bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-layer model for photomodulated thermoreflectance of semiconductor wafers
Authors: Christofides C. , Diakonos F. , Seas A. , Christou C. , Nestoros M. , Mandelis A.
Published: July 2002

A complete theoretical analysis of the laser photomodulated thermoreflectance signal from a two-layer semiconducting wafer is presented. It is shown that the electronic and thermal properties of a thin surface layer may be determined by using the measured induced photothermal signal. Several numerical simulations are performed in order to study the influence of various electronic, optical and thermal parameters of the two-layer sample on the photomodulated thermoreflectance signal. The influence of the upper layer as well as the influence of the substrate on the signal are also discussed and parameter regimes are identified, where the characterization of the thin overlayer may be possible using this technique.
Lateral damage extension during masked ion implantation into GaAs
Authors: Faye M. M. , Vieu C. , Assayag G. Ben , Salles Ph. , Claverie A.
Published: July 2002

We have studied both experimentally and theoretically the generation of damage in GaAs due to ion implantation through mask openings of small dimensions. We show that it is possible to master the generation of damage, i.e., the amorphization phenomenon in the direction perpendicular to the ion beam and close to the mask edges. A theoretical model is used to simulate the ion implantation process and damage accumulation through Ti–Au masks. After comparing the shape of the crystalline/amorphous interfaces as revealed by cross-sectional electron microscopy with our simulations, this model is used to predict the evolution of the two-dimensional damage distributions beneath the mask edges as functions of ion beam and implantation mask parameters. Undamaged regions of nanometer dimensions can be preserved even when using masks of reasonable dimensions (100–200 nm). This can be done only by adjusting the ion beam parameters through the accurate simulation of the two-dimensional damage generation.
The effect of arsenic overpressure on the structural properties GaAs grown at low temperature
Authors: Lagadas M. , Hatzopoulos Z. , Tsagaraki K. , Calamiotou M. , Lioutas C. , Christou A.
Published: July 2002

The structural properties of GaAs grown by molecular-beam epitaxy at low temperatures have been investigated by scanning electron microscopy, transmission electron microscopy, and high-resolution x-ray double-crystal rocking curves as a function of arsenic overpressure during growth. It was found that surface smoothness and excess arsenic incorporation both depend strongly on growth temperature and on As/Ga flux ratio. For each growth temperature there is a ‘‘window’’ in the flux ratio which results in smooth surfaces. As-grown layers have an increased lattice constant in the growth direction. This relative lattice expansion increases with flux ratio at a constant growth temperature and eventually saturates. Transmission electron micrographs have revealed the presence of arsenic precipitates in material annealed at 600 °C. Increasing the As4 pressure during growth results in increases in precipitate diameter by almost 50% while their density and shape remain constant. Based on these observations a model has been developed to explain the lattice expansion dependence on arsenic overpressure.
Formation and characterization of Si/SiO2 multilayer structures by oxygen ion implantation into silicon
Authors: Hatzopoulos N. , Siapkas D. I. , Hemment P. L. F. , Skorupa W.
Published: July 2002

Buried double oxide structures in Si have been produced by the sequential high- and low-energy implantation of oxygen ions at 2 MeV and 90 keV, respectively. Each implantation step was followed by a high-temperature anneal at 1300 °C for 6 h. Fourier transform infrared reflection spectroscopy has been used in order to characterize the as-implanted and annealed samples. Rutherford backscattering spectroscopy/channeling analysis was also carried out for selected samples. The morphology of the two buried layers is the same as for the single energy implants. No interaction or transport of oxygen between the two layers is observed. The in-between buried Si layer as well as the Si overlayer are of high crystal quality and could be potentially used as waveguiding layers, in a Si-based optical waveguiding structure.
Fluxon pinning in annular Josephson junctions by an external magnetic field
Authors: Vernik I. V. , Keil S. , Thyssen N. , Doderer T. , Ustinov A. V. , Kohlstedt H. , Huebener R. P.
Published: July 2002

The behavior of long annular Josephson junctions with various numbers n of trapped fluxons is studied in the presence of a uniform external magnetic field parallel to the plane of the junction’s tunnel barrier. We report on measurements of the magnetic dependencies of the critical current and current–voltage characteristics with different n. Experimental results are explained by the model assuming that a barrier-parallel magnetic field produces potential wells for fluxons and antifluxons at the opposite locations. We also present numerical simulations using the perturbed sine-Gordon equation with an additional magnetic-field-induced term. Good agreement is found between the measured critical current versus magnetic-field dependencies and the numerical simulations.
Polar Kerr effect in Heusler alloys
Authors: van Ek J. , Huang W. , MacLaren J. M.
Published: July 2002

Theoretical spectra for the magneto-optical Kerr effect in polar geometry have been obtained for four series of L21 Heusler alloys Fe2 YZ and Co2 YZ, with Y=V, Cr, Mn, Fe, Co, Ni and Z=Al or Ga. Selected spectra are presented. Trends are identified in the relation between electronic structure and the polar Kerr angle spectrum for L21. Further it is shown that the large Kerr rotation in the C1b compound PtMnSb cannot be attributed to a few states close to the Fermi level.
Magnetization reversal in cobalt and nickel electrodeposited nanowires
Authors: Ounadjela K. , Ferré R. , Louail L. , George J. M. , Maurice J. L. , Piraux L. , Dubois S.
Published: July 2002

We have investigated the magnetization reversal processes in arrays of sub-micron Ni and Co wires by means of magnetization and torque experiments together with micromagnetic calculations. The wires were produced by electrodeposition in the cylindrical pores of track-etched polymer membranes. Diameters in the range 35–400 nm have been studied. The arrays of Co and Ni nanowires display different magnetic behaviors. Particular emphasis is given to the competing shape and crystal magnetic anisotropies that exist in the Co nanowire system. In both systems, explaining the experimental results requires domain formation, except for the smallest diameters where single domain behavior occurs.
Dynamic control of instability thresholds in yttrium-iron-garnet
Authors: Mar D. J. , Carroll T. L. , Pecora L. M. , Rachford F. J.
Published: July 2002

Yttrium-iron-garnet (YIG) is an important material used in microwave limiters and other devices. In this work, we use dual microwave (1–4 GHz) drives to study the behavior of magnetostatic and spin-wave modes in YIG spheres and rectangular films. The samples are placed in a dc magnetic field and driven by cw and pulse-modulated microwave excitations at magnetostatic mode frequencies. A second microwave drive applied to the sample excites additional spin-wave modes that can interact with those arising from the original excitation and thereby affect the transmission characteristics at the primary frequency. For both films and spheres, we observe a significant decrease in transmission of the primary when the secondary frequency is tuned to approximately half that of the primary drive. This decrease is observed both in the steady state behavior and in the initial overshoot transient associated with pulse modulation of the primary excitation.
Schottky barrier formation for Fe on GaAs(001) and the role of interfacial structure (abstract)
Authors: Jonker B. T. , Kneedler E. M. , Thibado P. , Glembocki O. J. , Whitman L. J. , Bennett B. R.
Published: July 2002

Spin injection into a semiconductor is expected to utilize a ferromagnetic material as the electrical contact. One system of potential interest consists of Fe contacts to a GaAs-based device structure. We have used photoreflectance spectroscopy to measure the Schottky barrier height formed for Fe films on GaAs(001) as a function of the GaAs surface reconstruction. The samples were grown by molecular-beam epitaxy in a multichamber UHV facility which includes chambers for photoelectron diffraction and scanning tunneling microscopy. These techniques were used to obtain a detailed atomic model of the Fe/GaAs interface, enabling us to correlate the measured barrier heights with the interfacial structure, and further discuss the results in the framework of several models for Schottky barrier formation. For the As-rich 2×4 and c(4×4) surfaces, we conclude that the interface is formed by Fe bonding with an As monolayer after dissolution of the surface As dimers and some Ga into the Fe film. The samples were removed from the UHV system after coating with 50 Å of Au for the photoreflectance measurements. While the pure Schottky model predicts a value of 0.43 eV, we find nearly identical barrier heights of 0.8 eV for both the 2×4 and c(4×4) surfaces. This is consistent with the similar interfacial structure resulting from Fe growth on each surface, and the value of 0.8 eV is in agreement with that predicted by the effective work function model assuming predominantly Fe–As bonding. We compare these results with similar data for growth on the Ga-rich 4×6 reconstruction and on ZnSe(001) surfaces.
Microstructure and strain relaxation in organometallic vapor phase epitaxy of strain-compensated GaInP/InAsP multilayers on InP(001)
Authors: Desjardins P. , Marchand H. , Isnard L. , Masut R. A.
Published: July 2002

The various mechanisms responsible for the strain relaxation of strain-compensated GaInP/InAsP multilayers grown on InP(001) using low-pressure organometallic vapor-phase epitaxy (LP-OMVPE) were investigated using a combination of transmission electron microscopy (TEM), high-resolution x-ray diffraction (HRXRD), and reciprocal lattice mapping. We examined separately the effect of the misfit strain f as well as the total strain energy εT on the strain relaxation mechanisms. We also investigated the effect of the growth temperature Ts on roughening. For the structures composed of a small number of superlattice periods, N=10, TEM and HRXRD indicate that strain relaxation occurs essentially through non-homogeneities at the interfaces for increasing misfit strain f values (at least up to |f|=1%, the largest strain used in these experiments). In comparison, when the magnitude of the misfit strain is kept constant, increasing the number of periods eventually leads to a massive generation of dislocations in the multilayer. For |f|=0.75%, coherency breakdown was observed around the 14th–15th period in a 50-period sample. However, the strain-compensated multilayer structures can be in a metastable state since all layers are perfectly flat and no dislocations are visible in a 20-period sample with the same misfit strains in the layers. Finally, we observed that the growth temperature Ts had a drastic effect on the morphology of the layers: increasing Ts from 620 to 680 °C while keeping all other growth parameters constant introduced large periodic lateral thickness modulations as well as dislocation clusters in the structures. Diffraction contrast analysis in plan-view TEM indicates significant anisotropy with the features elongated in the [11̄0] direction. These results could be used as guidelines for the design of highly perfect and reliable device structures grown by LP-OMVPE.
Electric field and equivalent circuit in all-film capacitors
Authors: Joubert Ch. , Béroual A. , Rojat G.
Published: July 2002

This work shows that the electric field enhancement of the plate edges in all-film capacitors has only a very small effect on the current value through the capacitor although this enhancement could lead to dielectric breakdown at low frequency for high voltage. On the other hand, the electric field along the plates depends on frequency and on the resistivity of the plates. Consequently, the surface admittance Ys of the capacitor will also vary with frequency. This latter parameter has been described by an equivalent circuit with lumped elements. Thanks to this equivalent circuit and to previous work, it will be possible to determine with good accuracy the current distribution in metallized capacitors.
Nonlinear magneto-optical response of Co/Cu multilayered nanowires (abstract)
Authors: Kirilyuk A. , Rasing Th. , Doudin B. , Ansermet J.-Ph.
Published: July 2002

Wire-shaped magnetic multilayers are known to exhibit a giant magnetoresistive effect.1 On the other hand, producing the regularly structured submicron objects is one of the challenges of nanotechnology. A detailed characterization of such objects has become a problem of primary importance, in particular their interfaced properties, which strongly influence the magnetoresistive behavior. In this paper, we test the magnetization sensitive second harmonic generation (MSHG) technique on Co/Cu multilayered nanowires. As a new magneto-optical technique, MSHG was recently demonstrated to have the strong combination of interface sensitivity with very large magneto-optical effects.2 Therefore, it might be a suitable method for the characterization of these multilayer interfaces. The samples were prepared by alternative electrodeposition of the two metals resulting in a multilayers grown perpendicularly to the wire axis, with a layer thickness of few nm. MSHG measurements showed a large nonlinear optical response of these samples which is clearly due to their complicated structure. Magnetic response of MSHG roughly followed the usual Kerr-effect behavior, however, with some nonmonotonic effects in intermediate fields. This might be related to a nonsynchronous reversal of the Co layers. In addition, we make an attempt to correlate the observed MSHG signals with a multiple-scattering modeling of the interfaces.
Morphology and magnetization changes upon annealing of Fe/Cu(100): An in situ scanning tunneling microscopy and magneto-optical Kerr effect study
Authors: Mentz E. , Weiss D. , Ortega J. E. , Bauer A. , Kaindl G.
Published: July 2002

Morphology changes and irreversible reorientation of the easy magnetization axis upon annealing of thin epitaxial fcc-iron films grown at 100 K on Cu(100) were studied in situ with scanning tunneling microscopy and the magneto-optical Kerr effect. An irreversible spin reorientation from in-plane to out-of-plane is observed for 6 monolayers thick iron films annealed to 350 K. This change in magnetic anisotropy is accompanied by considerable smoothing of the iron film surface. Annealing to higher temperatures leads to a strong increase of the coercive field by up to a factor of 6. The surfaces of these films are atomically flat and display some scattered needle-like protrusions and dislocation lines.
Schottky barrier heights of metals contacting to p-ZnSe
Authors: Koide Yasuo , Kawakami T. , Teraguchi N. , Tomomura Y. , Suzuki A. , Murakami Masanori
Published: July 2002

Schottky barrier heights (SBH’s) of a variety of metals (In, Cd, Nb, Ti, W, Cu, Ag, Au, Ni, Pt, and Se) contacting to p-ZnSe grown by a molecular beam epitaxy method were measured by an internal photoemission, capacitance–voltage (C–V), and/or current density–voltage (J–V) method. The internal photoemission method could not measure accurately the SBH’s of these metals due to a strong interference of the monochromatic incident light in the p-ZnSe epilayer. The C–V method measured a SBH value of 1.23 eV for the Au contact and 1.13 eV for the Ni contact, but did not measure the SBH’s of other metals due to strong hysteresis of the C–V curves. The SBH’s of these metals were successfully measured by the J–V method to be 1.2±0.1 eV. The present experiment showed that the SBH values were independent of the work functions of the contact metals, indicating that the Fermi-level could be pinned at the p-ZnSe/metal interface. In addition, turn-on voltages conventionally used to evaluate the electrical properties of the contact metals were found to be very sensitive to the SBH values.
The atomic structure of {101̄0} inversion domain boundaries in GaN/sapphire layers
Authors: Potin V. , Ruterana P. , Nouet G.
Published: July 2002

Using high resolution electron microscopy and extensive image simulation, the atomic structure of the inversion boundaries has been determined in GaN layers grown on sapphire by electron cyclotron resonance assisted molecular beam epitaxy. They form nanometric domains (5–20 nm) limited by {101̄0} planes crossing the whole epitaxial layer. These small dimensions excluded the use of more conventional microscopy methods such as convergent beam diffraction for their characterization. For image simulations, up to 10 models including the well known Austerman–Gehman and Holt ones were considered for the boundary atomic structure. The overall agreement with the observed contrast was reached for a Holt type model of structure in which the boundary plane contains two wrong bonds per atom. Although the overall atomic configuration is neutral in {101̄0} boundary planes, it is expected to be highly energetic in ionic materials like GaN due to the difference in Ga–Ga and N–N bond length and local charge difference. This may probably explain the small size of the domains observed in the present samples.
Airy function analysis of Franz–Keldysh oscillations in the photoreflectance spectra of In1−xGaxAsyP1−y layers
Authors: Hall D. J. , Hosea T. J. C. , Lancefield D.
Published: July 2002

Room temperature photoreflectance (PR) has been performed on three In0.32Ga0.68As/ In0.76Ga0.24As0.53P0.47 tensiley strained single-quantum-well structures, with heavy Zn modulation p-doping (5×1017 cm−3) in the quaternary barriers, which are lattice matched to an InP substrate. The PR spectra exhibit strong, well-defined, and regular Franz–Keldysh oscillations (FKO) associated with the barrier layers. We study the FKO in detail, comparing two different techniques of analyzing them to obtain a measure of the built-in electric field: (i) the conventional simple graphical asymptotic technique; and (ii) least-squares fitting to the experimental spectra using the recently proposed electromodulation model based on complex Airy functions. In the second method, the PR spectra are best described by the sum of two Airy function expressions representing degenerate heavy- and light-hole band edges. Good fits are obtained without the need to use an empirical energy-dependent broadening term to account for the effects of nonflatband modulation and nonuniform fields. The results are consistent with FKO originating from heavy- and light-hole transitions under the same electric field, but having a partial destructive interference effect in the PR spectrum. The fitted field value of ∼17 kV/cm is essentially the same as that obtained in the graphical analysis which assumed that the FKO were heavy-hole dominated. However, contrary to previous suggestions, neither the heavy- nor light-hole contributions dominate the actual FKO spectrum.
The diamond C13/12C isotope Raman pressure sensor system for high-temperature/pressure diamond-anvil cells with reactive samples
Authors: Schiferl David , Nicol Malcolm , Zaug Joseph M. , Sharma S. K. , Cooney T. F. , Wang S.-Y. , Anthony Thomas R. , Fleischer James F.
Published: July 2002

By using a thin C13 diamond chip together with a C12 diamond chip as sensors, the diamond Raman spectra provide the means to measure pressure precisely (±0.3 GPa) at any temperature (10–1200 K) and simultaneous hydrostatic (or quasihydrostatic) pressure (0–25 GPa) for any sample compatible with an externally heated diamond-anvil cell. Minimum interference between the Raman spectrum from the diamond anvils and those of the pressure sensors is obtained by measuring pressures with the Raman signal from the C13 diamond chip up to 13 GPa, and that from the C12 chip above 10 GPa. The best crystallographic orientation of the diamond anvils is with the [100] direction along the direction of applied force, in order to further minimize the interference. At 298 K, the pressure dependence of the C13 diamond first-order Raman line is given by ν(P)=νRT+aP for 91 at. % C13 diamond, where νRT(13C)=1287.79±0.28 cm−1 and a(13C)=2.83±0.05 cm−1/GPa. Analysis of values from the literature shows that the pressure dependence of the Raman line of C12 diamond is best described by the parameters νRT(12C)=1332.5 cm−1 and a(12C)=2.90±0.05 cm−1/GPa. The temperature dependence of the diamond Raman line is best described by ν(T)−νRT=b0 for T⩽200 K, and ν(T)−νRT=b0+b1.5Tk1.5 for 200 K⩽T⩽1500 K, where Tk=T−200 K. For 91 at. %C13 diamond, the parameters are b0=0.450±0.025 cm−1;b1.5=−(7.36±0.09)×10−4 cm−1 K−1.5; and for C12 diamond, the parameters are b0=0.467±0.033 cm−1,b1.5=−(7.56±0.10)×10−4 cm−1 K−1.5. Although no quantitative theoretical models are available for calculating the Raman shift as a function of temperature, the excellent fits to the data suggest that the Tk1.5 dependence above has a physical basis.
Low-frequency noise and performance of GaN p-n junction photodetectors
Authors: Kuksenkov D. V. , Temkin H. , Osinsky A. , Gaska R. , Khan M. A.
Published: July 2002

We report on low-frequency noise characteristics of visible-blind GaN p-n junction photodetectors. Carrier hopping through defect states in the space charge region, believed to be associated with dislocations, is identified as the main mechanism responsible for the dark conductivity of the photodiodes. Under reverse bias, the dark current noise has the 1/f character and obeys the Hooge relation with α≈3. Under forward bias, we observe generation-recombination noise related to a trap level with the activation energy of 0.49 eV. Under illumination, detectivity is found to be shot noise limited. The noise equivalent power of a 200×200 μm2 photodetector is estimated at 6.6×10−15 W/Hz1/2 at a bias of −3 V.
Structural and optical characterization of epitaxial waveguiding BaTiO3 thin films on MgO
Authors: Beckers L. , Schubert J. , Zander W. , Ziesmann J. , Eckau A. , Leinenbach P. , Buchal Ch.
Published: July 2002

Epitaxial waveguide structures of c-axis oriented BaTiO3 thin films on MgO(001) have been grown by pulsed laser deposition. The structural properties of the samples have been characterized by Rutherford backscattering spectrometry/ion channeling (RBS/C), x-ray diffraction, and atomic force microscopy. We found excellent crystalline quality even up to thicknesses of a few microns. This has been confirmed by RBS/C minimum yield values of 2%–3%, a full width at half maximum of 0.36° of the BaTiO3 (002) rocking curve, and a rms roughness of 1.1 nm for a 950 nm BaTiO3 film. The out-of-plane refractive index was measured to be close to the extraordinary bulk value with the birefringence being about one third of the bulk value. Waveguide losses of 2.9 dB/cm have been demonstrated.
Voltage-induced evolution of emission spectra in organic light-emitting diodes
Authors: Kalinowski J. , Di Marco P. , Fattori V. , Giulietti L. , Cocchi M.
Published: July 2002

A simple model for the voltage-induced alteration of emission spectra from single- as well as multilayer organic light-emitting diodes (LEDs) based on low-molecular-weight materials is reported. The relative contribution of the redshifted emission spectrum component of a dopant or a separate film in multilayer structure can be changed by electric-field-induced quenching of its excited states and/or voltage evolution of the recombination zone. The effect is demonstrated on N,N′-diamine (TPD) single-layer LEDs nonuniformly doped with a perylene bisimide pigment (PBP) and double heterostructure TPD/Alq3/PBP with 8-hydroxyquinoline aluminum, Alq3. While in the first class of LEDs the color changes from red to blue dependent on the applied voltage and average concentration of the PBP dye, the second-type structures operate in the red-green range as the blue emission from TPD is eliminated. Analytic considerations are presented relating the relative contribution of different color emission bands to the driving current and applied electric field as well as to the composition and structure parameters of LEDs such as dopant concentration or thickness of the layers.
Characterization of optically active defects created by noble gas ion bombardment of silicon
Authors: Deenapanray P. N. K. , Perret N. E. , Brink D. J. , Auret F. D. , Malherbe J. B.
Published: July 2002

The modification in the G-line (969.5 meV) and the C-line (789.4 meV) photoluminescence (PL) intensities were studied as a function of the fluence, energy, and mass of the bombarding ions (He, Ne, Ar, and Kr). The intensities of the luminescent lines induced by 1 keV Ne bombardment were found to decrease with increasing dose after reaching a maximum at about 1×1012 ions/cm2. Considerable reductions in the intensities of the G- and C-lines were also recorded during bombardment using heavier noble gas ions and they have been attributed to the higher rates of nuclear energy deposition with increasing bombarding ion mass. The incident ion energy at which the PL intensities of the spectral lines reached their maximum values was found to be dependent on the ion mass and fluence. We have explained the decrease in PL intensities of the G-line and C-line to be due to the introduction of increased amounts of nonradiative recombination centers with increasing incident ion dose and mass. Further, the integral sum of defects induced during bombardment as a function of projected ion range and excitation depth of the Ar-ion laser has been used to qualitatively describe the decrease in the intensities of the two lines.
Soft magnetic properties of Co–Cr–O granular films
Authors: Morikawa Takeshi , Suzuki Motofumi , Taga Yasunori
Published: July 2002

Excellent soft magnetic properties have been achieved for Co–Cr–O granular films with nanoscaled grains of Co distributed in the amorphous intergranular regions of Cr2O3. A rapid decrease in coercivity has been observed with the volume fraction of Co, and the minimum value of 0.39 Oe was obtained at 75.5 vol % of Co without postannealing. This value is one order of magnitude smaller than those of Co–Al–O and Co–Si–O sputtered films. The saturation magnetization, in-plane anisotropy field, and permeability below 800 MHz are 11.8 kG, 80 Oe, and 141, respectively. Because of its large saturation magnetization and in-plane anisotropy, the resonance frequency is estimated at 2.9 GHz. Therefore, Co–Cr–O granular films can be used in the high frequency devices operating over 2 GHz.
The design and analysis of axial field multipole impulse magnetizing fixtures
Authors: Riley C. D. , Jewell G. W. , Howe D.
Published: July 2002

A methodology for the design of fixtures for the multipole magnetization of axially magnetized permanent magnet rings is presented. The methodology, which is based on three-dimensional analytical field calculation techniques, embraces the calculation of the fixture electrical parameters, the electrical simulation of the fixture performance and the calculation of the level of saturation achieved in the permanent magnet. The methodology is validated by means of an experimental case study on a 6 pole isotropic NdFeB magnet ring.
Stress reduction in silicon dioxide layers by pulsing an oxygen/silane helicon diffusion plasma
Authors: Charles C. , Boswell R. W.
Published: July 2002

A low pressure, high density helicon reactor used to deposit silicon dioxide (SiO2) from a mixture of oxygen/silane has been pulsed with a constant “on” time of 500 μs and a duty cycle varying from 10% to 100%. Over this range, the deposition rate changes by only a factor of 2.5 implying that deposition is continuing in the postdischarge with a time constant of 1 ms. For duty cycles of 30% and above, the films show good characteristics but the 10% duty cycle has a somewhat higher “p etch” implying some porosity. The pulsing reduces the compressive stress by at least a factor of two. This is correlated with the reduction in the measured plasma potential and density implying that for the present conditions, the stress is determined by the energy and number of ions striking the growing film. For the low ion energies considered here (⩽50 eV) a simple model using a temporal evolution of the ion energies and fluxes measured in an argon plasma suggests that the compressive stress would decrease in the postdischarge with a time constant of about 80 μs, assuming that the total stress is the integral of the stress over the on and “off” periods. The experimental results in oxygen/silane plasmas show that this is probably an upper limit and the actual decay time may be considerably less.
A theoretical model for gas metal arc welding and gas tungsten arc welding. I.
Authors: Haidar J.
Published: July 2002

A recently developed theory for predicting arc and electrode properties in gas metal arc welding (GMAW) has been generalized to include arc–electrode interfaces, variation of surface tension pressure with temperature, Marangoni forces and handling of weld pool development in stationary gas tungsten arc welding (GTAW). The new theory is a unified treatment of the arc, the anode, and the cathode, and includes a detailed account of sheath effects near the electrodes. The electrodes are included as dynamic entities and the volume of fluid method is used to handle the movement of the free surface of the molten metal at one electrode. Predictions can be made of the formation and shape of the welding droplets as a function of time in GMAW and also of weld pool development in GTAW, accounting for effects of surface tension, inertia, gravity, arc pressure, viscous drag force of the plasma, Marangoni effect and magnetic forces, and also for wire feed rate in GMAW. Calculations are made of current densities, electric potential, temperatures, pressures and velocities in two dimensions, both in the arc and also within the molten metal and solid electrodes. Calculations are presented for GMAW and GTAW for an arc in argon and the results are compared with experimental temperature measurements for the plasma and the electrodes.
Combinatorial approaches toward patterning nanocrystals
Authors: Vossmeyer T. , Jia S. , DeIonno E. , Diehl M. R. , Kim S.-H. , Peng X. , Alivisatos A. P. , Heath J. R.
Published: July 2002

A scheme for generating complex, spatially separated patterns of multiple types of semiconducting and/or metallic nanocrystals is presented. The process is based on lithographic patterning of organic monolayers that contain a photolabile protection group and are covalently bound to SiO2 surfaces. The process results in spatially and chemically distinct interaction sites on a single substrate. Nanocrystal assembly occurs with a high selectivity on just one type of site. We report on the production of binary, tertiary, and quatemary patterns of nanocrystals. We highlight and discuss the differences between nanocrystal/substrate assembly and molecule/substrate assembly. Finally, we investigate the assembled structures using photoluminescence and absorption spectroscopy.
Theoretical investigation of mid-infrared interband cascade lasers based on type II quantum wells
Authors: Mu Yao-Ming , Yang Rui Q.
Published: July 2002

The band structures and optical gain in mid-infrared type II InAs/GaInSb/AlSb interband cascade lasers were investigated using an eight-band k⋅p model. Threshold carrier densities with different electric fields were obtained at 80 and 300 K. Implications of the calculated results on device performance are discussed.
Exchange coupling in Fe50Mn50/Ni81Fe19 bilayer: Dependence on antiferromagnetic layer thickness
Authors: Sang H. , Du Y. W. , Chien C. L.
Published: July 2002

We have determined the dependence of exchange coupling on the antiferromagnetic layer thickness (tAF) in FeMn/NiFe bilayers, where the FeMn layer varies from 0.5 to 32 nm, and the NiFe is constant at 30 nm. For tAF>3 nm, the exchange field Hex varies as (1/tAF)0.3 at both 80 and 300 K, whereas the coercivity is insensitive to tAF. The value of Hex displays a simple angular dependence of cos θ.
Assignment of the Raman active vibration modes of β-Si3N4 using micro-Raman scattering
Authors: Honda Katsuya , Yokoyama Shino , Tanaka Shun-ichiro
Published: July 2002

The vibrational modes of the β-Si3N4 Raman active bands appearing in the wave number region between 110 and 1100 cm−1 were determined by the polarization and the crystallographic orientation dependence of the Raman intensities of the bands using micro-Raman spectroscopy. The laser beam was focused onto a single grain with the shape of a regular hexagon or elongated hexagon on the sintered ceramic plate. The three intense bands appearing at about 185, 208, and 230 cm−1 were attributed to the vibrational modes of E2g, Ag, and E1g, respectively. The remaining peaks were also assigned to the irreducible representations.
In situ
thin-film texture determination
Authors: Litvinov Dmitri , O’Donnell Thomas , Clarke Roy
Published: July 2002

A kinematic theory of reflection high energy electron diffraction (RHEED) is presented for textured polycrystalline thin films. RHEED patterns are calculated for arbitrary texture situations and for any general crystallographic orientation that may be encountered in thin-film growth. It is shown that the RHEED pattern can be used as a fast and convenient tool for in situ texture characterization. The approach also permits quantitative extraction of angular dispersion parameters which are useful for optimizing thin-film growth conditions.
Effect of impurities in the CdS buffer layer on the performance of the Cu(In, Ga)Se2 thin film solar cell
Authors: Kylner A.
Published: July 2002

The highest efficiencies of Cu(In, Ga)Se2 (CIGS) thin film solar cells have been achieved when incorporating a thin CdS buffer layer grown by chemical bath deposition (CBD). The reason for this success has recently been discussed in terms of a pure Cd-doping effect in the CIGS layer. Such a model suggests that the bulk properties of the CBD-CdS buffer layer would be of minor importance. In this work, CBD-CdS layers having different bulk properties (i.e., concentrations of incorporated impurities) were employed in a number of CIGS solar cells. To further explore the bulk versus interface properties, half of these CIGS devices were subjected to a pre-deposition of an additional intermediate ultrathin layer. Moreover, CIGS devices made with CBD-CdS layers of different thickness were fabricated. Both standard and temperature dependent current–voltage (I–V) measurements were performed. The results indicate that the bulk properties of the CBD-CdS buffer layer indeed play an important role in the formation of the CdS/CIGS heterojunction. By increasing the impurity concentration or the thickness of the CBD-CdS layer, the open-circuit voltage Voc was observed to substantially increase. This favorable effect was counteracted by the appearance of a crossover effect in the I–V characteristics for devices with the highest impurity concentration or thickness of the CBD-CdS layer. The pre-deposition of an ultrathin layer did not affect these results. The presence of the crossover effect was strongly correlated to the appearance of trap-assisted tunneling in addition to the thermally assisted tunneling. The observed crossover effect was suggested to originate from too high a number of impurities (defect states) in the bulk of the CdS layer.
Magnetoelastic anisotropy in NdFeB permanent magnets
Authors: de Groot C. H. , de Kort Kees
Published: July 2002

The influence of magnetoelastic effects on the anisotropy in NdFeB permanent magnets has been investigated. A relation between the magnetostriction coefficients and the magnetoelastic anisotropy is derived for crystals with tetragonal symmetry. Transmission electron microscopy experiments show that stress and strain are present at the boundaries of the Nd2Fe14B grains. The origin of the stress is discussed and an order-of-magnitude calculation of its effect on the anisotropy is provided for thermally induced stress. The calculations are in good agreement with the measured influence of stress on coercivity. Both experiment and calculations show that homogeneous stresses have only a small influence on the coercivity.
Influence of natural and initial acoustoelastic coefficients on residual stress evaluation: Theory and experiment
Authors: Duquennoy Marc , Ouaftouh Mohammadi , Ourak Mohamed , Xu Wei-Jiang
Published: July 2002

The velocities of the ultrasonic bulk waves propagating in a prestressed material with orthorhombic symmetry are formulated in two frames of reference. From these velocities, acoustoelastic coefficients are theoretically determined with the natural and initial coordinates. For each acoustoelastic coefficient, the differences between its expressions according to the two systems of coordinates are then theoretically calculated and experimentally determined. From these results, the error on the residual stress evaluation due to the use of natural acoustoelastic coefficients rather than those defined with the initial coordinates is estimated.
Heat capacity near first order phase transitions and the magnetocaloric effect: An analysis of the errors, and a case study of Gd5(Si2Ge2) and Dy
Authors: Pecharsky V. K. , Gschneidner K. A.
Published: July 2002

The heat capacity measured in an adiabatic heat pulse calorimeter with nonzero heat capacity suffers from intrinsic errors in the vicinity of a first order phase transition. When these errors are carried over into the calculation of the magnetocaloric effect, the latter also suffers from large systematic errors. The sources of the intrinsic errors in the heat capacity near the first order phase transition temperature and the procedures to minimize them are discussed. The experimental heat capacity data of Gd5(Si2Ge2) and ultra pure Dy, both of which exhibit first order phase transition, are used to confirm the theoretical conclusions.
A high energy x-ray and neutron scattering study of iron phosphate glasses containing uranium
Authors: Karabulut M. , Marasinghe G. K. , Ray C. S. , Waddill G. D. , Day D. E. , Badyal Y. S. , Saboungi M.-L. , Shastri S. , Haeffner D.
Published: July 2002

The atomic structure of iron phosphate glasses containing uranium has been studied by complementary neutron and x-ray scattering techniques. By combining x-ray and neutron structure factors, detailed information about different pair interactions has been obtained. Most of the basic structural features such as coordination numbers and O–O and P–O distances in uranium containing glasses are the same as those in the base glass of batch composition 40Fe2O3–60P2O5 (mol %). However, the Fe–O distances change slightly with the addition of uranium. The observed structural parameters support a structural model in which the waste elements occupy voids in the Fe–O–P network, hence, not altering the basic structure of the parent iron phosphate glass.
Raman scattering from Ge nanostructures grown on Si substrates: Power and limitations
Authors: Kolobov A. V.
Published: July 2002

The analysis of Raman spectra obtained for different germanium nanostructures grown on silicon substrates is presented. Comparison of these spectra with a Raman spectrum of a silicon wafer reveals a one-to-one correspondence of features located around 229, 300, and 435 cm−1. It is argued that the peaks observed at these frequencies and often ascribed to Ge nanostructures are, in fact, coming from the Si substrate. The erroneous ascription of the peaks makes the corresponding conclusions incorrect.
Coercive and anisotropy fields in patterned amorphous FeSi submicrometric structures
Authors: Vélez M. , Morales R. , Alameda J. M. , Briones F. , Martı́n J. I. , Vicent J. L.
Published: July 2002

Amorphous FexSi1−x films have been prepared on Si substrates in order to fabricate submicrometric magnetic structures with soft magnetic behavior. The magnetic properties compositional dependence of the unpatterned samples has been analyzed to select the Fe content (x=0.7) with the lowest coercive and anisotropy fields values. Arrays of Fe0.7Si0.3 lines have been fabricated by electron beam lithography combined with a liftoff technique, with typical dimensions of 200 nm linewidth and 1 μm line spacing. These arrays present coercive fields parallel to the line direction as small as 9 Oe.
Magnetic properties of MnBi1−xRx (R=rare earth) systems
Authors: Saha S. , Huang M. Q. , Thong C. J. , Zande B. J. , Chandhok V. K. , Simizu S. , Obermyer R. T. , Sankar S. G.
Published: July 2002

MnBi crystallizes in a NiAs-type hexagonal crystal structure, exhibits a high uniaxial anisotropy, and is potentially useful as a permanent magnet material. We have examined the effect of partial substitution of Bi with rare earth elements on the magnetic properties of MnBi. MnBi1−xRx (R=Nd, Dy) were prepared by mechanically alloying powders of the constituent elements at liquid nitrogen temperature followed by heat treatment. X-ray diffraction and magnetic measurements were performed on powder samples to characterize the samples. We found that in MnBi1−xNdx, coercivity (at room temperature) increases from 0.7 kOe to 6.6 kOe for x=0.0 and 0.3, respectively. In MnBi1−xDyx the coercivity increases from 0.7 kOe to 7.9 kOe for x=0.0 and 0.3. The increase in coercivity may be in part due to the increase in the crystal field anisotropy as Nd or Dy is introduced and in part due to the finer particle size. A magnet made from MnBi shows coercivity of ∼17 kOe. A very fine particle size is considered to be the reason for this high coercivity.
Uniaxial magnetic anisotropy of epitaxial Fe films on InAs(100)-4×2 and GaAs(100)-4×2
Authors: Xu Y. B. , Freeland D. J. , Tselepi M. , Bland J. A. C.
Published: July 2002

The evolution of the uniaxial magnetic anisotropy of ultrathin epitaxial Fe films grown on InAs(100)-4×2 and GaAs(100)-4×2 has been studied in situ by means of the magneto-optical Kerr effect. In Fe/InAs(100)-4×2, the uniaxial magnetic anisotropy easy axis direction along [011] was found to be rotated 90° compared with that of Fe/GaAs(100)-4×2 along [01̄1]. Real-time reflection high energy electron diffraction measurements of Fe/InAs(100)-4×2 show that the lattice constant of the epitaxial Fe films relaxes remarkedly faster along the [01̄1] direction than along the [011] direction in the same thickness range where the uniaxial magnetic anisotropy occurs. These results suggest that the symmetry-breaking atomic scale structure of the reconstructed semiconductor surface gives rise to the uniaxial magnetic anisotropy in a ferromagnetic metal/semiconductor heterostructure via surface magneto-elastic interactions.
Study of 360° domain walls in NiFe/NiO film by tip–sample interaction on magnetic force microscope
Authors: Zhao T. , Hou C. , Fujiwara H. , Cho H. , Harrell J. W. , Khapikov A.
Published: July 2002

The 360° domain wall loops were studied using a magnetic force microscope (MFM) in a glass/Ta(10 nm)/NiFe(12 nm)/NiO(6 nm)/Ta(5 nm) multilayered film fabricated by magnetron sputtering. The film had an enhanced coercivity but no M–H loop shift due to the thin NiO layer. It was found that the local magnetic field exerted by a MFM tip can strongly affect the 360° domain wall, and this interaction was used to probe the structure and dynamics of the wall. The repeated scanning action of the tip causes the transformation from a 360° wall loop into a 180° wall loop. The magnetization distributions for these domain walls were derived from the MFM images. The mechanism of the irreversible change of wall structures caused by the tip–sample interaction was investigated based on micromagnetic analysis. The study of the transformation process provided a better understanding of the structures of the 360° walls. This work suggests that the tip–sample interaction could be a useful tool for studying the domain wall structures.
Compound semiconductor oxide antireflection coatings
Authors: Knopp K. J. , Mirin R. P. , Bertness K. A. , Silverman K. L. , Christensen D. H.
Published: July 2002

We report the development of high quality, broad-bandwidth, antireflection (AR) coatings using the low index provided by wet thermally oxidized Al0.98Ga0.02As. We address the design criteria, fabrication, and characterizations of AR coatings composed of surface and buried oxide layers on GaAs. We show, using native-oxide dispersion data, that surface oxide coatings can be designed to offer a nearly zero minimum of reflectance and a reflectance of <1% over bandwidths as large as 500 nm. Surface coatings having a reflectance minimum of 0.4% and a reflectance of <1% over >250 nm have been experimentally demonstrated at a design wavelength of 1 micrometer. Additionally, buried oxide coatings can be designed with an AlxGa1−xAs matching layer of any composition to exactly match the admittance of any substrate with effective index between 2.5 and 3.5. We have demonstrated buried oxide coatings, also designed for 1 micrometer, having a reflectance minimum of 0.4% and a reflectance of <1% over 21 nm. The calculated optical scattering loss from measured roughness data indicates that reflectance minima as low as 10−4 % are ultimately achievable with native-oxide antireflection coatings.
Size distribution and electroluminescence of self-assembled Ge dots
Authors: Vescan L. , Stoica T. , Chretien O. , Goryll M. , Mateeva E. , Mück A.
Published: July 2002

In this article we study the electroluminescence of p-i-n diode structures with Ge dots consisting of coherent three-dimensional small (pyramids) and larger (dome) islands. The Ge dots are formed through strain-induced islanding. The diode structures, including one layer with Ge dots, were deposited on Si mesas with variable areas in order to study the influence of limited area deposition on self-assembling. It was observed that the reduction of deposited area improves island uniformity. The combined analysis of island distribution and electroluminescence spectra has lead to the conclusion that domes in small diodes have a smaller Si content or are less relaxed than domes in larger diodes. The diodes are found to emit up to room temperature near the optical communication wavelength of 1.3 microns.
Change of TO and LO mode frequency of evaporated SiO2 films during aging in air
Authors: Brunet-Bruneau A. , Fisson S. , Vuye G. , Rivory J.
Published: July 2002

The microstructural modifications, induced by atmospheric water vapor, in the silica network of porous amorphous SiO2 films, evaporated by electron gun and capped by a nonporous TiO2 layer, have been investigated. We have taken advantage of infrared ellipsometric measurements to extract the infrared complex dielectric function of the SiO2 films in the 600–5000 cm−1 range, from which we deduce the actual values of the frequencies of the Si–O–Si stretching vibrational modes [transverse optic (TO) and longitudinal optic (LO)]. The TO and LO frequency shifts are studied experimentally and reproduced by simulation. Experimental data show the increase as a function of time in air of both TO and LO frequencies, and of the refractive index in the visible range. The film is modeled as a mixture of two constituents: a silica matrix, with variable density, and pores, into which water can penetrate. We show that the TO frequency is mainly sensitive to the silica matrix density, while the LO frequency gives information about the pore volume fraction. We first demonstrate that our deposition method leads to films having initially a dense silica matrix, characterized by a high visible refractive index (1.543 as compared to 1.458 for fused silica). Second, we show that the strong relaxation of the silica network (characterized by the decrease of the silica matrix refractive index from 1.543 to 1.475), due to the water penetration in the pores, is accompanied by a decrease of the pore volume fraction (from ∼30% before venting down to ∼15% after air exposure).