bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation of epitaxial CoSi2 films on (001) silicon using Ti-Co alloy and bimetal source materials
Authors: Hsia S. L. , Tan T. Y. , Smith P. , McGuire G. E.
Published: July 2002

Using coevaporated Ti-Co alloy and sequentially evaporated Ti-Co bimetallic layer source materials, CoSi2 films have been grown on (001) Si. The film resistivity and resistance thermal stability are excellent. The CoSi2 are epitaxial single-crystal films containing antiphase domains in the Ti-Co bimetallic layer cases and are polycrystalline films containing a substantial portion of epitaxial grains in the Ti-Co alloy cases. The epitaxial or substantially epitaxial nature of these CoSi2 films is the reason for the excellence in the film resistivity and resistance thermal stability. We believe that the epitaxial nature of the CoSi2 films results from two roles played by Ti. In the first, Ti served as a getterer for removing the native oxide layer on the Si wafer surfaces, which causes the nucleation of CoSi2 grains with random orientations. In the second, Ti silicides formed in the early stage of the annealing process served as Co diffusion barriers preventing Co2Si and CoSi formation, which would also lead to the formation of randomly oriented CoSi2 grains. Models of the interfacial structure of the epitaxial CoSi2 film and Si substrate have been constructed for [001] and [111] orientations. These models revealed that antiphase boundaries serve the role of relieving the lattice mismatch between the epitaxial CoSi2 film and Si substrate.
A 3-dimensional model for low-pressure chemical-vapor-deposition step coverage in trenches and circular vias
Authors: IslamRaja M. Mazhar , Cappelli M. A. , McVittie J. P. , Saraswat K. C.
Published: July 2002

A general analytical model has been developed to calculate particle transport and spatial step coverage evolution within 2-dimensional and 3-dimensional microelectronic device structures during low-pressure chemical vapor deposition. The model can account for spatially dependent nonunity reactive ‘‘sticking probabilities,’’ anisotropic source fluxes, and trench ‘‘shadowing’’ effects. There is no restriction on the initial and evolving shape of the structure. Model results are compared to direct Monte Carlo simulations for step coverage on rectangular trenches, and are found to more accurately describe the observed experimental step coverages during phosphorous-doped silicon dioxide glass film deposition. We present here, for the first time, detailed calculations of step coverage in circular vias for a wide range of reactive sticking probabilities.
Effect of microstructure on the corrosion behavior of NdFeB and NdFeCoAlB magnets
Authors: Camp F. E. , Kim A. S.
Published: July 2002

We have previously shown that the corrosion behavior of ternary NdFeB magnets is affected by their oxygen, carbon, and nitrogen contents. The corrosion was measured in autoclaves which give accelerated testing environments of high heat and humidity. In this study, we relate this corrosion behavior in both NdFeB and NdFeCoAlB magnets to effects upon their microstructure. When the oxygen and carbon contents are low in ternary NdFeB magnets, a thick Nd-rich phase (α-Nd and/or NdOx) forms along grain boundaries and their triple junctions. As the oxygen and carbon contents increase, the Nd-rich coating along the boundaries becomes thinner and agglomerates into the triple junctions. With thin grain boundaries, the pathways for corrosion propagation are hindered, thus improving corrosion resistance. With increases in oxygen, the α-Nd and unstable NdOx are changed to stable Nd2O3. Nitrogen increases also aid in the process of conversion to Nd2O3 which leads to better corrosion resistance. Two different features are seen in the microstructure of NdFeCoAlB magnets. The grain boundary phase (Nd3Co) is very stable. Any oxygen in the system appears as fully oxidized Nd2O3. Easily corroded α-Nd and NdOx are not detected. All of these factors combine to produce excellent corrosion resistance in this variety of NdFeCoAlB magnets.
Magnetic properties of (110)[001] grain oriented 6.5% silicon steel
Authors: Honma H. , Ushigami Y. , Suga Y.
Published: July 2002

Grain oriented 6.5% silicon steel with orientation (110)[001] was prepared by the secondary recrystallization technique for the first time. Magnetic properties of grain oriented 6.5% silicon steel have previously not been reported because of poor secondary recrystallization. In this paper, the magnetic properties of this grain oriented 6.5% silicon steel are reported. The degree of the deviation from (110)[001] orientation, indicated by B8/Bs, was almost the same as in conventional material. B8/Bs were 1.45/1.80–1.61/1.80. Core losses of 9-mil thick materials were W10/50 = 0.25–0.35 w/kg. The frequency dependence of losses was compared with that of nonoriented 6.5% silicon steel and grain oriented 3% silicon steel. Grain oriented 6.5% silicon steel had the lowest at a flux density of Bm = 1.0 T, and a frequency of 50–1000 Hz. The superiority of grain oriented 6.5% silicon steel was remarkable in the higher frequency region. The hysteresis loss of grain oriented 6.5% silicon steel was smaller than that of grain oriented 3% silicon steel. The anomaly factor of eddy current loss was large. If the number of effective 180 domain walls is increased, very low loss will be obtained.
Structural and magnetic properties of diode radio-frequency sputtered Cr/Co multilayers
Authors: Boher P. , Giron F. , Houdy Ph. , Beauvillain P. , Chappert C. , Veillet P.
Published: July 2002

Structural properties of diode radio-frequency (rf)-sputtered Cr/Co multilayers have been investigated using in situ kinetic ellipsometry, grazing x-ray reflection, x-ray diffraction, and nuclear magnetic resonance. Results have been correlated with magnetic characterizations obtained by SQUID susceptometry. Interdiffusion along ≂10 Å occurs at the ‘‘Cr on Co’’ interface. The Co on Cr interface appears sharper. X-ray diffraction and nuclear magnetic resonance show that Co layers grow with bcc structure when the Co layer thickness dCo is lower than ≂15 Å. In this case, the films are strongly textured with the Cr bcc (100) direction perpendicular to the plane of the substrate. A better structural coherence is observed for the thinner layers. When dCo is thicker than ≂15 Å, a mixture of hcp and fcc Co phases appears by x-ray diffraction and nuclear magnetic resonance. The good structural quality of the films is confirmed by the occurrence of satellite peaks in the x-ray diffraction patterns. The Co magnetic moment extracted from the hysteresis loops measurements is coherent with these changes of the cobalt structure with layer thickness. In plane magnetization measurements show a antiferromagnetic coupling of the Co layers through the Cr layers.
Trapping and trap creation studies on nitrided and reoxidized-nitrided silicon dioxide films on silicon
Authors: DiMaria D. J. , Stathis J. H.
Published: July 2002

Alternative gate insulators for silicon-based technologies involving nitridation or reoxidation-nitridation of silicon dioxide layers are shown to be inferior to as-grown oxide in terms of charge trapping over a wide range of fields under uniform electron-injection conditions. Although nitrided layers seem to suppress trap generation more effectively than does silicon dioxide, background trapping in the as-fabricated oxynitride layers formed near their interfaces is greatly increased. The apparent reduction in trapped charges universally reported in reoxidized-nitrided oxides under high-field injection conditions is shown to be due to a decrease in occupation of these sites at fields exceeding 8 MV/cm.
Band-to-band optical absorption in narrow-gap Hg1−xCdxTe semiconductors
Authors: Chu Jun-hao , Mi Zheng-yu , Tang Ding-yuan
Published: July 2002

The infrared absorption spectra have been measured at temperatures from 4.2 to 300 K for thin bulk samples of Hg1−xCdxTe with compositions, x, from 0.170 to 0.443 with thicknesses smaller than 10 μm. The spectral range of the investigations includes the exponential absorption edges and intrinsic absorption spectra determined by electron transitions from the valence band to the conduction band. From the measured absorption spectra, the energy-band gap was determined by the photon energy at the absorption transition point between the Urbach exponential region and the Kane region. The data were fitted by a calculation of the intrinsic optical transition using Kane’s theory. The dependence of the energy-band gap on composition and temperature was determined in the alloy composition range from x=0 to 0.443 and for x=1, for temperatures from T=4.2 to 300 K. The absorption coefficient, αg, at the band-gap energy versus composition and temperature, and the exponential rule satisfied by the absorption edge are also obtained.
Strain effects on normal incidence hole intersubband absorption in a p-type semiconductor quantum well
Authors: Teng D. , Lee C. , Eastman L. F.
Published: July 2002

We theoretically investigate the effects of strain on normal incidence hole intersubband absorption in p-type semiconductor quantum well. We found that for the normal incident light the most prominent transition probability and absorption coefficient are from the first heavy hole subband to the first light hole subband. Since the strain in the quantum well layer changes the energy split between the first heavy and light hole subbands, the absorption coefficient peak can be tailored to occur at a desired wavelength (say λ=10 μm) by changing the strain.
Influence of temperature and backside roughness on the emissivity of Si wafers during rapid thermal processing
Authors: Vandenabeele Peter , Maex Karen
Published: July 2002

The influence of temperature and roughness on the backside emissivity of Si wafers was studied. In situ measurements were done in two commercial rapid thermal processing systems. An experimental setup was built for in situ emissivity measurements of wafers with a polished or nonpolished backside. The emissivity of double-side polished wafers was measured for temperatures ranging from 300 to 700 °C and at wavelengths of 1.7 and 3.4 μm. It was found that the absorption coefficient α of lightly doped silicon is described by the equation α=4.15×10−5λ1.51T2.95 exp(−7000/T) cm−1, for wavelengths λ ranging from 1.5 to 5 μm and temperatures T ranging from 673 to 973 K (λ in μm, T in K). The backside emissivity of Si wafers with different roughnesses was measured. At temperatures above 600–700 °C the wafers are opaque and the emissivity is only slightly dependent on backside roughness. At lower temperatures the wafers are partially transparent and the emissivity is strongly dependent on the backside roughness of the wafer due to light trapping in the bulk of the wafer. For the latter case a new model was developed to simulate the emissivity of wafers with a rough backside at low temperatures.
Effect of annealing temperature on the hole concentration and lattice relaxation of carbon-doped GaAs and AlxGa1−xAs
Authors: Höfler G. E. , Höfler H. J. , Holonyak N. , Hsieh K. C.
Published: July 2002

Measurements of the hole density in carbon-doped GaAs and AlxGa1−xAs as a function of the annealing temperature are presented. It is shown that after sample annealing at low temperatures (T<550 °C), the hole concentration increases in all samples doped ≥1×1019 cm−3 with a simultaneous decrease in the hole mobility. However, sample annealing at higher temperatures (T≳600 °C) results in a reduction of the hole concentration in all samples doped with carbon at concentrations higher than ≊5×1019 cm−3. The reduction in hole concentration is also accompanied by an increase in lattice parameter of the carbon-doped epilayer. The observed changes in the electrical and microstructural properties are explained in terms of two different mechanisms: (1) the passivation of carbon acceptors by the incorporation of hydrogen during growth, and (2) the change in the lattice site location of carbon atoms upon annealing. Direct determination of the lattice site location of carbon in samples doped ≥5×1019 cm−3 showed that the fraction of interstitial carbon after annealing at T≳600 °C is at least 70% higher than in the as-grown samples.
Calculations of the Faraday rotation in RE-substituted iron garnets
Authors: Xu You , Zhang Guoying , Duan Mingqian
Published: July 2002

The calculations of the Faraday rotation (FR) induced by the rare-earth (RE) ions in the RE-substituted iron garnets with RE=Ce, Pr, and Tb, based on quantum theory are presented. The FR contributed by the RE ions is caused mainly by the intraionic electrical dipole transitions between the 4fn and 4fn−1 5d configurations. The crystal field (CF) and the superexchange interaction are two crucial factors in determining the FR contributed by the RE sublattice. For the Ce3+ ion in Ce:YIG, the energy differece between the lowest CF split 5d level and the lowest CF split 4f level is 1.415 eV, hence the FR contributed by the Ce3+ ion at 633 and 1150 nm wavelengths have different signs. The splitting of the excited and ground configurations induced by CF is smaller for the Pr3+ and Tb3+ ions in Pr:YIG and Tb:YIG. Therefore, the FR at 633 and 1150 nm wavelengths in these garnets have the same sign. The theoretical results are in good agreement with experiments.
Method for reduction in surface generation current in polycrystalline-silicon-gate metal-oxide-semiconductor devices
Authors: Sheu Yea-Dean , Hawkins Gilbert A.
Published: July 2002

Generation current from interface states at the Si/SiO2 interface is a dominant noise source in metal-oxide-semiconductor type solid-state imagers. A simple method to reduce the interface state density of the solid-state imagers is described. The method involves annealing at 450 °C in the presence of an unpatterned aluminum film over the layer of passivation oxide of a completed device. In comparing this method to the conventional process of annealing with patterned aluminum, this method results in an order of magnitude reduction in surface generation current. It is believed that Al reacts with trace water in the chemical-vapor-deposition oxide and generates active hydrogen. Hydrogen diffuses through the oxide and polycrystalline-silicon gate to the Si/SiO2 interface and passivates the interface states.
Time-resolved temperature measurements during rapid solidification of Si-As alloys induced by pulsed-laser melting
Authors: Kittl J. A. , Reitano R. , Aziz M. J. , Brunco D. P. , Thompson M. O.
Published: July 2002

The solidification of Si-As alloys induced by pulsed-laser melting was studied at regrowth velocities where the partition coefficient is close to unity. The congruent melting temperatures T0 of Si-As alloys were determined using a temperature measurement technique developed for this work, and was confirmed with T0 measurements using three other methods. The time-resolved temperature measurement uses a thin-film platinum thermistor, below and electrically isolated from the Si-As alloy layer, to directly measure the temperature during solidification. The other techniques compared the results of heat flow simulations with the fluence dependence of the peak melt depth obtained by transient conductance, the fluence dependence of the melt duration determined from time-resolved reflectivity and transient conductance, and the fluence threshold for the initiation of melting. This combination of measurements in conjunction with Rutherford backscattering spectrometry permitted the determination of the solid-liquid interface temperature, velocity and partition coefficient, the latent heat of fusion and T0 for Si-4.5 at. % As and Si-9 at. % As alloys. The values of T0 determined by all four independent methods were consistent, indicating overall agreement between the direct experimental measurements and the analyses based on heat flow simulations. T0 was determined to be 1565±25 K for 4.5 at. % As and 1425±25 K for 9 at. % As. In addition, the enthalpy of fusion was determined to be independent of composition for the range studied. The values obtained in this work are compared with previous measurements.
Small-angle neutron-scattering studies of Ce(Fe1−xAlx)2
Authors: Yang Y. S. , Gaulin B. D. , Fernandez-Baca J. A. , Ali Naushad , Wignall G. D.
Published: July 2002

Ce(Fe1−xAlx)2, with x=0, 0.06, and 0.14, has been studied by small-angle neutron scattering in order to investigate the instability of the ferromagnetic state in the pure material to small amounts of magnetic disorder. The pure material (x=0) displays a ferromagnetic transition near TC=225 K, and more complicated behavior below ∼100 K. The disordered alloys, x=0.06 and x=0.14, display ferromagnetic-like states over relatively narrow temperature ranges, and an additional low-temperature phase. The ferromagnetic-like state in the x=0.14 alloy appears to be qualitatively different from that exhibited by the other two. High-wave-vector-resolution measurements on the x=0.06 sample indicate that its low-temperature ordered phase is ferrimagnetic in origin.
Valence band structures of heavily doped strained GexSi1−x layers
Authors: Fu Y. , Jain S. C. , Willander M. , Loferski J. J.
Published: July 2002

Valence-band structure and density-of-state (DOS) effective mass are calculated for heavily p-type-doped Si and strained GexSi1−x layers grown on (100) Si. At low doping values calculations have been made by earlier authors and the results agree with the published values of DOS mass by these authors. The Fermi energy EF measured from the valence-band edge has been calculated using our values of DOS effective mass. At 1020 cm−3 doping level the value is 77.3 meV compared with 80 meV determined from luminescence experiments. The value of EF obtained by using the normally accepted value of 0.57 for the hole effective mass in Si is more than the experimental value by a factor about 2, i.e., the calculations reduce the discrepancy from 100% to less than 4%. The effective barrier height of (p+-Ge0.42Si0.58)/(p-Si) internal photoemission photo detector has also been calculated using our calculated value of the DOS hole mass. The calculated value is in reasonable agreement with the experimental value. The values of the band parameters and of deformation potentials are known only for small values of k and energies. These values have been used at large doping concentrations where k and energy are also large. The agreement of the theory with experiment suggests that the values of the parameters are valid approximately at higher energies also.
A microstructural study of porous silicon
Authors: Berbezier I. , Halimaoui A.
Published: July 2002

Porous silicon (PS) films obtained from lightly and heavily doped p-type silicon have been investigated by transmission electron microscopy. Silicon crystallites with nanometric dimensions have been evidenced in both types of porous silicon layers. High resolution observations revealed lattice disorder even for low-porosity (65%) samples. Concerning PS layers obtained from lightly doped substrates, it is shown that increasing porosity leads to a crystallite size reduction and to the deterioration of the material crystallinity. When the porosity is increased up to a value of about 85%, silicon crystallites with a mean diameter of less than 3 nm and an amorphous phase are clearly imaged.
β-FeSi2 in (111)Si and in (001) Si formed by ion-beam synthesis
Authors: Oostra D. J. , Bulle-Lieuwma C. W. T. , Vandenhoudt D. E. W. , Felten F. , Jans J. C.
Published: July 2002

Ion-beam synthesis of β-FeSi2 is demonstrated both in (111) Si and (001) Si substrates by 450 keV Fe ion implantation at elevated temperatures using a dose of 6×1017 Fe/cm2 and subsequent annealing at 900 °C. The structure of the buried layers has been analyzed using Rutherford backscattering spectrometry, x-ray diffraction, and (cross-section) transmission electron microscopy. In (111) Si an epitaxial layer is formed consisting of grains with lateral dimensions of approximately 5 μm. Epitaxy of β-FeSi2 (110) and/or (101) planes parallel to the (111) Si substrate plane is observed. In (001) Si a layer is formed consisting of grains with lateral dimensions of typically 0.5 μm. Several grain orientations have been observed in this material, among others β-FeSi2 {320}, {103}, and {13,7,0} parallel to (001) Si. Selected (111) Si samples were investigated optically using spectroscopic ellipsometry, and near-infrared transmittance and reflectance spectroscopy. The results confirm that the β-FeSi2 layer has an optical band gap of 0.87 eV. The ellipsometry results indicate that the layers formed by ion-beam synthesis are more dense than those formed by surface growth techniques. Hall measurements show that the β-FeSi2 layers obtained are p type. Mobilities observed are 1–4 cm2/V s at room temperature and approximately 25 cm2/V s at liquid-nitrogen temperature. These results show that the electrical properties of ion-beam-synthesized β-FeSi2 is comparable with those of surface-grown material. The results confirm that optoelectronic applications of β-FeSi2 are limited.
A proposal of nanoscale devices based on atom/molecule switching
Authors: Wada Yasuo , Uda Tsuyoshi , Lutwyche Mark , Kondo Seiichi , Heike Seiji
Published: July 2002

This paper proposes a very small switching device, called an atom relay, which would supersede present metal-oxide-semiconductor devices for the next decade. The basic configuration of an atom relay consists of an atom wire, a switching atom, and a switching gate, with total dimensions below 10 nm, and an operation speed at more than terahertz level. The operation principle of the atom relay is that a switching atom is displaced from the atom wire by the electric field supplied from the switching gate, and the atom relay exhibits an ‘‘off’’ state. The switching characteristics of the atom relay are demonstrated by simulation, and it is shown that the electron propagation is successfully cut if a gap of about 0.4 nm is formed in the atom wire by the displacement of the switching atom. A self-relay structure, in which the switching atom is displaced by the electric field from the atom wire itself, enables a dynamic memory cell, and the functions are ascertained by simulation. Fundamental logic circuits as NAND and NOR gate constitutions are also proposed. These logic and memory circuits can integrate a supercomputer in a 200-μm-square of area, with 107 gates of logic circuit and 109 bits of memory, and operate at more than 1012 Hz. The atom relay is evaluated on the basis of the characteristics necessary for integrated circuit devices, together with several nanoscale devices, and is found to be the most promising candidate device for future integrated circuits.
Micrograin structure influence on electrical characteristics of sputtered indium tin oxide films
Authors: Higuchi Masatoshi , Uekusa Shinichiro , Nakano Ryotaro , Yokogawa Kazuhiko
Published: July 2002

The relationship between micrograin structures and electrical characteristics of sputtered indium tin oxide (ITO) films was investigated. Micrograin structures were observed by a high resolution scanning electron microscope. Electrical characteristics were evaluated by four point probe resistance measurement and Hall effect measurement. Low resistivity ITO films had domain structures. One domain consisted of many sputter grains having the same orientation. The resistivity decreased with increasing domain size. The domain boundary might cause scattering for conduction electrons. Therefore, larger domain ITO films had a higher Hall mobility. The minimum resistivity was 1.8×10−4 Ω cm, deposited at a sputtering voltage of −250 V and a 250 °C deposition temperature. The electron conduction mechanism in domain structured ITO films was taken into consideration.
A new figure of merit and methodology for quantitatively determining defect resolution capabilities in deep level transient spectroscopy analysis
Authors: Doolittle W. A. , Rohatgi A.
Published: July 2002

This article provides guidelines for accurately comparing the defect resolution capabilities of different deep level transient spectroscopy (DLTS) analysis techniques. Additionally, guidelines for choosing measurement parameters such as the proper weighting times and temperature range in order to achieve maximum energy resolution are presented. The modulation function method is compared to the correlation method of linear predictive modeling (CMLPM) transient analysis technique, and conventional DLTS spectrum methods. It is found that for extremely high signal to noise ratio transients (high trap concentration), the modulation function method is superior to all other methods. As the signal to noise ratio is reduced to a more practical range, CMLPM resolves closely spaced defect levels better than any other method examined. For very low signal to noise ratios, the effects of noise can only be overcome by the inherent averaging present in the more conventional DLTS spectrum analyses. To demonstrate the applicability of this figure of merit, a discussion of the resolution problems involved in distinguishing between simultaneously present Ti and Mo impurities in Si is also presented. Finally, CMLPM is also found to be superior to the inverse laplace transform technique.
Estimation of the spontaneous emission factor for microdisk lasers via the approximation of whispering gallery modes
Authors: Chin Mee Koy , Chu Daniel Y. , Ho Seng-Tiong
Published: July 2002

Microdisk structures have been used to achieve low-threshold lasing. For these microcavity lasers, the spontaneous emission coupling factor β is an important parameter since it determines the threshold current of the laser. Theoretical calculation of β based on the exact solution of the modes in a microdisk is complicated. A simple, approximate method for solving the waveguide modes and the density of states is developed here, using conformal transformation and the Wentzel–Kramers–Brillouin approximation and taking into account the effect of the disk thickness. We find that the β value for a microdisk laser is smaller than that of an ideal laser that has a cylindrical waveguide structure with a strong index guiding. A considerably high value of β can still be achieved, however, in a microdisk laser.
Electrical studies of charge build-up and phonon-assisted tunneling in double-barrier materials with very thick spacer layers
Authors: Goodings C. J. , Mizuta H. , Cleaver J. R. A.
Published: July 2002

Double barrier heterostructure materials containing very thick, low-doped layers adjacent to the barriers have been studied. In particular, charge build-up in the quantum well has been investigated along with the effects on this of a magnetic field perpendicular to the barriers. Phonon-assisted tunneling has also been observed from both localized and delocalized states in the emitter.
Determination of para- and ferromagnetic components of magnetization and magnetoresistance of granular Co/Ag films (invited)
Authors: Stearns Mary Beth , Cheng Yuanda
Published: July 2002

Several series of thin films (∼3000 Å) were fabricated by coevaporation of Co and Ag in a dual e-beam UHV system at substrate temperatures, TS, between 300 and 500 K. The composition was varied between 23 and 54 at. % Co. The maximum measured magnetoresistance was 31% at room and 67% at nitrogen temperature. Magnetization and Rutherford backscattering measurements showed that the Co and Ag atoms are completely segregated for films deposited at TS≥400 K. The magnetoresistance (MR) and magnetization curves have been analyzed taking into account both the paramagnetic (PM) and ferromagnetic (FM) contributions that are observed to be present. The films having ≤32 at. % Co are entirely paramagnetic. The fraction of Co atoms in the ferromagnetic component increases monotonically with increasing TS and/or at. % Co. The average radii of the PM granules in the films having an MR effect of 25%–31% are ∼20–22 Å with a minimum average separation of ∼10 Å. The large MR values attained in both the PM and FM components are attributed to the small effective domain sizes, ∼40–50 Å. The MR values are large because the MR effect varies inversely as the mean-free-path for magnetic boundary scattering which is the average distance between domain boundaries. The variation of the resistive and magnetic behavior of the films was also studied as a function of the annealing time for an annealing temperature of 300 °C. A striking behavior seen in these films is the long relaxation times when a FM component is present.
Synthesis and properties of α″-Fe16N2 in magnetic particles
Authors: Bao Xiaohua , Metzger Robert M. , Carbucicchio Massimo
Published: July 2002

The α″-Fe16N2 phase was synthesized by a nitriding, quenching, and tempering process starting from Fe2O3. Acicular γ-Fe2O3 was first reduced to α-Fe under H2, then it was converted to γ-austenite at 650–700 °C by nitriding using a NH3-H2 mixed gas. A martensitic transformation to α′-martensite occurred when the sample was quenched in liquid nitrogen. Finally, the α″-Fe16N2 phase was formed from the α′-martensite by tempering in the temperature range 120–200 °C. In order to increase the extent of the martensitic transformation, acicular Fe2O3 particles with 10–15 at % MnO2 were prepared by oxidizing Fe++ and Mn++ in alkaline aqueous solution. When a quenched sample, which contains martensite, α-Fe, and austenite, was tempered at 200 °C for different times, the magnetic moment first increased (transformation of α′-martensite to α″-Fe16N2); the highest magnetic moment was obtained at about 60 min of tempering. Using even longer tempering times, the magnetic moment decreased (decomposition of α″-Fe16N2 to α-Fe and γ′-Fe4N). Only a very small amount of γ′-Fe4N was formed after tempering at 120 °C for one week. The average coercivity of the samples nitrided at 650–700 °C is about 200–300 Oe; the sample magnetization is about 170 emu g−1. Mössbauer spectroscopy and x-ray diffraction confirm the presence of α″-Fe16N2. The estimated mole fraction of α″-Fe16N2 (which should be called more simply Fe8N) is 0.30–0.40. The low coercivity of the final product indicates that sintering of the acicular particles occurred, presumably because the silica protective coating of the particles did not resist the high temperatures needed to form austenite.
Magnetotransport and electron subband studies of edge delta-doped Al0.27Ga0.73As/GaAs single quantum wells
Authors: Kim T. W. , Yoo K-H. , Lee K-S. , Kim Y. , Min S-K. , Yom S. S. , Lee S. J.
Published: July 2002

Shubnikov–de Haas and Van der Pauw Hall effect measurements at 1.5 K have been carried out to investigate the existence of a two-dimensional electron gas and to determine subband energies in a Si-delta-doped Al0.27Ga0.73As/GaAs single quantum well. The fast Fourier transformation results for the S-dH data indicate clearly the occupation of two subbands in edge delta-doped Al0.27Ga0.73As/GaAs quantum wells. Capacitance-voltage profiling and temperature-dependent photoluminescence measurements have been performed to characterize the properties of edge delta-doped Al0.27Ga0.73As/GaAs quantum wells. Using these experimental results and a self-consistent numerical method which took into account the exchange-correlation effects, the electron subband energies were determined. These results indicate that edge delta-doped Al0.27Ga0.73As/GaAs single quantum wells are similar to the asymmetrical potential wells occupied by relatively high electron carrier densities.
Modeling of hardening at very high strain rates
Authors: Bodner S. R. , Rubin M. B.
Published: July 2002

A modification of the Bodner–Partom elastic-viscoplastic constitutive model is proposed to account for strain rate dependence of the evolution of hardening. The suggested procedure is for the rate of hardening in the hardening evolution equation to be a direct function of total strain rate. Material constants are determined for oxygen-free-electronic copper and simulations of response behavior at very high rates of straining appear to be consistent with test observations.
Response linearity of Nb tunnel junction detectors for photon energies from 1.5 to 6.4 keV
Authors: Rando N. , Peacock A. , van Dordrecht A. , Hübner P. , Videler P. , Salmi J. , Suni I.
Published: July 2002

Recent experimental results show a linear energy response in high quality Nb-Al-AlOx-Nb superconducting tunnel junction detectors for photon energies between 1.5 and 6.4 keV. The experimental data are based on both direct x-ray illumination and on the escape and re-absorption of fluorescent photons created in the junction electrodes and in the silicon substrate. The observed linearity of the energy response raises questions on the validity of some theoretical models which describe the relaxation process occurring in a superconducting thin film after x-ray photoabsorption. Such models generally predict nonlinear effects due to large quasiparticle number densities and short recombination times.
Silicide formation in Co/amorphous Si multilayers
Authors: Wang W. H. , Wang W. K.
Published: July 2002

We studied interfacial reactions in Co/amorphous Si(a-Si) multilayers by transmission electron microscopy. We found that an intermixed layer of amorphous cobalt silicide formed in the as-deposited state. To explain the solid-state amorphization reaction, two parameters were used. They were the thermodynamic driving force (heat of formation) and the interfacial energy. The initial amorphization reaction in Co/a-Si multilayers was thermodynamically and kinetically favored. However, the formed amorphous interlayer remained about 1 nm thick and did not grow thicker with increasing modulation period and annealing temperature. The reason for this phenomenon was that the amorphous interlayer acted as a diffusion barrier to impede the amorphization reaction in Co/a-Si multilayers. Co2Si phase was always the preferred phase in the crystallization process for different average compositions of the multilayers. The mechanism that controlled the phase selection in Co/a-Si interfacial reaction was interpreted by using the model of modified heat of formation.
Heterojunction bipolar transistors in AlGaInP/GaAs grown by metalorganic vapor phase epitaxy
Authors: Yow H. K. , Houston P. A. , Button C. C. , Lee T.-W. , Roberts J. S.
Published: July 2002

A series of low-pressure metalorganic vapor phase epitaxy grown AlxGa0.52−xIn0.48P/GaAs single heterojunction bipolar transistors with x=0, 0.18, 0.30, 0.40, and 0.52 have been studied. These devices consistently exhibit near-ideal characteristics with base and collector current ideality factors close to unity except in the ungraded AlInP case. A low conduction band spike height which is believed to be due to unintentional grading at the emitter-base junction of the partially graded devices (intentionally graded down close to the Ga0.52In0.48P lattice-matched composition) provides effective hole current suppression over the whole range of compositions. Base bulk recombination current was determined to be the current-gain limiting mechanism for x≤0.3 in this heterojunction bipolar transistors (HBT) system. The common-emitter dc current gain showed a maximum value of 380 at JC=4×103 A/cm2 for x=0.18 which translates to a minority electron diffusion length of 1.1 μm. These are the highest values reported for a base sheet resistance of 440 Ω/⧠.
Effect of magnetization profiles on the torque of magnetic coupling
Authors: Huang Der-Ray , Chiou Gwo-Ji , Yao Yeong-Der , Wang Shyh-Jier
Published: July 2002

The torque of magnetic coupling is sensitive to the number of magnetic poles, the magnetization profiles, the magnetic field strength of magnets, and the separation distance between the magnets of the coupling. For different multipole magnetic couplings with the same magnetic field strength, the torque of magnetic coupling is proportional to the number of magnetic poles at small separation distance, but it is inversely proportional to the number of magnetic poles as the separation distance becomes large. The theoretically calculated values of the torque of magnetic coupling are in excellent agreement with the experimentally measured values.
Local and nonlocal density functional studies of FeCr
Authors: Singh David J.
Published: July 2002

Local spin density approximation and generalized gradient approximation calculations are reported for ferromagnetic B2 FeCr. Both shear elastic constants are found to be positive, indicating metastability of the B2 structure. Both the moments of the Fe and Cr layers are ferromagnetically aligned, with the Fe moment being considerably depressed. This is in contrast to the behavior for thicker films. The evolution of the behavior as a function of the layer thickness is addressed using supercell calculations. It is found that Fe monolayers in bcc Cr are ferromagnetically aligned with the neighboring Cr moments, while thicker Fe layers are antiferromagnetically aligned with the adjacent Cr layers.
Mechanism of negative-bias temperature instability in polycrystalline-silicon thin film transistors
Authors: Maeda S. , Maegawa S. , Ipposhi T. , Nishimura H. , Ichiki T. , Mitsuhashi J. , Ashida M. , Muragishi T. , Inoue Y. , Nishimura T.
Published: July 2002

It is found that 0.1 V-order threshold voltage shift (Vth shift) takes place in polycrystalline-silicon thin film transistors during negative-bias temperature stress (−BT stress), while the Vth shift in the case of positive-bias temperature stress is negligibly small. The Vth shift caused by −BT stress has an exponential dependence on the stress gate bias and reciprocal of temperature. Moreover, it also has a close relation with the grain size of poly-Si films and the hydrogenation process. However, it is independent of the gate insulator materials. Some models previously proposed for amorphous silicon TFTs could not explain these results. A new model is proposed based on a reaction between hydrogen and the SiO2 network at and near the poly-Si/SiO2 interface to clarify the mechanism and for consistent interpretation of the experimental results. Furthermore, the model has been verified qualitatively.
Lower temperature crystallization and ordering in sol-gel derived Pb(Sc0.5Ta0.5)O3 powders and thin layers
Authors: Liu Donhang , Payne David A.
Published: July 2002

Lead scandium tantalate was prepared by a polymeric sol-gel method, and the crystallization into the perovskite phase was determined at significantly reduced temperatures (600–700 °C). The extent of B-site cation ordering was also determined as a function of cooling conditions. Results are reported for the integration of capacitors on silicon with a capacitance density greater than 200 fF/μm2. A preferred (111) orientation was obtained, with (1/21/21/2) B-site cation partial ordering. No evidence was obtained for A-site Pb vacancy ordering. Results are reported for x-ray diffraction and selected-area electron diffraction investigations.
Characteristics of a GaAs-InGaAs quantum-well resonant-tunneling switch
Authors: Guo Der-Feng , Laih Lih-Wen , Tsai Jung-Hui , Liu Wen-Chau , Hsu Wei-Chou
Published: July 2002

A switching device, with a p-type delta-doped sheet in the center of an InGaAs-GaAs quantum well, has been fabricated and demonstrated. An N-shaped negative-differential-resistance phenomenon resulting from the resonant-tunneling effect through the miniband is observed in the current-voltage measurement. From the experimental results, it is seen that the temperature plays an important role in device operation. The influences of temperature upon the peak-current voltage, valley-current voltage, peak-current density, and valley-current density are studied and discussed.
Noise performance of bound-to-miniband transition III-V quantum-well infrared photodetectors
Authors: Wang Daniel C. , Bosman Gijs , Wang Yeng H. , Li Sheng S.
Published: July 2002

Dark current noise measurements between 10 and 105 Hz were carried out at T=77 K on three different types of III-V quantum-well infrared photodetectors designed for 8–12 μm IR detection. These devices have superlattice barriers leading to miniband transport in the extended conduction band. For frequencies between 102 and 104 Hz, noise plateau levels stemming from the trapping and detrapping of electrons in the quantum-well bound states are observed. From the measured noise data the low-bias electron diffusion length, the bias-dependent noise gain, the electron trapping probability, and the thermal electron generation rate are calculated.
Evidence of type-II band alignment at the ordered GaInP to GaAs heterointerface
Authors: Liu Q. , Derksen S. , Lindner A. , Scheffer F. , Prost W. , Tegude F.-J.
Published: July 2002

Interfacial characteristics of Ga0.51In0.49P/GaAs heterostructures grown by metal-organic vapor-phase epitaxy in the temperature range from 600 °C to 730 °C were studied. Photoluminescence (PL) measurements have been used for this purpose. A PL peak with an energy of about 1.425 eV (870 nm) was continuously observed in samples containing the GaInP-to-GaAs interface. Excitation power dependent PL measurements show that this peak belongs to an excitonic recombination. Furthermore, a strong blue-shift of this PL-peak energy was observed as the excitation power increased. We attribute the 870 nm peak to the radiative recombination of spatially separated electron-hole pairs and suggest the type-II band alignment at the ordered GaInP to GaAs heterointerface under growth conditions reported here. Further investigations using x-ray diffraction measurements and simulations with dynamical theory show that the lower and upper interfaces are not equivalent. This explains the absence of type-II transition in most GaAs-to-GaInP lower interfaces.
Phase stability and transformations in pure and lanthanum modified lead zirconate ceramics
Authors: Dai Xunhu , Xu Z. , Viehland Dwight
Published: July 2002

Lanthanum modified lead zirconate with La to 8 at. % were prepared using mixed oxide method and followed by hot press sintering. The basic characterization included: dielectric spectroscopy, Sawyer–Tower polarization measurements, and temperature dependent selected area electron diffraction techniques. An intermediate rhombohedral ferroelectric phase (FER), characterized by square hysteresis loops, was found to exist in lead zirconate below the dielectric maximum. Strong 1/2{110} superlattice reflections associated with M-type oxygen octahedral rotation is found in the intermediate rhombohedral ferroelectric state. In addition, 1/x{110} incommensurate reflections were observed within a narrow temperature range between the FER phase and the low-temperature orthorhombic antiferroelectric state. With increasing La content, the temperature stability range of the incommensurate antiferroelectric phase was increased, at the expense of the FER phase. Incommensuration is suggested to result from a competition between ‘‘broken’’ dipolar and antiferroelectric sublattice interactions.
Excitation dynamics of luminescence from porous silicon
Authors: Ventura P. J. , do Carmo M. C. , O’Donnell K. P.
Published: July 2002

Time-resolved luminescence of transiently excited porous silicon reveals the dynamics of the excitation-luminescence cycle in which localized excitons efficiently generate orange-red luminescence from this material at room temperature. Upon transient excitation with nanosecond pulses, both blue and orange luminescence bands appear coincidently. At long time delays, the orange luminescence dominates. We show here that the time-delayed orange luminescence is thermally activated by hopping-related processes. The activation energy for a 500 ns delay is 36 meV. Our observations are in principle capable of resolving outstanding contradictions in the porous silicon luminescence literature. Quantum confinement on a wire fluctuating width provides the strong energy disorder which facilitates exciton localization.
Generation of thermally induced defects in buried SiO2 films
Authors: Zvanut M. E. , Chen T. L. , Stahlbush R. E. , Steigerwalt E. S. , Brown G. A.
Published: July 2002

We show that annealing of the buried oxide layer used for device isolation generates point defects in the SiO2 film and that this defect generation is independent of temperature above 1000 °C. Electron paramagnetic resonance data obtained on thermally grown buried oxides and those fabricated by ion implantation indicate that the defect is intrinsic to the structure of SiO2 and is associated with an oxygen deficient environment. The similarity in the generation of the defects studied here and the formation of SiO observed in earlier studies of low pressure high temperature oxidation suggests that the formation of the buried oxide defect is related to the reduction of SiO2 and the release of SiO.
Stoichiometric low-temperature GaAs and AlGaAs: A reflection high-energy electron-diffraction study
Authors: Missous M.
Published: July 2002

By careful control of the arsenic beam supply during the growth of GaAs and Al0.42Ga0.56As at low temperatures (∼200 °C), very strong and sustained reflection high-energy electron-diffraction (RHEED) oscillations have been observed. Both the period and intensity of the RHEED oscillations are shown to be a strong function of the arsenic overpressure with the former increasing with increase arsenic supply, reflecting a decrease in the number of atoms taking part in the two-dimensional (2D) growth mode, and the latter decreasing with increasing arsenic supply, reflecting the creation of a barrier to 2D growth by the excess arsenic. Under exact stoichiometric conditions, the quality of the GaAs and AlGaAs is comparable to those grown at high temperatures. It is therefore surmized that nonstoichiometry in low-temperature-grown GaAs can be overcome leading to the growth of stoichiometric low temperature materials possessing properties similar to those of conventional high-temperature-grown layers.
Franz–Keldysh oscillations in modulation spectroscopy
Authors: Shen H. , Dutta M.
Published: July 2002

In the presence of an electric field, the dielectric constant of a semiconductor exhibits Franz–Keldysh oscillations (FKO), which can be detected by modulated reflectance. Although it could be a powerful and simple method to study the electric fields/charge distributions in various semiconductor structures, in the past it has proven to be more complex. This is due to nonuniform fields and impurity induced broadening, which reduce the number of detectible Franz–Keldysh oscillations, and introduce uncertainties into the measurement. In 1989, a new structure, surface–undoped–doped (s-i-n+/s-i-p+) was developed, which allows the observation of a large number of FKOs and, hence, permitting accurate determination of electric fields. We present a review of the work on measuring electric fields in semiconductors with a particular emphasis on microstructures using the specialized layer sequence. We first discuss the general theory of modulation techniques dwelling on the approximations and their relevance. The case of uniform field, obtained with this specialized structure as well as that of the nonuniform field, are addressed. The various experimental techniques are also briefly reviewed. We then summarize the various experimental results obtained in the last few years using these special structures and FKOs and find that, even in this short period, good use has been made of the technique and the structure. This is followed by a brief review of the work on nonuniform fields. In this case, the work on actual device structures has significant technological implications. Important issues such as metallization and processing, the effects of surface treatment and thermal annealing, Schottky barrier heights of different metals, piezoelectric fields in (111) grown strained InGaAs/GaAs quantum wells, and Fermi level in low-temperature grown GaAs have been studied using this structure. This structure has also been used to study the dynamics of photomodulation, revealing the nature of the cw photoreflectance.