bluuebunny/crossref_metadata_2025 huggingface dataset:
Material properties of compositional graded InxGa1−xAs and InxAl1−xAs epilayers grown on GaAs substrates
Authors: Chyi J.-I. , Shieh J.-L. , Pan J.-W. , Lin R.-M.
Published: July 2002

The residual strain, crystallographic tilt, and surface topography of InxGa1−xAs and InxAl1−xAs (0<x<0.3) epilayers grown on GaAs substrates are investigated. The residual strain of the InxAl1−xAs grown on graded InyAl1−yAs is shown to be strongly dependent on the thickness of the underlying-graded buffer layers and is larger than that of the InGaAs of the same structure. The crystallographic tilt of the InGaAs epilayers with respect to GaAs substrate is found to be strongly dependent on the growth temperature as well as the layer structure of the underlying buffer layer, while that of InAlAs is insensitive to these two factors. This behavior is attributed to the different roughness of the growth front between these two material systems and is consistent with the observation by atomic force microscopy.
Interaction of Ni90Ti10 alloy thin film with 6H-SiC single crystal
Authors: Levit M. , Grimberg I. , Weiss B.-Z.
Published: July 2002

Interfacial reactions, phase formation, microstructure, and composition, as functions of heat treatments (400–800 °C) were investigated in Ni90Ti10 alloy thin film coevaporated on an n-type 6H-SiC (0001) single-crystal substrate. The study was carried out with the aid of Auger electron spectroscopy, x-ray diffraction, and analytical transmission electron microscopy. The interaction was found to begin at 450 °C. Ni and C are the dominant diffusing species. The reaction zone is divided into three layers. In the first layer, adjacent to the SiC substrate, the presence of Ni-rich silicide, Ni2Si, and C precipitates, was observed. The second layer is composed mainly of TiC, while the third consists of Ni2Si. This composite structure, consisting of the silicide as a low resistivity ohmic contact, and of the carbide as a diffusion barrier, promises high-temperature stability crucial to ohmic contact development for SiC technology. Factors controlling phase formation in the Ni–Ti/SiC system are discussed.
Thermally stimulated depolarization current investigation of the relaxation behavior of polymers with chlorocyclohexyl side groups
Authors: Díaz-Calleja R. , Sanchis M. J. , Alvarez C. , Riande E.
Published: July 2002

The considerable dielectric activity exhibited by acrylate polymers containing chlorocyclohexyl side groups is investigated by using both global and partial thermally stimulated depolarization current (TSDC) techniques. The critical interpretation of the spectra obtained by TSDC experiments on poly(2-chlorocyclohexyl acrylate) (P2CCA), together with previous results related to the dynamics of 2-chlorocyclohexyl acetate, model compound of the side group of P2CCA, seem to suggest that conformational transitions about O–Cy and CH–C*O* bonds, rather than chair to reverse chair transition in the cyclohexyl group, may be involved in the absorption exhibited by these polymers in the glassy state. Attempts are also made to link the relaxation spectra obtained by TSDC techniques with the ac dielectric spectra.
Near band edge absorption spectra of narrow-gap III–V semiconductor alloys
Authors: Krishnamurthy Srinivasan , Chen A.-B. , Sher A.
Published: July 2002

Near band edge absorption spectra of the narrow-gap semiconductor alloys InxTl1−xP, InxTl1−xAs, and InxTl1−xSb were calculated and compared with those of HgxCd1−xTe. To test accuracy, we compared the calculated absorption spectra in GaAs with experimental results and found good agreement. Within 50 meV from the absorption edge, the absorption coeffi cient of InxTl1−xP is found to have about the same magnitude as that in HgxCd1−xTe and GaAs, whereas that in InxTl1−xAs and InxTl1−xSb is much smaller. This result and other merits found from previous studies indicate that InxTl1−xP has a potential to compete favorably with HgxCd1−xTe for long-wavelength infrared applications.
B-doped fully strained Si1−xGex layers grown on Si(001) by gas-source molecular beam epitaxy from Si2H6, Ge2H6, and B2H6: Charge transport properties
Authors: Lu Q. , Sardela M. R. , Bramblett T. R. , Greene J. E.
Published: July 2002

B-doped Si1−xGex layers with Ge fractions, determined by Rutherford backscattering spectroscopy, ranging from 0 to 0.28 and B concentrations, from quantitative secondary-ion spectroscopy measurements, between 5×1016 and 4×1019 cm−3 were grown on Si(001) at temperatures Ts=475–575 °C by gas-source molecular beam epitaxy from Si2H6, Ge2H6, and B2H6. Film thicknesses ranged from 200 nm for alloys with x=0.28 to 800 nm with x=0.05 to 1.4 μm for Si. Structural analyses by high-resolution x-ray diffraction and reciprocal lattice mapping combined with transmission electron microscopy showed that all films were fully strained, with measured relaxations of only ≂4×10−5, and exhibited no evidence of dislocations or other extended defects. The hole conductivity mobility μc,h in these layers increased continuously with increasing Ge concentrations, whereas the Hall mobility decreased yielding a Hall scattering factor that ranged from 0.75 for Si to 0.26 for Si0.72Ge0.28 but was not strongly affected by B concentration. μc,h, with CB=2×1018 cm−3, varied from 110 cm2 V−1 s−1 for Si0.95Ge0.05 to 158 cm2 V−1 s−1 for Si0.72Ge0.28, compared to 86 cm2 V−1 s−1 for Si, in good agreement with Boltzmann transport model calculations accounting for changes in the valence-band structure due to the effects of both alloying and biaxial in-plane compressional strain.
Effects of surface treatments and metal work functions on electrical properties at p-GaN/metal interfaces
Authors: Ishikawa Hidenori , Kobayashi Setsuko , Koide Y. , Yamasaki S. , Nagai S. , Umezaki J. , Koike M. , Murakami Masanori
Published: July 2002

In order to examine the possibility of preparing a nonreacted (nonalloyed) Ohmic contact to p-GaN, the effects of GaN surface treatments and work functions of the contact metals on the electrical properties between the metal contacts and p-GaN were investigated. A contamination layer consisting of GaOx and adsorbed carbons was found on the GaN substrate grown by metalorganic chemical vapor deposition. The contamination layer was not completely removed by sputtering the GaN surface with Ar and N ions where the ion densities were ∼10−2 μA/cm2. Although the contamination layer was partially removed by immersing in a buffered HF solution, little improvement of the electrical properties of the GaN/metal interfaces was obtained. Most of the contamination layer was removed by annealing the Ni and Ta contacts at temperatures close to 500 °C. These annealed contacts exhibited slightly enhanced current injection from the contact metal to the GaN. The present surface treatment study indicated that removal of the contamination layer did not significantly reduce the contact resistance. On the other hand, the resistance decreased exponentially with increasing the metal work functions, where Pt, Ni, Pd, Au, Cu, Ti, Al, Ta, and Ni/Au were deposited on the GaN. This result suggests that the Schottky barrier height at the p-GaN/metal interface might not be pinned at the GaN surface. The present study concluded that a contact metal with a large work function is desirable for nonreacted Ohmic contacts to p-GaN. However, these contacts did not provide the low contact resistance required for blue laser diodes.
Nanocrystalline Fe–M–B–Cu (M=Zr,Nb) alloys with improved soft magnetic properties
Authors: Makino Akihiro , Bitoh Teruo , Inoue Akihisa , Masumoto Tsuyoshi
Published: July 2002

The soft magnetic properties of the nanocrystalline Fe–M–B (M=Zr,Nb) alloys, which exhibit a high saturation flux density (Bs) above 1.5 T as well as a high effective permeability (μe) above 30 000 at 1 kHz, were found to be improved by adding small amounts of Cu and by optimizing the chemical composition. The addition of Cu to the alloys decreases the bcc grain size. The excellent soft magnetic properties (a high μe of 100 000 at 1 kHz combined with a high Bs of 1.53 T) can be achieved in the region where small grain size, as well as nearly zero-magnetostriction are obtained, which is attained in the compositional range around Fe84Nb3.5Zr3.5B8Cu1. The soft magnetic properties can be further improved by low temperature annealing before the crystallization treatment, probably as a result of a decreased grain size distribution in the crystallized state. Consequently, the μe reaches the maximum value of 120 000 for the nanocrystalline Fe84Nb3.5Zr3.5B8Cu1 alloy.
Epitaxial structure and magnetic anisotropies of metastable single crystal Co0.70Mn0.30 film
Authors: Zhao H. W. , Chen Y. , Zhu W. R. , Dong G. S. , Jin X. F. , Lu M. , Zhai H. R.
Published: July 2002

Alloy films of Co0.70Mn0.30 were grown on GaAs (001) substrates by molecular beam epitaxy at room temperature. The metastable tetragonally distorted single crystal structure was confirmed by reflection high energy electron diffraction and x-ray diffraction measurement, which exhibited a 2.87 Å in-plane lattice parameter. Ferromagnetic resonance measurements and theoretical fitting were performed and showed that, with a Mn capping layer, a uni-directional anisotropy existed. In addition, a fourfold tetragonal magnetocrystalline anisotropy as well as a uniaxial term in the film plane was also confirmed. The hysteresis loops recorded by longitudinal magneto-optical Kerr-effect also demonstrated the existence of different kinds of in-plane magnetic anisotropy. The origins of the anisotropy are explained tentatively.
Stress effects on exchange coupling field, coercivity, and uniaxial anisotropy field of NiO/NiFe bilayer thin film for spin valves
Authors: Han De-Hua , Zhu Jian-Gang , Judy Jack H. , Sivertsen John M.
Published: July 2002

The effects of uniaxial stress on the exchange coupling field, Hex, coercivity, Hc, and uniaxial anisotropy field, HK, in NiO/NiFe bilayers were experimentally studied. The NiO/NiFe bilayers were deposited onto a Si(100) wafer using radio frequency reactive sputtering. Samples of the bilayers were externally and constantly uniaxial stressed (either tensile or compressed) using a specially designed sample holder with a fixed radius of curvature. The hysteresis loops of the stressed NiO/NiFe bilayer samples were measured in situ along the easy and hard axes of the NiFe films using a vibrating sample magnetometer. The composition of the NiFe film in the NiO/NiFe bilayer was characterized as Ni80.2Fe19.8 using x-ray photoelectron spectroscopy. The Hc and HK of the bilayers were significantly affected by the stress, while the Hex was apparently not changed by the same stress. The large changes in the coercivity in the stressed NiO/NiFe bilayer were produced by the change of the effective uniaxial anisotropy field of the bilayer. We conclude that the control and reduction of both intrinsic and external stress are very important in the fabrication of spin-valve giant magnetoresonance heads and sensors.
Photoelectrochemical conversion in a WO3 coated p-Si photoelectrode: Effect of annealing temperature
Authors: Yoon Ki Hyun , Shin Choul Woo , Kang Dong Heon
Published: July 2002

The photoelectrochemical properties of a p-type silicon (100) electrode coated with tungsten oxide thin film were investigated as a function of annealing temperature. The variation in the annealing temperature affected the photocurrent of a WO3/p-Si electrode. A maximum photocurrent was obtained when the 500 Å WO3 thin film coated p-Si electrode was annealed at 350 °C for 1 h. A further increase in the annealing temperature and film thickness degraded the photocurrent. This can be explained in terms of electrical resistivity, carrier concentration, and depletion layer width. A WO3 thin film deposition on the p-Si shifted the flatband potential of the p-Si electrode by 0.3 V in the anodic direction, resulting in an improvement in conversion efficiency. These results are supported by x-ray diffraction, Auger electron spectroscopy, and capacitance measurements.
Single and multiband modeling of quantum electron transport through layered semiconductor devices
Authors: Lake Roger , Klimeck Gerhard , Bowen R. Chris , Jovanovic Dejan
Published: July 2002

Non-equilibrium Green function theory is formulated to meet the three main challenges of high bias quantum device modeling: self-consistent charging, incoherent and inelastic scattering, and band structure. The theory is written in a general localized orbital basis using the example of the zinc blende lattice. A Dyson equation treatment of the open system boundaries results in a tunneling formula with a generalized Fisher-Lee form for the transmission coefficient that treats injection from emitter continuum states and emitter quasi-bound states on an equal footing. Scattering is then included. Self-energies which include the effects of polar optical phonons, acoustic phonons, alloy fluctuations, interface roughness, and ionized dopants are derived. Interface roughness is modeled as a layer of alloy in which the cations of a given type cluster into islands. Two different treatments of scattering; self-consistent Born and multiple sequential scattering are formulated, described, and analyzed for numerical tractability. The relationship between the self-consistent Born and multiple sequential scattering algorithms is described, and the convergence properties of the multiple sequential scattering algorithm are numerically demonstrated by comparing with self-consistent Born calculations.
Nanocrystalline carbide/amorphous carbon composites
Authors: Voevodin A. A. , Prasad S. V. , Zabinski J. S.
Published: July 2002

Nanocrystalline TiC/amorphous carbon (a-C) composite films were synthesized at near room temperature with a hybrid process combining laser ablation of graphite and magnetron sputtering of titanium. Film microstructure was investigated by x-ray photoelectron spectroscopy, x-ray diffraction analyses, and transmission electron microscopy. Mechanical properties were evaluated from nanoindentation, scratch, and friction tests. The films consisted of 10 nm sized TiC crystallites encapsulated in a sp3 bonded a-C matrix. They had a hardness of about 32 GPa and a remarkable plasticity (40% in indentation deformation) at loads exceeding their elastic limit. They were also found to have a high scratch toughness in addition to a low (about 0.2) friction coefficient. The combination of hardness and ductility was correlated with film phase composition and structural analyses, using concepts of nanocomposite mechanics. The properties of the TiC/a-C composites make them beneficial for surface wear and friction protection.
Model for trap filling and avalanche breakdown in semi-insulating Fe:InP
Authors: Corvini P. J. , Bowers J. E.
Published: July 2002

A self-consistent two-carrier numerical model for steady-state current flow in n-semi-insulating-n InP structures allows the treatment of avalanche breakdown in addition to trap filling. Band-to-band impact ionization is included as a source term in the continuity equations. Carrier diffusion, nonlinear velocity field characteristics, and Shockley-Read-Hall recombination through the traps are also included, and the effects of each on the field and trapped carrier distributions are calculated. (The progress of trap filling predicted by the traditional drift-only theory is also calculated.) With impact ionization, hole accumulation near the cathode redistributes the space charge and contributes to positive feedback for avalanche breakdown. The model predictions are consistent with experimentally observed catastrophic breakdown and allow the development of design guidelines for avoiding device failure.
Multilayer film stability
Authors: Sridhar N. , Rickman J. M. , Srolovitz D. J.
Published: July 2002

We apply a linear stability analysis to examine the effect of misfit stress on the interface diffusion controlled morphological stability of multilayer microstructures. The stresses could be the result of misfit strains between the individual film layers and/or between film and substrate. We find that misfit between the layers in the film can destabilize the multilayer structure in cases where the thinner layer is elastically stiffer than the thicker layer. The rate at which these instabilities develop increase with increasing misfit and decreasing interfacial energy. Even when there is no misfit between layers, the misfit between the multilayer film and substrate can destabilize the interfaces. This type of instability occurs whether the thinner layers are stiffer or more compliant than the thicker ones. By appropriate choice of the elastic moduli mismatch between layers and relative layer thicknesses, the presence of an interlayer misfit can suppress the instability caused by the substrate misfit. We present stability diagrams that can be used to design stable, multilayer films using all of the degrees of freedom commonly available in multilayer film deposition.
Study of defects in GaN films by cross-sectional cathodoluminescence
Authors: Zaldivar M. Herrera , Fernández P. , Piqueras J.
Published: July 2002

Cathodoluminescence (CL) in the scanning electron microscope has been used to study cross-sectional samples on GaN epitaxial films grown on sapphire. Increased CL emission, attributed to the presence of stacking faults and decorated dislocations, is observed in a region of the buffer layer close to the film–substrate interface. In the epilayers also a region of enhanced emission is observed which is partially caused by Si doping and in which structural defects are involved. Cross-sectional CL appears as a useful method to reveal features of the spatial distribution of luminescence, not detectable by plan-view measurements.
Magnesium acceptor levels in GaN studied by photoluminescence
Authors: Viswanath A. Kasi , Shin Eun-joo , Lee Joo In , Yu Sungkyu , Kim Dongho , Kim Baeyong , Choi Yoonho , Hong Chang-Hee
Published: July 2002

Magnesium doped GaN epitaxial layers were grown by metal-organic chemical vapor deposition on sapphire substrate. Energy levels of these acceptors were investigated by systematic photoluminescence measurements in the temperature range of 12–300 K. Magnesium concentration was varied from <1×1019 to higher than 5×1019 cm−3. Photoluminescence measurements were made on the as-grown and annealed samples. We have observed various transitions related to donor to acceptor and their phonon replicas, conduction band to acceptors, and free excitons. Their dependence on temperature, concentration of the magnesium impurity and annealing conditions was discussed. In our study, two important observations were made. First, very deep level luminescence was not observed even in the highly magnesium doped as-grown samples. Second, free exciton transitions including valence band splittings were observed for the first time in the Mg-doped materials, demonstrating the high quality of the samples.
Effect of Ga substitution on the structure and magnetic properties of Ho2Fe17−xGax (x=0–8) compounds
Authors: Wang Fangwei , Shen Bao-gen , Zhang Panlin , Cheng Zhao-hua , Zhang Junxian , Gong Huayang , Liang Bing , Sun Xiangdong , Yan Qiwei
Published: July 2002

The Rietveld analysis of x-ray diffraction data shows that Ho2Fe17−xGax (x=0–8) solid solutions crystallize with the Th2Ni17 structure for x⩽2, the Th2Zn17 structure for x⩾4, and the two coexisting phases for x=3. The unit-cell volume increases linearly with a slope of 8.7 Å3 per Ga atom. At lower gallium concentrations, Ga atoms occupy the 18f, 18h sites in the Th2Zn17 structure (or 12j, 12k sites in the Th2Ni17 structure) and avoid the 6c, 9d sites. For higher Ga content (x⩾6) Ga atoms still avoid the 9d site, but preferentially occupy 6c and 18f sites for x=6–8 while the occupancy factor of the 18h site decreases. Gallium substitution decreases the saturation magnetization linearly with a slope of 2.5μB. The compounds Ho2Fe17−xGax (x=7,8) possess uniaxial magnetocrystalline anisotropy at room temperature. Doping first increases, then decreases the Curie temperature until this finally rebounds for x>6. The “anomalous” enhancement of the Curie temperature for x=7, 8 may be correlated with the variation of the Fe–Fe bond lengths.
Wave propagation in layered piezoelectric structures
Authors: Mesquida A. A. , Otero J. A. , Ramos R. R. , Comas F.
Published: July 2002

In the present work we analyze wave propagation in piezoelectric layered structures with special emphasis on semiconductor layered systems. The mathematical and physical formulation of the problem is stated according to the phenomenological theory of continuous media. The general solution of the system of equations for a piezoelectric material with cubic symmetry is given. The boundary conditions for mechanical displacements and elastic stresses, as well as the electrical quantities, are analyzed. The problem is split into two coupled system of second order ordinary differential equations: one for the displacement v1(z) and the electric potential ψ(z), and the other for the two remaining mechanical displacements v2(z) and v3(z). The dispersion relations are expressed through determinants because of their complexity, and the dispersion curves are numerically obtained. The obtained results, in the form of propagating waves along the structure, are discussed.
Experimental and computer simulation studies of diffusion mechanisms on the arsenic sublattice of gallium arsenide
Authors: Schultz M. , Egger U. , Scholz R. , Breitenstein O. , Gösele U. , Tan T. Y.
Published: July 2002

Interdiffusion experiments with GaAsP/GaAs and GaAsSb/GaAs superlattice samples were performed at various temperatures and arsenic vapor pressures. From the depth-concentration profiles effective diffusion coefficients were calculated. The dependence of these effective diffusion coefficients on the ambient arsenic pressure led to the conclusion that the interdiffusion process is governed by a substitutional-interstitial diffusion mechanism. The good agreement of the effective diffusion coefficients of the GaAsP/GaAs and GaAsSb/GaAs samples with each other and the agreement with arsenic self-diffusion data from the literature is an indication that phosphorus and antimony have good tracer properties to investigate arsenic self diffusion. Comparing our results with sulfur in-diffusion experiments from the literature we conclude that the kick-out mechanism governs self-diffusion on the arsenic sublattice in GaAs. Our results are in contradiction to arsenic self-diffusion experiments which indicated a vacancy mechanism.
Pr K-edge x-ray absorption fine structure analysis of the local structure of Pr in fluorozirconate glasses
Authors: Braglia M. , Dai G. , Mosso S. , Pascarelli S. , Boscherini F. , Lamberti C.
Published: July 2002

In order to investigate the local structure around Pr active centers in fluorozirconate optical fibers, we report a Pr K-edge x-ray absorption fine structure study, on two series of the compounds based on the composition 53ZrF4–20SrF2–4YF3–3AlF3–20NaF (mol %) (ZSYAN). In the first series, different doping contents of PrF3 from 500 to 5000 ppmw were assessed in order to investigate the possible correlation between local structure and concentration quenching. In the second series, Pr3+ was introduced in different forms of dopants, PrF3, PrCl3, Pr6O11, and Pr2S3, with the concentrations of 500 and 5000 (or 10 000) ppmw. In both series, we find that no first shell Pr–Pr signal is present, even in samples having a Pr3+ concentration of 10 000 ppmw, i.e., one order of magnitude greater than what is universally recognized to be the concentration quenching limit (≈1000 ppmw). In the second series, we have observed that whatever the Pr precursor, the first shell environment around Pr3+ in ZSYAN samples is always the same as in the PrF3 model compound. Similar measurements performed on 53ZrF4–20BaF2–4LaF3–3AlF3–20NaF (mol %) (ZBLAN), doped with PrF3 from 500 to 10 000 ppmw, although more noisy, qualitatively confirm the results obtained from ZSYAN.
Valveless pumping using traversing vapor bubbles in microchannels
Authors: Jun Thomas K. , Kim Chang-Jin “CJ”
Published: July 2002

Pumping of fluids in microchannels using the movement of a single or multiple vapor bubble(s) is proposed, analyzed, and demonstrated. The pumping mechanism requires no micromechanical moving parts for actuation by utilizing asymmetric heating which creates a variation in vapor pressure and surface tension due to the heater-induced temperature gradient along the channel. A heat and mass transfer analysis was performed to understand the pumping mechanism and estimate the pumping capability of the micropumping device. To verify the concept and our analysis, a pumping device with a transparent microchannel with a hydraulic diameter of 3.4 μm was fabricated on a silicon wafer using surface micromachining. Experimental results with the first generation device have shown pumping of isopropanol at velocities as high as 160 μm/s (0.5 nl/min flow rate) with a pressure head of approximately 800 Pa.
Effect of probe geometry on the Hall response in an inhomogeneous magnetic field: A numerical study
Authors: Liu S. , Guillou H. , Kent A. D. , Stupian G. W. , Leung M. S.
Published: July 2002

The effect of probe geometry on the classical Hall response to a weak perpendicular inhomogeneous magnetic field is studied numerically. An electric potential equation based on a classical model of the two-dimensional Hall effect is solved numerically for a generalized flux distribution to find the Hall response function. We find that the magnitude and shape of this response function is strongly affected by probe geometry. Asymmetric cross-shaped Hall probes, with one narrow voltage lead, have a strongly peaked response more localized than in symmetric probe arrangements. This suggests novel lithographic patterns that may improve the spatial resolution of Hall magnetometry and scanning Hall probe microscopy.
Charge distribution function of dust particles in low temperature plasmas
Authors: Gordiets B. F. , Ferreira C. M.
Published: July 2002

The charge distribution function (CDF) of dust particles in low temperature laboratory plasmas is obtained as a function of the discrete particle negative charge Z=0, 1, 2,…, and various plasma parameters. An analytical expression for the CDF is derived both for Maxwellian and non-Maxwellian plasma electrons taking into account the processes of electron attachment and detachment (secondary electron emission), and recombination of positive ions with the dust particles.
Effects of oxygen-related defects on the leakage current of silicon p/n junctions
Authors: Murakami Yoshio , Satoh Yuhki , Furuya Hisashi , Shingyouji Takayuki
Published: July 2002

The reverse-bias leakage characteristics of silicon pn junctions have been investigated with particular attention to the effects of various types of oxygen-related defects, such as oxygen precipitates, oxidation induced stacking fault, and grown-in defects. The effects of oxygen-related defects on the leakage current of pn junctions in intrinsic gettering wafers and precipitation annealed wafers have been investigated quantitatively, and the field oxidation temperature used to form pn junctions has been found to be an important factor in determining the pn junction leakage current because oxygen-related defects are formed during low temperature field oxidation. It has also been found that grown-in oxidation induced stacking faults degrade the leakage characteristics. Grown-in defects that are well known to degrade the oxide breakdown characteristics were found to have some effects on the increase of the leakage current. In addition, it is recognized that the leakage current of pn junctions formed in wafers that have a relatively high concentration of interstitial oxygen has a transient component, caused by oxygen-related hole traps, which might also be the origin of the 1/f noise observed in pn junctions.
Analysis of photoluminescence from porous silicon: Particle size distribution
Authors: Yorikawa H. , Muramatsu S.
Published: July 2002

A model for the photoluminescence (PL) spectrum of porous silicon has been recently proposed by the present authors under the assumption that porous silicon consists of various size nanoparticles, and the particle size has been found to obey the logarithmic normal distribution. In this paper, the model is thoroughly investigated to demonstrate how the peak energy and the full width at half maximum for the PL spectra depend on excitation energy and to compare their dependence with available experimental data. By further applying this model to experimental data reported in the literature, an attempt is made to extract information about particle size distribution in porous silicon. The result is discussed within the context of sample preparation conditions.
A study of the photothermal signal produced by a series of subsurface cylinders in opaque materials
Authors: Sanchez-Lavega A. , Salazar A. , Terrón J. M. , Ocáriz A.
Published: July 2002

We present a theoretical and experimental study of the photothermal signal produced by a series of buried cylinders in an optically opaque sample. The models are valid for any value of the thermal properties of the cylinder and sample (the extreme cases treated are isolating and conducting cylinders), and the modulated excitation can be extended (plane wave generation) or punctual (spherical wave generation). The model results are compared with measurements performed on calibrated samples using two photothermal techniques: “mirage” deflection and infrared radiometry. Semiquantitative agreement between theory and experiment is found showing that this methodology can be used as a tool for the nondestructive evaluation of close subsurface cylinders, which represents a further step to the photothermal study of some materials of interest as the fiber-reinforced composites.
Energy level alignment at interfaces of organic semiconductor heterostructures
Authors: Hill I. G. , Kahn A.
Published: July 2002

We have used ultraviolet photoelectron spectroscopy (UPS) to investigate the interfaces of two organic semiconductor heterostructures. UPS was used to determine the relative energies of the highest occupied molecular orbitals of the organic semiconductors, and to measure the interface dipoles at each interface. The two systems studied were the 4-4′-N,N′-dicarbazolyl-biphenyl(CBP)/tris(8-hydroxy-quinoline)aluminum (Alq3) interface, and the copper phthalocyanine (CuPc)/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′biphenyl-4,4″diamine (α-NPD) interface. The assumption of a common vacuum level was found to be valid, within experimental uncertainty, for both interfaces.
Magneto-optical properties of a sputtered Bi-substituted Dy–Fe garnet-ferrite/hematite bilayer
Authors: Furuya A. , Yoshikawa H. , Tanabe T. , Yamamoto M. , Tailhades P. , Bouet L. , Despax C. , Presmanes L. , Rousset A.
Published: July 2002

Rapid thermal annealing was performed after a Bismuth-substituted Dy iron garnet-ferrite (Bi2DyFe4GaO12) was deposited on a quartz glass substrate having a hematite underlayer, and the effect of the hematite underlayer on the magneto-optical characteristics of the garnet ferrite film was evaluated. The presence of the underlayer greatly improved the magneto-optical effect of the garnet film, but the amount of improvement depended on the hematite phase. Big improvement was obtained using underlayer phases that are easily crystallized, i.e., Fe3O4 and γ-Fe2O3 which have a spinel structure, but not with α-Fe2O3 which has a corundum structure.
Study of magnetization processes and the generation of magnetoacoustic and Barkhausen emissions in ferritic/pearlitic steel
Authors: Lo C. C. H. , Scruby C. B.
Published: July 2002

Magnetoacoustic emission (MAE), Barkhausen emission (BE), and hysteresis loop measurements have been made on steel specimens heat treated to produce ferritic/pearlitic structures with various volume fractions of ferrite Vα. It was found that in the low-field region the BE activity decreases whereas the MAE signal increases with Vα. Increasing Vα also results in a decrease in the initial peak height of MAE profiles and the formation of outer peaks in BE profiles. It was observed by Lorentz microscopy that in ferrite non-180° domain wall motion occurs readily in the low-field region, while in pearlite significant 180° domain wall motion takes place only when a reversed field was applied. These results can account for the variations in the MAE and BE profiles with Vα. The sensitivity of the MAE and BE profiles to Vα enables both techniques to be exploited for monitoring the ferrite content of steel components.
Microwave reflections from a vacuum ultraviolet laser produced plasma sheet
Authors: Kelly K. L. , Scharer J. E. , Ding G. , Bettenhausen M. , Kuo S. P.
Published: July 2002

A pulsed, 193 nm excimer laser is utilized to photoionize the organic gas tetrakis- dimethylamino-ethylene (TMAE). The laser ionizes a plasma sheet with a width of 7.8 cm and an adjustable thickness of 0.7–1.4 cm. The axial scale length of the plasma density is a function of TMAE neutral pressure and is typically 50 cm. X-band (10 GHz) microwaves are incident on the plasma with the electric field polarized parallel to the laser beam axis. The power reflection coefficient and the phase of the reflected signal are studied as a function of time. A monostatic homodyne detection system with a response time of 10 ns is utilized to determine the amplitude and phase of the reflected wave. The peak plasma density is ne≈4×1013 cm−3, sufficiently above the critical density (ncrit=1.2×1012 cm−3) to produce reflections comparable to a conducting sheet placed in the same position as the plasma. A computer model is developed to interpret and optimize the plasma conditions which provide the highest backscatter and phase-stable reflection coefficient for the longest period of time. The presence of axial density gradients causes the reflected wave to be scattered through a wide angle. As the gradients relax, the backscatter reflection coefficient increases to a value of nearly 100%. The plasma density and two-body recombination coefficient are measured by means of microwave backscatter plasma reflectivity and Langmuir probes.
On the kinetic mechanism of grain boundary wetting in metals
Authors: Glickman E. E. , Nathan M.
Published: July 2002

The thermodynamic condition characteristic of grain boundary wetting (GBW) causes an imbalance between grain boundary (GB) and solid–liquid interphase surface tensions γGB and γSL. This creates in turn a force acting at the root of the GB groove, and pointing into the solid. The “indentation” action of this force is suggested to cause stress-driven self-diffusion into the GB. This process removes the solid atoms from the groove cavity and causes their deposition along the GB (“internal solution”). Assuming that the GB acts as a perfect sink, this “self-indentation-internal solution” mechanism can account for a number of GBW features: the non-Mullins grooving morphology and linear kinetics, the origin of the singular stress field at the wetting front, the expansion of the solid under GBW, the influence of external stress on GBW, the GBW transitions with temperature, and the fast atomic penetration of the liquid metal ahead of the groove root.
Ensemble Monte Carlo calculation of hole transport in bulk 3C–SiC
Authors: Bellotti Enrico , Nilsson Hans-Erik , Brennan Kevin F. , Ruden P. Paul
Published: July 2002

In this article the first calculation of hole transport in the 3C phase of SiC is presented. The salient features of the model are the full band-structure computed by the empirical pseudopotential method, a numerically calculated hole-phonon scattering rate and the impact ionization transition rates. The coupling constants necessary to determine the scattering rates have been determined either from available data in the literature or by fitting the calculated mobility values to low field experimental results. The impact ionization transition rates have been determined directly from the band-structure based on a wave-vector dependent dielectric function. The steady state drift velocity as a function of the applied electric field strength is computed for different field directions and doping concentrations. The calculated results show the presence of an anisotropy in the drift velocity for the field applied along different directions, similar to what is found in silicon. The maximum values of the velocity are 1.63×107 cm s−1 and 1.43×107 cm s−1 for the (100) and (111) field directions, respectively. High field transport has also been studied. The calculated ionization coefficients show no appreciable anisotropy for the field applied along different directions. The second valence band contributes the most to the impact ionization rate. It is further found that the ionization threshold is relatively soft.
Numerical study of hysteresis and morphology in elongated tape particles
Authors: Seberino Christian , Bertram H. Neal
Published: July 2002

A three-dimensional micromagnetic model with cubic discretization is used to examine switching field dependence on particle size, aspect ratio, external field angle, and shape morphology for typical tape particles. For rectangular particles, discretization along the particle axis gives accurate switching fields for all radii approximately less than an exchange length. Switching fields are independent of aspect ratio for 5:1 and greater rectangular particles except for sufficiently small radii where uniform switching occurs. Spherocylinders, cylinders, and rectangular parallelepipeds give virtually the same switching field at least for all radii less than two exchange lengths. For radii approximately equal to an exchange length, the corresponding switching field of an ellipsoid is nearly twice that of these other shapes. Numerical results for approximate ellipsoids agree with analytical predictions and previous fine element method calculations.
Magnetic properties of sol-gel derived Ni–Zn ferrite thin films on yttria stabilized zirconia buffered Si(100)
Authors: Bae Seung-Young , Kim Chul-Sung , Oh Young-Jei
Published: July 2002

The effects of yttria stabilized zirconia (YSZ) buffer layers for the sol-gel derived Ni–Zn ferrite thin films on SiO2/Si(100) substrates were investigated. The 500 Å thick YSZ buffer layers were found to effectively suppress the diffusion of Si into the ferrite thin films. As a consequence, films with lower Hc and higher Ms could be obtained by annealing at higher temperature (900 °C), which was not accomplished in the films without the buffer layer due to the Si diffusion. The angular variation of Hc with respect to the direction of applied field shows that a films without the buffer layers deviates more from the domain wall motion, thus, the diffused Si was suggested to segregate at the grain boundaries of the ferrite film and to reduce the intergranular exchange coupling.
The effects of chemical composition on electron delocalization and magnetic ordering in ilvaite, Ca[Fe2+, Fe3+][Fe2+]Si2O7O(OH)
Authors: Dotson Cherie R. , Evans B. J.
Published: July 2002

Substitutional impurities are expected to have significant influences on long-range, order–disorder phenomena associated with magnetic ordering and electron delocalization in mixed-valence solids. These influences have been investigated by means of superconducting quantum interference device susceptometry and Fe57 Mössbauer spectroscopy on more than 40 mineral specimens of the mixed-valence solid ilvaite, Ca[Fe2+, Fe3+][Fe2+]Si2O7O(OH), with manganese contents between 0.2 and 10 wt %. At 295 K, the Fe57 Mössbauer spectra are quite diverse, exhibiting different numbers of lines, varying degrees of resolution and line broadening. The complexity of the spectra correlate well with increasing Mn content. At 160 K, the spectra are remarkably similar and are characterized by three overlapping subspectra. The Isomer shifts are: 1.09, 1.07, and 0.64 mm s−1, respectively, which may be unambiguously assigned to 2 Fe2+ and 1 Fe3+ species. The magnetic ordering transition near 120 K, associated with the ordering of the moments of the Fe2+ and Fe3+ ions involved in electron delocalization, is not evident in the susceptibility data. Further, the susceptibility versus temperature curves exhibited no qualitative changes with varying Mn content. The magnetic transition associated with the ordering of the magnetic moments of the Fe2+ ion, not involved in electron delocalization, is clearly evident in the susceptibility data and exhibits a systematic dependence on Mn content, occurring at 27, 26, 25, and 23 K for Mn contents of 0.23, 0.70, 0.90, and 0.97 wt %. For Mn contents above 1 wt %, this magnetic transition is not observed.
Thermal relaxation of the pinned layer magnetization in NiO spin valves
Authors: Taylor Roger H. , O’Barr R. , Yamamoto S. Y. , Dieny B.
Published: July 2002

A variation of the switching field of the pinned layer (PL) magnetization in NiO based spin valves was observed as a function of the rate of variation of the applied field. This phenomenon was characterized at low frequencies (0.4–120 Hz) using quasistatic magnetoresistance (MR) transfer curve testing and measurements of the relaxation of the resistance around the quasistatic switching field of the pinned layer (10−1–105 s). The influence of temperature was investigated between room temperature and 125 °C. The relaxation data were fit using a model based on an Arrhenius law thermal activation with a single relaxation time. The variation in the switching field of the pinned layer as a function of the applied field rate was compared to the Kurkijärvi model. The fit parameters in both models are the energy barrier height and the thermally activated volume of nucleation. The parameters derived from both types of experiments agree remarkably well. The understanding of this low frequency magnetic behavior of spin valves allows one to gain insight into the long-term relaxation of the magnetization of the PL in MR readback heads, as well as into the magnetic response of this layer to very short (ns) pulses of field or high frequency (100 MHz) excitations.
Structure and magnetic properties of a two-dimensional c(2×2) Mn–Co/Cu(001) surface alloy
Authors: Choi B.-Ch. , Bode P. J. , Bland J. A. C.
Published: July 2002

The structure and magnetic properties of epitaxially grown Mn on fcc Co/Cu(001) are studied using in situ magneto-optical Kerr effect and low-energy electron diffraction. A two-dimensional c(2×2) Mn–Co(001) surface alloy stabilizes in the range of 0.3–0.8 ML of Mn, in which Mn is ferromagnetically coupled to the fcc Co(001). Above one monolayer of Mn, the c(2×2) superstructure disappears and the Mn overlayer no longer has long range ferromagnetic order. Furthermore, it is shown that the strengths of the magnetic anisotropies and magneto-optic response are correlated with the MnCo surface alloy formation.
Pinning effect of the grain boundaries on magnetic domain wall in FeCo-based magnetic alloys
Authors: Yu R. H. , Basu S. , Zhang Y. , Parvizi-Majidi A. , Xiao John Q.
Published: July 2002

We have studied the dynamics of grain growth and the pinning effect of grain boundaries on magnetic domain walls in FeCo soft magnetic alloys. It has been found that grain growth takes place at temperatures above 600 °C. The activation energy for grain growth in a disordered state at 820 °C is about 57.4±0.5 kcal/mole. The effect of grain size on magnetic properties has been singled out by keeping the same ordering parameter (S=0 and 0.88) for all samples studied. Microstructural characterization and magnetic measurements indicate that the grain size significantly affects the magnetic coercivity. A linear relationship between the coercivity and the reciprocal of the grain size has been universally found regardless of the heat-treatment histories. Lorenz microscopic observation demonstrates that grain boundaries act as pinning sites for the magnetic domain wall movement.
Vacuum ultraviolet and x-ray luminescence efficiencies of Y3Al5O12:Ce phosphor screens
Authors: Baciero A. , Placentino L. , McCarthy K. J. , Barquero L. R. , Ibarra A. , Zurro B.
Published: July 2002

Phosphor screens are used in many applications to convert incident radiation to visible wavelengths that can be readily measured by modern detectors. For many such systems it is necessary to tailor the phosphor screen thickness to the wavelength range of interest in order to maximize the efficiency. Here, we describe the experiments made to determine the luminescent efficiency of thin Y3Al5O12:Ce (P-46) phosphor screens (1–22 mg cm−2) in the vacuum ultraviolet and x-ray ranges. We use current and previous measurements to obtain fitting parameters for a granular unidimensional radiation transfer model developed. In the model, we take into account the effects of scattering and absorption of the luminescent light emitted and we discuss the effects of grain size and shape on the model. Finally, we find that a screen thickness of ∼1.7 mg cm−2 is optimum for incident radiation between 1.2 and 124 nm but this thickness becomes wavelength dependent at shorter wavelengths (0.025–1.2 nm).
Modeling of enlarged back gaps in vertical giant magnetoresistance read heads
Authors: Anderson J. M. , Pohm A. V.
Published: July 2002

To fully exploit the capability of vertical head sensors, it is necessary to increase the gap immediately after the front contact region located at the air bearing surface. Because of fringing of the flux as it leaves the sensor and passes to the shields, the effective gap is less than the physical gap. An analysis has been performed using an electrical, SPICE, analog model for the field equations to determine the relationship between the change in the physical gap and the effective gap in terms of the signal decay along the sensor [A. V. Pohm, R. S. Beech, J. M. Anderson, and W. C. Black, IEEE Trans. Magn. 33, 2392 (1997)]. Three enlarged half gaps were considered: one with an abrupt increase, one with a slope of two, and one with a slope of one. The analysis considered mostly back gaps which were ten times larger than the front gaps, going from 0.1 to 1.0 μm, for example. The results show an abrupt step yields an effective slope of 0.9 and a physical step with a slope of 1 yields an effective slope of 0.75. The effective slopes are being used to more accurately model the decay of signal flux along vertical head sensors.
Grating-coupled surface plasmons at microwave frequencies
Authors: Hibbins A. P. , Sambles J. R. , Lawrence C. R.
Published: July 2002

This work presents a detailed investigation of electromagnetic coupling to the surface plasmon-polariton (SPP) at microwave frequencies. We have recorded the wavelength-dependent reflectivity from a metallic sinusoidal diffraction grating of pitch 15 mm. In order to minimize the problems associated with nonplanar incident wavefronts, we have developed an apparatus that collimates the incident beam. We illustrate resonant coupling to the SPP at wavelengths of the order of 10 mm. The wavelength-dependent reflectivities recorded have been successfully fitted using a differential formalism of conical diffraction with a single set of grating parameters describing the grating profile and metal permittivity.
Carrier concentration and mobility in GaN epilayers on sapphire substrate studied by infrared reflection spectroscopy
Authors: Li Z.-F. , Lu W. , Ye H.-J. , Chen Z.-H. , Yuan X.-Z. , Dou H.-F. , Shen S.-C. , Li G. , Chua S. J.
Published: July 2002

We report the measurement of carrier concentration and mobility of metalorganic chemical vapor deposited GaN thin films on the sapphire substrate by an infrared reflection technique. By fitting with the experimental data we obtain all the parameters of the lattice vibration oscillators and of the plasmon. From the plasmon frequency and the damping constant we have derived the carrier concentration and the electron mobility. The concentration agrees with the Hall data very well while the mobility values are smaller than that of the Hall measurement by a factor of about 0.5. We attribute such mobility lowering to the increase of scattering for the electrons coupling with the incident photons.