bluuebunny/crossref_metadata_2025 huggingface dataset:
Intersubband all-optical limiter based on thermally induced intervalley transfer
Authors: Meyer J. R. , Vurgaftman I. , Hoffman C. A.
Published: July 2002

We propose an all-optical limiter based on the thermally induced intervalley transfer of electrons from Γ-valley states with forbidden normal-incidence intersubband interactions to L-valley states which absorb strongly. Detailed modeling of the device performance in the short-pulse regime (⩽100 ns) yields that the output intensity for a limiter with only 10% insertion loss at low excitation levels will remain clamped over a dynamic range of 25–40.
In situ infrared characterization of the silicon surface in hydrofluoric acid
Authors: Chazalviel J.-N. , Ozanam F.
Published: July 2002

The surface of silicon in hydroflouric acid (HF) is coated with covalently attached hydrogen (SiHx groups, with x=1,2,3), with probably a very small concentration of SiF bonds. In contrast with a recent claim by Niwano et al. [J. Appl. Phys. 79, 3708 (1996)], we show that the concentration of SiF bonds is as yet beyond infrared detection limits. Our analysis indicates that the band at 2230 cm−1 observed by these authors actually arises from electrolyte absorption.
Energy levels in finite barrier triangular and arrowhead-shaped quantum wires
Authors: Gangopadhyay Samita , Nag B. R.
Published: July 2002

Energy eigenvalues and energy shifts are calculated for triangular and arrowhead-shaped GaAs/Ga0.6Al0.4As quantum wires taking into account the finite value of the barrier potential. Calculations were made by using the eigenfunctions of an infinite-barrier right-angled isosceles triangular wire. Calculated values are compared with available experimental results. An equivalence relation is also examined for the estimation of eigenvalues of triangular and arrowhead-shaped quantum wires by knowing the values for rectangular wires.
Measurement of the stray field emanating from magnetic force microscope tips by Hall effect microsensors
Authors: Thiaville A. , Belliard L. , Majer D. , Zeldov E. , Miltat J.
Published: July 2002

We describe the use of micronic Hall sensors as magnetic-field profilometers with submicron resolution. The procedure involves the deconvolution of Hall voltage maps produced by scanning the field source over the sensor, with a scanning probe microscope. The response function of an infinite Hall cross is calculated analytically in the two-dimensional case, using conformal mapping techniques. Various methods of deconvolution of the Hall voltage maps are presented and compared. The calculated response function is used for the deconvolutions, and different effective sensor sizes are tried. It is shown that the remaining main uncertainties come from the ignorance of the true response function of the sensor, ascribed to the charge depletion phenomenon that is known to occur at the sensor edges. The method is applied to thin-film magnetic force microscope tips for which a precise knowledge of the tips field at sample location proves crucial to image interpretation. Maximum fields in the range 10–100 Oe are found at a distance known to be about 100 nm from the tip contact surface, depending on the tip coating thickness and magnetization direction.
Effect of high-lying minibands on superlattice vertical transport
Authors: Lei X. L. , da Cunha Lima I. C. , Troper A.
Published: July 2002

The role of high-lying minibands in superlattice vertical transport is investigated using a nonparabolic balance-equation approach extended to include multi-miniband occupation. We find that the existence of high-lying minibands results in a decrease of the electron temperature, a reduction of the peak drift velocity and a slowdown of the velocity-drop rate in the negative differential mobility (NDM) regime, in comparison with those predicted by a single miniband model. Numerical calculations of steady-state transport in typical GaAs-based superlattices at room temperature, show that these effects become significant when the strength of the electric field gets close to or falls in the NDM regime.
Red laser induced upconversion luminescence in Er-doped calcium aluminum germanate garnet
Authors: Zhang Xiao , Yuan Jianhui , Liu Xingren , Jouart Jean Pierre , Mary Gérard
Published: July 2002

In this work, we have studied the spectroscopic and dynamic properties of the green (4S3/2→4I15/2) and blue (2P3/2→4I11/2) upconversions for a series of Er3+ doped Ca3Al2Ge3O12, induced by a red tunable laser excitation. The green emission is due to either an energy transfer involving two ions excited in the 4I11/2 multiplet or an excited state absorption from the 4I13/2 level, depending on whether the laser excitation wavelength is tuned on 4I15/2→4F9/2 or 4I13/2→4F5/2. The blue emission mainly results from a three-step absorption process through the 4I13/2 and 4S3/2 multiplets. The temperature and concentration effects have been analyzed for the green and blue upconversions.
Interplay of electrons and phonons in heavily doped GaN epilayers
Authors: Demangeot F. , Frandon J. , Renucci M. A. , Meny C. , Briot O. , Aulombard R. L.
Published: July 2002

Raman spectroscopy is used to analyze the effect of electrons on the lattice dynamics of unintentionally heavily doped GaN. The deposition temperature of GaN buffer layers on sapphire substrates is found to have an important influence on the presence of free carriers in GaN layers, evidenced by plasmon coupling to the A1(LO) phonon. Data from infrared measurements are used to calculate the Raman line shape of q=0 coupled A1(LO)-plasmon modes in a dielectric approach and give a good fit of the L−(q=0) component observed in Raman spectra. In particular, the fitting procedure applied to spatially resolved micro-Raman measurements reveals an inhomogeneous concentration of electrons on the scale of hexagonal microcrystallites. Partial screening of phonons with wave vectors differing from the q=0 transfer of incident and scattered photons is invoked to explain LO-like scattering over the whole spectral range of optical phonons, attributed to charge density fluctuations on account of its polarization properties.
Efficient multilayer electroluminescence devices with poly(m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene) as the emissive layer
Authors: O’Brien D. , Bleyer A. , Lidzey D. G. , Bradley D. D. C. , Tsutsui T.
Published: July 2002

The use of a new highly luminescent conjugated polymer as an emissive layer in single and multilayer electroluminescence devices is reported. Poly(m-phenylenevinylene-co-2,5 -dioctyloxy-p-phenylenevinylene) [PmPV-co-DOctOPV] was prepared via a Wittig synthesis reaction. The resulting polymer has a high photoluminescence quantum efficiency in the solid state with an emission spectrum peaked at 506 nm (2.45 eV) in the green. Electroluminescence devices were fabricated with an ITO anode and a MgAg cathode. Three different structures were studied: (i) single layer devices containing only PmPV-co-DOctOPV; (ii) double layer devices with PmPV-co-DOctOPV and an evaporated film of 1,3-bis(4-tert-butylphenyl-1,3,4-oxadiazoyl) phenylene [OXD-7] as an electron transport layer; (iii) triple layer devices containing PmPV-co-DOctOPV, OXD-7 and in addition a polyvinylcarbazole hole transport layer. Electroluminescence external quantum efficiencies for these devices were found to be up to 0.08%, 0.55%, and 1%, respectively, corresponding to luminous efficiencies of ≈0.5, ≈3, and ≈6 lm/W and power efficiencies of 8.5×10−5, 5.9×10−4, and 6.0×10−4 W/W.
Electroreflectance of surface-intrinsic- n+-type doped GaAs
Authors: Wang D. P. , Huang K. M. , Shen T. L. , Huang K. F. , Huang T. C.
Published: July 2002

The electroreflectance spectra of surface-intrinsic- n+-type-doped GaAs were measured at various bias voltages (Vbias). Results revealed many Franz–Keldysh oscillations (FKOs) above the band-gap energy, which have been attributed to a uniform electric field (F) in the undoped layer below the surface. However, there has been no other evidence for the uniformity of F in the undoped layer. Since it is known that F can be deduced from the periods of the FKOs, the relations between F and Vbias can, thereby, be obtained. The nearly linear relation, thus found, confirms the existence of a nearly uniform field in the undoped layer. From the plot of F against Vbias,the values of the thickness of the undoped layer and the barrier height can also be evaluated.
Synthesis and electron field emission of nanocrystalline diamond thin films grown from N2/CH4 microwave plasmas
Authors: Zhou D. , Krauss A. R. , Qin L. C. , McCauley T. G. , Gruen D. M. , Corrigan T. D. , Chang R. P. H. , Gnaser H.
Published: July 2002

Nanocrystalline diamond films have been synthesized by microwave plasma enhanced chemical vapor deposition using N2/CH4 as the reactant gas without additional H2. The nanocrystalline diamond phase has been identified by x-ray diffraction and transmission electron microscopy analyses. High resolution secondary ion mass spectroscopy has been employed to measure incorporated nitrogen concentrations up to 8×1020 atoms/cm3. Electron field emission measurements give an onset field as low as 3.2 V/μm. The effect of the incorporated nitrogen on the field emission characteristics of the nanocrystalline films is discussed.
Different “average” nuclear magnetic resonance relaxation times for correlation with fluid-flow permeability and irreducible water saturation in water-saturated sandstones
Authors: Borgia G. C. , Brown R. J. S. , Fantazzini P.
Published: July 2002

Fluid-flow properties of porous media, such as permeability k and irreducible water saturation Swi, can be estimated from water 1H nuclear magnetic resonance (NMR) relaxation data, but there are basic questions regarding data processing and interpretation. We found that Swi and k are better estimated if different forms of “average” relaxation time are used. NMR longitudinal relaxation data for a suite of 106 water-saturated clean sandstones were used. Sandstones represent a specialized class of porous media, where even for small porosity, substantially all pore space is connected. The sandstones exhibit distributions of relaxation times ranging over factors from at least 10 to more than 103. We tried several forms of “average” relaxation time T. One family of Ts is 〈Tp〉1/p, where lim p→0 gives the geometric mean. The best estimator we found for Swi uses a form of average relaxation time only, rather than relaxation time cutoff. The time used can be any of several forms of T, giving more emphasis to short times than the geometric mean does. On the contrary, the best T for estimating permeability without other information is precisely the geometric mean. The best estimates of permeability came from fits of ln (k/φ) using Ts with emphasis at slightly longer times. While Swi is better estimated by using all the data points (starting from our minimum 0.4 ms), k is better estimated by starting at a few ms, that is by ignoring a non-negligible fraction of the signal for some samples. These results can be obtained also by using computations that do not need to invert multiexponential relaxation data, and good results are obtained even with only a few data points. The results are compatible with the reasonable picture, where high surface-to-volume pores, giving signal components with short relaxation times but not contributing to the permeability, are important in determining the fraction of the wetting phase which remains trapped in the solid matrix after displacement with a nonwetting phase.
Observation of suppressed radiative recombination in single quantum well p-i-n photodiodes
Authors: Nelson Jenny , Barnes Jenny , Ekins-Daukes Nicholas , Kluftinger Benjamin , Tsui Ernest , Barnham Keith , Foxon C. Tom , Cheng Tin , Roberts John S.
Published: July 2002

We have measured electroluminescence (EL) spectra of GaAs/InGaAs and AlGaAs/GaAs single quantum well (QW) p-i-n photodiodes at temperatures between 200 and 300 K and forward biases close to the open circuit voltage. Integrated EL spectra vary like eqV/nkT with an ideality factor n=1.05±0.05 over five decades, indicating purely radiative processes. The spectra are calibrated into absolute units enabling comparison to be made with the predictions of a theoretical model. For each temperature and bias we calculate the EL spectrum and radiative current expected in the detailed balance limit, integrating the theoretical emission spectrum over the surface of the device, in order to establish the quasi-Fermi potential separation, Δφf, in the QW and, where possible, in the host material. For the GaAs/InGaAs cell we are able to model emission from the QW and the host material simultaneously. We find that, in all cases, the QW emission is overestimated by theory if it is assumed that Δφf=V. QW emission corresponds instead to a value of Δφf which a few tens of mV less than V. In contrast, emission from the host material, where visible, is well fitted by the model with Δφf=V at all biases and temperatures. We attribute the variation in Δφf to irreversible thermally assisted escape from the QWs.
Two-dimensional nuclear magnetic resonance study of a hydrated porous medium: An application to white cement
Authors: Dolinšek J. , Apih T. , Lahajnar G. , Blinc R. , Papavassiliou G. , Pintar M. M.
Published: July 2002

Structure and dynamics of a hydrated white cement were investigated by the T1-weighted lineshape, two-dimensional (2D) exchange, and 2D separation of inhomogeneous and homogeneous lineshapes nuclear magnetic resonance (NMR) techniques. T1 weighting of the proton spectrum eliminates the strong bulk water signal from the pores so that the weaker spectrum of the solid cement matrix becomes observable. The proton spectrum of the solid cement fraction exhibits a characteristic Pake doublet shape and shows a close similarity to the powder spectra of pure calcium hydroxide and the crystalline water of gypsum. The 2D exchange NMR spectrum of white cement demonstrates the existence of slow chemical exchange processes of protons in the solid matrix on a sub-kHz frequency scale. This exchange is orders of magnitude slower than the exchange between the surface and bulk water. The 2D NMR separation of inhomogeneous and homogeneous lineshapes technique demonstrates that the absorption lines of a set white cement are inhomogeneously broadened due to the existence of a distribution of static local magnetic fields. The major contribution to these fields comes from the dipolar fields of the paramagnetic electrons whereas the magnetic susceptibility differences of the liquid and solid fractions add a small part at the solid-liquid boundary where the direction of the external magnetic field is changed.
Thermal annealing of Pd/InAlAs Schottky contacts for transistor buried-gate technologies
Authors: Chuang H. F. , Lee C. P. , Tsai C. M. , Liu D. C. , Tsang J. S. , Fan J. C.
Published: July 2002

We studied the thermal reaction of Pd/In0.52Al0.48As contacts using capacitance–voltage (C–V), current–voltage, Auger electron spectroscopy, and x-ray diffraction analyses and compared the results to those for Pd/Al0.25Ga0.75As and Pd/In0.53Ga0.47As contacts. The thickness of InAlAs consumed by the reaction during annealing was calculated directly from the measured C–V profiles. Pd starts to react with InAlAs at a temperature of 100 °C, lower than it does with AlGaAs. For thermally annealed Pd/InAlAs and Pd/AlGaAs, both compositional and structural changes were found at the Pd/semiconductor interface. However, in heat-treated Pd/InGaAs samples, compositional changes but no structural changes were observed.
Design issues in ionized metal physical vapor deposition of copper
Authors: Grapperhaus Michael J. , Krivokapic Zoran , Kushner Mark J.
Published: July 2002

The filling of deep vias and trenches with metal for interconnect layers in microelectronic devices requires anisotropic deposition techniques to avoid formation of voids. Ionized metal physical vapor deposition (IMPVD) is a process which is being developed to address this need. In IMPVD, a magnetron sputter deposition source is augmented with a secondary plasma source with the goal of ionizing a large fraction of the metal atoms. Application of a bias to the substrate results in an anisotropic flux of metal ions for deposition. The ion flux also contributes to “sputter back” of metal deposits on the lip of the via which could lead to void formation. In this article, we describe and present results from a two-dimensional plasma model for IMPVD using a dc magnetron and an inductively coupled auxiliary ionization source. The scaling of copper IMPVD is discussed as a function of buffer gas pressure, sputter source, and source geometry. We show that the deposition rate of metal on the substrate will be reduced as pressure increases due to the increase in diffusive losses. We also show that the sputtering of the auxiliary coils can be a significant issue in IMPVD systems, which must be addressed in tool design.
Influence of oxygen on the viscosity of Zr–Al–Cu–Ni metallic glasses in the undercooled liquid region
Authors: Kübler A. , Eckert J. , Gebert A. , Schultz L.
Published: July 2002

The viscosity of Zr–Al–Cu–Ni metallic glasses prepared as melt-spun ribbons, slowly cooled bulk samples, and mechanically alloyed powder exhibiting different oxygen contents was measured above the glass transition temperature Tg using parallel-plate rheometry. For rapidly quenched ribbons, oxygen contents ⩽0.8 at. % do not drastically affect the viscosity of the glassy phase, but changes in the composition of the metallic constituents are more significant. For slowly cast samples, oxygen levels above 0.3 at. % lead to an increase in viscosity due to formation of crystalline phases. In contrast, mechanical alloying provides completely amorphous powders even for oxygen contents above 2 at. %. This allows measurement of the influence of oxygen on the viscosity of single-phase glassy alloys for high oxygen levels, revealing a significant increase in viscosity with increasing oxygen content. The viscosity in the undercooled liquid can be fitted well by a Vogel–Fulcher–Tammann (VFT) relation. Independent of their oxygen content, the samples exhibit a small VFT temperature T0 and a high melt viscosity which are characteristic of a strong glass.
Single crystallites in “planar polycrystalline” oligothiophene films: Determination of orientation and thickness by polarization microscopy
Authors: Vrijmoeth J. , Stok R. W. , Veldman R. , Schoonveld W. A. , Klapwijk T. M.
Published: July 2002

Thin films of evaporated oligothiophenes (α-nT, n=4–8) show a “planar polycrystalline” structure: each of the individual crystallites has a random azimuthal orientation, the (a,b) face of its unit cell is aligned with the surface plane. We introduce a technique to determine the orientation and thickness of such aligned thiophene crystals by optical polarization microscopy. Due to the optical birefringence of the crystal, it appears with different colors in the microscope dependent on its orientation and thickness. To support the method proposed, we solve Maxwell’s equations and obtain quantitative agreement with the observed colors. The organic crystal shows biaxial anisotropy. For unsubstituted quaterthiophene, α-4T, we find effective refractive indices nb=1.84±0.1 and na=1.61±0.1 for waves under normal incidence. Our conclusions are fully confirmed by atomic force microscopy with molecular resolution. Our analyses result in a simple recipe to obtain the directions of the a and b crystal axes from the optical experiment.
Solar cell efficiency and carrier multiplication in Si1−xGex alloys
Authors: Wolf M. , Brendel R. , Werner J. H. , Queisser H. J.
Published: July 2002

Crystalline Si1−xGex compounds offer the possibility for tuning the electronic energy band structure with the chemical composition of the alloy in order to adapt the material for devices utilizing the energy of solar photons at an optimum. We concentrate on the efficiency enhancement due to carrier multiplication by impact ionization. We calculate the internal quantum efficiency and the possible solar cell efficiency for this material system. The number of impact-generated charge carriers is obtained by a simulation of the competing carrier–carrier and carrier–photon scattering processes. These calculations show that the wave vector dependence of the scattering processes is unimportant for good agreement between theoretical and experimental quantum efficiencies in Si and Ge. Finally, we calculate solar cell efficiencies under the ideal assumption of unity collection efficiency and radiative recombination only. Impact ionization enhances the theoretical conversion efficiency by 0.5 percentage point; this improvement is curtailed by the strong phonon emission probability of hot carriers.
Nonlinear ultrasonic parameter in quenched martensitic steels
Authors: Hurley D. C. , Balzar D. , Purtscher P. T. , Hollman K. W.
Published: July 2002

We have determined the nonlinear ultrasonic parameter β and the ultrasonic longitudinal phase velocity vL for a series of martensitic steel specimens which varied in carbon content. The specimens were measured in the as-quenched state to ensure that the carbon was present primarily as an interstitial in the martensite. β increased monotonically with carbon content and hardness over the range 0.10–0.40 mass % C (39.0–57.5 Rockwell C hardness). However, vL remained virtually the same for all specimens. Therefore we conclude that β is sensitive to microstructural variations between the specimens, but vL is not. X-ray diffraction experiments indicate that the dislocation density in the specimens is large (∼1011–1012/cm2) and increases with increasing carbon content. These results support the hypothesis that the observed increase in β can be attributed to dislocation-related effects in the specimens.
Thermal stability of epitaxial aluminum on In0.53Al0.47As Schottky diodes grown by molecular beam epitaxy
Authors: Pilkington S. J. , Missous M.
Published: July 2002

Current–voltage and capacitance–voltage techniques have been used to characterize the electrical properties of annealed epitaxial aluminum contacts to In0.53Al0.47As grown by molecular beam epitaxy. These as-deposited diodes were found to have electrical characteristics that were dominated by thermionic emission, with an ideality factor of 1.06–1.08 and a barrier height of 0.55–0.56 eV. As the anneal temperature is increased, there is a slight increase in the value of the barrier height, which is believed to be related to an increasing interfacial reaction occurring, promoting the formation of AlAs. For anneals above 400 °C, the electrical characteristics start to degrade rapidly. For comparison, conventionally evaporated Au/InAlAs diodes were also characterized. These diodes had an ideality factor of 1.14–1.18 and a barrier height of 0.63–0.67 eV. Although the electrical characteristics showed little variation in the forward direction, the reverse characteristics exhibited a significant variation between diodes. Upon annealing, the characteristics show variations between diodes, with the characteristics significantly degraded for anneals of 300 °C, showing the poor thermal stability exhibited by conventionally evaporated contacts. The observed characteristics for the epitaxial aluminum contacts to InAlAs, compared with those from conventionally evaporated gold contacts, have implications for the Schottky gate contact in the manufacture of InAlAs–InGaAs high electron mobility transistors.
Effects of prior hydrogenation on the structure and properties of thermally nanocrystallized silicon layers
Authors: Achiq A. , Rizk R. , Gourbilleau F. , Madelon R. , Garrido B. , Pérez-Rodrı́guez A. , Morante J. R.
Published: July 2002

Nanocrystalline silicon layers have been obtained by thermal annealing of films sputtered in various hydrogen partial pressures. The as-deposited and crystallized films were investigated by infrared, Raman, x-ray diffraction, electron microscopy, and optical absorption techniques. The obtained data show evidence of a close correlation between the microstructure and properties of the processed material, and the hydrogen content in the as-grown deposit. The minimum stress deduced from Raman was found to correspond to the widest band gap and to a maximum hydrogen content in the basic unannealed sample. Such a structure relaxation seems to originate from the so-called “chemical annealing” thought to be due to Si–H2 species, as identified by infrared spectroscopy. The variation of the band gap has been interpreted in terms of the changes in the band tails associated with the disorder which would be induced by stress. Finally, the layers originally deposited with the highest hydrogen pressure show a lowest stress—which does not correlate with the hydrogen content and the optical band gap—and some texturing. These features are likely related to the presence in these layers of a significant crystalline fraction already before annealing.
Thermal diffusivity of lead iodide
Authors: Silva T. S. , Alves A. S. , Pepe I. , Tsuzuki H. , Nakamura O. , d’Aguiar Neto M. M. F. , da Silva A. Ferreira , Veissid N. , An C. Y.
Published: July 2002

The thermal diffusivity and thermal conductivity have been determined for lead iodide PbI2, at room temperature, using the photoacoustic spectroscopy. The result shows a thermal diffusivity αs=(25.0±0.4)×10−3 cm2/s, a value very close to other semiconductors of current technological importance. The electrical conductivity is also measured and discussed.
Magnetic phase diagrams of holmium determined from magnetoresistance measurements
Authors: Gebhardt Jeffrey R. , Ali Naushad
Published: July 2002

The magnetic phase diagrams of holmium in the magnetic field-temperature plane have been determined using electrical resistance as a function of temperature and electrical resistance as a function of applied magnetic field (magnetoresistance). Two phase diagrams were constructed corresponding to magnetic fields applied along the a and c axes. For temperatures below 80 K and an applied field along the a axis, our H–T phase diagram agrees well with that of Jensen and Mackintosh. In a temperature range of 30–100 K we observe two transitions which correspond to the boundaries of the predicted helifan phase. However, at higher temperatures there are differences with the work of Willis et al., most notably in the behavior of the Néel temperature splitting. We observe a splitting of the Néel temperature above 1 T with one feature remaining nearly constant in temperature as the field is increased. For a c-axis applied field, our phase diagram is very similar to the results of Willis et al., a splitting of the Néel temperature transition above 0.5 T is observed. At temperatures below 18 K and magnetic field less than 1.5 T, an anomaly associated with a devil’s staircase is seen.
Magneto-optic effect of the ferromagnetic diluted magnetic semiconductor Ga1−xMnxAs
Authors: Ando K. , Hayashi T. , Tanaka M. , Twardowski A.
Published: July 2002

Magnetic circular dichroism (MCD) of a ferromagnetic diluted magnetic semiconductor Ga1−xMnxAs films was measured to clarify their electronic structure. Strong enhancement of MCD by Mn substitution indicated a strong sp–d hybridization. The p–d exchange interaction was concluded to be antiferromagnetic, which was in contrast with the reported ferromagnetic p–d exchange in very diluted paramagnetic GaAs:Mn bulk crystals. The change of the character of the p–d exchange interaction is due to the unique feature of Ga1−xMnxAs that the relative abundance of the neutral and ionized Mn acceptors controls the p–d exchange.
Growth kinetics and optical properties of self-organized GaN quantum dots
Authors: Widmann F. , Daudin B. , Feuillet G. , Samson Y. , Rouvière J. L. , Pelekanos N.
Published: July 2002

Self-organized GaN islands of nanometric scale were fabricated by controlling the Stranski–Krastanov growth mode of GaN deposited by molecular beam epitaxy on AlN. Evidence for ripening of dots under vacuum has been observed, resulting in changes in dot size distribution. We also show that in superlattice samples, consisting of multiple layers of GaN islands separated by AlN, the GaN islands are vertically correlated provided that the AlN layer thickness remains small enough. The luminescence peak of GaN dots is blueshifted with respect to bulk emission and its intensity does not vary with temperature, both effects demonstrating the strongly zero-dimensional character of these nanostructures.
Ferromagnetic tunnel junctions with plasma-oxidized Al barriers and their annealing effects
Authors: Sato M. , Kikuchi H. , Kobayashi K.
Published: July 2002

Cross-geometrical Ni–Fe/Co/Al–AlO/Co/Ni–Fe/Fe–Mn/Ni–Fe tunnel junctions were fabricated by magnetron sputtering. To form the insulating layer, an Al layer was reverse sputtered in an atmosphere of either oxygen or oxygen–argon mixture at low power after deposition. The oxidization time necessary to form an AlO barrier was much shorter than that by natural oxidization, lasting for only a half to a few minutes. By adding argon to oxygen, the oxidization was slowed down and high MR ratios were obtained for a wide range of time. A magnetoresistance (MR) ratio of 16% was observed in the as-deposited junction when the barrier was oxidized in oxygen plasma for 35 s. In addition, the MR ratio increased to 24% by annealing at 300 °C. In as-deposited junctions, the tunnel resistances were increased by increasing the plasma oxidization time, but the MR ratios gradually decreased. The estimated tunnel barrier width increased and the barrier height decreased with the plasma oxidization time. After annealing, the MR ratio increased only for those junctions oxidized for short times. This suggests that the remaining Al between the AlO and the Co surface plays an important role in the effects of annealing.
The effect of silicon on the glass forming ability of the Cu47Ti34Zr11Ni8 bulk metallic glass forming alloy during processing of composites
Authors: Choi-Yim H. , Busch R. , Johnson W. L.
Published: July 2002

Composites of the Cu47Ti34Zr11Ni8 bulk metallic glass, reinforced with up to 30 vol % SiC particles are synthesized and characterized. Results based on x-ray diffraction, optical microscopy, scanning Auger microscopy, and differential scanning calorimetry (DSC) are presented. During processing of the composites, a TiC layer forms around the SiC particles and Si diffuses into the Cu47Ti34Zr11Ni8 matrix stabilizing the supercooled liquid against crystallization. The small Si addition between 0.5 and 1 at. % increases the attainable maximum thickness of glassy ingots from 4 mm for Cu–Ti–Zr–Ni alloys to 7 mm for Cu–Ti–Zr–Ni–Si alloys. DSC analyses show that neither the thermodynamics nor the kinetics of the alloy are affected significantly by the Si addition. This suggests that Si enhances the glass forming ability by chemically passivating impurities such as oxygen and carbon that cause heterogeneous nucleation in the melt.
Contact electrification studies using atomic force microscope techniques
Authors: Gady B. , Reifenberger R. , Rimai D. S.
Published: July 2002

Contact electrification measurements using atomic force microscopy techniques were performed using micrometer-sized spheres made of polystyrene and flat substrates of either freshly cleaved, highly oriented pyrolytic graphite (HOPG) or a 0.2-μm-thick Au[111] film grown on mica. The polystyrene/HOPG interaction exhibited significant electrostatic charging when compared to the polystyrene/Au system. This result is consistent with qualitative expectations of contact charging based on a triboelectric series of common materials. The observed contact electrification is also consistent with electronic charge transfer between materials, rather than an ionic or material transfer mechanism.
Quantitative interfacial profiles in CdTe/Mn(Mg)Te heterostructures
Authors: Charleux M. , Rouvière J. L. , Hartmann J. M. , Bourret A.
Published: July 2002

We have measured by high resolution transmission electron microscopy (HRTEM) the width of interfaces in two II–VI heterostructures: CdTe/MnTe and CdTe/MgTe, as a function of the growth mode. A critical review of the different parameters involved in the direct determination of the chemical profile by HRTEM enables us to precisely determine the sensitivity and accuracy of the methods on these particular materials. The measured interface width is of the order of 2.5–3 monolayers (ML) and is compatible with an exchange mechanism involving the monolayer being grown and the last deposited monolayer. Several growth procedures were compared: conventional molecular beam epitaxy and atomic layer epitaxy (ALE). In the case of saturated and oversaturated ALE the inverse MnTe/CdTe interface is no longer planar. A destabilization of the growth front occurs when one or more Mn monolayers per cycle are deposited, through the formation of MnTe islands. Thermal interdiffusion seems to be negligible in the case of Mn. The present HRTEM values for the interface widths extend the results obtained by magneto-optical measurements to higher concentration values and confirm the exchange mechanism. The higher value obtained by x-ray reflectivity (4.7 ML) is explained by the large difference of the average volume on which the measurement is performed.
Self-assembled InAs/GaAs quantum dots studied with excitation dependent cathodoluminescence
Authors: Tang Y. , Rich D. H. , Mukhametzhanov I. , Chen P. , Madhukar A.
Published: July 2002

We have examined the optical properties of self-assembled InAs quantum dots (QDs) with polarization sensitive and time-resolved cathodoluminescence (CL) techniques. The InAs QDs were formed via self-assembly during molecular beam epitaxial growth of InAs on unpatterned GaAs(001). CL spectra exhibited a two-component line shape whose linewidth, intensity, and peak positions were found to be temperature and excitation dependent. The two components are found to be consistent with state filling of the QDs, resulting in emission involving ground state and excited state excitonic transitions. The luminescence intensities and lineshapes of the QD and wetting layer (WL) excitonic transitions were analyzed with constant excitation and time-resolved CL for various temperatures and excitation levels to study the thermal activation, re-emission, and recombination kinetics of carriers. Thermal quenching of the QD ground state and excited state components in the 105–175 K range is correlated with a rise in the WL emission, showing that the WL carrier distribution is fed partially by thermal reemission of carriers from the QDs. A more rapid thermal quenching of the QD and WL excitonic emissions in the 185–300 K range is consistent with thermal reemission of carriers into the GaAs matrix. Time delayed CL spectra show that carriers in the excited states rapidly feed the ground state during the decay phase of the luminescence, further confirming that the two components are derived from the same QD. The CL decay time for the QD luminescence was found to be wavelength and temperature dependent, owing to thermal reemission into the WL. The CL decay times ranged from 0.1 to 2.0 ns. A polarization anisotropy was found in CL spectroscopy, revealing the importance of the stress anisotropy in both the formation of QDs and their subsequent optical emission.
Theoretical modeling of nonequilibrium optical phonons and electron energy relaxation in GaN
Authors: Tsai Chin-Yi , Chen Chih-Hsiung , Sung Tien-Li , Tsai Chin-Yao , Rorison Judy M.
Published: July 2002

The decay of the zone-center longitudinal optical (LO) phonon in GaN into a transverse optical (TO) phonon and a longitudinal acoustic (LA) phonon is theoretically investigated. Its decay into two LA phonons is forbidden. A theoretical model is presented to study the effect of nonequilibrium LO and TO phonons on the electron energy relaxation rate. The electron energy relaxation time is calculated, and its value is shown to strongly depend on the finite lifetimes of both LO and TO phonons. The individual contributions of A1 mode and E1 mode optical phonons in the overall electron energy relaxation processes are also discussed.
Quantum waveguide theory of serial stub structures
Authors: Jin G. J. , Wang Z. D. , Hu A. , Jiang S. S.
Published: July 2002

The electronic behaviors in quantum wires with serial stubs are studied. A general theory of quantum waveguide based on transfer matrix method is developed and is used to treat periodic stub structures, serial stub structures with a defect stub, and Fibonacci stub structures. A number of interesting physical properties in connection with electronic transmission, energy spectra, and charge density distributions in these structures, are found theoretically. In particular, we find that whether there are periodicity and symmetry in the transmission and energy spectra depends on the commensurability of the length parameters. If one length ratio is incommensurate, then the transmission and energy spectra do not exhibit periodicity and symmetry even for periodic stub structures. In particular, the quasiperiodic behaviors are shown in Fibonacci stub structures proposed by us whenever the length parameters are commensurate. The experimental relevance is also addressed briefly.
Numerical simulations of the electrical characteristics and the efficiencies of single-layer organic light emitting diodes
Authors: Malliaras G. G. , Scott J. C.
Published: July 2002

We have used numerical methods to calculate the current and the efficiency of single-layer organic light emitting diodes, taking into account field-dependent mobilities, diffusion, and thermionic injection at the metal/organic interfaces. It is shown that when the barrier for charge injection from the metal to the organic is lower than 0.3 eV, the contact is, for practical purposes Ohmic, supplying the bulk of the organic layer with a space charge limited current. Determination of the carrier mobility by fitting experimental current–voltage data is discussed and the voltage dependence of the current and the efficiency are analyzed. The recombination profile is shown to evolve with voltage, shifting towards the electrode which injects the carrier with the lower mobility. The numerical results are compared with experimental data from poly[2-methoxy,5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV) based devices.
The effects of surface ripples on sputtering erosion rates and secondary ion emission yields
Authors: Carter G.
Published: July 2002

The rate of erosion by non-normal incidence ion bombardment of a sinusoidally rippled surface is calculated analytically for an arbitrary dependence of the sputtering yield on the angle of ion incidence and spatial derivatives of the surface gradient as a function of the ratio of the ripple amplitude to wavelength. It is shown that, relative to the value for a flat surface, the erosion rate first increases and then decreases as this ratio increases, but before this decrease can occur, interpeak shadowing of the incident ion flux changes the ripple habit to sawtooth form and the erosion rate reaches a steady-state value. The influence of this variation on the sputtered atom yield and, therefore, on secondary ion yield is also evaluated and shown to compare favorably with experimental studies of the behavior of this parameter.
Time dependent study of a multi-ionic xenon plasma
Authors: Sobral H. , Schinca D. , Gallardo M. , Duchowicz R.
Published: July 2002

The dynamics of a low pressure xenon discharge plasma was analyzed by using a collisional-radiative model. The calculations reproduce the main features of the temporal evolution of xenon spontaneous emission lines experimentally observed under the pulsed regime. The analysis includes transitions belonging to several xenon species (Xe I–Xe VIII). More than 100 measured lines were considered in this study. The pressure and discharge voltage dependence of the temporal emission distribution (time delay, shapes, and intensity maximum position), obtained from the calculation, are in agreement with the experimental results. The main excitation-deexcitation processes are discussed.
Inhomogeneous electric field-induced Raman scattering at ZnSe/GaAs interfaces
Authors: Pagès O. , Erguig H. , Laurenti J. P. , Certier M. , Bormann D. , Khelifa B. , Briot O.
Published: July 2002

The activation of inhomogeneous electric field-induced Raman scattering (IEFIRS) at ZnSe/GaAs interfaces is investigated. The consideration of the inhomogeneous character of the electric field brings additional conditions on top of those predicted by the microscopic and perturbative approaches. These conditions concern the relative orientations of (i) the wave vector of the vibrational modes in the matter, (ii) the direction of the electric field at the junction, and (iii) the gradient vector of the electric field magnitude in the space charge regions. As latter vector has opposite directions in the layer and the substrate, IEFIRS is only observed from one side of the junction for a given scattering geometry.
Fabrication, modeling, and direct evanescent field measurement of tapered optical fiber sensors
Authors: Moar P. N. , Huntington S. T. , Katsifolis J. , Cahill L. W. , Roberts A. , Nugent K. A.
Published: July 2002

Tapered optical fibers have been fabricated using a flame elongation technique. The evanescent field distribution surrounding the taper region has been measured directly using a scanning near field optical microscope and results are found to be in good agreement with predictions made using a finite difference beam propagation method.
Dynamics of a coplanar-electrode plasma display panel. II. Cell optimization
Authors: Rauf Shahid , Kushner Mark J.
Published: July 2002

Plasma display panels (PDPs) are a leading technology for large-area flat panel displays. As a result, there is significant interest in improving their efficiency, luminosity, and lifetime. In this article, results from a two-dimensional model are used to investigate the consequences of operating conditions, gas mixture, cell dimensions, and material properties on the visible light generation capacity (luminosity and efficiency) of a coplanar-electrode PDP cell sustained in He/Ne/Xe gas mixtures. Of the species that dominantly lead to the generation of visible light (Xe*, Xe**, and Xe2*), Xe2* makes the largest contribution for our conditions since its UV radiation is optically thin and Xe2* is efficiently generated from the long-lived xenon metastable. Significant improvements could be made in PDP light generation efficiency by choosing operating conditions that favor production of Xe2*, such as increasing gas pressure to enhance the three-body collision processes that generate Xe2*. Gas mixtures with more Ne (or less He) were found to produce more visible light at higher efficiency since electron transport in Ne is less collisional than He and Xe2* is produced more efficiently in three body collisions with Ne. PDP light emission characteristics are sensitive to the spacing between the dielectrics and there is an optimum spacing where both total visible light output and efficiency are high. It was found that PDP cells do not generate visible light efficiently during the postavalanche discharge phase due to low values of E/N (electric field/total gas density) in the bulk plasma region. Slight improvements can be made in light generation efficiency be choosing conditions for which PDP cell spends less time in the discharge phase in each cycle.
Simulations of stress evolution and the current density scaling of electromigration-induced failure times in pure and alloyed interconnects
Authors: Park Young-Joon , Andleigh Vaibhav K. , Thompson Carl V.
Published: July 2002

An electromigration model is developed to simulate the reliability of Al and Al–Cu interconnects. A polynomial expression for the free energy of solution by Murray [Int. Met. Rev. 30, 211 (1985)] was used to calculate the chemical potential for Al and Cu while the diffusivities were defined based on a Cu-trapping model by Rosenberg [J. Vac. Sci. Technol. 9, 263 (1972)]. The effects of Cu on stress evolution and lifetime were investigated in all-bamboo and near-bamboo stud-to-stud structures. In addition, the significance of the effect of mechanical stress on the diffusivity of both Al and Cu was determined in all-bamboo and near-bamboo lines. The void nucleation and growth process was simulated in 200 μm, stud-to-stud lines. Current density scaling behavior for void-nucleation-limited failure and void-growth-limited failure modes was simulated in long, stud-to-stud lines. Current density exponents of both n=2 for void nucleation and n=1 for void growth failure modes were found in both pure Al and Al–Cu lines. Limitations of the most widely used current density scaling law (Black’s equation) in the analysis of the reliability of stud-to-stud lines are discussed. By modifying the input materials properties used in this model (when they are known), this model can be adapted to predict the reliability of other interconnect materials such as pure Cu and Cu alloys.
Highly coercive SmCo5 magnets prepared by a modified hydrogenation-disproportionation-desorption-recombination process
Authors: Kubis M. , Handstein A. , Gebel B. , Gutfleisch O. , Müller K.-H. , Schultz L.
Published: July 2002

The hydrogenation-disproportionation-desorption-recombination (HDDR) process was applied to SmCo5 using extreme conditions, namely high hydrogen pressures and reactive milling under hydrogen. Investigations on the hydrogen absorption behavior of SmCo5 by differential scanning calorimetry under hydrogen pressures between 1 and 7 MPa showed absorption events due to an interstitial absorption at about 100 °C and a disproportionation reaction at about 600 °C. X-ray diffraction showed the disproportionation of SmCo5 into Sm hydride and fcc-Co. A favorable effect of high hydrogen pressures on the disproportionation reaction was observed which can be explained by a decrease of the free enthalpy of the samarium hydride for increasing hydrogen pressures. Reactively milled SmCo5 showed also the products of the disproportionation reaction. The recombination to the original SmCo5 phase on hydrogen desorption in a subsequent heat treatment in vacuum was successful for both methods. However, Sm2O3, Sm2Co17, and Sm2Co7 were detected as minor phases. Maximum coercivities μ0JHC of 2.1 and 4.7 T were achieved for high pressure and reactively milled HDDR powders, respectively. The high coercivities originate from the high anisotropy field of the SmCo5 phase in combination with the grain refinement due to the HDDR treatment.
Magnetic field induced optical transmission study in an iron nanoparticle ferrofluid
Authors: Wu K. T. , Yao Y. D. , Wang C. R. C. , Chen P. F. , Yeh E. T.
Published: July 2002

Optical transmission of an iron nanoparticle ferrofluid has been investigated as a function of incident optical wavelengths between 450 and 700 nm and applied magnetic fields up to 200 Oe. The transmittance decreases as the thickness and concentration of the Fe ferrofluid films increase. This effect is more manifest with shorter incident wavelength. The decrease of transmittance for sample prepared by electrochemical method is much more rapidly than that of samples prepared by laser ablation technique. However, the magnetic field effect for these samples is vise versa. The aspect ratio of the Fe nanoparticles we prepared is roughly between 1 and 2. The decrease of the transmittance between 575 and 450 nm is explained by a classical electrostatic calculation based on the Mie/Drude formalism.