bluuebunny/crossref_metadata_2025 huggingface dataset:
On complex demodulation and time-frequency distributions for broadband reflectometry signals
Authors: Bizarro João P. S. , Figueiredo António C.
Published: July 2002

Complex demodulation is used to generate a positive joint time-frequency distribution for broadband reflectometry signals, the so-called demodulogram, which is actually the dual of the well-known spectrogram and which, for a given signal, is obtained by squaring the modulus of the inverse Fourier transform of the signal frequency-windowed Fourier spectrum. The demodulogram and its properties are discussed, and the former is applied to the analysis of typical reflectometry data, being shown, in particular, that it can indeed yield what is strictly defined as the instantaneous frequency of discrete-time reflectometry signals.
An improved instrument for measuring time-resolved lanthanide emission and resonance energy transfer
Authors: Xiao Ming , Selvin Paul R.
Published: July 2002

Time-resolved fluorescence, particularly of millisecond lanthanide emission, is of considerable interest because of potentially increased sensitivity when short-lived autofluorescence is a problem. Lanthanides have also previously been shown to be excellent donors in resonance energy transfer measurements, where a lanthanide donor transfers energy to a conventional fluorophore. An instrument for the detection of micro- to millisecond lanthanide luminescence and delayed fluorescence, amidst a background of large prompt fluorescence, is described. Time-delayed emission spectra can be measured with 16 μs dead time while suppressing prompt fluorescence by 25 000-fold. This is accomplished by using a mechanical chopper placed very close to the entrance slit of a diffraction spectrometer, and a charge coupled device detector at the exit port. Excited-state lifetimes can also be measured with a 22 μs dead time while reducing detector ringing due to prompt fluorescence to an insignificant level. This is accomplished by high-voltage gating of a photon-counting photomultiplier tube in combination with a fast constant-fraction discriminator and amplifier. These improvements, which utilize all commercially available equipment, reduce the dead time by a factor of 10 for delayed emission measurements and a factor of 5 for lifetime measurements over a previously described instrument. The current instrument is particularly well suited for lanthanide-based resonance energy transfer measurements.
A low stray light, high current, low energy electron source
Authors: Adelt M. , Körber R. , Drachsel W. , Freund H.-J.
Published: July 2002

A design of an electron gun system is presented whose stray light emission is reduced by about three orders of magnitude compared to a regular low-energy electron diffraction gun. This is achieved by a combination of a BaO cathode run at rather low temperature and a 30° tandem parallel-plate analyzer used as an optical baffle. The system provides a high beam current of several microampers at 50 eV beam energy. The system can be used down to ∼10 eV.
Electron impact ionization in a microion trap mass spectrometer
Authors: Kornienko Oleg , Reilly Peter T. A. , Whitten William B. , Ramsey J. Michael
Published: July 2002

A microscale ion trap mass spectrometer (r0=0.50 mm, z0=0.55 mm) with an electron gun for electron impact ionization of gaseous samples is described. Operated in the mass-instability mode, the trap had a m/z range from 40 to 400 Da. For single scans, peak widths of less than 0.2 Da were obtained.
Neutral helium beam probe
Authors: Karim Rezwanul
Published: July 2002

This article discusses the development of a code where diagnostic neutral helium beam can be used as a probe. The code solves numerically the evolution of the population densities of helium atoms at their several different energy levels as the beam propagates through the plasma. The collisional radiative model has been utilized in this numerical calculation. The spatial dependence of the metastable states of neutral helium atom, as obtained in this numerical analysis, offers a possible diagnostic tool for tokamak plasma. The spatial evolution for several hypothetical plasma conditions was tested. Simulation routines were also run with the plasma parameters (density and temperature profiles) similar to a shot in the Princeton beta experiment modified (PBX-M) tokamak and a shot in Tokamak Fusion Test Reactor tokamak. A comparison between the simulation result and the experimentally obtained data (for each of these two shots) is presented. A good correlation in such comparisons for a number of such shots can establish the accurateness and usefulness of this probe. The result can possibly be extended for other plasma machines and for various plasma conditions in those machines.
Designs for an asymmetric gradient set and a compact superconducting magnet for neural magnetic resonance imaging
Authors: Crozier Stuart , Luescher Kurt , Hinds Gavin , Roffmann Wolfgang U. , Doddrell David M.
Published: July 2002

Imaging of the head and neck is the most commonly performed clinical magnetic resonance imaging (MRI) examination [R. G. Evans and J. R. G. Evans, AJR 157, 603 (1991)]. This is usually undertaken in a generalist MRI instrument containing superconducting magnet system capable of imaging all organs. These generalist instruments are large, typically having a bore of 0.9–1.0 m and a length of 1.7–2.5 m and therefore are expensive to site, somewhat claustrophobic to the patient, and offer little access by attending physicians. In this article, we present the design of a compact, superconducting MRI magnet for head and neck imaging that is less than 0.8 m in length and discuss in detail the design of an asymmetric gradient coil set, tailored to the magnet profile. In particular, the introduction of a radio-frequency FM modulation scheme in concert with a gradient sequence allows the epoch of the linear region of the gradient set to be much closer to the end of the gradient structure than was previously possible. Images from a prototype gradient set demonstrate the effectiveness of the designs.
Self-excitation and operational characteristics of the crossed-field secondary emission electron source
Authors: Saveliev Y. M. , Sibbett W. , Parkes D. M.
Published: July 2002

We have investigated the crossed-field secondary emission (CFSE) electron source which is of a magnetron type with smooth cylindrical electrodes and axial applied magnetic field. It initiates at the negative slope d|U|/dt<0 of the high voltage pulse U∼10–40 kV, but further current production is maintained by a self-sustained secondary electron emission regardless to the voltage pulse shape. The output electron beam is tubular with a thin ∼1 mm wall. This article is concerned mainly with the identification of the mechanisms governing the excitation and generation of the electron beam and with the determination of the principles upon which the “optimal” CFSE electron source should be designed. We have demonstrated that the CFSE diode starts operation in a self-excitation regime (i.e., without application of the primary current) provided there is a partial trapping of the multiplying electrons inside the diode boundaries. The required axial decelerating force can be established with the use of either axial electric or nonuniform magnetic fields. Amongst all of the practical methods tested (shifting of the anode with respect to the cathode, double diode, diodes with ferromagnetic parts, use of the nonuniform external magnetic field), the diode with a ferromagnetic ring insert inside the cathode cylinder has been shown to be the most successful. It has generated an ∼240 A electron beam with a perveance of ∼85 μA/V3/2. The operating range of the CFSE diode is limited by both low and high magnetic fields. The lower limit arises from a necessity to comply with a Hull cutoff condition. The upper limit is determined by the time required for development of an electron avalanche. A secondary electron emission mechanism of current production in the CFSE diode allows the diode to operate in an oscillating regime when the applied magnetic field is higher but close to the Hull cutoff value. It has thus been possible to generate 100% density modulated electron beams at a modulation frequency of ∼107 Hz in our present experiments with the possibility of further increases up to ∼108 Hz. A geometrical scaling law for the CFSE diodes has been deduced empirically. It states that the perveance of the output electron beam is proportional to the geometrical factor X=(Dk/de)(Ld/de−0.8), where Dk is the cathode diameter, de is an effective diode gap, and Ld is the diode length. The scaling law provides a tool for designing the CFSE diodes and predicting the ultimate beam currents. For a practical size of device, this electron current could be as high as ∼1 kA.
Theory and simulations of electron beam ion source in reflex mode
Authors: Donets E. D. , Donets E. E. , Syresin E. M.
Published: July 2002

The electron beam ion sources (EBIS) proposed in JINR are applied for generation of extremely highly charged ions. The intensity of extracted ion beams and ion charge states are determined by the current and current density of used electron beams. In the normal mode of EBIS operation each electron passes through the region of an ion trap only one time and then it picks up by the collector. The number of extracted ions during one pulse is proportional to the number of fast electrons in the trap region. The idea of EBIS operating in reflex mode, when electron passes many times through the trap region is directed to the increase of the number of the electrons in the trap region and consequently to the increase of an ion output. The experimental results of an EBIS operation in the reflex mode are obtained during several of the last years. The theoretical simulations of EBIS in this mode are discussed in this article.
Enhanced production of multi-charged ions using pulse-modulated microwave in a 2.45 GHz electron cyclotron resonance source
Authors: Kato Yushi , Kobayashi Shigeki , Ishii Shigeyuki
Published: July 2002

Fundamental phenomena of an electron cyclotron resonance (ECR) multi-charged ion source (2.45 GHz) have been experimentally studied. The ECR plasma is confined in the mirror field superimposed by the octupole magnetic field. An ECR zone and potential well are formed near the bottom of the mirror trap. Multi-charged ions pass through the extractor at the mirror end, and the charge state distributions of extracted ions are investigated. Pulse-modulated microwave produces the ECR plasma with afterglow, relaxes the potential well, and then enhances the extracted multi-charged ion currents. Time-averaged Ar4+−9+ currents increase, even when the microwave power is nearly equal to or lower than the continuous microwave. The rectangular pulse width and the duty ratio are typically about 0.05 ms and 50%, respectively. The origin of enhanced production by pulse-modulated microwave is discussed by taking account of plasma parameters. Measurement of plasma parameters by a Langmuir probe and dependence of current increment on the ionic charge suggest the relaxation of the potential well.
Ion source of multiply charged C60 fullerene and fullerene linear accelerator
Authors: Hattori Toshiyuki , Hayashizaki Noriyosu , Matsui Shinjirou , Tomizawa Hiromitsu , Yoshida Toru , Gates Stephen John , Sasa Kimikazu , Majima Satoshi , Isokawa Katsushi , Ito Takashi , Okamura Masahiro , Yamada Satoru
Published: July 2002

Production and acceleration system of multiply charged C60 and C70 fullerene ions are being designed for basic studies on cluster physics. Multiply charged fullerene ions are produced by electron impacts. Ions selected by the analyzing magnet are accelerated up to 100 kV through an acceleration column. The ion source generates C60+(8.4 nA), C602+(4.5 nA), C603+(1.6 nA). C60+, C602+, and C603+ are accelerated up to 100, 200, and 300 keV, respectively. The alternating phase focus (APF)-interdigital H-mode (IH) linac was designed to accelerate C60+ ion (ε=1/720) and C70+ ion (ε=1/840) from 0.2 to 3.2 keV/amu with an APF and an operation frequency of 10 MHz.
Ion source for ion beam deposition employing a novel electrode assembly
Authors: Hayes A. V. , Kanarov V. , Yevtukhov R. , Hegde H. , Druz B. , Yakovlevitch D. , Cheesman W. , Mirkov V.
Published: July 2002

A rf inductively coupled ion source employing a novel electrode assembly for focusing a broad ion beam on a relatively small target area was developed. The primary application of this ion source is the deposition of thin films used in the fabrication of magnetic sensors and optical devices. The ion optics consists of a three-electrode set of multiaperture concave dished grids with a beam extraction diameter of 150 mm. Also described is a variation in the design providing a beam extraction diameter of 120 mm. Grid hole diameters and grid spacing were optimized for low beamlet divergence and low grid impingement currents. The radius of curvature of the grids was optimized to obtain an optimally focused ion beam at the target location. A novel grid fabrication and mounting design was employed which overcomes typical limitations of such grid assemblies, particularly in terms of maintaining optimum beam focusing conditions after multiple cycles of operation. Ion beam generation with argon and xenon gases in energy ranges from 0.3 to 2.0 keV was characterized. For operation with argon gas, beam currents greater than 0.5 A were obtained with a beam energy of 800 eV. At optimal beam formation conditions, beam profiles at distances about equal to the radius of curvature were found to be close to Gaussian, with 99.9% of the beam current located within a 150 mm target diameter. Repeatability of the beam profile over long periods of operation is also reported.
Development of a compact multicusp ion source of He+
Authors: Nishiura M. , Sasao M. , Wada M. , Hamabe M. , Kuroda T. , Guharay S. K.
Published: July 2002

A filament-type helium ion source has been developed for the purpose of alpha particle diagnostics and as a test beam for electrostatic quadrupole transport experiments. We have obtained preliminary results of He+ beam current, ∼60 mA/cm2. The normalized brightness of this beam is ∼1010 A/(m rad)2. The normalized emittance (90%) measured by multislit faraday cup emittance meter is ∼0.6 mm mrad below 100 V, 10 A arc discharge. The effects of plasma parameters in the source on the beam emittance and on the beam perveance are discussed.
Electron cyclotron resonance light source assembly—A vacuum-ultraviolet radiation source based on an electron cyclotron resonance plasma
Authors: Grübling P. , Hollandt J. , Ulm G.
Published: July 2002

A vacuum-ultraviolet radiation source based on an electron cyclotron resonance plasma was developed. Electron cyclotron resonance light source assembly (ELISA) is a 10 GHz monomode source of compact design featuring a tunable cavity and permanent magnets which can be axially positioned. The radiation emission of the source can be detected simultaneously in the vacuum-ultraviolet spectral range via a toroidal grating monochromator and in the x-ray spectral range via a Si(Li) detector. Spectroscopic investigations of a krypton and a neon plasma in the 9–27 nm spectral range show that ELISA produces complex line spectra of highly ionized atoms by applying only 12 W microwave power.
Ion-beam characteristics of novel helicon ion sources for different plasma parameters
Authors: Hong I. S. , Hwang Y. S. , Lee G. H. , Kim D. Y. , Won H. Y. , Eom G. S. , Choe W.
Published: July 2002

A high-current ion source requires a high-density plasma source to provide sufficient ions to be accelerated. For a continuous high-power ion source, a new concept ion source using a helicon plasma source has been developed. A compact high-density helicon plasma is generated with very high-power efficiency, and ion beams are extracted from the plasma source. With various plasma parameters, extracted ion-beam characteristics are studied in a helicon ion source for the first time. Plasma parameters, especially plasma density, are shown to be strongly correlated with the extracted beam characteristics.
Large area double-sided YBa2Cu3O7−δ films grown by single-source metal-organic chemical vapor deposition
Authors: Lu Z. , Truman J. K. , Johansson M. E. , Zhang D. , Shih C. F. , Liang G. C.
Published: July 2002

Thin films of YBa2Cu3O7−δ were sequentially grown on both sides of 3 in. diameter LaAlO3 substrates by a single-source MOCVD technique to facilitate the fabrication of double-sided high temperature superconductor microwave devices. YBCO films on both sides of the LaAlO3 substrates had thicknesses greater than 4000 Å, Tc’s of 87–88.5 K, and microwave surface resistances as low as 23 mΩ at 94 GHz and 77 K (scaling to 260 μΩ at 10 GHz). It was found that the substrate temperature, brought to the desired value by radiant heating, played a critical role in the deposition of very low surface resistance YBCO films. Microwave bandpass filters with a 19-pole forward coupled configuration were fabricated from these films. The filters showed excellent performance with dissipation losses as low as 0.5 dB.
Investigation of many-body effects in one-side modulation-doped InP-InGaAs heterostructure
Authors: Cardoso A. J. C. , Morais P. C. , Cox H. M.
Published: July 2002

A one-side modulation-doped quantum well of InGaAs lattice matched to InP has been used to investigate many-body effects in a quasi-two-dimensional (2D) electron gas. The sample was grown by vapor levitation epitaxy (VLE). Low-temperature photoluminescence (PL) measurements were carried out under different optical excitation intensities using 5145 Å line from an argon ion laser. By increasing the laser intensity over five orders of magnitude a 13.2 meV blue shift in the PL line was observed to occur for a single asymmetric quantum well having a 2D electron gas density as low as 0.9×1011 cm−2. The observed blue shift is explained in terms of the reduction of both band bending (of the order of 1.8 meV) and many-body effects (of the order of 11.4 meV) due to the reduction of the 2D electron gas density in the quantum well.
Electric field screening in a multiple-quantum-well optically addressed spatial light modulator
Authors: Smith S. L. , Hesselink L. , Partovi A.
Published: July 2002

The transmission and photocurrent of a multiple-quantum-well optically addressed spatial light modulator are measured to study the screening dynamics of the internal electric field under uniform illumination. The response is modeled with a numerical device simulator which solves the conventional drift-diffusion transport equations. Evidence is found for bipolar transport, and the photocurrent response at low internal fields is shown to decay due to the transit time of carriers across the quantum well region. For large internal fields the photocurrent response saturates and the corresponding screening rate is estimated using an analytical device model.
Size distribution of end-of-range dislocation loops in silicon-implanted silicon
Authors: Pan G. Z. , Tu K. N. , Prussin S.
Published: July 2002

The size and distribution of end-of-range dislocation loops in silicon implanted with 50 keV 1016 Si/cm2 and annealed at 750 °C for various times have been studied by transmission electron microscropy. The normalized distribution profile of the dislocation loops has been found to be invariant with time, based on density and size measurement. The profile is quite different from the conventional distribution profile of Oswald ripening in grain growth and precipitate coarsening. Measurement of the total number of interstitials bound in the extrinsic loops shows that the ripening is a conservative process. An explanation for the particular distribution profile is attributed to the stress field associated with the dislocation loops.
SiOx luminescence from light-emitting porous silicon: Support for the quantum confinement/luminescence center model
Authors: Cooke D. W. , Bennett B. L. , Farnum E. H. , Hults W. L. , Sickafus K. E. , Smith J. F. , Smith J. L. , Taylor T. N. , Tiwari P. , Portis A. M.
Published: July 2002

Measurements of hydrogen loss and luminescence as a function of annealing temperature in porous silicon suggest that luminescence is attributable to electron-hole recombination in SiOx surface layers with an intensity that is dependent upon the surface hydrogen content. The luminescence is composed of three Gaussian bands similar to those found in amorphous SiO2. X-ray photoelectron spectroscopy and scanning electron microscopy show porous silicon has SiOx on the surface, which is comprised of many particles of about 10 nm size. Collectively, the data strongly support the previously proposed quantum confinement/luminescence center model.
Influence of dislocation loops created by amorphizing implants on point defect and boron diffusion in silicon
Authors: Chao H. S. , Griffin P. B. , Plummer J. D.
Published: July 2002

An experiment has been performed in which wafers with boron-doped, buried marker layers were implanted with 100 keV, 2×1015 cm−2 Si. This is an amorphizing implant. A second implant of B was introduced prior to any post-implant annealing such that the implanted B was completely contained within the preamorphized region. After the implants, samples were annealed at various temperatures for various times and secondary-ion mass spectroscopy was used to obtain the dopant profiles. It was found that the buried marker layer exhibited normal transient-enhanced diffusion behavior. However, the B in the preamorphized region did not experience any significant amount of motion. This suggests that the solid phase regrowth of the amorphous layer did not cause a redistribution of the dopant atoms within that layer, and also the plane of dislocation loops that form at the amorphous/crystalline interface is an effective barrier against the interstitial damage diffusing upwards from the nonamorphized tail of the amorphizing Si implant. The same behavior was observed when As or P implants are used instead of the B implant. This type of behavior has been simulated using a model considering the growth of stacking faults bounded by dislocation loops.
Substrate dependence in the growth of epitaxial Pb1−xLaxTiO3 thin films
Authors: Kim Y. , Erbil A. , Boatner L. A.
Published: July 2002

The metalorganic chemical vapor deposition (MOCVD) technique has been applied to the growth of epitaxial Pb1−xLaxTiO3 (PLT) thin films with x=0.28. By first introducing an initial TiO2 layer, three-dimensional epitaxial PLT films were grown on the (100) surface of MgO substrate. For both KTaO3 (100) and Al2O3 (0001) substrates, heteroepitaxy was achieved without the introduction of TiO2 as the initial, intervening layer between the PLT film and the substrate. On Al2O3 substrates, PLT films with a [111] preferred orientation were grown with a good epitaxial in-plane relationship.
Submicron and low-temperature ohmic contacts on δ-doped GaAs
Authors: Piotrowicz P. J. A. , England J. M. C. , Cleaver J. R. A. , Stanley C. R. , Holland M. C.
Published: July 2002

Practical nonalloyed ohmic contacts on δ-doped GaAs have been compared for AuGeNi (88:12:5) /and Cr metallizations to show the importance of metallization type for minimizing the contact resistance. They are shown to have low contact resistances even at 4.2 K and for contact sizes down to 240 nm diam. The effect of heating AuGeNi contacts to 270 °C is shown to be beneficial for large-area contacts but not for submicron contacts, implying that nonuniformity is introduced.
Room-temperature visible photoluminescence from silicon-rich oxide layers deposited by an electron cyclotron resonance plasma source
Authors: Kim Keunjoo , Suh M. S. , Kim T S. , Youn C. J. , Suh E. K. , Shin Y. J. , Lee K. B. , Lee H. J. , An M. H. , Lee H. J. , Ryu H.
Published: July 2002

Highly split, visible light emissions at room temperature were observed in the range from 335 to 650 nm in silicon-rich oxide films deposited in the plasma phase of a mixture of silane and oxygen. The mechanism of the light emissions is classified into two categories. The photoluminescence bands at both 365 and 469 nm are related to the intrinsic defects of the E′ center and the neutral oxygen vacancy, respectively. However, the relatively sharp peaks at 403 and 535 nm are correlated with the development of polycrystalline core of Si-enriched parts.
Influence of the interfaces on the anisotropic magnetoresistance of Ni/Co multilayers
Authors: Lesmes F. , Salcedo A. , Freijo J. J. , Garcia D. , Hernando A. , Prados C.
Published: July 2002

Anisotropic magnetoresistance in Ni is found to increase abruptly when Co impurity layers are inserted. Some experiments carried out in different Ni/Co multilayers indicate that interfaces are responsible for the magnetoresistance enhancement.
Valence band spectroscopy in V-grooved quantum wires
Authors: Goldoni G. , Rossi F. , Molinari E. , Fasolino A. , Rinaldi R. , Cingolani R.
Published: July 2002

We present a combined theoretical and experimental study of the anisotropy in the optical absorption of V-shaped quantum wires. By means of realistic band structure calculations for these structures, we show that detailed information on the heavy- and light-hole states can be singled out from the anisotropy spectra independently of the electron confinement, thus allowing accurate valence band spectroscopy.
(110) oriented GaAs/Al0.3Ga0.7As quantum wells for optimized T-shaped quantum wires
Authors: Gislason H. , So/rensen C. B. , Hvam J. M.
Published: July 2002

High control of (110) oriented GaAs/Al0.3Ga0.7As quantum wells is very important for the growth of optimized T-shaped GaAs/AlGaAs quantum wires. We investigate theoretically and experimentally 20–200 Å wide (110) oriented GaAs quantum wells grown on (110) oriented substrates and cleaved edges. Photoluminescence transition energies are found to be in good agreement with theory for all well widths. The mean well width is controllable to 1 monolayer accuracy and an effective well width fluctuation of 3.7 Å is derived from the photoluminescence linewidths. The growth rate calibration of the aluminum content of the barrier material agrees within 1% with an estimate from the bound exciton emission of (110) Al0.3Ga0.7As epilayers.
Lateral composition modulation in AlAs/InAs short period superlattices grown on InP(001)
Authors: Mirecki Millunchick J. , Twesten R. D. , Follstaedt D. M. , Lee S. R. , Jones E. D. , Zhang Yong , Ahrenkiel S. P. , Mascarenhas A.
Published: July 2002

Spontaneous lateral composition modulation as a consequence of the deposition of a (AlAs)n/(InAs)m short period superlattice on an InP(001) substrate is examined. Transmission electron microscopy images show distinct composition modulation appearing as vertical regions of In- and Al-rich materials alternating in the [1̄10] projection. The periodicity of the modulation is 130 Å, and is asymmetric. The transmission electron and x-ray diffraction patterns from the structure exhibit distinct satellite spots which correspond to the lateral periodicity. Transmission electron microscopy images show that the individual superlattice layers possess cusplike undulations, which directly correlate with the composition modulation. Composition modulation in this sample appears to be coupled to morphological and compositional instabilities at the surface due to strain.
Alternating current conductivity in doped and undoped poly(o-methoxyaniline)
Authors: Lepienski C. M. , Faria R. M. , Leal Ferreira G. F.
Published: July 2002

We report on ac conductivity measurements carried out on poly(o-methoxyaniline) films, of high molecular weight, at different doping levels. The results are typical of a disordered medium showing a constant conductivity at low frequencies and rising for higher values. In order to interpret the results, we use a model which takes into account that the sample is composed of an insulating disordered matrix sprinkled with conductive islands generated by doping, as indicated by energy dispersive x-ray microanalysis. It comes out of the fitting that the doping induces an increase in the carrier mobility, instead of increasing the density of carriers.
Er/O and Er/F doping during molecular beam epitaxial growth of Si layers for efficient 1.54 μm light emission
Authors: Ni W.-X. , Joelsson K. B. , Du C.-X. , Buyanova I. A. , Pozina G. , Chen, W. M. , Hansson G. V. , Monemar B. , Cardenas J. , Svensson B. G.
Published: July 2002

Er, together with oxygen or fluorine as co-dopants, has been incorporated into Si during molecular beam epitaxial growth using co-evaporation of Si and Er containing compounds. The Er doping concentration using both Er2O3 and ErF3 can reach a level of ∼5×1019 cm−3 without precipitation, which is at least one order of magnitude higher than a previously reported solid solubility limit for Er in Si. Growth, structural, and luminescence characterization of these Er/O and Er/F doped Si samples are reported. In particular, 1.54 μm electroluminescence has been observed from Er/O doped Si layers at room temperature through hot electron impact excitation.
The effect of the microstructure on the magnetic interactions in CoFe–AgCu granular films: From demagnetizing to magnetizing interactions
Authors: Batlle X. , Franco V. , Labarta A. , Watson M. L. , O’Grady K.
Published: July 2002

The sign and strength of the dominant magnetic interactions in Co34Fe8Ag54Cu4 granular films were modified by changing the microstructure through annealing. Magnetic force micrographs showed that in the as-cast sample the magnetic moments of neighboring grains tended to be arranged parallel along a direction out of the film plane, forming elongated magnetic clusters that were themselves aligned antiparallel, with dominant demagnetizing interactions. This is a direct evidence that an uncompensated antiferromagneticlike microstructure is stabilized below the volume percolation threshold. However, in the sample annealed at 750 °C the particle growth led to large in-plane ferromagneticlike clusters with dominant magnetizing interactions. Thus, in this letter we present direct correlation of interactions effects with magnetic measurements and show that ΔM plots correlate with changes in the magnetic microstructure in these systems.
Hydrogen induced silicon surface layer cleavage
Authors: Lu Xiang , Cheung Nathan W. , Strathman Michael D. , Chu Paul K. , Doyle Brian
Published: July 2002

Physical mechanisms of hydrogen induced silicon surface layer cleavage were investigated using a combination of microscopy and spectroscopy techniques. The evolution of the silicon cleavage phenomenon is recorded by a series of microscopic images. The underlying hydrogen profiles under (between 250 and 500 °C) annealing are characterized by secondary-ion-mass spectroscopy and hydrogen forward scattering experiments. An idea gas law model calculation suggests that internal pressure of molecular hydrogen filled microcavities is in the range of Giga-Pascal, high enough to break silicon crystal bond. A dose threshold, which prevents cleavage, is observed at 1.6×1017 cm−2 for 40 kV hydrogen implantation.
Electrically pumped tunable terahertz emitter based on intersubband transition
Authors: Xu Bin , Hu Qing , Melloch Michael R.
Published: July 2002

An electrically pumped three-level system was designed and fabricated using an AlGaAs/GaAs multiple quantum well structure. Under appropriate biases, the structure emits terahertz (THz) radiation, as a result of diagonal (or interwell) intersubband transition. The emission spectra were resolved using an external Fourier transform infrared spectrometer. The center frequency of the emission was voltage tunable. The emission spectra and the power-voltage relation showed clear evidence that the middle level was depopulated by fast longitudinal optical-phonon scattering, thus, a population inversion between the two upper levels is feasible.
Operation of high-sensitivity radio frequency superconducting quantum interference device magnetometers with superconducting coplanar resonators at 77 K
Authors: Zhang Y. , Zander W. , Schubert J. , Rüders F. , Soltner H. , Banzet M. , Wolters N. , Zeng X. H. , Braginski A. I.
Published: July 2002

We have developed a novel coplanar resonator serving as a tank circuit for radio frequency washer superconducting quantum interference devices (SQUIDs). Two coplanar lines surrounded the circular flux concentrator washer, 13.4 mm in diameter. The SQUID, 3.5 mm in diameter, was coupled to the concentrator in the flip-chip configuration. All parts were fabricated from single-layer epitaxial YBa2Cu3O7 films on LaAlO3 substrates. In this layout, the resonant frequency could be set between 600 and 850 MHz. With SQUID loops of 100×100 μm2 (SQUID) inductance Ls=150 pH or 10×500 μm2 (Ls=260 pH), we determined at 77 K white flux noise levels Sφ1/2 of 1.0×10−5 and 1.5×10−5φ0/Hz, respectively. Both figures corresponded to the field resolution of about 30 fT/Hz.
Influence of oxygen background pressure on crystalline quality of SrTiO3 films grown on MgO by pulsed laser deposition
Authors: Kalyanaraman R. , Vispute R. D. , Oktyabrsky S. , Dovidenko K. , Jagannadham K. , Narayan J. , Budai J. D. , Parikh N. , Suvkhanov A.
Published: July 2002

We have systematically investigated the effect of oxygen partial pressure (PO2) on the crystalline quality of SrTiO3 films grown on MgO (001) substrates using pulsed laser deposition and established optimized conditions for the growth of high-quality epitaxial films. The crystalline quality is found to improve significantly in the O2 pressure range of 0.5–1 mTorr, compared to the films deposited at higher pressures of 10–100 mTorr. The x-ray diffraction rocking curves for the films grown at PO2 of 1 mTorr and 100 mTorr yielded full width at half-maximum (FWHM) of 0.7° and 1.4°, respectively. The in-plane x-ray φ scans showed epitaxial cube-on-cube alignment of the films. Channeling yields χmin were found to be <5% for the 1 mTorr films and ∼14% for 100 mTorr films. Thermal annealing of the SrTiO3 films in oxygen further improves the quality, and the 1 mTorr films give FWHM of 0.13° and χmin of 1.7%. In-plane misorientations of the annealed SrTiO3 films calculated using results of transmission electron microscopy are ±0.7° for 1 mTorr and ±1.7° for the 10 mTorr film. The high temperature superconducting (high-Tc) Y1Ba2Cu3O7−δ films grown on these SrTiO3/MgO substrates showed a χmin of 2.0% and transition temperature of ∼92 K, indicating that SrTiO3 buffer layers on MgO can be used for growth of high-quality Y1Ba2Cu3O7−δ thin film heterostructures for use in high-Tc devices and next generation microelectronics devices requiring films with high dielectric constants.
High-temperature persistent spectral hole burning of Eu3+-doped SiO2 glass prepared by the sol-gel process
Authors: Nogami Masayuki , Abe Yoshihiro
Published: July 2002

Persistent spectral hole burning was observed at temperatures higher than 77 K in SiO2 glass doped with the Eu3+ ions. The Eu3+-doped SiO2 glass was prepared using the sol-gel process of Si(OC2H5)4 and EuCl3⋅6H2O. A persistent spectral hole was burned in the excitation spectrum of the F07→5D0 transition of Eu3+ using a Rhodamine 6G dye laser, of which the hole width and depth were 1.6 cm−1 and ∼20% of the total intensity, respectively, at 77 K. Hole depth decreased with increasing temperature and disappeared above ∼130 K. A possible mechanism for the hole burning is related to the local structure around Eu3+ and the residual OH and H2O in the glass.
Many-body optical gain of wurtzite GaN-based quantum-well lasers and comparison with experiment
Authors: Park Seoung-Hwan , Chuang Shun-Lien
Published: July 2002

The optical gain of wurtzite InxGa1−xN/In0.02Ga0.98N and GaN/AlxGa1−xN quantum well (QW) lasers taking into account many-body effects is investigated. The valence band structures are calculated as a function of strain and well thickness. The inclusion of compressive strain shows better lasing performance because of the increase of the subband energy separation in the valence band. Our theoretical gain spectra of In0.15Ga0.85N/In0.02Ga0.98N QW lasers are in good agreement with measured ones reported by Nakamura, IEEE J. Sel. Top. Quantum Electron. 3, 712 (1997). It is also shown that there is a universal relation governing the dependence of the band-gap renormalization on the two-dimensional carrier density for GaN-based QW lasers as there is for the infrared III-V systems.
X-ray emission from metallic (Al) and dielectric (glass) targets irradiated by intense ultrashort laser pulses
Authors: Saemann A. , Eidmann K.
Published: July 2002

We measured in a large spectral range from 180 eV to 20 keV the x-ray emission from aluminum and glass targets irradiated by 150 fs laser pulses at intensities of ≈1017 W/cm2. Unexpected large differences are observed at photon energies above ≈1 keV, where the Al target emitted almost 100 times more x rays than glass. Important for this result is a high prepulse to main pulse contrast.
Local variation of room-temperature thermal conductivity in high-quality polycrystalline diamond
Authors: Reichling M. , Klotzbücher T. , Hartmann J.
Published: July 2002

Local thermal conductivity inside grains of high-quality polycrystalline diamond grown by chemical vapor deposition is measured at room temperature with micrometer spatial resolution. An effective conductivity is determined by choosing experimental conditions where the measured heat flow extends over adjacent grains. It is found that the effective conductivity may vary by a factor of 2, depending on the averaged volume and position on the sample. These variations are attributed to different grain structures present at the investigated locations. Local conductivity within a single grain varies by ±30% and a maximum value of 2350 W/mK is found. Local thermal conductivity data are related to the crystalline quality and impurity content determined by micro-Raman measurements.
Study of the optical properties of fused quartz after a sequential implantation with Si and Au ions
Authors: Oliver A. , Cheang-Wong J. C. , Crespo A. , Hernández J. M. , Solı́s C. , Muñoz E. , Espejel-Morales R. , Siejka J.
Published: July 2002

Implantation of Au ions into Si-implanted fused quartz strongly enhances the photoluminescence (PL) intensity around 630 nm measured after subsequent sample annealing at 900 °C. This effect is attributed to the enhancement of the formation of Si nanocrystals by the presence of Au ions and not by ion-implantation-induced defects. This conclusion was deduced by monitoring the defect formation in fused silica by 2 MeV Si ion implantation with doses ranging from 2×1016 to 1×1017 Si/cm2. Some of the 4×1016 Si/cm2-implanted samples were reimplanted at a similar depth with 10 MeV Au ions at doses of 4×1016 and 1.2×1017 Au/cm2. The absorption spectroscopy, electron paramagnetic resonance and PL measurements show the presence of B2 and E′ matrix point defects in as-prepared Si-implanted samples. As these defects disappear after annealing at 600 °C, the presence of a strong PL peak in samples implanted and annealed at 900 °C strongly suggests that the observed luminescence is produced by Si nanoparticle formation.
Dielectric anomaly in strontium bismuth tantalate thin films
Authors: Takemura Koichi , Noguchi Takehiro , Hase Takashi , Miyasaka Yoichi
Published: July 2002

The authors have succeeded in observing the dielectric anomaly in ferroelectric strontium bismuth tantalate (SBT) thin films, and studied how it varies with film composition. For a stoichiometric SBT film, the dielectric anomaly was diffused, and the Curie temperature (Tc) was approximately 260 °C, which is lower than that for the bulk ceramic. A SBT film with excess Bi composition showed Tc of approximately 300 °C, and Tc for Sr deficient SBT films was higher than 340 °C. The composition dependence of remanent polarization and coercive field near room temperature for the SBT films reflects the composition dependence of the Tc values.
Neutralized (NH4)2S solution passivation of III–V phosphide surfaces
Authors: Yuan Z. L. , Ding X. M. , Lai B. , Hou X. Y. , Lu E. D. , Xu P. S. , Zhang X. Y.
Published: July 2002

Synchrotron radiation photoelectron spectroscopy has been used to investigate III–V phosphide GaP and InP (100) surfaces treated with a neutralized (NH4)2S solution. Compared to the conventional basic (NH4)2S solution treatment, a thick sulfide layer with P–S bond and strong Ga–S (In–S) bond of high thermal stability is formed on the neutralized (NH4)2S-treated GaP (InP) (100) surfaces. The possible passivation mechanisms of the two (NH4)2S solutions to III–V phosphide surfaces are also discussed.
Effect of the end-of-range loop layer depth on the evolution of {311} defects
Authors: Raman R. , Law M. E. , Krishnamoorthy V. , Jones K. S.
Published: July 2002

The interactions between end-of-range dislocation loops and {311} defects as a function of their proximity were studied. The dislocation loops were introduced at 2600 Å by a dual 1×1015 cm−2, 30 keV and a 1×1015 cm−2, 120 keV Si+ implantation into silicon followed by an anneal at 850 °C for 30 min. The depth of the loop layer from the surface was varied from 2600 to 1800 Å and 1000 Å by polishing off the Si surface using a chemical–mechanical polishing (CMP) technique. A post-CMP 1×1014 cm−2, 40 keV Si+ implantation was used to create point defects at the projected range of 580 Å. The wafers were annealed at 700, 800, and 900 °C, and plan-view transmission electron microscopy study was performed. It was found that the number of interstitials in {311} defects decreased as the projected range damage was brought closer to the loop layer, while the number of rectangular elongated defects (REDs) increased. Experimental investigation showed that REDs are formed at the end of range. It is concluded that the interstitials introduced at the projected range are trapped at the end of range. The REDs are formed due to the interactions between the interstitials and the pre-existing dislocation loops.
Origin of anomalous polarization offsets in compositionally graded Pb(Zr, Ti)O3 thin films
Authors: Brazier Mark , McElfresh M. , Mansour Said
Published: July 2002

Ferroelectric films with composition gradients normal to the substrate have recently been reported to exhibit anomalously large polarization offsets when hysteresis loops are driven with an ac voltage and measured using a Sawyer–Tower (ST) circuit. These offsets have been reported in graded Pb(Zr, Ti)O3 (over 400 μC/cm2) and graded (Ba, Sr)TiO3 (30 μC/cm2) films. In this work, it was found that the offset observed in graded Pb(Zr, Ti)O3 films can be attributed to development of a dc voltage across the ac-voltage driven film, rather than a polarization offset. Recognition of this result reduces these previously reported offset values by a factor of Cs/Cref, where Cref and Cs are the reference and sample capacitances, respectively, used in the ST circuit. These dc-voltage offsets still represent a phenomenon which may lead to novel device applications.
BaBi
4
Ti
4
O
15
ferroelectric thin films grown by pulsed laser deposition
Authors: Satyalakshmi K. M. , Alexe M. , Pignolet A. , Zakharov N. D. , Harnagea C. , Senz S. , Hesse D.
Published: July 2002

BaBi 4 Ti 4 O 15 (BBiT) is an n=4 member of the Bi-layer-structured ferroelectric oxide family (Aurivillius phases). BBiT thin films with preferred orientations have been grown on epitaxial conducting LaNiO3 electrodes on (001) SrTiO3 by pulsed laser deposition. Cross-section electron microscopy analysis reveals that the films consist of ct-axis oriented regions and mixed at- and ct-axis oriented regions. The mixed at- and ct-axis oriented regions show high surface roughness due to the rectangular crystallites protruding out of the surface, whereas the ct-axis oriented regions show a smooth surface morphology. In the mixed at- and ct-axis oriented regions, the BBiT films exhibit saturated ferroelectric hysteresis loops with remnant polarization Pr of 2 μC/cm2 and coercive field Ec of 60 kV/cm and no polarization fatigue up to 108 cycles. The regions having ct-axis orientation with a smooth surface morphology exhibit a linear P–E curve. The results show that the ferroelectric properties of a planar capacitor consisting of BBiT depend on the crystalline orientation of the film.
Independent control of ion energy and flux in plasma-enhanced diamond growth
Authors: Teii Kungen
Published: July 2002

Ion energy and flux incident upon a positively biased substrate in an inductively coupled plasma (ICP) have been analyzed during diamond growth at a pressure of 20 mTorr. An electrically floated characteristic of the ICP source allowed a shift up of the plasma potential by the biasing. For the substrate bias (Vb) above 20 V, the ion energy remained constant, while the ion flux was shown to decrease with increasing Vb. The diamond film grown with a high ion flux was composed of well-coalesced large scale islands as compared to that with a low ion flux. The results provide a way to control ion energy and flux independently and its advantage for ion-assisted diamond growth.
2 THz bandwidth electrical pulses on Au and YBa2Cu3Ox transmission lines
Authors: Osbahr Christian J. , Larsen Britt H. , Holst Thorsten , Shen Yueqiang , Keiding So/ren R.
Published: July 2002

An amorphous photoconductive GaAs switch is laser ablated directly onto a superconducting YBa2Cu3Ox coplanar transmission line structure. Illuminating the switch with femtosecond laser pulses generates electrical pulses containing frequency components up to 2 THz. The pulses are detected by photoconductive sampling and have a full width at half maximum of 0.4 ps without deconvolution. Measurements have been performed on both YBa2Cu3Ox and Au/YBa2Cu3Ox transmission lines and we find attenuation and dispersion coefficients to be larger on YBa2Cu3Ox at 77 K than on Au. Furthermore, the integrated photoconductive switch is used to demonstrate that subpicosecond pulses can propagate through small gaps in the transmission lines with minimum attenuation and dispersion.
Accommodation of nonstoichiometry in (100) fiber-textured (BaxSr1−x)Ti1+yO3+z thin films grown by chemical vapor deposition
Authors: Stemmer Susanne , Streiffer S. K. , Browning Nigel D. , Kingon Angus I.
Published: July 2002

We have investigated the microstructural accommodation of nonstoichiometry in (BaxSr1−x)Ti1+yO3+z thin films grown by chemical vapor deposition. Films with y=0.04 and y=0.15 were studied by high-spatial resolution electron energy-loss spectroscopy, revealing changes in chemistry and local atomic environment both at grain boundaries and within grains as a function of titanium content. We find that excess titanium in the samples with y=0.15 segregates to the grain boundaries in addition to being partially accommodated in the grain interior.
Negative magnetoresistance of γ-Fe2O3 observed in γ-Fe2O3/Ag granular nanocomposites
Authors: Tang Jinke , Feng Li , Wiemann Joan A.
Published: July 2002

The magnetoresistance of (γ-Fe2O3)xAg100−x (x=50–90), a granular system where insulating magnetic nanoparticles and nonmagnetic metal are intimately mixed, has been studied at room temperature. For high silver concentration (x⩽70), samples are metallic and exhibit ordinary positive magnetoresistance. Below the percolation threshold, which is 11.4 vol % of silver (x=71), samples are insulators. Our data suggest that direct tunneling between silver grains across γ-Fe2O3 barriers dominates at low temperature and variable range hopping becomes the main transport mechanism at high temperature. The variable range hopping within γ-Fe2O3 is believed to be associated with the presence of Fe2+ impurities which is determined by Mössbauer spectroscopy. Negative magnetoresistance up to −2% is found in (γ-Fe2O3)xAg100−x (x⩾72) in an applied field of 10 kOe at room temperature. It is proposed that the observed negative magnetoresistance is due to the field-dependent hopping rate of electrons from Fe2+ to Fe3+ which is enhanced due to the alignment of their moments by an applied magnetic field.