light generated in an array of particles complicates the extraction of quantitative information about light generation efficiency. Here we demonstrate that by coupling SCL with modern digital imaging manipulation techniques, we can extract quantitative information about the beam energy dependence of phosphor cathodoluminescence with high spatial resolution. A number of common phosphors are examined and a wide range of behavior is observed. We find that materials like ZnS:Ag and SrGa2S4:Eu display uniformly high nonradiative surface energy losses, while other materials like ZnO:Zn have surface losses which are spatially very nonuniform.
Shear birefringence measurements on polymer thin films deposited on quartz resonators
Authors: Domack Arno , Johannsmann Diethelm
Published: July 2002

We have measured the second-order stress birefringence in a 2 μm film of poly-(methyl-methacrylate) sandwiched between the top electrode of a quartz resonator and a second, semitransparent gold layer. The gold–film–gold system forms an optical Fabry–Perot resonator with characteristic dips of reflectivity at certain angles of incidence, which are the coupling angles of the different optically resonant modes. When modulating the driving voltage of the quartz resonator with a frequency in the kHz range and referencing the detection electronics to the modulation, shifts in the coupling angle in the range of 10−3° are easily detected. They result from a pseudo-dc component in the stress-optic signal, which is proportional to the square of the shear strain. This second-order stress birefringence is related to normal stresses in the film via the stress-optic law. We compare our findings to predictions from the theory of finite elasticity.
Ferroelastic domain switching of Pb3(PO4)2 single crystals
Authors: Lim Ae Ran , Chang Jin-Hae , Jeong Se-Young
Published: July 2002

We investigated the ferroelastic domain switching due to stress in Pb3(PO4)2 single crystals grown by the Czochralski method. The temperature dependence of the stress–strain hysteresis was also studied. From this result, it was determined that the Pb3(PO4)2 single crystal undergoes a phase transition near Tc=180 °C. In other words, the phase transition due to an external mechanical stress applied to Pb3(PO4)2 crystals causes a transformation from the ferroelastic to the paraelastic phase. From these results, it was confirmed that Pb3(PO4)2 crystals have ferroelastic characteristics below Tc.
Comparison of electron cyclotron resonance and radio-frequency inductively coupled plasmas of Ar and N2: Neutral kinetic energies and source gas cracking
Authors: Materer N. , Goodman Rory S. , Leone Stephen R.
Published: July 2002

Neutral mean kinetic energies and the amount of neutral source gas cracking of particles emanating from an electron cyclotron resonance (ECR) plasma source and a radio-frequency inductively coupled plasma (ICP) source are compared as a function of applied power by modulated beam time-of-flight analysis for Ar and N2 gases. For both sources, the source chamber is maintained at 0.07 Pa by a constant flow of either Ar or N2, while the applied power is varied from 50 to 250 W and from 50 to 550 W for the ECR and the ICP sources, respectively. For the Ar plasmas, the neutral Ar mean kinetic energies are similar and range between 0.07 and 0.15 eV, with the ICP being somewhat higher. In the case of the N2 plasmas, the ICP mean energies are also somewhat higher than those found for the ECR source. The mean energies range between 0.02 and 0.26 eV and 0.02 and 0.40 eV for the N2 and N species, respectively. The N:N2 flux ratio for the ICP source increases to a plateau of approximately 0.021±0.002 after an applied power of 250 W is reached, while for the ECR plasmas, the N:N2 flux ratio varies in a more complex fashion with a much higher value of 0.17±0.04.
Passivation of an n-type InP surface with an As2S3 film
Authors: Mada Yoichi , Wada Kazumi
Published: July 2002

Passivation of an n-type InP surface using an As2S3 film has been investigated. The properties of As2S3/InP interfaces were evaluated by characterizing metal–insulator–semiconductor (MIS) diodes with an evaporated As2S3 film as an insulator. In the inversion region, the capacitance–voltage (C–V) curve of the diode shows deep depletion, while an illuminated curve shows an inversion ledge. A remarkable feature of the MIS C–V curve is a decrease of injection-type hysteresis after a long period of time. This may be associated with the decomposition of As2S3. The minimum interface state density estimated using the Terman method is 1–2×1011 eV−1 cm−2. X-ray photoelectron spectroscopy was also used to study the As2S3/InP interface, and it was deduced that the interface appears to be passivated by In–S bonds.
Biexciton emission from thick ZnSe epilayer grown by molecular beam epitaxy
Authors: Hauksson I. S. , Kawakami Y. , Fujita Sz. , Fujita Sg. , Galbraith I. , Prior K. A. , Cavenett B. C.
Published: July 2002

Photoemission under high photoexcitation in thick ZnSe epilayer grown by molecular beam epitaxy (MBE) has been investigated using time-resolved photoluminescence spectroscopy. The presence of biexcitons is demonstrated under picosecond pulsed excitation as biexciton emission is observed 4 meV below the free exciton emission. The rise time of the biexciton emission follows the rise time of the free exciton and decays with a single exponential form with shorter lifetime than the free exciton. This clear observation of biexciton luminescence in MBE grown ZnSe epilayer, suggests that spontaneous emission commonly observed at lasing threshold under optical pumping at low temperatures is due to biexciton recombination.
Impact of the transmission line properties of a metal-ultrathin silicon dioxide-semiconductor field-effect transistor on the extracted inversion-layer thickness
Authors: Kol’dyaev V. I. , Clerix A. , Deferm L. , Van Overstraeten R.
Published: July 2002

The effective thickness of the inversion layer, expressed in terms of the inversion charge centroid, is determined by capacitance measurements in the weak, moderate and strong inversion regimes using a new model for the gate-to-channel capacitance which takes into account quantum and transmission line effects in all the regimes. It is shown that the extracted inversion-layer thickness, using the capacitance-voltage method in a typical frequency range of 10–100 kHz, is unambiguous only for short channel metal-silicon dioxide-semiconductor field effect transistors with channel length Lm less than 1 μm. For long channel devices having Lm>(5–10) μm the extraction is very difficult due to the dominant impact of the transmission line effect on the measured capacitance value. The effective inversion-layer thickness is found to follow a 1/Ninvm law where Ninv is the inversion-layer concentration (m≅1/3 in the weak inversion regime, m=1/2–1 in moderate inversion regime and m=1/3 in the strong inversion regime). In the strong inversion regime the effective inversion-layer thickness is exactly equal to the real inversion-layer centroid which can be rather accurately described, as is shown, using the electric quantum limit approximation. In the moderate and weak inversion regimes the dependence can be explained in terms of the real average charge centroid and an effective conducting area under the gate which is smaller than the real area due to fluctuations of the surface potential. The surface potential fluctuation can be attributed to dopant distribution fluctuations.
Magnetic properties enhancement of α-Fe/Nd2Fe14B-type nanocomposites by Co substitution
Authors: Chang W. C. , Wu S. H. , Ma B. M. , Bounds C. O. , Yao S. Y.
Published: July 2002

The phase evolution and magnetic properties of melt-spun Nd9.5(Fe1−xCox)85.5B5 (x=0, 0.05, 0.1, and 0.15) nanocomposites have been investigated. It was found that Co substitution for Fe, i.e., x=0.05–0.15, improves the Curie temperature (Tc), remanence (Br), and maximum energy product (BH)max of the materials obtained. Co substitution for Fe was found to promote grain coarsening, after thermal processing, in materials with a high Co concentration. Wohlfarth remanence analysis suggests that dilute-Co substitution for Fe increases the strength of the exchange coupling between the soft magnetic phases, α-(Fe, Co) and Nd2(Co, Fe)17, and the 2:14:1 phases. The increase in the saturation magnetization of α-(Fe, Co) and 2:14:1 phase, in conjunction with the enhanced exchanged coupling, presumably, result in an increased (BH)max of Nd9.5(Fe0.95Co0.05)85.5B5 and Nd9.5(Fe0.90Co0.10)85.5B5 samples. Once above a critical Co concentration, for the compositions studied, the strength of exchange coupling interactions between magnetically soft and hard phases does not seem to vary significantly with Co content. Moreover, both the irreversible loss of induction and reversible temperature coefficient of induction (conventionally referred to as α) of fully processed materials decrease with increasing Co concentration.
Small-scale laboratory measurement and simulation of a thermal precursor shock
Authors: Grun J. , Burris R. , Joyce G. , Slinker S. , Huba J. , Evans K. , Manka C. K. , Barthel J. R. , Wiehe J. W.
Published: July 2002

A shock precursor in a thin thermal layer above a 3000 K hot surface is measured in a small-scale laboratory experiment. Precursor and thermal layer characteristics are diagnosed using spectroscopy, interferometry, and dark-field shadowgraphy. The experiment is successfully simulated with a code previously validated using large-scale high-explosive tests, thereby demonstrating its applicability to events that differ in energy by 12 orders of magnitude. The simulation shows that a small-scale laboratory experiment can be a good surrogate for infrequent, expensive, and hard to diagnose large-scale tests.
Photoreflectance and time-resolved photoreflectance in delta-doped superlattices
Authors: de Sousa D. F. , Bell M. J. V. , Nunes L. A. O.
Published: July 2002

In this paper we report a detailed study of photoreflectance (PR) in δ-Si:GaAs superlattices. PR spectra yield three contributions: (1) from intrinsic GaAs; (2) Franz–Keldysh oscillations (FKO) due to the surface electric field, whose value is explained by a simple capacitor model; and (3) FKO attributed to the buffer/superlattice interface. We observed the photovoltaic effect due to the probe beam in the PR arrangement and achieved surface voltage reductions up to 30%. We also present a time-resolved PR technique, which allows us to discriminate these contributions in the time domain and gives us the PR characteristic decay times.
Wet oxidation of amorphous Si0.67Ge0.25C0.08 grown on (100) Si substrates
Authors: Bair A. E. , Atzmon Z. , Alford T. L. , Smith David J.
Published: July 2002

Wet oxidation annealing of thin films of amorphous Si0.67Ge0.25C0.08 was performed over the temperature range from 700 to 950 °C. Changes in composition and microstructure were assessed using Rutherford backscattering spectrometry and transmission electron microscopy. A nearly pure layer of SiO2 with approximately 1 at. % carbon was formed, with Ge being rejected from the oxide at all temperatures. At low temperatures, the oxide formed was very thin. Ge piled up at the oxide/film interface and the thin film microstructure remained amorphous. At higher temperatures, a network of nanocrystals was observed which was believed to provide a grain boundary diffusion path for Ge which had redistributed throughout the remaining layer. It is proposed that the Ge layer had inhibited oxidation at the lower temperatures, whereas its removal resulted in increased oxidation rates at higher temperatures. Annealing at 950 °C for 5 and 6 h resulted in an epitaxial transformation and a single crystal structure. This process occurred as a result of the silicon being removed from the substrate by the oxide front which served as a sink. Germanium then diffused into the vacancies in the substrate forming a new epitaxial layer.
Photoluminescence from Ge clusters embedded in porous silicon
Authors: Liu Feng-Qi , Wang Zhan-Guo , Li Guo-Hua , Wang Guang-Hou
Published: July 2002

Visible photoluminescence (PL) and Raman spectra of Ge clusters embedded in porous silicon (PS) have been studied. The as-prepared sample shows redshifted and enhanced room temperature PL relative to reference PS. This result can be explained by the quantum confinement effect on excitons in Ge clusters and tunnel of excitons from Si units of the PS skeleton to Ge clusters. One year storage in dry air results in a pronounced decrease in PL intensity but blue-shifted in contrast to reference PS. This phenomenon correlates to the size decrease of macerated Ge clusters and occurrence of “quantum depletion” in Ge clusters. Consequently, only excitons in Si units contribute to PL.
Spatial characteristics of electron swarm parameters in gases
Authors: Date H. , Ventzek P. L. G. , Kondo K. , Hasegawa H. , Shimozuma M. , Tagashira H.
Published: July 2002

The results of a Monte Carlo simulation study focusing on the spatial characteristics of swarm parameters in an isolated electron swarm and the distribution of the electron density and flux taking into account the anode boundary are presented. The spatial variation of the swarm parameters is interpreted using conventional swarm theory, and potential problems with the measurement of arrival-time spectra of electrons are considered.
Thermionic electron emission from narrow band-gap semiconductors under picosecond laser excitation
Authors: Mao Samuel S. , Mao Xianglei , Yoo Jong H. , Greif Ralph , Russo Richard E.
Published: July 2002

A model is presented to relate picosecond laser induced thermionic electron emission to the carrier and phonon dynamics of semiconductors. Silicon is chosen as a model material since its optical and thermal properties are well characterized. The temporal profiles of thermionic emission current and total electron yield are obtained as a function of incident laser fluence for different surface conditions, below the silicon surface melting threshold. Two distinct regimes have been found for the dependence of the electron yield on the laser fluence due to thermionic emission and this behavior is related to Auger recombination.
Irreversibility and magnetization dynamics in samples with helical anisotropy
Authors: Rodríguez M. , López E. , Sánchez M. C. , Aroca C. , Sánchez P.
Published: July 2002

Amorphous ribbons with helical induced anisotropy show outstanding properties such as their capability to be magnetized through a single Barkhausen jump. This phenomenon is due to the helical anisotropy distribution induced in the samples by current annealing. The anisotropy direction rotates 180° continuously from one surface of the ribbon to the other. The Barkhausen jumps arise from the chirality conservation between the magnetization and the anisotropy direction during the magnetization process. In this work we report a study of the dynamics of the irreversible magnetization processes that these samples show. The existence of closure domains at the end of the ribbon and their variation when the applied field is up to a critical value make the reversal process. The dynamics of the wall, which is responsible of the reversal, has been studied through a conventional Sixtus–Tonks experiment.
He–Xe laser magnetometry
Authors: Emile Olivier , Poirson Jérôme , Bretenaker Fabien , Le Floch Albert
Published: July 2002

The 3.51 μm He3–Xe136 laser shows promise for applications to mean field magnetometry. A careful analysis of the locking mechanisms is performed and experimentally verified. The high gain of the transition and the high mass of the Xe136 atom permits one to increase the magnetometer scale factor by a factor of 3 compared with Ne20. Moreover, in the general case, the width of the locking region is divided by the same factor.
Roughness effects on the critical fracture toughness of materials under uniaxial stress
Authors: Palasantzas George
Published: July 2002

The Griffith criterion is applied for the calculation of the critical fracture toughness upon which the formation of a rough self-affine crack (which is characterized by the rms roughness amplitude σ, the correlation length ξ, and the roughness exponent H) commences. For large crack sizes R≫ξ, the stress field singularity close to the crack tip involves the value −1/2 in both the strong and weak roughness limit. In the latter limit, the fracture toughness K remains close to the classical value K≈2(γE)1/2 with γ the surface tension and E the Young modulus, while in the strong roughness limit it becomes significantly large [>2(γE)1/2] following the asymptotic behavior K≈2(γE)1/2 (σ/ξH)1/2.
Optical and compositional study of silicon oxide thin films deposited in a dual-mode (microwave/radiofrequency) plasma-enhanced chemical vapor deposition reactor
Authors: Etemadi R. , Godet C. , Perrin J. , Seignac A. , Ballutaud D.
Published: July 2002

This study is focused on the deposition and growth of amorphous silicon oxide optical thin films in a dual-mode (microwave/rf) reactor. The optical, chemical, and structural properties of a-SiOx:H films have been studied as a function of different parameters (substrate temperature, microwave power, gas flow, dilution, addition of rf plasma) by in situ UV-visible and in situ infrared (IR) ellipsometry, Fourier transform IR transmission spectroscopy, nuclear analysis, and VUV absorption spectroscopy. We have shown that it is possible to increase the density of the films and decrease their hydrogen content by depositing at higher temperature (T⩾200 °C) and/or using a helium dilution and/or using the ion bombardment effect of the rf plasma. In situ IR ellipsometry has revealed water absorption on the as-deposited silicon oxide films, just after exposing them to the ambient atmosphere. The lower is the film density and the higher is the water absorption. UV absorption spectroscopy has evidenced an absorption peak at 6.3 eV, with an amplitude dependent on the hydrogen content. Since the films are interesting for optical multilayers, we have also studied the effect of UV irradiation, showing the decrease of O–H groups (reduction of the hydrogen content) and a reoxydation of the silicon oxide films. The thermal stability studies reveal an effusion of H2 and H2O molecules for temperatures above 300 °C.
Spectral selectivity of cermets with large metallic inclusions
Authors: Arancibia-Bulnes C. A. , Ruiz-Suárez J. C.
Published: July 2002

In order to address the problem of the optical reflectance of cermets beyond the quasistatic dipolar limit, we consider Monte Carlo multiple scattering simulations as well as a radiative transfer study. Slabs of alumina with inclusions of Cu, Au, Co and Cr are studied; reflectance coefficients are calculated as a function of wavelength, considering metal inclusion diameters up to 2.0 μm. We show that despite the incoherent diffuse propagation of light inside the slab, strong selectivity is achieved by particle diameters around 0.2 μm, even for very low metal concentrations.
Pulse broadening in femtosecond x-ray diffraction
Authors: Tomov I. V. , Chen P. , Rentzepis P. M.
Published: July 2002

Advances in femtosecond laser technology make feasible the generation of femtosecond x-ray pulses. An analysis of Bragg diffraction of subpicosecond x-ray pulses is presented. It is shown that the three-dimensional nature of the diffracting crystal grating and the finite size of the x-ray beam will lead to a considerable broadening of the diffracted femtosecond x-ray pulse.
Effects of wafer thermostability and wafer-holding materials on optical loss in GaAs annealing
Authors: Wu Y. S. , Feigelson R. S. , Route R. K. , Zheng D. , Gordon L. A. , Fejer M. M. , Byer R. L.
Published: July 2002

A periodic structure of bonded GaAs wafers has been proposed for quasi-phase-matched second-harmonic generation. However, current bonding processes used to fabricate these structures often lead to unacceptably high optical losses. When commercial semi-insulating GaAs wafers were bonded at 850 °C, increases in the free-hole concentration (thermal conversion) were found to be a major cause of excess optical loss. This conversion depended on both the GaAs source and the materials comprising the sample holder. We found that quartz and sapphire could not be used in contact with GaAs wafers because they stuck during the bonding process. On the other hand, As-charged graphite holders did not stick and worked well under our bonding conditions.
Nonlinear stability analysis of the growth surface during diamond chemical vapor deposition
Authors: Mahalingam Pushpa , Dandy David S.
Published: July 2002

The morphological stability of the solid–gas interface under conditions of diffusive transport of reactant species towards the surface during the chemical vapor deposition process is analyzed using linear and nonlinear perturbation theories. The Landau coefficient, which represents the nonlinear growth rate, is calculated using the direct method of undetermined coefficients. A dispersive relation is derived which relates the effects of species diffusive transport towards the growing interface, surface diffusion, and geometrical factors with the stability of perturbations on the interface. The resulting relation is applied to the diamond chemical vapor deposition process. Linear and nonlinear instability of the interface is obtained for diamond chemical vapor deposition conditions. Linear instability increases but the Landau coefficient becomes larger, indicating greater nonlinear stability as the reactor pressure increases, although both linear and nonlinear analyses suggest more stability as the reactor temperature increases. However, during typical diamond growth conditions, it is predicted that the diamond–gas interface is unstable to both infinitesimal and finite amplitude disturbances.
Discharge induced enhancement of diamond nucleation on Si via hot filament chemical vapor deposition
Authors: Cui J. B. , Shang N. G. , Fang R. C.
Published: July 2002

A new pretreatment method involving a direct current discharge was used to enhance diamond nucleation on mirror polished silicon substrates employing hot filament chemical vapor deposition. A nucleation density of 108/cm2 was obtained. The nucleation density on Si substrates pretreated by the discharge at room temperature is similar to that treated at high temperature. The discharge process was monitored by optical emission spectroscopy. Raman scattering, scanning electron microscopy, and x-ray photoelectron spectroscopy were employed to analyze the pretreated Si substrate. The results suggest that a thin carbon layer formed during the discharge pretreatment process which contributes to the enhancement of diamond nucleation.
In situ Fourier transform infrared spectroscopy and stochastic modeling of surface chemistry of amorphous silicon growth
Authors: Wetterauer U. , Knobloch J. , Hess P. , Houle F. A.
Published: July 2002

We present a combined experimental and theoretical study of the formation of silicon hydride surface species during the F2 laser (157 nm) chemical vapor deposition of amorphous hydrogenated silicon (a-Si:H). A one-dimensional stochastic model has been applied to simulate the growing surface. The predictions of the model were compared to the results of Fourier transform infrared transmission spectroscopy providing submonolayer resolution. We present a qualitative analysis of the bonding configuration in the initial stage of film growth on H-terminated Si(111). The high sensitivity and resolution of the spectroscopic method allowed us to distinguish between four surface species by a deconvolution of the observed feature around 2100 cm−1 during the deposition process. The theoretical results for the evolution of the different surface species and bulk hydrogen are in close agreement with these spectra. Consistence between mass spectrometric data and simulation was also achieved for the growth rate and bulk hydrogen content as a function of disilane partial pressure and laser intensity. The good agreement between experiment and simulation obtained in this work indicates that it is possible to describe the main features of the complex chemical system of the growing film with a model based on a few dominant surface reactions.
Finite bias spin dependent tunneling: A nonequilibrium Green’s function approach
Authors: Wang Xindong
Published: July 2002

The nonequilibrium Green’s function theory is applied to a model spin dependent tunneling system at finite bias. In the limit of zero bias, the present result agrees with that of Landauer’s formula. For finite bias, it is found that as the bias increases, the magnetoresistance ratio decreases.
Local distribution on magnetic properties in grain-oriented silicon steel sheet
Authors: Enokizono Masato , Tanabe Ikuo , Kubota Takeshi
Published: July 2002

Grain-oriented silicon steel is the most important soft magnetic material used as core material of large transformers, large rotating machines, and pole transformers. Total loss of grain-oriented silicon steel tends to become lower with an increasing degree of texture. However, the material which has a higher degree of texture ordinarily contains larger grains, and the materials which have larger grains show higher total loss due to increased eddy current loss. As the gauge is reduced, the rate of decrease in total loss becomes lower for the thinner gauge due to increased hysteresis loss. However, the investigation of local magnetic properties due to grain situation in this sheet was not discussed from the viewpoint of the distribution of localized magnetic properties, for example, iron loss, hysteresis loop, behavior H vector (magnetic field strength) and B vector (Magnetic flux density). This paper describes the distribution of magnetic properties in high oriented silicon steel sheet, which are loss, hysteresis loop, and locus of field strength.
Analysis of magnetic behavior of exchange-enhanced SmFeCo magnets
Authors: Dahlgren M. , Grössinger R. , Cornejo D. R. , Missell F. P.
Published: July 2002

A series of densified exchange-enhanced SmFeCo magnets was produced by mechanical alloying, followed by a heat treatment. The nanocrystalline phases and the grain size distribution of the samples were characterized from x-ray diffraction patterns. Magnetic measurements in terms of different models were analyzed for the temperature behavior of the magnetization and coercivity. One analytical formulation was derived from the random anisotropy model predicting that with increasing Mr/Ms ratio a decreasing coercivity occurs. Another numerical approach, which is based on a micromagnetic calculation using finite element techniques was also applied. Attempts were made to improve the existing model derived from the finite element technique. The nucleation model was found to be unsatisfactory for describing the temperature-dependent coercivity.
Optimization of coils for detecting initial rotor position in permanent magnet synchronous motor
Authors: Wakao S. , Onuki T. , Tatematsu K. , Iraha T.
Published: July 2002

This article describes the shape optimization of detective coils for initial rotor position estimation in the drive system of permanent magnet synchronous motors. The characteristics of magnetic circuits in rotating machinery are very complicated. The magnetic flux distribution in the air gap is nonsinusoidal due to the influence of slots, magnets, and magnetic saturation, etc. By utilizing the slot ripple influence and taking the effects of both magnetic saturation and permanent magnets into consideration, an optimization of the detective coil by means of a genetic algorithm combined with a hybrid finite element and boundary element method is reported. This produces a global optimal solution in a shorter time.
Computational micromagnetic investigation of magnetization reversal in Nd–Fe–B nanocomposite magnets
Authors: David S. , Kevorkian B. , Toussaint J. C. , Givord D.
Published: July 2002

As a complement to the experimental analysis of magnetization reversal in a two-phase system, a numerical micromagnetic three-dimensional calculation is developed and tested. It is applied to a set of 64 nanometer-scale hard (Nd2Fe14B) and soft (Fe3B) cubic grains. Calculations reproduce qualitatively the experimentally observed processes and allow the relation between the structure and the coercivity mechanism in nanocomposite hard magnetic materials to be better understood.
Determination of the size, optical index and density of particles in an argon–silane radio-frequency discharge
Authors: Plain A.
Published: July 2002

We have determined, by laser light scattering experiments at three different angles (90°±9°) and by use of the Mie theory, the temporal evolution of particle size (in the range 40–110 nm) in an argon silane radio frequency discharge. The results are in good agreement with values previously determined by transmission electron microscopy. The evolution of the optical indices (absorption and refraction) has been calculated as a function of the particle size at λ=514 nm. We have also shown that these variations are independent of the electrical charges of the particles. The particle density in the discharge is about 108 cm−3.
Effects of Ga substitution on structure and magnetocrystalline anisotropy of Tm2Fe17
Authors: Shen Bao-gen , Cheng Zhao-hua , Wang Fang-wei , Yan Qi-wei , Tang Hong , Liang Bing , Zhang Shao-ying , de Boer F. R. , Buschow K. H. J. , Ridwan S.
Published: July 2002

A detailed investigation of the structure and magnetic properties of Tm2Fe17−xGax (x=0, 1, 2, 3, 4, 5, 6, 7, and 8) compounds has been performed by means of x-ray-diffraction, neutron-diffraction, magnetization, and ac-susceptibility measurements. Crystal-structure studies have shown that the prepared samples are single phase with the hexagonal Th2Ni17 for x⩽3 and the rhombohedral Th2Zn17 structure for x⩾5. In Tm2Fe13Ga4 the Th2Zn17 structure coexists with the Th2Ni17 structure. Substitution of Ga for Fe in Tm2Fe17 leads to an increase of the unit-cell volume, which is linear with the Ga concentration. In Tm2Fe17−xGax, the saturation magnetization at 1.5 K decreases linearly with increasing Ga content with a rate of 2.3 μB per substituted Ga atom. The Curie temperature is found first to increase with increasing Ga content, going through a maximum value of 485 K at about x=3, then to decrease. Between x=6 and 7, a minimum value of TC is reached and for higher x values TC increases again. X-ray-diffraction measurements on magnetically aligned Tm2Fe17−xGax powders show that the compounds with x⩽6 have an easy-plane type of magnetic anisotropy, whereas the compounds with x⩾7 exhibit easy c-axis anisotropy at room temperature. All Tm2Fe17−xGax compounds exhibit a spin-reorientation transition at low temperature, except for the sample with x=6, which shows an easy-magnetization direction perpendicular to the c axis in the temperature range from 5 to 300 K. For x⩽5, the spin-reorientation temperature is found first to increase with x and then to decrease, having a maximum value of 211 K at about x=3. In the samples with x⩾7, an easy-plane anisotropy was observed at low temperature, but an easy-axis preference of the magnetization at room temperature was observed. The results obtained for Tm2Fe17−xGax indicate that the mutually competing Tm- and Fe-sublattice anisotropies both change their sign with increasing Ga concentration.
Transport of sputtered atoms in facing targets sputtering geometry: A numerical simulation study
Authors: Senthil Nathan S. , Mohan Rao G. , Mohan S.
Published: July 2002

The transport of sputtered Ag atoms in the facing targets sputtering geometry studied by Monte Carlo simulation is presented. The atoms are ejected with a cosine distribution and undergo many collisions with the background argon gas atoms. The collisions are simulated with a Born–Mayer interaction potential. The model predicts the flux of Ag atoms arriving at the substrate, their energy, and angular distribution. These results have been compared with experimental results reported earlier. Similar computations have been extended to study the complex transport of multicomponent Y, Ba, and Cu in oxygen gas during YBa2Cu3O7−x sputter deposition.
Measurement of sound velocity and thickness of thin samples by time-resolved acoustic microscopy
Authors: Hänel Volker
Published: July 2002

Simultaneous measurement of sound velocity and thickness of thin specimens is a well-known technique in time-resolved acoustic microscopy. However, the applicability is restricted, because a substrate in contact with the back of the sample is required. Furthermore, the substrate must have an uncoated area, because a reference signal from the substrate’s surface is needed. To overcome these performance limitations, a technique has been developed that focuses the lens on the front and afterwards on the back of the sample. An equation for the longitudinal sound velocity of thin samples has been analytically deduced using this approach. Having calculated the sound velocity, the thickness of the sample directly follows using known relations. Additionally, the developed method has been experimentally applied to thin polyvinylidene fluoride film. The results confirm the capability of the method.
Fabrication of a periodically poled laminar domain structure with a pitch of a few micrometers by applying an external electric field
Authors: Yamada Masahiro , Saitoh Masaki
Published: July 2002

The fabrication of a fine-pitch laminar domain structure in a lithium niobate (LiNbO3) single crystal is reported. Special emphasis is placed on how to fabricate the structure homogeneously, in which it was found that the application of multiple pulses is beneficial, and how to fabricate the structure in a relatively thick substrate, in which it was found that the application of a accumulated static field on the multiple pulses is useful. The following questions are discussed: Why the inverted domain structure corresponds to the structure of the electrode on the +z face, and why the application of multiple pulses is useful in the fabrication of a homogeneous laminar domain structure.
Nonlinear fundamental photothermal response in three-dimensional geometry: Theoretical model
Authors: Mandelis Andreas , Salnick Alex , Opsal Jon , Rosencwaig Allan
Published: July 2002

A general three-dimensional theoretical model for fundamental and harmonic response generation as a result of periodic heating of a system consisting of a nonlinear layer with temperature-dependent thermal conductivity and specific heat and a linear substrate is developed. Analysis of the fundamental component of the surface temperature shows that the nonlinear thermal conductivity alone does not affect the phase of the thermal-wave field. The efficiency of the thin nonlinear layer as an energy conversion filter that drives the harmonic response of the substrate is shown by the analysis of the limiting cases of the theoretical model.
Investigation of collection efficiencies much larger than unity in a-Si:H p-i-n structures
Authors: Main C. , Zollondz J.-H. , Reynolds S. , Gao W. , Brüggemann R. , Rose M. J.
Published: July 2002

An apparent quantum efficiency much larger than unity is observed under reverse bias voltage conditions, in hydrogenated amorphous silicon p-i-n structures. High collection efficiencies are measured for low-level probe illumination of the device n side with red light, with simultaneous bias illumination from the device p side with strongly absorbed blue light. The photogating effect responsible varies experimentally with reverse bias voltage, and collection efficiencies for the probe excitation of up to 50 are obtained. Detailed computer simulations corroborate such high values of quantum efficiency and the underlying mechanisms for the effect are revealed. We present the influence on quantum efficiency of bias light wavelength and photon flux, probe light photon flux, applied voltage, and defect density.
Pseudomorphic growth of ultrathin cubic 3C–SiC films on Si(100) by temperature programmed organometallic chemical vapor deposition
Authors: Hofmann Jens , Veprek Stan , Heindl J.
Published: July 2002

The large lattice mismatch between 3C–SiC and silicon of about 20% is reduced to only ⩽0.4% when the commensurability of five 3C–SiC to four Si lattice planes with one misfit dislocation parallel to the film/substrate interface can be utilized for the pseudomorphic growth. We report on a successful preparation of several nanometer thin pseudomorphic films and some of their structural properties. We further show that the formation of voids in the silicon substrate due to Si evaporation, which is one of the important reasons for structural imperfections of heteroepitaxial 3C–SiC films, can be strongly suppressed by our recently developed deposition technique.
Hysteretic behavior of magnetic particles with dipole interaction
Authors: Lu Jing Ju , Lin M.-T. , Kuo Chien Cheng , Huang Huei Li
Published: July 2002

Energy barrier height and coercivity variation of a pair of interacting particles has been studied as a function of the bonding angle β for 0°⩽β⩽π/2 and applied field using the fixed step Monte Carlo simulation approach supplemented by the perturbation method. Coercivity Hc(β) of the system has local maxima at β=0 and π/2, and a global minimum at β≃65°, and remains depressed compared to the noninteracting case for β⩽30° at any temperature. The barrier height behaves similarly. Due to the random nature of distribution dipole interaction, on average, leads naturally to a reduction in coercivity and energy barrier height in comparison with the noninteracting case.
Magnetic design for low-field tunability of microwave ferrite resonators
Authors: Dionne Gerald F. , Oates Daniel E.
Published: July 2002

An advanced microstrip resonator design consisting of a ferrite substrate formed in a closed magnetic path (toroidal structure) is shown to produce substantially improved tunability over previous versions that operated in an external magnetic field. Magnetically tunable toroidal resonators with tunability approaching 10% in magnetic fields of 60 Oe have been demonstrated using conventional ferrite compositions. Calculations with a hysteresis-loop theory developed specifically for modeling the low-field magnetization of ferrites were successfully fitted to resonator frequencies measured as a function of magnetic field. Performance predictions for resonators with ferrites optimized for low-field magnetization properties suggest that tunability approaching 15% is possible at magnetic fields of less than 5 Oe.
Oxygen precipitation in silicon: Experimental studies and theoretical investigations within the classical theory of nucleation
Authors: Kelton K. F. , Falster R. , Gambaro D. , Olmo M. , Cornara M. , Wei P. F.
Published: July 2002

Quantitative measurements of the oxygen precipitate rate as a function of annealing were made in Czochralski-grown silicon wafers that contained different initial concentrations of oxygen. All wafers were annealed at 1000 °C for 15 min to ensure that the initial cluster-size distributions were identical in all samples of the same composition prior to the multi-step annealing treatments used for the precipitation studies. The experimental data are compared with numerical predictions for time-dependent nucleation within the classical theory of nucleation. Quantitative agreement is obtained between the measured and calculated densities of oxygen precipitates for nucleation temperatures greater than 600 °C, but only over a narrow range of oxygen composition. Below 600 °C, the measured density for all samples is orders of magnitude larger than is predicted from the model. Further, the measured data show an anomalously small temperature dependence for the induction time for nucleation that does not scale with the diffusion coefficient, as expected from the classical theory of nucleation. Fundamentally, the classical theory of nucleation cannot explain the time-dependent nucleation of oxygen precipitates for temperatures below 650 °C. A possible reason is given.