bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct observation of a scaling effect on effective minority carrier lifetimes
Authors: Schöfthaler Martin , Rau Uwe , Werner Jürgen H.
Published: July 2002

The effective minority carrier lifetime in semiconductors with recombination inhomogeneities depends on the spatial distribution of recombination sites and not only on their absolute number. We use time-resolved microwave reflection measurements to monitor the carrier recombination in silicon wafers with a periodic metallization pattern on one surface. This contact scheme simulates the distribution of sites of enhanced carrier recombination. The experiments reveal a sensitive dependence of the effective minority carrier lifetime on the interdistance and the size—i.e., the scaling—of the metallization pattern at fixed metallization area ratio. This so-called scaling effect occurs whenever the size and the interdistance of recombination sites are comparable to the minority carrier diffusion length. Our experiments are in good agreement with results from a new analytical three-dimensional simulation which is based on the solution of the electronic transport equations in Fourier space. Our model is applicable to the optimization of the backside ohmic contact pattern for high efficiency solar cells.
Characterization of magnetohydrodynamic effects in a plasma opening switch
Authors: Spanjers G. G. , Yadlowsky E. J. , Hazelton R. C. , Moschella J. J.
Published: July 2002

Plasma dynamics are characterized in a long-conduction time plasma opening switch [Phys. Fluids B 4, 2368 (1992)] operated with densities and currents near the theoretical intersection of the magnetohydrodynamic (MHD) [IEEE Trans. Plasma Sci. PS-19, 400 (1991)] and electronmagnetohydrodynamic (EMH) [Phys. Fluids B 3, 1908 (1991)] regimes. In agreement with MHD theory, a hydrodynamic snowplow is observed to translate axially and reach the load end of the switch at opening. The axial motion of the front agrees with one-dimensional analytic predictions if the carbon plasma possesses an average ionization of 1, but is significantly less than theory if higher ionization levels are present. The axial motion of the plasma center-of-mass is significantly less than that expected from theory for any ionization level. The reduced center-of-mass motion is attributed to an ion loss mechanism in the unconfined switch plasma, an effect also observed in the total particle inventory, which saturates while the source plasma flux remains relatively constant. An ion lifetime of 450±15 ns (290±23 ns) for Z=1 (Z=2) results in a predicted center-of-mass motion in agreement with the measurements. Only minimal EMH effects are observed indicating, in contrast to theoretical predictions, that the switch is MHD dominated in this regime.
Supralinear photoconductivity of copper doped semi-insulating gallium arsenide
Authors: Schoenbach K. H. , Joshi R. P. , Peterkin F. , Druce R. L.
Published: July 2002

We report on the intensity dependent supralinear photoconductivity in GaAs:Si:Cu material. The results of our measurements show that the effective carrier lifetime can change over two orders of magnitude with variations in the intensity of the optical excitation. A threshold intensity level has been observed and can be related to the occupancy of the deep copper level. Numerical simulations have also been carried out to analyze the trapping dynamics. The intensity dependent lifetimes obtained from the simulations match the experimental data very well. Finally, based on the nonlinear intensity dependence of the effective lifetimes, a possible low-energy phototransistor application for the GaAs:Cu material system is presented.
Magnetic and transport measurements of Tl-1223 superconductors
Authors: Zheng D. N. , Johnson J. D. , Jones A. R. , Campbell A. M. , Liang W. Y. , Doi T. , Okada M. , Higashyama K.
Published: July 2002

Magnetic measurements have been performed on silver clad Tl-1223 and Tl-2223 tapes and a Tl-1223 single crystal to study the critical current properties of these samples. The Tl-1223 tape is shown by both magnetic and transport experiments to exhibit a highly granular nature, much more severe than that seen in the Tl-2223 tape, and in Bi-2223 tapes fabricated by similar rolling and pressing techniques. This characteristic is explained in terms of the poor grain alignment in the Tl-1223 tape resulting from the spherical morphology of Tl-1223 powders, in contrast with the micaceous and easily aligned Bi-2223 and Bi-2212 materials. However, evidence of a small number of strong intergrain links which remain superconducting at 77 K and 8 T is clear. The Tl-1223 single crystal exhibits clear anisotropic behavior, and this, together with the lack of good crystallographic alignment in the tapes is used to explain the variations in irreversibility line behavior between sample forms. The ratio of the irreversibility fields measured with field parallel and perpendicular to the ab plane is approximately 8. A clear ‘‘fish tail’’ feature is also observed in magnetic hysteresis curves obtained from the Tl-1223 single crystal.
Amorphous alloy formation by mechanical alloying and consecutive heat treatment in Fe50B50 powder mixture
Authors: Balogh J. , Kemény T. , Vincze I. , Bujdosó L. , Tóth L. , Vincze G.
Published: July 2002

Equiatomic composition powder mixture of pure Fe and B was mechanically alloyed in a controlled atmosphere vibration mill. Amorphous alloy phase formation was established by Mössbauer spectroscopy and by transmission electron microscopy. The variation of the average boron concentration and the homogeneity of the amorphous phase during the alloying process was followed by Mössbauer spectroscopy. At the early stage of the milling process a broad concentration distribution (from 20 to 50 at. % B) was observed. Upon further milling the average B concentration of the amorphous component was gradually enhanced and the concentration heterogeneity decreased. The B concentration of the amorphous phase could be further enhanced by heat treating the milled powder mixture. The highest boron content alloy attained in this study is Fe60B40.
Photoconductive gain and generation–recombination noise in quantum-well photodetectors biased to strong electric field
Authors: Shadrin V. D. , Mitin V. V. , Choi K. K. , Kochelap V. A.
Published: July 2002

The influence of the nonuniform photogeneration on the electric-field distribution is considered for quantum-well photodetectors under drift velocity saturation. We found that spatial nonuniformity of photogenerated electrons due to attenuation of the infrared flux induces strong electric-field domains. The electric-field domains formation is accompanied by degradation of the signal-to-noise ratio. We obtained that domain structures undergo realignment at certain threshold voltage as a result of feedback influence of the quantum well recharging on the photogeneration rates which in turn cause the additional electric-field redistribution. The realignment manifests itself in a steplike change of photoconductive gain and quantum efficiency of photoabsorption at threshold bias voltage and is followed by considerable increase of generation–recombination noise.
Domain configurations due to multiple misfit relaxation mechanisms in epitaxial ferroelectric thin films. III. Interfacial defects and domain misorientations
Authors: Speck J. S. , Daykin A. C. , Seifert A. , Romanov A. E. , Pompe W.
Published: July 2002

Interfacial defect theory is applied to the epitaxial ferroelectric system consisting of a tetragonal ferroelectric such as BaTiO3 or PbTiO3 grown onto a cubic (001) substrate. The interfacial defects that result from the diffusionless paraelectric to ferroelectric (PE→FE) phase transition are treated under the constraint that no misfit dislocations are generated during or as a result of the transition. The domain pattern develops to provide strain relief in the film. The interfacial defects for the ...c/a1/c/a1... domain pattern include coherency edge dislocations and coherency wedge disclinations. Interfacial defects for the ...a1/a2/a1/a2... domain pattern include coherency edge and screw dislocations. Far-field strain states for both domain patterns can be predicted from the interfacial defect content. From the twinning geometry, general expressions are derived for the far-field rotations of the crystal axes of individual domains for the ...a1/c/a1/c... and the ...a1/a2/a1/a2... domain pattern. The geometrically predicted rotation angles for ...c/a1/c/a1... domain pattern are verified by x-ray-diffraction and transmission electron diffraction for epitaxial PbTiO3 films grown on (001) SrTiO3 substrates.
Phase formation in Ni/InP contacts
Authors: Mohney Suzanne E. , Chang Y. Austin
Published: July 2002

The reaction of Ni films with InP is of technological interest because of the importance of Ni in ohmic contacts to n-type InP. However, numerous discrepancies are found in the literature concerning the Ni/InP reaction. In this study, bulk and thick film diffusion couples are examined in light of the Ni–In–P phase diagram, and many of the apparent discrepancies in the literature are clarified. Segregation of In and P into Ni–In and Ni–P binary phases is found to be an intermediate stage of the reaction, which is most easily observed in the reaction of thicker Ni layers with InP substrates. However, the Ni2InP ternary phase is found in the final stage of the reaction of Ni films on InP substrates, as predicted by the phase diagram. This sequence of phase formation differs dramatically from that observed in the other Ni/III–V systems that have been examined, and these differences are discussed.
Nonlinear bulk acoustic waves in anisotropic solids: Propagation, generation, and reflection
Authors: Zhou Shengqing , Jiang Wenhua , Shui Yongan
Published: July 2002

In this article, we address the nonlinear properties of bulk acoustic waves in anisotropic solids. While ordinary driving waves are obtained by means of the quasiwave mode concept in the discussion on propagation problems, we focus our analysis on the driving wave satisfying the resonant condition which exhibits the characteristics of three-dimensional spatial accumulation. Then a theory as the basis for solving nonlinear boundary value problems is developed. The novel analysis is presented for nonlinear reflection of bulk acoustic waves at a free surface of a generally anisotropic solid, which reveals that the surface may generate freely propagating second-harmonic waves, and that the reflective second-harmonic waves increase two dimensionally in the bulk. This technique can also be applied to wave reflection problems at other kinds of planar boundaries. Some numerical calculation is carried out for the case in an isotropic solid.
Experimental and theoretical study of dipole emission in the two-dimensional photonic band structure of the square lattice with dielectric cylinders
Authors: Suzuki Toshio , Yu Paul K. L. , Smith David R. , Schultz Sheldon
Published: July 2002

Dipole radiation in a two-dimensional periodic dielectric structure is studied both theoretically and experimentally in this article. For the two-dimensional square lattice structure with dielectric cylinders, the emission power of an electric dipole in the photonic band structure is computed numerically by a combination of the plane wave method, the dyadic Green’s function, the Poynting theorem, and the triangular integration method. The theoretical results are compared with experimental measurements in a microwave frequency region. The computed radiation spectra in the photonic band structure show good agreement with the experimentally measured results.
Void formation during film growth: A molecular dynamics simulation study
Authors: Smith Richard W. , Srolovitz David J.
Published: July 2002

Two-dimensional, nonequilibrium molecular dynamics simulations have been applied to study the structure of thin films grown on single-crystal Lennard-Jones substrates. The principal microstructural features to develop within these films are single vacancies and small voids which tend to be slightly elongated and to be aligned in the growth direction. Both the void volume and the mean surface roughness of the films are found to be decreasing functions of substrate temperature and deposition kinetic energy. Voids are shown to form as a consequence of both surface roughness and shadowing effects. The attraction between deposited atoms and the sides of surface depressions lead to the formation of outgrowths from the sidewalls of the surface depression. These outgrowths shadow the open void beneath them and continue to grow across the voids by interaction with the depositing atoms until a continuous bridge is formed that closes off the void. Since this bridging mechanism leaves behind a surface depression above the closed-off void, new voids tend to form above it. This leads to the alignment of voids along the film growth direction. The spacing of the resultant void tracks is correlated with the wavelength of the surface roughness. Increasing temperature and deposition kinetic energy enhancing surface mobility leads to an increase in the wavelength of the surface roughness and hence an increase in the spacing between void tracks. Edge dislocations tend to form within voids as a natural consequence of the void bridging process, however nondislocated voids are also observed.
Open-circuit voltage characteristics of InP-based quantum well solar cells
Authors: Anderson Neal G. , Wojtczuk Steven J.
Published: July 2002

Open-circuit voltage characteristics of InP-based multiple-quantum-well (MQW) solar cells are studied experimentally and theoretically. Experimental results are presented for spectral response and terminal characteristics of In0.53Ga0.47As/InP MQW cells and compared with results from a corresponding bulk p-i-n control cell. Open-circuit voltages measured for these cells, and for other InGaAs/InP and InAsP/InP MQW cells reported in the literature, are then analyzed using a simple ideal theory for MQW cells which attributes open-circuit voltage reductions to increased radiative recombination in the quantum wells. The large (0.19–0.33 V) measured open-circuit voltage reductions which accompany introduction of the quantum wells are shown to agree with predictions from the ideal theory to within ∼0.03 V on average. Finally, implications of this work for the design of efficient InP-based MQW solar cells are discussed.
Photoreflectance of Cu-based I–III–VI2 heteroepitaxial layers grown by metalorganic chemical vapor deposition
Authors: Shirakata Sho , Chichibu Shigefusa
Published: July 2002

Photoreflectance (PR) spectroscopy has been developed as a powerful technique for the characterization of the heteroepitaxial layer of the Cu–III–VI2 semiconductors having the chalcopyrite structure. PR measurements have been carried out in the energy region near the fundamental absorption edge at 77 K for the heteroepitaxial layers of CuAlSe2, CuGaSe2, CuGaS2, and CuAlS2 grown on GaAs and GaP substrates by means of the low-pressure metalorganic chemical vapor deposition. Crystal quality, stress, and the crystallographic orientation have been well characterized, based on the analysis of the PR spectra in terms of transition energy, intensity, and broadening parameter.
One-flux analysis of current blocking in double-heterostructure bipolar transistors with composite collectors
Authors: McKinnon W. R.
Published: July 2002

The flux method of McKelvey, Longini, and Brody [Phys. Rev. 123, 51 (1961)] is used to analyze current blocking in double-heterostructure bipolar transistors with composite collectors (CC). The effects of electron accumulation in the spacer layer are included; these are shown to impose an intrinsic limit on the simplest CC design. The analysis is extended to include compositional or band-gap grading, which can substantially reduce the current blocking at high currents, though it has less effect at lower currents.
The dependence of magnetoacoustic emission on magnetizing frequency in nickel and mild steel
Authors: Ng D. H. L. , Yu C. C. , Qin C-D. , Lo C. C. H. , Jakubovics J. P.
Published: July 2002

Magnetoacoustic emission (MAE) measurements have been made on nickel and mild steel samples subjected to a magnetizing field of various frequencies f. In both samples, the MAE signals are found to increase with increasing f. The experimental results are in good agreement with the predictions of a model relating the generation of MAE to domain wall nucleation-annihilation and domain wall motion. When f is large, the contribution of wall motion to the generation of MAE is predicted to be dominant. Conversely, when f is small, MAE is generated mainly by domain wall nucleation-annihilation processes. At high frequencies, the amount of MAE from both nickel and mild steel samples is found to vary linearly with f0.5. As f decreases, the MAE deviates from the f0.5 relationship for f<10 Hz in mild steel, and for f<2 Hz for nickel. The results suggest that in nickel, wall motion remains the dominant source of MAE generation over a wider frequency range than in steel.
Surface deformations and domains in Terfenol-D by scanning probe microscopy
Authors: Holden A. P. , Lord D. G. , Grundy P. J.
Published: July 2002

This paper reports the first direct measurements of the deformations in the surface of a ferromagnet caused by magnetostrictive lattice strains in neighboring magnetic domains. Observations of the {011} and {112} surfaces of twinned Terfenol-D (Tb0.3Dy0.7Fe2) crystals by atomic force microscopy reveal the deformation domains previously imaged by optical microscopy and predicted by recent theory. The surface topography has been imaged in the remanent and ac demagnetized states and measurements of the deformation gradients have enabled the magnetostriction constant in the 〈111〉 direction to be estimated as 1.63×10−3 to within 8%. All theoretically predicted deformation domains have been observed and, in particular, the exact compatibility of such deformations across the growth twin boundaries is clearly revealed. The magnetic domains associated with the observed deformations are inferred from magnetic force microscopy observations which yield complex contrast patterns.
Suppression of polarization switching in triglycine sulfate crystals
Authors: Kugel V. D. , Rosenman G.
Published: July 2002

Polarization switching in triglycine sulfate crystals with one of the polar surfaces covered by a thin dielectric film (1 μm) has been found to be highly diminished in a 50 Hz electric field. We assume that the depolarization field caused by the film significantly moderates the process of the nucleation and (or) sideways motion of 180° domain walls. When crystals with both conductive electrodes were connected to the Sawyer–Tower circuit through the capacitor, which was equivalent to the dielectric film, the polarization reversal did occur. We assume that in the case of the conductive electrode, a nonuniform distribution of the electrical charge at the electrode/ferroelectric interface and an injection current through the interface trigger off polarization reversal. Analysis of the energetic state of the crystals has been performed.
Removal of halocarbons from air with high-power microwaves
Authors: Nusinovich G. S. , Milikh G. M. , Levush B.
Published: July 2002

Local removal of halocarbons from air by using high-power microwave sources is investigated. The destruction of halocarbons can be caused by free electrons produced in a freely localized discharge initiated in the air by a focused microwave beam. After fast cooling, the electrons can attach to halocarbon molecules and destroy them. Operating regimes and their efficiencies are analyzed, and the corresponding parameters of microwave sources are specified. The availability of microwave sources for such processes is discussed.
Photoluminescence spectra of deformed Si-Ge alloy
Authors: Tanaka K. , Suezawa M. , Yonenaga I.
Published: July 2002

We measured photoluminescence (PL) spectra of deformed bulk Si-Ge alloys and found peculiar peak shifts of D1 and D2 lines depending on the deformation and annealing temperatures. Alloy crystals were grown by the Czochralski method. Specimens were deformed by compression at temperatures between 700 and 900 °C in an argon atmosphere. PL spectra were measured at 4.2 K. Peak positions of D1 and D2 lines depended on the deformation temperatures; they were at higher energies at higher deformation temperatures. On the other hand, those of D3 and D4 did not depend on the deformation temperature. The magnitudes of peak shifts of D1 and D2 lines were proportional to t2/3 at small t due to isothermal annealing ( t: the duration of annealing) at around 650 °C. The activation energy was determined to be 2.5 eV, which was much smaller than that of self-diffusion. These results were interpreted as being due to the change of alloy composition around dislocations caused by the elastic interaction between dislocations and constituent atoms, i.e., Si and Ge, in which process point defects generated during deformation were thought to play a crucial role.
Heteroepitaxial structures of SrTiO3/TiN on Si(100) by in situ pulsed laser deposition
Authors: Vispute R. D. , Narayan J. , Dovidenko K. , Jagannadham K. , Parikh N. , Suvkhanov A. , Budai J. D.
Published: July 2002

High-quality ceramics based heteroepitaxial structures of oxide-nitride-semiconductors, i.e., SrTiO3/TiN/Si(100) have been fabricated by in situ pulsed laser deposition. The dependence of substrate temperature and oxygen partial pressure on the crystalline quality of the SrTiO3 films on Si with epitaxial TiN template has been examined. We found that epitaxial growth occurs on TiN/Si(100) above 500 °C, initially at a reduced O2 pressure (10−6 Torr), and followed by a deposition in the range of 5–10×10−4 Torr. X-ray diffraction (Θ, ω, and Φ scans) and transmission electron microscope (TEM) results revealed an excellent alignment of SrTiO3 and TiN films on Si(100) with a cube-on-cube epitaxy. Rutherford backscattering and ion channeling results show a channeling minimum yield (χmin) of ∼13% for the SrTiO3 films. High-resolution TEM results on the SrTiO3/TiN interface show that the epitaxial SrTiO3 film is separated from the TiN by an uniform 80–90 Å crystalline interposing layer presumably of TiNxO1−x (oxy-nitride). The SrTiO3 film fabricated at 700 °C showed a high relative dielectric constant of 312 at the frequency of 1 MHz. The electrical resistivity and the breakdown field of the SrTiO3 films were more than 5×1012 Ω cm and 6×105 V cm−1, respectively. An estimated leakage current density measured at an electric field of 5×105 V/cm−1 was less than 10−7 A/cm2.
The effect of interference on magneto-optics in magneto-optical layered structures
Authors: Wang Y. , Zheng W. M. , Zhou S. M. , Chen L. Y. , Zhu W. R. , Qian D. L. , Zheng Y. X. , Zhang H. Y. , Huang J.
Published: July 2002

In this article the magneto-optics of magneto-optical (M-O) layered structures have been studied theoretically and experimentally. For the air/M-O/air configuration, an analytic expression between the apparent complex Faraday rotation and the eigenvalue φ̃F=e1φ̃F′ can be obtained, when the M-O layers are semitransparent and weakly magnetic. The interference factor e1 is a function of the optical constants and the M-O layer thickness d, and the light wavelength. In these structures, the apparent Faraday rotation consists of two parts. One oscillates as a function of the M-O layer thickness and the other is proportional to the layer thickness. The oscillation period and the amplitude are determined by the optical constants of the M-O layers. For the air/M-O/reflector configuration, the Kerr rotation φ̃k oscillates as a function of the M-O layer thickness and approaches a constant as the thickness d→∞. If the M-O layers are semitransparent and weakly magnetic, the apparent Kerr rotation can be expressed as φ̃k=e2φ̃F′. For ultrathin metallic magnetic bilayered films the Kerr rotation is proportional to the M-O layer thickness and the enhancement factor is a function of the optical constants of the M-O layer and NM reflector. The magneto-optics of a Co spinel ferrite film, Co/Cu, Fe-Ni/Cu, and Co/Si structures have been studied experimentally.
Reflective twisted nematic liquid crystal displays. I. Retardation compensation
Authors: Tang S. T. , Yu F. H. , Chen J. , Wong M. , Huang H. C. , Kwok H. S.
Published: July 2002

Reflective twisted nematic displays where the output polarizer is eliminated are analyzed and demonstrated. The absence of the rear output polarizer has many advantages, including much higher brightness and higher resolution. In this article, we examine the solution space for the design of such reflective displays. Both the cases of a simple reflective display and the use of a retardation film within the display to compensate for color dispersion are explored. It is shown that excellent black and white displays can be obtained using film compensation. Experimentally, over 90% reflectance from a compensated reflective display has been obtained with low color dispersion. The optical response of this reflective display was also found to be quite fast and suitable for video rate applications.
Exchange anisotropy in polycrystalline and epitaxial (001)-oriented NiO/NiFe bilayers grown by ion beam sputtering (abstract)
Authors: Michel R. P. , Chaiken A , Wang C. T.
Published: July 2002

The connection between the spin structure of antiferromagnetic NiO and the exchange anisotropy observed in NiO/NiFe bilayers is not well understood. For instance, the NiO bulk-terminated (001) surface is compensated, and therefore simple models predict no exchange bias in (001)-oriented NiFe/NiO bilayers.1 Using a newly developed ion-beam sputtering (IBS) process to deposit NiO exchange-coupled films,2 we have simultaneously grown polycrystalline and epitaxial NiO/NiFe bilayers. NiO grown on NiFe/MgO is polycrystalline, while NiO grown directly on MgO is epitaxial. The in-plane orientation of the epilayers was confirmed using (hk0) x-ray diffraction. The exchange anisotropy in epitaxial (001)-oriented bilayers is about half as large as that observed in polycrystalline bilayers. The size of the exchange anisotropy does not depend on the orientation of the bias field with respect to the in-plane NiFe/NiO crystallographic direction, indicating that the same interfacial spin structure is achieved regardless of the bias field direction. These results show that the surface NiO spin structure is different from that of the bulk, and is uncompensated at the interface independent of the crystalline orientation of the bilayer. Results on epitaxial Co/NiO and NiFe/NiCoO bilayers will also be discussed.
Microstructural and magnetic studies of Mn–Al thin films
Authors: Kuo P. C. , Yao Y. D. , Huang J. H. , Shen S. C. , Jou J. H.
Published: July 2002

Mn–Al thin films with high coercivity and high saturation magnetization were successfully fabricated by rf magnetron sputtering with properly controlled chemical composition, substrate temperature, and annealing temperature. A high coercivity of about 3000 Oe and a saturation magnetization of about 420 emu/cc have been achieved. We have observed that during annealing at 410 °C, the nonmagnetic ε phase with a grain size of roughly 100 nm transforms into a metastable ferromagnetic τ phase with a platelike grain size of roughly 300 nm. From the continuous measurement of the stress of the films in vacuum as a function of temperature, we observed a compression stress during heating below 220 °C, and a tension stress above 220 °C during cooling. The structure phase transformation from ε to τ phases was related to the stress variation from compression to tension. The high coercivity can be explained by the high magnetocrystalline anisotropy constant of the τ phase and the magnetoelastic energy arises from the residual stress of Mn–Al films after the shear transformation.
On the local magnetic moments at X sites in XFe2 compounds: Pressure effects (X=Y, Lu, Zr, Hf)
Authors: Tovar Costa M. V. , de Oliveira N. A. , Troper A.
Published: July 2002

Adopting a simple model to describe the intermetallic cubic Laves phase compounds XFe2 (X=Y, Lu, Zr, Hf) we discuss the effect of an external applied pressure on the local magnetic moments at both sublattices. We assume that pressure acts on the system, modifying the widths of the sublattice bands and the spin-dependent hybridization between d states of the two sublattices. Using realistic densities of states we obtain, in a self-consistent way, the local d-magnetic moments at X and Fe sites which turn out to be antiparallel. As pressure increases one shows that the magnitude of the local magnetic moment at the X site increases until saturating, whereas the local magnetic moment at the Fe site decreases until saturating. Similarly to the experimental data, which show that the hyperfine field at the X site increases with applied pressure, our theoretical model predicts that the magnetic moment at the X site also increases with pressure. The calculated hyperfine and magnetic moments at the Fe site are also consistent with available experimental data.
Lateral confinement by low pressure chemical vapor deposition-based selective epitaxial growth of Si1−xGex/Si nanostructures
Authors: Vescan L. , Dieker C. , Souifi A. , Stoica T.
Published: July 2002

Among the growth approaches being considered currently to realize quantum dots and quantum wires is the selective epitaxial growth on patterned substrates. With this technique the feature size and geometry are mainly limited by the lithographic process. With optical lithography we achieved a lateral dimension of ⩾0.4 μm. Therefore, to further reduce the lateral dimension, but still using optical lithography, the tendency toward facet formation during selective epitaxial growth was investigated. Si0.70Ge0.30 multiple quantum well structures with Si0.935Ge0.065 spacers and buffers were deposited on (001) Si. The buffer thickness was varied so as to achieve facet junction. While on large areas the Si0.935Ge0.065 buffer was relaxed, for dots ⩽300 μm or narrower the structures remained strained even for buffer thicknesses exceeding by a factor of two–three the critical thickness of large area. In dots and wires where facet junctioning has taken place a rounded region between facets (approximately 50 nm broad) in the quantum well layers was observed. In wires oriented parallel to 〈100〉 sidewalls self-organized wire formation and vertical correlation of these growth induced wires was observed. The photoluminescence of all dots and wires down to the lowest achieved dimension and including the self-organized wires is strong, with the integral intensity normalized to the surface coverage for 100 nm dots exceeding by a factor of 50 the emission from unpatterned areas.
Current bistability and switching in weakly coupled superlattices GaAs/AlGaAs
Authors: Rasulova G. K. , Efimov Yu. A. , Murzin V. N.
Published: July 2002

Current–voltage characteristics of 30 period 390 Å GaAs/110 Å Al0,3Ga0,7As superlattices are studied. Features revealed in multistable current branches of the hysteresis loops made it possible to estimate gradual changes in the size and shape of the domain boundary and reconstruct the final shape of the domain boundary. It was concluded that a considerable current bistability observed in the investigated superlattices is due to the domain boundary expansion. Switching between bistable current states in each hysteresis loop is detected. The switching time depends on the direction of the current jump. The switching time from a high- to low-current state is about 10−7 s; the time of the reverse switching to the high-current state is about 10−6 s. The observed switching is attributed to the shrinkage and expansion of the domain boundary size.
Lateral oxidation of buried AlxGa1−xAs layers in a wet ambient
Authors: Langenfelder T. , Schröder St. , Grothe H.
Published: July 2002

The lateral oxidation of buried AlxGa1−xAs layers with high Al content (x=0.8–1) is investigated, using an oxidation process in a wet N2+H2O ambient at 370–450 °C. The oxidation is clearly selective and significantly affected by process temperature, material composition, AlxGa1−xAs layer thickness, and the geometry of the oxidized structures. An asymptotic oxide growth with constant activation energies for the reactive process and for the transport mechanism is observed. The experimental oxidation behavior coincides well with a model of self-blocking pores.
Shock jump equations for unsteady wave fronts
Authors: Sano Yukio
Published: July 2002

First, the Rankine–Hugoniot (R–H) relations are generalized for unsteady shock wave fronts of infinitesimal risetime. The equation for particle velocity has a term of strain rate, while that for stress contains terms of strain rate and acceleration. Next, shock jump equations of general form for particle velocity, stress, and specific internal energy are derived. They involve the combined effect of strain wave form, its change with time, and the path in time of strain. The effect, as well as the terms of strain rate and acceleration in the generalized R–H equations, indicates uncertainty about the applicability of familiar R–H equations to the shock fronts. Finally, jump equations for specific strain waves are evaluated: Jumps in the three quantities are influenced greatly by the effect. If both the wave form and the path are linear, then the equations are of the same form as the R–H equations.
Comment on “The analysis of piezoelectric/piezomagnetic composite materials containing ellipsoidal inclusions” [J. Appl. Phys. 81, 1378 (1997)]
Authors: Nan Ce-Wen
Published: July 2002

A method previously developed for predicting the coupled magnetoelectric effect in a piezoelectric–piezomagnetic composite containing ellipsoidal inclusions is a general, explicit method and not as Huang and Kuo [J. Appl. Phys. 81, 1378 (1997)] reviewed. After correcting two obvious mistakes in Huang and Kuo’s numerical results for effective coupled magnetoelectric coefficients, their results appear to be the same as one type of previous results.
Photofield interface impurity spectroscopy in blocked impurity band Si:B structures
Authors: Pasquier S. , Mény C. , Asadauskas L. , Léotin J. , Aronzon B. A. , Rylkov V. V. , Conédéra V. , Fabre N. , Régolini J. L. , Morin C.
Published: July 2002

The far infrared spectral response of photoconductive boron doped silicon blocked impurity band structures was measured at electric field values up to 7 kV/cm and sample temperatures in the range of 4–15 K. The structures are constructed of adjacent layers of pure (NA=5×1013 cm−3) and boron doped (NA=1018 cm−3) silicon inserted between highly doped ohmic contact layers. A broad-band photoresponse is observed which is attributed to the impurity band of the doped layer. In addition, several peaks of response are observed, even at the lowest sample temperatures, which are attributed to optical transitions from ground states to excited states of isolated impurities. It is shown that these transitions originate from a thin layer at the interface between the doped and the pure silicon regions. When the electric field is oriented towards the doped layer, the photocurrent peaks are attributed to the tunneling of photoexcited holes across excited states into near neighbor atoms within a hole depleted delta sheet, followed by relaxation to the ground state. Then hopping transport occurs across the doped layer to the ohmic contact. In this scheme, the hole depleted layer is generated by the effect of the applied electric field and involves only 109–1010 cm−2 impurities. This study provides evidence of a new mechanism for extrinsic photoconductivity at interfaces.
Photocurrent and photoluminescence measurements in the near-band-edge region of 6H GaN
Authors: Kornitzer K. , Thonke K. , Sauer R. , Mayer M. , Kamp M. , Ebeling K. J.
Published: July 2002

The optical transitions near the band edge in molecular-beam epitaxy grown 6H GaN films on sapphire have been studied by photocurrent (PC) and photoluminescence (PL) measurements. The low-temperature PC spectra show well-resolved free-exciton transitions, which allow us to determine the A, B, and C free-exciton energies and the corresponding direct band gaps. PL measurements were performed to check the assignment of the excitonic transitions and to derive strain-dependent excitonic parameters from the energetic position of the free A exciton transition, which serve as an input to the fit function for the PC spectra. Variation of the sample temperature between 5 and 300 K allows us to extract the temperature dependence of the band gap.
Colossal magnetoresistance and Mössbauer studies of the manganites La1−xSnxMn0.985Fe0.015O3+δ
Authors: Li Z. W. , Morrish A. H. , Zhou X. Z. , Dai S.
Published: July 2002

Colosssal magnetoresistance (CMR), Mössbauer spectra, and phase compositions of the iron-doped manganites, La1−xSnxMn0.985Fe0.015O3+δ (x=0.3 and 0.5), have been studied. A small amount of iron doping modifies the CMR and other magnetic properties of the manganites. La0.7Sn0.3Mn0.985Fe0.015O3+δ exhibits CMR; the magnetoresistance coefficient with H=20 kOe and T=225 K is about 80%. La0.7Sn0.3Mn0.985Fe0.015O3+δ consists of two phases, (LaSn2+)MnO3, or ABO3, and (LaSn4+)2Mn2O7, or A2B2O7. The ABO3 phase has a transition from paramagnetism to ferromagnetism at T=225 K. The CMR has its origin in the ABO3 phase.
Photonic band gaps in metallic microcavities
Authors: Kitson S. C. , Barnes W. L. , Sambles J. R.
Published: July 2002

We investigate the mode structure of optical microcavities made using metallic mirrors. By giving one of the mirrors wavelength scale periodic texture we show that cavity modes, in particular the antisymmetric coupled surface plasmon polariton mode, may be blocked by a photonic band gap. We compare experimental and theoretical results and use them to indicate how changing the type of texturing may be used to control the modes of the microcavity. The relevance of the reported work to microcavity devices such as the light emitting diode is also discussed.
Mapping of crystal defects and the minority carrier diffusion length in 6H–SiC using a novel electron beam induced current technique
Authors: Tabib-Azar M. , Hubbard S. M. , Schnabel C. M. , Bailey S.
Published: July 2002

We report on a mapping technique used to correlate the structure of defects with their electrical characteristics in semiconductors. Interesting results that directly and clearly show the influence of micropipes on the minority carrier diffusion length were obtained in both p- and n-type 6H–SiC. The method is based on electron-beam induced current (EBIC) measurements in planar structures. Values of hole diffusion length in defect free regions of n-type 6H–SiC, with a doping concentration of 1.7×1017 cm−3, ranged from 1.46 to 0.68 μm. These values were reduced to below 0.1 μm at the center of large defects. In addition, measurements on p-type 6H–SiC resulted in electron diffusion lengths ranging from 1.42 to 0.8 μm which also showed drastic reductions near defects. Our planar mapping technique measures diffusion lengths along a linescan. This linear map is then overlaid onto the EBIC image, allowing direct visualization of defects and their effect on minority carrier diffusion lengths.
Photoluminescence and Raman study of compensation effects in Mg-doped GaN epilayers
Authors: Eckey L. , von Gfug U. , Holst J. , Hoffmann A. , Kaschner A. , Siegle H. , Thomsen C. , Schineller B. , Heime K. , Heuken M. , Schön O. , Beccard R.
Published: July 2002

The compensation of Mg-doped GaN is systematically studied by low-temperature photoluminescence and Raman spectroscopy using a series of samples with different Mg concentrations. Strongly doped samples are found to be highly compensated in electrical measurements. The compensation mechanism is directly related to the incorporation of Mg. Three different deep donor levels are found at 240±30, 350±30, and 850±30 meV from the conduction band, each giving rise to deep unstructured donor-acceptor pair emission.
Optical heterodyne detection at a silver scanning tunneling microscope junction
Authors: Kokubo T. , Gallagher Alan , Hall J. L.
Published: July 2002

The 435 MHz beat signal between two modes of a He–Ne laser (633 nm) has been detected in the current of a scanning tunneling microscope (STM), using a silver probe to enhance the sensitivity using its plasmon resonance. The difference frequency between this optical beat and a radio frequency voltage applied to the junction has also been detected in the STM current. The difference frequency and heterodyne signals, as well as the direct current (dc) probe expansion, fit a calculation of modulated and dc thermal expansion of the probe tip, once one assumes that a surprisingly large fraction of the laser beam is absorbed at a small structure at the probe end. The beat signal due to current rectification is not observed for the experimental conditions, but the potential for utilizing the plasmon resonance to obtain such a signal using a silver probe is evaluated for other, feasible conditions where it might provide exceptional sensitivity.
Electric field dependence of piezoelectric properties for rhombohedral 0.955Pb(Zn1/3Nb2/3)O3– 0.045PbTiO3 single crystals
Authors: Liu Shi-Fang , Park Seung-Eek , Shrout Thomas R. , Eric Cross L.
Published: July 2002

The electric field dependence of the piezoelectric properties of rhombohedral 0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3 crystals were investigated as a function of orientation with respect to the prototypic (cubic) axes. For 〈111〉 oriented fields, depolarization and subsequent domain reorientation resulted in an apparent maximum in the piezoelectric coefficients occurring at ∼5 kV/cm, followed by nonhysteretic dij saturation, indicating a single domain state under bias. By extrapolation, single domain values for the piezoelectric coefficients d33 and d31 were determined to be 125 and −35 pC/N, respectively. The hydrostatic piezoelectric coefficient dh for single domain crystals was calculated to be ∼55 pC/N, coincident with the experimentally determined values under hydrostatic pressure. For 〈001〉 oriented fields, piezoelectric coefficients d33〈001〉 and d31〈001〉 as high as 2250 and −1000 pC/N were determined, respectively. Although a high value of dh〈001〉 (∼250 pC/N) was expected, the experimentally determined value was only ∼50 pC/N. A change of polar vector within the crystal lattice was discussed in relation to the volume strain associated with an E-field induced phase transition and the possible origin of the discrepancy in hydrostatic dh values.
Connection between hydrogen plasma treatment and etching of amorphous phase in the layer-by-layer technique with very high frequency plasma excitation
Authors: Vetterl O. , Hapke P. , Houben L. , Finger F. , Carius R. , Wagner H.
Published: July 2002

The formation of microcrystalline silicon was investigated employing the layer-by-layer technique with very high frequency plasma excitation. Etching of the deposited layers was found to be a dominant effect during hydrogen plasma treatment. Microcrystalline growth occurred when the thickness of the deposited layer was reduced to a certain value in each cycle. The etching stopped when a noticeable crystalline volume fraction was produced; for long hydrogen treatment times no film growth was observed.
Effect of nonstoichiometry on the thermoelectric properties of GeBi4Te7
Authors: Kuznetsov V. L. , Kuznetsova L. A. , Rowe D. M.
Published: July 2002

Polycrystalline samples of GeBi4Te7 with different compositional deviations from stoichiometry have been prepared and characterized by Seebeck coefficient and electrical resistivity measurements over the temperature range 100–750 K. At room temperature all samples exhibit n-type conductivity and a high concentration of electrons in the range 1.7×1019–2.4×1020 cm−3. The width of the energy band gap of GeBi4Te7 was 0.21±0.02 eV as determined from the temperature dependence of electrical resistivity in the intrinsic region. A dominant point defects model is proposed to explain the influence of compositional nonstoichiometry on the transport properties.
Preisach modeling of a magneto-optical medium with perpendicular magnetization
Authors: Fry Richard A. , Lou Jie , Reimers Ann , Bennett Lawrence H. , Torre Edward Della
Published: July 2002

Magneto-optical Kerr effect measurements on a (0.3 nm Co/0.6 nm Pt)15 multilayer film with perpendicular magnetization showed Kerr rotation hysteresis loops that are very square, but asymmetric: on the descending major loop, while the magnetization decreases rapidly once the applied field approaches the coercive field, there is a long tail as saturation is approached. Modeling of this behavior was performed using a Preisach-based approach. To account for the known changes in magnetization behavior as a function of the number of bilayers, the Preisach density function was replaced by the sum of as many Gaussian distributions as there were bilayers in the material. The parameters of these Gaussian functions were systematically varied. The resulting model shows a good fit to both the experimental major loop and a first-order reversal curve.
Neutron powder diffraction of carbon-coated FeCo alloy nanoparticles
Authors: Scott John Henry J. , Chowdary Krishna , Turgut Zafer , Majetich Sara A. , McHenry Michael E.
Published: July 2002

Neutron powder diffraction is used to study the order–disorder transformation in carbon-coated FexCo1−x nanoparticles produced using a radio frequency plasma torch. The nanoparticles, nominally Fe50Co50, are produced from alloy powder and acetylene precursors by gas-phase nucleation from the plasma. The resulting nanoparticles undergo an order–disorder transformation near 730 °C, passing from an ordered B2 (CsCl) structure to a disordered A1 (body-centered-cubic) structure upon heating, similar to the transformation seen in bulk equiatomic FeCo. Although it is very difficult to quench the disordered state in bulk samples, the extreme cooling rates present in the plasma reactor produce metastable disordered nanoparticles. Neutron powder diffractograms acquired during a heating–cooling cycle at 27, 500, 710, 800, 710, and 400 °C indicate the particles relax to their equilibrium ordered state upon heating first, disorder as they pass through the transformation temperature, and reorder upon cooling.
Low core losses and soft magnetic properties of Fe–Al–Ga–P–C–B–Si glassy alloy ribbons with large thicknesses
Authors: Mizushima Takao , Makino Akihiro , Yoshida Shoji , Inoue Akihisa
Published: July 2002

The structure, soft magnetic properties, and core losses were investigated for a Fe77Al2.14Ga0.86P8.4C5B4Si2.6 glassy alloy with a sheet thickness in a wide range from 30 to 240 μm prepared by the melt-spinning technique. The maximum thickness (tmax) for glass formation and the thermal stability (ΔTx) of the supercooled liquid region defined by the difference between the crystallization temperature (Tx) and the glass transition temperature (Tg) are about 220 μm and 35 K, respectively. The saturation magnetization (σs) of this glassy alloy is about 1.5 T. The effective permeability at 1 kHz is as high as 12 000 in a thickness of 30 μm and it maintains high values above 4400 up to a thickness of tmax. The coercive force is kept at a low level, under 3 A/m up to tmax. This glassy alloy also shows low core loss values of 0.1–0.3 W/kg at f=50 Hz and Bm=1.0 T in the thickness from 30 to 220 μm. On the other hand, a Fe78Si9B13 amorphous alloy shows almost the same low core losses as that of the glassy alloy only in a thickness of less than 70 μm. The difference between the dependence of the soft magnetic properties and the core loss on the thickness of these alloys should arise from their difference in ability of glass forming. The Fe–Al–Ga–P–C–B–Si glassy alloys should be very useful for inductive applications because of their thick ribbon shape and good soft magnetic properties in addition to low core loss.