bluuebunny/crossref_metadata_2025 huggingface dataset:
Properties of infinite layer Ca1−xSrxCuO2 films oxygen doped by ion implantation
Authors: Wong A. S. , Ma Q. Y. , Dosanjh P. , Carolan J. F. , Hardy W. N.
Published: July 2002

Infinite layer Ca1−xSrxCuO2 (CSCO) films, grown by pulsed laser deposition with x=0.66 and 0.9, were doped by implanting O+ ions with an accelerating energy of 70 keV and a dose of 3×1016 cm−2. The resistivity of the films, in the temperature range 4–200 K, decreased by over two orders of magnitude compared with the original as-grown film. Annealing the as-grown films in an atmosphere of O2 at 500 °C resulted in only a factor of 2 decrease in this temperature range. A magnetic anomaly resembling a diamagnetic transition was observed in the 85–90 K range in implanted films which was not observed in as-grown films. A comparison of the properties of implanted films, before and after annealing, suggest that increased oxygen content alone is not responsible for the observed changes, and thus defect creation resulting from implantation may also play a significant role.
Diamond nucleation and growth on TaN2
Authors: Wang Zhiyong , Yao Jianghong , Han Li , Chen Guanghua
Published: July 2002

High quality diamond films have been deposited on tantalum nitride (TaN2) materials, which are used as heating elements for high-speed thermal printing heads, by using the microwave plasma chemical vapor deposition technique via a particular pretreatment, intercurrent treatment, and shutdown process. Scanning electron microscopy, x-ray diffraction, and Raman spectroscopy were employed to investigate the structure and quality of the films. A highly adherent film-substrate system was obtained. The possibility of chemical vapor deposition diamond films being used as protective layers of thermal printing heads is indicated. The low-temperature pretreatment with a methane-rich hydrogen plasma plays an important role in nucleating diamond crystallites on the TaN2 substrate, thus enhancing the nucleation density and making it possible to form a continuous diamond film on TaN2. A speculation is proposed to interpret the effect.
On the theory of thermal stresses in a thin bonding layer
Authors: Cherepanov Genady P.
Published: July 2002

Thin bonding layers made of solders or metal-filled adhesives are widely used in electronics. They are mostly responsible for the integrity and reliability of computers and other electronic devices. Major concerns are due to thermal stresses arising in the process of electronic packaging. In this article, the analytical theory of thermal stresses is constructed under some natural simplifying assumptions. The assumptions include the thermoelastic behavior of the bonded materials and thermoelastic–plastic behavior of a bonding layer, after the latter is deposited and the composite system is cooled to an operational temperature. The closed system of governing partial differential equations for thermal stresses in a thin bonding layer is derived for any in-plane shape of the layer. Some particular problems are solved in an explicit form and the implication of the solutions for the prediction of planar voids and cracks formation are discussed.
The effect of post deposition low energy plasma bombardment on the ultra thin hydrogenated silicon oxide films
Authors: Bao Tien-I , I Lin
Published: July 2002

The effect of the low energy (30 eV) Ar plasma on the property of the deposited ultra thin a-SiOX:H (0≤X≤2) films is investigated by alternate deposition and post-deposition Ar plasma treatment processes in a rf hollow oval magnetron system using an in situ ellipsometer and infrared absorption spectroscopy. The results show that the low energy Ar plasma bombardment has no effect on the stoichiometric oxide film but is able to cause hydrogen bond breaking and desorption, and reduce the thickness of the hydrogenated thin film.
Stoichiometry of Cd(S,Se) nanocrystals by anomalous small-angle x-ray scattering
Authors: Ramos Aline , Lyon Olivier , Levelut Claire
Published: July 2002

In Cd(S,Se)-doped glasses the optical properties are strongly dependent on the size of the nanocrystals, but can be also largely modified by changes in the crystal stoichiometry; however, the information on both stoichiometry and size is difficult to obtain in crystals smaller than 10 nm. The intensity scattered at small angles is classically used to get information about nanoparticles sizes. Moreover the variation of amplitude of this intensity with the energy of the x ray—‘‘the anomalous effect’’—near the selenium edge is related to stoichiometry. Anomalous small-angle x-ray scattering has been used as a tentative method to get information about stoichiometry in nanocrystals with size lower than 10 nm. Experiments have been performed on samples treated for 2 days at temperatures in the range 540–650 °C. The samples treated at temperatures above 580 °C contain crystals with size larger than 4 nm. For all these samples the anomalous effect has nearly the same amplitude, and we found the stoichiometry x=0.4 for the CdSxSe1−x nanocrystals. This agrees with the previous results obtained by scanning electron microscopy and Raman spectroscopy. The results are also confirmed by measurements of the position of the optical absorption edge and by wide-angle x-ray scattering experiments. For the sample treated at 560 °C, the nanocrystal size is 3 nm and the stoichiometry x=0.6 is deduced from the anomalous effect. For samples treated at lower temperatures the anomalous effect is not observable, indicating an even lower selenium content in the nanocrystals (x≳0.7). We observed differences in the Se content of nanocrystals for different heat treatments of the same initial glass. These results may be very helpful to interpret the change in the optical properties when the temperature of the treatments decreases in the range 560–590 °C. In this temperature range, compositional effects seem to be of the same order of magnitude as the effects of the quantum confinement.
The function estimation in measuring temperature-dependent thermal conductivity in composite material
Authors: Huang Cheng-Hung , Yan Jan-Yuan
Published: July 2002

An inverse analysis utilizing the conjugate gradient method of minimization and adjoint equation is used successfully to solve the inverse problem in estimating the temperature-dependent thermal conductivity of a composite material. No prior information is available on the functional form of the unknown thermal conductivity in the present study, thus, it is classified as the function estimation in inverse calculation. The accuracy of the inverse analysis is examined by using the simulated exact and inexact measurements that were obtained within the medium.
Lattice constants and thermal expansion of AlxGa1−xAs:Te
Authors: Bak-Misiuk J. , Leszczynski M. , Domagala J. , Zytkiewicz Z.
Published: July 2002

An influence of Te doping on the lattice parameter and the thermal expansion of AlxGa1−xAs epitaxial layers was examined by high-resolution x-ray diffractometry over a temperature range 77–673 K. For doped AlxGa1−xAs layers the thermal expansion coefficients were found to be larger relative to undoped samples of the same Al content. This phenomenon is attributed to the change of anharmonic part of lattice vibrations by free electrons and/or ionized tellurium atoms. An increase of thermal expansion caused by doping is a factor which should be taken into account in lattice constant measurements at 295 K. We propose an interpretation of the lattice expansion (examined at room temperature by other authors and by us) of GaAs caused by Te doping in terms of three factors: (i) ‘‘size’’ effect (bigger Te atoms with respect to As), (ii) free electrons via the deformation potential of the conduction-band minimum occupied by these electrons, (iii) increase of the thermal expansion.
γ-ray-induced optical attenuation in Ge-doped-silica fiber image guides
Authors: Griscom David L.
Published: July 2002

Optical attenuation in fiber optic image guides comprising germanium-doped-silica-core/ pure-silica-clad picture elements was investigated in the range 420-1100 nm during and after room-temperature 60Co γ irradiation. The induced absorption spectra of the picture elements were successfully fitted by linear combinations of three known defect bands in irradiated Ge-doped SiO2: the Ge(1) center, the GeX center, and the nonbridging oxygen hole center (peak positions of 4.4, 2.6, and 2.0 eV, respectively). Dose for dose, the relative strengths of these three bands differed greatly between two nominally identical image guide samples from the same manufacturer. Defect production kinetics were followed at four dose rates ranging from 1.1 to 180 Gy/h. These kinetics and the post-irradiation anneal behaviors are interpreted in terms of the previously reported tendency of the Ge(1) center to undergo a thermal conversion to GeX. The net effect on absorption at 550 nm is a dose-rate-independent linear growth up to ∼1000 Gy(Si) followed by saturation, with essentially no post-irradiation thermal decay at 22 C. At 650 nm, the kinetics are similar except that there is an ‘‘inverted’’ dose-rate dependence (lower losses at higher dose rates) and a small post-irradiation increase in absorption with time at 22 C. Irrespective of dose rate, these image guides are unsuitable for use in multimeter lengths if there is a likelihood of a lifetime radiation exposure of ∼100 Gy (worst sample) or ∼1000 Gy (best sample).
Raman and infrared spectroscopy of ferroelectric Pb(Ti0.48Zr0.52)O3 films deposited by pulsed laser ablation
Authors: Yakovlev V. A. , Mattei G. , Iembo A. , Fuso F. , Arimondo E. , Allegrini M. , Leccabue F. , Watts B. E.
Published: July 2002

Pb(Ti0.48Zr0.52)O3 films produced by pulsed laser ablation deposition have been locally examined for their homogeneity and thickness through a comparative use of Raman and infrared spectroscopy. Raman scattering intensity appears to be an oscillating function of the position of the point under measurements. The observed oscillations were explained by light interference effects in the film and used to obtain the thickness profile and the refractive index dispersion of the film. The intensity distribution in Raman spectra across the film differs from that of the target. This difference is larger at the center than near the edge of the film.
Transverse beam profiles on traveling-wave optical parametric generation in KTiOPO4 crystals
Authors: Nishikawa Tadashi , Uesugi Naoshi
Published: July 2002

Transverse beam profiles on traveling-wave optical parametric generation in KTiOPO4 crystals pumped by short optical pulses are investigated. The transverse beam profile deformation and splitting due to the back-conversion in the traveling-wave parametric propagation process is clarified for the first time from experimental observations and calculations. The transverse beam profile becomes ring shaped when there is no walk-off and deforms by expanding to the walk-off direction when walk-off exists. The deformation of the profile can be reduced by using a short crystal or decreasing the pump intensity due to the subsequent reduction in back-conversion. However, in these cases, the conversion efficiency becomes very low (3%–4%). We can improve the conversion efficiency while keeping good transverse beam profiles by using the two-crystal beam overlapping technique. The optimum conditions for this technique are clarified through calculations and it is shown that conversion efficiency of more than 18% can be achieved when they are used.
Numerical analysis of photorefractive InP:Fe at large fringe contrast
Authors: Wolffer N. , Gravey P. , Coquillé R.
Published: July 2002

A finite difference method is used to analyze the behavior of photorefractive InP:Fe at high fringe contrast under externally applied field. The evolution with increasing fringe contrast m of the electron-hole resonance, which occurs in the presence of a continuous field, is studied. As m increases, the field tends to concentrate in a small zone and reaches very large local values. On the other hand, the resonance loses height and widens. For illuminations closed to the resonance, the two-wave mixing gain presents the usual aspect of a decreasing function of m. However, for other illuminations, it can be larger at a small pump-to-probe ratio than at a large one. When an ac field technique is used, nonlinearities drastically reduce the two-wave mixing gain, even at small fringe contrast. As m increases, the space-charge field tends to take a square shape. Even for rather small m values (0.1), the gain presents a maximum near the dc field resonant illumination, which is not predicted by the linear theory. Finally, when drift is the dominant process for the grating formation, the applied field limits the amplitude of the space-charge field, independently of an optimized dopant concentration which allows high gain in the small m approximation.
Effects of grain size and preferred orientation on the electromigration lifetime of Al-based layered metallization
Authors: Kondo Seiichi , Deguchi Osamu , Hinode Kenji
Published: July 2002

Ten kinds of Al-based layered metallizations were fabricated and investigated to find out how a refractory metal underlayer affects the crystallographic characteristics of an Al layer and, in consequence, the electromigration resistance of the metallization. Both Al grain size and Al (111) preferred orientation were compared with electromigration lifetime by using three interconnect linewidths. A W group underlayer was found to change the Al grain size while maintaining the preferred orientation. A WN underlayer, in particular, increases the Al grain size more than a SiO2 underlayer does, resulting in long electromigration lifetime. A Ti group underlayer, in contrast, improves Al (111) preferred orientation while maintaining the grain size. The crystallographic characteristics of the layered interconnects were plotted on constant-lifetime-line figures indicating the direction of the next layered metallization.
Nuclear magnetic resonance relaxation study of wettability of porous rocks at different magnetic fields
Authors: Bonalde I. , Martín-Landrove M. , Benavides A. , Martín R. , Espidel J.
Published: July 2002

The wettability of the rock surface plays a very important role in the transport of fluid in porous media. The combined Amott-USBM method currently used for the determination of these values is not able to produce reliable estimates when the surface has an intermediate wettability. The nuclear magnetic resonance (NMR) technique could provide an efficient method if the parameters affected by the wettability could be established in well defined model systems. To obtain information about the effect of wettability changes upon the NMR parameters, T1 relaxation time measurements were carried out on 1H and 2H at two different magnetic fields. A large number of well defined consolidated and unconsolidated samples, with different degrees of wettability were used in this study. The results obtained showed that the T1 values, as well as the relaxation rate distribution parameters, such as the distribution width, are affected by changes in the wettability. A dependence of the T1 wettability data on the magnetic field intensity was also observed.
1.7–1.9 μm InxGa1−xAs/InyAl1−yAs light-emitting diodes lattice-mismatched grown on GaAs
Authors: Murti M. R. , Grietens B. , Van Hoof C. , Borghs G.
Published: July 2002

InxGa1−xAs/InyAl1−yAs based light-emitting diodes emitting in the wavelength range 1.7–1.9 μm have been grown nonlattice matched on GaAs. Electroluminescence spectra are measured at 77 K and the injection level dependence has been studied. Mechanisms that broaden the lineshape are discussed. The position of the band gap and the electron temperature are derived by fitting the spectra. Band gap narrowing was observed as a function of the injected carrier density.
Highly photosensitive helium diluted amorphous silicon 1.5 eV band gap: Role of pressure
Authors: Hazra Sukti , Middya A. R. , Ray Swati
Published: July 2002

Highly photosensitive low band gap (≥1.5 eV) hydrogenated amorphous silicon (a-Si:H) films have been developed by plasma enhanced chemical vapor deposition using helium dilution of the process gas, silane at substrate temperature 210 °C. Low band gap of a-Si:H films is achieved by reducing bonded hydrogen content and hence by increasing compactness of the films. At the optical gap ∼1.5 eV, a-Si:H film has high mobility lifetime product, ημτ (8×10−5 cm−2 V−1) and photosensitivity (≳7×104) values. The defect density of the material is as low as 7.8×1015 cm−3 eV−1 and hydrogen content is 4.54 at. %. Low band gap amorphous silicon germanium (a-SiGe:H) films reported so far do not have such high ημτ and photosensitivity values at 1.5 eV optical gap. Thus, this low band gap a-Si:H films can be used as intrinsic layer in the bottom cell of a multijunction solar cell replacing a-SiGe:H alloy films.
Photoluminescence studies on very high-density quasi-two-dimensional electron gases in pseudomorphic modulation-doped quantum wells
Authors: Li Wei , Wang Zhanguo , Song Aimin , Liang Jiben , Xu Bo , Zhu Zhanping , Zheng Wanhua , Liao Qiwei , Yang Bin
Published: July 2002

Photoluminescence studies on highly dense quasi-two-dimensional electron gases (2DEGs) in selectively Si δ-doped GaAs/In0.15Ga0.85As/Al0.25Ga0.75As quantum wells (Ns=4.24×1012 cm−2) are presented. Five well-resolved photoluminescence lines centered at 1.4194, 1.4506, 1.4609, 1.4695, and 1.4808 eV were observed, which are attributed to the recombinations of 2DEG with holes in the n=1 heavy-hole sub-band. The sub-band separations clearly exhibit the feature of the investigated quantum wells structure. The linewidths of these peaks are in the range of 2.2–3.4 meV, indicating the high quality of the structures. The reason for discrepancy between the calculated and observed linewidth is given. Their dependence on the excitation intensity and temperatures are also discussed.
-charge-induced acceleration of ions emitted by laser-irradiated surfaces
Authors: Peurrung A. J. , Cowin J. P. , Teeter G. , Barlow S. E. , Orlando T. M.
Published: July 2002

Pulsed-laser-irradiated surfaces sometimes emit positive ions at energies several volts higher than one would expect, even at modest (<0.1 J/cm2) fluences. A mechanism that can account for this phenomenon is discussed. Intense surface photoemission of electrons during the laser pulse leads to the formation of a space-charge layer near the surface. If the laser fluence were constant, the ions would accelerate and subsequently decelerate as they pass through this steady potential well. As the laser pulse ends, however, some ions may undergo extended acceleration as this space-charge layer moves away from the surface. The maximum possible ion acceleration is analytically calculated and the acceleration for a range of realistic experimental parameters is numerically predicted.
Study of a〈001〉 and a〈110〉 screw dislocations viewed end-on in crystal Ba(NO3)2 by birefringence topography
Authors: Ge Chuan-Zhen , Wu Zhen-Hua , Wang Hai-Wen , Qi Ming , Ming Nai-Ben
Published: July 2002

The screw dislocations with Burgers vector a〈001〉 and a〈110〉 in Ba(NO3)2 crystals have been studied by birefringence topography. In combination with the results obtained previously [C.-Z. Ge, N.-B. Ming, K. Tsukamoto, K. Maiwa, and I. Sunagawa, J. Appl. Phys. 69, 7556 (1991); C.-Z. Ge, H.-W. Wang, and N.-B. Ming, ibid. 74, 139 (1993)], it has been shown that not only a〈111〉, but also a〈110〉 screw dislocations viewed end-on are visible; however, a〈100〉 screw dislocation viewed end-on are always invisible in barium nitrate crystals. The intensity distributions and extinction rules of birefringence images of a〈110〉 screw dislocation viewed end-on are determined both experimentally and theoretically.
Recovery of vacuum ultraviolet irradiated metal-oxide-silicon systems
Authors: Druijf K. G. , de Nijs J. M. M. , van der Drift E. , Granneman E. H. A. , Balk P.
Published: July 2002

This article is concerned with the instability and recovery of the large negative midgap voltage induced in metal-thermally grown SiO2-silicon capacitors with transparent Al gate upon vacuum ultraviolet exposure by a Kr lamp (hν=10 eV). Evidence is presented from high-frequency capacitance–voltage measurements, showing that this midgap voltage instability is caused by the generation of unstable fast donor states, stable and unstable slow donor states, and drifting ions. Two fundamentally different mechanisms contribute to the gradual recovery of the samples: the disappearance of the unstable centers (fast and slow donor states and mobile ions) and hole emission from slow states and from mobile ions arriving at the substrate or at the gate. The annealing of unstable fast donor centers is inhibited when the states are charged positively. It is argued that the unstable fast and slow donor states and the drifting ions have an identical microscopic nature: all are associated with a H atom attached to an O atom in the network. The unstable fast donor states are attributed to H atoms trapped at or very close to the Si substrate, the unstable slow donor states to H trapped somewhat further away. Annealing of these unstable slow states is explained from the gradual disappearance of the atomic H from the oxide. It is shown that the stable slow states are related to near-interfacial O vacancies, activated by first trapping a hole.
Measurements of the electrohydrodynamic instability in planar geometry using gallium
Authors: Mayberry C. S. , Schamiloglu E. , Donohoe G. W.
Published: July 2002

The electrohydrodynamic (EHD) instability in planar geometry was observed using gallium as the working fluid. Electric fields in the range of 0.12–0.26 MV/cm were applied in pulse durations up to 14.4 ms. The instability of the gallium surface was observed using a box camera and pulsed backlighting. The measured wavelengths of the instability ranged from 1.2 to 17.0 mm, consistent with theoretical predictions provided a field enhancement factor of 1.5 is assumed on the meniscus of the gallium. A transverse magnetic field of about 0.6 T was found to completely dampen the instability along the magnetic field at an applied electric field of 0.2 MV/cm; the transverse direction was unaffected. This result puts limits on the usefulness of an EHD ion source in magnetically insulated diodes for inertial confinement fusion studies.
A field-assisted emission model of interface states in heterostructure devices
Authors: Mohajerzadeh S. , Selvakumar C. R. , Noel J. P. , Houghton D. C.
Published: July 2002

We present a physical model to study the interface states in p-n heterostructures at different ambient temperatures. Field-assisted emission of such states is considered as the source for the linear increase in charge concentration at the p-n interface with the applied reverse voltage. The high frequency capacitance-voltage technique is used to study the charging and discharging of interface states in an MBE-made sample at different temperatures and different biases. The experimental results show good agreement with the prediction of our model.
X-ray focusing optics using flat or bent crystals with two-dimensionally modulated surfaces
Authors: Chang W. Z. , Förster E.
Published: July 2002

Calculations of focus characteristics for x-ray focusing systems using either flat or bent crystals with two-dimensional modulated surfaces are presented. Based on the wave-optics approach, the analytical formulas are derived for elliptical Fresnel zone structures (flat crystals) and linear Bragg zone structures (toroidally bent crystals). It is shown that for a given Si(111) reflection and Ti Kα radiation, a micron-size focal spot can be achieved by using only one flat crystal with a modulated surface. The peak intensity is then nearly 100 times higher than that of the flat crystal without modulations. However, for a toroidally bent crystal with both Fresnel and Bragg diffraction amplitudes, the latter becomes dominant, since, due to the bending, the rapid oscillations of the Fresnel diffraction amplitude vanish. It is also shown that a focal spot of 0.3 μm for a bent crystal with linear Bragg zone structures is approximately four times narrower than that of an unmodulated surface. From the analysis, the modulated surfaces for cylindrically and spherically bent crystals are deduced to be a combination of Bragg and Fresnel zones.
Growth textures of thick sputtered films and multilayers assessed via synchrotron transmission Laue
Authors: Vill M. A. , Rek Z. U. , Yalisove S. M. , Bilello J. C.
Published: July 2002

The growth textures of thick sputtered Mo metallizations and Mo/W multilayers, were characterized via a synchrotron white-beam (WB) x-ray transmission Laue technique. Transmission x-ray diffraction studies of Mo specimens up to 61 μm thick were performed with WB synchrotron radiation; while the practical thickness limit for similar observations using a conventional laboratory Cu K(α) x-ray source is ten times smaller. This unique approach used polychromatic x rays to simultaneously produce diffraction from a wide spread of orientations of many crystallographic planes for all the grains within a relatively large specimen volume (≊60×106 μm3). These patterns were obtained for polycrystalline 31- and 61-μm-thick Mo/W multilayer specimens, and a 35-μm-thick-monolithic Mo foil specimen. In all three cases the alignment of specimen grains was similar to what would be expected for single-crystal transmission patterns, except that the recorded intensity distributed was less localized. The WB transmission images were indexed using a reciprocal space construction for the Laue case. In the multilayers, the grains were oriented out-of-plane such that 〈110〉 crystallographic planes were aligned in the direction of sputter growth, while in the monolithic Mo specimen 〈111〉 crystallographic planes were so aligned, i.e., perpendicular to the deposition substrate. A spread in orientation of ∼5° was measured in the multilayer specimens, while the monolithic Mo specimen showed a spread of ∼30° when compared to a perfect single-crystal orientation. Preferred orientation was also observed within the plane of growth to varying degrees for all three samples.
Specific heat of YCo12B6 and GdCo12B6 intermetallics
Authors: Nahm Kyun , Kim Chul Koo , Mittag M. , Jeong Yoon Hee
Published: July 2002

The specific heat of polycrystalline intermetallics YCo12B6 and GdCo12B6 which crystallize in the hexagonal SrNi12B6 structure have been measured in the temperature range of 80 K<T<250 K. The results are successfully explained using the Weiss and the Néel molecular field models and the Ginzburg–Landau theory. The implications of the present results are also discussed.
Temperature variation of ultrasonic attenuation and nonlinearity parameter in LiF and NaF
Authors: Lambade S. D. , Sahasrabudhe G. G. , Rajagopalan S.
Published: July 2002

The temperature dependence of ultrasonic absorption and nonlinearity parameter has been investigated in the range 80–300 K for LiF and NaF. We have estimated the attenuation of ultrasound in these materials for longitudinal and shear waves along the [100], [110], and [111] directions using Mason’s pure mode formulation [in Physical Acoustics, edited by W. P. Mason and R. N. Thurston (Academic, New York, 1964–65), Vol. 3B, Chap. 6] and also using a computer program recently developed by us which is based on the interaction of the sound wave with a much wider spectrum of the phonon modes and which also incorporates the refinements suggested by Merkulov, Kovalenok, and Konovodchenko [Sov. Phys. Solid State 11, 2241 (1970)] regarding the dependence of Mason’s integral on the cutoff frequency supported by the lattice. The results obtained using our program are in better agreement with the experimental variation of attenuation than those obtained using Mason’s pure mode scheme in most cases. The nonlinearity parameter β defined by Breazeale and Philip (in Physical Acoustics, Vol. 17, Chap. 1) has also been estimated for longitudinal waves along the [100], [110], and [111] directions in this temperature range. The variation of β with temperature in these materials is found to be similar to that measured for NaCl.
A radiometric and electrical characterization of low pressure dc positive column sulfur discharges
Authors: Gibson N. D. , Lawler J. E.
Published: July 2002

The dc positive column sulfur (S) and S–Ar discharges have been studied over a wide range of operating conditions. Even though the equilibrium partial pressure of S2 is low in the 50 °C–200 °C temperature range investigated here, the primary radiating molecule in these discharges is S2. Electronic collisions dissociate S8 to produce the radiating S2 molecules at densities far in excess of the equilibrium density. The majority of the spectral emission occurs in the 2800–3500 Å range. The spectrum observed from these discharges corresponds nicely to that obtained from dc discharges at much higher temperatures. The radiation output has been studied as a function of input power, wall temperature, buffer gas pressure, discharge radius, and cold spot temperature. The absolute efficiency of the discharge in generating near-ultraviolet radiation has been measured and the results are presented for many areas of the multidimensional parameter space. The maximum efficiency of ultraviolet and visible emission obtained was approximately 17%.
Photothermal mirage characterization of vertical interfaces separating two different media
Authors: Sanchez-Lavega A. , Forge Ph. , Salazar A. , Ocariz A.
Published: July 2002

A theoretical and experimental study about the ability of the photothermal modulated mirage technique to characterize vertical interfaces (thermal resistances) separating two different opaque and thermally thick media with thermal conductivities larger than the medium surrounding the sample and with any value in their thermal diffusivities has been carried out. For the calculation of the mirage deflection, a theoretical model that incorporates the mirage parameters (sizes of probe and pump beams, probe beam height) and which is valid for any value of the thermal resistance characterizing the interface is presented. The model is validated by experimental measurements on a fabricated sample having a controlled interface width that separates two pieces of copper and steel. Fitting the model calculations and the experimental data allows one to retrieve the value of the thermal resistance.
Monte Carlo simulation of transverse electron focusing
Authors: Hornsey R. I.
Published: July 2002

A Monte Carlo simulation for transverse electron focusing has been developed to allow the investigation of the individual and combined effects of injector and collector widths, specularity coefficient, injection divergence angle, and electron mean free path. A complete determination of the focusing characteristics requires the simultaneous inclusion of all of these parameters, but no previous analytical treatment has achieved this. A systematic study of these effects is presented here using the Monte Carlo simulation and results from the simulation are compared with experimental data, both for single-crystal metals and for devices fabricated in a two-dimensional electron gas by ion implantation. Several previous authors have reported experimental characteristics for which the apparent specularity coefficient derived from the magnitudes of the first two electron focusing peaks is anomalously high. Using the simulation developed here, it is now shown that this effect arises from the combination of a short mean free path and highly divergent electron injection. It is also confirmed that the mean free path can be determined with reasonable accuracy from the electron focusing characteristics even when the injection is divergent.
Electronic properties of cesium on 6H-SiC surfaces
Authors: van Elsbergen V. , Kampen T. U. , Mönch W.
Published: July 2002

The adsorption of cesium on clean {0001} surfaces of n- and p-6H-SiC samples at low temperatures was investigated by using Auger electron, x-ray photoelectron, and ultraviolet photoelectron spectroscopy as well as a Kelvin probe. At clean surfaces the Fermi level is pinned at 1.2 eV above the valence-band maximum and the ionization energy measures 5.7 and 5.8 eV on Siand become metallic after the deposition of the first Cs layer. For submonolayer coverages, Cs-induced surface donors form at 2.96 eV above the valence-band maximum. They are due to covalent Cs–Si bonds. The barrier height of Cs/6H-SiC Schottky contacts was found as 0.57±0.05 eV with n-type and 2.28±0.1 eV with p-type doped samples. These results confirm the concept that the continuum of metal-induced gap states determines the barrier heights of ideal metal-semiconductor contacts.
Transient current across insulating films with long-range movements of charge carriers
Authors: Iwamoto Mitsumasa
Published: July 2002

Based on the ion hopping model, the dielectric relaxation phenomena in insulating films with long-range movements of charge carriers have been analyzed without neglecting space-charge field and diffusion. The transient current flowing across insulating films with long-range movements of charge carriers (impurity ions) which are blocked at the electrodes was analyzed. It was found that the time-dependent transient current first decreases, then increases, and finally decreases again, possibly due to the space-charge field formed in the films. Thermally stimulated depolarization current across insulating films due to impurity ions was also analyzed. In order to clarify the nonmonotonic time dependence of the aforementioned transient current, the time-dependent current was calculated without taking into account the space-charge field. It was found that the current monotonously decreases in this case. Finally, we concluded that the calculation method based on the ion hopping model is helpful for a better understanding of the dielectric relaxation phenomena in insulating films with long-range movements of charge carriers.
Study of the initial growth phase of chemical vapor deposited diamond on silicon(001) by synchrotron radiation
Authors: Geier S. , Hessmer R. , Preckwinkel U. , Schweitzer D. , Schreck M. , Rauschenbach B.
Published: July 2002

Very thin diamond films (thickness ∼0.1 μm) have been investigated by x-ray diffraction pole figure measurements using synchrotron radiation in order to understand the mechanism of heteroepitaxial diamond nucleation and the first steps of film growth on silicon(001) substrates. The diamond layers consist of an epitaxially aligned component with a crystallographic orientation identical to the substrate. The initial orientational spread of the grains around the perfect epitaxial orientation prior to any modification by a subsequent textured growth step has been determined. In the studied temperature range for the nucleation step the misalignment decreases slightly with increasing temperature. Besides the epitaxial crystallites their corresponding twins of first and second order have been found. The intensity distribution of the pole figures indicates that the process of twinning plays a dominating role in the initial growth stage which shows a tendency to become even more pronounced for higher substrate temperatures.
Interpretation of photocurrent correlation measurements used for ultrafast photoconductive switch characterization
Authors: Jacobsen R. H. , Birkelund K. , Holst T. , Uhd Jepsen P. , Keiding S. R.
Published: July 2002

Photocurrent correlation measurements used for the characterization of ultrafast photoconductive switches based on GaAs and silicon-on-sapphire are demonstrated. The correlation signal arises from the interplay of the photoexcited carriers, the dynamics of the bias field and a subsequent recharging of the switch. By using both photocurrent measurements and terahertz spectroscopy we verify the importance of space-charge effects on the carrier dynamics. Photocurrent nonlinearities and coherent effects are discussed as they appear in the correlation signals. An analysis based on a simple model allows an estimate of the carrier lifetimes associated with the photoconductive switching process. We illustrate how pulse propagation can be studied sequentially using this technique and how a minor modification of the experimental setup enables the study of screening from long-lived carriers. We emphasize in what ways the different techniques of measuring ultrashort electrical pulses are sensitive to different aspects of the pulse forming mechanisms.
Ion energy, ion flux, and ion mass effects on low-temperature silicon epitaxy using low-energy ion bombardment process
Authors: Shindo Wataru , Ohmi Tadahiro
Published: July 2002

In low-temperature (300–350 °C) silicon epitaxy employing low-energy inert-gas ion bombardment on a growing film surface, the effects of ion bombardment energy and ion flux as well as that of ion species on the crystallinity of a grown silicon film have been experimentally investigated. It is shown that the energy dose determined by the product of ion energy and ion flux is a main factor for epitaxy that compensates for the reduction in the substrate temperature. Large-mass, large-radius ion bombardment using Xe has been demonstrated to be more effective in promoting epitaxy at low substrate temperatures than Ar ion bombardment. Thus, low-energy, high-flux, large-mass ion bombardment is the direction to pursue for further reducing the processing temperature while preserving high crystallinity of grown films.
Dominant recombination center in electron-irradiated 3C SiC
Authors: Son N. T. , Sörman E. , Chen W. M. , Singh M. , Hallin C. , Kordina O. , Monemar B. , Janzén E. , Lindström J. L.
Published: July 2002

Deep level defects and their role in carrier recombination processes in electron-irradiated 3C SiC have been studied by photoluminescence (PL) and optically detected magnetic resonance (ODMR). An isotropic ODMR spectrum, with a g value of 2.0061±0.0002 and an effective electron spin S=1/2, is observed in irradiated 3C SiC films. From the spectral dependence studies of the ODMR signal, the defect is shown to be a deep level center related to a radiation-induced PL band with a zero-phonon line at 1.121 eV. Due to the competition between different carrier recombination channels, this ODMR spectrum can also be observed as a decrease of any other PL emissions from the sample, indicating its dominant role in recombination processes.
Effect of size on levitation force in a magnet/superconductor system
Authors: Yang Z. J. , Hull J. R.
Published: July 2002

We consider a model system consisting of an infinitely long magnetic dipole line placed symmetrically above an infinitely long superconducting strip. Using the Meissner effect of superconductors, we derive analytical expressions of the levitation forces acting on the dipole line. At lowest-order approximation, we discuss the possible application of our model system to estimate the upper limit of the levitation forces in some magnetic bearing systems. In one example, the model correctly calculated the vertical vibration frequency of an experimental superconducting bearing.
m=3 Ashkin–Teller-like cubic model on an FCC lattice
Authors: Fisch Ronald
Published: July 2002

Monte Carlo simulations have been used to study a model consisting of three Ising models with nearest neighbor exchange J on a face-centered-cubic lattice, which are coupled together by a constraint. The constraint requires that the vector sum of the four spins located at the corners of any elementary tetrahedron of the lattice not add to zero. Finite-size scaling analysis of the Monte Carlo results for L×L×L lattices with L=16, 32, and 64 has been used to find that Tc /J=10.84±0.01. The finite-size scaling functions for the magnetization and magnetic susceptibility near Tc are consistent with standard m=3 Heisenberg critical exponents, as predicted by renormalization group theory. Strengthening the four-spin constraint increases the value of the effective negative Ashkin–Teller four-spin coupling, and drives Tc to infinity.
Competition between spin-Peierls phase and three-dimensional antiferromagnetic order in CuGe1−xSixO3
Authors: Renard J. P. , Le Dang K. , Veillet P. , Regnault L. P. , Dhalenne G. , Revcolevschi A.
Published: July 2002

The effect of substitution of Si to Ge in CuGe1−xSixO3 single crystals has been studied by magnetic susceptibility, Cu nuclear magnetic resonance, and neutron scattering measurements. The phase diagram in the (x,T)-plane is characterized by an initial rapid decrease of the spin-Peierls temperature, TSP, on increasing x and by the occurrence of a three-dimensional long range ordered antiferromagnetic phase at T=TN in the range 0.005≤x≤0.05. This phase diagram is qualitatively discussed. The neutron scattering data for x=0.007 show that the dimerization which takes place below TSP is not suppressed below TN.
Composition dependence of giant magnetoresistance in (La1−xYx)2/3Ca1/3MnOδ (0≤x≤1)
Authors: Li Zisen , Zeng X. T. , Wong H. K.
Published: July 2002

We have prepared a series of polycrystalline bulk samples of (La1−xYx)2/3Ca1/3MnOδ (0≤x≤1), and studied their magnetization M, resistivity ρ, and low-field magnetoresistance (MR) in the temperature range 10 ≤T≤300 K. The ferromagnetic transition temperature of the samples Tc decreases with increasing x. For each sample, both ρ and MR have peak values at a temperature close to Tc. The peak value of ρ increases rapidly with increasing x while the peak value of MR has a maximum at x∼0.4. We observed MR value [defined as [ρ(0)−ρ(B)]/ρ(0)] as high as 91% at B=0.85 T. Some of the results could be explained by the semicovalent exchange theory of Goodenough [Prog. Solid State Chem. 5, 145, (1971)].
Magnetism in URhSi
Authors: Prokeš K. , Brück E. , Buschow K. H. J. , de Boer F. R. , Sechovský V. , Svoboda P. , Hu X. , Maletta H. , Gortenmulder T. J.
Published: July 2002

Our neutron-powder-diffraction experiment revealed that URhSi crystallizes in the orthorhombic TiNiSi (space group Pnma) structure and orders ferromagnetically at low temperatures with the U magnetic moments of 0.11 μB aligned along the c axis. Anomalies in the temperature dependence of the magnetic susceptibility, specific heat and electrical resistivity indicate that URhSi orders below 9.5 K. The enhanced Cp/T value (extrapolated to 0 K) of 186 mJ/mol K2 can be partially reduced in magnetic fields, which indicates a considerable magnetic contribution even at very low temperatures. The ferromagnetic ground state is documented also by magnetization measurements at low temperatures. The high-field magnetization data obtained on oriented powder reveal a strong magnetocrystalline anisotropy. All the results obtained on polycrystalline samples classify URhSi as an itinerant 5f ferromagnet with very reduced U magnetic moments.
Ab initio calculations of spin-dependent transport properties (invited) (abstract)
Authors: Mertig I.
Published: July 2002

Ab initio calculations of transport properties for ferromagnetic dilute alloys and magnetic multilayers are reported. The electronic structure of the considered system is calculated within the frame of density functional theory using local spin density approximation. The scattering of the electrons by impurities is described self-consistently by means of the Green function of the perturbed system. As a result the spin anisotropy of the scattering can be discussed without any free parameter. The transport is described quasiclassically, that is, the linearized Boltzmann equation is solved taking into account the anisotropy of the Fermi surface and the anisotropy of electron-impurity scattering. The method is applied to calculate the residual resistivity of dilute ferromagnetic Fe and Co alloys which are in very good agreement with experiment. Giant magnetoresistance (GMR) was computed for Fe/Cr multilayers for CIP (current in plane) and CPP (current perpendicular to plane) geometry. It can be shown that GMR exists for spin-independent scattering (Fe impurities in Cr layers) determined by the electronic structure of multilayers in ferro- and antiferromagnetic configuration. The consideration of spin-dependent scattering (say Cr impurities in Fe layers) enhances or reduces the GMR effect in dependence of the spin-anisotropy. It can be shown especially that combination of defects with different spin-anisotropy can produce inverse GMR. Finally, calculations are presented for GMR in granular Cu(Co) systems. From the investigation becomes obvious, that the effect is strongly influenced by the spin anisotropy of scattering.
Magnetic and recording properties of monolayer and multilayer thin-film media by using composite targets
Authors: Lal Brij B. , Bourez A. , Russak Michael A.
Published: July 2002

A single chamber deposition process (N-process) to produce multilayer media was developed using a newly designed magnetron cathode. The key feature of this cathode is its use of a composite target and magnetron design which permits specific areas of the target to be independently activated. Magnetic properties, read/write performance, and media-noise of single-layer (SL) and multilayer (ML) media produced with this cathode were compared to media fabricated using a standard two chamber process (S-process). CoCrTa:SL-media sputtered using the N-process produced ∼400–600 Oe higher Hc, ∼24% lower noise, and ∼3 dB higher SNR at 136.4 KFCI recording density in comparison to SL media sputtered by S-process. X-ray diffraction analysis indicated a higher degree of in-plane c-axis orientation in N-process SL media. Sputtering the magnetic layer at elevated temperature due to zero dwell time between Cr and magnetic layer deposition and the formation of preferred in-plane orientation in the magnetic layer contributed to the increase in Hc of N-process media. ML-media fabricated by two different sputtering processes with Cr-interlayer (Cr-IL) of 5–15 Å (1-chamber and 2-chamber) showed similar noise reduction: ∼30% at 153.4 KFCI recording density. In addition, for the comparable Hc and Mrt, the ML media sputtered by N process produced lower pulse width (PW-50), better overwrite (OW), and lower bit shift (BS).
Comparison of resonance conditions in double-barrier structures and triple-barrier structures
Authors: Zhao X. D. , Yamamoto H. , Chen Z. M. , Taniguchi K.
Published: July 2002

A theoretical analysis has been carried out to compare the resonance condition in the asymmetric double-barrier structure with that in the symmetric triple-barrier one. It is found that the triple-barrier structure may be considered as two quasidouble-barrier structures whose resonance condition may decide the resonance levels in the triple-barrier structure. It is confirmed that two modes exist in the triple-barrier structure: one is normal mode consisting of doublet and the other is degenerated mode of singlet. The critical condition between the two modes is given and its physical meaning is examined. Moreover, it is confirmed that both the normal mode and the degenerated mode exist also in symmetric n-fold barrier structures (n≥3).
Dopant-induced lattice dilation in n-type InP homoepitaxial layers
Authors: Ferrari C. , Franzosi P.
Published: July 2002

n-type Si- and Sn-doped InP homoepitaxial layers have been grown on (001) Fe-doped semi-insulating InP substrates by metal organic vapor phase epitaxy and liquid phase epitaxy, respectively. The net carrier concentration has been determined by conventional Hall measurements, while the total dopant concentration has been evaluated by secondary ion mass spectrometry. The change in lattice constant has been carefully measured by x-ray double crystal diffractometry as a function of the dopant concentration. A lattice dilation has been observed for both dopants, but the Sn doping has been found to be about eight times less effective than the Si doping. The results could not be explained by Vegard’s law.