bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of the very low-frequency characteristics of dielectric materials: A surface potential approach
Authors: Pouillès V. , Lebey T. , Castelan P.
Published: July 2002

A new approach using surface potential measurements for low-frequency characterization of dielectric materials is proposed. In fact, isothermal surface potential decay measurements are often used to characterize both surface and bulk properties of dielectric materials. A lot of work has been devoted to correlate these potential decays to physical mechanisms, when superficial conductivities can be neglected. In most cases, charge injection has been considered; however, following either sample thicknesses or low applied electric field, these decays may be related to the absorption current of the material. A close correlation relating the derivative of the surface potential to the absorption current is established. The frequency characterization is then achieved using Fourier transformation. In order to validate it, this method is applied to different kinds of material such as coatings or epoxy based composites. It allows one to determine, for composite materials, the existence of very low-frequency relaxation associated with interfacial phenomena. A final validation is achieved using the Kamers–Kronig test. This new approach provides the study of the reinforcement morphology or volume fraction on the permittivity spectra. It may also be used in different physical fields. An example concerning corrosion detection is given but this technique may also be applied in the study of electrical aging.
Low-temperature photoluminescence of epitaxial InAs
Authors: Lacroix Y. , Tran C. A. , Watkins S. P. , Thewalt M. L. W.
Published: July 2002

Photoluminescence studies as well as reflectance and transmittance measurements were performed on high-purity epitaxial InAs grown by metal-organic chemical-vapor deposition. We report the optical identification of excitonic, donor, and acceptor impurity related transitions at a temperature of 1.4 K. Measurements at higher temperature and in the presence of magnetic fields up to 7 T support these identifications. We find the excitonic band gap at 415.65±0.01 meV according to the minimum in the polariton reflectance feature. The donor–acceptor-pair and acceptor-bound exciton transitions for three different acceptors are observed by photoluminescence, and we tentatively associate one of them to a double acceptor formed by a Ga impurity on an As lattice site. A donor-bound exciton transition is observed with a binding energy of 0.42 meV. The magnetic field dependence yields values of the electron effective mass and g factor of (0.026±0.002)m0 and −15.3±0.2, respectively, in good agreement with values obtained by other techniques. Furthermore, we report a deep luminescence band of unknown origin at ∼375 meV, related to drastic temporal changes in the band-edge photoluminescence intensity.
A comparative study of Ge nanocrystals in SixGeyOz alloys and SiOx/GeOy multilayers
Authors: Zacharias M. , Weigand R. , Dietrich B. , Stolze F. , Bläsing J. , Veit P. , Drüsedau T. , Christen J.
Published: July 2002

Amorphous SixGeyOz alloy films of different chemical composition and SiOx/GeOy multilayers are prepared by dc magnetron sputtering. The formation of Ge nanocrystals in the alloy matrix is induced by annealing at temperatures ranging from 700 to 900 °C. In contrast, the crystallization in the Ge sublayers in the multilayer structures occurs at 500 °C. Using high resolution transmission electron microscopy, we can directly image the Ge nanoclusters in the as-prepared films as well as the Ge nanocrystals after crystallization in the annealed alloy and multilayer samples. Raman spectra of the as-prepared and annealed films demonstrate the strong similarity between the alloy and the multilayer films. In both cases, the crystallization of the Ge phase is observed by the narrowing and disappearing of the amorphous mode around 270 cm−1 and the appearance and increase of the mode below 300 cm−1, typical for nanocrystallized Ge. The Ge cluster formation is stimulated by a high oxygen content of the alloy films. If the Si atoms are not saturated with oxygen, Si–Ge alloy regions are formed, resulting in an increase of the crystallization temperature of the films. A weak broad Raman mode at 390 cm−1 is found as a signature of these regions. Films containing small Ge nanocrystals show visible room temperature photoluminescence around 3.03 and 2.34 eV. Quantum confinement or the presence of luminescence centers (Si20, Ge20) at the surface of the Ge nanocrystals are discussed as possible origins of the bright blue luminescence. However, a peak shift to higher energies with decreasing crystal size is not detectable, contrary to the predictions of the quantum confinement model.
Vacancy production by 3 MeV electron irradiation in 6H-SiC studied by positron lifetime spectroscopy
Authors: Kawasuso Atsuo , Itoh Hisayoshi , Ohshima Takeshi , Abe Koji , Okada Sohei
Published: July 2002

The vacancy production in 6H-SiC by 3 MeV electron irradiation at room temperature was studied using positron lifetime spectroscopy combined with annealing experiments. It was found that the trapping rates of positrons in vacancies increased linearly with the fluence in the initial stage of irradiation. After the linear increase, the trapping rates were found to be proportional to the square root of the fluence. The linear and nonlinear fluence dependences of the trapping rates are explained by the reduction of vacancies due to recombination with interstitials during irradiation. The positron trapping rate for the admixture of silicon vacancies and divacancies showed a tendency to saturate in the higher fluence range. The trapping rate for carbon vacancies decreased after reaching a maximum. These results are explained in terms of the shift of the Fermi level due to the irradiation process. It was found that, for the lightly irradiated specimen, an annealing stage caused by recombination between close vacancies and interstitials was observed. However, such an annealing stage was not observed when using a heavily irradiated specimen. These different results are explained as the reduction of interstitials due to the recombination with vacancies and long-range migration of interstitials to sinks during irradiation.
Photoluminescence from indented MgO crystals using a near ultraviolet/visible Raman microscope
Authors: Chaudhri M. Munawar , Sands Howard S.
Published: July 2002

Using a Renishaw ultraviolet/visible Raman microscope and excitation radiation of 325 nm from a HeCd laser, photoluminescence investigations have been made in the wavelength region 325–726 nm of indentended MgO crystals of 99.9% and 99.99% purity. It was found that the plastic deformation caused considerably enhanced photoluminescence from within the indentation as well as from the plastically deformed region around it. In both types of crystal, the photoluminescence consisted of a strong band at 375 nm and a much weaker and broader band at 525 nm. The strongest luminescence was obtained from within the indentation, clearly indicating the absence of radiative quenching. On annealing the indents at 1000 °C in vacuum for 2 h, the relative intensity of the 375 nm band decreased, whereas the relative intensity of the 525 nm band increased. It has been suggested that the 375 and the 525 nm bands are probably due to the F+ and F centers, respectively, and that the relative number density of the F+ centers increases with increasing plastic strain. The effect of the annealing is to convert F+ to F centers, and to cause an overall decrease in the total number density of color centers.
High resolution x-ray diffraction and scattering measurement of the interfacial structure of ZnTe/GaSb epilayers
Authors: Li C. R. , Tanner B. K. , Ashenford D. E. , Hogg J. H. C. , Lunn B.
Published: July 2002

The surface and interface structures of ZnTe epilayers grown by molecular beam epitaxy on GaSb (001) substrates under different conditions have been investigated by high resolution x-ray diffraction and grazing incidence scattering. Reciprocal space mapping around the symmetrical diffraction reciprocal point 004 and asymmetrical diffraction point 1̄1̄5 showed that the ZnTe epilayers, in the samples investigated, were fully strained to the substrate. The crystalline quality of the ZnTe epilayer grown on a substrate annealed in a Zn flux was very good, while evidence for an interfacial layer, of thickness varying from 2–20 nm, was found when the substrate was annealed in a Te flux prior to growth. This is attributed to Ga2Te3 formation at the interface. The interfacial layer roughens the interface and surface, and both crystal truncation rod measurements and grazing incidence x-ray reflectivity show the surface roughness to be about 4 nm. Such a rough surface and interface is also inferred from the broader distribution along the transverse direction in reciprocal space maps. A shorter lateral correlation length is found for the roughness of the sample containing the interfacial layer. The disappearance of interference fringes is attributed to nonuniformity of the interfacial layer.
Influence of the substrate on growth and magnetoresistance of La0.7Ca0.3MnOz thin films deposited by magnetron sputtering
Authors: Vlakhov E. S. , Chakalov R. A. , Chakalova R. I. , Nenkov K. A. , Dörr K. , Handstein A. , Müller K.-H.
Published: July 2002

Off-axis radio frequency magnetron sputtering was employed to grow La0.7Ca0.3MnOz (LCMO) thin films onto three different types of substrates. The substrate strongly influences the structure and the colossal magnetoresistance effect of the obtained films. Single-crystalline thin films were prepared on LaAlO3 (100) substrates, showing a low value of residual resistivity and a metal–insulator transition at a temperature of up to Tpeak=290 K. The latter value of the transition temperature is one of the highest reported so far on thin films of the La–Ca–Mn–O system. Films deposited onto Y-stabilized ZrO2 substrates and onto MgO substrates are polycrystalline and less textured. These films are characterized by a large negative magnetoresistance ratio MR=[R(H)−R(0)]/R(0) measured for small values of the magnetic field H. For H=1.5 kOe, the MR was found to be approximately −30%, −20%, and −8% at temperatures of 20, 77, and 180 K, respectively. The magnetoresistance of polycrystalline LCMO films shows two contributions, a low-field MR and a high-field MR, which are different in their dependence on temperature. The low-field MR is related to the magnetization process and can be attributed to scattering processes in domain walls or to tunneling of polarized charge carriers between the grains of the polycrystalline films.
Strain relaxation and exciton localization effects on the Stokes shift in InAsxP1−x/InP multiple quantum wells
Authors: Aı̈t-Ouali A. , Yip R. Y.-F. , Brebner J. L. , Masut R. A.
Published: July 2002

Photoluminescence (PL) and optical absorption studies have been performed on strained-layer InAsxP1−x/InP (001) (x<0.27) multiple quantum wells grown by low pressure metal-organic vapor phase epitaxy. The series contains samples with both coherently strained and partially relaxed multilayers, where the relaxation is characterized by misfit dislocations. The PL transition line shape at low temperature and at low excitation intensity as well as the evolution of its peak energy with temperature are characteristic of the recombination of band tail localized excitons induced by potential fluctuations. The redshift of the PL peaks relative to the absorption peaks is attributed to two factors: band tail localization and thermalization. The low temperature PL spectra were fitted with an analytical model for the emission line shape, proposed by Ouadjaout and Marfaing [Phys. Rev. B 46, 7908 (1992)]. This allowed us to quantify the PL peak redshift due to band tail localization. After accounting for this effect, the residual energy difference, which we define as the Stokes shift, shows a very strong correlation with the degree of structural relaxation in the multilayers measured by high resolution x-ray diffraction and transmission electron microscopy. This allows the separation of the strain release contribution from the thermalization processes responsible for the Stokes shift.
The roles of injection and mobility in organic light emitting diodes
Authors: Malliaras G. G. , Scott J. C.
Published: July 2002

Numerical methods have been used to solve the bipolar current problem for a single emitting layer between electrodes with explicit injection characteristics. We consider ohmic and tunneling contacts at the anode and/or cathode for various ratios of hole to electron mobility. Diffusion is included and found to have minimal effect on the recombination efficiency. The recombination profile is dictated mainly by the ratio of mobilities, less so by contacts. Maximum efficiency is obtained for two ohmic contacts. When the injection is imbalanced, higher efficiencies are achieved when the majority carrier has the lower mobility. At sufficiently high voltages, the current tends towards balance, allowing for maximum efficiency.
Magnetic properties of TbCo3B2 studied by neutron diffraction, magnetization, and ac-susceptibility measurements
Authors: Caspi El’ad N. , Pinto Haim , Kuznietz Moshe , Ettedgui Hanania , Melamud Mordechai , Felner Israel , Shaked Hagai
Published: July 2002

Neutron-diffraction measurements on TbCo3B2, with the isotope B11, show ferromagnetic structure below 35(2) K. Ordered magnetic moments of Tb and Co were found to be in the basal planes directed parallel to each other. The Tb ordered moment is significantly smaller than its free-ion value. The Co ordered moment is nearly zero. Ac-susceptibility and SQUID magnetometer measurements agree with the neutron results.
Evidence of spin disorder at the surface–core interface of oxygen passivated Fe nanoparticles
Authors: Del Bianco L. , Hernando A. , Multigner M. , Prados C. , Sánchez-López J. C. , Fernández A. , Conde C. F. , Conde A.
Published: July 2002

Hysteresis, thermal dependence of magnetization, and coercivity of oxide coated ultrafine Fe particles prepared by inert gas condensation and oxygen passivation have been studied in the 5–300 K range. The results are found to be consistent with a spin-glasslike state of the oxide layer inducing, through exchange interaction with the ferromagnetic core, a shift of the field cooled hysteresis loops at temperatures below the freezing at approximately 50 K.
Characterization of scanning tunneling microscopy and atomic force microscopy-based techniques for nanolithography on hydrogen-passivated silicon
Authors: Fontaine P. A. , Dubois E. , Stiévenard D.
Published: July 2002

A comparison between scanning tunneling microscope (STM) and atomic force microscope (AFM) nanolithography techniques based on local oxidation of silicon is proposed. This work deals with the three different near-field microscopy techniques, namely, STM, AFM in contact mode (CM-AFM), and tapping mode (TM-AFM), all of them operated in air. The thickness and width of oxide stripes are studied as a function of the applied probe–sample voltage, the speed of the probe and the setpoint (current, applied force, and vibration amplitude for STM, AFM contact, and tapping, respectively). The advantages and drawbacks of each technique are analyzed, establishing TM-AFM as the best candidate for scanning probe microscope nanolithography.
Temperature dependence of the barrier height of Pt/n-GaAs Schottky diodes
Authors: Hübers H.-W. , Röser H. P.
Published: July 2002

The dependence on temperature of the Schottky barrier height of Pt on n-GaAs is reported. Two different behaviors are observed. The Schottky contacts of one group have a mean barrier height of 1.018 eV and a temperature coefficient of −0.23 meV/K while the contacts of the other group have a significantly lower barrier height of 0.922 eV which is almost independent of the temperature. These results are interpreted on the basis of recent models of Fermi level pinning. While the Fermi level in the diodes of the first group is pinned to a charge neutrality level, it is pinned by defects in the diodes of the second group.
Transverse domain structure related giant magnetoimpedance in nanocrystalline Fe73.5Cu1Nb3Si13.5B9 ribbons
Authors: Guo H. Q. , Kronmüller H. , Dragon T. , Chen C. , Shen B. G.
Published: July 2002

A sensitive field- and frequency-dependent magnetoimpedance (MI) has been observed in nanocrystalline Fe73.5Cu1Nb3Si13.5B9 ribbons. A maximum value for the MI ration ΔZ/Z=[Z(H)−Z(Hmax)]/Z(Hmax), of more than 400% was obtained in these nanocrystalline ribbons (annealed for 3–5 h at 550 °C). A peak in the field dependence of the MI ratio ΔZ/Z was observed in nanocrystalline samples, but not in the as-quenched amorphous samples. The sensitivity attained a value larger than 60% Oe−1 in the field range 3–7 Oe at 800 kHz for the nanocrystalline ribbons. Domain observation experiments show that the transverse domain structure observed in nanocrystalline samples is responsible for their giant MI effects, and the domain wall movements in the transverse domain pattern region are dominant during the initial magnetization processes in both longitudinal and transverse directions of the applied field. The correlation between the magnetization processes and the giant MI effects is discussed briefly based on the view of a modified skin effect.
Noise properties of ferromagnetic tunnel junctions
Authors: Nowak E. R. , Merithew R. D. , Weissman M. B. , Bloom I. , Parkin S. S. P.
Published: July 2002

We report measurements of voltage fluctuations in magnetic tunnel junctions which exhibit both high and low magnetoresistance (MR). The voltage noise power normalized to the square of the junction bias voltage was 10−14/Hz at a frequency of 1 Hz in a high MR junction. Low MR junctions had significantly higher noise power at 1 Hz and the origin of the noise was not magnetic. In these junctions, random telegraph noise was observed over a wide range of temperatures and junction biases. The results are consistent with a two-channel model of conduction, one of which is spin independent and gives rise to large noise. A noise measuring technique provides evidence for bias-dependent current-path rearrangements. The data support the existence of an inhomogeneous (filamentary-like) current-flow pattern across the tunnel junction associated with the spin-independent channel.
Modeling radiation induced structural evolution in nonmetallic compounds
Authors: Ossi P. M. , Pastorelli R.
Published: July 2002

To interpret and moreover to predict whether the structure of the near surface region of an ion bombarded crystalline system will be able to withstand irradiation, or it will undergo radiation induced amorphization is a difficult problem. The atomistic segregation charge transfer model is adopted here to interpret the stage of nucleation of a crystalline, or an amorphous structure in binary films of covalently bonded materials, ion bombarded under conditions favoring the formation of dense collision cascades. A nonequilibrium compositional profile is supposed to develop at the cascade–matrix interface, following bombardment induced interface segregation of one of the film constituents. Such a local stoichiometry change is associated with the development of a nonequilibrium electronic density profile. Interatomic interactions leading to the formation of amorphous, or crystalline nuclei are treated by local charge transfer reactions. Qualitative differences are found both concerning the energetics of charge transfer reactions and the bonding features of the nuclei, with a set of 14 compounds undergoing amorphization and 12 compounds remaining crystalline upon ion bombardment.
Electron beam induced current and remote electron beam induced current assessment of chemical vapor deposited diamond films
Authors: Cremades A. , Piqueras J.
Published: July 2002

In the present work, electron beam induced current (EBIC) has been applied to characterize several kinds of chemical vapor deposition diamond films. Regions of enhanced carrier recombination are detected in plan-view observations of thin films as well as in cross sections of thick films. Remote EBIC (REBIC) has been applied to obtain information about charged defects present in the samples. The dependence of EBIC and REBIC contrast on the contact configuration used, and on the observation conditions has been analyzed. Cathodoluminescence images of the same samples have been recorded for comparison.
Temperature dependence and annealing effects on spin dependent tunnel junctions
Authors: Sousa R. C. , Sun J. J. , Soares V. , Freitas P. P. , Kling A. , da Silva M. F. , Soares J. C.
Published: July 2002

The temperature and annealing effects on junctions with high (10–13 MΩ μm2) and low (25–30 kΩ μm2) resistance-area products were studied. Junction tunneling magnetoresistance (TMR) is almost unchanged and above 20% up to 200 °C. A sharp and reversible TMR decrease is observed between 200 and 220 °C and is due to the exchange loss in the pinning layer. Junction TMR increases from 22% to 26% in high resistance-area product samples (resistance decreases a factor of 2), and from 22% to 37% (resistance increases 30%) in low resistance-area product samples, upon anneal up to 200–230 °C. Rutherford backscattering (RBS) analysis of the oxygen distribution in as-deposited samples indicates oxygen asymmetry in the barrier. This asymmetry and asymmetry in barrier parameters, found in as-deposited samples, disappear after anneal at 200 °C. Two regimes for the TMR dependence on anneal are proposed. The first up to 200 °C, where TMR increases, as barrier is homogenized and polarization near the top electrode increases. The second, above 200 °C in low-resistance junctions, where TMR increase is related with barrier height increase.
CrPt
3
thin film media for perpendicular or magneto-optical recording
Authors: Leonhardt T. D. , Chen Y. , Rao M. , Laughlin D. E. , Lambeth D. N. , Kryder M. H.
Published: July 2002

The magnetic properties of CrPt3 L12 ferrimagnetic thin films have been studied. Films were produced by sputtering multilayers of Cr and Pt onto silicon nitride coated silicon substrates. The as-deposited films are nonmagnetic. An anneal at ∼800 °C results in ferrimagnetic behavior with a perpendicular easy-axis. X-ray diffraction and transmission electron microscopy (TEM) measurements show that (111) CrPt3 is the only crystalline phase present after annealing. Rocking curves with a full width at half maximum as low as 1.8° indicate good crystallographic orientation. Magnetic properties of the films vary with composition, annealing temperature and time, layer thickness, and sputtering conditions. The films exhibit large coercivities, Hc, that can be tuned in the range 1500–8000 Oe. Saturation magnetization, Ms, is typically 150–200 emu/cc. Squarenesses, S, as high as 0.99 have been found. A uniaxial magnetic anisotropy constant, Ku, of up to 8×106erg/cc was achieved. TEM micrographs show a 35 nm average grain size and complete interdiffusion of the Cr and Pt. Magneto-optical hysteresis loops at 632.8 nm wavelength reveal Kerr rotations of about 0.21° when the films are overcoated with a quarter-wavelength dielectric.
High-field electrical conduction in polyimide films
Authors: Tu N. R. , Kao K. C.
Published: July 2002

Electrical conduction current in polyimide films has been measured at high fields (>1.4 MV cm−1) using a Au metal (M)–polyimide film (P)–p-silicon (S)–MIS structure. On the basis of the current–voltage characteristics coupled with the capacitance–voltage characteristics measured under various conditions, it is found that electrical conduction at high fields is mainly due to Fowler–Nordheim type tunneling injection of electrons from the Au gate electrode if it is negatively biased, or from the PI/Si contact if the Au gate electrode is positively biased. The conduction current is strongly dependent on the concentration and the centroid location of the trapped charges. Computer simulation reveals that the trap concentration is of the order of 6×1017 cm−3 with its centroid located near the electron injecting contact, and that the trap ledge occurs only when the concentration of the net negatively trapped electron charge is large or the location of its centroid is close to the injecting contact. The decay of the dark charging current after the application of a step-function direct current field is associated mainly with the time-dependent trap-filling process. The photoconduction under an ultraviolet light illumination is due mainly to the photogeneration of free holes. Polyimide is very sensitive to the environmental humidity. Electrical conduction current increases and the breakdown strength decreases rapidly with increasing humidity.
Characterization and modeling of a microwave driven xenon excimer lamp
Authors: Ametepe Joseph D. , Diggs Jessie , Manos Dennis M. , Kelley Michael J.
Published: July 2002

We designed, constructed, and studied a novel probe-coupled 2.45 GHz microwave arrangement to drive a Xe excimer lamp. Electrical efficiency and output power in the 160–200 nm range (Xe second continuum) both increased with pressure and input power up to 1500 Torr and 600 W (42.5 W/cm3), respectively. The maximum in-range optical power was 60 W, more than 80% of the total. Cooling with liquid nitrogen boil off rather than room air more than doubled output power. Model calculation and experimental measurement of the angular distribution of emission find considerable intensity well away from the surface normal. Correcting experimental measurements of excimer lamp output power accordingly brings good agreement with the 20%–40% electrical efficiency predicted by theory.
Near edge x-ray absorption fine structure characterization of polycrystalline GaN grown by nitridation of GaAs (001)
Authors: Lübbe M. , Bressler P. R. , Braun W. , Kampen T. U. , Zahn D. R. T.
Published: July 2002

The phase composition and microcrystalline structure of thin gallium nitride (GaN) grown by nitridation of (001) oriented gallium arsenide (GaAs) was investigated by near edge x-ray absorption fine structure (NEXAFS) spectroscopy. The GaN layer was grown by the interaction of atomic nitrogen produced by a rf-plasma source with the clean GaAs surface at a temperature of 700 °C. In this way a GaN film thickness of ≈100 Å was obtained after 6 h of nitridation. Using surface sensitive NEXAFS at the nitrogen K edge, the partial nitrogen p density of states was determined. Comparing the data to reference spectra of hexagonal and cubic GaN, the amount of cubic GaN in the nitrided film was estimated to be 20%–25%. Varying the angle of polarization of the synchrotron radiation with respect to the sample surface, the geometric anisotropy of the GaN film, and thus its crystalline structure, was probed, providing information on the orientation of the GaN microcrystallites. The results from the polarization dependent measurements suggest that the c axes of the hexagonal GaN crystallites in the film are mainly oriented parallel to the (001) direction of the GaAs substrate. The c axes of roughly 45% of the crystallites are tilted by 90° and lie parallel to the surface plane.
Transmission near-field scanning microscope for infrared chemical imaging
Authors: Dragnea Bogdan , Preusser Jan , Schade Wolfgang , Leone Stephen R. , Hinsberg William D.
Published: July 2002

We report transmission infrared near-field scanning microscopy (IR-NSOM) imaging of chemically amplified photoresist polymers patterned by ultraviolet exposure. Chemical specificity was attained using infrared wavelengths tuned to the 3 μm OH stretch absorption band of the polymer, a band sensitive to the chemical changes characteristic of the lithographic photochemical process of this material. Contrast mechanisms are discussed together with the IR-NSOM specifics, such as the fabrication of an infrared near-field probe with high throughput, which lead to an attainable resolution of λ/10 and a transmission sensitivity of 1%.
Giant magnetic viscosity in SmCo5−xCux alloys
Authors: Téllez-Blanco J. C. , Turtelli R. Sato , Grössinger R. , Estévez-Rams E. , Fidler J.
Published: July 2002

Hysteresis loops in SmCo5−xCux as-cast and annealed magnets (1<x⩽3) were measured at room temperature using a pulsed-field magnetometer and a static vibrating sample magnetometer. At field sweeps between 2 and 17 (GA/m)/s a giant—in this field range nearly constant—magnetic viscosity Sv was obtained. The viscosity is approximately proportional to the Cu concentration and to the coercive field. A heat treatment at 1273 K for three weeks changes the microstructure and enhances strongly the intensity and the time dependence of the coercivity. The decay of the inverse of the coercive field Hc with time follows a simple ln(t) relation between 10−5 and 10 s. Data analysis allows a density of stacking faults N to be extracted for each sample. Sv (and Hc) increases with N which increases with x.
GaAs wafer bonding by atomic hydrogen surface cleaning
Authors: Akatsu Takeshi , Plößl Andreas , Stenzel Heinz , Gösele Ulrich
Published: July 2002

A method of large-area wafer bonding of GaAs is proposed. The bonding procedure was carried out in an ultrahigh vacuum. The wafer surfaces were cleaned at 400 and 500 °C by application of atomic hydrogen produced by thermal cracking. The wafers were brought into contact either immediately after the cleaning, or at temperatures as low as 150 °C, without application of a load, and successfully bonded over the whole area. High-resolution transmission electron microscopy revealed that the wafers could be directly bonded without any crystalline damage or intermediate layer. From a mechanical test, the fracture surface energy was estimated to be 0.7–1.0 J/m2, which is comparable to that of the bulk fracture. Furthermore, this bonding method needs no wet chemical treatment and has no limits to wafer diameter. Moreover, it is suitable for low temperature bonding.
Time-resolved spectroscopy of excitonic luminescence from GaN homoepitaxial layers
Authors: Yamada Yoichi , Sasaki Chiharu , Kurai Satoshi , Taguchi Tsunemasa , Sugahara Tomoya , Nishino Katsushi , Sakai Shiro
Published: July 2002

Recombination dynamics of excitonic transitions in GaN homoepitaxial layers has been studied by means of time-resolved luminescence spectroscopy. An excitation-density-dependent transition of the dominant radiative recombination process at 3.471 eV from donor-bound excitons to biexcitons was clearly observed in the temporal behavior. In addition, double-exponential decay of biexciton luminescence was observed, which is one of the characteristics of biexciton luminescence at high excitation densities.
Modeling of rapid particle growth by coagulation in silane plasma reactor
Authors: Kim Kyo-Seon , Kim Dong-Joo
Published: July 2002

The rapid particle growth by coagulation of particles in silane plasma reactor was analyzed, considering the Gaussian distribution function for particle charges. The model equations for particle growth were based on the experimental observations that the large predator particles of a few hundred nms are quite monodisperse and are composed of many small, tiny protoparticles of a few nms. The effects of process conditions such as protoparticle size, residence time, and mass generation rates of predator and protoparticles on particle growth in plasma reactor were analyzed theoretically. Based on the Gaussian distribution function of particle charges, most of the large predator particles in plasma reactor are found to be charged negatively, but some fractions of small, tiny protoparticles are in neutral state or even charged positively. Significant amount of negative charges in plasma reactor exist on the protoparticles. The predator particles charged negatively are believed to coagulate very fast with the protoparticles charged positively. The predator particle size and average charge per predator particle increase with time. The protoparticle concentration increases at first by the faster generation rate than the disappearance rate by coagulation and decreases later by the faster coagulation rate with predator particles and reaches the steady state. The predator particle concentration increases with time in the beginning as predator particles are generated and reaches the steady state later by the balance of generation and disappearance rates. As the protoparticle size decreases, predator particle size increases more quickly by the faster coagulation with protoparticles of higher concentration. As the residence time increases, or as the mass generation rate of protoparticles increases, the protoparticle concentration and the predator particle size increase more quickly. As the mass generation rate of predator particles increases, the predator particle concentration increases and the number of protoparticles per predator particle decreases and the predator particle size becomes smaller. The model results of rapid particle growth by coagulation in this study are found to be in close agreement with the published experimental results.
Magnetic properties of MnZn ferrites prepared by soft chemical routes
Authors: Razzitte A. C. , Jacobo S. E. , Fano W. G.
Published: July 2002

MnZn ferrites with different composition have been synthesized by soft chemistry routes using oxalate precursors with the partial replacement of water by acetone as the solvent. Toroids were sintered at 1250 °C for 3 h in O2/N2 atmosphere. Sintering conditions significantly changed macroscopic characteristics as density and initial permeability for different composition. Variation of the complex permeability with frequency have been measured over a wide range of frequency, up to 1.8 GHz where dispersion is attributed to spin resonance. No additives for increasing densification were employed. Samples exhibit the highest permeability values for Mn0.8Zn0.2Fe2O4. An anomalous effect is observed when copper is added to MnZn ferrites. The initial permeability (μi) values do not exhibit much variation with temperature, except near Tc where it falls sharply. Curie point and magnetic losses are related with composition.
The magnetic anisotropy and domain structure of permalloy antidot arrays
Authors: Yu C. T. , Jiang H. , Shen L. , Flanders P. J. , Mankey G. J.
Published: July 2002

The magnetic properties of antidot arrays in permalloy films were studied using magnetic force microscopy (MFM), the magneto-optical Kerr effect (MOKE), and torque magnetometry. New observations of the magnetic domain structure, magnetization reversal process, and magnetic anisotropy are presented. Magnetic domains were imaged during magnetization reversal to identify the magnetization switching processes with the field applied along the diagonal and the edge of the hole mesh. A four-fold anisotropy related to the confinement of domains by the hole mesh was observed by torque magnetometry at intermediate fields.
A magnetic and crystallographic study of (Sm/Gd)2(Fe/Si)17Cz solid solutions
Authors: Ezekwenna Peter C. , Marasinghe G. K. , Nam Joong-Hee , James W. J. , Yelon W. B. , Ellouze M. , I’Héritier Ph.
Published: July 2002

The crystallographic and magnetic properties of SmyGd2−yFe17−xSix (0⩽x⩽3 and y=1 and 1.5) solid solutions and their interstitial carbides have been investigated using x-ray diffraction and magnetic measurements. The SmyGd2−yFe17−xSix samples crystallized in the rhombohedral Th2Zn17 structure with less than 5 mol % of impurities. The unit cells of the mixed rare-earth (R) samples are smaller than those of Sm2Fe17 and Gd2Fe17. The carbided samples contain up to a total of 15 mol % of free iron, an iron silicide, and/or cubic Si5C3. The unit cells of the carbided samples are 1%–4% larger than those of the parent samples. For a given silicon concentration, the Curie temperatures (Tc) of SmyGd2−yFe17−xSix intermetallics are higher than those of the two end members. For example, the Tc of SmGdFe17 (280 °C) is approximately 160° and 80° higher than that of Sm2Fe17 and Gd2Fe17, respectively. The Tc measured for the SmyGd2−yFe17−xSix samples, 280–290 °C, are among the highest values observed for a R2Fe17−xMx intermetallic where M is a substituent other than cobalt. Except in the case of SmGdFe16SiCz (z unknown), the Tc of the carbided samples are 20%–25% higher than those of the parent samples. A Tc of 426 °C and a magnetization of 120.6 emu/g observed for SmGdFe16SiCz are the highest values measured for the intermetallics investigated herein. As determined by x-ray diffraction studies of magnetically aligned samples, the easy axis of magnetization is parallel to the c axis.
Crystallization of an amorphous layer in P+-implanted 6H-SiC studied by monoenergetic positron beams
Authors: Uedono Akira , Tanigawa Shoichiro , Ohshima Takeshi , Itoh Hisayoshi , Yoshikawa Masahito , Nashiyama Isamu , Frank Thomas , Pensl Gerhard , Suzuki Ryoichi , Ohdaira Toshiyuki , Mikado Tomohisa
Published: July 2002

Depth distributions and species of defects were determined from measurements of Doppler broadening spectra of annihilation radiation and lifetime spectra of positrons for 6H-SiC implanted with 200 keV P+ at a dose of 1×1015 cm−2. The annealing behavior of an amorphous layer was divided into four stages. Stages I (100–500 °C) and II (500–1100 °C) were identified as the relaxation of amorphous networks and the agglomeration of open spaces owing to rearrangements of atoms, respectively. In states III (1100–1500 °C) and IV (1500–1700 °C), corresponding to the recrystallization of the amorphous layer, the mean size of the open volume of defects decreased with increasing annealing temperature; these defects were identified as open spaces adjacent to extended defects. Vacancy-type defects were found in the subsurface region (<100 nm) at high concentration even subsequent to an annealing at 1700 °C. The annealing behavior of defects in the specimens irradiated at elevated temperatures is also discussed.
The atomic and magnetic structure of NdMn(6−x)FexGe6 solid solutions
Authors: Han J. , Marasinghe G. K. , James W. J. , Chen M. , Yelon W. B. , Dubenko I. , Ali N.
Published: July 2002

The magnetic and crystallographic properties of induction-melted NdMn(6−x)FexGe6 intermetallics (x=0, 1.0, and 1.5) in the temperature range of 30–475 K have been studied by x-ray and neutron diffraction techniques and SQUID measurements. All of the samples crystallized in the YCo6Ge6-type structure (P6/mmm). A small amount (<5 mol%) of Nd(MnFe)2Ge2 is present as an impurity. As expected, iron replaces manganese at the 3g site. The unit cell volume decreases slightly with increasing iron content at an average rate of 1.3% per substituted atom. Lattice parameters a and c contract at a rate of 0.6% and 0.2% per substitution atom, respectively. The net magnetization of these samples decreases rapidly with increasing iron content. According to neutron diffraction data, the magnetic moment of the iron sublattice couples negatively with ferromagnetically coupled manganese and neodymium moments. Addition of iron suppresses the spin reorientation processes observed in NdMn6Ge6. Whereas the net moment in NdMn5Fe1Ge6 slowly cants away from the c-axis with increasing temperature, the easy direction of NdMn4.5Fe1.5Ge6 is approximately parallel to the c-axis in the temperature range mentioned above.
Monte Carlo calculations of the elastic moduli and pressure-volume-temperature equation of state for hexahydro-1,3,5-trinitro-1,3,5-triazine
Authors: Sewell Thomas D. , Bennett Carl M.
Published: July 2002

Isothermal-isobaric Monte Carlo calculations were used to obtain predictions of the elastic coefficients and derived engineering moduli and Poisson ratios for crystalline hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). The elastic coefficients were computed using the strain fluctuation formula due to Rahman and Parrinello [J. Chem. Phys. 76, 2662 (1982)]. Calculations were performed as a function of temperature (218 K⩽T⩽333 K) and hydrostatic pressure (0 GPa⩽p⩽4 GPa). The predicted values of the moduli and Poisson ratios under ambient conditions are in accord with general expectations for molecular crystals and with a very recent, unpublished determination for RDX. The moduli exhibit a sensitive pressure dependence whereas the Poisson ratios are relatively independent of pressure. The temperature dependence of the moduli is comparable to the precision of the results. However, the crystal does exhibit thermal softening for most pressures. An additional product of the calculations is information about the pressure-volume-temperature (pVT) equation of state. We obtain near-quantitative agreement with experiment for the case of hydrostatic compression and reasonable, but not quantitative, correspondence for thermal expansion. The results indicate a significant dependence of the thermal expansion coefficients on hydrostatic pressure.
Dielectric behavior of Ba(Ti1−xZrx)O3 single crystals
Authors: Yu Zhi , Guo Ruyan , Bhalla A. S.
Published: July 2002

In this article, we report the successful growth of Ba(Ti1−xZrx)O3 (x=0.05–0.2) single-crystal fibers by the laser-heated pedestal growth technique. A single-phase perovskite structure of the materials has been identified by the x-ray diffraction technique. The phase diagram for Ba(Ti1−xZrx)O3 single crystals is established for x⩽0.2. Dielectric properties as function of temperature and frequency and room-temperature hysteresis loops are measured. The remnant polarization (Pr) and coercive fields (Ec) are obtained and compared for both single crystals and ceramics. The small dielectric relaxation has been observed for the lower-temperature phase transition (around 40 °C at 1 kHz) of the sample with x=0.08. The relaxation times follow the Arrhenius law with τ0=0.4×10−13 s and Erelax=0.53 eV. A common feature of the low-temperature relaxation mode in the sample is also observed, which follows the Arrhenius law with the τ0=0.8×10−10 s and Erelax=0.46 eV.
Infrared organic light emitting diodes using neodymium tris-(8-hydroxyquinoline)
Authors: Khreis O. M. , Curry R. J. , Somerton M. , Gillin W. P.
Published: July 2002

We have studied the photoluminescence and electroluminescence of neodymium tris-(8-hydroxyquinoline) and have found evidence, from the Stark splitting of the neodymium emission, for two isomers of the molecule. Following sublimation it appears that one of these isomers predominates. Photoluminescence can be excited through absorption into the organic ligands and there appears to be efficient coupling between the singlet and triplet exciton levels in the ligand and the internal levels of the neodymium. We can obtain bright infrared electroluminescence from the intraatomic levels within the neodymium at wavelengths of 900, 1064, and 1337 nm.
Electrode-sheath voltages in high-pressure mercury arcs
Authors: Kloss A. , Schneidenbach H. , Schöpp H. , Hess H. , Hitzschke L. , Schalk B.
Published: July 2002

Electrode-sheath voltages (ESVs) were determined as a function of time in high-pressure ac mercury arcs running at different frequencies in the range 50 Hz–5 kHz with sinusoidal wave forms. Besides the experimental investigations, a one-dimensional model was used to describe the arc-column properties. Measurements of the voltage across the arcs were compared with model calculations for the arc-column voltage only. The calculated voltages are mostly smaller than the measured ones, and it was concluded that the difference should correspond to the ESV. This voltage drop was compared with values obtained in a completely different way, namely, by measuring the voltage at different lengths of the arcs which were otherwise identic, and extrapolating it to zero length. As had been shown before, at 50 Hz this voltage drop has a very pronounced time behavior during a half cycle. The investigations were extended to higher frequencies, and the obtained differences are discussed. The field strengths were derived from the rise of the measured voltage versus the discharge lengths; a quantitative agreement with the calculated field strengths was obtained by choosing an appropriate pressure.
Light scattering measurements in a dilute microemulsion
Authors: Kim Mahn Won , Dozier William D. , Klein Rudolf
Published: July 2002

We have measured the mutual diffusion coefficient and static light scattering intensity at small angle of a water-in-oil microemulsion at low (0.005–0.04) minor component volume fraction. The system studied was AOT/water/decane at 25 °C. We found a linear dependence on volume fraction for both quantities, with virial coefficients of −17 and −11, respectively, for the static structure factor and mutual diffusion coefficient. Using available expressions for these coefficients as a function of the parameters of a model potential consisting of an attractive square well and a hard core, these results can be shown to be in agreement with those previously obtained by neutron scattering.
The efficient evaluation of configuration interaction analytic energy second derivatives: Application to hydrogen thioperoxide, HSOH
Authors: Lee Timothy J. , Handy Nicholas C. , Rice Julia E. , Scheiner Andrew C. , Schaefer Henry F.
Published: July 2002

We present an efficient reformulation of the analytic configuration interaction (CI) energy second derivative. Specifically, the Z-vector method of Handy and Schaefer is used to avoid solving the second order coupled perturbed Hartree–Fock (CPHF) equations. We have incorporated translational–rotational invariance into the new method. We present a more efficient method for the evaluation of the Y matrix contribution. The procedure which has been implemented can accommodate very large basis sets and CI expansions for any general restricted Hartree–Fock (RHF) reference wave function. As a test case, we apply the new procedure to the HSOH molecule using a double zeta plus polarization basis set. This leads to 50 contracted Gaussian basis functions and 116 403 configurations in the CI expansion. Harmonic vibrational frequencies and infrared intensities are predicted for HSOH and its deuterated isotopomers. The analytic method described herein requires only 56% of the central processor unit time used by a numerical method.
Velocity modulation infrared laser spectroscopy of negative ions: The ν3, ν3+ν1−ν1, ν3+ν2−ν2, and ν3+2ν2−2ν2 bands of cyanate (NCO−)
Authors: Gruebele Martin , Polak Mark , Saykally Richard J.
Published: July 2002

We have measured 132 transitions in the ν3 (CN stretching) fundamental and the corresponding bending and stretching hotbands of the cyanate anion, ranging from P(52) to R(66), with a tunable diode laser using the velocity modulation technique. The spectra were fit to the standard linear triatomic molecule rotation–vibration Hamiltonian, yielding effective molecular parameters for the (0000), (0110), (0200), (1000), (0001), (0111), (0201), and (1001) states. The equilibrium rotational constant was determined to be Be =0.385 933(116) cm−1. A comparison with condensed phase results is presented. As previously oberved for N− 3, cyanate is detected with high vibrational excitation in the bending mode, most likely as a consequence of the formation mechanism.
Velocity modulation diode laser spectroscopy of negative ions: The ν1, ν1+ν2−ν2, ν1+ν3−ν3 bands of thiocyanate (NCS−)
Authors: Polak Mark , Gruebele Martin , Saykally Richard J.
Published: July 2002

149 transitions in the ν1 band (CN stretch) and the corresponding bending and stretching hot bands of thiocyanate (NCS−) have been measured using velocity modulation spectroscopy with a tunable diode laser. The data were fit to an effective rotation–vibration Hamiltonian, yielding spectroscopic parameters for the (000), (100), (010), (110), (001), and (101) vibrational states. The band origin is ν1=2065.9312(13) cm−1 and the equilibrium rotational constant is calculated to be 0.197 438(61) cm−1. NCS− was prepared in a NH3/CS2 discharge, and unlike the recently studied case of NCO−, vibrational excitation in excess of the rotational temperature (650±200 K) was not observed.