bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical determination of strains in heterostructures: GaAs/Si as an example
Authors: Landa G. , Carles R. , Fontaine C. , Bedel E. , Muñoz-Yagüe A.
Published: July 2002

Raman and photoluminescence spectroscopies are used to characterize crystalline quality and interfacial strain in heterostructures. The effect of a biaxial stress on electronic and vibronic energies is reviewed and then applied to the case of a GaAs layer. Measurements on GaAs grown on Si(100) by molecular-beam epitaxy are made over a wide temperature range (4→700 K). The evolution of the strain is deduced from the shift of both the energy-band gaps and the long-wavelength transverse and longitudinal-optical-phonon frequencies. The sensitivity of the Raman probe is dramatically enhanced by excitation under resonant conditions at the E1 edge of GaAs. The measurements confirm the anisotropy of the strain and demonstrate that both its sign and value at room temperature result from a balance between two reverse phenomena: the thermal expansion and the lack of complete relaxation of the lattice mismatch during growth.
The effect of the pattern shape of Hall devices in the electrical characterization of Si-implanted GaAs
Authors: Hyuga Fumiaki
Published: July 2002

We show that the Greek-cross pattern effectively reduces the error due to finite-sized contact in Hall-effect measurements. This pattern enables measurements of areas as small as 4×4 μm, which is comparable to the n-channel area of GaAs field-effect transistors. We find that the sheet carrier concentration agrees well with the value derived from the capacitance-voltage method using this pattern. This indicates that the small Greek-cross pattern is suitable for electrical property characterization of active layers.
Magnetic properties of RMn2 compounds (R=heavy rare earths)
Authors: Labroo Sunil , Ali Naushad , Robinson Paul
Published: July 2002

Magnetic and electrical properties of RMn2 compounds, where R=Gd–Tm, have been investigated via SQUID magnetometry, ac-susceptibility, thermal-expansion, electrical-resistivity, and magnetoresistance experiments from 4.2 K to room temperature. All samples with R=Gd, Tb, Dy, Ho, Er, and Tm are found to order ferromagnetically at Tc =109, 54, 35, 24, 15, and 8 K, respectively. The paramagnetic-to-ferromagnetic phase transitions in case of Gd and Tb compounds are of first order, shown as a sharp volume change in the thermal-expansion curve, but the transitions in the cases of other compounds are second order in nature. The first-order transitions are similar in nature to that in the case of YMn2, where the transition is believed to be associated with the collapse of the Mn moment. The transition temperatures in RMn2, where R=Gd–Tm, are in fairly good agreement with the prediction by the de Gennes rule. In GdMn2 and TbMn2, the Mn moment is found to be finite and it appears that Mn and Gd or Tb sublattices order simultaneously, but in the case of RMn2, with R=Dy–Tm, the Mn moment is small. The magnetoresistance at 4.2 K is found to be large and negative, as expected in a localized ferromagnetic metal. In the case of RMn2, for R=Dy–Tm, a T2 dependence is observed in the low-temperature resistivity, consistent with the electron-magnon scattering in a ferromagnet.
Field effect on the gain coefficient in a GaAs-AlGaAs single-quantum-well laser
Authors: Huang F. Y.
Published: July 2002

The gain coefficient of a GaAs-AlGaAs single-quantum-well laser in an applied static electric field is calculated. The quasi-bound-state energy and the lifetime are obtained by using the tunneling resonance technique. The principal field effects on the laser emission are (i) shift of the laser wavelength, (ii) reduction and broadening of the gain spectrum, and (iii) increase of the threshold current. Based on the field-dependent shift of the peak gain position, a possible wavelength tuning mechanism for the quantum-well laser is proposed.
A neutron backscattering study of lattice deformations in silicon due to SiO2 precipitation
Authors: Magerl A. , Schneider J. R. , Zulehner W.
Published: July 2002

Czochralski-grown dislocation free silicon crystals containing approximately 7.5, 9.9, and 18×1017 cm−3 oxygen atoms have been annealed at 750, 1050, and 1200 °C for times varying between 20 and 216 h. Neutron transmission spectra measured at reflection 111 in backscattering geometry reveal mean lattice contractions of the order of 5×10−5 and lattice parameter fluctuations up to Δd/d≊5×10−4. The measuring time for one spectrum is 15 min and an on-line characterization of the defects produced during annealing is feasible. A combination of this technique with neutron small angle scattering, γ-ray diffractometry, and diffraction experiments with high-energy synchrotron radiation opens new possibilities for investigations of oxygen precipitation in silicon crystals as used in very-large-scale integration semiconductor device technology.
Raman study under resonant conditions of defects near the interface in a GaAs/Si heterostructure
Authors: Mlayah A. , Carles R. , Landa G. , Bedel E. , Fontaine C. , Muñoz-Yagüe A.
Published: July 2002

A Raman study has been performed, under resonant conditions, on a GaAs bevelled-edge layer grown on a Si substrate to characterize the optical and crystalline properties of the epilayer near the interface. According to the geometrical characteristics of the sample, a theoretical expression for the Raman intensities profile has been established and compared to the experimental data. This fitting procedure enables us to investigate the absorption coefficient of the GaAs layer due to the disorder-induced softening of the E1 edge. A quantitative analysis of the lattice disorder has been carried out on both longitudinal and transverse optical modes by studying the Raman line-shape evolution versus the laser spot position on the bevel edge. From this study, we have followed the recovery of the crystalline quality of the epilayer while going away from the interface, and evaluated the ‘‘Raman thickness’’ of the dislocated layer. Using the spatial correlation model as a relationship between the disorder amount and the outcoming effects on the Raman line peaks, we have estimated the dislocation density at the heterostructure interface.
Thermal ionization of excitons in GaAs/AlGaAs quantum well structures
Authors: Colocci M. , Gurioli M. , Vinattieri A.
Published: July 2002

Photoluminescence and photoluminescence excitation spectra have been performed on GaAs/AlGaAs quantum well structures in the temperature range 4–300 K. Sharp exciton resonances are present up to room temperature and can be ascribed to localized excitons for T≤50–70 K and to free excitons at higher values of T. Nevertheless, a line-shape analysis of the PL spectra clearly shows the presence of band-to-band recombination. A fit based on a simple statistical model reproduces with high accuracy the photoluminescence spectrum line shape and allows to evaluate the relative densities of excitons and free carriers generated by the exciton dissociation. We find that the ratios of the relative densities can be interpreted on the basis of the law of mass action for describing the thermal equilibrium between excitons, electrons, and holes.
Electroluminescence from Gunn domains in GaAs/AlGaAs heterostructure field-effect transistors
Authors: Zappe Hans P. , Moglestue C.
Published: July 2002

Electroluminescence from the drain-edge of GaAs/AlGaAs heterostructure metal-semiconductor field-effect transistors under high applied drain bias is observed. The visible light emission is seen through Monte Carlo simulations to be correlated with Gunn domains in the sample, in which the high-field region gives rise to both impact ionization current and the luminescent emission.
On the calculation of the electromagnetic force field in the circular stirring of metallic melts
Authors: Saluja N. , Ilegbusi O. J. , Szekely J.
Published: July 2002

A mathematical representation has been developed to represent the electromagnetic force field used to impart rotary motion to a cylindrically shaped molten metal pool. Problems of this type arise in the electromagnetic stirring of continuous casting systems. The formulation given here introduces two new facets which have been neglected by previous investigations, namely, an allowance for the nonlinear dependence of the magnetic flux intensity on the radial distance, and the coupling between the melt velocity and the induced field. It is shown that neglecting these factors may involve errors in the 20%–50% range.
Duality and fluxonics in superconducting devices
Authors: Kadin A. M.
Published: July 2002

The concept of electrical duality is reviewed, with a focus on the symmetry between the motion of charge and the motion of magnetic flux. This approach is especially illuminating for the case of a superconductor, in which flux is quantized in units of Φ0=h/2e. In a two-dimensional superconductor, a quantized vortex or fluxon can be viewed as the dual of an electron. Based on this analogy, new superconducting ‘‘fluxonic’’ device concepts are proposed, including optical detectors and transistors. Finally, the implications of this picture for low- and high-temperature superconductors are evaluated.
Germanium/gallium arsenide alloys grown by molecular-beam epitaxy
Authors: Baird R. J. , Holloway H. , Tamor M. A. , Hurley M. D. , Vassell W. C.
Published: July 2002

Metastable germanium/gallium arsenide alloys have been grown by molecular-beam epitaxy on (100)-oriented GaAs substrates. Double-crystal diffractometry shows that the layers have crystalline perfection comparable to that of good crystals of GaAs. Electron mobilities in the range 500–1500 cm2 V−1 s−1 are observed from epilayers with electron concentrations in the range 1017–1019 cm−3. Hole mobilities were relatively small at <40 cm2 V−1 s−1 with concentrations in the range 3×1017–3×1020 cm−3. We find substantial nonstoichiometry of the GaAs component with As/Ga atom ratios ranging from 0.94 to 1.17. As-deficient through moderately As-rich alloys exhibit a positive deviation from Vegard behavior that changes to a negative deviation with further increase in the As content. The closeness of lattice match with GaAs substrates allows the epilayer/substrate combination to retain a coherent interface with epilayer thicknesses up to at least 1.5 μm. This is demonstrated by measurements of tetragonal distortion and of curvature that are in excellent agreement with predictions for an interface without misfit dislocations. We have not observed any indication of phase separation in layers grown at temperatures up to 580 °C.
Magnetic and thermomagnetic analysis of metal evaporated tape
Authors: Speliotis D. E. , Judge J. P.
Published: July 2002

Metal evaporated (ME) tape is being developed as an advanced medium for high-density magnetic tape recording. It is produced by evaporation in an oxygen atmosphere, and consists of very small Co and CoNi crystallites intermingled with oxides of Co and Ni. The angle of incidence of the vapor flux is varied continuously during deposition, imparting to the film a distinct curved columnar structure. Previous studies have measured the angular dependence of the coercivity and remanence out of the plane of the tape in order to determine the preferred direction of magnetic anisotropy and the mechanism of magnetization reversal in these films. These studies, however, do not take into account the demagnetizing field. In this study, we used a two-dimensional vector vibrating-sample magnetometer (VSM) to obtain the correct angular dependence of the coercivity and of the remanent magnetization. Our results show that magnetization reversal near the easy axis proceeds by incoherent rotation, and not by domain-wall displacement as suggested by other investigators. We also measured the torque characteristics in the three principal planes and used them to determine the magnitude and angle of the uniaxial magnetic anisotropy in these films. To understand the origin of the magnetic anisotropy, we studied the temperature dependence of Ms and Hc over the range of −100 to +100 °C. We found that Ms is essentially independent of temperature in this range, while Hc has a very large negative temperature coefficient (about 6 Oe/°C). This implies that it is primarily the crystalline anisotropy of the Co and CoNi crystallites which controls the coercivity of these films, and not the shape of the distinct columnar structure or any shape contribution of the crystallites themselves.
Correlations between film properties and microwave surface resistance for YBa2Cu3O7−δ thin films on MgO
Authors: Young K. H. , Sun J. Z.
Published: July 2002

Epitaxial YBa2Cu3O7−δ thin films grown by laser ablation on MgO (100) substrates were investigated for microwave applications. By systematically varying the growth conditions, we obtained films with various microstructures, low-frequency superconducting properties, and microwave performance. The surface resistances were determined from a measured unloaded quality factor in a 8.6-GHz microstrip resonator. Surface resistance was found to correlate most directly with the degree of grain alignment as revealed by electron channeling and x-ray diffraction studies. Films grown at optimal conditions gave a scaled surface resistance of 0.6 mΩ at 77 K and 10 GHz.
Transport dynamics of a 19-MeV, 700-kA electron beam in a 10.8-m gas cell
Authors: Sanford T. W. L. , Halbleib J. A. , McAtee W. H. , Mikkelson K. A. , Mock R. C. , Poukey J. W. , Welch D. R.
Published: July 2002

Measurements and modeling show that the 19-MeV, 700-kA HERMES III electron beam can be stably transported over 10.8 m in a 65-cm diam drift cell filled with low-pressure air or N2. A pressure of 100±20 Torr maximizes the energy transport efficiency at 71±2.4%. At 100 Torr, the pressure is high enough to generate a net current sufficient to produce magnetic pinch forces that contain the radial oscillations of the beam, but low enough to prevent significant loss from inelastic collisions and beam instabilities. Comparison of the model results with the measurements show that the modeling can be used to predict qualitatively beam behavior in the cell.
Near-surface defects associated with 2.0-MeV 16O+ ion implantation in n-GaAs
Authors: Tin C. C. , Barnes P. A. , Bardin T. T. , Pronko J. G.
Published: July 2002

Using the technique of deep-level transient spectroscopy (DLTS), we have studied the formation of the defects in the near-surface region of n-GaAs implanted with different doses of 2.0-MeV 16O+ ions. Our results show that EL6 and the U band are the principal defect centers in room-temperature-implanted samples. In the case of samples implanted at 200 °C, DLTS results also show the presence of hole levels in addition to the EL6 and the U band. When the samples were subjected to rapid thermal annealing after room-temperature ion implantation, hole traps were the major defect centers in the near-surface region. Just below the surface region, however, limited recovery has occurred as shown by the re-emergence of electron traps. However, the defect structure is still different from that of the unimplanted samples. These results can be used to show the extent of lattice recovery after rapid thermal annealing. A mechanism is proposed to explain the evolution of the defects in MeV ion implanted n-GaAs.
A theory of the magnetic field from current monopoles
Authors: Ferguson A. Stewart , Durand Dominique
Published: July 2002

A method for calculating the magnetic fields of volume currents by using point current sources (monopoles) is derived. A model is developed for a point source that is consistent with Maxwell’s equations. An arbitrary arrangement of point sources in a conducting volume is shown to generate no magnetic field outside the conductor if the volume surface coincides with an equipotential surface of the sources. This allows such sources to be added to any problem without affecting the magnetic field, and simplifies the calculation of the magnetic field generated by an arbitrary point source in both spherical and semi-infinite volume conductors. The magnetic field generated by current monopoles located within these volume conductors is then derived. The formulation greatly simplifies the analysis of the magnetic field generated by higher-order current sources, notably dipoles, in these volumes.
Numerical investigation of the kinetics and chemistry of rf glow discharge plasmas sustained in He, N2, O2, He/N2/O2, He/CF4/O2, and SiH4/NH3 using a Monte Carlo-fluid hybrid model
Authors: Sommerer Timothy J. , Kushner Mark J.
Published: July 2002

Capacitively coupled radio-frequency (rf) glow discharges are standard sources in plasma assisted materials processing. Theoretical analyses of rf discharges have been hampered by the computational difficulty of simultaneously resolving nonequilibrium electron transport and plasma chemistry. We have developed a hybrid Monte Carlo-fluid simulation that can simulate nonequilibrium electron transport while executing with the speed of a fluid simulation. An electron Monte Carlo simulation (EMCS) is used to calculate the electron energy distribution (EED) as a function of position and phase in the rf cycle. Collision rates and transport coefficients are calculated from the EED and used in a self-consistent fluid model (SCFM) of charged particle behavior and a neutral chemistry/transport model. Electric fields from the SCFM are cycled back to the EMCS, and the process is iterated until convergence. All pertinent heavy particle (charged and neutral) reactions can be included as well as collisions of electrons with ions, excited states, and reaction products. The hybrid model is applied to a variety of gas mixtures of interest to materials processing.
Molecular beam epitaxy growth and characterization of CdxHg1−xTe (0.4<x<1) based quantum wells
Authors: Monterrat E. , Ulmer L. , Mallard R. , Magnea N. , Pautrat J. L. , Mariette H.
Published: July 2002

CdxHg1−xTe epilayers and CdxHg1−xTe/CdyHg1−yTe quantum well heterostructures have been grown by molecular beam epitaxy. The various parameters of the growth technique have been carefully optimized: control of the thickness by the observation of the oscillations of the reflection-high-energy electron diffraction, the alloy composition by measuring the Cd sticking coefficient in our growth procedure, and the quality of the interfaces by performing transmission electron microscopy. The photoluminescence spectra at 5 K of thick CdxHg1−xTe epilayers are dominated by bound exciton recombination below the fundamental band gap Eg. By contrast, the main emission in the quantum wells photoluminescence spectra is due to an intrinsic recombination of the confined carriers in their respective ground state. The influence of CdxHg1−xTe well thickness and the CdyHg1−yTe barrier composition of this quantum well transition energy are presented.
Heteroepitaxial growth and characterization of SrF2/(100)InP
Authors: Barrière A. S. , Elfajri A. , Guégan H. , Mombelli B. , Raoux S.
Published: July 2002

The composition of SrF2 thin films deposited on carbon plates by sublimation under ultrahigh vacuum has been investigated mainly by Rutherford backscattering (RBS) of α particles as a function of the substrate temperature during the condensation phase and of the thickness of the layers. It is shown that for TS = 270 °C the obtained films are stoichiometric, homogeneous, and quasi-insensitive to air exposure. The structure of such layers grown on (100)InP single crystals has been deduced from the study of channeling phenomenon. It is demonstrated that the RBS minimum yield χmin, measured on SrF2, decreases with increasing thin-film thickness, varying from 100 to 400 nm. For a 400-nm-thick SrF2 thin film at 350 nm from the interface χmin=0.05 showing that the layer is well crystallized and presents the same orientation as the substrate.
Ion energy distributions on surfaces exposed to plasmas: An experimental and theoretical investigation
Authors: Heim D. , Störi H.
Published: July 2002

The plasma sheath is a space region between a plasma and the surface of a solid next to the plasma. The plasma sheath influences the plasma assisted deposition processes of thin films. It is also a determining factor for particle extraction as a plasma diagnostic tool. Ions were extracted from a plasma, produced by a dc hollow cathode discharge. The extracted ions were mass analyzed and simultaneously their energy distributions were measured. During each experiment, the discharge parameters were kept constant, whereas the sheath voltage and therefore the thickness of the sheath in front of the extraction orifice was varied. In this way the influence of the sheath on the composition of the ion flux and on the energy distributions of the various ion species could be observed. The gases used were hydrogen, hydrogen-argon, and hydrogen-silan mixtures in the pressure range of 0.08–1 mbar. The ion current densities were about 1 mA/cm2 and the sheath voltages were varied from floating potential to 160 V. A numerical model was developed to calculate the potential distribution in the sheath and the energy distributions of the various ion species. These calculations were done by an iterative method to obtain a self consistent solution. By comparing the results of the calculations with the measured ion energy distributions and ion intensities, conclusions about collision processes of ions and neutrals could be made. The investigations show that due to collision processes, the composition of the ion flux passing the sheath is considerably changed and the energy distributions of the various ion species can be explained by a few simple collision processes.
Characterization of heavily carbon-doped GaAs grown by metalorganic chemical vapor deposition and metalorganic molecular beam epitaxy
Authors: Stockman S. A. , Höfler G. E. , Baillargeon J. N. , Hsieh K. C. , Cheng K. Y. , Stillman G. E.
Published: July 2002

Carbon-doped GaAs with carbon concentrations ranging from 2×1017 cm−3 to 2.6×1020 cm−3 has been characterized by variable temperature Hall effect measurements, secondary ion mass spectrometry (SIMS), and double-crystal x-ray diffraction (DCXD). The samples studied were grown by metalorganic chemical vapor deposition (MOCVD) and by metalorganic molecular beam epitaxy (MOMBE). The hole mobility is dominated by degenerate conduction for hole concentrations ≥1×1019 cm−3, and the 77 K resistivity is typically 30%–35% lower than at 300 K in these samples. The mobilities of C-doped p+-GaAs are found to be significantly higher than for Zn- or Be-doped p+-GaAs for doping concentrations in excess of 2×1018 cm−3. The maximum achievable hole mobilities for C-doped material grown by the two techniques are nearly identical, indicating that neither MOCVD nor MOMBE has an inherent advantage over the other for producing low-resistivity p-type GaAs. SIMS analysis and Hall effect measurements reveal that the total carbon concentration, [C], is higher than the as-grown hole concentration, p, in the most heavily doped samples. DCXD measurements show general agreement with the lattice mismatch predicted by Vegard’s law. However, for [C]≳1020 cm−3 a discrepancy between the predicted and measured mismatch suggests that partial lattice relaxation or the presence of interstitial carbon may need to be considered in order to adequately describe the lattice contraction.
Mössbauer study of amorphous and nanocrystalline Fe73.5Cu1Nb3Si13.5B9
Authors: Zhou X. Z. , Morrish A. H. , Naugle D. G. , Pan R.
Published: July 2002

The superior soft magnetic material, Fe73.5Cu1Nb3Si13.5B9, formed by partially crystallizing amorphous ribbons upon annealing, has been investigated using x-ray diffraction and Mössbauer spectroscopy. The microstructure of the annealed ribbons consists mainly of DO3 α-FeSi nanocrystallites and an amorphous phase that lies in the grain boundaries. This residual amorphous phase has a Curie temperature of about 600 K; when it becomes a paramagnet the coercivity of the ribbon increases dramatically. It follows that, in addition to the nanocrystallite grain size and the random anisotropy, the coupling between the nanocrystallites by the amorphous grain boundaries is important for the achievement of the excellent soft properties. When Cu is replaced by Ag or Ni, a higher annealing temperature is required to produce nanocrystals.
The spectrum of energy levels of the Ga-vacancy/deuterium complexes in p-GaAs
Authors: Amore Bonapasta A. , Bonanni B. , Capizzi M. , Cherubini L , Emiliani V. , Frova A. , Sacks R. N. , Sarto F.
Published: July 2002

Energy levels of the complexes formed by Ga vacancies binding one or more deuterium (or hydrogen) atoms are investigated by low-temperature photoluminescence. Since the as-grown, nondeuterated material is unintentionally doped p type, the emission is possible because of ‘‘internal’’ recombination from a D donor in an adjacent bond-center position and the ground levels of the different possible vacancy configurations, with zero, one, or more D atoms trapped in its dangling bonds. At 2 K, the transitions occur at 1.14, 1.19–1.22, 1.33–1.30, and 1.34 eV, and the last two have never been observed before. The given energy ranges correspond to laser excitation increasing from the lowest to highest level, for a given D treatment of the material. We explain the new transitions and the multiplet character of some of the bands in terms of the possible different ways of accommodating the D atoms inside the vacancy. The level separation in our data is less than that predicted theoretically for the ‘‘bare’’ vacancy with a fully unrelaxed lattice.
Particle–particle interactions in dusty plasmas
Authors: Boufendi L. , Bouchoule A. , Porteous R. K. , Blondeau J. Ph. , Plain A. , Laure C.
Published: July 2002

High densities of submicron particles have been created in an Ar/SiH4 parallel plate radio-frequency (rf) discharge. The particles were collected and measured by electron microscopy and the mean particle diameter was found to be 230±60 nm. Laser scattering from the dense clouds of such particles showed that the concentration was 1×108 cm−3. A laser Doppler anemometer was used to measure the particle velocity distribution and hence the mean particle mass. This is consistent with the specific density of the hydrogenated amorphous silicon. The mean velocities of particles were measured at two different gas flows when the discharge was extinguished, so that the particles are neutral and do not interact, and the particles move with the gas velocity. However, during the discharge the particles have almost no mean axial velocity, even though the gas flow is as large as before. This is due to the strong interparticle interactions that keep the particle cloud, as a whole, stationary. The charge on the particles is estimated, leading to a value of the Coulomb coupling parameter of Γ=10. This large value suggests that the particle cloud can be viewed as a Coulomb liquid.
Microstructural investigations of Bi doped Fe garnet thin films for magneto-optical recording
Authors: Rogers D. J. , Eppler W. R. , Laughlin D. E. , Kryder M. H.
Published: July 2002

Gd1.6Bi1.4Fe3.8Ga1.2O12 garnet films for short wavelength magneto-optical recording are investigated. These films have high Faraday rotation (up to 1.3 °/μm), good corrosion resistance and good magnetic properties (Hc≳1 kOe with hysteresis loop squareness of 1) but poor noise performance. This paper shows that the noise is a result of the grain size being of the order of the recorded domain size and that the grain size and distribution are determined by the initial density of garnet nuclei. In the course of this study, it was discovered that these films contain a high density of Bi globules which precipitate out of the garnet matrix. It is believed that these globules contribute to the high coercivity of sputtered garnet films.
Physical properties of contamination particle traps in a process plasma
Authors: Carlile Robert N. , Geha Sam S.
Published: July 2002

This article describes the results of measurements of the plasma parameters, plasma potential Φ, positive ion density n+ , electron density ne, electron temperature Te, and electron energy distribution function (EEDF) f(ε) within and near the electrostatic particle traps in an argon process plasma. The measurements have been made with a tuned Langmuir probe at a constant set of system parameters: 250 W, 13.56 MHz power, 15 mTorr, and 10 sccm flow of Ar. The electrostatic traps have precisely defined boundaries characterized by a sharp increase in Φ of about 6 V as we have described elsewhere. We show here that during an initial mapping period of 0–10 min after the rf power is turned abruptly on (10 min are required to map 48 points), n+ is continuous across a trap boundary as are ne and Te. kTe/e≊6.5 eV uniformly throughout the mapped region. If the mapping is repeated later at t=30–40 min while n+ is still continuous and does not show any change in its values in time, ne has decreased by 42% within the trap compared to the region outside the trap which we call the ambient plasma region. Also, kTe/e has increased within the trap to 8 eV while the ambient plasma remains at 6.5 eV. If the mapping is again repeated at t=60–70 min, ne has decreased by 56% of its value in the ambient plasma and kTe/e has increased within the trap to 10 eV. This picture is consistent with the postulation that we have made previously that the trap initially contains few particles and is created by the system configuration and not particles; the trap then slowly fills with negatively charged particles. In order to maintain charge neutrality within the trap ne must decrease. Also, since the particles allow recombination to occur on their surfaces, the electron temperature must increase within the trap in order that generation can keep up with increased recombination. Finally, the EEDF appears to be Maxwellian in the ambient plasma but non-Maxwellian within the trap. In both cases, the EEDF has an attenuated high energy tail.
Lateral modulations in zero-net-strained GaInAsP multilayers grown by gas source molecular-beam epitaxy
Authors: Ponchet A. , Rocher A. , Emery J.-Y. , Starck C. , Goldstein L.
Published: July 2002

Compressive GaInAsP multiple quantum wells (MQW) grown by gas source molecular-beam epitaxy present altered structural and optical characteristics when tensile GaInAsP barriers are used instead of lattice-matched ones. An alternate tensile/compressive GaInAsP MQW has been examined by transmission electron microscopy. A strong lateral modulation of thickness, strain, and probably chemical composition was shown. This modulation exhibits pronounced anisotropy, with a periodicity of about 50 nm along the [110] direction. Although its origin is not fully accounted for yet, it seems to allow partial elastic relaxation of tensile layers. Based on this analysis, a schematic description of distortion modulation is proposed.
Anomalous giant magnetostriction of amorphous (SmFe2) (1−x) at. % B (x) at. % alloy in a low magnetic field
Authors: Kim Jai-Young
Published: July 2002

RFe2 laves phase intermetallic compounds are promising materials for magnetostrictive applications. To obtain a larger magnetostriction in a low magnetic field, the influence of boron addition on the giant magnetostriction of an amorphous (SmFe2) (1−x) at. % B (x) at. % alloy has been examined. In the amorphous alloy, the saturation magnetostriction anomalously increases, while the saturation magnetization decreases with the increase in x. The highest saturation magnetostriction of −670×10−6 at 10 kOe and its effective magnetostriction of −490×10−6 at 0.3 kOe can be obtained for amorphous (SmFe2) 99.26 at. % B 0.74 at. % alloy. This effective giant magnetostriction obtained in a low magnetic field is larger than those reported in previous researches. I attribute this anomalous giant magnetostriction in a low magnetic field to the increment of elastic energy in the amorphous (SmFe2) (1−x) at. % B (x) at. % alloy.
High-field effective linewidth and eddy current losses in moderate conductivity single-crystal M-type barium hexagonal ferrite disks at 10–60 GHz
Authors: Truedson John R. , McKinstry Kevin D. , Kabos Pavel , Patton Carl E.
Published: July 2002

The losses associated with the high-field tail region of the ferromagnetic resonance (FMR) absorption curve were investigated at 10, 19, 35, and 60 GHz for 0.10–1.75-mm-thick c-plane circular disks of flux-grown single-crystal M-type barium ferrite materials. A conventional high-field effective linewidth analysis of the data yielded an effective linewidth which increased with the square of the disk thickness and linearly with frequency, dependencies which indicate a predominant eddy current loss process. Based on these results, an eddy current loss analysis of the tail region was done, based on the insulator FMR response and eddy current losses driven by the FMR response. This analysis leads to a new noninvasive technique for the determination of the microwave conductivity in moderate conductivity ferrites. One obtains the conductivity from an appropriate analysis of the FMR absorption tail in the same way that analysis of the magnetic loss tail yields a high-field effective linewidth. Based on this technique, the microwave conductivity of these flux-grown barium ferrite single-crystal materials was determined as a function of frequency and found to increase linearly from 0.033±0.004 Ω−1 cm−1 at 10 GHz to 0.10±0.02 Ω−1 cm−1 at 60 GHz. These results are consistent with a measured dc conductivity of 0.03–0.05 Ω−1 cm−1.
Nano-indentation studies of ultrahigh strength carbon nitride thin films
Authors: Li Dong , Chung Yip-Wah , Wong Ming-Show , Sproul William D.
Published: July 2002

Carbon nitride (CNx) thin films were prepared by dc magnetron sputtering of a graphite target in a nitrogen ambient onto Si(100) substrates held at ambient temperatures. The films are amorphous with a small volume fraction of nanocrystallites. All CNx coatings grown to a thickness of 1.5 μm are adherent and smooth. Nanoindentation studies showed clear dependence of hardness and effective modulus on deposition process parameters (nitrogen pressure, target power, and substrate bias). Most striking is the observation that CNx films can be synthesized with yield strength exceeding 5 GPa.
Overlayer-induced perpendicular anisotropy in ultrathin Co films (invited)
Authors: Engel Brad N. , Wiedmann Michael H. , Falco Charles M.
Published: July 2002

We have used in situ polar Kerr effect measurements to study the magnetic anisotropy of MBE-grown X/Co/Y trilayers, where X and Y are combinations of the nonmagnetic metals Ag, Au, Cu, or Pd. The competition between the perpendicular anisotropy of the initial underlayer X/Co interface and the in-plane shape anisotropy of the Co film allows us to adjust the total anisotropy of the uncovered Co to be in plane and of moderate strength. In this way, we can measure hard-axis (perpendicular) polar hysteresis curves in situ as a function of overlayer Y coverage, and directly deduce the anisotropy field. Polar hysteresis curves were measured in situ for systematically varied Co and overlayer Y layer thicknesses 2 Å≤tCo≤20 Å and 0 Å≤tY≤100 Å. We find, for particular combinations, the magnitude of the X/Co/Y perpendicular anisotropy is strongly peaked at ∼1 atomic layer overlayer Y coverage.
Electronic structure and magneto-optical properties of MnBi and MnBiAl
Authors: Jaswal S. S. , Shen J. X. , Kirby R. D. , Sellmyer D. J.
Published: July 2002

MnBiAl films are of considerable current interest for possible magneto-optical applications because of their perpendicular anisotropy, large polar Kerr rotation, reduced grain size, and structural stability. We report here experimental and theoretical studies of the effect of Al alloying on the electronic structure and magneto-optical properties of MnBi. Our measured spectral dependencies of the polar Kerr rotation in the two systems are similar. We carried out relativistic self-consistent spin-polarized electronic structure calculations on MnBi and MnBiAl, and the calculated densities of states are in good agreement with available x-ray photoemission spectroscopy data for both materials. The spin magnetic moment of Mn increases and the overall orbital moment decreases with the addition of Al to MnBi. The latter effect implies the lowering of spin-orbit interactions upon alloying MnBi with Al, and the two effects together suggest that the spectral dependencies of the Kerr rotation should be similar, as is observed experimentally.
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enhancement of Co-adsorbed γ-Fe2O3 particles via surface treatment with sodium polyphosphate
Authors: Spada F. E. , Parker F. T. , Berkowitz A. E. , Cox T. J.
Published: July 2002

The coercive force HC of partially reduced, acicular γ-Fe2O3 particles having an adsorbed Co layer (4 wt % Co; Fe+2/Fe+3≊0.09; 2000 Å×240 Å particle dimensions; 36 m2/g specific surface area) increases substantially following surface treatment with (NaPO3)n≊14⋅Na2O. The maximum observed effect occurs at a polyphosphate-to-iron oxide weight ratio P/Fe near 0.5, denoted (P/Fe)max, where HC of the Co-adsorbed oxide increases from 687 to 1096 Oe immediately following treatment. The latter value increased to 1215 Oe after a four month storage in an argon environment containing less than 1 ppm H2O and O2, suggesting that the polyphosphate-oxide interaction is sensitive to moisture or oxygen. Co-adsorbed particles containing no Fe+2 in the oxide core also exhibit HC enhancement following surface treatment, but the change in HC is substantially less than that of particles containing Fe+2. The general behavior of polyphosphate-treated Co-adsorbed particles is observed to be similar to that of treated berthollide particles having no adsorbed Co layer: (i) the magnitude and sign of change in HC varies with P/Fe, reaching a peak value at (P/Fe)max, (ii) the magnitude of increase in HC at (P/Fe)max depends on the Fe+2 content of the particle core, (iii) saturation magnetization of the particles treated at (P/Fe)max is essentially unchanged from that of the untreated precursor, (iv) HC of specimens treated at (P/Fe)max returns to the value of the untreated precursor when the polyphosphate coating is removed via washing with water, and (v) particle dissolution occurs when P/Fe≫(P/Fe)max. Studies of anisotropy field distributions show that there is no obvious bimodal character to the very broad distribution of anisotropy fields found in randomly oriented specimens of surface-treated particles.
Effect of As4/Ga flux ratio on electrical and optical properties of low-temperature GaAs grown by molecular beam epitaxy
Authors: O’Hagan S. , Missous M.
Published: July 2002

Hall-effect, near-band-edge infrared absorption, and photoluminescence measurements have been carried out on undoped and Si- and Be-doped GaAs layers grown by molecular beam epitaxy at a substrate temperature of 250 °C, under As4/Ga flux ratios varying from As-rich to stoichiometric growth conditions. Dopant concentrations at or above ∼1×1019 cm−3 appear to reduce the incorporation of excess arsenic as both antisite and interstitial defects at all flux ratios at this growth temperature, but only under stoichiometric conditions for Si doping of 1×1018 cm−3. The effect is attributed to dopant influencing the dissociation of the As4 molecule and the incorporation of excess As atoms into the crystal. Highly doped n-type material with excellent electrical and optical properties, and high electrical quality p-type material have been achieved by moving towards stoichiometric growth conditions. This is believed to be due to further reduction of formation of compensating defects, Ga vacancies in the n-type case, As antisites in the p type. A photoluminescence peak at 1.24 eV suggests the formation of SiGa−VGa pair defects in the highly Si-doped material.
Effect of composition and temperature on electric fatigue of La-doped lead zirconate titanate ceramics
Authors: Jiang Q. Y. , Subbarao E. C. , Cross L. E.
Published: July 2002

Composition and temperature of ferroelectric La-doped lead zirconate titanate ceramics influence its electric fatigue behavior, defined as the degradation of the electrical properties under the action of an ac field applied for a long time. Compositions of rhombohedral symmetry exhibit little or no fatigue compared with those of tetragonal and orthorhombic symmetry. At temperatures higher than the dielectric maximum, no fatigue effect was detected. Compositions close to phase boundaries (FE-AFE, FE-FE, or FE-PE) display significant fatigue behavior. Electric fatigue arises from the pinning of domains by space charges or injected carriers or from microcracking. The former (which are charge related) is accompanied by smaller strains and is recoverable by thermal and electrical treatment, while the latter (arising from microcracking) arises from large incompatible stresses between grains and is a permanent damage. The understanding of the mechanism of electric fatigue gained in the present study provides guidelines for enhancing the long-term reliability of devices based on ferroic materials.
Magnetite coating prepared by ferrite plating on expanded polytetrafluoroethylene membrane for medical use
Authors: Zhang Q. , Itoh T. , Abe M.
Published: July 2002

As the first step to the study of increasing biocompatibility of expanded polytetrafluoroethylene (EPTFE) membranes, cardiovascular patches of EPTFE substrates were coated with Fe3O4 layer, which is expected to accelerate proliferation of endothelial cells. The ferrite layers were formed by the thin liquid film ferrite plating method at T=75–90 °C and pH (in oxidizing solution)=6.9–7.4. At a fixed pH value, a single-phase Fe3O4 layer is obtained when T is higher than a threshold temperature, which increases with an increasing pH value; below the threshold temperature, the layer has an impurity phase of γ-FeOOH. The obtained Fe3O4 layers are polycrystalline with no preferred orientation, having magnetization and coercive force which agree with those reported for bulk samples.
Effect of oxygen incorporation on magnetoresistance in Co/Cu multilayers
Authors: Kagawa K. , Kano H. , Okabe A. , Suzuki A. , Hayashi K.
Published: July 2002

The background pressure dependence of magnetoresistance (MR) ratio of Co/Cu multilayer films and the effect of the Fe buffer layer were studied. Secondary ion mass spectrometry (SIMS) measurement of the films and in situ quadrupole mass spectrometry (QMS) analysis in conjunction with background pressure dependence of MR ratio confirmed that the oxidation of Co/Cu gives a crucial influence to the giant magnetoresistance (GMR) effect. The oxidation of Co/Cu weakens the antiferromagnetic coupling, and results in the low MR ratio. The Fe buffer layer absorbs the residual oxygen, and this leads to the increase of the MR ratio. The MR ratio of (Co1 nm/Cu1 nm)100/Fe2.5 nm film deposited under low background pressure and high deposition rate so as to suppress oxidation reached over 80% at RT and 160% at 2 K.
Determination of thickness of multiple layer thin films by an x-ray-diffraction technique
Authors: Chaudhuri J. , Hashmi F.
Published: July 2002

An x-ray-diffraction method for determining the thickness of multiple layer thin films grown on a single-crystal substrate is presented. The equations, based on the kinematical theory of x-ray diffraction and the mosaic crystal model, were developed. As an example of the application of the method, thicknesses of a double heterostructure system, namely AlAs/AlGaAs/GaAs, were determined. Good agreement was obtained between the results from x-ray measurement and scanning electron microscopy data, demonstrating the high precision of this technique.
Quantum transmitting boundary method in a magnetic field
Authors: Leng Manhua , Lent Craig S.
Published: July 2002

A numerical algorithm for the solution of the two-dimensional effective-mass Schrödinger equation for current-carrying states, the quantum transmitting boundary method, is extended to magnetotransport problems where a magnetic field is applied. Boundary conditions appropriate for such states are developed and a solution algorithm based on the finite-element method is constructed. The algorithm is valid for general device shapes, general potential profiles, and multiple leads of general orientations. The technique is applied to a quantum channel with a single scatterer, an antidot, in the channel. Magnetic quasibound states (MQBS) are formed around the scatterer and MQBS-induced resonant reflection is observed.
Synchrotron radiation x-ray absorption of ion bombardment induced defects on diamond (100)
Authors: Huang L. J. , Bello I. , Lau W. M. , Lee S.-T. , Stevens P. A. , DeVries B. D.
Published: July 2002

The surface defect structures on diamond (100) surfaces induced by 500 eV neon ion bombardment and by subsequent annealing were studied in situ with x-ray absorption near-edge structure (XANES) spectroscopy using 250–800 eV synchrotron radiation and with low energy electron diffraction. Ex situ x-ray photoemission spectroscopy (XPS) was also used to characterize the defective layer. Significant changes in the XANES spectra were identified for the defects induced by ion bombardment and subsequent annealing. The diamond discrete exciton absorption at 289.0 eV was clearly suppressed even at the lowest ion fluence used in this study, i.e., 3×1014/cm2, and no such exciton could be observed at 7×1014/cm2. However, the changes in the multi-maxima shape-resonance absorption structure in the range of 290–310 eV indicated that a loss of the diamond long range order required a fluence of 1×1015/cm2. The structural changes were also manifested by the transformation of gap state absorption typical of clean 2×1 surfaces to the π* absorption typical of amorphous carbon. XPS showed that the defective layer was about 2 nm thick. For all samples prepared with the bombardment conditions in the study, both the XANES and XPS data also indicated no phase transformation from defective layers to graphite even after annealing to a temperature of 1100 °C.
Controlling high frequency chaos in circular YIG films
Authors: Peterman D. W. , Ye M. , Wigen P. E.
Published: July 2002

High power ferromagnetic resonance experiments involving thin circular YIG films produce chaotic oscillations with broad band frequency spectra in the 0.5–25 MHz range. To eliminate chaotic oscillations, a delayed feedback of the ac component of the ferromagnetic resonance signal was used to modulate one of the system parameters, the applied static magnetic field. The chaotic oscillations were converted to oscillations of reduced periodicity, and ultimately the quiescent state, as the gain in the feedback loop was increased. Similar results have been obtained in numerical simulations.
Epitaxial ferromagnetic MnAs thin films grown by molecular-beam epitaxy on GaAs: Structure and magnetic properties
Authors: Tanaka M. , Harbison J. P. , Park M. C. , Park Y. S. , Shin T. , Rothberg G. M.
Published: July 2002

We have studied structural and magnetic properties of epitaxial MnAs thin films with various thicknesses (L=1.0–200 nm) on GaAs substrates. The MnAs thin films were grown at 200–250 °C on an As-rich disordered c(4×4) (001) GaAs surface by molecular-beam epitaxy (MBE). The growth direction of the MnAs was found to be along the [1̄100] axis of the hexagonal unit cell. X-ray spectra of the MnAs at room temperature have two peaks, indicating that the present MBE-grown MnAs films consist of the hexagonal ferromagnetic phase and orthorhombic paramagnetic phase. Magnetization measurements revealed that the MnAs thin films have perfectly square hysteresis characteristics with relatively high remnant magnetization Mr=300–567 emu/cm3 and low coercive field Hc=65–926 Oe, compared with those of epitaxial MnGa and MnAl thin films reported previously.