bluuebunny/crossref_metadata_2025 huggingface dataset:
Band alignment strategy for efficient optical modulation in quantum-confined Stark effect devices
Authors: Yip R. Y.-F. , Masut R. A.
Published: July 2002

Strategies designed to optimize the performance of the multi-quantum well stack in optical modulator devices based on the quantum-confined Stark effect can be derived from an observation that the effective mass of the active quantum well material is significantly smaller in the conduction band than that in the valence band. This suggests that significant improvements in the overall performance of the devices may be achieved by engineering the band alignment of quantum well and barrier to accommodate the difference in the effective masses.
Intrinsic chaos in a dc field biased quantum heterostructure
Authors: McNary A. Jason , Puri Ashok
Published: July 2002

A closed, quantum, double barrier, GaAs/AlGaAs heterostructure is made chaotic by adding a nonlinear potential term, α〈Q(t)〉, to the time-dependent Schrödinger equation, and the dynamical behavior of an electron cloud moving in the heterostructure biased by a dc electric field is examined numerically. Using phase-space diagrams, power spectrums, and Lyapunov exponents, both qualitative and quantitative measures of the chaos in the system were taken. In general, for all values of α, the nonlinearity parameter, the Lyapunov exponent, λ, increases as the applied dc field, β, increases. However, for values of α⩽1.376, we notice a sharp drop in λ for the value of β =−9.2×107 V/m corresponding to an average dc voltage of −.085 eV in the central well. This first order type transition to high values of λ for α>1.376 corresponds to a similar increase in the mean charge trapped in the heterostructure and in the average nonlinear potential in the central well for that dc field. This behavior is attributed to the fact that for α⩽1.376 and β=−9.2×107 V/m, the field effects dominate, but for α>1.376, the nonlinearity term dominates.
Dynamics of the single barrier heterostructure hot electron diode
Authors: Reklaitis A. , Stasch R. , Asche M. , Hey R. , Krotkus A. , Schöll E.
Published: July 2002

The dynamics of the bistable and oscillatory behavior of a heterostructure hot electron diode is investigated both theoretically and experimentally. Monte Carlo simulations are performed accounting for the scattering to subsidiary conduction band valleys of GaAs and AlGaAs. S-shaped current-voltage characteristics and self-sustained current oscillations in an external cavity are obtained in good agreement between theory and experiment.
Magnetic measurements on electrochromic Ni-oxide-based films
Authors: Ragan D. D. , Svedlindh P. , Granqvist C. G.
Published: July 2002

Electrochromic Ni-oxide-based films were prepared by cathodic electrodeposition and, for comparison, also by reactive sputtering. The films were characterized by cyclic voltammetry, spectrophotometry, and infrared absorption spectroscopy. Explorative magnetic measurements, performed at low temperatures using SQUID technology, exhibited a well-defined Curie–Weiss behavior with a transition temperature that was depressed by proton extraction.
Correlation between giant magnetoresistance and magnetic interactions in a CoAg multilayered/granular system
Authors: Viegas A. D. C. , Geshev J. , Schelp L. F. , Schmidt J. E.
Published: July 2002

Magnetizing, demagnetizing, and remanent magnetization curves for [Co(12 Å)/Ag(60 Å)]×15 multilayered/granular films thermally treated have been measured. The changes of the giant magnetoresistance and the interaction effects have been explained as a result of the structural, morphological, and magnetic evolution of the samples as a function of the thermal treatment. It has been inferred that for samples annealed at temperatures lower than 360 °C, the changes of the giant magnetoresistance come from the misalignment of the magnetic moments of the Co particles formed during annealing. A strong correlation between the giant magnetoresistance and the magnetic interaction effects has been found for samples annealed at temperatures higher than 360 °C: the giant magnetoresistance is degraded as the demagnetizing interparticle interactions are increased.
Virtual charge method for electrostatic calculations in metallic tip and semiconducting sample systems
Authors: de la Broı̈se X. , Lannoo M. , Delerue C.
Published: July 2002

We calculate the electrostatic field between a metallic tip and a semiconductor surface by replacing the electrodes by a set of virtual charges, adjusted to fit the boundary conditions on the surfaces. The boundary conditions on the semiconductor side of the surface are obtained by using the result of a direct integration of the Poisson equation. The results of the method are compared to those given by a one-dimensional model. It is then applied to get some information which is useful in the theory of near field microscopy: Curvature of the energy bands at the semiconductor surface, influence of the tip shape, range of the electric field, effect of a dielectric insulating layer.
Phonon-limited inversion layer electron mobility in extremely thin Si layer of silicon-on-insulator metal-oxide-semiconductor field-effect transistor
Authors: Shoji Masanari , Horiguchi Seiji
Published: July 2002

Phonon-limited inversion layer electron mobility in extremely thin (100) Si layers of silicon-on-insulator field-effect transistors has been studied at 300 K using a relaxation time approximation and a one-dimensional self-consistent calculation. For the Si layer thickness tSi of more than approximately 5 nm, the mobility behavior as a function of an effective vertical electric field is found to be almost identical with that of bulk Si inversion layers. For a thickness of less than that, however, the mobility behavior is considerably affected by the change in the electronic structures due to a confinement effect. As the Si layer thickness decreases, the phonon-limited electron mobility μph increases to a maximum at tSi of ∼3 nm and decreases monotonically. The increase in mobility results from the increase of the fraction of electrons in the lowest energy subband that has a higher mobility than other subbands. The mobility decrease in the extremely thin tSi region is attributed to the enhancement of phonon scattering rates caused by a reduction of the spatial widths of the subbands.
The influence of residual gas pressures on the degradation of ZnS powder phosphors
Authors: Swart H. C. , Trottier T. A. , Sebastian J. S. , Jones S. L. , Holloway P. H.
Published: July 2002

Phosphor powder of ZnS:Cu,Al,Au has been subjected to electron bombardment (2 keV, 2 mA/cm2) in residual gas pressures ranging from 0.6×10−8 to 7.0×10−8 Torr. Auger electron spectroscopy and cathodoluminescence (CL), both excited by the same electron beam, were used to monitor changes in the surface chemistry and cathodoluminescent brightness versus vacuum conditions during electron bombardment. A direct correlation between the surface reactions and the degradation of CL brightness was observed. The formation of a nonluminescent ZnO layer on the surface of the phosphor was largely responsible for the degradation of the ZnS. The aging of the phosphor was not only a function of the charge per unit area (Coulomb dose) bombarding the surface, but also a function of residual gas pressure and composition. In particular, H2O had the greatest effect on the rate of degradation. The results are interpreted in terms of an electron-beam stimulated surface chemical reaction.
Effects of bandwidth limitations on the localized state distribution calculated from transient photoconductivity data
Authors: Webb D. P. , Main C. , Reynolds S. , Chan Y. C. , Lam Y. W. , O’Leary S. K.
Published: July 2002

The possible effects of experimental bandwidth limitation on the accuracy of the energy distribution of the density of localized states (DOS) calculated from transient photoconductivity data by the Fourier transform method is examined. An argument concerning the size of missing contributions to the numerical Fourier integrals is developed. It is shown that the degree of distortion is not necessarily large even for relatively small experimental bandwidths. The density of states calculated from transient photodecay measurements in amorphous arsenic triselenide is validated by comparing with modulated photocurrent data. It is pointed out that DOS distributions calculated from transient photoconductivity data at a high photoexcitation density are valid under certain conditions. This argument is used to probe the conduction band tail in undoped a-Si:H to energies shallower than 0.1 eV below the mobility edge. It is concluded that there is a deviation in the DOS from exponential at about 0.15 eV below the mobility edge.
Interlayer coupling in Co/Cu/permalloy/Cu multilayers
Authors: Edelstein A. S. , Bussmann K. M. , Turner D. C. , Chopra Harsh D.
Published: July 2002

The interlayer coupling, domain structure, and microstructure of ion beam sputtered multilayers, were investigated by magnetization, optical microscopy using magnetic colloids, small and wide angle x-ray scattering, and atomic force microscopy measurements. The study focused on multilayers that were composed of ten repeats of Co(1.5 nm)/Cu(tCu)/permalloy(2.6 nm)/Cu(tCu) with tCu=2.6 and 5.0 nm. The top surface rms roughness of the multilayers was 0.31 and 0.96 nm for samples with tCu=2.6 and 5.0 nm, respectively. The maximum peak to peak roughness in a 1 μm2 scan was 3.0 and 8.9 nm for samples with tCu=2.6 and 5.0 nm, respectively. The lateral feature size was 30–40 nm and 70 nm for samples with tCu=2.6 and 5.0 nm, respectively. The multilayers with tCu=2.6 had sharper, flatter interfaces than those with tCu=5.0 nm. Magnetization measurements and the domain structure at room temperature showed that when tCu=2.6 nm the Co and permalloy layers were ferromagnetically coupled, while if tCu=5.0 nm, the Co and permalloy layers were largely uncoupled. Twin walls present for tCu=2.6 nm clearly indicated locked-in domains of the ferromagnetically coupled Co and permalloy layers; for tCu=5.0 nm, such coupled wall structures were almost nonexistent. Modeling the magnetization, it is estimated that the ferromagnetic coupling only has to be greater than 0.0047 mJ/m2 for the Co and permalloy layer magnetizations to be nearly parallel at all field values. Since not much coupling is needed, the coupling in the samples with tCu=2.6 nm could be due to the magnetostatic coupling which exists even in perfectly flat films or to the magnetostatic coupling of surface magnetic dipoles created by roughness.
Electron mobility in In0.5Ga0.5P
Authors: Nag B. R. , Das Madhumita
Published: July 2002

The Hall mobility of electrons is calculated for In0.5Ga0.5P by using the experimental values of the effective mass and the band gap, and the estimated values of other constants. The experimental results are explained by taking the alloy scattering potential and the acoustic phonon deformation potential to be 0.435 and 12 eV, respectively. It is concluded that the experimental samples had impurity concentrations lying mostly between 5 and 15 times the electron concentration.
Coercive Sm2Fe17N3: A model pinning system created by heavy ion irradiation
Authors: Dempsey N. M. , Rao X. L. , Coey J. M. D. , Nozières J. P. , Ghidini M. , Gervais B.
Published: July 2002

Swift high energy (5 GeV) Pb ion irradiation has been employed to create defects in the high performance permanent magnet material Sm2Fe17N3. Total ion fluences in the range from 1011 to 2×1013 ions/cm2 were studied. In Mössbauer spectra at 26 and 300 K, both a paramagnetic doublet and a broad ferromagnetic contribution characteristic of an amorphous or strongly disordered phase are found to be induced by irradiation. Magnetization reversal was studied and coercivity is found to increase upon irradiation, up to fivefold at 5 K (μ0Hc=1.5 T) for the highest ion fluence. Initial magnetization curves of the irradiated samples are characteristic of material with a pinning type coercivity mechanism with a maximum initial depinning field of 0.4 T for the highest fluence. Magnetization measurements made parallel and perpendicular to the ion beam direction suggest that the defects created are highly anisotropic in shape. The temperature dependence of the coercivity has been analyzed within the framework of the Gaunt model of domain wall pinning.
Magnetostriction and susceptibilities of twinned single crystals of Terfenol-D
Authors: Zhao X. G. , Lord D. G.
Published: July 2002

Measurements are presented of the dc magnetostriction, and both the ac and dc susceptibility along the three orthogonal axes of [112̄], [111], and [11̄0] from twinned single crystals of Terfenol-D as a function of varying compressive stress and magnetic field mutually applied along the [112̄] rod axis. The data are used in an attempt to analyze the magnetization processes occurring in such a specimen. The experimental results show clearly the two characteristics of the magnetization process corresponding to (i) the magnetostrictive “jump” effect, and (ii) the slow changes of magnetostriction occurring above the “jump.” It is estimated that in stage (i) of the magnetization process, the domains aligned along the [111] axis by the applied stress rapidly and massively redistribute their moments both to the [111̄] direction closest to the applied field as well as to the [1̄11̄] and [11̄1̄] directions; and during stage (ii), the domains occupying the [1̄11̄] and [11̄1̄] directions redistribute to the [111̄] successively by non-180° domain-wall motion. However, two fine magnetization processes are found in the field range of the jump effect indicating that some domains must redistribute from the [111] to the [1̄11̄] and [11̄1̄] initially, then to the [111̄] even when the field is only increased in the initial stage (i). The nonzero susceptibilities measured along the [111] and [11̄0] suggest that both the volumes of parent and twin material as well as the probabilities for domain redistribution from one easy direction to the [1̄11̄] and [11̄1̄] might be different.
Unusual substitutional properties of Ru in the La0.7Sr0.3Mn1−xRuxO3 system
Authors: Manoharan S. Sundar , Ju H. L. , Krishnan Kannan M.
Published: July 2002

La 0.7 Sr 0.3 Mn 1−x Ru x O 3 ,  0⩽x⩽0.2 samples were prepared both in bulk and thin film formed using pulsed laser deposition. Magnetization measurements of bulk samples indicate a marginal decrease in Tc from 366 to 335 K up to 15% Ru substitution in Mn site. X-ray diffraction of the 〈100〉 oriented epitaxial films indicate a monotonic increase in the lattice parameter with Ru doping. The magnetoresistive behavior of the Ru substituted films are comparable with the parent La0.7Sr0.3MnO3 films. Unlike other elemental substitution in Mn sites, the effect of Ru to stabilize magnetic ordering at elevated temperatures is exceptional.
Isotope effect and Tc in manganites and high Tc oxides
Authors: Kresin Vladimir Z. , Wolf Stuart A.
Published: July 2002

Oxygen isotope effect in manganites (cuprates) is caused by the dynamic Jahn–Teller effect on the bridging oxygen ion (e.g., the ion between Mn3+ and Mn4+ in manganites). This effect leads also to the vibronic reduction and effective decrease of the ferromagnetic transition temperature.
Thermal transport in Sr1−xCaxRuO3
Authors: Shepard M. , Henning P. F. , Cao G. , Crow J. E.
Published: July 2002

We have studied the thermal conductivity K(T,B) (6<T<250 K, 0<B<15 T) of single crystals of SrRuO3, a ferromagnet with Tc=160 K, and isostructural but paramagnetic CaRuO3, as well as the intermediate crystals (Sr, Ca)RuO3 for the purpose of probing charge carrier (Ke), phonon (Kp), and magnon (Km) excitations in these systems. The contributions of electron and phonon scattering processes to the thermal conductivities have been estimated using a Wiedemann–Franz relationship to estimate Ke and thus obtain Kp at higher temperatures. Roughly 50% of the thermal conductivity is estimated to be phononic near room temperature. Amorphous behavior is observed in the doped samples, and no break in the thermal conductivity is seen at the ferromagnetic onset. A comparison of magnetothermal conductivity and magnetoresistivity measurements at lower temperatures in SrRuO3 and CaRuO3 reveals that additional scattering mechanisms have become important. These investigations argue for the importance of phonon and magnon contributions, in addition to electronic contributions, to the magnetic behavior in Sr1−xCaxRuO3. Results will be discussed in relation to recent theoretical work on these systems and measurements on related systems.
Effect of Al substitution on the local environments and magnetic properties of partially nitrogenated (Er0.5Pr0.5)2Fe17 permanent magnets
Authors: Harris V. G. , Fatemi D. J. , Suresh K. G. , Rama Rao K. V. S.
Published: July 2002

Extended x-ray absorption fine structure measurements of the Fe K, and Pr and Er LIII absorption edges, were carried out to elucidate the relationship between the local structure and magnetism in Al substituted, partially nitrogenated (Er0.5Pr0.5)2Fe17 permanent magnets. We find that the nitrogenation acts to dilate both the Fe–Fe and the (Pr,Er)–Fe bonds, thus raising the TC via a magnetovolume effect. However, nitrogenation of Al-substituted samples acts to decrease the average Fe–Fe bond distance, thus reducing the exchange on the Fe sublattice and lowering TC relative to the nitrogenated parent compound. This is opposite to the trend measured in systems when N is absent.
Evolution of magnetism in Nd(Co1−xSix)2 and Ho(Co1−xSix)2
Authors: Cuong T. D. , Havela L. , Sechovský V. , Arnold Z. , Kamarád J.
Published: July 2002

We report on a comparative study of the effects of Si doping and of external pressure on magnetism and related phenomena in Nd(Co1−xSix)2 and Ho(Co1−xSix)2 compounds. The lattice volume in the Ho system is almost composition invariable for x up to 0.1, whereas an increase of ≈6% is observed in the Nd system. Si substitutions for Co cause a dramatic increase of TC in the Ho system for x⩽0.075 and a gradual reduction of TC in Nd analogs, whereas the Co moment becomes suppressed for x>0.05 in both cases. Negative pressure effects both on TC and the Co magnetic moment, although quantitatively different, are found in both systems. Expected changes of electronic structure and their influence on the hierarchy of exchange interactions and formation of the Co moment are discussed.
Canted antiferromagnetism and spin glasslike behavior in a family of two-dimensional organic/inorganic nanocomposites
Authors: Gı̂rţu M. A. , Wynn C. M. , Fujita W. , Awaga K. , Epstein A. J.
Published: July 2002

We report magnetic properties of a new class of hybrid organic/inorganic layered materials, Cu2(OH)3(CmH2m+1COO), m=7, 9, and 11. The frequency dependence of the linear ac susceptibility and the irreversibility in the field-cooled/zero-field-cooled magnetization show that the geometrical frustration of the triangular lattice is strong enough to cause glassiness. The strong peak in the second harmonic of the nonlinear ac susceptibility indicates that frustration is not sufficient to prevent the system from developing a spontaneous moment. We propose that the interplay of Heisenberg antiferromagnetic exchange and Dzyaloshinskii–Moriya interaction leads to the unusual coexistence of glassiness and canted antiferromagnetism.
Structural and magnetic properties of RSr2Fe3O9 (R=La, Y, Pr, and Gd)
Authors: Awana V. P. S. , Dou S. X. , Felner I. , Nowik I. , Malik S. K. , Mehta Apurva , Singh Rajvir , Narlikar A. V. , Yelon W. B.
Published: July 2002

We have investigated the RSr2Fe3O9 (R=La, Y, Pr, and Gd) materials by several experimental techniques, including x-ray and neutron diffraction, magnetic susceptibility, and Mossbauer spectroscopy measurements. All materials studied are single phase and crystallize in the hexagonal perovskite structure. Magnetic susceptibility studies reveal that, for R=La and Pr, the Fe ions order antiferromagnetically at about TN=190 K. Short range magnetic correlations are observed up to 250 K. For R=La and Pr, Fe57 Mossbauer studies reveal two inequivalent magnetic sextets below 190 K, with the area ratio 2:1. Above TN, one singlet is observed with an isomer shift value typical to Fe4+. For R=Y and Gd, the magnetization curves do not lend themselves to easy determination of the magnetic transition due to an extra magnetic phase which exists up to 350 K. This phase is a result of deficiency of oxygen. The Mossbauer57 spectra at 300 K indicate two superimposed singlet lines, and contain a small magnetic sextet. There is no indication that the R sublattice in RSr2Fe3O9 (R=Pr and Gd) orders magnetically down to 1.5 K.
Magnetic and structural characterizations of HCP permalloy films grown by molecular beam epitaxy
Authors: Huang J. C. A. , Hu Y. M. , Yu C. C.
Published: July 2002

An hcp metastable phase of Ni80Fe20(11̄00) structure has been grown as high-quality epitaxial films on Co(11̄00) and Cr(211) planes. The epilayers of Ni80Fe20 grow as hcp structure up to a critical thickness of ∼100 Å, and the further layers transform to twinned fcc structure. Magneto-optical Kerr effect study indicates that the hcp Ni80Fe20(11̄00) layers exhibit strong crystalline induced uniaxial magnetic anisotropy with easy axis along the hcp[0001] direction. The subsequent fcc Ni80Fe20 layers show decreasing coercive fields as the film thickness increased. For the thick fcc film, interestingly, the magnetic easy axis shifts 90° with respect to that of the underlying hcp layers. The result is explained by the shape anisotropy effect due to the formation of quasi-1D columnar islands directed perpendicular to the hcp[0001] direction.
Spin-flop tendencies in exchange-biased Co/CoO thin films
Authors: Borchers J. A. , Ijiri Y. , Lee S.-H. , Majkrzak C. F. , Felcher G. P. , Takano K. , Kodama R. H. , Berkowitz A. E.
Published: July 2002

In order to study the antiferromagnetic (AFM) spin structure near the interface of exchange-biased bilayers, polarized neutron diffraction measurements were performed on a series of (111) Co (7.5 nm)/CoO (X nm) and CoO (X nm) thin films where X=20, 40, and 100 nm. In these samples, field cooling through the Néel temperature of the AFM increases the component of the CoO moment perpendicular to the applied field, relative to the parallel component. The subsequent application of a 500 Oe field perpendicular to the cooling direction rotates both the Co and CoO moments. Experiments on CoO films without Co showed a smaller difference between the parallel and perpendicular CoO moments in response to cooling and applied fields. Exchange coupling between the Co and CoO layers is apparently responsible for the increased projection of the AFM moments perpendicular to the cooling field.
Two-photon absorption and nonlinear refraction of lanthanum sulfide-gallium sulfide glasses
Authors: Zhou Zhong Hua , Hashimoto Tadanori , Nasu Hiroyuki , Kamiya Kanichi
Published: July 2002

The two-photon absorption coefficient (β) and nonlinear refractive index (γ) of the binary La2S3–Ga2S3 and ternary MS–La2S3–Ga2S3 (MS=Ag2S and Na2S) glasses were measured at 532 nm by the Z-scan method. The β of the glasses was ∼40 cm GW−1 and nearly unchanged with La2S3 content in the binary glasses, while it increased, or decreased with the substitution of Ag2S, or Na2S, respectively, for La2S3 in the ternary glasses. The γ of the glasses increased, with the rising La2S3 content in the binary glasses, and increased, or decreased with the addition of Ag2S, or Na2S, respectively, in the ternary glasses. The results were compared with the third-order nonlinear optical properties measured by third harmonic generation and discussed on the basis of their linear optical properties.
Deposition of hydrogenated diamond-like carbon films under the impact of energetic hydrocarbon ions
Authors: Zhang Qing , Yoon S. F. , Rusli  , Ahn J. , Yang H. , Bahr D.
Published: July 2002

In this article we studied the influence of bombardment energy of hydrocarbon ions on the properties of hydrogenated diamond-like carbon (DLC) films using x-ray reflectivity, Raman spectroscopy, and Fourier-transform infrared. The DLC films were prepared with an electron cyclotron resonance system using H2 and CH4 gases and the ion energy was tunable through a rf-induced dc bias voltage. It was observed that the surface roughness is increased and C–H bonded hydrogen concentration is decreased with increased ion energy, whereas the mass density, hardness, and sp3/sp2 ratio exhibited optimum values. A thin SiC layer was found to form between the DLC films and silicon substrates. Two proposed carbon deposition mechanisms, i.e., the shallow implantation (subplantation) model and the adsorbed layer model, are examined based on the results obtained in this study. Our results indicate that ion bombardment energy is a critical factor in determining the film properties and the ion subplantation could be an important deposition process even for hydrogenated DLC films.
Well depth fluctuation of Si1−xGex/Si quantum well structures studied by conductance–voltage technique
Authors: Zhang Sheng-kun , Jiang Zui-min , Qin Jie , Hu Dong-zhi , Lu Fang , Wang Xun
Published: July 2002

A conductance method is proposed to study the interfacial band offset and the well depth fluctuation of SiGe/Si quantum well structures. Based on an equivalent circuit model, the frequency-dependent and temperature-dependent conductance–voltage (G–V) characteristics of quantum well structures are analyzed. It is revealed that corresponding to each well a conductance peak will appear in the G–V curve and meanwhile a capacitance step will appear in the capacitance–voltage curve. By this conductance method the position of the Fermi level as functions of applied bias voltage and temperature can be obtained in a wide voltage range. The conductance method is found to be an effective method to measure not only the band offset of single quantum well but also the well depth fluctuation of multiple quantum wells.
Planar quantum transistor based on 2D–2D tunneling in double quantum well heterostructures
Authors: Simmons J. A. , Blount M. A. , Moon J. S. , Lyo S. K. , Baca W. E. , Wendt J. R. , Reno J. L. , Hafich M. J.
Published: July 2002

We report on our work on the double electron layer tunneling transistor (DELTT), based on the gate control of two-dimensional–two-dimensional (2D–2D) tunneling in a double quantum well heterostructure. While previous quantum transistors have typically required tiny laterally defined features, by contrast the DELTT is entirely planar and can be reliably fabricated in large numbers. We use a novel epoxy-bond-and-stop-etch flip-chip process, whereby submicron gating on opposite sides of semiconductor epitaxial layers as thin as 0.24 μm can be achieved. Because both electron layers in the DELTT are 2D, the resonant tunneling features are unusually sharp, and can be easily modulated with one or more surface gates. We demonstrate DELTTs with peak-to-valley ratios in the source-drain I–V curve of order 20:1 below 1 K. Both the height and position of the resonant current peak can be controlled by gate voltage over a wide range. DELTTs with larger subband energy offsets (∼21 meV) exhibit characteristics that are nearly as good at 77 K, in good agreement with our theoretical calculations. Using these devices, we also demonstrate bistable memories operating at 77 K. Finally, we briefly discuss the prospects for room temperature operation, increases in gain and high speed.
Tip–sample capacitance in capacitance microscopy of dielectric films
Authors: Goto Kazuya , Hane Kazuhiro
Published: July 2002

The tip–sample capacitance in the scanning capacitance microscopy (SCM) of dielectric films is described through theoretical calculations based on the method of images. The results are explained with the charge density distribution in the tip–sample system. Furthermore, capacitance signals in the tapping mode SCM of dielectric films are experimentally investigated and found to be in good agreement with the simulation results.
Study of the expansion of a plasma generated by electron-beam evaporation
Authors: Bésuelle E. , Nicolaï J.-P.
Published: July 2002

The stationary expansion of an ionized uranium vapor, created in an electron-beam evaporator, is studied in this article. Using an electrostatic probe, the electron temperature, the electron density, and the plasma potential have been measured along the vertical axis in the evaporator. The analysis, made by moving the probe along the vertical axis, first shows that the plasma is thermalized during its expansion in the vacuum chamber and, second, that the plasma density decreases exponentially. The plasma expansion is theoretically described using a fluid model which takes into account the charge exchange process. In solving the system, consisting of three equations (continuity, momentum conservation, and energy conservation), good agreement with the experimental data is obtained.
Departures from equilibrium near the copper anode of an argon transferred arc
Authors: Bouaziz M. , Gleizes A. , Razafinimanana M.
Published: July 2002

Spectroscopic measurements on an argon transferred arc have been performed in the plasma region just above the copper anode, by means of the arc interruption technique. This technique, based on the recording of the transient evolution of total spectral line intensities, allows one to determine the difference Te−Th between the electron temperature and the heavy particle temperature, existing in steady state. During the extinction of the low arc current, the evolution of some argon lines presents first a sudden increase before a slow general decrease, whereas the evolution of copper lines shows first a very fast decrease followed by a slow extinction. These phenomena have been interpreted considering that argon excited-state populations are strongly correlated with ions and electrons (Saha equilibrium) whereas the copper excited-state populations are correlated with the ground-state density (Boltzmann equilibrium). The difference Te−Th deduced from both kinds of measurements varies between 500 and 800 K.
16 MeV-electron-induced defects in iron-doped indium phosphide
Authors: Kamta M. , Schwab C. , Domngang S. , Engel J.-P.
Published: July 2002

Diverse experimental approaches such as microwave saturation, photoexcitation, and phototransients, in combination with powerful multiparameter data analysis, have enabled us to unravel several intertwinning spectra of paramagnetic defects in 16 MeV-electron-irradiated semi-insulating InP:Fe, aimed at simulating ion implantation damage in this III-V substrate material. Some spectra have already been reported and identified in as-grown or electron-irradiated samples, such as the cubic FeIn and trigonal FeIn-Ini,In centers or in fast-neutron-irradiated samples, such as the tetragonal FeIn-Ini,P pair. Two of the new centers discovered have symmetries closely related to those of the former trigonal and tetragonal pairs, suggesting that they derive from the same defects, but in an environment modified by the particle-induced lattice damage. In this regard, it is noteworthy that these centers indeed show anisotropy effects that can be traced to the geometrical configuration of the electron irradiation. Surprisingly, no experimental evidence was found for the PIn isolated phosphorus antisite spectrum, although the latter defect might nevertheless be part of some of the new centers.
Relaxation process of excited carriers in luminescent and nonluminescent porous silicon
Authors: Ohmukai Masato , Tsutsumi Yasuo
Published: July 2002

We investigated photoluminescence (PL) and photoacoustic spectra of several porous silicon samples and proposed a relaxation process for excited carriers in luminescent and nonluminescent porous silicon. The carriers, generated by the exciting light, relax through a nonradiative process and then recombine resulting in light emission in the energy range from 1.7 to 2.2 eV. From the fact that PL peak energy depends linearly on the excitation energy, light emission occurs through radiative centers such as surface-localized states. If the nonradiative centers are distributed in this energy range, PL efficiency is noticeably quenched.
Electron-stimulated reaction of C60 with a model etching gas, SF6
Authors: Coquel Jean-Marc , Hunt Michael R. C. , Šiller Lidija , Palmer Richard E.
Published: July 2002

High-resolution electron energy loss spectroscopy has been used to characterize changes induced in C60 films due to electron-stimulated reaction with SF6, a standard etching gas. A monolayer of SF6 was adsorbed at low temperature on 4 monolayer thick C60 films grown on Au(110). The C60 vibrational modes and SF6 vibrations observed were essentially unshifted from the pristine solid state and gas phase values, respectively, indicating a van der Waals-type interaction between SF6 and C60. These surfaces were subsequently irradiated with electrons of energy between 15 and 230 eV, stimulating a reaction between C60 and the adsorbed SF6. At low electron beam energy and electron dose (E=15 eV, 1.5×10−3 C cm−2), the electron-stimulated reaction of SF6 with C60 induces some disorder within the films, and subtle changes are observed in the vibrational spectra. At higher beam energy and electron dose (E=230 eV, 4.7×10−3 C cm−2), significant changes occur in the films as demonstrated by the appearance of strong disorder. The resulting material appears to have a graphite-like local structure. Several new vibrations are observed and may be due to the formation of graphitic sheets.
Origin of the optical gap dependence of a-SiGe solar cell stability
Authors: Terakawa Akira , Matsunami Hiroyuki , Kiyama Seiichi , Tsuda Shinya
Published: July 2002

The effect of the optical gap (Eopt) on the thermal recovery behaviors of a-SiGe solar cells was systematically investigated. It was found that the time constant and the activation energy required for the thermal annealing of metastable defects do not strongly depend on Eopt. The independence of the activation energy from Eopt suggests that the variation of Eopt causes little energy difference in the trap depth of hydrogen, which should diffuse to remove metastable defects during network relaxation by annealing. It was also shown that the degradation ratio for narrower Eopt cells exhibits greater temperature dependence after prolonged light soaking. This tendency was analytically attributed to the Eopt dependence of the light-induced defect creation process and the Eopt independence of the thermally induced annealing process.
Comparative study of the relaxation behavior at very low frequencies of acrylate polymers with pendant 1,3-dioxane rings in their structure
Authors: Dı́az-Calleja R. , Sanchı́s M. J. , Guzmán J. , Riande E.
Published: July 2002

The dielectric activity of poly {5-[(acryloxy)methyl]-5-methyl-1,3-dioxacyclohexane} (PAMMD), poly{5-[(acryloxy)methyl]-5-ethyl-1,3-dioxacyclohexane} (PAMED) and poly{5-[(acryloxy) methyl]-2-phenyl-5-ethyl-1,3-dioxacyclohexane} (PAEDP) is studied by thermostimulated discharge current (TSDC) techniques. The global TSDC curves corresponding to PAMMD, PAMED, and PAEDP exhibit subglass absorptions centered in the vicinity of −105, −100, and −120 °C, respectively. The dielectric activity of PAEDP in the glassy region is negligible in comparison with that of both PAMMD and PAEDP which have nearly similar relaxation strengths. Prominent peaks associated with the glass–rubber transition are located at 41, 38, and 57 °C for PAMMD, PAMED, and PAEDP, respectively. The real and loss components of the complex dielectric permittivity in the frequency domain (10−10–10−2 Hz) were calculated at several temperatures from elementary peaks obtained by partial TSDC curves in the glassy region. The differences observed in the spectra are discussed in terms of previous molecular dynamics studies carried out in model compounds of the flexible side groups of these polymers.
Statistical–mechanical calculations of thermal properties of diatomic gases
Authors: Gordillo-Vázquez Francisco J. , Kunc Joseph A.
Published: July 2002

The impact of rotational–vibrational dynamics of molecules on the molecular partition functions, law of mass action and thermodynamic functions of partially dissociated diatomic gases is discussed. A group of 11 gases, expected to have their partition functions the most sensitive to the molecular rotational–vibrational properties, is selected for rigorous and detailed studies, and the partition functions, dissociation degrees and free energies of the gases are calculated (using various models of molecular rotational–vibrational dynamics) and compared in a broad range of temperature and particle density.
Modeling and three dimensional simulation of the neutral dynamics in an air discharge confined in a microcavity. II. Analysis of the wall and geometry effects
Authors: Eichwald O. , Bayle P. , Yousfi Y. , Jugroot M.
Published: July 2002

This article is devoted to the analysis of the confinement effects of the neutral dynamics generated by a short-gap (0.5 mm) discharge inside three different microcavity geometries (cylinder, cube, and bricklike) filled with air at atmospheric pressure (760 Torr) and ambient temperature (293 K). The discharge is modelled by two mathematical functions representing the Joule heating and the momentum transfer between charged and neutral particles. Their spatio-temporal evolution are taken from experimental results with 470 ns for the duration and 13.5 W for the maximum injected power. The neutral gas evolution is described by the classical transport equations and solved by a powerful numerical monotonic upstream-centered scheme for conversion laws. Because of the microcavity dimensions considered, particular care has been used in the analysis of the thermal and hydrodynamics boundary layers which condition the gas–solid interaction in terms of viscous slip effects and thermal exchanges. The results presented show the microcavity geometry effects on the distribution of the initial cylindrical pressure wave as soon as it reaches the lateral walls. They show the specificity of the cube and bricklike microcavities due to the delayed reflections on the corners leading to a more heterogeneous gas behavior than in the case of the cylindrical microcavity. We also discuss the specific gas behaviors near the wall resulting from heat exchange and viscous stress.
Electromagnetic surface wave modes in nonrelativistic electron–positron plasmas
Authors: Cho Sang-Hoon , Lee Hee J.
Published: July 2002

Electromagnetic surface waves propagating on the plane interface (x=0) between the electron–positron plasma and vacuum are investigated by the specular reflection procedure. The transverse electromagnetic modes are studied in terms of dispersion relation both in the presence and absence of an applied magnetic field. The analytic modes for some limiting cases are derived and discussed with the aid of some numerical analysis. In the presence of an applied magnetic field (B0=B0ŷ) directed perpendicular both to the interface normal and the wave vector, the cold electromagnetic surface wave dispersion relation shows that possible modes appear only when the frequency (ω) and the wave vector (k) satisfy the condition Ω2<ω2<Ω2+ωp2 and c2k2>Ω2 (Ω is cyclotron frequency).
Thermal dissociation process of hydrogen atoms in plasma-enhanced chemical vapor deposited silicon nitride films
Authors: Maeda Masahiko , Itsumi Manabu
Published: July 2002

Annealing characteristics of plasma-enhanced chemical vapor deposited silicon nitride films have been evaluated. Annealing temperature dependence for N–hydrogen bonds decreases monotonically with increasing temperature, while that for Si–hydrogen bonds initially increases and then decreases. The annealing time dependence of the absorption coefficients indicate the N–hydrogen bonds have two bonding configurations: one dissociates from 400 °C, and the other above 600 °C. The dissociation of the weaker configuration follows the first-order reaction, and the estimated activation energy is about 0.45 eV. For the Si–hydrogen bonds, the increase of the absorption coefficient in the initial stage of annealing is due to the recombination of hydrogen released atoms from the N–hydrogen bonds with Si dangling bonds, which occurs because of the dense structures of the films. By subtracting out this increase due to the recombination reaction, it is found that the dissociation reaction for the Si–hydrogen bonds is also a first-order reaction, and the estimated activation energy is 0.60 eV.
Calculation of carrier density in separate confinement heterostructure layer
Authors: Kurakake H.
Published: July 2002

Recently, it has been discussed that the carrier overflow into the separate confinement heterostructure (SCH) layer has a significant impact on the performance of semiconductor lasers. To clearly determine the essential parameters related to carrier overflow, the numerical relationship between the carrier densities in the active and SCH layers was calculated, based on the assumption that the quasi-Fermi levels are invariant and the electron and hole densities are equal. The calculated result conformed to that obtained with a laser simulator which was based on the drift-diffusion model. These results suggest that the relationships in the product of the effective state densities of the conduction and valence bands, and the band gap between the active and SCH layers, are essential for the carrier overflow.
Temperature induced perpendicular magnetic anisotropy in Co/Cu/Co trilayers
Authors: Dinia A. , Persat N. , Danan H.
Published: July 2002

The evolution of the magnetic anisotropy between room temperature and 50 K has been studied using torque magnetometry on a hcp (0001) Co (0.8 nm)/Cu (1.5 nm)/Co (0.8 nm) trilayer prepared by ultrahigh vacuum evaporation. At 300 K this sample presents an easy plane of the magnetization in the film plane, with a very small effective anisotropy constant Keff (≈−4.91×105 erg/cm3). By cooling the sample, the easy magnetization direction becomes perpendicular to the film plane. Keff is positive below 288 K and increases continuously upon decreasing the temperature. At 50 K, the effective anisotropy constant reaches about 2.4×107 erg/cm3. This strong increase of the effective anisotropy upon decreasing the temperature can be explained by a strong increase of surface anisotropy term. Magnetization measurements have revealed the existence of one magnetically dead monolayer at each interface, indicating a strong intermixing in our Co/Cu interfaces at 300 K. Thus the evolution of the magnetism of the intermixed region as a function of the temperature may be at the origin of the strong increase of the effective anisotropy.
Fabrication and magnetic investigation of Y or Gd containing CoFe2O4 nanocrystalline films
Authors: Yan Chunhua , Cheng Fuxiang , Peng Zuoyan , Xu Zhigang , Liao Chunsheng
Published: July 2002

The exertion for carrying out the substitution of Fe3+ ion in CoFe2O4 spinel by Y3+ and Gd3+ ions has been put forth in the present article. Y or Gd containing CoFe2O4 nanocrystalline films with average particles size of 65 nm were synthesized by sol-gel method, and then the magnetic properties of the products were investigated at room temperature. To improve the crystallization of Gd-doped cobalt spinels, we buffered them on about a 20-nm-thick CoFe2O4 thin film. However, the analysis on the results for both x-ray diffraction and magnetic measurement showed that it is difficult to perform the doping of Y3+ ions inside cobalt spinel ferrite. The proper doping amount of magnetic Gd3+ ion can enlarge saturation magnetization (Ms) and coercive force (Hc) of resultant nanocrystalline films, whereas the doping of Y only reduces them.
Effect of iron on oxygen precipitation in nitrogen-doped Czochralski silicon
Authors: Zhang Xiwen , Yang Deren , Fan Ruixin , Zhang Jinxin , Que Duanlin
Published: July 2002

The effect of iron on oxygen precipitation in nitrogen-doped Czochralski (NCZ) silicon was investigated by Fourier transform infrared spectroscopy at room temperature or at liquid helium temperature. The experiments revealed that the oxygen precipitation could be enhanced by the contamination of iron in common Czochralski (CZ) silicon, or by the doping of nitrogen in NCZ silicon. In NCZ silicon, iron did not affect the precipitation of oxygen during annealing at high temperatures. After preannealing at 750 °C, the oxygen precipitation in NCZ silicon was suppressed due to the addition of iron. It is concluded that the generated iron nitride, which is related to an optical absorption line at 669 cm−1, emits self-interstitial silicon atoms to impede the nucleation of oxygen precipitates at low temperatures.
Laser-induced structural relaxation and crystallization phenomena in the picosecond time scale in GeSbO thin films
Authors: Solis J. , Morilla M. C. , Afonso C. N.
Published: July 2002

Structural relaxation phenomena in the ps time scale have been observed upon pulsed laser irradiation of (GeSb)O amorphous films. The transformation of the irradiated surfaces has been temporally resolved by means of in situ optical techniques with ns and ps resolution. The results show the occurrence of structural relaxation phenomena in a time shorter than 600 ps in the as-deposited material or about 200 ps in thermally relaxed samples. In the latter case, a substantial amount of crystalline material is nucleated at the initial stage of the transformation.
X-ray irradiation induced bulk space charge in stabilized a-Se x-ray photoconductors
Authors: Haugen C. , Kasap S. O. , Rowlands J.
Published: July 2002

The time-of-flight (TOF) transient photoconductivity technique is used with an appropriate method and algorithm, based on the time dependence of the transient photocurrent i(t), to probe the spatial distribution of x-ray induced net bulk space charge in stabilized a-Se (a-Se:0.3%As doped with 10 to 20 ppm Cl) x-ray photoconductors with long carrier lifetimes (hole lifetime of ∼500 μs and electron lifetime of ∼750 μs from interrupted TOF). It is observed that x-ray irradiation of biased a-Se photoconductors results in a net negative space charge distribution. The spatial variation of the space charge density ρ(x) was found to decay exponentially from the irradiation surface. The spatial dependence of ρ(x) did not follow the photon absorption profile as closely as observed previously when only unbiased a-Se layers were irradiated. However, the changes in the spatial distribution of ρ(x) with the x-ray mean photon energy exhibited the same tendency as the photon absorption profile. The distribution of the trapped charge and rate of detrapping was examined as a function of temperature, photon energy, and with infrared illumination. The total negative space charge was found to decrease with time following a power law dependence of the form N∼t−n, where n≈0.3 and was independent of the irradiation intensity, which indicates that electron detrapping occurs from an energy distribution of deep localized states. Using an infrared (IR) filter that allows photon energies smaller than 1.1 eV to pass, it is shown that IR illumination increases the rate of detrapping and confirms that electrons are trapped in deep electron trapping states between 0.6 and 1.1 eV in the density of states diagram of a-Se [M. Abkowitz Philos. Mag. Lett. 58, 53 (1988)]. The mean detrapping current was found to be ∼10 pA, orders of magnitude smaller than the expected dark current. However, the trapped charge modifies the electric field at the top metal electrode causing it to become more injecting and leading to an increase in the dark current. The increase in the dark current is a possible origin for the persistent x-ray photocurrents recently reported for a-Se x-ray photoconductors.