bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic self-defocusing of nanosecond laser pulses in bulk ZnSe
Authors: Milward J. R. , Ji W. , Kar A. K. , Pidgeon C. R. , Wherrett B. S.
Published: July 2002

A large refractive nonlinearity (−2×10−20 cm3) has been observed in polycrystalline ZnSe across a range of wavelengths just below the band edge, using nanosecond laser pulses. The transmitted spatial profiles are measured, and theoretical fits give the spectral response of the nonlinear refractive cross section at low irradiances. A comparison with band filling theory indicates that the dynamic Burstein–Moss shift is the dominant mechanism responsible for this refractive nonlinearity.
Improved bounds on the effective conductivity of high-contrast suspensions
Authors: Torquato S. , Rubinstein J.
Published: July 2002

Conventional upper and lower bounds on the effective conductivity σe of two-phase composite media diverge from one another in the infinite-contrast limits (α=∞ or 0). We have derived a generally nontrivial upper bound on σe for suspensions of identical spheres when the spheres are superconducting, i.e., the upper bound does not necessarily become infinite in the limit α→∞. Similarly, a generally nontrivial lower bound on σe is derived for the aforementioned suspension when the spheres are perfect insulators, i.e., the lower bound does not necessarily vanish in the limit α→0. The bounds are computed for two models: simple cubic arrays and random arrays of spheres.
The effects of laser illumination and high energy electrons on molecular-beam epitaxial growth of CdTe
Authors: Wu Y. S. , Becker C. R. , Waag A. , Bicknell-Tassius R. N. , Landwehr G.
Published: July 2002

We report the results of a detailed investigation on the Te-stabilized (2×1) and the Cd-stabilized c(2×2) surfaces of (100) CdTe substrates. The investigation demonstrates for the first time that both laser illumination and, to a greater extent, high-energy electron irradiation increase the Te desorption and reduce the Cd desorption from (100) CdTe surfaces. Thus it is possible by choosing the proper growth temperature and photon or electron fluxes to change the surface reconstruction from the normally Te-stabilized to a Cd-stabilized phase.
Comparative study of phosphosilicate glass on (100) silicon by furnace and rapid isothermal annealing
Authors: Thakur R. P. S. , Singh R. , Nelson A. J. , Ullal H. S. , Chaudhuri J. , Gondhalekar V.
Published: July 2002

Annealing experiments were carried out on phosphosilicate glass (PSG) films deposited on (100) silicon substrates by using a low-pressure chemical vapor deposition technique. Rapid isothermal processing and conventional furnace heating were used to study the electrical, structural, and mechanical characteristics of these films and the results of the two processes compared. A refractive index of 1.457 was obtained in the rapid isothermal annealing cycle of 800 °C/15S, but was 1.419 for the furnace annealing cycle (i.e., 800 °C/65S). Spreading resistance analysis has shown that the junction depth remains unchanged for an 800 °C/15S rapid isothermal annealing cycle. Stress measurements show that rapid isothermal annealing leads to less strain compared to furnance annealing. The x-ray photoelectron spectroscopy analysis shows that as compared to furnance annealing, rapid isothermal annealing provides a chemically homogenous interface. High-frequency capacitance voltage (C-V) measurements show that furnance-annealed samples are leaky and a higher concentration of oxygen-related defects are present in the PSG/Si interface. On the other hand, because of a relatively clean interface, a well behaved C-V characteristic is observed in the rapid isothermal annealed samples. In summary, as compared to furnace annealing, rapid isothermal annealing resulted in superior structural, mechanical, and electrical properties of PSG films on Si substrates. A plausible explanation of such behavior may be attributed to the difference in the radiation spectra of the two sources of energy.
Range statistics and Rutherford backscattering studies on Fe-implanted In0.53Ga0.47As
Authors: Gulwadi Sadanand M. , Rao Mulpuri V. , Simons David S. , Holland O. W. , Hong Won-Pyo , Caneau Catherine , Dietrich Harry B.
Published: July 2002

Single-energy Fe implantation at energies in the range 50 keV–2 MeV to achieve a peak Fe concentration of 1–2×1018 cm−3 is performed into undoped (n-type) InGaAs layers grown on InP:Fe. The first four statistical moments of the Fe profiles measured by secondary-ion mass spectrometry are determined. The Pearson IV distribution calculated from these moments matches the implant-profile closely. Samples implanted with Fe to doses in the range 5x1012 –2×1015 cm−2 at 380 keV are analyzed by Rutherford backscattering measurements to study ion-induced damage. For 380-keV implants, amorphization begins at a dose of ≊3×1013cm−2.
Characterization of GaAs grown on Si epitaxial layers on GaAs substrates
Authors: Adomi K. , Strite S. , Morkoç H. , Nakamura Y. , Otsuka N.
Published: July 2002

GaAs has been grown on 9 and 18 Å thicknesses of epitaxial Si which was grown on GaAs (100) substrates. The GaAs on Si interface was characterized by cross-sectional transmission electron microscopy. A 9 Å thickness of Si on GaAs is pseudomorphic while 18 Å of Si is relaxed. Antiphase domains (APDs) were observed to annihilate near the GaAs on Si interface. Annihilation ocurred within 100 Å of the interface for the 9 Å thickness of Si and around 1500 Å for the 18-Å Si case. From a detailed analysis of the APD shapes and sizes, we deduce that GaGa bonds are energetically favored in the {111} planes and that two separate APD annihilation mechanisms occur. The growth mode of epitaxial Si on GaAs was also studied by in situ high-energy electron diffraction.
Magnetic properties of Fe73.5Cu1Nb3(SixB1−x)22.5 (x=0.5–0.8) alloys with ultrafine grain structures
Authors: Noh T. H. , Pi W. K. , Kim H. J. , Kang I. K.
Published: July 2002

The metalloid effect on magnetic properties and microstructures of iron-based alloys with ultrafine grain structures was investigated. For Fe73.5Cu1Nb3(SixB1−x)22.5 (0.5≤x≤0.8) alloys with 10–15-nm grain diameters, the permeability increased with Si content. The alloys with x=0.5 and 0.6 subjected to the optimum heat treatment had two different ferromagnetic phases which were composed of an α-Fe(Si) solid solution with high Curie temperature and an interfacial phase with low Curie temperature. However, in high Si alloys with x=0.7 and 0.8, only a phase with high Curie temperature appeared. This result suggests that in the high Si composition range, a complete transformation from the amorphous phase to the α-Fe(Si) single phase was achieved. The disappearance of the interfacial phase which would certainly disturb the exchange coupling between the α-Fe(Si) phase grains would be considered to be one of the factors for the increased permeability.
Barkhausen transition irregularities in soft magnetic films
Authors: Soohoo R. F.
Published: July 2002

Barkhausen transition irregularities have been observed in a dozen samples of uniaxial Permalloy films using the resonance enhancement technique. For some of the samples, as many as 20 absorption transitions were observed with threshold field clustered about Hc and ranging from 1 to 3 Oe. For other samples, only one or two transitions were observed. The latter cases were of particular interest to our study of magnetization reversal, because these transitions were associated with domain-wall motion thresholds of reversed edge domains in the remanent state after saturation in the easy direction. Although the defect structure of a given sample was clearly deterministic, our results showed that the threshold fields for motion of an edge domain wall fluctuated for different passes of the switching field through Hc, depending on the degree to which the specimen was truly saturated prior to the transition. These fluctuations were typically about 20% of the average value. It was further found that if great care were taken to insure that each pass through Hc was preceeded by the same degree of saturation, these deviations were greatly reduced to less than 5%. These results show that Barkhausen transitions are deterministic rather than noisy in nature.
Domain-wall dynamic instability
Authors: Yuan Samuel W. , Bertram H. Neal
Published: July 2002

Computer simulation is utilized to follow the dynamic evolution of a one-dimensional Bloch domain wall. The Landau–Lifshitz equation with Gilbert damping is solved with the applied field along the domain magnetization. Walker’s solution is verified for the case where the anisotropy field is much larger than the Walker critical field. When the critical field is comparable to the anisotropy field, the wall motion exhibits instability. For an external field lower than the Walker critical field, after some transient behavior, the steady-state Walker-type wall configuration is reached; when the applied field is above the Walker limit, the initial single wall gradually evolves into an odd number of Bloch walls with consecutively opposite senses. Wall motions for soft ferrite material with different dampings are also investigated. For very small damping the stability holds well. However, for medium damping, it is found that below some critical field, smaller than Walker’s, the steady-state solution is stable. Above that field, the single wall first develops a frontal wake which enlarges until instability when an odd number of walls form.
Electronic structure and magnetic properties of Co-rich ternary compounds: YCo10V2
Authors: Jaswal S. S.
Published: July 2002

Ternary compounds of Fe and/or Co with the ThMn12 structure are of interest as possible permanent-magnet materials. We have studied the electronic and magnetic structures of YCo10V2 using self-consistent spin-polarized calculations. The calculations employ the linear-muffin-tin-orbitals method in the local-density approximation. The Co 3d states dominate the density of states near the Fermi energy with an average exchange splitting of about 1 eV. The calculated magnetization per formula unit for one of the V-atom configurations (8.11 μB/FU) is in reasonable agreement with the experimental value (7.19 μB/FU). The Co moments for the three sites are quite different (μf = 0.75 μB, μi = 1.39 μB, μj = 0.91 μB). The V and Y atoms develop moments opposite to those of Co due to interatomic exchange interactions with the Co atoms [μ(V) = 0.50 μB, μ(Y) = 0.29 μB].
On the threshold of parametric instability of spin waves in a film with a rough surface
Authors: Cherepanov V. B.
Published: July 2002

The threshold of parametric excitation of spin waves in a ferrite film with a rough surface is calculated. The influence of the surface on the threshold is twofold: (i) the normal component of the wave vector of a spin wave is quantized and (ii) surface roughnesses scatter a spin wave elastically. The elastic scattering conserves energy of spin waves and mixes phases of spin waves with different wave vectors. The threshold of parametric excitation hth depends on the relationship between the film thickness d and the mean free path l. The threshold in a thick film (d≫l) is close to that in an unbounded sample of ferromagnet hth0: (hth/hth0)2 = 1 + (πl/d)2. In a thin film (d≪l) the threshold is close to that in a randomly inhomogeneous ferromagnet (hth/hth0)2 ∼ γs/γ, where γ is the spin-wave damping and γs is the frequency of elastic scattering of spin waves on surface roughnesses.
Microstructure and magnetic properties of low-neodymium Nd-Fe-B-Si magnets produced from HP gas atomized powder
Authors: Wronski Z. S.
Published: July 2002

Fine spherical powder with tetragonal Nd2Fe14B phase was prepared by high-pressure gas atomization. Magnetic powder with iHc = 7.0 kOe was obtained from the Nd9Fe78B11Si2 alloy. Thus, the Nd content was lower than the one used for preparation of either sintered or melt-spun material. Three distinctive microstructures were found in the atomized particles: dendritic, microcrystalline, and amorphous. The amorphous phase crystallizes at 601 °C. The Curie temperature Tc of the fully recrystallized powder is 311 °C. Bonded magnets were prepared from either atomized or melt-spun annealed powders. The iHc was improved by 37% after optimal annealing of the atomized powder. The observed effects were related to the microstructure refinement. From the initial magnetization curve, it is presumed that the generation of iHc is mainly due to the random-strength pinning mechanism of the domain walls.
Recent neutron studies of high-T
c materials (invited) (abstract)
Authors: Mook H. A.
Published: July 2002

Neutron scattering results on high-temperature superconductors have included measurements of the magnetic ordering of the Cu and rare earth ions and studies of fluctuations of the magnetic moments. The fluxoid lattice of YBa2Cu3O7−δ has also been observed using neutron diffraction techniques. The most recent studies of these topics will be reviewed with particular emphasis placed on measurements of the magnetic fluctuations for the (La1−xSrx)2CuO4 system.
Change in magnetic state of Fe+silica gel nanocomposites due to low temperature treatment in ammonia
Authors: Shull R. D. , Ritter J. J. , Swartzendruber L. J.
Published: July 2002

Homogeneous gelled composites of iron and silica containing 11–40 wt. % Fe were prepared by room temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst). Previous electron microscopy, x-ray diffraction, and Mössbauer effect data showed these bulk materials are comprised of nanometer-sized regions of iron compounds embedded in a silica gel matrix. They were also all paramagnetic below 300 K. Here the effect on the magnetic state of these nanocomposites following a low temperature (T<400 °C) treatment in 1 atm of ammonia (after a prior anneal in 1 atm of hydrogen) is presented, along with the dependence on the H2 pretreatment. In all cases the room temperature Mössbauer spectra for the material in the NH3-treated and H2-pretreated conditions were similar. However, when treated in H2 at 770 °C (2 h) the Mössbauer spectra also contained a significant component having a large isomer shift (∼1.3 mm/s) and quadrupole splitting (∼3.2 mm/s). This material was also slightly ferromagnetic at all temperatures below 300 K. For materials pretreated in hydrogen below 400 °C, a threefold enhancement in the magnetic susceptibility was measured following treatment in ammonia. In addition, both the field and temperature dependence of the susceptibility indicated the presence of spin-glass behavior at 10 K for NH3-treated samples containing up to 40% Fe. At room temperature, these latter ammonia-treated nanocomposites were either superparamagnetic (Fe contents, CFe, up to 25%) or ferromagnetic (CFe≳25%).
The magnetostatic contribution to perpendicular magnetic anisotropy in CoCr with grain-boundary segregation
Authors: Snyder J. E. , Kryder M. H.
Published: July 2002

The magnetostatic contribution to perpendicular magnetic anisotropy caused by nonmagnetic grain-boundary layers around the crystals in a hexagonal array of hexagonal columnar crystals has been calculated. Using film thickness and crystal diameter data, and the results of grain-boundary segregation calculations, values of normalized magnetostatic anisotropy constant were calculated, and compared to experimental data on the variation of normalized net anisotropy Knet/2πM2s. It is concluded that the contribution of magnetostatic effects to the variation of normalized anisotropy is negligible. Indeed, for all the films of this study, one can use the continuous thin-film value ( − 2πM2s) for magnetostatic contribution to anisotropy, with at most a 2% error. It is predicted that magnetostatic effects only become appreciable when the nonmagnetic and magnetic dimensions become comparable in size. It is concluded that mechanisms other than magnetostatic effects must be found in order to explain the perpendicular anisotropy in CoCr sputtered films.
Time resolved thermomagnetic recording process in GdTbFeCo films
Authors: Shieh H-P. D.
Published: July 2002

The dynamics of domain formation in a GdTbFeCo film during a full cycle of thermomagnetic (TM) recording has been studied by a time-resolved polarized-light microscope. The microscope has 5 ns temporal resolution and sub-μm spatial resolution. Nucleation generally occurs within 15 ns after a write laser pulse. Domain growth and contractive rates are strong functions of irradiated laser power. Domain contractive rates are much higher than growth rates in all cases. The domain growth, shrinkage, and stabilization processes are very reproducible regardless of whether writing is started from nucleation or from a seed domain. A bias field affects dynamic domain size, but not growth rates.
90° walls in bulk ferromagnetic materials
Authors: Aharoni Amikam , Jakubovics J. P.
Published: July 2002

The differential equations for the minimal energy state of a one-dimensional 90° wall in an infinite cubic crystal are used to establish two self-consistency criteria for Ritz models of such a wall, or for its computation. Numerical minimization of the energy is carried out for the physical parameters of Fe, but with various values of Ms. The results show a structure with very little surface charge (a ‘‘Néel wall’’) for rather small Ms. This turns (via some intermediate structures with a mixed charge) to one that has almost only surface charge (a ‘‘Bloch wall’’) for larger Ms (such as the actual value in Fe).
Thermal stability of molybdenum–silicon nitride–silicon Schottky diodes
Authors: Park Heungsoo , Helms C. R.
Published: July 2002

Previously our group [Sobolewski et al., Appl. Phys. Lett. 54, 638 (1988)] had demonstrated metal–thin insulator-silicon Schottky diode structures which allow the Si Schottky barrier height to be adjusted over nearly the full range of the silicon band gap by an appropriate choice of insulator thickness and metal. However, previous attempts to achieve a structure with a high barrier height to a p type that is stable above 400 °C (using primarily titanium) have failed. In this paper we report on results for molybdenum, which has a stable tie line to SiO2 and Si3N4 in a metal–silicon–oxygen (nitrogen) ternary phase diagram which leads to a more stable system.
Optical spectroscopy analysis of YBa2Cu3O7 at room temperature and at 10 K
Authors: Fournier J. , Fournier P. G. , Kaddouri A. , Dunet H.
Published: July 2002

Optical spectra of sputtered excited particles emitted during fast Kr+ ion bombardment of YBa2Cu3O7 above and below the critical temperature Tc∼90 K were recorded between 190.0 and 679.3 nm. Intensities and positions of lines are discussed and interpreted. The two optical spectra exhibit discrete lines which are attributed to neutral excited Cu, Ba, and Y and a few lines attributed to ionic Ba and Y. The relative intensities are the same at 10 K and at room temperature, while the absolute intensity increases by 40%. A small continuum attributed to oxide is also observed in the two spectra.
Tunneling spectroscopy on metal-insulator-silicon structures with very thin insulating layers
Authors: Balk P. , Ewert S. , Schmitz S. , Steffen A.
Published: July 2002

Tunneling measurements were performed at 4.2 K to investigate metal-insulator-silicon junctions with very thin silicon oxide or silicon nitride layers (2–5 nm) as tunneling barriers. For the measurements at 4.2 K highly degenerate P- or B-doped Si samples were used. Silicon oxide layers were produced by oxidation of Si in dry oxygen at 600 °C or by plasma deposition in an oxygen glow discharge. Silicon nitride films were prepared by nitridation in a NH3 discharge. The effects of interface states, the Si doping density, the metal of the gate electrode and the tunneling barrier material on the tunneling conductivity were studied systematically. Changes in the conductivity were found to be related to changes in the density of interface states, caused by annealing processes or by high-field stress. Vibrational modes of phonons and impurities in the structures were detected by inelastic electron tunneling spectroscopy. Using this approach a direct correlation between the increase in the density of interface states and the decrease of the density of Si-H groups was established.
Experimental investigation of Fe-(SiO2) granular films
Authors: Kanai Y. , Charap S. H.
Published: July 2002

Fe-(SiO2) granular films were fabricated by sputtering with a mosaic target. The fractional area of iron on the target was varied from 0.29 to 0.42. The volume fraction of iron was found from the saturation magnetization to be within 0.03 of that predicted by the target composition. The sample coercivities ranged from Hc = 130 to 950 Oe at room temperature and 690 to 2390 Oe at 77 K. Characteristics relevant for recording media were investigated for the highest-coercivity sample. The squareness values for hysteresis loops measured in and out of the plane of the film, Mr/Ms = 0.54 and 0.43, respectively, indicate that the granules have a uniaxial anisotropy and the easy axes tend to lie more in the plane of the film. The rotational hysteresis was measured at room temperature; the rotational hysteresis integral was calculated to be Ir = 1.56. Because the magnetization reversal is assisted by thermal fluctuation, the low temperature value of Ir was estimated by scaling the measured value by the ratio of the coercivities measured at 77 K and at room temperature. The value of Ir ≊ 0.6 is closest to the theoretical value for the coherent rotation mechanism. The relative decay of magnetization between 30 and 3600 s had a maximum value of ΔM/Ms = 0.10 for a reversal field near Hc. This value falls within the range of the values reported for conventional particulate media, which suggests that a stable transition can be recorded in this material.
Be diffusion mechanism in GaAs investigated by slow positron beam
Authors: Lee Jong-Lam , Wei Long , Tanigawa Shoichiro , Kawabe Mitsuo
Published: July 2002

The slow positron beam technique was applied on undoped and Be-doped GaAs to study the effects of Be impurities on both creation and migration of Ga vacancies, VGa, during annealing. It is observed that a monovacancy of VGa is created in Be-doped GaAs to result in enhanced Coulombic interaction between As vacancy, VAs, and Be acceptor, BeGa. In undoped GaAs, the formation of divacancies, VGa-VAs, is dominant. The migration depth of vacancies is shorter in Be-doped GaAs than in undoped GaAs. This suggests the existence of Ga interstitials, IGa, in the Be diffused layer which interact with VGa introduced from the surface. Based on these observations, we suggest the kick-out mechanism for Be diffusion in GaAs.
Si—O—Si strained bond and paramagnetic defect centers induced by mechanical fracturing in amorphous SiO2
Authors: Munekuni Shuji , Dohguchi Nobuyuki , Nishikawa Hiroyuki , Ohki Yoshimichi , Nagasawa Kaya , Hama Yoshimasa
Published: July 2002

Mechanical stress is applied to high-purity amorphous SiO2 samples by means of fracturing. The electron-spin-resonance spectra suggest that the formation of a majority of paramagnetic defects is from cleavage of Si—O—Si bridges in the glass network, but there are some sample-to-sample variances in fracture-induced paramagnetic defects, suggesting cleavage of differing chemical bonding states in the samples. Nonstoichiometric bonds, ≡Si—Si≡ and ≡Si—O—O—Si≡, are assumed to be one reason for the sample dependency. Formation of Si—O—Si strained bonds from mechanical fracturing is confirmed from sequential γ-ray irradiation and heat annealing experiments. The Si—O—Si strained bond is approximately annealed at about 300 °C. By comparing the fracture-induced defects for glass preforms and optical fibers, the change in chemical bonding state can be analyzed. Analysis of mechanical-fracture-induced defects is a strong technique for elucidation of the chemical bonding state of silica glass.
The negative differential resistance characteristics of double-barrier interband tunneling structures
Authors: Houng M. P. , Wang Y. H. , Shen C. L.
Published: July 2002

The tunneling mechanism responsible for negative differential resistance in GaSb/AlSb/GaSb/AlSb/InAs double-barrier interband tunneling structures was studied systematically. It was found that the peak current results from resonant interband tunneling and that the spacers make a significant contribution to the valley current. Furthermore, the optimal thicknesses for the GaSb well and AlSb barriers were predicted to be 65 and 10 Å, respectively, which agrees fairly well with experimental results. Our theoretical results give some useful design principles for this type of interband tunneling device.
On the artificial creation of the EL2 center by means of boron implantation in gallium arsenide
Authors: Morante J. R. , Pérez-Rodríguez A. , Samitier J. , Romano-Rodríguez A.
Published: July 2002

In the present work, an analysis of the dark and optical capacitance transients obtained from Schottky Au:GaAs barriers implanted with boron has been carried out by means of the isothermal transient spectroscopy (ITS) and differential and optical ITS techniques. Unlike deep level transient spectroscopy, the use of these techniques allows one to easily distinguish contributions to the transients different from those of the usual deep trap emission kinetics. The results obtained show the artificial creation of the EL2, EL6, and EL5 defects by the boron implantation process. Moreover, the interaction mechanism between the EL2 and other defects, which gives rise to the U band, has been analyzed. The existence of a reorganization process of the defects involved has been observed, which prevents the interaction as the temperature increases. The activation energy of this process has been found to be dependent on the temperature of the annealing treatment after implantation, with values of 0.51 and 0.26 eV for the as-implanted and 400 °C annealed samples, respectively. The analysis of the optical data has corroborated the existence of such interactions involving all the observed defects that affect their optical parameters.
Dynamic compaction of high-temperature superconducting YBa2Cu3Ox powders by laser-driven shocks
Authors: Darquey P. , Kieffer J. C. , Gauthier J. , Pépin H. , Chaker M. , Champagne B. , Villeneuve D. , Baldis H.
Published: July 2002

Laser-driven shocks are successfully used to densify high-temperature superconducting YBa2Cu3O7 ceramic powders. Pressures of 10 GPa on a nanosecond time scale were generated by producing a confined plasma with a high-intensity laser pulse. Significant consolidation is generated up to 300 μm below the irradiated surface (1 cm2). Magnetic susceptibility measurements indicate that the pinning of the intergranular vortices is strongly affected by the shock propagation. Recovery of superconductivity in compacted material is obtained with a postshock oxygen annealing.
Attenuated total reflection method of excitation of the surface polariton in the Kretschmann configuration
Authors: Seshadri S. R.
Published: July 2002

In the attenuated total reflection method of excitation of the surface polariton, the plane-wave reflectivity has a singular behavior for angles of incidence which nearly lead to the phase-matching condition. A technique for the elimination of the singularity leads to the usual two canonical equations governing the excitation of the surface polariton. The characteristics of the antiresonance in the angular response of the reflectivity are treated, and the effect of a small loss in the metal on these characteristics is also discussed.
Positron mobility in thermally grown SiO2 measured by Doppler broadening technique
Authors: Kong Y. , Leung T. C. , Asoka-Kumar P. , Nielsen B. , Lynn K. G.
Published: July 2002

The positron mobility in thermally grown SiO2 is deduced from Doppler broadening lineshape data on a metal-oxide-semiconductor sample for positrons implanted into the oxide layer. The fitted mobility is ∼13(10)×10−3 cm2/s V. This value is between that of the electron and hole mobilities in the same system and is two orders of magnitude smaller than the previous estimate from positron measurements.
Calculation model for the optical constants of amorphous semiconductors
Authors: Adachi Sadao
Published: July 2002

We present a phenomenological expression for the calculation of the optical constants, the refractive index, the extinction coefficient, and the absorption coefficient, of amorphous (a) semiconductors. Our proposed model requires four parameters, namely Eg (optical energy gap), Ec (high-energy cutoff), D (nondirect-transition strength), and Γ (damping). The damping parameter Γ is considered to reflect the distribution of electronic states rather than being the consequence of disorder broadening. Analyses are presented for a-Si, a-Ge, and a-GaAs, and results are in satisfactory agreement with the experimental data over the entire range of photon energy (1.5–6.0 eV).
A comment on dielectric theory: Differential equations and permittivity
Authors: Kliem H.
Published: July 2002

For an explanation of the non-Debye response (NDR) several models have been developed during the last years. Some of these models use differential equations with time-dependent coefficients for the polarization or for the dielectric decay function. In this article it is investigated whether these differential equations may be used for an explanation of NDR. The conclusion is that they are not suitable.
High-energy Si implantation into InP:Fe
Authors: Nadella Ravi K. , Rao Mulpuri V. , Simons David S. , Chi Peter H. , Fatemi M. , Dietrich H. B.
Published: July 2002

High-energy Si implantations were performed into InP:Fe at energies ranging from 0.5 to 10 MeV for a dose of 3×1014 cm−2, and at 3 MeV for the dose ranging from 1×1014 to 2×1015 cm−2. The first four statistical moments of the Si-depth distribution, namely range, longitudinal straggle, skewness, and kurtosis, were calculated from the secondary-ion mass spectrometry (SIMS) data of the as-implanted samples. These values were compared with the corresponding trim-89 calculated values. SIMS depth profiles were closely fitted by Pearson IV distributions. Multiple implantations in the energy range from 50 keV to 10 MeV were performed to obtain thick n-type layers. Variable temperature/time halogen lamp rapid thermal annealing (RTA) cycles and 735 °C/10-min furnace annealing were used to activate the Si implants. No redistribution of Si was observed for the annealing cycles used in this study. Activations close to 100% were obtained for 3×1014-cm−2 Si implants in the energy range from 2 to 10 MeV for 875 °C/10-s RTA. Transport equation calculations were used to interpret low activation results for high dose Si implants. Polaron electrochemical C–V profiling was used to obtain carrier concentration depth profiles. The lattice quality of the as-implanted and annealed material was evaluated by performing x-ray rocking curve measurements.
Flow-orientation effect in batch-produced Langmuir–Blodgett films: Observation of the unsteady flow around the stagnation point
Authors: Tabe Yuka , Ikegami Keiichi , Kuroda Shin-ichi , Saito Kazuhiro , Saito Mitsuyoshi , Sugi Michio
Published: July 2002

The in-plane anisotropy of the Langmuir–Blodgett films of a merocyanine-fatty acid mixed system has been further studied for two different cases of batch production with the substrates aligned face to face and side by side, respectively, to clarify the limitations of the flow-orientation model based on the ideal-fluid approximation. For the side-by-side case, satisfactory coincidence is found between the model and experiment. The actual dichroic behavior deviates from that predicted for the face-to-face case with smaller intersubstrate distances. It was speculated that the stagnation point is associated with anomaly in velocity which is responsible for this discrepancy. In order to examine this, the flow of the monolayer on the water surface during the deposition has been observed using sulphur powder as marker in addition to the optical measurements. The actual flow deviates from that expected from the model and is associated with unsteady motion around the stagnation point as previously speculated. It is indicated that the ideal-fluid approximation is inappropriate for the face-to-face case with smaller distances, leading to the prediction inconsistent with the actual dichroic behavior.
Contributions to the technique and interpretation of the photothermal beam deflection experiment
Authors: Woelker M. , Bein B. K. , Pelzl J. , Walther H. G.
Published: July 2002

Two aspects of the photothermal beam deflection experiment, which are essential for a quantitative interpretation and for an improved performance of the experiment are studied here: The alignment of the probe laser beam with respect to the pump beam and to the solid surface under investigation and, second, the smoothing of experimental data by convolution filtering.
Distribution mechanism of voids in Si-implanted GaAs
Authors: Chen Samuel , Lee S.-Tong , Braunstein G. , Ko K. Y. , Tan T. Y.
Published: July 2002

Voids, formed by the condensation of an excess of implantation-induced vacancies, have been recently identified as the defect directly responsible for dopant diffusion and electrical activation anomalies in Si-implanted and annealed GaAs and GaAs/AlGaAs superlattice materials. Depending on the implanted dose, voids can be distributed either throughout the implanted region or in two bands. We have examined the origin of this void distribution difference. In the as-implanted sample associated with the latter case, a buried continuous band of amorphous GaAs has formed. GaAs formed by the recrystallization of amorphous GaAs does not contain excess vacancies and therefore cannot form voids. However, on either side of the amorphous layer, the excess vacancies can condense to form the observed banded distribution of voids. In the as-implanted sample associated with the former case, a continuous amorphous GaAs layer did not form, and therefore, upon annealing, voids are seen throughout the implanted region.
Optical effects of multilayer thin-film structures during zone-melting recrystallization with an infrared heat source
Authors: Wong Peter Y. , Miaoulis Ioannis N.
Published: July 2002

Processing multilayer thin-film structures, such as silicon-on-insulator (SOI) wafers, with a radiant line heat source has been used extensively to recrystallize and improve the quality of specific films in the structure. The radiation heat transfer has been found to have the greatest impact on the success of the process. The amount of reflection and absorption of radiation emitted by the infrared heat source is influenced by the layering scheme and thicknesses of the films when these thicknesses are on the same order of magnitude as the wavelengths of the radiation. A two-dimensional numerical heat transfer model has been developed to account for the optical nature of the multilayer films during infrared processing. A numerical parametric study was conducted to evaluate the significance of varying the thickness of different layers of SOI wafers during zone-melting-recrystallization processing with a graphite strip heater emitting in the infrared spectrum. Results indicate that varying either the capping or insulating silicon oxide film causes significant fluctuations in the reflectivity and temperature profiles of the film. Increasing the thickness of the Si film results in small fluctuations of the reflectivity. A linear increase of the maximum temperature attained and the melt zone width was calculated for increasing thickness of the Si film. Sensitive layering schemes, where small variations in thickness may result in significant changes in reflectivity, were identified. The optical effects on the width and structure of the melt zone were investigated.
A study on the c-axis alignment of Nd-Fe-B die-upset magnets (abstract)
Authors: Kawai T. , Tanigawa S. , Tokunaga M.
Published: July 2002

It is necessary to get higher c-axis alignment of Nd2Fe14B grains in Nd-Fe-B magnets for higher-energy products. During the hot working process, where melt-spun flakes were hot pressed followed by die-upsetting, hot-pressed bodies showed roughly isotropic structure while c-axis alignments were developed during die-upsetting. In this report, we studied the relationship between the reductions during upsetting and the degree of the c-axis alignments and also tried to get the formula of this relationship. We prepared samples having different reductions and evaluated remanence, c-axis alignment and strains of each flake in the samples. c-axis alignments were evaluated using x-ray diffraction.1 Strains were measured using the sizes of each deformed melt-spun flake. In the range of 50%–70% reduction, remanence was lower in the surface layer than in the center. The difference in remanence in these two regions became smaller as the reduction became larger. The distribution of c-axis alignments and strains showed similar phenomena. Then we could propose the following formula showing the relationship between reduction and c-axis alignment: (σ − σ0)/(σi − σ0) = (1 − εz)4, where σ is standard deviation of c axis alignment and σ0, σi denotes the σ value at enough large strain and initial value of σ, respectively. εz means strain parallel to pressing axis.
Comparative measurements of the field dependence of Young’s modulus and shear modulus in Fe-based amorphous wire
Authors: Atalay S. , Squire P. T.
Published: July 2002

In this paper we present measurements of the field dependence of Young’s modulus E(H) and shear modulus G(H) for amorphous wires of composition Fe77.5B15Si7.5. E(H) was measured by a vibrating reed method and G(H) by means of a torsional pendulum. For as-cast wire the maximum change in either modulus is about 2%, and the form of the field dependence is consistent with the rotation of moments in the outer sheath from a radial to an axial direction, analogous to the moment rotation model for transversely field annealed ribbons. The small magnitude of the modulus changes is a consequence of the rather high anisotropy of a few kJ/m3 associated with the casting stress. The modulus changes in current annealed wires are much greater than in the as-cast state, with ratios Emin/Emax approaching the largest values found in transversely field annealed ribbons. This is consistent with the reduction of the radial stress by annealing, but also indicates the retention of a substantial proportion of off-axis moments. The significance of these results in the light of present understanding of domain structure in amorphous wires is discussed.
Thermodynamic and magnetic properties of the tetravalent rare-earth perovskite system BaPrO3
Authors: Cao G. , Yuen Tan , Pernambuco-Wise P. , Crow J. E. , O’Reilly J. W. , Kuric M. V. , Guertin R. P. , Rosov N. , Lynn J. W.
Published: July 2002

The temperature dependence of the magnetic susceptibility χ(T) and heat capacity C(T), the temperature and field dependence of the magnetization M(T,H), and neutron diffraction have been measured in the tetravalent perovskite system BaPrO3. A clear anomaly, observed in C(T), χ(T), and neutron scattering data at 11.7 K is attributed to antiferromagnetic alignment of Pr. Magnetization indicates that this ordering is insensitive to magnetic fields with TN being independent of H for H<12 T. High-field isotherms show an upward kink in slope at 12.7 T independent of temperature for T<9.5 K, suggestive of a spin reorientation transition. 119Sn Mössbauer data for BaPr0.95Sn0.05O3 indicate a quadrupole splitting at 300 K that broadens below TN due to a hyperfine field.
Magnetic and structural properties of Co/Cr multilayers with in-plane anisotropy
Authors: Wang D. , Sellmyer D. J.
Published: July 2002

Previously we studied the effect of Cr spacer layer on in-plane coercivity in Co/Cr multilayered films. It was found that the high coercivity is related to the easy axis in-plane orientation and the character of the composition modulation. In this paper, the dependence of the saturation magnetization as a function of Co layer thickness is used to study the interface structure. The layer structure is confirmed by small angle x-ray diffractometry for typical films. The magnetic interaction among the grains is studied by examining deviation from the behavior predicted by the Wohlfarth model for noninteracting single-domain particles.
Folded phonon spectra of progressively annealed amorphous Si/Ge superlattices
Authors: Kumar Sunil , Trodahl H. J.
Published: July 2002

Raman scattering from folded acoustic modes of amorphous Si/Ge superlattices has been employed to study thermally induced structural changes in samples of different periods. For a superlattice with 13.5-nm period, the position and intensity of the folded phonon doublet, obtained as a function of annealing temperature, reveal (i) the diffusion of Si into the Ge layers up to an anneal temperature of 500 °C without much effect on Si/Ge interfaces, (ii) a severe degradation of Si/Ge interfaces at an anneal temperature of 600 °C, followed by crystallization of the superlattice, and (iii) a nearly complete intermixing of the layers after the 700 °C anneal. A superlattice with a larger period (P=20.5 nm), on the other hand, retains its layered structure even after an anneal of 700 °C.
Tunneling stabilized, magnetic force microscopy with a gold-coated, nickel-film tipa)
Authors: Moreland John , Rice Paul
Published: July 2002

Tunneling stabilized magnetic force microscopy (TSMFM) is done with a scanning tunneling microscope having a flexible magnetic tip. TSMFM can be used to generate maps of magnetic records with submicrometer resolution. We find that Au-coated, Ni-film tips made from a free-standing 0.5-μm-thick Ni film can be used as a noninvasive probe for imaging magnetic bit patterns on the surfaces of computer hard and floppy disks, and computer tape. This variant of scanning tunneling microscopy shows promise as a viable tool for diagnostic use in the magnetic recording industry.
Roles of Ti-intermetallic compound layers on the electromigration resistance of Al-Cu interconnecting stripes
Authors: Lee C.-C. , Machlin E. S. , Rathore H.
Published: July 2002

Various suggestions for the origin of the beneficial effect of TiAl3 layers on the electromigration resistance of Al-Cu interconnect stripes have been evaluated experimentally. The leading candidate for this effect, the shunting of current through the intermetallic compound layer that spans opens (voids) in the Al-Cu layer, cannot account for the correlation found in the present investigation between the median time to failure and the median rate of increase of electrical resistance in the Al-Cu layer of different multilayer interconnect configurations. When the TiAl3 layer lies above the Al-Cu conducting layer, the major enhancement of median lifetime to failure is an effect the TiAl3 layer has on the ‘‘electromigration resistance’’ of the Al-Cu layer. This effect arises from the overlayer acting to decrease the rate of mass transport in the Al-Cu layer rather than from any effect on the grain size or texture of the Al-Cu layer. It is believed that the TiAl3 overlayer acts to produce triaxiality in the local stress fields developed at mass sinks and sources. This triaxiality of the stress field should act to increase the local yield strengths. A model analysis shows that the yield strengths required to reduce the mass flux by the observed amount are reasonable for a near triaxial state of stress. Only when the TiAl3 layer is below the Al-Cu layer is it possible that shunting is the cause of the enhancement in the median time to failure relative to that for a single Al-Cu interconnect layer.
Investigation by laser-induced fluorescence of surface vaporization during the pulsed CO2 laser irradiation of a titanium sample in an ambient gas
Authors: Hermann J. , Boulmer-Leborgne C. , Dubreuil B. , Mihailescu I. N.
Published: July 2002

This work deals with the study of laser-induced surface vaporization in the presence of an ambient gas, in the conditions where a plasma develops at the gas-material interface. In the experiment, a pulsed CO2 laser is focused onto a titanium target in a cell containing He, Ar, or N2 from a few tenths of Torr up to atmospheric pressure. The temporal and spatial distributions of Ti atoms vaporized from the surface are measured by laser-induced fluorescence spectroscopy. Strong evaporation is observed in helium up to several hundred Torr. In this gas, the Ti atom plume expands according to the propagation of a blast-wave. On the contrary, a very small quantity of material is vaporized in the presence of Ar or N2, because a highly absorbing breakdown plasma develops in this case, as soon as the surface vaporization threshold is reached, shielding the target from subsequent laser heating. These results corroborate our previous analysis based on the spectroscopic observation of the plasma.
Strain, doping, and disorder effects in GaAs/Ge/Si heterostructures: A Raman spectroscopy investigation
Authors: Mlayah A. , Carles R. , Leycuras A.
Published: July 2002

The present work is devoted to a Raman study of GaAs/Ge/Si heterostructures grown by the vapor-phase epitaxy technique. We first show that the GaAs epilayers are submitted to a biaxial tensile strain. The strain relaxation generates misfit dislocations and thus disorder effects which we analyze in terms of translational invariance loss and Raman selection rules violation. The first-order Raman spectra of annealed samples exhibit an unexpected broadband we identify as due to scattering by a coupled LO phonon-damped plasmon mode. This is corroborated by an accurate line-shape analysis which accounts for the recorded spectra and makes evident the presence of free carriers within the GaAs layers. Their density is estimated from the deduced plasmon frequency and also using a method we have presented in a previous work.
Optical waveguides formed by deuterium passivation of acceptors in Si doped p-type GaAs epilayers
Authors: Zavada J. M. , Weiss B. L. , Bradley I. V. , Theys B. , Chevallier J. , Rahbi R. , Addinall R. , Newman R. C. , Jenkinson H. A.
Published: July 2002

We report on the use of plasma hydrogenation of Si doped, p-type GaAs crystalline samples to form infrared waveguides through acceptor passivation. Epilayers grown by liquid phase epitaxy were exposed to a deuterium plasma for ninety minutes at three different temperatures. Secondary-ion mass spectrometry (SIMS) analysis indicated that the deuterium concentrations in the crystals after plasma exposure were nearly equal to the acceptor level and extended to depths between 2.0 and 4.0 μm. Reflectivity measurements showed that the epilayers had passivated regions whose thicknesses corresponded to those determined by SIMS analysis. Laser coupling experiments at 1.15 μm showed optical waveguiding in each sample and lowest propagation losses were on the order of 35 dB/cm. At a wavelength of 1.523 μm, only the sample processed at the highest temperature exhibited laser guiding and losses were considerably higher.
Application of the tight-binding Green’s function method to interface roughness in resonant tunneling heterostructures
Authors: Henrickson Lindor E. , Hirakawa Kazuhiko , Frey Jeffrey , Ikoma Toshiaki
Published: July 2002

Tight-binding Hamiltonian matrix elements are derived including the effects of a space-dependent effective mass. The matrix elements are formulated for a single-particle effective-mass Hamiltonian in the nearest-neighbor approximation. The resulting Hamiltonian matrix has been used to investigate the effect of heterointerface roughness in double barrier resonant tunneling structures. Calculations show that a resonant level splitting in the transmission characteristic can occur when the lateral scale of the heterointerface roughness becomes longer than the de Broglie wavelength of the resonant energy. The observed level splitting is explained in terms of an averaging of transmission through parallel resonant structures having different well widths.