bluuebunny/crossref_metadata_2025 huggingface dataset:
Reflection and transmission of Bleustein–Gulyaev surface waves around a corner
Authors: Kiel/czyński P. , Pajewski W.
Published: July 2002

The reflection and transmission of Bleustein–Gulyaev surface waves incident normally upon the corner of a homogeneous piezoelectric wedge formed by two stress-free and metallized planes are investigated both by theory and experiment. Using a reciprocity relation for linear piezoelectric medium and surface terms of the appropriate Green’s function, the reflection AR and transmission AT coefficients of the Bleustein–Gulyaev wave have been determined as a function of the wedge angle θ and electromechanical coupling coefficient k15 of the piezoelectric body. The validity of the applied analytical method was verified by comparing experimental results obtained for lithium iodate (LiIO3) wedge-shaped specimens. The results calculated numerically for the power reflection ‖AR‖2 and transmission ‖AT‖2 coefficients show considerably better agreement with the experimental ones than those obtained previously.
High electron mobility in the selectively doped heterostructures grown by normal pressure metalorganic chemical vapor deposition
Authors: Mori Y. , Nakamura F. , Watanabe N.
Published: July 2002

GaAs/AlGaAs heterostructure selectively doped with Si was grown in a conventional normal pressure metalorganic chemical vapor deposition using trimethyl metals and arsine. Mobility of the two-dimensional electron gas as high as 8100, 148 000, and 401 000 cm2/V s was obtained at 300, 77, and 5 K, respectively, for a sample with an undoped Al0.3Ga0.7As spacer layer of 100 Å and with a sheet electron density of about 5×1011 cm−2. The heterostructure with the spacer layer thinner than 100 Å shows high mobility comparable to the best values obtained by molecular-beam epitaxy. The influence of growth parameters on the electron mobility in the heterostructure is described in detail.
Sputtering and native oxide formation on (110) surfaces of Cd1−xMnxTe
Authors: Neff H. , Lay K. Y. , Abid B. , Lange P. , Lucovsky G. , Bachmann K. J.
Published: July 2002

Native oxides on the surface of Cd1−xMnxTe (0≤X≤0.7) have been analyzed on the basis of XPS measurements. Depth profile analysis revealed a significant increase in the thickness at higher Mn concentrations and a strong Mn segregation to the surface, respectively. Sputter-induced damage on cleaved (110)-oriented surfaces was analyzed by photoreflectance and photoluminescence measurements. The damage was found to be larger on CdTe than on the alloy. Thermal annealing showed nearly complete restoration for the surface of the alloy, while CdTe revealed irreversible modifications in the near-surface regime upon sputtering and post annealing.
Amplification of acoustic waves in piezoelectric semiconductors: Effect of nonlinearity in electron effective mass and collision frequency
Authors: Ghosh S. , Sinha D. K.
Published: July 2002

This paper essentially seeks to provide a theoretical construct to the experimental studies on the acoustoelectric amplification in piezoelectric semiconductors showing that the attenuation of an acoustic wave crosses over to its amplification when electron drift velocity exceeds the acoustic wave velocity. In this paper, first, a phenomenological theory is developed to study the relative change in electron mass and collision frequency when an acoustic wave propagates through piezoelectric semiconductors. Then, analytically, we have shown that this nonlinearity in electron mass and collision frequency arising due to the electric vector of the field associated with the wave can also explain this behavior. Numerical computations are made for a typical case of n-InSb at 77 K.
Absolute photoabsorption cross-section measurement of the Kr2F excimer at 248 nm
Authors: Hakuta Kohzo , Komori Hiroshi , Mukai Naruhiko , Takuma Hiroshi
Published: July 2002

The photoabsorption of Kr2F in the excited electronic state 42Γ has directly been measured at 248 nm observing its fluorescence suppression induced by KrF laser radiation. The suppression signals have been measured in the laser power density range from 10 to 90 MW/cm2. The analysis has led us to conclude the following: the upper state of the absorption process is a bound electronic state and the photoabsorption cross section is (1±0.2)×10−18 cm2 which is about 1/5 of the value assumed in the current kinetic model of the KrF laser medium.
ArF excimer-laser-stimulated growth of polycrystalline GaAs thin films
Authors: Donnelly V. M. , McCrary V. R. , Appelbaum A. , Brasen D. , Lowe W. P.
Published: July 2002

The growth of GaAs thin films by a laser photodecomposition process is reported. Ga(CH3)3 and As(CH3)3 mixtures are photolyzed above (100) Si and GaAs substrates by a 193-nm argon fluoride excimer laser beam directed normal to the substrate surface. Gas-phase products diffuse to, and stick on the surface where further laser irradiation leads to the formation of polycrystalline GaAs, with grain sizes between ∼20 and 2000 Å. Deposited films contain ∼1–3 at. % carbon, as determined by Auger electron spectroscopy. Surface irradiation by the transmitted beam improves the optical constants of the film to values approaching bulk GaAs, but annealing is not sufficient to promote epitaxial growth at fluences as high as 70 mJ/cm2, for average substrate temperatures up to 400 °C. Laser irradiation of the GaAs substrates exposed to BCl3 or As(CH3)3 immediately prior to deposition removes the native oxide and carbon contamination.
Analysis of the effective stresses acting on twinning partial dislocations in silicon
Authors: Yasutake Kiyoshi , Shimizu Shinji , Kawabe Hideaki
Published: July 2002

The growth process of a twin from a source under a constant applied stress was investigated by computer simulation. Based on the high-voltage electron microscopic observations of the stressed silicon crystals, the thickness of the twin was neglected in the model, and the partial dislocation group containing slip dislocations was considered as well as the simple group of identical partial dislocations. It is found that the effective stress acting on the partials near the twin tip is close to the applied stress for all the partial dislocation groups considered in this paper.
Improvement in the surface photovoltage method of determining diffusion length in thin films of hydrogenated amorphous silicon
Authors: Moore A. R. , Lin Hong-sheng
Published: July 2002

Two modifications in the method of determining diffusion length in a-Si:H films by surface photovoltage are presented; one calculational, the other experimental. In the calculation a known theory together with data at three dc-bias light levels are used to compute the correct diffusion length, as well as the space-charge density at all light levels. Experimentally, provision for illuminating a thin sample with blue light from the rear in addition to the normal red and monochromatic light from the front serves to eliminate the disturbing effect of a back surface barrier. The improved results are illustrated.
Transmission-electron microscope studies of Au-Ni-Ge based ohmic contacts to GaAs-AlGaAs MODFET device
Authors: Rai A. K. , Ezis A. , McCormick A. W. , Petford-Long Amanda K. , Langer D. W.
Published: July 2002

A technique is developed to obtain cross-sectional transmission-electron microscope specimen of a specifically desired region of semiconducting devices. A Au-Ge-Ni based metallization scheme is used to obtain low resistivity ohmic contact to GaAs-AlGaAs based modulation-doped field-effect transistors device. Cross-sectional electron microscopy and energy dispersive x-ray analysis revealed that a uniformly alloyed region (Ni-Ge-As) can be obtained through the proposed metallization scheme. Transmission-electron diffraction, high-resolution electron microscopy, and optical-diffraction analyses are employed to determine the various phases in the contact region. Movement of contact materials both parallel and perpendicular to the device surface is observed at the metallization edges after alloying. Contacts stressed at elevated temperatures to the point of incipient instability are found to exhibit a rougher metal/semiconductor interface.
NMR and Mössbauer study of R2TM14B with R=Y, La, Th, and TM=Fe, Co
Authors: Erdmann K. , Deppe P. , Rosenberg M. , Buschow K. H. J.
Published: July 2002

Mössbauer spectroscopy and nuclear magnetic resonance have been used in order to determine the hyperfine fields at Fe, Y, and B sites in the R2TM14B compounds with R=Y, La, Th, and TM=Fe or Co.
A study on textures of nonoriented silicon steel
Authors: Liu Puru , Li Fulin
Published: July 2002

A study was made of the following: texture parameters (Tp), pole figures, inverse pole figures and magnetic properties for a series of nonoriented silicon steels, e.g., W10, W14, W18,and DW2. Among these materials, DW2 designed by WISCo., is one of the recent products containing phosphorus and lower silicon. The microstructure of DW2 is mainly of columnar grain, and microstructure of the others is mainly of equiaxed grain. It has been found that there are quite strong textures in each of these nonoriented silicon steels. They are {111} or {334} components in W10-W18, and {100} and {334} in DW2. The percentage of {1k0} in DW2 is more than 58%.
Strong influence of magnetic surface anisotropies on the Damon-Eshbach mode frequency of ultrathin Fe(110) layers (abstract)
Authors: Hillebrands B. , Güntherodt G.
Published: July 2002

Epitaxially grown Fe(110) layers on W(110) with thicknesses between 8 and 150 Å have been investigated in situ by Brillouin scattering. A strong increase of the frequency of the Damon-Eshbach (surface spin wave) mode by a factor of up to 3 has been observed with decreasing layer thickness below 60 Å. The frequency increase is accompanied by a strong line width broadening. In good agreement with earlier observations,1–3 the change over of the easy axis of the saturation magnetization from [001] to [1-10] could directly be observed at a critical thickness dc=95±5 Å with an applied residual magnetic field of 30 G. The anomalous behavior of the Damon-Eshbach mode is quantitatively described by a model fit, which takes explicitly into account magnetic surface anisotropies. The magnetic in-plane surface anisotropy constant Ks,p=2.4×10−2 erg cm2 has been calculated from the literature value of the bulk anisotropy constant K1, the residual applied magnetic field and the critical thickness dc, using the homogeneous magnetization approximation.2 The fitted value of the magnetic out-of-plane anisotropy constant Ks=2.8±0.7 erg/cm2 is smaller than the value obtained for the case of Fe(110) on GaAs(110).1 While for Fe thicknesses larger than 30 Å the film quality is not very crucial for the spin-wave spectra, 10-Å-thick films show a strong increase of the Damon-Eshbach mode frequency with increasing density of steps on the surface. Since these steps are aligned parallel to the [001] direction as observed by LEED, we attribute the frequency increase to additional uniaxial surface anisotropies induced by the steps. On the other hand, the formation of islands leads to a frequency decrease due to reduced anisotropies.
Permanent magnets based on replacing nickel with iron in Ni5Sm and its boride derivatives (abstract)
Authors: El-Masry N. A. , Stadelmaier H. H.
Published: July 2002

In hexagonal Ni5R and its boride derivative Ni4RB (R=rare earth atom) nickel can be replaced with iron up to a limit of Fe:Ni=1:1 when R=Sm. At this ratio, FeNi is electronically equivalent to Co, and the magnetic properties are comparable to those of the cobalt compounds, but the moments and Curie temperatures tend to be lower. The moments and Curie temperatures increase with the Fe:Ni ratio. The Fe fraction can be extended when some of the Sm is replaced by Er, but never to the point of complete elimination of Ni. In the boron alloys the Co4RB type is still stable at Fe:Ni=1.67; above Fe:Ni=3:1, Fe14R2B dominates.
Domain walls subject to quenched impurities (invited)
Authors: Kardar Mehran
Published: July 2002

Interfaces between domains become rough as a result of pinning to impurities. The degree of roughening can be characterized by the exponent describing the scaling of fluctations with the size of the domain wall. Current understanding of the roughening process will be reviewed. The simple Imry–Ma scaling arguments are expected to give the correct roughening exponent in systems subject to random-field impurities, but they clearly fail in the case of random bonds. Possible reasons for this failure are examined. A novel class of impurities, with long-range power-law interactions with the interface, are considered. By changing the exponent of the power law, this model interpolates between random bonds and random fields.
A neutron diffraction study of the spin-glass-like system Y2Mo2O7 (abstract)
Authors: Reimers J. N. , Greedan J. E.
Published: July 2002

It has been reported recently that the crystalline, magnetically concentrated material Y2Mo2O7 displays behavior normally associated with spin-glass or cluster glass systems.1 This compound is widely reported to have the cubic pyrochlore structure, space group Fd3m, but no detailed structure determination had been reported. We have determined the crystal structure of Y2Mo2O7 from a powder profile analysis of neutron diffraction data and confirm the space group and structural assignment. That is, all of the 88 atoms in the cubic unit cell fully occupy the Wyckoff positions normally associated with the fully ordered cubic pyrochlore structure: Y in 16d, Mo in 16c, O1 in 48 f, and O2 in 8b. Alternative models with disorder between Y and Mo or with O2 randomly occupying 1/4 of the 32c sites gave poorer agreement indices than the fully ordered model. Thus, to within the limits of accuracy of this study, Y2Mo2O7 is a spin-glass-like material with negligible atomic scale disorder. To date, this situation is unique among all known concentrated, crystalline spin-glass systems.
Anomalous magnetization of GaAs
Authors: Datta T. , Barrientos A. , Amirzadeh J. , Jones E. R.
Published: July 2002

SQUID susceptometric measurements of bulk GaAs in the temperature range 5<T<400 K, show the longitudinal dc susceptibility χ(T) is not independent of temperature. Over this range of T the net susceptibility is diamagnetic, but with different sensitivity to temperature change. For T≲50 K, χ(T) changes rapidly with T. Occasional discontinuous jumps in χ(T) are also observed. Above ∼150 K, χ(T) is less sensitive to T. Large qualitative and quantitative difference may be seen in the data obtained under different experimental protocols. We anticipate that the low-temperature magnetic behavior of other semiconductors of this type will also be anomalous. This is particularly important because these materials are extensively employed in many electronic devices.
Mössbauer study of spin reorientation in Y1.8Er0.2Fe14B
Authors: Abe M. , Liou S. H. , Chien C. L. , Koon N. C. , Das B. N. , Callen E.
Published: July 2002

Fe57 Mössbauer spectra were taken from 300 K down to 4.2 K for two single-crystal samples of Y1.8Er0.2Fe14B, one cut with its [001] axis normal to the surface, the other with its diagonal of [100], [010], and [001] axes normal to the surface. The former sample has shown that the angle between the Fe spin and the [001] increases continuously below ∼90 K. This qualitatively agrees with the spin reorientation (i.e., the easy axis of magnetization rotates away from the [001] toward [100] on cooling below ∼90 K) which have already been observed by torque magnetometry. The latter sample has shown that the Fe spin actually rotates continuously; abrupt change of Fe spin direction, which have been already suggested to occur in Er2Fe14B, does not occur in Y1.8Er0.2Fe14B. The samples were so thick that the intensity of strong absorption lines were saturated. We have derived a formula with which the saturation effect can be corrected in order to determine the angle of the Fe spin direction correctly.
Magnetic properties of aligned NdDyFeCoB magnets (abstract)
Authors: Song Lin , Wang Yizhong , Yu Boliang , Feng Minying , Wang Zhenxi , Yang Xianchen , Wu Jinhua
Published: July 2002

Sintered magnets (Nd1−xDyx)16.5(Fe0.85Co0.15)76.5B7 with 0≤x≤0.2 were prepared. In the temperature range 4.2–450 K, the magnetic hysteresis loops with parallel and perpendicular to aligned directions were measured applying a field up to 15 T. The temperature dependence of the permanent magnetic parameters for the magnets with various Dy concentrations was shown. Using a modified Sucksmith-Thompson plot, the magnetic anisotropy fields of (Nd1−xDyx)2(Fe0.85Co0.15)14B were determined from the experimental results of sintered magnets. As Dy concentration increases, the anisotropy field of the magnets apparently increases, while the anisotropy field of the magnets decreases linearly. Substituting Dy for Nd, there is no effect on Curie temperature of the magnets, which was determined from that of SmCo-type magnets, there exists a transition from nucleation dominated at a low magnetizing field to pinning dominated at a high magnetizing field.
Performance and analysis of amorphous silicon p-i-n solar cells made by chemical-vapor deposition from disilane
Authors: Hegedus Steven S. , Rocheleau R. E. , Buchanan W. , Baron B. N.
Published: July 2002

The photovoltaic performance of amorphous silicon p-i-n solar cells made by chemical-vapor deposition (CVD) from disilane is reported and analyzed. Intrinsic layers were deposited at rates from 0.2 to 50 Å/s at temperatures from 380 to 460 °C with and without boron doping. Device performance was insensitive to substantial differences in disilane purity. A cell efficiency of 4% was achieved. The primary limitation to higher efficiency was low fill factor (<50%) due to high series resistance (>18 Ω cm2). Analysis of the series resistance indicated a contact-related resistance of 4–12 Ω cm2 and a photoconductive resistance composed of intrinsic layer thickness-independent (10 Ω cm2) and thickness-dependent terms. Analysis of the voltage dependence of the current collection indicated a fill factor of 60% would be expected in the absence of series resistance. The maximum short-circuit current of 12.5 mA/cm2 (normalized to 100 mW/cm2) resulted with a boron-doped i layer deposited at 440 °C at 3.3 Å/s. Modeling of the collection efficiency indicated collection widths up to 0.33 μm for boron-doped and 0.24 μm for undoped p-i-n devices. In order to achieve high-efficiency cells using CVD from disilane, the limitations imposed by low photoconductivity, a high density of states, and restricted cell design imposed by the high deposition temperatures would have to be overcome.
Temperature-dependent 27Al quadrupole splitting in YbCuAl
Authors: Lysak M. , Reyes A. P. , Tien Cheng , van Kalkeren G. , MacLaughlin D. E. , de Boer F. R.
Published: July 2002

The quadrupole splitting of the 27Al NMR spectrum in the intermediate-valent (IV) compound YbCuAl has been measured as a function of temperature between 2 and 300 K. The splitting HQ between maxima of ±3/2⇄±1/2 powder-pattern satellites varies from (222±2) Oe at 10 K to (245±2) Oe above 80 K, in contrast to the nearly constant value (268±3) Oe observed in the nonmagnetic reference compound YCuAl over the same temperature range. Measurements at 4.2 K for applied magnetic fields between 2.3 and 8.2 kOe confirm that the splitting is strictly first-order quadrupolar in origin, i.e., that the temperature variation is not an artifact of the large 27Al magnetic line broadening in this system at low temperatures. The observed variation of HQ is two orders of magnitude larger than the corresponding anomalous lattice constant variation over approximately the same temperature range. This suggests considerable temperature dependence of the Yb elecronic structure, which is apparently an important contributor to the electric field gradient (EFG) at Al sites in YbCuAl. A similar temperature variation of the EFG at Yb sites has been attributed to crystal-line electric field (CEF) splitting of the Yb ionic states, but there is no indication of a CEF splitting in inelastic neutron scattering. The EFG therefore appears to reflect the breakup of the IV ground state with increasing temperature.
Potential flow model for the hydromagnetic Rayleigh–Taylor instability in cylindrical plasmas
Authors: Hwang C. S. , Roderick N. F.
Published: July 2002

A potential flow model has been developed to study the linear behavior of the hydromagnetic equivalent of the Rayleigh–Taylor instability in imploding cylindrical plasmas. Ordinary differential equations are obtained for both (r,z) and (r,θ) disturbances. The model allows the study of the dynamic effects of the moving plasma on the development of the instability. The perturbation equations separate into a geometric part associated with the motion of the interface and a nongeometric part associated with the stability of the interface. In both planes the geometric part shows growth of perturbations for imploding plasmas. The surface is also unstable in both planes for plasmas being imploded by magnetic fields. Analytic solutions are obtained for constant acceleration. These show that the short wavelength perturbations that are most damaging in the (r,z) plane are not affected by the motion of the interface. In the (r,θ) plane the growth of longer wavelength disturbances is affected by the interface motion.
Negative differential resistance in a strained-layer quantum-well structure with a bound state
Authors: Lee G. S. , Hsieh K. Y. , Kolbas R. M.
Published: July 2002

Strained-layer resonant tunneling structures of pseudomorphic GaAs-AlAs-In0.09Ga0.91As-AlAs-GaAs are studied both experimentally and with a simple quantum mechanical model. The energy spectrum of the unbiased structure includes the usual resonant states and also bound states due to the narrow-band gap InGaAs well. Strong tunneling (and negative differential resistance) are observed for the resonant states. We observe for the first time tunneling associated with a ‘‘bound-state’’ energy level which result in a zero differential conductance feature. Qualitative agreement between the experimental and calculated results are shown.
Influence of xenon on the self-reversal maxima of the Na-D emission lines in high-pressure sodium lamps
Authors: Jongerius M. J.
Published: July 2002

We report on the influence of xenon buffer gas on the shape of the self-reversed Na-D lines emitted by high-pressure sodium (HPS) arcs. In a previous paper we have reported experimental results on the collisional broadening of the Na-D line wings by xenon at high temperatures [M. J. Jongerius, J. Phys. B 20, 3345 (1987)]. These results are used to calculate the relative wavelength positions Δλr and Δλb of the red and blue Na-D self-reversal maximum, respectively, as functions of the Na/Hg amalgam temperature. The additional broadening by xenon leads to a strong asymmetry in the shape of the Na-D emission lines and a distinct maximum in Δλr/Δλb because of the Na-Xe satellite at 590.9 nm. The presence of this maximum in Δλr/Δλb is confirmed by experiments in which the shape of the Na-D emission lines is measured as a function of the arc power. The maximum value of Δλr/Δλb can be used as a measure of the xenon pressure in the arc. Furthermore, Δλr and Δλb can be used to obtain values for the sodium and mercury pressures in the burning arc. In the conventional method (J. J. de Groot and J. A. J. M. van Vliet, in Proceedings of the 2nd International Symposium on Incoherent Light Sources, Enschede, 1979, summary p. 30), the Na-D line broadening by the xenon buffer gas is not taken into account. We propose a modification of the conventional method to account for the influence of the collisional Na-D line broadening by xenon.
ZnSe/ZnS heteroepitaxial growth using an intermediate strained-layer superlattice buffer
Authors: Yokogawa Toshiya , Ogura Mototsugu , Kajiwara Takao
Published: July 2002

High-quality ZnSe layers have been successfully grown on ZnS layers with an intermediate ZnSe-ZnS strained-layer superlattice (SLS) by low-pressure metalorganic vapor-phase epitaxy. If we use the SLS buffer between the ZnSe and ZnS layer, the excitonic-emission line in the photoluminescence spectra of the ZnSe epilayers due to the free exciton becomes stronger and emissions related to misfit dislocations weaker compared with those of the ZnSe layer grown directly on the ZnS layer. Reflective high-energy electron diffraction measurements also showed that these (100) ZnSe layers grown on the SLS buffer were single crystals. With double-crystal x-ray diffraction, it is supposed that the misfit strain was completely relaxed by using the SLS buffer in only 0.6-μm-thick ZnSe layer grown on the ZnS layer. From these results, we have found that the ZnSe-ZnS superlattice buffer is very useful for obtaining a good ZnSe/ZnS heterostructure.
Influence of dispersion on the transmission characteristics of supertwisted nematic effects in liquid-crystal displays
Authors: Breddels P. A. , van Sprang H. A. , Bruinink J.
Published: July 2002

The transmission properties of director configurations with a 180° twist and low tilt (∼1°) and of director configurations with a 270° twist and high tilt (∼25°) were calculated and compared with experimental data. It was found that the dispersion in the birefringence must be taken into account in order to properly calculate transmission wavelength spectra. This appeared to be of special importance for materials with large birefringence.
Luminescence and excitation energy transfer in monolayer assemblies incorporating organic dye molecules. II
Authors: Yonezawa Yoshiro , Möbius Dietmar , Kuhn Hans
Published: July 2002

Excitation energy transfer in the monolayer assembly is studied in which the same combination of organic dye molecules as in our preceding paper [J. Appl. Phys. 62, ▪▪▪ (1987)] is employed, but donor molecules are organized into the J aggregate, which consists of large arrays of densely packed chromophores and is characterized by a strong and narrow absorption band (J band) considerably red shifted from the corresponding monomer absorption band and a sharp resonance fluorescence. The acceptor molecules can be dissolved in the J aggregate, resulting in quenching of the resonance fluorescence and appearance of sensitized fluorescence of the acceptor. Quenching of resonance fluorescence by the acceptor obeys the Stern–Volmer relation with a temperature-dependent Stern–Volmer constant. Lowering the temperature from T=293 to 77 K causes the decrease of both the Stern–Volmer constant and the intensity ratio of the sensitized fluorescence to the resonance fluorescence. Comparison of the Stern–Volmer constants and luminescence lifetimes of the donor in the isolated and aggregated states reveals that energy transfer from the J aggregate occurs far more efficiently than from the monomer. The observed temperature dependence is analyzed under an assumption that the rate constant of radiative transition is proportional to 1/T. Such an assumption is rationalized by the model that the coherent exciton extending over a certain domain moves over about 3000 donor molecules, occasionally reaching the vicinity of an acceptor.
Amplification of bipolar current flow by charge induced from an insulated gate electrode
Authors: Xu J. , Shur M. , Hack M.
Published: July 2002

We present the results of a two-dimensional simulation of the amplification of an ambipolar current flow in a four-terminal crystalline silicon device by charge induced into this flow from an insulated gate electrode. We show that although a bipolar current flow in a crystalline device can indeed be modulated by a gate field, the magnitude of this modulation is less than in amorphous silicon double-injection field-effect transistors. Our proposed structure also has an interesting feature in that the gate field redistributes the current flow between its terminals.
Self-consistent theory including nonstationary phenomena for carrier transport in the AlGaAs/GaAs high electron mobility transistor
Authors: Fjeldly Tor A. , Johnsen Lars
Published: July 2002

An analytical formulation is developed for carrier transport in the two-dimensional conducting channel of the AlGaAs/GaAs high electron mobility transistor (HEMT). The problem related to current saturation is also addressed, and a simple model for the pinchoff region is proposed. The present description is based on a self-consistent treatment of electric field, carrier velocity, and carrier density in the potential well. It has previously been shown that important nonstationary effects such as velocity overshoot are reasonably well described within the framework of the relaxation time approximation of the Boltzmann transport equation. Justified by the same approximation, it is shown here that a simple and manageable theory for nonuniform transport in the HEMT can be formulated on the basis of the transient behavior of the average carrier velocity in the homogeneous case. The required homogeneous transient responses to step changes in the electric field can readily be obtained from the space invariant Boltzmann equation, and can be calculated once and for all by advanced methods, such as the Monte Carlo technique. Specific calculations of stationary current-voltage characteristics and of associated transport variables for a HEMT are presented.
Structural and electrical properties of silicon nitride films prepared by multipolar plasma-enhanced deposition
Authors: Boher P. , Renaud M. , Van Ijzendoorn L. J. , Barrier J. , Hily Y.
Published: July 2002

A new system of dielectric deposition using a multipolar plasma enhanced by a hot filament has been used to deposit multipolar plasma chemical vapor deposition silicon nitride films on various substrates (GaAs, Si, GaInAs, etc.). Using in situ kinetic ellipsometry during the depositions, the flow ratio SiH4/N2 has been optimized to form as dense silicon nitride as possible. The density variation has been attributed to a variable amount of oxygen in the films certainly in the form of silicon dioxide. Using Rutherford backscattering and spectroscopic ellipsometry, the amounts of oxygen have been measured precisely. Using infrared absorption, we have demonstrated the low hydrogen concentration of our films compared to plasma-enhanced chemical vapor deposition ones. At constant flow ratio, we have demonstrated the effect of the deposition rate on the stoichiometry of the films. Films deposited at very low deposition rates (<10 Å/min) are quasistoichiometric and produce important reductions of GaInAs/Si3N4 interface densities of states compared to higher deposition rate films (>24 Å/min). The conduction mechanism appears ionic in oxygen-rich silicon nitride films and controlled by a Poole–Frenkel effect in the case of moderate deposition rates.
An analytic method to determine the effect of source modeling errors on the apparent location and direction of biological sources
Authors: de Munck J. C. , van Dijk B. W. , Spekreijse H.
Published: July 2002

Evoked potentials (EPs) and electroencephalograms (EEGs) can be used to determine the location, the direction, and strength of electrical brain activity. For this purpose mathematical models are used which describe regions in the head with different conductivity. In most models, the sources are described by mathematical (current) point dipoles. However, EPs and EEGs are generated with more extensive cortical areas. In this study an analytic method is described to calculate the effect of source extension on the potential distribution measured at the scalp and also on the difference between the location of the extended source and the location of the equivalent point dipole. General formulas are derived which express in spherical harmonics the potential distribution that results from a circularly symmetric extended source. It is shown that for sources that obey specific symmetries the influence of source extension on the potential distribution is a fourth-order effect in the distance between electrode and the origin (the middle point of the head), and a second-order effect in the extension. It is also shown that for such sources the error in localization (i.e., the distance between the position of the equivalent dipole and the center of the extended source) is zero when Geselowitz’s method [IEEE Trans. Biomed. Eng. BME-12, 164 (1965)] is used. Because in volume conductor models the relation between source and potential is given by Poisson’s equation, it is suggested by the authors that the results of the present study may be extended to applications in other fields of physics as well.
Magnetic susceptibility in the spin glass system, [(CH3)3NH]CO1−xNixCl3 2H2O (abstract)
Authors: Rubenacker Gerald V. , Haines Donald N. , Drumheller John E.
Published: July 2002

The pseudo-one-dimensional mixed system [(CH3)3NH]Co1−xNixCl3 2H2O was found to exhibit spin-glass behavior below a characteristic temperature evidenced by the onset of time-dependent thermoremanent magnetization. In order to confirm the spin-glass behavior we now have studied the ac susceptibility of this series in zero field using an ac SQUID susceptometer. Along the weak ferromagnetic axis (a axis), the ac susceptibility shows a cusp at the spin-glass transition temperature. Increasing the measurement frequency from 80 to 160 Hz increases the slope of the susceptibility below the transition temperature but no significant shift in the transition temperature itself is seen for this small frequency range. Spin-glass behavior is seen only along the a axis and a typical antiferromagnetic transition is seen along the c axis.
Variation of magnetic properties with composition in (Fe-Ni)76Si8B16 amorphous ultrafine particles
Authors: Ohnuma S. , Mitera M. , Graham C. D. , Masumoto T.
Published: July 2002

The composition dependence of magnetic properties in ultrafine (Fe1−x Nix )76 Si8 B16 amorphous particles has been investigated. Samples were prepared by sputter deposition onto sputter-etched polyimide substrates. The particle diameter was held constant at about 800 Å, and the L/D ratio was approximately 4. The room-temperature saturation magnetization (Ms ) decreases with increasing Ni content and extrapolates to zero at about x=0.9. All the samples showed positive anisotropy (Ku ). This anisotropy varied linearly with M2s, suggesting that its origin is largely the shape anisotropy of the particles. Furthermore, the measured perpendicular coercive field varied linearly with Ku /Ms , as predicted by a simple Stoner–Wohlfarth single domain model. However, the absolute value of Hc and its angular dependence cannot be explained by the coherent rotation model.
Magnetism of metastable phases: Band theory and epitaxy (invited)
Authors: Marcus P. M. , Moruzzi V. L.
Published: July 2002

Total-energy band calculations are used to analyze the magnetic phases of metallic elements as functions of volume. The calculations utilize a fixed-spin-moment procedure, which is described and justified as a natural generalization of density-functional theory. This procedure finds the ground-state energies of electronic systems under two constraints, and hence determines the system energy as a function of two variables—volume and magnetic moment. The energy function is used to find the ferromagnetic phases and their ground-state properties, including bulk moduli and magnetic susceptibilities. The systems studied are fcc Fe, fcc Co, bcc Ni, fcc Pd, and bcc Mn, each of which undergoes a phase transition for small changes of the lattice constant from equilibrium (zero-pressure) values.
Modified magnetic properties in laser welded materials
Authors: Smith N. , Bird R.
Published: July 2002

The magnetic properties of laser welded magnetic materials can be modified by selecting material composition, and by setup parameters. The modification depended upon the original material composition, annealed coercive force, and the size of the weld size. Original grain boundaries were reformed in the silicon iron samples only, which also had the smaller increase in coercive force. The coercive force of cobalt-vanadium iron with a lower initial value was increased more than similar samples with a higher initial coercive force value. Double welds doubled the increase in coercive force of single welds. Magnetic fields of the intensity used in this test were not found to have a large effect upon the changes in coercive force due the laser welding process. In prior tests, stronger alternating magnetic fields reduced coercive force in cobalt-vanadium iron alloys.
Spin-wave resonance in FeNiBSi films
Authors: Xia J. , Ryu J. S. , Vittoria C.
Published: July 2002

Ferromagnetic resonance (FMR) measurements have been performed on thin films of Fe80−xNixB15 Si5 alloys (x=5, 40, and 60). The effective magnetization (4πMeff ), magnetic anisotropy field (HA), and g factor were measured. The values of g for all the films ranged between 1.90<g<2.11. Spin-wave resonance (SWR) excitations were observed for H, the magnetic field, applied perpendicular to the film plane. SWR fields were found to obey the n2 law and yielded exchange constants in the range of A=0.2–1.9×10−6 ergs/cm. The nearest-neighbor exchange parameter J was deduced from A and implies that J varies within a factor of 2 as the value of x changes. In one of the nickel-rich samples, the SWR fields appear to be linear with n implying nonuniform distribution of magnetization in the film.
Temperature hysteresis and time-dependent effect in the initial susceptibility of rapidly solidified Monel (abstract)
Authors: Atzmony U. , Swartzendruber L. J. , Bennett L. H.
Published: July 2002

The temperature dependence of the ac susceptibility for a Cu-Ni alloy near the Monel composition (28 at. % Cu) was previously reported.1 A careful recording of this temperature dependence revealed the existence of temperature hysteresis for both the as-spun condition and after various isothermal anneals. The hysteresis is exhibited by higher susceptibility values during the cooling-down period compared to the warming up. The difference exists over the entire temperature range, though it is most pronounced around the peak. The peak for the as-spun sample is broader and therefore the effect is more clearly observed there. The time dependence decrease of the ac susceptibility was followed at room temperature for both the as-spun and the annealed samples and was found to have a long time constant. The decrease could be abruptly generated by ‘‘shaking’’ the sample with an external disturbing magnetic field. The hysteresis is attributed to the fact that above the peak and just below the Curie temperature, the sample is magnetically soft and is magnetized parallel or antiparallel to the alternating small magnetic field. Due to relaxation effects below the ac susceptibility peak (and the corresponding increase of the anisotropy), the direction of the magnetization tends to rotate to the local easy direction, but since the relaxation time constant is long compared to the measuring period, it partially retains its previous direction and thus the susceptibility remains higher during the cooling duration. The anisotropy keeps it at the lower value when the temperature is increased again.2
Thermal aging and storage characteristics of rapidly solidified Nd-Fe-B powder (abstract)
Authors: Welker E. E. , Guthrie M. S.
Published: July 2002

Various magnetic products are now being produced from rapidly solidified Nd-Fe-B powder. Some types of magnets included injection molded, calendered, compression molded and hot pressed. A major factor leading to this increased use is the inherent stability of the powder. This paper reviews the test results of the following studies: (a) Rapidly solidified Nd-Fe-B powder stored at ambient conditions for a period of 11 months. Oxygen and nitrogen are monitored along with magnetic induction (Br). (b) Nd-Fe-B powder was tested for 6 months at 23 and 38 °C in humidity of 50% and 80%, respectively. Magnetic induction, Hci and Bd were monitored. (c) Nd-Fe-B powder was tested at elevated temperatures including 155 °C for 696 h. Magnetic induction and oxygen were monitored. The test results show the rapidly solidified Nd-Fe-B powder to be exceptionally stable requiring no special handling under normal conditions.
Nonequilibrium relaxation of the Ising square lattice at the percolation threshold
Authors: Landau D. P. , Wansleben S.
Published: July 2002

The time-dependent behavior of a site-diluted Ising square lattice at the percolation limit is studied by Monte Carlo simulations. Contrary to theoretical predictions, we find a stretched exponential decay M(t)=A exp[−(t/τ)n] with a temperature-dependent exponent n.
Perturbations in the adaptive refinement of boundary elements
Authors: Jayakumaran S. , Hoole S. Ratnajeevan H.
Published: July 2002

Error estimation and the associated adaptive refinement of boundary element meshes in solving Poissonian magnetic fields is described. The percentage change in the potential or its gradient in two consecutive boundary element solutions is employed as a measure of error.
Attaining the ideal detection limit using random multiplication
Authors: Jindal R. P.
Published: July 2002

The effect of measurement time and the detection system bandwidth product in determining the signal-to-noise ratio is formulated. Results are derived for both cases when the detector is operated in the deterministic limit and the random limit. A comparison suggests that for level detection, when the measurement accuracy is limited by the finite time constant of the detection system, the random limit operation results in superior performance. For pulse detection, the random limit operation makes it possible to approach the ideal performance limit for large time bandwidth products. Also, the random scheme can attain the ideal performance limit even in the presence of gain fluctuations of the detector. The principle of random multiplication therefore provides an alternative approach to realizing ideal photodetectors.
Optical properties of ion-beam-deposited ion-modified diamondlike (a-C:H) carbon
Authors: Orzeszko S. , Woollam John A. , Ingram David C. , McCormick A. W.
Published: July 2002

Diamondlike carbon (DLC) is a hard, semitransparent material usually containing varying amounts of hydrogen. These materials have numerous potential applications, including use as coatings for infrared optics, and as such, the effects of damaging irradiation is of practical interest. In this paper we present results of variable angle spectroscopic ellipsometric (VASE) studies of ion-beam-deposited DLC films. These films have been further modified by directing 1-MeV gold ions, as well as 6.4-MeV fluorine ions, through the DLC and into the underlying silicon substrates, and the percentage of hydrogen in the film was measured versus fluence using proton recoil analysis. Optical analysis was performed assuming the Lorentz oscillator model. Three versions were used: one oscillator, two oscillator (with one fixed in energy), and two oscillator with all parameter variable. The latter model fits the VASE data extremely well, and the two oscillators can be interpreted as involving π to π* and σ to σ* band transitions. With ion modification the oscillators shift to lower photon energy, consistent with reduction in hydrogen concentration and possible increased graphitization.
Kinetics and mechanism of laser-induced photochemical deposition from the group 6 hexacarbonyls
Authors: Jackson Robert L. , Tyndall George W.
Published: July 2002

A quartz crystal microbalance has been used to measure the rates of photochemical deposition from Cr(CO)6, Mo(CO)6, and W(CO)6 induced by a focused frequency-doubled argon ion laser (257 nm). The kinetic data indicate that deposition occurs via single-photon dissociation of the hexacarbonyl in the gas phase, yielding products that subsequently condense on the substrate surface. This interpretation is supported by the observation of material deposited well outside the area of the substrate that is directly illuminated by the laser beam. A calculation of the deposit thickness versus radial distance from the center of the laser beam, using a simple gas-phase model of the deposition process, provides an adequate fit to the observed thickness profile of a typical deposit. The presence of ripples in material deposited within the focal spot of the laser beam suggests that secondary photoinitiated dissociation of the gas-phase photoproducts occurs after they condense on the substrate surface. Adsorption measurements performed with the microbalance show that the group 6 hexacarbonyls do not adsorb on SiO2 and nickel surfaces under the conditions of the deposition experiments, up to their respective room-temperature vapor pressures.
Silver distribution in wet-sensitized As10Ge22.5Se67.5 films after electron beam exposure
Authors: Liang Yong-Cheng , Tada Kunio
Published: July 2002

In photodoping, it is well known that the silver concentration of the exposed region is somewhat higher at the edge than at its interior. This is termed the ‘‘edge sharpening’’ effect. Exposure by electrons or ions has been reported to produce no such effect, which leads to the conjecture that it might be dependent on the kind of irradiation involved. However, we have shown here by electron probe microbeam analysis, without ambiguity, that the edge-sharpening effect does occur in regions of a chalcogenide glass film As10Ge22.5Se67.5 that is coated with silver selenide and exposed by electrons of 25 keV. Furthermore, we have observed, for the first time, that silver concentration at the periphery of the exposed region increases with dosage. It has then been shown that such a silver distribution is not due to the temperature gradient of a highly focused electron beam. Finally, exposure by electron beam has been found to induce surface-topological changes in the exposed regions at a dosage of about 1 C/cm2, which is similar to photoinduced surface-topological changes studied recently. These characteristics have an important effect on patterning in the submicron range by electron beam lithography using the induced silver doping effect.
Absorption, fluorescence, and electron spin resonance investigation of tervalent cerium activated LaMgAl11O19
Authors: Viana B. , Aka G. , Vivien D. , Lejus A. M. , Théry J. , Derory A. , Bernier J. C. , Garapon C. , Boulon G.
Published: July 2002

Single crystals of the magnetoplumbite (MP)-like compounds La1−xCex MgAl11O19 (0≤x≤1) have been grown from the melt by the flame fusion method. These crystals have been characterized by optical absorption and fluorescence which occur between 4f and 5d levels as well as by electron spin resonance at 20 K. For dilute samples (x≤0.15) all of these studies agree with a distribution of cerium between two kinds of sites with C2v point symmetry but with different optical spectroscopic properties whose occupation depends on the cerium content (x value). There is no evidence of any cerium in the regular MP lanthanide site with D3h symmetry. Several structural models which could account for these results are discussed.
Three-dimensional solid-phase-epitaxial regrowth from As+-implanted Si
Authors: Horiuchi M. , Tamura M. , Aoki S.
Published: July 2002

Solid-phase-epitaxial (SPE) regrowth from selectively As+-implanted amorphous Si is analyzed by cross-sectional transmission electron microscopy. SPE regrowth in the (100) wafers proceeds into both vertical and lateral directions simultaneously. For (111) samples, SPE regrowth in the diagonal direction, i.e., 〈100〉 direction near the mask edge, is dominant. In addition to the end of range defects and projected range defects, in the SPE process, defects of a third type are generated just beneath the implantation mask edge. Differences in mask material which control the applied stress at the mask edge have little influence on edge defect generation. Substrate orientation and mask pattern direction play important roles in the edge defect formation mechanism.