bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of the lattice strain and chemical composition of semiconductor heterostructures by high-resolution x-ray diffraction
Authors: De Caro Liberato , Giannini Cinzia , Tapfer Leander
Published: July 2002

In order to determine the strain field of highly mismatched semiconductor heterostructures by high-resolution x-ray diffraction with high accuracy, we derived a new second-order approximation of the incidence parameter considering an arbitrary lattice deformation. Our calculations show that, almost independently from the substrate orientation and the considered material system, for low Miller-index reflections a lattice mismatch greater than 0.004 is the value for which quadratic corrections must be considered. The quadratic approximation increases the range of validity by one order of magnitude, i.e., to a lattice mismatch up to 4%. In addition, the analytical expression which relates the strain components measured by x-ray diffraction to the lattice mismatch is derived for semiconductor epitaxial layers grown on arbitrarily oriented substrate crystals. Using Vegard’s rule, our formula allows us to determine the chemical composition of ternary compounds even for low-symmetry substrate orientations. We show that in this case shear strain components have a non-negligible weight in the determination of the chemical composition of ternary compounds. Several examples considering III–V, II–VI, and IV–IV semiconductor material systems are reported and discussed.
Dipole-exchange spin wave spectra of exchange-coupled magnetic multilayers calculated by transfer matrix formalism
Authors: Rojdestvenski I. V. , Cottam M. G. , Slavin A. N.
Published: July 2002

We employ a transfer matrix formalism to study dipole-exchange spin waves in a perpendicularly magnetized multilayered structure consisting of identical magnetic layers alternated by nonmagnetic spacers, assuming both intralayer and interlayer exchange interactions. The interlayer exchange and the anisotropies at the magnetic layer boundaries are taken into account via linearized Hoffman boundary conditions. The bulk-mode bands for the case of infinite multilayers and the bulk and surface modes for semi-infinite multilayers are derived. Applications are made to recent ferromagnetic resonance data for Co/Pt multilayers. The extension of our theory to finite multilayers is discussed.
Free excitonic transitions in GaN, grown by metal-organic chemical-vapor deposition
Authors: Smith M. , Chen G. D. , Lin J. Y. , Jiang H. X. , Khan M. Asif , Sun C. J. , Chen Q. , Yang J. W.
Published: July 2002

The dynamic processes of the free excitonic transitions in GaN grown by metal-organic chemical-vapor deposition (MOCVD) have been studied. The recombination lifetimes of the A and B excitons have been measured at different temperatures and excitation intensities, from which radiative recombination lifetimes of about 0.35 and 0.3 ns for the A and B excitons, respectively, have been obtained. An increase in excitation power has resulted in a drastic enhancement in the radiative decay rate as well as in the exciton photoluminescence quantum yield, suggesting the excitonic transitions may provide gain for laser actions in GaN. The high quality as well as high purity of the investigated MOCVD sample has been demonstrated by the observations of (1) the free A- and B-excitonic transitions, (2) excited states of the free excitons, (3) narrow free excitonic emission linewidths (1.7 meV at 10 K), (4) low electron concentration, and (5) high electron mobilities (∼600 cm2/V s).
Damage in ion implanted silicon measured by x-ray diffraction
Authors: Milita S. , Servidori M.
Published: July 2002

The x-ray rocking curve analysis was used to investigate damage accumulation with increasing dose in silicon implanted with 50 keV and 1 MeV 11B+ ions, and 50 keV, 180 keV, and 0.7 MeV 28Si+ ions. The damage buildup was studied by following the maximum value of lattice strain normal to the surface (ε⊥) and the depth integral of ε⊥. The ε⊥ profiles were obtained with a dynamical diffraction formalism by using automated best fits to experimental x-ray rocking curves. It is shown that, for doses below the amorphization limit, the damage increases sublinearly with dose and not very differently for B and Si ions. The sublinearity results from intercascade recombination of point defects produced under bombardment. The small differences, if real, in the sublinearities observed for the two ions could be explained by the different forms of aggregation to which the surviving defects evolve when the dose approaches the amorphization threshold. However, the study of damage growth must stop at the upper dose at which a continuous buried amorphous layer begins to form. In fact, starting from this dose, a simple semikinematical diffraction model shows that the determination of a unique peak value of ε⊥ depth profile, and hence of its integral, is not possible. This is the consequence of the fact that the sample behaves as an x-ray interferometer when an embedded amorphous layer is present. The analysis reported here is compared with similar studies recently published in the literature.
Quasineutrality in semiconductors
Authors: in ’t Hout S. R.
Published: July 2002

A general analysis of the semiconductor transport equations is performed in order to establish under which conditions the often used, but rarely verified, quasineutrality assumption of negligible space charge in the so-called ‘‘neutral regions’’ in a semiconductor holds. The small-signal deviation from exact neutrality is calculated for all four nonequilibrium states, that is, injection, accumulation, extraction and exclusion, for one-dimensional current flow in a semi-infinite homogeneous semiconductor. It is shown how the deviation from neutrality depends on the material parameters. Contrary to injection and exclusion, the small-signal deviation from neutrality for accumulation and extraction depends strongly on the current density. A range of current densities for which quasineutrality holds is given. Significant new results are obtained for the intermediate- and large-signal regime. For high-level injection and accumulation in n-type material, quasineutrality is shown to hold whenever small-signal quasineutrality holds at the same material parameters and current density. For high-level extraction, the deviation from neutrality is found to increase compared to the small-signal case. The corresponding current density range for which quasineutrality holds is given. The high-level exclusion case is still an open issue.
Effects of bar length on switching field of nanoscale nickel and cobalt bars fabricated using lithography
Authors: Kong Linshu , Chou Stephen Y.
Published: July 2002

The switching behavior of isolated nanoscale nickel and cobalt bars, which were fabricated using electron-beam lithography, was studied as a function of bar length. The bars have a 35 nm thickness, a 100 nm width, and a length varying from 200 nm to 5 μm. Magnetic force microscopy showed that except for the Ni bars with a length equal to or less than 250 nm, all other as-fabricated bars were single domain. Unlike the bar width dependence, the switching field of the single-domain bars was found to first increase with the bar length, then decrease after reaching a peak. The peak switching field and the corresponding bar length are 640 Oe and 1 μm for Ni and 1250 Oe and 2 μm for Co, respectively. The nonmonotonic length dependence suggests that the magnetization switching may be quasicoherent in the short bars and incoherent in the long bars, and that the exchange coupling is much stronger in Co bars than in Ni bars. Furthermore, the switching field of 1-μm-long Co bars was found to increase monotonically as the bar width decreases, reaching 3000 Oe at a 30 nm width.
Kinetic Ising system in an oscillating external field: Stochastic resonance and residence-time distributions
Authors: Sides S. W. , Ramos R. A. , Rikvold P. A. , Novotny M. A.
Published: July 2002

Experimental, analytical, and numerical results suggest that the mechanism by which a uniaxial single-domain ferromagnet switches after sudden field reversal depends on the field magnitude and the system size. Here we report new results on how these distinct decay mechanisms influence hysteresis in a two-dimensional nearest-neighbor kinetic Ising model. We present theoretical predictions supported by numerical simulations for the frequency dependence of the probability distributions for the hysteresis-loop area and the period-averaged magnetization, and for the residence-time distributions. The latter suggest evidence of stochastic resonance for small systems in moderately weak oscillating fields.
Colossal magnetoresistance of the variable range hopping regime in the manganites
Authors: Viret M. , Ranno L. , Coey J. M. D.
Published: July 2002

Evidence for variable range hopping (VRH) is presented in resistivity measurements in thin films and bulk samples of different substituted x=0.3 manganese perovskites. The necessity to modify the Mott VRH model is shown and we have developed a model based on magnetic localization hopping, which is used to reproduce temperature and field dependence of resistivity in these mixed-valence compounds.
Enhancement of resonant tunneling current at room temperature
Authors: Kim Gyungock , Roh Dong-Wan , Paek Seung Won
Published: July 2002

Enhancement of the resonant peak current through the ground quantum-well state at room temperature is observed at a low voltage by adding the second quantum-well structure to the AlAs/GaAs (001) double-barrier single quantum-well heterostructure. The peak to valley current ratio increases slightly with the increasing temperature up to room temperature, and shows stable characteristics up to high temperature in this AlAs/GaAs (001) triple barrier heterostructure. The results indicate that the optimum alignment of the Fermi level with the lined-up quantum-well states can greatly improve the resonant peak current in the low voltage range, and therefore device characteristics.
Modeling of three-peak current–voltage characteristics with two resonant tunneling diodes connected in series
Authors: Gan Kwang-Jow , Su Yan-Kuin , Wang Ruey-Lue
Published: July 2002

The conditions for three-peak current–voltage (I–V) characteristics with two discrete resonant tunneling diodes (RTDs) connected in series are discussed and analyzed. With suitable design, one can obtain three-peak I–V characteristics with two series-connected RTDs, and some equations are derived to help the design. The effect of parasitic resistance on the I–V characteristics is investigated. A large-signal model of the RTD is developed by using the piecewise-linear approximation technique, and this model can be easily implemented in PSpice. The simulated I–V curve of the series circuit is in good agreement with the measured results. The results in this paper can be applied to the design of multipeak I–V curve with less RTD structure in comparison with the traditional device that stacks the same RTDs to obtain a multipeak I–V curve.
Polarization switching in ferroelectric cathodes
Authors: Rosenman G. , Shur D. , Garb Kh. , Cohen R. , Krasik Ya. E.
Published: July 2002

A new mechanism of polarization switching and electron emission in ferroelectric cathodes is proposed. Surface flashover plasma of a ferroelectric origin was observed on a polar ferroelectric surface [D. Shur, G. Rosenman, and Ya. E. Krasik, Appl. Phys. Lett. 70, 574 (1997)]. Simultaneous measurements of switched charge and plasma density show that expanding surface plasma represents a dynamic switching electrode. Direct measurements of ion/electron emission currents and surface analysis implemented by different analytic tools indicate that electrons and ions from the surface plasma contribute to spontaneous polarization screening. The high energy of charged particles emitted from the surface plasma is ascribed to a high surface potential during polarization switching.
X-ray photoemission studies and energy-band diagrams of (In,Se)–CuInSe2/SnO2 heterostructures
Authors: Massé G. , Djessas K.
Published: July 2002

Possible photovoltaic effects in CuInSe2(and related materials)/SnO2 thin film structures are discussed with the CuInSe2 layer grown by close-spaced vapor transport while the growth temperature increases. This procedure can particularly induce a photovoltaic effect having the positive polarity on SnO2, making this type of structure interesting in CuInSe2-type solar cells. The discussion is supported by electron-beam induced current studies and x-ray photoelectron spectroscopy. The band offsets of SnO2–CuInSe2 and CuInSe2–(In,Se) heterojunctions were determined, and energy-band diagrams drawn. (In,Se) involves InSe or In2Se3 phases, and possibly Cu-poor phases, which form in the layer when increasing the growth temperature. Finally, the photovoltaic effect, having the positive polarity on SnO2, is ascribed to either a shallow (from SnO2) p-n CuInSe2 homojunction or a shallow p CuInSe2-n (In,Se) heterojunction. The n-type doping of CuInSe2 could be due either to diffusions during the growth or stoichiometry variations. Taking into account the heavy doping of n SnO2 and p CuInSe2, the energy-band diagrams of these shallow junctions can be compared to a Schottky diode with a barrier increased by the p-type doping.
Use of Rietveld refinement for elastic macrostrain determination and for evaluation of plastic strain history from diffraction spectra
Authors: Daymond M. R. , Bourke M. A. M. , Von Dreele R. B. , Clausen B. , Lorentzen T.
Published: July 2002

Macrostrain variations in engineering components are frequently examined using neutron diffraction, at both reactors and pulsed sources. It is desirable to minimize the sampling volume in order to maximize the spatial resolution, although this increases the required measurement time. At reactors, macrostrain behavior is inferred from a single lattice reflection (deemed to be representative of the bulk response). At a pulsed source, a complete diffraction pattern is recorded and accordingly it is natural to fit the entire diffraction spectra using a Rietveld [J. Appl. Cryst. 2, 65 (1969)] refinement. This means that an idealized crystal structure is fit to the measured distorted crystal structure, which includes deviation of the measured lattice reflections from the ideal due to elastoplastic strain anisotropies, which are dependent on the particular lattice reflection (hkl) considered. We show that elastic macrostrains calculated from lattice parameter changes in Rietveld refinements (without accounting for hkl dependent anisotropies) are almost identical to the bulk elastic response and are comparable to the response obtained from a single lattice reflection typically used by practitioners at a steady state source. Moreover good refinements on the complete pattern are obtained with short measurement times compared to what is required for good statistics for single reflections. By incorporating a description of the elastic strain anisotropy expected in cubic materials into the Rietveld code, an empirical prediction of plastic strain history is possible. The validity of these arguments is demonstrated by analysis of a uniaxial tensile load test and a reanalysis of previously reported data taken on a deformed stainless steel ring. The plastic strain predictions compare favorably with a finite element model.
Grazing incidence reflectivity and total electron yield effects in soft x-ray absorption spectroscopy
Authors: Alders D. , Hibma T. , Sawatzky G. A. , Cheung K. C. , van Dorssen G. E. , Roper M. D. , Padmore H. A. , van der Laan G. , Vogel J. , Sacchi M.
Published: July 2002

We report on a study of grazing incidence absorption and reflection spectra of NiO in the region of the Ni 2p edge. The aim is to evaluate the distortion of the near edge spectrum by the critical angle behavior of individual components within the spectrum. This can be used to improve the separation of multiplets and enhance low spectral weight line shapes like charge transfer satellites. The measured spectra have been compared with calculations using an optical model.
Comparison of two methods for describing the strain profiles in quantum dots
Authors: Pryor C. , Kim J. , Wang L. W. , Williamson A. J. , Zunger A.
Published: July 2002

The electronic structure of interfaces between lattice-mismatched semiconductors is sensitive to the strain. We compare two approaches for calculating such inhomogeneous strain—continuum elasticity [(CE), treated as a finite difference problem] and atomistic elasticity. While for small strain the two methods must agree, for the large strains that exist between lattice-mismatched III-V semiconductors (e.g., 7% for InAs/GaAs outside the linearity regime of CE) there are discrepancies. We compare the strain profile obtained by both approaches (including the approximation of the correct C2 symmetry by the C4 symmetry in the CE method) when applied to C2-symmetric InAs pyramidal dots capped by GaAs.
Dopant-sensitive luminescence spectroscopy of yttrium aluminum garnet by high-current electron pulses
Authors: Solomonov V. I. , Mikhailov S. G. , Shulgin B. V. , Krylov S. I. , Deykoon A. M.
Published: July 2002

Pulsed cathode luminescence (PCL) spectroscopy is developed to detect dopant-d elements: Cr, Mn, Co, Fe, V, and Ni in aluminum garnet crystals. PCL is excited by 2 ns electron pulses with high current density 700±100 A/cm2 and the energy of electrons 180±20 keV. In contrast to conventional spectroscopies, which require low temperatures to suppress thermal quenching and produce measurable intensity of luminescence, our method works at room temperatures. The high-current PCL spectroscopy allows us to discriminate the new lines and bands of d elements which are inactive in photo- and x-ray luminescence. A large portion of the PCL spectrum can be taken simultaneously due to the high intensity of luminescence.
Optical spectroscopy of Er3+ and Yb3+ co-doped fluoroindate glasses
Authors: Ribeiro C. T. M. , Zanatta A. R. , Nunes L. A. O. , Messaddeq Y. , Aegerter M. A.
Published: July 2002

Optical absorption, Stokes, and anti-Stokes photoluminescence were performed on Er3+–Yb3+ co-doped fluoroindate glasses. For compounds prepared with a fixed 2 mol % ErF3 concentration and YbF3 contents ranging from 0 to 8 mol %, important upconversion processes were observed as a function of temperature and photon excitation energy. Based on the experimental data, two mechanisms for the upconversion (or anti-Stokes photoluminescence) processes were identified and analyzed in detail. At high Yb contents, the upconversion mechanisms are mostly determined by the population of the F5/22 levels of Yb3+ ions (or I11/24 levels of Er3+ ions, by energy transfer) regardless of the photon excitation energy and temperature of measurement. Moreover, green and red light emission have similar intensities when a large Yb3+ content is present.
High-power continuous-wave operation of a InGaAs/AlGaAs quantum dot laser
Authors: Maximov M. V. , Shernyakov Yu. M. , Tsatsul’nikov A. F. , Lunev A. V. , Sakharov A. V. , Ustinov V. M. , Egorov A. Yu. , Zhukov A. E. , Kovsh A. R. , Kop’ev P. S. , Asryan L. V. , Alferov Zh. I. , Ledentsov N. N. , Bimberg D. , Kosogov A. O. , Werner P.
Published: July 2002

A 1 W continuous-wave laser operation via the ground state of vertically coupled InGaAs quantum dots (VCQDs) in an AlGaAs matrix is demonstrated. VCQDs are directly revealed in transmission electron microscopy images of the laser structure. Ninety-six percent internal quantum efficiency is realized. The laser gain maximum shifts significantly with drive current towards higher photon energies in agreement with the relatively broad size distribution of VCQDs.
Effect of target composition on properties of NdBa2Cu3O7−δ superconducting thin films grown by pulsed laser deposition: A comparative study
Authors: Li Yijie , Tanabe K.
Published: July 2002

High quality epitaxial NdBa2Cu3O7−δ (NBCO) superconducting thin films have been deposited on SrTiO3 (100), LaAlO3 (100), and NdGaO3 (110) substrates by pulsed laser deposition using slightly Ba-rich Nd1−xBa2+xCu3O7−δ (x=0.00, 0.03, 0.05, and 0.10) targets. The influence of target composition on the properties of thin films, including superconductivity, epitaxial structure, and surface morphology have been systematically investigated. In both four-probe and dc magnetization measurements, the Tc0 of 91–93 K is routinely achieved from targets with x=0.00, 0.03, and 0.05. The highest is 94 K for the films corresponding to the target of Nd0.97Ba2.03Cu3O7−δ. The critical current density Jc in a 1000 Å thick, 5 μm wide bridge is 3.8×106 A/cm2 at 77 K in zero magnetic field. X-ray diffraction (XRD) analysis demonstrates that the structure of films sensitively depends on the substrate deposition temperature. When the substrate deposition temperature decreases from 900 to 800 °C, the orientation of NBCO films gradually changes from pure c-axis to a-axis, which is similar to the results observed for YBa2Cu3O7−δ films. For pure c-axis oriented films deposited at 850–900 °C, the omega scan x-ray rocking curve of the (005) reflection gives the very small value (∼0.05°) of the full width at half maximum. In the films deposited from the targets with x=0.05 and 0.10, very weak peaks corresponding to BaO, BaCuO2, and Ba2CuO3 are detected by XRD, while only (123) phase is observed in the films deposited from the targets with less Ba-rich compositions. The surface morphology of the film was studied by high-resolution scanning electron microscope and atomic force microscope (AFM) in detail. It was found that the surface morphology, featuring mainly square-shaped out-growths, is sensitive to the deposition parameters, especially the substrate temperature. Smooth nearly out-growth-free surface with relatively low density (∼106/cm2) of droplets has been obtained by optimizing the deposition parameters. AFM images demonstrate that with the increase of the barium content in the target the surface morphology of films changes from very smooth to somewhat rough.
Resonant tunneling in asymmetrical double-barrier structures under an applied electric field
Authors: Miyamoto Kiyoshi , Yamamoto Hiroaki
Published: July 2002

Resonant tunneling is studied theoretically for the asymmetrical rectangular double-barrier structures with deep well when a dc bias field is applied. Analytical expressions for the transmission coefficient and the resonance condition are derived by assuming that the effect of the space charge buildup in the well is neglected. It is found that the resonant tunneling with unity transmission (unity resonance) occurs when both the phase difference condition for resonance (PDCR) and the maximum condition for peak value hold simultaneously, while occurs the under-unity resonance that the peak value of transmission coefficient at resonance is below unity when only PDCR holds. The values of transmission peak and the resonance energy are investigated by two methods, Airy function analysis and multistep rectangular approximation. Wave-functions of an electron at the resonance level are calculated and the confining phenomenon is confirmed. Moreover, the effect of the right-hand side barrier height on the transmission coefficient characteristics is investigated.
Superconducting tunnel junction base electrode planarization
Authors: Thomas C. D. , Ulmer M. P. , Ketterson J. B.
Published: July 2002

We have investigated the planarizing effects of multilayered superconducting thin films. Atomic force microscopy measurements indicate that coating a standard niobium base electrode with alternating layers of aluminum and niobium significantly reduces the film’s overall surface roughness. Planarized films such as these were used as the base electrodes of superconducting tunnel junctions that show vastly improved leakage characteristics over conventional junctions fabricated under the same conditions.
New infrared absorption bands related to interstitial oxygen in silicon
Authors: Hallberg T. , Murin L. I. , Lindström J. L. , Markevich V. P.
Published: July 2002

In addition to the well-known spectrum of Czochralski-grown silicon, two infrared bands at 560 and 648 cm−1 are found to arise from the interstitial oxygen in silicon. The assignment of the bands is performed using the studies of their oxygen isotope shift and temperature dependence. The 560 cm−1 band is attributed to the hot counterpart of the mode at 518 cm−1 and the 648 cm−1 band to the combination of the far-infrared mode at 29 cm−1 with the Raman active mode at around 600 cm−1.
Characterization of random rough surfaces by in-plane light scattering
Authors: Zhao Y.-P. , Wu Irene , Cheng C.-F. , Block Ueyn , Wang G.-C. , Lu T.-M.
Published: July 2002

The reciprocal space structures of Si backside rough surfaces were studied using an in-plane light scattering technique. A diode array detector was used to collect the angular distribution of the scattered intensity. The diffraction profiles are interpreted based on the theory of diffraction from a self-affine rough surface. Roughness parameters such as interface width, lateral correlation length, and the roughness exponent, are extracted from the profiles and are compared to that obtained by real space imaging techniques such as atomic force microscopy and stylus profilometry. Factors that limit the accuracy of light scattering measurements are discussed.
Optical characterization of AlN films grown by plasma source molecular beam epitaxy
Authors: Krupitskaya Regina Y. , Auner Gregory W.
Published: July 2002

Thin films of AlN were grown on Si(111), sapphire (11̄02), and sapphire (0001) substrates by plasma source molecular beam epitaxy (PSMBE). Optical transmission spectra of PSMBE grown AlN/sapphire show neither significant band edge distortions nor deep level absorption in the spectral range 0.2–6.5 μm. Furthermore, there is no evidence of deep levels in the spectra of AlN/Si in the range 2.5–25 μm. The optical phonon modes of AlN/Si films were directly observed in infrared transmission and 80° angle of incidence reflectance spectra. Longitudinal optical (900 cm−1) and transverse optical (680 cm−1) modes are shifted toward each other comparable to previously reported bulk values. Optical spectra of thick textured film samples with low degree of orientation have phonon energies close to bulk values (probably due to strain relaxation).
Voltage switching and oscillations in a single barrier heterostructure hot-electron diode
Authors: Krotkus A. , Reklaitis A. , Geizutis A. , Asche M.
Published: July 2002

S-type negative differential conductivity and relaxation oscillations in a single-barrier n−-GaAs/Al0.34Ga0.66As/n+-GaAs heterostructure hot-electron diode are studied experimentally and by ensemble Monte Carlo simulations at 300 and 77 K lattice temperature. The influence of interband impact ionization on the instability is investigated. The frequency is found to be limited by a slow electron dispersal from the accumulation layer adjacent to the n−-GaAs/Al0.34Ga0.66As interface to less than 10 GHz. The maximum frequency of the oscillations observed experimentally was about 0.5 GHz on account of the parasitic capacitance of the investigated device structure.
Near-field diffraction by a slit in a thick perfectly conducting screen flying above a magneto-optical disk
Authors: Shih O. W.
Published: July 2002

A two-dimensional analysis is presented for the near-field diffraction of a plane wave incident upon a slit in a thick perfectly conducting screen flying above a magneto-optical disk. The angle and polarization of the incident wave are arbitrary. The screen thickness, slit width, and fly height are all on the order of a wavelength or less. The disk is treated as a thin-film multilayer stack on a semi-infinite substrate. Any combination of the thin-film layers can be magneto-optically active. Thus, polarization is not necessarily preserved. The fields and their polarizations within and around the slit are determined by rigorously solving Maxwell’s equations with the appropriate boundary conditions imposed by both the slit and the disk. The solution, which involves Fourier transforms and mode expansions, is based upon a method developed previously by other researchers to investigate scattering systems composed of slits in thick conductors only. In this article, their method is extended to include systems that also have thin-film multilayer media which can change polarization. The extended theory, numerical issues, and example calculations are presented and discussed.
Theory of magnetic-field-sensitive metal–oxide–semiconductor field-effect transistors
Authors: Dobrovolsky V. N. , Krolevets A. N.
Published: July 2002

Magnetic-field-sensitive metal–oxide–semiconductor field-effect transistors (MAGFETs) are used as magnetic field sensors. The Hall effect in the high-resistance channel region near the drain ensures a high magnetic field sensitivity of the transistor. In this case, the channel is nonuniform, and this fact presents difficulties when developing a Hall effect theory (the well-known theory of the Hall effect for uniform wafers is now inapplicable). Here, we propose a method that reduces the theory of galvanomagnetic effects in the MAGFETs with nonuniform channels to that for uniform wafers. Basing on this method, as well as on the conformal mapping and the Hall field symmetry, we calculate the Hall electromotive force in MAGFET channels with the Hall taps and split drain. The Hall current in MAGFETs with the split drain and split source is also obtained. It is shown that MAGFET magnetoresistance may be high.
Low energy, ion-induced electron and ion emission from stainless steel: The effect of oxygen coverage and the implications for discharge modeling
Authors: Walton S. G. , Tucek J. C. , Champion R. L. , Wang Yicheng
Published: July 2002

Absolute yields of electrons and negative ions resulting from positive ions impacting stainless steel have been determined as a function of impact energy for clean and oxygen-covered surfaces. Photoelectron emission has been used to monitor the effect of oxygen coverage on the surface work function. The emission of negative ions and electrons has been described with an excitation mechanism similar to that used in the Menzel–Gomer–Redhead model. We illustrate the implications of the present observations for discharge modeling and diagnostics by citing a parallel-plate, rf discharge in oxygen as an example.
Ripening suppression and large photoluminescence blueshift in aligned InGaAs quantum dots on a vicinal (100) GaAs substrate
Authors: Kim Yong , Min Byong Don , Kim Eun Kyu
Published: July 2002

We observe ripening suppression in aligned InGaAs quantum dots (QDs) along multiatomic steps on a vicinal (100) GaAs substrate. By varying the growth interruption times, a study of QD morphologies by atomic force microscope reveals a clear Ostwald ripening suppression effect on QD formation. On the other hand, we observe a regular ripening for InGaAs QDs on an exact (100) substrate. In addition, n-QD chains aligned along multiatomic steps are observed. An inhomogeneously broadened photoluminescence (PL) spectrum with larger spectral width is obtained for a QD ensemble on a vicinal (100) substrate as compared to those on an exact (100) substrate although statistical analyses of QD size distributions show similar size dispersions. The spectral broadening is attributed to the lateral quantum coupling of aligned n-QD chains. PL spectra for QDs on vicinal (100) substrates show a larger rate of blueshift with increasing interruption times, which is interpreted as a lack of the PL compensation effect by size enlargement of the QDs during growth interruption. Thus the result supports the ripening suppression effect which is observed in the morphology study.
Photon mode localization in disordered arrays of vertical cavity surface emitting lasers
Authors: Golshani A. , Pier H. , Kapon E. , Moser M.
Published: July 2002

Coupled arrays of vertical cavity surface emitting lasers with controlled optical disorder were realized by patterning the reflectivity of the top cavity mirrors. The size of each square array element, defined by photolithography, determines the local cavity oscillation frequency and loss. Optical disorder across the array can therefore be introduced by varying the size of the lasing elements. The localization of photon modes and the array coherence were investigated as a function of the degree of disorder and the array dimensionality. We found that a higher degree of disorder leads to more localized coherent modes and consequently to broader far-field patterns. However, the effects of disorder reduce considerably as the array dimensionality increases from one to two by adding array rows, leading to more effective suppression of mode localization.
Transition of the anisotropic magnetoresistance of permalloy films from the negative to the positive along with thickness
Authors: Sung G. , Park C.-M. , Shin K.-H.
Published: July 2002

The anisotropic magnetoresistance of dc magnetron-sputtered Ni80Fe20 permalloy films showed the evolutionary transition of the sign, from the negative magnetoresistance to the positive magnetoresistance (PMR), as the thickness increases. A similar phenomenon with the angle between the field and the current was recently reported. In our study, however, the transition occurred in the transverse measurement at the fixed angle of 90°. Ni80Fe20 films were deposited on Si substrates in a magnetic field of 300 Oe at the ambient temperature and at the deposition rate of 4 Å/s. Because the PMR was known to be peculiar to the longitudinal measurement, it was investigated why the PMR in the transverse measurement occurred. By a vibrating sample magnetometer and magnetic force microscope, the PMR curve in the transverse measurement was found to be related to the canted hysteresis loop peculiar to the stripe domains caused by the perpendicular anisotropy. Unanticipated magnetostrictive strain by the compositional fluctuation and/or the effect of surface roughness was suggested as a plausible mechanism of the perpendicular anisotropy.
Study of the low temperature thermal properties of the geometrically frustrated magnet: Gadolinium gallium garnet
Authors: Tsui Y. K. , Kalechofsky N. , Burns C. A. , Schiffer P.
Published: July 2002

Gadolinium gallium garnet, Gd3Ga5O12 (GGG) has an extraordinary low temperature phase diagram. Although the Curie–Weiss temperature of GGG is about −2 K, GGG shows no long-range order down to T∼0.4 K. At low temperatures GGG has a spin glass phase at low fields (⩽0.1 T) and a field-induced long-range order antiferromagnetic state at fields of between 0.7 and 1.3 T [P. Schiffer et al., Phys. Rev. Lett. 73, 2500 (1994), S. Hov, H. Bratsberg, and A. T. Skjeltorp, J. Magn. Magn. Mater. 15–18, 455 (1980); S. Hov, Ph.D. thesis, University of Oslo, 1979 (unpublished), A. P. Ramirez and R. N. Kleiman, J. Appl. Phys. 69, 5252 (1991)]. However, the nature of the ground state at intermediate fields is still unknown, and has been hypothesized to be a three-dimensional spin liquid. We have measured the thermal conductivity (κ) and heat capacity (C) of a high-quality single crystal of GGG in the low temperature regime in order to study the nature of this state. The field dependence of κ shows that phonons are the predominant heat carriers and are scattered by spin fluctuations. We observe indications in κ(H) and C(H) of both the field induced ordering and the spin glass phase at low temperatures (T⩽200 mK).
Mössbauer spectroscopy study of the rhombohedral phase Y2Fe17Nx with intermediate nitrogen content (0⩽x⩽2.8)
Authors: Yang D. P. , Budnick J. I. , Hines W. A. , Zhang Y. D.
Published: July 2002

Mössbauer spectra were obtained at 15 K from samples of rhombohedral Y2Fe17 and its nitrides Y2Fe17Nx, with nominal nitrogen contents of x=0.6, 1.2, 1.8, 2.4, and 2.8. The spectra have been analyzed using a model which is consistent with the crystallographic structure and with magnetization that lies along a general direction in the hexagonal basal plane. The hyperfine field values for the four crystallographically inequivalent Fe sites change differently upon nitrogenation. For each of the Y2Fe17Nx samples with intermediate N content, the spectrum can be described as a superposition of two components, one from the nitrided shell of the sample particle and one from the outer portion of its unnitrided core. These results, along with earlier 57Fe nuclear magnetic resonance measurements, yield a consistent picture for the two-phase configuration of Y2Fe17Nx and its hyperfine field behavior.
Effect of Ga substitution on the Sm sublattice anisotropy in Sm2Fe17−xGax (0⩽x⩽8)
Authors: Zhang Y. D. , Budnick J. I. , Hines W. A. , Shen B. G. , Cheng Z. H.
Published: July 2002

In order to study the unique behavior of the Sm sublattice anisotropy, magnetization curves along both the easy and hard directions were obtained at 10 K for Sm2Fe17−xGax (0⩽x⩽8) powders which were aligned using either static- or dynamic-field alignment methods. The Fe sublattice contribution was isolated by making similar measurements on Y2Fe17−xGax (0⩽x⩽8), as the presence of Y eliminates the rare-earth sublattice contribution. The results show that the Sm sublattice anisotropy is dominant for x⩾1. It increases with increasing Ga content, reaching a (broad) maximum of 1.8×107 erg/cm3 for x≈2–3, and then decreases and becomes negative for x≈5.5. This work shows that the unique behavior of the magneto-crystalline anisotropy for Sm2Fe17−xGax arises from the variation of the Sm-sublattice contribution with Ga substitution.
Enhancement of thermoelectric power factor in composite thermoelectrics
Authors: Bergman David J. , Fel Leonid G.
Published: July 2002

The analytical properties of macroscopic transport coefficients of two-component composites are first used to discuss the thermoelectric power factor of such a composite. It is found that the macroscopic power factor can sometimes be greater than the power factors of both of the pure components, with the greatest enhancement always achieved in a parallel slabs microstructure with definite volume fractions for the two components. Some interesting examples of actual mixtures are then considered, where the components are a “high quality thermoelectric” and a “benign metal,” leading to the conclusion that considerable enhancement of the power factor is often possible, with but a modest reduction in the thermoelectric figure of merit, compared to those of the high quality thermoelectric component. Two possibilities for fabricating real composites with such improved thermoelectric properties emerge from this study: a parallel slabs microstructure of benign metal and high quality thermoelectric, and a sintered collection of benign metal grains, each of them coated by a thin shell of high quality thermoelectric.
Green photoluminescence band in γ-irradiated oxygen-surplus silica glass
Authors: Sakurai Yuryo , Nagasawa Kaya
Published: July 2002

Previous studies have reported the existence of a green photoluminescence (PL) band in oxygen-deficient silica glass when exposed to ultraviolet (UV) light. Oxygen deficient defects (Eδ′) were thought to be the origin of this PL band. In this article, we describe the characteristics of a green PL band in oxygen-surplus silica glass (excited by visible and UV light). At room temperature, we observed the full width at half maximum and lifetime of this PL band to be about 0.2 eV and 300 ns, which differed from the previously reported values of 0.4 eV and 20–30 ns, respectively. We propose that the peroxy radical (O3≡Si–O–O↑,↑: unpaired electron) or other oxygen surplus defects are the cause of this green PL band rather than oxygen deficient defects.
Formation, characterization, and properties of a new boron-carbon-nitrogen crystal
Authors: Yao B. , Liu L. , Su W. H.
Published: July 2002

Amorphous boron-carbon-nitrogen (BCN) powders were produced by mechanically milling a mixture of hexagonal boron nitride (h-BN) and graphite powders for 120 h in an argon atmosphere. By annealing the amorphous BCN powders isothermally under 4 GPa at temperatures above 880 K, a new B–C–N crystal with a tetragonal structure was obtained. Its lattice constants decrease with the increasing annealing temperature and are a=1.685 nm and c=0.537 nm, respectively at an annealing temperature of 1170 K. The conductivity measurement suggests the new B–C–N phase is a semiconductor with a very small band gap and the band gap changes depending on the temperature in the temperatures between room temperature and 840 K.
Lateral scalability limits of silicon conduction channels
Authors: Trellakis A. , Ravaioli U.
Published: July 2002

We have investigated the lateral scalability limits of the conduction channels in several metal-oxide-semiconductor (MOS) structures, at room temperature, with the goal to understand for which geometries and under which operating conditions a narrow channel approaching the quantum-wire limit can maintain reasonable isolation. A wide range of calculations were carried out using an efficient two-dimensional self-consistent model based on the coupled Schrödinger and Poisson equations. We found that a good trade-off in performance and manufacturability is obtained with a T-shaped gate metallization. Our calculations show that if one uses a highly doped substrate for this system, a quasimonomode quantum wire can be realized at room temperature with a sufficiently high confining barrier. Because of random dopant fluctuations affecting reproducibility and threshold behavior, it would be desirable to eliminate doping in the channel when the width approaches nanometer scale. However, the same MOS structures implemented using an undoped substrate exhibits various problems, the most serious one being a very low confining potential. Considerable improvement is obtained if an undoped layer is grown on top of a highly doped substrate, leading to reasonable confinement and threshold control.
Characterization of CaTiO3-modified Pb(Mg1/3Nb2/3)O3 dielectrics
Authors: Chen X. M. , Lu X. J.
Published: July 2002

Pb(Mg 1/3 Nb 2/3 )O 3 dielectrics modified by CaTiO3 were prepared by a solid-state reaction method, and microstructure analyses were performed together with the dielectric characterization. Solid solutions with cubic perovskite structures were obtained for compositions of 0–60 mol %CaTiO3, and the orthorhombic perovskite structures were observed for compositions near to the end member of CaTiO3 in the present system. The dielectric loss and temperature coefficient could be pronouncedly reduced by incorporating CaTiO3 into Pb(Mg1/3Nb2/3)O3. Low-loss dielectrics (tan δ∼10−4 at 1 MHz) with dielectric constants of 120–262 and small temperature coefficient (τε∼−960 to −1100 ppm/ °C) were obtained, and further improvement of dielectric properties could be expected through structural modifications. Good microwave dielectric properties, ε=172.6 and Qf=1930 GHz, were achieved in a composition of 0.4Pb(Mg1/3Nb2/3)O3/0.6CaTiO3, where the temperature coefficient of resonant frequency τf was estimated as 470 ppm/ °C.
Interferometric characterization of single-mode bent optical fibers
Authors: El-Diasty F.
Published: July 2002

Multiple beam Fizeau fringes in transmission have been used to demonstrate and to measure the change of principal stresses in the cladding of single-mode optical fibers due to bending. The study represents a nonlinear relation between the difference of maximum radial values of cladding’s refractive indices with the radii of curvature in the cladding of the bent optical fibers. The limiting acceptable bending radius has been investigated.
Photonic band in two-dimensional lattices of micrometer-sized spheres mechanically arranged under a scanning electron microscope
Authors: Miyazaki Hideki T. , Miyazaki Hiroshi , Ohtaka Kazuo , Sato Tomomasa
Published: July 2002

Two-dimensional photonic crystals of dielectric spheres with a 2.1 μm diameter have been fabricated by arranging individual spheres using a micromanipulation technique in a scanning electron microscope. A buildup of photonic bands from whispering gallery modes has been observed as the number of spheres increased, by measuring the transmission spectra for lattices composed of various numbers of spheres. The photonic band dispersion curves were experimentally obtained for a finite system made of 91 spheres from the transmission spectra for oblique incidence in the near-infrared region. They were in good agreement with the results of a numerical calculation for an infinite lattice. Since this mechanical manipulation technique enables us to control the arrangement of individual optical wavelength-sized scatterers, it provides a new way to systematically investigate various photonic band effects.
Studies of ambient dependent electrical behavior of nanocrystalline SnO2 thin films using impedance spectroscopy
Authors: Varghese Oomman K. , Malhotra L. K.
Published: July 2002

Impedance spectroscopy has been used to investigate the properties of sol-gel derived ultrafine grained tin oxide thin films subjected to various ambients and temperatures. The impedance measurements have been carried out in the frequency range 250 kHz–10 mHz and Cole–Cole plots drawn. Equivalent circuit models have been suggested and fitting of the experimentally obtained data done on the basis of the approach of “universal dielectric behavior” introduced by Jonscher. The films have shown strong interaction with water vapor in the atmosphere at room temperature (300 K) as reflected from the changes in the values of resistive and capacitive components in the equivalent circuits at different humidity levels. The conduction mechanism appears to be due to the transfer of protons through the physisorbed layer of water molecules. A low frequency spur has been observed in the impedance diagram at medium and high humidity levels and attributed to the migration of adsorbed ions towards the electrode sample contact region under the influence of the electric field. These ions accumulate in the electrode sample contact region, giving rise to a non-Debye capacitance. The spur in the impedance diagram disappears on heating the films to 373 K due to the desorption of water molecules from the surface. The films thereafter exhibit semiconducting behavior till 473 K. A reversal in behavior has been observed during further increase of temperature due to the conversion of O2− ions to highly active O− ions which abstract electrons from the grain bulk. At very high temperatures in excess of 573 K, the spur in the impedance diagram reappears, but this time due to accumulation of the adsorbed O− ions, thereby giving rise to another non-Debye capacitance in the electrode sample contact region. The spur disappears in the presence of ethanol due to removal of oxygen ions by ethanol molecules. The films on float glass have exhibited large impedance changes in presence of ethanol but negligible changes in presence of other reducing gases like H2 and liquefied petroleum gas.
Continuous wave infrared laser action, self-frequency doubling, and tunability of Yb3+:MgO:LiNbO3
Authors: Montoya E. , Sanz-Garcı́a J. A. , Capmany J. , Bausá L. E. , Diening A. , Kellner T. , Huber G.
Published: July 2002

The laser performance of the nonlinear Yb3+:MgO:LiNbO3 crystal is assessed under Ti:sapphire laser pumping. Spectral properties related to infrared laser action such as the emission and absorption cross sections have been evaluated. Different crystallographic configurations have been used. Laser oscillation in the near infrared region has been observed along the crystallographic c axis and along the type I phase matching direction for infrared to green generation. High slope efficiency in the infrared region (62%) and the possibility of low laser thresholds (14.7 mW) are demonstrated. By using a birefringent filter, broadband tuning from 1030 to 1070 nm is obtained in this system. In addition, 58 mW of green power by self-frequency doubling of infrared laser emission is achieved, revealing the potential of LiNbO3:MgO as a self-frequency-doubling material.