bluuebunny/crossref_metadata_2025 huggingface dataset:
Upconversion in Er-implanted Al2O3 waveguides
Authors: van den Hoven G. N. , Snoeks E. , Polman A. , van Dam C. , van Uffelen J. W. M. , Smit M. K.
Published: July 2002

When pumped with a 1.48 μm laser diode, Er-implanted Al2O3 ridge waveguides emit a broad spectrum consisting of several distinct peaks having wavelengths ranging from the midinfrared (1.53 μm) to the visible (520 nm). In order to explain these observations, three different upconversion mechanisms are considered: cooperative upconversion, excited state absorption, and pair-induced quenching. It is found that for samples with a high Er concentration (1.4 at. %), cooperative upconversion completely dominates the deexcitation of the Er3+ ions. For a much lower concentration (0.12 at. %), the influence of cooperative upconversion is strongly reduced, and another upconversion effect becomes apparent: excited state absorption. These conclusions are based on measurements of the luminescence emission versus pump intensity, and also on measured luminescence decay curves. The upconversion coefficient is found to be (4±1)×10−18 cm3/s; the excited state absorption cross section is (0.9±0.3)×10−21 cm2. It is shown that in spite of these upconversion effects, a high fraction of the Er3+ can be excited at low pump powers. For pump powers between 2 and 10 mW, the optimum Er concentration is calculated. The results show that for an Er concentration of 0.5 at. %, more than 2 dB/cm net optical gain is achievable at a pump power less than 10 mW.
Non-thermal plasma remediation of SO2/NO using a dielectric-barrier discharge
Authors: Sun Wanming , Pashaie Bijan , Dhali Shirshak K. , Honea Frank I.
Published: July 2002

Here we report the experimental and simulation results of removal of SO2 and NO from a simulated coal combustion gas using a dielectric-barrier discharge. A Monte Carlo calculation is done to determine the transport properties of the flue gas. Following this, a detailed plasma chemistry calculation is done which takes into account the pulsed nature of the dielectric-barrier discharge. The computed results are in good agreement with the experimental data. We find that in low concentrations (400 ppm for SO2 and 100 ppm for NO) the dielectric-barrier discharge was capable of removing 99% of the SO2 and NO. Due to temperature dependence of the heavy particle reactions, there is a decrease in NO removal as the energy density is increased past an optimum value.
Ultrahigh vacuum SQUID magnetometry study of the magnetic properties of Co/Co-oxide thin films
Authors: Spagna S. , Maple M. B. , Sager R. E.
Published: July 2002

Exchange coupling between Co and Co-oxide in bilayer and trilayer thin-film structures has been studied using a recently developed ultrahigh vacuum SQUID magnetometer (UHVSQM) system. Using this novel technique, newly deposited Co films from a separate molecular beam epitaxy facility were transported into the UHVSQM without breaking vacuum and magnetically characterized before and after in situ room temperature oxidation. Field-cooled hysteresis measurements performed at low temperatures indicate the presence of a complex antiferromagnetic Co-oxide domain structure as a result of oxidation. The stability of the magnetic structure is determined by competition between frustration generated at the interface when the ferromagnetic Co moments are reversed and stabilizing antiferromagnetic anisotropy which becomes large only at low temperatures (<150 K).
Magnetic phase diagram of the FexMn0.6−xAl0.40 alloys series
Authors: Pérez Alcázar G. A. , Zamora Ligia E. , Bohórquez A. , González E. , González J. M.
Published: July 2002

This work shows preliminary results on the study of FexMn0.6−xAl0.40 alloys. It includes measurements of the temperature dependence of the ac susceptibility, zero field cooled magnetization, and Mössbauer spectroscopy. From the results obtained, the occurrence of reentrant spin glass behavior from a spin glass phase to an antiferromagnetic one for 0.2≤x≤0.35 and from a spin glass phase to a ferromagnetic one for the x=0.55 sample is proposed. Also, a normal spin glass transition was detected in the Fe range corresponding to 0.35≤x≤0.5. These results allow us to construct a magnetic phase diagram for the alloy series.
Shock profile induced by short laser pulses
Authors: Couturier S. , de Rességuier T. , Hallouin M. , Romain J. P. , Bauer F.
Published: July 2002

Standard 25-μm-thick polyvinilydene fluoride (PVDF) piezoelectric gauges and new 450-μm-thick P(VDF 70%, TrFE 30%) piezoelectric copolymer have been used to record shock profiles at the back face of metallic targets irradiated by laser pulses of 2.5 and 0.6 ns duration at a 1.06 μm wavelength. The records are fully explained with simplified space–time diagram analysis. The pressure profile applied at the front face of the target has been determined from these records combined with numerical simulations of wave propagation through the target. A numerical code describing the interaction of laser with matter (FILM) has also been used for computing the applied pressure. Both methods lead to very close results. The peak pressure dependence on incident laser intensity is determined up to 30 GPa at 1012 W/cm2.
Mode interactions in chirowaveguides
Authors: Kamenetskii E. O.
Published: July 2002

Chirality may provide the coupling, and hence the conversion, between two eigen polarization modes: TE and TM modes. In this paper an analysis of mode interaction in chirowaveguides is realized, taking into account longitudinal components of polarization currents. Special consideration is given to so-called induced polarization effects which are caused by abrupt discontinuities of longitudinal polarization currents on the boundary of chiral–nonchiral media. Because of induced polarization effects, an asymmetry in mode coupling may take place. The method of investigation may be a powerful tool for the analysis of mode interactions due to both material and geometric effects.
Effect of annealing on the defect structure in a-SiC:H films
Authors: Friessnegg T. , Boudreau M. , Brown J. , Mascher P. , Simpson P. J. , Puff W.
Published: July 2002

The annealing behavior of amorphous, hydrogenated silicon carbide films in the range 400–900 °C was studied by optical characterization methods, 15N hydrogen profiling, and defect profiling using a variable energy positron beam. The films were deposited in an electron cyclotron resonance chemical vapor deposition system using ditertiary butyl silane [SiH2(C4H9)2] as the monosource for silicon and carbon. As-deposited films were found to contain large concentrations of hydrogen, both bonded and unbonded. Under rapid thermal annealing in a N2 atmosphere, the bonded hydrogen effuses giving rise to additional Si–C bond formation and to film densification. After annealing at high temperatures in N2, a marked decrease in the total hydrogen content is observed. After annealing in vacuum, however, the hydrogen effusion promotes void formation in the films.
A dielectric mixing law for porous ceramics based on fractal boundaries
Authors: Calame J. P. , Birman A. , Carmel Y. , Gershon D. , Levush B. , Sorokin A. A. , Semenov V. E. , Dadon D. , Martin L. P. , Rosen M.
Published: July 2002

The effect of porosity on the complex dielectric permittivity of microwave sintered zinc oxide at room temperature and 2.45 GHz is reported. The predictions of conventional Maxwell–Garnet theory and the effective medium approximation are in poor agreement with the experimental results. Various methods are employed to investigate the system in an effort to come up with new mixing laws, including combinations of these two analytic theories and finite difference electromagnetic simulations of representative microstructures. A model that assumes the existence of dielectrically inactive, fractal-geometry boundaries between ceramic grains provides an excellent description of the results with no free parameters. It gives physical insight into the experimentally observed mixing law.
Photoluminescence studies of InGaAs/InAlAs strained double quantum wells
Authors: Shen W. Z. , Shen S. C. , Chang Y. , Tang W. G. , Chen J. X. , Li A. Z.
Published: July 2002

Photoluminescence measurements have been performed on a In0.82Ga0.18As/In0.52Al0.48As strained double quantum wells with narrow well widths of 38 and 48 Å. The band offset ratio Qc in this narrow quantum well structures was found to be 0.70±0.01. Based on a steady-state photoluminescence model, the temperature and laser excitation dependences of the luminescence intensity allow us to conclude that the thermal quenching of luminescence is controlled by the quantum transport of the photogenerated carriers for temperature below 125 K, while the dominant luminescence quenching mechanism above 150 K is a thermally activated detrapping of carriers from the wells into barriers, followed by nonradiative recombination in the barriers.
Effect of tribological wear on ultraviolet laser interactions with single crystal NaNO3 and CaCO3
Authors: Shin J.-J. , Kim Myoung-Won , Dickinson J. T.
Published: July 2002

We report measurements of the neutral and ion emissions accompanying laser irradiation of cleaved and abraded NaNO3 and CaCO3. In both materials, abraded surfaces yield especially intense positive ion emissions during laser irradiation at low fluences (i.e., fluences well below those required for surface damage and/or the formation of a fluorescent plume). Abraded NaNO3 also yields extremely intense neutral emissions (NO, O2) derived from NO3− decomposition. Measurements of neutral molecules released during abrasion show significant anion-derived emissions from both materials (CO2 from CaCO3; NO from NaNO3). We attribute the effect of abrasion on the laser-induced emissions to the production of easily photoionized electron traps during abrasion. Such traps are expected to strongly enhance photoinduced ion emission from both materials. Because the NO3− ion undergoes dissociative electron attachment, photoionizable defects also enhance NO emission during laser irradiation. In contrast, the CO32−anion does not appear to undergo dissociative electron attachment and no enhancement of CO2 emission is observed during laser irradiation.
The ambipolar Auger coefficient: Measured temperature dependence in electron irradiated and highly injected n-type silicon
Authors: Jonsson P. , Bleichner H. , Isberg M. , Nordlander E.
Published: July 2002

From free-carrier absorption measurements of spatially and temporally resolved excess carrier concentrations in the n-base of electron irradiated, low doped, highly injected p-i-n type diodes, the effective ambipolar lifetime has been extracted with respect to different injection levels and temperatures. By measuring lifetimes up to injection levels of 3×1017 cm−3, the ambipolar Auger coefficient Ca and its temperature dependence has been determined in the range of 300–420 K. In accordance with recent publications, the room temperature value of the Auger coefficient was found to be about three times higher than one of the most commonly used values. Further on, the ambipolar Auger coefficient is empirically estimated to vary with temperature as Ca(T)=1.1×10−30(T/300)1.8, were T is the absolute temperature. Also the ambipolar diffusion coefficient has been investigated and compared with different models. The results are important and useful in simulation of high-injected silicon devices, i.e., solar cells and power devices.
Isoconcentration studies of antimony diffusion in silicon
Authors: Nylandsted Larsen A. , Kringho/j P. , Hansen J. Lundsgaard , Shiryaev S. Yu.
Published: July 2002

The diffusion of Sb in Si at concentrations around its solid solubility has been studied by isoconcentration experiments. The samples, grown by molecular-beam epitaxy, had constant Sb121 background dopings and a Sb123 spike embedded in this background. The diffusion was followed as a function of Sb background concentration at two different temperatures of 872 and 1019 °C by secondary ion mass spectrometry, differential Hall/resistivity measurements, and transmission electron microscopy. At concentrations exceeding the solid solubility Sb precipitates and interstitial-type dislocation loops were observed. At these concentrations the diffusivity decreased with increasing Sb background concentration. At concentrations below both the solid solubility and the intrinsic carrier concentration, for the highest diffusion temperature of 1019 °C, the diffusivity increases with increasing Sb background doping. This behavior is discussed considering mobile Sb2V complexes.
Growth, structure, and magnetic properties of γ-Fe2O3 epitaxial films on MgO
Authors: Gao Y. , Kim Y. J. , Thevuthasan S. , Chambers S. A. , Lubitz P.
Published: July 2002

Single-crystal epitaxial thin films of γ-Fe2O3(001) have been grown on MgO(001) using oxygen-plasma-assisted molecular beam epitaxy. The structure and magnetic properties of these films have been characterized by a variety of techniques, including reflection high-energy electron diffraction (RHEED), low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy and x-ray photoelectron/Auger electron diffraction (XPD/AED), vibrating sample magnetometry, and ferromagnetic resonance. Real-time RHEED reveals that the film growth occurs in a layer-by-layer fashion. The γ-Fe2O3(001) film surface exhibits a (1×1) LEED pattern. The growth of γ-Fe2O3 films at 450 °C is accompanied by significant Mg outdiffusion. AED of Mg KLL Auger emission reveals that Mg substitutionally incorporates in the γ-Fe2O3 lattice, occupying the octahedral sites. Magnetic moments are ∼2300 G and ∼4500 G for γ-Fe2O3 films grown at 250 °C and 450 °C, respectively. The high magnetic moment for the films grown at 450 °C could be attributed to the high degree of structural order of the films and Mg substitution at octahedral sites.
Effects of crystal texture on exchange anisotropy in NiO spin valves
Authors: Lee Sang-Suk , Hwang Do-Guwn , Park C. M. , Lee K. A. , Rhee J. R.
Published: July 2002

The effects of crystal texture on magnetoresistance, exchange anisotropy field Hex and coercive field Hc in the NiO350 Å/NiFe50 Å/Cu20 Å/NiFe50 Å spin valve films deposited on Corning glass, MgO(100), MgO(110), and MgO(111) by rf and dc magnetron sputtering techniques were investigated. The Hex and Hc of the NiO350 Å/NiFe50 Å bilayers grown on MgO(111) and Corning glass were almost the same 90–93 Oe and 56–60 Oe, respectively. The Hex of the bilayer film on MgO(100) decreased to 68 Oe, however, the Hc of that increased to 114 Oe. The steplike magnetoresistance curve of the spin valve film on MgO(110) disappeared. The x-ray scan of the NiO film deposited on Corning glass shows weakly preferred crystal orientation of NiO(200), (111) and (220) texture, and the NiO films on the MgO single crystals have been grown (100), (110), and (111) epitaxial textures. These results show that there is no clear trend of increasing Hex for films with greater (111) texture. The large Hc of the bilayer on MgO(100) is probably due to interface roughness.
Observation of the ferromagnetic domain structure in exchange-coupled antiferromagnetic/ferromagnetic thin films (abstract)
Authors: Kief M. T.
Published: July 2002

Exchange-coupled antiferromagnetic/ferromagnetic (AFM/FM) thin films are known to exhibit many interesting and potentially useful properties, including a unidirectional anisotropy.1 Despite nearly three decades of research, the nature of the AFM/FM interface exchange interaction is still not well understood, particularly in polycrystalline thin films. Simple predictions suggest an exchange-coupling strength 10–100 times stronger than generally observed. The models of Mauri et al.2 and Malozemoff3 resolve this inconsistency by considering domains and reversal within the AFM layer. Clearly, to understand this phenomena we need an accurate description of the AFM’s bulk and interface spin structure and its action under the influence of a coupled FM film. In an effort to better understand the AFM/FM system, we have been studying NiO/NiFe. In this report, we examine the magnetic domain structure and reversal process in the NiO/NiFe system by imaging of the NiFe magnetic domain structure. This is accomplished using our newly developed Kerr microscope. This microscope uses a phase detection method that allows high spatial resolution (0.5 μm) and good magneto-optic contrast. In addition, we can discriminate the magnetization vector components in the sample plane to develop a vector map of the magnetization. Kerr images indicate that the magnetic domain structure in the NiFe is complicated by the NiO interaction. The magnetization assumes a large scale “ripplelike” structure with domains walls perpendicular to the easy direction. This surprising result may result from a local dispersion in the NiO pinning direction. We will present a simple model of this interaction and relate it to the other current standard models.
Line-focus acoustic microscopy measurements of acoustic properties of LiTaO3 crystal plates with an inversion layer
Authors: Tourlog Ailie , Achenbach Jan D. , Kushibiki J.
Published: July 2002

Inversion layers of 2 μm and 30 μm thickness were formed at the negative dipole-end (−c) face of a Z-cut LiTaO3 crystal plate by proton exchange followed by heat treatment. The phase velocities of leaky surface acoustic waves (LSAWs) on both the positive (+c) and negative (−c) faces of the plate were measured by line-focus acoustic microscopy versus the angle of propagation, at a frequency of 225 MHz, and the results were compared with theoretical predictions. It is shown that for an inversion layer of 30 μm thickness the phase velocities of LSAWs on the +c and −c faces are the same, and almost equal to the velocities of the virgin plate. This suggests that the material constants, which are decreased due to the proton exchange, are restored to the original values by the heat treatment.
Ferrite synthesis in microstructured media: Template effects and magnetic properties
Authors: O’Connor C. J. , Buisson Y. S. L. , Li S. , Banerjee S. , Premchandran R. , Baumgartner T. , John V. T. , McPherson G. L. , Akkara J. A. , Kaplan D. L.
Published: July 2002

Inverse micelles and organogels provide novel environments to synthesize ferrite particles. The fluid microstructure provides a template for the synthesis. Our experiments with ferrite synthesis in inverse micelles indicate the formation of superparamagnetic nanoparticles. Of interest is the encapsulation of these particles in polymer microspheres. The encapsulation is done using simple polymer precipitation in the micellar nonsolvent. The process results in a polymer-ferrite composite exhibiting supermagnetism. Low temperature spin glass properties of the composite are characterized through SQUID measurements. These composites have a superparamagnetic blocking temperature of 16 K and follow Curie–Weiss law at temperatures above 60 K with the fitted parameters: C=0.941 emu/g K, θ=−287 K, and TIP=0.0001 emu/g. Since the polymer used is polyphenol, a highly functionalizable material, the composite is well suited for applications in magnetic bioseparations and magnetic coatings.
Influence of the doping profile and deep level trap characteristics on generation-recombination noise
Authors: Godoy A. , Jiménez-Tejada J. A. , Palma A. , Cartujo P.
Published: July 2002

Generation-recombination (g-r) noise originated by the fluctuating occupancy of deep level traps in the depletion regions of a junction field effect transistor (JFET) has been thoroughly analyzed. A numerical simulator of this type of device, which allows us to calculate all relevant electrical magnitudes in the structure, was used to calculate the noise power spectral density. To check our simulator, comparisons with experimental data of g-r noise in JFETs were made, showing very good agreement. Our numerical results have provided useful information about the influence on noise of several aspects such as the modelling of the shallow doping profile, and the relative contribution of each junction of the device on the total noise. In addition, the energetic position of deep levels in the band gap and the relative concentration of deep and shallow impurities were all combined to quantify the noise spectral density and to locate the region of the structure that contributes the most to the g-r noise. This report clearly shows the importance of including all technological parameters for a correct interpretation and study of low-frequency noise in JFETs.
Structural changes of the metallic glass Zr65Al7.5Cu27.5 during glass transition and in the undercooled liquid region
Authors: Schumacher H. , Herr U. , Oelgeschlaeger D. , Traverse A. , Samwer K.
Published: July 2002

The metallic glass Zr65Al7.5Cu27.5 exhibits a wide temperature range in the undercooled liquid region. This offers the possibility of examining the phenomena that occur during glass transition. We investigated this glass by x-ray diffraction and x-ray absorption spectroscopy (XAS) at room temperature and at elevated temperatures below and above the glass transition temperature. The total number of nearest neighbors determined by x-ray diffraction remained unchanged. Using XAS irreversible changes in the short range order (SRO) towards the SRO of crystalline Zr2Cu is observed in the nearest neighbor shell which is consistent with the x-ray diffraction results. The long range order was not affected by these changes. From the temperature dependence of the atomic mean-square relative displacements σ2, Debye temperatures were determined and they compare well with low temperature specific heat measurements. Above Tg the slope of σ2 increases drastically and is attributed to anharmonic effects.
Structural and spectroscopic magneto-optic studies of Pt–Ni multilayers
Authors: Angelakeris M. , Poulopoulos P. , Vouroutzis N. , Nyvlt M. , Prosser V. , Visnovsky S. , Krishnan R. , Flevaris N. K.
Published: July 2002

Pt m – Ni n multilayers were grown by e-beam evaporation under ultrahigh vacuum; m and n will depict the number of atomic planes of the corresponding constituents in one modulation period and, in this work, they were varied between 1 and 14. X-ray diffraction and conventional electron microscopy experiments revealed a fcc polycrystalline structure with excellent modulation. Magneto-optic properties, studied at room temperature between 1.5 and 5.2 eV, were found to depend strongly on both m and n. Perpendicular anisotropy was exhibited for samples with thin Ni layers and, unexpectedly, for thicker Ni layers when Pt layers became considerably thin (m∼2). Comparison of the magneto-optic features of two similar samples grown on glass and polyimide revealed that the choice between glass and polyimide does not affect magneto-optic properties.
Size and temperature dependence of electrical resistance and thermoelectric power of Bi2Te2Se1 thin films
Authors: Damodara Das V. , Selvaraj S.
Published: July 2002

Thin films of Bi2Te2Se1 of various thicknesses have been deposited on clean glass plates using the flash evaporation technique. Electrical resistance and thermoelectric power measurements have been carried out on these films in the temperature range 300–485 K. The thickness dependences of electrical resistivity and thermoelectric power of the films have been analyzed using the effective mean-free path model. The thickness dependence of activation energy of the films is explained by Seto’s polycrystalline model. Various material parameters such as mean-free path and Fermi energy have been calculated from the analysis of experimental data. The thermoelectric power factor of the films has been calculated using the measured electrical resistivity and thermoelectric power values.
Dielectric behavior of nanocomposites of lead sulphide and ferroelectric glass ceramics
Authors: Kundu T. K. , Mukherjee M. , Chakravorty D. , Cross L. E.
Published: July 2002

Glass ceramics containing micrometer-sized lead zirconate titanate have been synthesized and nanometer-sized lead sulphide (PbS) phase has been grown within these by treatment in hydrogen sulphide gas. The PbS particle sizes vary from 4.4 to 15.3 nm. Dielectric measurements on the nanocomposites have been carried out in the temperature range 120–300 K. The dielectric constant values for composites containing PbS exhibit drastic increase as compared to those of the glass ceramic samples. For specimens with PbS size varying from 4.4 to 15.3 nm the corresponding volume fraction of PbS being 0.07 and 0.14 the values range from 900 to 3500 at a frequency of 1 kHz. Bergman’s [D. J. Bergman, Phys. Rev. C 43, 377 (1978)] analytical formulation with some density functions has been used to calculate the value of effective dielectric permittivity for the various nanocomposites. The theoretical values do not show any agreement with the experimentally observed data. However, more spectral weight functions need to be tried before any conclusion can be drawn regarding the mechanism responsible for the present experimental data.
Self-organization of pores in SiC/Si composite structures
Authors: Parkhutik Vitali P.
Published: July 2002

Anodic dissolution of a structure composed of the chemically vapor deposited amorphous SiC film and silicon wafer results in the formation of a system of hexagonally ordered pore colonies. This intriguing effect takes place when the structure SiC/Si is subjected to a short-time (several seconds) anodization in 48% HF/ethylene glycol solution. As a result of this procedure the SiC layer is converted into a porous one with diameters of the pores not exceeding 30–50 nm. Continuing the anodization process beyond the complete conversion of SiC layer into a porous one results in porous structure formation in an underlying Si wafer. It appears that pores in silicon form colonies arranged in hexagonal order. Characteristic distance between neighboring colonies increases with the anodic current density applied during the electrochemical process. It is assumed that the effect of self-ordered pore colonies growth is due to the accumulation of mechanical stress at SiC/Si interface due to a lattice mismatch and dynamic pressure developed in the pores during the anodic dissolution reaction.
Magnetostrictive and magnetic properties of the pseudobinary compounds PrxTb1−xFe2 and Pr0.15TbxDy0.85−xFe2
Authors: Li Yangxian , Tang Chengchun , Du Jiang , Wu Guangheng , Zhan Wenshan , Yan Weili , Xu Guizhi , Yang Qingxin
Published: July 2002

The synthesis and magnetic and magnetoelastic properties of PrxTb1−xFe2 and Pr0.15TbxDy0.85−xFe2 are reported. The study on the crystalline structure, magnetization, and Curie temperature shows that PrFe2 has a lattice parameter of 7.463 Å and a magnetic moment of 4.93 μB/f.u. With the addition of Pr, the anisotropy of TbFe2 decreases and the optimal magnetostriction with low anisotropy occurs on the Tb rich side for TbxDy1−xFe2. In addition, measurement of oriented samples has been carried out to analyze the magnetostrictive behavior.
Artificial dielectrics: Nonlinear properties of Si nanoclusters formed by ion implantation in SiO2 glassy matrix
Authors: Vijayalakshmi S. , Grebel H. , Iqbal Z. , White C. W.
Published: July 2002

The nonlinear optical properties of Si nanoclusters formed by ion implantation into an SiO2 glassy matrix and followed by annealing have been studied at λ=532 nm and λ=355 nm by use of Z-scan and pump-probe techniques. These have been compared to the nonlinear properties of laser-ablated Si films. At relatively large intensities (>1 MW/cm2), the absolute nonlinear values for these isolated nanoclusters were comparable to those obtained for laser-ablated samples although opposite in sign. Laser-ablated samples showed a much larger effect at relatively low intensities (<1 MW/cm2), while the ion-implanted films showed almost none. Lifetime constants were in the range of 3–5 ns for all samples.
Circumferential magnetization curves of Co-rich amorphous wires under tensile stress
Authors: González M. T. , Garcı́a K. L. , Valenzuela R.
Published: July 2002

Circumferential magnetization curves were obtained for low, negative magnetostriction amorphous wires of nominal composition (Co0.94Fe0.06)72.5B15Si12.5, under tensile stress up to 250 MPa, by using the magnetoimpedance effect. The various magnetization processes, i.e., spin rotation, domain wall bulging, and domain wall displacement were resolved by frequency measurements in the 5 Hz–13 MHz range, and by varying the applied field amplitude between 0.28 and 12 A/m (root mean square). The results show that reversible magnetization processes (domain wall bulging and spin rotation) are damped by tensile stress, while the irreversible process (domain wall displacement) is enhanced by stress. These results are interpreted in terms of an increase in anisotropy and a reorienting of magnetization in circumferential domains, both as a result of the stress-induced anisotropy.
Nanolithography by selective chemical vapor deposition with an atomic hydrogen resist
Authors: Mitsui Toshiyuki , Hill Eric , Ganz Eric
Published: July 2002

We report the fabrication of Al nanostructures using selective chemical vapor deposition (CVD) growth and an atomic hydrogen resist. A scanning tunneling microscope is used to pattern the hydrogen terminated surface by local removal of hydrogen atoms. The high selectivity of the CVD process limits Al growth to the uncovered regions. We demonstrate the fabrication of Al features as small as 2 nm.
Impact of the gate material on the interface state density of metal–oxide–silicon devices with an ultrathin oxide layer
Authors: Lundgren Per
Published: July 2002

Aluminum, chromium, gold and polycrystalline silicon contacted metal–oxide–silicon devices with ultrathin (∼3 nm) thermal oxides have been compared regarding their electrical characteristics. The impact of the gate material on the features of the interface state density displays itself mainly as a change in the passivation dynamics during heat treatment of what appears to be one and the same as-grown dangling bond defect in all cases.
In situ reflection high-energy electron diffraction observation of epitaxial LaNiO3 thin films
Authors: Chen P. , Xu S. Y. , Zhou W. Z. , Ong C. K. , Cui D. F.
Published: July 2002

Epitaxial LaNiO3 (LNO) thin films were grown on (001) SrTiO3 substrates by laser molecular-beam epitaxy. The growth process of the LNO films was monitored by in situ reflection high-energy electron diffraction (RHEED). Clear RHEED patterns and the intensity oscillation of RHEED were observed during the epitaxial growth process. The morphology of the films was studied by atomic force microscopy. The results show that the films grown by this method have a nanoscale smooth surface with the root-mean-square surface roughness smaller than 7 nm on an area of 1×1 μm2. X-ray diffraction patterns indicate that the crystalline LNO films exhibited preferred (00ℓ) orientation. The resistivity of the thin film is 0.28 mΩ cm at 278 K and 0.06 mΩ cm at 80 K, respectively.
Coherence-based edge diffraction sharpening of x-ray images: A simple model
Authors: Margaritondo G. , Tromba G.
Published: July 2002

Recent, striking examples of x-ray image sharpening are based on Fresnel edge diffraction. A simple model shows that image sharpening can be achieved under less stringent conditions as could be thought. The extreme spatial coherence of third-generation synchrotron sources appears helpful but not strictly necessary, and high time coherence is not an essential requirement. These conclusions, consistent with the results of other authors, strongly enhance the potential impact in medical x-ray applications, materials science and other fields.
Structure and magnetic properties of RCo7−xZrx (R=Pr or Er, x=0–0.8)
Authors: Huang M. Q. , Drennan M. , Wallace W. E. , McHenry M. E. , Chen Qun , Ma B. M.
Published: July 2002

Alloys of composition RCo7−xZrx (R=Pr or Er and x=0−0.8) were synthesized and characterized in the temperature range of 10–1273 K in fields up to 5 T. As with the SmCo7−xZrx system studied earlier in our laboratory, the effects of Zr doping on the stability of the TbCu7 phase and the increase in the anisotropy field HA are also observed in the systems of PrCo7−xZrx and ErCo7−xZrx. Nearly single phase TbCu7 materials were formed in as-cast alloys when x=0.1–0.2. In the case of R=Pr, HA changes from almost planar for x=0 to uniaxial with Ha∼100 kOe for x⩾0.2 at room temperature (RT). In the case of R=Er, HA for the x=0.1 composition is almost two times larger than that of the Zr-free alloys, which shows strong uniaxial anisotropy at both RT and 10 K. Spin reorientation behavior (when R=Pr) and R–M antiparallel coupling (when R=Er) were also observed.
Spin fanning configurations in antiferromagnetic layers with a uniaxial anisotropy and torque transfer characteristics from top to bottom spins
Authors: Fujiwara H. , Sun M.
Published: July 2002

Spin configurations along the thickness direction generated in antiferromagnetic (AF) layers with an in-plane uniaxial anisotropy have been calculated by numerical method. It is confirmed that when the bottom-surface anisotropy energy constant is less than 1/4 of 180°-wall energy density, σw, a sudden change or switch of the spin configuration occurs at a critical angle of the top spins when the AF-layer thickness, t, is greater than a critical value, tc. This means that no hysteresis occurs with a rotation of the top spin for t<tc. Spin configurations with a folded or periodic structure which Néel discussed have been revealed to be unstable without microscopic defects or irregularities which pin the spin angle at some depth from the top surface. It is found that when the bottom-surface anisotropy is lower than 1/4σw, the torque sustainable by the top-surface spins does not exceed 1/2σw. It is also found that the effect of top-spin rotation penetrates substantially down as deeply as a few times the 180°-wall thickness. The effect of the AF-easy axis tilting out of the plane has also been studied.
The optically active center and its activation process in Er-doped Si thin film produced by laser ablation
Authors: Ishii Masashi , Ishikawa Tetsuya , Ueki Tatzuo , Komuro Shuji , Morikawa Takitaro , Aoyagi Yoshinobu , Oyanagi Hiroyuki
Published: July 2002

The local structure of erbium-doped silicon produced by the laser ablation technique is investigated by Er LIII-edge x-ray absorption fine structure analysis. The combined analysis of extended x-ray absorption fine structure analysis and an x-ray absorption near-edge structure simulation based on multiple-scattering theory reveals the most probable atomic coordination of the optically active center; Er bonded with six oxygen atoms has a C4v symmetry. The optical activation process of this system is also discussed. The Si target with 10 wt% Er2O3 has two kinds of local structures, C-rare-earth Er2O3 grain and another Er phase incorporated in Si. The laser ablation homogenizes these phases, and deposits a new single-phase structure of the octahedron (Oh point group) on the substrates. In this phase, the optical transition probability is low due to the forbidden 4f transition of Er in the crystal field originating from the higher-order symmetry of O. After annealing, degradation of the symmetry from Oh to C4v results in a crystal field suitable for inducing sufficient radiation transition.
Ultrafast ablation with high-pulse-rate lasers. Part II: Experiments on laser deposition of amorphous carbon films
Authors: Rode A. V. , Luther-Davies B. , Gamaly E. G.
Published: July 2002

Ultrafast pulsed laser deposition is a novel technique for depositing particle-free, thin solid films using very high repetition rate lasers. The process involves evaporation of the target by low energy laser pulses focused to an optimum intensity to eliminate particles from the vapor. This results in films with very high surface quality while the very high repetition rate increases the overall deposition rate. Here we report an experimental demonstration of the process by creating ultrasmooth, thin, amorphous carbon films using high repetition rate Nd:YAG lasers. Both a 10 kHz, 120 ns Q-switched Nd:YAG laser, or a 76 MHz 60 ps mode-locked Nd:YAG laser were used in the experiments. The number of particles visible with an optical microscope on the carbon film deposited using the mode-locked laser was less than one particle per mm2. Scanning electron microscopy images demonstrated that the deposited film had a very fine surface texture with nanoscale irregularities. Atomic force microscopy surface microroughness measurements revealed a saturation-like behavior of the root-mean-square roughness at <12 nm over the whole deposited surface area for 10 kHz Q-switched laser evaporation; and almost at the atomic level (<1 nm) for the 76 MHz mode-locked laser evaporation. Raman spectroscopy of the deposited films indicated that they consisted of a mixture of sp3 and sp2 bonded amorphous carbon. The thickness of the amorphous carbon film deposited simultaneously on two 4 in. silicon wafers varied by only ±5% over an area of ∼250 cm2. The deposition rate was ∼2–6 Å/s at a distance of ∼150 mm from the target, which is 10 to 25 times higher than that achieved with conventional high energy low repetition rate nanosecond lasers.
Model of point-of-use plasma abatement of perfluorinated compounds with an inductively coupled plasma
Authors: Fiala Ales , Kiehlbauch Mark , Mahnovski Sergej , Graves David B.
Published: July 2002

Point-of-use plasma abatement (PPA) has been proposed as one way to eliminate perfluorinated compound (PFC) emission from various tools used in integrated circuit manufacturing. PPA employs a high density plasma between the process tool turbomolecular pump and the backing pump. Oxygen is added to the process tool effluent upstream of the PPA tool. The mixture of oxygen and PFC-containing tool effluent enters the PPA tool and the PFCs are converted to products that can be scrubbed downstream of the backing pump. In this article, we present a model for the PPA tool operation, illustrating the principles with a mixture of C2F6/O2. A plasma model is coupled to a neutral transport and reaction model, including electron-impact molecular dissociation and subsequent gas phase chemistry.
Characteristic red photoluminescence band in oxygen-deficient silica glass
Authors: Sakurai Yuryo , Nagasawa Kaya , Nishikawa Hiroyuki , Ohki Yoshimichi
Published: July 2002

We studied a red photoluminescence (PL) band at about 1.8 eV with full width at half maximum of 0.2–0.4 eV in a series of oxygen deficient-type silicas before and after γ irradiation. The decay lifetime of the PL was estimated to be about 200 ns. The PL excitation peak was found to be located at 2.1 eV. The intensity of the 1.8 eV band was enhanced by about 100 times after γ irradiation up to a dose of 10 MGy. These results suggest that the 1.8 eV PL is associated with oxygen deficient states in silica glass, which were introduced during manufacture and were enhanced further by the γ irradiation. Comparison of the PL properties was made with other luminescent Si-based materials in terms of the peak energy, lifetime, and temperature dependence.
Moss–Burstein and plasma reflection characteristics of heavily doped n-type InxGa1−xAs and InPyAs1−y
Authors: Charache G. W. , DePoy D. M. , Raynolds J. E. , Baldasaro P. F. , Miyano K. E. , Holden T. , Pollak F. H. , Sharps P. R. , Timmons M. L. , Geller C. B. , Mannstadt W. , Asahi R. , Freeman A. J. , Wolf W.
Published: July 2002

Degenerately doped (>1019 cm−3) n-type InxGa1−xAs (x∼0.67) and InPyAs1−y (y∼0.65) possess a number of intriguing electrical and optical properties relevant to electro-optic devices and thermophotovoltaic devices in particular. Due to the low electron effective mass of these materials (m*<0.2) and the demonstrated ability to incorporate n-type dopants into the high 1019 cm−3 range, both the Moss–Burstein band gap shift and plasma reflection characteristics are particularly dramatic. For InGaAs films with a nominal undoped band gap of 0.6 eV and N=5×1019 cm−3, the fundamental absorption edge increased to 1.27 eV. InPAs films exhibit a shorter plasma wavelength (λp∼5 μm) in comparison to InGaAs films (λp∼6 μm) with similar doping concentrations. The behavior of the plasma wavelength and the fundamental absorption edge are investigated in terms of conduction band nonparabolicity and Γ-L valley separation using detailed band structure measurements and calculations.
Effective material properties in twinned ferroelectric crystals
Authors: Erhart Jiřı́ , Cao Wenwu
Published: July 2002

Without external fields, ferroelectric materials will have multidomain configuration in the ferroelectric state. Detailed analysis found that twinning may not be treated as random since the number of orientations for the domain walls are limited in a given symmetry change during a ferroelectric phase transition. In each finite region of a large crystal or in small crystallites, a particular set of twins is favored under certain boundary conditions, which consists of only two of the low temperature variants. Statistic models of random distribution of domains do not apply for calculating the physical properties of such twin structures. However, one could derive the two domain twin properties by using the constitutive equations and appropriate mechanical boundary considerations. This paper presents a theoretical analysis on such a two-domain twin system, including its global symmetry and effective material properties resulting from different twinning configurations. Numerical results are derived for LiNbO3 and BaTiO3.
Evolution of phase composition and associated properties in the process of growth of thin films
Authors: Kukushkin S. A. , Osipov A. V .
Published: July 2002

A complete set of equations describing the evolution of phase composition of an island ensemble at the stage of the Ostwald ripening in the process of thin-film growth is derived and its solution is found. The size distribution function of islands in solid solutions is obtained for different types of growth mechanisms. The general approach to control phase composition and associated properties (electric, optical, mechanical, and others) in formed multicomponent systems is elaborated. It is shown that the most favorable conditions to control these properties occur at the Ostwald ripening stage and also during nonlinear phenomena, such as self-excited oscillations or self-organization. The relationship between size and composition of the new phase nuclei formed in the course of condensation of solid solution films is revealed. A set of equations describing the evolution of the size distribution function of the properties in island films of solid solutions during changes in external parameters of the system is derived and solved. We show that substantial dependence of composition on external parameters is revealed for the sizes of islands with radius R⩽10−8 m, irrespective of the type of substance. For island-type films of stoichiometric composition, the diagram for coexistence of appropriate properties is plotted in the concentration-temperature coordinates that allows to us determine the conditions required to obtain the properties. Probable periodic changes in time and space of various properties on multicomponent systems undergoing the first order phase transition are predicted.
Modeling and characterization of a strained Si/Si1−xGex transistor with δ-doped layers
Authors: Geux Linda S. , Yamaguchi Ken
Published: July 2002

A two-dimensional model of a strained Si/Si1−xGex transistor with δ-doped layers was developed. A semiclassical drift diffusion model is used to study the effects of different conduction-band offsets and variation of the distance between the Si channel and an n-type δ-doped layer as well as the thickness of this δ-doped layer at room temperature. We found that a large conduction-band offset, or a large Ge concentration, confines electrons more strongly to the Si channel. These factors raise the drain current when the doping level per unit area is constant. The area between the Si channel and the δ-doped layer and the δ-doped layer itself forms a barrier to electrons donated by the donor atoms in the δ-doped layer. Hence, the smaller the distance between the Si channel and the δ-doped layer and the thinner the δ-doped layer, the larger the number of electrons in the Si channel. Through the present analysis, an optimum design concept is clarified for device applications of Si/Si1−xGex systems.
i
th
layer electrodynamics: A canonical approach
Authors: Beeli Pieder
Published: July 2002

Exact analytic expressions for the fields and the power dissipated in the ith layer of an n-layered structure are derived under steady-state and normal incidence via continuum electrodynamics. Via a transmission-line analog, we recursively propagate the surface wave impedance backward. We incorporate a canonical approach via three transfer functions that recursively propagates the field forward. The results apply exactly for an arbitrary number of layers, composed of arbitrary uniaxial materials, and having layers of arbitrary thicknesses. We consider examples of the electrodynamics of a superconducting thin film atop a dielectric and backed by a normal metal as a function of the thickness of the dielectric.
Design of a InP/In1−xGaxAsyP1−y/In0.53Ga0.47As emitter-base junction in a Pnp heterojunction bipolar transistor for increased hole injection efficiency
Authors: Ekbote S. , Cahay M. , Roenker K.
Published: July 2002

Starting with Burt’s envelope function theory, we calculate the transmission coefficients of holes across an InP/In1−xGaxAsyP1−y/In0.53Ga0.47As heterointerface while varying the width and gallium and arsenic fractions of the InP lattice-matched quaternary compound (x=0.47y). While comparing our results to the case of an abrupt InP/In0.53Ga0.47As interface, we find that the transmission coefficients of both heavy- and light-holes can be enhanced significantly for a 60-Å-wide quaternary layer with an arsenic fraction y=0.4 (x=0.188). This should lead to an enhanced hole injection efficiency of Pnp heterojunction bipolar transistors using the heterointerface analyzed here as an improved design of the emitter-base junction.
Selective nanoscale growth of titanium on the Si(001) surface using an atomic hydrogen resist
Authors: Mitsui Toshiyuki , Curtis Rob , Ganz Eric
Published: July 2002

Nanoscale titanium structures are fabricated on a patterned Si(001)-(2×1) surface using an atomic hydrogen resist. The patterning is achieved by removing small areas of hydrogen with a scanning tunneling microscope. The large chemical reactivity of the bare Si surface compared to the hydrogen passivated surface provides selective area growth of titanium clusters grown by chemical vapor deposition using TiCl4. Titanium growth by chemical vapor deposition is normally limited by chlorine passivation of the bare Si surface. However, by removing the chlorine with the scanning tunneling microscope, the growth can be resumed.
Electrodynamic properties of coplanar waveguides made from high-temperature superconducting YBa2Cu3O7−δ electrodes on nonlinear dielectric SrTiO3 substrates
Authors: Findikoglu A. T. , Reagor D. W. , Rasmussen K. O/. , Bishop A. R. , Gro/nbech-Jensen N. , Jia Q. X. , Fan Y. , Kwon C. , Ostrovsky L. A.
Published: July 2002

We present a comprehensive study of broadband (0–2 GHz) electrodynamic properties of coplanar waveguides made from high-temperature superconducting thin-film YBa2Cu3O7−δ electrodes on nonlinear dielectric single-crystal SrTiO3 substrates. The waveguides exhibit strong dielectric nonlinearities, in addition to temperature-, dc-bias-, and frequency-dependent dissipation and refractive index. By using parameters determined from small-signal (linear) transmission characteristics of the waveguides as a function of dc bias, we develop a model equation that successfully predicts and describes large-signal (nonlinear) behavior.