bluuebunny/crossref_metadata_2025 huggingface dataset:
Modelling and design of quantum dot cellular automata
Authors: Fu Y. , Willander M.
Published: July 2002

Research aimed at design of electronic computers that contain nano-size components has been developing very rapidly. In this work we study the quantum dot cellular automata (QCA) by the full Schrödinger equation in which the antisymmetric property of the many-electron state is included. We have concluded that because of the quantum wave characteristics, a localized electronic state appears only at very high gate voltages in a dual-gate Si quantum dot structure. The Coulomb exchange interaction due to the wave function overlapping between two electrons reduces the total energy of the electron pair. These two factors must be carefully studied for a functional QCA.
Matrix method for the x-ray rocking curve simulation
Authors: Kim In , Ryu Sang-Wan , Choe Byung-Doo , Kim Hee-Dae , Jeong Weon Guk
Published: July 2002

A matrix representation is developed for the solution of the Takagi–Taupin equations of x-ray diffraction. By the virtue of its unimodular property, the solution matrix substantially reduces the calculation time for the superlattice (SL) structure with a large periodicity. Also, the simplified form of the solution makes it easier to understand and quantify the inherent properties of the x-ray diffraction such as the interference fringes and the SL peaks.
Transient behavior of a-Si(C):H bulk barrier color detectors
Authors: Zhu Q. , Sterzel J. , Schneider B. , Coors S. , Böhm M.
Published: July 2002

The photocurrent transient of a-Si(C):H n-i-p-i-n/p-i-n-i-p color detectors under pulse illumination at different wavelengths has been investigated. In contrast to amorphous silicon Schottky diodes and p-i-n/n-i-p structures, the photocurrent decay after the end of the steady state illumination is barely influenced by the applied bias voltage. Moreover, a reversal of the photocurrent direction can be observed under certain bias when the light is being switched on. It is suggested that these properties of n-i-p-i-n/p-i-n-i-p structures are mainly attributed to the accumulation and trapping of the photogenerated carriers near the central barrier.
Observation of near-surface electrically active defects in n-type 6H–SiC
Authors: Doyle J. P. , Schöner A. , Nordell N. , Galeckas A. , Bleichner H. , Linnarsson, M. K. , Linnros J. , Svensson B. G.
Published: July 2002

In n-type 6H–SiC epitaxial layers grown by vapor phase epitaxy, we find that in contrast to the majority of the epitaxial layer, where electrically active defects are observed with a concentration less than 1×10−13 cm−3, a region near the front surface contains defects with concentrations approaching 1014 cm−3. A relationship between the near-surface defects and metallic impurities is suggested by a Ti concentration of 1×1016 cm−3 in this region. The high concentration of near surface defects is found to significantly reduce the carrier lifetime.
Measurements of relative BCl density in BCl3-containing inductively coupled radio frequency plasmas
Authors: Fleddermann C. B. , Hebner G. A.
Published: July 2002

The relative density of BCl radicals in inductively coupled plasmas has been studied using laser-induced fluorescence (LIF), and the BCl excited state has been studied using plasma-induced emission (PIE). Measurements were made as a function of input power, pressure, position, and as a function of gas ratio for industry-relevant metal-etch gas mixtures containing BCl3, Cl2, Ar, and N2. LIF was used to measure the ground state BCl population, whereas PIE monitored the BCl A1Π excited state; the LIF and PIE intensities varied differently as the plasma parameters were changed. Between 150 and 400 W input power at 20 mTorr pressure, there was no variation in BCl density, indicating that the dissociation fraction for BCl3 to BCl was constant with power. No significant interactions between BCl3 and Cl2 or Ar were evident in the LIF measurements. However, the BCl density was suppressed by addition of nitrogen to the plasma. The BCl density was radially uniform for all gas mixtures, but axial measurements showed a slight decrease in BCl density near the upper electrode. After running the reactor with a BCl3/N2 mixture, BCl was observed for up to an hour after the discharge was switched to Cl2: this is attributed to buildup of BN films on reactor surfaces and subsequent etching of the film by Cl.
Beam characteristics of short-pulse radiation with electromagnetic missile effect
Authors: Wang Gang , Wang Wen Bing , Liang Chang Hong
Published: July 2002

Beam characteristics such as beamwidth and beam intensity of short-pulse radiation with electromagnetic missile effect, not restricted to the electromagnetic missile, are generally range dependent in pulse propagation. An effective measure to study such beam characteristics is to investigate the local drop rate of the energy pattern in the beam profile and the local decay rate of energy in the pulse beam. It is shown that both the energy decay rate and the energy drop rate are definitely determined by the time–space source parameters so that the beam characteristics can be readily evaluated and properly controlled. Moreover, the underlying relation between the slow decay rate and the energy drop rate are constructed, which shows perfect harmony in achieving slower-energy decay and higher-energy concentration in the beam.
Computation of the nonlinear magnetic response of a three dimensional anisotropic superconductor
Authors: Žutić Igor , Valls Oriol T.
Published: July 2002

Many problems in computational magnetics involve computation of fields which decay within a skin depth δ, much smaller than the sample size d. We discuss here a novel perturbation method which exploits the smallness of ε≡δ/d and the asymptotic behavior of the solution in the exterior and interior of a sample. To illustrate this procedure we consider the computation of the magnetic dipole and quadrupole moments of an anisotropic, unconventional, three dimensional superconductor. The method significantly reduces the required numerical work and can be implemented in different numerical algorithms. © 1998 American Institute of Physics.
Long wavelength spin dynamics in La0.53Ca0.47MnO3
Authors: Rhyne J. J. , Kaiser H. , Luo H. , Xiao Gang , Gardel M. L.
Published: July 2002

The magnetic structure and dynamics in the colossal magnetoresistance (CMR) class perovskite La0.53Ca0.47MnO3 have been studied by elastic and inelastic neutron scattering. This composition is near the 0.5 Ca transition from a metallic ferromagnet to an insulating antiferromagnet. Powder neutron diffraction on these samples showed coexisting ferromagnetic and antiferromagnetic phases at low temperature and a splitting of the lattice parameters of the antiferromagnetic phase near TN, reflecting the onset of the charge ordered state. Inelastic scattering measurements of the ferromagnetic excitations exhibited well-defined spin waves of resolution width below Tc. The spin waves exhibited conventional Heisenberg behavior with dispersion of the form E=Dq2+Δ with spin stiffness D(T=0 K)≈135 meV Å2 and an energy gap Δ≈0.1 meV. The value of the spin wave stiffness D is similar to that found for other ferromagnetic perovskite materials, and it renormalized with temperature in a manner consistent with the magnetization.
Exchange coupling of NiFe/FeRh–Ir thin films
Authors: Yuasa S. , Nývlt M. , Katayama T. , Suzuki Y.
Published: July 2002

We report unusually large room temperature coercive field Hc induced in Ni0.81Fe0.19 films grown on the (001) magnetically compensated surface of antiferromagnetic single crystal CsCl-type FeRh0.95Ir0.05 alloy thin layers on MgO(001) substrates. The specimens were prepared by conventional rf sputtering in Kr atmosphere, at 450–500 °C for FeRh–Ir and 20–200 °C for NiFe. The epitaxial growth of NiFe/FeRh–Ir and atomically flat terraces of FeRh–Ir were confirmed by x-ray reflectometry and scanning tunneling microscopy. For a NiFe(2.7 nm)/FeRh–Ir(22 nm) film, a maximum Hc of 810 Oe is observed. The 1/tNiFe dependence of Hc for larger NiFe thicknesses indicates that the large coercive field originates from the magnetic interaction at NiFe/FeRh–Ir interface. The large Hc combined with high corrosion resistance make FeRh based alloys a promising candidate for application as pinning layers in spin-valve devices.
Study on the in-plane uniaxial anisotropy of high permeability granular films
Authors: Li W. D. , Kitakami O. , Shimada Y.
Published: July 2002

We have investigated the uniaxial magnetic anisotropy of the high resistivity Fe–Al–O granular films prepared by sputtering. It is found that application of external dc field during film deposition and post-annealing treatments give rise to no serious effects on the magnetic anisotropy. In contrast, the incident angle of sputtered particles to the substrate greatly affects the strength and the direction of the anisotropy. These results suggest that the magnetic anisotropy is induced by a shape anisotropy due to anisotropic morphology in the films. The temperature dependence of the anisotropy constant, however, does not obey the usual shape anisotropy relationship Ku(T)∝Ms(T)2. In the present article, we propose an anisotropic coupling model in order to explain the uniaxial anisotropy found in these granular films, where the magnetic coupling between Fe grains is assumed to be anisotropic in the film plane due to the presence of low Tc intergranular phase which bridges the gap between Fe grains in one direction but not in the orthogonal direction. This model satisfactorily explains the temperature dependence of Ku(T)and Ms(T) for all Fe–Al–O granular films prepared in the present experiment. This agreement between the experiments and the theory implies that the magnetic anisotropy of the granular films is remarkably influenced by the magnetism and spatial distribution of the low Tc intergranular phase.
A Mössbauer spectral study of Tb2Fe17 and the Tb2Fe17−xSix solid solutions
Authors: Hautot Dimitri , Long Gary J. , Ezekwenna P. C. , Grandjean F. , Middleton D. P. , Buschow K. H. J.
Published: July 2002

The Mössbauer spectra of a series of rhombohedral Tb2Fe17−xSix solid solutions, with x equal to 0, 1, 2, and 3, have been measured as a function of temperature. Although the spectra of Tb2Fe17 change substantially upon cooling from 295 to 85 K, it has been possible to fit them with a consistent seven sextet model corresponding to a basal magnetization. The spectral analysis yields reasonable hyperfine parameters and the expected changes with temperature. The resulting weighted average effective iron recoil mass of 66 g/mol and the Mössbauer temperature of 395 K are typical of this type of intermetallic compound. In addition, the isomer shifts and the hyperfine fields observed for the crystallographically distinct iron sites in Tb2Fe17 agree well with those expected from the differences in the Wigner–Seitz cell volumes and the near-neighbor environments of the four sites. The spectra of the silicon substituted solid solutions have been fit with the same model and similar hyperfine parameters, but with a binomial distribution of near-neighbor environments. The weighted average hyperfine field decreases slowly with increasing silicon content, whereas the isomer shift increases.
Effects of excitons on solar cells
Authors: Zhang Yong , Mascarenhas Angelo , Deb Satyen
Published: July 2002

We have studied the effects of excitons on the two key parameters of a Si solar cell: the dark-saturation current and short-circuit current. We have found that the effect of excitons on the dark-saturation current is very sensitive to the boundary condition for excess excitons at the edge of the depletion region. With the assumption of near equilibrium between the electrons and excitons, we find that the exciton effect is rather small, which is contrary to the conclusion of significant reduction in the dark-saturation current made in previous work with the assumption of no excess excitons at the edge [J. Appl. Phys. 79, 195 (1996)]. The results for the short-circuit current are very similar to the previous work. However, the analytical results for the carrier concentrations and the corresponding currents are now presented in a simple way in which the physical meaning of each individual term is elucidated or revealed. Furthermore, we have found, for practical purposes, very accurate approximate solutions for the carrier concentrations and corresponding currents. Our conclusion is that the major effect of excitons on the Si solar cell performance relies on whether the effective diffusion length (L1) of the coupled electron–exciton system is significantly greater than that of the electron itself (Le).
Trajectory displacement effect in particle projection lithography systems: Modifications to the extended two-particle theory and Monte Carlo simulation technique
Authors: Jansen G. H.
Published: July 2002

The extended two-particle model for statistical interactions in particle beams, developed by Jansen [Coulomb Interactions in Particle Beams (Academic, Boston, 1990)], has been refined to improve the accuracy of the predictions of the trajectory displacement effect in particle beam projection systems. The original theory was developed for probe forming systems, such as electron and ion scanning microscopes and Gaussian or shaped beam lithography systems. Fit functions are used within the theory to express part of the numerical output into explicit analytical prescriptions. These functions were found to become inaccurate for the relatively wide beams typically used in the more recently developed projection type lithography systems. New fit functions are presented which extend the applicability of the theory to the wide beams and doublet configurations used in projection systems. The Monte Carlo program MONTEC, used to verify the results of the analytical theory, has been modified as well to account for the first order space charge magnification effect. This effect could be ignored for the relatively small spots of Gaussian and shaped beam systems, but would yield a significant overestimation of the trajectory displacement effect—assumed to be identical to the remaining blur after refocusing—for the wide images used in projection type of systems. The refined analytical theory and the modified MONTEC program have been used to evaluate the impact of statistical interactions on the performance of the SCALPEL electron projection system and a hypothetical ion projection lithography system, representing a simplified model of the IMS ALG-1000 (He+) system. The analytical predictions are in good agreement with the Monte Carlo results. An estimate of the total system resolution, determined by the combined effect of statistical interactions and geometrical aberrations, indicates that maximum attainable beam current for a 0.18 μm design rule is about 25 μA for the SCALPEL and 0.2–0.3 μA for the ion projection system, leading to an ∼10× higher throughput for the former taking the difference in resist sensitivities for electrons and ions into account.
X-ray photoelectron diffraction from (3×3) and (√3×√3)R 30° (0001)Si 6H–SiC surfaces
Authors: King S. W. , Ronning C. , Davis R. F. , Busby R. S. , Nemanich R. J.
Published: July 2002

High-resolution (±1°) x-ray photoelectron diffraction (XPD) patterns were obtained along high symmetry azimuths of the (3×3) and (√3×√3)R30° reconstructed (0001)Si 6H–SiC surfaces. The data were compared to XPD patterns obtained from (7×7) Si (111) as well as to models proposed for the (3×3) and (√3×√3)R30° 6H–SiC reconstructions. Forward scattering features similar to those observed from the (7×7) Si (111) were also observed from the (√3×√3)R30° 6H–SiC (0001)Si surface. Additional structures were found and attributed to the substitution of carbon atoms for silicon. Unlike (1×1) and (7×7) Si (111) surfaces, the XPD patterns of (3×3) and (√3×√3)R30° SiC (0001)Si surfaces are different which is due to the presence of an incomplete bilayer of Si on the (3×3) surface. The most significant difference with the Si system is the equivalence of the [101̄0] and [011̄0] azimuths in the (3×3) structure. These results are consistent with a faulted Si bilayer stacking sequence which was proposed based on scanning tunneling microscopy observations.
Numerical study of argon ions transported across the sheath in electron cyclotron resonance discharges
Authors: Zhong X. X. , Wu J. D. , Sun J. , Wu C. Z. , Li F. M.
Published: July 2002

The transport of argon ions through the sheath as well as through the neutral region in electron cyclotron resonance discharges in argon gas is studied by means of Monte Carlo simulation, with the emphasis on the sheath potential and the characteristics of argon ions in the sheath. The evolution of the energy, velocity, and angle distributions of the ions in the sheath and the dependence of the distributions on gas pressure and substrate bias are investigated primarily concerned with collisional sheath. It is found that the effect of the pressure on the ion behavior in the sheath is not the same as in the neutral region, however, the substrate bias greatly influences the ion behavior mainly due to ion-neutral elastic collisions within the sheath. Our results also show that the ion motion in crossing the sheath tends to be parallel to the applied magnetic field, and an ion flux with good directionality can be expected by imposing on a negative voltage substrate.
Correlation between fatigue and piezoelectric properties in (Pb, La) (Zr, Ti)O3 thick films
Authors: Bobnar Vid , Kutnjak Zdravko , Levstik Adrijan , Holc Janez , Kosec Marija , Hauke Tilo , Steinhausen Ralf , Beige Horst
Published: July 2002

The correlation between the ferroelectric fatigue and variations in piezoelectric properties in a 8/65/35 (Pb, La) (Zr, Ti)O3 thick film was studied by simultaneous measurements of the polarization hysteresis loop, the strain-field loop, the piezoelectric constant, and the dielectric constant during cycling. It was found that the fatigue reflects itself in the variations of the piezoelectric constant and the induced strain. Namely, during cycling both quantities decrease for the same relative amount as the remanent polarization. In addition, the effective electrostriction constant as a function of switching cycles was determined.
Comparison of Cu gettering to H+ and He+ implantation-induced cavities in separation-by-implantation-of-oxygen wafers
Authors: Zhang Miao , Lin Chenglu , Duo Xinzhong , Lin Zixin , Zhou Zuyao
Published: July 2002

Well-defined bands of cavities have been formed beneath the buried oxide (BOX) layer of two sets of separation-by-implantation-of-oxygen (SIMOX) wafers by H+ and He+ implantation. The gettering of Cu impurities, which were implanted into the top Si layer at different doses (5×1013, 5×1014, and 5×1015/cm2), to the cavities has been studied by secondary ion mass spectroscopy and cross-sectional transmission electron microscopy. The results indicated that the cavities induced either by H+ or He+ implantation are effective gettering centers for Cu in SIMOX wafers, and up to 4×1015/cm2 Cu has diffused through the BOX layer and been captured by the cavities. The gettering efficiency of cavities increases with the decrease of Cu implantation doses and the increase of annealing temperatures. He+ ion implantation is found to be more suitable for cavity formation and impurity gettering than H+ ion implantation.
Studies of ZnSe-based semiconductor thin films using grazing incidence x-ray scattering and diffraction
Authors: Huang S. , Soo Y. L. , Ming Z. H. , Kao Y. H. , Na M. H. , Chang H. C. , Lee E. H. , Luo H. , Peck J. , Mountziaris T. J.
Published: July 2002

Grazing incidence x-ray scattering and x-ray diffraction techniques have been employed to investigate the microstructures in various ZnSe-based semiconductor thin films grown on GaAs substrates by molecular beam epitaxy and metalorganic chemical vapor deposition methods. The results are also used for a comparison of the interfacial roughness and overall quality of the II–VI thin films prepared by these two different growth methods. Structural parameters such as the interfacial roughness and layer thickness obtained from the scattering measurements and lattice constants obtained from the x-ray diffraction pattern around the GaAs(004) peak can be correlated with the film deposition rate, compound composition, and lattice strain in the epilayers. We thus demonstrate that x-ray scattering techniques in conjunction with diffraction measurements are useful tools for nondestructive characterization of buried interfaces in semiconductor layer materials.
Trapping of Si interstitials in boron doping background: Boron clustering and the “+1” model
Authors: Huang M. B. , Mitchell I. V.
Published: July 2002

Boron transient enhanced diffusion (TED) in Si predoped with boron isotope atoms has been studied using secondary ion mass spectroscopy and channeling nuclear reaction analysis. Si crystal was first implanted with B11 ions of various doses and subsequently annealed at 1100 °C for 2 h to produce a uniform B11 doping concentration. The B11-doped sample and a Si control sample were then implanted with 40 keV B10 ions. Compared to the B11-free sample, B10 TED in the B11-doped sample is much retarded during the initial anneal at 750 °C for 1 h, while more broadening in the B10 profile occurs for the B11-doped sample after a second anneal at 950 °C for 15 min. The effect of B11 doping on B10 TED is discussed in terms of the trapping of Si interstitials in B11 doping background. The amount of trapped Si interstitials, for a certain B10 dose, increases with B11 doping concentration, but no “missing” Si interstitials are found for a B11 doping level of 7.8×1017 cm−3. From our data we show that it is also possible to test the “+1” model from the knowledge of B clustering mechanisms.
Doping of ZnSe during molecular beam epitaxial growth using an atomic phosphorus source
Authors: Calhoun L. C. , Park R. M.
Published: July 2002

Zinc selenide films were doped with phosphorus during molecular beam epitaxial (MBE) growth by employing a novel, valved, three-zone solid-source radio frequency (rf)-cracker unit manufactured by Oxford Applied Research (OAR). Optical emission spectroscopy analysis of the plasmas produced in the rf chamber of the unit showed that the apparatus was capable of generating a flux of atomic phosphorus. By suitably adjusting the operating conditions of the unit, phosphorus concentrations in ZnSe epilayers were varied over the range of 1×1016 cm−3 to high-1018 cm−3, in a highly controlled and reproducible fashion. Phosphorus atoms, in contrast to P4 molecules, were found to be highly chemically reactive at the growing ZnSe surface at a normal growth temperature (around 300 °C), and the OAR unit was found to be eminently suitable for the provision of atomic P for MBE growth. Doping using atomic P was found, at least in lightly to moderately doped ZnSe, to provide an acceptor state that gave rise to a neutral-acceptor bound exciton emission located at 2.7919 eV, in keeping with that of other substitutional acceptors in ZnSe. Also, compensation in atomic P doped ZnSe appears to occur by virtue of the formation of shallow donor states rather than via the production of deep level states, which dominate in the case of most of the previously reported phosphorus-doped ZnSe studies.
Microstructure and magnetic properties of Fe100−xPtx alloy films
Authors: Kuo Chih-Ming , Kuo P. C. , Wu Huan-Chao
Published: July 2002

Polycrystalline Fe100−xPtx alloy thin films with Pt composition x=25–67 at. % were prepared by dc magnetron sputtering on natural-oxidized silicon wafer substrates, then postannealed in vacuum at various temperatures and over different periods. The effects of film composition, magnetic layer thickness, annealing temperature, and annealing time on the magnetic properties parallel and normal to the film plane were investigated. Films with high in-plane coercivity could be obtained by annealing the films in the temperature range between 400 and 650 °C. Optimum in-plane squareness and coercivity of the film occurred at a film composition of Fe50Pt50 with thickness of about 200 nm after annealing at 600 °C for 30 min. The analyses of transmission electron microscopy diffraction patterns indicated that this annealed Fe50Pt50 film was nearly entirely in the fct γ1-FePt phase. The average grain size of this Fe50Pt50 film was about 85 nm. The grain size was almost independent of the cooling rate after annealing, but the magnetic hardening mechanism and coercivity of the annealed film was largely dependent on the cooling rate.
Structural evolution during deposition of epitaxial Fe/Pt(001) multilayers
Authors: Hufnagel T. C. , Kautzky M. C. , Daniels B. J. , Clemens B. M.
Published: July 2002

We have investigated the structure of epitaxial Fe/Pt(001) multilayers deposited by direct current magnetron sputtering. In these multilayers, the structure of the Fe layers depends on their thickness: Thick (tFe>22 Å) Fe layers are body-centered cubic (bcc), while thin (tFe<12 Å) Fe layers are face-centered cubic (fcc). Ex situ x-ray diffraction reveals that the unstrained lattice parameter of bcc Fe in epitaxial multilayers is significantly greater than that of bulk bcc Fe, possibly due to alloying with Pt. This suggests that the observed “fcc Fe” is actually an intermixed fcc Fe–Pt interfacial layer. To investigate this possibility, we have performed grazing-incidence x-ray scattering in situ during deposition of epitaxial Fe/Pt(001) multilayers. The structure of Fe(001) layers as thin as 10 Å is bcc, strained due to epitaxial mismatch with the Pt(001) underlayer. Additional Fe deposition results in relaxation of the bcc Fe lattice parameter toward its bulk value. Deposition of Pt onto a 50 Å thick bcc Fe(001) layer has little effect on the Fe, other than to increase its lattice parameter slightly. In contrast, deposition of Pt onto a 20-Å-thick bcc Fe(001) layer results in a partial transformation of the Fe to a fcc structure. We propose that this transformation is the result of intermixing of Pt into the previously deposited Fe layer, resulting in the formation of a fcc Fe-Pt alloy layer.
Berreman 4×4 matrix method for reflective liquid crystal displays
Authors: Stallinga S.
Published: July 2002

The Berreman 4×4 matrix method is extended in order to calculate the optical behavior of reflective liquid crystal displays. Expressions for the reflection matrix and the reflection coefficient are derived. Possibly present uniaxial and/or biaxial retarders can be included in the calculation. Conservation of energy and time reversal invariance of Maxwell’s laws imply an important symmetry relation. This relation is used to reduce the computational time with a factor of 2. The new numerical method is illustrated by calculations of a direct-view twisted nematic effect, without and with a compensating biaxial retarder.
Light-induced transmission nonlinearities in gallium selenide
Authors: Ferrer-Roca Ch. , Bouvier J. , Segura A. , Andrés M. V. , Muñoz V.
Published: July 2002

The intensity of a He–Ne laser (633 nm, 5 mW) transmitted by different GaSe samples is observed to change in correlation with a Nd-yttrium–aluminum–garnet laser pulse (532 nm, 7.8 ns, 3 mJ) which excites them. Such time response has been attributed to a nonlinear optical effect, i.e., a decrease in the refractive index due to the exciton screening by the photogenerated carriers. A calculation of the absorption coefficient and refractive index at different carrier concentrations has led to a reconstruction of transmittance transients which fully agree with the experimental data at different incident intensities and temperatures.
Magnetic aftereffect in a bimodal Co/Pt magneto-optical medium
Authors: Fry Richard A. , Bennett Lawrence H. , Torre Edward Della
Published: July 2002

Magneto-optical Kerr effect measurements were used to perform magnetic aftereffect studies of the reversal behavior in a bimodal magneto-optical medium with perpendicular magnetic anisotropy. This (0.3 nm Co/1.2 nm Pt)15 multilayer film has shown both the Kerr rotation (θK) and ellipticity (εK) to exhibit major hysteresis loops with two distinct field-dependent steps. However, whereas both steps in the θK loop occur in the same direction, in the εK loop they occur in opposite directions. Thus, upon traversing the descending major loop, at the first magnetic reversal, εK increases even though the magnetization decreases. Magnetic aftereffect in this material was investigated in the vicinity of each magnetization step by measuring both θK and εK over time. The data clearly show the time dependence of each of the two separate reversals, including the changes in εK which occur in opposite directions.
Xenon–neon gas proportional scintillation counters: Experimental and simulation results
Authors: Dias T. H. V. T. , Santos F. P. , Rachinhas P. J. B. M. , Borges F. I. G. M. , dos Santos J. M. F. , Conde C. A. N. , Stauffer A. D.
Published: July 2002

When gas proportional scintillation counters (GPSC) are used to detect very low energy x rays, the addition of the light noble gas neon to the usual xenon filling improves the collection of primary electrons that originate near the detector window. However, xenon–neon mixtures have lower electroluminescence yields than pure xenon. Increasing the scintillation electric field jeopardizes the energy resolution because of the additional fluctuations introduced by electron multiplication. In this work we investigate the effect of a limited amount of charge multiplication on the electroluminescence yield and the energy resolution R of a xenon–neon GPSC using both Monte Carlo simulation and experimental measurements. We consider xenon–neon mixtures with 5%, 10%, 20%, 30%, 40%, 50%, 70%, 90%, and 100% Xe at a total pressure of 800 Torr. Comparing the experimental and Monte Carlo data for 5.9 keV x rays, we conclude that optimum value of R is reached in a region of weak ionization with a charge gain of less than 2. By extrapolating the experimental results for R to infinite light yield we obtain the intrinsic energy resolution Rint for 5.9 keV x rays in all mixtures. From these results we can predict Fw values, where F is the relative variance in the number of primary electrons (the Fano factor) and w is the mean energy required to produce a primary electron. From a comparison between Monte Carlo and experimental electroluminescence yields, F and w values are estimated for 5.9 keV x rays in the various mixtures.
Bidirectional stress on a p-metal–oxide–silicon capacitor
Authors: Ziane Dj. , El-Hdiy A. , Salace G.
Published: July 2002

Tunneling Fowler–Nordheim bidirectional electron injections were performed on a metal–oxide–silicon capacitor under constant voltage, and the study of generated defects behavior was investigated. It was found that defects generated within the oxide are of the same physical nature and are mainly composed of neutral traps. The charge and discharge of these traps depend on the oxide field polarity. At the oxide–silicon interface, the saturation of interface state generation is maintained by the alternate change of the stress field polarity. This saturation is linked to the Si−δ–H+δ or other Si−δ–A+δ polar bonds at the silicon/oxide interface.
Tunable epitaxial growth of magnetoresistive La2/3Sr1/3MnO3 thin films
Authors: Fontcuberta J. , Bibes M. , Martı́nez B. , Trtik V. , Ferrater C. , Sánchez F. , Varela M.
Published: July 2002

We report on the growth of epitaxial La2/3Sr1/3MnO3 thin films on buffered Si(001) substrates. We show that a suitable choice of the buffer heterostructure allows one to obtain epitaxial (00h), (0hh), and (hhh) manganite thin films. The magnetotransport properties are investigated and we have found that the low-field magnetoresistance is directly related to the width of the normal-to-plane rocking curves, irrespective of the film orientation. The magnetic anisotropy of these films has also been determined.
Current–voltage characteristics and electrical transport properties of grain boundaries in La1−x(Sr/Ca)xMnO3
Authors: Todd N. K. , Mathur N. D. , Isaac S. P. , Evetts J. E. , Blamire M. G.
Published: July 2002

Grain boundaries in doped lanthanum manganites have been shown to have a large low-field magnetoresistance. However, the mechanism of electrical transport across grain boundaries, and the origin of the low-field magnetoresistance, are not well understood. Models based on scattering at domain walls, spin-polarized tunneling and depression of the Curie temperature due to strain near the grain boundary have all been proposed. This article reports detailed studies of the transport properties of artificial grain boundaries formed in a variety of thin films grown on bicrystal substrates. Resistance versus field sweeps on all grain boundary devices showed strong low-field magnetoresistance and the effect of individual domain motion at the grain boundary has been observed in single grain boundaries. In all cases, current versus voltage characteristics were highly non-ohmic, and reminiscent of an electron tunneling process. However, the magnetic dependence of the current–voltage characteristics implies that the magnetoresistance may be unrelated to tunneling.
Polarization anisotropy in quasiplanar sidewall quantum wires on patterned GaAs (311)A substrates
Authors: Santos Paulo V. , Nötzel R. , Ploog K. H.
Published: July 2002

The polarization anisotropy of the photoluminescence from quasiplanar quantum wires fabricated on the sidewall of [01̄1]-oriented mesas on the (311)A GaAs surface is investigated using a modulation technique that combines high spatial (∼1 μm) and polarization resolution (<0.1%). Due to their low symmetry, (311)A-oriented quantum wells are intrinsically anisotropic with the fundamental transition preferentially oriented along the [2̄33] direction. The anisotropy contribution from the lateral confinement in the [01̄1] wires opposes that from the vertical confinement. With increasing lateral confinement, the wire anisotropy initially reduces and then changes its sign. The lateral extent of the electronic wave functions involved in the anisotropic transitions is obtained from a comparison of the results with a theoretical model.
Suppression of the surface-inversion layer of p-type InAs
Authors: Lachenmann S. G. , Friedrich I. , Förster A. , Schäpers Th. , Kastalsky A.
Published: July 2002

The resistivity of spatially selectable regions on p-type InAs is increased by epitaxial growth of a larger band-gap material, i.e., InAlAs, on top of InAs. Due to this InAlAs layer, the formation of a two-dimensional electron gas in the InAs layer is suppressed. This is demonstrated experimentally and supported by calculations of the conductance and valence band profile. At low temperature (4.2 K), the resistance of p-InAs coated with InAlAs and InGaAs is increased by a factor of 180 compared with bare p-InAs.
Lower critical fields of Bi2Sr2CaCu2O8+d and YBa2Cu3O7−d single crystals
Authors: Tochihara S. , Yasuoka H. , Mazaki H. , Osada M. , Kakihana M.
Published: July 2002

Temperature dependence of the lower critical fields Hc1(T) of Bi2Sr2CaCu2O8+d and YBa2Cu3O7−d single crystals has been investigated for both orientations of the applied fields, parallel and perpendicular to the c axis. The magnetization curves of the samples were measured using a superconducting quantum interference device, and Hc1,c(T) (H∥c axis) and Hc1,ab(T) (H⊥c axis) were obtained using the modified Kim–Anderson critical-state model. From the results, we found both Hc1,c(T) and Hc1,ab(T) for a Bi2Sr2CaCu2O8+d single crystal had an anomalous upturn below about 40 K, while a YBa2Cu3O7−d single crystal did not show any anomaly down to about 10 K. Discussion on a remarkable anomaly Hc1,ab(T) at low temperatures is made using the formulas for anisotropic Hc1 in the Ginzburg–Landau theory. The result suggests that the temperature dependence of the coherence length in the ab plane plays an essential role. We may insist that the anomalous upturn of Hc1,ab for a Bi2Sr2CaCu2O8+d single crystal at low temperatures is attributed to the enhancement of the superconducting order parameter in the domain between the CuO2 layers, as theoretically predicted in terms of a proximity effect. Penetration-depth ratios λc2(0)/λc2(T) and λab2(0)/λab2(T) were also calculated using a theory based on incoherent quasiparticle hopping. Plots of the ratios as a function of the reduced temperature qualitatively reproduce the prediction by the hopping model, and suggest that both Bi2Sr2CaCu2O8+d and YBa2Cu3O7−d single crystals seem to possess the nature of d-wave pairing.
Purification of epitaxial InAs grown by liquid phase epitaxy using gadolinium gettering
Authors: Krier A. , Gao H. H. , Sherstnev V. V.
Published: July 2002

Very pure InAs epitaxial layers of high quantum efficiency have been grown by liquid phase epitaxy using Gd gettering of the growth solution. The residual carrier concentration was reduced to ∼6×1015 cm−3 and the peak photoluminescence intensity increased considerably (between 10 and 100 times) using this technique. The low-temperature (4 K) photoluminescence spectra exhibited sharp bound exciton and donor-acceptor lines. The linewidth of the exciton was measured to be only 3.8 meV which is narrower than for undoped epitaxial InAs grown by other techniques.
Gain dependence of the noise in the single electron transistor
Authors: Starmark B. , Henning Torsten , Claeson T. , Delsing P. , Korotkov A. N.
Published: July 2002

An extensive investigation of low frequency noise in single electron transistors as a function of gain is presented. Comparing the output noise with gain for a large number of bias points, it is found that the noise is dominated by external charge noise. For low gains we find an additional noise contribution which is compared to a model including resistance fluctuations. We conclude that this excess noise is not primarily due to resistance fluctuations. For one sample, we find a low minimum charge noise of qn≈2×10−5 e/Hz at a frequency of 4.4 kHz.
The origin of shallow etch pit defects in low dislocation density GaP crystals
Authors: Okada H. , Kawanaka T. , Ohmoto S.
Published: July 2002

The defect corresponding to shallow etch pits (S-pits) on GaP wafer has been revealed. It is a vacancy-type Frank dislocation loop which surrounds a stacking fault. It is proposed that this loop is formed by the condensation of excess gallium vacancies during cooling of the GaP crystal. The phosphorus precipitates on the dislocation loop result from the condensation of the emitted phosphorous interstitials accompanied by the growth of the loop. Stoichiometry control is necessary to suppress the formation of S-pit defects in low dislocation density compound semiconductor crystals.
Quantitative characterization of advanced porous ceramics based on a probabilistic theory of ultrasonic wave propagation
Authors: Fan Qiulin , Takatsubo Junji , Yamamoto Shigeyuki
Published: July 2002

A straightforward nondestructive method based on the probabilistic theory of ultrasonic wave propagation [JSME Int. J., Ser. A, Mech. Mater. Eng. 39, 266 (1996)] was developed to quantitatively evaluate porosities, pore shapes, and pore sizes in advanced porous ceramics merely by measuring the ultrasonic delay time and pulse width. The extensive ultrasonic measurements and image microanalyses were conducted in advanced porous alumina, sialon, and zirconia with different porosities. A universal equation was established for porous ceramics, clarifying the intrinsic relationships between ultrasonic characteristics (propagation time and pulse width) and pore distribution (porosity, pore shape, and pore size). The critical volume fraction porosity were estimated separately as approximately 0.06, 0.11, and 0.10 in these ceramics using image microanalysis techniques, at which the transition from the continuous to discontinuous pore phase takes place during sintering. An excellent agreement of two useful corollaries with the data on the above nondestructive and destructive examinations validates the quantitative applicability of the probabilistic theory to pore characterization of advanced ceramics, metals, and their combinations.
Dynamics of pulsed-power chlorine plasmas
Authors: Malyshev M. V. , Donnelly V. M. , Colonell J. I. , Samukawa S.
Published: July 2002

Characteristics of chlorine, transformer-coupled pulsed plasmas are reported. Time dependencies of electron (ne), positive ion (ni+), and negative ion (ni−) densities and electron temperatures (Te) were measured with a Langmuir probe and microwave interferometry at 240 and 500 W input powers, and pressures between 3 and 20 mTorr. During the OFF portion of the power modulation, ne decreases rapidly as Cl− is formed by dissociative attachment of Cl2. The formation of Cl− is accelerated at high Cl2 densities (at high pressures and low powers). At 10 mTorr and higher pressures, an ion–ion plasma forms near the end of the OFF portion of the cycle, the sheath collapses, and Cl− reaches the wafer. Te decays rapidly in the OFF period and increases with a similar time constant at the beginning of the ON cycle if electrons are present at a sufficiently high level. If ne is very low at the beginning of the ON cycle, such as at high pressure (10 mTorr), then Te exhibits a spike at the beginning of the ON period. In a comparison study, plasma induced damage is reduced when aluminum is etched under similar source power modulation conditions.
Band gap determination of semiconductor powders via surface photovoltage spectroscopy
Authors: Gal D. , Mastai Y. , Hodes G. , Kronik L.
Published: July 2002

Surface photovoltage spectroscopy (SPS) is introduced as a powerful tool for band gap determination of semiconductor powders. The main advantage of SPS is that scattering and reflection do not interfere with the spectra. Therefore, it does not suffer from the inherent limitations of transmission/reflection based spectroscopies, most notably diffuse reflectance spectroscopy (DRS). The principles of the approach are presented and its usefulness is demonstrated by comparing it with DRS for band gap determination of GaAs, InP, CdTe, CdSe, and CdS semiconductor powders.
Experimental and theoretical investigations on nonlinear resonances of composite buckled microbridges
Authors: Nicu L. , Bergaud C.
Published: July 2002

Theoretical and experimental investigations have been conducted to study the nonlinear vibrations of a clamped–clamped beam with the effect of the initial deflection taken into consideration. A preliminary study analyzed the linear vibrations of a composite microcantilever beam. A very good agreement is found between theoretical and experimental values of the first three resonant frequencies of composite cantilever beams by taking into account the effect of the gold layer on the shift of the resonant frequencies. With this preliminary result, an exact solution is given to the linear problem associated with the nonlinear vibrations of initially buckled clamped–clamped structures using a direct analytical model. Several experimental multimode dynamic responses of composite buckled beams are also given to illustrate the validity and accuracy of this analytical model. Then, the influence of the interaction between resonant modes and the microstructure imperfections (nonideal boundary conditions, composite morphology, etc.) on the shift between the theoretical and experimental values of the resonant frequencies is considered.
Point defect injection during nitrous oxidation of silicon at low temperatures
Authors: Skarlatos D. , Tsoukalas D. , Giles L. F. , Claverie A.
Published: July 2002

In this work we use dislocation loops and boron-doped δ layers to monitor the interstitial injection during nitrous oxidation of silicon at low temperatures (850–950 °C). The interstitials captured by the loops are measured using transmission electron microscopy. The number of Si atoms released after oxynitridation was calculated from the difference between the total number of atoms stored in the loops for oxidizing and inert ambient. We found that this number is larger compared with the same dry oxygen oxidation conditions, although the oxidation rate is lower during oxynitridation in comparison to dry oxidation. Analysis of the earlier data leads to higher supersaturation ratio of interstitials in the former case. This result is also confirmed by measuring the diffusivity enhancement of boron δ layers during oxidation under both ambients.
Electron energy and potential distribution in a diode with a ferroelectric cathode
Authors: Dunaevsky A. , Krasik Ya. E. , Felsteiner J. , Krokhmal A.
Published: July 2002

We study the time and space resolved energy of charged particles emitted from the plasma formed on the surface of polarized and unpolarized ferroelectric cathodes under the application of driving pulses having either a positive or negative polarity. It is found that the energy of the emitted charged particles does not exceed the amplitude of the driving voltage and is independent of the initial polarization state of the ferroelectric. In addition, data concerning the energy of the electrons under the application of an accelerating pulse of 25 kV, and data concerning the electron beam uniformity and the time resolved potential distribution inside the anode–cathode gap of the electron diode are presented. We discuss our experimental results within the framework of the incomplete surface discharge model.
Investigation of phase transition thresholds by nonlinear transient laser gratings on silicon surfaces
Authors: Frass A. , Lomonosov A. , Hess P. , Gusev V.
Published: July 2002

Nonlinear transient laser gratings were excited by crossing two 180 ps Nd:YAG laser pulses (355 nm) at the silicon surface. The launched counterpropagating surface acoustic wave trains were detected outside the source with a cw laser probe-beam-deflection setup. Phase transitions occurring within the Gaussian fluence distribution of the grating induced characteristic changes in the amplitude of the corresponding grating oscillations involved and higher harmonics were found in the frequency spectrum. In the molten surface region, the amplitude decreased compared to the thermoelastic excitation, and second-harmonic generation was observed, whereas ablation generated an asymmetric profile with higher amplitudes and even the third harmonic could be detected. An accurate method is introduced to determine the ablation threshold (340 mJ/cm2 for silicon) from the transition of linear to nonlinear behavior in the distorted non-Gaussian amplitude distribution.
Predictions of magnetism in transition metal aluminides
Authors: Papaconstantopoulos D. A. , Hathaway K. B.
Published: July 2002

The Stoner Model has been used to investigate the stability of ferromagnetism in FeAl and the analogous series 3d transition metal aluminide compounds. Stoner factors, N(EF)I, were calculated from augmented plane wave (APW) electronic structure calculations based on local density approximation (LDA), using the Janak formalism. The Stoner criteria [N(EF)I>1] predicts ferromagnetism in FeAl for lattice parameters larger than 5.44 a.u., which is less than the experimental lattice constant of 5.50 a.u., but greater than the APW–LDA minimum energy lattice constant. The Stoner factors for ordered transition metal aluminides in the B2 (CsCl) structure agree reasonably well with those calculated using a rigid band approach, starting from FeAl and shifting EF to simulate the remainder of the series. These Stoner factors predict ferromagnetism for CrAl and MnAl and, assuming the rigid band approximation, for FeAl alloyed with Cr and Mn to the level of Fe0.8Cr0.2Al and Fe0.6Mn0.4Al.