bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation of magnetic, magnetomechanical, and electrical properties of the Tb0.27Dy0.73Fe2−xCox system
Authors: Dhilsha K. R. , Rama Rao K. V. S.
Published: July 2002

X-ray-diffraction studies carried out on Tb0.27Dy0.73Fe2−xCox (x=0,0.5,1.2,2.0) revealed that all compositions investigated retained the C-15 cubic Laves phase structure. Magnetization and electrical resistivity measurements were carried out in the temperature range 77–800 K. Anomalies observed in the resistivity measurements have been identified corresponding to spin reorientation transitions occurring in this system. Some of these transitions have also been observed in the magnetization data. Magnetostriction and magnetomechanical coupling coefficient were measured on samples of x=0, 0.5, and 1.2 prepared by arc melting as well as arc melting followed by zoning using an induction furnace. Magnetostriction and coupling coefficient were found to decrease with increase in cobalt concentration. Magnetostriction data on zoned samples revealed a change in the easy direction of magnetization from [111] to [100] in the case of x=0.5.
Analysis of GaAs Schottky/tunnel metal–insulator–semiconductor diode characteristics based on an interfacial layer model
Authors: Ikoma Hideaki , Ishida Toshiki , Sato Kenji , Ishikawa Toshifumi , Maeda Keiji
Published: July 2002

Current-voltage (I-V) characteristics of GaAs Schottky and tunnel metal–insulator–semiconductor diodes were measured at various temperatures between 100 and 400 K and analyzed on a basis of an interfacial layer model recently proposed by Maeda, Umezu, Ikoma, and Yoshimura [J. Appl. Phys. 68, 2858 (1990)]. The ideality factor n obtained from the forward I-V characteristics increased from about 1.05–1.1 to 2–3 as the temperature was decreased from 400 to 100 K. This could be interpreted as an increase in the electron occupation ratio of the interface states with decreasing temperatures. Precise measurement of the n value of the I-V curve revealed anomalous behavior as a function of the forward current, which was not observed in either amorphous or crystalline Si Schottky diodes. This seems to be related to the presence of a density peak in the interface state distribution like those proposed by Spicer, Newman, Spinat, Liliental-Weber, and Weber, [J. Vac. Sci. Technol. A 8, 2084 (1990)]. The interfacial Fermi level was demonstrated to move from the metal Fermi level as applied voltage is increased, but it hardly moved relative to the conduction band bottom. This is considered to be the first direct evidence of Fermi level pinning in I-V characteristics.
Study and improvement of anomalous interface states of metal-oxide-semiconductor structures induced by rapid thermal post-oxide annealing
Authors: Hsieh J. C. , Fang Y. K. , Chen C. W. , Tsai N. S. , Lin M. S. , Tseng F. C.
Published: July 2002

Anomalous interface states were caused by post-oxide rapid thermal annealing in an n+ polycrystalline silicon metal-oxide-semiconductor capacitor. These anomalous interface states have been investigated using high/low frequency capacitance/gate voltage (C/V) measurements. An additional annealing process (450 °C, 30 min in 90% N2/10% H2 mixed gas) was found to improve the anomalous interface states. The improved results were identified using a constant current injection stress test.
A comparison of the thermal and near band-gap light-induced recoveries of EL2 from its metastable state in semiinsulating GaAs
Authors: Alvarez A. , Jiménez J. , Chafai M. , Bonnafé J. , González M. A.
Published: July 2002

The reverse transition from the EL2 metastable state to the ground state is achieved by thermal annealing above 140 K. An optical recovery of the ground state photoresponse (1–1.3 eV) can be also done exciting with near band-gap light. We present herein a photocurrent study of the photoquenching transients starting from either a thermal recovery situation or an optically recovery one. It is seen that the recovered states are not the same for both of them. While the thermal recovery restores the ground state of EL2, the optical recovery restores another state, labeled EL2r, which gives a different photoquenching transient as compared to the ground EL2 level.
Thermal decomposition of native oxide on Si(100)
Authors: Miyata N. , Shigeno M. , Arimoto Y. , Ito T.
Published: July 2002

We investigated the thermal decomposition of native oxide on Si(100) under ultrahigh vacuum using high-resolution x-ray photoelectron spectroscopy (XPS). The native oxide was formed by wet chemical treatment (HCl/H2O2/H2O), a widely employed procedure for preparing atomically clean surfaces. XPS measurements revealed that high temperature heating (≳700 °C) leads to a remarkable alteration in Si 2p and O 1s spectra. After heating to 700 °C, the Si3+ structure increases and the O 1s full-width-at-half-maximum decreases. After heating to 800 °C, the Si4+ and O 1s intensity decreases but the Si2+ intensity remains almost unchanged. We suggest that the formation of volatile SiO is related to the Si3+ structure produced by high temperature annealing.
Neutral and ion emissions accompanying pulsed excimer laser irradiation of polytetrafluoroethylene
Authors: Dickinson J. T. , Shin Jaw-Jung , Jiang W. , Norton M. G.
Published: July 2002

The neutral and charged species emitted by pulsed-laser irradiation of polytetrafluoroethylene (PTFE) at 248 nm in vacuum have been examined. In particular, the species and properties of the emission products produced at typical fluence regimes used in the pulsed-laser deposition of PTFE thin films have been characterized. The relative intensities of the major products as well as their dependence on laser fluence are presented, and a simple model is used to fit the observed fluence dependence. Evidence that the major neutral component, the monomer (C2F4), is formed from a thermally activated unzipping reaction is presented. The ionic species are derived from the neutral decomposition products, apparently ionized by electron collisions in the weak plasma generated at the target surface.
Preparation of p-type silicon films by plasma decomposition of a SiH4/H2/BF3 gas mixture
Authors: Kakinuma H. , Mohri M. , Tsuruoka T.
Published: July 2002

p-Type microcrystalline Si films have been prepared by rf decomposition of SiH4/H2/BF3 gases. The gas composition, substrate temperature, and rf power dependence of film conductivity σ are investigated. The σ increases with increasing BF3 and saturates or takes a maximum at [BF3]/[SiH4]∼1. A fairly high σ value of ∼1.2×10−2 S/cm is obtained for the low rf power (∼0.1 W/cm2). Increasing rf power decreases σ, which is in contrast to the SiH4/H2/B2H6 system. The incorporation rate of boron from gas phase into the film is found to be low (∼4%) by use of secondary ion mass spectroscopy. Increasing hydrogen increases σ. Temperature is found to be the most effective deposition parameter. The σ monotonically increases with an increase of substrate temperature TS. Transmission electron microscopy observation shows that films with higher σ contain higher density or larger size Si crystallites. The crystalline size increases by a short time (≤15 min) annealing at 400 °C. The deposition parameter dependence of σ in comparison to the SiH4/H2/B2H6 system is discussed. The low temperature annealing effect is discussed in terms of film structure.
Defect structure of strontium titanate thin films
Authors: Feil W. A. , Wessels B. W.
Published: July 2002

The electrical and optical properties of deep-level defects in SrTiO3 thin films grown by organometallic chemical-vapor deposition were investigated. Transient photocapacitance spectroscopy was used to determine the deep-level defect structure. Metal/insulator/semiconductor diodes were used for these measurements. The dominant defects in the SrTiO3 consisted of a series of deep-level trapping states with energies in the range of Ev+2.4 to Ev+3.15 eV and a series of shallower traps near the conduction-band edge. The defect concentration ranged from 1014 to 1018 cm−3 in the as-grown films.
Giant magnetoresistance in soft magnetic NiFeCo/Cu multilayers
Authors: Jimbo Mutsuko , Tsunashima Shigeru , Kanda Tatsuya , Goto Shoji , Uchiyama Susumu
Published: July 2002

Giant magnetoresistance effect and crystal structure have been investigated for Ni66Fe16Co18/Cu multilayers with Fe and NiFeCo buffer layers prepared by rf magnetron sputtering. For both buffer layers, the multilayers show magnetoresistance, which oscillates as a function of Cu thickness. The maximum magnetoresistance ratios at the first peak are 35% and 25% and those at the second peak are 15% and 12% for the Fe and NiFeCo buffer layers, respectively. The multilayers with the NiFeCo buffer show weaker antiferromagnetic coupling than those with the Fe buffer. The NiFeCo (5 nm) [NiFeCo(1.5 nm)/Cu(2.2 nm)]30 multilayer with uniaxial magnetic anisotropy shows a MR ratio of 12%, which reaches 90% of the saturation with a field as low as 20 and 30 Oe in the easy and hard directions, respectively.
X-ray double and triple crystal diffractometry of mosaic structure in heteroepitaxial layers
Authors: Holý V. , Kuběna J. , Abramof E. , Lischka K. , Pesek A. , Koppensteiner E.
Published: July 2002

X-ray diffraction in thin layers containing small randomly placed defects is described by means of the kinematical diffraction theory and optical coherence formalism. The method enables us to calculate both the diffracted intensity and its angular distribution, so that it can be used for simulating double crystal and triple crystal x-ray diffractometry experiments. The theory has been applied to experimental data obtained from diffractometry measurements of an epitaxial ZnTe layer with mosaic structure after several steps of chemical thinning. A good agreement of the theory with experiments has been achieved.
General solution to the hygrothermoelastic interface problem with discontinuity between dissimilar anisotropic media
Authors: Chang R. C. , Chao C. K.
Published: July 2002

A general solution to the hygrothermoelastic problem associated with an interface discontinuity between dissimilar anisotropic media is presented. The complex variable representation of Lekhnitskii [Theory of Elasticity of an Anisotropic Elastic Body (Holden-Day, San Francisco, CA, 1963)] is extended into the hygrothermoelastic problem by means of five complex functions including three stress functions, one temperature function, and one moisture function. A special technique of analytical continuation is used to deal with the continuous conditions of dissimilar media. Based on the Hilbert problem formulation, the linear relation between the boundaries of discontinuity can be derived into a type of Cauchy integral, which makes the solution in a compact version. The result shows that the heat or moisture flux exhibits an inverse square-root singularity at the tips of discontinuity while the nature of singularity of the hygrothermal stresses possess the same trig-log character as those obtained for the elastic problem.
Current conduction in an implant isolated GaAs/AlGaAs heterostructure
Authors: Henderson T. , Liu W. , Kim T. S.
Published: July 2002

We report the current-voltage characteristics of an oxygen implant-isolated region of a GaAs/AlGaAs heterostructure between 25 and 295 K. Current conduction was dominated by bulk rather than metal contact limited effects. Bulk conduction in the implant-isolated region is due to several different mechanisms; at low electric fields a resistive current; at low temperatures and high electric fields, field ionization current proportional to V2 exp(−V0/V); and at high temperatures and high electric fields, Poole–Frenkel conduction proportional to V exp(aV1/2/rkT−qφb/rkT). The resistive current can further be separated into two components, one of which dominates at low temperatures and is proportional to V exp(−B/T1/4), and the other which dominates at high temperatures and is proportional to V exp(−Ea/kT). These results are applicable to any GaAs-based structure rendered semi-insulating by an isolation implant, and describe the current conduction characteristics from zero bias to breakdown. Such a description is critical to understanding leakage currents in GaAs/AlGaAs devices such as high-performance heterojunction bipolar transistors, field-effect transistors, and laser diodes that commonly employ implant isolation as a part of the fabrication process.
Growth of zinc blende-GaN on β-SiC coated (001) Si by molecular beam epitaxy using a radio frequency plasma discharge, nitrogen free-radical source
Authors: Liu H. , Frenkel A. C. , Kim J. G. , Park R. M.
Published: July 2002

We report the growth of zinc blende-GaN epitaxial films on β-SiC coated (001) Si substrates using a molecular beam epitaxy approach in which the reactive nitrogen species are generated in a remote 13.56 MHz rf plasma discharge, nitrogen free-radical source. We postulate, based on optical emission spectroscopy studies of the remote plasma, that, in our study, nitrogen atoms are the species primarily responsible for efficient nitridation. The zinc blende nature of the GaN films was confirmed by in situ reflection high-energy electron diffraction, ex situ x-ray diffraction, and ex situ low-temperature photoluminescence analyses. Our zinc blende-GaN film growth rates (∼0.3 μm/h) are higher than those reported to date that involve the use of electron cyclotron resonance type reactive nitrogen sources.
Observation of contact holes by atomic force microscopy with a ZnO whisker tip
Authors: Kado Hiroyuki , Yamamoto Shin-ichi , Yokoyama Kazuo , Tohda Takao , Umetani Yukihiro
Published: July 2002

We have developed a new precise imaging technique for atomic force microscopy (AFM) and observed complicated surface structures of integrated circuit (IC) devices by this technique. This technique consists of a hopping mode operation and use of a zinc oxide whisker as a probing tip with high aspect ratio. In this operation, topographic data of the surface structures are obtained under a constant repulsive force at each measuring point, and after each measurement the probing tip is withdrawn from the surface and moved to the next measuring point. The contact holes with a diameter of ∼1 μm and a depth of 1.5 μm fabricated on IC devices have been successfully imaged in this AFM imaging technique.
Ladderlike ionization of the mercury atom in Hg-Ar low-pressure discharge and its modeling
Authors: Wani Koichi
Published: July 2002

A collisional-radiative model for hydrogen plasma has been reviewed in order to evaluate the ionization process through the high-lying levels of the mercury atom. Experimentally observed population densities of the mercury atom near its series limit have shown an x*−6 dependence (x* is the effective principle quantum number) as is predicted from the collisional-radiative model for an ionizing plasma. This fact suggests that a ladderlike ionization process accounts for a missing ionization process found in a previous model calculation for the mercury-argon low-pressure discharge [J. Appl. Phys. 68, 5052 (1990)]. An effective rate coefficient for the ladderlike excitation and ionization has been determined and introduced into the model equation. Electric and optical characteristics as well as the population densities of the excited atoms in the positive column have been calculated and compared with the experimental result. Significant improvement in agreement is obtained. The ladderlike excitation and ionization via high-lying levels of the mercury atom have been shown to occupy 10%–20% of the total ionization rate in the discharge considered.
Calculation of lifetime dependence of Er3+ on cavity length in dielectric half-wave and full-wave microcavities
Authors: Lin C. C. , Deppe D. G.
Published: July 2002

Angular spontaneous emission rates and lifetimes of Er3+ in Si/SiO2 microcavities are calculated for cavity lengths close to full and half-wavelengths. For a half-wavelength cavity, the calculations show that the spontaneous lifetime is sensitive to the variation of cavity length, and agrees very well with the experimental data measured by Vredenberg, Hunt, Schubert, Jacobson, Poate, and Zydzik [Phys. Rev. Lett. 71, 517 (1993)]. This variation comes from a coupling rate change into the cavity normal. For a full wavelength cavity, the lifetime is less sensitive to cavity length due to strong coupling of spontaneous emission to a fundamental waveguide mode.
Aging studies of atomic layer epitaxy ZnS:Mn alternating-current thin-film electroluminescent devices
Authors: Abu-Dayah A. , Wager J. F.
Published: July 2002

A study of the aging behavior of atomic layer epitaxy (ALE) ZnS:Mn alternating-current thin-film electroluminescent (ACTFEL) devices is undertaken by monitoring the internal charge-phosphor field (Q-Fp) and capacitance-voltage (C-V) characteristics as a function of aging time. The electrical properties of ALE ZnS:Mn ACTFEL devices are asymmetrical with respect to the applied voltage pulse polarity; the aging characteristics of such devices are also asymmetric. The electrical asymmetries are attributed to differences in the interface state densities at the top and bottom phosphor/insulator interfaces. The presence of Cl at the indium tin oxide interface and its tendency to migrate into the ZnS during aging is believed to be the major cause of the observed asymmetrical aging characteristics.
Erbium in crystal silicon: Segregation and trapping during solid phase epitaxy of amorphous silicon
Authors: Custer J. S. , Polman A. , van Pinxteren H. M.
Published: July 2002

Solid phase epitaxy of Er-implanted amorphous Si results in segregation and trapping of the Er, incorporating up to 2×1020 Er/cm3 in single-crystal Si. Segregation occurs despite an extremely low Er diffusivity in bulk amorphous Si of ≤10−17 cm2/s, and the narrow segregation spike (measured width ≊3 nm) suggests that kinetic trapping is responsible for the nonequilibrium concentrations of Er. The dependence of trapping on temperature, concentration, and impurities indicates instead that thermodynamics controls the segregation. We propose that Er, in analogy to transition metals, diffuses interstitially in amorphous Si, but is strongly bound at trapping centers. The binding enthalpy of these trapping sites causes the amorphous phase to be energetically favorable for Er, so that at low concentrations the Er is nearly completely segregated. Once the concentration of Er in the segregation spike exceeds the amorphous trap center concentration, though, more Er is trapped in the crystal. We also observe similar segregation and trapping behavior for another rare-earth element, Pr.
Photoinduced scanning tunneling microscopy of insulating diamond films
Authors: Mercer T. W. , Carroll D. L. , Liang Y. , DiNardo N. J. , Bonnell D. A.
Published: July 2002

Insulating diamond films were imaged by scanning tunneling microscopy (STM) utilizing photoinduced bulk carrier transport to establish tunneling currents. General comparisons of topographic STM images and atomic force microscopy images acquired on the same sample demonstrate that submicrometer structures obtained in the images can be correlated. This observation establishes that the topography of an insulating surface such as diamond can be imaged under illumination by STM. The results suggest that local electronic structure of illuminated insulating surfaces can be probed using scanning tunneling spectroscopy.
Optical properties of disordered GaAs/(Al,Ga)As distributed Bragg reflectors
Authors: Floyd P. D. , Merz J. L.
Published: July 2002

The variation of the reflectivity, stopband width, and penetration depth of disordered GaAs/(Al,Ga)As distributed Bragg reflectors (DBRs) is analyzed by simulating interdiffusion of Al and Ga at the interfaces within the DBR. The results show that important optical parameters are unchanged for short diffusion lengths, but drastically degrade for larger diffusion lengths. The results indicate the limits of thermal processing for device structures containing GaAs/AlGaAs DBRs.
Electrical resistivity and local magnetic order in random anisotropy amorphous ferromagnets
Authors: Amaral V. S. , Sousa J. B. , Moreira J. M. , Barbara B. , Filippi J.
Published: July 2002

In ferromagnetic systems with random anisotropy (RA), long-range magnetic order is destroyed and the correlation function of the magnetization exhibits an exponential form e−r/Rf, where the correlation length Rf corresponds to the size of Imry–Ma domains. Also, the law of approach to magnetic saturation follows a 1/√H law [ferromagnet with wandering axis (FWA) regime]. We have calculated the effect of magnetic correlations on the electrical resistivity and magnetoresistance of a RA ferromagnetic system at low temperatures. We find that in zero magnetic field RA introduces a positive contribution into the electrical resistivity, which increases with the anisotropy to exchange ratio, D/𝒯.  In the FWA regime the magnetoresistance has a logarithmic dependence on the magnetic field with a slope Δρ/(ρ ln H) inversely proportional to Rf. These features have been observed in an experimental study of the magnetoresistance at low temperatures (T≊4 K) of the amorphous series (DyxGd1−x)Ni in which the D/𝒯  ratio can be varied from near zero up to one from GdNi to DyNi. A quantitative analysis of the results enables the direct determination of the correlation length Rf and of the volume of structural correlations Ωc. We find that Rf varies from about 12 Å in DyNi up to 300 Å in (Dy0.1Gd0.9)Ni and Ωc is the same throughout the series (Ωc≊500 Å3) in excellent agreement with the values determined from magnetic measurements on the approach to saturation. These results provide an independent confirmation of the exponential decay with distance and field dependence of magnetic correlations in random anisotropy systems.
Sign reversal of the linear electro-optic effect in the chiral nematic phase
Authors: Komitov L. , Lagerwall S. T. , Stebler B. , Strigazzi A.
Published: July 2002

The chiral nematic phase aligned with its helical axis along one preferred direction, being parallel to the confining substrates, exhibits a linear electro-optic effect when an electric field is applied normally to the substrates. The effect is considered to be of flexoelectric origin. A sign reversal of the effect is found when the helix of the chiral nematic phase changes its handedness, thus revealing the relationship between the sign of the linear electro-optic effect and the helix handedness. Here, we present and discuss a simple model of this relationship.
Characterization of deep level defects in thermally annealed Fe-doped semi-insulating InP by thermally stimulated current spectroscopy
Authors: Kuriyama K. , Tomizawa K. , Kashiwakura Mari , Yokoyama K.
Published: July 2002

The evaluation of deep defects in unannealed and annealed Fe-doped semi-insulating InP was studied using thermally stimulated current (TSC) and photoluminescence (PL) methods. Four TSC peaks with an ionization energy of 0.16, 0.23, and 0.42 eV, respectively, are observed clearly at an annealing temperature ranging from 400 to 700 °C. A 0.42 eV trap is associated with a deep phosphorous vacancy complex defect. 0.16 and 0.32 eV traps are related to the 917 and 888 nm PL emissions, respectively. These emissions are attributed to the deep-donor-acceptor pair transition and exciton bound to a deep level acceptor, respectively, while the 0.23 eV trap is likely to be buried in a broad TSC peak observed in a starting sample.
Ellipsometry of anisotropic substrates: Re-examination of a special case
Authors: De Smet D. J.
Published: July 2002

When the index of refraction of a bulk material is determined using ellipsometry, an erroneous result is obtained if the material is anisotropic, and we have assumed that it is isotropic. This is not surprising. What is surprising is that, in the case of a uniaxial material with its optic axis perpendicular to the surface, the apparent index of refraction obtained from ellipsometric measurements does not lie between the ordinary and extraordinary indices of refraction and is therefore not even a good estimate of the actual index of refraction experienced by light in the anisotropic material. In addition, even though the actual index of refraction experienced by the light depends on the angle of incidence, making ellipsometric measurements at different angles of incidence does not resolve this problem.
Studies on diamondlike carbon films for antireflection coatings of infrared optical materials
Authors: Zhang G. F. , Guo L. J. , Liu Z. T. , Xiu X. K. , Zheng X.
Published: July 2002

Diamondlike carbon (DLC) films were directly deposited onto germanium (Ge) and zinc sulphide (ZnS) slices by a capacitively coupled 13.56 MHz rf glow discharge plasma with Ar-C2H2 gas mixtures. The IR transmittance was measured using a Fourier-transform infrared spectrometer. The maximum values of the IR transmission of Ge and ZnS with DLC films on both sides are 99% and 95.8%, respectively, which come up to the theoretical values. A nonuniform DLC film, of which the refractive index gradually changes along the thickness, has been successfully deposited onto a Ge slice for the first time and the IR transmission of a nonuniform DLC film coated onto both sides of a Ge substrate at the wider wave band of 2.5–15 μm is over 85%.
Diagnostics of YBa2Cu3O7−δ laser plume by time-of-flight mass spectrometry
Authors: Berardi V. , Amoruso S. , Spinelli N. , Armenante M. , Velotta R. , Fuso F. , Allegrini M. , Arimondo E.
Published: July 2002

The time-of-flight mass spectrometry has been used to investigate the formation of charged species in the laser ablation of YBa2Cu3O7−δ (YBCO) target. Mass spectra were taken both in high vacuum and in oxygen environment for two values of the laser energy density (0.74 and 2.5 J cm−2). The analysis of the charged species in the plume has been carried out as a function of the distance between the target and the spectrometer axis and the relative abundance of Cu, Y, Ba, CuO, YO, and BaO ions has been measured and discussed. In this paper the formation of heavy species and YBCO clusters is also discussed. The plume range has been evaluated as a function of the absorbed energy at a distance from the target plane close to that one at which good quality high Tc superconducting films are obtained, the plume composition has been found to be characterized by the presence of a series of oxide clusters originating from the interaction between the ablated material and the environmental oxygen. This confirms that the interaction between the plume and the environmental oxygen plays an important role both in the formation of heavy charged species and in the capture of the oxygen required to obtain the correct stoichiometric composition of the deposited film.
Coercivity and switching field of single domain γ-Fe2O3 particles under consideration of the demagnetizing field
Authors: Fulmek Paul L. , Hauser Hans
Published: July 2002

The coherent rotation of the spontaneous polarization in ellipsoidal ferromagnetic single domain particles is calculated under consideration of the magnetocrystalline anisotropy and the true, inner field. Magnetization curves and switching fields are computed for uniaxial and cubic magnetocrystalline and shape anisotropy. Considering only the magnetic energy inside the particle, the resulting switching fields are lower than those predicted by the Stoner–Wohlfarth theory, and they are qualitatively and quantitatively comparable to the results of other models. The comparison to measurements on isolated γ-Fe2O3 particles shows excellent agreement.
Temperature dependence of photoluminescence in InAsP/InP strained multiple quantum wells
Authors: Zhao Y.-G. , Masut R. A. , Brebner J. L. , Tran C. A. , Graham J. T.
Published: July 2002

The photoluminescence spectra of InAsP/InP strained multiple quantum wells have been experimentally determined in the temperature range 7–300 K. In order to understand the temperature behavior of the photoluminescence, a theoretical calculation is presented that takes into account the temperature-induced variations in band gap, carrier effective mass, biaxial strain, and exciton binding energy. The results show that the energy of the transition E1H between the n=1 electron subband and the n=1 heavy-hole subband changes as a function of temperature, and depends mainly on the evolution of the strained band gap of the InAsP layers. This is because in the temperature range 7–300 K the variations of the electron subband energy and the exciton binding energy are much less than those of the strained band gap, while the variation of the heavy-hole subband energy can be neglected. These results also explain why, for a lattice-matched quantum well, the variation of exciton peak energies with temperature follows that of the forbidden energy gap of the bulk material in the well.
Amorphization and recrystallization of 6H-SiC by ion-beam irradiation
Authors: Heera V. , Stoemenos J. , Kögler R. , Skorupa W.
Published: July 2002

Amorphization of 6H-SiC with 200 keV Ge+ ions at room temperature and subsequent ion-beam-induced epitaxial crystallization (IBIEC) with 300 keV Si+ ions at 480 °C have been studied by Rutherford backscattering spectrometry/channeling and transmission electron microscopy analysis. Experimental results on amorphous layer thicknesses have been compared with trim calculations in association with the critical energy density model. Density changes during amorphization have been observed by step height measurements. Particular attention has been directed to the crystal quality and a possible polytype transformation during the IBIEC regrowth. The IBIEC process consists of two stages and results in a multilayer structure. In the initial phase an epitaxial growth of 6H-SiC has been obtained. With increasing IBIEC dose the epitaxial growth changes to columnar growth and is stopped by polycrystallization of 3C polytype in the near-surface region.
Electric and photoelectric properties of diode structures in porous silicon
Authors: Pačebutas V. , Krotkus A. , Šimkienė I. , Viselga R.
Published: July 2002

Diode properties of structures containing a porous silicon layer have been investigated. The structures have been fabricated by depositing metal or indium-tin-oxide layers either directly on the ‘‘as-anodized’’ silicon wafer or on the wafers with upper, nanocrystalline part of the porous layer removed. Different behavior was observed in both cases: (1) The photovoltaic effect is absent in diodes from as-anodized wafers but appears in diodes from the cleaned wafers; (2) the diode ideality factor is close to 2 for ‘‘cleaned’’ diodes and increases in diodes containing an upper porous silicon layer. Energy-band diagrams for the structures are proposed and it is concluded that the electrical transport in those structures is limited by the carrier recombination in the depletion layer.
Influence of the idle region on the dynamic properties of window Josephson tunnel junctions
Authors: Monaco R. , Costabile G. , Martucciello N.
Published: July 2002

The processes developed in the last years for the fabrication of high quality, reliable all-refractory Josephson tunnel junctions open a window in an insulating region, called the idle region, which provides the passivation of the base electrode all around the junction itself. We report experimental data showing that the phase velocity of electromagnetic waves in the junction barrier can be considerably modified by the presence of the insulating region surrounding the window Josephson tunnel junctions. We have investigated this phenomenon in unidimensional junctions, measuring the asymptotic voltage of the Fiske resonances in the presence of an externally applied magnetic field. We have considered the effect of an idle region: (i) along the long junction dimension and (ii) at the extremities of the junction. When comparing the behavior to that of a bare junction, in the first case the phase velocity has been found to increase and to be affected by dispersion, while in the latter case it decreased and was frequency independent. In order to interpret the main features of the phenomena observed experimentally in terms of basic mechanisms, we propose two simple models for the two different geometrical configurations.
Resonant behavior, symmetry, and singularity of the transfer matrix in asymmetric tunneling structures
Authors: Gildenblat G. , Gelmont B. , Vatannia S.
Published: July 2002

We investigate the energy dependence and symmetry properties of the transfer matrix describing the tunneling and reflection processes in multiple asymmetric localized potential barriers. In particular, we show that certain quasi-bound states result in Lorentzian peaks of the diagonal matrix element of the transfer matrix in the energy region where tunneling is impossible. The change of symmetry properties of the transfer matrix in this energy range is established and related to a characteristic behavior of the phase of the reflection amplitude. This analysis is verified using quasi-classical theory of asymmetric double barriers and numerical calculations of the transfer matrix in electrically biased finite superlattices.
Two-region model for hydrogen trapping in and release from graphite
Authors: Haasz A. A. , Franzen P. , Davis J. W. , Chiu S. , Pitcher C. S.
Published: July 2002

A new model has been developed for hydrogen retention and trapping in and release from graphite. Two different regions in the graphite with different hydrogen transport and trapping behaviors are distinguished, the bulk region within, and the surface region on graphite crystallites. The model incorporates new experimental results related to atom diffusion and recombination on inner surfaces. Recombination is explained from a fundamental viewpoint by linking it to diffusion using a classical expression. The model is applied to a number of reemission and thermal desorption experiments, in particular, the reemission of hydrogen atoms during irradiation with energetic hydrogen ions and the formation of HD during irradiation with H+ and D+ or during thermal desorption of graphite that was preimplanted with H+ and D+ ions with different energies.
Point defects in irradiated Li3VO4 single crystal
Authors: Miki T. , Murata T. , Ishii T. , Ebina Y. , Sakata S. , Fujii I.
Published: July 2002

Radiation-induced defects in a new second harmonic generation material, lithium vanadate single crystal, have been studied by electron paramagnetic resonance (EPR) and optical absorption measurements. EPR detects two intense signals due to intrinsic O− and impurity-associated CO−3 trapped-hole centers at 77 K, but no such signal in the case of trapped-electron centers. The EPR signal of impurity Fe3+ ions is suppressed by 77 K irradiation. Low-temperature irradiation produces three optical absorption bands peaking at 3.14, 2.0, and 1.56 eV. It has been concluded that some of the free electrons produced by irradiation are trapped by Fe3+ ions at low temperatures. After annealing at room temperature all the optical absorption bands disappear, although the EPR signals of CO−3 trapped-hole centers and unidentified electron-type centers at g=1.96 remain stable up to 330 K.
Photoreflectance study on the behavior of plasma-induced defects deactivating Si donors in GaAs
Authors: Nakanishi Hideo , Wada Kazumi , Walukiewicz W.
Published: July 2002

The behavior of Ar plasma-induced defects deactivating Si donors in GaAs has been studied. We have applied photoreflectance spectroscopic analysis combined with stepwise wet etching to the depth profiling of defects and succeeded in determining the concentrations and profiles of defects in the sub-surface layer for the first time. We found that the point defects responsible for deactivation undergo electronically enhanced diffusion under the circumstance of photoexcited carriers, demonstrating that the ultraviolet light from plasma is a cause of the deep penetration of defects far beyond the stopping range of ions. We also found that the generation of these point defects is enhanced by photoexcited carriers and that diffusing point defects are trapped by the background defects or impurities, forming immobile complexes. We propose a model in which self-interstitials are the most probable point defects responsible for deactivation.
Overheated metastable states in pulsed laser action on ceramics
Authors: Mazhukin V. I. , Smurov I. , Flamant G.
Published: July 2002

Volume overheating of solid and liquid phases in pulsed laser evaporation of superconducting ceramics is analyzed by numerical simulation. The mathematical model includes the processes of heating (with a volume energy release), melting-solidification, and evaporation. It is shown that the maximum values of overheating of the solid phase (with respect to its melting point) exceed 100 degrees and those of the liquid phase exceed several hundred degrees (with respect to the surface temperature). The times of existence of these metastable states are tens and hundreds of nanoseconds, respectively. The dynamics of the processes are analyzed in a wide range of variation of the absorption coefficient (i.e., laser wavelength). It is shown that the probability of explosive decay of the metastable states in the solid phase increases with laser wavelength, whereas for the metastable states in the liquid phase the overheating parameters exhibit a maximum versus laser wavelength.
Temperature dispersion of refractive indices in β-BaB2O4 and LiB3O5 crystals for nonlinear optical devices
Authors: Ghosh Gorachand
Published: July 2002

The thermo-optic coefficients, i.e., the variation of refractive index with temperature (dn/dT), determine the temperature characteristics of nonlinear optical laser devices. A lack of proper dn/dT data prevents the assessment of the temperature-tuning behavior of various parametric three-wave interactions in barium borate and lithium triborate crystals. These dn/dT values are analyzed critically by use of a physical model based on the thermal expansion coefficient, the temperature coefficient of the excitonic band gap, and an isentropic band gap. The computed optical constants from the thermo-optic coefficients and the room temperature Sellmeier coefficients are used to calculate the refractive indices for any operating temperature and wavelength along the principal axes of both crystals. Temperature sensitivities are evaluated and thoroughly compared to the experimental values. Also, this study interprets the existing nonlinear optical devices, in particular, the recently introduced femtosecond traveling-wave parametric generations in a lithium triborate crystal, more accurately expressed in the range of −40 to 320 °C. A type I second-harmonic generation of a 1.48 μm laser diode with an output power of 100 mW is observed at a noncritical phase-matching temperature of 50 °C in this crystal. It is confirmed here that the thermo-optic coefficients are both wavelength and temperature dependent and the data at the band edge are especially important to account for the temperature-tuning phase-matched characteristics.
Quantum vortex motion in high-T
c superconductors
Authors: García A. , Zhang X. X. , Tejada J.
Published: July 2002

Magnetic relaxation experiments at low temperatures were performed in different zero-field-cooled (ZFC) and field-cooled (FC) high-Tc superconductors (HTSCs): TlBaCaCuO (2212 and 2223 phases, polycrystalline and thin-film samples), (Hg,Tl)BaCaCuO (1223 phase, polycrystalline material), and (Bi,Pb)SrCaCuO (2212 phase, single crystal). For each system and in the whole temperature range investigated, the relaxation curves obtained after both cooling processes are linear with the logarithm of time. The temperature dependence of the relaxation rate normalized to the first magnetization value, R=‖d(M/M0)/d ln(t)‖, follows a trend which is common to all systems: R decreases linearly with decreasing temperature down to a value, which is called the crossover temperature, below which it levels off to a T-independent plateau. This behavior gives evidence of a transition in the mechanism responsible for the relaxation process at low temperatures, from thermally activated (linear dependence on T) to quantum vortex motion (T-independent regime). The experimental values for the crossover temperatures and normalized relaxation rates compare fairly well to numerical estimates in the framework of the theories of quantum vortex motion in layered HTSCs. Finally, the transition from one regime into another was studied in two samples of the TlBaCaCuO, 2223 phase, system in order to investigate the influence of dissipation on the quantum process. A clear conclusion on this point could not be drawn from these kinds of measurements.
Superconducting hysteresis and the Preisach model
Authors: Mayergoyz I. D.
Published: July 2002

The critical state (Bean) model for superconducting hysteresis is discussed for arbitrary specimen shapes and complex flux distributions. It is demonstrated that this model exhibits the ‘‘wiping out’’ property and ‘‘congruency’’ property of minor hysteresis loops. This leads to the conclusion that the critical state model is a particular case of the Preisach model. For this reason, it is suggested to use the Preisach model for the description of superconducting hysteresis in very general situations.
Effect of ion beam mixing on microstructure and magnetic properties of Gd-Co multilayer films
Authors: Choe G. , Walser R. M.
Published: July 2002

This work investigated the effect of ion beam mixing on the magnetic and structural characteristics of Gd-Co multilayer films. Multilayer films with bilayer periods (BP) from 10 to 300 Å were mixed by 150 to 300 keV Ar+ ion doses of 1016 cm−2. The magnetizations of the as-deposited multilayers were dominated by oxidation, and by the asymmetric redistribution of Co atoms. Ion beam mixing eliminated much of these effects, but the degree was strongly dependent on the layer thickness. The small BP films were completely mixed, with amorphous nanoparticles, but the large BP films contained small, microcrystals in an amorphous matrix due to incomplete mixing. The magnetic moment of ion beam mixed films were strongly dependent on the ion beam energy and the layer thickness.
New estimation of surface anisotropy
Authors: Hu Xiao , Kawazoe Yoshiyuki
Published: July 2002

A micromagnetic study for the spin–reorientation transition in ultrathin magnetic films is reported. Phase diagrams of the magnetization configuration are presented. Scaling relations among the film thickness, exchange coupling, and magnetic anisotropies are revealed. Formulas are given for the energy stored in the film per unit area, which enable one to evaluate the surface anisotropy by ferromagnetic resonance techniques.
Interaction of light with a nonlinear spin wave in a normally magnetized ferromagnetic film
Authors: Stashkevich A. A. , Cottam M. G. , Slavin A. N.
Published: July 2002

A strong collinear magneto-optical interaction of guided light with an intensive forward volume spin wave (SW) has been studied in a normally magnetized film. A numerical perturbation technique has been developed to quantify the effects, which accompany the optical diffraction. Extensive numerical simulations of the magneto-optical diffraction by the SW affected by nonlinear automodulation were carried out. The parameters of interaction related to automodulation mechanisms were varied within a wide range of values, covering most practical situations. Specifically, the automodulation frequency assumed values of 1 MHz<F<100 MHz, and the index of phase modulation, linear in modulation instability threshold, the values of 1<v<4.
Interdiffusion of multilayer ferroelectric thin films using YBa2Cu3O7−x as an intermediate layer
Authors: Wang J. P. , Ling Y. C. , Lin I. N. , Lin W. J. , Tseng T. Y.
Published: July 2002

Interdiffusion between layers in multilayer ferroelectric thin films was studied from their elemental depth profiles using secondary ion mass spectrometry. Among the films deposited on YBa2Cu3O7−x/CeO2/Si substrates, the interdiffusion is most pronounced for [Sr0.5Ba0.5]Nb2O6 (SBN) films, less marked for Pb0.97La0.03(Zr1−xTix)0.9925O3 (PLZT, x=0.54 or 0.34) films, and is least significant for BaTiO3 films. Higher substrate temperature used for growing SBN films is believed to be the main cause of interdiffusion. The larger proportion of cationic vacancies existing in PLZT films is another possible source inducing interdiffusion. Using YBa2Cu3O7−x/SrTiO3 as substrates substantially reduces the interdiffusion between layers. This is ascribed to the better crystallinity of the YBa2Cu3O7−x layers deposited on SrTiO3 substrates. These results indicate that both the characteristics of ferroelectric films and the underlying YBa2Cu3O7−x layers substantially modify the interdiffusion behavior between the layers.
Diamond film quality: Effects of gas phase concentrations on the Raman spectra
Authors: Harris Stephen J. , Weiner Anita M. , Prawer Steven , Nugent Kerry
Published: July 2002

A series of diamond films have been grown, all at Tsub=1110 K and atomic H concentrations [H]=3×10−10 mol/cm3. The acetylene and methyl radical concentrations at the substrate surface were varied independently by factors of up to 4 and 10, respectively, by adjusting the pressure and the input methane concentration, and the effects of these variations on the Raman spectra were examined. The linewidth of the 1332 cm−1 diamond feature, the sp2 nondiamond carbon feature, and the luminescence yields all increased with increased [CH3]sub, but varying the [C2H2] had no observable impact. A quantitative relationship is provided between the sp2 content and [CH3]sub. The luminescence peak, which is the most sensitive feature to [CH3]sub, is due to an impurity from the filament. Using micro-Raman spectroscopy, large variations are found in the Raman spectra of adjacent crystals and within a single crystal. We attribute these fluctuations to inhomogeneous and anisotropic strains.
Effect of e-beam irradiation on a p-n junction GaN light emitting diode
Authors: Li X. , Gu S. Q. , Reuter E. E. , Verdeyen J. T. , Bishop S. G. , Coleman J. J.
Published: July 2002

A GaN p-n junction structure was grown on a (0001) sapphire substrate by atmospheric pressure metalorganic chemical vapor deposition using a double buffer layer. The resulting light emitting diode (LED) was further exposed to a low-energy electron beam source. The effect of e-beam exposure on the room-temperature electroluminescence spectra of the LED is reported. It is found that the electroluminescence spectral features change dramatically as a function of the electron-beam exposure time and current density. This is attributed to changes in active Mg concentration. The origin of each electroluminescence band is discussed.
Thin film of lithium niobium oxynitride as ionic conductor
Authors: Nguyen Le Quang , Truong Vo-Van
Published: July 2002

A novel glassy ionic conductor, lithium niobium oxynitride, was prepared in form of thin films and characterized. Thin-film deposition was carried out by rf magnetron sputtering of LiNbO3 target in nitrogen-containing atmosphere. These films exhibit ionic conductivity of two orders higher than that of LiNbO3 films, the room-temperature conductivity being about 5×10−7 S cm−1. Film structure is highly cross linked, leading to activation energy as low as 0.5 eV. Film transmittance is higher than 85% in both the visible and solar ranges, making them also suitable for use in electrochromic devices.