bluuebunny/crossref_metadata_2025 huggingface dataset:
Status of the CO2 laser ion source at CERN
Authors: Fournier P. , Grégoire G. , Kugler H. , Haseroth H. , Lisi N. , Meyer C. , Ostroumov P. , Schnuriger J.-C. , Scrivens R. , Rodriguez F. Varela , Wolf B. H. , Homenko S. , Makarov K. , Satov Y. , Stepanov A. , Kondrashev S. , Sharkov B. , Shumshurov A.
Published: July 2002

A laser ion source using a CO2 laser focused onto a solid target is under study at CERN for the production of high currents of highly charged heavy ions, for possible use in the preinjector for the large hadron collider. A new expansion and extraction layout was installed in this test facility, improving the alignment and making the target to extraction distance more flexible. A two solenoid beam transport system was studied for providing the matching of the beam to a radio-frequency quadrupole. An electrostatic beam transport using gridded electrostatic lenses was designed and constructed as an alternative to a magnetic system. Results show an increased overall current transmission for the electrostatic case. Investigation of the laser parameters required for the production of 1.4×1010 Pb25+ ions in a 5 μs pulse, has been performed using the TIR-1 laser facility at power densities up to 1014 W cm−2 for a focal spot size of 65 μm. The results of the latest scaling are presented.
Preliminary tests for the electron cyclotron resonance ion source coupled to a laser ion source for charge state enhancement experiment
Authors: Gammino S. , Ciavola G. , Torrisi L. , Celona L. , Wolowski J. , Woryna E. , Parys P. , Làska L. , Kràsa J. , Shirkov G. D.
Published: July 2002

At the Laboratori Nazionali del Sud we have designed a hybrid ion source, consisting of a laser ion source as first stage, which gives intense currents of electrons and of multiply charged ions, followed by an electron cyclotron resonance (ECR) ion source as a second stage, which should act as a charge state multiplier. The ECR ion source coupled to a laser ion source for charge state enhancement (ECLISSE) experiment has been funded by INFN and preliminary experiments have been carried out at IPPLM in Warsaw, in order to confirm the beneficial effects of the axial magnetic field of the ECR ion source on the extraction of the ions from the LIS, as foreseen by calculations. The description of the ECLISSE experiment and of the results of the preliminary tests will be reported.
Measurement of angular divergence and ion species ratios of an rf-driven multicusp ion source for diagnostic neutral beam by Doppler shift spectroscopy
Authors: Yoo S. J. , Yang H. L. , Hwang S. M.
Published: July 2002

The ion species ratios as well as the angular divergences are measured by using a Doppler shift spectroscopy of Hα spectral lines, which originate from several different ions, such as H2+ and H3+ as well as H+, and are spectrally well resolvable from each other on the measured spectral window of detection system. The angular divergences of the ion beam components are determined from the linewidths of the measured emission lines, and the ratio of mixed species is deduced from the intensity ratio of each peak. The ion species ratios measured by the Doppler shift spectroscopy are cross checked by a mass analyzing magnet. The measurements are performed varying the input rf power and the operating source pressure.
Compensating acceptors and donors in nitrogen δ-doped ZnSe layers studied by photoluminescence and photoluminescence excitation spectroscopy
Authors: Zhu Z. , Brownlie G. D. , Horsburgh G. , Thompson P. J. , Wang S. Y. , Prior K. A. , Cavenett B. C.
Published: July 2002

The compensating acceptors and donors in nitrogen δ-doped ZnSe epilayers grown by molecular beam epitaxy using a nitrogen rf-plasma source are studied by means of photoluminescence (PL) and photoluminescence excitation spectroscopy (PLE). The temperature dependence of PL and PLE spectra obtained from the nitrogen δ-doped layers is investigated in detail, and a deep acceptor and a deep donor with ionization energies of ∼170 and ∼88 meV are reported for the nitrogen δ-doped layers. These two deep centers are assigned to N clusters, i.e., NSe-Zn-NSe for the deep acceptor and NSe-NZn for the deep donor.
The role of the surface in transient enhanced diffusion
Authors: Lim D. R. , Rafferty C. S. , Klemens F. P.
Published: July 2002

The enhanced diffusion of impurities that is seen following ion implantation is rapidly quenched, hence, the name transient enhanced diffusion (TED). The quenching of TED is associated with the annealing of implant damage, either by the diffusion of point defects to the bulk or to the surface. It is variously assumed that either the surface or the bulk is the predominant annealing site. In this work, we explore these assumptions by observing the reduction of TED in a buried marker, when the surface is etched to bring it closer to implanted damage. The results show a considerable reduction in TED with surface etching, demonstrating that the surface plays a key role in annealing implant damage. Numerical modeling allows extraction of the surface recombination length of interstitials at 800 °C. Only a value of 0.1 μm is consistent with the data. All but a very small fraction of implanted interstitials are recombined at the surface with this value.
Femtosecond demodulation source for high-resolution submillimeter spectroscopy
Authors: Goyette Thomas M. , Guo Wei , De Lucia Frank C. , Swartz John C. , Everitt Henry O. , Guenther B. D. , Brown Elliott R.
Published: July 2002

A new continuously tunable submillimeter source for spectroscopy and other high-resolution applications has been developed. In this source the optical spectrum of a mode-locked femtosecond laser is downconverted into the submillimeter region by the demodulation process of a photoconductive switch. The power generated is subsequently radiated into free space by an antenna which is integrated along with the switch on low-temperature grown GaAs. The very high resolution is ultimately traceable to the cavity length of the laser and the stable mode-lock frequency which results. Among the most important attributes of the sources are straightforward absolute frequency calibration, very high spectral purity, and the potential for spectral multiplexing.
Wide and continuous wavelength tuning in a vertical-cavity surface-emitting laser using a micromachined deformable-membrane mirror
Authors: Larson M. C. , Harris J. S.
Published: July 2002

Continuous wavelength tuning of 15 nm was achieved micro-electromechanically in an InGaAs/AlGaAs vertical-cavity surface-emitting laser operating near 960 nm. The device utilizes a micromachined deformable-membrane top mirror suspended by an air gap above a p-i-n diode quantum well active region and bottom mirror. Applied membrane-substrate bias produces an electrostatic force which reduces the air gap thickness and therefore tunes the lasing wavelength. Typical tuning bias ranged between 15 and 23 V, and the minimum threshold current was 35 mA for pulsed room-temperature operation.
Carrier traps in a GaAs/AlxGa1−xAs single electron transistor
Authors: Sakamoto T. , Nakamura K.
Published: July 2002

We study carrier traps in a single electron transistor fabricated from a GaAs/AlxGa1−xAs heterostructure. In the heterointerface, there are many kinds of defects, which induce various trap levels in the band gap of AlxGa1−xAs or GaAs. The current through the transistor switches between two states because of trapping and detrapping of a single electron. The gate voltage dependencies of the switching indicate how the traps are spatially distributed. The possibility of the existence of a trap with multilevels is also discussed.
Magnetotransport, magneto-optical, and electronic subband studies in InxGa1−xAs/In0.52Al0.48As one-side-modulation-doped asymmetric step quantum wells
Authors: Kim T. W. , Jung M. , Lee D. U. , Yoo Keon-Ho , Yoo Kyung-Hwa
Published: July 2002

Shubnikov–de Haas (SdH), Van der Pauw Hall-effect, and cyclotron resonance measurements on InxGa1−xAs/In0.52Al0.48As asymmetric step quantum wells were carried out to investigate the electrical properties of a free electron gas and to determine the effective mass of the electron gas, the subband energies, and the wave functions in the InxGa1−xAs quantum wells. The SdH measurements at 1.5 K demonstrated clearly the existence of a quasi-two-dimensional electron gas in the InxGa1−xAs quantum wells, and the fast Fourier transformation results for the SdH data indicated clearly the occupation by that gas of two subbands in the quantum wells. The electronic effective masses determined from the slopes of the main peak absorption energies as a function of the magnetic field were 0.0477 and 0.053 me for the first excited and the ground subbands, respectively. The electronic subband energies and the wave functions in the InxGa1−xAs/In0.52Al0.48As step quantum wells were calculated by a self-consistent method taking into account exchange-correlation effects together with the strain and nonparabolicity effects. The results of the cyclotron resonance measurements qualitatively satisfied the nonparabolicity effects of the step quantum well. These results can help improve understanding for the application of InxGa1−xAs/In0.52Al0.48As step quantum wells in optoelectronic devices.
Simulation of electron transport across charged grain boundaries
Authors: Srikant V. , Clarke D. R. , Evans P. V.
Published: July 2002

The I–V (current density-electric field) characteristics of low-angle grain boundaries consisting of periodic arrays of charged dislocations are computed using a quasiclassical molecular dynamics approach. Below a critical value of the grain boundary misorientation, the computed I–V characteristics are linear whereas above they are nonlinear. The degree of nonlinearity and the voltage onset of nonlinearity are found to be dependent on the grain boundary misorientation.
Improvement of SiGe oxide grown by electron cyclotron resonance using H2O vapor annealing
Authors: Tchikatilov D. , Yang Y. F. , Yang E. S.
Published: July 2002

The influence of low-temperature annealing in H2O vapor on electron-cyclotron-resonance (ECR) grown SiGe oxides is reported. Annealing the oxides in H2O vapor at 280 °C for 3 h 20 min, applied after annealing in forming gas at 450 °C for 30 min, has several important effects: It reduces oxide leakage current by up to four orders of magnitude, decreases the density of interface states, and results in a low fixed oxide charge density of −5.0×1010 cm−2 in comparison to those of the films subjected to annealing in forming gas only. In addition, higher cumulative dielectric breakdown fields up to 8 MV/cm have been achieved. From the results obtained it is evident that vapor annealing is beneficial for ECR-grown SiGe oxides.
Charge sensitivity of superconducting single-electron transistor
Authors: Korotkov Alexander N.
Published: July 2002

It is shown that the noise-limited charge sensitivity of a single-electron transistor using superconductors (of either SISIS- or NISIN-type) operating near the threshold of quasiparticle tunneling, can be considerably higher than that of a similar transistor made of normal metals or semiconductors. The reason is that the superconducting energy gap, in contrast to the Coulomb blockade, is not smeared by the finite temperature. We also discuss the increase of the maximum operation temperature due to superconductivity and the peaklike features on the I–V curve of SISIS structures.
The removal of nitrogen during boron indiffusion in silicon gate oxynitrides
Authors: Ellis K. A. , Buhrman R. A.
Published: July 2002

The effect which nitrogen has upon boron diffusion through oxynitride gate dielectrics has been studied with secondary ion mass spectroscopy of polycrystalline silicon–oxynitride–silicon structures, and with x-ray photoelectron spectroscopy of silicon–boron–oxynitride–silicon structures. These studies show that the indiffusion of boron results in removal of nitrogen from the oxynitride. This indicates that the role of nitrogen in suppressing boron penetration is chemical in nature rather than diffusive.
Effect of growth-rate ratio on surface morphology of homoepitaxial diamond (100) and (111)
Authors: Rawles Robin E. , Morris William G. , D’Evelyn Mark P.
Published: July 2002

Homoepitaxial diamond (100) and (111) films were grown side by side in a hot filament reactor at 875 °C with and without oxygen. The growth rates and surface morphologies were monitored by in situ Fizeau interferometry and atomic force microscopy, respectively. Penetration twins were observed on the (100) surface at a low (100)-to-(111) growth-rate ratio but not at a high value, as expected. Surprisingly, penetration twins were not observed on the (111) surfaces, which were dominated by contact twins in both cases. Implications of these results for microstructural control of diamond films and the effect of oxygen additions are discussed.
Sol–gel epitaxial growth of La1−xCaxMnO3 with colossal magnetoresistance effect
Authors: Bae Seung-Young , Wang Shan Xiang
Published: July 2002

The successful sol–gel growth of colossal magnetoresistive La-Ca-Mn-O powder and epitaxial thin film is reported. Homogeneous powders were prepared at a temperature of 315 °C and epitaxial thin films were obtained on MgO(100) and LaAlO3(100) through heat treatment at 700–950 °C. A transition from semiconductor (negative dρ/dT) to metal (positive dρ/dT) was observed at 225 K, which was located in proximity to the magnetic transition temperature (Tc) of 228 K. A peak magnetoresistance ratio of ∼900% occurred at 220 K.
Scanning electron-acoustic imaging of residual stress distributions in aluminum metal and ZrSiO4 multiphase ceramics
Authors: Zhang B. Y. , Jiang F. M. , Shi Y. , Yin Q. R. , Qian M. L.
Published: July 2002

The scanning electron-acoustic imaging technique has been used in the characterization of the residual stress field distributions existing in the subsurface in aluminum disks and 20 vol % SiC ( w)/ZrSiO4 multiphase ceramics left by Vicker’s indentation. The experimental results reveal that the distribution areas are the plastic-elastic interchange zones. The electron-acoustic signal generation mechanism in the samples are discussed.
Lateral and vertical ordering in multilayered self-organized InGaAs quantum dots studied by high resolution x-ray diffraction
Authors: Darhuber A. A. , Holy V. , Stangl J. , Bauer G. , Krost A. , Heinrichsdorff F. , Grundmann M. , Bimberg D. , Ustinov V. M. , Kop’ev P. S. , Kosogov A. O. , Werner P.
Published: July 2002

We have studied multiple layers of self-organized InGaAs-islands grown on GaAs by x-ray diffraction reciprocal space mapping. We found an anisotropy of the dot spacing in [100] and [110] direction consistent with an ordering of the dots in a two-dimensional square lattice with main axes along the [100] direction and a lattice parameter of 55 nm. The nearly perfect vertical alignment (stacking) of the dots was deduced from the diffraction peak shape.
Optical amplification with neodymium-doped chalcogenide glass fiber
Authors: Mori Atsushi , Ohishi Yasutake , Kanamori Terutoshi , Sudo Shoichi
Published: July 2002

Optical amplification at 1.083 μm with neodymium-doped chalcogenide fiber is observed. The thermal and spectroscopic properties of a new chalcogenide glass system are measured in view of its potential use as a rare-earth host. A maximum internal gain of 6.8 dB is achieved for a pump power of 180 mW.
High quality large-area CdTe(211)B on Si(211) grown by molecular beam epitaxy
Authors: Rujirawat S. , Almeida L. A. , Chen Y. P. , Sivananthan S. , Smith David J.
Published: July 2002

We describe the growth of high quality CdTe(211)B layers by molecular beam epitaxy on nominal Si(211) substrates. Prior to CdTe deposition, thin ZnTe(211)B buffer layers were grown to preserve the homo-orientation. Large-area CdTe(211)B layers were routinely obtained by optimizing the growth parameters. From x-ray diffraction, we observed the presence of twin-free CdTe(211)B layers. One 8 μm thick CdTe epilayer had a near-surface etch pit density of 1.5×105 cm−2, which surpassed the best value reported for CdTe(211)B grown on GaAs(211)B, GaAs/Si(211), or Si(211) substrates.
Use of guided spontaneous emission of a semiconductor to probe the optical properties of two-dimensional photonic crystals
Authors: Labilloy D. , Benisty H. , Weisbuch C. , Krauss T. F. , Houdré R. , Oesterle U.
Published: July 2002

We describe an experimental setup, which allows assessing the optical properties of two-dimensional photonic crystals combined with a waveguide geometry, and etched into a light-emitting (GaAs/InGaAs) semiconductor. By means of a guiding layer, the spontaneous emission of the material is used as a built-in source to probe the properties of the etched microstructure, conveniently compared to the usual measurement schemes. We show polarized transmission and coefficients largely depending on the photonic crystal orientation.
Photostimulated luminescence in Eu2+-doped fluoroaluminate glasses
Authors: Qiu Jianrong , Shimizugawa Y. , Iwabuchi Y. , Hirao K.
Published: July 2002

Photostimulated luminescence phenomenon was observed in a fluoroaluminate glass doped with Eu2+. When the x-ray irradiated glass was excited by a He–Ne laser (633 nm), photostimulated luminescence at 400 nm due to the 5d–4f transition of Eu2+ ions was observed. X-ray absorption spectra showed that a part of Eu2+ converted to Eu3+ upon the irradiation of x-ray. By comparing with the investigations on photostimulable luminescence phosphor BaFBr:Eu2+, the luminescence appears to result from the photostimulated recombination of holes and electrons at traps, which leave electrons in a long-lived excited state.
SiGe quantum dots prepared on an ordered mesoporous silica coated Si substrate
Authors: Tang Y. S. , Cai S. , Jin G. , Duan J. , Wang K. L. , Soyez H. M. , Dunn B. S.
Published: July 2002

This letter reports a new way of preparing wafer sized SiGe quantum dots on an ordered mesoporous sol gel silica coated Si. It was found from x-ray diffraction that very good regular layers of mesoscopic sized SiGe quantum dots can be formed in the silica. Initial low temperature photoluminescence measurements show much improved light emission of the buried dots. This technique is a potential low cost method for producing quantum dot arrays.
Low-noise YBa2Cu3O7−x single layer dc superconducting quantum interference device (SQUID) magnetometer based on bicrystal junctions with 30° misorientation angle
Authors: Beyer J. , Drung D. , Ludwig F. , Minotani T. , Enpuku K.
Published: July 2002

We have fabricated and characterized a low-noise direct-coupled magnetometer based on a 100 pH YBa2Cu3O7−x dc superconducting quantum interference device (SQUID) on a 10 mm×10 mm SrTiO3 bicrystal substrate with 30° misorientation angle. The thin films were deposited by hollow cathode discharge sputtering and patterned using conventional photolithography and Ar ion beam etching. The SQUID magnetometer was operated using direct-coupled flux-locked-loop electronics with bias reversal. The sensor had a usable voltage swing of 39 μV and a white magnetic field noise of 32 fTHz−1/2 with a 1/f corner at 2 Hz, including electronics and environmental noise. The voltage versus flux (V−Φ) characteristic showed a pronounced distortion on the negative slope. Numerical simulations were performed to explain the distorted V−Φ characteristic. Measurements of magnetocardiograms demonstrate the suitability of this sensor for biomagnetic applications.
Surface transverse waves on langasite
Authors: Koskela J. , Lehtonen S. , Plessky V. P. , Salomaa M. M.
Published: July 2002

Surface transverse waves (STW) are numerically simulated on langasite. As our method of investigation, we compute the admittance of an infinite synchronous resonator. The resonance and antiresonance frequencies are evaluated as functions of the crystal cut, temperature, and the thickness of the aluminium electrodes. For optimal parameters, we demonstrate a vanishing first-order temperature coefficient at room temperature and find a coupling strength several times stronger than that for Rayleigh waves in ST quartz. In particular, we find that the sensitivity of the resonance frequency to variations in the thickness of the aluminium electrodes is significantly lower in langasite than in ST quartz.
Atomic structure of faceted planes of three-dimensional InAs islands on GaAs(001) studied by scanning tunneling microscope
Authors: Hasegawa Y. , Kiyama H. , Xue Q. K. , Sakurai T.
Published: July 2002

The three-dimensional (3D) island structure was prepared by molecular beam epitaxy for the lattice mismatched InAs/GaAs(001) system and its images showing atomic structure on faceted planes were taken in situ by ultrahigh vacuum scanning tunneling microscopy (STM). The (113), (114), and (215) faceted planes are observed for the 3D islands. Based on the STM images, atomic structural models are proposed for the faceted surfaces. The surface structure of the (113) faceted planes we propose is different from those observed on the flat GaAs(113) surface.
Valence and atomic size dependent exchange barriers in vacancy-mediated dopant diffusion
Authors: Nelson J. S. , Schultz P. A. , Wright A. F.
Published: July 2002

First-principles pseudopotential calculations of dopant-vacancy exchange barriers indicate a strong dependency on dopant valence and atomic size, in contrast to current models of vacancy-mediated dopant diffusion. First-row elements (B, C, N) are found to have exchange barriers which are an order of magnitude larger than the assumed value of 0.3 eV (the Si vacancy migration energy).
Positive giant magnetoresistance in a Fe3O4/SrTiO3/La0.7Sr0.3MnO3 heterostructure
Authors: Ghosh K. , Ogale S. B. , Pai S. P. , Robson M. , Li Eric , Jin I. , Dong Zi-wen , Greene R. L. , Ramesh R. , Venkatesan T. , Johnson M.
Published: July 2002

A very large positive magnetoresistance (MR) has been discovered in a Fe3O4/SrTiO3/ La0.7Sr0.3MnO3 heterostructure for the transport perpendicular to the layer planes and applied magnetic field in the film plane. The observed MR features do not show any obvious correlation with the hysteresis behavior of the ferromagnetic bilayers. A possible explanation of these results is given in terms of the relative differences in the majority and minority spin bands of the two ferromagnetic layers and the field induced modifications of domain structures therein.
Current heating in polymer light emitting diodes
Authors: Tessler N. , Harrison N. T. , Thomas D. S. , Friend R. H.
Published: July 2002

We present an investigation of current-induced heating in polymer light emitting diodes. Using short electrical pulse measurements, we were able to quantify the temperature rise in the active region. We consider that heating effects play a major role in limiting the maximum efficiency of devices and in initiating degradation mechanisms. Heating and heat sinking are also discussed in the context of electrically pumped polymer lasers.
Coherent acoustic phonons in PbTe quantum dots
Authors: Thoen E. R. , Steinmeyer G. , Langlois P. , Ippen E. P. , Tudury G. E. , Brito Cruz C. H. , Barbosa L. C. , Cesar C. L.
Published: July 2002

Femtosecond pump-probe studies at 1.5 μm of PbTe quantum dots in a glass matrix have revealed oscillations due to a spheroidal acoustic mode. The wavelength dependence of the observed frequencies indicates size selectivity. The frequency and damping of the mode as a function of wavelength agree with the predictions of an acoustic continuum model that suggests that the dominant damping mechanism is radiative loss from the dots to the surrounding glass.
Free-carrier generation in amorphous semiconductors by intense subgap excitation
Authors: Tanaka Keiji
Published: July 2002

Photoconduction spectra in three amorphous semiconductors have been measured as a function of light intensity. In As2S3, shrinkage of photoconductive response peak from 2.7 to 2.0 eV occurs with an increase in the light intensity from 10−3 to 108 W/cm2. In a-Si:H, intense illumination just enhances photocurrents, and intermediate features appear in As2Se3. The anomalous red shift of the photoconduction spectrum discovered in As2S3 can be accounted for as arising from a wide valence-band tail and slow hole transport.
Effects of annealing on self-organized InAs quantum islands on GaAs(100)
Authors: Mo Q. W. , Fan T. W. , Gong Q. , Wu J. , Wang Z. G. , Bai Y. Q.
Published: July 2002

Self-organized InAs islands on (001) GaAs grown by molecular beam epitaxy were annealed and characterized with photoluminescence (PL) and transmission electron microscopy (TEM). The PL spectra from the InAs islands demonstrated that annealing resulted in a blueshift in peak energy, a reduction in intensity, and a narrower linewidth in the PL peak. In addition, the TEM analysis revealed the relaxation of strain in some InAs islands with the introduction of the network of 90° dislocations. The correlation between the changes in the PL spectra and the relaxation of strain in InAs islands was discussed.
Zero-dimensional excitonic properties of self-organized quantum dots of CdTe grown by molecular beam epitaxy
Authors: Terai Yoshikazu , Kuroda Shinji , Takita Kôki , Okuno Tsuyoshi , Masumoto Yasuaki
Published: July 2002

The successful growth of self-organized quantum dots (QDs) of CdTe on ZnTe (100) surface by molecular beam epitaxy is reported. Atomic force microscope measurements on the uncapped samples revealed the formation of CdTe QDs with typical dot diameters of 20±2 nm and heights of 2.7±0.3 nm at 3.5-ML-thick CdTe deposited. The intensity of photoluminescence (PL) from the capped QDs was higher than CdTe/ZnTe single quantum wells (SQWs) by a few orders of magnitude at 4.2 K, and exhibited a thermal quenching with an activation energy of 110 meV, which is about twice as large as those in SQWs. In time-resolved PL measurements, the decay time was almost independent of temperature below 20 K. This is interpreted as due to the zero-dimensional excitonic properties in QDs.
In situ scanning tunneling microscopy study of C-induced Ge quantum dot formation on Si(100)
Authors: Leifeld O. , Müller E. , Grützmacher D. , Müller B. , Kern K.
Published: July 2002

Deposition of submonolayer coverages of C on Si(100) prior to Ge growth leads to the formation of Ge quantum dots below the critical thickness for Ge islanding on bare Si(100). In situ scanning tunneling microscopy reveals a high density of irregularly shaped islands for Ge coverages from 2.5 to 4 ML. Island sizes are broadly distributed between 10 and 25 nm. Keeping the C coverage constant and increasing the Ge coverage from 2.5 to 4 ML, the islands increase in height but their density remains constant (∼1011 cm−2). At a Ge coverage of 5.8 ML, formation of larger (105)-faceted islands is observed. Their density is reduced by a factor of 4 compared to smaller Ge coverages. Transmission electron microscopy shows that the nonfaceted islands are preserved after Si capping.
Multimode Er-doped fiber for single-transverse-mode amplification
Authors: Sousa J. M. , Okhotnikov O. G.
Published: July 2002

We demonstrate that spatial distribution and doping level of the active ions can be designed to provide efficient fundamental mode selection in multimode erbium-doped fiber at high output power. As a result an improvement in scaling fiber devices with single-transverse-mode output to higher power levels is achieved through an increase in mode–field area. It was found that pump-induced guiding contribute the light propagation in a multimode fiber amplifier.
Long wavelength (1.3 and 1.5 μm) photoluminescence from InGaAs/GaPAsSb quantum wells grown on GaAs
Authors: Dowd P. , Braun W. , Smith David J. , Ryu C. M. , Guo C.-Z. , Chen S. L. , Koelle U. , Johnson S. R. , Zhang Y.-H.
Published: July 2002

Room temperature photoluminescence at wavelengths between 1.2 and 1.5 μm has been observed in samples consisting of InGaAs/GaPAsSb quantum well structures grown on GaAs. The emission wavelength is varied primarily by changing the composition within the GaPAsSb layer. It is proposed that such long wavelength emission results from a spatially indirect interband transition in the type-II quantum wells where the electron and hole wave functions have large spatial overlap.
Anisotropic electrical transport property in La4BaCu5O13+δ and La4BaCu4NiO13+δ epitaxial thin films
Authors: Venimadhav A. , Herle P. Subramanya , Vedawyas M. , Shivakumara C. , Hegde M. S.
Published: July 2002

Epitaxial films of La4BaCu5O13+δ and La4BaCu4NiO13+δ oxides are grown with a-b plane parallel to (100) of LaAlO3 and SrTiO3 by pulsed-laser deposition. The conductivity measurements performed along the c direction using LaNiO3 as the electrode show metallic behavior whereas they show semiconducting behavior in the a-b plane. Anisotropic transport property of these thin films is explained on the basis of nearly 180° connected Cu–O–Cu chains with an average Cu–O distance of 1.94 Å along the c direction and nearly 180° and 90° connected Cu–O–Cu chains in the a-b plane with short and long Cu–O distances ranging from 1.863 to 2.303 Å. YBa2Cu3O7−x has been grown along (00l) on La4BaCu5O13+δ and shows a Tc of 88 K.
Band-gap control of GaInP using Sb as a surfactant
Authors: Shurtleff J. K. , Lee R. T. , Fetzer C. M. , Stringfellow G. B.
Published: July 2002

The use of surfactants to control specific aspects of the vapor-phase epitaxial growth process is beginning to be studied for both the elemental and III/V semiconductors. To date, most reported surfactant effects for semiconductors relate to the morphology of the growing films. However, semiconductor alloys with CuPt ordering exhibit much more dramatic effects. The change in the CuPt order parameter induced by the surfactant translates into a marked change in the band-gap energy. Previous work concentrated on the effects of the donor tellurium. Te is less than ideal as a surfactant, since the change in band-gap energy is coupled to a large change in the conductivity. This letter presents the results of a study of the effects of an isoelectronic surfactant on the ordering process in GaInP. Sb has been found to act as a surfactant during organometallic vapor-phase epitaxial growth. At an estimated Sb concentration in the solid of 1×10−4, order is eliminated, as indicated by the band-gap energy. Surface photoabsorption (SPA) data indicate that the effect is due to a change in the surface reconstruction. Adding Sb leads to attenuation of the peak at 400 nm in the SPA spectrum associated with [1̄10] P dimers. The addition of Sb during the growth cycle has been used to produce a heterostructure with a 135 meV band-gap difference between two layers with the same solid composition.
Quantum dots induced by strain from buried and surface stressors
Authors: Davies John H.
Published: July 2002

Quantum dots can be induced in a quantum well by strain from a buried, self-assembled dot or from a stressor on the surface. The elastic fields are shown to be significantly different in their ability to trap carriers. There is no dilation around a buried dot, which therefore has no effect on electrons, and the axial strain repels both holes and excitons. A stressor on the surface, in contrast, produces dilation that can confine all carriers. In both cases, the piezoelectric potential may trap electrons and holes separately in space, storing long-lived spatially indirect excitons.
Effective microwave surface resistance of gold-contacted YBa2Cu3O7−x thin films
Authors: Zaitsev A. G. , Schneider R. , Geerk J. , Linker G. , Ratzel F. , Smithey R.
Published: July 2002

The microwave surface resistance (Rs) of YBa2Cu3O7−x (YBCO) films sputtered simultaneously on both sides of CeO2 coated sapphire wafers of 3 in. diameter was measured using the disk resonator technique at a frequency of 1.92 GHz. By deposition of Au layers of various thicknesses on the unpatterned YBCO side of the disk resonator, we studied the effective Rs of the Au-contacted YBCO films. Although the Au layer was not directly exposed to the microwave power, it dramatically increased the effective Rs of the YBCO film. For example, Rs(77 K)=16.5 μΩ of a 300-nm-thick YBCO film increased to 85 μΩ by the deposition of a 0.1-μm-thick Au layer. The increase of the Au thickness to 1.2 μm resulted in a further enhancement of the effective Rs up to 560 μΩ. We explain this effect in terms of the impedance transformation model. According to this model the effective Rs of the Au/YBCO bilayer decreases with increasing YBCO film thickness. However, Au layers with thicknesses above 1 μm considerably enhance the effective Rs even for thick YBCO films (700–800 nm). A higher quality of the YBCO films (in terms of shorter London penetration depths) reduces the effect of a Au layer, while a low electrical resistivity of this layer leads to a further increase of the effective Rs.
Second-order, high-temperature superconducting gradiometer for magnetocardiography in unshielded environment
Authors: Zhang Y. , Panaitov G. , Wang S. G. , Wolters N. , Otto R. , Schubert J. , Zander W. , Krause H.-J. , Soltner H. , Bousack H. , Braginski A. I.
Published: July 2002

By employing high-temperature superconducting quantum interference device (SQUID) magnetometers, we have assembled a second-order gradiometer for magnetocardiography (MCG) in unshielded environment. With this high-temperature superconductor (HTS) SQUID system, we demonstrated its diagnostic relevance for MCG in terms of signal-to-noise ratio, spatial resolution, frequency bandwidth, rejection of environmental disturbances, and long-term stability. The electronically balanced gradiometer consists of three HTS radio-frequency SQUIDs with superconducting coplanar resonators, mounted in axial gradiometric arrangement with a baseline of 7.5 cm. The system achieves a common mode rejection for axial homogeneous fields of about 104 without any mechanical balancing, and a white noise about 130 fT/√Hz at 77 K, with an 8×8 mm2 flux pickup area. MCG maps above volunteers’ chests have been recorded in unshielded environment in a bandwidth of about 130 Hz. We showed the influence of several notch filters (suppressing the power line frequency) on the quality of the MCG signals.
Formation of Ti3+ in sapphire by co-implantation of Ti and O ions
Authors: Morpeth L. D. , McCallum J. C.
Published: July 2002

We have demonstrated that co-implantation of Ti and O ions into c-axis oriented α-Al2O3 substrates followed by thermal annealing can substantially increase the fraction of Ti ions present in the optically active 3+ oxidation state. Evidence for the presence of Ti3+ is given by the observation of strong luminescence over the wavelength range ∼600–900 nm under Ar laser excitation and by the presence of the characteristic absorption band at ∼450–550 nm. We have studied the luminescence intensity as a function of implantation and annealing parameters and have found that there is a strong dependence on the ratio of implanted Ti and O and on the annealing temperature.
Neutron diffraction study of the reduction of NiAl2O4
Authors: Ustundag E. , Clausen B. , Bourke M. A. M.
Published: July 2002

The reduction of a solid NiAl2O4 cylinder to a metal-ceramic composite consisting of Ni particles in an Al2O3 matrix was monitored in situ at 1220 °C with neutron powder diffraction. The reaction kinetics was determined with a time resolution of 30 min. The reduction is associated with a volume shrinkage. A comparison of finite element calculations and the changes in the measured lattice parameters suggests that creep has relaxed the residual strains that would otherwise result from the volume shrinkage. The data also indicate that structural evolution in unreduced NiAl2O4 via a change in the cationic sublattice towards inverse spinel occurred and that led to a variation in lattice parameters.
Optical absorption and anomalous photoconductivity in undoped n-type GaN
Authors: Chung S. J. , Jeong M. S. , Cha O. H. , Hong C.-H. , Suh E.-K. , Lee H. J. , Kim Y. S. , Kim B. H.
Published: July 2002

Photoconductivity and optical absorption measurements were employed to analyze deep levels in undoped n-type GaN films grown on sapphire substrate by metalorganic chemical vapor deposition. At room temperature, the photoconductivity measurement exhibits a broad level at around 1.90 eV. Similarly, the optical absorption spectrum shows a deep level located at 1.87 eV within the band gap, which is best described by a transition from a donor charge-transfer level to the conduction band, according to Lucovsky theory. A persistent photoconductivity whose behavior is distinctive from that of previously reported work for n- or p-type GaN epitaxial films was observed. The photocurrent quenching and decreased dark current in the persistent photoconductivity effect suggest that metastable electron states are formed in the band gap to trap electrons which tunnel out the potential barrier with long recovery time.
Matrix dependence of strain-induced wavelength shift in self-assembled InAs quantum-dot heterostructures
Authors: Yeh N.-T. , Nee T.-E. , Chyi J.-I. , Hsu T. M. , Huang C. C.
Published: July 2002

We report on the matrix-dependent strain effect in self-assembled InAs quantum-dot heterostructures using photoluminescence measurements. A series of samples were prepared to examine the effect of quantum dot position with respect to the so-called strain-reducing layer (SRL). Since the SRL reduces the residual hydrostatic strain in the quantum dots, long emission wavelength of 1.34 μm is observed for the InAs quantum dots with an In0.16Ga0.84As SRL. The dependence of the emission wavelength on the thickness of the cap layer on SRL also indicates the importance of the role of matrix in the strain relaxation process of the dots. Using In0.16Al0.84As instead of In0.16Ga0.84As as the SRL, a blueshift in wavelength is observed because the elastic stiffness of In0.16Al0.84As is higher than that of In0.16Ga0.84As and less strain is removed from the dots with In0.16Al0.84As SRL.
Low-energy electron beam focusing in self-organized porous alumina vacuum windows
Authors: Doll T. , Vuckovic J. , Hochberg M. , Scherer A.
Published: July 2002

Micromachined, micron-thick porous alumina membranes with closed pore endings show high electron transparency above an energy of 5 keV. This is due to the channeling of electrons along the negatively charged insulating pores after surmounting the thin entrance layer. We also find a sharp hightransparency energy window at energies as low as 2 keV which may be the result of a local maximum of channeling, as predicted by simulations, and positive charge up of the entrance layer causing electron electrostatic focusing. Applications for these membranes range from atmospheric electron spectroscopy to self-assembled, nanoscale, large-area electron collimators.
Effect of multidirection rubbing on the alignment of nematic liquid crystal
Authors: Kim Young Jin , Zhuang Zhizhong , Patel Jay S.
Published: July 2002

We have investigated the alignment properties of liquid crystals induced by multiple rubbing of the surfaces in different directions. Experiments were carried out using homeotropic and hybrid-aligned samples. It is experimentally found that the alignment of the liquid crystals is along neither of the rubbing directions, but instead lies along an axis intermediate between these two directions, and that the direction depended on the relative strength of rubbing along the two axes. A model that assumed the grooves along two rubbing directions is proposed, and the relation between the orientation of the liquid crystal and the relative rubbing strength is analyzed. We found that this model can explain the observed experimental results.
Differential geometry of GLp,q(1|1)
Authors: Çelik Salih
Published: July 2002

We introduce a construction of the differential calculus on the quantum supergroup GLp,q(1|1). We obtain two differential calculi, respectively, associated with the left and the right Cartan–Maurer one-forms. We also obtain the quantum superalgebra of GLp,q(1|1). Although all of the structures we obtain are derived without an R matrix, they nevertheless can be expressed using an R matrix.
Dissociation dynamics of the water molecule on the Ã 1B1 electronic surface
Authors: Yang X. F. , Hwang D. W. , Lin J. J. , Ying X.
Published: July 2002

Photodissociation of H2O, D2O, and HOD on the Ã 1B1′ surface through 157.6 nm excitation has been studied using the H(D) atom Rydberg tagging time-of-flight technique. Vibrational state distribution has been measured for the OH/OD product from the photodissociation of the H2O, D2O, and HOD molecules. Comparisons of our results with previous theoretical calculations and experimental results obtained using the laser induced fluorescence (LIF) technique have been made. Experimental results in this work indicate that the relative populations for vibrationally excited OH(v⩾2) products measured using LIF are significantly underestimated, suggesting that LIF as a technique to quantitatively measure vibrational distributions of reaction product OH is seriously flawed. The experimental results presented here are in rather good agreement with previous theoretical calculations. However, our results indicate that the calculated vibrational populations for the higher vibrational states of OH are still somewhat overestimated. Relative branching ratio of H and D productions from HOD has also been estimated. This complete set of data for the photodissociation of H2O should provide an excellent testing ground for improving the theoretical potential energy surface of the first electronically excited state of the water molecule.
Why two qubits are special
Authors: Vollbrecht K. G. H. , Werner R. F.
Published: July 2002

We analyze some special properties of a system of two qubits, and in particular of the so-called Bell basis for this system, and discuss the possibility of extending these properties to higher dimensional systems. We give a general construction for orthonormal bases of maximally entangled vectors, which works in any dimension, and is based on Latin squares and complex Hadamard matrices. However, for none of these bases the special properties of the operation of complex conjugation in Bell basis hold, namely that maximally entangled vectors have up-to-a-phase real coefficients and that factorizable unitaries have real matrix elements.
On the Lewis metric
Authors: Gariel J. , Marcilhacy G. , Santos N. O.
Published: July 2002

We revisit Lewis solution giving a mechanical interpretation to its field equations. The structure of the equations that the metric coefficients satisfy can be associated to the motion of a classical particle in a central field.
Polymer and cathode emission studies of polymer-based light-emitting diodes under strong electrical pulse excitation
Authors: Levi Ofer , Yakimov Aharon V. , Nassar Hesham , Davidov Dan , Pfeiffer S. , Hörhold H. H.
Published: July 2002

We present spectroscopic and time-resolved studies of polymer-based light-emitting diodes under strong pulse excitations. The devices were fabricated using soluble poly(p-phenylene vinylene) (PPV) derivatives. Above some threshold voltage, new emission lines were observed in addition to the electroluminescence (EL) emission associated with the polymer. These new emission lines are attributed to excitation of the cathode material by collisions with holes at electric fields above 3.2 MV/cm. Time-resolved studies of the cathode emission and the polymer EL show similar time delays between the EL emissions and the onset of the voltage pulse. This supports the idea that the polymer EL emission at high fields and the cathode emission are governed by hole transport. Current–voltage characteristics were studied up to 6 MV/cm. The data can be described by the space charge limited current model for electric fields up to 2.7 MV/cm but show deviation from this model at higher electric fields. EL quantum efficiency decreases for fields above 2.5 MV/cm. This decrease is attributed to exciton dissociation at high fields. Based on the results in this article we suggest that hybrid EL devices containing layers of polymers and inorganic nanoparticles operating under high field excitation may have some advantages for future electrically pumped lasers.
Photoreflectance characterization of the plasma-induced damage in Si substrate
Authors: Wada Hideo , Agata Masashi , Eriguchi Koji , Fujimoto Akira , Kanashima Takeshi , Okuyama Masanori
Published: July 2002

Si surface damage induced during Ar and CHF3/CF4/Ar plasma processing has been characterized by photoreflectance spectroscopy (PRS). The photoreflectance (PR) signal intensity decreases drastically as the rf input power increases in the Si substrate exposed to the plasma. The recovery of the plasma-induced damage is confirmed by PRS after subsequent annealing over 500 °C. We found, from the shift of the PR spectra, the strain at the Si surface induced by the plasma treatment might be due to the introduction of the ions, C, F, or H, not to those of Ar. The depth profile of the defect density in the Si substrate is estimated quantitatively by analyzing the depth profile of the PR signal intensities.
Particle irradiation of doping superlattices: An optical investigation
Authors: Brink D. J. , Kunert H. W. , Sieberhagen R.
Published: July 2002

Optical properties of α- and β-particle irradiated doped superlattices (npnp) were studied by photoluminescence, ellipsometry, and Raman spectroscopy. Irradiation causes a substantial shift of the main emission band. Single-wavelength ellipsometry shows a small drop in extinction coefficient, but no measurable change in real index of refraction or in depletion width between n and p layers. Thermal annealing at 450 K results in the formation of a new emission band at 1.51 eV. High temperature annealing reverses all radiation induced changes.
Optical properties of arsenic ions implanted GaAs/AlGaAs V-grooved quantum wires
Authors: Zhao Q. X. , Willander M. , Lu W. , Liu X. Q. , Shen S. C. , Tan H. H. , Jagadish C. , Zou J. , Cockayne D. J. H.
Published: July 2002

Asymmetric double GaAs/AlGaAs V-grooved quantum wires, grown by low pressure metalorganic chemical vapor deposition, are studied using photoluminescence (PL) spectroscopy. The structure was selectively treated by ion implantation at different arsenic (As) doses after growth. The ion implantation strongly reduces the efficiency of the emissions from the implanted well regions or even quenches the PL emissions from certain well regions due to irradiation damage. Wire emission is clearly resolved in the samples after treatment by low dose implantation. The temperature dependence of the wire emission intensity shows an enhancement at a temperature of around 45 K. The wire emission peak with a shoulder at its high energy side at low temperatures develops into double peaks in a temperature region between 20 and 140 K, and the high energy transition component dominates the PL spectra at temperatures above 140 K. The deduced energy separation between two peaks is about 10 meV. With further increasing temperatures the wire emission related to the light hole state can be observed at temperatures above 150 K. Deduced splitting between the heavy and light states is about 35 meV in our structures.
Luminescence of Eu(fod)3 in a homologic series of simple alcohols
Authors: Schuurmans Frank J. P. , Lagendijk Ad
Published: July 2002

We present measurements on the luminescence of Eu(fod)3 dissolved in a series of (deuterated) alcohols and mixtures hereof, to test local-field corrections on spontaneous emission. The alcohols used are CH3(CH2)mOH, with m ranging from 0 for methanol to 9 for 1-decanol. Accordingly, the refractive index varies from 1.33 to 1.44. To test the influence of the intrinsic molecular vibrations of the alcohols on the luminescence properties, also deuterated alcohols are used: CH3(CH2)mOD, with m=0,1,3. Although the chemically homologic series of alcohols was carefully selected, small variations in the local environment of Eu(fod)3 complex upon changing the host (alcohol), blur most of the local-field effect on spontaneous emission.