bluuebunny/crossref_metadata_2025 huggingface dataset:
Ferromagnetic semiconductor (In,Ga,Mn)As with Curie temperature above 100 K
Authors: Slupinski T. , Munekata H. , Oiwa A.
Published: July 2002

We have grown (InyGa1−y)1−xMnxAs ferromagnetic semiconductor layers with Mn composition of x up to 0.13 on InP substrates by molecular beam epitaxy. Near the lattice-matched composition, i.e., y∼0.53, the Curie temperature increases linearly with the ferromagnetically effective Mn composition xeff, following the empirical equation TC=1300×xeff. We obtained Curie temperatures above 100 K when x is relatively high (x>0.1; xeff⩾0.08) and the hole concentration is of the order of 1019 cm−3.
Photothermal probing of inhomogeneously modulated transparent thin films
Authors: Wright O. B. , Li Voti R. , Matsuda O. , Larciprete M. C. , Sibilia C. , Bertolotti M.
Published: July 2002

The problem of the photothermal modulation of optical beams passing through multilayer films is an extremely complex one owing to the inhomogeneously modulated refractive index combined with multiple optical reflections inside the sample. This problem has so far not been given an exact analytical treatment in the field of photothermal probing. We consider here such a treatment for normal-incidence optical probing in reflectance of photothermally modulated single-layer thin-film samples with arbitrary optical constants. The validity of the method is demonstrated by application to a thin transparent film of silica on a silicon substrate.
Substrate-induced mechanical and dielectric properties of a ferroelectric thin film
Authors: Lahoche L. , Lorman V. , Rochal S. B. , Roelandt J. M.
Published: July 2002

We propose herein a phenomenological model for the dielectric and mechanical behavior of ferroelectric PbTiO3 film on an MgO oxide substrate. The model takes into account the superstructure imposed on the epitaxial film by the substrate, the dislocation formation mechanism, and the specific mechanical boundary conditions. It is shown that nonhomogeneous film–substrate coupling related to superstructure formation is one possible mechanism for elastic stress relaxation in thin films. Stability regions in the thickness–temperature plane are obtained for different ferroelectric states formed during the deposition process and the subsequent cooling to room temperature. In ultrathin films, the superstructure formation mechanism favors a phase with a nontrivial unit cell multiple as well as a polarization direction perpendicular to the substrate. For films with a thickness of greater than 100 nm, two-dimensional clamping serves to stabilize the typical c domains. For intermediate thicknesses, several a type states may become stable. Numerical modeling of the residual strain level in the film is performed for three qualitatively different thicknesses.
Measurement of Cl and Br photofragment alignment using slice imaging
Authors: Rakitzis T. Peter , Kitsopoulos Theofanis N.
Published: July 2002

Slice imaging is used to measure directly the alignment of Br(2P3/2) and Cl(2P3/2) photofragments from the photodissociation of Br2 and Cl2, respectively, at 355 nm, using an intensity-normalization method that does not require the wavelength scanning of the probe laser. In both cases the C 1Π(1u) state is optically excited. The Br photofragments are maximally aligned and populate mJ=±1/2 only, whereas the Cl photofragments populate both the mJ=±1/2 and mJ=±3/2 by 80% and 20%, respectively. These results show that Br2 dissociates adiabatically, whereas nonadiabatic transitions occur to the A 3Π(1u) during Cl2 dissociation.
Square-lattice photonic band-gap single-cell laser operating in the lowest-order whispering gallery mode
Authors: Ryu Han-Youl , Kim Se-Heon , Park Hong-Gyu , Hwang Jeong-Ki , Lee Yong-Hee , Kim Jeong-Soo
Published: July 2002

Square-lattice photonic band-gap lasers are realized at room temperature from single-cell photonic crystal slab microcavities fabricated in InGaAsP quantum-well materials emitting at 1.5 μm. This single-cell photonic band-gap laser operates in a class of two-dimensional mode to be classified as the smallest possible whispering gallery mode with genuine energy null at the center. The low-loss nondegenerate mode with modal volume of 0.1 (λ/2)3 demonstrates a spectrometer-limited below-threshold quality factor >2000 and a theoretical quality factor >10 000. The other class of photonic crystal lasers is also observed outside the photonic band gap of the square lattice, operating in the dipole mode.
Efficient characterization of ultrathin AlOx insulating barriers in magnetic tunnel junctions fabricated by masked rf plasma oxidation technique
Authors: Park J. H. , Lee G. S. , Yang J. Y. , Yoon K. S. , Kim C. O. , Hong J. P. , Kim H. J.
Published: July 2002

AlO x insulating barriers in magnetic tunnel junctions were prepared by a masked rf plasma oxidation process to reduce direct ion-bombardment effect on the barrier. In our method, the root-mean-square value of 2.1 Å in the barrier and the magnetic resistance ratio up to 32% were observed. In addition, the surface plasmon resonance spectroscopy (SPRS) measurement was performed to efficiently determine optimum oxidation time, dielectric properties, and oxidation states inside thin barrier. The SPRS results revealed that the dielectric value and thickness of the optimum barrier were found to be 1.3576+i1.4488 and 16.3 Å, respectively, with a thickness confirmation by a high resolving transmission electron microscope measurement.
Algorithms and software for ordinary differential equations and differential- algebraic equations, Part I: Euler methods and error estimation
Authors: Hindmarsh Alan C. , Petzold Linda R.
Published: July 2002

Department Editor: William J. Thompson
Nondestructive control of weldings using the mirage detection
Authors: Lepoutre F. , Fournier D. , Boccara A. C.
Published: July 2002

Small defects in edge-to-edge weldings of thin pieces of stainless steel have been detected by the mirage effect. A simple 1D theoretical model shows the physical parameters influencing the experimental results. Using the phase of both the transverse and normal deviations at various frequencies allows for the separation of optical and thermal effects, of surface and subsurface defects, and for the determination of the depths and widths of the subsurface defects.
Particle size effects on magnetic properties of BaFe12−2xTixCoxO19 fine particles
Authors: Kubo O. , Ido T. , Yokoyama H. , Koike Y.
Published: July 2002

Particle size effects on magnetic properties of BaFe12−2xTixCoxO19 fine particles were investigated. Saturation magnetization of these particles decreased with decreasing particle size. This result can be explained with the existence of a nonmagnetic thin layer, several angstroms in thickness, on the surface of the particles. Particle size dependence of coercivity for these particles, below 1000 Å in size, follows the superparamagnetism theory. For the decrease in coercivity above 2000 Å, multidomain nucleation is inferred as one of the possible mechanisms. Particle shape dependence of coercivity is well explained with the coherent rotation model, considering shape anisotropy.
A new method of measuring the thermal time constant of junction lasers
Authors: Fujita Osamu
Published: July 2002

A new method of measuring the thermal time constant of a junction laser’s active region by using the output light power frequency response is presented. The thermal time constant τ can be calculated from the maximum value of the phase shift which is measured at a lower frequency than the cutoff frequency (1/2πτ). This method is highly suitable for use under high drive current and/or high temperature conditions.
Intrinsic anisotropy of Tb-Fe films prepared by magnetron Co sputtering
Authors: van Dover R. B. , Hong M. , Gyorgy E. M. , Dillon J. F. , Albiston S. D.
Published: July 2002

Amorphous Tb-Fe thin films have been produced by dual-gun dc magnetron cosputtering, which allows a moderate range of compositions (spanning about 20 at. %) to be deposited in a single run under identical conditions. The films exhibit clean M-H loops and torque curves which are somewhat anomalous but show consistent trends. Films deposited at room temperature with a composition in the range 17–30 at. % Tb showed a positive intrinsic anisotropy of roughly 3×106 erg/cm3, resulting in an easy axis of magnetization that is perpendicular to the plane of the film, while films with >37 at. % Tb had a small negative intrinsic anisotropy (ca. −2×105 erg/cm3). The temperature dependence of the anisotropy indicates that neither uniaxial stress nor shape anisotropy can account for most of the intrinsic uniaxial anisotropy observed, so we hypothesize that the large anisotropy must be due to pair ordering or a local anisotropy field.
Rapidly quenched FexTa100−x alloys
Authors: Chien C. L. , Liou S. H. , Ha B. K. , Unruh K. M.
Published: July 2002

Binary alloys of FexTa100−x were made by high-rate sputtering. A sharply defined boundary separates the composition ranges in which crystalline alloys (93≤x≤100) and amorphous alloys (15≤x≤90) are formed. The amorphous alloys with Fe concentrations up to the threshold of xc≂65 are nonmagnetic. For the samples with x>xc, the magnetic ordering temperature (Tc) varies monotonically with Fe content to 200 K for a-Fe90Ta10. However, the crystalline alloys, with slightly higher Fe content, are strongly ferromagnetic. Their Tc’s are several hundred degrees higher, accompanied by large increases in magnetic moment and hyperfine fields. This discontinuity illustrates the sensitivity with which the magnetic properties of Fe-transition metal alloys depend on structure. In contrast, no discontinuity has been observed in Co-Ta alloys.
Effect of SF6 fluorine donor on the multikilojoule HF chemical laser initiated longitudinally by intense electron beams
Authors: Fujioka Tomoo , Kannari Fumihiko , Suzuki Tadashi , Obara Minoru
Published: July 2002

The effects of fluorine donor additives and initiation level on the HF laser output characteristics in a strong and short pulse initiation regime are experimentally and theoretically investigated. Typical parameters of the circular e beam used in the experiment are 2 MV, 60 kA, 65 ns FWHM, and 10 cm in diameter, and the e beam is injected longitudinally. The addition of SF6 gas to a mixture of F2/O2/H2 is found to increase not only the specific laser energy but also the electrical laser efficiency. The maximum laser energy obtained from the low-pressure mixture of F2/SF6/O2/H2=200/84/60/45 (Torr) is 4.4 kJ in a 70 ns FWHM with an electrical laser efficiency of 203% and a chemical laser efficiency of 18.6%. Based on the experimental results, the scaling of the HF laser was established in terms of specific HF laser energy and electrical laser efficiency.
Validity of self-bias voltage measurements on insulating electrodes in radio-frequency dry etching systems
Authors: de Vries C. A. M. , van den Hoek W. G. M.
Published: July 2002

It is demonstrated that the measured value of the self-bias voltage on electrodes with an insulating target plate in a radio-frequency dry etching system is strongly influenced by the configuration of the electrode, and may have no straightforward relation with the actual self-bias voltage between the target surface and ground. The mechanism allowing measurement of the self-bias voltage is identified as plasma-induced surface conduction on the insulating target plate assisted by high electric fields at the target plate edge.
Observation of single-carrier space-charge-limited flow in nitrogen-doped α-silicon carbide. II. Electrical noise
Authors: Tehrani S. , Hench L. L. , Van Vliet C. M. , Bosman G.
Published: July 2002

Noise spectra of α-SiC in the presence of space-charge-limited flow are attributed to trapping noise. In the ohmic regime, SΔI∝ I20 and in the ohmic and low-voltage quadratic regime SΔI∝I0‖V0‖ as required by the theory. The trapping levels are determined from the slope of the time constants versus 1/T; the results are in fair agreement with those obtained from the current-voltage characteristic. The magnitude of the noise requires a modulation mechanism, such as caused by mobility fluctuations in the temperature range where ionized impurity scattering dominates.
Crystal orientation dependence of the electrical transport and lattice structure of zinc selenide films grown by metalorganic chemical vapor deposition
Authors: Stutius W. , Ponce F. A.
Published: July 2002

ZnSe films have been grown on 〈100〉, 〈110〉, and 〈111〉B GaAs surfaces, using the metalorganic chemical vapor deposition technique. The electrical transport properties and lattice structure characteristics of these films have been studied using a van der Pauw technique and high-resolution transmission electron microscopy. The electrical and structural properties of these films vary significantly with growth direction. In (100) layers, the observed defects are similar in nature to isolated compensated centers. For other orientations, observed properties are best described in terms of extended defects and grain boundaries.
Spatial characteristics of the optogalvanic effect in striated rare-gas discharges
Authors: May Randy D.
Published: July 2002

The spatial variation of the optogalvanic (OG) signal resulting from visible laser irradiation of rare-gas positive column discharges has been investigated for the case of standing striations. The OG signal was observed to periodically change polarity as the laser was scanned transversly across the bright and (relatively) dark regions of the column. Transitions involving the 3P2,0 metastable levels were found to be everywhere opposite in polarity to those originating on freely radiating levels. The effects of trapped resonance level emission, and collisions of the second kind involving metastables are proposed to be important in determining the OG signal polarity for excitation in certain discharge regions.
Crystallization characteristics of Co-Zr metallic glasses from Co52Zr48 to Co20Zr80
Authors: Altounian Z. , Shank R. J. , Strom-Olsen J. O.
Published: July 2002

We present a crystallization study of melt-spun Co-Zr metallic glasses. The primary crystallization products are all metastable or unstable except around the equiatomic composition where the stable phase CoZr is produced. Alloys at the composition Co25Zr75 show a crystallization characteristic that is dependent on heating rate.
Electron density in low-pressure Na-Ne discharges deduced from laser absorption experiments
Authors: Cornelissen H. J. , Merks-Eppingbroek H. J. H.
Published: July 2002

Laser absorption experiments are described to measure the radial sodium ground-state distribution in the positive column of a low-pressure Na-Ne discharge. The radial electron density distribution is obtained by combining the results with a particle balance. The electron densities at the tube axis ne(0) derived in this way are approximately a factor of 2.5 higher than the ones derived from a direct determination with Langmuir probes. Measuring the sodium profiles as a function of the distance to a probe reveals quantitatively the perturbative effect on the plasma of the glass capillary that supports the probe. This partly explains the observed discrepancy in ne(0). The electron temperature Te is deduced from the laser absorption data using the measured electrical conductivity of the plasma. This value of Te agrees very well with the value as measured with the probe.
Intense magnetization generated by standing waves in a plasma
Authors: Chakraborty B. , Khan Manoranjan , Bhattacharyya B.
Published: July 2002

Nonlinear increments of displacements of electrons in a cold unmagnetized electron plasma due to two strong elliptically polarized electromagnetic waves have been studied. These results have been used to find the induced magnetization due to standing waves formed by the two counterpropagating elliptically polarized electromagnetic waves. The induced magnetization has been found to be strong enough for interest in the investigation of laser-plasma interaction. For an Nd-glass laser (λ=1.06 μm, power flux∼1016 W/cm2) interacting with a plasma (N∼1017/cm3), the induced magnetic field is found to be in the megagauss region. The principal effect of the high magnetic field is on the electron thermal conduction. The heat transport is inhibited due to the generated field.
Semiconductor-laser thermal time constant
Authors: Esman R. D. , Rode D. L.
Published: July 2002

Thermal time constants of semiconductor lasers (‘‘test lasers’’) are measured by transient heating of test laser chips by use of another laser (‘‘pump laser’’). Pump laser emission impinging on the test laser p contact generates local heating. Thus, current-independent thermal responses are studied. Thermal time constants of 5.6 and 7.2  μs are measured and tend to confirm heat diffusion as the mechanism governing transient laser thermal behavior for times in the several microsecond range.
Oxygen doping in close-spaced-sublimed CdTe thin films for photovoltaic cells
Authors: Hsu T. M. , Jih R. J. , Lin P. C. , Ueng H. Y. , Hsu Y. J. , Hwang H. L.
Published: July 2002

Efficient CdTe/CdS thin film solar cells have been the recent focus, in which the CdTe layers were reported by close-spaced sublimation, and oxygen was used to control the  p-type conductivity of the deposited films. Both the fundamental gap and the impurity level were determined by the wavelength modulation reflectance spectroscopy, which demonstrates that while oxygen atoms have an ionization energy of about 0.1 eV, they do not behave as a simple shallow acceptor. This finding is supported by the electrical characterization. The oxygen concentration incorporated in the CdTe thin films were found to be in the range of 1019–1020 cm−3 by the IR measurements, while a carrier concentration between 1010 and 1012 cm−3 was obtained by Hall measurements.
Molecular reactions at plasma-polymerized film surfaces
Authors: Buss Richard J.
Published: July 2002

The ability of a variety of molecules to stick to the surface of a growing plasma-deposited polymer film was tested by a new technique. An inductively coupled rf plasma of an organic molecule was sampled through an orifice into a vacuum chamber. The beam of all plasma species intersected a substrate and plasma polymer was deposited. A second beam of a probe species was scattered from the surface of the film during deposition in the vacuum chamber and the IR spectra of the final film and the deposition rate were measured to determine the effect of the scattering. Molecules which are not attached to the film include 2-vinylpyridine, deuterium, C2D2, toluene, and C2Cl4. The molecules O2 and NO readily attach to the growing film while having no effect on the deposition rate. It was also determined that ion bombardment of the film surface does not play a significant role in the deposition process.
Particle number and pressure determination in high-pressure lamps
Authors: Schäfer R. , Stormberg H.-P.
Published: July 2002

An experimental setup is described which makes it possible to determine the particle number per unit length and the pressure in high-pressure mercury lamps. For this purpose the absorption of the frequency-doubled 514.5-nm line of an Ar+ laser by the mercury ground state is used. The method has been applied to measure the spatial variation of the particle number along the axial direction of a high-pressure arc and also to determine the time-dependent particle number during the warm-up and cool-down phases of a lamp. Furthermore the frequency dependence of the pressure fluctuations in a high-pressure lamp during an ac cycle has been measured for sinusoidal as well as pulsed supply voltage. The results are in agreement with calculations from a numerical model.
Inhomogeneous carbon bonding in hydrogenated amorphous carbon films
Authors: Grill A. , Meyerson B. S. , Patel V. V. , Reimer J. A. , Petrich M. A.
Published: July 2002

Hard-carbon films prepared by the rf-plasma decomposition of acetylene have been investigated by high-resolution 13C nuclear magnetic resonance spectroscopy, x-ray photoelectron spectroscopy (XPS), and the H(15Nα,γ)C nuclear resonant reaction. It was found that the ratio of sp2:sp3 bound carbon was 1.6, and that virtually all sp3 carbon atoms are, in fact, bound to one or more hydrogen atoms. Bulk layers contain about 40% hydrogen; however, results of the measurements of the hydrogen concentration, as well as those of XPS, confirm that the composition and properties of these carbon films are a strong function of their distance from the initial growth interface, and are spatially varying over the first 40 nm.
The P-T magnetic phase diagram of SmMn2Ge2
Authors: Gyorgy E. M. , Batlogg B. , Remeika J. P. , van Dover R. B. , Fleming R. M. , Bair H. E. , Espinosa G. P. , Cooper A. S. , Maines R. G.
Published: July 2002

SmMn2Ge2 shows reentrant ferromagnetism. Using flux grown single crystals we find that on cooling through the Curie temperature (Tt1=350 K) the material becomes ferromagnetic (FM). Upon further cooling, at Tt2=150 K an antiferromagnetic (AFM) phase is reached which is stable until Tt3=100 K when there is a transition to FM. The transitions at Tt1 and Tt2 are first order with a temperature hysteresis of about 4 K. Using ac susceptibility to determine the various transition temperatures as a function of pressure (P) we find that Tt3 rapidly decreases with increasing P and at relatively low pressures falls below our measuring range (90 K). Tt2 increases nearly linearly with P and reaches 330 K and 11 kbar. Tt1, drops slowly with increasing P and at 11 kbar the high temperature FM region ends. An x-ray determination of the lattice parameters (at atm pressure) shows that in the AFM region the a0 lattice parameter is decreased, discontinuously changing at Tt2 and Tt3. The c0 lattice parameter is essentially unchanged going through this transition region. These observations indicate that the magnetic properties of SmMn2Ge2 are very sensitive to interatomic spacing. Measurements with a differential scanning calorimeter show that, on heating, the transition at Tt2 is endothermic.
Competition between second harmonic generation and one- and two-photon absorption in the anthracene/9,10-dihydroanthracene mixed crystal
Authors: Bree A. , Leyderman A. , Taliani C.
Published: July 2002

Second harmonic generation (SHG) in single crystals of 9,10-dihydroanthracene (DHA) is reported. If the axes of the optical indicatrix are labeled X, Y, and Z (with X the fast ray) then Y-polarized SHG is observed at room temperature using Z-polarized laser excitation at 687.9 nm; in this YZ section both the incoming and outgoing rays behave ordinarily (no walk-off problem). There was some anthracene impurity present in all samples, and since anthracene absorbs near 340 nm, SH production could be followed by observing the resulting anthracene fluorescence; anthracene bands were observed as peaks or dips in the fluorescence response curve. These observations can be accounted for using a simple model in which SHG is purely a property of DHA, the presence of anthracene acting only to attenuate the conversion efficiency, which was always held <1% to avoid crystal damage.
Energy balance of the high-pressure mercury discharge with sodium- and scandium-iodide additives
Authors: Dakin James T. , Gilliard Richard P.
Published: July 2002

A high-pressure Hg discharge with NaI and ScI3 additives is studied through model calculations and spectroscopic diagnostics. The computer model solves the coupled energy, radiation, and chemistry equations for the discharge on a self-consistent basis. The wall-stabilized discharge is assumed to be in local thermodynamic equilibrium and to exhibit radial symmetry and axial homogeneity. From a prescription of the boundary conditions, the model predicts radial variations in discharge quantities, such as the temperature, chemistry, and radiation field. Predictions are checked by directly comparing model results with experimental data for a horizontal discharge tube. The arc tube is rotated about its own axis so as to suppress convective effects. Necessary input to the model includes oscillator strengths and broadening constants for a large number of Hg, Na, and Sc spectral lines, as well as scattering cross sections and thermochemical data. The required data have been gathered from available sources and determined internally from the experiment. The model is used to study mechanisms in the energy balance which contribute to radial spreading of the discharge. In the absence of the NaI and ScI3 additives, resonance radiation transport plays a major role in the spreading of the high-pressure Hg discharge. With the additives, this effect is diminished, and the more uniform ohmic heating associated with the low ionization potential of Na becomes important.
Al-doped and Sb-doped polycrystalline silicon obtained by means of metal-induced crystallization
Authors: Gong S. F. , Hentzell H. T. G. , Robertsson A. E. , Hultman L. , Hörnström S.-E. , Radnoczi G.
Published: July 2002

Thin-film multilayer structures of a-Si/Al/a-Si and a-Si/Sb/a-Si were deposited by electron-beam evaporation. The microstructure and the electrical properties of as-deposited and annealed (T<1370 K) thin films were determined. A p-n junction was formed between polycrystalline silicon (poly-Si) doped with Sb and a p-type Si substrate. Al and Sb were found to induce crystallization of a-Si at 600 and 700 K, respectively. After annealing to 1370 K for 60 min, the resistivities 7.0×10−3 Ω cm for the Al-Si sample and 1.4×10−2 Ω cm for the Sb-Si sample were obtained. Passivation of poly-Si grain boundaries by Sb is proposed.
Ampèrian recoil and the efficiency of railguns
Authors: Graneau Peter
Published: July 2002

In this paper the mechanical efficiency of the railgun is defined as the force accelerating the armature-projectile combination divided by the total electrodynamic force generated in the gun. The energy expended in a shot may then be equated to the ohmic loss plus the kinetic energy that would have been developed in the absence of mechanical losses. In this way it can be shown that the overall energy efficiency can never be greater than the square of the mechanical efficiency. Comparing calculations with experimental data makes it clear the reported disappointing performance of railguns is due to some ill-understood mechanical deficiency. A simple experiment is described which reveals buckling and distortion of the rails by recoil action. This explains the mechanical inefficiency. In relativistic electromagnetism, the recoil force should act ‘‘on the magnetic field’’ and absorb field-energy momentum. The Ampère–Neumann electrodynamics, on the other hand, requires the recoil forces to reside in the railheads and push the rails back toward the gun breech. Experiment confirmed the latter mechanism.
Generation of divacancies in silicon irradiated by 2-MeV electrons: Depth and dose dependence
Authors: Svensson Bengt G. , Willander Magnus
Published: July 2002

Czochralski-grown silicon samples (n-type) have been irradiated by 2-MeV electrons at room temperature to doses in the range of 1015–1016 cm−2. The generation of divacancies (V2) has been studied as a function of bombardment dose and sample depth using deep-level transient spectroscopy (DLTS). In the DLTS spectra, a level ∼0.41 eV below the conduction band is attributed to V2 in accordance with previous assignments made by other authors. It is shown that the generation rate of V2 is not only determined by the primary electron energy, but also depends on the bombardment dose. Initially, the divacancy concentration depends linearly on dose while at high doses a gradual decrease in the generation rate occurs. The concentration of V2 increases significantly with depth close to the surface (≤3 μm) while for larger depths a constant value is reached. The experimental depth profiles are qualitatively compared with a theoretical curve. This curve is calculated assuming vacancy diffusion, and it is speculated that a substantial amount of the divacancies are formed by vacancy-vacancy pairing. Furthermore, in some samples the depth profile of a level ∼0.23 eV below the conduction band is also monitored and based on these results the level scheme for V2 is discussed.
Interdiffusion and structural relaxation in Mo/Si multilayer films
Authors: Nakajima Hideo , Fujimori Hiroyasu , Koiwa Masahiro
Published: July 2002

Interdiffusion and structural change on annealing of sputter-deposited Mo/Si and Mo(N)/Si(N) multilayer thin films have been investigated over the temperature range from 674 to 1027 K. X-ray diffractometry shows that in the as-deposited Mo/Si multilayers the Mo is bcc with the stacking of (110) plane parallel to the substrate, and the Si is amorphous, while in the as-deposited Mo(N)/Si(N) multilayers, both Mo and Si nitrides are amorphous. The interdiffusivities have been determined from the decay rate of satellite peak intensity around (000). The activation energies for the interdiffusion in Mo/Si and Mo(N)/Si(N) multilayers are 105±5 and 351±88 kJ mol−1, respectively. A drastic decrease in the satellite peak intensity on annealing is observed in the Mo/Si multilayer films, which is interpreted to be due to interdiffusion and structural relaxation. On the other hand, a remarkable increase in the satellite intensity is found for the nitride multilayer films, which is explained by crystallization into β-Mo2N/α-Si3N4. The modulation wavelength decreases by 8%–12% after anneal. The decrease in the thickness of annealed Mo/Si multilayer films is also found by a depth profilometer.
Enhanced magnetic coercivity in magnetic granular solids
Authors: Xiao Gang , Chien C. L.
Published: July 2002

We have fabricated magnetic granular Fe-SiO2 solids over a wide metal volume fraction range 15%–100%. The microstructure of the samples has been characterized by using transmission electron spectroscopy, electron diffraction, and Mössbauer spectroscopy. Giant magnetic coercivity, as high as 2500 Oe, has been observed in samples with nanometer particle size. The behaviors of coercivity across the metal volume fraction range are attributed to the change of particle size and percolation effect.
The 1.380- and 1.360-eV photoluminescence transitions in undoped InP grown by molecular-beam epitaxy
Authors: Iliadis A. A. , Ovadia Shlomo
Published: July 2002

Unintentionally doped InP grown by molecular-beam epitaxy was examined by photoluminescence spectroscopy, secondary ion mass spectroscopy, and x-ray topography, in order to clarify the origin of the 1.380-eV emission band and the 1.360-eV emission peak. The 1.380-eV emission band was found to consist of four transitions which were attributed to band-to-acceptor (B-A0) and donor-to-acceptor (D0-A0) transitions due to Ca and Mg impurity atoms, inadvertently incorporated during growth. The 1.360-eV sharp emission was found to be an excitonic transition related to a deep donorlike phosphorus vacancy complex. This emission was also found to be a doublet with a split energy (ΔES) varying with the position of the laser beam on the sample, and it was attributed to internal elastic strain.
Electron attachment rate constants of HBr, CH3Br, and C2H5Br in N2 and Ar
Authors: Wang W. C. , Lee L. C.
Published: July 2002

The electron attachment rate constants of bromine compounds in the buffer gases of N2 and Ar (∼250 Torr) were measured as a function of E/N (or mean electron energy). The measured electron attachment rate constants of HBr, CH3Br, and C2H5Br show maximum values of 1.05×10−9, 1.08×10−11, and 9.3×10−11 cm3/s at mean electron energies of 0.55, 0.4, and 0.8 eV, respectively. The electron drift velocities for the gas mixtures of CH3Br in N2 and Ar were also measured.
Direct Bloch line optical observation (invited)
Authors: Thiaville A. , Boileau F. , Miltat J. , Arnaud L.
Published: July 2002

A new technique for observing domain walls and lines in materials with perpendicular anisotropy is presented. It employs laser light in a laser scan microscope and uses anisotropic diffraction by the wall/line magnetic structure. The contrast obtained can be understood simply and a numerical computation shows good agreement with experimental results. Wall configurations with numerous lines may be obtained when nucleating parallel stripes upon reduction to zero of a saturating in-plane field.
Identification of paramagnetic AsGa and optical EL2 centers in semi-insulating gallium arsenide
Authors: Wang Guangyu , Zou Yuanxi , Benakki S. , Goltzene A. , Schwab C.
Published: July 2002

After a brief recall of our main results obtained during a recent theoretical cluster calculation on anion antisite-related defects in GaAs, we discuss the consequences of their resonance parameters. Experimentally, we perform a detailed analysis of the electron paramagnetic resonance data in plastically deformed undoped and Cr-doped semi-insulating materials in conjunction with simultaneous EL2° optical absorption measurements. Combining theoretical calculations and experimental results, we are able to identify the ‘‘As+Ga’’ spectrum as a superposition of spectra ascribed to the isolated As+Ga and to its binary and ternary vacancy complexes, whereas the ternary complex AsGaVGaVAs only is believed to be the probable configuration for EL2.
Determination of displacements in ultrasonic waves by scanning tunneling microscopy
Authors: Heil J. , Wesner J. , Grill W.
Published: July 2002

A method for the determination of the displacements in ultrasonic waves on surfaces of solids is presented. The technique is based on the scanning tunneling microscope and the measurements are performed in real space. The design of a low-temperature microscope capable of operating at liquid-helium temperature is shown. A simple theory is given, which describes the observations in a satisfactory way.
Peak parameters from peak area to height ratio in thermally stimulated depolarization and thermoluminescence
Authors: Christodoulides C. , Apekis L. , Pissis P.
Published: July 2002

A method is described for using the area under a peak and the peak height in evaluating the activation energy and preexponential factor of thermally stimulated depolarization or thermoluminescence peaks. The accuracy achieved by the mathematical approximation used is discussed and the use of the method is demonstrated by applying it to a number of experimental peaks.
Mössbauer and neutron diffraction studies of Y2(Fe1−xMnx)14B
Authors: Pringle O. A. , Marasinghe G. K. , Long Gary J. , James W. J. , Yelon W. B. , Xie D. , Grandjean F.
Published: July 2002

We have used Mössbauer spectroscopy and neutron diffraction to study a series of Y2(Fe1−xMnx)14B samples in the composition range from x=0.0 to 0.4. Y2(Fe0.6Mn0.4)14B is paramagnetic at both room temperature and 85 K. The iron quadrupole splitting in this paramagnetic compound allows us to place an upper limit on the quadrupole shift in the magnetic Y2(Fe1−xMnx)14B alloys. Refinement of Y2(Fe1−xMnx)14B neutron diffraction data have been used to give the site occupancies of manganese on the transition-metal sublattice. Both neutron diffraction patterns and Mössbauer effect spectra indicate a marked preference for the manganese to occupy the 8j2 site, which is the largest volume transition-metal site. Both experimental techniques give completely consistent results for the site occupancies in Y2(Fe1−xMnx)14B.
High-speed measurement of the response time of a GaAs/AlxGa1−xAs multiquantum-well long-wavelength infrared detector
Authors: Bethea C. G. , Levine B. F. , Hasnain G. , Walker J. , Malik R. J.
Published: July 2002

We have measured the response time of a GaAs/Alx Ga1−x As multiquantum well infrared detector (at a wavelength of λ=6.4 μm). The intrinsic rise time is determined to be less than 300 ps.
Preparation and characterization of (111)-oriented Fe3O4 films deposited on sapphire
Authors: Fujii T. , Takano M. , Katano R. , Bando Y. , Isozumi Y.
Published: July 2002

Films of various iron oxides including FeO, Fe3 O4 , and α-Fe2 O3 were formed on the α-Al2 O3 (0001) surface by a reactive vapor deposition method and characterized by x-ray diffraction and conversion electron Mössbauer spectroscopy (CEMS). The formation range for each phase was determined as a function of the substrate temperature (Ts ) and oxygen partial pressure (PO2 ). Typically, the deposition of (111)-oriented epitaxial magnetite films could be performed at low temperatures of Ts =523∼623 K and PO2 =1.0–5.0×10−4 Torr. Good stoichiometry of the as-grown films were confirmed by CEMS, and the Verwey transition was clearly detected by measurements of resistivity and CEMS down to 77 K. However, the large lattice mismatch between the substrate and Fe3 O4 resulted in a columnar particle growth. The initially grown phase on such a mismatched substrate was specifically characterized by depositing the Mössbauer active isotope, 57 Fe, only at the deepest layers. On the other hand, by depositing 57 Fe only in the topmost layers, surface layers of well-crystallized films have been found to be rather stable against oxidation.
Effect of biaxial strain on exciton transitions of AlxGa1−xAs epitaxial layers on (001) GaAs substrates
Authors: Logothetidis S. , Cardona M. , Tapfer L. , Bauser E.
Published: July 2002

From the analysis of spectroscopic ellipsometry measurements in the temperature range between 16 and 300 K, the exciton splittings and shifts due to lattice mismatch in the AlxGa1−xAs/GaAs alloy system grown by liquid-phase epitaxy are obtained. The temperature dependence of the Ev10 and Ev20 exciton transitions shows that the difference in the expansion coefficients of the two materials does not play a significant role in the induced internal stress. The internal stresses are evaluated from an x-ray double-crystal diffraction technique. The shear deformation potential values of AlxGa1−xAs determined from the experimental results are in disagreement with those found in the literature.
rf-diode-sputtered iron nitride films for thin-film recording head materials
Authors: Wang S. , Kryder M. H.
Published: July 2002

We have obtained iron nitride films with a saturation magnetization of 21 kG, coercivity less than 1 Oe, and hard axis permeability of 4000. The saturation magnetostriction decreased as input power increased, or as the N2/Ar flow rate ratio decreased. X-ray analysis revealed that an increase in input power or a decrease in the flow rate ratio resulted in less γ′-Fe4N phase in the films deposited. The coercivity was also very sensitive to these variations. The iron nitride films did not show structural changes under vacuum annealing from 150 to 300 °C, but their coercivity decreased slightly. The coercivity increased rapidly after annealing above 350 °C, however.
Melt-spun Pr2(Fe1−xCox)14B permanent magnet materials
Authors: Fuerst C. D.
Published: July 2002

The substitution of cobalt for iron is known to increase the Curie temperature and stabilize the magnetic properties of melt-spun Nd2(Fe1−xCox)14B ribbons. The preparation of optimum permanent magnets from cobalt-substituted materials is, however, complicated by the need for nonstoichiometric starting materials which vary with the level of cobalt concentration. This paper contains the results of selecting compositions which optimize the room-temperature properties of melt-spun ribbons containing Pr2(Fe1−xCox)14B as the principal constituent. The hard magnetic properties of 11 cobalt concentrations were examined spanning the range x=0 to 1, inclusive. For cobalt concentrations up to x=0.5, the remanence and energy product did not change substantially from the x=0 values of 8 kG and 12 MGOe, respectively. Remanence and energy product decreased monotonically to their endpoint values of ∼5 kG and ∼5 MGOe for cobalt-rich alloys (x>0.5). The coercivity increased smoothly from 13 kOe for x=0 to 25 kOe for x=1. There was no evidence of the down turn in Hci normally associated with decreasing anisotropy.
A new approach to high-efficiency multi-band-gap solar cells
Authors: Barnham K. W. J. , Duggan G.
Published: July 2002

The advantages of using multi-quantum-well or superlattice systems as the absorbers in concentrator solar cells are discussed. By adjusting the quantum-well width, an effective band-gap variation that covers the high-efficiency region of the solar spectrum can be obtained. Higher efficiencies should result from the ability to optimize separately current and voltage generating factors. Suitable structures to ensure good carrier separation and collection and to obtain higher open-circuit voltages are presented using the (AlGa)As/GaAs/(InGa)As system. Efficiencies above existing single-band-gap limits should be achievable, with upper limits in excess of 40%.
A study of new surface passivation using P2S5/(NH4)2S on GaAs Schottky barrier diodes
Authors: Hwang K. C. , Li Sheng S.
Published: July 2002

A new surface passivation using P2S5/(NH4)2S on GaAs Schottky barrier diodes formed by Au and Al contacts was investigated, and the results were compared with those of (NH4)2Sx- and NH4OH/HF-treated devices. With new surface treatment, the effective barrier heights for both the Al- and Au-GaAs Schottky diodes were found to vary with the work function of metals, which is clear evidence of the lower surface state density in these diodes. Results of the I-V and C-V measurements show that P2S5/(NH4)2S-passivated diodes have lower reverse leakage current and higher effective barrier height than that of the (NH4)2Sx-treated diodes.
Oxidation of metastable single-phase polycrystalline Ti0.5Al0.5N films: Kinetics and mechanisms
Authors: McIntyre D. , Greene J. E. , Håkansson G. , Sundgren J.-E. , Münz W.-D.
Published: July 2002

Metastable single-phase, NaCl-structure, polycrystalline Ti0.5Al0.5N alloy films have been shown to exhibit much better high-temperature (750–900 °C) oxidation resistance than polycrystalline TiN films grown under similar conditions. The Ti0.5Al0.5N alloys, ≂3 μm thick, were deposited at temperatures between 400 and 500 °C on stainless-steel substrates by dc magnetron sputter deposition in mixed Ar+N2 discharges with an applied negative substrate bias Vs of either 0 or 150 V. Oxidation in pure O2 initially occurred at a rate that varied parabolically with time. The oxide overlayers consisted of two partially crystalline sublayers, the upper one Al-rich and the lower one Ti-rich, with no measurable N concentrations in either. Inert-marker transport experiments showed that oxidation proceeded by the simultaneous outward diffusion of Al to the oxide/vapor interface and inward diffusion of O to the oxide/nitride interface. The oxidation rate constant K increased with oxidation temperature Tox at a rate much higher than would be predicted from a simple exponential dependence due to changes in the oxide microstructure (increased crystallinity) with increasing Tox. At Tox ≥850 °C, O transport became the rate-limiting step. After oxidation times, ranging from 6 h at 750 °C to 7 min at 900 °C, oxide crystallites, exhibiting a tetragonal rutile TiO2 structure, were observed in Vs=0 samples to grow at an accelerated rate up through cracks in the oxide overlayer. The formation of these crystallites was postponed until a much later stage in oxide-overlayer development for samples grown with Vs=150 V.
Short-range ordering in a three-dimensionally frustrated magnet, Tb2Mo2O7, by wide- and small-angle neutron diffraction
Authors: Greedan J. E. , Reimers J. N. , Penny S. L. , Stager C. V.
Published: July 2002

Tb2Mo2O7 is an ordered crystalline oxide with the pyrochlore structure. This material undergoes a magnetic transition at 25 K which is similar to that of a spin glass. Neutron diffraction data indicate the presence of short range magnetic order via broad, diffuse reflections which persist above 25 K. Below 25 K the diffuse peaks increase in intensity but do not narrow. Fourier analysis of the data yield an approximation to the radial distribution function which is interpreted in terms of spin–spin correlations up to fourth neighbors. The first, third, and fourth neighbor interactions are ferromagnetic but the second neighbor interaction is strongly antiferromagnetic and involves only a Tb-Mo coupling. SANS data show magnetic scattering only at large Q values, reenforcing the view that antiferromagnetic interactions dominate. The Q-integrated magnetic SANS intensities show a temperature dependence similar to that for the correlation functions.
High-coercivity magnetic hard disks using glass substrates
Authors: Kogure T. , Katayama S. , Ishii N.
Published: July 2002

A new structure for sputtered thin-film high-density longitudinal magnetic recording media using glass substrates has been proposed. The coercivity of Co alloy films is increased considerably by depositing a thin Ti layer between the Cr underlayer and the glass substrates. X-ray diffraction analysis indicates a change in the preferred orientation of the Cr layer. Transmission electron microscopy reveals that there is a significant change in the microstructure of the columnar Cr layer when Ti coating is used. It is proposed that the Ti film prevents oxygen from the glass substrates from reacting with the Cr and Co alloy magnetic layers.
On the effect of the barrier widths in the InAs/AlSb/GaSb single-barrier interband tunneling structures
Authors: Chen J. F. , Yang L. , Wu M. C. , Chu S. N. G. , Cho A. Y.
Published: July 2002

The dependence of the interband tunneling current on AlSb barrier widths is studied in the InAs/AlSb/GaSb single-barrier diode structures. The experimental results show that the peak current density displays an exponential dependence on the barrier width. The Wentzel-Kramers-Brillouin approximation combined with the k⋅p two-band model were used in analyzing the energy level in the AlSb barrier through which the peak tunneling currents occur. The energy level thus obtained (0.48±0.10 eV above the valence band edge of the AlSb) agrees with the valence-band offset (0.40±0.15 eV) between the AlSb and the GaSb obtained by x-ray photoemission measurement reported by Gualtieri et al. [Appl. Phys. Lett. 49, 1037 (1986)]. By adjusting the barrier width properly, we obtained a high peak current density of 24 kA/cm2 (with a peak-to-valley ratio of 1.4) and a high peak-to-valley ratio of 4.5 (with a peak current density of 3.5 kA/cm2) at room temperature. In addition, the peak-current voltages for different AlSb barrier widths were calculated and compared with the measured results.
Effective nonlinear response in dilute nonlinear granular materials
Authors: Hui P. M.
Published: July 2002

We consider the nonlinear response of a granular material in which a concentration p of spherical grains of nonlinear material are embedded in a linear host. An approximation, based on the Maxwell–Garnett formula, is proposed for the effective nonlinear susceptibility which is applicable for arbitrary nonlinearity in the dilute limit of embedded grains. The nonlinear susceptibility is found to be χeff=p[3ε0A/(ε0B +2ε0A)]2‖3ε0A/(ε0B +2ε0A)‖βχB, where ε0A is the dielectric constant of the linear host, and the displacement and electric field in the nonlinear grains are related by D=ε0BE+χB‖E‖βE. Previous results on cubic nonlinearity are recovered as a special case of the present approximation.
Structure and magnetic properties of Co/Al multilayered films
Authors: Mitsuzuka T. , Kamijo A. , Igarashi H.
Published: July 2002

We report on an investigation of Co/Al multilayered films (MLFs). The samples had periodic layered structures and had in-plane easy axes. Nonmagnetic CoAl compound was formed at the Co/Al interfaces. Co/Al MLFs had a perpendicular interface anisotropy. This interface anisotropy energy (Ks) was estimated to be 0.25 erg/cm2. Taking account the volume contribution of Co layers, it was suggested that perpendicular magnetism would appear when the remaining unmixed Co layer thickness was under 6 Å. However, this surface anisotropy energy vanished when the Co layers were thinner than 9 Å. In Co/Al MLFs with thin Co layers, the island growth of Co on the initial stage was considered to be the reason for Ks=0.