bluuebunny/crossref_metadata_2025 huggingface dataset:
Orthogonal pinning of two ferromagnetic layers in a synthetic spin valve
Authors: Beach Robert S. , McCord Jeffrey , Webb Patrick , Mauri Daniele
Published: July 2002

A method for pinning synthetic antiferromagnet (SAF) based spin valves is presented, which allows pinning to be established perpendicular to the setting field H. As the SAF(AP1/Ru/AP2) magnetizes, its layer moments undergo a spin–flop transition for a magnetic field H=HSF and align with H for H=HSat. The magnetization of layer AP1 is perpendicular to H for a field H=HGold. This field corresponds to the maximum giant magnetoresistance GMR on HSF<H<HSat for an unpinned SAF spin valve. SAF-pinned samples [antiferromagnet/AP1/Ru/AP2] set using H=HGold are mixed domain, with AP1 magnetized in two directions perpendicular to H, corresponding to left- and right-handed flopping. The spin–flop handedness may be controlled for a viable process. The technique offers substantial advantages to sensor stabilization schemes that rely on additional pinned ferromagnet/antiferromagnet layers, because these layers may be pinned orthogonal to the SAF without regard to the blocking temperatures of the respective antiferromagnets.
Heavy fermions in Kondo lattice compounds (invited)
Authors: Steglich F. , Rauchschwalbe U. , Gottwick U. , Mayer H. M. , Sparn G. , Grewe N. , Poppe U. , Franse J. J. M.
Published: July 2002

Even in the absence of both magnetic order and superconductivity, Ce-based Kondo lattice systems show distinct anomalies in the low-temperature specific heat and thermopower. These features can be understood, in the frame of a microscopic treatment of the asymmetric Anderson model, by taking into account the periodicity of the Ce sites. A substantial variation in the observed phenomena between CeAl3 and normal (n) state CeCu2Si2 on the one hand as well as CeRu2Si2 and CeCu6 on the other highlight the importance of realistic band structure calculations for the quasiparticles (‘‘heavy fermions’’) in these different compounds. The temperature dependence of the upper critical magnetic field Bc2(T) for the heavy-fermion superconductor CeCu2Si2 (Tc≂0.6 K) shows a flat maximum at ∼0.2 Tc, which is ascribed to coherence-derived structure in the n-state quasiparticle density of states. A dc Josephson effect with a critical pair current of ordinary size is observed for the first time on a weak link between CeCu2Si2 and Al. This proves that CeCu2Si2 is a superconductor with dominant spin-singlet pairing. No Josephson effect can be found, however, between heavy-fermion superconducting UPt3 and Al, Nb, or UPt3 as counterelectrodes, in accord with a possible anisotropic (L≥1, S=0 or 1) pairing in UPt3. For this material, the thermal conductivity in the superconducting state approaches an asymptotic κS=αT2 law as T→0, with α=32 mW/cm K3, and the thermopower above TC shows a temperature dependence similar to that of the ‘‘spin fluctuation’’ system UAl2.
Revealing of defects in InP by shallow (submicron) photoetching
Authors: Weyher J. L. , Giling L. J.
Published: July 2002

Indium phosphide samples (n and p type) have been photoetched in solutions consisting of CrO3-HF-H2O, i. e., in diluted Sirtl or Sirtl-like mixtures under laser or white light illumination. The method proved to be very sensitive for the delineation of defects and inhomogeneities in substrates and epitaxial layers. In n-type InP an etch depth of 0.05–0.3 μm is sufficient to reveal dislocations, growth striations, precipitates, and linear defects or to delineate epijunctions in cross sections. In p-type material, removal of 0.5–1 μm is required to obtain a clear picture of the defects. In contrast to n-type material the results for p-type InP are similar with and without illumination. The method has been calibrated with Huber etch.
Analysis of fluorescence line shapes for free-to-bound transitions in semiconductors
Authors: Bartram R. H. , Vassell M. O. , Zemon S.
Published: July 2002

A theoretical model is presented for the physical description of free-to-bound transitions to deep levels in semiconductors and for the synthesis of their fluorescence line shapes. The analysis takes account of broadening by emission and absorption of lattice quanta, by the continuum of initial electronic states, and by the random distribution of impurity sites or other inhomgeneous mechanisms. The vibrational contribution is developed for linear coupling to a single vibrational mode in the harmonic approximation, and for linear coupling to many modes having a range of vibrational frequencies, including the specialization of the latter results to the ‘‘narrow coupling’’ limit. The electronic contribution is specified only for thermal broadening by conduction-band states. The contribution of inhomogeneous mechanisms is modeled by Gaussian distributions. Formulas for the line-shape function and its initial moments are provided for use in fitting and interpreting experimental photoluminescence data. Use of the ‘‘narrow coupling’’ results to synthesize the temperature dependence of measured line shapes can prescribe the parameters of a single-frequency configurational-coordinate model.
Carrier conduction in ultrathin nitrided oxide films
Authors: Suzuki Eiichi , Schroder Dieter K. , Hayashi Yutaka
Published: July 2002

Electrical conduction in ultrathin nitrided oxide films on silicon has been investigated by comparing it with that in ultrathin oxide. Significant current enhancement in the nitrided oxide and partial recovery of the enhancement by O2 annealing were observed, which can be satisfactorily explained by a two-step tunneling via traps generated in the nitrided oxide during nitridation. From the comparison between theoretical calculations of tunneling current and experimental data for the nitrided oxide and the SiO2, it is found that the effective barrier height for electrons at an aluminum-nitrided oxide interface decreases from 3.1 to 2.5 eV for an as-grown nitrided oxide and to 2.75 eV for a nitrided oxide annealed in O2. The nitrogen-related traps are estimated to be situated at the surface portion within 32–34 Å from the metal-insulator interface with the energy level of 2.87 eV for the former and 3.32 eV for the latter below each conduction-band edge.
Interaction of a vircator microwave generator with an enclosing resonant cavity
Authors: Benford J. , Price D. , Sze H. , Bromley D.
Published: July 2002

Interaction between a vircator microwave generator and a resonant cavity is produced by enclosing the virtual cathode inside a right circular cylinder. By varying the cavity length, frequencies corresponding to the TM01n cavity modes are tuned to resonate with the broad- band oscillating virtual cathode. Nonresonant frequencies are suppressed, decreasing the bandwidth of the emitted radiation by a factor ≥5. Measured microwave power is about 4 times that from a detuned cavity.
An electroreflectance study of CdTe
Authors: Enloe W. S. , Parker J. C. , Vespoli J. , Myers T. H. , Harper R. L. , Schetzina J. F.
Published: July 2002

Electroreflectance measurements are reported for bulk and epitaxial CdTe layers grown by molecular-beam epitaxy (MBE). Spectra are analyzed to determine the E0, E1, and E1+Δ1 transition energies as well as the associated broadening energies. The broadening energies are found to correlate with the structural quality of the epilayers. Additional spectral features near the band edge (1.45–1.51 eV) and in the UV (3.1–4.3 eV) are discussed. The extremely sharp spectra obtained for the MBE-grown epitaxial layers indicate high-quality material.
Magnetization and magnetic anisotropy of R2Co14B and Nd2(Fe1−x Cox)14B measured on single crystals
Authors: Hirosawa S. , Tokuhara K. , Yamamoto H. , Fujimura S. , Sagawa M. , Yamauchi H.
Published: July 2002

Magnetization and anisotropy field of R2Co14B with R being La, Nd, and Gd and of Nd2(Fe1−xCox)14B have been measured on single-crystal specimens. The cobalt sublattice has a planar anisotropy: K1=−1.30 MJ/m3 at 4.2 K for La2Co14B with the [110] direction being the room-temperature easy direction of magnetization. The rare-earth ions in R2Co14B experience a smaller exchange field than in R2Fe14B, which results in a rapid reduction of the rare-earth sublattice magnetization and anisotropy with increasing temperature. Because of the planar anisotropy of cobalt and the weaker R-Co interactions than R-Fe, the anisotropy field of Nd2(Fe1−xCox)14B turns out to be almost independent of the cobalt concentration in a temperature range between 300 and 500 K.
Influence of Bi and Mn on the green luminescence of ZnO ceramics
Authors: García J. A. , Remón A. , Piqueras J.
Published: July 2002

The effect of the addition of Bi and Mn on the photoluminescence from ZnO ceramics has been investigated. The effect of the presence of impurities on the green luminescence band can be compared to the effect of oxidizing treatments. A narrow green band has been observed in Mn-doped samples.
Magnetic behavior of Y-Ba-Cu-O superconductors prepared under different oxygen anneals
Authors: Datta T. , Almasan Carmen , Estrada J. , Violet C. E. , Gubser D. U. , Wolf S. A.
Published: July 2002

In our study of three YBa2 Cu3 oxides prepared with different oxygen anneals, we observed that the temperature derivative of the magnetization or susceptibility χ(T) of these high-Tc superconductors has a maximum at approximately the transition temperature, Tc . This temperature as estimated by the peak value of (∂χ/∂T)H , ranged between 55 and 65 K. The most oxygen-poor sample showed the lowest Tc and the most oxygen-rich sample showed the highest Tc . In the subcritical region, a novel irreversibility is observed in the field-cooled warm-up and cool-down behaviors.
Structural comparison of amorphous Cu50Zr50 alloys prepared by proton irradiation, melt spinning, and mechanical alloying
Authors: Lee Dokyol , Cheng J. , Yuan M. , Wagner C. N. J. , Ardell A. J.
Published: July 2002

X-ray diffraction was used to evaluate the structures of amorphous samples of Cu50Zr50 prepared by three different methods, namely, melt spinning, mechanical alloying, and irradiation by 2-MeV protons. The results demonstrate conclusively for the first time that, within experimental error, the short-range atomic distribution in an amorphous alloy is the same, regardless of how it is prepared.
ac impedance measurements and V-I characteristics for Co-, Mn-, or Bi-doped ZnO
Authors: Smith Agnès , Baumard Jean-François , Abélard Pierre , Denanot Marie-Françoise
Published: July 2002

ZnO has been doped with various oxides, mainly bismuth, cobalt, and manganese oxides. The aim of the present approach is to look at the influence of each dopant on the electrical behavior of ZnO in a system which is more simple and chemically purer in comparison with commercial varistors. It appears that cobalt or manganese additions do not substantially modify the bulk conductivity of ZnO. Also these dopants by themselves do not lead to a varistor effect when the material is subjected to a potential sweep. On the other hand, bismuth-based compositions behave like varistors. The best performances have been recorded for a simultaneous doping with manganese and bismuth.
Current distribution during the breakdown in a coaxial electrode system
Authors: Feugeas J. , von Pamel O.
Published: July 2002

The breakdown stage in a coaxial electrode system (similar to Mather-type plasma focus) was studied in hydrogen for a wide range of pressures through the fast discharge of the energy stored in a parallel plate capacitor. A peak current Iep of 18.5 kA was reached in 60 ns for a 20 kV capacitor charge voltage, giving the typical initial current rise time obtained in normal plasma focus discharges. Thin current sheaths (≲2 mm thick) in the insulator region with current density ≳10 kA/cm2 were measured. These current sheaths always showed a well-defined filamentary structure with the number of filaments depending linearly on Iep , with an observed current limit per filament If depending on the pressure P as I2f∼1/P.
Derivation and correction of the Tsu–Esaki tunneling current formula
Authors: Bandara K. M. S. V. , Coon D. D.
Published: July 2002

The theoretical basis of the Tsu–Esaki tunneling current formula [Appl. Phys. Lett. 22, 562 (1973)] is examined in detail and corrections are found. The starting point is an independent particle picture with fully antisymmetrized N-electron wave functions. Unitarity is used to resolve an orthonormality issue raised in earlier work. A new set of mutually consistent equations is derived for bias voltage, tunneling current, and electron densities in the emitter and collector. Corrections include a previously noted kinematic factor and a modification of emitter and collector Fermi levels. The magnitude of the corrections is illustrated numerically for the case of a resonant tunneling current-voltage characteristic.
Generalized formulas for reflected pulse response of multilayered structures
Authors: Chern E. James , Nielsen Hatsumi T. C.
Published: July 2002

To accurately evaluate the integrity of the interfaces of a multilayered structure using ultrasonic pulse echo methods, we must understand the wave propagation in the structure and identify the appropriate interface signal. However, the signal is often complicated by the material and defect characteristics of the preceding interfaces. We have derived generalized mathematical formulas for the reflected mode time and the pulse amplitude response of an arbitrary n-layered structure for use in modeling the wave propagations in multilayered structures and identifying the critical structural interfaces. The time and echographic amplitude responses are calculated by computer using routines programmed for a given specimen. A comparison of the results of laboratory measurements of various glass and aluminum specimens with the results from computer-simulated experiments have verified the validity of the formulas.
Ion scattering analysis programs for studying surface and interface structures
Authors: Kido Yoshiaki , Koshikawa Takanori
Published: July 2002

We have synthesized ion scattering analysis programs for studying surface and interface structures. The probing ion energies can be selected in a wide energy range from 10 keV up to several MeV. One of the present programs allows the simulation of both random and aligned spectra from perfect or partially disordered heteroepitaxial films. For defect profiling, the following types are available: (1) asymmetric Gaussian, (2) exponential reduction shape, and (3) step function. Another is used to analyze the structures of islands formed on the top surfaces of substrates. The targets have basically multielemental and multilayered structures comprising the units of amorphous/single crystal. These simulation programs are applied to disorder profiling of Ar+-implanted LiNbO3, mechanically polished Mn-Zn ferrite, and GaAs/Si heteroepitaxial films, and to characterizing the Cu-Si islands on Si substrates.
Thermoelectric power of annealed β-Ag2Se alloy thin films: Temperature and size effects—possibility of a new (β2) phase at low temperatures
Authors: Damodara Das V. , Karunakaran D.
Published: July 2002

Thermoelectric power of annealed β-Ag2Se thin films of different thicknesses has been measured both while heating and cooling by the integral method. It is found that it remains practically constant (in β-Ag2Se phase) during heating while it is a function of temperature while cooling. The thermoelectric power in both heating and cooling cycles is a function of inverse thickness of the films. The difference in behavior between Ag2Se films during heating and cooling is attributed to the possible transformation to monoclinic phase during cooling from the original orthorhombic phase during heating. The inverse thickness dependence has been explained by the size effect theories. Important material parameters like carrier concentration, Fermi energy, effective mass of carriers, and energy dependence of the mean free path have been evaluated for the β-Ag2Se (orthorhombic) phase.
Magnetic and transport properties of Fe/Cr superlattices (invited)
Authors: Barthélémy A. , Fert A. , Baibich M. N. , Hadjoudj S. , Petroff F. , Etienne P. , Cabanel R. , Lequien S. , Nguyen Van Dau F. , Creuzet G.
Published: July 2002

We describe the magnetic and transport properties of Fe(001)/Cr(001) superlattices grown on GaAs (001) by molecular-beam epitaxy and characterized by reflection high-energy electron diffraction (RHEED), Auger spectroscopy, x-ray diffraction, and electron microscopy. For Cr layers thinner than about 30 Å the magnetic behavior reveals strong antiferromagnetic couplings between the Fe layers across the Cr layers. Polarized neutron diffraction experiments confirm the existence of an antiferromagnetic superstructure. We discuss the origin of the antiferromagnetic (AF) coupling. The Fe/Cr superlattices with AF interlayer coupling exhibit a giant magnetoresistance: when an applied field aligns the magnetizations of the Fe layers, the resistivity drops by a factor of 2 for some samples. This giant magnetoresistance can be ascribed to the spin dependence of the electron scattering by interfaces. We compare our results with the predictions of two recent theoretical models.
Pressure and temperature dependence of the first-order Raman mode of diamond
Authors: Tardieu A. , Cansell F. , Petitet J. P.
Published: July 2002

The fundamental Raman mode of diamond has been measured as a function of P and T up to 15 GPa and 400 °C. The calculated values of pressure coefficient (2.64 cm−1/GPa) and temperature coefficient (0.9×10−2 °C/GPa) of diamond show that this material can be used as a pressure sensor in a diamond anvil cell at high temperature. The Grüneisen coefficient (γ) is calculated and found to be equal to 0.9. From this value of γ, the atomic volume of diamond is obtained in the whole range of pressure and temperature.
Device-quality wide-gap hydrogenated amorphous silicon films deposited by plasma chemical vapor deposition at low substrate temperatures.
Authors: Hishikawa Yoshihiro , Tsuge Sadaji , Nakamura Noboru , Tsuda Shinya , Nakano Shoichi , Kuwano Yukinori
Published: July 2002

Hydrogenated amorphous silicon (a-Si:H) films have been fabricated by a plasma chemical vapor deposition (plasma-CVD) method at low substrate temperatures (Ts : 80 or 50 °C) to obtain wide-gap films. Device-quality wide-gap films (photoconductivity under AM-1.5, 100 mW/cm2 illumination ∼10−5 Ω−1 cm−1, ratio of photoconductivity and dark conductivity ∼106, and Tanc’s gap ∼2 eV) are obtained at low Ts, by optimizing the plasma parameters or by diluting the material gas (SiH4) with H2. Experimental results suggest that lowering the deposition rate of a-Si:H films is an important factor in obtaining high-quality films at low Ts using a plasma-CVD method.
The effect of high-temperature heat treatment on the electrical power factor and morphology of silicon germanium-gallium phosphide alloys
Authors: Min Gao , Rowe D. M.
Published: July 2002

The results of an experimental investigation into the effect of high-temperature heat treatment on the electrical resistivity and Seebeck coefficient of silicon germanium-gallium phosphide alloys and the accompanying changes in morphology are reported. The specimens were heat treated for different periods of time and quenched or allowed to cool slowly in air. Their morphology was investigated using scanning electron microscopy and electron spectroscopy for chemical analysis. The electrical resistivity and Seebeck coefficient were measured as a function of temperature from room temperature to about 800 K. Substantial changes in the electrical properties and in morphology were observed after 15 min and 4-h periods of heat treatment, respectively. A less significant decrease in electrical resistivity accompanied further heat treatment. However, the Seebeck coefficient remained almost constant which resulted in an increase in the electrical power factor. Prior to heat treatment, the alloy consisted of host matrix and a silicon-rich black phase. Heat treatment was accompanied by the appearance of a white phase which was rich in germanium and gallium phosphide. Although in general it was not possible to correlate the changes in electrical properties with the morphology. The highest power factor obtained in this investigation corresponded to a morphology which consisted of the host matrix and a significant proportion of white phase.
Filament-assisted diamond growth kinetics
Authors: Harris Stephen J. , Weiner Anita M. , Perry Thomas A.
Published: July 2002

We have measured the growth rate of chemical vapor deposition diamond films under filament-assisted conditions using a microbalance. The pressure was varied from 20 to 200 Torr, and the ratio R of CH4 to H2 was varied from 0.2% to 1%. Raman spectra showed only diamond features for our films. We found that for R≳0.2%, where filament carburization was not an issue, the growth rate scaled as Rα, where α is an empirical constant that varies from about 1 at 20 Torr to about 0.5 at 200 Torr. A comparison of these results to predictions of our gas-phase/gas-surface model for diamond growth shows that the model accurately predicts both the value of α and how α varies with pressure. Reasons for the success of our very simple model are discussed.
Analysis of cyclic deposition of diamond
Authors: Wang Hai , Frenklach Michael
Published: July 2002

A cyclic process of diamond deposition was investigated using a detailed surface kinetic model. The process modeled in this study was comprised of two alternating steps: a growth period in a methane-helium mixture, and an etching period in a hydrogen-oxygen-helium mixture. The model included a detailed description of gas-surface reactions in terms of elementary chemical steps. Experimental data available in the literature were simulated first and a fair agreement was obtained between the model and experiment. The model was then subjected to an analysis by varying the time periods of the cycle. The computational results predict a significant increase in the average growth rate of diamond films containing low sp2 carbon component for shorter total cycles, and longer the growth period of the cycle.
Magnetic and structural characterization of Nd5Fe17
Authors: Landgraf F. J. G. , Missell F. P. , Rechenberg H. R. , Schneider G. , Villas-Boas V. , Moreau J. M. , Paccard L. , Nozières J. P.
Published: July 2002

A new stable compound in the Nd–Fe system has been identified as Nd5Fe17. The crystal structure belongs to the hexagonal space group P63/mcm, with a=2.0214(8) nm and c=1.2329(8) nm, and with 12 formula units per unit cell. This compound is a ferromagnet with Tc = 503 K and magnetization 162 emu/g at 4.2 K. Mössbauer spectra have been measured and fitted with five Fe sites, yielding an average Beff=26.5 T at room temperature, from which an average Fe moment of 1.83 Bohr magnetons is deduced. X-ray powder diagrams from magnetically aligned samples suggest basal anisotropy.
Damage coefficient associated with free exciton lifetime in GaAs irradiated with neutrons and electrons
Authors: Parenteau M. , Carlone C. , Khanna S. M.
Published: July 2002

The lifetime τ of the free exciton at T=17 K in irradiated GaAs has been determined by the reflectance of layers grown by metalorganic chemical vapor deposition and of semi-insulating samples grown by the liquid-encapsulated Czochralski method. The samples had been irradiated with 1 MeV and thermal neutrons, and with 7 MeV and white electrons up to 7 MeV. The linear increase of τ −1 as a function of the fluence is explained by a simple kinetics model, which applies especially well to the epitaxial layers. The damage coefficient kτ associated with this lifetime has been measured. For the defects created by all types of radiation, the radius associated with the cross section for the capture of excitons is three to ten times the radius of the free exciton. The exciton transition energy is found to decrease as its reflectance structure is broadened.
Simplified closed-form trap-assisted tunneling model applied to nitrided oxide dielectric capacitors
Authors: Fleischer S. , Lai P. T. , Cheng Y. C.
Published: July 2002

Conduction has been studied in ultrathin nitrided oxide, re-oxidized nitrided oxide, and nitrogen-annealed nitrided oxide film capacitors in which the nitridation step was performed by a low-partial-pressure nitridation technique. Results indicate that, as well as some degree of barrier lowering due to the build-up of nitrogen at the injecting interface, a trap-assisted mechanism could be responsible for the enhanced conduction exhibited by the nitrided oxide devices. A simplified closed-form trap-assisted tunneling model is employed that produces a fit to the data with a trap depth of 2.1 eV. The difference between this trap model and a model requiring numerical integration was negligibly small (∼2%).
Electrostatic and current transport properties of n+/semi-insulating GaAs junctions
Authors: Darling Robert B.
Published: July 2002

The steady-state charge-balance-related properties of semiconductor junctions between highly doped n-type epitaxial layers of GaAs and semi-insulating GaAs substrates are examined. Specific results are obtained for the three most common defect compensation mechanisms within the semi-insulating material: (i) a deep donor interacting with a shallow acceptor, typified by the case of the EL2 defect and background carbon, respectively; (ii) a shallow donor and a deep acceptor, as would occur for heavy levels of silicon and chromium; and (iii) a deep donor and a deep acceptor, as would be typical of EL2 and light levels of either chromium or a complementary antisite defect. Electrostatic properties, including Fermi-level positions, built-in potentials, asymptotic electric-field profiles, and junction capacitance are analytically derived based upon Hall/Shockley–Read models of the defect states and these are additionally compared against numerical solutions which implement the same models. Junction boundary conditions that pertain to the high-level injection case normally encountered in these junctions are also analytically derived and verified by numerical simulation. Limitations on the applicability of standard step-profile p-n–junction theory are discussed, as are necessary considerations for proper numerical modeling.
Entropy fluxes, endoreversibility, and solar energy conversion
Authors: De Vos A. , Landsberg P. T. , Baruch P. , Parrott J. E.
Published: July 2002

A formalism describing the conversion of radiation energy into work can be constructed in terms of energy and entropy fluxes. This leads, among others, to the Landsberg efficiency, a formula expressing an upper bound to photothermal solar energy conversion. The relationship between the above approach and the viewpoint of endoreversible thermodynamics is investigated. An extension to photovoltaic solar energy conversion is formulated. A generalization of the Landsberg formula, for a solar energy converter with arbitrary bandgap, is presented.
Near-blue photoluminescence of Zn-doped GaS single crystals
Authors: Aono Tomoyoshi , Kase Kunio , Kinoshita Akira
Published: July 2002

The photoluminescence and optical absorption properties of Zn-doped GaS crystals prepared by the iodine vapor transport method are reported. Undoped GaS crystals are also used for comparison. Zn substituted for a Ga site acts as an acceptor having a deep energy level and forms a complex with the iodine coactivator, which can be the luminescence center. When increasing the charged amount of Zn, the near-blue emission band of 2.47 eV at 97 K becomes dominant, while the 2.17-eV emission band due to the Ga vacancies, which is dominant in the undoped crystals, gradually vanishes because of the reduction of the Ga vacancies. It is thus described that Zn is a promising dopant in GaS for near-blue-light-emitting devices.
Chemical and compositional changes induced by N+ implantation in amorphous SiC films
Authors: Laidani N. , Bonelli M. , Miotello A. , Guzman L. , Calliari L. , Elena M. , Bertoncello R. , Glisenti A. , Capelletti R. , Ossi P.
Published: July 2002

The effects of 30 keV N+ implantation in amorphous silicon carbide films deposited on silicon substrates by rf sputtering over a fluence range of 1×1016–2×1017 ions cm−2, are studied by means of x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and infrared (IR) absorption techniques. The ion-induced modifications of these films have been investigated on the basis of the chemical state evolution of Si, C, and N (using XPS and AES) and on the basis of the vibrational features of the films components (using IR absorption). The results show that implanted N bonds Si selectively, substituting the C atoms in the silicon carbide, and the C substitution by N results in a composite layer of carbonitrides and free C. An ion-induced C transport has also been observed and correlations are established between the formation of silicon carbonitrides and the dynamical behavior of the C in the implanted layer. The latter appears as a superposition of (a) a chemically induced atomic redistribution, required by local stoichiometry and space-filling possibilities in an amorphous network, and (b) a radiation-induced redistribution, a mechanism that is prevailing at low-fluence implantation.
Temperature dependence of species concentrations near the substrate during diamond chemical vapor deposition
Authors: Corat E. J. , Goodwin D. G.
Published: July 2002

Measurements have been made of the temperature dependence of CH3, CH4, and C2H2 very near the substrate during filament-assisted diamond growth. CH3 was detected using (2+1) resonance-enhanced multiphoton ionization (REMPI), and CH4 and C2H2 concentrations were measured using sampling mass spectrometry. A strong dependence of the CH3 REMPI signal on substrate temperature was observed, which at low temperatures may be characterized as having an activation energy of approximately 4±1 kcal/mole. Methane and acetylene, on the other hand, are relatively independent of substrate temperature. These results are most likely due to recombination of methyl to methane or ethane in the cool gas layer near the substrate or on the surface at low substrate temperatures.
The role of negative ions in the formation of particles in low-pressure plasmas
Authors: Choi Seung J. , Kushner Mark J.
Published: July 2002

Large particles (tens of nm to tens of μm in diameter) are problematic in low-pressure (<1 Torr) plasma processing (etching, deposition) discharges because they can contaminate the product and can perturb electron transport. Although the source of these particles has been studied by a number of groups, a definitive explanation is still lacking. In this paper, we theoretically investigate the role of negative ions in the formation of large clusters, the precursors to particles, in low-pressure plasmas. We find that the formation of particles requires a critically large cluster. Forming the critically large cluster requires longer residence times in the plasma than is usually possible if clustering involves only neutral particles. We propose that negatively charged intermediates, which are trapped in electropositive plasmas, increase the average residence time of clusters to allow the growth of critically large clusters.
Radiative and nonradiative transitions in GaAs:Er
Authors: Fang Xiao M. , Li Yabo , Langer Dietrich W.
Published: July 2002

Erbium doped GaAs was grown by metal-organic chemical vapor deposition using a novel liquid precursor: tris(n-butylcyclopentadienyl)erbium [Er(C4H9C5H4)3]. The morphology was excellent at growth temperatures near 620 °C. Based on a simple model for the excitation dependence of the emission, the nonradiative Auger-type process was estimated to be nearly five times that of the energy transfer process from bound excitons to the Er3+ ions that subsequently resulted in the Er-related light emission. Temperature induced quenching of the emission was found to be dominated by transitions with an activation energy of 74 meV.
Ferroelectric properties of Cd1−xZnxTe solid solutions
Authors: Benguigui L. , Weil R. , Muranevich E. , Chack A. , Fredj E. , Zunger Alex
Published: July 2002

Measurements of the spontaneous polarization P, x-ray diffraction, birefringence, dielectric constant at different frequencies, and specific heat Cp of the Cd0.9Zn0.1Te alloy are presented. The results demonstrate that this system exhibits a diffuse, second-order ferroelectric transition. The transition is of order-disorder type as deduced from the dielectric measurements. It is found that: (a) The birefringence is proportional to P, as expected from a system with a piezoelectric paraelectric phase; (b) the heat capacity is given by Cp=(Tc/C)‖(PdP/dT)‖, where C is the Curie constant. One of the main phenomena observed in these solid solutions is the instability of the ferroelectric phase: Once the neighborhood of the transition temperature is reached, the transition disappears upon subsequent cooling. A hypothesis for this instability is offered in terms of a two-state configuration-coordinate diagram.
Demagnetized-state dependence of Henkel plots. I. The Preisach model
Authors: Vajda Ferenc , Della Torre Edward , McMichael R. D.
Published: July 2002

The interpretation of interaction through Henkel plots and Δm plots has become popular in recent years; however, the demagnetized state is often not specified. In this paper, the demagnetized state dependence of Henkel plots is calculated using the classical Preisach model, the moving model, and the complete-moving-hysteresis model. In the calculation of the virgin remanence curve, ac, dc+, and dc− demagnetized states were used. The resulting collection of Henkel plots contains examples of ‘‘up’’ and ‘‘down’’ deviations from the Wohlfarth line by changing the demagnetized state only.
Vertical drift mobility of excited carriers in multi quantum well structures
Authors: Fraenkel A. , Finkman E. , Maimon S. , Bahir G.
Published: July 2002

Measurements of the vertical mobility of photo-excited carriers in quantum well infrared detectors are reported. The low temperature mobility is extracted by means of a photo geometrical magnetoresistance technique. This approach is most suitable for the multi quantum well structure geometry. Both rectangular and stepped well structures were studied. They exhibit asymmetry with regard to bias polarity for both gain and mobility. This asymmetry is shown to be contributed by both well structure, and growth induced inequivalent interfaces. The low field mobility is governed mainly by unscreened ionized impurity scattering in the barriers. The well interfaces act as additional anisotropic scattering centers. We observe clear correlation between the extracted drift velocity and the optical gain. It is postulated that anisotropic well recapture causes polarity dependent energy distribution above the barriers. In addition, we conclude that intervalley scattering of the hot electrons has a substantial influence on the detector performance.
Evaluation of the Coulomb energy for single-electron interface trapping in sub-μm metal-oxide-semiconductor field-effect transistors
Authors: Mueller H. H. , Wörle D. , Schulz M.
Published: July 2002

Capture and emission time constants are measured for a set of individual interface traps in different metal-oxide-semiconductor field-effect transistors (MOSFETs) by random telegraph signals. The data are evaluated to extract the Coulomb energy induced by the transfer of a single electron into an interface trap. A unified Coulomb energy of the order of several hundred millivolts independent of trap-specific properties is found, which is proportional to temperature and decays logarithmically with inversion carrier density in the MOSFET channel. The Coulomb energy found is in quantitative agreement with the theoretical modeling. The Coulomb effect is large compared to the trap lowering by the electric field and to the residual entropy change.
Characterization of nanocrystallites in porous p-type 6H-SiC
Authors: Shor J. S. , Bemis L. , Kurtz A. D. , Grimberg I. , Weiss B. Z. , MacMillian M. F. , Choyke W. J.
Published: July 2002

We report the formation of porous p-type 6H-SiC. The existence of uniformly dispersed pores was confirmed by transmission electron microscopy, with interpore spacings in the range of 1–10 nm. The porous film as a whole is a single crystal. Luminescence peaks above the normal band gap of 6H-SiC have been observed in the porous layer, but were not distinguished in the bulk SiC substrate. Quantum confinement is discussed as a possible mechanism for the luminescence effects.
Giant magnetoresistance related transport properties in multilayers and bulk materials (invited)
Authors: Sato H. , Henmi H. , Kobayashi Y. , Aoki Y. , Yamamoto H. , Shinjo T. , Sechovsky V.
Published: July 2002

A systematic comparison of magnetoresistance, Hall effect, thermal conductivity, and thermoelectric power has been made on systems exhibiting giant magnetoresistance (GMR), Co/Cu/Ni(Fe) multilayers, and AgCo granular alloys, for examples. Each property exhibits field dependence characteristic of the GMR and justifies its own merit in characterizing the conduction-electron scattering responsible for the GMR. The comparison was extended to intermetallic compounds such as REGa2 and RECo2 (RE: rare earth element) which also show a large magnetoresistance.
Charge separation and photovoltaic conversion in polymer composites with internal donor/acceptor heterojunctions
Authors: Yu G. , Heeger A. J.
Published: July 2002

The photosensitivity of semiconducting polymers can be enhanced by blending donor and acceptor polymers to optimize photoinduced charge separation. We describe a novel phase-separated polymer blend (composite) made with poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene], MEH-PPV, as donor and cyano-PPV, CN-PPV, as acceptor. The photoluminescence and electroluminescence of both component polymers are quenched in the blend, indicative of rapid and efficient separation of photogenerated electron-hole pairs with electrons on the acceptor and holes on the donor. Diodes made with such a composite semiconducting polymer as the photosensitive medium show promising photovoltaic characteristics with carrier collection efficiency of 5% electrons/photon and energy conversion efficiency of 0.9%, ∼20 times larger than in diodes made with pure MEH-PPV and ∼100 times larger than in diodes made with CN-PPV. The photosensitivity and the quantum yield increase with reverse bias voltage, to 0.3 A/W and 80% electrons/photon respectively at −10 V, comparable to results obtained from photodiodes made with inorganic semiconductors.
In situ measurement of size and density of submicron aerosol particles
Authors: Schleicher B. , Künzel S. , Burtscher H.
Published: July 2002

The density of nanometer sized particles of different materials (Pd, Ag, Co, Carbon, oil droplets and oil burner exhaust) suspended in a carrier gas is determined in situ as function of particle radius via the measurement of particle mobility and inertia using a differential mobility analyzer and a low pressure impactor. From the density and size of agglomerated particles, information on the structure of the agglomerates can be obtained; in particular a fractal-like dimension can be determined. Whereas the densities of carbon particles and of particles in oil burner exhaust have the expected values, the densities of the metal particles are too low compared to the bulk values. The results for the fractal-like dimensions of the agglomerates can be explained by assuming a mixture between monomer-cluster and cluster-cluster agglomeration in the free-molecular regime. The fractal-like dimension of ferromagnetic Cobalt agglomerates provides evidence that particles consisting of magnetic materials tend to form chainlike agglomerates.
Modeling the diffusion of grown-in Be in molecular beam epitaxy GaAs
Authors: Hu J. C. , Deal M. D. , Plummer J. D.
Published: July 2002

The diffusion of Be in GaAs is studied in samples which are molecular beam epitaxy GaAs with grown-in Be. The Be diffusion profiles of samples annealed under various conditions are obtained using secondary ion mass spectrometry. SUPREM-IV.GS, a simulator for GaAs and Si processing technology, is used to compare the experimental results with our models and to extract parameters. The Be diffusion profiles show a kink feature and a time-dependent Be diffusivity which are successfully simulated. The intrinsic Be diffusivity, the Ga interstitial diffusivity, and the equilibrium concentration of Ga interstitials, all as a function of temperature, are obtained from this study: D+1Be=0.17 exp(−3.39 eV/kBT) cm2 s−1, DI=6.4×10−5 exp(−1.28 eV/kBT) cm2 s−1, and CI*int=4.7×1028 exp(−3.25 eV/kBT) cm−3. The role of nonequilibrium Ga point defects in the anomalous Be diffusion behavior is addressed.
Behavior of charge carriers and excitons in multilayer organic light-emitting diodes made from a polysilane polymer as monitored with electroluminescence
Authors: Suzuki Hiroyuki , Hoshino Satoshi
Published: July 2002

Using electroluminescence (EL) as a monitor, we have investigated the behavior of charge carriers injected from electrodes and excitons generated by the recombination of charge carriers in multilayer organic light-emitting diodes (LEDs) using poly(methylphenylsilane) (PMPS) as a hole transporting material. Our multilayer LEDs have two or three functional organic layers including Coumarin 6 [3-(2′-benzothiazolyl)-7-diethylaminocoumarin, abbreviated as C6] and/or tris-(8-hydroxyquinoline) aluminum layers as well as a PMPS layer. When the LEDs were fabricated, two parameters of the C6 layer were changed, the layer thickness (30–120 nm) and the dye concentration (1–100 wt %). We employed a combined analysis of the dependence of the EL spectra on the thickness and dye concentration of the C6 layer, the dye-selective fluorescence spectra and the current–voltage–EL characteristics, to reveal the thickness of the electron–hole capture zone and the behavior of charge carriers and excitons during operation in these LEDs.
High frequency (1–1200 MHz) magnetoimpedance in CoFeSiB amorphous wires (abstract)
Authors: Paramonov V. P. , Antonov A. S. , Lagarikov A. N. , Panina L. V. , Mohri K.
Published: July 2002

Magnetoimpedance (MI) in Co-based amorphous alloys has been shown to be a very sensitive, quick-response new method for measurements of magnetic fields. At present, the experimental results on MI in amorphous wires are obtained for frequencies f<200 MHz. For quick-response magnetic heads used in high density magnetic recording, the carrier current frequency of the MI element is needed to be increased up to 1 GHz to detect recorded signals of 50–100 MHz. Here we present the experimental data on MI in CoFeSiB amorphous wires for a broad waveband of 1–1200 MHz. The experimental technique is based on the measurement of the complex reflection coefficient from a coaxial waveguide having an amorphous wire as an internal conductor. This method avoids the radiation effects, inevitable at high frequencies, f≳100 MHz, in simpler techniques utilizing an oscilloscope or an impedance analyzer with a lead wire. From the data on reflection coefficient, the real and imaginary parts of the wire impedance Z and permeability μ are found as functions of a frequency and an external longitudinal field. The impedance versus field behavior changes with increasing the frequency. For f<1 MHz the absolute value of the impedance ‖Z‖ decreases with increasing the field. As the frequency is increased, a maximum appears in the impedance-field dependence. In the case of high frequencies, f≳800 MHz, ‖Z‖ increases with the field. For all frequencies, a higher sensitivity is seen in small fields less than 2 Oe. The sensitivity has a maximum of about 100%/Oe at the frequency of 600 MHz, and it is still very high (∼20%/Oe) up to f∼1 GHz. These results are in satisfactory agreement with the theoretical ones based on the skin effect in a magnetic wire with a tensor rotational permeability.
Reply to ‘‘Comment on ‘Evidence of enhanced epitaxial crystallization at low temperature by inelastic electronic scattering of mega-electron-volt heavy-ion-beam irradiation’ ’’ [J. Appl. Phys. 80, 4235 (1996)]
Authors: Nakata Jyoji
Published: July 2002

Enhanced epitaxial crystallization by mega-electron-volt (MeV) heavy-ion-beam irradiation cannot be explained only by the diffusion-limited model. The validity of data showing the absence of dose rate effect on MeV ion-beam-induced epitaxial crystallization (IBIEC) rate is examined in relation to the self-ion-beam annealing effect in MeV As+-ion-beam irradiation in the crystalline Si substrate. The reliability of the experimental results showing the critical thickness dependence in the diffusion-limited model is discussed in detail. It is pointed out that there is room to question the existence of critical thickness. Two guidelines for constructing an accurate IBIEC model are proposed.
Properties of a submicrometer x-ray beam at the exit of a waveguide
Authors: Jark Werner , Di Fonzo Silvia , Lagomarsino Stefano , Cedola Alessia , di Fabrizio Enzo , Bram Andreas , Riekel Christian
Published: July 2002

This report discusses the properties of a 13-keV submicrometer x-ray beam exiting from a waveguide. Waveguides for this spectral regime can be constructed by enclosing a low-absorbing material between highly absorbant metals. Best performance is found for about 0.1 μm guiding layer thickness. Measurements of the photon beam size close to the exit and of the intensity distribution far from the exit will be presented. From these data one derives a beam size at the exit which is identical to the guiding layer thickness. This number being in the submicrometer range offers interesting perspectives for microscopy experiments in the hard x-ray range.
Size dependence of switching thresholds for pseudo spin valve MRAM cells
Authors: Everitt B. A. , Pohm A. V. , Daughton J. M.
Published: July 2002

For magnetoresistive random access memory (MRAM) to be a successful new memory technology, very dense cells with high output are necessary. As bit sizes shrink, it becomes advantageous to employ memory modes in which the uniaxial anisotropy lies along the length of the bit. For bits smaller than approximately 0.2 μm in width, curling across the bit becomes small, and the magnetization may be treated as a uniform single domain. We have modeled “pseudo spin valve” MRAM cells in the single-domain approximation. Calculations in this size regime for asymmetric giant magnetoresistive (GMR) sandwich material reveal that switching thresholds are sensitive to bit size. Submicron cells 0.4 μm×2.2 μm in size have been patterned from asymmetric GMR sandwich material and switching thresholds compare favorably with the single-domain model. These “pseudo spin valve” memory cells are very promising for nonvolatile, high speed, high-density memories.
Properties of chemically deposited Cu2S films on porous silicon
Authors: Dhar S. , Chakrabarti S.
Published: July 2002

The electrical and optical properties of Cu2S films, deposited on porous Si from a chemical bath, are presented. Annealing of the deposited film in air, at an optimized temperature of 150 °C, is shown to be essential to get films with the lowest values of electrical resistivities. The film is more than 60% transparent in the wavelength range of 600–1200 nm. Films deposited on porous Si produce Schottky-like diodes. The best diode characteristics are obtained after annealing the film at 150 °C. The diodes show both photoresponse and electroluminescence properties. Luminescence from porous Si, with a top coating of annealed Cu2S, is about 70% of that obtained when no coating was applied.