bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal conductivity and magnetic transitions in Mn3+/Mn4+ manganites
Authors: Hejtmánek J. , Jirák Z. , Arnold Z. , Maryško M. , Krupička S. , Martin C. , Damay F.
Published: July 2002

We report on a comparative study of three manganites: Pr0.5Sr0.5MnO3, Pr0.5Ca0.5MnO3, and Pr0.5Sr0.3Ca0.2MnO3, which all exhibit the same Mn3+/Mn4+=1:1 concentration but show a very distinct magnetic behavior. The Pr0.5Sr0.5MnO3 sample is a ferromagnet (FM) below TC=265 K and becomes the A-type antiferromagnet at TN=140 K. In Pr0.5Ca0.5MnO3 the charge order develops at TCO=245 K and the CE-type antiferromagnetism occurs below TN=175 K. The Pr0.5Sr0.3Ca0.2MnO3 sample exhibits the FM transition at TC=225 K and the CE type antiferromagnetism and charge ordering occur simultaneously at TN=TCO=160 K (185 K) upon cooling (heating), respectively. The study is complemented by the data on two systems of 30% Mn4+, the ferromagnetic metal Pr0.7Sr0.1Ca0.2MnO3, and insulator Pr0.85K0.15MnO3, both with similar Curie temperature TC∼140 K. The electrical resistivity, thermoelectric power, thermal conductivity, and volume thermal expansion data are analyzed with respect to observed magnetic transitions and reveal the significantly distinct ground state of the A-type and CE-type antiferromagnets. We suppose that phonons and charge carriers are coupled via the Jahn–Teller effect and the CMR effect observed in manganites originates thus from a mutual interplay of a strong electron–phonon interaction and spin order.
Frustration in the paramagnetic phase of spin-density-wave CrFeV alloys
Authors: Galkin V. Yu. , Ortiz W. A. , Fawcett E.
Published: July 2002

The magnetic susceptibility χ(T) of the spin-density-wave (SDW) alloys (Cr+2.7% Fe)1−xVx (x=0.07, 0.29, 0.61, 0.9, 1.28, and 2.0% V) and of the paramagnetic alloys (x=5 and 10% V) measured in a field, H=100 Oe, in the temperature range, T=5–400 K, was found to exhibit hysteresis between the field-cooled and zero-field-cooled states, starting at an irreversibility limiting temperature Ti well above the Néel temperature TN in the SDW alloys, and also in the paramagnetic alloy (Cr+2.7% Fe)95V5. This behavior is attributed to the frustration of moments on the Cr atoms associated with the formation of a local-SDW phase above the bulk Néel transition.
Magnetic transitions and oxygen content of Ca3Ru2O7
Authors: McCall S. , Cao G. , Crow J. E. , Guertin R. P.
Published: July 2002

Magnetization and magnetoresistivity of as-grown, oxygenated, and argon annealed single crystals of Ca3Ru2O7 are presented for 5<T<300 K and 0<H<7 T. As-grown Ca3Ru2O7 has been shown to have an antiferromagnetic metallic phase for 48<T<56 K and 0<H<4 T, located between the high-temperature paramagnetic metallic and the low-temperature antiferromagnetic insulating phase. Details of the field dependence of the magnetoresistance of as-grown Ca3Ru2O7 show slight variations between the H–T metal-insulator and paramagnetic-antiferromagnetic phase boundaries as determined from magnetization and from magnetoresistance measurements, but well-characterized samples with oxygen addition or deletion show much more extensive variations. Oxygenation lowers the electrical resistivity and sustains the 6 T first-order field-induced magnetic transition for T≪TM. However, magnetization is linear through the same field region, and argon annealed samples, with decreased oxygen content, show the same linearity of the magnetization. The results underscore the importance of oxygen content in the ruthenates, in analogy with the role of oxygen content in the cuprates.
Phase diagram of a highly diluted, disordered Ising system: The Al-rich, Al–Fe system
Authors: Restrepo J. , Pérez Alcázar G. A. , González J. M.
Published: July 2002

Disordered solid solutions containing a highly diluted magnetic element in which competitive exchange interactions are present can allow the observation of magnetic ordering (ferromagnetism, antiferromagnetism, and spin-glass behavior) at dilutor element contents different from those corresponding to ordered alloys. In this work we present a simulational study of the magnetic properties of the Al-rich, Al–Fe disordered alloy system, which meets the above mentioned requirements and is therefore a good candidate to show differences between the magnetic phase diagrams corresponding to the ordered and disordered states. Our simulations were carried out on the basis of a randomly site-diluted Ising model, using the Monte Carlo method as energy minimization tool. In our model, the spins of the Fe atoms are assumed to be coupled by a direct nearest-neighbor ferromagnetic exchange J (depending on the Al concentration), and by an Al-mediated antiferromagnetic superexchange interaction −αJ. Our results suggest the presence of ferromagnetic order for Al concentrations higher than those for which that order disappears in ordered alloys and the occurrence, for Fe atomic percentages between 10% and 25%, of a re-entrant spin glass phase whose freezing temperature decreases with the degree of dilution.
Exchange narrowing effects in the EPR linewidth of Gd diluted in Ce compounds
Authors: Venegas Pablo A. , Netto Paulo R. S.
Published: July 2002

Anomalous thermal behavior on the EPR linewidths of Gd impurities diluted in Ce compounds has been observed. In metals, the local magnetic moment EPR linewidth, ΔH, is expected to increase linearly with the temperature. In contrast, in CexLa1−xOs2 the Gd EPR spectra show a nonlinear increase. In this work, the mechanisms that are responsible for the thermal behavior of the EPR lines in CexLa1−xOs2 are examined. We show that the exchange interaction between the local magnetic moments and the conduction electrons are responsible for the narrowing of the spectra at low temperatures. At high temperatures, the contribution to the linewidth of the exchange interaction between the local magnetic moments and the Ce ions has an exponential dependence on the excitation energy of the intermediate valent ions. A complete fitting of the EPR spectra for powdered samples is obtained.
Ground state properties of a high-spin Mn12O12 molecule in an organic compound
Authors: Wang S. Y. , Zou Liang-Jian , Gong X. G. , Zheng Q. Q. , Lin H. Q.
Published: July 2002

The electronic structure and the magnetism of a Mn12O12 molecule at ground state have been studied by density functional theory with a local spin density approximation. We have found that the magnetic moments of Mn ions in the tetrahedron and those of Mn ions in the crown of the Mn12O12 molecule align antiferromagnetically. The average moment per Mn ion is about 3.07μB in the tetrahedron and 4.07μB in the crown. The total spin amounts to 20.0μB which is in agreement with recent experimental results. The significant difference of magnetic moments between Mn ions at two sites is attributed to the different exchange splitting of 3d orbitals. However, the charge difference between the two kinds of Mn ions is as small as 0.22 electrons. The charge density and the spin density exhibit strong directional dependence, which indicates the strong anisotropy in this molecule.
Trial on-silicon micromagnetoelastic devices
Authors: Takezawa M. , Yamaguchi M. , Ishiyama K. , Arai K. I.
Published: July 2002

This paper discusses two new kinds of micromagnetoelastic devices made on silicon wafer whose function is to control the permeability of magnetostrictive soft magnetic thin films by voltage-controlled elastic strain. One is piezoelectric type and the other is electrostatic type. Structure, fabrication process, characteristics, and maximum possible output are discussed. The feasibility of these devices has been clarified although the rate of obtained permeability change was less than 1%. These device characteristics could be improved to 11% for piezoelectric type and to 80% for electrostatic type by the optimization of device dimensions and reduction of process damage to the magnetic film.
Polycrystalline and laminated La0.7Sr0.3MnO3 films made by pulsed laser deposition
Authors: Cadieu F. J. , Rani R. , Qian X. R. , Cadieu C. F. , Chen Li , Mendoza W. , Shaheeen S. A.
Published: July 2002

Highly smooth and particulate free relatively thick polycrystalline La0.7Sr0.3MnO3 films out to 6 μm thick have been grown by shadowed pulse laser deposition at 30 Hz (248 nm, 500 mJ). Such films exhibited a low field and high field magnetoresistance as previously reported by a number of groups. Multilayer films made with alternating 8-nm-thick barium ferrite and 16-nm-thick La0.7Sr0.3MnO3 laminations exhibited the same high field response, but did not exhibit the low field magnetoresistance component. This is consistent with the low field component arising from intergrain scattering. Patterned films have been simultaneously deposited onto R-plane, C-plane, and polycrystalline alumina substrates. The films were postannealed at 750 K in 200 Torr O2 for 45 min. The low field magnetoresistance was most pronounced for the single layer films made onto C-plane sapphire and then polycrystalline alumina substrates. The films exhibited a broad maximum in resistivity versus temperature at 162, 210, and 218 K for films grown onto R-plane sapphire, C-plane sapphire, and alumina substrates, respectively. Restricted grain size films made with La0.7Sr0.3MnO3 and barium ferrite laminations exhibited a single linear magnetoresistance response of −0.43%/kOe for in plane applied fields of ±10 kOe.
Giant magnetoresistance and remanence in granular CoCu codeposited films
Authors: Viegas A. D. C. , Geshev J. , Schmidt J. E. , Ferrari E. F.
Published: July 2002

We present experimental magnetoresistance, magnetization, and remanence data for CoCu granular films obtained by electron beam codeposition, subsequently submitted to a thermal treatment. The magnetization measurements show coexistence of superparamagnetic and ferromagnetic (or blocked superparamagnetic) Co grains. By modeling the experimental magnetization curves, the distribution function of the superparamagnetic particle size, the volume fraction of the blocked part of the Co particles, and their saturation magnetization are obtained. δM plots are constructed using the remanence curves. A correlation between the changes of the δM parameters and those of the giant magnetoresistance, caused by the annealing, is discussed.
Magnetoresistance in single Fe(001) ultrathin films
Authors: Yuasa S. , Katayama T. , Nývlt M. , Suzuki Y. , Yori T.
Published: July 2002

Magnetoresistance in Au(001)-sandwiched Fe(001) ultrathin films was investigated at T=1.6 K. Effects of both the Fe layer and the Au cap layer were investigated using wedge-shaped Fe and Au cap layers. The total thicknesses of the films were kept much thinner than the mean free path of the conduction electrons in order to enhance the magnetoresistance by electron channeling between the surface and the substrate. The Fe wedge-shaped films in the range dFe=0–4 Å showed a granular-type giant magnetoresistance (GMR) because of a discontinuity in the Fe layer. The films with dFe=4–25 Å also showed that the GMR component through the Fe layer is continuous. In the Au cap wedge-shaped films, the GMR component nearly vanished at dAu cap=0. The MR ratio increased linearly with dAu cap in the range dAu cap=0–50 Å and saturated at dAu cap∼130 Å. Moreover, the CuO overlayer on the Au cap wedge-shaped film resulted in a 25% decrease in the MR ratio in the whole dAu cap range. These phenomena can be interpreted as differences in spin flip probability for electron reflections on the surface and at the interfaces.
Angle-resolved magnetic and transport properties of Pr0.7Sr0.3MnO3 thin films
Authors: Wolfman J. , Prellier W. , Simon Ch. , Mercey B.
Published: July 2002

In this communication, the variation of the resistance and of the magnetization with respect to the angle between the applied magnetic field and the substrate normal is discussed for a Pr0.7Sr0.3MnO3 thin film at 30 K. An angular hysteresis is observed for these properties and this behavior is correlated with the magnetic domain wall motion. It is shown that the number of domain walls is controlled through the rotation of the applied magnetic field, and that these domain walls play an important part in the colossal magnetoresistance behavior of these films. Furthermore, a model describing the reversal of the magnetization with the rotation of the magnetic field has been developed. This model enables one to extract the dependence of the resistance upon the direction of the magnetization with respect to the crystalline axis.
Microwave and magnetooptic measurements of nonlinear dispersive magnetostatic waves in a yttrium-iron-garnet-gadolinium-gallium-garnet waveguide
Authors: Tsai C. S. , Young D. , Nikitov S. A.
Published: July 2002

Nonlinear propagation characteristics of magnetostatic surface waves (MSSWs) with carrier frequencies ranging from 0.5 to 7.0 GHz and magnetostatic forward volume waves (MSFVWs) with carrier frequencies ranging from 0.5 to 5.0 GHz in yttrium iron garnet-gadolinium gallium garnet waveguides have been measured in a delay line configuration using both microwave and guided-wave magnetooptic (MO) methods. The latter, involving noncollinear guided-wave MO Bragg interaction, was employed for the first time. For the MSSW, nonlinear characteristics including power limiting, discrete sideband modulation, continuum sideband modulation, and group splitting were observed and measured. Among such nonlinear characteristics, continuum modulation spectrum was observed and measured for the first time. For the MSFVW, nonlinear characteristics including parametric instability and soliton formation were also observed and measured. The MO data obtained by scanning the location of the incident light beam right beneath the input transducer and along the MSW propagation path has shown that the nonlinear characteristics observed were related to propagation and not to electromagnetic to MSW conversion effects at the input. Theoretical estimates are in a fair agreement with the experimental data. Finally, compression of an input pulse of 60 rf cycles (20 ns in width) to an output pulse of 6 rf cycles (2 ns in width) using a 3 GHz MSFVW pulse was also demonstrated.
Analysis of optical gain and threshold current density of wurtzite InGaN/GaN/AlGaN quantum well lasers
Authors: Yeo Y. C. , Chong T. C. , Li M. F. , Fan W. J.
Published: July 2002

The valence subband structures, density-of-states, and optical gain of (0001) wurtzite InxGa1−xN/GaN quantum wells (QWs) are studied using a numerical approach. We used the effective-mass parameters of GaN and InN derived using the empirical pseudopotential method. By varying the well width and mole fraction of In in the well material, the effects of quantum confinement and compressive strain are examined. A narrower well width and a higher In mole fraction in the well lead to transverse electric enhancement and transverse magnetic suppression of the optical gain. From the relationship between the optical gain and the radiative current density, we obtain the transparent current density for a single QW to be 200 A/cm2. The InGaN/GaN/AlGaN separate confinement heterostructure multiple QW (MQW) laser structure is then analyzed. It is shown that a suitable combination of well width and number of QWs should be selected in optimizing the threshold current density in such MQW lasers.
Alternative (Pd,Ti,Au) contacts to (Pt,Ti,Au) contacts for In0.53Ga0.47As
Authors: Chor E. F. , Chong W. K. , Heng C. H.
Published: July 2002

(Pd,Ti,Au) contacts, Ti/Pd/Au and Pd/Ti/Pd/Au, to In0.53Ga0.47As have been investigated for applications in InP-based devices. A thin interfacial Pd layer, with an optimum thickness of ∼100 Å, was found to be essential in contact to p+-In0.53Ga0.47As but undesirable to n+-In0.53Ga0.47As. Pd (100 Å)/Ti (200 Å)/Pd (200 Å)/Au (2000 Å) and Ti (200 Å)/Pd (200 Å)/Au (2000 Å) yielded, respectively, a minimum specific contact resistance (ρc) of 1.68×10−6 Ω cm2 and 2.54×10−7 Ω cm2 to p+- and n+-In0.53Ga0.47As(p+=n+=1×1019 cm−3). (Pd,Ti,Au) contacts have shown to outperform (Ti,Pt,Au) counterparts in ρc. In addition, it has been seen that contact anneal beyond 400 °C should be avoided for thin base InP-based heterojunction bipolar transistors.
Interface effects on the magnetization reversal of Tb/Fe multilayers
Authors: Kim W. S. , Kleemann W. , Tappert J. , Keune W.
Published: July 2002

The magnetization reversal of multilayers {Tb/Fe/Ag}10(S1), {Fe/Tb/Ag}10(S2), and {Fe/Tb}10(S3) is measured at room and low temperatures using polar magneto-optical Kerr (MOKE) spectroscopy and SQUID magnetometry. The Ag blocking layers in S1 and S2 strongly suppress the perpendicular magnetic anisotropy (PMA) induced by the Fe/Tb interfaces, whereby the Fe-on-Tb interfaces give rise to much stronger PMA than the Tb-on-Fe ones. MOKE ellipticity in the near-infrared reveals an uncoupled soft-magnetic contribution in all samples, S1–S3. It is attributed to amorphous iron segregated within the bulk of the α-Fe layers and contributing to about 10% of their volume.
Electromigration study of Al/low dielectric constant polymer and Al/SiO2 dioxide line structures
Authors: Wang Pei-hua , Ho Paul S. , Pellerin John , Fox Robert
Published: July 2002

Electromigration reliability is a key concern for implementation of low dielectric constant (low κ) materials for on-chip interconnects. To address this problem, we have carried out a comparative study of electromigration performance for Al(0.5 wt% Cu) line structures formed with tetraethylorthosilicate (TEOS)-based SiO2 and a low κ polymeric dielectric, poly(arylene ether) (PAE). The test structures consisted of 800 μm long single-level line structures with adjacent extrusion monitor lines. Resistometric measurements were performed to measure electromigration lifetime. To supplement lifetime measurements, x-ray diffraction and transmission electron microscopy were used to measure overall grain size and grain orientation distributions, and electron backscattering diffraction was applied to determine local grain orientation and grain boundary characteristics. In addition, scanning electron microscopy was used to examine void morphology. Microstructural analysis has revealed a larger grain size and enhanced 〈111〉 texture in the PAE passivated Al lines compared to TEOS SiO2 passivated lines. The difference in microstructure is attributed to a high temperature process used to cure the PAE which is absent from the processing of the TEOS SiO2 samples. The improved electromigration performance can be attributed to the larger grain size and decreased grain misorientation in the PAE passivated lines.
Role of xenon additive in microwave plasma-assisted (H2+CH4) chemical vapor deposition of diamond thin film
Authors: Hosomi Takeshi , Maki Tetsuro , Kobayashi Takeshi , Yoshizako Yuji , Taniguchi Michio , Sugiyo Masato
Published: July 2002

We have examined in detail the role of the Xe additive in microwave plasma-assisted {CH4+H2} chemical vapor deposition (CVD) of diamond film. Effects of Xe addition were evident in the increased growth rate (about 50% increase for 1% Xe), without degradation of the crystallinity, and in the morphological change from the cubic to platelet grain structures. Based on the results of measuring the plasma emission spectra, Raman shift, and microwave plasma impedance, the favorable effects of Xe addition were attributed to its low dissociation and excitation energies (8.28 eV for Xe radicals), which are sufficient for the formation of CH3 but not CH2 radicals. Namely, the addition of Xe caused the CVD plasma to have higher density and lower temperature. This resulted in a large amount of atomic hydrogen and CH3 (precursors for diamond crystallization) and also a smaller amount of CH2, leading to the growth of a good-quality diamond film with a high deposition rate.
Atomic structure and magnetic properties of MnFe2O4 nanoparticles produced by reverse micelle synthesis
Authors: Carpenter Everett E. , O’Connor Charles J. , Harris Vincent G.
Published: July 2002

Using the aqueous cores of reverse micelles as nanoreactors, nanoparticles (d∼10 nm) of the mixed ferrite MnFe2O4 were produced. Seven processing trials were performed where the concentration of ammonium hydroxide, reaction temperature, and the oxidizing agent were varied. All trials result in Mn-ferrite particles with varying chemistry and structure. The Mn concentration in the resulting ferrite is strongly enhanced by both the presence of H2O2 as an oxidizing agent and a surplus of ammonium hydroxide. The increased Mn concentration correlates with a higher fraction of octahedrally coordinated Mn cations. When near-stoic amounts of ammonium hydroxide are used, the resulting ferrites are nearly stoichiometric with a more equitable distribution of Mn cations on the octahedral and tetrahedral sublattices. In all ferrite nanoparticles, the Mn cations have a preference for octahedral site occupancy that is larger than the 20% measured in bulk Mn-ferrite. We attribute the cation filling trends to the stabilization of excess trivalent Mn during processing.
Magnetism of nanophase metal and metal alloy particles formed in ordered phases
Authors: Carpenter Everett E. , Seip Candace T. , O’Connor Charles J.
Published: July 2002

In general, the intrinsic magnetic properties of a single metallic elemental can be increased by forming alloys containing one or two additional metals. In this article, metallic cobalt, cobalt/platinum alloys, and gold-coated cobalt/platinum nanoparticles have been synthesized in reverse micelles of cetyltrimethlyammonium bromide. Magnetic characterization of all samples demonstrate that the particles containing platinum and gold exhibit a higher blocking temperature and larger coercivities relative to pure cobalt nanoparticles of the same size. The dc susceptibility of a sample of 15 nm cobalt nanoparticles exhibit a blocking temperature of 70 K and coercivity, Hc, of 1800 G at 2 K. When equimolar quantities of cobalt and platinum were combined and reduced in the reverse micelle, the blocking temperature increased to 130 K and Hc at 2 K is reported as 2700 G. When additional platinum is added, however, the blocking temperature dropped to 100 K and coercivity at 2 K decreased to 2000 G. Addition of a gold coating to the equimolar CoPt nanoparticles further reduced the blocking temperature to 30 K and coercivity of the sample at 2 K decreased to 1000 G. Above the blocking temperature, all samples exhibit superparamagnetic behavior.
Use of Rietveld refinement to fit a hexagonal crystal structure in the presence of elastic and plastic anisotropy
Authors: Daymond M. R. , Bourke M. A. M. , Von Dreele R. B.
Published: July 2002

When multiple elastic diffraction peaks are obtained from an x-ray or neutron source, data analysis is commonly performed using a Rietveld refinement applied to the entire pattern, rather than simply performing single peak fits. In the simplest case the crystal structure is assumed to be ideal despite the presence of stresses which, coupled with the elastic and plastic anisotropy of individual grains, can result in a nonisotropic response of the polycrystal. A first step to account for this anisotropy in the refinement is to include an anisotropic strain parameter. In an earlier work [J. Appl. Phys. 82, 1554 (1997)] we included elastic anisotropy into a Rietveld refinement and discussed its validity in the elastic and plastic regimes for a cubic crystal structure. Here we extend the discussion to include anisotropy in hexagonal crystal structures. The agreement between single peak fits and the Rietveld refinement modeled single peak positions is considered for hexagonal close packed beryllium in the presence of an applied compressive load, in both the elastic and plastic regime (to ∼1% plastic strain). Agreement is found to be good in the elastic and early plastic regime, where only basal slip is assumed to be active. At higher loads, where prism slip and/or basal fracture are also assumed to be active, agreement is poorer for some diffraction planes. The implications for residual stress measurements are considered.
Dopant induced ablation of poly(methyl methacrylate) at 308 nm
Authors: Lippert T. , Webb R. L. , Langford S. C. , Dickinson J. T.
Published: July 2002

Poly(methyl methacrylate) (PMMA) is highly resistant to laser ablation at 308 nm. Either very high fluences or absorbing dopants must be used to ablate PMMA efficiently at this wavelength. We investigate two dopants, pyrene and a common solvent, chlorobenzene, using time-of-flight mass spectroscopy. Both compounds improve the ablation characteristics of PMMA. For both dopants, the first step in ablation is an incubation process, in which absorption at 308 nm increases due to the production of C=C bonds along the polymer backbone. Incubation at 308 nm is similar to that observed for shorter ultraviolet wavelengths in previous studies. The principal ablation products and their corresponding temperatures are consistent with a photothermal ablation mechanism.
Thickness mode material constants of a supported piezoelectric film
Authors: Lukacs Marc , Olding Tim , Sayer Michael , Tasker Ron , Sherrit Stewart
Published: July 2002

New thick film ceramic processing techniques and microelectromechanical systems require material characterization of a piezoelectric film supported by a substrate. An analytical solution of the one-dimensional wave equation for multiple layered systems driven in the thickness mode is presented. The impedance across the piezoelectric layer is derived and expressed in terms of the material properties of the two materials. This includes the open-circuit elastic stiffness c33D, the clamped permittivity ε33S and the h33 piezoelectric constant of the piezoelectric layer and the elastic stiffness csD of the substrate. The properties are expressed as complex variables in order to account for the losses within the materials. The material parameters of the solution are extracted from experimental results using a modified Levenberg–Marquardt technique. The capabilities of this nondestructive technique are demonstrated using experimental and simulated impedance spectra of lead zirconium titanate sol gel composite coatings within the thickness range of 15–70 μm on aluminum and on platinum electroded silicon substrates and by simulating the mass loading of a quartz thickness resonator. The analytical solution allows for the extension of the Institute of Electrical and Electronics Engineers free resonator impedance technique to supported films and provides a method for determining the material parameters of a piezoelectric coating.
Transition of carrier distribution from a strained to relaxed state in InGaAs/GaAs quantum well
Authors: Wang P. Y. , Chen J. F. , Wang J. S. , Chen N. C. , Chen Y. S.
Published: July 2002

This work investigates the transition of carrier distribution from the strained to the relaxed state in In0.2Ga0.8As/GaAs quantum well by measuring capacitance voltage and analyzing x-ray diffraction. According to those results, there is carrier confinement in the In0.2Ga0.8As quantum well with well thickness less than the critical thickness. Increasing the well thickness beyond the critical thickness leads to a significant carrier depletion around the quantum well. Double-crystal x-ray rocking curves reveal that when InGaAs well thickness increases beyond the critical thickness, the interference pattern disappears and relaxation begins to occur from near the bottom InGaAs/GaAs interface while the top interface still remains strained. Results obtained from the critical thickness determined from x-ray diffraction correspond to the transition of carrier distribution, illustrating that the capacitance–voltage measurement is a rather effective means of determining the critical thickness.
Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures
Authors: Ambacher O. , Smart J. , Shealy J. R. , Weimann N. G. , Chu K. , Murphy M. , Schaff W. J. , Eastman L. F. , Dimitrov R. , Wittmer L. , Stutzmann M. , Rieger W. , Hilsenbeck J.
Published: July 2002

Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face AlxGa1−xN/GaN/AlxGa1−xN and N-face GaN/AlxGa1−xN/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15<x<0.5) and thickness between 20 and 65 nm demonstrate the important role of spontaneous and piezoelectric polarization on the carrier confinement at GaN/AlGaN and AlGaN/GaN interfaces. Characterization of the electrical properties of nominally undoped transistor structures reveals the presence of high sheet carrier concentrations, increasing from 6×1012 to 2×1013 cm−2 in the GaN channel with increasing Al-concentration from x=0.15 to 0.31. The observed high sheet carrier concentrations and strong confinement at specific interfaces of the N- and Ga-face pseudomorphic grown heterostructures can be explained as a consequence of interface charges induced by piezoelectric and spontaneous polarization effects.
The roles of N and P type Si(100) substrates in the nucleation and growth of textured diamond films by hot filament chemical vapor deposition
Authors: Chen J. S. , Yeh W. Y. , Lin W. T. , Liu W. Y. , Lu M. F. , Hwang J. , Chang H.
Published: July 2002

P- Si (100) and n-Si(100) substrates had quite different responses to the same process parameters used in the modified four-step diamond growth method, i.e., pretreatment, heating, bias enhanced nucleation (BEN) and bias texture growth (BTG), which has been developed to grow textured diamond films by hot filament chemical vapor deposition. At the pretreatment step, a bright blue plasma discharge induced the formation of damaged voids randomly distributed on the surfaces of p-Si(100) and n-Si(100). The damaged voids on p-Si(100) are several microns in size and 3 μm in depth. In contrast, the size and depth of the damaged voids on n-Si(100) are in nanometer scale, approximately two orders of magnitude lower than those on p-Si(100). At the BEN step, carburization occurred along with the possibility of diamond nucleation. Unfacet nuclei of micron scale distributed around the edge of damaged voids all over the p-Si(100) substrate. In contrast, a great number of small nuclei of nanometer scale spread and covered all the damaged voids around the outer edge of the n-Si(100) substrate. The continuous textured diamond film grown on p-Si(100) had better diamond quality than that on n-Si(100) at the BTG step. The textured diamond film on p-Si(100) was flat, however, that on n-Si(100) was under stress in convex shape. Ion bombardment at the BTG step resulted in the enhancement of the growth of textured diamond and in the degradation of diamond quality through the formation of amorphous carbon. P-Si(100) is considered better than n-Si(100) to be the substrate for textured diamond deposition.
Ultrafast all-optical modulation of interband-light pulses by ultrashort intersubband light pulses in semiconductor quantum wells
Authors: Neogi A. , Yoshida H. , Mozume T. , Wada O.
Published: July 2002

We analyze the optical modulation of an interband resonant light pulse by intersubband resonant light pulses in a three-level quantum well system. The origin of the third order self-saturation optical nonlinearity resulting in the modification of the interband light absorption is assumed to lie in the carrier density modulation induced by the interband and intersubband transitions. The optical nonlinearity can be optimized to achieve the maximum modulation by applying control light pulses shorter than the intersubband energy relaxation time but longer than the intersubband phase relaxation time. We have also experimentally investigated the material system suitable for optical communication regime and have observed that the In0.53Ga0.47As/AlAs0.56Sb0.44 quantum well lattice matched to InP yielding an intersubband transition as short as 1.4 μm is the most suitable system for the proposed modulation scheme.
Removal of end-of-range defects in Ge+-pre-amorphized Si by carbon ion implantation
Authors: Chen Peng-Shiu , Hsieh T. E. , Chu Chih-Hsun
Published: July 2002

Carbon ion implantation was employed to annihilate the end-of-range (EOR) defects in Ge+-pre-amorphized Si. Experimental results showed that the efficiency of EOR defect removal depends on the Ge+-pre-amorphization conditions, the location of projected range (Rp) of carbon implant and subsequent annealing conditions. The best defect removal occurred when Rp of carbon implantation was brought close to the amorphous/crystalline (a/c) interface generated by Ge+-pre-amorphization. The higher the annealing temperature, the better the interstitial gettering efficiency of carbon atoms was observed. However, transmission electron microscopy investigation revealed the emergence of hairpin dislocations when dose and accelerating voltage of Ge+ implantation were high. In specimens without carbon implantation, the hairpin dislocations could be readily removed by a 900 °C, 30 min anneal. For carbon-implanted specimens, the density of hairpin dislocations increased when Rp of carbon implantation was close to the (a/c) interface. The glide motion of hairpin dislocations was affected by Ge+-pre-amorphization conditions and was inhibited by the SiC complexes formed in the vicinity of dislocations so that they became rather difficult to anneal out of the specimens.
Temperature effects on the switching of small ultrathin magnetic structures
Authors: Chui S. T.
Published: July 2002

Mechanisms for the switching of the magnetization in ultrathin small structure are summarized. The temperature dependence of the switching field for different sizes and anisotropies is studied and found to be significant. Much of the current interpretations of finite temperature experimental results on giant magnetoresistance and tunnel structures in terms of zero temperature micromagnetics calculations have not considered this problem.
Effect of stress anisotropy on hysteresis and Barkhausen noise in amorphous materials
Authors: Appino C. , Durin G. , Basso V. , Beatrice C. , Pasquale M. , Bertotti G.
Published: July 2002

Hysteresis, power losses, and the Barkhausen effect are investigated in an Fe-based highly magnetostrictive amorphous material, as a function of applied stress. By means of the static and dynamic Preisach model, and of existing theories of the Barkhausen effect, the results are shown to be compatible with the existence of a characteristic structural length δc, playing a role similar to that of grain size in crystalline materials. At low applied stresses, where the magnetization process is dominated by quenched-in stresses σi, δc is identified with the typical wavelength of σi fluctuations. The theoretical analysis leads to the estimate δc∼70–100 μm and 〈σi〉∼3.5 MPa.
Solutions to micromagnetic standard problem No. 2 using square grids
Authors: Lopez-Diaz L. , Alejos O. , Torres L. , Iniguez J. I.
Published: July 2002

Solutions to micromagnetic standard problem No. 2 are presented in this article and the main features of the computational techniques used are discussed. Values of coercivity and remanence have been obtained in the range 0.8<d/lex<20.0. A continuous transition from uniform to nonuniform rotation is observed leading to a steady decrease in the coercivity as the ratio between the scaled geometry and the exchange length is increased. A sharp transition in the remanent states is found at d/lex=9.0. The nonuniform reversal mechanism is discussed and is found to consist on transitions between in-plane S-shaped patterns. The results are interpreted qualitatively.
In situ investigation of the magnetization processes in amorphous glass-covered wires by ferromagnetic resonance measurements
Authors: Chiriac H. , Colesniuc C. N. , Óvári T.-A. , Ticuşan M.
Published: July 2002

The magnetization processes in amorphous glass-covered wires with positive (Fe based), negative (Co based), and nearly zero magnetostriction (Co based with a small amount of Fe addition) are analyzed by ferromagnetic resonance (FMR) measurements. A second peak in the low field range of the derivative resonance spectra has been observed for all samples. A clear relationship between the shape of this peak and the type of magnetization process that occurs in these wires was found. The obtained results show that FMR measurements offer new possibilities of investigating the magnetization processes in amorphous glass-covered wires.
Simulation of the interband s–d and intraband s–s electron–phonon contributions to the temperature dependence of the electrical resistivity in Fe/Cr multilayers
Authors: Almeida B. G. , Amaral V. S. , Sousa J. B. , Colino R. , Schuller I. K. , Moschalkov V. V. , Bruynseraede Y.
Published: July 2002

High-resolution electrical resistivity (ρ,dρ/dT) measurements were performed in three series of [Fe30 ÅCrt Å] multilayers in the temperature range 15–300 K, with an applied magnetic saturation field (7.5 kOe). The samples were deposited by molecular beam epitaxy on MgO substrates and by sputtering on MgO and Si substrates. For T<50 K the ideal resistivity follows ρi=β T3, indicating the dominance of phonon-assisted interband s–d scattering in this temperature range. For T>150 K the resistivity attains the classical regime with ρ∝T. To simulate the observed ρi(T) we have used a model that takes into account intraband s–s and interband s–d electron–phonon scattering, written as ρsd=A×f1(T) and ρss=B×f2(T), where f1 and f2 are functions only of the temperature, A and B are sample-dependent constants and ρi=ρsd+ρss. The model predicts that ρi∝T3 at low temperatures and ρi∝T at high temperatures as observed in our multilayers. The experimental curves of ρi and dρ/dT are well reproduced in the whole temperature range (15–300 K) and from the fits to these curves A and B are determined for each sample. By plotting B vs A we find that each point from all the multilayers falls in a straight line indicating that B is proportional to A. The simulated resistivity thus predicts that ρi=βf(T) where f(T)=α1×f1(T)+α2×f2(T) is a function only of the temperature, as observed experimentally.
Grain size-dependent electrical properties of nanocrystalline ZnO
Authors: Nan Ce-Wen , Tschöpe A. , Holten S. , Kliem H. , Birringer R.
Published: July 2002

Undoped and Bi-doped nanocrystalline ZnO was prepared by the inert gas condensation method. Samples of different grain sizes were obtained by annealing treatments at various temperatures. The dc and ac electrical conductivity of undoped and Bi-doped nanocrystalline ZnO was investigated as a function of grain size. In dc measurements, nanocrystalline ZnO of small grain size (<20 nm) exhibited time dependent currents similar to those found in other dielectric solids and which are characteristic for space charge limited conduction. However, unusually long transient times of up to 300 s were observed. Similar samples of larger grain sizes (>40 nm) did not exhibit time dependent conduction but exhibited ohmic behavior. This difference in dc conductivity is discussed in terms of the grain size dependence of the average trap density. A small amount of Bi dopant had no measurable effect on the electrical behavior in the small grain size range but resulted in higher specimen resistance in the large grain size range. ac electrical properties were characterized by impedance spectroscopy. The impedance spectra of undoped and Bi-doped samples of small grain sizes were rather similar and exhibited two semicircles. In contrast, the impedance spectra of samples with larger grain sizes were single, depressed circles with much higher impedances for the Bi-doped sample, in agreement with the dc measurements. A consistent assignment of the contributions due to bulk conductivity, porosity, and grain boundary effects to the overall impedance is presented.
Arrhenius analysis optical transitions in strained InGaAsP quantum wells
Authors: Lúcio A. D. , Cury L. A. , Matinaga F. M. , Sampaio J. F. , Bernussi A. A. , de Carvalho W.
Published: July 2002

Strained InGaAsP/InP single quantum wells grown by low pressure metalorganic vapor phase epitaxy are studied by photoluminescence. We demonstrate that the analysis of the Arrhenius plot, specially modified to fit the temperature dependence of the integrated photoluminescence intensity, can be used as a complementary technique in order to identify different optical transitions that take place in more complex photoluminescence spectra.
Long-term conduction behavior of white-light emitting ZnS-based phosphor films
Authors: Lee Yun-Hi , Kim Young-Sik , Ju Byeong-Kwon , Sung Man-Young , Oh Myung-Hwan
Published: July 2002

To investigate systematically the causes of the aging of thin film electroluminescent devices, time-dependent current–voltage characteristics of doubly doped ZnS:Pr, Ce, Mn layer has been investigated under a direct current high field as one of the accelerated aging methods. The surface roughness along to the direction perpendicular to the indium-tin-oxide–glass substrate and the high peak-to-valley roughness are assumed to be the main sources for current fluctuations during the measurement. It was observed that the leakage current level after long-term stressing increased or decreased depending on post-treatment indicating that the bulk-controlled conduction was the dominant mechanism determining long-term behavior and this mechanism is not sensitive to the variations in the deposition parameters. The experimental results indicate the fact that the long-term conduction behavior of ZnS-based film may be related to defect redistribution after lowering barrier height during initial stressing. Finally, we suggest that an enhancement of contact adhesions via surface smoothing of the upper interface can contribute to the long-term stability.
The influence of mass transport processes on the response to gas mixtures of field-effect devices with large-area catalytic metal gates
Authors: Johansson M. , Ekedahl L.-G. , Lundström I.
Published: July 2002

A model of how response maps of gas mixtures obtained from large-area field-effect devices with thin catalytic metal gates are affected by the mass transport processes in the gas above the gate has been derived. The model is based on a numerical calculation of the local analyte concentration in the vicinity of the gate of the device. It is found that the catalytic activity of the gate metal may give rise to substantial lateral variations in the analyte concentration at the gate. The varying analyte concentration offers an explanation to the shape of the response maps obtained in earlier experiments. The model is utilized to predict how the analyte mass fraction at the gate, and thus the response maps, should be affected by the gas velocity, the catalytic surface area, and the local consumption rate of analyte at the gate. It is also shown that the geometry of the measurement cell that surrounds the sensing device during measurements may affect the response pictures. The use of the model in connection with distributed chemical sensing and determination of reaction rates is briefly discussed.
Thermally stimulated polarization and depolarization phenomena in PbWO4 single crystals
Authors: Suszyńska M. , Macalik B. , Nikl M.
Published: July 2002

Thermally stimulated polarization and depolarization current (TSPC and TSDC, respectively) measurements were exploited to characterize the dielectric relaxation of PbWO4 single crystals of different origin, i.e., grown by different methods and containing different amount of impurities. In addition to the dc conduction current, measured by the TSPC runs, at least one TSDC band was detected for the investigated specimens. Activation parameters characterizing the polarization and conduction processes were evaluated, and the possible nature of the related defects has been discussed. The results obtained for nominally pure crystals show the presence of orientation polarization related to some permanent dipoles. In the less pure samples, in addition to the high-temperature interfacial polarization, a low-temperature band was tentatively ascribed to a space charge polarization related to the impurities.
Local crystallography and stress voiding in Al–Si–Cu versus copper interconnects
Authors: Keller R. R. , Kalnas C. E. , Phelps J. M.
Published: July 2002

We compare the local crystallographic orientations associated with stress voids in Al–1Si–0.5Cu (wt %) with those in pure copper interconnects. Orientations were sorted by whether grains were immediately adjacent to voids. Grains adjacent to voids in Al–Si–Cu showed a 〈111〉 fiber texture that was slightly stronger than those in intact regions. This is in contrast to copper, which showed weaker local 〈111〉 texture around voids. We postulate the difference to be due to the relative effectiveness of the diffusion paths available in the lines. For Al–Si–Cu, the presence of defects associated with precipitates may allow more rapid diffusion than grain boundaries. Voiding in copper, which is free from such defects, depends more on grain boundary structure.
Particle-in-cell and Monte Carlo simulation of the hydrogen plasma immersion ion implantation process
Authors: Kwok Dixon Tat-Kun , Chu Paul K. , Wood Blake P. , Chan Chung
Published: July 2002

Hydrogen plasma immersion ion implantation into a 200-mm-diam silicon wafer placed on top of a cylindrical stage has been numerically simulated by the particle-in-cell (PIC) and transport-and-mixing-from-ion-irradiation (TAMIX) methods. The PIC simulation is conducted based on the plasma comprising three hydrogen species H+, H2+, and H3+ in a ratio determined by secondary ion mass spectrometry. The local sputtering losses and retained doses are calculated by the Monte Carlo code TAMIX. The combined effect of the three species results in a maximum retained dose variation of 11.6% along the radial direction of the wafer, although the implanted dose variation derived by PIC is higher at 21.5%. Our results suggest that the retained dose variations due to off-normal incident ions can partially compensate for variations in incident dose dictated by plasma sheath conditions. The depth profile becomes shallower toward the edge of the wafer. Our results indicate that it is about 34% shallower at the edge, but within a radius of 6.375 cm, the depth of the peak only varies by about 5%. For plasma implantation process design, a combination of PIC and TAMIX is better than the traditional practice of using PIC alone.
Reflectivity investigations as a method for characterizing group III nitride films
Authors: Shokhovets S. , Goldhahn R. , Gobsch G. , Cheng T. S. , Foxon C. T. , Kipshidze G. D. , Richter Wo.
Published: July 2002

A highly sensitive method for characterizing heteroepitaxial group III nitride films on various substrates is developed on the basis of reflectivity studies below and just above the fundamental absorption edge. By analyzing the envelopes of the reflectivity spectrum, the influence of a buffer layer, and/or a nonabrupt substrate/film interface is verified, the refractive index of the active layer can be calculated independently of both the active layer thickness and the interface properties, and finally, qualitative conclusions about the interface properties are deduced. On this basis a suitable model for data fitting is established which contains only a small number of adjusted parameters. Moreover, the correlation between the refractive index and the thickness of the active layer is removed. From the fit, the energy dispersion of the refractive index and the thickness for both the active layer and the interlayers are obtained. The ability and high accuracy of the method is demonstrated by applying it to the investigation of hexagonal GaN films grown on GaAs and 6H–SiC substrates.