bluuebunny/crossref_metadata_2025 huggingface dataset:
Grain morphology and magnetic properties of CoCrPt/Cr(Si,Ti) films sputtered at room temperature
Authors: Choe G.
Published: July 2002

CoCrPt films were deposited on Cr-Si and Cr-Ti underlayers at room temperature with variations in Si and Ti concentration. Magnetic properties including Hc and S* were improved by Cr-Si and Cr-Ti underlayers. CoCrPt films deposited on a Cr-Si underlayer showed uniformly grown Co grains without large chains of grouped fine grains that were observed for the films deposited on a Cr underlayer, thus resulting in the increased S*. The Cr-Ti films showed changes in crystallography with increasing Ti content and thus affected the growth texture of Co grains. It is suggested that high performance CoCrPt recording media can be fabricated at room temperature by modifying the Cr underlayer films.
Magnetic properties of melt-quenched Ni-rich amorphous and bcc Zr–Ni alloys (abstract)
Authors: Kiss László F. , Varga Lajos K. , Bakonyi Imre
Published: July 2002

It has been demonstrated recently that the structure of melt-quenched Zr–Ni ribbons in the vicinity of 90 at. % Ni was strongly dependent on the quenching rate: at the highest quenching rate, the alloys Zr10Ni90 and Zr9Ni91 could be prepared with an amorphous structure whereas at lower quenching rates the Zr9Ni91 alloy was prepared as a solid solution of Zr in Ni with a body-centered cubic (bcc) structure. In a preliminary measurement we have established a Curie point (Tc) of 66 K for the a-Zr9Ni91 ribbon which was much smaller than the value Tc=235 K reported for an a-Zr10Ni90 ribbon. This discrepancy has motivated us to perform the present detailed magnetization study in the temperature range 5 to 300 K. It was also of interest to compare the Curie point of the amorphous and bcc phase with the same chemical composition. The real component χ′ of the AC susceptibility as well as the magnetization isotherms up to H=18.5 kOe have been investigated. The measurements of χ′ yielded T=41, 66, and 70 K for a-Zr10Ni90, a-Zr9Ni91, and bcc-Zr9Ni91 ribbons, respectively. The para-ferromagnetic transitions were found to be fairly sharp although in some of these samples several transitions differing by a few degrees only could be observed. In some of the samples, however, very faint magnetic transitions indicated by a small broad peak could also be detected around 250 K. This feature can be attributed to some residual phases of either an unknown Zr–Ni intermetallic compound or of Ni(Zr) solid solution precipitates.

The close coincidence of this peak with the Curie temperature reported by Kaul suggests that his value (Tc=235 K) cannot be representative of the amorphous Zr–Ni matrix but it rather may originate from ferromagnetic precipitates. The magnetization isotherms for the amorphous and bcc-Zr9Ni91 alloy indicated the development of a significant magnetization around the Tc values deduced from χ′. The saturation magnetization at the lowest temperature amounted to about 7 emu/g in both structural modifications. This value is reasonable in view of the reduction of Tc with respect to pure fcc-Ni. The present Tc data indicate that the critical concentration for the onset of magnetic ordering in the Zr–Ni system should be slightly below 90 at. % Ni. It can be concluded from this study that the magnetic behavior of Zr–Ni alloys at about 90 at. % Ni content is characteristic for very weak itinerant ferromagnets and the results will be analyzed in this framework with the help of Arrott plots.
Measurement of clottability of fibrin polymers using magnetic orientation
Authors: Iwasaka M. , Ueno S. , Tsuda H.
Published: July 2002

Fibrin polymers, as a kind of diamagnetic material, are oriented parallel to the direction of magnetic fields. We investigated the polymerization of partially digested fibrinogens in an 8 T magnetic field in order to observe the clotting ability of various sizes of fibrinogen fractions using a magnetic orientation technique. We purified high-molecular weight fraction (F1) and low-molecular weight fraction from human fibrinogen (F2). Fibrin gels were formed in an 8 T magnetic field for 9 h, and transmittancies were measured to evaluate the degree of magnetic orientation. The results show that a lack at the C-terminal half of one Aα chain did not affect the magnetic orientation of fibrin. We also investigated the effect of the digestion of fibrinogen by plasmin on the magnetic orientation of fibrin. The result shows that partially digested fibrin molecules also orient in an 8 T magnetic field. However, the degree of magnetic orientation significantly decreases when fragment X and fragment Y appear.
Influence of the plastic anisotropy on the magnetic properties of a nonoriented 3% silicon iron
Authors: Hug E. , Hubert O. , Clavel M.
Published: July 2002

Modifications of the magnetic properties of a nonoriented 3% SiFe alloy with plastic strains are reported. The samples have been tested along rolling and transverse directions lying in the sheet plane by means of a suitable experimental system of measurements using a 50 Hz sinusoidal magnetic field. The magnetic properties markedly deteriorate when measurements are carried out in the direction of the applied stress. This phenomenon is less important perpendicular to the applied stress. Quasistatic experiments (f=0.1 Hz) have been accomplished to separate the total losses into hysteresis and dynamic components. Hysteresis losses strongly increase and dynamic losses sensitively decrease with the plastic deformation whatever the direction in sheet plane. Compression-tension fatigue tests carried out have shown that the material displays a strong kinematic strengthening behavior representative of internal and long range stresses in the strained sample. The degradation of the magnetic properties of nonoriented 3% SiFe alloys with plastic strains is the result of two effects: one due to dislocations and another due to internal stresses. The latter display along the direction of magnetic measurements the same effects as an outside elastic stress.
Transmission electron microscopy and reflected high-energy electron-diffraction investigation of plastic relaxation in doped and undoped ZnSe/GaAs(001)
Authors: Rosenauer A. , Reisinger T. , Franzen F. , Schütz G. , Hahn B. , Wolf K. , Zweck J. , Gebhardt W.
Published: July 2002

We report on reflected high-energy electron-diffraction and transmission electron microscopy plane-view investigation of the dislocation structure in doped and undoped ZnSe/GaAs(001) grown by molecular-beam epitaxy and metal-organic vapor-phase epitaxy. The thicknesses of the investigated layers vary between 60 and 900 nm. Several stages of dislocation formation are found which occur at distinct layer thicknesses. Frank partial dislocations (up to 500 nm), Shockley partial dislocations (between 130 and 400 nm) with a maximum density at 300 nm, and perfect 60° dislocations (above 300 nm) are observed in samples with perfectly smooth surface. The formation of Shockley partial dislocations is strongly anisotropic which might be due to the higher mobility of α-type dislocations. An increased roughness of the growing surface yields a suppression of Shockley partial dislocations and an irregular dislocation network with dislocations inclined to the 〈110〉 directions. A regular dislocation network with straight dislocations is found in Cl-doped samples.
Neutron diffraction structural study of Ce2Fe17−xGax
Authors: Luo H. , Hu Z. , Yelon W. B. , Mishra S. R. , Long G. J. , Pringle O. A. , Middleton D. P. , Buschow K. H. J.
Published: July 2002

Six samples of Ce2Fe17−xGax with nominal Ga content x equal to 0, 0.3, 0.5, 0.7, 1.0, 2.0 have been studied by powder neutron diffraction at room temperature. Both crystalline and magnetic refinements have been carried out. All six samples adopt the Th2Zn17-type rhombohedral structure. The only additional phase found is α-iron. Gallium atoms are found to have high affinity for the iron 18h site, and are absent from the 9d and 18f sites. The Ga substitution for Fe leads to an expansion of both the a and c axes. The Curie temperature increases from 238 K for Ce2Fe17 to 406 K for Ce2Fe15Ga2. Magnetic refinements on the samples with x=0.3, 0.5, 0.7, 1.0, and 2.0 reveal that the magnetic moments of the four Fe sites are in the basal plane and that their values increase with increasing Ga content.
Cobalt-doped carbon nanotubes: Preparation, texture, and magnetic properties
Authors: Lafdi K. , Chin A. , Ali N. , Despres J. F.
Published: July 2002

The nested carbon nanotubes discovered by Ijima and co-workers as a by-product of fullerene production extended the dimension and geometries of fullerenes into the domain of nanoparticles. These nanotubes are fiberlike structures consisting of concentric graphite sheets nested along the axis extending as long as several micrometers, and diameters of these nanotubes are limited to less than 100 nm. In this paper we report preparation, structural and magnetic properties of cobalt-doped carbon nanotubes. We observed a tubular structure made of coaxial graphite sheets. The inside, of these carbon nanotubes, was filled with cobalt particles. The magnetic properties of the Co-doped nanotubes were measured using a SQUID magnetometer. The undoped (pure) carbon nanotubes exhibit diamagnetic behavior. The Co-doped nanotubes exhibit superparamagnetic behavior with saturation of magnetization at about 0.5 T and coercive field of 750 G.
Experimental studies of nonlinearities in MR heads
Authors: Xing Xinzhi , Bertram H. Neal
Published: July 2002

Magnetoresistive (MR) heads have been widely developed for high density digital magnetic recording. The nonlinear MR head response causes nonlinear distortion in the playback signals, and this results in a degradation of system performance. In this work, techniques for identifying the nonlinearity due to the playback process have been developed. Different levels of nonlinear distortion in the playback signals were observed for different head/disk combinations. The behavior of the MR nonlinear distortion as a function of flying height and recording density was also examined.
Bi–DyIG films: Enhancement of the Dy contribution to the Faraday rotation
Authors: Guillot M. , Le Gall H. , Gouzerh J. , Desvignes J. M. , Artinian M.
Published: July 2002

The Faraday rotation of epitaxial garnet films of composition {R3−xBix}[Fe2](Fe3)O12 with R=Dy, Y, and high Bi content (x≂1.30) has been measured in the temperature range 10 K<T<600 K at λ=1155 nm. The analysis of the data in terms of the sublattice magnetization reveals a strong contribution of the {Dy} sublattice. The ‘‘electric’’ dipole Faraday rotation per Dy ion has been determined and compared to its value in the bismuth-free dysprosium garnet. It turns out that Bi gives rise to a pronounced enhancement of this contribution. This result is compared with that observed for other Bi-substituted rare-earth iron garnets in the visible band.
New magnetic phases of Fe on fcc Co(001) and Ni(001)
Authors: O’Brien W. L. , Tonner B. P.
Published: July 2002

A sequence of three distinct magnetic phases of Fe are found for growth on both fcc Co(001) and Ni(001) using x-ray magnetic circular dichroism. For Fe coverages below 5 ML the films are ferromagnetic, with the magnetic moments aligned perpendicular to the surface for growth on Ni and parallel to the surface for growth on Co. Between 5 and 11 ML the Fe films are nonferromagnetic at room temperature. Above 11 ML the Fe films are once again ferromagnetic. An identical sequence of magnetic phase transitions is known to occur for Fe growth on Cu(001). Based on this comparison and on the nearly identical lateral lattice constants of fcc Co, Ni, and Cu, we conclude that Fe growth on fcc Co(001) and Ni(001) follows the same sequence of crystalline phase transitions as Fe growth on Cu(001).
Microstructure and magnetic properties of Cu0.8(Fe1−xCox)0.2 alloy powders manufactured by a mechanical alloying process
Authors: Yoo Yong Goo , Yu Seong Cho , Kim Won Tae
Published: July 2002

Changes in structural and magnetic properties during mechanical alloying and heat treatment were studied in Cu0.8(Fe1−xCox)0.2 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) alloys using x-ray diffractometry, transmission electron microscopy, and magnetometry. Supersaturated solid solutions with a fcc crystal structure were obtained in all alloys by mechanical alloying. The grain sizes of the solid solutions were about 20 nm. Magnetization of the supersaturated solid solutions could be explained by a mixture of two types of particles showing paramagnetism and ferromagnetism. The variation in magnetization with Co content in the solutions was similar to the Slater–Pauling curve. Fe–Co with a bcc structure precipitates during annealing of the Cu0.8Fe0.1Co0.1 alloy. With increasing annealing temperature ferromagnetic behavior becomes dominant due to a coarsening of the precipitates.
The magnetomechanical effect in electrolytic iron
Authors: Devine M. K. , Jiles D. C.
Published: July 2002

The effect of stress on the magnetization of high purity iron has been studied systematically at different locations on the M,H plane corresponding to a variety of magnetic states of the material. The results confirm earlier studies that show the sign of the derivative dM/dσ changes at different locations on the M,H plane, and can even change as a result of a monotonic increase of stress either in compression or tension. The observed behavior can be explained on the basis of a recent theory which predicts that the irreversible changes in magnetization resulting from changes in applied stress cause the magnetization to approach the anhysteretic magnetization curve at the given applied field strength.
Ferromagnetism of Y2(Fe1−yCoy)17−xAlx
Authors: Ohkubo N. , Kanematsu K.
Published: July 2002

Magnetization and x-ray measurements of alloy systems, Y2(Fe1−yCoy)17−xAlx (y=1/3,2/3), were performed and the magnetic and crystallographic data are shown with the results of the systems where y=0 and 1.0. The crystal structure of all alloys changes from hexagonal Th2Ni17 type to rhombohedral Th2Zn17 type with increasing aluminum concentration. The average magnetic moments of the 3d metal atom of all alloy systems decrease with increasing aluminum concentration. The Curie temperatures of the alloy systems where y=1/3, 2/3, and 1.0 decrease smoothly with increasing aluminum concentration. On the other hand, the system where y=0 at first increases and then decreases. These magnetic properties are discussed by Stoner’s model.
Viscosity of Hg0.84Zn0.16Te pseudobinary melt
Authors: Mazuruk K. , Su Ching-Hua , Sha Yi-Gao , Lehoczky S. L.
Published: July 2002

An oscillating-cup viscometer was developed to measure viscosity of molten HgZnTe ternary semiconductor alloys. Data were collected for the pseudobinary Hg0.84Zn0.16Te melt between 770 and 850 °C. The kinematic viscosity was found to vary from approximately 1.1 to 1.4×10−3 cm2 s−1. A slow relaxation phenomena was also observed for temperatures from the melting point of 770 to ∼800 °C. Possible mechanisms for this effect are discussed.
Electrical characteristics of heteroprocessed Pb(Zr0.52Ti0.48)O3 films
Authors: Jung Hyung Jin , Kim Tae Song
Published: July 2002

Sol-gel derived 10% Pb excess Pb(Zr0.52Ti0.48)O3 (PZT) layers placed at the top and bottom of rf-sputtered PZT films are very effective in decreasing the microcracks occurring on the surface of rf-sputtered PZT films during postannealing at 600, 650, and 700 °C. The deposition of PZT films is conducted on Pt/Ti/SiO2/Si(100) substrates at room temperature. The surface microstructure and crystallographic structure of buffered PZT films are investigated through scanning electron microscopy and x-ray-diffraction analysis. In addition, electrical measurements are also examined through hysteresis, dielectric constant and loss, J–V, and fatigue measurements. The P–E hysteresis measurements show an increase in the remanent polarization (12.1, 22.45, 34.64 μC/cm2) and a decrease in the coercive fields (108, 72.9, 68.3 kV/cm) with the increase of postannealing temperature. Dielectric constant and tan δ are varied from 523 to 1330 and 0.0274 to 0.0738 with the increase of annealing temperature. By applying 1 MHz rectangular pulse up to 5×1010 cycles for fatigue test, the 33% reduction of polarization is also observed.
Application of the finite-element method and the finite-difference method to photothermal inspections
Authors: Goch G. , Reigl M.
Published: July 2002

Theoretical description of photothermal processes require analytical solution to the thermal diffusion equation. This solution takes great efforts, and in many cases an analytical expression is not accessible. In this article, additional solution methods for the thermal diffusion equation, especially for photothermal problems, are presented. These methods are the finite-element method and the finite-difference method; the latter is described in detail. They enable a closed theory for all photothermal processes using a modulated light source and measurements in the equilibrium state. The numerics are verified by comparison with some analytically solved problems. The main discussion topic is surface hardened steel samples. Here, some well-known models for hardness profiles and a new approach, calculated by the finite-difference method, are discussed.
Modeling a backgated GaAs metal–semiconductor–metal photodetector
Authors: Hurd C. M. , McKinnon W. R.
Published: July 2002

We use a two-dimensional, drift-diffusion calculation to illustrate the physics behind the recently described GaAs metal–semiconductor–metal photodetector with an ohmic backgate provided by a p-doped layer. We calculate the transient response of this structure to a pulse of illumination. According to these simulations, the speed of the falling side of the response is improved by the backgate, which removes photogenerated holes from the active layer, but the degree of improvement depends on the chosen contact. The fastest fall time is found in the current at the cathode when both the cathode and backgate are grounded. We show why this is faster than the response of the current at either the anode or the backgate, and why this advantage is practically lost when the backgate is biased.
A physical interpretation of the far-field directivity pattern of lithium niobate bar transducers
Authors: Assaad Jamal , Bruneel Christian
Published: July 2002

In acoustic imaging and nondestructive testing applications, the quality of images obtained using linear arrays is closely related to the far-field directivity pattern of an elementary transducer bar. But, for some values of width-to-thickness ratio W/T (W being the width and T the thickness) these transducers exhibit parasitic modes with resonance frequencies close to their working frequency. In order to analyze and identify these modes and their parasitic radiations, the finite element method is used with the help of an original extrapolation method to compute the far-field radiation pattern. Measured far-field directivity patterns for (YZw)36° transducer bar of LiNbO3 are presented for a large spectrum of W/T. To get a better understanding of the elementary transducer behavior, an approximate directivity pattern is obtained using a simple method and is compared to the numerical and measured directivities. This simple physical model is based on Sommerfeld’s formula. It is shown that for some values of W/T, harmonic vibrations at the surface of transducer can be coupled to the fluid domain and produce a parasitic radiation (for example, for W/T=1, the first harmonic vibration introduces a strong parasitic radiation).
Low temperature electroluminescence spectroscopy of high electron mobility transistors on InP
Authors: Sylvestre A. , Aniel F. , Boucaud P. , Julien F. H. , Crozat P. , De Lustrac A. , Adde R. , Jin Y. , Praseuth J. P.
Published: July 2002

Electroluminescence spectroscopy of short gate high-electron-mobility transistors (HEMTs) on InP substrates is performed at cryogenic temperatures. Electroluminescence is a reliable tool to investigate impact ionization as compared to studies based on gate current which depend on the weakness of the intrinsic gate current intensity. In on-state biased devices, a low energy (0.7–0.9 eV) recombination band is observed which is related to radiative recombination of carriers created by impact ionization in the low band gap InGaAs channel. The evolution of the luminescence intensity versus bias applied to the device shows that the electroluminescence intensity and impact ionization depend on two competing parameters: the electric field in the gate–drain access area and the drain current intensity. We show that the so-called ‘‘kink’’ effect, which is a noticeable increase of the output conductance and which is observed at relatively moderate drain bias (600–750 mV) in our devices, is not correlated with impact ionization. The electroluminescence of the device in the off state is also investigated. This study allows the direct observation of impact ionization in the off-biased channel of InP-based HEMTs. In this low current regime, the electroluminescence intensity follows the electric field, i.e., the drain–gate voltage until breakdown of the device occurs. The voltage breakdown of the device in the off state is discussed in terms of impact ionization in the InGaAs channel due to hot carriers originating from the gate leakage current. Finally, a method for reducing or avoiding impact ionization in these devices, i.e., for increasing the device reliability, is discussed.
Photoluminescence and its decay of the dye/porous-silicon composite system
Authors: Li Peng , Li Qingshan , Ma Yurong , Fang Rongchuan
Published: July 2002

Photoluminescence (PL) and its decay in the composite system of dye/porous silicon, consisting of oxidized porous silicon (Psi) and impregnated dyes, have been studied. We find that the PL spectra of the dyes are similar to that of dyes in ethanol solution while being quite different from those of dye films absorbed on the surface of c-Si. The line shape of the spectra is more symmetric for impregnated dyes than that in solution. The PL decay of the impregnated dyes has been measured. The lifetime is about 100 ps for rhodamine B impregnated in as-prepared Psi and it becomes much shorter for that in the annealed Psi layers. The influence of absorbed dyes on the PL decay of Psi has also been measured. It shows that impregnated dyes provide new nonradiative processes for the deactivation of the excited Psi.
Epitaxial Si(001) grown at 80–750 °C by ion-beam sputter deposition: Crystal growth, doping, and electronic properties
Authors: Lee N.-E. , Xue G. , Greene J. E.
Published: July 2002

Epitaxial undoped and Sb-doped Si films have been grown on Si(001) substrates at temperatures Ts between 80 and 750 °C by ultrahigh-vacuum Kr+-ion-beam sputter deposition (IBSD). Critical epitaxial thicknesses te in undoped films were found to range from 8 nm at Ts=80 °C to ≳1.2 μm at Ts≥300 °C, while Sb incorporation probabilities σSb varied from unity at Ts≲550 °C to ≂0.1 at 750 °C. These te and σSb values are approximately one and one to three orders of magnitude, respectively, higher than reported results achieved with molecular-beam epitaxy. Temperature-dependent transport measurements carried out on 1-μm-thick Sb-doped IBSD layers grown at Ts≥350 °C showed that Sb was incorporated into substitutional sites with complete electrical activity and that electron mobilities in films grown at Ts≥400 °C were equal to the best reported results for bulk Si.
Hole-trapping sites and the mechanism of the photostimulated luminescence of the x-ray storage phosphor RbI:Tl+
Authors: Rogulis U. , Dietze C. , Pawlik Th. , Hangleiter Th. , Spaeth J.-M.
Published: July 2002

Radiation damage centers generated by x irradiation at 80 and 300 K in the storage phosphor RbI:Tl+ were investigated with optically detected magnetic resonance and optical measurements. It is shown that the intensity of the photostimulated luminescence depends on the concentration of Tl2+ centers. Bleaching into any of the seven identified Tl2+ absorption bands destroys the Tl2+ centers and leads to a Tl+ emission as well as to VK centers. Proportionally to photodestruction of Tl2+ centers, the photostimulated luminescence is decreased. The traps for information storage in RbI:Tl+ are F centers and Tl2+ centers.
Plastification during sorption of polymeric thin films: A quartz resonator study
Authors: Domack Arno , Johannsmann Diethelm
Published: July 2002

Employing impedance analysis of quartz resonators, we have monitored the complex viscoelastic compliance of a 1.67-μm-thick film of poly(isobutylene) during swelling in toluene vapor. The optical thickness was simultaneously measured by ellipsometry. The acoustic model used for analysis is discussed. We can quantitatively derive the shear modulus. The results show a decrease of the glass transition temperature of about 60 °C. We suggest that the method is of high value for the investigation of sorption and drying kinetics in the regime of low solvent content.
Screening effects on the degree of ionization and hole mobility for p-type Si and Ge
Authors: Chun S. K.
Published: July 2002

The degree of ionizations for Si and Ge with boron doping are calculated by taking into account the screening effect on the impurity energy level. The hole mobilities are then calculated as a function of doping concentration using the relaxation time approximation. Due to the screening effect on the impurity energy level, the degree of ionization increases over the entire temperature range. The ionized impurity scattering increases while the neutral impurity scattering decreases, and therefore the calculated mobilities for both Si and Ge agree well with the experimental data.
Magnetism and structure of Dy/Fe multilayers studied by 57Fe and 161Dy Mössbauer spectroscopy
Authors: Tappert J. , Keune W. , Brand R. A. , Vulliet P. , Sanchez J.-P. , Shinjo T.
Published: July 2002

Dy(48 Å)/Fe(40 Å) multilayers were investigated by 57Fe and 161Dy Mössbauer spectroscopy in zero external field between 4.2 and 300 K. The 57Fe spectra could be analyzed in terms of a pure bcc-Fe contribution and an intermixed interface contribution. The latter is assigned to an amorphous Dy1−xFex interface alloy with an estimated most-probable composition x≊0.78. The bcc-Fe phase shows a broad spin-reorientation transition from a preferentially in-plane-oriented Fe-spin direction near 300 K to a more out-of-plane-oriented spin texture at 4.3 K characterized by an average misalignment angle of ∼52° relative to the sample-plane normal. The value of the average 161Dy hyperfine field at 4.2 K (583.0 T) is slightly larger than the value of bulk Dy metal, but is smaller than those of Dy–Fe intermetallics. The average orientation of Dy moments which was found to be random at 4.2 K may be the reason for the misalignment of Fe magnetic moments at low temperature. The magnetic ordering temperatures of the Dy layers were determined by superconducting quantum interference device magnetometry to be TC=110±15 K and TN=200±20 K, the latter value being in agreement with that estimated from 161Dy spectra.
Photoluminescence study of Al doping in GaAs grown by molecular-beam epitaxy
Authors: Qurashi Umar S. , Iqbal M. Zafar , Andersson T. G.
Published: July 2002

A detailed photoluminescence investigation of the effects of Al doping on GaAs grown by molecular-beam epitaxy is reported. Materials with 0.1%, 1%, and 3% Al doping have been studied. Viewed as an AlxGa1−xAs alloy semiconductor, our study on these materials presents results on the optical characteristics of the lowest Al composition material reported to date. Photoluminescence spectra show three major peaks. Detailed measurements on the dependence of these spectra on temperature and excitation power have been carried out to characterize the various transitions responsible for the luminescence peaks. A near-band-edge exciton peak, a carbon acceptor-related free-to-bound transition, and a defect complex-related luminescence peak, probably involving silicon, are identified as the main features of our luminescence spectra. The integrated luminescence intensity shows a strong quenching with the increase in Al doping, thus supporting the increase of nonradiative deep-level centers with Al doping, concluded from our earlier reported deep level transient spectroscopy study. The relative intensity of the excitonic peak shows a stronger decrease than that of the other luminescence features. New data on the shift of the excitonic transition energy with Al content x in AlxGa1−xAs, for the lowest x values yet reported, are provided by our photoluminescence study.
Evaluation of diffusion length from a planar-collector-geometry electron-beam-induced current profile
Authors: Luke Keung L.
Published: July 2002

The subject of this article is the determination of the minority carrier diffusion length L from a planar-collector-geometry electron-beam-induced current (EBIC) profile. Among extant techniques based on the analysis of a ln[IEBIC(x1)xα1] vs x1 plot, (x1 is beam-to-collector distance, and α is a constant), we find two major problems that demand immediate attention. First, the most widely used technique is found to be based on an invalid assumption, which results in the intrusion of large systematic errors into the values of L and surface recombination velocity sT. Therefore, this technique in its present form is no longer usable. Second, all these techniques are asymptotic (x1≳2L), a matter of great concern to experimentalists because large x1 means small signal-to-noise ratio and the proximity of adjacent active regions. We devise a nonasymptotic technique, based also on the analysis of a ln [IEBIC(x1)xα1] vs x1 plot, to evaluate L from a region close to the collector, as close as one-half, but no farther than two diffusion lengths away. This technique is applied to four separate sets of experimental EBIC profile data derived from three independent publications. In each case, L evaluated is either the same or nearly the same as the one obtained by curve fitting. We also show how this technique can be used to simultaneously evaluate L and η (=sTL/D,D=diffusion coefficient).
Influence of the laser wavelength on the microstructure of laser ablated Ba0.5Sr0.5TiO3 films
Authors: Tcheliebou Frédéric , Baik Sunggi
Published: July 2002

Barium strontium titanate (Ba0.5Sr0.5TiO3 or BST) films were deposited on the (1 1̄ 02)-oriented sapphire substrates using a Nd:YAG laser operating in its second and third harmonics (532 and 355 nm, respectively) and a KrF excimer laser (248 nm). In the same growth conditions, the overall films have the cubic perovskite structure and show randomly oriented polycrystalline BST for the films deposited at laser wavelengths of 532 and 355 nm. The films deposited with the radiation of 248 nm wavelength are preferentially oriented along the [111] direction. Scanning electron micrographs reveal that the film surface is covered by particles whose size and density are laser wavelength dependent. The better film morphology (smaller and fewer particulates) is obtained with a laser wavelength of 248 nm. BST absorbance spectrum measured in the region covering the laser wavelengths used (UV-visible) shows strong variation, with high absorption coefficient in the UV. The influence of the processing variables such as the laser characteristics, and target physical properties during the ablation deposition is discussed. In the high peak power density regime (∼108 W/cm2), we have found that the laser wavelength, through the target absorption coefficient, affects strongly the particulate size and density. Therefore, short laser wavelength is proven as an alternative way to solve the large particulate issue in the case of BST thin films.
Magneto-optical dispersion of Si1−xGex epitaxial layers and Si/Ge superlattices
Authors: Vergöhl Michael , Dettmer Klaus , Kessler Franz Rudolf
Published: July 2002

The magneto-optical dispersion of Si1−xGex mixed crystal films on (001)-Si substrate and an asymmetric (Si16Ge10)33 superlattice were investigated. Optical measurements at B=0 (reflectivity and wavelength modulated reflectivity) were performed in order to fit the film thickness, the value of x, and the field free optical constants. Using these optical constants at B=10 T, the off-diagonal element of the dielectric tensor was evaluated numerically from the complex magneto-optical Kerr effect in the spectral region from about 1.5 up to 4.0 eV photon energy. The spectra for the off-diagonal element of the dielectric tensor and the spectra of the differential dielectric function were modelled with analytical functions following the theoretical background. Within the framework of a multi-component fitting of the dielectric tensor, a significant improvement of resolution of the relevant interband critical points (E1, E1+Δ1, E0′) is performed. The dependencies of the relevant transition energies as well as the sum-g-factors on the composition parameter x were determined. From the difference between the ḡ value of E1 and that of (E1+Δ1), the concentration dependence of the interband matrix element was calculated as Π(x)=0.06+0.3×x2. For the superlattice, a value quite similar to that of Π(x=0.42) was found.
Noise correlation measurements in bipolar transistors. I. Theoretical expressions and extracted current spectral densities
Authors: Jarrix S. , Delseny C. , Pascal F. , Lecoy G.
Published: July 2002

Low-frequency noise measurements are performed on classical Si bipolar transistors and on AlGaAs/GaAs heterojunction bipolar transistors (HBTs) mounted in a common-emitter configuration. Expressions for the spectral densities are derived taking into account a correlation between base and collector noise sources ib and ic. Values of emitter series resistances and of the base ideality factor are determined from these noise measurements. Then the spectral densities related to ib and ic as well as the cross-spectrum are extracted. In the case of classical Si transistors, the excess noise is attributed to the current noise source ib. The effect of the base series resistance is shown on the white noise. For the HBTs, the white noise is not reached. The excess noise is attributed to the correlated current noise sources ib and ic. From the analysis of the current spectral densities with base current the increase of correlation with bias is revealed. The extracted current spectral densities provide the foundation for the calculation of the coherence function associated to base and collector currents. This coherence function is presented in part II. The shape of the curves shows correlation phenomena to be mainly related to generation-recombination components. Also in part II, the spectral densities associated with the correlated and uncorrelated part of the collector current are extracted and studied versus bias and geometry.
Excited-state absorption measurement in Ge-doped SiO2 glass
Authors: Fujimaki Makoto , Seol Kwang Soo , Ohki Yoshimichi
Published: July 2002

Optical absorption change in the microsecond order in oxygen-deficient Ge-doped SiO2 glass was measured as a function of time just after photon irradiation from a KrF excimer laser. The absorption above 3 eV was found to decay with a similar time constant as that of the luminescence at 3.1 eV. From this, it is confirmed that the observed absorption change is due to the excitation of electrons from the lowest excited triplet state to an upper state. By taking account of the energy range of the absorption, there is a high possibility that the upper state is the conduction band.
High electronic excitations and ion beam mixing effects in high energy ion irradiated Fe/Si multilayers
Authors: Bauer Ph. , Dufour C. , Jaouen C. , Marchal G. , Pacaud J. , Grilhé J. , Jousset J. C.
Published: July 2002

Mössbauer spectroscopy (57Fe) shows evidence for mixing effects induced by electronic energy deposition in nanoscale Fe/Si multilayers irradiated with swift heavy ions. A decrease in the mixing efficiency with electronic stopping power is reported; a threshold is found, under which iron environment modifications no longer occur. The kinetics of Fe–Si phase formation after irradiation suggests the existence of three regimes: (i) for high excitation levels, a magnetic amorphous phase is formed directly in the wake of the incoming ion and an almost complete mixing is reached at low fluence (1013 U/cm2); (ii) for low excitation levels, a paramagnetic Si-rich amorphous phase is favored at the interface while crystalline iron subsists at high fluences; (iii) for intermediate excitation levels, saturation effects are observed and the formation rate of both magnetic and paramagnetic phases points to direct mixing in the ion wake but with a reduced track length in comparison to U irradiation. The measured interfacial mixing cross section induced by electronic energy deposition suggests that a thermal diffusion process is mainly involved in addition to damage creation.
A theoretical analysis of the picosecond and sub-picosecond infrared-absorption spectroscopy of hot holes in germanium
Authors: Supancic P. , Schullatz M. , Hohenester U. , Kocevar P. , Rota L.
Published: July 2002

We present an ensemble Monte Carlo study of the thermalization and relaxation of photoexcited heavy and light holes in gallium doped p-type germanium for doping densities of 1×1016 and 7×1016 cm−3. We analyze recently published data obtained by picosecond excite-and-probe absorption spectroscopy in the mid infrared, i.e., with excitation energies of 124 and 136 meV, and at lattice temperatures of 30, 80, and 300 K. The Monte Carlo simulations contain all types of intraband and interband hole–hole and hole–optical–phonon scatterings, as well as free-carrier-induced ionizations of neutral acceptors and captures of free holes by ionized acceptors. Good agreement is found between the simulated and the measured absorption changes as functions of time. Quite generally it turns out that the initially created nonthermal features in the heavy- and light-hole distribution functions persist to times of typically twice the pump–pulse duration, thereby noticeably contributing to the initial nonlinear response. For longer times, the practically thermalized distributions give absorption changes which exclusively reflect the cooling dynamics of the heavy holes.
Improved pseudomorphic high electron mobility transistor structures on InGaAs substrates
Authors: Hoke W. E. , Lyman P. S. , Mosca J. J. , Hendriks H. T. , Torabi A. , Bonner W. A. , Lent B. , Chou L.-J. , Hsieh K. C.
Published: July 2002

Single and double pulse doped pseudomorphic high electron mobility transistor structures with 110-Å-thick InGaAs channel layers have been grown on InxGa1−xAs substrates (x=0.04; 0.065) and GaAs substrates. For In0.23Ga0.77As channel layers, higher electron mobilities were obtained on In0.04Ga0.96As substrates due to reduced strain. Transmission electron microscopy micrographs on a GaAs-based sample exhibited a roughened selectively doped heterojunction but no detected misfit dislocations. Pseudomorphic structures with In0.27Ga0.73As channel layers were also grown on In0.065Ga0.935As substrates with good transport and optical properties. The properties of the analogous structure grown on GaAs were severely degraded. Transmission electron microscopy micrographs on the GaAs sample showed a very rough selectively doped heterojunction with misfit dislocations.
Dynamic emission of screw dislocations from a propagating crack tip
Authors: Tsai Y. Z. , Lee Sanboh
Published: July 2002

The dynamic emission of screw dislocations from a propagating crack tip has been investigated. The crack velocity and velocity of each dislocation are assumed to be proportional to the third power of the stress intensity factor and the effective stress, respectively. The stress intensity factor for the ith dislocation emission is derived according to the spontaneous emission criterion. It is found that the stress intensity factor for dislocation emission is independent of crack mobility but the number of dislocations emitted decreases with increasing crack mobility. The number of emitted dislocations and the size of the plastic zone decrease but the maximum stress intensity factor for dislocation emission increases with increasing initial crack length. The size of the dislocation-free zone increases with increasing initial crack length for steady state. The dislocation–crack system reaches the steady state when the dislocations and crack tip move at the same velocity.
Scaling approaches to magnetization measurements of melt-textured (Y1−xPrx)Ba2Cu3O7−δ
Authors: Xiao Y. G. , Yin B. , Li J. W. , Zhao Z. X. , Fu X. K. , Ren H. T. , Xiao L.
Published: July 2002

Scaling approaches to magnetization measurements are explored on a set of melt-textured (Y1−xPrx)Ba2Cu3O7−δ samples with different Pr concentrations. When normalized with corresponding scaling parameters Hm, Mm, and Jm, respectively, magnetic hysteresis loops and related critical current densities for different temperatures from 4.5 to 75 K are all found to exhibit scaling behavior. It is shown that for low scaled field (h) region, this behavior can be described qualitatively well based on an extended critical-state model. The effective activation energy extracted from magnetic relaxation measurements can also be qualitatively described by a scaling relation within the framework of collective-pinning theory. In addition, Pr doping is found not to influence the scaling approaches themselves up to the highest level investigated in the present work. However, the scaled magnetic hysteresis loop width, critical current density, and effective activation energy are all increased by Pr doping, further suggesting the enhancement of flux pinning for our samples. Furthermore, experimental results and fitting parameters are discussed as well with respect to relevant scaling background models.
The effects of grain boundary diffusion anisotropy on via electromigration failure
Authors: Ghiti A. , O’Neill A. G.
Published: July 2002

Electromigration performance of multilevel interconnect vias is investigated using a three-dimensional computer model. The model uses the finite-element method to obtain self-consistently the temperature and current density distributions in order to calculate electromigration fluxes. The model includes the polycrystalline grain structure of the tracks as well as stress-migration and concentration gradient backfluxes. While in single level systems, failure can be analyzed with two-dimensional models because the fluxes are homogeneous, the inclusion of the third dimension along the track thickness is necessary for multilevel systems. In addition to the effects of hot spots, current crowding, and microstructure, it is found that the anisotropy of the grain boundary diffusion plays an important role in determining the locations of void formation. The microstructural details of the track at the interface with the via, including grain boundary geometry, are very important for electromigration studies.
Magnetic localization of free exciton magnetic polarons in diluted magnetic semiconductors
Authors: Miao J. , Stirner T. , Hagston W. E.
Published: July 2002

A model of free exciton magnetic polarons (EMPs) in diluted magnetic semiconductors is proposed on the basis of the localization of the hole in the magnetic potential well created by its own exchange field. The corresponding calculations of the free EMP binding energies, as a function of Mn2+ ion concentration and temperature, are performed. The results are in good agreement with recent experimental measurements. It is found that free EMPs are stable up to relatively high temperatures (T≲30 K) over a wide range of Mn concentrations x without any other primary localization, e.g., alloy potential fluctuations. The dependence of the critical temperature of free EMP formation on the Mn concentration is determined for various diluted magnetic semiconductor materials, and is in good agreement with the experimental results for Cd1−xMnxTe. The suppression of free EMPs under the application of an external magnetic field is also considered.
Generation of energetic He atom beams by a pulsed positive corona discharge
Authors: Lo Shui-Yin , Lobo Julio D. , Blumberg Seth , Dibble Theodore S. , Zhang Xu , Tsao Chun-Cheng , Okumura Mitchio
Published: July 2002

Time-of-flight measurements were made of neutral helium atom beams extracted from a repetitive, pulsed, positive-point corona discharge. Two strong neutral peaks, one fast and one slow, were observed, accompanied by a prompt photon peak and a fast ion peak. All peaks were correlated with the pulsing of the discharge. The two types of atoms appear to be formed by different mechanisms at different stages of the corona discharge. The fast atoms had energies of 190 eV and were formed at the onset of the pulsing, approximately 0.7 μs before the maximum of the photon peak. The slow peak, composed of electronically metastable He atoms, originated 30–50 μs after the photon pulse, and possessed a nearly thermal velocity distribution. The velocity distribution was typical of an undisturbed supersonic expansion with a stagnation temperature of 131 K and a speed ratio of 3.6. Peak intensities and velocities were measured as a function of source voltage, stagnation pressure, and skimmer voltage.
Electrical and photoluminescence properties of CuInSe2 single crystals
Authors: Schön J. H. , Arushanov E. , Kloc Ch. , Bucher E.
Published: July 2002

Electrical and photoluminescence measurements have been carried out on CuInSe2 single crystals. The observed temperature dependence of the Hall coefficient in n-type CuInSe2 single crystals is explained in assuming the existence of an impurity band. The values of the activation energy of the shallow donors, their concentration, and the concentration of the compensating acceptors were calculated. The values of the activation energy of the deep donors (80±10 meV and 110±10 meV) were estimated on the basis of the photoluminescence measurements. The concentration dependence of the activation energy of the shallow donor level and the variation of the Coulomb interaction as a function of the carrier density are determined.
Polarimeter for soft x rays (abstract)
Authors: Kachel T. , Holldack K. , Schäfers F. , Packe I.
Published: July 2002

During the past few years the magnetic circular dichroism effect (MCD) has become an increasingly important tool for studying magnetic materials and their surfaces. The employment of MCD is based on the use of circularly polarized soft x rays, which can be obtained from electron storage rings. Circularly polarized x rays appear either as off-plane bending magnet radiation or radiation from special undulator devices, e.g., crossed or helical ones. However, an unsolved problem has been the independent and simple measurement of the polarization of this radiation. At BESSY, we have developed a new compact polarimeter. The instrument consists of four major elements. There are (i) a pinhole to select the desired part of the photon emission cone and (ii) a 100-period Cr/C multilayer to serve as a monochromator with a spectral resolution power of 50. The angle of incidence is set to 15.9° to meet 718 eV photon energy that corresponds to a maximum MCD effect close to the Fe 2p threshold excitation energy. There are also (iii) a 50 nm Fe film on 3 μm Mylar foil to show the MCD effect in photon transmission (the foil is mounted under 45° to the photon beam and can be magnetized in the plane of the Fe film) and (iv) a Si-pin diode to measure the transmitted photon flux. The assembly fulfills the following requirements: (1) insensitive to linearly polarized and unpolarized light, (2) performance at high heat load, (3) reliability and long term stability also in air, (4) wavelength sensitivity, and (5) fast readout capability. In future work the polarimeter can be employed for the characterization of helical undulators such as the BESSY II UE56, which cover the range of the Fe 2p threshold. With foils of other transition metal or rare-earth materials, the sensitive photon energy range can be extended.