bluuebunny/crossref_metadata_2025 huggingface dataset:
Defect formation and annealing behavior of InP implanted by low-energy N15 ions
Authors: Rauhala E. , Ahlgren T. , Väkeväinen K. , Räisänen J. , Keinonen J. , Saarinen K. , Laine T. , Likonen J.
Published: July 2002

Defect formation has been studied in nitrogen-implanted III–V compound semiconductor material InP. Sulphur-doped n-type (100) InP samples were implanted at room temperature with 30 keV 15N+ ion doses of 1014–1016 N atoms cm−2. The implanted samples were subjected to isochronal vacuum annealing in the temperature range of 450–650 °C. The annealing behavior of nitrogen atom distributions, implantation-induced displacements of indium atoms, vacancy-type defects, and damage annealing were studied by nuclear resonance broadening, secondary ion mass spectrometry, ion backscattering and channeling, and slow positron annihilation techniques. Doses above 1015 N atoms cm−2 were found to produce amorphous layers extending from the surface to depths beyond the deposited energy distribution, up to 110 nm. The depth of an amorphous layer was observed to depend on the implantation dose. Temperature and dose-dependent epitaxial regrowth starting from the amorphous-crystalline interface was observed at 575 °C. The damage and vacancy concentration distributions were correlated with the implanted nitrogen distribution in the case of the highest implantation dose at 1016 N atoms cm−2; disorder annealing and loss of nitrogen behave in a like manner with increasing temperature. For the lower doses, however, almost no redistribution or loss of nitrogen arose despite structural damage and vacancy annealing. Interpretation of the positron data allowed an identification of two types of vacancies. The type of the vacancy defects was found to depend on the implantation dose and annealing temperature; monovacancies were formed in the In sublattice at the low implantation doses, while the highest dose produced divacancies. The annealing tended to convert the monovacancies into divacancies, which were recovered at high temperatures depending on the implantation dose.
Laser performance of Coumarin 540A dye molecules in polymeric host media with different viscosities: From liquid solution to solid polymer matrix
Authors: Costela A. , Garcia-Moreno I. , Barroso J. , Sastre R.
Published: July 2002

Photophysical parameters and lasing properties of Coumarin 540A dye molecules are studied in solutions of increasing viscosity, from liquid solutions in 1,4-dioxane to solid solutions in poly(methyl methacrylate). The fluorescence quantum yield and lasing efficiencies decrease as the viscosity of the solution increases, reflecting the strong influence of the rigidity of the medium on the radiative processes. The photodegradation mechanisms acting on the fluorophores are analyzed by following the dependence of laser induced fluorescence and laser output on the number of pump laser pulses. The fluorescence redistribution after pattern photobleaching technique is used, and Fick’s second law is applied to study the diffusion of dye molecules in the highly viscous polymer solutions. The diffusion coefficients of the dye molecules as a function of the increased viscosity of the medium are determined.
The initiation of high current density switching in electrically conductive polymer composite materials
Authors: Duggal Anil R. , Sun F. G.
Published: July 2002

In this paper, the idea that high power switching in conductor-filled polymer composite materials is triggered by the injection of a high energy density at the electrical contact interfaces is tested. A simple theoretical model for the electrical contact interface is developed and used to predict that the time required for switch initiation should be inversely proportional to the square of the product of average initial current and contact resistance. This scaling prediction is tested by means of switching experiments where the switch initiation time is monitored as a function of average initial current level and contact resistance. It is found that the experimental data are consistent with the theory over a variation of more than three orders of magnitude in both initial current level and contact resistance. In addition, it is determined that the data are consistent with the interpretation that polymer ablation is the physical event which triggers switching.
Effect of majority carrier space charges on minority carrier injection in dye doped polymer light-emitting devices
Authors: Berleb S. , Brütting W. , Schwoerer M. , Wehrmann R. , Elschner A.
Published: July 2002

By blending suitable dyes into poly(N-vinylcarbazole) we have fabricated devices which emit light in the whole visible spectrum. Their current–voltage characteristics can be described by space-charge limited currents with effective trapping of the charge carriers by the dye molecules, while the light intensity shows a Fowler–Nordheim-like behavior as a function of the external electric field. However, the anodic space charge changes the field distribution inside the device and leads to a cathodic field enhancement which has to be considered in the Fowler–Nordheim equation. We were able to model the electroluminescence characteristics by assuming tunneling of the minority charge carriers through a triangular barrier. The obtained barrier heights showed a strong dependence on the dye molecules, suggesting that the injection of minority charge carriers takes place directly into the lowest unoccupied molecular orbital of the chromophors. Using poly(p-phenylenevinylene) and an oxadiazole starburst molecule as additional hole and electron transport layers, respectively, luminance of 250 cd/m2 and external quantum efficiency of 1% could be achieved.
Dielectric study in nanocrystalline Bi4Ti3O12 prepared by chemical coprecipitation
Authors: Jiang A. Q. , Li G. H. , Zhang L. D.
Published: July 2002

Bi 4 Ti 3 O 12 nanoparticles were synthesized by chemical coprecipitation and crystallized in the perovskite structure after calcining at 520 °C. The dielectric properties were measured in bulk nanocrystalline compacts with different grain sizes. It was found that there were three peaks in the curves of the dielectric response as a function of temperature. The first peak shifts to higher temperature with decreasing grain size, which is considered to originate from the polarization of the ions in the conductive (Bi2O2)2+ layers across the potential barrier of the weak conductive perovskitelike layers. The second peak, contributed by the polarization of the defect dipoles on the grain surfaces, especially Bii.–Vo″ dipoles, decreases gradually in intensity and finally disappears with increasing grain size. The last one, corresponding to a ferroelectric phase transition temperature, increases at first with decreasing grain size from 56 to 25 nm, then decreases with further decreasing grain size, and the mechanism is correlated with a competing effect of the released internal stresses and the clamped domain walls due to the diffusion of oxygen vacancies.
Resonant magnetic quantum tunneling through thermally activated states
Authors: Fernández J. F. , Bartolomé J. , Luis F.
Published: July 2002

A theory of the magnetic relaxation of a large spin (S=10) having a large uniaxial magnetocrystalline anisotropy is outlined. The theory explains magnetic relaxation observed in Mn12 acetate. The joint action of local fields and a fourth order distortion of the magnetocrystalline anisotropy is necessary to account for the tunneling that is observed, between the m=−4 and m=4 unperturbed states. Even in resonance, tunneling takes place mainly incoherently in Mn12 acetate. It proceeds through the lowest energy state doublet which is not blocked by longitudinal local fields. The relaxation rate Γ has been calculated using a master equation. The model gives magnetization hysteresis loops and ac magnetic susceptibility curves which are in quantitative agreement with experimental results. For temperatures below 0.5 K approximately, nonresonant tunneling from the ground state becomes the dominant relaxation mechanism.
Magnetic and structural properties of commercial Sm2(Co,Fe,Cu,Zr)17-based magnets
Authors: Chen X. , Liu J. F. , Ni C. , Hadjipanayis G. , Kim A.
Published: July 2002

A newly developed Sm(Co,Fe,Cu,Zr)z magnet, with z≈7, has a lower temperature coefficient of coercivity β, as compared with other commercial magnets with z≈7.7. It was found that the initial magnetization curve for the newly developed magnet is typical of pure uniform domain wall pinning, while for the commercial magnets nucleation and domain wall pinning occur simultaneously, although domain wall pinning is still the dominant mechanism. It was also found that the magnetic viscosity parameter Sv has similar temperature dependence with the coercivity. Transmission electron microscope studies showed that the smaller cell size contributes to the low temperature coefficient of coercivity.
Magneto-impedance effect in high permeability NiFeMo permalloy wires
Authors: Vázquez M. , Garcı́a-Beneytez J. M. , Sinnecker J. P. , Li Lin
Published: July 2002

The giant magneto-impedance effect (GMI) has been measured in commercial HyMu80® permalloy wires, submitted to different treatments (annealing and cold-drawing). Although hysteresis measurements show quasibistable magnetic behavior, GMI variations as high as 160% can be found in the frequency range 1–10 MHz. The frequency dependence of the GMI is also analyzed in terms of a nonhomogeneous anisotropy distribution.
Probing the magnetic microstructure of an amorphous GdFe system with magnetic anomalous small angle x-ray scattering
Authors: Fischer P. , Zeller R. , Schütz G. , Goerigk G. , Haubold H.-G. , Pruegl K. , Bayreuther G.
Published: July 2002

The combination of x-ray magnetic circular dichroism (X-MCD) with anomalous small angle x-ray scattering (ASAXS) allows to determine size distributions and correlations lengths in the nanometer range of magnetic precipitates in granular systems. Results on an amorphous GdFe system with a pronounced perpendicular magnetic anisotropy are reported. Standard ASAXS measurements taken at the Fe K edge provide information on the electronic structure yielding a correlation maximum at 75 Å and an average particle radius of 6 Å. The magnetic scattering curves (MASAXS) were obtained at the corresponding Gd L3,2 edges. Their intensities can be explained by magnetic contributions to the anomalous scattering factors f′ and f″. A change of sign in the MASAXS is observed between profiles taken at the L3 and the L2 edge and with the change of circular polarization as expected from X-MCD.
Giant magnetoresistance in granular CuFeNi alloys
Authors: Martins C. S. , Rechenberg H. R. , Missell F. P.
Published: July 2002

Giant magnetoresistance (MR) was studied in alloys of composition Cu80Fe10Ni10, Cu80Fe15Ni5, Cu80Fe5Ni15, and Cu80Fe20, produced by melt spinning and annealed in the temperature range 300–600 °C. Materials were characterized by Mössbauer spectroscopy at several temperatures. At both 78 and 300 K, spectra for all CuFeNi compositions showed a singlet associated with superparamagnetic Fe nanoparticles, but at 4.2 K the sextet characteristic of ferromagnetic Fe was visible. From the hyperfine field reduction we estimated the radius of the Fe particles. Magnetization data for Cu80Fe10Ni10 at 300 K were compared with a model which features a log-normal distribution of noninteracting superparamagnetic particles. Thus the mean magnetic moment and the particle density were determined. The magnetoresistance ratio MR was compared to a model which considers bulk and interface scattering.
Metallic needle artifacts in magnetic resonance imaging
Authors: Liu Haiying
Published: July 2002

A better understanding of the appearance of a biopsy needle and its image artifacts as well as its interaction with various sequence parameters in magnetic resonance (MR) imaging is beneficial for its application in MR image guided surgical procedures. A standard analytical solution to an infinite long cylindrically shaped object immersed in a uniform magnetic field has been used to predict MR image artifacts. In a more practical setting, we investigated the characteristics of MR image artifacts associated with a finite length metallic needle, specifically at the tip of a biopsy needle when it is approximately parallel to the main magnetic field. The origin of the needle tip artifact, which is exhibited as a blooming ball, was studied using MR imaging techniques and numerical simulation employing finite element method. Satisfactory agreement between theory and experiment has been achieved. Results showed that the image artifacts associated with biopsy needle are present and dependent on MR imaging parameters, but the artifacts can be reduced if optimal imaging parameters were used.
Magnetic interactions in Fe–Ba hexaferrite nanocomposite materials
Authors: Montero M. I. , Cebollada F. , Morales M. P. , González J. M. , Hernando A.
Published: July 2002

Results are presented on the hysteretic behavior of Fe-rich, composite Ba hexaferrite–Fe samples prepared by ball milling. The most remarkable feature of these samples was the observation of loops which were displaced in the negative sense of the field axis. Similar to this, the field evolution of the isothermal and the demagnetization remanences evidenced the achievement upon the application of 8 kOe fields of nonequivalent values of both quantities. Our results are discussed considering the different magnetic hardness of the two phases forming the sample and the occurrence of interphase (dipolar) interactions.
Heat diffusivity of Nd1−xSrxMnO3−δ and La1−xCaxMnO3−δ compounds
Authors: Liebe J. , Kang H. , Haupt L. , Mandal P. , Medvedeva I. V. , Rao G. H. , Bärner K. , Poddar A. , Muruguraj P. , Fischer R. , Gmelin E. , Gommert E. , Helmolt R. v. , Wecker J.
Published: July 2002

The thermal diffusivity of bulk and epitaxial layer samples of the giant magnetoresistance compounds Nd0.67Sr0.33MnO3−δ and La0.67Ca0.33MnO3−δ is investigated in a comparative study for temperatures between 35 and 350 K in order to detect preparational differences and to correlate the thermal transport with magnetization M and resistivity ρ. We find for Nd–Sr an anomaly of the heat diffusivity D near 145 K (80 K La–Ca) which we attribute to a temperature-induced canted metal to AFM insulator transition and a second anomaly in D(T) near 250 K (300 K) which we assign to the transition to paramagnetism. The low-temperature anomaly can be a step of up to five times in the heat conductivity, which makes these compounds potential magnetic field activated heat switches, as this transition can also be induced by a magnetic field.
Electrical and optical characterization of ion-implanted CuGaSe2 single crystals
Authors: Schön J. H. , Arushanov E. , Kulyuk L. L. , Micu A. , Shaban D. , Tezlevan V. , Fabre N. , Bucher E.
Published: July 2002

CuGaSe 2 single crystals were doped with B and Ge by ion implantation and analyzed by Hall effect, photoluminescence, and reflected second harmonic generation measurements. After ion implantation the crystals exhibited a destroyed surface structure. Measurements of the band edge photoluminescence and the second harmonic generation before and after annealing clearly reflected the thermal healing of the implantation-induced crystalline defects. As expected the analysis of the electrical measurements showed the incorporation of donor levels due to implantation with B and Ge, which led to electrically compensated samples. However no n-type conductivity could be obtained. This is ascribed to self-compensation of the defects for implantation ion doses higher than 1015 cm−2.
Singular perturbation theory of a thin film surface-polariton polarizer for a planar optical waveguide
Authors: Sletten Mark A. , Seshadri S. R.
Published: July 2002

An on-line polarizer of the order of a millimeter in length is designed by locating a thin metal film suitably close and parallel to a dielectric film waveguide. The polarizer transmits the transverse electric guided mode of the dielectric film with negligible insertion loss. As a consequence of the phase-matched wave interaction between the surface polariton supported by the metal film and the transverse magnetic guided mode of the dielectric film, the transverse magnetic guided mode is severely attenuated resulting in a high extinction coefficient. A singular perturbation theory using multiple space scales is developed for the analysis of this thin film surface polariton polarizer for a planar dielectric film optical waveguide and illustrative numerical results are presented. The sensitivity of the performance of the polarizer to small changes in the various physical parameters is examined.
Ultrafast absorber saturation process and short pulse formation in injection lasers
Authors: Zaitsev S. V. , Gordeev N. Yu. , Soshnikov M. P. , Massa J. S. , Buller G. S.
Published: July 2002

The nature of lasing threshold in passively Q-switched GaAs/AlGaAs lasers with saturable absorbers formed by heavy ion implantation is investigated in this article. After studying various laser characteristics, including threshold current density, differential quantum efficiency, spectral output, and picosecond time-resolved emission, we conclude that the origin of the Q-switching is unlikely to be caused by spontaneous emission or mode locking, and that collective coherent radiation effects may contribute to the onset of lasing.
Dynamic composite elastic medium theory. Part I. One-dimensional media
Authors: Kaelin Bruno , Johnson Lane R.
Published: July 2002

Wave propagation in stratified media may be described by scattering theory, effective medium theory or ray theory, depending upon the frequency range. We present a dynamic composite elastic medium theory which describes wave propagation at all frequencies. In the first part of the series we consider randomly layered one-dimensional media and in the second part media with three-dimensional inclusions. Non-self-consistent and self-consistent methods using the scattering functions of the individual layers are formulated which allow the calculation of phase velocity, attenuation and wave forms. In the low frequency limit only the self-consistent method agrees with the Reuss average and in the high frequency limit it yields the correct ray theory average velocity. The comparison with complete numerical solutions shows that our theory predicts the coherent wave through randomly layered media. Hence, the dynamic composite elastic medium theory can be used to compute frequency dependent elastic properties of randomly layered media without calculating the complete wave propagation solution.
Space-charge effects and current self-quenching in a metal/CdS/LaS cold cathode
Authors: Mumford P. D. , Cahay M.
Published: July 2002

We analyze the importance of space-charge effects in the cathode to anode gap region of a recently proposed metal/CdS(cadmium sulfide)/LaS(lanthanum sulfide) cold cathode. Our approach is based on an ensemble Monte Carlo description of electron transport assuming ballistic injection across the CdS and LaS layers. Under this approximation, the energy spectrum of the injected beam entering the air gap can be determined exactly as a function of the applied bias across the CdS layer. The effects of shot noise in the injected current are taken into account. For some of the biasing conditions considered here, space-charge effects are quite drastic and lead to dynamical effects which are responsible for the onset of current self-quenching similar to the Child–Langmuir regime of operation of thermionic cathodes. The limiting anode current density is found to be much larger than the Child–Langmuir limit. In the presence of strong space-charge effects, large oscillations in the minimum of the electrostatic potential in front of the cathode lead to oscillations in the measured anode current within the tens of GHz frequency range.
Role of hydrogen surface coverage during anodic plasma deposition of hydrogenated nanocrystalline germanium
Authors: Poulsen Per R. , Wang Mingxiang , Xu Jun , Li Wei , Chen Kunji , Wang Guanghou , Feng Duan
Published: July 2002

Nanocrystalline Ge:H films were deposited on the anode of a conventional plasma-enhanced chemical vapor deposition system using highly H2-diluted GeH4 as source gas. The structural changes with substrate temperature, hydrogen dilution, and rf power were investigated by x-ray diffraction and Raman scattering. The hydrogen content and H bonding configuration were determined by infrared absorption spectroscopy. It was found that increased rf power mainly favors nucleation of grains while increased H2 dilution mainly favors the subsequent growth of grains. Transitions from an amorphous to a crystalline and then back to an amorphous phase are observed as the substrate temperature is increased. The amorphous phase at high temperatures and variations in the preferred crystallite orientations are explained by changes in the diffusion length of adsorbed precursors due to changing hydrogen coverage on the growing surface.
Effects of reorientation of molecules in a nonlinear-optical photoconducting polymer on formation of an antiguide structure
Authors: Sassa Takafumi , Umegaki Shinsuke
Published: July 2002

When the poled nonlinear-optical photoconducting polymer is irradiated by the pump light, the generated space charges screen the poling electric field and induce reorientation of the nonlinear-optical molecules. It results in reduction of refractive indices and of a nonlinear-optical coefficient of the polymer. Due to the former reduction, the antiguide structure is so made as to confine the leaky lightwaves. We confirmed the reduction by observing the second-harmonic generation from the pump-irradiated polymer and compared the self enhancement of the Cerenkov-radiation type second-harmonic power generated from a thin-film waveguide.
Field-induced silver injection into sodium diborate glass
Authors: Doi Akira , Ohsugi Masahiro
Published: July 2002

Field-induced silver injection into glass was studied in the time domain. It was found that injection feasibility was different from sample to sample, that silver injection became progressively less favorable with cycles of silver deposition/injection, and that it would occur by field evaporation from the silver anode caused by an effective field of ∼108 V/cm within an insulating electric double layer formed near the anode.
Growth, annealing and oxidation of the Ni/MgO (001) interface studied by grazing incidence x-ray scattering
Authors: Barbier A. , Renaud G. , Robach O.
Published: July 2002

We report an in situ grazing incidence x-ray scattering study of the Ni/MgO (001) interface during its formation at room temperature. In-plane measurements reveal that the interface is sharp and that the epitaxial relationship is complex. In the early stages of growth, up to one monolayer of “on site” Ni, in-plane strained to the lattice parameter of MgO, is observed. An oxygen epitaxial site is found. For the on-site fraction the interfacial and Ni interlayer distances were determined. For coverages larger than one monolayer two distinct lattices exist: expanded Ni (001) and Ni (110). The latter exhibits several orientations with respect to the substrate depending on the thickness. The Ni (110) orientations disappear by annealing at high temperature, leaving only the Ni cube/cube orientation. The layer was also almost fully transformed into NiO (001) by high temperature oxidation.
Molar fraction and substrate orientation effects on carbon doping in InGaAs grown by solid source molecular beam epitaxy using carbon tetrabromide
Authors: Lubyshev D. I. , Micovic M. , Cai W. Z. , Miller D. L.
Published: July 2002

Carbon incorporation in InGaAs and GaAs is systematically studied in solid-source molecular beam epitaxy (MBE) as a function of carbon tetrabromide pressure, indium molar fraction, and substrate orientation. The maximum attainable free carrier concentration in GaAs and InGaAs lattice matched with InP was 2×1020 cm−3. The etching effect of CBr4 on growth rate reduction, surface morphology, and growth mechanism is clarified. A comparative study of carbon incorporation as function of substrate orientation and polarity was undertaken by the growth of GaAs and In0.53Ga0.47As on (n11)A and B (n=2, 3, 5) and (100) oriented substrates. Free carrier concentration and mobility measurements showed no carbon autocompensation in GaAs but strong amphoteric behavior for In0.53Ga0.47As grown on arsenic terminated planes. Measurement of hole concentration as function of indium molar fraction in InxGa1−xAs shows that carbon tetrabromide can be used as an effective acceptor doping percursor for indium molar fraction x less than 80% in solid-source MBE.
Atomic layer epitaxy and molecular beam epitaxy of CdTe/MnTe superlattices: A structural and optical study
Authors: Hartmann J. M. , Kany F. , Charleux M. , Samson Y. , Rouvière J. L. , Mariette H.
Published: July 2002

Atomic layer epitaxy (ALE) is investigated together with conventional molecular beam epitaxy (MBE) for the growth of CdTe/MnTe superlattices. A systematic structural and magneto-optical study demonstrates that: (i) all Mn atoms incident on the surface get incorporated; however, when a quantity superior or equal to 1 monolayer of Mn is sent onto the surface per ALE cycle, the growth front roughens, leading to the formation of MnTe islands, (ii) optimized atomic layer epitaxy allows us to obtain at 280 °C CdTe/MnTe superlattices with a better control than in conventional MBE, but does not prevent the exchange between Cd and Mn atoms from occurring at the interfaces, (iii) low temperature ALE (200 °C and lower) seems to be a promising way of obtaining more abrupt interfaces. A precise value of the ratio of the elastic coefficients 2c12/c11 is otherwise inferred through this study for zincblende MnTe (1.12).
Electron spin resonance study of aquated hexachloro-rhodate complexes in AgCl microcrystals
Authors: Vercammen H. , Ceulemans T. , Schoemaker D. , Callens F. , Vandenbroucke D.
Published: July 2002

The electron and hole trapping and detrapping properties of three types of rhodium centers were determined by electron spin resonance in Rh3+-doped AgCl microcrystals. Three paramagnetic Rh2+ centers were identified after ultraviolet irradiation: RhCl64−, and the singly or doubly aquated RhCl5(H2O)3− and RhCl4(H2O)22−. All three defects have a 4d7 orbital ground state in a strong (distorted) octahedral field. The RhCl64−, RhCl5(H2O)3−, and RhCl4(H2O)22− centers act as deep electron traps (lifetime at 300 K greater than 50 min) and as recombination centers. These properties of the rhodium centers are consistent with photographic experiments.
Optical properties of InxGa1−xN alloys grown by metalorganic chemical vapor deposition
Authors: Shan W. , Walukiewicz W. , Haller E. E. , Little B. D. , Song J. J. , McCluskey M. D. , Johnson N. M. , Feng Z. C. , Schurman M. , Stall R. A.
Published: July 2002

We present the results of optical studies of the properties of InxGa1−xN epitaxial layers (0<x<0.2) grown by metalorganic chemical vapor deposition. The effects of alloying on the fundamental band gap of InxGa1−xN were investigated using a variety of spectroscopic techniques. The fundamental band-gap energies of the InxGa1−xN alloys were determined using photomodulation spectroscopy measurements and the variation of the fundamental band gap was measured as a function of temperature. The effects of pressure on the band gap for InxGa1−xN samples with different alloy concentrations were examined by studying the shift of photoluminescence (PL) emission lines using the diamond-anvil pressure-cell technique. The results show that PL originates from effective-mass conduction-band states. Anomalous temperature dependence of the PL peak shift and linewidth as well as the Stokes shift between photoreflectance and PL lines is explained by composition fluctuations in as-grown InGaN alloys.
Silicon interface layers at GaAs/AlGaAs heterojunctions
Authors: De Franceschi Silvano , Altomare Fabio , Beltram Fabio , Lazzarino Marco , Sorba Lucia , Franciosi Alfonso
Published: July 2002

The tunability of the band offset in buried GaAs/AlGaAs(001) heterojunctions through insertion of Si interface layers is tested by means of transport techniques. We find that Si diffusion and segregation play a dominant role in spite of the relatively low growth temperatures employed (500 °C), so that band-offset modification will be hindered in practical devices.
Bonding at the CdSe/SiOx (x=0,1,2) interfaces
Authors: Masson D. P. , Landheer D. , Quance T. , Hulse J. E.
Published: July 2002

The chemistry occurring at the CdSe/Si, CdSe/SiO, and CdSe/SiO2 interfaces was investigated by looking at very thin tapered films (0–10 nm) of thermally evaporated CdSe with x-ray photoelectron spectroscopy. The analysis of the attenuation of the x-ray photoelectron signals along the tapered film was used to measure the electron mean free paths in as-deposited CdSe. The electron mean free path was found to increase with the photoelectron energy from 1.5 nm at 720 eV to 2.3 nm at 1200 eV. Our data suggest an island growth mechanism for CdSe on the Si substrate and a more uniform growth on silicon oxide. In the early growth, Se is first adsorbed on the surface creating sites where Cd subsequently adsorbs. Interdiffusion is observed for CdSe on the Si and SiO substrates after a vacuum anneal at 390 °C. The main result of this interdiffusion process is the formation of Si–Se bonds. Similar interdiffusion processes on thermal SiO2 substrates are expected but were too small to be detected.
Subpicosecond hot carrier cooling in amorphous silicon
Authors: White J. O. , Cuzeau S. , Hulin D. , Vanderhaghen R.
Published: July 2002

Measurements of the cooling rate of hot carriers in amorphous silicon are made with a two-pump, one-probe technique. Pump photons at 2 eV create free carriers and pump photons at 1.42 eV heat the carriers up to 1.2 eV/pair. The experiment is simulated with a rate-equation model describing the energy transfer between a population of hot carriers and the lattice. An energy transfer rate proportional to the temperature difference is found to be consistent with the experimental data. An energy transfer rate independent of the temperature difference is inconsistent with the data. This contrasts with the situation in crystalline silicon and GaAs. The measured cooling time, 0.2 ps, is sufficient to explain the absence of avalanche effects in amorphous silicon at fields below 106 V/cm.
Operation and design of metal-oxide tunnel transistors
Authors: Rendell R. W. , Buot F. A. , Snow E. S. , Campbell P. M. , Park D. , Marrian C. R. K. , Magno R.
Published: July 2002

The current–voltage (I–V) characteristics in the ballistic limit of metal-oxide tunnel transistors are calculated as a function of temperature, potential barrier height, gate insulator thickness, aspect ratio, and oxide-channel shape. The saturation (‘knee’) point and three modes of current transport across the device are discussed. For a given aspect ratio, the output impedance improves with increase in tunnel-oxide width, accompanied by slight decrease of gate transconductance. The net result is a significant improvement in the transistor gain. The gate transconductance improves with decrease in gate-insulator thickness, while approximately maintaining the output impedance. The net result is also a significant improvement in the transistor gain. Thus for a given aspect ratio, further device optimization to increase the transistor gain can be carried out by either increasing the tunnel oxide width or decreasing the gate insulator thickness. In practice, one preferably does both. A numerical study of the device performance of tapered-oxide devices is undertaken. We find that uniform-oxide channel design is generally superior to tapered-oxide channel designs.
Micro-Raman and photoluminescence investigation of ZnxCd1−xSe thin film under high pressure
Authors: Li W. S. , Shen Z. X. , Shen D. Z. , Fan X. W.
Published: July 2002

Micro-Raman and photoluminescence (PL) studies of a ZnxCd1−xSe (x=0.68) thin film sample have been carried out under high pressure at room temperature. In the PL spectra, the PL energy gap shift exhibits sublinearity with pressure and a least-square fitting to the experimental data gives pressure coefficients of α=0.082 eV/GPa and β=−0.0052 eV/GPa2. The second-order pressure coefficient β of the energy gap obtained experimentally is anomalously large and its origin was explained by alloy potential fluctuation with increasing pressure. From the Raman spectra, the first-order pressure coefficient was also calculated using least-square fitting. The low energy tail of the longitudinal-optical phonon was found to develop with pressure and the line shape change with pressure is interpreted in terms of a “spatial correlation” model.
Noise characteristics of radio frequency sputtered amorphous silicon carbide films
Authors: Choi W. K. , Han L. J. , Chua L. G.
Published: July 2002

Noise measurements of radio frequency sputtered hydrogenated (a-SiC:H) and unhydrogenated (a-SiC) amorphous silicon carbide films have been carried out. Two Lorenztian components were found in the noise spectra of the a-SiC:H and a-SiC films. Discrete traps, created by the sputtering process, were suggested to be responsible for the Lorenztian spectra observed. As only a relatively small amount of hydrogenation was achieved in our a-SiC:H films, passivation of the discrete traps was therefore not significant. This accounts for the Lorenztian spectra of the a-SiC:H films. We propose that furnace annealing has reduced the discrete traps substantially so that only the 1/f noise was observed in the annealed samples.
Stress-induced alignment and reorientation of hydrogen-associated donors in silicon
Authors: Gorelkinskii Yu. V. , Nevinnyi N. N. , Abdullin Kh. A.
Published: July 2002

Electron paramagnetic resonance (EPR) studies have been made of the stress-induced alignment and the subsequent recovery of the double donor (AA1 EPR center) that is formed in float-zone silicon following hydrogen implantation and annealing for ∼20 min at a temperature above ∼300 °C. The obtained data compare with those of the thermal double donor (NL8 EPR center). The activation energy for atomic reorientation of the (HDD+) AA1 defect is (2.3±0.1) eV. The reorientation rate is greater than that of the (TDD+) NL8 defect formed in Czochralski Si by a factor of 104. Both centers have C2v symmetry and piezospectroscopic measurements reveal a large compressional strain along their [001] (three-axis) direction. However, in contrast to the NL8, the core of the AA1 defect produces also large compressional strain along its one-axis parallel to [110] direction. These data demonstrate unambiguously that the two centers have different molecular structures, in spite of their similar EPR spectra and electrical properties. It is suggested that the two centers have similar core structures (a 〈001〉-oriented self-interstitials complexes), while the outer shell structure incorporates hydrogen or oxygen atoms, respectively.
Stark ladders as tunable far-infrared emitters
Authors: Donovan K. , Harrison P. , Kelsall R. W.
Published: July 2002

A superlattice of GaAs/Ga1−xAlxAs quantum wells forms a Stark ladder under the influence of a perpendicular electric field. A two level incoherent emitter system, formed by radiative intersubband transitions between adjacent wells, is investigated as a tunable far-infrared radiation source. Intersubband transition rates are calculated at 4, 77, and 300 K for applied fields from 0 to 40 kV cm−1. It is shown that the quantum efficiency of the radiative emission reaches a maximum at low temperatures for a field of 32 kV cm−1. Under these conditions the emission wavelength is 38 μm with an estimated power output of 1.1 mW.
Crystallinity and magnetoresistance in La1−xCaxMnO3 thin films
Authors: Gillman E. S. , Li M. , Dahmen K.-H.
Published: July 2002

The crystallinity of La1−xCaxMnO3 (LCMO) thin films can be controlled by preparing the films on substrates with different lattice parameters. Closely lattice matched LCMO thin films on LaAlO3 (LAO) substrates exhibit a high degree of crystallinity and have magnetoresistance (MR) and MR ratio [R(H=0 T)-R(H=6 T)]/R(H=6 T) that is sharply peaked in the vicinity of the metal-insulator transition temperature (TMI). Notably, films grown on LAO(011) have TMI shifted up in temperature ∼20 K higher than the same films on LAO(001) substrates. LCMO films on Al2O3 and Y-ZrO2, that are not as closely lattice matched, have a lower degree of crystallinity, less sharply peaked MR and a nearly constant MR ratio below TMI over a broad temperature range.
Measurements of the absolute external luminescence quantum efficiency in ZnSe/ZnMgSSe multiple quantum wells as a function of temperature
Authors: Westphäling R. , Ullrich P. , Hoffmann J. , Kalt H. , Klingshirn C. , Ohkawa K. , Hommel D.
Published: July 2002

We report on a novel experimental setup using a miniature integrating sphere fitted into a cryostat to measure spectrally resolved the absolute external photoluminescence quantum efficiency ηlum in the temperature range from 6 K to beyond room temperature. Direct access to ηlum is achieved by an uncomplicated method to calibrate the spatially integrated emission spectra absolutely. The fraction of light coupled out of the sphere into the detection system is in good approximation independent of the emitted wavelength and the emission characteristics of the investigated samples, due to their smallness compared with the total sphere area. This fact is proved by theoretical calculations for the sphere throughput and the irradiance within the sphere. To demonstrate the capabilities of this setup, we investigate in high quality ZnSe/ZnMgSSe 10× multiple quantum wells, the temperature dependence of the quantum efficiency, simultaneously for the quantum well and the barrier luminescence. Within a thermal activation model, three processes are found to contribute to the thermal quenching of ηlum of the quantum well luminescence.
Fresnoite: A new ferroelectric mineral
Authors: Foster M. C. , Arbogast D. J. , Nielson R. M. , Photinos P. , Abrahams S. C.
Published: July 2002

Fresnoite, Ba2TiOSi2O7, has been predicted structurally to be a new ferroelectric. Detection of both ac and dc dielectric hysteresis verifies the prediction. The spontaneous polarization Ps estimated from the hysteresis at 1.2 MV m−1 ac is ∼0.2 C m−2 at 295 K, comparable with the minimum Ps observed in one-dimensional ferroelectrics. A reproducible calorimetric anomaly with entropy change 0.19(3) J mol−1 K−1 at 433(2) K in polycrystalline material coincides with a small dielectric and pyroelectric anomaly previously reported in single crystals; an entropy change ∼0.2 J mol−1 K−1 at 810(5) K also accompanies a dielectric anomaly observable in ceramic samples at 805(5) K. Both calorimetric anomalies are ∼60 K wide. Observation of dielectric hysteresis at 875 K shows that neither anomaly corresponds to the Curie temperature; both are likely associated with small changes in atomic position, not with symmetry changes. Melting onset in fresnoite is 1703(1) K with undercooling as deep as 435 K.
Magnetic properties of R2Co15Al2 compounds with R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm
Authors: Shen Bao-gen , Cheng Zhao-hua , Zhang Shao-ying , Wang Jing-yun , Liang Bing , Zhang Hong-wei , Zhan Wen-shan
Published: July 2002

An investigation of the structure and the magnetic anisotropy of R2Co15Al2 (R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm) compounds has been performed by means of x-ray diffraction and magnetization measurements. X-ray diffraction shows the prepared compounds to be single phase, having the hexagonal Th2Ni17-type structure for R=Y, Ho, Er, and Tm and the rhombohedral Th2Zn17-type structure for R=Ce, Pr, Nd, Gd, Tb, and Dy. Except for the Ce compound, the unit-cell volumes of R2Co15Al2 compounds decrease in accordance with the lanthanide contraction. Substitution of Al for Co in R2Co17 leads to a decrease of the saturation magnetization at 1.5 K and Curie temperature. The exchange-coupling constants JCo–Co and JR–Co have been calculated by using the method based on magnetic ordering temperature. It is found that the JR–Co has a small dependence on the R elements and is almost not affected by the Al substitution. The Ce compound is found to exhibit an anomalous lattice parameter and magnetic characteristic, which are relative to the mixed-valence behavior of the Ce ion. X-ray diffraction measurements on magnetically aligned R2Co15Al2 powders show that the compounds with R=Pr, Nd, Gd, Tb, Dy, and Ho have an easy-plane type of magnetic anisotropy, whereas the compounds with R=Y, Ce, Sm, Er, and Tm exhibit an easy-axis type of magnetic anisotropy at room temperature. The compounds R2Co15Al2 with R=Pr and Ho exhibit a spin-reorientation transition and the spin-reorientation temperature is found to be 531 and 431 K, respectively. A strong uniaxial anisotropy is observed in Sm2Co15Al2 compound with a magnetocrystalline anisotropy field of 84 kOe at room temperature.
Infrared vibration spectra of hydrogenated, deuterated, and tritiated amorphous silicon
Authors: Sidhu L. S. , Kosteski T. , Zukotynski S. , Kherani N. P.
Published: July 2002

This article presents infrared absorption data of amorphous silicon alloys in which the hydrogen isotopes deuterium and tritium have been substituted for hydrogen. Silicon–deuterium and silicon–tritium vibration frequencies are related to silicon–hydrogen vibration frequencies by simple mass relationships. The silicon–deuterium wagging vibration is broadened and blueshifted due to strong coupling to the amorphous silicon network vibrations.
LP
01
-mode output beam from a micro-sized hollow optical fiber: A simple theoretical model and its applications in atom optics
Authors: Yin Jianping , Zhu Yifu
Published: July 2002

We calculate electric field distributions of the output beam from a LP01 mode in a micro-sized hollow optical fiber under the Fresnel approximation and find that the output beam characteristics in free space is similar to that of a TEM01* doughnut laser beam. We propose a simple theoretical model, a modified TEM01* doughnut beam, to simulate the free-space output beam from the LP01 mode in hollow fibers, and discuss potential applications of the LP01-mode output beam in atomic guiding, funneling, cooling, and trapping. Our study shows that this model is in good agreement with the numerical results derived from the Fresnel diffraction theory (except near the weak first-order diffraction ring around the zero-order diffraction peak). The model can be conveniently used to calculate the optical potential, gradient force, light-induced acceleration, spontaneous-emission rate, and quantum tunneling loss in atomic guiding, funneling, and trapping with the LP01-mode output beam in free space. It can be also used to study the optical-potential evaporative cooling of cold atoms in a gravito-optical trap constructed by the LP01-mode output beam.
Low temperature absorption and luminescence of oligothiophene single crystals
Authors: Schön J. H. , Kloc Ch. , Laudise R. A. , Batlogg B.
Published: July 2002

The optical properties of high-temperature-phase α-quaterthiophene and α-hexathiophene single crystals were investigated by photoacoustic and photoluminescence spectroscopy at low temperatures. The absorption and fluorescence spectra of both materials consist of a series of similar well-resolved lines, which are explained by vibronic progression of the 0–0 transition. Five vibronic modes are identified and ascribed to the out of phase translation of adjacent monomer units, ring deformation, and ring breathing modes. The energies of the out of phase translation and the antisymmetric ring deformation are slightly smaller for the hexamer than for the tetramer, whereas the other modes were independent of the conjugation length. Only a small Stokes shift is observed in these high quality crystals (18 cm−1 for α-quaterthiophene and 10 cm−1 for α-hexathiophene) and is ascribed to a shallow defect state below the bottom of the excitonic band.
Raman, infrared and x-ray diffraction study of phase stability in La1−xBaxMnO3 doped manganites
Authors: Roy Chaitali , Budhani R. C.
Published: July 2002

Formation of different crystallographic phases and their characteristic Raman and infrared spectra are studied in the manganite system La1−xBaxMnO3 for x in the range of 0⩽x⩽1, and with variations in oxygen stoichiometry. Synthesis of the end member LaMnO3 in pure argon environment leads to the formation of Jahn–Teller distorted, antiferromagnetic orthomanganite. While the observed Raman modes in this compound are primarily due to internal vibrations of MnO6 octahedra, the infrared (IR) spectra show an absorption edge in addition to the IR active phonons. The oxygen rich LaMnO3 is rhombohedral and has fewer zero-wave-vector phonon modes. In the barium substituted compositions with 0<x⩽0.25, a single phase rhombohedral compound of decreasing rhombicity is formed. A further increase in x leads to the ideal cubic perovskite structure for which a factor group analysis yields no Raman active and three IR active phonons of F1u symmetry. The compound with x=0.35, shows faint Raman modes of hexagonal BaMnO3 (P63mc noncentrosymmetric group) whose presence is not seen in x-ray diffraction data. For x>0.35, all three techniques used here show the formation of a two-phase mixture of La0.65Ba0.35MnO3 and hexagonal BaMnO3. An attempt has been made to correlate the progressive increase in symmetry of the crystal structure, shift of IR absorption edge to lower energies, and disappearance of phonon modes with x and δ with the reported electrical and magnetic behavior of this hole-doped manganite.
A dual stage slider–suspension design for nanospaced recording
Authors: Liu Bo , Sheng Gang , Hua Wei , Yuan Zhimin , Cheng Ming , Liu Gui Rong
Published: July 2002

This article presents a novel design of dual stage slider suspension for 5–8 nm spaced recording. The primary slider-suspension design is the same as what has been used in the current hard disk drives. However, slider’s ABS is redesigned with two side channels and a V-shaped deflector to prevent airborne particles from entering the interface between read/write pad and disk surface. The secondary slider suspension is a W-shaped microstructure fabricated by semiconductor technology. Details of computer simulation and evaluations of the new design were conducted and are reported, including a flying performance study of the primary slider design, a particle trajectory study, suspension structure analysis, and the mechanical durability estimation of the read/write pad.
Thermally assisted resonant quantum tunneling of magnetization in Fe8 clusters
Authors: Zhang X. X. , Hernandez J. M. , del Barco E. , Tejada J. , Roig A. , Molins E. , Wieghardt K.
Published: July 2002

Magnetic ac susceptibility of randomly arranged crystallites (powder sample) and well-oriented Fe8 crystallites has been measured as a function of applied magnetic field, temperature, and frequency. The Fe8 clusters, made of eight iron ions and with formula [(tacn)6Fe8O2(OH)12]8+, have a ground state S=10. From the magnetization data M(H) with the dc field, H, applied parallel and perpendicular to the easy axis of the aligned sample and χ″(T) data for different frequencies, the anisotropy field Han∼5 T and energy barrier U/kB=26.9 K were obtained. Peaks in the curves of χ′(H) were clearly observed at fields Hn=nH0, with n=0,±1 for powder sample where H0=2.4 kOe; and n=0, ±1, ±2, for an oriented sample where H0=2.2 kOe. These peaks appearing at Hn=nH0 in the curves of χ′(H) can be well described by the thermally assisted resonant quantum tunneling of magnetization.
Measurement of the spin polarization of LaSrMnO
Authors: Osofsky M. S. , Nadgorny B. , Soulen R. J. , Broussard P. , Rubinstein M. , Byers J. , Laprade G. , Mukovskii Y. M. , Shulyatev D. , Arsenov A.
Published: July 2002

A new method for determining the transport spin polarization, point contact tunneling from a low temperature superconductor into a ferromagnet, is used to determine the spin polarization of several LaSrMnO thin films and crystals. The Andreev process and its utility in measurements of spin-polarization are described. Preliminary results for the spin polarization of LSMO are presented.