bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo simulation of electron transport in alternating-current thin-film electroluminescent devices
Authors: Bhattacharyya K. , Goodnick S. M. , Wager J. F.
Published: July 2002

An ensemble Monte Carlo simulation of electron transport in bulk ZnS at different electric fields is presented. Scattering mechanisms associated with polar optical phonons, acoustic phonons (through deformation potential coupling), intervalley scattering, and impurities (neutral and ionized), are included in a nonparabolic multivalley model. Simulation indicates that the polar optical phonon and intervalley scattering mechanisms are dominant, whereas neutral and ionized impurity scattering are of no significance in determining the high-field electron transport in bulk ZnS. The simulated results show that approximately 26% of the electrons possess total energies exceeding 2.1 eV, the threshold energy for Mn impact excitation, at an electric field of 1 MV/cm. This fraction of electrons with energies exceeding 2.1 eV is estimated to be 50% and 65% at electric fields of 1.5 and 2.0 MV/cm, respectively. Transient overshoot effects are found to be of negligible importance in the operation of alternating-current thin-film electroluminescent (ACTFEL) devices. The steady-state electron distribution at high fields is sufficiently energetic to explain the observed efficiency of ACTFEL devices. No evidence for a significant electron population with energies in excess of 5 eV is found, even during the brief nonstationary regime, and thus very few carriers possess sufficient energy to induce band-to-band impact ionization.
Mechanical alloying process of Fe-Cr powders studied by magnetic measurements
Authors: Koyano T. , Takizawa T. , Fukunaga T. , Mizutani U. , Kamizuru S. , Kita E. , Tasaki A.
Published: July 2002

A mechanical alloying process for a mixture of elemental Fe and Cr powders with the Cr compositions 20–70 at. % was investigated through the measurements of x-ray diffraction, magnetization, and 57Fe Mössbauer spectrum. We show that magnetic studies provide more detailed information about the alloying process occurring during ball milling than the conventional diffraction techniques in this particular system. A final product after ball milling was identified as a high-temperature phase of the α solid solution, regardless of compositions studied. Powders subjected to milling in Ar gas atmosphere for 200 h were further ball milled in N2 atmosphere. The presence of N2 gas has caused a partial amorphization. The amorphous phase thus produced is found to be paramagnetic at room temperature.
Transport mechanism in semiconducting glassy silicon vanadates
Authors: Ghosh A.
Published: July 2002

Measurements are reported for the electrical conductivity of the semiconducting glassy silicon vanadates in the temperature range 80–400 K. The experimental results have been analyzed with reference to the existing theoretical models of hopping conduction. The analysis shows that at high temperatures the conductivity data are consistent with Mott’s model [J. Non-Cryst. Solids 1, 1 (1968)] of phonon-assisted polaronic hopping, while Mott’s variable range hopping model [Philos. Mag. 19, 835 (1969)] is valid at low temperatures. The small polaron model of Schnakenberg [Phys. Status Solidi 28, 623 (1968)] is also found to be consistent with the temperature dependence of the conductivity. The temperature dependence of the conductivity can also be interpreted in the framework of the percolation model proposed by Triberis and Friedman [J. Phys. C 18, 2281 (1985)]. The analysis of the conductivity data also indicates that the hopping in these vanadate glasses occurs in the nonadiabatic regime.
Radio frequency pumped superconducting quantum interference devices with two quantization loops
Authors: Krivoy G. S. , Koch H.
Published: July 2002

Conventional rf SQUIDs (superconducting quantum interference devices) consist of a superconducting loop interrupted by one weak link, and when read out via a tank circuit they produce output signals in the order of a few tens of microvolts. When the single weak link is replaced by a dc SQUID (a superconducting loop with two junctions) a so-called double quantization loop SQUID is formed. In this paper it is shown theoretically, and supported experimentally with thin-film devices of three different designs, that the general behaviour can be explained by a simplified picture of the fixed critical current of the single junction of the rf SQUID being replaced by an effective critical current of a dc SQUID loop that is varied by the applied magnetic flux (i.e., we have a rf SQUID with a variable β parameter). A more detailed analysis of the experimental data and the theory of this novel device reveals its interesting nonlinear character and shows the deviations from this simple picture. It is demonstrated experimentally that output signals in the order of ≥1 mV can be obtained with a suitable read-out mode for the tank circuit, a clear advantage when compared to conventional rf SQUIDs. With respect to dc SQUIDs, the coupling to integrated planar input coils will be unproblematic since no microwave resonances should occur in the structures.
A combined plasma-surface model for the deposition of C:H films from a methane plasma
Authors: von Keudell A. , Möller W.
Published: July 2002

The deposition of C:H layers by an electron-cyclotron-resonance plasma from methane was investigated. C:H was deposited at a methane pressure of 1.6 Pa and a substrate temperature between room temperature and 700 K. The film composition, morphology, and structure were investigated by high-energy ion beam analysis and scanning electron microscopy. A combined plasma-surface model for thin-film deposition is proposed, which includes the electron-induced dissociation of methane in the plasma and a growth model. The dominant reactions for film growth are the adsorption of the radical CH3, the direct incorporation of the ions, and the etching reactions with atomic hydrogen from the plasma. A consistent description for the deposition of hydrocarbon layers emerges. It compares favorably with measurements on the temperature dependence of the film growth and the influence of variable gas flow through the reactor on the growth rate and the film morphology.
High-resolution magneto-optic measurements with a Sagnac interferometer (invited)
Authors: Kapitulnik A. , Dodge J. S. , Fejer M. M.
Published: July 2002

A technique for measuring the Faraday effect and the magneto-optic Kerr effect has been developed. In a Sagnac interferometer, two optical beams follow identical paths in opposite directions. Effects which break time-reversal symmetry, such as magneto-optic effects, will cause destructive interference between the two beams. By measuring the phase shift between circular polarization states reflected from a magnetized sample, the polar magneto-optic Kerr effect is measured to an accuracy of 3 μrad, with a spatial resolution of 2 μm. The interferometric technique provides a number of advantages over conventional polarizer methods, including insensitivity to linear birefringence, the ability to completely determine the magnetization vector in a region, and the ability to sensitively measure magneto-optic effects without an external field. It is also shown that this device has great potential if incorporated into a near-field optical device. Some of the considerations for the design of a near-field Sagnac magneto-optic sensor are introduced and the advantages of the device are discussed. Some preliminary experiments are shown.
Surface recombination velocity measurements at the silicon–silicon dioxide interface by microwave-detected photoconductance decay
Authors: Stephens A. W. , Aberle A. G. , Green M. A.
Published: July 2002

This article presents measurements of the effective surface recombination velocity Seff at the Si–SiO2 interface of thermally oxidized p-type silicon wafers as a function of carrier injection level. The experiments cover a large range of injection levels and substrate resistivities, using the ‘‘microwave-detected photoconductance decay’’ method. A minimum in Seff has been observed experimentally. The experimental results for Seff are compared with calculations based on an extended Shockley–Read–Hall formalism which includes surface band bending effects due to oxide charges.
Loss mechanisms in polyimide waveguides
Authors: Kowalczyk T. C. , Kosc T. , Singer K. D. , Cahill P. A. , Seager C. H. , Meinhardt M. B. , Beuhler A. J. , Wargowski D. A.
Published: July 2002

Waveguide losses in thin film polyimides using waveguide loss spectroscopy and photothermal deflection spectroscopy as a function of cure cycle and structure were studied. Fluorinated sidegroups on the polyimide backbone lead to decreases in birefringence and absorption. The primary waveguide loss mechanism is absorption, not scattering. Waveguide losses as low as 0.4 dB/cm at 800 nm have been measured. Losses as low as 0.3 dB/cm at 1300 nm can be inferred from the photothermal deflection spectroscopy.
Optical excitation and thermal recovery of the 78 meV/203 meV acceptors in GaAs
Authors: Suezawa Masashi , Sumino Koji
Published: July 2002

Optical excitation spectra and thermal recovery after excitation of the 78 meV/203 meV acceptors in heat-treated GaAs doped with boron (B) and aluminum were determined by measurements of optical absorption of 78 and 203 meV acceptor states and of optical absorption due to localized vibration of antisite B. Antisite B was generated due to annealing of B-doped GaAs at high temperatures. The 203 meV band in the specimen annealed at 1200 °C was found to have two threshold energies, i.e., 0.76 and 1.14 eV. The 78 and 203 meV bands in the specimen annealed at 600 °C were found to have three threshold energies, i.e., 0.52, 0.80, and 1.08 eV. The magnitude of these energy levels was interpreted to be above the top of valence band. Defects responsible for these energy levels were generated due to annealing of specimens. Thermal recovery of optically excited defects was also investigated. Both acceptor states were found to recover between 50 and 150 K, and the 203 meV acceptor state was found to recover between 150 and 220 K in GaAs annealed at 1200 °C. Both acceptor states recovered between 30 and 100 K in GaAs annealed at 600 °C. Activation energies of recovery stages of 150–220 K in a specimen annealed at 1200 °C and 30–100 K in a specimen annealed at 600 °C were determined to be 0.58 eV and 46 meV, respectively.
Ultraviolet light induced annihilation of silicon dangling bonds in hydrogenated amorphous silicon nitride films
Authors: Warren W. L. , Seager C. H. , Kanicki J. , Crowder M. S. , Sigari E.
Published: July 2002

We report results of electron paramagnetic resonance, photothermal deflection spectroscopy, and capacitance-voltage measurements on amorphous hydrogenated silicon nitride (a-SiNx:H) thin films exposed to ultraviolet (UV) illumination. It has been previously shown that exposure to UV light activates silicon dangling-bond defects, i.e., K0 centers, in a-SiNx:H thin films. Here, we demonstrate that the initially UV-activated K0 center can be irreversibly annihilated at long illumination times. Because this effect seems to scale with H content of the measured films, we propose that hydrogen may be passivating the K0 defects during the extended UV exposure. We also show that films subjected to long UV exposures trap charge as efficiently as those having much larger K0 concentrations. A few possibilities to explain this effect are discussed.
Effects of donor doping on deep traps in In0.5Ga0.5P grown by liquid phase epitaxy
Authors: Kwon Ho Ki , Kwon S. D. , Choe Byung-Doo , Lim H.
Published: July 2002

The properties of deep traps in undoped, Si-, S-, Se-, and Te-doped In0.5 Ga0.5 P layers grown on GaAs substrates by liquid phase epitaxy are investigated by deep level transient spectroscopy, thermally stimulated capacitance, and Hall measurements. Only one kind of deep trap is observed in undoped layer. Among the doped layers, it is only in the S-doped layer that the deep trap concentration is increased and the persistent photoconductivity is observed. Furthermore, the deep trap properties in undoped and S-doped layers are nearly the same. Considering the amount of residual S atom in the undoped layer, it is suggested that the deep trap in the undoped layer may result from the residual S impurity.
On the data analysis of light-biased photoconductance decay measurements
Authors: Aberle Armin G. , Schmidt Jan , Brendel Rolf
Published: July 2002

The use of bias light is common practice today in photoconductance decay (PCD) measurements to analyze semiconductor samples with injection-level dependent recombination parameters (i.e., surface recombination velocity and/or bulk lifetime). Recently, it has been shown on theoretical grounds that the previously reported recombination parameters from light-biased PCD experiments are not the actual properties of the investigated sample, but so-called differential recombination parameters [R. Brendel, Appl. Phys. A 60, 523 (1995)]. In the present article the theory relevant to light-biased PCD measurements is discussed in detail and subsequently applied to monocrystalline silicon wafers with nitride and oxide passivated surfaces in order to verify the deviations between the differential and actual surface recombination velocities. Special emphasis is paid to the experimental fact that the injection level cannot be reduced below a minimum value due to signal-to-noise problems.
Sensitivity analysis for the determination of recombination parameters in Si wafers using harmonic carrier generation
Authors: Schönecker A. , Eikelboom J. A. , Burgers A. R. , Lölgen P. , Leguijt C. , Sinke W. C.
Published: July 2002

The sensitivity of a contactless measurement procedure to determine the minority-carrier recombination parameters in Si wafers is analyzed. The measurement is based on the harmonic generation of excess carriers, whose time dependence is measured either by μ-wave reflection or free-carrier absorption. Both the measurement equipment and the model used to extract the surface recombination velocity and the bulk carrier lifetime are introduced and the sensitivity of the parameter fit procedure is examined. The results of this uncertainty analysis are applied to typical experimental situations, e.g., bulk minority carrier lifetime measurement of a monocrystalline wafer with equal surfaces or a bulk minority carrier lifetime mapping done on an as-grown multicrystalline Si wafer before it obtained saw damage removal. The method proves to be very useful, if the interpretation of the results is carefully done and an appropriate experiment is chosen to produce the best possible results.
Structural investigation of two carbon nitride solids produced by cathodic arc deposition and nitrogen implantation
Authors: Merchant A. R. , McCulloch D. G. , McKenzie D. R. , Yin Y. , Hall L. , Gerstner E. G.
Published: July 2002

Carbon nitride materials have been the focus of research efforts worldwide. Most materials studied have been amorphous, with only a few groups claiming to have found a crystalline material. We investigate the structure of amorphous carbon nitride solids produced by two different techniques: cathodic arc deposition and high dose nitrogen implantation of glassy carbon. Transmission electron microscopy, electron energy loss spectroscopy, energy filtered electron diffraction, Rutherford backscattering, infrared, and Raman spectroscopy are all used to derive structural information. It was found that the carbon nitride materials prepared by both techniques have similar structures and [N]/[C] ratios (0.3). The materials appear to have a primarily sp2 bonded carbon structure with a smaller bond length than found in amorphous carbon. This is explained by nitrogen substituting into ‘‘rings’’ to a saturation level of about one nitrogen per three carbon atoms. We have found no evidence for a C3N4 crystalline structure, nor any amorphous derivative of it.
Interlayer exchange coupling in amorphous/crystalline NiFe2O4 thin-film bilayers
Authors: Korenivski V. , van Dover R. B. , Suzuki Y. , Gyorgy E. M. , Phillips J. M. , Felder R. J.
Published: July 2002

Amorphous/crystalline bilayers of NiFe2O4 exhibit interlayer magnetic exchange coupling, which results from an interaction between a spin-glass material and a ferrimagnetic material. The observed effect is reminiscent of the well-known exchange coupling effect between an antiferromagnet and a ferromagnet, which is widely used in applications where field biasing of thin magnetic films is desirable.
Characteristics of condensed gas moderators for the generation of very slow polarized muons
Authors: Morenzoni E. , Prokscha Th. , Hofer A. , Matthias B. , Meyberg M. , Wutzke Th. , Glückler H. , Birke M. , Litterst J. , Neidermayer Ch. , Schatz G.
Published: July 2002

Motivated by the possibility of using condensed gas moderators to produce very slow (epithermal) polarized muons we have studied the moderator properties of rare gas solids and solid N2 thin films as a function of the growing parameters (deposition temperature and rate, heat treatment) and of the film thickness. For Ar and Kr moderators the moderation efficiency is found to depend on the growth temperature and an annealing effect is observed. The dependence can be interpreted in terms of the changes in granularity and porosity of the condensed gas layers when the growth parameters are varied. From the thickness dependence of the moderation efficiency the escape depth of the very slow muons was determined. Its large value indicates a suppression of electronic energy loss mechanisms for the epithermal muons. The time dependent decay properties of the moderators were also investigated and found to depend solely on the rest gas pressure. At a pressure of 10−10 mbar the moderation efficiency remains stable over a period of days.
Synthesis and optical properties of MoS2 and isomorphous nanoclusters in the quantum confinement regime
Authors: Wilcoxon J. P. , Newcomer P. P. , Samara G. A.
Published: July 2002

Highly crystalline nanoclusters of hexagonal (2H polytype) MoS2 and several of its isomorphous Mo and W chalcogenides have been synthesized with excellent control over cluster size down to ∼2 nm. These clusters exhibit highly structured, bandlike optical absorption and photoluminescence spectra which can be understood in terms of the band-structures for the bulk crystals. Key results of this work include: (1) strong quantum confinement effects with blue shifts in some of the absorption features relative to bulk crystals as large as 4 eV for clusters ∼2.5 nm in size, thereby allowing great tailorability of the optical properties; (2) the quasiparticle (or excitonic) nature of the optical response is preserved down to clusters ≲2.5 nm in size which are only two unit cells thick; (3) the demonstration of the strong influence of dimensionality on the magnitude of the quantum confinement. Specifically, three-dimensional confinement of the carriers produces energy shifts which are over an order of magnitude larger than those due to one-dimensional (perpendicular to the layer planes) confinement emphasizing the two-dimensional nature of the structure and bonding; (4) the observation of large increases in the spin-orbit splittings at the top of the valence band at the K and M points of the Brillouin zone with decreasing cluster size, a feature that reflects quantum confinement as well as possible changes in the degree of hybridization of the electronic orbitals which make up the states at these points; and (5) the observation of photoluminescence due to both direct and surface recombination. Several of these features bode well for the potential of these materials for solar photocatalysis.
Bipolar charge and current distributions in organic light-emitting diodes
Authors: Scott J. C. , Karg S. , Carter S. A.
Published: July 2002

The electron and hole charge distributions and the luminance profile in organic light-emitting diodes (OLEDs) depend upon the bulk properties of the emissive layer, as well as on the injection characteristics at the anode and cathode interfaces. We address the problem of separating the relative contributions of hole injection, electron injection, and recombination to the overall performance of single layer OLED devices. Using the approach of Parmenter and Ruppel [J. Appl. Phys. 30, 1548 (1959)], and including Langevin recombination, expressions are derived for the current–voltage and radiance–current dependencies in terms of electron and hole mobility, luminescence yield, and a “current balance” factor. When one carrier dominates the current flow, as in many practical cases, it is possible to obtain a simple asymptotic relationship which permits a test of the assumptions required to obtain the analytic solution. Experimental data from poly(2-methoxy-5(2′-ethyl)hexoxy-phenylenevinylene) diodes fabricated with various anode and cathode materials are evaluated in the context of this analytical approach.
Low-temperature refractive indices of Cd1−xMnxTe and Cd1−yMgyTe
Authors: André R. , Dang Le Si
Published: July 2002

The refractive indices of Cd1−xMnxTe and Cd1−yMgyTe (0<x<0.28 and 0<y<0.70) were determined by analyzing interference fringes in reflectivity spectra of thin films grown by molecular beam epitaxy. It is shown that their dispersions in the transparent region at 4 K are well described by a Sellmeier relation. These results have been applied to grow CdMnTe/CdMgTe Bragg mirrors incorporated in a II–VI microcavity operating in the strong exciton-photon coupling regime.
Study of defects in electron irradiated CuInSe2 by positron lifetime spectroscopy
Authors: Polity A. , Krause-Rehberg R. , Staab T. E. M. , Puska M. J. , Klais J. , Möller H. J. , Meyer B. K.
Published: July 2002

CuInSe 2 was studied in the as-grown state and after low-temperature (4 K) 2 MeV electron irradiation. The positron bulk lifetime of 235 ps was measured for the unirradiated sample. The positron bulk lifetime was theoretically calculated and is in good agreement with the experimental value. In addition, the defect-related lifetimes for mono-, di-, and trivacancies are theoretically determined. An increased average positron lifetime indicated after electron irradiation the appearance of open-volume defects, most probably of divacancy type. The disappearance of this defect was observed during annealing below 250 K. Other defects were formed leading to a divacancy signal at least stable up to 600 K in the temperature range above 450 K.
Anisotropic complex permittivity measurements of mono-crystalline rutile between 10 and 300 K
Authors: Tobar Michael Edmund , Krupka Jerzy , Ivanov Eugene Nicolay , Woode Richard Alex
Published: July 2002

The dielectric properties of a single crystal rutile (TiO2) resonator have been measured using whispering gallery modes. Q factors and resonant frequencies were measured from 300 to 10 K. Q factors as high as 104, 105, and 107 were obtained at 300, 80, and 10 K, respectively. Using the whispering gallery mode technique we have determined accurately the loss tangent and dielectric constant of monocrystalline rutile and obtained much more sensitive measurements than previously reported. We show that rutile exhibits anisotropy in both the loss tangent and permittivity over the range from 10 to 300 K.
Multilayer thermionic refrigerator and generator
Authors: Mahan G. D. , Sofo J. O. , Bartkowiak M.
Published: July 2002

A new method of refrigeration is proposed. Cooling is obtained by thermionic emission of electrons over periodic barriers in a multilayer geometry. These could be either Schottky barriers between metals and semiconductors or else barriers in a semiconductor superlattice. The same device is an efficient power generator. A complete theory is provided.
Probing local magnetic disorder by investigating spin dependent photoelectron scattering
Authors: Wende H. , Freeland J. W. , Chakarian V. , Idzerda Y. U. , Lemke L. , Baberschke K.
Published: July 2002

We present here systematic temperature dependent magnetic extended x-ray absorption spectroscopy (MEXAFS) measurements. This fine structure can be explained by the spin dependent scattering of the photoelectron and yields information about the local magnetic surroundings of the absorbing atom. We find a strong temperature dependence at temperatures far below the Curie temperature. This shows that the intensity of the MEXAFS signal is not only determined by the value of the magnetic moment but also by the relative vibrations of the backscattering atom to the absorbing atom.
The simulation of switching in polycrystalline ferroelectric ceramics
Authors: Hwang Stephen C. , Huber John E. , McMeeking Robert M. , Fleck Norman A.
Published: July 2002

A polarization switching model for polycrystalline ferroelectric ceramics has been developed. It is assumed that a single ferroelectric crystallite in a ceramic, which is subjected to an electric field and/or a stress, undergoes a complete polarization change and a corresponding strain change if the resulting reduction in potential energy exceeds a critical value per unit volume of switching material. The crystallite’s switch causes a change in the interaction of its field and stress with the surrounding crystallites, which is modeled by the Eshelby inclusion method to provide a mean field estimate of the effect. Thus the model accounts for the effects of the mean electric and stress fields arising from the constraints presented by surrounding crystallites as well as the externally applied mechanical and electrical loads. The switching response of the ceramic polycrystal is obtained by averaging over the behavior of a large number of randomly oriented crystallites. The model, along with the linear dielectric, elastic, and piezoelectric behavior of the material, is implemented in a computer simulation. A fit to experimental electric displacement versus electric field, strain versus electric field, and strain versus stress curves of a ceramic lead lanthanum zirconate titanate PLZT at room temperature is used to obtain material parameters. The model then successfully predicts the electric displacement and strain hysteresis loops for the PLZT under varying electric fields with a constant applied stress.
A combined x-ray specular reflectivity and spectroscopic ellipsometry study of CeO2/yttria-stabilized-zirconia bilayers on Si(100) substrates
Authors: Méchin L. , Chabli A. , Bertin F. , Burdin M. , Rolland G. , Vannuffel C. , Villégier J.-C.
Published: July 2002

The combination of x-ray specular reflectivity (XRSR) with spectroscopic ellipsometry measurements constitutes a new and useful tool for the characterization of thin films. We illustrate this statement with measurements made on CeO2/yttria-stabilized-zirconia (YSZ) bilayers deposited onto silicon substrates, CeO2 being on top of YSZ. CeO2/YSZ buffer layers are notably appropriate for the epitaxial growth of high temperature superconducting films or all sorts of oxide films on Si substrates. XRSR was used to measure the thickness of each layer of CeO2/YSZ bilayers deposited on silicon. The results showed remarkably well defined and numerous oscillations, thus revealing low interface and surface roughness. Using a modeling and fitting process with the measured data, we extracted an interface roughness between YSZ and CeO2 in the range of 0.2–0.5 nm and a CeO2 surface roughness of 1.9 nm (root mean square values). Moreover the simulation curve fitted very well if a thin amorphous SiO2 layer was assumed to lie at the interface between YSZ and Si. The optical properties of YSZ and CeO2 in the 0.25–1.7 μm wavelength range, which are strongly dependent upon the composition and deposition parameters, were determined by the combination of spectroscopic ellipsometry measurements with XRSR results, and comparisons with the literature were made. Overall our results showed very good quality of the CeO2/YSZ oxide bilayers grown on silicon substrates.
Compositional dependence of the giant magnetoresistance in FexRh1−x thin films
Authors: van Driel J. , Coehoorn R. , Strijkers G. J. , Brück E. , de Boer F. R.
Published: July 2002

In this article we report on the magnetic and transport properties of FexRh1−x thin films, prepared by evaporation in high vacuum, in the composition range 0.41<xFe<0.59. Upon annealing (at a temperature of 870 K or higher) a certain volume fraction transforms to the ordered CsCl-type (α′) FeRh phase. Close to room temperature this phase shows a hysteretic transition between the antiferromagnetic (AF) and the ferromagnetic (F) state for samples with xFe<0.5, which gives rise to a magnetoresistance (MR) effect. Although the magnetic transition was never found to be complete, it is shown that the full MR ratio can be obtained by extrapolation of the measured MR ratio as a function of the relative change of the magnetization at the transition. The AF→F transition is only observed for films with xFe<0.505±0.015, for which the α′ phase with this (fixed) composition is present together with a nonmagnetic Rh-rich fcc-type phase, as is shown from a combination of x-ray diffraction, Mössbauer spectroscopy, and magnetization studies. This observation, which was not expected from the phase diagrams available from the literature, can explain our finding that the full MR ratio observed for films in this compositional range is independent of the Fe content. The full MR ratio is 85±6% at room temperature, which is very close to the MR ratio observed for bulk FeRh, implying a high structural quality of the films grown. X-ray diffraction and transmission electron microscopy were used to quantify microstructural aspects such as the grain diameter and strain. The highest MR ratio actually observed is 58%, in a 4400 kA/m field at 275 K. The full MR increases to approximately 150% at 250 K, and, as estimated from the resistivity of F and predominantly AF samples with almost the same composition, to 680±100% at 4.2 K.
Upconversion lasing of a thulium-ion-doped fluorozirconate glass microsphere
Authors: Fujiwara Hideki , Sasaki Keiji
Published: July 2002

Blue and red upconversion lasing of a Tm3+-doped fluorozirconate glass microsphere was demonstrated. Tens-of-micrometer-sized, genuinely spherical glass particles were produced by a melting method with a burner. The single microsphere was pumped by a focused beam of a Nd:YAG laser operating at 1064 nm. Three-photon-excited lasing emission could be observed in 480 and 800 nm regions at room temperature, and their lasing thresholds were determined to be 20 and 5 mW, respectively. Applications of the microspherical upconversion laser to near-field scanning optical microscopy are also discussed.
Electron-beam irradiation-induced nonlinearity in silicate glass films and fabrication of nonlinear optical gratings
Authors: Nakanishi Makoto , Sugihara Okihiro , Okamoto Naomichi , Fujimura Hisashi , Egami Chikara
Published: July 2002

Electron-beam (EB) irradiation on rf-sputtered silicate glass thin films can produce second-order nonlinearity. EB-poled germanosilicate glass film shows a maximum nonlinear optical (NLO) coefficient of d33=0.33 pm/V. It is found that the nonlinearity is homogeneous throughout the film layer and depends on both the EB current and dose. Moreover, a NLO grating, i.e., χ(2) grating, is realized using a periodic EB writing technique.
Electronic states and effective negative electron affinity at cesiated p-GaN surfaces
Authors: Wu C. I. , Kahn A.
Published: July 2002

The electronic structure of, and the effects of cesium (Cs) and oxygen (O) adsorption on, the p-GaN(001) surface are investigated via photoemission spectroscopy. Bulk- and surface-sensitive photoemission measurements, and oxygen exposure of clean surfaces, demonstrate the existence of filled surface states which extend ∼0.6 eV above the valence band maximum. The valence band maximum measured after the removal of the surface states gives a downward band bending and electron affinity equal to 1.2±0.2 and 3.3±0.2 eV, respectively. The surface dipole layer induced by exposure to oxygen followed by Cs deposition lowers the vacuum level by 2.8±0.3 eV with respect to the valence and conduction band edges. Under these conditions, the vacuum level is approximately 0.7 eV below the conduction band minimum of the bulk, corresponding to the effective negative electron affinity at this surface.
Optimizing utilization efficiencies in electronegative discharges: The importance of the impedance phase angle
Authors: Entley W. R. , Langan J. G. , Felker B. S. , Sobolewski M. A.
Published: July 2002

We have investigated the operating conditions that result in the greatest utilization efficiencies (UEs) of NF3, CF4, and C2F6 in a capacitively coupled GEC reference cell. We have also independently measured the rf electrical characteristics and optical emission spectra of the plasmas. To avoid inadvertently attributing changes in the UE, discharge impedance, rf currents, or atomic emission intensities to parasitic losses in the matching network or rf delivery system, the rf generator was adjusted to ensure that the same amount of power was dissipated within each discharge. For the NF3 plasmas, argon was used as a diluent and both the NF3 concentration and reactor pressure were varied. For the CF4 and C2F6 based plasmas, the gas compositions were fixed (86 mol % CF4/O2 and 50 mol % C2F6/O2) and the reactor pressure was varied. The greatest NF3 UEs occurred within a narrow range of NF3 partial pressures. The greatest CF4 and C2F6 UEs occurred within a narrow range of reactor pressures. For all mixtures, operating conditions that yielded the highest UEs also yielded the brightest plasmas, the lowest impedance magnitudes, the greatest fraction of current flowing to the grounded electrode, and impedance phase angles within a narrow window centered near φpe=−40°. Within this region, plasma power is most efficiently utilized to dissociate the source gas and excite the atoms that emit light. Collapsed plasmas, observed for high pressure highly electronegative conditions, exhibited very low UEs. At optimal operating conditions the UE of the fluorinated source gases were found to decrease in the order: NF3>C2F6>CF4. The results of this study suggest that the baseline corrected fluorine atom emission intensity (703.7 nm), the magnitude of the discharge impedance, or phase angle of the discharge impedance could be monitored to determine the relative fluorinated source gas UE in an arbitrary plasma reactor as the operating conditions are varied. The concept of an ideal NF3 partial pressure could prove to be a useful strategy to prevent the formation of collapsed plasmas at high reactor pressures while maintaining high NF3UEs.
Metalorganic chemical vapor deposition of high mobility AlGaN/GaN heterostructures
Authors: Keller S. , Parish G. , Fini P. T. , Heikman S. , Chen C.-H. , Zhang N. , DenBaars S. P. , Mishra U. K. , Wu Y.-F.
Published: July 2002

In this article, we discuss parameters influencing (a) the properties of thin AlxGa1−xN layers grown by metalorganic chemical vapor deposition and (b) the electrical properties of the two-dimensional electron gas (2DEG) forming at the AlxGa1−xN/GaN heterojunction. For xAl>0.3, the AlxGa1−xN layers showed a strong tendency towards defect formation and transition into an island growth mode. Atomically smooth, coherently strained AlxGa1−xN layers were obtained under conditions that ensured a high surface mobility of adsorbed metal species during growth. The electron mobility of the 2DEG formed at the AlxGa1−xN/GaN interface strongly decreased with increasing aluminum mole fraction in the AlxGa1−xN layer and increasing interface roughness, as evaluated by atomic force microscopy of the surfaces prior to AlxGa1−xN deposition. In the case of modulation doped structures (GaN/AlxGa1−xN/AlxGa1−xN:Si/AlxGa1−xN), the electron mobility decreased with decreasing thickness of the undoped spacer layer and increasing silicon doping. The electron mobility was only moderately affected by the dislocation density in the films and independent of the growth temperature of the AlxGa1−xN layers at xAl=0.3. For Al0.3Ga0.7N/GaN heterojunctions, electron mobility values up to 1650 and 4400 cm2/V s were measured at 300 and 15 K, respectively.
Two-dimensional electron gases in Ga-face and N-face AlGaN/GaN heterostructures grown by plasma-induced molecular beam epitaxy and metalorganic chemical vapor deposition on sapphire
Authors: Dimitrov R. , Murphy M. , Smart J. , Schaff W. , Shealy J. R. , Eastman L. F. , Ambacher O. , Stutzmann M.
Published: July 2002

We report on the growth of nominally undoped GaN/AlxGa1−xN/GaN (x<0.4) high mobility heterostructures with N-face or Ga-face polarity on sapphire substrates by plasma-induced molecular beam epitaxy (PIMBE) and metalorganic chemical vapor deposition in order to study the formation and electrical transport properties of polarization induced two-dimensional electron gases (2DEGs). By depositing a thin AlN nucleation layer on the sapphire substrates before the growth of a GaN buffer layer by PIMBE, we were able to change the polarity of the wurtzite films from N to Ga face. The switch in the polarity causes a change in the sign of the spontaneous and piezoelectric polarization directed along the c axis of the strained AlGaN barrier. As a consequence the polarization induced 2DEG is confined at different interfaces in heterostructures with different polarities. The transport properties of the 2DEGs in Ga- and N-face heterostructures were investigated by a combination of capacitance–voltage profiling, Hall effect, and Shubnikov-de Haas measurements. Dominant electron scattering mechanisms are studied in order to provide the knowledge necessary for further improvements of the electron transport properties and performance of AlGaN/GaN based “normal” (based on Ga-face heterostructures) and “inverted” (based on N-face heterostructures) high electron mobility transistors.
Effect of coating thickness on thermal stresses in tungsten-coated optical fibers
Authors: Shiue Sham-Tsong , Lien Pin-Tzu , He J.-L.
Published: July 2002

This study investigates the effect of coating thickness on the thermal stresses in tungsten-coated optical fibers. Theoretical results indicate that the maximum normal stress in the tungsten coating decreases with increasing coating thickness. However, the maximum shear stress at the interface of the glass fiber and tungsten coating increases. Eight samples of tungsten-coated optical fibers with coating thicknesses of 58, 75, 101, 128, 158, 383, 557, and 1013 nm, respectively, are immersed in liquid nitrogen for one day. Experimental results show that thermal stresses will either break or delaminate the tungsten coating. The crack density decreases with increased coating thickness, while the delaminated area of tungsten coating increases. The theoretical results can explain the break and delamination of the tungsten coating in the optical fiber. To minimize the break and delamination of the tungsten coatings in the optical fibers, the optimal thickness of the tungsten coating is about 158 nm.
Eddy current losses in ferromagnetic laminations
Authors: Serpico C. , Visone C. , Mayergoyz I. D. , Basso V. , Miano G.
Published: July 2002

It is demonstrated through the comparison of analytical, numerical, and experimental results that the existence of excess eddy current losses can be explained by the peculiar nature of the nonlinear diffusion of electromagnetic fields in magnetically nonlinear laminations. The essence of this peculiar nature is that nonlinear diffusion occurs as inward progress of almost rectangular profiles of magnetic flux density of variable height. Approximating actual profiles of magnetic flux density by rectangular ones, the problem of nonlinear diffusion can be treated analytically by using a simple model. The accuracy and the limit of applicability of the rectangular profile model are discussed by comparing its predictions with finite elements numerical solutions of nonlinear diffusion equation as well as with experimental results.
Optical properties of partially damaged ion implanted SiC layers by use of effective medium models
Authors: Wendler E. , Peiter G.
Published: July 2002

6H-SiC was implanted at room temperature with B+ ions and an energy-dose program which results in almost homogeneously damaged layers of about 0.43 μm thickness. The damage concentration was detected by Rutherford backscattering spectrometry with channeling. Optical transmission and reflection spectra were measured in the photon energy range 0.5 eV⩽ℏω⩽3.2 eV. It is shown that effective medium approximations can be used to analyze the measured spectra assuming the implanted layer to consist of a physical mixture of crystalline and defective crystalline or of defective crystalline and amorphous material. The distribution of the amorphous volume fraction and that of defective crystalline material is determined by fitting of the measured optical spectra. The observed dose dependences of these volume fractions are successfully applied to study the mechanisms of damage formation.
Importance of indium tin oxide surface acido basicity for charge injection into organic materials based light emitting diodes
Authors: Nüesch F. , Forsythe E. W. , Le Q. T. , Gao Y. , Rothberg L. J.
Published: July 2002

The influence of the indium tin oxide (ITO) electrode surface acido basicity on organic electroluminescent device characteristics is studied. As measured by photoelectron spectroscopy, acid and base treatments produce large work function shifts of the oxide compared to standard ITO treated by an oxygen plasma or water. The current onsets for triphenyldiamine (TPD) single layer diodes sandwiched between ITO and a silver electrode are in qualitative agreement with the work function of the hole injecting oxide electrode. However, saturated photovoltage measurements on single layer diodes built on acid and water treated ITO disagree with the work function obtained from photoelectron experiments. This is explained by protonation of the diamine layer close to the electrode surface producing a charged double layer. It suggests that even acidic surface hydroxyl groups of the oxide electrode are able to protonate the diamine layer, explaining the importance of plasma cleaning to remove surface hydroxyls. The same ITO treatments are investigated in tris(8-hydroxyquinoline) (Alq) aluminum single layer diodes as well as hybrid TPD/Alq light emitting devices. Implications of metal–oxide surface acido basicity on electrode treatment and molecular material design are discussed.
An optical study of self-assembled InxGa1−xAs/GaAs quantum dots embedded in a two-dimensional electron gas
Authors: Ribeiro E. , Cerdeira F. , Brasil M. J. S. P. , Heinzel T. , Ensslin K. , Medeiros-Ribeiro G. , Petroff P. M.
Published: July 2002

We studied the low temperature (77 K) photomodulated reflection and transmission as well as the photoluminescence at 2.2 K of a self-assembled InxGa1−xAs quantum dot layer grown on a (100) GaAs substrate in the vicinity of a two-dimensional electron gas. The dot layer was grown without rotation of the substrate in order to achieve a gradual variation of the In concentration along the wafer diameter. This resulted in an increase in the density of quantum dots along the In concentration gradient, which is reflected in a characteristic dependence of the relative intensities of the spectral lines. A consistent assignment of the optical structure observed in all spectra leads to an estimate of the average value of the Fermi energy in the conduction band of the wetting layer (EF≃13.4 meV). The variation of this Fermi energy along the composition gradient can be obtained from the spectra, and an estimate of the gradient of the density of quantum dots along this direction can be made. A careful comparison of the variation of the critical energy of the different lines suggests that the average quantum dot size depends on the In molar fraction of the alloy, which is seen to vary more or less linearly across the wafer diameter.
Phenomenological scaling laws for ‘‘semidilute’’ macromolecule solutions from light scattering by optical probe particles
Authors: Phillies George D. J. , Ullmann Gregory S. , Ullmann Kathleen , Lin Thy-Hou
Published: July 2002

Polymer solution dynamics may be inferred from light scattering spectra of dissolved optical probe particles. We compare a variety of probes in solutions of several polymers. In the ‘‘overlapping’’ concentration/molecular weight regime, the Stokes–Einstein equation fails by up to a factor of 2, while the probe diffusion coefficient D follows a scaling law D/D0=exp(−aMγcνRδ) (c, M, and R are the polymer concentration, molecular weight, and the probe radius, respectively). Experimentally, γ=0.8±0.1, ν=0.6–1.0, and δ=−0.1 to 0, contrary to the theoretical predictions γ=0 and δ=1. With very high molecular-weight polymers, we observe a further ‘‘entangled’’ regime, characterized by huge (104) failures of the Stokes–Einstein equation and the appearance of ‘‘fast’’ modes in the scattering spectrum.
The use of van der Waals forces to orient chemical reactants: The H+CO2 reaction
Authors: Buelow S. , Radhakrishnan G. , Catanzarite J. , Wittig C.
Published: July 2002

The 193 nm excitation of the CO2 ⋅ HBr van der Waals complex causes HBr to dissociate rapidly, thereby promoting the endothermic reaction of atomic hydrogen with the adjacent CO2. The reactants are highly oriented by the CO2 ⋅ HBr equilibrium geometry.
A cluster Monte Carlo method for the simulation of anisotropic systems
Authors: Zannoni C.
Published: July 2002

A Monte Carlo method for studying anisotropic systems with a new type of boundary conditions is proposed. The missing interactions at the sample box surface are replaced by interactions with ghost molecules. The orientations of these ghost particles are sampled from a ‘‘least biased’’ singlet distribution obtained self-consistently from the order parameter inside the sample. The method is tested on the Lebwohl–Lasher model with 125 and 1000 particles. Results for the transition temperature and order parameters are in good agreement with those obtained from simulation of much larger systems. The method is fairly general and should be applicable in a variety of simulations. Here we show that it can also be used to study director fluctuations in nematogen models.
Quadratic configuration interaction. A general technique for determining electron correlation energies
Authors: Pople John A. , Head-Gordon Martin , Raghavachari Krishnan
Published: July 2002

A general procedure is introduced for calculation of the electron correlation energy, starting from a single Hartree–Fock determinant. The normal equations of (linear) configuration interaction theory are modified by introducing new terms which are quadratic in the configuration coefficients and which ensure size consistency in the resulting total energy. When used in the truncated configuration space of single and double substitutions, the method, termed QCISD, leads to a tractable set of quadratic equations. The relation of this method to coupled-cluster (CCSD) theory is discussed. A simplified method of adding corrections for triple substitutions is outlined, leading to a method termed QCISD(T). Both of these new procedures are tested (and compared with other procedures) by application to some small systems for which full configuration interaction results are available.