bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear effects of ionization on surface waves on a plasma–metal interface
Authors: Ostrikov K. N. , Yu M. Y. , Azarenkov N. A.
Published: July 2002

Nonlinear effects associated with density modulation caused by wave-induced ionization in magnetized plasmas were studied. The ionizing surface waves propagate at the interface between the plasma and a metallic surface. It is shown that the ionization nonlinearity can be important for typical experimental conditions.
Pulsed-laser deposition of diamond-like carbon: Relations between laser fluence, velocity of carbon ions, and bonding in the films
Authors: Koivusaari K. J. , Levoska J. , Leppävuori S.
Published: July 2002

In the pulsed-laser deposition process, high intensity laser pulses expel material from a solid target and form expanding plasma near the solid surface. The expansion of the plasma produces a forward-directed beam of ionized and neutral species with typical energies of 1–100 eV. In this study, amorphous diamond-like carbon (DLC) thin films were deposited onto silicon substrates at room temperature using an XeCl excimer laser (wavelength 308 nm, pulse length 20 ns) with laser fluences in the range 5–45 J cm−2, on a pyrolytic graphite target. The effect of laser fluence on the velocity and kinetic energy distribution of carbon ions was measured by time-of-flight (TOF) spectrometry using a system based on a Faraday cup with biased grids, and a multichannel plate based particle detector. We have found high kinetic energies, up to 500 eV, for expelled atomic species. In order to study the effect of the energy of the arriving ions on the structure of DLC, the bonding of carbon atoms in films, deposited under similar conditions as in the TOF measurements, was studied by x-ray absorption near-edge structure.
Hot-electron transport through Au/CaF2/Si(111) structure studied by ballistic electron emission spectroscopy
Authors: Sumiya Touru , Honda Katsuya , Miura Tadao , Tanaka Shun-ichiro
Published: July 2002

The influence of calcium fluoride (CaF2) [about two monolayers (ML)] in Au/CaF2/Si(111) heterostructure on hot-electron transport across the CaF2 intralayers has been studied using ballistic electron emission microscopy (BEEM) and Fourier transform infrared spectroscopy (FTIR). The BEEM current–voltage spectra show that the electron transport property is strongly affected by the CaF2 intralayers. The threshold voltage V0 for the onset of the BEEM current for an insulating CaF2 intralayer, which is about 3.6 V, is obtained only for the sample in which CaF2 was deposited at 700 °C. In contrast, the threshold voltage of the sample in which CaF2 was deposited at 550 °C is determined to be about 0.74 V. The FTIR spectra of these CaF2 layers show that Ca–Si–F bonds exist in the latter, but not in the former. The existence of Ca–Si–F bonds implies that the CaF2 heteroepitaxial growth at 550 °C is unsuccessful in obtaining a high-quality CaF2 layer and will induce many defects in the CaF2 layer and/or at the interface. The defect-induced states in the CaF2 intralayers allow hot electrons to travel through the intralayers even below 3.6 eV and lie in a position of the threshold voltage for the onset of the BEEM current to be about 0.74 V.
Powder agglomeration patterns at acoustic driving observed by sonoluminescence technique
Authors: Korotchenkov O. A. , Goto T.
Published: July 2002

A sonoluminescence (SL) technique gives a sensitive probe to detect spatial agglomeration patterns produced in powder mixtures subjected to acoustically driven plate. Studies of the surface-integrated SL intensity with increasing driving amplitude yield data about variations of the packing density on the surface of vibrating powders. Imaging of spatial distributions of the SL intensity enables direct measurement of the size of agglomeration patterns. The size of the patterns is found to be ∼102 μm in a 3-μm-sized powder at a driving frequency of about 4 MHz. The distribution of the average sonoluminescence intensity across the pattern is attributed to the density gradient while rapid variations in the SL intensity across the pattern are suggested to give the particle velocity distribution within the pattern. The temporal changes of the SL intensity can be used to study time evolutions of dense powder arrangements.
Postdeposition relaxation of internal stress in sputter-grown thin films caused by ion bombardment
Authors: Nowak R. , Yoshida F. , Morgiel J. , Major B.
Published: July 2002

A hitherto unreported postdeposition method of relaxing ultrahigh internal stresses prevailing in sputter-grown thin films is the subject of the present research. A significant reduction of stress has been confirmed by x-ray diffraction and independent substrate deflection measurements for HfN films treated with Si+ ions of various energies (450 keV, 500 keV, and 1.1 MeV). The particular sequence of the performed experiments has allowed us to deduce the most likely scenario of the resulting stress relaxation. We argue that the observed reduction of internal stress had been caused by structural changes, namely the transport of interstitial defects occurring within the thermal spikes induced by ion bombardment and an increase in vacancy concentration. Simple theoretical considerations proved that the existence of an amorphous silicon interlayer formed right under the HfN film during the bombardment cannot be the cause of the observed stress relaxation. The employed Auger electron spectroscopy, transmission electron microscopy, and x-ray diffraction technique confirmed that the ion bombardment did not seriously affect either the composition, dislocation structure, or texture of nitride films. The relative softening of HfN after ion bombardment was found to be caused by amorphization of the substrate directly under the film. The penetration of incident particles deeper than predicted by a numerical simulation was attributed to a distinct crystallographic texture determined experimentally for HfN films. It was confirmed that modification with Si+ ions does not affect the resistivity of HfN films.
Evolution of Ge islands on Si(001) during annealing
Authors: Kamins T. I. , Medeiros-Ribeiro G. , Ohlberg D. A. A. , Stanley Williams R.
Published: July 2002

The evolution of the shape and size distributions of Ge islands on Si(001) during annealing after deposition has been studied at different temperatures and effective coverages. The initial distributions of square-based pyramids, elongated “hut” structures, faceted “dome-shaped” islands, and much larger “superdomes” depends on the deposition conditions. During annealing after deposition, the islands coarsen over a limited range of times and temperatures. Those pyramidal-shaped islands that grow transform to faceted, dome-shaped islands as they become larger. Initially dome-shaped islands that dissolve transform to a pyramidal shape as they become smaller during the process of dissolving. Outside of this coarsening regime, the islands can achieve a relatively stable, steady-state configuration, especially at lower temperatures. At higher temperatures, intermixing of Si into the Ge islands dominates, decreasing the strain energy and allowing larger islands to form. At lower and intermediate temperatures, the initial wetting layer is metastable, and some Ge transfers to the islands during the early stages of annealing.
Propagation of non-thermal phonons induced by α-particle bombardment in BaF2
Authors: Bravin M. , Clegg P. , Booth N. E. , Bruckmayer M. , Djotni K. , Esposito E. , Houwman E. P. , Kraus H. , Salmon G. L.
Published: July 2002

We have measured the time dependence of the flux of non-thermal phonons generated by α particles in a BaF2 single crystal of mass 1 g using a Series Array of Superconducting Tunnel Junctions. Taking advantage of the almost perfect elastic symmetry of BaF2, we approximate the equations of quasidiffusive propagation to give a simple diffusion equation characterized by an effective diffusion coefficient D=KL8/9, where K is a material constant and L the distance between the phonon source and the point of detection. We use this model to perform pulse-shape analysis on our data and show that agreement is excellent, and highly preferred over a ballistic pulse shape. The distributions of values of K determined for different lengths L are found to overlap strongly, in support of the law D=KL8/9. We suggest that pulse-shape analysis may provide an alternative to time-difference measurements to achieve position sensitivity in a phonon-mediated detector based on a BaF2 absorber.
Denuded zone and diffusion length investigation by electron beam induced current technique in intrinsically gettered Czochralski silicon
Authors: Spiga S. , Castaldini A. , Cavallini A. , Polignano M. L. , Cazzaniga F.
Published: July 2002

Electron beam induced current (EBIC) technique is successfully used to characterize intrinsically gettered Czochralski silicon. The impact of three different sequences of thermal treatments, typically used in ultralarge scale integration device manufacturing, on the denuded zone (DZ) formation and oxygen precipitation in the bulk is evaluated. EBIC technique is applied in a nonstandard configuration, where a Schottky diode is evaporated on the wafer cross section, for the direct observation of the DZ and oxygen related defects in the silicon bulk. The reduction of minority carrier diffusion length, due to the formation of recombination centers after oxygen precipitation, is also estimated by EBIC in planar collector geometry. The DZ determination by EBIC technique is in good agreement with surface photovoltage measurements and microscopical inspections after chemical etching.
Doping effect of charged porphyrin derivative into multilayered conducting polymer heterostructure by self-assembly method
Authors: Yamaue Takahiro , Kawai Tsuyoshi , Onoda Mitsuyoshi , Yoshino Katsumi
Published: July 2002

Optical and electrical properties of molecularly doped multilayered heterostructures of conducting polymers, which were prepared by alternative dipping of a substrate into a solution of a negatively charged conducting polymer and a mixed solution of a positively charged conducting polymer and a positively charged porphyrin, have been studied. In spite of the low doping level of the porphyrin, remarkable photocurrent enhancement was observed in the doped multilayered heterostructure. This doping effect was explained in terms of efficient photoinduced charge separation at the interfaces between negatively and positively charged conducting polymers and also at the interface between porphyrin derivative and conducting polymers. The photoconduction was also found to be dependent on the polarization of excitation light.
Theory of potential modulated microwave reflectivity at semiconductor surfaces
Authors: Natarajan Arun , Nellore Anoma , Searson Peter C.
Published: July 2002

Microwave reflectivity can be used to probe the electrical properties of the semiconductor/solution interface by measuring the reflectivity response to a modulation in the band bending. We use a multiphase stratified media model to calculate the microwave reflectivity for a semiconductor in contact with a solution. The reflectivity change produced by such systems is related to the frequency of the microwave source, the thickness of the semiconductor, the thickness of the space charge layer of the semiconductor, the dielectric constants, and conductivities of the various media. The sensitivity factor of this model is compared to experimental results for silicon surfaces.
Optical interference in magneto-optic Kerr-effect measurements of magnetic multilayers
Authors: Maat S. , Shen L. , Hou C. , Fujiwara H. , Mankey G. J.
Published: July 2002

Hysteresis loops of exchange-biased permalloy/Fe50Mn50/permalloy trilayers on glass were measured as a function of Fe50Mn50 and permalloy thicknesses with the longitudinal Kerr effect employing a coherent light source. Kerr signals originate from both permalloy layers and give a superposition of hysteresis loops. In vibrating sample magnetometer or looptracer measurements the contribution of a particular layer to a major hysteresis loop cannot be identified. With the Kerr setup presented it is possible to identify the contribution of each layer individually, since the finite optical path through the trilayer gives rise to optical attenuation and interference. For an increasing total thickness of the trilayer, the signal of the buried permalloy layer will become weaker due to attenuation. Kerr measurements of trilayers up to 40 nm show a superposition of two equally oriented loops. Major loops for trilayers of thickness greater than 40 nm show a superposition of two oppositely oriented hysteresis loops. The transition is dependent only on the total thickness of the trilayer, rather than the thickness of each individual layer. This unusual effect can be explained by the phase difference of the two Kerr signals. Additional measurements performed from the glass side of the samples and measurements of a Fe50Mn50/permalloy bilayer confirm that the sense of a hysteresis loop can change for a buried layer due to optical effects.
Wet oxidation of Ti34Si23N43
Authors: Kacsich T. , Gasser S. , Tsuji Y. , Dommann A. , Nicolet M.-A.
Published: July 2002

The wet oxidation of magnetron-sputtered x-ray-amorphous Ti34Si23N43 films at temperatures from 500 to 1000 °C and ambient pressure was investigated by backscattering spectrometry, x-ray diffraction analysis, and step profilometry. Up to 800 °C, the oxidation yields x-ray-amorphous oxide scales of atomic composition Ti20Si13O67. At 500 and 550 °C the oxide growth kinetics is parabolic. At 700 and 800 °C, the oxidation kinetics is well described by a logarithmic time dependence. At 1000 °C, the oxide scale has a layered structure with a crystalline TiO2 layer on top of Ti–Si–O.
Analysis of conduction mechanisms in annealed n-Si1−xCx:H/p-crystalline Si heterojunction diodes for different doping concentrations
Authors: Marsal L. F. , Pallares J. , Correig X. , Orpella A. , Bardés D. , Alcubilla R.
Published: July 2002

We fabricated and characterized annealed n-type amorphous Si1−xCx on p-type crystalline silicon heterojunction diodes with three different base doping concentrations: NA∼1016, 1018, and 1020 cm−3. The conduction mechanisms were determined by analyzing the temperature dependence of the current–voltage characteristics. The results show that the diodes with low doping concentrations (1016 cm−3) are ideal, because the phosphorous slightly diffuses into the crystalline silicon, whereas diodes with higher doping concentrations (1018–1020 cm−3) are dominated by multitunneling capture emission. The increase in the base acceptor doping concentration also causes excess current over the ideal diode current at low forward bias and an increase in the leakage reverse current.
Ultrasonic pulse compression with one-bit time reversal through multiple scattering
Authors: Derode Arnaud , Tourin Arnaud , Fink Mathias
Published: July 2002

We present experimental results that demonstrate the feasibility of one-bit time reversal even through high-order multiple scattering. A short ultrasonic pulse is transmitted and propagates through a random set of steel rods. The scattered waves are recorded on a 128-channel array, time reversed, and retransmitted through the same medium. The time-reversal mirror takes advantage of multiple scattering to compress the scattered waves into a pulse and focus it back onto the source. Paradoxically, we show that the results are even better when the scattered signals are digitized over one bit. Both temporal and spatial resolutions remain unchanged, while the compressed pulse is amplified by 12 dB, and the signal-to-noise ratio is lowered by 1.2 dB. A statistical model is developed, and its predictions are found to be in good agreement with the experimental results.
Modeling of the power-dependent velocity of microwave magnetic envelope solitons in thin films
Authors: Zaspel C. E. , Kabos P. , Xia H. , Zhang H. Y. , Patton C. E.
Published: July 2002

Recently obtained single soliton solutions to the nonlinear Schrödinger equation with additional higher order nonlinear and dispersion terms have been used to model the properties of microwave magnetic envelope solitons in thin films and, in particular, the recently observed power-dependent soliton velocity in these films. The modeling is based on an empirical scaling between the dimensionless amplitude parameters in the higher order nonlinear Schrödinger (HONLS) equation and the pulse power levels in the experiment. Based on this scaling, the amplitude-dependent soliton velocity from the HONLS equation solution is shown to match the velocity versus power response from the experiment.
Explicit Green’s function of a boundary value problem for a sphere and trapped flux analysis in Gravity Probe B experiment
Authors: Nemenman I. M. , Silbergleit A. S.
Published: July 2002

Magnetic flux trapped on the surface of superconducting rotors of the Gravity Probe B (GP-B) experiment produces some signal in the superconducting quantum interference device readout. For the needs of GP-B error analysis and simulation of data reduction, this signal is calculated and analyzed in this article. We first solve a magnetostatic problem for a point source on the surface of a sphere, finding the closed form elementary expression for the corresponding Green’s function. Second, we calculate the flux through the pick-up loop as a function of the source position. Next, the time dependence of a source position, caused by rotor motion according to a symmetric top model, and thus the time signature of its flux are determined, and the spectrum of the trapped flux signal is analyzed. Finally, a multipurpose program of trapped flux signal generation based on the above results is described, various examples of the signal obtained by means of this program are given, and their features are discussed. Signals of up to 100 fluxons, i.e., 100 pairs of positive and negative point sources, are examined.
Transmission properties at microwave frequencies of two-dimensional metallic lattices
Authors: Navarro Enrique A. , Martinez-Pastor Juan , Such Vicente
Published: July 2002

The transmission properties of different metallic photonic lattices (square and rectangular) have been experimentally studied. A numerical algorithm based on time domain finite differences has been used for simulating these photonic structures. The introduction of defects in the two-dimensional metallic lattice modifies its transmission spectrum. If metal rods are eliminated from (or added to) the lattice, extremely narrow peaks are observed at some particular frequencies below (or above) the band pass edge.
Sensitized near-infrared luminescence from polydentate triphenylene-functionalized Nd3+, Yb3+, and Er3+ complexes
Authors: Klink S. I. , Hebbink G. A. , Grave L. , Van Veggel F. C. J. M. , Reinhoudt D. N. , Slooff L. H. , Polman A. , Hofstraat J. W.
Published: July 2002

Hexa-deutero dimethylsulfoxide (DMSO-d6) solutions of terphenyl-based Nd3+, Yb3+, and Er3+ complexes functionalized with a triphenylene antenna chromophore exhibit room temperature near-infrared luminescence at wavelengths of interest for the optical telecommunication network (∼1330 and ∼1550 nm). The sensitizing process takes place through the triplet state of triphenylene as can be concluded from the oxygen dependence of the sensitized luminescence. A significant fraction of the excited triphenylene triplet state is quenched by oxygen, instead of contributing to the population of the luminescent state of the lanthanide ion. The luminescence lifetimes of the triphenylene-functionalized lanthanide complexes ((2)Ln) are in the range of microseconds with a lifetime of 18.6 μs for (2)Yb, 3.4 μs for (2)Er, and 2.5 μs for (2)Nd in DMSO-d6. These luminescence lifetimes seem almost completely dominated by the vibrational quenching by the organic groups in the polydentate ligand and solvent molecules, which leads to low overall luminescence quantum yields.
Growth of undulating Si0.5Ge0.5 layers for photodetectors at λ=1.55 μm
Authors: Lafontaine H. , Rowell N. L. , Janz S. , Xu D.-X.
Published: July 2002

Si 0.5 Ge 0.5 /Si multiquantum-well structures are grown using a production-compatible ultrahigh vacuum chemical vapor deposition system. The structures are designed in order to obtain dislocation-free undulating strained layers used as the absorbing layers in photodetector structures. The Si/SiGe/Si stack on a silicon-on-insulator wafer is used as the waveguiding layer. Transmission electron microscopy and photoluminescence are used to characterize the undulating layers. A photoluminescence emission corresponding to the band edge “no phonon” transition is measured at a wavelength beyond 1.55 μm. Preliminary data from metal–semiconductor–metal photodetectors fabricated with this material show a responsivity of approximately 0.1 A/W at the telecommunication wavelength of λ=1.55 μm.
Deep hole traps in Be-doped Al0.5Ga0.5As layers grown by molecular beam epitaxy
Authors: Szatkowski J. , Płaczek-Popko E. , Sierański K. , Hansen O. P.
Published: July 2002

Deep hole traps in p-type Al0.5Ga0.5As grown by molecular beam epitaxy have been studied by the deep-level transient-spectroscopy method applied to samples with a Schottky diode configuration. Five hole traps, labeled as H0 to H4, were found. For traps H1, H3, and H4 the activation energies for emission were ET1=0.14 eV, ET3=0.40 eV, and ET4=0.46 eV, respectively. Hole emission from trap H2 was dependent on the external electric field. The emission rate obeyed the Poole–Frenkel relation. When extrapolated to zero electric field, the thermal activation energy for hole emission was ET2,0=0.37 eV. Capture cross sections for traps H1 and H4 were thermally activated with energy barriers EB1=0.04 eV and EB4=0.18 eV, respectively.
Inelastic electron tunneling experiments in Bi1−xSbx alloys
Authors: Nagao Jiro , Hatta Eiji , Mukasa Kōichi
Published: July 2002

Inelastic electron tunneling experiments have been performed in Bi0.94Sb0.06–Al oxide-Al junctions. A strong peak observed at 1.9 mV in the second derivative tunnel spectra is closely related to the estimated effective Debye temperature, besides intervalley acoustic and optical phonon structures. This structure clearly corresponds to the electron–intravalley acoustic phonon coupling.
Theoretical and experimental studies in n-type modulation-doped InxGa1−xAs/InyAl1−yAs/InP magnetic sensors
Authors: Ben Jazia A. , Mejri H. , Hassen F. , Maaref H. , Guillot G.
Published: July 2002

We present a theoretical study of the quantized electronic states in both n-type uniformly doped and δ-doped double quantum well (DQW) InxGa1−xAs/InyAl1−yAs heterostructures. The shape of the confining potential, the subband energies and their occupancies, the eigen envelope wave functions, and the Fermi energy in the quantum wells were calculated self-consistently taking into account exchange-correlation and strain effects. The influence of the doping contents, the thicknesses of the barrier, and the channel sheet layers on the confinement properties of these heterostructures will be discussed. By optimizing the parameters of the InxGa1−xAs/InyAl1−yAs heterostructures to be used as Hall magnetic sensors, we have demonstrated that the parallel conduction in the InyAl1−yAs barrier can be suppressed and the density of the two-dimensional electron gas in the InxGa1−xAs channel is reduced as well. Hall data, obtained on molecular beam epitaxy grown Si modulation-doped InxGa1−xAs(x=0.75)/InyAl1−yAs DQWs, are used as an experimental support to validate our calculation. A deep lying defect has been detected through the observation of persistent photoconductivity (PPC) at low temperature. We have also studied the effects of this PPC on the transport properties of the heterostructures investigated. A detailed analysis of all these results will be presented.
Atomic structure, magnetism, and transport properties of damaged La0.67Ca0.33MnO3−δ processed via high-energy ball milling
Authors: Liou Michael B. , Islam Shaheen , Fatemi D. J. , Browning V. M. , Gillespie D. J. , Harris V. G.
Published: July 2002

A La0.67Ca0.33MnO3−δ alloy was produced via a low-temperature solid state reaction in a high-energy ball mill. The milled alloy is found to exhibit the paramagnetic-to-ferromagnetic (P–F) transition at 150 K. However, the widely reported insulator-to-metal (I–M) transition that usually accompanies the P–F transition is totally suppressed. Anneals at 500–1000 °C in air (t=8 h) are found to lower the resistivity but not restore the I–M transition. The suppression of the I–M transition is attributed to anion defects in the structure that have not been annihilated during heat treatments. Extended x-ray absorption fine structure is employed to track the evolution of the atomic structure around the Mn cations at various stages of processing.
Nonlinear propagation of Bessel–Gauss ultrasonic beams
Authors: Ding Desheng , Wang Shanjin , Wang Yaojun
Published: July 2002

We investigate theoretically the properties of the fundamental and second harmonic components of the Bessel beam with a finite aperture, the Bessel–Gauss beam, whose transverse profile is given by the product of Bessel and Gaussian functions. The analysis is based on the linearized and quasilinear solutions of the Khokhlov–Zabolotskaya–Kuznetsov nonlinear wave equation. The analytical and approximate expressions are derived for the fundamental and the second harmonic generation in this beam. It is thereby demonstrated that under certain circumstances, the second harmonic in the Bessel–Gauss beam is nearly radially nondiffracting, and that the beamwidth is approximately one-half of that of the fundamental. This result is an extension to the previous work on the nonlinearity of the Bessel beam, where the infinite extent of the beam has been assumed.
Above-barrier states in GaAs–AlGaAs superlattices studied by photoconductivity and photoreflectance
Authors: Fan J. C. , Chen Y. F. , Lin D. Y. , Huang Y. S. , Chen M. C. , Lin H. H.
Published: July 2002

We report that the quasibound states at the above-barrier region in AlGaAs–GaAs superlattices can be clearly observed at room temperature by photoconductivity as well as photoreflectance measurements. We provide concrete evidence to confirm that free-carrier confinement at barrier layer does exist. It is also found that the barrier-width dependence of the above-barrier transition energies can be described quite well by the modified Messiah’s calculation. However, the simple calculation using the constructive interference condition can only explain the transitions at lower energies, and fails with increasing transition energy.
The thermal refractive index coefficients of 7.5 mol % Nb:KTiOPO4 crystals
Authors: Zhang D. Y. , Shen H. Y. , Liu W. , Zhang G. F. , Chen W. Z. , Zhang G. , Zeng R. R. , Huang C. H. , Lin W. X. , Liang J. K.
Published: July 2002

The principal refractive indices and thermal refractive index coefficients for Nb:KTiOPO4 (KTP) crystal grown by the top-seed flux method from a starting material containing 7.5 mol % Nb have been measured using the autocollimation method. Using measurements made at wavelengths of 0.53975, 0.6328, 1.0795, and 1.3414 μm, in the temperature range from ambient to 188 °C, it is possible to determine the constants of the Sellmeier equations and the thermal refractive index coefficients as a function of wavelength. Also, the temperature acceptance bandwidth at 90° second harmonic generation noncritical phase matching direction was calculated. Comparison of these results with those for undoped KTiOPO4 [K. Rato, IEEE J. Quantum Electron QE-28, 1974 (1992)] and KNbO3 [B. Zysset, I. Biaggo, and P. Gunter, J. Opt. Soc. Am. B 9, 380 (1992)] implies that Nb-doped KTP crystals are suitable for blue coherent light devices.
Global model and scaling laws for inductively coupled oxygen discharge plasmas
Authors: Chung T. H. , Yoon H. J. , Seo D. C.
Published: July 2002

For inductively coupled oxygen rf discharge plasmas, a global model which consists of the energy and particle balance equations is formulated. The energy balance equation includes energy losses for electron-neutral elastic collision, excitation, ionization, dissociation, pair production, and charged particle wall loss. Particle balance equations are written for all species of interest. For a specified discharge length and diameter, absorbed power, pressure, electron temperature-dependent reaction rate coefficients and surface recombination constants, we solve these equations to determine the densities of all species and the electron temperature. We measure these parameters by the Langmuir probe method. According to the prevailing particle loss mechanism, the parameter space can be divided into a volume recombination-loss-dominated region and an ion-flux-loss-dominated region. Based on the global model equations, the scaling laws of plasma variables with the control parameters for the ion-flux-loss-dominated region are estimated and compared with the experimental results.
Study of direct tunneling through ultrathin gate oxide of field effect transistors using Monte Carlo simulation
Authors: Cassan E. , Galdin S. , Dollfus P. , Hesto P.
Published: July 2002

Direct tunneling gate currents of ultrathin gate oxide thickness metal oxide semiconductor field effect transistors (MOSFETs) are modeled in a two-step calculation procedure based on the treatment of physical microscopic data acquired during Monte Carlo device simulation. Gate currents are obtained by weighting the carrier perpendicular energy distribution at the Si/SiO2 and N+-poly–Si/SiO2 interfaces by the electron transmission probability, which is calculated by the one-dimensional Schrödinger equation resolution with the transfer-matrix method. The procedure is applied to a 0.07 μm gate length and 1.5 nm gate oxide thickness transistor, for which the gate and drain voltage influences on gate currents are studied by assuming at first a uniform gate oxide layer. It is shown that the maximum gate current is obtained for one of the two static points of complementary metal oxide semiconductor inverters: VGS=VDD and VDS=0, which raises a severe problem of standby power consumption. The contribution of hot carriers to the tunnel current is evaluated and is found to be small in case of such ultrathin oxide n-MOSFETs: contrary to thick (>5 nm) gate oxide transistors, the maximum gate current is not linked to the carrier energy peak in the channel but is located near the source well where the electron concentration is the largest. Oxide thickness fluctuations are then considered by meshing the oxide surface area and assuming a Gaussian law for the local oxide thickness deviation to the mean value. It is shown that a correct agreement is achieved with experimental published data when the oxide film nonuniformity is included in the calculation. Gate currents mapping for different bias conditions are given and analyzed, which show that very high current densities run through the oxide layer in the vicinity of weak points. An estimate of the surface through which flows the major part of the current is made, and a link between the highly nonuniform current leakage and the soft-breakdown mechanism of the oxide layer is proposed.
Linear electroabsorption in semi-insulating GaAs/AlGaAs asymmetric double quantum wells
Authors: Aguilar María , Carrascosa M. , Agulló-López F. , Agulló-Rueda F. , Melloch M. R. , Nolte D. D.
Published: July 2002

Electroabsorption has been investigated in semi-insulating asymmetric GaAs/AlGaAs double quantum wells presenting high linear Stark responses, adequate for photorefractive applications. We have used the envelope function approximation to calculate the linear Stark shifts of the energy levels and select a suitable structure for the experimental study. The experimental data indicate that the response to the applied field critically depends on a complicated interplay of effects that compete or cooperate to suppress or enhance the electroabsorption. For positive field polarity, the competing contributions of the overlapping e1–hh1 and e1–hh2 transitions partially cancel the electroabsorption despite large linear Stark shifts. On the other hand, small negative fields induce large electroabsorption because the Stark shifts of the two transitions have opposite signs.
Oxygen-related defects in O+-implanted 6H–SiC studied by a monoenergetic positron beam
Authors: Uedono Akira , Tanigawa Shoichiro , Ohshima Takeshi , Itoh Hisayoshi , Aoki Yasushi , Yoshikawa Masahito , Nashiyama Isamu
Published: July 2002

Vacancy-type defects and their annealing properties for O+- or N2+-implanted 6H–SiC were studied using a monoenergetic positron beam. For ion-implanted specimens with a dose of 1×1013 cm2, the mean size of open volume of defects was estimated to be close to the size of divacancies. Annealing processes of the damage were identified to be agglomerations of vacancy-type defects due to migrations of carbon vacancies (100–400 °C), silicon vacancies (400–800 °C), and vacancy complexes such as divacancies (800–1000 °C), and the annealing out of the open spaces adjacent to extended defects (1000–1400 °C). From a comparison between the annealing behaviors of defects for the O+-and the N2+-implanted specimens, the migration and the agglomeration of vacancy-type defects were found to be suppressed in the O+-implanted specimen. This fact was attributed to the formation of complexes between vacancy-type defects and oxygen. For ion-implanted specimens with a dose of 1×1015 cm2, annealing behaviors of amorphous regions were also studied.
Low-temperature defect chemistry of oxides. II. Analytical relations
Authors: Sasaki K. , Maier J.
Published: July 2002

The low-temperature defect chemistry of oxides is considered, characterized by frozen-in interaction with the ambient oxygen and reversibility of internal interactions, in particular the redistribution of electronic carriers. Analytical relations describing ionic and electronic defect concentrations are derived for various conditions. The presence of redox-active, i.e., deep-level, dopants proves to be of special interest in this context. The analytical relations permit the detailed discussion of the dependencies of the charge carrier concentrations on the control parameters. Such analytical relations are useful for understanding and tailoring defect concentrations and thus related properties of electroceramics.
Segregation and diffusion of phosphorus from doped Si1−xGex films into silicon
Authors: Kobayashi S. , Iizuka M. , Aoki T. , Mikoshiba N. , Sakuraba M. , Matsuura T. , Murota J.
Published: July 2002

Segregation and diffusion of P from in situ doped Si1−xGex (0.25⩽x⩽0.8) epitaxial films into Si at 750–850 °C were investigated using secondary ion mass spectroscopy and differential resistance measurements. It was found that the surface P concentration in the diffused layer in Si was higher than the P concentration in the Si1−xGex film, which signifies the segregation of P from the Si1−xGex film into Si. The segregation coefficient, defined as the ratio of the active P concentration in the Si to that in the Si1−xGex film, was about 2.5 at 800 °C in the case of the Si0.75Ge0.25 film as a diffusion source and increased with increasing Ge fraction. It was found that the P diffusion profiles in Si were normalized by x/t, even though the segregation of P occurred, the diffusion coefficient of P depended on the surface P concentration, and the high concentration diffusion characteristics of P in Si were similar to those reported by using a conventional diffusion source.
Ion channeling and x-ray diffraction studies of epitaxial Fe on GaAs(001)
Authors: Lallaizon C. , Lépine B. , Ababou S. , Guivarc’h A. , Députier S. , Abel F. , Cohen C.
Published: July 2002

Epitaxial 80-nm-thick films of Fe were grown at room temperature by molecular beam epitaxy on GaAs(001) and on a reacted layer composed of a mixture of (Fe2As+Fe3Ga1.8As0.2) resulting from the solid-state interdiffusions in a (20 nm Fe)/GaAs(001) structure annealed at 450 °C. The partially relaxed strains induced by the lattice mismatch were studied using ion channeling, Rutherford backscattering, and x-ray diffraction. Both channeled angular scans and x-ray diagrams showed that the 80 nm Fe films are not fully relaxed. The average crystalline quality of the Fe films is better on GaAs(001) (χmin=3%) than on the reacted layer (χmin=7%) mainly because of the mosaic structure of the reacted layer. The strains parallel to the surface in the Fe films were found to be compressive in the Fe/GaAs(001) heterostructure and tensile in the Fe/(Fe2As+Fe3Ga1.8As0.2)/GaAs(001) one. Therefore, our work shows that the presence of a reacted layer at the interface in a Fe/GaAs structure does not impede the epitaxial growth of Fe but can invert the kind of strain appearing in the Fe film.
Simulation of boundary condition influence in a second-order ferroelectric phase transition
Authors: Cao Wenwu , Tavener Simon , Xie Shumao
Published: July 2002

Using a two-dimensional Ginzburg–Landau model and the finite-element computational method, we have calculated stable domain configurations resulting from a second-order ferroelectric phase transition for a finite-sized system. The boundary conditions applied here correspond to fully charge compensated situations, either by surface electrodes or by the injection of charges (or defects) near the sample surface. The domain wall thickness of a finite system without surface electrodes was found to become thinner as it approaches sample surfaces. This is distinctively different from that of an infinite system for which a planar wall assumption can be used. The orientation of the macroscopic polarization of a finite system without surface electrodes was found to be determined by its aspect ratio. A size effect was observed when all the dimensions were reduced simultaneously. The relaxation process in the formation of domains and the switching process have also been simulated for charge neutral boundary conditions using a time dependent Ginzburg–Landau model. The simulation results verified that the surfaces are the favored nucleation sites for domain switching.
Transport parameters and sound propagation in an air-saturated sand
Authors: Tizianel Julian , Allard Jean F. , Castagnède Bernard , Ayrault Christophe , Henry Michel , Moussatov Alexei , Gedeon Antoine
Published: July 2002

Measurements at ultrasonic frequencies of the transmission coefficient and the sound speed in layers of quarry sand saturated by air and helium are performed at different pressures. The measurement of surface impedance at audible frequencies is also performed for air-saturated layers. Evaluation of transport parameters can be obtained from these measurements. Close sets of parameters can be obtained at high and low frequencies with the model by Pride et al. [Phys. Rev. B 47, 4964 (1993)].
Nonuniform quantum well infrared photodetectors
Authors: Wang S. Y. , Lee C. P.
Published: July 2002

A nonuniform quantum well infrared photodetector (QWIP) structure is proposed. By changing the doping concentration and barrier width of each quantum well, the electric field distribution can be tailored. The nonuniform QWIPs show excellent performance compared with conventional uniform structures. The dark current is about an order of magnitude lower and the background limited temperature increases to 77 K. A detectivity of 2.0×1010 cm Hz1/2/W and a responsivity of 0.25 A/W at 8.2 μm have been obtained for these devices. A simple picture of electric field distribution within the structure is described to explain the results.
Atomistic simulation of the vapor deposition of Ni/Cu/Ni multilayers: Incident adatom angle effects
Authors: Zhou X. W. , Wadley H. N. G.
Published: July 2002

Molecular dynamics simulations have been used to explore the effects of incident adatom angle upon the atomic scale structure of Ni/Cu/Ni multilayers grown by vapor deposition under controlled incident atom energy conditions. For incident atom energies of 1 eV or less, increasing the incident angle increased interfacial roughness, resulted in void formation in the nickel layer, and intermixing at the interfaces between metal layers. The interfacial roughness that formed during low impact energy oblique angle deposition was significantly reduced by substrate rotation during growth. However, rotation had no beneficial effects upon interfacial mixing. The use of a higher incident atom energy (⩾5 eV/atom) resulted in flatter interfaces and eliminated voids under oblique incidence conditions, but it also caused more severe interfacial mixing by an atomic exchange mechanism. When low (thermal) impact energies were used to deposit the first few monolayers of each new metal layer, intermixing by the exchange mechanism during subsequent hyperthermal energy deposition could be significantly reduced. Using this modulated incident energy growth strategy, films with little interfacial roughness and intermixing could be grown over a wide range of incident angles with or without substrate rotation.
Electronic properties of CuGaSe2-based heterojunction solar cells. Part I. Transport analysis
Authors: Nadenau V. , Rau U. , Jasenek A. , Schock H. W.
Published: July 2002

This article presents a systematic study on the electronic transport mechanisms of CuGaSe2-based thin film solar cells. A variety of samples with different types of stoichiometry deviations, substrates and buffer layers is investigated. We propose two transport models, namely tunneling enhanced volume recombination and tunneling enhanced interface recombination, which allow to explain the observed features for all devices under consideration. The doping level of the absorber layer turns out to be the most decisive parameter for the electronic loss mechanism. The doping is influenced by the type of stoichiometry deviation as well as by the Na content of the substrate. High doping levels result in tunnel assisted recombination. The best solar cells display the lowest tunneling rates. For these devices treatments of the absorber surface by air-annealing and/or the deposition temperature of the CdS buffer layer are decisive for the final device performance. We use the investigation of the open-circuit voltage relaxation to verify the assumptions on the dominant loss mechanism in the different devices.
Photoluminescence properties of Zn-doped heterostructures having native-oxide layers
Authors: Tinkham B. P. , Dupuis R. D.
Published: July 2002

The compound semiconductor industry has recently developed “native oxides” comparable to that of SiO2. Most research on these new oxides has been done with AlGaAs compounds. In this work we compare oxides generated from AlGaAs to those generated from InAlP in terms of their effects on the optical properties of GaAs (p type) active regions. Photoluminescence measurements were taken on both types of samples and comparisons were made before and after oxidation. Secondary ion mass spectroscopy was used to identify the chemical changes that occurred during oxidation.
Charge injection and transport in films of CdSe nanocrystals
Authors: Ginger D. S. , Greenham N. C.
Published: July 2002

We have studied charge injection and charge transport in thin disordered films of CdSe nanocrystals between metal electrodes. Current–voltage characteristics of these devices are investigated as a function of electrode material, nanocrystal size, and temperature. We measure the photocurrent response of these structures and find that the photocurrent action spectra follow the quantum-confined absorption spectra of the nanoparticles. For dissimilar top and bottom electrodes, we find that the devices are highly rectifying. High work function materials such as gold and indium-tin oxide are found to be poor electron injectors, consistent with the estimated conduction and valence band levels of the nanocrystals. We observe that the current–voltage characteristics exhibit a history and time dependence which is characteristic of persistent photoconductivity, with current at constant bias decaying with time according to a stretched exponential form. We propose a model based on space-charge limited current dominated by mobile electrons which slowly fill deep traps. Numerical simulations show that the model is able to describe the observed time dependence. We also find that the conductivity is strongly temperature dependent, and is qualitatively consistent with an activated hopping process at temperatures above 180 K. We use the data and simulations to estimate the electron mobilities to be in the range of ∼10−4–10−6 cm2 V−1 s−1 and the trap densities to be approximately 2×1016 cm−3.
Electrical characteristics of thin Ta2O5 films deposited by reactive pulsed direct-current magnetron sputtering
Authors: Kim J.-Y. , Nielsen M. C. , Rymaszewski E. J. , Lu T.-M.
Published: July 2002

Room temperature deposition of tantalum oxide films on metallized silicon substrates was investigated by reactive pulsed magnetron sputtering of Ta in an Ar/O2 ambient. The dielectric constant of the tantalum oxide ranged from 19 to 31 depending on the oxygen percentage [P(%)=PO2/(PO2+PAr)] used during sputtering. The leakage current density was less than 10 nA/cm2 at 0.5 MV/cm electric field and the dielectric breakdown field was greater than 3.8 MV/cm for P=60%. A charge storage as high as 3.3 μF/cm2 was achieved for 70-Å-thick film. Pulse frequency variation (from 20 to 200 kHz) did not give a significant effect in the electrical properties (dielectric constant or leakage current density) of the Ta2O5 films.
π/2 and 5π/2 twisted bistable nematic liquid crystal display
Authors: Xie Z. L. , Dong Y. M. , Xu S. Y. , Gao H. J. , Kwok H. S.
Published: July 2002

A π/2 and 5π/2 twist bistable nematic liquid crystal cell is optimized in optics by a parameter space approach. The cell possesses good contrast ratio, a preferable dΔn value, a wide viewing angle, and black–white display. Three switching wave forms can be used to switch the cell between π/2 and 5π/2 twist states. Controlling the selection voltage amplitude can provide gray scales for the cell.
Electronic excitation induced mass transport on 200 MeV F107+14 ion irradiated Si surface
Authors: Singh J. P. , Singh R. , Kanjilal D. , Mishra N. C. , Ganesan V.
Published: July 2002

Modification on Si (111) surface resulting from 200 MeV Ag107+14 ion irradiation at an incidence angle of 15° with respect to the surface normal has been investigated by atomic force microscope (AFM) and x-ray diffraction techniques. Root mean square roughness measured using AFM was found to increase from 0.96 nm in the pristine sample to 18.33 nm in the sample irradiated with an ion fluence of 5×1013 ions/cm2. This spectacular increase in surface roughness is ascribed to the displacement of Si atoms driven by the creation of unsaturated dangling bonds on the Si surface during swift heavy ion irradiation. At a fluence of 5×1013 ions/cm2, a new type of ditch and dike structure was observed. These structures were distributed over the irradiated surface. Each ditch is followed by a dike. This is attributed to the cumulative effect of ion irradiation arising due to overlapping of ion-induced damaged zones and electronic excitation-induced shear motion of the atoms towards the surface.
Microtexture and electromigration-induced drift in electroplated damascene Cu
Authors: Proost J. , Hirato T. , Furuhara T. , Maex K. , Celis J.-P.
Published: July 2002

In this work, the electromigration (EM) performance of electroplated damascene Cu is investigated by drift experiments on Blech-type test structures in both polycrystalline and bamboo microstructures. For the first, microtexture data were obtained from electron backscatter diffraction as well. While both bonding areas and 10 μm wide lines were found to have a predominantly random grain orientation, the drift studies indicated the importance of strongly segregating impurities in controlling Cu grain-boundary EM. For the bamboo lines, the impact of different barrier layers has been investigated, comparing Ta, TaN, and TiN. Drift was shown to proceed in all cases at the metallic Cu barrier interface, but faster for the Ta as compared to the TaN and TiN barriers. Cu drift data were finally compared to available literature results and to our previous drift studies on Al(Cu).