bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical activation and diffusivity of ion-implanted Zn acceptors in GaN under high-pressure, high-temperature annealing
Authors: Suski T. , Jun J. , Leszczyński M. , Teisseyre H. , Strite S. , Rockett A. , Pelzmann A. , Kamp M. , Ebeling K. J.
Published: July 2002

Optical activation of Zn ions implanted to an epitaxial film of GaN was performed by means of sample annealing at N2 overpressure up to 1.6 GPa. By applying pressure we avoided GaN decomposition and could increase the annealing temperature up to 1550 °C versus a limit of 1000–1100 °C at ambient pressure. The Zn-acceptor-related photoluminescence (PL) intensity in implanted samples is maximized by annealing above 1350 °C after which the Zn PL intensity exceeds epitaxially doped GaN:Zn with comparable Zn concentration by factor 15. High-pressure annealing causes a significant diffusion of implanted Zn atoms in GaN films. It is also possible to diffuse Zn from the external source into the implanted/unimplanted layers. High dislocation densities strongly accelarate the Zn diffusion.
High power radio frequency generation by relativistic beams in dielectric structures
Authors: Schoessow P. , Conde M. E. , Gai W. , Konecny R. , Power J. , Simpson J.
Published: July 2002

We have studied the interaction of a high current electron beam with dielectric loaded waveguides as a source of electromagnetic radiation. A unique high current photoinjector-based electron linac was used to generate the drive beam for these experiments, and the fields generated were diagnosed using a trailing probe (witness) beam from a second photocathode gun. Traveling wave dielectric structures with luminal (vphase=c) frequencies of 15 and 20 GHz were used. The radio frequency power levels generated in these initial experiments were very large—up to 11 MW.
The influence of surface currents on pattern-dependent charging and notching
Authors: Hwang Gyeong S. , Giapis Konstantinos P.
Published: July 2002

Surface charge dissipation on insulator surfaces can reduce local charging potentials thereby preventing ion trajectory deflection at the bottom of trenches that leads to lateral sidewall etching (notching). We perform detailed Monte Carlo simulations of pattern-dependent charging during etching in high-density plasmas with the maximum sustainable surface electric field as a parameter. Significant notching occurs for a threshold electric field as low as 0.5 MV/cm or 50 V/μm, which is reasonable for the surface of good insulators. The results support pattern-dependent charging as the leading cause of notching and suggest that the problem will disappear as trench widths are reduced.
Pulsed laser-induced single event upset and charge collection measurements as a function of optical penetration depth
Authors: Melinger Joseph S. , McMorrow Dale , Campbell A. B. , Buchner Stephen , Tran Lan Hu , Knudson A. R. , Curtice W. R.
Published: July 2002

We use picosecond laser pulses to investigate single event upsets and related fundamental charge collection mechanisms in semiconductor microelectronic devices and circuits. By varying the laser wavelength the incident laser pulses deposit charge tracks of variable length, which form an approximation to the charge tracks resulting from high energy space particle strikes. We show how variation of the charge track length deposited by laser pulses allows the mechanisms of charge collection in semiconductor devices to be probed in a sensitive manner. With the aid of computer simulations, new insight into charge collection mechanisms for metal–semiconductor field effect transistor (MESFET) devices and heterojunction bipolar transistor devices is found. In the case of the MESFET we point out the correlation between charge collection in the device and the ensuing single event upset in the composite circuit. In favorable cases, we show how probing circuits with tunable laser pulses can estimate a charge collection depth, which is a circuit parameter important for the prediction of error rates for circuits operating in a space-radiation environment.
Defect structure of carbon rich a-SiC:H films and the influence of gas and heat treatments
Authors: Friessnegg T. , Boudreau M. , Mascher P. , Knights A. , Simpson P. J. , Puff W.
Published: July 2002

A comprehensive study of carbon rich a-SiC:H films using optical absorption measurements, Fourier transform infrared spectroscopy, thermal desorption measurements, atomic force microscopy, and positron lifetime and Doppler-broadening techniques suggests that open volumes are formed in the films, due to incomplete breaking of the source molecule during film deposition. These open volumes are interconnected and can effectively trap gases from the ambient, during the film growth or after deposition. With increasing temperature the gases are desorbed from the internal surfaces of these open volumes and are released from the sample. This increases the areal density of the defects and is observable in positronium formation and annihilations of positrons with surface electrons. The growth of a nanocrystalline structure is observable upon annealing. At sufficiently high temperatures thermal breaking of Si–H and C–H bonds occurs and results in irreversible structural changes and film densification due to new C–C bond formation in the film.
Investigation of the evolution of single domain (111)B CdTe films by molecular beam epitaxy on miscut (001)Si substrate
Authors: Xin Y. , Browning N. D. , Rujirawat S. , Sivananthan S. , Chen Y. P. , Nellist P. D. , Pennycook S. J.
Published: July 2002

A comprehensive view of the microstructure of (111)B CdTe films grown on miscut (001)Si substrates by molecular beam epitaxy has been obtained by transmission electron microscopy and scanning transmission electron microscopy. It is found that in the initial growth stage, CdTe nucleates with a dominance of one particular domain: a domain with (111)B polarity and orientation of [11−2]CdTe//[1−10]Si, although there are also some other domains of different polarity and orientation. The dominance of one type domain is due to the reduction of the surface symmetry by using the miscut substrate and by using optimum growth conditions. As the growth proceeds, a single-crystal film is produced by the dominating domain overgrowing the minority domains nucleated at the film–substrate interface. This results in the final film of single-crystal character having (111)B polarity with [11−2]CdTe along [1−10]Si.
Electronic characterization of N,N′-bis(2-phenylethyl)perylene-3,4:9, 10-bis(dicarboximide) and its application to optical disks
Authors: Mizuguchi Jin
Published: July 2002

The title compound is a commercial black pigment based on the perylene skeleton. As evaporated, it exhibits a brilliant red color (absorption maximum: 500 nm). However, it undergoes a color change from red to black (absorption maxima: 473 and 610 nm) when exposed to acetone vapor or heated above 100 °C for several seconds. The electronic spectra have been characterized on the basis of the crystal structure and intermolecular interactions. The red color (500 nm) is due to randomly oriented, individual molecules in the amorphous phase; whereas the black phase is characterized by a quasiordered system which gives two absorption bands: one is due to individual molecules (473 nm) and the other is due to exciton coupling effects (610 nm). The color change is attributed to the appearance (“on” state) or disappearance (“off” state) of the absorption band around 610 nm and is applicable to optical disk systems based on an AlGaInP diode laser of 635 nm. An information storage system has been developed in which the absorption at 635 nm is switched on (red–black) or switched off (black–red) on irradiation with laser in the presence of a hydrazone compound that induces a phase change.
A prism coupler technique for characterizing thin film II–VI semiconductor systems
Authors: Peiris F. C. , Lee S. , Bindley U. , Furdyna J. K.
Published: July 2002

This article has two objectives. First, we determine the indices of refraction n of a series of molecular beam epitaxy-grown Zn1−xCdxSe epilayers using a technique based on coupling of evanescent waves via a prism into a semiconductor film. Highly precise values of n, and their dependence on the alloy composition x, are obtained for photon energies below the band gap of the alloy material. And second, we use these results to demonstrate the usefulness of the prism coupler method as a very reliable, convenient, and accurate tool for simultaneous determination of composition of semiconductor alloys in thin film form (since n depends on composition), and the film thickness. This method determines the film thickness with typical uncertainty of less than 0.5%.
Abnormal electrical behavior and phase changes in implanted p+- and n+-Si channels under high current densities
Authors: Huang J. S. , Liao C. N. , Tu K. N. , Cheng S. L. , Chen L. J.
Published: July 2002

The p+- and n+-Si channels were prepared by implanting a dose of 5×1015 ions/cm2 of BF2+ and As+ at 40 keV, respectively, into the n-Si and p-Si substrates, followed by a 900 °C-30 min annealing. Nickel contacts (cathode and anode) to these channels were electron-gun evaporated. Electrical current was applied gradually to 80 mA to these channels, corresponding to current densities of 106–107 A/cm2. For the p+-Si, the resistance responded by increasing to a maximum, then decreasing until a precipitous drop took place. For the n+-Si, the resistance increased by a less amount compared to the p+-Si, but also dropped abruptly. The resistance drop is permanent in the p+-Si channels, but not in the n+-Si channels. Mechanisms responsible for these resistance changes in terms of Joule heating, high field effect and junction leakage are proposed. For the precipitous drop in channel resistance of the p+-Si, scanning electron microscopy and transmission electron microscopy showed that a NiSi2 line bridging the cathode and anode contacts had been formed. No silicide line formation in the n+-Si channels was observed.
Investigation of magnetic properties of Tb2Fe17−xMnx compounds
Authors: Wang Yin-gang , Yang Fuming , Chen Changpin , Tang N. , Lin Pinghua , Wang Qidong
Published: July 2002

The effect of Mn on structure and magnetic properties of Tb2Fe17−xMnx compounds has been investigated. Substitution of Mn for Fe does not change the structure of Tb2Fe17. All compounds crystallize in the Th2Ni17-type structure. The Curie temperature of Tb2Fe17−xMnx compounds decreases monotonically with increasing Mn content. The saturated magnetic moment and the magnetic moment of the transition metal sublattice of the Tb2Fe17−xMnx compounds decrease with increasing Mn concentration. A point without zero moment similar to the compensation point was observed in the temperature dependence of magnetization for the Tb2Fe17−xMnx compound with x=4.0, which is explained by a three-sublattice model.
Modeling and manufacturability assessment of bistable quantum-dot cells
Authors: Governale M. , Macucci M. , Iannaccone G. , Ungarelli C. , Martorell J.
Published: July 2002

We have investigated the behavior of bistable cells made up of four quantum dots and occupied by two electrons, in the presence of realistic confinement potentials produced by depletion gates on top of a GaAs/AlGaAs heterostructure. Such a cell represents the basic building block for logic architectures based on the concept of quantum cellular automata (QCA) and of ground state computation, which have been proposed as an alternative to traditional transistor-based logic circuits. We have focused on the robustness of the operation of such cells with respect to asymmetries derived from fabrication tolerances. We have developed a two-dimensional model for the calculation of the electron density in a driven cell in response to the polarization state of a driver cell. Our method is based on the one-shot configuration-interaction technique, adapted from molecular chemistry. From the results of our simulations, we conclude that an implementation of QCA logic based on simple “hole arrays” is not feasible, because of the extreme sensitivity to fabrication tolerances. As an alternative, we propose cells defined by multiple gates, where geometrical asymmetries can be compensated for by adjusting the bias voltages. Even though not immediately applicable to the implementation of logic gates and not suitable for large scale integration, the proposed cell layout should allow an experimental demonstration of a chain of QCA cells.
Simulation of dynamic compaction of metal powders
Authors: Kumar D. Roshan , Kumar R. Krishna , Philip P. K.
Published: July 2002

This article presents numerical studies on the deformation of particles during dynamic compaction of metal powders. The analysis of the process is based on a micromechanics approach using multiple particle configurations. The material considered is elastoviscoplastic with interparticle friction. Two-dimensional studies on particles in close packed arrangement were carried out using plane strain conditions for deformation and thermal response. The finite element method using an explicit dynamic analysis procedure was used for the simulations. The influence of speed of compaction, strain hardening, strain rate dependency, interparticle friction and size of the powder particles on the final shape and temperature variations within the particles were analyzed. The studies offer useful information on the shape and temperature variations within the particles. The results provide a better understanding of the dynamic compaction process at the micromechanics level.
Investigation of diffuse interfaces using time-resolved acoustic spectroscopy
Authors: Richardson C. J. K. , Ehrlich M. J. , Wagner J. W.
Published: July 2002

A theoretical and experimental investigation is conducted on the frequency-dependent reflectivity of ultrashort acoustic elastic waves from a diffuse interface. One-dimensional theoretical predictions are obtained from a transfer-matrix representation of the extended diffusional interface. Experimental results are obtained with time-resolved thermal and acoustic spectroscopy applied to a sputter-deposited two-layer thin film sample of aluminum and gold on a glass substrate. For the first time, variations in the spectral character of reflected acoustic waves, as measured with time-resolved spectroscopy, are quantified using the transfer function between successive arrivals at the free surface. This transfer function is used as a basis for quantitatively determining the nanometer sized extent of a diffuse interface.
Structure of self-assembled layers on silicon: Combined use of spectroscopic variable angle ellipsometry, neutron reflection, and atomic force microscopy
Authors: Styrkas D. A. , Keddie J. L. , Lu J. R. , Su T. J. , Zhdan P. A.
Published: July 2002

Neutron reflection (NR), spectroscopic ellipsometry (SE), and atomic force microscopy (AFM) have been used to characterize the structure of self-assembled octadecyltrichlorosilane (OTS) layers on silicon. The first two of these techniques rely on modeling of the experimental data and may thus result in the unrealistic representation of the composition and structure at the interface. Ambiguities arise from model-dependent analysis complicated by the lack of sufficient external constraints to converge nonunique solutions to a unique one. We show in this work that AFM measurements provide extra constraints to allow us to obtain a physical description closer to the actual structure of the film. It was found that “the simpler the better” modeling strategy very often employed during the fitting of ellipsometric and neutron reflection data is, therefore, not necessarily the best way to obtain a reliable description of the interfacial structure. Our AFM findings necessitated the refit of both neutron and ellipsometric data that were previously described by a single-layer model. Interpretation of the structure of thin layers that is based only on indirect measurements such as SE, NR, and x-ray reflection techniques may be, therefore, misleading. A combined analysis of SE, NR, and AFM data suggests that the OTS film may comprise a rough layer, with pinholes down to bare silicon oxide surface, consisting at least of mono-, bi- and trilayers of OTS molecules.
Spatial variation of activation energy in undoped high-resistivity bulk GaAs
Authors: Reichel C. , Siegel W. , Kühnel G.
Published: July 2002

In undoped semi-insulating GaAs grown by the liquid encapsulated Czochralski or vertical gradient freezing technique mesoscopic electrical nonuniformities correlated to the cellular structure of dislocations exist which are mainly caused by the enrichment of the deep defect level EL2 in the dislocation-rich cell walls. In undoped GaAs crystals with a transition semi-insulating/medium resistivity in the transition region, the resistivity fluctuations between cell walls and cell interiors are much more pronounced (up to three orders of magnitude) and must be caused by different donors. A point contact technique developed for the detection of such nonuniformities was used to measure activation energies separately in cell walls and in cell interiors. In this way, it could be shown that also other defects or impurities than the EL2 are accumulated in the cell walls so that different donor species dominate the electrical properties of cell walls and of cell interiors.
Ensemble Monte Carlo study of electron transport in wurtzite InN
Authors: Bellotti Enrico , Doshi Bhautik K. , Brennan Kevin F. , Albrecht John D. , Ruden P. Paul
Published: July 2002

Electronic transport in wurtzite phase InN is studied using an ensemble Monte Carlo method. The model includes the full details of the first five conduction bands derived from the pseudopotential method and a numerically calculated impact ionization transition rate using a wave-vector- dependent dielectric function. Calculated results for electron transport at both low and high electric field are presented and compared with available results from simpler methods. The dependence of the relevant transport properties on the parameters is discussed, in particular in regards to the uncertainties in the band structure and coupling constants. It is found that at a field of 65 kV/cm that the peak electron drift velocity is 4.2×107 cm/s. The peak velocity in InN is substantially higher than in GaN. The velocity field curve presents a noticeable anisotropy with respect to field direction. The peak velocity decreases to 3.4×107 cm/s for a field of 70 kV/cm in the direction perpendicular to the basal plane. The electron velocity at the onset of impact ionization reaches a value slightly lower than 2.0×107 and 1.0×107 cm/s for fields applied in the direction parallel and perpendicular to the basal plane. The low field mobility has been determined to be in the range of 3000 cm2/V s. The determination of the ionization coefficients is somewhat uncertain due to the lack of knowledge of the high energy phonon scattering rates. Nevertheless, the calculations presented here of the ionization coefficients and quantum yield provide a reasonably accurate estimate of the ionization process.
Magnetization reversal mechanisms in small antiferromagnetic particles
Authors: Zianni X. , Trohidou K. N.
Published: July 2002

We discuss the magnetization reversal mechanisms in small antiferromagnetic particles at temperatures below the blocking temperature. Our calculations have been done with the Monte Carlo simulation technique. We consider classical Heisenberg exchange interactions between the spins and distinct anisotropy for the core and the surface of the particles. For the surface anisotropy we consider two cases for the easy axis orientation: (a) along a fixed direction, which is the same in the core of the particles and (b) radial at each surface site. We found two magnetization reversal mechanisms in both cases of surface anisotropy. In the first mechanism, the switching field reverses all spins in the particle and the magnetization relaxes to the opposite remanence after removing the applied field. A phenomenological model is also developed and the magnetization reversal is explained in terms of the surface anisotropy and the surface exchange field. In the second mechanism, the particle magnetization vanishes at a switching field which distorts the surface spin alignment and induces an opposite moment in the antiferromagnetic core of the particle. The main feature of this mechanism is that it causes a reversible effect, i.e., the remanence is restored as soon as the applied field is removed. In this case the switching field exhibits strong temperature dependence.
Simple thermodynamic diagrams for real refrigeration systems
Authors: Gordon J. M. , Ng K. C. , Chua H. T.
Published: July 2002

The thermodynamic performance of real irreversible cooling and refrigeration systems (chillers) can be summarized in simple rectangular temperature-entropy diagrams, in analogy to classic pedagogical examples for idealized reversible devices. The key to translating complex dissipative losses into this graphical framework is the process average temperature—a factor that can be calculated from nonintrusive experimental measurements, for converting entropy production into lost work. An uncomplicated thermodynamic model is used to transform the governing chiller performance equations into an easily-interpreted graph. Examples based upon actual data from commercial work-driven (reciprocating) and heat-driven (absorption) chillers are presented, and are used to highlight the predominance of internal dissipation in determining chiller efficiency. With the thermodynamic diagram representation, the relative roles of each irreversibility source, as well as the reversible and endoreversible limits, become transparent.
Magnetic structure and microstructure of die-upset hard magnets RE13.75Fe80.25B6 (RE=Nd, Pr): A possible origin of high coercivity
Authors: Volkov V. V. , Zhu Y.
Published: July 2002

In situ transmission electron microscopy magnetizing experiments combined with Lorentz magnetic microscopy in Fresnel–Foucault modes were used to characterize the magnetic structure of die-upset, high energy-product hard magnets Nd13.75Fe80.25B6 and Pr13.75Fe80.25B6. Experimental observations indicate a well-aligned grain structure and quasiperiodic nonaligned “extended defect” layers transverse to press direction. The local remanence of the “defect” layers is far from saturation when the external field is removed. The layers are enriched with inclusions of approximate composition Nd7Fe3, generally with a polygonal shape, and are associated with the original ribbon interfaces. They may be responsible for a high coercivity mechanism, since the motion of reverse domains can be impeded by these layers, even when they are nucleated. Thus, a delayed nucleation of reversed domains seems to be a limiting factor for magnetization reversal and coercivity force. Both Lorentz magnetic imaging and high-resolution microscopy highlight the role of magnetocrystalline anisotropy for domain wall-grain boundary interactions and pinning. Local remanence was estimated directly from magnetic moment sensitive Foucault images.
The relationship of thermal expansion to magnetocrystalline anisotropy, spontaneous magnetization, and Tc for permanent magnets
Authors: Chen Christina H. , Walmer Marlin S. , Walmer Michael H. , Gong Wei , Ma Bao-Min
Published: July 2002

The materials studied here include SmCo5, GdCo5, Sm2TM17, Gd2TM17, Nd9.4Pr4.6Fe80B6, Alnico, SrFe12O19, and PtCo (TM=transition metal). For the first time, the thermal expansion data α(⊥) and α(∥) for the eight materials from as low as −180 °C to as high as 1250 °C have been reported. α(⊥) is the thermal expansion coefficient in the base plane and α(∥) is the coefficient in the c axis. All anisotropic materials tested show thermal expansion anisotropy (TEA). The degree of TEA is defined as γ=α(⊥)/α(∥). The γ has been observed to be related to the degree of magnetocrystalline anisotropy field HA. SmCo5 has the highest HA (350 kOe) and the highest γ (2.5). All the materials show thermal anomaly with a changing slope for α(∥) near Tc and near phase transformation temperatures. The magnitude of thermal anomaly has been observed to be related to the magnitude of spontaneous magnetization (SMZ). The most distinct thermal anomalies are observed for Nd9.4Pr4.6Fe80B6, in which both α(⊥) and α(∥) show a deep valley near Tc, and a peak at around the spin-tilt temperature. Nd–Fe–B type material has the highest SMZ and the largest thermal anomaly of all the permanent magnets. Spontaneous magnetostriction occurs mainly in the c axis for RE–Co alloys (RE=rare earth), and in all directions for Nd–Fe–B type alloys. A rubber band model is used to describe the spontaneous magnetostriction force.
Micromagnetic simulation method with N1.5 scaling
Authors: Ribeiro Filipe , Freitas P. P. , Martins José Luı́s
Published: July 2002

A thin permalloy film with dimensions of 1 μm×2 μm×20 nm was simulated with a new three-dimensional method. The method is designed in such a way that the computing time scales roughly as N1.5 in the number N of discretization elements used to describe the system. The Poisson equation is solved by a relaxation procedure and the energy is minimized with a damped molecular dynamics algorithm.
A torque transducer based on local bands of naturally stabilized remanent circumferential magnetization
Authors: Garshelis I. J. , Jones C. A.
Published: July 2002

A simple construction for “polarized ring”-type torque transducers is described. Instead of residing within a separate magnetoelastically active ring, bands of circumferential remanent magnetization are established in the shaft itself. Coercivity, crystal anisotropy, and the closed circumferential configuration combine to stabilize the polarization. Symmetry precludes the appearance of magnetic fields from an untorqued shaft. Stress anisotropy associated with applied torque tilts the average easy axis into a helical orientation with chirality and helix angle proportional to torque direction and amplitude. Fields, associated with the divergence of the axial component of this magnetization, then arise in the space around each such band. Suitable shaft materials have high enough anisotropy and coercivity, and low enough magnetostriction to prevent these fields from significantly magnetizing proximate shaft regions. Experimental transducers, using heat treated alloy steel shafts, perform adequately for many industrial applications.
Detection efficiency of microparticles in laser breakdown water analysis
Authors: Saito Mitsunori , Izumida Shin-ya , Onishi Kin-ya , Akazawa Jun
Published: July 2002

Polystyrene and silica particles in pure water were successfully detected to a size as small as 0.04 μm by the laser breakdown method. In conventional experimental conditions, detection efficiency has been extremely low, since breakdown occurs only when a particle happens to be present in a narrow focal region during a short laser pulse duration. Through the use of a long focal length lens, which expanded the high power density region without inducing water breakdown around the focal point, efficiency was notably enhanced. Ultrasonic aggregation of particles was also observed to enhance efficiency.
Theoretical and numerical model for estimating unknown magnetic parameters in studying ferromagnetic and antiferromagnetic coupled films
Authors: Sun M. , Fujiwara H. , Kim J. , Hou C. H.
Published: July 2002

A new method of estimating unknown magnetic parameters for coupled thin films of ferromagnetic and antiferromagnetic materials is introduced. It combines convenient torque and spin angle measurements with strength of mathematical models and numerical techniques. Description of this approach is outlined and test results for several simulated examples are presented.
Effect of biquadratic exchange coupling on magnetoresistance and magnetization process in magnetic bilayer systems
Authors: Kuo C. C. , Lin M.-T. , Huang H. L.
Published: July 2002

An improved model is proposed to deal with the magnetic bilayer systems taking into account the contribution of the anisotropy energy and biquadratic exchange coupling to elaborate on the evolution of the magnetoresistance (MR) ratio and magnetization process. The results indicate that the characteristic behavior of the MR ratio depends distinctly on both the biquadratic coupling constant and the layer thickness. The profile of the MR ratio was found to vary from an inverted bell shape to a concave pyramid with increasing biquadratic coupling strength, and decays sharply with the layer thickness. This model calculation helps us to provide a venue for further understanding the MR or giant magnetoresistance behavior of the magnetic multilayer system.
High-speed characterization of submicrometer giant magnetoresistive devices
Authors: Russek S. E. , Oti J. O. , Kaka Shehzaad , Chen Eugene Y.
Published: July 2002

A microwave test structure has been designed to measure the high-speed response of giant magnetoresistive (GMR) devices. The test structure uses microwave transmission lines for both writing and sensing the devices. Pseudo-spin-valve devices, with line widths between 0.4 and 0.8 μm, were successfully switched with pulses whose full width at half-maximum was 0.5 ns. For small pulse widths τpw the switching fields are observed to increase linearly with 1/τpw. The increase in switching fields at short pulse widths is characterized by a slope which, for the current devices, varies between 4 and 16 μA s/m (50–200 Oe ns). The magnetoresistive response during rotation and switching was observed. For small rotations (∼45° between layer magnetizations) the GMR response pulses had widths of 0.46 ns, which is at the bandwidth limit of our electronics. For larger rotations (∼90°) the response pulses broadened considerably as the magnetic layers were rotated near the unstable equilibrium point perpendicular to the device axis. Full 180° switches of both soft and hard layers were observed with switching times of approximately 0.5 ns.
Specular and diffuse electron scattering at interfaces in metal spin-valve structures
Authors: Keavney David J. , Park Sung-Kyun , Falco Charles M. , Slaughter J. M.
Published: July 2002

We have made thickness dependent in situ resistivity measurements on sputtered metal films in real time during film deposition. These measurements allow the separation of the bulk and finite-size contributions to the resistance. The metals studied were Co, Cu, Ni80Fe20, and Ta, deposited in differing orders to isolate the scattering at interfaces relevant to common spin-valve structures. We see sources of excess diffuse scattering in bilayers of Ta with the 3d metals, regardless of the deposition order. We see a similar effect for Co on Cu, but not for Cu on Co. In some cases, we see significant changes in film resistance from overlayers as thin as 2 Å. These results show that the two Cu interfaces in a spin valve are not equivalent, and that other interfaces in the structure may be important. We discuss the origin of the diffuse scattering in terms of interdiffusion or a roughening transition during the initial deposition and island coalescence.
Evaluation of self-bias potential distribution on a powered electrode of supermagnetron plasma apparatus
Authors: Kinoshita Haruhisa , Nomura Shyuji , Nakagawa Yukito , Tsukada Tsutomu
Published: July 2002

The distribution of self-bias potential (Vdc) on a powered electrode of a supermagnetron plasma system, where both electrodes are supplied phase-shifted radio frequency (rf) currents, was measured using five electrical probes buried in the electrode. For comparison, the same was measured for a conventional magnetron plasma system. Measurements were carried out with an Ar discharge at the pressure region of 4–50 mTorr. The data obtained with the supermagnetron and the conventional magnetron plasm systems were used to map the Vdc distribution on the powered electrodes of each plasma source. When the phase difference between rf currents applied to the two parallel electrodes of supermagnetron plasma system were changed, a drastic change of the Vdc is observed. The uniformity of the Vdc distribution is greatly improved when the phase difference between rf currents is varied from 0° to about 180°. The further increase of phase difference between two rf currents causes a decrease of the uniformity of the Vdc distribution. The uniformity of the Vdc obtained on the electrode of supermagnetron plasma source with 180° phase-shifted currents is observed to be superior to that of the conventional magnetron plasma source.
A neutron powder diffraction study of the system La2(Co1−xFex)16Ti for x=0.0, 0.2, and 0.4
Authors: Liang J. K. , Huang Q. , Santoro A. , Liu Q. L. , Chen X. L.
Published: July 2002

The nuclear and magnetic structures of La2(Co1−xFex)16Ti have been studied by neutron powder diffraction for the compositions x=0.0,0.2,0.4. The compound crystallizes with the symmetry of space group R3̄m. Site occupancy results show significant departures from a purely statistical distribution, with the Ti atoms located in sites which interact weakly with La, and with the Co atoms occupying preferentially the sites with strong La interaction. These results are consistent with the enthalpies of formation of equiatomic alloys of rare earth and 3d elements. The spin ordering at all compositions and temperatures is ferromagnetic. At room temperature the moments are aligned along the c axis for the compositions x=0.0 and x=0.2, and form an angle of about 57° with c for x=0.4. This tilt angle is a function of temperature and the moment is aligned along c at 11 K, moving gradually towards the a,b plane as the temperature increases.
Optimization of bimodal nitrogen concentration profiles in silicon oxynitrides
Authors: Dang Sanjit Singh , Takoudis Christos G.
Published: July 2002

Since the presence of nitrogen is responsible for highly desirable properties of silicon oxynitrides in the ultra-large-scale integration era, the amount, position, and concentration profile of N in these films are of great interest. In this regard, we have studied several processing sequences (using successive oxynitridation/oxidation/oxynitridation steps) in order to obtain a bimodal N concentration profile with one peak close to the dielectric/Si interface and the other near the top surface of the dielectric. At 900 °C and 1 atm, it is found that the first oxynitridation step is required to incorporate less than a “critical” amount of N so that a bimodal profile is eventually obtained. Suggestions on how to optimize the concentration and profile of N within the silicon oxynitride film through process–property relationships are presented. Further, the flexibility in N incorporation offered by a mixed ambience of nitrous oxide (N2O) and nitric oxide (NO) coupled with the observation that pure NO processing incorporates about one order of magnitude more nitrogen than pure N2O processing could indeed result in novel approaches in designing and optimizing the chemical (electrical and physical) properties of the oxynitrides. Such results may have significant implications for microelectronic applications of the silicon oxynitridation technology.
Microstructure and interfacial states of silicon dioxide film grown by low temperature remote plasma enhanced chemical vapor deposition
Authors: Park Young-Bae , Rhee Shi-Woo
Published: July 2002

The properties of SiO2 film and the Si/SiO2 interface formed by remote plasma enhanced chemical vapor deposition with the addition of chlorine into SiH4–N2O have been investigated. With the chlorine addition, the deposition rate increased at low deposition temperatures but at temperatures above 150 °C, it decreased because of the desorption of surface halide species. Chlorine at the Si/SiO2 interface prevented further subcutaneous oxidation and formed strong, terminal site Si–Cl bonds which reduced the interface state density. The substitution reaction of O and H with Cl in the bulk oxide film leads to a disordered film structure and decreased hydrogen concentration. The surface roughness increased and the refractive index decreased with increased Cl2 addition. With chlorine addition of less than 6 vol %, the interface trap density (located at ∼Ev+0.3–0.4 eV) significantly decreased down to the 1–3×1011 eV cm2 level at the Si midgap. At high chlorine partial pressure and temperature, the local interface trap density (located at ∼Ev+0.7–0.8 eV) increased due to increased structural disorder resulting from breakage of the Si–O bond.
Growth of epitaxial CoSi2 on SiGe(001)
Authors: Boyanov B. I. , Goeller P. T. , Sayers D. E. , Nemanich R. J.
Published: July 2002

A technique for achieving epitaxial growth of (001)-oriented CoSi2 on strained epitaxial layers of Si1−xGex(001) is described. The technique is based on a variation of the template method, and is designed to control the local environment of Co atoms at the CoSi2/SiGe interface. The effects of the Co–Ge interactions on the interfacial reaction and the epitaxial orientation and the morphology of the silicide film were investigated. This reaction was found to cause pitting in (001)-oriented CoSi2 films, and to stabilize the (221¯) orientation for films codeposited under conditions where CoSi2(001) growth is achieved on Si(001) substrates. The (221¯)-oriented CoSi2 films were islanded after annealing at 700 °C. The islands were terminated by (1¯11) and (110) facets inclined at 15.8° and 19.5°, respectively, from CoSi2 [221¯] towards CoSi2 [114]. These results were interpreted in terms of reduction of interfacial and surface energies, and geometric effects. Silicide films up to 730-Å-thick were deposited and annealed up to 900 °C. The films were stable against agglomeration, and retained tensile stress in the CoSi2 layer after annealing at 700 °C. The rms roughness of the CoSi2 films was comparable to that of the Si(001) substrate—less than 15 Å over areas as large as 20×20 μm2. Films annealed at 900°C were severely agglomerated.
Compensation introduced by defect complexes in p-type ZnSe
Authors: Ren Tian-Ling , Zhu Jia-Lin , Zhu Ziqiang , Yao Takafumi
Published: July 2002

Defect complexes in N-doped and As-doped ZnSe are studied by using the discrete-variational local-density-functional method within a cluster model. Based on the difference of formation energy between two complexes, it is found that the NSe–Zn–VSe complex is a more efficient acceptor compensator than the NSe–Znint complex in N-doped ZnSe, while the AsSe–Znint complex is a more efficient acceptor compensator than the AsSe–Zn–VSe complex in the As-doped ZnSe. The NSe–Zn–NSe complex with a 170 meV acceptor level and the NSe–NZn complex with an 88 meV donor level are respectively identified. The existence of donor states of N molecules in ZnSe is confirmed.
Scanning tunneling spectroscopy study of erbium doped GaSb crystals
Authors: Hidalgo P. , Méndez B. , Piqueras J. , Plaza J. , Diéguez E.
Published: July 2002

Er doped GaSb single crystals have been studied by scanning tunneling spectroscopy (STS) and cathodoluminescence (CL) in a combined scanning electron microscope–scanning tunnelling microscope system. The surface band gap in doped samples has been found to be about 0.5 eV while in undoped crystals the gap is close to the bulk value. Inhomogeneities in the local electronic properties of the doped crystals are studied by a correlation of the CL images and STS data.
n-
Si
/i-p-i
SiGe/n-Si structure for SiGe microwave power heterojunction bipolar transistor grown by ultra-high-vacuum chemical molecular epitaxy
Authors: Zhang Jinshu , Jin Xiaojun , Jia Hongyong , Chen Peiyi , Tsien Pei-Hsin , Feng M. X. , Lin Q. Y. , Lo Tai-Chin
Published: July 2002

The n-Si/i-p-i SiGe/n-Si structure, grown by ultra-high-vacuum chemical molecular epitaxy, was analyzed by cross-sectional transmission electron microscopy and secondary ion mass spectroscopy. It is shown that no defects are observed in the n-Si/i-p-i SiGe/n-Si structure, the interfaces between the SiGe layer and the n-Si layers are clear and planar, both the Ge and boron atoms are uniformly distributed in the p-SiGe, and the profiles of boron and Ge are abrupt from the n-Si to the SiGe layer. A high-performance microwave power SiGe heterojunction bipolar transistor was fabricated using the n-Si/i-p-i SiGe/n-Si structure. Therefore, ultra-high-vacuum chemical molecular epitaxy is one of the most promising methods for the growth of the Si/SiGe strained epilayers.
Current transfer at superconducting Bi2Sr2CaCu2O8+δ–Ag joints
Authors: Kuršumović A. , Baranowski R. P. , Glowacki B. A. , Evetts J. E.
Published: July 2002

Composite reaction textured Bi2Sr2CaCu2O8+δ superconducting samples were made by partial melt processing, with aligned MgO fibers as seeding and texturing interfaces. The electric contacts were in situ processed thick silver tapes of the same width as the sample. The voltage drop at the metal–superconductor joint was measured in liquid nitrogen during continuous current ramp in self fields and applied fields. The contact resistivity and current transfer length were estimated by applying a transmission line model for current transport across the joint. The calculated and estimated transfer length, corresponding to >95% current transfer, occurs over only 0.75 mm of the contact in the case of a 0.125-mm-thick Ag current lead. At currents, I, below critical, IC, this approach was satisfactory. However, at higher currents nonohmic behavior is observed. The apparent contact resistance increased with increasing current, and with applied magnetic field. This contribution to the contact resistance arises from flux flow effects in the superconductor and is shown to vary nonlinearly with both increasing current and increasing applied field. Consequently the current transfer length increased progressively over the whole contact, approaching a rather uniform current distribution at about I∼12IC. The results are compared with a nonlinear finite element model of current flow in an in situ current element; the agreement of the model with experiment is good.
Magnetization processes in Co/Cu multilayers with low magnetoresistive hysteresis
Authors: Chapman J. N. , Rose J. , Aitchison P. R. , Holloway H. , Kubinski D. J.
Published: July 2002

We have used transmission electron microscopy to study magnetization processes in Co/Cu multilayers with the Cu spacer layer thickness close to 9 Å. The films show giant magnetoresistance (GMR) values ≈25%, saturation fields of 1–2 kOe, and very little magnetoresistive hysteresis; they are of interest as position sensors. While the Cu thickness was chosen to correspond to the first antiferromagnetic maximum, magnetic images taken throughout a magnetization cycle attest to the fact that the antiferromagnetic coupling is far from complete. Detailed analysis of image sequences and the corresponding low angle diffraction patterns suggests that the coupling is dominated by a biquadratic component. This is consistent with the relatively low value of GMR. Furthermore, the well-defined and relatively simple domain processes which are observed over the low field regime (±50 Oe) explain why little hysteresis is observed.
Dielectric studies of Zn1−xMnxSe epilayers
Authors: Su J. S. , Wang J. C. , Chen Y. F. , Shen J. L. , Chou W. C.
Published: July 2002

We report on the dielectric properties of Zn1−xMnxSe(0⩽x⩽0.78) epilayers, by capacitance and dissipation factor measurements at temperature 5 K<T<475 K and frequency 20 Hz<f<100 kHz. A Debye-like relaxational contribution to the dielectric response is observed, which requires the presence of charge redistribution. The relaxation is found to be a thermally activated process, and the activation energies obtained from the dissipation factor and capacitance are in good agreement. The capacitance is found to increase with a decrease in test frequency. From our results it is established that the dielectric response is caused by carrier hopping among structural defects. A monotonic variation is found in the relationship between the activation energy and the Mn concentration. This monotonic variation is interpreted in terms of the four-center model, in which the number of Mn atoms appearing in the nearest-neighbor sites of the defect can have four possible configurations. The measured defect behavior reflects the overall average of all energy levels involved in the center, and the result is weighted by the relative concentrations.
Infrared optical properties of SrBi2Ta2O9 ferroelectric thin films
Authors: Huang Zhiming , Yang Pingxiong , Chang Yong , Chu Junhao
Published: July 2002

Infrared optical constants of strontium bismuth tantalate (SBT) ferroelectric thin films have been derived from infrared spectroscopic ellipsometric measurements (2.5–12.5 μm). By fitting the experimental pseudodielectric constants 〈ε1〉 and 〈ε2〉 with a three-phase model (Air/SBT/Pt), the optical constants and thickness of the films have been obtained, and an intrinsic vibrational frequency of the films at 802 cm−1 has also been acquired. The possible contribution to the vibrational frequency has been discussed.
Volume plasmons in porous silicon
Authors: Sato S. , Rath S. , Akiyama S. , Nozaki S. , Morisaki H.
Published: July 2002

Plasmon loss features of porous silicon (PS) layers consisting of Si nanoparticles with average diameters of 3.6–4.5 nm were studied using x-ray photoelectron spectroscopy. The volume plasmons of the Si nanoparticles in the as-prepared PS layers have the same energy as that of bulk Si (17.4 eV). The surfaces of the PS layers were sputter etched by argon (Ar) ion bombardment to remove any hydrogen passivation. The plasmon energy for the surface-sputtered PS layers increases from 17.4 eV to either ∼17.9 or 18.5 eV depending on the size of the nanoparticles, while the energy for bulk Si remains the same after the surface sputtering. A qualitative model with the plasma frequency modified from the free electron model was introduced to explain the observed characteristic features in the volume plasmons.
Hole transport via dangling-bond states in amorphous hydrogenated silicon nitride
Authors: Shannon J. M. , Morgan B. A.
Published: July 2002

Current transport through good quality undoped silicon-rich silicon nitride is normally limited by the contacts. However, if the concentration of silicon dangling bonds is increased by current stressing or raising the nitrogen content, current transport becomes bulk controlled and well described by the Poole–Frenkel effect in a defect band. Using thin-film diode structures on either p-type or n-type silicon substrates, we show that the I/V characteristics can only be explained if current transport in the defect bands within the silicon-rich nitride is via the movement of holes. Hole transport through defect states is probably a common feature in all the amorphous silicon-rich alloys.
Perturbative analytical solutions of the electric forward problem for realistic volume conductors
Authors: Nolte Guido , Curio Gabriel
Published: July 2002

Exact analytical solutions for the surface potential due to a current source are available only for special volume conductors. Here we derive approximate analytical solutions exploiting the fact that for the upper convexity of the head corrections to the spherical approximation are small compared to its radius. First we approximate the real surface, defined in terms of an angular dependent “radius,” by a finite sum of spherical harmonics, regarding everything but the zeroth component (the sphere) as a small perturbation. Inserting this formulation into the standard surface integral equation allows us to analytically construct and invert the integral operator as a Taylor expansion with respect to the perturbation. Remarkably, for finite order of perturbation the integral operator and its inverse, expressed as a matrix in the basis of spherical harmonics, is sparse. Furthermore, without loss of generality the solution due to a current dipole can be expressed as a set of sums over a single index. Explicit formulas and examples will be presented for one shell, for an approximation of the surface up to second order of spherical harmonics, and up to first order of the perturbation. By comparing the results to the exact solution for a prolate spheroid we estimate the performance for realistic deformations.